The syntheses of water network systems are usually performed by minimizing the total annual cost. In this contribution, Mixed Integer Nonlinear Programming (MINLP) syntheses of water networks are performed by using various economic objectives, in order to investigate their effects on the structural, environmental, and economic characteristics of optimal water networks. Batch-semicontinuous and isothermal continuous water networks were analyzed during this study. Significant differences between optimal networks were obtained when using different economic objectives. Minimization of freshwater costs produced highly integrated designs with high levels of water reuse, regeneration reuse or recycling, but low profitability. In contrast, maximization of the internal rate of return resulted in highly profitable designs with low investment and a low level of water integration. Either minimization of the total annual cost, maximization of the net present value, or maximization of the annual profit produced designs with intermediate or high levels of integration between water using operations, and modest profitability. These criteria produced compromise solutions with proper trade-offs between the profitabilities and sustainabilities of water network designs.

This work presents a formalized methodology for salt's separation from three component electrolytic systems. The methodology is based on the multi-variant modelling block of a generalized crystallization process, with options for simulating the boundary conditions of feasible equilibrium processes and the elements of crystallization techniques. The following techniques are considered: cooling crystallization, adiabatic evaporative-cooling crystallization, salt-out crystallization, isothermal crystallization, and a combination of the mentioned techniques. The multi-variant options of the crystallization module are based on different variable sets with assigned values for solving mathematical models of generalized crystallization processes. The first level of the methodology begins with the determination of salt crystallization paths from a hypothetical electrolytic AX-BX-H2O system, following by an examination of salt-cooling crystallization possibilities. The second level determines feasible processes by the communication of a feed-system with the environment through a stream of evaporated water, or introduced water with introduced crystallized BX salt. The third level determines the value intervals of the variables for feasible processes. The methodological logic and possibilities for the created process simulator are demonstrated on examples of sodium sulphate separation from the NaCl-Na2SO4-H2O system, using different salt concentrations within the feed system.

This contribution describes the use of sequential and simultaneous strategies for the synthesis of heatintegrated process water networks (HIPWNs). The former strategy consists of the water network (WN) model by Ahmetovic and Grossmann (2011) , which determines the minimum freshwater consumption and the stage-wise model by Yee et al. (1990) for the synthesis of a heat exchanger network (HEN), both of which are performed sequentially. In the recently introduced approach by Ahmetovic and Kravanja (2012) both networks are solved simultaneously by applying a combined (WN-HEN) model. This combined model is formulated as a non-convex mixed integer nonlinear programming (MINLP) problem with the objective function defined as the total annual cost (TAC). As, on the one hand, the combined model in this simultaneous approach enables the obtaining of appropriate trade-offs between freshwater, utilities, and investment, it is, on the other hand, very difficult to solve, due to its nonconvex and nonlinear nature. The synthesis of HIPWNs thus still remains a big challenge. The development of efficient solution strategies is necessary in order to accomplish this task. In this paper, syntheses of HIPWNs are carried out using the above-mentioned strategies. It is worth pointing out, that in the simultaneous approach the problem can be solved directly as one system or the WN solved first to provide a good initial point followed by the overall heat-integrated process water network (HIPWN) problem after. The proposed strategies were tested on a literature Case-Study. The solutions were obtained with the minimum consumption of freshwater and utilities, and TAC was significantly improved, when compared to those reported in the literature. The solutions obtained clearly indicate that the proposed strategies can be successfully applied for the synthesis of HIPWNs.

In this paper we address the synthesis problem of distributed wastewater networks using mathematical programming approach based on the superstructure optimization. We present a generalized superstructure and optimization model for the design of the distributed wastewater treatment networks. The superstructure includes splitters, treatment units, mixers, with all feasible interconnections including water recirculation. Based on the superstructure the optimization model is presented. The optimization model is given as a nonlinear programming (NLP) problem where the objective function can be defined to minimize the total amount of wastewater treated in treatment operations or to minimize the total treatment costs. The NLP model is extended to a mixed integer nonlinear programming (MINLP) problem where binary variables are used for the selection of the wastewater treatment technologies. The bounds for all flowrates and concentrations in the wastewater network are specified as general equations. The proposed models are solved using the global optimization solvers (BARON and LINDOGlobal). The application of the proposed models is illustrated on the two wastewater network problems of different complexity. First one is formulated as the NLP and the second one as the MINLP. For the second one the parametric and structural optimization is performed at the same time where optimal flowrates, concentrations as well as optimal technologies for the wastewater treatment are selected. Using the proposed model both problems are solved to global optimality.

In this work we address the water consumption optimization of second generation bioethanol production plants from lignocellulosic switchgrass when using thermo-chemical, thermo-biochemical, or biochemical routes considering corn-based ethanol as a reference. To optimize the water consumption a three stage method is used. First, energy consumption is optimized in the production processes, which reduces the cooling needs of the processes and thus, the water losses by evaporation and drift in the cooling tower. Next, a number of technologies are considered to partially substitute the use of water as cooling agent. Finally, the optimal water networks for each of the ethanol production processes are designed by determining water consumption, reuse, and recycle and the required treatment using a superstructure optimization approach. The resulting water consumption ratios range from 1.5 to 3 gal/gal, which are in the range or even below the amount of water needed for gasoline production and with low or no water ...

We propose a general superstructure and a model for the global optimization for integrated process water networks. The superstructure consists of multiple sources of water, water-using processes, wastewater treatment, and pre-treatment operations. Unique features are that all feasible interconnections are considered between them and multiple sources of water can be used. The proposed model is formulated as a nonlinear programing (NLP) and as a mixed integer nonlinear programing (MINLP) problem for the case when 0–1 variables are included for the cost of piping and to establish optimal trade-offs between cost and network complexity. To effectively solve the NLP and MINLP models to global optimality we propose tight bounds on the variables, which are expressed as general equations. We also incorporate the cut proposed by Karuppiah and Grossmann to significantly improve the strength of the lower bound for the global optimum. The proposed model is tested on several examples. © 2010 American Institute of Chemical Engineers AIChE J, 2011

In this paper we study the simultaneous energy and water consumption in the conceptual design of corn-based ethanol plants. A major goal is to reduce the freshwater consumption and wastewater discharge. We consider the corn-based ethanol plant reported in Karuppiah, et al. AICHE J. 2008, 54, 1499−1525. We review the major alternatives in the optimization of energy consumption and its impact in water consumption. Next, for each of the alternatives we synthesize an integrated process water network. This requires closing the loops for process and cooling water and steam, and implementing the proper treatment for the water streams. We show that minimizing energy consumption leads to process water networks with minimum water consumption. As a result, freshwater use is reduced to 1.54 galwater/galethanol, revealing that it is potentially possible to achieve levels of freshwater consumption that are significantly lower than the ones in current industrial operation and that wastewater discharged can also be reduced.

In this paper, a computer aided analysis and synthesis of the crystallization processes from multicomponent electrolyte systems were studied. In addition, the vacuum crystallization processes with adiabatic cooling of the system are presented. The cooling process of a multicomponent electrolyte system can be considered as a process with the concentration of the system and/or the crystallization of the solid phase from the system. Requirements for multivariant options of the process simulator are the result of practical needs in the design of new processes or the improvement of exploitation processes. According to this, there are needs for a simulation of a simple flashing of the system as well as for the vacuum cooling crystallization processes with the cyclic structure. The possibilities of the created process simulator are illustrated on three component electrolyte systems. Application of the process simulator for any other electrolyte systems requires only an update of the thermodynamic model, and physico-chemical properties related to electrolyte system.

Determination of process structure of thermal utilization of mineral mat­ters from waste streams, is a multi-variant problem. These processes are energy-intensive and it is very important to determine the process structures for realization of the required processes in the starting phases of process development. The structure of the process system, beside the system equilibrium, depends on vector parameters of the feed stream. In this work a newly developed methodology for determination of process variants of thermal utilization of mineral salts from a hypothetical three-component AX-BX-H2O system is presented. The methodology is created on starting synthesis problem for which a set of types of process units for realization process and type of desired crystal product is determined. The methodology includes process decomposition in two subsystems: concentration (saturation) subsystem and crystallization subsystem. Concentration of feed stream is realized in isothermal conditions of water evaporation and crystallization process using various techniques: isothermal water evaporation, cooling of solution in vacuum and cooling of solution through contact surface. Determination of physical feasible processes is performed by simulation of the process superstructure in which each particular process structure is a special case of the created process superstructure. Realization of mentioned activity is provided by creating algorithms and programming software (process simulator) in which the equation system of the superstructure mathematical model is solved for various variants of set of specified variables. The created methodology and possibilities of the created process simulator are presented in the illustrative case study of waste stream utilization of the NaCl-KCl-H2O system. In addition to this, for conditions of total heat integration of subsystems is demonstrated that a small change of salt concentration of feed stream can require transfer non-cyclic in cyclic process structure.