When something is left over from a chemical process or reaction, my understanding is that that's called a residue, not a residual.

"Residual" to me has a financial meaning, but, googling it, I find a scientific/mathematical meaning also -- but not for this kind of chemical context.

"Residue may be the material remaining after a process of preparation, separation, or purification, such as distillation, evaporation, or filtration. It may also denote the undesired by-products of a chemical reaction."

https://en.wikipedia.org/wiki/Residue_(chemistry)

https://en.wikipedia.org/wiki/Residual

Unless really describing a result of a chemical reaction in the context of chemistry, I wouldn't choose to use the word residue, and the sterilization of products is not IMHO chemistry. And in a non-chemical context, a residue would usually be understood to apply to some solid matter, and not to a gas.

Residual is usually an adjective, and although it also has some uses as a noun, none of them are applicable here.

I would translate your sentence as "After sterilizing products with ethylene oxide, some of this gas can remain in the packaging." or possibly "After sterilizing products with ethylene oxide, some residual gas can remain in the packaging."

amw's points in #2 are well taken.

Thanks for your answers, seems to be more complicated than I thought...

Even the text at the link in #4 predominantly uses "residue." The uses of "residual" there sound a little odd to me, but I am not trained in the relevant tech/science.

amw's suggestion of rewording so as to avoid either term seems like a workable idea in this case. If it were, or is, a case of solid matter being left over, I'd certainly prefer "residue" for that.