Tubewall temperatures

Reformers are fired to maintain a required process gas outlet temperature. Most modern reformers are top fired. In a top-fired reformer, the burners are located at the top of the furnace and fire downward. Process gas flows downward through catalyst-filled tubes. This flow of process gas and flue gas allows the highest flue gas temperature when the in-tube process gas temperature is lowest and the lowest flue gas temperature when the in-tube process gas temperature is highest. This results in tube-wall temperatures that are uniform over the tube s length and since the average tubewall temperature is lower this reduces tube cost and increases tube life. 

Heat flux is defined as heat input per unit of time per square unit of inside tube surface. A low heat flux provides extra catalyst volume and lower tubewall temperatures. This increases the reforming reaction conversion and increases tube life. A high heat flux reverses these effect, but reduces the number of tubes. The flux is highest at the zone of maximum heat release and then drops to a relatively low value at the tube outlet. 

The piping design limits variations in gas flow to the tubes and burners to +2.5% to keep tubewall temperatures uniform. The PSA offgas flow is available to the burners at only about 3 psig. If preheated combustion air is used, the differential air pressure across each burner is typically less than 2 inches of water. The distribution is aided with symmetrical piping. 

Note that the highest tubewall temperature occurs at the outlet, which is quite common for this type of furnace. Also note that at the tube outlet, the tubewall temperature is very close to the required process gas outlet temperature. This is because the heat flux at the outlet is quite low, having fallen steadily for the last two-thirds of the tube. 

Therefore, the tubewall temperature is minimized for the required process gas outlet temperature, which in turn minimizes the required tubewall thickness. 

It is important to obtain good flow distribution for all reformer streams. The piping should be designed sueh that the variation in gas flow to the reformer tubes and to the burners does not exceed plus or minus 2.5 percent. Otherwise, the tubewall temperatures may not be sufficiently uniform. 

The program can be operated when the temperatures of the reaction mixture or of the tubewall are known. Application of the model is illustrated with examples. 

The reforming reaction equilibrium is favored by high temperature. At the pressure levels used in hydrogen plants, the reformer process gas exit temperature typically runs between 1500 and 1700°F. Lower temperatures give insufficient conversion. Higher temperatures increase metallurgical requirements, tubewall thickness, and fuel consumption. 

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