Ch 9 Gas Separation by Membranes Separation of solvent and solute in SFE-processes Problems: High pressure ( > 100 bar) Solution of Carbon Dioxide in Polymers Influence on glass transition point Retentate Feed Permeate Membrane Flow sheet of a membrane separation
Retentate OC CO2 Permeate Membrane Process
Separation by membranes p = 2.0 MPa active dense layer 1.5 mole CO2 kg/(m2 h) < 0.06 wt.-% 1.86 wt.-% P = 18 MPa, T = 323 K GKSS-membrane (organic, active dense layer)
Literature overview Reference Membrane System T, P RO, (Polyamid) 473 K, 30 MPa Wagner (1986) Kaffein Kapton® (Polyimide) Ethanol, Petroleum compounds Semenova et al. (1992, ´93, ´94) 423 K, 15 MPa Composite with Nafion® PEG, Triglycerides Sarrade et al. (1996, ´97) 333 K, 31 MPa Ceramic , NTGS-2100 (Silikon) PEG 400-600 313 K, 20 MPa Nakamura et al. (1994)
Membranes Flat sheet membranes • ROMACO, high pressure RO, (Polyamide, Pall Rochem) • PAN-Fluorinated Polymer (FP), GP, (GKSS) • PEI-FP, GP, (GKSS) • PVDF-FP, GP, (GKSS) • 6-FDA-4MPD/DABA 4:1 (Polyimide, crosslinked with ethylene glycol, University of Heidelberg) • Al2O3-TiO2, (Inocermic)
Membranes 6FDA-4MPD/DABA 4:1
Membranes Tubular membranes • Carbone membrane, (20 nm, Le Carbone-Lorraine) • ZrO2- TiO2, (Schuchmacher) • Al2O3-TiO2 -FP, (US-Filter, GKSS)
Membranes • Mechanisms of membrane transport
Membranes • Pore diameter and thickness of inorganic gas separation membranes, after van Veen et al. (1996). Inorganic Membranes
Membranes • Classification of ceramic membranes (Bonekamp, 1996).
Membranes Polymeric, Nonporous Membranes States of polymers
Effect of Temperature on the Polymeric Structure • Specific volume and free volume as a function of temperature for an amorphous polymer: A: specific volume of a liquid; B: specific volume of a glassy polymer; C: specific volume of a crystal solid; W: van der Waals volume; Tg: glass transition temperature; Tm: melting temperature.
Membranes Influences on Membrane Properties Effect of Temperature on the Polymeric Structure Effect of Pressure on the Polymeric Structure Swelling and Plasticization of Polymers Aging of Polymers
Gas Permeation through Membranes Steady state flux J : Pe : effective permeability coefficient, (integral value over the whole membrane). with Fugacity coefficient : z: compressibility factor.
Gas Permeation through Membranes The permeability coefficient for ideal–gas conditions Pressure - normalized flux Q, "membrane permeability": Separation factor for a binary mixture of component A and B:
Stofftransport Gaspermeation: P = DH: Permeationskoeffizient, D = Diffusionskoeffizient, H = Henry-Koeffizient. Trennfaktor: Einheit von P:
Gas Permeation through Membranes Joule-Thomson effect Definition of the Joule-Thomson coefficient: Joule-Thomson coefficient of carbon dioxide
Gas Permeation through Porous Membranes • A: Hagen-Poiseuille's flow, • B: Knudsen flow, • C: surface flow, • D: multilayer adsorption, • E: capillary condensation, • F: molecular sieving. Transport mechanisms through porous membranes
Gas Permeation through Porous Membranes Hagen-Poiseuille Flow The term p/RT has to be replaced by the mean density rm resulting in the following relation for Hagen-Poiseuille's flow of carbon dioxide through mesoporous membranes:
Gas Permeation through Porous Membranes Surface Diffusion and Capillary Condensation Surface diffusion is a poorly understood phenomenon The total molar surface flux is calculated by: with pore length l, porosity e, and density r of the solid. The surface diffusion coefficient Ds is a function of the amount of gas q adsorbed on the surface. The effective surface coverage qe of gas can be described by adsorption isotherms, for monolayer adsorption: Langnuir isotherm: For multilayer adsorption BET isotherm:
Gas Permeation through Porous Membranes • Permeability of carbon dioxide through vycor glass, after Rhim and Hwang, (1975). Maximum permeability: point where capillary condensation takes place.
Gas Permeation through Porous Membranes The capillary condensation pressure (pt) can be predicted by the Kelvin equation: where t represents the thickness of the adsorbed layer. For non-cylindrical capillaries the term 2 cosq (r-t)/r2has to be replaced by another relation.
Gas Permeation through Porous Membranes Adsorption Isotherms at Sub- and Supercritical Conditions • Isotherms of the total amount of adsorbed carbon dioxide on two different silica gels. Silica gel with 10 nm mean pore diameter, -- 308.15 K, -- 318,15 K, data replotted from Bamberger (1996) silica gel with 1 nm mean pore diameter, -- 313,15 K, -- 333,15 K, data replotted from Ozawa and Ogino (1972).
Gas Permeation Through Nonporous Membranes The flux Ji of component i is given by Fick’s law: Temperature dependence of the permeability coefficient P = D S: Temperature dependence of diffusivity and solubility:
Gas Permeation Through Nonporous Membranes Sorption of gases in all types of amorphous polymers shows that the solubility of CO2 increases with decreasing temperature (van Krevelen, 1990): Transport mechanism of penetrants through polymers differs below and above the glass transition of polymers. The diffusion of penetrants through glassy polymers is a highly non-linear function depending on the state of the polymer.
Concentration Polarization Concentration polarization at steady state conditions; left: normal concentration polarization; right: gel-layer formation
Concentration Polarization A liquid layer forms on the retentate surface of the membrane when the solubility ís reached Change in concentration when CO2 is removed by a membrane process 2 phases 1 phase Solubility of fatty acid ethyl esters in carbon dioxide (Riha, 1996).
Membrane Test Cell Membrane
Experimenatal Membrane Test Tubular membrane test cell.
Pure Gas Permeation • TEOS, pressure-normalized CO2 flux vs. upstream pressure –– 34 °C, –– 49 °C,–– 66 °C –– 0.1 MPa 25 °C manufacturer, p = 0.3–1.9 MPa, 20-mm TC, membrane #2, values taken after 30 minutes.
Membranes Inorganic Membranes: Titania-Alumina Composite Membranes Schematic representation of titania-alumina membrane cross-sections • TEOS • Substructure: commercially available g-Al2O3 membrane of 18 mm in diameter, mean pore diameter of 5 nm, porosity 50%, surface roughness appr. 0.2 mm. Substructure modified by tetraethylorthosilicate (TEOS) treatment.
Polymeric Membranes An intermediate ultrafiltration layer of polyetherimide (PEI) is applied to a polyester support fleece. The PEI-layer is then coated with a selective layer of poly (tetrafluoroethylene) (PTFE). PEI-Teflon Membranes • Chemical structure of polyetherimide (PEI), trademark Ultem by GE. • Repeat unit of poly (2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole) [PDD] / PTFE, commercially available under the trademark (AF 2400 Du Pont)
Gas Permeabilities of PTFE-Polymers 1 Barrer = 10-10 (cm3 cm)/(cm Hg s cm2) Gas Teflon® PTFE AF2400 CO2 2800 12 O2 990 4.2 H2 4100 # He 2700 # H2 2200 9.8 N2 350 1.4 CH4 340 # C2H4 350 # C2H6 180 # Structure of Monomers of the AF2400-polymer
Pure Gas Permeation • TEOS membranes: pressure-normalized CO2 flux vs. upstream pressure, –– #1 increasing pressures, –– #1 decreasing pressures, –– #2 increasing pressures, p = 0.3–1 MPa; T = 50 °C, 20-mm TC.
Pure Gas Permeation: Organic Membranes • PEI-TE10x pressure-normalized CO2 flux as a function of upstream and permeate pressure for increasing upstream pressures (solid symbols), and decreasing upstream pressure (open symbols): –– 1 MPa transmembrane pressure difference,–– downstream pressure at atmospheric pressure, T = 50 °C, 47-mm TC.
Pure Gas Permeation: Organic Membranes • CO2 flux at different upstream pressures as a function of transmembrane pressure difference for PEI-TE1x, –– 7 MPa, –– 9 MPa, –– 12 MPa, –– 14 MPa, –– 15.9 MPa, –– 18.1 MPa, T = 50 °C, 47-mm TC.
Influence of Repeated Use • PAN-AF2400-Membran, 50 °C
CO2-Permeate Flow of PEI-AF2400-1x Related to PEI-AF2400-10x • –– increasing retentate Pressure (Feed side), • –– decreasing retentate pressure (Feed side), T = 50 °C, 47-mm test cell.
Diffusion Coefficient of CO2 • 7-m Teflon AF 2400 layer on a PEI-AF2400-10x Membran, 50 °C, p = 1 MPa; 50 °C, p2 = atmospheric pressure; 35 °C, p2 = atmospheric pressure (after Merkel et al. (1999).
CO2 Permeate Flow TEOS - membrane 2-HHU-1 Sartorelli 2001
TEOS-Membrane: Feed: P = 23 MPa; T = 60° C Sartorelli 2001
Separation Factors TEOS-Membrane 2-HHU-1 Sartorelli 2001
Permeate Flow AF 2400 - Membrane Permeate flows of CO2 at 50 °C, 60 °C and 70 °C, Exp. series: 2-FP-X10 (18 MPa/50 °C) and 3-FP-X10 (23 MPa/60 °C).
CO2 Permeate Flow: AF 2400 membrane Sartorelli 2001
Separation Factor: AF 2400 membrane Sartorelli 2001