Why must the CO2-enriched methanol from low-temperature methanol washing be subjected to cryogenic treatment before being sent to the flash tower for flashing?
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We know that absorption is an exothermic reaction; after methanol absorbs CO2 in the decarboxylation tower, the temperature of the CO2-enriched methanol (the absorbent) rises. High temperature and low pressure facilitate flashing, allowing the gas to be more easily separated from the absorbent (methanol). So why is it still necessary to cool it to -34 degrees using an ammonia cooler before flashing? :loveliness:
Cooling is carried out first to prevent CO2 and H2S from desorbing, which could affect the recovery of CO and H2.
Firstly, it is related to the evaporation temperature of liquid ammonia; the lowest temperature achievable by ammonia coolers is -40 degrees Celsius. Only by transferring cooling capacity to the solution before CO2 separation can the shallow cooling provided by ammonia cooling be used for the deep cooling after CO2 separation. Secondly, the decomposition of H2 is greatly affected by pressure while temperature has little impact; the situation is reversed for CO2. This ensures that as little CO2 as possible evaporates in the medium-pressure flash system. The less vapor that evaporates, the smaller the heat loss, and the more CO2 product gas can be recovered by the subsequent systems. Third is the work done by the flash vapor compressor; allowing a large amount of waste gas CO2 to circulate within the compressor is a tremendous waste.
The solubility of CO and H2 in methanol solutions is primarily determined by pressure, with temperature having little effect on their solubility. The solubility of H2S and CO2 in methanol solutions depends not only on pressure but also increases significantly at low temperatures; therefore, deep cooling to -34 degrees using an ammonia cooler followed by flashing is carried out in order to maximize the recovery of CO and H2 absorbed in the solution within the flash tank, while minimizing the desorption of H2S and CO2 in that same tank.
Not all of the incoming flash vapor has been cooled with ammonia, and the portion of flash vapor that has been cooled with ammonia is the rich methanol from one column being sent to another column. After flashing, it returns to the top of the CO2 concentration tower. If the temperature is not low, how can it be ensured that the H2S emissions from the tower top do not exceed the specified limits?
1. Obtain a cooling source with a lower temperature. 2. Achieve a staged flash evaporation effect, which reduces the release of CO2 while facilitating the release of H2; this reduces losses of useful gases and lowers the work required by the compressor
Reply 1# lbzhayh: Since the temperature of the rich methanol is high, it first undergoes cryogenic treatment, and then flash evaporation is carried out to obtain a colder feedstock. In rich methanol, in addition to substances such as carbon dioxide, there are also useful gases like hydrogen and carbon monoxide. By lowering the temperature of the rich methanol and then reducing the pressure, these useful gases can be recovered, which helps to prevent the release of carbon dioxide. Finally, carbon dioxide is distilled out in a flash tower
So, is it true that the lower the temperature, the better? How can we ensure that, while maintaining the recovery of useful gases, the purity and quantity of the resulting carbon dioxide product meet the specified standards?
Taking into account the temperature difference for heat exchange as well as the load on the ice machine, ammonia coolers can generally reduce the solution temperature to around -34 or -35; beyond that, it reaches its limit. To achieve even lower temperatures, higher-grade pipeline materials would be required, and pursuing further low temperatures would only result in unnecessary additional costs.
It is mainly to consider reducing carbon dioxide and hydrogen sulfide, as their release affects the composition of the recycle gas.