Sugar mill supersaturation and crystalisation steps 1

[Gerdler]

Im working on a small project on a sugar mill. Not completely related to my project but something that have sparked my interest and I have not been able to find literature on is the function of the supersaturation prior to the crystallisation step.

After a multiple-effect evaporator of perhaps 5-8 effects the juice is concentrated to about 70-75% dry substance (mostly sugar) and then the supersaturation and crystallisation takes place. From what I understand it water is being boiled of under low pressure in three steps that form 3 different products, A, B and C. A being the white sugar that is most wanted.

Question:
What im wondering is if the supersaturation in steps A, B and C functions as evaporator effects with the steam from the prior step being used in the second step or if new steam is supplied to each of the supersaturation steps?

Maybe there is no standard operating procedure. But Im interested in any experience you have.

 

[rmw]

I'm getting out on a limb here because my specialty is on the steam and heat transfer side of the process, not the sugar making per se, but the A, B, & C strikes are a function of the seeding that is done to initiate the crystallization. Your A strike takes the sugar that wants to crystallize the easiest, and the B & C strikes follow from what is left. Even the molassas that is left has a little sugar left in it but it uneconomical to try to get any more out. Your A strike is the highest purity sugar and some of it is used as seed for the subsequent strikes.

I don't think that at the brix of the evaporator effects that the sugars are ready to crystallize or that would be a problem there. The evaporators just take highly dilluted cane juice and boil off the water until the mixture is concentrated enough to start the sugar making process. So what you have here is concentrated cane juice, nothing more.

That is the reason for the vacuum pan operation, to boil off more water from the juice at low temperatures (low vacuums and low steam pressures) to make the juice into syrup and bring the syrup into supersaturation.

Some sugar will crystallize from the juice naturally, but it is a lot faster and a lot surer process if the seed sugar is added to the supersaturated syrup give the nucleation points for the crystals to begin to from around.

Here is a site that can help a lot with terminology and definitions.

http://www.sugartech.co.za/definitions/index.php

Walk around under the centrifuges and find the molassas stream and (with a clean washed hand of course) reach out and touch a sample to taste. That is what is left when most all the sugar is gone.

rmw