Azeotrope solvents

The equihbrium shown in equation 3 normally ties far to the left. Usually the water formed is removed by azeotropic distillation with excess alcohol or a suitable azeotroping solvent such as benzene, toluene, or various petroleum distillate fractions. The procedure used depends on the specific ester desired. Preparation of methyl borate and ethyl borate is compHcated by the formation of low boiling azeotropes (Table 1) which are the lowest boiling constituents in these systems. Consequently, the ester—alcohol azeotrope must be prepared and then separated in another step. Some of the methods that have been used to separate methyl borate from the azeotrope are extraction with sulfuric acid and distillation of the enriched phase (18), treatment with calcium chloride or lithium chloride (19,20), washing with a hydrocarbon and distillation (21), fractional distillation at 709 kPa (7 atmospheres) (22), and addition of a third component that will form a low boiling methanol azeotrope (23). 

Some solvent mixtures can be very difficult and energy intensive to separate because of the closeness of boiling points and the formation of azeotropic mixtures [45]. Azeotropic or extractive distiUation can be used for azeotropic solvent mixtures and solvents which have very low relative volatihties ]43, 45]. Azeotropic and extractive distillation involves the addition of another solvent, known as an entrainer, which will form its own azeotrope with one of the components to be separated ]45]. However, the additional solvent required for azeotropic and extractive distillation can also generate more wastes depending on how easily the entrainer itself can be recycled and reused. 

Pervaporation (PV) is a membrane-based process used to separate aqueous, azeotropic solvent mixtures. This is done using a hydrophihc, non-porous membrane that is highly selective to water. Figure 3.9 shows a typical PV system that produces a dehydrated solvent stream (retentate) from a solvent/water feed. 

There are two main methods for synthesis of polyimides. In one, the dianhydride and diamine are separately dissolved in A,lV-dimethylacetamide or A-methylpyrrollidinone. The solutions are mixed for 24 h to afford poly(amic) acid. The mixture is then refluxed for a further 24 h, using an azeotroping solvent to distill off water. Alternatively, the diamine is mixed with the dianhydride as its ester-acid, and heated at 190 °C in the presence of o-dichlorobenzene as azeotroping solvent. There are reported improvements113. A useful intermediate used in the manufacture of polyamide and imide engineering plastics is 1,3-bis(3-aminophenoxybenzene). 

The most common ketals are dioxolane and dioxane these form more readily and have fewer problems with enol ether contamination. Sometimes, however, they can be difficult to hydrolyze, particularly when the molecule contains sensitive functional groups. The reaction is acid catalyzed, with some of the more common acids being para-toluenesulfonic, pyridinium para-toluenesulfonic, and eamphorsulfonie. The eluninated water can be removed either with an azeotroping solvent (such as toluene or benzene) or by the addition of a dehydrating agent such as trimethylorthoformate. 

An enumeration of some of the specifications which must be met by an efficient solvent follows. Obviously, the solvent should be noncorrosive to the equipment and should not react with the feed to form undesired products. It should produce a sufficient change in the volatilities of the components to be separated so that these components may be separated with a reasonable number of plates at an economical reflux ratio. The solvent should have an appropriate boiling point relative to the components of the feed to be separated. An azeotropic solvent should have a volatility near the major component desired in the overhead product and an extractive solvent should have a volatility lower than the major component to be withdrawn in the bottom product. The solvent should not be toxic, and it should be available in sufficient quantities at a reasonable price. 

Synthesis of symmetric squaraines is generally achieved by the condensation of one equivalent of squaric acid, with two equivalents of the respective nucleophiles, under reflux in an azeotropic solvent. In this extremely versatile one-pot synthesis, same substituents are attached on the 1,3-position of squaric acid to yield symmetric squaraines. Nucleophiles like pyrroles [7], phenols [7], azulenes [14] and N,N-dialkylanilines [15] condense with squaric acid, and a representative example is given in Scheme 1. Nucleophiles obtained by the elimination of HI from 2-methyl substituted quinolium, benzothiazolium, and benzoselenazolimn iodide also react with squaric acid to yield 1,3-disubstituted squaraines [16], as shown in Scheme 2 for the synthesis of 5. 

Thermotropic Properties. Glycophorin can be incorporated into multilamellar liposomes by hydration of a glycophorin-lipid film prepared by rotary evaporation of a mixture of glycophorin and lipid in an azeotropic solvent (chloroform/methanol/water, 81 15 14% v/v bpt 52.6 C). VVe found that the azeotropic solvent mixture facilitated uniform dispersion of the glycophorin in the lipid (28). 

By azeotropic dehydration. An azeotropic solvent, such as toluene or xylene is used to remove water. 

The selection of the solvents is very important, because the compounds to be extracted must all be miscible in them. The solvents properties including dielectric constant in order to obtain the desired polarity, the boiling point, the miscibility and the purity must all be known. Sometimes it is important to use azeotropic solvents to obtain a more complete... 

The above thermodynamic analysis of the polycondensation reveals that PLIAs having high molecular weights may be produced when the condensed water is efficiently removed to a level of 1 ppm from the polymerization system without evaporation of the L-lactide monomer present in equilibrium. The ordinary reaction conditions that may allow the effective removal of the water may involve (1) a temperature range of 180-200 °C (2) a low pressure below 5 torr and (3) a long reaction time in the presence of an appropriate catalyst and, in some cases, azeotropic solvent for removing water efficiently. ... 

In 1995, as mentioned above, Ajioka et al. first succeeded in synthesizing PLLA having higher than 100 kDa by using diphenyl ether as an azeotropic solvent.In this polycondensation system the high boiling solvent was refluxed at reduced pressure to azeotropically distil the condensed water and to make it adsorb on molecular sieves. This method is especially effective for the co-polycondensation of L-lactic acid with other monomers (hydrojgrl-acids and diol/dicarbojq lic acid combinations). 

Tape casting Planar 100 800 Low viscosity (Pa s), azeotropic solvents, various organics... 

Once the polycondensation has been completed, the polymer is rigorously freed from any basic or nucleophilic contaminants such as the catalyst. Next, the polymer is carefully dried. The use of an azeotropic solvent in the first stage greatly aids the final drying step. This step is often completed by the addition of a drying agent such as phosphorus pentoxide or polyphosphoric acid. " ... 

Levin, E. (1958) Fish flour and fish meal by azeotropic solvent processing. Food Technol. 13, 122-135. 

While azeotropes with as many as five components are known, only binary azeotropes are commonly used in solvent cleaning. There are a few three-component cleaning azeotropic solvents patented (and fewer still commercialized). 

Binary azeotropes can be and are commercially disrupted (not "broken") But, that requires designed chemical processes involving additional solvent components and very specific conditions to do so Individual azeotropic solvent components can t be liberated from an azeotrope by attraction of some soil components. 

Reaction step quench Distillation azeotropic solvent drying Aqueous extraction phase splits... 

The selective absorption of solvents in a gel is fundamentally determined by the chemical structure of the primary monomer. However, to improve selectivity, it is extremely important to control the crosslinking structure. In this section, gel membrane application is introduced as an example of practical use. However, further usefulness is expected in the area of membrane separation for organic solvent mixtures where many azeotropic solvent mixtures exist. For practical plication, it is necessary to study the relationship between the chemical or crosslinking structure that has selective absorption, and the mechanism of the solvent, which permeates through the membrane. 

A method of obtaining high ortho resole structures has been reported in a patent issued to Monsanto.Toluene or xylene is used to form an azeotropic solvent with the high ortho content promoted by the continuous removal of the water by distillation. These materials have important applications as binders and molding resins. 

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