Ethylene oxide formation

Electrochemical Process. Several patents claim that ethylene oxide is produced ia good yields ia addition to faradic quantities of substantially pure hydrogen when water and ethylene react ia an electrochemical cell to form ethylene oxide and hydrogen (206—208). The only raw materials that are utilized ia the ethylene oxide formation are ethylene, water, and electrical energy. The electrolyte is regenerated in situ ie, within the electrolytic cell. The addition of oxygen to the ethylene is activated by a catalyst such as elemental silver or its compounds at the anode or its vicinity (206). The common electrolytes used are water-soluble alkah metal phosphates, borates, sulfates, or chromates at ca 22—25°C (207). The process can be either batch or continuous (see Electrochemicalprocessing). 

At lower temperatures (260°C) higher operating pressures (5 bar) and high C2H4 to 02 ratios (Fig. 8.42) ethylene oxide formation and C02 formation both exhibit electrophobic behaviour over the entire Uwr range 47 Both rates vary by a factor of 200 as UWr is varied by 0.6 V (p varies between 3 and 0.015). The selectivity to ethylene oxide exhibits two local maxima 47 More interestingly, acetaldehyde appears as a new product47... 

Even for the well-known ethylene oxide formation, improvements in space-time yield were reported. A value of 0.78 t/(h m ) using an oxidative modified silver was obtained, which exceeds considerably the industrial performance of 0.13-0.261 h m [159]. 

However, these investigations also point out that we need a proper definition of space-time yields for micro reactors. This refers to defining what essentially the reaction volume of a micro reactor is. Here, different definitions lead to varying values of the respective space-time yields. Following another definition of this parameter for ethylene oxide formation, a value of only 0.13 t h m is obtained -still within the industrial window [159, 162, 163]. 

Beneficial Micro Reactor Properties for Ethylene Oxide Formation... 

GP 2] [R 2] New microstructured silver platelets have no initial activity for ethylene oxide formation [26, 40]. After treatment with the OAOR process, a small increase in activity was observed. After 1000 h of operation under oxygen conditions, larger amounts of ethylene oxide were produced. 

These mostly negative effects are explained as being due to the mechanistic characteristics of ethylene oxide formation, having a reaction order lower than one [102],... 

Emission rate enhancement, 14 852 Emissions. See also Emission Emissions. See also Fugitive emissions from the chloride process, 25 63 dioxin and furan, 13 181 effect of fuels on, 26 719-721 from ethylene oxide formation, 10 653-654... 

Let us consider a more complicated case, similar to Temkin s proposed system (5) for ethylene oxide formation, but without any prior assumption about the direction of any step in it. The overall reaction space remains the same as in Example 6, but there are additional intermediates. In particular, acetaldehyde (CH3CHO) is an intermediate which is not bound to the catalyst. Its role still requires clarification, as indicated in recent studies by Wachs and Chersick (24), but, whether or not Temkin s scheme proves to be correct, it illustrates our method. The steps are as follows ... 

Preceding the discussion of the two proposed mechanisms of ethylene oxide formation, the different forms of oxygen and their suggested roles in the oxidation process will be reviewed briefly. 

Although these reactions have been researched extensively and are the subjects of numerous patents, the precise reaction mechanism is not fully understood. The controversy has mostly centered on the nature of the oxygen species responsible for ethylene oxide formation (103). The results of various surface characterization studies indicate that there are at least three types of adsorbed oxygen species on silver monoatomic chemisorbed oxygen, diatomic (molecular) oxygen, and subsurface oxygen. The first results from a dissociative adsorption of oxygen on a silver surface ... 

In contrast to lead, the possible poisoning by metallic elements, derived from the vehicle system, is not easily documented. Many formulations of automotive catalysts contain both base and noble metals, but the detailed effect of such combinations on the particular reactions is rarely known with precision. One study was concerned with the effect of Cu on noble metal oxidation catalysts, since these, placed downstream from Monel NOx catalysts, could accumulate up to 0.15% Cu (100). The introduction of this amount of Cu into a practical catalyst containing 0.35% Pt and Pd in an equiatomic ratio has, after calcination in air, depressed the CO oxidation activity, but enhanced the ethylene oxidation. Formation of a mixed Pt-Cu-oxide phase is thought to underlie this behavior. This particular instance shows an example, when an element introduced into a given catalyst serves as a poison for one reaction, and as a promoter for... 

What safety precautions should you take with the ethylene oxide format tion discussed in Example 4-6 With the bromine cyanide discussed in Example 4-11 ... 

In order to assess whether a catalyst optimized for epoxybutene formation was active and selective for ethylene oxide formation as well as whether a state of the art ethylene oxide catalyst (11) was active and selective for epoxybutene production, the two different catalysts were evaluated and the data summarized in Table 3. The ethylene oxide catalyst showed excellent performance (even at one atmosphere pressure) for the formation of ethylene oxide, yet was virtually inactive... 

In the presence of ethylene oxide, formaldehyde, and acetaldehyde, the rate of ethylene consumption remains unchanged. There is also no inhibition of ethylene oxide formation at its gas-phase concentration of about 1%. This is probably due to the blocking off of the most active surface sites. 

The electron work function is related to oxygen pressure A = ylogC0t Substituting this value into the equation for the rate of ethylene oxide formation, we obtain... 

When y = 1 the reaction rate will be independent of oxygen concentration. It is also necessary to take into account the effect of reaction products on kinetics of the reaction. The rate of ethylene oxide formation is known to be inhibited both by ethylene oxide and C02. The measured shifts of with adsorption of these substances on silver have shown... 

Equation (2.57) fits the data with an average error of 5%. The model based on molecular oxygen gives an equation with an average error of 10%. However, the difference in fit is not great, and the exact mechanism of ethylene oxide formation is still uncertain. 

Figure 2.14 Test of Eq. (2.56) for ethylene oxide formation (From Ref. 11.) Reproduced with permission of the American Institute of Chemical Engineers. Copyright 1971 AlChE. Aii rights reserved. 

Poisoning can affect the selectivity as well as the rate of conversion, and mild poisoning may be beneficial. The oxidation of ethylene is carried out using silver catalysts that are deliberately poisoned with chlorine compounds, and the selectivity is improved, because the total oxidation reaction is suppressed more than the rate of ethylene oxide formation [14]. The presence of sulfur compounds changes the selectivity for competitive hydrogenation, such as the hydrogenation of acetylenes or diolefins in the olefins [15]. 

Figure I Comparison of predicted relative rate of ethylene oxide formation based on Equation (k) with experimental data (lines are predicted rates) (from Klugherz and Harriott [37]). 

Ag Au ethylene oxidation higher selectivity of ethylene oxide formation... 

The first step involves formation of ethylene oxide, which subsequently reacts with water. The overall reaction yield is low (65-75%), because during ethylene oxide formation a considerable amount of ethylene is oxidized to CO2. 

Hernandez Carucd, J.R., Halonen, V., Eranen, K., Wama, J., Ojala, S., Huuhtanen, M., Keiski, R., and Salmi, T. (2010) Ethylene oxide formation in a microreacton from qualitative kinetics to detailed modeling. 

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