Intrinsic deactivation rate

TT-mordenite, which contained 11.2 wt % alumina, was dealuminated by A A heating for 4 hours at 700° C and then leaching under 6N HC1 reflux to 0.1 wt % alumina content (1) (aluminum-deficient H-mordenite). The intrinsic cracking activities of the parent H-mordenite and acid-leached H-mordenites are proportional to aluminum content (2) (i.e., the associated Bronsted acid centers). The deactivation rate of the aluminum-deficient H-mordenite is much lower than that of the parent H-mordenite. 

In the following paragraphs, we will investigate the relationship between the observed deactivation rate constant kd obs and the intrinsic deactivation rate constant kd. 

Potential pitfalls exist in ranking catalysts based solely on correlations of laboratory tests (MAT or FFB) to riser performance when catalysts decay at significantly different rates. Weekman first pointed out the erroneous conversion ranking of decaying catalysts in fixed bed and moving bed isothermal reactors (1-3). Phenomena such as axial dispersion in the FFB reactor, the nonisothermal nature of the MAT test, and feedstock differences further complicate the catalyst characterization. In addition, differences between REY, USY and RE-USY catalyst types exist due to differences in coke deactivation rates, heats of reaction, activation energies and intrinsic activities. 

Estimates of Model Parameters. The reactor models for FFB, MAT and riser include important features for translating the MAT and FFB data to steady state riser performance. A series of key parameters specific to a given zeolite and matrix component are needed for a given catalyst. Such key parameters are intrinsic cracking anc( coking activities (kj, A ), activation energies and heats of reaction (Ej, AHj), coke deactivation rate (exponents nj), and axial dispersion in the FFB unit (DA). Other feedstock dependent parameters include the inhibition constants (kHAj), the coking constants (XAj), and the axial molar expansion factor (a). 

When one metal ion is used as a donor for sensitizing the emission of a second accepting metal ion, the characteristic lifetimes r of their excited states, which are related to their deactivation rates by r = k l, are affected by the metal-to-metal communication process. This situation can be simply modeled for the special case of an isolated d-f pair, in which the d-block chromophore (M) sensitizes the neighboring lanthanide ion (Ln) thanks to an energy transfer process described by the rate constant k 1 ". In absence of energy transfer, excited states of the two isolated chromophores decay with their intrinsic deactivation rates kxl and kLn, respectively, which provides eqs. (32) and (33) yielding eqs. (34) and (35) after integration ... 

This situation is encountered for a large number of d-f pairs, because the intrinsic deactivation rates of the d-block donor kM are often considerably larger than the deactivation of the Ln-centered 4f excited states. For instance, it occurs for the Ru Yb... 

The replacement of Ru11 with Cr111 as the donor in [CrYb(115)3]6+ illustrates this second situation. The combination of the intrinsic deactivation rate of the Cr-centered donor levels kCr = 2.7 x 102 s-1 (in [CrGd(115)3]6+), with the rate of energy transfer fceCtr Yb = 2.4 x 102 s-1 gives k = kCr + = 5.1 x 102 s 1. This remains... 

Low conversion levels are recommended to compare intrinsic activities or deactivation rates in PFRs since these are disguised due to the integral reactor behavior. For example, a factor of two in activity yields, for a first order irreversible reaction, pairs of conversion levels of 90 and 99%, 30 and 51% or 5 and 9.75%, depending on the used space velocity. 

On the other hand, uniform or homogeneous catalyst poisoning presumes that the poison precursor species has full access to the catalyst interior before deactivation begins. There is no dif-fusional resistance for this species. This will be more likely to occur when the pores are large, the catalyst pellets small, and the intrinsic deactivation rate is low. In addition smaller poison precursor molecules will be able to diffuse more rapidly into the catalyst interior. Here the Thiele modulus for poison laydown h will be small, and in the limit, zero. 

A series of CoMo/Alumina-Aluminum Phosphate catalysts with various pore diameters was prepared. These catalysts have a narrow pore size distribution and, therefore, are suitable for studying the effect of pore structure on the deactivation of reaction. Hydrodesulfurization of res id oils over these catalysts was carried out in a trickle bed reactor- The results show that the deactivation of reaction can be masked by pore diffusion in catalyst particle leading to erro neous measurements of deactivation rate constants from experimental data. A theoretical model is developed to calculate the intrinsic rate constant of major reaction. A method developed by Nojcik (1986) was then used to determine the intrinsic deactivation rate constant and deactivation effectiveness factor- The results indicate that the deactivation effectiveness factor is decreased with decreasing pore diameter of the catalyst, indicating that the pore diffusion plays a dominant role in deactivation of catalyst. 

Fig.4. Intrinsic deactivation rate constant and apparent deactivation rate constant v.s. average pore diameter.

Although the deactivation of Industrial catalysts is often due to two or more different causes, the modeling of simultaneous deactivation phenomena has not been widely studied (refs. 1, 2). The occurrence of two different deactivation processes not only adds another level of complexity to the determination of the intrinsic kinetic behavior but also complicates the interpretation of the experimental results. In our previous studies regarding the thloresistance of naphtha reforming catalysts (refs. 3, 4) we have shown that the activity decay caused by the presence of sulfur compounds in the feed is often accompanied by coking. In this situation, the thioresistance cannot be obtained in a simple way from the deactivation curves. The characteristics of the sulfur poisoning have to be deduced from the overall deactivation rate. 

From the values of TOF, the increasing order of activity for the fresh catalysts in the hydrogenation of ethylbenzene is Pd < Ni < Pt < Rh < Ru, Concerning the deactivation process, the intrinsic order of sulfur resistance appears to be Ru Rh Ni > Pd > Pt. On the other hand, the half deactivation time and the catalyst life decrease in the order Rh > Ru Pd Ni > Pt. This difference is due to the fact that the lifetime of a particular catalyst is an extensive property, which depends both on the deactivation rate constant (k l and the initial number of exposed metal atoms (N). Finally, we want to point out the small differences in the activity found for the fresh catalysts (CRu/CPd - 1.6), as compared with the greater values of their sulfur resistance (CRu/CPd = 6.5 or CRu/CPt = 12.5). 

Although, formally, the integral in Eq. (2.9) is over the range [0,00], the domain of integration may be shortened via three mechanisms.4 First, the effective lifetime of the wavepacket on the excited-state potential energy surface is limited by radiative decay rate and/or the collisional deactivation rate of the excited electronic state these effects can be represented by a phenomenological lifetime, T 1. Second has an intrinsic decay that... 

Generally, as the Thiele modulus of main reaction is smaller than unity, the order of deactivation is equal to unity and, therefore, the apparent deactivation rate constant can he extracted from the slope of -In Xs(t) versus t Krishnaswamy and Kittrell (ref. 10) have shown that the relationship between the intrinsic deactivation rate constant k and the apparent deactivation rate constant kda can be expressed as... 

In the simplest case, the catalytic activity is proportional to the number of active sites Nj, intrinsic rate constant and the effectiveness factor. Catalyst deactivation can be caused by a decrease in the number of active sites, changes in the intrinsic rate constant, e.g. changes in the ability of surface sites to promote catalysis and by degradation in accessibility of the pore space. When the reaction and deactivation rates are of different magnitudes, the reactions proceed in seconds while the deactivation can require hours, days or months, and moreover the deactivation does not affect the selectivity, the concept of separability is applied. The reaction rates and deactivation are treated by different equations. A quantity called activity, (a) is introduced to account for changes during the reaction. 

In situ poisoning experiments were carried out on MgY zeolite by doping the toluene/methanol mbdure with either an acid (acetic acid) or a lase (3,5-dimethyl pyridine). Fresh catalysts were initially tested for about 3 h using a pure toluene/methanol mixture before introducing the doped feed. The activity was defined as a = ro/ro, where r and r(t) are the reaction rates at zero time and time t, respectively. The MgY activity diminished with time on stream when using undoped reactants because of the formation of carbonaceous deposits. Hence, when a teic confound is added into the feed, a simultaneous deactivation process by coke and poison takes place. The activity decay caused by 3,5-dimethyl pyridine (3,5-DMP) alone can not be obtained directly fiom the experimental data. To estimate the poison intrinsic effect, it can be assumed that both effects are additive, which implies that the overall deactivation rate is a simple sum of each individual rates (hypothesis of independence) [19]. According to mechanistic deactivation models [20], the overall deactivation rate is expressed as follows ... 

These equations are again based on a pseudo steady-state approximation so that intrinsically the deactivation rate must be much slower than the diffusion or chemical reaction rates. The equations can be easily solved, as in Chapter 3, and the result substituted into the definition of the effectiveness factor, with the following results ... 

---