Vertical Crystalliser Design Parameters
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. You can do an online rapid design for your particular vertical crystalliser right now.
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
- A-massecuite : 8 hours
- B-massecuite : 24 hours
- C-massecuite : 40 hours
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
- A-massecuite : 0.75 rpm
- B-massecuite : 0.5 rpm
- C-massecuite : 0.25 rpm
The torque required to operate a C massecuite vertical crystalliser is given below
T = PK· r2· l(N/n)n
- Tis the required torque [kNm]
- PKis a constant, and has a value between 15 and 20 for C massecuite
- ris the radius of the rotor [metre]
- lis the height of the massecuite in the crystalliser [metre]
- Nis the rotation speed of the rotor [rpm]
- nis the behavior index, and has a value of 0.75