Chapter 6. Membrane Process (Carrier Mediated Transport)

Transcription

1 National November 17, 2015 (Wed) Chapter 6. Membrane Process (Carrier Mediated Transport) Chang-Han Yun / Ph.D.

2 Contents 6.1 Introduction 6.2 Osmosis Contents Contents 6.3 Pressure Driven Force 6.5 Other Driving Force 2

3 Membrane supply an interphase between two phases Liquid as a membrane Liquid membrane or Liquid film separates two phases from each other. Differences in solubility and diffusivity in the liquid film occur separation Carrier Contained at the inside of the membrane Ability to complex with a specific solute to enhance the flux of solute Characteristic of a facilitated or carrier Reversible chemical reaction(complexation) process + Diffusion process 1 Diffusion = rate-limiting (fast reaction) mainly occur in most of case 2 Reaction = rate-limiting (slow reaction and relatively fast diffusion) 3

4 [Table 6-17] Diffusivities in carrier mediated systems System Mobile carrier system Solvent swollen or gel system Fixed carrier D(cm 2 /s) > 10 7 Mobile or fixed type carrier(<figure 6-29>) Mobile type : Carrier dissolved in the liquid Carrier-solute complex diffuses across the membrane. Higher diffusivity in the mobile system Fixed type : Carrier bound chemically or physically to a solid polymer Very restricted mobility Solute jumps or 'hops' from one site to the other <Figure 6-29> Schematic drawing of a mobile carrier system (left) and a fixed carrier system (right). 4

5 Liquid membrane <Figure 6-30> Schematic drawing of two types of liquid membrane - left : supported liquid membrane(slm) - right: emulsion liquid membrane(elm) Types of liquid membrane(see <Figure 6-30>) 1) Immobilized Liquid Membrane(ILM) or Supported Liquid Membrane(SLM) Liquid film : immobilized within the pores of a porous membrane Role of porous membrane : framework or supporting layer for the liquid film Preparation : impregnating a hydrophobic porous membrane with a suitable organic solvent. 5

6 2) Preparation of ELM (<Figure 6-31>) 1 Mix 2 immiscible phases vigorously with surfactant to stabilize the emulsion form stable emulsion droplets (droplet size : μm) 2 Add emulsion to aqueous phase form W-O-W emulsion Oil phase = Liquid membrane Phase 1 and phase 2 : generally aqueous solutions Liquid membrane phase : organic phase (immiscible with water) Solubility of organic to water : very important factor to stabilize system <Figure 6-31> Preparation of ELM. 6

7 Selectivity of LM : Low only used in some specific applications Solubility & Diffusivity Selectivity Diffusivity : components size = similar diffusivity = not much difference Solubility : difference in distribution coefficient of species i among W-O-W Difference in distribution coefficient of species i among W-O-W Selectivity Without carrier, difference in diffusivity and solubility = low low selectivity Carrier- or Facilitated transport (<Figure 6-32>) Carrier : Accelerate the transport of specific component Selectivity 7

8 U-tube experiment Bottom of an U-tube : organic (higher density than water) One arm of the U-tube : aqueous KCl solution Other arm of the U-tube : pure water Organic phase : chloroform containing carrier(18-crown ether-6 : a high affinity to KCl salt) Salt transport by Δc KCl from high concentrated solution to pure water phase Without carrier : very low transport of salt ( KCl solubility to chloroform = very low) With carrier : form a reversible complex with the salt transport of K <Figure 6-33> 8-crown-6 complexed with a potassium ion. [Figure 6-32] U-tube experiment to demonstrate facilitated transport. 8

9 Carrier- Carrier molecule(c) : Enhance transport of component A (A+C complex AC) Coupled transport (<Figure 6-34>) 2 components are involved in carrier mediated-transport Co-transport : 2 components are moving in the same direction Counter-transport : 2 components are moving in opposite directions Decomplexation in the opposing phases Transport : low high concentration Real driving force : concentration gradient of complex(c) Diffusive transport (without carrier) Facilitated transport uncoupled <Figure 6-34> Transport mechanism in a liquid membrane. Left : Diffusive transport(without carrier) Right : Facilitated transport(with carrier C) coupled 9

10 Transport mechanism 1 Dissolving solute of aqueous phase 1(feed phase) into the membrane 2 Complexation between the carrier and the solute at the phase 1 interface 3 Diffusion of carrier-solute complex to opposite interface through liquid membrane 4 Decomplexation at phase 2(stripping phase or receiving phase) interface 5 Releasing solute from the membrane to phase 2 6 Diffusion of free carrier back to phase 1 interface through membrane <Figure 6-35> The mechanism of carrier- in liquid membranes with mobile carriers. 10

11 Basic feature of carrier- Complexation reaction = reversible Affinity between the carrier and the solute = very high Strong complex slow release Weak complex only limited facilitation occurs selectivity =low Bond energies of these reversible complexes = range of kj/mol Similar bond energy : H-bonding, acid-base interactions, chelation, π bond interactions Effects contribute to the transport of component A 1 Rate of complexation / decomplexation at the two interfaces 2 Diffusion of the complex (and the free solutes) across the membrane Another characteristics of facilitated transport Flux = not proportional anymore to the driving force At (very) low concentrations in feed phase, still appreciable fluxes can be obtained. 11

12 <Example> O 2 and N 2 transport through water film Carrier(Co compound) : complexation with O 2, but no complexation with N 2 Without carrier (un-coupled transport) transport of O 2 and N 2 by free diffusion p solubility Flux of O 2 and N 2 partial pressure(concentration) Solubility : O 2 > N 2 flux : O 2 > N 2 With carrier(coupled transport) N 2 flux : no change ( no complexation with carrier) O 2 flux : enhanced <Figure 6-36> Oxygen and nitrogen Flux through water with and Without carrier (cobalthistidine) 12

13 <Example> Coupled transport nitrate ion (NO 3 ) : see <Figure 6-37> Carrier : many ion-exchange components Complexing agents for anions : Tertiary amines or quarternary ammonium salts Charge density on the anion affinity between anion anion-exchange component Affinity sequence of various anions (anion quarternary ammonium salt) I > NO 3 > NO 2 > Cl > H 2 PO 4 2 > HSO 4 > SO 4 2 > HCO 3 > PO 4 3 > CO 3 2 Count-anion to exchange with nitrate : Cl (Feed : NO 3 Cl ; Strip : Cl NO 3 ) Transport of NO 3 against its own driving force Actual driving force = Δc of Cl in membrane Affinity(NO 3 carrier) Affinity(Cl carrier) Very high Cl in strip phase Easy decomplexation Equilibrium reaction : RCl + NO 3 R NO 3 + Cl <Figure 6-37> Counter-current transport. Cl concentration in phase 2 (strip phase) is very high in comparison to the low NO 3 concentration in the feed (phase 1). 13

14 Aspects of separation O 2 and N 2 transport through water film(<figure 6-38>) Mechanisms contribute to the total O 2 flux through the membrane Coupled transport : A + C AC Free diffusion Follow normal Fick s law Total flux of component A : sum of the two contributions (6-85) Fick's law Carrier-mediated diffusion <Figure 6-38> Schematic drawing of the concentration profiles arising from free O 2 diffusion via Fick's law (curve b) and by facilitated diffusion (curve a). 14

15 Equilibrium constant of the complexation reaction, (6-86) Average carrier concentration in the membrane, c = c C + c AC,o (6-87) where c C = concentration of free carrier c AC,o = concentration of complexed carrier at a certain point in the membrane Eq(6-86) and (6-87) (6-85) and <assume> c A,l c AC,l 0 Total flux of component A, (6-88) Define partition(distribution) coefficient k = c Ao /c Af where c Af = concentration of component A in the feed Eq(6-88) (6-89) Two limiting cases from [Figure 6-38] and Eq(6-85) : Rate Determining Step 1 1 st term(fickean diffusion) Reaction rate = low c AC,o c A,o 2 2 nd term(diffusion of the complex) Reaction rate = fast c AC,o c A,o 15

16 Damköhler number Ratio between reaction rate and diffusion rate Define the 2 nd Damköhler number = l 2 /(D t 0.5 ) where t 0.5 = half life of the complexation reaction (reaction time constant) D = diffusion coefficient of the free component l = membrane thickness Reaction time constant(t 0.5 ) D/l 2 2nd Damköhler number[l 2 /(D t 0.5 )] 1 2 nd Damköhler number[l 2 /(D t 0.5 )] 1 Reaction = rate determining Free diffusion = rate determining Reaction rate = very fast Reaction rate = slow Diffusion of free permeant = neglected Diffusion of complex = neglected <Example> l = 10 μm, t 0.5 = 10 7 sec, D = 10 9 m 2 /sec Damköhler number = range of

17 Flux ratio of 10 (total flux/fickean flux) facilitated transport = rate-determining step (region II) <Figure 6-39> Schematic drawing of the ratio of the total flux to the Fickean flux as a function of the Damköhler number. 17

18 For coupled process to transport NO 3 Chemical reaction : RCl + NO 3 R NO 3 + Cl Equilibrium constant(k) : (6-90) where subscript o : refer to organic phase subscript w : refer to aqueous phase Total NO 3 in organic = [NO 3 ] o + [RNO 3 ] o Solubility of free ions in organic phase([no 3 ] o ) = very low Total NO 3 in liquid membrane [RNO 3 ] o Distribution coefficient on feed side (6-91) and distribution coefficient on strip(permeate) side (6-92) K = / = Ratio of distribution coefficient K = / = high carrier is very selective 18

19 Overall transport Nitrate flow in the boundary layer (J bl ) (6-93) J i, which is determined by the ease of complexation, J i = k 1 [NO 3 ] w k 1 [NO 3 ] m (6-94) where k 1 and k 1 = rate constants [NO 3 ] w and [NO 3 ] m = interfacial [NO 3 ] in aq.(w) and org. phase(m) respectively Nitrate flux through the membrane phase(j m ) (6-95) Under steady-state, J b1 = J i = J m = overall flux J dc/dx = Δc/Δx and Eq(6-93), (6-94), (6-95) (6-96) where δ = thickness of the boundary layer and l= membrane thickness. 19

20 [NO 3 ] w = f(t) constant (6-97) where V = total feed volume and A = membrane area <Assume> rate of complexation = very fast (6-96) (6-98) By dividing Eq(6-96) by k 1 and neglecting 1 in the denominator Eq(6-98) & Eq(6-96) (6-99) 1 Diffusion process through liquid membrane = predominant boundary layer phenomena = neglect permeability coefficient, P = k NO3 - D m /l 2 Diffusion process through boundary layer = predominant P = D bl / δ Eq(6-99) Eq(6-97), and integration with the BCs BC 1 : c = c o at t = 0 & BC 2 : c = c at t = t (6-100) 20

21 Liquid membrane development Main components of SLM (Supported Liquid Membrane) Support membrane Organic solvent Carrier Free liquid film = very unstable use porous membrane as a framework(slm) SLM : unstable with time For high stability, Hydrophobic(PE, PP, PVDF) as support For high flux, Porosity of support = high Membrane should be as thin as possible ( Flux [thickness] -1 ) [Table 6-18] Some porous membranes frequently used as supports for supported liquid membranes (SLM). Preparation technique Stretching Phase inversion Material Polypropylene (Celgard) Polytetrafluoroethylene (Gore-Tex) Polypropylene (Accurel) Polyethylene 21

22 Choice of organic solvent Requirements of organic solvent to apply to SLM systems Solubility in the aqueous phase = extremely low Volatility = low Solvent for both the carrier and the carrier-solute complex Viscosity = not much high Carrier or carrier-solute complex increases the viscosity of organic solvent Stokes-Einstein equation, : Viscosity Diffusivity (6-101) [Table 6-19] Viscosities at T = 298 K of some solvents used in LM processes Solvent o-dichlorobenzene l-octanol dibutylphthalate o-nitrophenyl octyl ether o-nitro diphenylether Viscosity(g/(cm s))

23 By carrier concentration : two effects = counteracting Eq(6-89) : Flux Viscosity Diffusity Flux Very severe problem with SLM which causes the process to cease Loss of the organic phase unstability of the liquid film with time Essential for the solubility of the organic phase in the aqueous phase Shear forces generated by feed flow emulsification of the organic phase form small emulsion droplets diffuse out of the organic phase eventually the organic phase is completely removed. (<Figure 6-42>) Carrier loss, and Osmotic effects Involving high ion strength generate high osmotic pressure differences 23

24 Approach to solve these problems Gelation of the liquid membrane phase Change organic liquid to highly swollen crosslinked polymer(gel) Negative effect on diffusion coefficient Stability of the layer improved dramatically Gelled liquid layer Obtained by adding a small amount of a polymer to the organic phase Polymers : PVC, PAN, PMMA <Figure 6-42> Schematic representation of the emulsification of the organic phase in supported liquid membranes. 24

25 Choice of carrier Choice of the carrier = key factor in facilitated transport Ratio of the distribution coefficients determine selectivity In fact, every specific solute its own specific carrier Selection of the carrier : very important and very difficult From liquid extraction much information about carrier can be obtained. Classification of carrier molecules oximes (tertiary) amines crown ethers cobalt complexes calixarenes [Table 6-20] Structures of various carriers 25

26 Application Classification of application : depend on species to be separated Cations Anions Gases Organic molecules Ion separation Wide range of carriers is available Easily removed via facilitated transport Cations to be recovered by LM : Cu 2+, Hg 2+, Ni 2+, Cd 2+, Zn 2+, Pb 2+ Anions to be recovered by LM : NO 3, Cr 2 O 2 7, uranyl [UO 2 (SO 4 ) 2 2 ] Gas separation : completely different type of class of facilitated transport Separation of O 2 from N 2 Removal of H 2 S from natural gas NH 3, NO x and SO 2 from waste gases Separation of organic mixtures Separation of hydrocarbons aliphatic/aromatic(benzene/hexane) isomeric xylenes Removal of phenol from waste water 26

27 Summary of carrier Items Membranes Thickness Characteristics Supported Liquid Membranes (SLM) Emulsion Liquid Membranes (ELM) Fixed carrier membranes Solvent swollen membranes μm (SLM), 0.1 l μm (ELM) Pore sizes Non-porous (liquid!) Driving force Concentration difference Separation principle Affinity to carrier (carrier ) Membrane material Hydrophobic porous membrane Applications Removal of specific ions Cations (Cd, Cu, Ni, Pb) Removal of gases O 2 /N 2 separation Separation of organic liquids Removal of phenol Anions (nitrate, chromate) Removal of H 2 S, CO 2, SO 2, CO, NH 3 27