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 (126.96.36.199.2); the technical side will be dealt with here [41-45].
Rectification can be carried out:
Laboratory equipment for the two important types is depicted in Fig. 2.51.
Discontinuous rectification is characterised by non-recurring feed and a separation column. To generate a reflux, an overhead condenser is inserted and a cold trap cools the obtained distillate. For analytical-preparative purposes in the laboratory, an apparatus with annulus columns has proved to be successful. This apparatus has a vacuum of up to 0.01 mbar, an output of 1-50 ml/h and low hold-up volume. Larger rectification units have tray columns, sieve plates and packing material with a vacuum of up to 0.1 mbar and outputs of several litres per hour.
On the one hand, the simple, easy to control mode of operation, which is especially called for in the laboratory or with trial runs, is of advantage. On the other hand, the long time of direct contact, the high energy requirements and the difficult automation, as product and distillate change continuously, have turned out to be of a disadvantage.
Therefore, partly continuous rectification is a mode for special applications. In this case, the mixture to be separated is introduced into the flask as distillate is removed. The mixture is fed into the flask or just above the flask and proceeds at boiling temperature. The condensation heat from the overhead condenser can be employed as a heat source. After a certain time when the flask is filled, the feed is stopped and the high-boiling constituents are separated discontinuously. This technology is used for removing the volatile first runnings or the solvent from high-boiling components. Again, the mode of operation is simple; the long thermal exposure of the flask contents is not desirable.
Continuous rectification does not suffer from these disadvantages. Here, a continuous feed between amplifier and stripping column exists. In industrial applications, the following criteria should be considered in operating a rectification unit at economical cost :
- reduction of the operation pressure increases relative volatility and reduces the energy input
- optimisation of reflux ratio (minimum reflux)
- in vacuum rectification, the low pressure loss per plate permits energy savings
- the heat of the leaving distillate and especially of the bottom flow can be utilised for preheating the feed
- the usage of heating pumps in applications with high energy demand.
For heating pumps, employing a direct product stream, e.g. vapour recompression, results in a higher difference between head and sump temperature than the usage of an external compression fluid. After adjustment of the operating conditions, a head and a sump product form. The reduced thermal exposure is of advantage, as well as the low energy requirements and the high output. Methods developed on a laboratory scale can be transferred to semi-industrial and industrial units.
If multi-compound mixtures are present, it is necessary to introduce the bottom product into a further continuous unit. Mixtures with n components require n-1 separation columns. This technology is used for processes on a large industrial scale. In the flavour industry, mixtures with three or more components are first separated into two to three fractions and then subjected to a discontinuous separation process. The after-run may be separated from its high-boiling constituents by thin-film evaporators.
The essential parameters for the construction of a column are:
- vapour loading coefficient: this value is characteristic for the specific throughput
- the height of theoretical plates: this value shows the separation efficiency HETP
- the pressure loss per plate: important for usage under vacuum.
The columns can be structured into three basic types: tray, filling bodies and packed columns [47-49].
Tray columns are available in lengths up to 100 m and with very large diameters. The interfacial area in the column is generated by the vapour which permeates through the holes with high velocity and collides with the descending liquid to form dispersions. Tray columns show a broad range of vapour loading capacity with high separation efficiency, even with small loads. The number of theoretical plates is, depending on the technical construction, relatively high; the same applies to capacity. The loss of pressure with 2-5mbar per plate has a negative impact on the operation under vacuum.
A number of different constructions have been developed for tray columns; the essential ones are still bubble-cap, valve, sieve and grating plates. Bubble-cap plates are the oldest development and, due to the high production costs, they are rarely used today.
Sieve plates have also been known for a long time and possess fine drillings. The vapour flows through them and comes into exchange with the liquid, thus forming a bubble layer. This reduces the loss of pressure, but, as a result of their construction, they also have a lower vapour loading coefficient. The danger of encrustation is, however, high, as the drillings are blocked easily. Cross-flow sieve plates and crosscurrent sieve plates are available from a number of manufacturers and they are optimally designed to meet individual requirements.
Column fillings with irregular packings have found increasing application. The cylindrical Raschig rings have been replaced by Pall rings, Berl saddles, Intalox and grating rings. Due to their high surface, these columns possess good separation efficiency and show small loss of pressure. The disadvantage is the poor distribution of the liquid, especially with larger column diameters. This can be countered by the insertion of a special distribution device. Since the phase transfer should use the high surface area of the fillings, the wetting characteristics of these materials are important. Adequate packing material can ensure appropriate wetting. Apart from different metals, glass, porcelain, ceramics, carbon and plastics have been used .
The ordered structure of the regular packings column with uniform flow canals allows a precise phase distribution and small loss of pressure  (Fig. 2.52). The very large surface area results in a high number of theoretical plates. Since regular packings ensure a good distribution of the liquid, a high vapour loading is encountered. Due to the capillary mechanism of the net structure, the packings also work at very low vapour loading. Again, columns with bigger diameters should have a good liquid distribution device and wetting of the packings also has to be taken into account. These regular packings are prone to encrustations and expensive to produce.
As a result of their positive characteristics, packings have found a broad range of application in the flavour industry . Both regular and irregular packings result in a small HETP value with very low pressure loss in the column. These are important features for the use of these packings in the flavour industry.
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