Homogeneous separation precipitation

Because of the extremely high pressures (15,000 to 45,000 psig), ethylene exists in the liquid phase and polymerization occurs in solution. Owing to high temperatures (typically >200 °C), polyethylene is also dissolved in monomer and the reaction system is homogeneous. LDPE precipitates only after the reaction mass is cooled in post-reactor separation vessels. Relative to other processes, reactor residence times are very short (<30 seconds for the autoclave process and <3 min for the tubular process) (7). 

Figure 11.4 Separation of nitrate and bromide in cabbage-lettuce extract on an amino phase. Conditons sample, 20 jxl of extract after homogenization, protein precipitation and filtration column, 25cm x 3.2 mm i.d. stationary phase, LiChrosorb NH2, 5ixm mobile phase, 1 ml min 1% KH2PO4 in water adjusted to pH 3 with H3PO4 UV detector, 210 nm. Peaks 1 = nitrile (from nitrate converted during sample treatment) 2 = bromide 3 = nitrate.

For HDL-C measurements, similar developments have taken place with direct homogeneous assays, which eliminate the need for precipitation and separation steps (Okazaki et al. 1997 Reed 1997 Warnick, Nauck, and Rifai 2001). The differing proportions of lipoproteins (and their cholesterol content) in the plasmas of the various laboratory species mean that the suitability of these homogeneous and precipitation methods must be checked if meaningful results are to be obtained, particularly when lipoprotein antibodies are used (Warnick and Albers 1978 Hoffmann et al. 1985 Sjoblom and Eklund 1989 Tschantz and Sunahara 1993 Escola-Gil et al. 1999 Ensign et al. 2006). 

Z. Dlsche I should like to mention an observation which may be pertinent in this respect. When lens homogenates are precipitated with 80% acetone at 0 , glutathione is separated from the proteins and remains in solution. When, however, the precipitation is carried out with 90% acetone, glutathione is fixed on the protein due to some changes in the latter. When the protein is redissolved and reprecipitated at 80% only a small fraction of the fixed glutathione goes into the supernatant. 

The complexity of petroleum products raises the question of sample validity is the sample representative of the total flow The problem becomes that much more difficult when dealing with samples of heavy materials or samples coming from separations. The diverse chemical families in a petroleum cut can have very different physical characteristics and the homogeneous nature of the cut is often due to the delicate equilibrium between its components. The equilibrium can be upset by extraction or by addition of certain materials as in the case of the precipitation of asphaltenes by light paraffins. 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

The identification of camphene is best carried out by its conversion into isobomeol under the influence of acetic acid in the presence of sulphuric acid. In order to effect this conversion, 100 grams of the fraction containing the terpene in substantial quantity are mixed with 250 grains of glacial acetic acid and 10 grams of 50 per cent, sulphuric acid. Tne mixture is heated for two to three hours on a water-bath to a temperature of 50° to 60°. At first the liquid separates into two layers, bat soon becomes homogeneous and takes on a pale red colour. Excess of water is added, and the oil which is precipitated, and which contains the isobomeol in the form of its acetate, is well washed with water repeatedly. It is then saponified by heating with alcoholic potash solution on a water-bath. The liquid is then evaporated and extracted with water, and the residue recrystallised from petroleum ether. 

The precipitated metallic hydroxides or hydrated oxides are gelatinous in character, and they tend to be contaminated with anions by adsorption and occlusion, and sometimes with basic salts. The values presented in Table 11.2 suggest that many separations should be possible by fractional precipitation of the hydroxides, but such separations are not always practical owing to high local concentrations of base when the solution is treated with alkali. Such unequal concentrations of base result in regions of high local pH and lead to the precipitation of more soluble hydroxides, which may be occluded in the desired precipitate. Slow, or preferably homogeneous, precipitation overcomes this difficulty, and much sharper separations may be achieved. 

Any strong acid that may be present is first neutralised. Then, by selecting an appropriate base, whose conjugate acid has a Ka of about 10 5, the equilibrium for the tripositive cations will be forced to the right the base is too weak, however, to remove the hydroxonium ions from the equilibrium of the dipositive cations. Since a large excess of the basic ion is added, a basic salt of the tripositive metal usually precipitates instead of the normal hydroxide. Acetate or benzoate ions (in the form of the sodium salts) are the most common bases that are employed for this procedure. The precipitation of basic salts may be combined with precipitation from homogeneous solution, and thus very satisfactory separations may be obtained. 

Precipitation of calcium oxalate may also be made from homogeneous solution either by use of urea as reagent or by the use of dimethyl oxalate. Both procedures lead to satisfactory separations from magnesium. 

In addition to the insoluble polymers described above, soluble polymers, such as non-cross-linked PS and PEG have proven useful for synthetic applications. However, since synthesis on soluble supports is more difficult to automate, these polymers are not used as extensively as insoluble beads. Soluble polymers offer most of the advantages of both homogeneous-phase chemistry (lack of diffusion phenomena and easy monitoring) and solid-phase techniques (use of excess reagents and ease of isolation and purification of products). Separation of the functionalized matrix is achieved by either precipitation (solvent or heat), membrane filtration, or size-exclusion chromatography [98,99]. 

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