Title of article: UPDATE 1-Exxon has alky upset at Joliet refinery

TORONTO, Aug 6 (Reuters) - Exxon Mobil (XOM.N) said it had an upset at an alkylation unit at its 238,600 barrel per day refinery in Joliet, Illinois, on Thursday which resulted in two people being taken to the hospital.

Exxon Mobil said the cause was a propane leak at about 12:30 p.m. CDT (1730 GMT) that resulted in a release of hydrofluoric acid.

Impact on the refinery's operations was not immediately available.

Hydrofluoric acid is used in alkylation units, which make high-octane blending components for gasoline. It can cause severe burns and cause significant damage to heart, lungs and bones in humans.

An explosion in an alkylation unit at Citgo Petroleum Corp's Corpus Christi, Texas, refinery on July 19, triggered a release of hydrofluoric acid vapor within the refinery. One worker was severely injured and remains hospitalized. (Reporting by Janet McGurty; Additional reporting by Erwin Seba in Houston; Editing by Marguerita Choy)