What 5 properties influence the selection of separation equipment
- capacity
- efficiency
- pressure drop
- cost
- operating range
List the main requirements for a tray
- it should provide intimate mixing between liquid and vapour streams
- it should be suitable for handling the desired rates of vapour and liquid without entrainment or flooding
- it should be stable in operation
- it should be reasonably easy to erect and maintain
- it should have as low a pressure drop as possible (vacuum distillation)
Define entrainment
The carryover of liquid droplets in the gas phase to the plate above
Define flooding
Occurs when there is excessive entrainment or when there is excessive liquid backup in the down comer
List three types of liquid flow over trays
Cross-flow
Reverse
Double-pass
What main factors must be considered when comparing the performance of plates
- capacity
- cost
- efficiency
- operating range
- pressure drop
Give an overview of capacity in plate types
There is little difference in the capacity rating of the three types of plates
Give an overview of efficiency in plate types
Will be almost the same when operating - no real distinction can be made
Give an overview of pressure drop in plate types
The late pressure drop will depend on the detail design of the plate - in general sieve give the lowest pressure drop, then valves and finally bubble caps
Give an overview of cost in plate types
Bubble caps are most expensive. The relative cost will depend on material of construction used
Give an overview of operating range in plate types
Operating range means the range of vapour and liquid rates over which the plate will operate satisfactorily
What are the main features that influence column performance
- Liquid and vapour velocities
- physical properties of the liquid and vapour
- extent of entrainment of liquid by rising vapour streams
- hydraulics of the flow of liquid and vapour across and through the tray
Describe the sectional construction of a plate
The plate sections are supported on a ring welded round the vessel wall and on beams - usually angle or channel sections
Describe stacked plates
This configuration is used when the diameter of the column is too small for a man to enter to assemble the plates. Each plate is fabricated complete with the down comer and joined to the plate above and below using screw rods. The plates are installed in the column shell as an assembly of approximately 10 plates. They are not fixed to the vessel wall, so there is no positive seal at the edge of the plate, and a small amount of leakage will occur
Describe down comers
Simplest and cheapest form of construction. The down comer channel is formed by a flat plate, called an apron, which extends from the outlet weir.
What are the basic requirements of a plate contacting stage
- provide good vapour-liquid contact
- provide sufficient liquid hold up for good mass transfer
- have sufficient area and spacing to keep the entrainment and pressure drop within acceptable limits
- have sufficient downcomer area for the liquid to flow freely from plate to plate
Describe packed columns
Cylindrical shell of the column is filled with packing. The packing can consist of rings, saddles or any other shape, which is designed to give a good contact area between the liquid and the vapour.
The requirements of a packing is that it should…
- provide a large surface area
- have an open structure - give low resistance to gas flow
- give uniform liquid distribution on the packing surface
- give uniform vapour flow across the column cross section
What categories can packing be divided into
Broken solids
Grids
Shaped packing
List two classes of distillation columns
- plate columns
- packed columns
List 3 types of cross flow trays
Sieve Plate
Bubble - Cap
Valve Paltes
