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
| Massecuite | Minimum | Average | Maximum |
|---|---|---|---|
| A | 3.72 | 8.81 | 15.68 |
| B | 2.30 | 14.70 | 28.13 |
| C | 17.37 | 43.80 | 95.87 |
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
where