Silver Catalyst Operation

Reactors consist of a 4-m wide catalyst bed, filled with a 3-cm layer of 0.5-to 5-mm silver granules or a pack of silver gauzes. Space velocity through the bed is as high as 300,000 h to minimize formaldehyde loss. Process gas at 560-680 C must be rapidly cooled in a waste heat boiler irmnediately it leaves the reaction zone to avoid thermal decomposition of the product. 

The product contains up to 50% formaldehyde after distillation. By recirculating the vent gases, the operating temperature can be reduced and the methanol conversion increased. This results in an increase in the concentration of formaldehyde in the product to 55%. Methanol conversion is, however, limited to 70% in a single reactor. If a second reactor and additional air is added to give a higher methanol conversion, distillation can be avoided, although yields are reduced. 

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Unsteady-State Direct Oxidation Process. Periodic iatermption of the feeds can be used to reduce the sharp temperature gradients associated with the conventional oxidation of ethylene over a silver catalyst (209). Steady and periodic operation of a packed-bed reactor has been iavestigated for the production of ethylene oxide (210). By periodically varyiag the inlet feed concentration of ethylene or oxygen, or both, considerable improvements ia the selectivity to ethylene oxide were claimed. 

Ethylene is currently converted to ethylene oxide with a selectivity of more than 80% under commercial conditions. Typical operating conditions are temperatures in the range 470 to 600 K with total pressures of 1 to 3 Mpa. In order to attain high selectivity to ethylene oxide (>80%), alkali promoters (e.g Rb or Cs) are added to the silver catalyst and ppm levels of chlorinated hydrocarbons (moderators) are added to the gas phase. Recently the addition of Re to the metal and of ppm levels of NOx to the gas phase has been found to further enhance the selectivity to ethylene oxide. 

Lefort A process for making ethylene oxide by oxidizing ethylene in the presence of a silver catalyst. Invented and developed in the 1930s by T. E. Lefort at the Societe Frangaisc de Catalyse. For maty years, refinements of this basic process were operated in competition with the ethylene chlorohydrin process, but by 1980 it was the sole process in use. 

The reduction of silver ion in a sulfite solution is catalyzed by silver, (Sheppard, 3 James, 12) but the catalysis is less pronounced than that operating in the reactions of the developing agents already considered. The sulfite reaction is very slow at 20°, even in the presence of large amounts of colloidal silver. Data obtained at 59.8° indicate a dependence of rate upon the 0.75 power of the silver ion concentration and upon the first power of the sulfite ion concentration. The reaction, even in the presence of silver catalyst, is further catalyzed by copper salts. 

Mechanisms There is a derth of knowledge about the mechanisms operative in selective oxidation reactions. The only exceptions are the reactions of ethylene to ethylene oxide on supported silver catalysts and of propylene to acrolein on bismuth molybdate type catalysts. For the latter, it is well established through isotopic labeling experiments that a symmetric allyl radical is an intermediate in the reaction and that its formation is rate-determining. Many studies simply extrapolate the results substantiated for this case to other reactions. New ideas on mechanisms are presented by Oyama, et oL, Parmaliana, et aL, and Laszlo. 

Fig. 1. Process for making formaldehyde, using a silver catalyst- Developed and operated by Imperial Chemical Industries Limited and available through Davy McKee (Oil) and Chemicals Ltd. London. England...

Heterogeneous oxidative processes operate at high temperatures (250-450 6C) and are useful for the synthesis of acrolein and acrylic acid from propylene over bismuth molybdate catalysts, the synthesis of maleic and phthalic anhydrides from the oxidation of benzene (or C4 compounds) and naphthalene (or o-xylene) respectively over vanadium oxide,101 arid the synthesis of ethylene oxide from ethylene over silver catalysts.102... 

Several companies are working on the direct oxidation of propene for instance Lyondell is operating a pilot plant in Newtown Square, PA, and intends to commercialize the technology by 2010. Shell Chemical is also working on a direct route to PO production, based on variations of the gold and silver catalysts it uses to make ethene oxide. 

As far as finding improved catalysts or operating conditions is concerned, a variety of experimental studies have been conducted. Promotion by alkali metal salts [11], especially cesium [12,13], has been shown to increase the EO selectivity. Similarly, it is observed that addition of chlorine to the catalyst results in higher EO selectivity [14-18]. Atmospheric pressure experiments in our laboratory have indicated that Cu-Ag bimetallic catalysts are more effective than pure Ag [19]. Furthermore, it is observed that after addition of Cs and Cl to the Cu-Ag bimetallic catalyst, it outperforms the corresponding promoted silver catalyst [20]. Published experimental studies have t)q)ically focused on a limited number of catalyst components, whereas one could potentially explore a wide variety of catalyst combinations using theoretical tools. 

Formaldehyde is nowadays one of the major produced chemicals due to its uses in many fields of chemical industry [1]. The commercial production started in 1890 using metallic copper catalysts. In 1910 copper catalysts were replaced by silver catalysts with higher yields [2]. Although the first report of the excellent catalytic behavior of iron molybdates in selective oxidation of methanol to formaldehyde is of 1931, the related industrial process based on them only went into operation in 1940-50 [1]. A recent report [3] shows that iron molybdates and silver catalysts are nowadays equally used as industrial catalysts for formaldehyde production. 

The oxidation of mixtures of ethylene with ethylene oxide, formaldehyde and acetaldehyde on a stationary surface of a silver catalyst that was under operation for several hundred hours is of a different nature. The reaction rates for mixtures of radioactive ethylene with ethylene oxide, as well as with formaldehyde and acetaldehyde, are shown in Fig. 7. Oxidation proceeded under dynamic conditions at 220°. 

Boreskov et al. Fluidized catalyst operation using silver inside porous alumina (24)... 

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