Mixing a liquid storage tank with gas purge?

[Dereks19]

Our plant is bringing in a new hydrocarbon raw material that is consistently all over the map in reactives content. We would like to adjust each shipment with reactives (small adjustments ~3% of feed, very compatible, identical density) as it arrives to bring it within our specification range. Adjustment would take place in a 180,000lb storage tank with no agitation.

Is there any literature available on using a nitrogen purge or sparge ring at the bottom of the vessel to mix two liquids without any added agitation? I've looked all over but can't seem to find much in this area.

 

[Latexman]

Have you thought of putting a recirculation loop on the tank. There's tons of literature on that.


Good luck,
Latexman

 

[25362]

If the viscosity of the liquids isn't high, I assume the whole exercise is to ensure maximum homogeinity in minimum time. Otherwise, natural diffusion in a small tank as the one commissioned (100 m³?) could do the job in hours, right ?

If the volume ratio between the equal-density liquids to be mixed is large, one could try injecting the "smaller volume" into the "larger volume" in proportioned dosing, by using external circulation (plus in-line mixing elements for added turbulence) to the mixing tank, as suggested by Latexman, or when transferring the new lot to the tank.

 

[Montemayor]

Dereks19:

Latexman and 25362 are recommending what is normally done in industrial applications. I've re-trofitted recirculation pumping systems on heavy & light polymer storage tanks in order to eliminate troublesome and leaky side-mounted agitators and the results were a success. The last modification was done in 1998. All that is involved is a eductor-type device that is immersed inside the tank. By using the velocity head created by the recirculation pump, a suction is created in the mixing device that allows for throrough and effective mixing. I used an suction steam heater in my applications and it also heated the contents - killing two birds with one stone.

I would not use Nitrogen as the driving medium because its introduction would involve control instrumentation to protect the tank that would be expensive as well as the consumption of the N2. However, I would use the N2 as an inert tank blanket if the contents merit it.

Regards

Art Montemayor
Spring, TX

 

[Dereks19]

A recirculation loop w/ static mixer was one of the original ideas, but out of fear I may be killing a mosquito with a truck instead of a fly swatter, I wanted to look into a quick and easy purge option. As 25362 says, natural diffusion will most likely mix the contents to zero concentration gradient within a matter of hours. Looks like a loop is the surefire bet though.

Thanks for the help!

Derek Stevens
Pleasant Prairie, WI

 

[Montemayor]

Dereks19:

I want to make sure we have our terms straight:

A static mixer is an in-line device (a "wide spot" in the line) that has internal packing that creates turbulent mixing of two fluids introduced upstream of the device.

The eductor-type mixer I'm referring to is more of a "dynamic" device. It is just as cheap as a static mixer except that it is not an in-line device. Rather, it is placed directly inside the tank; it has no moving parts and uses the velocity head created by the recirculating pump's action to draw ("suck&quot in tank contents into its side inlet and mix turbulantly with the recirculating fluid. For tank purposes, it is far more effective than an in-line static mixer. We found such a device creates internal mixing very similar to a mechanical agitator and although it is not as energy efficient as a mechanical device such as an agitator, it is self-contained, has no fugitive emissions (shaft leaks), and is maintenance free (no moving parts or gear boxes). I hope I've explained the difference between both types.

Regards



Art Montemayor
Spring, TX