Z. Fonyo

Rigorous simulation of energy integrated and thermally coupled distillation schemes for ternary mixture

Abstract Sharp (99% purity) separation of a ternary mixture, characterised by near uniformly distributed volatility, by direct separation sequence without, with forward, and with backward energy integration, by indirect separation sequence without, with forward, and with backward energy integration, by sloppy separation sequence with forward and with backward energy integration, and by thermally coupled sloppy separation sequence (Petlyuk system) is rigorously modelled and optimised. Three feed compositions, namely (Case 1) equimolar A/C ratio with 10% B, (Case 2) equimolar feed, and (Case 3) equimolar A/C ratio with 80% B are compared. Comparison is based on total annual costs (TACs) using European and American price systems. The savings in TAC of Petlyuk are uniformly about 28–33% in all the three cases, while the savings of the energy integrated systems increases together with increasing ratio of B in the feed. In Case 1, Petlyuk system is the winner, with 33% savings, but a conventional energy-integrated system is handicapped by just a very few percent. In Cases 2 and 3, Petlyuk system is not amongst the best structures. In Case 2, either the conventional energy-integrated systems or the energy-integrated sloppy structures win with 35–41%; while Case 3, the energy-integrated sloppy structures are the best with about 51% savings.

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.

THE USE OF NONLINEAR PROGRAMMING TO OPTIMAL WATER ALLOCATION

Abstract Environmental protection, shortage of fresh-water and rising costs for wastewater treatment are all convincing motives for reducing fresh-water consumption and wastewater discharge of the chemical, petrochemical, petroleum refining and other process industries. Maximizing water reuse, regeneration re-use, and regeneration recycling within the chemical plant, as well as optimal distribution of waste streams for end-of-pipe treatment can reduce fresh-water usage and wastewater discharge, while they are also significant in shrinking capital investment in wastewater treatment systems. Optimal assignment and design of water consuming, regenerating, and treatment systems is a complicated task that can be mathematically formulated as mixed integer non-linear programming (MINLP). In the present article the superstructure based ‘Cover and Eliminate’ approach with NLP is applied with the tools of the GAMS/M1NOS/CONOPT package and compared to previous results. After introducing the problem in the context of chemical process synthesis, a mathematical model is described and the use of the methodology is explained. Experience with the use of GAMS is discussed. Several case studies are solved including basic examples from the literature and their variants. The main conclusion is that the application of the mathematical programming for the optimal water allocation problem is essential owing to the broad variety of the specification opportunities. The complex nature of re-use, regeneration re-use, and recycling with multiple pollutants and multiple treatment processes cannot be simultaneously taken into account by conceptual approaches. It is also shown that the assumption on the independency of contamination rates, generally applied in earlier works, are not necessarily valid; and the NLP approach can deal with the more reliable specifications.

Rigorous modelling and optimization of hybrid separation processes based on pervaporation

Hybrid separation processes are becoming more and more important in the practice if membrane technologies are also involved. In this work, a systematic investigation is completed for three sequence alternatives of distillation and pervaporation. These are the following: pervaporation followed with distillation (PV+D), distillation followed with pervaporation (D+PV), two distillation columns and a pervaporation unit between them (D+PV+D). The hybrid separation process alternatives are evaluated with rigorous modelling tools, but first, a rigorous simulation algorithm is determined for the pervaporation. The three hybrid separation processes are rigorously modelled with CHEMCAD, and optimized with the dynamic programming optimization method for the case of the separation of ethanol-water mixture. The objective function is the total annual cost (TAC). The energy consumption is also investigated. The selection of the ethanol-water mixture has two motivations: (i) it is quite often studied and well known, and (ii) to make biofuel (ethanol) production more economical, membrane technologies might also be applied. The results are compared with each other and with the classical separation completed with heteroazeotropic distillation. The optimized TAC shows that the distillation column followed with pervaporation is the most economical hybrid separation process alternative. Its TAC is about 66% of that of the classical separation.

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.

Novel hybrid separation processes for solvent recovery based on positioning the extractive heterogeneous-azeotropic distillation

The solvent recovery is often the separation of highly non-ideal mixtures. The separation and recovery of quaternary solvent mixtures coming from printing and medicine factories are investigated. The components of the six quaternary mixtures studied in form of heterogeneous and homogeneous azeotropes of minimum boiling point. The mixtures can be categorised into three groups according to their VLLE behaviours. After studying their behaviours, three novel hybrid separation processes based on the extractive heterogeneous-azeotropic distillation, a special kind of new distillation, are developed and recommended for each group of solvents. In the novel processes, beside the extractive heterogeneous-azeotropic distillation, ordinary distillations and phase separation units are also used utilising the fact that the combination of such units, the hybrid processes, offer a higher variety of the possibilities for the separation of highly non-ideal mixtures. The separation processes are verified experimentally and the agreements between the simulated and measured data prove to be rather favourable. A strategy is recommended for the use of the novel hybrid processes developed according to the VLLE behaviour of the non-ideal quaternary mixtures.

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.

Environmental and economic comparison of waste solvent treatment options

The sustainable development and consumption need more efficient use of natural resources. As a consequence, the use of industrial solvents demands their recovery instead of end-of-pipe treatment. It is not always clear, however, which treatment alternative should be applied. Based on an industrial case study, the environmental and economic evaluation and comparison of the treatment alternatives of a non-ideal solvent mixture containing azeotropes is investigated for determining the preferable option. For the recovery of the industrial solvent mixture, two different separation alternatives are evaluated: a less effective alternative and a novel design based on hybrid separation tools. An end-of-pipe treatment alternative, incineration, is also considered and the split of the solvent mixtures between recovery and incineration is investigated. The environmental evaluation of the alternatives is carried out using ‘Eco-indicator 99 life-cycle impact assessment methodology’. Economic investigation is also accomplished. The economic features clearly favour the total recovery, however, the environmental evaluation detects that if a recovery process of low efficiency is applied, its environmental burden can be similar or even higher than that of the incineration. This motivates engineers to design more effective recovery processes and reconsider the evaluation of process alternatives at environmental decision making.

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.