In a medium saucepan, combine the rhubarb with the port, vinegar, sugar and orange zest and bring to a boil. Remove the pan from the heat and let steep for 30 minutes. Cover and simmer over moderately low heat, stirring often, until the rhubarb is just tender, about 5 minutes. Discard the orange zest. Transfer to a blender and puree. Season with salt and cayenne.