A brief description of the CO2 Removal System is as follows. Refer to Figure 1. Syn-Gas enters the base of the CO2
Absorber (101-E) from the upstream process Condensate Separator (102-F). Prior to that, heat exchangers exist, which have
cooled the gas to about 110-140 Deg F (43-60 Deg C). Lean Amine solvent is pumped to the top of the CO2 Absorber, and
flows downward, counter-current to the Syn-Gas in the column. The Rich amine solution exits the CO2 Absorber, with
recovered CO2 chemically bound and dissolved and passes through the lean/rich solvent heat exchanger (109-C), being
heated, prior to entering the CO2 Stripper (Regenerator) columns (102-EA/EB). These columns are equipped with reboiler
heat exchangers (105-C and 111-C) to supply heat to the solvent to remove the CO2 from the solution. The pressure of the
solvent is let-down, through the inlet valves, as the rich solution divides into the CO2 Stripper columns. The CO2 Absorber in
Ammonia plants typically operates at a pressure of 350-400 psig (24.6-28.1 kg/cm2 g), while the CO2 Stripper column
commonly operates at 8-16 psig (0.6-1.1 kg/cm2 g).

The regenerated amine solution exits the CO2 Stripper, returning through the lean/rich solvent exchanger (109-C), being
cooled, while heating the rich solution which enters the CO2 Strippers. The sizing of this exchanger and its associated heat
recovery affects the thermal efficiency of the CO2 Removal process. After passing through further external cooling (generally
using cooling water) the lean amine solvent returns to the lean solution pump, where mechanical work is supplied to raise the
solvent pressure, pumping the solution once again to the CO2 Absorber, in a continuously recirculating chemical process, often
referred to as a recycle process. Recycle processes are among the most difficult to accurately model with process
engineering software, often referred to as process simulation software.

In Figure 2, shown as Example 1, an Ammonia plant CO2 Removal System operates in a plant at 1550 Short Tons Per Day
(STPD) Ammonia production (1406 mt/d). Figure 2 illustrates a typical modern CO2 Removal System, modeled with WinSim
Inc. “Design II” software. Design II can simulate CO2 Removal Systems using MDEA based solvents, including mixed amine
thermodynamics and fluid properties predictions, and equilibrium calculations, including Piperazine Protonation and Piperazine
Carbamate formation.
As presented in Figure 2, Syn-Gas enters the CO2 Absorber column, 101-E, containing 18.3 mole percent CO2 (wet basis),
and the treated overhead Syn-Gas is reduced to 82 ppmv CO2 content, using a trayed Absorber column. The CO2 Absorber
includes 20 two-pass sieve trays to achieve the CO2 removal. The Absorber trays have active area 76.5% of total area, and
10% open area. Lean MDEA solution at 112 Deg F (44.4 Deg C) after heat loss is pumped at 3906 gpm (14.78 m3/min) to
the CO2 Absorber.

The rich MDEA solution is heated through the Absorber in recovery of the CO2 acid gas to an outlet temperature of 167 Deg
F (75 Deg C). At operating conditions modeled, the rich aMDEA solution holds 0.639 moles of CO2 per mole of combined Pz
and MDEA amines. The rich solution is then heated with the 109-C lean/rich solvent exchanger to 225 Deg F (107 Deg C). The
rich solution is then divided by means of two pressure let-down valves into two parallel CO2 Stripper columns (102-EA/EB)
operating at 30.4 psia (2.14 kg/cm2 abs). The rich MDEA solution flashes as it enters the trayed CO2 Stripper columns. The
Stripper trays have active area 76.5% of total area, and the top 7 trays have 5% open area, with the bottom 10 trays having
7% open area.
Next Page
Back to Process Engineering and Technology Articles