The Role of Process Integration in Process Synthesis

Transcription

1 The Role of Process Integration in Process Synthesis Jeffrey J. Siirola Purdue University, West Lafayette, Indiana Carnegie Mellon University, Pittsburgh, Pennsylvania "Process integration" and "process synthesis" have had somewhat different interpretations at different times and in different places especially Europe and the US. These definitions have ranged from being nearly synonymous as a holistic systems approach to process design (for example the works of Smith and El-Halwagi), to particular efficiency-improving resource conservation aspects of process design (as exemplified by the family of Pinch Technologies for the reuse of heat, power, solvents, water, hydrogen, etc. pioneering by Linnhoff, Manousiouthakis, and others), to integration being various subcomponents of synthesis (as in the works derived from the approaches of Rudd or Douglas). This presentation is a discussion of synthesis and integration from the point of view of this last tradition. In that view, process design is one of the intermediate steps in the industrial chemical innovation sequence which follows need identification and product design and precedes equipment acquisition, construction, commissioning, and operation. Process design is sometimes broadly defined to include chemistry selection and refinement, flowsheet development, equipment design, and plant design, but also more narrowly as just the flowsheet development step. Either way, the issues found at each and every step of the innovation process are addressed, often multiple times and at different levels of detail using engineering design problem-solving approaches. A wide variety of such problem-solving approaches have been described, but most can be restated as a multistep procedures consisting of goal formulation followed by an iterative sequence of the generation, analysis, evaluation, and optimization of alternatives, concluding with the selection of the "best" alternative found. In the chemical process design context, process synthesis is that step within the flowsheet design sequence which involves the generation of alternatives (closely coupled with the accompanying analysis, evaluation, and optimization). A number of approaches to generating flowsheet alternatives (process synthesis) have been proposed over the decades. These include modification of existing flowsheets recorded in the literature to meet the present problem, mathematical economic optimization of a superstructure of redundant processing options resulting in a flowsheet after elimination of unused equipment and interconnections, and finally procedures to build the flowsheet from scratch given only the goals of the process, chemical engineering and related principles, and physical property, economic, business, and other relevant data (sometimes assumed). The goal of process synthesis then is the identification of the tasks that need to be accomplished, the chemical and physical phenomena to be exploited, the conditions under which these phenomena should best be exploited, and the details of and the interconnections among equipment designed to exploit phenomena and accomplish these tasks (all of this represented diagrammatically as a flowsheet) to convert available raw materials into the desired product.

2 In approaching process synthesis, especially in this last systematic manner, a number of different procedures have been suggested, but almost all some form of hierarchical decomposition to focus attention on different aspects of the process in an orderly manner. The typical hierarchy consists of establishing the chemical reaction sequence that leads from available raw materials to the desired product, establishing the preferred conditions (feed purity, temperatures, pressures, phases, catalysts, solvents, diluents, etc.) to carry each chemical reaction, determining the basic flow of the chemical species among the various reactions, determining separations and purifications that may be required to meet reaction feed purity and product fitness for use requirements, and doing whatever is required so that physical conditions required by the reactors and by the separators are met. The first two steps (reaction sequence and preferred reaction conditions) may have been performed by specialists (chemists) prior to flowsheet synthesis. Even so, one of the first tasks of the process designer (possibly in conjunction with the process chemist) is to determine if the desired reaction conditions are such that each reaction in the reaction sequence must be implemented in its own reactor equipment, or whether multiple consecutive reactions may be implemented in the same reactor and if so the resulting consequences and constraints. If feasible, the collocation of multiple reactions in the same reactor is the first instance of "process integration" in this approach to process flowsheet synthesis for the purpose of process simplification (reduced reactor count). The next step in the flowsheet design sequence is the establishment of the basic flow patterns among the sources of raw materials, the various reactors, and the products (and wastes). Part of this pattern, or "species allocation" is trivial driven by the source and destination of specific chemical species. Part is driven by the chemical consequences of contaminants in source streams to reactors which may result in additional reactions. Such consequences may be estimated through iterative consultations with the process chemist and redefine purity requirements for reactor feed streams. Finally, in situations of incomplete reaction conversion for kinetics, equilibria, or stoichiometry reasons, the fate of unreacted reactants appearing in reactor effluents must be decided. If chosen, the recovery and recycle such unreacted reactants is the second instance of process integration, this time for the purpose of improving raw material consumption efficiency. The next step in the flowsheet design sequence involves the identification of tasks to implement both the reaction path and the species allocation previously determined. This step too is hierarchical and tends to concentrate in order on separations (to achieve composition goals), phase (to best meet the required conditions for products and reactor feeds and separator inputs), temperature and pressure (again to best meet the required conditions for phase, products, and reactor feeds and separator inputs), and sometimes other properties specific to certain phases such as particle size and size distribution, shape, dispersion, etc. A variety of technologies exist for each of these tasks (reaction, separation, phase change, temperature change, pressure change, etc.) the understanding and exploitation of which is the domain of chemical engineering. These technologies are based on exploiting physical and chemical phenomena by establishing the conditions and the geometry such that these phenomena will inevitably accomplish the desired task at the necessary rate to meet process objectives. The relevant phenomena are quite varied and include many variants of chemical reaction, interphase mass transfer, heat transfer, fluid dynamics, mechanical operations, etc. In most cases the phenomena are driven by

3 thermodynamics (a gradient or force field) sometimes set up using an available "utility" (combustion fuel, hot oil, steam, cooling water, refrigeration, compressed air, electricity, etc.). Both the equipment in which the processing tasks take place and the utilities used to provide the driving forces cost money. A process design has many objectives including desired production quantity, product fitness for use, health and safety of producers, consumers, and neighbors, environmental impact minimization, social responsibility, etc. Foremost among these must also include economics because absent this, resources cannot be allocated for construction and operation of the plant. There are many factors which impact process economics, but among the most significant typically are the capital cost of the production facility and the continuing operating costs for raw materials, utilities, labor, etc. This is why process synthesis (and its related analysis, evaluation, and optimization) are so focused on economics. The design activities associated with the process synthesis steps of task identification and equipment design that that can most improve economics turn out again to take advantage of various activities of "process integration". We will briefly mention six. First is integration of complimentary tasks. Given properly designed equipment, most process tasks can be accomplished independently driven by application of an external utility. Enthalpy change (temperature change, phase change, supply/removal of heat of reaction) is the archetypical example. Every task of that type may be driven by its own utility selected given physical and economic constraints. However, if certain conditions are met, some tasks requiring heat may be driven directly by other tasks requiring rejection of heat (heat exchange networks). Systematic approaches to discovering better heat exchanger networks was one of the first process synthesis subproblem studied dating to the 1960's. The driver is usually savings in operating (utility) costs rather than equipment (heat exchanger) capital costs which sometimes actually increase compared with unintegrated designs. There are many factors which complicate heat exchanger network synthesis (safety concerns over leaks, unique materials of construction and heat transfer coefficients, exponential equipment cost correlations, process control issues, etc.), but the general concept is well accepted and most process designs employ some form of heat integration. One of the key analytical insights was concept of the "pinch" which places constraints on which exothermic tasks and which endothermic tasks should not be "matched" together even if feasible, if thermodynamic maximal utility reduction is to be achieved. Similar task integrations directly related to utility minimization have also been developed for power recovery (especially related to compression and expansion of gasses), water reuse (fairly common in countercurrent solids wash/filtration circuits), dirty solvent reuse (mass separation networks, however, much less common in actual practice), and similar situations. Second is the adjustment of task conditions at one level of the design hierarchy (say the specification of separation tasks) to increase the opportunity for integration of complimentary tasks of the type discussed above. An obvious example is distillation sequences that may be designed if the species allocation is such that different components in some stream are to be sent separately to different destinations. There are now numerous heuristic and algorithmic procedures for selecting the order of separations and for optimizing the performance of equipment to implement such a sequence of separation tasks. But each separation task is generally driven by some utility. Relatively straightforward optimization can be applied to minimize the overall separations (and associated heat transfer) equipment cost and utility

4 operating costs. The various associated enthalpy-changing costs can also be heat-integrated, and as these complementary tasks tend to involve sensible heat, the resulting integration savings are important, but relatively small (10-15% net present cost). However, if the separation sequence optimization and the associated enthalpy-changing task integrations are done simultaneously, then often very different operating conditions are found with separation units operating at much higher and lower temperatures, but also so that latent heat is recovered. The resulting economic savings tend to be relatively larger (35-40% net present cost), although constraints arise as tasks are performed under more extreme conditions. A third type of integration is found when more than one phenomenon is brought to bear either sequentially or simultaneously to accomplish a task. Examples of sequential implementation include such familiar strategies as filtration followed by drying and sequenced reactors of different types (CSTR, PFR) or geometries or operating conditions to improve conversion or selectivity. Examples of parallel phenomena implementation include the addition of mass separation agents to favorably impact activity coefficients (the entrainer in azeotropic distillation or the solvent in extractive distillation) and the combining of separation with reaction (to overcome equilibrium limitations, remove catalyst poisons, avoid competitive sequential reactions, make azeotropes disappear because of the interaction with reaction equilibria, or react away components so that they do not need to be separated). For many tasks, performance is enhanced or achieved not by a utility but by the action of a material (solvent, diluent, catalyst, or mass separation agent (solvent in absorption, extraction, and extractive distillation, entrainer in azeotropic distillation, sorbent in adsorption, etc.)). A fourth type of integration is exploited when a material or species already in the process such as a feed, an intermediate, or a product is used for such a purpose rather than adding new species. Discovering such opportunities is a matter of search before external compositions are considered, sometimes resulting in overall flowsheet simplification. After all process tasks have been identified, they can usually be implemented by designing a piece of equipment to perform each separately. Depending on the operating conditions, and the various internal and external flow patterns, and consequences of loss of degrees of freedom, it is sometimes possible to employ a fifth type of integration, the combination of multiple tasks into fewer numbers of equipment. Some examples are fairly common (reactive distillation, the "hyphenated" separation-identification processes of analytical chemistry (GC-MS, etc.)), while others are more or less accidental. However when they are found, the economic savings can be substantial (80% net present cost in the Eastman methyl acetate example). Finally, sometimes task accomplishment involves simultaneous integrated phenomena such as reaction, mass transfer, heat transfer, and mixing and fluid flow. Each of those phenomena may involve gradient fields (concentration, temperature, pressure, etc.), volumes or areas over which these forces act, and phenomenological details wrapped up in coefficients (kinetics constants, heat transfer coefficients, mixing parameters, friction factors, etc.), and each may scale differently with increasing production rate. Sometimes in order to optimize the balance among the various factors for a desired production rate, some unusual design elements, geometries, conditions, or parallelism may be incorporated to change in a manner different from common

5 practice a particular gradient, area, surface-to-volume ratio, etc. Examples include HiGee distillation (to reduce the height equivalent of a theoretical plate), exothermic microreactors (for greater heat transfer area to reaction volume), centrifugal molecular distillation (volatilization with very short thermal contact). These techniques are sometimes called process intensification (as is the previous integration strategy of task combination). The motive, however, is almost always a better balance of integrated phenomenological rates that do not scale together conveniently in conventional equipment geometries, although sometimes the equipment is of physically smaller size (if not higher capital cost). In the chemical process design view presented here, process synthesis is the generation of flowsheet alternatives. However, to generate flowsheet alternatives which will prove to be economically superior, the process synthesis will incorporate a number of elements of process integration including the selective integration of reaction steps, component recycle within the flowsheet, integration of complementary enthalpy changing (and solvent and mass separating agent utilization) tasks to minimize utilities, coordination of task operating conditions to optimize the opportunity for such complementary task integration, integration of multiple phenomena when necessary to accomplish tasks, integration of multiple tasks into single equipment, and adjustment of design elements, geometries, and conditions to better balance multiple integrated phenomena rates if necessary for scale up. All of these process intensification techniques are currently in use in industrial chemical process synthesis.