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Wet Bleaching Process:
As wet bleaching implies, the oil in contact with the bleaching agent contains some water during the reaction time. The amount is maxium 0.5%, but even this amount of water would create filteration problem if it is left in the mixture. For this reason a wet bleaching plant has an extra drying vessel between the bleaching reactor operates at a medium vaccum level.
To maintain the correct water content, the bleaching reactor operates at a vaccum level.
So why we use wet bleaching ? The simple answer is economics. If wet bleaching utilises the earth more sfficiently, then you use less earth, and consequently will have lower earth costs, and lower losses of oil retained in the spent earth.
One way of obtaining the right amount of water in the oil is to by-pass the drying stage in euther a neutralising, or demugging plant, and feed directly to the bleaching reactor.
Another technique is to intensively mix a small amount of a dilute acid solution into the feed to bleaching. This not only sets the water content, but it also removes soaps which can lead to a further reduction in the amount of bleaching agent needed.
The filters are arranged so that the oil can be recycled. Either through the reactor, or a via a pre-coat tank, or fed to the other filter when cleaning is required. Oil in the filter cake is reduced by steam blowing. The oil in the steam is separated in a cyclone and returned to a slop oil tank.
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