application of demester pad at a knock out drum 1

[farahnaz77]

where is the demister pad used in a knock out drum; in other word, when there is no necessity to use them?

 

[TD2K]

I'm not sure what you are asking.

The demister pad is intended to remove entrained liquids from a gas stream. On a vertical vessel, it's located above the feed entrance. On a horizontal vessel, it's located near the gas outlet which would be at the other end from the inlet.

"when there is no necessity to use them". If your particular system doesn't care if you have some liquid in the gas, you don't need a demister pad. It's your call as the process engineer.

 

[MortenA]

Consider use of other demisting equipment such as vanes and cyclones that has a higher efficiency.

Best regards

Morten

 

[BenThayer]

if your vessel is large enough and the gas flow rate slow enough, this would be the criteria of if you need a demister pad.

other considerations:

1) orientation (do you have the gas/liquid entering tangentially)

2) what % carryover can you accept?

3) see equation 18-155 in Perry's fifth edition

if U in your vessel can be 0.15*U (calculated), you may not need a demister.

check out websites for various manufacturers too.

 

[tkdhwjd]

To answer your first question...
It is typically used under the outlet nozzle. Be sure to meet the minimum distance criteria in between the outlet nozzle and the top of the demister to avoid mal-distribution of flow.

To answer your second question...
You will have to calculate/estimate % carry over and evaluate whether the downstream equipment can manage the % carry over. If your downstream equipment requires lower entrainment rate, then you will have to install properly designed demister.

 

[25362]

To MortenA, I beg to disagree.

For gas-liquid systems -the case now under consideration- both cyclones and demister pads have their useful applications depending on the liquid/vapor load and particle sizes.

Cyclones function by using centrifugal force; demisters are impingement separators.

Demisters are effective entrainment separators in evaporators, absorption towers, distillation columns, etc. Their function is based mainly in increasing the size of drops so that gravity drip off can rapidly occur without the drops being carried away by the gas.

Knitted mats with a surface area of 400 m²/m³ have shown 99.9+ % removal efficiency. Even general purpose standard mats have shown removal efficiencies of 99.5+%. For services containing solids or "dirty materials" fish-bone woven mats with a free volume of 99% and a surface area of 160-230 m²/m³ have been reported to operate with 90+% removal efficiency. Mesh demisters excel at low loads of about hundreds of ppm by volume, and particle sizes of above 10 [μ]m.

Then there are silicone impregnated (polyester or glass) fibre mist eliminators with diameters down to 1.3 [μ]m to catch submicron acid fogs.

Zig zag vanes are the preferred separator for the same loads as demisters, but for larger particle sizes reaching a size of 0.5-1 cm.

Cyclones, particularly wet cyclones, are useful for high loads and a wider range of particle sizes. From published data, only when the particle dimensions are 10 times the "cut size" the collecting efficiency reaches 99%. Cut size is defined as the diameter of particles of which 50% are collected. To avoid re-entrainment special design provisions are taken.

There is a plethora of literature on the subject. And new developments are offered by manufacturers of separators every now and then.