Commercial pervaporation membranes

A list of typical commercial pervaporation membranes [23] is given in Table 3.1. Commercial hydrophilic membranes are very often made of polyvinyl alcohol (PVA), with differences in the degree of crosslinking. Commercial hydrophobic membranes often have a top layer in polydimethyl siloxane (PDMS). However, a wide variety of membrane materials for pervaporation can be found in the literature, including polymethylglutamate, polyacrylonitrile, polytetrafluoroethylene, polyvinylpyrrolidone, styrene-butadiene rubber, polyacrylic acid, and many others [24]. A comprehensive overview of membrane materials for pervaporation is given by Semenova et al. [25],... 

Ortiz I. 2008. Recovery of key components of bilberry aroma using a commercial pervaporation membrane. Desalination 224 34-39. 

Various experiments, carried out with different photocatalytic reactors and commercial pervaporation membranes, by varying the relative weight of pervaporation with respect to photocatalysis, have demonstrated that the intensification factor depends solely on the parameter 5 previously defined, as is apparent in Fig. 3.7. 

The intensification factor vs 5 (adapted from Camera-Roda and Santarelli, 2007).The various symbols indicate experiments carried out with different photocatalytic reactors and different commercial pervaporation membranes. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

Pervaporation Membranes Pervaporation has a long history, and many materials have found use in pervaporation experiments. Cellulosic-based materials have given way to polyvinyl alcohol and blends of polyvinyl alcohol and acrylics in commercial water-removing membranes. These membranes are typically solution cast (from... 

So far, the separation of azeotropic systems has been restricted to the use of pressure shift and the use of entrainers. The third method is to use a membrane to alter the vapor-liquid equilibrium behavior. Pervaporation differs from other membrane processes in that the phase-state on one side of the membrane is different from the other side. The feed to the membrane is a liquid mixture at a high-enough pressure to maintain it in the liquid phase. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, maintaining it in the vapor phase. Dense membranes are used for pervaporation, and selectivity results from chemical affinity (see Chapter 10). Most pervaporation membranes in commercial use are hydrophyllic19. This means that they preferentially allow... 

Both Mitsui [26] and Sulzer [27] have commercialized these membranes for dehydration of alcohols by pervaporation or vapor/vapor permeation. The membranes are made in tubular form. Extraordinarily high selectivities have been reported for these membranes, and their ceramic nature allows operation at high temperatures, so fluxes are high. These advantages are, however, offset by the costs of the membrane modules, currently in excess of US 3000/m2 of membrane. 

Both of the current commercial pervaporation processes concentrate on the separation of VOCs from contaminated water. This separation is relatively easy, because organic solvents and water have very different polarities and exhibit distinct membrane permeation properties. No commercial pervaporation systems have yet been developed for the separation of organic/organic mixtures. However, current membrane technology makes pervaporation for these applications possible, and the process is being actively developed by a number of companies. The first pilot-plant results for an organic-organic application, the separation of methanol from methyl tert-butyl ether/isobutene mixtures, was reported by Separex in 1988 [14,15], This is a particularly favorable application... 

The principal problem hindering the development of commercial systems for organic/organic separations is the lack of membranes and modules able to withstand long-term exposure to organic compounds at the elevated temperatures required for pervaporation. Membrane and module stability problems are not... 

Table 3.1 Pervaporation membranes used in commercial applications [23].

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

These developments result from the introduction of composite membranes, originally developed in the 1970s primarily, for desalination by reverse osmosis. Application of the same membrane fabrication techniques to pervaporation membranes radically improved their performance and spurred commercial utilization. Today, pervaporation and vapor permeation plants are widely used to dehydrate volatile organics and separate other mixtures, primarily in the pharmaceutical and fine chemical industries. 

Pervaporation membrane reactors are not a recent discovery. The use of a PVMR was proposed in a U.S. patent dating back to 1960 [3.6]. Though the technical details on membrane preparation and experimental apparatus were rather sketchy, the basic idea was described there, namely, the use of a water permeable polymeric membrane to drive an esterification reaction to completion. A more detailed description of a PVMR can be found in a later European patent [3.7], which described the use of a flat membrane (commercial PVA or Nafion ) placed in the middle of a reactor consisting of two half-cells. The reaction studied was the acetic acid esterification reaction with ethanol. For an ethanol to acetic acid ratio of 2, liquid hourly space velocities (LHSV) in the range of 2-5, and a temperature of 90 °C complete conversion of the acetic acid was reported. The use of PVMR for this reaction shows promise for process simplification, as indicated schematically in Figure 3.2, which shows a side-by-side comparison of a conventional and a proposed PVMR plant for ethyl acetate production. 

As permselective barriers, synthetic membranes have been employed in a variety of applications, which include dialysis, mirofiltration, ultrafiltration, reverse osmosis, pervaporation, electrodialysis, and gas separation. Synthetic membranes also find special applications as permselective barriers for ion-spedfic electrodes, biosensors, controlled release, and tissue-culture growth. Some commercial polymer membranes are listed in Table 5.20. 

Since the pioneering work of Tehennepe et al. [152] in 1987, many efforts have been made filling the polymeric matrix with zeolites in order to improve their stability. There are several companies that offer pervaporation organic membranes and composite membranes such as Sulzer Chemtech [153]. Commercial pervaporation and vapor permeation installations utilize polymeric membranes, like PVA (Sulzer Chemtech), polyimide (Vaperma), per-fiuoropolymers (MTR and Compact Membrane Systems), and polyelectrolytes (GKSS) or ceramic membranes, like zeolite A (Mitsui, Mitsubishi, Inocermic) and silica... 

Membrane distillation is similar to pervaporation since phase change is involved in the process. When feed liquid (usually water) is in contact with a nonwetted porous hydrophobic membrane, water does not enter into the pores because the feed Hquid is maintained below a threshold pressure, the liquid penetration pressure of water. Only water vapor permeates through the pores from the feed to the permeate side. The driving force is the vapor pressure drop from the feed to the permeate side, since the permeate temperature is maintained below the feed temperature. Commercial hydrophobic membranes made of polypropylene, poly(vinylidene fluoride) and poly-... 

The current commercial zeolite membranes, developed for pervaporation, are not yet useful in gas separations (H2/CO2 selectivity for NaA membranes of Mitsui and Inocermic are 6 and 5.6, respectively) because of the presence of large inter-crystalline defects. They, furthermore, participate in the separation process. During pervaporation the water fills the intra-crystalline and intercrystalline pathways. However, much effort is in progress to produce defect free zeolitic membrane also for gas separations. In this chapter the application of zeolite membranes in gas separations is reported and deeply discussed. The main strategic methods used for the membrane preparation and mass transport through zeolite membranes are also dealt with. 

Urtiaga, A., Casado, C., Asaeda, M. and Ortiz, I. 2006. Comparison of SiO3-ZrO2-50% and commercial SiOj membrane on the pervaporative dehydration of organic solvents. Desalination 193 97-102. 

The commercial PVA membranes GFT-1005 and Tl-b (PVA-based) were used by Bendict et al. (2003, 2006) in a stirred batch reactor coupled with an ESU pervaporation cell for the esterification of lactic acid/succinic acid and ethanol. In these two studies, two solid catalysts Amberlyst XN-1010 and Nation NR50 were used. The kinetics of pervaporation were studied to obtain a correlation for the flux of water... 

More recently, composite polymeric catalytic membranes consisting of a dense layer of a mixed-matrix of tiny particles of Amberlyst-35 dispersed in PVA cross-linked with maleic add cast over a commercial PVA membrane (PERVAP 1000), were effidently used in the pervaporation-assisted esterification of acetic acid and ethanol. After 8 h of reaction, a 60% increase in conversion was observed for the catalytic membrane configuration, compared to an inert membrane/fiuidized-bed configuration. 

A layout consisting of a semibatch tank reactor, loaded with a sulfonic acid functionalized poly(styrene divinylbenzene) copolymer (Amberlyst) as catalyst and connected to an external pervaporation module equipped with a commercial PVA membrane, has been used for esterification studies of acetic acid/isopropanofi ° and lactic acid/ethanofi systems. A similar arrangement was reported by Lauterbach and Kreis for the propionic acid/propanol esterification. 

Buchaly et al propose a hybrid process combining reactive distillation and pervaporation for the propionic add/propanol esterification. The membrane module equipped with a commercial PVA membrane is located in the distillate stream in order to selectively remove the produced water. The desired product, n-propyl propionate is removed at the bottom of the distillation column, which is packed with Amberlyst 46 as catalyst, in a reactive zone. 

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