HDM Reactions

Because porosity and metal-storage capacity associated with it are crucial for HDM reactions, carbon supports may offer some advantages compared with y-Al203 supports. Thus, various methods suitable for tailor-making of the surface properties of carbons have been developed and successfully applied. 

For both the carbon and y-Al203-supported catalysts, a small amount of metal will be removed with the aid of H2 and H2S not requiring the involvement of catalyst,whereas the predominant portion of metal will be released with the aid of a catalyst. Apparently, this portion of metals will deposit on the exterior of a catalyst in the form of fine particles. 

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A better understanding of the chemical nature of the metal compounds, the mechanisms of HDM reactions, and metal deposition phenomena would establish a basis for developing improved hydroprocessing catalysts and reactors. A goal of research in this area is to develop catalysts with greater metals tolerance and operational life in reactors. 

In Section IV, the kinetics and mechanisms of catalytic HDM reactions are presented. Reaction pathways and the interplay of kinetic rate processes and molecular diffusion processes are discussed and compared for demetallation of nickel and vanadium species. Model compound HDM studies are reviewed first to provide fundamental insight into the complex processes occurring with petroleum residua. The effects of feed composition, competitive reactions, and reaction conditions are discussed. Since development of an understanding of the kinetics of metal removal is important from the standpoint of catalyst lifetime, the effect of catalyst properties on reaction kinetics and on the resulting metal deposition profiles in hydroprocessing catalysts are discussed. 

It is convenient to compare quantitatively the relative rates of the HDM reactions and the diffusion of metal species as revealed by the metal... 

The effect of HDM reaction selectivity variations on the metal distribution parameter at the reactor entrance for Ni-T3MPP follows. 

B. Hydrodemetallation of Petroleum Oils and Residuum 1. Kinetics and Mechanism of HDM Reactions... 

Pazos et al. (1983) proposed a consecutive HDM reaction mechanism similar to the model compound studies in which part of the vanadium deposition occurs from species V) not originally present in the oil ... 

The apparent HDM reaction orders greater than unity have been attributed to the presence of more than one class of metal compounds reacting with different rates (Oleck and Sherry, 1977 Cecil et al., 1968). Just as in hydrodesulfurization, the simultaneous occurrence of several first-order reactions with different rates can lead to an apparent reaction order greater than unity (de Bruijn, 1976). Wei and Hung (1980) theoretically demonstrated conditions whereby two first-order reactions give rise to apparent second-order kinetics. 

Catalyst surface activity may be manipulated to alter the ratio of HDM activity to metal compound diffusivity with a predictable impact on optimum pore size (Howell etal., 1985). Lowering the intrinsic surface activity by varying the quantity, chemical composition, or distribution of active catalytic metals will increase the Ni and V penetration into the catalyst. The lower surface activity catalysts may be able to tolerate a smaller pore size (higher total surface area) and still maintain an acceptable performance for the HDM reactions. 

The presence of V3S4 crystals can only be attributed either to an autocatalytic mechanism of this type or the migration of the deposited metals. It is known that deposited Ni and V sulfides possess some catalytic activity (see Section IV). Slurry processes have been proposed which utilize Ni and V deposited from the oil onto a slurry material (Bearden and Aldridge, 1981). Studies have appeared in the literature demonstrating that nearly all of the transition metals are catalytically active for HDS reactions and presumably for HDM (Harris and Chianelli, 1984). Rankel and Rollmann (1983) impregnated an alumina catalyst base with Ni and V and concluded that these sulfides display an order of magnitude lower activity than the standard Co-Mo sulfide catalyst for HDS reactions, but exhibited similar activity for HDM reactions. 

Any process variable which increases the HDM reaction rate will decrease the effectiveness factor and hence the distribution parameter. Effects of hydrogen partial pressure and reaction temperature on the deposited metal profiles were obtained by Tamm et al. (1981) and are shown in Figs. 45 and 46. Consistent with the HDM reaction mechanism, both higher temperature and hydrogen enhance the reaction rate (see Section IV) and, therefore, decrease the distribution parameter. 

The manner in which Ni and V sulfide deposits accumulate on individual catalyst pellets depends on the kinetics of the HDM reactions as influenced by catalyst properties, feed characteristics, and operating conditions. The dynamic course of deactivation of catalytic reactor beds is also determined by the kinetics of the HDM reaction. The lifetime and activity of a reactor bed are directly related to the details of the metal deposit distribution within individual pellets. This section will review deactivation behavior of reactor beds in light of our understanding of the reaction and diffusion phenomena occurring in independent catalyst pellets. Unfortunately, this is an area of research which remains mostly proprietary with too little information published. What has been published is generally lacking in detail for the same reason. 

The model formulated by Ahn and Smith (1984) considered partial surface poisoning for HDS and pore mouth plugging for HDM reactions. The conservation equations with first-order reactions for metal-bearing and sulfur-bearing species were based on spherical pellet geometry rather than on single pores. Hence, a restricted effective diffusivity was employed... 

Haynes apd Leung (1983) formulated a similar configurational diffusion model combining the effects of active site poisoning as well as pore plugging on the HDM reaction. In this case the reaction form in the conservation equation is multiplied by a deactivation function which accounts for the loss of intrinsic activity, (1 - ) is frequently chosen, where x is the fractional coverage of the sites. Other forms of the site deactivation function have been discussed by Froment and Bischoff (1979). The deactivation was found to depend on a dimensionless parameter given by... 

As discussed in Section IV, Agrawal and Wei (1984) and Ware and Wei (1985b) have successfully modeled experimental deposit profiles by using the theory of coupled, multicomponent first-order reaction and diffusion. Wei and Wei (1982) employed this theory to evaluate the influence of catalyst properties on the shape of the deposit profile. Agrawal (1980) developed a model for the deactivation of unimodal and bimodal catalysts based on the consecutive reaction path. These approaches represent a more realistic consideration of the HDM reaction mechanism than first-order kinetics and will, accordingly, be discussed in more detail. 

Agrawal (1980) adopted the grain model of Sohn and Szekely (1972) to model the deactivation of a bimodal catalyst for the HDM reaction. The schematic in Fig. 59 illustrates the proposed physical structure of the catalyst pellet. The macrospherical pellet of radius Ra is composed of numerous microspheres of radius / , where the number of microspheres per unit volume is given by... 

Fig. 60. Initial HDM reaction rate versus micropore radius and grain size at a fixed porosity for the macroporous catalyst (Agrawal, 1980).

Agrawal (1980) also computed the effect of time on stream on the HDM reaction rate for various cases of bimodal and unimodal catalysts. These comparisons are shown in Fig. 61. As is evident, improvements in stability and overall activity rather than initial activity are gained, whether unimodal or bimodal catalysts are used, by increasing the micropore size. The relative capacity of the catalysts can be visualized as the area under the curves in Fig. 61. 

There have been numerous studies on HDM catalyst deactivation. They differ in the mechanisn of deactivation, in the kinetics used for the HDM reaction, in the expression employed to describe diffusivity or pore structure. These different approaches can lead to quite different conclusions as to the catalyst properties that yield optimum overall activity (3-9). 

The apparent activation energy (Ea) for the HDM reaction, for catalysts A and C, were 18 kcal/gmol and 13.9 kcal/gmol respectively. Romay [11] obtained 19 kcal/gmol for the HDM of an hexane deasphalted oil of Boscan crude, for the temperature range 350-415 0, on a Al/Co/Mo extrudate. 

First order kinetics for the HDM reaction, with reference to vanadium concentration in the liquid phase. This assumption finds support in the literature [13). 

Romay [11] obtained q 0.4, at 400 C and 1000 psig of hydrogen pressure, for a fresh extrudate (1/16 ). Hiemenz [14] and other authors have reported similar values for the HDM reaction. 

Figure 3. HDM reaction mechanism of model compound vanadyl-tetraphenylporphyrin.

Prerequisite for hydrodemetallisation is the diffusion of the large porphyrins into the catalyst porous texture prior to the sequential reaction mechanism. Diffusion of these large molecules can be limited by geometric exclusion and hydrodynamic drag. When the solute molecular size is significant as compared to the pore size, a restrictive factor can be introduced to account for the reduction in difftisivity. As a consequence, clarification of detailed HDM reaction kinetics may be obscured by diffusion limitations. 

The reaction kinetics of model compound VO-TPP on a sulfided wide-pore V/Si02 catalyst was studied. This HDM reaction was carried out with sulfided catalysts, which is a realistic approach to the HDM process. The catalyst properties are summarized in Table 1. 

The two-site reaction kinetics model proposed by Bonn [1] was used to evaluate the kinetic parameters. Activation energies and pre-exponential factors were determined from experiments between 570-630 K at 10 MPa. In order to decrease the strong inter-correlation between pre-exponential factors and activation energies, the reparametrisadon method of Kittrell [4] was used. Values for the pre-exponential factors at a reference temperature and activation energies are presented in Table 2. Experimental and theoretical details on HDM reaction kinetics will be published elsewhere [5]. 

Figure 3. HDM reaction mechanism of model compound vanadyl-tetraphenyl-porphyrin. (TPP tetraphenylporphyrin, TPC tetraphenylchlorin, TPiB tetra-phenylisobacteriochlorin)...

HDM Reaction Kinetics. Intrinsic reaction kinetic parameters for the... 

The HDM reaction was carried out at industrial conditions, 553 K and 9.0 MPa Hj pressure. At certain time intervals a liquid sample was taken from the autoclave and analysed ex-situ with UV-Vis spectroscopy to determine the porphyrin concentrations (9). 

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