Dry Tray Pressure Drop And Hydraulic Tray Pressure Drop

Hello everyone

In the book "Working guide to process equipment"by Liebermann it is written that

When the dry tray pressure drop is significantly less than the hydraulic tray pressure drop, the tray will start to leak or weep and tray efficiency will be adversely affected.

Well, with the example you present it is obvious that you know absolutely nothing about distillation tray hydraulics. You need to refer to some introductory material before you delve into the details of tray design. Do that, then come back. I've never found Norm Liberman very good when it comes to design. His and his wife's material is mostly directed at operators and general information.

Bobby

Sir, Could you please recommend any good book for distillation tray hydraulics?.....I am not a design person but at least I want to understand tray hydraulics thoroughly. I am a Chemical engineer and have gone through basics of distillation column from Treybal and Geancoplis. These books mostly dealt with mass transfer and tray hydraulics is not in detail. 

Please reply

Thanks & Regards!

Sandeep

I suggest you start with:

Henry Kister,

Ludwig volume 2,

Coulson & Richardson volume 6,

hi ,

let you take a look at this material.

hope this is helping you

Breizh

You might visit Koch-Glitsch website. Long ago they published tray design manuals. If you don't find one, let me know. I might have a digital copy. And Kister is one of the best authorities on distillation. Not sure what he offers for tray hydraulics. I didn't mean any disrespect with my earlier reply. But it seems to have gotten your attention. The subject is not a simple one to understand.

Bobby

Henry Kister discusses tray design (chapter 6) and tray efficiency (chapter 7) in his book Distillation Design.

The classic Glitsch Ballast Tray Design Manual can still be downloaded at Koch-Glitsch website:

www.koch-glitsch.com/Document%20Library/Bulletin-4900.pdf

Stephen Hall's Rules of Thumb for Chemical Engineers may also be of interest.

Hi ,

The document in reference above .

Good luck

Breizh

Thank you all for your kind responses....I am going through these sources....But I was thinking someone would be kind enough to pinpoint my mistakes in the above example....As I found only   separation tower design ...have discussed hydraulics.....Though I have not gone through these sources in detail...It will take time...

Thanks & Regards to all of you !

Sandeep

"Give a man a fish and he will eat for a day. Teach a man to fish and he will always eat."

This is a good philosophy, and one to which I adhere.

Bobby

http://facstaff.cbu....s/distill7.html

May be good to read !

Breizh

What diagram?

Here vapor pressure drop stands for what? Is it total pressure drop of vapor across the tray or it is only dry tray pressure drop due to orifice?

Neither.

It is the dynamic pressure (0.5*ρ*u_h²) of the vapor exiting the sieve holes that must be sufficient to counteract the static head of the liquid on the tray.

It is not the dry tray pressure drop that matters. In fact sieve holes are punched either down or up, usually down. A sieve tray with holes punched downward will have a higher dry tray pressure drop that an identical one with the holes punched upward. But that does not mean that a tray with downward punched holes would have less weeping.

Equation (6.31.a) of the socalled Fair's model is in my opinion wrong, as dry tray pressure drop h_d is not what matters, and moreover the RHS is wrong because the liquid on the tray is aerated so its head is less than its height.

Note also what Kister wrote on page 300: The mechanism of weeping is not well understood.

So unless you want to become an expert on the subject (requiring searching for, and reading of, many scientific papers on the subject), I suggest you spend your time on more interesting and more important aspects of distillation or chemical engineering.

If you would ever need to design a tray yourself you would be using dedicated software that would warn if weeping (or entrainment or whatever) is not within proper design range and recommend adjusting number and/or diameter of holes or whatever. Or you simply send your specifications (tray loadings, fluid properties, max deltaP, et cetera) to a tray vendor.

http://www.cheresour...-pressure-drop/

https://www.cheresou...llation-column/

Edited by PingPong, 07 May 2018 - 10:18 AM.

I have reattached the diagram

What you state in the diagram is nonsens.

Use your common sense: what does the liquid really feel? is that the pressure drop over the metal plate underneath it? why would that be? and why would the liquid care what the pressure underneath the tray (inlet of the sieve holes) is? it doesn't.

The liquid only feels the dynamic pressure (0.5*ρ*u_h²) of the vapor exiting the holes and pushing against it in upward direction.

That dynamic pressure can be in the same order of magnitude as the dry pressure drop but that does not mean that they are equivalent.

If two properties happen to have about the same numerical value that does not mean that they can be interchanged in all kind of formulas. They remain two different properties from a physical point of view.

I would be disapointed if Henry Kister even today would think that dry tray pressure drop matters for weeping.

It is the dynamic pressure of the vapor that matters.

Hello,

The subject is very interesting. I would like to add some material for discussion.

Regards,

Nikolai

Interesting, but useless. The physics are well understood. And have been for many years. What one must be cautious of is an expert author in a book makes a statement without any derivation of the actual fact. People tend to take such experts at their word, foolishly. And at their peril. The foolishness is oft quoted to support the claim. Again, with no supporting calculations or experiments.

As to this discussion, sieve trays are rarely used. When they are, the tray vendor designs them based on owner supplied process data.

Bobby

Useless indeed.

Aspen states that:

"Weeping can start to occur on a tray when the dry hole pressure loss drops below 0.015 kPa."

They must be joking, 0.015 kPa is equal to only 1.5 mm water column, or say 2 mm hydrocarbon column.

That is absurdly low.

Another funny statement:

"weepFactor is an adjustable parameter enabling the user to adjust the weeping effect"

Great, and how is the user supposed to figure out what value to give it?

And this one:

"Note that Aspen HYSYS Dynamics assumes the liquid is present as clear liquid."

What is the use of a dynamic simulator that ignores that the fluid volume on the tray is about half vapor bubbles and only about half actual liquid?

Aspen is always good in claiming that they can calculate, model or predict just about everything, but if one could look behind their screen one would probably see that they are almost as clueless as most of their users.

In the book 'Distillation Design' by Henry Kister 

In section 6.2.5. 'Factors affecting flooding'

It is written that Low pressure favors high vapor velocities and low liquid flow rates and therefore spray regime dispersions.

I can understand at low pressure density of vapor will go low and hence vapor velocity will be high. But How it will favor low liquid flow rate?

Also, In section 6.2.4 'Froth entrainment flooding'

It is written that ' At high liquid rates,cross flow of vapor in opposite direction to the liquid can build up froth near tray inlet and center. This channels more vapor to the tray outlet region, thus accelerating the cross flow.'

Please explain this phenomenon.

Thanks & Regards!

Sandeep

6.2.5.

Low pressure means large column diameter, which means long outlet weir length. With liquid flow rate is meant: volumetric liquid flow divided by weir length, experessed in m³/h / m weir length or gpm / inch weir length. You can see such for example at x-axis of figure 6.15 and others, and in the text here and there.

6.2.4.

The liquid height on a tray is highest at tray inlet (downcomer exit) and lowest at tray outlet (weir).

The vapor has some preference bubbling through less resistance.

Make a drawing of the side view of a couple of trays above each other, draw arrows for vapor and liquid flows and it should become clear.

 

I have reattached the diagram

 

"at the tray floor, the static liquid head tends to force liquid down through the perforations. The vapor pressure drop counteracts the downward force and acts to keep liquid on the tray. Weeping take place when the liquid head on the tray exceeds the pressure drop that is holding the liquid on the tray."

 

Here vapor pressure drop stands for what? I asked this question to henry kister and He confirmed that it is dry pressure drop.

 

Now, my doubt was  why weeping will take place when the liquid head on tray exceeds the dry tray pressure drop? 

As below hole there will be always a higher pressure of vapor than the pressure at above the hole (i.e. on tray floor). [Higher by dry pressure drop]

 

For example in this attached diagram, dry pressure drop is x while liquid head is 2x. But still pressure below the hole is higher than pressure above the hole. That's why liquid should not weep through the hole. 

 

@PingPong - "Equation (6.31.a) of the socalled Fair's model is in my opinion wrong, as dry tray pressure drop h_d is not what matters, and moreover the RHS is wrong because the liquid on the tray is aerated so its head is less than its height."

 

If  you do not agree with (6.31 a), then do you agree with (6.31 c) which include the aeration factor as f?

 

I thought I am doing very fundamental mistake, That's why I wanted to clarify my doubts.

 

Anyway Once again thanks to all of you !

 

Hi, Maybe Im beating the dead horse! but If I can just put in my two cents' worth  , I think there is something misleading in the sketch   ! when weeping/dumping happens, no more pressure above weeping tray will be 7 psig as normal condition. That is the reason during pressure survey of a column, if the pressure drop (pressure difference of top and bottom of a column) is too low, weeping/dumping can be the culprit! So, in your diagram, if we change pressure above the weeping tray to 11(because 4 psig hydraulic pressure drop no more happens when vapor can not pass through the liquid inventory on the tray because of weeping!), then it makes sense that liquid from above tray pours on lower tray! If you are worry that where that vapor flow goes, there are two ways: This low vapor flow passes through only part of the holes that not weeping and/or through the Downcomer when liquid level on the trays is not enough to seal it. In fact, The only obstacle that keeps the liquid on a tray, is the force of up going vapor through the holes. This force comes from dry pressure drop! what do you think?

Edited by Lawless, 17 July 2019 - 06:37 AM.

 

I have reattached the diagram

 

"at the tray floor, the static liquid head tends to force liquid down through the perforations. The vapor pressure drop counteracts the downward force and acts to keep liquid on the tray. Weeping take place when the liquid head on the tray exceeds the pressure drop that is holding the liquid on the tray."

 

Here vapor pressure drop stands for what? I asked this question to henry kister and He confirmed that it is dry pressure drop.

 

Now, my doubt was  why weeping will take place when the liquid head on tray exceeds the dry tray pressure drop? 

As below hole there will be always a higher pressure of vapor than the pressure at above the hole (i.e. on tray floor). [Higher by dry pressure drop]

 

For example in this attached diagram, dry pressure drop is x while liquid head is 2x. But still pressure below the hole is higher than pressure above the hole. That's why liquid should not weep through the hole. 

<Vapor pressure drop> would be the sum of (1)total pressure drop of vapor passing thru orifices (i.e., dry pressure drop) and (2) that passing thru the liquid channel as bubblets including other pressure drops thru vapor-liquid mixed channels.  If the column pressure is 7 psig, the dry pressure drop  4 psi, and liquid channel pressure drop 2 psi respectively, then the column bottom pressure would be 13 psig.  

On each tray, the two-phase liquid tends to downflow thru the orifice hole and the vapor upflow thru the liquid channel.  On the top face of the orifice the driving force of the vapor flow is pressure force (13-4 = 9 psig), which sholud be slighly higher than that of the liquid flow (7psig +2 psi =9psig) to prohibit the liquid from weeping.

The liquid head on each tray shall be based on clean(vapor-free) single phase.  Then total pressure drop of the column shall be 2 psi(from static_clean liquid head) plus 4 psi dynamic (pressure drop).   Total 4 psi pressure drop(dry pressure drop) prevent the liquid from weeping. If the liquid head is larger than 4 psi as equivalent clean liquid head, then weeping may start.

The word "significantly" in the Libermann's might mean "too small enough not to flow freely through the liquid channel".

Stefano Kim//Sept 22,2019

Edited by sgkim, 22 October 2019 - 07:25 PM.