Bring a pot of salted water to a boil. Trim the root off of the bunch of celery, separate the stalks, and remove the leaves. Wash the stalks under cold running water to remove any dirt. Pat dry.
Cut the stalks into slices that are about ¼-inch thick. Once the pot of water is boiling, add the celery and cook for 3 minutes.
Remove the pot from heat, and use a slotted spoon to remove the celery stalks (pick up as much as you can with each spoonful) and transfer them to a large bowl of ice water. If the ice begins to melt, add more cubes. Let them sit for a few minutes so that the cooking stops. Drain the celery and pat dry.
Arrange the celery in a single layer on a baking sheet lined with parchment paper. Transfer the baking sheet to the freezer, placing it in an area where it can lay flat and won't be disturbed. Let the celery freeze for one to two hours until frozen solid. This step prevents the celery from sticking together and forming clumps.
Transfer the frozen celery pieces to a freezer-safe bag. Seal the bag tightly, and label with the date. Store frozen celery for up to 12 months in the freezer. Use frozen celery straight from the freezer in soups, stews, casseroles, and blended cold preparations like smoothies or juices.
I recommend blanching celery before freezing for the best quality, taste, and texture. But you can technically skip the blanching step. If you do, frozen raw celery will only last about 2 months.