1 ׀ INTRODUCTION
Energy conservation remains the prime concern for many process industries considering the rising energy cost and environmental limitations. In order to increase the profitability of the industries and reduce their environmental impacts, several methods for analyzing energy systems of new and existing plants have been developed. Among the various process integration methods used to minimize excessive heat energy consumption in different industrial processes, pinch analysis is most commonly used . Pinch technology has made possible the design of new plants with optimum energy and capital costs, and performance improvement of existing processes . The technique has been used globally to target hot and cold energy requirement for crude distillation units (CDU) and other processes . Based on pinch analysis, the heat exchanger network retrofitting is envisaged as one of the promising options for reducing energy consumption which could lead to enhanced economic and environmental sustainability .
Literature is rich on the various industrial application areas of the pinch analysis technique. For instance, its application for general heat integration of distillation columns has been reported by Ajay and Amiya , and for internally heat integrated distillation columns by Nakaiwaet al. . Al-Riyami et al. studied the effects of changing the pinch temperature of a fluid catalytic cracking plant on the hot and cold utilities and the area of the heat exchanger networks. Ajao and Akande investigated the energy integration of the crude pre heat train of Kaduna refinery where they found out the optimum pinch temperature for the pre-heat train using pinch Analysis techniques. Salomeh et al. used the Heat-Int software which is based on methods of Pinch Technology to design, optimize and improve the integrated heat exchanger network of crude oil preheating process in distillation unit in Arak refinery. With the aid of the Heat-int software, Al-Mutairi and Elkawad also carried out energy integration of a heat exchanger network in a plant refinery plant using pinch analysis and investigated the effects of pinch temperature on changes in hot and cold utilities and on the area of heat exchangers. Revamping projects using pinch design method conducted for existing oil refineries to improve their operation and achieve more energy savings have been reported . In addition, the stage model has been applied to many CDUs as in the work of Promptak et al. .
A previous study conducted by Akande indicated that several possibilities exist for energy saving in the Nigerian industrial sector such as the plant refineries. On heat exchanger network synthesis (HENS), first described and formulated by Masso and Rudd , extensive reviews have been contributed by a number of workers such as Linhoff and Flower , Nishida et al. , Papoulias and Grossmann , Linhoff and Ahmad , Yee et al. , Furman and Sahinidis , , Morar and Agachi , Klemeš and Kravanja , and Klemeš et al. . But, despite the advancement in methodologies and tools of process modelling and optimization, a major challenge in HENS problems is how to develop superior models and algorithms that can optimally obtain optimal heat exchanger network (HEN) with lower total annualized cost.
Among the several design targets for HEN synthesis proposed previously by different workers are the findings of Hohmann , Raghavan , Linnhoff and Flower , Papoulias and Grossmann , Cerda et al. , and O’Younget al. which demonstrated the prediction of either the minimum utilities required for a specified minimum temperature difference (Tmin), or the minimum number of units for specified utilities, independent of area. But, the study conducted by Colberg and Morari developed a pair of transshipment nonlinear programs (NLP) to simultaneously calculate the area and capital cost targets for HEN synthesis, making it possible to evaluate the trade-off between the area and number of units before synthesis. Basically, Colberg and Morari formulated the transhipment model of Papilias and Grossman as a NLP for targeting the area on HENS. The NLP model of Colberg and Morari is able to target for both restricted matches and those that are not restricted.
The HINT (Heat-Integration) is non-commercial software developed by Department of Chemical engineering and Environmental Technology, University of Valladolid, Spain that is capable of handling design of small heat exchanger network . It is based on the principle of pinch analysis, a reliable method that has been used in the optimization of HENs. In the present study, the goal is to apply pinch analysis using HINT software in energy conservation and optimization of crude distillation unit (CDU) of Kaduna Refining and Petrochemical Company (KRPC) Ltd and compare results with the NLP technique of Colberg and Morari .