15.6 Simulations

SIMULINK provides a nice modeling environment for simulating very simple process flow sheets. For rigorous process design and analysis, chemical process flow sheeting packages such as ASPEN PLUS and HYSYS should be used. Flow sheets can quickly be developed using standard "icons" that represent unit operations, such as reactors and separators. This next example illustrates the use of SIMULINK to solve a flow sheet similar to Example 15.1.

Table 15-1. H₂ Make-up and Vapor Recycle as a Function of H₂ Fraction in Purge Stream

H2 purge frac, y7,H	Toluene conv., XT	H2 make-up, n2	Recycle, n8
0.3	0.7	1.515	18.96
0.4	0.7	1.786	13.57
0.5	0.7	2.174	10.11
0.6	0.7	2.778	7.46

Example 15.6: A Simple Recycle System

Here we consider a slightly different steady state than that shown in Figure 15-3. In Figure 15-23 the make-up feedstream (stream 1) has 0.9 mole fraction A. The CSTR has a molar holdup of N = 200 moles and a reaction rate constant of k = 0.6 hr^-1. One of our objectives is to compare the responses of the flow sheets with and without a recycle stream, to disturbances in the fresh feed (stream 1) composition. A SIMULINK .mdl for the recycle system dynamics is shown in Figure 15-24, where the mixing point, the CSTR, and the separator are "masked" functions. The CSTR is "unmasked" in Figure 15-25, to reveal the modeling equation

Equation 15.6

Consider a step change in the feed composition of A from 0.9 to 1.0 mole fraction. The results for the recycle system are compared with the "straight through" system without recycle in Figure 15-26. Notice that the recycle system has a tremendous change in the separator product stream (recycle flow) compared to the no-recycle case. As discussed in Section 15.2 (and Example 15.2), the recycle system has a large gain and time constant compared to the system without recycle.