Solvent vapour stripping

Now lower the paper-strip, which has become saturated with the solvent vapour molecules, so that it dips about 5 mm. into the solvent. Then close the cylinder, and set it aside as before for 12-18 hours, by which time the solvent will have risen about three-quarters of the height of the strip. 

This experiment requires less time than the former, for the paper strip comes into equilibrium with the solvent vapour much more rapidly, and can then be inserted into the solvent without intermediate drying. 

Group IIIB. Ni, Co, Mn, and Zn The metals are present as chlorides in dilute hydrochloric acid. The solvent used is acetone containing 5 per cent (v/v) water and 8 per cent (v/v) hydrochloric acid (d 1.18). The separation is conducted in an atmosphere saturated with respect to the solvent. The strip is dried (after a solvent movement of about 25 cm), exposed to ammonia vapour, and then sprayed with a saturated alcoholic solution of alizarin containing 0-1 per cent rubeanic acid and 1 per cent salicyladoxime. The following results are obtained (cf. also Fig. VI.5b). 

The test solution should not contain more than 5 per cent (v/v) concentrated hydrochloric acid and must have a pH < 2. It is spotted on a paper strip and allowed to evaporate for 10-15 minutes. Diffusion of the solvent takes place in an atmosphere saturated with respect to the vapour of a saturated solution of methyl acetate in water, and the temperature is maintained constant at 22°. The solvent moves sufficiently far in 20-30 minutes to effect a complete separation. After evaporation of the solvent, the strip is made alkaline by exposure to ammonia vapour and then sprayed with a 1 per cent solution of diphenyl-carbazide in alcohol. Mercury is indicated by a narrow blue band in the dry solvent front. 

Selenium and tellurium The elements are present as selenite and tellurite in dilute nitric acid solution. The mixture is spotted upon paper and dried thoroughly in the air. The solvent is dry n-butyl alcohol containing 4 per cent (v/v) of dry methanol. The atmosphere in the separation vessel is saturated with respect to the solvent vapour and the relative humidity is also maintained at 50 per cent by means of a saturated solution of calcium nitrate. The solvent is allowed to diffuse 8-10 cm down the strip (c. 2 hours). After evaporation of the solvent, the strip is sprayed with 0 5m tin(II) chloride in dilute hydrochloric acid. The tellurium is indicated by a black band (RF 0 1) and the selenium as an orange band (RF 0 5). It is possible to detect 1-5 pg of Se in the presence of 1 mg of Te by this method (see also Fig. VI.5g). 

The air in the chamber is saturated with the solvent vapour. For keeping an approximately constant temperature, the walls of the chamber and its bottom are covered with strips of filter paper and the chamber is covered with close-fitting lid. Usually, the solvent front is allowed to ascend about 10 cm above the origin before the plate is removed. The solvent front is carefully marked with a sharp pencil. The solvent in the layer evaporates within a few minutes. Heat can be applied if necessary, and then the plate is ready for location of the compounds. 

Note. The period of 5-8 hours recommended above for attaining an equilibrium between the vapour molecules of the mixed solvent and those absorbed by the paper strip is essential if accurate R values are required for identification of mixed amino-acids. To illustrate the separation, as in the above experiment, this period may be reduced to about 2 hours. 

Scandium, thorium and the rare earths The metals are present as nitrates in dilute nitric acid solution. The solvent is 2-methyltetrahydroxyfuran (tetrahydrosylvan) containing 5 per cent (v/v) water and 10 per cent (v/v) nitric acid (d. 1 42). The mixture is spotted upon paper and dried thoroughly in the air. The relative humidity inside the extraction vessel is maintained at 80 per cent by means of a saturated solution of ammonium chloride. After the solvent has diffused about 15 cm down the solvent strip, it is allowed to evaporate, and the strip is placed for about 10 minutes in an atmosphere of ammonia vapour. The paper chromatogram is then sprayed with an alcoholic solution of alizarin and finally with 2m acetic acid. The following results are obtained. 

Steam distillation is used when normal distillation is not an option, due to thermal sensitivities. By adding water or steam, the boiling points of compounds can be depressed, allowing them to evaporate at lower temperatures. After distillation the vapours are condensed and typically yield two phases water and organics. These can then be easily separated on the basis of their different densities. On a simple level, this can be seen every day in fresh black coffee, where small amounts of organic, water-immiscible oils float on the surface. Steam distillation is employed on an industrial scale in the manufacture of essential oils (e.g. lavender, eucalyptus and fruit oils) that are used in the food and flavour industries. In this area, the use of water is highly desirable as a natural solvent. Steam distillation is also used in petroleum refineries and petrochemical plants, where it is commonly referred to as steam stripping . 

An alternative, more specialized procedure for small-scale use involves the use of vaporizing devices in which a high concentration of an insecticide is dispersed on a solid carrier from which during heating (mosquito mats) or burning (smokes or mosquito coils), or sometimes at mom temperature (impregnated plastic strips), an insecticidal vapour is produced. When used in such products PBO acts more as a solvent or evaporation retardant which evens out the rate of volatilization than as a synergist. 

It needs a vapour pressure well above or below that of the solvent being recaptured and no azeotrope with it. If the scrubbing liquid boils below the solvent, comparatively little solvent will need to be evaporated in the stripping column (e.g. methanol stripped from water) while if the solvent is less volatile, the stripping column will need to remove large amounts of water when recapturing dimethylformamide (DMF). 

For the very low concentration of dissolved solvent commonly found in air stripping, the value of -y can be treated as "y . Values of "y are available for a large number of single solvents in water (Table 3.8). They can be obtained from vapour-liquid equilibrium data and, for solvents such as hydrocarbons and chlorinated hydrocarbons that are very sparingly soluble in water, from solubility data. 

Steam stripping is not suitable for the water-miscible, high-boiKng solvents listed in Table 3.8. These have lower values of than the vapour pressure of water at 25 °C, which is 23.3 mmHg. In addition to these, cyclohexanol and butyl Cellosolve require a lot of stripping stages and may be better removed from water by extraction. 

In a more practical system, there would be involatile residue in the solvent and the vapour pressure of the solvent would be "yxp i. Until most of the solvent had been stripped out, y is likely to be close to unity. 

Eventually when all the solvent in a batch process is stripped out, the vapour would be all steam. 

Stripping. This technique is particularly useful for the recovery of low concentrations of solvents with low water solubilities and/or high volatilities with respect to water. Recapture of the solvent from the air stream can be accomplished by an adsorption process, such as activated carbon, as described above. This is a limitation of the system as the concentration of the solvent in the vapour phase may be very low resulting in excessive cost and complexity. The use of steam as the stripping medium results in easier methods for liberation of the solvent. Thus, for solvents which are immiscible with water then several methods, such as decanting, are now more appropriate as the proportion of solvent to water has increased. 

MEMBRANE VAPOUR PERMEATION SOLVENT STRIPPING OF ARTIFICIAL SYNTHETIC POLYISOPRENE LATICES AND OTHER LATICES... 

---