Secondary & Auto-thermal Reforming Catalysts
A secondary reformer is an auto-thermic unit located directly after a steam reformer in a typical ammonia plant while when oxygen is introduced as the oxidizing agent into the reformer instead of air, this unit is called auto-thermal reformer, as can be seen in some methanol plants today. In a secondary reformer, reformed gas of steam reforming unit is initially mixed with air. Such mixing has two purposes, first, it will supply the proper amount of nitrogen (for synthesis of ammonia) into the system and secondly, rapid reaction of the process gas with air provides the heat required for additional reforming. The secondary reformer in ammonia plants will typically be air-blown.
Two important series of reactions take place in the secondary reformer. Firstly, the oxygen component of the air is consumed by combustion with hydrogen and methane. The overall reactions in the combustion zone can be considered as follows:
CH4 + 3/2 O2 → CO + 2 H2O ΔHr° = - 519.76 kJ/mol
H2 + 1/2 O2 → H2O ΔHr° = - 242.0 kJ/mol
Secondly, in the catalyst zone, heterogeneous endothermic and reversible reactions take place. Some of the residual methane from the primary reformer undergoes further reforming while water gas shift reaction takes place as well.
CH4 + H2O ↔ CO+ 3 H2 ΔHr° = - 206.100kJ/mol
CO + H2O ↔ CO2 + H2 ΔHr° = - 41.150 kJ/mol
Both reactions reach equilibrium at a bed outlet temperature of about 1000 °C. The methane content of gas leaving the secondary reformer is usually in the range 0.2– 0.5 % while oxygen is completely removed.
The main problems in the operation of a secondary reformer are associated with the high temperatures generated by the combustion reactions where the temperature at the point of mixing air or oxygen and tubular reformer effluent may reach 1200 °C or more. The gas distributor must be well designed so that the process gas and air are mixed as rapidly and thoroughly as possible. This extremely high temperature needs a refractory type catalyst to withstand this temperature without encountering catalyst fusion. The catalyst bed itself is protected from the very high temperatures generated by the homogeneous combustion of air, by a layer of refractory material that is placed on top of the large, temperature-resistant catalyst particles. The rest of the bed is filled with secondary reforming catalyst.
The catalyst can operate for several years provided that the reacting gases are well mixed and that no hot spots (which in extreme cases could reach 1500 °
C) are allowed to develop in the catalyst bed. The gas distributor plays an important part in protecting the catalyst. The temperature of the catalyst bed in the secondary reformer falls as gas passes through the bed. At the inlet to the bed, the temperature is extremely high due to the highly exothermic combustion reaction in which all of the oxygen is consumed. The temperature of the bed towards the outlet then falls as the endothermic reforming reaction takes place. It is interesting to note that when the temperature of the primary reformer outlet falls and the slippage of methane increases, the temperature of the outlet of the secondary reformer also falls.
In comparison with primary reforming catalyst, the nickel content is lower. Secondary reforming catalyst contains about 5-10 % nickel, supported on highly stable and temperature-resistant α–alumina, magnesium aluminate or calcium aluminate carriers, in the form of rings. Additional catalyst used at the top of the bed as a heat guard is in the form of large solid cylinders of α-alumina may contain 9% nickel oxide.
KhTD Secondary & Auto-thermal Reforming Catalysts
KhTD Co. has introduced two series of secondary reforming catalyst based on rugged heat-resistant and high crush strength catalyst carriers. Below, we have presented a brief description and technical specifications of our both manufactured grades which are categorized according to the carrier type and their sizes.