In this section we study the simulation and control of the tert-amyl methyl ether (TAME) process as a typical example of a reactive distillation system. There are two feedstreams: (1) methanol and (2) a mixture of reactants and inert components.
The C5 feedstream to the TAME process contains about 24 mol% reactive isoamylenes: 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B). The remaining components are pentanes and pentenes (largely isopentane, iC5), which are inert in the TAME reaction. TAME is the highest-boiling component, so it leaves in the bottoms stream from the reactive distillation column. The lighter C5s leave in the distillate stream along with a significant amount of methanol.
Methanol forms minimum boiling azeotropes with many of the C5s. The reactive column operates at 4 bar, which is the optimum pressure that balances the temperature requirements for reaction with those for vapor-liquid separation. At this pressure, isopen-tane and methanol form an azeotrope at 339 K that contains 26 mol% methanol. Therefore the distillate from the reactive column contains a significant amount of methanol, which must be recovered.
Since the iC5/methanol azeotrope is pressure-sensitive (79 mol% iC5 at 10 bar and 67 mol% iC5 at 4 bar), it is possible to use a pressure-swing process with two distillation columns, operating at two different pressures, to separate methanol from the C5 components. An alternative separation process for this system is extractive distillation, which is studied in this chapter.
Figure 9.7 gives the flowsheet of the process. There is a prereactor upstream of the reactive distillation column C1. The flowsheet contains three distillation columns (one reactive), and there are two recycle streams (methanol and water). The design of the prereactor and reactive column are based on a study by Subawalla. and Fair.1
The prereactor is a cooled liquid-phase tubular reactor containing 9544 kg of catalyst. The C5 fresh feed (flowrate 1040kmol/h) and 313 kmol/h of methanol are fed to the reactor.
1H. Subawalla and J. R. Fair, Design guidelines for solid-catalyzed reactive distillation systems, Ind. Eng. Chem. Research 38, 3693 (1999).
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