Membrane Separation technology

FRANCISCO RAMALHOSA | RICARDO SILVA 1

O V E R V I E W A B O U T M E M B R A N E S E P A R A T I O N B M E 2 0 1 5

Introduction

Membrane technology is a growing subject in a broad range of applications, from

industrial to medical applications. The main feature that is exploited is the ability of a

membrane to control the permeation rate of a chemical species trough the membrane.

In the first chapters, this review provides a general introduction to membrane

science, covering the basic topics to all membrane processes, such as types of membrane

and transport mechanisms. In the following chapters, the main industrial membrane

separation processes are reviewed, such as reverse osmosis, nanofiltration, ultrafiltration,

microfiltration, pervaporation and electrodialysis. In the last chapters a more considerate

attention is given to the reverse osmosis and nanofiltration processes, plus the recent

advancements of both.

Historical Development

In 1748 Abbe Nolet discovered the phenomenon of osmosis in natural membranes

and describes it as the permeation of water through a diaphragm [1]. Until the beginning

of XX century, membranes were only used as laboratory tools to develop physical and

chemical theories. Van Hoff in 1887 developed his limit law using membranes made by

Moritz Traube, Traube was the first to produce artificial semipermeable membranes by

putting droplets of glue in tannic acid. This kind of semipermeable membrane also was

used by Maxwell in developing the kinetic theory of gases.

Later in 1907 Bechhold introduced term ultrafiltration forcing solutions at

pressures up to several atmospheres through membranes prepared by impregnating filter

paper with acetic acid collodion nitrocellulose.

By the early 1930s microporous collodion semipermeable membranes were

commercially available recurring on Bechholds technique, in the next few years others

kinds of polymers were used like cellulose acetate for example. In 1950 Hassler

introduces the first concept of membrane desalination [1].

At World War II membranes were important to test the drinking water, because

water supplies in Europe had broken down, and filters to test for water safety were needed.
http://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/wiki/Tannic_acid



These kind of filters were developed by the US Army, later Millipore Corporation

exploited the semipermeable membranes becoming the largest microfiltration membrane

producer.

Until 2003 there wasnt significant membrane separation industry, because

membranes were too slow, unselective, and expensive. Early 1960 Loeb-Sourirajan

developed a process that transformed membrane separation to an industrial process, these

membranes consisted in an ultrathin, selective surface film on a much thicker and more

permeable microporous support, this kind of process uses the anisotropic reverse osmose,

and it was 10 times higher than that of any membrane available in the market, and made

reverse osmosis a new potentially method to desalt water.

The work of Loeb and Sourirajan with the economic financiation of Office of

Saline Water, resulted in the commercialization of reverse osmosis and at the same time

in the development of ultrafiltration, microfiltration and medical separation process. In

1945 W.J. Kolf [6] had demonstrated the first artificial kidney, only after 20 years the

technology was used on a large scale. Since then more than 800 000 people are sustained

by artificial kidneys and also the development of the membrane blood oxygenator made

possible an open-heart surgery, helped millions of people. In 1966 Alex Zaffaroni

founded Alza, a company dedicated to the developing of membranes that controlled drug

delivery systems, improved the efficiency and safety of drug delivery.

Between 1960 and 1980 the LoebSourirajan technique suffered some progresses

including interfacial polymerization and multilayer composite for making high-

performance membranes. Using membranes with selective layers as thin as 0.1 m, large-

membrane-area spiral-wound, hollow-fine-fiber, capillary, and plate-and-frame modules

were developed, improving the membrane stability. In the 1980s Monsanto Prism

developed the membrane gas separation process in industrial scale, like the membrane for

hydrogen separation. Gas separation technology is expanding quickly, Cynara and

Separex produced membranes to separate carbon dioxide from natural gas, also

pervaporation systems for dehydration of alcohol are now available, and others

pervaporation applications are at the early commercial stage.



Membrane Science Technology

Introduction

The primary characteristic that is exploited in membrane separation, is the ability

of a membrane to control the permeation rate of a chemical species. But the goal varies

according to the application. In controlled drug delivery the goal is to moderate the

permeation rate of a drug from a reservoir to the body, instead, in separation applications,

the goal is to allow one component of a mixture to permeate the membrane freely, while

hindering permeation of other components.

Types of Membranes

A membrane is nothing more than a discrete, thin interface that moderates the permeation

of chemical species. This interface may be molecularly homogeneous, that is, completely

uniform in composition and structure, or it may be chemically or physically

heterogeneous, for example, containing holes or pores of finite dimensions or consisting

of some form of layered structure [1].

The principal types of membrane are shown schematically in Figure 1.1 and are

described briefly below.



Symmetrical membranes

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. The difference between a conventional filter, are the extremely

small pores, ranging between 0.01 to 10 m in diameter. Therefore, particles larger than

the largest pores are blocked by the membrane [2]. Only molecules that differ considerably

in size can be separated effectively by microporous membranes, for example, in

ultrafiltration and microfiltration. Microporous membranes have much higher fluxes than

nonporous membranes, being widely used in industrial processes.

Nonporous Membranes

This 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 [1]. Nonporous membranes can separate permeants of similar size if their

concentration in the membrane material differs significantly. Therefore, the solubility of

the components of a mixture in the membrane material is directly related to their transport

rate within the membrane. However, these membranes are rarely used in industrial

processes because the transmembrane flux through these relatively thick membranes is

too low for practical separation processes. Thus, they are only used in laboratory work.

Sometimes, to improve the flux, these membranes have an anisotropic structure.

Electrically Charged Membranes

Electrically charged membranes are most commonly very finely microporous,

with the pore walls carrying fixed positively or negatively charged ions, being referred to

as anion-exchange membrane and cation-exchange membrane, respectively.

Separation is accomplished by exclusion of ions of the same charge as the fixed

ions of the membrane structure.



Anisotropic Membranes

High transport rates are desired in membrane separation, for economic reasons,

therefore the membrane should be as thin as possible, since the transport rate of a species

through a membrane is inversely proportional to the membrane thickness.

Membrane fabrication technology limits manufacture of films to about 20 m

thickness. However, in the last 30 years there were major improvements to produce

anisotropic membrane structures. These membranes consists in an extremely thin surface

layer supported on a much thicker and porous substructure. The surface layer determines

the separation proprieties and permeation rates, while the substructure only provides

mechanical support.

Since the high fluxes provided by anisotropic membranes are so great, almost all

commercial processes use such membranes.

Ceramic and Metal Membranes

The majority of membranes used commercially are polymer-based, however other

materials like ceramic and metal are being investigated. Metal membranes are, for

example, being tested for the separation of hydrogen from gas mixtures. Also, ceramic

membranes are being used in ultrafiltration and microfiltration separations, where a

solvent resistance and thermal stability are required.

Membrane Processes

The four developed industrial membrane separation processes are microfiltration,

ultrafiltration, reverse osmosis, and electrodialysis, differing principally in the average

pore diameter of the membrane filter. Microfiltration membranes filter colloidal particles

and bacteria from 0.1 to 10 m in diameter, ultrafiltration membranes can be used to filter

dissolved macromolecules from 100 to 1000 . In reverse osmosis membranes, the

membrane pores are so small, from 5 15 in diameter, that they are within the polymer



that form the membrane, the transport occurs via statistically distributed free volume

areas.

The difference between reverse osmosis, ultrafiltration and microfiltration is in

pore diameter, it produces important differences in the way the membranes are used. The

liquid flow through a pore (q) is given by Poiseuilles law as:

=4

128

is the pressure difference across the pore, is the liquid viscosity and l is the pore

length.

The flux, per unit membrane area, is the sum of all the flows through the individual

pores, is given by:

= 4

128 (1.2)

Where N is the number of pores per square centimeter of membrane.

For membranes of equal porosity () the number of pores per square centimeter is

proportional to the inverse square of the pore diameter:



= 4

2 (1.3)

The flux is given by combining Equations (1.2) and (1.3):

J=P

32

Fluxes are proportional to the square of these pore diameters, the permeance

(flux per unit pressure difference (J /_p)) of microfiltration membranes for example is

higher than that of ultrafiltration membranes, which is much higher than that of reverse

osmosis membranes. These differences significantly impact the operating pressure.

Carrier Facilitated Transport

One of the most important industrial membranes to be developed is carrier

facilitated transport which employs liquid membranes containing a carrier agent.

Facilitated transport is concerned with the reversible and selective reaction between the

solute or component and the carrier. This reaction normally takes place throughout the

liquid membrane phase in the feed side, the carrier agent acts as a shuttle to selectively

transport one particular component from the feed and then diffuses across the membrane

to release the permeant on the product side of the membrane. Water is required on both

the feed and permeate side of the membrane for high selectivity to be observed, probably

as a consequence of the active participation of water in the chemical reactions that

enhances solubilities. Facilitated transport membranes can be used to separate gases and

metal ions (coupled transport). Membrane transport is then driven by a difference in the

gas partial pressure across the membrane to separate gases, or driven by flow of hydrogen

or hydroxyl ions to separate metal ions, the ion-exchange reaction occurs at a liquid-liquid

interface since metal ions are not soluble in the organic membrane phase.

One simple example is the hemoglobin, on the feed side hemoglobin reacts with oxygen

to form oxyhemoglobin, which then diffuses to product side, there the oxygen is liberated

and hemoglobin is re-formed. The hemoglobin then diffuses back to the feed side of the

membrane to repeat the process. In the coupled transport case one of the species moved



against its concentration gradient, provided the concentration gradient of the other

species. In example, the carrier is an oxime that forms an organic soluble complex with

copper ions. On the feed side two oxime carrier molecules pick up a copper ion, liberating

two hydrogen ions at the same time. The copperoxime complex then diffuses to the

product side, where the higher concentration of hydrogen ions makes the reaction

reversed. The copper ion is liberated to the permeate solution, and two hydrogen ions are

picked up, then the reformed oxime molecules turns back to the feed side.

Facilitated transport carrier particular species, so extremely high selectivity

membranes are needed, however there is no commercial applications in this area because

there are problems with the physical and chemical instability of the liquid membrane and

the carrier agent. Some potential solutions have been studied to make carrier facilitated

transport viable in a commercial and industrial way.



Transport Mechanisms

Introduction

There are two models that describes the mechanism of permeation in membranes,

the solution-diffusion model and the pore-flow model. Permeation in reverse osmosis and

pervaporation occurs by molecular diffusion and is described by the solution-diffusion

model, while the pore-flow model defines the microfiltration and ultrafiltration, as shown

in Fig. X. There are also an intermediate model, in which nanofiltration falls into.

Solution-Diffusion Model

Diffusion, the basis of the solution-diffusion model, is the process by which

matter is transported from one part of a system to another by a concentration gradient. In

an isotropic medium, the individual molecules are in random motion, with no preferred

direction. However, if there is a concentration gradient of permeate molecules in the

medium, a transport of mass will occur from the high to the low concentration region.

The pores, in this model, are tiny spaces between polymer chains caused by thermal

motion. So, the pores appear and disappear during the motion of permeants traversing

the membrane.



This phenomenon is theoretically described by Ficks law of diffusion, which

states

=

where is the flux of a component i (g/cm2s) and / is the concentration gradient

of a component i. The diffusion coefficient, (cm2/s) is a measure of the mobility of

the individual molecules.

In practical applications, high fluxes are achieved by creating large concentration

gradients in the membrane.

This model is universally well accepted and supported, since it provides simple

equations that accurately link the driving forces of concentration and pressure with flux

and selectivity. However, it has been unsuccessful at providing a connection between the

nature of the membrane material and the membrane permeation proprieties.

Pore-Flow Model

The pore-flow model is usually described by a pressure-driven convective flow

through very small pores. Separation occurs because one of the permeants is filtered from

some of the pores, while other permeants pass through. While the pores in the solution-

diffusion model vary from position and volume during the motion of the permeants, in

the pore-flow model the pores are large and fixed and do not fluctuate in position or

volume.



The fundamental equation that describes this model is Darcys law, which is

=

where K is a coefficient regarding the nature of the medium, ci is the concentration of a

component i in the medium and dp/dx is the pressure gradient present in the porous

medium.

The pore-flow model is much less developed, compared to the solution-diffusion

model. There are many characteristics hard-to-compute that affect the permeation.

Measurements of permeation through ideal uniform-pore-diameter membranes made by

the nucleation track method are in good agreement with theory. Unfortunately,

industrially useful membranes have nonuniform tortuous pores and are often anisotropic

as well.

Intermediate Model

The intermediate zone between the previous models, seems to occur with

membranes with very small pores, ranging from 5 to 10 . Apparently this transition is

in the nanofiltration range, with membranes having good filtration to divalent ions and

organic solutes, but rejection to monovalent ions in the 20-70% range.

The transition from reverse osmosis membranes to ultrafiltration membranes is

shown in Figure X.

Membrane Separation technology

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