Hot Lime Softener Blowdown

[dg47]

I have a question on the blowdown from 3 hot lime softeners at a facility. I'm a wastewater consultant. For environmental reasons, we intend to capture and dewater the lime solids from the softeners. The blowdown is at 22 psi and 230F. About a 10% lime slurry. There is a continuous blowdown of 100 gpm from the combined units or about 60,000 lb/h. The solids will be separated in a thickener tank and the overflow of soft water will be recycled back to the system. Do I need a flash tank before the thickener? I think so; however, I have an purge blowdown that can be quite high for 20-30 seconds from a 6" pipe wide open every couple hours. The flash tank would be very large if I have to size for the high purge. Any suggestions? Thank you.

 

[ApC2Kp]

dg47,

It sounds like you need to have two blowdown tanks - one smaller size blowdown tank for the 100 gpm continuous duty, and then a second one somewhat larger size for the 20-30 seconds of purge blowdown. It might not be too much larger for the 20-30 seconds duration, maybe 2X-3X size? The larger intermittent blowdown tank could have a reserve volume of cold liquid for reducing the amount of flash.

Check with

http://www.pennseparator.com

for blowdown tanks sizing.

 

[shahyar]

1. It is better to name that "desludging" rather than "blow down" to not confusing people in boiler industry.
2. If you switch from continuous desludging to intermittent one you can have thicker sludge ( more than 15% up to 25%)with lower water loss. So there is no need to use thickener at all. But if you want to trap that small amount of water (in intermittent desludging), you need to use belt filter or centrifuge.
3. I am familiar with HLS, but I don't understand why you have two types of desludging: "purge blow down"( 6"? wow) and the other one.
4. The sludge is not just lime. It is mainly Calcium Carbonate and residual lime (unreacted) and Mg(OH)2 and unreacted MgO if you used MgO too.

Hope this help