From the same source.
How is biodiesel made?
Biodiesel is made through a chemical process called transesterification whereby the glycerin is separated from the fat or vegetable oil. The process leaves behind two products -- methyl esters (the chemical name for biodiesel) and glycerin (a valuable byproduct usually sold to be used in soaps and other products).
I stand corrected on the soap issue.
Evidently, that was a bad example. My point though is still valid. Much of what is sometimes thought of as waste, is not really wasted.
I looked up cottonseed and found that it is already used as cattle-feed and to produce food-oil for human consumption, often used to make potato chips, among other things. If that oil were used for fuel, either we would need to replace it with something else for what it is used for now, or do with out those things.
Also the amount of cottonseed oil produced last year works out to about 129 million gallons, which although it is a large amount of oil, it would only amount to less than one half of one percent of our diesel needs, and that is if it was all converted to fuel.