Rectification (process engineering)
The rectification also counterflow distillation called, is a thermal separation process for separating a homogeneous solution of two or more substances.
The rectification is a thermal separation process and provides an extension of the distillation or a series connection is many distillation steps. The main advantages of rectification are that the system can be operated continuously and that the separation effect in comparison with the distillation by a multiple greater, as the vapor is in countercurrent contact with the liquid several times in succession. The column works in a more energetically efficient, technically less complex and space-saving manner than a series connection of single distillations.
At each of these additional contact surfaces, a mixture with a different concentration condenses from the vapor mixture according to the phase equilibrium , while as a result of the condensation that is released, a mixture with a higher concentration of the more volatile component gradually evaporates from the liquid phase.
If a liquid mixture is evaporated, the concentrations of the individual substances in the gas and in the liquid phase are determined by the temperature and the pressure. If the contact is long enough, an equilibrium is established. Only in the case of the pure substance and an azeotropic mixture is the equilibrium composition in the gas phase and in the liquid phase the same. In all other cases, the concentration of the low boiler in the gas phase is higher than in the liquid phase. If - as a model - equilibrium can be established on a column tray, one speaks of a theoretical tray or a theoretical stage; a column made up of such stages is an ideal column.
Example of an ideal column
As an example, the conditions in an ideal column with two trays, which are in ideal equilibrium, should be considered. The substances to be separated, a low boiler A and a high boiler B, should have the same enthalpy of vaporization . An evaporator in the sump ensures complete evaporation. Total condensation takes place in the head. From this it follows: Incoming and outgoing material flows are not to be taken into account. The upward gas flow and the downward liquid flow are always the same, the column is operated with so-called “infinite reflux” and two so-called theoretical trays.
The gas stream from the evaporator and the liquid stream from the second tray meet on the first tray. The gas stream partially condenses, with a larger proportion of the condensate being attributable to the high boiler and a smaller proportion to the low boiler, so that the proportion of the low boiler in the remaining gas flow increases while that of the high boiler decreases. The heat of condensation released in the process in turn contributes to the fact that the low boiler preferably passes into the gas phase on this first tray and thus an enrichment of the low boiler takes place in the gas stream. This equilibrium of high boilers and low boilers in the gas stream continues to rise to the second tray and is again subject in the same way to the depletion process of the high boiler in the gas stream or the enrichment of the low boiler, so that the high boiler is further separated from the low boiler. The separation efficiency of this principle increases the further the boiling points of the relevant components are apart and the more theoretical plates such a column has.
This can also be illustrated using a so-called rectification diagram in which the two components are shown as a function of their new mass equilibrium in the respective theoretical soils.
The description in theoretical stages is based on a primarily thermodynamic limitation of a column with trays. The description of a column as being mass transfer limited is closer to physical reality.
If a thermodynamic limitation is assumed, a tray efficiency of max. 0.7 reached because the equilibrium state is not reached. For columns with packings, the idea of equilibrium levels no longer has any real equivalent. Only the number of theoretical steps per meter can be specified here. A distillation column in the laboratory with a structured carbon fabric packing achieves up to 50 theoretical ( mixer-settler ) stages per meter. A slot-bottom distillation column , on the other hand, only has 0.7 to 2 theoretical stages per meter.
A detailed modeling and measurement of the mass transfer is difficult. The separation efficiency of a column is therefore usually given by the manufacturers in theoretical stages for a specific mixture and with infinite reflux.
For the design of columns with packings, HTU-NTU calculations (HTU: height of transfer unit, NTU: number of transfer units) are used. However, the HTU must be determined experimentally.
The design equations for columns are so good that even large columns are built purely on the basis of laboratory values.
Separation of azeotropic mixtures
As with distillation, only non- azeotropic mixtures can be separated under normal conditions . If it is necessary to separate an azeotropic mixture, the azeotropic point must be shifted. It must not be in the concentration and temperature range of the system. A shift occurs, for example, by changing the operating pressure or by adding an auxiliary substance.
Operation of columns
A distinction is made between a continuous and a discontinuous ( English batch ) mode of operation.
- In the continuous mode of operation, the mixture to be separated is continuously fed in and the top or bottom product is continuously withdrawn. The composition (at the top, bottom, bottom) remains the same, the column is in a quasi-steady state of equilibrium.
- In the discontinuous mode of operation, a certain amount of mixture is initially introduced and the rectification column is started up until an equilibrium has been established on all (theoretical) trays. This happens with an infinite return. Then the desired top or bottom product is removed. The composition changes with the extraction, there is a shift in equilibrium. If the product no longer meets the requirements, the batch is canceled and a new batch is started.
- The two-pressure process (also called pressure swing distillation ) uses the pressure dependency of the azeotrope. It is used, for example, to separate THF and water or ethanol and water.
- With extractive rectification (Distex process), the position of the azeotrope is influenced by the addition of an auxiliary substance . E.g. a mixture of benzene and cyclohexane can be separated by adding aniline .
- In the case of azeotropic rectification , an auxiliary substance is used which, in the case of close-boiling mixtures, forms a new azeotrope with one of the substances and acts as an azeotrope converter in the case of azeotropic mixtures.
The McCabe-Thiele method
The basis for the McCabe-Thiele method is the equilibrium diagram of the basic mixture of substances. Since in practice rectification is generally carried out at constant pressure, the isobaric equilibrium diagram is the basis of the McCabe-Thiele method. It is created by applying the mole fractions of a component (usually lower boiling) in the liquid phase to the corresponding mole fractions of the same component in the gas phase.
The Ponchon Savarit Method
In contrast to the McCabe-Thiele method, the energy balances are not neglected in the Ponchon-Savarit method. The graphic evaluation takes place in the enthalpy-composition diagram (H (x, y)).
The multi-stage distillation in columns is a frequently used and well-mastered method of thermal material separation in the chemical industry. Other methods are used if the properties of the material do not permit multi-stage distillation.
The petroleum refining uses the multi-stage distillation columns. Crude oil is also stripped in the column with steam and at the same time chemically changed.
In so-called air separators, the liquefied air is separated into its components by rectification using the Linde process . Here are nitrogen , oxygen and argon recovered from the air. In very large systems, a helium / neon mixture is also obtained, which in a second step is divided into pure neon and helium. Other constituents of the air, which are only found in very small quantities in the air, are generally not produced because there are other, cheaper methods of obtaining them. As an alternative to air separation, adsorption processes are sometimes used, especially for smaller systems .
Use in the laboratory
When synthesizing substances and isolating or purifying natural substances in the laboratory , mixtures of liquids with small boiling point differences must be separated by rectification. Different separation columns are used depending on the size of the boiling difference. A frequently used rectification column is the Vigreux column .
- Klaus Sattler: Thermal separation process . 3. Edition. Wiley-VCH, Weinheim 2001, ISBN 3-527-30243-3 , pp. 124 .
- Klaus Sattler: Thermal separation process . 3. Edition. Wiley-VCH, Weinheim 2001, ISBN 3-527-30243-3 , pp. 156-159 .
- Klaus Sattler: Thermal separation process . 3. Edition. Wiley-VCH, Weinheim 2001, ISBN 3-527-30243-3 , pp. 95-97 .
- Animation of theoretical soils in ChempaPedia.
- Klaus Sattler: Thermal separation process . 3. Edition. Wiley-VCH, Weinheim 2001, ISBN 3-527-30243-3 , pp. 198-199 .