Alcohols, dehydration with membranes

Water can be removed from methanol by a membrane of polyvinyl alcohol cross-linked with polyacrylic acid, with a separation factor of 465.204 A polymeric hydrazone of 2,6-pyridinedialdehyde has been used to dehydrate azeotropes of water with n- and /-propyl alcohol, s- and tort butyl alcohol, and tetrahydrofuran.205 The Clostridium acetobutylicum which is used to produce 1-butanol, is inhibited by it. Pervaporation through a poly(dimethyl-siloxane) membrane filled with cyclodextrins, zeolites, or oleyl alcohol kept the concentration in the broth lower than 1% and removed the inhibition.206 Acetic acid can be dehydrated with separation factors of 807 for poly(4-methyl-l-pentene) grafted with 4-vinylpyridine,207 150 for polyvinyl alcohol cross-linked with glutaraldehyde,208 more than 1300 for a doped polyaniline film (4.1 g/m2h),209 125 for a nylon-polyacrylic acid membrane (5400 g/m2h), and 72 for a polysulfone.210 Pyridine can be dehydrated with a membrane of a copolymer of acrylonitrile and 4-styrenesulfonic acid to give more than 99% pyridine.211 A hydrophobic silicone rubber membrane removes acetone selectively from water. A hydrophilic cross-linked polyvinyl alcohol membrane removes water selectively from acetone. Both are more selective than distillation.212... 

The dried mucous membrane of the large intestine may be used as source of erepsin. The tissue is dehydrated with alcohol, and finally dried over sulphuric add. The product obtained withstands a temperature of loo for more than an hour without deterioration and shows great activity. 

The performance data showed that this was an excellent PV membrane for alcohol dehydration. However, the structure and the performance of these novel composite membranes varied with conditions of membrane preparation, snch as hydrolysis degree of PAN membrane, content of cross-linking agent, and heat-cnring temperature. The results indicated that the separation factor and the permeation rate of this novel composite membrane increase with the increase in operating temperatnre. At the same time, the PV properties can be adjusted by changing the structnre of the top layer and the middle layer. The membrane had a better PV performance of a separation factor more than 1410 and 0.33 kg/m h for the 90 wt% EtOH aqueous solution and a separation factor of 5000 and flux of 0.43 kg/m h for the 90 wt% i-PrOH aqueous solution. 

Ionic polymers with carboxylic and sulfonic acid functional groups have extremely high water permeabilities. Carboxy methyl cellulose/poly-acrylic acid blends show good water permeability and excellent selectivity for water over low alcohols (Table 1). These membranes can be effectively used to dehydrate alcohols [8]. Selectivity for water over alcohol increases with decreasing polarity of the alcohol. The water fluxes can be correlated with water vapor pressure and are not a function of the nature of the alcohol. 

While the GFT pervaporation process for alcohol dehydration is based on a polyvinyl alcohol membrane of high hydrophilicity, polyion complex membranes have been recently developed to replace polyvinyl alcohol. The membrane material consists of polyanion (polyacrylic acid, molecular weight 5(X),(X)0, cross-linked with diepoxide) and polycation (PCA-107, the structure... 

The selective separation of water from aqueous solutions of isopropanol or the dehydration of isopropanol can be carried out with different membranes, which contain polar groups, either in the backbone or as pendent moieties. For the dehydration of such a mixture, poly(vinyl alcohol) (PVA) and PVA-based membranes have been used extensively. PVA is the primary material from which the commercial membranes are fabricated and has been studied intensively for pervaporation because of its excellent film forming, high hydrophilicity due to -OH groups as pendant moieties, and chemical-resistant properties. On the contrary, PVA has poor stability at higher water concentrations, and hence selectivity decreases remarkably. 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

Hydrophilic membranes with a preferential permeation of water are mainly used for the dehydration of organic solvents with an emphasis on azeotropic mixtures. Membranes for the removal of small alcohol molecules like methanol and ethanol are also of a hydrophilic nature. 

This beneficial effect of fluorination on hydrolytic stability has also been demonstrated with the synthetic prostaglandin SC-46275 (Fig. 70). This compound possesses an anti-secretory activity that protects the stomach mucous membrane. However, its clinical development was too problematic because of the instability of the tertiary allyl alcohol in acidic medium (epimerisation, dehydration, etc.). A fluorine atom was introduced on the C-16 methyl to disfavour the formation of the allylic carbocation. This fluorinated analogue possesses the same biological activity, but does not undergo any degradation or rearrangement, and itepimerises only slowly [165]. 

Assay preparation Weigh and mix the contents of not fewer than 20 Capsules. Transfer an accurately weighed portion of the mixture, equivalent to about 20 mg of omeprazole, to a 100-ml volumetric flask, add about 50 ml of Diluent, and sonicate for 15 min. Cool, dilute with Diluent to volume, mix, and pass through a membrane filter having 0.45 /im or finer porosity. [Note Bubbles may form just before bringing the solution to volume. Add a few drops of dehydrated alcohol to dissipate the bubbles if they persist for more than a few minutes]. 

Separation of isopropanol (IPA) and water by pervaporation has also reached production scale. Much of the current capacity is devoted to azeotrope breaking and dehydration during IPA synthesis. Recently, anhydrous isopropanol has become a preferred drying solvent in the semiconductor industry, where chip wafers are first washed with ultrapure water, then rinsed with the alcohol to promote uniform drying. The water-laden isopropanol generated can be conveniently reused after dehydration by pervaporation. Unlike with pressure-driven membrane processes such as RO or UF, particulates and nonvolatile substances such as salts are not carried over during pervaporation. This helps maintain the effectiveness of contamination control. 

Besides producing mixed-hydrocarbons (ultra-clean diesel), F-T process can also selectively produce mixed-alcohols (oxygenated fuel). The addition of mixed-alcohols into gasoline can effectively reduce HC and CO emissions. However, before directly used as fuels or blended with conventional fuels, the water content in the as-produced F-T mixed-alcohols must be reduced below 0.5wt.%. This dehydration step is essential but difficult since most of the contained alcohols form azeotropies with water. In our group, we studied the dehydration performance of microwave synthesized NaA zeolite membrane toward F-T produced mixed-alcohols [24, 25]. The membrane also showed excellent pervaporation performance toward dehydration of simulated F-T produced mixed-alcohols. The permeate consisted of only water and little methanol (< 10%) in aU the range of feed composition. This result confirmed that NaA zeolite membrane based pervaporation (or vapor permeation) process could be an effective technology for dehydration of F-T produced mixed-alcohols. 

The same as F-T produced mixed-alcohols, low purity bio-ethanol extracted from fermentation broth must be refined into high purity fuel grade ethanol. The pervapo-ration dehydration pilot plant based on NaA zeolite membrane was set up by Mitsui Engineering Shipbuilding Co., Ltd. (MES) in 1999. Recently, a pilot-scale NaA zeolite membrane based vapor permeation dewatering installation has been setup in our group with a handling capacity of 250 L/D. This installation can continuously produce 225 L 99.7wt.% ethanol per day. Meanwhile the permeate is nearly pure water. 

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