My question is related to an amine absorber used to strip out H2S from fuel gas or recycle gas. We know thatthe optimum temperature difference between gas and amine is to be maintained at 8 to 11 degrees Celsius.
1) Why does absorption decrease when delta T is below 8 & above 11? Please answer for individual case.
2) Moreover, does only delta T matter or do individual temperatures of gas and lean amine also matter for determining absorption efficiency ?
3) If individual temperatures do matter then what should be the optimum range of both?
Answers
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15/01/2020
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Ganesh Maturu, Self, maturu.ganesh@gmail.com
I think you understood in wrong way. It is always advisable to maintain amine temperature higher than HC gas by 10-15 deg F (5-10 deg C) just to avoid foaming.
1. If the amine temperature is lower or just above Gas temperature, there is possibility of HC condenses in Amine because of lower temperature difference. HC in Amine creates foaming tendency and separation becomes poor in the Absorption column.
2. It is always advisable to maintain lower temperature for HC gas as much as possible as H2S absorption efficiency into amine is good at lower temperatures. This is the reason we see some hydrotreaters Cold separator vapor is cooled using Cooling water and maximize HC cooling and send it to Scrubber.
3. Based on HC temperature, Lean amine temperature is maintained 5-10 deg C above HC temperature.
4. If we increase lean amine temperature very high, again absorption efficiency comes down. However some units, Lean amine temperature is near to air cooler outlet temperature and HC temperature is near to water cooler outlet temperature to reduce CAPEX of providing water cooler at Lean amine air cooler outlet. During this case, we may see DT more than 10 deg C (HC outlet is 40 deg C and Amine temperature is 55-60 deg C). However this is not the optimum Scrubber performance as high temperature reduces scrubber efficiency but since CAPEX and Cooling water Opex reduced by avoiding Lean amine cooler, some compromize can be done.
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09/01/2020
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Pronab Mistry, ADNOC Gas Processing, pronab88@yahoo.com
I assume you are talking about MDEA absorption.
1.General rule of thumbs is to maintain 5 C difference between Gas and lean amine temperature, to avoid hydrocarbon condensation in MDEA solvent. However many cases it is not sufficient 5 C to avoid hydrocarbon condensation and many operatorss prefer to maintain a safe zone which will be a 8 C or more.
2. At low temperature of absorption of acid gas to amine is favorable and reverse is not.
3. If you set one temperature, say amine temperature 45 C then feed gas also will be set accordingly 5 or 8 C difference.
There are many technical papers avaialbe in the net, please go through them for better understanding of amine system.
BRE is one of them:
https://www.bre.com/PDF/Design-and-Operation-of-a-Selective-Sweetening-Plant-Using-MDEA.pdf
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08/01/2020
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Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
Your question is bit confusing. I believe you mean H2S absorption from fuel gas by amine and not strippinig.
1. The lean amine temperature is set generally 5 °C higher compared to sour gas feed. This is to avoid the hydrocarbon condensation inside the column. If the gas condenses in the column then it will increase the foaming and therefore loss of capacity and separation efficiency. The absorption is depending on the temperature, the lower the temperature, the higher the acid gas pickup and vice versa. The lower temperature difference between lean amine and sour gas feed will help to cool the absorber and improves the acid gas pick up. The higher temperature difference will increase the temperature and reduces the absorption efficiency.
2. As explained above the delta T is important for different reason. The absorber temperature profile will be depending on the gas and liquid phase feed temperature to absorber and acid gas pickup. Higher feed temperature will lead to higher temperature bulge in the absorber and lower absorption efficiency. on the other had if we reduce the lean amine temperature below 25 °C then the viscosity of liquid phase becomes higher. Higher viscosity means higher resistance for mass transfer i.e. lower absorption efficiency.
3. There is no optimum range, the gas feed temperature is depending on the upstream unit. The liquid feed temperature is set 5°C higher than the gas feed temperature.
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08/01/2020
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Zoltan Fodor, MOL Group / Production Technology, zfodor@mol.hu
Hi,
1. Absorption does not decrease by low dT. If dT is too low, hydrocarbons can condense into the amine which contaminates it. This can lead to foaming issues.
2. Temperature does have an influence. The lower the better the absorption.
3. Optimally you keep amine temperature as low as possible but above 25 degC just because of viscosity increase.
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