Distillation and Evaporation PDF

5. Distillation Definition Distillation is the process in which a liquid is vapourised (turned into vapour), re condensed (turned back into a liquid) and collected in a container The boiling point of mixtures depends upon mole fraction of the component present i.e : a) In pure substan...

5. Distillation Definition Distillation is the process in which a liquid is vapourised (turned into vapour), re condensed (turned back into a liquid) and collected in a container The boiling point of mixtures depends upon mole fraction of the component present i.e : a) In pure substances the temperature remains constant during distillation process so long as both vapor and liquid are present. b) In miscible liquid mixture the temperature increases throughout process because composition of vapor changes continuously. 1: Heat source 2: Still pot 3: Still head 4: Thermometer/Boiling point temperature 5: Condenser 6: Cooling water in 7: Cooling water out 8: Distillate/receiving flask 9: Vacuum/gas inlet 10: Still receiver 11: Heat control 12: Stirrer speed control 13: Stirrer/heat plate 14: Heating (Oil/sand) bath 15: Stirrer bar/anti-bumping granules 16: Cooling bath. Types of distillation Simple distillation Fractional distillation Steam distillation Vacuum distillation Destructive distillation Find out; Øwhat they are ØDifferences between them ØApplication of each type 6. Evaporation Introduction Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which instead occurs on the entire mass of the liquid Theoretically evaporation means free escape of vapours from the surface of a liquid below its boiling point. As evaporation is a very slow process, therefore a liquid is usually boiled / heated to speed up this process. Factors affecting evaporation üTemperature üSurface area üAgitation üAtmospheric aqueous pressure üType of product required üEconomic factors How do these factors affect evaporation? Temperature The rate of evaporation is directly proportional to the temperature; thus, the higher the temperature, the higher the rate of evaporation. However, maximum evaporation is attained at the boiling point of the liquid. Thermostable preparations can be evaporated at higher temperatures whiles thermolabile preparations will have to be evaporated at low temperatures Many substances like glycosides, alkaloids, hormone, enzymes snd antibiotics may get decomposed even at temperature below 100 degrees Celsius and hence they should be evaporated at low and careful temperature. Extremely heat sensitive substances are evaporated under reduced pressure. Temperature Time required for evaporation is very critical. Exposure to a higher temperature for a shorter period of time may be less destructive than a lower temperature with exposure for a longer period of time. Therefore, evaporation should be carried out for a short time at low temperature Evaporation in some cases may be carried out at a low controlled temperature till a concentrated product is obtained. Surface area The rate of evaporation is directly proportional to the surface area of the content vessel exposed. The greater the surface exposed to evaporation, the higher the rate of evaporation Agitation During evaporation, the upper layers of the liquid to be evaporated has a tendency to form a scum or a layer which lowers the rate of evaporation Stirring or agitating the solution will prevent the decomposition of the preparation at the bottom of the container Atmospheric aqueous vapour pressure If the atmospheric aqueous vapour pressure or the moisture contents in the air are high, the rate of evaporation will be slow but if it is less, the rate of evaporation will be high. Hence, the rate of evaporation can be increased by free circulation of warm air over the evaporating pan Atmospheric pressure on the liquid under evaporation The rate of evaporation is inversely proportional to the atmospheric pressure on the surface of the liquid. If the atmospheric pressure on the liquid is reduced to half, then the rate of evaporation will be doubled. Due to this reason, many instances of evaporation is done under reduced pressure, i.e. by producing vacuum over the surface of the liquid Type of product required The selection of the method and apparatus to be used for evaporation depends upon the type of product required. Evaporating pans or stills may be used to produce liquids or dry prodcuts Economic factors While selecting the method and apparatus to be employed in evaporation, due consideration must be given to cost of labour, fuel, floor area and materials. The recovery of the solvents and the utilization of ‘waste’ heat are also important factors which contribute significantly to reduction of overall cost