Noncatalytic Synthesis Gas Production by Reduction of ZnO with

Article pubs.acs.org/IECR

Noncatalytic Synthesis Gas Production by Reduction of ZnO with Methane in a Dilute Phase Pneumatic Conveying Reactor A. Afshar Ebrahimi,* H. Ale Ebrahim, and A. H. Faramarzi Department of Chemical Engineering, Amirkabir University of Technology, Tehran Polytechnic, Petrochemical Center of Excellency, Tehran, 15875-4413, Iran ABSTRACT: A continuous dilute phase pneumatic conveying reactor has been constructed for management of the ZnO reduction by methane. Preheated zinc oxide powder was fed and mixed with methane with specific ratio continuously. The reactor features an upward flow of pure CH4 laden with ZnO particles, which moved through the reactor. The heating zone length was 184 cm in which the reactants converted to synthesis gas and zinc. The maximum chemical conversion of methane to synthesis gas was 94% at the reactor temperature of 1398 K and methane flow rate of 2.7 L/min. Analysis of the product gases revealed high quality synthesis gas production without the influence of methane cracking or other undesired side reactions. Reactor temperature, solid reactant preheating, and residence time in the hot zone had significant influence on the chemical conversion. A steady-state one-dimensional reactor model was generated, and the result of simulation was successfully compared with experiments. Ale Ebrahim and Jamshidi.9 The nature of gaseous products and H2/CO molar ratio of produced synthesis gas using thermogravimetry and online gas analysis with a mass spectrometer was investigated by Ale Ebrahim and Jamshidi as well.10 The results showed that the H2/CO ratio is about 2, which indicates that an economical integrated plant can be expected by combining a metallurgical and a petrochemical unit. However, there was only a 3% syngas concentration due to a small single ZnO pellet with an excess gas stream in TG.10 The mechanism of reaction between CH4 and ZnO has also been studied by Su et al.11 Reaction 1 can be handled in different reactor configurations, using different sources of energy for supplying the required process heat of this endothermic reaction. Steinfeld et al. used a horizontal gas−particle vortex flow reactor in a high-flux solar furnace.12 Up to 90% chemical conversion on the solid reactant at about 1573 K was reported. However, the conversion of methane to synthesis gas was incomplete due to the reaction being affected with considerable excess methane that was required for acceptable lifting of the ZnO particles in the horizontal solar reactor. Kraeupl and Stienfeld reached 96% chemical conversion of methane to synthesis gas in the same solar reactor configuration at 1676 K using excess solid reactant.13 Ao et al. explored reaction 1 in an alkali molten salt reactor at the temperature range of 1123−1223 K in batch mode. They have found that the molten salt plays a reaction medium role and has a feasible thermal storage capacity.14 The H2/CO molar ratio was reported to be 2 at 1198 K. The chemical conversion of methane became higher at 1223 K, but the product gaseous molar ratio was reported at 2.7. Reaction 1 has been carried out in a packed-bed reactor in our previous work.15

1. INTRODUCTION Synthesis gas is one of the most important intermediate commodities in the petrochemical plants.1 Synthesis gas is produced by partial oxidation of methane. The oxygen donor for this partial oxidation can be supplied from various sources. For instance, syngas can be produced from the steam reforming of methane (H2O is the donor of oxygen). This is the most common industrial method in petrochemical units. This traditional catalytic process suffers mainly from high energy consumption due to a highly endothermic reaction, high investment costs, catalyst deactivation as well as H2/CO ratios greater than 5, which is not suitable for all important downstream processes.2 Synthesis gas can be also produced by a partial oxidation method (pure oxygen is the donor of oxygen).3 This method is an exothermic reaction and requires pure oxygen as one of the reactants, thus, resulting in the process becoming too expensive. The major problems of this method include catalyst deactivation (due to coke deposition) and heat wave and hot spot formation through the fixed bed reactors, which also leads to severe catalyst deactivation. This applies, in particular, to Ni-based catalysts.4 Recent research has focused on the noncatalytic production of synthesis gas, that is, via direct reduction of metal oxides by methane.5,6 For example, employing ZnO as the donor of oxygen, leads to synthesis gas and zinc production. The overall reaction may be described as follows: ZnO + CH 4 → Zn(g ) + CO + 2H2 ΔH = 320.2 kJ/mol

(1)

The above reaction has been analyzed thermodynamically by Steinfeld et al.7 The kinetics of ZnO reduction by CH4 at the chemical control regime has been studied on the thermogravimetric (TG) scale at moderate temperatures extensively.8 The effect of mass transfer and bulk flow on the reduction of ZnO by methane at higher temperatures was also studied by © 2012 American Chemical Society

Received: Revised: Accepted: Published: 3271

October 1, 2011 January 22, 2012 February 6, 2012 February 17, 2012 dx.doi.org/10.1021/ie2022515 | Ind. Eng. Chem. Res. 2012, 51, 3271−3278

Industrial & Engineering Chemistry Research

Article

In the present work, a dilute phase pneumatic conveying reactor (aerosol flow reactor) is employed for handling reaction 1. The reaction is surveyed experimentally in the reactor and a mathematical model is presented for the reaction through the reactor. The main significance of this work is its proposing and applying a continuous reactor configuration for a noncatalytic synthesis gas production. Therefore, it might be a suitable approach toward industrial production.

The maximum reformed methane flow was 744 cc/min at 1268 K, and H2/CO molar ratios were close to 2. Zinc is another interesting product of reaction 1. This metal is widely used in the galvanization, pharmaceutical, and food industries. It is also an attractive solid energy carrier. Zinc can be reoxidized with steam that, in turn, can produce high purity hydrogen even in far destinations. In addition, it can be used in the Zn/air fuel cells to generate electricity. Berman and Epstein surveyed the kinetics of hydrogen production in the oxidation of liquid zinc with water vapor.16 Vishnevetsky and Epstein investigated production of hydrogen by hydrolysis of zinc in batch experiments at atmospheric pressure.17 High purity hydrogen has been achieved in these experimental tests. Thus, a thermochemical cycle, where zinc oxide is reduced by methane and the elementary zinc is reoxidized (for instance with water vapor), can be established. Therefore, the Zn would be recycled and is just used as a flexible storage of energy. Ao et al. studied the reaction of methane with ZnO in the absence and presence of CO2 in a small packed bed reactor. They have reported that ZnO functions as a catalyst and metallic zinc plays the role of an intermediary as well as an intermediate product in the process.18 An investigation on the economic evaluation of the solar coproduction of zinc and synthesis gas has been done by Steinfeld and Spiewak.19 Owing to the sixth power dependence between particles, the van der Waals attractive interactions are dominant for fine particles. In addition, electrostatic forces, liquid bridging, mechanical interlocking and local sintering might become important under certain conditions.20,21 Thus, the bubbling fluidized bed regime cannot be achieved for submicrometer ZnO particles in this study. So a continuous pneumatic conveying regime may be suitable for handling the reaction in which the solid reactant particles are completely conveyed upward by gas flow.

2. EXPERIMENTAL SETUP The experimental set up of process is shown schematically in Figure 1. This setup consists of three major parts: powder feeder system, reactor and downstream separators. There is a ZnO powder feeder where the powder is preheated electrically in the hopper of the feeder system (with capacity of containing 3.5 kg ZnO powder). The preheated ZnO powder is fed to a crushing section by means of a screw. A fast rotating brush grinds the agglomerates of ZnO exiting from the screw feeder. The details of particle feeder system, particle size distributions in the reactor, physical properties of the particles in the reactor and hydrodynamic of the two phase flow are beyond the scope of this paper and requires separate investigations. Flows of inlet gases were controlled manually with rotameters. A portion of the total inlet methane flow was conducted to the crushing section for better performance of the crusher and mixing the powder with gas simultaneously. The other portion (the main fluidizing flow) joined the two-phase flow exiting from the crushing-mixing section for promoting the lifting of the particles and to avoid plugging or sedimentation. The final gas-laden particle flow entered to the bottom of a vertical cylindrical reactor. The inlet gas flows were also preheated electrically. The reactor body is made of a heat resistance steel alloy. Inner diameter, length and wall thickness of the reactor

Figure 1. Flow sheet diagram of the complete experimental setup. The reactor temperature and reactants flow rates are controlled. 3272

dx.doi.org/10.1021/ie2022515 | Ind. Eng. Chem. Res. 2012, 51, 3271−3278


Article

Figure 2. Continuous pneumatic conveying of ZnO particles without recirculation in the Plexiglas model with inner diameter of 2.5 cm.

verified the high purity zinc oxide powder. Purging, preheating the powder, and increasing the reactor temperature to the desired point under inert flow usually took 1 h during the experiments. After a steady state condition was established, the flow of nitrogen was completely substituted with pure methane flow (99.99% purity). Simultaneously, the preheated powder was fed to the crushing-mixing section by a screw conveyer at a specific rate. ZnO feeding rate was adjusted over stoichiometry with respect to methane flow. Once particles injected into the hot reactor, bubble formation in the water scrubbers (in downstream) became much stronger. The increase of bubble formation implied the presence of reaction in the hot zone. This is because product gaseous mole numbers are greater than reactants gaseous mole numbers (see reaction 1). The outlet products entered through the water-cooled double pipe steel condenser with 1 m length where the zinc vapor was condensed. Two-stage water scrubbers captured the remainder of the solid particles downstream. Finally, the outlet gases were led to a gas analyzer (Rayleigh Co,WQF-510 FTIR spectrometer) for online gas analysis.

were 2.5 cm, 200 cm and 6 mm respectively. The reactor was vertically mounted in an electrical furnace. Length of the heating zone of furnace was 184 cm. Three K type thermocouples sensed the outer skin temperature of the reactor. Another K type thermocouple was inserted from the top of the reactor for sensing the inner temperature of the top part of reactor and for controlling the temperature during experimental tests. The outlet products from the reactor entered to a 1 m horizontal double pipe steel condenser and cooled with water. Therefore, the Zn vapor exiting the reactor was solidified in the water cooled condenser. There were water scrubbers in two stages after the condenser to capture the remainder particles that escaped from the condenser. The composition of outlet gases was quantitatively analyzed on line by FTIR. 2.1. Experimental Tests. The reactor design evolved from an initial proposal resulted from flow visualization experiments carried out in two reactor geometries. Clear Plexiglas models were built for purpose of visualizing two-phase fluidization behavior in cold condition. the main scope for cold tests is to obtain a rough estimation for adjusting the gas−solid rates for complete pneumatic conveying. The vertical Plexiglas model pipe with inner diameter of 2.5 cm and 2 m length shows the best lifting of ZnO aggregates with gas flow. Figure (2) shows that the outlet ZnO particles from the mentioned Plexiglas model is in a complete continuous pneumatic conveying regime without any sedimentation or recirculation of ZnO powder. During all experiments, the reactor was heated to reach the desired temperature while the inert nitrogen flowed to purge oxygen from the reactor. Total nitrogen flow during the startup was about 3 L/min at room temperature and atmospheric pressure. ZnO powder (PARZINOX 19984) with 99.8% purity, a reported mean particle size of 0.4 μm and specific surface area of 4.6 m2/g was preheated electrically up to 500 °C in the hopper of the feeder. Figure 3 is the SEM of the zinc oxide powder used in the tests. Submicrometer size of the ZnO primary particles can be observed in this figure. Also XRD tests

3. REACTOR MODELING In this section, the gas−solid reaction in the dilute phase pneumatic conveying reactor is modeled and simulated in the stiochiometric condition. The uniform mixture of particle solids in gas phase with a plug flow is assumed as the primary assumption for modeling. The plug flow assumption for upward flow of gas−solid mixture is a common assumption especially in the riser of the FCC (fluid catalytic cracking) units. Heavy fuel oil is converted to lighter hydrocarbons such as gasoline and diesel in a FCC riser by cracking on 80 μm zeolite catalyst particles in an upward pneumatic conveying flow of gas−solid mixture. Since the ZnO primary particles are nonporous, the shrinking particle model has been assumed for the reaction of methane with ZnO particles. While the reaction takes place between gas phase and solid phase in a shrinking particle 3273



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Figure 3. SEM image of the ZnO powder used in the experimental tests.

Figure 4 shows an element of the reactor in which the molar conservations will be considered for modeling. The inlet molar flow of methane into the reactor can be defined as follows:

manner, the mentioned mixture is propelled upward with a variable velocity Ulift. The modeling consists of three major parts as follows: (1) molar balancing of the reactants and products in the differential element (Figure 4) in order to obtain the differential

FCH40 =

π P Dia 2Ulift0 4 RT

(2)

Parameters are defined in the nomenclature. Ulift0 is the inlet velocity of methane in the entrance of reactor. The molar conservation of methane in the element shown in Figure (4) may be written as follows: FCH4|Z − FCH4|Z + dZ = kCCH4 ds

(3)

where k is the reaction rate constant, ds is differential surface of the spherical particles in the element, and CCH4 is the methane concentration. The physical and chemical properties of the ZnO used in refs 8 and 9 are the same as the ZnO properties used in this work. Thus reaction rate constants of refs 8 and 9 are applicable for this work. The reaction rate constant grows significantly by increasing the temperature. For instance, the reaction rate constants at 1123, 1273, and 1373 K are 0.0072, 2.532, and 186.33 cm/sec, respectively.8,9 Considering stoichiometric conditions, the following relations may be written for the molar flows:

Figure 4. Element of the noncatalytic dilute phase pneumatic conveying reactor for developing the model equations.

equation for variation of methane concentration through the reactor (2) gaseous volume conservation in the differential element (Figure 4) in order to obtain the differential equation for variation of gaseous velocity inside the reactor (3) combining the above parts and applying the shrinking particle behavior for gas−solid reaction 3274

P(yCH ) P(1) π π 4 Dia 2Ulift0 − Dia 2Ulift 4 4 RT RT P(yZn ) P(y ) (g) π π = Dia 2Ulift CO = Dia 2Ulift 4 4 RT RT ( ) P y H2 1π Dia 2Ulift = 24 RT

(4)

FZnO0 = FCH 40

(5)



Article

The gaseous volume conservation in the element of Figure 4 for obtaining the variation of gas velocity inside the reactor, is as follows:

Inserting eq 2 into eq 3 and considering CCH4 = P(yCH4)/(RT), ds = π/4Dia2 dZ(1 − ε)3/r results in π P Dia 2 (UliftyCH |Z − UliftyCH |Z + dZ ) 4 4 4 RT PyCH π 3 4 =k Dia 2 dZ (1 − ε) RT 4 r

π Dia 2(Ulift|Z − Ulift|Z + dZ ) 4 PyCH π 4 Dia 2 dZ (1 − ε) =k RT 4 ⎛ ⎞ 3 ⎜ 16 28 2 65 ⎟ × ⎜ − 28P − 2P − 65P ⎟⎟ r ⎜ 16P ⎝ RT RT RT RT ⎠

(6)

where ε is the void space in the differential element in the reactor. Equation 6 after algebraic manipulations becomes as follows: d(Ulift yCH ) 4

dZ

=−

3kyCH (1 − ε) 4

r

above equation can be simplified to the following equation:

(7)

9kyCH (1 − ε) dUlift 4 = dZ r

The above equation contains variable parameters such as upward mixture velocity, methane mole fraction, void space of the element, and ZnO particle radius in the differential element of the reactor. Equation 4 leads to the following equations: U yCO = yZn = lift0 − yCH (g) 4 Ulift

U yH = 2 lift0 − 2yCH 2 4 Ulift

(13)

(14)

Equation 14 is obtained by the gaseous volume conservation in the element of the reactor, shown in Figure 4. Therefore the gas velocity through the reactor increases as a result of the progress of reaction. Differentiating eq 7 results as follows:

(8)

Ulift

dyCH

4

+ yCH

dZ

(9)

3kyCH (1 − ε) dUlift 4 =− 4 dZ r

(15)

Inserting eq 14 into eq 15: U yCH + yCO + yH + yZn = 4 lift0 − 3yCH 4 2 (G) 4 Ulift

dyCH

(10)

dZ

considering the molecular weight of the components, the following equation may describe the total gas and solid molar conservation: π P Dia 2Ulift0 (16 + 81) RT 4 π P = Dia 2Ulift (16yCH + 28yCO + 2yH 4 2 4 RT P(yCH ) π 4 + 65yZn ) + Dia 2Ulift 81 (g) 4 RT

=−

3 + 9yCH kyCH (1 − ε) 4 4 Ulift r

(16)

Thus, eqs 14 and 16 needed to be solved simultaneously. The boundary conditions of these equations are Z=0

at

yCH = 1;

r = r0 ;

4

FCH4 = FZnO0 = FCH 40 ;

Ulift = Ulift0

(17)

The shrinking particle model has been considered for the reaction of the primary zinc oxide particles with methane. The solid conversion was calculated through the reactor accordingly to the following equation:

(11)

the above equation can be simplified to the following equation: U 97 lift0 = 97yCH + 28yCO + 2yH + 65yZn 4 2 (g) Ulift

4

X=1− (12)

FZnO FZnO0

(18)

Equations 14 and 16 were solved by the Runge−Kutta method through the reactor. A computer code has been prepared for this purpose in MATLAB. The following equation describes the void space used in the modeling:

By inserting eqs 7 and 8 into these equations, the total molar balance will be satisfied. The solid phase particle reactants are aggregates of the primary zinc oxide particles (primary particle diameter is 0.4 μm). Porosity of aggregates of fine particles (nanoparticles) was studied in fluidization mode elsewhere,22,23 where it was found that the porosity of the aggregates were higher than 0.99. Therefore, we assume that the ZnO primary particles in the aggregates may be considered separate primary particles due to very high porosity of the aggregates. Thus, the parameter r is being considered as primary particle radius (0.4 μm) in all simulations. Gas velocity is a function of Z in view of the fact that there is an increase in the mole number of the gaseous products in relation to the mole number of gaseous reactants.

FCH4(16) ρCH

ε= FCH4(16) ρCH

4

FZn (65) FH2(2) F (28) (g) + CO + + ρCO

4

F (28) + CO + ρCO

ρH

ρZn

2

FH2(2) ρH

2

+

(g)

FZn

(65)

(g)

ρZn

(g)

F (81) + ZnO ρZnO

(19)

The use of the relation Fi = π/4Dia2Ulift(P(yi)/(RT)) for gaseous components in the above equation leads to the 3275



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Table 1. Operational Conditions and Chemical Conversion of Methane to Synthesis Gas in the Experimental Runs with Results of Simulationsa run no.

reactor temp K

test duration (min)

ZnO feeding rate (g/min)

inlet CH4 flow rate (L/min)

outlet CH4 mole fraction

H2 mole fraction

CO mole fraction

H2/CO molar ratio

Average reactants temp inside reactor obtained by simulation K

I II III IV V VI

1303 1303 1343 1343 1398 1398

33 36 40 37 43 45

27 28 27 28 28 29

4 2.7 4 2.7 4 2.7

0.45 0.38 0.36 0.29 0.12 0.06

0.37 0.42 0.42 0.49 0.62 0.64

0.18 0.2 0.22 0.22 0.26 0.30

2.05 2.1 1.9 2.22 2.38 2.13

1098 1100 1119 1123 1203 1207

a

Gas flow rates were calculated at room temperature and atmospheric pressure. Solid reactant and gaseous reactant were preheated to 773 and 423 K, respectively.

flow rate of solid reactant is greater than the heat capacity and mass flow rate of the gaseous reactant, the preheating of the solid reactant is significantly more effective than the preheating of the gaseous reactant. Because of the highly endothermic nature of the reaction and because of the absence of particles recirculation in the two phase flow, it is expected that the inside temperature at the entrance of the reactor is lower than the other sections of the reactor. The produced synthesis gas has high quality without any impurities such as CO2. Furthermore, the H2/CO molar ratio at the best conversion was 2.13 (run VI). The outlet gaseous spectrum (peak heights of CH4 and CO in FTIR) did not change during the steady-state experiments considerably. It should be noted that nonpolar symmetric molecules such as H2 cannot be detected by FTIR. Since there was no side product in the outlet gaseous spectra, the hydrogen mole fraction was obtained by calculating the difference and was checked by the mass balance of carbon and hydrogen. No carbon deposition was observed among the solid products or inside the reactor after the experiments. The conversion of zinc oxide to zinc was incomplete because the reaction was affected with excess ZnO. Larger reforming capacity may be reached by increasing the gas-laden particle residence time in the hot zone (for instance recirculation the effluents or increasing the heating zone length). The reformed methane flow rate for the best chemical conversion of methane was 2.7 L/min in the continuous dilute phase pneumatic conveying reactor with 2.5 cm inner diameter. This value is about 14 times greater than the maximum reformed flow rate in our previous work (a batch packed-bed reactor with inner diameter of 5 cm).15 Higher temperatures (over 1523 K on the outer skin of the reactor) were avoided, as such high temperatures approach the maximum operating temperature of the materials construction. Considering Table 1, the chemical conversion of methane was increased by decreasing the inlet gas flow. This clearly means that the residence time of the gas−particle mixture in the reactor plays significant role in chemical conversion. 4.1. Results of Simulation. In this section, eqs 8 and 10 are solved by the Runge−Kutta method and the results of simulations are compared with experimental data in order to obtain a more detailed sight into the noncatalytic dilute phase pneumatic conveying reactor. The experimental runs are chosen for comparison with the simulations. In these comparisons, particle temperature (T) is the fitting parameter; that is, the mean temperature of the particles along the reactor was found by trial and error when the experimental data were fitted by the model. For instance, Figure 5 shows a close agreement between the computer code result and experimental data at the

following relation for the void space after algebraic manipulations: ε=

4Ulift0 − 3yCH Ulift 4 PyCH

81

4 4Ulift0 − 3yCH Ulift + Ulift 4 RT ρZnO

(20)

Equation 20 shows the variation of the void space through the reactor. This equation implies the dependence of the void space to the gas velocity and methane concentration also.

4. RESULTS AND DISCUSSION Chemical conversion of methane to synthesis gas in the dilute phase pneumatic conveying reactor is shown in Table 1. The experimental tests were carried out under pure inlet methane flow. The top point reactor temperature, ZnO feeding rate and stoichiometric ratio of the reactants are also indicated in Table 1. The loading ratio of the particles to gas was kept over the stiochiometric ratio (see eq 1) to ensure methane conversion. Although the solid to gas ratio was over the stoichiometric ratio, no plugging or sedimentation was observed during experimental tests. This was mainly due to increase of the gas velocity in hot reactor and increase of the product gaseous volume with respect to gaseous reactant. XRD pattern of the collected solids from the downstream indicates that the outlet solids were mainly unreacted ZnO. Reactor outer skin temperatures, under approximate steady state conditions, ranged between 1373 and 1488 K and were measured by three K type thermocouple beside the vertical reactor (see Figure 1. The reactor’s inside top temperatures were measured between 1303 and 1398 K during these experiments. As expected, best results were obtained at higher temperatures and when the reactants were preheated. Preheating of the reactants (especially solid reactant) plays a significant role in chemical conversion. For instance, the maximum conversion of methane to synthesis gas was 30% at 1398 K without preheating the reactants. But the degree of chemical conversion of CH4 to synthesis gas, when the solid reactant was preheated up to 773 K, reached 94% at the inner reactor temperature of 1398 K. This experiment showed that the mixture temperature (especially solid temperature) is the most important parameter in chemical conversion. The preheating of pure methane flow shall be controlled accurately; otherwise, methane cracking is inevitable. Therefore, the inlet methane flow temperature never exceeded 423 K (although methane cracking is expected at higher temperatures) to ensure that there was no methane cracking in the inlet flows before the reactor. Since the heat capacity and mass 3276



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5. CONCLUSION A continuous dilute phase pneumatic conveying reactor for coproducing synthesis gas and zinc was tested in the temperature range 1303−1398 K in an electrical furnace. During the experiments, maximum chemical conversion of methane (94%) was achieved under continuous steady state conditions, and high quality synthesis gas was obtained. Therefore, this reactor configuration can be employed for synthesis gas production via noncatalytic reaction of CH4 + ZnO. The process mentioned in this work also offers the possibility of zinc production as an attractive and flexible green energy carrier. Future work is being focused on scaling-up the reactor for the coproduction of zinc and synthesis gas and replacing the electrical furnace with natural gas flares (similar to flares of catalytic steam reforming in petrochemical plants) as the energy source for this reaction, which is feasible in the regions with huge natural gas reservoirs.

■

Figure 5. Reactant and product mole fractions vs axial reactor coordinate. A comparison between simulation results (continuous lines) and experimental run VI (points).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

best fit value of T = 1207 K. This value indicates that the average particles temperatures inside the hot zone of the reactor are 1207 K for run VI. Behavior of gaseous reactant and products are also predicted along the reactor in Figure 5 and compared with experimental results of run VI. The right side column in Table 1 shows the average temperatures of solid reactant within the hot zone, obtained by fitting experimental results with those of simulations. This column shows the effects of reactor temperature and methane flow rate (residence time of reactants). Figure 6 shows the effect of temperature in constant flow rate for two experiments that are simulated in the reactor. This figure shows the chemical conversion growth by increasing reactor temperature in the simulations and experiments. Moreover Table 1 shows that by increasing the methane flow rate, the outlet methane mole fraction increases due to a decrease of methane residence time in the reactor.

The authors declare no competing financial interest.

■

NOMENCLATURE CA = methane concentration, mol/cm3 ds = π/4Dia2 dZ (1 − ε)3/r = differential surface of the spherical particles in the element Dia = reactor diameter, cm F = molar flow rate, mol/sec Fx0 = molar flow rate of component x at the entrance of the reactor, mol/sec k = reaction rate constant for reaction 1, cm/sec Mi = molecular weight of component i, g/mol P = reactor pressure, atm R = universal gas constant, 82.057 (cm3·atm)/(mol·K) r = ZnO particles radius in the reactor, cm T = temperature K Ulift = upward mixture velocity in the reactor, cm/sec

Figure 6. Simulation results of run II (dashed lines) and run IV (continuous lines). Effect of reactor temperature on chemical conversion in a constant methane flow rate. 3277



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(21) Xu, C.; Zhu, J. Experimental and theoretical study on the agglomeration arising from fluidization of cohesive particlesEffects of mechanical vibration. Chem. Eng. Sci. 2005, 60, 6529. (22) Wang, X. S.; Palero, V.; Soria, J.; Rhodes, M. J. Laser-based planar imaging of nano-particle fluidization: Part IDetermination of aggregate size and shape. Chem. Eng. Sci. 2006, 61, 5476. (23) Wang, X. S.; Palero, V.; Soria, J.; Rhodes, M. J. Laser-based planar imaging of nano-particle fluidization: Part IIMechanistic analysis of nanoparticle aggregation. Chem. Eng. Sci. 2006, 61, 8040.

X = solid conversion yA = methane mole fraction Z = reactor coordinate, cm ε = void fraction of two phase flow (see Figure 4) ρ = (PMi)/(RT) = density of component i, g/cm3

■

REFERENCES

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Noncatalytic Synthesis Gas Production by Reduction of ZnO with

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