Article pubs.acs.org/IECR
Perfluoro−coated Hydrophilic Membranes with Improved Selectivity Yu Huang, Richard W. Baker, and J. G. Wijmans* Membrane Technology & Research, Inc., 39630 Eureka Drive, Newark, California 94560 ABSTRACT: Most polymeric solvent dehydration pervaporation membranes are made from cross-linked hydrophilic polymers such as cellulose esters or polyvinyl alcohol. At low feed water concentrations, these membranes have very good water/ethanol selectivities. However, the membranes absorb water and swell significantly at higher feed water concentrations, leading to a significant loss of selectivity. We have found that coating these hydrophilic membranes with a thin hydrophobic perfluoropolymer layer can effectively prevent the swelling of the underlying hydrophilic membranes. These coated membranes display a higher selectivity than either the hydrophilic base membrane or the perfluoro coating material alone, when measured with the same feed solution. This counterintuitive result is explained by analyzing the transport behavior of the coated membrane.
1. INTRODUCTION Development of economical cellulose-to-ethanol conversion technology will transform the transportation fuel market. Six U.S. demonstration plants, each producing 5−20 million gal/y of cellulosic ethanol will come online in 2012. Several more are expected to start up in 2013. If these demonstration projects are successful, full-scale plants will follow. To meet the Department of Energy’s 2022 cellulosic bioethanol production target, as many as 30 plants, each producing an average of 50 million gal/y of ethanol, must be built.1 The demonstration plants being constructed all use distillation to convert dilute ethanol-containing fermentation broth to azeotropic ethanol (90−93 wt % ethanol). A molecular sieve adsorption system is then used to produce 99.7 wt % dry ethanol. Replacement of the molecular sieve units by membranes in these next generation bioethanol plants is a major opportunity for the membrane industry. MTR has been developing this technology with Dr. Lee Vane of the U.S. EPA for a number of years. The low energy distillation-membrane process we are developing is described in a number of publications.2−4 The current membranes used to dehydrate ethanol are based on hydrophilic polymers such as polyvinyl alcohol, cellulose esters, chitosan, and the like.5−8 These polymers have very good selectivities and reasonable permeances when used to separate water from ethanol solutions containing 0−20 wt % water. However, at higher feed water concentrations, the membranes lose selectivity because of excessive water sorption, leading to swelling and plasticization. Recently, we and others9−11 have shown that perfluoropolymer (PFP) membranes have useful properties for this separation. The sorption of ethanol/water by these polymers is less than 1 wt % at all feed concentrations. As a consequence, membranes made from these polymers maintain constant fluxes and selectivities at all feed water concentrations. However, permeances and selectivities are at the low end of what is needed for a commercial process. This paper describes the properties of multilayer composite membranes consisting of a hydrophilic polymer membrane, overcoated with a thin PFP protective layer. We will show that this combination has a better selectivity than either the base hydrophilic membrane material or the PFP topcoat material. © 2012 American Chemical Society
2. THEORETICAL BASIS Membrane permeation in pervaporation is quantified in terms of membrane permeance and selectivity using the solution-diffusion equation.12,13 P ji = i (pif − pip ) (1) S 3 2 where ji is the molar flux (cm (STP)/cm ·s), S is the membrane thickness, pfi and ppi are the partial vapor pressures of component i on the feed side and permeate side of the membrane, and Pi is the permeability of the membrane material, usually expressed in Barrer (1 × 10−10·cm3(STP) cm/(cm2·s·cmHg)). This equation describes both gas permeation (where pfi and ppi are the gas phase partial pressures) and pervaporation (where pfi is the vapor pressure of component i in equilibrium with the feed liquid). Because the thickness of the selective layer in composite membranes is difficult to measure, membranes are typically characterized by permeance (P/ i S ) which is expressed in gas permeation units or gpu (1 × 10−6 cm3(STP)/(cm2·s·cmHg)). When the permeate pressure is very small compared to the feed pressure, as is the case for all the experimental data reported in this paper, eq 1 can be simplified to P ji = i pif (2) S The membrane separation performance is given as the ratio of the permeances or permeability of components i and j: P /S P αij = i = i Pj/S Pj (3) where αij is the selectivity of the membrane for component i over component j. In this paper, component i represents water, and component j represents ethanol. Special Issue: Baker Festschrift Received: Revised: Accepted: Published: 1141
August 2, 2012 October 18, 2012 October 19, 2012 October 19, 2012 dx.doi.org/10.1021/ie3020654 | Ind. Eng. Chem. Res. 2013, 52, 1141−1149
Industrial & Engineering Chemistry Research
Article
Figure 1. Chemical structures of the perfluoropolymers Teflon AF, Hyflon AD, and Cytop.
in all solvents except a few perfluorinated compounds and are unaffected by almost all other chemicals including acids and alkalis and strong oxidizing agents. These characteristics make them ideal membrane materials for use in hostile environments. The permeation properties of these three materials were reported in an earlier paper.9 Cytop has the highest selectivity but lowest permeability. Teflon AF has the lowest selectivity but highest permeability. Hyflon has intermediate properties. We made trial membranes from all of these polymers, and the Hyflon membranes appeared to have the best balance of properties for ethanol dehydration, so for this work, we used the Hyflon AD polymer. The PFP membrane from this point onward corresponds to a Hyflon AD membrane. A schematic diagram of a PFP-coated cellulose ester membrane is shown in Figure 2.
The permeance and selectivity described above are intrinsic properties of the separation membranes and are universally used to describe permeation through gas separation membranes. We prefer to report pervaporation data as permeances and selectivities in the same way because these terms are normalized for driving force, and results obtained at different conditions or driving forces can be compared.13 This procedure also allows pervaporation and vapor permeation data from different studies to be compared.
3. EXPERIMENTAL METHODS 3.1. Membrane Materials. The cross-linked hydrophilic membranes used in this work are cellulose ester-based membranes custom-made for MTR. All the hydrophilic membrane samples used in this work were from the same batch of production; therefore, the thickness of the selective layer is expected to be within a narrow range for all cellulose ester membranes tested. The pure PFP membranes and the PFP topcoats on cellulose ester base membrane were made at MTR via a dip coating method using a laboratory film coating apparatus of the type developed by Riley and described in reference 14. Dilute solutions containing 0−1% perfluoropolymer were dissolved in a perfluoro solvent; this hydrophobic perfluoro solvent has no effect on the hydrophilic cellulose ester layer during coating. After coating, the solvent was removed via evaporation by passing the membrane through an oven at about 100 °C. The thickness of the perfluoro polymer layer depends on the coating solution concentration. The higher the PFP concentration in the coating solution, the thicker the PFP coating layer. For all coating solutions, the final PFP coating layer thickness is in the submicrometer range. For pure PFP membranes, the coating layer thickness can be estimated from the measurement of the nitrogen gas permeance; however, this gas permeation technique is not applicable to top coating of PFP onto a very low nitrogen permeance cellulose ester membrane (
