Preliminary Separator Selection

Approximate Particle Sizes From Liquid Full Cone Spray Nozzles  

Nozzle Size, In. Operating Pressure, Psig Approx. Mean Particle Size, Microns 

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The Sylvan Chart [2] of Figure 4-3 is useful in preliminary equipment selection, although arranged primarily for dust separations, it is applicable in the appropriate parts to liquid separations. Perry [23] presents a somewhat similar chart that is of different form but contains much of the same information as Figure 4-1 and 4-lA. 

A fairly efficient method of selective detection of active forms of oxygen by means of sensors is the preliminary separation of their mixture with the aid of filters designed for the purpose, filters that vigorously de-excite this or that form of oxygen. It has been mentioned in Section 5.4 that freshly atomized films of Ag efficiently absorb O-atoms from the gaseous phase, mildly de-excite 02 molecules. To de-excite the singlet oxygen molecules proper, use should be made of filters with... 

Boto and Bunt [465] used thin-layer chromatography for the preliminary separation of chlorophylls and phaeophytins from seawater, and combined this with selective excitation fluorometry for the determination of the separated chlorophylls a, b, and c, and their corresponding phaeophytin components. An advantage of the latter technique is that appropriate selection of excitation and emission wavelengths reduces the overlap among the emission spectra of the various pigments to a greater extent than is possible with broadband excitation and the use of relatively broadband filters for emission. 

Very often the liquids to be processed may be contaminated with substances detrimental to some types of zeolites consequently a complete knowledge of the process stream composition and physical properties must be available before preliminary sieve selection can be made. In the absence of prior knowledge of separation factors, competitive co-adsorption, environmental stability, regeneration techniques, or irreversible zeolite contamination, zeoli te contamination, zeolite specification must be proceded by time-con-... 

In addition to TMS ethers, dimethylsilyl ethers [36] were studied for the analysis of alcohols and phenols. They were prepared by reaction with reagents analogous to those used for TMS ethers [dimethylmonochlorosilane and tetramethyldisilazane in pyridine (1 3 9)] and they provide shorter retention times in comparison with TMS ethers on Apiezon L. Grant [37] converted phenols into bromomethyldimethylsilyl ethers. In addition to a higher sensitivity, selective detection, permitting, e.g., phenolic substances to be determined in tars without any preliminary separation, was even achieved by using an ECD. 

When the distribution ratio is not highly favorable, it is still possible to obtain a quantitative and selective separation through the use of a countercurrent liquid extraction approach. Although such approaches are no longer practical, having largely been supplanted by instrumental techniques such as preparative HPLC and continuous solvent extraction, countercurrent separations are conceptually useful. These approaches can be applied to preliminary separation of complex mixtures or in the isolation of compounds that do not perform well in LC because of undesirable interaction with the stationary phase (irreversible adsorption, denaturation, etc.). For these reasons, most applications of countercurrent separations involve the isolation of natural or biochemical products from plant or animal extracts. As will be described below, countercurrent extractions form the theoretical basis for LLE cartridges. 

In summary, separator selection calls for clear problem definition, in broad terms thorough cataloging of process information and preliminary and tentative equipment selection, followed by refinement of the initial selections through tests on an increasingly larger scale. Reliability, flexibility of operation, and ease of maintenance should be weighed heavily in the final economic evaluation rarely is purchase... 

Moderate to high selectivity. Often a wavelength can be found at which the analyte alone absorbs, thus making preliminary separations unnecessary. Furthermore, where overlapping absorption bands do occur, comections based on... 

Serious interference in the determination of iridium is caused by Rh and Pt. The selectivity of the method can be improved by preliminary separation of Ir(IV) by extraction with tetraphenylarsonium chloride (CHCI3). If large amounts of Pt, Pd, Ru and Os are present, it is advisable to separate these metals, e.g., by selective reduction of Pt to the metal, extraction of Pd as its dimethylglyoximate, and distillation of Ru and Os as the tetroxides. 

The method is not selective. Other platinum metals also form floatable ion-associates with SCN and Capri Blue. Certain other metals, such as Au, Ag, Cu, Hg, and W, also interfere, and hence a preliminary separation of Ru is necessary. For example, distillation of the tetroxides allows Ru and Os to be isolated from practically all other metals. Then, extraction from thiocyanate medium [14] separates Os and Ru, and one after the other can be determined, e.g., with Capri Blue. 

Although preliminary separations may be required, in other instances the precipitation step in gravimetric analysis is sufficiently selective that other separations are not required. The pH is important because it often influences both the solubility of the analytical precipitate and the possibility of interferences from other substances. For example, calcium oxalate is insoluble in basic medium, but at low pH the oxalate ion combines with the hydrogen ions to form a weak acid. 8-Hydroxy-quinoline (oxine) can be used to precipitate a large number of elements, but by controlling pH, we can precipitate elements selectively. Aluminum ion can be precipitated at pH 4, but the concentration of the anion form of oxine is too low at this pH to precipitate magnesium ion. 

The frontal chemical concentration method can also be used for the intermediate concentration trace amounts of heavy components [60] in the analysis of gaseous monomers (ethylene, propylene). The concentration was conducted on a short intermediate column containing diethanolamine and pure carbon dioxide was used as the carrier gas. The method permits three steps to be integrated into a single run preliminary separation, concentration of heavy trace components and analytical determination of the composition of the concentrate. The concentration of the heavy trace components to be determined was 10" %. Trace analytical methods based on selective retention of the main component are becoming more common in chromatography. 

The determination of an ion or molecule by direct po-tentiomctric measurement is rapid and simple, requiring only a comparison of the potential developed by the indicator electrode in the test solution with its potential when immersed in one or more standard solutions of the analyte. Because most indicator electrodes are selective, preliminary separation steps are seldom required. In addition, direct potentiometrie mea.sure-ments are rapid and readily adapted to the continuous and automatic monitoring of ion activities. 

Extraction is seldom the sole method used to purify a compound, but it is a rapid and versatile technique that can be used to achieve a preliminary separation prior to a final purification step. Separation of components by extraction depends upon the difference in solubility of a compound in two mutually insoluble phases. Mathematical aspects of extraction are formulated in terms of a simple distribution law, K = CJC, which states that at equilibrium a solute will distribute itself between two immiscible phases, a and 6, such that the ratio of concentrations in the two phases is a constant at a given temperature. The constant K is called the partition or distribution coefficient. If a substance dissolved in solvent b is to be extracted into a second solvent a), it is obviously advantageous to choose solvent a such that the value of K will be as large as possible. Unfortunately, there is no sure way of predicting K, and the organic chemist relies on the rule that like dissolves like and his previous experience in selecting the best solvent system for an efficient extraction. 

After the preliminary separation, the next (and most critical) step is the selection of a suitable crystallization solvent. Ideally, the solvent either should not dissolve the impurities at all or should dissolve and retain all of them, while depositing all the desired compound. The desired compound should be much more soluble in hot solvent than in cold solvent. Some commonly used solvents are listed in Table 2-1 in order of decreasing dielectric constant. 

For samples containing several substrates, a preliminary separation should be considered or the total must be determined. In the case of L-amino acid assay the use of decarboxylating enzymes acting selectively on different amino acids is an attractive possibility, and enzyme electrodes of this type are known for L-tyrosine, L-phenylalanine, and L-tryptophan. These sensors will be coupled with a carbon dioxide base sensor. 

Since conclusions reached in forensic laboratories are used in the criminal justice system, they must leave no room for doubt. Thus, analytical methods must meet strict criteria. They must be extremely selective, reproducible, sufficiently sensitive, and suitable for qualitative and quantitative analysis. It is also highly desirable for the method to call for the minimum number of pretreatment steps and to be applicable to compound mixtures without preliminary separation of their components. Nuclear magnetic resonance (NMR) spectroscopy meets these criteria. It is well... 

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