Ana M. Eliceche

Optimisation of azeotropic distillation columns combined with pervaporation membranes

The main objective of this work is the analysis and optimisation of azeotropic distillation columns, when a liquid side stream with the distributing non-key component is treated in a pervaporation membrane and the retentate is recycled to the column. The objective is to separate the pure distributing non-key component using a pervaporation membrane, thus helping to improve the purity in the top and/or bottom products. The operating conditions of the column such as reflux ratio, product and side draw flowrates and pressure are selected optimally to minimise the operating cost of the hybrid system. The simulation of the pervaporation membrane units is implemented solving the differential–algebraic equation system of mass transport and energy balances. The optimisation of the operating conditions of the azeotropic distillation column in the hybrid distillation/pervaporation system for Methyl tert-butyl ether production is presented. Numerical results with a significant reduction in the operating cost are reported.

Environmental life cycle impact as a tool for process optimisation of a utility plant

The purpose of this paper is to minimize environmental life cycle impact when a detail process modelling is available. A methodology is presented to calculate the optimum operating conditions of an ethylene process utility plant. The overall environmental impact is calculated as a weighted sum of global warming, acidification, eutrophication, photochemical oxidation, ozone depletion, human toxicity and ecotoxicity. The battery limits of the plant are extended to include the relevant environmental impacts corresponding to the imported electricity generated in thermoelectric, hydroelectric and nuclear plants. A mixed integer non-linear programming problem is formulated and solved in GAMS. Significant reductions in environmental impact particularly in global warming, the most relevant category, are obtained choosing the pressure and temperature of high, medium and low pressure steam headers together with the selection of optional drivers and boilers. Improvements achieved simultaneously in natural gas and electricity consumption and operating cost are also reported.

Optimal operation of selective membrane separation processes for wastewater treatment

Abstract The objective of this work is to identify and select optimally the main operating conditions of selective membrane separation processes for wastewater treatment. The advantage of this technology is that it reduces the amount of metals disposed into the environment, by removing them from wastewater and concentrating them simultaneously in the stripping stream for re-use in the plant that generates the effluent. The operating conditions are evaluated solving an optimisation problem, where the set of algebraic and differential equations that represent the component mass balances are formulated as equality constraints, together with the separation objectives that are posed as inequality end point constraints. The optimisation leads to significant improvements in terms of the flow rate of the effluent treated, showing the potential of the methodology presented. Insight on the dynamic behaviour of the selective membrane separation processes is gained through the sensitivity analysis.

Bi-objective minimization of environmental impact and cost in utility plants

Abstract A methodology to minimize potential environmental impact and operating cost in the selection of the operating conditions of a steam and power plant is presented. A bi-objective mixed integer nonlinear programming problem is formulated and solved in GAMS. Different strategies are implemented successfully to generate the Pareto curve such as: minimum distance to the utopia point, ɛ-constraint, weighted sum and global criterion. An analysis of the Pareto curve allows the identification of two regions where it is cheaper and more expensive respectively, to reduce the potential environmental impact, providing relevant information to support a decision making process. The economical valorisation of greenhouse gases emissions reduction was also carried out, showing the region of the Pareto curve in which the income would compensate the increment in operating cost, leading to a reduction of the potential environmental impact with no extra cost.

Flexibility analysis of an ethylene plant

It is desirable to know the variations around the normal feed conditions that a real plant can handle ensuring feasible operation and to detect the plant bottlenecks for these perturbations. This information is provided by a flexibility analysis of the real plant. Perturbations on total flow rate and ethane composition are expected in the feed of an ethylene plant. Due to the fact that only two uncertain parameters are contemplated, the maximum feasible displacement from the nominal point in different directions of the uncertain space can be evaluated. The maximum displacement is determined solving a nonlinear programming problem where the main operating conditions of the plant are the optimization variables. An active capacity constraint indicates a plant bottleneck. Therefore the flexibility analysis of a real plant allows the quantification of the perturbations that the plant can accommodate for different directions of interest, associating bottlenecks with scenarios in which they will take place.

Optimal design of membrane processes for wastewater treatment and metal recovery

Different continuous operating modes of a nondispersive solvent extraction (NDSX) process with membrane modules of hollow fibre for wastewater treatment and metal recovery are evaluated. Cocurrent and countercurrent flow of aqueous and organic phases in the membrane modules with alternative product extraction locations for the removal and recovery of hexavalent chromium from wastewater are studied. The design of NDSX process in a countercurrent continuous operation is addressed for the first time in this work and the improvement achieved is reported. The membrane area required in the countercurrent operation is reduced significantly compared with the cocurrent operation, requiring also less investment in pumps and pumping cost leading to the minimum total cost. Thus a significant improvement in the design of this new and cleaner technology is presented. # 2003 Elsevier Ltd. All rights reserved.

Continuous operation of membrane processes for the treatment of industrial effluents

Abstract The feasibility of operating a Non-Dispersive Solvent Extraction (NDSX) process in a continuous mode is analysed for the first time in this work. A configuration is proposed for the treatment of industrial effluents and metal recovery. The pollutant is removed from the effluent in the extraction module and simultaneously it is recovered in the stripping module, for recycling and re-use in the plant that generates the effluent. Thus, the amount of pollutant disposed of into the environment is reduced. The optimum membrane areas and the operating conditions are calculated solving a non-linear programming problem that includes the differential equations representing the composition profiles in the membrane modules. A particular application of this technology is evaluated, to remove Cr(VI) from surface treatment effluents, recover and concentrate it for recycle and reuse. The optimum operating conditions and areas of the extraction and stripping membranes, calculated at different Cr(VI) effluent compositions, are reported.

Multi Objective Optimization Using Life Cycle Environmental Impact and Cost in the Operation of Utility Plants

Abstract Environmental and economic objective functions are used simultaneously to select the operating conditions of a steam and power plant. Different methodologies to solve multi objective optimization problems were implemented successfully. The life cycle potential environmental impact and the operating cost of the power plant are minimized simultaneously. A methodology is presented to estimate the potential environmental impacts during the most important life cycle stages associated with imported fuel and electricity in the utility plant. Mixed Integer Non Linear multi objectives problems are formulated and different strategies are implemented and successfully solved in GAMS.

Feasible operating region of natural gas plants under feed perturbations

Abstract The main objective of this work is to find the set of scenarios for variable feed conditions which the plant can accommodate operating in the feasible region. A rigorous simulation of the natural gas plant has been implemented. The solubility of carbon dioxide in mixtures of light hydrocarbons is predicted. Carbon dioxide solidification in the coldest sectors of the plant is not desired. Therefore a constraint for each of the top plates of the demethanizer column is added, in such a way that the operating conditions are chosen to avoid freezing conditions. The nominal point that represents the normal feed conditions is known. The maximum displacements from the nominal point, in different directions of the uncertain space, are found solving a nonlinear programming problem. The main operating conditions of the plant are the optimization variables. In the numerical example presented, the carbon dioxide and methane feed flowrate are assumed as uncertain parameters. The minimim of the maximum displacement from the nominal point is found. The corresponding direction and active constraints which limit the flexibility are reported. The feasible region in the two dimensional space is found.

Evaluation of design flexibility in distillation columns using rigorous models

Abstract A methodology is presented for the evaluation of the flexibility and bottlenecks detection of a given distillation column design, using rigorous simulation models. The flexibility index is calculated as proposed by Swaney and Grossmam (1985), solving a nonlinear programming (NLP) problem in the direction of each vertex in the uncertain space, in which the objective is the maximization of the displacements. The control variables, reflux and product flow rates, are the optimization variables of the NLP. The purity specifications, recovery and maximum equipment capacities are posed as the constraints for feasibility of the NLP. Numerical results will be presented for the case of a debutanizer column. Uncertainties are considered in components feed flow rate, maximum allowed vapor velocity, heat transfer coefficients of condenser and reboiler, and cooling water inlet temperature. Great physical insight can be gained from the NLP solutions in the directions of the vertices, detecting bottlenecks and the worst combinations of uncertain parameters.