Liquid with solvent circulation

FigJ.8 FI manifold for liquid-liquid extraction with solvent circulation. S. aqueous sample SG. phase segmenton SP. phase separator R, restrictor or impedance coil P. pump V. V. valve (two positions) OS. oiganic solvent D. detector [26]. 

Selection of Solvent When choice is possible, preference is given to liquids with high solubilities for the solute a high solubility reduces the amount of solvent to be circulated. The solvent should be relatively nonvolatile, inexpensive, noncorrosive, stable, nonviscous, nonfoaming, and preferably nonflammable. Since the exit gas normally leaves saturated with solvent, solvent loss can be costly and may present environmental contamination problems. Thus, low-cost solvents may be chosen over more expensive ones of higher solubility or lower volatility. 

Description Hydrocarbon feed is preheated with hot circulating solvent and fed at a midpoint into the extractive distillation column (EDC). Lean solvent is fed at an upper point to selectively extract the aromatics into the column bottoms in a vapor/liquid distillation operation. Nonaromatic hydrocarbons exit the column top and pass through a condenser. A portion of the overhead stream is returned to the column top as reflux to wash out any entrained solvent. The balance of the overhead stream is the raffinate product, requiring no further treatment. 

The above process was improved by carrying out the photochemical chlorination of ethylene carbonate in the presence of an initiator, with a high-pressure Hg lamp at 70-100 °C, and with continuous circulation of the liquid through an illuminated side-arm. It has been found that ethylene carbonate can be photochemically converted to the tetrachloro derivative in the absence of a solvent, and the process has been performed on a 200 L scale. 

The designer first must select the solvent liquid to be used and then specify its circulation rate. The greater the solubility of solute in the solvent, the lower will be the necessary liquid rate. The minimum solvent rate possible is that flow which produces a concentration of solute in the effluent liquid which is in equilibrium with the solute concentration in the entering gas stream. Of course, an infinite number of theoretical stages is required at minimum solvent flow. The economic optimum design results from a balance between the solvent circulation rate and the depth of packing in the absorber. 

Pervaporation is a process in which organic solvent water mixture or organic solvent mixture can be separated by partial vaporization through a nonporous permeate selective membrane. In this process liquid feed mixture circulates in contact with the active nonporous side of the membrane while a vacuum is applied on the other side of the membrane. A phase change of membrane-selective permeate takes place in the membrane. The membrane-selective permeate diffuses through the membrane and desorbs on the posterior side of the membrane. Later, it evaporates with the help of a vacuum from the posterior side of the active nonporous membrane. The transport of the permeate through a nonporous permeate-selective membrane is quite complex. This could be explained in three steps, which are as follows ... 

The reaction takes place at low temperature (40-60 °C), without any solvent, in two (or more, up to four) well-mixed reactors in series. The pressure is sufficient to maintain the reactants in the liquid phase (no gas phase). Mixing and heat removal are ensured by an external circulation loop. The two components of the catalytic system are injected separately into this reaction loop with precise flow control. The residence time could be between 5 and 10 hours. At the output of the reaction section, the effluent containing the catalyst is chemically neutralized and the catalyst residue is separated from the products by aqueous washing. The catalyst components are not recycled. Unconverted olefin and inert hydrocarbons are separated from the octenes by distillation columns. The catalytic system is sensitive to impurities that can coordinate strongly to the nickel metal center or can react with the alkylaluminium derivative (polyunsaturated hydrocarbons and polar compounds such as water). 

The concept of extractive reaction, which was conceived over 40 years ago, has connections with acid hydrolysis of pentosans in an aqueous medium to give furfural, which readily polymerizes in the presence of an acid. The use of a water-immiscible solvent, such as tetralin allows the labile furfural to be extracted and thus prevents polymerization, increases the yield, and improves the recovery procedures. In the recent past an interesting and useful method has been suggested by Rivalier et al. (1995) for acid-catalysed dehydration of hexoses to 5-hydroxy methyl furfural. Here, a new solid-liquid-liquid extractor reactor has been suggested with zeolites in protonic form like H-Y-faujasite, H-mordenite, H-beta, and H-ZSM-5, in suspension in the aqueous phase and with simultaneous extraction of the intermediate product with a solvent, like methyl Aobutyl ketone, circulating countercurrently. 

Solvent Systems. The problem of sulfur deposition in sour gas producing wells has resulted in the development of new solvent systems for treating plugged wells or for continuous circulation to prevent deposition. The hazards of using carbon disulfide as a sulfur solvent have resulted in the development of techniques based on either disulfide liquids (e.g. Merox) or amines both of which are capable of combining chemically with the deposited sulfur and carrying it to the surface (8). 

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