Pervaporation membrane testing

Some efforts have already been made to develop ceramic pervaporation membranes, especially silica and zeolite membranes, which are both hydrophilic membranes. Silica pervaporation membranes have been developed by ECN, The Netherlands. The membranes were tested in a pilot installation of 1 m2 membrane surface at Akzo Nobel and other companies in the Netherlands [34, 35]. 

Most binary system/pervaporation membrane combinations are tested as above with low permeate pressures. The following relationship can be extracted from such results for both fast and slow permeating components ... 

Example 26.3. Laboratory tests of a pervaporation membrane exposed to liquid with 90 weight percent ethanol and 10 percent water at 60°C showed a flux of 0,20 kg/ra -h and a permeate compostion of 7.1 percent ethanol when the downstream pressure was 15 mmHg. (a) Calculate the permeability of the membrane to ethanol and to water at the test conditions and the selectivity for water. (6) Predict the local permeate composition for 90 percent ethanol and 60°C if the downstream pressure is kept at 30 mm Hg by a water-cooled condenser. What is the condensing temperature (c) Calculate the local permeate composition for 95 percent, 99 percent, and 99.9 percent ethanol at 60°C and 30 ram Hg assuming the permeabilities are the same as for part... 

In 2012, Yeh and co-workers first applied ENMs in water/ethanol pervaporation [112]. As shown in Fig. 13.16, the separation membrane system consists of a cross-linked PVA hydrophilic top layer, a cellulose nanofibrous buffer layer, an ENM layer with high porosity and fully interconnected pore structure, and a PET support layer. In water/ethanol pervaporation test, the separation factor of the new membrane could reach 80.8, and flux was 765 kg/m h, which were significantly higher than conventional membranes (e.g., Sulzer 1210). Using ENMs promised the novel pervaporation membrane a higher permeate flux than conventional ones, and using the cellulose nanofibrous layer prevented the penetration of the barrier layer. 

Example 18.4-1 Volatile organic carbon (VOC) recovery You want to reduce trichloroethylene concentration from 0.1 % to 0.001% in a stream of water flowing at 11/sec.To do so, you are testing a pervaporation membrane which is 1.3 pm thick (excluding the support). The membrane has the following properties... 

Bowen et al. [94] made a B-MFI membrane on a monohthic support. The pervaporation fluxes and selectivities of several alcohol/water mixtures were comparable to similar tubular-based B-MFI membranes, demonstrating the scale-up, although, for pervaporation, the quality requirements are much more forgiving. Kuhn etal. tested a multicharuiel high-silica MFI membrane for ethanol/water separation. The membrane was supphed by NGK Insulators and, also, in this case, the multicharuiel membrane measures up to its tubular counterparts [95] (Figure 10.8). 

Recently, a novel hydrophylic polymer membrane based on poly(allylamine hydrochloride) (PAA.HC1)/PVA, crosslinked with GA, has been also tested for methanol dehydration by pervaporation technique [33], Even if the reported results show a small selectivity of the last type of membrane, the blend s composition, the curing degree and the process conditions (temperature, feed concentration, etc.) could be used to obtain a better separation of methanol. 

PVA-PAcr.Ac. membranes have been tested also for ethanol separation from ethanol/toluene mixture, by using pervaporation technique. The reported data concerning the separation process characteristics are presented in table 8. 

Recently [43] Gao et al. applied a zeolite-fiOlled polyvinyl alcohol (PVA) membrane in esterification and acetalization reactions. Zeolites NaA, KA and CaA as well as NaX were loaded into PVA up to 27 wt% and the composites tested in selective water removal during reaction. A pervaporation cell with a membrane area of 22.9 cm was coimected to a collection system kept at a vacuum of 0.1 mm Hg. A sulfonated resin was used as Bronsted acid catalyst in the esterification mixture (120 ml). Figure 28 shows the progress of the esterification of salicylic acid and methanol at 60°C. The reaction is accelerated considerably as a result of the water removal. 

The lack of methods for a fast and reliable assessment of membrane quality is stiU one of the outstanding issues in zeolite membrane development. The usual meaning of the term quality relates to the ability of the membrane to carry out a given separation, therefore, is a system-specific property and the universal membrane quality test does not exist. In general, specific permeation measurements at different temperatures, either of single gases (or vapors) or of multicomponent mixtures in the gas or liquid (pervaporation) phase, provide extremely useful information on the effective pore structure of the membrane, on the... 

Tetrahydrofuran, THF, is an important industrial solvent and forms an azeotropic mixture at 5.3 wt% with water (see Table 10.3). To separate water/THF, Li et al. [148] tested the pervaporation performance of different hydrophihc zeolite membranes, zeolite A, zeohte Y, MOR, and ZSM-5. The preliminary test showed that the separation factor increased as the Si/Al ratio of the zeohte decreased, except for the case of zeolite A. This fact is probably due to the lower quality of this membrane with respect to the others since in the permeation of triisopropylbenzene (TIPB), showed the highest flux, 3.1 g/m h, indicating the presence of nonselective defects. Therefore, the best results were obtained with zeolite Y, rendering a separation factor of 300 with a water flux of 2.24 kg/m h at 60°C. The water flux increased with water concentration in the feed, up to a value of 15 wt%, indicating that the zeolite was saturated, as was the same for the case of water/ethanol mixtures in zeolite A, previously described. At the same time, the separation factor decreases as water concentration decreased. The stabihty of the membrane was also studied, showing a stable performance after 35 h of operation. 

Pervaporation can also operate in batch mode, and this is done typically when testing membranes for small plants and for some larger multipurpose plants. Batch pervaporation systems are robust, well proven, and flexible in operation. The pumparound rate on batch systems is normally set high to give a low permeate quantity per pass. Pervaporative cooling effects are small, and such systems can be built with a single preheater and unheated modules (Fig. 3). 

Performance of a specific membrane with a particular feed is typically determined in a laboratory experiment. A heated quantity of feed is run over the membrane in a batch pervaporation test, and samples of feed and permeate are taken periodically and analyzed. Permeate rate is also measured. The temperature of the circulating liquid is thermostatically controlled and the permeate pressure is kept low, perhaps 5-lOmbar. Overall flux rates for permeating and nonpermeating components are determined by mass balance and plotted together with permeate composition against the feed composition. 

In an early study of pervaporation using n-heptane at 1 atm and 99°C the flux was inversely proportional to the thickn s of the dense polymer film, as expected, but the flux increased only slightly as the downstream pressure was decreased from 500 to 50 mm Hg. This is consistent with Eq. (26.38) and a value of 5 or more for pC. Other studies with pure feed liquids have given similar results, and direct measurements have shown very nonlinear concentration profiles in the membrane. However, in commercial applications of pervaporation, the liquid feed usually has a low concentration of the more permeable species, so the swelling of the membrane and the resulting nonlinear effects are not as pronounced as when testing pure liquids or splutions of high concentration. 

Similar TS-1 films have been applied for phenol hydroxyl-ation reaction to dihydroxybenzenes (hydroquinone and catechol) [354] and catalytic oxidation of styrene to benzaldehyde and phenylacetaldehyde [355] with hydrogen peroxide as oxidant in batch-type membrane reactors. The dihydroxybenzenes and phenylacetaldehyde selectivity values increased with in-framework Ti content. In order to reduce the TS-1 membrane costs, Chen et al. [356] have successMly synthesized TS-1 on mullite tubes by replacing TPAOH with TPABr/EtjNH system (4% of the initial cost). The catalytic activity was tested in the probe reaction of isopropyl alcohol oxidation with hydrogen peroxide under pervaporation condition at 60°C. In general, future work on TS-1 film catalysts is required to improve mass transfer resistances and reaction conversion without compromising selectivity. 

In a kilogram-scale reaction-pervaporation unit, the method has been tested extensively. The apphed membranes showed high permeability and selectivity towards water during the whole reaction period. Besides that, the membranes appeared to be thermally and chemically stable for the reaction conditions applied. For this specific application the energy savings as compared to conventional methods are estimated to be more than 40%, and the reactor efficiency can be increased by at least 30% [99, 100]. 

The desulfurization of liquid fuels using pervaporation has been increasingly investigated over the last few years [84]. As middle distillates contain mainly aromatic sulfur compounds, desulfurization membranes tend to make use of developments in aromatic-aliphatic separation. The most frequently used membrane materials investigated for the desulfurization of liquid hydrocarbon mixtures are polyurea-polyurethane, polysiloxane. Nation, cellulose triacetate, and poly-imide [84]. In addition to a range of processes for the desulfurization of naphtha fractions patented by ExxonMobil, Transionics, and Marathon Oil, only the S-Brane process developed by W. R. Grace and Sulzer has been tested beyond the laboratory scale [84]. 

In a distinct vein, BC/chitosan membranes have been tested for pervaporative separation of binary aqueous-organic mixtures (ethanol/water) [111]. The substantially high pervaporative separation index (350 kg. x.m. h 0 and low activation energy (10 kj.mol ) are indicative of the high potential of BC/chitosan membranes in the pervaporative separation of ethanol/water azeotrope. Targeting to mimic the intrinsic antimicrobial properties of chitosan on BC nanofibrils, nanostructured BC nanocomposite membranes were obtained by surface functionalization with aminoalkyl groups (Figure 2.14) [114]. These bioactive nanostructured membranes also presented improved mechanical and thermal properties and may be useful for biomedical applications. 

Pervaporation was next carried out to test the selectivity of polyaniline membranes toward acetic acid-water mixtures. Different feed ratios of acetic acid and water were pervaporated through both undoped and doped polyaniline, as shown in Fig. 33.18, where the feed water content is plotted versus the permeant water content. For comparison the vapor-liquid equilibrium curve for acetic acid-water [78] is plotted just above the line of no separation. From Fig. 33.18 it is clear that undoped polyaniline has a small preference for permeating water over acetic acid at essentially any composition. However, this small preference at an average water permeability of about 0.5 g mm/(m--h) is too low to have any utility. More interesting is fully HCl-doped polyaniline, which permeates water over acetic acid in a much more selective fashion. In fact, even with a mixture of 859f acetic acid-15% water, at least 93 wt % of the permeant was water. It should be pointed out that when undoped membranes are used in the presence of acids, they will partially dope the polyaniline, the extent depending on the pH of the acid used. However, when a... 

One of them employs membrane-based separation processes connected to the esterification reaction. In this respect, vapor permeation and pervaporation process have been tested and dn-ee different layouts have been reported for ethyl lactate production. In one of them, membrane module is located outside the reactor unit and the retenate is recirculated to the reactor." " In another scheme, the membrane module is placed inside the reactor, but the membrane does not participate in the reaction directly and simply acts as a filter," " and in the third configuration, membrane itself participates in die reaction catalysis (catalytic membrane reactor)." Different hydrophilic membranes, such as polymeric, ceramic, zeolites and organic-inorganic hybrid membranes were tested. ... 

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