Countercurrent rectification

If a waste contains a mixture of volatile components that have similar vapor pressures, it is more difficult to separate these components and continuous fractional distillation is required. In this type of distillation unit (Fig. 4), a packed tower or tray column is used. Steam is introduced at the bottom of the column while the waste stream is introduced above and flows downward, countercurrent to the steam. As the steam vaporizes the volatile components and rises, it passes through a rectification section above the waste feed. In this section, vapors that have been condensed from the process are refluxed to the column, contacting the rising vapors and enriching them with the more volatile components. The vapors are then collected and condensed. Organics in the condensate may be separated from the aqueous stream after which the aqueous stream can be recycled to the stripper. 

Plate column Cross-flow, countercurrent Integral Liquid and/or gas Absorption, rectification, stripping... 

REFLUX. In distillation processes in which a fractionating column is used, the term reflux refers to the liquid that has condensed from the rising vapor and been allowed to flow back down the column toward the still. As it does so. it comes into intimate contact with the rising vapor, resulting in improved separation of the components. The separation resulting from contact of the countercurrent streams of vapor and liquid is called rectification or fractionation,... 

A rectification zone in which the dissolved bntenes fraction is displaced from the solvent in countercurrent flow with butadiene t apdr, which is obtain by controlled reboiling of the extract. These butenes, which contain butadiene, are returned to the absorption zone. A sidestream is drawn off at a level where the olefin content is practically nil, the acenlene content low, and the 1,3-butadiene content a maximum. Separation takes place in a column with about 45 travs operatins at the bottom around 75 C at 0.7. 10 Pa. 

A general scheme for the countercurrent ion-exchange separation process with flow reversal is analogous to the schemes for such two-phase separation processes as rectification, chemical exchange, etc. (Fig.l). 

In mass transfer apparatus one of two processes can take place. Multicomponent mixtures can either be separated into their individual substances or in reverse can be produced from these individual components. This happens in mass transfer apparatus by bringing the components into contact with each other and using the different solubilities of the individual components in the phases to separate or bind them together. An example, which we have already discussed, was the transfer of a component from a liquid mixture into a gas by evaporation. In the following section we will limit ourselves to mass transfer devices in which physical processes take place. Apparatus where a chemical reaction also influences the mass transfer will be discussed in section 2.5. Mass will be transferred between two phases which are in direct contact with each other and are not separated by a membrane which is only permeable for certain components. The individual phases will mostly flow countercurrent to each other, in order to get the best mass transfer. The separation processes most frequently implemented are absorption, extraction and rectification. 

In a similar manner to that for the absorption column a linear relationship between the compositions of the two phases can be found for extraction and rectification. To illustrate this we will look at a rectification column. The basic process of rectification is when boiling a multicomponent mixture the vapour generated flows upwards countercurrent to the condensate which falls down the column. As the condensate is colder than the vapour, the components with higher boiling points, the least volatile, condense. They release their enthalpy of condensation to the components with the lower boiling points, the so called more volatile components, which are vaporized. This causes the vapour to become rich in the more volatile components while the less volatile components make up the liquid. The... 

The equipment for continuous distillation can only separate one stage in the equilibrium diagram. Countercurrent distillation, also called rectification, has found widespread application with normal pressure and coarse vacuum distillation when complex mixtures or components with small relative volatility factor are to be separated. The fundamentals are discussed above (2.1.3.3.2) the technical side will be dealt with here [41-45]. 

Since most continuous extraction methods use countercurrent contacts between two phases, one a light liquid and the other a heavier one, many of the fundamentals of countercurrent gas absorption and of rectification carry over into the study of liquid extraction. Thus questions about ideal stages, stage efficiency, minimum ratio between the two streams, and size of equipment have the same importance in extraction as in distillation. 

The tedious separate steps of vaporization and condensation are therefore combined in practice by the process of countercurrent distillation (rectification). In a distillation column, the ascending hotter vapor is flowing past the cooler reflux, i.e., a part of the condensate that is flowing back (Fig. 14.19). The close contact between the reverse flows favors a fast entropy and energy exchange. Temperature and composition are therefore close to the particular equilibrium (which depends upon the height in the column) but never reach it. 

According to the previous chapters the reader may come to the conclusion that countercurrent columns are dominant as has been shown for mass transfer equipment used in the areas of rectification, absorption, and extraction. However, this is not true because the continuous transport of solid granular material is much more difficult in comparison to a fluid. Therefore, nearly all adsorbers are fixed beds which are operated batchwise. As a rule, at least two fixed beds are installed in continuously operated industrial processes. The first bed is used for the adsorption step whereas in the second the adsorbates is removed or desorbed at the same time. The duty of the two beds is changed when the adsorption capacity is exhausted. Sometimes several beds are arranged to cany out pressurization and depressurization steps. 

The terms "number of stages N" and "height equivalent to a theoretical plate HETP" are used in chromatography however, their meanings are quite different in comparison to continuously operated countercurrent columns (absorption, extraction, rectification). Here, the column length or height must be sufficient to draw apart the bands of components or fractions. This reqnires favorable eqnUibria, certain retardation differences, and limited axial dispersion. 

If phase I is generated by phase transition or phase reversal from phase II in a heat exchanger on top of a counterflow column, a reflux principle is used instead of the simple flow-through principle (see Fig. 1-51). Phase I is no longer a fresh feed to the column but is the reflux generated by converting phase II into phase I this is the case in countercurrent distillation or rectification. 

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