Recycling calculations

Since this book is concerned primarily with one kind of equipment at a time, all that need be done here is to call attention to the existence of the abundant literature on these topics of recycle calculations and flowsheet simulation. 

Metcalfe and Perkins (7 ) and Perkins (75 ) combined the recycle calculations with the design specifications to solve simultaneously equations of the form... 

In addition, convergence calculations may be combined simultaneously with design specifications. The usual methods would be to embed the design in a convergence loop and meet the design specification in each recycle calculation. A quasi-Newton method convergence calculation in ASPEN will allow a simultaneous, more efficient solution for the more difficult problems. 

Chemical processes more often than not contain recycle, a feature that complicates their analysis. Recycle often occurs, as in the styrene process where unreacted ethylbenzene is recovered and recycled back to the reactor as a physical mass flow. Recycle also occurs in the form of heat exchange (again in the styrene process) and sometimes as information, e.g., a specification that two variable temperatures must equal each other. The sequential-modular solution strategy is based upon knowing all inputs to a module and using these to calculate all outputs. When an input stream to a module is the output of a downstream module (i.e., there is recycle), calculations cannot be performed for the upstream module because one of its inputs is not yet known. This is illustrated in Fig. 4.7 unit 1 cannot be calculated because input stream 4 is the output of unit 2 nor can unit 2 be calculated because input stream 2 is an output of unit 1. This same problem of circular reasoning was encountered in Example 1. This dilemma in the sequential modular solution scheme can be... 

Suppose that the filtrate were discarded instead of being recycled. Calculate the production rate of crystals. What are the benefits and costs of the recycling ... 

Bypass and recycle calculations are approached in exactly the same manner the flowchart is drawn and labeled, and overall balances and balances around the process unit or the stream mixing point following the process unit are used to determine unknown variables. 

The convergence of recycle calculations is almost always better if a good initial estimate of the tear stream is provided. 

Do not let recycle streams confuse you. The steps in the analysis and solution of material balance problems involving recycle are the same as described in Table 2.4. With a little practice in solving problems involving recycle, you should experience little difficulty in solving recycle problems in general. The essential point you should grasp with respect to recycle calculations in this chapter is that the processes such as shown in Fig. 2.4 or 2.16 are in the steady state. 

Other modules take care of equipment sizing and cost estimation, perform numerical calculations, handle recycle calculation (described in more detail below), optimize, and serve as controllers (executives) for the whole set of modules so that they function in the proper sequence. Figure 5.11 illustrates the information flow in a fiowsheeting package composed of modules. 

Understand how to follow the calculation sequences implemented automatically by ASPEN PLUS, CHEMCAD, and PRO/II for recycle calculations and when satisfying design specifications. For recycle calculations, the reader should be able to alter the calculation sequence by specifying tear streams. 

Hot gases from the toluene hydrodealkylation reactor are cooled and separated as shown in the flowsheet of Figure 4.32. In a steady-state simulation, can the composition of the recycle stream be determined without iterative recycle calculations Explain your answer. 

Understand the advantages of performing optimization and converging recycle calculations and design specifications simultaneously, as implemented using an infeasible path optimization algorithm. 

A methanol synthesis loop with a stoichiometric feed of CO and H2 is to be designed for a 95% overall conversion of CO. All the methanol formed leaves in the product stream. Not more than 2% of the CO and 0-5% of the H2 emerging from the reactor is to leave in the product stream—the remainder is recycled. Calculate the single pass conversion, the recycle ratio and the composition of the product. 

The previous chapters served as an introduction to multicomponent distillation. Matrix methods are efficient, but they still require a fair amount of time even on a fast conputer. In addition, they are simulation methods and require a known number of stages and a specified feed plate location. Fairly rapid approximate methods are required for preliminary economic estimates, for recycle calculations where the distillation is only a small portion of the entire system, for calculations for control systems, and as a first estimate for more detailed simulation calculations. 

Table A7.1 Excel Spreadsheet Designators and Formulas for Countercurrent Flow with Recycle Calculations...

Edie, F. C. and Westerberg, A. W., Decision Variable Selection to Avoid Hidden Singularities in Resulting Recycle Calculations for Process Design, Chem. Engr. Journal 2, 114 (1971). 

Process simulation software typically will produce a flowsheet, but this is probably not a true process flow diagram and should not be used as one in a report. There are two reasons for this. First, the simulator-produced flowsheet is unlikely to follow all the conventions of PFDs given in Chapter 1 regarding equipment symbols, line crosses, labels, and so on. Second, the process simulated is not the true process. One unit in the process simulator (such as a distillation column) maybe several pieces of equipment (tower, condenser, condensate tank, etc.) that need to be shown on a PFD. It is common to simulate a sin e unit (such as a process-process heat exchanger) as two units to decouple the recycle calculations. Some units, such as a storage tank, require no calculations in the process simulator and thus are not shown on its flowsheet. However, the simulator flowsheet is essential in the appendix of the report if the simulator report is included. Care should be taken to use the same stream numbers in the simulation flowsheet as in the PFD whenever possible. 

First catalyst bed exit gas recycle calculations also give steady-state exit gas compositions and quantities. These allow calculation of ... 

Appendix Y Cooled first catalyst bed exit gas recycle calculations... 

Third catalyst bed exit gas recycle calculations (Fig. Y.2) are done as described above, but all three beds must be included in the calculations. Computer calculations are required. 

One of the authors would specifically like to thank his son George Davenport and his nephew Andrew Davenport for their help with (i) wet sulfuric acid and (ii) cooled catalyst bed exit gas recycle calculations. 

Schott, N. R., L. Lak, and G. Smoluk. 1977. Recycle Calculations in Plasticating Extrusion. Society of Plastics Engineers Technical Papers, 32, 43 6. 

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