MEXICO CITY, Oct 20 (Reuters) - Mexican conglomerate Alfa reported on Thursday that it had almost doubled its third-quarter net profit from a year earlier, boosted by a strong performance from its petrochemical subsidy Alpek.

The Monterrey-based company, whose business lines also include food and telecommunications, said quarterly net profit leaped 95% from a year earlier to reach 4.86 billion pesos ($241.47 million), Revenue rose 28%.

Earnings before interest, tax, depreciation and amortization (EBITDA), or core earnings, edged up 4% to 9.21 billion pesos.

Chief Executive Alvaro Fernandez said Alfa benefited from "higher-than-expected industry margins" in Alpek's polyester and expanded polystyrene unit, as well as its newly purchased PET sheet and resins business.

The company said the subsidiary, which makes plastics, chemicals and polyesters, had taken a 2.37-billion-peso hit from an inventory adjustment and lower prices for petrochemicals.

Alpek, which reported its results in a separate release earlier on Thursday, said overall volume had reached an all-time high thanks to new business and solid demand, while core earnings were boosted by new business in the Middle East.

It said polyester demand had been strong throughout the quarter with high margins in Asia, though demand for polypropylene, used in plastic packaging, parts and textiles, dipped toward the end of the period.

Food subsidiary Sigma saw revenue rise 10% from a year earlier as it saw growth across all regions, though its core earnings fell 13%, impacted by the depreciation of the euro and high energy prices in Europe, Alfa said.

Alfa, which is spinning off its stake in telecommunications unit Axtel, said it was expecting the new unit, to be called "Controladora Axtel," to go public by the end of 2022.

Alfa executives have said they wanted to split the company due to a "conglomerate discount" undervaluing its operations within the wider group. It counted Axtel as a discontinued operation in its third-quarter results. ($1 = 20.1271 pesos at end-September) (Reporting by Kylie Madry and Aida Pelaez-Fernandez; Writing by Valentine Hilaire, Editing by Sarah Morland and David Gregorio)