The Study On Revamp And Optimization Of Rectisol Unit In A Coal-to-Methanol Plant

The production of coal-to-methanol process includes four main steps:coal gasification, gas purification, methanol synthesis and refining. In addition to the effective gases CO and H2, there are other gas impurities in the raw gas made from coal gasification, such as acid gases CO2, H2S, COS, etc., which may cause catalyst poisoning during methanol synthesis reaction. Therefore, it is necessary to purify the raw gas. At present, NHD and Rectisol are two main gas purification technologies used in domestic coal-to-methanol process. Considering technical advancement and operation cost, Rectisol process is superior to NHD technology.As a kind of physical solvent processes, Rectisol process can remove acid gases, such as CO2, H2S, COS from raw gas, using cold methanol as solvent. The principle of Rectisol is the strong acid gas absorption ability of methanol at a low temperature. Nowadays, almost all Rectisol process units in the world are developed from Linde and Lurgi process. In China, Dalian University of Technology started calculation of Rectisol processes from1983. As a result, a new simulation software RPS was developed successfully, along with a new energy-saving Rectisol process, which can save about10%of cold load and equipment investment compared to Linde process.In this work, the Rectisol process of a coal-to-methanol plant is analyzed. In this process, CO2concentration of the CO2gas product is less than98%, and the production of CO2is too low. According to the requirements of the plant, it is necessary to revamp the original process for the purpose of increasing CO2production and concentration with the CO2recovery up to40%, which can meet the production demands of downstream process. At the same time, it is better to decrease energy consumption of the process. Based on the strong simulation function and integrated physical property database of Aspen Plus software, the whole process simulation is completed using PSRK physical property. Then the revamped schemes are proposed as follows:two flash tanks are added in the reabsorption tower to reduce CO2loss so as to solve the problems of low CO2production and concentration, the ammonia cooler of medium pressure tower is revamped and the mole flow of methanol circulating is reduced to optimize the process. The optimization scheme results in the improvement of CO2production, reduction of energy consumption as well as reduced operation cost. The revamped and optimized schemes proposed in this paper can meet the requirements of the plant on the basis of meeting the original process index. This study can provide theoretical guidance for the technical revamp of the plant.