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].