Use and Applications of Membranes

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-615

Use and Applications of Membranes

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Process Engineering Guide:

Use and Applications of
Membranes

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

GENERAL

3

4.1
4.2
4.3
4.4

What is a Membrane Process?
What does a Membrane look like?
Why use Membranes?
Membrane Types and Polymers Used

3
5
6
7

5

REVERSE OSMOSIS

7

5.1
5.2
5.3
5.4
5.5
5.6

Principles of Reverse Osmosis
Limitations
Performance
Costs
Worked Example
Applications

7
8
9
9
9
10

6

MICROFILTRATION AND ULTRAFILTRATION

11

6.1
6.2

Microfiltration
Ultrafiltration

11
13


7

PERVAPORATION

15

7.1
7.2
7.3
7.4
7.5

Classes of Application
Characteristics
Costs
Example - Lurgi Design
Application - Stripping Organics from Water

15
16
16
16
18

8

GAS SEPARATION AND VAPOR PERMEATION

19

8.1
8.2

Gas Separation
Vapor Permeation

19
19

9

LESS COMMON MEMBRANE PROCESSES

20

9.1
9.2
9.3
9.4

Dialysis
Electrodialysis
Electrolysis
Salt Splitting

20
20
21
22

10

BIBLIOGRAPHY

22

TABLES
1

UTILITY CONSUMPTION AND COST COMPARISON

18


FIGURES
1

MEMBRANE "FILTER" PROCESS

4

2

DEAD END FILTRATION

4

3

CROSSFLOW FILTRATION

4

4

PLATE AND FRAME MODULE

5

5

SPIRAL WOUND MODULE

5

6

REVERSE OSMOSIS OF SEA WATER

7

7

THE REVERSE OSMOSIS SYSTEM AND NOMENCLATURE

8

8

TYPICAL EFFECT OF FEED PRESSURE ON PERMEATE

8

9

CANADIAN SITE CLEAN-UP

11

10

LANDFILL LEACHATE TREATMENT PROCESS

11

EFFECT OF PRESSURE DIFFERENCE, VELOCITY AND
SOLIDS CONTENT ON FLUX

12

12

RETENTION OF SOME "DIAFLOW" MEMBRANES

14

13

EUROPEAN DYES PROCESS

15

14

FLUX AS A FUNCTION OF CONCENTRATION

17

15

THE LURGI PERVAPORATOR PILOT PLANT

17

16

LURGI PERVAPORATION PLANT COMPARED WITH
ENTRAINER DISTILLATION PLANT

18

17

LIQUID EFFLUENT CLEAN-UP

18

18

GAS PERMEATION PILOT PLANT - MERCEDES BENZ

20

19

ORGANIC STORAGE TANK VENT SCRUBBING

20

11


20

ELECTRODIALYSIS

21

21

SALT SPLITTING

22


0

INTRODUCTION/PURPOSE

Membrane processes are a large family of techniques that can provide
spectacular separations in certain cases. Unfortunately such cases are relatively
rare. Therefore the number of engineers familiar with the principles involved and
opportunities open is relatively small.
The purpose of this Process Engineering Guide (PEG) is help the reader
consider if there might be a benefit from successfully employing a membrane
technique for a separation task. The aim is not to make the reader an expert but
to get to the stage where the ideas can sensibly be taken to one of the experts.

1

SCOPE

This Process Engineering Guide presents an overview of membrane processes
and, for the more common ones, some ball-park throughput and cost data, as
well as worked examples. It does not cover the design of membranes.

2

FIELD OF APPLICATION

This Guide applies to the process engineering community and others involved in
process development in GBH Enterprises worldwide.

3

DEFINITIONS

For the purposes of this Guide, the following definitions apply:
Osmotic
pressure

The applied pressure required to prevent the flow of a
solvent across a membrane which allows the passage
of the solvent but not the solute and which separates a
solution from pure solvent.

Permeate

The stream which passes through (permeates) the
membrane, i.e. the efflux which has passed through the
membrane.

Recovery(S)

Mass ratio of flowrates of Permeate stream to feed stream.

Rejection(R)

1 - ratio of concentrations of solute in Permeate to feed (this
term is usually specific to Reverse Osmosis (RO)).


Retentate

The stream which is retained by the membrane, i.e. the
efflux which has not passed through the membrane.

4

GENERAL

4.1

What is a Membrane Process?

A membrane process is where a fluid mixture is placed on one side of a thin
sheet whose properties are such that one or more components in the mixture
pass through it more easily than others. The actual process occurring can be
adsorption, solution, diffusion, evaporation or a combination of these. However,
many membrane processes can be regarded as "fine filters". Some are able to
filter out or fractionate at molecular level, as illustrated in Figure 1.

FIGURE 1

MEMBRANE "FILTER" PROCESS

Other membrane processes such as gas separation, pervaporation, dialysis and
electrodialysis also operate in the lower regions of Figure 1. Processes operating
in the upper reverse osmosis (RO) region or the lower ultrafiltration (UF) region
are sometimes called Nanofiltration (NF).


Dead end filtration (see Figure 2) is that commonly used for in-line filters and
laboratory filtrations. All of the feed must either pass through the filter medium or
build up as a filter cake. Membrane processes do not usually operate this way.

FIGURE 2

DEAD END FILTRATION

In crossflow filtration (see Figure 3), the feed travels across the face of the filter
medium. Some material passes through the medium to form the Permeate. Not
all permeable material is allowed to pass through. It carries the concentrated
solute or solids out of the filtration (or membrane) unit in the form of a
concentrate or Retentate.
FIGURE 3

CROSSFLOW FILTRATION


4.2

What does a Membrane look like?

In its most usual form, the membrane is a flat sheet of polymer base with a thin
layer of semipermeable material on one side. These layers might be the same
material but are usually different, the base layer being porous, to allow easy fluid
flow whilst supporting the active layer. The semi-permeable layer is very thin to
give good mass transfer rates.
Sheet membranes can be mounted in a plate and frame arrangement like a plate
heat exchanger (see Figure 4). However, they are more commonly wound into a
spiral giving a cylindrical appearance (see Figure 5). The required duty
influences the choice of configuration and construction details.
A second common form is extruded tubular material, in which case the
membrane is usually homogeneous. The tube diameter can vary from under 1
mm to several mm. These are usually secured into a cylindrical housing to result
in what looks like a miniature shell and tube heat exchanger.
A third common form is more like a normal shell and tube heat exchanger. Here
the metal tubes are covered in sheet membrane material which they support, and
they have many holes in them to allow the Permeate to pass through. This
arrangement is good for high viscosity liquids where other arrangements have
excessive pressure drops at the fluid velocities needed to maximize the
Permeate flow rate.
FIGURE 4

PLATE AND FRAME MODULE


FIGURE 5

SPIRAL WOUND MODULE

4.3

Why use Membranes?

(a)

Low energy use
Depending on the application, a membrane process employed to
concentrate an aqueous solution can use as little at 1% of the energy of
an evaporation process.

(b)

Novel separations
Separations can be performed which are not possible by other means. For
example, certain azeotropes can be separated, aromats can be separated
from aliphats, and fractionations on molecular weight are possible.

(c)

Waste recovery
Effluents, both liquid and gaseous, can be cleaned up using membrane
extraction processes more economically than by using other technologies.


(d)

Displacement of chemical equilibrium
In certain reactions, byproducts can be removed continuously, leading to
improved reaction conversions; e.g. esterifications, where water is
continuously removed to favor the forward reaction.

4.3.1 Reasons for Benefits
Most membrane processes do not require a change in state in the process fluid.
Energy input is only required to overcome the Osmotic Pressure and flow losses.
Hence they have a very low energy input compared to evaporation, and
especially when compared to distillation using a high reflux ratio and minimal
heat integration with other process items.
Membranes involve a number of transport processes and these are not limited by
vapor liquid equilibria.
4.3.2 Constraints on Use of Membranes
There are a number:
•

In reverse osmosis, the Osmotic Pressure and membrane strength limit
the concentration of dissolved inorganic salts to about 5 wt% (although
higher molecular weight species can be concentrated more).

•

Membranes are not compatible with all chemicals, and are usually prone
to fouling by small particles and by microbiological activity.

•

Membranes need to be tested before accurate design in any new
application is possible.

•

Membranes sometimes have to be developed for new separation
processes which adds to the cost and time needed.

•

Membranes are not always economic when alternative techniques exist,
especially on a large scale as the normal process plant economy of scale
does not apply due to their modular construction.

However the world market for membranes is over $3 billion/year, and growing
about 15%/year (2005 figures). Most of this is in dialysis membranes (see 9.1).


4.4

Membrane Types and Polymers Used

Membranes come in 2 main physical forms, porous and non-porous. Porous
membranes, as used for microfiltration, have microscopic holes in them. In nonporous ones, as used in reverse osmosis, the separating layer is a dense film
without holes.
They come in as many polymer types as can be processed into a membrane.
Polymer choice is dictated by the separation needed and processing conditions.
Cellulose acetate, polyvinyl alcohol, and polysulfones are the commonest
polymers used. Some specialized ceramic membranes are also available.

5

REVERSE OSMOSIS

Osmosis has been observed for centuries. Reversing the process to produce a
purified water from a contaminated one was first observed around 1900. It
became a commercial process in the 1960s when asymmetric membranes were
developed which could produce potable water from brackish water or sea water
at a good rate. Figure 6 shows the reverse osmosis of sea water
diagrammatically.
FIGURE 6

REVERSE OSMOSIS OF SEA WATER

Membranes for these duties are now highly developed and available as
proprietary items in a variety of package types.


5.1

Principles of Reverse Osmosis

All solutions exhibit an Osmotic Pressure, but it is very low unless the molecular
weight of the solute is small:

Sea water has an Osmotic Pressure of about 27!bar g.
Purification systems are characterized by their yield and product quality. In RO
systems, these are expressed as the Recovery (S) and Rejection (R).
Both salt and water traverse the membrane, but by different mechanisms and
under different driving forces. The water flow rate is approximately proportional to
the difference between the applied pressure and Osmotic Pressure differences
between feed and Permeate sides, whereas the salt transport is concentration
driven, and is independent of the applied pressure.
Therefore, RO membrane give their best quality product at high transmembrane
pressure and low product Recovery. As the applied pressure drops, the water
rate drops, and the Permeate salt concentration increases (see Figure 8). As the
Recovery increases, the outlet concentration increases, and so the salt transport
rate at the outlet increases, deteriorating the average product quality. The
deterioration of product quality depends on the membrane selectivity.


FIGURE 7

THE REVERSE OSMOSIS SYSTEM AND NOMENCLATURE

Using the symbols given in Figure 7, S and R are defined as:

FIGURE 8 TYPICAL EFFECT OF FEED PRESSURE ON PERMEATE


5.2

Limitations

Modern RO membranes are usually capable of withstanding a transmembrane
pressure difference of 70 bar (1000 psi) and temperatures of about 70°C. Some
are able to withstand low levels of free chlorine, but many are not. All these
membranes are very sensitive to fouling and pretreatment is absolutely essential.
This will involve removal of fine particles, removal of sparingly soluble salts which
might precipitate out as the feed concentration increases, and pH adjustment.
Increasing Osmotic Pressure limits the concentration to which a solution can be
concentrated using RO. As Osmotic Pressure varies with temperature and
molecular weight, no specific upper concentration can be quoted, but a
concentration such that a driving force of at least 100!psi is left would be
reasonable. For NaCl, using 1000 psi feed pressure this would be about
8.0!wt%.
5.3

Performance

The production rate (flux) will drop from an initial value and become fairly stable,
typically at 50 - 100 kg/m2 h at 40°C and 40 bar pressure difference. Periodic
cleaning with a commercial cleaning solution will be necessary to maintain this.
Rejection varies with the dissolved species, but typical figures would be the
following, which are for a commercial Desal-5 membrane:

These performances are based on certain feed flow rates. If the flow rate is
reduced, the performance deteriorates as the boundary layer resistance
increases. This is known as membrane polarization.
Membrane flux varies exponentially with temperature, and it is worthwhile
operating close to the membrane temperature limit if possible.


5.4

Costs

Membrane spiral wound modules cost typically $15 - 35 per m2 before
installation. However, the pumping, storage and control equipment costs have to
be added to this. This cost will then typically be at least doubled by the need to
provide feed pretreatment. The details vary with each application, but a total
installed cost in the order of $ 35 - 65 per m2 membrane area should be
anticipated.

5.5

Worked Example

How much membrane area is needed to produce 1200 m3/day of pure water
(Na2SO4 concentration < 500 ppm) from process water containing 2% dissolved
Na2SO4 at 50°C using reverse osmosis? A single stage process is envisaged
using a feed pressure of 35 bar g and a Recovery of 30%. The membrane has
the following characteristics:

Retentate concentration is given by:


Hence the product specification is achieved using this membrane and assumed
Recovery.
Other Osmotic Pressures can now be calculated:

Note:
The concentration in use is much higher than that in the specification, so a test to
examine the effect of this on Rejection would be necessary. The Recovery might
then have to be reduced to keep the Permeate in specification as the Rejection
would probably be less under these conditions.


5.6

Applications

5.6.1 Site clean-up - Canada
A large site in Canada with a problem; a dump of gypsum and 500 million gallons
of ponds contaminated with ammonia, phosphates and dinitrotoluene, the legacy
from a previous phosphates plant complex.
Rainfall was causing these materials to be washed out into local water courses.
Lime treatment and passage through ponds did not improve the runoff water to a
level suitable for irrigation or cooling tower makeup. Evaporation and reverse
osmosis were examined as means to purify the treated water. RO was selected
as it offered the lowest overall cost (see Figure 9).
FIGURE 9

CANADIAN SITE CLEAN-UP

A system of pretreatment was developed which removes suspended solids, kills
algae and reduces hardness. The reverse osmosis final stage then produces
water to specification, with the Retentate being recycled at present to the ponds.
Water is produced at a rate of 335 Imperial gpm (91 m3/h). This represents
removal of water from the ponds at over 5 times the rainfall rate, and would
empty the ponds in about 3 years. Frequent changes in the condition of the feed
water make it difficult to pretreat consistently. However, the RO system is
working very well, and the Permeate has been used as cooling tower makeup
water, having < 200 ppm dissolved solids. The concentrate, with > 28000 ppm
solids, will in future be evaporated to dryness, contributing to their target of
becoming a zero discharge site. The membranes used are from Fluid Systems
(San Diego) type 8221HR which are cellulose acetate, spiral wound units.


5.6.2 European Landfill (see Figure 10)
Reverse osmosis can take quite crude waste streams and ground waters and
turn them into potable quality. Similar plants exist taking contaminated water and
cleaning it, e.g. taking wash water from plating plants to produce a concentrated
effluent and clean wash water for reuse.

FIGURE 10 EUROPEAN LANDFILL LEACHATE TREATMENT PROCESS

Flow: 35 m3/h Total cost: $USD 3/m3 Effectiveness: > 95% of BOD, COD and Cl
- removed

6

MICROFILTRATION AND ULTRAFILTRATION

6.1

Microfiltration

Microfiltration (MF) is a process which can remove particles from suspension
down to about 0.02 microns. Hence it uses porous membranes with pore
diameters suitable to retain the solute, or the dissolved species to be removed. It
might be selected such that some materials pass through the membrane, hence
giving a fractionation.
It is pressure driven with the differential pressure quite low, up to 2 or 3 bar.
Asymmetric membranes are used to maximize the flux.
In contrast to dead-end filters (see Figure 2) such as strainers, belts filters, etc.,
these filters operate with a continuous, recycling flow of feed across the
membrane face. This is known as crossflow filtration (see Figure 3).


The flow minimizes the boundary layer thickness and maximizes the flux. Hence
the feed can be allowed to increase to very high levels of suspended solids, the
limiting factor usually being pressure drop along the membrane due to the
viscosity of the slurry. A diagram is given in Figure 11.

FIGURE 11 EFFECT OF PRESSURE DIFFERENCE, VELOCITY AND
SOLIDS CONTENT ON FLUX

The flux increases with increasing crossflow velocity and with increasing
transmembrane pressure drop until a plateau is reached. The flux also varies
with solids content and temperature. Hence experimentation is usually needed
before a system can be designed. It is normal to arrange more than one filtration
stage for large changes in concentration to optimize flowrates and pressure
drops. Also the optimum membrane geometry will vary for different
Concentrations, and later stages handling high viscosity slurry will have different
membranes and modules than earlier ones.
Fouling is a perennial problem with membranes, and provision has to be made to
allow for regular cleaning. This will be done by taking the unit off-line and
circulating cleaning agents. These are usually surfactant solutions with
appropriate cleaning additives.
6.1.1 Costs
MF membranes are more expensive than RO ones as they are arranged in
tubular form. Complete in a housing, they cost from 350 to 650 $USD/m2. Costs
of pumps, tanks, piping and instrumentation have to be added to this. The
specific costs gets less up to modules of about 10 m2, and with bulk purchases,
but the economy of scale is not high.
Fluxes depend on all the above variables, but values in the region of 300 to 1000
kg/h/m2 are typical.

6.1.2 Worked Example
Design an MF system to concentrate 3.6 m3/h of a slurry from cf = 5 % w/v to cr =
20% w/v using a single stage process and MF modules each containing 19 tubes
in parallel, 1" ID and 42" long, giving 1.62 m2 filtration area. The maximum
allowed transmembrane pressure difference is 3 bar. Assume Permeate
pressure is 0 bar g.
Laboratory trials have resulted in the following data:
Flux J = 2 × 10-5 V 0.75 . ln (cg/cr).dP.e-0.3dP m3/sec.m2
Retention = 100%
Pressure drop at 1 m/s = 0.2 bar
Note:
This model follows the gel layer model where fouling and concentration
polarization are modeled by regarding them as the formation of a physical layer
which limits the upper concentration which the Retentate can reach, cg. Assume
a value of cg = 30%
Assume operation in a circuit where the feed is approximately at product
concentration.
This design will look at pressure differences ΔP in the region of 1 to 3 bar, and
velocities V from 1 to 3 m/s.
For 1 m/s with the modules in parallel, the pressure drop down the tubes is small.
However at 3 m/s and/or modules in series, the effective ΔP will be much less
than the feed pressure, leading to a low average flux.
Ideally the flow over a membrane will be turbulent. However, at the high viscosity
which slurries like this one can reach, this is often not possible. Calculation
shows that in this case, for a slurry viscosity of 360 cP, Re = 71 at 1 m/s, i.e. the
flow is streamline.


(a) Modules in parallel, V = 1 m/s, 1 bar g feed

(b) Modules in parallel streams each containing 4 modules in series, with V =
3 m/s, and 3!bar g feed

Clearly this is a better design, and further alternatives could be explored
using more modules in series (but at necessarily lower velocity due to
pressure drop) and then considering the total costs allowing for the modules,
pump, pipework, recirculation tank (if any), instrumentation, power
consumption, etc.
A further option is to split the duty into 2 stages, the first from say 5% to 10% w/v
slurry. This is more effective as the first stage works at a higher flux due to less
polarisation, the 3 bar pressure is generated twice, and flows can be better
optimised. Although the cost of two pumping and recirculation systems has to be
borne, this is frequently a more cost-effective arrangement.
6.2

Ultrafiltration

Ultrafiltration is arranged in crossflow mode rather like MF. However, the
membrane is usually non-porous or has very fine pores, and so a larger
transmembrane pressure drop is needed to achieve a realistic flux.
Typically, the flux of UF units would be in the range 20 - 150 kg/m2/h. The
pressure drop used would be from 2 to 10 bar.

UF membranes are usually characterised by their 'molecular weight cutoff'
(MWCO). This is determined by passing solutions of globular proteins, which are
spherical molecules, through the membrane, and finding the relationship
between retention and molecular weight. The MWCO is defined as the molecular
weight which has a retention of 90% under the test conditions. In practice, you
can only use this to select which membranes to test, as performance varies with
more than just molecular weight (for example, molecular shape and pH), and two
different membranes with the same MWCO may give different results for your
system. Also, the sharpness of the cutoff varies between membranes, and
depends on the pore size distribution.
UF is often used for oil-water separations. If the oil in water is a stable emulsion,
it can be removed down to about 10 - 50 ppm. If the feed is over about 2% oil, a
centrifugal pretreatment is usual.
Figure 12 shows how some commercial UF membranes retain test solutions of
varying molecular weights.
FIGURE 12 RETENTION OF SOME "DIAFLOW" MEMBRANES


6.2.2 Application – European Dye Manufacturer
European Dye Manufacturer makes a range of reactive dyes.The dyes are made
as an aqueous solution, then precipitated by the addition of salt and filtered, The
filter cake is re-slurried and the larger part of the production is evaporated to
dryness. However, many customers prefer the product as a strong solution, and
this can only be made if all the salt and other impurities are removed.
Therefore a process was developed where the solution is passed over a
membrane which was chosen to allow the salt and other small molecules to
Permeate, but to retain the larger dye molecules, whose molecular weight is
typically 650 to 1300.
The process has been refined and enlarged over the years. It started production
as a 50 m2 membrane area installation, making 300 te/yr of liquids. It now stands
as a 375 m2 plant, making 2000 te/yr (see Figure 13).
The process of making these liquid dyes would not be possible without this
nanofiltration process.
FIGURE 13

EUROPEAN DYES PROCESS


7

PERVAPORATION

Pervaporation (PV) is a permeation process in which the Permeate evaporates
as it passes through the membrane.
It uses non-porous membranes, and the mechanism is one of solution of one or
more species in the feed into the membrane, followed by diffusion through the
membrane. The phase change occurs inside the membrane, as the material
leaves the non-porous active layer.
Hence a separation can occur based on solubility differences for the feed
materials in the membrane. The primary driving force is concentration difference
not pressure, and so upstream pressure does not affect the process much,
whereas downstream pressure has a strong effect on pervaporation rate.
7.1

Classes of Application

Three classes of pervaporation have emerged:
(a)

Drying of alcohols, ketones and organic acids
These often exhibit an azeotrope which poses problems for conventional
processes. PVA membranes have been used in pervaporation processes
to dry sub-azeotropic mixtures to points above the azeotrope. Ethanol
drying plants are the largest PV plants in operation, with capacities up to
50,000 te/yr. The feed cannot be more than 20% water to avoid damage
of this membrane material (other materials do not have this limit, but have
a poorer selectivity). Studies have shown that PV is economically similar
to azeotropic distillation at this scale, and considerably better at smaller
ones.

(b)

Stripping organics from water
These operate at under 10% organics in water. Examples of stripping
benzene, CFCs, and solvents have been reported but no large
installations are presently in operation.

(c)

Separating mixtures of organics
Some interesting separations are possible which are presently only
possible by adsorption and solvent extraction. For example, separation of
aromatics from aliphatics, and alcohols from similar ketones has been
reported.


7.2

Characteristics

As latent heat has to be supplied, the feed cools as pervaporation proceeds. As
flux declines at lower temperatures, it is necessary to remove the feed and
reheat it if a significant amount of Permeate is required.
Low pressure drop on the Permeate side is important, and so modules tend to be
specifically made spiral wound or plate and frame types.
7.3 Costs
Due to the inherent complexity of the plant with vacuum pump, condensers and
other auxiliaries, PV plant can typically cost $USD 350 per m2 membrane area,
but for a special membrane this can reach $USD 250000 per m2.
The membrane flux depends heavily on the outlet concentration, and so
therefore low target concentrations lead to large membrane areas. Typically, a
PVA membrane will pass 1 kg/h/m2 of water from a solution of 1% water in
ethanol, falling to zero at 0.1 % water.
Consequently, many studies have shown that hybrid plants are the most
economic option, with conventional distillation being used for the bulk of the
separation, and PV being used for surpassing the azeotrope.
Treatment of 3 m3/h effluent containing 1000 ppm benzene, to discharge at < 10
ppm benzene, has been shown to cost a total of about $USD 3.5 per m3. Another
company quotes treatment of 40 te/day of effluent containing 1000 ppm
trichlorethane as costing capital of 1500 to 3500 $USD/m3/day capacity, and
running costs of $USD 1.5 – 3.5 per m3 effluent.
7.4

Example - Lurgi Design

The following example is based on work published by Dr. Ulrich Sander of Lurgi
GmbH.
Laboratory experiments have to be carried out initially to establish the flux
obtained and the relationship between separation coefficient and feed
concentration, all at different temperatures. Flux is strongly dependent on
temperature. Strictly, these also need to be measured for different Permeate
pressures, although again only the flux should be sensitive to this. In nonobvious
cases, different membrane materials, perhaps even similar ones but from
different sources, or made by different methods, need to be screened in the
laboratory.

Lurgi have made many PV alcohol dehydration plants, and a typical plant takes
94 wt% ethanol, aiming to dehydrate it to 99.5 wt%. The heat and mass balance
shows that the feed cools too much before the product specification is met for
this to be a single stage process. The design can have up to 10 stages, i.e. the
feed is preheated to 100°C, then removed at 90 - 95°C for reheating to 100°C,
before returning to the next stage, 10 times. The Permeate from each
stage is condensed at 10 - 15 mbar. Lurgi prefers plate and frame type units
made from stainless steel. The membrane chosen is PVA composite, which is
always used for dehydrations like this.
The relationships between Permeate flux and composition and the feed
composition at various temperatures can be shown on a graph. From the mass
and heat balance, lines can be drawn on it to show the temperature and
composition profile, assuming perfect mixing, no polarisation and equating the
sensible heat change in the feed to the latent heat in the water permeated.
These show that a single stage process is ineffective as it works at too low a
temperature and hence too low an average flux. Hence the feed is allowed to
cool to, say, 90°C, after which it is removed from the first stage, reheated to
100°C, and returned to the second stage.
Clearly, there is some optimum between keeping the feed hot with many
reheating stages, and hence minimum membrane area, with the converse. The
calculation needs to be done several times to determine this optimum. Figure 14
shows that about 5 stages would result if the feed were removed when the
temperature drops to 90°C or the flux drops to 0.1 kg/h/m2.


FIGURE 14

FLUX AS A FUNCTION OF CONCENTRATION

The area can be roughly calculated by considering each stage as working at an
average flux between the values for the inlet and outlet streams for the stage.
Using this method, the following areas are derived for a unit making 500 kg/h of
dehydrated alcohol:
Stage
Area/m2

1
41.0

2
58.7

3
42.9

4
67.2

5
130.6

Total area: 340 m2

Figure 15 shows a typical PV plant flowsheet.


FIGURE 15 THE LURGI PERVAPORATOR PILOT PLANT

Figure 16 shows how a PV/distillation hybrid system compares to an entrainer
distillation system for an azeotropic mixture such as ethanol/water. Table 1
shows their relative economics.
FIGURE 16 LURGI PERVAPORATION PLANT COMPARED WITH
ENTRAINER DISTILLATION PLANT


TABLE 1 UTILITY CONSUMPTION AND COST COMPARISON

7.5

Application - Stripping Organics from Water

Pervaporation can remove organics from aqueous wastes. Whether it is the most
cost effective way of doing this for any particular waste cannot easily be decided,
but needs evaluating (see Figure 17).
FIGURE 17 LIQUID EFFLUENT CLEAN-UP


8

GAS SEPARATION AND VAPOR PERMEATION

8.1

Gas Separation

Gas separation is a diffusion process, and hence is done by porous membranes.
It has been commercially in use for many years. The technology breakthrough
which revolutionized this was the use of a silicone layer on polysulphone
membranes to plug pinholes which cause severe performance loss with gaseous
materials. Monsanto invented this, and their patented 'PRISM' system is
installed worldwide, typically in ammonia plants. Flowrates up to 84,000 m3/h
are processed.
Separation occurs mainly due to molecular weight differences. Hence it is most
commonly used in extracting hydrogen from mixed streams. However, water,
H2S, CO2 and O2 all diffuse more quickly than N2 and CH4, say, in PRISM
membranes.
Gas separation is also used to separate nitrogen from air, and +2500 units
worldwide produce this at lower cost than a cryogenic process. They usually
operate with high feed pressure. Membranes made from polyimide or polyether
ketone allow higher operating temperatures of 150°C and 200°C respectively,
than the PRISM ones which are limited to 100°C.

8.2

Vapor Permeation

Vapor permeation differs from gas separation in that it is a solution/diffusion
process and uses non-porous membranes, at close to atmospheric pressure,
with the Permeate under vacuum.
These systems are much less common than gas separation, but are being
offered for duties such as scrubbing organic-laden air caused by breathing of
stock tanks. Units extracting vinyl chloride monomer from air are operating in the
USA.
Fluxes are lower than for gas separation due to the higher molecular weight of
the Permeate and the nature of the membrane, although appropriate membrane
choice such that relative solubilities favour the desired extraction can improve
them.


8.2.1 Application - Mercedes Benz
A good study of extracting valuable dimethyl isopropylamine from exhaust air
was carried out by Mercedes Benz. They studied the optimization of refrigeration
temperature and vacuum pressure to produce a design which scrubbed their
casting shop exhaust air from 4000 ppm to under 1000 ppm (which then went to
a caustic scrubber). The plant processes 1000 m3/h air, cost a total of $USD
200K, produces 99% DMIP for re-use, and paid for itself in under 3 years (see
Figure 18).

8.2.2 Application - Process Vent Clean-up
Vapor separation membranes can extract organic materials from air streams to
produce a vent suitable for atmospheric emission. If the specification is very low,
scrubbing might be needed as a polishing stage. Most projects would be justified
on environmental grounds, but for high value materials, a cost-effective Recovery
can be demonstrated over and above this (see Figure 19).
FIGURE 18 GAS PERMEATION PILOT PLANT - MERCEDES BENZ


FIGURE 19 ORGANIC STORAGE TANK VENT SCRUBBING

9

LESS COMMON MEMBRANE PROCESSES

9.1

Dialysis

Dialysis accounts for the majority of membrane material used in the world today.
It is a concentration driven process, where the process fluid passes over a
membrane, the other side of which is swept by a suitable fluid, the 'dialyte'. The
membrane is chosen such that undesired materials in the process fluid will pass
through, driven by their low concentration in the dialyte, where they might react
or just be diluted. Other materials are retained by the membrane, and so the
process fluid is purified.
The mass transfer coefficient is usually low, even if high flow rates across the
membrane surface are used. The membranes are usually not very selective, and
so the process is not likely to be useful to large scale processes.
However over 100,000 people owe their lives to dialysis; it is the separating
process used in artificial kidney machines to remove toxins from blood.


9.2

Electrodialysis

The membrane process uses an electric field as the driving force for the
separation. It therefore has characteristics quite unlike the other membrane
processes. It is only loosely related to dialysis, in that process fluid sweeps both
sides of the membrane.
Membranes can be made such that they are very selective to ionic charge
differences. Cationic membranes allow the passage of positively charged ions,
but reject negatively charges ones. An electrodialysis unit (ED) consists of a
large stack of alternating cationic and anionic membranes (see Figure 20). If an
electric field is impressed over the whole stack, ions try to move towards the
oppositely charged end of the stack. However, the membranes interfere with
this, with the result that the channels between the membrane pairs become
alternately enriched and depleted in the ionic species. If the two types of product
stream are physically separated as they flow out of the cell, a separation has
been achieved.
A European Institute has done a lot of work on ED, in their assessment of using
ED to purify dilute aqueous waste, (that 8% sodium nitrate solution could be
concentrated to 18%. The unit also produced a depleted stream containing 1%
sodium nitrate. The power consumption was 0.26 kWh per kg of NO3 in the
concentrate.
ED appears to be more economic than RO in some circumstances, and is used
in many large brackish water purification plants worldwide. It has also been used
to remove acids from effluent water, remove salt from sugar and cheese whey
solutions, and remove tartaric acid from wine.
It can concentrate the feed material to a much higher concentration than RO as it
does not suffer from Osmotic Pressure limitations. Hence, sodium chloride can
be concentrated up to about 20 wt%. This is used in Japan as a preconcentration
stage for the production of solid salt.


FIGURE 20 ELECTRODIALYSIS

9.3

Electrolysis

Membranes have been used in brine electrolysis since the invention of the
diaphragm cell. The modern way to make chlorine is using a membrane cell in
the plate and frame arrangement.

9.4

Salt Splitting

A variant on electrolysis and electrodialysis is salt splitting (see Figure 21), where
a salt such as sodium nitrate is electrolyzed and combined with water to produce
nitric acid and sodium hydroxide solution, using a bipolar membrane. It was
estimated that 11% waste sodium nitrate solution could be converted into 3717
te/yr of 65% nitric acid and 2360 te/yr of 50% caustic soda for a variable cost of
$USD 330 per te of NaOH, from a plant costing $USD7.92M. This plant consists
of evaporation as well as the membrane operation.


FIGURE 21 SALT SPLITTING

11

BIBLIOGRAPHY

"Introduction to Membrane Processes" by Mulder
"Handbook of Industrial Membrane Technology" by Porter
"Effective Industrial Membrane Processes" by Turner
"Membrane Separation Technology" by Scott
"Membrane Processes" by Rautenbach & Albrecht
"Handbook of Industrial Membranes" by K Scott


Use and Applications of Membranes

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