July 11 will mark 7-Eleven’s 90th birthday — and the convenience store chain is ready to celebrate. Participating 7-Eleven stores will give away free small Slurpees, fittingly from 11 a.m. to 7 p.m. today. To further celebrate, as part of Slurpee Week, customers who purchase seven Slurpees between July 7 and July 18 and scan the 7-Eleven mobile app will receive 11 free Slurpees. Participants must join 7-Eleven’s customer loyalty program, 7Rewards to be eligible for the offer. The 7‑Eleven mobile app is available on the App Store and Google Play. Free Slurpee Day has been offered annually on July 11 since 2002. The frozen, carbonated beverage that 7-Eleven dubbed the Slurpee was invented in 1959 by Omar Knedlik, a Kansas City Dairy Queen owner. When his soda fountain machine broke down, he improvised by putting his sodas in a cooler to stay chilled. These frozen, slushy drinks got the attention of customers…