What is pervaporation (PV)?

1. Feed preparation and heating. The feed is brought to temperature before it reaches the membranes. In Petro Sep's AZEO-SEP® units the feed is heated, in the batch tank or ahead of the first module, and a feed pump moves it; AZEO-SEP operates at 75 to 85 °C (167 to 185 °F). A hotter feed permeates faster.
2. Sorption. On the feed side, the component with the stronger affinity for the membrane dissolves into its dense surface: water into a hydrophilic membrane, organics into a hydrophobic one.
3. Diffusion. The dissolved molecules diffuse across the membrane. The permeate's composition is set by how readily each component dissolves in the membrane and diffuses through it, not by the vapor-liquid equilibrium, so a component at low concentration in the feed can be highly enriched in the permeate.
4. Desorption under vacuum. At the downstream face the permeate evaporates into the permeate side, which a vacuum pump holds at low pressure. The difference between the permeating component's partial pressure on the feed side and on the vacuum side drives the transfer. In AZEO-SEP's continuous units the modules sit in vacuum chambers.
5. Condensing the permeate. The vapor is condensed to a liquid. AZEO-SEP condenses the permeate ahead of the vacuum pump, which removes the gases that do not condense; VOC-SEP® condenses the vapor leaving the vacuum pump, recovering the organics as a liquid.
6. Reheating between stages. The heat that evaporates the permeate comes out of the liquid feed, so the feed cools along the membrane and the flux falls with it. Plants therefore usually split the membrane area into modules in series with heaters between them; in AZEO-SEP's continuous units, inter-stage heaters supply the heat of vaporization for a higher removal of the permeating component.
7. The retentate. What does not cross leaves the last module as the retentate, concentrated in the components the membrane holds back. In solvent dehydration that is the dried solvent, the product; in VOC removal it is the treated water.

In a batch unit the retentate returns to a heated feed tank, and steps 2 to 5 repeat with each pass until the target purity is reached. The terms used here are defined in the membrane glossary.

The membrane decides which component moves: what crosses, and how readily, depends on each component's affinity for the membrane and its diffusion rate through it, not on the vapor-liquid equilibrium. Petro Sep's AZEO-SEP brochure says the separation is mainly due to the components' difference in polarity, not in volatility. Vacuum is the usual way to create the driving force, and Petro Sep's systems use a vacuum pump; a sweep gas or a temperature difference can do the same job. Temperature matters as well: in a 2024 pilot study, a hotter feed raised the flux and shortened the time needed to reach the target dehydration.

The steps are the same in both; the membrane's affinity sets the direction. Petro Sep's cellulosic ethanol presentation sums it up: AZEO-SEP takes the water out, and VOC-SEP, the opposite technology, keeps the water in the feed. Industrial pervaporation membranes are generally composites, a thin, dense selective layer coated on a porous support. A third kind, organic-organic pervaporation, separates one organic from another (methanol from MTBE is a studied case).

At an azeotrope the vapor and the liquid have similar compositions, so distillation alone can go no further: ethanol and water form an azeotrope at 95.6% ethanol by mass, and isopropanol and water form another. Pervaporation does not depend on that equilibrium, so a suitable membrane can take the separation past the azeotropic point; in the AZEO-SEP brochure's words, its membrane selectivity "is not affected by azeotrope formation". For ethanol and water, two distillation routes past the azeotrope, extractive and azeotropic distillation, both need high energy, and azeotropic distillation uses highly toxic solvents. AZEO-SEP adds no chemicals. The options are compared under breaking azeotropes.

In industrial solvent purification it is applied mainly to dehydrating ethanol and isopropanol. Petro Sep's AZEO-SEP brochure adds these uses: recovering spent solvents in pharmaceutical and chemical plants, where solvents often form azeotropes with water; raising the throughput of distillation columns whose feed components differ little in volatility; raising the yield of equilibrium-controlled reactions by removing water continuously with an AZEO-SEP and distillation hybrid, as when esters form from acids and alcohols; and drying solvents captured in activated carbon beds and desorbed with steam. VOC-SEP returns valuable organics from waste streams to the process and concentrates wastewater, cutting the volume sent for disposal. See solvent recovery and purification and VOC removal from process water and wastewater.

Ethanol, MTBE, THF, methyl acetate and ethyl acetate appear on both lists: which membrane applies depends on whether water or the organic is the component to remove. Both brochures invite enquiries about solvents that are not listed.

The pilot's goal was to bring organics down far enough for sewer discharge instead of incineration, and it reached sewer-discharge levels on the three feeds. Flux is not a single figure: it depends on the membrane, the mixture and the temperature.

In the hybrid, the column does the first separation and the membrane takes the overhead past the azeotrope to the final dehydration. Petro Sep also states that a hybrid unit on a column with low volatility differences "will increase throughput and reduce operating costs". VOC-SEP units can be designed for batch or continuous operation, depending on the volume to be treated, and continuous units may need several modules in series to reach the desired separation. Petro Sep supplies skid-mounted units on a turnkey basis; system choices for a given plant are covered under pervaporation systems.

• Distillation separates components that differ in volatility and boiling point. It can bring ethanol from a fermentation broth close to the azeotrope, but it cannot pass an azeotrope on its own, and columns separating components with low volatility differences need high reflux.
• Molecular sieves are widely used adsorbents for drying ethanol. The adsorbent has to be regenerated, and the desorption step needs high temperature, low pressure or both, which weighs on process cost.
• Pervaporation runs continuously, needs no added chemicals and combines readily with other processes. Only the permeate evaporates, so the latent heat goes to the permeate alone; heating the feed, the vacuum pump and cooling the condenser use energy too. It can be added to an existing entrainer-distillation or molecular-sieve unit rather than replacing it.

• Solids. When dissolved or undissolved solids are present in the feed, one practice is to evaporate it first and use vapor permeation instead.
• Heat. Evaporating the permeate cools the feed; the temperature drop limits the size of a module, and staged reheating needs heat exchangers between modules.
• Membrane stability. The membrane has to keep its transport properties despite swelling and concentration polarization. Organic-organic mixtures are harder for polymeric membranes, because polymers swell much more in pure organic solution.
• The minor component. Studies of distillation-pervaporation hybrids for azeotropes found it expedient to let the membrane extract the component at lower concentration in the feed. VOC-SEP suits feeds with 0.2% to 10% VOCs.
• Volatility, for VOC-SEP. The organics should boil below water for optimum performance, and the degree of separation is limited by the equilibrium of the VOCs between the liquid and vapor phases.
• Scale and cost. A 2022 study names production capacity and installation cost as the limitations of pervaporation.

The case studies are the Virginia hybrid refinery and the Sarnia bioethanol and ABE plant. Petro Sep's Company Profile also shows a VOC-SEP unit in Canada and a food-grade hybrid AZEO-SEP, AQUA-SEP and VOC-SEP project in China, without dates. Its project table lists completed solvent projects at various US locations from 2005 to 2008 (25,000 to 35,000 L/day), a completed 2012 solvents project in Virginia of 200,000 L/day, a 50,000 L/day biofuel plant in Ontario from 2010, and 2023 bioethanol projects of 100,000 L/day each in Canada and Malaysia, listed as ongoing; the table does not name the technology used.

Petro Sep started in 1999 as a membrane research and development company. A 2002 paper by C. K. Yeom, M. Kazi and F. U. Baig, written at Petrosep Membrane Research Inc. in Oakville, "Simulation and Process Design of Pervaporation Plate-and-Frame Modules for Dehydration of Organic Solvents" (Membrane Journal), modeled the dehydration of organic solvents in commercial-scale plate-and-frame pervaporation modules, which hold a stack of membrane sheets. It simulated continuous and batch operation. For a fuller treatment of the process, see Fakhir U. Baig's chapters "Pervaporation", in Advanced Membrane Technology and Applications (Wiley, 2008), and "Pervaporation and Hybrid Vacuum Membrane Distillation Technology and Applications", in Membrane Separation Principles and Applications (Elsevier, 2019).

Vapor permeation suits streams that are already vapor, such as the top streams of fractionation columns. More in What is vapor permeation (VP)?, on vapor permeation systems and in What is membrane distillation (MD)?; Petro Sep's vacuum membrane distillation platform is AQUA-SEP.