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Description of the rectification process

Description of the rectification process: rectification is the process of separation of mixtures of mutually soluble components, which differ in boiling point by repeated countercurrent contacting of non-equilibrium liquid and vapor. The contacting is carried out, usually in the Columns in Belleville or countercurrent packed contact devices — steam from the bottom up, the liquid from the top down. Patrol unit is a vertical steel pipe located inside the contact devices. In the columns of Belleville contact occurs stepwise in separate stages, called plates (sieve, retaining, valve, etc.), usually by bubbling steam through a layer of liquid or spray by mixing, or otherwise providing the most effective heat and mass transfer. In the packed columns is continuous contact between the vapor and liquid in a layer of packing tape with a developed surface, which is filled with a column (gravel, rings, springs, nets, etc.). The liquid, relatively low-boiling components of the rich, which has a relatively low temperature, goes to the upper contact device. Pairs, rich high-boiling components having a higher temperature, the device enters the contact below. At the contact device liquid and vapor tend toward equilibrium by heat and mass transfer. If the equilibrium between the vapor and liquid, leaving the contact device is achieved, the contact device is called the theoretical stage or a theoretical plate. A simple distillation («moonshine») provides a one-time good contact of the liquid and vapor, and is equivalent to one theoretical stage.

The actual plate efficiency of industrial columns have 0.3 … 0.8 theoretical stages. For packed columns is the value, called the height equivalent to theoretical plate, — the height of the nozzle layer, mass transfer efficiency is equivalent to one theoretical stage. This height can be 100 … 600 mm. At the contact device pairs enriched low-boiling components, and high-boiling liquid. Passing a number of successive stages, liquid and vapor reaches the specified concentrations of the components. At the top of the column is concentrated low-boiling components, at the bottom — high. By increasing the number of steps, we can obtain any desired precision components division. At the height of the column density components vary sometimes quite nonlinear.

In continuous distillation apparatus material is introduced at about the middle height of the column, ie on that plate, where the concentration of the components are approximately equal to those of raw materials. On top of the column selected distillate rich in low-boiling components. The remainder are taken from the bottom, rich in high-boiling components. Couples with the upper plate of the column are cooled in a condenser, a part of in the form of vapor or fluid as shown in the distillate, the remainder is returned to the column as a liquid. The liquid from the bottom plate is heated in the reboiler, the liquid is selected as the bottom product (residue), and the rest in the form of steam is returned to the column. The ratio of the mass flow rate of fluid flowing from the condenser to the column to the mass flow of distillate is called the reflux ratio. The ratio of the mass flow of vapor from the boiler to the mass flow balance is the number of steam. These numbers characterize the operation of the upper (higher power) and lower (lower power) sections of the column. The higher the reflux (and steam), the number, the easier (fewer steps) accuracy is achieved by a given separation of the mixture by rectification, but also increase the specific energy costs and reduced productivity of the column. Reflux (and steam), the number can not be less than a certain minimum at which a given accuracy of rectification is not achieved by an arbitrarily large number of steps.

For the periodic rectification in the boiler appropriate volume (called a cube column) is loaded portion of the raw materials in the process of rectification is not added and the bottoms of the continuously changing the composition of the raw material to a predetermined high-boiling residue. Accordingly, on top of the column selected distillate composition changing over time. If the number of mixture components is small (2 … 5), and the number of stages and reflux ratio are sufficient for a relatively clear separation, the composition of the distillate and the temperature of the upper plate is changed in steps, first the distillate consists of a very concentrated low-boiling component (called first pillar) , followed by a short transition period, when the distillate is a mixture of variable composition, in which the concentration of the first component decreases and the concentration of the second component increases, then the concentrated distillate consists of a second component, etc. for all components. Distillate transitional periods are traditionally called bad cuts, it is mixed with the next portion of raw materials. If the sharpness of separation is small and / or large number of components (oil mixture), the aliasing of the distillate becomes invisible, the composition of the distillate and the temperature of the upper plate is continuously changing.

Multi-component mixtures can be separated into individual components of the re-distillation of narrow fractions of distillates containing even a small number of components.

Features distillation of petroleum mixtures are due to the quality requirements of the fractionation and the fact that oil mixtures are composed of thousands of components. Multicomponent mixtures of oil causes a continuous part in the distillate of the periodic rectification of any practicable number of stages and reflux ratio. The quality of fractionation is determined by results of simple distillation (standard ASTM D86) of samples of this fraction, the temperature of 5% and 95% distillation. Standards for petroleum products is determined appropriate, that the overlap of 95% and temperatures of 5% between adjacent distillation fractions should be no more than 10 … 15C. For example, if 95% of the gasoline fraction obtained by this column, is distilled to no more than the D86 at 180C, then 5% of the diesel fraction obtained in the same column, should drive away on the D86 at least in the 170S.

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