Application of columns with side withdrawal in schemes for benzene recovery from benzene-containing fractions of various compositions

Objectives. The work set out to evaluate the energy and economic efficiency of applying complex columns with side withdrawal in extractive distillation schemes of benzene–cyclohexane–toluene mixtures of various compositions using N-methylpyrrolidone as the entrainer.

Methods. A method based on the transformation of graphs representing flowsheets was used for the synthesis of extractive distillation schemes incorporating columns with side withdrawal. Determination of optimal scheme parameters was performed by scanning the range of variable changes with a specified step according to the criterion of minimizing total energy consumption in the column reboilers. The Non-Random Two Liquid local composition equation was used for modeling vapor–liquid equilibrium. The computational experiment was carried out using the Aspen Plus v.10 software package.

Results. Six flowsheets incorporating columns with side withdrawal (flowsheets of subset I) were generated using the graph method based on five flowsheets for extractive distillation of the benzene–cyclohexane–toluene mixture incorporating columns with a side section (flowsheets of subset Θ). Optimal parameters of the synthesized flowsheets were determined for separating the studied mixture of two initial compositions according to the criterion of total energy consumption. These compositions are simplified analogs of fractions produced in the processes of catalytic hydrotreating-hydrocracking of pyrolysis gasoline (Composition 1) and during catalytic steam dealkylation of pyrolysis gasoline (Composition 2). Columns with side withdrawal in each flowsheet of subset I were calculated with both vapor-phase and liquid-phase side streams. Extractive distillation schemes of different structures were compared according to the criteria of total energy consumption in column reboilers (∑Q_reb) and total annual cost (TAC).

Conclusions. It is established that the energy and economic efficiency of flowsheets incorporating columns with side withdrawal does not significantly depend on the phase state of the side stream. When separating the benzene–cyclohexane–toluene mixture of both considered compositions, the minimum ∑Q_reb value was found to correspond to Flowsheet I2.2-V. This flowsheet consists of an extractive distillation column, an N-methylpyrrolidone regeneration column with a side withdrawal of a vapor-phase stream above the feed plate, and a distillation column for the benzene–toluene mixture. The minimum TAC value is characteristic of Flowsheet I1.1, which includes an extractive column with a side withdrawal of a stream below the feed plate in the vapor phase (for Composition 1) or in the liquid phase (for Composition 2). For separating the studied mixture of both considered compositions, it is advisable to use Flowsheet I1.1 with a liquid-phase side withdrawal stream from the extractive column, as in this case, withdrawing the side stream is simpler and regulating its flow rate and composition is easier.

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