Page 352 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering
P. 352
P1: GLQ Final Pages
Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57
Membranes, Synthetic, Applications 287
salt-containing feed and almost salt-free permeate transmembrane osmotic pressure difference between the
streams,
π, must be overcome to drive water to the per- feed and product water (∼50 psi), the extrapolated water
meate side. The osmotic pressure difference,
π, between flux is essentially zero.
two solutions of different concentration is the pressure dif- Application of RO for water production is now a
ference that exists when there is no difference in chemical well-accepted and economical process even for higher
potential of water on the two sides of the membranes. concentration seawater with osmotic pressures of over
Neglecting convection effects, the solution-diffusion 300 psi. Malta, for example, has evolved an economical
model gives the following expressions for water (1) and reliable application of this technology to produce 60%
salt (2) molar fluxes through a membrane with a selective of its potable water supply (Lamendolar and Tua,
layer thickness of L and a transmembrane pressure drop 1995).
+
p (Merten, 1966): Rejections of other ions besides Na and Cl are tun-
−
ˆ
J A = D A K A V A [
p −
π]/LRT, (9a) able characteristics of the reverse osmosis membranes that
depend upon the intrinsic nature of polymer separating
J B = D B K B
C B /L, (9b) layer and how it has been processed. In general, bivalent
ions like Ca 2+ and SO 2− are more easily rejected than are
where D A and K A and D B and K B are the diffusion coef- 4
monovalent ones like Na and Cl .
+
−
ficient and partition coefficients for water and salt in the
membrane, respectively. The partial molar volume of wa-
ˆ
ter, V A , is generally well approximated by the pure com-
ponent molar volume. The observed salt rejection coeffi- II. MEMBRANE MATERIALS, GEOMETRY,
cient is given in terms of external bulk salt concentrations AND PACKAGING
2
(moles/cm ) and known fluxes as shown below:
A. Membrane Material Selection
C B j B j B
R o = 1 − = 1 − ≈ 1 − . (10)
C bulk C bulk C bulk ˆ Membranes used for separation are thin selective barriers.
B B j v B j A V A
They may be selective on the basis of size and shape,
Increasing the (
p −
π) term in Eq. (9a) clearly in-
chemical properties, or electrical charge of the mate-
creases rejection, since the flux of solvent (water) in-
rials to be separated. As discussed in previous sections,
creases proportionally to this factor, while the flux of salt
membranes that are microporous control separation pre-
is essentially independent of it, within the accuracy of the
dominantly by size discrimination, charge interaction, or
approximations of the model. A typical example of such
a combination of both, while nonporous membranes rely
behavior is shown in Fig. 4 as a function of feed pressure
on preferential sorption and molecular diffusion of indi-
◦
at 25 C for a brackish water feed with low salt concen-
vidual species. This permeation selectivity may, in turn,
tration (0.5 wt % or 0.16 mol %). As expected based on
originate from chemical similarity, specific complexation,
Eq. (9a), when the applied transmembrane
p equals the
and/or ionic interaction between the permeants and the
membrane material, or specific recognition mechanisms
such as bioaffinity.
A membrane material should meet several criteria: it
should be chemically and physically stable under anti-
cipated operating conditions, have the permselectivity re-
quired for a given process design, and be conveniently
fabricated into membrane form. Polymers are the most
frequently used membrane materials as they offer a wide
spectrum of properties. Specialty membranes made of in-
organic materials such as ceramics, metals, and carbon are
also available. Their ability to withstand extreme tempera-
tures and harsh chemical conditions enables their deploy-
ment in applications not addressed by polymeric mem-
branes. Membranes used in the life sciences are designed
to contact delicate biological or biochemical materials; a
high degree of biocompatibility and hydrophilicity is nec-
2
FIGURE 4 Flux in GFD (gal/ft /day) and rejection of NaCl at 25 C
◦
for atmospheric pressure permeate with increasing applied feed essary to minimize nonspecific interaction and the conse-
pressure with a 5000 mg/L salt feed. The membrane is an asym- quent degeneration in membrane performance or damage
metric polyamide. to the biological material.