5 replies to this topic

[Shaikat]

I have the following information of a packed column (for stripping colored water using sulfar dioxide gas):

Column Dia = 6 inch
Column Height = 6 feet
Packing = 1/4 inch Plastic Rasching Ring
Packing Height = 3 feet
Pressure Drop = 0.2-0.6 inch water per foot of packing
Vapor density = 1.2 kg/m3
Liquid density = 1000 kg/m3
Liquid viscosity = 1 centistokes

I need to calculate 1. Liquid Superficial Velocity &
2. Vapor Superficial Velocity

Can anyone help me ? I just need to know the procedure or the name of the references which may contain the procedure.

Thanks

[Art Montemayor]

Shaikat:

You don't need a "Procedure". What you need is to get back to basics.

Find out the definition of "Superficial velocity". You will probably realize that all that you have to do is apply the formula:

Fluid flow rate / Cross-sectional Area

It is that simple. I suspect that you have not studied your textbook or you were asleep when your instructor described superficial velocities. You certainly do not need all the data that you are proposing to use.

[Shaikat]

Shaikat:

You don't need a "Procedure". What you need is to get back to basics.

Find out the definition of "Superficial velocity". You will probably realize that all that you have to do is apply the formula:

Fluid flow rate / Cross-sectional Area

It is that simple. I suspect that you have not studied your textbook or you were asleep when your instructor described superficial velocities. You certainly do not need all the data that you are proposing to use.

I know the formula of superficial velocity & yes,i've studied the textbooks.But my point is - in most cases we have the inlet flow rates & the desired compositions of the outlet streams & we have to calculate the size of the tower.But i am given with the above information and asked to determine the required flowrates.You see, this is completely opposite to what we usually do. I'm sorry that i didn't make myself clear in the first place.

[clarenceyue]

Shaikat:

I believe Art is absolutely spot on, you have not been paying attention during your classes, especially, on fluid mechanics.

Have you tried looking at Ergun's equation perhaps? I may be wrong in assuming that Ergun's equation might apply, but at least that's a start.

I hope that by telling the starting point, you can begin to start looking at the right direction.

Clarence

[katmar]

Shaikat,

As you have said you have learned from your past exercises, and as Art has specified in his reply, you need the fluid flow rates to calculate the superficial velocities. Where can you calculate the fluid flow rates from? The only way would be to back calculate from the pressure drop. As you have a range of pressure drops you will get a range of flow rates and therefore a range of superficial velocities.

But to get from the pressure drop to the flow rates you also will need the gas to liquid ratio and you do not specify this. Maybe your instructor wants you to calculate the required gas to liquid ratio. If this is so, this is a rather involved problem you have been given.

If you need help on calculating the pressure drop I suggest you start by looking in your texts for the Generalized Pressure Drop Correlation by Eckert.

[djack77494]

Shaikat,
As you should know by now, you have definitely not defined a problem. If you have a problem here, it is underspecified. It's kind of like saying I want to get to Rome, so which direction do I go in? If you're in Paris you might go south and east; from Athens you'd go north and west. You must provide additional information such as a flowrate (vapor or liquid) and enough other information to eventually be able to calculate the other flow. A full pressure profile (inlet and outlet pressures) would be useful for calculating one of the flowrates if the other and a full mechanical description of your column were available.