Page 153 - Gas Purification 5E

Alhnolaniines for Hydrogen Sulfide and Carbon Dioxide Renioval 141

10. Check to see that TR assumed in Step 6 matches TR calculated by equation 2-38. If the
temperatures match, the rich solution equilibrium composition and temperature have
been correctly determined. Go to Step 13 to determine the actual design balance.
11. If TR calculated by equation 2-38 is less than the TR assumed in Step 6, the lean amine
circulation rate can be reduced. Assume a new TR between TR calculated and TR
assumed, and repeat Steps 6 through 12 until a match is obtained.
12. If the calculated TR is greater than the TR assumed in Step 3, the circulation rate is too
low and must be increased. Assume a new TR between TR calculated and TR assumed,
and repeat Steps 6 through 12 until a match is obtained.
13. The design heat and material balance is obtained after a converged rich amine solution
loading and temperature have been determined. Set the design rich solution loading ta
75% of the equilibrium loading and repeat Steps 8 and 9 to determine the design TR and
rich and lean amine flows:

(2-39)

If the temperature of the product gas is high enough to affect its HIS content (assuming
equilibrium with the lean solution) consider reducing the amine solution concentration.
Also, 75% of the equilibrium rich solution loading may be higher than desired as highly
loaded rich amine solutions can be very corrosive. Refer to the maximum recommended
rich amine loadings in Chapter 3 for guidance on maximum loadings.

Amine Regenerator Calculations

Amine regenerator design calculations that can be performed readily without a computer
are based on a series of enthalpy balances. The basis for these balances is empirical data on
the amount of steam required to strip the rich amine. These data are usually available in the
form of: 1) the pounds of stripping steam required to strip one gallon of rich solution or; 2)
the moles of water vapor per mole of acid gas in the regenerator overhead just upstream of
the regenerator condenser. Since the water vapor is normally condensed and returned to the
regenerator as reflux, the moles of water vapor per mole of acid gas leaving the regenerator
is often called the ”reflux ratio.” There is a direct relationship between these two empirical
measures of amine regenerator energy input.
Most rich amines can be adequately stripped with 0.9 to 1.2 pounds of steam per gallon of
rich solution. See Figure 2-91, which is based on empirical MEA performance data collect-
ed by Fitzgerald and Richardson (1966A, B). Implicit in the data of Fitzgerald and Richard-
son is the assumption that the leadrich exchanger approach (lean amine temperature out
minus rich amine temperature in) is about 40°F. When the energy requirement is stated in
terms of reflux ratio, the empirical data indicate that supplying enough reboiler energy to
generate about 1 to 3 moles of reflux water per mole acid gas is usually sufficient to ade-
quately strip most rich amine solutions. Use of either type of empirical stripping data is
straightforward. The following calculation strategy is based on the availability of plant per-
formance data based on pounds of stripping steam per gallon of rich solution, as there are
more data of this type in the literature. Results can be qualitatively checked by calculating
the moles of reflux water per mole acid gas.
The enthalpy balances required to size an amine regenerator are indicated in Figure 2-95.
The balances begin with a direct calculation of the reboiler duty using a selected value for
pounds of steam per gallon of rich solution based on Figure 2-91. This is followed by a cal-
culation of the ledrich exchanger duty. An enthalpy balance around the regenerator is then