Total Liquid Collector vs Partial Draw Tray in Packed Columns
A tower may need to collect all liquid leaving an upper packed bed, or withdraw only part of that liquid as a side stream. These duties require different internal arrangements.
Confusing a total collector with a partial draw tray can cause unstable liquid levels, non-representative product withdrawal or inadequate flow to the packed bed below.
What Is a Total Liquid Collector?
A total collector intercepts substantially all liquid descending from the upper packed bed.
The collected liquid may then be:
- Sent to an external pump-around loop
- Mixed with an intermediate feed
- Measured
- Withdrawn completely
- Sent to a lower redistributor
- Divided between product draw and internal reflux
A total collector normally uses a deck with protected vapor risers so liquid cannot fall directly through the vapor passages.
What Is a Partial Draw Tray?
A partial draw arrangement removes only a specified portion of the descending liquid while allowing the remaining liquid to continue downward.
Its duty may include:
- Side-product withdrawal
- Pump-around circulation
- Sampling
- Intermediate cooling
- Liquid inventory control
The challenge is obtaining the required draw without disturbing the flow or composition supplied to the lower section.
Why Representative Composition Matters
Liquid leaving a packed bed may not be perfectly uniform across the tower diameter.
If a partial draw collects mainly from one region, the withdrawn stream may not represent the average liquid composition.
Potential consequences include:
- Off-spec side product
- Incorrect analyzer reading
- Unstable draw control
- Different composition entering the lower bed
- Difficult material-balance reconciliation
A mixing or total-collection step may be required when representative composition is important.
Liquid-Level Control
The collector must maintain enough liquid inventory for stable withdrawal while avoiding backup into the vapor risers.
Review:
- Normal liquid level
- High-level condition
- Minimum operating level
- Draw-control response
- Drain capacity
- Surge volume
- Vapor-riser height
- Start-up inventory
Vapor Flow Still Passes Upward
Whether collection is total or partial, vapor must pass through the internal with acceptable pressure drop.
The design should evaluate:
- Vapor-riser area
- Local vapor velocity
- Entrainment
- Riser-cap arrangement
- Interaction with the upper packed bed
Questions for the Process Designer
Before selecting the internal, define:
- Must all descending liquid be collected?
- What fraction is withdrawn?
- Must the side draw be compositionally representative?
- Is the remaining liquid redistributed?
- Is mixing required?
- What happens during minimum flow?
- Where does overflow go?
- Is external level control provided?