Why must methane gas be compressed first before being dried
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The exit pressure after the methanation unit is approximately 2–3 MPa. Why is it necessary to increase the pressure to around 12 MPa before sending it to the drying unit? Won’t such a high pressure increase the cost of equipment investment? Is it because when the pressure is low, the water absorption driving force is insufficient to meet the drying requirements? So compress first and then dry? Drying is carried out using the triethylene glycol method. Hero ORZ
Reply to 1# Primeval Forest: It is well known that high pressure facilitates adsorption. However, if drying is carried out before compression, some of the water exists in the form of steam, which cannot be effectively adsorbed by the molecular sieve; as a result, the drying effect is not optimal. Over time, this can have an impact on subsequent stages involving cryogenic fluids and chemical processes. It’s just my personal opinion; if any expert out there has valuable insights, I would appreciate it if they could share them.
After the gas is compressed, the density of water vapor increases and the temperature rises as well. When compressed gas is cooled, its relative humidity increases. As the temperature continues to drop until the relative humidity reaches 100%, water droplets form and precipitate from the compressed gas. Therefore, compressing the gas first and then drying it facilitates water removal; the triethylene glycol method is used for drying.
This post was last edited by lflsedin on 2011-11-20 at 14:00. Whether to dry first and then compress, or compress first and then dry, depends on the pressure before compression and the grid connection pressure, in order to conduct an investment cost analysis. Generally speaking, it is more cost-effective to dry first and then compress; high pressure leads to a sharp increase in equipment costs.
We first compress and then dry, using 3A molecular sieve as the dehydration adsorbent. Due to the low liquefaction temperature, a higher dehydration temperature is required, and molecular sieves possess strong selective adsorption capacity as well as high adsorption properties at low water vapor partial pressures.
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Reply to 4# Franktian: I agree with what was said on floor 4. Considering various factors at present, for our new projects we are adopting the method of drying first and then compressing; I think this is a reasonable approach.
It depends on what purpose your process is intended for Methanation produces a large amount of water vapor, essentially to saturated levels; if gas is only transported through pipelines, low-pressure dehydration should be sufficient to meet the requirements. Cooling after high pressure can itself condense some water; moreover, the partial pressure of water vapor does not increase, so its effect on dehydration is relatively limited.