A computational procedure for systematic analysis of water reuse , regeneration and recycle in retrofit design of refinery water network systems

Water is a key element in the operation of petroleum refineries and petrochemical plants. Scarcities in freshwater supply and increasingly stringent rules on wastewater discharges have emerged as issues of major concerns to plant operators, along with an increased awareness in the need to support sustainable development initiatives and minimisation of water footprint. In line with these developments, a local refinery has included water reuse as part of its technology agenda, with the aim of minimising freshwater consumption and wastewater generation, through the approaches of water reuse, regeneration (i.e., treatment), and recycle (W3R). In particular, this paper is a report on a systematic procedure for carrying out a conceptual feasibility study to develop effective W3R strategies for water using processes and water treatment operations at the said refinery. The main thrust of the paper is to discuss the formulation and solution of a mathematical model with optimisation procedure in order to determine the optimal retrofit of the refinery water network systems. The integrated model explicitly considers the incorporation of water minimisation approaches and strategies by first postulating a superstructure that embeds all feasible flowsheet alternatives for the implementation of potential W3R opportunities using a modified version of the state-task network (STN) representation. Subsequently, a nonlinear programming (NLP) model was formulated based on the superstructure to determine the continuous variables of total stream flowrates and compositions. The key input to the model was the comprehensive water balance and wastewater characterisation data that was developed and conducted earlier. Computational studies were performed using the GAMS modeling language platform to obtain a feasible and optimal retrofit structure of the water network. The outcome from this modeling study was the development of W3R options inclusive of pretreatment requirement for reuse, which shall be further evaluated technoeconomically and subsequently implemented, where operationally-feasible