Small diameter tower internal orifice type gravity pan distributor

1. Pan distributor is belong to column internals

Column internals play very important role in effective mass transfer performance. So they need to be designed carefully for both process and mechanical point. Meanwhile, the engineers put great effort into structured packing and random packing development in the recent years. The column internals include mist eliminators, feed pipes, distributors, re-distributors, support plates, bed limiters, collector trays, etc.

2. Pan distributor is a kind of orifice gravity liquid distributor

Orifice type gravity distributor usually shows as pan type. An orifice pan distributor consists of a pan. There are circular or rectangular risers for vapor flow and perforation for liquid flow in the pan.Orifice type distributor can be designed to have a large number of drip points to provide superior liquid distribution, though it may suffer from corrosion and plugging. Orifices are sized to maintain a minimum liquid head at desired turndown conditions & to avoid distributor overflow during turn up condition. Very small orifice diameters (<4mm) are avoided to prevent fouling.

3. Pan distributor application circumstances

4. Important points in designing Pan liquid distributor:

There are some important points to be checked before finalizing the liquid distributor design.

• Maximum and Minimum head:

Maximum liquid head is the actual liquid head on the distributor deck at turn up condition. The overall height of the liquid distributor (trough / riser) shall be higher than this maximum head. Please refer to the following formula.

Riser (Trough) height = {(Maximum head) 1.10 + 25} mm

Minimum head on the distributor is always to be estimated as per requirement at turn down condition. Liquid vortexing and entrainment may happen at low liquid head as insufficient resistance to orifices. Also considerable mis-distribution can occur if minimum heads are fallen short of. So the minimum liquid head on the distributor shall be around 35mm. Punched orifices, instead of drilled orifices, are always recommended in the direction of liquid flow.

• Pressure drop and open area requirement for vapor:

Sufficient gas passage area must be provided by the liquid distributor according to gas load, to avoid liquid entrainment or high pressure drop. A minimum of 12% open area is used when very low open area is required for gas. The pressure drop of liquid distributor shall not exceed a value of 20mm water column. Well, it finally depends on gas load, liquid load and a particular service.

• Flow variation:

Flow variation will increase due to low liquid head at the time of low liquid loads and / small column diameters. Flow variation is a strong function of mechanical points such as plate levelness, distributor deck flatness etc. Liquid mis-distribution and entrainment may occur on below packed bed with excessive flow variation. So we need to minimise the excessive flow variation. The liquid distributor or re-distributor can allow maximum of 12% flow variation.

5. Pan distributor production laser cutting machine

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