Solvent adsorption designer" solvents

A convenient notation for classifying mixtures employed in liquid-liquid extraction is C/, where C is the number of components and the number of partially miscible pairs. Mixtures 3/1, 3/2, and 3/3 are called Type I, Type II, and Type III by some authors. A typical 3/1 three-component mixture with only one partially miscible pair is furfural-ethylene glycol-water, as shown in Fig. 3.10, where the partially miscible pair is furfural-water. In practice, furfural is used as a solvent to remove the solute, ethylene g yco, from water the furfural-rich phase is called the extract, and the water-rich phase the raffinate. Nomenclature for extraction, leaching, absorption, and adsorption always poses a problem because, unlike distillation, concentrations are expressed in many different ways mole, volume, or mass fractions mass or mole ratios and special solvent-free designations. In this chapter, we will use V to represent the extract phase and L the raffinate phase, and y and x to represent solute concentration in these phases, respectively. The use of V and L does not imply that the extract phase in extraction is conceptually analogous to the vapor phase in distillation indeed the reverse is more correct for many purposes. 

In summary, expensive "designer" solvents can be recovered from air emissions by adsorption into activated carbon which can be desorbed with steam. However, the size of the adsorber bed will be fairly large because of the lower level of adsorption on activated carbon. 

Note that adsorption isotherms are not available at the reference of Footnote 23 for "designer" solvents. 

The removal of AAT sulfur compounds from the diesel fuel feedstock by extractive desulfurization (EDS), either adsorption or solvent extraction, is a viable alternative to HDS." This approach is based on the polarity difference between the AAT family of compounds and the hydrocarbons found in the diesel fraction. Because AATs comprise several dozen different compounds, they represent a range of solvent polarities, in some cases quite similar to the aromatic compounds found in diesel fuel. The critical polarity difference between individual hydrocarbon and thiophenic compounds is therefore variable, and in some cases may be insufficient to allow a functional separation by extraction. Polarity difference can, however, be increased by oxidizing the thiophenic sulfur to the corresponding more polar mono- and dioxides, the alkylated-aryl-thiophene-sulfoxide or sulfone (AATS). This in turn facilitates the separation, and several EDS processes therefore are designed to oxidize the AAT before extraction of the sulfur." ... 

After introducing (8.32) into (8.30) and rearranging, we obtain the following equation for the relative adsorption of Fof / with respect to component 1 (by convention, the solvent is designated component 1) ... 

The discussion so far has been confined to systems in which the solute species are dilute, so that adsorption was not accompanied by any significant change in the activity of the solvent. In the case of adsorption from binary liquid mixtures, where the complete range of concentration, from pure liquid A to pure liquid B, is available, a more elaborate analysis is needed. The terms solute and solvent are no longer meaningful, but it is nonetheless convenient to cast the equations around one of the components, arbitrarily designated here as component 2. 

Other types of regenerators designed for specific adsorption systems may use solvents and chemicals to remove susceptible adsorbates (51), steam or heated inert gas to recover volatile organic solvents (52), and biological systems in which organics adsorbed on the activated carbon during water treatment are continuously degraded (53). 

Adsorption The design of gas-adsorption equipment is in many ways analogous to the design of gas-absorption equipment, with a solid adsorbent replacing the liqiiid solvent (see Secs. 16 and 19). Similarity is evident in the material- and energy-balance equations as well as in the methods employed to determine the column height. The final choice, as one would expect, rests with the overall process economics. 

Scott and Kucera [4] carried out some experiments that were designed to confirm that the two types of solute/stationary phase interaction, sorption and displacement, did, in fact, occur in chromatographic systems. They dispersed about 10 g of silica gel in a solvent mixture made up of 0.35 %w/v of ethyl acetate in n-heptane. It is seen from the adsorption isotherms shown in Figure 8 that at an ethyl acetate concentration of 0.35%w/v more than 95% of the first layer of ethyl acetate has been formed on the silica gel. In addition, at this solvent composition, very little of the second layer was formed. Consequently, this concentration was chosen to ensure that if significant amounts of ethyl acetate were displaced by the solute, it would be derived from the first layer on the silica and not the less strongly held second layer. 

Engineering Considerations To effect the good engineering design of an activated carbon adsorption system, it is first necessary to obtain information on the following the actual cubic feet per minute (ACFM) of air to be processed by the adsorber, the temperature of gas stream, the material(s) to be absorbed, the concentration of the material to be adsorbed, and if the intended application is air pollution control such as odor control - then the odor threshold of the material to be adsorbed. In addition, data is needed on the presence of other constituents in the gas stream, and whether or not solvent recovery is economical. 

Solvent reeovery systems would also neeessitate the speeifieation of eondenser duties, distillation tower sizes, holding tanks, piping, and valves. It is important to note that the engineering design of an adsorption system should be based on pilot data for the partieular system. Information ean usually be obtained direetly from the adsorbent manufaeturer. The overall size of the unit is determined primarily by eeonomie eonsiderations, balaneing the operating eosts against the eapital eosts. 

The adsorption process generally is of an exothermal nature. With increasing temperature and decreasing adsorbate concentration the adsorption capacity decreases. For the design of adsorption processes it is important to know the adsorption capacity at constant temperature in relation to the adsorbate concentration. Figure 11 shows the adsorption isotherms for several common solvents. 

Adsorption is influenced by the surface area of the adsorbent, the nature of the solvent being adsorbed, the pH of the operating system, and the temperature of operation. These are important parameters to be aware of when designing or evaluating an adsorption process. 

---