Temporary poisons

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. 

The most common oxygenic compounds encountered in ammonia synthesis are carbon monoxide, carbon dioxide, and molecular oxygen. The equivalence per oxygen atom of these compounds has been established by Larson and Tour and Almquist and Black. This is due to the methanation activity of the ammonia synthesis catalyst  

Carbon dioxide will undergo a similar reaction, though more slowly, and molecular oxygen will be hydrogenated too. Thus in the major part of the catalytic reactor, the sole oxygen-containing compound will be water. 

With respect to the interaction between water and iron surfaces, reference is made to the preceding chapters, in particular Chapters 3-5. 

From thermodynamical data it may be shown that, in ammonia synthesis gas, water is required in percent amounts in order to oxidize bulk metallic iron to the oxide (magnetite or wustite). On the other hand, it is very clear from the poisoning studies to be discussed later in this chapter that water in the low ppm range exerts a definite poisoning action. 

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Fouling Salt formation can build-up on the catalyst surface effectively limiting accessibility. Ammonium bisulfate can form at low temperatures. This foulant can be removed by increasing temperature and is considered a temporary poison. 

Its content of oxygen compounds, which represent temporary poisons, must satisfy certain limitations, such as O, CO2 < 1 ppm CO, H1O, CO CO2 < 2 ppm. The requisite purity levei is reached by repeating the liquid nitrogen scrubbing or methanation treatment, as required. Moisture is then removed by drying or by cryogenics. 

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. 

Sulfur, chlorine, and phosphorus are classed as temporary poisons in reforming reactions, because the activity of the catalyst is recovered when the poison... 

Moreover, the kinetics are further complicated by the high level of water in the reactor inlet, deriving from its high concentration in both fresh and recycled ethanol, that acts as a temporary poison for the catalyst and strongly reduces the reaction rate [6]. 

V. Addition of other species (for example species electronegative in character, which act as selective or non-selective temporary poisons). ... 

Poisons are the same as for magnetite and to approximately the same degree. Catalyst recovers after temporary poisons (H2O, O2, CO, CO2) are removed. 

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. 

Answer After about two days the reactivity gain will exceed the capacity of the horizontal rod system in the older piles, and the reactor must be kept subcrltlcal with the vertical safety rods and/or supplementary control or temporary poison. 

Answer The xenon builds up after the pile is shut down cbsorling the react vlty forr.erly held by the temporary poison. Thus, in the operating period after poison discharge it is possibi.e to attain turnarc-iund at a high power level. . ith only the horizontal, rod controls, and, ohoroforo, nerr.it a rapid rise to the ccrailibriam power level. 

Answer Yes - The near-infinite variety of pile conditions is broker, down into a series of charts according to major factors such as power level, length of previous outage, time operated, amount of temporary poison, etc. Accuracy of approximate case calculated for a given ra.nge of conditions is of the same order as the rod system calibration.. 

Push - ("Eanfordese") - The term refers to the discharge of temporary poison that has been charged into a reactor to gain ontrol of reactivity during a start-up... 

Hun (or Poison Run) A short period (few hours) of operation of a reactor after a sxarTi-up with ter.porary poison charged in the active zone. After the reactivity has reached a maximuni and is decreasing the reactor is shut down so the temporary poison can be discharged. 

Sometimes, as in the case for all temporary poisoning or nutritional deficiency, the clinical symptoms are the direct expression of the biochemical lesion no specific anatomical lesion may be detected. Table 1 shows how the biochemical lesions may be classified according to their origin. 

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