Houston-based Westlake Chemical Corp. is increasing the capacity of the new chlor-alkali plant to be built at the company’s vinyls manufacturing complex in Geismar.

The chlor-alkali facility’s capacity will now be 350,000 electrochemical units a year, up from 250,000. Chlor-alkali is made by passing an electric current through a solution of water and salt. The process results in an electrochemical unit consisting of 1 ton of chlorine, 1.1 tons of caustic soda and 0.03 tons of hydrogen, according to Euro-Chlor, a chlor-alkali industry group.

Construction costs are now estimated at $370 million to $420 million, up from the $250 million to $300 million projected for the smaller plant. Site work is underway and the plant is targeted for start-up in the second half of 2013. The plant will create approximately 100 permanent positions and 400 to 500 construction jobs.