A fundamental understanding of the reaction kinetics is essential in process development, scale-up, and design. Determination of detailed reaction kinetics for OSR is very challenging, particularly for liquid hydrocarbons since liquid hydrocarbons are complex mixtures of hundreds of components, and each one undergoes several different reactions. In addition to the reactions associated with reforming, other reactions may take place: water-gas shift, carbon formation, methanation, hydrocracking, dehydrocycliza-tion, dehydrogenadehydrocycliza-tion, ring opening, hydrogena-tion, etc. Also, the activity of the catalyst changes rapidly during the reaction and the kinetics vary with catalyst, fuel composition, and operating conditions. It would be desirable to develop predictive models to account for variations in these parameters.
6.4.1. Hydrocarbons
The kinetics of OSR of hydrocarbons are mainly described by two approaches: (1)
Combination of individual reactions and (2) Fundamental. These two approaches are dis-cussed in the following sections.
6.4.1.1. Combination of Individual Reactions Approach
In OSR, a series of reactions (Eqs. 6.2e6.14) are likely to take place depending on the reac-tion condireac-tions. However, some reacreac-tions such as Boudouard reaction, methanation reaction, decomposition reaction, partial combustion reaction, and CO2 reforming are generally ignored in the kinetic modeling of OSR of hydrocarbons for simplification. Only reactions such as steam reforming, total combustion, and water-gas shift reaction that significantly affect the kinetics of OSR of hydrocarbons are considered. In this approach, researchers adopt the reaction kinetics for these significant reac-tions from published literature to develop a kinetic model for OSR of hydrocarbons. A number of models can be found for total combustion, steam reforming, and water-gas shift reaction. Researchers use different combi-nations of rate expressions for these individual reactions. They make some modification in the original form of rate expressions to reflect their reaction conditions or catalysts. For example, kinetic models for steam reforming and water-gas shift from Xu and Froment [65] over Ni-based catalyst are widely used by researchers for SR and WGS steps in the OSR of methane, while the steam reforming model from Tottrup [66]is widely used for higher hydrocarbons.
The combination of individual reactions used by various researchers and the corresponding rate expressions for these individual reactions are summarized in Table 6.3. The approach is also explained by an example below from Pacheco et al.[67].
Pacheco et al. [67] developed and validated a pseudo-homogeneous mathematical model for OSR of isooctane and subsequent water-gas shift. They assumed two-step mechanism for OSR of isooctane: combustion of isooctane
6. OXIDATIVE STEAM REFORMING
144
followed by steam and CO2reforming, and water-gas shift reaction. Their model uses Langmuire HinshelwoodeHougeneWatson (LHHW) kinetics in which the expressions were obtained from literature for combustion, steam reforming, CO2
reforming, and WGS to determine the kinetic parameters from isooctane OSR experimental data over Pt on ceria.Table 6.4 shows the rate
expression used by Pacheco et al. [67] in the reformer modeling. The parameters of these rate expressions are summarized inTable 6.5.
Shigarov et al. [78] modeled OSR of diesel assuming that two reactions (combustion of hydrocarbons and steam reforming of hydrocar-bons) occur simultaneously in the reactor, but the reaction rate is controlled by the external TABLE 6.3 Summary of the Combination of Individual Reaction Approach
Research group (fuel) Individual reactions considered Kinetic rate equations adopted from Halabi et al.[68]; Hoang et al.[69e70]
(methane)
CH4þ 2O2¼ CO2þ 2H2O CH4þ H2O¼ CO þ 3H2
COþ H2O¼ CO2þ H2
CH4þ 2H2O¼ CO2þ 4H2
Ma et al.[71]; Trimm and Lam[72]
Xu and Froment[65]
Xu and Froment[65]
Xu and Froment[65]
Nah and Palanki[73](heptane) C7H16þ 7H2O¼ 7CO þ 15H2
CH4þ H2O¼ CO þ 3H2
COþ H2O¼ CO2þ H2
CH4þ 2H2O¼ CO2þ 4H2
Tottrup[66]
Xu and Froment[65]
Xu and Froment[65]
Xu and Froment[65]
Rabe et al.[10](gasoline) CmHnþ mH2O¼ mCO þ (m þ n/2)H2
CH4þ H2O¼ CO þ 3H2
COþ H2O¼ CO2þ H2
CO2þ 4H2¼ CH4þ 2H2O
Tottrup[66]
Papadias et al.[74](gasoline) CmHnþ (m þ n)/2O2¼ mCO2þ (n/2)H2
CmHnþ mH2O¼ mCO þ (m þ n/2)H2
COþ H2O¼ CO2þ H2
Power law rate expression Dubien et al.[75]
Wheeler et al.[76]
TABLE 6.4 Rate Equations Used in the Reforming Model by Pacheco et al.[67]; Reprinted from Pacheco et al. [67], Copyright (2003), with permission from Elsevier.
Reaction Expression for ri T (C) Catalyst Reference
C8H18þ 16O20 8CO2þ 9H2O
r1 ¼ k1PiC8PO2 800e900 Ni/Al2O3 [77]
C8H18þ 8H2O0 8CO þ 17H2
r2 ¼ k2
P2:5H2
PiC8PH2O P3H2PCO=K1
ð1 þ KCOPCOþ KH2PH2þ KiC8PiC8þ KH2OPH2O=PH2Þ2
!
500e750 Ni/MgAl2O3 [65]
C8H18þ 8CO20 16CO þ 9H2
r3 ¼ k3PiC8PCO2 1 P2COP2H2 K3PiC8PCO2
!
800e900 Ni/Al2O3 [77]
C8H18þ 16H2O0 8CO2þ 25H2
r4 ¼ k4
P3:5H2
PiC8P2H2O P4H2PCO2=K4
ð1 þ KCOPCOþ KH2PH2þ KiC8PiC8þ KH2OPH2O=PH2Þ2
!
500e750 Ni/MgAl2O3 [65]
COþ H2O0 CO2 þ H2
r5 ¼ k5
PH2
PCOPH2O PH2PCO2=K5
ð1 þ KCOPCOþ KH2PH2þ KiC8PiC8þ KH2OPH2O=PH2Þ2
!
500e750 Ni/MgAl2O3 [65]
KINETICS 145
mass transfer of hydrocarbons. However, the kinetic model ignored other side reactions (carbon formation, methanation, etc.) that may be occurring during OSR of diesel. The surface concentration of hydrocarbons participating in each competing reaction (combustion and SR) was determined by the ratio of kinetic rates of both reactions. They also estimated that hydro-carbon bulk-surface flux (or mass transfer) determines the overall reaction rate, not the oxygen concentrations. Their reaction model is similar to the two-step reaction mechanism for OSR. Interestingly, the model predicts that steam-reforming reaction initiates even at the top of the catalyst bed, but is dominated by the combustion reaction (see Fig. 6.13). The high reaction rate of steam-reforming reaction at the inlet of the reactor bed may extend the catalyst life because it helps minimize hot spots, which can be detrimental to the catalyst.
Models that treat the kinetics to the major reactions independently fail to capture the effects of one reaction on others. For example, highly exothermic combustion reactions at the initial stage of the catalyst bed not only generate temperature gradient across the catalyst bed, but also produce highly active intermediate
species. Both the temperature gradient and highly active intermediate species affect other reactions, whether they occur in parallel or sequentially. Also, these kinetic models assume that the kinetics for the SR of hydrocarbons dictate the reaction in most of the reactor. There-fore, there is a need for a kinetic model to describe the OSR reaction process that is not based on individual reactions.
6.4.1.2. Fundamental Approach
Dorazio and Castaldi [79] extended the n-heptane homogeneous reaction mechanism (which is readily available) to larger hydrocar-bons by includingn-tetradecane decomposition reactions such as partial oxidation, steam reforming, and cracking reactions. A simplified transition state theory approach was used to calculate the kinetic parameters which were not available in open literature. A sensitivity analysis was also performed to identify the dominating reaction paths during the reforming reaction of n-tetradecane. This was done by omitting reac-tions sequentially and then observing the effects on conversion and product species concentra-tions. If the conversion or product species concentrations are greatly affected by omitting TABLE 6.5 Results of the Kinetic Parameter
Regression for the Rate Equations Used in Table 6.4; Reprinted from Pacheco et al.
[67], Copyright (2003), with permission from Elsevier.
Parameter Pre-exponential
factor
Activation energy (kJ/mol) k1(mol/(gcats bar2)) 2.58Eþ 08 166.0 k2(mol bar0.5/gcats)) 2.61Eþ 09 240.1 k3(mol/gcats bar2)) 2.78E 05 23.7 k4(mol bar0.5/(gcats)) 1.52Eþ 07 243.9 k5(mol/(gcats bar)) 1.55Eþ 01 67.1 KH2O(dimensionless) 1.57Eþ 04 88.7*
* Heat of adsorption of water (DHH2O).
FIGURE 6.13 Distribution of apparent reaction rates along the catalyst bed during OSR of diesel (O/C¼ 1.0, S/C
¼ 1.67, and inlet temperature ¼ 346 C); Reprinted from Shigarov et al. [78], Copyright (2009), with permission from Elsevier.
6. OXIDATIVE STEAM REFORMING
146
a reaction, then it means that the reaction repre-sents an important step in the mechanism of reforming of n-tetradecane. The model agrees with the experimental values at a range of temperatures and space velocities. From sensi-tivity analysis, it was observed that cracking of tetradecane, oxidation reactions of resulting