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.

Residence Time

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

Residence Times [hr]

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

A-massecuite : 8 hours

B-massecuite : 24 hours

C-massecuite : 40 hours

Cooling

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 m^{2}/m^{3}.

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/m^{2}/K. A value acceptable for design is 18 W/m^{2}/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

Torque Requirements

The torque required to operate a C massecuite vertical crystalliser is given below

T = PK· r^{2}· l(N/n)^{n}

where

T

is the required torque [kNm]

PK

is a constant, and has a value between 15 and 20 for C massecuite

r

is the radius of the rotor [metre]

l

is the height of the massecuite in the crystalliser [metre]