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Heterogeneous oxidation of zinc vapor by steam and mixtures of steam and carbon dioxide
Accepted Manuscript Heterogeneous Oxidation of Zinc Vapor by Steam and Mixtures of Steam and Carbon Dioxide Luke J. Venstrom, Paul Hilsen, Jane H. Davidson PII: DOI: Reference:
S0009-2509(18)30150-7 https://doi.org/10.1016/j.ces.2018.03.020 CES 14094
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
29 August 2017 24 January 2018 7 March 2018
Please cite this article as: L.J. Venstrom, P. Hilsen, J.H. Davidson, Heterogeneous Oxidation of Zinc Vapor by Steam and Mixtures of Steam and Carbon Dioxide, Chemical Engineering Science (2018), doi: https://doi.org/ 10.1016/j.ces.2018.03.020
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Heterogeneous Oxidation of Zinc Vapor by Steam and Mixtures of Steam and Carbon Dioxide Venstrom, Luke J.1 Department of Mechanical Engineering, Valparaiso University 1900 Chapel Drive, Valparaiso, IN 46383 [email protected] Hilsen, Paul Department of Mechanical Engineering, University of Minnesota—Twin Cities 111 Church Street, Minneapolis, MN [email protected] Davidson, Jane H. Department of Mechanical Engineering, University of Minnesota—Twin Cities 111 Church Street, Minneapolis, MN [email protected] ABSTRACT The kinetics of the heterogeneous oxidation of zinc vapor by water vapor were measured in a tube flow reactor for temperatures from 800 to 1100 K, zinc vapor partial pressures up to 0.39 atm, and water vapor partial pressures up to 1.0 atm. The results extend the prior data for oxidation of zinc by water vapor from zinc partial pressures on the order of 0.01 atm to higher values appropriate for fuel production via the Zn/ZnO thermochemical cycle. Measured oxidation rates span 10-7 to 10-5 mol cm-2 s-1. A second order, reversible reaction rate expression
-
is developed
from regression of the data and a numerical model of advective and diffusive mass transfer. The kinetic parameter
-
is a non-monotonic function of temperature with a
negative activation energy for temperatures between 800 and 1050 K, consistent with prior studies. In a second set of experiments, the rate of the heterogeneous oxidation of 1
Corresponding author, e-mail: [email protected].
zinc vapor by mixtures of water vapor and carbon dioxide was measured. The product gas is hydrogen rich due to faster surface reaction kinetics for oxidation with water vapor than with carbon dioxide. We conclude that it is preferable to split water and carbon dioxide in separate reactors rather than co-produce H2 and CO in a single reactor for production of synthesis gas in the Zn/ZnO solar thermochemical redox cycle. Keywords: solar, energy, fuel, mass transfer, metal oxidation, kinetics Highlights:
The heterogeneous oxidation of Zn(g) provides rapid conversion of Zn to ZnO.
The oxidation of Zn(g) by H2O proceeds at 10-7–10-5 mol cm-2 s-1 at 800–1100 K.
The kinetics of oxidation of Zn(g) with H2O is fit to
The oxidation of Zn(g) with H2O is 20 times faster than with CO2 at 1100 K.
-
.
INTRODUCTION The heterogeneous oxidation of zinc vapor by water and carbon dioxide (reactions 1 and 2) plays an important role in a variety of processes including zinc smelting (Clarke and Fray, 1978;Clarke, 1979;Clarke and Fray, 1979;Cox and Fray, 2003;Lewis and Cameron, 1995a;Lewis and Cameron, 1995b;Osborne et al., 2001), steelmaking (Bronson et al., 2017), and solar thermochemical fuel production via the Zn/ZnO redox cycle (Venstrom and Davidson, 2011;Venstrom and Davidson, 2013). (1) (2) In zinc smelting and steelmaking, the oxidation of zinc vapor is an undesirable side reaction in the recovery of zinc from zinc-laden gas streams that include carbon dioxide, carbon monoxide, and water vapor. Zinc partial pressures are low in these processes,
2
typically below ~0.05 atm. For solar fuel production, Zn oxidation is the requisite water and/or carbon dioxide splitting step, and high zinc partial pressure is desired. In both applications, process development benefits from a kinetic model. A number of studies of the heterogeneous oxidation of zinc vapor have developed kinetic expressions for low zinc partial pressure conditions applicable for smelting and steel making. Kinetic expressions are available for zinc, water vapor, and carbon dioxide partial pressures less than ~0.05 atm (Bronson et al., 2017;Clarke and Fray, 1978;Clarke and Fray, 1979). Here we review expressions for Zn(g) oxidation by H2O(g), which is the focus of the present work. Clarke and Fray (1979) measured oxidation rates in a 10-mm-i.d. tube reactor for temperatures between 773 and 1173 K, Zn(g) partial pressures up to 0.05 atm, and for stoichiometric oxidation where the partial pressures of Zn(g) and H 2O(g) are equal. The data were fit to a semi-empirical reaction rate expression: (3) where
is the partial pressure of zinc vapor at chemical equilibrium and
temperature-dependent parameters (Clarke and Fray, 1979). The parameter
and
are
decreased
from 781 K to 1077 K, implying a negative activation energy over this temperature range. Bronson et al. (2017) used a larger, 40-mm-i.d. reactor to obtain data between 600 and 1000 K for Zn(g) partial pressures up to 0.01 atm, and H2O(g) partial pressures up to 0.05 atm. They fit the data to a second-order reversible reaction rate expression: (4)
3
The temperature-dependent kinetic parameter, , was given by an Arrhenius expression with an activation energy of 13.7 kJ/mol and a pre-exponential factor of 32.9 mol m-2 s1
atm-2. In the solar Zn/ZnO thermochemical redox cycle, the heterogeneous oxidation of
zinc vapor was first proposed by Venstrom and Davidson (2011) as a means to avoid the rate limitations of the diffusion of reactants across the ZnO layer that forms on the surface of solid and liquid zinc (Abanades, 2012;Ernst et al., 2009;Loutzenhiser et al., 2009;Lv et al., 2010;Nicodemus and Davidson, 2015;Vishnevetsky and Epstein, 2007). Reviews of the Zn/ZnO solar thermochemical cycle are provided by (Agrafiotis et al., 2015;Romero and Steinfeld, 2012;Steinfeld, 2005). In the first step of the cycle, process heat supplied via concentrated solar radiation drives the endothermic dissociation of ZnO: (5) Complete thermal dissociation of ZnO is possible at temperatures of 2100–2300 K (Muller et al., 2006). Alternatively, the reduction of ZnO can be completed at lower temperatures carbothermally (Brkic et al., 2016;Perkins et al., 2008;Wieckert et al., 2007) or electrothermally (Schroeder et al., 2011;Venstrom et al., 2009), but thermal reduction is favored environmentally. Fuel production is via reactions 1 and 2. The rapid kinetics promised by the oxidation of zinc vapor are beneficial to quickly reach high conversions of Zn to ZnO. Moreover, even in oxidation reactors in which the original intent was to oxidize solid or liquid zinc, heterogeneous oxidation is an important reaction pathway when Zn vapor is present (Nicodemus and Davidson, 2015;Weibel et al., 2014). In a practical process, fuel production will be favored by zinc partial pressures much higher than those in the 4
smelting or steel making processes or than those available at temperatures below the Zn melting temperature of 693 K. For example, raising the zinc partial pressure from 0.01 atm to 0.3 atm increases the specific rate of oxidation by CO2 at a concentration of 0.5 atm by two orders of magnitude from 3.7×10-6 to 1.1×10-4 mol cm-2 s-1 at 1100 K (Venstrom and Davidson, 2013). A kinetic model was developed for the heterogeneous oxidation of Zn(g) by carbon dioxide for the higher Zn(g) partial pressures anticipated for fuel production in the Zn/ZnO thermochemical cycle (Venstrom and Davidson, 2013). For Zn partial pressures up to 0.36 atm, and CO2 partial pressures up to 0.45 atm at 800 to 1150 K, the rate is given by: (6) -2
-1
The activation energy is 44 kJ/mol and the pre-exponential factor is 920 mol m s atm-2. Oxidation rate increases with temperature due to higher zinc saturation pressure as well as kinetics via the Arrhenius-based parameter
-
. For zinc partial pressures below
0.01 atm, Bronson et al. (2017) report a similar activation energy of 50.2 kJ/mol but a lower pre-exponential factor, 406 mol m-2 s-1 atm-2. In the present study, we fill a gap in the literature for the rate of the heterogeneous oxidation of Zn(g) with H2O at Zn(g) partial pressures greater than 0.05 atm that are meaningful for fuel production, as opposed to the zinc smelting and steel making processes. Oxidation rates are measured from 800 to 1100 K, Zn(g) partial pressures up to 0.39 atm and H2O(g) partial pressures up to 1.0 atm. A numerical model of convective mass transfer is applied to decouple the intrinsic surface kinetics from the measured global reaction rates. The kinetics of water splitting is compared to the previously published kinetics of carbon dioxide splitting (Venstrom and Davidson, 2013). This 5
comparison is used to interpret new measurements of heterogeneous oxidation of Zn(g) in mixtures of H2O and CO2. The goal of this second set of experiments is to determine the relative amounts of H2O and CO2 in the reactant stream that will lead to a 2:1 ratio of H2to-CO in the product gases. A 2:1 ratio is desirable for the production of high-quality hydrocarbon fuels in the Fischer-Tropsch process (Dry, 1996). APPROACH Experiments are performed in a quartz tube flow reactor specifically designed to evaluate the kinetics of the heterogeneous oxidation of Zn(g). (The reactor is solely for measurement of kinetics and is not, as has been inferred by others (Weibel et al., 2017), intended as a scalable reactor for practical or efficient fuel production.) The facility was used previously to study the oxidation of Zn(g) and CO2 (Venstrom and Davidson, 2013). Here we provide an overview of the reactor and describe the operating conditions for the present study. The reactor, depicted in Fig. 1, is constructed of concentric quartz tubes positioned horizontally within an electric furnace (Thermolyne 79500). Zinc is evaporated in the outer tube. The Zn(g) reacts with H2O(g) and/or CO2 on the inner surface of the central tube, referred to as the oxidation tube. ZnO first forms as a monolayer on the quartz surface. The layer thickness grows over the course of an experiment. Rates of ZnO production are determined gravimetrically, and the total H2 and CO production are measured by analyzing the composition of the product gas. Mass transfer in the reactor is modeled by solving the conservation of mass, momentum, and species in an axisymmetric domain using a finite volume numerical method, an approach
6
that captures the effects of both diffusion to the reaction site and advective mixing of the reactants at the shear layer between the inlet flows on the centerline and in the annulus.
Figure 1. The isothermal tube flow reactor for measurement of the kinetics of the heterogeneous oxidation of Zn(g). The reactor is not drawn to scale; dimensions are in millimeters. Prior to each experiment, the reactor is purged by a flow of Ar at 523 K. After the purge, the oxidation temperature and inlet concentration of reactants are set for each experiment. Reaction conditions are maintained to within ±10 K of the set point temperature in the 300 mm reaction zone. Temperature is measured with a chromelalumel thermocouple. Zinc vapor is generated by flowing high-purity Argon (Ar, grade 5.0) over a pool of liquid Zn (99.999% pure, metals basis) contained in the outer quartz tube. The Zn(g)-Ar gas mixture flows into the 3.9 mm i.d. oxidation tube where it is mixed with a coaxial flow of H2O(g) or pre-mixed H2O(g) and CO2 (99.9% pure). Steam is produced by either saturating a flow of Ar with distilled and degassed H2O(l) (Toxidation ≤ 900 K or in a boiler (Reimers Model JR99) fed with distilled H2O(l) (Toxidation > 900 K). All gas flows, are controlled with mass flow controllers to within 1% of the desired flow rate. The mass of ZnO(s) deposited is measured in approximately 5 cm increments along the longitudinal axis of the inner quartz tube. A monolayer of ZnO is formed on 7
the quartz tube very quickly and then the ZnO forms layer by layer. Thus, the material of the tube is unlikely to impact the overall rate of the heterogeneous formation of ZnO. ZnO is removed from the tube via an acid wash, and the mass of ZnO is determined by weighing the quartz tube prior to and after the acid wash. Elemental Zn(s) and C(s) do not deposit on the tube. Hydrogen, which would indicate Zn co-deposition, is not evolved during ZnO removal and analysis of the hydrochloric acid solution using atomic absorption spectroscopy confirms that carbon is not co-deposited (Venstrom and Davidson, 2013). As the product gas mixture flows out of the reactor, unreacted Zn(g) and H2O(g) are condensed out of the gas flow. The outlet gas composition, including concentrations of H2, CO, and unreacted CO2, is measured with a Raman laser gas analyzer (RLGA, ±0.1 mol%). Data are collected at steady state, based on product gas concentrations, for 6–30 minutes, ensuring that the mass of ZnO(s) deposited along the tube is detectable. The overall rate of oxidation is expressed as (7) where
is the inner diameter of the oxidation tube (3.9 mm),
section over which the ZnO mass, ZnO, and
, is measured,
is the length of the
is the molecular mass of
is the time over which H2 and CO are produced in the experiment.
The surface kinetics are extracted from the overall rate of oxidation using a numerical finite volume model to account for mass transfer in the reactor. The model domain corresponds to the isothermal region of the oxidation tube from z=0 to z=200 mm and from r=0 to r=1.95 mm. The model assumes steady-state, axisymmetric, and isothermal flow, and calculates the velocity, pressure, and the concentrations of reacting
8
species in the flow by solution of the differential conservation equations of mass, momentum, and species in the reactor. The assumption of steady-state flow conditions is reasonable for the present study despite the growth of the ZnO layer on the tube walls because the tube diameter decreases by at most 5% for the amount of mass deposited in the present study. The model is described in greater detail in the prior study of CO2 oxidation (Venstrom and Davidson, 2013). A summary is provided here. The model assumes ideal gas behavior with spatial changes in density and viscosity due to changes in composition. The viscosity of the reacting flow is calculated using a mass-average of the pure species viscosities, and the viscosities of the pure species are evaluated using tabulated data (Yaws, 2010). The diffusion of species is modeled using a Fickian constitutive relationship in which the effective diffusion coefficients for the reacting species are calculated using a mixing law (Mills, 2001). The binary diffusivities of the reacting species are estimated according to the Chapman-Enskog theory. The boundary conditions include the Zn(g) and H2O inlets at z=0, the product mixture outlet flow at z=200 mm, symmetry on the centerline, and heterogeneous Zn(g) oxidation on the tip of the capillary tube and the inner surface of the oxidation tube. Fully-developed laminar velocity profiles for tube flow and annular flow are applied at the H2O and Zn(g) inlets, respectively. At the outlet, flow is assumed to be normal to the tube cross-section and the pressure is atmospheric. The no-slip condition is applied at the tip of the capillary tube and the inner surface of the oxidation tube. The heterogeneous reaction at these surfaces is modeled by equation 8 (Venstrom and Davidson, 2013): (8) where
is the deposited mass flux and
is the rate of the
heterogeneous oxidation of Zn(g). The heterogeneous reaction rate is a function of the 9
local partial pressures of the reacting species (i.e., the partial pressures at the surface) and also of the oxidation temperature (see Table 1). The deposited mass flux gives rise to a suction velocity at the reacting surfaces that is normal to the surfaces for which the magnitude is given by (9) where
is the mass-density of the gas mixture. To extract kinetic parameters from the experimental data, regression of the
numerically predicted and measured Zn(g) oxidation rates is achieved by adjusting the value of the kinetic parameter(s) to minimize a root-sum-squared error objective function.
(10) The index indicates summation over all axial locations of the experimental data. Minimization of the objective function is achieved using GenOpt (Wetter, 2011). Table 1 lists the kinetic expressions that were considered and the respective kinetic parameters. The phenomenological Langmuir-Hinshelwood and precursor mechanism reaction rate expressions account for the surface adsorption and subsequent reaction of gas species, and have been successfully applied to heterogeneous reaction systems that involve a solid product (Masel, 2001). They feature multiple temperaturedependent kinetic parameters. In the empirical variable-order reaction rate expression, the order of the oxidation with respect to Zn(g) and CO2 varies through the kinetic parameters
and
. Temperature-dependence in the variable-order kinetic expression is
included through a third kinetic parameter. The second-order expression represents the special case in which
=
= 1 in the variable-order expression and the special case in
10
which the denominators of the Langmuir-Hinshelwood and precursor mechanism rate expressions are approximately unity. The physical interpretation of the latter case is that the surface coverage of the adsorbed species is low, a probable condition at the high temperatures considered in the present study. Table 1. List of kinetic expressions considered for the oxidation of Zn(g) by H2O(g) and their associated kinetic parameters Kinetic Expression Kinetic Kinetic Model Parameters Second-order
-
LangmuirHinshelwood Precursor mechanism nth-order
RESULTS AND DISCUSSION Heterogeneous Zn(g)-H2O Oxidation Kinetics Table 2 lists the conditions of the H2O oxidation experiments, including the oxidation temperature, the reactance mass flow rates and partial pressures at the reactor inlet, and the Reynolds number for the reacting flow based on the oxidation tube inner diameter. Experiments were conducted at a total pressure of ~1.0 atm from 800 to 1100 K. The partial pressure of zinc at the reactor inlet spanned two orders of magnitude from 0.002 atm at 800 K to 0.385 atm at 1100 K. The partial pressure of Zn was maintained below the zinc saturation pressure to preclude the possibility of zinc condensation. The water vapor partial pressure varied between 0.028 atm and 1.0 atm.
11
Table 2. The oxidation temperature, Reynolds number, and mass flow rate and partial pressure of the reacting species at the inlets in the water-splitting experiments. The balance of mass in the gas phase is Argon. Zn(g) Inlet H2O(g) Inlet Red Experiment (K) (×106 kg/s) (atm) (×106 kg/s) (atm) 1 800 26 3.34 0.002 0.69 0.028 2 900 17 1.43 0.017 1.39 0.028 3 900 17 2.07 0.018 1.30 1.000 4 1000 15 2.21 0.052 0.40 1.000 5 1050 24 1.90 0.228 1.83 0.696 6 1100 27 2.02 0.231 1.34 1.000 7 1100 32 2.79 0.385 2.00 0.509
Measured and numerically predicted rates of the heterogeneous oxidation of Zn(g) by H2O(g) are compared in Figure 2 along the length of the oxidation tube. The model predictions match the measured reaction rates for all conditions considered in the present study. At all temperatures, the rate of oxidation increases to a maximum within 10 mm of the inlet as the flow develops and the Zn(g) and H2O(g) mix. The oxidation rate then decreases in the axial direction as reacting species are consumed. Higher temperatures favor rapid Zn(g) oxidation. At 1100 K, the peak oxidation rate for the conditions in the present study is 1.5×10-5 mol cm-2 s-1. At 1050 K, the peak oxidation rate drops an order of magnitude to 3×10-6 mol cm-2 s-1, and at 900 K the peak oxidation rate drops another order of magnitude to 3×10-7 mol cm-2 s-1. The non-linear changes with respect to temperature are a result of both the exponential temperature-dependence of the zinc saturation pressure and, as will be shown, the non-linear temperature-dependence of the global kinetics.
12
Figure 2. The rate of heterogeneous Zn(g) oxidation by H2O(g) along the length of the reactor. The curves represent the regression of the reactor model to the data.
The importance of decoupling mass transfer and surface reaction kinetics to develop a kinetic expression is highlighted in Fig. 3. In this figure, the partial pressures of Zn(g), H2O(g), and H2 calculated in the model are compared at the surface of the oxidation tube where the reaction occurs and in the bulk flow along the flow path using the oxidation conditions at 1000 K as an example. The bulk flow concentration is calculated at any axial location from the radial distributions of concentration according to equation 11:
(11) Mass transfer is important at any axial location where the partial pressures at the reaction surface and in the bulk flow are different. Surface and bulk flow partial pressures differ significantly in the first ~40 mm of the oxidizer tube where the flow is developing and the measured reaction rates (depicted in Fig. 2) are fastest. The Zn(g) surface partial pressure is as much as 0.01 atm lower than the bulk flow partial pressure (50% 13
difference). The H2O(g) surface partial pressure is as much as 0.26 atm lower than the bulk (94% difference), and the H2 surface partial pressure is as much as 0.004 atm larger than the bulk (20% difference). These results show that it would introduce substantial error to use bulk flow partial pressures when evaluating surface reaction kinetics. In the present study, the partial pressures at the reacting surface are used to evaluate the reaction kinetics so that the kinetics are free from error due to mass transfer. Regression of the measured reaction rate data and the reactor model are best achieved using a second-order, reversible reaction rate expression for the surface kinetics with a linear dependence on both the Zn(g) and H2O(g) partial pressures: (12) Temperature dependence of the reaction rate is captured in the parameter, the equilibrium constant,
-
, and
. The equilibrium constant is for the overall water-splitting
reaction (equation 1), and is calculated from existing thermodynamic databases (Antti Roine). With the alternative expressions featuring multiple kinetic parameters (Table 1), each of which represents an additional degree-of-freedom in the regression, the fit of the data was not improved enough to be considered statistically significant.
14
Figure 3. Partial pressures of (a) Zn(g), (b) H2O(g), and (c) H2 at the surface of the oxidation tube and in the bulk flow along the reacting flow path at 1000 K.
15
The values of the kinetic parameter
-
are provided in Table 3 for each
oxidation temperature and are plotted in Arrhenius form in Fig. 4. The kinetic parameter decreases from 200 mol m-2 s-1 atm-2 at 800 K to 3.4 mol m-2 s-1 atm-2 at 1050 K, and then increases to ~16 mol m-2 s-1 atm-2 at 1100 K. The experiments were repeated at 900 K and 1100 K to confirm this trend. At 900 K, the values of the kinetic parameter determined via regression of the repeated experiments are 59 and 60 mol m-2 s-1 atm-2, and at 1100 K, the values are 14.5 and 17.5 mol m-2 s-1 atm-2. These values are within experimental uncertainty.
Table 3. Kinetic parameter for the heterogeneous oxidation of Zn(g) by H 2O(g) -
Experiment 1 2 3 4 5 6 7
(K) 800 900 900 1000 1050 1100 1100
(mol m-2 s-1 atm-2) 199 59 60 6.8 3.4 14.5 17.5
Figure 4. Arrhenius plot of the temperature dependent kinetic parameter (mol m-2 s-1 atm2 ) determined via regression of the reactor model and the measured reaction rates for H2O. The best-fit line is a linear regression of the data between 800 K and 1050 K. 16
The decrease in the value of the kinetic parameter from 800 to 1050 K implies that for the oxidation of Zn(g) by H2O(g), the activation energy is -110 kJ mol-1 (see Fig. 4). Negative activation energies have been reported for other heterogeneous reaction systems, e.g. the chemical vapor deposition of TiO 2 (Jones et al., 1998;Kang et al., 2000) and cracking of n-alkanes over a zeolite (Wei, 1996), as well as for Zn(g) oxidation (Clarke and Fray, 1979). For the same kinetic expression (equation 12) and over a similar range of temperature, Bronson et al. (2017) report a positive activation energy of 13.7 kJ/mol. We attribute this difference to the fact that Bronson et al. assume fully developed and radially mixed conditions in a model of their reactor. We find flow development and radial diffusion to be important in a much smaller diameter reaction tube. Zinc and water vapor must spread from their respective inlets at the annulus and centerline of the tube, respectively, throughout the cross section of the oxidation tube over the first ~5 mm along the flow path as is evident in the rise of the rate of the oxidation in Fig. 2. Once developed, radial gradients in the concentrations of the species show that diffusion also affects oxidation rates. Thus it is possible that the Bronson et al. kinetic model includes the effect of mass transfer and consequently does not produce the negative activation energy found in the present work and in Clarke and Fray (1979). A mechanism most often referred to as the precursor mechanism is one plausible chemical pathway for Zn(g)-H2O(g) oxidation that could explain the trends observed in the kinetic parameter,
-
, of equation 12. In the precursor mechanism, H2O(g) first
adsorbs to the reaction surface and then reacts with Zn(g) from the gas phase (Masel, 2001). The overall reaction rate expression for the precursor mechanism is
17
(13) where
is the temperature-dependent reaction rate parameter for the oxidation of
adsorbed H2O and Zn(g) and H2O(g) on ZnO. If
is the equilibrium constant for the adsorption of
≪ 1, then the denominator of equation 13 is approximately unity,
and equations 12 and 13 become identical with
-
. In this case, the
apparent activation energy of the second-order kinetic parameter,
, is equal to the sum
of the activation energy of the reaction rate parameter and the heat of adsorption of H2O: (14) Adsorption is an exothermic process (
< 0). Thus, if the magnitude of the heat of
adsorption is larger than the activation energy
, the apparent activation energy
will
be negative, as is observed in the present study (Fig. 3). Data for the heat of adsorption of H2O on polycrystalline ZnO support this hypothesis. The heat of adsorption depends on the surface coverage of H2O. At low coverage, the heat of adsorption is -100 kJ/mol. At 80% coverage, it increases to a maximum of -160 kJ mol-1 (Nagao et al., 1978). The apparent activation energy of -110 kJ mol-1 in the present study falls on the low end of the reported range of heat of adsorption, consistent with a low activation energy of the elementary chemical conversion step compared to the heat of adsorption of H2O. Combining the apparent activation energy from the present study and the reported heat of adsorption reported by Nagao et al. (1978) suggests that the activation energy of the oxidation of adsorbed H2O by Zn(g) is between 15 and 75 kJ mol-1. It is perhaps coincidental but nonetheless interesting that the activation energy measured for the oxidation of Zn(g) by CO2,
18
45 kJ/mol, falls within this same range (Bronson et al., 2017;Clarke and Fray, 1978;Venstrom and Davidson, 2013). Comparison of the Heterogeneous Oxidation of Zn(g) by H2O and CO2 The rate of the heterogeneous oxidation of Zn(g) by H2O(g) is compared to that for oxidation by CO2. The rate of the oxidation of Zn(g) by CO2 is evaluated using equation 6, developed in our prior work (Venstrom and Davidson, 2013). The rate of oxidation of Zn(g) by H2O(g) is evaluated using equation 12. Figure 5 is a plot of the rate of oxidation as a function of temperature from 800 K to 1100 K, the range of temperatures included in the experiments from which the kinetic expressions were developed. For each temperature, the partial pressures of Zn(g), H2O(g), and CO2 are set equal to the saturation pressure of zinc at the corresponding temperature, i.e., and
, and the partial pressure of the H2 or
CO product is taken to be zero. At 1100 K, H2O oxidizes Zn(g) eighteen times faster than CO2. The rate of Zn(g) oxidation by H2O is 1.4×10-5 mol cm-2 s-1 while the rate of Zn(g) oxidation by CO2 is 7.6×10-7 mol cm-2 s-1. This significant difference in surface reaction kinetics holds across the temperature range considered. Even though the rate of Zn(g) oxidation decreases in both the H2O and CO2 systems at 800 K, H2O(g) still oxidizes Zn(g) 1000 faster than CO2 at a rate of 1×10-7 mol cm-2 s-1 compared to 1×10-10 mol cm-2 s-1.
19
Figure 5. Comparison of the rate of H2O-splitting and CO2-splitting via the heterogeneous oxidation of Zn(g) for partial pressures of Zn(g), H 2O(g), and CO2 all set equal to the zinc saturation pressure.
Oxidation of Zn(g) by mixtures of H2O and CO2 Given that the surface reaction kinetics of Zn(g) oxidation by H 2O is substantially faster than that for oxidation of Zn(g) by CO2, the possibility of co-producing H2 and CO in a single reactor via simultaneous oxidation of Zn(g) by H 2O and CO2 becomes a challenge. In Figure 6, we present the results of experiments conducted with a co-feed of H2O and CO2. In these experiments, a constant 1.23 mmol/min flow of H2O(g) was premixed with a flow of CO2 varied between 1.23 and 36.9 mmol/min to obtain H2O:CO2 ratios between 1:4 and 1:75. Mass transfer does not contribute to the relative rates of H2 and CO production. For 900 K to 1000 K, the diffusion coefficient for H2O in Ar is ~50% greater than the diffusion coefficient for CO2 in Ar. This difference in the diffusion coefficients is minimal compared to the large concentrations of CO 2 relative to H2O developed in the reactor for the range of H2O:CO2 ratios considered. For H2O:CO2 less than 1:4, the CO2 concentration exceeds the H2O concentration by approximately the value of the H2O:CO2 ratio. Larger concentrations of CO2 in the bulk flow lead to more 20
rapid mass transfer of CO2 to the reacting surface than H2O. The significant difference in surface reaction kinetics (Fig. 5) is thus the more likely explanation for the data in Fig. 6. The consequence of the higher surface reaction rates with H2O than with CO2 is a product gas mixture that is excessively rich in H2. Figure 6 shows the H2:CO ratio in the product gas mixture at 900 and 1000 K for the molar ratios H2O:CO2 established in the reactant feed gas. To obtain a H2:CO ratio of 2:1 at 900 K, about 80 times more CO2 than H2O must be supplied in the reactant feed gas. At 1000 K, 20 times more CO2 than H2O must be provided. Thus, to achieve the desired H2:CO ratio, the product stream necessarily contains large amounts of unreacted CO2. For a Fischer-Tropsch process, unreacted CO2 would have to be separated from the product gases to avoid poisoning the catalysts used in the process (Dry, 1996).
Figure 6. The relative amount of hydrogen and carbon monoxide in the product gas (H2:CO product ratio) for reactant gas mixtures with the indicated relative amounts of steam and carbon dioxide (H2O:CO2 reactant ratio). The relatively small amounts of CO produced via the oxidation of Zn(g) by mixtures of H2O and CO2 can be attributed partly to the water gas shift reaction. 21
(15) According to Le hatlier’ pri ciple, the excess CO2 in the reactive flow drives reaction 15 towards its reactants. Thus, some of the unreacted CO2 reacts with H2 produced when Zn(g) is oxidized by H2O to form CO. The water gas shift reaction cannot explain the hydrogen-rich product measured in the present experiments. The most probable explanation is the substantially different rates of the heterogeneous oxidation of Zn(g) by H2O and CO2. Other studies that have oxidized Zn by mixtures of H2O and CO2 report similarly H2-rich product gas streams. Both Clarke (1979) and Bronson et al. (2017) measure more rapid oxidation of Zn(g) by H2O than CO2. Stamatiou et al. (2010) report more rapid production of H2 in the oxidation of Zn particles in a thermogravimetric analyzer for temperatures from 673 to 1173 K. At these temperatures, the Zn particles vaporize and Zn(g) is subsequently oxidized. CONCLUSION In the present work, measured rates of the heterogeneous oxidation of Zn(g) with H2O are presented from 800 to 1100 K, Zn partial pressures up to 0.39 atm, and H2O(g) partial pressures up to 1.0 atm. These are the first experiments for water oxidation of Zn(g) conducted at the partial pressures of reactants that would be needed for meaningful rates of production of hydrogen via the solar thermochemical Zn/ZnO redox cycle. In a second set of experiments, heterogeneous oxidation of zinc is carried out in mixtures of steam and carbon dioxide for H2O:CO2 ratios between 1:4 and 1:75. A tube flow reactor provided a controlled experimental environment. A model of the advective and diffusive mass transfer in the reactor was applied to extract the intrinsic
22
surface kinetics from the measured rates because mass transfer slowed the measured rates. The kinetics are best fit with the second order, reversible kinetic expression in equation 12. The finding of a negative activation energy is consistent with reported activation energies for other heterogeneous reaction systems, including for Zn(g) (Clarke and Fray, 1979). Additional study is needed to understand the underlying chemical mechanisms for this surface reaction. The precursor mechanism is one plausible chemical pathway. Rates of Zn(g) oxidation by H2O(g) are on the order of 10 -7–10-5 mol cm-2 s-1, about twenty times the rate of oxidation with CO2 at 1100 K. Consistent with this finding, the heterogeneous oxidation of Zn(g) in mixtures of H2O and CO2 produces a hydrogen-rich mixture of H2 and CO. To obtain a 2:1 ratio of H2:CO, twenty times more CO2 than H2O must be fed to the reactor. This operating condition leads to product gas streams with excess CO2, an unfavorable situation for syngas production because CO2 poisons the catalysts (Dry, 1996). We thus recommend separation of the oxidation reactors for production of H2 and CO, at least for downstream processing of synthesis gas to liquid fuels. A reactor specifically designed for high volume fuel production via the heterogeneous oxidation of zinc could provide rapid rates of H2 or CO production and thus rapid and complete conversion of Zn to ZnO. As pointed out in prior work by two of the authors (Venstrom and Davidson, 2011), reactors for fuel production require high surface area for the reaction to take place and ideally high Zn partial pressure to maximize fuel production. The approach of using ZnO particles as a substrate for the reaction first suggested in 2011 by the present authors and later demonstrated by Weibel
23
et al. (2014) is a promising approach. A challenge in reactors in which Zn is volatilized from Zn particles mixed with ZnO particles will be to ensure that the heterogeneous oxidation reaction takes place preferentially on the surface of the ZnO particles, rather than on the surface of the Zn particles. The formation of an oxidation layer on the Zn particle will reduce the partial pressure of Zn in the reactor. Alternately, zinc vapor can be introduced to the reactor as was done in the present work. FUNDING This work was funded by the University of Minnesota Initiative for Renewable Energy and the Environment and the National Science Foundation through a Graduate Fellowship awarded to Luke Venstrom.
24
NOMENCLATURE
it
MFC
diameter, mm activation energy, kJ/mol objective function for mass transfer model optimization enthalpy change, kJ/mol diffusive mass flux, kg/m2-s reaction rate parameter, mol/m2-s-atm2 equilibrium constant mass, kg mass flow rate, kg/s advective mass flux, kg/m2-s molecular weight, g/mol Mass flow controller partial pressure, atm radial coordinate, mm area-specific reaction rate, mol cm-2 s-1 oxidation tube inner surface radius, mm universal gas constant, m3-Pa/mol-K time, s temperature, K axial velocity, m/s magnitude of the suction velocity due to heterogeneous reaction, m/s axial coordinate, mm indicates a change in a quantity mass fraction of species in the gas mixture stoichiometric coefficient
Subscripts ads CO CO2 d deposition eq H2 H2 O i j m model p
refers to the adsorption of H2O on ZnO surface refers to CO product refers to CO2 reactant duration of H2 or CO production refers to ZnO mass deposited in the heterogeneous oxidation of Zn(g) refers to chemical equilibrium refers to H2 product refers to H2O reactant index for experimental data points index for gas-phase species, e.g., Zn(g), H2O, etc. refers to the gas mixture refers to reaction rate predicted in numerical model reaction rate parameter for the oxidation of adsorbed H 2O and Zn(g) refers to the radial coordinate direction 25
overall mass flow rate tot refers to Zn reactant, or the Zn inlet Zn refers to ZnO product ZnO Superscripts o at the inlet of the reactor or control volume refers to chemical equilibrium eq oxidation (in reference to temperature) ox
26
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Kang, B., Lee, S., Boo, J., 2000. Growth of TiO2 thin films on Si(100) substrates using single molecular precursors by metal organic chemical vapor deposition, Surface and Coatings Technology 131(1-3), 88-92. Lewis, L. A., Cameron, A., 1995a. Oxidation Kientics of Zinc Vapor in CO:CO 2 Mixtures: Part II. Application of Plug Flow Concepts, Metallurgical and Material Transactions B 26B, 919-924. Lewis, L. A., Cameron, A. M., 1995b. Oxidation Kinetics of Zinc Vapor in CO:CO 2 Mixtures: Part I. Comparison with Past Literature, Metallurgical and Material Transactions B 26B, 911-918. Loutzenhiser, P. G., Galvez, M. E., Hischier, I., Stamatiou, A., Frei, A., Steinfeld, A., 2009. CO2 splitting via two-step solar thermochemical cycles with Zn/ZnO and FeO/Fe3O4 redox reactions II: Kinetic analysis, Energy and Fuels 23(5), 2832-2839. Lv, M., Zhou, J., Yang, W., Cen, K., 2010. Thermogravimetric analysis of the hydrolysis of zinc particles, International Journal of Hydrogen Energy 35(7), 2617-2621. Masel, R. I., 2001. Chemical Kinetics and Catalysis. Wiley-Interscience. Mills, A. F., 2001. Mass Transfer. Prentice Hall. Muller, R., Haeberling, P., Palumbo, R. D., 2006. Further advances toward the development of a direct heating solar thermal chemical reactor for the thermal dissociation of ZnO(s), Solar Energy 80(5), 500-511. Nagao, K., Yunoki, H., Muraishi, H., Morimoto, T., 1978. Differential Heat of Chemisorption. 1. Chemisorption of Water on Zinc Oxide and Titanium Oxide, Journal of Physical Chemistry 82(9), 1032-1035. Nicodemus, J. H., Davidson, J. H., 2015. Mechanisms of hydrolysis in a transverse jet zinc aerosol reactor, Chemical Engineering Science 122, 514-522. Osborne, J. M., Rankin, W. J., McCarthy, D. J., Swinbourne, D. R., 2001. The Oxidation of Zinc Vapour in CO-CO2-N2 Gas Mixtures, Metallurgical and Material Transactions B 32B, 37-45. Perkins, C., Lichty, P. R., Weimer, A. W., 2008. Thermal ZnO dissociation in a rapid aerosol reactor as part of a solar hydrogen production cycle, International Journal of Hydrogen Energy 33(2), 499-510. Romero, M., Steinfeld, A., 2012. Concentrating solar thermal power and thermochemical fuels, Energy and Environmental Science 5(11), 9234-9245.
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Schroeder, R., Neves, P., Matthews, L., Leatzow, D., Kondratko, J., Will, J., Duncan, S., Sheline, W., Lindeke, N., Palumbo, R., 2011. Solar Thermal Electrolytic Process for the Production of Zn From ZnO: The Electrolysis of ZnO From 1275–1500 K, Journal of Solar Energy Engineering 133(4), 041013. Stamatiou, A., Loutzenhiser, P. G., Steinfeld, A., 2010. Solar Syngas Production via H2O/CO2-Splitting Thermochemical Cycles with Zn/ZnO and FeO/Fe3O4 Redox Reactions, Chemistry of Materials 22(3), 851-859. Steinfeld, A., 2005. Solar Thermochemical Production of Hydrogen: A Review, Solar Energy 78, 603-615. Venstrom, L., Krueger, K., Leonard, N., Tomlinson, B., Duncan, S., Palumbo, R. D., 2009. Solar thermal electrolytic process for the production of Zn from ZnO: an ionic conductivity study, Journal of Solar Energy Engineering 131(3), 031005. Venstrom, L. J., Davidson, J. H., 2011. Splitting Water and Carbon Dioxide via the Heterogeneous Oxidation of Zinc Vapor: Thermodynamic Considerations, Journal of Solar Energy Engineering 133(1), 011017. Venstrom, L. J., Davidson, J. H., 2013. The Kinetics of the Heterogeneous Oxidation of Zinc Vapor by Carbon Dioxide, Chemical Engineering Science 93, 163-172. Vishnevetsky, I., Epstein, M., 2007. Production of hydrogen from solar zinc in steam atmosphere, International Journal of Hydrogen Energy 32, 2791-2802. Wei, J., 1996. Adsorption and cracing of n-alkanes over ZSM-5: Negative activation energy of reaction, Chemical Engineering Science 51(11), 2995-2999. Weibel, D., Jovanovic, Z. R., Galvez, E., Steinfeld, A., 2014. Mechanism of Zn particle oxidation by H2O and CO2 in the presence of ZnO, Chemistry of Materials 26(22), 64866495. Weibel, D., Jovanovic, Z. R., Steinfeld, A., 2017. Exploiting kinetics to unravel the role of a ZnO diluent in the production of CO via oxidizing Zn particles with CO 2, Chemical Engineering Science 165, 96-107. Wetter, M., 2011. GenOpt: Generic Optimization Program, Lawrence Berkeley National Laboratory 3.1.0. Wieckert, C., Frommherz, U., Kraupl, S., Guillot, E., Olalde, G., Epstein, M., Santen, S., Osinga, T., Steinfeld, A., 2007. A 300 kW Solar chemical pilot plant for the carbothermic production of zinc, Journal of Solar Energy Engineering 129(2), 190-196. Yaws, C., 2010. Yaws' Transport Properties of Chemicals and Hydrocarbons. Knovel.
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Highlights:
The heterogeneous oxidation of Zn(g) provides rapid conversion of Zn to ZnO.
The oxidation of Zn(g) by H2O proceeds at 10-7–10-5 mol cm-2 s-1 at 800–1100 K.
The kinetics of oxidation of Zn(g) with H2O is fit to
-
.
The oxidation of Zn(g) with H2O is 20 times faster than with CO2 at 1100 K.
30
S0009-2509(18)30150-7 https://doi.org/10.1016/j.ces.2018.03.020 CES 14094
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
29 August 2017 24 January 2018 7 March 2018
Please cite this article as: L.J. Venstrom, P. Hilsen, J.H. Davidson, Heterogeneous Oxidation of Zinc Vapor by Steam and Mixtures of Steam and Carbon Dioxide, Chemical Engineering Science (2018), doi: https://doi.org/ 10.1016/j.ces.2018.03.020
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Heterogeneous Oxidation of Zinc Vapor by Steam and Mixtures of Steam and Carbon Dioxide Venstrom, Luke J.1 Department of Mechanical Engineering, Valparaiso University 1900 Chapel Drive, Valparaiso, IN 46383 [email protected] Hilsen, Paul Department of Mechanical Engineering, University of Minnesota—Twin Cities 111 Church Street, Minneapolis, MN [email protected] Davidson, Jane H. Department of Mechanical Engineering, University of Minnesota—Twin Cities 111 Church Street, Minneapolis, MN [email protected] ABSTRACT The kinetics of the heterogeneous oxidation of zinc vapor by water vapor were measured in a tube flow reactor for temperatures from 800 to 1100 K, zinc vapor partial pressures up to 0.39 atm, and water vapor partial pressures up to 1.0 atm. The results extend the prior data for oxidation of zinc by water vapor from zinc partial pressures on the order of 0.01 atm to higher values appropriate for fuel production via the Zn/ZnO thermochemical cycle. Measured oxidation rates span 10-7 to 10-5 mol cm-2 s-1. A second order, reversible reaction rate expression
-
is developed
from regression of the data and a numerical model of advective and diffusive mass transfer. The kinetic parameter
-
is a non-monotonic function of temperature with a
negative activation energy for temperatures between 800 and 1050 K, consistent with prior studies. In a second set of experiments, the rate of the heterogeneous oxidation of 1
Corresponding author, e-mail: [email protected].
zinc vapor by mixtures of water vapor and carbon dioxide was measured. The product gas is hydrogen rich due to faster surface reaction kinetics for oxidation with water vapor than with carbon dioxide. We conclude that it is preferable to split water and carbon dioxide in separate reactors rather than co-produce H2 and CO in a single reactor for production of synthesis gas in the Zn/ZnO solar thermochemical redox cycle. Keywords: solar, energy, fuel, mass transfer, metal oxidation, kinetics Highlights:
The heterogeneous oxidation of Zn(g) provides rapid conversion of Zn to ZnO.
The oxidation of Zn(g) by H2O proceeds at 10-7–10-5 mol cm-2 s-1 at 800–1100 K.
The kinetics of oxidation of Zn(g) with H2O is fit to
The oxidation of Zn(g) with H2O is 20 times faster than with CO2 at 1100 K.
-
.
INTRODUCTION The heterogeneous oxidation of zinc vapor by water and carbon dioxide (reactions 1 and 2) plays an important role in a variety of processes including zinc smelting (Clarke and Fray, 1978;Clarke, 1979;Clarke and Fray, 1979;Cox and Fray, 2003;Lewis and Cameron, 1995a;Lewis and Cameron, 1995b;Osborne et al., 2001), steelmaking (Bronson et al., 2017), and solar thermochemical fuel production via the Zn/ZnO redox cycle (Venstrom and Davidson, 2011;Venstrom and Davidson, 2013). (1) (2) In zinc smelting and steelmaking, the oxidation of zinc vapor is an undesirable side reaction in the recovery of zinc from zinc-laden gas streams that include carbon dioxide, carbon monoxide, and water vapor. Zinc partial pressures are low in these processes,
2
typically below ~0.05 atm. For solar fuel production, Zn oxidation is the requisite water and/or carbon dioxide splitting step, and high zinc partial pressure is desired. In both applications, process development benefits from a kinetic model. A number of studies of the heterogeneous oxidation of zinc vapor have developed kinetic expressions for low zinc partial pressure conditions applicable for smelting and steel making. Kinetic expressions are available for zinc, water vapor, and carbon dioxide partial pressures less than ~0.05 atm (Bronson et al., 2017;Clarke and Fray, 1978;Clarke and Fray, 1979). Here we review expressions for Zn(g) oxidation by H2O(g), which is the focus of the present work. Clarke and Fray (1979) measured oxidation rates in a 10-mm-i.d. tube reactor for temperatures between 773 and 1173 K, Zn(g) partial pressures up to 0.05 atm, and for stoichiometric oxidation where the partial pressures of Zn(g) and H 2O(g) are equal. The data were fit to a semi-empirical reaction rate expression: (3) where
is the partial pressure of zinc vapor at chemical equilibrium and
temperature-dependent parameters (Clarke and Fray, 1979). The parameter
and
are
decreased
from 781 K to 1077 K, implying a negative activation energy over this temperature range. Bronson et al. (2017) used a larger, 40-mm-i.d. reactor to obtain data between 600 and 1000 K for Zn(g) partial pressures up to 0.01 atm, and H2O(g) partial pressures up to 0.05 atm. They fit the data to a second-order reversible reaction rate expression: (4)
3
The temperature-dependent kinetic parameter, , was given by an Arrhenius expression with an activation energy of 13.7 kJ/mol and a pre-exponential factor of 32.9 mol m-2 s1
atm-2. In the solar Zn/ZnO thermochemical redox cycle, the heterogeneous oxidation of
zinc vapor was first proposed by Venstrom and Davidson (2011) as a means to avoid the rate limitations of the diffusion of reactants across the ZnO layer that forms on the surface of solid and liquid zinc (Abanades, 2012;Ernst et al., 2009;Loutzenhiser et al., 2009;Lv et al., 2010;Nicodemus and Davidson, 2015;Vishnevetsky and Epstein, 2007). Reviews of the Zn/ZnO solar thermochemical cycle are provided by (Agrafiotis et al., 2015;Romero and Steinfeld, 2012;Steinfeld, 2005). In the first step of the cycle, process heat supplied via concentrated solar radiation drives the endothermic dissociation of ZnO: (5) Complete thermal dissociation of ZnO is possible at temperatures of 2100–2300 K (Muller et al., 2006). Alternatively, the reduction of ZnO can be completed at lower temperatures carbothermally (Brkic et al., 2016;Perkins et al., 2008;Wieckert et al., 2007) or electrothermally (Schroeder et al., 2011;Venstrom et al., 2009), but thermal reduction is favored environmentally. Fuel production is via reactions 1 and 2. The rapid kinetics promised by the oxidation of zinc vapor are beneficial to quickly reach high conversions of Zn to ZnO. Moreover, even in oxidation reactors in which the original intent was to oxidize solid or liquid zinc, heterogeneous oxidation is an important reaction pathway when Zn vapor is present (Nicodemus and Davidson, 2015;Weibel et al., 2014). In a practical process, fuel production will be favored by zinc partial pressures much higher than those in the 4
smelting or steel making processes or than those available at temperatures below the Zn melting temperature of 693 K. For example, raising the zinc partial pressure from 0.01 atm to 0.3 atm increases the specific rate of oxidation by CO2 at a concentration of 0.5 atm by two orders of magnitude from 3.7×10-6 to 1.1×10-4 mol cm-2 s-1 at 1100 K (Venstrom and Davidson, 2013). A kinetic model was developed for the heterogeneous oxidation of Zn(g) by carbon dioxide for the higher Zn(g) partial pressures anticipated for fuel production in the Zn/ZnO thermochemical cycle (Venstrom and Davidson, 2013). For Zn partial pressures up to 0.36 atm, and CO2 partial pressures up to 0.45 atm at 800 to 1150 K, the rate is given by: (6) -2
-1
The activation energy is 44 kJ/mol and the pre-exponential factor is 920 mol m s atm-2. Oxidation rate increases with temperature due to higher zinc saturation pressure as well as kinetics via the Arrhenius-based parameter
-
. For zinc partial pressures below
0.01 atm, Bronson et al. (2017) report a similar activation energy of 50.2 kJ/mol but a lower pre-exponential factor, 406 mol m-2 s-1 atm-2. In the present study, we fill a gap in the literature for the rate of the heterogeneous oxidation of Zn(g) with H2O at Zn(g) partial pressures greater than 0.05 atm that are meaningful for fuel production, as opposed to the zinc smelting and steel making processes. Oxidation rates are measured from 800 to 1100 K, Zn(g) partial pressures up to 0.39 atm and H2O(g) partial pressures up to 1.0 atm. A numerical model of convective mass transfer is applied to decouple the intrinsic surface kinetics from the measured global reaction rates. The kinetics of water splitting is compared to the previously published kinetics of carbon dioxide splitting (Venstrom and Davidson, 2013). This 5
comparison is used to interpret new measurements of heterogeneous oxidation of Zn(g) in mixtures of H2O and CO2. The goal of this second set of experiments is to determine the relative amounts of H2O and CO2 in the reactant stream that will lead to a 2:1 ratio of H2to-CO in the product gases. A 2:1 ratio is desirable for the production of high-quality hydrocarbon fuels in the Fischer-Tropsch process (Dry, 1996). APPROACH Experiments are performed in a quartz tube flow reactor specifically designed to evaluate the kinetics of the heterogeneous oxidation of Zn(g). (The reactor is solely for measurement of kinetics and is not, as has been inferred by others (Weibel et al., 2017), intended as a scalable reactor for practical or efficient fuel production.) The facility was used previously to study the oxidation of Zn(g) and CO2 (Venstrom and Davidson, 2013). Here we provide an overview of the reactor and describe the operating conditions for the present study. The reactor, depicted in Fig. 1, is constructed of concentric quartz tubes positioned horizontally within an electric furnace (Thermolyne 79500). Zinc is evaporated in the outer tube. The Zn(g) reacts with H2O(g) and/or CO2 on the inner surface of the central tube, referred to as the oxidation tube. ZnO first forms as a monolayer on the quartz surface. The layer thickness grows over the course of an experiment. Rates of ZnO production are determined gravimetrically, and the total H2 and CO production are measured by analyzing the composition of the product gas. Mass transfer in the reactor is modeled by solving the conservation of mass, momentum, and species in an axisymmetric domain using a finite volume numerical method, an approach
6
that captures the effects of both diffusion to the reaction site and advective mixing of the reactants at the shear layer between the inlet flows on the centerline and in the annulus.
Figure 1. The isothermal tube flow reactor for measurement of the kinetics of the heterogeneous oxidation of Zn(g). The reactor is not drawn to scale; dimensions are in millimeters. Prior to each experiment, the reactor is purged by a flow of Ar at 523 K. After the purge, the oxidation temperature and inlet concentration of reactants are set for each experiment. Reaction conditions are maintained to within ±10 K of the set point temperature in the 300 mm reaction zone. Temperature is measured with a chromelalumel thermocouple. Zinc vapor is generated by flowing high-purity Argon (Ar, grade 5.0) over a pool of liquid Zn (99.999% pure, metals basis) contained in the outer quartz tube. The Zn(g)-Ar gas mixture flows into the 3.9 mm i.d. oxidation tube where it is mixed with a coaxial flow of H2O(g) or pre-mixed H2O(g) and CO2 (99.9% pure). Steam is produced by either saturating a flow of Ar with distilled and degassed H2O(l) (Toxidation ≤ 900 K or in a boiler (Reimers Model JR99) fed with distilled H2O(l) (Toxidation > 900 K). All gas flows, are controlled with mass flow controllers to within 1% of the desired flow rate. The mass of ZnO(s) deposited is measured in approximately 5 cm increments along the longitudinal axis of the inner quartz tube. A monolayer of ZnO is formed on 7
the quartz tube very quickly and then the ZnO forms layer by layer. Thus, the material of the tube is unlikely to impact the overall rate of the heterogeneous formation of ZnO. ZnO is removed from the tube via an acid wash, and the mass of ZnO is determined by weighing the quartz tube prior to and after the acid wash. Elemental Zn(s) and C(s) do not deposit on the tube. Hydrogen, which would indicate Zn co-deposition, is not evolved during ZnO removal and analysis of the hydrochloric acid solution using atomic absorption spectroscopy confirms that carbon is not co-deposited (Venstrom and Davidson, 2013). As the product gas mixture flows out of the reactor, unreacted Zn(g) and H2O(g) are condensed out of the gas flow. The outlet gas composition, including concentrations of H2, CO, and unreacted CO2, is measured with a Raman laser gas analyzer (RLGA, ±0.1 mol%). Data are collected at steady state, based on product gas concentrations, for 6–30 minutes, ensuring that the mass of ZnO(s) deposited along the tube is detectable. The overall rate of oxidation is expressed as (7) where
is the inner diameter of the oxidation tube (3.9 mm),
section over which the ZnO mass, ZnO, and
, is measured,
is the length of the
is the molecular mass of
is the time over which H2 and CO are produced in the experiment.
The surface kinetics are extracted from the overall rate of oxidation using a numerical finite volume model to account for mass transfer in the reactor. The model domain corresponds to the isothermal region of the oxidation tube from z=0 to z=200 mm and from r=0 to r=1.95 mm. The model assumes steady-state, axisymmetric, and isothermal flow, and calculates the velocity, pressure, and the concentrations of reacting
8
species in the flow by solution of the differential conservation equations of mass, momentum, and species in the reactor. The assumption of steady-state flow conditions is reasonable for the present study despite the growth of the ZnO layer on the tube walls because the tube diameter decreases by at most 5% for the amount of mass deposited in the present study. The model is described in greater detail in the prior study of CO2 oxidation (Venstrom and Davidson, 2013). A summary is provided here. The model assumes ideal gas behavior with spatial changes in density and viscosity due to changes in composition. The viscosity of the reacting flow is calculated using a mass-average of the pure species viscosities, and the viscosities of the pure species are evaluated using tabulated data (Yaws, 2010). The diffusion of species is modeled using a Fickian constitutive relationship in which the effective diffusion coefficients for the reacting species are calculated using a mixing law (Mills, 2001). The binary diffusivities of the reacting species are estimated according to the Chapman-Enskog theory. The boundary conditions include the Zn(g) and H2O inlets at z=0, the product mixture outlet flow at z=200 mm, symmetry on the centerline, and heterogeneous Zn(g) oxidation on the tip of the capillary tube and the inner surface of the oxidation tube. Fully-developed laminar velocity profiles for tube flow and annular flow are applied at the H2O and Zn(g) inlets, respectively. At the outlet, flow is assumed to be normal to the tube cross-section and the pressure is atmospheric. The no-slip condition is applied at the tip of the capillary tube and the inner surface of the oxidation tube. The heterogeneous reaction at these surfaces is modeled by equation 8 (Venstrom and Davidson, 2013): (8) where
is the deposited mass flux and
is the rate of the
heterogeneous oxidation of Zn(g). The heterogeneous reaction rate is a function of the 9
local partial pressures of the reacting species (i.e., the partial pressures at the surface) and also of the oxidation temperature (see Table 1). The deposited mass flux gives rise to a suction velocity at the reacting surfaces that is normal to the surfaces for which the magnitude is given by (9) where
is the mass-density of the gas mixture. To extract kinetic parameters from the experimental data, regression of the
numerically predicted and measured Zn(g) oxidation rates is achieved by adjusting the value of the kinetic parameter(s) to minimize a root-sum-squared error objective function.
(10) The index indicates summation over all axial locations of the experimental data. Minimization of the objective function is achieved using GenOpt (Wetter, 2011). Table 1 lists the kinetic expressions that were considered and the respective kinetic parameters. The phenomenological Langmuir-Hinshelwood and precursor mechanism reaction rate expressions account for the surface adsorption and subsequent reaction of gas species, and have been successfully applied to heterogeneous reaction systems that involve a solid product (Masel, 2001). They feature multiple temperaturedependent kinetic parameters. In the empirical variable-order reaction rate expression, the order of the oxidation with respect to Zn(g) and CO2 varies through the kinetic parameters
and
. Temperature-dependence in the variable-order kinetic expression is
included through a third kinetic parameter. The second-order expression represents the special case in which
=
= 1 in the variable-order expression and the special case in
10
which the denominators of the Langmuir-Hinshelwood and precursor mechanism rate expressions are approximately unity. The physical interpretation of the latter case is that the surface coverage of the adsorbed species is low, a probable condition at the high temperatures considered in the present study. Table 1. List of kinetic expressions considered for the oxidation of Zn(g) by H2O(g) and their associated kinetic parameters Kinetic Expression Kinetic Kinetic Model Parameters Second-order
-
LangmuirHinshelwood Precursor mechanism nth-order
RESULTS AND DISCUSSION Heterogeneous Zn(g)-H2O Oxidation Kinetics Table 2 lists the conditions of the H2O oxidation experiments, including the oxidation temperature, the reactance mass flow rates and partial pressures at the reactor inlet, and the Reynolds number for the reacting flow based on the oxidation tube inner diameter. Experiments were conducted at a total pressure of ~1.0 atm from 800 to 1100 K. The partial pressure of zinc at the reactor inlet spanned two orders of magnitude from 0.002 atm at 800 K to 0.385 atm at 1100 K. The partial pressure of Zn was maintained below the zinc saturation pressure to preclude the possibility of zinc condensation. The water vapor partial pressure varied between 0.028 atm and 1.0 atm.
11
Table 2. The oxidation temperature, Reynolds number, and mass flow rate and partial pressure of the reacting species at the inlets in the water-splitting experiments. The balance of mass in the gas phase is Argon. Zn(g) Inlet H2O(g) Inlet Red Experiment (K) (×106 kg/s) (atm) (×106 kg/s) (atm) 1 800 26 3.34 0.002 0.69 0.028 2 900 17 1.43 0.017 1.39 0.028 3 900 17 2.07 0.018 1.30 1.000 4 1000 15 2.21 0.052 0.40 1.000 5 1050 24 1.90 0.228 1.83 0.696 6 1100 27 2.02 0.231 1.34 1.000 7 1100 32 2.79 0.385 2.00 0.509
Measured and numerically predicted rates of the heterogeneous oxidation of Zn(g) by H2O(g) are compared in Figure 2 along the length of the oxidation tube. The model predictions match the measured reaction rates for all conditions considered in the present study. At all temperatures, the rate of oxidation increases to a maximum within 10 mm of the inlet as the flow develops and the Zn(g) and H2O(g) mix. The oxidation rate then decreases in the axial direction as reacting species are consumed. Higher temperatures favor rapid Zn(g) oxidation. At 1100 K, the peak oxidation rate for the conditions in the present study is 1.5×10-5 mol cm-2 s-1. At 1050 K, the peak oxidation rate drops an order of magnitude to 3×10-6 mol cm-2 s-1, and at 900 K the peak oxidation rate drops another order of magnitude to 3×10-7 mol cm-2 s-1. The non-linear changes with respect to temperature are a result of both the exponential temperature-dependence of the zinc saturation pressure and, as will be shown, the non-linear temperature-dependence of the global kinetics.
12
Figure 2. The rate of heterogeneous Zn(g) oxidation by H2O(g) along the length of the reactor. The curves represent the regression of the reactor model to the data.
The importance of decoupling mass transfer and surface reaction kinetics to develop a kinetic expression is highlighted in Fig. 3. In this figure, the partial pressures of Zn(g), H2O(g), and H2 calculated in the model are compared at the surface of the oxidation tube where the reaction occurs and in the bulk flow along the flow path using the oxidation conditions at 1000 K as an example. The bulk flow concentration is calculated at any axial location from the radial distributions of concentration according to equation 11:
(11) Mass transfer is important at any axial location where the partial pressures at the reaction surface and in the bulk flow are different. Surface and bulk flow partial pressures differ significantly in the first ~40 mm of the oxidizer tube where the flow is developing and the measured reaction rates (depicted in Fig. 2) are fastest. The Zn(g) surface partial pressure is as much as 0.01 atm lower than the bulk flow partial pressure (50% 13
difference). The H2O(g) surface partial pressure is as much as 0.26 atm lower than the bulk (94% difference), and the H2 surface partial pressure is as much as 0.004 atm larger than the bulk (20% difference). These results show that it would introduce substantial error to use bulk flow partial pressures when evaluating surface reaction kinetics. In the present study, the partial pressures at the reacting surface are used to evaluate the reaction kinetics so that the kinetics are free from error due to mass transfer. Regression of the measured reaction rate data and the reactor model are best achieved using a second-order, reversible reaction rate expression for the surface kinetics with a linear dependence on both the Zn(g) and H2O(g) partial pressures: (12) Temperature dependence of the reaction rate is captured in the parameter, the equilibrium constant,
-
, and
. The equilibrium constant is for the overall water-splitting
reaction (equation 1), and is calculated from existing thermodynamic databases (Antti Roine). With the alternative expressions featuring multiple kinetic parameters (Table 1), each of which represents an additional degree-of-freedom in the regression, the fit of the data was not improved enough to be considered statistically significant.
14
Figure 3. Partial pressures of (a) Zn(g), (b) H2O(g), and (c) H2 at the surface of the oxidation tube and in the bulk flow along the reacting flow path at 1000 K.
15
The values of the kinetic parameter
-
are provided in Table 3 for each
oxidation temperature and are plotted in Arrhenius form in Fig. 4. The kinetic parameter decreases from 200 mol m-2 s-1 atm-2 at 800 K to 3.4 mol m-2 s-1 atm-2 at 1050 K, and then increases to ~16 mol m-2 s-1 atm-2 at 1100 K. The experiments were repeated at 900 K and 1100 K to confirm this trend. At 900 K, the values of the kinetic parameter determined via regression of the repeated experiments are 59 and 60 mol m-2 s-1 atm-2, and at 1100 K, the values are 14.5 and 17.5 mol m-2 s-1 atm-2. These values are within experimental uncertainty.
Table 3. Kinetic parameter for the heterogeneous oxidation of Zn(g) by H 2O(g) -
Experiment 1 2 3 4 5 6 7
(K) 800 900 900 1000 1050 1100 1100
(mol m-2 s-1 atm-2) 199 59 60 6.8 3.4 14.5 17.5
Figure 4. Arrhenius plot of the temperature dependent kinetic parameter (mol m-2 s-1 atm2 ) determined via regression of the reactor model and the measured reaction rates for H2O. The best-fit line is a linear regression of the data between 800 K and 1050 K. 16
The decrease in the value of the kinetic parameter from 800 to 1050 K implies that for the oxidation of Zn(g) by H2O(g), the activation energy is -110 kJ mol-1 (see Fig. 4). Negative activation energies have been reported for other heterogeneous reaction systems, e.g. the chemical vapor deposition of TiO 2 (Jones et al., 1998;Kang et al., 2000) and cracking of n-alkanes over a zeolite (Wei, 1996), as well as for Zn(g) oxidation (Clarke and Fray, 1979). For the same kinetic expression (equation 12) and over a similar range of temperature, Bronson et al. (2017) report a positive activation energy of 13.7 kJ/mol. We attribute this difference to the fact that Bronson et al. assume fully developed and radially mixed conditions in a model of their reactor. We find flow development and radial diffusion to be important in a much smaller diameter reaction tube. Zinc and water vapor must spread from their respective inlets at the annulus and centerline of the tube, respectively, throughout the cross section of the oxidation tube over the first ~5 mm along the flow path as is evident in the rise of the rate of the oxidation in Fig. 2. Once developed, radial gradients in the concentrations of the species show that diffusion also affects oxidation rates. Thus it is possible that the Bronson et al. kinetic model includes the effect of mass transfer and consequently does not produce the negative activation energy found in the present work and in Clarke and Fray (1979). A mechanism most often referred to as the precursor mechanism is one plausible chemical pathway for Zn(g)-H2O(g) oxidation that could explain the trends observed in the kinetic parameter,
-
, of equation 12. In the precursor mechanism, H2O(g) first
adsorbs to the reaction surface and then reacts with Zn(g) from the gas phase (Masel, 2001). The overall reaction rate expression for the precursor mechanism is
17
(13) where
is the temperature-dependent reaction rate parameter for the oxidation of
adsorbed H2O and Zn(g) and H2O(g) on ZnO. If
is the equilibrium constant for the adsorption of
≪ 1, then the denominator of equation 13 is approximately unity,
and equations 12 and 13 become identical with
-
. In this case, the
apparent activation energy of the second-order kinetic parameter,
, is equal to the sum
of the activation energy of the reaction rate parameter and the heat of adsorption of H2O: (14) Adsorption is an exothermic process (
< 0). Thus, if the magnitude of the heat of
adsorption is larger than the activation energy
, the apparent activation energy
will
be negative, as is observed in the present study (Fig. 3). Data for the heat of adsorption of H2O on polycrystalline ZnO support this hypothesis. The heat of adsorption depends on the surface coverage of H2O. At low coverage, the heat of adsorption is -100 kJ/mol. At 80% coverage, it increases to a maximum of -160 kJ mol-1 (Nagao et al., 1978). The apparent activation energy of -110 kJ mol-1 in the present study falls on the low end of the reported range of heat of adsorption, consistent with a low activation energy of the elementary chemical conversion step compared to the heat of adsorption of H2O. Combining the apparent activation energy from the present study and the reported heat of adsorption reported by Nagao et al. (1978) suggests that the activation energy of the oxidation of adsorbed H2O by Zn(g) is between 15 and 75 kJ mol-1. It is perhaps coincidental but nonetheless interesting that the activation energy measured for the oxidation of Zn(g) by CO2,
18
45 kJ/mol, falls within this same range (Bronson et al., 2017;Clarke and Fray, 1978;Venstrom and Davidson, 2013). Comparison of the Heterogeneous Oxidation of Zn(g) by H2O and CO2 The rate of the heterogeneous oxidation of Zn(g) by H2O(g) is compared to that for oxidation by CO2. The rate of the oxidation of Zn(g) by CO2 is evaluated using equation 6, developed in our prior work (Venstrom and Davidson, 2013). The rate of oxidation of Zn(g) by H2O(g) is evaluated using equation 12. Figure 5 is a plot of the rate of oxidation as a function of temperature from 800 K to 1100 K, the range of temperatures included in the experiments from which the kinetic expressions were developed. For each temperature, the partial pressures of Zn(g), H2O(g), and CO2 are set equal to the saturation pressure of zinc at the corresponding temperature, i.e., and
, and the partial pressure of the H2 or
CO product is taken to be zero. At 1100 K, H2O oxidizes Zn(g) eighteen times faster than CO2. The rate of Zn(g) oxidation by H2O is 1.4×10-5 mol cm-2 s-1 while the rate of Zn(g) oxidation by CO2 is 7.6×10-7 mol cm-2 s-1. This significant difference in surface reaction kinetics holds across the temperature range considered. Even though the rate of Zn(g) oxidation decreases in both the H2O and CO2 systems at 800 K, H2O(g) still oxidizes Zn(g) 1000 faster than CO2 at a rate of 1×10-7 mol cm-2 s-1 compared to 1×10-10 mol cm-2 s-1.
19
Figure 5. Comparison of the rate of H2O-splitting and CO2-splitting via the heterogeneous oxidation of Zn(g) for partial pressures of Zn(g), H 2O(g), and CO2 all set equal to the zinc saturation pressure.
Oxidation of Zn(g) by mixtures of H2O and CO2 Given that the surface reaction kinetics of Zn(g) oxidation by H 2O is substantially faster than that for oxidation of Zn(g) by CO2, the possibility of co-producing H2 and CO in a single reactor via simultaneous oxidation of Zn(g) by H 2O and CO2 becomes a challenge. In Figure 6, we present the results of experiments conducted with a co-feed of H2O and CO2. In these experiments, a constant 1.23 mmol/min flow of H2O(g) was premixed with a flow of CO2 varied between 1.23 and 36.9 mmol/min to obtain H2O:CO2 ratios between 1:4 and 1:75. Mass transfer does not contribute to the relative rates of H2 and CO production. For 900 K to 1000 K, the diffusion coefficient for H2O in Ar is ~50% greater than the diffusion coefficient for CO2 in Ar. This difference in the diffusion coefficients is minimal compared to the large concentrations of CO 2 relative to H2O developed in the reactor for the range of H2O:CO2 ratios considered. For H2O:CO2 less than 1:4, the CO2 concentration exceeds the H2O concentration by approximately the value of the H2O:CO2 ratio. Larger concentrations of CO2 in the bulk flow lead to more 20
rapid mass transfer of CO2 to the reacting surface than H2O. The significant difference in surface reaction kinetics (Fig. 5) is thus the more likely explanation for the data in Fig. 6. The consequence of the higher surface reaction rates with H2O than with CO2 is a product gas mixture that is excessively rich in H2. Figure 6 shows the H2:CO ratio in the product gas mixture at 900 and 1000 K for the molar ratios H2O:CO2 established in the reactant feed gas. To obtain a H2:CO ratio of 2:1 at 900 K, about 80 times more CO2 than H2O must be supplied in the reactant feed gas. At 1000 K, 20 times more CO2 than H2O must be provided. Thus, to achieve the desired H2:CO ratio, the product stream necessarily contains large amounts of unreacted CO2. For a Fischer-Tropsch process, unreacted CO2 would have to be separated from the product gases to avoid poisoning the catalysts used in the process (Dry, 1996).
Figure 6. The relative amount of hydrogen and carbon monoxide in the product gas (H2:CO product ratio) for reactant gas mixtures with the indicated relative amounts of steam and carbon dioxide (H2O:CO2 reactant ratio). The relatively small amounts of CO produced via the oxidation of Zn(g) by mixtures of H2O and CO2 can be attributed partly to the water gas shift reaction. 21
(15) According to Le hatlier’ pri ciple, the excess CO2 in the reactive flow drives reaction 15 towards its reactants. Thus, some of the unreacted CO2 reacts with H2 produced when Zn(g) is oxidized by H2O to form CO. The water gas shift reaction cannot explain the hydrogen-rich product measured in the present experiments. The most probable explanation is the substantially different rates of the heterogeneous oxidation of Zn(g) by H2O and CO2. Other studies that have oxidized Zn by mixtures of H2O and CO2 report similarly H2-rich product gas streams. Both Clarke (1979) and Bronson et al. (2017) measure more rapid oxidation of Zn(g) by H2O than CO2. Stamatiou et al. (2010) report more rapid production of H2 in the oxidation of Zn particles in a thermogravimetric analyzer for temperatures from 673 to 1173 K. At these temperatures, the Zn particles vaporize and Zn(g) is subsequently oxidized. CONCLUSION In the present work, measured rates of the heterogeneous oxidation of Zn(g) with H2O are presented from 800 to 1100 K, Zn partial pressures up to 0.39 atm, and H2O(g) partial pressures up to 1.0 atm. These are the first experiments for water oxidation of Zn(g) conducted at the partial pressures of reactants that would be needed for meaningful rates of production of hydrogen via the solar thermochemical Zn/ZnO redox cycle. In a second set of experiments, heterogeneous oxidation of zinc is carried out in mixtures of steam and carbon dioxide for H2O:CO2 ratios between 1:4 and 1:75. A tube flow reactor provided a controlled experimental environment. A model of the advective and diffusive mass transfer in the reactor was applied to extract the intrinsic
22
surface kinetics from the measured rates because mass transfer slowed the measured rates. The kinetics are best fit with the second order, reversible kinetic expression in equation 12. The finding of a negative activation energy is consistent with reported activation energies for other heterogeneous reaction systems, including for Zn(g) (Clarke and Fray, 1979). Additional study is needed to understand the underlying chemical mechanisms for this surface reaction. The precursor mechanism is one plausible chemical pathway. Rates of Zn(g) oxidation by H2O(g) are on the order of 10 -7–10-5 mol cm-2 s-1, about twenty times the rate of oxidation with CO2 at 1100 K. Consistent with this finding, the heterogeneous oxidation of Zn(g) in mixtures of H2O and CO2 produces a hydrogen-rich mixture of H2 and CO. To obtain a 2:1 ratio of H2:CO, twenty times more CO2 than H2O must be fed to the reactor. This operating condition leads to product gas streams with excess CO2, an unfavorable situation for syngas production because CO2 poisons the catalysts (Dry, 1996). We thus recommend separation of the oxidation reactors for production of H2 and CO, at least for downstream processing of synthesis gas to liquid fuels. A reactor specifically designed for high volume fuel production via the heterogeneous oxidation of zinc could provide rapid rates of H2 or CO production and thus rapid and complete conversion of Zn to ZnO. As pointed out in prior work by two of the authors (Venstrom and Davidson, 2011), reactors for fuel production require high surface area for the reaction to take place and ideally high Zn partial pressure to maximize fuel production. The approach of using ZnO particles as a substrate for the reaction first suggested in 2011 by the present authors and later demonstrated by Weibel
23
et al. (2014) is a promising approach. A challenge in reactors in which Zn is volatilized from Zn particles mixed with ZnO particles will be to ensure that the heterogeneous oxidation reaction takes place preferentially on the surface of the ZnO particles, rather than on the surface of the Zn particles. The formation of an oxidation layer on the Zn particle will reduce the partial pressure of Zn in the reactor. Alternately, zinc vapor can be introduced to the reactor as was done in the present work. FUNDING This work was funded by the University of Minnesota Initiative for Renewable Energy and the Environment and the National Science Foundation through a Graduate Fellowship awarded to Luke Venstrom.
24
NOMENCLATURE
it
MFC
diameter, mm activation energy, kJ/mol objective function for mass transfer model optimization enthalpy change, kJ/mol diffusive mass flux, kg/m2-s reaction rate parameter, mol/m2-s-atm2 equilibrium constant mass, kg mass flow rate, kg/s advective mass flux, kg/m2-s molecular weight, g/mol Mass flow controller partial pressure, atm radial coordinate, mm area-specific reaction rate, mol cm-2 s-1 oxidation tube inner surface radius, mm universal gas constant, m3-Pa/mol-K time, s temperature, K axial velocity, m/s magnitude of the suction velocity due to heterogeneous reaction, m/s axial coordinate, mm indicates a change in a quantity mass fraction of species in the gas mixture stoichiometric coefficient
Subscripts ads CO CO2 d deposition eq H2 H2 O i j m model p
refers to the adsorption of H2O on ZnO surface refers to CO product refers to CO2 reactant duration of H2 or CO production refers to ZnO mass deposited in the heterogeneous oxidation of Zn(g) refers to chemical equilibrium refers to H2 product refers to H2O reactant index for experimental data points index for gas-phase species, e.g., Zn(g), H2O, etc. refers to the gas mixture refers to reaction rate predicted in numerical model reaction rate parameter for the oxidation of adsorbed H 2O and Zn(g) refers to the radial coordinate direction 25
overall mass flow rate tot refers to Zn reactant, or the Zn inlet Zn refers to ZnO product ZnO Superscripts o at the inlet of the reactor or control volume refers to chemical equilibrium eq oxidation (in reference to temperature) ox
26
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Highlights:
The heterogeneous oxidation of Zn(g) provides rapid conversion of Zn to ZnO.
The oxidation of Zn(g) by H2O proceeds at 10-7–10-5 mol cm-2 s-1 at 800–1100 K.
The kinetics of oxidation of Zn(g) with H2O is fit to
-
.
The oxidation of Zn(g) with H2O is 20 times faster than with CO2 at 1100 K.
30