Grease and flour 10-inch tube pan. Mix together the flour and baking powder. Cream together butter, shortening and sugar until light and fluffy. Beat in the eggs one at a time. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Stir in key lime juice and vanilla extract. Pour batter into prepared pan. Bake in a preheated oven for 90 minutes or until a toothpick inserted into the center of the cake comes out clean. Allow to cool in pan 10 minutes. Turn it out onto wire rack. While warm, prick top of cake with toothpick. Pour Key Lime Glaze over warm cake. Cool completely. Key Lime GlazeIn small saucepan, combine sugar, butter and key lime juice. Bring to boil. Continue to boil, stirring constantly, for 1 minute. Remove from heat. Key Lime JuiceFresh key limes are great when they are available but frequently they are difficult to find. Florida Key West is a producer of bottled Key Lime Juice available retail and wholesale.