Acid oils are fractions of Crude vegetable oils, those are rejected during Refining of Vegetable oil, like soya, sunflower, rapeseed etc. to produce Refined Oil. Crude vegetable oil is not stable and becomes rancid in few days time. It has a very low shelf life. Refined oil has a long shelf life, hence it is in great demand. Animal fats are also not stable and become rancid in few days time. Recycled Restaurant Oil also becomes rancid due to frying. All these have upto 10% Free Fatty Acids, which make these non-edible. Acid Oils having 10% Free Fatty Acids, cannot be used directly for manufacture of BioDiesel, unless its Free Fatty Acid content is reduced below 3%.
This is achieved by Glycerolysis of Acid Oils. Glycerine obtained from BioDiesel manufacturing plant as by-product, is mixed with Acid Oil and heated with acid catalyst. After few hours Free Fatty Acids react with Glycerine and form mono-glycerides and di-glycerides (Oil is tri-glyceride). Once Free Fatty Acid content of Acid Oil is reduced, it can be used or sold to BioDiesel Manufacturers. There is acute shortage of Acid Oil of this grade and BioDiesel Manufacturers pay good price for it. Unprocessed Acid Oil has very low price and can be used only for manufacture of Washing Soap.
PFAD: Palm fatty acid distillate (PFAD) is a by-product from palm oil refinery process, comprising mostly fatty acids (70% as palmitic acid) and also valuable minor components such as vitamin E, phytosterol and squalene.
Biodiesel is methyl or ethyl ester of fatty acid made from Soya Acid Oil, Cotton Seed Acid Oil or Rice Bran Acid Oil or Animal fats (Tallow). Since edible oils are in short supply in India, the main raw materials for manufacture of biodiesel, are Soya Acid Oil, Cotton Seed Acid Oil, Rice Bran Acid Oil, Palm Stearine, Used Cooking Oil and Animal Fat.

Physical Characteristics of Processed Acid Oils:
Specific gravity: 0.87 to 0.89
Viscosity @ 40°C: 30 to 50
Sulfur, wt%: 0.0 to 0.0024
Cloud point °C: -11
Pour point °C: -15
Iodine number: 60 to 135
Acid Oil is sold in 35 kg carboys, 225 kg MS Barrels and Bulk in Tankers.
Description: Processed Acid Oil is a waste product of Oil Refining Industry, slightly yellow in color, oily liquid with a slight aromatic odor and a bitter taste.
Applications: It is commonly used for manufacture of BioDiesel and in preparation of cattle feed.
Standards and Specifications: Currently there is no standard and it should meet buyer’s requirements.
Transport information: Non-Hazardous for air, sea and road freight.
Personal protection: Safety glasses, adequate ventilation.
Consumer Safety: For Acid Oil all safety precautions are same as that of Vegetable Oil.

Acid Oil Glycerolysis does not require economy of scale. There is no minimum size for a facility. Small decentralised facilities do not require dedicated technical staff support, they can be operated by locally trained non-technical staff.
Small plants are more energy and capital efficient than their bigger centralised counterparts. A 180 tons / year facility will use 500 watts per litre of processed acid oil produced, whereas a 20,000 tons /year plant will require 400 watts per litre or more. The rising demand for processed acid oil is so far been met by large centralised BioDiesel plants, usually 20,000 tons / year, or more; but as biodiesel becomes better known, small decentralised of processed acid oil units will proliferate.
Within the next ten years the aggregate production of small decentralised processed acid oil plants will be greater than the aggregate production of the larger centralised units. The number of small decentralised plants will grow not only because of cost and environmental considerations, but also because such units can be manufactured locally without a high-tech input, and can make use of a greater variety of locally available feed stocks.
Small scale processing of acid oil generates a win-win situation, in which low tech inputs are transformed into a superior, environmentally friendly raw material, without the need for large capital expenditure, or expensive technology transfers. As a bonus, de-centralisation of raw material supply will reduce distribution costs, stop gross local product transfer, and what is more, empower people, making them self-sufficient. The know-how and the hardware are available. All we need now is a bit of word spreading.
The price of Processed Acid Oil will be essentially same as that of the Palm Stearin, Animal Tallow or Used Cooking Oil currently used for manufacture of BioDiesel. The sale of the glycerine covers not only the cost of the alcohol and the catalyst, but the energy input, and labour, as well. Apart from Acid oil, there are a number of oils which are not stable and hence non-edible.