This is so called the main process of any Ammonia production facility (or any other product that require mass production of Hydrogen gas from methane reforming). Normally reforming reaction takes place in two equipments, known as primary and secondary reformers. In some plant pre-reformer is also installed later on. Pre-reforming technology was first incorporated in Ammonia plants in the early 90’s. This technology was used in connection with revamps of existing plants were the objective was to decrease energy consumption and/or increase throughput. In recent years a number of grass-roots ammonia plants have been designed with a prereformer to optimize the size of the tubular reformer. Lets first study the dynamics of reforming reaction, then to the significance of all three sub reactors.
Reforming reactions can be taken place as follows;
CnHm + nH2O ==> nCO + (m/2 + n)H2 ------ General reaction for hydrocarbon in feedstock
For methane, it can be extracted as follows;
CH4 + H2O ==> CO + 3H2 (dH = +206 kJ/mol)
Along with following water-gas shift reaction as follows;
CO + H2O ==> CO2 + H2 (dH = -41 kJ/mol)
That also results in following resultant reaction.
CH4 + 2H2O ==> CO2 + 4H2 (dH = +165 kJ/mol)
There is a strong belief that above reaction also takes place as direct one. But that doesn't matter, as far as reaction equilibrium and energy balance is concerned. All above reactions are reversible reactions. Allow me to share some equilibrium data in next post.
Being strongly endothermic in nature and producing higher moles than reactants, reforming reaction is highly favorable in high temperature and low pressures. Typically temperatures of reforming reactors are held above 500°C and may go upto 1000°C. These reaction temperatures may vary based on reactant concentrations, referred as steam to carbon ratio. In Ammonia plants it's a very important control parameter. Attached is a graphical representation of S/C ratio impact over temperature versus Methane reforming.
It is one of the most economic method of Hydrogen production on industrial level. Steam reforming of Methane consists of three reversible reactions, two strongly endothermic reforming reactions (1) and (3) the moderately exothermic water-gas shift reaction (2).
CH4 + H2O ===> CO + 3H2 ΔH @ 25C = +206.2 kJ/mol (~ +88650 BTU/lb-mol) ---------------------(1)
CO + H2O ===> CO2 + H2 ΔH @ 25C = -41.2 kJ/mol (~ -17710 BTU/lb-mol) ----------------------(2)
CH4 + 2H2O ===> CO2 + 4H2 ΔH @ 25C = +165 kJ/mol (~ +70940 BTU/lb-mol) -----------------------(3)
It should be emphasized that CO2 is not only produced via the shift reaction (2), but also directly via the steam reforming reaction (3). This implies that reaction (3) is not just the 'overall reaction'.
Due to its endothermic character, reforming (overall) is favored by high temperatures. And as reforming is accompanied by a volume expansion so it also favored by low pressures. Increasing the amount of steam will enhance the CH4 conversion, but requires an additional amount of energy to produce the steam. But reduction in steam may also cause coke formation during the reaction. So an optimum approach has to be taken. In practice, steam-to-carbon ratios around 3 are applied, which corresponds to a varying steam-to-gas ratio (due to difference in gas composition around the world). Due to ease in control, steam-to-gas ratio are maintained indirectly to maintain S/C. Typically temperatures 800°C (1470 °F) along pressure of 30 bar (435 psig) are maintained for optimum catalyst and process performance at primary furnace outlet.
Now there come the introduction of second constituent, Nitrogen, most abundantly available component in the world. Reforming process is optimized by using compressed air for following purpose;
- To introduce Nitrogen in the process.
- To further increase the reforming process temperature by burning Hydrogen.
Its a win-win situation. Industrially Nitrogen is introduced in the system at the expense of precious Hydrogen gas. Its a simple combustion reaction as follows;
H2 + (1/2)O2 ===> H2O ΔH @ 25C = -286 kJ/mol (~ -122960 BTU/lb-mol) -----------------------(3)
Amount of air is dictated by Hydrogen-to-Nitrogen at the inlet of Ammonia converter. This equipment is termed as secondary reformer. Temperatures are as high as 1200°C (2192°F) depending upon the amount of air introduced. But due to the reforming reaction still in progress, temperature reduced upto 930°C (1706°F) at the outlet of secondary reformer. Methane compositions are less than 1% at that point. Catalyst of both reformers is based on Nickel. Although Nickel has the lowest activity per surface area available in its group, but industrially economically available. Rest of the candidates has following activity relative to Nickel.
| Catalyst metal content (wt%) Relative rate | |
| Ni (16) | 1.0 |
| Ru (1.4) | 2.1 |
| Rh (1.1) | 1.9 |
| Pd (1.2) | 0.4 |
| Ir (0.9) | 1.1 |
| Pt (0.9) | 0.5 |
The activity of a catalyst is related to the metal surface area (i.e., the number of active sites). This implies that, generally, the catalytic activity benefits from a high dispersion of the metal particles. There is an optimum beyond which an increase in Ni-content does not produce any increase in activity, usually around 15–20 wt% (depending on support structure and surface). Apart from the amount of available metal surface area, also the structure of the available surface area strongly influences the catalyst activity. For instance, the close-packed surface of nickel is less active than the more open surface.
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