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Batch distillation: In many instances processes are carried out in batches, and it is more convenient to distil each batch separately. In these cases the whole of a batch is run into the boiler of the still and, on heating, the vapour is passed into a fractionation column. As with continuous distillation, the composition of the top product depends on the still composition, the number of plates in the column and on the reflux ratio used. When the still is operating, since the top product will be relatively rich in the more volatile component, the liquid remaining in the still will become steadily weaker in this component. As a result, the purity of the top product will steadily fall.
1. This method of operating a batch still requires a continuous increase in the reflux ratio to maintain a constant quality of the top product. 2. An alternative method of operation is to work with a constant reflux ratio and allow the composition of the top product to fall. engineering-resource.com
One of the added merits of batch distillation is that more than one product may be obtained. Thus, a binary mixture of alcohol and water may be distilled to obtain initially a high quality alcohol. As the composition in the still weakens with respect to alcohol, a second product may be removed from the top with a reduced concentration of alcohol. In this way it is possible to obtain not only two different quality products, but also to reduce the alcohol in the still to a minimum value. This method of operation is particularly useful for handling small quantities of multi-component organic mixtures, since it is possible to obtain the different components at reasonable degrees of purity. engineering-resource.com
Operation at constant reflux ratio If the same column is operated at a constant reflux ratio R, the concentration of the more volatile component in the top product will continuously fall. Over a small interval of time dt , the top-product composition with respect to the more volatile component will change from xd to xd + dxd, where dxd is negative for the more volatile component. If in this time the amount of product obtained is dDb, then a material balance on the more volatile component gives:
Plate columns: In order to translate these quantities into an actual design the following factors should be considered: (a) The type of plate or tray. (b) The vapour velocity, which is the major factor in determining the diameter of the column. (c) The plate spacing, which is the major factor fixing the height of the column when the number of stages is known.
Types of trays: It should provide intimate mixing between the liquid and vapour streams, that it should be suitable for handling the desired rates of vapour and liquid without excessive entrainment or flooding, that it should be stable in operation, and that it should be reasonably easy to erect and maintain. The arrangements for the liquid flow over the tray depend largely on the ratio of liquid to vapour flow.
Types of flow: (a) Cross-flow. Normal, with a good length of liquid path giving a good opportunityfor mass transfer. (b) Reverse. Downcomers are much reduced in area, and there is a very long liquid path. This design is suitable for low liquid–vapour ratios. (c) Double-pass. As the liquid flow splits into two directions, this system will handle high liquid–vapour ratios.
Sieve or perforated trays: These are much simpler in construction, with small holes in the tray. The liquid flows across the tray and down the segmental downcomer.
bubble-cap tray: This is the most widely used tray because of its range of operation. The individual caps are mounted on risers and have rectangular or triangular slots cut around their sides. The caps are held in position by some form of spider, and the areas of the riser and the annular space around the riser should be about equal.
Valve trays: These may be regarded as a cross between a bubble-cap and a sieve tray. The construction is similar to that of cap types, although there are no risers and no slots. It may be noted that with most types of valve tray the opening may be varied by the vapour flow, so that the trays can operate over a wide range of flowrates. Because of their flexibility and price, valve trays are tending to replace bubble-cap trays.
Factors determining column performance (a) Liquid and vapour velocities. (b) Physical properties of the liquid and vapour. (c) Extent of entrainment of liquid by rising vapour streams. (d) The hydraulics of the flow of liquid and vapour across and through the tray.
Operating ranges for trays: For a given tray layout there are certain limits for the flows of vapour and liquid within which stable operation is obtained.
Foaming: Foaming refers to the expansion of liquid due to passage of vapour or gas. Although it provides high interfacial liquid-vapour contact, excessive foaming often leads to liquid buildup on trays. In some cases, foaming may be so bad that the foam mixes with liquid on the tray above. Whether foaming will occur depends primarily on physical properties of the liquid mixtures, but is sometimes due to tray designs and condition. Whatever the cause, separation efficiency is always reduced.
Entrainment: Entrainment refers to the liquid carried by vapour up to the tray above and is again caused by high vapour flow rates. It is detrimental because tray efficiency is reduced: lower volatile material is carried to a plate holding liquid of higher volatility. It could also contaminate high purity distillate. Excessive entrainment can lead to flooding.
Weeping/Dumping: This phenomenon is caused by low vapour flow. The pressure exerted by the vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping. That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency.
Flooding: Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. The increased pressure from excessive vapour also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above. Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.
