Separation of process solvent

Figure 1. Separation of process solvent 92-03-035 boiling cuts diluted 1 1 with pyridine with specific metal detection. Conditions 100A fi-Styragel column eluent, pyridine flow rate, 0.5 mL/...

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. 

Subsequent separation of this solvent imposes substantial capital and operating cost penalties. A Bayer AG patent (37) claims use of a solvent in which DNT is soluble, but in which the TDA is practically insoluble. This allows separation and recycle of the solvent to the reactor without any distillation process. 

Supercritical fluids have also been used purely as the solvent for polymerization reactions. Supercritical fluids have many advantages over other solvents for both the synthesis and processing of materials (see Chapter 6), and there are a number of factors that make scCCH a desirable solvent for carrying out polymerization reactions. As well as being cheap, nontoxic and nonflammable, separation of the solvent from the product is achieved simply by depressurization. This eliminates the energy-intensive drying steps that are normally required after the reaction. Carbon dioxide is also chemically relatively inert and hence can be used for a wide variety of reactions. For example, CO2 is inert towards free radicals and this can be important in polymerization reactions since there is then no chain transfer to the solvent. This means that solvent incorporation into the polymer does not take place, giving a purer material. 

Microdistillation with mass spectral analysis of the distillate yielded valuable information about the SRC s studied. Although only 2-17% of the SRC s were volatile under the conditions used, the nature of the distillate defined the completeness of process solvent separation, solvent separation parameters, and degree of depolymerization of the coal. Also, the distillate contains stable reaction intermediates between liquid products and coal itself. 

In addition to the stationary and mobile phases, separations obtained in TLC are affected by the vapor phase, which depends on the type, size, and saturation condition of the chamber during development. The interactions of these three phases as well as other factors, such as temperature and relative humidity, must be controlled to obtain reproducible TLC separations. The development process with a single (isocratic) mobile phase is complicated because of progressive equilibration between the layer and mobile phase and separation of the solvent components of the mobile phase as a result of differential interactions with the layer, which leads to the formation of an undefined but reproducible mobile phase gradient. 

Reverse osmosis employs a semipermeable membrane that allows passage of the solvent molecules, but not those of the dissolved organic and inorganic material. A pressure gradient is applied to cause separation of the solvent and solute. Any components that may damage or restrict the function of the membrane must be removed before the process is performed. Capital investment and operating costs depend on the waste stream composition. 

The mixture to be separated contains [U02] and Pu(TV) nitrates, as well as metal ions such as 3gSr. Kerosene is added to the aqueous solution of metal salts, giving a two-phase system (i.e. these solvents are immiscible). Tributyl phosphate (TBP, a phosphate ester) is added to form complexes with the uranium-containing and plutonium ions, extracting them into the kerosene layer. The fission products remain in the aqueous solution, and separation of the solvent layers thus achieves separation of the fission products from Pu- and U-containing species. Repeated extractions from the aqueous layer by the same process increases the efficiency of the separation. 

The separation and subsequent recovery of one liquid from another is perhaps one of the most regular activities within the chemical industry and a number of methods have been developed to achieve this process. Two principal categories can be identified of relevance to solvents which are the recovery of a solvent from water, which includes drying, and the separation of one solvent from another. The techniques relevant to each case are discussed below. 

Liquid-liquid extraction. Liquid-liquid extraction is primarily used when distillation is impractical or too costly. For example when the relative volatilities of the two components are less than 1.3. In much the same way as with separation of solvent and water, liquid-liquid extraction is a process of separating components based on their distribution between two immiscible liquids. A good example of this technology is the separation of aromatic and aliphatic hydrocarbons by washing with a mixture of diethylene glycol and water which selectively extracts the more polar aromatics [31]. However, the application of liquid-liquid extraction to the separation of two solvents is relatively rare. This is primarily driven by economics as further stages are required such as distillation to achieve complete recovery of the solvent. 

Recoverability. In all liquid-extraction processes, it is necessary to remove the extracting solvent from the two products resulting from the separation. This is important not only to avoid contamination of the products with the solvent but also to permit reuse of the solvent in order to reduce the cost of operation. In practically every instance, the recovery process is ultimately one of fractional distillation, and the relative volatility of the solvent and substance to be separated must be high in order that this may be carried out inexpensively. The existence in the system of azeotropes involving the solvent must be checked particularly, since their presence may prevent separation of the solvent by ordinary distillation means. A very complete, indexed list of azeotropes which has recently been compiled is most convenient for at least initial studies of recoverability (4). The question as to whether the solvent or the components which are separated by the extraction process should be the more volatile is an important one. In most extraction processes, the quantity of solvent... 

On the other hand, from an environmental point of view, the use of organic solvents should be minimized because of their harmful environmental impacts. In addition, they are usually expensive and require separation from the product. The need for high yields with easy separation to produce products of higher purity while using environmentally friendly processes has led to a search for new technologies and new solvents. Any alternative to organic solvents should dissolve reaction substrates and reduce the excess alcohol inhibition effect and, at the same time, avoid a difficult separation of the solvent. In this regard, supercritical fluids (SCFs) have been put forward and have shown potential (Romero et al 2005). Further discussion on the use of SCFs, as a reaction medium, is covered in Section 6.5. 

Extraction is carried out batchwise or continuously. For batch extraction, rotating extractors are frequently used, serving both for extraction and the subsequent separation of the solvent from the extraction residue by distillation. Batch extractors are simple and robust, but have a limited capacity, and the periodic filling and emptying is time consuming. Consequently, for bulk materials continuous countercurrent extractors with a capacity of up to several 1000 td are widely used to process oilseeds, but they are also used for coffee and tea (Figure 3.3.54). 

Formal Separation of the Solvent Effects We consider the association process... 

In a solvent extraction system, the extract solvent phase is highly enriched in the solutes preferentially extracted. From this extract, the solvent stream has to be recovered for reuse at the other end of the cascade simultaneously, a highly solute-enriched stream is obtained as the product after separation of the solvent. The solute concentration of this latter stream is (much) higher than that of the feed stream (see Figures 8.1.36 and 8.1.37(a)). This is obviously one of the ultimate product streams from the overall extraction process. 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. In absorption, a gas mixture is contacted with a liquid solvent which preferentially dissolves one or more components of the gas. Absorption processes often require an extraneous material to be introduced into the process to act as liquid solvent. If it is possible to use the materials already in the process, this should be done in preference to introducing an extraneous material for reasons already discussed. Liquid flow rate, temperature, and pressure are important variables to be set. 

Eliminate extraneous materials for separation. The third option is to eliminate extraneous materials added to the process to carry out separation. The most obvious example would be addition of a solvent, either organic or aqueous. Also, acids or alkalis are sometimes used to precipitate other materials from solution. If these extraneous materials used for separation can be recycled with a high efficiency, there is not a major problem. Sometimes, however, they cannot. If this is the case, then waste is created by discharge of that material. To reduce this waste, alternative methods of separation are needed, such as use of evaporation instead of precipitation. 

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