Coke-conversion selectivity

A theory has been developed which translates observed coke-conversion selectivity, or dynamic activity, from widely-used MAT or fixed fluidized bed laboratory catalyst characterization tests to steady state risers. The analysis accounts for nonsteady state reactor operation and poor gas-phase hydrodynamics typical of small fluid bed reactors as well as the nonisothermal nature of the MAT test. Variations in catalyst type (e.g. REY versus USY) are accounted for by postulating different coke deactivation rates, activation energies and heats of reaction. For accurate translation, these parameters must be determined from independent experiments. 

This work provides conclusive evidence that transient catalyst characterization tests can result in erroneous catalyst ranking. For example, USY catalysts show higher activity than REY catalysts in the laboratory tests but lower activity in a steady state riser. Although emphasis in this paper is placed mainly on the coke-conversion selectivity, the analysis is also extended to yields of other FCC products. 

In this paper, we will first illustrate the mathematical models used to describe the coke-conversion selectivity for FFB, MAT and riser reactors. The models also include matrix and zeolite contributions. Intrinsic activity parameters estimated from a small isothermal riser will then be used to predict the FFB and MAT data. The inverse problem of predicting riser performance from FFB and MAT data is straightforward based on the proposed theory. A parametric study is performed to show the sensitivity to changes in coke selectivity and heat of reaction which are affected by catalyst type. We will highlight the quantitative differences in observed conversion and coke-conversion selectivity of various reactors. 

A parameter kc representing coke-conversion selectivity, is defined as ... 

For each test, the first column represents the observed catalyst activity and the second column represents coke conversion selectivity given by Equation 25. 

From the above table, the FFB test would rank the catalysts with respect to decreasing catalyst activity in the order C, B, A. The MAT test on the other hand would rank them in the order C, A, B. However, the true ranking, as seen in the riser test, is A, C, B. Thus, both MAT and FFB rank the catalysts incorrectly with respect to catalyst activity. Although the relative magnitude of kg parameters is different, the rankings by coke conversion selectivity is correct for all tests. These results are not unexpected and can be explained by the mathematical model presented above. 

REY Catalysts. REY catalysts always give lower FFB conversions than MAT conversions (Figure 1). To simulate the observed conversion differences in the MAT and FFB units for REY catalysts, the intrinsic cracking activity kj is increased at constant coking activity Aj. This choice of parameters is a first order approximation for the activity and coke-conversion selectivity variation of equilibrium catalysts. Parameters summarized in Table V are used as the initial starting point. 

In Figure 2, we compare the coke-conversion selectivity behavior as a function of activity for MAT, FFB and riser reactors. The kc is relatively flat for the FFB test, with correspondingly higher negative slopes for MAT and steady state risers. The riser results show the steepest slope indicating that even though the kg observed in the FFB test is relatively independent of activity, the coke selectivity improves with activity in a riser. 

Figure 2 Predicted coke-conversion selectivity as a function of catalyst activity (crackability) for REY catalysts.

In Figure 6, we show the results of a simulation in which the cracking activity kj was varied while keeping the coking activity Aj constant (similar to the simulation above for REY catalysts). It is interesting to note that for the choice of the parameters noted above, kg values in MAT and riser closely trace each other. On the other hand, FFB coke-conversion selectivity, kg, has somewhat lower slope. 

In the preceding sections we concentrated mainly on the coke-conversion selectivity aspects of various reactors. In this section we will briefly discuss the selectivities of major FCC products gasoline, LCO, HCO, and light gases in both transient and steady state reactors. Weekman (1-3) has looked at steady state and transient reactors for gasoline selectivity shifts. However, he did not include axial dispersion effects in his analysis which are important for laboratory FFB reactors, and are accounted for here. 

FCC catalyst testing prior to use in commercial reactors is essential for assuring acceptable performance. Purely correlative relations for ranking catalysts based on laboratory tests, however, can be erroneous because of the complex interaction of the hydrodynamics in the test equipment with the cracking kinetics. This paper shows how the catalyst activity, coke-conversion selectivity and other product selectivities can be translated from transient laboratory tests to steady state risers. Mathematical models are described which allow this translation from FFB and MAT tests. The model predictions are in good agreement with experimental data on identical catalysts run in the FFB, MAT and a laboratory riser. 

The effect of temperature on the conversion, selectivity and yield after 3 hours on stream is shown in figure 2. In each case a catalyst mass of 0.1g boria on alumina catalyst was tested. With increasing reaction temperature the oxime conversion increased, however, maxima in lactam selectivity and yield were observed at a reaction temperature of 300 C. At higher temperatures excessive coking and side reactions were thought to occur,... 

However there is a particular effect of coke on selectivity experimentally shown constant conversion. Coke decreases the gasoline and coke selectivity and increases the gas selectivity. 

It is well known in industrial media that a large catalyst to oil circulation ratio (C/0) enhances the gasoline yield and selectivity, as the coke yield is kept constant. This effect cannot be described by kinetics if we consider that is the same for all reactions. In this case the curve yield versus conversion would remain unaffected by the C/0. To demonstrate the effect of coke on selectivity, a second set of experiments was done where the variations of conversion were obtained by variation of the equilibrium catalyst mass in the MAT (initial c=0). The comparison of the two sets shown on the figure 4, demonstrates unambiguously that the coke drops the gasoline and coke selectivitres and enhances the gas selectivity. This is a direct laboratory representation of the industrial effect of C/O. In industrial plants, the final coke content of the catalyst is inversely proportional to the C/O because the coke yield is maintained constant to equilibrate the thermal balance of the plant. 

To take into account this particular effect of coke ori selectivity it is necessary to use different 4>jj tor each reaction. A direct determination of all parameters Aij, Bij, kij by optimisation would be impossible because of their targe number. The analytic resolution of the equations giving jj is r>ot possible because of the existence of successive reactions. An experimental method to obtain the 4>ij functions may be to get data with and without initial coke content at lower conversions (for 4>ij) and to study the gasoline cracking (for 4>2j)... 

Zeolite OH groups (mmol/g) metal cation (wt.%) conversion (%) p-ET coke amount selectivity (mg/g) (wt.%) ... 

Figure 3. Partially coked and non-coked samples in pseudocumene conversion. Selectivity for isomerization products versus conversion and relationship between conversion and the number of external acid sites.

Table 1 presents the n-hexane conversion, selectivity to isomers and coke deposited after reaction for catalysts prepared by using two different platinum precursors tetraammine platinum nitrate and hexachloroplatinic acid. Both materials were calcined at different temperatures after platinum addition. For both platinum precursors, run under standard operational conditions, the optimum calcination temperature for catalytic activity was 500 °C. The amount of coke is small and the TPO profiles of the coked samples (not shown) are similar for all catalysts. Coke is completely burnt off at temperatures below that at which the catalyst was calcined after the metal addition. This is an important feature, because regeneration procedures would not affect the metal function. 

Relative acidity at175 C Si02 [%] Na20 [%] Conversion [%] Selectivities Ethene Ether [%] [%] C(coke) [%]... 

Laboratory studies indicate that a hydrogen-toluene ratio of 5 at the reactor inlet is required to prevent excessive coke formation in the reactor. Even with a large excess of hydrogen, the toluene cannot be forced to complete conversion. The laboratory studies indicate that the selectivity (i.e., fraction of toluene reacted which is converted to benzene) is related to the conversion (i.e., fraction of toluene fed which is reacted) according to ... 

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