Sweet As
The Options
Air and water discharges from carbon plant. (click to enlarge)
More than 160 tonnes of steel was used to construct the decolourisation plant (click to enlarge)
COP Outcome development and evaluation
Any replacement process had to satisfy the 12 resource consents governing the operations of the plant. Engineers at Chelsea evaluated various alternatives, including a brand new char process which would have been more efficient but didn't deliver other significant benefits. A fluoridised pond system, in which liquid sugar is dosed through settling ponds that are then discharged into the water system, was morally out of the question. “While this would have involved less capital outlay it was totally unacceptable to discharge like this," says New Zealand Sugar's project engineer Gerhard van Niekerk (TIPENZ).
It was decided to go with a granular activated carbon decolourisation process which utilised “adsorption". This chemical process leaves colourants in the sugar after the first stage of refining. In the next stage the colourants, which are polar, attract each other in a similar way to particles in home water filtering systems, so all that ends up at the bottom is the liquid sugar.
Mr Van Niekerk explains that this is a very efficient process, the activated carbon offering over 10 times more surface area than bone char for the same volume. The carbon, which is made from coal, is extremely porous, with an estimated surface area of 1000 square kilometres per cubic metre. The plant uses about 160 tonnes a year, representing a surface area equivalent to that of the South Island. The activated carbon is contained in five large columns through which the amber-coloured sugar liquor is pumped.
The process has the potential to create mountains of discarded carbon – no real challenge to the engineers at Chelsea who installed a natural-gas-fired multi-hearth kiln as part of a carbon regenerating system.
“We put new carbon in a column which runs for four weeks decolourising the sugar but when the colour starts changing we know we have to dump and regenerate it. We drain and flush the unit, retrieve the sugar and pass the carbon through the kiln at a rate of about half a tonne an hour, heated up to 950 degrees Celsius."
The colourants are separated by means of air movement and the carbon is then moved into a quench tank, which cures it to its original state. “We only lose five percent in the process due to mechanical breakage, because carbon is very soft. We pick that five percent up through the cyclone as bottom ash," says Mr Van Niekerk.
