Adsorption desulfurization performance and adsorption-diffusion study of B2O3 modified Ag-CeOx/TiO2-SiO2

doi:10.1016/j.jhazmat.2018.09.037

Journal of Hazardous Materials

Volume 362, 15 January 2019, Pages 424-435

https://doi.org/10.1016/j.jhazmat.2018.09.037 Get rights and content

Highlights

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The loading of B₂O₃ improved CeO_x dispersion and then further facilitated Ag dispersion.
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D_s obtained by the batch experiment was applied to the prediction of breakthrough curves.
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It was good agreement between the simulated and experimental data.

Abstract

In this work, the adsorption desulfurization performance and adsorption diffusion study of B₂O₃ modified Ag-CeO_x/TiO₂-SiO₂ adsorbent were investigated. The adsorption desulfurization performance was studied by batch and fixed bed tests. The homogeneous surface diffusion model (HSDM) was employed to investigate the adsorption and diffusion behavior of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in diesel. It was found that the addition of B₂O₃ promotes the dispersion of CeO_x species and then further facilitates the dispersion and oxidation of Ag species resulting in higher adsorption desulfurization activity. Ag species are in state of Ag, Ag₂O and Ag₂O₂, among which, Ag₂O and Ag₂O₂ are found to be the active centers.

The kinetics of adsorption desulfurization of model diesel fuel was investigated to provide guiding significance for the prediction of breakthrough curves of fixed-bed adsorption columns. The batch kinetic experiment modeled by HSDM model indicates that surface diffusion controls the main rate. The surface diffusion coefficient D_s determined by batch adsorption experiments is independent of operation conditions, which can be used to directly predict the breakthrough behavior in fixed bed adsorption. The modified HSDM model is proposed to describe the breakthrough behavior. Results indicate that the breakthrough time is affected by bed height, flowrate and influent concentration.

Introduction

With the increaseof the demand for clean energy, the removal of sulfur compounds from fuels has attracted more and more attention in the recent years. As we know, the presence of sulfur compounds will not only cause environment pollution and acid rain which is bad for human health, but also lead to catalyst poisoning. Therefore, it’s necessary to develop an effective technique to remove the sulfur compounds from the source. Traditional hydrodesulfurization (HDS) [[1], [2], [3]] is the major industrial method to remove the sulfur compounds such as sulfides, mercaptans, disulfides and thiols. However, the dibenzothiophene (DBT) and its alkyl derivatives are still recalcitrant to the HDS process. Simultaneously, the HDS technique requires higher temperature, pressure, and more hydrogen which results in high operation cost [4,5].

Adsorption desulfurization [[6], [7], [8]] is a promising technology for the desulfurization due to milder operation conditions and its simplicity. A variety of metals such as silver [[9], [10], [11]], copper [[12], [13], [14], [15], [16]], nickel [[17], [18], [19]] and cerium [[20], [21], [22]] are widely acted as active components supported on various inorganic supports [8,[23], [24], [25]]. Among them, Ag exhibits the best adsorption desulfurization performance. However, the desulfurization performance of most of these adsorbents can not meet the standard of sulfur amount in fuels due to the presence of steric hindrance of alkylated sulfur compounds and the strong competitive adsorption between polyaromatic hydrocarbons and sulfur compounds [[26], [27], [28], [29]]. Therefore, a variety of techniques of adsorbent modification were studied to enhance the adsorption desulfurization performance. As we know, the surface properties of adsorbent such as the surface acidity and surface defects caused by incorporating other hetero atoms are the key factors to influence the adsorption desulfurization performance. Xu et al. [30] found that the doping of CeO_x can dramatically facilitate the dispersion of Ag species and changed the valence state of Ag species resulting in the improvement of the adsorption desulfurization performance. The B₂O₃-modified Ag/TiO₂-Al₂O₃ by adsorbents reported by Li et al. [31] exhibited higher adsorption desulfurization performance due to the more weak acid sites. Tatarchuk et al. [32] found that the loading of TiO₂ could provide more active sites and improved the dispersion of Ag components to give higher desulfurization activity. So we attempted adulterating B₂O₃ to adjust the surface properties of adsorbent.

In recent years, most researchers paid attention to the preparation and modification of adsorption desulfurization adsorbents to enhance the adsorption performance. However, few researchers systematically focused on the study from the preparation of adsorbents to engineering aspects such as isotherm, kinetics and adsorption-diffusion behavior of the adsorption desulfurization process which are important to the design of adsorber. Fixed bed adsorption technology has been widely applied in separation and purification field and other fields due to its simplicity and high efficiency. Therefore, designing and optimizing the production process and operating conditions of fixed bed are important. The determination of breakthrough curves under different conditions obtained through experiment not only cause high cost but also need long period. Therefore, it is necessary to prepare a low-cost adsorbent with good desulfurization performance, and develop a suitable mathematical model to predict the fixed bed adsorption behavior to systematically study the diffusion and mass transfer of adsorption desulfurization. The HSDM model has been successfully applied in different systems [[33], [34], [35], [36], [37]]. The adsorption and diffusion behavior of p-nitrophenol in the structured column predicted by the modified HSDM model was investigated by Shao et al. [33]. It was found that the breakthrough behavior of p-nitrophenol can be successfully predicted by HSDM model and found that the structured column could improve bed capacity utilization and the mass transfer rate.

In this work, the adsorption desulfurization performance and adsorption-diffusion study of B₂O₃ modified Ag-CeO_x/TiO₂-SiO₂ were investigated. The characteristics of adsorbent with and without B₂O₃ modification were investigated by N₂-physisorption, TEM, HAADF STEM, EDX, XPS, O₂-chemisorption to reveal the mechanism of B₂O₃ modification and the relationship between the structure and performance of adsorbent. Simultaneously, the batch equilibrium and kinetic experiment were inspected to provide model parameters for the prediction of breakthrough curves in fixed bed using HSDM model. Finally, the HSDM model was proposed to predict the breakthrough behavior and explained the diffusion behavior in fixed bed for adsorption desulfurization.

Section snippets

Preparation of adsorbent

The Ag-B₂O_3/CeO_x/TiO₂-SiO₂ adsorbents were prepared by multi-step impregnation method. Firstly, the SiO₂ power (Qingdao Xinchanglai silica gel Co., Ltd.) was activated via calcination at 823 K for 2 h. The B₂O_3/CeO_x/TiO₂-SiO₂ support was prepared by co-impregnation method with 1:1:2:44.3 M ratio of B:Ce:Ti:Si with Ti component, B component and Ce component incorporating successively. Briefly, 0.6 g calcined SiO₂ power was immersed in 10 mL ethanol solution of tetrabutyl titanate (Tianjin

Theory

The HSDM model assumes that surface diffusion is dominant rate limiting step. Simultaneously the external film mass transfer is also considered.

The equations of HSDM model are listed as follows [41]:

The mass balance equation of the moving phase in the column is: $\frac{\partial c (z, t)}{\partial t} = D_{L} \frac{\partial^{2} c (z, t)}{\partial z^{2}} - u \frac{\partial c (z, t)}{\partial z} - \frac{3 (1 - ε)}{ε R} k_{f} [c (z, t) - c_{s} (z, t)]$

The mass balance equation of particle can be represented as: $\frac{\partial q (r, z, t)}{\partial t} = D_{s} [\frac{\partial q^{2} (r, z, t)}{\partial r^{2}} + \frac{2}{r} \frac{\partial q (r, z, t)}{\partial r}]$

The moving phase and particle concentration is correlated with Freundlich equation [42]: $q (r = R, z, t)$

Pore structural characterization of the adsorbents

Fig. 1 shows the N₂ physical adsorption isotherm and pore diameter distribution of supports and adsorbents. Table 1 shows the corresponding porous parameters. The N₂ physical adsorption isotherm curves of all samples are attributed to be mixed type H1 and H3 loops. Simultaneously, there only exist slight differences between B₂O₃-modified Ag-CeO_x/TiO₂-SiO₂ adsorbents and unmodified adsorbent, indicating that the B₂O₃ particles don’t block the pore structure of adsorbents. The presence of

Conclusions

The B₂O₃ modified Ag-CeO_x/TiO₂-SiO₂ adsorbent was prepared by novel co-impregnation method for the removal of the organosulfur compounds from diesel fuel. The results proved that B₂O₃ modified Ag-CeO_x/TiO₂-SiO₂ adsorbent exhibited an enhancement for the desulfurization performance for CN-IV standard diesel fuel in batch experiment. The doping of B₂O₃ can promote the dispersion of Ag species on Ag-B₂O₃/CeO_x/TiO₂-SiO₂ adsorbent without blocking the pore structure of adsorbents. Ag species on

Acknowledgment

We acknowledge the support of the National Natural Science Foundation of China (No. 21376055).

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Adsorption desulfurization performance and adsorption-diffusion study of B2O3 modified Ag-CeOx/TiO2-SiO2

Highlights

Abstract

Introduction

Section snippets

Preparation of adsorbent

Theory

Pore structural characterization of the adsorbents

Conclusions

Acknowledgment

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Adsorption desulfurization performance and adsorption-diffusion study of B₂O₃ modified Ag-CeO_x/TiO₂-SiO₂