effect of seeding in crystallysers
effect of seeding in crystallysers
(OP)
Dear all,
Can anyone tell me more about seeding techniques and the effect in evaporative crystallizers ?
Thanks
Can anyone tell me more about seeding techniques and the effect in evaporative crystallizers ?
Thanks





RE: effect of seeding in crystallysers
Calculations are not so simple, however you have to solve a system of differential equations form mass, energy and class number of crystals including nucleation rate, growth rate, agglomeration and attrition rate as well.
m777182
RE: effect of seeding in crystallysers
But still its unclear to me:
"The consequence is that you have less nuclei in your solution and more supersaturation is turned to the growth of crystals".
How come that less nuclei results in crystal growth (volume increasing).Does seeding acts as an actuator? Can you give me a specific calculation eg. ammonium-sulphate/water? How can we investigate and monitored the seeding by experiment especially for ammonium-sulphate crystalization?
Thanks
RE: effect of seeding in crystallysers
A seed acts apparently as a mean to lower the activation energy for a nucleation: the solubility of a particle depends (besides on T) on its radius; the smaller it is the more is soluble. Consequently smaller particles need higher supesaturation for phase change. Adding seeds means that you start actual growth immediatelly.
Regretfully I cannot give you calculations here because it is not a one-two-three task, but the ammonium sulphate is a system that has been published in many papers.
The most simple experiment would be to track the crystall size distribution with adding different amounts of seeds preferentially of equal size- a dust or fines of your produst will be fine, but keep temperature profiles, supersaturation profiles and aggitation constant!
m777182
RE: effect of seeding in crystallysers
RE: effect of seeding in crystallysers
I agree that swirling and high turbulence increase the secondary nucleation so it should be kept on minimum by using efficient aggitators with low power input; however you should keep some mixing to allow the heat transfer and homogeinity of the suspension. Operating at low supersaturation makes your production rate low and by low supersaturation you can get a different crystal shape, mostly more perfect shape. The trend today is towards spherical dense crystalls with high bulk density.So the good policy might be a fines trap:separating and dissolving fines and recirculating slightly undersaturated solution.Interesting topics,isn't it?
m777182
RE: effect of seeding in crystallysers
Basically, we are looking at the same crystallizer... external recirculation - and thermal destruction - of fines you say, and I agree. Adding that the unsaturated solution should be injected in the best possible location, generally deep down in the vessel, since it is hot. The problem is, in real life you cannot always thermally destroy the fines : think to anhydrous sodium sulphate, for example. So you have to work on both issues. BTW, I meant reducing the volume of the supersaturation zone, not reducing the supersaturation in terms of % excess solute - of course you have to compromise, somehow.