Industrial water reuse: RO + membrane distillation to near-ZLD

A chemical-site water-reuse train of the kind large industrial water users build when the intake is salinising and freshwater is getting scarce: reverse osmosis makes reuse-grade water, the RO concentrate is warmed by low-grade waste heat and pushed further by membrane distillation, and the MD concentrate is crystallised to salt. Overall water recovery comes out at 94.9%.

The point of the flowsheet is why MD is there at all. RO is pressure-driven, so its ceiling is osmotic pressure -- and this feed hits that wall hard: at 25 bar the same 75% recovery is infeasible, and the model says so with a number rather than a shrug (osmotic pressure 27.2 bar against a 25 bar feed, a structured SPEC_THERMODYNAMICALLY_IMPOSSIBLE error, which is why the feed here runs at 45 bar). Membrane distillation is driven by a vapour-pressure difference instead, so only vapour crosses and there is no osmotic ceiling at all: it takes the RO concentrate from 2.0 mol% to 9.7 mol% salt, recovering 81% of the water RO had to leave behind, and its distillate is solvent-only.

The waste-heat coupling is not decoration. At the RO concentrate's own 25 C the MD flux is about 1 kg/m2/h and the module is pointless; warmed to 60 C it runs at 21.5 kg/m2/h, inside the 10-50 kg/m2/h band real DCMD modules achieve. That is the entire commercial case for MD -- it is a heat-driven process, so it is only cheap where low-grade heat is already being vented.

Honesty note: the membrane coefficient and the film heat-transfer coefficients are measured module properties supplied as inputs, not correlations -- they depend on the membrane's porosity, tortuosity and thickness and on the module hydrodynamics, so the flux is only as good as those numbers. The reported temperature-polarisation coefficient (0.72 here) is the fraction of the measured driving force that actually reaches the membrane, and it is the thing a bulk-temperature model gets wrong. No membrane-wetting model: pore wetting by surfactants is what ends a real MD module's life and is not predicted here. The salt is NaCl only -- a real industrial reuse stream carries silica, which is usually what actually caps recovery; screen that separately with the silica saturation tool, since this flowsheet's thermo does not carry it.