How acrylic acid is made

The stuff that lets a diaper swallow a flood begins as a sharp-smelling liquid made from propylene and air.

stations: 6
things upstream: 7
raw sources: 4
steps from the ground: 4

The short version

A two-stage oxidation of propylene gives this acrid, colorless liquid. Propylene, air and steam first form acrolein over a catalyst, and a second reactor takes that on to the acid. The hot gas is cooled and the product captured in water or a solvent. Extraction and distillation then finish it for diaper polymers, paints and adhesives.

Mix propylene, air and steam

Steam does two jobs in this feed. The second-stage catalysis needs it, and it dilutes the gas away from an explosive mixture. Some designs recycle waste gas to replace part of it, because steam costs money and too much makes the product harder to recover.

Goes in
Propylene Air
Oxidize propylene to acrolein

The gas passes over a catalyst in the first reactor. Most of the propylene stops at acrolein, an acrid-smelling intermediate, with a little of the final product as a by-product. Hot spots in the catalyst bed are the danger here, so heat has to be kept under control.
Send acrolein onward

Outlet gas from the first stage goes straight into a second reactor with nothing separated out. A different catalyst carries the acrolein the rest of the way. Some designs add fresh oxygen or steam at this point.
Cool and capture the vapor

The product leaves as a gas. Cooling condenses it into a water solution, or a suitable solvent absorbs it instead.
Recycle the leftover gas

What remains is mostly nitrogen, carbon dioxide and carbon monoxide, with some unreacted propylene and acrolein. Part of it can go back to the reactors as a diluent.
Extract and distill

Purification uses extraction, distillation or similar steps to isolate the product from its water solution. Then polymer makers take over. Diaper plants blend it with sodium hydroxide and polymerize it into sodium polyacrylate.