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COMMENTARIES Modern Chemical Engineering in the Framework of Globalization, Sustainability, and Technical Innovation† 1. Introduction: Chemical and Related Process IndustriessAt the Heart of a Great Number of Scientific and Technological Challenges Chemical and related industries, including process industries such as petroleum, pharmaceutical and health, agriculture and food, environment, textile, iron and steel, bituminous, building materials, glass, surfactants, cosmetics and perfume, and electronics, are evolving considerably at the beginning of this new century, because of unprecedented market demands and constraints stemming from public concern over environmental and safety issues, and sustainable considerations. To respond to these demands, the following challenges are faced by the chemical and process industries, involving complex systems both at the process scale and at the product scale. (a) For the production of commodity and intermediate products such ammonia, sulfuric acid, calcium carbonate, ethylene, methanol, and ethanol, patents usually do not concern the products, but rather the processes. And now, these processes are not selected based on economic exploitation alone. Rather, the compensation resulting from the increased selectivity and savings linked to the process itself must be considered. With high bulk chemicals, the problem becomes complex, because other factors, such as health, safety, and environmental aspects (including nonpolluting technologies, reduction of raw materials and energy losses, and product/byproduct recyclability), must be considered. The trend toward global-scale facilities soon will require a total or partial change of technology: facilities will no longer be capable of being built “just a bit bigger” if one must handle throughputs never before seen in chemical and related industries. Therefore, we are faced with the need for a change in technologies to scale-up the reliability of new processes from the current semi-work scale to a vast scale in which there is no previous experience. Thus, for such highvolume bulk chemicals, the client will buy innovative processes that are nonpolluting and perfectly safe, which might involve membrane technologies and/or microtechnologies that are concerned with microstructured mixers or reactors. (b) Progression from traditional intermediate chemistry to new specialities, active material chemistry, and related industries involves the chemistry/biology interface of the agriculture, food, and health industries. Similarly, it involves the upgrading and conversion of petroleum feedstocks and intermediates and the conversion of coal-derived chemicals or synthesis gas into fuels, hydrocarbons, or oxygenates. This progression is driven by the new market objectives, where sales and competitiveness are dominated by the end-use properties of a product, as well as its quality. Indeed, end consumers generally do not judge products * Tel.: +33(0)3 83 17 50 77. Fax: +33 (0)3 83 32 73 08. E-mail:
[email protected]. † This paper was originally scheduled to be part of the special issue, “Membrane Engineering for Process Intensification” (Vol. 46, No. 8).
according to technical specifications, but rather according to quality features, such as size, shape, color, aesthetics, chemical and biological stability, degradability, therapeutic activity, solubility, mechanical, rheological, electrical, thermal, optical, magnetic characteristics for solids and solid particles, touch, handling, cohesion, friability, rugosity, taste, succulence, and sensory properties. This control of the end-use property, expertise in the design of the process, continual adjustments to meet the changing demands, and speed in reacting to market conditions are the dominant elements. Indeed, these high-margin products, which involve customer-designed and perceived formulations, require new plants, which are no longer optimized to produce one product at good quality and low cost. Actually, the client buys the product that is the most efficient and the first on the market. He will have to pay high prices and expect a large benefit from these short-lifetime and high-margin products. Moreover, the need is for multipurpose systems that can be easily cleaned and easily switched over to other recipes (flexible production, small batches modular setups, etc.). It is important to note that, today, 60% of all products sold by chemical companies are crystalline, polymeric, or amorphous solids. These complex materials must have a clearly defined physical shape to meet the designed and desired quality standards. This also applies to pastelike and emulsified products. Actual developments require increasingly specialized materials, active compounds, and special effect chemicals, which are much more complex, in terms of molecular structure, than traditional, industrial high-bulk-volume chemicals. The aforementioned considerations must be taken into account in the modern chemical engineering of today. But how? We shall try to answer this question in presenting, successively, the current complementary approach for chemical engineering, which involves the organization of scales and complexity levels, then the current tools for the success of this approach, and the four parallel tracks met for investigations in the topic. 2. Today’s Chemical and Process Engineering Approach: Organizing Scales and Complexity Levels Thus, chemical and process engineering is concerned with understanding and developing systematic procedures for the design and optimal operation of chemical, pharmaceutical, food, cosmetics, and process systems, ranging from nanosystems and microsystems to industrial-scale continuous and batch processes, all within the concept of the chemical supply chain.1 This chain begins with chemical or other products that industry must synthesize and characterize at the molecular level. The molecules are then aggregated into clusters, particles, or thin films. These single or multiphase systems form microscopic mixtures of solid, pastelike, or emulsion products. The transition from chemistry and biology to engineering involves the design and analysis of production units, which are integrated into a
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Figure 1. Scales and complexity levels in process engineering. (Reprinted with permission from ref 8; copyright Berkeley Electronic Press, 2003.)
process and become part of a multiprocess industrial site. Finally, this site is part of the commercial enterprise driven by market considerations and demands the inclusion of the product quality, once again, within the framework of sustainability. In this supply chain, it should be emphasized that product quality is determined at the nanoscale and microscale levels and that a product with a desired property must be investigated for both structure and function. This will help make the leap from the nanoscale level to the process level. The key to success is to obtain the desired end-use properties, and then to control product quality, by controlling the microstructure formation. A thorough understanding of the structure/property relationship at both the molecular level (e.g., surface physics and chemistry) and the microscopic level (e.g., coupling reaction mechanisms and fluid mechanics) is of primary importance to be able to design production processes that ensure the customer quality requirements. Moreover, most of chemical processes are nonlinear and nonequilibrium, belonging to the so-called complex systems for which multiscale structure is the common nature. This requires an integrated system approach for a multidisciplinary and multiscale modeling of complex, simultaneous, and often coupled momentum-, heat-, and mass-transfer phenomena and kinetic processes that are happening on different scales: (1) Different time scales (10-15 to 108 s) are used, from femtoseconds and picoseconds for the motion of atoms in a molecule during a chemical reaction, to nanoseconds for molecular vibrations, hours for operating industrial processes, and centuries for the destruction of pollutants in the environment. (2) Different length scales (10-9-106 m) are encountered in industrial practice and are shown in Figure 1, with approaches on the nanoscale (molecular processes, active sites), the microscale (bubbles, droplets, particle wetting, and eddies), the mesoscale for unit operation (reactors, exchangers, columns), the macroscale for production units (plants, petrochemical complexes), and the megascale (atmosphere, oceans, and soils, e.g., up to thousands of kilometers for the dispersion of emissions into the atmosphere). Therefore, organizing scales and complexity levels in process engineering is necessary to understand and describe the events at the nanoscale and microscale and to better convert molecules to useful and required products at the process scale. This leads one to organize levels of complexity, by translating molecular processes into phenomenological macroscopic laws to create and control the required end-use properties and functionality of products manufactured by continuous or batch processes (transforming molecules into money).
Moreover, any progress in the analysis of multiscale structures in chemical engineering, not to mention any breakthrough in understanding complex systems, is bound to contribute to the formation of a new knowledge base for the process industry, which requires a complex system and multiscale methodology.3 This multiscale approach is also encountered in biotechnology and bioprocess engineering, to better understand and control biological tools such as enzymes and microorganisms and to manufacture products. In such cases, it is necessary to organize the levels of increasing complexity from the gene with known properties and structure, up to the product-process couple, by modeling coupled mechanisms and processes that occur on different scales, as shown in Figure 2. This concerns approaches on the nanoscale (molecular and genomic processes, and metabolic transformations), the microscale (respectively, enzymes in integrated enzymatic systems, biocatalyst environment, and active aggregates), the mesoscale for unit operation (bioreactors, fermenters, exchangers, separators, etc.), and macroscales and megascales (respectively, for units and plants, and for the interaction with the biosphere). For illustration, biology’s catalysts, enzymes, are protein molecules that substantially accelerate the biochemical reaction in the cell. Understanding an enzyme at the molecular nanolevel means that it may be tailored to produce a particular end-product at the product and process mesoscales and macroscales. This leads to considerable opportunities to apply genetic-level controls to make better biocatalysts and novel products, or to develop new drugs and new therapies and biomimetic devices while responding to societal challenges. Moreover, advances in genomics mean that customized chemical products are likely to become more relevant, and very soon. Thus, the ability to think across length scales makes chemical engineers particularly well-poised to elucidate the mechanistic understanding of molecular and cell biology and its larger-scale manifestation, i.e., decoding communications between cells in the immune systems.4 In food process engineering today, there is significant scope for such approaches in regard to linking scale to model process physics, process (bio)chemistry and process microbiology from the molecular and cellular scale to the full process plant scale.5 I have defined this approach as “triplet-molecular ProcessesProduct-Process Engineering (3PE)”: an integrated system approach of complex multidisciplinary nonlinear and nonequilibrium phenomena occurring on different length and time scales (managing complexity).2 Therefore, in addition to the basic and irreplaceable notions of unit operations, coupled heat, mass, and momentum transfers,
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Figure 2. Organizing levels of complexity underline the new view of biochemical engineering. (Reprinted with permission from ref 8; copyright Berkeley Electronic Press, 2003.)
the traditional tools of chemical reaction engineering, as well as the fundamentals of chemical and process engineering (separation engineering, catalysis, thermodynamics, process control, economic considerations, etc), this integrated multidisciplinary and multiscale approach is beneficial and has considerable advantages for the development and the success of this engineering science, in terms of concept and paradigms. The work of today’s chemical and process engineers clearly involves a strong multidisciplinary collaboration with physicists, chemists, biologists, mathematicians and instrumentation specialists. Moreover, developing new concepts within the framework of what could be called “physical-bio-chemical engineering” totally justifies the qualification of process engineering as an extension of chemical engineering and takes on its full meanings.6,7 In such a context, biology is included as a foundation science of process engineering, along with physics and chemistry, to involve chemical engineers with the basic concepts of genetics, biochemistry, and molecular cell biology, which will serve well the needs of the fine chemical, pharmaceutical (such as chiral compounds), and biotechnology industries. Improving both the design and evaluation of complex systems for the production of real products requires further research into strategies, methodologies, and tools. We will see that they are oriented toward the acquisition of basic data in thermodynamics, kinetics, rheology, and mass, heat, and momentum transport, and toward the conception of new integrated operations that incorporate the coupling and uncoupling of elementary processes (transfer, reaction, separation) or combining several functions in one piece of equipment, clearing the way for smaller, lessexpensive installations, which requires improved knowledge of process modeling, automation, and control. This requires mathematical models and scientific instrumentation, which provide useful basic data that can be treated using powerful computational tools. For example, the treatment of generalized local information increasingly demands the help of computational fluid dynamics (CFD) and computational fluid mixing (CFM). This has been the case for a long time in combustion, automotive, aeronautic, and spatial applications, especially for the knowledge, control, stability of flows, and the characterization and improvement of transfer phenomena. However, because of recent rapid advances in software programs (e.g., CFDLIB, FLUENT, PHOENICS, FLOW 3 D, FIDAP, FLOW MAP, ANSYS CFX5.6), CFD and CFM are becoming more important every day for the design and scaleup of (new) equipment or for multifunctional unit operations. For
example, CFD is used for the simulation of flow phenomena and processing generalized local information, or for understanding the impact of complex flow geometries on mixing and reaction phenomena at the micro eddy scale, or for obtaining information on the detailed quantitative flow pattern in single and multiphase flows, mixing and axial mixing, mass- and heattransfer coefficients, drop- and bubble-size distribution, and so on; it can be used in bubble columns and centrifugal extractors, or for investigating the effect of the pumping direction of an axial flow impeller, the feed rate and the number of feed inlets on the operation of continuously fed stirred tank reactors (CSTRs), or for the numerical simulation of the complex hydrodynamics of multiphase catalytic gas-liquid-solid reactors, or for modeling sieve tray hydraulics; or it can be used for analyzing local hydrodynamics parameters of both liquid and gas phase in the riser of external loop airlift reactors, or for simulating flow in complex geometries such as reactor internals (industrial distributor devices), or for describing the observed flux behavior or to predict the exiting stream temperatures of fluids in direct contact membrane distillation modules of smaller as well as larger dimensions, and so on. Calculations can be performed for any geometric complexity and for single and multiphase flow, provided that physical models are available. Nevertheless, the use of this tool becomes possible only when the calculation time is acceptable, i.e., less than few days. Thus, CFD is a good link between laboratory experiments, conducted with common fluids such as air, water, organics, and hydrocarbons, and industrial operations involving complex fluids, and severe temperature and pressure conditions. An important advantage of CFD techniques is that the geometry and scale effects are automatically taken into consideration. Imaging complements CFD and CFM, especially for validation purposes complementarily to experimental data obtained with sophisticated techniques such as laser Doppler velocimetry (LDV) and particle image velocimetry (PIV), computerautomated radioactive particle tracking (CARPT), or positron emission tomography (PET) and positron emission particle tracking (PEPT) for opaque systems. 3. Today’s Tools for the Success of Chemical and Process Engineering for Modeling Complex Systems at Different Scales More precisely, it should be underlined that the 3PE approach is now possible thanks to significant simultaneous breakthroughs in three areas: molecular modeling (both theory and computer
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simulations), scientific instrumentation and noninvasive measurement techniques, and powerful computational tools and capabilities for information collection and processing.8 At the nanoscale, molecular modeling assists in maintaining better control of surface states of catalysts and activators, obtaining increased selectivity and facilitating asymmetrical syntheses (such as chiral technologies) or explaining the relationship between structure and activity at the molecular scale to control crystallization, coating, and agglomeration kinetics. Also, atomic force microscopy (AFM) is a versatile and useful technique to study engineering processes at the molecular and nanoscale levels. This is due to its tremendous surface microscopic capabilities, including three-dimensional (3D) topography of nanoparticles fabrication, metrology, analysis of microscopic phase distribution in polymers, and thin film, mechanical, and physical property characterization. Moreover, AFM, in conjunction with colloid probes, coated colloid probes, and cell probe techniques, allow direct quantification of the forces of interaction between (coated) colloids/cell and planar surfaces of process materials. The approach has a big role in the development of (bio)fouling-resistant membranes by correctly identifying membranes with low fouling properties. At the microscale, computational chemistry is very useful for understanding complex media such as non-Newtonian liquids, molten salts, supercritical fluids, multiphase dispersions, suspensions, and, more generally, all systems whose properties are controlled by rheology and interfacial phenomena such as emulsions, colloids, gels, froths, foams, hydrosoluble polymers, and particulate media (such as powders, aerosols, and charged and viscous liquids). Computational chemistry also helps us understand fractal structures of porous media and their influence on mass and heat transfer and on chemical and biological reactions. In addition, noninvasive measurement techniques such as spectroscopic or monochromatic ellipsometry (diffusing wave spectroscopy), nuclear magnetic resonance (NMR), computerautomated radioactive particle tracking, tomographic techniques, etc., coupled with two-dimensional (2D) or 3D image reconstruction techniques, are very helpful in obtaining hydrodynamics data at the time and space microscale. Among the available tomography techniques, electrical tomography (either resistance and capacitance) seems the most promising technique for the dynamic flow imaging purpose, i.e., to visualize gas holdup in gas-liquid systems such as stirred tanks, or in bubble columns, and the flow patterns inside packed beds, or to visualize the transient phenomena and hydrodynamic characteristics in bubble columns, and in two- and three-phase fluidized beds, or even both to visualize solid distribution in solid-liquid stirred tank and to monitor phase dispersion and separation for liquid-liquid processes relevant to the pharmaceutical industry. Electrical tomography is also used for the quantification of mixing with chemical reaction in batch stirred vessel reactions. NMR coupled with magnetic resonance imaging (MRI) presents a noninvasive means to obtain specific information about the structural heterogeneity of materials and porous media. For multiphase flow in catalytic porous media, this sophisticated technique leads to simultaneous measurement of the parameters characterizing hydrodynamics, mass transfer, and chemical conversion. This technique leads also to the time- and space-dependent droplet separation kinetics in granular bed filters. At the mesoscales and macroscales, CFD is required for the design of new operating modes for existing equipment such as reversed flow, cyclic processes, unsteady operations, extreme conditions (such as high temperature and pressure) technologies, and supercritical media. CFD is also required for the design of
new equipment or unit operations. It is especially useful when rendering process step multifunctional with higher yields in coupling chemical reactions with separation or heat transfer. It provides a considerable economic benefit. More generally, CFD is of great assistance concerning the design of new equipment, based on the new principles of coupling or uncoupling elementary operations (transfer, reaction, separation). At the production unit and multiproduct plant scale, dynamic simulation and computer tools for the simulation of entire processes are more and more required and are applied to analyze the operating conditions of each piece of the production unit equipment. They are used to predict the material flows, as well as the states and residence times within individual pieces of equipment, to simulate the entire process, in terms of time and energy costs. New performances (product quality and final cost) resulting from any change due to a blocking step or a bottleneck in the supply chain will be predicted within a few seconds. Many different scenarios may be tested within a short time, allowing rapid identification of an optimal solution. For instance, the simulation of an entire production year (steady-state flow sheet simulation) may require only a few minutes on a desktop computer. Clearly, such computer simulations enable the design of individual steps and the structure of the entire process at the megascale, and they place the individual process in the overall context of production. Thus, integrating the previous considerations, what trends and means are at the disposal of chemical engineering? 4. Chemical and Process Engineering: Quo Vadis? Our future approach is characterized by four main parallel and simultaneous objectives. These concern a total multiscale control of the processes to increase selectivity and productivity, a process intensification using multifunctional equipment, new operating modes or operating with microstructured equipment, manufacturing of end-use properties and the application of multiscale and multidisciplinary computational chemical engineering modeling and simulation to real-life situations. 4.1. Total Multiscale Control of the Process To Increase Selectivity and Productivity. This necessitates the “intensification” of operations and the use of precise nanotechnology and microtechnology design. This is the case of molecular information engineering, where, for example, instead of using porous supports for heterogeneous catalysts, synthetic materials with targeted properties are now conceived and designed. The development of an effective catalyst (a complex system) in both composition and functionality is central to a successful catalytic process. The ability to better control its microstructure and chemistry allows for systematic manipulation of the catalyst’s activity, selectivity, and stability. 4.1.1. Nanotailoring and Microtailoring of Materials with Controlled Structure. Through control of the pore opening and crystallite size and/or proper manipulation of stoichiometry and component dispersion, the ability now exists to engineer novel structures at the molecular and supramolecular levels via nanostructure synthesis.9 This led to the creation of nanoporous and nanocrystalline materials. Both of these materials possess an ultrahigh surface-to-volume ratio, which offers a greatly increased number of active sites for performing catalytic reactions. Nanocrystalline processing includes tailoring size-dependent electronic properties, homogeneous multicomponent systems, defect chemistry, and excellent phase dispersion. Nanocrystalline catalysts have greatly improved catalytic activity over conventional systems and multifunctionalities necessary for complex
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applications. For example, the catalytic activity of structuresensitive reactions, such as photocatalysis over titania, used for the decomposition of chemical wastes (such as chloroform and trichloroethylene) is dependent not only on the number of active sites, but also on the crystal structure, interatomic spacing, and crystallite size of the catalytic material. By varying the crystal size and phase through molecular engineering, it is possible to manipulate and optimize the catalyst design. Also, the high volume fraction of surface/interfacial atoms in nanocrystalline materials presents a great opportunity to control the surface chemistry and defect concentration. The design of such materials, with flexible control of stoichiometry and electronic properties that are important for redox catalysis can be exploited in a variety of catalytic processes. Moreover, vapor-phase and wet-chemical synthetic approaches have led to unprecedented control of material structures at the atomic and molecular levels, and these approaches have created ensembles of such features in the shape of nanocrystalline systems involving crystallite-size tuning. Now, complex nanocomposite systems can be built to fulfill the various roles required for the reaction mechanism and conditions. Nanocomposite processing and tailoring also lends itself readily to intelligent combinatorial approaches in material design and rapid catalyst screening.10 Also, through supramolecular templating, nanoporous systems can now be derived with well-defined pore size and structure, as well as compositional flexibility in the form of particles and thin films. Nanoporous structures have many possibilities in materials applications, with further development in molecular engineering, in areas such as surface functionalization of inorganic structures and the extension of supramolecular templating to organic systems. Self-assembly of nanostructured building blocks (e.g., nanocrystals) and combining porosities on different length scales will lead to interesting hierarchical structures. Systems with multiple levels of intricacies and design parameters offer the possibility of simultaneously engineered molecular, microscopic, and macroscopic material characteristics, which, in response to societal demands, may lead to the construction of such advanced systems as biomimicking medical implants or electronic/photonic devices. Nanotechnology and functionalized membranes for reactions and water-treatment applications represent other illustrations of the nanotailoring and microtailoring of materials with controlled structure. Membranes functionalized with appropriate molecules or nanostructured metals provide applications ranging from tunable flux and separation, as well as toxic metal capture, to toxic organic destruction. Moreover, if the selected macromolecule is a biomolecule (such as a polypeptide), then, in addition to creating a highly charged field in the membrane pores, conformational changes (such as helix-coil) can be utilized to conduct tunable nanofiltration separations with macroporous membranes at low pressure. Also, the complete dechlorination of trichloroethylene and selected polychlorinated biphenyls (PCBs) by membrane-based nanosized metals at room temperature can be mentioned.11 In addition, the next generation of membranes for reverse osmosis purification of water, which relies on partitioning selectivity and mobility contributions, requires optimization of membrane materials and structures, i.e., the formulation of hybrid materials comprising blends of organic and inorganic materials to combine the best characteristics of each of the components. In the field of homogeneous catalysis, a supramolecular fine chemistry has been recently established. It extends the principle of self-organization of the enzyme (catalyst) molecule to
nonbiological systems, using supramolecular compounds as catalysts for the shape selection of molecules. Such catalysts are formed in situ by self-organization, i.e., chemical bionics.12 Generally, in regard to the tailoring of materials with controlled structure, the previous approaches imply that chemical engineers should and will go down to the nanoscale to control events at the molecular level. At this level, new functions such as self-organization, regulation, replication, and communication have been observed and can be created by manipulating supramolecular building blocks. The latest advances in nanotechnology have generated materials and devices with new physical, chemical, and biochemical characteristics for a wide variety of applications. Moreover, it should be emphasized that, with their broad training in chemistry, physical chemistry, processing, systems engineering, and product design, chemical engineers and researchers are in a unique position and have a pivotal role in this technological revolution, with regard to the management of complex systems. 4.1.2. Increase Selectivity and Productivity by Supplying the Process with a Local “Informed” Flux of Energy or Materials. At a higher microscale level, detailed local temperature and composition control through staged feed and heat supply or removal would result in higher selectivity and productivity than the conventional approach, which imposes boundary conditions and lets a system operate under spontaneous reaction and transfer processes. Therefore, finding some means to convey energy at the site by supplying the process with a local “informed” flux of energy, where it may be utilized in an intelligent way, is a challenge. Such a focused energy input may be achieved using ultrasonic transducers, laser beams, or electrochemical probes; however, a more fundamental approach is still required to progress in this direction. Some type of feedback between the process and the energy source is needed to convey the exact amount of energy, at the precise location where it must be utilized to promote transfer or reaction. Driving the elementary processes within the unit is a challenge, but combining microelectronics and elementary processes, such as tuning the selectivity by controlling catalytic reactions at the surface of electronic chips, is a direction that should be explored more. 4.1.3. The Necessity To Increase Information Transfer in the Reverse Direction, from Process to Man. It is highly desirable to develop a variety of intelligent sensors, visualization techniques, image analysis, and on-line probes giving instantaneous and local information about the state of the process. This opens the way to a new “smart chemical and process engineering” and requires close computer control, relevant models, and arrays of local sensors and actuators. Fieldprogrammable analog arrays, coupled with microreactor technology, promise to change the way plants are built, as well as the methods by which their processes are designed and controlled. Rapid progress is noticeable in this area, although sensors for opaque materials and particulate solids in bulk systems are still scarce. 4.2. Process Intensification: Design of Novel Equipment Based on Scientific Principles, New Operating Modes, and New Methods of Production. Process intensification refers to complex technologies that replace large, expensive, energyintensive, and polluting equipment or processes with versions that are smaller, less costly, more efficient, less polluting, or combine multiple operations into a single apparatus or into fewer devices.
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Process intensification involves sustainable-related dimensions, reducing material usage, energy consumption, and waste generation, i.e., producing much more with much less, thus also enhancing the corporate image of chemistry, because costs reductions of 10%-30% are obtained by optimizing the process. The progress of basic research in chemical engineering has led to a better understanding of elementary phenomena and it is now possible to imagine and utilize new operating modes of equipment or design novel equipment based on scientific principles. 4.2.1. Process Intensification Using Multifunctional Reactors. Such is the case with the “multifunctional” equipment that couples or uncouples elementary processes (transfer-reactionseparation) to increase productivity and/or selectivity, with respect to the desired product, and to facilitate the separation of undesired byproducts. Recently, extractive reaction processes involving single units that combine reaction and separation operations have received considerable attention, because they offer major advantages over conventional processes, because of the interaction of reaction, mass, and energy transfer. Thermodynamic limitations, such as those of azeotropes, may be overcome and the yield of reactions increased. The reduction in the number of equipment units leads to reduced investment costs and significant energy recovery or savings. Furthermore, improved product selectivity leads to a reduction in raw material consumption and hence, operating costs. Globally, process intensification through the use of multifunctional reactors permits significant reductions in both investment and plant operating costs. Cost reductions up to 30% could be obtained by optimizing the process. In an era of limited profit margins, it allows chemical producers more leverage in regard to competition in the global marketplace. Moreover, in an increasingly commoditized market, this may be the determining factor for success. 4.2.1.1. Reactive Separation Processes Involving Unit Operation Hybridation. A great number of reactive separation processes that involve unit operation hybridation exist. The concept of reactive or catalytic distillation has been successfully commercialized, both in petroleum processing, where packed-bed catalytic distillation columns are used, and in the manufacture of chemicals where reactive distillation is often used.13-15 Catalytic distillation combines reaction and distillation in one vessel, using structured catalysts as the enabling element.16 The combination results in a constantpressure boiling system, ensuring precise temperature control in the catalyst zone. The heat of reaction directly vaporizes the reaction products for efficient energy utilization. By distilling the products from the reactants in the reactor, catalytic distillation breaks the reaction equilibrium barrier; it eliminates the need for additional fractionation and reaction stages, while increasing conversion and improving product quality. Both investment and operating costs (lower energy usage) are much lower than with conventional reaction, followed by distillation. The use of reactive distillation in the production of fuel ethers (such as tertiary amyl methylether (TAME), methyl tertiary butyl ether (MTBE), or methyl acetate) clearly demonstrates some of the benefits. Similar advantages have been realized for the production of high-purity isobutene; for aromatic alkylations; for the reduction of benzene in gasoline and in reformate fractions; for the production of isopropyl alcohol via the hydration of propylene; for the selective production of ethylene glycol, which involves a great number of competitive reactions; and for the selective desulfurization of fluid catalytic cracker gasoline fractions; as well as for various selective hydrogena-
tions. Extraction distillation is also used for the production of anhydrous ethanol, or for the production of isopropyl alcohol via the hydration of propylene. The next generation of commercial processes using catalytic distillation technology will be in the manufacture of oxygenates and fuel additives, or in the synthesis of a range of fatty acid esters used in the manufacturing of cosmetics, detergents and surfactants, or in the recovering of lactic acid from fermentation broth. A comprehensive and general overview of reactive distillation, status, and future direction is presented in Sundmacher and Kienle.17 Liquid maldistribution (large-scale and small-scale maldistribution) in packed-columns distillation column is probably one of the most described effects that influence the efficiency of the column. A general trend is the emergence of multifunctional packings and their application in combined systems such as catalytic distillation,18 or multiphase catalytic processes.19 In such a trend, an experimental and modeling study of the hydrodynamic and mass-transfer performances of modular catalytic structured packings has been presented recently.20 It is interesting to note that the proposed model proved to be able to account correctly for geometrical variations and is very capable of predicting the performance of modular catalytic structured packings. Monolith is an example of a structured catalyst or reactor; the borders vanish for this catalyst application, either as a catalyst or as a functional internal reactor. The use of these structured reactors allows decoupling of the chemistry, transport phenomena, and hydrodynamics, and the like to tailor the reactor/catalyst independently to satisfy optimal operation conditions. Monoliths can be used both for co-current and counter-current operation in gas-liquid reactions. They can combine the advantages of the slurry and trickle-bed reactor and eliminate the disadvantages such as discontinuous operation, stirring energy input, and catalyst attrition or ineffective catalyst use, liquid maldistribution, and local hotspots that may develop and cause runaways. They can be contacted in virtually any desired way, opening up new processing routes.21,22 An ingenious design is the integration of mixing and catalyst in the same monolith stirrer reactor, demonstrating that it is possible to immobilize enzymes on the monolithic structure. The monolith stirrer reactor features highly effective internal and external mass-transfer results, whereas the entire immobilized enzyme is used effectively.23 In the laboratory, monolithic structures provide a tool for scaling down catalyst testing units, with a wink to microreactor technology and combinatorial catalysis. It is expected that monoliths will be increasingly applied to chemical and biochemical conversion processes, from bulk and fine chemical production processes and in cleanup processes. Indeed, the motivation of the use of monoliths (or capillary channels in other configurations), and the proof of concept, have been established with great certainty, often leading to very accurate predictions of the performance.24,25 Moreover, it will be shown later that one of the major objectives of the research on novel (micro)structured reactor beds and catalytic support is process intensification, or herein, among others, a marked reduction of the size of the current reactor units is pursued. This involves monolith reactors, beadstring reactors, ceramic foam packings, composite structured packing reactors, three levels of porosity reactors, etc. A global optimization analysis of a general class of perforated monolithic bed reactors has been presented for the case of an isothermal first-order reaction and for laminar flow conditions.26 The
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Figure 3. Investigations on advanced membrane technology, for both material design (hardware) and processing methods (software).
resulting design rules indicate how a given amount of catalyst material should best be perforated or distributed in space, as a function of the available inlet pressure. It is shown that, in application, the more-advanced reactor designs involving beds with ultrasmall flow-through pores, proposed in the past years, the productivity gain of these advanced concepts can be as large as a factor of up to 100. An alternative reaction-separation unit is the chromatographic reactor. It uses differences in the adsorptivity of the different components involved, rather than differences in their volatility. It is especially interesting as an alternative to reactive distillation when the species involved exhibit small volatility differences, are nonvolatile, or are sensitive to temperature, as in the case of small fine chemical or pharmaceutical applications. There are several classes of reactions to which reactive chromatography is applied. The widest class of reactions is given by esterification reactions catalyzed by acidic ion-exchange resins or by immobilized enzymes, as the polarity difference between the two products (ester and water) makes their separation easy on many different adsorbents. Other applications include transesterifications, alkylation, etherification, (de)hydrogenations, and reactions involving sugars. Reactive chromatography has also been used for methane oxidation. In all these applications, special care must be devoted to the choice of the solid phase for sorption selectivity, sorption capacity, and catalytic activity. Typical adsorbents used include activated carbon, zeolites, alumina, ion-exchange resins, clays, and immobilized enzymes. In regard to the coupling of reaction and crystallization, myriads of basic chemicals, pharmaceuticals, agricultural products, and ceramic powders and pigments produced by reactive crystallization-based processes exist: these are processes that combine crystallization with extraction to solution mine-salts. These separation processes are synthesized by bypassing the thermodynamic barriers imposed on the system by the chemical reactions and the solubility of the components in the mixture. By combining crystallizers with other unit operations, the stream compositions can be driven to regions within the composition space where selective crystallization can occur. It is possible to selectively crystallize desired solid products after a reaction step and to use compound formation to affect
the separation of a mixture and a systematic method to synthesize flow sheets to separate binary mixtures by crystallizer-extractor hybrids has been presented where the use of decanters, counter-current extraction, and fractional countercurrent extraction is discussed for several phase behaviors, including complex systems with multiple reactions.27a The complementary nature of crystallization and distillation is also explored. Hybrids provide a route to bypass thermodynamic barriers in the composition space that neither the distillation, which is blocked by azeotropes and hindered by tangent-pinches in the vapor-liquid composition space, nor the selective crystallization, which is prevented by eutectics and hampered by solid solutions and temperature-insensitive solubility surfaces, can overcome when used separately.27b Extractive and adductive crystallization are solvent-based techniques that require distillation columns. They are applied to high-melting, close-boiling systems. Extractive crystallization uses a solvent to change the relative solubility of the solutes to affect separations. The distillation column is used to create solvent swings and recycle the solvent. Commercial examples include solvent dewaxing, solvent de-oiling, and the separation of sterols. Another advantage of such crystallization-distillation hybrid separation processes is that they do not require the addition of solvents, which may increase the process flows, create waste streams, propagate throughout a chemical plant, and require costly separation and recycle equipment. 4.2.1.2. Membrane Technologies Respond Efficiently to the Requirement of So-Called Process Intensification. They allow improvements in manufacturing and processing, such as substantial decreases in the ratio of equipment size to production capacity, as well as the energy consumption and/or waste production, in addition to easy control and scaleup, all of which result in less-expensive, sustainable technical solutions. The papers by Drioli and co-workers28-30 have documented the stateof-the-art, include the progress made in membrane technology, in regard to both hardware (materials design) and software (processing methods), as schematically shown in Figure 3, and present perspectives on integrated membrane operations for sustainable industrial growth. The first studies on membrane reactors used membranes to distribute the feed of one of the reactants to a packed bed of
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catalysts, i.e., to improve the selectivity in partial oxidation reactions. Other methods such as the immobilization of biocatalysts on polymeric membranes have attempted selective removal of the product from the reaction site to increase the conversion of product-inhibited or thermodynamically unfavorable reactions. With such membrane bioreactors, provided that membranes with a suitable molecular-weight cutoff are used, chemical reaction and physical separation of biocatalysts (and/ or substrates) from the products can happen in the same unit. Substrate partitioning at the membrane/fluid interface can be used to improve the selectivity of the catalytic reaction toward the derived products with minimal side reactions. Bioreactors based on the hollow-fiber design are used to produce monoclonal antibodies for diagnostic tests, to mimic biological processes, or to produce pure enantiomers, when a membrane separation is combined with an enianto-specific reaction. In regard to general membrane reactors, the result is a more-compact system with higher conversion. This technology is adapted to the increasing demand for food additives, feeds, flavors, fragrances, pharmaceuticals, and agrochemicals. Phase-transfer catalysis can also be performed in membrane reactor configurations by immobilizing the appropriate catalysts in the microporous structure of the hydrophobic membrane. Catalytic membrane reactors are also proposed for selective product removal to remove equilibrium limitations, i.e., catalytic perm-selective or non-perm-selective membrane reactors, packedbed (catalytic) perm-selective membrane reactors, fluidized bed (catalytic) perm-selective membrane reactors. Therefore, the reaction step can be improved through the integration of a separation step, such as, for example, that in membrane reactors for dehydrogenation reactions where hydrogen is withdrawn from the reaction mixture using perm-selective palladium membranes, thereby shifting the reaction equilibrium to the desired products. Alternatively, the separation step also can be improved, because of the integration of a reaction step, such as, for instance, that in membrane reactors for the catalytic partial oxidation of methane using perm-selective dense perovskite membranes for the air separation. Here, the high consumption rate of oxygen at the permeate side helps to increase the chemical potential difference of oxygen across the membrane and, hence, the oxygen fluxes.31 Moreover, current perovskite membranes potentially open a completely new way of performing high-temperature operations with oxygen demands.32,33 Problems such as thermodynamic instability of the membrane under reducing operating conditions and safety can be handled by appropriate reactor or process design. Therefore, a novel fluidized-bed membrane reactor has been proposed for hydrogen production by steam reforming of methane, where both perovskite oxygen perm-selective membranes and palladium-based hydrogen perm-selective membranes are integrated in two sections: a partial oxidation bottom section and a reforming/ shift top section, because of a difference in operating temperature of the respective membranes. The experimental results have clearly shown the improvements in methane conversion, completion of the water shift (lower CO selectivity), and hydrogen production field.34 More generally, the development of such membrane reactors for high-temperature applications only became realistic in the past few years, with the development of high-temperatureresistant membranes (i.e., palladium membranes) mainly for dehydrogenation reactions, where the role of the membrane is simply hydrogen removal.
In addition, a new field of chemical and process engineering is now wide open with the coupling of supercritical fluids and membrane concepts to the design of very attractive and powerful processes, to improve the transfer, reaction, and handling of highly viscous liquids. This is the case, for example, for enzymatic membrane reactors where the use of supercritical fluids (with low viscosity and high diffusivity, which result in faster reaction kinetics) improves the enzyme action, with respect to liquid conditions, several times and, in addition, the enzyme membrane enhances its activity many-fold, using a crossflow configuration in the reactor operation.35 The recent design strategy, formulated with the intent to provide concrete benefits in manufacturing and processing, also should be emphasized; this strategy involves the use of nanofiltration membranes to separate products of ionic-liquid-mediated reactions from the ionic liquid used to enhance the reaction.36 The major interest of all the processes created thus far is to safeguard the environment and the products. This is particularly essential when the processes are of a biological nature.37-39 For more general applications, material scientists must solve the problem of providing inorganic membranes of perfect integrity, that have mechanical and thermal stability, and that will allow large fluxes of desired species. Next, chemical engineers must resolve the heat-transfer problem that now threatens successful scaleup. Therefore, it might seem reasonable to expect membrane reactors that combine oxygen transfer membranes with selective catalytic layers for the partial oxidation of hydrocarbons. However, a continuous research effort in the dynamics of these processes and in the study of advanced control systems applied to integrated multimembrane operations is still necessary. And, finally, in regard to the use of membranes for process intensification, the special issue of Chemical Engineering Research and Design on the process developments in membrane contactors40 should be recommended. It highlights the growing importance of membrane processes, and the recent developments with integrated and hybrid membrane operations. Important challenges in the topic are presented which offer interesting prospects for process industries and in new nontraditional areas, such as regenerative medicine or interfacing electronics. A deeper material analysis and characterization of the structure and properties, as well as modeling in design at the molecular and nanoscale levels, become essential for high control of the process performance and an advanced knowledge of membrane functions for process intensification. This supports the recent creation of the European Network of Excellence “NanoMemPro”, whose title, “Expanding Membrane Macroscale Applications by Exploring Nanoscale Material Properties” (http://www.nanomempro.com), is quite comprehensive. 4.2.1.3. Multifunctional Reactors: Classification and Limitations. Multifunctional reactors are not new to the chemical and process industry; they have been used for absorption or extraction with chemical reaction for several decades. However, only recently have reactors and equipment that incorporates several functions into one unit been formally classified as being multifunctional. The great benefits obtained in integrating the progress of knowledge at different time and length scales have been acknowledged by the process industries. This was illustrated by the first international symposium on multifunctional reactors41 with a presentation of research and development in the main domains of reaction and heat exchange, reaction and membrane separation, reaction and sorption, reaction and power generation, reactions and distillation, reaction and catalyst regeneration, and the use of nontraditional structured
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packing. This was also illustrated by the second symposium on multifunctional reactors,42 which shows that, although two areassreactive distillation and membrane reactorssstill dominate the subject, several others, such as chromatographic reactors, are catching up rapidly and exotic newcomers (for example, those involving the electrochemical processes in fuel cells) are emerging. Moreover, this was recently illustrated by the Third International Symposium on Multifunctional Reactors,43 with contributions on reactive distillation, adsorptive reactors, membrane reactors, auto thermal reaction systems, and structured reactor configurations. And, finally, it was created in 2005 on that topic a Working Party of the European Federation of Chemical Engineering regrouping a great number of academic and industrial partners that are more and more concerned with intensified processes and equipment. It seems clear that, to achieve optimal performances with multifunctional reactors, it is important to lead a scientific approach and classification that shows where and at which scale the integration of functionalities occurs, i.e., in the case of catalytic reactors, either at the catalyst level (multifunctionality combining catalytic properties and engineered catalyst microstructure), at the reaction interface (multifunctionality chemical reaction and enhanced inter phase transport), at the intra-reactor level (multifunctionality chemical reaction and intra reactor unit processes such separation or heat transfer), or at the inter-reactor level (multifunctionality with recirculation of solids). However, it should be emphasized that the use of hybrid technologies that are encountered in a great number of multifunctional reactors is limited by the resulting problems with control and simulation. For example, the interaction between simultaneous reaction and distillation introduces more-complex behavior, involving the existence of multiple steady states and output multiplicities corresponding to different conversions and product selectivity than those achieved in conventional reactorsand ordinary distillation columns. This leads to interesting challenging problems in dynamic modeling, design, operation, and strong nonlinear control. Indeed, the response of a reactive separator with marginal changes in design parameters, such as feed position, feed flow, number of stages, height, type of packing or plates, may be drastic and unforeseen, and, consequently, the simulation of this hybrid equipment should be based on reliable models with high accuracy. Their control requires sophisticated model predictive control, robust control, and adaptative control, where mathematical predictive control may be required to run 50-500 times faster than real time. There is also an increasing awareness that the full potential of multifunctional reactors may only be realized if the reaction and the unit operation integrated are properly harmonized. Too much integration can even exert a negative influence, requiring detailed modeling of the underlying processes and careful selection of the chemical and physical system properties and operation conditions. It is even suggested that many of the potential benefits can be achieved by a partial approach to multifunctionality and that further integration has a low increment value.44 4.2.2. Process Intensification Using New Operating Modes and/or Application of External Driving Forces. The intensification of processes is obtained by new modes of production that are also based on scientific principles. New operating modes have been studied in the laboratory and/or pilot stage: reversed flow for reaction-regeneration, unsteady operations and cyclic processes, extreme conditions, pultrusion, low-frequency vibrations to improve gas-liquid contacting in bubble columns, hightemperature and high-pressure technologies, and supercritical
media, as previously stated, are now seriously considered for practical applications. Reactors can be operated advantageously with moving thermal fronts that are created by periodic flow reversal. Low-level contaminants or waste products such as volatile organic compounds (VOCs) can be efficiently removed in adiabatic fixed beds with periodic reversal by taking advantage of higher outlet temperatures generated in earlier cycles to accelerate exothermic reactions. Energy and cost savings are affected by this substitution of internal heat transfer for external exchange. Some attractive options for improved catalytic reactor performance via novel modes of operation include the following: periodic (symmetric) operation of packed beds with exothermic reaction, coupling of an exothermic and endothermic reaction in a periodically operated (asymmetric) packed bed, and induced pulsing liquid flow in trickle beds to improve liquid-solid contacting at low liquid mass velocities in the co-current downflow mode.45,46 Also, such periodic operation, with respect to liquid flow, may help in getting process intensification for gas-limiting reactions or for petroleum applications where filtration and bed plugging are serious threats.47 Moreover, nonconventional research on magnetic-driven process intensification is worth noting for mini-trickle-bed reactors, especially in nonpetroleum applications, such as those in fine and pharmaceutical chemical processes. It has been shown that a positive gradient in homogeneous magnetic fields promotes larger values of liquid holdup (and, thus, wetting efficiency in the trickle-flow regime) and two-phase pressure drop.48 Also, when high conversions are required and the gaseous byproduct of the reaction is known to inhibit the rate, as in hydrodesulfurization, or in selective hydrogenation, counter-current flow operation of traditional trickle beds is now preferred.49 Also, improving the product selectivities in a parallel-series reaction by feeding one reactant through the reactor via stagewise reactant dosing will be applied.50 Note that the use of ultrasonic and microwave technologies can enhance the rates and improve the selectivities of catalytic reactions.51 In the gas-to-liquid process to convert natural gas to syngas via partial oxidation, note the recent concept of the reverse-flow catalytic membrane reactor, which uses a perovskite membrane to integrate the recuperative heat exchange inside the membrane reactor, as well as to reduce the costs associated with cryogenic air separation for partial oxidation.52 More generally, it is strongly suggested that, in the coming years, more investigation should be done with regard to alternative sources and forms of energy that can be utilized to achieve drastic improvements in the efficiency of chemical and related processes as potential means for process intensification (the energy of the electric field, electromagnetic radiations (light and microwaves), and high gravity fields, and the energy of flow cavitation and supersonic shockwaves).70 4.2.3. Process Intensification Using Microengineering and Microtechnology. Currently, production modes are increasingly challenged by decentralization, modularization, and miniaturization. Microtechnologies that have been developed, especially in Germany (i.e., IMM, Mainz, and FZK-Karlsruhe) and in the United States (i.e., at the Massachusetts Institute of Technology (MIT) and DuPont), lead to microreactors, micromixers, microseparators, micro-heat exchangers and microanalyzers, making accurate control of reaction conditions possible, with respect to mixing, quenching, and the temperature profile. Microfabrication techniques and scaleup by replication have shown spectacular advances in the electronics industry, and, today, also in microanalysis by biological and chemical applications. Microfabricated chemical systems are now expected to have
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Figure 4. Schematic of the IMM micromixer for high-throughput screening (HTS). (Reprinted with permission from ref 57; copyright Wiley-VCH, 2000.)
many advantages for chemical kinetic studies, chemical synthesis, and, more generally, process development. Indeed, the reduction in size and integration of multiple functions has the potential to produce structures with capabilities that exceed those of the conventional macroscopic systems and to add new functionality, while potentially making mass production at low cost possible. Miniaturization of chemical analytic devices in micro-totalanalysis system (µTAS) represents a natural extension of microfabrication technology to biology and chemistry, with clear applications in combinatorial chemistry, high-throughput screening, and portable analytical measurement devices. Also, the merging of µTAS techniques with microreaction technology promises to yield a wide range of novel devices for reaction kinetic and micromechanism studies, as well as on-line monitoring of production systems. Microreaction technology is expected to have several advantages for chemical production.53-55 The high heat- and masstransfer rates that are possible in microfluidic systems allow reactions to be performed under more aggressive conditions with higher yields than conventional reactors. Also, new reaction pathways considered too difficult for application in conventional macroscopic equipment, such as the direct fluorination of aromatic compounds, could be pursued because, if the microreactor fails, the small amount of chemicals that may be released accidentally could be easily contained. The presence of integrated sensor and control units could allow the failed microreactor to be isolated and replaced while other parallel units continued production. In addition, these inherent safety characteristics could allow a production scale system of multiple microreactors, enabling a distributed point-of-use synthesis of chemicals with storage and shipping limitations, such as highly reactive and toxic intermediates such as cyanides, peroxides, and azides. Microchemical systems for combinatory synthesis and screening of small molecules and systems for nucleic acid synthesis and detection have already revolutionized drug discoveries in pharmaceutical companies. Similarly, rapid screening of small molecules and systems for nucleic acid synthesis pathways can lead to analogous productivity increases in chemical industry
laboratories. Experimentation at the conventional bench-scale level is limited by the high costs of reagents and safety concerns, which the small volumes and inherent safety characteristics of the microreactors can effectively eliminate. Moreover, scaleup to production by replication of microreactors units used in the laboratory would eliminate costly redesign and pilot-plant experiments, thereby shortening the development time from the laboratory to commercial-scale production. This approach is particularly advantageous for pharmaceutical and fine chemical industries where production amounts are often less than a few metric tons per year. It will be shown later that others more recently have begun to apply microchannel technology to largerscale applications, such as methane steam reforming and the production of propylene oxide and hydrogen peroxide (via industrial partnerships with, respectively, Degussa-IMM, accentus-FMC, and UOP-IMM), as referenced in the work of Tonkovich et al.56 Small devices are already used for process chemistry testing, such as catalyst testing. Chemical detection is the rate-limiting step in most techniques, because detailed product information must be obtained using sequential screening. However, with the continual advances in µTAS and microfabrication techniques, these macroscopic test systems can be replaced by PC-cardsized microchemical systems that consist of integrated microfluidic, sensor, control, and reaction components that require less space and utilities and produce less waste. Moreover, the small dimensions imply laminar flow, making it feasible to fully characterize heat and mass transfer and extract chemical kinetic parameters from the sensor data. As an illustration, a new concept was proposed for highthroughput screening (HTS) experiments for rapid catalyst screening, based on dynamic sequential operations with a combination of pulse injections and micromachined elements.57 The principle used for the test microreactor is a combination of pulse injections of the catalyst and the substrate, a static IMM interdigital micromixer with negligible volume and a residence time of
