PROCESS TRENDS

Process Intensification: Transforming Chemical

Emerging equipment, processing techniques, and operational methods promise spectacular improvements in process plants, markedly shrinking Andrzej I. Stankiewicz, DSM Research/Delft University their size and dramatically boosting their efficiency. of Technology These developments may result in the extinction Jacob A. Moulijn, Delft University of Technology of some traditional types of equipment, if not whole unit operations.

oday, we are witnessing important fication, no matter how we define it, does not new developments that go beyond seem to have had much impact in the field of “traditional” . stirring technology over the last four centuries, at many universities and or perhaps even longer. But, what actually is industrialT research centers are working on novel process intensification? equipment and techniques that potentially could In 1995, while opening the 1st International transform our concept of chemical plants and Conference on Process Intensification in the lead to compact, safe, energy-efficient, and envi- , Ramshaw, one of the pio- ronment-friendly sustainable processes. These neers in the field, defined process intensifica- developments share a common focus on “process tion as a strategy for making dramatic reduc- intensification” — an approach that has been tions in the size of a so as to around for quite some time but has truly emerged reach a given production objective (2). These only in the past few years as a special and inter- reductions can come from shrinking the size of esting discipline of chemical engineering. individual pieces of equipment and also from In this article, we take a closer look at pro- cutting the number of unit operations or appa- cess intensification. We define what it involves, ratuses involved. In any case, the degree of re- discuss its dimensions and structure, and review duction must be significant; how significant recent developments in process-intensifying de- remains a matter of discussion. Ramshaw vices and methods. speaks about volume reduction on the order of 100 or more, which is quite a challenging What is process intensification? number. In our view, a decrease by a factor of One of the woodcuts in the famous 16th two already bears all attributes of a drastic century book by Georgius Agricola (1) illus- step change and, therefore, should be consid- trates the process of retrieving gold from gold ered as process intensification. ©Copyright 2000 ore (Figure 1). The resemblance between some On the other hand, Ramshaw’s definition is American Institute of the devices shown in the picture (for in- quite narrow, describing process intensifica- of Chemical Engineers. All rights reserved. stance, the stirred vessels O and the stirrers S) tion exclusively in terms of the reduction in Copying and and the basic equipment of today’s chemical plant or equipment size. In fact, this is merely downloading permitted process industries (CPI) is striking. Indeed, one of several possible desired effects. Clear- with restrictions. Agricola’s drawing shows that process intensi- ly, a dramatic increase in the production ca-

22 January 2000 Chemical Engineering Progress dustry, however, process developers still often opt for conventional shell- and-tube units, even in cases where plate or spiral heat exchangers could easily be applied. Process intensification concerns only engineering methods and equip- ment. So, for instance, development of a new chemical route or a change in composition of a catalyst, no mat- ter how dramatic the improvements they bring to existing technology, do not qualify as process intensification. We, therefore, offer the following definition: Process intensification consists of the development of novel apparatuses and techniques that, compared to those commonly used today, are ex- pected to bring dramatic improve- ments in and process- ing, substantially decreasing equip- ment-size/production-capacity ratio, energy consumption, or waste pro- duction, and ultimately resulting in cheaper, sustainable technologies. Or, to put this in a shorter form: any chemical engineering develop- ment that leads to a substantially smaller, cleaner, and more energy- efficient technology is process intensification! As shown in Figure 2, the whole field generally can be divided into two areas: • process-intensifying equipment, such as novel reactors, and intensive mixing, heat-transfer and mass-trans- fer devices; and • process-intensifying methods, such as new or hybrid separations, in- ■ Figure 1. 16th century technology for retrieving gold from ore (1). tegration of reaction and separation, heat exchange, or phase transition (in so-called multifunctional reactors), pacity within a given equipment certain established technologies and techniques using alternative energy volume, a step decrease in energy hardware. Usually, these have been sources (light, ultrasound, etc.), and consumption per ton of product, or applied on a limited scale (at least in new process-control methods (like in- even a marked cut in wastes or comparison with their potential) and tentional unsteady-state operation). byproducts formation also qualify as have not yet generally been recog- Obviously, there can be some process intensification. nized as standard by the chemical en- overlap. New methods may require Not surprisingly, process intensifi- gineering community. A typical ex- novel types of equipment to be devel- cation, being driven by the need for ample is the compact oped and vice versa, while novel ap- breakthrough changes in operations, (3,4). These exchangers have been paratuses already developed some- focuses mainly on novel methods and widely used for quite a long time in times make use of new, unconven- equipment. But, it also encompasses the food industry. In the chemical in- tional processing methods.

Chemical Engineering Progress January 2000 23 TRENDS

Process Intensification

Equipment Methods

Equipment for Equipment for Carrying Out Operations Multifunctional Hybrid Alternative Other Chemical Reactions not Involving Reactors Separations Energy Sources Methods Chemical Reactions

Examples Spinning Disk Reactor Static Mixers Reverse-Flow Membrane Absorption Centrifugal Fields Supercritical Fluids Static Mixer Reactor Compact Heat Reactors Membrane Ultrasound Dynamic (Periodic) (SMR) Exchangers Reactive Distillation Adsorptive Distillation Solar Energy Reactor Operation Static Mixing Catalysts Microchannel Heat Reactive Extraction Microwaves (KATAPAKs) Exchangers Reactive Electric Fields Monolithic Reactors Rotor/Stator Mixers Chromatographic Plasma Technology Microreactors Rotating Packed Beds Reactors Heat Exchange (HEX) Centrifugal Adsorber Periodic Separating Reactors Reactors Supersonic / Membrane Reactors ■ Reactor Reactive Extrusion Figure 2. Process intensification and Jet-Impingement Reactive Comminution its components. Reactor Fuel Cells Rotating Packed-Bed Reactor

Process-intensifying One of the more important disad- equipment vantages of static mixers is their rela- Our earlier comment that Agricola’s tively high sensitivity to clogging by woodcut shows how little stirring . Therefore, their utility for re- technology has progressed is not en- actions involving slurry catalysts is tirely true. In fact, the technology of limited. Sulzer solved this problem stirring has been greatly intensified (at least partially) by developing during the last 25 years, at least as structured packing that has good stat- far as liquid/liquid and gas/liquid ic-mixing properties and that simulta- systems. Surprisingly, this was neously can be used as the support achieved not by improving mechani- for catalytic material. Its family of cal mixers but, quite the opposite, by open-crossflow-structure catalysts, abandoning them — in favor of stat- so-called KATAPAKs (6) (Figure 4a), ic mixers (5). These devices are fine are used in some gas-phase exother- examples of process-intensifying mic oxidation processes traditionally equipment. They offer a more size- carried out in fixed beds, as well as in and energy-efficient method for mix- catalytic distillation. KATAPAKs ing or contacting fluids and, today, have very good mixing and radial serve even wider roles. For instance, heat-transfer characteristics (6). Their the Sulzer (Winterthur, Switz.) SMR main disadvantage is their relatively static-mixer reactor, which has mix- low specific geometrical area, which ing elements made of heat-transfer is much lower than that of their most tubes (Figure 3), can successfully be important rival in the field, monolith- applied in processes in which simul- ic catalysts (7) (Figure 4b). taneous mixing and intensive heat removal or supply are necessary, ■ Figure 3. Proprietary reactor-mixer is a clas- Monolithic catalysts such as in nitration or neutralization sic example of process-intensifying equipment. Monolithic substrates used today reactions. (Photo courtesy of Sulzer.) for catalytic applications are metallic

24 January 2000 Chemical Engineering Progress ■ Figure 4. ements, using the latter as gas/liquid (a) Packing with dispersing devices. The in-line units integrated catalyst offer additional advantages: (photo courtesy of • low investment costs, because Sulzer.), and (b) monolithic in-line monolithic reactors are ready- catalyst (photo to-use modules that are installed as courtesy of part of the pipelines; Corning). • compact plant layout (in-line monolith reactors can even be placed underground, say, in cement ducts — see Figure 5); • ability to meet much higher safety and environmental standards or nonmetallic bodies providing a of conventional packed-bed systems; than conventional reactors (such as, multitude of straight narrow channels • high geometrical areas per reac- for instance, by placing the reactor of defined uniform cross-sectional tor volume, typically 1.5–4 times unit beneath ground level); shapes. To ensure sufficient porosity more than in the reactors with partic- • very easy and quick replacement and enhance the catalytically active ulate catalysts; (e.g., in case of catalyst deactivation) surface, the inner walls of the mono- • high catalytic efficiency, practi- simply by swapping a piece of lith channels usually are covered with cally 100%, due to very short diffu- pipeline, instead of having to unload a thin layer of washcoat, which acts sion paths in the thin washcoat layer; old and load new catalyst; as the support for the catalytically ac- and • the possibility of distributing tive species. • exceptionally good perfor- multiple feed points along the reac- The most important features of the mance in processes in which selec- tor; and monoliths are: tivity is hampered by mass-transfer • easy attainment of a near-to- • very low drop in sin- resistances. plug-flow regime. gle- and two-phase flow, one to two Monolithic catalysts also can be In a modeling study of an industri- orders of magnitude lower than that installed in-line, like static mixing el- al gas/liquid process, Stankiewicz (8)

Side-Stream (Optional)

Monolithic Catalyst

Heat Exchange (Optional) Reaction Dispersing, Mixing

■ Figure 5. Cross-flow monolithic structure. (Illustration courtesy of Corning.)

Chemical Engineering Progress January 2000 25 PROCESS DESIGN TRENDS

gives a spectacular example of an ap- microreactors. The very high heat- years, Pacific Northwest National proximately 100-fold reduction in re- transfer rates achievable in microre- (Richland, WA) has actor size from replacing a conven- actors allow for operating highly demonstrated microchannel heat ex- tional system with an in-line mono- exothermic processes isothermally, changers in a planar sheet architec- lithic unit. which is particularly important in car- ture that exhibit high heat fluxes and One of the problems in monolith rying out kinetic studies. Very low re- convective-heat-transfer coefficients. reactors, especially for gas-phase cat- action-volume/surface-area ratios make The reported values of heat-transfer alytic processes, is difficult heat re- microreactors potentially attractive for coefficients in microchannel heat ex- moval due to the absence of radial processes involving toxic or explo- changers range from Å10,000 to dispersion. Monolith channels are sive reactants. The scale at which Å35,000 W/m2K (4, 12). fully separated from each other and, processes using batteries of multiple therefore, the only heat microreactors become economically Rotating devices mechanism is the conductivity and technically feasible still needs to Almost as high heat-transfer coef- through the monolith material. For be determined, though. ficients are achievable in the spinning highly exothermic gas-phase reac- The geometrical configuration of disk reactor (SDR) (13). This unit tions, so-called HEX reactors devel- microchannel heat exchangers (stacked (see Figure 7) developed by oped by BHR Group, Ltd. (Cranfield, cross-flow structures) resembles that Ramshaw’s group at Newcastle Uni- U.K.) (9) present a promising option. of the cross-flow monoliths in Figure versity (Newcastle, U.K.) primarily is In these reactors, one side of a com- 6, although the materials and fabrica- aimed at fast and very fast liquid/liq- pact heat exchanger is made catalyti- tion methods used differ. The chan- uid reactions with large heat effect, cally active, either by washcoating or nels in the plates of microchannel such as nitrations, sulfonations, and by introducing catalytically active el- heat exchangers are usually around 1 polymerizations (e.g., styrene poly- ements (such as pellets or structured mm or less wide, and are fabricated merization (14)). In SDRs, a very thin packings). A ceramic cross-flow via silicon micromachining, deep X- (typically 100 µm) layer of liquid monolith structure developed by ray lithography, or nonlithographic moves on the surface of a disk spin- Corning Inc. (Corning, NY) (10) micromachining. Over the past few ning at up to approximately 1,000 (Figure 6) also potentially can be rpm. At very short residence times used as a catalytic reactor/heat ex- (typically 0.1 s), heat is efficiently re- changer, e.g., for carrying out two moved from the reacting liquid at chemical processes (exo- and en- heat-transfer rates reaching 10,000 dothermic) within one unit. Com- W/m2K. SDRs currently are being pared to conventional fixed-bed reac- commercialized. tors, such reactors offer much better Other reactors especially dedicated heat-transfer conditions — namely, to fast and very fast processes worth heat-transfer coefficients typically of mentioning include: the supersonic 3,500–7,500 W/m2K, and heat-trans- gas/liquid reactor developed at Prax- fer areas of up to 2,200 m2. air Inc. (Danbury, CT) (15) for gas/liquid systems and the jet-im- Microreactors pingement reactor of NORAM Engi- Even higher values of heat-trans- ■ Figure 6. Concept of an in-line catalytic neering and Constructors (Vancouver, fer coefficients than those in the HEX reactor (8). BC) (16,17) for liquid/liquid systems. reactors can be achieved in microre- actors. Here, values of up to 20,000 ■ Figure 7. W/m2K are reported (11). Microreac- Feed Schematic of the tors are chemical reactors of extreme- spinning-disk ly small dimensions that usually have reactor. a sandwich-like structure consisting Products of a number of slices (layers) with micromachined channels (10–100 µm in dia.). The layers perform various Heat Exchange functions, from mixing to catalytic reaction, heat exchange, or separa- tion. Integration of these various functions within a single unit is one of the most important advantages of

26 January 2000 Chemical Engineering Progress The former employs a supersonic ■ Figure 8. shockwave to disperse gas into very Axis Centrifugal adsorber tiny bubbles in a supersonic in-line (23). (Drawing courtesy of Bird Liquid Feed mixing device, while the latter uses a Liquid Effluent Engineering.) system of specially configured jets and baffles to divide and remix liq- L uid streams with high intensity. Adsorbent Feed Adsorbent Effluent Rotor/stator mixers (18), which are aimed at processes requiring very fast Centrifugal Field w2R mixing on a micro scale, contain a w high-speed rotor spinning close to a motionless stator. Fluid passes through the region where rotor and Fresh Adsorbent stator interact and experiences highly pulsating flow and shear. In-line rotor/stator mixers resemble centrifu- gal and, therefore, may simul- Liquid Feed taneously contribute to pumping the . Rotational movement and centrifu- gal forces are used not only in SDRs. High gravity (HIGEE) technology, which Imperial Chemical Industries (London) started working on in the late 1970s as a spinoff from a NASA research project on microgravity en- vironment (19,20), has developed Liquid Effluent into one of the most promising branches of process intensification. HIGEE technology intensifies mass- transfer operations by carrying them Adsorbent Effluent out in rotating packed beds in which high centrifugal forces (typically 1,000 g) occur. This way, heat and Chong Zheng’s group also has high capacities (typically 10–50 momentum transfer as well as mass achieved successes in crystallization m3/h). transfer can be intensified. The rotat- of nanoparticles: very uniform 15–30 ing-bed equipment, originally dedi- nm crystals of CaCO3 have been Process-intensifying cated to separation processes (such as made in a rotating crystallizer at pro- methods absorption, extraction, and distilla- cessing times 4–10 times shorter than As highlighted in Figure 2, most tion), also can be utilized for reacting those for a conventional stirred-tank process-intensifying methods fall into systems (especially those that are process (22). Another interesting ex- three well-defined areas: integration mass-transfer limited). It potentially ample here, also undergoing commer- of reaction and one or more unit op- can be applied not only to gas/liquid cialization, is a centrifugal adsorber erations into so-called multifunction- systems, but also to other phase (Figure 8) developed at Delft Univer- al reactors, development of new hy- combinations including three-phase sity of Technology (Delft, The brid separations, and use of alterna- gas/liquid/ systems. Recently, Netherlands) (23). This is a new con- tive forms and sources of energy for Chong Zheng’s group at the HI- tinuous device for carrying out ion- processing. Let’s now take a closer GRAVITEC Center (Beijing) has suc- exchange or adsorption processes. look at each of these areas. cessfully applied rotating (500–2,000 Using a centrifugal field to establish rpm) packed beds on a commercial countercurrent flow between the liq- Multifunctional reactors scale for deaeration of flooding uid phase and the adsorbent enables These can be described as reactors in Chinese oil fields. There, rotating use of very small (10–50 mm) adsor- that, to enhance the chemical conver- machines of Å1 m dia. replaced con- bent particles and design of extreme- sion taking place and to achieve a ventional vacuum towers of Å30 m ly compact separation equipment higher degree of integration, combine height (21). with very short contact times and at least one more function (usually a

Chemical Engineering Progress January 2000 27 PROCESS DESIGN TRENDS

) that conventionally lower capital investment (30). Also, a membrane unit). Yet, practically no would be performed in a separate reverse process to the one described large-scale industrial applications have piece of equipment. A widely known above, that is, combination of reac- been reported so far. The primary rea- example of integrating reaction and tion and , has been stud- son for this most definitely is the rela- in a multifunctional unit ied for benzene oxidation to cyclo- tively high price of membrane units, is the reverse-flow reactor (24). For hexane and for methanol synthesis although other factors, such as low exothermic processes, the periodic (31,32). The number of processes in permeability as well as mechanical flow reversal in such units allows for which reactive distillation has been and thermal fragileness, also play an almost perfect utilization of the heat implemented on a commercial scale important role. Further developments of reaction by keeping it within the is still quite limited — but the poten- in the field of material engineering catalyst bed and, after reversion of the tial of this technique definitely goes surely will change this picture. flow direction, using it for preheating far beyond today’s applications. Multifunctional reactors may inte- the cold reactant . To date, re- Numerous research groups are in- grate not only reaction and heat trans- verse-flow reactors have been used in vestigating other types of combined re- fer or reaction and separation but also three (24): SO2 actions and separations, such as reac- combine reaction and phase transi- oxidation, total oxidation of hydrocar- tive extraction (33,34), reactive crystal- tion. A well-known example of such a bons in off-gases, and NOx reduction. lization (35), and integration of reac- combination is reactive extrusion. The recent introduction of inert pack- tion and sorption operations, for in- Reactive extruders are being increas- ing for heat exchange (25) has lead to stance, in chromatographic reactors ingly used in the industries. a “sandwich” reactor; it consists of (36,37,38) and periodic separating re- They enable reactive processing of three zones — a catalyst bed between actors, which are a combination of a highly viscous materials without re- two beds of packing of heat-accumu- pressure swing adsorber with a period- quiring the large amounts of lating material. The reverse-flow prin- ic flow-forced packed-bed reactor (39). that stirred-tank reactors do. Particu- ciple also has been applied in rotating larly popular are twin-screw extrud- monolith reactors, which are used in- Membrane reactors ers, which offer effective mixing, the dustrially for removal of undesired Today, a huge research effort is de- possibility of operation at high pres- components from gas streams and voted to membrane reactors (40). The sures and , plug-flow continuous heat regeneration (26). membrane can play various functions characteristics, and capability of mul- Studies also have been carried out on in such reactor systems. It, for in- tistaging. Most of the reactions car- employing reversed-flow reactors for stance, can be used for selective in- ried out in extruders are single- or endothermic processes (27). situ separation of the reaction prod- two-phase reactions. New types of Reactive (catalytic) distillation is ucts, thus providing an advantageous extruders with catalyst immobilized one of the better known examples of equilibrium shift. It also can be ap- on the surface of the screws, howev- integrating reaction and separation, plied for a controlled distributed feed er, may allow carrying out three- and is used commercially (28). In this of some of the reacting species, either phase catalytic reactions (47). case, the multifunctional reactor is a to increase overall yield or selectivity Fuel cells present another example distillation column filled with catalyt- of a process (e.g., in fixed-bed or of multifunctional reactor systems. ically active packing. In the column, fluidized-bed membrane reactors Here, integration of chemicals are converted on the cata- (41,42)) or to facilitate and electric power generation takes lyst while reaction products are con- (e.g., direct bubble-free oxygen sup- place (see, for instance, Ref. 48). Si- tinuously separated by fractionation ply or dissolution in the liquid phase multaneous gas/solid reaction and (thus overcoming equilibrium limita- via hollow-fiber membranes (43,44)). comminution in a multifunctional re- tions). The catalyst used for reactive In addition, the membrane can enable actor also has been investigated (49). distillation usually is incorporated in-situ separation of catalyst particles into a fiberglass and wire-mesh sup- from reaction products (45)). Finally, Hybrid separations porting structure, which also provides the membrane can incorporate catalyt- Many of the developments in this liquid redistribution and disengage- ic material, thus itself becoming a area involve integration of mem- ment of vapor. Structured catalysts, highly selective reaction-separation branes with another separation tech- such as Sulzer’s KATAPAK, also are system. The scientific literature on cat- nique. In membrane absorption and employed (29). The advantages of alytic membrane reactors is exception- stripping, the membrane serves as a catalytic distillation units, besides the ally rich (see, for instance, Ref. 46) permeable barrier between the gas and continuous removal of reaction prod- and includes many very interesting liquid phases. By using hollow-fiber ucts and higher yields due to the ideas (such as heat- and mass-integrat- membrane modules, large mass-trans- equilibrium shift, consist mainly of ed combination of and fer areas can be created, resulting in reduced energy requirements and dehydrogenation processes in a single compact equipment. Besides, absorp-

28 January 2000 Chemical Engineering Progress Acetic Acid Methanol Catalyst

Methyl Acetate

Acetic Distillation Acid Methyl Acetate Extractive Catalyst Distillation Water Reactive Distillation Azeo Reaction

Reactive Methanol Distillation

Distillation Solvent Entrainer Water Heavies

Conventional Task-Integrated Water

■ Figure 9. Task-integrated methyl acetate column is much simpler than conventional plant. (Drawing courtesy of Eastman Chemical (76). tion membranes offer operation inde- the membrane than in the pressure- some fine-chemical processes from pendent of gas- and liquid flow rates, driven processes; batchwise to continuous operation. without entrainment, flooding, chan- • less membrane fouling, due to neling, or foaming (50,51). larger pore size; and Use of alternative forms Membrane distillation is probably • potentially lower operating tem- and sources of energy the best known hybrid, and is being peratures than in conventional evapo- Several unconventional processing investigated worldwide (52,53). The ration or distillation, which may en- techniques that rely on alternative technique is widely considered as an able processing of -sensi- forms and sources of energy are of im- alternative to and tive materials. portance for process intensification. evaporation. Membrane distillation Among hybrid separations not in- For instance, we already have dis- basically consists of bringing a volving membranes, adsorptive dis- cussed the potential benefits of using volatile component of a liquid feed tillation (55) offers interesting ad- centrifugal fields instead of gravitation- stream through a porous membrane vantages over conventional methods. al ones in reactions and separations. as a vapor and condensing it on the In this technique, a selective adsor- Among other techniques, research other side into a permeate liquid. bent is added to a distillation mix- on sonochemistry (the use of ultra- Temperature difference is the driving ture. This increases separation abili- sound as a source of energy for force of the process. Foster et al. ty and may present an attractive op- chemical processing) appears to be (54) name four basic advantages of tion in the separation of the most advanced. Formation of mi- membrane distillation: or close-boiling components. Ad- crobubbles (cavities) in the liquid re- • 100% rejection of ions, macro- sorptive distillation can be used, for action medium via the action of ul- molecules, colloids, cells, and other instance, for the removal of trace im- trasound waves has opened new pos- nonvolatiles; purities in the manufacturing of fine sibilities for chemical syntheses. • lower operating pressure across chemicals; it may allow switching These cavities can be thought of as

Chemical Engineering Progress January 2000 29 PROCESS DESIGN TRENDS

high energy microreactors. Their ■ Figure 10. Single-unit collapse creates microimplosions Flooding Tank with very high local energy release distillation plant for hydrogen peroxide (temperature rises of up to 5,000 K Coolling Water (77). (Drawing and negative of up to courtesy of Sulzer.) 10,000 atm are reported (56)). This Vacuum may have various effects on the re- acting species, from homolytic bond Direct Condenser breakage with free radicals forma- tion, to fragmentation of polymer Cooling Water chains by the shockwave in the liq- uid surrounding the collapsing bub- ble. For solid-catalyzed (slurry) sys- tems, the collapsing cavities addi- tionally can affect the catalyst sur- face — this, for example, can be Column used for in-situ catalyst cleaning/re- juvenation (57). A number of sono- Product have been Lamella-Type Separator developed and studied (58). Sono- chemistry also has been investigated in combination with other tech- niques, e.g., with electrolysis for ox- Intermediate Product idation of phenol in wastewater (59). The maximum economically and Steam technically feasible size of the reac- Climbing Film Evaporator tion vessel still seems to be the de- termining factor for industrial appli- Condensate cation of sonochemistry. Solar energy also may play a role in chemical processing. A novel high- Feed temperature reactor in which solar en- ergy is absorbed by a cloud of react- ing particles to supply heat directly to range of processes, including painting, plications tested so far in the labora- the reaction site has been studied coating, and crop spraying. In these tory and on industrial scale include: (60,61). Experiments with two small- processes, the electrically charged methane transformation to acetylene scale solar chemical reactors in which droplets exhibit much better adhesion and hydrogen, destruction of N2O, re- thermal reduction of MnO2 took place properties. In boiling heat transfer, forming of heavy residues, also are reported (60). Other studies electric fields have been successfully CO2 dissociation, activation of organ- describe, for example, the cycloaddi- used to control nucleation rates (66). ic fibers, destruction of volatile or- tion reaction of a carbonyl compound Electric fields also can enhance pro- ganic compounds in air, to an olefin carried out in a solar fur- cesses involving liquid/liquid mix- conversion to synthesis gas, and SO2 nace reactor (62) and oxidation of 4- tures, in particular liquid/liquid extrac- reduction to elemental sulfur. chlorophenol in a solar-powered fiber- tion (67) where rate enhancements of optic cable reactor (63). 200–300% have been reported (68). Other methods Microwave heating can make Interesting results have been pub- A number of other promising tech- some organic syntheses proceed up to lished concerning so-called Gliding niques do not fall within the three 1,240 times faster than by conven- Arc technology, that is, plasma gener- categories we have discussed. Some tional techniques (64). Microwave ated by formation of gliding electric already are known and have been heating also can enable energy-effi- discharges (69,70,71). These dis- commercially proven in other indus- cient in-situ desorption of hydrocar- charges are produced between elec- tries. For instance, supercritical fluids bons from zeolites used to remove trodes placed in fast gas flow, and (SCFs) are used industrially for the volatile organic compounds (65). offer a low-energy alternative for processing of natural products. Be- Electric fields can augment process conventional high-energy-consump- cause of their unique properties, rates and control droplet size for a tion high-temperature processes. Ap- SCFs are attractive media for mass-

30 January 2000 Chemical Engineering Progress transfer operations, such as extraction Dynamic (periodic) operation of that is, combinations of reactions and (72) and chemical reactions (73). chemical reactors has interested re- one or more unit operations, will play Many of the physical and transport searchers for more than three decades. a dominant role in the future, process- properties of a SCF are intermediate In many laboratory trials, the inten- intensive, sustainable CPI. Has the between those of a liquid and a gas. tional pulsing of flows or concentra- evolution of chemical engineering Diffusivity in an SCF, for example, tions has led to a clear improvement of thus reached the point in which tradi- falls between that in a liquid and a product yields or selectivities (75). tional unit operations will give way to gas; this suggests that reactions that Yet, despite a great amount of re- these hybrid forms and become ex- are diffusion limited in the liquid search, commercial-scale applications tinct? Our answer to this question is phase could become faster in a SCF are scarce, and limited mainly to the both no and yes. phase. SCFs also have unique solubil- reverse-flow reactors we have already No, because the development of ity properties. Compounds that are discussed. One of the main reasons is these new, integrated apparatuses and largely insoluble in a fluid at ambient that dynamic operation requires in- techniques is and will remain deeply conditions can become soluble in the vestments to synchronize nonstation- rooted in the knowledge of the basic, fluid at supercritical conditions. Con- ary and stationary parts of the process. traditional unit operations. More than versely, some compounds that are So, in general, steady-state operation that, further research progress in pro- soluble at ambient conditions can be- is less expensive. There are cases, cess intensification will demand a come less soluble at supercritical however, in which dynamic operation parallel progress in fundamental unit- conditions. SCFs already have been may prove advantageous, despite the operation-based knowledge. There- investigated for a number of systems, tradeoffs involved (76). fore, traditional unit operations will including reactions, Diels- not disappear, at least not from chem- Alder reactio