Mass transfer and separation technology Massöverföring och separationsteknik ( MÖF-ST ) , 7 sp

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1 Mass transfer and separation technology Massöverföring och separationsteknik ( MÖF-ST ) , 7 sp 15. Membrane separations Ron Zevenhoven Åbo Akademi University Thermal and Flow Engineering Laboratory tel ; ron.zevenhoven@abo.fi 15.1 Overview, Membrane units 2/34

2 Membrane separations Picture: SH06 Membrane separations involve the separation of a feed (stream) into components using a semi-permeable barrier through which the components move with different velocity. The products are referred to as permeate and retentate; a so-called sweep stream can be used to remove the permeate. The feed, permeate and retentate are usually gases or liquids but may be solids as well. Typically for membrane separations 1) retentate and permeate are miscible; 2) the separating agent is a semi-permeable barrier; and 3) sharp separations are difficult to achieve otherwise. 3/34 Industrial application examples Table: SH06 4

3 Example: Natural gas separation Picture: 5/34 Membrane materials The semi-permeable barrier is either A thin, non-porous polymer film, or A porous polymer, ceramic or metal, or A liquid or gas Natural polymers may be wood, cotton, rubber Many suitable synthetic organic polymers have been developed since ~ 1930 During the process the membrane should not disintegrate (dissolve, break,...) Picture: SH06 6/34

4 Permeance Important for a membrane is High permeance for a given species to be separated A high ratio of permeance for the species to be separated Permeance P M can be compared to a mass transfer coefficient: for a membrane with thickness l M (m) and driving force Δc (mol/m 3, kg/m 3 ) or Δp (Pa/m), it can be defined for a certain transport rate N i of species i per m 2 area (or flux ) as N i P M,i M driving force P M, i driving force where P Mi is the permeability for species i, and P Mi is the permeance The unit barrer is widely used for permeability, where 1 barrer = kmol m/(m 2 s Pa) 7/34 Membrane types Membranes can be dense (= non-porous) or (micro-) porous In dense membranes, pores are < a few Å (1Å = 0.1 nm), so that most species must dissolve and diffuse through the material. (In crystalline materials diffusion can be difficult and proceeds primarily through amorphous regions.) In micro-porous membranes, pores of µm are large compared to molecule size. (Low selectivity for small molecules.) Technical membranes are usually composites of thin, dense film on a supporting, much thicker micro-porous material permeance for a species can be high even if permeability is low. Polymer membrane types Picture: SH06 8

5 Membrane modules Important membrane shapes for use are flat sheets, tubes, fibres, or (ceramic) monoliths Common membrane units are available a compact modules that support the membrane and guide input / output flows Pictures: SH Transport in membranes 10

6 Transport in membranes /1 Mass transfer across membranes is determined by (for pore sizes large to non-existing) convection, which can almost always be described by Hagen-Poiseuille laminar tube flow, corrected for void fraction (voidage) gas or liquid diffusion in pores, based on Fick s Law and corrected for the effect of voidage and tortuosity (both leading to collisions of diffusing species with the pore wall), for gases Knudsen diffusion solid diffusion, where the gas or liquid component is absorbed, diffuses through the membran and desorbs at the other end. Transport in membranes: (a) Bulk flow (convection), (b) Pore diffusion, (c) Restricted diffusion, (d) Solution diffusion. a-c: Porous membrane; d: Dense membrane) Picture: SH06 11 Transport in membranes /2 For dense membranes, the so-called solution-diffusion model describes the concentration profile across a membrane by assuming phase equilibrium at the fluid-membrane surfaces For gas separations, external mass transfer resistances can be neglected compared to membrane diffusion resistance Concentration / partial pressure profiles for transport through membranes: (a) Porous membrane, liquids (b) Dense membrane, liquids (a) Porous membrane, gases (b) Dense membrane, gases Picture: SH06 12

7 Transport in membranes /3 The most important flow patterns are based on mixing (which may be limited in practice), co-current flow, counter flow or cross flow For fast permeation rates the cross-flow model may be applied For example with pressures p P and p F, transfer of n moles of species A, membrane area A M (m 2 ) for permeability P MA : θ= cut fraction of feed A M x x RA FA P MA M y y p A P A dn x p A F Pictures: SH06 13/ Membrane separation processes 14

8 Dialysis Consider a liquid feed at pressure p 1, containing solutes A and B and insoluble, dispersed matter (colloidal) in a solvent; at the other side of the membrane pressure is p 2 and a sweep flow removes permeate Solutes A and B are transported through the membrane by dialysis Dialysis (from Greek δια = through and λυρις = to loosen ) If p 1 = p 2, solvent can pass the membrane by osmosis (p 1 > p 2 stop or will prevent osmosis) Colloidal matter cannot pass the membrane Picture: SH06 Important applications: Reverse osmosis Electrodialysis / electro-osmosis Thermal osmosis 15/34 Electrodialysis Electrodialysis has been used since ~1900 to improve dialysis rates in electrolyte solutions Currently, electro-dialysis is used for separating (aqueous) electrolyte solutions into concentrate ( brine ) and diluate ( desalted water ) Ion-selective membranes are used: anion-selective membranes carry a positive charge and permit passage of negative ions (anions), vice versa for cation-selective membranes Different products are collected in different compartments A DC current is applied using two chemically neutral electrodes Picture: SH06 16

9 Reverse osmosis Pictures: SH06 Osmosis (from Greek oσμως = push ) implies to transfer of solvent through a membrane more permeable for solvent than for solute. The osmotic pressure π can be estimated by π = RT c/m, with solute (dissolved salt) concentration c and average molar mass M for the solute (dissolved salt ions). Important application: de-salination of sea water or brackish water 17/34 Gas permeation Gas permeation allows for separating light species (< 50 g/mol) from small amounts of more heavy species, at high feed gas pressure Permeate-side pressure is much lower, typically 1 atm Usually dense membranes, sometimes micro-porous Examples: H 2 removal from methane O 2 enrichment in air CO 2 removal Natural gas purification Removal of pollutants from air Important alternative for cryogenic distillation Picture: SH06 18/34

10 Pervaporation In pervaporation the phase state on either side of the membrane is different For a liquid feed mixture with solute species A and B, keeping the pressure p 2 below the vapour pressure for species A (may be near vacuum) increases the selectivity for A Inside the membrane, a liquid zone and a vapour zone can be distinguished, and swelling may occur Evaporation of A has a heat effect: operate adiabatically or with heat transfer Examples: Separation of water from alcohols, esters, ketones, other organics Cyclohexane benzene separation Pictures: SH06 19/34 Ultrafitration, microfiltration /1 Ultrafiltration, microfiltration are (like reverse osmosis) pressure driven Micro-porous membranes allow water and small molecules to pass, retaining large molecules (e.g. ultrafiltration: 1000 < MWCO <100000, molecular weight cut-off) and particles (e.g. microfiltration: > µm) While reverse osmosis produces primarily purified solvent, ultra/micro-filtration recovers solutes Artificial kidney 1 atm ~ 14.7 psi (pound per square inch) Picture, table: SH06 20/34

11 Ultrafitration, microfiltration /2 Microfiltration Batch ultrafiltration Single-stage continuous feed-and -bleed ultrafiltration Multi-stage continuous feed-and -bleed ultrafiltration 21/34 Osmotic power plant Saltkraft (Norway) Source: Modern Power Systems November 2007, p 8 22/34

12 15.4 A few examples (old exam & dhb07) 23 Old exam question 430 (May 2012) 24

13 Old exam question 430 (May 2012) Picture on previous page!! 25 Old exam question 430 answer /1 26

14 Old exam question 430 answer /2 27 Self-study Example 1 Source: dhb07 28

15 Self-study Example 2 Source: dhb07 29 Self-study Example 2 (cont d) Source: dhb07 30

16 Self-study Example 3 Source: dhb07 31 Self-study Example 4 Source: dhb07 32

17 Self-study Example 4 Source: dhb07 33 Sources #15 dhb07 A.B. De Haan, H. Bosch Fundamentals of industrial separations 2nd Ed., TU Eindhoven / U Twente, the Netherlands (2007) Ch. 11 SH06 J.D. Seader, E.J Henley Separation process principles John Wiley, 2nd edition (2006) Chapter 14 T68 R.E. Treybal Mass transfer operations McGraw-Hill 2nd edition (1968) p ÖS97 G. Öhman, H. Saxén Transportprocesser Åbo Akademi University (1997) /34