Theory

Steam reforming can be described by the irreversible reaction(s)

CnHm + nH2O => nCO + (n + m2 ) H2

combined with the reversible reactions

CH4 + H2O = CO + 3H2

CO + H2O = CO2 + H2

The total of these reactions is strongly endothermic and in the presence of a nickel catalyst, the gas mixture will be close to equilibrium at the exit of the furnace. The molar steam to carbon ratio, temperature and the pressure largely determine the final composition of the gas leaving the reformer.
It is evident that higher temperatures result in less methane and more carbon monoxide in the equilibrated gas. A large surplus of steam favours both low methane and low carbon monoxide, whereas high pressure increases the methane content.



Process Description

The methane feed is expected to be containing certain pollutants and, in this regard, a proper methane stream pretreatment is performed.
The purified methane stream is mixed with water vapor at fixed hydrocarbon-steam ratio, under dedicated control.
The methane-steam mixture is heated to reach the optimum SMR reactor inlet temperature. This temperature will be indirectly controlled by varying the heat-recovery duty of a boiler and consequently the furnace flue gas temperature at the inlet of a Economizer.
Then, the methane-steam stream enters the SMR Reactor where the most of the methane is converted to hydrogen and a carbon monoxide and dioxide (carbon oxides coexist is equilibrium with a certain ratio depending on the temperature: the monoxide is more present at high temperature while the dioxide at low temperature).
The SMR Reactor will be consisting of several tubes containing catalyst fixed beds and externally heated by the flame of special design multi-burners using both off-gas from the methane purification section and part of the methane feed as fuel (the overall reaction is highly endothermic and needs heat input). Usage of off-gas as fuel is aimed to minimize the emission of waste products from the Package and optimize the heat balance and consumptions.
The outlet temperature of the reactor guarantees a good compromise between the SMR reactions conversions and the reactor catalyst life. As a consequence of this relative high temperature, the content of carbon monoxide is relatively high. Since this monoxide is quite hard to be separated from the process stream, it must be converted in the dioxide, which is much easily removed. To do that the so called shift reactors are necessary. The content of the carbon monoxide (CO) should be reduced at the inlet of the downstream PTSA purification unit (this is important to reach the required hydrogen purity, under a wide range of possible operating conditions).
The gas stream is feed to the purification Pressure/Temperature Swing Adsorption (PTSA) Package
In this section the hydrogen is purified, by adsorption on triple-bed sieves. However, part of the produced hydrogen is recycled for the regeneration, at low pressure, of the adsorption beds. The so produced off-gas is send to the Equalizer (equalizing is necessary since the composition of the off-gas is changing during the adsorption-regeneration cycles) and then used as fuel in the SMR furnace burners.