Deactivation in Hydrodemetallization

Samples of used residue hydrodemetallization catalysts prepared by hydrotreating a Safanyia atmospheric residue have been characterized and tested using model compounds in order to investigate the initial deactivation of the catalyst Samples containing 4 to 10 wt % carbon and less than 200 wt ppm V or 10 to 15 wt % carbon and 1.3 wt % V have been obtained from tests in batch and continuous flow reactors respectively. It is shown that in the early stage of the catalyst deactivation a small amount of vanadium is more deactivating than a large amount of carbon. 

Carbon and metal sulfide deposits are the two main causes of deactivation of residue hydrodemetallization (HDM) catalysts. During a catalytic test, the metals contained in the feed (Ni, V) are slowly deposited on the catalyst surface leading to the build up of large particles of metal sulfides which ultimately plug the catalyst pores. Carbon, on the other hand, is known to accumulate quickly on the catalyst surface within the first days of a run until a steady state is reached (1-20). At the beginning of a run, a strong deactivation of the residue HDM catalyst rapidly occurs to which both types of deposits may contribute. However at the present time it is not clear whether this initial deactivation is mainly due to coke or metal sulfide deposits. 

The present study has shown that during the initial deactivation of a residue hydrodemetallization catalyst, coke formation is very rapid but is far less poisoning than small amount of vanadium well dispersed in the grain volume. This suggests that metal deactivation predominates over coke deactivation at the beginning of a run of a resid hydrotreater. 

Some studies of potential commercial significance have been made. For instance, deposition of catalyst some distance away from the pore mouth extends the catalyst s life when pore mouth deactivation occurs. Oxidation of CO in automobile exhausts is sensitive to the catalyst profile. For oxidation of propane the activity is eggshell > uniform > egg white. Nonuniform distributions have been found superior for hydrodemetallation of petroleum and hydrodesulfurization with molybdenum and cobalt sulfides. Whether any commercial processes with programmed pore distribution of catalysts are actually in use is not mentioned in the recent extensive review of Gavrillidis et al. (in Becker and Pereira, eds., Computer-Aided Design of Catalysts, Dekker, 1993, pp. 137-198), with the exception of monolithic automobile exhaust cleanup where the catalyst may be deposited some distance from the mouth of the pore and where perhaps a 25-percent longer life thereby may be attained. 

We have taken catalytic hydrodemetallation (HDM) of crude oil for optimization of the pore structure of catalyst supports. Crude oil contains small quantities of nickel and vanadium in the form of porphyrins. During the demetallation process the metals are deposited on the catalyst in the form of sulfides. These sulfides cause irreversible fouling. In order to describe this deactivation, a realistic model of pore structure has to be employed. Therefore, we have used a three-dimensional network with a maximum connectivity of 18 (see Fig. 1). As in practice the connectivities are less than ten, we have used average connectivities 3, 6 and 9 in our calculations. 

Activities (HDC/HDS/hydrodemetallization [HDM]) were measured at constant operating conditions in a 0.05 L EBR with 100% of each of the different VR feed. The results obtained on catalyst deactivation are shown in Table 10.3. Comparing the... 

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