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There are two main types of bleaching clays used in refining: neutral clay and acid-activated clay.
Neutral clay is good for reducing the red color (carotenoids) from oil but it is not as effective in removing chlorophyll, soap, and trace impurities from the oil.
For lower quality crude oils, especially those containing high levels of chlorophyll, phospholipids, and soap, activated clay may be necessary.
For good results, crude oil must meet minimum standards for clay to be effective, such as less than 3PPM phosphorus, 100PPM soap, 0.1% moisture. These are polar in nature and therefore become attached to the active sites of the bleaching clay. This results in fewer sites left for adsorbing the impurities. Thus, additional amounts of clay are required to remove impurities.
Considering clay adsorbs as much as 35% of its weight or 0.35 lb of oil per pound of clay, losses can be significant. Performance of clay depends on the quality of the crude, clay dosage (0.5% by weight minimum), degree of mixing (120 RPM), contact time (30-45 mins), operating temperature (212-230 F), and operating pressure (50 Torr).
Bleaching is typically performed under vacuum to prevent oxidation which may lead to decomposition of products (aldehydes, ketones, polymers, non-triglycerides, chlorophyll), some of which make the oil ten times more phone to photo-oxidation.
Citric or phosphoric acid (50-300 PPM) is typically a necessary addition to improve the removal of trace metals and to hydrolyze part of the soaps. The use of acid in the presence of heat creates the real risk of isomerization and is a concern.
If you are unsure of the impurities in your crude and their affinity to clay, perhaps clay is not always the best alternative for your process.