Refinery acid-gas treating: MDEA/PZ absorber-stripper

Simultaneous CO2 AND H2S removal from a sour natural-gas / refinery off-gas stream by a piperazine-promoted MDEA solvent — the mixed-amine chemistry legacy tools reach for heavy rate-based/OLI add-ons to model. A high-pressure absorber contacts the sour gas with lean MDEA/PZ solvent (both acid gases absorb into the amine); the rich amine is heated and let down to a low-pressure steam stripper that drives the acid gases back off as a concentrated acid-gas stream and regenerates the lean solvent. Runs on the new 'enrtl-mdea-pz' electrolyte package: a generalized speciation (MDEA protonation + piperazine carbamate/dicarbamate + CO2/HCO3-/CO3-- + H2S/HS- + water) with Davies activity — it correctly reproduces piperazine's promotion (the blend holds more CO2 at a given partial pressure than MDEA alone). Honesty notes: this is an EQUILIBRIUM capacity/selectivity model — NOT rate-based, so PZ's kinetic CO2 promotion and MDEA's kinetic H2S-over-CO2 selectivity are not captured; the absorber/stripper are Kremser shortcut columns; the PZ carbamate constants and the H2S path are screening-grade (see flowsim/solver/thermo/enrtl_blend.py); and the lean-amine loop is left OPEN (the regenerated solvent is a product, not closed back onto the absorber — the same honest simplification the 'co2-from-natural-gas' example makes), with makeup steam as the reboiler surrogate. What it genuinely computes: deep sweetening of the gas, the rich CO2/H2S amine loadings, the concentrated acid-gas overhead, and a fully regenerated lean solvent.