Packed Column Distillation Flashcards
Two Packing Types?
Random and Structured
Random is where discrete pieces of the same geometrical shape are “dumped” or randomly packed in a column shell.
Structured or systematic is where they are arranged. crimped layers of Wire Mesh, Corrugated wire mesh, or corrugated sheets are stacked in the column.
Two types of Rings?
Rascig or Pall Rings
Rascig is “Perfect cylinders”, they are cheaper but pool with liquid
Pall has edges and angles, helps V & L interact, surface area evenly distributed and minimises obstructions and liquid hold up
To maximise efficiency we need to
maximise specific surface area
i.e. surface area per volume
What is HETP?
- Equation
- relationship between this and a packed column?
Height equivalent to theoretical plates
= column length / no of theoretical plates
Smaller the HEPT the more efficient the packed column is
Efficiency generally increases…
Random and Structured Packing
(random) … when particle size decreases
(Structured) … when space between layers decreases
Spreading the surface uniformity not only maximises the efficiency but
( 4 things)
- improves V - L contact
- promotes uniform distribution of V and L
- minimises stagnant liquid pooling
- minimises wetting of packing surfaces
Dewetting of packing surfaces at low liquid rates…
reduces efficiency
What is a trade off?
Efficiency and Capacity
Increased particle ring size or space between adjacent layers minimises resistance to vapour upflow, which therefore increases capacity.
But increasing this void space decreases efficiency as it decreasing with increasing packing size and space between layers.
Structured vs Random
Structured has smaller packing factor and less lateral movement of liquid
random has larger packing factor and more lateral movement of liquid
They both have an increasing amount of liquid hold up as liquid flow rate increases
Random Packings with small sized packings (large packing factors) mean more vapour flowed down and reduces capacity and efficiency
What is the flood point?
The point at which the velocity of the upflowing vapours obstructs the downflowing reflux and the column loads with liquid.
Three reasons for Flooding? (Packed Bed)
- Increase in pressure drop across bed
- packing particles non uniform
- Velocity of gas or liquid too high
Symptoms used to recognise a flood? (Packed Bed) (5 of them)
- liquid on top of bed
- Excessive entrainment of liquid
- Sharp rise in pressure drop
- Sharp rise liquid hold up
- Sharp drop in efficiency
Poor tray efficiency is caused by…
flooding and dumping
Flooding on tray column happens…
due a lack of downcomer clearance
They trays above will flood while trays below will run dry
When the distance of the downcomer is too big what happens? (2 things)
Either
- (penetration) the falling liquid has enough momentum to go through the holes of the tray below
- (premature downcomer backup) downcomer is unsealed and vapour may flow up the downcomer and the tray above floods
Best type of sealed downcomer?
A high capacity tray with a dynamic seal.
A restriction in the downcomer provides sufficient pressure drop on the liquid that a small head builds up and vapour cannot bypass
What does N stand for?
Number of moles transferred per unit volume
What does NTU stand for? and definition?
The number of transfer units
Measure of difficulty of separation.
Relates the change in bulk phase composition to the average driving force.
What does HTU stand for? and definition?
The height of transfer units
The height of packing at which gives a change in composition equivalent to one transfer unit.
A direct measure of column efficiency.
Equation to get the height (H) of the column?
H = NTU x HTU
What value compares the efficiency between packed and trayed colums?
HEPT
As HETP decreases what happens?
- packing efficiency increases
- packing surface area per unit volume increases
- particle size (random) decreases
- Channel size (structured) decreases
Therefore separation efficiency increases
