Prereactor And Reactive Column

There are two fresh feedstreams. One is methanol. The other is a mixture of C5 components that contains reactive isoamylenes plus other C5 paraffins, naphthenes, and olefins. This C5 stream typically comes from a petroleum refinery light-ends unit that separates light hydrocarbon components generated in catalytic cracking into various streams. Because the boiling points of all of the C5 components are quite similar, it is uneconomical to separate out the isoamylene reactants from the other C5s. The C5 fresh feedstream contains 24 mol% reactive isoamylenes. The remaining components are pentanes and pentene (largely isopentane), which are inert in the TAME reaction.

8.3.1 Base Case Design of Reactive Column

Subawalla and Fair1 presented a design of a prereactor and reactive distillation column. Figure 8.23 gives the flowsheet with a pressure-swing methanol recovery section. Figure 8.24 displays the flowsheet with an extractive distillation methanol recovery section. Note that the prereactor and reactive column are identical in both flowsheets.

The prereactor is a cooled tubular reactor containing 9544 kg of catalyst. All of the C5 fresh feed is fed to the prereactor along with a portion of the total methanol. The C5 fresh feed flowrate is 1040 kmol/h. The fresh feed of methanol coming into the process is 232 kmol/h, but there is a large methanol recycle stream from the recovery section. The flowrate of the recycle methanol is 318 kmol/h. The total methanol (550 kmol/h) is split between the prereactor (315 kmol/h) and the reactive distillation column (235 kmol/h).

The reactor effluent is fed into a 36-stage reactive distillation column (C1) on stage 28. Aspen notation of numbering stages from the top is used, with stage 1 being the reflux drum. The Subawalla and Fair design1 has catalyst present on stages 7-23. The reactor effluent is fed five trays below the reactive zone. A methanol stream is fed at the bottom of the reactive zone (stage 23). The flowrate of the methanol feed to the reactive column is 235 kmol/h.

Figure 8.25 gives composition profiles in reactive column C1. Note the low concentration of methanol in the lower part of the column and the fairly high concentration of one of the reactants (2M2B) in the same zone. This is below the reactive trays, which are stages 7-23. Figure 8.26 shows the temperature profile. The reflux ratio is 4, which gives a bottoms purity of 99.2 mol% TAME and a distillate impurity of 0.1 ppm TAME.

Swing Reactor
Figure 8.23 The TAME process with pressure-swing methanol recovery.

Reactor r 326 K 2.5 atm

1342 kmol/h 0.235 MeOH

Reactor

Pressure Swing Distillation

Water Makeup (24 kmol/h)

Water Recycle = B3 1026 kmol/h

C5 Product = D20 0001 Me0H

830 kmol/h 0.0001 MeOH 0.029 H,0

MeOH Recycle = D3

Figure 8.24 The TAME process with extractive distillation methanol recovery.

Water Makeup (24 kmol/h)

Water Recycle = B3 1026 kmol/h

C5 Product = D20 0001 Me0H

830 kmol/h 0.0001 MeOH 0.029 H,0

1342 kmol/h 0.235 MeOH

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  • taneli keskitalo
    Why do you need a prereactor for the reactive distillation?
    8 years ago

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