Permanent Poisons

Permanent poisoning is caused by compounds, the presence of which in the synthesis gas results in an activity decline. The activity is not regained when the compound is removed from the synthesis gas. The poisoning action is frequently due to the irreversible formation of surface and/or bulk compounds with iron or with one of the promoters, in particular potassium. The deposition of solid material, as discussed briefly in Section 8.2.3, can also lead to permanent poisoning. 

Sulfur is the most common of the permanent poisons. In plants based on gasification of sulfur-containing feedstocks, in particular coal, traces of carbonyl sulfide and hydrogen sulfide may reach the synthesis converter. In plants based on natural gas, the sulfur content of the feed is very low, and furthermore the copper-based low-temperature shift catalyst and the nickel-based clean-up metha-nation catalyst act as efficient guards by irreversibly absorbing these traces of sulfur. In the earlier days of ammonia synthesis, the lubrication oil for the compressors was a common source of sulfur, but today this problem is widely recognized and examples of sulfur poisoning in the industry are rare. 

Sulfur is chemisorbed very strongly by the iron crystal surface. Benard et suggest a value of -190kJmor for the enthalpy for formation of a sulfide 

Analysis of spent industrial catalysts discharged from the inlet layer of the catalyst bed has revealed sulfur in concentrations ranging from 1000 to 5000 ppm. If all the sulfur is present as a monolayer, this would correspond to an iron metal surface area from 2 to 10m g . It may also indicate that the major part of the catalyst surface is the iron surface and not, as often suggested by Emmett (see Section 8.4), made up by promoter phases. 

Chlorine is also a serious poison for the synthesis catalyst, particularly for ammonia plants using hydrogen produced by electrolysis plants. Chlorine reacts with the potassium promoteri and forms potassium chloride, which may migrate in the catalyst particle or diffuse into the gas stream, depending on the temperature. 

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Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

These metals permanently poison the FCC catalyst by lowering the catalyst activity, thereby reducing its ability to produce the desiretl products. Virtually all the metals in the FCC feed are deposited on the cracking catalyst. Paraffinic feeds tend to contain more nickel than vanadium. Each metal has negative effects. 

If the catalyst surface is slowly modified by chemisorption on the active sites by materials which are not easily removed, then the process is frequently called poisoning. Restoration of activity, where possible, is called reactivation. If the adsorption is reversible then a change in operating conditions may be sufficient to reactivate the catalyst. If the adsorption is not reversible, then we have permanent poisoning. This may require a chemical retreatment of the surface or a complete replacement of the spent catalyst. 

Removal of the metal contaminants is not usually economical, or efficient, during rapid regeneration. In fact, the deposited metals are believed to form sulfates during removal of carbon and sulfur compounds by combustion that produce a permanent poisoning effect. Thus, if fixed-bed reactors are to be used for residuum or heavy oil hydrodesulfurization (in place of the more usual distillate hydro-desulfurization) it may be necessary to first process the heavier feedstocks to remove the metals (especially vanadium and nickel) and so decrease the extent of catalyst bed plugging. Precautions should also be taken to ensure that plugging of the bed does not lead to the formation of channels within the catalyst bed which will also reduce the efficiency of the process and may even lead to pressure variances within the reactor because of the distorted flow patterns with eventual damage. 

Another important issue is the permanence of the catalyst inhibition or poisoning. The permanence of catalyst inhibition is dependent on the mechanism of the chemical interaction of the poison with the catalyst. Catalyst inhibition and the resulting reduction in reaction rate could result from competition between the poison and the preferred reactant at the catalytic site, either because of a high affinity of the poison for the catalyst site or because of its slow reaction once on the catalyst site. If the affinity is too high, as when the poison actually reacts with the catalyst to form a new compound, the catalyst is permanently poisoned. If the inhibition is only related to a slow rate of reaction, it may be possible to remove the poison from the catalyst surface and restore catalyst activity. 

Reforming catalysts may suffer deactivation either in reversible or irreversible mode. Deactivation is irreversible when catalyst is exposed to permanent poisons or very high temperature during regeneration. Directional concentration of specific permanent poisons in... 

Hydrogen sulfide causes a permanent poisoning of iron catalysts. Methane does not poison ammonia catalysts under normal synthesis conditions. Equilibrium data (Browning, De Witt, and Emmett, 77 Browning and Emmett, 78) should be mentioned in this connection. 

The catalyst is poisoned by CO, C02, and H20 so they must be rigorously removed upstream in the hydrogen synthesis process. Oxygen molecules are permanent poisons. Other poisons such as sulfur, arsenic, halides, and phosphorous must be carefully removed upstream in as much as they too are permanent poisons. 

The make-up gas must be free of sulfur, arsenic and phosphorus compounds, as well as chlorine and. in general halogenated derivatives which constitute permanent poisons. 

The catalyst is sensitive to sulfur and arsenic poisoning (the Utter being a permanent poison). Natural gas must, therefore be desulfurized. Carbon and coke deposits also damage the catalyst and must be removed by steam or by burning off with air. 

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. 

Roles of Coke and Metals on Catalyst Deactivation. The model compound activity test (Figure 4) and the diffusivity test (Figure 6) clearly show that both coke and metal sulfides have a responsibility for decreasing intrinsic reaction rate and effective diffusivity. However, the former test suggests that coke and metal sulfides do not independently affect the active sites. As shown in Figure 4, in the third bed, coke rapidly covered more than 70% of the original active sites at the start of the run. However, after the run, the ratio of the coke-covered active sites to the original ones dropped drastically to less than 10%, while that of the metal-poisoned active sites became around 70%. This indicates that the metal sulfides deposit on part of the active sites which coke initially covers and permanently poison them. 

But for the same catalyst batch, the same pattern of declining conversion was observed, suggesting that permanent poisoning did not occur. 

From the standpoint of daily capacity, the greatest application of fluidized bed catalysis is to the cracking of petroleum fractions into the gasoline range. In this process the catalyst deactivates in a few minutes, so that advantage is taken of the mobihty of fluidized catalyst to transport it continuously between reaction and regeneration zones in order to maintain its activity some catalyst also must be bled off continuously to maintain permanent poisons such as heavy metal deposits at an acceptable level. 

All licensors agree on the necessity of hydrotreating the feed to lower the level of poisons for the platinum-based reforming catalyst. Temporary poisons are sulfur and nitrogen, while As, Pb, and other metals are permanent poisons. Proper conditions of hydrogen, pressure, temperature, and space velocities are able to reduce these poisons to the acceptably low levels of modern catalysts. Numerous process design modifications and catalyst improvements have been made in recent years. 

Steam reforming catalysts are poisoned by sulfur, arsenic, chlorine, phosphorus, copper and lead. Poisoning results in catalyst deactivation however, sulfur poisoning is often reversible. Reactivation can be achieved by removing sulfur from the feed and steaming the catalyst. Arsenic is a permanent poison therefore, feed should contain no more than 50 ppm of arsenic to prevent permanent catalyst deactivation by arsenic poisoning 13]. 

Catalyst deactivators are carbon monoxide, carbon dioxide, and sulfur compounds. Depending on the source of hydrogen, all of these contaminants may be present. Sulfur is a permanent poison therefore, the level in the hydrogen feed must be less than 1 ppm to ensure satisfactory catalyst life [2]. Carbon monoxide and carbon dioxide are deactivators that inhibit the hydrogenation reaction but do not permanently poison the catalyst. Reducing the quantity of the contaminants in the feedstock will restore catalyst activity. 

Sulphur compounds, halides, phosphorus and arsenic are permanent poisons to ammonia catalysts (22). However, in most plants the upstream low-temperature shift catalyst and the Ni-based methanation catalyst both serve as efficient guards by irreversibly adsorbing traces of such compounds. Thus, permanent poisons are normally not a severe problem. 

The cold or hot reactivity of the water-cooled loading Is not to exceed the coinblned negative reactivity worth associated with the Horizontal Control Rod System the xenon present In the reactor Inserted supplementary control devices and the poison pieces or poison columns under Irradiation. That Is the reactivity In excess of that coiqpensated by Inserted supplementary and permanent poisons Is not to exceed the strength of the Horizontal Control Rods, 1 5 per cent k/k In the... 

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