6 replies to this topic

[AFFAN]

Guide me for determining "HETP, Flood point corelations for. packed towers using rachig rings as packing material. Actually i want to redesign distillation system
consisting of three towers.
in a batch distillation system if i am hot having liquid flow rate and vapor flow rate, what is the alternative method of detrmining it?

[siretb]

Have you the heat duty at reboiler and at condenser ?
OR heat load at reboiler and reflux ratio ?
OR distillate rate and reflux ratio.

any couple of data fully specifying the column will do. I assume you know the pressures and temperatures.

[sgkim]

Hi, Affan

Many people surely have tried to derive any correlation for the estimation of HETP and flooding point. But none of the process variables such as heat duties, reflux ratio, distillate rate, temperature, pressure, etc. does seem to be any variable to determine HETP or flooding point. Several empical correlations based on experiments can be found to estimate them from the parameters like specific packing area, surface tension, viscosity, molecular mass, densities of liquid and vapor, liquid to vapor ratio, and so on.

On these subject you would better refer to the Kister, H.Z. "Distillation Design", MGH, 1992.

sgkim

[AFFAN]

Have you the heat duty at reboiler and at condenser ?
OR heat load at reboiler and reflux ratio ?
OR distillate rate and reflux ratio.

any couple of data fully specifying the column will do. I assume you know the pressures and temperatures.

i am at the moment only having reflux ratio and distillate rate accodingly.
plus again there is no rule of thumb available about determining "HETP" for "Raschig Rings" as these are available for other packings!

[siretb]

Well, if you know pressure, temperature, of course the system (components), having reflux ratio, distillate rate and the kind of packing you want to use should enable you to get a fair guess of what the HETP is going to be. First you calculte how close you are to flooding, and then derivr the HTU/HETP from correlations.

I agree with sgkim: these will be empirical correlations, but that would be better with nothing.

If you need more help, please provide as much details as possible: specifications, what kind of packing you wish to use
I hope this

[AFFAN]

Well, if you know pressure, temperature, of course the system (components), having reflux ratio, distillate rate and the kind of packing you want to use should enable you to get a fair guess of what the HETP is going to be. First you calculte how close you are to flooding, and then derivr the HTU/HETP from correlations.

I agree with sgkim: these will be empirical correlations, but that would be better with nothing.

If you need more help, please provide as much details as possible: specifications, what kind of packing you wish to use
I hope this

Hello
Thx all of u for your interest in my question, As i mentioned earlier that i am going to redesign a distillation system consisting of three towers carrying out concenteration ( the batch one), Extractive and Vaccum distillation respectively. the whole system is based on the most old style packings of "Raschig rings ceramic"
presently i am designing i'st tower the batch one. but problem lies with unavailability of data and any empirical relationships particularly for rachig rings to calculate HETP , Flooding point etc.

if u people can provide me some guidance in this regard it will be really helpful.

[sgkim]

Hello
Thx all of u for your interest in my question, As i mentioned earlier that i am going to redesign a distillation system consisting of three towers carrying out concenteration ( the batch one), Extractive and Vaccum distillation respectively. the whole system is based on the most old style packings of "Raschig rings ceramic"
presently i am designing i'st tower the batch one. but problem lies with unavailability of data and any empirical relationships particularly for rachig rings to calculate HETP , Flooding point etc.

if u people can provide me some guidance in this regard it will be really helpful.

Hi, AFFAN

Raschig rings were used decades ago. You would better refer to the following two quite old correlations for Raschig rings:
(1)Murch, Ind. Eng.Chem,45,2616 (1953):
HETP(m)=C1*G^C2*dc^1.24*Z^(1/3)*α*μ/ρ

(2)Ellis, Burmingham Univ. Eng. 5(1),21(1953):
HETP(m)=18*dr+12m(G'/L'-1)

where C1,C2 are constants, G=mass velocity of vapor(kg/m^2/sec), G',L'=vapor & liquid mass rates, dc=column diameter(m), dr=ring diameter(m), Z=packed depth(m), α=relative volatility, μ=liquid viscosity(N-sec/m2), ρ=liquid density(kg/m3), m=average slope of equilibrium curve, and
C1=7.43*10^5 and C2= -0.37 for 12.5mm , C1=1.26*10^5, C2= -0.24 for 25mm, C1=1.80*10^5, C2= -0.10 for 50mm Raschig rings

In rather recent reference, Frank,Chem.Eng 111, 14, Mar 1977, gave definitive HETP values:
0.46 m for 25 mm, 0.66 m for 38 mm, 0.9 m for 50mm, for small diameter shells (<0.6m) HETP=shell dia; and for vacuum columns add 0.1 m to these heights additionally.

No one would blame them for the reliabilities of the figures or correlations.
For the subjects of flooding points, however, you can refer to any textbook in hand.

sgkim