Melt the butter in a large pot over medium heat. Cook the celery and onion until tender, about 3 to 5 minutes. Stir in the remaining ingredients and bring to a boil. Reduce heat, cover and simmer for 70 to 80 minutes or until the peas are tender. Puree the soup until smooth, using an immersion blender. Or, wait for the soup to cool and puree in batches in a regular blender.