Large-pore Catalysts Research Articles

Modelling of the methane steam reforming reactor with large-pore catalysts

Methane steam reforming is an endothermic process requiring high quantities of heat using catalysts with low effectiveness factors. Intraparticle forced convention can be promoted by using large-pore structured catalysts, in order to reduce intraparticle gradients and consequently increase the efficiency of the process. In order to evaluate the effect of this convective phenomenon, in the steady-state regime, three reactor models (two dimensional and one-dimensional models) were analysed: the pseudo-homogeneous model (PH), the heterogeneous model which considers diffussions as the only mechanism of transport inside the solid (HTd) and the heterogeneous model which also includes the intraparticle convection (HTdc). Moreover, wall temperatures that must be used for getting the same final conversion of methane through the two heterogeneous models were also calculated

Effect of pore structure of nickel-molybdenum/alumina catalysts in hydrocracking of coal-derived and oil sand derived asphaltenes

In this paper effect of catalyst pore structure is studied by using four unimodal Ni-Mo/Al{sub 2}O{sub 3} having different median pore diameters (MPD, 120-730 {Angstrom}) for hydrocracking (425{degrees} C, 1 h, 4.9 MPa H{sub 2}) of the asphaltenes derived from Akabira coal (Aka-Asp) and Athabasca oil sand bitumen (Aosb-Asp). The asphaltene conversions increased with increasing catalyst MPD up to 290 {Angstrom} and then remained constant or decreased up to 730 {Angstrom}. Aka-Asp has a much smaller molecular size but a higher aromaticity than Aosb-Asp, whereas conversion of the former appeared to be more sensitive to pore size. Higher oil yields were obtained with large-pore catalysts, while the highest oil yield from Aosb-Asp was still remarkably lower than that from Aka-Asp. Maximum hydrodeoxygenation activity appeared at an MPD of 150 {Angstrom} for runs of both asphaltenes, while an MPD of 290 {Angstrom} corresponded to the highest activity for hydrodesulfurization of the sulfur-rich Aosb-Asp. Characteristics of used catalysts are also discussed.

Effect of intraparticle convection on the steady-state behavior of fixed-bed catalytic reactors

The effect of intraparticle convection on the behavior of fixed-bed reactors containing large-pores catalysts (e.g. selective oxidation catalysts) is analysed by comparing three reactor models : pseudohomogeneous model ( PH), heterogeneous diffusion ( HT d ) and heterogeneous diffusion/convection model ( HT dc ). Runaway diagrams were constructed based on the “maxima-curve”, allowing system properties to change with temperature and on the method of isoclines. It is shown that the “maxima-curve” method fails in the case of the HT dc model since then the “maxima-curve” P bm = f( T bm ) has two maxima. The stable region of operation predicted by the HT dc model. Energy savings can be obtained by using large-pore catalysts since then it is possible to operate at lower wall/feed temperatures than those used with conventional catalysts. New catalysts can be designed to take advantage of the improvement on the effectiveness factor due to intraparticle convection and energy savings due to the use of more favorable operating conditions.

Hydroprocessing of directly liquefied biomass with large-pore catalysts

Different extracted fractions of directly liquefied biomass were hydroprocessed in order to determine the effects on the products from pore volume distribution of the catalysts used. Desalted, directly liquefied biomass from the PERC process was consecutively extracted with isooctane and xylene. The hydroprocessing was performed in a downflow reactor with sulfided catalysts. The sulfided CoMo catalyst (Ketjen 742) with narrow pores gave the best deoxygenation of the catalysts tested at 350 o C. The oil with the lowest oxygen content (extracted with isooctane) showed the lowest deoxygenation and change in molecular weight distribution

Effect of Pore-Size Distribution on the Catalytic Performance for Coal Liquefaction. II. Carbonaceous and Metallic Deposition on the Catalyst

Abstract The behavior of carbonaceous and metallic depositions on catalysts during hydroliquefaction of coal was investigated by using several kinds of catalyst supports with different pore-size distribution values in order to obtain the fundamental data for designing long-life catalysts. Higher concentration of carbonaceous deposition was observed on large-pore and bimodal catalysts than was observed on the comparative small pore and unimodal catalysts, respectively. However, the carbonaceous deposit was distributed more evenly inside large pores than it was inside small pores. The micropores of the bimodal catalysts were more effectively used than were those of the unimodal catalysts. Thus, the unimodal catalysts with small pores suffered more severe pore-mouth poisoning than the bimodal or the large-pore catalysts. The penetration behavior of the metallic components was found to be different for each metal. Discrete minerals such as clay could not penetrate into the catalyst pores and were deposited on the external surface of the catalyst particle. While, the metallic components coordinated with organic compounds such as phenolates or chelates deposited inside the macro- and mesopores. Easy access of the large asphaltenic molecules to the active sites resulted in high coal-liquefaction activity for the catalyst. This was accompanied by large amounts of both carbonaceous and metallic depositions. It is concluded that large-pore or bimodal pore structure is required for both high liquefaction activity and suppression of pore-mouth poisoning.

Dependence of resid processing selectivity on catalyst pore size distribution

In the hydroprocessing of resid, catalyst selectivity and the relationship between metals and sulfur distribution in the resid are key factors in achieving required product specifications. Using gradient elution chromatography as a method of analysis, we have found that catalyst pore size distribution can significantly affect selectivity for different metal and sulfur components in residua. A small pore catalyst was observed to be more selective for non-asphaltene sulfur and metals, while a large pore catalyst was found to be very selective for asphaltene metals and sulfur. This variation of catalyst selectivity is a result of the concentration of metals and sulfur in different portions of the resid. For typical Middle Eastern residua, about 70–75% of the sulfur was found to be present in low molecular weight less aromatic GEC fractions, while 95% of the metals were in highly aromatic high molecular weight fractions. Thus the variation of selectivity with pore size was found to result from concentration of the sulfur and metal species in molecules of different size.

Effects of Pore Size Distribution on the Catalytic Performance for Coal Liquefaction. I. The Activity and Selectivity of the Catalyst

Abstract Coal liquefaction experiments were carried out by using seven catalysts with different pore structures. As for the unimodal catalysts, the total conversion increased with the pore radius of the catalyst because of the lesser pore-diffusional limitation. However, the ratio of oil to oil-plus-asphaltene decreased with the pore radius. In the case of a large-pore catalyst, heavy components with a large molecular size and a low reactivity can approach the intra-particle active sites and occupy them to prevent the hydrocracking of asphaltene to oil. Bimodal catalysts with micropores (r<10 nm) and macropores (r>300 nm) gave higher values of both the coal liquefaction and the oil yields than did the unimodal catalysts. The presence of macropores seems to enhance the pore diffusion and increase the efficiency of the micropore use, while mesopores with radii between 10 nm and 300 nm did not make such a high contribution to the pore diffusion as was shown for the macropores. For the removal of heteroatoms, there was little difference in the performance among the catalysts with different pore structures. It is suggested that the active sites for the heteroatom removal are deactivated easily by the heavy components.