Application Research on Energy Recovery Technology in Low-Temperature Methanol Wash Process
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1. Process characteristics: With the widespread use of the low-temperature methanol washing process, companies such as Lurgi and Linde have developed various process flows tailored to different feedstocks and gasification methods. By optimizing existing processes and improving equipment, these approaches achieve higher efficiency in gas purification as well as more efficient and rational use of energy. 1. The Lurgi process: Since 1954, when the German company Lurgi built the world’s first low-temperature methanol wash unit at Sasol’s synthetic fuel plant in South Africa, nearly a hundred such units have been put into operation abroad to date. The low-temperature methanol washing process flow of Lurgi Company is: gasification – shift – desulfurization – decarburization, with the shift step located between desulfurization and decarburization. Since there is no intermediate methanol cycle to provide the cooling required by the system, all of the cooling capacity must be supplied by other processes within the entire production facility. The Lurgi process is characterized by absorption in H2S and CO2 separation columns; four-stage cooling of the feed gas, with a complex ammonia cooling process; the use of shell-and-tube heat exchangers, low requirements for HCN, and easy cleaning of the heat exchangers; it has a relatively complex process flow, high cooling load, and high electricity consumption. Due to the low absorption temperature, the circulation rate of the methanol solution is relatively high; compared with the Linde process, it has slightly higher energy consumption and a larger volume for the absorption tower. Since the cooling capacity of the system is supplied from the outside, operation adjustment is relatively flexible, and the operational flexibility of the tower is improved through the design of new tray types. The company’s newly designed low-temperature methanol washing unit integrates the relevant equipment together, utilizing liquid level and gravity to transport the fluid, which reduces the number of pumps and pipelines and lowers the investment cost for the unit. 1.2 Linde process: The low-temperature methanol washing process developed by Linde involves desulfurization and decarburization being carried out in one step; that is, decarburization takes place immediately after desulfurization, with no shift reaction occurring between them, and selective desulfurization and decarburization are performed in a single step after the shift. This process utilizes patented equipment – high-efficiency coiled tube heat exchangers – thereby improving heat exchange efficiency. Especially in the case of combined heat exchange involving multiple fluid streams, this design allows for a more compact layout of the equipment, reduces energy consumption, and saves space. The Linde process is characterized by segmented absorption of H2S and CO2 in separate towers; primary pre-cooling of the feed gas with simple ammonia cooling; the use of coil-type heat exchangers, which require high standards for HCN and are difficult to clean; a relatively simple process flow with low heat consumption. To facilitate maintenance and cleaning, the company improved the design of the high-efficiency coil-type heat exchanger; the improved heat exchanger is divided into 2 parts, each made of a different material. In addition, to address the problems that arose during production, the company took the following improvement measures: first, it installed a system pre-washing stage to remove impurities such as NH3 and HCN from the feed gas; second, it increased the volume of the feed gas separator to lower the temperature of the feed gas entering the system; third, it used high-concentration methanol vapor from the top of the methanol water column for concentration, thereby reducing the amount of water carried along by the methanol circulating in the system.
2. Related research on the domestic low-temperature methanol washing process: In the 1970s, China began research on the low-temperature methanol washing process. Through the efforts of numerous research institutions, including Sinopec Group and the Shanghai Research Institute of Chemical Industry, significant achievements have been made in the fundamental theoretical research on the low-temperature methanol washing process, as well as in process calculation and calorific value estimation. These achievements have laid an important foundation for the development of this technology in China. For example, compared with the process developed by Lze Company, the low-temperature methanol washing process using a six-tower configuration developed by relevant Chinese departments can significantly reduce the equipment investment required. 3. Research and Application of Low-Temperature Methanol Wash Process in China 3.1 Process Research and Equipment Manufacturing. Research on the low-temperature methanol wash process in China began in the 1970s. Institutions such as the Lanzhou Design Institute of Sinopec, the Research Institute of Nanjing Chemical Industry Co., Ltd. under Sinopec, Zhejiang University, the Shanghai Chemical Industry Research Institute, Dalian University of Technology, and Beijing University of Chemical Technology have carried out extensive work in areas such as fundamental theoretical research, chemical process simulation calculations, thermodynamics and determination of basic data, as well as mathematical models for gas-liquid equilibrium calculations, achieving certain progress in these fields. Currently, many large-scale acid gas purification units in China have adopted the low-temperature methanol washing process, and extensive experience has been accumulated in aspects such as design, construction, installation, and operation. In recent years, the country has also made remarkable progress in the manufacturing of equipment for the low-temperature methanol washing process. Dalian Iceberg Group Jinzhou Heavy Machinery Co., Ltd. manufactured 23 types of super-large complete sets of equipment, including towers, heat exchangers, and separation tanks, for the plants introduced by Shanghai Coking Co., Ltd. Key equipment such as the HzS concentration tower, the reformer gas absorption tower, and the gas methanol absorption tower utilize 3.5Ni low-temperature steel processing technology. 3.2 Low operating costs: Methanol solutions possess good chemical and thermal stability, low viscosity and low corrosivity, so no defoamers are required. Although the low-temperature methanol washing process requires a higher initial investment, its low energy consumption and high purification efficiency give it better technical and economic advantages over the NHD process; as a result, this process is preferred in most new installations today.
4. Energy recovery schemes in the low-temperature methanol washing process 4.1 Startup control of turbine pumps To maintain the stable performance of turbine pumps and extend their service life, it is necessary to make a series of adjustments to their startup process, so that the pumps can be warmed up effectively and their rotation speed can be kept away from the critical speed range, thereby preventing excessive vibration of the turbine rotor and extending the lifespan of the pumps. The increase in turbine speed can be roughly divided into the idle speed range, the critical speed range, the operational range, and overspeed. The wwsos governor is used as the controller during startup to regulate the turbine so that it can pass through each stage steadily, thereby ensuring the smooth operation of subsequent equipment and normal production of the process. 4.2 Overall structure of the turbine pump unit The overall structure of the turbine pump unit includes the energy recovery component (i.e., the main elements of the turbine pump), the valves at the inlet and outlet of the turbine pump, as well as pressure sensors for monitoring pressure; it also includes gate valves, emergency shut-off valves, and flow meters used in the process flow. 4.3 Lubrication system section: Since the thrust bearings and central bearings of the turbine pump are subjected to enormous pressure from the rotor during rotation, both types of bearings must be properly lubricated and under a certain oil pressure level in order to enable the turbine pump to operate and the rotor to rotate. Therefore, a reliable lubrication system is necessary to supply oil to the turbine pump continuously in order to maintain pressure, and this pressure must be above a certain value; hence, appropriate low oil pressure alarm settings are required. And when the lubrication pump fails to provide sufficient oil volume and pressure, it is necessary to immediately stop the operation of the turbine pump in order to reduce wear on the central bearing and thrust bearing. 4.4 Interlock logic: Since the modified liquid-rich energy recovery system represents an improvement over the original process by recovering energy from that process, the modified turbine pump units must not interfere with the normal operation of the original process. In other words, in the event of a failure in the turbine pumps, there must be appropriate safety interlocks in place to ensure that the process can switch back to the configuration used before the modification. 5. Conclusion With the construction of large-scale facilities for coal-based synthetic ammonia, coal-based methanol, coal-based dimethyl ether, coal-based natural gas, and coal-based hydrogen production, the low-temperature methanol washing process, as one of the key technologies in modern coal chemical industry, holds broad application prospects. At present, most of the low-temperature methanol washing processes used in China are imported technologies, resulting in high investment costs. In order to reduce costs, enhance market competitiveness, and maximize efficiency, domestic manufacturers and research institutions should intensify their research on this process, focusing on independent innovation, integrated innovation, as well as the introduction, adaptation, and further innovation of existing technologies. They should increase investment in research and development and technological upgrades for this process, and develop advanced low-temperature methanol washing technologies and equipment of international standard that possess independent intellectual property rights. This will help to reduce costs, improve market competitiveness, enhance the technical and economic efficiency of such facilities, and continuously refine the low-temperature methanol washing systems that have already been localized through practical production experiences.