Page 372 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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

Membranes, Synthetic, Applications 307

4. Water Vapor Separation amount of air also leaks through the pores and carries
the water vapor away from the membrane. In this way,
Increasing environmental awareness and rising costs of
the accumulated water vapor in the membrane sublayer is
energy and chemical supplies have helped spur interest
continually swept away, thereby preventing condensation
in membrane processes as a means of recovering those
and loss of water removal capability. Micropores normally
resources. Although vapor separation is closely related
considered defects in the membrane are in fact a necessary
to gas permeation in mechanism, the presence of con-
feature in this membrane design. Very low dew points can
densable components permits unique process designs and
be reached with this membrane unit with no additional
opportunities for energy recovery.
power input and no moving parts. The inherent reliability
Water vapor separation from air is an example where
and simplicity of this product makes it attractive for instru-
significant energy recovery is possible. Many industries,
mentation applications and at locations that are difficult
including pulp-and-paper, textile, and food processing,
to access.
use large amounts of energy to dry their products. The
water vapor in the exhaust air from the dryer carries with
it the latent heat of vaporization. Reclaiming this latent 5. Organic Vapor Separations
heat could substantially reduce the net thermal energy re-
Organic vapor separation from air is a means of controlling
quirement of the drying process. Heat exchange between
pollutionandrecoveringvaluableresources.Anindication
incoming and exhaust streams is inefficient because only
of the environmental problem is the more than 30 mil-
sensible heat is recovered, and because the temperature
lion tons per year of volatile organics emitted in 1975
differential is usually small. Condensing the water vapor
from all stationary sources in the United States. These
by recompressing the moist exhaust air would release the
included petroleum refineries, chemical plants, and de-
latent heat, but this approach is also inefficient because
fective solvent storage and conveyance facilities. Much of
most of the energy is expended in compressing the major
the vapors emitted—hydrocarbons, chlorinated solvents,
component—air. A membrane-aided vapor-compression
alcohols, and ketones—are potentially recoverable with
process shown in Fig. 19 could be a more efficient alterna-
membranes highly permeable to organics and relatively
tive. Moisture from the dryer exhaust would pass through a
impermeable to air. A scheme for treating a solvent-laden
hydrophilic membrane and be compressed. The latent heat
air stream from a drying oven is shown in Fig. 20, where
liberated can be used to preheat the feed air for the dryer. It
liquid solvent is recovered by compressing the permeate
has been estimated that more than half of the energy used
stream, and hot air is recycled to the drying oven.
in a paper drying machine can be reclaimed in this way.
In gas- or vapor-phase chemical reactors, the product
Economic feasibility of this scheme hinges on the relative
stream typically contains some residual reactants and one
costs of energy and that of the membrane system.
or more inert gases. At the end of the reaction cycle the
Already commercialized is an innovative membrane
product is recovered—for example, by condensation—
module capable of in-line drying of air. Moist air is fed
and the remaining gases are vented to the atmosphere or
to one side of an inherently water-permeable membrane,
flared. Significant quantities of reactants can be lost in this
which has a low density of surface pores. While water va-
way, some adding to the environmental burden of the pro-
por diffuses through the membrane preferentially, a small
cess. Membrane systems have been designed to address
both issues. By selecting a membrane more permeable to
the reactant than to the inert gases, a highly concentrated
reactant stream can be collected as permeate and recycled

FIGURE 19 Concept of membrane-assisted recompression for FIGURE 20 A membrane system for solvent recovery and waste
energy recovery from drying operations. (Bend Research, Inc.) heat capture. (Membrane Technology and Research, Inc.)