14/11/2008
Donal Keane
Introduction and history • Pervaporation is a membrane technology used to seperate liquid mixtures • It is a part of a larger family including Microfiltration, Ultrafiltration, Nanofiltration, Reverse Osmosis and Gas Separation • Pervaporation (Permeation + Evaporation) was first defined by Kober in 1917 after observing that some liquids spontaneously evaporated in sealed dialysis bags
• A liquid feed with two components to be separated is passed over the membrane surface and one component is able to pass through the membrane preferentially • The underside of the membrane is held under vacuum causing the liquid passing across to vaporise • The fraction of the feed that passes across the membrane is called the permeate, the fraction that fails, the retentate • The permeate is condensed, the retentate is recycled to the feed tank to allow further separation to occur • It is often run as a batch operation with the run terminated once the required composition in the retentate has been achieved
J = permeate flux, g/m2/hr m = mass of permeate, S = membrane surface area t = permeation time
yi = the weight of component i in permeate yj = the weight of component j in permeate xi = the weight of component i in retentate xj = the weight of component j in retentate αi = the separation factor for component i with respect to j
• The most important is the removal of water from concentrated ethanol. Typically the ethanol feed consists of about 10% water. The PV process removes water as permeate, to produce ethanol containing less than 1% water • Problems associated with azeotropic distillation are avoided. Azeotrope = ‘boil without change’. Entrainers needed during distillation to break. Ethanol water 96%. • Sulzer Chemtech are the leader in the field with more than 100 plants worldwide • The second commercial application of PV is the removal of small amounts of volatile organic compounds (VOC’s) from contaminated water using a hydrophobic membrane • Organic/organic separations are also possible but are at an early stage Pervaporation in Ireland • CIT, DIT benchtop pervaporators • Pfizers, Cork • 30m2 membrane area, 9 modules in series. ( largest plant 2400m2) PVA membranes from Sulzer, Feed predistilled to azeotrope
• There are two types of separation membranes used today 1) pore flow microporous membranes and 2) dense solution diffusion membranes which are classified according to the mechanism of separation • Simplistically, pore flow membranes separate by molecular filtration/sieving in which one of the smaller molecules move through some of the pores whereas some larger molecules are excluded, H2O = 0.296 nm, EtOH =0.43nm • In solution-diffusion one of the permeates has a larger solubility and diffusivity across the membrane • Controversy has existed in modelling mechanisms and categorizing membranes. Researchers have suggested that micropores arise in dense polymers due to fluctuation in of the polymer chains due to thermal energy. • However these pores are temporal and solution diffusion is now accepted
1.Pore flow
2. Solution Diffusion
Sulzer Pervap 2510 • Solution diffusion type membrane Consisting of a Poly vinyl alcohol (PVA) selective layer on Poly acrylo nitrile (PAN)supported on non woven fabric • Annealed/crosslinked
Type A zeolite membrane Inocermic • Zeolites are crystalline microporous aluminosilicates •This is a pore flow type membrane grown hydrothermally • on substrates •Pore diameter ~0.4 nm, H2O = 0.296 nm, EtOH =0.43
Amorphous microporous silsesquioxane prepared by ‘sol-gel’ method • This is a pore flow type membrane deposited on substrates by casting/coating from a silica gel
Sulzer bench top unit used for testing membrane stability and performance • Permeate and retentate heating bath with temperature control composition measured by density feed tank (2L) recycling pump ( 80 L/hr) flowmeter test cell (6 inch), upper and lower part attached by 4 clamps Porous steel support, O ring Valves, thermometer, safety switches
2 % w/w dissolved in water, cast on a glass plate, dried for 24 hr and peeled from substrate
• Alginate is a natural water soluble polysaccharide obtained from seaweed and is used widely in the food industry as a thickener • It has a block polymer consisting of two acid residues • It is a very hydrophillic material and its pervaporation performance has been shown to exceed PVA and other polyscaccharides such as cellulose • Despite its potential, alginate has yet to be commercialized due to mechanical instability
Performance • In a typical 8 hour experiment, permeate and retentate samples are taken every hour • With a feed of 78 % the alginate membrane removed 8% water and showed high selectivity (>98% water in permeate)
Temp/de g
Mass permeate/ g
60
14.2
70
24.4
• Alginate shows superior performance to Sulzer 2200 80 40.6 • However due to mechanical instability, the membranes can be used only once Effect of temperature on flux, PV2200, 67% EtOH feed
9 Inorganic membranes do not swell, whereas polymeric membranes do(which leads to a loss of selectivity) 9 Inorganic membranes are more chemically stable than polymeric membranes, allowing separations of strong solvents or low pH mixtures 9 Inorganic membranes are stable at high temperatures in which higher fluxes can be achieved • On the other hand inorganic membranes cost significantly more to produce than polymer membranes and can be brittle • Inorganic membranes can have other problems too such pore blocking in amorphous silica membranes due to leeching For this reason composite or hybrid membranes consisting of a polymeric/organic component and an inorganic component are being actively researched
Mixed Matrix Membranes y
y y
Vol. Adsorbed(cc/g STP)
y
The simplest composite membrane consists of a polymeric base material in which inorganic material (filler) is dispersed and locked into polymer matrix These hybrid materials have been described as class 1. Ideally the filler particles would increase mobility of the component that is more permeable in the polymer, while decreasing the compont that is less permeable The introduction zeolite and mesoporous silica (~ 3 nm pore size) has been shown to increase flux with little or no decrease in selectivity Disadvantages include interface voids (non selective macropores) and particle flocculation •Mesoporous SiO2 synthesized from a modified Stober method for use as stationary phases in chromatography columns •Pore size tailored from ~ 2-25nm •Particle size tailored ~ 40nm-5um
0.18 0.17 0.16 0.15 0.14 0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 0
100
200
300
Pore diameter(Å)
J. P. Hanrahan, D. A. Keane, M. P. Copley, J. D. Holmes, M. A. Morris, PCT IE08/000074.
• Synthesized spherical porous silica particulates sonicated in water and dispersed in polymer solution prior to casting • Particles observed to be locked into matrix but not in an ordered manner, flocculation • At present membranes mechanically unstable for use in pervaporator, need to be cast on supports (anisotropic membrane type) • The effect of pore size, particle size and loading on flux and selectivity is to be studied in a systematic study
• Class 2 hybrid materials involve the organic-inorganic interaction at the molecular level with strong ionic/covalent bonding and can avoid nonselective voids at the organic-inorganic interface and the agglomeration of inorganic particles • This can be achieved by the ‘sol-gel’ process where liquid inorganic metal oxide precursors are hydrolysed and condensed under mild conditions, and can form a homogeneous solution with a chosen polymer • GmBH patent : US7410065, Aug 2008 • Addition of inorganic oxide (titania,zirconia) precursors • Temperature stability of PVA increased from 105 to 150 deg
• Silsesquioxanes/ormosils are a hybrid organic/inorganic material in which the organic group is bonded to the siloxane backbone • ERC have shown remarkable hydrothermal stability in a sol-gel synthesized ormosil membrane . •This was achieved by the introduction of hydrophobic –CH3 groups into silica reducing siloxane hydrolysis, thus maintaining the microporosity which otherwise tends to be lost in aqueous solution due to silica dissolution or “leaching”. • The introduced hydrophobicity did not decrease flux or selectivity(over 2 yrs).
Energy Research Centre (Holland) 2008:
•POSS materials are classified by having cage structures as opposed to SS which have random or ladder structures • The materials are compatible with polymers due to the organic end groups (tethers) which may be covalently or non covalently bonded to the organic molecular chains.
• Flux
and selectivity of pristine NaAlg superior to PVA (Sulzer Pervap 2200) • Unsupported pristine membranes mechanically unstable, can be used only once • Mixed matrix (inorganic filler) membranes unstable Goals: 1. Cast membranes on support (non-woven fabric) 2. Investigate effect of pore and particle size of SiO2 filler particles on flux and selectivity 3. Investigate Class 2 hybrid (molecular) membranes