5 replies to this topic

[vne]

All,

Please help me with a question on superficial velocity calculation.

To determine the superficial velocity in a mol sieve bed, GPSA suggests the following formula:
ΔP/L = B μ Vg + C ρ Vg^2 (Eq 20-21, GPSA 12th)

In which B and C are constants representing for sieve type (e.g. 1/8'' or 1/16'').

How to calculate superficial velocity (Vg) if both 1/8'' and 1/16'' are used in one bed (e.g. the upper part of the bed contains 1/8'', and the rest contains 1/16'')?

Thank you

[Art Montemayor]

VNE:

I don't know how you define it, but I have always defined and understood superficial velocity to be the velocity inside a vessel when calculated as if there is/are NO INTERNALS inside the vessel.

Therefore, all you need to calculate the superficial velocity is the actual vapor flow rate through the vessel and the internal diameter of the vessel. The actual volumetric flow rate divided by the cross-sectional flow area (assuming an empty vessel) equals the superficial velocity.

Do we agree?

By the way, are you using the Ergun equation (the equation of choice) for calculating the pressure drop in an adsorbent bed?

[vne]

Art,

Sorry if my question is confusing. I'm applying Ergun equation for calculating tower diameter of dehydration tower using solid desiccant (with Delta P/ L is presumed).

Tower diameter can be determined by superficial velocity which is the outcome of Ergun equation (above). However Ergun equation as specified in GPSA 12th is for bed with 1 kind of mol sieve. I'm wondering how to calculate superficial velocity if 2 kinds of mol sieve (e.g. both 1/8'' and 1/16'') are used in the same bed.

Thank you

[breizh]

Vne ,
To me the equation is related to the flow (velocity ) through the bed .

DP/Z = a *U +B*U ^2 with
a =fct of ( viscosity ,specific area, porosity and hK =coefficient # 5 kozeny model)
b= fct of ( density of fluid , specific area , porosity and hB #0.3 cte de Burke -plumber)

first part is rfelated to laminar flow and second to turbulent flow


Hope this helps

Breizh

[Zauberberg]

Superficial velocity through the bed will always be the same - for a constant diameter vessel - regardless of size of the Mol Sieve adsorbent. Art has given the correct answer to you.

As for the Ergun equation issue, you have to apply different coefficients for different adsorbents (size) and see if the criteria for minimum pressure drop of 0.01 PSI/ft has been met for all desiccant layers. That' how it works.

See attached spreadsheet, and even better you can create your own based on published coefficients for Ergun equation, and seek for the optimum solution.

Attached Files

•  Adsorber Pressure Drop.xls   585KB   328 downloads

[vne]

All,

It's clear for me now.

Thank you very much.