# 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.

## 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]
MassecuiteMinimumAverageMaximum
A3.728.8115.68
B2.3014.7028.13
C17.3743.8095.87

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 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

## Torque Requirements

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

T = PK· r2· 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]
• N
is the rotation speed of the rotor [rpm]
• n
is the behavior index, and has a value of 0.75