Coke Formation Rate

The amount of coke that is being deposited on a catalyst has been traditionally followed with conventional microbalances. However, due to the inherent limitations of this equipment, in which it is almost impossible to avoid feed by-pass effect and diffusional effects, this technique has not been very useful to determine coking kinetics as a function of feed composition. A recycle electrobalance reactor has been designed to avoid this undesirable effect . The unit was designed to operate up to 500°C and at atmospheric pressure. 

---

The hydrogen content in the gas influences the coke formation rate [7] as well. In a recent article Hou et al. [19] showed the influence of the removal of hydrogen on coking rates in a membrane steam reformer using palladium membranes. The need of a minimum concentration of hydrogen is of special importance when operating a membrane steam reformer, because it limits the process conditions at which such a reactor can be operated. 

The results show that the specificities of catalyst deactivation and it s kinetic description are in closed connection with reaction kinetics of main process and they form a common kinetic model. The kinetic nature of promotor action in platinum catalysts side by side with other physicochemical research follows from this studies as well. It is concern the increase of slow step rate, the decrease of side processes (including coke formation) rate and the acceleration of coke transformation into methane owing to the increase of hydrogen contents in coke. The obtained data can be united by common kinetic model.lt is desirable to solve some problems in describing the catalyst deactivation such as the consideration of coke distribution between surfaces of metal, promoter and the carrier in the course of reactions, diffusion effects etc,. 

The catalyst activity is determined by the coke extraction rate relative to the coke formation rate. The coke extraction rate is determined by the solubility of the coke compounds in the reaction mixture and the diffusivity of the extracted compounds through the porous catalyst. The coke formation on the catalyst occurs from the hexenes, and more significantly, from hexene oligomers formed in the fluid phase catalyzed by traces of peroxide impurities [3]. A detailed mathematical model is presented here to interpret the results presented in our earlier paper [2] and to develop a better understanding of the underlying physicochemical processes. 

TEOM can be used to continuously monitor the mass of a catalyst sample during operation. The fixed-bed characteristics of the microbalance coupled with the online GC and/or MS measurements therefore allow for simultaneous determination of catalytic reaction rates and selectivities and coke formation rates (2). TEOM has been used to provide continuous recording of such data at temperatures up to 973 K and pressures up to 60 bar. 

As regards the coke formation rate (Fig. 2a) it can be seen that an increase in the pH2 produces a shift of the rc maximum to longer times. In this way, the increase in the pm causes a reduction in the rcmax value and in the deactivation of the catalyst, so that an increase in the residual coking rate can be observed. The absence of H2 in the feed produces a smaller quantity of coke and an increase in the deactivation rate, the residual coke formation rate being much lower than in the other cases. 

With respect to H2 production (results not shown), a behavior parallel to that of the coke formation rate is observed. In this case it can be seen how an increase in the pH2 reduces both the quantity of H2 generated and the deactivation rate. 

It can be seen in Figure 4 that an increase in the reaction temperature leads to an increase in the initial reaction rate and a reduction in the initial period of very low coke production. A decrease in the residual coke formation rate can also be observed, as well as in the quantity of coke formed. 

As regards H2 production, figure 4a shows that an increase in the temperature leads to an increase in the initial hydrogen production rate. However, it also produces a faster deactivation of the catalyst and a reduction in the residual Hj production, the same as occurs with the final coke formation rate. These results are in agreement with previous results presented in the literature [18,20]. 

In a fundamental study of coke formation on a chromia-alumina dehydrogenation catalyst, the catalyst activity and coke formation rate were measured in a differential reactor [20]. The equation for the rate of coking allowed for the decrease in rate with increase in coke level and the effects of reactants and products ... 

Note that coke deposition in the MTO catalyst can lead to the coverage of part of the active sites and reduce the catalyst activity. When the coke content is sufficiently high, the methanol conversion shows a rapid decrease and most of the active sites have been covered and the catalyst becomes deactivated. From Fig. 18, we can find that in a wide range of coke content (0-8.87 wt%) the catalyst in the microreactor can maintain a high methanol conversion (>98.5%). This implies that the complete conversion of methanol can be realized with a small mass of active catalyst (and thus, a small amount of active sites) in MTO reaction. Therefore, it is important to know the coke formation rate in MTO process. Based on the data reported in Fig. 18, we can estimate the coke formation rate ... 

Figure 20 The coke formation rate as a function of the coke content. The coke formation rate is normaiized by the cataiyst ioading, and the experimental conditions are the same as in Fig. 19.

The third parameter of critical importance is the catalyst-to-methanol ratio. This parameter is the key to control the circulation rate of catalyst between reactor and regenerator in pilot-scale setup. Normally it is hard to derive the relation between the catalyst-to-methanol ratio and reaction results via direct measurement in the microscale experiments as there is no circulation. However, by analyzing the coke formation in the MTO reaction, we can predict the optimal catalyst-to-methanol ratio. FromEq. (15), we can obtain the coke formation rate. We assume that the coke formation rate can be directly used in the pilot-scale experiments. Thus, the mass flow rate of catalyst required to transport this amount of coke is estimated as following ... 

The coupled furnace-reactor simulation requires an accurate description of the heat transfer from the furnace to the reactor. The global radiative heat transfer from the furnace to the reactor was calculated by the zone method (Fig. 12.5.A-2, Left) proposed by Hottel and Sarofim [1967], To take into account the local influence of radiative heat transfer, CFD simulations of the furnace were carried out using a radiative heat transfer model for short distances [De Marco and Lockwood, 1975], Knowledge of the local flue gas composition is required to calculate the heat release by combustion in each flue gas volume element and the absorption coefficients for radiation. Coupled CFD simulations of the reactor tubes and furnace predict the process gas conversion and the product yields, as well as coke formation rates. 

The authors used the Amoco model and compared it with a model developed by Lee and Groves (1985). They parameterized the Lee model to match the more complicated Amoco model by adjusting the heat of reaction and coke formation rate constant. They also studied non-linear multivariable control of process variables that showed much inteiactioa... 

The catalytic cracking of n-dodecane on HZSM-5 zeolite at 400-450°C under supercritical and subcritical pressures (0.1. 0 MPa) was also studied in [189]. The activity of the catalyst and its stabilization toward deactivation decrease with increasing pressure, and the catalyst preserves a substantially higher activity if the feed rate exceeds 4ml/min under supercritical conditions. The contribution of supercritical extraction to the activity of the HZSM-5 catalyst increases with increasing hydrocarbon feed rates and decreasing catalytic activities, and reaches the maximum when the coke formation rate equals to the coke removal rate by a supercritical hydrocarbon. 

Annaland TV, Kuipers JAM, van Swaaij WPM A kinetic rate expression for the time-dependent coke formation rate during propane dehydrogenation over a platinum alumina monolithic catalyst, Catal Today 66(2—4) 427—436, 2001. 

Reactions were stopped when aromatics yield became about 40 C-mol%. BET total surface area and coke content were also measured about used zeolites of H-Si-Al-Ga and H-Si-Al-Ga(TPA,KOH). Coke formation rate and total surface area loss per milligram of coke are shown in Table 3. Coke formation rate and total surface area... 

On the other hand, as shown in Table 3, the rate of coke formation of H-Si-Al-Ga(TPA,KOH) treated at 343 K was faster than that of H-Si-Al-Ga, and consequently the catalyst life became shorter (Fig.l). Since SiOj/AljOj ratio of H-Si-Al-Ga(TPA,KOH) decreased with the treated temperature (Table i), it is suggested that the excess treatment with KOH solution increased the acid density of external surface and consequently the coke formation rate on them. 

---