4 replies to this topic

[Nina1119]

Hello,

I am trying to design a HCl scrubber and I also need to calculate the water blowdown rate for the scrubber. The concentration of HCl in is 200ppm (flow rate: 25000acfm) and scrubbing solution is 25% NaOH (Flow rate: 400gpm). It is a 8ft diameter unit with 12ft packed bed height. I have calculated the chemical consumptions for this design. But I do not know how to calculate the water blowdown rate. Can somebody educate me on this?

Please help.

Thank you!

[breizh]

Hi

Why do you mean by water blow down , it should be the liquid collected in the sump underneath the column , a mixture of ( NaOH+ water + Nacl) ?The strategy should be to replace the scrubbing solution by fresh caustic soda solution when you reach let say ph 8 or 9 .

Hope this helps

Breizh

Edited by breizh, 19 October 2011 - 06:21 PM.

[kkala]

Attached "HCLscr.xls" could offer some further help, according to my understanding. Under stable condition (flow, temperature, humidity) of ingoing gas, blowdown is what "remains" from the ingoing liquid, that is water + soda solution (increase of it will increase blowdown). A heat and material balance is needed to specify liquid streams and their temperatures.
Blowdown should have a rate high enough to prevent precipitation of formed NaCl (or any other possible salt), but it should also have a generous margin to account for variations in conditions.
There may be some rule of thump about the blowdown of HCl scrubbers, not known to me, like the ones concerning blowdown from cooling towers or boilers.
For want of something better, one could consider (for example) that input water flow should cover the case of gas temperature increase by 3 oC. This extra water flow has to be added to blowdown under normal conditions.
Probably scrubbing solution is the 400 GPM, while make up caustic is that solution of 25% NaOH. Additional water inlet is assumed, depending on gas temperature. The scheme in "HCLscr.xls" reflects this understanding.

Attached Files

•  HCLscr.xls   25.5KB   252 downloads

[Nina1119]

Thank you for all the replies. Special thanks for the spread sheet and a detailed explaination @kkala. It was really helpful.

[kkala]

For heat and material balance mentioned in previous post by kkala, equilibrium vapor pressure of water is needed in function of temperature. Water97_v13.xla is very good for this, see http://www.cheresour...ater-and-steam/. In case it is not convenient (e.g. when the recipient does not have it), correlation in attached "WaterVP.xls" can be useful, taken from Engineering toolbox. As seen, error below 140 oC is less than 0.5%. Water vapor pressure in the correlation is expressed as absolute, not gauge pressure.

Attached Files

•  waterVP.xls   29.5KB   121 downloads