1.3 Hydrodesulfurization (HDS) Process
1.4.2 Processes Employing Adsorptive Desulfurization
Adsorptive desulfurization technology is considered promising evidenced by the fact that two processes, IRVAD and Philips S–Zorb, were developed based on it (Irvine and Varraveto 1999, Velu et al. 2003, Pawelec et al. 2011).
S Zorb process: In the S-Zorb process, desulfurization takes places in presence of hydrogen so as to accelerate the reaction between sulfur compounds and adsorbing agent. However, as compared to HDS process, the hydrogen consumption is much lesser. Different configurations of process flow have been used such as fluidized bed, moving bed and slurry column to transport the adsorption agent through reaction column and regeneration column. Regeneration of the adsorbent is carried out by subjecting it to air oxidation followed by processing by sulfur collector. The sorbent is purged with nitrogenand returned to its original state using hydrogen – before returning it to the reaction column. A possible chemical mechanism of adsorption in the S Zorb process is as follows: The hydrogen attacks sulfur compounds and weakens the atomic bonds of the sulfur atoms, which promotes their reactive adsorption on to the sorbent. In this mechanism, benzothiophene after desulfurization is converted into ethyl benzene. It is claimed that S zorb process does not involve any loss of hydrogen through olefins hydrogenation.
Figure 1.5: Adsorptive desulfurization – IRVAD (Irvine and Varraveto, 1999)
IRVAD process: Irvine (1998) and Irvine and Varraveto (1999) have reported an
adsorptive desulfurization process called IRVAD. This process is claimed to remove wide spectrum of organosulfur compounds from various refinery streams including FCC gasoline. This process employs a moving bed of solid sorbent. This bed is brought in countercurrent contact with sulfur rich hydrocarbon stream. The desulfurized hydrocarbon stream emerges from the top of absorber. The sorbent loaded with sulfur compounds leaves from bottom. The spent sorbent is sent to the regeneration chamber where organosulfur compounds absorbed on the surface of sorbent are desorbed. The sorbent employed in this process is alumina, which can operate upto 240oC at low pressure with hydrocarbon/sorbent weight ratio of 1.4.
This technology has been tested in pilot plant experiments in order to desulfurize the FCC feedstock (1276 ppm) and coker naphtha (2935 ppm Sulfur) which resulted in a 90% decrease in sulfur content. The IRVAD method is limited by the sorbent capacity and its selectivity, as adsorption of sulfur compounds occurs parallel to the surface of the sorbent. For example, dibenzothiophene gets attached parallel to the surface of the catalyst via π–electron of the aromatic ring (Xiaoliang et al., 2000). So, the sorbent requirement for effective operations is very high. Other limitation that has hampered commercial application of IRVAD adsorptive desulfurization technology is high pressure hydro-treatment required to eliminate organosulfur compounds concentrated on the adsorbent. However, optimization of some properties of adsorbent and process conditions (e.g. sorbent particle size and reactivation step temperature) can increase the efficiency and potentially make the process commercially viable. Table 1.4 summarizes some of the researchers conducted for the sulfur removal with different types of adsorbents at optimized condition.
1.5 O
XIDATIVED
ESULFURIZATION(ODS)
Oxidative desulfurization has been introduced as a new technology for deep desulfurization of diesel oil, and has proven to be an attractive alternative to hydrodesulfurization.
It does not require expensive hydrogen, but instead uses (relatively much cheaper) oxidant such as peracids and hydrogen peroxide.
In the ODS process, the hydrocarbon fraction containing sulfur compound is extracted from the feedstock, instead of converting to hydrogen sulfide as in HDS process.
ODS process operates near atmospheric pressure and relatively mild temperature (<100oC), and as a result, capital cost of the process is substantially lower than HDS process.
Table 1.3: Summary of Sulfur removal by Hydrodesulfurization process
Sulfur Compound Catalyst Operating condition % Sulfur removal Reference DBT Al2O3–ZrO2 supported CoMo catalyst Temp:350o C
Pressure: 8 MPa
90 % Sintarako et al. 2015 DBT NiMo catalysts supported on
Al2O3/MgO
Temp = 300o C Pressure = 3 MPa
75 % Mogica–Betancourt
et al. 2014 BT, DBT NiMoW catalyst dispersed by diatomite Temp = 340o C
Pressure = 4 MPa
63.8 % Di and Chenguang 2013
Alkyl DBTs Diesel CoMoP/nanoAl2O3, CoMoB/nanoAl2O3, CoMo/nanoAl2O3, and
CoMoP/microAl2O3
Temp = 310oC Pressure = 3 MPa
CoMo/ nanoAl2O3
= > 98 %
Rashidi et al. 2013
Alkyl DBTs Diesel Al2O3–TiO2, Al2O3–TiO2–SiO2
Supported Bimetallic Pt–Pd Catalysts
Temp= 330 oC Pressure= 5 MPa
Pt–Pd/ Al2O3– TiO2–SiO2 = 95 %
Wan et al. 2009
Alkyl DBT NiW/TiO2–Al2O3 Temp = 350 oC
Pressure = 5 MPa
100 % Duan et al. 2009 Alkyl DBTs Diesel P and Ni–Al2O3 supported Mo
Oxycarbides
Temp= 340 oC Pressure= 4 MPa
50 % Costa et al. 2009 Thiophene FeS–MoS supported
on Al2O3 and carbon
Temp= 280 oC Pressure = 0.1 MPa
30 % Hubaut et al .2007 Alkyl DBTs Co–Mo supported
on MCM–41
Temp = 350
Pressure = 4.5 MPa
57% Turaga et al. 2003
Notation: DBT: Dibenzothiophene; BT: Benzothiophene; HDS: Hydrodesulfrurization; Temp: Temperature
Table 1.4: Summary of Sulfur removal by Adsorptive desulfurization process
Sulfur compound Adsorbent Operating conditions % Sulfur
removal Reference DBT Mesoporous carbon (CMK–3) Temp = 25 oC
67 % Shi et al., 2015 Hydrodesulfurized
diesel fuel Activated carbon Temp= 25 oC 90 % Selvavathi et al., 2009 DBT Mesoporous aluminosilicates
Temp = 25 oC
Oil : Adsorbents ratio=
20 mL/g
87 % Tang et al., 2009
4–6 DMDBT Y zeolite Temp = 60 oC 97 % Tang et al., 2008
4–6 DMDBT NiMoP/Al2O3 + NaY zeolites Temp= 340 oC
Pressure = 40 (atm) 56 % Richard et al., 2007 Thiophene Activated Carbon
Two step process Temp= 70
Pressure = 1.5 (atm)
88 % Sano et al., 2005
DBT sulfone Alumina Temp = 200 oC 30 % Larrubia et al., 2002