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Can you recommend an agitator that will fully suspend and dissolve salt?
Q: I am looking to dissolve fine salt in water in a continuous process. The aim is to produce near-saturated brine without any suspended solids.
Fine Salt
Mass flow rate = 1000 kg/hr
Bulk density = 1200kg/m3
Temperature = 60-70 degrees C
Water
Mass flow rate = 4000 kg/hr
Temperature = 20-50 degrees C (water is generally at 50 degrees But sometimes at lower temp of 20 degrees)
Dynamic Viscosity = 1.003 0.55 cP
Pressure = 4 Bar
Total flow rate into the tank = 5 m3/hr
Residence time of 5 minutes
Total flow rate out of tank= 5 m3/hr
Vessel is under atmospheric pressure. I am having difficulty in selecting a stirrer that can both fully suspend the salt while also dissolving it. Can you recommend an agitator and a desired operating speed?
A:
If I understand the question correctly, the problem is feeding 1000 kg/hr fine salt and 4000 kg/hr water through a 0.417 cubic meter vessel. If 5 cubic meters per hour go through the vessel with only a five minute residence time, the vessel must be 5 cubic meters divided by 12 or about 0.4 cubic meters. At that size a direct drive portable mixer, operating at 1500 or 1800 rev/min depending on the frequency of your power is the most appropriate option. The mixer should be either 190 or 250 watt mixer, with an appropriately sized axial flow hydrofoil or propeller. The mixer should be mounted at an angle that limits the depth of the surface vortex to less than 1/4 the distance to the impeller. If the mixer draws air into the liquid, the mixing efficiency will be reduced.
The problem may be over-specified. Properly suspending the salt and keeping the vessel well mixed will not assure that all of the salt will dissolve in the desired length of time, especially over the range of temperatures mentioned. The mixing intensity has less of an effect on the rate of dissolution than the saturation concentration and the particle size. As long as the mixer keeps the solids in suspension, more mixing intensity will increase only slightly the rate of dissolution.
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