pressure of this stream back to that of the reactor inlet. Hence the presence of the compressor in the recycle loop C. Unit Operations One of the major contributions to the practice of chemical engineering is the concept of the unit operations. This concept was developed by Arthur D. Little and Warren K. Lewis in the early 1900,s. Prior to its development, chemical engineering was, to a large extent, practiced along the lines of specific process technologies. For instance, if distillation was required in the manufacture of acetic acid, it became a problem in acetic acid distillation. The fact that a similar distillation might be required for the manufacture of, say, acetaldehyde was largely ignored. What Little and Lewis did was to show that the principles of distillation(as well as many other processing operations)were the same regardless of the materials being processed. So, if one knew how to design distillation columns, one could do so for acetic acid, acetaldehyde, or other mixture of reasonable volatility with equal facility. The same proved to be true of ot operations such as heat transfer by two-fluid heat exchangers, gas compression, liquid pumping, gas absorption, liquid-liquid extraction, fluid mixing, and many other operations common to the chemical industry The case for chemical reactors is less clear. Each reaction system tends to be somewhat unique in terms of its reaction conditions(temperature, pressure, type of catalyst, feed compo- sition, residence time, heat effects, and equilibrium limitations ) Thus, each reaction system must be approached on its own merits with regard to the choice of reactor type and design. However, the field of chemical reaction engineering has undergone substantial development over the past few decades, the result being that much of what is required for the choice and design of reactors is subject to a rational and quantitative approach D. Modes of Process Operation As mentioned previously, there are several modes of process operation. The one that has been most widely studied by chemical engineers is that of the steady-state operation of continuous processes. The reasons have also been discussed. In this mode of operation we assume that the process is subject to such good control that as feed materials flow into the process as constant flow rates, the necessary reactions, separations, and other operations all take place at conditions that do not vary with time. The amount of material in each item of equipment(its inventory)does not vary with time, pressures and liquid levels are constant. Nor do temperatures, compositions and flow rates at each point in the process vary with time. From the standpoint of the casual observer, nothing is happening. However, finished product is flowing out of the other end of the process into the product storage tank The next most common mode of process operation is known as batch operation. Here every processing operation is carried out in a discrete step. Reactants are pumped into a reactor
-18- pressure of this stream back to that of the reactor inlet. Hence the presence of the compressor in the recycle loop. C. Unit Operations One of the major contributions to the practice of chemical engineering is the concept of the unit operations. This concept was developed by Arthur D. Little and Warren K. Lewis in the early 1900's. Prior to its development, chemical engineering was, to a large extent, practiced along the lines of specific process technologies. For instance, if distillation was required in the manufacture of acetic acid, it became a problem in acetic acid distillation. The fact that a similar distillation might be required for the manufacture of, say, acetaldehyde was largely ignored. What Little and Lewis did was to show that the principles of distillation (as well as many other processing operations) were the same regardless of the materials being processed. So, if one knew how to design distillation columns, one could do so for acetic acid, acetaldehyde, or any other mixture of reasonable volatility with equal facility. The same proved to be true of other operations such as heat transfer by two-fluid heat exchangers, gas compression, liquid pumping, gas absorption, liquid-liquid extraction, fluid mixing, and many other operations common to the chemical industry. The case for chemical reactors is less clear. Each reaction system tends to be somewhat unique in terms of its reaction conditions (temperature, pressure, type of catalyst, feed composition, residence time, heat effects, and equilibrium limitations). Thus, each reaction system must be approached on its own merits with regard to the choice of reactor type and design. However, the field of chemical reaction engineering has undergone substantial development over the past few decades, the result being that much of what is required for the choice and design of reactors is subject to a rational and quantitative approach. D. Modes of Process Operation As mentioned previously, there are several modes of process operation. The one that has been most widely studied by chemical engineers is that of the steady-state operation of continuous processes. The reasons have also been discussed. In this mode of operation we assume that the process is subject to such good control that as feed materials flow into the process as constant flow rates, the necessary reactions, separations, and other operations all take place at conditions that do not vary with time. The amount of material in each item of equipment (its inventory) does not vary with time; pressures and liquid levels are constant. Nor do temperatures, compositions, and flow rates at each point in the process vary with time. From the standpoint of the casual observer, nothing is happening. However, finished product is flowing out of the other end of the process into the product storage tanks. The next most common mode of process operation is known as batch operation. Here, every processing operation is carried out in a discrete step. Reactants are pumped into a reactor
mixed, and heated up to reaction temperature. After a suitable length of time, the reactor turned off by cooling it down. It now contains a mixture of products, by-products, and unreacted reactants. These are pumped out of the reactor to the first of the various separation steps possibly a batch distillation or a filtration if one of the products or byproducts is a solid. Each batch step has a beginning, a time duration, and an end. (Most activities around the home are batch in nature-cooking, washing clothes, etc. a third mode is cyclic operation. From the standpoint of flows in and out of both the process and individual items of equipment, operation is continuous. However, in one or more items of equipment, operating conditions vary in time in a cyclical manner. a typical example is reactor whose catalyst deactivates fairly rapidly with time due to, say, coke formation on the catalyst. To recover the catalyst activity, it must be regenerated by being taken out of reaction operation. The coke is removed either by stripping by blowing an inert gas over the catalyst or, in the more difficult cases, by burning the coke off with dilute oxygen in an inert gas carrier Two characteristics of cyclical operation become apparent. First off, if the process is to be operated continuously but it one or more items of equipment must be taken off line for regen- eration of one sort or another, we must have at least two of such items available in parallel. One is on line while another is being regenerated. The second characteristic is the is that operation conditions in the cyclically operated equipment must vary with time. If catalyst activity decrease with time, then something must be done to maintain the productivity of the reactor. Usually this is achieved by raising the reactor temperature. Thus, a freshly regenerated reactor will start off at a relatively low temperature; the temperature will be raised during the cycle, and the reactor will be taken off line when no further benefit is to be obtained by raising the temperature any further (The catalyst may melt, for instance.)
-19- mixed, and heated up to reaction temperature. After a suitable length of time, the reactor turned off by cooling it down. It now contains a mixture of products, by-products, and unreacted reactants. These are pumped out of the reactor to the first of the various separation steps, possibly a batch distillation or a filtration if one of the products or byproducts is a solid. Each batch step has a beginning, a time duration, and an end. (Most activities around the home are batch in nature - cooking, washing clothes, etc.). A third mode is cyclic operation. From the standpoint of flows in and out of both the process and individual items of equipment, operation is continuous. However, in one or more items of equipment, operating conditions vary in time in a cyclical manner. A typical example is reactor whose catalyst deactivates fairly rapidly with time due to, say, coke formation on the catalyst. To recover the catalyst activity, it must be regenerated by being taken out of reaction operation. The coke is removed either by stripping by blowing an inert gas over the catalyst or, in the more difficult cases, by burning the coke off with dilute oxygen in an inert gas carrier. Two characteristics of cyclical operation become apparent. First off, if the process is to be operated continuously but it one or more items of equipment must be taken off line for regeneration of one sort or another, we must have at least two of such items available in parallel. One is on line while another is being regenerated. The second characteristic is the is that operation conditions in the cyclically operated equipment must vary with time. If catalyst activity decrease with time, then something must be done to maintain the productivity of the reactor. Usually this is achieved by raising the reactor temperature. Thus, a freshly regenerated reactor will start off at a relatively low temperature; the temperature will be raised during the cycle; and the reactor will be taken off line when no further benefit is to be obtained by raising the temperature any further. (The catalyst may melt, for instance.)
IIL. PROCESS MATERIAL BALANCES Revised October 10.1999
-20- III. PROCESS MATERIAL BALANCES Revised October 10, 1999
Material balances result from the application of the law of conservation of mass to individual items of equipment and to entire plants(or subsections thereof). When the mass conservation equations are combined with enough other equations(energy balances, equilibrium relationships, reaction kinetics, etc. for an individual item of equipment(such as a reactor or a distillation column), the result is a mathematical model of the performance of that equipment item The model can be dynamic or steady state, depending upon how it is formulated For the present let us confine our attention to steady-state models. Such equipment models are generally nonlinear and must be solved by iterative procedures, usually with the help of a computer. However, if we make enough simplifying assumptions, linear models will result starting point for many applications, one which is pursued in subsequent sections of these noter o While these will not be as accurate as the more rigorous nonlinear models, they are a goo Individual models can be combined to represent the performance of an entire chemical plant or sections thereof. For instance, one might start by modeling the performance of the reaction section.When this is in hand, one can then add other parts of the plant such as the separation and purification sections. As will be seen, if we limit ourselves to simple, linear equipment models, then the overall process or flowsheet model will also be linear. This we can solve using a spreadsheet for instance. Not only that, we can solve rather large flowsheet model with a relatively modest amount of effort. This is one motivation for using linear models If the equipment models are nonlinear, we almost always require the use of a computer Indeed, there are special-purpose programs that have been developed just for solving process flowsheet models. These programs are generally known as steady-state process simulators or flowsheeting programs A. The Stream Summar Before discussing equipment characterization in detail, it is necessary to consider the characterization of the streams entering and leaving each item of equipment. While there is no unique way of doing this, the following characterization is typical. Each stream is represented by a vector of various quantities. If there are nc components of significance in the stream, then the first nc entries in the vector will be fin -the molar flow rate of the component i in stream n Additional quantities that may be included in this vector are the total molar flow rate of stream Tn -the temperature of stream n
-21- Material balances result from the application of the law of conservation of mass to individual items of equipment and to entire plants (or subsections thereof). When the mass conservation equations are combined with enough other equations (energy balances, equilibrium relationships, reaction kinetics, etc.) for an individual item of equipment (such as a reactor or a distillation column), the result is a mathematical model of the performance of that equipment item. The model can be dynamic or steady state, depending upon how it is formulated. For the present let us confine our attention to steady-state models. Such equipment models are generally nonlinear and must be solved by iterative procedures, usually with the help of a computer. However, if we make enough simplifying assumptions, linear models will result. While these will not be as accurate as the more rigorous nonlinear models, they are a good starting point for many applications, one which is pursued in subsequent sections of these notes. Individual models can be combined to represent the performance of an entire chemical plant or sections thereof. For instance, one might start by modeling the performance of the reaction section. When this is in hand, one can then add other parts of the plant such as the separation and purification sections. As will be seen, if we limit ourselves to simple, linear equipment models, then the overall process or flowsheet model will also be linear. This we can solve using a spreadsheet for instance. Not only that, we can solve rather large flowsheet models with a relatively modest amount of effort. This is one motivation for using linear models. If the equipment models are nonlinear, we almost always require the use of a computer. Indeed, there are special-purpose programs that have been developed just for solving process flowsheet models. These programs are generally known as steady-state process simulators or flowsheeting programs. A. The Stream Summary Before discussing equipment characterization in detail, it is necessary to consider the characterization of the streams entering and leaving each item of equipment. While there is no unique way of doing this, the following characterization is typical. Each stream is represented by a vector of various quantities. If there are nc components of significance in the stream, then the first nc entries in the vector will be fi,n - the molar flow rate of the component i in stream n. Additional quantities that may be included in this vector are: Fn - the total molar flow rate of stream n, Tn - the temperature of stream n
Pn-the pressure of stream n, Rn -V/F, (the mols of stream n that are vapor / (the total mols of stream n)[V/F=0, stream is liquid V/F= 1, stream is vapor;0 V/F 1, stream is a two-phase mixture of liquid and vapor n- mass density of stream n, Mwn-average molecular weight of stream n, Wn- the total mass flow rate of stream n Hn -the total enthalpy of stream n, Sn-the total entropy of stream n, and Gn -the total free energy of stream n Not all of these quantities have to be included in the stream vector for all problems. Only the f are needed for linear material balance calculations. However, most process simulation programs include all of the variables listed above in the stream characterization vector as well as mol fractions and weight fractions of the individual components. Since the total mass balance around each item of equipment is easy to check, we will include Wn in the stream vector(and will need Mwn to calculate it from Fn) a typical summary is shown in the Table IlI-l below. It is for the Ammonia Synthesis Loop whose PID was presented in Fig. Il-1 of the previous chapter. It was computed using the Chapter VIL. The details of the spreadsheet solution are given in AppendixD que developed in EXCEL spreadsheet program based on the linear material solution technique developed in B. Equipment Characterization 1. Reactors The reactor is the heart of almost any chemical process. A simple reactor is shown schematically in Fig. IlI-1. The input or feed stream STin contains the reactants along any inerts, feed impurities, and diluents (all of which have usually been premixed in a mixer). One or more chemical reactions take place in the reactor and the reaction products, any remaining unreacted reactants, and the inerts, diluents, and feed impurities leave in the reactor output or effluent stream S tout ummary TI ST3 ST6 ST7 st9
-22- Pn - the pressure of stream n, Rn - V/F, (the mols of stream n that are vapor)/(the total mols of stream n) [V/F = 0, stream is liquid; V/F = 1, stream is vapor; 0 V/F 1, stream is a two-phase mixture of liquid and vapor] n - mass density of stream n, Mwn - average molecular weight of stream n, Wn - the total mass flow rate of stream n, Hn - the total enthalpy of stream n, Sn - the total entropy of stream n, and Gn - the total free energy of stream n. Not all of these quantities have to be included in the stream vector for all problems. Only the fi,n are needed for linear material balance calculations. However, most process simulation programs include all of the variables listed above in the stream characterization vector as well as mol fractions and weight fractions of the individual components. Since the total mass balance around each item of equipment is easy to check, we will include Wn in the stream vector (and will need Mwn to calculate it from Fn). A typical summary is shown in the Table III-1 below. It is for the Ammonia Synthesis Loop whose PID was presented in Fig. II-1 of the previous chapter. It was computed using the EXCEL spreadsheet program based on the linear material solution technique developed in Chapter VII. The details of the spreadsheet solution are given in Appendix D. B. Equipment Characterization 1. Reactors The reactor is the heart of almost any chemical process. A simple reactor is shown schematically in Fig. III-1. The input or feed stream STin contains the reactants along any inerts, feed impurities, and diluents (all of which have usually been premixed in a mixer). One or more chemical reactions take place in the reactor and the reaction products, any remaining unreacted reactants, and the inerts, diluents, and feed impurities leave in the reactor output or effluent stream STout. Stream Summary Comp ST1 ST3 ST4 ST6 ST7 ST8 ST9