Low Temperature Water Gas Shift Process
The water-gas shift reaction (WGSR) is a reversible slightly exothermic chemical reaction with an equilibrium constant that decreases with increasing temperature.
CO + H2O ↔ CO2 + H2 ΔHo298 = -41.09 kJ mol -1
The exothermic essence of this reaction demonstrates that it is desirable to perform the water gas shift reaction at low temperatures to obtain high CO conversion. However, to achieve sufficient high reaction rates, it is necessary to operate at higher temperatures, even though the equilibrium composition is not so favorable under such conditions. Furthermore, since the number of moles does not change in the reaction, the equilibrium composition is independent of the total pressure.
Water gas shift process continues to maintain an important role in refinery and petrochemical industry, as an integral part for industrial production of hydrogen for the refinery plants and the manufacture of ammonia as well as for the adjustment of the carbon monoxide to hydrogen ratio (CO:H2
ratio) in syngas for several applications, such as synthesis of methanol.
It should be noted that, in ammonia or hydrogen plants, it is essential that even trace levels of carbon oxides removed from the reformed gas (After steam and secondary reformers) in order to avoid the poisoning of the ammonia synthesis catalysts or have a high purity hydrogen in the downstream units.
Since the reaction is exothermic and reversible, WGSR is, for thermodynamic and kinetic reasons, normally performed in two steps. Industrially, WGSR is carried out at high temperature (583-803 K) and low temperature (473-523 K) shift reactions. Before introduction the feed gas stream into LTS reactor, the effluent from HTS unit should be first cooled down to about 190-220◦
Low Temperature Water Gas Shift Catalyst
KhTD LTS catalyst has a high activity toward converting remaining content of carbon monoxide presents in the HTS outlet stream. Inlet CO concentration may vary between 1 and 5% (dry Molar Basis) depending on the operational parameter and the performance of the HTS catalysts which installed upstream.
The CO content should be reduced down to 0.2-0.45% (Dry Molar Basis). This catalyst is generally constituted of a ternary mixture of copper oxide as the active metal, zinc oxide and alumina as the structure forming agents. The copper should be reduced prior to be used for the conversion of the feed stream.
The low temperature shift reaction is operated adiabatically in industrial scale, hence the temperature will be raised along the length of the reactor. The inlet temperature may be 190-230°C and the total pressure does not usually exceed 40 bar. As stated before, it is desirable to perform WGSR at lower temperatures to obtain a high CO conversion. Therefore, new generations of LTS catalysts operate at the temperatures below 200°C. Another advantage of operation at lower temperatures is to avoid catalyst deactivation due to sintering.
KhTD LTS Catalysts are manufactured through a sophisticated co-precipitation process in which all of the constituents uniformly dispersed in the catalyst texture which is the unique characteristic of such catalyst preparation method. In addition, the complete and comprehensive control over the physical and chemical properties of the materials can be achieved during the synthesis using the automated equipment. Such process will ultimately provide a mesoporous structure with nano-sized copper particles which further exhibits a superior performance for the carbon monoxide removal during the process.
Here the optimization and precise control of the synthesis parameters during the catalyst production will result in well distribution of the three containing oxide in Nano-metric scale.
In general, with the same amount of copper content the smaller copper crystallite size gives more available reaction sites and higher catalytic activity. Furthermore, the particle growth during operation, namely sintering as well as poisoning, are considered as the main reasons of catalyst deactivation. These phenomena may be inhibited through covering copper particles with zinc and aluminum oxides. This shall ensure a longer lifetime and more stability with time in terms of catalyst activity and by-product formation.
In conclusion, KhTD LTS catalysts superior features summarized as follows:
High and stable catalytic activity for CO conversion
A comparable catalyst activity versus current commercial catalysts
Significant prolonged catalyst lifetimes
A high resistance against sulfur as a permanent catalyst deactivator
High mechanical strength and stability