Diesel hydrotreater with closed H2 recycle + amine wash

Diesel (an n-hexadecane carrier with a dibenzothiophene sulfur slice) and hydrogen react over a trickle-bed-equivalent reactor (dibenzothiophene + 2 H2 -> biphenyl + H2S, the direct-desulfurization route; C12H8S + 2 H2 balances, and the 3 H2 this example once carried destroyed two hydrogen atoms per molecule) after a fired-heater preheat, at a treat-gas rate of ~3 mol H2 per mol oil (~380 Nm3/m3) — the hydrogen excess that keeps the catalyst wet with H2, not the 0.06 stoichiometric need. The effluent is cooled to 520 K (a real unit's feed/effluent exchange) before the hot high-pressure separator takes the gas off; a cold separator at 290 K condenses the carried-over oil; an amine wash (a fixed-recovery separator — the same simplification the carbon-capture example uses, not a full electrolyte amine model) scrubs H2S from the recycle gas, and a compressor closes the loop with a 3% purge. The separator liquids are let down to 3 atm: a low-pressure separator flashes off the dissolved hydrogen and most of the H2S as sour gas, and a six-stage steam stripper (Kremser, ~10 mol% stripping steam) finishes the desulfurized diesel. Every hydrogen atom is accounted for: makeup = reaction consumption + purge + sour gas + stripper overhead. History, because it changed the solver: this flowsheet used to run 6 mol/s of makeup on 50 mol/s of oil. All of that hydrogen dissolved in the hot liquid, both separators produced no gas, and the recycle the stream table showed was a stale tear iterate the loop had accepted as converged — the flowsheet now clears an edge its unit did not feed, so a loop can no longer converge onto a stream it fabricated. The refluxed stripper column that fed on it, asked to hold supercritical H2S in a 600 K bottoms under a 239 K H2S reflux, lost a fifth of its hydrogen and was gated unconverged for the same reason.