Separation processes Dr Separation processes Dr. Hassan Sawalha Chemical Engineering Department An-Najah National University

Two words: permselective and evaporation Pervaporation Two words: permselective and evaporation Two phase system Liquid feed and vapor permeate. Asymmetric composite membrane Selective for species A Species B usually has some finite permeability. The dense film is in contact with the liquid side Vacuum vaporization

Water selective: Hydrophilic membrane

Organic selective: hydrophobic membrane

Applications dehydration of ethanol Pervaporation is best applied when the feed solution is dilute in the main permeant dehydration of ethanol dehydration of other organic alcohols, ketones, and esters (3) removal of organics from water. (4) separation of organic mixtures, e.g., benzene-cyclohexane, is receiving much attention.

Hybrid process distillation-pervaporation for removal of water from ethanol. (retentate) Pervaporation unit 95.6 % ethanol 25 % ethanol (permeate) 60 % ethanol 40% water The distillate purity is limited because of the 95.6 wt% ethanol in water azeotrope.

Transport equations in pervaporation Because phase change and nonideal-solution effects in the liquid feed Simple equations i.e. for dialys is do not apply to pervaporation. A particularly convenient PV model is that of Wijmans and Baker They express the driving force for permeation in terms of a partial-vapor-pressure difference. Because pressures on the both sides of the membrane are low, the gas phase follows the ideal-gas law.

Therefore, at the upstream membrane surface (I), permeant activity for component i is expressed as:

Flux

Gas Permeation the feed gas at high pressure PI, contains some low-molecular-weight species (MW < 50) higher-molecular-weight species. Usually a sweep gas is not used, permeate side of the membrane is maintained at a much lower pressure, P2, often near-ambient pressure. The membrane is often dense permselective for certain of the low-molecular-weight species.

Applications (1) separation of hydrogen from methane; (2) adjustment of H2-to-CO ratio in synthesis gas (3) O2 enrichment of air (4) N2 enrichment of air; (5) removal of C02; (6) drying of natural gas and air

Transport equation In dense membranes species absorbed at the surface then transported through the membrane by one or more mechanisms. Permselectivity depends on both membrane absorption and the membrane transport rate. Usually all mechanisms are formulated in terms of a partial-pressure

Flux

solution

Ultrafiltration Ultrafiltration and microfiltration are more commonly used for recovering the solutes

Rejection

Concentration factor

Process Configurations An ultrafiltration process is commonly conducted in one of four configurations or combinations: (1) batch ultrafiltration, (2) continuous bleed-and-feed ultrafiltration, (3) batch diafiltration (4) continuous bleed-and-feeddiafiltration.

Batch Ultrafiltration

Separation with ultrafiltration

HW

Continuous Feed-and-Bleed Ultrafiltration A large fraction of the retentate is recycled at steady state Bleed is that portion of the retentate that is not recycled, but is withdrawn as product retentate At startup the entire retentate is recycled Until the desired retentate concentration is achieved, At which time bleed is initiated

The advantages and disadvantages of feed-and-bleed operation The single-pass mode is usually unsuitable for ultrafiltration because the main product is the concentrate rather than the permeate (as in reverse osmosis) High yields of permeate are required in order to adequately concentrate solutes in the retentate a single-pass ultrafiltration requires a very long membrane path or a very large membrane area

The advantages and disadvantages of feed-and-bleed operation with the high recycle ratio, the concentration of solutes on the retentate side is high resulting in the lowest flux, Larger membrane area

Solution: Multistage continuous feed-and-bleed ultrafiltration where the retentate (bleed) from each stage is sent to the next stage, while the permeates from the stages are collected into a final composite permeate the final and highest concentration is only present in the final stage.

Diafiltration Involves the addition of solvent (usually water) to the retentate, followed by filtration. Additional solvent dilutes the retentate so as to increase the flux. Thus ultrafiltration is employed to a certain limiting concentration of solutes, followed by diafiltration to further enhance solute separation. The final retentate may not be very concentrated in retained solutes, but it contains a smaller fraction of permeable solutes

MICROFILTRATION microfiltration is a pressure-driven, microporous membrane process used to retain matter commonly of 0.1-10 microns. the matter may include large colloids, small and solid particles, blood cells, yeast, bacteria and other microbial cells, and very large and soluble macromolecules

Membrane structures for microfiltration screen filters that collect retained matter on the surface depth filters that trap particles at constrictions within the membrane

depth filters include: relatively thick, high-porosity (80-85%) castcellulose-ester membranes having an open, tortuous, sponge-like structure; thin, low-porosity (nominal 10%) polyester or polycarbonate track-etch membranes of a sieve-like structure with narrow distribution of straight through,cylindrical pores. The latter have a much sharper cutoff, resulting in enhanced separation factors

Common modes of microfiltration.

Transport equations Equations for computing TFF microfiltration are those developed for ultrafiltration. This includes batch, continuous feed-and-bleed, and diafiltration operation modes. Equations for DEF microfiltration are those for conventional, batch, solid-liquid, slurry filtration, frequently referred to as cake filtration.

Transport equations DEF microfiltration

improvements in yield by a combined operation in which: Constant-flux operation is employed in Stage 1 up to a limiting pressure drop, followed by (2) Constant-pressure operation in Stage 2 until a minimum flux is reached

Constant-Flux Operation

Constant-Pressure Operation