Stirred Tank for Precipitation

[david207]

I am looking to size a stirred tank with an inlet stream of brine containing CaCl₂ impurity and another inlet stream with Na₂CO₃ used to precipitate all or most (>99%) of the calcium out as CaCO₃. Can a rough estimate of the volume of the stirred tank be made without experimental data and if so how?
Thanks

 

[ashtree]

David,

In broad terms yes.

If it is your intent to have most or all of the chemical reaction to occur in this tank you will need to know the chemical reaction rates and your flow rates in order to calculate a hydraulic detention time. Your reaction rates are going to depend on things like process temperature, relative concentrations, intensity of mixing. At normal room temperatures i would think that 10-15 mins detention time would be enough.

If your intent is purely just to mix the chemicals this could be done a lot quicker and a minute or two detention time would be heaps if well mixed.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[david207]

Thank you for your reply.

Yes it would be my intent for most of the chemical reaction to occur in the tank. Do you know where I could find data for the chemical reaction rates? Not having much luck finding reaction rate data so far.

 

[bimr]

One aspect is the chemical reaction rate, another aspect is the time that it will take for the material to settle out of the tank.

Chemical precipitation equipment for something like this in the water treatment business normally has a 60 minute retention time.

 

[itdepends]

Make sure the Na2CO3 is already dissolved before you add it to the reactor or you'll coat the solid Na2CO3 with CaCO3.

I would mix the two streams in one tank and use a second tank for solid/liquid separation if required. The reaction will be very fast, the tank residence time will be more about allowing sufficient time for mixing/preventing short circuiting rather than the precipitation time. I assume you have access to the waste stream? A quick lab test will give you an idea of how fast the reaction is. Have the waste stream mixing, add enough CaCl2 to be in excess (say 5% excess), sample and filter 30 seconds later and check the residual CaCl2 level. My bet is that you would already be at equilibrium.

If you don't have access to a lab to analyse for calcium- take your sample, filter and then add more Na2CO3 and visually check for additional precipitation.

If you don't have access to the waste stream- make up an artificial one. Both chemicals are available at your supermarket (Damp rid/damp remover is normally CaCl2 and washing soda is Na2CO3)

Before you get too carried away though- how important is the answer (i.e. how big is your waste stream, what's the concentration of CaCl2, what's your target concentration).

If it's a small flow then the cost will be relatively insensitive to the required residence time. The larger the flow the higher the justification for lab/pilot testing.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"