4 replies to this topic

[drahyden]

Hello,

I'm new at posting so I´ll try to be brief and clear with my question:

I'm evaluating the preformance of an oxidation tower from an exhausted caustic regeneration system,

System description:

A caustic stream of 8.8%wt NaOH in water () enters from the botom of the tower and floods the column up to the point of a chimenay tray, where it exists the tower. Aproximately 1 ft above the caustic stream feed two arm air difussers with 17, 1/8" diameter holes feed plant air in to the process. The counter pressure caused by the level control of the hydraulic column is expected to be 0.719 barg

I can read the air pressure at the air line, 6" prior of it entering the tower -> 3.6 barg

Also I can read the pressure at the exit gas stream of the dome of the tower -> 2.82 barg

My question is: With this data how can I calculate the air flowrate to the tower without having the discharge coefficient or the pressure distribution of the spargers in the column?


Thak you in advance for your feedback.

[breizh]

Consider these resources to support your query .It may help.

Breizh

[S.AHMAD]

1. If the flowrate of air is an important operating parameter, then a flowmeter should be installed.
2. Estimation of flowrate from operating data is possible but it will not be as accurate as a flowmeter
3. I suggest that you ESTIMATE the flowrate by using orifice equation. However you need to determine the pressure drop across the sparger hole (orifice) by taking into consideration the tower system pressure and hydraulic head of solution above the sparger.

Edited by S.AHMAD, 07 March 2012 - 08:15 PM.

[drahyden]

Thank you for your feedback,

S. Ahmad, as I discovered recently we do have a volumetric flow meter on the air supply, however, we suspect the indication is mistaken because the instrument datasheet, says it's a rotameter, whose operating conditions are 6.3 barg and 32 °C. Under this conditions the scale goes from 1-100% where 100% equals 30.1 Normal Cubic Meters per Hour. The deal is that under actual conditons the reading is 21%. I know rotameters are afected by process conditions, but how can i correct the data?

[S.AHMAD]

Drahyden
1. There are three possibilities of the root-cause of the problem:
a. Incorrect reading of flowmeter
b. Air flowrate higher than design
c. Both
2. As mentioned earlier, you can estimate the flowrate using the orifice equation, but you need to know the pressure drop across the sparger that can be estimated from operating data. Accuracy of calculated flowrate would depend on the accuracy of pressure drop estimate.
3. The flowrate measured by rotameter is a function of fluid density. For air, the density is depending on temperature and pressure. From the data given, design pressure is 6.3 barG but the actual is 3.6 barG. Therefore, the flowrate must be corrected for density variation.