A systematic method to customize an efficient organic Rankine cycle (ORC) to recover waste heat in refineries

Abstract

Organic Rankine cycles (ORCs) convert low temperature waste heat into power. When there are multiple waste heat sources in a refinery, operability and safety considerations may make it more practical to use hot water as the medium to recover waste heat. The hot water stream can then release the heat to the organic working fluid in an ORC system. In this paper, how to customize an efficient ORC for a heat exchanger network (HEN) to optimally recover multiple strands of waste heat is investigated. Because the heat exchanger network structure, the hot water loop, and ORC system interact with each other, the coordination and synthesis of these systems ought to be considered simultaneously to maximize the energy performance. A methodology is proposed using the waste heat composite curve (WHCC) and grand composite curve (GCC) to diagnose inefficiencies in an existing heat exchanger network. In addition, the WHCC can be used to solve the problem of the tradeoff between waste heat quality and quantity recovered with an intermediate stream. WHCCs are classified into two types, and procedures for designing the recovery network for each type are presented while considering the interaction with working fluid selection. The methods proposed in this paper can help engineers diagnose problems with the original heat exchanger network, and determine the flowrate of hot water, the structure of the waste heat recovery network, the best working fluid and the operating conditions of ORC system in an integrated manner. The ideas are applied to an illustrative case study in collaboration with Sinopec. The case study shows the effectiveness of this method and compares different extents of heat exchanger network modification.