BAROMETRIC DISTILLATION and the PROBLEM of NON-CONDENSABLE GASES by Eiki Martinson

BAROMETRIC DISTILLATION AND THE PROBLEM OF NON-CONDENSABLE GASES by Eiki Martinson

A Thesis Submitted to the Faculty of The College of Engineering and Computer Science in Partial Fulﬁllment of the Requirements for the Degree of Master of Science

Florida Atlantic University Boca Raton, Florida December 2010

ACKNOWLEDGEMENTS

Thanks are due first to Michael Levine, originator of the Barometric Distillation concept, for providing generous financial support, energetic motivation, and a vivid example of success as a professional inventor. Once again I am indebted to my advisor, Dr. Daniel Raviv, who always keeps the faith, even when his students have lost it. My research partners Brandon A. Moore, Thomas J. Kelly, and John D. Morris poured out a flood of ideas, built sometimes improbable but always magnificent apparatus, and designed ingenious software and elegant experiments. This thesis would have been impossible without them. Finally, I apologize to the inhabitants of FAU’s Science and Engineering building, who tolerated with good humor a ragged crew of lunatics spilling 50 gallons of water at a time onto their floors. The mess is cleaned up for good this time, I promise.

iii ABSTRACT

Author: Eiki Martinson

Title: Barometric Distillation and the Problem of Non-Condensable Gases

Institution: Florida Atlantic University

Thesis Advisor: Daniel Raviv

Degree: Master of Science

Year: 2010

Barometric distillation is an alternative method of producing fresh water by desalination. This proposed process evaporates saline water at low pressure and consequently low temperature; low pressure conditions are achieved by use of barometric columns and condensation is by direct contact with a supply of fresh water that will be augmented by the distillate. Low-temperature sources of heat, such as the cooling water rejected by electrical power generating facilities, can supply this system with the latent heat of evaporation. Experiments are presented that show successful distillation with a temperature difference between evaporator and condenser smaller than 10. Accumulation of dissolved gases coming out of solution, a classic problem in low- pressure distillation, is indirectly measured using a gas-tension sensor. The results of these experiments are used in an analysis of the specific energy required by a production process capable of producing 15 liters per hour. With a 20 difference, and neglecting latent heat, this analysis yields a specific energy of 1.85 kilowatt-hour per cubic meter, consumed by water pumping and by removal of non-condensable gases.

iv DEDICATION

When I was very young my father brought home a complicated machine and let me take it apart, ensuring that one day I would end up here. This work is dedicated to him—and to my mother, for being patient with the wayward engineers in her life. Contents

1 Introduction 1 1.1 The Increasing Demand for Water ...... 2 1.2 Current Desalination Technology ...... 4 1.2.1 Multi-Stage Flash Distillation ...... 5 1.2.2 Vapor Compression ...... 6 1.2.3 Multi-Eﬀect Distillation ...... 8 1.2.4 Reverse Osmosis ...... 10 1.2.5 Electrodialysis ...... 12 1.2.6 Comparison of Current Technologies ...... 13 1.2.7 Use of Low-Availability Heat ...... 14

2 The Barometric Distillation Process 17 2.1 The Torricelli Column ...... 18 2.2 Temperature Maintenance ...... 22 2.3 Evaporator and Condenser Design ...... 25 2.3.1 Maximizing Surface Area ...... 25 2.3.2 Applying the Venturi Eﬀect ...... 28 2.4 The Problem of Non-Condensable Gases ...... 29 2.4.1 Methods of Gas Removal ...... 31 2.5 Sources of Heat ...... 34

v 2.6 Application of Alternative Energy ...... 36 2.7 Other Methods of Barometric Distillation ...... 37 2.7.1 Atwell’s Patent ...... 38 2.7.2 University of Florida Project ...... 39 2.7.3 Seawater Solar Barometric Distillation ...... 40 2.7.4 Low Temperature Thermal Desalination ...... 41

3 Theoretical Analysis 43 3.1 Vaporization ...... 43 3.2 Henry’s Law ...... 45 3.3 Energy Eﬃciency ...... 47 3.3.1 Cost of Supply Water Pumping ...... 48 3.3.2 Estimating Pipe Friction ...... 50 3.3.3 Cost of Non-Condensable Gas Removal ...... 52 3.4 Environmental Impact of Brine Discharge ...... 54 3.5 Availability of Waste Heat ...... 56

4 Experimental Apparatus 57 4.1 Mechanical System ...... 57 4.1.1 Torricelli Columns ...... 59 4.1.2 Heat Sources ...... 62 4.1.3 The Vapor Conduit ...... 62 4.1.4 Supply Water Injection ...... 65 4.1.5 Vacuum System ...... 68 4.2 Data Acquisition and Control ...... 69 4.2.1 Temperature ...... 73 4.2.2 Pressure ...... 73

vi 4.2.3 Flow ...... 74 4.2.4 Total Dissolved Gas ...... 74 4.2.5 Control Actuators ...... 75

5 Experiments and Results 78 5.1 Distillation Experiments ...... 79 5.2 Total Dissolved Gas Experiments ...... 81 5.2.1 Degassing and Regassing ...... 81 5.2.2 Step Response ...... 83

6 Analysis 86 6.1 Achievable Pumping Ratio ...... 86 6.2 TDG Sensor Characterization ...... 88 6.3 Non-Condensable Gas Extraction Rate ...... 90 6.4 Supply Water Pumping Eﬃciency ...... 92 6.5 Eﬃciency of Gas Extraction ...... 94 6.5.1 The Dissolved Gas Contribution of the Evaporator ...... 96 6.5.2 Rate of Water Vapor Extraction ...... 98 6.5.3 Properties of the Gas Mixture ...... 99 6.5.4 Vacuum Pump Power ...... 100

7 Conclusions 103 7.1 Future Work ...... 105

A Octave Source Code 119 A.1 Evaluation of the Colebrook Formula ...... 119 A.2 Determining the Pumping Ratio ...... 120 A.3 Comparing Step Responses ...... 121

vii A.4 Demonstrating Sensor Response to Input ...... 122 A.5 Calculating Pump Power Across a Range of Reinjection Depths . . . 123

viii List of Figures

1.1 Multi-Stage Flash Distillation ...... 5 1.2 Vapor Compression ...... 7 1.3 Multi-Eﬀect Distillation ...... 9 1.4 Reverse Osmosis ...... 12

2.1 The Torricelli Column ...... 18 2.2 Eﬀect of Temperature on Vapor Pressure of Water ...... 19 2.3 Distillation Using a Pair of Torricelli Columns ...... 20 2.4 Eﬀect of Salinity on Vapor Pressure of Water ...... 22 2.5 Some Methods of Increasing Surface Area ...... 26 2.6 Application of Eductor to Vapor Connection ...... 28 2.7 Temperature Dependence of Solubility ...... 30 2.8 Salinity Dependence of Solubility ...... 31 2.9 Direct Gas Extraction ...... 32

3.1 Control Volume for Pumping Energy Calculation ...... 49 3.2 Control Volume for Gas Extraction and Re-injection ...... 53

4.1 Top of Apparatus ...... 58 4.2 Bottom of Apparatus ...... 59 4.3 Foot Portion of Column and Reservoir, Condenser Side ...... 61 4.4 Condenser End of Vapor Conduit Equipped with Bubbler ...... 63

ix 4.5 Realization of Tilted Vapor Conduit with Heat Exchanger ...... 66 4.6 Vapor Conduit, Supply Manifolds, and Instrumentation ...... 67 4.7 Cabinet Housing Supply Pumps and Instrumentation ...... 70 4.8 Instrument Drawer Under Construction ...... 71 4.9 DC Power Drawer ...... 72 4.10 Detail of Instrument Drawer Showing Pump Relays ...... 76

5.1 Temperatures During Distillation ...... 79 5.2 Vacuum Pump Degassing Performance ...... 83 5.3 Step Response of Total Dissolved Gas Sensor ...... 85

6.1 Experimental Pumping Ratio ...... 88 6.2 Comparison of Experimental and Modeled Step Responses ...... 89 6.3 Proposed Original, Modeled, and Experimental Degassing Behavior . 91 6.4 Pump Power Required for Gas Reinjection ...... 101

x List of Tables

1.1 2005 Worldwide Desalination Capacity by Technology ...... 13 1.2 Performance of Operating Desalination Facilities ...... 14

3.1 Enthalpy of Vaporization for Water ...... 44 3.2 Henry’s Law Coefficients ...... 45 3.3 Sechenov Salt-Effect Coefficients for Aqueous NaCl ...... 46 3.4 Roughness Values for Typical Pipe and Duct Materials ...... 51 3.5 Discharge Salinity for Representative Values of R ...... 55

xi Chapter 1

Introduction

Earth’s supply of fresh water is one of the most fundamental limits to the growth of human civilization and prosperity. Nearly one billion people around the world are still without adequate sources of clean drinking water [1]. Most of them live in developing nations, where population growth will soon place even greater demands on water resources. Unfortunately, despite the enormous volume of water on the planet, accessible freshwater is comparatively rare: only 2.5% of water on earth is fresh, and of that two-thirds is frozen year-round and thus impractical for use [2]. Sea water is, of course, plentiful; moreover, the fastest growth in world population is projected to occur within 120 miles of a coastline, where over half of the world’s population already lives [3]. Here increasing demand coincides with limitless potential, suggesting that desalination technology will become a major contributor to water supply. However, existing desalination methods require too much energy to be cost- eﬀective in many markets. These technologies are also complex and require large capital investments, making them unsuitable for underdeveloped regions. This work explores an alternative approach to desalination, here referred to as Barometric Distillation because it employs the principle of operation used in the ﬁrst barometers as a way of achieving sub-atmospheric pressure, permitting distillation

1 at comparatively low temperature. With appropriate design this process should be able to produce water at many different scales, for large cities or rural villages, in the First World or the Third. Such systems may be configured to use waste heat from power plants and industrial processes, or low-temperature heat from other sources. It is relatively simple to operate and maintain and can be constructed from low-cost “off-the-shelf” components.

1.1 The Increasing Demand for Water

Global water demand has been increasing for two fundamental reasons: first, the rise in population, and second, an increase in water use per person (associated with economic growth) [4]. Over the course of the twentieth century, water demand increased at a rate approximately twice that of the population [5], and global population itself more than doubled in the latter half of the century alone [6]. These reasons combined produced a nearly 10-fold increase in water withdrawals worldwide from the year 1900 to 2000 [5]. There is little reason to believe that the rise in water consumption will cease in the near future. One study has predicted a worldwide increase in water demand for domestic purposes alone of almost 7% per capita over the next 15 years [7], the implications of which are made worse by the likelihood of population gain over the same period. Furthermore, human uses of water goes far beyond drinking and bathing; water is a indispensable input to the production of food, processing of raw materials of all kinds, and the manufacture of finished goods. Agriculture currently is responsible for 90% of total water consumption worldwide (although it causes only two-thirds of water withdrawals; the difference is explained by the relatively large amount of evaporative losses in agriculture relative to industrial or

2 domestic use) [4]. While it has become much more efficient—per capita use of water for food production was reduced by a factor of two from 1961 to 2001 despite increases in per capita food production itself—and further efficiency gains are by no means impossible—there remain 840 million undernourished people in the world; growing enough food to provide these people adequate levels of nutrition as well as supplying the projected increase in world population will likely require the development of additional water resources beyond contributions from increased irrigation efficiency [8]. Perhaps the greatest future pressure on world water supplies will come as a result of raising the inhabitants of the Third World to First World standards of living. Such an ambitious goal is made all the more daunting by the projection that the largest portion of world population growth is to occur in underdeveloped nations [9]; surely their soon-to-be-born citizens will want the same level of prosperity. If this project is to succeed, a much higher worldwide average of water use per capita will be necessary—in 1995, per capita domestic water use of underdeveloped countries was only 55% that of the developed world, to say nothing of water used for irrigation [7]. Although some of the shortfall can be supplied by improved use of existing groundwater, construction of dams and reservoirs, and wastewater recycling, desalination will have a role to play in the future. Already desalination produces more than 35 million cubic meters of water every day [10]. Growth in desalination capacity grew by roughly 7% per year from 2000 to 2005 [11], and one study has indicated that, in ten water-scarce countries, a further growth of 200% will be necessary by the year 2025 to meet domestic water needs alone, without increasing stress on groundwater resources [12].

3 1.2 Current Desalination Technology

Desalination technologies currently in use can be divided into two broad categories. The first of these is thermal desalination; these methods use heat transfer to accomplish a phase change. Most thermal processes produce water vapor by evaporating saline feedwater, leaving behind a concentrated salt solution; the vapor is condensed in such a way as to keep the condensate separate from the brine. Any process meeting this description can be referred to as distillation. An alternative type of thermal desalination, although one which has as yet not achieved commercial success, freezes rather than evaporates the feed water to achieve a similar separation of fresh product water and brine [13]. Distillation results in extremely pure water and is also effective in removing some impurities other than salt. Membrane techniques comprise another major category of desalination methods; these processes make use of some type of membrane or filter to separate fresh water from concentrated brine. The most successful of these technologies is Reverse Osmosis (RO), in which feed water is made to pass through a membrane impermeable to salt; this is similar to the process of osmosis which occurs in the cells of living creatures, but in the opposite direction. Although the water produced by RO is usually somewhat less pure than that produced by distillation, this process uses less energy per volume of product, a benefit that has lead to great succcess in the desalination marketplace. A brief discussion of the mature, competitive technologies used for desalting seawater and brackish water will give a baseline for comparison against the barometric distillation process proposed in this work. Although there exist many possible variations on each of these methods and some areas of potential complexity are neglected, the following discussion should convey some general understanding of the state of the art in the desalination field.

4 Vacuum system

Steam Heat input exchanger

Feedwater Return input to boiler

Condensate collection tray Vapor Fresh water output

Flash chambers Concentrated brine

Figure 1.1: Multi-Stage Flash Distillation (redrawn by the author from [15])

1.2.1 Multi-Stage Flash Distillation

Multi-Stage Flash distillation, or MSF, is one of the simplest and most widespread of thermal desalination methods. In this process (figure 1.1), preheated feedwater passes through a heat exchanger where it receives additional heat from steam extracted from a power plant turbine or produced in a boiler. This hot seawater is introduced to the first of several flash chambers in which reduced pressure causes it to flash into vapor, which comes into contact with tubes carrying relatively low-temperature water, transferring heat to them and thereby condensing. Liquid condensate drips from the tubes, is caught in a tray and extracted for use. The low-temperature water carried in the inside of the tubes is the original input feedwater to the MSF process; this is what causes that water to be preheated [14]. Liquid water remaining in the first flash chamber passes to a second one, in which it has another opportunity to evaporate as each subsequent stage is at lower pressure; the greater the number of stages, the better the efficiency of the overall process [15]. A significant benefit of MSF distillation over other thermal methods is the relative lack of scale formation on the heat-exchanging surfaces (in this case, the condensing

5 tubes). Scaling is a fouling phenomenon in which the mineral compounds of calcium carbonate, magnesium hydroxide, and calcium sulfate, commonly found dissolved in most feedwaters, crystallize onto heat transfer surfaces [16]. Although solubility of

solids generally increases with temperature, some salts (CaSO4 is the most troubling example) dissolved in water exhibit locally retrograde solubility, or solubility decreasing with increased temperature in some temperature range; these salts precipitate out of solution onto evaporator surfaces [17]. The layer of scale which is eventually deposited there has a high thermal resistance and can reduce flow rates as well, progressively ruining the effectiveness of the heat exchanger. In the case of MSF distillation, evaporation largely takes place in the volume of the flash chambers rather than on heat-transfer surfaces; scale precipitates in the chamber where it has a much lower impact on efficiency and is more easily removed. Unfortunately, this advantage, along with the relative simplicity and low capital cost of MSF, have not been enough to offset this method’s poor energy efficiency in comparison with newer technologies like RO. As a result MSF has lost its former dominance in the industry and is now mostly used in areas such as the Persian Gulf that enjoy cheap oil but suffer expensive water [14].

1.2.2 Vapor Compression

Vapor compression methods supply the heat necessary to evaporate feedwater by compressing the resulting vapor, rather than by heat transfer with process steam or some other outside source [18]. In the VC process, feedwater enters a pressure chamber, where it is sprayed onto a bank of tubes carrying higher-temperature water, causing it to evaporate. A compressor extracts the resulting water vapor from the chamber and introduces it into the inside of the evaporator tubes, where—now under increased pressure—it condenses, transferring the latent heat of condensation to the

6 Preheating Feedwater input

Vapor

Fresh water output Brine

Compressor (mechanical pump or steam ejector)

Figure 1.2: Vapor Compression (redrawn by the author from [15])

feedwater on the outside of the tubes. This condensate exits the pressure vessel and can preheat the initial feedwater before exiting the process as freshwater product [14]. A mechanical compressor or pump can supply compression in a VC process, in which case it is named Mechanical Vapor Compression, or MVC. Alternatively, a thermal source can provide the energy, with compression achieved using a steam ejector; such a process is known as Thermal Vapor Compression, or TVC [15]. A steam ejector is an example of an eductor, a device in which a moving fluid (the eductant) passes through some type of constriction or nozzle, gaining velocity but losing pressure in an example of the Venturi effect. A port connected to this constricted region provides suction and any fluid drawn into this port will be raised in pressure and mixed with the eductant [19]. In the case of TVC, steam from a boiler or power-plant turbine is used as the eductant, drawing vapor from the outside of the evaporator tubes, compressing it, and supplying it to the inside of those tubes along with the eductant steam. Part

7 of the resulting condensate returns to the original steam boiler to replace that used in the ejector; the remainder is the product of the TVC desalination system [18]. A potential application of eductor technology to the Barometric Distillation process, using liquid water instead of steam as an eductant, is described in section 2.3.2. MVC is currently one of the more efficient distillation methods, capable of producing water at a cost of 8-9 kWh/m3 [20]. Low-temperature operation is a practical benefit of MVC as this greatly reduces corrosion, allowing the use of lower cost metals in construction [14]; it also prevents a loss of efficiency due to fouling (these advantages will later be shown to exist for the Barometric Distillation process as well). However, MVC is not applicable to large scales at present; most operating plants are designed to produce less than 5000 m3/day [14].

1.2.3 Multi-Eﬀect Distillation

InMulti-Effect Distillation (MED, or alternatively MEE, for Multi-Effect Evaporation) water evaporates in multiple stages, with vapor from each stage condensing in the next stage, where it contributes its latent heat to produce more vapor for yet another stage [15]. As with MSF distillation, MED uses steam from a power plant or separate boiler as a heat source; this primary steam transfers heat to a flow of feedwater, condenses, and is returned to its source. The feedwater evaporates and flows into the second stage, where this vapor performs the same role as the primary steam did in the first stage. Depending on the difference in temperature between the available primary steam and the input seawater, this process can be continued for any number of additional stages, known as effects; practical implementations usually comprise between 10 and 16 effects [21]. Most operating examples of MED consist of a series of horizontal-tube falling-film heat exchangers. Vapor from the last effect enters the tubes and condenses on the

8 Vapor to next effect Steam ejector High-pressure steam (if available)

Vacuum system

Low pressure steam

Feedwater input Concentrated brine rejected Brine to Brine to previous effect previous effect Product water

Figure 1.3: Multi-Eﬀect Distillation [22]

inside, while brine from the next effect is sprayed onto the tubes from above, partially evaporating; the vapor flows into tubes of the next effect [23]. For the first effect, the input vapor is the primary steam, while vapor produced in the last effect enters a heat-rejection condenser, where input seawater is sprayed onto the tubes, condensing it; the remaining brine is then pumped to the penultimate effect to be evaporated. A vacuum system, either mechanical or using a steam ejector, removes any vapor produced in this final condenser, after which it can be used to preheat the feedwater before it enters the MED chamber (the preheater is not shown in figure 1.3) [14]. MED plants have in the past suffered from significant scaling problems on the surfaces of the hot tubes, a problem which resulted in the widespread adoption of MSF instead [15]. Since evaporation occurs on the outside of the tubes, scale is deposited there; this is particularly a problem as mechanical methods available for cleaning the inside of heat-exchanger tubes (such as circulating sponge-rubber balls through the tubes of an on-line MSF plant [24]) cannot be used in MED operations [25]. Research in this area is ongoing, but the magnitude of the scaling problem can be limited by a

9 combination of high wetting rates (high flow rates over the heat-transfer tubes) [23], use of polymeric anti-scaling additives, and limitation of evaporator temperature to no more than 70 [25]—a set of design improvements collectively referred to as Low-Temperature Multi-Effect Distillation, (the name of a proprietary process) [21]. Where higher-pressure steam is available—extracted from an intermediate stage of a turbine, for example, as is common for providing steam to various industrial processes—an MED system can be equipped with a steam ejector connected to the evaporator-condenser chambers to lower their pressure. The output of the ejector is introduced into the first effect as low-pressure steam would be in other MED plants; this technique is therefore a hybrid of the MED and TVC processes [21]. A further refinement of such hybrids is the substitution of turbine-driven compressors for the steam ejectors, which suffer from low adiabatic efficiency [26]. Multi-effect distillation (particularly the LT-MED variant) is one of the most promising thermal desalination methods, with overall energy efficiencies approaching those of reverse osmosis while enjoying superior product quality and somewhat more relaxed pretreatment requirements [21].

1.2.4 Reverse Osmosis

Reverse osmosis separates water from brine by elevating the pressure of saline feedwater beyond its osmotic pressure, which is directly related to the concentration of salt in the solution (among other factors) [15]. This high pressure causes water to move across a semi-permeable membrane in the opposite direction of natural osmosis, leaving salt ions behind [27]. RO requires pressures in the range of 50 to 80 atm for seawater input and 10 to 25 atm for brackish input; raising feedwater to such a pressure at acceptable ﬂow rates requires considerable power, but this pumping is the only signiﬁcant consumer of energy in the RO process [14].

10 In comparison with distillation techniques, reverse osmosis produces water of lower quality—higher in Total Dissolved Solids (TDS), a measure of salinity—but with greater energy efficiency. Operators of RO plants must often apply pre-filtering and various other treatments to feedwater since contaminants such as sand or biological organisms can greatly shorten the operating life of the membranes [15]. Higher salinity seawater requires higher operating pressure than brackish water in the case of RO, but salinity has little effect on the operation of thermal distillation plants (although it will result in faster scale formation and, therefore, increased maintainance costs). Despite these problems, the superior energy efficiency of RO has helped to earn it a dominant position in the present desalination market; although a mature technology, research into membrane improvements, energy recovery systems, and novel applications is ongoing, making RO a “moving target” for competing technologies. One important development behind the current success of reverse osmosis is the use of turbines and other methods to recover some of the energy consumed by the high-pressure pumps; much of this energy is still present in the rejected brine stream in the form of relatively high pressures and flow rates [28]. Numerous devices exist that can exploit this energy, from centrifugal designs such as turbines to techniques such as reciprocating or rotary work exchangers [29]. Energy recovery and other techniques are continuing to improve the specific energy consumption of RO facilities; the recently constructed SWRO plants at Ashkelon, Palmachim, and Hadera, all in

Israel, are designed to consume less than 4 kWh/m3 [30]. Some researchers are investigating the use of naturally-available pressure diﬀerences to drive membrane processes, as in the case of submarine reverse osmosis [31], which substitutes hydrostatic pressure found in the deep sea for that provided by pumps. A similar application is the joint Israel-Jordan Red-Dead Conveyer project, which aims to convey water from the Red Sea to the Dead Sea via canal and use the natural 422

11 Low-pressure pump Membranes Post-treatment

Feedwater input

Pre-treatment Energy recovery

Product water High-pressure pump Brine rejected

Figure 1.4: Reverse Osmosis (redrawn by the author from [14])

meters of elevation diﬀerence to provide high pressure seawater to a RO facility; the resulting brine would be rejected to the Dead Sea, replenishing its declining waters [32].

1.2.5 Electrodialysis

Electrodialysis is an alternative membrane technology using ion-exchange membranes to separate salts and other ionic components from water; in contrast with RO, ED achieves separation not by raising feedwater pressure but by applying a DC voltage across a stack of membranes, pulling anions and cations away from the pure water stream [33]. As with RO, the specific energy of desalination increases with salinity (higher voltage is required), but at an even less favorable rate; for this reason ED has only been cost effective for desalting brackish waters; despite some significant benefits over the RO process:

Membrane fouling is greatly reduced in ED due to the Electrodialysis Reversal (EDR) process, in which the electrical polarity across the stack is reversed periodically, switching the brine and pure-water channels and removing charged deposits [34].

Higher brine concentrations can be achieved [34], making ED suitable as part of a process to obtain dry salt as a valuable byproduct of desalination [35].

12 Table 1.1: 2005 Worldwide Desalination Capacity by Technology [10] Technology Fraction of Total Reverse Osmosis 46% Multi-Stage Flash 36% Vapor Compression 5% Electrodialysis 5% All others 5% Multi-Eﬀect Distillation 3%

Since water does not ﬂow through ED membranes, they can tolerate a greater amount of suspended solids such as sand (such particles are very destructive to RO membranes), reducing the need for pre-ﬁltration [15].

However, the unsuitability of the ED process for higher salinity feedwaters, as well as its inherent inability to remove anything but ionic species (micro-organisms, for instance, are not removed by ED) has limited its application. Although electrodialysis provides only 5% of all desalination capacity in the world, it represents 13.7% of brackish water desalination capacity [10] due to its advantages in that salinity range.

1.2.6 Comparison of Current Technologies

Two processes account for the great majority of currently operating desalination capacity: Multi-Stage Flash distillation and Reverse Osmosis; in recent years RO has taken a steadily increasing lead over MSF. A few other methods are also in practical use, as seen in table 1.2.6 showing the percentage of desalinated water produced by each of the major processes in 2005, worldwide. Performance characteristics, including energy per unit water and the product salinity, are provided for the same leading technologies in table 1.2.6; salinity of the feed water is also listed where available since product salinity and especially speciﬁc

13 Table 1.2: Performance of Operating Desalination Facilities Process Location Speciﬁc Energy Feed TDS Product TDS Reference (kWh/m3) (PPM) (PPM) LT-MED1 Marshall 2.0 — — [36] Islands LT-MED2 Tianjin, 4.55 — — [37] China MSF Kuwait 25.74 — — [38]

MVC Sardinia, 8.5 38 000 < 5 [39] Italy SWRO Ashkelon, < 3.9 40 679 < 80 [40] Israel EDR Tenerife, 0.9 1 200 300 [41] Spain 1 This facility uses waste heat from a diesel generator which is not included in the specific energy total. Cooling the engine in this way improves its thermal efficiency; this effect is not counted. 2 Steam is extracted from a power-plant turbine with the resulting loss of electrical power counted against efficiency.

energy (in the case of RO and ED) depend on this parameter. Data in the table comes from operating experience for practical desalination facilities and should not be taken as a theoretical maximum; better implementations of any of these processes may exist and many other practical examples may not be able to achieve the same performance, so this comparison should be taken only as a rough guide to what is possible with each method. Also, the product TDS parameter should not be taken as a limit of the technology in question—in most cases better water quality can be purchased at increased expense in energy or in additional treatment requirements.

1.2.7 Use of Low-Availability Heat

Steam power plants provided roughly 82% of world electricity generation in 2005 [42]. Whether combustion of fuels or nuclear ﬁssion is the primary source of energy, all

14 these facilities reject waste heat to the environment due to limitations imposed by the second law of thermodynamics. Typically this heat rejection is accomplished by evaporative means as with cooling towers or by heat transfer with seawater, in the case of coastal power plants. It is commonplace in the literature to propose taking advantage of this potential energy resource, no matter which desalination process is being investigated. Indeed, nearly all of the processes described above can benefit, in varying degree, from the application of such “free” but low-in-thermodynamic-availability heat sources. Waste heat can supply steam to thermal desalination plants using MSF, MED, or other processes. However, waste heat is waste for a reason; it is too low in temperature for the power plant to efficiently operate another steam cycle using it, in which case it is too low in temperature to be suitable without supplement for most of the major thermal desalination technolgies [14]. Integration of power and desalination facilities can mitigate this; for example, a power plant can release higher-temperature heat at some cost in efficiency, although the combined effiency of power and water production may be higher. Such optimization can be a political problem, however, in areas where water and electricity are supplied by independent authorities [14]. A different source of waste heat is the exhaust stream from a combustion process; these high-temperature gases can be made to exchange heat with seawater used as input to a distillation process. Combinations of diesel generators with MED desalination have provided cogeneration of electricity and fresh water on some Caribbean and Pacific islands for decades [43], [44]; in such installations water circulating through the cooling jacket of the diesel engine also recovers heat for desalination use. Exploiting waste heat in this way offers the extra benefit of increasing the thermodynamic efficiency of the engine itself; in some cases improving it from 40% to 80% [44]. Using flue gases from larger fossil-fuel electricity-generating plants has been

15 considered as well. The decreased temperature of the exhaust after the heat exchange is yet another advantage as it reduces evaporative losses of the cooling water used in some types of Flue Gas Desulfurization (FGD) scrubbers (which reduce sulfur dioxide emissions), saving this lost water as well as providing more water from the distillation process itself [45].

16 Chapter 2

The Barometric Distillation Process

Barometric distillation lowers the boiling temperature of water by introducing it into a low-pressure environment. This reduces the energy required to distill a particular volume of water for two reasons:

1. Water to be distilled will not have to be raised as high in temperature, although this is a minor beneﬁt as the majority of the heat needed to boil water, even when starting from standard temperature and pressure, is necessary to supply the latent heat of evaporation, not to raise the water to 100 .

2. Low-temperature boiling enables the exploitation of sources of heat normally rejected as waste in industrial processes like power generation, so that the cost of heat can be considered free when analyzing such a system; in other words it allows the use of energy from a lower-availability source.

One way of creating a low-pressure environment while providing a means for water to enter and exit that environment makes use of barometric pressure and gravity in a concept dubbed the “Torricelli Column” after Evangelista Torricelli, inventor of the

17 Low pressure

10 meters

Figure 2.1: The Torricelli Column barometer.

2.1 The Torricelli Column

The earliest type of barometer was made by ﬁlling a closed-bottom tube with mercury, then inverting that tube into a reservoir full of mercury so that the top is sealed and the bottom is open and submerged beneath the mercury surface in the reservoir. Mercury will ﬂow out of the tube until equilibrium is attained, at which point the

18 14

Vapor Pressure of Pure Water [kPa] 2

0 0 10 20 30 40 50 Temperature []

Figure 2.2: Effect of Temperature on Vapor Pressure of Water [46] weight of mercury in the column is balanced by barometric pressure on the free-surface in the reservoir. The equilibrium height depends on local atmospheric conditions but is usually close to 760 mm at sea level. Such barometers can be made with other liquids as well; since water is much less dense than mercury the equilibrium height of the column in a water barometer will be roughly 10 meters, as seen in figure 2.1. Torricelli used this apparatus to prove the existence of vacuum—the top of the column is empty but has previously been full of water; air cannot enter the sealed tube; thus he concluded that the empty space contains nothing at all. Actually, this is not strictly true; although the principle is sound, a hypothetical perfect vacuum would cause the water to evaporate, filling the space with vapor and raising the pressure above zero. The Torricelli column never attains zero pressure but instead remains at low pressure.

19 Vapor Non-condensable gas extraction

Evaporator Condenser

10 meters

Supply pump

Feedwater Freshwater reservoir reservoir

Figure 2.3: Distillation Using a Pair of Torricelli Columns

20 This equilibrium pressure is referred to as the vapor pressure. All liquids (and even solids) have some non-zero vapor pressure; for any substance placed in a vacuum, some molecules will evaporate, causing an increase in the pressure of the surrounding environment; when the vapor pressure of that substance at that temperature is reached, the system is in equilibrium. Figure 2.2 is a plot of the vapor pressure of pure water versus temperature, showing the direct relationship between the two. This temperature dependence is crucial to the operation of barometric distillation as investigated in this research. Distillation requires two steps: evaporation and condensation. A pair of Torricelli columns can be configured in such a way as to make one of them an evaporator and one a condenser (figure 2.3). The column to be used as evaporator is filled with water which is salty or otherwise contaminated; the condenser is filled with pure fresh water; both columns are connected to each other at the top so that water vapor can flow across. To ensure that evaporation occurs in the evaporator and condensation in the condenser, it is necessary to have a temperature difference between the columns. If water in the evaporator is warmer than that in the condenser, it will have a higher vapor pressure and will evaporate, seeking equilibrium. This will raise the pressure in the evaporator higher than that in the condenser, causing vapor to flow in the direction of the condenser, where it meets water of lower temperature to which it transfer heat, causing the vapor to condense. As long as the temperature difference (denoted ∆T ) is maintained water will be transferred from the evaporator to the condenser, augmenting the available supply of pure water. However, higher temperature in the evaporator does not guarantee a higher vapor pressure for that water in a desalination operation since the effect of salinity cannot be neglected. Vapor pressure of water depends not only on temperature but also on the amount of solids dissolved in it, and the effect of this dependence works against

21 2.34

2.32 [kPa]

2.3

2.28

2.26

2.24

2.22

2.2

2.18 Vapor Pressure of Saline Water at 20 2.16 0 20 40 60 80 100 120 Salinity [parts per thousand]

Figure 2.4: Eﬀect of Salinity on Vapor Pressure of Water [47]

the barometric distillation process. As seen in figure 2.4 (generated from empirical correlations given by [47]), vapor pressure decreases with increased salinity; for this reason, two low-pressure chambers at equal temperature will undergo distillation in the wrong direction, from fresh water to salt water. For desalination operations, therefore, the effect of salinity will have to be overcome by applying a greater temperature difference than otherwise would be required.

2.2 Temperature Maintenance

Vaporizing a given amount of liquid requires an amount of energy known as the enthalpy of vaporization or latent heat of vaporization; although dependent on temperature this quantity is often given at the boiling temperature for atmospheric pressure—for

22 water at 100, the enthalpy of vaporization is 2257 kJ/kg [48]. Therefore, for every kilogram of vapor produced a 100 evaporator loses 2257 kilojoules of heat and for every kilogram of condensate the condenser gains the same 2257 kilojoules. The temperature difference between chambers could not be maintained, and distillation would quickly cease, if heat were not provided to one and removed from the other. One of the features of Barometric Distillation, as it is understood and investigated in this paper, is the method by which that heat is transferred. Unlike the multiple effect or vapor compression methods, evaporation in BD does not occur on the outside of tubes or similar structures carrying hot water or steam; unlike multi-stage flash, condensation does not occur on the surface of tubes carrying feedwater to be preheated. In this process simply pumping more hot feedwater to the evaporator (as is proposed often enough by other researchers in the field of low-pressure distillation—see section 2.7) and more cold fresh water to the condenser (less commonly proposed) maintains ∆T ; adding this water to the top of a Torricelli column pushes water down and out into the reservoir, as atmospheric pressure can only support a column 10 meters tall. Due to the loss of heat to vaporization, water exiting the evaporator will be colder than when it entered. Water exiting the condenser will be warmer by the same amount (if the same amount of water is pumped into the condenser as the evaporator). Since one goal of this process is to exploit low-temperature sources of heat, ∆T will probably not be large: it may be 20 or less, leaving little margin for the temperature to change before both chambers are brought into equilibrium. This may require large volumes of water to be supplied to the chambers. For example, at a specific heat of

4.2 kJ/kg·K, supplying enough energy to replace that lost by the evaporation of one kilogram with no more than a 5 change across the column will require the addition

23 of hot feedwater with a mass equal to:

(1 kg) (2257 kJ/kg) m = = 107 kg (2.1) (5 K) (4.2 kJ/kg·K)

The condenser, supplied with the same amount, will experience a gain of 5. Under these conditions, 107 times more water will have to be supplied to each chamber than will be produced as distillate—the ratio of supply water to product water is a critical parameter of the BD process and will be denoted R in later analysis. Condensers employing this type of heat exchange are referred to as direct-contact condensers in the literature, since they bring vapor into direct contact with cooling water. They enjoy lower capital cost and fewer problems with scaling or corrosion in comparison with condensers that transfer heat through tubes or other surfaces [49]. Most importantly for the barometric distillation process, however, there is no temperature drop or thermal resistance across a heat-exchanger wall in these direct-contact condensers, allowing smaller temperature diﬀerences to drive the distillation process [49]. These beneﬁts exist also for the direct-contact evaporator used in this process. The higher-temperature saltwater supplied to it has two functions: the portion that evaporates is analogous to the feedwater in a conventional evaporator, while the portion that does not is analogous to the heating water passing through the tubes. Similar designs serve both as evaporators and condensers in open-cycle Ocean Thermal Energy Conversion (OTEC) systems, which supply warm water from the surface of the ocean to an evaporator and cold water from some greater depth to a condenser; a turbine extracts energy from the vapor as it passes from one to the other [50].

24 2.3 Evaporator and Condenser Design

Despite the simplicity of direct-contact heat exchangers, their design is not trivial by any means; the geometry of the chambers and the nature of the liquid flows therein is critical to the successful operation of this desalination system. Indeed, some practical realizations of barometric distillation fail to produce any water at all (at reasonable values of ∆T ) without first applying some optimizations, as was discovered during the course of this research. However, many potential improvements impose some energy penalty of their own, so it is necessary to understand the overall system efficiency when considering any of them. Electricity consumed by the supply water pumps is the largest share of the total energy consumed by the barometric distillation process if the enthalpy of vaporization is not considered (refer to section 6.4 for analysis of this). The volume flow rate through these pumps required to achieve a given rate of water production is the ratio R. This ratio, unfortunately, cannot simply be decided upon—it depends on ∆T and on the characteristics of the evaporator, condenser, and the vapor conduit connecting them. Poorly designed chambers may fail to take complete advantage of the available temperature difference; very effective chambers may bring the temperature difference so low that the process stalls (while consuming more energy in the pumps than is necessary); optimal chambers may be cheated by an undersized vapor conduit.

2.3.1 Maximizing Surface Area

One way to improve the eﬀectiveness of the evaporator and condenser is to increase the surface area of the liquid water in them. In both chambers, that surface area depends on the nature of the ﬂow of supply water. In the worst case, the supply pumps inject this water at or under the free surface in the barometric column, yielding

25 (a) Spray nozzles (b) Vortex against chamber (c) Packing material walls Figure 2.5: Some Methods of Increasing Surface Area

a surface area equal to the cross-sectional area of the chamber. Performance can be improved over this simple design by using the empty space above the water level, although raising water by this additional height results in some energy cost. One such method features nozzles that spray water into the chamber. The surface area of this configuration depends on the design of the nozzle; flows ranging from an atomized mist to a rain of large droplets can be achieved. Unfortunately, there is a pressure drop across any such nozzle, which means that the supply water pump consumes more energy. Also, the small orifices found in such nozzles are susceptible to fouling, which might require frequent maintenance. This is particularly true of the evaporator as it contains salty or otherwise contaminated water and operates at higher temperature. Another way to increase surface area is to flow water against the inside of the (presumably) cylindrical chambers in vortex fashion. Water supplied to evaporator and condenser would enter through ports angled in such a way as to direct multiple

26 streams of water around and down the walls of the chamber—this water would spread out into a thin sheet coating the chamber walls, in which the total surface area is the cross-sectional area of the chamber plus the circumference of the chamber multiplied by the height of the vortex. Although not capable of providing such high surface areas as a fine mist, this method probably enjoys a smaller head loss and a better resistance to fouling. Alternatively, surface area can be improved by filling the evaporator and condenser chambers with some packing material featuring a high surface-area-to-volume ratio (figure 2.5c. Supply water would be pumped to the top of a stack of such material (called a packed bed) and made to flow down as a fluid film through spaces inherent in the bed. A packed bed can be implemented using pellets of different shapes, for which many designs are commercially available, including the original Raschig rings—short lengths of tube packed randomly into distillation columns. Other random packings such as saddles, spheres, perforated rings, and various exotic shapes are manufactured for this purpose in metal, plastic, or ceramic materials [51]. Recently chemical engineers have tended toward the use of structured packings instead; these are made of regular arrangements of smooth, textured, or perforated sheets (typically metal) stacked into layers; the angle of these sheets with respect to the flow axis alternates from layer to layer, causing the falling fluid films on each sheet to mix effectively and providing a high surface area [52]. Although more expensive, structured packings often impose a lower pressure drop on the fluid flow and can therefore improve the energy efficiency of the overall process [51]. In each of the methods discussed above, the ultimate surface area is proportional to the height at which the supply water is injected (relative to the level of water inside the column). Greater height means a greater volume of sprayed droplets in the first case, a taller vortex in the second case, and a greater area of wetted pellets or other

27 Condenser supply water

Nozzle Vapor flow

Supply water and condensing vapor Eductor

Evaporator Condenser

Figure 2.6: Application of Eductor to Vapor Connection

media in the third case. However, as previously mentioned, such improvement comes at the price of increased head loss in the water supply circuit.

2.3.2 Applying the Venturi Eﬀect

Another potential improvement to this style of barometric distillation processes is the inclusion of an eductor, a device that uses the Venturi effect to produce suction, as discussed in section 1.2.2. Supply water on its way to the condenser would first pass through the eductor, accelerating as it passes through a nozzle. As seen in figure 2.6, the suction port at the throat of the eductor would be connected to the evaporator. Vapor from the evaporator would be drawn through this eductor, compressing it and mixing it very effectively with the cold supply water, causing it to condense. Barometric distillation equipped with an eductor in this manner may be able to operate with a lower temperature difference. Indeed, barometric condensers equipped

28 with eductors are already used in a variety of chemical engineering applications [53]. Although eductors were not experimented with in the course of this research, their application to this type of desalination deserves further investigation.

2.4 The Problem of Non-Condensable Gases

One obstacle to successful commercial implementation of barometric and other low- pressure distillation schemes is the presence of dissolved gases in most sources of water. A body of water exposed to the atmosphere will accept into solution oxygen, nitrogen, and the rest of the gases found in air; at equilibrium the solution will approximately obey:

Henry’s Law. At constant temperature, the solubility of a given gas in a given liquid is directly proportional to the partial pressure of that gas [54].

Sources of water to be used in low-pressure distillation systems (in this case, both evaporator and condenser supplies) should contain the equilibrium amount of a particular dissolved gas according to Henry’s law, that is, an amount proportional to the partial pressure of that gas in the atmosphere. When water from such a source enters an environment of lower pressure, the partial pressures of the gases in that environment are now lower; to reach equilibrium excess gas will come out of solution, forming bubbles and rising to the surface. For this barometric distillation system, water circulating from both the evaporator and condenser reservoirs will cause dissolved gases to accumulate in the Torricelli chambers. The water pumped into the chambers will ﬂow down and out of the column carrying with it a relatively lower concentration of dissolved gases. Water vapor is one of these gases, but the water vapor coming out of solution will condense and ﬂow down through the condenser column, so the contribution to total pressure in the chambers due to water vapor will

29 20 Nitrogen 18 Oxygen Argon 16

4 Solubility in Fresh Water [ml/kg] 2

0 0 5 10 15 20 25 30 35 40 Temperature []

Figure 2.7: Temperature Dependence of Solubility in Fresh Water [55] remain constant. The other gases present in the atmosphere—and consequently, in the source water—cannot condense in the temperature range in question and so the overall pressure will continue to rise. For this reason such gases are referred to as non-condensable gases in the literature of chemical engineering and desalination. Solubility of gases in water decreases with increasing temperature, as seen in figure 2.7 for the three largest components of air: oxygen, nitrogen, and argon. This temperature dependence implies that the colder condenser supply will transport gases into the low-pressure system at a higher rate than the evaporator supply will. The presence of salt in the water to be distilled will also decrease the solubility of gases in that water [56]; this effect is shown in figure 2.8 for water at 20. Therefore, efforts to minimize the problem of non-condensable gases should concentrate first on the condenser.

30 12 Nitrogen Oxygen Argon 10

Solubility in Fresh Water [ml/kg] 2

0 0 5 10 15 20 25 30 35 40 Salinity [Parts Per Thousand]

Figure 2.8: Salinity Dependence of Solubility at 20 [55]

2.4.1 Methods of Gas Removal

Due to the accumulation of non-condensable gases occurring in any implementation of the Barometric Distillation process, it will be necessary to apply one of several methods for removal of non-condensable gases, each of which will impose some additional energy cost. One initially considered method would use Torricelli columns, one each for the hot and cold sources, to extract dissolved gases from water before supplying that water to the evaporator or condenser. Water with dissolved gases would be circulated through the low pressure columns, and vacuum pumps would evacuate the top of the chambers. However, any system that degasses water before that water enters the distillation chambers must be capable of extracting all of the dissolved gases from solution, or

31 Vacuum pump

Vacuum pump

(a) To the Atmosphere (b) Reinjected Into the Water Column

Figure 2.9: Direct Gas Extraction

gases will still accumulate and an additional degassing system will be required. A separate Torricelli degasser would also spoil the energy eﬃciency of the entire system:

Water on each side would have to be pumped through a degassing column as well as through the distillation column, approximately doubling the energy cost due to water pumping.

Completely degassing water during a single pass through a degassing column would require extremely high surface area. Most methods of increasing surface area (atomization through a nozzle, tall waterfall-like formations, etc.) impose an additional pumping cost.

Using multiple passes through degassing columns instead would multiply the pumping cost by the number of passes.

Another idea is to extract gases directly from the distillation column by means of a vacuum pump. Evacuation could occur periodically: as when, for example, a

32 sensor indicates that pressure in the chamber has increased beyond some threshold; or continuously, using some relatively low-power pump that removes gas at the same rate as it accumulates, possible also under feedback control of some sensor. This evacuation capability could also be used to lower pressure in the system, raising the barometric columns prior to operation. Since this function will be necessary anyway, giving it another use as a degassing system will reduce overall complexity and capital expense. Furthermore, this technique would not require circulating water twice as with the pre-degassing concept. However, such a vacuum pump would still require energy. The energy cost of vacuum pumping increases as the difference between chamber pressure and atmospheric pressure increases. For this distillation system, the operating pressure will certainly be below 10% atmosphere (most experiments to date have taken place at 3-5% atm). Figure 2.9a depicts direct evacuation of gases to the atmosphere, in which case the pump is working against almost 1 atm of pressure. An alternative, as seen in figure 2.9b, is to extract the gas by means of the vacuum pump and reinject it at some depth in the water column, relying on the downward flow of water to carry bubbles down and out of the apparatus. In this mode the back-pressure on the vacuum pump is proportional to depth below the surface, which is considerably less than a full atmosphere. Re-injection of gases has the additional advantage of capturing water vapor that is extracted from the distillation chamber along with non-condensable gases and condensing it in the fresh water column; simple evacuation to the atmosphere would produce a lower efficiency by rejecting this water vapor. What depth the bubbles need to be re-injected at is unfortunately something of an open question at present—unfortunately because the depth determines the energy efficiency of this method. Answering this question is the goal of a parallel research project conducted at the University of Michigan by Makiharju and Schulz [57].

33 Finally, it should be considered that gases can be prevented from entering the low- pressure system from the condenser supply by placing a heat exchanger there instead of directly mixing supply and product water. This method will suffer an additional loss of thermal efficiency due to the heat transfer coefficient across the solid heat- exchanger surfaces, but it is possible that a greatly reduced rate of non-condensable gas introduction could more than compensate for this; such a heat exchanger would also enable the use of non-pure cooling water, such as that drawn from the deep sea.

2.5 Sources of Heat

Desalination by this process requires large (relative to the amount of product water) volumes of both hot saline water and cold fresh water. As was briefly mentioned in section 1.2.7, one possible source of feedwater is the seawater used for open-loop cooling in coastal power plants; this water is drawn from the ocean, passed through the steam condenser, and returned at a somewhat higher temperature, perhaps as high as 15 warmer [58]. For the year 2000, in the United States alone, such facilities drew nearly 60 billion gallons of water per day from the ocean, coastal bodies of water, and saline ground-water resources [59]. Temperatures of these discharged cooling streams vary depending on regulatory conditions and the sensitivity of the local marine environment, but a temperature rise of 8 is a reasonable value for coastal nuclear plants, for instance [60]. Applying barometric distillation or a similar technology to such a facility may provide an additional benefit of reducing the discharge temperature by the amount necessary to supply the enthaply of vaporization to the evaporator. Such an implementation of BD also requires no modification to the power cycle—the desalination process can simply accept as much power-plant cooling water as is available at the current discharge

34 temperature, providing fresh water and passing most of the cooling water on to the sea at lower temperature, albeit slightly higher salinity. However, sources of “cold” are just as important to applications of the barometric distillation process as sources of heat; the condenser must be cooled by large flow rates of lower-temperature freshwater. This suggests that barometric distillation would be most useful as a way of augmenting existing water supplies. Large surface bodies of water such as lakes or reservoirs may be suitable; they are not infinite heat sinks but, if large enough, the steady-state temperature rise resulting from such use may be acceptable. Perhaps more promising is the application of fresh groundwater from aquifers; outflow from the condenser column could be returned to the aquifer, replenishing the existing supply. This idea is particularly valuable in coastal areas where power plant discharge is available and where many aquifers are already overdrawn and suffering from saline intrusion from the nearby ocean; injecting additional freshwater at the coast allows more water to be drawn from elsewhere in the aquifer without the threat of overall salinization [61]. Aquifer replenishment has been applied successfully for years, often by injecting reclaimed or treated wastewater and relying on the aquifer to provide final purification [62]. However, use of groundwater as a heat sink may raise environmental concerns of its own; also, the additional energy required to pump water up from the depth of an aquifer may make this impractical. There is no reason why a barometric distillation facility could not use a combination of heat sources and sinks. For instance, a supply of hot water from a power plant could be further raised in temperature using solar collectors; even if purpose-built collectors are not employed due to capital cost or the additional pumping energy required, optimization of the process may include painting the evaporator black or otherwise attempting to increase solar energy absorption. Similarly, the condenser can in any

35 case be equipped with ﬁns as another means of shedding heat to the environment. Further investigation will have to reveal if these supplementary techniques are worth the additional capital expense they impose.

2.6 Application of Alternative Energy

Alternative energy and desalination are a natural fit for some fundamental reasons. One of the persistent problems with renewable energy is the need to store the output produced during times when, for example, the sun is shining or the wind is blowing, and to release that energy when demanded. Batteries, hydrogen, flywheels, supercapacitors, pumped-storage hydropower, etc., all impose some efficiency loss and can present other undesirable effects as well, such as the toxic by-products of battery manufacturing and disposal. Using alternative energy sources to supply desalination operations, however, minimizes many of the problems of energy storage, as desalinated water can be considered (in the abstract) as stored “energy”—where the energy stored is that which would have been spent to desalinate the same amount later. This form of energy storage avoids many of the efficiency losses encountered in more direct methods; water storage in tanks, natural reservoirs, or towers is already a common feature of municipal water supply systems around the world. One way to power Barometric Distillation with alternative energy is to augment the evaporator heat supply with solar thermal collectors. This would be of particular benefit in underdeveloped countries, in which the following combination of conditions often prevails:

Inadequate water resources and a great need to develop new sources of water to supply growing populations.

36 High levels of average insolation [63].

Underdeveloped industrial infrastructure and electrical-generating capacity— therefore a lack of electricity for RO desalination, extracted or back-pressure steam for thermal methods, and cooling water for BD or similar technologies.

Another way to harness alternative energy sources to this type of barometric distillation is to supply power to the pumps by a wind turbine; shaft power from the turbine could be used directly without intermediary electrical generation. Such reduction in complexity and capital cost is an important advantage in underdeveloped regions, and pumping by wind power is a long-established technology. Depending on the nature of the heat source and other factors, it may be most economical to simply let distillation cease at times when wind energy is not available, and buffer supply variations by storing some of the produced water; the aquifer-injection mode discussed in section 2.5 does this inherently. Ocean waves are another source of energy for pumping. Any of the various floating- body, submerged-body, or Oscillating Water Column (OWC) techniques could power pumps via mechanical linkage or fluid power [64]. Alternatively, something similar to OWC could pump surface seawater into the evaporator directly; a water column with an open end in the sea could take the place of the supply pipe; when waves pass by the height of that column would rise and seawater would flow into the chamber. Unfortunately, this would require careful site-selection and would only work in cases where colder fresh water is available near the coast.

2.7 Other Methods of Barometric Distillation

Attempts at using barometric columns to produce low pressure for distillation purposes are a reliable presence in the literature of desalination; patents on similar ideas have

37 been issued and papers exploring this technology have been published for more than half a century. One of the first variations on this theme is that of Snyder, who received a U.S. patent in 1949 for a system of vacuum distillation supplied with heat by a solar collector drawing water from the evaporator column. Flow through this collector, for which several alternate designs are given, is driven by natural convection resulting from the solar heat. Cooling of the condenser is accomplished by placing fins on the outside of the apparatus to increase heat transfer to the environment. This invention uses no pumps to supply water to evaporator or condenser; non-condensable gases are not considered in the patent [65]. In 1981 another patent was granted to Humiston for a barometric distillation system using temperature differences between water from deep in the ocean and water at the surface. As the condenser cooling water is itself salty it is isolated from the distillate, circulating through some arrangement of tubes; as such it is not a direct-contact condenser and would suffer from some amount of thermal resistance. Humiston’s patent also claims that a turbine placed in the vapor conduit can recover enough energy from vapor flow to more than offset the energy consumed by the evaporator and condenser supply pumps, supplying net energy for other uses. Again the problem of dissolved gases or methods of its mitigation are not described [66]. The ideas of Humiston can be seen as a variation on Ocean Thermal Energy Conversion, a concept that has enjoyed much interest as a means of energy production, although some investigators have pursued desalination as a secondary goal [50].

2.7.1 Atwell’s Patent

Atwell was awarded a patent related to barometric distillation in 1985; his invention also includes two columns, one as evaporator and one as condenser. This process does

38 not rely on any pre-existing temperature diﬀerence nor does it use pumps to supply either chamber with water. Instead the entire system is initialized by evacuating the chambers with a vacuum pump, drawing saline and fresh water up into the columns. The two chambers are connected by a fan or compressor which moves water vapor from one side to the other, lowering pressure over the evaporator and raising it over the condenser. The fresh water in the condenser absorbs the heat of vaporization, becoming warmer, and passes down through the column. The possibility of non- condensable gases accumulating in the low-pressure system is mentioned and it is suggested that the vacuum pump would be used to maintain low pressure [67]. One important aspect of the Atwell patent is the use of annular or otherwise contacting ducts containing the two barometric columns; this permits the outgoing, warmer condensate to preheat the incoming evaporator supply [67]. This last feature could be a possible enhancement to the barometric distillation process described in this paper; it would improve eﬃciency in implementations including a blower-assisted vapor conduit under conditions in which the condenser exit temperature exceeds the evaporator inlet temperature. Such blower conduits were built and experiments were conducted without success (see section 4.1.3), although without using the annular column design or any other form of heat-exchange between condenser discharge and evaporator supply.

2.7.2 University of Florida Project

Al-Kharabsheh and Goswami of the University of Florida describe the results of investigations (including numerical simulations [68] and physical experiments [69]) into a proposed process of low-pressure solar-heated vacuum distillation. Their system comprises an evaporator column supplied with heat from a solar collector, through which saline feedwater does not circulate; instead the collector operates in a closed-loop

39 wirh a coil or other type of heat exchanger inside the evaporator. Only enough water is fed to the system to replace the amount distilled plus the amount of brine that is withdrawn to prevent the evaporator contents from becoming too saline, which would reduce the vapor pressure beneath that of the condenser as well as contributing to scaling [68]. In this process the condenser receives no cooling water; latent heat of vaporization is shed to the atmosphere by means of cooling fins on the outside of the condenser tube. The experimental apparatus used in Al-Kharabsheh and Goswami’s research includes a condenser and cooling fins made of copper to improve heat transfer to the environment. These authors are also aware of the problem of non-condensable gases, providing for their removal by “periodically flushing the system and restarting it” [68].

2.7.3 Seawater Solar Barometric Distillation

A group working in Italy and Egypt, headed by Mario Reali, has published results from a theoretical analysis of a somewhat different solar-heated barometric distillation process, which they refer to as Seawater Solar Barometric Distillation, or SW-SBD. Their technology uses barometric columns located inside underground pits so that the surface of the water inside the column is just above ground level; these columns are supported by reservoirs approximately 10 meters below the ground and capable of being raised or lowered to adjust column height in response to changes in temperature or solar radiation [70]. Seawater entering the SW-SBD plant is heated first by energy-recovery heat exchangers (in which water vapor is cooled and condensed at the end of the distillation process) and further heated to evaporation by a field of solar collectors. The resulting mixture of hot brine and vapor is sent to a de-misting device to separate the two (the brine exits through another barometric column and is also used to preheat the

40 incoming seawater); the vapor, condensed in the energy-recovery heat exchanger, exits into a fresh-water reservoir at the bottom of another barometric column [71]. Before entering the series of heat exchangers and solar collectors, incoming seawater in the SW-SBD process passes through a venting system for removal of dissolved gases, which comprises a low-pressure chamber evacuated by a mechanical pump or, alternatively, by a steam ejector, depending on the local availability of electricity or of steam, among other considerations [70]. Reali has also published an analysis of a two-stage SW-SBD design with improved thermal eﬃciency [71].

2.7.4 Low Temperature Thermal Desalination

Recently operational experience with low pressure distillation has been gained from a practical implementation of a plant driven by a naturally-available temperature difference. This plant, located on the island of Kavaratti, India and operated by India’s National Institute of Ocean Technology, is an example of what its creators have dubbed Low Temperature Thermal Desalination, or LTTD [72]. In this process warm water from the surface of the sea is drawn into a low-pressure evaporator which discharges through a barometric column; the condenser is cooled by cold seawater from deep in the ocean. The Kavaratti plant and an associated experimental facility on a floating barge off Chennai, India have operated using a temperature difference of roughly 20 between evaporator and condenser, enabling the on-shore plant to produce 100 tons of fresh water per day [73]. Floating LTTD or OTEC operations benefit from having to pump cold water a minimum distance (only the vertical depth to the surface, rather than some horizontal distance inland as well); the shore facility has nearly the same benefits due to the unusual undersea geology around Kavaratti island, which features deep water close to shore. Nevertheless, NIOT reports requiring 30% more energy per unit of water

41 produced than a reverse osmosis operation would, although they believe that larger- scale plants will enjoy improved eﬃciency and predict that a 10 million liter per day operation will be 25% more eﬃcient than RO. The institute is planning to build similar facilities on neighboring islands; perhaps these will support the results of NIOT’s analysis [72].

42 Chapter 3

Theoretical Analysis

3.1 Vaporization

Vaporization of water, or of any substance, can occur by three routes:

Evaporation is vaporization occuring at the liquid-gas interface when some molecules happen to acquire enough energy through molecule-to-molecule collisions to escape from the liquid body.

Boiling occurs throughout a body of liquid raised to its boiling temperature; it is characterized by the formation of bubbles which break the surface, releasing vapor.

Sublimation is direct vaporization from a solid (a well-known example is the carbon dioxide vapor escaping from the surface of dry ice, although this will occur with most substances, including water). This type of vaporization will not be of further interest here.

Both surface evaporation and boiling of water can occur in a Barometric Distillation process; in either case the energy required is the same latent heat of evaporation, denoted as ∆Hvap and most often expressed in units of energy per mole or per unit

43 mass. This quantity has an inverse relationship with temperature, as seen in table 3.1, but remains relatively constant at temperatures below 100.

Table 3.1: Enthalpy of Vaporization for Water [48]

Temperature ∆Hvap ()(kJ/kg) 20 2 454.1 40 2 406.7 60 2 358.5 80 2 308.8 100 2 257.0 140 2 144.7 180 2 015.0 220 1 858.5 260 1 662.5 300 1 404.9 340 1 027.9 374.14 0

One can show that heat of vaporization is the largest component of energy use in conventional distillation. The speciﬁc heat of water in the temperature range of

0 to 100 is roughly 4.18 kJ/kg·K [74]. Therefore, raising water by 75 K, from room temperature to the boiling point, requires speciﬁc enthalpy equal to:

∆H = c ∆T = (4.18 kJ/kg·K)(100 K) = 314 kJ/kg (3.1) m p

The energy required to then complete the phase change for that one kilogram of water (2257 kJ) is larger by more than a factor of seven than the energy needed to elevate it to 100; suggesting that supplying the latent heat from a “free” source can make a large diﬀerence in overall energy use.

44 3.2 Henry’s Law

Henry’s law can be mathematically expressed by a variety of formulas with associated proportionality constants for diﬀerent gases (referred to as “Henry’s law coeﬃcients”) tabulated by various authors. One expression of Henry’s law relates concentration to partial pressure: c k = (3.2) H p where the partial pressure p has units of atm (absolute), and c is the concentration in moles per liter. kH, the Henry’s law constant, therefore has units of atm. Values of kH for the major constituent gases of the atmosphere can be found in table 3.2; the

0 superscript 0 in kH indicates that these coeﬃcients apply at standard temperature and pressure and zero salinity.

Table 3.2: Henry’s Law Coeﬃcients for Gases in Water [75]

0 −d ln kH Gas kH d(1/T ) (mol/L·atm) (K) −3 O2 1.3×10 1 500 −4 N2 6.1×10 1 300 −4 H2 7.8×10 500 Ar 1.4×10−3 1 500

The temperature dependence of solubility can be modeled by correcting the Henry’s law coeﬃcients using the van’t Hoﬀ equation, integrated as:

−d ln k 1 1 k = k0 exp H − (3.3) H H d (1/T ) T T 0 where T 0 is standard temperature, usually taken as 293 K, although it should be

0 consistent with the temperature used in determining KH. An example application of this temperature correction relationship is found in section 6.5.

45 Salinity also reduces the solubility of gases, due to what is known as the “salting-out eﬀect”. This behavior can be understood using the Sechenov equation [76]:

0 kH log = kscsalt (3.4) kH

0 where kH is the Henry’s law coefficient at zero salinity, kH is the corrected Henry’s law coefficient, ks is a Sechenov coefficient specific to the gas and salt in question, and csalt is the salinity expressed as concentration, or molarity of salt in water, typically with units of moles per liter. Table 3.2 lists values of the Sechenov coefficient for various gases in aqueous NaCl solutions. As with the temperature dependence correction, the Sechenov equation is applied to a practical example in section 6.5.

Table 3.3: Sechenov Salt-Eﬀect Coeﬃcients for Aqueous NaCl [77]

Gas ks (L/mol)

O2 0.141 N2 0.134 H2 0.103 Ar 0.133

In a body of water at equilibrium with the atmosphere, the dissolved gas partial pressure of each air constituent is equal to the partial pressure of that gas in the atmosphere itself. For example, the partial pressure of nitrogen is 0.78 atm both in the atmosphere and in any water in equilibrium with it; the partial pressure of oxygen under these conditions is 0.21 atm [78]. The concentrations of these gases in water at equilibrium with a STP atmosphere is determined using Henry’s law:

−3 −6 atm·L mol mol L cO2 = pO2 kH,O2 = (0.21 atm) 1.3 × 10 / = 273 × 10 / (3.5)

−4 −6 atm·L mol mol L cN2 = pN2 kH,N2 = (0.78 atm) 6.1 × 10 / = 476 × 10 / (3.6)

46 Due to the diﬀerent solubilities of these gases in water as expressed in the Henry’s law coeﬃcients, oxygen consitutes approximately 36% of the dissolved gas even though it makes up only 21% of the atmosphere.

3.3 Energy Eﬃciency

To evaluate this technology and compare it to existing or future methods of water desalination, it is necessary to estimate its eﬃciency, or the amount of water produced for a certain amount of energy expended. The water production rate is trivially related to the previously discussed pumping ratio thus:

Q m˙ Q = s m˙ = s (3.7) d R d R

where Qd is the rate of water transferred (by volume) via distillation, Qs is the volume flow rate of supply water, and R is the pumping ratio. Since water under this range of conditions can be expected to be incompressible, the same ratio can also relate the equivalent mass flow rates m˙ s and m˙ d. Obviously R is the critical parameter; but one cannot simply demand a ratio of 100:1 or 50:1 and achieve arbitrarily high efficiency. The pumping ratio depends on temperature, geometry of the evaporator and condenser, and the nature of the flows within; it is probable that experimental results are the best way to answer this question. At steady-state one can consider not only rates of water production, but also the mass of supply water necessary to produce a desired total mass of distillate:

m m = s (3.8) d R

47 3.3.1 Cost of Supply Water Pumping

If a temperature difference is assumed to be available “for free”, from power plant waste heat or a similar source, the energy cost of distillation by this process is due to pumping the supply water to the top of the apparatus. Analyzing this cost begins with the energy equation of fluid mechanics, simplified by assuming water under these conditions to be incompressible and viscous work (due to shear stress at the control surface) to be negligible:

p V 2 p V 2 hpump = + + z − + + z + hfriction (3.9) γ 2g out γ 2g in

The symbol h denotes head generally; with appropriate subscripts indicating that head is provided by a pump or lost to friction; changes in pressure and velocity at the inlet are represented by the term p/γ, the pressure head, and by the term V 2/2, the velocity head [79]. All terms in eq 3.9 are in units of length. Since γ (the speciﬁc weight, γ = ρg) can be assumed to be constant and the velocity at the inlet (the reservoir free surface) is zero, this equation reduces to:

p − p V 2 h = out in + (z − z ) + out + h (3.10) pump γ out in 2g friction

If the control volume is chosen as in ﬁgure 3.3.1, with the inlet at the height of

the free surface in the reservoir, the pressure diﬀerence pout − pin is negative and the pressure head term cancels the elevation diﬀerence from the free surface of the reservoir to the free surface inside the chamber, by the fundamental hydrostatic relation [79]:

p2 − p1 = −γ(z2 − z1)

Therefore eq. 3.10 can be simplified by defining an elevation difference as the additional

48 Supply pipe

Pump

Control surface

Figure 3.1: Control Volume for Pumping Energy Calculation height above the chamber’s free surface, as seen in ﬁgure 3.3.1, producing the following relation: V 2 h = ∆z + out + h (3.11) pump 2g friction

The power expended by the pump is:

2 Vout mgh˙ pump mg˙ (∆z + 2g + hfriction) Ppump = = (3.12) ηpump ηpump

49 wherem ˙ is the mass ﬂow rate through the control surface and ηpump is the eﬃciency of the pump. For the Barometric Distillation process, the total power needed to pump supply water is the sum of the power used by the evaporator and condenser pump:

m˙ sghpump m˙ sghpump PS = + (3.13) ηpump evap ηpump cond

Here the subscript s inm ˙ s indicates that this mass flow rate is of the supply line for that side of the system. In the special case where the head loss, supply flow rate, and pump efficiency are the same for both, eq. 3.13 reduces to:

2m ˙ Sghpump PS = (3.14) ηpump

In steady-state the amount of water produced is denoted by md, so the energy required to pump enough supply water to produce a given amount of water is:

E m gh m gh S = s pump + s pump md ηpumpmd evap ηpumpmd cond

or, substituting with eq. 3.8:

E Rgh Rgh S = pump + pump (3.15) md ηpump evap ηpump cond

3.3.2 Estimating Pipe Friction

Head loss due to friction in pipe ﬂow problems can be related to the geometry of the ﬂow and a dimensionless factor using the Darcy-Weisbach equation [81]:

L V 2 h = f (3.16) friction d 2g

50 Table 3.4: Roughness Values for Typical Pipe and Duct Materials [80] Material Condition (mm) Steel New 0.045 General Rust 2.0 Iron Cast, new 0.30 Wrought, new 0.045 Brass Drawn, new 0.002 Sheet metal Duct 0.03 Glass or Plastic Drawn Tube 0.002 Rubber Smooth 0.01 Concrete Very smooth 0.04 Rough, visible 2.0 where L/d is the ratio of length to internal diameter for the pipe, V is the average velocity, and f is the dimensionless friction factor. For the BD system as depicted in ﬁgure 3.3.1, the length L is equal to:

L = 10 m + ∆z

The method of obtaining f depends on which of two flow regimes prevails in the pipe: laminar or turbulent flow. These regimes are characterized by the Reynolds Number, the value of which depends on the flow conditions and geometry in question:

ρV d Re = (3.17) d µ

in which µ is the coefficient of viscosity for the fluid. Above a Reynolds number of approximately 2300, flows enter a transitional regime before becoming entirely

turbulent at Red = 4000, roughly.

51 For laminar ﬂow, an exact solution for the friction factor exists:

64 f = (3.18) Red

For transitional or turbulent ﬂow, f can be obtained by using the Colebrook formula [79]:

1 /d 2.51 1/2 = −2.0 log + 1/2 (3.19) f 3.7 Redf

The friction factor under turbulent conditions also depends on roughness height , or the height of features on the inside surface of the pipe. Values for corresponding to typical pipe materials, manufacturing methods, and conditions are given in table 3.3.2. Equation 3.19 has been plotted in the well-known form of the Moody chart for a range of values of /d (or relative roughness); alternatively the Colebrook formula can be solved numerically. A script written in the GNU Octave numerical computation language is provided that evaluates this formula iteratively (appendix A.1); section 6.4 includes an example of its use.

3.3.3 Cost of Non-Condensable Gas Removal

As discussed in section 2.4.1, non-condensable gases can be extracted from the low pressure chambers by using a vacuum pump to evacuate the condenser. These gases can be rejected to the atmosphere or re-injected into the column, using the water ﬂow to carry bubbles down and out of the system. In either case, work will have to be done; this cost can be estimated by considering the vacuum pumping as an isentropic compression and correcting for irreversibilities by an eﬃciency factor. For polytropic, internally reversible compression of an ideal gas and neglecting kinetic and potential

52 Vacuum pump

Control surface

Figure 3.2: Control Volume for Gas Extraction and Re-injection energy changes, the speciﬁc work (work per unit mass) is given by [48]:

" # w −nR T p (n−1)/n = m 1 2 − 1 (3.20) m n − 1 p1

where T1 and p1 are the temperature and pressure at the inlet, respectively (and p2 is the outlet pressure), Rm is the speciﬁc gas constant for the mixture of non-condensable gases, and n is the polytropic exponent. For isothermal compression in which the evacuated gas remains at the inlet temperature throughout, n is equal to 1; for the more realistic (and more conservative) asumption that the compression is adiabatic n is equal to k, the ratio of constant-pressure and constant-volume speciﬁc heats for the gas in question: c k = p cv

The outlet pressure p2 depends on which mode of gas extraction is chosen; for evacuation to the atmosphere p2 = 1 atm; in the case of re-injection p2 will be less than 1 atm, depending on the injection depth. A control volume for this case is depicted in

53 ﬁgure 3.2. Again the fundamental hydrostatic relation can be used to relate pressure to elevation; in this case the re-injection depth under the free surface in the chamber,

indicated as ∆zr, is the relevant elevation diﬀerence, and the speciﬁc weight is that of water:

p2 = γwater∆zr

Since equation 3.20 provides the speciﬁc work of the compression, the power required by the vacuum pump is:

w P = m˙ (3.21) gas m gas

where m˙ gas is the mass flow rate of gas through the vacuum pump. For pressure inside the chambers to remain constant, this rate must be equal to the rate at which gases dissolved in the supply water come out of solution; it can be related to the supply water flow rate and, using the ratio R, to the water production rate. An empirical relationship between the gas extraction rate and the water flow rate is obtained in section 6.3 using experimental results from the Barometric Distillation apparatus. Also, the energy efficiency equations developed in this section are applied to the results of distillation experiments in section 6.5.

3.4 Environmental Impact of Brine Discharge

One obstacle to industrial-scale desalination is concern raised over possible ecological damage caused by discharge of concentrated brine to the environment. This is particularly signiﬁcant if the desalination plant discharges not to the open ocean but to some intermediary body of water, such as a bay, estuary, shallow-water sea, etc. These waterways, being smaller in volume and somewhat protected from the ocean,

54 may experience a significant change in salinity or other water quality factors due to desalination operations. Without treatment or dilution, discharge water from RO membranes usually has a salinity in the range of 60,000 to 70,000 PPM, 66% to 94% higher than that of average seawater. The U.S. Environmental Protection Agency considers 10% or less increase to be acceptable [82]. Some newer RO plants are classified as high water recovery, which produces a discharge even higher in TDS; the SWRO plant at Ashkelon, Israel produces a waste stream of 73,500 PPM [83] and some recently-developed membranes reject water at 90,000 PPM TDS [82]. Discharge salinity is somewhat improved in the case of thermal distillation methods like Multi-Stage Flash, which historically has returned water with a 10% to 50% concentration increase [84]. In contrast, Barometric Distillation produces an effluent stream at much lower salinity. The change in concentration depends only on the pumping ratio thus:

C 1 D = (3.22) C0 1 − 1/R

where CD is the concentration of the discharge, C0 that of the incoming stream of seawater (or other water to be distilled; the concentration here applies to any contaminant), and R is again the familiar pumping ratio. The percent increase in salinity predicted by eq. (3.22) for representative values of R is given in table 3.4.

Table 3.5: Discharge Salinity for Representative Values of Pumping Ratio R R Increase in TDS 50 2.04% 100 1.01% 150 0.67% 200 0.50% 300 0.33%

55 Depending on the future of environmental regulation, damage caused by brine discharge may be an increasing concern for operators of desalination plants. Possible means of mitigation include pumping eﬄuent through pipelines to multiple distributed sites in the ocean or diluting brine with additional seawater to lower the salinity of the combined stream. Such measures will impose additional costs on SWRO operations that will not apply to Barometric Distillation. Thus, although the large amount of water required for temperature maintainance of the evaporator and condenser is, in general, a disadvantage of the BD process, the relatively low-salinity waste stream might compensate somewhat.

3.5 Availability of Waste Heat

Although the energy efficiency estimate given above assumes the heat of evaporation is provided by waste heat sources considered free, it would be a mistake to consider these sources unlimited. A practical barometric distillation plant should be designed with capacity appropriate to the source of waste heat being exploited. One easy-to-understand example of such a consideration is the case of a coastal power plant outputting a given flow rate of warm seawater at a given temperature; suppose also that a supply of relatively cold fresh water at a known temperature (as from a local aquifer) is available. Given this value for ∆T , it should be possible to estimate the pumping ratio (chiefly determined by the available temperature difference) from experimental or other studies (this thesis is a first step towards such a goal; see section 6.1). A relation between R and temperature would enable the dimensions and other design parameters of a proposed BD system to be intelligently chosen; they would also provide an estimate of the output of the system.

56 Chapter 4

Experimental Apparatus

This research made use of a functional barometric distillation system, constructed from common plumbing parts and “off the shelf” hardware, an approach allowing for easy maintenance and rapid implementation of new ideas. It also serves to demonstrate the suitability of this technology for low-capital-cost water production on a small scale or in underdeveloped areas. Although the research apparatus is scaled-down in volume, energy requirements, and flow rates, one parameter cannot be scaled: the 10 meter separation between the free surface in the reservoirs and that in the chambers. For this apparatus, placing the top chambers on the fourth floor and the bottom reservoirs on the second floor of a university building (with floors separated by five meters each) supplies the necessary 10-meter height difference.

4.1 Mechanical System

Two nearly-identical subsystems are the core of the distillation apparatus used in these experiments: the evaporator and condenser columns. On the fourth ﬂoor, at the top of the apparatus, are the vacuum chambers in which the phase changes occur,

57 Vortex injection wands Vapor conduit

Condenser

Evaporator

Labview Water level interface

Breakout box for sensors

Vacuum pump

Instrumentation DC power supply Hot water heater computer for sensors

Figure 4.1: Top of Apparatus

a connection between the chambers to conduct vapor from one to the other, and a vacuum pump for evacuating the system. On the second story reside the two reservoirs supplying the evaporator and condenser columns and two water pumps for raising this water to the top of the apparatus and injecting it into the vacuum chambers. Components on the fourth and second ﬂoors are connected by ﬂexible vinyl hose comprising most of the height of the Torricelli columns and of the supply water lines.

58 Evaporator Original breakout box, pump relays column (replaced by instrumentation drawer)

Plumbing loop Evaporator Pump Condenser from heater and reservoir cabinet reservoir recirculation pump

Figure 4.2: Bottom of Apparatus

4.1.1 Torricelli Columns

At the top of the columns are vacuum chambers made from six-inch diameter clear PVC pipe to permit visual observation of the fluid inside. In both cases, the top of this length of pipe is fitted with an opaque coupler for the purpose of increasing the cross-sectional thickness in that segment, providing the following benefits:

Eight holes were drilled radially through the coupler and pipe in this segment

59 and tapped to receive brass plumbing ﬁxtures, providing vacuum-tight ports through which supply water can be introduced or in which sensors are placed to measure conditions in the chambers. A thicker cross-section here means that the holes are longer and thus make a better seal with the male ﬁxtures threaded into them.

Increased thickness allows a groove to be machined into the end face of the vacuum chamber to accept an O-ring. Any ﬂat plate can then be used to seal the top of either chamber, relying on atmospheric pressure to push the plate down and compress the O-ring.

Also fitted with a similar opaque coupler and threaded instrumentation ports is the bottom of each vacuum chamber. A series of reducing bushings is glued into this segment to reduce the diameter of the pipe such that it can accept a three-inch threaded steel fitting, the other end of which is barbed to accept the main Torricelli column hose. Two hose clamps per column are used to reinforce the interface between barbed fitting and hose, both to achieve a better seal and to provide additional structural support forthe significant weight of the 10-meter hose. That hose itself is a three-inch vacuum-rated clear PVC hose reinforced with a continuous coil of embedded steel wire to resist collapse under atmospheric pressure. It descends approximately 10 meters to the second floor, where it is fitted to an assembly of rigid PVC pipe dubbed a “foot”. This foot provides a structurally sound point of attachment for the hose and a place in the column to include a flowmeter (section 4.2.3) and, in some configurations, a valve for closing the bottom of the column. Foot segments for evaporator and condenser reside in standard 55-gallon polyethylene drums, graduated in gallons and liters for volume measurement, which perform the

60 Condenser column

Turbine flowmeter

Supply line to chamber

Supply line to pump

Rigid column foot

Reservoir

Support cart

Figure 4.3: Foot Portion of Column and Reservoir, Condenser Side role of reservoirs for the recirculating water. Polystyrene plugs, two inches thick, were fabricated according to the interior dimensions of the drums (with holes for column foot and supply line) and ﬂoated on the surface of the reservoirs in distillation experiments as a way of further insulating the water reservoirs and preventing unwanted heat transfer with the atmosphere.

61 4.1.2 Heat Sources

Heat is added to the evaporator supply by conventional residential hot water heaters. Due to the absence of a 220V electrical outlet near the apparatus, two of the largest commercially-available 110V water heaters (the General Electric GE02P06SAG), providing 1440 W each, are used instead. One of these heaters is located next to the evaporator reservoir; a small pump circulates water from the tank through this heater and back to the reservoir. Another heater resides next to the evaporator column on the fourth ﬂoor; water from the evaporator supply line passes through this heater directly on the way to being injected into the column. Experiments quickly revealed that domestic water heaters are not designed for vacuum service; gas leaked into the system through the heater’s pressure relief valve. This necessitated a modiﬁcation that would be extremely dangerous if used under any other circumstances: the relief valve was removed and the threaded port plugged. It should be stressed that this is only safe because the desalination apparatus, and the water heater with it, always operates under very low pressure. Both heaters together add a maximum of heat to the distillation system of 2.88 kW. If the enthalpy of vaporization (section 3.1) is 2400 kJ/kg, this 2.88 kW should be capable of continuously producing no more than:

1 −3 Q <= (2.88 kJ/s) kg/kJ = 1.2 × 10 kg/s = 4.32 L/hour (4.1) 2400

4.1.3 The Vapor Conduit

Several designs for conduits to carry vapor from evaporator to condenser were implemented and tested. All were constructed from PVC and brass plumbing

62 components ﬁtted to clear acrylic plates which sealed the top of the columns.

Figure 4.4: Condenser End of Vapor Conduit Equipped with Bubbler

An obvious design for the vapor conduit is a simple length of straight pipe with elbows at both ends to connect with the top plate of each column. The diameter of this pipe should be selected according to the designed-for production rate to avoid a signiﬁcant pressure drop across the vapor conduit. A variation of this design sent vapor down a vertical extension of the conduit into an open tee-ﬁtting submerged under

63 the water surface in the condenser; this “bubbler” concept was believed to promote condensation by forcing the vapor into contact with cold condenser water. However, this introduces back-pressure against the vapor flow; also, the large bubbles formed have less liquid-to-gas contact area than droplets of water in the vapor space would have. Consequently, although this design was built and tested, all results published here use the simple conduit without the bubbler feature. Another concept experimented with during the course of this investigation is to assist vapor flow by means of a pump or fan in the vapor path in an attempt to raise pressure on the condenser side and lower it on the evaporator side, which should improve the rate of water production. Such a conduit was built with a modular design in which axial blowers are sealed, using epoxy, to lengths of clear PVC terminated by flange fittings. These modules are then bolted, on both ends, to additional segments containing elbows which attach to the column sealing plates, making a complete vapor conduit with an interchangeable center section. Two such blower modules were built, using Attwood Marine’s 12-volt DC-powered “bilge blowers” originally intended for ventilating marine engine compartments and bilges; the modules include their Turbo 3000 and Turbo 4000 series blowers, with three and four inch diameter rotors respectively. However, these blowers are not able to achieve a significant change in pressure in this application. They also provide an increased surface area on which vapor is able to condense; due to the geometry of the blowers liquid water rapidly collects around the fan blades, dramatically increasing drag and ruining blower performance. To address this last issue, a new vapor conduit was constructed as a way of mounting a blower in a tilted pipe, so that the product of any premature condensation would run into the condenser end. This meant that the evaporator end of the conduit had to be higher, however. As a precaution against condensation in this vertical

64 section of pipe (which would be wasted as it would fall into the evaporator column again), a heat exchanger was constructed in the center of the conduit to prevent the vapor from losing temperature during this part of its travel to the condenser. This heat exchanger was a length of copper pipe fed with hot water from the evaporator supply, which then continued on to the evaporator injectors. This conduit’s downward-sloping portion accepts the same blower modules as the non-sloping conduit. To serve as a control during experiments an additional module was constructed without a blower, incorporating instead a vacuum-rated ball valve to isolate the evaporator from the condenser. This last module is mounted in the apparatus seen in ﬁgure 4.5. Finally, the most successful vapor conduit investigated in this study is simply a length of straight pipe with a ball valve in the center. Although separating the chambers from each other is unnecessary during practical operation, it is useful for many sorts of experiments, some of which are detailed in chapter 5.

4.1.4 Supply Water Injection

Water is pumped from reservoir to chamber using two inexpensive centrifugal pumps. These were mounted on rubber bushings in the pump cabinet at the second ﬂoor and equipped with small hoses through which water could be introduced to the priming port of the pump, which proved necessary for starting the pumps from a completely dry state. The inlets to these pumps were connected via hose to rigid wands placed in the reservoirs. Several methods of introducing water to the evaporator and condenser were considered and experimented with. As discussed in section 4.1.1, the top of each column is equipped with eight threaded ports above the water surface that can be used for injection. All injection methods tested to date have used four of these ports

65 Heat exchanger inlet Vacuum pump port Heat exchanger copper pipe Vapor conduit valve

Evaporator top Heat exchanger outlet Condenser top

Figure 4.5: Realization of Tilted Vapor Conduit with Heat Exchanger

distributed uniformly (spaced by 90◦ from each other) around the circumference of the chamber and located at an equal elevation. One simple technique is to pump water directly through into the chambers through these ports. In practice this proved unsatisfactory, as the four water streams were of such velocity that they met in the middle of the evaporator, producing a sort of barrier of liquid water that traps vapor underneath, keeping it from ﬂowing to the condenser. This phenomenon eﬀectively reduces the surface area to something on the order of the cross-sectional area of the chamber, wasting the additional height of the injectors relative to the free surface that was purchased at the expense of increased head loss.

66 Cross-conduit valve Valve and line to vacuum pump

Supply manifold Evaporator pressure indicator supply inlets (2 of 4) Differential Condenser pressure sensor pressure sensor

Figure 4.6: Vapor Conduit, Supply Manifolds, and Instrumentation

All experiments with this method of injection failed to produce any measurable water output. A more promising variation would use some form of nozzle to break the water stream into a spray of droplets. The design of these nozzles is neccessarily a compromise between pressure drop across the nozzle and the size of the droplets produced; a more restrictive nozzle can make a higher-surface-area spray but at the expense of increased pumping energy. Fouling considerations further complicate nozzle selection. This method was not investigated experimentally in this study. Yet another technique is that of “vortex injection”; supply water is introduced to the vacuum chamber in such a way as to ﬂow in a thin sheet against the chamber wall. In one realization of this idea the velocity of the supply water ﬂow has a tangential as

67 well as axial component, with the result that a particular unit of water will follow a roughly helical path down and around the vacuum chamber. Apparatus implementing this idea was constructed and tested in the evaporator (see ﬁgure 4.1), proving itself the best of all water-injection devices experimented with during this research. The mechanical design made use of four plastic wands parallel to the axis of the column and descending from the water injection ports at the top of the evaporator. These wands were pierced at intervals along their length and adjusted such that the emerging water streams contacted the chamber wall with the axial and tangential components discussed above. This method gave the supply water the desired vortex-like ﬂow and kept the center of the column clear so that vapor could pass through to the condenser.

4.1.5 Vacuum System

In most experimental configurations, gases are evacuated from the system through a port at the top of the vapor conduit that is guaranteed to be above water level. A valve is attached to this port, permitting the chambers to be isolated from the vacuum system and any leaks that may exist in it. A hose connects the outlet of this valve to an assembly of PVC fittings constituting a water trap; water which has condensed on the way to the vacuum pump flows into this reservoir and can be periodically removed. This water trap is connected by vacuum hose to a Welch 2562B-01 wobbling-piston pump; this unit can bring the entire distillation apparatus from atmospheric conditions to working pressure in a few minutes and conveniently requires no lubricating oil. It is also more than capable of extracting non-condensable gases at the rate at which they accumulate, maintaining chamber pressure during operation. Experiments revealed that this pump is unable to start at the low pressure present during operation; to accommodate this a tee-fitting and valve are connected in between the pump and

68 the rest of the system; opening the valve brieﬂy introduces air from the atmosphere, raising the pressure somewhat and allowing the pump to start.

4.2 Data Acquisition and Control

For research purposes the apparatus is extensively instrumented with a variety of sensors, the output of which is acquired and displayed by a personal computer equipped with National Instruments data acquistion hardware and using NI’s popular Labview software. Although this sophisticated combination of instrumentation and data acquisition is very convenient for the researcher, it is anticipated that most of this complexity would not be strictly necessary in many production implementations of the process, an important consideration for service in underdeveloped areas. Sensors associated with the evaporator and condenser chambers are naturally located on the fourth floor, which is also host to the data-acquisition hardware. This consists of a Dell desktop PC equipped with a National Instruments PCI-6229 card capable of digitizing up to 32 analog inputs at a resolution of 16 bits and a rate of 250 kilosamples per second [85]. It also provides 4 analog outputs and 48 digital I/O pins used, in this application, to control the supply water pumps and the pneumatic foot-valves (section 4.2.5). Signals to and from this card are carried over a cable to a National Instruments SCB-68 connector block, which breaks-out the 68-pin connector on the card to rows of screw terminals; these are in turn wired to twist-lock bulkhead connectors in a custom breakout box that accept cables from the sensors themselves. Power is distributed to many of the sensors by wiring housed in this breakout box and is provided by a 12-volt DC supply kept nearby; these features can be seen in figure 4.1. Sensors and control electronics are also located on the second floor, for monitoring

69 DC power drawer

Instrument drawer

19-inch rack cabinet

Original manual controls

Hose for pump-priming

Supply pumps

Extension cord storage

Figure 4.7: Cabinet Housing Supply Pumps and Instrumentation conditions in and around the reservoirs and for operating the supply pumps. This hardware is largely housed in the 19-inch-rack cabinet that also contains the supply pumps; a rack-mounted instrumentation drawer connects to the fourth-ﬂoor breakout box via a 24-conductor umbilical cable terminated by bulkhead connectors on both ends. Wiring contained in this drawer (ﬁgure 4.10) routes these signals to bulkhead connectors on a front panel, to which the sensor cables are connected. As with the

70 Signal connector to Barometer Mains inlet and circuit DAQ computer breaker

Flow sensor DC power Switched mains connectors inlets outlets for pumps

Temperature sensor TDG sensor Relay board for pumps connectors connector

Figure 4.8: Instrument Drawer Under Construction

fourth-ﬂoor breakout box, the instrument drawer distributes power at various DC voltages to the sensors as needed. This drawer is also the location of relays used to control the supply pumps (section 4.2.5). Power is provided by a DC power drawer mounted in the rack and containing four open-channel power supplies; one outputting 12 volts and three at 24 volts each; so many supplies are necessary since the ﬂowmeters (section 4.2.3) use current-loop

71 Mains inlet and Four open-frame circuit breaker power supplies

Banana jacks Neon indicators

Safety ground

Figure 4.9: DC Power Drawer signaling and consequently each require their own power supply. These units are wired to neon indicators and banana jacks on the front panel of the power drawer; patch cables are used to connect these outlets to the power inlets on the instrument drawer’s panel.

72 4.2.1 Temperature

Both evaporator and condenser chambers are equipped with Omega ON-970-44005 thermistors, which each consist of a five-inch-long temperature sensing probe attached to a body threaded for 1/8 NPT. These are mounted in threaded bushings inserted through the bottom coupler (again using the additional thickness in this section to ensure a good seal) so that the sensing tip is immersed in the column of water. Temperature in both reservoirs is measured with two more of the same Omega thermistors, attached to floats made of PVC pipe fittings which locate the tip of the sensor roughly 10 inches below the water surface. Ambient temperature is measured using a thermistor calibrated against an Omega bench temperature meter; this sensor is located on the fourth floor and is typically hung from the frame of the apparatus.

4.2.2 Pressure

Each chamber contains a thin-film pressure transducer (Omega PX603-30VAC5V) powered from the fourth-floor 12-volt DC supply; these sensors output 1-5 volts proportional to a pressure range of 0-30 inches of mercury or 0-1 atm. The chamber pressure sensors are threaded into the upper instrument ports in the evaporator and condenser. Detecting the relatively small difference in pressure between evaporator and condenser during some experiments is difficult due to the wide measurement range of these sensors. To directly measure the pressure drop across the vapor conduit a differential pressure transducer, Omega model PX771A-100WCDI, is connected between both chambers. Like the absolute pressure sensors, this device outputs 1-5V over a range of 0 to 0.042 atm.

73 To serve as a baseline, the atmospheric pressure can be measured using a barometer manufactured by Omega (model PXM02MD0-880MBARA5T). This unit is mounted inside the instrument drawer on the second ﬂoor, with its pressure port facing an opening to reduce the sensor’s time constant. In practice this minor design consideration is unnecessary, however, as changes in the atmospheric pressure have been almost unmeasurably small during the course of any experiment.

4.2.3 Flow

Flow through the columns is measured by two turbine flowmeters manufactured by Seametrics (model number WTP102-200-18-2). These devices are bolted to flanges in the rigid “foot” sections at the bottom of each column (figure 4.3). Flow readings are signalled to the DAQ hardware by an analog current-loop operating in the standard 4-20 mA range; because of this method of communication each flow meter requires its own 24 volt DC power supply in the loop. Two identical open-frame power supplies are installed in the DC power drawer for this purpose; they are placed in series with the current loops by connecting patch cables to banana jacks on the front of the instrument drawer.

4.2.4 Total Dissolved Gas

Due to the impact of the non-condensable gas problem on overall distillation eﬃciency, it is desirable to measure how much dissolved gas is contained in some given body of water and to measure how fast this gas comes out of solution when brought into the low-pressure environment of the evaporator and condenser chambers. Many methods are available for measurement of dissolved oxygen concentration due to the importance of this parameter to human health and to aquatic ecology; however, the sum of all

74 non-condensable gases is of greater interest in low-pressure distillation research. Gas tension sensors are one type of device that can directly measure the total partial pressure resulting from all gases dissolved in water. Typically, they are fully or partially submerged and contain a pressure chamber separated from the water by a gas-permeable membrane; gases will diffuse through this membrane until the inside of the sensor is at equilibrium with the pressure of gases dissolved in the water [86]. Therefore, a pressure sensor reading the inside of the gas tension device measures the total dissolved gas partial pressure, which can be related to concentration using Henry’s law (section 3.2). This research made use of a T900 TDG pressure probe, manufactured by In-Situ Inc. The output of the T900 is an analog voltage level proportional to the gauge total dissolved gas pressure, or the difference between total dissolved gas partial pressure and the ambient pressure. During experiments this probe was usually left to float in the reservoir of interest and its cable was terminated by a bulkhead connector plugged into a port in the front panel of the instrument drawer. The sensor also features a replaceable membrane cartridge, necessary as fouling of the membrane by algae or other contaminants can increase the time constant of the sensor even though the equilibrium pressure will not be affected [87]. Experience revealed that this probe responds to changes in gas concentration slowly even with pristine membranes, a well- known limitation of gas tension devices addressed by researchers in oceanography [88]. Efforts to compensate for this delay through sensor characterization are detailed in section 6.2.

4.2.5 Control Actuators

Early experiments conducted using this apparatus made use of a “low-tech” means of control: commands shouted down from the fourth ﬂoor to a collaborator on the second

75 ﬂoor who operated pumps and valves by hand. This scheme was eventually automated using Labview so that most of the parameters of the system could be controlled from the fourth ﬂoor PC, either automatically (implementing a feedback controller, for example) or manually. A custom graphical interface was developed as a Labview VI program, using the visual programming and front panel environment provided by that software; this also controlled all aspects of data acquisition and provided virtual dials, running plots, and other types of indicator for temperatures, pressures, and other parameters of the system.

Solid-state relays

Switched mains outlets Transistor driver board for supply pumps

Figure 4.10: Detail of Instrument Drawer Showing Pump Relays

Crydom solid-state relays driven by discrete bipolar transistors, whose bases are

76 driven in turn by logic-level outputs from the National Instruments hardware, switch the two water supply pumps. Both foot units are equipped with valves capable of sealing the bottom of the Torricelli column. These can be used to initialize the system without the use of a vacuum pump; with both foot-valves closed, water is pumped into the columns until the level reaches the very top of the vacuum chambers, at which point the vapor conduit is placed onto the apparatus, compressing the O-rings either by its own weight, by additional force applied manually by the experimenters themselves, or by fixing the conduit to the columns by mechanical means. Opening the valve then causes water to flow out from the chambers until the level reaches its ten-meter-equilibrium height; as soon as the free surface begins to descend the pressure of the outside atmosphere is enough to keep the vapor conduit sealed against the O-ring. Most often, however, the vacuum pump is used instead to evacuate the system prior to operation. Each foot-valve consists of a pneumatically operated plumbing balloon—a tool designed to allow plumbers to carry out repairs on a section of some pipe network which lacks valves or other means of stopping flow. Applying pressurized air to this device causes a rubber balloon to inflate, sealing the pipe in which it is inserted. For service as foot-valves these ballons are placed in tee-sections at the bottom of the column which have been modified to hold them in place; air can be supplied by a compressor and tank combination and controlled by a pair of solenoid valves, energized by relays driven directly from the National Instruments hardware. The vacuum pump used for system initialization and non-condensable gas removal resides on the fourth floor and is operated by hand, as is the heating-loop pump on the second floor, the state of which changes too infrequently during typical experiments to justify equipping it with remote control.

77 Chapter 5

Experiments and Results

Experiments performed during this study were designed to investigate various aspects of the barometric distillation process. The most fundamental of these is simply to operate the device as a desalinator and measure the performance either directly, by observing the change in volume of either reservoir, or indirectly, by measuring evaporator or condenser temperature, either of which can be easily related to the quantity of distillate. Another set of experiments used the Total Dissolved Gas sensor (section 4.2.4) to investigate the effect of non-condensable gases; these experiments were designed to lead to an estimate of the transport rate of such gases into the chamber, so that the magnitude of this problem in production systems can be understood. It is unfortunately not enough to measure the degassing rate directly as the TDG sensor has a poor transient response and requires a correction to be applied to produce a usable result for this analysis. To that end the sensor is characterized by measuring its step reponse. Unless otherwise noted the configuration of the apparatus is kept the same in all experiments detailed in this chapter. In each case the same sensors and instrumentation electronics (as detailed in section 4.2) are used. The apparatus is fitted with the most

78 60 Evaporator Tank Temperature Condenser Tank Temperature 55 Ambient Temperature

50 ]

35 Temperature [

20 0 5 10 15 20 25 30 Time [min.]

Figure 5.1: Temperatures During Distillation

sucessful injection method, the vortex-wand injectors (section 4.1.4), and the same vapor conduit, one using a straight section of pipe with a valve in the center, the status of which can be open or closed depending on what is being measured in the particular experiment.

5.1 Distillation Experiments

A typical experiment intended to investigate the desalination performance of the system is conducted according to the following procedure:

1. The bottom reservoirs are each ﬁlled with a known volume of water; a temperature diﬀerence is established by heating the evaporator tank and, optionally, by placing a quantity of ice in the condenser tank (in this case the volume of water must

79 be measured again after the ice melts completely).

2. Once the desired temperature diﬀerence is achieved, the vacuum pump is turned on. The vapor-conduit valve is left open so that both sides can be evacuated by one pump. It may be necessary to physically pull the top plates down onto the chambers to ensure that the O-rings are properly seated; once the chamber pressure drops slightly the pressure from the atmosphere is enough to maintain the seal.

3. As the chambers are evacuated, water rises into the two Torricelli columns until the free surface is close to the top (the exact height depends on the original water level in the reservoirs). Visible boiling may occur, depending on temperature, and non-condensable gases will come out of solution as bubbles.

4. Once the equilibrium height (with pump running) is attained, the vacuum pump is switched oﬀ and the valve separating the pump from the vapor conduit is closed, preventing any gas leaks through the vacuum pump. The water level will drop and typically oscillate brieﬂy before coming to rest at a somewhat lower level due to gases coming out of solution and raising the chamber pressure.

5. Using the Labview GUI, both supply pumps are turned on to recirculate reservoir water through the chambers. Distillation begins at this point and data is recorded for later analysis; of particular importance is the temperature of evaporator and condenser as this is a measurement of the quantity of distillate.

6. The experiment ends when the temperature diﬀerential is too small to support distillation. This is a limitation of the available heat source, which is undersized for the ﬂow rates and volumes present in this apparatus, making steady-state distillation impossible.

80 Alternatively, it is possible, more for demonstration than for research purposes, to open and close the vapor-conduit valve during distillation operation. Closing this valve abruptly stops the visible boiling in the evaporator, and opening it again causes the boiling to re-appear, providing a dramatic demonstration of the pressure-reducing eﬀect of the condenser on the evaporator. Temperature behavior of the evaporator and condenser during a typical distillation experiment is presented in ﬁgure 5.1. The initial volume recorded for this experiment is 150 L in each reservoir. During the experiment both heaters and the recirculating pump for the reservoir heater were in operation, which explains why the condenser temperature is seen to change much more than the evaporator temperature; the heaters are keeping evaporator temperature closer to constant.

5.2 Total Dissolved Gas Experiments

All measurements taken with the total dissolved gas sensor are calibrated in units of millimeters of mercury relative to one atm; in other words, if the subject water comes to equilibrium with a standard atmosphere enviroment, the TDG sensor will read zero. Any TDG partial pressure above zero indicates supersaturated water, while any reading below zero is of partially degassed water. Since degassed water is of greater interest in barometric distillation studies, most readings taken using this sensor occur in the 0 to -400 mmHg range.

5.2.1 Degassing and Regassing

To measure the degassing rate, water is circulated from the reservoir through one Torricelli column (either evaporator or condenser—they are identical for the purposes of this experiment) while the vacuum pump is operated. These experiments proceed

81 thus:

1. The TDG sensor is placed in a reservoir and the data acquisition and control system is started; observation of the TDG signal over a period of several minutes conﬁrms that the gas concentration has reached steady-state. During this time water is circulated through the column to ensure a uniform gas concentration. A lid is placed on top of the chamber to prevent the water from becoming super-saturated with dissolved gas (as it can if the water is sprayed into open air), but the chamber remains at standard pressure.

2. Force is momentarily applied to the lid while the vacuum pump is running until atmospheric pressure seals the chamber; the vapor-conduit valve is kept closed to isolate the other column from the experiment. This continues until the water level in the column has reached maximum.

3. Reservoir water is circulated through the chamber by activating the appropriate supply pump, while the vacuum system continues evacuation.

4. Non-condensable gases come out of solution in water as it passes through the low- pressure environment of the chamber; these gases are rejected to the atmosphere by the vacuum pump. After a noticeable delay, readings of the TDG sensor begin to fall.

5. The vacuum pump is turned oﬀ. TDG concentration continues to fall, before leveling oﬀ and eventually reaching equilibrium.

Experiments reveal that this method of degassing can easily result in non-condensable gas levels below the range of the TDG sensor; the operator should cease degassing at some level signiﬁcantly higher than the minimum of −400 mmHg to account for the

82 50

-50

-100

-150

-200

-250

-300

Relative TDG Partial Pressure [mmHg] -350

-400 0 2 4 6 8 10 12 14 16 Time [min.]

Figure 5.2: Vacuum Pump Degassing Performance as Measured by TDG Sensor

time delay and ensure that TDG levels come to equilibrium within the measurable range, if the ﬁnal partial pressure is of interest. Overall volume did not change measurably during any degassing trials, despite the loss of small amounts of gas and water vapor to the atmosphere. The average chamber temperature was 22.1 and the average chamber pressure was 0.048 atm or 4864 Pa; both remained approximately constant during this trial.

5.2.2 Step Response

Degassing experiments make obvious a limitation of TDG measurement: the poor transient response of the sensor. Results presented in ﬁgure 5.2 indicate a delay of greater than one minute before the TDG sensor begins to respond to the decreasing gas concentration in the reservoir. The true rate of non-condensable gas removal at

83 various concentration levels is therefore highly uncertain. However, degassing measurements obtained from a slow sensor can be improved, not in real-time, but after an experiment has been completed, if a full characterization of that sensor is performed and used to correct the unprocessed data. The best way to do this is to obtain the time-domain impulse response of the system in question; the transformation of this response into the frequency domain is the transfer function of the system. For some types of sensor a reasonable approximation of an impulse is possible: striking an accelerometer with a hammer, for example. Producing an impulse of dissolved gas concentration, however, is difficult (how can a infinite concentration be applied for an infinitesimally short time?); worse, a very slow sensor with a time constant on the order of minutes is likely to produce an unmeasurably small response, missing the event entirely. Obtaining a step response is a more practical approach. This can be done using two bodies of water at different levels of TDG concentration; the sensor is placed in one and allowed a long time to reach equilibrium. While recording its output, the sensor is pulled from one body of water and placed in the other with as rapid a movement as possible; given the very slow transient response of the TDG sensor this is an adequate approximation of a step function in gas concentration. This experiment is conducted according to the following procedure:

1. One reservoir is degassed by sealing the corresponding chamber, turning on the vacuum pump and circulating water through the column. The TDG sensor is used to monitor the progress of the degassing.

2. Before the gas concentration falls below the range of the sensor, the degassing is stopped. A long time is allowed to pass to conﬁrm that the system is at steady-state and that the TDG measurement remains within range.

84 50

-50

-100

-150

-200

-250

-300

Relative TDG Partial Pressure [mmHg] -350

-400 0 10 20 30 40 50 60 Time [min.]

Figure 5.3: Step Response of Total Dissolved Gas Sensor

3. The experiment begins when the TDG sensor is pulled from the degassed reservoir and immediately placed in the other water supply, which has been allowed to remain at a higher concentration level; usually one very close to the equilibrium level expected at standard temperature and pressure.

4. Data is recorded, again for a long time so that the sensor can achieve a new equilibrium at this higher concentration.

A clearly overdamped transient response is seen in ﬁgure 5.3.

85 Chapter 6

Analysis

6.1 Achievable Pumping Ratio

As previously noted, the pumping ratio R depends on the geometry of the distillation columns, surface area and other characteristics of the liquid flows inside, and on the temperature difference ∆T . Of these factors it is probable that ∆T has the largest effect on the ratio. It is also the easiest parameter to adjust; in fact, in most distillation experiments that begin with a pre-established temperature difference ∆T varies naturally as the heat sources are not sufficient to maintain high evaporator temperature once distillation begins. Figure 5.1 shows the results of one such experiment. Water flow rates in both columns were recorded as well and remained roughly constant during the experiment; from this and the temperature history it is possible to determine not only the total amount of distillate produced but the rate of distillation versus time or, more interestingly, versus ∆T . Temperature decrease in the evaporator is related to the amount of energy lost to evaporation just as temperature increase in the condenser is related to the amount of water condensed (see section 3.1). Of the two, the condenser temperature is preferable to use in determining R as temperature change in the

86 condenser is caused entirely by phase change; the evaporator temperature curve is distorted by the addition of 3 kW by the heaters. Distillate production rate is related to condenser temperature by:

dV dT C = V (t) c p (6.1) dt dt ∆Hvap

In theory the derivative of temperature used here can be calculated using numerical methods from the experimental data; in practice simple methods of numerical differentiation such as finite-difference are not suitable as small amounts of noise in the temperature record yield large amounts of noise in the derivative; low-pass filtering or other techniques will have to be employed. One method known to yield good results in such cases is the Savitsky-Golay filter [89]. An Octave script using this filter to obtain a cleaner derivative of temperature is included in appendix A.2. The result of running this script on the data presented in figure 5.1 is shown in figure 6.1; the ratio varies between 400 and 100 during the course of the experiment. As expected, raising ∆T lowers R, but only up to a point—the ratio is seen to be approaching a lower bound of approximately 120 as the limits of the apparatus become significant. Further, as ∆T is reduced to 5 degrees or less, the ratio rises sharply, suggesting that no distillation will take place below some threshold temperature difference. It should be understood that these limits (R > 120, ∆T > 5) are probably valid only for this particular apparatus, although other Barometric Distillation systems will probably be governed by similar limiting phenomena.

87 450

400

350

R 300

250

200 Pumping Ratio

150

100

50 0 5 10 15 20 25 30 35 Temperature Diﬀerence ∆T []

Figure 6.1: Experimental Pumping Ratio

6.2 TDG Sensor Characterization

A transfer function for the TDG sensor can be obtained using an experimental step response (ﬁgure 5.3). One method of doing this is to assume that the sensor can be adequately represented by a ﬁrst-order system (a strong assumption given the shape of the plot), and measure the appropriate system parameters from the plot or by inspecting the data. The time constant, or time the step response takes to reach 63% of its steady state value, is measured at 360 seconds, and the DC gain is one. This approach yields a transfer function of:

1 H(s) = (6.2) 360s + 1

88 The equivalent time-domain impulse response is:

1 h(t) = [e−1/360tu(t)] (6.3) 360

-50

-100

-150

-200

-250

-300

Relative TDG Partial Pressure [mmHg] -350 Experimental First-Order Model -400 0 5 10 15 20 25 30 35 40 Time [min.]

Figure 6.2: Comparison of Experimental and Modeled Step Responses

Octave can produce a step response from equation 6.2 using the functions tf() and lsim(), which together can simulate system response to an arbitrary input function. This approach was selected instead of using the Octave function step() in order to reproduce the response to a step function of non-unity magnitude and non-zero initial value, as in the experiment, where the step function has an initial value of -376 mmHg, a ﬁnal value of 7 mmHg, and a time delay of 80 s (this is when the step change in dissolved gas concentration was applied). The Octave code to produce the equivalent step response from the ﬁrst-order model is provided in appendix A.3.

89 Figure 6.2 is a plot of step response for the ﬁrst-order model, showing good agreement with the experimental step response. The time scale has been reduced to 40 minutes in order to emphasize the diﬀerence between the experimental and modeled results.

6.3 Non-Condensable Gas Extraction Rate

It is theoretically possible to correct the degassing data plotted in figure 5.2 by deconvolving this with the modeled impulse response of the sensor (equation 6.3). Unfortunately, deconvolution is extremely sensitive to noise in the experimental signal and produces nearly useless results in this case without applying more advanced techniques such as Wiener deconvolution. However, obtaining the true degassing curve in this case is easier because of the particular shape of the measured TDG curve and because the sensor is so well modeled by a first-order system. After the initial delay is overcome, the measured degassing curve declines with nearly a constant slope, although the curve does flatten over time by a small amount. This suggests that the true degassing curve might be well described by an exponential of the form: y = 760e−0.0014t − 760 (6.4) in which the rate -0.0014 was determined by trial-and-error, and the amplitude of 760 is due to one atmosphere equalling 760 millimeters of mercury; an expression like eq. 6.4 will approach a value of -760. A reading of −760 mmHg (supposing that the sensor could measure partial pressures below -400 mmHg) would indicate that all dissolved gas had been removed from the subject water, so any degassing process should indeed approach this limit asymptotically. This proposed true degassing curve can be verified by using it as input to the

90 0 Proposed Original First-Order Model Result Experimental Measurement -100

-200

-300

-400

-500 Relative TDG Partial Pressure [mmHg]

-600 0 2 4 6 8 10 12 14 16 Time [min.]

Figure 6.3: Proposed Original, Modeled, and Experimental Degassing Behavior model obtained in section 6.2. Once again, an Octave script (appendix A.5) using the lsim() function can obtain model responses to arbitrary input functions; figure 6.3 is the output of this program. The response of the first-order model to the proposed original function is very similar to the experimental measurement; the only significant difference is an additional time delay of approximately 50 seconds in the experimental results. Since the sensor step response does not show the sort of delay that would be typical of higher-order models, it is likely that this delay is not a result of the sensor but rather reflects some aspect of the flows encountered during degassing. Although the raw measured data is plotted from the time when the flow meter first indicated that the full flow rate had been acheived (four seconds after the pump was turned on), water exits the column at the bottom of the reservoir while the sensor measures near the top of the reservoir; some time must be required for a change in TDG partial

91 pressure to reach the sensor. Such a delay is not encountered in the step response experiments, in which the sensor is quickly thrust into a fully-mixed, fully-equilibrated quantity of water.

6.4 Supply Water Pumping Eﬃciency

Applying the analysis developed in section 3.3 to the data seen in ﬁgure 5.1 can produce an estimate of the distillation eﬃency of the BD apparatus studied here, or one similar to it. The following assumptions are applied:

A temperature diﬀerence of greater than 20 is available, yielding a pumping ratio of approximately 120.

Flow rates averaging 8 gallons per minute, or 30.3 liters per minute, exist in both evaporator and condenser lines (this is realistic for the current apparatus).

Both sides of the system have identical pump eﬃciencies of ηpump = 80% and equal head losses (to be calculated below).

Supply water is injected at a point one meter above the height of the water column, or ∆z = 1 m.

The supply line has an internal diameter of one inch and is made of smooth plastic (similar to the experimental apparatus) with a roughness height of = 0.0015 mm.

To begin it is necessary to determine the Reynolds number of the supply line flow (eq. 3.17). V is the average velocity over the cross-section of the flow, determined by dividing the flow rate by the cross-sectional area (that of a circular duct with 1 inch

92 internal diameter): −6 3 Q 505 × 10 m /s V = = = 1.00 m/s πr2 π (0.0127 m)2

ρV d (998 kg/m3) (1.00 m/s) (0.0254 m) Red = = = 25300 µ 1.003 × 10−3 Ns/m2

This value is well into the turbulent range, so the Colebrook formula can be used to determine the friction factor; running the Octave script colebrook.m (appendix A.1) provides a value of f = 24.6 × 10−3. Head loss due to friction is given by the Darcy-Weisbach equation (eq. 3.16):

2 m 2 L V −3 11 m (1.00 /s) hfriction = f = 24.6 × 10 = 0.54 m d 2g 0.0254 m 2 (9.81 m/s2)

Observe that L = 11 m; this is the 10 meter height of the water column plus one meter for ∆z, as assumed above. The total head required of the pump is thus:

2 m 2 Vout (1.00 /s) hpump = ∆z + + hfriction = 1 m + + 0.54 m = 1.59 m 2g 2 (9.81 m/s2)

The energy eﬃciency for this apparatus, where energy is consumed only by the supply pumps and neglecting the cost of removing non-condensable gases, is therefore:

m 2 ES 2RgHpump 2 (120) (9.81 /s ) (1.59 m) = = = 4.68 kJ/kg md ηpump 0.8

This value is speciﬁc not only to the particular apparatus but also to the ﬂow rate

− 3 selected at the beginning, 8 gallons per minute (505 × 10 6 m /s), which was used to determine the friction factor; it is important to consider not only how much energy is required to produce a given mass of water but also how much power is needed to provide a desired distillation rate. This apparatus under the same assumptions should

93 require pumping power equal to:

−6 m3 kg 3 m 2 2m ˙ Sghpump 2 (505 × 10 /s) (998 /m ) (9.81 /s ) (1.59 m) PS = = = 19.7 W ηpump 0.8

for a production rate of:

L Qs 30.3 /min Q = = = 0.25 L/min = 15 L/hour d R 120

6.5 Eﬃciency of Gas Extraction

It must be emphasized that all analysis in the previous section neglects completely the cost of removing non-condensable gases. Using the relations developed in section 3.3.3 and the non-condensable gas extraction rate measured in section 6.3, an estimate can be obtained for this additional cost. Given a known rate of decline for some particular dissolved gas partial pressure, it is possible to determine a volume ﬂow rate through the extraction path (vacuum pump, reinjection tube, etc.) using the ideal-gas law. For some gas i, Henry’s law states:

ci pi = kH or, taking the time derivative:

c˙i p˙i = (6.5) kH

The number of moles of i is related to concentration by the total volume of water VT :

ni = VT ci n˙ i = VT c˙i

n˙ i = VT kHp˙i (6.6)

94 where n˙ i is the rate at which moles of i come out of solution. This rate is enough to determine the head required of the extraction pump using the methods of section 3.3.3. This analysis will also assume that only oxygen and nitrogen are signiﬁcant, as together they account for more than 98% of the atmosphere by volume [78], and that they are present in extracted gas in the same proportion as their relative proportions in the atmosphere, expressed as:

p˙O2 = 0.21p ˙TDG p˙N2 = 0.78p ˙TDG (6.7)

where p˙TDG is the rate of change of TDG partial pressure, equal to the time derivative of equation 6.4: dy =p ˙ = −1.06e−0.0014t (6.8) dt TDG

At t = 0, when the TDG partial pressure is zero relative to the atmosphere (in other words, when the source water is fully equilibrated), the rate of change is:

mmHg −3 atm p˙TDG = −1.06 /s = −1.39 × 10 /s

Therefore, the rates of partial pressure of oxygen and nitrogen are:

−3 atm −3 atm p˙O2 = 0.21 −1.39 × 10 /s = −0.295 × 10 /s

−3 atm −3 atm p˙N2 = 0.78 −1.39 × 10 /s = −1.10 × 10 /s

A straightforward application of equation 6.6 can determine the rate of moles of gas coming out of solution. Here the Henry’s law constants are taken at 20, the 2 diﬀerence between this and the actual temperature of the condenser is assumed to

95 have negligible eﬀect.

n˙ O2 = VT kH,O2 p˙O2 (6.9)

−3 −3 −6 = (150 L) 1.3 × 10 mol/L·atm −0.295 × 10 atm/s = −50.5 × 10 mol/s

n˙ N2 = VT kH,N2 p˙N2 (6.10)

−4 −3 −6 = (150 L) 6.1 × 10 mol/L·atm −1.10 × 10 atm/s = −101 × 10 mol/s

And the total rate for the mixture of gases is:

−6 mol n˙ NCG,cond = 159 × 10 /s (6.11)

The negative sign is dropped, meaning that this is no longer the amount of gas being lost from the reservoir, but the amount of gas passing through the extraction circuit

6.5.1 The Dissolved Gas Contribution of the Evaporator

However, the extraction rate measured in the degassing experiment applies only to

the condenser side (indicated by the subscript in n˙ cond); during the experiment the evaporator was sealed oﬀ to prevent distillation from occuring and to keep temperature and pressure as constant as possible. During actual desalination operation the evaporator will contribute non-condensable gases as well, so the value in equation 6.11 is too low. Simply doubling the condenser rate would be a crude approximation of the total; as solubility of gases declines with temperature and salinity (see section 2.4), less gas will be extracted from the evaporator than from the condenser. One way to correct the gas extraction rate calculated above for temperature and salinity is to adjust the Henry’s law coeﬃcients for nitrogen and oxygen using equations 3.3 and 3.4, which will result in lower rates of gas coming out of solution.

96 The temperature correction, assuming again that the condenser is 20 hotter than the evaporator, is:

0 1 1 0 kH,O ,T = k exp (1500 K) − = 0.72k 2 H,O2 313 K 293 K H,O2 0 1 1 0 kH,N ,T = k exp (1300 K) − = 0.75k 2 H,N2 313 K 293 K H,N2

Correcting for salinity requires ﬁrst that the salinity of evaporator feed water is known. This depends on the application and site of the barometric distillation process; seawater varies considerably in salinity from location to location, to say nothing of the possibility of desalinating brackish waters. In further analysis, the assumption is made that ocean water of 3.5% salinity by mass, or 35 parts per thousand, is supplied to the evaporator; this value of salinity is found at surface level in many places in the

earth’s oceans [90]. This salinity is equivalent to a molar concentration of 0.62 mol/L. Under this assumption, equation 3.4 yields:

k k k = H,O2,T = H,O2,T H,O2,T,S 10(0.141)(0.62) 1.22 k k k = H,N2,T = H,N2,T H,N2,T,S 10(0.134)(0.62) 1.21

Substituting values for the temperature correction provides ﬁnal values for the adjusted Henry’s law coeﬃcients:

0.72 k = k = 0.59k H,O2,T,S 1.22 H,O2 H,O2 0.75 k = k = 0.62k H,N2,T,S 1.21 H,N2 H,N2

These corrections are then applied to the mole ﬂuxes determined in equations 6.9

97 and 6.10 to obtain the rate of gas extracted from the evaporator:

−6 −6 mol s mol s n˙ evap,O2 = 0.59n ˙ O2 = 0.59 50.5 × 10 / = 29.8 × 10 /

−6 −6 mol s mol s n˙ evap,N2 = 0.62n ˙ N2 = 0.62 101 × 10 / = 62.6 × 10 /

Therefore, the total mole ﬂux of non-condensable gases for the apparatus is:

−6 mol −6 mol n˙ NCG = (159 + 29.8 + 62.6) × 10 /s = 251 × 10 /s (6.12)

6.5.2 Rate of Water Vapor Extraction

Water vapor is an additional and very signiﬁcant component in the gas mixture being evacuated from the chamber. Although extracting water vapor along with the non-condensable gases is not necessary or even desirable, the vacuum pump obviously cannot choose which molecules to evacuate; the presence of water vapor in the chamber means that some of it will be removed through the extraction path. The rate of such extraction can be estimated using the ratio of the vapor pressure of water at the chamber temperature to the non-condensable gas partial pressure. That vapor pressure is:

pH2O,295 K = 0.026 atm

The total pressure in the chamber is 0.048 atm. Since all gases have been assumed ideal and the total pressure is the sum of all partial pressures (by Dalton’s Law), partial pressure fractions are equal to mole fractions as in the following expression:

p n H2O = H2O = 118% ptotal − pH2O ntotal − nH2O

98 So the molar ﬂow rate of water is:

−6 mol −6 mol nH˙2O = 1.18n ˙ NCG = 1.18 251 × 10 /s = 296 × 10 /s

Note that the flow rate of water is 118% of the total non-condensable flow rate, but that condenser and evaporator do not make separate contributions to the water vapor flux as they did for oxygen and nitrogen; water vapor flowing from the evaporator through the vapor conduit condenses until the condenser pressure is equal to the vapor pressure at the condenser temperature.

6.5.3 Properties of the Gas Mixture

Mixed gas ﬂows through the vacuum pump with a total molar rate of:

−6 mol −6 mol n˙ m = (251 + 296) × 10 /s = 547 × 10 /s (6.13) and the ideal gas law provides the corresponding volume ﬂow rate:

−6 mol J n˙ mRT (547 × 10 /s) (8.314 /K ˙mol) (295 K) −6 3 Q = = = 276 × 10 m /s (6.14) m p 4864 Pa where the temperature and pressure used are their average values in the condenser during degassing; since the extraction circuit draws directly from the condenser and most of the dissolved gas ﬂux is due to the condenser this assumption is probably reasonable. It will be necessary to determine the molar mass of the gas mixture; this is simply

99 the weighted average of molar masses for three molecules under consideration:

n˙ O2 n˙ N2 n˙ H2O Mm = MO2 + MN2 + MH2O n˙ m n˙ m n˙ m 32 (50.5 + 29.8) + 28 (101 + 62.6) + 18 (296) = g/mol 547 −3 = 22.81 × 10 kg/mol

This result leads to the mass ﬂow rate:

−6 mol −3 kg −6 kg m˙ m =n ˙ mMm = 547 × 10 /s 22.81 × 10 /mol = 12.47 × 10 /s

6.5.4 Vacuum Pump Power

Equation 3.20 can be used to determine the specific work (and consequently power, by multiplying this value by the mass flow rate) required of the vacuum pump. The specific gas constant is:

R 8.314 J/K·mol R = = = 364 J/K·kg m −3 kg Mm 22.81 × 10 /mol

This compression is assumed to be adiabatic; therefore the polytropic exponent n should be equal to the speciﬁc heat ratio k; taking the value for air as an estimate gives n = 1.4. If gases are rejected to the atmosphere instead of re-injected into the column, the speciﬁc work is:

" # w nR T p (n−1)/n = m 1 2 − 1 (6.15) m n − 1 p1 " 0.4/1.4 # 1.4 (364 J/K·kg) (295 K) 1 = − 1 0.4 0.048

3 = 519 × 10 J/kg

100 9

Vacuum Pump Power [W] 2

0 0 1 2 3 4 5 6 7 8 9 10 Depth [m]

Figure 6.4: Pump Power Required for Gas Reinjection

The necessary pump power, taking the isentropic eﬃciency to be ηs = 80%, is given by: −6 w m˙ 12.47 × 10 kg/s m 3 J Pgas = = 519 × 10 /kg = 8.09 W (6.16) m ηs 0.80

If extracted gas is instead re-injected, the speciﬁc work and pump power will vary

depending on depth from Pgas = 0 at zero depth to Pgas = 8.09 W at the maximum depth of 10 meters, where pressure is equal to one atmosphere. An Octave script has been written to perform the calculation over this depth range; the result of this is plotted in ﬁgure 6.4. Taking the result of equation 6.16 and adding it to the power required by the water supply pump yields a value for the total power consumed by a BD process at a similar scale as the experimental apparatus, with all the assumptions listed above,

101 and rejecting gases directly to the atmosphere. That power is:

Ptotal = PS + Pgas = 19.7 W + 8.09 W = 27.8 W (6.17)

Since this analysis applies to a system producing 15 liters per hour, the energy required to produce a given amount of water is:

J 27.8 /s 3 E = = 6.67 × 10 J/L = 1.85 kWh/m3 (6.18) 4.167 × 10−3 L/s

102 Chapter 7

Conclusions

It would be a mistake to take the calculated value of specific energy in the last chapter too seriously; especially wrong would be to conclude from it that Barometric Distillation is twice as efficient as Reverse Osmosis. In all analysis of potential desalination technologies, it is crucial to compare “oranges to oranges”. Those comparing estimates of energy efficiency and overall cost must insist on equivalent assumptions. For example, an RO system making use of the greater-than osmotic pressure naturally available in the deep ocean (the osmotic pump concept [91]) can theoretically operate with no energy expenditure at all, but for this to be compared against conventional RO systems the cost of pumping the fresh water product some distance to shore has to be accounted for [15]. Similarly, to compare costs for a proposed process derived from theoretical analysis or from measurement of an experimental system to those obtained from operational experience of a commercially mature process is dangerous; it is likely that such a comparison will be unfairly favorable to the experimental or theoretical process. There are many additional energy expenditures necessary in an operating facility that are not captured by this analysis. For instance, head loss in the supply water circuit will probably be higher in reality with the effect of fittings, valves, and

103 nozzles or similar surface-area-increasing devices. The calculation presented here also assumed that evaporator and condenser supply waters were available in pools under the apparatus, an arrangement that existed in the experimental apparatus but probably will not exist in reality, where additional pumps may be necessary to bring these feedwaters to the distillation plant. This extra cost will be particularly high where the condenser freshwater supply is drawn from a deep aquifer. Furthermore, all analysis was based on a new, pristine system, without the friction and loss of cross-sectional area caused by inevitable fouling. Although it is easy to argue that such fouling is likely to occur at a slower rate in this process due to lower-temperature operation and the use of direct contact heat-exchangers, the specific energies cited in table 1.2.6 are, once again, published mean values from operational facilities and therefore already include this effect. Also, the pumping ratio R, plotted vs. ∆T in figure 6.1, should be slightly higher to account for the effect of salinity on vapor pressure; all experiments with this apparatus made use of the same fresh water in evaporator and condenser. This phenomenon will increase the specific energy of barometric distillation somewhat. Finally, and most crucially, the large saline heat sources and freshwater heat sinks envisioned as driving the BD process may not be common in the real world, or may be too difficult to exploit. In this case other means of supplying and removing the latent heat of vaporization may need to be developed, and many of these methods, such as solar-thermal collectors, will require additional pumping energy. All unaccounted-for effects are not on the negative side of the ledger, however. For instance, it is probable that significant improvements in the design of the evaporator and condenser are possible; changes such as the vortex-wand injection on the evaporator side were seen to greatly increase the effectiveness of the system; it is hard to believe that these first simple attempts have exhausted the pool of possible optimizations.

104 Any improvement in this area that manages to reduce R can have significant effects on energy efficiency. It is also worth noting that this process has some advantages beyond merely the use of low-temperature heat; the apparatus is mechanically simple, requiring no more moving parts than a pair of low-pressure water pumps and a vacuum pump for non-condensable gas removal. Direct-contact evaporation and condensation obviates the need for metallic tubes as heat-transfer surfaces—a BD system can be constructed from a broad variety of low-cost, corrosion-free materials. As mentioned above, this mode of heat exchange combined with the potential for low-temperature operation should reduce the costs associated with scale prevention and removal in MED and similar processes. Also, as discussed in section 3.4, rejected brine is naturally diluted by the direct-contact evaporator according to the ratio R, which may reduce brine disposal costs associated with current desalination facilities.

7.1 Future Work

Before development of the barometric distillation process can continue it is most important to answer this question: do temperature differences of the sort assumed in the specific energy analysis exist, and can they be exploited practically? Large volumes of high-temperature saline water exist in many places, at virtually every coastal power-generating plant, for instance—but equivalently sized sources of cold freshwater, able to tolerate some rise in steady-state temperature without ecological damage, are probably harder to find; worse, they have to be found near the heat supply. Even if these conditions are met, some potential sources of water for the condenser may be costly to use; for example, drawing water from an aquifer will impose significant additional head loss on the supply pump circuit. A possible application of BD that

105 sidesteps this problem is to draw water from a coastal aquifer near a power plant and supply it directly to the local water grid, augmented with distillate from this process and using the grid itself to shed excess heat. Answering this question, among others, will require the specialized knowledge of water-supply engineers and hydrogeologists, among other technical experts; careful study by economists would be as welcome. Beyond this fundamental question, there are many almost-obvious improvements to be applied to the process itself. In particular, optimizations of the evaporator and condenser design should be explored; the use of packed beds in similar applications in the chemical industry may point the way towards significant reductions in the pumping ratio R, and consequently, in the specific energy of the BD process (see section 2.3.1 for more on these methods). Similar gains may be made by investigating the vapor conduit, which is very likely undersized in the current apparatus; it may be that this is why the system is unable to reach a lower value for R than 120, regardless of the temperature difference applied to it. Beyond this, other enhancements are worth investigating: the failure of the axial blowers (section 4.1.3) in this particular apparatus does not in any way exhaust the possibilities of this method. Other types of compressors should be analyzed as possible drivers of a BD process designed to work with lower ∆T . The application of eductors to the vapor conduit is particularly promising, although like other compressors they are not “free” with respect to specific-energy; the eductor will cause another pressure drop in the condenser supply circuit. Yet another future design requirement that will probably warrant investigation is the inclusion in the vapor conduit of a demisting device or mist eliminator; these are used in thermal desalination methods to prevent aerosolized droplets of water, entrained in the vapor flow, from traveling across from evaporator to condenser and so contaminating the product water. Such phenomena were not significant in this

106 preliminary research due to the relatively long, relatively narrow vapor conduit, but in an optimized production facility the connection between chambers should be designed to present minimal impedance to vapor flow, which may lead to a significant exchange of liquid water and consequent need for a demister of some type [92]. As it will introduce an impedence of its own, the designer will have to aim at a trade-off between increasing specific energy or increasing TDS; not enough is known about these details of the BD process to make an intelligent choice as yet. Finally, investigation of degassing methods, including the injection of gas bubbles into the condenser column, should be brought to completion as there is currently no answer to two crucial questions: what is the minimum injection depth at which bubbles will be carried down and out of the system by the condenser flow itself; and can removal of non-condensable gases be made more efficient by this method? Hopefully, additional research aimed at finding answers to these questions will be forthcoming. Indeed, other investigators have already started expanding on this work, based on patent applications and early publications by the author and others [93]. Although the desalination industry is understood to be settling on reverse osmosis as a default technology and expects to see few significant developments in thermal desalination in future, processes like Barometric Distillation may have something to offer, even if it is discovered that they can be effectively applied only to some specific situations. In any case, absent a clear demonstration of its infeasibility, it would be a mistake to abandon inquiry into this promising technology.

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118 Appendix A

Octave Source Code

Several aspects of this research made use of GNU Octave, a free, open-source software package for performing numerical calculations. It uses a scripting language mostly compatible with that of Matlab; indeed, code written for one is likely to work in the other. Those scripts previously referenced in the text and used to obtain a result or prepare output for plotting are included here.

A.1 Evaluation of the Colebrook Formula

#!/usr/bin/octave -qf % colebrook.m % This script iteratively evaluates the Colebrook formula % for pipe friction. % Eiki Martinson, MSEE Thesis, July 2010

clear; clc; f_initial = 0.035; %Initial value guessed at by aiming at the %middle of the Moody chart Re = 25300; epsilon = 0.002; d = 25.4; %Last two values both in millimeters r = epsilon / d; %Roughness ratio f = f_initial; for i = 1:10

119 f = (2.0 * log10((r/3.7) + (2.51/(Re*f^0.5))))^-2; end

A.2 Determining the Pumping Ratio

#!/usr/bin/octave -qf % ratio.m % Calculates achieved pumping ratio from history of % condenser temperature. % Eiki Martinson, MSEE Thesis, August 2010

clear; clc; close all; outfile = "achieved-ratio.dat"; infile = "../data/06-03-28-distill-reduced.dat";

%converts Fahrenheit to Celsius function out = f2c(in) out = in .- 32; out = out .* (5/9); endfunction

cutoff = 2; %cutoff first samples - pump was not running yet increment = 1; actual_data = load(infile); time = actual_data(:,1); lengthtime = length(time); time = time(cutoff:increment:lengthtime); tempC = actual_data(cutoff:increment:lengthtime,5); %condenser %temperature tempE = actual_data(cutoff:increment:lengthtime,4); %evaporator %temperature flowC = actual_data(cutoff:increment:lengthtime,8); %condenser column %flow rate tempC = f2c(tempC); tempE = f2c(tempE);

p = 4; n = 31; F = sgolay(p,n,1); %uses a p-order, n-window Savitsky-Golay filter %to clean up noisy first derivative tempchange = sgolayfilt(tempC,F);

120 flowC = flowC .* (3.7854/60); %convert Gal/min %to L/s

C_p = 4.18; %specific heat volume(1) = 150; %initial volume latentheat = 2400; %latent heat of evaporation deltaTemp = tempE - tempC; %difference between evaporator and %condenser temperature deltaTemp = deltaTemp(2:length(deltaTemp));

for i = 2:length(time) volumechange(i) = C_p * tempchange(i) * volume(i-1) / latentheat; volume(i) = volume(i-1) + volumechange(i); volumerate(i) = volumechange(i)/(time(i)-time(i-1)); pumpratio(i-1) = flowC(i)/volumerate(i); end

fd = fopen(outfile, "wt"); fprintf(fd, "#Temperature Ratio\n"); for i = 1:length(pumpratio) fprintf(fd, "%4.3f ", deltaTemp(i)); fprintf(fd, "%3.1f\n", pumpratio(i)); end fclose(fd);

A.3 Comparing Step Responses

#!/usr/bin/octave -qf % stepresp.m % Outputs step response of first-order model for comparison % Eiki Martinson, MSEE Thesis, July 2010

clear; clc; outfile = "step-comparison.dat"; infile = "../data/07-03-27-step1-reduced.dat";

delay = 80;

sys = tf([1], [360 1]);

121 t = 0:3599; x = [zeros(delay, 1); ones(length(t)-delay,1)]; x = x * 383; %magnitude of input step, %7 mmHg-(-376 mmHg)=383 mmHg x = x - 376;

[output, B] = lsim(sys, x, t, [-376*360]); actual_data = load(infile); time = actual_data(:,1); TDG = actual_data(:,13); fd = fopen(outfile, "wt"); fprintf(fd, "Time TDG-Actual TDG-Modeled\n"); for i = 1:2400 fprintf(fd, "%4.3f ", time(i)); fprintf(fd, "%3.3f ", TDG(i)); fprintf(fd, "%3.3f\n", output(i)); end fclose(fd);

A.4 Demonstrating Sensor Response to Input

#!/usr/bin/octave -qf % slopedemo.m % Demonstrates output of 1st order model for sensor % Eiki Martinson, MSEE Thesis, August 2010

clear; clc; outfile = "slope-demonstration.dat"; infile = "../data/07-03-22-degas.dat";

sys = tf([1], [360 1]);

t=0:911; x = 760*exp(-0.0014 * (t+33)) - 760; %33 second time shift %as the initial pressure %is not zero.

122 [output, B] = lsim(sys, x’, t, [-34*360]);

actual_data = load(infile); TDG = actual_data(:,13); TDG = TDG(10:length(TDG)); %Pump wasn’t started until %10 seconds after data collection %began. fd = fopen(outfile, "wt"); fprintf(fd, "Time TDG-Proposed TDG-Modeled TDG-Measured\n"); for i = 1:length(t) fprintf(fd, "%4.3f ", t(i)); fprintf(fd, "%3.3f ", x(i)); fprintf(fd, "%3.3f ", output(i)); fprintf(fd, "%3.3f\n", TDG(i)); end fclose(fd);

A.5 Calculating Pump Power Across a Range of

Reinjection Depths

#!/usr/bin/octave -qf % depth.m % Outputs vacuum pump power vs. reinjection depth plot % Eiki Martinson, MSEE Thesis, September 2010

clear; clc; outfile = "reinjection-depth.dat";

n = 1.4; %polytropic exponent p1 = 0.048; %inlet pressure in atm Rm = 364; %specific gas constant in J/(K*kg) T1 = 295; %inlet temperature in K gamma = 9.789; %specific weight of water in kN/m^3 massflow = 12.47e-6; %mass flow rate in kg/s eta = 0.80; %efficiency

123 d = 0:0.05:9.85; %depth in meters p2 = d*gamma*0.009869 + 0.048; %pressure in atm swork = ((n*Rm*T1)/(n-1)) .* ((p2./p1).^((n-1)/n)-1); power = swork.*(massflow/eta); fd = fopen(outfile, "wt"); fprintf(fd, "Depth[m] Power[W]\n"); for i = 1:length(d) fprintf(fd, "%1.2f ", d(i)); fprintf(fd, "%1.2f\n", power(i)); end fclose(fd);

124