A direct method for incorporation of tray-efficiency matrix in simulation of multicomponent separation processes

Both experimental and estimated multicomponent tray efficiencies are known to become unbounded and exhibit bizarre variation from tray to tray in a distillation column even for ideal systems. Besides lacking the physical significance, they pose numerical difficulties in computation of the component tray efficiencies from the point efficiencies whenever the latter assume negative values (Lockett, M. J. (1986). Distillation tray fundamentals. Cambridge: Cambridge University Press). The use of the Stefan–Maxwell approach for estimation of point efficiency leads to a matrix form. Recently, the tray-efficiency matrix for well-mixed, dispersed and plug flow of liquid has been obtained in terms of point-efficiency matrix similar to binary systems. However, there are no method to use directly the tray-efficiency matrix in simulation. Here, we present a method for direct incorporation of the efficiency matrix in the well-known Naphtali–Sandholm method. We have studied the convergence characteristics of the modified and the original methods for distillation, absorption and extraction columns. The results show the modified one is superior to the original method.     

19万t/a醋酸乙烯精馏装置的技术改造

应用计算机模拟了进料热状况对醋酸乙烯精馏系统的影响,并计算了不同回流比下的塔板数、能耗、冷却负荷等参数值,通过改变进料热状况消除塔内恒浓区,优选出最佳精馏方案,即进料气化率20％、同流比为2、理论板数为63。应用复合斜孔塔板代替筛板塔板,对醋酸乙烯精馏塔进行了技术改造。改造后装置生产能力提高至19万t／a,节能20％,产品质量稳定,醋酸乙烯精馏塔运行周期延长至4年。     

Investigation of ethyl acetate reactive distillation process

In this paper, an extensive study of ethyl acetate synthesis by homogeneously catalyzed reactive distillation is presented. Reactive distillation is a promising operation whereby reaction and separation take place within a single distillation column. The synergistic effect of this combination has the potential to increase conversion, improve selectivity and facilitate separation tasks. The feasibility of ethyl acetate synthesis is examined using the reactive distillation lines diagram. A completely rate-based simulator DESIGNER developed within the framework of a large European research project is used in order to predict concentrations, temperatures and other important process variables. In order to validate theoretical predictions, a set of reactive distillation experiments is performed in a glass tray column with 80 bubble cap trays. The concentrations and temperature profiles computed by DESIGNER agree well with the experimental data.     

Design and control of acetic acid dehydration system via heterogeneous azeotropic distillation

Acetic acid dehydration is an important operation in the production of aromatic acid, such as terephthalic acid or in the manufacture of cellulose acetate. Although acetic acid and water does not form azeotrope, but using simple distillation to separate these two components is not practical. The reason is because the system has tangent pinch on the pure water end, thus it is more customary in industry to use an entrainer via a heterogeneous azeotropic distillation column system for the separation. In this study, a suitable entrainer is selected from three candidate acetates through rigorous steady-state simulation of this system. Optimum process design and operating condition are determined to keep high-purity bottom acetic acid composition and also keep a small acetic acid loss through top aqueous draw. Furthermore, the overall control strategy of this column system is proposed to hold both bottom and top product specifications in spite of feed rate and feed composition load disturbances. The proposed overall control strategy is very simple requiring only one tray temperature control loop inside the heterogeneous azeotropic column.     

香茅油间歇精馏塔的工艺计算

对300t/a香茅油间歇精馏塔进行物料衡算,根据物料衡算的结果对精馏塔的操作温度、最小理论板数、理论板数、最小回流比、操作回流比和塔径等进行计算,获得塔体的工艺和设备参数。     

带有侧线采出回流的部分透热精馏的节能优化

以正丙醇-异丙醇体系为例，研究了带有侧线采出回流的部分透热精馏操作。在该操作中，精馏段侧线采出气相，经塔外冷凝后回流至塔内采出板上方；提馏段某塔板被同轴的夹套式中间再沸器环绕，侧线采出该板处的气相回流至塔内采出板上方。通过单因素分析和响应面法对精馏段和提馏段操作的相关工艺参数分别进行了模拟优化，并对相应操作的热力学性能和分离性能的变化进行了分析。最终优化结果表明：达到规定的分离效果，带有侧线采出回流的部分透热精馏相较于绝热精馏有效能损失降低了26.5%。带有侧线采出回流的部分透热精馏操作通过合理分配能量、降低对热剂和冷剂的品位要求和提高能量利用率，最终达到节能目的。     

Understanding process intensification in cyclic distillation systems

The most effective separation possible in distillation columns takes place in the hydrodynamic regime where there is perfect displacement of the liquid and vapor streams. This can be achieved in distillation equipment with separate phase movement (SPM). Such an innovative route for process intensification in distillation is called cyclic distillation. The required process conditions are the lack of liquid outflow from the tray during vapor admission and the lack of liquid mixing in adjacent trays upon outflow of liquid. Remarkably, the throughput of such a column that operates in a controlled cycle mode is two or more times higher than the throughput reachable with conventional operation, at equivalent separation performance.In this study, a theoretical stage model with perfect displacement is proposed and the theory of the process working lines is developed. An adequate mass transfer model is also described along with the mode of calculation of tray columns operating in the cyclic operation mode. Sensitivity analysis was used to determine the effect of the key model parameters. The theoretical developments were implemented at industrial scale and subsequent testing showed an increase in the separation efficiency of 2-3 times as compared to the standard process.     

一种改进的多元精馏塔动态模型

综合了精馏塔的物料及能量平衡、塔板水力学方程、塔板效率及再沸器动态特性,给出一种改进的非理想多元物系精馏塔通用动态数学模型.增加了汽液平衡计算,从而弥补了文献[1]所提出的动态模型的缺陷,提高了其精度和通用性.模型仿真算法采用Gear积分方法,稳定性好,对解刚性比很高的精馏塔微分方程组十分有效.通过对某丙烯腈萃取精馏塔的仿真表明,该动态模型能够较好地反映对象的动态形为.     

Performance and applications of flow-guided sieve trays for distillation of highly viscous mixtures

With a particular focus on the distillation of highly viscous or self-polymerized mixtures, this study reports the hydrodynamic and mass transfer performance of two flow-guided sieve trays, including their pressure drop, entrainment, weeping and tray efficiency, obtained experimentally with an air-water/oxygen system in a Φ600 mm plexiglass column. The results show that the 8 mm hole flow-guided sieve tray tested shows better characteristics than the 7 mm flow-guided hole tray in terms of pressure drop and mass transfer. Then we present practical industrial examples of applications of the flow guided trays for distillation of viscous mixtures, i.e., the separation of vinyl acetate (VAC) from a polyvinyl acetate (PVAC) polymer solution with dynamic viscosity μ=50,000 mPas, the separation of highly unsaturated C5 mixtures by extractive distillation, and the distillation of thick, condensed and highly viscous fermentation mixtures made from fermented mash. It is demonstrated that flow-guided sieve trays with relatively large holes are an excellent candidate for distillation of mixtures with suspended solids, or concentrated/self-polymerized polymer solutions.     

Simulation and analysis of extractive distillation process in a valve tray column using the rate based model

Valve trays are becoming popular in the chemical process industries owing to their flexibility to handle a wide range of vapor throughputs. Using the rigorous rate based model, the importance of the non-equilibrium approach is demonstrated for a typical extractive distillation process in a Glitsch V-1 valve tray column. Simulation results based on an in-house developed code indicated that the rate based model predictions for a valve tray column operation showed significant differences relative to the equilibrium model. Even small errors in product purities translated into nonoptimal feed stage locations and inaccurate number of stages required. The counter-intuitive effect of high reflux ratio on separation is explained.     

Hydrodynamic model for controlled cycling in tray columns

The paper describes a model for controlled cycling in tray columns. Gas phase dynamics and hydrodynamic relations are included in the proposed model. The model has been simulated on a digital computer.Gas phase dynamics is found to play an important role for cyclic operation of distillation columns, while as the effect of gas phase dynamics can be ignored for absorption columns.In a 5-tray distillation column reasonable separation between liquid and vapour flow periods can be achieved, but with increasing number of trays the switch-over from one period to another becomes less ideal. For a ten tray column the flow situation is unacceptable.The simulations also show that the plug flow condition in the liquid flow period is most difficult to satisfy in the top and bottom of the column. The risk of flooding on some trays and other trays running dry increases when the numbers of trays increases.Finally it is shown that the hydrodynamic behaviour may be improved by changing the geometry of some of the trays.     

混合粒子群算法用于共沸精馏塔的最优设计

提出了一种适于求解混合整数非线性规划问题的混合粒子群优化算法,并将其与化工过程模拟软件相结合,用于共沸精馏塔的最优设计。优化模型以年度总费用最小为目标,以精馏段板数、提馏段板数和回流比为优化变量,并引入流体力学约束使得优化结果更具实际价值,并以效率更高的"轮盘赌"式策略处理整数变量,约束处理采用Deb方法。最终以C++实现优化算法,C#编制界面,通过商业模拟软件Aspen Plus计算粒子适应度,将本方法用于一个醋酸甲酯/甲醇/水三元共沸组成的分离案例,所获最优年度总费用优于文献结果。     

基于传热/传质的乙烯裂解过程脱甲烷塔进料瓶颈识别及流程重构策略

精馏塔进料的组成与温度会影响塔内质量交换和能量利用,不恰当的进料会导致全塔的分离及用能效果变差。针对多进料精馏塔的组合进料问题提出一种识别不合适进料位置的方法,基于塔板的传热温差和传热量、传质浓度差和传质量计算方法提出应用传热/传质复合曲线识别精馏塔进料瓶颈的方法,并将其应用于裂解装置脱甲烷塔进料瓶颈的识别,采用调整进料位置的流程重构策略实现去瓶颈的操作。流程模拟及瓶颈分析结果表明所提出的方法能识别出脱甲烷塔的进料瓶颈,重构流程的方法能实现去瓶颈的操作,并使全塔的传质传热特性、分离效果变好,能耗降低。     

Design and control of reactive distillation for hydrolysis of methyl lactate

Esterification of raw lactic acid from fermentation broth and then hydrolysis of lactate ester in reactive distillation column is an effective process for purification of lactic acid. Reactive distillation for hydrolysis of methyl lactate is studied. First, the thermodynamic properties and reaction kinetics are analyzed; then the reactive distillation column is designed based on the objective function total annual cost. The effects of tray number of rectifying section, tray number of reactive section, and feed location on the total annual cost are investigated. And a dual-temperature control structure is proposed for the optimal reactive distillation column, and the results show that it works quite well for this hydrolysis system.     

The second law optimal state of a diabatic binary tray distillation column

A new numerical procedure to minimize the entropy production in diabatic tray distillation columns has been developed. The method was based on a least square regression of the entropy production at each tray. A diabatic column is a column with heat exchangers on all trays. The method was demonstrated on a distillation column separating propylene from propane. The entropy production included contributions from the heat transfer in the heat exchangers and the mass and heat transfer between liquid and vapor inside the distillation column. It was minimized for a number of binary tray distillation columns with fixed heat transfer area, number of trays, and feed stream temperature and composition. For the first time, the areas of heat exchange were used as variables in the optimization. An analytical result is that the entropy production due to heat transfer is proportional to the area of each heat exchanger in the optimal state. For many distillation columns, this is equivalent to a constant driving force for heat transfer. The entropy production was reduced with up to 30% in the cases with large heat transfer area and many trays. In large process facilities, this reduction would ideally lead to 1-2 GWh of saved exergy per year. The most important variable in obtaining these reductions is the total heat transfer area. The investigation was done with a perspective to later include the column as a part in an optimization of a larger process. We found that the entropy production of the column behaved almost as a quadratic function when the composition of the feed stream changed. This means that the feed composition is a natural, easy variable for a second law optimization when the distillation column is a part of a process. The entropy production was insensitive to variations in the feed temperature.     

萃取精馏复合溶剂的复配选择

提出按溶剂之间复配形成氢键的情况和修正的UNIFAC模型相结合的方法选择分离共沸体系的萃取精馏复合溶剂。准确测量了共沸体系与萃取精馏单溶剂和复合溶剂的汽液平衡数据,结果表明采用的萃取精馏单溶剂和复合溶剂的选择方法具有较好的准确度,模拟值与实验值比较,相对偏差均小于9%。通过综合比较得到分离乙酸乙酯-乙醇、乙醇-水和环己烷-苯3个共沸体系的最佳复合溶剂及配比。在最佳复合溶剂组成下,原共沸体系的相对挥发度达到最大值,该值优于组成复合溶剂的单溶剂所达到的分离效果。在最佳复合溶剂组成下,改变原料体系组成,复合溶剂组成对原料体系相对挥发度的影响趋势基本相同,这表明复合溶剂在萃取精馏塔内不同原料组成下均能较好地增加体系的相对挥发度,为工业上在萃取精馏过程中使用复合溶剂提供了理论依据。     

A modified model of computational mass transfer for distillation column

The computational mass transfer (CMT) model is composed of the basic differential mass transfer equation, closing with auxiliary equations, and the appropriate accompanying CFD formulation. In the present modified CMT model, the closing auxiliary equations [Liu, B.T., 2003. Study of a new mass transfer model of CFD and its application on distillation tray. Ph.D. Dissertation, Tianjin University, Tianjin, China; Sun, Z.M., Liu, B.T., Yuan, X.G., Liu, C.J., Yu, K.T., 2005. New turbulent model for computational mass transfer and its application to a commercial-scale distillation column. Industrial and Engineering Chemistry Research 44, 4427-4434] are further simplified for reducing the complication of computation. At the same time, the CFD formulation is also improved for better velocity field prediction. By this complex model, the turbulent mass transfer diffusivity, the three-dimensional velocity/concentration profiles and the efficiency of mass transfer equipment can be predicted simultaneously. To demonstrate the feasibility of the proposed simplified CMT model, simulation was made for distillation column, and the simulated results are compared with the experimental data taken from literatures. The predicted distribution of liquid velocity on a tray and the average mass transfer diffusivity are in reasonable agreement with the reported experimental measurement [Solari, R.B., Bell, R.L., 1986. Fluid flow patterns and velocity distribution on commercial-scale sieve trays. AI.Ch.E. Journal 32, 640-649; Cai, T.J., Chen, G.X., 2004. Liquid back-mixing on distillation trays. Industrial and Engineering Chemistry Research 43, 2590-2597]. In applying the modified model to a commercial scale distillation tray column, the predictions of the concentration at the outlet of each tray and the tray efficiency are satisfactorily confirmed by the published experimental data [Sakata, M., Yanagi, T., 1979. Performance of a commercial scale sieve tray. Institution of Chemical Engineers Symposium Series, vol. 56, pp. 3.2/21-3.2/34]. Furthermore, the validity of the present model is also shown by checking the computed results with a reported pilot-scale tray column [Garcia, J.A., Fair, J.R., 2000. A fundamental model for the prediction of distillation sieve tray efficiency. 1. Database development. Industrial and Engineering Chemistry Research 39, 1809-1817] in the bottom concentration and the overall tray efficiency under different operating conditions. The modified CMT model is expected to be useful in the design and analysis of distillation column.     

间歇蒸馏的优化操作

本文研究了间歇蒸馏操作的最佳经济收益问题.在二元物系的系统模型中考虑了塔板的持液及冷凝器的持液,并考虑了塔板效率.用多项式拟合相平衡关系.选择回流比作为控制函数,应用最优控制理论导出计算步骤,用数值方法叠代出使经济收益函数最大的回流比函数.将解出的最优操作方案与两种传统的操作方案进行了比较.为检验最优操作方案的可实施性,在一筛板塔上进行了实验,证明优于传统操作.     

Simulation and analysis of dehydration distillation column based on distillation mechanism integrated with neural network

In an industrial solvent dehydration distillation column (SDDC) model, the Murphree efficiency represents the separation ability of a distillation tray and the SDDC model’s performance depends on the value accuracy of the Murphree efficiency. Because there are many operation conditions having nonlinear effect on Murphree efficiency, it is difficult to determine its value. To develop a precise and robust SDDC model, a novel hybrid model combining distillation mechanism with neural network is proposed. In the SDDC hybrid model, the neural network is employed to model the nonlinear relationship between the operation conditions and Murphree efficiency, which is embedded into the SDDC mechanistic model. The results showed a good predicting and robust performance of the hybrid model under different operation conditions. Based on the hybrid model, the effect of the operation conditions on SDDC was analyzed to obtain some useful guiding rules for the SDDC operation.     

Process improvement on separating C4 by extractive distillation

C4 mixtures are usually separated by extractive distillation with acetonitrile (ACN) and N,N-dimethylformamide (DMF) methods in industry. These two methods were modified to improve the extractive distillation process in this work. Vapor–liquid equilibrium (VLE) models for the ACN and DMF methods were established with the Wilson equation. In terms of the models the extractive distillation processes were improved by means of a computer simulation software. The improvement to the ACN method was such that liquid load in the second extractive distillation column was significantly decreased and there were few changes on the equipment. Improvement to the DMF method was also made by replacing tray configuration and decreasing the times of butadiene phase changes. Both improvements are closely connected with the C4 practical production and are easily accomplished.