Impact of Thickness of Pd/Cu Membrane on Performance of Biogas Dry Reforming Membrane Reactor Utilizing Ni/Cr Catalyst

Abstract

The present study pays attention to biogas dry reforming for the purpose of producing H₂. It is known that biogas contains approximately 40 vol% CO₂, causing a decrease in the efficiency of power generation due to its lower heating value compared to natural gas, i.e., CH₄. We suggest a hybrid system composed of a biogas dry reforming membrane reactor and a high-temperature fuel cell, i.e., a solid oxide fuel cell (SOFC). Since biogas dry reforming is an endothermic reaction, we adopt a membrane reactor, controlled by providing a non-equilibrium state via H₂ separation from the reaction site. The purpose of the present study is to understand the effect of the thickness of the Pd/Cu membrane on the performance of the biogas dry reforming membrane reactor with a Pd/Cu membrane as well as a Ni/Cr catalyst. The impact of the reaction temperature, the molar ratio of CH₄:CO₂ and the differential pressure between the reaction chamber and the sweep chamber on the performance of the biogas dry reforming membrane reactor with the Pd/Cu membrane as well as the Ni/Cr catalyst was investigated by changing the thickness of the Pd/Cu membrane. It was revealed that we can obtain the highest concentration of H₂, of 122,711 ppmV, for CH₄:CO₂ = 1:1 at a reaction temperature of 600 °C and a differential pressure of 0 MPa and using a Pd/Cu membrane with a thickness of 40 μm. Under these conditions, it can be concluded that the differential pressure of 0 MPa provides benefits for practical applications, especially since no power for H₂ separation is necessary. Therefore, the thermal efficiency is improved, and additional equipment, e.g., a pump, is not necessary for practical applications.

1. Introduction

H₂ is thought to be a promising fuel for addressing the global warming issue. Several countries, including Japan, are developing technologies not only to produce H₂ but also to create systems using H₂ as a fuel. The present study focuses on biogas dry reforming to produce H₂. Biogas consists of CH₄ (55–75 vol%) and CO₂ (25–45 vol%) [1]. It is mainly produced via fermentation by means of the action of anaerobic microorganisms on raw materials such as garbage, livestock excretion and sewage sludge. In 2020, 1.46 EJ of biogas was produced in the world, indicated as about five times greater than the amount produced in 2000 [2]. Therefore, we believe that biogas can be expected to be an attractive source material for producing H₂.

Biogas is known to generally be used as fuel for gas engines, as well as microgas turbines [3]. Biogas includes about 40 vol% CO₂, meaning that the efficiency of power generation decreases due to its lower heating value compared to that of natural gas, i.e., CH₄. We have suggested a hybrid system composed of a biogas dry reforming membrane reactor and a high-temperature fuel cell, i.e., a solid oxide fuel cell (SOFC) [4,5,6]. SOFCs can utilize not only H₂ but also CO, which is a by-product, as fuel via biogas dry reforming. Consequently, this system can be used in a wider operation area.

Biogas dry reforming has been investigated by several researchers [7,8,9,10,11,12]. The catalyst used for biogas dry reforming is important. Ni-based catalyst types are known as the most well-known catalyst types used for biogas dry reforming. A Ni/Ru bimetallic catalyst developed by Miao et al. [7] achieved 100% conversion of CO₄ and CH₂ over 750 °C and a H₂/CO ratio of 1.0 within a temperature from 773 K to 800 °C. Ni/Mg-Al₂O₃, which was produced by Shaffner et al. [8], demonstrated 80% conversion of CO₄ and CH₂ at 500 °C and a H₂/CO ratio of approximately 1.0 within a temperature from 350 °C to 800 °C. The Ni-SiO₂@SiO₂ core–shell produced by Kaviani et al. [9] showed an increase in CO₄ and CH₂ conversions from 20% to 70% and 30% to 90%, respectively, as the temperature increased from 500 °C to 700 °C. The H₂/CO ratio increased from 0.6 to 0.8 as the temperature increased from 450 °C to 700 °C. Ni/MgO/Al₂O₃ with a modifier (Gd, Sc, La), which was produced by Ha et al. [10], realized CH₄ conversion of 50% and CO₂ conversion of 95%. In addition, the H₂/CO ratio exhibited a range of 0.8 to 1.0. Ni/Co/Al₂O₃ produced by Hajizadeh et al. [11] demonstrated an increase in CH₄ conversion from 5% to 100% as the temperature rose from 400 °C to 550 °C. If the pressure is higher, the rising rate of CH₄ conversion increases with the reaction temperature. The Ni/Mg/La/Al catalyst, which was developed by Calgaro et al. [12], achieved CH₄ and CO₂ conversions of 70% and 80%, respectively, at 750 °C. In addition, the H₂/CO ratio was 0.85 at 750 °C.

According to previous studies [7,8,9,10,11,12], Ni-based catalysts have been well investigated for biogas dry reforming. In addition, they have reported that some Ni alloy catalysts are promising because they prevent carbon deposition, which decreases the performance of pure Ni catalysts. Though many types of Ni alloy catalysts have been developed and tested, Ni/Cr catalysts have not been examined well, except for in our previous study [5]. Though Cr is one metal which is used in other reaction processes, there are few studies on biogas dry reforming with Cr as a catalyst, making the use of Ni/Cr as a catalyst for biogas dry reforming an innovative aspect. Consequently, we adopt a Ni/Cr catalyst for biogas dry reforming in order to understand the characteristics of Ni/Cr catalysts.

Additionally, biogas dry reforming is an endothermic reaction, making it a meaningful issue to conduct the process at lower temperatures in order to improve the thermal energy efficiency of the biogas dry reforming process. For this purpose, using a membrane reactor is one promising approach because H₂ production is enhanced due to the non-equilibrium state provided by H₂ separation from the reaction site [5]. Our previous study [5] reported the experimental investigation of a biogas dry reforming membrane reactor using a Pd/Cu membrane as well as a Ni/Cr catalyst. Compared to a pure Ni catalyst, the concentration of H₂ produced using the Ni/Cr catalyst was higher. Though the effects of the reaction temperature, the molar ratio of CH₄:CO₂ and the differential pressure between the reaction chamber and the sweep chamber on the performance of the biogas dry reforming membrane reactor with a Pd/Cu membrane as well as a Ni/Cr catalyst were investigated [5], the effect of the thickness of the Pd/Cu membrane has not been examined yet. In the previous study [5], the thickness of the Pd/Cu membrane was 20 μm. A thinner H₂ separation membrane can be expected to offer a stronger H₂ separation performance due to its lower H₂ permeation resistance. However, we have to consider the strength of the H₂ separation membrane under larger differential pressures between the reaction chamber and the sweep chamber, as well as during high-temperature operation.

Therefore, the purpose of this study is to understand the impact of the thickness of the Pd/Cu membrane on the performance of the biogas dry reforming membrane reactor with a Pd/Cu membrane as well as a Ni/Cr catalyst. The impact of the reaction temperature, the molar ratio of CH₄:CO₂ and the differential pressure between the reaction chamber and the sweep chamber on the performance of the biogas dry reforming membrane reactor with a Pd/Cu membrane as well as a Ni/Cr catalyst is investigated when changing the thickness of the Pd/Cu membrane. In this study, a molar ratio of CH₄:CO₂ = 1.5:1 simulates the composition of biogas.

The reaction scheme for CH₄ dry reforming (DR) is described as follows:

CH₄ + CO₂ ↔ 2CO + 2H₂ + 247 kJ/mol

(1)

Furthermore, the following reactions are considered the phenomena that play out in this study:

CO₂ + H₂ ↔ CO + H₂O + 41 kJ/mol

(2)

CO₂ + 4H₂ ↔ CH₄ + 2H₂O − 164kJ/mol

(3)

CH₄ + H₂O ↔ CO + 3H₂ − 41 kJ/mol

(4)

where Equation (2) indicates a reverse water–gas shift reaction (RWGS), Equation (3) indicates a methanation reaction and Equation (4) indicates steam reforming of CH₄. As to carbon deposition, the following reactions are considered:

CH₄ ↔ C + 2H₂ + 75 kJ/mol

(5)

2CO ↔ C + CO₂ − 173 kJ/mol

(6)

CO₂ + 2H₂ ↔ C + 2H₂O − 90 kJ/mol

(7)

CO + H₂ ↔ C + H₂O − 131 kJ/mol

(8)

2. Experiment

2.1. Experimental Apparatus and Procedure

Figure 1 shows a schematic drawing of the experimental apparatus used in the present study. The experimental set-up is composed of a gas cylinder, mass flow controllers (S48-32; manufactured by HORIBA METRON Inc. Beijing, China), pressure sensors (KM31), valves, a vacuum pump, a reactor consisting of a reaction chamber and a sweep chamber and gas sampling taps. This study installs the reactor in a furnace. This study controls the temperature in the furnace using far-infrared heaters (MCHNNS1; manufactured by MISUMI, Schaumburg, IL, USA). This study controls CH₄ gas (purity over 99.4 vol%) and CO₂ gas (purity over 99.9 vol%) using mass flow controllers and mixes them before introducing them into the reaction chamber. This study measures the pressure of the mixed gas at the inlet of the reaction chamber by means of pressure sensors. This study controls Ar gas (purity over 99.99 vol%) by means of a mass flow controller and measures the pressure of the Ar gas by means of a pressure sensor. This study provides Ar as the sweep gas. This study suctions the exhausted gas at the outlet of the reaction chamber and the sweep chamber by means of a gas syringe through the gas sampling tap. This study measures the concentration of sampled gas by means of a TCD gas chromatograph (manufactured by GL Science, Tokyo, Japan). The TCD gas chromatograph and the methanizer’s minimum resolution is 1 ppmV. This study measures the gas pressure at the outlet of the reactor by means of a pressure sensor. The gas concentration and pressure at the outlet of reaction chamber and sweep chamber are measured, respectively.

Figure 2 shows the details of the reactor, which consists of a reaction chamber, a sweep chamber, and a H₂ separation membrane. The reaction chamber and the sweep chamber are made of stainless steel with a scale of 40 mm × 100 mm × 40 mm. The space of reaction in the reaction chamber is 16 × 10⁻⁵ m³. A porous Ni/Cr (Cr: 35 wt%) catalyst is added into the reaction chamber. The average hole diameter of the Ni/Cr catalyst is 0.8 mm. We know from the procedure brochure that the porosity of the Ni/Cr catalyst is 0.93. The mass of the filled Ni/Cr catalyst is 74.6 g. According to the manufacturer’s brochure for the Ni/Cr catalyst (Sumitomo Electric Toyama Co., Ltd., Imizu, Japan), the Ni/Cr catalyst is produced by the following processes: (i) foam resin, (ii) conductive treatment, (iii) electrodeposition, (iv) heat disposal and (v) alloying treatment. This study has adopted a commercial catalyst due to its high versatility and easy installment within industry. The selected commercial catalyst is produced considering its strength and cost, with the result that there is no selection without 35 wt% of Cr.

Figure 3 shows a photo of the Ni/Cr catalyst added into the reactor used in the present study. Figure 4 shows a photo of the details of the Ni/Cr catalyst. A Pd/Cu membrane (Cu: 40 wt%; manufactured by Tanaka Kikinzoku, Tokyo, Japan) is chosen as a H₂ separation membrane. From the manufacturer’s brochure for the Pd/Cu membrane (manufactured by Tanaka Kikinzoku, Tokyo, Japan), the Pd/Cu membrane is produced by the following processes: (i) dissolving of the alloy metals of Pd/Cu and (ii) flatting and thinning of the alloy metal film. We change the thickness of the Pd/Cu membrane by 20 μm, 40 μm and 60 μm. This study hypothesizes that the characteristics of the membrane reactor depend on the balance between the reaction performance of the catalyst and the separation performance of the H₂ separation membrane. Therefore, the present study investigates the impact of the thickness of the Pd/Cu membrane on the performance of the membrane reactor, changing the initial reaction temperature, the molar ratio of CH₄:CO₂ and the differential pressure between the reaction chamber and the sweep chamber. Figure 5 shows a photo of the Pd/Cu membrane with a thickness of 60 μm. This study measures the temperatures at the inlet, the middle and the outlet of the reaction and the sweep chamber by means of K-type thermocouples. This study controls the initial reaction temperature and sets using a far-infrared heater, confirmed by means of the thermocouples. This study collects the measured temperature and pressures by means of a data logger (GL240; manufactured by Graphic Corporation, Orland Park, IL, USA).

Table 1. Each parameter under the experimental conditions set in this study.

Parameter	Value
Initial reaction temperature (pre-set reaction temperature) [°C]	400, 500, 600
Pressure of supply gas [MPa]	0.10
Differential pressure between the reaction chamber and the sweep chamber [MPa]	0, 0.010 and 0.020
Molar ratio of CH4:CO2 provided (flow rate of CH4:CO2 provided [NL/min])	1.5:1, 1:1 and 1:1.5 (1.088:0.725, 0.725:0.725, 0.725:1.088)
Feed ratio of sweep gas to supply gas [-]	1.0

lists the experimental parameters applied in the present study. This study changes the molar ratio of CH₄:CO₂ provided to 1.5:1, 1.1 and 1:1.5. The molar ratio of CH₄:CO₂ simulates biogas in the present study. According to our previous study [13], the feed ratio of sweep gas to supply gas, defined as the flow rate of sweep gas divided by the flow rate of supply gas composed of CH₄ and CO₂, has been set at 1.0, indicating the optimum feed ratio of sweep gas to supply gas [13]. The differential pressure between the reaction chamber and the sweep chamber is varied between 0 MPa, 0.010 MPa and 0.020 MPa. This study measures and confirms this differential pressure using pressure sensors installed at the outlet of the reaction chamber and the outlet of the sweep chamber. The initial reaction temperature, e.g., the initial temperature of the reactor, is changed to 400 °C, 500 °C and 600 °C. We measure the initial reaction temperature by means of thermocouples before supplying not only the mixed gas of CH₄ and CO₂ but also the sweep gas into the reactor. This study detects the gas concentrations at the outlet of the reaction chamber and the sweep chamber by means of an FID gas chromatograph (GC3220; manufactured by GL Science) and a methanizer (MT221; manufactured by GL Science, Tokyo, Japan). We show the mean data among five trials for each experimental condition in the next figures. The distribution of each gas concentration ranges under 10%.

2.2. Assessment Procedure for the Performance of the Reactor

We evaluate the reaction and separation performances of the proposed membrane reactor using the gas concentration at the outlet of the reaction chamber and the sweep chamber. According to these data, the CH₄ conversion (X_CH4), CO₂ conversion (X_CO2), H₂ yield (Y_H2), H₂ selectivity (S_H2) and CO selectivity (S_CO) are calculated. These assessment factors can be defined as follows:

X_CH4 = (C_{CH4, in} − C_{CH4, out})/(C_{CH4, in}) × 100

(9)

X_CO2 = (C_{CO2, in} − C_{CO2, out})/(C_{CO2, in}) × 100

(10)

Y_H2 = (1/2)(C_{H2, out})/(C_{CH4, in}) × 100

(11)

S_H2 = (C_{H2, out})/(C_{H2, out} + C_{CO, out}) × 100

(12)

S_CO = (C_{CO, out})/(C_{H2, out} + C_{CO, out}) × 100

(13)

where C_{CH4, in} indicates the concentration of CH₄ at the inlet of the reaction chamber [ppmV], C_{CH4, out} indicates the concentration of CH₄ at the outlet of the reaction chamber [ppmV], C_{CO2, in} indicates the concentration of CO₂ at the inlet of the reaction chamber [ppmV], C_{CO2, out} indicates the concentration of CO₂ at the outlet of the reaction chamber [ppmV], C_{H2, out} indicates the concentration of H₂ at the outlet of the reaction chamber and the sweep chamber [ppmV], and C_{CO, out} indicates the concentration of CO at the outlet of the reaction chamber [ppmV].

Furthermore, H₂ permeability (H) and permeation flux (F) can be calculated as follows:

H = (C_{H2, out, sweep})/(C_{H2, out, sweep} + C_{H2, out, react}) × 100

(14)

$$F={\displaystyle \frac{P\left(\sqrt{P_{react,ave}}-\sqrt{P_{sweep,ave}}\right)}{\delta }}\times 100$$

(15)

where C_{H2, out, sweep} indicates the concentration of H₂ at the outlet of the sweep chamber [ppmV], C_{H2, out, react} indicates the concentration of H₂ at the outlet of the reaction chamber [ppmV], P indicates the permeation factor [mol/(m·s·Pa^0.5)], P_{react, ave} indicates the average pressure of the reaction chamber [MPa], P_{sweep, ave} indicates the average pressure of the sweep chamber [MPa] and δ indicates the thickness of the Pd/Cu alloy membrane [m].

Moreover, the thermal efficiency of the membrane reactor (η) is calculated, which is defined as follows:

$$\eta ={\displaystyle \frac{Q_{\mathrm{H}2}}{\left(W_{\mathrm{S}.\mathrm{C}.}+W_{\mathrm{R}.\mathrm{C}.}+W_{\mathrm{p}}\right)}}\times 100$$

(16)

where Q_H2 indicates the heating value of H₂ produced, which is based on the lower heating value [W], W_R.C. indicates the amount of pre-heat of the supply gas provided into the reaction chamber [W], W_S.C. indicates the amount of pre-heat of the sweep gas provided into the sweep chamber [W] and W_p indicates the pump power to provide the differential pressure between the reaction chamber and the sweep chamber [W].

3. Results and Discussion

3.1. Impact of the Initial Reaction Temperaature and the Molar Ratio of CH₄:CO₂

To investigate the impact of the thickness of the Pd/Cu membrane on the reaction characteristics as well as the H₂ separation performance, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11 show the concentration of H₂, CO, CH₄ and CO₂ at the outlet of reaction chamber and the sweep chamber at differential pressures of 0 MPa, 0.010 MPa and 0.020 MPa, respectively. The initial reaction temperature (reaction temperature) is varied between 400 °C, 500 °C and 600 °C in Figure 6, Figure 8 and Figure 10. In addition, the molar ratio of CH₄:CO₂ is changed to 1.5:1, 1:1 and 1:1.5. Since we have focused on the phenomena of the transportation of H₂ from the reaction chamber to the seep chamber in the present paper, Figure 7, Figure 9 and Figure 11 show the concentration of H₂ in the seep chamber. The initial reaction temperature (reaction temperature) is changed to 400 °C, 500 °C and 600 °C in these figures. In addition, the molar ratio of CH₄:CO₂ is changed to 1.5:1, 1:1 and 1:1.5.

According to Figure 6, Figure 8 and Figure 10, we can observe the concentration of H₂ at the outlet of the reaction chamber rises with a rise in the reaction temperature. Since DR is an endothermic reaction, as explained by Equation (1), the reaction progresses with the rise in the reaction temperature well. The H₂ production rate within DR increases with the rise in the reaction temperature according to theoretical kinetic research [14]. This observed tendency is obtained regardless of the other parameters, i.e., the molar ratio of CH₄:CO₂, the thickness of the Pd/Cu membrane and the differential pressure between the reaction chamber and the sweep chamber. Additionally, we would like to substantiate the effect of the thickness of the Pd/Cu membrane on the characteristics of the membrane reactor in the following section.

We can observe from Figure 7, Figure 9 and Figure 11 that the concentration of H₂ at the outlet of the sweep chamber rises with the rise in the reaction temperature. The concentration of H₂ at the outlet of the reaction chamber is higher at a higher reaction temperature, resulting in the driving force to permeate the Pd/Cu membrane being stronger due to the large H₂ partial pressure difference between the reaction chamber and the sweep chamber, indicating the big difference in the concentration of H₂ between the reaction chamber and the sweep chamber. As a result, the concentration of H₂ at the outlet of the sweep chamber becomes higher.

As seen from Figure 6, Figure 8 and Figure 10 and Figure 7, Figure 9 and Figure 11, the thickness of the Pd/Cu membrane which exhibits the highest concentration of H₂ 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 H₂ is lower when the thickness of the Pd/Cu membrane is thinner, resulting in H₂ being separated well. The highest concentration of H₂ not only in the reaction chamber but also in the sweep chamber is obtained for the thickness of 40 μm regardless of the molar ratio of CH₄:CO₂ at a differential pressure of 0 MPa. The differential pressure of 0 MPa results in a permeation flux of 0 mol/(m²·s), meaning that the driving force for H₂ separation is mainly the difference in the concentration of H₂ between the reaction chamber and the sweep chamber. The concentration of H₂ in the sweep chamber for the thickness of 20 μm is the highest among the investigated thicknesses at the differential pressure of 0.020 MPa, while the concentration of H₂ at the outlet of the reaction chamber for the thickness of 20 μm is the lowest among the investigated thicknesses regardless of the molar ratio of CH₄:CO₂, as exhibited in Figure 10. The permeation flux at the differential pressure of 0.020 MPa is 7.07 × 10⁻⁴ mol/(m²·s), indicating the largest value among the investigated differential pressures. As a result, the impact of pressure on the H₂ separation performance is the greatest. In addition, since the thickness of 20 μm is the thinnest among the investigated thicknesses, the penetration resistance of H₂ for the Pd/Cu membrane is the lowest. As a result, it can be claimed that the H₂ produced in the reaction chamber penetrates via the Pd/Cu membrane well under the combination of conditions of a differential pressure of 0.020 MPa and a thickness of 20 μm. Regarding the differential pressure of 0.010 MPa, the optimum thickness of the Pd/Cu membrane which obtains the highest concentration of H₂ in the reaction chamber, as well as that in the sweep chamber, is not clear. It is also influenced by the molar ratio of CH₄:CO₂. An impact of the H₂ separation performance on the reaction mechanism, including DR, as well as the other reactions, as shown in Equations (2) and (3), is thought to exist. A kinetic study considering the gas separation is future work for our study.

Comparing the molar ratio of CH₄:CO₂, the concentration of H₂ for a molar ratio of CH₄:CO₂ = 1:1 is the highest among the conditions investigated in this study. The molar ratio of CH₄:CO₂ = 1:1 means the stoichiometric ratio for FR is fulfilled, as shown in Equation (1), resulting in H₂ presumably being produced easily. The kinetic pressure in the cases of CH₄:CO₂ = 1.5:1 and 1:1.5 is 3.18 × 10⁻⁴ Pa, while that in the case of CH₄:CO₂ = 1:1 is 2.03 × 10⁻⁴ Pa. The differential pressure between the reaction chamber and the sweep chamber is much bigger than the kinetic pressure, so it is believed that the effect of an increase in static pressure and a decrease in kinetic pressure on the H₂ separation performance is low.

It can be seen from Figure 7, Figure 9 and Figure 11 that the concentration of H₂ at the outlet of the sweep chamber in the case of the molar ratio of CH₄:CO₂ = 1:1 is higher compared to the other molar ratios, except for at a differential pressure of 0.020 MPa. The concentration of H₂ in the reaction chamber is higher at a higher reaction temperature, causing the driving force needed to permeate the Pd/Cu membrane to be stronger, resulting in a higher concentration of H₂ in the sweep chamber. As for the differential pressure of 0.020 MPa, since the differential pressure is too large, the separation rate of H₂ might be higher than the rate of production of H₂ by the Ni/Cr catalyst. Therefore, it is believed that an effective non-equilibrium state cannot be obtained. According to Figure 6, Figure 8 and Figure 10, the concentration of H₂ at the differential pressure of 0.020 MPa is relatively lower than that at the other differential pressures. We also understand the lower production performance of H₂ at the differential pressure of 0.020 MPa due to this tendency.

3.2. Effect of Differential Pressure between the Reaction Chamber and the Sweep Chamber

To investigate the effect of the thickness of the Pd/Cu membrane on the reaction characteristics in the reaction chamber, Figure 12 exhibits the concentration of H₂, CO, CH₄ and CO₂ for the differential molar ratios of CH₄:CO₂. In Figure 12, the reaction temperature is 600 °C. Additionally, the differential pressure between the reaction chamber and the sweep chamber is changed to 0 MPa, 0.010 MPa and 0.020 MPa. Moreover, Figure 13 shows the concentration of H₂ in the sweep chamber to examine the effect of the thickness of the Pd/Cu membrane on the H₂ separation performance for the different molar ratios of CH₄:CO₂. In Figure 12, the reaction temperature is 600 °C. Additionally, the differential pressure between the reaction chamber and the sweep chamber is changed to 0 MPa, 0.010 MPa and 0.020 MPa.

According to Figure 12, we can see that the concentration of H₂ 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 CH₄:CO₂. Moreover, we can observe from Figure 13 that the concentration of H₂ at the outlet of the sweep chamber rises with a decrease in the differential pressure between the reaction chamber and the sweep chamber regardless of the molar ratio of CH₄:CO₂, which follows the tendency observed in Figure 12. Moreover, the highest concentration of H₂ at a differential pressure of 0 MPa is obtained for a thickness of the Pd/Cu membrane of 40 μm for the reaction chamber as well as the sweep chamber. The kinetic pressure in the cases of CH₄:CO₂ = 1.5:1 and 1:1.5 is 3.18 × 10⁻⁴ Pa, while that in the case of CH₄:CO₂ = 1:1 is 2.03 × 10⁻⁴ Pa. The differential pressure between the reaction chamber and the sweep chamber is much higher than the kinetic pressure, so it is believed that the effect of the increase in static pressure with the decrease in kinetic pressure on the H₂ separation performance is low. Since the differential pressure of 0 MPa means the permeation flux is 0 mol/(m²·s), the driving force of H₂ separation is mainly the difference in the concentration of H₂ between the reaction chamber and the sweep chamber. The concentration of H₂ at the outlet of the reaction chamber for a thickness of 40 μm is the highest among the investigated thicknesses shown in Figure 12, so it is thought that the concentration of H₂ at the outlet of the sweep chamber is the highest for a thickness of 40 μm, as displayed in Figure 13, after the penetration of H₂ through the Pd/Cu membrane.

3.3. Comparison of the Assessment Factors Investigated in This Study

To examine the reaction and separation performances of the proposed membrane reactor with a Ni/Cr catalyst as well as a Pd/Cu membrane,

Table 2. Comparison of CH₄ conversion, CO₂ conversion, H₂ yield, H₂ selectivity, CO selectivity, H₂ permeability, permeation flux and thermal efficiency (a): CH₄:CO₂ = 1.5:1; (b): CH₄:CO₂ = 1:1; (c): CH₄:CO₂ = 1:1.5).

(a)
CH4 Conversion [%]	CO2 Conversion [%]	H2 Yield [%]	H2 Selectivity [%]	CO Selectivity [%]	H2 Permeability [%]	Permeation Flux [mol/(m2·s)]	Thermal Efficiency [%]
400 °C
0 MPa
12.9	−13.1	1.00 × 10−3	2.02 × 10−3	100	0	0	1.36 × 10−2
0.010 MPa
9.14	−7.46	4.28 × 10−4	8.77 × 10−4	100	58.5	5.00 × 10−3	2.40 × 10−3
0.020 MPa
8.30	−6.19	1.78 × 10−4	3.47 × 10−4	100	0	7.07 × 10−3	2.40 × 10−3
500 °C
0 MPa
16.2	−17.3	2.62 × 10−1	5.24 × 10−1	99.5	1.62	0	2.74
0.010 MPa
7.44	−4.76	5.06 × 10−2	9.48 × 10−2	99.9	1.65 × 10−1	2.50 × 10−3	5.39 × 10−1
0.020 MPa
8.05	−5.69	4.34 × 10−2	8.65 × 10−2	99.9	1.92 × 10−1	3.54 × 10−3	4.62 × 10−1
600 °C
0 MPa
11.1	−5.40	1.69	3.15	96.9	6.22 × 10−1	0	14.8
0.010 MPa
39.2	−49.1	1.12	2.19	97.8	5.19 × 10−2	5.00 × 10−4	9.90
0.020 MPa
25.1	−29.3	7.04 × 10−1	1.33	98.7	1.54 × 10−1	7.07 × 10−4	6.20
(b)
400 °C
0 MPa
9.37	−4.37	1.51 × 10−3	3.03 × 10−3	100	0	0	1.70 × 10−2
0.010 MPa
11.0	−6.02	2.07 × 10−4	4.19 × 10−4	100	48.3	5.00 × 10−3	1.20 × 10−3
0.020 MPa
18.2	−13.2	3.49 × 10−4	6.90 × 10−4	100	28.6	7.07 × 10−3	2.80 × 10−3
500 °C
0 MPa
9.09	−3.33	3.93 × 10−1	7.51 × 10−1	99.2	3.38	0	3.37
0.010 MPa
6.73	−1.60	6.26 × 10−2	1.20 × 10−1	99.9	0	2.50 × 10−3	5.54 × 10−1
0.020 MPa
10.0	−4.92	5.41 × 10−2	1.10 × 10−1	99.9	0	3.54 × 10−3	4.79 × 10−1
600 °C
0 MPa
13.7	−5.73	1.51	2.84	97.2	1.06 × 10−1	0	11.0
0.010 MPa
18.4	−12.1	6.56 × 10−1	1.32	98.7	5.49 × 10−1	5.00 × 10−4	4.78
0.020 MPa
21.4	−14.8	7.97 × 10−1	1.51	98.5	2.76 × 10−1	7.07 × 10−4	5.82
(c)
400 °C
0 MPa
10.0	−2.50	1.34 × 10−4	2.76 × 10−4	100	0	0	1.20 × 10−3
0.010 MPa
11.4	−3.40	5.15 × 10−4	9.89 × 10−4	100	48.6	5.00 × 10−3	2.37 × 10−3
0.020 MPa
12.3	−4.03	2.75 × 10−4	5.56 × 10−4	100	0	7.07 × 10−3	2.47 × 10−3
500 °C
0 MPa
10.1	−2.20	2.57 × 10−1	5.11 × 10−1	99.5	1.46 × 10−1	0	1.82
0.010 MPa
10.0	−2.43	5.15 × 10−2	1.06 × 10−1	99.9	2.43 × 10−1	2.50 × 10−3	3.64 × 10−1
0.020 MPa
8.21	−1.23	5.87 × 10−2	1.27 × 10−1	99.9	0	3.54 × 10−3	4.16 × 10−1
600 °C
0 MPa
22.9	−10.9	1.95 × 10−1	3.68 × 10−1	99.6	2.05	0	1.12
0.010 MPa
18.7	−7.66	4.60 × 10−1	9.44 × 10−1	99.1	4.07 × 10−1	5.00 × 10−4	2.68
0.020 MPa
49.5	−28.1	5.38 × 10−1	1.00	99.0	6.74 × 10−4	7.07 × 10−4	3.12

shows a comparison of the CH₄ conversion, CO₂ conversion, H₂ yield, H₂ selectivity, CO selectivity, H₂ permeability, permeation flux and thermal efficiency for the different reaction temperatures, molar ratios of CH₄:CO₂ and differential pressures between the reaction chamber and the sweep chamber.

According to Table 2, we can see that CH₄ and CO₂ conversion exhibits a positive value and a negative value, respectively. From this result, it can be claimed that reactions consuming CH₄ and producing CO₂ occur. Additionally, it can be seen from Table 2 that the CO selectivity is larger than 90%, indicating that CO is produced more compared to H₂. These phenomena can be explained as follows:

(i)

H₂ is produced as explained by Equation (1) or Equation (5).

(ii)

CO is produced after H₂ is consumed via Equation (2).

(iii)

Carbon and CO₂ are produced after some of the CO is consumed via Equation (6).

Since Equation (6), shown in (iii), is an exothermal reaction, CO can be produced easily. After the experiments in this study, carbon was observed, which can be explained as shown in Figure 14. The mass of the Ni/Cr catalyst utilized in the present study was increased by 9.1 g after the experiments, compared to its initial weight of 74.6 g. This is due to the carbon deposition shown in Equation (6). According to a survey of the literature by us, many previous studies have conducted XPS analyses on the catalyst, e.g., Ni/Al and so on [15,16,17,18,19,20], not the deposited carbon. There are few studies investigating XPS analyses of the deposited carbon. Therefore, we think a lot of the research within this research field potentially does not place emphasis on XPS analyses to identify the nature of carbon deposition. We would like to put aside carbon deposition itself for the purpose of explaining the reaction mechanism in this paper. We think clarification of the nature of carbon deposition in a study belongs to another research field. In the near future, we would like to investigate and discuss the nature of carbon deposition in a new paper. As for the H₂O formation shown in Equation (2), this study has confirmed this phenomenon through observation by means of the gas bag illustrated in Figure 15. The colored part, which is different from the other area in the red circle illustrated in Figure 15, indicates H₂O formation. In addition, the numerical simulation results enabled by the commercial software COMSOL Multiphysics Ver. 6.2, which includes the simulation codes from Equations (1)–(4) alongside a 3D model, indicate H₂O formation. For example, the concentration of H₂O at the outlet of the reactor for a molar ratio of CH₄:CO₂ = 1.5:1 at 400 °C, 500 °C and 600 °C is 6228 ppmV, 28,946 ppmV and 33,614 ppmV, respectively. We would like to report the details of the numerical simulation in the near future.

According to the results and the discussion in this study, we can obtain the highest concentration of H₂ of 122,711 ppmV with CH₄:CO₂ = 1:1, at a reaction temperature of 600 °C and a differential pressure of 0 MPa and using a Pd/Cu membrane with a thickness of 40 μm. Under these conditions, the kinetic rate is 0.86 mol/(m³·s), and the permeation flux is 0 mol/(m²·s). Since the penetration resistance of H₂ is thought to be lower when the thickness of the Pd/Cu membrane is thinner, H₂ is separated well. However, the performance of the Ni/Cr catalyst is not high in the present study, resulting in the H₂ separation rate being too high at a thickness of 20 μm to reach the non-equilibrium state for dry reforming. In other words, it is necessary to match the H₂ separation rate of the Pd/Cu membrane and the H₂ production rate of the Ni/Cr catalyst for a high H₂ yield. Therefore, a thickness of 40 μm is the best thickness in this study since the H₂ separation rate of 40 μm matches the H₂ production rate of the Ni/Cr catalyst under the conditions investigated in this study. In addition, the CH₄ conversion, CO₂ conversion, H₂ yield, H₂ selectivity, CO selectivity, H₂ permeability and thermal efficiency are 13.7%, −5.73%, 1.51%, 2.84%, 97.2%, 1.06 × 10⁻¹% and 11.0%, respectively. We have reviewed previous studies and summarized the reports as well as this study in

Table 3. Comparison of the performance of the membrane reactors reported in previous studies and that of this study.

Membrane Type	Thickness of Membrane [μm]	Catalyst Type	Reaction Temperature [°C]	CH4 Conversion [%]	References
Pd/Ag/Cu	15.6	Ni/CeZrO2	550	65.2	[21]
Pd	17	Ru/ZrO2-La2O3	450	28	[22]
Pd/Ag	75	Rh/CaO-SiO2	550	35	[23]
Pd	50	Rh/γ-Al2O3	800	98	[24]
Pd0.78/Ag0.22 hollow fiber	~1.5	Ni-SiO2/CeO2	600	66	[25]
Pd/Cu	40	Ni/Cr	600	34.6	This study

.

The CH₄ conversion in this study, shown in this table, follows the optimum experimental conditions, providing the highest concentration of H₂. According to the comparison of the CH₄ conversion, we cannot claim that a thinner membrane provides greater CH₄ conversion. It might be thought that the type of combination of the H₂ separation membrane and the catalyst decides the optimum thickness of the H₂ separation membrane, which is the same as in this study. In addition, the performance of the Ni/Cr catalyst utilized in the present study is not high compared to that in previous studies. To enhance the performance of H₂ production, thermal efficiency and CH₄ conversion in the proposed membrane reactor, the present study proposes using other types of catalysts. This study examined a Ni/Cr alloy catalyst. Though Ni is a popular catalyst for DR, Ru is also used for DR [26,27,28]. There are no reports on Ni/Ru/Cr alloy catalysts being used for DR nor membrane reactors. We would like to study using Ni/Ru/Cr catalysts to enhance the performance of DR in the near future.

4. Conclusions

We have investigated, for further understanding, the effect of the thickness of the Pd/Cu membrane on the performance of a biogas dry reforming membrane reactor with a Pd/Cu membrane as well as a Ni/Cr catalyst. The impact of the reaction temperature, the molar ratio of CH₄:CO₂ and the differential pressure between the reaction chamber and the sweep chamber on the performance of the biogas dry reforming membrane reactor with a Pd/Cu membrane as well as a Ni/Cr catalyst has been investigated, varying the thickness of the Pd/Cu membrane. Consequently, the following conclusions were obtained:

(1)

The concentration of H₂ at the outlet of the reaction chamber increases with an increase in the reaction temperature irrespective of the molar ratio of CH₄:CO₂, the thickness of the Pd/Cu membrane or the differential pressure between the reaction chamber and the sweep chamber. Additionally, the concentration of H₂ at the outlet of the sweep chamber also rises with a rise in the reaction temperature.

(2)

The highest concentration of H₂ in the reaction chamber as well as the sweep chamber is obtained for a thickness of 40 μm regardless of the molar ratio of CH₄:CO₂ at a differential pressure of 0 MPa. The concentration of H₂ in the sweep chamber for a thickness of 20 μm is the highest among the investigated thicknesses at a differential pressure of 0.020 MPa. But the concentration of H₂ at the outlet of the reaction chamber for a thickness of 20 μm is the lowest among the thicknesses investigated regardless of the molar ratio of CH₄:CO₂. On the other hand, the optimum thickness of the Pd/Cu membrane to obtain the highest concentration of H₂ in the reaction chamber and in the sweep chamber at a differential pressure of 0.010 MPa is not clear. The optimum thickness is varied with a change in the molar ratio of CH₄:CO₂ at a differential pressure of 0.010 MPa. The reaction mechanism might be complex at a differential pressure of 0.010 MPa. We think the solution for deciding the optimum Pd/Cu membrane thickness at a differential pressure of 0.010 MPa is to adopt a catalyst which has higher performance compared to the Ni/Cr catalyst used in the present study. We have the idea of adopting a Ni/Cr/Ru catalyst instead of a Ni/Cr catalyst.

(3)

The concentration of H₂ at the outlet of the reaction chamber and the sweep chamber relatively increases with a decrease in the differential pressure between the reaction chamber and the sweep chamber regardless of the molar ratio of CH₄:CO₂, respectively.

(4)

The reaction occurring in the present study is proposed as follows: (i) H₂ is produced via the reaction shown in Equation (1) or Equation (5). (ii) CO is produced after H₂ is consumed via Equation (2). (iii) Carbon and CO₂ are produced after some of the CO is consumed via Equation (6).

(5)

We can obtain the highest concentration of H₂ of 122,711 ppmV in the case of CH₄:CO₂ = 1:1, a reaction temperature of 600 °C, a differential pressure of 0 MPa and using a Pd/Cu membrane with a thickness of 40 μm. Under these conditions, the kinetic rate is 0.86 mol/(m³·s), and the permeation flux is 0 mol/(m²·s). In addition, the CH₄ conversion, CO₂ conversion, H₂ yield, H₂ selectivity, CO selectivity, H₂ permeability and thermal efficiency are 13.7%, −5.73%, 1.51%, 2.84%, 97.2%, 1.06 × 10⁻¹% and 11.0%, respectively.