Energy & Fuels 2006, 20, 1287-1293
1287
Model of Fluidized Catalytically Cracked (FCC) Gasoline Photochemical Desulfurization Reactor Lei Wang* and Ben-Xian Shen School of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai, 200237, People’s Republic of China
Shu-Zhen Li Jiangsu Chang Zhou Higher Vocational School of Construction, Changzhou, 213016, People’s Republic of China ReceiVed August 22, 2005. ReVised Manuscript ReceiVed March 5, 2006
Photochemical desulfurization reactor has been investigated in the process of fluidized catalytically cracked (FCC) gasoline refined by extraction and photochemical techniques. A mathematical model of the photochemical desulfurization reactor has been constructed based on photon quantum absorption theory and rational simplification. At the same time, the basic equation about radiotechnology transfer and local volumetric rate of energy absorption model are introduced in the photochemical desulfurization reactor. The model is validated through the experimental data from photochemical desulfurization.
1. Introduction Photochemistry developed into its maturity in the 20th century, and many photochemical processes have been applied to production; however, few have been applied to petroleum processing. The main obstacles to the industrial application of the process are (i) the low-oxidation activity and low selectivity of the sulfides present in fuel oils, (ii) the difficulties in separation and recovery of the catalysts after the reaction, and (iii) the low efficiency of H2O2 utilization. In addition, the removal of sulfur from transportation fuels has been mandated by governments around the world to reduce atmospheric pollution by sulfur oxides; therefore, research has particularly focused on innovative routes for desulfurization approaching cleaner technology and compliance with increasingly stringent laws. For these reasons, fluidized catalytically cracked (FCC) gasoline and diesel oil has been desulfurized via the utilization of photochemistry processes, and fine results were obtained at the laboratory at the East China University of Science and Technology (ECUST).1,2 By utilizing a semiconducting light catalyzer, the desulfurizing result was better improved and the residence time was shortened. Currently, most researchers have been focused on studying the light catalyzer and its mechanism and reaction process; however, it is not reported that the mathematical model of the photochemical desulfurization reactor has been applied to petroleum processing techniques. However, the application of photocatalysis for the removal of sulfur from FCC gasoline on an industrial scale can be * Author to whom correspondence should be addressed. Tel.: +86-2164253346. Fax: +86-21-64252610. E-mail address: WLLSZ119110@ 163.com. (1) Mohamed, A. I.; Shen, B. X.; Chen, Q. A design for photochemical desulfurization and solvent extraction for light oil. Hung. J. Ind. Chem. 2002, 30 (3), 161-165. (2) Ibrahim, A. I.; Shen, B. X.; Zhou, W. Desulfurization of FCC gasoline by solvent extraction and photooxidation. Pet. Sci. Technol. 2003, 21 (9), 1555-1573.
assisted by the development of both new photoreactor designs and mathematical models. Generally, effective photo utilization is a crucial performance criterion, which governs the economic viability of a particular photocatalytic oxidation reaction design. The photochemical desulfurization reactor differs greatly from the general reactor. The variety of light sources, photon spread, absorption, eradiation, and the reactor geometry, as well as the location of the reactor relative to the light source, can exert direct influence on the photochemistry reaction process. A radiotechnology transfer equation and a light intensity distribution equation must be introduced to construct a photochemistry reactor model; thus, it is much more complicated to build a photochemistry reactor model than to construct the traditional reactor. As the research in photocatalysis progresses and more active photocatalysts are developed, there will be issues in photoreactor design. Photocatalytic reactor design is particularly challenging, in that an effective photoreactor must simultaneously and efficiently contact the activating light, the solid catalyst, and the FCC gasoline while providing high photon utilization in a physically compact vessel, so photoreactors with the following specifications still need to be developed and assessed: (i) they must have adequate light distribution throughout the photoreactor volume; (ii) there must be sufficient mass-transfer rates between the liquid phase and the photocatalyst surface; and (iii) they must be inexpensive and easy to fabricate and operate. Herein, we report a mathematical model of a batch reactor. In the model, we start from the photon quantum and take into account the reactor geometry, the variety of the light sources, the exterior traits of the catalyzer, and the influence of light intensities on chemical reaction. The mathematical modeling of such a reactor is essential to aid the scale-up design and optimization of this type of reactor. The modeling was developed using parameters that can be estimated easily from real systems, and model solutions can be obtained with minimal computational effort. The mathematical model of the batch reactor was
10.1021/ef050268h CCC: $33.50 © 2006 American Chemical Society Published on Web 03/23/2006
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Wang et al.
validated with results from the photochemistry desulfurizaion reaction. 2. Experimental Section A stainless steel photochemical desulfurization reactor (16 cm in diameter × 25 cm in height) was used with an ultraviolet radiation lamp and quartz pipe inside it and an aerating board at the bottom, and the aerating quantity was adjusted by a flow meter. Sulfide-enriched FCC gasoline (by solvent extraction) and 10 g of immobilized titanium dioxide were added into the reactor. The photochemical reaction occurred under ultraviolet irradiation. Sampling then was to occur at different timea, and an analysis was conducted to measure its sulfur contents. 3. Building a Mathematical Model of the Photochemical Desulfurization Reactor The mathematical model of the photochemical desulfurization reactor must be constructed using a radiant energy transfer equation, as well as a heat quantity equation, momentum transfer equation, quality transfer equation, and reaction kinetics equation. 3.1. Hypothesis of the Mathematical Model. The model is defined by the following assumptions: (1) The reactor conditions are constant temperature and steady state. (2) The photochemical reaction occurs at the surface of the catalyzer. (3) The catalyzer surface, which is perpendicular to the direction of light transmission, is considered to be the valid reaction surface. (4) Only radial diffusion of the reactant and the radial divergence of ultraviolet light is taken into account, and axial diffusion of the reactant and the axial divergence of ultraviolet light is considered to be negligible. (5) The reactant, catalyzer, and solvent absorb ultraviolet light in the form of photons without accumulation. (6) The absorption of ultraviolet light by the reaction product and the intermediate product is considered to be negligible. (7) Influence caused by the attenuation of ultraviolet light is considered to be negligible. (8) Ultraviolet light is simplified as a beeline, and it is fused with the reactor axes. 3.2. Radiant Energy Transfer Equation. The photochemical reaction velocity is dependent on the local volumetric rate of energy absorption (LVREA), and the LVREA is dependent on the radiant energy distribution inside the photochemical reactor. The factors that affect the radiant energy distribution inside a photochemical reactor include (i) the radiation wavelength of the light source, (ii) the geometry of the reactor, (iii) the relative locations of the reactor and the light source, and (iv) the mixed character of the reaction system. To confirm the light source model and radiant energy transfer model, an energy equation of radiant energy transfer must be devised.3 Currently, there are two models of light source: the incidence model, which is applied to the exterior light source of the reactor, and the emission model, which is applied to the interior light source. In this experiment, the light source is located in the interior of reactor, so a radial emission model is selected. This means the light source emits photons perpendicular to the symmetry axes of the reactor. Figure 1 illustrates the light source radial emission model of a photochemical reactor.4 Taking into account the flexible dispersion of photons by the medium, the basic equation of radiant energy transfer5 in the
Figure 1. Radial emission model for the light source in a photochemical reactor.
tentative direction of the incidence photons is given as eq 1:
∫ pI dl
dI + θI + βI ) y0 + β dl
(1)
where I is the radiation intensity at x (given in candela (cd)), l the ultraviolet transmitting distance along the radius (given in centimeters), θ the radiant energy absorption coefficient of the mediums (given in units of cm2/kg), β the radiant energy dispersion coefficient of the mediums inward, y0 the change in the light intensity of the catalyzer-granule emission along transmission distance x, and p the phase function. The reaction happens at normal temperature, so the emission item y0 can be neglected. The expression β∫ pI dl represents the inward diffusion quantity of radiant energy by the medium. The total radiant energy transiting the microsurface in unit time is
B)
∫λλ ∫l Iλ dl dλ 2
(2)
1
where B is the total radiant energy across the surface (given in joules), λ the ultraviolet wavelength (given in nanometers), λ1 the minimum ultraviolet wavelength across a macrosurface (given in nanometers), λ2 the maximum ultraviolet wavelength across a macrosurface (given in nanometers), and Iλ the radiation intensity at ultraviolet wavelength λ (given in units of cd). The LVREA is given as
E)
∫λλ ∫l RmIλF1 dl dλ 2
(3)
1
where E is the LVREA (given in units of J cm-3 s-1), Rm the average absorption coefficient of solute (given in units of cm2/ kg) and F the systemic density (expressed in terms of kg/m3). The reaction velocity and LVREA can be linked by photon efficiency:
φ)
(-rA) w (-rA) ) φE E
(4)
where φ is the quantum efficiency (given as a percentage) and -rA is the reaction rate (given in units of µg g-1 s-1). The LVREA can be calculated through measurement of the reaction velocity, and then the radiant energy distribution inside the reactor can be obtained. Conversely, the reaction velocity can be calculated by measuring the LVREA. 3.3. Photochemical Reaction Dynamics Equation and Mass-Transfer Equation. 3.3.1. Photochemical Reaction Dynamics.6,7 Photochemical desulfurization is comprised of two stages: an extraction process and photo-oxidation with ultra(3) Alfano, O. M.; Romeno, R. L.; Cassano, A. E. Radiation field modelling in photoreactors homogeneous media. Chem. Eng. Sci. 1986, 41 (3), 421-444. (4) Li, L. Photochemical reactor. Chem. React. Eng. Technol. 1995, 11 (4), 386-395. (5) Huang, R. Q.; Liu, T.; Li, Zh. M., et al. Study on radiation transport in heterogeneous photocatalytic reactor. J. Yunnan Normal UniV., Nat. Sci. 2001, 21 (5), 32-36.
FCC Gasoline Photochemical Desulfurization Reactor
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Figure 2. Regression curve of experimental data of the photochemical reaction of a simulating system.
violet irradiation from a high-pressure mercury lamp. During the extraction process, sulfur-containing compounds are transferred from the light oil to the solvent. In the photo-oxidation reaction, the sulfur-containing compounds are photo-oxidized and photodecomposed in the solvent via ultraviolet irradiation from a high-pressure mercury lamp. First of all, we may assume that the temperature has no influence on photochemistry; the most useful measure of reaction rate for the reactant sulfide then is8
-rA ) -
dcA k1k2cA ) dt 1 + k2cA
(5)
where cA is the mass fraction of reactant (given in units of µg/ g), t the reaction time (given in hours), k1 the reaction rate constant at the catalyzer surface, (given in units of s-1), and k2 the absorption rate constant at the catalyzer surface, (given in units of s-1). When k2cA is ,1, the reaction order of the photochemistry catalyzing desulfurization is 1.
-rA ) -
dcA ) kcA dt
(6)
where k ) k1k2 and k is the reaction rate constant (given in units of s-1). Certain proportions of thiol, sulfur, ether, and thiophene are added into normal octane as the simulating system; photochemical reaction occurs at temperatures of
