Z. Szitkai

Energy savings of integrated and coupled distillation systems

Abstract Direct separation sequence without, with forward, and with backward energy integration, indirect separation sequence without, with forward, and with backward energy integration, sloppy separation sequence without (preflash system), with forward, with backward, and with double energy integration, and thermally coupled sloppy separation sequence (Petlyuk system) are compared with short-cut and rigorous modelling. Based on theoretical considerations and economically evaluated rigorous case studies for ternary mixtures it is demonstrated that in the most cases the Petlyuk system is not superior to the energy integrated configurations even in energy savings. According to the energy consumption of sharp separation determined by short-cut methodology, all the sloppy sequence structures are equivalent. According to the energy losses determined by short-cut methodology, derived here, the energy-integrated structures win almost everywhere in the studied conditions that include a range of relative volatility ratios and the whole feed composition triangle. According to rigorously simulated and optimised results, together with controllability studies, the advantageous application of the thermally coupled (Petlyuk) systems is constrained to a very small range of relative volatility ratio, feed composition, and price structure. This small range is situated somewhere around balanced relative volatility ratio A/B to B/C, small amount of the middle component B, balanced presence of the two swing components A and C in the feed, and high energy costs to investment costs ratio or slow depreciation rate.

Optimization of hybrid ethanol dehydration systems

Abstract Hybrid ethanol dehydration systems are modeled and optimized using MINLP. The systems consist of a distillation column for approaching the ethanol/water azeotrope and of a pervaporation unit for producing pure ethanol. The optimal design and operating parameters including number of trays, feed location, reflux ratio, number of membrane sections in series and the number of membrane modules in each section are determined. Regression equations fitted to solutions of differential equations are employed for modeling the membrane modules. Quadratic and exponential regression, as well as metric and linear interpolation are studied for approximating the integral membrane model; the exponential approximation is selected. A new mathematical representation of the superstructure of the membrane subsystem is suggested and applied. A successive refinement method with non-increasing number of binary variables is developed and successfully applied. Computational experiences with gams/dicopt are presented. Using our new membrane superstructure representation, the hybrid system can be optimized effectively. The optimization method developed is also successfully applied for process intensification of an industrial scale dehydration plant. Compared to the existing plant, 12% savings in the total annual cost can be achieved by applying 32% additional membrane surface, in consequence of a radical decrease in the reflux ratio (3.3:1.4) in the column, and of producing less concentrated alcohol in the distillate. Sensitivity of the total annual cost to the specified ethanol yield, overall membrane surface and membrane replacement cost is studied. Total permeate recycling is found to be more economical, compared to partial recycling.

Rigorous MINLP model for ethanol dehydration system

Abstract Ethanol dehydration systems are modeled and optimized using MINLP. The systems consist of a distillation column for approaching the ethanol/water azeotrope and a pervaporation system for producing pure ethanol. Rigorous modeling equations are employed for modeling both the column and the subsystem of membrane modules. Quadratic and exponential regression, as well as metric and linear interpolation are studied for approximating the integral membrane model. A method for radically decreasing the multiplicity of equivalent structures covered by the superstructure is suggested and applied. A successive refinement method with non-increasing number of binary variables is developed and successfully applied.

Handling of removable discontinuities in MINLP models for process synthesis problems, formulations of the Kremser equation

This paper presents a new method for handling removable discontinuities in non-convex mixed integer non-linear programming (MINLP) models for chemical process synthesis and design. First, the occurrences of different kinds of discontinuities in design equations are discussed. Then methods so far developed for handling discontinuities in an MINLP design environment are summarized. In the main part of the paper, a new method is presented for handling removable discontinuities. Our new method is compared to five conventional literature methods applied to three mass exchange network synthesis problems of different size, where the Kremser equation is used for calculating the number of equilibrium stages. In addition, a heat exchange network synthesis problem is considered where the logarithmic mean temperature difference is calculated rigorously. Our method proved to be much faster than the other methods examined and shows less sensitivity to the change of initial values in terms of optimal objective function value and solution time.

Optimal water use and treatment allocation

Abstract Water consumption and emission can be decreased by optimal allocation of water re-use, regeneration and re-use. regeneration and recycle in water consuming mass exchange networks and by applying optimally distributed effluent treatment processes in case of several emission sources of different contamination levels. Several example problems taken from the literature have been reproduced and solved by GAMS/MINOS/CONOPT package. In most cases our optimal solution is identical to those of others; in some other cases, however, we found better solutions. We also found our method more flexible than those based on targeting and conceptual design. Some example problems involve constraints and/or specifications of those kinds the referred methods cannot deal with. Namely, the constant mass load specification is substituted by either mass load proportional to the water flow rate (involving constant concentration shift) or some other, even more realistic, non-linear functions. The optimal systems have different structures according to the applied mass load relations.

A New MINLP model for mass exchange network synthesis

Abstract Based on the principles of a simultaneous optimisation model developed by Yee and Grossmann (1990) for heat exchanger network synthesis, a robust optimisation model for mass exchange network synthesis has been developed. Superstucture, essential modelling equations, and example problems with their solutions are presented. The new model is fairly linear and is applicable to systems of both packed and staged vessels, and multicomponent problems as well.

Comparison of different mathematical programming techniques for mass exchange network synthesis

Abstract In this paper two design techniques, based on mathematical programming, for the synthesis of mass exchange networks (MENs) are compared. Problems not generally dealt with in the literature, and associated with these techniques, are highlighted. A method is presented for generating several feasible initial solutions to avoid accepting poor local solutions as final designs. A method of handling the discontinuity of the Kremser equation used for determination of the number of stages is also discussed. The new method does not rely on the use of integer variables. In addition, a method of generating MINLP solutions that feature integer stage-numbers is also presented. It is shown that insight-based superstructures assuming vertical mass transfer may fail to include the optimal structure of the MEN. Solutions of several MEN synthesis example problems are presented. Advanced pinch and mathematical programming-based solutions are compared. Both simple and advanced capital costing functions are used for the estimation of the total annual cost (TAC) of the MENs.

Solution of MEN synthesis problems using MINLP: Formulations of the Kremser equation

Publisher Summary One way of solving mass-exchange network (MEN) synthesis problems is to formulate the synthesis task as a mixed-integer non linear programming (MINLP) problem. The solution of this optimization problem delivers the desired MEN. In most cases, the objective function of the optimization problem is the total annual cost. Assuming linear-phase equilibrium relations, the capital investment calculations of staged mass exchangers are most commonly based on the Kremser equation. Discontinuous functions, such as the Kremser equation, are difficult to formulate in MINLP model. A new method is suggested for overcoming the difficulties arising from the discontinuity of the Kremser equation. The method is reliable and faster than the big-M, multi-M, simple logic, and convex-hull formulations applied for this particular problem. The method is tested on a small and a large MEN synthesis problem. The solutions for four well-known MEN synthesis problems are also presented.

Short-cut design of batch extractive distillation using MINLP

Abstract An automatic design method for batch extractive distillation, one of the most important techniques for separating low relative volatility or azeotropic mixtures, is presented. Example calculations are performed to the acetone-methanol mixture using water as entrainer. The NLP and MINLP problems are solved with applying GAMS DICOPT++.

Optimisation of distillation and pervaporation system for ethanol dehydration

Ethanol dehydration systems consisting of a distillation column and pervaporation modules are rigorously modelled and optimised using MINLP. The optimal design and operating parameters including number of trays, feed location, reflux ratio, number of membrane sections in series and the number of membrane modules in each section are determined. A method for radically decreasing the number of equivalent structures covered in the superstructure is suggested and applied. Computational experiences with GAMS DICOPT are presented. With full structural multiplicity, the solver was not able to determine the optimal structure; but with reduced multiplicity the optimal structure has been determined. Optimal structures with and without distillation column are presented.