Design parameters for vertical crystallisers, including residence time, cooling rate, cooling surface area to volume ratio, heat transfer co-efficients, rotations speed and torque requirements are given below.
The is a wide variation in residence times in installed crystallisers in the various factories. Southern African plant data summarised in the table below shows these variations
The residence times that can be used for designing new vessels are
C-massecuite can (and should) be cooled to below 40°C, the cooling rate can be as much as 7°C/hr while the massecuite is still hot, this cooling rate reduces rapidly as the massecuite cools and gets more viscous.
The cooling surface to volume ratio can be in the range 1 to 2 m2/m3.
The cooling water fow velocity should be in the range 0.9 to 2.0 m/s in order to achieve good heat transfer without excessive pressure drop and scaling.
The overall heat transfer co-efficient (OHTC) for C-massecuite in a stirred vertical crystalliser with static cooling elements is in the range 10 to 20 W/m2/K. A value acceptable for design is 18 W/m2/K.
The usual height of massecuite in the vertical crystalliser is between 9 and 11 metres
Speed of rotation of the stirrer are as follows
The torque required to operate a C massecuite vertical crystalliser is given below
T = PK· r2· l(N/n)n