Online switching of entrainers for acetic acid dehydration by heterogeneous azeotropic distillation

Traditionally, a foreign chemical such as isobutyl acetate (IBA) is used as an entrainer in a heterogeneous azeotropic distillation (HAD) column to separate acetic acid (HAc) and water. However, the loss of this entrainer may contaminate the process of manufacturing terephthalic acid because of the recycle of product streams to an oxidation reactor and other parts of the process. In the study, use of p-xylene (PX), a component indigenous to the terephthalic acid process and already present in the feed to the HAc dehydration column, as the entrainer for the separation of HAc and water is investigated. Optimization results show that using the HAD process with PX as the entrainer not only overcomes the shortcomings of IBA as the entrainer but also requires less energy consumption. Moreover, we demonstrate that it is possible to dynamically transform the HAD column with IBA as the entrainer into the HAD column with PX as the entrainer, without the need of shutdown and startup. The key to successful implementation of such a dynamic transition is to maintain a proper PX inventory inside the HAD column. While simple sidedraw flow control can be used, it will create PX imbalance if there is a flow measurement bias. A temperature control scheme was found to accelerate such a transition and maintain HAc purities at their designed values.     

溶剂脱水塔共沸精馏过程模拟与优化

针对溶剂脱水塔醋酸分离难的问题,利用剩余曲线图(RCM)法研究醋酸-水-NBA三元共沸物体系,理论分析回流组分的改变对溶剂脱水塔操作的影响.运用ASPEN PLUS软件模拟某石化公司的脱水塔装置,结合实际分析精馏过程的各种影响因素,和优化相应的操作参数.回流比在0.76,水回流率在0.0285,酯相回流中的116流股比率控制在0.9时,模拟结果最佳.该结果对生产会有一定的指导作用.     

稀醋酸共沸精馏脱水工艺的模拟

醋酸是一种重要的化工原料,其水溶液广泛存在于各类工业过程中。由于醋酸与水会缔合使得两者的相对挥发度不大,生产中用于分离醋酸/水的工艺能耗较高,因此研究者和工业界都在寻求更好的分离方法。本文以醋酸正丙酯做挟带剂,应用Aspen Plus流程模拟软件,汽相逸度采用Hayden-O’Connell方程,液相活度系数采用NRTL方程计算,实现了稀醋酸共沸精馏脱水工艺的模拟。本文详细讨论了全塔理论板数、进料位置、回流比等因素对脱水塔性能的影响。结果表明,在理论板数50块,回流比3.2,醋酸进30块板,挟带剂与进料比0.15条件下,塔釜得到醋酸浓度高于95%。模拟结果对醋酸脱水工艺的设计和改造具有重要的指导意义。     

乙酸丁酯反应精馏过程模拟设计

反应精馏偶合了反应和精馏两种单元操作,通过精馏促进反应,可以提高反应转化率和收率,为可逆反应的化工过程生产提供了新的设计途径。基于严格热力学分析计算,利用计算机模拟和优化手段。提出了乙酸丁酯反应精馏、分离纯化的生产流程。采用UNIQUAC方程表征乙酸-正丁醇-乙酸丁酯-水四元非理想体系的汽液平衡,首先,根据实验数据回归了热力学模型中的交互作用参数,并预测了体系中5个共沸物组成,模型的计算结果与实际数据吻合。基于平衡级模型,提出了由平衡反应器、反应精馏塔、倾析器和纯化塔构成的可行流程,对提出的设计流程进行了模拟、优化,得到了操作工艺参数。模拟结果对工业过程的设计和改造具有一定的指导意义。     

Comparison of pressure-swing distillation and extractive distillation with varied-diameter column in economics and dynamic control

Pressure-swing distillation and extractive distillation are two common methods for azeotrope separation. The economics and controllability are two crucial factors for evaluating the feasibility of a separation process. A varied-diameter column (VDC) was used in the process design to evaluate its economics and controllability. Five azeotropic systems were investigated in order to compare the economics of pressure-swing distillation and extractive distillation with a VDC. Results indicate that pressure-swing distillation with a VDC saves more money than extractive distillation. The dynamic control were evaluated in the acetone-methanol system for both processes with a VDC. The improved control structure for pressure-swing distillation with a VDC can handle ±20% disturbances effectively, while the improved control structure for extractive distillation with a VDC can only handle ±10% disturbances. A comparison of the two methods from the viewpoint of economics and controllability demonstrates that pressure-swing distillation is more suitable when using a VDC.     

Experimental investigation of conventional control strategies for a heterogeneous azeotropic distillation column

In this work, a laboratory scale sieve plate distillation column was constructed to investigate the conventional control strategies of an isopropyl alcohol (IPA), cyclohexane (CyH) and water (H₂O) heterogeneous azeotropic distillation column. Steady state process analysis showed that the optimal operation point should be located at a critical reflux, a transition point at which the distillation path switches from a route that passes through IPA+H₂O azeotrope to one that passes through IPA+CyH azeotrope. At this critical reflux, a high purity IPA product can be obtained with minimum energy consumption and maximum product recovery. However, the steady state is extremely sensitive to feed disturbances. A good control strategy must be able to maintain a steady column temperature profile that shows a plateau near 70°C to ensure passage around IPA+CyH azeotrope. In this study, an inverse double loop control strategy is recommended. Through experimental testing, the proposed control strategy was demonstrated to keep the product IPA purity at the desired high-purity level under all feed disturbance changes while other conventional control strategies fail.     

Design and control of an ethyl acetate process: coupled reactor/column configuration

In this paper, design and control of a realistic coupled reactor/column process to produce ethyl acetate is studied. The process design is more complicated because the ethyl acetate product is neither the lightest nor the heaviest component in the system. A search procedure is proposed to obtain the optimum process design and operating condition of this process. The optimum process design is the one that minimize the Total Annual Cost (TAC) of this process while satisfying the stringent product impurity specifications. The optimum overall process design includes a continuous-stirred tank reactor (CSTR) coupled with a rectifier, a decanter, another stripper, and a recycle stream. After the process design is established, the next step is to use dynamic simulation to test the appropriate control strategy for this process. Sensitivity analysis is performed to obtain the suitable temperature control points for the columns. The proposed control strategy is very simple containing only one temperature control loop in each column. This recommended simpler control strategy uses the ratio of acetic acid feed rate to ethanol feed rate to control the 5th stage temperature of the rectifier and uses the stripper reboiler duty to control the 5th stage temperature of the stripper. The proposed control strategy does not need any on-line composition measurements and can properly hold product purity in spite of feed flow rate and feed composition disturbances. For small deviations of the product impurity compositions during disturbances, a slow cascade outer composition loop structure can be implemented using off-line composition measurements from the quality lab.     

Control of reactive distillation process for production of ethyl acetate

In this paper, plant-wide control for the production of ethyl acetate using reactive distillation is studied. Four important issues are considered in developing control schemes, including: (1) economics; (2) steady-state deviation of key product purities; (3) controllability in terms of degree of oscillation and settling time; and (4) feasible region of disturbances for effective control. Starting with two basic control schemes that have been studied before, new control schemes are developed to improve the operability of the process. These new control schemes have evolved from the basic schemes by making a trade-off between optimal design and control or by selecting sensor locations using closed-loop sensitivity analysis. It is found that, while being subjected to fluctuations in the composition of the acid feed or in the production flow-rate, sensor location based on traditional open-loop sensitivity causes a larger overshoot and steady-state deviation of key product purities. Sensor location on the basis of a closed-loop sensitivity analysis provides a better alternative for feedback control. The resulting scheme for control is found to be effective in reducing the steady-state deviation and in promoting good control performance.     

恒沸精馏乙酸丁酯和正丁醇过程的优化

乙酸丁酯和正丁醇是两种重要的有机化工原料,广泛应用在化工和制药行业中.在青霉素提取过程中产生大量的含乙酸丁酯、正丁醇、水的混合物,如何将其分离,有实际的意义.本文介绍皂化处理方法和恒沸精馏分离方法,通过利用3A分子筛优化恒沸精馏工艺过程,从而达到节能降耗20%的目的.     

Two-point control of a reactive distillation column for composition and conversion

Reactive distillation is a hybrid process with dual process objectives: reactant conversion and product composition. Control schemes for reactive distillation frequently neglect the effect of the principal operating parameters on the reactant conversion, and this has a detrimental effect on the overall process profitability. An ETBE reactive distillation column has been used as a case study to show how a two-point control configuration, which recognises the importance of both composition and conversion, can be developed and implemented for a reactive distillation process. The combined composition and conversion control configuration was tested using SpeedUp dynamic simulations and proved to be effective in maintaining a high isobutylene conversion despite process disturbances. The two-point control scheme also had superior disturbance rejection capability, especially for feed rate changes, and composition set-point sensitivity compared with a one-point control scheme.     

Role of multiplicity in reactive distillation control system design

The impact of steady-state multiplicities on the control of a simulated industrial scale methyl acetate reactive distillation (RD) column is studied. At a fixed reflux rate, output multiplicity, with multiple output values for the same reboiler duty, causes the column to drift to an undesirable steady-state under open loop operation. The same is avoided for a fixed reflux ratio policy. Input multiplicity, where multiple input values give the same output, leads to “wrong” control action under feedback control severely compromising control system robustness. A new metric, rangeability, is defined to quantify the severity of input multiplicity in a steady-state input–output (IO) relation. Rangeability is used in conjunction with conventional sensitivity analysis for the design of robust control structures for the RD column. Results for the two synthesized control structures show that controlling the most sensitive reactive tray temperature results in poor robustness due to low rangeability causing “wrong” control action for large disturbances. Controlling a reactive tray temperature with acceptable sensitivity but larger rangeability gives better robustness. It is also shown that controlling the difference in the temperature of two suitably chosen reactive trays further improves robustness of both the structures as input multiplicity is avoided. The article brings out the importance of IO relations for control system design and understanding the complex dynamic behavior of RD systems.     

萃取精馏的条件及选择萃取剂的研究

采用UNIFAC改进模型和MATLAB计算软件,对萃取精馏过程中萃取剂选择、萃取剂性能和萃取条件的影响进行了研究。通过对甲醇-丙酮、甲醇-乙酸甲酯和苯-环己烷等多个共沸体系在不同萃取剂和不同萃取条件下的相对挥发度的计算比较,确定了体系进行萃取精馏分离时合适的萃取剂分别为DMSO、水和DMF等,讨论了原料组成和溶剂比对相对挥发度的影响。结果表明:溶剂比是重要的影响因素之一,溶剂比越大,被分离体系的相对挥发度越大;原料组成对相对挥发度的影响较弱,随着重组分含量的增加,体系的相对挥发度稍有下降。本文还对文献数据进行了关联和比较,计算结果与文献符合良好,表明所开发的程序具有一定的可靠性和适用性。     

Combined internal model and inferential control of a distillation column via closed-loop identification

A procedure for designing distillation control systems with spe cified nominal properties is presented. The desired behaviour of the control system for both setpoint changes and disturbances in the feed flow rate and the feed composition can be specified. Both types of specifications can be handled because the disturbances can be inferred from the behaviour of the inventory control system. The control system is realized as a combined internal model and inferential control (CIMIC) system. A disturbance rejecting and decoupling (DRD) control structure is obtained as a special case. The performance of the control system is demonstrated experimentally on a pilot-scale distillation column. For comparison, experiments with pure internal model control (IMC) are also illustrated. A preliminary model of the distillation column was determined from step tests carried out in open-loop operation, but the final model used in the control system designs was obtained via a control-relevant closed-loop identification.     

Multivariable control of double-effect distillation configurations

Heat-integrated double effect distillation column design is promising from the viewpoint of energy conservation. However, the control of the system is very difficult in the sense that the system is nonlinear, multivariable and interacting. A new model-based control scheme developed by Han and Park (AIChE J. 1993, 39 (5), 787) to deal with these difficulties has been applied to these heat-integrated distillation configurations (feed-split configuration, heavy-split configuration). Our simulation results indicate that the control scheme is able to overcome the severity of interactions and shows good control performance for a heat integrated double-effect distillation column with high purity specification.     

APC技术在醋酸回收装置的应用

分析醋酸回收装置共沸精馏系统运行现状,在DCS常规控制基础上,应用先进工艺控制(APC)技术。通过克服萃取液蒸发器进料量、进料组分、加热蒸汽压力波动等干扰,稳定主蒸塔温度分布,同时优化主蒸塔中部温度、提馏段灵敏板温度,实现"卡边"优化控制。实测同等稳定负荷情况下,装置加热蒸汽单耗下降1.8%。     

节能型甲基异丙基酮精馏过程的控制研究

在酸-醛法合成甲基异丙基酮(MIPK)的反应粗产物中,水与MIPK、二异丙基酮(DIPK)等均形成共沸物,使得MIPK的分离精制变得非常困难,文中采用剩余曲线图法对丙酮-MIPK-水三元物系进行分析,提出了通过利用中间分离器在一个精馏塔中实现丙酮和水同时脱除的节能型精馏过程,并运用奇异值分解(SVD)方法对提出的节能型精馏过程进行分析,选择了适宜的控制方案,建立了相应的动态模拟模型,对控制系统的动态特性进行了分析.结果表明,文中提出的控制方案具有较好的抗扰动能力.     

醋酸/水萃取精馏的模拟

利用PRO／Ⅱ软件对醋酸／水的萃取精馏进行了模拟。采用Hayden-O’Connell方程计算逸度系数，NRTL方程计算活度系数。模拟结果显示，与普通精馏相比，萃取精馏工艺的再沸器热负荷降低了51．8％，生产每吨醋酸耗费蒸汽量由8．9t降至4．6t，说明在同样的分离指标下，萃取精馏法分离醋酸／水要优于普通精馏法。     

Using MPC to control middle-vessel continuous distillation columns

The use of model predictive control (MPC) in middle-vessel continuous distillation column (MVCC) is discussed. It is shown that using a 5 × 5 MPC implementation (where all levels are included in MPC as integral process variables) allows using a smaller middle-vessel, particularly when disturbances can be measured: a good performance is ensured without having the middle vessel drained or overfilled. Also, it is shown that MPC practically circumvents the issue of tuning the middle-vessel level controller. Furthermore, the MVCC design makes conventional decentralised control perform comparably to MPC.     

Overall control strategy of a coupled reactor/columns process for the production of ethyl acrylate

Ethyl acrylate (EA) is widely used in industry as a precursor for varnishes, adhesive, and finishes of papers and textiles. This important ester can be produced directly from ethanol (EtOH) and acrylic acid (AA) via esterification reaction with the presence of sulphuric acid as homogeneous catalyst. The proposed design flowsheet of this process includes a CSTR reactor coupled with a rectifier and an overhead decanter. In order to further purify the final EA product, another stripper is needed with its top vapor recycled back to decanter. The simplest and industrial easily applicable overall control strategy will be investigated with only one tray temperature control loop in each of the two columns. The final proposed overall control strategy of this process is found to be different than another similar coupled reactor/columns process published earlier [I-L. Chien, Y.P. Teng, H.P. Huang, Y.T. Tang, Design and control of an ethyl acetate process: coupled reactor/column configuration, J. Proc. Cont. 15 (2005) 435–449]. Both EtOH and AA feed flow rates are used as manipulated variables in the overall control strategy with CSTR heat duty left as throughput manipulator for the overall process. The final EA product with stringent specifications of 0.1 wt% EtOH and 0.005 wt% AA impurities can be achieved with this proposed overall control strategy despite feed flow rate and feed composition disturbances.     

Model predictive control of reactive distillation for benzene hydrogenation

Benzene hydrogenation via reactive distillation is a process that has been widely adopted in the process industry. However, studies in the open literature on control of this process are rare and seem to indicate that conventional decentralized PI control results in sluggish responses when the reactive distillation column is subjected to disturbances in the feed concentration. In order to overcome this performance limitation, this work investigates model predictive control (MPC) strategies of a reactive distillation column model, which has been implemented in gPROMS. Several MPCs based upon different sets of manipulated and controlled variables are investigated where the remaining variables remain under regular feedback control. Further, MPC controllers with output disturbance correction and, separately, with input disturbance correction have been investigated. The results show that the settling time of the column can be reduced and the closed loop dynamics significantly improved for the system under MPC control compared to a decentralized PI control structure.