Catalysts arsenic poisoning

Catalysts commonly lose activity in operation as a result of accumulation of materials from the reactant stream. Catalyst poisoning is a chemical phenomenon, A catalyst poison is a component such as a feed impurity that as a result of chemisorption, even in smaH amounts, causes the catalyst to lose a substantial fraction of its activity. For example, sulfur compounds in trace amounts poison metal catalysts. Arsenic and phosphoms compounds are also poisons for a number of catalysts. Sometimes the catalyst surface has such a strong affinity for a poison that it scavenges it with a high efficiency. The... 

Arsenic does not combine directly with carbon, silicon or boron. The reaction with metals to form definite arsenides or alloys is described no pp. 57-78. The presence of small quantities of arsenic or of its compounds in certain catalysts has a poisoning effect. The first traces added to the catalyst have the greatest effect thus the activity of 0-35 g. of platinum was reduced linearly by the addition of arsenic up to 0-7 mg., this quantity reducing the catalytic activity to 45 per cent, of its original value the addition of 10 mg. of arsenic, however, depressed the activity only to 26 per cent, of the original value.3 Vanadium catalysts are poisoned by the presence of arsenic, although the action is slow arsenic pentoxide is formed.4... 

Arsenic pentoxide catalyses the reaction between sulphur dioxide and oxygen,9 the amount of sulphur trioxide formed reaching 54 per cent, at 660° C. The reaction consists in the alternate reduction of the pentoxide to arsenious oxide by the sulphur dioxide and reoxidation to the pentoxide, so that arsenious oxide acts similarly. The catalytic activity is less than that of ferric oxide, but the latter is activated by addition of arsenic pentoxide the maximum amount of conversion increases from 69-5 to 78-5 per cent, and occurs at a temperature 63° lower than is required in the absence of the promoter. Arsenic pentoxide does not activate catalysts which act rapidly, such as vanadium pentoxide. Platinum and silver catalysts are poisoned by arsenic pentoxide.10... 

Poisons for the nickel catalyst are sulfur, arsenic, chlorides or other halogens, phosphates, copper and lead. A 15 percent nickel catalyst is poisoned at 775°C if the gas contains as little as 0.005 percent (50 ppm) sulfur. 

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 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. 

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... 

Under certain conditions, especially when feeds derived from coal gasification are used, irreversible arsenic poisoning of the ferrochrome catalyst is likely to occur. When operated within normal parameters, expected life for this catalyst ranges between 1 and 3 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]. 

Catalysts surface poisoning by arsenic Mechanism of triphenylarsine interaction with alumina supported nickel. 

The reaction of tiiphenylarsine with alumina supported catalyst leads to drastic decrease of the benzene hydrogenation rate. The benzene hydrogenation at 373 K on the arsenic poisoned Ni/Al203 catalysts is completely suppressed for As/Ni, ratio greater than ca. 0.3. 

The metal function of the catalyst is poison by sulfur, as discussed later. The elimination of sulfur from the naphthas is carried out in the hydrodesulfurization vmit, in which the sulfur compounds are hydrogenolyzed, producing H2S, which is separated. This unit contains a catalyst, usually Co-Mo, Ni-Mo, or Ni-W supported on alumina. This catalyst also eliminates the nitrogen as NH3 and retains the arsenic and metals present in the feed. 

The steam reforming catalysts are very sensitive to some impurities in the feedstock such as sulfur, arsenic, halogens, phosphorous and lead etc., even with very low contents. Generally, sulfur content is required to be below 0.5 ml m . Halogen such as chlorine, poisoning role is similar to sulfur, has the same limited content. Arsenic poisoning is permanent and irreversible. Thus, the restriction for arsenic is very strict. The steam reforming catalysts must be replaced when they are seriously poisoned by arsenic. 

The demand for such stronger acid had been small, until the emergence of the dyestuffs industry in the 1870 s. Then, Rudolf Messel (1848 - 1920) in London, and Clemens Alexander Winkler (1838 - 1904) in Freiberg, worked on the process. A key part of their invention was the use of powdered platinum as the catalyst. The German-bom Messel had been a chemist in the firm Squire, Chapman, and Company from 1875 to 1878. The improved, yet still flawed, technology was first installed at their sulfuric acid plant at Silvertown in 1876. In 1878, Messel became a partner in the renamed firm Spencer, Chapman and Messel, where he was Managing Director until 1916. In 1875, developments by Winkler had led to another Contact plant to be opened in Frieberg. While the new plants were competitive versus the expensive Nordhausen acid, the process was still costly and difficult to operate. Impurities in the system, especially arsenic, poisoned the catalyst. Except for a few special cases, the Contact process was still only a technical curiosity. 

Permanent deactivation can occur when the catalyst is poisoned by arsenic or flumine ot damaged due to exposure to moisture or high gas temperatures. 

Poisons for the nickel catalyst are sulfur, arsenic, chlorides or other halogens, phosphates. and copper or lead. A 15 percent nickel catalyst is poisoned at 775°C, should the gas contain 0.005 percent sulfur. This is equivalent to reaction of all the nickel on the surface of the crystallites 1 micron in diameter. For lower operating temperatures, the amount required for poisoning is even lower. When using naphtha as a feedstock, 0.5 ppm of sulfur (w/w) in the naphtha is the maximum allowed concentration for operation at 775°C. 

Shale oil contains large quantities of olefinic hydrocarbons (see Table 8), which cause gumming and constitute an increased hydrogen requirement for upgrading. Properties for cmde shale oil are compared with petroleum cmde in Table 10. High pour points prevent pipeline transportation of the cmde shale oil (see Pipelines). Arsenic and iron can cause catalyst poisoning. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Most commercial methanator catalysts contain nickel, supported on alumina, kaolin, or calcium aluminate cement. Sulfur and arsenic are poisons to the catalyst, which can also be fouled by carry-over of solvent from the CO2 removal system. 

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