aMDEA closed solvent loop (activated-MDEA acid-gas removal)

The activated-MDEA (aMDEA) acid-gas removal process BASF developed at Ludwigshafen -- methyldiethanolamine promoted with piperazine -- run as a genuinely CLOSED solvent loop: the regenerated lean amine is cooled, pumped back to absorber pressure and returned to the absorber, with a small side bleed and a demin-water/amine makeup. The companion 'mixed-amine-acid-gas-treating' showcase deliberately leaves that loop open (the regenerated solvent is a product); closing it is what this example adds, and it changes what the model can tell you. Sour gas at 50 bar (5% CO2, 3% H2S in methane) is contacted with lean solvent; the rich amine is heated, let down to 1.8 bar and stripped; the lean solvent returns through a cooler and a pump; a 3% bleed purges degradation products; makeup replaces what leaves. One Wegstein tear on the lean stream closes it. The headline result is one no open-loop model can produce: the lean-solvent composition is not specified anywhere in the flowsheet -- it is the fixed point of the loop's own water and amine balance -- and it converges to 87.7 mol% water / 10.2% MDEA / 2.0% piperazine, which is 41 wt% MDEA and 5.9 wt% PZ (about 3.6 and 0.7 mol/L at a typical 1.04 g/cm3 solvent density), inside the concentration window BASF's own aMDEA patents claim. Also computed: 114 mol/s of circulating solvent for 100 mol/s of sour gas, a rich loading of 0.36 mol CO2 + 0.21 mol H2S per mole of amine, treated gas that is pure methane, a 39 mol/s acid-gas overhead, and 0.23% hydrocarbon slip into it. Sweeping the stripping steam traces the loop's water balance: more steam carries more water overhead, so the circulating inventory shrinks from 225 to 53 mol/s across the sweep -- a closed-loop coupling that simply does not exist in an open-loop model. Balance closure is the precondition for an acceptance test under VDI 2048 (control and quality improvement of process data by correction calculation, for operation and acceptance tests in energy technology and the chemical industry); the same variance-weighted least-squares correction that standard is built on is available on solved results through the data-reconciliation tool. Honesty notes: (1) equilibrium, not rate-based -- piperazine's kinetic CO2 promotion and MDEA's kinetic H2S selectivity are not captured, and both columns are Kremser shortcut contactors; (2) there is no overhead reflux condenser -- the stripper runs on external stripping steam as a reboiler surrogate, so the acid gas leaves wet (79 mol% water) and the makeup is correspondingly large at 14 mol/s of mostly demin water; adding a reflux drum that returns that water is the next increment, and it needs a real reboiler rather than a steam feed to keep the water balance closed; (3) the piperazine carbamate constants and the H2S path are screening-grade; (4) solvent degradation and heat-stable salts are not modeled, so the 3% bleed is a specified purge, not a computed degradation rate; (5) this is the process class BASF commercialized, not a reproduction of any BASF plant design.