BLEACHED OIL FILTRATION:
The main purposes of the bleaching process is the removal (complete or partlal) of color bodies to a certain lovibond color rating, but ¡n that process a number of unwanted non fat components should be removed as well. The color bodies can be removed by the heat bleaching effect, chemical oxidizing or absorption on natural or acidactivated clays, non-activated clays, silica or activated carbón. The "other" components that need to be removed sometimes require a totally different approach. Heat or chemical treatments are not normally used alone for decolorising edible oils because of the risk of changing the structure in the unsaturated fatty acid or producing colored degradation producís. Sometimes two or more bleachingsteps are used, each one with a special task.
The bleaching process also removes traces of soap, gums, metal, and more recently, chemical compounds such as dioxine, PAH and PCB, which nave been identified as toxic compounds and should be removed to a level that complies with the international agreed upon standards. After the crude oil is cleaned by filtration or separation (decanter centrifuge), itis further processed following the route as shown in diagram 1. A selection can be made from the so called chemical or caustic refining process or the steam or physical refining process. With the increase in environmental restrictions and the water pollution probiem imposed by the caustic refining process, the world trend is towards physical refining or a combination process.
DIAGRAM 1PHYSICAL REFINING
CHEMICAL OR CAUSTIC REFINING
CRUDE OIL STORAGE GUM CONDITIONING
BLEACHING FILTRATION PRE-TREATED OIL STORAGE
CRUDE OIL STORAGE
PRE-TREATED OR NEUTRALIZED AND BLEACHED STORAGE
Liquid Filtration Consultants Lochem BV
Bleaching can be done by batch or continuous process whereas the specialityoils and fats will be processed in batch or semi continuous process. The bulk oils such as Soybean, Palm, Sunflower, Rape, etc will follow the continuous refining route. In both cases the oil is mixed with adsorbents (clay, carbón, etc) and the system is kept under vacuum. After allowing the oil/clay mixture enough residence time to perform the bleaching action, the mixture is filtered to remove theadsorbent and other solid impurities.
For the filtration process and filtration rate the particle size distribution is of great importance. A high proportion of the finest particles will obstruct the clear filtration, decrease the filtration rate and/or interfere with the growth of the filter cake, resulting in a high oil retention in that same filter cake. The use of filter aid (Kieselguhr orPerlite) was often practiced to improve the filtration performance of poor filtering clays, but since the oil losses are always given in a percentage of the cake, there was a common interest among all refiners to avoid the use of filter aid.
The bleaching clay and other adsorbent producers from their side focused on the combination of filterability and bleach ability resulting in prominent brandslike:
• Tonsil.... FF (fast filtration).
• Filtrol .... SF (speed filtration).
• Norit..... HF (high filtration)
Whatever clay, adsorbent or mixture is used, the process is a so called precoat filter application.
In precoat filtration our main objectives are:
1) To provide a septum or precoat layer which is "tight" enough to retain all the suspended solids from the liquid to befiltered.
2) To provide this same septum with máximum porosity so that the máximum quantity of suspended solids can be retained before this septum becomes blocked. When the septum is blocked cleaning becomes necessary, which in turn causes an interruption in flow. Obviously when the suspended solids are smaller in size than the pores of the cake, the solids will pass through. Fortunately...