Calculation of thermal transfer
The thermal transfer coefficient calculation for the slurry is performed as if the liquor is alone. This process is cautious as:
- - the presence of crystals scrapes and activates the boundary layer;
- - crystal conductivity is between 1 and 10 W.m-1.°C-1 and is greater than that of the liquid (0.1 at 0.5 W.m-1.°C-1).
The thermal transfer coefficients vary from 200 to 2,000 W.m-2.°C-1 according to the properties of the liquid phase (thermal conductivity and viscosity). We refer to the calculation method in section 2.2.2 of [DUR 16].
Agitation and attrition
Agitation plays a double role:
- - to homogenize the suspension, so that, in continuous, non-fluidized crystallizers, the residence time is precise for a given size;
- - to homogenize the liquid phase in order to avoid local supersaturations other than the mean supersaturation in the device.
The draft tube system with a marine impeller is the best as it allows for a considerable internal recirculation flow. Attrition in a draft tube is negligible if the clearance between the tube and the impeller is greater than or equal to 2 cm and if the peripheral velocity of the impeller is less than 8 m.s-1.
Nonetheless, the effects of attrition are still a danger in the following rare instances:
- the crystals are particularly fragile due to the their jagged form or their weak cohesion;
- the installation includes a variety of slurry pumps (multilevel crystallizers).
Fragments resulting from attrition only come from crystals of a minimum size. Indeed, crystals of a size smaller than 100 pm faithfully follow their fluid streams and do not collide with solid bodies (agitator impeller, pump wheel). The size of fragments resulting from abrasion is in the order of 1100 pm.