chemical plant

I'd say you need to delve into some of the catalogs of the various retailers out there such as Walthers who have just such items in kit form.

walthers.com

Both Plastruct and Evergreen have stuff that will be handy.  I think the Plastruct catalog even shows either an oil refinery or chemical plant as an example of the use of their stuff.  In addition to the tubes and rods and stuff, one of them has valves and such.  Also one has ladders, beams, etc.  Go to a big hobby shop that has a good stock of both and look over what they have, or find their web sites.  Knowing what is available will help planning.

Also visit a drug store or department store and look at all the bottles for  shampoo, cleaning products, etc.  Some of those should work for tanks and reactors.

Also check Walthers for what is already available in refinery and chemical plant stuff.  That stuff could be added to a larger scratchbuilt plant.

I suggest you lay paw to the excellent reference manual Styrene Modeling by Evergreen Scale Modeling. There's a peek in PDF: http://www.evergreenscalemodels.com/Book.htm

And one tip I cannot stress too strongly: THINK OUTSIDE YOUR SCALE. Tea bags can become filter mesh or chain-link fence depending upon the scale it which it's used. Wallboard, scored, snapped, and glued edge-on, makes excellent rubble. One man's electronic trash is another's prize gizmo. Remember: anything is fair game.