Crystallizer piston with scraped walls [ULL 86]
The device is a horizontal cylinder fitted with helical scrapers. It is equipped with a double envelope. The force of pressure causes the crystals and the liquor to progress in the same direction.
The coolant circulates counter-current to the suspension in the envelope.
The exchange surface is in the order of 1 m2 per linear m. The scrapers turn slowly (5-20 rev. mn-1). The calculation and experience show that the transfer coefficient varies from 100 to 300 W.m-2.°C-1.
Small and relatively uniform crystals can also be obtained.
The production of a tube is in the order of a dozen tons per day. Note that the device can easily accept a difference in temperature of 15°C between the slurry and the coolant.
The equations that were established for the cooled crystallizing vat also apply here by replacing Дт by Ax/V, where V is the slurry’s rate of progression.
However, the movement of the scrapers and the presence of crystals lead to an axial dispersion within the liquid, and rather than writing as for the cooled crystallizing vat:
we will write:
We will make our first calculation neglecting the axial dispersion. From the profile obtained in this way for X, we will iterate on the complete equation. During the first calculation, we assume a linear temperature profile for the coolant.