We found that if we remove the extraction line and the desorption line, it will turn into a much more simple system.
Do I need the second "the" in this sentence?
There are different ways of phrasing it with slightly different meanings:
the extraction line and the desorption line
the extraction and desorption line
the extraction and desorption lines
I'll take the red jacket and the blue jacket
I'll take the red and blue jacket
I'll take the red and blue jackets
This could mean any of the following, depending on the context:
You don't need it, but it will be clearer if you include it.
If you omit it, there is a possible interpretation that "extraction line and desorption line" is interpreted as one single item rather than two. This is probably not very likely, because of the repetition of "line", and it may be that readers' knowledge of the subject would make it even less likely; but you can avoid any risk of misunderstanding by including the second "the".