Separation Difficulties

A brief discussion of sohd-liquid phase equihbrium is presented prior to discussing specific crystalhzation methods. Figures 20-1 and 20-2 illustrate the phase diagrams for binary sohd-solution and eutectic systems, respectively. In the case of binary solid-solution systems, illustrated in Fig. 20-1, the liquid and solid phases contain equilibrium quantities of both components in a manner similar to vapor-hquid phase behavior. This type of behavior causes separation difficulties since multiple stages are required. In principle, however, high purity... 

Some of the disadvantages of the Stille reaction, e. g. the low reactivity of some substrates, separation difficulties in chromatography, and the toxicity of tin compounds, have been ameliorated by recent efforts to improve the procedure. Curran has, in a series of papers, reported the development of the concept of fluorous chemistry, in which the special solubility properties of perfluorinated or partly fluorinated reagents and solvents are put to good use [45]. In short, fluorinated solvents are well known for their insolubility in standard organic solvents or water. If a compound contains a sufficient number of fluorine atoms it will partition to the fluorous phase, if such a phase is present. An extraction procedure would thus give rise to a three-phase solution enabling ready separation of fluorinated from nonfluorinated compounds. 

Finally, radiochemical methods of analysis may be used to follow rates of detritiation. This method is particularly useful for very slow reactions (where it is impractical to collect data for any appreciable extent of reaction) as an initial rate approach may then be employed. Separation difficulties, at least for aqueous solutions, may be overcome by using the freeze-drying method or the more recent countercurrent dialysis and Sephadex gel filtration techniques. ... 

The program can be modified slightly, to calculate the minimum analysis time required to isolate 5 gram of material, at a given inlet pressure, from a series of mixtures of differing separation difficulty. The results obtained are shown as curves relating separation time to separation ratio in figure 5. 

As the alkylation catalyst ages, its titratable acidity decreases because it becomes diluted with water from the feed stock and with water and complex hydrocarbon oils produced by side reactions. The catalyst is ordinarily discarded when its titratable acidity drops to 85 to 88%. If the catalyst is used further, the quality of the alkylate deteriorates rapidly, and the hydrocarbon-acid emulsion becomes so stable that separation difficulties are encountered in the acid settler. 

Separation difficulties from H3P03 and POCI3 sometimes occur. 

Sulfuric acid alkylation units have excellent safety reports [18], The main concerns are the H2S04 and the large inventories of light hydrocarbons. The incidents that have been reported include emulsion-separation difficulties leading to acid carryover, acid runaway resulting in low-quality alkylate and requiring shut down of the olefin feed, or tight emulsion in the net effluent water wash with the consequence of carryover of the alkaline water into the separation section. 

Lowers relative volatility and increases separation difficulty. This raises reflux and stage requirements and reboiler and condenser duties. 

Partially ethylated alditol acetates may be used as alternative sugar derivatives when separation difficulties are encountered with particular combinations of the corresponding methylated compounds (74BMS263 75MI4). 

The case studies were selected to reflect different separation difficulties and different contents of the intermediate component of the quaternary mixtures. The mixtures considered are displayed in Tables 1-2. The mole fraction of 0.05 was shown to be a borderline value for use of side-stream columns for ternary separations (Tedder and Rudd, 1978). The total feed flowrate for all cases was 45.5 kmol/h. Since the feed involves a hydrocarbon mixture, the Chao-Seader correlation was used for the prediction of thermodynamic properties. The design pressure for each sequence was chosen such that all condensers could be operated with cooling water. 

The interchangeable use of the terms flours and starches for many of the storage hydrocolloids listed in Table I is a result of the separation difficulty. A flour is the crude, milled portion of that plant structure rich in starch. Starches are the granules, predominantly carbohydrate, which are separated from the storage structure. Some separations are relatively effective, as with corn (maize) and potato starches. Wheat starch cannot be separated as cleanly as com starch, but sufficiently so that a distinction may be made between the starch and the flour. For tapioca, sago, arrow-root, and rice, the dividing Une is hazy. 

Derivatization reactions can be performed either pre- or postcolumn. As outlined by Brinkman, there are important advantages to using the postcolumn techniques whenever possible (68). First, the analytes can be separated in their original form, which often permits the adoption of published separation procedures. Second, artifact formation is generally not a serious problem, in contrast to precolumn derivatization, where it increases the separation difficulty and causes problems with quantitation. Third, the reaction does not need to be complete and the reaction products need not be stable the only requirement is reproducibility. Several reaction principles have been extensively applied. These include true chemical derivatization such as with dansyl chloride or o-phthalaldehyde UV irradiation, which can convert the analyte of interest into a more easily detectable species solid-phase reactions, including catalytic reactions such as with the use of immobilized enzymes and chemiluminescence techniques. 

Solids feed preparation usually plays a crucial role in leaching processes. The use of line particles usually win spied up diffusive transfer of solutes from the solid to the extract. However, if too fine a size is used, extract may not be able to flow Ibrongh solid beds as rapidly as desired, flow pressure drop may be excessive and cause undesirable bed compaction, displacement instability will he enhanced, extract cling will increase, solid-liquid separation difficulty may increase, and excessive amocrats of intracellular colloidal material may be released. Therefore, the use of a compromise particle size is almost invariably desirable. Except for flaked oilseeds, where flake thicknesses of the order of 0.2 mat are necassery to obtain adequate oil release, particle diameters or thicknesses in the 2-5 mm range usually represent a good choioe for industrial scale extractions. 

A facile route to tetrakisfluorophosphine complexes of zero-valent nickel, NiL4, which avoids the separation difficulties encountered in displacing carbon monoxide from nickel carbonyl, involves displacement... 

Y Sc are not lanthanides, but their chemical properties are so nearly identical that they occur in the same ore bodies and have the same separation difficulties as those of the lanthanides. Hence, they are included in the rare earth category. 

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