Separation of Organic-Water Solutions

Let us consider the equations that describe the equilibrium extraction of a material from one phase to another. In practice, equilibrium concentrations in two-phase systems are described in terms of a distribution coeffieient (sometimes called a partition coefficient), which relates the equilibrium concentrations of a species in each of the two phases. Physical chemistry texts usually 

This simplified equation is more generally employed by the groups discussing ethanol-water separation studies and we use it in the subsequent discussion. 

For the case of a constant distribution coefficient, Treybal (1968) shows that the theoretical minimum solvent-to-feed ratio necessary to extract all of a component from a feed stream is equal to the inverse of the distribution coefficient. Recall, however, that the theoretical minimum value of the solvent-to-feed ratio requires an infinitely tall column. In actual practice a greater solvent-to-feed ratio, typically 1.3 to 1.5 times minimum, is employed. We present some elementary but informative graphical design procedures to illustrate the relationship between the distribution coefficient and the operation of the supercritical fluid extraction process for separating ethanol-water. 

We shall use an operating line with a solvent-to-feed ratio 1.3 times the minimum (see figure 8.12). 

The low distribution coefficients, the attendant requirement of recycling CO2 containing very little ethanol in order to achieve a high recovery of ethanol from the feed stream, and the inability to achieve the separation of ethanol from the extract stream by pressure letdown required the development of this SCF extraction-distillation process. The diagram shown in figure 8.14 pictorially summarizes that an old distillation technique can be combined with new supercritical CO2 extraction to solve the separation problem supercritical CO2 can extract the ethanol from the feed stream, distillation can separate and regenerate the solvent for recycle, and vapor compression can achieve energy efficiency. 

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Supercritical Fluid Process Development Studies SEPARATION OF ORGANIC-WATER SOLUTIONS Ethanol—Water... 

Another widely used approach in this area is a sol-gel process. In order to create surface roughness after deposition of thin films, a secondary component is included in the sol-gel deposition process which can be removed later by dissolution in hot water or sublimation. The removal of the secondary components gives porous structures. Subsequent lluorinated silane coating can render these sol-gel processed films superhydrophobic [81-83]. Microporous structures can be created through phase separation of organic polymer solutions and then used as a template for sol-gel processing of porous silica substrates. Ruorosilane treatment of these substrates produces superhydrophobic surfaces [84]. 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

As already discussed, chromatography on silica or alumina stationary phases nowadays should be restricted to the separations of organic soluble compounds. Of course, also ionic and other water-soluble substances have been and will be separated with such phases. But in order to obtain highly efficient separations (i.e., elution of solutes without tailing) the eluent composition has to be selected and adjusted carefully. Such polar components are better separated by applying tended phases or ion exchange or ion pair chromatography. 

The mixture is cooled and transferred to a 1-1. beaker. After addition of 300 ml. of cold water, the aqueous phase is acidified with 35-40 ml. of acetic acid. The layers are separated and the water solution is extracted with three 75-ml. portions of ether. The organic solutions are combined, washed with 100 ml. of water, and dried over anhydrous magnesium sulfate. The low-boiling solvents are removed by distillation at atmospheric pressure, and the residue is distilled under reduced pressure through a short (15-cm.) Vigreux column. After a 1-5 g. forerun, the product is collected at 125-135°/3-S mm. (Note 4). The yield is 66-74 g. (70-78%). 

T HE SEPARATION, CONCENTRATION, AND FRACTIONATION of organic solutes in aqueous solutions by reverse osmosis are of practical interest from the points of view of water purification and collection of samples for environmental analysis. Although many experimental data on the separation of organic solutes are available in the literature (1-2), very few fundamental works have been accomplished so far. We have been studying this subject in the framework of the preferential sorp-... 

Concentration Plus Solvent Transfer. Concentration of the organic solutes is essential to the determination of many organic contaminants present in water at very trace levels. The solvent transfer is needed for implementation of the separation and detection schemes that do not tolerate the water matrix. For bioassay work, concentration and solvent transfer are also needed because the amounts are too low for direct testing of the water solutions, and dimethyl sulfoxide. (DMSO) is the preferred solvent. In bioassay studies that involve animal exposure, the concentration scheme must accommodate very large volumes of water. Theoretically and practically, these elements of the analytical and bioassay methodologies can be achieved by using solid adsorbents, especially synthetic polymers. 

Pervaporation is a membrane separation process where the liquid feed mixture is in contact with the membrane in the upstream under atmospheric pressure and permeate is removed from the downstream as vapor by vacuum or a swept inert gas. Most of the research efforts of the pervaporation have concentrated on the separation of alcohol-water system [1-20] but the separation of acetic acid-water mixtures has received relatively little attention [21-34]. Acetic acid is an important basic chemical in the industry ranking among the top 20 organic intermediates. Because of the small differences in the volatility s of water and acetic acid in dilute aqueous solutions, azeotropic distillation is used instead of normal binary distillation so that the process is an energy intensive process. From this point of view, the pervaporation separation of acetic acid-water mixtures can be one of the alternate processes for saving energy. 

Distillation with steam in plant operations is a relatively expensive process because it requires large volumes of steam and cooling water. For laboratory work, however, it is a very suitable method for the smooth separation of organic products, which are volatile with steam, from non-volatile inorganic impurities and high molecular tarry byproducts. The separation of isomers is also possible in those instances where only one of the isomers is volatile with steam (e.g., o- and p-nitrophenols, page 147). Volatile acids and basic compounds can be separated from each other, and from neutral products, by successive steam distillations from acid and alkaline solution. 

Similarly, Sano et al. [1994] added colloidal silica to a stirred solution of tetrapropylammonium bromide and sodium hydroxide to synthesize a hydrogel on a stainless steel or alumina support with a mean pore diameter of 0.5 to 2 pm. The composite membrane is then dried and heat treated at 500 C for 20 hours to remove the organic amine occluded in the zeolite framework. The silicalite membranes thus obtained are claimed to be free of cracks and pores between grains, thus making the membranes suitable for more demanding applications such as separation of ethanol/water mixtures where the compound molecules are both small. The step of calcination is critical for synthesizing membranes with a high permselectivity. 

Membranes having effective pore sizes between 0.001 and 0.01 pm are used in nanofiltration. NF is placed between reverse osmosis and ultrafiltration, and because of that it is sometimes considered as loose reverse osmosis. Typical operating pressures for NF are 0.3-1.4 MPa. The process allows to separate monovalent ions from multivalent ions, which are retained by NF membrane. The process can be used for separation of organic compounds of moderate molecular weight from the solution of monovalent salts. The very well-known application in nuclear industry is boric acid recovery from contaminated cooling water in nuclear reactor. There are some examples of nanofiltration applications and studies done with the aim of implementation in nuclear centers described in literature. Some of them are listed in the Table 30.4. 

Separation of organic/organic mixtures Extraction of water from an aqueous solution of ethanol Extraction of water from solutions of ethanol, acetic acid Separation of benzene/ -hexane mixtures Separation of xylene isomers... 

The separation of organic molecules out of the soil solution onto the solid phase is called partitioning. The ratio of a molecule s concentrations in the water and SOM phases is a constant, the partition coefficient 1 d ... 

Chemical complexation is most useful for the separation of organic solutes from water when the solute has certain physical properties. Some of the most important criteria favoring the use of complexation are the... 

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