Recovery of 2-propanol (IPA) from spent cleaning and carrier-solvent streams is evaluated with the Solvent Recovery ROI Model for IPA Users: Three Operating Scenarios. The model basis uses 99.9 wt% virgin 2-propanol at 1.50 USD/kg, hazardous waste disposal at 0.85 USD/kg for D001 ignitable liquid under 40 CFR 261.21, electricity at 0.11 USD/kWh, and 240 operating days/year. Purity is assessed by gas chromatography with flame ionization detection, water by ASTM E203 Karl Fischer titration, density by ASTM D4052, and flash point by ISO 2719. The three scenarios differ in feed volume, water content, equipment configuration, and product specification.

Simple payback is defined as installed capital divided by annual net savings. Annual net savings are calculated from recovered IPA mass multiplied by the virgin solvent price, plus avoided hazardous waste disposal, minus electrical utilities, regeneration energy, filter consumables, maintenance, and supervision. Because the closed-cup flash point of 2-propanol is 12 °C and the lower explosive limit is 2.0 vol%, all distillation and storage equipment is bonded and grounded according to NFPA 77, and receivers are placed in forced ventilation. Equipment marking, vent sizing, and electrical classification must be verified against local fire codes and are not replaced by this model.

2-Propanol forms a minimum-boiling azeotrope with water at approximately 87.9 wt% IPA and 80.37 °C at 101.3 kPa. The latent heat of vaporization of 2-propanol at the normal boiling point is approximately 663 kJ/kg, and the liquid specific heat capacity is approximately 2.56 kJ/kg·K. A single-stage still without reflux has a theoretical minimum energy demand near 0.22 kWh/kg for heating from 25 °C to 80 °C and vaporizing IPA; actual systems consume 0.5–0.9 kWh/kg after thermal losses, reflux, and condenser cooling. Fractional columns with reflux and molecular sieve drying consume more energy per kilogram but enable reuse in applications requiring <0.2 wt% water.

What Operating Limits Dictate Payback for a 1,000 L/Month Batch Still?

A 1,000 L/month benchtop cleaning operation generates spent 2-propanol containing 85 wt% IPA, 10 wt% water, and 5 wt% dissolved rosin, solder flux, and particulate metal oxides. The recovery system is a 50 L glass-lined batch still with a single theoretical stage, no reflux, a 6 kW heater, and a 2 m² shell-and-tube condenser. Because simple distillation cannot exceed the isopropanol-water azeotrope of 87.9 wt% IPA at 80.37 °C and 101.3 kPa, the overhead cut is terminated when the vapor temperature reaches 82 °C. This limit holds product water at 12–14 wt%, which is acceptable only for non-optics wiping applications that do not require dry IPA.

Overhead temperature variation of ±3 °C during the cut changes distillate water content by approximately 5 wt%. Batch-to-batch variation in feed water from 8 wt% to 14 wt% shifts the endpoint by 20–30 min. Pre-filtration through a 5 µm polypropylene bag is required to prevent flux solids from forming a thermally insulating layer on the reboiler bottom. The still processes 20 batches/month, each with 50 L feed and a complete cycle of 5 h: 1 h heat-up, 3 h distillation, and 1 h cooling and bottom discharge. Heat-up is limited to 3 °C/min to avoid foam formation from flux residues. Silicone-based antifoams are avoided because they can contaminate the distillate.

Annual feed volume is 12,000 L; at 0.82 kg/L the total spent solvent mass is 9,840 kg. The feed contains 8,364 kg IPA, of which 70% is recovered as 5,855 kg. Distillate mass at 86 wt% IPA is 6,808 kg, leaving 3,032 kg of still bottoms for disposal. Avoided virgin IPA purchase is 8,783 USD/year. Avoided disposal is 8,364 USD/year minus 2,577 USD/year, yielding 5,787 USD/year. Electricity for 240 batches/year at 6 kW for 5 h per batch is 7,200 kWh/year, costing 792 USD/year. Filtration consumables and supervision add 3,100 USD/year. Net annual savings are approximately 10,677 USD, and simple payback for the 45,000 USD installation is 3.9–4.2 years. Published industrial data for this exact single-stage batch configuration is limited; the model is most sensitive to disposal fee and feed water content.

Vapor-Liquid Equilibrium and Molecular Sieve Drying in a 5,000 L/Month Precision Optics Line

Spent 2-propanol from a 5,000 L/month precision optics cleaning line contains 90 wt% IPA, 7 wt% water, and 3 wt% nonvolatile polishing compounds. The product specification requires 99.8 wt% minimum IPA and 0.2 wt% maximum water because residual water creates drying spots and particle deposition. The recovery unit is a 250 L 316L stainless steel batch column with structured packing, a glass-lined reboiler, and a reflux ratio of 2:1. The overhead is condensed at 20 °C. A first cut below 80 °C is collected separately and diverted to fuel blending when methanol exceeds 0.5 wt%; the main fraction is collected between 80.3 °C and 82 °C. Column operation is maintained at 101.3 kPa; reboiler temperature is limited to 110 °C to prevent discoloration from thermal degradation of nonvolatile residues. Waste is pre-filtered through a 5 µm bag and a 1 µm cartridge to protect the structured packing from fouling.

Water polishing after distillation uses a 100 kg bed of 3A molecular sieve at a liquid hourly space velocity of 0.5 h⁻¹. The bed is regenerated with dry nitrogen at 230 °C for 6 h when product water reaches 0.2 wt%. The breakthrough capacity is approximately 40,000 L of processed distillate under the stated feed water load. If the sieve bed is exposed to free water or operated above 60 °C during adsorption, capacity loss exceeds 15%. Distillate quality is verified by ASTM E203 for water and by gas chromatography for organic impurities. The glass-lined reboiler is rinsed with warm water every 20 batches to remove accumulated polishing compound residue. The installation is equipped with a 3 kW vacuum pump for leak testing and a 5 kW chiller for condensation.

Annual feed volume is 60,000 L; at 0.81 kg/L, total feed mass is 48,600 kg. The feed contains 43,740 kg IPA, of which 92% is recovered as 40,241 kg. Distillate mass at 99.8 wt% IPA is 40,322 kg, leaving 8,278 kg of residue. Avoided virgin IPA purchase is 60,362 USD/year. Avoided disposal is 41,310 USD/year minus 7,036 USD/year, yielding 34,274 USD/year. Total electrical load is approximately 34,242 kWh/year, including reboiler duty at 0.72 kWh/kg distillate, vacuum pump, chiller, and sieve regeneration; utility cost is 3,767 USD/year. Labor and maintenance add 11,000 USD/year. Net annual savings are 79,869 USD, and simple payback for the 180,000 USD installation is 2.2–2.4 years.

Continuous recovery in a 50,000 L/month semiconductor carrier-solvent loop is justified by metallic contamination limits and hazardous waste throughput rather than by solvent price alone. Spent 2-propanol from wafer edge-bead removal and cleaning contains 92 wt% IPA, 6 wt% water, and 2 wt% dissolved photoresist and silicon fines. The recovery train consists of a wiped-film evaporator operating at 120 rpm and 120 °C maximum metal temperature, a 2.5 m packed rectification column at 40 kPa absolute, and a pervaporation unit for final water removal to 0.1 wt%. The reduced-pressure column shifts the overhead azeotrope to approximately 91–93 wt% IPA before pervaporation. The pervaporation feed is held at 70 °C with a permeate pressure below 2 kPa; exceeding 75 °C causes membrane swelling and loss of water selectivity. Metallic contamination is controlled by electropolished 316L stainless steel piping, PTFE gaskets, and point-of-use 0.05 µm filtration, because dissolved metals in reclaimed carrier solvent can affect wafer yield at concentrations below 50 ppb.

Photoresist residues create the main process conflict. If the wiped-film evaporator metal temperature exceeds 120 °C, dissolved photoresist components crosslink and form a hard film on heat transfer surfaces. The rotor speed of 120 rpm and residence time below 2 min suppress this fouling. The system is run continuously for 168 h/week with supervisory control and data acquisition for column feed rate, reboiler level, overhead temperature, and pervaporation permeate pressure. Reboiler level is maintained at 60% to avoid exposing heat transfer surfaces. A gas chromatograph sample is drawn every 8 h to confirm IPA purity; water is measured every 4 h by ASTM E203. The pervaporation membrane flux is maintained at 0.4–0.6 kg/m²·h; a flux drop below 0.3 kg/m²·h triggers regeneration or replacement. The recovery train is not recommended for feed containing amine-based strippers because trace amines degrade the pervaporation membrane and may induce stress corrosion cracking in stainless steel at reboiler temperatures.

Annual feed volume is 600,000 L; at 0.82 kg/L, total feed mass is 492,000 kg. The feed contains 452,640 kg IPA, of which 95% is recovered as 430,008 kg. Distillate mass at 99.9 wt% IPA is 430,438 kg, leaving 61,562 kg of residue. Avoided virgin IPA purchase is 645,012 USD/year. Avoided disposal is 418,200 USD/year minus 52,328 USD/year, yielding 365,872 USD/year. Electrical load with heat integration and steam recompression is approximately 175,653 kWh/year; utility cost is 19,322 USD/year. Labor and maintenance add 38,000 USD/year. Net annual savings are 953,562 USD, and simple payback for the 1,200,000 USD installation is 1.2–1.4 years.