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Calculate liquids level at distillation bottom tray

Calculate liquids level at distillation bottom tray

Hello Everyone,

Would like to get some idea and guidance to determine the liquid level in distillation column at the bottom tray.

1. What is the practical way to calculate the remain liquid height at the bottom tray of the column?
2. How can i get the pressure losses at the Nozzles (N1 and N2), is there any book that can give the way to get the K factor?

I have attached a sketch of the column.


RE: Calculate liquids level at distillation bottom tray

The liquid level on all trays is a fixed value. It fixed by the weir height. Check the tray drawing and find the height of the weir that is next to the downcomer.

RE: Calculate liquids level at distillation bottom tray

My apology, not really sure with your point don1980.

The flow fron the column to the reboiler is driven by the elevation changed between the column and the reboiler.

Regarding the question i posted above, Ii was trying to find the idea to correlate the Bernouli's equation (P = rho* g* h) to determine the height of the remain liquid at the column outlet nozzle N1 (chimney tray), but i was not really sure if taken into account the dP created by elevation changed, hydraulic losses through pipe and nozzles.

RE: Calculate liquids level at distillation bottom tray

It sounds like, from @C26M13's second post, that the level in the reboiler and the column bottom sump is set by an overflow weir in the reboiler and not by an overflow weir in the column bottoms sump.
In this case, agreed the level in the column bottom tray / sump will be a function of all of the following
a)Overflow crest at the overflow weir in the reboiler
b)Pressure drop at the reboiler liquid feed nozzle and the column exit nozzle
c)Friction losses in the transfer piping from the column to the reboiler
d)Friction losses across the tube bundle in the reboiler.

For the K values for the exit and feed nozzles, these may be found in Perry Chem Engg Handbook or Crane TP410. Pls note these K values are applicable for flow in the highly turbulent region where the friction factor has sort of plateued out. If the Nre is say less than 1E4 or so, you may use the 2K or Darby 3K method to get better estimates for K. Perry also has estimates for the overflow crest at the weir in the reboiler (Manning formula).

RE: Calculate liquids level at distillation bottom tray

Thank you @georgeverghese.

Appreciate for your help to make it more clear the idea i was trying to raise. The points you have mentioned above are the steps i am thinking can be linked to the Bernoulis' equations.

I already calculated the transfer piping dP using HYSYS model and loss in nozzles (entrance and exit) using hL = K * Vel^2 / 2*g (Kentrance 0.5 and Kexit 1.0).

To determine the liq. level at the chimney tray?

RE: Calculate liquids level at distillation bottom tray

I think the sketch is incorrect. It shows liquid flow from the bottom tray to the reboiler, rather than flow from a sump to the reboiler. Such a design doesn't make sense.

RE: Calculate liquids level at distillation bottom tray

C26M13 - there's no significant difference between the first sketch and the revised one. It still doesn't make sense, unless there is something very unusual about this application, such as the presence of solids/heavy tar in the column sump, which you don't want to feed into the reboiler. Is the reboiler outlet line connecting to the column above the bottom tray?

Reboilers are almost always fed from the liquid that collects in the bottom of the column, and not liquid extracted directly from the bottom tray downcomer (as shown in your sketch). And the reboiler outlet line connects to the column below the bottom tray.

Show the location of the reboiler outlet line (where it connects to the column relative to that bottom tray), and explain whether there's a reason for feeding the reboiler from the bottom tray downcomer. You show a line coming off the bottom of the column. Where does that go, and why is that not the feed for the reboiler?

RE: Calculate liquids level at distillation bottom tray

To determine the level at the chimney tray, this liquid level elevation will be = elevation of the liquid overflow crest at the reboiler product overflow weir + (all friction losses in Pa / (rho *g)) as you have previously indicated. Looks like you've got all the terms I had indicated accounted for except (a) and (d). (d) may be a small contribution to the total dp since this is a pool boiling kettle type reboiler.

Since this is a side reboiler, it is strange why this reboiler liquid product joins the final liquid product leaving the bottom sump - maybe so because this side reboiler product liquid also meets the composition spec for the bottoms liquid product ??

RE: Calculate liquids level at distillation bottom tray

This scheme where the reboiler product vapor return ties in UPSTREAM of the chimney tray requires an additional pressure drop term to be included to those suggested earlier
(e) the pressure drop in the entire vapor return path which should include the following (i) the vapor exit nozzle losses (ii) friction drop in the vapor return line to the reboiler (iii)entrance nozzle loss at the column (iv) chimney tray riser and slot losses. This subtotal loss in [Pa] can be added to convert to static head at the chimney tray as discussed previously.

RE: Calculate liquids level at distillation bottom tray

Thanks a lot @don1980 and @georgeverhese for you explanation and points to give ideas for the calculation.

The reason reboiler liquids join the final product leaving the sump is as @georgeverhese mentioned before, this liquids product from Reboiler also meet product specification to be sent to the storage tank.

For my HYSYS model, since the tansfer piping is declining from the column to the reboiiler there is no pressure loss, just pressure gain I got from HYSYS model. Is that correct?
Pressure drop is obtained from Weir height, Nozzle (in and out) and pressure drop in tube bundle is neglected.

@georgeverhese: Why do I need to include also the pressure loss from vapor return line? What is the correlation between this term to the liquid height?

Appreciate for your comment and inputs. Thank you.

RE: Calculate liquids level at distillation bottom tray

There is no need to included these additional terms in (e) if the vapor space pressure in the reboiler is the same as that for the vapor space at the feed chimney. But it should be apparent that reboiler vapor space pressure is actually higher than chimney tray vapor space pressure, by an amount equal to all the terms in (e).

By the way, the way the sketch shows, ALL the liquid arriving at the chimney tray ( from above) goes to this side reboiler. So how can there be a liquid product line from the column bottoms sump also, since there is no liquid feed to this bottom section from the chimney tray above ?

RE: Calculate liquids level at distillation bottom tray

Thank You Everyone for sharing and putting your ideas into my post.

This already done and from this post i got the ideas to complete this. This basically to ensure the the liquid level will not go higher that the Risers and also not fall below the diameter of the liquid outlet nozzle.


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