精对苯二甲酸装置溶剂脱水塔水相回流对分离效果影响的研究

Study on aqueous reflux in the acetic acid dehydration distillation column

在醋酸脱水共沸精馏系统中,水相回流不仅能调节物料平衡,而且能控制塔顶出料中的醋酸浓度。在醋酸脱水系统设计和控制的研究中,水相回流往往给定一个固定的流量,很少被用作控制变量,水相回流与精馏塔的进料变化及出料组成的关系未见报道。本文利用Aspen Plus流程模拟软件模拟计算精对苯二甲酸(PTA)装置溶剂脱水共沸精馏系统。利用三元相图分析醋酸-水-醋酸正丁酯(HAC-H_2O-NBA)体系中,进料组成变化与水相回流和酯相回流之间的关系。在稳态模型的基础上模拟分析进料中水含量和杂质对二甲苯(PX)对水相回流的影响,结果与相图分析一致。研究结果表明:对于某一特定的出料要求,当进料水含量高于69%时,水相回流可以降低为零,如果此时进料组成发生变化,只要调节酯相回流即可保证精馏塔的分离要求,而当进料水含量低于69%时,水相回流需要进行调节以保证塔顶采出水中醋酸含量合格;不能让杂质PX在塔内累积过多,以免因水相回流大量增加而使塔的能耗增加。本文的研究很容易扩展到其它具有特定出料要求的醋酸脱水共沸精馏系统中,研究结果将为实际生产过程中精馏塔出料组成和能耗的控制提供参考。

Aqueous reflux is important in acetic acid azectropic dehydration system.It can not only adjust the mass balance,but also control the concentration of acetic acid in the top product of the column.Aqueous reflux is often given a fixed value when design and control a acetic acid azectropic dehydration system,and rarely be used as a control variable.The relationship between aqueous reflux and feed composition change or the specification of product has not been reported.In this paper,the solvent dehydration azeotropic distillation column in the production of pure terephthalic acid（PTA） is simulated by using Aspen Plus software.The HAC-H_2O-NBA ternary phase diagram is used to analyze the relationship of feed composition change,aqueous reflux and organic reflux.Base on the established steady-state model,the influence of feed component and p-xylene（PX） in aqueous reflux is simulated and analyzed,and the result is consistent with the ternary phase diagram analysis.From analysis and simulation,we conclude that the aqueous reflux can reduce to zero when the water composition in feed exceed 69%for a given specifications of product,and the specifications of the top and bottom product can be properly held as long as the organic reflux is regulated in spite of feed composition changed at this time.But if the water composition in feed blow 69%,the aqueous reflux must also be adjusted correspondingly in order to ensure acetic acid in aqueous taken out from the top of the column qualified.Besides,the excessive accumulation of PX should be avoided,or it will increase energy consumption due to the increasing of aqueous reflux.This study can be easily extended to other acetic acid azeotropic dehydration system with specific requirement of product,and the result obtained will be helpful to control the product composition and energy consumption in actual production.