Process, absorption

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. In absorption, a gas mixture is contacted with a liquid solvent which preferentially dissolves one or more components of the gas. Absorption processes often require an extraneous material to be introduced into the process to act as liquid solvent. If it is possible to use the materials already in the process, this should be done in preference to introducing an extraneous material for reasons already discussed. Liquid flow rate, temperature, and pressure are important variables to be set. 

The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

This is the familiar absorption process illustrated by the appearance of an aqueous solution of copper sulphate as blue due to the absorption of the complementary colour, red, by the solution. 

Both emission and absorption processes rely on the background radiation, which is present throughout the universe and which has a wavelength distribution characteristic of a black body and a temperature of about 2.7 K. This radiation is a consequence of the big bang with which the universe supposedly started its life. 

We have seen in Section 5.2.1.4 that there is a stack of rotational energy levels associated with all vibrational levels. In rotational spectroscopy we observe transitions between rotational energy levels associated with the same vibrational level (usually v = 0). In vibration-rotation spectroscopy we observe transitions between stacks of rotational energy levels associated with two different vibrational levels. These transitions accompany all vibrational transitions but, whereas vibrational transitions may be observed even when the sample is in the liquid or solid phase, the rotational transitions may be observed only in the gas phase at low pressure and usually in an absorption process. 

Figure 9.17 Absorption and fluorescence spectra of rhodamine B in methanol (5 X 10 mol 1 ). The curve marked is for the J 2 absorption (process 8 in Figure 9.18) and that marked 5 for process 1. (Reproduced, with permission, from Dienes, A. and Shank, C. Y, Chapter 4 in Creation and Detection of the Excited State (Ed. W. R. Ware), Vol. 2, p. 154, Marcel Dekker, New York, 1972)...

Depending on the method of pumping, the population of may be achieved by — Sq or S2 — Sq absorption processes, labelled 1 and 2 in Figure 9.18, or both. Following either process collisional relaxation to the lower vibrational levels of is rapid by process 3 or 4 for example the vibrational-rotational relaxation of process 3 takes of the order of 10 ps. Following relaxation the distribution among the levels of is that corresponding to thermal equilibrium, that is, there is a Boltzmann population (Equation 2.11). 

In a two-photon absorption process the first photon takes the molecule from the initial state 1 to a virtual state V and the second takes it from V to 2. As in Raman spectroscopy, the state V is not an eigenstate of the molecule. The two photons absorbed may be of equal or unequal energies, as shown in Figures 9.27(b) and 9.27(c). It is possible that more than two photons may be absorbed in going from state 1 to 2. Figure 9.27(d) illustrates three-photon absorption. 

Figure 9.28 A two-photon (or more) absorption process may be monitored by (a) measuring total, undispersed fluorescence or (b) counting the ions produced by a further photon (or photons). T is a virtual state...

The purpose of such scmbbing operations may be any of the following gas purification (eg, removal of air pollutants from exhaust gases or contaminants from gases that will be further processed), product recovery, or production of solutions of gases for various purposes. Several examples of appHed absorption processes are shown in Table 1. 

Discussion of the concepts and procedures involved in designing packed gas absorption systems shall first be confined to simple gas absorption processes without compHcations isothermal absorption of a solute from a mixture containing an inert gas into a nonvolatile solvent without chemical reaction. Gas and Hquid are assumed to move through the packing in a plug-flow fashion. Deviations such as nonisotherma1 operation, multicomponent mass transfer effects, and departure from plug flow are treated in later sections. 

Equimolar Counterdiffusion. Just as unidirectional diffusion through stagnant films represents the situation in an ideally simple gas absorption process, equimolar counterdiffusion prevails as another special case in ideal distillation columns. In this case, the total molar flows and are constant, and the mass balance is given by equation 35. As shown eadier, noj/g factors have to be included in the derivation and the height of the packing is... 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

The diffusion coefficient depends upon the characteristics of the absorption process. Reducing the thickness of the surface films increases the coefficient and correspondingly speeds up the absorption rate. Therefore, agitation of the Hquid increases diffusion through the Hquid film and a higher gas velocity past the Hquid surface could cause more rapid diffusion through the gas film. 

Another use is in various extraction and absorption processes for the purification of acetylene or butadiene and for separation of aHphatic hydrocarbons, which have limited solubiHty in DMF, from aromatic hydrocarbons. DMF has also been used to recover CO2 from flue gases. Because of the high solubiHty of SO2 iu DMF, this method can even be used for exhaust streams from processes using high sulfur fuels. The CO2 is not contaminated with sulfur-containing impurities, which are recovered from the DMF in a separate step (29). 

In general, siUca has proved to be a good material of constmetion for the burner. Cast iron, steel, or graphite was sometimes used. Gaseous HCl produced by this method is very pure and can be used to manufacture pure hydrochloric acid by the adiabatic absorption process (33) or falling film absorption process (34). 

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. 

Fig. 2. Refrigerated absorption process for the production of LPG and natural gasoHne Hquids.

Less propane and butanes are produced compared to natural gas Hquids by the adsorption process than are obtained normally for the same gas by the oil-absorption process. Because adsorption efficiency increases with a decrease in temperature, the adsorption cycle should operate at the lowest temperature that is economically feasible. 

Anhydrous lithium hydroxide [1310-65-2], LiOH, is obtained by heating the monohydrate above 100°C. The salt melts at 462°C. Anhydrous lithium hydroxide is an extremely efficient absorbent for carbon dioxide (qv). The porous stmcture of the salt allows complete conversion to the carbonate with no efficiency loss in the absorption process. Thus LiOH has an important role in the removal of carbon dioxide from enclosed breathing areas such as on submarines or space vehicles. About 750 g of lithium hydroxide is required to absorb the carbon dioxide produced by an individual in a day. 

Principles in Processing Materials. In most practical apphcations of microwave power, the material to be processed is adequately specified in terms of its dielectric permittivity and conductivity. The permittivity is generally taken as complex to reflect loss mechanisms of the dielectric polarization process the conductivity may be specified separately to designate free carriers. Eor simplicity, it is common to lump ah. loss or absorption processes under one constitutive parameter (20) which can be alternatively labeled a conductivity, <7, or an imaginary part of the complex dielectric constant, S, as expressed in the foUowing equations for complex permittivity ... 

The dielectric permittivity as a function of frequency may show resonance behavior in the case of gas molecules as studied in microwave spectroscopy (25) or more likely relaxation phenomena in soUds associated with the dissipative processes of polarization of molecules, be they nonpolar, dipolar, etc. There are exceptional circumstances of ferromagnetic resonance, electron magnetic resonance, or nmr. In most microwave treatments, the power dissipation or absorption process is described phenomenologically by equation 5, whatever the detailed molecular processes. 

In the early 1970s, air pollution requirements led to the adoption of the double contact or double absorption process, which provides overall conversions of better than 99.7%. The double absorption process employs the principle of intermediate removal of the reaction product, ie, SO, to obtain favorable equiUbria and kinetics in later stages of the reaction. A few single absorption plants are stiU being built in some areas of the world, or where special circumstances exist, but most industriali2ed nations have emission standards that cannot be achieved without utili2ing double absorption or tad-gas scmbbers. A discussion of sulfuric acid plant air emissions, control measures, and emissions calculations can be found in Reference 98. 

More recentiy, sulfuric acid mists have been satisfactorily controlled by passing gas streams through equipment containing beds or mats of small-diameter glass or Teflon fibers. Such units are called mist eliminators (see Airpollution control methods). Use of this type of equipment has been a significant factor in making the double absorption process economical and in reducing stack emissions of acid mist to tolerably low levels. 

The curve in Figure 21 represents SO2 equiUbrium conversions vs temperature for the initial SO2 and O2 gas concentrations. Each initial SO2 gas concentration has its own characteristic equiUbrium curve. For a given gas composition, the adiabatic temperature rise lines can approach the equiUbrium curve but never cross it. The equiUbrium curve limits conversion in a single absorption plant to slightly over 98% using a conventional catalyst. The double absorption process removes this limitation by removing the SO from the gas stream, thereby altering the equiUbrium curve. 

Absorption processes are categorized based on the mechanism of absorption, as either chemical or physical. In addition, a number of hybrid absorption processes featuring both chemical and physical solvents have been developed. 

Fig. 3. Flow diagram for a chemical absorption process where the horizontal lines within the towers represent trays or packing.

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