Utilization of Energy Input to the Amine Stripper
March 2, 2011
Occasionally ProMax users ask how to find out how much of the reboiler duty is used for heating the rich amine liquid stream and how much of the duty is used for the acid gas desorption reaction.
It would be very difficult if not impossible to determine the exact amount of duty used for heating and for stripping since the heat of reaction is not a constant but varies based on loading, temperature, and pressure. In addition, there is also a third component to the duty added to the reboiler, and that is generating diluent steam within the tower to lower the effective partial pressure of the acid gases. This amount of heat is then used to condense the steam to reflux water (condenser duty).
To get an estimate of the split in energy usage, you could do an energy balance:
Total Reboiler Duty - Condenser Duty - (Cp Lean Amine)*(Mass Flow Lean Amine)*(Reboiler Temp - Feed Temp)
The remaining duty is an estimate of the desorption part of the duty.
Alternatively, the heat of absorption calculated by putting the equivalent amount of acid gas into solution is an estimate of the desorption part of the duty. ProMax can be used to get the heat of absorption by simulating the lab technique employed by GPA Research Reports RR-85, RR-102, RR-108, RR-114, RR-127, and RR-130, which report experimentally measured values for heat of reaction for the various amines. To do this, create an amine stream of the weight fraction of interest at the reboiler temperature and a pressure high enough that when the amine and acid gas are mixed and cooled, no vapor is present (this may be 300 psia or higher). Add the appropriate number of moles of acid gas per mole of amine in that stream in a mixer then cool the mixer effluent back to the original reboiler temperature in a heat exchangermaking sure no vapor is present at the final temperature. The duty required to accomplish this cooling is the heat of absorption for that many lbs of acid gas, and is an estimate of the desorption part of the reboiler duty.
Yet another way to determine an estimate the desorption part of the reboiler duty would be to feed the rich amine to a Divider block with temperature and pressure change set to 0. Determine the amount of duty required to achieve removal of acid gas such that the vapor contains the amount of acid gas in the condenser overhead stream and the liquid contains the amount of acid gas in the lean amine. The calculated Divider block duty is an estimate of the desorption part of the reboiler duty.
Estimated values obtained by these methods will not be exactly the same, but will likely be within 10% of each other. It should be noted that this simplified analysis treats the entire stripper complex as a black box and does not account for all of the compounds such as hydrocarbons, water, amine, etc. that enter the stripper as a liquid in the rich amine and exit in the vapor from the condenser. To be completely thermodynamically accurate, you would also have to account for the compounds that enter in the vapor phase of the rich amine feed flashed down to column pressure and exit in the lean amine, and the temperature difference between the condenser overhead and the feed temperature. Not taking those minor effects into account is what causes the difference between the heat of desorption and the required duty by energy balance.