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Gas Liquid Reactions Gasliqnew.doc 3-12-05 CHAPTER 13 Gas-Liquid Systems This chapter will focus on the systems where a dissolved gas contacts a liquid. In most cases the dissolved gas will undergo a reaction in the liquid phase. Systems where no reaction occurs are known as physical absorption while the systems where reaction occurs are referred to as chemical absorption. First we provide some examples of application areas where gas-liquid systems are encountered in the field of environmental reaction engineering. This is followed by a discussion of the types of equipment used for gas- liquid contacting. The theory of gas-liquid mass transfer and design of such systems then follow. 1. Application Areas FCC Off Gas Cleaning FCC (Fluid catalytic cracking) off gas from the catalyst regenerator is a major source of pollutants in oil refineries. These gases contain SO2 (quantity depends on the sulfur content of feed), and NOx. These gases are treated in a spray column. The spray is water with the other additives such as alkali, caustic soda or lime. A typical complete flowsheet for the process is shown in Figure 1. The design also removes particulate matter present in the off gas as well as NOx. The pressure drop is low due to the open design. FIGURE 1. FCC Off-Gas purification (to be added) Wet Gas Scrubbing This process is useful for highly diluted SO2 from flue gases. The process uses a concentrated sodium sulfite solution. The absorbed SO2 is thermally released from the bisulfite solution in an evaporator/crystallizer. SO2 emerges as a concentrated gas containing 95% SO2 by volume. This can be converted to elemental sulfur, liquid SO2 or sulfuric acid. A typical flowsheet is shown in Figure 2. FIGURE 2. SO2 removal by wet gas scrubbing H2S removal and spent caustic oxidation Caustic solutions are often used to treat H2S and mercapten containing gas streams. This is an example of a gas-liquid reaction. The sulfides are formed as a result of reaction of caustic solution with the sulfur bearing compounds. This waste stream is highly toxic with high COD. Hence, the spent caustic solution has to be oxidized to form thiosulfate/sulfate. The treated stream is then suitable for wastewater treatment unit. A typical flow chart for spent caustic oxidation is shown in Figure 3. A series of mechanically agitated vessels are used as reactor here. FIGURE 3. Spent caustic oxidation Other applications include air/steam stripping of VOC from wastewater, ozone treatment, etc. 2. Common types of reactors. Common reactors and their brief merits and demerits are discussed below: 1. Packed columns: Liquid and gas flow over a column filled with packings with the flow usually in a countercurrent manner. The design of such equipment need pressure drop calculations and establishment of flooding limits in addition to a mass transfer and reaction analysis. A column is said to flood when the gas flow is sufficient to blow the liquid against gravity and the column can not be operated at these flow rates. Gas pressure drop is usually low with packed columns. A variety of packings are available to suit the system requirements. The system is not suitable if the gas contains suspended solids. 2. Bubble columns: Mixing is caused by the gas flow alone and no additional energy input is necessary. The advantages are simplicity in construction, no moving parts and minimum energy requirements compared to other contactors. Disadvantage is the high pressure drop of the gas due to hydrostatic head of the liquid. Considerable backmixing of the liquid occurs here. Liquid hold up is high in these reactors and hence these are suitable for slow reactions in the liquid. 3. Mechanically agitated contactors: the liquid is agitated by an impeller. System has large liquid holdup and consumes considerable power. Very efficient mixing, moving parts require maintenance. Very suitable for small scale systems in pollution prevention and also for batch or near batch operations. As an example we note that the black liquor oxidation (Na2S solution) is carried out in a series of stirred vessels. 4. Spray columns: Liquid is dispersed in the form of fine spray into an empty column with gas upflow. The system is suitable for highly soluble gases, can also handle gas streams containing suspended impurities. Ultra low pressure drop for the gas phase. Some limitations are energy input to spray the liquid; low mass transfer efficiency due to bypassing and droplet coalescence and liquid entrainment in the gas phase. As an example, we note that the spray columns are used for SO2 removal from FCCU flue gases because the gas stream contains suspended solids and also because SO2 has a high solubility in the aqueous phase. 3. Fundamentals of gas-liquid mass transfer The essential concepts to describe the gas-liquid mass transfer are: (i) Henry’s law (ii) The two film model Henry’s law: Henry’s law describes the equilibrium concentration in the liquid exposed to a gas containing a solute. Various forms are used for Henry’s law. These were summarized in Chapter 3 and students should recall them at this stage. Two common definitions will be used here. (1) y = mx where m is the ratio of mole fraction in the gas to that liquid at equilibrium. (2) P = HC where H is the ratio of gas phase partial pressure to concentration in the liquid at equilibrium. The unit for H is then Pa.m3/mole or atm.m3/mole. Two film model: The basic postulate is the existence of stagnant boundary layers on either side of the gas- liquid interface. Hence the terminology “two” film and these are referred to as gas film and liquid film. Concentration gradients are assumed to exist only in the films. Outside the film, the phases are assumed to be at uniform concentration and are referred to as gas bulk and liquid bulk. A mass transfer driving force is defined for transport in the gas film liquid films individually. In the next section, we show how to combine this into an “overall” mass transfer coefficient. y =mx Gas I I phase Liquid phase xI y x x* y* Gas Liquid film film where x* = hypothetical equilibrium value corresponding to y = y / m xI, yI are the interfacial mole fractions on the liquid side and gas side of the interface respectively. Interface is assumed to be in equilibrium. The mole fractions in the bulk gas are denoted as y and that in the bulk liquid as x. yI mxI by Henry’s law assuming interface to be in equilibrium NA = mass transfer rate per area = k y y yI k x xI x where kx and ky are mass transfer coefficients based on mole fraction driving force. * Overall coefficient is defined as N A K x x x we find from the above equations: 1 1 1 K x mk y k x Alternative way of writing the transfer rate is: * N A K y y y y* = mx = hypothetical gas phase value corresponding to the bulk liquid. 1 1 m K y k y k x y y* K * x * Note that N A K x x x K x y y m m m K Hence K x y m Mass transfer coefficients can also be based on concentration driving force: * N A k L CI CL k x k LCT CT = total molar concentration in the liquid Mass transfer coefficients for the gas phase is often based on gas phase partial pressure. N A k g p pI k y k g P The overall coefficient KL based on concentration driving force is then given as 1 1 1 K L k L kG H Similarly the coefficient KG based on gas phase partial pressure as a driving force is given as 1 1 H K G k g k L where H is in units of Pa m3/mole. Various definitions and inter-relationships should be noted. Example 1: At a point in a gas absorption column, we have air and water in contact. The solute is H2S and the partial pressure of H2S in the air is 5% and the concentration of H2S dissolved in water is zero. Find the rate of mass transfer if the gas side and liquid side mass transfer coefficients are as follows: -4 kL= 2x10 m/s -6 2 kg = 6x10 mole/m Pa s Also find the overall transfer coefficient and express in terms of (a) an overall partial pressure driving force and (b) a liquid phase concentration driving force. Solution: Henry’s law solubility coefficient of H2S is needed and the value used here is 1950 Pa m3/mol. Overall transfer coefficient based on partial pressure driving force is KG and is calculated as 1 1 H K G k g k L -7 2 KG =1x10 mole/Pa m s -7 5 Rate of absorption = KG PG = 1 x 10 x 0.05 x 1 x 10 = 5.09x10-4 mole/m2 s Similarly -4 KL =1.95 x10 m/s which is close to kL. Hence it is seen that the major resistance to transport lies in the liquid side of the film for this case. Gas-liquid Reaction : First order reactions. Model equations for transport in the film. d 2 A D k A (1) A dx 2 1 where k1 is the rate constant for the reaction. It is convenient to normalize the equation by defining the following dimensionless variable. A x a , y A * Dimensionless representation of Eq (1) is then d 2 a Ha 2 a dy 2 k where Ha2 is 2 1 DA Ha is an important dimensionless group for gas – liquid reactions and is known as the Hatta number.
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