Diseño del proceso
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Diseño del proceso

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diseño del proceso del libro de synthesis, analysis and evaluation de seider.

diseño del proceso del libro de synthesis, analysis and evaluation de seider.

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Diseño del proceso Diseño del proceso Document Transcript

  • 1.1 Primitive design problems The designs or retrofit of chemical processes begins with the desire the produce profitably chemicals that satisfy the societal needs that arise in the broad spectrum of industries that employ chemical engineers. These include the manufacture of petrochemicals, petroleum products, industrial gases, foods, pharmaceuticals, polymers, coatings, electronic materials, and other chemicals and biochemical, as show in table 1.1, which lists typical process designs, several of which are used to illustrate the design strategies presented in this book. Note that although many of the projects were studied by students of the senior author in recent years, the projects were provided by industrial consultants and are usually close approximations to those under by their companies. Note also that many of the process designs are directed to meet environmental regulations, which are becoming increasingly strict. Thanks in part to the growing awareness of the public, many of the design projects involve the redesign, or retrofitting, of existing chemical processes to solve environmental problems and the adhere to stricter standards of safety, as discussed in sections 1.3 y 1.4. Designs projects have many points of origin. Often they result from the explorations of chemists, biochemists, and engineers in research labs to satisfy the desires of customers for chemicals with improved properties for many applications (e.g., textiles, carpets, plastic tubing). In this respect, several well-known products, such as Teflon (polytetrafluoroethylene), were discovered by accident. (At du Pont, a polymer residue that had accumulated in a lab cylinder of tetrafluoroethylene was found to provide a slippery surface for cookware capable of withstanding elevated temperatures, among many similar applications.) In other cases, an inexpensive source of a raw material(s) becomes available and process engineers are called on to design processes that use this chemical, often with new reaction paths and methods of separations. Other designs problems originated when new markets are discovered, especially in developing countries of Southeast Asia and Africa. Yet another source of design projects are engineers themselves who often have a strong feeling that a new chemical or route to producing an existing chemical may be very profitable. Typical primitive design problem Consider, as a typical example, the need to manufacture vinyl chloride, A monomer intermediated for the production of polyvinyl chloride, An important plastic that is widely used for rigid plastic piping, fittings, and similar products. A typical primitive problem statement is as follows: “An opportunity has arisen to satisfy a new demand for vinyl chloride monomer, on the order of 800 million pounds per year, in a petrochemical complex on the Gulf Coast, given that existing plant owned by the company producces 1 billion pounds per years of this commodity chemical. Because vinyl chloride
  • monomer is an extremely toxic sbstance, it is recommended that all new facililities be designs carefully to satisfy governmental health and safety regulations.” Process design team To addres a problem statement, a small designs team may be created, often involving a chemical engineer, a chemist and possibly a biochemist, as appropriate. Other expertise is added to team to address specific areas. Thet team undertakes to asses the primitive problem statement and to follow a sequence of well-recognized steps in process design, as described in the next section. Before turning to these steps, however, consider an important source of problemas statements, at least for most designs students. Industrial consultants At many universities, design projects originate as courtesy of a person(s) from a local chemical company, often referred to as industrial consultant(s) because many chemical engineers, with the encouragement of their companies, spend considerable time answering questions and advising students as their designs proceed. Although chemical companies must protect their trade secrets, most are able to create projects that are remarkably close to the processes they design and operate. Examples of many primitive design problem statements prepared by industrial consultants for student design groups at the University of Pennsylvania are given in Appendix VIII. (Furthermore, a report titled Process Design Projects at Penn: 100 problem statements (Seider and Kivnick, 1994), provides problem statements for which student designs have been completed, many of which are available using an interlibrary loan) Each project presents a current situation followed by an opportunity that could lead to a profitable solution. In most cases, guidelines, constraints, and some specifications are provided to limit the scope of the investigations. Usually, key references to journal articles, patents, or reports give considerable information with which to begin working toward a solution. These problem statements are of course very primitive. As the design team tackles the project, the problems are gradually reformulated to reflect new information as it is collected. This process is quite iterative, with much new information generated as the iterations proceed. To become acquainted with the design process, the next section introduces many of the steps. 1.2 steps in designing and retrofitting chemical processes In examining the steps commonly practiced in designing a chemical process, it is important to keep in mind that design problems are open ended and many have many solutions that are attractive and near optimal. Furthermore, no two designers design a complex process following exactly the same steps. In fact, to capture the Know-how of experienced designer’s step be tracked so that others can learns to apply them when working on designs of similar processes.
  • Design in the most creative of engineering activities, with many opportunities to invent imaginative new processes. It is also the essence of engineering, differentiating an engineer from scientist. Whereas chemical engineers engaged in process development exercise creativity in formulating experiments and theories to uncover and explain the mechanisms of processing operations (often involving complex reaction kinetics with heat and mass transfer in various flow fields), process designers face additional challenges in creating complex flow sheets and selecting operating conditions to produce desired products with a high degree of selectivity, little recycle, and low utility costs. Process designs are rarely straightforward or routine; rather, they involve innovative approaches to integrated processes that are more profitable, as well as easily controlled, environmentally sound, and operationally safe. Turn now to Figure 1.1, in which the principal steps in designing and retrofitting chemical processes are shown. Many of these steps are discussed throughout the five parts of this book, as indicated in the figure. In the paragraphs that follow, emphasis is placed on the first two steps, followed by a brief introduction to the others. Assess primitive problem As stated previously, process design begins with a primitive design problem that expresses the current situation and provides an opportunity to satisfy a societal need. Normally, the primitive problem is examined by a small design team, which begins to assess its possibilities, refine the problem statement, and generate more specific problems. In so doing, a key question concerns the sources of raw materials. Are they available in house, are they readily available outside, or do they need to be manufactured? For the latter, a process to manufacture the raw materials will be designed if the process turns out to be attractive, or its design will be included in the process design for the chemical of interest. When assessing the primitive design problem, it is common to gather information on the scale of the process, usually based on a preliminary assessment of current production, projected market demand, and current and projected selling prices. Also, an approximate location for the plant must be selected (e.g., a location along the Mississippi river, as opposed to the gulf coast, the Baja peninsula, or Tokyo Bay). Plant locations are not restricted to single country. Finally, it must be emphasized that most primitive design problems have special circumstances associated with them. Trough meetings with technical management of an engineering group, business and technical leaders of the organization that may be purchasing the services of the design team, and others who may possess special knowledge of the processing area, the primitive problem is refined and alternative specific problems created. Consider the primitive design problem to manufacture vinyl chloride monomer, presented in the previous section. The design team generates several specific problems, or alternatives, the solutions of which might solve the primitive problem. This is typically one of the most creative aspects of processes design, often involving brainstorming sessions in which ideas are generated, with no idea rejected initially regardless of how of radical or unconventional it may be. Indeed, the success of the design effort.
  • To satisfy the need for an additional 800 MM lb/yr (MM in American engineering units is thousand-thousand or 1 million) of vinyl chloride, the following plausible alternatives might be generated. Alternative1. A competitor`s vinyl chloride plant, which produces 2 MMM lb/yr of vinyl chloride and is located about 100 miles away, might be expanded to produced the required amount, which would be shipped by truck or rail in tank car quantities. In this case, the design team projects the purchase price and designs storage facilities. This might be the simplest solution to provide the monomer required to expand the local polyvinyl chloride (PVC) plant. Alternative2. Purchase and ship, by pipeline from a nearby plant, chlorine from the electrolysis of NaCl solution, react the chloride with ethylene to produce the monomer and HCI as a by-product. Alternative3. Because the existing company petrochemical complex produces HCI as a by- product in many processes (e.g., in chloroform ad carbon tetrachloride manufacture) at a depressed price because large quantities are produced, HCI is normally available at a low price. Reactions of HCI with acetylene, or ethylene and oxygen, can be used to produce 1.2- dichloroethane, an intermediate that can be cracked to produce vinyl chloride. Alternative4. Design an electrolysis plant to produce chlorine. One possibility is to electrolyze the HCI available from within the petrochemical complex, to obtain H2 and Cl2. React chlorine, according to alternative 2. Elsewhere in the petrochemical complex, react hydrogen with nitrogen to form ammonia or hydrogen with CO to produce methanol. These are typical of the large number of alternatives that can be generated as a base on which to begin the engineering of process. At this stage, the alternatives often have not been studied carefully, and hence it is important to recognize that as the engineering work proceeds, some alternatives will be rejected and new alternatives may be generate. This is a crucial aspect of process engineering. On the one hand, it is important to generate promising alternatives. On the other hand, to meet the competition with a plant designed and built in a timely fashion, it is important not to generate too many alternatives that require extensive engineering efforts to be evaluated. Survey literature When generating alternative specific problems, design teams in industry have access to company files as well as to the open literature. These resources can provide helpful leads to specific problems, as well as thermo physical property and transport data, possible flow sheets, equipment descriptions, and process models. If the company has been manufacturing the principal chemicals, or related chemicals, information available to the design team provides an excellent starting point, enabling the team to consider variations to current practice very early in the design cycle. In spite of this, even when designing a next generation plant to expand the production of a chemical, or retrofitting a plant to eliminate bottlenecks and expand its production, the team may find many opportunities to improve the processing
  • technologies. Several years normally separate plant starts and retrofits, during which technological changes are often substantial. For this reason, it is important to make a thorough search of the literature to uncover the latest data, flow sheets, equipment, and models that may lead to a more profitable design. Several literature resources are widely used by process design teams, including the Stanford research institute (SRI) designs reports, encyclopedias, handbooks, indexes, and patents, many of which are available electronically, with an increasing number available on the world wide web (internet). SRI DESIGN REPORTS SRI, A consortium of several hundred chemical companies, publishes documentation of many chemical processes in considerable detail. Their reports provide a wealth of information, especially, for inexperienced designers, but unfortunately they are usually not available in university libraries because of high subscription costs. Most industrial consultants have access to these reports, however, and may be able to provide helpful information to student design teams, especially those that carry out some of the design work in company libraries. ENCICLOPEDIAS Three very comprehensive, multivolume encyclopedias contain a wealth of information concerning the manufacture of most chemicals. Collectively, these encyclopedias describe the uses for the chemicals, the history of manufacture, typical process flow sheets and operating conditions, and related information. The three encyclopedias are the Kirk-Othmer encyclopedia of chemical technology (1991), the encyclopedia of chemical processing and design (Mcketta and Cunningham, 1976), and Ullman’s Encyclopedia of industrial chemistry (1988), for a specific chemical or substance, it is not uncommon for one or more of these encyclopedias to provide 5 to 10 pages of pertinent information. Although the encyclopedias are update too infrequently to provide the very latest technology, the information they contain id normally very helpful to a design team when beginning to asses a primitive design problem. Many other encyclopedias may also be helpful, including the McGraw-hill Encyclopedia of science and technology (1987), Van Nostrand’s Scientific Encyclopedia (considine, 1988), the encyclopedia of fluid mechanics (cheremisinoff, 1986), and the encyclopedia of materials science and engineering (bever, 1986). Patents Patents are important sources of which the design team must be aware, to avoid the duplication of designs protected by patents. After the 17 years that protect patented processes in the United States are over, patents are often helpful in the design of next generation processes to produce the principal chemicals, or chemicals that have similar properties, chemical reactions, and so on. Patents from the United States, Great Britain, Germany, Japan, and other countries are available in major libraries, from which copies can be ordered, and increasingly, copies can be obtained by fax transmission. Recently, patents have begun to be added to the World Wide Web.
  • Process creation After assessing the primitive problem and surveying the literature of the most promising of the specific problems (alternatives), the design team begins the process creation step, As show in figure 1.1, this involves the assembly of a preliminary database that is comprised of thermo physical property data, including vapor-liquid equilibrium data, flammability data, toxicity data, chemical prices, and related information needed for preliminary process synthesis. In some cases, experiments are initiated to obtain important missing data that cannot be estimated accurately, especially when the primitive problem does not originate from laboratory study. Then, preliminary process synthesis begins with the design team creating flow sheets involving just the reaction, separation, and the temperature and pressure-change operations, And selecting process equipment in a task-integration step. Only those flow sheets that show a favorable gross profit are explored further; the others are rejected. In this way, detailed work on the process is avoided when the projected cost of the raw materials exceeds that of the products. These steps are described in detail in chapter 2, in which a flow sheet of operations is synthesized to address alternative 2 for the problem of increasing the production of vinyl chloride. Development of base case To address the most promising flow sheet alternatives, the design team is usually expanded, or assisted by specialized engineers, to develop base-case designs. This usually involves the development of just one flow sheet for each favorable process. As described in Chapter 2, the design team deigns by creating detailed process flow sheet, accompanied by steady-state material and energy balances, and a list of the major equipment items. A material balance table shows the state of each stream; that is, the temperature, pressure, phase, flow rate, and composition, plus other properties as appropriate. In many cases, the material and energy balances are performed at least in part, by a computer-aided process simulator, such as ASPEN PLUS, HYSYS, CHEMCAD, or PRO/II. Then, the design team seeks opportunities to improve the designs of the process units and to achieve more efficient process integrations, applying the methods of heat and power integration- for example, by exchanging heat between hot and cold streams. For each base-case design, three additional activities usually take place in parallel. Given the detailed process flow sheet, the design team refines the preliminary database to include additional data such as transport properties and reaction kinetics, feasibilities of the separations, matches to be avoided in heat exchange (i.e., for bidden matches), heuristic parameters, equipment sizes and costs as a function of throughput, and so on. This is usually accompanied by pilot-plant testing of confirm that the various equipment items will operate properly and to refine the database. If unanticipated data are obtained, the design team may need to revise the flow sheet. In some cases, equipment vendors run tests as well as generate detailed equipment specifications. To complement these activities, a steady-state simulation model is prepared for the base-case design. Process simulators are often useful in generating databases because of their extensive data banks of pure-component properties and physical property correlations for ideal and non ideal mixtures. When these are not available,
  • simulation programs can in regress experimental data taken in the laboratory or pilot plant for empirical or theoretical curve fitting. As shown in figure 1.1 in developing a base-case design, the design team checks regularly to confirm that the process remains promising. When this is not the case, the team often returns to one of the steps in process creation or develops the data base-case design. Finally, before leaving this topic, the reader should note that process creation and development of a base-case design are the subjects of Part One, entitled Process Invention heuristics and analysis (chapters 1-4). Detailed process synthesis using algorithmic methods While the design team develops one or more base-case design, detailed process synthesis may be undertaken using algorithmic methods as described in part two. These methods create and evaluated separation trains for recovering species in multicomponent mixtures (chapter 5), located and reduce energy usage (chapter 6), and create and evaluate efficient networks of the exchangers with turbines for power recovery (chapter7). With the results of these methods, the design team compares the base case with other promising alternatives and many cases identify flow sheet that deserve to be developed along with, or in place of, the base-case design. More specifically, chapter 6 discusses second law analysis, which provides an excellent vehicle for screening the base-case design or alternatives for energy efficiency. In this analysis, the lost work is computed for each process unit in the flow sheet. When large losses are encountered, the design team seeks methods to reduce these losses. Finally, in chapter 7, algorithmic methods are used to synthesize networks of heat exchangers, turbines, and compressors to satisfy the heating, cooling, and power requirements of the process. These methods, which place emphasis on the minimization of utilities such as steam and cooling water. Are used by the design team to provide a high degree of heat and power integration in the most promising processes. Plant wide controllability analysis and optimization An assessment of the controllability of the process is initiated after the detailed process flow sheet has been completed, beginning with the qualitative synthesis of control structures for the entire flow sheet, as discussed in chapter 12. Then measures are used that can be applied before the equipment is sized in the detailed design stage, to assess the ease of controlling the process and the degree to which it is inherently resilient to disturbances. These measures permit alternative processes to be screened for controllability and resiliency with little effort and, for the most promising processes, they identify promising control structures. Subsequently, control systems are added and rigorous dynamic simulations are carried out to confirm the projections sing the approximate measures discussed previously. This is the subject matter of chapters 13 and 1, which, together with chapter 12, comprise part four “plant wide controllability assessment.”
  • DETAILED DESIGN, EQUIPMENT SIZING AND COST ESTIMATION, PROFITABILITY ANALISYS, AND OPTIMIZATION After completing the base-case design, the design team usually receives additional assistance in carrying out the detailed design, equipment sizing and capital-cost estimation, profitability analysis, and optimization of the process. Each of these topics is covered in a separate chapter in part three. Although these chapters describe methods for completing rigorous cost estimates and profitability analyses, it is important to recognize that the more approximate methods are often sufficient to distinguish between alternatives during process creation (part one) and detailed process synthesis (part two). Throughout these parts of the book, references are made to approximate methods included in chapters 9 and 10. While carrying out these steps, the design team formulates start-up strategies to help identify the additional equipment that is usually required. In some cases, using dynamic simulators, the team extends the model for the dynamic simulations of the control system and tests start- up strategies, modifying them when they are not implemented easily. In addition, the team often prepares its recommendations for the initial operating strategies after star-up has been completed. Another crucial activity involves an analysis of the reliability and safety of the proposed process, as discussed in section 1.4. This often involves laboratory and pilot-plant testing to confirm that typical faults (valve and pump failures, leaks, etc.) cannot propagate through the plant to create accidents, such as explosions, cloud of toxic vapor, or fires. Often, HAZOP (hazard and operability) analyses are carried out to check systematically all of the anticipated eventualities. When the detail design stage is completed, the feasibility of the process is checked to confirm that the company´s profitability requirements have been met. If this proves unsatisfactory, the design team determines whether the process is still promising. If so, the team returns to an earlier step to make changes that it hopes will improve the profitability otherwise, the process design is rejected. Written process design report and oral Presentation Reporting and presentation are key steps in selling company management on the need to proceed with the design. As seen in chapter 15, the written design report is a work in progress as the design evolves. The process design report is the basis for the final, detailed design of the plant, suitable for construction. Final design, construction, start-up, and operation In creating the final design, much detailed work is done, often by contractors, using many mechanical, civil, and electrical engineers. They complete equipment drawings, piping diagrams, instrumentation diagrams, the equipment layout, the construction of a scale model, and the preparation of bids. Then the constructions phase is entered, in which engineers and project managers play a leading role. The design team may return to assist in plant start-up and operation. Note that these two activities are placed in dashed boxes in figure 1.1 because they are often not the responsibilities of chemical engineers.
  • Summary With the preceding description of figure 1.1 the reader should have gained a good appreciation of the subjects to be learned in process design and how this text is organized to describe the design methodologies in the context of many kinds of design problems. Having completed the coverage on assessing the primitive problem and surveying the literature, for the next steps, beginning with process creation, the reader can turn to chapter 2. Before leaving this chapter, however, discussions are provided on environmental protection in section 1.3, important safety considerations in section 1.4, engineering ethics in section 1.5, and the role of the computer in process design in section 1.6. These are key concerns of a design team throughout the design process, and consequently, these sections provide background for many of the discussions in the chapters that follow. 1.6 Role of computers Many calculations in process design do not require detailed algorithms because they involve simple equations and graphic procedures that can be carried out quickly without the complications of computers. In some circles, designers take pride in making quick and effective decisions using heuristics and back-of-the-envelope calculations. Indeed, in the earliest steps in figure 1.1, calculations are often quite approximate and the sources of data not terribly extensive. However, it does not take long for design teams to seek some computational assistance, if for no other reason than to access the extensive data banks associated with process simulators. As more data are obtained and flow sheets become more complicated, designers use a combination of computer resources involving spreadsheets, mathematical packages, and process simulators, both steady state and dynamic. In this section, the objective is to introduce briefly these three computational aids, with emphasis on their role in the design process. Table 1.4 is a listing of the more widely used computer programs that have been found useful in process design. A much more extensive list of chemicals engineering software is the 1997 CEP software directory of the AICHE, which describes more than 1,700 commercial computer programs. Spreadsheets In the world of the personal computer, spreadsheets such as Microsoft ´s excel, lotus 1-2-3, and Corel’s Quattro pro have become as easy to use as word processors and communication packages. Most engineers can enter tables of data ad program the spreadsheets to evaluate arithmetic expressions with very little effort. Whole columns and rows are manipulated and graphed, and results are stored and annotated without the complex formatting instructions that company procedural languages like FORTRANT. For this reason, many engineers have switched from procedural languages to spreadsheets, even for the implementation of iterative algorithms- for example, in the solutions of nonlinear equations. In the design area, spreadsheets are widely used for profitability analysis. Given the total installed cost of the equipment and the unit’s costs of the products, by-products, and the raw materials, as well as the utilities, spreadsheets compute the total capital investment, the cost sheet, and various
  • profitability measures, such as the return on investment and the net present value. Results from spreadsheets are readily plotted in any of a variety of graphs, and for the more difficult computational problems, a spreadsheet can be linked to a procedural language. For example, visual basic for applications (VBA) is available as macro language for Microsoft Excel. Mathematical packages Many kinds of engineering calculations can be carried out quickly and efficiently with symbolic and numeric mathematical packages. Examples are the analysis of linear systems (MATLAB), symbolic manipulations in algebra and calculus (MATHEMATICA, MAPLE), numerical integration of ordinary differential equations (COLNEW and ODEPACK); numerical solutions of partial differential equations (PDECOL), bifurcation analysis of nonlinear systems (AUTO), solution of mathematical programs in optimization (GAMS), and statistical analysis. Optimization involves the solution of linear programs (LPS), nonlinear programs (NLPs), mixed- integer linear programs (MILPs), and mixed-integer nonlinear programs (MINLPs). In this book, several optimization problems are solved with GAMS, which is introduced in Appendix VII. A less important, but growing, analysis in design involves the controllability of a process, which is often assessed by linearizing about a steady state, in this book; MATLAB is used for this purpose. Process simulators Computer-aided process design programs, often referred to as process simulators, flow sheet simulators, or flow sheeting package, are widely used in process design. The facilities of Computational guidelines With the broad array of computational packages available, especially to universities where they can be licensed at relatively low cost, a design team can lose valuable time in learning to use package when the calculations can be completed rapidly using simple equations and graphics. Computer packages should be used only when they can be applied easily and when their rigorous models for equipment and thermo physical properties are justified. Normally, ease of use is based on experience, which grows rapidly with exposure. As engineers and students gain familiarity with computer packages, they are often used very affectively throughout the design process, providing a real advantage to a design team. There is one basic premise, however, that must be observed: In process design, to permit a company to be
  • competitive, it is crucial that deadlines be met. To the extent that computer packages can accelerate the completion of the project, possibly with more rigorous results, they should be used. However, the deadlines must be met, with or without the computer. When computational difficulties are encountered, it is important that the design team find other methods for obtains results, even if they are more approximate.