Pervaporation membrane fouling

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

The advantages of membrane distillation are associated with relatively lower energy costs as compared to competing processes such as distillation, reverse osmosis, and pervaporation (PV) much lower membrane fouling as compared with microhltration (MF), ultrahltration, and RO a quantitative rejection of nonvolatile solutes from the feed stream lower operating pressure and temperatures, without sacrihcing flux as compared with competing processes. 

In the production of bioethanol, sugar is fermented, yielding low concentrated alcohol solutions that are recovered from the broth by ultrafiltration or pervaporation membranes. Ultrafiltration and pervaporation of bioethanol from fermentation broth are IG biofuel processes. As in all bioreactor-coupled membrane processes, membrane fouling and drop in permeate fluxes during continuous operation are the main concerns. 

Fouling occurs mainly in micronitration/ultraiiltration where porous membranes which are implicitly susceptible to fouling are used. In pervaporation and gas separation with dense membranes, fouling is vinually absent. Therefore, pressure driven processes will be emphasized but also here the type of separation problem and the type of membrane used in these processes detemiine the extent of fouling. Roughly three types of foulant can be distinguished ... 

Figure 20-48 shows Wijmans s plot [Wijmans et al.,/. Membr. Sci., 109, 135 (1996)] along with regions where different membrane processes operate (Baker, Membrane Technology and Applications, 2d ed., Wiley, 2004, p. 177). For RO and UF applications, Sj , < 1, and c > Cl,. This may cause precipitation, fouling, or product denatura-tion. For gas separation and pervaporation, Sj , >1 and c < ci. MF is not shown since other transport mechanisms besides Brownian diffusion are at work. 

Concentration polarization can dominate the transmembrane flux in UF, and this can be described by boundary-layer models. Because the fluxes through nonporous barriers are lower than in UF, polarization effects are less important in reverse osmosis (RO), nanofiltration (NF), pervaporation (PV), electrodialysis (ED) or carrier-mediated separation. Interactions between substances in the feed and the membrane surface (adsorption, fouling) may also significantly influence the separation performance fouling is especially strong with aqueous feeds. 

Analytical pervaporation is the process by which volatile substances in a heated donor phase evaporate and diffuse through a porous hydrophobic membrane, the vapour condensing on the surface of a cool acceptor fluid on the other side of the membrane. Surface tension forces withhold the fluids from the pores and prevent direct contact between them. A temperature difference that results in a vapour pressure difference across the membrane provides a strong driving force for the separation, which also occurs in the absence of a temperature gradient. Evaporation will occur at the sample surface if the vapour pressure exceeds that at the acceptor surface. One important feature of pervaporation modules used for analytical purposes is the air gap between the donor phase and the hydrophobic membrane, which avoids any contact between them and reduces the problems associated with fouling of the membrane. 

Fadeev AG, Meagher MM, Kelley SS, Volkov VV. 2000. Fouling of poly[-l-(trimethylsilyl)-l-propyne] membranes in pervaporative recovery of butanol from aqueous solutions and ABE fermentation broth. J. Membr. Sci. 173 133-144. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

When pervaporation is used to clean up end-of-pipe effluents, there is a possibility of contamination of the effluent with oil emulsions and such material fouls the membrane surface, severely reducing its capacity to pass solvents. 

Fortunately another transport resistance, which is extremely important in the filtration processes and in reverse osmosis, namely fouling, is of no concern in pervaporation or vapor permeation with polymeric membranes. In these membranes no pores exist that can be blocked by any precipitation out of the liquid or vapor phase. Even if precipitation, e.g. of salts in dehydration processes, does occur the growth of the salts crystals may attack and eventually destroy the separating layer of the membrane, but will usually not influence the flux of water to the membrane. 

A novel membrane module design. Top Cross-sectional view of a membrane-coated channel. Bottom Channel flow pattern. High surface-area modules could reduce the relative cost of pervaporation. The short, narrow, non-uniform, multi-path channels that create higher shear ate formed. The turbulence promoting pathways could go a long way to mitigate fouling in ultrafiltration and microfiltration membrane systems. 

A promising technique for residual monomer removal is pervaporation, as no additional chemicals are needed for this membrane process and the energy costs are typically low. It has been shown that pervaporation can remove a considerable amount of acryhc monomer from polymethylmethacrylate (PMMA) latexes [15]. Apparently, the Hmiting factor for mass transfer does not occur in the polymer particles, mainly because of the high specific area of the polymer-water interface as compared to the membrane area. Although the high initial costs, as well as fouling of the membrane surface with the polymer particles, are potential drawbacks, pervaporation may thus be expected to provide a viable alternative. 

In order to minimize fouling problems of hydrophobic membranes traditionally used in MD, Zwijnenmberg et al. (2005) proposed the use of dense pervaporation (PV) type membranes and developed a process called solar-driven PV. This process permits processing seawater and brackish water without pre-treatment giving constant fluxes in time and producing high quality water in a single step. 

The main problem in membrane processes, especially for UF and MF separations, is the decrease of permeate flux caused by concentration polarization and fouling, whereas other membrane processes such as gas separation and pervaporation are less affected. Different approaches have been studied to reduce fouling. Hybrid systems using different types of membrane operations (e.g., distillation, dialysis, NF, pervaporation, and osmosis) prevent microbial fouling, offering a strong potential for the use of new types of thin-film composite membranes. 

Ionic membranes are characterised by the presence of charged groups. Charge is, in addition to solubility, diffusivity, pore size and pore size distribution, another principle to achieve a separation. Charged membranes or ion-exchange membranes are not only employed in electrically driven processes such as electrodialysis and membrane electrolysis. There are a number of other processes that make use of the electrical aspects at the interface membrane-solution without the employment of an external electrical potential difference. Examples of these include reverse osmosis and nanofiltration (retention of ions), microfiltration and ultrafdtration (reduction of fouling phenomena), diffusion dialysis and Donnan dialysis (combination of Dorman exclusion and diffusion) and even in gas separation and pervaporation charged membranes can be applied... 

An overview of mass transfer correlalion.s in membrane processes can be found in ref. J In microflltration and ulirailllration. the diffusion coefficients of the retained macromolecuies, or suspended panicles are small relative to those which apply to the retained components in reverse osmosis, gas separation and pervaporation. In addition, the fluxes in microflltration and ultrafiltration are large relative to those in pervaporation and gas separation. Hence, the consequences of concentradon polarisation in the case of microfiltradon and ultrafiltration are very severe. The consequences of fouling will be discussed later. 

---