What are the advantages and disadvantages of using lean methanol washing exclusively, using lean methanol washing in some cases and semi-lean solution washing in other cases?

Has anyone done a comprehensive comparison in this area?

1. Advantages of the semi-poor liquid process: (1) Less cooling capacity is required. The semi-poor liquid absorption process helps to reduce the system temperature, resulting in a lower temperature of the poor methanol entering the absorption tower, which in turn enhances the solubility of acidic gases in methanol ； At the same time, due to the higher amount of CO2 dissolved in methanol per unit mass, the methanol-rich stream can reach lower temperatures during low-pressure flashing and stripping flashing. By exchanging heat with cold and hot methanol, the temperature of methanol in the absorption tower can be reduced further, which in turn decreases the volume of methanol-poor stream that needs to be circulated in the system and lowers heat consumption. (2) The circulation volume of lean methanol decreases. In the semi-poor liquid absorption process, the lean methanol in the absorption tower is replaced by semi-lean methanol, thereby reducing the circulation volume of lean methanol. (3) Since the flow rate of lean methanol in the semi-lean liquid absorption process is low, this means that the amount of methanol circulating into the thermal regeneration system is small; as a result, the diameter of the thermal regeneration tower and the methanol-water separation tower is smaller compared to that in the full-lean liquid absorption process. (4) Less nitrogen is consumed. The N2 consumption is related to factors such as the amount of enriched methanol entering the enrichment tower and the temperature. In the absorption process using completely lean liquid, the circulation volume of enriched methanol is large, resulting in relatively higher N2 consumption. (5) Low consumption of steam, circulating water, and methanol, resulting in low operating costs. In the semi-poor liquid absorption process, since the circulation volume of lean methanol is low, less steam is required for its thermal regeneration. The consumption of circulating water is primarily used for cooling the top condenser in the thermal regeneration system and the lean methanol. In the semi-lean liquid absorption process, the low circulation volume of lean methanol means that less circulating water is required as well. The methanol loss in low-temperature methanol washing units is mainly due to the amount carried away by the purified gas, exhaust gas, CO2, and wastewater. A water wash system is installed before the exhaust gas and CO2 are discharged to recover methanol; the low temperature in the system reduces the saturated vapor pressure of methanol in the purified gas, thereby decreasing the methanol consumption in the system. In the semi-poor liquid absorption process, the temperature of the methanol absorption liquid is low, so methanol loss is slightly reduced.

Disadvantages of the semi-poor liquid process: (1) The amount of circulating methanol is relatively high. In the semi-lean solution absorption process, the CO2 absorption section of the scrubber uses semi-lean solution instead of some of the fully lean solution. Since the absorption efficiency of the semi-lean solution is lower than that of the fully lean solution, the circulation volume of the semi-lean solution is higher than that of the fully lean solution being replaced. (2) The power consumption is relatively high. The power consumption of the lean methanol pump and the rich methanol pump in the semi-lean liquid absorption process is less than that in the full-lean liquid absorption process. However, due to the addition of one stream of semi-lean methanol and one set of semi-lean methanol pumps, its total energy consumption is higher than that of the full-lean liquid absorption process. Furthermore, in the semi-poor liquid process, the total methanol circulation volume is high, the amount of vapor generated by flashing is large, and as a result, the power consumption of the recycle gas compressor increases accordingly. (3) The process is relatively complex, and the investment is high. The methanol circulation system of the semi-poor liquid absorption process includes an additional semi-poor liquid line and a semi-poor methanol pump; the heat recovery from within the heat exchange system as well as the supplementation of external cooling capacity make this system more complex, resulting in slightly higher investment costs for the installation.