In this paper, we propose a general superstructure and a model for the global optimization for the design of integrated process water networks. The superstructure consists of multiple sources of water, water-using processes, wastewater treatment and pre-treatment operations. The unique features are first, that all feasible interconnections are considered between them, including water re-use, water regeneration and re-use, water regeneration recycling, local recycling around process and treatment units. Second, multiple sources of water of different quality that can be used in the various operations are included. Third, the superstructure incorporates both mass transfer and non-mass transfer operations. The proposed model of the integrated water network is formulated as a Nonlinear Programming (NLP) and as a Mixed Integer Nonlinear Programming (MINLP) problem for the case when 0-1 variables are included to model the cost of piping and/or selection of technologies for treatment. The MINLP model can be used to find optimal network designs with different number of streams in the piping network. In this work, we propose to represent the bounds on the variables as general equations obtained by physical inspection of the superstructure and using logic specifications needed for solving the model. We also incorporate the cut proposed by Karuppiah and Grossmann (2006) to significantly improve the strength of the lower bound for the global optimum. The proposed model is tested on the several illustrative examples, including large-scale problems.

In this paper we study the simultaneous energy and water consumption in the conceptual design of cornbased 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.

No abstract

No abstract

No abstract

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 discharge depending on the process. Further reduction can be obtained by using air cooling and if the water released from the crop can be properly recovered and treated. Under these conditions the water consumption ratios range from 0.4 to 1.7 gal/gal and with no or low water discharge.

No abstract

No abstract