Ghasem D. Najafpour. Biochemical Engineering and Biotechnology

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Cost, always important, is difficult to quantify because the actual selling price of the

same module design varies widely, depending on the application. Generally, high-

pressure modules are more expensive than low-pressure or vacuum modules. Hollow fibre

modules are significantly cheaper per square metre of membrane than spiral-wound or

plate-and-frame modules but can only be economically produced for very-high-volume

applications that justify the expense of developing and building the spinning and module

fabrication equipment. An estimate of module manufacturing cost is given in Table 16.2;

the selling price is typically two to five times higher.

Two other major factors determining module selection are concentration polarisation

control and resistance to fouling. Concentration polarisation control is a particularly impor-

tant issue in liquid separations such as reverse osmosis and ultrafiltration. Hollow-fine-fibre

modules are notoriously prone to fouling and concentration polarisation and can be used in

reverse osmosis applications only when extensive, costly feed solution pretreatment

removes all particulates. These fibres cannot be used in ultrafiltration applications at all.

Another factor is the ease with which various membrane materials can be fabricated into

a particular module design.

16,18

Almost all membranes can be formed into plate-and-frame,

spiral-wound and tubular modules, but many membrane materials cannot be fabricated into

hollow fine fibres or capillary fibres. Finally, the suitability of the module design for high-

pressure operation and the relative magnitude of pressure drops on the feed and permeate

sides of the membrane can be important factors.

4,11

The types of module generally used in

some of the major membrane processes are listed in Table 16.2.

Example

An ultrafiltration plant is required to treat 50m

/day of protein-containing waste stream.

The waste contains 0.05% by weight of protein which has to be concentrated to 2%

to allow recycling to the main process stream. The tubular membranes to be used are

374 BIOCHEMICAL ENGINEERING AND BIOTECHNOLOGY

TABLE 16.3. Parameters for membrane module design

Parameter Hollow fine Capillary Spiral-wound Plate-and- Tubular

fibres fibres frame

Manufacturing cost ($/m

) 2–10 5–50 5–50 50–200 50–200

Concentration polarisation Poor Good Moderate Good Very good

fouling control

Permeate-side High Moderate Moderate Low Low

pressure drop

Suitability for high- Yes No Yes Marginal Marginal

pressure operation

Limitation to specific types Yes Yes No No No

of membrane material

Ch016.qxd 10/27/2006 10:52 AM Page 374

available as 30 m

modules. Pilot-plant studies show that the flux J through these mem-

branes is given by:

where C

is the concentration of protein in kg/m

. However, owing to fouling the flux never

exceeds 0.04m/h.

Estimate the minimum number of membrane modules required for the operation of this

process (a) as a single feed and bleed stage and (b) as two feed and bleed stages in series.

Assume operation for 20 hours each day.

Solution

(a) As a single feed and bleed stage:

• Let Q

be the volumetric flow rate of feed, Q

the volumetric flow rate of concen-

trate, C

the solute concentration in the feed, C

the solute concentration in the con-

centrate, F the volumetric flow rate of membrane permeate, and A the required

membrane area.

• We assume there is no loss of solute through the membrane.

• First we check the concentration (C

) at which the flux becomes fouling-limited:

That is, below this concentration the membrane flux is 0.04m/h.

This does not pose a constraint for the single stage, as the concentration of solute (C

)

will be that of the final concentrate (20kg/m

Conservation of solute gives:

⫽ Q

(a)

A fluid balance gives:

⫽ F ⫹ Q

(b)

Combining these equations and substituting known values:

Thus, 10 modules will almost meet the specification for a single-stage process.

As two feed and bleed stage in series:

In addtition to the symbols previously defined, let Q

be the volumetric flow rate of

retentate at the intermediate point, C

be the concentration of solute in the retentate at this

2 438 0 02

30..ln⫽⫽AA

 1 m

004 002

..ln⫽

4 kg/m

 ⬇

⫽ 002

.ln

MEMBRANE SEPARATION PROCESSES 375

Ch016.qxd 10/27/2006 10:52 AM Page 375

point, F

and F

be the volumetric flow rate of membrane permeate in the first and second

stages respectively, and A

and A

the required membrane area in these respective stages.

Conservation of solute now gives:

⫽ Q

(c)

A fluid balance on stage 1 gives:

⫽ Q

⫹ F

(d)

A fluid balance on stage 2 gives:

⫽ Q

⫹ F

(e)

Substituting given values in equations (d) and (e):

(f)

(g)

The procedure now is to use trial and error to estimate the value of C

that gives the

optimum values of A

and A

• If C

⫽5 kg/m

, the A

⫽63 m

and A

⫽23 m

. That is, an arrangement of three modules –

one module is required.

• If C

⫽4 kg/m

, the A

⫽55 m

and A

⫽31 m

. That is, an arrangement of two modules – one

module is almost sufficient.

• If C

⫽ 4.5 kg/m

, the A

⫽ 59 m

and A

⫽ 27 m

. That is, an arrangement of two

modules – one module meets the requirement. This arrangement is that requiring the

minimum number of modules.

16.11 MEMBRANE FOULING

A limitation to the more widespread use of membrane separation processes is membrane

fouling, as would be expected in the industrial application of such finely porous materials.

Fouling results in a continuous decline in membrane permeation rate, an increased rejection

of low molecular weight solutes and eventually blocking of flow channels. On start-up of a

process, a reduction in membrane permeation rate to 30–10% of the pure water permeation

rate after a few minutes of operation is common for ultrafiltration. Such a rapid decrease

may be even more extreme for microfiltration. This is often followed by a more gradual

125

0 0625 0 00811

A⫽⫹

125

002

.ln⫽⫹

376 BIOCHEMICAL ENGINEERING AND BIOTECHNOLOGY

Ch016.qxd 10/27/2006 10:52 AM Page 376

decrease throughout processing. Fouling is partly due to blocking or reduction in effective

diameter of membrane pores, and partly due to the formation of a slowly thickening layer

on the membrane surface.

16,17

The extent of membrane fouling depends on the nature of the membrane used and on the

properties of the process feed. The first means of control is therefore careful choice of

membrane type. Secondly, a module design that provides suitable hydrodynamic conditions

for the particular application should be chosen. Process feed pretreatment is also important.

In biological applications pretreatment might include prefiltration, pasteurisation to destroy

bacteria, or adjustment of pH or ionic strength to prevent protein precipitation. When mem-

brane fouling has occurred, back-flushing of the membrane may substantially restore the

permeation rate. However, this is seldom totally effective so that chemical cleaning is even-

tually required. This involves interruption of the separation process, and possibly quite sub-

stantial time losses due to the extensive nature of cleaning required. Thus, a typical cleaning

procedure would involve: flushing with filtered water at 35–50 ⬚C to displace residual reten-

tate; recirculation or back-flushing with cleaning agent, possibly at elevated temperature;

and rinsing with water to remove sterilising solution. More recent approaches to the control

of membrane fouling include the use of more sophisticated hydrodynamic control affected

by pulsated feed flows or non-planar membrane surfaces, and the application of further per-

turbations at the membrane surface such as continuous or pulsated electric fields.

16.12 NOMENCLATURE

Concentration of solute in feed stream, mg⭈l

⫺1

Concentration of solute in permeate, mg⭈l

⫺1

C Solute concentration, mg⭈l

⫺1

J Membrane permeation rate, mg⭈l

⫺1

⭈h

⫺1

⌬P Pressure difference applied across the membrane

⌬P Difference in osmotic pressure across the membrane

Resistance of the membrane

Resistance of layers deposited on the membrane

R⬘

Resistance of the film layer

Resistance of the cake

e Void volume of the cake

Mean particle diameter

r Specific resistance of the deposit

V Total volume filtered

Volume of particles deposited

Bulk concentration of particles in the feed

Membrane area

Resistance of all particles deposition

Resistance removed by cross-flow

Flux obtained experimentally or from the film-model

b Fraction of filtered particles remained in the filter cake

MEMBRANE SEPARATION PROCESSES 377

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REFERENCES

1. Coulson, J.M. and Richardson, J.F., “Chemical Engineering”, vol. 2, 5th edn. Butterworth-Heinemann, 2002.

2. Strathmann, H., J. Membr. Sci. 9, 121 (1981).

3. Lonsdale, H.K., J. Membr. Sci. 10, 81 (1982).

4. Backer, R.W., “Membrane Technology and Applications”, McGraw-Hill, 2000.

5. Rautenbach, R. and Albrecht, R., “Membrane Processes”, John Wiley, Chichester, 1989.

6. Mulder, M., “Basic Principles of Membrane Technology”, 2nd edn. Kluwer Academic Publishers, 2000.

7. Larcy, R.E., “Hanion Techniques for Chemical Engineers” (P.A. Schweitzer, ed.), 2nd edn. McGraw-Hill,

New York, 1988.

8. Belfort, G., In “Advanced Biochemical Engineering” (H.R. Bungay and G. Belfort eds). John Wiley,

New York, 1987.

9. Frankenfeild, J.W. and Li, N.N., In “Handbook of Separation Technology” (R.W. Rousseau ed.). Wiley,

New York, 1987.

10. Bowen, W.R., Hilal, N., Lovitt, R.W. and Williams, P.M., J. Membr. Sci. 110, 233 (1996).

11. Bowen, W.R., Hilal, N., Lovitt, R.W. and Williams, P.M., J. Membr. Sci. 110, 229 (1996).

12. Bowen, W.R., Hilal, N., Lovitt, R.W. and Williams, P.M., Colloid Interface Sci. 180, 350 (1996).

13. Bowen, W.R., Mohammed, A.W. and Hilal, N., J. Membr. Sci. 126, 91 (1997).

14. Hilal, N., Bowen, R.W., Lovitt, R.W. and Wright, C.J., J. Engng Life Sci. 2 (5), 131 (2002).

15. Hilal, N., Al-Zoubi, H., Darwish, N.A., Mohammed, A.W. and Abu Arabi, M., Desalination 170, 281 (2004).

16. Hilal, N., Ogunbiyi, O., Miles, N.J. and Nigmatullin, R., Separation Sci. Technol. 40, 1957 (2005).

17. Bowen, W.R., Hilal, N., Lovitt, R.W. and Wright, C.J., In “Surface Chemistry and Electrochemistry of

Membrane Surfaces”, vol. 79, (T.S. Sørensen, ed.). Surfactant Science Series, Marcel Dekker, USA, 1999.

18. Bowen, W.R., Hilal, N., Jain, M., Lovitt, R.W., Mohammad, A.W., Sharif, A.O., Williams, P.M. and Wright,

C.J., In “Comprehensive Chemical Kinetics”, vol. 37, “Application of Kinetic Modelling” (R.G. Compton and

G. Hancock, eds). Elsevier Science B.V., 1999.

16.13 CASE STUDY: INORGANIC ZIRCONIA ␥-ALUMINA-COATED

MEMBRANE ON CERAMIC SUPPORT

Abstract

Sol–gel is one of the most useful techniques for preparation of inorganic membranes with

fine pores in the nanometer range (1–5nm). The sol is a stable suspension of colloidal solid

particles within soft uniform solution. The gel was obtained by hydrolysis with open reflux

in 24 hours at 85–90 ⬚C. The advantage of sol–gel technology is the ability to produce

378 BIOCHEMICAL ENGINEERING AND BIOTECHNOLOGY

This case study was contributed by:

Nidal Hilal,

Ghasem Najafpour,

Abdul Latif Ahmad

Reader in Chemical Engineering, Director of Centre for Clean Water Technologies, University of

Nottingham, Deputy Director of Research/School of Chemical, Environmental and Mining

Engineering, University Park, Nottingham, NG7 2NR, United Kingdom.

Biochemical Engineering, Program Chairman of Biotechnology, Faculty of Chemical Engineering,

Noshirvani Institute of Technology, University of Mazandaran, Babol, Iran.

Membrane Technology, School of Chemical Engineering, Engineering Campus, Universiti Sains

Malaysia, Seri Ampangan, 14300 Nibong Tebal, S.P.S., Pulau Pinang, Malaysia.

Ch016.qxd 10/27/2006 10:52 AM Page 378

highly pure ␥-alumina and zirconia membranes at medium temperatures of about 700 ⬚C

with uniform pore size distribution in a thin film. However, the major disadvantage of this

process is that the membrane is sensitive to heat treatment, resulting in cracking on the film

layer. Successful crack-free products were produced, but they needed special care and time

for suitable heat curing. Only ␥-alumina membranes have the disadvantage of poor chemi-

cal and thermal stability. There was no opportunity to carry heat treatment at very high tem-

peratures above 700 ⬚C, where at 900 ⬚C it was expected the transformation of

␥-aluminium from ␥;␪;␣-alumina may take place. Successful coating on supported

membrane products was obtained using ZrO

. Suitable supported and unsupported zirconia

alumina membranes were developed and fabricated using the sol–gel technique. More than

100 samples of successful and unsuccessful membranes for demonstration were fabricated.

The successful results are presented and discussed. In this project, the porous ceramic sup-

ports and the metal-oxide-coated membranes were fabricated in the research laboratory at

the Universiti Sains Malaysia.

16.13.1 Introduction

Membrane separations represent a new type of unit operation, which, ultimately, is expected

to be replaced by significant proportions of conventional separation processes. The advantage

of membrane separation lies in its relatively low energy requirements. The reason for low

energy consumption is that, unlike conventional processes such as distillation, extraction

and crystallisation, it generally does not feature phase transition. The rapid development of

membranes in wastewater treatment encourages the development and fabrication of an inor-

ganic membrane. The results expand knowledge and produce various types of ceramic

membrane.

The ceramic membrane has a great potential and market. It represents a distinct class of

inorganic membrane. In particular, metallic coated membranes have many industrial appli-

cations. The potential of ceramic membranes in separation, filtration and catalytic reactions

has favoured research on synthesis, characterisation and property improvement of inorganic

membranes because of their unique features compared with other types of membrane. Much

attention has focused on inorganic membranes, which are superior to organic ones in thermal,

chemical and mechanical stability and resistance to microbial degradation.

Alumina membranes have recently received considerable attention for their use in a wide

range of applications. Alumina is the most cost effective and widely used material in the

family of engineering ceramics. The raw materials from which this high-performance tech-

nical grade ceramic is made are readily available and reasonably priced, resulting in good

value for the cost in fabricated alumina shapes, with an excellent combination of properties

and an attractive price. For commercial membrane applications, ␣-Al

and ␥-Al

are

the most common membranes. ␣-Al

membranes are well known for their thermal and

hydrothermal stabilities beyond 1000 ⬚C.

An inorganic membrane can be prepared by various methods such as sol–gel, phase sep-

aration and leaching.

2,3

The sol–gel process is considered the most practical method among

those used to prepare inorganic membrane. Sol–gel processing is a simple technology in

principle but requires considerable effort to become of practical use. The advantage of this

MEMBRANE SEPARATION PROCESSES 379

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technology is the ability to produce high-purity ␥-alumina membranes at low temperatures

(400–600 ⬚C). The pore size distribution is very narrow and additional elements can be

added.

4,5

However, only ␥-alumina membranes have the disadvantage of a poor chemical and

thermal stability. Below pH 4, they dissolve to form Al

3⫹

ions; above pH 12, the membrane

will form Al(OH)

⫺

. ␥-Al

has a highly detective spinal structure and can be considered

as an AlO

species, which dissolves more slowly, in low and high pH of the aqueous phase

as strong hydroxides behave.

Meanwhile, at higher temperatures (600–900 ⬚C), pore growth occurs. Above 850 ⬚C,

the transformation of ␥-alumina upon heating: ␥;␪;␣-alumina. The stable ␣-Al

phase forms above 1000 ⬚C by nucleation and growth.

In the transition of alumina

derived from boehmite gels, the pores are as large as the grains. These pores are unable to

stop grain boundary migration when ␣-Al

forms above 1100 ⬚C. Hao et al.

stated that

the phase transformation from ␥-Al

to ␪-Al

occurred at 900 ⬚C for pure alumina

membranes. With further heat treatment, 1200 ⬚C and higher, a complete transformation of

the ␪-Al

to the high temperature ␣-Al

phase is observed.

Foreign additives can affect the phase transformation as well as the pore structure at high

temperatures. To inhibit dissolution of alumina species or pore growth, the particles are

coated with an inert component such as zirconia, titania and lanthana.

It has been reported

that the effect of zirconia addition in alumina retarded the grain growth behaviour of the

alumina membrane. It was found that the ZrO

prevents abnormal growth of Al

during

heat treatment.

1,3

The effect of zirconia on the phase transformation could be explained by

380 BIOCHEMICAL ENGINEERING AND BIOTECHNOLOGY

Start

Literature

survey

Synthesis of

inorganic

membrane

Selection of

membrane

support

• Membranes are

produced

• Characterised

membrane

Characterisation of the

membranes

Upgrading the

experimental

parameters

Yes

• Test the

membrane

• Check the capabilities

for the emulsified oil

FIG. 16.21. Overall scheme and stages involved for fabrication of inorganic membrane.

Ch016.qxd 10/27/2006 10:52 AM Page 380

the fact the presence of the zirconia on the ␥-alumina surface may reduce the possibility of

the nucleation of ␣-alumina, thus raising the phase transformation temperature.

In this case study, a zirconia–alumina membrane has been developed using the sol–gel

technique with and without support.

6,7

The porous ceramic was prepared to fabricate the

membrane support. A thin film of aluminum and zirconium were formed on the porous

ceramic support. Unsupported membrane was also prepared. The unsupported membrane

was not strong enough to hold a high-pressure gradient; it was very fragile and not useful

MEMBRANE SEPARATION PROCESSES 381

FIG. 16.22. Non-uniform porous media of the ceramic membrane support.

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for independent use. The supported membrane on a highly porous ceramic was quite strong,

but the coating covered the solid surface. The ceramic support was strong enough to hold any

mechanical forces. Apart from the ceramic support, poly-vinyl alcohol (PVA) was prepared

to be used as a binder in the preparation of zirconia-alumina membranes to avoid the forma-

tion of cracks on the membrane surface. The overall process in the fabrication of membranes

382 BIOCHEMICAL ENGINEERING AND BIOTECHNOLOGY

FIG. 16.23. Alumina coated on the ceramic membrane.

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is shown in Figure 16.21. The uniform coating and crack-free membranes were examined

by scanning electron microscopy (SEM) and light microscop (LM) photographs shown in

Figures 16.22–16.25. The main part of the research activities was the preparation and

development of an inorganic membrane with and without support (zirconia-coated on

␥-alumina). The membrane surface was characterised by SEM and LM.

In preparing the membrane, a clear sol was obtained by the addition of acid into the alu-

minum sec-butoxide sol to peptise the sol and obtain a stable colloid solution. Aluminum

monohydroxides formed by the hydrolysis of aluminum alkoxides, which are peptisable

to a clear sol. Peptisation was performed by the addition of acid and heat treatment for a

sufficient time. It was found that stable sols cannot be obtained when the concentration of

the peptisation acid is too low. The critical range for inorganic acids such as nitric, hydro-

chloric and perchloric acids is 0.03–0.1 mole/mole of hydroxide. In this study, nitric acid

was used as the peptising agent. The resulting sols are poured into Petri dishes and dried in

an oven at a controlled drying rate to obtain a gel layer. The molar ratio of zirconia salt

MEMBRANE SEPARATION PROCESSES 383

FIG. 16.24. Zirconia coated on the ceramic membrane.

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