Step 2. Calculate recycle rate of lean amine leaving the bottom of the tower. Obtain
from the lab data or design specification its composition in terms of residual H 2 S,
water, MEA (or DEA), and hydrocarbon (if any).
Step 3. Develop the material balance over the tower.
Step 4. Using the data in Step 3, set the external tower top reflux ratio as 2:1, and
calculate the heat balance over the tower top to find the condenser duty.
Step 5. Calculate the heat entering with the rich amine feed before the preheat
exchangers (see previous section above).
Step 6. Using plant data for lean amine temperature in and out of the preheat
exchangers, calculate its duty. Add the duty to the enthalpy from Step 5 to
give feed enthalpy into the tower.
Step 7. Calculate the overall heat balance over the stripper to find the reboiler duty.
Remember to add in the heat of dissociation which is equal to the heat of reaction
in the contactor (see previous section above).
Step 8. Saturated 50 psig steam is usually used as the heating medium. Calculate the
amount of steam from its enthalpy data (steam tables) and the reboiler duty.
Step 9. If the steam usage is excessive, check overhead stream and the lean amine
concentration. It is possible that a high volume of water is being evaporated. If
this is the case, reduce the reboiler duty to maintain the amine concentration and
H 2 S concentration.
Removing Degradation Impurities from MEA
Although MEA (monoethanolamine) is the most efficient absorbent in the amine
family, it has one major shortcoming. It is readily degraded by certain sulfur
compounds such as carbonyl sulfide (COS) and by carbon disulfides. Both these
compounds are found in significant quantities in gases from cracking processes
such as thermal crackers and catalytic crackers.
It is difficult if not impossible under normal refinery conditions to remove these
sulfides from the gas. What can be done and is the normal practice is to remove the
product of degradation and return the “clean” amine to the system. Two items of
equipment are added to the process to achieve this. These are a filter and a
reclaimer. The filter is a normal leaf-type filter contained in two filter casings.
These are piped up in parallel with one on stream and the other shut down for
cleaning and as spare.
Reclaimers are really kettle-type reboilers. They take as feed a portion of the
lean amine leaving the stripper. This stream is vaporized and the vapor returned to
the stripping tower. The residue or sludge is the product of degradation and is
dumped to waste. Reclaimers can be designed to operate continuously or on a batch
basis. Steam is used as the heating medium. As in the case of the stripper reboilers,
the heating steam temperature to the reclaimer is also carefully controlled. Thus the
steam medium is saturated 50 psig steam. It must be remembered that the duty to
condense the vaporized amine from the reclaimer must be added to the stripper
Refinery Gas Treating Processes
513
from the lab data or design specification its composition in terms of residual H 2 S,
water, MEA (or DEA), and hydrocarbon (if any).
Step 3. Develop the material balance over the tower.
Step 4. Using the data in Step 3, set the external tower top reflux ratio as 2:1, and
calculate the heat balance over the tower top to find the condenser duty.
Step 5. Calculate the heat entering with the rich amine feed before the preheat
exchangers (see previous section above).
Step 6. Using plant data for lean amine temperature in and out of the preheat
exchangers, calculate its duty. Add the duty to the enthalpy from Step 5 to
give feed enthalpy into the tower.
Step 7. Calculate the overall heat balance over the stripper to find the reboiler duty.
Remember to add in the heat of dissociation which is equal to the heat of reaction
in the contactor (see previous section above).
Step 8. Saturated 50 psig steam is usually used as the heating medium. Calculate the
amount of steam from its enthalpy data (steam tables) and the reboiler duty.
Step 9. If the steam usage is excessive, check overhead stream and the lean amine
concentration. It is possible that a high volume of water is being evaporated. If
this is the case, reduce the reboiler duty to maintain the amine concentration and
H 2 S concentration.
Removing Degradation Impurities from MEA
Although MEA (monoethanolamine) is the most efficient absorbent in the amine
family, it has one major shortcoming. It is readily degraded by certain sulfur
compounds such as carbonyl sulfide (COS) and by carbon disulfides. Both these
compounds are found in significant quantities in gases from cracking processes
such as thermal crackers and catalytic crackers.
It is difficult if not impossible under normal refinery conditions to remove these
sulfides from the gas. What can be done and is the normal practice is to remove the
product of degradation and return the “clean” amine to the system. Two items of
equipment are added to the process to achieve this. These are a filter and a
reclaimer. The filter is a normal leaf-type filter contained in two filter casings.
These are piped up in parallel with one on stream and the other shut down for
cleaning and as spare.
Reclaimers are really kettle-type reboilers. They take as feed a portion of the
lean amine leaving the stripper. This stream is vaporized and the vapor returned to
the stripping tower. The residue or sludge is the product of degradation and is
dumped to waste. Reclaimers can be designed to operate continuously or on a batch
basis. Steam is used as the heating medium. As in the case of the stripper reboilers,
the heating steam temperature to the reclaimer is also carefully controlled. Thus the
steam medium is saturated 50 psig steam. It must be remembered that the duty to
condense the vaporized amine from the reclaimer must be added to the stripper
Refinery Gas Treating Processes
513