3.1. Impact of Initial Reaction Tempreature and Molar Ratio of CH4:CO2
To investigate the impact of thickness of Pd/Cu membrane on the reaction characteristics in the reaction chamber,
Figure 5,
Figure 7 and
Figure 9 [M1] show the concentration of H
2, CO, CH
4 and CO
2 at the outlet of reaction chamber at the differential pressure of 0 MPa, 0.010 MPa and 0.020 MPa, respectively. In these figures, the initial reaction temperature (reaction temperature) is changed by 400 ℃, 500 ℃ and 600 ℃. In addition, the molar ratio of CH
4:CO
2 is changed by 1.5:1, 1:1 and 1:1.5. Moreover,
Figure 6,
Figure 8 and
Figure 10 show the concentration of H
2 at the outlet of the sweep chamber to investigate the impact of thickness of Pd/Cu membrane on the H
2 separation characteristics at the differential pressure of 0 MPa, 0.010 MPa and 0.020 MPa, respectively. In these figures, the initial reaction temperature (reaction temperature) is changed by 400 ℃, 500 ℃ and 600 ℃. In addition, the molar ratio of CH
4:CO
2 is changed by 1.5:1, 1:1 and 1:1.5.
According to
Figure 5,
Figure 7 and
Figure 9, it is seen that the concentration of H
2 at the outlet of reaction chamber increases with the increase in the reaction temperature. Since DR is an endothermic reaction as shown in Equation (1), the reaction is progressed with the increase in the reaction temperature well. H
2 production rate of DR increases with the increase in reaction temperature according to the theoretical kinetic study [
14]. This tendency is obtained irrespective of the molar ratio of CH
4:CO
2, the thickness of Pd/Cu membrane and the differential pressure between the reaction chamber and the sweep chamber.
It is seen from
Figure 6,
Figure 8 and
Figure 10 that the concentration of H
2 at the outlet of the sweep chamber increases with the increase in the reaction temperature. Since the concentration of H
2 at the outlet of the reaction chamber is higher at higher reaction temperature, it is thought that the driving force to penetrate Pd/Cu membrane is bigger due to the large H
2 partial pressure difference between the reaction chamber and the sweep chamber, i.e. large concentration difference of H
2 between the reaction chamber and the sweep chamber, resulting in the higher concentration of H
2 at the outlet of the sweep chamber.
According to
Figure 5,
Figure 7 and
Figure 9 and
Figure 6,
Figure 8 and
Figure 10, the thickness of Pd/Cu membrane which exhibits the highest concentration of H
2 at the outlet of the reaction chamber and the sweep chamber depends on the differential pressure between the reaction chamber and the sweep chamber. Generally speaking, the penetration resistance of H
2 is lower when the thickness of Pd/Cu membrane is thinner, resulting that H
2 is separated well. At the differential pressure of 0 MPa, the highest concentration of H
2 in the reaction chamber as well as the sweep chamber is obtained for the thickness of 40 μm irrespective of molar ratio of CH
4:CO
2. Since the differential pressure of 0 MPa means the permeation flux of 0 mol/(m
2・s), the driving force of H
2 separation is the concentration difference of H
2 between the reaction chamber and the sweep chamber mainly. At the differential pressure of 0.020 MPa, the concentration of H
2 in the sweep chamber for the thickness of 20 μm is the highest among the investigated thicknesses, while the concentration of H
2 at the outlet of the reaction chamber for the thickness of 20 μm is the lowest among the investigated thicknesses irrespective of the molar ratio of CH
4:CO
2 as shown in
Figure 9. The permeation flux at the differential pressure of 0.020 MPa is 7.07×10
-4 mol/(m
2・s) which is the largest among the investigated differential pressures, resulting that the effect of pressure on the H
2 separation performance is the largest. In addition, since the thickness of 20 μm is the thinnest among the investigated thicknesses, the penetration resistance of H
2 of Pd/Cu membrane is the smallest. As a result, it can be claimed that the H
2 produced in the reaction chamber penetrates via Pd/Cu membrane well under the combination condition of the differential pressure of 0.020 MPa and thickness of 20 μm. As to the differential pressure of 0.010 MPa, the optimum thickness of Pd/Cu membrane which obtains the highest concentration of H
2 in the reaction chamber as well as that in the sweep chamber is not clear. It is also influenced by the molar ratio of CH
4:CO
2. The impact of H
2 separation performance on the reaction mechanism including DR as well as the other reactions as shown in Equations (2) – (3) is thought to be existed. The kinetic study considering the gas separation is the future work in this study.
Comparing the molar ratio of CH4:CO2, the concentration of H2 in case of the molar ratio of CH4:CO2 = 1:1 is the highest among the investigated conditions. Since the molar ratio of CH4:CO2 = 1:1 is the stoichiometric ratio of FR as shown in Equation (1), it is thought that H2 is produced easily. The kinetic pressure in case of CH4:CO2 = 1.5:1 and 1:1.5 is 3.18×10-4 Pa, while that in case of CH4:CO2 = 1:1 is 2.03×10-4 Pa. Since the differential pressure between the reaction chamber and the sweep chamber is much larger than the kinetic pressure, it is thought that the impact of increase in static pressure with the decrease in kinetic pressure on the H2 separation performance is small.
It is seen from
Figure 6,
Figure 8 and
Figure 10 that the concentration of H
2 at the outlet of the sweep chamber in case of the molar ratio of CH
4:CO
2 = 1:1 is higher compared to the other molar ratios except for the differential pressure of 0.020 MPa. Since the concentration of H
2 in the reaction chamber is higher at higher reaction temperature, it is thought the driving force to penetrate Pd/Cu membrane is bigger, resulting in the higher concentration of H
2 in the sweep chamber. As to the differential pressure of 0.020 MPa, the differential pressure is too high, resulting that the separation rate of H
2 might be higher than the production rate of H
2 by Ni/Cr catalyst in the reaction chamber. As a result, it is thought that the effective non-equilibrium sate can not be obtained. Comparing the concentration of H
2 at the outlet of the reaction chamber shown in
Figure 5,
Figure 7 and
Figure 9, the concentration of H
2 at the differential pressure of 0.020 MPa is relatively lower than that at the other differential pressures. We can also know the lower production performance of H
2 at the differential pressure of 0.020 MPa according to this tendency.
3.2. Impact of Differential Pressure between the Reaction Chamber and the Sweep Chamber
To investigate the impact of thickness of Pd/Cu membrane on the reaction characteristics in the reaction chamber,
Figure 11 shows the concentration of H
2, CO, CH
4 and CO
2 for the differential molar ratio of CH
4:CO
2. In this figure, the reaction temperature is 600 ℃. In addition, the differential pressure between the reaction chamber and the sweep chamber is changed by 0 MPa, 0.010 MPa and 0.020 MPa. Moreover,
Figure 12 shows the concentration of H
2 in the sweep chamber to investigate the impact of thickness of Pd/Cu membrane on the H
2 separation characteristics for the different molar ratio of CH
4:CO
2. In this figure, the reaction temperature is 600 ℃. In addition, the differential pressure between the reaction chamber and the sweep chamber is changed by 0 MPa, 0.010 MPa and 0.020 MPa.
According to
Figure 11, the concentration of H
2 at the outlet of the reaction chamber relatively increases with the decrease in the differential pressure between the reaction chamber and the sweep chamber irrespective of the molar ratio of CH
4:CO
2. In addition, we can see from
Figure 12 that the concentration of H
2 at the outlet of the sweep chamber increases with the decrease in the differential pressure between the reaction chamber and the sweep chamber irrespective of the molar ratio of CH
4:CO
2, which follows the tendency observed in
Figure 11. Moreover, the highest concentration of H
2 at the differential pressure of 0 MPa is obtained for the thickness of Pd/Cu membrane of 40 μm for the reaction chamber as well as the sweep chamber. The kinetic pressure in case of CH
4:CO
2 = 1.5:1 and 1:1.5 is 3.18×10
-4 Pa, while that in case of CH
4:CO
2 = 1:1 is 2.03×10
-4 Pa. Since the differential pressure between the reaction chamber and the sweep chamber is much larger than the kinetic pressure, it is thought that the impact of increase in static pressure with the decrease in kinetic pressure on the H
2 separation performance is small. Since the differential pressure of 0 MPa means the permeation flux of 0 mol/(m
2・s), the driving force of H
2 separation is the concentration difference of H
2 between the reaction chamber and the sweep chamber mainly. Since the concentration of H
2 at the outlet of the reaction chamber for the thickness of 40 μm is the highest among the investigated thicknesses shown in
Figure 11, it is thought that the concentration of H
2 at the outlet of the sweep chamber for the thickness of 40 μm is the highest, as shown in
Figure 12, after the penetration of H
2 through Pd/Cu membrane.
3.3. Comparison of Assessment Factor among the Investigated Experimental Conditions
To investigate the performance of proposed membrane reactor using Ni/Cr catalyst and Pd/Cu membrane,
Table 2 lists comparison of CH
4 conversion, CO
2 conversion, H
2 yield, H
2 selectivity, CO selectivity, H
2 permeability, permeation flux and thermal efficiency for the different reaction temperature, the molar ratio of CH
4:CO
2 and the differential pressure between the reaction chamber and the sweep chamber.
According to
Table 2, we can see that CH
4 conversion and CO
2 conversion exhibits the positive value and the negative value, respectively. From this result, it can be claimed that the reaction consuming CH
4 and that producing CO
2 are occurred. In addition, it is seen from
Table 2 that CO selectivity is higher than 90 %, indicating that CO is produced more compared to H
2. These phenomena can be explained as follows:
- (i)
H2 is produced by Equation (1) or Equation (5).
- (ii)
CO is produced after consuming H2 via Equation (2).
- (iii)
Carbon and CO2 are produced after consuming a part of CO via Equation (6).
Since Equation (6) shown in (iii) is an exothermal reaction, CO can be produced easily. After the experiments in this study, the carbon was observed which can be explained as shown in
Figure 13. The weight of Ni/Cr catalyst used in this study has been increased by 9.1 g after experiments, compared to the initial weight of 74.6 g. This is due to a carbon deposition shown in Equation (6). As to H
2O formation shown in Equation (2), this study has confirmed it by the observation using the gas bag shown in
Figure 14. The colored part, which is different from the other area in the red circle shown in
Figure 14, indicates H
2O formation. In addition, the numerical simulation result using the commercial software COMSOL Multiphysics which includes the simulation codes on Equations (1), (2), (3) and (4) with 3D model indicates H
2O formation. For example, the concentration of H
2O at the outlet of reactor in case of the molar ratio of CH
4:CO
2 = 1.5:1 at 400 ℃, 500 ℃ and 600 ℃ is 6228 ppmV, 28946 ppmV and 33614 ppmV, respectively. The authors would like to report the detail of the numerical simulation in the near future.
According to the investigation in this study, the highest concentration of H
2 is 122711 ppmV in case of CH
4:CO
2 =1:1 at the reaction temperature of 600 ℃ and the differential pressure of 0 MPa using the Pd/Cu membrane whose thickness of 40 μm. Under this condition, the kinetic rate is 0.86 mol/(m
3・s) and the permeation flux is 0 mol/(m
2・s). In addition, CH
4 conversion, CO
2 conversion, H
2 yield, H
2 selectivity, CO selectivity, H
2 permeability and thermal efficiency is 13.7 %, -5.73 %, 1.51 %, 2.84 %, 97.2 %, 1.06×10
-1 % and 11.0 %, respectively. To improve the performance of H
2 production as well as thermal efficiency of the proposed membrane reactor, this study proposes using the other type of catalyst. This study has investigated Ni/Cr alloy catalyst. Though Ni is a popular catalyst for DR, Ru is also used for DR [
15,
16,
17]. There is no report on Ni/Ru/Cr alloy catalyst used for not only DR but also membrane reactor. This study would like to study Ni/Ru/Cr catalyst in order to enhance the performance of DR in the near future.