Design of a Vent Condenser 1

[Timewrap]

Dear All,

I have a vessel operating at atmospheric pressure and the fluid is Ethanol. The operating temperature of this vessel will be about 90 deg C and ethanol boils at 78.4 deg C at atm. pressure. THis tank will be drained out to another tank quickly as soon it is filled up. To minimise the loss of ethanol, I considering to put up a vent condenser on a 4" nozzle which exists on top of the vessel.

Can somebody throw some light on the rate of evaporation of ethanol from the tank. i'm trying to calculate the area required for the heat load.

The tank capacity is 1200 gallon and the diameter is 8'.

thx in advance for your help

Timewrap

 

[sshep]

Hey Timewrap,

Tank vent condensers have been discussed from a practical standpoint several times in this forum, including installation options recently (thread124-261117).

The breathing rates through the vent are based on heat and mass transfer to the tank, the subject is well covered by API 2000. Since the evaporation rate of a stangant tank is related to the heat flux at the tank wall from the environment (radiation and conduction), you can possibly tweek these calculations if needed- an appendix documents the assumptions. If you have an N2 purge (or blanket pad valve leaks), then this will be an extra load to the breathing on a stagnant tank.

Also, a floating roof seems to be an alternative that you haven't discussed.

best wishes,
sshep

 

[katmar]

The ethanol is going to flash and cause safety and environmental issues, so you have to do something about it. A vent condenser may be the correct option but you have not described your process in sufficient detail for anyone to give constructive advice.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Timewrap]

Dear sshep and Katmar,

Thanks for the reply..

I've referred to API 2000, there is a table 2A which lists the thermal venting requirement for a specific capacity of the tank. Is that the correct table I'm looking at??

My tank volume is 1200 gallon, it means that I can have thermal venting rate of 60 SCFH of air, is that correct.

Thx

Timewrap

 

[Compositepro]

It is not possible to have 90C ethanol in an atmospheric tank.

 

[BenThayer]

how do you intend to operate this tank at a temperature greater than the boiling point AND at atmospheric pressure?

by definition, if you are going to have the tank at 90°C, the vapor pressure will be ~23 psia.

if you pump the ethanol into the tank, the contents are going to flash almost immediately as it enters the tank and cool to ~78°C. The rate you pump in and the % flash will dictate your vent condenser sizing.

am i missing something?

 

[sshep]

I didn't catch the 90C point before. The situation in this case is that you will need to calculate the mass flashing at the feed rate to the tank (amount which must vaporize to cool the remaining liquid to the boiling point), and this (plus vapor space expansion) would be the initial load to the condenser. Clearly though, an atmospheric tank is not an acceptable case for storage of rundown this hot. You will need to cool your rundown to some reasonable temperature BEFORE it goes to storage.

best wishes,
sshep