3 replies to this topic

Hi,

Could anyone tell me if there are guidelines available or suggest rules of thumb for the piping lengths from the top of the column to the condenser, reflux drum and back into the column. And also the same for the reboiler. Also what types of valves should be considered. Any feed back would be great.

Thanks.

[AlertO]

i think the condensor and reboiler should be located near the column as much as possible due to pressure drop minimization.
Then, the line size of OVHD line will be considered. The following criteria is an example for line sizing around the column.

For Overhead Line Guidelines:
(deltaP/100’)MAX = 0.070 P^1/2 psi
vMAX =100 / v^1/2 (fps) – not to exceed 60-80 fps

where P’ = pressure at the overhead receiver, psia
where v = density of vapor at flowing temperature and pressure, lb/ft3

For columns operating at very low pressure where minimum pressure drop is critical, use the following guideline:

(P/100’)MAX = 0.035 P^1/2 psi

where P’ = pressure at the overhead receiver, psia
For Bottom Line to reboiler:
(P/100’)MAX = (0.0062) (l – v) - 0.06 (psi)

where l = density of liquid at bottom of column, lb/ft3
v = density of vapor in reboiler return line, lb/ft3
For reboiler return line
Kettle Type Reboiler: (deltaP/100’)MAX = 0.20 psi

Thermosyphon (or once-through, non-pumped): (deltaP/100’)MAX = (0.0062) (l - v) - 0.016 (psi)

where l = density of liquid in return line to column, lb/ft3
v = density of vapor in return line to column, lb/ft3

However, the location of them should be considered about its level too.

[katmar]

This is rather a wide topic to answer here, and you should try to get hold of the book on Distillation Design by Henry Kister if you want the full details. But here are a few pointers.

Vapour lines: The important thing is the pressure in the column, not the pressure drop in the line. Set your pipe pressure drop to suit the pressure range you need in the column.

Condensate lines: I have seen these lines cause problems where the velocity is too high and vapour (or even air) is drawn down the pipe. This affects instrument readings and air in the column is obviously not a good thing. The section of pipe above the reflux nozzle should (in my opinion) be designed to be self venting. Kister discusses this in detail.

In general reboiler piping is kept short and no valves are used. But I have used a reactor to reboil a column and the two units were about 15 metres apart, so this rule is not unbreakable. But it did take some trial and error to get the piping right.

i think the condensor and reboiler should be located near the column as much as possible due to pressure drop minimization.
Then, the line size of OVHD line will be considered. The following criteria is an example for line sizing around the column.

For Overhead Line Guidelines:
(deltaP/100’)MAX = 0.070 P^1/2 psi
vMAX =100 / v^1/2 (fps) – not to exceed 60-80 fps

where P’ = pressure at the overhead receiver, psia
where v = density of vapor at flowing temperature and pressure, lb/ft3

For columns operating at very low pressure where minimum pressure drop is critical, use the following guideline:

(P/100’)MAX = 0.035 P^1/2 psi

where P’ = pressure at the overhead receiver, psia
For Bottom Line to reboiler:
(P/100’)MAX = (0.0062) (l – v) - 0.06 (psi)

where l = density of liquid at bottom of column, lb/ft3
v = density of vapor in reboiler return line, lb/ft3
For reboiler return line
Kettle Type Reboiler: (deltaP/100’)MAX = 0.20 psi

Thermosyphon (or once-through, non-pumped): (deltaP/100’)MAX = (0.0062) (l - v) - 0.016 (psi)

where l = density of liquid in return line to column, lb/ft3
v = density of vapor in return line to column, lb/ft3

However, the location of them should be considered about its level too.