Process simulation recycles

Orbach, O. Crowe, C. M., "Convergence Promotion in the Simulation of Chemical Processes with Recycle - The Dominant Eigenvalue Method", Can. J. of Chem. Eng. (1971) 49 503-513. 

The final example to illustrate our plantwide control design procedure comes from Luyben and Tyreus (1998), who present design details of an industrial process for the vapor-phase manufacture of vinyl acetate monomer. This process is uniquely suited for researchers pursuing process simulation, design, and control studies. It has common real chemical components in a realistically large process flowsheet with standard chemical unit operations, gas and liquid recycle streams, and energy integration. 

Note that this 12-line calculation would have required setting up a recycle and three stream-adjusts in a process simulator, illustrating that simple problems can often be solved more easily by hand or spreadsheet calculations if the boundaries for mass balances are chosen carefully. 

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. 

Optimization of a large process simulation model is intrinsically difficult, particularly if there are multiple recycles. As noted in Section 1.9.9, the solution algorithms... 

Note This problem can be solved without using process simulation software. Start the mass balance at the reactor inlet (after the recycle streams have been added) and assume 100 kgmol/h of benzene at this point. 

The fusel-rich stream leaving the decanter is fed to the fusel sector where the stream is washed with water to recover about 96 % of the incoming ethanol. The resulting water-rich stream is recycled to the hybrid column. To do this, an overall amount of 363 kg/h wash-water and seven separation steps are necessary. The conceptual design of a cross-flow operation is performed using DISTIL, while process simulation is done in Hysys. 

The process model was built using PETROX, a proprietary sequential-modular process simulator from PETROBRAS. The simulation comprises 53 components and pseudocomponents and 64 unit operation modules, including 7 distillation columns and a recycle stream. All modules are built with rigorous, first-principles models. For optimization applications, PETROX was linked to NPSOL, an SQP optimisation algorithm. 

Outline an overall process flow sheet and material balance including solvent recovery and recycle. This should be done with the aid of process simulation software. [See Seider, Seader, and Lewin, Product and Process Design Principles Synthesis, Analysis, and Evaluation, 2d ed. (Wiley, 2004) and Turton et ah. Analysis, Synthesis, and Design of Chemical Processes, 2d ed. (Prentice-Hall, 2002)]. In the flow sheet include methods needed for controlling emissions and managing wastes. Carefully consider the possibility that impurities may accumulate in the recycled solvent, and devise methods for purging these impurities, if needed. 

Most processes involve a recycle stream. The reason is that all the reactants do not react, and businesses cannot afford to throw the rest away. Furthermore, any leftovers have to be disposed of in an environmentally friendly manner, which costs money. Thus, engineers take the unreacted reactants and put them back in the start of the process and try again. This makes the mass balances a little more complicated, and it leads to iterative methods of solution, which are described in this chapter. The hrst part of this chapter uses Excel to solve mass balances with recycle streams. Situations in which the energy balances affect the mass balance are treated in Chapters 6 and 7, because these are best done using a process simulator such as Aspen Plus . 

Many process simulators use this procedure because it is quite easy to implement. You only need to have a module or subroutine for a mixer, a reactor, and a separator, and the equations for each of these are quite simple. One problem that arises, though, is that the procedure takes many iterations to converge if the conversion is low. And, if there are interlocking recycle streams, convergence may not occur at all. However, it is quite a good scheme, and you can apply it using a spreadsheet. 

The inlet stream is compressed to 4000 psi with an isentropic compressor. The stream is mixed with the recycle stream and heated to 900°F, the reactor temperature. In the reactor, there is a pressure drop of 30 psi. The outlet is cooled to 80 F and the liquid and vapor phases are separated. The vapor phase goes to recycle, and 0.01 percent of it is used as purge. A recycle compressor then compresses the rest from 3970 to 4000 psia. In a real process, the heat transfer to preheat the feed to the reactor uses the effluent from the reactor, usually inside the same vessel. In process simulators, though, it is useful to begin as shown in Figure 7.1 to help convergence. NRTL thermodynamics was chosen and is justihed a posteriori. 

Process simulation with recycle and phase equilibrium. Chapter 7, p. 91. 

In the CRC scenario (cf. Sect. 1.2), CHEOPS [409], described in Subsect. 5.3.5, is used for the simulation of the overall PA6 production process. CHEOPS uses different existing simulation tools to carry out the overall process simulation by an a-posteriori runtime integration approach. The task of CHEOPS is to perform all partial simulations, each with the appropriate tool, to exchange simulation results between them, and to converge recycle streams which may occur in the flowsheet. 

Figure 3.6 presents the final Process Simulation Diagram of the HDA process. Disregarding the loop created by the flowsheet controller, three recycle loops may be identified heat integration around the reactor, recycle of hydrogen and recycle of toluene. The last two loops have a common part from the mixer up to the flash. As a result, the two loops may be solved by only one tear stream. Hence, we have three loops but only two tear streams, as for example the exit streams from mixer and reactor. However, a further simplification is possible. We may break the loop around the heat... 

An important phenomenon has been observed in the operation of many chemical plants with recycle streams. The same phenomenon has been observed and quantified in numerical simulation studies of industrial processes with recycles. A small change in a load variable causes a very large change in the flow rates around the recycle loop. We call this the snowball effect. 

The examples for experimental validation of the SMB model are based on the extended model (Figure 6.37) that takes into account the fluid dynamic effect of piping, especially recycle lines and other peripheral equipment such as measurement devices. From point of process simulation these are additional elements of the plant that have to be regarded within the overall flow sheet. 

A. Boldizar, Simulated Recycling-Repeated Processing and Ageing ofLDPE in proceedings of R 95, Geneva, Switzerland, 3, pp. 10-18 (1995). 

Be able to use the process simulators systematically during process creation, following sequences similar to those illustrated later in this chapter for a toluene hydrodealkylation process. The reader will learn to simulate portions of the process (the reactor section, the distillation section, etc.) before attempting to simulate the entire process with its recycle loops. Many examples and exercises enable the reader to master these techniques. 

Figure 4.10 Process with recycle (a) simulation flowsheet (b) ASPEN PLUS simulation flowsheet.

Figure 4.12a shows a simulation flowsheet with two recycle loops for ASPEN PLUS. Flowsheets for CHEMCAD and PRO/II are identical except for the subroutine (or model) names for the units. Note that no recycle convergence units are shown. This is typical of the simulation flowsheets displayed by most process simulators. The flowsheet for HYSYS.PIant is an exception because the recycle convergence unit(s) are positioned by the user and appear explicitly in the flowsheet. For ASPEN PLUS, CHEMCAD, and PRO/Il, to complete the simulation flowsheet, either one or two convergence units are inserted, as described below. Note that a single convergence unit suffices because stream S6 is common to both loops, as illustrated in Figure 4.12b. Stream S6 is tom into two streams, S6 and S6, with guesses provided for the variables in S6. Since no units are outside of the loops, all units are involved in the iterative loop calculations. The calculation sequence is...

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