Recovery of liquid CO₂ from cleaning solutions without phase change using ultrafiltration and microfiltration membranes

Recovery of liquid and supercritical fluid (SCF) CO₂ in the application of CO₂ as processing solvent in extraction, chemical, and biochemical reactions is an important issue. Membrane-based separation may provide an option in recovering CO₂ in liquid and/or SCF phase without expensive, energy intensive recompression. The potential application of microporous inorganic membranes in separating liquid CO₂ without phase change was investigated. A high-pressure membrane filtration unit was designed, built, and tested. Microporous stainless steel (0.2 μm) and ceramic (0.02 μm, and 1000 Dalton) tubular elements were retrofitted into in-house designed, high-pressure housings to test the functionality of the system using Triton X-100 solute in liquid CO₂ as feed. In-house designed fiberoptic cells coupled with a UV-VIS spectrophotometer was used for on-line measurement of solute Triton X-100 in the feed and permeate streams. The fiberoptic cell coupled with UV-VIS spectrophotometer was capable of providing on-line concentration measurement of Triton X-100 at operating pressures up to 100 bar. Two different path-length fiberoptic cells, 5.0 mm and 0.5 mm, were designed and built to cover a wide range of solute concentrations. Membrane filtration experiments with ceramic membranes showed that 1000-Dalton membrane offered a solute rejectivity of about 80%. The coarse ceramic membrane (0.02 μm) had a lower rejectivity, which was about 55%. The 0.2-μm stainless steel membrane provided very little solute rejectivity.