MEMBRANE TECHNOLOGY OTHER TECHNIQUES

1. MEMBRANE DISTILLATION OSMOTIC DISTILLATION MEMBRANE EXTRACTION MEMBRANE TECHNOLOGY OTHER TECHNIQUES

2. Feed side Membrane Permeate side To TL x = 0 x = L Figure 1. Temperature profile across a homogeneous membrane OTHER TECHNIQUES MEMBRANE DISTILLATION Introduction • Most membrane transport processes are isothermal processes with either concentration pressure or electrical potential difference as the driving force.In this case we have a thermal process. (1) • When a membrane separates two phases held at different temperatures, heat will flow from the high-temperature side to the low-temperature side. This transport of heat can be described by Fourier’s equation, where the heat flow is related to the corresponding driving force, the temperature difference.

3. OTHER TECHNIQUES MEMBRANE DISTILLATION Definition • Is a process in which two liquid or solutions at different temperatures are separated by a porous membrane. • The liquid or solution must no wet the membrane otherwise the pores will be filled for capillary force. This implies that non-wettable porous hydrophobic membranes must be used in the case of aqueous solutions. • Membrane distillation is a type of low temperature, reduced pressure distillation due at the use porous hydrophobic polymers. • The material in this case is not wetted by the liquid feed and thus liquid penetration and transport across the membrane is prevented, provided the feed site pressure does not exceed the minimum entry pressure for the pore size distribution of the particular material.

4. Air/vapour Feed H2O T1 Permeate H2O T2 Hydrophobic porous membrane Liquid water Liquid water T1>T2 Figure 2. Schematic representation membrane distillation OTHER TECHNIQUES MEMBRANE DISTILLATION Schematic representation • Such transport occur in a sequence of three steps: • Evaporation on the high-temperature side. • Transport of vapour molecules through the pores of the hydrophobic porous membrane. • Condensation on the low-temperature side. • Membrane distillation is one of the membrane processes in which the membrane is no directly involves in separation the only function of the membrane is to act as a barrier between the twos phases. Selectivity is completely determined by the vapour liquid equilibrium involves. This means that the component with the highest partial pressure will show the highest permeation rate. (M. Mulder, 1997.Basic Principles of Membrane Technology)

5. OTHER TECHNIQUES MEMBRANE DISTILLATION Schematic representation 2 6 1 5 3 4 Figure 3. Fractionation by membrane distillation, 1, porous hydrophobic membrane polymer; 2, feed; 3, vapour space; 4, cooling water; 5, chilled wall; 6, condensed droplets. (K.Scott & R. Hughes, 1996.Industrial Membrane Separation Technology)

6. OTHER TECHNIQUES MEMBRANE DISTILLATION Process parameters Figure4. Contact angles of liquid droplets on a solid (nonporous) material. • The main requirement is that the membrane must no be wetted. If wetting occurs, the liquid will penetrated spontaneously into the pores of the membrane. • The wettability is the determined by the interaction between the liquid and the polymeric material, with no wetting occurring at low affinity. Information about wettability can be obtained by contact angle measurements, i.e. a drop of liquid is placed upon a nonporous flat surface and the contact angle is measured. • For low affinity the contact angle  will have values > 90º. • If the material is porous, the liquid will penetrate into the pores wetting occur ( < 90º). • This can be described by the Laplace equation: (2)

7. OTHER TECHNIQUES MEMBRANE DISTILLATION Membrane materials • The membrane hydrophobic materials typical are: • Polypropylene • PTFE (polytetrafluroethylene) • PVDF (polyvinylidenedifluoride)

8. OTHER TECHNIQUES MEMBRANE DISTILLATION Some applications The applications are determined by the wettability of the membrane, which implies that mainly aqueous solutions containing inorganic solutes can be treated. The surface tension of these solution differs little from that of water. • The production of pure water : • Laboratories • Semiconductor industry • Desalination of the seawater • Concentration of aqueous solutions • Removal of compound organic volatiles (VOC´s) • Contaminated surface water (benzene) • Fermentation products and volatile bioproducts (ethanol, butanol, acetone or aroma compounds).

9. OTHER TECHNIQUES MEMBRANE DISTILLATION Advantage & drawback Advantage • The major advantage of membrane distillation is that, with compact modules equipped with hollow fibres, a high surface area per unit liquid volume for mass transport is accessible and thus high overall permeation rates are attainable. Drawback • The main limitation in a MD is the surface tension of the solution that must be similar at tension surface water. • The greater practical limitation in membrane distillation applications is the required pressure upstream of the membrane.

10. OTHER TECHNIQUES OSMOTIC DISTILLATION Introduction • Similar to membrane distillation. • Both phases at the same temperature. • The partial pressure gradient due to the osmotic pressure is the driving force. • The osmotic pressure is risen by adding appropriate compounds to the receiving phase.

11. OTHER TECHNIQUES OSMOTIC DISTILLATION Transport mechanism • Osmotic distillation is a separation process in which a liquid mixture containing a volatile component is contacted with microporous, non-liquid-wettable membrane whose opposite surface is exposed to a second liquid phase capable of absorbing that component. Feed Channel Dilute Feed In Concentrated Feed Out Semipermeable Membrane Concentrated Brine In Diluted Brine Out Brine Channel Figure 5. In osmotic distillation, a semipermeable membrane acts as a vapor gap that allows migration of volatiles in a single direction. Chemical Engineering Progress, 1998

12. OTHER TECHNIQUES OSMOTIC DISTILLATION Some applications • This technique to play an important role in the processing of foods, pharmaceutical and biological products. Examples: • Fruit juice enrichment. • Alcohol removal from wine and beer.

13. OTHER TECHNIQUES OSMOTIC DISTILLATION Transport mechanism (solute concentration) The driving potential for such transport is the difference in vapor pressure of each component over each of the contacting liquid phases. If the sole or primary volatile component in solution is the solvent, then evaporation of solvent from the solution of higher vapor pressure into that of lower vapor pressure will result in concentration of the former and dilution of the latter. If the solvent vapor pressure over the liquid being concentrated drops to a value equal to that over receiving phase, no further transport will occur. Increasing Water Vapor pressure Diluted Brine Out Concentrated Brine In Membrane Dilute Feed In Concentrated Feed Out Decreasing Water Vapor pressure Figure 6. Mechanism of osmotic distillation through a microporous hydrophobic membrane

14. OTHER TECHNIQUES OSMOTIC DISTILLATION Transport Mechanism (Alcohol removal ) In this case, the solute of interest is evaporated from the feed at the membrane surface, transported by vapor diffusion through the membrane pores, and condensed into a strip liquid on the opposite face of the membrane. Most commonly, the stripping liquid is pure water or an aqueous solution containing a lesser concentration of the solute being transferred. Increasing alcohol partial pressure Dilute Alcohol Solution Out Water In Membrane Water Alcohol Wine In Dealcoholized Wine Out Decreasing alcohol partial pressure Figure 8. Mechanism of selective alcohol removal by evaporative pertraction Chemical Engineering Progress, 1998

15. OTHER TECHNIQUES OSMOTIC DISTILLATION Process design • The objective of an OD process for concentration of an aqueous feed is to remove water from the feed via transfer of a large fraction of the water into a saline strip solution, yielding a product of the desired solute concentration and diluted strip. The essential design parameters are: • The required plan capacity in daily volume of feed to be concentrated. • The solute concentrations in the feed and final concentrated. • The water vapor pressure/concentration relationship for the feed stream. • The water vapor pressure/salt concentration relationship for the strip solution. • The intrinsic water vapor permeability of the OD membrane, expressed as:

16. OTHER TECHNIQUES OSMOTIC DISTILLATION Process design The process employs partial batch recycle on the feed side (to minimize large feed viscosity changes through the membrane) and continuous countercurrent recycle plus evaporative reconcentration of the brine strip. Figure 9. Typical OD system for juice concentration Chemical Engineering Progress, 1998

17. OTHER TECHNIQUES OSMOTIC DISTILLATION Process design In the figure 10, we can see the following, the wine from a chilled storage tank is continuously recycled through the shell side of the OD contactor array, while strip solution (water) from a second storage tank is continuously recycled. Figure 10. Dealcoholization of ferments by evaporative pertraction Chemical Engineering Progress, 1998

18. OTHER TECHNIQUES OSMOTIC DISTILLATION Advantage & drawback Advantage • Retains flavours and fragrances better than thermal techniques. • The solutes are concentrated at low temperature and pressure, with minimal thermal or mechanical damage to or loss of those solutes (thermal labile) Drawback • Is a process very expensive for water removal from solution than more-conventional processes such as distillation, UF, and RO.

19. OTHER TECHNIQUES MEMBRANE EXTRACTION Introduction • These membrane processes are generally referred to as membrane contactor. (other names: pertraction, gas adsorption, membrane based solvent extraction, liquid-liquid extraction, hollow-fiber contained liquid membrane, etc.) • The separation performance in these processes is determined by the distribution coefficient of a component in two phases and the membrane acts only as an interface, similar to membrane distillation. • In general, it is not the enhanced mass transfer but rather the large area per volume as can found in hollow fiber and capillary modules, that makes this process more attractive than convectional dispersed-phase contactor

20. gas liquid liquid gas liquid liquid a) b) c) OTHER TECHNIQUES MEMBRANE EXTRACTION Membrane contactor Figure 11 .Schematic drawing of various membrane contactor.

21. OTHER TECHNIQUES MEMBRANE EXTRACTION Transport mechanism • If a component i is transferred from feed phase to the permeate phase three steps can be considered in general: • Transport from feed phase to the membrane. • Diffusion through the membrane • Transfer from the membrane to the permeate phase The flux of component i is expressed in terms of an overall mass transfer coefficient with If the mass transfer resistance is completely in the membrane phase then the first equation is reduces to

22. Porous membrane Feed Permeate Gas phase Liquid phase Gas phase Liquid phase Gas phase boundary layer Liquid phase boundary layer Membrane OTHER TECHNIQUES MEMBRANE EXTRACTION Gas-liquid membrane contactor Porous membrane Feed Permeate Gas phase Liquid phase Gas phase Liquid phase Gas phase boundary layer Liquid phase boundary layer Membrane a) b) Figure 12. Gas-liquid contactors with a non-wetted membrane (left side) and a wetted membrane (right side ) and the corresponding concentration profile.

23. OTHER TECHNIQUES MEMBRANE EXTRACTION Liquid-liquid membrane contactor • This process is characterised by two liquid stream separated by a porous or nonporous membrane. • In case of a porous membrane the feed phase may either wet or not wet the membrane. • Firstly we will consider the case where the feed is an organic solvent from which a solute has to be removed while the permeate phase is an aqueous phase.

24. Porous membrane Porous membrane Feed Permeate Feed Permeate Liquid phase Liquid phase Liquid phase Liquid phase OTHER TECHNIQUES MEMBRANE EXTRACTION Liquid-liquid membrane contactor Gas phase Liquid phase Gas phase Liquid phase Liquid phase boundary layer Liquid phase boundary layer Liquid phase boundary layer Liquid phase boundary layer Membrane Membrane a) b) Figure 13. Liquid-liquid membrane contactors with a wettable liquid feed (left side) and a non-wettable liquid feed phase (right side ) and the corresponding component concentration profile.

25. dense membranes Thin toplayer Porous membrane Gas phase Liquid phase Gas phase Liquid phase OTHER TECHNIQUES MEMBRANE EXTRACTION Nonporous membrane contactors Gas-liquid and liquid-liquid membrane contactors due to shear stresses, osmotic flow, and pressure gradients. This may be overcome by using either nonporous membrane contactors (G-L, L-G or L-L contactor) or by applying a coating onto the porous membrane. l1 l2 Gas phase Liquid phase Gas phase Liquid phase Liquid phase boundary layer Liquid phase boundary layer Liquid phase boundary layer Liquid phase boundary layer Membrane Membrane a) b) Figure 14. Gas-liquid membrane contactors with a composite membrane (porous membrane left) and dense membrane (right)

26. OTHER TECHNIQUES MEMBRANE EXTRACTION Some application • G-L CONTACTORS • CO2 and H2S from natural gas • CO2 from biogas • O2 transfer (blood oxygenation, aerobic fermentation) • CO2 transfer (beverages) • NH3 from air (intensive farmery) • L-G CONTACTORS • Volatile bioproducts (alcohol, aroma compounds) • O2 removal from water • L-L CONTACTORS • Heavy metals • Fermentation products (citric acid, acetic acid, lactic acid penicillin)

27. OTHER TECHNIQUES MEMBRANE EXTRACTION Advantage & drawback Advantage • The used of the hollow Fiber membranes given a bigger area to flux. Drawback • Instability of the system, the pressure applied must no be greater than the wetting pressure or the liquid penetration may occur.