Fundamentals of Hydrotreating Part 2

Catalysts

Most of the hydrotreating catalysts in commercial use are supported on y-alumina (Y-Al2O3), sometimes with small amounts of silica (SiO2) or phosphorus (P). Preparation of the support is a very important step during catalyst manufacture to achieve a material with a high surface area and an appropriate pore structure. This high surface area is required to disperse the active metals and promoters uniformly. The typical active metals are molybdenum (Mo) and tungsten (W) sulfides, modified by a promoter: either cobalt (Co) or nickel (Ni) sulfide. The main function of the promoter is to increase the activity of the active metal sulfide substantially. The amount of each component in a commercial catalyst depends on the application desired. In general, the specifications of the feed and the desired product quality will determine which catalyst (or combination of catalysts) will be used.

CoMo and NiMo/Y-Al2O3 are the preferred catalysts, for several reasons; they are cheap, highly selective, easy to regenerate, and resistant to poisons. Although being more effective for HDN and HDA, NiW catalysts are seldom used for commercial hydrotreating applications since they are much more expensive than NiMo catalysts. CoMo//-Al2O3 catalyst is recommended for HDS, and NiMo//-Al2O3 or NiCoMo//-Al2O3 for HDN. NiMo catalysts possess higher hydrogenation activity than CoMo catalysts and hence are more suitable for saturation of aromatic rings, although both catalysts will remove both sulfur and nitrogen.

Since the physical and chemical composition of petroleum and its fractions varies considerably depending on their origin, there is not a universal catalyst for hydrotreatment of all the feeds to achieve the desired target in terms of impurities removal and conversion. Thus, the properties of catalysts for hydrotreatment of light and middle distillates are different from those used to hydrotreat heavy oils. Whereas for light distillate hydrotreating the chemical composition of the catalytic surface and the specific surface area are the most important parameters, since metal and coke deposition are not crucial, in the case of heavy feeds, porosity is the determinant as to suitable catalyst activity and life. In both cases the role of the support is crucial. For heavy oil hydrotreat-ments, support acidity and porosity have to be designed carefully to accomplish the optimum catalyst performance. Acidity must be strictly balanced to perform hydrocracking at a desired reaction extent, but not so much as to produce excessive coking. The acidity of the support is provided primarily by silica, zeolite, and/or phosphorus. In the case of porosity, when hydrotreating light and middle distillates, minimum pore size is required to overcome most diffusional restrictions. However, for hydrotreating heavy oils, pore size needs to be designed properly to handle the complex large molecules (e.g., asphaltenes) contained in such feeds. The ability to adjust pore size to concentrate pores around a particular diameter has a great impact on the hydrotreat-ing catalyst activity either at the beginning of operation (start-of-run) or at the middle or end of operation (middle-of-run or end-of-run).

The content of hydrocrackable compounds (Chyc) can be approximated by the feed concentration as follows:

Usually, hydrotreating catalysts are prepared in the oxide state (e.g., CoOMoO3/Y-Al2O3) and they must be activated by converting the metals from oxide form to sulfide form to achieve the maximum activity of the catalyst. This step, also called presulfiding- is carried out by four routes (Marroqu-n et al., 2004):

1. With a nonspiked feedstock, in which sulfiding is conducted with the same sulfur from the normal feedstock

2. With a H2/H2S mixture, carried out in the gas phase and most practiced in laboratory experiments

3. With a spiked feedstock, in which sulfiding is done primarily by the sulfur of the spiking agent

4. Ex situ sulfiding, which has been reported to have the same or better activity and stability than in situ sulfiding

In liquid – phase sulfiding, the hydrocarbon carrier aids in wetting and hence in providing a better distribution of sulfur across the bed and sulfiding the catalyst evenly. The hydrocarbon also serves as a sink of heat generated, allowing for better control of the exothermic reaction between sulfur and the metal of the catalyst. This allows for more rapid presulfiding. The sulfiding reaction is highly exothermic, and much care must be taken to prevent excessive temperatures during activation to prevent permanent catalyst deactivation.

It should be remembered that a spiking agent is a sulfur-containing organic compound that releases H2S at a much lower temperature than do the sulfur compounds present in normal feedstocks. There are various spiking agents reported in the literature and frequently used for activation of HDS catalysts, such as carbon disulfide (CS2) , dimethyl sulfide (DMS), dimethyl disulfide (DMDS), butanethiol, ditertiary nonyl polysulfide (TNPS), ethyl mercaptan (EM), dimethyl sulfoxide (DMSO), and n-buthyl mercaptan (NBM). Among them, DMDS has demonstrated better behavior during laboratory and commercial presulfiding.

Before presulfiding, two main steps are recommended to achieve the optimal catalyst activity:

1. Catalyst drying, because due to the hygroscopic nature of the alumina carrier, the catalyst can take up water, and when heating up in wet conditions with oil, the catalyst can be damaged mechanically

2. Catalyst soaking, which is done to wet the catalyst particles properly to prevent the presence of dry areas in the catalyst bed, which eventually lowers the overall activity

The shape and size of hydrotreating catalysts vary depending on the manufacturer. These parameters are important in achieving good catalyst performance and must be matched by the properties of the feed, the process technology, and the type of reactor. As for the size of catalyst, there is a limit to the decrease in particle size [e.g., -Lin. (0.8mm)], after which particles disintegrate. In addition, such small catalyst particles will cause AP problems in fixed-bed reactors. On the other hand, the most common commercial shapes of hydrotreating catalysts are sphere, pellet, cylinder, bilobular, trilobular, and tetralobular. The size and shape of the catalyst particles are usually defined to minimize pore diffusion effects in the catalyst particles and pressure drop across the reactor.

During hydrotreating operation, the performance of the catalyst is measured primarily by the following criteria:

• Initial catalyst activity: measured at the start-of-run condition, and corresponds to the reactor temperature required to achieve the quality of product desired

• Catalyst stability: measured under middle-of-run and end-of-run conditions, and is determined by the rate of temperature increase required to maintain product quality

• Product quality: controlled during the complete operation of the catalyst, and is an indication of the ability of the catalyst to produce products with the specifications desired

During hydrotreating of heavy feeds, the catalyst exhibits a certain degree of deactivation, depending on the nature of the feed, the type of reactor, and reaction conditions. The two main causes of catalyst deactivation are coke deposition and metals deposition. Coke is generally formed by thermal condensation, catalytic dehydrogenation, and polymerization reactions. The main coke precursors are asphaltenes. Coke formation is very rapid during start-of-run, after which it rises to an equilibrium level. During middle-of-run the total amount of coke remains almost constant. In general, the maximum coke laydown is about 20wt%. Deactivation by coke is temporal since catalyst activity can be restored by regeneration. The recovery of catalyst activity can be about 90% by in situ regeneration and 95 to 97% by ex situ regeneration.

On the contrary, deactivation by metals (mainly Ni and V) is not reversible, and when the catalyst has been deactivated by metals, it needs to be replaced. The deposition of metals takes place at the pore entrances or near the outer surface of the catalyst.

To compensate for catalyst deactivation the reactor temperature needs to be increased continuously to keep the product quality at the level desired. However, exposing the catalytic bed at high temperatures will cause catalyst support sintering, which is another reason for loss of irreversible catalyst.