Role of Flocculants in Sugar Industry | Flocculant Dosing Online Calculator
Introduction of flocculants:
Flocculants are a reagent added to a dispersion of solids in a liquid to bring together the fine particles to from floc.
The flocculants are generally classified as natural flocculants and Synthetic flocculants.
They are water soluble anionic, cationic, non ionic polymers . Non ionic polymers adsorb on the suspended particles The most common natural flocculants are
a) Starch derivatives: Mostly pregelatinized hence water soluble . They are corn or potato, starch are natural starches, anionic oxides starches or amine treated cationic starches . They used of this classes of product has decreased in water treatment but remain important in the paper industries .
b) Polysaccharides: Usually guar ,gums & mostly used in acidic medium .
c) Alginate: Anionic & used in potable water treatment
Polyacrylamides: The most common polymers are those based on Polyacrylamide. Which is a a nonionic polymer. Their effect is due to bridging between particles polymer chains. Polymer can be given anionic character by co polymerizing acrylmide with acryl amide acid,. Cationic polymer are prepared by co polymerizing acryl amide with a cationic monomer. All available acryl amide based polymers have a specific amount of ionic monomer giving a certain degree of ionic characters.
Action of flocculants:
a) Dispersion of polymer in solution phase.
b) Transport of polymer to solid phase.
c) Adsorption of polymer on solid surface.
d) Collision of particles being adsorbed polymer to permit formation.
Advantages of flocculants in clarification:
a) Rate of settling is fast.
b) Batter compactness in mud.
c) Improved clarity of juice.
d) Reduction in color formation and mud volume.
e) Due to thickening of mud lesser sugar loss in filter cakes.
f) Efficiency of vacuum filter improve.
g) Clarifier capacity increased by 10-20 %.
h) 0.3 to 0.5 rise in purity achieved from mixed juice to clear juice.
i) Reduction in filter cake pol by 0.3 to 0.5 units.
Flocculents in sugar industry:
a) A number of synthetic flocculating agents are using in the sugar industry clarification process. These flocculants are synthetic high molecular weight polyacrylamides which are partially hydrolysed. Their efficiency as settling aids depends on the molecular weight which ranges between 7 to 10 million and the degree of hydrolysis.
b) These compounds serve as bridges among particles of precipitate and thus bring about formation of bigger aggregates of flocs.
c) The most common flocculants used for juice clarification as well as filter feed conditioning are hyolyzed polyacrylamide compounds, Which are anionic types. Cationic type of flocculents have much more limited use, mainly as decolorants in syrup flotation processes.
d) They are added as 0.05 % concentrated solution in water at the rate of 1.5 to 5 ppm on juice,
e) The dosing to be connectec to the treated juice in the pipe connecting flash tank to clarifier.
f) In the trayless clarifier has reduced the time of juice retention to 45 to 60 minutes. For improving settling rate and increasing the clarifier through put the addition of synthetic flocculant is essential.
g) Thus the introduction of synthetic polyacrylamide as settling aids has been a significant advance in the process of clarification.
|Molecular weight increase||Poor solubility, more viscous solutions, chain more shear sensitive, Higher unit cost, Higher optimum dose, bridging favored, large flocs and faster settling but higher sediment volume|
|Temperature Change||Complex effects|
|Dose increase||Better flocculation up to optimum|
|Shear increase||Breakdown of long chain, irreversible|
|Particle surface area increase||Increase flocculants consumption, ultra fines, susceptible to over dosing and restabilisation.|
|Suspension pH||Non ionic little affected, ionization and extension of anionic at alkaline pH- converse for catatonics, also alters particle charge, combined effects on flocculation are complex.|
|Suspension ionic strength||Similar to suspension pH- can promote or inhibit flocculation, Excess salt lowers solubility, coils polymer|
|Polymer charge density increase||Extends polymer solution under suitable conditions, decrease adsorption on to particle of same sign.|
Clarification in sugar process:
a) The clarification step in sugar manufacture is designed to remove insoluble suspended impurities from the mixed juice .
b) The formation of cane juice floc is based on Ca3(PO)4 and protein. Its effective density is varied by surface charge effects (Zeta potential).
c) Flocculation has been described by Crees et al. as a two-stage reaction, the first step being the scavenging of individual particles into micro flocs, and the second being the agglomeration of these into macro-flocs that settle relatively rapidly.
d) The nature of colloid particles must be collected into agglomerates. The intended effect of juice clarification by sedimentation is to achieve 1. higher clarity of juice 2. compact volume of mud.
e) The higher clarity of juice means fewer suspended solids. Flocculants improve juice clarity by decreasing suspended solids. The compactness of mud can reduce the retention time of juice inside the clarifier thus reducing the color formation and also sucrose degradation.
Flocculant preparation and dosing:
a) The flocculant tanks in either stainless steel or suitable MS tank with epoxy painting. This eliminates the potential contact between flocculant molecules and iron, which can quickly result in degraded flocculant.
b) The preparation tank should be located directly above the stock holding tank. The preparation tank having stirrer and its recommended speed of 10-15 rev/min. Otherwise air sparger is one of the best option for preparation of flocculent.
c) Always ensure that in-line valves are fully open when dropping the solution from preparation tank to holding tank. Because molecules will be sheared if the solution is passed through partially open valves.
d) It having two tanks are sized to provide 4 to 8 hours supply of 0.05% to 0.2% concentration. The flocculant demand equivalent to 1.5 to 4 ppm on juice. The concentarion and dosing ppm may be veariy according to type of clarifier and dosing process. The final dosing solution concentarion to be maintain 0.05%.
e) A minimum of four hours of preparation should be allowed before the flocculant solution is dropped to the holding tank. A mixing time of two hours should be considered as the absolute minimum mixing time and should only be adopted as an emergency procedure.
f) The water supply must be of good quality with a pH greater than 8, of low hardness and be less than 50ºC. It is recommended that cooled condensate with 8 pH.
g) It is essential that all the flocculant powder is wetted before reaching the water in the mixing tank to prevent the formation of large lumps or ‘oysters’ of flocculant, which are difficult to dissolve fully, and can block pipes and screens.
h) Suitable disperser units consist of a funnel hopper and a variable aperture arrangement designed to ensure wetted individual particles of flocculant powder.
i) Flocculant solution will become ineffective after 18 hours, so fresh solution must be prepared after a long process stoppage.
j) The most suitable pump for flocculant is a progressing cavity pump operating at slow speeds with a variable frequency drive.
k) In some designers flocculant preared by 0.2% concentration and the dilute to 0.03 to 0.05% solution by inline mixer with condensate water or clear juice while solution passing from flocculent pump to flash tank. It will helpful to increase the useful life of the flocculant and reduce the water dilution.
Flocculant preparation and holding tank volume and dosing pump calculation.
In this we take one example and find the above parameters.
|Flocculant Dosing for clarifier|
|3||Concentration of flocculant||C||0.05||%||0.05 to 0.2 %|
|4||Retention time||T||4||hrs.||4 to 8 hrs.|
|1||Flocculant Quantity||Q = TCH x R / 1000||0.42||Kg/hr|
|2||Flocculent flow rate||Qr = Q x 100/ C||840||Lt/ hr|
|3||Pump capacity required||Qr x 1.1||924||Lt/ hr||10% extra|
|4||Preparation tank capacity||(Qr x T/1000) x 1.1||3.696||M3||10% extra|
|5||Holding or stock tank capacity||(Qr x T/1000) x 1.2||4.032||M3||20% extra|
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