Paths combination for HENs retrofit Abdelbagi Osman a, * , M.I. Abdul Mutalib a , M. Shuhaimi a , K.A. Amminudin b a Chemical Engineering Programme, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia b Process Control and Systems Department, Saudi Aramco, Dhahran, Saudi Arabia article info Article history: Received 13 December 2008 Accepted 1 April 2009 Available online 8 April 2009 Keywords: Heat exchanger networks Retrofit Path analysis Heat integration abstract With the significant increase in fuel price, energy conservation projects should be reviewed and analyzed properly. Retrofit of heat exchanger networks (HENs) is among the common projects to reduce the plant operational cost. This paper introduced a new procedure using the path analysis approach for HEN ret- rofit. The developed procedure attempts at generating options for retrofit solution. These options are cre- ated by combining the available utility paths in HEN systematically. Instead of relying on a single path, the heat load could be shifted from HEN utilities using a set of paths simultaneously. To ensure feasible heat transfer between the hot and cold streams throughout, Exchanger Minimum Approach Temperature (EMAT) is maintained while shifting the heat load. The available exchangers’ pressure drop is considered in calculating the heat transfer coefficients. HEN devices are subjected to some additional area without any topological modification. In addition, the investment of such area could be recovered in a short span of time. The calculation of this approach encountered some iteration which has been overcome by math- ematical programming. Demonstration example showed some options to be reasonable retrofit solutions. Energy savings ranged between $150 K and $450 K per year with payback time of less than 2 years were possible. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Heat exchanger networks (HENs) are widely used in many pro- cess industries for the purpose of maximizing heat recovery and hence reducing utility consumption. The hot utility for processes heating is often extracted from a utility system where fuel firing takes place. Previously, HENs system has been well studied in terms of grass-root design. However, retrofit studies are still ac- tively pursued to further improve energy recovery. It was reported that 70% of the projects conducted in the industry involved process retrofits [1]. The pressure from increasing energy price and tighter environmental limits for process emission continue to provide the fresh perspective for reviewing retrofit projects. Industry is often reluctant to undertake grass root projects to resolve energy prob- lems in view of the economic uncertainties; beside the pressure of providing quick return to shareholders. Therefore, it is important to continue looking for solutions that could give quicker returns. The HEN retrofit options are presented in this paper using path analysis approach. Path analysis in retrofit is commonly used in practice due to its powerful thermodynamic insights [2]. But the use in developing retrofit options is relatively new as it optimizes the system within the identified paths. These options are generated by shifting the heat load from HEN utilities through individual util- ity paths as well as combined paths. In this work, EMAT has been set to be a minimum driving force for individual exchangers instead of DT min in HEN while shifting the heat load. According to Smith, once the HEN has been designed using the pinch method, DT min will no longer be a constraint for heat recovery during the optimization process using loop and path [3]. However, impractical temperature driving force for individual exchanger must be avoided. 2. Background The setting up of the retrofit targets using pinch analysis was first developed by Tjoe and Linnhoff [4]. Retrofit of heat exchanger networks could be classified either as a major or minor retrofit. The major one incorporates topological modification of HEN where new device(s), re-sequencing and/or re-piping are considered. However, the minor retrofit projects involve additional area to the existing HEN. In the literature, it was found that most of the retrofit studies for HEN featured reconstruction and topological changes mainly. 2.1. HEN retrofit and path analysis approach Energy consumption of an existing HEN could be decreased without changing the network structure. This is possible through the exploitation of the utility path as mentioned earlier. However, 1359-4311/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2009.04.006 * Corresponding author. Address: Faculty of Engineering and Technology, University of Gezira P.O. Box 20, Wad Medani, Sudan. Tel.: +60 175206659. E-mail address: abdelbagi2001@yahoo.com (A. Osman). Applied Thermal Engineering 29 (2009) 3103–3109 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng