Investigating Control Schemes for an Ideal Thermally Coupled Distillation Column (ITCDIC)

The control schemes of an ITCDIC are addressed. A modified IMC scheme (M‐IMC) is proposed to overcome model/plant mismatch of the Internal Model Control scheme (IMC). Predictive PID control (P‐PID) and Adaptive Predictive control (AP‐PID) schemes are also presented to improve effectively the response speed of the multi‐loop PID control (M‐PID) and eliminate its residual error. A detailed comparative investigation on the above five control schemes was performed. Simulation results demonstrate all the schemes are able to keep two end products within their specifications. M‐IMC is the best one with the fastest response speed. AP‐PID is the second choice since it is better at dealing with sudden set‐point transitions and complex external disturbances than P‐PID. M‐PID cannot compete with AP‐PID and P‐PID due to its slow servo response speed and large residual error. IMC ranks last as it is extremely sensitive to changes in the operating conditions.     

Design Optimization of a Crude Oil Distillation Process

In this work, the optimal design of a crude oil distillation unit was made. Since the method used here is based on the general optimization framework, it is possible to design the process systematically. The results of this study gave better performances than that of the existing process. The process in this study is currently in operation and has the capacity of 150,000 BPSD. Feed locations, heat duties of pumparounds and operating conditions of the preheat train are selected as variables, and the energy consumption, operating cost and annual cost are formulated as objective functions. The optimal feed locations of both the main column and stabilizer are obtained by solving rigorous models and mixed integer nonlinear programming. Considering the results of the optimal feed tray, the cost optimization is carried over to obtain the optimal annual cost, which considers both the capital and operating cost, which is decreased from US$10,649,000/year to USdollar;9,185,230/year.     

Identification and Control of an Industrial Binary Distillation Column: A Case Study

The challenging problem of identification and control of an industrial binary distillation column is addressed in this paper. Process identification represents an alternative to modeling and is shown to be the appropriate procedure for predictive control design. The predictive controllers based on the identified model ensure stability and high performance for a wide operating range of the industrial distillation column.     

A New Configuration for a Fully Thermally Coupled Distillation Column with a Postfractionator and Separated Main Columns

The installation of a postfractionator into a fully thermally coupled distillation column (FTCDC) significantly improves the distillation column efficiency, but the column operation is more difficult than for the original FTCDC. A modified configuration of the postfractionator FTCDC having sectionalized main columns is proposed for operational improvement, and its performance is examined through the HYSYS simulation of a BTX fractionation process. By setting different pressures for the various sections of the main columns, the proposed distillation column facilitates easy vapor transfer between the sections without the need to utilize compressors. The outcome of a dynamic simulation on the column shows that the control of the column pressure is easily undertaken and the specifications of three products in the BTX process can be separately controlled to facilitate ease of column operation.     

A Method for the Design of Divided Wall Columns

A divided wall column has been modeled as a Petlyuk column with no heat transfer across the column wall. The feed to the column has been generalized as a mixture of saturated liquid and vapor (0 <q <1). The pinch compositions at connection points, the Fenske equation and the Gilliland correlations, which have been commonly used by previous investigators to design divided wall columns, have not been applied. In contrast, a shortcut method based on Underwood's equations has been introduced. Moreover, it is shown that the split of the internal reflux over both sides of the middle wall of the column is bounded and a method for choosing the proper value of the split ratio is suggested. This is a novel approach not attempted before by investigators.     

内部热耦合精馏塔基于非理想物系的最优评价

Internal thermally coupled distillation column(ITCDIC) is a frontier in energy saving distillation research. In this paper, the optimal assessment on the energy saving and the operating cost for ITCDIC of nonideal mixture is explored. An evaluating method is proposed, and the pertinent optimization model is then derived. The ethanol-water system is studied as an illustrative example. The optimization results show that the maximum energy saving in ITCDIC process is about 35% and the maximum operating cost saving in ITCDIC process is about 30%,as compared with a conventional distillation column(CDIC) under the minimum reflux ratio operating; the optimal operating pressure of the rectifying section is found to be around 0.25 MPa; the effects of the feed composition,operating pressure and the heat transfer rate on operation are also found and analyzed. It is revealed that ITCDIC process possesses high energy saving potential and promising economical prospect.     

Modeling and Behavior Analyses of Internal Thermally Coupled Air Separation Columns

The internal thermally coupled air separation column (ITCASC) is a frontier of air separation energy savings research. The first principles model of the ITCASC is proposed and is based on the material balance, energy balance, and the related thermal coupling relationship which can be used for further control and design studies. The related solution procedures are given. Detailed behavior analyses are carried out. The research results reveal that the ITCASC process has the perfect separation effect and exhibits different behaviors compared to the conventional air separation column (CASC). The optimization model and the related operation optimization are further explored to gain insight on the energy saving potential of the ITCASC process. Optimization research results show that the ITCASC has about 47 % energy saving potential compared to the CASC process.     

Dividing Wall Distillation Columns: Optimization and Control Properties

The optimal design of dividing wall columns is a non‐linear and multivariable problem, and the objective function used as optimization criterion is generally non‐convex with several local optimums. Considering this fact, in this paper, we studied the design of dividing wall columns using as a design tool, a multi‐objective genetic algorithm with restrictions, written in Matlab^TM and using the process simulator Aspen Plus^TM for the evaluation of the objective function. Numerical performance of this method has been tested in the design of columns with one or two dividing walls and with several mixtures to test the effect of the relative volatilities of the feed mixtures on energy consumption, second law efficiency, total annual cost, and theoretical control properties. In general, the numerical performance shows that this method appears to be robust and suitable for the design of sequences with dividing walls.     

热偶精馏过程的优化计算及控制

建立了TCS热偶精馏过程优化设计的数学模型，编制了逐板计算的计算机程序，根据严格逐板计算的结果，运用差分法求出灵敏度系数，对TCS热偶精馏进行了灵敏度分析，确定灵敏板的位置即找到最优控制点，在此基础上选择最优控制方案，实现TCS热偶精馏塔的最优设计。对苯-甲苯-乙苯物系进行了优化设计，提出了本流程最优控制方案。     

Control of Ideal Heat Integrated Distillation Columns

The operation of an ideal heat integrated dlstillation columan (HIDiC) is addressed .Five Kinds of control configurations, i.e. single-loop control, multi-loop control, multivariable internal model control (IMC), modified multlvariable internal model control (MIMC) and nonlinear process model-based control (NPMC), are designed and applied to the process. Simulation results demonstrate that all of the above control confaguratloam ere valid for product quality control. NPMC control configuration is found to be the best one among all the alternatives. It can readily realize setpoint transitions and conduct effectively against external disturbance. MIMC control configuration ranks second in the row for its regulatory responses to feed composition disturbances with relatively extended setting time, Next comes from the multi-loop control configuration, which is moee or lees handicapped by its greater deviations and overshootings. IMC control configuration can not compete with the multi-loop control configuration because it is extremely sensitive to operating condition changes. Single-ioop control configuration is the worst one among all the mentioned control configurations. Its responses for the uncoutrolled end products are extremely sluggish.     

Eco-efficient Separation of Mono- and Dichloroacetic Acid by Thermally Coupled Extractive Distillation

Due to its high reactivity, monochloroacetic acid (MCA) is an essential intermediate used in the manufacturing of a wide variety of chemicals. Its industrial production process is highly energy- and capital-intensive, requiring costly purification steps to remove dichloroacetic acid (DCA) impurities. Here, an innovative eco-efficient process for MCA-DCA separation is proposed based on extractive distillation in a fully thermally coupled system (a dividing-wall column (DWC) or an equivalent side-rectifier configuration) using a new effective solvent: diethylene glycol dipentylether. The design was optimized by performing sensitivity analyses and varying key operating and design parameters. The study proves that separating MCA-DCA by an extractive DWC (or thermally coupled) is feasible, efficient, controllable, economic, and sustainable.     

Experimental Investigation of Decentralized Process Control for Reactive Dividing-Wall Columns

The reactive dividing-wall column (RDWC) presents a highly integrated process that enables significant reductions in investment costs and energy consumption. However, the high degree of integration of this apparatus causes numerous interactions between kinetics, vapor-liquid equilibrium, and mass transfer. To ensure a reliable operation of the RDWC, suitable control schemes need to be developed and experimentally validated. A decentralized control scheme for the RDWC is presented and for the first time experimentally investigated on an RDWC pilot plant. A comparison of experimental and simulated data is carried out and shows good agreement.     

Offset‐Free Inferential Feedback Control of Distillation Compositions Based on PCR and PLS Models

An offset‐free inferential feedback control strategy for distillation composition control using principal component regression (PCR) and partial least squares (PLS) models is presented in this paper. PCR and PLS model based software sensors are developed from process operational data so that the top and bottom product compositions can be estimated from multiple tray temperature measurements. The PCR and PLS software sensors are then used in the feedback control of the top and bottom product compositions. With this strategy the problem of substantial time delay in composition analyzer based control and of substantial bias in single tray temperature control can be overcome. A practically very important issue in software sensor based feedback control is that static control offsets often exist due to a static estimation bias, especially when the process operating condition changes. A technique for eliminating the static estimation bias and the resulting static control offsets through mean updating of process measurements is proposed in this paper. Applications to a simulated methanol‐water separation column demonstrate the effectiveness of this control strategy.     

Sidestream Analysis and Optimization of Full Tower Internal Thermally Coupled Air Separation Columns

An appropriate sidestream is the key to improve the yields and purities of products in the cryogenic air separation process. The sidestream of a full tower internal thermally coupled air separation column is characterized. Sensitivity analysis indicates that heat duty distribution, purities, and yields of the products are strongly influenced by the withdrawn position and the flow rate of the sidestream. Optimization models are proposed by adopting the minimum flow rate of the low‐purity liquid nitrogen product and with maximum flow rates of the oxygen product or nitrogen product as optimization target. The optimization research allows selecting the optimum operating conditions with different production requirements.     

Adaptive Robust Generic Model Control of High‐Purity Internal Thermally Coupled Distillation Column

The internal thermally coupled distillation column (ITCDIC) is a frontier in energy‐saving distillation research. The process inside a high‐purity ITCDIC is of great nonlinear dynamics, which trouble the conventional control schemes. A multivariable adaptive robust generic model control (ARGMC) is presented to solve the difficulties, where an ARX model is derived and a recursive least squares estimation (RLSE) method is introduced. The benzene/toluene system is studied as an illustrative example. The results of ARGMC are compared with the robust generic model control (RGMC) and traditional PID control in detail. The performances in both servo control and regulatory control confirm the accuracy and validity of ARGMC for the high‐purity ITCDIC process.     

Optimal Designs of Multiple Dividing Wall Columns

Since the optimal design of dividing wall columns (DWC) is a highly nonlinear and multivariable problem, an appropriate solving tool is required. In this paper a multi‐objective genetic algorithm with restrictions is considered to design columns with dividing walls. Also, a methodology is proposed for sizing the DWC. The proposed design methodology allows achieving appropriate designs for columns with two dividing walls. As expected, the physical structures that allow the use of one or two dividing walls are not so different from each other and, as a consequence, the difference in the total annual costs for both systems depends mainly on the energy requirements.     

苯-环己烷萃取精馏过程模拟和优化研究

以UNIFAC方程为物性计算方法,使用化工过程模拟系统PRO/对苯-环己烷萃取精馏分离过程进行模拟和优化研究。采用双塔联用工艺,对萃取剂种类、萃取剂用量、温度、加入位置,原料液进料温度、进料位置,两塔的总理论板数分别进行了优化。结果表明:以糠醛为萃取剂,糠醛用量与原料液用量摩尔比为2.2、加入温度为298.15℃、在塔1的第7块板加入,原料液进料温度为353.85℃、在塔1的第25块板进料,塔1的总理论板数为35块,塔2总理论板数为10块时,分离过程达到最优。此时,塔1的环己烷产品纯度为0.9953,塔2的苯产品纯度为0.9890,循环糠醛纯度为0.9985。     

Modeling and Analysis of Internal Heat Integrated Distillation Columns

A generalized steady-state model is being developed for an internal heat integrated distillation column (IHIDiC). A procedure incorporating the Newton-Rapheon method is devised for solving the model equations. Separation of an ethanol-water binary mixture is simulated and analyzed with the model, Two pinch points are found within the process, making the separation ex-tremely difllcult and expensive. Two sharp fronts in the temp-erature and the composition profiles are being observed. With the introduction of heat integration, satisfactory separation may be obtained in a limited number of stages with lower reflux ratios. Increasing the pressure difference between the rectifylng and the strlpping sectlons, however, would bring about a reduced relative volatility between the two components involved, creating adverse separation performances. It is obvious that optimigation of the IHIDiC is of prime importance.