UNIT 11 MEMBRANE SEPARATION
Membrane Separation
Structure
11.1
Introduction Objectives
11.2 11.3
General Aspects of Membrane Process Some Important Membrane Processes Reverse Osmosis (RO) Nanofiltration (NF) Ultrafiltration (UF) Microfiltration (MF) Dialysis Electrodialysis (ED) Gas Separation Prevaporation Liquid Membrane Processes
11.4
Mechanisms of Separations through Membranes Sieving Solution-Diffusion Preferential Sorption-Capillary Flow Donnon Effect Knudsen Flow Surface Flow Facilitated Diffusional Transport Active transport
11.5 11.6
Osmotic Phenomena Reverse Osmosis Process Basic Equations Concentration Polarization
11.7 11.8 11.9
Dialysis Electrodialysis Applications Desalination and Water Treatment Protein Recovery Production of Table Salt Hemodialysis Ion Selective Membrane Electrode Specific Gas Probes Detection and Analysis of Particulate Contamination Microbiological Analysis
11.10 Summary 11.11 Terminal Questions 11.12 Answers
11.1
INTRODUCTION
In Unit 1 of this course, two types of classifications for different separation methods were proposed–one based on the property leading to separation and the other on equilibrium and rate processes. Membrane separations figure under the category of molecular geometry in one (sub-Sec. 1.6.6) and in the rate processes under first (subSec. 1.7.2). Inclusion of membrane separations under these two classifications very aptly defines the principles behind the membrane separations. Membrane processes utilize semipermeable/ permselective membranes to achieve separations of various chemical substances which can be either in solid, liquid or in gaseous forms. Membrane processes can be used to separate chemical substances in various sizes from microscopic to molecular level. For example, membranes can be used to separate 5
Other Separation Methods
suspended particles from a turbid solution, separation of dissolved solutes from saline waters, separation of toxic metabolic products from blood, separation of a gas from a mixture of gases and so on. The membrane separations are different from many of the other separation processes like solvent extraction, chromatographic methods involving partition, adsorption, ion exchange etc. which involve an equilibrium between the substances getting separated and the separating phases. The onset of equilibrium in these process restricts the extent of separation achievable and the equilibrium is shifted suitably to achieve the desired level of purity. Also, in some of these processes, a reversal of equilibrium is often necessary to regenerate the separating medium so that it can be reused. In these processes which are equilibrium governed, we always talk of how much separation is accomplished in terms of partition coefficients or distribution ratios. In contrast, membrane processes are rate governed processes processes wherein we talk not only of relative quantity of the substance separated but also about the rate at which the separation is taking place. Also, membrane processes are continuous separation processes and there is no need of membrane regeneration as is required in equilibrium governed processes. Because of some of its inherent advantages, the membrane processes are replacing some of the well known separation techniques for technological applications. The present unit is devoted to membrane separations. It starts with a simplified picture of membrane separation, familiarity with some of the general terms and a classification of different membrane processes. The important characteristics of these processes are briefly discussed. This is followed by a description of the different mechanisms of separation by membrane processes. The processes like osmotic phenomena, reverse osmosis, dialysis and electrodialysis are explained in detail. Membrane separations have multidisciplinary applications and some of these are highlighted towards the end of the unit. Objectives
After studying this Unit, you should be able to •
explain the general aspects and importance of various membrane processes,
•
understand and explain the various mechanisms involved in membrane based separations,
•
define the terms like product flux, solute retention percentage, percent recovery, membrane fouling and concentration polarization,
•
explain osmosis, reverse osmosis, dialysis and electrodialysis processes and how they differ from each other, and
•
enumerate some of the important applications based on membrane separations from different areas of science and technology.
11.2
GENERAL ASPECTS OF MEMBRANE PROCESS
Before we discuss the different types of processes, it will be necessary to get a clear picture of a simple membrane process and be familiar with the general terms in usage. It may be equally important to know the criteria which evaluate the process under consideration. A generalised and simplified schematic of membrane separation process is given in Fig. 11.1. 6
Other Separation Methods
suspended particles from a turbid solution, separation of dissolved solutes from saline waters, separation of toxic metabolic products from blood, separation of a gas from a mixture of gases and so on. The membrane separations are different from many of the other separation processes like solvent extraction, chromatographic methods involving partition, adsorption, ion exchange etc. which involve an equilibrium between the substances getting separated and the separating phases. The onset of equilibrium in these process restricts the extent of separation achievable and the equilibrium is shifted suitably to achieve the desired level of purity. Also, in some of these processes, a reversal of equilibrium is often necessary to regenerate the separating medium so that it can be reused. In these processes which are equilibrium governed, we always talk of how much separation is accomplished in terms of partition coefficients or distribution ratios. In contrast, membrane processes are rate governed processes processes wherein we talk not only of relative quantity of the substance separated but also about the rate at which the separation is taking place. Also, membrane processes are continuous separation processes and there is no need of membrane regeneration as is required in equilibrium governed processes. Because of some of its inherent advantages, the membrane processes are replacing some of the well known separation techniques for technological applications. The present unit is devoted to membrane separations. It starts with a simplified picture of membrane separation, familiarity with some of the general terms and a classification of different membrane processes. The important characteristics of these processes are briefly discussed. This is followed by a description of the different mechanisms of separation by membrane processes. The processes like osmotic phenomena, reverse osmosis, dialysis and electrodialysis are explained in detail. Membrane separations have multidisciplinary applications and some of these are highlighted towards the end of the unit. Objectives
After studying this Unit, you should be able to •
explain the general aspects and importance of various membrane processes,
•
understand and explain the various mechanisms involved in membrane based separations,
•
define the terms like product flux, solute retention percentage, percent recovery, membrane fouling and concentration polarization,
•
explain osmosis, reverse osmosis, dialysis and electrodialysis processes and how they differ from each other, and
•
enumerate some of the important applications based on membrane separations from different areas of science and technology.
11.2
GENERAL ASPECTS OF MEMBRANE PROCESS
Before we discuss the different types of processes, it will be necessary to get a clear picture of a simple membrane process and be familiar with the general terms in usage. It may be equally important to know the criteria which evaluate the process under consideration. A generalised and simplified schematic of membrane separation process is given in Fig. 11.1. 6
Membrane
Membrane Separation
Permeate depleted with either A or B
Influent with substances A and B
Concentrate enriched with either A or B Fig. 11.1: A schematic representation representation of a simple membrane membrane process
In membrane separation processes, two bulk phases are separated by a barrier which is called the membrane. The membrane controls the passage of different chemical substances through it which depends upon the nature of the chemical substances and the membrane. This results in depletion of that substance in the permeate through the membrane. The membrane phase can either be nonporous, microporous, mesoporous or o
macroporous. By IUPAC classification, pores between 20 A and mesoporous and pores larger than 500
o
500 A are called called
o
A are called called macropores. A membrane can also either be a solid or a liquid. It can be a polymeric material, inorganic refractory compound or a metal or their composites. Permeation of chemical species through membranes is achieved by applying a driving force across the membrane. This can be in the form of hydrostatic pressure difference, temperature difference, concentration difference, partial pressure difference or electrical potential gradient. Application of different types of driving forces as mentioned above across various types of membranes gives a broad classification of membrane separation processes. Accordingly, the mechanism by which chemical species permeate depends on the type of membrane process. The permeation of chemical species across the membrane is kinetically driven by the application of mechanical, chemical or thermal work.
The performance of the membrane is defined in terms of two simple parameters, viz. transmembrane flux and solute retention or solute selectivity. Transmembrane flux or permeation rate is the volumetric or mass or molar flow rate of a fluid passing through unit area of membrane surface per unit time. Transmembrane flux of any species, i, is directly proportional to the driving force applied across the membrane and is inversely proportional to the effective thickness of the membrane. If the driving force is described in terms of hydrostatic pressure gradient (∆P) or partial pressure difference (∆ pi) or concentration difference (∆C i ), across the membrane for any species , i, then J i i
=
Pi
( ∆P or ∆ pi or ∆C i ) / δ
… (11.1)
where, J i is the transmembrane flux for species i, Pi is the permeability of species i, through the membrane and is the effective membrane thickness. The ratio of permeability of species i, to effective membrane thickness is sometimes called permeance. The retention or selectivity is a measure of the relative permeation rates of different components through the membrane. It is often expressed as selectivity coefficient , α .
7
Other Separation Methods
For two species A and B permeating across a membrane, the selectivity coefficient for species A with respect to B (α AB ) is given by as follows: A
aB
=
P A / P B
....
(11.2)
The selectivity coefficient can also be expressed as the ratio of concentration of the two species in the permeate to the concentrate stream. Accordingly, a BA =
(C A / C B)p / (C A / C B )c
... (11.3)
where, C A and C B represent the concentration of species A and B, respectively and the subscripts p and c represent the permeate and the concentrate, respectively.
11.3
SOME IMPORTANT MEMBRANE PROCESSES
The principal objective of a membrane separation process is separation of a desirable substance from a given mixture. Today, almost all the membrane processes use synthetic membranes. Although biological membranes carry out a number of complicated separations in all life processes, they are not widely used. There are a number of membrane processes which can be defined by considering the following aspects: •
Separation objective–the nature of the substance which needs to be separated from the nature of the mixture,
•
The nature of the species retained by the membrane,
•
The nature of the species permeating through the membranes,
•
The driving force needed to achieve the desired separation, and
•
The mechanism of separation.
There are several membrane separation processes which are of industrial importance as on today. They are briefly discussed below with respect to their principal characteristics. 11.3.1 Reverse Osmosis ( RO )
The Reverse Osmosis (RO) is a process wherein a relatively pure solvent is separated from a salt solution by using a semipermeable membrane by the application of hydrostatic pressure. The hydrostatic pressure can vary from 2 MPa to around 6 MPa depending upon the salt content of the feed mixture. The solvent permeates preferentially through the membranes whereas the solutes, particularly electrolytes and low molecular weight nonelectrolytes are retained by the membranes. For effectively retaining microsolutes having molecular weight less than 300 or effective o
size less than 10 A , reverse osmosis process is used. The process is used to produce relatively pure water or a concentrated solution of microsolutes from a given salt solution. A simple schematic of the process is given in Fig. 11.2. The most notable example of reverse osmosis process is the production of drinking water from naturally occurring saline waters. This process is discussed in more detail in subsequent sections of this unit.
8
Pressure in excess of osmotic pressure
Membrane Separation
Semipermeable membrane
Solution Pure water Fig. 11.2: The schematic representation of a reverse osmosis process
11.3.2 Nanofiltration (NF)
The process of nanofiltration is slightly different from the reverse osmosis process in the sense that the permeating species in this case is solvent as well as low molecular solutes or low valency solutes. This process also operates with hydrostatic pressure difference across a semipermeable membrane having pore sizes which are slightly larger than that of reverse osmosis membranes. The pore sizes of NF membranes are in the range of 10 -30 Å . The hydrostatic pressure used in this process can vary from 1.5 Mpa to 2 MPa. This process is essentially used to fractionate solutes based on valency of either cation or anion and also to separate various organic solutes of low molecular weights. Various applications of NF process are briefly outlined in latter part of this unit. 11.3.3 Ultrafiltration (UF)
The ultrafiltration (UF) is a process wherein the solvent along with microsolutes permeates through the membrane and macrosolutes are retained by the membranes. This process is similar to sieving and the driving force is the hydrostatic pressure across the membrane. The process is used to fractionate the solutes in a solution based on their size or molecular weight difference. Size or the molecular weight difference of the macrosolute retained by the membrane depends upon the pore size of the membranes. Microsolutes whose effective sizes are smaller than the pore size of the membranes permeate along with the solvent whereas macrosolutes whose effective sizes are larger than the pore size of the membranes are retained. The driving force used in ultrafiltration processes is of the order of 500 kPa or so. The membranes used o
o
in ultrafiltration processes have pore sizes ranging from around 30 A to about 200 A . Typical applications of the ultrafiltration process are the concentration of protein in milk for cheese making and separation of colloidal particles, oils and macromolecules from effluent waters as well as from surface waters. 11.3.4 Microfiltration (MF)
The microfiltration (MF) is a process mainly used for the separation of submicron size (< 0.1 µm) particulate matter from solution. This process also requires hydrostatic pressure gradient across the membrane and the pressure used is of the order of 100 kPa or so. The pore size of the membranes decide the size of the particulate matter retained. The process is similar to ultrafiltration and separation takes place by sieving. The essential difference between ultrafiltration and microfiltration is the size of the macrosolute retained, pore size of the membranes and the hydrostatic pressure needed across the membranes. Typical applications of microfiltration process are in the removal of bacteria from water samples and removal of submicron size suspended dust and particulate matters from gas streams. Removal of chemical oxygen demand from effluent waters is another important application of the MF process. 11.3.5 Dialysis
9
Other Separation Methods
Dialysis is a membrane process in which a microsolute permeates across a membrane as a result of concentration gradient from a given feed solution to another receiving solution with a common solvent. The microsolutes permeate across the membrane faster than the macrosolutes which are also present in the solution. The pore size of the membranes are chosen in such a way to block the passage of macrosolutes. The o
pore sizes of membranes used in dialysis typically ranges from around 20 A to o
60 A or so. Commonly, the fluid phase from which the microsolutes are removed is designated as feed phase whereas the fluid phase which receives the microsolutes is designated as dialysate. The most common application of dialysis process is the purification of blood to reduce the toxic end products of metabolism in a patient whose kidneys have failed. It is the single largest application of dialysis process and is known as hemodialysis or artificial kidney. This process is discussed in detail in subsequent section of this unit. 11.3.6 Electrodialysis (ED)
In electrodialysis (ED) process, electrolytes are removed from one solution to the other and it is used for desalination of saline waters. The cations and anions permeate through cation selective and anion selective membranes, respectively as a result of applied electrical energy. Even though reverse osmosis and electrodialysis are both useful for desalination, there is a fundamental difference between the two. In RO, the solvent permeates through the membranes and solutes, both electrolytes and nonelectrlytes, are retained by the membranes. In ED, electrolytes permeate through the membranes and, the solvent and nonelectrolytes generally do not permeate through the membranes. The solution from which electrolytes are removed gets depleted of salt and the solution which receives the solute gets enriched with salt. A simple schematic of an electrodialysis process is given in Fig. 11.3.
Concentrated feed solution AE, CE= Anion selective and cation selective membranes; A,C = Anion and cation, respectively. Fig. 11.3: A schematic representation of electrodialytic process
The process is used for desalination as well as production of table salt.The cation and anion selective membranes used in this process are ion exchange membranes which are nothing but ion exchangers in sheet form. Similar to cation exchange and anion exchange resins, there are cation exchange and anion exchange membranes. This process is discussed in detail in subsequent section of this unit.
10
Membrane Separation
11.3.7 Gas Separation
The concept of separating gases using membranes is more than 100 years old, but the widespread use of gas separation membranes has occurred only within the last decade. Membranes separate gases only if some components pass through the membranes more readily as compared to others. A typical membrane process for gas separation operates with hydrostatic or partial pressure difference across the membrane as the driving force. The feed gas mixture is fed to the membrane at an elevated pressure, where it permeates across the membranes. The other side of the membranes is held at a lower pressure. Separation is achieved because of differences in the selective permeation rates of the feed gas components. Components that permeate more rapidly across the membranes become enriched in the permeate stream while the slower permeating components are concentrated in the residual or the retentate at high pressure. The degree to which gaseous components are separated is governed by the ability of the membranes to discriminate between the two gases as well as the relative driving force for each component. The solubility of the gases in the membrane matrix and the diffusivity of the dissolved gas molecules are the main governing factors in the selectivity. Many gas separating membranes are considered to be dense membranes. The pore size of the membranes for use in gas separation membranes needs to be much smaller than the mean free path of the permeating gas molecules. Details regarding the mechanism involved in gas separation through membranes and the relation between mean free path of the gas molecules and membrane pore size will be dealt with in the next section. Important applications of gas separation using membranes are in the production of high purity nitrogen from air, oxygen enrichment from air and recovery of helium from natural gas. 11.3.8 Pervaporation
Pervaporation is a membrane separation process wherein a membrane separates liquid feed and vapour permeate phases. Some of the components in the liquid feed phase vapourises and permeates through the membranes and separation involves a phase change. A schematic of pervaporation process is given in Fig. 11.4. FeedFeed mixture mixtureAAand and BB
Permeate enriched with A or B
Permeate enriched wiht A or B vacuum
pervaporation membrane Pervaporation membrane Fig. 11.4: A schematic representation of pervaporation process
It is a versatile method of separation of polar and nonpolar liquid mixtures and the driving force for this kind of separation is the partial pressure difference of the permeating components across the membranes. The gaseous permeate is usually held at a low pressures and the liquid feed stream is at an elevated temperature to facilitate evaporation of permeating components. Pervaporation is an attractive process for separation of liquid mixtures which form azeotropes, i.e., where vapour and liquid have the same composition in equilibrium and standard distillation cannot achieve the separation. Important applications of pervaporation process are dehydration of alcohol
11
Other Separation Methods
and recovery and concentration of naturally occurring aroma compounds from dilute solutions. 11.3.9 Liquid Membrane Processes
The process as well as the mechanism involved in separation using liquid membranes are briefly outlined before. As it is by now familiar, a membrane is simply a barrier between two phases and if one component of a mixture moves through the membrane faster than the other, separation is accomplished. A membrane is basically defined on its separating performance and not necessarily it should be a solid. Liquids that are immiscible with the source (feed) and receiving (product) phases can also be used as membranes. Different solutes will have different solubilities and diffusion coefficients in a liquid. The product of the diffusivity and the solubility is known as the permeability coefficient which is proportional to the solute flux. The differences in the permeability coefficients produce a separation between solutes at constant driving force (concentration difference). Because the diffusion coefficients in liquids are typically orders of magnitude higher than in solid matrices, a larger solute flux can be obtained with liquid membranes. Further, the solute flux can be enhanced by adding a nonvolatile complexing agent (carrier) to the liquid membrane. This carrier can selectively and reversibly react with the solute. This reversible reaction provides a means of enhancing the solute flux and improving the selectivity at the same time. Liquid membranes are encountered in three basic configurations. In a bulk liquid membrane (BLM), a relatively thick layer of immiscible liquid is used to separate the source and the receiving phases. To provide thinner liquid membrane, the liquid can be impregnated in the pores of a microporous membrane. This configuration is known as an immobilised or supported liquid membrane (ILM or SLM). In the third configuration which possesses the largest interfacial areas, the receiving phase is emulsified in an immiscible liquid membrane. The emulsion liquid membrane (ELM) is also known as liquid surfactant membrane. The emulsion liquid membrane is then dispersed in the feed solution and the solute transport takes place from the feed phase to the internal receiving phase. Transport in supported liquid membranes
In the extraction of the metal ions , the carrier molecule in the membrane picks up metal ions from the feed solution forming a complex. This complex diffuses to the other side of the membrane where decomplexation occurs and the metal ions are released into the receiving phase. Free carrier then diffuses back across the membrane for use in another cycle. This coupled transport through SLM can take place by the following processes. * Co-transport * Coupled transport Coion transport in which both metal ions and its counterions are transported from the feed solution through the SLM and into the receiving phase is shown in Fig. 11.5 (a). If the carrier, C, is neutral, the driving force is the difference in distribution coefficients ( K d ) between the feed and receiving solutions . This is generally achieved by maintaining a concentration gradient of the counterion X − between the two solutions. This metal ion and counterion form a complex with the carrier , C, in the membrane. This complex diffuses to the other side of the liquid membrane and the metal ion and counterion are released into the receiver solution. The chemical reaction for this coupled transport is M n+ + nX + C (membrane)
12
extraction
CMXn (membrane)
... (11.4)
CMX n (membrane)
stripping
C (membrane) + M n+ + nX
... (11.5)
Membrane Separation
The liberated carrier molecule diffuses back to the feed solution -SLM interface, picks up another metal ion and the counterion and the process continues until the final equilibrium is attained. Counterion transport (Fig. 11.5 b) in which an acidic carrier, HC , loses a proton and forms a complex , MC, metal ion at the feed solution-SLM interface. This complex diffuses to the SLM-strip solution interface where it liberates the metal cation into the receiver phase and simultaneously picks up a proton from the strip solution. The regenerated carrier, HC, diffuses back to the feed solution-SLM interface, picks up another metal ion and the process continues. The acidic carrier molecule shuttles between the feed and the strip solution-SLM interfaces. Mn+ +
n HC (membrane)
MCn (membrane) +
n H+
extraction
MC n (membrane) +
stripping
n H+
n H+
... (11.6)
+ M n+
... (11.7) The driving force in a counter transport SLM system is the pH difference between the feed and the strip solution. For efficient transport, high feed and low strip solution, K d values are needed. The K d difference between the feed and strip solution is maintained by a pH gradient. For divalent metal ion and an acidic carrier, the reaction at the feed solution-SLM interface is as follows: M2+ + 2 HC → MC2 ( membrane ) + 2 H+ (aq)
... (11.8)
The equilibrium constant is given by the expression K d
[MC 2 ( membrane) ] [H + (aq)] = [HC ( membrane) ]2 [M 2+ ( aq) ]
… (11.9)
By inverting the concentration terms in Eq. 11.9 we get the equilibrium constant at the receiving side of the membrane. The overall equilibrium constant is achieved when there is no further metal ion transport from the feed to the receiving side and the following relationship applies. log [(M2+) receiving phase / (M2+) feed ] = 2 ∆ pH
… (11.10)
where, ∆ pH is the pH difference between the feed and the strip solution. If for example , the pH values for the feed and receiving solutions are 4 and 1, respectively , M2+ can be concentrated in the receiving solution to as high as 10 6 : 1 relative to its concentration in the feed. Feed phase M ++ X
Membrane phase
Receiver phase
C CMX C CMX
M ++ X
(a)
13
Other Separation Methods
M+ HC H+
M+
MC HC
MC
H+
(b) Fig. 11.5: Transport Mechanism: (a) Co-transport and (b) Counter transport
SAQ 1
We have a mixture of protein, sucrose and calcium chloride in a solution in water and we want to separate them from one another. Devise a combination of membrane processes for accomplishing the same. ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………..
11.4
MECHANISMS OF SEPARATION THROUGH MEMBRANES
The mechanism by which membrane exercises the property of selective permeation of certain chemical substances is of utmost importance for a clear understanding of membrane separation processes. The process of separation involves several fundamental physical phenomena as the constituents in the feed pass over the membrane surface, into it, through it and finally leave the membrane surface from the other side. A generalised treatment of various mechanisms applicable in different membrane processes are discussed in this section. 11.4.1 Sieving
The simplest type of membrane is one that functions merely as a sieve. If such a membrane has to be effective, the solvent molecules must be much smaller than the molecules or ions of the solutes and the pore size of the membranes must be intermediate between the two. This condition is easily attainable in the case of solutions where the solute is a macromolecule or a polymer where considerable size difference exists between solute and the solvent molecules. In ultrafiltration and microfiltration, sieving is one of the predominant mechanisms considered. However, no sharp distinction exists between water molecules and small inorganic ions and non-electrolytes and obviously sieving is not the mechanism by which a membrane can retain small inorganic ions and small non-electrolytes and allow permeation of water. 11.4.2 Solution-Diffusion
Solution-diffusion is often cited as the possible mechanism to explain selective permeation of a chemical substance through membrane which is nonporous, from a feed mixture where no appreciable size differences exist. For any chemical species to permeate from one side of the membrane to the other side, it has to dissolve or absorb in the membrane and subsequently diffuse through the membrane and get desorbed on
14
the other side. Consequently, certain components in the feed mixture get sorbed on the membrane in preference to other components due to chemical similarity or chemical preference which could give the required selective permeation property to the membrane.
Membrane Separation
Alternatively, when, in those cases where no significant changes exist in the sorption behaviour of the components of the feed mixture, selective permeation still can arise due to differences in their diffusion rates through the membranes. Chemical substances with higher molecular mass diffuse slowly through the membranes as compared to substances with lower molecular mass. The term permeation and diffusion is alternatively used to describe the transport of chemical species across the membranes. However, the distinction between them needs to be clearly understood. The term diffusion specifically refers to random molecular motion of a species yielding to a net transfer of that species from a region of higher concentration to a region of lower concentration. The term permeation refers to a much more general phenomenon of transfer of chemical species from one region to another under different kinds of driving forces, namely, concentration gradient, pressure gradient, electrical gradient or even temperature gradient. If the membrane is viewed as a porous body in which solvent but not the solute is adsorbed on the pore walls, this adsorbed solvent may so fill the pores such that there is no room for the passage of solute molecules. Solvent molecules may pass through by successive transfer from one adsorption site to the next. Since the association of the adsorbed solvent molecules with the adsorbing site needs not be completely broken down during the process, it requires relatively little energy, compared to that required for a solute molecule to invade the mass of adsorbed solvent molecules. In the case of desalination membranes, the adsorption of solvent molecules is brought out by hydrogen bonding. Such membranes are known to allow permeation of those solutes like ammonia, phenol, etc. which have hydrogen bonding tendencies similar to that of water. 11.4.3 Preferential Sorption-Capillary Flow
Preferential sorption-capillary flow mechanism was the fundamental approach for the practical development of reverse osmosis process for converting sea water into potable water. If the surface of the membrane in contact with the solution is of such a chemical nature that it has preferential sorption for water or preferential repulsion for solutes then a multimolecular layer of preferentially sorbed pure water could exist at the membrane surface. Continuous removal of this interfacial water can then be effected by letting it flow under pressure through membrane capillaries. This concept also defines a critical pore diameter required on the membrane surface for maximum rate of withdrawal of pure water. This is obviously twice the thickness, t , of the interfacial pure water layer. If the pore diameter is higher, then the rate of withdrawal of water will be higher but the solute separation will be lower since not only the adsorbed pure water but also bulk of the solution will also flow through the pores. If the pore diameter is smaller, the solute separation could be maximum but the rate of permeation will be reduced. The connecting pores in the interior bulk of the membrane from the surface could be and should be bigger. These requirements are essential from a practical point of view because, the total resistance to the permeating water will be low. 11.4.4 Donnan Effect
Ion selective membranes show selective permeability towards ions of a particular sign. Such membranes are ion exchangers in sheet form, a cation exchanger being selectively permeable towards cation and an anion exchange showing the same 15
Other Separation Methods
behaviour towards anions. The chemical structure of ion exchange membranes consists of a network of carbon atoms and styrene nuclei with a suitable functional group attached to the styrene nuclei. If the membrane is immersed in an aqueous medium, it absorbs water and swells due to the affinity between the functional groups and water. The functional groups dissociate to yield an electrically charged fixed group chemically bound to the hydrocarbon matrix and a mobile ion carrying the opposite charge. The latter are free to move under the influence of an applied electrical field or to exchange with other ions of similar charge diffusing into the membrane from the external solution. Cation selective membranes contain negatively charged groups fixed to the hydrocarbon matrix and a mobile cation. Anion selective membranes contain a positively charged group fixed to the hydrocarbon matrix and a mobile anion. When an ion selective membrane is in contact with an aqueous solution of an electrolyte, those ions having the charge similar to that of the fixed group on the membrane polymer matrix are designated as coions and those ions having charge similar to that of the mobile ions are designated as counter ions. When the electrical equilibrium is established between the ion selective membrane and the external electrolyte solution, within the membrane phase, the charges of the fixed group is electrically balanced with those of the counter ions from the external solutions as well as its own intrinsic counter ions. The coions from the external solution are more or less excluded from the polymer matrix. This type of exclusion is called the Donnan Exclusion and the selectivity of ion exchange membranes arises due to Donnan effect . 11.4.5 Knudsen Flow
Knudsen flow or knudsen diffusion is one of the possible mechanisms cited to explain the gas separation through membranes. When the mean free path of the gas molecules is small compared to the pore size of the membranes, the permeation of gas molecules is governed by viscous flow or hydrodynamic flow. The flow rate of the gas molecules in viscous flow is directly proportional to the pressure gradient causing the flow. Under such condition, the collision frequency between gas molecules exceeds greatly as compared to the collision frequency of gas molecules with the pore wall of the membranes. No specific separation of a particular gas component may be expected, because of complete molecular chaos. However, when the mean free path of the gas molecules is much larger than the membrane pore diameter, the collision between gas molecules becomes much fewer than the collision between gas molecules and the wall. The situation is known as rarefied gas and the flow under such conditions is usually referred to as “free molecular diffusion” or “Knudsen Flow”. In Knudsen flow the component gases flow through the membrane pores independently of each other. Thus, a separation takes place due to the differences in the molecular weight. This is the principle involved in the separation of U-235 isotope by means of gaseous diffusion process. 11.4.6 Surface Flow
Permeation rates of gaseous molecules through the membranes may be enhanced in some cases by surface diffusion mechanism. In surface diffusion, gas molecules adsorb to the surface of the pores of the membranes and then diffuse along the pore walls. High selectivity can be achieved in cases where preferential adsorption of one of the components occurs. Surface flow is the result of solution diffusion mechanism and is used to describe the preferential permeation rate of some gaseous components through membranes from a given feed mixture and is similar to preferential sorption capillary flow mechanism described previously for liquid feed systems. Separation of condensible gases and vapours from noncondensible permanent gases through membranes is often explained by surface flow. Such a separation is carried out at a 16
suitable temperature and pressure near or below the critical temperature of the gas. At high pressures, multilayer adsorption of gas molecules occurs at the pore walls reducing the size of the pores. At a particular pressure, the entire pore volume is filled with the permeating gas and the gas molecules are said to have condensed inside the membrane pores, effectively blocking the passage of permanent gases. Such a phenomenon is known as capillary condensation.
Membrane Separation
11.4.7 Facilitated Diffusional Transport
When a simple diffusion process is coupled with chemical reactions, the net permeation rate may be greatly affected. Such a diffusional permeation is called facilitated diffusional transport . Facilitated diffusion occurs in liquid membrane systems. This mechanism requires a carrier substance (X) inside the membrane which shuttles back and forth between the two sides of the membrane carrying the permeant molecule (A). The schematic of facilitated diffusion which occurs in liquid membranes is given in Fig. 11.6.
Fig. 11.6: A schematic representation of facilitated diffusion in liquid membranes
11.4.8 Active Transport
All the mechanisms discussed so far are for nonliving membranes, where permeation of chemical substances takes place in the direction from high concentration to low concentration. For living (biological) membranes, such as cell membranes the actual mechanism of permeation are generally quite complicated. In some cases, a substance may be transported through a cell membrane against its concentration gradient. This does not mean that the second law of thermodynamics is violated. In addition to diffusion process, there are other reactions occurring in the membrane that supplies energy to the system. Thus, the cell is doing work to move the substance through the cell membrane against a concentration gradient. Since the cell membrane is ‘actively’ transporting the matter, the term ‘active transport’ is used for such a process. A schematic of active transport mechanism is given in Fig. 11.7, and is quite similar to facilitated diffusion. The schematic explains the active transport of a chemical substance A through cell membrane from low concentration to high concentration with the help of enzymatic reactions at the two surfaces of the membranes. Enzyme E1 near the surface of low concentration of A promotes the chemical reaction between A and B to yield a compound AB within the cell membrane. Compound AB will diffuse through the membrane to the other side where there is another enzyme E2 .This enzyme does just the opposite of what E1 does and expedites the decomposition of AB into A and B.
17
Other Separation Methods
Low concentration of A
High concentration of A Metabolic pump
Fig. 11.7: A schematic representation of active transport in cell membranes
The component A diffuses out of the membrane to the high concentration side because there is a local build up of A inside the membrane. The carrier molecule B is too large to pass through the cell membrane and is contained within the membrane. The two enzymatic reactions constitute a metabolic pump which generates power to pump A against its concentration gradient. The accumulation of potassium ions in cells is a well known example of active transport. SAQ 2
Explain the importance of surface pore size for separation of salt and water by reverse osmosis process in the context of preferential sorption-capillary flow mechanism. ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………..
11.5
OSMOTIC PHENOMENA
When a salt solution is separated from pure solvent by a semipermeable membrane, a membrane that permits the passage of solvent but not the solute, it is observed that solvent tends to pass through the membrane into the solution and thereby, dilute it. The phenomenon is called osmosis. Before proceeding, it is necessary to define a quantity called the osmotic pressure (Π). For this purpose, consider Fig. 11.8.
Fig. 11.8: Osmotic pressure of salt solutions
A is a chamber open at one end and fitted with a movable piston B. The chamber is divided by means of a semipermeable membrane C, into two sections, of which the right one is filled with pure solvent, the other with some salt solution. Due to osmosis, the solvent will tend to pass through the membrane into the solution and displace the piston upward. The motion of the piston and the osmosis can be prevented, however, by the application of pressure to the piston in order to keep it in the original position. The mechanical pressure which must be applied to prevent osmosis of the solvent into the solution through semipermeable membrane is called osmotic pressure of the 18
solution. This pressure for a given solution depends on a number of several solution properties. But this pressure does not depend on the nature of the membrane so long as the membrane is truly semipermeable.
Membrane Separation
It is possible to derive, from purely thermodynamic considerations, an expression for osmotic pressure in terms of measurable solution parameters. At any constant temperature and pressure of one atmosphere, transfer of solvent into solution occurs because the molar free energy of the pure solvent, µ 0, (standard chemical potential of the solvent) is greater than the partial molar free energy of the solvent in the solution µ i . To bring an equilibrium between the two and thus, stop osmosis, it is necessary to increase the value of µ i by applying higher external pressure on the solution. If this increase in free energy is ∆ µ i , then the condition for osmotic equilibrium must be 0
= µ i + ∆ µ1
... (11.11)
µ i0 – µ i = ∆ µ1
... (11.12)
µ i
and hence The chemical potential ( µ i) of component, i, in a solution is defined in terms of Gibbs free energy (G) by the relation dG = S dT + V dP +Σi µ i d N i
... (11.13)
where, S is the entropy, T is the absolute temperature, V is the volume , P is the pressure and N i is the number of moles of component, i. From Eq. 11.13, µ I = (∂G / ∂ N i )T,P,N
… (11.14)
= (∂ G / ∂ P)T,
… (11.15)
and V
where, N represents the entire set of N ’s and N j represents all N ’s except N i.. Differentiating Eq. 11.13 with respect to N i (∂ 2G / ∂ N i ∂P)T,N = (∂µ i / ∂ P)T,N = (∂ V / ∂ N i )T,P,N = V i
… (11.16)
where, V i is the partial molar volume of component, i. The thermodynamic activity, ai of component, i is related to its chemical potential µ i by the relation µ i0
=
µ i * + RT ln ai
… (11.17)
where, R is a gas constant. In a binary aqueous salt solution system, let component, i represent water indicated by subscript w and let µ i = µ ∗ (chemical potential of water at a specified pressure and temperature). According to Eq. 11.17, the chemical potential of water µ w in the solution at pressure P1 is less than that of pure water. ∗ µ w
= µ w*
+ RT ln aw
… (11.18)
where, aw is the thermodynamic activity of water in solution. The equilibrium can be restored by increasing the pressure on the solution side to P2 such that the chemical potential of water is raised to that of pure water, µ w. The increase in chemical
19
Other Separation Methods
potential of water in the solution as the pressure is increased from P1 to P2 is obtained from Eq. 11.16. P2
P2
∫1
(∂µ w / ∂ P)T,N dP =
P
∫
V w dP
... (11.19)
P1
Since this increase, added to µ w must restore the chemical potential of water and according to Eq. 11.19 µ w +
∫
P2
∫
P2
P1
∫
V w d P = µ * w
… (11.20)
V w dP = µ *w − µw
… (11.21)
V w ( P2 − P1 ) = − RT ln aw
… (11.22)
P1
when V w is assumed constant. The pressure difference (P2 –P1) is by definition the osmotic pressure of the solution, usually represented by Π . Thus, V w Π
= – RT ln
aw
= – RT / V w lnaw
Π
… (11.23) … (11.24)
For calculating the activity of water in the solution, aw, , the vapour pressure data are needed. Often aw is calculated from the relation aw
= Pw /
*
Pw
... (11.25)
where, Pw is the vapour pressure of water in equilibrium with the solution at a specified temperature, while Pw* is that of pure water at the same temperature. If the solute is volatile, the partial vapour pressure of water must be used for Pw. Substituting Eq. 11.25 in Eq. 11.24, we get Π V w = RT ln ( Pw / P*w )
…(11.26)
Eq. 11.19 can be reduced to a simpler form from the special case of dilute solution of binary electrolyte obeying Raoult’s law. For such solution o
P1 / P1 = N 1
= (1– N 2)
… (11.27)
where, N 1 and N 2 denote the mole fraction of solvent and solute, respectively. Eq. 11.26 can be written in terms of solute, mole fraction as follows: Π V w
= − RT ln ( 1− N 2)
… (11.28)
If ln ( 1– N 2 ) be expanded in series, then for dilute solutions all terms beyond the first can be neglected, and ln (1– N 2) becomes – N 2 which is equal to n2 / n1 where, n2 is the number of moles of solute in n1 moles of solvent. Hence,
20
Π V w = RT n2 / n1
... (11.29)
and
... (11.30)
Π V w n1 = n2 RT
But V w n1 is the total volume of solvent containing n2 moles of solute, which for dilute solutions is essentially the volume of the solution. Consequently, Π V = n2 RT
Membrane Separation
… (11.31)
or alternately Π =
cRT
… (11.32)
where, c is the molarity of the solution. Eq. 11.32 is well known as van’t Hoff’s equation for ideal solutions. For electrolytes which ionise in solvent/ water, the observed osmotic pressure is more than what is predicted from molar concentrations data as the osmotic pressure is a colligative property of the solution which depends on the number of species in solution. Accordingly, a correction term known as van’t Hoff factor is introduced in Eq.11.25 as follows. Π = icRT
… (11.33)
The van’t Hoff factor is approximately equal to number of ions produced during ionization per molecule of the electrolyte.
SAQ 3
Which will have a higher osmotic pressure, 5% calcium chloride solution or 5% urea solution? ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………..
11.6
REVERSE OSMOSIS PROCESS
Osmosis is a natural process wherein solvent/water passes through a semipermeable membrane from the side with lower solute concentration to the higher solute concentration side until equilibrium of solvent/water chemical potential is restored. To reverse the flow of water, a pressure difference greater than the osmotic pressure difference is applied. As a result separation of water from salt solutions becomes possible. This phenomenon is termed reverse osmosis. It is also known as hyperfiltration. The process is generally evaluated in terms of parameters such as water flux, J w, solute separation, R and water recovery,Y . The definitions of these parameters are given below. The parameter water flux, J w, refers to the amount of permeating solvent/water per unit time per unit area of membrane. J w
=
Q p / A.t
… (11.34)
where, Q is the quantity of permeating solvent/water per time, t , through membrane area, A. The quantity Q may be expressed on weight basis, volume basis or molar basis. The separation of salt by membrane is commonly expressed as solute separation R, 21
Other Separation Methods
which is the fraction of salt in the feed that does not permeate through the membrane. R
= 1– C p / C f
… (11.35)
where, C p and C f denote solute concentration in the membrane permeated stream (See Fig. 11.1) and in the feed inlet stream, respectively. Water recovery Y , is commonly used to define the percentage of feed water that is
converted into pure water in the permeate and is calculated as follows: Y
= (q p / q f ).100
… (11.36)
where, q p and q f denote the quantity of permeate stream and feed inlet per unit time, respectively. . 11.6.1 Basic Equations
The process parameters as defined above depend on several factors such as the physical structure and the chemical nature of the membranes, pressure, temperature, nature and concentration of solute, flow rate of the feed salt solution along the membrane surface etc. The nature of the dependence of these factors on the process performance are analysed and a number of quantitative expressions are derived. A detailed description of their mathematical aspect is beyond the scope of this unit. A simplified expression for water flux and solute separation based on solution-diffusion mechanism is given below. In the solution-diffusion mechanism, the permeation of solvent as well as solute through the membrane arises due to the dissolution of the permeating species in accordance with an equilibrium distribution law and diffusion through membranes in response to pressure and concentration gradient. An expression for water flux is given by J w
=
Dw C w V w (P – Π ) / RT ∆ x
… (11.37)
where P and Π are respectively, the applied pressure and osmotic pressure of the feed solution, ∆ x is the thickness of the membrane, Dw and C w , respectively refer to diffusion coefficient and concentration of water in the membrane phase expressed as a volume ratio and which is a dimensionless quantity. __
The terms V w , R , T refer to partial molar volume of water, gas constant and temperature, respectively. , The quantity Dw C w V w / RT ∆ x can be considered a membrane constant and denoted by A whose magnitude depends on the physical structure as well as the chemical nature of the membrane and the temperature of the feed. Accordingly, Eq. 11.37 can be written as J w
=
A(P – Π )
… (11.38)
Eq. 11.38 also indicates that the observed water flux across a membrane not only depends on the value of membrane constant A, but also on the applied pressure and osmotic pressure of the feed which is a function of solute molarity, its nature and temperature. For a given applied pressure, if the solute concentration in the feed is higher, its osmotic pressure will be higher and consequently, the observed water flux will be lower. Alternatively, for a given feed solution with a definite feed concentration, higher applied pressure will improve the observed water flux across the membrane. It follows that for separating pure water from salt solution having higher solute concentration, like sea water, a higher operating pressure is needed to maintain
22
the same driving force whereas lower operating pressures are sufficient for feed solutions of lower solute concentrations.
Membrane Separation
The solute also diffuses through the membrane simultaneously with water and the solute flux, ( J S ) which is a measure of the quantity of solute, expressed in mass or molar basis, diffusing through the membrane per unit time per unit area, is given by J S
=
DS K S ∆C S / ∆ x
… (11.39)
where, ∆C S is the difference in solute concentration across the membrane, K S is the distribution coefficient for the solute between feed solution near the membrane surface and the membrane which is given again as a dimensionless parameter and DS refers to the diffusion coefficient of solute in the membrane phase. The quantity DS K S / ∆ x can be treated as solute permeation constant ( B) whose magnitude is characteristic of the membrane and the solute under consideration. Accordingly, Eq. 11.39 may be written as J S
=
B(∆C S )
… (11.40)
Eq. 11.40 indicates that the solute flux through the membrane increases with increase in solute concentration difference across the membrane which will manifest in lower observed solute separation. The magnitude of solute flux depends on the value of solute permeation constant ( B).The desirable properties of semipermeable membranes for use in reverse osmosis process is higher water flux and solute separation which is possible for a membrane with a higher value of A and a lower value of B. Consequently, the ratio of the two constants A/B is often used to describe the semipermeable character of a membrane. Higher the value of A/B, the more selective is the membrane for water against solute. The solute concentration in the permeate is determined by the relative flow of solute and water as follows. C p
= J S / J w
… (11.41)
Combining Eqs. 11.41, 11.40, 11.38 and 11.35, an expression for solute retention R can be obtained as given below: R = 1 – C p /C f
=
A (P – Π )/ A (P – Π ) + B
... (11.42)
It should be noted that water flux and solute flux are inversely proportional to the membrane thickness but solute retention R which is given in terms of the fluxes, is independent of membrane thickness. The solute retention increases with net effective pressure difference (P–Π ) and the ratio A/B. 11.6.2 Concentration Polarization
Concentration polarization results from the build up of highly concentrated solute on the membrane surface as compared to the bulk feed solution away from the membrane surface. This occurs because water permeation at the membrane surface leaves the more concentrated solute layer which must diffuse back into the bulk liquid. Concentration polarization increases the osmotic pressure at the membrane surface, which causes a reduction in water flux and an increase in salt diffusion across the membrane. If the concentration of sparingly soluble salts in the boundary layer adjacent to the membrane surface exceeds the solubility limits, precipitation or scaling will occur on the membrane surface. At such high concentrations, colloidal materials in the feed solution may also agglomerate and foul the membrane surface. SAQ 4
23
Other Separation Methods
What are the desirable properties of semipermeable membranes for use in reverse osmosis process? ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………..
11.7
DIALYSIS
Dialysis is a membrane separation process wherein solutes diffuse across a membrane barrier by means of a concentration gradient. The rate of diffusion of a solute through the membrane increases linearly with increase in concentration gradient and the proportionality constant, namely the diffusion coefficient, D, is a characteristic property of the solute for a given membrane. Diffusion coefficients decrease roughly in proportion to the square root of molecular weight of solutes and consequently, solutes with higher molecular weights diffuse slowly through the membranes. To achieve selective permeation of solutes by dialysis process, a combination of solute and membrane properties are chosen. The pore sizes of membranes are chosen in such a manner to exclude large size macrosolutes. Also, the process is employed for separating those solutes where an order of magnitude difference exists among diffusion coefficient values. Dialysis is a highly constrained process because the molecular diffusional rates are slow and if the receiving solutions (dialysate) are not continuously removed, the solute concentration on both sides of the membrane will tend to equalise, negating the driving force for separation. The permeant species is not recoverable in pure form and is necessarily more dilute in the dialysate than in the starting stream. For these reasons, dialysis has been limited to specialised uses in pharmaceutical and medical purposes where partial purification of the feed stream, rather than recovery of a product is intended. The most important parameter to characterize dialysis process is the diffusive solute flux ( J D) across the membrane which is given by … (11.43)
J D = K Dm ∆C/ ∆ x
where Dm denotes solute diffusion coefficient in the membrane, ∆C is the concentration gradient of solute across the membrane and K is the partition coefficient defined as the ratio of solute concentrations between membrane and external solutions on either side at equilibrium. Fig. 11.9 shows a typical concentration profile for diffusive transport across a dialysis process. Feed
Dialysate
C FM C F
C DM
24
Membrane Separation
Fig. 11.9: Typical concentration profile in dialysis process
In this example, the solute is less soluble in the membrane than in the external phases. The partition coefficient K is given by K = C FM / C F = C DM /C D
… (11.44)
The product KDm / ∆ x is often termed membrane diffusive permeability (Pm) of the solute. If two or more solutes are dialysing at the same time, the degree of separation or enrichment is proportional to the ratio of their permeabilities. The closer the diffusive permeability of a solute in the membrane is to that in free solution, the more rapid will be the dialytic transport. Pm = K D M / ∆ x
… (11.45)
J D = Pm ∆C
… (11.46)
and The above equations also indicate that the diffusive solute flux in dialysis process is inversely proportional to the membrane thickness but the degree of separation among different solutes (selectivity) is independent of membrane thickness. For this reason, dialysis membranes are always made as thin as possible consistent with the requirement of mechanical strength and reliability. Another parameter, namely, intrinsic membrane resistance ( R M ) is also defined in dialysis process. R M
= ∆ x / KD M
… (11.47)
R M value is characteristic of a particular membrane-solute system. The diffusive solute
flux is written in terms of resistance values as below. J D = ∆C/R M
… (11.48)
Solutions adjacent to membrane surfaces are rarely well mixed and the resistance to diffusive transport resides not just in the membrane but also in the fluid regions, termed stagnant boundary layers on both the dialysate and feed side. These stagnant boundary layers offer additional resistance to diffusive solute flux and the overall resistance ( R0) is the sum of all the resistances in series. R0 = R B +R M + R D
… (11.49)
where, R B and R D refer to resistances offered by feed and dialysate side boundary layers. SAQ 5
What are the limitations of dialysis process? ………………………………………………………………………………………….. …………………………………………………………………………………………..
11.8
ELECTRODIALYSIS
Electrolysis is a membrane process wherein preferential transport of cations and anions are achieved using cation and anion selective membranes as a result of an electrical driving force. Using this process, the concentration of ionic species can be increased or decreased so that practical concentration or depletion of an electrolyte solutions is possible. With ion selective membranes of recent origin which are more permeable to univalent cations or anions, electrodialysis process can be used to 25
Other Separation Methods
simultaneously separate and concentrate univalent ions from solutions containing mixtures of uni and multivalent electrolytes. The principle of this process has been already explained (refer Fig. 11.3). Generally, the feed electrolyte solution where cations and anions are removed, is termed dialysate and the receiving solutions where the ions are added is termed as brine. During electrodialysis process, a number of transport processes occur simultaneously which are illustrated in Fig. 11.10. A
C conter ion transport co ion transport
C+H2O A+H2O
diffusion
CA + H2O
osmosis
H2 O
+
AE,CE= Anion selective, cation selective membranes, C,A,CA = cation, anion, electrolyte Fig. 11.10: Various transport processes occurring in electrodialysis process
The counter ion (ion having opposite charge compared to that of the fixed group of the membrane) transport constitutes the major electrical ion movement in the process.Anion is the counter ion for anion selective membranes and cations are the counter ions for cation selective membranes.The counter ion permeation through the membranes carries with them a certain quantity of water by electro osmosis. The coion (ion having similar charge as that of the fixed group of the membranes) transport is comparatively small and is dependent upon the quality of the ion selective membranes and brine concentration. Anion is the coion for cation selective membranes and cation is the coion for anion selective membranes. Water is also transported electroosmotically with the coions. Diffusion of electrolytes occurs from the brine to the dialysate because in the electrodialysis process the brine is usually more concentrated than the dialysate. Water transport is also associated with electrolyte diffusion. Water transport due to osmosis takes place from the low concentration dialysate compartment into the higher concentration brine compartment. The efficiency of electrodialysis process in the depletion of ionic substances from the dialysate may be considerably reduced by the adverse effects, namely, coion transport , free electrolyte diffusion, electroosmotic water transport associated with counter ion movement and osmosis. The effects of these unwanted transfer processes can, however, be reduced, by the selection of the membranes and by the operational procedure. Considering the above transport processes, ion selective membranes for use in electrodialysis process should have a high selectivity for counter ions. The selectivity of ion selective membranes may be expressed in terms of transport number of counterions in the membrane phase ( t ) transport number is a measure of the fraction of the total current carried by the counter ion through the membranes. This transport number decreases as the salt concentration of the solution in contact with the membrane increases. The term perselectivity (P) is also defined to characterize the counter ion selectivity of membranes. P
26
=
t – t / 1– t
… (11.50)
where, t denotes the transport number of the ions in free solution.
Membrane Separation
The ion selective membranes also need to have a high electrical conductance when in equilibrium with the most dilute solution to be encountered in the process. The membranes should permit only a negligible rate of free electrolyte diffusion under the conditions of concentration difference expected in the process. As the concentration difference across the membrane increases, free diffusion of electrolyte also increases. The efficiency of electrodialytic process is given in terms of Coulomb efficiency (η ). η =
Number of equivalents of electrolytes displaced from dialysate to the brine Number of Faradays of electricity passed in the process
… (11.51) The unwanted transport processes, namely, coion transport, free electrolyte diffusion and water transport contribute to the decrease in Coulomb Efficiency. The rate of counter ion transfer flux ( J ) through membranes in electrodialysis process is given by J
= t i /F
… (11.52)
where, i is the current density (amperes per square centimetre area of membranes) and F is the Faraday constant. Similarly, the electrical ion fluxes (J) in the solution is given by J = t i / F
… (11.53)
Eqs. 11.45 and 11.46 indicate that the counter ion fluxes in solution as well as in the membranes increase with increase in current density. The flux of ions resulting from diffusion can be expressed in terms of Fick’s first law . [ J ]d = − D dc / d x
… (11.54)
At steady state, the combined electrical and diffusive flux through the membranes in the boundary layers equals the electrical flux through the membranes and the electrical flux through the membrane is the total flux. J = t i / FZ = − D dc /d x + t i / FZ
… (11.55)
Integration of Eq. 11.55 yields a simple relationship between boundary layer thickness, δ , current density, i , interfacial concentration, C o, and the bulk concentration , C b. C o
= C b + ( t − t ) iδ / DFZ
… (11.56)
Since t > t , i.e., the transport number of counter ions through membrane is more than in solution, the ion flux in solution will be lower compared to their flux through the membranes. The electrolytic movement of counter ions up to the membrane surface will be insufficient to meet the faster rate of its migration through the membranes. This difference will lead to depletion of counter ions on the dialysate side adjacent to membrane and a build up of counter ions on the brine side adjacent to the membranes. This is called concentration polarization and is schematically depicted in Fig.11.11.
27
Other Separation Methods
Fig. 11.11: Concentration profile in electrodialysis
With increase in current density, the concentration polarization becomes more steep and at sufficiently high current densities, the counter ion migration at layer adjacent to the membrane in the dialysate approaches zero. Since there will not be any ions to carry the current, any further current through the membranes will be carried by the hydrogen and hydroxyl ions formed by the ionization of water at the interface. The current at which this water splitting begins to occur is called the limiting current density (ilim), because any further increase in current density would cause loss of coulomb efficiency , increased electrical resistance and changes in the pH of the solutions. The limiting current density is described by setting C o = 0 in Eq. 11.56 in which case ilim
=
C D Z/ δ ( t – t )
… (11.57)
where C,D,Z and δ denote concentration of the ion in the bulk dialysate solution, diffusivity in solution, electronic charge and thickness of the boundary layers, respectively. SAQ 6
Define Coulomb Efficiency in electrodialysis process and mention the factors contributing to decrease in Coulomb efficiency. ………………………………………………………………………………………….. ………………………………………………………………………………………….. …………………………………………………………………………………………..
11.9
APPLICATIONS
The membrane processes are used for separation of chemical substances and are of immense interest for a wide range of commercial applications. The analytical applications of membranes are grossly over shadowed by the industrial applications of membranes. A complete and comprehensive discussion of all membrane based applications would be very exhaustive and is beyond the scope of this unit. However, a few major large scale applications of membranes will be discussed here. 11.9.1 Desalination and Water Treatment
Desalination of sea water (containing approximately 35000 to 40 000 milligrams of dissolved salts per litre) and brackish waters (containing approximately 5000 to 10000 milligram of dissolved salts per litre) to produce potable water is one of the widely known industrial applications of membrane processes. Reverse osmosis processes to a large extent has been successfully used for the last two decades in many parts of the world. In addition to desalination, reverse osmosis process is also
28
used for the treatment of municipal waste waters and effluents from the various chemical industries.
Membrane Separation
11.9.2 Protein Recovery
The most prominent use of ultrafiltration process is in the food industry, where it is used ,for example to recover proteins from cheese whey, to concentrate milk before cheese making and for fruit juice clarification.Other important applications of UF process is in the removal of colour , odour and bacteria from the surface waters for drinking water needs. The pore size of most of the UF membranes used for such applications are such that they remove virtually all the microorganisms present in the water and viruses to a large extent. 11.9.3 Production of Table Salt
Electrdialysis was first developed for the desalination of saline waters, in particular brackish water. The production of potable water is currently the most important industrial application of electrodialysis. One significant feature of electrodialysis is that the salts can be concentrated to comparatively high values (in excess of 18 to 20 %) . The production of table salt from sea water by the use of electrodialysis to concentrate sodium chloride up to 200 grams per litre prior to evaporation is a technique developed extensively and used in Japan. 11.9.4 Hemodialysis
The most important application of dialysis process is in the artificial purification of blood using a dialysis membrane and this process is called hemodialysis. In hemodialysis, the impure blood from the patients flows across a dialysis membrane and a physiological saline solution flows along the other side of membrane. This process replaces kidney function in three principal areas, namely, removal of waste metabolites, removal of excess body water and restoration of acid-base and electrolyte balances. The waste metabolites include urea, uric acid, the end product of protein metabolism and creatinine, the end product of muscle metabolism. 11.9.5 Ion Selective Membrane Electrode
The analytical applications of membranes are largely in the area of ion selective membrane electrodes and particulate analysis. For most of the ion selective membrane electrodes, the role of membranes is not to transport specific ions but to selectively adsorb on either side giving rise to measurable electrical potential difference. A particular membrane permits only a particular kind of ion to penetrate and adsorb. Ion selective membrane electrodes have many applications in water analysis and environmental monitoring. The principle of operation of membrane electrode is given in Fig.11.12.
29
Other Separation Methods
Fig. 11.12: Basic principle of ion selective membrane electrode for metal ion determination
The electrical potential developed across the membrane, measured using ion selective membrane electrode and reference electrode allows the determination of metal ion as per Nerst equation given below. E =
2.303 RT / n F log C / CS
... (11.58)
In principle, all factors associated with the use of the electrode, other than the ion concentration to be measured, remain constant and the measurement of the cell potential is directly proportional to the logarithm of ion concentration. A range of different types of membranes are used in ion selective membrane electrodes, including glass, solid state, heterogenous and liquid ion exchanger based. Table 11.1 gives a selection of commercial ion selective membrane electrodes in which all the four types are used. Table 11.1: Typical Membranes and Commercial Ion Selective Membrane Electrodes Ion electrode +
Membrane
Concentration range
Major interferences
–1
(mol L ) –14
H K+ Na+ FCl Br I -
Glass Valinomycin Glass LaF3 Ag2S /AgCl Ag2S /AgBr Ag2S / AgI
10 to 1 10–6 to 1 10 –6 to sat. 10 –6 to sat. 10–5 to 1 10–6 to 1 10–7 to 1
CN S2– Ag+ Cd2+ Pb2+ Cu2+ Ca2+ Ca2+ + Mg 2+ NO 3−
Ag2S/AgI Ag2S Ag2S CdS/Ag2S PbS/Ag2S CuS/Ag2S (RO)2PO2 /(RO) 3PO (RO)2PO2 / ROH R4N/ether
10–6 to 10 –2 10–7 to sat. 10–7 to 1 10–7 to 1 10–7 to 1 10–8 to sat. 10–5 to 10–1 10–7 to 1 10–5 to 1
None Ag+,H+ ,Li+ Ag+, H+, Li+ OH¯ , H+ Br –, I¯, CN¯, S2– I¯, CN¯ , S2– CN¯, S2– Table continued on next page I¯, S2– Hg 2+ Hg2+ Ag+, Hg2+, Cu 2+ Ag +, Hg 2+, Cu 2+ Ag +, Hg 2 +, S2– Zn2+,Fe2+,Pb2+,Ca2+ Cu2+,Zn2+,Fe2+,Ni2+,Pb2+ ClO4¯ , ClO3¯ , I¯ ,Br¯
Various types of glasses have been used as membrane electrodes in ion selective membrane electrodes. When these electrodes were developed originally, they were primarily used in measuring pH values. However, the scope of application has been widened to include many univalent cations such as sodium, potassium, lithium, ammonium, silver, rubidium, and caesium. Glass membranes are generally based on Na2O–Al2O3–SiO2 mixtures. Membranes selective to hydrogen ions are rich in SiO2 whereas membranes selective to alkaline metal ions have a higher content of Al2O3 . A variety of different solid state membranes are used for different ion detection. Many of these use a combination of Ag2S + AgX (where X can be chloride, bromide, or thiocyanide) in the form of a pressed disc and the electrode responds to X–. For the determination of cations (Mn+), the membrane material is a mixture of Ag2S + Mn /2 S.
30
Heterogenous membranes use similar active components as solid state devices but instead the active material is deposited into the pores of an inert support like silicone rubber or polyvinyl chloride plastic.
Membrane Separation
Ion selective electrodes based on liquid ion exchanger membranes immobilize an active species, (an organic molecule dissolved in a solvent), into the pores of an inert polymer. The typical active species are phosphate di esters for calcium ion detection, metel complexes for anion detection and neutral macrocyclic crown ethers for alkali metal detection. 11.9.6 Specific Gas Probes
Membranes are also used in specific gas probes for measuring dissolved gases and gas phase partial pressures. The specific gas analysis is based on the electrochemical oxidation or reduction of the gas at the appropriate electrode of an electrochemical cell giving rise to a current whose magnitude depends on the concentration of the gas consumed in the electrochemical reaction. The whole detection system is protected by the membrane which is permeable to the particular gas component of interest. The membrane can be polytetrafluoro ethylene, polyethylene, polypropylene, nylon or cellophane. The membranes typically have pore sizes of several microns. Portable devices for specific gas analysis are available for detection of gaseous oxygen, hydrogen sulphide, carbon dioxide, sulphur dioxide, nitrogen dioxide and hydrogen cyanide. 11.9.7 Detection and Analysis of Particulate Contamination
Microfiltration membranes offer a general way of removing particulate material from fluid streams and are routinely used in a range of analytical procedures to determine particulate contamination in gases and liquids. In general, these methods involve passing a representative sample of liquid or gas through a suitable membrane. All particulate matter which exceeds the membrane pore sizes are retained on the surface where the contaminants may then be analysed. 11.9.8 Microbiological Analysis
Microporous membranes are important for the detection of microorganisms in foods, beverages pharmaceutical products and potable water sources. The technique involves the filtration of the samples through a microfiltration membranes to trap the microorganism, then culturing the microorganism on the membrane and then counting the grown colonies. Another use of ultrafiltration is the preparation of plasma from the whole blood by removing particulates.
11.10
SUMMARY
Now let us summarise the important aspects of membrane separations which we have studied in this unit. Membrane processes use selective membranes for separation of various chemical substances which can be either in suspended or dissolved states and also in gaseous forms. Semipermeable membranes are physical barriers which allow permeation of certain chemical constituents across them and effectively preventing others from a feed mixture. Permeation of chemical substances requires a driving force which can be either a pressure gradient, electrical potential gradient or concentration gradient. Gaseous permeation, reverse osmosis, ultrafiltration and microfiltration processes utilise pressure gradient across the membrane. Dialysis and liquid membrane 31
Other Separation Methods
processes utilise concentration gradient. Electrodialysis process utilises electrical potential gradient as the driving force. Mechanism of permeation of chemical substances through membranes basically involve solution-diffusion and the permeating species dissolve in the membrane and subsequently diffuse across. Preferential permeation across the membrane arises due to the selective dissolution of the substances in the membrane and difference in their diffusion rates through the membrane matrix. Chemical preference between the membrane and the permeating substance, selective complexation within the membrane and size compatibility between them may contribute to the separation mechanism. Osmotic phenomena occurs due to the difference in chemical potential between pure solvent and solution and is the flow of solvent from pure solvent to solution through semipermeable membranes. Reverse osmosis involves separation of solvent from solution by application of pressure in excess of the osmotic pressure of the solution. Dialysis process involves separation of low molecular weight solutes from high molecular weight solutes by using concentration gradient across semipermeable membranes of appropriate pore size and thickness. Electrodialysis process involves separation of cations and anions using cation and anion selective membranes by application of electrical potential. Reverse osmosis and electrodialysis processes are mainly used for production of pure water from naturally occurring saline waters and industrial effluents. They are widely applied for producing drinking water and process water for industrial use as well as for effluent treatment. Dialysis is mainly used for artificial purification of blood from patients suffering from kidney failure. The protein metabolic products like urea, uric acid and creatinine, etc. are separated from the blood stream in dialysis process.
11.11
TERMINAL QUESTIONS
1.
Distinguish between osmosis and reverse osmosis, dialysis and electrodialysis membrane processes.
2.
What is facilitated diffusional transport mechanism and in which membrane process it is mainly explained?
3.
What are the parameters on which osmotic pressure of a salt solution depends? Calculate the osmotic pressure of 1000 ppm calcium chlorideat 300 K?
4.
Explain concentration polarization phenomena occurring in reverse osmosis process?
5.
What are important parameters to characterize dialysis process and discuss the role of membrane thickness?
6.
Discuss limiting current density and water splitting in electrodialysis process.
11.12
ANSWERS
Self Assessment Questions
32
1.
Protein, sucrose and calcium chloride and water can be separated from each other by a combination of ultrafiltration, electrodialysis and reverse osmosis processes. High molecular weight proteins can be separated from low molecular weight sucrose and calcium chloride by ultrafiltration process. The permeate from ultrafiltration process can be treated by electrodialysis to separate calcium chloride which is ionic in nature. The dialysate from electrodialysis process can be processed by reverse osmosis whereby sucrose can be concentrated and pure water permeates through the membranes.
2.
Separation of water and salt by reverse osmosis process is explained by preferential sorption-capillary flow mechanism. This mechanism involves formation of pure water layer at the membrane surface by preferential sorption and its subsequent withdrawal through membrane pores by application of pressure. To get maximum solute separation and permeation of water, the diameter of the pore should not exceed twice the thickness of pure water layer. If the pore size is more, the solute separation will be low and if the pore size is less , rate of permeation will be less but solute separation will be more.
3.
The osmotic pressure of a salt solution is given by
Membrane Separation
Π = iC RT
5% CaCl2 solution means 5 grams of calcium chloride in 100 grams of solution ∴ Molarity of CaCl2 solution = 5 × 1000 / 100 × 110.98
= 0.451 moles/ litre
van’t Hoff factor, i = 3, R = 0.082 litre atmosphere per degree per mole, T = 300 K ∴Osmotic pressure of CaCl2 solution = 3 × 0.451 ×0.082 × 300 = 33.28. atm Similarly, molarity of 5% urea solution = 5 × 1000 / 100 × 60.032 = 0.833 moles / litre∗ i for urea = 1 since it does not ionise ∴osmotic pressure of 5% urea solution = 1 × 0.833 × 0.082 ×300
= 20.49 atm ∴5% CaCl2 solution has higher osmotic pressure ∗In the above calculation the density of solutions is taken as unity for simplification. 4.
The desirable properties of semipermeable membranes in reverse osmosis process is maximum solvent permeation and minimum solute permeation. For a given solute solvent system, the solvent permeation rate through different membranes is decided by the pure water permeation constant (A) under identical operating conditions. Similarly, the solute diffusion rate for different membranes is decided by the solute permeation constant ( B) under identical solute concentration and operating conditions. The higher value of the ratio of the constants A / B indicates the most desirable property for a semipermeable membrane.
5.
Dialysis is a diffusive process and since the molecular diffusional rates of solutes are slow, the entire process tends to be slow and time consuming. If the receiving solutions (dialysate) are not periodically removed, the solute 33
Other Separation Methods
concentration on both the sides of the membranes tends to equilalise, negating the driving force for separation. The permeant species is not recoverable in the pure form and is necessarily more dilute in the dialysate than in the starting stream. 6.
Coulomb efficiency indicates the efficiency of the electrodialysis process and is defined as the ratio of number of equivalents of electrolytes displaced from dialysate to the brine, to the number of faradays of electricity passed. It is generally expressed in percentage. The unwanted transport processes, namely, coion transport, free electrolyte diffusion and water transport accompanying ion transport across the membranes contribute to the decrease in coulomb efficiency.
Terminal Questions
1.
Osmosis is a process in which pure solvent permeates through a semipermeable membrane when a pure solvent is separated from a solution by a semipermeable membrane. This occurs due to the difference in chemical potential gradient which is higher for pure solvent than in solution. Osmosis will occur when two solutions with different solute concentrations are separated by a semipermeable membrane and the flow of solvent will be from lower solute concentration side to higher solute concentration side. Reverse osmosis process is an extension of osmosis process wherein solvent is allowed to flow from the solution by the application of pressure in excess of the osmotic pressure of the solution. Dialysis process is driven by concentration gradient and involves molecular diffusion of solutes, ionic as well as nonionic. Electrodialysis process is driven by electrical potential gradient and involves mainly electrical migration of ions of either charge.
2.
When a simple diffusion process occurring across a semipermeable membrane is coupled with chemical reactions within the membrane, the net diffusional rate of permeating species through the membrane may be greatly enhanced. Such a diffusional permeation is called facilitated diffusional transport . Facilitated diffusion occurs in liquid membrane systems. This mechanism requires a carrier substance inside the membrane which shuttles back and forth between the two sides of the membrane carrying the permeating substance by combining with it. Forward reaction of chemical combination between the carrier and the permeating substance takes place on one side of the membrane and the backward reaction of dissociation into carrier and the permeant occurs on the other side.
3.
The osmotic pressure of a salt solution depends on its molar concentration (C ), temperature (T ), and the nature of the substance whether ionic or nonionic. If it is a strong electrolyte, the number of ions it dissociates into upon ionization per molecule (i) is also used in computing the osmotic pressure of the solution. If it is a weak electrolyte, the degree of dissociation is used to compute the osmotic pressure. The molar concentration of 1000 ppm calcium chloride (C ) = moles / litre T = 300 K , i = 3, R =
0.082 litre atm/degree/mole. ∴ Osmotic pressure of 1000 ppm calcium chloride = 3 × 9.01 × 10–3 × 0.082 × 300 = 0.665 atm. 34
9.01 × 10–3
