Mahdi Abdi-Khanghah Mostafa Adelizadeh 2*Zahra Naserzadeh 3*Zhien Zhang 4*

1 Department of Chemical Engineering,Petroleum University of Technology,Ahwaz,Iran

2 Department of Environmental Engineering,Azad University of Rudehen,Tehran,Iran

3 Industrial Safety and HSE Department,Faculty of Engineering,Kar Higher Education Institute,Qazvin,Iran

4 College of Chemistry and Chemical Engineering,Chongqing University of Technology,Chongqing 400054,China

Keywords:Catalytic hydro cracking Bi-and tri-metallic mesoporous catalyst Al-HMS n-Decane Mini-reactor

A B S T R A C T Bi-metallic(Pt–Sn and Sn–Ni)and tri-metallic(Pt–Sn–Ni)catalysts,supported on Al-containing hexagonal mesoporous silica(Al-HMS)(Si/Al=20)materials,were synthesized.N 2 adsorption–desorption,X-ray diffraction(XRD),Brunauer–Emmett–Teller(BET)test,and temperature programed desorption(NH3-TPD)were used to characterize physicochemical characteristics and textural properties of the Al-HMS catalysts.Catalytic performances on hydro-cracking of n-decane at different reaction conditions were studied in a microreactor.Comparison between Pt–Sn,Sn–Ni and Pt–Sn–Ni catalyst under different hydro-cracking conditions was made.The experimental results indicate that the proper balance between the acid and metal functions is the key in synthesizing a catalyst with a better performance in hydro-cracking.Tri-metallic catalyst exhibits the best catalytic performance in n-decane hydro-cracking than two bi-metallic catalysts.

1.Introduction

Due to increasing demand for high quality middle distillates,catalytic cracking and hydro-conversion have attracted extensive attention in the last decade[1–4].Hydrocracking and hydrogenation of n-decane can be obtained by using metal particles(e.g.Pt/Al2O3)and acid sites as bifunctional catalysts,with the acid sites providing the cracking function and metal sites promoting hydrogenation–dehydrogenation function[5].Hamoule et al.[6]investigated the catalytic performance of different bi-functional catalysts,changing the ratio of Si/Al from 5 to 30,since the molar Si/Al ratio influenced the structural regularity and surface acidity of catalyst.Acidity of Al-HMS was enhanced due to the reduction of Si/Al ratio.Because n-decane cracks to valuable product such as C4,C5and C6,the acidic sites resulted from increased Al content made the product distribution more desirable.

The growth of bi-and tri-metallic catalystsisan attainment of recent years in the field of catalytic hydro-conversion.In spite of the fact that in industrial practices,the main purpose of using tri-metallic catalyst is to facilitate the removal of sulfur content of feed.Further literature survey suggested that by adding a third metal,such as iridium,rhenium,or tin,the promotion effect on platinum catalyst for conversion and selectivity can be observed experimentally[7–16].Peyrovi et al.[17]characterized different bi-and tri-metallic catalysts supported on Al-containing hexagonal mesoporous silica(Al-HMS)with a constant Si/Al ratio,and catalyst application was investigated via n-heptane hydro conversion.The catalytic performances were found to be a function of interaction between metal phases,and the better performance was observed for the tri-metallic catalyst in comparison with bi-metalic one.

Due to the large and uniform pore sizes together with high surface area and wide applications of mesoporous materials as supports for catalysis,the silicate mesoporous materials have received extensive attention in the last decade.Then,Alothman[18,19]in a review emphasized on silicate mesoporous materials and their applications.The structure and performance of different mesoporous catalysts have shown a better hydro-conversion activity in comparison with the Al2O3supported catalyst[17].Regardless of zeolite supports in catalysis,organic groups in the mesoporous materials modify the surface properties,protect the surface from chemical attacks and tune the surface reactivity between 2 and 10 nm pore sizes[18].

This paper presents the synthesis of novel bi-and tri-metallic catalysts which are used for hydro-conversion for the first time.Also,CFD simulation of a mini-reactor which is used for n-decane hydroconversion has been carried out in this work.An experimental investigation of the effect of temperature and BET surface area of catalyst on reactant conversion and product distribution was performed.Experiments with different operating conditions of the hydro-conversion in a laboratory micro-reactor indicate that the tri-metallic(Pt–Sn–Ni)is the best catalyst for the conversion of n-decane under the optimum operating condition as compared with other bi-metallic catalysts.

2.Experimental

2.1.Catalyst preparation method

Impregnation method was used to synthesize the Al-HMS with the ratio of Si/Al=20 through two steps as described by Tanev et al.previously:a)support synthesis,and b)active phase addition[19].Mesopouros support(Al-HMS)preparation was done using a sol–gel method:tetraethyl orthosilicate(TEOS,Merck),aluminum isopropoxide(Merck),and dodecyl amine(Merck)as the support silica source,aluminum source,and surfactant were respectively mixed and stirred.In a typical preparation,tetraethyl orthosilicate(1.0 mol)was added under vigorous stirring to a solution of amine(0.27 mol)in ethanol(9.09 mol)and deionized water(29.6 mol).The solid product was filtrated and washed with water,then dried at 90°C overnight in ambient air,and finally calcined at 600°C for 5 h in the air flow.

One tri-metallic catalyst[(0.1)Pt–(0.1)Sn–(0.3)Ni]and two bimetallic catalysts[(0.1)Pt–(0.4)Ni]and[(0.1)Sn–(0.4)Ni]w ere prepared by the impregnation of Al-HMS(Si/Al=20)support with appropriate concentrations of H2PtCl6,SnCl2and Ni(NO3)2as the platinum,tin and nickel sources,respectively.For the presence of consistency between catalyst metal loadings,all metals such as Pt,Sn and Ni being specified with weight percentage in the brackets.Impregnated samples were dried overnight at 90 °C,and then calcined in air at 350 °C for 3 h.Calcination temperatures were selected according to the available methods for different synthesized catalysts[14].It should be noted that in the figures and tables,catalysts which are denoted as Pt–Sn–Ni,Pt–Ni,Sn–Ni,respectively,are(0.1 wt%)Pt–(0.1 wt%))Sn–(0.4 wt%))Ni,(0.1 wt%))Pt–(0.4 wt%))Ni,and(0.1 wt%))Sn–(0.4)Ni.

2.2.Characterization

To study the effect of physicochemical properties of the catalysts,the fresh mesoporous catalysts were analyzed by the surface characterization technique[20].The crystallinity and purity of crystals of the bifunctional Al-HMS catalyst were measured by X-Ray Pow der Diffraction(XRD)technique by means of STOE diffractometer(Germany).Nitrogen adsorption–desorption(physisorption)technique with using liquid nitrogen to maintain test temperature at−4 °C was used for bi-and tri-metallic catalysts and calculation based on nitrogen adsorption for the Brunauer–Emmett–Teller(BET)surface area in the range of relative pressures 0.03＜P/P0＜0.30 was implemented and the isotherm of nitrogen physisorption at P/P0=0.99 was used to obtain pore volume of synthesized mesoporous catalysts.

2.3.Catalytic evaluation reactor

The catalyst evaluation experiments were conducted in a multipurpose mini-reactor which was prepared and modified for the hydroprocessing of n-decane.The reactor setup consisted of four major sections:decane feed storage,hydrogen-n-decane mixer,Hitach MFC,and high pressure liquid chromatography(HPLC)feed pump.Additionally,it was included of a mini-reactor(10 mm inner radius),packed with bi-and tri-metallic meso-porous catalyst(catalyst particles are between 80 and 200 nm)supported on Al-HMS(Si/Al=20).An online gas chromatograph YOUNG LIN 6000(YL Instruments,China)equipped with a HID detector and connected to a personal computer were used for analyzing the products and storing the data(Fig.1).The reactor which is used for heterogeneous catalyst-gas reaction was placed in a temperature-controlled furnaces and the temperature of the catalytic bed was monitored by a thermocouple located at the center of the reactor.The specified molar ratio of hydrogen to hydrocarbon in feed(H2:HC)=1:8 controlled by the Hitachi mass flow controller(MFC).

The amount of 1 g bi-functional catalyst with using 5 g of silicon carbide(5 times the weight of catalyst)was packed in the middle of horizontal stainless steel mini-reactor and the catalytic bed was then reduced with a hydrogen flow of 57 ml·min−1at 450 °C for 2 h before the reaction.The n-decane and hydrogen gas at a rate of 20 ml·min−1were fed into the reactor at a specified molar ratio(10:1)using the mass flow controller and the liquid hourly space velocity(LHSV)of n-decane varying in the range of 1–3 h−1was arranged using a HPLC pump.After each experimental run(30 min)for the specified temperature,the reduction procedure was repeated for 20 min.Hydro-processing products were analyzed by the gas chromatograph with the HID detector using He as the carrier gas.

n-Decane conversion and product selectivity are crucial parameters for hydro-conversion and hydrogenation of n-decane.The operating and experimental conditions in Table 1 list different experimental results for n-decane conversion published in the literature and those in the present study.

3.Numerical Simulation

3.1.Geometry

ANSYS Workbench multi-physic was used for axisymmetric CFD model generation.Fig.2 illustrates the computational domain with specified boundary and dimensional conditions similar to the experimental reactor.A stainless steel 316 L horizontal mini-reactor was used with the radius and length of 10 mm and 100 mm of the physical properties of the porous media in reaction region is obtained from the Ergun equation:whereΔP is the pressure drop in bed,Δl is the bed length,μis the viscosity,ϕ is the bed porosity and dpis the particle diameter.

Fig.2.Computational grid domain and specified boundary conditions.a)3D physical domain,b)axisymmetric schematic of mesh and physical boundary.

Fig.2b shows the physical domain and boundary conditions used for reactor modeling.Grid is finer at the inlet region and near the reactor wall due to the larger gradients of temperature and species concentration.

3.2.Mathematical modeling

To develop a general form of catalytic hydro-conversion reactor model,the conservation equation of species,mass,energy and momentum were applied on the control volume using ANSYS 15.By using the plug flow theory,the boundary conditions 1–3 and the assumptions 4–7 were applied for the reactor.

(1)Inlet:velocity inlet condition;mass fraction of H2=0.11;mass fraction of C10H22=0.89

(2)Outlet:out flow condition,

(3)Wall:no slip condition,T=Tw

(4)The temperature of the reactor wall was maintained constant Tw;the equilibrium form for an isothermal condition

(5)Ideal gas was assumed for all species in reactor

(6)Laminar and incompressible flow obtained from the low Reynolds number(in the range of 10–100)

(7)Porous media was assumed for the catalytic region

Using the above assumptions,the governing equations are rewritten as follow s:

Species transport equation

Momentum equation

The porosity of catalyst(ϕ)appears in Eqs.(2)–(5),and the effective thermal conductivity of the bed(k)in Eq.(5).

3.3.Chemical reaction model

Here,the kinetic model deals with nine species:C1,C2,C3,C4,iC4,C5,iC5,C6+,and H2.They represent methane,ethane,propane,n-butane,iso-butane,n-pentane,iso-pentane,higher than hexane hydrocarbons and hydrogen respectively.This kinetic model was proposed to predict the light species production as the amount of light species,such as hydrogen and light hydrocarbons,affected the performance of downstream equipment.The reaction rate in the micro reactor is expressed as follow s:where(r)diis the reaction rate of hydro-cracking of n-decane,Pn-decaneis the partial pressure of n-decane and Ptotalis the total pressure in the mini-reactor.Rate constants illustrated in Table 2 were obtained from successive quadratic programming(SQP)[24].The C code of the reaction mechanism and source term were compiled,and then linked with the commercial Fluent software.

Table 2Rate constant of the proposed kinetic model[24]

3.4.Verification of the numerical model

The feed hydro-conversion was used for the verification of numerical simulation(Fig.3).Maximum error of experimental data used for the verification of reactor modeling is 2.5%which is calculated based on repeating each experiment for three times.Under the same operating conditions,the numerical predictions are in good agreement with the experimental data of the hydro-conversion of n-decane over trimetallic(Pt–Sn–Ni)supported on Al-HMS(Si/Al=20)with error＜4%.Also for some feed flow rates,reduction of conversion at 450°C relates to the reaction mechanism which is complex and the possible error in experimental measurements.Grid in dependency of reactor simulation was confirmed in terms of the hydro conversion of ndecane.To this end,four different numbers of mesh element(8000,16000,24000,and 28000)(2 mesh=8 mm)were considered and the simulated conversion of n-decane in 28000 mesh elements showed a deviation of 1%below the result of 24000 mesh.Hence,the grid of 24000 mesh elements was utilized for the subsequent modeling of hydro conversion reactor.

4.Results and Discussion

4.1.Catalyst characterization

The crystallinity of different synthesized catalyst was determined to by means of XRD characterization test.Fig.4 shows the XRD patterns of Al-HMS-20 material and corresponding metal loaded catalysts introduced as Sn–Ni,Pt–Sn,and Pt–Sn–Ni in the 2θ range of 1–80°.For synthesized Al-HMS-20 support and corresponding catalysts,drastic peak and intensive reflection were observed at SAXS test which informs the mesoporous structure of synthesized catalyst.To specify another understanding from XRD pattern,focus on the broadening of low angle peak is crucial since the mentioned peak was decreased with impregnating metal phase on Al-HMS-20 support.It is clear that for Al-HMS-20 mesoporous catalyst,partially collapse of the framework occurred during the metal addition and the effect of this phenomenon is significant for Pt–Ni/Al-HMS-20.In addition,Table 3 contains structural properties of bi-metallic(Sn–Ni),(Pt–Ni)and tri-metallic(Pt-Sn-Ni)catalysts supported on Al-HMS-20.

4.2.n-Decane hydro-conversion over mesoporous catalyst

Fig.3.Comparing the numerical results and experimental data for the conversion of n-decane for tri-metallic catalyst at different wall temperatures(300–500 °C),atmospheric pressure and inlet superficial velocities(0.0001–0.01 m·s−1).

Fig.4.XRD patterns of Al-HMS-20 material and corresponding metal loaded catalysts.

Table 3Structural properties of bi-metallic(Sn-Ni),(Pt-Ni)and Tri-metallic(Pt-Sn-Ni)catalysts,supported on Al-HMS(Si/Al=20)

Comparison of experimental selectivity of different types of products(aromatics,cracking,isodecane and hydrogenolisis)over three mesoporous(Pt–Sn,Sn–Ni and Pt–Sn–Ni)catalysts at reaction condition were shown in Fig.5 and the compositions of products were also presented.Fig.5 indicates that tri-metallic Pt-Sn-Ni catalyst supported on Al-HMS has the best selectivity for aromatics and isodecane in atemperature range of 400–500 °C.This is very promising as the high octane numbers have been shown to be directly related to the higher yields and higher selectivity for aromatics and branched hydrocarbon[25].

The cracking products of bi-and tri-metallic catalysts indicated that the selectivity of light hydrocarbons,such as C1–C2,increased with the increasing temperature(data not shown here).This was from the competition between cracking,hydrogenation and aromatization at high temperature,which were affected by different properties of catalyst.Various selectivities of light hydrocarbons imply the acidity of support and its declination due to the presence of Sn.This has been reported that with the addition of Sn at high temperature,the hydrocracking at the strong acid site is inhibited[26].

Fig.6 shows the 2D contour of temperature in the mini-reactor based on the simulation results.The exothermic hydro-conversion reactions are the main reason for rising the temperature in the radial direction.Also,the temperature variation along the reactor axis exists due to the occurrence of exothermic hydro conversion reaction.In the other word for the temperature distribution along the reactor axis,the higher the distance from the reactor inlet the higher the temperature.Moreover,since wall temperature is constant,reactor wall temperature maintained at 300°C as shown in the figure(blue line near the wall temperature).Also,symmetry of reactor temperature with respect to reactor axis is comprehensible which is consistent with the existence of axisymmetric condition implemented in the simulation.

Fig.5.Comparison of selectivity of different types of hydrocarbon produced at reaction conditions:LHSV=2 h−1,P=0.1 MPa,temperature=300–600 °C,H2/n-decane=8.

Fig.5(continued).

Fig.6.Temperature contours in the vertical cross-section of catalytic reactor for atri-metallic Pt–Sn–Ni catalyst at inlet velocity=0.001 m·s−1 and constant wall temperature 300 °C.

4.2.1.Effect of wall temperature

The effect of wall temperature is critical as a minor change in the wall temperature can leads to a massive change in the reaction rates.Fig.7 indicates the effect of temperature on the conversion efficiency of C10H22and H2as reactants,and the CH4concentration as the product which is conducted by simulation.The production rate and reactant conversion increased by about 16%due to a temperature rise of 200°C,the higher the temperature the higher the conversion of n-decane.At 500°C the mole fraction of n-decane and hydrogen is the low est at 0.03 and 0.02 respectively while methane has the highest fraction 0.2.Fig.8 demonstrates the effect of temperature on the simulated CH4distribution throughout the catalytic porous media.This finding is important as the occurrence of more conversion occurred by increasing temperature.

4.2.2.Effect of feed flow rate

The n-decane hydroconversions under several feed inlet velocities(0.0001,0.001,and 0.01 m·s−1)were investigated numerically over tri-metallic(Pt–Sn–Ni)Al-HMS-20 catalyst.An increase in the inlet velocity caused the reduction in the n-decane conversion(Fig.9).This result is against the result reported by Sabour et al.[27]for the LHSV effect on conversion over Al-HMS catalyst.Further,different types of hydrocarbon,saturated,branched and aromatic are reported in Figs.5 and 6,under an optimum inlet velocity condition(inlet velocity=0.0001 m·s−1or LHSV=2 h−1).It can be seen that the mole fraction contour of CH4at different inlet velocities indicates that the lower the inlet velocity the higher the production of cracking products(Fig.10)and observation of this phenomena is consistent with the increment of residence time of reactant in the catalytic bed.By comparing the results in Figs.8 and 10,it is found that the temperature effect on the contour of species distribution is greater than the one from inlet velocity in a hydro conversion process.

Fig.7.Effect of wall temperature on(a)the C10H22 and CH4 mole fraction,and(b)the H2 mole fraction(absolute velocity=0.001 m·s−1)in experiments.

Fig.9.Effect of feed flow rate on(a)the C10H22 and CH4 mole fraction,and(b)the H2 mole fraction(wall temperature=300°C)as demonstrated by experiments.

Fig.8.Contour of mole fraction of CH4 at(a)wall temperature=300 °C,and(b)wall temperature=500 °C(absolute inlet velocity=0.001 m·s−1).

Fig.10.Contours of mole fraction of CH4 at(a)absolute inlet velocity=0.0001 m·s−1,and(b)absolute inlet velocity=0.01 m·s−1(wall temperature=300 °C).

5.Conclusions

The catalytic hydroconversion of n-decane in a laboratory scale mini-reactor was investigated through numerical and experimental endeavors[28–30].Due to the combination of micro-and mesoporosity the catalytic activity was clearly improved as implied by N2adsorption–desorption.Unsteady,homogeneous and axisymmetric simulations of the catalytic performance of tri-metallic(Pt–Sn–Ni),supported on Al-HMS(Si/Al=20),were carried out for the minireactor setup.A good agreement was achieved between experimental data and the results from the numerical modeling with the following conclusions:

•The tri-metallic(Pt–Sn–Ni)catalyst,supported on Al-HMS(Si/Al=20),had the higher activity and selectivity than those of bi-and mono-metallic catalysts.

•The production of light hydrocarbon was reduced in the presence of Sn.

•An increase in the wall temperature produced the higher n-decane conversion.

•The impact of temperature on product distribution was greater than the feed flow rate effect.

•The wall temperature of 500 °C can be recommended for the hydrocracking over the introduced catalyst.