Methanol is one of the most important raw materials in the chemical industry for producing formaldehyde, acetic acid, methyl formate, etc., and it also has been used as the fuel for methanol automobiles(Li et al., 2010; Riaz et al., 2013). Along with the rapid development of the coal chemical industry in recent years, methanol has played as an important intermediate in the comprehensive coal utilization processes, including coal-to-olefins, coal-to-dimethyl ether, and other coal-to-chemicals conversions (Galadima and Muraza, 2015; Gao et al., 2018). Generally, the syngas from coal gasification technology is used to synthesize methanol via the catalytic processes. Coal-based syngas-to-methanol technology has been the main route for methanol production, especially in China, due to its special status energy resources: rich in coal, poor in oil and natural gas(Li et al., 2010).
Several works have focused on the energy utilization efficiency of coal-based syngas-to-methanol process since it is a remarkable energy-intensive process(Riaz et al., 2013; Bessa et al., 2012; Cui et al., 2017; Rashid et al., 2011; Sun et al., 2012). Taking the whole process of coal-to-olefins as an example, which contains six sub-processes, including coal mining, transportation, coal-to-methanol (CTM), methanol-to-olefins (MTO), products delivery, and carbon capture and storage (CCS), respectively(Gao et al., 2018). The subsection of coal-to-methanol consumes the most energy in the whole process of coal-to-olefins, which dominates 71.04% of total energy consumption and it is five times more than CCS subsection (13.81%)(Gao et al., 2018). Extensive efforts have been made to develop the energy-saving strategies of coal-to-methanol process, among which the purification technology of crude methanol has attracted much attentions. A five-column heat integrated methanol distillation scheme was proposed by adding a medium-pressure column, and its total energy loss can be conspicuously reduced by 21.5% comparing to the previous four-column purification scheme(Sun et al., 2012; Chien et al., 2005). Besides, a hybrid methanol purification process was built to improve energy efficiency, which elaborately combined heat pump distillation with double-effect thermal integration by designing an intermediate heater to shunt the heat load of the reboiler(Bessa et al., 2012; Douglas and Hoadley, 2006). Although these above-mentioned strategies contributed to energy conservation on certain subsections in syngas-to-methanol process, other energy-saving alternatives regarding the overall process should also be raised more attention.
The study of heat exchanger network (HEN) can obtain a comprehensive strategy of heat exchange for the whole process of syngas-to-methanol by considering energy utilization, energy efficiency, operation cost, equipment cost, etc. Energy consumption is able to be reduced obviously by using optimized heat exchange strategy according to the result of global HEN analysis(Kang and Liu, 2019). Currently, the optimal calculation of HEN is performed based on the pinch technology which was developed for analyzing the potential energy-saving and economic gain(Rashid et al., 2011). Some previous studies(Kang and Liu, 2019; Payet et al., 2018) have discussed that the energy consumption and operation cost of syngas-to-methanol process can be saved through restructuring HEN referring to the result of pinch analysis. However, the optimization of HEN was always carried out under the stable heat transfer operating conditions, which ignored some inevitable uncertainties arisen from various external and internal factors (for example, feed status, product output, heat transfer coefficients, fouling, and so on). In order to make the optimal HEN from pinch analysis more precise and practical, the flexibility analysis was introduced with considering those uncertain factors under real operation status(Payet et al., 2018; ZHU et al., 1996). However, only very few reports have focused on the study of the flexibility analysis of HEN for methanol production so far.
In this work, the process of syngas-to-methanol was firstly simulated with optimized HEN, which is conducted by pinch technology with the assumption of stable operating conditions. Then, the flexibility analysis of optimized HEN was implemented by using the heuristic method. The “downstream paths” approach was used to determine the variables for building flexible HEN. Besides, the flexible HEN was improved to meet the requirements for actual production with enough area margin and heat exchangers amount. This work proposed a feasible strategy to save energy and equipment cost as well as operation cost of syngas-to-methanol by performing the flexible analysis on optimized HEN.