Elsevier

Fuel

Volume 88, Issue 1, January 2009, Pages 87-94
Fuel

Sulfur removal from model diesel fuel using granular activated carbon from dates’ stones activated by ZnCl2

https://doi.org/10.1016/j.fuel.2008.07.019Get rights and content

Abstract

Samples of granular activated carbon (GAC) were produced from dates’ stones by chemical activation using ZnCl2 as an activator. Textural characteristics of GAC were determined by nitrogen adsorption at 77 K along with application of BET equation (Brunauer, Emmett and Teller) for determination of surface area. Pore size distribution and pore volumes were computed from N2 adsorption data by applying the nonlinear density function theory (NLDFT). FT-IR spectra of GAC samples were also obtained to determine the functional groups present on the surface. GAC samples were used in desulfurization of a model diesel fuel composed of n-C10H34 and dibenzothiophene (DBT) as sulfur containing compound. More than 86% of DBT is adsorbed in the first 3 h which gradually increases to 92.6% in 48 h and no more sulfur is removed thereafter. The adsorption data were fitted to both Freundlich and Langmuir equations to estimate the adsorption parameters. The optimum operating conditions for GAC preparation based on high adsorption capacity are Tcarb = 700 °C, θcarb = 3.0 h and R = 0.5. Moreover, the efficiency of sulfur removal by GAC is reduced when applied to commercial diesel fuel. Finally, linear regression of experimental data was able to predict the critical pore diameter for DBT adsorption (0.8 nm) and validating the reported impact of average pore diameter of activated carbon on the adsorption capacity.

Introduction

Due to the increasingly stringent environmental regulations on the sulfur content in transportation fuels, ultra-deep desulfurization of diesel fuel has become a very important research topic. Sulfur compounds are the main cause of acid rain and poisoning of catalysts in CO and NOx catalytic converters [1], [2]. With the new environmental protection agency (EPA) Tier II regulations to cut the diesel sulfur from current 500 parts per million by weight sulfur (ppmw S) down to 15 ppmw S by June 2006, refineries are facing major challenges to meet the new fuel sulfur specification along with the required reduction of aromatics contents [3]. Further lower sulfur limits are expected for highway and also for non-road diesel fuels in the near future [4].

Although, fuel cells can be ideally operated with zero sulfur fuels [4] such as methanol-based fuels which are used to avoid poisoning of catalytic converters [5], gasoline and diesel commercial fuels are preferred due to their high-energy content, availability, safety and ease of storage. The sulfur concentration preferably should be below 0.1–0.2 ppmw.

Hydrodesulfurization (HDS) is the conventionally applied process for reducing of organo-sulfur in gasoline, diesel and other intermediate distillates where Co–Mo/Al2O3 or Ni–Mo/Al2O3 is used as the catalyst. This process successfully removes many sulfur compounds such as thiols, sulfides, disulfides and some thiophene derivatives, but to much lower extent removes dibenzothiophene derivates due to the steric hindrance on the sulfur atom (refractory organosulfur compounds), such as 4,6-dimethyldibenzothiophene (4,6-DMDBT), which are present in diesel fuel with sulfur concentrations in the order 400 ppmw [6], [7]. Other ways of increasing the effectiveness of HDS for producing low sulfur product include the use of higher temperatures and pressures, more active catalysts or longer residence times [4]. However, these alternatives are costly to refineries. This fact has enthused research in deep HDS, as well as the development of alternative or additional desulfurization technologies for diesel fuels [8].

Extensive work has been done to reduce the sulfur content of diesel fuels from 500 to 15 ppmw including engineering of the HDS reactor size and volume and development of adsorbents to remove organic sulfur compounds from fuels either through complexation [9], [10], [11], [12], [13], [14], [15], [16], [17], van der Waals, electrostatic interactions [16], [18], [19] or reactive adsorption by chemisorption at elevated temperatures [20], [21] among many others, for instance, McKinley and Angelici have shown preferential adsorption of dimethyldibenzothiophene (DMDBT) and dibenzothiophene (DBT) over decane on Ag+ salts supported on silica [22]. Plentiful approaches have been adopted with developing the catalytic activity of the conventional Co(Ni)Mo(W)/Al2O3 catalysts include mesoporous materials such as MCM-41 [23], [24], HMS [25], [26], [27] SBA-15 [22], [28], [29]. The support in details has been analyzed in recent reviews [29], [30]. Very recently, organo-sulfur compounds have been successfully reduced using desulfurizing bacteria strain [31], [32], [33]. These materials were very promising in reduction of the sulfur content in fuels, but they are either work at high temperature or difficult and expensive to prepare.

One of the promising new approaches to phase out these problems is the selective adsorption of DBTs at ambient temperature and pressure [34]. The great challenge in development of an effective adsorptive desulfurization process is to develop an adsorbent with high selectivity and high capacity of the sulfur compounds. Recently, many attempts have been made to develop adsorbents for desulfurization of liquid hydrocarbon fuels. The adsorption mechanism and selectivity over various adsorbents are still unclear [35].

Activated carbon (AC) is widely used as an adsorbent of organic contaminants due to its porous nature and large specific surface area and can be a good candidate for HDS. Activated carbon was studied in the desulfurization of digester gas [36], flue gas [37], [38] and fuel oil [39], [40]. The activated carbon shows higher adsorptive capacity and selectivity for both sulfur and nitrogen compounds, especially for the sulfur compounds with methyl substituents, such as 4,6-dimethyldibenzothiophene [35]. A number of studies directed to the use of AC in desulfurization of fuels include mathematical simulation of fixed-bed reactor for removing H2S with impregnated activated carbon [41], HDS of straight run gas oil using activated carbon fiber (ACF) [42], [43] and ultra deep hydrodesulfurization (UD-HDS), where sulfur and nitrogen species were removed by adsorption from the straight run gas oil (SRGO) [21].

Carbon aerogels produced by sol–gel is also used as adsorbents in desulfurization of diesel fuel [44], where higher adsorption capacity as they exhibit a larger pore size [45]. Activated carbon is also used for oxidative removal of DBT in presence of H2O2 as oxidant, where very low sulfur residual were left in the oxidized oil [45].

This paper presents preparation of granular activated carbon simply obtained by carbonization of dates’ stones, a very cheap agricultural waste, by chemical activation employing ZnCl2 activator for removal of DBT from a model diesel fuel. This material has showed very promising results in phenol and dye removal [46], [47], [48]. Effect of the preparation conditions on DBT removal will also be investigated.

Section snippets

Preparation of GAC samples

Dates’ stones were obtained from local dates’ packaging factories and cleaned by washing several times with distilled water to remove the impurities and sweet contaminants. Dates’ stones were then grinded to a particle sizes ranging form 1.0 to 2.4 mm and sieved to different particle size fractions. An average of 1.71 mm particle size is chosen for preparation of GAC samples. These samples are prepared by impregnation with ZnCl2 as activator according to the procedure published elsewhere [49]. A

Nitrogen adsorption measurements

Generally, the shape of the nitrogen adsorption isotherms strongly depends on the ZnCl2 impregnation ratio used. The nitrogen adsorption isotherms of samples DDS-23 and DDS-24 are depicted in Fig. 1a. Both samples were prepared at 700 °C for 3 h with two different impregnation ratios, R. For the sample prepared at R = 0.5 (DDS-23) type I adsorption isotherm is obtained according to the IUPAC classification [51]. The major nitrogen uptake occurs at relative pressures less than 0.25. An almost

Conclusions and recommendations

  • Selective adsorption of DBT using activated carbon activated prepared by ZnCl2 activator and prepared at different conditions from dates’ stone is feasible, promising and worth further studying.

  • The obtained adsorption data can be represented by both Freundlich and Langmuir isotherms. However, Freundlich isotherm fits the data more accurately.

  • Beside higher Tcarb and θcarb lower R ratio is preferred over higher values for higher adsorption capacity of the GAC samples.

  • Activated carbon with higher

Acknowledgements

This research was funded by the deanship of scientific research, King Abulaziz University, Project No. 105/427. Special thanks to Mr M. Rajput for GC analysis, Dr. M. Abdul Al-Salam, for FT-IR measurements and Dr. Arafat for valuable discussion and help with interpretation of FT-IR data.

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