Contents lists available at ScienceDirect Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep A hybrid design combining double-effect thermal integration and heat pump to the methanol distillation process for improving energy efficiency Chengtian Cui a , Jinsheng Sun a, ⁎ , Xingang Li a,b,c a School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, PR China b National Engineering Research Center of Distillation Technology, Tianjin, 300072, PR China c Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, PR China ARTICLE INFO Keywords: Methanol distillation Hybrid design Double-effect plus heat pump Waste heat recovery Process optimization Economic evaluation ABSTRACT Despite the low energy efficiency of distillation, it remains the popular separation technology for methanol purification. Enlightened by progress in heat pump (HP) concepts, which have been proposed to upcycle waste heat and reduce energy consumption, this work introduces a hybrid methanol distillation process, which ela- borately integrates the HP with double-effect thermal integration by designing an intermediate heater to shunt the heat load of the reboiler. Simultaneously, a corresponding optimization function and schematic solution procedure based on pinch technology are proposed to minimize the operational expenditure. The calculation results demonstrate the validity of the optimization method. Compared with the popular 4-column double-effect methanol distillation scheme, the hybrid scheme can considerably reduce utility depletion as well as operating costs, with an acceptable payback period for the compressor. As a result, the hybrid design that gets the ad- vantage of both double-effect and HP is worth extending to the methanol community as well as to other in- dustrial plants. 1. Introduction As one of the most important and widespread thermal separation methods in the modern process industry, distillation has been widely applied in the petrochemical, chemical, metallurgic, food, and textile industries. Representing a large part of the global energy usage, it is estimated that approximately 43% of thermal energy is used for in- dustrial applications [1]. In particular, distillation alone is responsible for approximately 40% of the thermal energy consumption in the chemical process industry [2,3], which is the impetus for various en- ergy saving programs that have been launched for improving distilla- tion performance. One major drawback of distillation lies in its low thermodynamic efficiency, requiring consumption of high-quality en- ergy in the reboiler, while rejecting a similar amount of waste heat to the condenser at a lower temperature [4]. In order to improve energy efficiency in a distillation column, several heat pump (HP) candidates have been proposed to aid the upcycling of waste heat that is removed from the condenser and to reduce the consumption of valuable utilities [5]. It is predicted that under certain conditions, the margin of energy savings of heat pump assisted distillation (HPAD) can be approximately 20–50% [1]. In the methanol industry, many methods have been proposed for the synthesis of methanol [6,7]. However, these crude methanol products must be refined through a distillation scheme before further utilization [8–14]. To date, a 4-column double-effect methanol distillation scheme has been the most readily adopted and widely used in China [8,9]. By dividing the methanol refining column into a pressured column (PC) and an atmospheric column (AC), this double-effect scheme has been shown to considerably decrease energy consumption compared with earlier designs [12]. Although the 4-column scheme (designated as the prototype scheme) has resulted in a significantly greater decrease in energy consumption than that expected through double-effect thermal in- tegration, it still consumes a considerable amount of hot utility in the PC reboiler (approximately 80% of the total hot utility consumption) and cold utility in the AC condenser (over 75% of all cold utility con- sumption) [8,11]. The literature surveys [8–12] demonstrated that previous works are mainly focused on using sole double-effect config- uration to achieve higher energy efficiency, ignoring the application of HPAD. As a continuation of our previous efforts [8,11], we propose a hybrid 4-column methanol distillation scheme, combining double-effect and HP. The hybrid design attempts to make full use of the HP to cool part of the AC top vapor in parallel with the condenser, and it can upcycle the waste heat available for certain heat sinks at higher http://dx.doi.org/10.1016/j.cep.2017.06.003 Received 17 October 2016; Received in revised form 3 May 2017; Accepted 3 June 2017 ⁎ Corresponding author. E-mail address: jssun2006@vip.163.com (J. Sun). Chemical Engineering & Processing: Process Intensification 119 (2017) 81–92 Available online 21 June 2017 0255-2701/ © 2017 Elsevier B.V. All rights reserved. MARK