The Influence of Inlet Geometry on the Performance …contemporary-energy.net/Articles/v02n02a03_-Ivana...The Influence of Inlet Geometry on the Performance of a Methanol Steam Reforming

International Journal of Contemporary ENERGY, Vol. 2, No. 2 (2016) ISSN 2363-6440 ___________________________________________________________________________________________________________

___________________________________________________________________________________________________________ I. Ivanović, A. Sedmak, M. Milošević: “The Influence of Inlet Geometry on the Performance of a Methanol Steam Reforming …”, pp. 22–28 22

DOI: 10.14621/ce.20160203

The Influence of Inlet Geometry on the Performance of a Methanol Steam Reforming Reactor

Ivana Ivanović1*, Aleksandar Sedmak2, Miloš Milošević1

1Innovation Center, Faculty of Mechanical Engineering, University of Belgrade

Kraljice Marije 16, 11120 Belgrade 35, Serbia; [email protected] 2Faculty of Mechanical Engineering, University of Belgrade

Kraljice Marije 16, 11120 Belgrade 35, Serbia Abstract This study deals with geometry of the inlet part of a packed bed methanol steam reforming reactor, which is a component of the specific high temperature indirect internal reforming polymer electrolyte membrane fuel cell stack. Important elements of the reactor geometry are predefined as well as the inlet and the outlet boundary conditions, the catalyst volume and properties; it remains to be examined how the rest of geometry, a geometry that could be changed, should be modelled to achieve desired efficiency. In this initial stage of analysis, the flow was treated as steady, laminar, incompressible, with inserted porous media, with constant temperature and without chemical reaction. It was concluded that applied changes of the inlet channel and the inlet chamber geometry have no significant influence on pressure drop trough the reactor. Possible corrections of the inlet geometry, which result in more favourable flow distribution, were proposed.

1. Introduction The research in the field of polymer electrolyte membrane fuel cell (PEMFC) is intensifying in recent years. According to results so far it seems that PEMFCs are efficient clean portable energy source if continuous supply of hydrogen as fuel is ensured. The supply of hydrogen must be resolved in such a way to produce minimum carbon monoxide as a pollutant by-product. It is evident that necessity of supplying PEM fuel cells with hydrogen lunched a number of parallel series of studies related to methanol steam reforming in micro reactors, as methanol is known to be a good source of hydrogen.

The main differences in the approach to methanol steam reforming is where the process take place. It can be external, in the reformer which is independent of the fuel cell, or, it can be internal, direct or indirect. In the internal reforming fuel cell, heat of the system is used for reforming process which is endothermic. In the direct internal reforming fuel cell methanol, or other fuel, is supplied directly to the anode where the catalyst is placed. In the indirect internal reforming fuel cell, which is the subject of this study, the reformer exists, and it is placed in the fuel cell stack as well as other devices necessary for the reforming process.

Upon review [1], the first study analyzing reforming in indirect internal reforming high temperature fuel cell was an experimental study published in 2005 in Ref. [2]. The stack in Ref. [2] was composed of reformer, made of aluminum and packed with CuO/ZnO/Al2O3 catalyst, and two high temperature PEMFCs. Methanol water mixture was supplied from evaporator which was placed externally, outside of fuel cell stack. The reforming temperature was between 180-200℃. Resulting performance was lower than in the case of the same fuel cell stack directly supplied with the mixture of H2 and CO2.

Keywords: Methanol steam reforming reactor; Heterogeneous catalyst bed; Laminar flow; Computational fluid dynamics

Article history: Received: 15 April 2016 Revised: 31 October 2016 Accepted: 04 November 2016



The way in which packed bed micro methanol steam reformers developed during last decade can be seen from experimental and numerical studies given in Refs. [3] – [7]. In the work of Pattekar and Kothare from 2005 (Ref. [3]) the radial micro rector with integrated vaporizer was presented. The reactor was packed with commercial Cu/ZnO/Al2O3 catalyst as in Ref. [2]. Experimental study and numerical study, which used in-house FORTRAN code, were performed. It was demonstrated that that reformer produced hydrogen for up to 20W of power. The same type of the reformer was studied in Ref. [4]. Kinetics of the model and pressure drop are calculated using in-house Matlab code. The goal was to produce hydrogen for 24W and 72W of power which was achieved. The analysis of carbon monoxide as by-product was also presented in Ref. [4].

Design, extensive experimental analysis, and 3D thermal analysis of real micro reactor are presented in Ref. [5]. Copper based catalyst was again used in the form of packed bed. Special attention was paid to heat transfer and insulation of the reforming system. The study presented in Ref. [6] from 2015 is noteworthy since it gives an extensive experimental and CFD analysis of three packed bed reformers with different geometry: multi-channel, radial, and tubular. Full 3D CFD analysis with power low kinetic model was performed and results were compared to experimental results. Another comprehensive experimental and complete 3D CFD analysis of plate-type micro methanol steam reforming reactor is given in Ref. [7].

In this study the 3D CFD analysis was performed in order to determine the influence of free flow area geometry of the micro methanol steam reforming reactor on velocity distribution and pressure drop in packed bed reaction chamber of the reactor.

2. Model description The external dimensions and shape of the reformer stack correspond to the dimensions of the fuel cell stack. It is composed of three plates, illustrated in Figure 1a. A middle plate, with a thickness of 4 mm, contains a reaction volume and inlet and outlet channels. The initial geometry of the reaction volume with channels is presented in Figure 1b. The dimension of the reaction volume is 34×37.4×4 mm. The catalyst bed in the reaction volume is expected to be separated from the channels with stainless steel mesh. Diameters of the inlet and the outlet are 3 mm.

In the changeable part, there is 3.5 mm in length between the center of the inlet and the steel mesh, and 5.5 mm between the steel mesh and the center of the outlet. The inlet channel, or the outlet channel, consists of vertical pipe like geometry that leads to, or from, small chamber.

The reforming reactor is placed between fuel cell stack and the thick insulation wall. It is expected that it will be heated by the fuel cell and that the temperature of the top surface of the upper plate of the reactor will be between 180 and 240 ℃. When the heat transfer is included into calculations, with the constant temperature at the top boundary surface, insulation at the bottom boundary surface, and the flux to the surroundings at side boundary surfaces, temperature differences are very small, approximately 0.5 ℃, mostly at edges and corners of the plates. According to this results, heat transfer was excluded from calculations, temperature of the system is treated as constant with the value of 180 ℃.

The methanol-water mixture ratio 1:1.3, and flow rate at the inlet is from 2.923⋅10-6 m3/s and below. The catalyst particle diameter is 300 μm, bulk density is around 1.1 g/mL, and catalyst density is around 4.7 g/mL.

Figure 1. Methanol steam reformer stack (a.) and the initial geometry of the reaction volume with inlet and outlet channels (b.)



3. Numerical modelling In this first series of calculations chemical reaction have not been taken into consideration. Stainless steel mesh, which holds catalyst in place, was also excluded from geometry.

The flow was modeled as steady, laminar, viscous, with inserted porous media. Free flow in the inlet and the outlet channel is given by incompressible continuity equation ∇ = 0 (1)

and steady incompressible Navier-Stokes equations ∇ = ∇ − + ∇ + ∇ (2)

Flow in reaction volume, i.e. in the inserted packed bed catalyst porous media, is given by Brinkman equation ∇ = ∇ − + ∇ + ∇ (3)

where ε and k are porosity and permeability respectively. When heat transfer is included into calculations Temperature of the fluid was set to constant value of 180 ℃. The density of the methanol-water mixture was calculated from the ideal gas low relation = RT (4)

Molar mass of ideal gas mixture is given as sum of molar masses of its components = ∑ (5)

where xi are mole fractions of methanol and water.

Flow rate was imposed as laminar inlet boundary condition, and zero gage pressure was imposed at the outlet.

4. Results and discussions Simulations were carried out on desktop PC with Intel Core i5-2300 CPU on 2.8 GHz and 16 GB RAM memory. Since the heat transfer of the system has not been taken into consideration only the inner volume geometry of the reformer was used.

4.1. Results for initial geometry

The first set of calculations for the initial geometry were executed for inlet flow rates of 2.923 10-6 m3/s,

1.949⋅10-6 m3/s, and 0.974⋅10-6 m3/s. To get the feel of the flow, the streamlines of the flow field for the highest inlet flow rate are illustrated in Figure 2.

The fully developed laminar flow enters through a vertical pipe to a reformer chamber very close to a packed bed catalyst, continues through the catalyst porous media, end exits through an elbow of a horizontal channel, through the channel, and through vertical pipe. The dominate feature of the central part of the inlet chamber flow is recirculation (see Figure 2). Recirculation is present in the case of all inlet flow rates as illustrated in Figure 3, and, as expected, weakens with decreasing of flow rate intensity.

There will be more discussion about velocity field further in the text; important for this group of simulations is influence of inlet flow rate on pressure drop, which develops mainly in the porous media. It can be seen from Figure 4 that difference in pressure at the exit from the porous media is negligible compared to the difference at the entrance. As expected, higher inlet flow rate produces higher pressure in the inlet channel and consequently larger pressure drop in porous media. The flow rate difference of approximately 1⋅10-6 m3/s results in a pressure difference of approximately 40 Pa.

The gage pressures in the inlet and the outlet channel are illustrated in Figure 5 to demonstrate that the pressure differences in these parts of reformer are negligible compared to pressure difference in porous media. It can be seen from Figure 5 that the order of magnitude of these pressure drops is 10-2 Pa.

4.2. Potential corrections in inlet geometry

The fact is that structure of the flow field in the inlet and the outlet channel must depend, among other, on their geometry. Previously, it was demonstrated that the geometry of the inlet channel results in recirculation, and, that the pressure drop in two channels is negligible compared to the pressure drop in the porous media.

Some changes of the inlet channel are introduced and their influence on the flow field were examined. These changes were mostly focused on the recirculation.

First, the sidewalls of the inlet channel are rotated to tangent the inlet pipe. Thickness of the initial inlet channel is 4mm, and it has been narrowed so that the whole bottom surface was raised 2 mm, (b, Figure 6).

The simulations were executed only for the highest inlet flow rate. The recirculation was still present in the flow of the inlet channel.

As illustrated in Figure 7, pressure closest to the top of the chamber in the entrance region of the porous media is insignificantly higher than the pressure in the case of basic geometry. The pressure at the other levels is now



Figure 2. Streamline presentation of the velocity field for the inlet flow rate of 2.923⋅10-6 m3/s

Figure 3. Streamlines in the inlet channel and at the entrance to a porous media for inlet flow rates 2.923⋅10-6 m3/s (left), 1.949⋅10-6 m3/s (center), and 0.974⋅10-6 m3/s (right) at cross section y = 0

Figure 4. Values of velocity (left) and gage pressure (right) for three different inlet flow rates along the porous media in the cross-section y = 0 at z = 0.002 m



Figure 5. Gage pressure in the inlet channel (left) and in the outlet channel (right) for the highest inlet flow rate in the cross-section y = 0 at z = 0.002 m

Figure 6. Initial inlet geometry (a) compared with a change in the form of step (b), and a step combined with curved bottom surface (c)

Figure 7. Pressure for the step change in the entrance region of the porous media at different levels in cross section y = 0 compared with the pressure for the highest flow rate of the initial geometry at level z = 0.002 mm



Figure 8. Streamline presentation of the velocity field in the case of inclined step surface for the inlet flow rate of 2.923⋅10-6m3/s

a. b.

Figure 9. Velocity magnitude values in z cross sections of the inlet channel for initial geometry (a)

and for the step geometry (b) in the case of the highest inlet flow rate influenced by the presence of the step. This influence is obvious in first few millimeters of the length where the pressure rapidly approaches the pressure of the initial geometry.

In further attempt to improve geometry of the inlet channel, the inclined wide cylindrical form has been cut from the step geometry (see c, Figure 6). As illustrated in Figure 8, the result was flow liberated from recirculation. In addition, the influence of the step that was illustrated in Figure 7 was less apparent, but the pressure was insignificantly lower than the pressure in the case of original geometry.

The impression is that the geometry of the inlet channel has to be somewhere in between changed geometries

and the initial geometry. From Figure 9 left, it is obvious that the flow in the case of initial geometry is not evenly distributed at all levels of the inlet chamber. The best distribution is in the vicinity of the cross-section z = 1.5 mm, almost at the bottom of the reformer. There is a large gap caused by recirculation at z = 2.5 mm, near the entrance to a chamber, and the gap near the top wall is even larger (bottom left, Figure 9).

Step geometry is illustrated in Figure 9 right. As mentioned step starts at z = 2 mm. Velocity magnitude is higher according to shallower space of the chamber. There is the space with very high velocity near the entrance at level z = 2.5 mm but the flow in whole inlet chamber is better distributed than in the case of initial geometry.



5. Conclusions In this work, only flow field in the reformer was analyzed, heat transfer and chemical reaction were excluded from calculations. The flow field was examined for three different inlet flow rates and two changes in the inlet chamber geometry. It was demonstrated in which way changes in geometry of the inlet chambers influence flow field distribution and pressure drop.

Presented study is less then initial for this type of problems. In further studies heat transfer and chemical reaction must be introduced in the calculations. This includes numerous parameters with uncertain values which indicates that sensitivity analysis must be performed.

Acknowledgements This research was carried out under the NATO Science for Peace Project EAP.SFPP 984738, and the Ministry of Education and Science of the Republic of Serbia project III 43007 The authors gratefully acknowledge colleagues from Slovenian National Institute of Chemistry for help in resources as well as in suggestions and comments.

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[3] A. V. Pattekar and M. V. Kothare, "A radial microfluidic fuel processor," Journal of Power Sources, vol. 147, p. 116–127, 2005.

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The Influence of Inlet Geometry on the Performance …contemporary-energy.net/Articles/v02n02a03_-Ivana...The Influence of Inlet Geometry on the Performance of a Methanol Steam Reforming