An insulation material having a thermal conductivity of 2.01E-4 W/mC is used to insulate a steel tank containing liquid nitrogen which is to be maintained at -196C. At this temperature 198.6 kJ is required to vaporize each kilogram of nitrogen. Assume the tank is a sphere having an OD of 0.61 m, and that the outer temperature of the insulation is 20C. Likewise assume the conduction resistance of the tank and the internal convection thermal resistance is negligible. It is desired to limit the amount of nitrogen vaporized to less than 0.45 kg per day. Determine the required thickness (cm) of the insulation layer.