Environ. Sci. Technol. 2003, 37, 428-436
Application of an Isothermal, Three-Phase Catalytic Reactor Model To Predict Unsteady-State Fixed-Bed Performance J I Y A N G , † D A V I D W . H A N D , * ,‡ DAVID R. HOKANSON,‡ AND JOHN C. CRITTENDEN‡ Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, and Institute of Paper Science and Technology, 500 10th Street, NW, Atlanta, Georgia 30318
CatReac, a three-phase catalytic mathematical model, was developed for analysis and optimization of the volatile reactor assembly used in International Space Station water processor. This wet oxidation process is used to remove low molecular weight contaminants such as acetic acid, acetone, ethanol, 1-propanol, 2-propanol, and propionic acid, which are not removed by the other treatment processes. The Langmuir-Hinshelwood (Hinshelwood, C. N. The Kinetics of Chemical Change in Gaseous Systems, 3rd ed.; Oxford: London, 1933; pp 301-347) isothermal adsorption expression was successfully used to describe the reaction kinetics of compounds on the catalyst surface for the compounds mentioned above. Smallcolumn experiments combined with the use of the Arrhenius equation were successfully used to predict the LangmuirHinshelwood parameters under different temperatures for a temperature range from 93 to 149 °C. Full-scale and smallcolumn experiments were successfully used to validate the model predictions for unsteady-state fixed-bed operations.
Introduction Water for drinking, food preparation, and hygiene accounts for over 90% of the basic consumables (water, oxygen, and food) required for survival aboard the International Space Station (ISS). Support of a four-person crew aboard the ISS would require transport of approximately 46 100 lb of water per year with an additional 4400 lb per year required for payloads (1-3). The technical and economic limitations of this water transport require onboard recovery and reuse of the ISS aqueous waste streams. Recycling the water aboard the ISS involves various treatment processes. Three treatment technologies are utilized in the ISS water processor: (i) water first passes through a 0.5-µm filter to remove particulate matter; (ii) ion exchange and adsorption media are combined in multi-filtration beds for removal of ionic and organic compounds; (iii) catalytic oxidation technology known as volatile reactor assembly (VRA) follows the multi-filtration beds to remove constituents that are not removed via adsorption or ion exchange. The VRA is a three-phase co-current packed column that uses a stoichiometric excess of gas-phase oxygen as the * Corresponding author phone: (906)487-2777; fax: (906)487-2943; e-mail:
[email protected]. † Institute of Paper Science and Technology. ‡ Michigan Technological University. 428
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 2, 2003
oxidant and a noble metal catalyst. The ion exchange and adsorption-treated wastewater is heated, pressurized, and fed to the packed column along with an oxygen feed stream. The polar, low molecular weight organic compounds contained in the wastewater are oxidized into mineral acids or organic acids that are subsequently removed by ion exchange. This wet oxidation process is typically operated at temperatures and pressures around 120-140 °C and 370-450 kPa, respectively. A mathematical model called CatReac was developed to describe the impact of the VRA process variables on process performance and to provide optimum operation. CatReac describes the transport and series reactions of chemical contaminants in the VRA reactor. The model mechanisms include axial dispersion, advective flow, gas-to-liquid and liquid-to-solid mass transport, intraparticle mass transport by pore and surface diffusion, and series reactions of multiple contaminants on the catalyst surface. The model was developed to simulate contaminant destruction and byproduct formation for both steady- and unsteady-state scenarios. The Langmuir-Hinshelwood (L-H) expression is employed in CatReac to describe the reaction kinetics of contaminant destruction and byproduct formation and destruction on the catalyst surface. Previous work (4) showed that the L-H parameters determined from single-compound small-column experimental data could be used to predict steady-state multi-compound and full-scale performance at an operating temperature of 121 °C. This paper presents the application of CatReac for predicting performance under different operating temperatures and unsteady-state operation. In addition, a method for estimating reaction rate parameters using a group contribution method is presented and tested.
Model Development Figure 1 displays the transport and reaction mechanisms that are incorporated in the CatReac model. The model assumes the following: (1) the mobile gas and liquid phases are transported through the fixed bed by advection; (2) the mobile gas and liquid-phase flow rates are constant; (3) the liquid- and gas-phase fluxes may be described by the linear driving force approximation; (4) the intraparticle transport is caused by pore diffusion, and surface diffusion is not important (Surface diffusion is incorporated into CatReac, but it is set to zero in this paper because the inherent assumption is that the reactants would migrate to reactive sites before they would make a significant contribution to the flux); (5) the catalyst is completely wetted by the liquid phase; (6) a local equilibrium exists at the adsorbent surface; (7) the multi-compound adsorption equilibrium and chemical kinetics may be described by the L-H kinetic expression; (8) the homogeneous oxidation reactions in the liquid and gas phases are not considered in the model, but their influence would be incorporated into the kinetic parameters; (9) the pressure drop across the fixed bed is negligible as compared to the total pressure, and the pressure is time-independent; and (10) the fixed bed is isothermal. Considering the above mechanisms and assumptions, mass balance expressions were derived for each phase in the VRA reactor. The mass balance expressions were then converted to dimensionless form. The dimensionless transport of the chemical organic compound i in the mobile gas phase is 10.1021/es025846c CCC: $25.00
2003 American Chemical Society Published on Web 12/03/2002
j generated within the immobile catalyst. The initial condition for eq 7 is
yji(rj,zj,T)|T)0 ) 0
(8)
One of the boundary conditions that result from symmetry is
∂yji(rj,zj,T) ∂rj
|
jr)0
)0
(9)
A self-starting numerical solution for eq 7 that is based on an overall mass balance is FIGURE 1. Mass transport and reaction mechanisms surrounding a single catalyst particle.
1 + Art ∂C h v,i(zj,T) ∂C h v,i(zj,T) ) -Ari + Dgv,i 1 + Dgt ∂T ∂zj 3Stv,i[C h b,i(zj,T) - C h v,i(zj,T)] (1) The terms in the mobile gas-phase mass balance, from left to right, account for accumulation, advective flow, and mass transfer between gas and liquid phase, respectively. The initial and boundary conditions for eq 1 are
C h v,i(zj,T)|T)0,0
