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Module :
Membrane based separation processes
Dr. Sirshendu De
Professor, Department of Chemical Engineering
Indian Institute of Technology, Kharagpur
Keywords:
Separation processes, membranes, electric field assisted separation, liquid membrane, cloud point extraction, electrophoretic separation, supercritical fluid extraction
Membrane Based Separation Processes
3.1 Definition of a membrane:
It is an interface that separates the two phases and restricts the transport of various chemical species through it. Membrane can be homogeneous, heterogeneous, symmetric, asymmetric, charged, neutral.
3.2 Membrane Casting:
Common polymeric membrane material is Cellulose Acetate. Various steps of a typical casting process are:
- Solvent (acetone) is added to polymer. A viscous solution is prepared.
- It is put on a plate. Place another plate on top of it. The gap is of the order of ~ 0.25mm
- Give a one directional motion to the top plate. A thin film is produced on the bottom plate.
- Take out bottom plate and place it in ice-water bath, which releases the film.
- Heat treat the film in hot water.
Typical casting conditions are:
Casting solution : CA 25%, Frmamide 30% and Acetone 45%. Casting temperature : 25^0 C Evaporation time : 1 minute
3.4 Types of motion of molecules through barrier:
1) Permeation:
a) Dissolution of permeating molecules in the membrane b) Diffusion of dissolved molecules c) Desorption of penetrant molecules to the downstream side.
2) Knudsen diffusion (d/λ < 0.2):
Single gaseous molecules diffuse under rarefied conditions so that the mean free path is longer than the pore diameter.
3) Convection (d/λ > 20):
Viscous flow through the pores of ultrafiltration and microfiltration.
3.5 Categorization of such processes:
Reverse Osmosis:
Small solute particles to be separated. Molecular weight < 100 Pore size: 2 – 10 A^0 Pressure: > 25 atm. Permeation is main transport mechanism Example: Filtration of salt solution
Nanofiltration:
Particles to be separated with Molecular weight: 200 – 1000 Pore size: 5 – 20 A^0 Pressure: 15 – 25 atm.
Particle retention of salts. Example: Filtration of dyes, small molecular weight organics, etc.
Ultrafiltration:
Molecular weight of particles : 10^3 - 10^5 Pore size: 20 – 1000 A^0 Pressure: 6 – 8 atm. Transport Mechanism: Convection (main) + diffusion Example: Filtration of protein, Red blood cells, polymers, etc.
Microfiltration:
Molecular weight > 1 lakh Pore size: more than 1000 A^0 Pressure: 2 – 4 atm. Example: Filtration of clay solution, latex, paint, etc.
3.6 Useful definitions:
Some useful definitions are presented below.
(1) Osmotic Pressure:
Osmosis means craving for water.
Solution H 2 O
Δh ρgh (^) Osmotic
pressure
(2) Observed retention: (Selectivity of membrane)
This property indicates the extent of separation of a membrane can produce with respect to the solute concentration in the feed. Thus, observed retention is defined as, Ro = 1 − CCop (3.2) where, C (^) p = Solute concentration in permeate Co = Solute concentration in feed If Ro → 1.0, solute is completely retained by the membrane.
(3) Real retention:
Real retention is a constant that defines the partition of the solute concentration across the membrane, i.e., between the the membrane-solution interface and the permeate side. Since, this definition is not masked by any physical phenomenon like concentration polarization, (defined later) etc., this definition indicates the true separation efficiency of the solute by the membrane. Rr = 1 − CCmp (3.3) Here, C (^) m = Solute concentration in membrane solution interface. It may be mentioned here that since, membrane surface concentration of solute is always greater than the bulk concentration, real retention is always greater than observed retention. For complete solute retention, Rr = 1.0.
(4) Molecular Weight Cut-Off (MWCO):
Molecular weight cut off is another concept to characterize a membrane. In this case, generally neutral solutes of various molecular weights are considered to conduct experiments. The operating conditions of these experiments are low transmembrane pressure drop, high turbulence and low feed concentration. Experiments are conducted using each of these solutes and the observed retention values at the steady state are measured. The observed retention values are then plotted against the molecular weight of the solutes in a semi-log plot. The typical solutes are glucose (molecular weight 180), sucrose (molecular weight 342), various fractions of polyethylene glycol (molecular weights: 200, 400, 600, 1000, 1500, 2000, 4000, 60000, 10000, 30000), dextran (molecular weight 40000, 1,50,000), etc. The molecular weight at 90% solute retention indicates roughly the molecular weight cut off of the membrane. The molecular weight cut off curves are shown in Figs. 3.3 and 3.
Fig.3.3: Typical molecular cut off curve of a membrane
R 0
Log MW
(6) Estimation of retention & permeability:
Retention:
Observed retention (Ro ): Estimate by direct experimental measurement Real retention (Rr): One has to conduct batch experiments at high stirring speed, low feed concentration and low operating pressure. In that case, it is assumed that there is no formation of concentrated solute layer over the membrane surface and in absence of polarized layer, observed retention is almost same as retention retention.
Permeability:
Membrane permeability is measured by distilled water runs. Experiments are conducted using distilled water at various transmembrane pressure drops values. At various pressure drops, the water flux is measured. A plot of permeate flux versus operating pressure would be a straight line through the origin, as shown in Fig. 3.5. The slope of this curve indicates the permeability (Lp ) of the membrane. I may be noted here that the permeability of a membrane is its pressure history only. In other words, permeability of the membrane is independent of turbulence (stirring speed or cross flow velocity) in the flow channel. The unit of permeability is m (^2) m (^). Pa s^3. or (^) Pa s^ m..
Fig.3.5: A typical flux versus pressure plot for ditilled water as feed
Membrane modules:
The practical equipment where the actual membrane based separation occurs is known as membrane modules. The basic aim of development of these modules is to provide maximum membrane area in relatively smaller volume, so that the permeate flux i.e., the productivity of the system is maximum. These membrane modules are of four types, (i) plate and frame module, (ii) hollow fiber module, (iii) spiral wound and (iv) tubular modules. Each of these modules is described below:
Plate and frame modules
The heart of plate-frame module is the support plate that is sandwiched between two flat sheet membranes. The membranes are sealed to the plate, either gaskets with locking devices, glue or directly bonded. The plate is internally porous and provides a flow channel for the permeate which is collected from a tube on the side of the plate. Ribs or grooves on the face of the plate provide a feed side flow channel. The feed chanel can be a clear path with chanel heights from 0.3 to 0.75 mm. The higher channel
Slope=Lp
ΔP
J (m 3 /m 2 .s)
membrane surface, promoting high and stable flux and easy cleaning, especially when the objective is to achieve high suspended solids in the MF, UF or NF concentrate. Permeate is driven through the membrane to be directed out of the system or back into the process depending on the application. There are many advantages in tubular membrane configurations. Besides their rugged construction, they have a distinct advantage of being able to process high suspended solids, and concentrate product successfully and repeatedly to relatively high end point concentration levels without plugging. A common objective of an end-of-pipe waste treatment UF system is to reduce waste volume as much as possible to reduce concentrate hauling costs. For juice clarification applications, tubular membrane systems produce the greatest yields and the highest final suspended solids concentration levels. Tubular MF, UF and NF systems do not require significant prefiltration. Some tubular products have the ability to be mechanically cleaned with spongeballs. Spongeballs can be used in process, and are also used to enhance chemical cleaning by reducing time and cleaning chemicals. Tubular membranes are ideally suited to treatment of metalworking oily waste, wastewater minimization and recovery from industrial processes, juice clarification, treatment of pulp and paper industry waste, etc. Tubular membranes typically have life upto 2 to 10 years. The following figure (Fig. 3.5 b) shows some tubular membranes.
Fig.3.5b: Pictures of tubular modules
Hollow fiber module
In hollow fiber module, lots of hollow fibers (each fiber is a tubular module) are kept in a large pipe. Geometry allows a high membrane surface area to be contained in a compact module. This means large volumes can be filtered, while utilizing minimal space, with low power consumption. Hollow fiber membranes can be designed for circulation, dead end and single pass operation. Some of the many hollow fiber membrane filtration applications include, potable water treatment, juice clarification, wine filtration, dairy processing, etc. The advantages of such modules include reduction in space requirement, lowering in labor cost, lowering in chemical cost, delivery of high quality product water, etc. Hollow fiber membranes offer the unique benefits of high membrane packing densities, sanitary designs and, due to their structural integrity and construction, can withstand permeate back pressure thus allowing flexibility in system design and operation. Most KMS hollow fiber products are available in (i) 1" diameter laboratory test cartridges ranging up to 10" diameter for commercial products, (ii) Standard commercial cartridge lengths of 25", 43", 48", 60" and 72", (iii) Nominal separation ranges from 0.2 micron down to 1,000 MWCO, (iv)
Fig.3.5d: Flow pattern in hollow fiber module
Spiral wound module
In spiral wound membrane, membrane is cast as a film onto flat sheet. Membranes are sandwiched together with feed spacers (typical thickness 0.03 to 0.1 inch) and permeate carrier. They are sealed at each edge and wound up around a perforated tube. The module diameter ranges from 2.5 to 18 inch and length varies from 30 to 60 inch. The typical cross section of the spiral wound module is shown below:
Fig.3.5e: Flow pattern in spiral wound module
The application of spiral wound module includes, seawater desalination, brackish water treatment, potable water treatment, dairy processing, electrocoat paint recovery, protein separation, whey protein concentration, etc.
Therefore, it can be identified that the modeling of plate and frame and spiral wound module can be done by considering the flow through a rectangular channel. On the other hand, that for a tubular and hollow fiber module are done by considering flow through a tube.
(b) Facilitated transport:
Fig.3.7: Facilitated transport In this case also the chemical potential of a species in upstream is more than that in the downstream. However, as shown in Fig. 3.7, components to be transported are coupled with a carrier in the membrane phase. So, it is a special form of passive transport and very selective and at the same time, the transport is facilitated by the carrier component.
(c) Active transport:
Fig.3.8: Active transport
μ' A
μ^ '' A
A
A
B
AB
A + B → AB
μ' A
μ^ '' A
A μ ' A >μ'' A
A
B
AB
A + B → AB
As shown in Fig. 3.8, components are transported against driving force. Driving force for transport is provided by the activation energy of chemical reaction in the membrane phase (Living Cell). In this case, the upstream chemical potential is higher than that of downstream potential.
3.9 Description of transport process by phenomenological equation
The transport processes are generally described by the phenomenological equations. Such equation describes “cause-effect” relationship. It presents flux of anything is proportional to the driving force. The proportionality constant is the transport property specific to the system. Table 3.1 presents various phenomenological equations common in chemical engineering applications. Table 3.1: Examples of phenomenological equations Phenomenon relation Flux Driving force Constant of property Fick’s Law Mass Concentration difference D Ohm’s Law Electricity Electric potential difference^ R Fourier’s Law Heat Temperature difference K
In membrane separation process, driving forces may be interdependent. Thus, a concentration gradient across a membrane not only leads to flow of matter, can also cause build up of pressure. An example is osmosis.
