Studies On The Synthesis Strategy Of Reactive Distillation Using Stage Composition Lines

Reactive distillation (RD) is a combination of unit operations, chemical reaction and distillation. This type of process which combines reaction and separation seems to provide promising process alternatives today. Reactive distillation is the most important process intensification ways in today's chemical industry. The benefits obtained in applying reactive distillation are that the combination of reaction and separation into single unit can reduce the equipment investment and the energy requirement considerably, and in the case of a reversible reaction, the yield of the desired product can be increased. Many of these potential advantages are good for environment. Because of the reaction introduced in the distillation system, the problem of design and synthesis of reactive distillation processes becomes complex. Therefore, the studies of design and integration are very valuable for academic and practical research.In this paper, the stage composition lines method was selected for the design of reactive distillation. Stage composition lines are used to represent all possible liquid composition in a column section, for specified product compositions and for all reflux and reboil ratios. Stage composition lines are continuous loci in nature. As the intersection of a pair of rectifying and stripping stage composition lines is a necessary and sufficient condition for staged distillation column feasibility, every intersection is an exact solution and produces accurate column specification. Design of reactive distillation using stage composition lines avoids iterative procedure and can be easily implemented and generates multiple feasible designs, which is helpful to design optimization and cost evaluation.Using the residue curves, distillation lines, operation leaf, stage composition lines and reactive space, this paper discussed some key elements of the design and synthesis problem of reactive distillation such as, whether the reaction should be introduced in the distillation system, whether the reactive distillation process must be divided into reactive and nonreactive sections and, how to obtain the optimum design. The main works of this paper are as follows:1. The graphical theory of design using stage composition lines was established based on the studies of the properties of residue curves, distillation lines, pinch point curve and operation leaf. In order to introduce the stage composition into the reactive distillation system, we derived a set of transformed composition variables for treating equilibria in multicomponent, multi-reaction systems with or without inert components present. According to the studies on reaction space we know that: if it is necessary to separate a product that is not in the reaction space, a nonreactive distillation section must be added.2. The equations of stage composition lines and overall balance for conventional distillation column were derived in this paper. Furthermore the design strategies for multicomponent distillation using stage composition lines and the rules for constructing the stage composition lines map were presented. To test the performance, the design strategy proposed was applied to ideal and non-ideal multicomponent systems. The results show that the stage composition lines method is better than Fenske-Underwood shortcut method and coincide with the simulation results of Aspen plus.3. With the assumption that phase equilibrium and reaction equilibrium are achieved in each stage, the stage composition line equations for reactive distillation column are derived based on the transformed composition variables. To simplify these equations, we defined dimensionless parameters for reference components and transformed equilibrium constant. Based on the studies of the properties of stage composition lines, a design strategy for the synthesis of reactive distillation columns using stage composition line was presented. The design strategy was then applied to an ideal reactive system to test its performance. The results show that the stage composition lines method can be easily implemented and generates multiple feasible designs, which is helpful to design optimization and cost evaluation.