The parent company of SuperAmerica and the St. Paul Park refinery has a new chief executive.

Northern Tier Energy said Friday that co-founder and CEO Mario Rodriguez has resigned, effective immediately, replaced by co-founder Hank Kuchta, who had been president and chief operating officer.

In a statement, Rodriguez said that after completing the company's initial public offering followed by record financial results in two consecutive quarters, "the time was right for me to step down and to explore new opportunities."

Kuchta came to Northern Tier after serving as president and chief operating officer of Premcor, which operated four U.S. refineries, the company said in a news release. He was at Premcor during its IPO and 2005 sale to Valero. Earlier, he was business development manager for Phillips 66 Co.

Northern Tier Energy, based in Ridgefield, Conn., has 166 company-operated and 67 franchised SuperAmerica stores, which it acquired along with the Minnesota refinery from Marathon Oil Corp. in 2010.

The company is a master limited partnership, whose units first went on sale in July at a below-expected $14 per unit. They have since risen dramatically, after the two strong quarters, and closed at $27.11 per unit on Thursday.

As part of its CEO announcement Friday, the company reaffirmed its fourth-quarter guidance to investors.

David Shaffer • 612-673-7090