Flint Hills Resources, the refining and chemical subsidiary of Koch Industries, has signed a deal to purchase the propylene maker PetroLogistics for $2.1 billion, including debt. Parties owning 73% of PetroLogistics, including its board and private equity partners, have approved the transaction. Flint Hills expects to complete it by the end of the year. PetroLogistics operates the first in a wave of U.S. propane dehydrogenation plants. It opened in 2010 in Houston on the site of a former ExxonMobil ethylene cracker. The company had sales of $758 million last year. Flint Hills makes polypropylene at plants in Texas and Michigan.