The chief operating officer of Philadelphia Energy Solutions Inc. resigned quietly in April, only 13 months after he was brought in to run the sprawling former Sunoco refinery in South Philadelphia, a filing with the U.S. Securities and Exchange Commission reveals.

The company did not disclose in the filing this month the reason for David M. Ritter's termination as executive vice president. Ritter, in a telephone interview Wednesday, said he resigned for "personal reasons." Chief executive officer Philip L. Rinaldi said Ritter departed on "friendly terms."

According to the SEC filing, Ritter was paid a base salary of $525,000 and received severance of $1.3 million.

The turnover in the upper echelon comes at a delicate time for Philadelphia Energy Solutions. The company, a joint venture of the Carlyle Group and Sunoco Inc., filed in February for an initial public offering, and its operations are currently under SEC review.

In March 2014, PES announced Ritter's appointment to the newly created position. He also had a seat on the company's board.

Ritter, 59, had worked previously as an executive for energy giants Royal Dutch Shell and Mobil Oil Corp. He is also president of Haymarket Group L.L.C., a boutique energy-industry consulting firm in Spring, Texas.

"The breadth and depth of David's experience will be an incredible asset to PES," Rinaldi said in a news release when Ritter's appointment was announced.

Ritter had responsibility for running the 335,000-barrel-a-day Philadelphia complex, which is operated as two separate refineries, the Girard Point and Point Breeze facilities.

A Pennsylvania native who holds a bachelor's degree in civil engineering and an M.B.A. from Lehigh University, Ritter maintained residences in the Woodlands, Texas; and Blue Bell. His 13-month employment was costly, according to an amended registration statement the company filed on July 1 with the SEC. In addition to his salary and severance, Ritter received a $125,000 bonus when he started work, an $85,000 relocation reimbursement, and a $350,000 year-end incentive bonus.

Rinaldi said Wednesday that the top tier of his management team had been reconfigured among five executive vice presidents, including a new chief operations officer, Gregory G. Gatta.

Philadelphia Energy Solutions was formed in 2012 as a joint venture between Carlyle and Sunoco, which had threatened to shut down the 2.2-square-mile refinery complex, the largest petroleum-processing facility in the eastern United States.

Under new management, the refinery has become profitable partly because it shifted its source of crude oil from pricey imports to more economical domestic supplies delivered by rail. PES earned $143.6 million in net income last year on $13.3 billion in sales, according to its SEC filing.

The company's initial public offering is complicated because it is occurring at the same time that PES is spinning off its logistics operation into a separate publicly traded entity, PES Logistics Partners L.P. Rinaldi said it was a rare case when one company is undergoing two simultaneous IPOs.

The logistics firm's chief asset is the oil-train unloading terminal that operates under a long-term agreement with the refinery.