Start-up heater

After the hydrogen is purified, it is ready for use in a refinery hydrogenation process. In an ammonia plant, the hydrogen-nitrogen mix is sent to an ammonia converter (Figure 4.7), which requires a start-up heater. Since the material in the heater will be exposed to hydrogen only for a short period, the time dependent curves in API 941 should be consulted when selecting an alloy for the heater tubes. 

E - Gas from externa start up heater F — Quench gas inlets G - Pyrometer... 

Fig. 3.7. Methanol synthesis loop, (a) Recycle compressor (f>) heat exchanger, (c) reactor, (d) cooler/condenser, (e) methanol separator, (/) start-up heater...

During start-up, hot gas from the start-up heater is introduced through the cold by-pass pipe at the top of the converter. 

Circular Exhibit 7-3 illustrates a circular furnace that houses tubes mounted vertically or helically in the radiant seaion. These furnaces are generally used for such smaller duties as start-up heaters or reboilers. Inlet and oudei connections are usually located at the top or the bottom of the radiant section, depending on... 

A commercially available ammonia synthesis catalyst is usually supplied with the iron phase in the form of magnetite, which first must be reduced to metallic iron before the catalyst is used. The reduction time is typically from three to five days, although the actual time required is dependent on the plant design and on limitations of equipment, such as the start-up heater. The general principles of reduction are outlined below. More detailed information to suit a specific plant can be obtained from catalyst suppliers. The principal factors governing a plant reduction are the water content of the circulating gas, the gas flowrate, the reduction pressure, and the reduction temperature. 

During the early stages of catalyst reduction, the maximum rate of gas flow is limited by the thermal capacity of the start-up heater to maintain the temperature of the bed. As more catalyst becomes reduced, the exothermic synthesis reaction begins. The heat from this exotherm supplements the output from the start-up heater, and the gas rate can be increased. The rate of increase of a bed inlet temperature should be limited to about 5°C per hour until a maximum outlet temperature of 500°C is reached. During this procedure, the temperature of the remaining unreduced catalyst beds should be just below the reduction temperature that is about 350°C for typical catalysts. As more beds are reduced, gas rates can be increased and the reduction rate is faster despite the larger catalyst volumes in the lower beds. 

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