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Relevance of Multiphase Reaction Engineering to Modern Technological Challenges M. P. Dudukovic* Chemical Reaction Engineering Laboratory (CREL), Department of Energy, EnVironmental and Chemical Engineering (EECE), Washington UniVersity in St. Louis (WUSTL), Missouri 63130
The purpose of this report is to summarize the lecture given at the joint CAMURE-6 and ISMR-5 international symposium in Pune, India, in January 2007. The emphasis is on the pivotal role that reaction engineering has to play in addressing modern technological challenges. First the global challenges of reducing the environmental impact of our technologies are considered. Then the role of multiphase reaction engineering in enabling efficient transfer of molecular-scale discoveries to more benign and sustainable processes is outlined. Typical scaleup methodologies are introduced, and research needed for their further improvement is discussed. Examples of the importance of proper scale-up are provided. Introduction Chemical reaction engineering (CRE) was born as a unifying methodology in the 1960s to serve the needs of the chemical and petroleum industries in quantifying transport-kinetic interactions.1,2 The CRE discipline has matured in the sense that it generated tremendous accomplishments and value in these industries. In the last few decades, the methods of the discipline have also been utilized in the production of semiconductors, optical fibers, composite materials, specialty polymers, pharmaceuticals, etc. These new applications enriched the discipline but dissipated its focus by emphasizing the specifics of each new targeted technology. The Catalysis in Multiphase Reactors (CAMURE) symposium series was initiated December 7-9, 1994, in Lyon, France, to celebrate the success of the completion of the UNITE MIXTE Project, in which reaction engineers and organic and catalytic chemists worked together on process development. This was a joint venture involving governmental funding agency CRNS, private industry (e.g., Rhone Poulenc), and universities (e.g., Toulouse, Lyon, etc.). It provided a clear example of the necessities and value of such interdisciplinary interactions. This meeting resulted in the first of the series of CAMURE proceedings and was edited by Pierre Fouilloux and appeared in Catalysis Today3 in early 1995. Other conferences in the reaction engineering field were initiated in the 1980s and 1990s, in addition to the traditional international symposium in chemical reaction engineering (ISCRE), and many mushroomed into their own series (e.g., Engineering Foundation ECI CRE Series, gas-liquid-solid GLS series, multifunctional reactors series, microreactors series, and many bioreactor and biotechnology conferences). Most emphasized the scientific basis of their rather narrow topical area, and only a few focused on advancing the general reaction engineering methodology, which provides improved methods for minimizing risk in transferring (e.g., scaling-up) scientific discoveries to commercial applications. The increased ability to monitor phenomena on the molecular and nanoscale has brought a fascination with molecular and nanoscale research. While undoubtedly many important discoveries await us at these scales, some current pressing challenges require further development and implementation of rational methodologies for transfer of molecular and * Contact information. Telephone: 314-935-6021. Fax: 314-9354832. E-mail:
[email protected].
nanoscale discoveries to commercial practice. The time has come to open new frontiers in reaction engineering by focusing on process development and scale-up and on developing techniques for multiscale analysis that will reduce scale-up risks. Instead of just tailoring the existing reactors to meet somehow the needs of specific chemical or biological systems, the general methodology, based on fundamentals, should be further developed for selection of the right reactor type and for scale-up of all such systems. Reaction engineering meetings should converge back to deal with the core of the discipline. Organizing the first joint meeting of CAMURE (Catalysis and Multiphase Reaction Engineering) and ISMR (International Symposium on Multi-functional Reactors) in Pune, India, is the step in the right direction. Global Challenges: Reduction of Environmental Impact What are the global challenges facing us? Clearly, providing energy, food, shelter, and health care for a growing population without damaging the global environment irreversibly is the key challenge. Developed nations would like to increase the market for their products; the developing world rightly aspires to an improved standard of living. Facing the uncertainties of global warming and the certainties that some resources are finite, the mandate for the process industry is to improve the efficiency of the use of energy and material resources in meeting increased demands while dramatically reducing the environmental impact. We are the providers of items that make the life style in the developed world possible, and we must lead in reducing the environmental impact of wider adaptation of such a life style. The two key factors that affect the global environment and sustainability of our practices are the total number of people and their life style. Agricultural practices, clearing of forest for arable land, irrigation of deserts, the extent of use of herbicides and pesticides, etc., obviously are all important. Mining for finite mineral or energy resources via strip or deep-shaft techniques, etc., affects the environment. Energy utilization efficiency, drilling for oil in pristine areas and oceans, use of hydroelectric power, etc., all have an environmental impact. Different recreational activities, such as country skiing or driving a snow mobile, walking or using a dune buggy, have different environmental consequences. As important as all of the above are, it is the manufacture of all products, such as fuels, chemicals, plastics, pesticides, and pharmaceuticals, that makes alternate
10.1021/ie070371p CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2007
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life styles possible, and that is the realm of chemical and process engineering. This realm consists of all chemical and physical transformations (and that includes biological) of starting materials derived from nonrenewable and renewable resources into a variety of products on which we depend to support our life style. Let us take a brief global look at the damage to the environment created by our technological activities. The total pollution generated can be, in the first approximation, expressed as a product of four factors: (1) pollution generated per unit of energy used, which depends on the level of available technology and process efficiency, (2) energy used for GNP generation, which depends on market forces, (3) GNP/per capita, which is affected by economic growth, and (4) total population. One can further simplify this global view of the pollution problem, as shown in Table 1, and present the global impact on the environment as a product of consumption per capita, population, and process inefficiency () 1 - process efficiency). Since population and consumption per capita rise without bounds, it seems self-evident that global environmental impact can be best reduced by controlling population growth, by reducing consumption per capita, or by doing both. Population growth control is anathema to two major religions, and reduction in consumption per capita is heresy to the free market system. Neither is a politically correct solution. Hence, the long-term solution to our global challenges in reducing the environmental impact of our technological activities lies in increasing the overall energy and material efficiency of processes. The change to truly sustainable technologies will take time,4 but it is unquestionable that our best hope lies in application of “green” chemistry principles and implementation of innovative more efficient processes. Unfortunately, there is much evidence that we are risk-adverse in adopting more efficient technologies. The reason lies in the socioeconomic sphere. For decades, process engineers have been taught that their processes must be profitable. The desire to become more “green” in processing, which partly resulted form social pressures, is to this day constrained by the requirement to be profitable. For example, in attempting to reduce the damage to the environment by the process industry during the last 20 years, the focus in the United States was on the following activities:5 (1) better education of personnel at existing manufacturing facilities, which resulted in better operating practices and paid for itself; (2) retrofitting of existing facilities, which resulted in improved profitability due to waste elimination or recycle of a valuable material (otherwise offending facilities were closed); (3) installation of end-of-the-pipe cleanup equipment, when required by law or peer pressure, was also pursued provided the process remained economical. Otherwise the facilities were closed or
Table 2. The Rich and the Poora people
rich
poor
Total Population population growth (% annual) life expectancy fertility rate infant mortality (per 1000 births) literacy
0.97 billion 0.5 78 1.7 5.4 98
2.3 billion 1.8 58 3.7 82 59
surface area (sq km) energy use per capita (kwh) electricity per capita (kwh)
Environment 35 million 5350 8340
32 million 508 302
Economy GNI Per Capita (US $)
28 550
Technology and Infrastructure fixed and mobile phones 1250 (per 1000 people) PCs per 1000 people 470 paved roads (% of total) 93 a
450 39 6.9 13
Source: The World Bank Group.
moved offshore. Unfortunately, this route of “globalization” of manufacturing facilities almost always resulted in licensing of the old (“World War Two technology”) at locations with more favorable regulatory and labor cost climate. Instead, what we need is the development and installation of new, cleaner, and more efficient technologies. First, this requires more investment by governments and industry into basic research to generate new green chemistries and better catalysts. Further development of novel process concepts requires substantial capital expenditures. It is also perceived as involving considerable risk and for that reason is not pursued by corporations at the moment. By investing more in the science of scale-up, the risk of commercializing novel technology can be significantly reduced, and as a result, the willingness to implement new technology will increase. This is the challenge for multiphase reaction engineering in the next few decades. Developing the political will to provide tax incentives to those that implement cleaner sustainable processes anywhere in the world would go a long way toward reducing the environmental impact of our technologies. Another reason why eventually we will have to embrace the new more efficient process technologies is the fact that the enormous gap between the rich and poor nations, illustrated in Table 2, must be bridged if we are going to avoid serious social upheavals in the future. Using the current inefficient technology will not do it since this is too wasteful in energy usage. Current attempts to bring the standard of living in Asia and Africa upward, toward the American level, by building up their
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Table 3. Industrial Sectors That Rely on Multiphase Reactors synthesis and natural gas conversion (MeOH, DME, MTBE, paraffins, olefins. higher alcohols) energy (coal, oil, gas, nuclear power plants) bulk chemicals (aldehydes, alcohols, amines, acids, esters, LABs, inorganic acids) fine chemicals and pharmaceuticals (Ag chemustry, dyes, fragrances, flavors, pharmaceuticals) biomass conversion (syngas, methanol, ethanol, oils, high value added products) petroleum refining (HDS, HDN, HDM, dewaxing, fuels, aromatics, olefins) polymer and materials manufacture (polycarbonates, PPO, polyolefins, specialty plastics, semiconductors, etc.) environmental remediation (De-NOx, De-Sox, HCFCs, DPA, gren processes)
manufacturing base while using current technology is clearly unsustainable. Only new technologies, that we must start to develop and implement now, can accomplish this while reducing the environmental burdens. In order to raise the living standards of the poor, while positively impacting the environment and the world economy, we need novel process and product technologies for the following: distributed power systems; consumer products and health care devices; reliable and clean energy carriers; recyclable materials; efficient and sustainable production of chemicals, fuels, and materials.; efficient water purification and potable water generation systems. Power generation and production of liquid fuels and of commodity chemicals, by their sheer size have an enormous environmental impact. Yet, via globalization, it is precisely these types of plants that are being built in the developing world based on old technologies. It is not as much that ideas for improved processes are in short supply, rather it is the unwillingness of the companies to assume the risk and invest in novel processes. Moreover, the developing world is able to build processes using older technologies based on expired patents, taking advantage of lower labor costs, less expensive raw materials, and less stringent environmental regulations to stimulate process innovations. It may be necessary to adopt new global environmental regulations. Another way around that problem might come from progressive tax breaks to those that are willing to assume the risk of implementing potentially cleaner new manufacturing technology. The risk assumed in adopting new technologies can be dramatically reduced by further improvement in proper reaction engineering methodology for scale-up. Role of Reaction Engineering In all the current and future methods of pollution reduction, chemical reaction engineering plays a pivotal role.6,7 That is
true in retrofitting activities, in end-of-the-pipe treatment, and certainly in the development of cleaner new green processes. In particular, it is important to recognize that at the heart of chemical transformations in all process and energy industries is multiphase reactor technology, as over 99% of reactor systems require the presence of more than one phase for proper operation. This multiphase reactor technology spans numerous industrial sectors as illustrated in Table 3 (e.g., energy, syngas and natural gas conversion, production of bulk chemicals, fine chemicals and pharmaceuticals, biomass conversion, petroleum refining, polymer manufacture, production of materials such as semiconductors and optical fibers, environmental remediation) and generates a large contribution to the gross domestic product in the United States. In all of the above industrial sectors, to implement commercially a new technology and design a reactor for it, one needs to be able to predict how the molecules of reactants will get in contact with each other and react, and how the rates and selectivity in such systems will differ from those observed at the laboratory scale. Hence, this transfer of molecular-scale discoveries to commercial use is the key challenge. The more accurate our methods for scale-up are the lower the risk of commercialization of new technologies. The powerful CRE methodology, developed over the last 50 years, offers a rational way to quantifying reactor performance based on mass, energy, and momentum balances by relating the prevalent multiscale transport and kinetic phenomena (see Table 4). Understanding these multiscale transport kinetic interactions in multiphase systems is the key to the selection of the best reactor type for a given chemistry and catalyst and to successful scale-up.6-9 The choice of the proper reactor type and operating conditions for a given process chemistry is the key factor in determining volumetric productivity and selectivity. Hence, although the reactor typically represents between 5 and 15% of capital and operating costs of the plant, its choice determines the number and load on prereactor and postreactor separation units and dictates the cost of the whole process. That is why the choice of the proper reactor type is essential, and it should be based on a reactor model. Such a model must capture the events on a multitude of scales at the right level and provide the ability to scale test tube discoveries to commercial processes. The complexity arises from the fact that the interactions of events on various scales are dependent on the scale of the equipment. It is increasingly necessary, when using novel more active catalysts, to understand the change of the flow pattern with reactor scale and the interaction of it with mesoscale transport. Scale-up methodology that relies increasingly on fundamental principles, outlined in Table 4, rather than on empiricism is needed.
Table 4. Key Elements of Reaction Engineering Methodology objective: quantitative description of multiphase reactor performance as a function of input and operating variables for various reactor types method: application of conversion laws (mass, energy, momentum) and of appropriate constitutive equations in rate laws on a multitude of relevant scales multiscales involved and level of description depend on what is required to meet the objective scale involved
objective
molecular
kinetic forms
eddy of particle scale
local rate laws
reactor scale
flow pattern, mixing pattern, contacting pattern
process scale
overall performance and controllability
methods used (at increasing level of sophistication) empirical, mechanism based, elementary steps, transition state theory, and molecular dynamics empirical, micromixing model, approximate diffusion + reaction models, turbulence with reaction rate, Stefan Maxwell equations coupled with kinetics, DNS, axial dispersion, phenomenological model, validated CFD models steady-state models, transient models
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product tons per year
E factor waste/product ratio by weight
oil refining bulk chemicals fine chemicals pharmaceuticals
106-108 104-106 102-104 100-103
∼ 0.1 100
Table 5 illustrates the so-called E factor of various industries.10 Those that practice CRE at the high level of sophistication, such as the petrochemical industry, produce the fewest undesirable products per unit desired product. So-called hightech industries, which are really high value added industries, like the electronic industry used to be and the pharmaceutical industry is now, have terribly high E factors and are not high tech at all from the environmental standpoint. Until very recently, their high profitability allowed them to ignore CRE principles in reactor selection and operation, but this is likely to change for the better in the future. Future processes will have to pay more attention to atom efficiency. Another point that should be made is that a process designed based on green chemistry principles will commercially be green only if scaled up correctly. Whether the process will be successful or not depends on the selection of the proper reactor type and its proper operation. A number of new processes were abandoned due to failures in scale-up, something that clearly is not advertized or readily noted by the general public. Yet a systematic approach to risk assessment analysis for chemical processes has not been articulated to the author’s knowledge. In summary, we should reiterate that both green chemistry principles and life cycle analysis of reactants and products should guide us in determination of the most desirable and sustainable process chemistry and chemical transformations that are involved in creating the desired product.12,13 The center of each new process is a chemical reactor in which key chemical transformations take place.14 Once the right catalyst has been identified to promote the desired green chemistry, the selection of the reactor type to be used, and the way it is operated, dictate the number and size of separation units needed and to a great extent determine the burden on the environment and energy efficiency of the process. Proper reactor selection and operation leads to optimal plants, minimizes the pollution burden and environmental concerns, and helps maximize energy efficiency. It is clear that adopting green manufacturing principles requires both technological and political changes. Such changes should include a new global tax structure that encourages corporations to implement green manufacturing and creates incentives for improved profitability through green manufacturing. In addition, modern scale-up methodologies must be further developed to minimize risk of scale-up. Only then will corporations embrace new technology rather than copy inefficient solutions of the past. Role of Research and Development in Scale-Up The key function of engineers is to transfer scientific discoveries into new technologies and practice for the benefit of mankind. That is also the main role of a process R&D department within a company, which also must meet set profitability goals for the process. Let us now briefly consider an ideal process R&D approach and compare this to what happens in everyday practice. Ideally, we should seek the best chemistry that maximizes atom, mass, and energy efficiency. We should do life cycle analysis to determine the best procedure and reaction. Based on the understanding of the reaction
pathways involved, we should seek the reactor with the best flow pattern and phase contacting pattern to optimize the whole system. We should, using a system’s approach,8,15 examine opportunities for effective coupling of reaction and separation. Bench-scale experiments should then be scaled up in an interdisciplinary effort. Instead, at present, R&D in most process corporations proceeds as follows (see Table 6): The chemistry that will do the job is found by trial and error (combinatorial analysis, etc). Usually reactors that the company is familiar with are tried on a bench scale, and best operating conditions are sought by statistical approaches with limited understanding of the underlying phenomena. New process chemistries are tested on pilot plants, but this step is increasingly bypassed for “known” chemistries. Scale-up is done and plants are built largely relying on the knowledge of contractors (i.e., construction companies). Unfortunately, by and large, the knowledge base that they use is based on the 1950s correlations dressed up in Excel spreadsheets. Plants built in this manner, especially reactors with novel more active catalysts, invariably have “startup problems” that tend to last indefinitely and experimentation with full-scale plants continues and it is becoming a norm. An “optimized” operation of a poor reactor choice remains inferior to the properly selected and properly scaled-up reactor type, thus resulting in much more waste and lower efficiency. The key scale-up issue can be summarized as follows: Once the reaction system is successfully run in the laboratory to produce the desired conversion, yield, and selectivity, reproducing these results at a commercial scale is next.16-18 Pilot plants remain important in this endeavor, and the business of building them is apparently doing well.19 We essentially have two choices. Horizontal scale-up (scale-up in parallel or scale-up by multiplication or scale-out)) offers one alternative, while vertical scale-up offers another. Only the latter must account for the effect of equipment scale on the interplay of transport and kinetics. The former keeps the geometry, flow, contacting pattern, and flow regime the same but has to deal with the logistics of system integration and flow distribution. Hence, without proper understanding of the system, scale-up that relies solely on statistical approaches has a high likelihood of failure. Classical reactor types, used in various technologies mentioned in Table 3, include packed beds, wall catalyzed reactors, bubble columns, stirred tanks, and risers and fluidized beds. Before proceeding with scale-up of such a reactor type, we should carefully examine whether the new process offers opportunities for reactor multifunctionality. Novel designs attempt to combine reaction and separation via reactive distillation, catalytic distillation, in situ adsorption, and via membrane reactors. Let us now consider what approach to scale-up we currently use and what remains to be done to make scale-up more reliable. Key reactor scale-up considerations require us to match the mean residence times or mean contact times in multiphase systems. This requires the knowledge of phase holdups and flow rates and flow patterns. To ensure the same performance, we must also match the variance of residence times in the reaction environment or the covariance of sojourn times in different phases.20 This requirement is often neglected. Naturally we must also ensure the same heat transfer per unit volume as indicated in Table 7. Scale-Up in Parallel and Microreactors. Scale-up by multiplication is practiced routinely for wall-cooled tubular reactors packed with catalyst. Once the satisfactory performance of one tube is accomplished, the number of tubes needed for commercial production is determined. Optimal performance of the tube packed with catalyst with small dt/dp ratio requires
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Table 6. Ideal and Currently Practiced Process R&D Approach Ideal R&D Approach select the best chemistry (use green chemistry principles) select the most suitable reactor type (based on multiscale considerations) consider possibilities for separation-reaction multifunctionality Scale-up bench scale results with care based on as complete understanding of the system as possible to minimize risk Prevalent Current R&D Approach find chemistry that does the job by trial and error use familiar reactors and add needed separations test new process chemistries on pilot plant scale build the plant with minimal scale-up expenditures relying predominantly on contractors experiment with the plant to determine ‘best conditions’ via statistical analysis Table 7. Key Scale-Up Requirements match mean residence time or mean contact time match [or account for the change in] dimensionless variance of residence (contact) times match [or account for change in] covariance of sojourn times in different environments (phases) of the system match heat transfer per unit volume, or account for the change with change in scale of equipment.
proper understanding of heat transfer in such tubes. The longterm effort of Dixon21-23 has contributed significantly to advancing the fundamental understanding of heat transfer in such tubes. Successful scale-up requires reliable single tube data, good knowledge base for manifold and flow distributor design, and avoiding external heat-transfer limitations upon scale-up. For many decades, industrial chemists were experimenting with a catalytic tube of 1-2 on. in diameter of desired lengths (-1 m or longer). Once the feed temperature and composition and flow rate used (i.e., mean residence time) produce the desired result, scale-up is simple in principle. It requires using N tubes identical to the one used in the laboratory, packed with the same mass of the same catalyst particles and receiving the same feed at the same flow rate as the tube in the lab. The number of tubes N needed is given by the ratio of the desired commercial production rate and that achieved in the laboratory. Lurgi and other companies build reactors with up to 50 000 tubes!24,25 This ability to scale up in parallel is a great advantage that microreactors offer also. Let us look now at some other advantages of microreactors: (1) high surface to volume ratios and, due to small dimensions, enhanced mass- and heat-transfer coefficients by 1-2 orders of magnitude, (2) laminar flow conditions and low-pressure drop but with opportunities to make RTD narrow by introduction of another phase, (3) controllable RTD and back-mixing, (4) high volumetric productivity, (5) low manufacturing and operating costs, (6) increased safety due to small amount of material, and (7) scale-up in parallel (scale out). The MIT group of Jensen,26 among others,27-29 has recognized the importance of being able to manipulate multiphase systems in microreactors and has shown that one can get competitive performance for various reactions, separations, and in material synthesis. The achieved performance of the microreactor depends on the level of understanding of the chemical system and the ability to manipulate microreactor design to meet the reaction contacting requirements best.30 If the desired figures of merit are met, then scale-up in parallel ensues. Jensen and his co-workers have also shown that, via multichannel integrated design, in principle, scale-up to large production rates is possible even for highly exothermic reactions such as direct fluorination of aromatics.31 They also proved the advantages of tailoring the RTD in a microreactor for colloid silica synthesis from TEOS, ammonium hydroxide, ethyl alcohol, and deionized water.32 In laminar flow in microchannels, a wide particle size distribution (PSD) occurs and the standard deviation of the PSD
approaches that of the batch system (8 vs 5%) only at large sizes and large residence times (due to Taylor diffusion effect). In contrast, in segmented flow, achieved by the addition of gas bubbles, a more uniform size distribution is obtained with considerable lower standard deviation of the PSD at smaller sizes due to the much narrower RTD in the liquid, This effect of RTD on pdf of particle sizes was predicted by Pratsinis et al.33 long ago. Jensen et al.32 clearly demonstrated how to use it. In their comprehensive review paper at CAMURE-5 and ISMR-4, Hessel et al.34 summarized well the contacting principles in gas-liquid and gas-liquid-solid microreactors. They reviewed the characteristics of a variety of contacting patterns attempted and reported vastly improved mass- and heattransfer coefficients, much larger interfacial areas, controllable RTDs, increased volumetric productivity, and ease of scaleout. They offered demonstrations of successful bench-scale use in direct fluorinations, oxidations with fluorine, chlorinations, sulfonations, and hydrogenations. Enthused by potential for significant process intensification, a number of companies are involved in R&D for potential commercialization of microreactors. Degussa,35 Clariant,36 Velocys,37 and others are in hot pursuit of implementation of microreactor technology. Degussa is running a demonstration project for the evaluation of microreaction technology or DEMiSTM for propylene epoxidation with hydrogen peroxide. Clariant opened its Competence Centre for Microreactor Technology (CCMRT) to increase efficiency, improve safety, and reduce the costs of pharmaceutical synthesis. Velocys using microchannels, formed by a patented process in steel,38 proposed to replace the conventional packed catalytic tubes in methane reformers and in Fischer Tropsch synthesis.39 Many advantages of miniaturization in achieving process intensification have been well summarized by Charpentier.29 The following question then arises. With all their perceived advantages, and the technologies available to manufacture microreactors in silicon, in glass, and in steel or in other metals or materials, why are not they more widely used? The answer is that they require very fast reactions and active stable catalysts (usually these two do not go together). Most importantly, microreactors are, due to their small dimensions, more prone to fouling, clogging, and leaks between channels. Their reliability and life on stream is still unknown. All of these are potentially solvable problems on a case-by-case basis. However, the perceived risk factor is too large and the estimated cost
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advantage too small for companies to replace the existing installations with microreactors. Most likely acceptance of microdevices will occur in large numbers first in consumer products, distributed power systems, highly energetic fast reactions, in situ production of hazardous chemicals, and health care. Other applications might be slower. Scale-up in parallel was suggested for monoliths also,40 as the argument that the monolith performance can be predicted based on single-channel performance is often made. During the last four decades, monoliths made inroads only in few large commercial applications: in automobile exhaust (multiple distributed units), and in power plant gas cleanup (SCR of NOx) for which they are the largest known reactors (up to 1000 m2 in size). The work of Schmidt with catalytic monoliths in partial oxidations (e.g., conversion of ethanol to hydrogen) opens new avenues for generation of clean fuels but is currently at the laboratory stage.41-43 Applications in other areas, especially for gas-liquid-solid reactions, are few. The customer resistance factor is too large due to uncertainties of multiphase flow distribution and life of wall-deposited catalyst on stream. Some of these concerns may be justified. For example, studies in our laboratory by γ-ray tomography revealed that in gas-liquid flows uniform distribution in monolithic channels cannot be taken for granted as there is only a narrow window of flow conditions that allows it.44 Outside that window of operation, the flow distribution can be highly nonuniform, and considering the monolith to be a bundle of identical channels would not model reality properly. Hence, a lot of work remains to be done even in multiphase systems that utilize scale-up in parallel. Characteristic reactor heat and mass-transfer times must be determined for appropriate flow configurations. Manifold design and heat integration, which will allow scale-up in parallel from single-channel studies, need to be brought to higher scientific levels than the current semiempirical approach. Since one deals with laminar flows, both transport theories and computational methods are available to handle flow, transport, and reaction in these channels at the fundamental level. Vertical Scale-Up. For large production capacities, it will remain necessary to scale up the size of equipment to be used in commercial applications compared to the laboratory-scale examined. In this situation, it is essential to properly quantify the interplay of transport and kinetics to be able to quantitatively assess the effect of change in reactor scale on its performance. This inevitably requires validation of models on very large scale, especially in the energy sector, which is something that industry failed to provide so far. The field is very broad, and it is impossible in this short paper to do justice to all who contributed to it. The insightful text by Ramachandran and Chaudhari45 on three-phase systems and the comprehensive treatment of threephase fluidized systems by Fan46 remain excellent background material for treatment of multiphase systems. The advances in computation that enable us to incorporate more detailed fluid mechanics into reactor models are well outlined by Kuipers and van Swaiij47 and Ranade,48 while proper coupling of turbulence and kinetics is thoroughly treated by Fox.49 The ramification of combining improved physical understanding of three-phase reactors with computational modeling on improved reactor models was recently summarized.50,51 The insight and tools needed for understanding multiphase reactors has been provided by the work of Aris on residence time and contact time distribution in many environments52,53 and by Shinnar and coworkers.54,55 The theoretical foundation provided the needed background for model development and experimental validation. In what follows, the efforts mainly conducted in our CREL
(http://crelonweb.wustl.edu) on improving the vertical scaleup procedure are described. We consider, as example, two types of systems here. Those with phases that are fully mobile, and those in which one phase, (e.g., catalyst bed) is fixed in space. Reactors with Moving Phases. When we have a reactor system with two moving phases, it is important to be able to describe the flow pattern of each. In the past, we relied on ideal reactor assumptions of treating each phase as being either in plug flow or perfectly mixed. When reality did not conform to these assumptions, the axial dispersion model (ADM) was often used to match experimental observations. It has been recognized, however, that the ADM is not predictive, and that one needs more accurate flow and mixing models based on the physical phenomena that occur in the system. Since multiphase systems are always opaque, it was important to develop means to measure phase holdup and velocity distribution in them. Such data are essential for validation of CFD models. Fully predictive multiphase CFD codes for reactor scales of interest are not available at present due to the fact that one has to model multiphase turbulence and complex interphase interactions. Thus, the following three-step approach to multiphase reactor modeling and scale-up is suggested: (a) capturing the physics of flow by experimental means; (b) testing appropriate CFD models and validating their results experimentally; (c) developing physically based engineering models for flow and mixing to be coupled with kinetic schemes for the process of interest. Let us examine the advantages and limitations of this approach to scale-up, relative to current practice. To capture the physics of flow, we need to assess the flow pattern and mixing in multiphase systems with large volume fractions of phases. This is a difficult task since optically based methods do not work reliably. Nevertheless, we and others have shown that using radioactive isotopes one can generate important information about the flows.56-58 Use of γ-ray computed tomography (CT) yields the time-averaged density distribution at various elevations of the reactor.59-61 Computer-assisted radioactive particle tracking (CARPT) provides the full Lagrangian description of the flow, as instantaneous motion of the radioactive tracer particle is continuously monitored throughout the reactor.62-64 Monitoring the trajectory of tracer particle of the same size and density as the solids in the system reveals solids motion. A neutrally buoyant, fully wettable, particle, or a very small particle, follows the liquid motion. The Lagrangian information obtained by particle tracking directly provides the components of the eddy diffusivity tensor and, upon ensemble averaging, yields average velocity fields in the system.63,64 Experimentation on different diameter systems, at different operating conditions, and with different physical properties builds the needed data base for validation of appropriate computational fluid dynamics (CFD) codes. For scale-up purposes, the only practical codes are based on the Euler-Euler interpenetrating fluids model, and on occasion on a LagrangianEulerian simulation. Clearly, this requires validation of the closures used for the interphase interaction terms and for multiphase turbulence. CARPT/CT data provide a test of the validity of suggested closures. The question of how large a scale has to be employed to validate the code for industrial-size reactors is still open. Once a CFD code is validated for a specific system, it can be used to generate data for the development of a reactor model. In the presence of a very large number of chemical reactions in a multiphase system, coupling of species balances with CFD is still too time-consuming and impractical. Nevertheless, the physically based reactor model, developed from CFD results, which were validated by CARPT/CT data,
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can capture the key time constants for mixing and flow that now can be coupled with kinetics. This leads to more reliable prediction of reactor performance because the reactor flow model now contains the key physical features of the flow pattern and mixing. In contrast, the conventional approach most often assumes that the flow in the reactor is either plug flow or completely mixed (CSTR). The ADM is used to describe a flow pattern that clearly deviates from an ideal reactor flow pattern. Unfortunately, all too often, the axial dispersion does not have a firm physical basis and it is impossible to predict its extent in scale-up with the needed accuracy. Let us illustrate some applications of the proposed method as outlined above for predicting reactor performance and describe its successes and limitations. Slurry Bubble Columns. The bubble column is a very important and widely used reactor type. Current interest is focused on applications in generation of liquid fuels such as in methanol, dimethyl ether, and Fischer Tropsch synthesis. Due to excellent heat-transfer properties, the reactor can be operated close to isothermally. Very high superficial gas velocity, up to and in excess of 0.5 m/s, is needed to suspend a sufficient amount of catalyst in order to reach high volumetric productivity. The design of bubble columns by reputable contractors is based on the assumption that the liquid is perfectly mixed and gas is considered either in plug flow or also well mixed. When high conversions are desired, this leads to vastly inflated estimates of required reactor size and overestimates of capital costs. A better model is needed for scale-up and cost estimation. Moreover, it is essential to understand the time constants associated with reactor-scale liquid recirculation,65 caused by the tremendous difference in buoyancy forces due to the parabolic gas holdup profile in churn turbulent flow66 and the time constants associated with eddy diffusivity.67,68 Assessment of catalyst circulation and distribution is also vital. We have learned from tomography studies that the gas holdup profile is almost parabolic in churn turbulent flow.60,65 This profile drives, by buoyancy force differences, a single liquid recirculation cell (in a time-averaged sense), which is confirmed by CARPT studies.67-69 CARPT also provides the axial and radial eddy diffusivities from the Lagrangian tracer trajectories.67-69 When the model is applied to a pilot plant column (of the same diameter as the cold flow model) for methanol, DME, or FT synthesis, both gas-phase and liquid-phase tracer responses are well predicted at seven different elevations.70-72 Being able to predict, with no adjustable parameters, tracer responses indicates that the model is capable of predicting reactor performance for any system of linear kinetics. One should note that the CARPT/CT data were obtained on equipment of the same diameter in the cold flow model and that the gas holdup profile was experimentally determined in the hot pilot plant using the cross-sectional averaged holdup (measured by DP cell) and line-averaged holdup (measured by nuclear densitometry gauge). To have a proper scale-up tool, one needs to show that CFD can predict CT/CARPT data and then use CFD to generate the parameters of our engineering reactor model. The usually used Euler-Euler interpenetrating fluid equations were executed in CFDLIB (of Los Alamos73-75) and FLUENT.76 Closures are needed for liquid turbulence (e.g., bubble-induced turbulence) and for the drag.77 Originally, computations were based on experimentally observed or assumed average bubble diameter; later we incorporated the population balance with coalescence and breakup.78 This eliminates the need to assign bubble diameter and finally yields much improved prediction of
experimentally observed holdup profiles! We get good agreement in predictions of both time-averaged velocity with CARPT data and of holdup profiles with CT data and, most importantly, of the computed eddy diffusivities and CARPT data,73 This completes the validation of not only the mean flow field and holdup distribution but of the dynamic features of our model for bubble columns. In spite of the above-illustrated success of the suggested model in predicting the tracer responses and mixing in the column and of extensive studies of bubble columns of others (e.g., Joshi et al.,79 Ranade et al.,48 Krishna et al.,80 Mudde et al.,81 Fan et al.,82 Scouten et al.83), key questions regarding scale-up to very large diameters remain. This is due to the uncertainties in physics of multiphase gas-liquid flows in the churn turbulent regime in large vessels. There are no available data on large enough vessels to test the CFD predictions for liquid recirculation, back-mixing, and gas distribution in columns of very large diameters like the ones contemplated for commercial FT synthesis. Hence, scale-up risk remains if one extrapolates the current predictions obtained in columns of up to 1 m in diameter to 10 m diameter columns needed for FT in GTL. That uncertainty, due to lack of reliable theory for multiphase turbulence and for the breakup-coalescence phenomena, can only be reduced by development of a new reliable theory or by obtaining the needed data on column diameters larger than 1 m. Testing a large-scale cold flow model would provide the information needed for extension of the approach illustrated above. Using internals to enable scale-up in parallel may also provide an answer. In systems with a multitude of reaction time constants, improved understanding of the dynamics of bubble formation and interfacial area renewal is also essential. Hence, the bottleneck to risk-free scale-up of bubble columns remains the incomplete understanding of the large-scale effects on flow and mixing in churn-turbulent flows. Much more work is needed in this area. Liquid-Solid Riser. Liquid-solid risers have been considered as a reactor of choice for solid acid-catalyzed alkylation that would replace current systems in which HF or sulfuric acid is used as catalyst. The key design issue, due to rapid solid acid catalyst deactivation, is the flow pattern of liquid and solids in the riser, and the extent of back-mixing of the solid catalyst in the riser. Classical impulse response liquid tracer studies confirm that liquid is in plug flow. How about the solids? The working design assumption in the literature is plug flow of solids, or some leakage of solids backward interpreted by an empirical axial dispersion coefficient. Use of CARPT and CT provides the database coupled with CFD model for an improved model of solids flow and solids back-mixing in liquid-solid risers.84-87 A single radioactive particle, monitored by CARPT, exhibits a tortuous path through the riser during a single visit87 as shown in Figure 1. Multiplying the difference in subsequent positions with the sampling frequency yields instantaneous velocities. Averaging ∼2000 or more of such pathways yields a symmetric solids ensemble averaged velocity profile, with particles rising in the middle and falling by the wall87 as indicated in Figure 2. A fully developed symmetric flow pattern of solids is observed in the ensemble-averaged sense. CFD computations reveal a highly complex three-dimensional instantaneous flow structure (which explains the single-particle trajectory), but 75 s of averaging produces the symmetric flow pattern of rising solids in the middle and falling by the wall. The agreement between
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Figure 1. Lagrangian trace over 38 s (1900 positions) for Ul) 20 cm/s and S/L ) 0.15.
Figure 2. Averaged solids axial velocity for Ul) 20 cm/s and S/L ) 0.15.
simulations and data is excellent for solids velocity, solids holdup distribution, and granular temperature, i.e., solids kinetic energy.87 This provides us with the proper model for the riser. It consists of plug flow of liquid and fully developed solids velocity profile with superimposed axial and radial diffusivities. This model can be coupled with appropriate reaction and deactivation kinetics. CFD can predict all the model parameters. As a bonus, by monitoring the time of entry and exit of the tracer particle from the riser section by CARPT, one obtains the residence time distribution of the solids in the riser. This confirms that solids flow deviates sometimes significantly from plug flow. For example, while a liquid flow pattern is well described by 20-50 tanks in series, the solids flow pattern is approximated by two to six tanks in series depending on the operating conditions! In addition, full Lagrangian characterization of the solids flow in the liquid-solid riser has been accomplished. CFD validated with CARPT/CT data is capable
of providing all the model parameters and produces liquid and solids RTDs in agreement with experimental observations.87 The CFD model developed should be tested for other similar liquid-solids flows and extended to particles of different size and density to quality as a sound design base for liquid-solid systems. It turns out that in this case our proposed method produced a reactor-scale model that can be used for scale-up and design, since the scale-up factor in column diameter is small. However, the bottleneck here does not lie in the reactor scale but rather on particle scale. The reasons for rapid catalyst deactivation are not fully understood. Potential use of supercritical CO2 liquids to speed up product desorption is being investigated88 as well as the fundamentals of transport and reaction within micropores of the solid acid catalyst. Gas-Solid Riser. Let us consider now a circulating fluidized bed with a gas-solid riser. Gas-solid risers have been used in fluid catalytic cracking for many decades, but more recently, they were advocated as a reactor of choice for catalysts that can undergo an oxidation-reduction cycle and be made attrition resistant. For example, in producing maleic anhydride from butane on a vanadium catalyst, the hydrocarbon selective oxidation occurs in the riser and the catalyst regeneration by air oxidation is done in the fluid bed prior to catalyst recirculation.89-91 Clearly, the solids flow pattern is one of several critical issues (others include catalyst attrition properties and ability to withstand cycling, catalyst inventory, and residence time in the riser and in the regenerator) as it determines the oxidation state of the catalyst, and its change with the scale of the equipment must be understood. Reliable predictions of the true extent of solids back-mixing in the riser, or even the prediction of solids residence time distribution, were unavailable prior to the CARPT studies. The knowledge of how flow pattern, flow regime and mixing vary with reactor scale is also not well understood, and this can lead to failure of scale-up. Thus, a pilot plant may work well but the commercial plant will not if the mean contact time and its dimensionless variance are not reproduced well between the two. The fact that solid backmixing increases with reactor diameter must be properly accounted for during scale-up. Both riser flow and regenerator flow are important. In our laboratory, Bhusarapu92 completed a thesis on complete flow mapping of solids in a gas riser operated in the fast fluidization (FF) and discrete particle transport (DPT) regime. He repeated his experiments at the Sandia National Laboratory
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on a riser that can handle much larger solids fluxes. The results are explained in detail in his thesis92 and associated publications.93,94 The main findings are that only CARPT can provide the true residence time distribution of solids in any section of the riser. CARPT uses a single radioactive particle and is capable of precisely identifying the time when the tracer particle crosses the inlet plane and enters the region of interest and the time when the tracer particle leaves it either by crossing the exit plane or by crossing the inlet plane again but moving in the opposite direction. Time when the particle reenters is also detected. By repeating the experiment several thousands times, this allows one to account properly for residence time and first passage time distributions. As indicated by Shinnar and Naor95 and Nauman96 in these types of systems with open boundaries, it is not possible to uniquely determine the RTD from an impulse response experiment in which one injects a bunch of tagged particles. This is because one does not know the precise entry time of the injected particles, and whether they crossed the inlet plane several times, and does not know at the exit plane whether the tagged particles are crossing going upward or down. The differences in the first passage time distribution and its variance and the corresponding quantities for the real RTD can be substantial leading to wrong estimates of solids back-mixing.92 Moreover, interpreting the observed tracer responses with ADM could lead to wrong conclusions regarding which flow pattern (FF or DPT) produces larger back-mixing. One should instead use the macromixing index (i.e., mean trajectory length) of Villermaux,97 which is directly obtained from the CARPT particle trajectory as the ratio of the average trajectory length of the particle travel through the system to the length of the system. The CARPT/CT database presents a challenge for the CFD modelers. They need to develop a code that will be able to predict not just the mean but also statistical quantities of the CARPT documented Lagrangian flow field of solids. Regarding a phenomenological model to use, the CARPT/ CT data reveal an ensemble average pattern of solids upward flow in the middle and downward by the wall with much larger solids holdup at the wall.92-94 This seems to confirm the usually employed core annulus model for risers. More careful examination of the tracer particle trajectories indicates that only ∼20% of the particles travel straight up, while most, even in the core, exhibit closed-loop trajectories with repeated recirculation. The tracer particle residence time in the monitored section can span 3 orders of magnitude. The particle that moves straight up stays very short in the section; the one caught in numerous down drafts stays very long. This behavior persists even at elevations of 33 diameters above the distributor where the flow is presumably fully developed. Due to typically small reaction time constants in risers, a more appropriate phenomenological model for the riser should be sought that takes advantage of the observed statistics of particle motion. The difference between CARPT determined RTD and FPTD can be large, and the mean and the variance of these curves and, hence, the solids back-mixing inferred from the variance is vastly different, indicating that great caution is advised when interpreting the results from the literature that are not based on CARPT experiments! Stirred Tanks. Since stirred tanks are used in such a wide area of applications we have confirmed that CARPT is capable of faithfully capturing the mean flow features, the intensity of vorticity, and up to 80% of turbulent kinetic energy in singlephase flow, where CARPT data are compared to LDA, PIV, etc.98,99 It takes only 16 h for CARPT to obtain this information that takes months for other techniques to acquire. In addition,
CARPT data can be obtained in two- and three-phase flows98,99 where laser-based techniques do not work reliably. The goal is to validate CFD codes based on CARPT/CT data and use the codes to obtain an appropriate flow and mixing model for the reactor. However, we had limited success in getting EulerEuler codes to predict the observed CARPT data100 for either single-phase or gas-liquid flow99,101,102 or liquid-solid flows.102,103 We leave this challenge to CFD specialists. In the meantime, assuming that CFD codes will eventually be validated, we use them to generate the compartmental model for the stirred tank. All flows, steady and transients, are computed by CFD and provided for each compartment. Our compartmental model for the tank differs from others in that it includes both convective mean flows and transient flows by turbulence. The success of this model in prediction of the effect of the location of feed point on reactor performance has been documented.104 Extension of this approach to multiphase systems is in progress. Trickle Beds. Let us turn our attention now to scale-up of trickle bed reactors used in a number of industries. The celebrated rule of thumb is to keep constant LHSV, which guarantees the constant volumetric flow of liquid per volume of the catalyst. It is inferred then that the mean contact time is the same. The problem with this approach is that the mean contact time between solids and liquid may not be the same because the liquid-solid contacting on the particle scale increases with increasing liquid mass velocity, which increases with the increase in reactor scale in order to maintain constant LHSV. Hence, in the absence of reactor-scale maldistribution, scale-up is forgiving for liquid-limited reactions prevalent in the petroleum industry, such as HDS, since liquid-solid contacting increases with reactor scale at constant LHSV. Unfortunately, for gas-limited reactions, the effect is the opposite, as an additional resistance to the gas arrival to the solid is created at larger liquid velocities due to increased holdup and increased liquid-solid contacting. This points to the pitfalls of scale-up at constant LHSV if done without understanding when this rule of thumb is valid. One should also keep in mind that it is not just the mean contact time but its variance that should be preserved upon scale-up. Enhancement of trickle bed reactor performance is achievable by forced cycling of the feed.105 It can be predicted based on the proper model of the system and has been confirmed experimentally.106 Unfortunately, there is large inertia in industry to adoption of this simple but effective means of improving reactor performance. To improve the understanding of the steady and dynamic behavior of these systems, we run both CT scans and appropriate CFD models, hoping that this will result in enhanced phenomenological models for scale-up and design. Numerous reviews107,108 have been published on trickle beds. Operation of laboratory reactors has been quantified experimentally and via modeling.110-112 The key issues related to scale-up involve the prediction of two-phase flow distribution in the beds. While CFD can do that,113-115 the uncertainty of the predicted flow field remains due to the lack of knowledge of the voidage distribution and particle orientation in the bed. Much work remains to be done along these lines. For further improvements in scale-up, the relationships between liquid holdup distribution and liquid and gas fluxes on different packing are needed. Effect of packing on gas and liquid distribution must be quantified. Mesoscale particle wetting effects should be incorporated in a reactor model. CFD should be validated for liquid and gas flow distribution. How to assess, describe, and predict voidage distribution and voidage space
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8683 Table 8. Options for Improved Reactor Performance via Process Intensification reactive (catalytic) distillation dynamic operation (, e.g., swing adsorption) periodic (symmetric) operation of packed beds with exothermic reactions coupling of an exothermic and endothermic reaction in a periodically operated packed bed induced pulsing in trickle beds counter current flow in gas-liquid-solid catalyzed systems membrane reactors flowing solids adsorbent expanded solvents (especially carbon dioxide)
configuration or particle orientation in large beds are some of the remaining key challenges. Only when this is accomplished can CFD become truly predictive in trickle bed scale-up and design. Process Intensification Use of more active catalysts places increasing demands on increasing transport rates and improving the volumetric productivity of reactors. Microreactors provide one solution; other means of process intensification are available for vertical scaleup also. Table 8 summarizes some of the process intensification alternatives applicable on micro or larger scales. Some ideas involve coupling of reaction and separation and only a few will be mentioned here. Reactive and catalytic distillation has received the most attention in academic laboratories116,117 and has been implemented in industrial installations.118 In our laboratory we studied photochemical chlorination of toluene to benzyl chloride as desired product.119 Since with respect to toluene we have consecutive reactions and with respect to chlorine competitive reactions, it is clear that one needs plug flow of toluene in the liquid phase and crossflow, that is back-mixed, for chlorine. Commercially, to achieve the favorable flow pattern for the formation of the intermediate, the liquid phase flows through a series of bubble columns, with light wells, while chlorine is fed in parallel to each column to keep its concentration low. Hence, the liquid experiences plug flow by going through a series of columns, and in each column gas is close to well mixed at its exit composition. Typical selectivity of 90% or more can be obtained at toluene conversion 30% or lower. We have shown119,120 that by conducting the process in photoreactive distillation semibatch mode we can achieve selectivity of better than 95% at toluene conversion well above 90%. A continuous system would operate even better. Clearly, this idea can be implemented, in principle, on the microscale or macroscale, but this reaction may be too slow for the microscale. In running adiabatic packed beds for exothermic reactions, it is well-known that the temperature rise adversely affects the achievable exit conversion due to equilibrium limitations, since the equilibrium constant goes down with increased temperature. Hence, an idea patented by Cottrell in the 1930s, of swinging the bed feed from one side to the other to achieve a more favorable inverted U temperature profile, was embraced by many in the reverse-flow concept, which was commercially implemented in sulfuric acid manufacture, VOC abatement, etc.121,122 We have carried that idea further by coupling an exothermic reaction (like methane combustion) with an endothermic one (like methane re-forming) in periodic operation with feed switching from end to end. Modeling shows that there is a region of operability where complete conversion for both reactions and
high thermal efficiency can be achieved.123-125 This concept can be implemented on the microscale as well as on a large scale. In his doctoral thesis126 and related paper,127 Ramaswamy thoroughly examined the various possibilities of coupling exothermic and endothermic reactions and discussed the modes of operation that lead to improved energy efficiency and avoid hot spot formation. He found that the dimensionless heat generation and heat consumption rates of the two reactions and their dimensionless activation energies are key factors in determining which flow pattern is best. The relationships that emerged are complex and cannot be readily summarized in a single equation. However, for most cases, countercurrent flow reactors result in energy loss compared to cocurrent or directly coupled adiabatic reactors. Summary and Conclusions In order to ensure a continued rise in the standard of living of the increasing world population, one must reduce the global environmental impact of our manufacturing technologies. This requires novel more efficient and ultimately sustainable processes. In order to speed up the implementation of new efficient technologies, in addition to appropriate political moves, it is essential to reduce the risk of scale-up of new molecular discoveries to commercial practice. Here, further development of reaction engineering methodology is needed. Scale-up and design based on fundamentals should become a norm for microreactor systems. Critical scale-up of multiphase reactor types must also be based on increasing application of fundamentals at the molecular, single-eddy, or single-particle scale and reactor scale as advocated in this paper. Scale-up issues should be considered at the conceptual stage of new process development. This requires an interdisciplinary coordinated effort, which is increasingly missing even in our largest global chemical manufacturing corporations. The Center for Environmentally Beneficial Catalysis (CEBC) (cebc.ku.edu), directed by Professor Subramaniam, is trying to fill this void and illustrate how development of new sustainable technologies should be done. From the material covered in this paper the following conclusions emerge. Development of reliable, precise, and inexpensive microfabrication technologies has resulted in phenomenal advances in microreactors offering intriguing alternatives for highly energetic, fast, and hazardous processes, especially when distributed systems are desirable. Scale-up in parallel offers additional advantages as theoretical principles and computational tools needed to work with microscale systems are available. Advances in noninvasive monitoring of multiphase opaque flows make validation of practical CFD codes possible and open the door for rational and successful vertical scale-up of a large number of reactor types. Implementation of these advances brings us closer to cleaner and more efficient processes and facilitates commercialization of new cleaner technologies. Various forms of process intensification are available and should be considered for the increasing number of processes. Tax incentives for implementation of novel more efficient technologies globally are called for to provide a boost to modern process development techniques and practice. Acknowledgment The author appreciates the support of the National Science Foundation ERC grant (EEC 0310689) for the Center of Environmentally Beneficial Catalysis (CEBC) that provided the
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opportunity for collaborative work on cleaner processes. CREL industrial sponsors are acknowledged for their support over the years. Most of all, all ex-CREL students and co-workers have contributed extensively in generating the methods discussed in this paper. My colleagues and friends, M. Al-Dahhan, P.A. Ramachandran, and J. Gleaves are acknowledged for the inspiration they constantly provide. Literature Cited (1) Kramers, H.; Westerterp, K. R. Elements of chemical reactor design and operation; Academic Press: New York, 1964. (2) Levenspiel, O. Chemical reaction engineering; J. Wiley: New York, 1962. (3) Fouilloux, P.. Ed. European Symposium on Catalysis in Multiphase Reactors. Catal. Today 1995, 24 (1-2). (4) Huesemann, B. H. The limits of technological solutions to sustainable development. Clean Technol. EnViron. Policy 2003, 5, 21. (5) Chemical Manufacturers Association. Preventing pollution in the chemical industry. 1982-1990, CNA: Washington DC, 1992. (6) DeLasa, H., Dogu, G., Ravella, A., Eds. Chemical reactor technology for environmentally safe reactors and products; NATO ASI Series E Vol. 225; Kluwer Academic Publ.: Dordrecht, 1992. (7) Dudukovic, M. P.; Mills, P. L. Symposium on catalytic reaction engineering and environmentally benign processes. Ind. Eng. Chem. Res. 1999, 33 (121), 2885. (8) Krishna, R.; Sie, S. T. Strategies for multiphase reactors selection. Chem. Eng. Sci. 1994, 49, 4067. (9) Lerou, J. J.; Ng, K. M. Chemical reaction engineering: a multiscale approach to a multi-objective task. Chem. Eng. Sci. 1996, 51 (10), 1595. (10) Trost, B. M. The atom economy: a search for efficiency. Science 1991, 254, 1471. (11) http://chemsoc.org/pdf/gcn/atomeff.ppt#279,1, Atom Efficiency. (12) Douglas, J. M. Process synthesis for waste minimization. Ind. Eng. Chem. Res. 1992, 31, 238. (13) Allen, T. D.; Shonnard, D. T. Green engineering: enVironmentally conscious design of chemical processes; Prentice Hall: Upper Saddle River, NJ, 2002. (14) Tunca, C.; Ramachandran, P. A.; Dudukovic, M. P. Role of chemical reaction engineering in sustainable engineering principles. In Sustainability science and engineering: defining principles; Abraham, M. A., et al., Eds.; Elsevier: Boston, MA, 2006; p 331. (15) Westerterp, K. R. Multi-functional reactors. Chem. Eng. Sci. 1992, 47, 2195. (16) Bisio, A.; Kabel, R. L. Scaleup of chemical processes; J. Wiley: New Y.ork, 1985. (17) Euzen, J. P.; Trambouze, P.; Wauquier, J. P. Scale-up methodology for chemical proceses; Technip: Paris, 1993. (18) Trambouze, P.; Euzen, J. P. Chemical reactors-from design to operation; Techip Paris, 2004. (19) http://www.zeton.com/home.htm. (20) Dudukovic, M. P. Tracer methods in chemical reactors. Techniques and applications. Chemical Reactors and Design Techniques; Delasa, H., Ed.; NATO ASI Series, 1986; p 107. (21) Dixon, A. G. Heat transfer in fixed beds at very low (
