MEXICO CITY, Oct 20 (Reuters) - Mexican conglomerate Alfa (ALFAA.MX) raised its guidance for 2021 EBITDA (earnings before interest, taxes, depreciation, and amortization) on Wednesday, on the back of improved performance by its petrochemical subsidiary Alpek. The Monterrey-based firm, whose business lines include food and telecommunications, revised up its 2021 EBITDA guidance by 10% to $1.937 billion. "Alpek once again delivered better-than-expected EBITDA, boosted by sustained margin strength," Alfa said in an earnings statement. Alfa reported a drop in third quarter net profit to 2.491 billion pesos ($121 million), from 3.766 billion pesos a year earlier. The firm attributed the drop to an unfavorable comparison with the same period in 2020, when it benefited from a $95 million gain from discontinued operations related to the spin-off of its auto parts producer Nemak. Alfa"s quarterly revenue rose 17.7% to 79.552 billion pesos. Shares of Alfa closed down 1.14% at 14.74 pesos each before the earnings report was released. ($1= 20.642 pesos at end-September) Reporting by Noe Torres; editing by Richard Pullin Our Standards: The Thomson Reuters Trust Principles.