Solvent sublation

In solvent sublation an immiscible liquid is placed atop the main liquid to trap the material deposited by the bubbles as they exit (Sebba, Ion Flotation, Elsevier, New York, 1962). The upper hquid should dissolve or at least wet the material. With appropriate selectivity, the separation so achieved can sometimes be much greater than that with bubble fractionation alone. 

A small amount of collector (surfactant) or other appropriate additive in the liquid may greatly increase adsorption (Shah and Lemlich, op. cit.). Column performance can also be improved by skimming the surface of the liquid pool or, when possible, by removing adsorbed solute in even a tenuous foam overflow. Alternatively, an immiscible liquid can be floated on top. Then the concentration gradient in the tall pool of main hquid, plus the trapping action of the immiscible layer above it, will yield a combination of bubble fractionation and solvent sublation. 

Figure 8.4 Apparatus for sample preparation using physical separation. A - fractionation tube and trap for assisted distillation (A septum injector, B carrier gas inlet, C Florisil trap for collecting volatile pesticides) B, Shapiro-type freeze concentrator and C, apparatus for solvent sublation.

The solvent sublation procedure of Wickbold [18] is another method that has been used for the analysis of LAS present in seawater [19,20], The solvent sublation technique (gaseous stripping into organic solvent, often ethyl acetate) has also been used to isolate and concentrate nonionic surfactants, e.g. AEs and APEO in aqueous samples [21,22], The co-extracted interferences can be eliminated by cation/anion ion-exchange and alumina chromatography [23,24]. 

The chromatographic determination of AEs in the environment has been extensively reviewed [3,4], AEs have been extracted from the aqueous matrix, among others, by solvent sublation [65], LLE [66], SPE with Amberlite XAD-2 [67], cartridges [68-73] (i.e. C1 Ci8, GCB, C8) or Cis disks [74] and matrix solid-phase dispersion for bioconcentration studies in fish samples [75],... 

IP LLE methods are generally employed for the extraction of LASs from river waters and the solvent sublation method of Wickbold is still used for their extraction from seawater [85]. SPE methods making use of C18 and C8 phases are largely employed [85]. The amount of sorbent is optimized as a function of the degree of pollution and the average composition of river waters [85]. The performances of C18 disks and C18 cartridges are compared [85]. 

Figure 19 Solvent sublation (a) process mechanism and (b) an exemplary process scheme. (From Refs. 301 and 302.)...

Valsaraj, K.T., Thibodeaux, L.J., Lu, X.-Y. (1991) Studies in batch and continuous solvent sublation. III. Solubility of pentachlo-rophenol in alcohol-water mixtures and its effects on solvent sublation. Sep. Sci. Technol. 26(4), 529-538. 

According to the collection procedure for the enriched gas-liquid and/or gas-solid interfaces, adsorptive bubble separation processes or techniques can also be divided into two large categories (a) foam separation, which involves the production of foam in the process, and (b) nonfoaming adsorptive bubble separation, which involves no production of foam. Foam separation can be further subdivide into foam fractionation and flotation. Nonfoaming adsorptive bubble separation, however, can be further subdivided into bubble fractionation, solvent sublation, and noirfoaming flotation. Lemlich (84) and Wang (1,75) provided the definitions of these technical terms in their books. 

Solvent sublation involves the collection of the enriched material on the bubble surface in an immiscible liquid above the bulk liquid media. More specifically, it is a technique in which the material raised to the surface of a solution by flotation is collected and prevented from redispersing into the bulk phase. This is achieved by spreading a thin layer of an immiscible organic solvent on the surface of the solution which causes the floated material to dissolve or to become suspended. 

Karger et al. (80) have studied the effects of gas-flow rate and surfactant concentration on the rates of extraction of two dyes by solvent sublation. Because in solvent sublation there is no longer the necessity for producing a foam, they concluded that the technique should be possible to selectively remove ionic solutes or impurities from an aqueous phase at trace concentrations. 

Sheiham and Pinfold (81) have attempted to remove a cationic surfactant, hexadecyl-trimethylammonium chloride, by both solvent sublation and foam fractionation. A comparison (81) between the rates of surfactant removal by the two separation techniques shows that the separation of the cationic surfactant from 10 M solutions by solvent sublation into 2-octanol is slower but preferable if the foams are unstable. 

Solvent sublation would appear to have a potential for large-scale water quality control processes and a definite role in trace analytical separation (82). 

Type of flow pattern(s) involved in an adsorptive bubble separation system depends on the type of process used. For example, bubble fractionation involves two-phase (gas-phase and liquid-phase) bubble flow, while solvent sublation involves multiphase bubble flow in their vertical bubble cells. Foam fractionation involves a two-phase bubble flow in the bottom bubble cell, and a two-phase froth flow in the top foam cell. However, all froth flotation processes (i.e., precipitate flotation, ion flotation, molecular flotation, ore flotation, microflotation, adsorption flotation, macroflotation, and adsorbing colloid flotation) involve multiphase bubble flow and multiphase froth flow. 

Sheiham and T. A. Pinfold, The solvent sublation of hexadecyl-trimethyl-ammonium chloride. Separation Science 7(1), 43-50 (1972). 

In the present study the removal of three hydrophobic compounds of environmental significance was explored using two bubble columns of different diameters utilising solvent sublation and bubble fractionation in the countercurrent... 

In solvent sublation experiments the aqueous feed was introduced using a liquid distributor in the aqueous phase. The organic solvent on top of the aqueous phase was stagnant. In bubble fractionation experiments the top and the bottom effluent rates were monitored and adjusted as required. The aqueous feed in bubble fractionation experiments was introduced using the liquid distributor. The overall operation in both cases was thus one in which the aqueous phase was in continuous countercurrent contact with air. The air was in a once-pass-through mode. The air flow rate was measured using a soap bubble flowmeter and a rotameter. 

The organic solvent used in the solvent sublation experiments was a light mineral oil supplied by Fisher Scientific Co. It is immiscible with water, nontoxic, and has high affinity for the hydrophobic compounds considered for removal. 

Figure L Schematic of continuous bubble fractionation and solvent sublation. Table I. Relevant Properties of Compounds lJsed 7,10)...

The analysis of the data from a continuous steady state operation was described by Bruin et al. (4) for bubble fractionation and by Valsaraj and Thibodeaux (7) for solvent sublation. 

In the case of continuous solvent sublation the fractional removal in the aqueous phase at steady state is given by the ratio (7)... 

The above equation for solvent sublation applies since the entire aqueous section is used as a stripping section (Figure 1). 

Using Equations (4) and (5) the fractional removals in the stripping section of the aqueous phase in a bubble fractionation column and a solvent sublation column were obtained. 

Figures 2 and 3 show that for both bubble fractionation and solvent sublation, the fraction removed at any particular G/L is in the order NAPH > PCP > TCP. According to the models for bubble fractionation (4) and solvent sublation (5) the fractiuonal removal from the aqueous phase is primarily dependent on the "separation factor" S, given by (Hc+6Kd/d)(G/L) where d is the average bubble diameter. If G/L is held constant the degree of removal for any particular compound depends uniquely on the "effective partition constant",...

Figure 2. Effect of air/water volumetric flow ratio on the solvent sublation of PCP, TCP and NAPH on the small-scale 5 cm diameter column.

Hc+6Kd/d) between the air bubble and the aqueous phase. If a constant d of 0.1 cm is assumed and on utilizing Table I for the other parameters, we obtain values of (He+6Kyd) of 0.023, 0.011 and 0.005 for NAPH, PCP and TCP respectively. It is thus apparent that the degrees of separation are in the same order as the effective partition constants (or the separation factors) and are in accordance with the theoretical models. Previous work on semi-batch processes for both bubble fractionation and solvent sublation have led to similar conclusions (4,10). 

The effect of mineral oil volume on the degree of separation in solvent sublation was not significant. For example, at a constant aqueous depth and aqueous influent feed rate, an increase in organic solvent volume from 5 mL (0.3 cm depth) to 15 mL (0.9 cm depth) increased the steady state degree of separation for PCP from 0.38 to 0.42 at a G/L of 10 and from 0.49 to 0.50 at a G/L of 50. Just as in solvent extraction, beyond a certain organic solvent volume, the degree... 

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