# Variation of the Number of Heat Sources in Methane Dry Reforming: A Computational Fluid Dynamics Study

Authors: Sunggeun Lee; Hankwon Lim
Journal: International Journal of Chemical Engineering
 (2021)

---

## Abstract

To overcome the weak point of the gas type heating (failure in heating uniformly and persistently), liquid type molten salt as a concentration of solar energy was considered as a heat source for dry reforming. This high-temperature molten salt flowing through the center of the tubular reactor supplies necessary heat. The dependence on the number of heat source of the hydrogen production was investigated under the assumption of the fixed volume of the catalyst bed. By changing these numbers, we numerically investigated the methane conversion and hydrogen flow rate to find the best performance. The results showed that the methane conversion performance and hydrogen flow rate improved in proportion to the number of heating tubes. For the one heat source, the reactor surrounded by a heat source rather than that located in the center is the best in terms of hydrogen yield. In addition, this study considered the case in which the system is divided into several smaller reactors of equal sizes and a constant amount of catalyst. In these reactors, we saw that the methane conversion and hydrogen flow rate were reduced. The results indicate that the installation of as many heating tubes as possible is preferable.


---

## Body

## 1. Introduction

Extensive use of hydrocarbons (fossil fuels) as an energy source has resulted in the global increase of temperatures and abnormal climate changes. Recent studies have pointed out that carbon dioxide and methane (more harmful than carbon dioxide) are the chief greenhouse gases. Both the reduction of these greenhouse emissions and the sufficient energy supply to the energy demands let one accelerate to find alternative energy sources. Hydrogen is an attractive candidate that satisfies these requirements. However, complete freedom from carbon emissions seems impossible, and the minimization of such emissions is a challenging problem.There are many known ways to produce hydrogen: reforming, coal gasification, partial oxidation, and electrolysis. In reforming, there are also several fuels to reform, such as methane, ethanol, ammonia, and methanol. Among them, methane is widely used because it can be obtained naturally without much process as shale gas. The methane reforming method is categorized into several types: steam [1], dry (carbon dioxide) [2], partial oxidation [3], and autothermal reforming methods [4]. Methanol and biogas can also be used instead of methane. As for the methods, methane steam reforming is still the cheapest and most well-established reforming method despite its drawbacks, such as sintering and sulfur poisoning.Dry or carbon dioxide reforming has gained considerable research attention as both greenhouse gases, namely, methane and carbon dioxide, are simultaneously reformed into syngas [5–36]. This method produces hydrogen and carbon monoxide with ratios between 1 and 2. However, the most important drawback of this reaction is the formation of coke on the catalyst, reducing its activity and thus requiring the regeneration of the catalyst to maintain its reaction performance. Nickel catalyst was used for methane steam reforming to study carbon deposition on catalyst. 600°C to 800°C temperature range, 0.5, 1.0, and 2.0 of steam to methane ratio, and pressure dependence were investigated. The increase of the pressure and the steam to methane ratio results in the decrease of carbon formation. 800°C and 0.5 steam to methane ratio gave the maximum carbon deposition [37]. Interesting removal of carbon deposits was reported [38] for mixed methane reforming and coke suppression by promoters [39]. Because the production of hydrogen is a highly endothermic reaction, an appropriate amount of heat is needed. This necessary heat is derived mostly from the combustion of natural gases, leading to another emission of greenhouse gases. As such, an alternative method is required to produce heat without the emission of greenhouse gases.The sun is a well-known source of an enormous amount of energy. Almost all the energy that we use could be regarded as coming from the sun. Hence, effective and convenient methods have been developed to transfer solar energy: solar photochemical water splitting (artificial photosynthesis) and solar thermochemical water splitting using a parabolic, trough-type solar receiver [40–42]. In these developments, efficiency is important when one type of energy is transformed into another. For example, efforts to increase the efficiency of solar absorbers using coats of paint have been studied [43].Another way of applying solar energy is to use it to power the reforming processes. Various types of solar reforming were reviewed like ASTERIX, directly irradiated annular pressurized receiver, and CAESER [44], which include reforming by using solar thermal energy [45–51]. A shell-type heat exchanger reactor that circulates heat using a molten salt stored in a tank is discussed for the reforming [52]. Molten salt is a means for storing solar energy as thermal energy. Another well-known method for storing solar energy is in the form of hydrogen formed by the electrochemical splitting of water, called photovoltaic water electrolysis. Studies about solar water splitting have also reported on high efficiency methods, such as 30% solar-to-hydrogen efficiency [53]. Although the temperature produced by concentrating solar energy is relatively high for reforming, an alternative reactor using a membrane can reduce the temperature, which is sufficient for a parabolic trough [54–59].Other solar thermochemical applications for hydrogen production have also been reported [60–64]. Water splitting through concentrated solar energy with an oxygen-permeable membrane was analyzed [65]. To overcome the low-temperature gain in a parabolic trough, a membrane is applied. Studies have also conducted a CFD model analysis of a parabolic trough with a membrane reactor for molten salt reforming [66], especially for molten salt flowing over the outer surface as a heat source [67]. Some researchers have reviewed domestic and industrial applications of parabolic-trough-concentrating solar thermal collectors [68]. In the present model, the required heat was derived from solar energy, which is concentrated and stored in the molten salt.This paper discusses the heat source geometry and its effect on methane conversion. As the heat source is a liquid rather than a gas, its property is easy and it has uniform supply of heat over long distances and its heat is long lasting. We investigated dry reforming because both potential greenhouse gases (methane and CO2) can be reformed simultaneously into useful syngas despite the possibility of coke formation, and this reaction has not yet been commercially developed. Section2 presents the necessary equations and illustrates geometry of the reactor. With this information, the simulation results and discussions are provided in Section 3. The conclusions are presented in Section 4.


## 2. Numerical Methods: Governing Equations

In this section, we introduce the motivation for the study, schematic of the reactor, and mathematical background used for our simulation. Most equations have already been mentioned in literature [69]. The equations in this paper are limited to those required to help understand the present work.As a renewable and sustainable option, solar energy is one of the most attractive sources of energy. In this study, we used molten salt as the heat source for reforming. Figure1 shows the diagrammatic process starting from the source of the solar energy to the reforming reactor. In this study, the green-dashed-line box was the focus of our numerical analysis.Figure 1
Conversion of solar energy to hydrogen. The green-dashed box is the focus of this study.Figures2(a) depicts the frontal view of the reactor, and Figures 2(b) and 2(c) display reactors with one heating tube at the center and an outer shell-type heat source, respectively. Our simulation investigates the methane conversion and hydrogen yield (flow rate) by varying the number of heating tubes and their positions. To maintain consistency, we fixed the total amount (volume) of the catalyst. That is, reducing the radius of the tubes is the only way to increase the number of tubes. The results will be compared with those obtained by placing a single heating tube at the center. There are two cases for a single heating tube depending on its location: the center and outer shell.Figure 2
(a) Frontal view of the reactor and schematic of reactors with (b) one heating tube at the center and (c) outer shell-type heating.Figure3 shows the frontal views of the three geometries simulated to which our numerical simulation was applied. We classify the geometries into three classes and denote them as 1, 2, and 3. In geometry 1, the number of the heating tubes varied from two to eight (note that seven is not available due to the irrational number: 360°/7). We studied the effect of changing the distance between the heating tubes (or in other words, the distance of heating tubes from the center of the reactor). Rotational symmetry along the cylinder axis was used in the construction of the geometry for convenience. A more general (without rotational symmetry) study might be appropriate for further study. The radius of the heating tubes becomes smaller as the number of tubes increases. In geometry 2, one of the heating tubes is located at the center of the reactor, while the remainder surrounds its center. The simulation was conducted by varying the distance between the heating tubes. In geometry 3, only the positions of the heating tubes marked by arrows were varied.Figure 3
Frontal view of each reactor representing geometries 1, 2, and 3.The reactor specifications, such as size and radius, are listed in Table1.Table 1
Reactor specification.
Reactor length0.2 mReactor radius0.052 418 83 mHeating tube radius0.020 976 m (one), 0.014 83 m (two), 0.012 11 m (three), 0.010 488 m (four), 0.009 38 m (five), 0.008 563 m (six), 0.007 928 m (seven), 0.007 416 m (eight)Thickness of insulator0.005 mReactor radius for outer shell-type model0.048 040 4mOuter radius of shell-type model0.0564 588 m (0.008 418 4 m thick)The governing equations applied in this study are summarized as follows [69]. Note that the current study is focused on steady state only.Heating tube is as follows.Heat transport in the heating tube is formulated as(1)∇⋅−kht∇T+ρCpu⋅∇T=0.The molten salt flows are described by the Navier–Stokes equations, that is, (2) and (3). In this flow, laminar flow is assumed without loss of consistency:(2)ρu⋅∇u=∇⋅−pI+μ∇u+∇uT−2η3∇⋅uI,(3)∇⋅ρu=0.The reaction is highly endothermic and needs an insulating jacket to reduce the heat loss. The reactor covered by such an insulator satisfies the following:(4)∇⋅−ki∇T=0.Reformer bed is as follows.The behavior of the porous media is usually described by the Ergun equation [70], which is well suited to experiments for both low and high Reynolds numbers. In the present simulation, we applied a low Reynolds number by assumption, and the flow was described through Darcy’s law, according to which the velocity and pressure satisfy the following relation:(5)u=−κrefμref∇p.The equation of continuity is derived asfollows:(6)∇⋅ρu=Qm.The reformer bed satisfies heat capacity given by(7)ϵrefρCpf+1−ϵrefρCps=0,where subscripts f and s represent the discriminate fluid and solid, respectively.The thermodynamic law describing the reformer bed is given as(8)∇⋅−kref∇Tref+ρCpfu⋅∇Tref=Q.Heat exchange occurs during the reaction, and heat generationQ is given by rΔHr. A heat source with reaction rates r1 and r2 and the corresponding enthalpies H1 and H2 satisfy(9)∇⋅−kref∇Tref+ρCpfu⋅∇Tref=−r2⋅H1−r2⋅H2.The diffusion of species satisfies the Maxwell–Stefan diffusion equation [70]. Inside the reformer filled with the catalyst, the diffusion is described as(10)ρv⋅∇ωi+∇⋅−ρωi∑j=1nDij∇xj+xj−ωj∇pp−DiT∇TT=Ri,where xi is the mole fraction for species i and is formulated as xi=ωj/Mj∑jωj/Mj−1. The thermal diffusion coefficients are denoted by DiT, and the source term for each species is represented by Ri. The coefficients Dij represent the binary diffusion coefficients [70] depending on σij and ΩDij. Here, σij is the spherical molecule Lennard–Jones diameter of the spherical molecule and ΩDij is the collisional integral, which is dimensionless for molecular diffusion. Binary coefficients are explicitly formulated as(11)Dij=0.0018583T31Mi+1Mj1pσij2ΩDij,where Mi is the molecular weight of the species and Dij is temperature dependent; nevertheless, we assumed that these values are constant and calculated at 800 K. The above-mentioned source term is given by Ri=−r1MCH4+2r1MH2−r2MH2−r1MCO2−r2MCO2+2r2MCO+r2MCO. The convective flux is expressed as follows at the outlet:(12)n⋅−ρωi∑j=1nDij∇xj+xj−ωj∇pp−DT∇TT=0.The lists of parameters used in numerical simulation are summarized in Table2 [69].Table 2
Parameters used in numerical studies.
Parameters in the equationsuin,ht1m⋅s−1Heating tube inlet velocityTin,ht1000KHeating tube inlet temperaturepin,ref75PaReformer bed inlet pressurepref1.0×105PaReference pressureTin,ref800KReformer bed inlet temperaturehht100W⋅m−2⋅K−1Heat transfer coefficienta (heating tube)hj1W⋅m−2⋅K−1Heat transfer coefficientb (insulating jacket)Binary diffusion coefficients (800 K), Lee et al. [69, 71]ωCH4,in0.15Weight fractionsωH2,in0.005ωCO,in0.00001ωCO2,in0.42ωAr,in0.42999ρj24kg⋅m3DensityaCp,j1.9J⋅kg−1⋅K−1Heat capacitya of insulating jacketkj0.027W⋅m−1⋅K−1Thermal conductivityaCp,ref2800J⋅kg−1⋅K−1Heat capacitybkref0.1W⋅m−1⋅K−1Thermal conductivitybμb2.7×10−5Pa⋅sViscosityc of reformer bedϵref0.25Porosityκref1.0×10−9m2PermeabilitydH1247×103J⋅mol−1Enthalpy of reaction 1H2412×102J⋅mol−1Enthalpy of reaction 2aWe considered this value; for various values, see https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html andfor polyurethane foam,https://www.nuclear-power.net/nuclear-engineering/heat-transfer/heat-losses/insulation-materials/polyurethane-foam/. bhttps://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html, chttps://www.engineeringtoolbox.com/gases-absolute-dynamic-viscosity-d_1888.html, and dhttps://tel.archives-ouvertes.fr/tel-00848600/document.COMSOL Multiphysics 5.5 is used for solving these equations.


### 2.1. Reaction: Methane Dry (CO2) Reforming

In this study, we considered methane dry reforming, which may be expressed as follows [72]:(13)CH4+CO2⟶2CO+2H2,ΔH=247.3kJmol−1r1=k1⋅KCO2KCH4PCO2PCH41+KCO2PCO2+KCH4PCH42⋅1−PCOPH22K1PCH4PCO2,(14)CO2+H2⟶CO+H2O,ΔH=41kJmol−1r2=k2PCO2⋅1−PCOPH2OK2PCO2PH2.Rhodium catalyst, 0.5 wt.-% Rh/Al2O3 catalyst, was used for the range 600°C–800°C. The rhodium catalyst was selected because no carbon formation was shown and the deactivation of rhodium catalyst decreases as temperature increases.Here, k1 and k2 are the rate constants for each reaction, and KCO2, KCH4, K1, and K2 are the equilibrium constants. They are explicitly written as follows:(15)KCO2=2.64×10−7e376.41J⋅mol−11/RTPa−1KCH4=2.63×10−7e406.84J⋅mol−11/RTPa−1k1=6.45×106e−102.065J⋅mol−11/RTmol⋅s−1⋅m−3k2=9.28×10−2e−731.05J⋅mol−11/RTmol⋅s−1⋅Pa−1⋅m−3K1=1010×e34.011⋅e−258.598J⋅mol−11/RTPa2K2=68.78e−37500.3J⋅mol−11/RT


## 2.1. Reaction: Methane Dry (CO2) Reforming

In this study, we considered methane dry reforming, which may be expressed as follows [72]:(13)CH4+CO2⟶2CO+2H2,ΔH=247.3kJmol−1r1=k1⋅KCO2KCH4PCO2PCH41+KCO2PCO2+KCH4PCH42⋅1−PCOPH22K1PCH4PCO2,(14)CO2+H2⟶CO+H2O,ΔH=41kJmol−1r2=k2PCO2⋅1−PCOPH2OK2PCO2PH2.Rhodium catalyst, 0.5 wt.-% Rh/Al2O3 catalyst, was used for the range 600°C–800°C. The rhodium catalyst was selected because no carbon formation was shown and the deactivation of rhodium catalyst decreases as temperature increases.Here, k1 and k2 are the rate constants for each reaction, and KCO2, KCH4, K1, and K2 are the equilibrium constants. They are explicitly written as follows:(15)KCO2=2.64×10−7e376.41J⋅mol−11/RTPa−1KCH4=2.63×10−7e406.84J⋅mol−11/RTPa−1k1=6.45×106e−102.065J⋅mol−11/RTmol⋅s−1⋅m−3k2=9.28×10−2e−731.05J⋅mol−11/RTmol⋅s−1⋅Pa−1⋅m−3K1=1010×e34.011⋅e−258.598J⋅mol−11/RTPa2K2=68.78e−37500.3J⋅mol−11/RT


## 3. Results and Discussions

We investigated the effect of the geometry of the heating tube on methane dry reforming while maintaining a fixed total volume of the catalyst or heating tube. Thus, the radius of the heating tube decreases with the increase in the number of heating tubes. We attempted to arrange the heating tubes as symmetrically as possible and identified the designs that offer good methane convergence. The effect of the placement of the heating tubes in more general positions is of particular interest.The following is a comment about model validation and mesh independence. Before presenting the results and discussion, we briefly comment on the model validation and grid mesh independence of our model. As our simulation is based on a previous study [69], which has discussed these two steps, we have omitted these procedures.The temperature distribution of the reactor for geometry 1 is shown in Figure4. In a previous study [73], we determined the counter-current flow (the flow directions of heat and feedstocks are opposite) to be more relevant by comparing the methane conversion between the cocurrent and counter-current flow of the heating tube. Therefore, in the present study, we considered only the counter-current flow. A darker red color shows a higher temperature than the lighter red or blue color.Figure 4
Temperature distributions (sliced perpendicular (first and second line) and parallel (third and fourth line) to the axis) according to the number of heating tubes for geometry 1: the cases in which the distance between the heating tubes is closest (first and third line) and farthest (second and fourth line).As mentioned earlier, dry reforming is an endothermic reaction and requires a high temperature. As the area increases, the temperature volume increases (as the darker red region becomes darker) and the reaction becomes more violent than the adjacent lower temperature region. The figure shows that the darker red colored region increases in area as the number of heating tubes increases (to the right direction in the figure), and the heating tubes are located farther from the center (down the figure). Thermodynamically, uniformly distributed high temperatures (without much temperature gradient) promote better performance of the reactor. To achieve this, the geometrically optimized heat supply of the reactor must be determined. Figure4 indicates the conversion of methane and flow rate of hydrogen.The conversion of methane according to the number of heating tubes in geometry 1 is shown in Figure5. Here, the positions mean the distance between the centerlines of both inner and outer cylinders. Subsequent positions are determined by increasing the distance between the heating tubes or the distance from the center. First, the overall methane conversion is proportional to the number of heating tubes. In each case, however, the rate of increase is different. For a smaller number of heating tubes, the rate of increase decreases as the position of the heating tubes from each other increases. A two-heating-tube configuration has a turning point where the conversion of methane peaks and subsequently decreases.Figure 5
CH4 conversions for different heating tube configurations.The hydrogen yield/flow rate has almost the same behavior as methane conversion, as shown in Figure6.Figure 6
H2 flow rate according to the number of heating tubes.Figure3 shows two or three types of geometries for a given number of heating tubes. Because of the computational limitations due to error, the full range of data for geometry 1 is not yet available. Figure 7 shows the methane conversion and hydrogen flow rates in these geometries. In the case of four heating tubes, geometry 3 showed the highest methane conversion and hydrogen flow rate at the common position of 0.026 m. However, the results in Figures 5 and 6, which display a lower rate of increase, show that geometry 1 yields the highest performance. Geometry 2 showed the lowest performance among the three geometries. Such patterns apply to all the scenarios depicted in Figure 7.Figure 7
CH4 conversion and H2 flow rates with varying the number of heating tubes: the number of heating tubes increases from top to bottom (4, 5, 6, and 7) for each geometry.
(a)(b)In Figure7, both the methane conversion and hydrogen flow rate for each geometry increase as the number of heating tubes does. Moreover, both of them in geometry 3 are the highest for almost all of the cases and followed by geometry 1 and geometry 2 in turn. We see that the flow rate and the conversion grow as the position becomes large. This means that the performance becomes increased as the heating tubes become farther from the center within our simulation. However, we see that for the same distance, for example, 0.024 m, the conversion increases as the number of heating tubes but decreases for 7 numbers of heating tubes.These results can be compared to the results for a single heating tube. This case can be divided into two cases: the one case in which the heat source is located along the center and the other case in which the reactor is surrounded by a shell-type heat source. The numerical analysis of methane conversion for a single heating tube through the center is approximately 34.8% and the hydrogen flow rate is approximately2.8857×10−4mol/s. This is the lowest value obtained in this study. The methane conversion rate obtained when heat was supplied from the outer surface was 69.95%, and the hydrogen flow rate was 4.1165×10−4mol/s. The methane conversion rate of the outer shell heat source corresponds to the case where the distance was 0.032 m; however, the hydrogen flow rate of the outer shell-type heat source surpasses all the above-mentioned results. We applied the same amount of heat and catalyst for the outer shell-type heat source, effectively reducing the reactor size and leading to a more enhanced hydrogen flow rate.Let us consider the average velocity of the molten salt in each geometry. As demonstrated earlier, the radius of the heating tube decreases as the number of heating tubes increases. However, the average velocity of flow remains the same for all the studied cases. This may be due to the fact that the reactor tube in the study is too short for the velocity difference to discriminate manifestly.Hydrogen permeation membranes are used in the process in order to lower the temperature and separate the hydrogen. However, the scope of our study was limited to the geometrical variations. It remains to be determined where to place the membrane and which type of membrane we use.In another view of the problem, a reactor was divided into several smaller, separated reactors, and each reactor had one heating tube. Figure8 displays the concept of the separated reactors. When the reactor size needs to be reduced, it can be done by dividing one reactor into smaller units. There are many ways to develop this idea by considering the arrangement of the heat source. For instance, five types are shown in Figure 9, applicable to six reformers. Among them, the most practicable is shown in Figure 9(a), and this design of the heat source gives the most uniform heat distribution to all reactors with the least heat loss. Moreover, it is the most compact size, for both the given space and the given number of reactors.Figure 8
One reactor with a number of heating tubes is divided into the same number of reactors with one heating tube with an equal amount of catalyst.Figure 9
Heat source configurations for the realization of Figure8. The arrows indicate the flow direction.
(a)(b)(c)(d)(e)Owing to the many advantages of the reactor shown in Figure9(a), we applied that type of heat source to the following reformer. Figure 10 shows two types of reactors. Figure 10(a) (especially the green-dashed box) was studied in the previous segment, and Figure 10(b) is a new design based on the results indicated in Figure 9. The reactor in Figure 10(a) is divided into smaller reactors (shown in Figure 10(b) inside the dark dashed green box), keeping the catalysis volume and number of heating tubes constant.Figure 10
Realization of Figure8 (especially dark dashed green box in Figure 10(b)) for six reactors based on the heat analysis results of Figure 9. Green-dashed box in (A) was studied in detail in this manuscript.The methane conversion and hydrogen flow rate for Figure10(b) are shown in Figure 11. Compared with Figure 5, Figure 11 shows that the conversion and flow rates are similar to the case of which distance of the tubes is 0.024m. Therefore, this type of reactor performs worse than a single reactor with a varying number of heating tubes does.Figure 11
Methane conversion (left) and hydrogen flow rate (right).Molten salt can be used as a heat source in fluidized bed type (uniform mixing and temperature gradients but large reactor scale) or bubble column reactor (high heat and mass transfer). Our model, however, compared to these two reactors (which is not well understood due to complexity of reaction), is more tractable in that our model can be constructed in smaller reactor scale which raises heat transfer and more well-known reaction. Especially, our reactor overcomes gas type heat source using liquid (molten salt) because liquid has higher heat capacity and carries longer distance than gas. Our reactor is a collection of advantages of fluidized bed or column reactor and packed bed reactor.Interesting experimental and CFD study was investigated for the pressure drop of the packed bed reactor [7]. The reactor is filled with porous catalysts particle with pore size ranging from 0.2 mm to 2.0 mm. The increase of the pore size from 0.2 mm to 2.0 mm results in decreasing of the pressure drop by more than 3 times. In (1) in [7], we expect that large porosity approximation leads to v2 dependence in Δp and vice versa. In our simulation, we fixed the inlet and outlet pressure as a boundary condition shown in the table and simplified the fluid motion without catalysts structure. It would be interesting if we use velocity boundary condition instead of pressure one to study pressure drop.


## 4. Conclusions

In a reactor, the maintaining of a uniform high-temperature distribution is important for high methane conversion and hydrogen yield rates. Because of its highly endothermic nature, natural gas reforming for the generation of hydrogen requires heat. Compared to the gas, the liquid has larger specific heat capacity than the gas does. Then, the liquid heat source is superior to the gas one in that it heats up the reactor longer and more uniformly. This property makes it possible to design various reactors from small in sizes to their complicated structures as illustrated in the manuscript. We might expect that this improves the performance of the packed bed reactor for various designs.In this study, we used a liquid (molten salt) as the heat source because it has several advantageous over gaseous sources. The total mass of the heated molten salt in each model was kept unvaried (by keeping the total volume constant) while the number of heating tubes varied. The results indicate that it is better to divide the heat source as much as possible and distribute them uniformly inside the reactor to transfer heat uniformly.With the help of commercial COMSOL Multiphysics 5.5 software, we numerically verified that both the methane conversion and hydrogen flow rate are proportional to the number of heating tubes. For one heating tube, the reactor surrounded by the heat source surpasses all the reactors studied in the hydrogen yield.In addition, we considered dividing the reactor into smaller reactors, each with a heating tube, and the total sum of the catalyst in each reactor was equal to that before division. In this case, the methane conversion and hydrogen flow rate corresponded to the results of the reactor before division, in which the heating tubes are separated from each other by a particular distance. However, the overall performance of the separated reactors is lower than that of the one integrated reactor.Therefore, considering both the methane conversion and hydrogen yield, we suggest that installing as many heating tubes as possible is preferable when using liquid (molten salt) as the heat source in a packed bed reactor rather than separating the reactors into many pieces, even though the reactor surrounded by the heat source is the best in terms of hydrogen yield.


### 4.1. Summarizing

(i)
We used COMSOL Multiphysics to study a packed bed reactor of methane dry reforming with liquid heat source-molten salt. Replacing the gas type heat source by liquid molten salt brought together the advantages in fluidized bed or bubble column reactors having high heat transfer and packed bed reactor well understood. This liquid type heat source makes it possible to construct reactor in small scale. Using solar energy as a heat source gives us almost free of CO2 emission compared to gas type heat source.(ii)
Hydrogen production is proportional to the number of heating tube while the outer shell-type heating is better for one heating tube.(iii)
Occurrence of catalysts fowling forbids continuous working of reactor. There is a limitation in heat transfer compared to the fluidized bed. Changing the shape of the reactor, for example, like frustum of cone shape reactor with frustum heating tubes and finding best performance, would be an interesting study. Shell-type heating will be a good challenge because we have seen in this work that outer shell heating is best for one heating tube case. In addition, methane pyrolysis in packed bed type would be a future study for the application of reactor.


## 4.1. Summarizing

(i)
We used COMSOL Multiphysics to study a packed bed reactor of methane dry reforming with liquid heat source-molten salt. Replacing the gas type heat source by liquid molten salt brought together the advantages in fluidized bed or bubble column reactors having high heat transfer and packed bed reactor well understood. This liquid type heat source makes it possible to construct reactor in small scale. Using solar energy as a heat source gives us almost free of CO2 emission compared to gas type heat source.(ii)
Hydrogen production is proportional to the number of heating tube while the outer shell-type heating is better for one heating tube.(iii)
Occurrence of catalysts fowling forbids continuous working of reactor. There is a limitation in heat transfer compared to the fluidized bed. Changing the shape of the reactor, for example, like frustum of cone shape reactor with frustum heating tubes and finding best performance, would be an interesting study. Shell-type heating will be a good challenge because we have seen in this work that outer shell heating is best for one heating tube case. In addition, methane pyrolysis in packed bed type would be a future study for the application of reactor.



---

*Source: 4737513-2021-11-24.xml*