Page 384 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Membranes, Synthetic, Applications 319

FIGURE 31 An ultrafiltration electrocoat paint recovery system. [Warashina et al. (1985, September). Chemtech,
pp. 558–561.]

the favorable economics is the typical payback period of Another example of using ultrafiltration for wastewater
less than a year for an ultrafiltration installation in mod- treatment and resource recovery is the separation of oil–
ern automotive plants, where adoption of UF technology water emulsions generated from metal machining, oil field
is practically universal. wastes, and enhanced oil recovery effluents. Hydrophilic
The dairy industry was also an early beneficiary of ultra- membranes such as cellulose acetate are preferred because
filtration technology. Major uses include the concentration they are effective barriers to oil droplets and are less prone
ofwholemilkorskimmilkforcheesemaking(seeFig.32); tofouling.TheUFpermeatereadilymeetsdirectdischarge
the recovery, fraction, and desalting of whey protein con- standards. The oil-rich stream can be processed to reclaim
centrates, and volume reduction of raw milk at the farm to the oil, or disposed at reduced transportation cost because
decrease transportation costs. Recently, ultrafiltration has of its reduced volume.
been used to increase the protein content of skim milk. In the petroleum industry, dewaxing solvents are sepa-
This creates a richer taste that is normally associated with rated by ultrafiltration from dewaxed oils by chemically
part-skim milk. In other segments of the food process- resistantmembranesmadefrompolysulfoneorpolyimide.
ing industry, ultrafiltration is used to concentrate gelatin, In a related process, pentane is separated from deasphalted
clarify fruit juices, removing proteinaceous impurities heavy oil under conditions intermediate between reverse
from wines to improve shelf life, and recover protein from osmosis and ultrafiltration (ca. 15 bar applied pressure).
soybean processing and from fishing industry wash water. High-molecular-weight hydrocarbons in the oil form a
gel layer on the surface of a polysulfone support mem-
brane. This gel restricts passage of heavier hydrocarbons
but not pentane, which is recovered as permeate. To sep-
arate other hydrocarbon mixtures that do not contain gel-
forming components, polymeric additives would be used
as a rejecting barrier substitute.
Textile sizing agents such as polyvinyl alcohol may also
be reclaimed from hot process water. Here, both poly-
meric membranes and inorganic, dynamic membranes are
appropriate choices. Systems based on polymeric mem-
branes operate at lower fluxes and require less recircula-
tion pumping, and are somewhat more economical. Plants
3
with treatment capacities as high as 60 m per hour are in
operation.
Several important UF applications are still in the devel-
opment stage. For example, metal recovery from plating
wastes has been proposed by using a flocculant or a chelat-
FIGURE 32 A nine-stage ultrafiltration plant concentrating whole
ing polymer to bind the metal ions, then recovering the
milk from 8 to 40% total solids for cheesemaking. (APV Crepeco
Inc.) polymer complex by ultrafiltration. The metal value may