Properties, Synthesis & Applications

By:M.Wadah Jawich

Gas Separation Membranes 1. Types of Membranes

A. Isotropic Membranes

I. Microporous MembranesII. Nonporous, Dense MembranesIII. Electrically Charged Membranes

B. Anisotropic Membranes

C. Ceramic, Metal and Liquid Membranes

2. Membrane Processes

A. Developed membrane separation industrial technologies

I. Microfiltration and UltrafiltrationII. Reverse osmosis III. Electrodialysis

B. Developing industrial membrane separation technologies

I. Gas separationII. Pervaporation

C. To-be-developed membrane separation technologies

I. Carrier facilitated transport II. Membrane contactorsIII. Piezodialysis membrane

3. Gas Separation Membrane

Membrane Materials and Structure

I. Metal MembranesII. Polymeric MembranesIII. Ceramic and Zeolite Membranes IV. Mixed-matrix Membranes

Applications of Gas Separation Membranes

I. Natural Gas SeparationsII. DehydrationIII. H2 Separation

4. Ceramic Membranes for Gas Separation

Preparation of Ceramic Membranes

I. Slip Casting II. Tape Casting III. PressingIV. ExtrusionV. Sol-Gel Process VI. Dip CoatingVII. Chemical Vapor Deposition (CVD)

Industrial Ceramic MembranesA. Zeolite membranes B. Silica membranesC. Carbon membranes

5. Summary and Conclusion

Introduction In general, a membrane can be described as a permselective barrier or a

fine sieve.

Permeability and separation factor of a ceramic membrane are the two most important performance indicators .

For a porous ceramic membrane, they are typically governed by :

Thickness Pore size Surface porosity of the membrane.

What Does A Ceramic Membrane Consist Of?

Ceramic membranes are usually composite ones consisting of several layers of one or more different ceramic materials.

They generally have: A macroporous support One or two mesoporous intermediate layers And a microporous (or a dense) top layer.

The bottom layer provides mechanical support, while the middle layers bridge the pore size differences between the support layer and the top layer where the actual separation takes place.

Commonly used materials for ceramic membranes are Al2O3, TiO2, ZrO2,

SiO2 etc. or a combination of these materials. Most commercial ceramic membranes are in disc, plate or tubular

configuration in order to increase the surface area to volume ratio , which gives more separation area per unit volume of membrane element

SEM micrograph of a layered ceramic membrane for oxygen permeation

����A Brief Overview On The Development of Artificial Membranes

Systematic studies of membrane phenomena can be traced to the eighteenth century philosopher scientists.

Through the nineteenth and early twentieth centuries, membranes had no industrial or commercial uses, but were used as laboratory tools to develop physical/chemical theories.

The period from 1960 to 1980 produced a significant change in the status of membrane technology.

Building on the original Loeb–Sourirajan technique. Other membrane formation processes, including interfacial polymerization and multilayer composite casting and coating, were developed for making high performance membranes with selective layers as thin as 0.1 μm or less .

Methods of packaging membranes into large-membrane-area spiral-

wound, hollow-fine-fiber, capillary, and plate-and-frame modules were also developed

By 1980, microfiltration, ultrafiltration, reverse osmosis and electrodialysis were all established processes with large plants installed worldwide.

The principal development in the 1980s was the emergence of industrial membrane gas separation processes. The first major development was the Monsanto Prism membrane for hydrogen separation, introduced in 1980.

Within a few years, Dow was producing systems to separate nitrogen from air, and Cynara and Separex were producing systems to separate carbon dioxide from natural gas.

The final development of the 1980s was the introduction by GFT, a small German engineering company, of the first commercial pervaporation systems for dehydration of alcohol.

Gas separation technology is evolving and expanding rapidly; further substantial growth will be seen in the coming years

A- Isotropic Membranes

1- Microporous Membranes2- Nonporous, Dense Membranes3- Electrically Charged Membranes

B-Anisotropic Membranes

C- Ceramic, Metal and Liquid Membranes

A. Isotropic Membranes.

1- Microporous Membranes

A microporous membrane is very similar in structure and

function to a conventional filter. It has a rigid, highly voided

structure with randomly distributed, interconnected pores.

However, these pores differ from those in a conventional

filter by being extremely small, on the order of 0.01 to 10 μm in

diameter.

All particles larger than the largest pores are completely

rejected by the membrane. Separation of solutes by microporous

membranes is mainly a function of molecular size and pore size

distribution.

2- Nonporous, Dense Membranes.

Nonporous, dense membranes consist of a dense film

through which permeants are transported by diffusion under

the driving force of a pressure, concentration, or electrical

potential gradient.

The separation of various components of a mixture is

related directly to their relative transport rate within the

membrane, which is determined by their diffusivity and

solubility in the membrane material.

3- Electrically Charged Membranes.

Electrically charged membranes can be dense or

microporous, but are most commonly very finely microporous, with

the pore walls carrying fixed positively or negatively charged

ions.

A membrane with fixed positively charged ions is referred to

as an anion-exchange membrane because it binds anions in the

surrounding fluid. Similarly, a membrane containing fixed negatively

charged ions is called a cation-exchange membrane.

Separation with charged membranes is achieved mainly by

exclusion of ions of the same charge as the fixed ions of the

membrane structure, and to a much lesser extent by the pore size.

The separation is affected by the charge and concentration

of the ions in solution.

B. Anisotropic Membranes

The transport rate of a species through a membrane is

inversely proportional to the membrane thickness. High

transport rates are desirable in membrane separation

processes for economic reasons; therefore, the membrane

should be as thin as possible.

The advantages of the anisotropic membranes is higher

fluxes. The separation properties and permeation rates of the

membrane are determined exclusively by the surface layer; the

substructure functions as a mechanical support.

Anisotropic membranes consist of an extremely thin surface layer

supported on a much thicker, porous substructure. The surface

layer and its substructure may be formed in a single operation or

separately

C. Ceramic, Metal and Liquid Membranes

Ceramic membranes, a special class of microporous

membranes, are being used in ultrafiltration and microfiltration

applications for which solvent resistance and thermal

stability are required .

Dense metal membranes, particularly palladium

membranes, are being considered for the separation of

hydrogen from gas mixtures, and supported liquid films are

being developed for carrier-facilitated transport processes

A- Developed membrane separation industrial technologies

1- Microfiltration and Ultrafiltration2- Reverse osmosis 3- Electrodialysis

B- Developing industrial membrane separation technologies

1- Gas separation2- Pervaporation

C- To-be-developed membrane separation technologies

1- Carrier facilitated transport 2-Membrane contactors3-Piezodialysis membrane

A- Developed Membrane Separation Industrial Technologies

1- Microfiltration and Ultrafiltration

In ultrafiltration and microfiltration the mode of separation is molecular sieving through increasingly fine pores.

- Microfiltration membranes filter colloidal particles and bacteria

-Ultrafiltration membranes can filter dissolved macromolecules,

such as proteins, from solutions

2- Reverse osmosis.

In osmosis membranes the membrane pores are so small and

are within the range of thermal motion of the polymer chains that

form the membrane.

The accepted mechanism of transport through these

membranes is called the solution-diffusion model.

3- Electrodialysis

A charged membranes are used to separate ions from aqueous

solutions under the driving force of an electrical potential difference.

The process utilizes an electrodialysis stack, built on the filter-

press principle and containing several hundred individual cells, each

formed by a pair of anion and cation exchange membranes.

B- Developing Membrane Separation Industrial Technologies

1- Gas separation

In gas separation, a gas mixture at an elevated

pressure is passed across the surface of a membr-

ane that is selectively permeable to one com-

ponent of the feed mixture; the membrane permeate is enriched in this

species.

Major current applications of gas separation membranes are the

separation of hydrogen from nitrogen, argon and methane in ammonia

plants; the production of nitrogen from air; and the separation of carbon

dioxide from methane in natural gas operations

2- Pervaporation

In pervaporation, a liquid mixture contacts

one side of a membrane, and the permeate is

removed as a vapor from the other. The driving

force for the process is the low vapor pressure on

the permeate side of the membrane generated by

cooling and condensing the permeate vapor.

Pervaporation offers the possibility of

separating closely boiling mixtures or

azeotropes that are difficult to separate by

distillation or other means (the dehydration of 90–

95% ethanol solutions)

C- To-Be- Developed Membrane Separation Technologies

1- Carrier Facilitated Transport

It employs liquid membranes containing a complexing or carrier

agent. The carrier agent reacts with one component of a mixture on the

feed side of the membrane and then diffuses across the membrane to

release the permeant on the product side of the membrane.

2- Membrane Contactors

Membrane contactors are devices that allow a gaseous phase and a

liquid phase to come into direct contact with each other, for the purpose of

mass transfer between the phases, without dispersing one phase into

the other.

A typical use for these devices is the removal or dissolution of gases

in water.

3- Piezodialysis Membrane

If fixed-ions of both anion and cation species are attach to

a polymeric membrane, pressure can be used as the driving force

to transport both ions of a salt across a single membrane, leaving

a diluted aqueous stream on the pressurized side.

A zeolite-based piezodialysis membranes are being

developed for desalination processes and some medical

applications in urology and cardiology

Membrane Materials and Structure

I. Metal MembranesII. Polymeric MembranesIII. Ceramic and Zeolite Membranes IV. Mixed-matrix Membranes

Applications of Gas Separation Membranes

I. Natural Gas SeparationsII. DehydrationIII. H2 Separation

Theoretical Background

Both porous and dense membranes can be used as selective gas separation

barriers; Three types of porous membranes, differing in pore size, are shown in

the figure below.

If the pores size = 0.1 to 10 μm :

=> Gases permeate the membrane by convective flow, and no separation

occurs.

If the pores are < 0.1 μm:

=> The pore diameter is ≤ the mean free path of the gas molecules :

=> Diffusion through such pores is governed by Knudsen

diffusion, and the

transport rate of any gas is inversely proportional to the square

root of its

molecular weight.

If the pores are extremely small, of the order 5–20 A˚

=> gases are separated by molecular sieving.

=> Transport includes both diffusion in the gas phase and

diffusion of adsorbed

species on the surface of the pores (surface diffusion).

Membrane Materials and Structure

1- Metal Membranes*- The study of gas permeation through metals began with Graham’s observation of hydrogen permeation through palladium.

*- Hydrogen permeates a number of metals including palladium, tantalum, niobium, vanadium, nickel, iron, copper, cobalt and platinum.

*- In most cases, the metal membrane must be operated at high temperatures (>300 ◦C) to obtain useful permeation rates and to prevent embrittlement and cracking of the metal by adsorbed hydrogen.

*-Hydrogen-permeable metal membranes are extraordinarily selective, being extremely permeable to hydrogen but essentially impermeable to all other gases.

Hydrogen permeation through a metal membrane is believed to follow the multistep process illustrated in the figure

2- Polymeric Membranes*- Early gas separation membranes were adapted from the cellulose acetate membranes produced for reverse osmosis.

*- These membranes are produced by precipitation in water; the water must be removed before the membranes can be used to separate gases.

=> The capillary forces generated as the liquid evaporates cause collapse of the finely microporous substrate of the cellulose acetate membrane, destroying its usefulness.

*- This problem has been overcome by a solvent exchange process in which the water is first exchanged for an alcohol, then for hexane.

*- Experience has shown that gas separation membranes are far more sensitive to minor defects, such as pinholes in the selective membrane layer, than membranes used in reverse osmosis or ultrafiltration

3- Ceramic and Zeolite Membranes*- These microporous membranes are made from aluminum, titanium or silica oxides.

*- Ceramic membranes have the advantages of being chemically inert and stable at high temperatures, conditions under which polymer membranes fail.

*-This stability makes ceramic microfiltration/ultrafiltration membranes particularly suitable for food, biotechnology and pharmaceutical applications.

*- These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support.

- Ceramic membranes are prepared by the sol–gel process

- Zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support.

4- Mixed-Matrix Membranes*- The ceramic and zeolite membranes have exceptional selectivities for a number of important separations. However, the membranes are not easy to make and expensive for many separations.

*- One solution to this problem is to prepare membranes from materials consisting of zeolite particles dispersed in a polymer matrix.

*- These membranes are expected to combine the selectivity of zeolite membranes with the low cost and ease of manufacture of polymer membranes. Such membranes are called mixed-matrix membranes.

Applications of Ceramic Membranes

1- Natural Gas Separations

*- The major component of raw natural gas is methane, typically 75–90% of the total. Natural gas also contains significant amounts of ethane, some propane and butane, and 1–3% of other higher hydrocarbons. In addition, the gas contains undesirable impurities: water, carbon dioxide, nitrogen and hydrogen sulfide.

*- To minimize recompression costs at gas processing plants, impurities must be removed from the gas, leaving the methane, ethane, and other hydrocarbons in the high-pressure residue gas.

*-Carbon dioxide is best separated by glassy membranes (utilizing size selectivity)

*- Hydrogen sulfide, which is larger and more condensable than carbon dioxide, is best separated by rubbery membranes (utilizing sorption selectivity).

*- Propane and other hydrocarbons, because of their condensability, are best separated from methane with rubbery sorption-selective membranes.

The relative size and condensability (boiling point) of the principal components of natural gas. Glassy membranes generally separate by differences in size; rubbery

membranes separate by differences in condensability

2- Dehydration*- All natural gas must be dried before entering the national distribution pipeline to control corrosion of the pipeline and to prevent formation of solid hydrocarbon/water hydrates that can choke valves.

*- Currently glycol dehydrators are widely used. However, glycol dehydrators are not well suited for use on small gas streams or on offshore platforms, increasingly common sources of natural gas

*- Membrane processes offer an alternative approach to natural gas dehydration. Two possible process designs are available.

In the first design, a small one-stage system removes 90% of the water in the feed gas, producing a low-pressure permeate gas representing 5–6% of the initial gas flow. This gas contains the removed water

In the second design, the wet, low-pressure permeate gas is recompressed and cooled, so the water vapor condenses and is removed as liquid water. The natural gas that permeates the membrane is then recovered, but the capital cost of the system approximately doubles

Dehydration of natural gas is easily performed by membranes but high cost may limit its scope to niche

applications.

3- H2 Separation

*-It is desirable to develop inorganic zeolite membranes that are capable of highly selective H2 separation from other light gases (CO2, CH4, CO).

*-Currently used zeolite membranes have not been successful for H2 separation, because they either have zeolite pores too big for separating H2 from other light gases or have many non-zeolite pores bigger than the zeolite pores, so called defects.

*-To selectively separate H2 from other light gases (CO, CO2, CH4), the zeolite membrane will have to discriminate between molecules that are approximately 0.3-0.4 nm in size and 0.1 nm or less in size difference.

*-To accomplish this sieving we need to:

A- Synthesize zeolite membranes with small pore in this size range

B- Post-treat existing zeolite membranes to systematically reduce the pore size and/or the number of defects.

Preparation of Ceramic Membranes

I. Slip Casting II. Tape Casting III. PressingIV. ExtrusionV. Sol-Gel Process VI. Dip CoatingVII. Chemical Vapor Deposition (CVD)

Indusrtial Ceramic MembranesA. Zeolite membranes B. Silica membranesC. Carbon membranes

Ceramic Membranes For Gas Separation There are two types of ceramic membranes suitable for gas separations: (1)

dense and (2) porous, especially microporous, membranes.

Dense Ceramic Membranes are made from crystalline ceramic materials such as fluorites, which allow permeation of only oxygen or hydrogen through the crystal lattice. Therefore, they are mostly impermeable to all other gases, giving extremely high selectivity towards oxygen or hydrogen.

Microporous Ceramic Membranes with pore sizes less than 2 nm.

*- They are mainly composed of amorphous silica or zeolites.*- They are usually prepared as a thin film supported on a

macroporous ceramic support, which provides mechanical strength, but offers minimal gas transfer resistances.

*- In most cases, some intermediate layers are required between the macroporous support and the top separation layer to bridge the gap between the large pores of the support and the small pores of the top separation layer.

Preparation of Ceramic Membranes

In general, preparation of ceramic membranes involves several steps:

(1) Formation of particle suspensions.

(2) Packing of the particles in the suspensions into a membrane precursor with a certain shape such as flat sheet, monolith or tube

(3) Consolidation of the membrane precursor by a heat treatment at high temperatures.

A generalized flow sheet for preparation of ceramic membranes using various conventional methods

1- Slip Casting *- When a well mixed powder suspension (slurry) is poured into a porous mould, solvent of suspension is extracted into the pores of the mould via the capillary driving force or capillary suction. The slip particles are, therefore, consolidated on the surface of the mould to form a layer of particles or a gel layer.

2- Tape Casting

*- The process consists of a stationary casting knife, a reservoir for powder suspensions, a moving carrier and a drying zone. In preparing flat sheet ceramic membranes, the powder suspension is poured into a reservoir behind the casting knife, and the carrier to be cast upon is set in motion.

*-The casting knife gap between the knife blade and carrier determines the thickness of the cast layer. Other variables which are important include reservoir depth, speed of carrier and viscosity of the powder suspension.

*-The wet cast layer passes into a drying chamber, and the solvent is evaporated from surface, leaving a dry membrane precursor on the carrier surface.

3- Pressing

*- The particle consolidation into a dense layer occurs by an applied force. This easily handled pressure press method has been frequently employed in screening new ionic and mixed conducting materials for development of oxygen or hydrogen permeable ceramic membranes.

*- A special press machine is used to apply more than 100 MPa pressure to press powders into a compacted disc. The diameter of the disc is usually a few of cm, the thickness is often around 0.5 mm and the disc is dense after firing.

4- Extrusion

*- The extrusion process is similar to fibre spinning processes, but there are a few differences between extrusion and spinning.

In extrusion: a stiff paste is compacted and shaped by forcing it through a nozzle. A requirement is that the precursor should exhibit plastic behavior, that is at lower stresses behave like a rigid solid and deform only when the stress reaches a certain value called the yield stress.

In spinning: a viscous solution or suspension is transformed into a stable shape in a coagulation bath through a spinneret.

In addition, the precursor made by extrusion possesses a homogeneous structure over the cross section, while it shows an asymmetric structure if prepared through the spinning process.

5- Sol-Gel Process

*- The advantage of the sol-gel technique is that the pore size of the membrane can be desirably controlled, especially for small pores.

*- There are two main routes through which the sol-gel membrane is prepared:

(1) The colloidal route, in which a metal salt is mixed with water to form a sol. The sol is coated on a membrane support, where it forms a colloidal gel.

(2) The polymer route, in which metal–organic precursors are mixed with organic solvent to form a sol, which is then coated on a membrane support, where it forms a polymer gel.

5- Sol-Gel Process

*- The Colloidal sols are the colloidal solutions of dense oxide particles such as Al2O3, SiO2, TiO2 or ZrO2.

*- For gas separation based on molecular sieving effects, ceramic membranes with pore sizes less than 1 nm must be employed.

=> In this case, the membrane can be prepared through the polymer sol route using the γ-alumina membrane as a support.

*- It should be noted that in the polymer sol route, the pore size of the membrane prepared is determined by the degree of branching of the inorganic polymer.

*- Sols of very small particles are prepared through hydrolysis and condensation of their corresponding alkoxides.

=> The partial charges of the metal in the alkoxides and hydrolyses speed influence the hydrolysis behavior

6- Dip Coating

*- The critical factors in dip coating are the viscosity of the particle suspension and the coating speed or time.

*- The drying process starts simultaneously with the dip coating, when the substrate is in contact with a atmosphere that has a relative humidity below 100 %.

*- In a multiple step process, after calcinations of the first layer, the complete cycle of dipping, drying and calcination is repeated.

7- Chemical Vapor Deposition (CVD)

*- Chemical vapor deposition is a technique which modifies the properties of membrane surfaces by depositing a layer of the same or a different compound through chemical reactions in a gaseous medium surrounding the component at an elevated temperature.

*- CVD system which includes a system of metering a mixture of reactive and carrier gases, a heated reaction chamber, and a system for the treatment and disposal of exhaust gases.

*- The gas mixture (which typically consists of hydrogen, nitrogen or argon, and reactive gases such as metal halides and hydrocarbons) is carried into a reaction chamber that is heated to the desired temperature.

*- The deposition of coatings by CVD can be achieved in a number of ways such as thermal decomposition, oxidation and hydrolysis

Industrial Ceramic Membranes

In According to the IUPAC definition:

“Microporous membranes are referred to as those with a pore diameter smaller than 2

nm”

There are two main types of microporous membranes used in gas separations, namely crystalline zeolite membranes and XRD amorphous membranes such as silica, carbon, etc.

The practically useful crystalline microporous membranes have polycrystalline structures, consisting of many crystallites packed together without any crystallite (grain) boundary gap in the ideal case.

1- Zeolite Membranes

*- Zeolites are crystalline microporous aluminosilicate materials with a regular three dimensional pore structure, which is relatively stable at high temperatures.

*- They are currently used as catalysts or catalyst supports for a number of high temperature reactions.

*- The unique properties of zeolite membranes are:

(1) their size and shape selective separation behavior.

(2) their thermal and chemical stabilities, which are also the general

advantages of ceramic membranes.

*- Due to their ‘molecular sieve’ function, zeolite membranes can principally discriminate the components of gaseous or liquid mixtures dependent on their molecular size.

*- In order to perform the molecular sieving function, the membranes must have negligible amounts of defects and pinholes of larger than 2 nm.

2- Silica Membranes

*- Microporous silica (SiO2) membranes are prominent representatives of amorphous membranes.

*- The first successful silica membranes for gas permeation/separation with good quality and high flux were prepared in 1989 using a sol-gel method where SiO2 polymer sols were firstly prepared by acid catalysed hydrolysis of tetraethoxysilane (TEOS) in alcoholic solution.

*- The acid catalyst reduces hydrolysis but enhances polycondenstion rates during the sol preparation process resulting in a polymeric sol containing silica particles of fractal structure.

*- Chemical vapor deposition (CVD) is another method used in preparation of microporous silica membranes.

3- Carbon Membranes

*- Carbon membranes are inexpensive, highly selective due to their pores of molecular dimensions.

*- They are prepared basically by carbonizing organic polymers as starting materials at high temperatures under controlled conditions. It is expected that carbonized materials are stable at high temperatures and resist chemical attack.

*-The challenge for carbon membranes is how to increase the gas permeation rate.

One approach is to make the membranes on mesoporous substrates. For example, carbon membranes were prepared by ultrasonic

deposition of polyfurfuryl alcohol on a porous inorganic support, followed by pyrolysis at 473–873 K to convert the polymer layer to microporous carbon film.

Another approach is using asymmetric hollow fiber membrane precursors.

In general, a membrane can be described as a permselective barrier or a fine sieve.

There are several fields on which membrane technologies are used:

A. Developed membrane separation industrial technologies (microfiltration and ultrafiltration, reverse osmosis , and electrodialysis)

B. Developing industrial membrane separation technologies (Gas separation and pervaporation)

C. To-be-developed membrane separation technologies (Carrier facilitated transport , membrane contactors, and piezodialysis membrane)

There are a lot of applications of gas separation membranes ( natural gas separations, dehydration, and H2 separation)

Membrane materials include metal membranes, polymeric membranes, ceramic and zeolite membranes , and mixed-matrix membrane.

Gas separation has become a major industrial application of membrane technology only during the past 20 years. Gas separation technology is evolving and expanding rapidly; further substantial growth will be seen in the coming years.

Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required.

Ceramic membranes are usually composite ones consisting of several layers of one or more different ceramic materials.

There are two types of ceramic membranes suitable for gas separations: (1) dense and (2) porous, especially microporous, membranes.

Dense ceramic membranes are made from crystalline ceramic materials

Microporous ceramic membranes are mainly composed of amorphous silica or zeolites

Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes.

On the industrial level: There are two main types of microporous membranes used in gas separations, namely crystalline zeolite membranes and XRD amorphous membranes such as silica, carbon, etc

Zeolite membrane synthesis is an important new field for development of ceramic membrane, the specifications that zeolite have makes it a promising material for investigation.

Several researches are being held for the manufacturing of ceramic membranes for gas separation out of zeolite, and there are several other medical and military applications that will find its way to the market in the coming years

Thank you