فایل ورد کامل سولفورزدایی با جذب سطحی سوختهای جت و دیزلی با استفاده از ماده های جاذب سطحی Ag/TiOx–Al2O3 و Ag/TiOx–SiO2

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تعداد صفحات این فایل: ۲۷ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Adsorptive desulfurization of jet and diesel fuels using Ag/TiOx–Al2O3 and Ag/TiOx–SiO2 adsorbents~~en~~

Abstract

The objective of this work is to examine the performance of Ag/TiOx–Al2O3 and Ag/TiOx–SiO2 for adsorptive desulfurization of JP5, JP8, Off-Road Diesel (ORD), and Ultra Low Sulfur Diesel (ULSD) at ambient conditions. These adsorbents were observed to be effective for desulfurizing liquid hydrocarbon feeds that ranged from 7.5 to 1172 ppmw S and comprised of diverse sulfur compositions. In fixed bed continuous adsorption tests, 4 wt.%Ag/TiOx–Al2O3 demonstrated saturation capacities of 10.11, 6.11, and 7.4 mg S/g adsorbent for JP5 (1172 ppmw S), JP8 (630 ppmw S), and ORD (452 ppmw S), respectively. In equilibrium saturation experiments, the adsorbent was able to desulfurize ULSD down to less than 75 ppbw S and this was achievable even when the initial concentration of non-sulfur aromatics was greater than 25,000 times higher. Dispersing TiOx onto high surface area alumina and silica substrates increased the sulfur adsorption capacities of both. This enhancement in adsorption capacity increased still further with silver addition. Higher Ag loading (up to 12 wt.%Ag) on TiOx–Al2O3 proved beneficial to sulfur adsorption capacity when compared to TiO2 (up to 4 wt.%Ag), indicating that the mixed oxide supports were able to host more active silver oxides. The adsorption selectivity toward different sulfur compounds present in the fuels varied during the fixed bed adsorption tests. The adsorbent had the greatest affinity for BT’s and the least for TMBT’s and DMDBT’s (the selectivity order from the strongest to weakest adsorption was: BT > MBT > DMBT > DBT MDBT > TMBT DMDBT). Ag/TiOx–Al2O3 was thermally regenerable in air for multiple cycles at temperatures ranging from 110 to 450 °C. The effects of titanium loading, silver loading, and titanium precursor on sulfur adsorption capacity are also presented.

۱ Introduction

Sulfur and its derivatives are major contaminants in hydrocarbon fuels. Desulfurization of fuels has gained importance due to environmental concerns, and many countries are enacting laws limiting maximum sulfur emissions. For example, the maximum sulfur concentration in highway diesel in the US has been limited to 15 ppmw since 2006, down from 500 ppmw [1–۳]. Other transportation fuels are also being regulated to reduce sulfur content. Desulfurization also has a significant impact on the successful application of fuel cell technologies. Transportation fuels such as gasoline, diesel, and jet fuel are ideal for on-board fuel cell systems because of their high energy density, availability, and operational safety factors. However, the development of fuel cell systems is restricted due to the demand of ultra low sulfur feeds in their reformation systems [4]. Sulfur poisons precious metal electrodes in fuel cells and reforming catalysts, therefore only fuels with less than 100 ppb sulfur content are allowable in fuel cell systems such as Proton Exchange Membrane (PEM) fuel cells [1,5]. High temperature Solid Oxide Fuel Cells (SOFCs) typically require less than 30 ppm S in the feed prior to reforming [6].

Most of the organosulfur compounds are generally removed from hydrocarbon fuels by conventional Hydrodesulfurization (HDS) process in refineries. It is a catalytic process requiring high temperature (300–۴۰۰ C) and pressure (3–۶ MPa) in the presence of hydrogen [1]. In the recent years, refineries are facing higher amounts of sulfurous crude oil as feedstock due to the diminishing crude oil reserves and producing larger volume of products from high sulfur heavy oil fractions. This, along with the demand for ultra clean fuel, are increasing the desulfurization cost [7]. Especially for achieving the sulfur concentration tolerable to fuel cells, the HDS process has to increase its reactor volume and H2 consumption [8,9]. HDS also has less satisfactory performance in removing Poly Aromatic Sulfur Heterocycles (PASHs) [10]. Hence, it is necessary to develop an alternative or supplementary process to HDS for deep desulfurizing fuel. Among several alternative processes reported earlier [1,8,11–۱۷], desulfurization by direct adsorption of organosulfur species at ambient conditions has gained much attention [9,18–۲۴]. Adsorptive desulfurization provides great applicability and has several advantages such as its selectivity toward PASH, scalability, and the ability to desulfurize hydrocarbon fuels to near zero sulfur content. It also does not require hydrogen or any other auxiliary units. Adsorptive desulfurization can supplement HDS process as a polishing step and offers an alternative solution to the high cost of producing ultra clean fuels.

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