Carbon Membranes - Universiti Teknologi Malaysia · 2016-10-25 · 1Advanced Membrane Technology...

Encyclopedia of Membrane Science and Technology

Online ISBN: 9781118522318

DOI: 10.1002/9781118522318

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Carbon Membranes

Ismail, A. F.1,2*

and Salleh, W. N. W1,2

1Advanced Membrane Technology Research Centre, Universiti Teknologi Malaysia, 81310

Skudai, Johor Bahru, Malaysia. 2Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia,

81310 Skudai, Johor Bahru, Malaysia.

Abstract

1.0 THEORY OF CARBON MEMBRANE PROCESSES, GENERAL PROPERTIES

OF CARBON MEMBRANES

1.1. Principles and properties of carbon membranes

1.2. Separation mechanisms

1.3. Carbon membrane configurations/classifications

1.4. Challenges in carbon membranes development

1.4.1. Mechanical stability

1.4.2. Membrane aging

1.4.3. Sorption of trace components

2.0 METHODS OF CARBON MEMBRANE PREPARATION AND

CHARACTERIZATION - DESIGN OF CARBON MEMBRANE MODULES

2.1. Introduction

2.2. Precursor selection concepts

2.3. Precursor membrane preparation

2.4. Heat treatment process

2.4.1. Stabilization process

2.4.2. Carbonization process

2.5. Membrane characterization

2.6. Membrane performance

2.7. Membrane modules

3.0 CARBON MEMBRANE PROCESSES & APPLICATIONS

3.1. Gas separation

3.2. Membrane reactor

3.3. Pervaporation

3.4. Water treatment

References


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Abstract:

Carbon membranes are considered as a new relative addition to the mainstream of membrane

material research. It shows tremendous growth and development efforts in this research field

through significant numbers of publication in recent years. Carbon membranes can be fabricated

by heat treatment from various types of polymeric precursor membranes such as Polyimide and

derivatives, polyacrylonitrile, phenolic resin, polyacrylonitrile (PAN), polyfurfuryl alcohol

(PFA), polyetherimide (PEI), polyphenylene oxide (PPO), polyvinylidene chloride (PVDC), and

polyphthalazinone ether sulfone ketone (PPESK). Carbon membranes can be divided into two

categories namely supported and unsupported. The typical characterization methods that used for

carbon membranes can be classified into membrane performance evaluation and physical

characterization. Physical characterization included thermogravimetry analysis (TGA), X-ray

diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared

spectroscopy (FTIR), adsorption and sorption experiments, and elemental analysis. Further

explanation on each process parameter will be discussed in order to obtain carbon membranes in

highest quality. In addition, the performances of various types of carbon membranes that

developed by previous researchers for different gas separation applications were also

summarized. Furthermore, the potential applications and future directions of carbon membrane

in gas separation processes were identified in this review. Other potential processes for carbon

membrane include pervaporation, water treatment, and membrane reactor was also emphasized.

Although there are significant researches conducted since the carbon membranes inception, the

need to extensively improve the gas permeation properties of these membranes still remains

important if it become a viable membrane material for industrial applications.

Keywords: precursor; carbon membrane; stabilization; carbonization

1.0 THEORY OF CARBON MEMBRANE PROCESSES, GENERAL PROPERTIES

OF CARBON MEMBRANES

1.1. Principles and properties of carbon membrane

The concept of carbon membrane for gas separation was found in the early 1970s. Barrer

et al. compressed nonporous graphite carbon into a plug, called carbon membrane (1). Bird and

Trimm used poly (furfuryl alcohol) (PFA) to prepare supported and unsupported carbon

membrane. During the carbonization, they encountered shrinkage problems that lead to cracking

and deformation of the membrane. Hence, they failed to obtain a continuous membrane (2).

Carbon molecular sieves produced from the high temperature treatment (carbonization) of

polymeric materials have proved to be very effective for gas separation applications by Koresh


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and Soffer (3-5). Molecular sieve carbon can be easily obtained by carbonization of various

types of polymers such as Polyimide and derivatives, polyacrylonitrile (PAN), phenolic resin,

polyfurfuryl alcohol (PFA), and various coals such as coconut shell (3-5). They described that

the pore dimensions of carbon were dependent on the morphology of the organic precursor and

the chemistry of the heat treatment process (6).

In membrane process, the permeability and selectivity are the most basic properties of a

membrane that need to be considered. It is well known that the membrane performance appears

to be a trade-off between selectivity and permeability, i.e. a highly selective membrane tends to

have a low permeability (7, 8). Lu et al. (9) stated that, the higher the selectivity and the more

efficient the process, the lower the driving force (pressure ratio) required in achieving given

separation; thus resulting in lower operating costs for the separation system. The higher the

permeability, the smaller the membrane area required; hence the capital cost of the system is

lower. Therefore, some attempts have been made to prepare membranes that can surpass

Robeson’s upper bound. They have pointed out that the carbon membranes are potential to

exceed such an upper bound (10-13). The previous studies proved that, the limitations and

drawbacks of polymeric membranes can be resolved by using carbon membrane in the separation

system. The comparison between polymeric and carbon membranes are summarized in Table 1.1

(14). This chapter gives an overview about the basic principles of carbon membrane and the

challenges that faced by researcher in the development of carbon membranes.

Table 1.1: Carbon membrane versus polymeric membrane

Polymeric membrane Carbon membrane

Structural

property

Separation

mechanism

Solution diffusion Knudsen diffusion: < 0.1 μm

Surface diffusion: < 10 - 20 Å

Capillary condensation: > 30Å

Molecular sieving: < 6Å

Advantages Low production cost Excellent chemical stability

Surpass the trade-off between

permeability and selectivity

Excellent thermal stability

Can be used at extreme operation

Pore dimension and distribution can

be fine tuning by simple

thermochemical treatment

Disadvantages Poor thermal resistance

Poor chemical resistance

Arduous to beach the trade-off

Brittle

High production cost

Vulnerable to adverse effect from


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between permeability and selectivity exposure to organic contaminants

and water vapor

1.2. Separation mechanism

Various gas transport mechanism across carbon membranes have been proposed

depending on the properties of both the membranes and permeant (gas molecules). This includes

Knudsen diffusion, surface diffusion, capillary condensation/partial condensation, and molecular

sieving (15, 16). Details explanations of the separation mechanism occur across carbon

membrane are illustrated in Table 1.2. The transport mechanism exhibited by most of carbon

membranes is the molecular sieving mechanism.

1.3. Carbon membrane configurations/classifications

Carbon membranes can be divided into two categories: unsupported and supported

carbon membranes. Unsupported membranes are generally produced as flat films, capillary

tubes, or hollow fibers. Meanwhile, the supported membranes are generally in the form of flat

sheets or tubes (17, 18). Supported membranes, sintered metals, alumina, or graphite materials

normally used either in flat disk or tubular configurations. There are several methods to deposit

the polymer solution on the support material such as dip-coating (19-22), spin coating (11, 17,

23-25), spray coating (26), and ultrasonic deposition. Numerous types of supported and

unsupported carbon membranes have been reported in the literature (refer Volume II). Based on

the previous researches, the supported carbon membrane is the most popular configuration

because it is simple to prepare and lower cost. However, this type of configuration exhibits a

problem of uniformity in deposition of polymer solution on the support. The cycle of polymer

deposition/heat treatment need to be repeated several times in order to obtain an almost defect-

free membrane (17, 25, 27).

Nevertheless, the numbers of report on hollow fiber carbon membrane have increased in

2000s. The beneficial features that contributed by hollow fiber which posses high separation

efficiency, and offer flexibility where the separation can occur in two ways, either ‘inside-out’ or

‘outside-in’. Besides, it also posses high active surface area to volume ratios (>1000 m2/m

3), able

to operate at high pressure drops, low resistant to gas flow, and low production cost as compared

to other types of configuration (31, 32).

1.4. The Challenges in carbon membrane development

1.4.1. Mechanical stability

The mechanical properties of the membrane are essential in the operation and the

membrane module design. In the case of carbon membranes derived from polymeric material,


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the major disadvantage in the commercialization is brittleness, which means that they require

careful handling.

Table 1.2: Separation mechanism of carbon membranes (15, 16, 28-30)

Knudsen diffusion

Pore sizes: < 0.1 μm

Can occur either by concentration or by pressure gradients

The relative permeation rate of each component is inversely proportional to the square root of

its molecular weight.

The selectivity is equal to the square root of the molecular-weight ratio of the largest to

smallest gas.

Low selectivity and not practical to be applied in the industry separation process

Surface diffusion

Pore size: < 10 - 20 Å

Separate non-adsorbable or weakly adsorbable gases (O2, N2, and CH4) from adsorbable gases

(NH3, SO2, H2S, and CFCs)

In this mechanism, the diffusing species adsorb on the walls of the pore, and then readily

transport across the surface in the direction of decreasing surface concentration.

The molecules with larger molecular weight and with larger polarity are selectively adsorbed

on the membrane surface.

The concentration of adsorbed species is depends on the temperature, pressure, and the nature

of the surface.

Capillary condensation /

Partial condensation

Pore size: > 30Å

Capillary condensation mechanism occurs when one or more of the components in the feed

stream condense in the pores and diffuse across the membrane.

A very high selectivity of separation of the condensable component can be achieved by this

mechanism but the extent of removal of that component from the gas mixture is limited by the

condensation partial pressure of the component at the system temperature, the pore size and

the geometry of the membrane.

Molecular sieving


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Pore size: < 6Å

Separate gas molecules of similar sizes

effectively

If the membrane has pore sizes between the

diameter of the smaller and the larger gas molecules, the only smaller molecule can be

permeated

In 1999, Tanihara and coworkers (33) studied the mechanical property of carbon

membranes derived from asymmetric Polyimide hollow fiber membrane produced by UBE

Industries. The elastic modulus significantly increased and the breaking elongation decreased as

the heat treatment temperature is increased from 600 to 1000 ºC. The elastic modulus of the

resultant carbon membrane carbonized at 800 ºC showed 9 times larger than membrane treated at

270 ºC. It was demonstrated that the aromatic fragments growth and crosslinking between the

fragments was progressing during the heat treatment process. In addition, it also contributed by

the asymmetric structure of the membranes that turn into dense with increasing treatment

temperature. Similarly, the development towards the increased elastic modulus of the carbon

membranes also found in PAN-based carbon fiber studies (34, 35).

An early work on the preparation of flexible carbon membrane from sulfonated polymer

was claimed in 2002 (36). The precursor membranes were carbonized at low temperature of 450

ºC for 1.5 h. The results indicated that carbon membranes prepared at low carbonization

temperatures have the ability to separate C3H6/C3H8 and CO2/N2 gas pairs with high selectivity.

At carbonization temperature of 500 ºC, the derived carbon membranes were brittle and the gas

permeation measurement could not be performed (36, 37). Recently, a flexible carbon membrane

derived from sulfonated PPO was successfully prepared by Yoshimune and Haraya (38). The

mechanical property of this membrane measured by means of a bending test technique on the

glass tube. It was found that carbonization of carbon membrane in the range of 450 to 600 ºC

produced more flexible carbon hollow fiber membrane with a smaller bending radius. However,

for carbon membrane carbonized at 700 ºC, a less flexible and brittle carbon membrane was

obtained due to the shrinkage effect during heat treatment. It was found that the outer diameter of

the hollow fiber affected the mechanical property of the carbon membranes. Due to that, the

bending radius has increased with the outer diameter of the carbon hollow fiber membrane

increased.

In addition to these issues, Ismail’s group (30, 39) stated that brittleness problem can be

minimized by using PAN as a precursor membrane for carbon membrane, since this polymer

have been widely used in the production of high strength carbon fiber. Further study on the

preparation of carbon membrane with high mechanical properties would be a major breakthrough

in this field and would be given an opportunity for potential commercialization of the

membranes. There are some available and modified polymers that possess the potential to

produce carbon membrane with mechanical stability that fulfills the industrial qualifications.

Based on the authors’ knowledge, a study on the mechanical properties of the carbon

membranes are still lacking in the open literature. One of the problems is related to the


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experimental operation due to carbon membrane fragility. For brittle carbon membranes, the

breaking elongation is relatively short, even at a long gauge length, the membrane elastic

modulus is also high. Therefore, the incorrect measurement of the elongation of the membrane

sample normally occurred once there is a motion deformation in the initial stretching. There are

various types of damage of the membrane in the sample preparation and testing process, such as

picking a carbon hollow fiber membrane from a bundle, mounting the membrane sample to the

tensile machine, and adhering the membrane to a paper holder. Although these membranes have

poor mechanical properties, it is still capable of being constructed into the module for gas

permeation testing in the laboratory scale; and manifold studies on the gas permeation

performance of the carbon membrane have been reported in the literature (40).

1.4.2. Membrane aging

Carbon membranes present significant problems regarding the performance stability even

though it exhibit excellent chemical and thermal resistance. One of the problems faced by carbon

membranes is the vulnerability to oxidation. The permeation properties of the membranes would

drastically affected even with a small change in the size of pore constrictions, and the associated

changes can be quite significant on time scales of weeks to years (41, 42). Upon air exposure

(even at room temperature), oxygen atoms from air combine with some active sites can lead to

creation of oxygen surface groups. Thus, oxygen chemisorptions can eventually reduce the open

porosity towards gas transport and increase the number of the constrictions of the pore structure,

offering additional restriction to diffusion, which is known as carbon aging. During aging, the

rate of change in permeation properties is dependent on the size of the gas molecules and it is

significantly affected by the permeation of larger molecules due to the loss of free volume (42).

An extensive study was conducted by a research group from Japan (19, 43) to investigate

the stability of the carbon membranes by maintaining the membrane in the air at 100 °C for a

month. During this treatment, oxygen in air reacted with the membrane and formed oxygen-

containing functional groups, which continuously decomposed to CO2. Later, the effects of aging

on carbon membrane permeation properties associated with air oxidation were studied by Fuertes

and Menendez (44, 45). The derived carbon membranes were stored under various types of

environments such as dry air, humid air, propylene and nitrogen. The results showed that a rapid

loss in permeance with time was observed for carbon membrane stored in dry and humid air.

However, carbon membranes stored under propylene and nitrogen was protected from aging. It is

considered that oxygen chemisorptions into carbon membranes instead of water physisorption is

the main reason for the loss of permeance when a prepared carbon membrane is exposed to air.

These findings were in agreement with the results reported by Lagorsse and coworkers (46).

In order to protect and minimize the carbon membrane from aging, the regeneration

technique by exposure to pure propylene was proposed in 1994. Experimental results

demonstrated that most membrane recovery cases were significantly boosted by exposure to


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propylene and it was confirmed that this chemical is effective in removing absorbed groups.

During regeneration, the adsorbed compounds were easily removed from the carbon due to the

weak interaction forces on the pore wall of the carbon membrane (47). Besides functioning as

regeneration agent, propylene can also be used for storage purpose of the carbon membranes as

suggested by Menendez and Fuertes (45). However, the systematic studies on the stability and

durability of the derived carbon membranes are still not sufficient to be used as a reference.

1.4.3. Sorption of trace components

In general, carbon surface are hydrophobic but micropore of carbonized membrane wall

are partially covered with oxygen-containing functional groups in giving the membrane a

hydrophilic character (48). It has been reported that the selectivity of a typical membrane

decreases as the amount of sorbed water has increased. This is primarily due to the fact that

water vapor can be found in a large number of process streams. One of the approaches to this

problem is coating method with highly hydrophobic polymer to develop carbon composite

membrane (49). The composite membranes were designed to have high resistance to water vapor

to prevent decreasing in permeation flux. Kai et al. (50) reported that the problem of separation

performance decline under humid conditions was improved by incorporation of Cesium (Cs) in

polymeric precursor solution. The separation performance was evaluated using a CO2/N2 gas

mixture with controlled relative humidity at 40 °C and at atmospheric pressure. The results

showed that the Cs-incorporated carbon membranes had a higher CO2 permeance and separation

factor under humid conditions.

Membrane performance losses in terms of selectivity and flux were severed and occurred

with feed stream concentrations of organics as low as 0.1 ppm. Carbon membranes have limited

stability in some gases (CH4, H2, CO2, O2) at relevant temperatures and appeared less feasible for

precombustion decarbonization processes (51). Therefore, the modification of the carbon

membrane in terms of surface properties is the important aspect in developing carbon

membranes with high performance stability.

2.0 METHODS OF CARBON MEMBRANE PREPARATION AND

CHARACTERIZATION - DESIGN OF CARBON MEMBRANE MODULES

2.1. Introduction

Carbon membranes considered as one of the most popular topic among the membrane

researchers due to its spectacular separation performance. However, an intensive research and

development work should be carried out to ensure the reality of carbon membranes in industry.

This section briefly discusses the process taken in the preparation, characterization, and the

membrane module design of carbon membrane. Generally, the preparation of carbon membranes


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involves three important processes, including precursor selection, precursor membrane

preparation, and heat treatment process. Simultaneously, there are several factors that need to

consider in order to ensure the success of each process (39, 40).

2.2. Precursor selection concepts

In polymeric precursor materials selection, thermal and chemical stability is not the only

features that need to be consider. The promising combination of required permeability and

selectivity and good mechanical properties of the final carbon membrane were also required.

Many researchers had fabricated carbon membranes from a wide range of polymeric precursors.

The used polymer precursor should satisfy a number of criteria such as high aromatic carbon

content, high glass transition temperature, Tg, chemically stable and provide superior separation

properties (14, 32, 39, 52).

Frequent used polymers are polyimides and derivatives, phenolic resin, polyacrylonitrile

(PAN), polyfurfuryl alcohol (PFA), polyetherimide (PEI), polyphenylene oxide (PPO),

polyvinylidene chloride (PVDC), and polyphthalazinone ether sulfone ketone (PPESK). Many

studies reveal that the pore dimensions and the distribution in the microstructure carbon

membranes are dependent on the selection of polymer precursors. As reported by Tin et al. (53),

the chemical composition of polymer precursor is the crucial factor which determines the pore

population created in the carbon matrix. Table 2.1 shows the summary of several potential

candidates of polymeric precursors for carbon membranes previously studied in the literature.

Detail reviews on these polymeric precursors can be found elsewhere (39, 40, 54, 55).

2.3. Precursor membrane preparation

In flat sheet membrane preparation, the polymer solution is poured onto the top surface of

a glass mirror and cast by a casting knife. The solvent is then allowed to evaporate until the

membrane is fully vitrified, which is determined by a change in the appearance of the membrane.

The resulting polymeric film then removed by lifting the edge of the film with a razor blade to

ease the film off of the surface. If the film is complicated to come off, a few drops of water is

squirted under the exposed edge, which causes the film to lift off of the glass plate. The

polymeric film will be dried at room temperature to remove any residual solvent and water. Any

areas which have visual stress points will not using for permeation experiments (55). Flat sheet

membrane also can be fabricated as supported membrane by depositing polymer solution onto

the inorganic support (81).

In the fabrication of hollow fiber membrane, the process are more complicated than flat

sheet and cannot be approached by the same method that led to the successful casting of the flat

sheet configuration. In hollow fiber membrane fabrication, spinneret is the most important device


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that would be utilized to form hollow fiber. Figure 2.1 illustrates the side view and cross section

of the spinneret that have been used in the spinning process (82)

Table 2.1: Summary of polymer precursor selection

Polymer precursor Properties

Polyimide

(12,14,17,19,23,33,56-

61)

Widely used by previous researchers

Limited (expensive and need to synthesize manually in the

laboratory)

Good for H2/CH4, CO2/N2, O2/N2, CO2/CH4, N2/CH4, and C3H6/C3H8

separation

Gas transport through the membrane occurs according to the

molecular sieving mechanism

Commercially available polyimide; Kapton, Matrimid, and P84 co-

polymer (BTDA-TDI/MDI)

PEI (20,62-64)

Good chemical and thermal stability

Impressive separation factors

Give high carbon yield

Good for CO2/CH4 separation

PAN (27,39)

Conquer nearly 90 % of all worldwide sales of carbon fibers

Low O2/N2 selectivity

Phenolic resin

(13,22,44,65)

Inexpensive polymers

Good separation of hydrocarbon gas mixtures such as

alkenes/alkanes and n-butane/iso-butane

PFA (25,29,31,66)

Simple molecular structure and formation mechanism

Only can be used for preparing supported membrane

Good for O2/N2 separation

PPO (67,68) Excellent mechanical properties

PVDC (24)

Exhibit molecular sieve properties similar to Zeolite 5A

Good for O2/N2 separation

Polypyrrolone (69)

Involved two step of synthesis

Exhibit high permeability

PPESK (70-73) Exhibit high H2, CO2, and O2 permeability


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Good for H2/N2, CO2/N2, and O2/N2 separation

Membrane performance can be improved by manipulating sulfone

over ketone units molar ratio (S/K)

Polymer blend

(26,64,71,74-80)

Blending of two polymers with different thermal properties is

preferably used (eg: PPO/polyvinylpyrrolidone (PVP),

polyimide/polyethylene glycol (PEG), PEI/PVP, PAN/PVP,

polyimide/PVP, polybenzimidazole (PBI)/polyimide)

Blending polymer with inorganic materials (zeolite, silica, and

carbon nanotube) (eg: polyimide/multiwall carbon nanotubes

(MWCNTs), PEI/MWCNTs, PPESK/zeolite, polyimide/silica)

Increased the mesoporous volume

Enhanced the gas permeability

Figure 2.1: Schematic diagram of (a) side view and (b) cross section of the spinneret (82)

Two methods were used for the preparation of essentially defect-free asymmetric hollow

fiber membranes by the phase inversion process, which is by wet spinning process and dry/wet

spinning process. The dry/wet phase inversion process is commonly used in the literatures (12,

18, 52, 59, 68, 83). In this process, dense skin is created due to the solvent in the casting solution

desolvates rapidly into the coagulation bath and relatively little nonsolvent diffuses into the

casting solution at the point when the casting solution and nonsolvent come into contact. The

thickness of the skin layer increased gradually until the diffusion of solvent from the sublayer

solution through the dense skin layer into the coagulation bath is restrained. On the other hand,

the wet spinning process predominated when the length of air-gap is zero. This would produce

the outer skin layer with big pores owing to the lack of coalescence (84, 85). In coagulation bath,

both the inside and the outside of the polymer solution were in contact with a nonsolvent for the

polymer. An exchange of solvent and nonsolvent caused the polymer to precipitate and a

structure with a certain degree of porosity is formed.

2.4. Heat treatment process


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Further study on the preparation factors are crucially needed in order to obtain

reproducible carbon membrane with high separation efficiency. Many researchers agreed that the

heat treatment conditions impose strong effect on the pore population created in the carbon

matrix as well as their gas permeation properties of the final carbon membranes. Table 2.2

summarized the heat treatment conditions that recently applied into carbon membrane

preparation by various researchers along with the types of precursor and configuration.

2.4.1. Stabilization Process

Stabilization or pre-oxidation process is one of the steps involved in the fabrication of

carbon membrane and carried out before the carbonization process. Most of the prepared high

performance carbon membranes were successfully stabilized in air atmospheres between 150 and

460 ºC, depending on the type of polymer precursor used. It is believed that greater stability

membranes were obtained after stabilization under air atmosphere mainly due to the contribution

of oxygen in the dehydrogenation reaction. In fact, oxygen mainly acts as a dehydrogenation

agent in the conversion of C-C bonds to C=C bonds and generates oxygen-bearing groups in the

polymer backbone, such as –OH and C=O. These groups promote intermolecular crosslinking of

the polymer chains and provide greater stability to sustain high temperature in the subsequent

carbonization process. If the stabilization process is not completed throughout the entire

membrane cross section, a significant weight loss can occur at higher temperature (62).

Table 2.2: Heat treatment condition used by previous researchers

Precursor Configuration Stabilization and

Carbonization Condition

(Temperature, Heating rate,

Thermal soak time)

Atmosphere Ref(s)

PAN Hollow fiber 250 °C, 5 °C/min, 0.5 h Air (31)

500 - 800 °C, 3 °C/min, 0.5 h N2

PAN Flat sheet 450 - 950 °C, 1 °C/min, 2 h N2/Ar (86)

PEI/PVP Hollow fiber 300 °C, 3 °C/min, 0.5 h Air (87-89)

650 °C, 3 °C/min, 0.5 h N2

PFA Flat sheet

100°C, 5 °C/min, 1 h

400 - 800°C, 5 °C/min, 2 h

Ar

Ar

(25)

PFR Tube 150°C, 0.5 °C/min, 1 h

850°C, 0.5 °C/min, 1 h

Air

Ar

(90)

Phenolic

resin

Tube 700 – 1000 °C, 10 °C/min, 1 - 8

h

Vacuum

N2

(22)

6FDA /

BPDA-

Flat sheet 550 °C, 2 h Vacuum, Inert (91)


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DAM

polyimide

Matrimid Hollow fiber 400 °C, 2 °C/min, 0.5 h

900 °C, 5 °C/min, 5 h

N2, N2

saturated with

water, CO2

(59)

P84 co-

polyimide

Hollow fiber 100 °C, 2 °C/min, 0.5 h

900 °C, 5 °C/min, 0.083 h

N2 (61)

PPESK Flat sheet 460 °C, 3 °C/min, 0.5 h Air (73)

650 – 950 °C, 1 °C/min, 1 h Ar

PPO Tubular 700 °C, 5 °C/min Ar (67,92-94)

PFR/PEG Tube 800 °C, 0.5 °C/min, 2 h N2 (26)

The stabilization process also offers the potential to prevent the melting and fusion of the

polymeric membranes. This is to avoid excessive volatilization of carbon element in the

subsequent carbonization process (73). As a result, the final carbon yielded from the precursor

can be maximized (39). Moreover, Zhang et al. (71) suggested that the stabilization process

should be conducted in order to maintain the morphological structure of the thermoplastic

polymer precursor in the resultant microporous carbons after heat treatment. Stabilization of the

thermoplastic polymers in the air allows oxygen bridges to be created between aromatic

molecules that inhibit the rearrangement and growth of aromatic crystallites during carbonization

step. Thus, a porous structure with more open pores can be formed in the resultant carbon

membranes (72).

The stabilization process variables can significantly affect the performance of the resulted

carbon membrane. These variables include stabilization temperature, heating rate and thermal

soak time. The effect of stabilization temperature and thermal soak time was reported frequently

in the literature because an excess oxidation abruptly expands the pore size and decreases perm

selectivity for permeants larger than 0.4 nm. A number of studies also been done to optimize this

condition process in order to obtain carbon membrane of good performance (44, 62, 71-73). The

gas permeability and selectivity of the stabilized membranes were increased in the magnitude

compared to the precursor membranes. This was due to the formation of pore structure by cross

linking and volatilization of small gaseous molecules produced by minor thermal degradation

during stabilization. Additionally, as the stabilization temperature increased, the degree of

decomposition and crosslinking in the membrane increased, which resulted in different

micropore structures of the carbon membranes that can be used to tune gas separation properties

of the derived carbon membranes.

The stabilization process perform in carbon membrane preparation is also similar to the

stabilization process that carried out in carbon fiber preparation. Basically, during the

stabilization process, the precursor undergo numbers of physical and chemical changes due to a

variety of exothermic chemical reactions, including decomposition, cyclization,

dehydrogenation, oxidation, crosslinking and fragmentation. The cyclization reactions that


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convert PAN into an infusible stable ladder polymer are of most importance in PAN fibers (95).

The stabilization reaction involved in the precursor polymer could be estimated quantitatively by

means of FTIR spectroscopy. For PAN polymer, the reactions such as cyclization, crosslinking

and dehydrogenation were indicated by the observation of a new band at 1595 cm-1

. This band

represented the combination vibration of C=C and C=N stretching, and NH in-plane bending of

the ladder-frame structure of the stabilized PAN. Another band that contributed to the

stabilization of PAN fiber was at 1718 and 1660 cm-1

due to the oxygen uptake reactions.

Additionally, the 1595 cm-1

band corresponding to cyclization became distinct only after PAN

fiber was heated for 30 min in air atmosphere (96, 97). It was concluded that the effective

stabilization reaction can occur at certain temperatures under certain time of exposure in air

environment.

However, there are no further explanations and description on the scientific studies

concerning the mechanism of the stabilization reaction during the stabilization step in the

preparation of polymer based carbon membranes in the literature. Most of the studies focus on

the effect of stabilization parameters on the gas permeation properties of the carbonized

membranes, in terms of permeability and selectivity rather than on the chemical point of views.

For instance, the mechanisms and kinetics of the stabilization reaction involved in PAN based

carbon fiber have been studied. Therefore, similar studies on the mechanism of the stabilization

occurred in the preparation of stabilized polymeric precursor membrane need to be conducted to

have further understanding on carbon membrane formation process.

2.4.2. Carbonization Process

Carbonization process is the heart of carbon membrane fabrication and considered as

compulsory process. Generally, the carbonization process takes place after stabilization process.

At the initial stage of carbonization process, most chemical reaction and volatile emission

occurred. Typical volatile byproducts that have been produced are ammonia (NH3), hydrogen

cyanide (HCN), methane (CH4), hydrogen (H2), nitrogen (N2), carbon monoxide (CO), and

carbon dioxide (CO2). The hydrogen evolution was indicated by the formation of graphite like

structure. The evolution of byproducts is dependent on the type of polymer used and generally

causes a large weight loss (39, 98).

During the carbonization process, part of the heteroatoms originally presented in the

polymer structure was eliminated while leaving behind a crosslinked and stiff carbon matrix. As

a result of the rearrangement of the molecular structure starting from the polymeric precursor,

amorphous microporous structure of carbon membranes was created by the evolution of gases

(63). The pore structure that formed during the carbonization process determines the separation

properties of carbon membrane. These pores were non homogeneous and consist of relatively

wide openings with few constrictions. In general, the pores vary in size, shape and degree of

connectivity depending on the chemistry of the carbonization step and the polymer precursor


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morphology. The microstructure of carbon membranes (pore size, pore volume, and pore

distribution) could be tailored by controlling the conditions of carbonization process. The

carbonization temperature that known as final temperature (25, 64, 83, 93), heating rate (22, 62),

thermal soak time (12, 31, 83), and carbonization atmosphere (59, 60, 91) are the factors that

influences the carbonization process. Therefore, it is necessary to optimize the parameters in

order to gain the best carbon membranes for gas separation.

Carbonization is carried out typically from 500 to 1000 °C, between the decomposition

and graphitization temperature of the precursor and at a wide range of heating rate of 1 to 13

°C/min. The increasing temperature of carbonization will produced carbon membrane with

higher compactness, more turbostratic, higher crystallinity, higher density and smaller average

interplanar spacing between the graphite layers of the carbon (99, 100). Meanwhile, thermal soak

time during the carbonization process can be different depending on the final carbonization

temperature (22, 101). Thermal soak time is the period of time taken to hold the membrane at a

constant carbonization temperature before the membrane is cooled down to room temperature.

Previous studies reported that selectivity of carbon membrane would increase at high thermal

soak time (98, 100). Carbonization process can be applied either in vacuum, inert atmosphere

(He, N2, or Ar) or oxidative atmosphere (CO2). The chemical damage and undesired burn off of

the polymer precursor membrane during carbonization can be avoided by controlling the

carbonization atmosphere. Vu and Koros (83) postulated that the inert gas environment

accelerates the carbonization reaction by increasing the gas phase heat and mass transfer to form

a more open porous matrix. The carbonization under CO2 environment showed a more

permeable and exhibited high permselectivity than those carbonized in N2. In this section, it can

be concluded that the application of a reactive media could result in carbon structures with

higher microporosity, total volume of pores and BET specific surface area (59).

2.5. Membrane characterization

Few techniques on characterization commonly used in carbon membrane preparation are

shown in Table 2.3. The techniques included thermogravimetry analysis (TGA), scanning

electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction

(XRD), adsorption and sorption experiments, and elemental analysis.

2.6. Membrane performance

Gas permeation measurement is the most comprehensive tool to assess performance of

the membranes. The two important quantities which can be determined by permeation

measurement are the gas permeability, which is a measure of the amount of permeate that passes

through a certain membrane area in a given time for a particular pressure difference, and the

selectivity which is a measure of the membrane quality. The performance of the carbon


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membranes are depending largely on the nature of pore structure and pore size in the concerned

porous carbon membranes. The pore size distributions of the resultant carbon membrane are

normally obtained from the N2 adsorption isotherms.

Carbon membranes have a well-developed ultramicropore network that can separate

small gas pairs with minor difference in diameter, thus exhibit higher gas permeability and

selectivity than polymeric membranes. The separation mechanism that normally take placed in

the carbon membrane is listed in the Table 1.2. The pore structure of carbon membrane is mainly

composed of the disordered inter-connective nano-channels formed by the intergranular holes

(supermicropores) packed by the turbostratic carbon microcrystals and the space

(ultramicropores) existed between the carbon sheets. The disordered inter-connective nano-

channels in the pore structure of the carbon membrane result in high diffusion resistance of the

gas molecules penetrating through the membrane, thus lead to low gas permeability of the carbon

membranes. As such, how to further enhance the gas permeability of carbon membranes remains

a challenge, and it must be tackled to help speed up the commercial applications (102).

Before carry out the gas permeation test, the hollow fiber membranes were potted at both

ends to form a bundle consisting of 5 to 10 fibers. One end of the fiber bundle is sealed into a

stainless steel tube, while the other end is fully potted with the epoxy resin. Loctite E-30CL

epoxy adhesive is used as a potting resin. Figure 2.2 shows the results of typical carbon hollow

fiber membranes that has obtained.

Table 2.3: Membrane characterizations

Instruments Descriptions

TGA

To determine weight loss of a substance as a function of process

temperature

To observe the thermal decomposition behaviors of the precursor

membranes

SEM

To observe the morphological structure of the membrane (surface,

skin layer and cross section)

To determine the overall quality of the selective layer, the presence

of any macroporous/mesoporous cracks or pinholes

To evaluate the apparent changes in membranes dimensions as a

result of the heat treatment process

FTIR To detect the change of the functional groups and elemental in the

membranes when they were heated from room temperature to

carbonization temperature

XRD

To determine the crystalline structure of polymers

To estimate the interplanar distance (d-spacing) in the membrane

matrix by the well known Bragg equation

The most relevant peaks when examining polymer-based carbon are

http://en.wikipedia.org/wiki/Polymer


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the (002) and (100) peaks. The (100) peak is commonly used to

determine the crystal length while the (002) peak is related to the

distance between carbon atoms on the same plane

Adsorption and sorption

experiments

To determine the pore size distribution of the carbon membrane by

the HK (Horvath and Kawazoe) method

Elemental analysis

To measure the percentage composition of carbon, hydrogen,

nitrogen and oxygen in the carbon membrane as a function of

process temperature

Figure 2.2: Typical carbon hollow fiber membranes (103)

This bundle is then inserted in a stainless steel module. All fittings and nuts that used are

of Swagelok type to get a better prohibition of leakage during the gas permeation test. A simple

bubble flow meter was used to obtain the permeation properties of the gas owing to its suitability

for the measurement of small and wide range of flow rate (56). In the single gas permeation

system, pure gases are introduced into the system at feed pressures of 1 to 15 bars at ambient

temperature. The permeate side of the membrane is maintained at atmospheric pressure. The

permeance, P (GPU) and ideal separation factor (or selectivity, α) of the carbon membrane are

calculated from the permeation rate by using equations (2.1) and (2.2) respectively.


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where P/l is the permeance of the hollow fiber (cm3 (STP)/cm

2.s.cmHg), Qi is the volumetric

permeate flow rate of gas i at standard temperature and pressure (cm3

(STP)/s), p is the pressure

difference between the feed side and the permeation side of the membrane (cmHg), A is the

membrane surface area (cm2), n is the number of fibers in the module, D is an outer diameter of

hollow fiber (cm) and l is an effective length of hollow fiber (cm).

2.7. Membrane modules

In order to ensure the efficiency of a membrane process in a given application, the

geometry of the membrane and the way it is installed in a suitable module is also important. The

accomplish system of the membrane separation depends not only on the development of high

performance membrane but also on the design and construction of an efficient and economical

membrane module. For commercial applications of membranes, it is preferable to fabricate a

module with an asymmetric structure and capillary or hollow fiber configurations in order to

increase the rate of permeation of the products. In general, the characteristics of the modules that

must be considered in all system designs are the cost of production, maintenance, and efficiency.

The most challenging task that must be considered while fabricating these modules is improving

the poor mechanical stability of carbon membranes. This is more crucial in hollow fiber carbon

membranes, since they are self-supporting (39, 40).

Until today, only the laboratory scale tubular or hollow fiber membranes have been

described in detail. Industrial scale applications of carbon membranes were hampered by the

difficulty that involved in the large scale membrane production, poor mechanical stability of the

membranes and the poor reproducibility of the membrane modules. The assembly of large scale

modules described only in the patents by Soffer and coworkers (104-106). The only commercial

modules are now available from Carbon Membranes Ltd (Israel) with a hollow fiber (small

tubular) configuration with the packing density of 2000 m2/m

3. Furthermore, the novel carbon

membrane module of the Blue Membranes GmbH is based on “honey comb module” concept

and has a high packing density of 2500 m2/m

3 (107, 108). Selected examples of modules

fabricated by various researchers are shown in Figures 2.3 and 2.4.


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Figure 2.3: Module constructed by Ismail’s group (39)

Figure 2.4: Module constructed by Koros’s group (83)

3.0 CARBON MEMBRANE PROCESSES & APPLICATIONS

3.1. Gas separation

At present, membrane processes are employed in a wide range of applications and the

number of such applications is still growing. In order to compete with the conventional

processes, the performances of membranes in the existing markets need to be improved and

alternative materials must be developed. The utilization of membranes in industrial gas

separations has undergone substantial growth in several areas since 1980 when the first large

scale application was developed by Permea for H2 separations (109). In 2007, there are15 new

gas plants around the world and this number jumped to 24 for 2008, indicating substantial new

opportunities for membranes are available (110). It is expected that by 2020, the opportunities

for new membrane installations in natural gas treatment will increase (109, 111). The increasing

in the activities of carbon membrane research and a great separation performance of the carbon


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membrane will be the motivation principles in future. In Table 3.1 below, the results of carbon

membrane performance that have been reported by different investigators are shown.

In the literature, various reports on O2/N2 separation using carbon hollow fiber

membranes and supported carbon membranes are available (12, 21, 22, 44, 58, 61, 64, 68, 69, 72,

76, 90, 113, 117). This gas pair separation can be applied in air separation, oxygen-enriched air

for combustion processes, medical purposes and pure O2 and N2 production. Currently, the

largest industrial gas separation application is the processing of natural gas. It is owing to the

rising in the natural gas consumption worldwide and most of the membrane processes is applied

towards the CO2 removal. The total consumption of natural gas worldwide is approximately 95

trillion standard cubic feet a year and is expected to rise up to 182 trillion cubic feet in 20 years

onwards due to the increasing demand for energy around the world (118).

Although polymeric membranes are able to compete successfully with other

technologies, for example the amine scrubbing, improvement in membrane fabrication is

necessary to grab the broader market opportunity for carbon membranes. This is because some of

glassy polymeric membranes lose their selectivity and productivity in the presence of trace

quantities of condensable heavy hydrocarbon. Moreover, the extremely high partial pressure of

CO2 can cause the plasticization in the skin layer of a membrane (119). Carbon membrane also

has a potential to recover CO2 gas from power plants that burn fossil fuel and to avoid its

emission to the atmosphere. This application was recommended by Centeno and Fuertes (11),

when they had successfully prepared carbon membrane with high selectivity of CO2/N2. Similar

findings were also made in the studies of Fuertes and Menendez (44), Lua and Su (58), Powell

and Qiao (120), Zhang et al. (79), Favvas et al. (59), Rao et al. (64) and Liu et al. (72).

In addition, the power plants combined with coal gasification processes need a H2

separation step at about 300 to 500 °C. At high temperature operation, only a pressure driven

membrane separation process can isolate the H2 from other carbon compounds (CO2, CO). In this

case, organic polymeric membranes cannot resist very high temperatures and begin to

decompose with certain components. Thus, carbon membrane that is chemically inert and

thermally stable up to more than 500 °C is more suitable for this application (11, 20, 121).

Carbon membranes can also be applied to the H2 recovery from the gasification process without

further compression of the feed gas, while rejecting a substantial portion of the hydrocarbons

(122). Polymeric membranes incurred additional recompression costs because H2, as a fast gas,

exits the unit at the low pressure side (119).

Table 3.1: Results of carbon membrane performance reported by different investigators

Precursor O2/N2 CO2/CH4 CO2/N2 H2/CH4 H2/N2 He/N2 C2H4/C2H6 Ref(s)

PAN ~3.7 (31)

PEI 12.5 (63)

PEI 3.9 17.5 (64)

PEI/PVP 4.6 13.7 (64)


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Furthermore, the separation of light alkenes/alkanes and olefins/paraffins have

been recognized as a key technology by the petrochemical industry (44, 100,

123). Carbon membranes are promising candidates for separating light alkenes/alkanes and

olefins/paraffins, especially propene/propane separation and 1,3-butadiene/n-butane separation,

respectively. Hayashi et al. (43, 124) reported that special carbonized membrane is capable of

recognizing size differences between alkane and alkene molecules. It is expected more

superior than other methods, such as distillation, adsorption and absorption-based on

energy consumption. A recent study estimated that about 10,000 BTU of energy is used annually

for olefin-paraffin distillation (119). It is also possible to employ carbon membranes to separate

the isomers of hydrocarbons into normal and branched fractions (125).

3.2. Membrane reactor

PEI/MWCNTs 24.2 48.8 (64)

PEI 16.2 (111)

PFA 7.5 (25)

Phenolic resin 9.2 39 42 11 (44)

Sulfonated phenolic resin 12 54 180 (21)

Phenolic resin 6.8 103 28 (22)

6FDA/BPDA-DAM

polyimide

10 110 (91)

P84 co-polyimide 12.25 38.9 42.8 (61)

Polyimide (AP) 12.3 (12)

Matrimid 8.8 78 27 (14)

Matrimid 5.5 20.86 23.6 (59)

Kapton 476.74 (60)

Kapton 19.69 138.53 60.87 (58)

LARC-TPI polyimide 3.0-3.3 > 104 (112)

Polypyrrolone 180 16 (69)

PPES 5.5 16.3 22.6 (113)

PPESK 23.8-27.5 150.4-

213.8

(71)

PPESK 21.9 93.1 (72)

PPESK 11.3-24.6 67-

171.7

97.5-

266.1

(73)

PVDC 14 (24)

PPO 33 14 80 (67)

PPO 11.4 127 (68)

TMSPPO 10 102 (68)

PPO 138 (93)

PFR/PEG 9.7 20.2 693.6 406.9 (26)

PFR/CMS 12.8 17.86 504.93 471.27 (114)

PPESK/PVP 6.5 25.7 38 (79)

PPESK/zeolite 6.9 102.5 26.6 (79)

PI-BTCOOMe/Cs2CO3 43 (50)

PI/MWCNTs 3.3 4.1 (78)

PBI/Matrimid 6.67 56.41 28.06 466.1 (77)

SPAEK/Ag 67 224 (115)


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A carbon membrane reactor constitutes is one of the most promising applications of

carbon membranes. The performance of a carbon membrane for the dehydrogenation of

cyclohexane to benzene was examined by Itoh and Haraya (126). They reported that the

performance of their carbon membrane reactor for dehydrogenation was fairly good compared

with that of a normal reactor, i.e. functioning at equilibrium. On the other hand, Lapkin (127)

used a macroporous phenolic resin carbon membrane as a contactor for the hydration of propene

in a catalytic reactor. He found that the use of this porous contactortype reactor for his high-

pressure catalytic reaction is practical. In 2003, Coutinho et al. (18) claimed that the PEI-based

carbon membranes had a potential to be applied in catalytic reactor application.

The combination of reaction and separation at high temperature in a membrane reactor

offers interesting new possibilities. In a membrane reactor, the separate product and feed

compartments allow more ways to optimize selectivity and conversion (128). Zhang et al.

constructed a carbon membrane reactor (CMR) for methanol steam reforming and found CMR

gave a higher methanol conversion than the conventional fixed bed reactor (FBR) and produced

a CO free hydrogen stream (129). Carbon membrane was prepared by spraying a DMF solution

containing phenol formaldehyde novolac resin (PFNR) and polyethylene glycol (PEG) on a

PFNR based green membrane support, the details of which are given elsewhere. The sprayed

layer was then carbonized under an inert gas stream to 800 °C at a heating rate of 0.5 °C/min. As

the catalyst for the steam reforming reaction, laboratory made Cu/ZnO/Al2O3 composite catalyst

was used. The inner diameter of a tubular carbon membrane that used is similar with a stainless

steel tube of FBR (6mm). The permeation mechanism through the membrane is by the sieve

mechanism since the gas permeance decreased with an increasing kinetic diameter of the gas

molecules. The steam reforming consists of the following chemical reactions:

CH3OH + H2O = CO2 + 3H2 endothermic reaction

CO + H2O = CO2 + H2 exothermic reaction

CH3OH = CO + 2H2 endothermic reaction

The first and the third reactions are highly endothermic and the second reaction is highly

exothermic. The results showed that CH3OH conversion increased with an increasing

temperature. This is because; both CH3OH conversion reactions (first and third reaction) are

endothermic. Meanwhile, the CH3OH conversion of CMR is higher than for FBR. This is

because the equilibrium shifted to the right side in the first and the last reaction when H2 is

preferentially removed from the reactor through the membrane wall. It is practically no CO and

methanol found in the permeate through the carbon membrane.

3.3. Pervaporation


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Sakata et al. made porous carbon membrane plates (PCMP) to separate

benzene/cyclohexane azeotrope by pervaporation (130). PCMPs were prepared from phenol

resin powder (BELLPEARL S-870, Kanebo) by pressurize the powder in a hydraulic press. The

disk was heated at 300 oC for an hour in the air stream and then carbonized at 800

oC in two

different environments. The results showed that PCMPs prepared under N2 stream had only

micropores, while PCMPs prepared under CO2 stream had both micro and mesopores. The

separation factor (benzene/cyclohexane) of 2.8 was obtained by pervaporation at 60 oC. Flux of

pure benzene and pure cyclohexane was 0.34 kg/m2h and 0.57 kg/m

2h, respectively. Recently,

Tanaka and coworkers (131) have produced supported carbon membranes through the

carbonization of resorcinol/ formaldehyde resin at relatively low temperatures ranging from 400

to 600 °C. Pervaporation separations for water/alcohol (methanol, ethanol, and i-propanol) and

water/acetic acid mixtures were conducted to investigate the feasibility of using the microporous

carbon membranes for pervaporation. The permeation flux and separation factor of about 0.23

kg/m2h and 4150, respectively, for 10 wt % water/ipropanol mixture at 70 °C were obtained. The

membrane performance remains stable in dehydrating water/i-propanol system. Moreover, the

membrane stability in acid was improved by the sulfonation treatment. It is indicated that for 10

wt % water/acetic acid mixture at 30 °C, the permeation flux and separation factor were stable

0.12 kg/m2h and 70, respectively, for 196 h.

3.4. Water treatment

Polyvinylidene chloride (PVDC) and polyvinyl alcohol (PVA) microspheres were

carbonized on ceramic membrane to fabricate activated carbon membrane for coke furnace

wastewater treatment (132). A ceramic tube was dipped into a polymer latex containing 70 wt %

PVDC and PVA microspheres of 0.10 to 0.15 µm to form aggregates of polymeric microspheres

on and within (within the pores of) the ceramic pipe. The precursor was heated at 300 °C and

further to 750 °C for carbonization. Nitrogen adsorption applying Horvath and Kowazoe method,

the membrane was found to have a bimodal pore size distribution with micropores of 0.7 to 0.8

nm in diameter and mesopores of 2 to 20 nm. The molecular weight cut off of the membrane at

ca 10,000 Dalton. Coke furnace wastewater containing dissolved organic carbon (DOC) of 69

ppm and two peaks at the molecular weight of 400 and 10,000 Da was subjected to the filtration

by the activated carbon membrane. The result indicated that 32 % of the COD is removed by the

sieve mechanism. Most likely, the adsorption occurs in the micropores in the carbon particles

while the filtration takes place where the pores formed as the interstitial space between the

carbon particles.

The immobilization of biofilms on permeable membranes for the biodegradation of

pollutants had drawn an increasing interest for applications, where conventional treatment

technologies are difficult to apply. As one of such examples, Hu et al. demonstrated the

usefulness of membrane aerated biofilm reactor (MABR) for the wastewater treatment (133); a


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carbon membrane aerated biofilm reactor (CMABR) was constructed to remove organics and

nitrogen containing compounds simultaneously in one reactor. The biofilm was occurred when

the bacterial adherence to the membrane surface. Oxygen pumped into the lumen (left side) of

the membrane passes through the membrane wall and is utilized by the bacteria within the

biofilm to oxidize the pollutants as it penetrates into the biofilm. The reactor feed contained

glucose and ammonium chloride as the sources of organic carbon and nitrogenous component. In

addition, the feed contained a small amount of minerals as element nutrition. The experimental

membrane bioreactor contained 16 carbon tubes of length 20 cm with an inner and outer

diameter of 4.7 and 8.9 mm, respectively. The pore size was 2 µm.

The most important parameter in the operation of CMABR is the oxygen supply rate. If

the excess oxygen is provided, heterotrophic bacteria will grow and the organic carbon for

heterotrophic denifitrication will be in short supply. On the other hand, if the oxygen supply rate

is low, the heterotrophic bacteria will compete with nitrifying microorganisms. Therefore, the

supplies of the oxygen need to be controlled for the optimum operation of CMABR. The oxygen

transfer was controlled by changing the oxygen pressure. Under the aeration conditions, an

assumption was made that all carbon and nitrogen in the feed water are oxidized into CO2 and

NO3-. With this assumption, the maximum oxygen supply rate (OSR) at each run was evaluated

to be around 1 to 6 g/day. Based on the analysis, the COD and NH4+-N were removed effectively

over a period of 121 days and a stable operation was achieved for COD removal within 8 days.

However, a significant growth of biofilm from 68th day and the NO3--N concentration as well as

TN concentration decreased considerably. It is revealed that the thickness of biofilm became as

high as 3.3 mm, which starts to hinder the supply of oxygen through the biofilm.

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List of Figures

Figure 2.1: Schematic diagram of (a) side view and (b) cross section of the spinneret (82)

Figure 2.2: Typical carbon hollow fiber membranes (102)

Figure 2.3: Module constructed by Ismail’s group (39)

Figure 2.4: Module constructed by Koros’s group (83)

List of Tables

Table 1.1: Carbon membrane versus polymeric membrane

Table 1.2: Separation mechanism of carbon membranes (15,16,28-30)

Table 2.1: Summary of polymer precursor selection

Table 2.2: Heat treatment condition used by previous researchers

Table 2.3: Membrane characterizations

Table 3.1: Results of carbon membrane performance reported by different investigators

Carbon Membranes - Universiti Teknologi Malaysia · 2016-10-25 · 1Advanced Membrane Technology...

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