Scrubber Stack

[myering]

I am new to permitting and I am trying to get a rough estimate of stack diameter and hieght for permitting before we have chosen a manufacturer for the scrubbers that we are going to use. We are going to use a packed bed scrubber. I have the flow rates calculated from the process engineer and I was wondering if there is a good calculation for figuring out diameter and height from flow rates from an ethanol process.

 

[twobits]

You should have a limit on your pressure drop. That coupled with the efficiency of your bed or what other specs they are giving you for what the bed consists of would give you a diameter and depth of your bed based on your flow rate. That is where I would start.

 

[myering]

I am trying to find the outlet exhaust stack diameter and height, not the bed dimensions.

 

[MikeClay]

You need to size the diameter to give a reasonable pressure drop. Is the scrubber at or near atmospheric pressure? Or is it pressurized and the stack downstream of a pressure control valve? These items along with stack height are needed.

Once a stack diameter is estimated, dispersion modeling is normally performed to determine the height. Toxic, flammable, or odor limits should be considered. Once the height is known, recheck the pressure drop. Iterate until an acceptable solution is reached.

--Mike--

 

[mbeychok]

MikeLay's response was indeed the correct way to determine your stack diameter and height.

However, if all you want is a rough estimate of the diameter and assuming your process engineer has given you the volumetric flow rate up the stack (say in ft³/s), then assume that the linear velocity in the stack will be 75 ft/s and:

Area, ft² = (V, ft³/s) [÷] (50, ft/s)

Then, since Area = (Pi)(D² [÷] 4, you can calculate the diameter, D.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[mbeychok]

Sorry, but that should last line should read:

Then, since Area = (Pi)(D²) ÷ 4, you can calculate the diameter, D.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[0707]

go to

http://images.google.co.uk/imgres?i...r+stacks+dimensions&gbv=2&svnum=10&hl=en&sa=G

luismarques

 

[myering]

Thanks guys that gives me a good calculation for a rough estimate on permitting.

 

[MikeClay]

I'm a little concerned with the 75 ft/sec velocity if it is an atmospheric scrubber. There may not be enough pressure available for that flow.

--Mike--

 

[myering]

I actually used 1100cfm 1500ft./min to 15000cfm 4350ft./min depending on flow rate, and they are all atmospheric. Based this of air flow duct velocities.

 

[25362]

Natural draught at the bottom of the stack is created by the density difference between the ouside air and that of the flue gases multiplied by the height of the stack. One can then see that a high gas exit temperature and a tall chimney would help in creating the driving force to overcome internal resistances.