Solubility-based separation process

So, with small variations of pressure or temperature, a supercritical fluid can be moved from a density where it dissolves reasonably high amounts of a solute to another one where it precipitates (almost) everything it had dissolved before. These characteristics make the supercritical-fluid phase attractive for solubility-based separation processes. 

Before paired-ion chromatography is discussed, it is illustrative to consider another solubility-based separation which relies on a similar approach. This separation is ion-pair extraction. Ion-pair extraction uses two immiscible liquids (aqueous and organic) often in a separatory funnel. An ionized compound (Aij) that is water soluble can be made to favor solution into the organic phase during an extraction by using a suitable counter ion (Baq) to form a neutral ion pair. Since the ion pair behaves as though it is a nonionic, neutral species, it will prefer to reside in the organic liquid layer, and the entire process can be described by the equation ... 

For any membrane-based separation process to be successful, the membrane must possess two key attributes high flux and good selectivity. Flux, which depends directly on permeability, was treated in Sections 4.4.2 and 4.4.4. Selectivity depends in part on differences in permeant size and solubility in the membrane (Section 4.4.5). Separation will then occur because of differences in the transport rates of molecules within the membrane. This rate of transport is determined by the mobility and concentration of the individual components as well as the driving force, which is the chemical potential gradient across the membrane. 

Another example of the solvent based separation process is the process described by Hafner [19]. The process is meant to separate PVC from non-vinyl components such as fabric ( for example in vinyl upholstery ) or paper (such as in vinyl wallpaper ). PVC is dissolved in a solvent, solution is separated from the non - soluble components by filtration and then the polymer is precipitated by addition of a non -solvent. Solvent, non - solvent and plasticizers are recovered by fractional distillation. 

Typical units of Hi are atmospberes/mole fraction. If H2 > Hi, then, for the same partial pressure pig = p2g) or mole fraction in the gas phase, the mole fraction of gas species 2 in the liquid phase is less than that of gas species 1. Species 1 is thus more soluble in the liquid phase and therefore may be separated from species 2 by absorption in a suitable liquid. Gas absorption based separation processes utilize this preferential solubility of some gases in selected liquid absorbents. The values of Henry s constant H,- in units of atmospheres/mole fraction for a variety of gases in water are provided in the handbook by Perry and Green (1984). For some species, the values are given at a number of temperatures and values of Pig. The latter does indicate a weak dependence of Hi on p,-g. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

The technique of solvent extraction has long been used in organic chemistry for concentrating and purifying some substances. In the case of organic compounds, the separation process is simple, in many cases being based only on differences in the solubility of the compounds in different solvents. 

Surfactants are selected based primarily on the degree of solubilization. Other factors to be considered include toxicity, biodegradability, surfactant sorption, and surfactant solubility and compatibility with the separation process. Surfactants have the ability to lower the interfacial tension between water and the contaminant by as little as a factor of three to four orders of magnitude. Combined with a sufficient reduction in capillary forces, this allows pumped groundwater theoretically to move the DNAPL toward the recovery or extraction well. This is accomplished by injecting surfactant solution into the contaminated zone. Impacted groundwater characterized by an increase in the concentration of the contaminant is then recovered and treated. 

Polyvinyl alcohol (PVA), which is a water soluble polyhidroxy polymer, is one of the widely used synthetic polymers for a variety of medical applications [197] because of easy preparation, excellent chemical resistance, and physical properties. [198] But it has poor stability in water because of its highly hydrophilic character. Therefore, to overcome this problem PVA should be insolubilized by copolymerization [43], grafting [199], crosslinking [200], and blending [201], These processes may lead a decrease in the hydrophilic character of PVA. Because of this reason these processes should be carried out in the presence of hydrophilic polymers. Polyfyinyl pyrrolidone), PVP, is one of the hydrophilic, biocompatible polymer and it is used in many biomedical applications [202] and separation processes to increase the hydrophilic character of the blended polymeric materials [203,204], An important factor in the development of new materials based on polymeric blends is the miscibility between the polymers in the mixture, because the degree of miscibility is directly related to the final properties of polymeric blends [205],... 

The reaction in a homogeneous solution with a polar organic solvent in which the enzymes and substrates are both soluble, occurs often at the expense of the enzyme stability [4, 5]. Besides immobilised enzymes in organic solvents [6], emulsion reactors, especially enzyme-membrane-reactors coupled with a product separation by membrane based extractive processes [7-9] and two-phase membrane reactors [10-12], are already established on a production scale. 

Numerous processes have been proposed for extracting potash from felspar, leucite, alunite, and other minerals rich in this substance, but the cost is so great that very few proposals yet made ofier promise of successful competition with the Stassfurt deposits. This is even the case with alunite, where mere calcination to 1000° drives off water and sulphuric acid, leaving water-soluble potassium sulphate, and alumina. Humphry Davy in his paper On Some Chemical Agencies of Electricity (1807), indicated in Cap. Ill, found that when water was electrolyzed in cavities contained in celestine, fluorspar, zeolite, lepidolite, basalt, vitreous lava, agate, or glass, the bases separated from the acid and accumulated about the cathode. It is therefore probable that if water with finely divided potash minerals in suspension were electrolyzed, the alkali would be separated in a convenient simple way. 

The third main class of separation methods, the use of micro-porous and non-porous membranes as semi-permeable barriers (see Figure 2c) is rapidly gaining popularity in industrial separation processes for application to difficult and highly selective separations. Membranes are usually fabricated from natural fibres, synthetic polymers, ceramics or metals, but they may also consist of liquid films. Solid membranes are fabricated into flat sheets, tubes, hollow fibres or spiral-wound sheets. For the micro-porous membranes, separation is effected by differing rates of diffusion through the pores, while for non-porous membranes, separation occurs because of differences in both the solubility in the membrane and the rate of diffusion through the membrane. Table 2 is a compilation of the more common industrial separation operations based on the use of a barrier. A more comprehensive table is given by Seader and Henley.1... 

One possibility to separate the enantiomers of rac- 1-phenylethanamine is to form diastereomeric salts with an enantiomerically pure chiral acid, e.g. (R,R) tartaric acid or (S)-2-hydroxysuccinic acid. These can be separated from each other by recrystallisation as a consequence of their different solubilities. Note, however, that the separation process is not complete at this stage since the amines are now present as salts. The separated salts must be treated with a strong base, e.g. aqueous sodium hydroxide, to convert them back to the free amines which can then be extracted into an organic solvent. After drying the extract distillation of the solvent leaves the pure amine. 

For laboratory purposes, methanol is the usual solvent, and procedures based on its use have been described in detail (21, 39, 81). Of great practical usefulness for the separation of the weakly basic fraction is the solubility of certain alkaloidal acetates in chloroform, e.g., reserpine, ajmalicine, and aricine, whereas other acetates are insoluble in this solvent, e.g., ajmaline, yohimbine, and a-yohimbine. Since the anhyd-ronium alkaloids are extremely strong bases, they can only be extracted into an organic solvent in their tertiary base form at pH 11. For industrial purposes, the best process extracts water-moistened Rauwolfia root with hydrocarbons such as benzene, toluene, or xylene. In this procedure, only the weak bases are extracted. No complicated separation processes are involved, and reserpine is obtained in high yields (122). 

After the toxic hexavalent chromium(VI) is reduced to less toxic trivalent chromium(III) ion, lime or another base chemical is added to convert soluble chromium(III) to insoluble chromium hydroxide in accordance with Eq. (7). A water-solid separation process unit is needed to remove the insoluble chromium hydroxide from the pretreatment wastewater. What are the feasible water-solid separation processes available ... 

The preferred usage is when the complexing agent interacts with the solute of interest and has little or no interaction with the other components of the feed stream. Separation processes based on reversible chemical complexation provide an enhancement in the solubility of the selected solute through the complexation reaction. This approach can provide high enhancement of capacities and selectivities for dilute solutes, especially when the solute feed concentration is below 10%. 

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