On the exhaust gas recovery issue of two-stage evaporation ejectors
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In the design of the new CO2 stripping process at our plant, the ammonia content in the gas coming out of the last ejector in each of the two evaporation stages is 10% in the first stage and less than 1% in the second stage; the water content is over 85%. The design calls for the gas from these ejectors to be recycled to a pressure-normal absorption tower (with an ammonia concentration in the incoming liquid of 6%) for absorption. No surface coolers are provided for the exhaust gases from these two evaporation stages. I would like to ask: when using a surface cooler to condense and recover exhaust gases, is it better to vent them or send them to an atmospheric pressure absorption tower for recovery?
Reply to 1# Kid1987: The final exhaust gas from the second evaporation stage is not recovered. There is a theoretical basis for using 3 injectors in two-stage evaporation. The main consideration is the compression ratio of the ejector, with the requirement that the final exhaust pressure be at atmospheric pressure (1 atmosphere). In my opinion, if final exhaust recovery is used, it will affect the pressure of the final exhaust, thereby impacting the operation of the three injectors in the second stage. Industrial production focuses on input and output; with such low levels of emissions, is it economically viable to recycle them? Are environmental regulations that strict in your area?
It is acceptable to vent the exhaust gas in two stages, but the ammonia content in the exhaust gas from the first stage of evaporation is still relatively high. It is designed to use an atmospheric pressure absorption tower for absorption; alternatively, if a surface condenser is used to recover the liquid phase and send it to the process condensate tank while venting the exhaust gas, which approach is better?
It wasn’t done in the past; now, the exhaust gases from stage 1 and stage 2 are combined together, and an ultimate air cooler has been installed. The exhaust can then be discharged through a chimney, or it can also be sent to the absorption tower as before.
Nowadays, all designs include condensers, which are very beneficial for recycling!