Baffled Tank - Residence Time

[ChE51]

Hello,

I have a hot water recycling problem. I want to recycle water from our lagoon into a hot water tank that is used for our tote cleaning processes. In order to kill the 'bugs' in the water I have been told by our microbio engineer that at 85C (hot water temp used today) we would need to hold the water for 10-15mins. I am trying to design a tank that can assure this residence time. I was considering a baffled tank.....any suggestions?

 

[smile1]

You might want a second opinion on sterilizing the hot water. I've had bio grow where there was no way it could survive. 85C might kill the bugs your trying to grow, but not all of them. If there is an available surface, something will grow.

 

[ChE51]

Thanks for the tip...Do you have any suggestions/comments on the tank design?

 

[smile1]

Well, if you design the baffled tank to plug flow velocity, the residence time will be very close to that of a plug flow reactor. so, flowrate / baffled tank volume = 15 min. Typical designs would be a racetrack assembly, similar to that in a lagoon. Depending on the size, you may just want to run 2 or 3 CSTR in series, sort of like water heaters, and cascade them. I would chlorinate at the end, though.

 

[ChE51]

Are there any concerns/tips you have on designing such a baffled tank?.....it appears to be pretty straightforward....just to confirm the baffles would be in the vertical position so the previous stage(consists of 2 baffles) will overflow in the following stage and the baffles heights would remain the same from stage to stage...thanks for ur help Smile1

 

[pleckner]

You do not really give enough information as to how your system is actually set up. However, for biopharm plants, a constant circulation is maintained in the hot water system. Otherwise you will form a bio-film on the surface of the tank regardless of the high temperature. Most people design for a minimum of 5 ft/s in the piping. If this is sanitary piping with no deadlegs, you can get away with less (closer to 3 ft/s). If this is for standard piping (including the presence of deadlegs), then you should probably design for even higher velocities. Keeping it fully turbulent is really the key. I hope we are talking about a 316L SS system here and not carbon steel!

I also designed a system once that was approved by the Virginia department of health. In that design, we injected chlorine first then stored the water in a horizontal tank with a horizontal baffle. The water came in from the top at one end through a 90 degree elbow facing the head. The water was withdrawn at the same end but below the baffle. This forced the water to transverse the vessel length twice and avoided any chance of short circuiting.

If you are using just heat, the second design won't work because you will still get a bio-film formation. You must circulate the water and keep it moving.

 

[ChE51]

Okay pleckner here's a more detailed explanation.

Basically we have a onsite lagoon (just storm water) we want to recycle back into the plant. The only place in the plant that has the capacity for this water is the tote cleaning area. Currently we use city water which we heat in a pick heater then store in a 1200 USG tank and keep heated by adding steam. In order to kill the bugs I have been advised by our microbio chemist that I would have to hold the water with bugs at 85C for at least 10mins. Since our flow is 200USGPM the current tank is too small. I am designing the system for a residence time of 15mins(just to be sure) so therefore my tank is 200*15 = 3000USG. I want to design the system so that it is seemless in that it switches from lagoon water to city water without operator action. Currently we fill the tank when it reaches a low level of 20% of the tank volume and stop filling when it reaches a high level of 90%. I have just never designed a PF reactor/tank with baffles and want to be sure about the baffle configuration and spacing. The tank I am designing is square and has to have a floor area of 8ft by 5ft so for 3000USG I need a height around 15ft. I think the best configuration is to have the baffles in a vertical position so that if the tank were empty and being filled it would ensure the residence time because it would force the water to fill up the first chamber then overflow to the next. I am not sure how much spacing between the baffle ends and the tank wall would be. You seem to have some good experience in this field...what do you think of my design so far? The tank would also have a recirculation and could possibly include chlorination.

 

[pleckner]

As you describe it, you probably don't have much choice but to proceed as you are. However, the key is circulation.

If I understand you correctly, you want to fill the first compartment and let it spill over into the second. You would then pump out of the second compartment to clean your totes. Once you finish cleaning, the water flow into the first compartment would stop. The water would then just sit there.

OK, this is how I think it can be done. Fill the first compartment with water through a dip pipe that goes down to within 6" of the bottom. Have this first compartment on continuous circulation and heated. Allow the water to spill into the second compartment. After cleaning, you need to drain the second compartment because the heel will just be sitting there and grow bugs. After draining, you might want to steam it out as well and evaporate any remaining water. You might even think of air drying that compartment. At a minimum, you would need to drain some of the water out of the first compartment to drop its level so it doesn't spill over and you would need to keep the first compartment circulated. Alternatively, you can drain the entire tank, steam it and dry it for the next cleaning cycle (I like this better).

For baffle spacing, just provide enough surge time for your pump in the second compartment. I would think 2 or 3 minutes is enough. The second compartment should be large enough to provide your 15 min. retention time.

If you can chlorinate, then heating won't be necessary. Just keep it circulated.

If I think of anything else or find some flaw in my reasoning, I'll repost.

Good luck with your application.

 

[kewilson8]

Hi all

Was wondering if anyone could help me or point me in the right direction for info on chlorine drying systems. I am trying to design one for a chloralkali plant without much success. Any help would be greatly appreciated

 

[moltenmetal]

Your strategy would be just fine if it weren't for residence time distribution. You need a minimum residence time of 10 minutes, so designing a tank with a mean residence time of 15 minutes is a decent first guess- but I doubt that your baffle sections will even approximately give you the residence time distribution of plug flow. For that, you need fully-developed turbulence in all parts of the tank, with no opportunity for short-circuiting or back mixing. A tank of the sort you're describing will definitely permit some water to sneak through without being there for the full ten minutes. In my experience, baffle tanks can have very complex residence time distributions, and their effective mean residence times are far shorter than what you might expect. If you have to use a baffle tank, I doubt that 15 minutes mean RT will be nearly enough.

The good news is that at 200 gpm and 85 C for water, fully developed turbulence is relatively easy to come by in pipe, at surprisingly low velocities. Just for fun, calculate the size of pipe which will give you a Reynold's number of 10,000 at 200 gpm- and make sure you use the actual viscosity of water at 85 C (it's lower than at room temperature, which helps). Then consider making an accordion-type tank (i.e. lengths of pipe with mitred bends) with a mean residence time of 15 minutes. If that won't fit in your allotted floor space, consider doing some slightly more detailed calcs to account for the elbows etc. (see Levenspiel's omnibook or his text for more details) and you may be able to get away with a slightly smaller tank- but probably not by much if you want 90% of the water to be in there for at least 10 minutes. You'll have to meaure the added fabrication cost of this type of design against the savings in floor space that it may provide to you. The worst option, though, would be to build something which doesn't suit the intended purpose (i.e. lets incompletely sterilized water go through)