Research and application progress of catalytic distillation technology

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Hebei Chemical Industry Volume 30 Issue 8 August 2007 Research and Application Progress of Catalytic Distillation Technology Zhu Fuyong Liu Yuejin Huang Jianping Li Yibin Xie Zhongquan Li Zonghui Liu Yongjie (1. School of Chemical Engineering, Xiangtan University, Xiangtan 411105, Hunan； 2． Hunan Xiangwei Co., Ltd., Xiangtan 411105, Hunan) reviewed the catalytic distillation and its catalyst loading technology, simulation research and application, and discussed the existing problems and future development directions in the current research on catalytic distillation technology. two-column exchange method: Ion exchange: Lysine ； Prewashing TQ 032 A Catalytic distillation is to pack solid catalyst in the distillation tower in an appropriate form, so that the catalytic reaction and distillation separation can be carried out continuously in the same tower. It is a new process that strengthens reaction and separation by coupling separation and reaction. Since the catalyst is fixed in the distillation tower, it plays a role in catalyzing and promoting gas-liquid heat and mass transfer. Compared with the traditional process of separate reactions and separations, catalytic distillation has the characteristics of low investment, low operating costs, energy saving, and high yield. It has attracted increasing attention, and its research and application are becoming increasingly widespread. 1 Catalytic distillation catalyst filling technology Since the catalyst plays a role in catalyzing and promoting gas-liquid heat and mass transfer during the catalytic distillation process, the catalyst structure is not only required to have a high catalytic efficiency, but also to have a good separation effect. At present, the catalysts used for catalytic distillation are mainly ion exchange resins and molecular sieves. The catalyst must adopt a special filling method to meet the basic requirements of reaction and distillation. Xiao Jian et al. divided catalyst filling methods into two categories, namely fixed bed type and structured packing type. Both types of filling methods have successful application examples. The structured packing type catalyst filling method is more suitable for distillation operations. It has good gas-liquid contact, does not require special components in the tower, and has a high catalyst utilization rate. However, it is difficult to replace the catalyst in this filling method, and it needs to be manually replaced by entering the tower after shutting down. The new catalytic distillation packing used by Sulzer is Katpak type packing, and there are research reports on fluid mechanics performance testing and hot mold experiments. The reaction section adopts a special catalyst filling method, generally there are three methods: (1) Mixing granular catalysts and inert fillers. The advantage of this method is that the catalyst is easy to load and unload. However, the accumulation of fine particle catalysts in the tower results in excessive rising steam resistance. (2) Place the granular catalyst in a porous container to form a catalyst component. The porous container can be a braided fabric such as nylon wire, or a wire mesh made of aluminum, stainless steel, or other materials. This kind of catalyst component must be connected with the elastic component to form a catalytic distillation element and have a large opening space. This placement method is widely used, but because the catalyst is placed in a porous container, diffusion has a certain impact on the reaction, and the components are complex. (3) The catalyst particles are placed in the interlayer of metal corrugated wire mesh or flat wire mesh and the interlayer of porous plate frames. This placement method has good mass transfer effect, but it is troublesome to load and unload. Plate towers generally have two ways of loading catalysts: (1) Set up a reactor outside the plate tower to couple the tray to the reactor without affecting the gas and liquid channels on the tray, but this operation method is relatively complicated. (2) Put the catalyst into the downcomer. One method is to bundle the catalyst in a conventional downcomer. This method may affect the liquid channel and prevent the liquid in the downcomer from flowing down smoothly. Another method is to evenly distribute many porous downcomers with circular lower ends sealed on the tray, and the catalyst is placed in each downcomer. The disadvantage of this method is that there are many downcomers and it is inconvenient to repair. Due to the use of a long circular downcomer with a small cross-sectional area, the liquid in the downcomer is not easy to renew, which affects the reaction. In order to overcome the defect of difficult mass transfer within the catalyst components, a new method of suspended bed catalytic distillation was proposed. This method couples the suspended bed catalytic reaction with distillation separation. In the reaction section, the catalyst is agitated by the rising steam and remains suspended and dispersed in the liquid phase. * * Improved liquid-solid mass transfer and reaction rate. Liao Anping developed fluidized catalytic distillation to solve the problem of filling method, which allows the use of fine-particle high-efficiency catalysts, but it also has shortcomings, such as the circulation sedimentation problem of the catalyst. 2 Simulation of the catalytic distillation process The mathematical models of the catalytic distillation process mainly include stage models and differential models. The stage models can be divided into equilibrium stage and non-equilibrium stage models. 2.1 Equilibrium stage model Lavejkov et al. used ion exchange resin as a catalyst and under the conditions of total reflux operation, proposed a simplified mathematical model of the catalytic esterification reaction distillation process. When deriving the mathematical model it is assumed that: (1) The whole tower has N balance stages including reboiler and condenser. ； (2) Each equilibrium stage can be regarded as a small fully mixed reactor ； (3) The gas and liquid phases are in phase equilibrium ； (4) All the reaction heat is absorbed by the material, and there is no heat loss. ； (5) The process is a constant state ； (6) The reaction only occurs in the liquid phase ； (7) The reaction reaches chemical equilibrium. When solving, the differential equation was first converted into a difference equation using the two-point implicit method, and then it was solved in conjunction with the phase equilibrium and chemical equilibrium equations and converged using the 0 method. The temperature, composition, and gas-liquid flow rate distribution at each stage in the tower were obtained. The model is simple and has a small amount of calculation. However, due to the great simplification of the process, its use is greatly limited, and the accuracy of the calculation results is not high. However, this method can be used to make a preliminary estimate of the process, or to provide an initial value for a more rigorous calculation method. Based on the structural characteristics of the heterogeneous MTBE catalytic distillation process, Bao Jie et al. proposed a 4-parameter structural analysis model, established a 3-diagonal matrix improved relaxation method mathematical model equation and algorithm, and proposed new improvements in the convergence acceleration mechanism. All experiments in the catalytic distillation process of synthetic MTBE at Qilu Petrochemical Research Institute were simulated, including the 2 000 t/a industrial test, and the calculated results were in good agreement with the experimental results. After being modified by structural parameters and basic data, this mathematical model can in principle be used to simulate the heterogeneous catalytic distillation process of other structural systems. 2.2 Non-equilibrium stage model In the mid-1980s, Krishnamurthy and Taylor first proposed the non-equilibrium stage model for packed columns and plate columns. This model uses the double-membrane theory to describe the transfer process of the gas-liquid two phases on the inner tower plate at the phase interface. It is assumed that the gas-liquid two-phase interface is in equilibrium. The material balance and energy balance of the gas and liquid phases are separately carried out, and the model is established and coupled. This avoids the need to estimate tray efficiency or equal plate heights. Subsequently, Zheng Yuxiang, Xu Xien and others added reaction terms to the material balance and extended the model to the catalytic reaction distillation process. In deriving the model equations they assumed: (1) The process is a constant state ； (2) The reaction only occurs on the catalyst surface ； The temperature and concentration of the catalyst surface and the bulk liquid phase are the same ； (3) The gas-liquid phase interface is uniform, and the gas and liquid phases are completely mixed. ； (4) The mass transfer rate at each point in any stage is equal, and the mixing effect can be ignored ； (5) There are N levels of equilibrium stages in the whole tower, and the condenser and reboiler are regarded as equilibrium stages. Xu Xi'en et al. used self-tested gas film and liquid film mass transfer coefficients and used the Newton-Raphson algorithm to calculate the process. By gradually increasing the non-equilibrium series, the solution when there were fewer tower sections was used as the initial value when there were more tower sections. Until the iteration results no longer changed significantly, they finally completed the simulation of production processes such as MTBE. Wu Yanxiang and Tan Tianen et al. used a non-equilibrium rate model to simulate the catalytic distillation pilot process of the hydrolysis of methyl acetate, and examined the effects of factors such as reflux feed ratio, feed water-to-ester ratio, and feed position on the hydrolysis of methyl acetate. They used self-tested empirical correlations to calculate the gas-liquid phase mass transfer coefficient of the catalyst packing layer in the reactive distillation section. ； Calculate the mass transfer coefficient of stripping section packing based on Onta correlation ； Calculate reaction rates using self-tested macroscopic reaction kinetic equations combined with catalyst package efficiency factors ； The 5acobian matrix of the model equation is divided into two parts. One part is calculated by the analytical expression of the partial derivative function, and the other part is calculated by the difference approximation. The simulation calculation has achieved satisfactory results. Since the non-equilibrium model takes into account the gas-liquid two-phase mass transfer and heat transfer resistance, the number of equations * * Increase. Since the convergence domain shrinks drastically as the number of equations increases, the non-equilibrium model not only requires much more calculations than the equilibrium model, but the initial value settings also become very demanding. 2.3 Differential model In 1989, Zhang Ruisheng et al. proposed a mathematical model for the multi-component heterogeneous catalytic distillation process. This model is different from the previous stage model in that it is a distributed parameter model. When deriving the mathematical model it is assumed that: (1) No radial concentration gradient ； (2) The mass transfer resistance mainly exists at the gas-liquid interface, and the mass transfer resistance within the phase and between liquid and solid can be ignored. ； (3) All the reaction heat is absorbed by the material, and there is no heat loss. ； (4) The gas and liquid phases are in equilibrium at the interface ； (5) The reaction only occurs in the liquid phase of the reaction section ； (6) The process is a constant state. Select any micro-unit segment of the bed and perform material balance calculations for each segment to obtain the material balance differential equation of each component. In addition, the model also includes enthalpy balance equations, normalization equations, phase balance equations, kinetic equations, gas-liquid equilibrium constants, calculation formulas for molar enthalpy and activity coefficients of gas-liquid phases, etc. For fast reversible reactions where the reaction kinetics are unknown, the chemical equilibrium equation can be used instead of the kinetic equation. For reactions that have not reached chemical equilibrium, this method can be used to obtain the maximum conversion rate and equilibrium concentration distribution along the tower. The efficiency factor of the reaction can also be measured experimentally, and the efficiency factor can be used to correct the equilibrium reaction amount. The above are issues related to the simulation calculation of the catalytic distillation process. Compared with homogeneous reaction distillation, the simulation calculation of catalytic distillation is not sufficiently studied. Especially for the mass transfer and heat transfer issues in heterogeneous processes, most current articles only consider the transfer phenomenon between gas and liquid and ignore the impact of liquid-solid mass transfer. In addition, due to the strong nonlinearity of the reaction kinetic equation, reactive distillation may have a multi-steady state problem, which is also worthy of further research. 3 Application of catalytic distillation technology Since catalytic reaction distillation technology was successfully applied to the synthesis process of MTBE, many new catalytic distillation processes have been successfully developed and are increasingly used. Song Shaoguang and others have successfully applied this technology to the synthesis of propylene glycol ether. ； Catalytic distillation technology has been used successfully in the production of high-purity isobutylene. 3.1 Alkylation process 1985, C. R. &L Company has successfully applied catalytic distillation technology in the process of alkylating propylene and benzene to produce cumene. Currently, another important alkylation process in industry is the alkylation of isobutane. The common shortcomings of the two existing processes (sulfuric acid alkylation process and hydrofluoric acid alkylation process) are high energy consumption, serious equipment corrosion, high maintenance costs, and the need to invest in high refrigeration equipment. These shortcomings can basically be overcome by using catalytic distillation technology. At present, this process has achieved experimental results and is considered industrially feasible, but there is still a large gap in catalyst activity and selectivity. 3.2 The superposition process uses catalytic distillation technology to selectively superimpose olefin molecules. Because precise temperature control will reduce the formation of by-products such as dimers, trimers or polymers. The butene stacked catalytic distillation process has obtained industrial license. Catalytic distillation for selective hydrogenation of olefins can selectively hydrogenate unwanted olefin impurities, making them lose their chemical activity or facilitating their removal by distillation. This process can be used to remove butadiene from the raw materials of the alkylation unit to extend the life of the catalyst and improve the quality of the finished product. The technology of hydrogenating dicyclopentadiene to produce cyclopentene or cyclopentane has also been reported. 3.3 Esterification and ester hydrolysis Acetification and ester hydrolysis reactions are generally reversible reactions. Due to limitations of chemical equilibrium, the conversion rate is not high, and binary or ternary azeotropes are easily formed between the reactants and products, making the purification process of the final product very complicated. Using catalytic distillation technology, the boiling point difference between the reactants and products or the low-boiling azeotropes formed with the products in the esterification reaction system can be used to separate the generated products from the reaction area in time and continuously through distillation, so that the conversion rate can be obtained. * * Improved and streamlined processes. A typical esterification reaction is a new process for the synthesis of methyl acetate by reactive distillation developed by Eastman-Kodak Chemical Company. This process uses H2S04 as a catalyst, and the process flow is greatly simplified. Since using H2S04 as a catalyst has problems such as corrosion of equipment and environmental pollution, some scholars later used solid acids as catalysts to study various esterification reactions, such as the synthesis of methyl acetate, ethyl acetate, hexyl acetate, dioctyl phthalate, etc. In addition, catalytic distillation can also be used in dimerization, hydration, dehydration, oxidation, amine conversion and other fields. 4 Summary In summary, catalytic distillation is an important means to improve the process. But only when the conditions for reaction and distillation are suitable, the two can be coupled. In addition, the choice of catalyst type and filling method is also very important. Its development focuses on how to prepare efficient and applicable catalysts and develop reasonable catalyst filling methods. Catalytic distillation simulation research mainly focuses on model research and model solution of the process, trying to obtain the parameters required for engineering amplification through mathematical simulation methods. However, the catalytic distillation process is relatively complex, and the established model has poor epitaxial properties. When used in engineering scale-up, there is a certain gap between it and actual production. This aspect still needs in-depth research. SmithL A, Process for separating isobutene from C4 streams． US: 4 302 356, 1981 Xiao Jian, Liu Jiaqi. Progress in catalyst packing forms of catalytic distillation towers. Advances in Chemical Engineering, 1999, (2): 8-12 Moritz P.Hasse H. Fluid dynamics in reactive distillation packing Katapsk-S． Chem Eng. Sci. , 1999, (54): 1 367-1 374. Ellenberger J, Krishna RC—counter—current operation of structure catalytically packed distillation columns: pressure drop, holdup and mi * ng． Chem Eng. 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