282280 Optimal Design of Catalytic Distillation Column for the Hydrolysis of Methyl Acetate

Tuesday, October 30, 2012: 4:55 PM
Oakmont (Omni )
Huidong Zheng, Yanru Zeng, Suying Zhao and Jiaojiao Bai, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou, China

Methyl acetate (MA) is a byproduct in the production of purified terephthalic acid (PTA) and polyvinyl alcohol (PVA). As containing methanol (MeOH) and some other impurities, the purity of MA is not high enough for industrial applications. The mixture is usually hydrolyzed to produce methanol (MeOH) and acetic acid (HAc) by catalytic distillation (CD) in industry, which is a typical application of CD. Due to the complicated synergetic effect and interaction of chemical reaction and distillation in CD column, it is difficult to design and optimize the column configuration and operation parameters. In this paper, a systematical and universal design method was proposed for the optimal design of CD column for MA hydrolysis in order to minimize the equipment and operation cost.
The reaction kinetics and thermodynamics data of MA hydrolysis system including vapor –liquid equilibrium (VLE), liquid-liquid equilibrium (LLE) and reaction kinetic model for  different types of catalyst, have been investigated in our previous researches. Moreover, the comprehensive studies about the structure of catalyst packing and the industrial applications of MA hydrolysis with CD column have been carried out by our research group in the last decades. Based on this theoretical and practical foundation, a conceptual design method by combining the reactive residue curve maps (rRCM) method and reactive extractive curve maps (rExCM) method is proposed in this paper. The configuration of CD, feed location and operation parameters for different MA feed compositions can be obtained by this method. The proposed method is characterized by relying on a minimal set of information concerning the physicochemical properties of the system and the reaction, introducing a set of methods ranged from putting forward various solutions to the optimization, presenting a novel design and analysis process for the hydrolysis of MA.
Conceptual design is a three-step process, (1)feasibility analysis, which aims at discriminating the thermodynamically attainable specifications; (2)synthesis, which confirms the results obtained through the first step based on a more rigorous analysis and determines the column configuration able to achieve the goals formerly specified; (3)design step, which enables to determine the operating parameters of the process. The final results can then be considered as an initial value for the further design and optimization.
  Firstly, the rRCM method and rExCM method being part of graphical methods based on the thermodynamics and reaction kinetics of the reaction system, enable to analyse visually the synergetic effect and interaction of chemical reaction and distillation in reaction zone and the whole unit, which can further formulate the feasibility and screen several kinds of column configuration of a CD unit with a degree of freedom less or equal to 2, while the rRCM method applies to the RD column with single-feed and the rExCM method applies to the double-feed. Additionally, the two methods can also display the distillation boundaries and determine whether a pure stripping or rectifying section is required. While the feasibility of RD column consisting in nonreactive and reactive section can be analyzed with static analysis under the assumption of infinite separation efficiency and steady-state operation.
  Secondly, the composition profiles are deduced from the mass balance equations stage by stage under neglection of all the thermal effects in the synthesis step which is  based on boundary value design method. On the other hand, the boundary value design method is also applied to analyze the effect of CD column configuration under the different synergies between reaction and separation. Sequentially, combined with the first step, the general rule of synergies in the whole unit is obtained. The CD column configuration parameters corresponding to specified conditions such as total number of theoretical stages, location and size of rectifying/stripping section, location and size of reactive section, location of the feed plate(s) are optimized under different feed compositions in the second step.
Finally, the objective of the design step is to recalculate the operation parameters without modifying the column configuration by taking into account the heat balances. The steady-state simulation with MESH model in Aspen Plus is employed to optimize the operation parameters such as reflux ratio, heat duty and distillation/bottom flow. A non-equilibrium rate model of the CD process is built with multi-scale to verify and optimize the general rule obtained in the second step.
   Although several successful sets of CD column for MA hydrolysis are built, there still remain some problems, such as low hydrolysis conversion, high energy consumption and complex column configuration. The method proposed in this work can provide important reference and direction for the further improvement of MA hydrolysis and also other CD processes.

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