Process first 4 ingredients in a food processor until completely smooth, about 2 minutes. Pour mixture into a 13- x 9-inch dish. Freeze, uncovered, until just set but not completely frozen, 2 1/2 to 3 hours.

Rake through granita using a fork, breaking up any large chunks. Return mixture to freezer; freeze until completely set and frozen, 2 to 3 hours. Rake through granita until it is completely broken up and no large chunks remain.

Transfer granita to an airtight container. Freeze up to 2 weeks. Sprinkle servings with lime zest.