Consider Example 17.3, where a mixture of CO₂ and methanol is separated in a jacketed flash vessel. The inlet temperature of the heating medium is 90°C, and its specific heat is similar to that of water. There is no phase change of the heating medium in the jacket. The heat transfer option in the dynamic simulation is “LMTD,” and the heating-medium flowrate is manipulated to maintain the vessel outlet temperature at 70°C, as shown in Figure E17.5(a). Figure E17.5(a) also shows that the flow dynamics of the heating medium are neglected, assuming perfect control. So there are four controllers: FC-1101, PC-1101, LC-1101, and TC-1101. Tune these controllers by using both methods mentioned before (i.e., by using stability margins and by using an approximate process model) and by applying the various rules discussed previously. Then decrease the feed flow to 50 kmol/h in a single step. Show the differences in the transient response of the outlet temperature with the controllers being tuned by different rules.

Example 17.3

Figure E17.3(a) shows the schematic of a jacketed flash vessel in which a mixture of CO and methanol is being separated. The inlet temperature of the heating medium stream is 90°C and its specific heat is similar to that of water. There is no phase change of the heating medium in the jacket. Compare the transient response of the flash vessel outlet temperature and its level for the “constant duty” and “LMTD” options when the inlet feed flowrate is reduced to 10 kmol/h. Assume no change in the inlet temperature and flowrate of the heating medium. The vessel is well insulated; that is, heat loss to the environment may be assumed to be negligible.

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Feed 30°C 5 bar FC FC-1101 100 kmol/h MeOH = 0.9 YcO₂ = 0.1 CV-1101 DP = 1 bar V-1101 70°C 3.0 bar DATO TC-1101 Heating Fluid In 8 3 PC-1101 (PC (LC) LC-1101 5 Heating Fluid Out CV-1102 DP = 1 bar CV-1103 DP = 1 bar 4 6 Vapor Product Liquid Product