1.3 Hydrodesulfurization (HDS) Process
1.6.2 Photochemical desulfurization
In photochemical desulfurization, the sulfur compound present in liquid fuel is photo–
excited using light of appropriate wavelength. The transient photo-excited state of sulfur aromatic compound such as singlet state reacts with the oxidant. Reactive intermediates lead to molecular reaction products, such as sulfones and sulfoxides of aromatic sulfur compounds (Tao et al., 2009). These sulfones and sulfoxides are removed by adsorption method. both Both ultraviolet and visible light irradiation has been used for the improvement of ODS (Hirai et al., 1997; Shiraishi et al., 1998, 2001). High–pressure mercury and Xe–Hg lamps have also been used in the ultraviolet (UV) region (λ > 290 nm) for direct oxidation of DBT and its alkyl derivatives (Hirai et al., 1996, Tao et al., 2009). UV (λ > 290 nm) has been used for indirect photo–oxidation and photocatalytic oxidation of sulfur compounds (Paybarah et al., 1982). The indirect oxidation constitutes the in situ generation of the oxidizing agent. Titanium–based catalysts have been studied in photocatalytic oxidation (Abdel–Wahab and Gaber, 1998; Wang et al., 2006). The performance of photocatalyst-assisted oxidative desulfurization has been revealed to be better than oxidative process alone (Robertson and Bandosz 2006). The mechanism involved in photocatalytic oxidation of DBT using titanium–based (TS–1) catalyst has been reported by Juan and coworkers (2010). Ultraviolet light with wavelength < 280 nm has also been used to study the mechanisms involved in the photo–oxidation of sulfur compounds. Visible light has also employed with mixed–phase Fe2O3 photo–catalyst for removal of sulfur from model fuel (n–octane) under simulated solar irradiation. In this process, 92% sulfur removal has been achieved in 90 min. The kinetics of photooxidation followed pseudo first–order profile with an apparent rate constant of 0.0287 min–1 (Li et al., 2012). According to the authors, electron transfer from the α–
Fe2O3 conductive band to the β–Fe2O3 conductive band effectively separates the photo–generated electrons and holes, which thereby reduces their recombination. The α–Fe2O3–originating holes will then move from the valence band of α–Fe2O3 to β–
Fe2O3 phase, and then react with OH– to form OH radical to participate in the oxidation reaction. However, when the β–Fe2O3 phase content exceeded 36.6%, photocatalytic activity slightly decreased because its band gap is narrower than that of the α–Fe2O3 phase and the recombination rate of the electron–hole pairs during the β–
Fe2O3 phase is faster than that in the α–Fe2O3 phase.
1.7 M
ICROBIAL/ E
NZYMATICD
ESULFURIZATIONAn alternative means to the chemical methods of HDS and ODS for removal of sulfur from fossil fuel is the biological method of microbial or enzymatic desulfurization.
Microorganisms require sulfur for their growth and biological activities. Sulfur generally occurs in the structure of some enzyme cofactors (such as coenzyme A, thiamine and biotin), amino acids and proteins (cysteine, methionine, and disulfur bonds).
Several microorganisms, depending on their enzymes and metabolic pathways, have the ability to acquire and metabolize carbon and sulfur from different sources. Some microorganisms, such as Arthrobacter, Brevibacterium, Pseudomonas, Gordona, and Rhodococcus spp. can consume the sulfur in heterocyclic thiophenic compounds such as DBT, and reduce the sulfur content in fuel. For microbial desulfurization, two main pathways have been reported: (1) ring–destructive Kodama pathway (degradation) and, (2) sulfur–specific (desulfurization) 4S pathways.
Table 1.5: Summary of the literature in PTA–assisted oxidative desulfurization
Sl.
No Authors Model Sulfur Compound &
Solvent
PTA
(concentration) Reaction Conditions Oxidation System
Method of Treatment
% Reduction of Sulfur
compound 1. Mei et al.,
2003 DBT (400 ppm)
Toluene TOAB
(7.32 mM)
React Vol: 50 mL;
Temp: 75 ± 2 °C; pH:
NA; Time: 7 min; US Freq: 20 kHz
H2O2/ Phospho–
tungstic acid US/ Oxidation/
Extraction
98 % (After Extraction); 2.5%
(After oxidation)
2. Chen et al., 2007
Thiophene or 3–
methyl thiophene (500 ppm) n–Heptane
TBAB (0.5 g)
React Vol: 50 mL;
Temp: 50 oC Time: 120 min; Str:
1500 rpm; Metal oxide loaded molecular sieve
H2O2/ Formic acid
Oxidative
Desulfurization in H2O2
Best with formic acid
Thiophene:
78.4%
3–Methyl thiophene 82.3%
3. Li et al., 2010
Thiophene (800 µL/L ≈1200 ppm)
n–Octane
TBAB (0.1 g)
React Vol: 50 mL; pH:
12; Temp: NA; Time:
120 min; Air Flow 150 ml/min
Photo–oxidative
365 nm, 500 W US 80.6%
4. Chen et al., 2010
Thiophene, BT, DBT (960 ppm) Toluene
TOAB (0.1 g) Temp: 88 ± 2°C
Freq: 20 kHz H2O2/Phospho–
tungustic acid US/ Oxidation/
Extraction 88.4%
5. Zhao et al., 2007
Thiophene (~
1200 ppm); n–
Heptane
TBAB, TEAB, TMAB, TPAB (0.0116 mol/L)
React Vol: 24 mL;
Time:120 min; Temp:
50 oC H2O2/ HCOOH
US/Oxidation–
Extraction:
TPAB– 86.5%
TEAB– 42.37 %;
without PTA:
28.3%
No PTA: 28.4%
TMAB: 42.4%
TEAB: 70%
TPAB: 86.6%
TBAB: 94.7%
Table 1.5: Continued…
Sl.
No Authors Model Sulfur Compound &
Solvent
PTA
(concentration)
Reaction Conditions
Oxidation System
Method of Treatment
% Reduction of Sulfur compound
6. Wan and Yen, 2007
Benzothiophene; 2 – methyl benzo–
thiophene;
Dibenzothiophene;
4–MDBT, Toluene
MBAH; TOAB;
TOAF;
TODAB;
TBAB; TMAF;
MTAC (0.1 g)
Temp: 70 ± 2°C;
Time:10 min;
Freq: 20 kHz
H2O2/Phospho–
tungustic acid
US–assisted Oxidation followed by extraction
TOAF: 90.3%
TODAB: 56.9%
TOAB: 49.6%
TBAB: 38.3%
MBAC: 11.4%
MBAH: 11.1%
TMAF: 8.2%
7. Sachdeva and Pant,
2010 DBT (325 ppm);
n–Decane TOAB
(0.006g/g S)
React Vol: 120 mL; Temp: 70°C;
pH: NA; Stirring:
1000 rpm; Time:
200 min
H2O2/Phospho–
tungustic acid
Conventional Oxidative desulfurization
98%
8. Zhao et al., 2009
Thiophene, DBT (600 ppm); n–
Heptane (24 mL)
TBSB
(0.0123 mol/L)
React Vol: 36 mL;
Temp: 45°C;
Time: 90 min; pH:
NA; Stirring: ~200 rpm
H2O2/ HCOOH Mechanical
Stirring Thiophene: 86.4 % DBT: 97.5%
9. Zhao et al., 2008
Thiophene (500 ppm); n–Octane + Xylene (1:1 vol)
HTMAB or TBAB (1.5 g)
React Vol: 130 mL; Temp: 40°C;
pH: NA; Time:
150 min; US Freq:
NA
H2O2/ phospho–
tungstic acid
Mechanical Stirring
96.3% with HTMAB
91.5% with TBAB
Note: The concentration given in the table for S compound is by default in ppmw. Abbreviations: TPAB – tetrapropyl ammonium bromide, TEAB – tertraethyl ammonium bromide, TMAB – tetramethyl ammonium bromide, HTMAB – hexadecyltrimethyl ammonium bromide, TBAB – tetrabutyl ammonium bromide, MBAH – methyltributyl ammonium hydroxide, TODAB – tetraoctadecyl ammonium bromide, TOAF – tetraoctyl ammonium fluoride, MTAC – methyltributyl ammonium chloride, TBSB – tetrabutyl ammonium bisulfate. US – ultrasound