Membrane Technology & Separation Processes. (Electrodialysis & Concentration Polarization)

Transcription

1 Membrane Technology & Separation Processes (Electrodialysis & Concentration Polarization) Module- 23 Lec- 23 Dr. Shishir Sinha Dept. of Chemical Engineering IIT Roorkee

2 Mixing vs Separation Two substances a and b will mix or separate depending on free enthalpy of mixing(δg m ) ΔG m = ΔH m TΔS m where ΔH is the enhalpy of mixing and ΔS is the entropy of mixing. m m ΔG < 0:Spontaneous mixing ; m ΔG m > 0:Spontaneous separation In most cases and always when A and B are gases the mixing occurs spontaneously and minimum amount of energy, W = ΔG min m The actual energy requirement for the separation will bemany times greater than W min The actual energy requirement depends on the type of separation processes What is a membrane? A membrane is a physical barrier (no necessarily solid) that gives, or at least helps, the separation of the components in a mixture.

3 - Membrane processes are not based in thermodynamic equilibrium but based in the different transport rate of each species through the membrane. - The membrane market is still growing. In the decade, the sales related to membrane products and systems doubled. - In 1998, these sales were over 5000 million. Advantages Energy savings. The energy consumption is very low as there is no phase change. Low temperature operation. Almost all processes proceed at room temperature, thus they can deal with compounds that are not resistant at high temperatures. Recovery. Both the concentrate and the permeate could be recovered to use. Water reuse. When applied to recover water, they avoid the transport of large water volumes and permit the reduction of the Chemical Oxygen Demand (COD) loading in sewage plants. Compact operation. Which permits to save space. Easy scale-up. Because usually they are designed in modules, which can be easily connected. Automatic operation. The most of the membrane plants are managed by expert systems. Tailored systems. In many cases, the membranes and systems can be specifically designed according the problem.

4 Disadvantages High cost. Membranes (and associated systems) are costly, but for low selective separations. Lack of selectivity. In many cases, the separation factors are still insufficient. Low fluxes. The permeat flowrate available are still too low for some applications. Sensitive to chemical attack. Many materials can be damaged by acids, oxidants or organic solvents. Lack of mechanical resistance. Many materials do not withstand abrasion, vibrations, high temperatures or pressures. - The membrane operations more widely used are those based in applying a pressure difference between both sides of the membrane. Micro Filtration (MF) (10-0.1m) Bacteria, suspended particles Ultrafiltration (UF) ( m) Colloids, macromolecules Nanofiltration (NF) 5e -3-5.e -4 m Sugars, dyes, divalent salts Reverse Osmosis (RO) (1.e -4-1e -5 m) Monovalent salts, ionic metals Water Microfiltration (MF).

5 Ultrafiltration (UF). Nanofiltration (NF). Reversee osmosis (RO). - Although similar in appearance, the involved mechanisms in the separation can be very very different. Name of the membrane process as function of the particle size.

6 - There are other separation operations where a membrane is the responsible of the la selective separation of the compounds: Dialysis. Gas permeation (GP). Electrodialysiss (ED). Liquid membranes. Pervaporation. - In others, the membrane is not directly responsible for the separation but it actively participates in: Membrane extraction. Membrane distillation. Osmotic distillation. Type of filtration.

7 Simple scheme of a membrane module. - Synthetic membranes are solid barriers thatt allow preferentially to pass specific compounds due to some driving force.

8 (Very) Simple scheme for some mechanisms of selective separation on a porous membrane. - The separation ability of a synthetic material depends on its physical, chemical properties. Pore size and structure Design Chemical characteristics Electrical charge - The membranes can be roughly divided in two main groups: porouss and non porous.

9 - Porous membranes give separation due to... size shape charge...of the species. - Non porous membranes give separation due to... selective adsorption diffusion...of the species. Main parameters. - Rejection, R, if there is just one component (RO) R(%) C 100 A,f C C A,f A,p C C A,p A,f - Separation factor - Enrichment factor α A,B C C A,p A,f /C /C B,p B,f A B A C C A,p A,f

10 for two or more component Main parameters. - In RO, often we use the Recovery (Y) Q Y(%) Q p f Q : Permeate flowrate (m /s) p 3 Q : Feed flowrate (m /s) f Main parameters. - Passive transport in membranes. The permeate flux is proportional to a given driving force (some difference in a property). Flux(J) Constant (A) Driving Force (X) Driving forces: Pressure (total o partial) Concentration

11 Electric Potential Main parameters. Membrane processes and driving force. Process Feed phase Permeate phase Driving Force Microfiltration L L ΔP Ultrafiltration L L ΔP Nanofiltration L L ΔP Reverse Osmosis L L ΔP Dialysis L L Δc Electrodialysis L L ΔΕ Pervaporation L G ΔP Gas Permeation G G ΔP Main parameters.

12 - Permeate flux. In MF and UF, porous membrane model is assumed, where the a stream freely flows through the pore. Then, the transport law follows the Hagen-Poiseuille equation. J w Q A w m 2 r P 8 d J w : Solvent flux (m 3 /s m 2 ) Q w : Solvent flowrate (m 3 /s) A m : Membrane area (m 2 ) d: Membrane thickness (m) : Viscosity (Pa s) P: Hydraulic pressure difference (Pa) r: Pore radius (m) : Porosity : Tortuosity Main parameters. - The above model is good for cylindrical pores. However, if the membrane is rather formed by a aggregated particles, then the Kozeny-Carman relation works much better. J w Q A w m 3 KS2 P 2 1 d J W : Solvent flux (m 3 /s m 2 ) Q W : Solvent flowrate (m 3 /s) S: Particle surface area (m 2 /m 3 ) K: Kozeny-Carman constant A m : Membrane area (m 2 ) d: Membrane thickness (m) : Viscosity (Pa s)

13 - In the operations governed by the pressure, a phenomenon called concentration polarisation appears, which must be carefully controlled. This is due to the solute accumulation neighbouring the membrane surface. Formation of the polarisation layer. - Concentration polarisation. (It is not fouling!!!)

14 - Fouling: Irreversible reduction of the flux throughout the time. Pore size reduction by irreversible adsorption of compounds. Pore plugging. Formation of a gel layer over the membranee surface (cake).

15 - Membrane can be classified in several ways, but always there are arbitrary classifications. Structure: symmetric, asymmetric Configuration: flat, tubular, hollow fiber Material: organic, inorganic Surface charge: positive, negative, neutral...and even other divisions and subdivisions - Structure: Symmetric. Also called homogeneous. A cross section shows a uniform porous structure. Asymmetric. In a cross section, one can see two different structures, a thin dense layer and below a porous support layer. - Integral: the layers are continuous. - Composites: the active layer (thickness μm) is supported over a highly porous layer ( μm), sometimes both layers are of different materials.

16 Symmetric UF membrane of 0.45 m made of cellulose acetate (Millipore). Surface Cross section TM Symmetric ceramic membrane of 0.2 m made of alumina (Al O ) (Anopore ). 2 3

17 Asymmetric ceramicc membrane made of -Al O 2 3 (Membralox). UF integral asymmetric membrane made of polypropylene.

18 RO composite membranes. - Configuration and modules Configuration: geometric form given to the synthetic membranes. Module: name of the devices supporting one or several membraness (housing). The module seals and isolates the different streams. Thee geometry and specificc fluid movement through the confined space characterises each module. The type of flux, the transport mechanism and the membrane surface phenomena depend on the module design. - Configuration: Flat.

19 - The active layer is a flat. - Synthesised as a continuous layer. - Later, one can select a desired geometry (rectangle, circle,...) to be placed in the module. - Used in two kind of modules: plate-and-fra ame and spiral wound. - High surface area/volume ratio. Plate-and-Frame Membrane System. Consists of layers of membranes separated by corrugated structural sheets, alternating layers with feed material flowing in and retentate flowing outt in one direction, while permeate flows out in the other direction.

20 Spiral-wound module. Spiral-wound module.

21 - Configuration: Tubular. - It is like a tube. - Usually the active layer is inside. - The permeate crosses the membrane layer to the outsidee (this is, the feed flows inside). - Low surface are/volume ratio. - Several lengths and diameter (> >10 mm). - Modules grouping one or various membranes. Different types of tubularr modules.

22 Hollow fiber module. Cross section of hollow fiber (Monsanto). Comparison with a clip.

23 Hollow fiber cross section of polyamide for RO (DuPont). Hollow fiber made of polysulfone ( 1 mm) for UF (detail).

24 Hollow fiber cross section of 1 mm (Monsanto). Hollow fiber surface of polypropylene (Celgard).

25 - Comparison between modular configurations. Module Parameter Tubular Spiral-wound Hollow fiber Specific surface area (m 2 /m 3 ) Inside diameter or spread (mm) Flux (L/m 2 day) Production (m 3 /m 3 per module & day) Space velocity (cm/s) Pressure loss (bar) Pretreatment Simple Medium High Plugging Small Medium Elevated Replacement Easy Difficult Impossible Cleaning: Mechanical Possible Not possible Not possible Chemical Possible Possible Possible

26 - Comparison between modular configurations. Modular configurations and processes. Module Operation Tubular Spiral-wound Hollow fiber Reverse Osmosis A VA VA Ultrafiltration VA A NA Microfiltration VA NA NA Pervaporation A VA VA Gas Permeation NA VA VA VA = Very appropriate; A = Appropriate; NA = Not appropriate - Material: Organic. - Made of polymers or polymer blends. - Low cost. - Problems with their mechanical, chemical resistance. Temperature ph, Solvents Pressure

27 Polypropylene with 0.2 m pores (Accurel). Polytetrafluoroetylene with 0.2 m pores. Membrane Technology

28 Dialysis - Applied since the 70 s. - Low industrial interest. - Ions & species of low MW (< 100 Da). - Ionic Membranes (just like ED) ). - Driving Force: concentration gradient. - Slow and low selective. Membrane Technology Dialysis - Artificial kidney. - NaOH recovery in textile effluents, alcohol removal from beer, salts removal (pharmaceutical industry) ).

29 Membrane Technology Dialysis Looks not very important...?. GS PV ED HD MF RO UF Membrane and module markets Membrane Technology Electrodialysis (ED) - First applications back at 30 s. - Ion Separations. - Ionic Membranes (non porous).

30 - Driving Force: gradient in electrical potential. - Potential: 1-2 V. - Flat configuration. - Hundreds of anionic and cationic membranes placed alternatively. - Orthogonal electrical field. Membrane Technology Electrodialysis (ED) Membrane Technology Electrodialysis (ED)

31 Membrane Technology Electrodialysis (ED) - Ionic Membranes (non porous). - Based on polystyrene or polypropylene with sulfonic and quaternary amine groups. - Thickness: mm. - ED with reverse polarization (EDR). - ED at high temperature (60ºC). - ED with electrolysis. Membrane Technology

32 Electrodialysis (ED) - Required membrane area Mass balance (in equivalents) j da V zdc 0 m C Vc Cout Vc cin j Charge flow i: electric current density (A/m 2 ) A m : membrane surface (m 2 ) j F di i da m combining A T NVC cin cout zf V cin cout zf NAm i i η: global electrical efficiency (~0.5 commercial equipment) j: cation flow (eq/m 2 s) F: Faraday constant (96500 C/eq) N: number cells in the equipment z: cation charge (eq/mol) Membrane Technology

33 Electrodialysis (ED) - Then the required energy, E (J), is 2 E N UC I t N I RC t U C : potential gradient in a cell (V) R C : total resistance in a cell () as I ia m VC cin cout zf then 2 VC E N c z F R C t 2 VC czf ó P N RC P: required Power (J/s) Membrane Technology Electrodialysis (ED) 3 Where, the required specific energy, (J/m ), is E czf Ê VC R NV C t 2 C La cell resistance can be estimated from a model based on series of resistances where the resistances to transport are considered through two membranes and the compartments concentrate and diluted.

34 Membrane Technology Electrodialysis (ED) - How to determine operational i? Cation Transport i F t M z cd c D c DM i t F D F i t z cd cdm M t + If c = 0 DM i lim D Fz c t t M D

35 Usually: i = 0.8i lim t: transport number D: diffusion coefficient Membrane Technology Electrodialysis (ED) - Intensity Evolution versus applied potential Membrane Technology Electrodialysis (ED) - Fields of application: Water desalination. - Competing to RO. - Economically more interesting at very highh or very saltt concentrations. - Other fields of application: Food Industry. Treatment of heavy metal polluted water.

36 Membrane Technology Electrodialysis (ED) - Examples: Production of drinking water from salty water. Water softening. Nitrate removal. Lactose demineralization. Acid removal in fruit juice. Tartrate removal from wines. Heavy metal recovery. Production of chlorine and sodium hydroxide. Membrane Technology Electrodialysis (ED)

37 electrolytic Celll for the production of chlorine and sodium hydroxide with cationic membrane. Membrane Technology Electrodialysis (ED)

38 Electrolytic cell for the production of sulfuric acid and sodium hydroxide with bipolar membrane. Membrane Technology Electrodialysis (ED) Hydrogen fuel cell with a cationic membrane.