Optimal two point control of a binary distillation column

Optimal two point control and operating policies have been developed for a semi-works scale 10 tray distillation column separating methanol and water. Two controllers, based on different control philosophies were synthesized. They are:

• 1 The controller that will bring the product compositions to their desired values while keeping off-specification products to a minimum.
• 2 The controller that will bring the product compositions to their desired values while maintaining the average product composition of the control period at the desired level.

Two types of disturbances were studied, namely disturbances in the feed stream and changes in product specifications. The column operation was represented by a linearized dynamic mathematical model based on a component material balance. The solution of the problem was obtained by application of the minimum principle of Fontryagin and the modified Riccati transformation technique. Simplified optimal control schemes can be developed from the results of this work. These control schemes are simple enough to be used for direct digital computer control.     

无累积罐循环全回流间歇精馏三温控制操作

提出了通过塔顶、塔中上以及塔中3个温度控制进行操作状态转换的无累积罐循环全回流间歇精馏控制方法,并以理想物系--乙醇-正丙醇混合物为分离物系进行了实验研究。通过实验确定了温度控制条件为当塔顶温度稳定后且塔顶和塔中上温差为0.3 ℃时变全回流为全采出操作,当塔中温度升高1.0 ℃时停止全采出转为全回流操作。同时还考察了在不同乙醇投料浓度条件下这种操作的运行情况,发现不同投料浓度对塔顶产品平均纯度的影响并不显著,基本能保持在0.99。最后在相同的投料浓度和操作条件下,对比了这种新型控制方式和双温度控制方式,结果表明三温度控制方式比双温度控制方式操作时间减少了23.3 min,分离效率提高了23.95%,产品浓度提高了1.06%,产品收率提高了1.08%。     

Optimization and control of a reactive distillation process for the synthesis of dimethyl carbonate

Dimethyl carbonate is an environmentally benign and biodegradable chemical. Based on integration of reactive distillation and pressure-swing distillation technologies, a novel process for synthesis of dimethyl carbonate through transesterification with propylene carbonate and methanol has been developed by Huang et al. In this work, the optimization of this process was performed by minimizing the total TAC. The results show that the op-timal design flowsheet can save energy consumption by 18.6％with the propylene carbonate conversion of 99.9％. Then, an effective plant-wide control structure for the process was developed. Dynamic simulation results dem-onstrate that the temperature/flow rate cascade control plus with simple temperature control can keep not only product purity but also the conversion of the reactant at their desired values in the face of the disturbance in re-actant feed flow rate and feed composition.     

Calculation of the expected performance of reverse osmosis plants

The designer of a reverse osmosis plant must forecast the brine and product stream concentrations from the membrane salt rejection and the ratio of product to feed flows (product recovery). this paper reviews the derivation of equations which predict these concentrations, and furthermore, yield an average feed stream concentration between feed and brine which allows the forecasting of plant productivity at high recoveries. The derived equations are contrasted with other less accurate equations which have been used in the industry.     

Kaibel分壁精馏塔的压力补偿-温度控制

Kaibel分壁精馏塔（Kaibel divided-wall column,KDWC）是分离三组分混合物DWC的进一步强化形式,可在一个塔内实现四组分混合物的高纯度分离。建立有效的控制结构是KDWC的研究关键,研究了KDWC分离苯、甲苯、邻二甲苯和均三甲苯四组分体系的动态过程,首次将压力补偿机制引入KDWC的控制,建立了压力补偿-温度控制结构（PTC）,并考察了该结构与温度控制结构（TC）的控制效果。研究结果表明：TC可实现进料流量或组分发生±15%扰动后的平稳控制,少部分产品纯度不合格,最大余差为-0.007;PTC可实现进料流量或组分发生±20%扰动后的平稳控制,产品纯度均合格,最大余差约为-0.003,且调节时间缩短为5~8 h。压力补偿-温度控制结构通过对下侧线温度控制回路设定值的修正,有效地改善了控制效果。     

Hollow fiber reverse osmosis: experiments and analysis of radial flow systems

The performance of a hollow fiber reverse osmosis system was determined experimentally by measuring the fraction of feed recovered as product, φ, and the concentration reduction ratio, θ_p. A predictive model for these two quantities was developed which yields results that agree with the experimental data very well for applied pressures of 300–450 psig., feed rates from 75 to 525 cc/sec and feed concentrations up to 15000 ppm of sodium chloride.The concept of distributed sinks in a continuum is used for the shell side, and a diffusion model is used to describe solute transport across the fiber wall. The principal assumptions made are negligible concentration polarization, small radial pressure drop in the shell and small solute transport, compared with solvent transport, through the fiber. Simple closed form expressions for φ and θ_p, given by eqns (43a) and (51), were obtained which are accurate over the range of experimental conditions employed.     

Towards predictive simulation of single feed semibatch reaction crystallization

A population balance model is developed over single-feed semi-batch reaction crystallization of benzoic acid. The model is evaluated by comparison with experimental data, and simulations are carried out to advance the understanding of the process. The model accounts for chemical reaction, micro and mesomixing, primary nucleation, crystal growth and growth rate dispersion (GRD). Two mechanistic mixing models are evaluated: the segregated feed model and the engulfment model (E-model) with mesomixing. When the mixing is described by the E-model (engulfment model) and GRD is accounted for, the model quite well captures the influence of reactant concentrations, agitation rate, feed point location, feed pipe diameter, total feeding time and crystallizer volume, on the product weight mean size. When using the SF-model (segregated feed model) the results are less satisfactory. The kinetics of nucleation and crystal growth have a great impact on the results of the simulations, influencing the product weight mean size as well as the response to changes in the processing conditions. A new set of kinetic data for benzoic acid derived from semi-batch experimental results are presented.     

Design and Control of Extraction Distillation for Dehydration of Tetrahydrofuran

Design and control of an extractive distillation system for tetrahydrofuran (THF) dehydration with ethylene glycol as entrainer is investigated. The main module is a two‐column system containing an extractive distillation column, whose top stream is the desired product THF, and an entrainer recovery column. Economic analysis with total annual cost as the objective function is developed. Two kinds of control strategies are explored for THF dehydration. The responses reveal that the control structure with fixed reflux ratio cannot maintain the bottom liquid level of the entrainer recovery column, while the control scheme with fixed reboiler heat duty/feed flow ratio exhibits good control performance in spite of large deviations in feed flow rate and feed composition.     

乙酸戊酯酯化反应精馏过程系统控制模拟及分析

反应精馏技术是将反应和精馏两个单元操作耦合在一个设备中同时进行,其非线性给反应精馏过程的平稳操作和过程控制提出了更高的要求。目前,反应精馏过程控制研究都是针对特定反应体系进行的有针对性的研究,对于乙酸戊酯反应体系的控制研究少有报道。本文利用Aspen Plus和Aspen Dynamic 软件对乙酸戊酯的反应精馏过程进行动态模拟,针对塔釜产品组成要求提出温度控制和组成控制两种不同的控制方案,对两个控制系统分别进行±10%的进料流量扰动测试和酸醇比为1:1、1.2:1的进料组成扰动测试。研究了不同扰动对产品质量的影响并比较了两种方案在不同扰动干扰下的控制效果。结果表明:进料组成扰动会对两个控制方案造成较大影响;温度控制方案的控制稳定性要好于组成控制方案。     

乳酸提纯新工艺动态建模仿真和控制系统设计

基于非平衡级和拟均相假设,建立了乳酸提纯反应精馏新工艺实验装置的动态机理模型.通过改进的数值计算方法提高了模型的求解效率,设计并实现了包含物性估算系统的模型仿真平台,以促进新工艺的工业化应用.利用仿真平台对新工艺装置进行了动态特性分析,在此基础上设计了两种单端质量控制方案:直接物料平衡和间接物料平衡方案.在不同类型和幅度的过程扰动下,分析比较了两种控制方案的调节性能.结果表明直接物料平衡方案控制品质优于间接物料平衡方案,可在不同扰动情况下满足过程的产品质量和转化率的联合控制要求.非平衡级动态机理模型能够反映反应精馏过程的动态特性,分析发现反应精馏过程有着独特的过程特性,基于机理模型的仿真平台是分析反应精馏特性的有效工具.     

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。     

Controllability and energy efficiency of a high-purity divided wall column

The divided wall column system is a promising energy-saving alternative for separating multi-component mixtures. However, high energy efficiency and stable operations can only be achieved with careful design of steady state operation and control scheme. In this study, the effects of liquid split and vapor split ratios on the energy efficiency and controllability of a divided wall column system for separating ethanol, n-propanol, and n-butanol were investigated. A region with high energy efficiency was identified. However, relative gain analysis found that the performance of multi-loops composition control would be very poor. Dynamic tests showed that multi-loop temperature control cannot return the product compositions to the desired values in case of feed composition disturbances. Outside this region, composition control can compensate for external disturbances such as feed flow rate and feed composition changes but not changes in operating region caused by internal variations such as liquid and vapor splits. Offsets in the product purities were found if temperature controls were used and there are disturbances in feed composition or changes in operating region caused by upsets in liquid and vapor splits. There is a trade-off between energy efficiency and controllability. A composition + temperature cascade scheme was proposed to stabilize the operation with high energy efficiency. The proposed scheme was able to maintain high product purity and reject external disturbances in feed flow and composition changes as well as internal disturbances such as changes in liquid and vapor splits.     

熔融悬浮结晶法提纯湿法磷酸

使用熔融悬浮结晶法分离提纯湿法磷酸,研究了全回流稳态操作条件下重力洗涤塔内磷酸的纯化过程,考察了原料质量分数、洗涤塔中搅拌转速等操作条件对塔中产品纯度、杂质质量分数沿塔高分布的影响,并用发汗提纯模型进行了拟合。研究结果表明:采用的悬浮结晶装置对湿法磷酸有良好的分离提纯效果;结晶器进料质量分数、洗涤塔搅拌速率对熔融液中杂质质量分数沿塔高的分布有很大影响,进料质量分数增加,则产品纯度增加,适度的搅拌有助于保证洗涤塔的纯化效率,而过高的搅拌速率对分离提纯效果影响不大。在结晶器H3PO4进料质量分数为84%—86%、洗涤塔搅拌速率为10—20 r/min范围内,产品磷酸纯度可达到食品添加剂的标准,发汗提纯模型能很好地描述稳态操作时塔内的杂质质量分数分布规律,在半对数坐标中,杂质质量分数沿塔高呈线性分布。     

Deep desulfurization of full range and low boiling diesel streams from Kuwait Lower Fars heavy crude

Information on feed quality and, in particular, various types of sulfur compounds present in the diesel (gas oil) fractions produced form different crudes and their HDS reactivities under different operating conditions are of a great value for the optimization and economics of the deep HDS process. This paper deals with deep desulfurization of gas oils obtained from a new heavy Kuwaiti crude, namely, Lower Fars (LF) which will be processed in the future at Kuwaiti refineries. Comparative studies were carried out to examine the extent of deep HDS, and the quality of diesel product using two gas oil feeds with different boiling ranges. The results revealed that the full range diesel feed stream produced from the LF crude was very difficult to desulfurize due to its low quality caused by high aromatics content (low feed saturation) together with the presence of high concentrations of organic nitrogen compounds and sterically hindered alkyl DBTs. The low-boiling range gas oil showed better desulfurization compared with the full range gas oil, however, deep desulfurization to 50 ppm sulfur was not achieved even at a temperature as high as 380 °C for both feeds. The desulfurized diesel product from the low-boiling gas-oil feed was better in quality with respect to the S, N and PNA contents and cetane index than the full-range gas-oil feed.     

通过调整进出料热状况消除隔离壁精馏塔的黑洞

隔离壁精馏塔在同时控制4个质量指标（塔顶、侧线、塔底产品浓度以及侧线采出杂质比）时会产生设计与操作黑洞,黑洞的产生对隔离壁精馏塔的应用范围带来很大的限制。本文提出了一种通过调整进料和侧线采出的热状况对隔离壁精馏塔的黑洞进行填补的策略。这两个操作变量通过对全塔热平衡的影响,改善预分离塔与主塔之间的耦合关系,进而改善两者分离能力并最终消除黑洞,同时改善了隔离壁精馏塔的动态特性。针对进出料热状况的调整,本文结合牛顿法给出了简单的调整规则,调整会形成3种不同的方案,为不同能量供应条件下填补隔离壁精馏塔的黑洞提供了可能。通过使用Aspen Plus模拟理想三元物系分离,对黑洞填补方法进行稳态分析和动态响应验证,证明采用调整进出料热状况的方法可以有效的填补隔离壁精馏塔的设计与操作黑洞并使其动态特性得到改善。本文所提出的方法完善了填补黑洞的方法体系,增加了隔离壁精馏塔的灵活性与适应性。     

Cycling Zone Adsorption: Variable-Feed Mode of Operation

Abstract

Relatively large separations are possible using a single cycling zone adsorption column when operated with product recycle. Such a recycle may cause a large cyclic step-change in feed concentration, resulting in significant differences in alternating product stream concentrations. It is concluded that such a “variable-feed mode of operation” may allow the separation of compounds whose solid—fluid equilibrium distribution coefficients are only slightly affected by changes in a thermodynamic potential, or compounds which are difficult to separate by classical techniques such as distillation or extraction. It will also be apparent that the “variable-feed mode of operation” of a single zone (column) produces separations that could only be previously developed by a number of constant feed concentration zones operating in a series. Theoretical and experimental studies show that longitudinal diffusion is the primary cause of effluent concentration profile distortion, limiting the duration of peak product concentration flow.     

Reducing the energy demand of continuous distillation processes by optimal controlled forced periodic operation

In a continuous distillation process, it is generally believed that steady-state conditions represent the optimal operation mode from the viewpoint of required energy supply and that any disturbance of steady-state conditions (e.g. an oscillating profile of reflux ratio or reboiler heat rate) will cause a higher energy demand if the product specifications (expressed as average values) have to be fulfilled. However, this assumption has been disproven by some theoretical and experimental studies in the past, where the distillation process was studied under unsteady-state conditions, to reduce the energy demand. In these studies, both the reflux and the vapor flow rates have been controlled periodically. In the present paper, dynamic optimization methods are applied to find the optimal profiles for all possible control variables (flow rates of feed, products, reflux and vapor). Two examples are discussed. In the first example, an ideal ternary mixture is separated into two fractions, and in the second example, the same mixture is separated into three fractions using a column with a sidestream. For both examples, it is demonstrated that appropriate control profiles for the main control variables can reduce the required energy supply compared with steady-state conditions. The results were obtained on the basis of a simple mathematical model of a distillation column with the assumption of constant relative volatilities and equimolar evaporation and condensation. To generate the control profiles, a new software package for the solution of large-scale optimal control problems was used. The optimal control profiles represent a decoupling of feed supply, removal of distillate product, removal of bottom product, and, in the second example, withdrawing of the sidestream.     

Prediction of sulphur content in the industrial hydrotreatment process

The artificial neural network models were developed to determine sulphur content in the hydrotreatment product. Two models for two different types of feed were developed: light gas oil and vacuum gas oil. The developed ANN models use 6 input variables that are continuously measured in the process and are in accordance with the engineering knowledge and thermodynamics of hydrotreatment processes. Given models show good predictability of sulphur content in the hydrotreatment product and are, therefore, used in practice for continuous monitoring and optimization. This kind of application can be easily developed in any other hydrotreatment process with the available adequate historical data.     

Effect of choline and acetate substrates on bifurcation and chaotic behavior of acetylcholine neurocycle and Alzheimer's and Parkinson's diseases

In this work a novel two-enzyme-two-compartment model is developed in order to explore the dynamics, bifurcation, and chaotic characteristics of the acetylcholine neurocycle. The model takes into consideration the physiological events of the choline uptake into the presynaptic neuron and choline release in the postsynaptic neuron. The effects of feed choline concentrations, feed acetate concentrations as bifurcation parameters are studied. It was found that feed choline concentrations play an important role and have a direct effect on the acetylcholine neurocycle through a certain important range of parameters. The feed acetate concentrations have less effect. A detailed bifurcation analysis over a wide range of parameters is carried out in order to uncover some important features of the system, such as static bifurcation, dynamic bifurcation and chaotic behavior. These findings are related to the real phenomena occurring in the neurons, like periodic stimulation of neural cells and non-regular functioning of acetylcholine receptors. The results are compared to the results of physiological experiments and other published models. As there is strong evidence that cholinergic brain diseases like Alzheimer's disease and Parkinson's disease are related to the concentration of acetylcholine, the present findings are useful for uncovering some of the characteristics of these diseases and encouraging more well-directed physiological research coupled to useful mathematical modeling. It is concluded from the results in this paper that feed choline is more important factor than feed acetate in ACh processes.     

Spray Drying of Mannitol as a Drug Carrier—The Impact of Process Parameters on Product Properties

Powders intended for the use in dry powder inhalers have to fulfill specific product properties, which must be closely controlled in order to ensure reproducible and efficient dosing. Spray drying is an ideal technique for the preparation of such powders for several reasons. The aim of this work was to investigate the influence of spray-drying process parameters on relevant product properties, namely, surface topography, size, breaking strength, and polymorphism of mannitol carrier particles intended for the use in dry powder inhalers. In order to address this question, a full-factorial design with four factors at two levels was used. The four factors were feed concentration (10 and 20% [w/w]), gas heater temperature (170 and 190°C), feed rate (10 and 20 L/h), and atomizer rotation speed (6,300 and 8,100 rpm). The liquid spray was carefully analyzed to better understand the dependence of the particle size of the final product on the former droplet size. High gas heater temperatures and low feed rates, corresponding to high outlet temperatures of the dryer (96–98°C), led to smoother particles with surfaces consisting of smaller crystals compared to those achieved at low outlet temperatures (74–75°C), due to lower gas heater temperatures and higher feed rates. A high solution concentration of the feed also resulted in the formation of comparably rougher surfaces than a low feed concentration. Spray-dried particles showed a volume-weighted mean particle size of 71.4–90.0 µm and narrow particle size distributions. The mean particle size was influenced by the atomizer rotation speed and feed concentration. Higher rotation speeds and lower feed concentrations resulted in smaller particles. Breaking strength of the dried particles was significantly influenced by gas heater temperature and feed rate. High gas heater temperatures increased the breaking strength, whereas high feed rates decreased it. No influence of the process parameters on the polymorphism was observed. All products were crystalline, consisting of at least 96.9% of mannitol crystal modification I.