Mist Eliminator / Demister - Sizing & Efficiency Calculation

Mist Eliminators or Demister Pads are very common & important things for process engineers but they are often neglected. Here is some input from my side theory as well as practical knowledge & gains from them.

The importance of a demister pad comes from the efficiency of separation & its impact on either product recovery or the indirect impact on downstream processes. Usually later is more important than direct impacts & therefore they are generally neglected peace of equipments.

The separation of entrained liquid droplets from a vapor / gas stream is called mist elimination & the equipment used for the purpose is called De-mister. Generally, in a manufacturing plant, process engineers or production engineers do not focus on the benefits which can be derived from the improvement in demister pad or mist eliminators.

For example, suppose your outlet gas goes to a reactor with VLE amount of component i.e. Ammonia. Now if ammonia from the separator goes up due to mist carry over or poor efficinecy of de-mister pad then the conversion in the reactor will go down, resulting in overall impact on synthesis loop pressure & energy. Also the production will go down.

Similarly, if a separator in acidic gas service do not work properly the downstream equipment will corrode easily. That's why we need efficient mist separation.

What is mist.......? Mist is the fine droplets of liquids of various sizes. They may vary from 0.5 micron to few 100 of microns in size. For example, good spray system generally generate 20 - 1000 micron size droplets, while columns & tray do have ~8 - 100 micron size drops. Saturated Vapors generally have finest size of droplets depending on their velocity at generating point.

Sizing
Generally we use Souder's equation as used for phase separator Or for knock out drums. That is

Vd = k x [ (L-G)/G ]^0.5

L & G are liquid & gas densities.

where k is the important part & is called the capacity design factor. It depends on type of de-mister pad. Selection of a too low or too high k is always have a negative impact in case of demisters as the efficiency greatly depends on velocities.

In case of lower velocities, droplets have low momentum to get path impingement & coalescene & therefore avoid capture into bigger drops & thus escape from the pad. At higher velocities the vapors have sufficient kinetic energy to re-entrain them. Therefore, correct range of k selection is necessary.

Based on past experiences & designs a value of k = 0.42 is most suitable for many applications. So after choosing k get the design velocity & then find out the diameter of separator. Now for predicting efficiency of de-mister pad, calculate K inertial parameter as below

K = [ (L - G)/ Vd^2 ] / ( 9 x mu x D)

L & G are liquid & gas density
Vd - velocity of gas calculated above
D - Diameter of pad

Now use following curve to get E factor for above K value.

Now calculate specific area of pad as below

A = Specific Area x Thickness x 0.67 / PI()

Now calculate % Efficiency as below

Eff = 100 - 100 / e ^ ( 0.213 x A x E )

In the next part I will consider pressure drop calculation.