Page 383 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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318 Membranes, Synthetic, Applications

is used to recover wash water from dye ranges and caustic working industry, and organics recovery (e.g., ethylene
soda from scouring effluents. Dynamic membranes pre- glycol). Over the past decade, the number of applications
pared from zirconium hydroxide and polyacrylic acid, for and the scale of their implementation continue to grow.
example, are well suited for these applications because So has the range of nanofiltration membranes and systems
they can withstand high temperatures and wide pH ranges, available commercially.
and because performance can be restored by stripping and
reforming the membrane in lieu of cleaning.
C. Ultrafiltration
Many membranes exhibit good rejection toward low
concentrations of alcohols, aldehydes, esters, and other UF is a membrane process useful for separating macroso-
organic compounds. However, organic liquid mixtures are lutes according to differences in molecular size and shape.
as yet seldom separated by reverse osmosis because few The fundamentals controlling this process, involving hy-
membranes developed for desalination exhibit adequate drodynamic sieving, have been discussed in the earlier
chemical resistance. Moreover, the high osmotic pressures section on mechanisms. The membranes used in UF al-
associated with concentrated solutions can drastically re- low free passage of solvent and solutes with molecular
duce the effective driving force and thus the productiv- weights below several hundred daltons, while retaining
ity. As an example, fermentation alcohol containing about specieslargerthanacharacteristicmolecularweightcutoff
8–10% ethanol may be concentrated to only about 60% (MWCO). MWCO is a semiquantitative way of specifying
using present RO technology. As noted earlier, for such the size discrimination characteristics of an ultrafiltration
applications, the problem of high osmotic pressures can membrane (a common definition being that 90% of the so-
be resolved with another membrane process known as per- lutes with molecular weights exceeding the MWCOwould
vaporation (q.v.). be rejected by the membrane). Substances that are sepa-
A notable shift has occurred over the past decade rated effectively by ultrafiltration include colloids, soluble
toward operating RO systems at gradually lower pressures polymers, and dispersions with molecular weights from a
while maintaining the high productivity once associated fewthousandtoabout1milliondaltons.Ingeneral,species
with high-pressure systems. This is in large part an energy whose molecular weights differ by two orders of magni-
efficiency consideration; lower power consumption will tude or more may be fractionated by ultrafiltration.
make desalination by RO attractive to a broader range Diafiltration is a variation of ultrafiltration, in which
of the global population, for whom the supply of high- fresh solvent is added to the feed solution to replenish
quality drinking water will become increasingly critical the volume ultrafiltered, and in the process washes small
in the future. molecules such as salts away from the retained macro-
molecules. Using appropriate replenishing solutions, di-
afiltration is a common procedure to perform buffer ex-
B. Nanofiltration
change of proteins. Alternatively, a dilute solution may
Since the mid-1980s, ultralow-pressure reverse osmosis— be first ultrafiltered to concentrate the feed material, then
sometimesreferredtoas“nanofiltration”—systemsopera- diafiltered to purify the retentate. It is sometimes possi-
ting between 5 and 10 bars have gained considerable favor ble to fractionate a mixture of macrosolutes by sequential
for groundwater softening, organics removal, and even do- diafiltration with a series of membranes of progressively
mestic point-of-use water treatment. These systems em- lower molecular weight cutoff ratings.
ploy “loose RO” membranes with good rejection toward Electrocoat paint recovery in the automotive manufac-
color substances and organic compounds with molecular turing and metal finishing industries is a major UF appli-
weights of several hundred to about 1000 daltons, but only cation. Electrocoating refers to the process of depositing
moderately retentive of monovalent salts. These operating electrophoretic paint from an aqueous dispersion onto im-
characteristics meet the needs for aqueous separations wh- mersed, charged metal surfaces. Thin coatings with uni-
ere high productivity and low operating costs are crucial. form coverage in recessed areas are obtained. After coat-
Concerns about groundwater contamination and munic- ing, the metal part is freed of excess paint by rinsing with
ipal water supply quality have driven much of the growth water. To help the process operate consistently, the paint
of various water treatment schemes involving nanofiltra- dispersion is continually purified through an ultrafiltration
tion as a stand-alone process or in combination with RO loop as shown in Fig. 31. Water containing accumulated
and/or UF in a broad range of water treatment systems salts and additives is removed, and the recycled paint is
delivering precise purity levels and attractive process eco- reconstituted with fresh water and solvent and returned
nomics. Other established applications include corn syrup to the immersion tank. In this way, UF reduces the cost
concentration, recycling of water-soluble polymers, efflu- of wastewater treatment by minimizing water discharge
ent treatment for the food and beverage industry, metal and recovers valuable paint for reuse. An indication of