Firm that acquired Sunoco bids for pipeline operator Williams Cos.

The acquisitive Dallas company that gobbled up Sunoco in 2012 is making a bid to become an even bigger energy behemoth.

Energy Transfer Equity L.P. announced Monday that it had proposed an all-stock merger with Tulsa-based Williams Cos. Inc. that it valued at $53.1 billion, combining two of the country's largest pipeline companies.

Williams rejected the offer, which priced its stock at a 32 percent premium, as being inadequate. But the company's stock soared Monday, closing almost 26 percent higher, and Williams announced that it had launched a review of its strategic alternatives, including a merger.

Williams Cos. controls Williams Partners, which owns and operates more than 33,000 miles of pipeline networks, including the Transco system, one of two major interstate pipelines that serves the Philadelphia region.

The merger would transform Energy Transfer, which has expanded rapidly under the leadership of chairman Kelcy L. Warren, 59, a colorful Texas billionaire whose appreciation of Jackson Browne's music inspired him to finance a tribute album last year.

Energy Transfer Equity, which maintains an intertwined family of energy companies, acquired Sunoco Inc. in 2012 for $5.3 billion through its subsidiary, Energy Transfer Partners.

The acquisition included Sunoco's network of retail fuel outlets but also control of Sunoco Logistics Partners L.P., the Philadelphia pipeline company that is redeveloping the former Marcus Hook refinery into a complex for processing and shipping natural-gas liquids derived from Marcellus Shale natural gas.

Through Sunoco, Energy Transfer also owns a one-third non-operating interest in the Philadelphia Energy Solutions refinery in South Philadelphia. Carlyle Group is the operator and majority owner of the refinery.

Energy Transfer also bought Susser Holdings, which operates the Stripes gas station chain in Texas. The extended family of retail fuel outlets that Energy Transfer has assembled is being consolidated under a new company, Sunoco L.P., which analysts expect will eventually be divested.

Another Equity Transfer subsidiary, Regency Energy Partners, last year bought PVR Partners L.P. of Radnor for $5.6 billion, absorbing the former pipeline operations of Penn Virginia Resources into Regency's operations.

Energy Transfer Equity said it initially made its offer to Williams in a May 19 letter to chief executive Alan Armstrong after Williams Cos. proposed a merger with Williams Partners, the pipeline subsidiary for which it is general partner and owns 60 percent of units.

On Sunday, Williams Cos. announced it had rejected an offer from an unidentified suitor, causing Energy Transfer to go public.

"ETE is disappointed that, despite the best of intentions and its efforts to reach a friendly, negotiated combination, it is forced into a position to publicly confirm its offer for Williams," the company said in a statement.

Fitch, the debt-rating agency, said the proposed merger would not immediately affect the companies' bond ratings. But it said the merger would have "strategic positives" because in the energy infrastructure industry, "generally, bigger is better."