Comparison between Conventional and Metakaolin bi-functional Catalyst in the Hydrodesulfurization Operation

The present study investigates hydrodesulfurization (HDS) of gas oil with 9300 ppm (0.93 wt%) sulfur supplied from Al-Dura Refinery by using an economic catalyst prepared from raw mineral (kaolin clay) cemented by alumina as composite support alumina meta-kaolin (AMK). Characterization of the prepared catalyst was achieved by using Energy Dispersive X-Ray Analysis (EDAX), scanning electron microscopy (SEM), BET surface area, pore volume , Bulk density, X-ray diffraction analysis (XRD) and Fourier-transform infrared spectroscopy (FTIR). AMK was modified as a bifunctional catalyst with active metal (Co and Mo). The hydrodesulfurization (HDS) efficiency was evaluated and compared with the traditional catalyst (CoMo-Al2O3) in a hydrotreating reaction carried out in one stage reactor at temperature 375 oC, pressure 40 bar, LHSV 1hr-1, and H2/HC ratio 200 vol. ratio. 62.2% and 90% of hydrodesulfurization efficiency were achieved for prepared catalyst (CoMo-AMK) and commercial CoMo-Al2O3 respectively at the same operating conditions. middle distillates by a trickle-bed reactor. The catalyst performance after impregnation indicated maximum activity of the hydro-desulfurization reaction. The sulfur conversions were (93.0%) at 360 °C and 10.3 bar for (3Co - 10W)/γ-Al 2 O 3 . The interaction kinetics of the HDS were best fitted with a pseudo-first-order power law model with reasonable accuracy (0.90 < R 2 < 0.95) [9].

In addition, if the sulfur compounds are not removed it would lead to impairment of the final output specifications and the catalyst in the next units (such as catalytic cracking and reforming) will be subjected to poisoning. So, the development of new types of catalysts with highly hydro-desulfurization (HDS) activity is much desirable [3]. HDS Selective means converting organic sulfur to hydrogen sulfide by preservation of olefins. Mainly this concept which approach revolves around redesign a catalyst to be able to desulfurize and unsaturated the olefin. This can be complete by removing the active sites that saturate the olefin from the catalyst surface.
The end of this approach is that the sulfur has been changed to (H 2 S), then olefins species are mainly preserved to prevent losses in fuel quality [4]. Usually, bimetallic catalysts based on the active phase and used different promoters (i.e. supported metal on alumina or silica) are used in industry to increase activity and selectivity for HDS of refractory sulfur-containing molecules [5].
The choice of catalyst mainly depends on the desired conversion and the characterization of the treated feeds. Ideal hydro-desulfurization catalyst most be capable removing nitrogen, sulfur and atoms of metal from streams of refinery and improve new a specifications fuel, such as the cetane or octane numbers and content of aromatics, which are necessary for high quality fuels and meet the standards of environmental legislation [6].
It is known that feed characteristics vary greatly and that the physical properties and the number of impurities determine and select the type of catalysts. This indicates that there is no universal catalyst or suitable catalytic system for hydro-processing feedstock from dissimilar sources, with concerning these physical and chemical properties, a wide range of hydro-processing middle distillates by a trickle-bed reactor. The catalyst performance after impregnation indicated maximum activity of the hydro-desulfurization reaction. The sulfur conversions were (93.0%) at 360 °C and 10.3 bar for (3Co -10W)/γ-Al 2 O 3 . The interaction kinetics of the HDS were best fitted with a pseudo-first-order power law model with reasonable accuracy (0.90 < R 2 < 0.95) [9].
Mineral clays or modified mineral clays are used as commercial catalysts [10]. It has wide applications because its swelling, ion exchange properties, adsorption and high surface areas [11]. From the beginning of the petrochemical industries and petroleum refining, clays such as alumina silicate (zeolite) has been used as catalysts. Clays of different acidity can be obtained by calcining step (thermal treatment) prior to preparing the catalyst. The high temperature of calcining step the clay determines the concentration and type of hydroxyl group, and hence their acidity [12]. Clays supported on compound oxides of Al 2 O 3 and SiO 2 having good catalytic properties and the ability to support by active metals and promoters [13]. The clay that is widely use in the preparation of catalyst is kaolin and has many applications such as synthesis zeolite [14] and gamma-alumina [15]. The ability to modify that clay by an active metals (metal oxide) or intercalation with plate anion, complexion materials, and organic chemicals represented a breakdown in the catalytic chemistry of these materials, as it presented new possibilities in mastering properties and interaction, although not fully explored [16][17].
Most of the kaolin minerals are kaolinite (Al 2 Si 2 O 5 (OH) 4 ) belonging to phyllo-silicates and ideally consisting of continuous sheets of tetrahedral and octahedral as shown in Figure (1

).
Each tetrahedral consists of a silica atom coordinated into four (O) atoms and bonds to the neighbouring tetrahedron by sharing three angles (basic oxygen atoms) to form an infinite twodimensional "hexagon" mash pattern parallel to (x & y-axes). Each octahedral is formed from an Al atom and is coordinated by six oxygen atoms (Oxygen and OH) and is associated with octahedral dull by sharing edges [18].

Drying process
The effectiveness of drying processes can have a large impact on product quality and process efficiency. Apart from the apparent requirements for drying solids for a subsequent process, drying can also be performed to improve the handling properties of the catalysts, as in the case of catalyst packing and other processes involving feed flow and for stabilization of moisture sensitive materials in heat-and mass-transfer processes [23]. The drying process was carried out using (10 Nl/hr) nitrogen gas at temperature (150°C) and pressure (10 bar) for (5 hr) to clean the unit from oxygen and check leaks.

Sulfiding Experimental
Pre-sulfide was done at 30 bars pressure, 220˚C temperature by 10 Nl/hr Hydrogen gas for 4 hr. The sulfide operation includes passing a sulfide agent H 2 on the bed of catalysts in a carefully controlled procedure involving several temperatures. Where the agent is heated in the presence of H 2 to easily decompose the sulfur compound to form H 2 S required to complete the sulfide reactions. So to complete the sulfiding process increase the temperature to (330°C) for (12 hr) to achieving maximum performance from their catalysts.

Energy Dispersive X-Ray Analysis (EDAX)
The way EDAX analysis works is that the electron beam hits the inner shell of an atom, knocking off an electron from the shell, while leaving a positively charged electron-hole.
Second, its position is filled by another electron from a higher energy shell, and the characteristic X-ray is released [21].

Scanning Electron Microscopy (SEM)
The images of a sample produced by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the detected signal to produce an image [21]. Figure (6) shows images of kaolin before and after cementation by Al 2 O 3 to produced AMK, it was found that there is a difference in the morphology after the alumina cementing process [22].

BET Surface Area
Table (1) shows the surface area and pore volume of kaolin, AMK, CoMoAMK, and traditional catalyst (CoMo-Al 2 O 3 ). The result shows the surface area of kaolin was increased after preparation of the AMK due to thermal treatment and cementing by alumina then decreases after modifying by metal, and the surface area of the traditional catalyst's brought. The surface area decreases after evaluation process of the prepared catalyst in a limited percentage due to the effect of the reaction of the removal process [19][20][21][22][23].

Bulk density
It was determined by a random place 40 cm 3 of catalysts into a 2 cm diameter cylinder. The difference in weight between the cylinder filled with the catalyst and the empty one represents the weight of the catalyst. Bulk density is defined as the ratio of catalysts weight to sample volume. The bulk density of prepared catalyst (CoMo-AMK) was (0.914 gm/cm 3 ).

X-ray diffraction analysis (XRD)
X-ray diffraction analyses of of sample preparation for the analysis followed the methods of [24]. The results are presented in Figure (

Fourier-transform infrared spectroscopy (FTIR)
FTIR spectra of prepare catalyst is shown in Figure ( as well as confirming that the impregnation process was carried out with the required accuracy.
The results obtained are consistent with the publication of Granizo et al., [26].

Hydro-treating process
The hydro-treating of gasoil was done at temperature 375 o C, LHSV 1 h -1 , hydrogen pressure was 40 bars and H 2 /HC ratio 200 ml/ml. these conditions were applied on Iraqi gas oil feedstocks supplied from Al-Doura refinery in order to test the prepared CoMo-AMK efficiency to reduce the relatively high levels of sulfur content in feed to minimum levels as it is done with the commercial Hydrodesulfurization catalysts. The products liquid was collected after a 2 h stabilization period for run to reach the steady state from which sample was taken for sulfur content analysis.