Remaining water from evaporator is pumped to the light end column where it is separated, to obtain our required purity. In this column about 99.9% of water is removed. To obtain such high purity we have selected distillation column.
5.6.1: DISTILLATION column selection
Distillation is a unit operation most widely used to separate liquid mixture into more or less pure components .In distillation separation of constituents is based upon differences in boiling points
- Specify the degree of separation required: set product specification.
- Select the operating conditions: batch or continuous; operating pressure.
- Select the type of contacting device: plates or packing.
- Determine the stage and reflux requirements: the number of equilibrium stages.
- Size the column: diameter, number of real stages.
- Design the column internals: plate, distributors, packing supports.
CHOICE BETWEEN PLATE AND PACKED COLUMN
- Plate columns are designed to handle wide range of liquid flow rates without flooding than packed column.
- For large column heights, weight of the packed column is more than plate column.
- Periodic cleaning is easy in plate columns as compared to packed columns.
- When temperature change is involved, packing may be damaged.
- If the liquid causes fouling or contains solids, it is easier to make provision for cleaning in plate column
SELECTION OF PLATE COLUMN
- More positive contact by repeated mixing and separation.
- For the liquid causes fouling. So man ways are installed on plate for cleaning purposes.
- For the large temperature changes.
- Plates are mostly used for large diameter more than 0.6m.
- Easy to maintain than that of packed column.
- Suitable for very low liquid rates.
SELECTION CRITERIA OF TRAYS
Cost of plate depends upon material of construction used.
For mild steel, the ratio of cost between plates is
Sieve plate : valve plate : bubble-cap plate
3 : 1.5 : 1.0 .
Sieve tray > valve tray > bubble-cap tray
It is the ranges of liquid and vapor flow rates which must be above the weeping conditions and below the flooding conditions. Operating range flexibility comparison is
bubble cape tray > valve tray > sieve tray
For good design, sieve plate gives satisfactory operating range.
Bubble-cap tray > valve tray > sieve tray
CHOICE OF TRAY TYPE (SIEVE TRAY)
- Sieve plates are lighter in weight and less expensive. It is easier and cheaper to install.
- Pressure drop is low as compared to bubble cap trays.
- Maintenance cost is reduced due to ease of cleaning.