Ammonia catalyst poisons Oxygen compounds

The proportions of the two reforming reactions and shift conversion are so controlled that the gas mixture obtained contains nitrogen and hydrogen in the mole ratio (volume ratio) of 1 3. However, this mixture still contains 20-30% carbon dioxide resulting from the shift conversion reaction and traces of unconverted carbon monoxide. Carbon dioxide can yield carbonates and carbamates in the ammonia synthesis cycle, which are undesirable because they can deposit in the piping. In addition oxygen, and any of its compounds such as carbon monoxide, water, etc., are also ammonia catalyst poisons [13]. Consequently they must be removed. 

Temporary Poisoning of the Ammonia Catalyst by Oxygen-Containing Compounds... 

P. E. H0jlund Nielsen, The Poisoning of Ammonia Synthesis Catalysts by Oxygenic Compounds Lecture given at ACS Meeting, Washington D.C. (Sept. 1983). 

Before the synthesis gas enters the ammonia synthesis loop, essentially all of the oxygen compounds must be completely removed to (l) avoid poisoning the ammonia synthesis catalyst and (2) keep C02 from forming carbamates and ammonium carbonate in the synthesis loop. It is also advantageous to remove the inert gases (methane, argon, etc.) to achieve a higher synthesis conversion per pass.74... 

Sulfur, Phosphorus, and Arsenic Compounds. Sulfur, occasionally present in synthesis gases from coal or heavy fuel oil, is more tightly bound on iron catalysts than oxygen. For example, catalysts partially poisoned with hydrogen sulfide cannot be regenerated under the conditions of industrial ammonia synthesis. Compounds of phosphorus and arsenic are poisons but are not generally present in industrial synthesis gas. There are... 

Chlorine compounds. The permanent poisoning effect of chlorine compounds is two orders of magnitude worse than that of oxygen compounds. Concentrations of about 0.1 ppm are viewed as the uppermost allowable limit in order not to affect adversely the life of ammonia catalysts [384]. The deactivation effect is based at least in part on the formation of alkali chlorides that are volatile at the upper synthesis temperatures. 

The potential for ruthenium to displace iron in new plants (several projects are in progress [398] of which two 1850 mtpd plants in Trinidad now have been successfully commissioned [1488]) will depend on whether the benefits of its use are sufficient to compensate the higher costs. In common with the iron catalyst it will also be poisoned by oxygen compounds. Even with some further potential improvements it seems unlikely to reach an activity level which is sufficiently high at low temperature to allow operation of the ammonia synthesis loop at the pressure level of the synthesis gas generation. 

These catalysts are extremely sensitive to catalyst poisons, which reduce chemisorption of hydrogen and nitrogen on the active surfaces of the catalyst and thereby reduce its activity. Gaseous oxygen-, sulfur-, phosphorus-and chlorine compounds, such as water, carbon monoxide, carbon dioxide, the latter being reduced to water under ammonia synthesis conditions, are particularly troublesome in this regard. Catalyst poisoned with oxide compounds can be reactivated by reduction with pure synthesis gas. 

In the process of ammonia synthesis, the common toxic compounds that lead to poisoning and loss of catalytic activity are oxygen and oxygenous compounds (CO, CO2, H2O), and non-metallic compounds such as sulfur, phosphorus, arsenic and chlorine, etc. Toxic metals may be present in the catalysts themselves, while metallic compounds are rarely present in the reaction gas. Oxygen and oxygenous compounds are reversible poisons which cause temporary poisoning but sulfur, phosphorus, arsenic, chlorine and their compounds are irreversible poisons which cause permanent poisoning. 

A temporary poison lowers the activity of an ammonia synthesis catalyst by reversible adsorption onto the catalyst surface. Unsaturated hydrocarbons like ethylene may also react as a temporary poison according to studies by Nielsen, but generally, it is the oxygenic compounds that constitute the single most important poison for ammonia synthesis. 

If there is no poison in the synthesis gas, the poison will be removed from the surface and the catalyst will regain its activity. An oxygenic compound reacts in that manner with the catalyst surface [1]. Unsaturated hydrocarbons may also react as a reversible poison [1], but usually oxygenic compounds are the most important, temporary poisons for ammonia synthesis catalysts. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

After this bulk removal of the carbon oxides, the typical synthesis gas still contains 0.2-0.5vol% CO and 0.005 0.2 vol% CO2. All oxygen-containing compounds have to be reduced to a very low ppm level, as they are poisonous toward the ammonia synthesis catalysts. Methanation is the simplest method to reduce the concentrations of the carbon oxides well below 10 ppm, and is widely used in steam-reforming plants. 

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