Kinetic coke

Coke-Conversion Kinetics. Coke formation kinetics and gas oil conversion are represented by the following irreversible reactions (cracking and coking) ... 

Modified catalysts possess high activity and selectivity to mono-olefins. The major by-products are diolefins that can be controlled kinetically. Coke formation is also suppressed, and therefore, stability is greatly improved. Over modified catalysts, the major reaction pathways for both light and heavy paraffin dehydrogenation systems are simpler (Fig. 7). 

Coke Kinetic coke (produced by reaction scheme) Metal coke (produced by metal activity on catalyst)... 

Vary the reaction selectivities for reaction paths (3 parameters) that lead to coke lumps (kinetic coke and metal coke), gasohne (G lump) and VGO (PH, NH, AHs, AHrl, AHr2 and AHr2 lumps) deactivation activity factors (2 para-... 

KcoKE Deactivation function due to kinetic coke, unitless McoKE Deactivation function due to metal coke, unitless Qcoke Kinetic coke on catalyst, kg kinetic coke/kg catalyst Cmcoke Metal coke on catalyst, kg metal coke/kg catalyst METALS Metals composition on catalyst ppm metals/kg catalyst kcoke Activity factor due to kinetic coke, unidess mcoke Activity factor due to metal coke, unidess E Murphree stage efficiency factor... 

C, 0.356—1.069 m H2/L (2000—6000 fU/bbl) of Hquid feed, and a space velocity (wt feed per wt catalyst) of 1—5 h. Operation of reformers at low pressure, high temperature, and low hydrogen recycle rates favors the kinetics and the thermodynamics for aromatics production and reduces operating costs. However, all three of these factors, which tend to increase coking, increase the deactivation rate of the catalyst therefore, operating conditions are a compromise. More detailed treatment of the catalysis and chemistry of catalytic reforming is available (33—35). Typical reformate compositions are shown in Table 6. 

The hydrocarbon feed rate to the reactor also affects the burning kinetics in the regenerator. Increasing the reactor feed rate increases the coke production rate, which in turn requires that the air rate to the regenerator increase. Because the regenerator bed level is generally held constant, the air residence time in the dense phase decreases. This decrease increases the O2 content in the dilute phase and increases afterbum (Fig. 5). 

In the modern unit design, the main vessel elevations and catalyst transfer lines are typically set to achieve optimum pressure differentials because the process favors high regenerator pressure, to enhance power recovery from the flue gas and coke-burning kinetics, and low reactor pressure to enhance product yields and selectivities. 

The catalysts used in this CCR commercial service must meet several stringent physical property requirements. A spherical particle is required so that the catalyst flows in a moving bed down through the process reactors and regenerator vessel. These spheres must be able to withstand the physical abuse of being educated and transferred by gas flow at high velocity. The catalyst particles must also have the proper physical properties, such as particle size, porosity, and poresize distribution, to achieve adequate coke combustion kinetics. 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

There is no mention in these reviews of any industrial implementation of supercritical kinetics. Two areas of interest are wastewater treatment—for instance, removal of phenol—and reduction of coking on catalysts by keeping heavy oil decomposition products in solution. 

The kinetics of this reaction, which can also be regarded as an erosion reaction, shows die effects of adsorption of the reaction product in retarding the reaction rate. The path of this reaction involves the adsorption of an oxygen atom donated by a carbon dioxide molecule on die surface of the coke to leave a carbon monoxide molecule in the gas phase. 

As illustrated in Fig. 1, the activated carbon displays the highest conversion and selectivity among all the catalysts during the initial reaction period, however, its catalytic activity continues to decrease during the reaction, which is probably caused by coke deposition in the micropores. By contrast, the reaction over the CNF composites treated in air and HN03 can reach a pseudo-steady state after about 200 min. Similiar transient state is also observed on the CNFs and the untreated composite. Table 3 collects the kinetic results after 300 min on stream over catalysts tested for the ODE, in which the activity is referred to the BET surface area. The air-treated composite gives the highest conversion and styrene selectivity at steady state. 

Dumez, F.J. and G.F. Froment, "Dehydrogenation of 1-Butene into Butadiene. Kinetics, Catalyst Coking, and Reactor Design", Ind Eng. Chem. Proc. Des. Devt., 15,291-301 (1976). 

Using a "home made" aneroid calorimeter, we have measured rates of production of heat and thence rates of oxidation of Athabasca bitumen under nearly isothermal conditions in the temperature range 155-320°C. Results of these kinetic measurements, supported by chemical analyses, mass balances, and fuel-energy relationships, indicate that there are two principal classes of oxidation reactions in the specified temperature region. At temperatures much lc er than 285°C, the principal reactions of oxygen with Athabasca bitumen lead to deposition of "fuel" or coke. At temperatures much higher than 285°C, the principal oxidation reactions lead to formation of carbon oxides and water. We have fitted an overall mathematical model (related to the factorial design of the experiments) to the kinetic results, and have also developed a "two reaction chemical model". 

In this study the oil yield decreased with reaction time, as oil was polymerized at higher temperature for Miike, Taiheiyo and Hikishima coals. Thus the kinetic models (Case 2 or 3) which involve two steps of resini-fication and coking correlated data reasonably well for above coals, whereas for Morwell and Bukit Asam coals, Case 5 is more suitable. 

There are several factors that may be invoked to explain the discrepancy between predicted and measured results, but the discrepancy highlights the necessity for good pilot plant scale data to properly design these types of reactors. Obviously, the reaction does not involve simple first-order kinetics or equimolal counterdiffusion. The fact that the catalyst activity varies significantly with time on-stream and some carbon deposition is observed indicates that perhaps the coke residues within the catalyst may have effects like those to be discussed in Section 12.3.3. Consult the original article for further discussion of the nonisothermal catalyst pellet problem. 

Propylene cokage experiments followed by gravimetry have shown that higher is the 5A zeolite calcium content, higher are the cokage kinetics and carbon content inside the pores (Fig. 1). The total carbon contents retained in the porosity after desorption at 350°C of physisorbed propylene are 14.5% and 11% for 5A 86 and 5A 67 samples respectively. These carbon contents are relatively important and probably come from the formation of heavy carbonaceous molecules (coke) as it has been observed by several authors [1-2], The coke formation requires acid protonic sites which seems to be present in both samples but in more important quantity for the highly Ca-exchanged one (5A 86). 

Weisz and co-workers organized the kinetic experiments to study the key underlying phenomena of coke burning, independent of other complicating phenomena. Their first step in this direction was to recognize that... 

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