Ammonia catalyst poisons Reversible poisoning

Figure 8.3.1 is a typical process diagram for tlie production of ammonia by steam reforming. Tlie first step in tlie preparation of tlie synthesis gas is desulfurization of the hydrocarbon feed. Tliis is necessary because sulfur poisons tlie nickel catalyst (albeit reversibly) in tlie reformers, even at very low concentrations. Steam reforming of hydrocarbon feedstock is carried out in tlie priiiiiiry and secondary reformers. 

Pig. 15. Reversible poisoning of an Fe-AljOj-KiO synthetic ammonia catalyst by water vapor (after Emmett and Brunauer). The curve shows, first, the poisoning effect obtained by replacing, at zero time, the normal synthesis gas by a gas containing 0.32 per cent of water vapor and, second, the effect of changing back to a dry gas after 60 minutes. 

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. 

Remaining trace quantities of CO (which would poison the iron catalyst during ammonia synthesis) are converted back to CH4 by passing the damp gas from the scmbbers over a Ni methanation catalyst at 325° CO -t- 3H2, CRt -t- H2O. This reaction is the reverse of that occurring in the primary steam reformer. The synthesis gas now emerging has the approximate composition H2 74.3%, N2 24.7%, CH4 0.8%, Ar 0.3%, CO 1 -2ppm. It is compressed in three stages from 25 atm to 200 atm and then passed over a promoted iron catalyst at 380-450°C ... 

Methanation, that is, the transformation of CO to methane222 270-272 [Eq. (3.1), reverse process], was developed in the 1950s as a purification method in ammonia synthesis. To prevent poisoning of the catalyst, even low levels of residual CO must be removed from hydrogen. This is done by methanation combined with the water-gas shift reaction.214,273,274 In the 1970s the oil crises spurred research efforts to develop methods for substitute natural-gas production from petroleum or coal via the methanation of synthesis gas. ... 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

The steam requirements in an ammonia unit can be reduced by lowering the steam-to-carbon ratio to the primary reformer. However a number of drawbacks can exist downstream in the I I I S and LTS reactors. The drawbacks include By-product formation in the HTS, Pressure drop buildup in the HTS, Reversible poisoning of the LTS catalyst, and Higher CO equilibrium concentrations exiting the HTS and LTS reactors. 

Systematic studies were carried out in order to discover a suitable catalyst. Iron catalysts were especially tried, because it was known that iron catalyses the decomposition of ammonia, which is the reverse of the reaction being studied. It was discovered that iron alone was only slightly active but its activity could be improved (promoted) or worsened (poisoned) by additives. In their studies over 10 000 catalysts were prepared and over 4000 were tested. 

Some other results of other TPH experiments can be seen in Table 1. In atmospheric-pressure tests at 900°C with 500 ppm HjS in the gas phase, sulfur was not desorbed from Catalyst Al. The same phenomenon was noticed in the tests performed at 900°C under 5 bar pressure with Catalysts A2 and C. In addition, when the sulfur content of the catalyst beds was analyzed after TPH experiments, it was observed that only a small amount of sulfur was present on the catalyst. This observation indicates that sulfur adsorption is not completely reversible, but that part of the adsorbed sulfur remains on the catalyst. The effect of this phenomenon was also observed when a catalyst was regenerated by removal of HjS to the gas mixture in fixed-bed poisoning tests. The catalyst activity did not reach the original level (with no HjS in the gas) especially in ammonia decomposition. The analysis of the sulfur content of the bed showed that a small amount of sulfur was still present on the catalyst. 

Sulfur from SO2 can poison noble metal catalysts by its strong bonding with the metal, forming the metal sulfide and even penetrating into the bulk metal l 3. When alumina is used as the catalyst support, irreversible deactivation can result from the formation of Al2(S04)3 with concurrent substantial reduction in surface area and pore volume " . Similar activity loss with decreased surface area and pore volume accompanying sulfur accumulation in the catalyst can result from the formation and deposition of sulfates of ammonia, particularly at lower operating temperatures, but these effects can usually be reversed by heating 2,47... 

The activity of the fresh catalyst is decreased by impurities in the feed gas which block active sites or coat the entire catalyst. In ammonia synthesis, reversible poisoning by oxygen, argon, and methane is known. This can be alleviated by flushing with a pure gas that is free of these components. Irreversible poisoning is caused by... 

For example, oxygen and water vapor are poisons to ammonia s3mthesis catalysts. Because these poisons can be removed by reduction by H2 or by treatment using a fresh synthesis gas without poisons, this is reversible poisoning. However, poisoning caused by sulfide, chlorine, phosphorus and heavy metal is very difficult to be removed so the poisoning is irreversible. 

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. 

The active a-Fe in an ammonia synthesis catalyst is easily poisoned by water. Research shows that the effect of water vapor on the catalytic activity is still reversible when the water vapor content is as high as 27mol9c or more but the activity cannot be restored when the content of water vapor reaches 42mol%. 

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. 

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