Sunday, 29 June 2008

Purpose and Process Essence of Distillation

Purpose and Process Essence of Distillation

Distillation is the oldest and the most universal process of chemical technology and other branches of industry incorporating separation of mixtures.

Practically, all natural substances and substances produced in the chemical reactors are mixtures that do not have the properties required for using them in techniques and for household needs. These mixtures should be separated into components or groups of components.

Distillation has substantial advantages over other processes applied in order to separate a mixture, such as extraction, crystallization, semipermeable membranes, etc. As a rule, it is the most cost-effective process, so it may be used for mixtures with very diverse properties. This process is based on the fact that the composition of the boiling liquid and that of the vapor over it differ. Thus, if the boiling temperature is low (e.g., air separation), it is necessary to use low-temperature refrigerants and conduct the process at a higher pressure. If it is high (e.g., in separation of heavy oil fractions or metals), high-temperature heat carriers or fire preheating have to be used and the process is run under vacuum.

If the composition of the boiling liquid and that of the vapor over it are quite close (e.g., xylene isomers or isotope separation), there is substantial energy consumption, which results in high capital costs. It is impossible to conduct the distillation process in the case of azeotropic composition (i.e., if the composition of the boiling liquid and that of the vapor are identical).

If the detrimental chemical reactions take place at the boiling temperature of mixture (i.e., the mixture is thermolabile), it is also impossible to run the distillation process.

So if we use the basic theory of distillation and the methods that follow from it, the cost of separation of the mixtures of substances with close boiling temperatures can be decreased, and the problems of azeotropic and thermolabile mixtures separation may also be substantially overcome.On the whole, in this book, special attention will be paid to the problems mentioned above.

(a) A distillation column with condenser, reboiler, and reflux capacity;

(b) control volume (dotted line) for obtaining material balance equations for the top


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