1.3 Hydrodesulfurization (HDS) Process
1.7.1 Destructive Biodesulfurization (Oxidative C–C bond cleavage)
Figure 1.8: Sulfur specific DBT degradation via the Kodama pathway
1.7.2 4S pathway (Oxidative C–S bond cleavage)
This metabolic pathway results in sulfur removal from dibenzothiophene through 4 sulfur containing intermediates, viz. DBT-sulfoxide, DBT-sulfone, DBT-sulfinate and the final product hydroxybiphenyl, and hence, it draws its name as 4-S pathway. A peculiarity of this pathway is that the basic carbon skeleton of the sulfur compound remains unaffected, and thus, there is no reduction in calorific value of fuel during
desulfurization, as in case of the Kodama pathway. The schematic of this pathway is shown in Fig. 1.9.
This metabolic pathway for sulfur removal was first reported for Rhodococcus rhodochrous IGTS8 by Gallagher et al. (1993). Besides Rhodococcus rhodochrous IGTS8 (Kilbane and Jackowski 1992; Mohebali et al. 2007), other bacteria, which follow the 4S pathway are R. erythropolis D1 (Izumi et al. 1994; Ohshiro et al. 1994), Rhodococcus ECRD1 (Grossman et al. 1999), Gordona CYKS1 (Rhee et al. 1998), Xanthomonas (Constanti et al. 1994), Nocardia globelula (Wang and Krawiee 1994), Paenibacillus strain (Konishi et al. 1997), and Mycobacterium sp. (Li et al. 2003).
The 4S pathway proceeds via two cytoplasmic monooxygenases supported by flavin reductase and a desulfinase. Two step action of DBT monooxygenase catalyzes the sequential conversion of DBT to DBT sulfoxide, and DBT sulfoxide to DBT sulfone, respectively. These reactions require flavin mononucleotide for the activity, which is provided by the enzyme flavin mononucleotide oxidoreductase. Conversion of DBT sulfone to hydroxyl-phenyl benzene sulfonate is catalyzed by DBT sulfone monooxygenase. This reaction also requires reduced flavin mononucleotide which is provided by the flavin oxidoreductase enzyme. The fourth enzyme HPBS desulfinase completes the pathway by converting hydroxyl phenyl benzene sulfonate to HBP and sulfite. HBP is highly soluble in oil and readily diffuses back into the oil phase. Thus the fuel value of the oil is conserved during the 4-S pathway. There are several resistances that put limit on the overall kinetics of the 4S pathway. The main rate limiting step is the transfer of polyaromatic sulfur heterocycle from oil phase into the cell. The supply of reducing equivalents and enzyme turnover rate is another hindrance for kinetic of the 4S pathway.
The genes responsible for the4S pathway were discovered in the species Rhodococcus erythropolis IGTS8. However, in the past several years of research these genes have been cloned, sequenced and engineered from several microbial species. These have also been transferred to other species.
Most of the microorganisms showing 4S metabolic pathway are mesophiles, i.e. the ability of DBT desulfurization is higher at temperature around 30°C and decreases with higher temperatures. If the biodesulfurization process is to be integrated with conventional HDS process, thermophillic biodesulfurization at higher temperatures is desired. If biodesulfurization could be performed around 50°C, it would obviate the need to cool the HDS treated diesel oil to ambient temperature. In addition, contamination by undesirable bacteria, which affects the BDS process, would be avoided at high temperature. Only a few microorganisms, such as a Paenibacillus strain (Konishi et al. 1997), Bacillus subtilis WU–S2B (Kirimura et al. 2001), Mycobacterium sp. X7B (Li et al. 2003), Mycobacterium sp. GTIS 10 (Kayser et al.
2002), and Mycobacterium pheli WU–F1 (Furuya et al. 2001) are reported to show desulfurization at high temperature. Table 1.6 shows the summary of the literature on biodesulfurization follow 4S pathway.
Figure 1.9: Sulfur specific DBT degradation via the 4S pathway
Table 1.6: Summary of the literature on biodesulfurization
Model oil Microorganism Experimental system and conditions %DBT
removal Reference Hexadecane Gardonia alkanivorans RIPI90A Batch process with mechanical agitation (30oC, 120 rpm,
pH 7.0, 10 days), Initial DBT concn: 100 ppm
90% Mohebali et al.
2007 n–Hexadecane Mycobacterium sp. ZD–19 Batch process with mechanical agitation (30oC, 180 rpm,
pH range 6.5 – 7.5, 4–7 days), Initial DBT concn: 92 ppm
100% Chen et al. 2008
Hexadecane Rhodococcus erythropolis IGTS8 Batch process with mechanical agitation (30oC, 200 rpm, 24 h, oil:water volume ratio 1:1), Initial DBT concn: 50 ppm
80% Caro et al. 2007
n–Heptane Gardonia alkanivorans strain 1B Batch process with mechanical agitation (30 oC, pH– 7.5, 150 rpm, 168 h, water/oil ratio 10:1), Initial DBT concn:
100 ppm
63% Alves et al. 2008
n–tridecane Bacillus subtitlis WU–S2B Batch process with mechanical agitation (50oC, 140 rpm, 12 h), Initial DBT concn: 100 ppm
50% Kirimura et al.
2001 n–tridecane Mycobacterium phlei WU–F1 Batch process with mechanical agitation (50oC, 180 rpm,
3 days), Initial DBT concn: 150 ppm
99% Furuya et al.
2001 Decane Rhodococcus erythropolis (ATCC
53968)
Agar plate interface bioreactor (30oC, 40 rpm, 5 days), Initial DBT concn: 184 ppm
90% Oda and Ohta 2002
n–Hexadecane Bacterium, strain RIPI–22 Batch process with mechanical agitation (30oC, 100 rpm), Initial DBT concn: 100 ppm
77% Rashtchi et al.
2006 Methenol Lysinibacillus sphaericus DMT–7 Batch process with mechanical agitation (150 rpm, 37oC,
15 days), Initial DBT concn: 100 ppm
60% Bahuguna et al., 2011
* All of these microorganisms follow 4S pathway of biodesulfurization. # The model sulfur compound used in all studies in DBT
1.8 B
ASICC
ONCEPTS ANDP
RINCIPLES OFU
LTRASOUND ANDC
AVITATIONUltrasound essentially refers to the sound waves having frequency beyond the upper limit of human hearing range (16 Hz – 16 kHz), or typically in the range of 20 kHz up to approximately 500 MHz.. Sound waves pass through an elastic medium in the form of longitudinal wave, i.e. in series of alternating compressions and rarefactions regions generated due to small amplitude oscillatory motion of fluid elements in the direction of propagation of the wave. The amplitude of displacement of fluid elements depends the pressure amplitude of the ultrasound wave, which in turn depends on the energy of the wave.
The frequency (f) and the acoustic amplitude (PA,max) are the most important properties that characterize a sound wave. The bulk pressure in the liquid medium undergoes periodic (usually sinusoidal) variation during propagation of the ultrasound wave. In simple form, the pressure amplitude of the ultrasound wave (PA) and the bulk pressure in the medium (Pt) at any instance (time, t) for frequency f by the following equation:
( )
,maxsin 2
A A
P =P πft
( )
,maxsin 2
t o A
P =P −P πft
where PA,max is the pressure amplitude of the ultrasound wave and Po is the static pressure in the bulk liquid medium. Ultrasound waves can be generated in the medium using a transducer, which essentially converts one form of energy (electrical) into another form (mechanical). The piezo–electric crystal undergoes volume oscillations under influence of alternating voltage (or potential) applied across it.
These mechanical oscillations can be converted into sound energy, once the piezo–
electric element is coupled to a fluid medium such as water.
Cavitation is a secondary effect of ultrasound. The basic definition of cavitation phenomenon can be given as nucleation, growth and implosive transient collapse of
due to ultrasound propagation. Occurrence of cavitation phenomenon induced by the propagation of the ultrasound wave can be explained as follows: In oscillatory motion during propagation of the wave, the fluid elements in the bulk medium are pulled apart from each other. If the amplitude of the ultrasound wave is strong enough to overcome the Laplace pressure (2σ r) of the liquid medium, the bond between the two fluid elements can break with creation of a void or cavity between them. In the subsequent compression cycle, this cavity is annihilated, giving rise to extreme concentration of energy. Theoretically, for creation of a “cavity”, the acoustic pressure has to overcome the van der Waal’s distance between the two molecules.
This would require enormous pressure amplitude (> 10,000 bar) of the ultrasound wave. However, in practical situation, cavitation occurs at very low pressure amplitude as 1.2 bar. This is due to presence of nuclei or weak spots in the liquid that assist occurrence of cavitation phenomenon. These nuclei or weak spots could be small bubbles or particles already present in the liquid or these could also be gas pockets trapped in the crevices in the solid boundaries of the processor or ultrasound probe. Under the influence of pressure variation due to ultrasound, these gas pockets or bubbles expand in the rarefaction cycle giving rise to cavitation event. The cavitation bubbles in this case are filled with non-condensable gas, usually air. If the temperature of the bulk liquid is sufficiently high, local vaporization of the liquid may also occur resulting in formation of vapor bubble.