An Improved Linear Programming Approach for Simultaneous Optimization of Water and Energy

An optimization method based on Mixed Integer Linear Programming (MILP) has been developed for simultaneous optimization of water and energy (SOWE) in industrial processes. The superstructure integrates process thermal streams and optimizes the consumption of water while maximizing internal heat recovery to reduce thermal utility consumption. In this paper, additional concepts have been implemented in the superstructure to target the issues of the pulp and paper processes. Non-Isothermal Mixing (NIM) has been considered at different locations in order to reduce the number of thermal streams and decrease the investment cost by avoiding unnecessary investment on heat exchangers. The concepts of restricted matches and water tanks have been added to the superstructure to adapt it to the pulp and paper case studies. The Integer-Cut Constraint (ICC) technique has been combined with the MILP model to generate systematically a set of optimal solutions to support the decision-making for cost-effective configurations

Systems interactions analysis for the energy efficiency improvement of a Kraft process

There is a broad range of techniques available to the engineer and that should be considered in a retrofit project to enhance the energy efficiency of an operating process. The two best known techniques and most often utilized are internal heat recovery by means of process to process heat exchange and water reutilization by the application of systems closure measures. The development of a heat recovery program aided by Pinch Analysis is well documented [1]. Similar approaches such as Water Pinch are also available to assist in the development of a system closure program [2]. They are usually applied independently yet, the results generated by either of the techniques may restrict the options available to the other. To maximize the global benefit to the process, they should be applied in conjunction. There are other energy enhancing techniques, such as: the increment of the rate of condensates return to the utility system, the elimination of non-isothermal mixing for heating or cooling and the adjustment of the temperature or pressure levels of the utilities. These techniques are often ignored in energy retrofit projects yet, they can have a significant effect on the overall steam consumption but they may also limit the extent of internal heat recovery and system closure achievable. There are also vast amounts of heat at low potential in various process streams near ambient temperature which cannot practically be recovered by heat exchanger. Upgrading some of this heat to a useful level by means of a heat pump can, in some cases, yield significant energy gains. Absorption heat pumps can be attractive because of their specific characteristics [3]. Finally the availability of excess steam production capacity generated by an energy integration project can be used to produce electric power for sale thus generating revenues to offset investments costs required for the implementation of heat enhancing measures. The methodology which has been developed and is presented herein could certainly be applied effectively to most energy intensive product manufacturing processes. A wood chemical pulping process, where water plays an important role as material and energy transporting medium, has been chosen for this first illustrative application. Also, reducing its energy cost and its emissions of greenhouse gases is a priority of the pulp and paper (P&P) industry world wide [4]

Editorial: combined water and heat integration in the process industries

Water and energy are resources that are used in large quantities in different sectors (domestic, agricultural, and industrial). Based on data on global water and energy consumption in the world over the recent past, as well as forecasts for the coming years, a continuous trend of increasing water and energy consumption can be observed. ..

Base case process development for energy efficiency improvement, application to a Kraft pulping mill. Part II: Benchmarking analysis

A new procedure for benchmarking analysis has been developed to evaluate the energy efficiency of a chemical process. Benchmarking is performed to identify process inefficiencies before developing energy enhancement measures. The new procedure combines typical techniques, such as the comparison with current practice, with utilization of new performance indicators based on exergy and energy content and the targeting by Pinch Analysis and Water Pinch. All process sections and the steam and water utility systems are evaluated. The procedure consists of five phases. In the first phase the data required is compiled. The second phase consists of comparing the energy and water efficiency of the base case to the current practice of the industry. In the third phase, the new energy and exergy content indicators are used to analyze the efficiency of utilities systems and to quantify the heat rejected by the process. In the fourth phase the minimum energy and water requirements are determined. The last phase is a synthesis by which the inefficiencies are identified and guidelines established for process improvement. Interactions between the utilities systems and the process are developed. The procedure has been applied to an operating Kraft pulping mill in Eastern Canada. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved