Plate Columns and Comparison of Tray Types

    For purpose of distillation, plate columns and packed columns can be used. In plate columns each plate constitutes a single stage, or in packed columns where mass transfer is between a vapor and liquid in continuous countercurrent flow.

     In order to design a plate type distillation column, following factors must be considered:

1) The type of plate or tray
2) The vapor velocity, which is the major factor that determines the diameter of the column.
3) The plate spacing, which is the major factor fixing the height of the column when the number of stages is known.

Types of Trays:
     The main requirement of a tray is that it should

a) provide an intimate contact of vapor and liquid phases, because more the contact of these two, more will be the mass transfer which brings about more enrichment.

b) it should be capable of handling more desired flow rates of vapor and liquid without excessive entrainment or flooding.

c) be stable in operation or have flexibility in operation.

d) be reasonably easy to erect and maintain.

Arrangement of Flow:
     The arrangements for the liquid flow over the tray depend largely on the ratio of liquid to vapor flow. Three layouts are shown here, of which the cross-flow array is much the most frequently used.

a) Cross-Flow: Normal, with a good length of liquid path giving a good opportunity for 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 -vapor ratios.

c) Double-pass: As the liquid flow splits into two directions, this system will handle high liquid-vapor ratios. 

     The liquid reflux flows across each tray and enters the downcomer by way of weir, the hight of which largely determines the amount of liquid on the tray. The downcomer extends beneath the liquid surface on the tray below, thus forming a vapor seal. The vapor flows upwards through risers into caps, or through simple perforations in the tray. Weir and downcomer is shown in second figure as follows:

Types Of Trays:
    Purpose of tray is to provide an intimate contact of liquid and vapor, and to make a low drop of pressure. So far in industry, following three types of trays are usually used:

a) Seive 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. This type of tray offers a very low pressure drop and is cheaper than the rest of the two, but it brings about less vapor-liquid contact as compared to the other two.

     The general form of the flow on a sieve tray is typical of a cross-flow system. With the sieve plate the vapor velocity through the perforations must be greater than a certain minimum value in order to prevent the weeping of liquid stream down through the holes. At the other extreme, a very high vapor velocity leads to excessive entrainment and loss of tray efficiency.

b) Bubble Cap Trays:
                                   This is the most widely used tray because of it's range of operations, but is now-a-days unable to compete with the third type which offers more flexible 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 area of the riser and the annular space around the riser should be about equal. With small trays, the reflux passes to the tray below over two or three circular weirs, and with the larger trays through segmental downcomers.

     This type of tray provides good efficiency than seive tray, flexible in operation (i.e, can be used for a range of liquid-vapor flow rates) but  is most costly and offers great pressure drop as compared to the other two. Bubble cap trays are capable of dealing with very low liquid rates and are therefore useful for operation at low reflux ratios.

c) 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 vlave tray the opening may be varied by the vapor flow, so that the trays can operate over a wide range of flowrates (i.e, it provides more flexibility in operation than a bubble-cap tray).
 
     It is low is cost than a bubble- cap tray since it is simple in construction. Because of their flexibility and low price, valve trays are tending to replace bubble-cap trays. It operates at the same capacity and efficiency as sieve trays. It has high turn-down ratio, i.e, it can be operated at a small fraction of design capacity.