Isotherm and Thermodynamic Studies on the Removal of Sulfur from


Jan 17, 2019 - In recent years, fuel modifications, such as the production of ultralow sulfur diesel, have been mandated by international agencies to ...
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Isotherm and thermodynamic studies on the removal of sulfur from diesel fuel by mixing-assisted oxidative - adsorptive desulfurization technology Marvin L. Samaniego, Mark Daniel Garrido de Luna, Dennis C. Ong, Meng-Wei Wan, and Ming-Chun Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04242 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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Energy & Fuels

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Isotherm and thermodynamic studies on the removal of sulfur from diesel fuel by mixing-

2

assisted oxidative - adsorptive desulfurization technology

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Marvin L. Samaniegoa, Mark Daniel G. de Lunaa,b, Dennis C. Ongc, Meng-Wei Wand, Ming-

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Chun Lud,*

6 7

a

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Environmental Engineering Program, National Graduate School of Engineering, University of the Philippines, 1101 Diliman, Quezon City, Philippines

b

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Department of Chemical Engineering, University of the Philippines, 1101 Diliman, Quezon City, Philippines

11

c

School of Technology, University of the Philippines Visayas, Miagao, Iloilo 5023, Philippines

12

d

Department of Environmental Resources Management, Chia-Nan University of Pharmacy and

13

Science, Tainan 71710, Taiwan, E-mail: [email protected]

14 15

* Corresponding author

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Graphical Abstract

25 26

Highlights

27



PAC and alumina had homogeneous and heterogenous adsorption sites, respectively

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Adsorption activation energy implied sulfur chemisorption on powdered alumina

29



Chemical reaction and diffusion processes controlled the sulfur-PAC adsorption

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Adsorption process for both PAC and powdered alumina was endothermic

31



High and low temperature favored sulfur-PAC and -alumina adsorption, respectively

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Abstract

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In recent years, fuel modifications, such as the production of ultra-low sulfur diesel

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(ULSD), have been mandated by international agencies to limit gaseous sulfur emissions and

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reduce atmospheric pollution. In this study, raw diesel fuel was subjected to sequential (1) high

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shear mixing-assisted oxidative desulfurization and (2) adsorptive desulfurization. A detailed

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study on the isotherm and thermodynamics of sulfur removal was carried out using powdered

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activated carbon (PAC) and powdered alumina in batch adsorption experiments. Results showed

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that sulfur adsorption by PAC and powdered alumina followed the Langmuir (R2 = 0.9020) and

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the Freundlich (R2 = 0.8626) isotherm models, respectively. Adsorption of sulfur by powdered

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alumina was controlled solely by chemisorption, while adsorption by PAC was controlled by a

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combination of a chemical reaction and diffusion processes. For both powdered alumina and

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PAC, the positive values of the enthalpy of activation (ΔH) indicate that the adsorption process

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was endothermic. Negative ΔS and increasing ΔG values with increase in temperature indicates

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that lower temperatures favored sulfur adsorption by powdered alumina, while positive ΔS and

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decreasing ΔG values with increase in temperature indicate that sulfur adsorption by PAC was

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more favorable at high temperature.

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Keywords: Adsorption; desulfurization; diesel; high-shear mixing; isotherm; thermodynamics

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1. Introduction

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Oxidative desulfurization (ODS) is considered the most effective alternative and/or post-

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treatment method in removing sulfur from fossil fuels. Its advantage over other sulfur removal

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technologies, such as selective adsorption, extractive separation, and biodegradation, include its

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ability to produce low sulfur fuels at near ambient temperature and pressure1. These alternative

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methods have been developed in order to address the problems encountered in current sulfur

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removal methods, such as hydrodesulfurization, which has been the standard industrial-scale

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desulfurization

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hydrodesulfurization technology include higher sulfur content feedstock, arising from the

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declining supply of crude oil and more stringent guidelines set by the United States

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Environmental Protection Agency (U.S. EPA) which limit sulfur levels in diesel fuels to 15 ppm

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from the previous 400 to 500 ppm2. At the onset, conventional hydrodesulfurization technology

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already suffers from non-selective hydrogenation of olefins and aromatics, especially

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dibenzothiophene (DBT) and its derivatives3,4. With the growing demand for ultra-low sulfur

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fuels, this technology will have to operate at higher temperatures and pressures and will have to

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involve

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hydrodesulfurization is no longer adequate and cost-effective especially when large amounts of

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refractory sulfur compounds are to be removed6,7.

larger

technology

reactors

for

with

decades.

volumes

Pressing

5-15

issues

times

the

with

this

present

energy-intensive

capacity5.

Thus,

82

ODS takes advantage of the fact that sulfur compounds in fuels are more prone to

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oxidation compared to other hydrocarbon components. In ODS, sulfur compounds are converted

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to highly polar sulfoxides and sulfones that can be readily removed by a suitable technology8.

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Hereafter, the main challenge is to selectively separate the sulfur species with low polarity from

86

the non-polar liquid phase9. In the past decade, selective sulfur removal from fuels has been

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accomplished using various adsorbents, including activated carbon10, alumina11, and silica12.

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Activated carbon (AC) is a versatile and widely used adsorbent primarily for removal of

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undesirable chemical species in liquids or gases. AC is produced from carbonaceous materials

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such as wood, coconut shells, sugar, coal, and lignin. Its high surface area, well-developed

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microporosity, and wide spectrum of surface functional groups make AC an ideal adsorbent13.

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The heteroatoms of porous carbon surface, mainly composed of oxygen, hydrogen, nitrogen, and

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halogens bonded to the edges of the carbon layers, govern the AC surface chemistry14. Among

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the heteroatoms, the oxygen-containing functional groups known as surface oxides, which are

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most commonly formed on the AC surface, are responsible for the enhancement of the material’s

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performance in catalytic reactions and adsorption processes15. On the other hand, aluminum

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oxide, commonly known as alumina, is commercially produced by thermal dehydration of

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aluminum trihydrate, Al(OH)3 or gibbsite16. When the trihydrate is heated to approximately 400

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°C, it is converted to crystalline γ/η-alumina having small amounts of boehmite and surface area

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of about 250 m2 g-1. However, when heated rapidly to 400-800 °C, gibbsite will become

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amorphous in form, having a higher surface area of 300-350 m2 g-1. Alumina has good

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mechanical properties and high surface area, which makes it a versatile sorbent for different

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applications. It has been widely used to remove organic compounds from aqueous solutions17.

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The performance of amorphous acidic alumina and crystalline boehmite in removing DBT was

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evaluated in a published study18, where acidic alumina was identified as the adsorbent of choice

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for the selective DBT removal via ultrasound-assisted oxidative desulfurization (UAOD)

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process.

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In this study, desulfurization of diesel fuel was carried out by sequential (1) high shear

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mixing-assisted oxidative desulfurization and (2) adsorptive desulfurization using powdered

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activated carbon and powdered alumina adsorbents. In addition, the isotherm and

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thermodynamic parameters of both adsorbents were evaluated. This study was motivated by the

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fact that sulfur removal from transportation fuels, such as diesel, is an urgent goal of clean fuel

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research. The combustion of sulfur-containing fuels releases sulfur oxides (SOx) which are

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precursors of acid rain and cause other adverse environmental effects19. These oxides also poison

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automobile exhaust catalysts designed for nitrogen oxide (NOx) reduction20–23. Moreover, the

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presence of SOx in the atmosphere poses health threats. Exposure to SOx in the ambient air has

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been associated with the development of cancer, reduced lung function, increased incidence of

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respiratory symptoms and diseases, irritation of the eyes, nose, and throat, and premature

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mortality24,25.

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2. Materials and methods

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2.1 Chemicals and adsorbents

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Commercial diesel was purchased from Taichin Company, Taiwan. Tetraoctylammonium

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bromide ([CH3(CH2)7]4NBr, TOAB), phosphotungstic acid hydrate (H3PW12O40·20H2O, HPW),

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and industrial grade hydrogen peroxide (50% purity) were purchased from Hung Yao

126

Instruments Company, Taiwan. Powdered alumina (activated Al2O3, Brockmann I, standard

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grade, ~105 µm particle size, 7.285 nm pore size) was purchased from Aldrich Chemical Inc.

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Powdered activated carbon (PAC) (~44 µm particle size, 2.222 nm pore size) was purchased

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from Fluka Analytical. Previous work reported that PAC had a surface area of 846 m2 g-1 and

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micropore area of 399 m2 g-1, while powdered alumina had lower surface area of 129 m2 g-1

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which implies presence of mesopores (2-50 nm)21.

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2.2 Analytical methods

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The sulfur concentrations of all samples were analyzed as total sulfur using an X-ray

135

fluorescence sulfur-in-oil analyzer (SLFA-2100, Horiba Scientific). A calibration curve

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established between sulfur concentration and a correlation factor became the basis for the direct

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measurement of sulfur concentrations. Gas chromatography – sulfur chemiluminescence detector

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(GC-SCD) (G6603A, Agilent Technologies) was used for the analysis of actual diesel fuel. The

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GC-SCD identifies sulfur compounds in a liquid mixture and provides information on the

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selectivity of the adsorption process to remove sulfur compounds. Adsorbent specific surface

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area was analyzed using a Brunauer-Emmett-Teller (BET) analyzer (ASAP, Micromeritics).

142 143

The sulfur removal and the adsorption capacity for sulfur (qt) were computed using Eq. (1) and Eq. (2), respectively 𝑠𝑢𝑙𝑓𝑢𝑟 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 (%) =

𝑞𝑡(𝑚𝑔/𝑔) =

(

𝐶0 ― 𝐶𝑒 𝐶0

)

⋅ 100

(𝐶0 ― 𝐶𝑡) ∙ 𝑉 𝑀

(1)

(2)

144 145

2.3 Desulfurization experiments

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A mixture of 500 mL diesel fuel, containing 4 g phosphotungstic acid and an equal amount

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of hydrogen peroxide with 2 g of TOAB, were added to a glass reactor and subjected to rapid

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mixing at 353 K using a high shear mixer (T-25, Ultra-Turrax, China) at an agitation speed of

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12,000 rpm for 35 min. The mixture was then allowed to cool and the organic phase was

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decanted and subsequently used as the adsorbate during the adsorption experiments.

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Sulfur removal from diesel after oxidative desulfurization was carried out in batch

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experiments21 using an orbital water bath shaker (Gyromax 929, Amerex Instruments, Inc., USA)

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set at a constant agitation speed of 120 rpm. A known amount of adsorbent was placed in a 250-

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mL Erlenmeyer flask with 20 mL diesel fuel. The mixture was agitated at a pre-determined

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temperature and contact time and filtered using a 0.2 µm polypropylene membrane prior to total

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sulfur content analysis.

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Pre-determined amounts of powdered alumina and PAC adsorbents (1, 3, 5, and 7 g) were

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each added into separate 20 mL diesel fuel samples with initial sulfur concentration of 950 ppm

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at 313 K. The adsorption capacities were measured at specified contact times, and the

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equilibrium adsorption capacity was determined after 24 h of mixing. Sulfur adsorption by

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powdered alumina and PAC adsorbents were also investigated at different temperatures (293,

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298, and 313 K). All the computed adsorption capacities were used to fit various adsorption

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isotherms and thermodynamics models.

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3. Results and discussion

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3.1 Liquid fuel characteristics

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The specifications of raw diesel fuel are shown in Table 1. The calculated initial sulfur

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content of the raw diesel was 1,130 ppm. After oxidation, the amount of sulfur in diesel dropped

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to 950 ppm. GC-SCD chromatograms of raw diesel fuel, diesel fuel after oxidative

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desulfurization, and diesel fuel after adsorptive desulfurization are presented in Fig. 1. As shown

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in the figure, thiophenic compounds, such as benzothiophene (BT) and dibenzothiophene (DBT),

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were removed from raw diesel after oxidative desulfurization. In addition, the amount of sulfur

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removed after oxidative desulfurization reached 15.9%, which is more than the 13.3% sulfur

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removal from jet fuel obtained in a similar oxidative-adsorptive desulfurization study26.

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3.2 Adsorption kinetics and thermodynamics

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The effect of temperature on sulfur removal by powdered alumina and PAC adsorbents

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was investigated in the temperature range of 293 to 313 K, below the flash point of the diesel

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sample. The calculated adsorption capacities, qe, at different temperatures were obtained using

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the pseudo-first order and pseudo-second order kinetic models. These models are useful in

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determining the mechanism that governs the adsorption of adsorbate onto the adsorbent, as well

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as the rate-determining step of the adsorption process27. The Lagergren pseudo-first order kinetic

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model assumes that the rate-limiting mechanism of the adsorption process is physical

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adsorption28. On the other hand, in the pseudo-second order kinetic model, chemisorption is

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considered as the rate-limiting mechanism of the adsorption process29. A more detailed

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discussion on the kinetics of sulfur removal using powdered alumina and PAC is presented

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elsewhere21. Table 2 presents the calculated adsorption capacities, qe, at different temperatures as

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fitted into the pseudo-first and pseudo-second order reaction kinetic models according to Eq. (3)

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and Eq. (4), respectively: 𝑙𝑛(𝑞𝑒 ― 𝑞𝑡) = 𝑙𝑛𝑞𝑒 ― 𝑘1𝑡

(3)

𝑡 1 1 = + 𝑡 𝑞𝑡 𝑘2𝑞2𝑒 𝑞𝑒

(4)

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where k1 is the rate constant of pseudo first-order adsorption (min-1), k2 (g mg-1 min-1) is rate

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constant of pseudo second-order adsorption, qe and qt are the amount of metal ion adsorbed per

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gram of sludge (mg g-1) at equilibrium and at any time, t, respectively.

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The high coefficients of determination (R2>0.998) for the pseudo-second order kinetic

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model, as presented in Table 2, imply that the rate-limiting step in the adsorption of sulfur

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species on both PAC and powdered alumina was chemical adsorption. In addition, the adsorption

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capacities for both adsorbents increased at higher adsorption temperatures.

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The effects of temperature on the adsorption rate constants are better explained by the

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adsorption activation energy, Ea30. The pseudo-second order rate constant, k2, can be expressed

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as a function of temperature using the Arrhenius-type relationship, shown in Eq. (5): ln 𝑘2 = ln 𝐴 ―

𝐸𝑎 𝑅𝑇

(5)

200

where A is a constant called the frequency factor, R is the gas constant (8.314 J.mol-1 K-1), and T

201

is the temperature (K). The magnitude of the activation energy differentiates physical adsorption

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from chemical adsorption. For physisorption, the activation energy is usually no more than 4.2 kJ

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mol-1 since the forces involved are weak (van der Waals and electrostatic forces), and

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equilibrium is rapidly attained and is reversible because of the small energy requirement.

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Chemisorption, on the other hand, is specific and involves forces much stronger than in

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physisorption. For activated chemisorption, the activation energy is between 8.4 and 83.7 kJ mol-

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1,

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very rapidly31. The activation energy of adsorption derived from the slope of the linear plot of ln

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k2 versus 1/T (Fig. 2a) were 17.56 kJ mol-1 for powdered alumina (R2 = 0.9586) and -21.71 kJ

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mol-1 for PAC (R2 = 0.9757). Thus, the rate-limiting step of sulfur adsorption onto powdered

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alumina was chemisorption, involving exchange of electrons between the sulfur compounds and

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the binding sites of powdered alumina32. The negative value of Ea for PAC suggests a multistep

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mechanism wherein an increase in temperature shifts the equilibrium in favor of its endothermic

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direction33. This means that adsorption of sulfur by PAC was not controlled by chemisorption

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alone. To investigate this phenomenon, the activation energy of diffusion, E’, was calculated

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using Eq. (6). In addition, the intraparticle diffusion coefficient, D, was determined using Eq. (7)

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which was derived from Fick’s law:

while nonactivated chemisorption gives Ea values near zero because of the process occurring

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ln 𝐷 = ln 𝐷0 ―

()

𝐸′ 1 𝑅 𝑇

ln [1 ― 𝐹(𝑡) ] = ― 2

𝜋2𝐷 𝑟2

(6)

𝑡

(7)

218

where D0 is the pre-exponential factor and r is the particle radius, assuming spherical geometry

219

(m). The value of D (m2 s-1) obtained for 3 g PAC adsorbent at different temperatures, using Eq

220

(7), was used to calculate the value of D0 and E’ from Eq. (6). The graph of ln D versus 1/T (Fig.

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3) gave a D0 value close to zero and an E’ value of -24.095 kJ mol-1. Since the activation energy

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for diffusion was less than the adsorption activation energy (E’ < Ea), the rate-limiting step of

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sulfur adsorption onto PAC was a combination of both chemical reaction and diffusion

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adsorption. Similar result on the comparison of activation energy and adsorption activation

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energy was reported in another adsorption study34.

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To further understand the thermodynamics of adsorption, the thermodynamic activation

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parameters - enthalpy of activation (ΔH), entropy of activation (ΔS), and Gibbs free energy of

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activation (ΔG) - were determined using Eq (8), Eq. (9) and Eq. (10) and summarized in Table 3: 𝐾=

𝑞𝑒 𝐶𝑒

―∆𝐺 = 𝑅𝑇𝑙𝑛𝐾 𝑙𝑛𝐾 =

∆𝑆 ∆𝐻 ― 𝑅 𝑅𝑇

(8) (9) ( 10 )

229

where K is the ratio of the concentration of adsorbate in adsorbent, qe, to the concentration of

230

adsorbate in solution, Ce16. The plot of ln K versus 1/T shown in Fig. 2b gave high coefficients of

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determination for PAC (R2 = 0.9999) and powdered alumina (R2 = 0.9619). For both powdered

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alumina and PAC, the positive value of ΔH indicates that the adsorption process was indeed

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endothermic35. For PAC, a positive value of ΔS (79.380 J mol-1 K-1) reflects increased degrees of

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freedom of the adsorbed sulfur compounds towards the selected adsorbents36. On the contrary,

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the negative entropy for sulfur adsorption onto powdered alumina (-8.482 J mol-1 K-1) suggests a

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decrease in randomness in the adsorption process and imply that the process may be reversible37.

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Table 3 shows that positive values of ΔG were observed at all temperature levels when 1 g

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and 3 g of PAC and 1 g of powdered alumina were used. These positive ΔG values suggest that

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the adsorption process was not spontaneous36, and energy is required to overcome the activation

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energy and/or to form an activated complex in order for the adsorption process to proceed38. For

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both PAC and powdered alumina at 313 K, the value of ΔG became more negative as adsorbent

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dosage increased from a range of 1 to 7 g (Table 3). This means that increasing the adsorbent

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dosage, which consequently increases the number of active sites, leads to a more feasible and

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spontaneous adsorption process at 313 K and, in effect, results in higher sulfur removal. As a

245

rule of thumb, if ΔG becomes more positive as temperature increases, as in the case for 1 g

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powdered alumina in a temperature range of 293 – 313 K, then the lower temperature makes the

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adsorption easier31. On the contrary, if ΔG becomes more negative with an increase in

248

temperature, as observed for 3 g PAC in a temperature range of 293 – 313 K, the adsorption

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process becomes more favorable at high temperature35. This is consistent with the result obtained

250

for powdered alumina with negative ΔS and with PAC having positive ΔS.

251 252

3.3 Adsorption isotherms and performance comparison with previous studies

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Adsorption isotherm models are useful in understanding the interactions between

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adsorbate molecules and the active sites on the adsorbent surface, as well as determining the

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amount of adsorbate that can be removed by a known quantity of adsorbent27. The Langmuir39

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and Freundlich40 isotherm models have been widely used to analyze the equilibrium adsorption

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data. The Langmuir isotherm assumes that monolayer adsorption occurs between the adsorbate

258

and finite number of adsorbent active sites41. Furthermore, it assumes homogenous distribution

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of adsorbent active sites, and that no interactions occur between adsorbed molecules37. On the

260

other hand, the Freundlich isotherm describes adsorption on a heterogeneous surface42, with the

261

assumption that the stronger binding sites on a heterogeneous surface are occupied initially, and

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that the binding strength falls with a rise in the degree of site occupation43. The linear form of the

263

Langmuir and Freundlich isotherm equations are given by Eq. (8) and Eq. (9), respectively. A

264

plot of 1/qe versus 1/Ce was used to determine the Langmuir constants, and a plot of log qe versus

265

log Ce for the Freundlich constants: 𝐶𝑒 1 1 = + 𝑞𝑒 𝑞𝑚 𝐾𝐿𝑞𝑚 log 𝑞𝑒 = log 𝑘𝑓 +

1 log 𝐶𝑒 𝑛

(8)

(9)

266

where qe (mg g-1) is the amount of sulfur compound adsorbed at equilibrium, Ce (mg L-1) is the

267

remaining concentration of the solution at equilibrium, kL is the Langmuir adsorption constant

268

related to the affinity of binding sites, kf is an indicator of the adsorption capacity, and n is

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related to the magnitude of the adsorption driving force and to the distribution of the energy sites

270

on the adsorbent.

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Shown in Fig. 4 are the Langmuir and Freundlich plots of sulfur adsorption onto PAC and

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powdered alumina at 313 K, while the model parameters and statistical fits of the adsorption data

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are summarized in Table 4. Sulfur adsorption onto powdered alumina followed the Freundlich

274

model, with correlation factor, R2, higher than that obtained from the Langmuir model. This

275

confirms that heterogeneous and multilayer adsorption occurred by formation of covalent bonds

276

through electron sharing or exchange between sulfur and the available binding sites, facilitated

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by the mesopores present on the powdered alumina21. The calculated value of n (1.294) was

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greater than unity, which suggests that adsorption of sulfur onto powdered alumina was

279

favorable44. Similar study reported that DBT adsorption onto alumina follows the Freundlich

280

isotherm45. In contrast, sulfur adsorption onto PAC followed the Langmuir isotherm, with

281

correlation factor, R2, higher than that obtained from the Freundlich model. This implies that the

282

adsorption process took place on homogeneous sites, within the macro- and mesopores of PAC21,

283

that are identical and energetically equivalent46. The n value of 0.782 derived using Freundlich

284

isotherm was less than 1, rendering this isotherm inappropriate for the sulfur-PAC system. The

285

suitability of the Langmuir isotherm for sulfur adsorption onto PAC is confirmed by the

286

separation factor constant, RL, calculated using Eq. (10): 𝑅𝐿 =

1 (1 + 𝑘𝐿𝐶0)

(10)

287

where C0 is the initial concentration (mg L-1). The value of RL is used to determine if the

288

adsorption is unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL =

289

0) 37. In this study, RL (0.662) was between 0 and 1, indicating that the adsorption was favorable.

290

Using the Langmiur isotherm model, the maximum adsorption capacity of 6.31 mg g-1 for PAC

291

was obtained. Previous study reported that DBT adsorption onto synthesized mesoporous carbon

292

adsorbent and the multi-ring sulfur compound adsorption onto carbon materials both followed

293

the Langmuir isotherm47.

294

Table 5 presents the equilibrium adsorption capacity of the adsorbents used in this study

295

compared with other adsorbents used in related studies. As shown, the adsorption capacities of

296

the PAC and powdered alumina were higher than the reported values on sulfur removal using

297

various adsorbents. The higher adsorption capacity of powdered alumina was due to the presence

298

of mesopores which facilitated contact between sulfur molecules and the internal sites of the

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299

adsorbent, while the lower adsorption capacity of the PAC as compared with powdered alumina

300

was due to hindered access to micropores caused by saturation of the macro- and mesopores of

301

PAC with sulfur compounds during the adsorption process21.

302 303

4. Conclusions

304

In this study, the oxidative-adsorptive desulfurization of diesel fuel was conducted using

305

PAC and powdered alumina as adsorbents. The positive ΔH values for sulfur adsorption by PAC

306

and powdered alumina adsorbents confirmed the endothermic nature of adsorption. The negative

307

ΔS value, and increasing ΔG values with increase in temperature, for sulfur adsorption by

308

powdered alumina indicates that lower temperatures favor the adsorption process, and the rate-

309

controlling step for powdered alumina is apparently a chemical sorption process. The positive ΔS

310

value, and decreasing ΔG values with increase in temperature, for sulfur adsorption by PAC

311

indicates that the adsorption process is more favorable at high temperature. For PAC the rate-

312

controlling step is a combination of both chemisorption and intraparticle diffusion, showing that

313

the adsorption of sulfur onto PAC is a multistep process wherein an increase in temperature

314

shifts the equilibrium in favor of its endothermic direction. For significant adsorption to occur,

315

an increase in adsorbent dosage, both for powdered alumina and PAC, is necessary, as shown by

316

the more negative value of ΔG at higher adsorbent dosage. Sulfur adsorption onto powdered

317

alumina occurred through electron sharing or exchange between sulfur and the heterogeneous

318

binding sites on the powdered alumina, while adsorption onto PAC took place on homogeneous

319

sites.

320

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321

Declarations of interest

322

The authors declare no competing financial interest.

323 324

Acknowledgements

325

The authors would like to thank the Ministry of Science and Technology, Taiwan (Contract No.

326

MOST 99-2221-E-041-012-MY3) and the Department of Science and Technology, Philippines

327

for providing financial support for this research undertaking.

328 329

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494 495 496 497 498 499 500 501 502 503 504

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505 506 507 508 509 510 511 512 513

(a)

(b)

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(c)

514

Fig. 1. GC-SCD chromatograms of diesel in various stages (a) raw, (b) after oxidative

515

desulfurization, and (c) after adsorptive desulfurization

516 517

Fig. 2. (a) Arrhenius plot of the pseudo-second order kinetic model and (b) plot of ln K versus

518

1/T for sulfur adsorption by PAC and powdered alumina

519 520 521 522 523

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524 525 526 527 528 529 530 531

532 533

Fig. 3. Plot of ln D versus 1/T

534 535 536 537 538

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539 540 541 542 543 544 545 546

547

Fig. 4. (a) Langmuir and (b) Freundlich plots of sulfur adsorption by PAC and powdered

548

alumina

549 550 551 552 553 554 555 556

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557 558 559 560 561 562 563 564

Table 1. Physical and chemical properties of actual diesel fuel (supplied by TaiChin Company,

565

Taiwan) Property Cetane index Polycyclic aromatic carbon (%, m m-1) Flash point (°C) Water content (mg kg-1) Total contamination (mg kg-1) Kinematic viscosity at 40 °C (mm2 s-1)

Standard method ASTM D976 EN12916 ASTM D93 ISO12937 EN12662 ASTM D445

566 567 568 569 570 571 572 573 574 575 576

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Value 48 11 55 200 22 2.0-4.5

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577 578 579 580 581 582 583 584

Table 2. Equilibrium adsorption capacities of PAC and powdered alumina adsorbents at

585

different temperatures fitted into the pseudo-first and pseudo-second order reaction kinetics. Temperature (K)

Pseudo-first order k1 (min-1) qe (mg g-1) R2

Pseudo-second order k2 (g mg-1 min-1) qe (mg g-1)

R2

PAC 293

0.0135

0.9682

0.9433

0.0621

2.0214

0.9980

298

0.0144

1.0608

0.9212

0.0428

2.5393

0.9983

313

0.0168

1.2526

0.9135

0.0352

3.0057

0.9944

293

0.0794

2.355

0.9196

0.1149

4.722

0.9995

298

0.0655

2.429

0.9417

0.1178

4.726

0.9974

313

0.0835

1.829

0.9114

1.1774

4.758

0.9997

Powdered alumina

586 587 588 589 590 591 592

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30

593 594 595 596 597 598 599 600

Table 3. Thermodynamic parameters of sulfur adsorption by PAC and powdered alumina Adsorbent (g) PAC 1

Temperature (K)

Keq

313 293 303 313 313 313

0.264 0.452 0.640 0.875 1.074 1.426

293 298 313 313 313 313

0.287 0.287 0.291 1.162 1.983 2.017

3 5 7 Powdered alumina 1 3 5 7

ΔH (kJ mol-1) ΔS (J mol-1 K-1) -

-

25.187

79.380

-

-

0.558

-8.482

-

-

601 602 603 604 605 606 607

ACS Paragon Plus Environment

ΔG (kJ mol-1) 3.466 1.934 1.123 0.348 -0.185 -0.923 3.041 3.090 3.214 -0.391 -1.781 -1.826

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

31

608 609 610 611 612 613 614 615

Table 4. Isotherm parameters for sulfur adsorption by PAC and powdered alumina Isotherm model Langmuir qmax (mg g-1) kL (L mg-1) R2 Freundlich kf (mg g-1)/(mg L-1) n R2

PAC

Powdered alumina

6.31 5.381 x 10-4 0.9020

27.03 2.828 x 10-4 0.8345

8.373 x 10-4 0.782 0.8948

0.027 1.294 0.8626

616 617 618 619 620 621 622 623 624 625 626

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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32

627 628 629 630 631 632 633 634

Table 5. Adsorption capacities for sulfur by various adsorbents Adsorbent Alumina, acidic Alumina, basic Alumina, neutral Zinc oxide Zeolite 13X Polymeric resin XAD-16 Polymeric resin XAD-4 Alumina basic (Alcoa) Alumina neutral (Alcoa) Activated carbon (Calgon F-300) Activated carbon (Calgon F-400) Silica gel (6-20 mesh) Silica Activated carbon (Calgon) CMS-4K(AC molecular sieve 4K) CMS-4K-5h (CMS-4K activated at 1173K for 5 h) Activated alumina ZSM-5 (Si/Al = 20) PAC Powdered alumina

Adsorbate DBTO in toluene (500 ppm sulfur)

commercial diesel fuel (473 ppm sulfur)

commercial diesel fuel (150 ppm sulfur)

qe(mg g-1) 5.7 5.5 4.3 0 0.6 0.6 0.8 0.9 1.4 4.1 4.4 5.1 1.1 0.5

Reference

0.3

49

18

48

0.05 commercial diesel fuel (325 ppm sulfur) commercial diesel fuel (950 ppm sulfur)

635

ACS Paragon Plus Environment

0.38 0.32 6.31 27.03

50

This study