Document Type: Research Paper
Authors
^{1} Assistant Professor, Department of Applied Chemistry, Faculty of Chemistry, Urmia University, Urmia, Iran
^{2} Professor, Department of Applied Chemistry, Faculty of Chemistry, Urmia University, Urmia, Iran
^{3} Department of Applied chemistry, Faculty of chemistry, Urmia university, Urmia, Iran
^{4} Phd Candidate, Department of Applied Chemistry, Faculty of Chemistry, Urmia University, Urmia, Iran
Abstract
Keywords
1. Introduction
A new method for the removal of the sulfur content in fuel oil is needed today to meet the future legislation on sulfur content, e.g. lower than 50 ppm. The conventional method for reducing sulfur is catalytic hydrodesulfurization (HDS) under severe conditions (Brown et al., 1989). It has been reported that methyl substitution at 4 and 6positions of dibenzothiophene (DBT) remarkably retards the rate of HDS, and 4methyldibenzothiophene (4MDBT) or 4,6dimethyldibenzothiophene (4, 6DMDBT) are thus very difficult to convert due to their sterichindrance even under deep desulfurization conditions. In fact, the reactivities of DBT’s decrease in the order of DBT > 4MDBT > 4,6DMDBT (Babich and Moulijn, 2003; Nimwattanakul et al., 2006).
Sulfurbearing compounds available in most of oil fractions are so undesirable because of the adverse effects on process prosperity, catalyst performance, and environmental problems. They also reduce the performance of the engines during the combustion process. The high temperature and pressure for conventional hydrodesulfurization are the most restrictions upon this method. Beside these disadvantages of the hydrodesulfurization, using expensive cobalt molybdenum catalyst and high consumption of hydrogen are the other disadvantages of this method. According to the limitation imposed by the International Fuel Quality Centre (IFQC), the sulfur content of gas oil and gasoline must be reduced to 10 ppm before 2010 (Choi et al., 2004). The newest methods for the removal of sulfurbearing compounds are selective adsorption from transportation fuels. The major advances in petroleum refining as well as for fuel cell application will be acquired because of the ambient temperature and pressure accomplished by this method. However, all of the adsorptive materials used for the desulfurization process from the fuel derivatives were ineffective until now, and all of the studies on nonconventional adsorption have not yielded enough efficiency to scale up to industrial production (Breysse et al., 2003). Some of the selective and high capacity sulfur adsorbents that have been discovered are listed below. Zeolites containing cuprous or silver cations prepared by the ion exchange of zeolites using known ionexchange procedures are the class of adsorbents that bind thiophenic compounds selectively by πcomplexation (Rashidi et al., 2015; Zhizhong, 2008).
Various studies on oxidative desulfurization (ODS) have been reported. The reactivity of sulfur compounds for oxidation is increased with the increase of electron density on sulfur atom. The reactivities of DBT derivatives are influenced by the electron donation of substituted methyl groups, so the reactivity decreases in the order of 4,6DMDBT > 4MDBT > DBT, reversing the order of reactivities for HDS. After oxidation, the sulfur compounds are transformed to sulfones. The extraction of sulfones is considered to be a useful method for removal (Song, 2003; Teymouri et al., 2013).
In this work, the efficiency of nanostructured Niclinoptilolite adsorbent was investigated in the removal of sulfur from gas oil. Niadsorbents were prepared by a wetimpregnation method. Gas oil model containing 300 ppmW sulfur was prepared by dissolving a calculated amount of DBT in ndecane. The concentration of DBT in ndecane was determined by UVVisible spectrophotometer. To improve the adsorbent efficiency for sulfur compounds removal, a minimum amount of hydrogen peroxide was used as an oxidizing agent. Eventually, the effects of metal loading, type of support, and H_{2}O_{2} dosage on the efficiency of sulfur removal were studied.
Response surface methodology was employed to study the influence of experimental parameters on the desulfurization efficiency of gas oil by oxidative process using Niclinoptilolite. The sulfur removal efficiency was selected as the response for the optimization, and the functional relationship between the response and the most significant independent variables (factors) was established by means of the experimental design. Three main factors were chosen as independent variables: dosage of H_{2}O_{2}, treatment time, and the percentage of Nisupported on the clinoptilolite (Arcoya et al., 1997; Kyan and Oswald, 1997; Yetilmezsoy et al., 2009). The optimization of experimental parameters is usually assessed by the systematic variation of one parameter while the others are maintained constant. This approach is unable to predict the optimized conditions of the process. In this respect, experimental designs are appropriate tools for process optimization (Annadurai and Sheeja, 1998; Bezerra et al., 2008). In fact, the experimental design allows a considerable reduction in the number of experiments and a rapid interpretation. In the experimental design, it is possible to study a large number of factors and to detect the possible interactions between them. All the parameters are simultaneously applied in order to calculate their relative effect (Choi et al., 2004; Mansouri et al., 2014).
2. Experimental
2.1. Materials and regents
Clinoptilolite was obtained from Semnan deposits in northeast of Iran. Hydrogen peroxide, ndecane, dibenzothiophene, and Ni (NO_{3})_{2}.6H_{2}O were purchased from Merck (Germany).
In order to reduce the size of particles to 250420 μm using an uptodown method, the sample was ground and screened. The impurities of the ground materials were removed by using a magnet bar, by using ultrasonic cleaning, by refluxing, and by washing several times by deionized water. For the removal of the carbonate impurities, the ground material was dissolved in 1 N sodium acetate solution. The chemicals used were reagent grade and the solutions were prepared with deionized water. A series of Ni/clinoptilolite adsorbents was prepared using the incipient wetness impregnation technique. Supported Niadsorbents with different Ni loadings were prepared by impregnating an appropriate amount of Ni (NO_{3})_{2}·6H_{2}O onto supports followed by drying at 110 °C and calcining at 550 °C for 4 hrs in air.
2.2. Catalyst characterization
Scanning electron microscopy images were captured by (TESCAN) microscope of Ardabil University. The structural characterization of catalysts was investigated by Xray diffraction (XRD) technique to analyze the fresh and used catalysts qualitatively and quantitatively. The XRD patterns of the samples were obtained by using the Razi Metallurgical Research Center Xray diffractometer system.
2.3. Catalytic activity
The oxidation of the model sulfur compounds was conducted in a flask with a magnetic stirrer. A 100 mL portion of ndecane solution containing dibenzothiophene as sulfur compounds (300 ppm) and 1 gr. of the adsorbent were added to the flask. The mixture was stirred, and an amount of 5 wt.% H_{2}O_{2} was added. The reaction was continued for 60, 150, and 240 min. The evolution of oxidative desulfurization as was monitored using a UVVis spectrophotometer (TG+80, Urmia university). The performance of the process was evaluated by analyzing the response of dibenzothiophene oxidation percentage (yield) after a fixed stirring time:

(1) 
where, C_{0} is the initial DBT concentration, and C_{t} stands for the DBT concentration after treatment by Niclinoptilolite.
2.4. Design of experiments
Response surface methodology (RSM) was developed by Box and collaborators in the 50’s. This term was originated from the graphical perspective generated after fitting the mathematical model, and its use has been widely adopted in texts on chemo metrics. RSM consists of a group of mathematical and statistical techniques that are based on the fit of empirical models to the experimental data obtained in relation to experimental design (GhaffariMoghaddam et al., 2014). Toward this objective, linear or square polynomial functions are employed to describe the system studied and to consequently explore (modeling and displacing) experimental conditions until its optimization. In this technique, the main objective is to optimize the response surface that is influenced by process parameters. RSM also quantifies the relationship between the controllable input parameters and the obtained response surfaces. Process optimization by RSM is faster for gathering experimental research results than the rather conventional, time consuming one factor at a time approach (Annadurai and Sheeja, 1998; Aslan, 2008; Bas and Boyaci, 2007; Baş and Boyacı, 2007; Bezerra et al., 2008; Khataee et al., 2010; Yetilmezsoy et al., 2009) .
In order to design experiments to predict the optimum condition, BoxBehnken design (BBD) was used. The method suggests a point selection from the threelevel factorial arrangement, allowing the efficient estimation of the first and secondorder coefficients of the mathematical model. In Box–Behnken designs, the experimental points are located on a hyper sphere equidistant from the central point. Its principal characteristics are as follows:
1) It requires an experiment number according to N= 2k (k−1) + cp, where k is the number of factors, and (cp) represents the number of the central points;
2) All factor levels have to be adjusted only at three levels (−1, 0, +1) with equally spaced intervals between these levels.
To find the optimum conditions for the degradation of desulfurization, a Box Behnken design (BBD) was adopted to evaluate the combined effect of 3 factors (variables), i.e. dosage of H_{2}O_{2} as an oxidizing agent (00.25 mL), treatment time (60240 min), and the weight percentage of Ni (510). For three variables (n = 3) and two levels (high (+) and low (−)), the total number of experiments was 17 (listed in Table 1). The data set obtained from BBD was used for the optimization of the response of desulfurization percentage.
The response surface methodology was used to fit the experimental data. In the RSM, the responses can be simply related to chosen factors by firstorder or secondorder polynomial models. The firstorder model uses a loworder polynomial model, which is appropriate for describing a flat surface, according to the equation:
(2) 
where, y is the predicted response; β_{0} and β_{i} represent the constant term and coefficients of the linear parameters respectively; x_{i} stands for the variables, and ε is the random error.
If interaction terms are included, the firstorder model can then be represented as follows:
(3) 
where β_{ij} represents the coefficients of the interaction parameters x_{i} and x_{j}. A secondorder model may be studied in order to locate the optimum point. A secondorder model is given by:
(4) 
where, β_{ii} represents the coefficients of the quadratic parameter.
Statistical analysis was performed using the software package Minitab v.17 and a regression model was proposed. The analysis of variance (ANOVA) was performed based on the proposed model to find the interaction between the process variables and the response. The quality of the fit for the polynomial model was expressed by the coefficient of determination (R^{2}, R^{2}_{adj}), and statistical significance was checked by the Fvalue, Pvalue, and adequate precision in the same program. Model terms were selected or rejected based on the probability value with 95% confidence level (P > 0.05). Finally, threedimensional response surface plots and contour plots were drawn in order to visualize the individual and the interaction effects of the independent variables on desulfurization (Aslan, 2008).
According to the three factors selected for DOE, 17 experiments suggested by Minitab v.17 software.
Table 1
The 3factor Box Behnken design matrix (independent variables and their coded and actual levels: amount of H_{2}O_{2} as an oxidizing agent (mL), time of reaction (min), weight percentage of Ni (%)) and the experimental results (desulfurization (%)) from gas oil.
Run 
V(H_{2}O_{2}, mL) 
Time (min) 
Ni (wt.%) 
1 
0 
150 
10 
2 
0.125 
150 
7.5 
3 
0.125 
240 
5 
4 
0 
240 
7.5 
5 
0.125 
60 
10 
6 
0.25 
150 
10 
7 
0 
60 
7.5 
8 
0.125 
240 
10 
9 
0.125 
150 
7.5 
10 
0.125 
60 
5 
11 
0.25 
240 
7.5 
12 
0.25 
60 
7.5 
13 
0.25 
150 
5 
14 
0.125 
150 
7.5 
15 
0 
150 
5 
3. Results and discussion
3.1. Catalyst determination
Figures 1 and 2 show the observed XRD patterns for the clinoptilolite catalysts with different amounts of Ni supported on clinoptilolite. It is found that the metallic Ni peaks (2θ=36° and 44.5°) of lower metal loading catalysts are weaker and broader than that of the catalysts with higher metal loading. This suggests that the Ni crystallite size is smaller on lower Ni loading catalysts than on the catalysts with a high Ni content. Figure 3 illustrates the scanning electron microscopy of the Niclinoptilolite proving the nanostructure of the synthesized catalyst and the agglomeration synthesis of the catalyst.
Figure 1
XRD pattern of clinoptilolite catalyst coated with 7.5% Ni.
Figure 2
XRD pattern of clinoptilolite catalyst coated with 10% Ni.
Figure 3
Scanning electron microscopy (SEM) of the Niclinoptilolite.
3.2. RSM modeling and optimization of desulfurization treatment
Response surface designs are useful for modeling a curved quadratic surface to continuous factors. A response surface model can pinpoint a minimum or maximum response if one exists inside the factor region. Three distinct values for each factor are necessary to fit a quadratic function, so the standard twolevel designs cannot fit curved surfaces. Box Behnken design was used in this study for the optimization and modeling of the process. The BoxBehnken design is an alternative to central composite designs. One distinguishing feature of the BoxBehnken design is that there are only three levels per factor.
The 3factor BBD matrix and experimental results obtained for the desulfurization from gas oil are presented in Table 1. Based on the experimental design (Table 1), a response surface model relating the desulfurization efficiency to independent variables was constructed to fit the experimental data. The linear model in terms of coded variables that approximated the efficiency of desulfurization process may be written as follows:

(5) 
where, Y is a response variable of sulfur removal efficiency. The b_{i} are regression coefficients for linear effects; b_{ii} represents the regression coefficients for squared effects; b_{ik} is the regression coefficients for interaction effects, and x_{i} stands for coded experimental levels of the variables.

(6) 
The models explained perfectly the experimental range studied as can be seen from the comparison of the graphical representation of actual values with the predicted ones (Figure 4).
Figure 4
Comparison of the experimental results of desulfurization efficiency (CR%) with those calculated via BoxBehnken design resulted equation.
Table 2 shows the results of ANOVA analysis of the linear model for the desulfurization process of gas oil. Larger Fvalues and smaller Pvalues are an indication of the significance of the model.
Table 2
Analysis of variance (ANOVA) for the fit of MO removal efficiency from BoxBehnken design.
Source 
Degree of freedom (DF) 
sequential sum of squares 
Adjusted[M. N.2] sum of squares 
Adjusted mean of squares 
Fvalue 
Pvalue 
Regression 
7 
2708.06 
2708.055 
386.865 
13.98 
0.001 
Linear 
3 
2088.64 
584.958 
194.986 
7.04 
0.016 
Square 
2 
554.50 
554.502 
277.251 
10.02 
0.009 
Interaction 
2 
64.92 
64.915 
32.458 
1.17 
0.364 
Residual Error 
7 
193.78 
193.77 
27.682 
 
 
LackofFit 
5 
185.14 
185.141 
37.028 
8.57 
0.018 
Pure Error 
2 
8.64 
8.637 
4.318 
 
 
Total 
14 
2901.83 
 
 
 
 
The model Fvalue of 13.98 and Pvalue smaller than 0.001 implied the high significance of the model. There was only a 0.01% chance that the “Model Fvalue” could occur due to noise. Pvalues less than 0.05 indicated which model terms are significant. In this case, all the factors were highly significant because Pvalue is smaller than 0.05. The “Lack of Fit Fvalue” of 8.57 and Pvalue of 0.018 implied that the lack of fit was not significant relative to the pure error. The nonsignificant lackoffit indicated the good predictability of the model. The coefficient of determination (R^{2}) is defined as the ratio of the explained variable to the total variation and is a measure of the degree of fit. It was found that the predicted values matched the experimental values reasonably well with R^{2} = 0.9827. This implied that 98.27% of the variations for sulfur compounds removal were explained by the independent variables, and this also meant that the model did not explain only 1.73% of variation. The R^{2} of 0.9827 was in reasonable agreement with Adj. R^{2} of 0.9827, also indicating good predictability of the model (Table 3).
Table 3
Statistical measures and performance of the RSM using the analysis of variance (ANOVA).
S 
R^{2} 
R^{2}(adj.) 
5.261 
98.27% 
86.6% 
Threedimensional response surface plots computed by means of the response surface model for desulfurization are shown in Figures 58, revealing the predicted effects of factors upon response.
Figure 5
The response surface plot for desulfurization efficiency (CR%) as the function of Ni (wt.%) and treatment time (min).
Figure 6
The response surface plot for desulfurization efficiency (CR%) as the function of H_{2}O_{2} (mL) and Ni (wt.%).
Figure 7
The response surface plot for desulfurization efficiency (CR%) as the function H_{2}O_{2} (mL) and treatment time (minute).
Figure 8
The results of Pareto analysis
Figure 5 illustrates the effect of treatment time and the weight percentage of Nisupported on clinoptilolite on desulfurization efficiency (CR%) for an initial DBT concentration. It is observed that the removal efficiency was enhanced with increasing treatment time (min) and decreasing the weight percentage of Ni, and at least more than 74% sulfur removal was obtained after 150 minutes at a 5 wt.% of Nisupported on clinoptilolite. It is also obvious that both of these two independent variables are so important in sulfur removal. Since adding Ni on the surface of clinoptilolite occupies the active desulfurization sites on the catalyst, increasing the amount of Ni causes less efficiency of the Niclinoptilolite in the treatment process.
Figure 6 depicts the effectiveness of hydrogen peroxide and Ni in the desulfurization of the gas oil. As it can be seen, the addition of hydrogen peroxide as an oxidizing agent decreases desulfurization efficiency because of the conjuncture of nanoadsorbent and thus the reduction of the surface/volume ratio.
Figure 7 shows the effect of treatment time and the amount of H_{2}O_{2} (mL) on desulfurization process efficiency. As it is inferred from Figure 7, an increase in treatment time causes the maximum adsorption of sulfur compound on the adsorbent, so more sulfur compounds are transformed into sulfone in desulfurization process. The contour plots of all the factors involved in desulfurization process efficiency are shown in Figure 8 with so near proximity of the proposed information about the factors sensitivity in the process.
3.3. Determination of optimal conditions for methyl orange removal
The desired goal in terms of sulfur compound removal efficiency was defined as “maximize” to achieve the highest treatment performance. The optimum values of the process variables in the optimum conditions are 5 wt.%, 0 (mL), and 240 (min) for the amount of Nisupported on clinoptilolite, dosage of hydrogen peroxide (H_{2}O_{2}) as an oxidizing agent regent, and treatment time respectively. After verification through a further experimental test with the predicted values, the result indicates that the removal efficiency of maximum sulfur compounds was obtained when the values of each parameter were set as the optimum values. It implies that Response surface methodology gives the optimized conditions to obtain the maximum desulfurization efficiency in the adsorptive process using Niclinoptilolite.[M. N.3]
4. Conclusions
The RSM model constructed out of the experimental data correlated fairly well (R^{2}=0.9827 for response surface methodology). Furthermore, two validation experiments were performed and the response of the model was found in agreement with the experimental results. The nobility of the work was to use the best amount of materials and methods in order to modify the synthesized catalyst for effective proficiency in desulfurization process. The model indicated that treatment time had the highest positive effect on sulfur compound removal efficiency as observed through beta regression coefficients, whereas the dosage of hydrogen peroxide had less positive effects on desulfurization efficiency. Thus, both the oxidizing agent and the metal (Ni) loaded on clinoptilolite cause less efficiency in desulfurization process because of adverse interactions with the sulfur adsorption. Although an increase in treatment time enhances the efficiency of the process, it did not change after 240 minutes. Therefore, time is effective until all the active sites of Ni are occupied by sulfur compounds. Energy conservation in the process can be better achieved by increasing the treatment time and by reducing the amount of Nisupported catalyst; however, this will ultimately increase the reactor size. The methods used to optimize the proficiency of the process could be performed in each part of industrial desulfurization plants in order to purify the gas oil product from any unwanted sulfur compounds.
Acknowledgments
The authors thank the Urmia University Excellent Grant and support in all the steps of the research.
Nomenclature
ANOVA 
: Analyze of variance 
BBD 
: Box Behnken design 
DBT 
: Dibenzothiophene 
IFQC 
: International Fuel Quality Centre 
Ni 
: Nickel 
RSM 
: Response surface methodology 
SEM 
: Scanning electron microscopy 
UV 
: Ultra violet 
XRD 
: X ray diffraction 