Investigation of the operability for four-product dividing wall column with two partition walls

For separating some specific four component mixtures into four products, the four-product dividing wall column (FPDWC) with two partition walls can provide the same utility consumption with the extended Petlyuk configuration, although with structure simplicity. However, the reluctance to implement this kind of four product dividing wall column industrially also consists in the two uncontrollable vapor splits associated with it. The vapor split ratios are set at the design stage and might not be the optimal value for changed feed composition, thus minimum energy consumption could not be ensured. In the present work, a sequential iterative optimization approach was initially employed to determine the parameters of cost-effective FPDWC. Then the effect of maintaining the vapor split ratios at their nominal value on the energy penalty was investigated for the FPDWC with two partition walls, in case of feed composition disturbance. The result shows that no more than + 2% above the optimal energy requirements could be ensured for 20% feed composition disturbances, which is encouraging for industrial implementation.     

萃取精馏分离苯/环己烷共沸体系的控制策略

将常规萃取精馏、差压热耦合萃取精馏以及隔壁塔萃取精馏技术应用于以糠醛为萃取剂的苯和环己烷共沸物分离过程。在稳态模型的基础上,利用Aspen Dynamics软件进行控制研究,对三工艺流程提出了若干控制策略。结果表明,对于常规萃取精馏过程,再沸器热负荷与进料量比值控制结构在降低控制过程超调量方面表现出明显优势;对于差压热耦合萃取精馏过程,带有压力-补偿控温策略的方案控制效果更佳;而对于隔壁塔,则选择了无隔板下方气液分离比控制的结构来作为较优的控制策略。     

Design and Control of a Fully Thermally Coupled Distillation Column Modified from a Conventional System

A modified fully thermally coupled distillation column is proposed, with utilization of the existing distillation columns of the conventional system, and its control scheme is suggested here. The proposed distillation system is applied to a benzene‐toluene‐xylene (BTX) separation process, of which the system design and control performance evaluation are conducted using the HYSYS software. The performance of the suggested 3 × 3 control is examined in the set‐point tracking of product specification and the regulation for the changes of feed flow and composition. The pairings of three proportional‐integral control loops are the reflux flow and the specification of overhead product, the prefractionator vapor flow and that of the side product, and the vapor boil‐up rate and that of the bottom product. The multi‐variable controllability using various indices is investigated for the proposed control scheme, and the controllability is compared with that of the cross‐pairing between the control loops of the side and bottom products.     

热泵精馏隔壁塔分离宽沸程物系的模拟

将中间换热和热泵精馏两种精馏节能技术应用到隔壁塔中,提出了带中间换热器的热泵精馏隔壁塔流程,以解决隔壁塔在分离宽沸程物系时出现的塔顶与塔底温差过高而不宜应用热泵精馏的问题。利用精馏塔总复合曲线图,可确定中间产品塔板采出流股的相态,从而得到不同类型的热泵精馏隔壁塔流程。宽沸程物系分离实例的模拟计算结果表明,该类流程在主塔气液相流量较大的情况下具有较高的节能效率。     

Control structure selection of increased-pressure extractive distillation process for DMC-MeOH azeotropic mixture

Producing dimethyl carbonate (DMC) as a green chemical with the desired purity is important in the industry. Although studies on the steady-state design of energy-efficient extractive distillation processes are important for the purification of DMC-methanol (DMC-MeOH) azeotropic mixtures, the dynamic controllability of these processes is also critical in the case of feed condition changes, and it should be investigated carefully. Results of the limited studies in the literature show that changing the operating pressures in extractive distillation processes might have different effects on the dynamic controllability of different systems. Thus, in this study, alternative control strategies are developed for a recently proposed increased-pressure extractive distillation process to separate DMC-MeOH mixture. All control structures are designed using inferential temperature controllers, which have a general acceptance in industrial applications. Effects of different ratio controllers are investigated by evaluating the dynamic responses of control structures for disturbances in feed flowrate and composition. Two metrics including integral absolute error and steady-state deviation of purities are used in the evaluation of alternatives. Results of dynamic simulations show that a control structure including reflux ratio controller is not a suitable strategy for this process. It is demonstrated that a control structure including reflux to feed ratio controller for both distillation columns is necessary for the robust and efficient control of a pressure-increased extractive distillation process. These efficient dynamic results support the economic advantage of increased-pressure extractive distillation process separating DMC-MeOH azeotropic mixtures.     

Feasible separation regions for distillation II: A generalized distillation limit

The boundary of the feasible separation region consists of several curves that are related to specific types of operating modes of the column. Some of these curves create a well‐known product composition multitude, whereas other curves form a generalized distillation limit. The generalized distillation limit demarcates the sloppy splits (i.e., separations in which the composition of at least one product lies inside the composition space) from regions not accessible by distillation and depends on the thermodynamic state of the feed (a mixture of vapor and liquid in equilibrium or saturated vapor/liquid) and column equipment (total/partial condenser and total/partial reboiler). The mathematical equations describing the generalized distillation limit are obtained based on the relationships between the curves (which form the generalized distillation limit) and specific types of operating modes of the column as well as the material balances for the enriching and stripping columns. Furthermore, the vapor and liquid pinch‐point curves, which go through the feed composition point, are not dependent on the thermodynamic state of the feed and column equipment. In addition, an algorithm for determining the generalized distillation limit is obtained. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

Dynamic control analysis of interconnected pressure-swing distillation process with and without heat integration for separating azeotrope

Dynamic controls of pressure-swing distillation with an intermediate connection(PSDIC) process of ethyl acetate and ethanol separation were investigated.The double temperature/composition cascade control structure can perfectly implement effective control when ±20% feed disturbances were introduced.This control structure did not require the control of the flowrate of the side stream.The dynamic controllability of PSDIC with partial heat integration(PHIPSDIC) was also explored.The improved control structure can effectively control ±20% feed disturbances.However,in industrial production,simple controller,sensitive and easy to operate,is the optimal target.To avoid the use of component controllers or complex control structure,the original product purities could be maintained using the basic control structure for the PSDIC process if the product purities in steady state were properly increased,albeit by incurring a slight rise in the total annual cost(TAC).This alternative method without a composition controller combined with the energy-saving PSDIC process provides a simple and effective control scheme in industrial production.     

Energy management in multi stage evaporator through a steady and dynamic state analysis

Increasing energy demand, high cost of energy and global warming issues across the globe require energy-intensive industries, such as paper mills to improve energy efficiency. Multi-stage evaporators used to concentrate the black liquor in such mills form its most energy consuming unit and require a strong understanding of steady and unsteady state behavior to ensure energy savings. The modeling of nonlinear heptads’ effect system yielded a set of complex nonlinear algebraic and differential equations that are analyzed using Interior-point method and state space representation. Dynamic response of product concentration and system vapor temperatures along with system stability and controllability have been explored by disturbing the flow rate, concentration and temperature of feed, and fresh steam flow rate. Simulations predict that steam flow rate, feed flow rate and its concentration invariably are major controlling factors (in decreasing order) of vapor temperature and product concentration. The interactive behavior between different effects translates into slower responses of the effects with increasing separation from disturbance source. This steady state and transient study opens many new explanations to this relatively less explored area and helps to propose and implement industrial PID controllers to reduce steam consumption and control product quality.     

甲醇-苯共沸体系变压精馏分离工艺的动态控制

基于甲醇和苯共沸体系的压敏性,利用Aspen Plus和Aspen Dynamics软件对变压精馏分离该体系的稳态工艺进行了模拟和优化,研究了该工艺的动态特性,提出了控制产品纯度的3种控制结构：基础控制结构、比例控制结构和双比例与温度?组分联合控制结构,通过对控制结构添加±20%的组分和流量干扰测试控制结构的稳定性. 结果表明,基础控制结构基本能实现稳健控制,但不能解决组分干扰引起的产品纯度偏差过大等问题;比例控制结构可实现相对稳健的控制,但改进效果不显著;双比例与温度?组分联合控制结构在受到20%进料和组分干扰后,产品纯度能较快恢复至设定值的99.90%,实现稳健控制.     

不同热集成变压精馏工艺分离乙酸乙酯/正己烷共沸物的优化与控制

基于变压精馏分离乙酸乙酯/正己烷共沸体系两塔的温差,利用Aspen Plus软件,以年度总成本最小为目标函数,对部分及完全热集成变压精馏工艺进行了稳态模拟及优化。在此基础上,利用Aspen Dynamics软件开发了多种控制结构,通过引入不同进料流量及组成的扰动测试控制结构的有效性。结果表明,完全热集成变压精馏工艺比部分热集成变压精馏工艺的经济性稍好。动态响应结果表明,部分热集成变压精馏工艺的压力?补偿温度控制结构可有效处理不同程度的干扰,能有效提高控制结构对干扰的响应速度,缩短达到新稳态的时间,保证乙酸乙酯和正己烷产品纯度在99.9wt%之上;而完全热集成变压精馏工艺的组分?温度串级控制结构仅能处理较小的组分和流量干扰,实现稳健控制,无法处理较大的干扰。综合比较两种工艺的经济性和可控性,认为部分热集成变压精馏工艺分离乙酸乙酯/正己烷共沸体系优于完全热集成变压精馏工艺。     

Kaibel隔壁塔用于四组分精馏的模拟优化和实验研究

隔壁精馏塔由于其特殊的结构可在单塔内实现多组分高纯度分离的目的。本文针对Kaibel隔壁精馏塔（KDWC）分离四组分混合物的节能工艺进行了模拟优化和实验研究。以甲醇、乙醇、正丙醇和正丁醇（MEPB）为例,通过热力学分析建立了稳态模拟的“四塔模型”,并以塔内温度分布为依据对模型准确性进行了实验验证。提出了一种基于再沸器能耗的优化流程,以再沸器最小能耗为目标函数,对KDWC的液相分配比（RL）及整体结构进行了优化。分析了KDWC的节能原理并考察了中间组分含量对KDWC节能效果的影响。对比了KDWC与常规传统三塔序列的能耗并对二者的热力学效率进行了计算。结果表明：温度分布的模拟值与实验值趋于一致,且液相分配比（RL）是塔的重要操作参数;KDWC结构相比于传统三塔序列节能的重要原因是有效降低了中间组分（乙醇和正丙醇）的返混程度,且随着中间组分含量的增加KDWC节能效果越来越明显;当中间组分摩尔分数为80%时,KDWC可节能35.65%,可提高热力学效率26.11%。通过本文研究,为隔壁塔用于四组分精馏提供了基础实验数据并为其节能优化提供了理论指导。     

Temperature control of an ideal heat-integrated distillation column (HIDiC)

In this work, a novel temperature control scheme is derived for an ideal heat-integrated distillation column (ideal HIDiC), in which a temperature difference between two stages is designated as the controlled variable to circumvent the side-effect of continuous pressure variations in the rectifying section. For making an appropriate compensation to the changes in operating conditions, an inferential signal extracted from the feed stage, n/2+1, is used to adjust the set-point of the temperature difference controller. Control of two ideal HIDiCs, separating, respectively, a binary mixture of benzene and toluene and an ideal ternary mixture of hypothetical components A-C is studied. It is demonstrated that the proposed temperature control scheme can retain a stable operation around the vicinity of the nominal steady state with improved dynamic performance and tolerable steady state discrepancies in comparison with the direct composition control scheme. Moreover, the proposed temperature control scheme is also found to be quite robust to the selection of temperature measurements.     

Catalytic SO2 oxidation in a periodically operated trickle-bed reactor

Centaur^TM carbon (Calgon Carbon Corp.) was used as a catalyst for SO₂ oxidation in a trickle bed under periodic interruption of its liquid flow for cycle periods from 15 to 60 min, cycle splits (split being the proportion of a cycle in which liquid flows) from 0.0083 to 0.2, and feed temperatures about 80°C. SO₂ removal in the trickle bed was improved at longer periods. At a fixed period of 15 min, a maximum occurred with split. At high splits, platinized and unplatinized carbons performed indistinguishably. Results suggested kinetic rate control at low splits and oxygen mass transport control at high splits.     

Control structure comparison for three-product Petlyuk column

The focus of this paper is to investigate different control structures (single-loop PI control) for a dividing wall (Petlyuk) column for separating ethanol, n-propanol and n-butanol. Four control structures are studied. All the results are simulations based on Aspen Plus. Control structure 1 (CS1) is stabilizing control structure with only temperature controllers. CS2, CS3 and CS4, containing also composition controllers, are introduced to reduce the steady state composition deviations. CS2 adds a distillate composition controller (CCDB) on top of CS1. CS3 is much more complicated with three temperature-composition cascade controllers and in addition a selector to the reboiler duty to control the maximum controller output of light impurity composition control in side stream and bottom impurity control in the prefractionator. CS4 adds another high selector to control the light impurity in the sidestream. Surprisingly, when considering the dynamic and even steady state performance of the proposed control structures, CS1 proves to be the best control structure to handle feed disturbances inserted into the three-product Petlyuk column.     

Composition control and temperature inferential control of dividing wall column based on model predictive control and PI strategies

The dividing wall column (DWC) is considered as a major breakthrough in distillation technology and has good prospect of industrialization. Model predictive control (MPC) is an advanced control strategy that has acquired extensive applications in various industries. In this study, MPC is applied to the process for separating ethanol, n-propanol, and n-butanol ternary mixture in a fully thermally coupled DWC. Both composition control and temperature inferential control are considered. The multiobjective genetic algorithm function “gamultiobj” in Matlab is used for the weight tuning of MPC. Comparisons are made between the control performances of MPC and PI strategies. Simulation results show that although both MPC and PI schemes can stabilize the DWC in case of feed disturbances, MPC generally behaves better than the PI strategy for both composition control and temperature inferential control, resulting in a more stable and superior performance with lower values of integral of squared error (ISE).     

用于分离三氯氢硅合成过程中主副产品的隔壁塔模拟

以三氯氢硅合成过程中得到的主副产品混合物二氯二氢硅-三氯氢硅-四氯化硅为分离物系,提出采用隔壁塔代替常规精馏序列分离的新工艺.利用Aspen Plus软件对隔壁塔进行模拟,考察回流比、隔板位置、进料位置、侧线采出位置、液相分配比以及气相分配比对塔顶、侧线以及塔釜产品摩尔分数的影响,得到隔壁塔的最佳工艺参数,并通过模拟比较隔壁塔与常规精馏序列分离此混合物的能耗情况.模拟结果表明:当回流比为6、隔板位置为主塔的第8块板和第24块板、进料位置为预分馏塔的第10块板、侧线采出位置为主塔的第15块板、液相分配比为0.21、气相分配比为0.5时,隔壁塔的分离效果最佳,主产品三氯氢硅的摩尔分数为99.999%;相比于常规精馏序列,隔壁塔再沸器节能29.09%以上,冷凝器节能29.48%以上.     

Effects of Thermal Feed Quality on the Performance of a Divided Wall Distillation Column

The present work deals with the effect of thermal feed quality on the performance of a divided wall distillation column (DWC). The thermal condition of the feed alters the pressure profile across the two sides of the dividing wall, thereby affecting not only the mass transfer characteristics but also the hydrodynamics of a DWC. It was observed that the natural (feasible) vapor split ratio does not depend on the liquid split ratio and the reflux flow rate when the feed is saturated liquid or sub-cooled liquid (q ≥ 1). However, for q < 1, that is, for two phase (vapor-liquid), saturated vapor, or superheated vapor feed, the liquid split ratio and the reflux flow rate have profound effect on the feasible vapor split ratio, and the pressure profiles are altered significantly. For the stable operation of a DWC, the feed should be either saturated liquid or sub-cooled liquid or the feed quality may be manipulated to adjust the vapor split ratio.     

Design and control of acetonitrile/N-propanol separation system via extractive distillation using N-methyl pyrrolidone as entrainer

The extractive distillation process for separating a binary azeotrope of acetonitrile and n-propanol is investigated using N-methyl pyrrolidone as entrainer. On the basis of multiparameter optimization, a design with optimized operation conditions for the process is presented. Two dynamic control structures are proposed. Feed disturbances are used to evaluate the dynamic performance. The response results reveal that the control structure CS1 with temperature–composition cascade control loop can handle all the disturbances and maintain product purities, except one large transient deviation, while the improved control structure CS2 with average temperature control loop can overcome this obstacle and handle disturbances effectively.     

Reducing total annualized cost and CO2 emissions in batch distillation: Dynamics and control

In this contribution, the direct vapor recompression approach is introduced in a batch distillation operated at an unsteady state condition. This vapor recompressed batch distillation (VRBD) accompanies an isentropic compressor that runs at a fixed as well as variable speed. Aiming to ensure the optimal use of internal heat source, an open‐loop control policy is proposed for the VRBD that adjusts either the overhead vapor splitting or the external heat supply to the reboiler. Again, the variable speed VRBD additionally involves the manipulation of compression ratio. Developing two alternative configurations of VRBD column, the best heat integrated scheme is attempted to identify in the aspects of energy efficiency and total annualized cost for further advancement. A closed‐loop control algorithm for the best performing variable speed VRBD aiming to meet the end objective of relatively high‐purity product discharged at a constant composition is developed. The separation of a reactive system is considered to illustrate these results and demonstrate the effectiveness of the novel VRBD scheme. Performing simulation tests, it is investigated that the closed‐loop control operation substantially improves not only the distillate purity but also the total amount of product. Achieving significant improvement in thermodynamic efficiency and cost by the controlled heat integrated scheme over its conventional counterpart, finally the attractiveness of the VRBD column by investigating its potential to reduce the greenhouse gas (i.e., CO₂) emissions is shown. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2821–2832, 2013     

EXPERIMENTAL EVALUATION OF THE INFERENTIAL SYSTEM FOR DISTILLATION CONTROL

An inferential control scheme has been applied to control the bottoms—product composition of a laboratory-scale n-propanol-methanol, distillation column. The closed-loop performance of the inferential control system is compared with the conventional proportional-integral control system. The application of inferential control resulted in improved response for disturbances in feed composition and feed flow rate.