Page 378 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GLQ Final Pages
Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

Membranes, Synthetic, Applications 313

C. Osmotic Distillation One approach to delivering increased performance in a
membrane process is to complement one separation mech-
Osmotic distillation also removes the solvent from a solu-
anism with another. Vapor-arbitrated pervaporation is an
tion through a microporous membrane that is not wetted
example of this strategy. In bioseparations, as will be cov-
by the liquid phase. Unlike membrane distillation, which
ered in a later section, a similar integration of several pro-
uses a thermal gradient to manipulate the activity of the
cess enhancements in High-Performance Tangential Flow
solvent on the two sides of the membrane, an activity gra-
Filtration is responsible for dramatic improvement in sep-
dient in osmotic distillation is created by using a brine
aration efficiency of protein mixtures once considered un-
or other concentrated solution in which the activity of the
achievable by means of conventional ultrafiltration.
solvent is depressed. Solvent transport occurs at a rate pro-
portional to the local activity gradient. Since the process
operates essentially isothermally, heat-sensitive solutions
may be concentrated quickly without an adverse effect. V. LIQUID SEPARATIONS
Commercially, osmotic distillation has been used to de-
water fruit juices and liquid foods. In principle, pharma- Membrane processes have been applied successfully to a
ceuticals and other delicate solutes may also be processed wide variety of liquid separations. Table VIII lists a num-
in this way. ber of typical applications by industry and by technology.
In the following sections, the function and applications of
each process are illustrated by commercialized examples.
D. Vapor-Arbitrated Pervaporation The outlook of each technology segment is also discussed.

In certain cases it is desirable to selectively remove a vola-
tile solute from a solution that contains other, less volatile, A. Reverse Osmosis
solutes as well as the solvent. Some examples are the re-
duction of ethanol content from alcoholic beverages or ROoccurswhenasolutionispressurizedagainstasolvent-
from dilute alcoholic extracts of aromatic flavors and fra- selective membrane, and the applied pressure exceeds the
grances from plant sources such as fruits or flowers. Con- osmotic pressure difference across the membrane. Water is
ventional pervaporation would facilitate removal of water the solvent in most existing reverse osmosis applications;
from such mixtures while retaining ethanol and the higher the solutes may be salts or organic compounds.
molecular weight organics that comprise the characteristic Reverse osmosis for desalting seawater and brackish
aroma and flavor profile of the products of interest. On the water was the first industrial-scale application of modern
other hand, membrane distillation or osmotic distillation membrane technology. The principles and practice of RO
cannot retain the volatile components at all. technology are well established, with a worldwide desali-
A process referred to as vapor-arbitrated pervaporation nation capacity reaching 6.8 billion gallons of water per
addressestheseissuesbymanipulatingthetransmembrane day at the end of 1999. Several factors contributed to the
activity gradients of water and ethanol in a pervaporation success of reverse osmosis for desalination: the process
system. Using a permeate side sweep stream that con- is more energy-efficient than distillation, high-flux mem-
tains water vapor at a partial pressure corresponding to branes with good salt rejection have become more durable,
the activity of water on the feed side, permeation of water lower cost commodity products. Modern RO plants are
3
is halted while ethanol continues to diffuse through the capable of producing potable water at less than $1/m in-
membrane into the sweep stream and is removed. In this cluding all capital and operating costs.
way, the native permselectivity of the membrane system Seawater contains about 3.5 wt % of total dissolved
can be altered in a controlled fashion to extract one or solids (TDS) in most locations of the world. Typical RO
more volatile components from a solution. systems operate between 50 and 70 bars, and require
The concept of this process has been demonstrated for less than 10 kWh in energy to produce one cubic meter
lowering or increasing the alcohol content of distilled spir- of potable water with less than 0.05 wt % TDS. This is
3
its by using water vapor or ethanol vapor in the sweep substantially lower than the 15–16 kWh/m required for
stream, respectively. In either case, this vapor arbitration multistage flash distillation technology. Although thermal
action combined with the inherent selectivity of the mem- desalination is well established and reliable, the energy
brane resulted in virtually complete preservation of the advantage of reverse osmosis favors the overall eco-
3
subtle character of the beverage, but in a more concen- nomics of membrane systems as large as 75,000 m /day
trated form due to the lower net volume of the retentate in capacity. Figure 29 shows a seawater reverse osmosis
product (Lee, 1993). Similar results may be anticipated in desalination facility. Over the last two decades, reverse
other volume reduction applications involving high-value osmosis has captured an increasing share of the desalina-
volatile feedstocks. tion market previously dominated by distillation—even