Coke formation equilibria

The ODH of ethylbenzene to styrene is a highly promising alternative to the industrial process of non-oxidative dehydrogenation (DH). The main advantages are lower reaction temperatures of only 300 500 °C and the absence of a thermodynamic equilibrium. Coke formation is effectively reduced by working in an oxidative atmosphere, thus the presence of excess steam, which is the most expensive factor in industrial styrene synthesis, can be avoided. However, this process is still not commercialized so far due to insufficient styrene yields on the cost of unwanted hydrocarbon combustion to CO and C02, as well as the formation of styrene oxide, which is difficult to remove from the raw product. 

The processing of hydrocarbons always has the potential to form coke (soot). If the fuel processor is not properly designed or operated, coking is likely to occur (3). Carbon deposition not only represents a loss of carbon for the reaction but more importantly also results in deactivation of catalysts in the processor and the fuel cell, due to deposition at the active sites. Thermodynamic equilibrium calculations provide a first approximation of the potential for coke formation. The governing equations are ... 

Assuming that all steps in the formation of coke precursors are in equilibrium and that the final coke formation step is irreversible, the coke buildup rate [from Eq. (32)] can be expressed as... 

Several technologies differing in the method of heat supply have been developed.191-193 When superheated steam is used, it provides the necessary energy for the endothermic dehydrogenation, acts as a diluent to achieve favorable equilibrium conditions, and prevents coke formation by transforming carbon to carbon oxides. A new oxidative dehydrogenation process (Styro-Plus) is under development.194... 

The reactor temperature required to prevent coke formation varies considerably for the different processes. Table 2.1 summarizes the values calculated assuming thermodynamic equilibrium for 2,2,4-trimethylpentane reforming. Generally, the coking tendency increases in the following order at constant O/C ratio SR > ATR > POx. These calculations demonstrate that at steam to carbon ratios (S/C) > 2 and reaction temperatures > 600 °C, which is very common for hydrocarbon fuel processors, coke seems to be an unstable species especially under the conditions of steam reforming. 

A feed of 100 Ncm3 min-1 methane and 50 Ncm3 min-1 oxygen was introduced into the reactor at a pressure loss of < 2.5 mbar. The residence time of the reaction was 50 ms. 60% conversion was achieved along with a high carbon monoxide selectivity of 70% at 700 °C reaction temperature. Owing to the short residence times applied, no coke formation was observed and carbon monoxide selectivity was higher than expected from the thermodynamic equilibrium [46],... 

Both catalyst activity and tar formation are directly affected by the state of hydration of the phosphoric acid-kieselguhr type of catalyst. At the higher temperature it is more difficult to maintain proper hydration. Hydration control is required because the catalyst has an optimum water content which determines the activity and selectivity of the catalyst. The water-vapor pressure varies at different catalyst temperatures and it is important to keep the water content of the hydrocarbon in equilibrium with that of the catalyst. In those units where water of saturation in the feed is insufficient, additional water must be injected into the feed as catalyst requirements dictate. The solid phosphoric acid type of catalyst contains the proper amount of water when manufactured and the art of catalyst hydration has reached such a point that catalyst in properly operated polymerization units no longer fails from coke formation or loss of activity. 

It is also of interest to observe that the coke laydown observed experimentally is more than an order of magnitude, less than would be predicted from equilibrium calculations. That is, the amount of coke on the catalyst per liter of methane on Figure 4 at a given temperature and steam methane ratio is about 5% of that shown on Figure 2 formed under equilibrium conditions suggesting that coke formation is rate limited. 

Note that as a first approximation the effect of hydrogen is not taken into account, which implies that the model will hold only for a limited range of hydrogen pressures. As a driving force for the reaction we use the gas-phase concentration, Cq, of the coke precursor Q, is the equilibrium constant of adsorption of Q on the catalyst surface. The rate constant for coke formation, kc, depends on the amount of coke present on the surface ... 

Frequently the kinetic description of catalyst deactivation and coke formation is complicated by instationary reaction conditions prevailing during the respective experiments. In this paper two experimental methods are presented.which enable the determination of such kinetics avoiding this problem 4 Use of a concentration controled continuously operated recycle reactor 4 Experimentation at the thermodynamic equilibrium of the main reaction to determine the coke formation kinetics at well defined operating conditions... 

The methanol to DME ratio was found to increase with coke formation on the catalysts (Fig. 6b). This ratio was quite far from the ratio at chemical equilibrium, which was calculated at 425°C to be 0.47. The deviation from chemical equilibrium ratio was found to be larger at higher coke contents and on the externally precoked samples. This indicates that the methanol conversion to DME is not fast enough to reach equilibrium, probably due to the moderate external acidity of SAPO-34 and partly also due to the effect of diffusion of DME at the higher coke contents. MTO can be considered a simple sequence reaction Methanols DME- olefins... 

Important aspects in dehydrogenation entail the approach to equilibrium or near-equilibrium conversions while minimizing side reactions and coke formation. 

The removal of a product from a reaction mixture can shift the chemical equilibrium of the reaction and thus generate increased product yields or lead to similar yields, which are obtained at lower temperatures. The latter has the extra advantage of decreasing the extent of deleterious side reactions, such as coke formation. 

The fraction of CH4 feed is fed through the bottom section in such a way that the permeating O2 is able to generate CPO (catalytic partial oxidation) equilibrium conditions in the bottom section, the temperature of which is in turn favorable for the permselective O2 transport. The steam is also added to avoid coke formation in the bottom section. The top section is then fed with the remaining CH 4 and steam feed so that overall autothermal process is achieved when both sections are considered together. The endothermic heat demand of the top section is thus catered by the equilibrium mixture corning from the bottom section and the side feed of additional CH4 and steam. 

The above reactions are in equilibrium and the formation of coke via reactions (2.7) and (2.8) becomes less favoured as the temperature increases. However, coke formation via reactions (2.5) or (2.6) becomes increasingly important at higher temperatures and, depending on the nature of the feed, can rapidly deactivate the SR catalyst and block the reactor [12]. 

Carbon removal by the reverse of reactions 6, 7 and 8 is possible, and operating conditions are generally adjusted to ensure that the feed and product gas compositions are far from values that, thermodynamically, would favour carbon formation (critical carbon limit) [1]. However, the approach to equilibrium is kinetical 1y controlled and, depending on the feed and on local conditions in the reactor, coke formation can and does occur [1,3,4]. 

In contrast to the results at 400°C, no reduction in asphaltene molecular weight was observed for residence times up to 40 minutes for reactions conducted at 425°C (see Fig. 9.2). This means that at higher temperatures, polycondensation reactions proceed faster than decomposition reactions. At any temperature, the determined molecular weight of asphaltenes shows that it reaches equilibrium as the reaction proceeds. This implies that at a longer residence time, the molecular weight of the asphaltene fraction will not increase any further because after achieving equilibrium molecular weight, they become less soluble in the maltenes. This leads to their flocculation from the maltenes fraction (Fig. 9.2) and, finally, to coke formation. 

The investigation into the influence of paraffinic plastics on asphaltene chemistry during thermal cracking showed that pure plastics affect only the equilibrium of alkylation reactions by the increase of the paraffinic radicals in the reaction zone (Figure 9.18). This means that asphaltene decomposition will be slowed down. As such, there will be no decomposition to form aromatic cores without paraffinic periphery. This decelerates polycondensation and coke formation during the thermal treatment of mixtures of vacuum residue and plastics. However, it does not promote the cracking of the asphaltenes. 

Increasing the length of the n-paraffin increases the value of the equilibrium constant K. The presence of hydrogen is unfavorable because it decreases the conversion at thermodynamic equilibrium, but the process is carried out under hydrogen pressure to decrease deactivation by coke formation. 

Syngas can be obtained as equilibrium product of reaction (1). The main requirement for a syngas catalyst is therefore to activate methane and oxygen under conditions where the equilibrium composition is favorable, and where the catalyst is not active for total oxidation (reaction 2) or coke formation. 

Equilibrium considerations suggest that the production of a high Btu gas product from benzene without coke formation will require operation at several hundred atmospheres and at relatively low conversions of benzene per pass, using hydrogen partial pressures below those needed for stoichiometric conversion of benzene to methane. 

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