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Process Engineering Guide: GBHE-PEG-RXT-811

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Process Engineering Guide: Novel Reactor Technology CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 FUEL CELLS 3 4.1 Introduction 3 4.2 Examples of Fuel Cells 5 4.3 PAFC (Phosphoric Acid Fuel Cell) 5 4.4 AFC (Alkaline Fuel Cell) 5 4.5 DMFC (Direct Methanol Fuel Cell) 6 4.6 SPFC (Solid Polymer Fuel Cell) 6 4.7 PCFC (Proton Conducting Fuel Cell) 6 4.8 MCFC (Molten Carbonate Fuel Cells) and SOFC

(Solid Oxide Fuel Cells) 7 5 ULTRASONIC REACTORS 7 5.1 Introduction 7 5.2 Recommendation 7 5.3 Design of Sonoreactor 8



6 THIN FILM REACTORS 10

6.1 Introduction 10 6.2 Equipment 10

7 ELECTRON BEAM REACTORS 11 8 MEMBRANE REACTORS 12 8.1 Introduction 12 8.2 Circumstances Suggesting Consideration of Membrane

Reactors 12 8.3 Example of a Membrane Reactor 13 8.4 Modeling Membrane Reactors 14 8.5 A Typical Analysis of a Membrane Reactor 15 9 SHOCK TUBE REACTORS 17

10 REACTIVE DISTILLATION 17 10.1 Introduction 17 10.2 Example of Reactive Distillation 18 11 OTHER NOVEL REACTOR TECHNOLOGY 20 11.1 Heterogenous Catalytic Reactors 20 11.2 Liquid and Liquid/Gas Phase Reactors 22 11.3 Reactor/Separation Combinations 24



FIGURES 1 SONOREACTOR PILOT PLANT LOOP RIG 9 2 CROSS SECTION OF SPINNING CONE REACTOR 11 3 REACTIVE DISTILLATION SCHEME 19



0 INTRODUCTION/PURPOSE This Guide is one in a series on Reactor Technology produced by GBH Enterprises. In other Reactor Technology Guides, the design or analysis of conventional reactors are discussed. While, hopefully, these approaches would result in the specification of economically viable new processes, or in the improvement of the running costs of existing ones, it is to be expected that step changes in commercial performance will result from radical changes in the process, e.g. substitution of one diluent for another, or from novel reactor technology. 1 SCOPE This Guide contains brief notes on novel reactor technologies which range from devices which are already being exploited, albeit in a limited way, to others whose potential is clear but which require further considerable development. The aim of this Guide is to make the user aware of a range of novel technologies; it does not give detailed information on their design. 2 FIELD OF APPLICATION This Guide applies to GBH Enterprises engineering community worldwide and to development scientists working in conjunction with that process engineering community. 3 DEFINITIONS For the purposes of this Guide, no special definitions apply.



4 FUEL CELLS 4.1 Introduction To date, the incentive for developing fuel cell technology has centered on the potential for producing electrical energy from fuel at greater efficiencies than can be achieved using "more conventional" heat and power systems. However, the capital cost of these units is sufficiently high that applications are limited largely to specific areas, e.g. space technology, military equipment, hospitals etc. For example, the cost of a phosphoric acid fuel cell will have to fall by at least a factor of four in order to compete head on with gas turbine combined cycle power generation. The attractiveness of fuel cells for power generation is enhanced if the waste heat that is inevitably produced can usefully be used for domestic/commercial heating or further power generation (i.e. CHP). There are various types of fuel cell under development, each of which is generally known by the name of the electrolyte used in the cell. A useful introductory survey of fuel cell types is provided by Linden in “Handbook of Batteries and Fuel Cells”. It is generally accepted that seven major types of fuel cells are under active development but only two, the phosphoric acid fuel cell (PAFC), and alkaline fuel cell (AFC), are close to commercialization. However, both the molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) are under intensive development, predominately in the USA and Japan. Brief comments on the seven fuel cell types are given in 4.3 to 4.8 (inclusive). Alkaline fuel cells (AFC) were used for the space shuttle and have been developed for transport applications, but they are not sufficiently durable for widespread stationary power generation. Of the other types of cell, most development has been on phosphoric acid fuel cells (PAFC) for stationary power generation. Molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC) are potentially more efficient than PAFCs, but are at least ten years behind in development. In addition to the use of fuel cells specifically for heat and power generation, other areas of application can ultimately be envisaged within the chemical industry whereby the exothermic nature of "traditional" reactions is harnessed to generate electrical energy. In his review Prof. Steele (IC 16653) indicates the potential of "second generation" type of fuel cells, principally those utilizing a solid oxide electrolyte, for undertaking traditional oxidation or partial oxidation reactions. The following processes are cited as being suitable, in principle, for use in ceramic fuel cells:



Fuel cells of this type can also be used to undertake chemical reactions which are not possible in more conventional reactor systems, e.g. selective partial oxidations. This can be achieved by chemical activation of the catalytic surface through operation only slightly above the equilibrium potential. The first of the reactions cited above is described in some detail within Prof. Steele's report. The electrolyte used within ceramic fuel cells is normally composed of a solid solution of zirconia and yttria which permits migration of oxygen ions. Selective catalysts laid down on either side of the thin solid oxide electrolyte provide the anode/cathode over which the reactant/air pass. Various potential designs of the overall cell are discussed in the report. There are other potential applications for fuel cells within the chemical industry, especially where hydrogen is present as a waste byproduct, e.g. some chlorine/caustic manufacturing sites. The economics of such applications are regularly reviewed to assess their potential. GBH Enterprises have made a study of the markets and opportunities for fuel cells and should be contacted to discuss the economic evaluations made as a result of this investigation. 4.2 Examples of Fuel Cells A fuel cell is a device for converting the chemical energy of a fuel directly to electrical energy. In principle, a fuel cell can convert primary energy to electricity with greater efficiency than can be achieved with the conventional steam turbine or other forms of heat engines. However, it will be seen that achieving these high efficiencies in practical engineering systems can be difficult and so, to date, commercial applications have been limited.



4.3 PAFC (Phosphoric Acid Fuel Cell) The PAFC has a planar configuration incorporating two gas diffusion electrodes and the immobilized phosphoric acid electrolyte. The system usually operates at around 200°C at 7 bar with hydrogen as fuel. For hydrocarbon fuel sources an external reformer is thus required which inevitably limits the overall efficiency. Forty-six 40 kW PAFC units have undergone an extensive field test program equivalent to 300,000 hours of cumulative operating experience, and a 4.5 MW plant has been successfully operated by Tokyo Electric Power Company. The International Fuel Cell company (a consortium of United Technologies, Toshiba and Bechtel Corporation) are now offering 200 kW, and 11 MW PAFC units for sale at special rates (~$1,800/kW) to allow further evaluation of these units in a variety of locations. However, the value of electrical generation efficiency is unlikely to exceed 40% using natural gas as a feedstock, and so the market penetration of these units will remain very small, except possibly for some special CHP applications and for selected strategic situations requiring uninterruptable power supplies. 4.4 AFC (Alkaline Fuel Cell) This system has been extensively developed for space applications and for limited terrestrial applications, particularly in the military sector. A planar configuration is usually employed for the stack which incorporates aqueous KOH as the electrolyte. For terrestrial applications, AFC units are usually operated in the temperature range 60-80°C and can attain up to 65% efficiency using pure H2. Civil applications are considered to be limited by the requirement that both fuel and oxidant gases need to be rigorously scrubbed of oxides of carbon and so pure hydrogen supplies are needed. Moreover, high catalyst loadings are usually specified, which is a further disadvantage of the AFC system. These drawbacks are contested by the ELENCO consortium which is committed to developing hydrogen-air fuel cells for large city vehicles (buses, etc.) and for specific market niches where the environmental advantages of fuel cells can allow a higher cost per kW.



4.5 DMFC (Direct Methanol Fuel Cell) This fuel cell uses an acid electrolyte (usually H2SO4) in which the methanol fuel is dissolved. The cathode is of the air diffusion type and the anode (usually carbon activated by a noble metal catalyst) is immersed in the H2SO4 electrolyte. This system was under active development for transport applications by Shell and Esso for more than a decade (until around 1980) and more recently Hitachi has commenced investigations into this system. Details of the Shell program are available (“Power Sources for Electric Vehicles” ed B D McNicol and D A J Rand). An advantage of DMFCs is the prospect of producing compact systems ranging in size from a few watts up to several kilowatts for ambient temperature operation. However, the performance of DMFC systems to date has been disappointing due to the rapid poisoning of the anode electrocatalysts by the methanol oxidation products. This, and the need for high noble metal catalyst loading, will probably ensure that this system does not emerge from the development laboratories until a major breakthrough is made relating to the anode electrocatalyst. 4.6 SPFC (Solid Polymer Fuel Cell) These systems incorporate a planar configuration in which the polymer is sandwiched between the two porous electrode structures. Most development work has been based on the NAFION perfluorosulfonic acid polymeric ion exchange membrane originally manufactured for the chlor-alkali industry. This material is relatively expensive and so SPFC systems have been developed, essentially for space and defense applications (e.g. submarine propulsion units). It is believed that considerable operating experience has been accumulated for small units but little information is in the public domain. It should be noted that SPFC units can also be used in the electrolysis mode for the production of hydrogen and oxygen and relevant investigation will also benefit the associated fuel cell technology. Relatively high noble metal catalyst loadings also contribute to make this system expensive and so the recent announcement by Ballard Corporation (Canada) of a new low cost unfluorinated polymer is very significant. However, the system still operates on pure hydrogen and oxygen and so civil applications are still unlikely.



4.7 PCFC (Proton Conducting Fuel Cell) Central to the development of this system is the availability of a solid state proton conducting membrane that can operate at intermediate temperatures (300-400°C). In this range of temperatures it is hoped that the improved electrode kinetics will avoid the need for noble metal catalysts and yet provide a satisfactory fuel/electricity conversion efficiency. Many hydrated salts including hydronium beta Al2O3 exhibit useful conductivities for H3O+ ion but the transport of water through the membrane and sensitivity to changes in the partial pressure of H2O make it unlikely that these compounds can be developed into engineering materials. Non-hydrated inorganic proton conductors such as a-zirconium hydrogen phosphate are unlikely to exhibit high H+ conductivities due to the propensity of protons to form relatively stable hydrogen bonds with oxygen ions in the structure. Indeed it is noteworthy that the high temperature protonic transport in SrCe(Yb)O3, is believed to be associated with the migration of oxygen ions. The writer considers that the search for stable solid state proton electrolytes is unlikely to be very rewarding, but progress in this area can be followed by consulting the proceedings of the International Conference on Protonic Electrolytes which is held every two years. 4.8 MCFC (Molten Carbonate Fuel Cells) and SOFC (Solid Oxide Fuel

Cells) Both these high temperature systems are considered to be contenders for the second generation of fuel cells, with multi-kW demonstration units becoming available in the mid 1990’s. In situ reforming (and direct electrochemical oxidation with SOFC systems) promise higher fuel electricity conversion efficiencies together with high grade heat for industrial CHP applications. Moreover, operation at high temperatures does not require noble metal electrocatalysts. However, both systems face major scale-up uncertainties and the reader is referred to section 5 Prof. Steele’s report for a discussion of current performance and development problems associated with both high temperature systems.



5 ULTRASONIC REACTORS 5.1 Introduction Ultrasonics are well known in the lab for intensive contacting of immiscible liquids and solids in low and moderate viscosity liquids and for enhancing certain reaction rates and yields. However, the laboratory equipment does not lend itself to scale-up, and until recently (with the Harwell sonoreactor) there have been no recommendable production-scale sonoreactors available. 5.2 Recommendation The AERE Harwell reactor (Figure 1) should be considered where there is a need to: (a) Make a liquid-liquid or solid-liquid dispersion of small, uniform drop or

particle size. (b) Enhance the rate of a liquid-liquid or solid-liquid reaction or dissolution. (c) Promote reactions of the type reported to be successfully enhanced. (d) Generate local heating. (e) Break up friable particles or lyse microorganisms, degass a liquid. The reactor is new and its performance on a new process would need to be proven by experiment. The construction seems to be robust, but so far, of course, reliability is somewhat untested over long timescales. Given simple acoustic shielding, the ultrasound does not present a safety problem. The design should be done with care to avoid energy losses and to ensure that the energy and, hence, the cavitation field responsible for the effects are well distributed across the reactor. Arbitrary addition of transducers to a vessel or pipe is not recommended. The use of ultrasound for processing high viscosity liquids is not recommended.



5.3 Design of Sonoreactor Figure 1 shows a version of the Harwell sonoreactor. It is recommended that any new applications use an existing Harwell design, or that Harwell carry out a new design. The energy is transmitted to the fluid via several transducers fixed around a pipe section (6" NPSin the current model), designed and tuned to give cavitation across the whole section. Several banks of transducers can be used in series if necessary. The reactor can be batch (or batch loop) or continuous. In continuous mode the process fluids pass through the active zone, in which the cavitation occurs. The surrounding system can provide once-through or recirculating loop flow as desired. Residence time in the active zone will be established by experiment.



FIGURE 1 SONOREACTOR PILOT PLANT LOOP RIG



6 THIN FILM REACTORS 6.1 Introduction Thin film reactors are used: (a) For rapid heat transfer, for example with rapid, exothermic or endothermic

reactions or heat-sensitive materials (liquids, slurries, or solids). (b) For rapid reactions with vapor release. The design methods for film thickness, residence time distribution and heat flux are not well established but have been partially tested. Heat transfer can be via the surface behind the film, using a jacket or induction heating, or by evaporation from the free surface, in which case very high fluxes are achievable. The residence time distribution in the film can be calculated from theory for laminar films. With low viscosities on centrifugal devices the film can become quasi-turbulent (i.e. some circulation within the film) which provides a nearer approach to plug flow. 6.2 Equipment Falling-film and scraped- or wiped-film devices (e.g. Luwa) are available commercially. They provide moderately thin films, with somewhat uncontrolled liquid flow patterns. Thinner, faster moving films can be obtained with centrifugal film reactors, often spinning discs, cones or cylinders. The Alfa-Laval Centritherm is a nested spinning cone device with steam jackets on the cones, intended for evaporation of sensitive foods, but possibly adaptable to reaction use. Some reactor technology vendors have developed semi-tech scale spinning cone, cylinder and packed-bed reactors and obtained some design data and experience with, for example, cumene hydroperoxide cleavage, polycondensation, polymer devolatilization, and solid-phase reactions.



FIGURE 2 CROSS SECTION OF SPINNING CONE REACTOR

7 ELECTRON BEAM REACTORS At least one attempt has been made to develop an industrial scale process making use of electron beams as chemical reaction activators. In 1970 the Ebara Corporation tested a flue gas scrubbing process in which the removal of NOx and SOx was promoted by firing a beam of electrons into the flue gas. The electronic activation affects primarily the rate of NO oxidation by creating OH radicals and possible oxygen atoms. The oxidation of NO is slow at low NO partial pressures, being second order in NO and, very unusually, having a reverse temperature dependence, that is proceeding more slowly at higher temperatures. Furthermore, NO2 (brown) is photochemically dissociated to NO and O2. The idea for electron activation sprang from consideration of the reactions producing photochemical smogs (such as those which plagued the Tees Valley in the ’sixties).



It is necessary to add ammonia to produce a consistently recoverable solid product consisting of a mixture of ammonium sulfate and ammonium nitrate. This is recovered using bag filters or electrostatic precipitators. Since 1975 about ninety five publications have appeared in the literature (mainly in "Radiation Physics and Chemistry") and at least two US Research organizations have conducted evaluations. A pilot demonstration unit has been built and operated in the USA, 1985; a pilot unit treating 10,000 Nm3/hr of sinter plant exhaust operated in Japan 1978/8. The US plant used two 80 kW beam generators, which are described as electron accelerator/scanner packages. The equipment resembles a TV cathode-ray system (although much higher powered) firing a scanning electron beam through a window of thin "1 - mil" Titanium foil. Protection against secondary radiation such as X-rays needs to be a feature of the design. The power requirement is claimed to be 2 to 8 kW per 1000 actual cubic feet per minute of flue gas (1 Mrad dosage = 10 J/g of gas). 8 MEMBRANE REACTORS 8.1 Introduction The term "membrane reactors" is applied to a variety of reactors which use a semi-permeable membrane directly within a reactor system. This membrane selectively favors transport of some reactant or product species over others (to which it is preferably impermeable) and either directly influences the course of the reaction, or enables the reaction system to exist. The term "membrane reactor" does not include applications of semi-permeable membranes as a separate unit operation (e.g. a recycle loop). A broad classification of membrane reactors would be: (a) Inorganic membranes, e.g. metals, glasses. (b) Organic polymeric membranes e.g. polyethersulfones. (c) Liquid membranes, e.g. hydrocarbon oils. The rarity of such reactors suggests that the membrane costs, and added reactor complexity compounded by the mass transport resistance inherent in the membrane, have rarely been justified by the advantages afforded by such a system, It may also be associated with a relatively low level of awareness of their possibilities.



8.2 Circumstances Suggesting Consideration of Membrane Reactors The following are examples of circumstances which prompt consideration of membrane reactors: (a) Equilibrium limited reaction producing a diffusive product, particularly

hydrogen, or water. (b) One permeable reactant needs to be kept at low concentration throughout

the reaction to avoid side reactions (applies to some analytical techniques; can be true in supplying microbial metabolites, high concentrations being wasted or even toxic).

(c) It is desirable to separate the catalyst or enzyme from the reaction system,

either simply to retain it, or to prevent some component of the reaction system from adversely affecting performance or life.

(d) One reactant is much more dilute than the other and by avoiding simple

mixing of the two, downstream product separation can be vastly cheaper (e.g. sulfuric acid in a hydrocarbon liquid droplet to recover ammonia from a dilute effluent stream, an example of a liquid membrane).

(e) Ion selectivity is required to favor a desired electrode reaction or to avoid

mixing of an electrode product with the source electrolyte. It is worth noting that living cells are far more sophisticated membrane reactors than anything yet devised by man, combining functions such as ion selectivity, active transport of ions and molecules against concentration gradients, the electrical mediation of multiple interconnected and nested biochemical cycles, pH regulation, not to mention self-replication! However, as an example, consider a simple membrane permeable to one species - hydrogen. 8.3 Example of a Membrane Reactor Equilibrium - Limited Reactions Producing a Diffusive Product, Especially Hydrogen. While a cynical interpretation is that it is a phenomenon searching for a use, very much research effort and patent lawyer time has been invested in the ability specifically of hydrogen to diffuse through certain precious metals, including silver, platinum and palladium (and its more malleable alloys with ruthenium) at moderate to high temperatures.



Other work has attempted to utilize the relative diffusivities of, say, hydrogen and hydrocarbon species through semi-permeable glasses and polymeric materials. (Because of temperature limitations the polymeric materials are less studied for direct mediation in hydrogen producing or consuming reactions, but of course exceedingly important in separate hydrogen separation processes). Two papers (K Mohan et al, AlChE Journal 32 (1986) p 2083 and N Itoh, AlChE Journal 33 (1987) p 1576) have presented the results of mathematical modeling of cyclohexane dehydrogenation reactors utilizing porous glass and palladium alloy respectively to overcome the equilibrium limitation to full conversion to benzene. One of the earliest reactions studied was that of hydrocarbon steam reforming. This is an endothermic equilibrium limited reaction:

Over a nickel catalyst the atoms of C, H and O are all brought into an equilibrium distribution amongst the species CH4, CO, CO2, H2O and H2. Reaction (2) has a much smaller heat of reaction than reaction (1) so that high temperatures favor CO and H2: temperatures of the order of 1000°C are needed to effect near complete decomposition of methane. Such temperatures can be attained by internal combustion (using air or oxygen). In practice, conventional hydrogen plants settle for about 4% residual methane in the hydrogen product after shift and CO2 removal, or use additional separation. It is reasonably clear that by removing H2 from the equilibrating mixture (while continuing to supply heat) the reaction can be driven to higher conversions at any temperature than if the H2 is left in the mixture, in which case it increasingly drives the back-reactions as its concentration increases. In the limit at a modest temperature sufficient to allow catalyst activity, very high conversion is possible to produce H2 at a very low pressure (at least in the final stages). In practice C-laydown considerations intervene. The use of precious metal membranes has long been thought about as a means of removing H2 directly from the reaction. One such study and the results of experiments are described in M Oertal et al, Chem Eng Technol, 10 (1987) p 248. Other studies further integrate the separation and reactor functions by using Pd-Ru alloy as a catalytically active membrane.



8.4 Modeling Membrane Reactors The modeling of membrane reactors involves adding to a basic reaction model, catering as necessary for concentration and temperature variables through kinetic and thermodynamic models, the transport of at least one of the reactant or product species. A well designed membrane reactor will not be so limited in either transport rate or reaction rate that the two phenomena can easily be separated, except possibly where the sole purpose is one of reactant concentration control. In general, the model should simultaneously address both phenomena in order to achieve accurate simulation. Two strategies suggest themselves. One is to add to a known reaction scheme a consecutive reaction yielding a non-interfering product (for example a liquid phase product in a gaseous reaction system), and to search for any available analytical results or set such a scheme up in general reactor simulation software. The second is to devise a specific model for the particular system being studied. Although useful analytical results are unlikely to be available without simplification of the problem as presented, some such results are available from related problems such as bubbling fluidized bed reactors, where gas circulation between unreactive bubble phase and reactive catalyst phase should be combined with a reaction model and either transport or reaction rate or both can control the final conversion. Under certain circumstances fairly simple compound exponential decay expressions result, containing potential control by transport or reaction in a summed decay exponent. The appended model due to Itoh gives an example of the model form obtained in the relatively simple case of H2 alone permeating a plug flow reactor wall. Note that the root partial pressure dependence of H2 transport rate is specific to H2 transport through a particular metal membrane (experimentally verified). It also probably applies to H2 transport through other metal membranes in which transport is effected as protons. Therefore the "metal film limited" process involves a proton concentration gradient in the metal, and a modified equilibrium constant analogous to a Henry’s constant at each surface:

at both inlet surface and exit surface.



The reaction modeled is cyclohexane dehydrogenation to benzene. 8.5 A Typical Analysis of a Membrane Reactor



Where the Ui and Vi (i = C, H, B and A) are the flow rates of component i in the reaction and separation sides, respectively. VR is the gross volume of the reaction section. PTr and PTs are the total pressures in the reaction and separation sides, respectively. An expression for the disappearance rate of cyclohexane, rC, can be expressed by:



Where p i is the partial pressure of component i in the reaction side. Equations (3) to (8) can be numerically integrated by standard techniques selected from the Nag routines, or published methods such as that due to Runge-Kutta-Gill.



NOMENCLATURE: k = apparent rate constant, eq’ n (8), mol m-3 Pa-1 s-1 KB = adsorption equilibrium constant of benzene, Pa-1 Kp = equilibrium constant, Pa3 L = dimensionless length of reactor P i = partial pressure of component i on reaction side, Pa P Tr = total pressure on the reaction side, Pa P Ts = total pressure on the separation side, Pa rC = dehydrogenation rate of cyclohexane, mol m-3 s-1 T = absolute temperature, K U A ° = flow rate of argon at reaction-side inlet, mol s-1 U I ° = flow rate of component i at reaction-side inlet, mol s-1 V I ° = flow rate of component i at separation-side inlet, mol s-1 α H = permeation rate constant for hydrogen, mol s-1 Subscripts: A = argon B = benzene C = cyclohexane H = hydrogen i = component i 9 SHOCK TUBE REACTORS Consideration was given in the late 70’s to their application to naphtha cracking. Maximum conventional process gas temperatures of 850°C or so, were limited by the metallurgy of the cracking furnace tubes. The chemistry suggested that improved yield of ethylene was available at higher temperatures and consequent lower residence times. At temperatures of about 1100°C, a few milliseconds were required, and the shock tube offered these conditions followed by a partial quench. In addition, the size of the shock wave reactor would be very much smaller than the conventional furnace.



10 REACTIVE DISTILLATION 10.1 Introduction This is an operating technique applicable to equilibrium chemical reactions where the products have different volatilities. The reaction is carried out in a distillation column so that the products are separated from each other, thus allowing the reaction to proceed to completion as far as the reaction kinetics will allow. Advantage has been taken of this principle for many years in batch operation of a reaction pot with overhead distillation, but exploitation by reaction and separation together in a continuous distillation column is more recent. A North American Company has reduced the costs of its ethyl acetate process by 60% compared with previous technology, and it is exploiting reactive distillation in other processes. A quantitative treatment of reactive distillation is being developed by Professor M F Doherty and co-workers at the University of Massachusetts at Amherst. This treatment is not yet ready for general use, so the objectives of this Clause are limited to alerting GBHE's technologists to the principles and to encourage appropriate experimental development. 10.2 Example of Reactive Distillation Ethyl acetate may be produced as follows:



Batch process: In a batch process, consisting of a kettle reactor attached to a distillation column, the ethyl acetate will necessarily appear in the first distillate fraction as the EtOH/EtAc azeotrope. At atmospheric pressure this limits the conversion of ethanol to a maximum of about 54%. Also, further processing is needed to separate the acetate from the azeotrope. Continuous process: By carrying out the reaction in a reactive distillation column and by using excess acid, the EtOH/EtAc azeotrope may be avoided and pure acetate is produced. By virtue of their relative volatilities, the two products, water and acetate, are separated - the water descending and the acetate rising in the form of EtOH/EtAc azeotrope. As it rises, the azeotrope is contacted with acid and its ethanol is reacted to extinction. The absence of water in this part of the column prevents back reaction. Figure 3 illustrates the process as an outline flowsheet.



FIGURE 3 REACTIVE DISTILLATION SCHEME



11 OTHER NOVEL REACTOR TECHNOLOGY This note lists other types of reactor which are either unusual or novel, or sometimes both. Proprietary reactors which use well established technology, but are marketed as being novel, are not included. Some of this technology is promoted by particular companies that own the patents, while other technology just exists as patents, which may be in force or may have expired. Many of the ideas have been around for a long time and are technologies looking for a problem. 11.1 Heterogeneous Catalytic Reactors 11.1.1 Magnetically Stabilized Fluidized Bed Reactor (MSFBR) The MSFBR consists of a fluidized bed but with magnetically susceptible particles stabilized by an externally applied electric field. The magnetic supports can be coated with an enzyme or catalyst. The reactor has an advantage over a normal fluidized bed in that it has better mass transfer and can sustain high gas velocities without particle elutriation. Also, the bed can deal with a feed containing dust that is not magnetically susceptible. Of interest when:

• There is a need for heterogeneous catalysis. • There are suspended particles in the fluid stream. • No heat transfer is required. • Testing has been undertaken for effluent processing and reactions using

enzyme catalysts. Being promoted by EA Technology. 11.1.2 Flow Cycling Fixed Bed Catalytic Reactor This is a single bed gas/solid exothermic reactor where the gas flow direction is periodically reversed to maintain a high temperature zone in the middle of the reactor bed and lower temperatures at the inlet and outlet. Choice of particle size and bed dimensions and cycle time enable appropriate relationships between bed heat capacity, reaction rate and heat transfer. The temperature reduction between the middle of the bed and the outlet enables better equilibrium conversion to be obtained.



This reactor has advantages when:

• The reaction is exothermic and uses a heterogeneous catalyst. • The reaction is equilibrium limited by product formation. • Heat recovery is not important either because

o Small scale of plant - can’t afford complication o or exotherm is small or reaction is at low temperature

The reactor has been commercialized for gaseous waste treatment for removal of VOCs, N0x and S02. 11.1.3 Plate Reactors The catalyst is coated onto plates so that the reactor has a dual function as a plate heat exchanger. Of interest when: The Catalyst is highly active. The reaction is highly exothermic or endothermic i.e. tubular reactor is currently used. Small scale is appropriate. The Catalyst is regenerable or has a long life. 11.1.4 Regeneratively Heated Reactor - Houdry Catofin Process Uses two packed beds. One bed is heated and cleaned up using clean fuel and air while the other bed does the catalytic reaction. Beds are switched after a time interval. Used for endothermic reactions which give carbon laydown or fouling. Considered as alternative for GHR on some Ammonia plant flowsheets. 11.1.5 Concurrent Downflow Contactor - Patented 1976 - Birmingham

University Of advantage for thermally neutral reactions where heat exchange isn’t needed – avoids circulation loop. Must have complete conversion of inlet gas. Can be used for gas liquid reactions over fixed bed catalysts.



11.1.6 Non-Catalytic Packed Bed Reactor Homogeneous gas phase reaction proceeds within a packed bed of balls etc. It allows better distribution of the gases and avoids diffusion flames. Used for incineration to give low N0x combustion and more certain combustion. It can be controlled over a wide range of combustion conditions. It has the advantage that it is not classed as an incinerator by the Environmental Agency, although if will achieve the same effect. Has been considered as an option instead of using an incinerator. General application for burning low calorific value gas. Possible process application is HCN. It is commercially available from Thermatrix. 11.1.7 Catalytically Coated Surface Reactor Catalyst is applied to a stirrer in a stirred tank reactor or to a static mixer in a plug flow reactor to provide initiation of a reaction. Of interest when:

• Only a small amount of catalyst is needed - e.g. for initiation • No catalyst is required in the product - e.g. polymers

Further development could be to use an electrically heated stirrer or static mixer to maintain the catalyst hot without having to heat up all the reacting liquid. Was considered by GBHE. Possible applications are liquid phase hydrogenations.



11.1.8 Rapid Cycling Regeneration Reactor Each active catalyst molecule only turns over one molecule of reactant before being regenerated. The catalyst in this case is sometimes called a ’cataloreactant’. The surface layer of catalyst will be transformed from one state to another by performing the reaction and will then be regenerated by using another reactant. This can be done by moving the solid from one reaction vessel to another and back again, or by having two or more fixed catalyst beds in which the feeds are cyclically switched. Most academic work is done using the former system, but this is very expensive to build at full scale, so full scale plants are likely to be cycling fixed beds. The technology is applicable to reactions, where the selectivity would be low if the feeds were mixed together over the catalyst bed. The most typical application is oxidation reactions. It is only likely to be economic, where existing routes to a product are not viable or are very expensive. This type of reactor has been evaluated by GBHE for oxidation of HCI to chlorine and for the production of aniline. 11.1.9 Hot Spot Reactors/Catalytic Partial Oxidation Reactor Reactants mixed together within a catalyst bed or immediately upstream of a catalyst bed can give different products than reactants being premixed and undergoing homogeneous reaction before entering a catalyst bed. Of interest for:

• Exothermic reactions involving combination of homogeneous and heterogeneous reaction.

• When high selectivity is difficult to achieve at present.



11.2 Liquid and Liquid/Gas Phase Reactors 11.2.1 Porous Wall Rate Controlled Reactor The reaction rate between two liquid or a liquid and gas reactants is controlled by using a porous metal barrier to control the rate of mixing between reactants. It has an advantage over stirred pot reactors and other mixing reactors that the concentration of the reactant that passes through the porous barrier is always low in the reacting mixture. Of interest when:

• Two liquid or one liquid and one gaseous reactant. • There is a need to avoid high local temperatures, or a need to maintain

one • component in excess to avoid by-product formation. • Total exotherm can be accommodated in one (or two) adiabatic reactors.

(Not clear what the advantage is over an intensive reactor mixer). 11.2.2 Pulsatile Plug Flow Reactors Used to give plug flow without using long thin pipes by pulsating the flow. Plug flow is most important for product quality where solids are produced and it is important to minimize by-product formation by achieving a narrow residence time distribution. Two versions are available: Mackley Oscillatory tube AEA Fluidic Plug Flow Mixer This has been commercialized, but maximum size to date is 250ml.



11.2.3 Vortex Mixer This is a static mixer which uses the pressure drop in the fluids to give intensive mixing. It is claimed to give low local shear compared to other intensive mixers, but it is not clear why this should be so. Of interest when:

• Mixing liquids or liquids and gases. • Fast reactions when short mixing times are needed. • Rapid dispersion of small volumes are needed to avoid byproduct

formation. • Rapid uniform mixing is needed for instance in precipitation reactions.

11.2.4 Gas Lift Loop Reactor The recycle in a liquid/gas phase reactor is achieved by utilizing the lift from the gas, which could be a reactant or product. Applicable when:

• Liquid/gas phase reactors. • Kinetics are slow, so large liquid volumes are needed. • Liquid recycle is needed.

11.3 Reactor/Separation Combinations 11.3.1 Exothermic Catalytic Condensing Reactor This is a multi-functional reactor for a gaseous exothermic reaction, with a fixed catalyst bed that uses a cold surface to condense reaction product inside the reactor. The total heat from the reaction exotherm is taken out at the product condensation temperature, so the scope for heat recovery is limited. The heat transfer to the cold surface is by natural convection. This is applicable when:



• The reaction is exothermic and uses a fixed bed catalyst. • The reaction is equilibrium limited by product formation. • Heat recovery from gas is not important either because:

o The plant is a small scale one. o The reaction temperature is within, say, 80° C of the condensation

temperature. 11.3.2 Adsorbent Reactors Reaction products removed by adsorbent to give higher conversion. Alternatives are:

• Equilibrium-Limited Periodic Separating Reactor o a fixed bed using pressure swing.

• Gas/ solid/solid trickle flow adsorbent reactor

o the absorbent is circulated through a regeneration process. Applicable when:

• Low conversions with current technology and • Difficult to separate products with current technology.

11.3.3 Supercritical Fluid Reactors Reaction takes place at supercritical conditions of reactants or reactant medium. Applicable when:

• Reaction takes place in inert liquid medium. • Preparation of reactants is difficult. • Difficult to separate products from reactants or reaction medium.



11.3.4 Ceramic Electrochemical Reactors Oxygen can diffuse through ceramic oxides at high temperature to give low oxygen concentrations when it comes into contact with another reactant. This means that the other reactant is always in excess and avoids formation of combustion or other byproducts. Of interest for:

• Oxidation or dehydrogenation reactions using oxygen as a reactant. • Current technology gives poor selectivity. • Reaction is endothermic or mildly exothermic. • Reaction can operate at greater than 750°C.

Originally developed for solid oxide fuel cells. 11.3.5 Microwave Heating of Reactors The use of energy derived from electricity as a continuous heat source is generally uneconomic at the scale at which some processes work. However, the use of electrical derived heating can be a good option for intermittent use. Reactor vessels or the catalyst itself in fixed bed reactors can be heated using microwaves. This could give the following benefits:

• an alternative to the use of separate start-up heaters. • to avoid condensing steam on catalysts during start-up. • supplying heat evenly during catalyst regeneration so that heat does not

have to be supplied by the regeneration fluid stream. • to evenly preheat furnace boxes or maintain them at temperature during

temporary shut-downs. By tuning the microwave frequency, the heat can be directed specifically at the catalyst support or at the active species. Microwave units up to 1-2MW are available depending on microwave frequency.



11.3.6 Induced Field Catalytic Reactors By putting a catalytic reactor in an electric field, polar reactants may be made to line up with the catalyst at a constant predetermined orientation. This will enable a highly selective reaction or reaction where it would otherwise not occur. Applicable for:

• Liquid phase reaction • Polar reactants • Kinetics very slow using current technology.

11.3.7 Rotating Cone Calciner Applicable when:

• Small throughputs required. • Low residence time required. • Low residence time distribution required.