Calculate liquids level at distillation bottom tray
Calculate liquids level at distillation bottom tray
(OP)
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.
Thanks,
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.
Thanks,





RE: Calculate liquids level at distillation bottom tray
RE: Calculate liquids level at distillation bottom tray
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
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
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
RE: Calculate liquids level at distillation bottom tray
Appreciate for all your support.
Thanks
RE: Calculate liquids level at distillation 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
RE: Calculate liquids level at distillation bottom tray
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
(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
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
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
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.
regards