مدل دو گانه برای هدف یابی بازسازی فرایند، کاربردی برای فرایند خمیر و کاغذ A dual representation for targeting process retrofit, application to a pulp and paper process

The optimal design of a utility system should seek to meet the energy requirements of the process that it serves at a minimal cost. Pinch analysis [1] is a mature technology, which has been applied with success to the design of heat exchange networks (HEN) in a broad variety of industries including the pulp and paper industry [2]. In the targeting step performed before the HEN design, the minimum energy requirement to heat and cool all process streams to their functional specifications is first established. However, the method, as first proposed, is not well suited to tackle certain issues encountered when attempting to increase the energy efficiency of a process beyond the maximum internal heat recovery that can be achieved by implementing an optimised HEN. Furthermore, it only deals with the reduction of the heat requirement rather than reducing the process energy expenses. Heuristic rules were first proposed by the developers of pinch analysis to provide guidance for the selection and the appropriate integration in a process of energy converting equipment such as turbines and heat pumps [3,4] while the concept of balanced composite curves [5,6] also broadened the scope of conventional pinch analysis to this type of application and the systematic search for solutions was made possible by developments of adapted optimisation algorithms [7]. The introduction of exergy composite curves [8] brought a new perspective to the identification and evaluation of process enhancement opportunities involving energy upgrading and conversion. Maximising the flowrate of the cheapest utility leads to the creation of utility pinch points [5,6] and underscores the need to analyse simultaneously the utility and the process networks. Graphical techniques become impractical when cycles are concerned. A method based on the use of optimisation techniques [10,12,13] and the corresponding graphical representations [9] has therefore been proposed by Marechal and Kalitventzeff. The cost of energy is a very significant factor in pulp and paper manufacturing [16] and the industry has invested many efforts to reduce it over the year [17]. System closure, i.e. the internal reuse of excess process water, using simulation and observation [20,21] or optimisation techniques [22], often entails a significant reduction in energy cost [18,19] and should be a preliminary to any energy optimisation project. For energy analysis per se, Pinch analysis has now become a routine tool and incursions have been made in extensions of the technique to specific cases such as, temperature mitigation by process streams mixing [23,24] or evaporator trains optimisation [25]. Effect modelling and optimisation concepts have also been applied in design methodologies related to reactive systems [15], or combining energy efficiency and environmental concerns [14,26]. The purpose of this work has been to develop a new method based on pinch analysis techniques and optimisation to identify and evaluate, at a very early stage of the study, the opportunities for reducing energy costs by improving the energy conversion in the process.