The heated tube length in the reformer is 10 m and the

QUESTION:

The heated tube length in the reformer is 10 m and the external diameter of the tubes is 10.5 em. If the rate of heat transfer (Q) from the combustion gases in the firebox to the reformer gases were accomplished entirely by convection,? the following equation would apply: Q = UoA" tlTlm where Uo is an overall heat transfer coefficient based on the external surface area of the reformer tubes in the firebox, Ao is the total external surface area of the tubes, and tlTim is an average difference between temperatures of the heat source (combustion gases) and the heat sink (reformer reaction gases): where tlT1 and tlTz are differences in temperature between the reformer gas and combustion gas at the inlet and at the outlet of the firebox. If the combustion gases are assumed to have a constant 7Convection is a heat transfer mechanism involving bulk movement of fluid at an elevated temperature to a region of lower temperature. 598 Chapter 13 Steam Reforming of Natural Gas and Subsequent Synthesis of Methanol temperature in the firebox of 960C (i.e., they are perfectly mixed), and Vo = 50 BtuWPft2h),8 what is the required number of tubes in the firebox? In fact, a large fraction of the heat transferred to the tubes is accomplished by a mechanism other than convection. What is that mechanism? (Hint: Think of your experience in being close to a flame associated with combustion of natural gas.) What will consideration of this additional mechanism mean in terms of the number of tubes required in the firebox?