The best processing conditions and typical characteristics of the state of the art and efficient clarifier

Topics: Flocculation, Syrup, Sugarcane Pages: 5 (1104 words) Published: April 24, 2015
The best processing conditions and typical characteristics of the state of the art and efficient clarifier. 1. Introduction
The clarification process occurs in a clarifier. The mixed juice is kept in the clarifier and the impurities settle down. Two layers are formed; the upper layer contains a clear clarified juice and the bottom contains mud, which settles down due to a higher density. The clear juice (overflow) is sent to the evaporators to be converted to syrup and finally sugar. On the other hand, the mud (underflow) is pumped to the filtration station to recover the sucrose entrained in the mud.[1] 2. Purpose

The purposes of the clarification of sugar cane juice have been stated to be ideally, to obtain: a) Maximum elimination of non-sugars
b) Maximum elimination of colloids
c) Low turbidity of the juice
d) Minimum color formation
e) Maximum rate of settling
f) Minimum calcium content of juice
g) Suitable pH of juice to avoid inversion of sucrose or decomposition of reducing sugar.[2]

3. Condition in clarifiers
Temperature is an important factor. The temperature in the clarifier should not be too low. According to the kinetic theory of matter, at low temperature, the flocs formed will move slowly this will affect the settling rate. Temperature above boiling point is not good. Above boiling point, the juice will start to boil. This will create turbulences in the juice hence affecting the setting rate Usually a temperature between 95-97°C is kept in the clarifier. [1]

4. Types of clarifier
i. Multi tray clarifier - Rapid Dorr
The MultiTray clarifier design contains several compartments which main objective is to increase the area to enhance the mud settling. Volumetric capacity of this clarifier is in the order of 1.5 m3 /TCH. Juice residence times of almost 1.1 hours to settle very light flocs. Longer residence time, the more sucrose are exposed for inversion. The table below shows the percentage inversion occurring for different residence time and also when there is a change in pH. Robust to handle sudden process changes and variations in the juice quality.

Residence time (minutes)
Clarified Juice pH (pH at 20 °C)

6.7
7.0
7.3
180
0.83
0.42
0.21
40
0.19
0.09
0.05
30
0.14
0.07
0.04
15
0.07
0.04
0.02
7
0.03
0.02
0.01

The Rapi-Dorr clarifier is operated on the counter flow principle. This clarifier has four compartments each with its own clear juice and mud outlets that allow the settler to work as four independent clarifiers. It consists in a large cylindrical tank with a central shaft provided with scrapers that rotates at low speed (12 revolutions per hour). The scrapers convey the mud to the bottom of each compartment where it is withdrawn. The mixed juice that enters tangentially at the top of the tank arrives to a flocculation chamber where a part of the mud rises to the surface and a scraper removes it. After this, the incoming juice passes from a central tube into the different compartments, where the juice is clarified, through the annular duct in the center of the vessel. The clear juice is drawn off from each compartment by pipes at the upper part of each section. [3]

ii. Single Tray
Characterized by a short retention time that usually ranges between 20 to 45 minutes. Thus reduce loss of sucrose due to inversion. This design decreases the horizontal distance to be traveled by primary feed layer reducing cross flow. Cost of implementation and maintenance, which is lower than any multi-tray type clarifier that handles the same flow rate. However, the clarifier is less robust to any perturbation of the process compared to the multi-tray settlers. Therefore, the temperature of the juice, the pH and the flocculant addition require a tight control to guarantee good clarification.

The operation of the SRI clarifier consists in conveying the juice from the flash tank into a feed chamber, then, the juice is divided and directed to a feed launder...

References: [1]Seebaluck. V,(2015). Lecture notes; Clarification
[2]Komen.P.J, (1966)Factors in clarification. [online] Available from: http://www.assct.com.au/media/pdfs/20-66.pdf
[Accessed 22 Feb 2015]
[3]Santiago.A (2013) DESIGN AND IMPLEMENTATION OF A VERY SHORT RETENTION TIME FILTRATE CLARIFIER [online] Available from: http://etd.lsu.edu/docs/available/etd-10282013-134806/unrestricted/Grimaldo_thesis.pdf
[Accessed 22 Feb 2015]
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