Minimum Export Steam

A minimum export steam plant is defined as a plant that optimizes heat recovery in the plant to the maximum extent possible. This is often done when the value of steam is essentially zero or the price of the feedstock and fuel are exceptionally high. Minimum steam export is often achieved by first increasing the SMR process gas inlet temperature and then the combustion air preheat temperature, both of whieh reduce the fired duty of the SMR. Typical temperatures are 1150°F for the process gas and 900°F for the air preheat. These changes reduce the export steam to a low level, but typically not completely to zero. Additional modifications are required to reduce the export steam to an absolute minimum. 

Adding reaction steps to the process can further reduce the amount of feed and fuel required by the hydrogen plant. The first reaction step that can be added is a prereformer. The prereformer is an adiabatic reactor where the feed is heated to approximately 900°F and the gas is partially reformed over a catalyst bed and hydrocarbon components of ethane and heavier are converted to methane. The conversion of ethane plus components to methane is exothermic. If the feedstock is essentially methane (i.e., natural gas), the amount of heat required to drive the steam methane reforming reaction to equilibrium will produce a temperature decrease across the reactor. However, 

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According to Kellogg 27.9 GJ/t NH3 can be achieved in a natural gas based plant with minimum energy export, but with export of larger quantities of steam this value could probably be brought down to about 27 GJ/t NH3. Figure 107 shows a simplified flowsheet of the process [1069] with Selexol C02 removal unit. 

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