In sugar processing, poor vertical crystallizer design will often result in channeled massecuite flow and dead spaces within the vessel, leading to a corresponding reduction in crystallization efficiency. Good flow patterns, as well as sufficient retention time and cooling, are essential to achieve adequate crystallization. In this blog, we discuss five aspects of design that have a marked effect on crystallizer performance.
1. Vessel Dimensions
Aspect ratio plays a critical role in vertical crystallizer design. This is the ratio of the vessel height to its diameter. As aspect ratio increases, the following factors have to be taken into consideration:
• More material is required to manufacture a tall narrow vessel due to the higher shell surface area per unit of volume.
• The higher the aspect ratio, the more expensive the fabrication because of the additional banks of tubes needed.
• A tall narrow vessel will result in a better massecuite flow pattern at a higher velocity than a vessel with a low aspect ratio.
• Massecuite and water pumps must be sized to deliver at a higher pressure.
Vessels with low aspect ratios need higher stirrer torque loads. There is also an increased potential for creating dead volume and massecuite short circuiting, leading to poor performance.
2. Cooling Elements
Even with the use of agitation, the amount of heat lost through the shell of the vessel is not sufficient for extensive crystallization. Crystallizers must be fitted with cooling elements using cooled water, ideally in countercurrent to massecuite flow. There are two types of cooling elements in use today:
• Moving cooling elements – Moving cooling elements can be designed to act as both cooling and agitation devices. However, their design is more complex than that of fixed elements and present several disadvantages. For instance, moving cooling elements are subjected to higher mechanical stresses than static elements. This can result in stress fatigue damage, especially at weld joints. Draining water from the cooling water also requires complete drainage of massecuite from the vessel.
• Fixed cooling elements – This option is widely preferred in large vertical crystallizers. The most common arrangement of tube banks is in horizontal alignment rather than in spiral for. Small diameter tubes assist in moving the massecuite away from the cooling surface, while longer tube length is necessary to meet the cooling surface area required. Many manufacturers install tubes of 40 to 50mm in diameter to optimize the heat transfer surface area. Successive tube banks are often arranged at right angles to the previous bank.
3. Agitator Configuration
Agitation of the massecuite between tube banks is essential to prevent thermal layering and promote uniform temperature distribution. Agitator arms help to maximize the efficiency of heat transfer between the cooling water and massecuite. They also help to prevent dead spots and short circuiting.
Agitator arms are aligned as close to the cooling tube bank as possible to break up any layering of massecuite on the tube surface. They are often made from angle iron in the form of a rectangular box that is effective in clearing the massecuite from tube banks above and below the agitator arm.
Other agitator designs incorporate a solid object attached to the arm. This causes extensive drag as it moves through the massecuite, breaking up thermal wakes and achieving temperature uniformity.
4. Agitator Drive Design
Agitator drive design plays an important role in vertical crystallizer performance. In general, operators have their choice between two drive options. They are:
• Hydraulic drives – This drive mechanism requires a hydraulic drive pack, which can be sized to operate several crystallizers. For a hydraulic motor to be mounted directly onto the drive shaft, it would need to be large enough to accommodate the high torques and low speeds required. In addition, hydraulic drives are generally difficult to maintain and develop issues with oil leaks when they start to wear. As a result, they are not popular as a drive mechanism for vertical crystallizers.
• Electric drives – Electric drives are the most common type of drive used for vertical crystallizers. The electric motor employs a gear reducer to provide a straightforward solution for agitator operation. Gear reducers have traditionally used a worm and crown wheel system, but these are being replaced by planetary gear reducers. The planetary gear reducers are more compact, have no alignment issues and are less prone to breakdown. Their high mechanical efficiency also enables a smaller drive to be installed.
5. Massecuite and Cooling Water Flow Paths
Even in the most efficient vessel shapes, cooling elements and agitators cannot completely overcome the inherent instability of the massecuite flow. As the massecuite cools, it becomes more viscous. This results in reduced flow and further cooling.
Hotter material within the massecuite will flow faster, and the contrast in flow and viscosity between hot and cold layers increases the tendency for dead spots and short circuits to form. This problem can be reduced with the use of baffles to direct flow and eliminate preferential flow paths.
For optimal efficiency, tube banks should be designed for countercurrent flow of cooling water. Other design considerations include the elimination of air locking and ensuring the pressure drop through the system is acceptable for effective temperature control of the system. In summary, effective design of continuous vertical crystallizers should include controlled cooling, and uniform flow of massecuite to ensure even temperature distribution and the elimination of dead spots and short circuiting.