Double wet-scrubber?? 2

[xiangqi]

My plant has a spray dryer for clay product. The dust is intercepted by cyclones at moderate efficiency (around 85% - my estimation) due to small particle size distribution. The system has a Venturi type scrubber to reduce the PM (particulate matters) emissions (regulation: 100 mg/m3).

From mass balance, if scrubber is running at 95% efficiency, the PM is about 300-400 mg/m3, well above the government requirement level (on 100,000 m3/hr air flow).

The plant is reluctant to spend lot of capitals to install bag-house to replace the cyclones. I am thinking about a cheap way - put another scrubber into the system, right before the existing scrubber. If the new scrubber can have 90% efficiency, the PM emission level will be controlled arond 20 mg/m3.

Do you think it is doable? Any other alternatives (low capital)? Many thanks for your reply and input!!

 

[mbritt]

There is hope –
Allow me to check some assumptions:
1. The clay is neutrally charged – without inclusions that would allow it to become polar.
2. The clay will become soft when it becomes wet.
3. The process is at ambient temperatures.
4. The venturi is reticulating water with neutral pH.
5. The venturi was designed with a 0.5 second or longer contact time after the initial interaction between air and liquid.

Comments:
1. The cyclone is a good choice for a first stage in particulate removal.
2. A venturi scrubber is ideal on gases that dissolve well in water but not so good as a particulate removing device.
3. A mist scrubber following the venturi is a viable solution in this application, but to be effective it will require a vessel large enough to provide 5 seconds retention time and a relatively slow gas velocity. Have more details if you are interested.

 

[siretb]

I think you can try a high energy Venturi scrubber.
Before putting another scrubber, check if it is not possible (ID Fan) to increase the specific energy at the existing Venturi Scrubbre (by changing the L/G liquid to gas ratio) This will cost you pressure drop (==energy) but it will reduce de d50 size of the Venturi.

What would not work , would be to have a second scrubber of the same technology, with same operating parameters (energy, pressure drop, ...) because the fine particles that would have escaped the first scrubber would also escape the second.

 

[xiangqi]

Many thanks to the kindly reply.

To mbritt: Yes, the clay is neutrally charged, but the temperature is at about 65 oC - coming from the spray dryer. You raised good point about the contact time in the Venturi, and I am not sure we have 0.5 sec contact time. The air flow is very high (maybe higher than design due to plant running at a lower inlet air temperature thus needs more hot air into dryer), at about 140,000 m3/hr (@65oC). The Venturi tune might have a size of 0.9 m X 0.7 m and 6 meters long. This ends up with about 0.1 second contact time.

However, we do have a mist eliminator, which is a large water tank with stack on top. I need to measure that to see if it has 5 second residence time(5 second ~ 194 m3, but it might be just OK (6 meter diameter X 8 meter heighth).

Is that typical design criteria for scrubber (the residence time)? What is typicall efficiency for dust removal? (suppose 95% > 10 um).

 

[xiangqi]

Thanks for your input, siretb!

Energy is not a problem. I already sent the request to meaure the pressure drop through Venturi. The emission gas has a very high tempperature (57-59 oC) which carries lots of vapor from the system. The problem is the limited fresh make-up water. I know the right L/G ratio is about 4-10 gal/1000 scft gas, the water flow to Venturi is at 950 liter/min and 62 oC (about 3 gal/1000 scft). So water is not enough and also hot! A recommendation for plant is to increase both the recirculation water and make-up water.

The most important is the emission level. I need to have a 98-99% scrubber efficiency to control the PM to 100 mg/m3 level. Add another scrubber is just playing the statistic game. If one scrubber can operate at 90% efficiency, then 2 scrubber can help me get to the 99%. Of course I will do some optimization first in the upstream process to limit the dust sent to scrubber.

What do you expect the efficiency for Venturi scrubber with mist elimilator? Could it get up to 99%?

 

[siretb]

Xiangqi

We can tell you whether or not you will achieve 90, 95 or 99% efficiency if we know the inlet PSD (particule size distribution)
If you know your Venturi design, it is possible to calculate the d50 of the Venturi. Many correlations are available (unfortunately yielding different answers). Among them Calvert, Morishima, .....
Again, it all depends on your PSD. I would tend to say, that if you want a good job done, the pressure drop would be in the range of 1000-1500Pa.
For very high efficiencies, be careful about the secondary re-entrainement (droplets of not-so-clean water beeing re-entrained from the venturi)

We cannot help much more now. Given the PSD and Venturi design & operating conditions, it could be possible to calcultate the expected pressure drop and efficiency.
Regards

 

[xiangqi]

Hi Siretb,

The PSD is: D50 @ 50um, about 20% less than 20 um, and 5% less than 10 um. The Specific gravity of the material is about 2.5.

Thanks!

 

[siretb]

Well, if you consider you might catch particles down to 1-2 microns, then it's not certain from your distribution that you'll achieve 99%. We would need to know the tail of your PSD (%mass below 1 micron, % mass below 2 microns).
Should be safe for 95% (of course you'll understand that a thorough design job needs to be done, and that, on the basis of what we know we cannot garantee it! This is well beyond of what can be done in this forum.)

What you have to do, if you stay with the Venturi design is
calculate the d50 of the Venturi for different configurations, and compare against your known distribution
Check that energy/pressure drop is OK
Do not forget the water bleed and how you monitor the particle load in your waters.
Keep in mind that your venturi efficiency will be much gas-load dependent. (low flow rate ---> low efficiency).
There are workarounds for this issue (variable throat, ....)
Remember for a given PSD (pressure drop <----> efficiency)

 

[xiangqi]

Thank you.

 

[Topumps]

Xiangqi,

There is another type of wet scrubber (WEV) better compared to Venturi, less pressure drop and water consumption, and higher separation efficiency and smaller foot print. If you have PSD data, the separation efficiency can be calcualted based on pressure drop. This scrubber employs a special design atomiser to generate a "tornado" type of vortex inside the scrubber chamber and suck the scrubbing liquor from bottom sump to scrub the gas through the chamber. All mist eliminators can be installed inside one vessel. It can achieve more than 99% efficiency for fractional microns of particles.

For more informaton, you can search WEV scrubber on google.