Process simulation programs

Although most process simulation programs feature a graphical user interface (GUI) that creates a drawing that resembles a PED, printouts of these drawings are very seldom used as actual process flow diagrams. The unit operations shown in the process simulation usually do not exactly match the unit operations of the process. The simulation may include dummy items that do not physically exist and may omit some equipment that is needed in the plant but is not part of the simulation. 

The most commonly used commercial process simulation programs are listed in Table 4.1. Most of these programs can be licensed by universities for educational purposes at nominal cost. 

Note Contact the website to check the full features of the most recent versions of the programs. 

Process simulation programs can be divided into two basic types  

Sequential-modular programs in which the equations describing each process unit operation (module) are solved module-by-module in a stepwise manner. Iterative techniques are then used to solve the problems arising from the recycle of information. 

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Given the first type of simulation, it is advantageous to be able to design a system of RO modules that can achieve the process objective at a minimal cost. A model has been iategrated iato a process simulation program to predict the stream matrix for a reverse osmosis process (132). In the area of waste minimization, the proper placement of RO modules is essential for achieving minimum waste at a minimum cost. Excellent details on how to create an optimal network of RO modules is available (96). 

A process-simulation program almost always contains a physical property service, because the quaflty of process design ultimately depends on the way in which the laws of physics and chemistry are appfled to the problem. Accordingly, the quaflty of this service is an important consideration to the user of a flow-sheeting system. 

Tlic heat duty is best calculated with a process simulation program hi will account for phase changes as the fluid passes throiigli ilic ctioke. It will balance the enthalpies and accurately predict the change m tcnipcrature across the choke. Heat duty should be checked for vanoits combinations of inlet temperature, pressure, flow rate, and outlet temper ature and pressure, so as to determine the most critical combination. 

The design of any of the distillation processes discussed requires choosing an operating pressure, bottoms temperature, reflux condenser temperature and number of trays. This is normally done using any one of several commercially available process simulation programs which can perform the iterative calculations discussed in Chapter 6. 

Detailed discussion of these programs is beyond the scope of this book. For a general review of the requirements, methodology and application of process simulation programs the reader is referred to the books by Husain (1986), Wells and Rose (1986), Leesley (1982), Benedek (1980), Mah and Seider (1980), Westerberg et al. (1979) and Crowe et al. (1971) and the paper by Panelides (1988). 

The UNIQUAC equation is not given here as its algebraic complexity precludes its use in manual calculations. It would normally be used as a sub-routine in a design or process simulation program. For details of the equation consult the texts by Reid et al. (1987) or Walas (1984). 

Procedures for the sequencing of columns are also available in the commercial process simulator programs for example, DISTIL in Hyprotech s suite of programs (see Chapter 4, Table 4.1). 

Leesley, M.E. Process Plant Design by Computer, Process Technology, Nov. 1973, p. 403. Kehat, E., Shacham, M. Chemical Process Simulation Programs-1, Process Technology, Jan./Feb. 1973, p. 35. 

Design of extraction processes and equipment is based on mass transfer and thermodynamic data. Among such thermodynamic data, phase equilibrium data for mixtures, that is, the distribution of components between different phases, are among the most important. Equations for the calculations of phase equilibria can be used in process simulation programs like PROCESS and ASPEN. 

A number of general microbiological growth media are available and may be used in a process simulation program. In general, when selecting a medium for use, the following considerations should be made ... 

Technically, COSMO-RS meets all requirements for a thermodynamic model in a process simulation. It is able to evaluate the activity coefficients of the components at a given mixture composition vector, x, and temperature, T. As shown in Appendix C of [Cl 7], even the analytic derivatives of the activity coefficients with respect to temperature and composition, which Eire required in many process simulation programs for most efficient process optimization, can be evaluated within the COSMO-RS framework. Within the COSMOt/ierra program these analytic derivatives Eire available at negligible additionEd expense. COSMOt/ierra can Eilso be csdled as a subroutine, Euid hence a simulator program can request the activity coefficients and derivatives whenever it needs such input. 

Equation (9.1) is the preferred method of describing membrane performance because it separates the two contributions to the membrane flux the membrane contribution, P /C and the driving force contribution, (pio — p,r). Normalizing membrane performance to a membrane permeability allows results obtained under different operating conditions to be compared with the effect of the operating condition removed. To calculate the membrane permeabilities using Equation (9.1), it is necessary to know the partial vapor pressure of the components on both sides of the membrane. The partial pressures on the permeate side of the membrane, p,e and pje, are easily obtained from the total permeate pressure and the permeate composition. However, the partial vapor pressures of components i and j in the feed liquid are less accessible. In the past, such data for common, simple mixtures would have to be found in published tables or calculated from an appropriate equation of state. Now, commercial computer process simulation programs calculate partial pressures automatically for even complex mixtures with reasonable reliability. This makes determination of the feed liquid partial pressures a trivial exercise. 

How to determine any HC liquid viscosity. For the viscosity of most any HC, see Fig. A-3 in Crane Technical Paper No. 410 [3], If your particular liquid is not given in this viscosity chart and you have only one viscosity reading, then locate this point and draw a curve of cP vs. temperature, °F, parallel to the other curves. This is a very useful technique. I have found it to be the more reliable, even when compared to today s most expensive process simulation program. Furthermore, I find it to be a valuable check of suspected errors in laboratory viscosity tests. If you don t have the Crane tech paper (available in any technical book store), then get one. You need it. I have found that most every process engineer I have met in my journeys to the four corners of the earth has one on their bookshelf, and it always looks very used. 

The triumph of this chapter is tray efficiency determination. Today we have commercial chemical engineering process simulation programs,... 

You may easily obtain these K values from a process simulation program for your particular case. If you do not have this source, use Chap. 2 of this book or the program RefFlsh, which is included in the com-... 

McDonnell Douglas Automation Co., "General Process Simulation Program" Box 516, St. Louis, MO 63166. 

The object of the presentation that follows is not to enable you to go out and simulate complex processes—the treatment is too brief and general for that—but simply to give you a sense of the structure of process simulation programs and calculations and to provide an idea of what such programs can do. 

In the process simulations we have discussed so far. the direction of flow of information corresponded to the direction of flow of the process streams—from feeds to products and around cycles. This mode of information flow is appropriate if the object is to calculate the output of a process for a given input and set of process parameters (temperatures, pressures, etc.) however, it often happens that a desired output is specified and input or process unit parameters required to achieve this output are to be calculated. A feature of process simulation programs called a design specification is used for calculations of this type. 

Bhaskara Reddy Penumalli, A Comprehensive Tvo-Dimensional VLSI Process Simulation Program, BICEPS, IEEE Tnns. Electron Devices, ED-30, No. 9, 986-992 (1983). 

Considering the number of permutations of container, closure system, and other product attributes that must be encompassed in a process simulation program, it should be evident that only in the simplest of situations would a single set of media fills be adequate to provide coverage of all aseptic processes performed. Where multiple lines are present in the facility, each should be considered independently. Process simulation results of one line are not predictive of results on another because the contamination rate is primarily dependent upon human performance. Even identical equipment in two clean rooms designed to the same standard will not give uniform results unless the aseptic technique of the operators is at the same level of performance. 

Tailored process simulation programs for aseptic processing. 

In cases in which the reaction quickly proceeds to equilibrium, the yields are easily estimated as the equilibrium yields. Under these circumstances, the only possibilities for process optimization are to change the temperature, pressure, or feed composition, so as to obtain a different equilibrium mixture. The calculation of reaction equilibrium is easily carried out using commercial process simulation programs. 

Close to the critical conditions, these equations should not be used. The procedure for calculation of polytropic work of compression or expansion close to the critical point is more complex (Shultz, 1962), and it is easiest to make such calculations using process simulation programs. 

Most of these programs are able to automatically extract stream data from process simulation programs, although great care should be taken to check the extracted data. There are many possible pitfalls in data extraction for example, not recognizing changes in the CP of a stream or partial vaporization or condensation of a stream, any of which could lead to a kink in the stream T-H profile. See Smith (2005) for more information on data extraction. 

Most flowsheet calculations are carried out using commercial process simulation programs. The process simulation programs contain models for most unit operations as well as thermodynamic and physical property models. All the commercial programs feature some level of custom modeling capability that allows the designer to add models for nonstandard operations. 

Subprograms and data banks for equipment sizing and costing. Process simulation programs enable the designer to consider alternative processing schemes. 

Each of the commercial process simulation programs contains a large data bank of pure component compounds. Most of the pure components are organic compounds, but inorganic compounds and electrolytes are also included. 

The methods used to converge recycle loops in the commercial process simulation programs are similar to the methods described in Section 1.9. Most of the commercial simulation programs include the methods described below. 

When there are multiple recycles present, it is sometimes more effective to solve the model in a simultaneous (equation-oriented) mode rather than in a sequential modular mode. If the simulation problem allows simultaneous solution of the equation set, this can be attempted. If the process is known to contain many recycles, then the designer should anticipate convergence problems and should select a process simulation program that can be run in a simultaneous mode. 

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