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The pressure at the top is maintained at 1.2–1.5 atm [2] so that the distillation can be carried out at close to atmospheric pressure, and therefore it is known as the atmospheric distillation column. [3] The vapors from the top of the column are a mixture of hydrocarbon gases and naphtha, at a temperature of 120 °C–130 °C.
Vacuum distillation or distillation under reduced pressure is a type of distillation performed under reduced pressure, which allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This technique separates compounds based on differences in their boiling points.
Pressure-swing distillation is essentially the same as the unidirectional distillation used to break azeotropic mixtures, but here both positive and negative pressures may be employed. This improves the selectivity of the distillation and allows a chemist to optimize distillation by avoiding extremes of pressure and temperature that waste energy.
Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize. It uses distillation to fractionate.
However, ordinary distillation below the boiling point is not practical because a layer of vapor-rich air would form over the liquid, and evaporation would stop as soon as the partial pressure of the vapor in that layer reached the vapor pressure. The vapor would then flow to the condenser only by diffusion, which is an extremely slow process.
Overall the pressure-swing distillation is a very robust and not so highly sophisticated method compared to multi component distillation or membrane processes, but the energy demand is in general higher. Also the investment cost of the distillation columns is higher, due to the pressure inside the vessels.
A process of heating oil shale to produce shale oil, oil shale gas, and spent shale is commonly called retorting. Airtight vessels to apply pressure as well as heat are called autoclaves . In the food industry , pressure cookers are often referred to as "retorts", meaning " canning retorts" for sterilization under high temperature (116–130 °C).
The pressure in a space cannot be in equilibrium with the temperatures of the walls of both subspaces. It has an intermediate pressure. Then the pressure is too low or the temperature too high in the first subspace, and the water evaporates. In the second subspace, the pressure is too high or the temperature too low, and the vapor condenses.