Chemical separation Liquid-phase

Nonadsorptive retention of contaminants may occur when chemicals reach the subsurface as a separate liquid phase or are adsorbed on suspended particles or orgaiuc residues. Contaminated suspended particles originating from sludge disposal or polluted runoff, for example, can represent a substantial hazard to the subsurface environment. 

In the phase separation model we take advantage of the fact that micellization has much in common with the formation of a separate liquid phase. At low concentration the chemical potential of the dissolved surfactants can be described by... 

Porous membrane - Membrane made of porous material. When it separates liquid phases its performance depends on the size of the pores and chemical properties of the material. If pore size is much larger than the molecular dimensions, the membrane exerts no influence on transport of individual components of separated liquids and only prevents mixing by convection. For smaller pores it selectively controls transport of species between the phases discriminating them by the size and/or charge. See also - membrane system, - membrane electrode. 

Azeotropes can also be homogeneous (single liquid phase) or heteroge-neous (multiple liquid phases). In a heterogeneous azeotrope the repulsive forces between different molecules in the liquid phase are strong enough to overcome the entropy increase due to mixing such that the liquid splits into two or more separate liquid phases. At equilibrium the chemical potential for each component is still the same in all phases ... 

Gold, silver, mercury, and platinum metals, as well as Se and Te, can be precipitated from acid solution in the elemental form by reduction with chemical reagents such as zinc, NH2OH, N2H4, SO2, or formic acid. In the trace analysis of high purity mercury the sample (about 100 g) is dissolved in HNO3 and the solution is warmed in the presence of formic acid. First of all, nitric acid, then mercury, is reduced. The mercury forms a separate liquid phase, and the impurities remain in the aqueous solution [102]. In the trace analysis of silver, the sample is dissolved in nitric acid, then formic acid and mercury are added. The silver liberated on reduction dissolves in the mercury to form an amalgam [102]. 

Within the separated liquid phases, Si02 and Al203 were analyzed according to conventional chemical methods. Na20 was determined by flame photometry. Crystalline reaction products were identified by Debye-Scherrer diagrams the composition of the mixtures was determined by comparison with diagrams of test samples. 

If we mix three members of very different chemical types (such as water, benzene, mercury) we would find three separate liquid phases, each containing one of the species practically pure, containing less (often much less ) than 1 mol% of each of the other two. 

The thin film model thus far has been applied to the volatilization of chemicals dissolved in water. Chemical volatilization can also occur from a layer of nonaqueous phase liquid (NAPE) floating on a water surface (or spilled on the ground). NAPE refers to a liquid, such as a solvent or a liquid fuel, that does not readily dissolve in water and hence tends to remain as a separate liquid phase. There are two kinds of NAPE light NAPL... 

Analysis of such cuts by spectrometry requires a preliminary separation by chemical constituents. The separation is generally done by liquid phase chromatography described in article 3.3.5. 

Nearly every chemical manufacturiag operation requites the use of separation processes to recover and purify the desired product. In most circumstances, the efficiency of the separation process has a significant impact on both the quality and the cost of the product (1). Liquid-phase adsorption has long been used for the removal of contaminants present at low concentrations in process streams. In most cases, the objective is to remove a specific feed component alternatively, the contaminants are not well defined, and the objective is the improvement of feed quality defined by color, taste, odor, and storage stability (2-5) (see Wastes, industrial Water, industrial watertreati nt). 

Manufacture and Processing. PytomeUitic acid and its dianhydtide can be synthesized by oxidizing dutene [95-93-2] (1,2,4,5-tettamethylbenzene). Liquid-phase oxidation using strong oxidants such as nittic acid, chromic acid, or potassium permanganate produces the acid which can be dehydrated to the dianhydtide in a separate step. This technology is practiced by AUco Chemical Co., a part of International Specialty Chemicals. 

R. L. Barton ( Sizing Liquid-Liquid Phase Separators Empirically, Chemical Engineering, July 8, 1974, Copyright (1974) McGraw-Hill, Inc., used with permission) provides the following quick method for sizing liquid-liquid phase sepai ators empirically. 

Barton, R. L., Sizing Liquid-Liquid Phase Separators Empirically, Chemical Engineering, July 8, 1974. 

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). 

We have seen above in two instances, those of liquid-liquid phase separation and polymer devolatilization that computation of the phase equilibria involved is essentially a problem of mathematical formulation of the chemical potential (or activity) of each component in the solution. 

Wahl and Deck were able to obtain an estimate of an assumed second-order rate coefficient ( 10 l.mole" .sec at 4°C) using a separation procedure based on the extraction of Fe(CN)e by a chloroform solution of Ph AsCl, in the presence of the ions Co(CN)g and Ru(CN)6, to reduce the exchange between the iron species in the two liquid phases. A similar estimate was obtained using a precipitation method in the presence of the carrier Ru(CN)6. A direct injection technique was used as short reaction times were necessary. Wahl has reviewed the large induced exchanges occurring in the chemical separation methods. The extraction procedure when the carriers Co(CN)6 and Ru(CN) are present provides the most satisfactory method of separation. ... 

The problem of transport of molecules through swollen gels is of general interest. It not only pertains to catalysis, but also to the field of chromatographic separations over polymeric stationary phases, where the partition of a solute between the mobile phase (liquid phase) and a swollen polymeric stationary phase (gel phase) is a process of the utmost importance. As with all the chemical and physicochemical processes, the thermodynamic and the kinetic aspect must be distinguished also in partition between phases. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

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