4. Introduction
• In a dialysis membrane-separation process, the feed is a liquid,
at pressure P1, containing solvent, solutes of type A, and solutes
of type B and/or insoluble, but dispersed, colloidal matter.
• A sweep liquid or wash of the same solvent is fed at pressure P2
to the other side of the membrane.
• The membrane is thin with micropores of a size such that solutes
of type A can pass through by a concentration driving force.
• Solutes of type B are larger in molecular size than those of type
A and pass through the membrane only with difficulty or not at
all.
5. Introduction
• Colloids do not pass through the membrane.
• With pressure P1 = P2, the solvent may also pass through
the membrane, but by a concentration-driving force
acting in the opposite direction.
• The transport of the solvent is called osmosis.
• By elevating P1 above P2, solvent osmosis can be
reduced or eliminated if the difference is higher than the
osmotic pressure.
6. Contd…
• The products of a dialysis unit (dialyzer) are a liquid
diffusate (permeate) containing solvent, solutes of
type A, and little or none of type B solutes; and a
dialysate (retentate) of solvent, type B solutes,
remaining type A solutes, and colloidal matter.
• Ideally, the dialysis unit would enable a perfect
separation between solutes of type A and solutes of
type B and any colloidal matter
7.
8. Example
• When dialysis is used to recover sulfuric acid (type A
solute) from an aqueous stream containing sulfate salts
(type B solutes), the following results are obtained
• Thus, about 64% of the H2SO4 is recovered in the
diffusate, accompanied by only 6% of the CuSO4, and no
NiSO4.
9. Applications
• Recovery of chromic, hydrochloric, and hydrofluoric acids from
contaminating metal ions.
• Removal of alcohol from beer to produce a low-alcohol beer.
• Recovery of nitric and hydrofluoric acids from spent stainless steel pickle
liquor.
• Removal of mineral acids from organic compounds.
• Removal of low-molecular-weight contaminants from polymers.
• Hemodialysis, in which urea, creatine, uric acid, phosphates, and chlorides
are removed from blood without removing essential higher-molecular
weight compounds and blood cells in a device called an artificial kidney
10. Membrane Types & Modules
• Typical microporous-membrane materials used in dialysis are
hydrophilic, including cellulose, cellulose acetate, various acid-
resistant polyvinyl copolymers, polysulfones, and
polymethylmethacrylate.
• Typically less than 50 mm thick and with pore diameters of 15
to 100 Ȧ
• The most common membrane modules are plate-and-frame
and hollow-fiber.
• Dialysis membranes can be thin because pressures on either
side of the membrane are essentially equal.
11. Contd…
• A common dialyzer is the plate-and-frame type. For
dialysis, the frames are vertical and a unit might
contain 100 square frames, each 0.75 m 0.75 m on
0.6-cm spacing, equivalent to 56 m2 of membrane
surface.
• Recent dialysis units utilize hollow fibers of 200-mm
inside diameter, 16-mm wall thickness, and 28-cm
length, packed into a heat exchanger- like module to
give 22.5 m2 of membrane area.
12. Contd…
• In a plate-and-frame dialyzer, the flow pattern is nearly
countercurrent.
• Because total flow rates change little and solute
concentrations are small, it is common to estimate solute
transport rate by assuming a constant overall mass-
transfer coefficient with a log-mean concentration-
driving force
13. Rate Equation
• The differential rate of solute mass transfer across
the membrane is
• where Ki is the overall mass-transfer coefficient, in terms of
the three coefficients
• where kiF and kiP are mass-transfer coefficients for the feed-
side and permeate-side boundary layers (or films)
14. Contd…
• If a solute does not interact with the membrane material,
effective diffusivity De , is the ordinary molecular-diffusion
coefficient, which depends only on solute and solvent
properties.
• In practice, the membrane may have a profound effect on
solute diffusivity if membrane–solute interactions such as
covalent, ionic, and hydrogen bonding; physical adsorption
and chemisorption; and increases in membrane polymer
flexibility occur.
• Thus, it is best to measure PMi experimentally using process
fluids.
15. • Transport of solvents such as water, usually in a direction
opposite to the solute, can be described in terms of Fick’s law
• It is common to measure the solvent flux and report a so-
called water-transport number, which is the ratio of the
water flux to the solute flux, with a negative value indicating
transport of solvent in the solute direction
• Design parameters for dialyzers are best measured in the
laboratory using a batch cell with a variable-speed stirring
mechanism on both sides of the membrane so that external
mass-transfer resistances, 1/kiF and 1/kiP are made negligible
16. Electrodialysis
• Electrodialysis is an electrically driven membrane
separation process.
• Electrodialysis refers to an electrolytic process for
separating an aqueous, electrolyte feed into
concentrate and dilute or desalted water by an
electric field and ion-selective membranes.
17. • In Electrodialysis, cation exchange membranes are
alternated with anion exchange membranes in a
parallel array to form solution compartments of
thickness ∼1 mm.
• The cation-selective membranes (C) carry a negative
charge, and thus attract and pass positively charged
ions (cations), while retarding negative ions (anions).
• The anion-selective membranes (A) carry a positive
charge that attracts and permits passage of anions.
Both types of membranes are impervious to water.
18. • The net result is that both anions and cations are concentrated in
compartments 1, 3 and 5, from which concentrate is withdrawn,
and ions are depleted in compartments 2 and 4, from which the
demineralized solution is withdrawn.
19. • A direct-current voltage causes current to flow through the
cell by ionic conduction from the cathode to the anode.
• A cell pair or unit cell contains one cation-selective
membrane and one anion-selective membrane.
• A commercial electrodialysis system consists of a large
stack of membranes in a plate-and-frame configuration,
contains 100 to 600 cell pairs.
• In a stack, membranes of 0.4 to 1.5 m2 surface area are
separated by 0.5 to 2 mm with spacer gaskets
20. • The total voltage or electrical potential applied
across the cell includes
1. The electrode potentials
2. Over voltages due to gas formation at the two electrodes
3. The voltage required to overcome the ohmic resistance of
the electrolyte in each compartment
4. The voltage required to overcome the resistance in each
membrane,
5. The voltage required to overcome concentration polarization
effects in the electrolyte solutions adjacent to the
membrane surface.
21. Applications
• Production of drinking water by the desalination of sea-water or
brackish water.
• Production of sodium chloride for table salt
• Recovery of nickel and copper from electroplating rinse water
• deionization of cheese whey, fruit juices, wine, milk, and sugar
molasses
• Separation of salts, acids, and bases from organic compounds
• Recovery of organic compounds from their salts
23. Introduction
• When miscible solutions of different concentration are
separated by a membrane that is permeable to the
solvent but nearly impermeable to the solute, diffusion
of the solvent occurs from less concentrated to the more
concentrated solution, where solvent activity is lower.
• The diffusion of solvent is called Osmosis.
• Osmotic transfer of water occurs in many plant and
animal cells
24.
25. Introduction
• The transfer of solvent can be stopped by increasing the
pressure of the concentrated solution until the activity of
solvent is same on both sides of the membrane.
• If pure solvent is on one side of the membrane, the
pressure required to equalize solvent activities is the
osmotic pressure of the solution.
• If pressure higher than the osmotic pressure is applied,
solvent will diffuse from concentrated solution to dilute
solution. This Phenomenon is called Reverse Osmosis
27. Basic Terminology
• Feed water: Supply water that is fed into the RO system to be
treated
• Permeate: A portion of the feed water that passes through a series
of membranes and is returned as purified water.
• Concentrate: A portion of the feed water that is rejected by the
membrane and contains the solution of impurities that have been
filtered out of the permeate.
• Water flux: The rate of permeate production typically expressed as
the rate of water flow per unit area of membrane (e.g., gallons per
square foot per day)
• Recovery rate: The ratio of permeate flow to feed water flow, which
indicates the overall water efficiency of the system
29. RO Membrane Properties
• More than 50% of RO modules use cellulose acetate
membrane.
• It has high permeability for water and low permeability for
dissolved salts.
• Its limitation are:
I. Smaller allowable pH range of 4.5-7.5 (beyond this range, cellulose
acetate becomes prone to hydrolysis)
II. Susceptibility to biological attack (degradation due to growth of
microbes)
III. Reduction of solvent flux because of compaction or mechanical
compression of membrane at high pressure difference
30. RO Membrane Properties
• Another common membrane materials in RO systems
are thin film composite (TFC) membranes.
• TFC membranes are not chlorine-tolerant but can
tolerate harsh chemical environments and wide ranges in
water temperature and pH, and are less vulnerable to
compaction than CA membranes.
• TFC membranes generally have higher water flux than CA
membranes because the layers are extremely thin, which
creates more water transport through the membrane
material.
31. Applications
• Potable water from sea or brackish water
• Ultra pure water for food processing and
electronic industries
• Pharmaceutical grade water
• Water for chemical, pulp & paper industry
• Wastewater treatment
• Municipal and industrial waste treatment
32. Mechanism
• The mechanism of water and salt transport in
reverse osmosis is not completely understood.
• One theory is that water and solutes diffuse
separately through the polymer by a solution-
diffusion mechanism.
• In this mechanism concentration of water in dense
polymer is assumed to be proportional to the activity
of water in the solution
33. Mechanism
• On the low-pressure side of the dense layer, activity is
essentially unity if nearly pure water is produced at 1 atm.
• On the high-pressure side, activity would be
slightly less than 1.0 at atmospheric pressure
1.0 at atmospheric pressure
slightly greater than 1.0 at higher pressures
• The upstream pressure is generally set at 20 to 50 atm above
the osmotic pressure of the feed solution.
• At these pressures, activity of water “aW“ is only a few percent
greater than for pure water at 1 atm and change in activity
and concentration across the membrane are small
34. • The Driving force for water transport is the difference
in activity, which is proportional to pressure
difference in osmotic pressures of feed and product.
• The equation for water flux is
Dw is the diffusivity in the membrane
Cw is the average water concentration (g/cm3)
Vw is the partial molar volume of water (cm3/gmol)
35. • The flux of solute is assumed proportional to the
difference in solution concentration, the diffusivity
and a solubility or distribution coefficient.
36. Concentration Polarization
• Nearly complete rejection of solute by the membrane
leads to a higher concentration at the membrane surface
than in bulk solution and this effect is called
Concentration polarization.
• Concentration polarization reduces the flux of water
because the increase in osmotic pressure reduces the
driving force for water transport.
• The solute rejection decreases both because of the lower
water flux and the greater salt concentration at the
surface increases the flux of solute
37. Concentration Polarization
• Equations for concentration polarization have been derived for simple
cases such as laminar flow of feed solution inside hollow fibres.
• Consider a membrane with a water flux Jw when the bulk solute
concentration is cs and f is the fraction of solute rejected.
Diffusion of solute away from membrane surface is characterized by mass
transfer Coefficient kc and driving force csi – cs .
At steady state, diffusional flux equals the amount of solute rejected per
unit area
Jw cs f = kc (csi – cs)
• Polarization factor is defined as
Г =
Jw f
kc
=
(csi – cs)
cs
38. Factors that impact on RO performance
Basic effects of:
• Temperature
• Pressure
• Recovery rate
39. Temperature Effects
• RO permeate flow is strongly dependent on the
temperature of the feed water.
• The higher the temperature the higher the permeate
flow rate.
• Why? Lower viscosity makes it easier for the water to
permeate through the membrane barrier
• RULE OF THUMB – for every 1˚C the permeate flow
will increase ~ 3%
40. Temperature Variation on Salt flux
• Solute rejection declines with a temperature rise because
of the osmotic pressure increase with temperature.
• Increasing temperature increases salt passage more than
water passage
• Generally you will get better rejections at lower
temperatures
• RULE OF THUMB – salt flux increases 6% for 1˚C increase
41. Pressure Effects
• Water passage increases with pressure. Solute
rejection rises with pressure, since solvent flux
increases and solute diffusion does not.
• Higher flow of water through the membrane will tend
to promote more rapid fouling, the single greatest
cause of membrane failure.
• Membrane element manufacturers usually provide
limits with regard to the maximum applied pressure to
be used, as a function of feed water quality.
42. Recovery rate Effects
• As recovery is increased, concentration of solute in the
concentrate stream increases, resulting in increased
osmotic pressure which must be overcome.
• Membrane flux declines with increasing solute
concentration at high recovery rate.
• Large systems typically have recovery rates between
40% and 60%. In other words, for every 10 gallons of
feed water entering the system, 4 to 6 gallons of
purified permeate water are produced
45. Introduction
• Pervaporation is a separation process in which one or
more components of a liquid mixture diffuse through
a selective membrane, evaporate under low pressure
on downstream side and are removed by a vacuum
pump or a chilled condenser.
• The process differs from other membrane processes
in that there is a phase change from liquid to vapor
in the permeate.
46.
47. Introduction
• The driving force in the membrane is achieved by
lowering the activity of the permeating components at
the permeate side.
• Components in the mixture permeate through the
membrane and evaporate as a result of the partial
pressure on the permeate side being held lower than the
saturation vapor pressures.
• The driving force is controlled by applying a vacuum on
the permeate side
49. • Composite membranes are used with dense layer in
contact with the liquid and the porous supporting layer
exposed to the vapor.
• Pervaporation is favored when the feed solution is dilute
in the main permeant because sensible heat of the feed
mixture provides the permeant enthalpy of vaporization
• The phase change occurs in the membrane and the heat
of vaporization is supplied by the sensible heat of the
liquid conducted through the thin dense layer.
50. • The decrease in temperature of the liquid as it
passes through the separator lowers the rate of
permeation and this usually limits the application
of pervaporation to removal of small amounts of
feed.
• Commercial units generally use flat-sheet
membranes stacked in filter press arrangement.
Spiral-wound membranes could also be used.
51. Applications
• Dehydration of ethanol or the production of high purity
ethanol by a hybrid process which also incorporates
distillation.
• Such separations use cellulose-acetate-based composite-
membranes, with an active layer of polyvinyl alcohol.
• Membranes used for ethanol purification are also suitable for
dehydration of many other organic solvents including
methanol, isopropanol, butanol, MEK, acetone and
chlorinated solvents.
52. Applications
• Removal of volatile organic contaminants from water
using silicone rubber or organophilic polymers for the
membrane.
• The separation of close-boiling organic mixtures like
benzene–cyclohexane is receiving much attention.
• Separating benzene from cyclohexane consisting of a
cellulose acetate support matrix and incorporating
polyphosphonates to improve the preferential
permeability of benzene.
58. Mechanism
• The flux of each component is proportional to the concentration
gradient and the diffusivity in the dense layer.
• However, the concentration gradient is often non-linear because
the membrane swells appreciably as it absorbs liquid.
• Diffusion coefficient in the fully swollen polymer may be 10 to 100
times the value in unswollen polymer.
• When the polymer is swollen mainly by absorption of one
component, the diffusivity of other components is increased also.
• This interaction makes it difficult to develop correlations for
membrane permeability and selectivity.
59. Mechanism
• Models for transport of permeant through a membrane by
pervaporation have been proposed, based on solution-
diffusion.
• They assume equilibrium between the upstream liquid and
the upstream membrane surface & between the
downstream vapor and its membrane side.
• Membrane transport follows Fick’s law, with a permeant
concentration gradient as the driving force.
• However, because of phase change and non ideal-solution
feed, simple equations do not apply.
Notas del editor
The liquid feed, which is DCE saturated with water (0.2 wt%), is preheated to 90C at 0.7 atm and sent to a PVA membrane system, which produces a retentate of almost pure DCE (<10 ppm H2O) and a permeate vapor of 50 wt% DCE. Following condensation, the two resulting liquid phases are separated, with the DCE-rich phase recycled and the water-rich phase sent to an air stripper, steam stripper, adsorption unit, or hydrophobic, pervaporation membrane system for residual DCE removal.
For removal of VOCs (e.g., toluene and trichloroethylene) from wastewater, pervaporation with hollow-fiber modules of silicone rubber can be used, as shown in Figure 14.23c. The retentate is almost pure water (<5 ppb of VOCs) and the permeate, after condensation, is (1) a water-rich phase that is recycled to the membrane system and (2) a nearly pure VOC phase.
