Emissions of methanol synthesis off-gas
Thread Content
In the synthesis loop, excessive venting of purge gas results in losses of useful gas, while too little venting increases compression power consumption. Is there an accurate method for calculating the amount of purge gas that should be vented? I would appreciate some guidance from the experts.
Generally, the emission amount of synthetic purge gas is controlled based on the composition of the circulating gas and the system pressure. The specific standards vary from person to person. Here, it’s generally 0.4MP, at around eight thousand cubic meters per hour.
Generally, it depends on the tower pressure, the gas composition, and the requirements of the process you are using. Our plant uses the Lurgi process, with gas production being the main objective and methanol being a by-product. The synthesis tower is of the U-tube type; the tower pressure is 5.0 MP. The compressor is an electrically driven reciprocating compressor, with a circulation volume of 220,000 and a blowdown volume of around 2,000, which is approximately 10%. If the gas composition is high and the compression ratio is large, more can be fed in to maintain the tower pressure. A lower gas composition allows for less delivery; it entirely depends on the pressure in the tower.
I’m sorry; there was a mistake in the previous post – the removal volume is over 20,000. Please understand. :L :L
Actually, this isn’t difficult; it’s calculated based on the amount of inert gas present. You should be able to do the calculation with this hint
The purge gas volume = fresh gas volume multiplied by the percentage of inert gases in the fresh gas / percentage of inert gases in the purge gas – is that it?
Yes, it’s possible to make an approximate estimate using this formula; in practice, adjustments can be made as appropriate based on the system pressure
How much fresh air do you have? If the circulation ratio is calculated at 4:1, then 55,000 units of fresh air should be used. If it’s really 20,000 as you said, then the amount of waste gas emitted would be too high, right?
The vent volume is a matter of inert gas balance: as much gas enters as much is vented out. The magnitude of the vent flow rate depends on the level of inert gas in the circulating air – it’s quite simple. When the catalyst is highly active, the inert gas can be used at a higher level, resulting in a lower flow rate of vent gas. As the catalyst’s activity declines, it becomes necessary to use the inert gas at a lower level, which in turn increases the vent gas flow rate.
It mainly depends on the amount of inert gases present in the syngas; when there is an excess of inert gases, CO, CO2, and H2 occupy less volume at a given pressure, resulting in an H/C ratio lower than 2.0. This, in turn, affects the synthesis process. Therefore, by calculating the H/C ratio required for synthesis and controlling the amount of inert gases, it is possible to determine the appropriate amount of off-gas to be released. The above are my personal opinions; I have only been in the methanol industry for a short time, so please forgive any shortcomings in what I’ve said!!:handshake
I strongly agree with the opinion from the 9th floor. Our exhaust volume is also around 10% of the fresh air volume, and the calculation using the aforementioned formula yields a similar result.
This is the formula; to calculate the quantity accurately, it is necessary to have precise knowledge of the composition. In actual operations, the accumulation of inert gas results in an increase in pressure, and quantification can be done based on the operating conditions
It is adjusted based on system pressure and the content of gases such as methane; it’s not as much as calculated