Absorption typical separations

Separation processes are based on some difference in the properties of the substances to be separated and may operate kinetically, as in settling and centrifugation, or by establishing an equilibrium, as in absorption and extraction. Typical separation processes are shown in Table 6.1. Better separations follow from higher selectivity or higher rates of transport or transformation. The economics of separation hinges on the required purity of the separated substance or on the extent to which an unwanted impurity must be removed (Figure 6.13). 

Finally, the interaction between the dipole and quadrupole of donor and acceptor molecules [13] is generally much weaker than the dipole-dipole interaction. The dipole—quadrupole term [/ (r) r-8] is typically 10—100 times weaker than the dipole—dipole term, though if the acceptor absorption spectrum is symmetry-forbidden (and so weak) but not spin-forbidden, the dipole transition moment for the acceptor is small [127]. Such is the case for energy transfer between rare-earth ions in tungstates typically separated by 1.7 nm [146]. The kinetics of dipole—quadrupole energy transfer are discussed in Chap. 4, Sect. 2.6. 

PITC (phenylisothiocyanate) Aabs = 254 nm. Phenylthiocarbamyl amino acid derivatives are moderately stable at room temperature (1 day). PITC reacts well with both primary and secondary amino acids. Reaction time is approximately 5 minutes at room temperature. Excess reagent must subsequently be removed under vacuum. Also, for hydrolyzed samples, hydrochloric acid must be completely removed prior to derivatization. As a result, even though the actual reaction time is reasonably fast, the total time for various sample manipulations can add up to 2 hours. This is partially compensated by the extremely fast separation possible (12 minutes). Detection is by UV absorption only. Detection limits are typically in the high picomole range. Short column life can result due to unreacted PITC getting into the column. Unlike some of the other reagents, PITC quantifies tyrosine and histidine very well. PITC analysis is available as a commercially prepackaged system dubbed Pico-Tag by Waters Corporation. Representative references include 184-188. See Fig. 11 for a typical separation. 

In addition to characterization of molecular and macroscopic electro-optic activity, it is important to define optical loss. Optical loss can be influenced both by absorption and by scattering effects. In order to minimize overall loss, it is important to understand the independent contributions made by scattering and absorption. To separate these effects, we need to determine the contributions made by both chromophore and polymer host to the optical absorption at device operating wavelengths. Chromophore interband electronic absorption can be measured on resonance by traditional UV-Visible spectrometry however, we will typically be concerned with optical absorption at telecommunication wavelengths of 1.3 and 1.55 microns where such techniques do not provide accurate information. Total optical absorption at 1.3 microns is occasionally determined by both the interband electronic absorption of the chromophore and by C-H vi-... 

The mechanics and applications of multiphase flow has been an area of continuing interest to chemical, environmental, and civil engineers (23,77). The multiphase flow patterns may be classified as bubble flow, plug flow, stratified flow, wave flow, slug flow, annular flow, spray flow, and froth flow. Typical sketches of these various flow patterns are shown in Fig. 3. They are self-explanatory. In the field of absorptive bubble separation processes, only multiphase bubble flow and froth flow are of interest to the process engineer. 

Figure 33-1 5 Typical separation by MECC. (a) Some test compounds 1 = methanol, 2 = resorcinol, 3 = phenol, 4 = p-nitroaniline, 5 = nitrobenzene, 6 = toluene, 7 = 2-naphthol, 8 = Sudan III capillary, 50-p.m inside diameter, 500 mm to the detector applied voltage, ca. 15 kV detection UV absorption at 210 nm. (b) Analysis of a cold medicine 1 = acetaminophen, 2 = caffeine, 3 = sulpyrine, 4 = naproxen, 5 = guaiphenesin, 10 = noscapine, 11 = chloropheniramine and tipepidine applied voltage, 20 kV capillary, as in (a) detection UV absorption at 220 nm. (From S. Terabe, Trends Anal. Client., 1989,8, 129.)...

Rapid scan infrared spectra may be simulated by observing the kinetic changes in intensity at a series of different wavelengths, typically separated by 1 to 10 cm , and concentrating on those regions of the spectrum corresponding to the absorption of reactants, any intermediates, and products. 

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. 

It is clear from Figure 7.18 that progressions and sequences are not mutually exclusive. Each member of a sequence is also a member of two progressions. However, the distinction is usefiil because of the nature of typical patterns of bands found in a band system. Progression members are generally widely spaced with approximate separations of cOg in absorption and co" in emission. In contrast, sequence members are more closely spaced with approximate separations of cOg — co". 

The butane-containing streams in petroleum refineries come from a variety of different process units consequently, varying amounts of butanes in mixtures containing other light alkanes and alkenes are obtained. The most common recovery techniques for these streams are lean oil absorption and fractionation. A typical scheme involves feeding the light hydrocarbon stream to an absorber-stripper where methane is separated from the other hydrocarbons. The heavier fraction is then debutanized, depropanized, and de-ethanized by distillation to produce C, C, and C2 streams, respectively. Most often the stream contains butylenes and other unsaturates which must be removed by additional separation techniques if pure butanes are desired. 

Absorption. Oil absorption is another process used for recovery of LPG and natural gas Hquids from natural gas. Recovery is enhanced by loweriag the absorption temperature to —45°C and by keeping the molecular weight of the absorption oil down to 100. Heat used to separate the product from the absorption oil contributes to the cost of recovery. Therefore, this process has become less competitive as the cost of energy has iacreased. A simplified flow diagram of a typical oil-absorption process is shown ia Figure 2. 

The special case involving the removal of a low (2—3 mol %) mole fraction impurity at high (>99 mol%) recovery is called purification separation. Purification separation typically results in one product of very high purity. It may or may not be desirable to recover the impurity in the other product. The separation methods appHcable to purification separation include equiUbrium adsorption, molecular sieve adsorption, chemical absorption, and catalytic conversion. Physical absorption is not included in this Hst as this method typically caimot achieve extremely high purities. Table 8 presents a Hst of the gas—vapor separation methods with their corresponding characteristic properties. The considerations for gas—vapor methods are as follows (26—44). 

Gas contact is typically carried out in absorption towers over which the alkaline solutions are recirculated. Strict control over the conditions of absorption are required to efficiendy capture the NO and convert it predominantly to sodium nitrite according to the following reaction, thereby minimizing the formation of by-product sodium nitrate. Excessive amounts of nitrate can impede the separation of pure sodium nitrite from the process. 

Direct evidence for the competition of two counteracting contributions to the transient absorption changes stems from the temporal evolution of the transmission change at 560 nm. From Figure 10-3 it can be seen that the positive transmission change due to the stimulated emission decays very fast, on a time scale of picoseconds. On the other hand the typical lifetime of excitations in the 5, slate is in the order of several hundred picoseconds. Therefore, one has to conclude that the stimulated emission decay is not due to the decay of the. Sj-population (as is typically the case in dye solutions). The decay is instead attributed to the transiei.i build up of spatially separated charged excitations that absorb at this wavelength. 

---