An overview of the progress in catalytic distillation technology

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I have edited it for you; please check it. 【Title】An Overview of the Progress in Catalytic Distillation Technology 【Authors】Wang Junmei, Qiu Ruchen 【Institution】School of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, Shandong Province 【Journal Name】Tianjin Chemical Industry 【Issue】July 2007 【Article ID】1008—1267(2007)04—0011—04 【Classification Number】CN TQ028．1+3 【Document Code】A 【Keywords】Catalytic distillation； Loading method ； Mathematical Model [Abstract] A review of the progress in catalytic distillation processes is presented. It introduces catalytic distillation columns, the methods of catalyst loading, and the current research status of the mathematical models for catalytic distillation columns. An overview of the progress in catalytic distillation technology: The reaction technology that combines catalytic reactions with distillation separation is known as catalytic distillation technology. This technology was first proposed by Bacchaus in 1921. In the mid-1970s, Eastman-Kodak Chemicals was the first company to industrialize a homogeneous reaction-distillation process that combined esterification with extractive distillation; by the late 1970s, this technology was extended to heterogeneous systems as well. The American company Chemical Research&Licensing began developing catalytic distillation technology in 1978, and in 1979 it succeeded in using this technology to synthesize methyl tert-butyl ether (MTBE); this technology attracted widespread attention. 1 Catalytic distillation columns: Depending on the way in which reaction and distillation are coupled, catalytic distillation columns come in two structural forms. In one form, the reaction and distillation occur simultaneously, with the chemical reaction taking place on the tray surfaces or within a catalytically active packing layer ； Another approach involves alternating between catalytic reactions and distillation separation; in a catalytic distillation column, the reactants in both the reaction section and the distillation section first undergo reactions in the reaction section, after which the products proceed to the distillation section for separation. Various structures for catalytic distillation columns have been developed abroad; the currently successful ones include the CR&L structure, IFP structure, Chevron structure, and Kuhrle structure, among others ； In addition, efforts are also being made to develop catalytic distillation columns of the frame-plate type and packed-grid type. Many research institutions in China, such as Nanjing University and the Qilu Petrochemical Research Institute, have made significant progress in this area. 2 Catalyst Loading Technology: Given that the catalyst plays a role in catalysis and mass transfer during catalytic distillation, it is required that the catalyst structure possess high catalytic efficiency as well as good separation efficiency. Therefore, the structural design and installation of the catalyst bed in the reaction section are crucial. To achieve the optimal coupling between catalytic reactions and distillation separation and to ensure stable operation of the entire catalytic distillation column, the principles for designing and selecting the packing arrangement in the reaction section are as follows: a. Provide a uniform spatial distribution for the catalyst to prevent it from being crushed due to swelling. b. To provide sufficient surface area and residence time for the catalytic reaction. c. Provide a smooth flow path for the vapor-liquid two phases to ensure high mass transfer efficiency. Based on the above principles, the packing methods for catalytic distillation columns are divided into four types: tray packing, packed packing, suspended packing, and bulk catalyst packing. 2.1 Catalyst packing method in plate towers: The catalyst particles are placed directly on the tray surfaces. The upward and downward flow of gas and liquid causes the catalyst on the trays to become fluidized, resulting in a uniform distribution of the catalyst throughout the reaction zone, high catalytic efficiency, and good contact between gas, liquid, and solid phases. However, the bed has a low porosity, leading to high pressure drops and an increased risk of damage. The catalyst bed is designed in the form of tray packings with a concentric circular sleeve structure. Arc-shaped downcomers similar to those in conventional distillation towers are installed on both sides of the inner cylinder; they serve as channels for the liquid phase to enter and exit the tray. The catalyst is placed on both sides of the outer annular tube; multiple partitions are installed inside the tube, on which glass beads of a certain height are placed or porous grids are used, serving to support the catalyst and distribute the liquid. With this special structure, the catalyst loading amount can be selected arbitrarily, allowing the catalytic distillation column to be applicable to both fast and slow reactions. For this method of filling the catalyst into the downcomer, uOP has made corresponding improvements. The downcomer is led outside the tower, and it is arranged alternately with ordinary tray plates. The catalyst bed is equipped with steam channels; the area of these channels can account for 1%–30% of the total bed area, and their diameter should be no less than 3 cm (preferably no less than 5 cm). Screens are placed on the upper and lower surfaces of the catalyst to securely support the catalyst bed, which has yielded good results. In some cases, the catalyst is placed on jet-type countercurrent packing trays; such trays are known as countercurrent jet packing trays, or JCPT trays. Compared to previous designs, the catalytic reactive distillation column utilizing this tray changes the natural flow of liquid on the trays, ensuring that the catalyst surface is continuously refreshed ； The tray is equipped with a clear liquid layer, allowing the catalyst to be fully immersed in the liquid phase, which facilitates reactions in the liquid phase ； The gas does not pass through the liquid layer, which prevents contact between the catalyst and the gas phase; this overcomes the problem of excessive pressure drop when gas passes through the catalyst bed. At the same time, it helps to extend the lifespan of the catalyst for certain chemical systems ； On each tray, the liquid undergoes reactions and distillation separation in several catalytic reaction and mass transfer units, which increases the contact time between the liquid and the catalyst, accelerates the catalytic reactions, and improves the conversion rate ； Due to the extraction effect of the gas, the flow of the liquid is enhanced, allowing the reaction heat to be released promptly, which is beneficial for distillation separation and catalyst protection ； The liquid holdup on the tray can be adjusted to ensure that the catalyst is completely immersed in the liquid phase, thereby allowing control over the amount of catalyst used and the reaction residence time. By using JcPT trays to upgrade the methanol distillation system, its production capacity increased significantly from 60 kt/a to 160 kt/a. Due to its advantages such as high processing capacity, efficiency, and stable operation, the JcPT tray is highly suitable for separation processes such as methanol distillation. 2.2 Method of packing filled catalysts: The method of packing filled catalysts involves placing the catalyst in small bags made of glass fiber, covering them with stainless steel corrugated mesh, and then rolling them into cylinders to form bundled packages. This structure is easy to install and remove, and it possesses high strength. The size of the catalyst structure can be adjusted as needed; during installation, the corrugated wire meshes of adjacent catalyst layers are arranged in a staggered pattern to ensure even gas-liquid distribution. The drawback is that the catalyst is wrapped in a layer of glass cloth, which results in high mass transfer resistance inside the catalyst package during catalytic distillation; as a result, the efficiency of the catalyst cannot be fully utilized. The American company Koch has developed a new type of catalyst packing method called Katamax. The catalyst is placed in the interlayer formed by two corrugated sheets, and then it is bundled into brick-like units which are arranged neatly inside the tower. Tests have shown that the efficiency of this catalyst packing method is over 75%, with mass transfer performance comparable to that of conventional distillation towers. In 1999, Sulzer introduced the Katapak-S type and Katapak-Sp type catalyst packing methods. It is claimed to outperform Katamax in terms of performance and efficiency. In this design, catalyst particles are placed in the interlayer between two metal corrugated screens, creating transverse channels that facilitate full contact between the gas and liquid phases; this ensures complete wetting of the catalyst, thereby greatly improving the efficiency of the catalytic reaction. The mass transfer process operates in a similar manner to that of conventional structured packing, and the interlayer can be made from various materials. This design is suitable not only for the production of corrosive products but also allows for catalyst regeneration within the tower when its activity declines. When the catalyst becomes completely deactivated and needs to be replaced, it can be filled back into the mesh. 2.3 Suspended charging method: There are two variants of suspended catalytic distillation (SCD). In one variant, the catalyst is suspended in the feed and introduced into the tower from the upper part of the reaction section; it then moves downward together with the liquid to the separator. The clear liquid resulting from this process is sent to the stripping section, while the catalyst can be reused. The other is that the catalyst is suspended by rising steam on the screens in the tray. The main advantage of suspended catalytic distillation is that the catalyst can be added or removed in the form of a suspension, without affecting the normal operation of the distillation tower; this reduces the resistance to mass and heat transfer, thereby improving catalyst efficiency ； Its main disadvantages include difficulties in separating the catalyst from the product and maintaining stable operation, loss of the catalyst during flow, and increased equipment costs. In the literature, compared with traditional processes, the use of suspended catalytic distillation for the synthesis of linear alkylbenzenes offers unique advantages. The experimental results show that at 20°C, the use of a PW heterogeneous catalyst can accelerate the reaction and increase the yield of the target product, linear alkylbenzene. 2.4 Catalyst Bulk Packing The catalyst packing is primarily obtained by mixing the catalyst active material with ion exchange resin, reinforcing materials, pore-forming agents, binders, and additives, and then processing it into saddle-shaped and ring-shaped packings. The main production methods are emulsion polymerization, block polymerization, and precipitation polymerization. The catalyst filler has catalytic properties as well as the function of separating bulk fillers ； The catalytic distillation column has the highest efficiency per unit volume ； The reaction section features a large specific surface area and porosity, as well as a low bed pressure, which creates favorable conditions for gas-liquid contact ； The catalyst has advantages such as easy installation and removal, low cost, and convenient operation. However, due to the swelling properties of polymer materials, in some reactant systems, the catalyst filler can swell and crush against one another, resulting in poor thermal stability and difficulties in catalyst processing. 3 Mathematical models: The simulation of the catalytic distillation process includes steady-state simulation and unsteady-state simulation. The mathematical models for steady-state simulation can be divided into equilibrium-stage models and non-equilibrium-stage models. 3.1 Mathematical model of the equilibrium stage: When establishing a mathematical model for the equilibrium stage, the following assumptions are generally made: (1) Chemical reactions occur only in the liquid phase ； (2) Complete mixing of the vapor and liquid phases on each tray ； (3) The vapor-liquid two-phase stream leaving the tray is in thermodynamic equilibrium and phase equilibrium ； (4) The entire process is operation in a steady state. Similar to conventional distillation, the mathematical model of reactive distillation includes the following equations: the material balance equation (M-equation), the vapor-liquid equilibrium equation (E-equation), the normalization equation (S-equation), and the enthalpy balance equation (H-equation). In addition, there is also the reaction kinetics equation (R-equation). For each equilibrium stage in the distillation column, a set of MESHR equations can be formulated. When solving the mathematical model, formulas for the molar enthalpies of the vapor and liquid phases as well as the phase equilibrium constants are also required. In principle, apart from taking into account the terms related to the generation or disappearance of a component due to chemical reactions in the component mass balance equations, as well as the heat of reaction terms in the heat balance equations and solving these together with the expressions for reaction rates or chemical equilibrium, the mathematical model for reactive distillation differs little from that of a simple distillation process. However, it is precisely because of the reaction speed that this set of model equations exhibits strong non-linearity. The rigorous calculations for reactive distillation can be roughly divided into four categories: the tridiagonal matrix method, the relaxation method, the Newton-Raphson method, and the homotopy extension method. The three-diagonal matrix method does not require derivative calculations; it can employ the Thomas method, which requires less storage and is faster to compute. However, when there are large differences in the boiling points of the components, convergence is often slow or may not occur at all. This algorithm is mainly suitable for systems with low degree of non-ideality, a reaction order not greater than 1, and low conversion rates ； The relaxation method has good stability, but its convergence speed is very slow, requiring a large number of iterations ； The most commonly used method is Newton-Raphson; its advantages are fast convergence and great flexibility regarding the iterative variables. The downside is that it requires a high initial value for the iteration variable, and an appropriate damping factor must be chosen; otherwise, the convergence speed is very slow, or convergence may not occur at all ； The advantage of the homotopy extension method is its good convergence and versatility, while the disadvantages are high requirements for initial values, longer computation time compared to the Newton method, and the need for substantial computer memory. It can be applied to reversible chemical reactions that are kinetically controlled systems. 3.2 Unbalanced stage model The unbalanced stage model is also known as the reaction-diffusion model. The biggest difference from the equilibrium-stage model is that the two-membrane theory is used to describe the phase interface between the vapor and liquid phases. The mass and heat transfer between the vapor and liquid phases are calculated using equations of heat and mass transfer rates. This allows for a more accurate description of the actual conditions inside the tower, while avoiding the need to introduce stage efficiency. Since the unsteady-state stage model takes into account the heat and mass transfer resistances between the vapor and liquid phases, the number of equations in the mathematical model increases significantly, the degree of non-linearity of these equations rises, the convergence region shrinks sharply as the number of equations increases, and higher initial values for the iterations are required; therefore, it is not easy to solve this problem using the Newton method. Qian Wei and others, in their simulation calculations of the non-equilibrium stage process for the catalytic distillation synthesis of isopropylbenzene, built upon the traditional non-equilibrium stage model by assuming isotemperature conditions among the vapor, liquid, and solid phases, thereby simplifying the energy balance equations. They also imposed reasonable constraints on the ranges of the variable values, and then used the Newton-Raphson method for calculation, which enabled smooth and rapid convergence. In summary, more in-depth research is needed in China regarding catalytic distillation, both in terms of theory and practical application, compared to foreign countries: it is necessary to develop one’s own catalytic distillation systems ； Further improve the efficiency of catalysts and develop new catalyst fillers ； Strengthen the development of new processes for catalytic distillation ； Develop new general mathematical models and computer applications ； Further promote the application of catalytic distillation in the chemical industry. This post was last edited by zhangyong6404 on 2009-2-26 12:40.]