Parallel Passage Reactor (PPR)를 이용한 메탄올 수증기 개질반응

추수태, 남석우, 한종희, 윤성필, 임태훈, 홍성안

한국과학기술연구원, 연료전지연구센터

Methanol Steam Reforming Using Parallel Passage Reactor (PPR)

Soo-Tae Choo, Suk-Woo Nam, Jonghee Han, Sung-Pil Youn,

Tae-Hoon Lim and Sung-Ahn Hong

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST)

Introduction

Hydrogen-fueled automobiles using fuel cells such as polymer electrolyte membrane fuel cells (PEMFCs) are under great development in an effort to reduce atmospheric emission [1,2], since fuel cell-driven vehicles do not emit any environmental pollutants such as NOx, SOx, and hydrocarbons.  However, direct use of gaseous hydrogen in fuel cell vehicle is hindered by problems of hydrogen storage, safety, and refueling that led to the development of efficient hydrogen storage systems or alternative fuels processors from which hydrogen is able to be produced onboard.  Among the many available liquid fuels, methanol is recommended as the most interesting fuel due to rapid dynamic reforming response and high reactivity at relatively lower reaction temperature [3-5].

It has been well known that methanol typically produces hydrogen through two reaction routes as written in the following reaction equations: steam reforming (SR) [Eq. (1)] and partial oxidation (POX) [Eq. (2)].

CH3OH + H2O → 3H2 + CO2     :R1	(1)

CH3OH + 0.5O2 → 2H2 + CO2	(2)

CH3OH + 0.25O2 + 0.5H2O → 2.5H2 + CO2	(3)

CO2 + H2 → CO + H2O	:R2	(4)

The endothermic steam reforming of methanol is well catalyzed over reduced copper-based catalysts by an external heat supply to produce hydrogen. In addition, the exothermic oxidation reaction is faster than the steam reforming reaction over the same catalyst system.  However, these respective reactions remain some great drawbacks to be applicable to the fuel cell vehicles due to their limitation on heat supply, and low hydrogen fraction of reformate that is fed as a composition containing H2-rich gas to anode of fuel cell stack.  In order to overcome these problems, therefore, the combination of two reactions, partial oxidation reforming of methanol, [Eq. (3)], may offer attractive options since an automobile reformer can be designed in a smaller scale with rapid response with respect to system loadings.  The reaction involves producing hydrogen from methanol and water while co-feeding with oxygen.  The ratio of the three reactants can vary and are often chosen so that the overall reaction can be either thermal-neutral or only modestly exothermic.

The present study has carried out as a part of the development of a dual-bed POX reformer for methanol using two commercial catalysts for a reforming and an oxidation reactions.  Prior to the design of a methanol POX reformer, first of all, the structured catalyst system needs to be investigated in order to induce the real design factors for applying the commercial catalysts.  For this purpose, this work has employed metal fiber mat, which is used as material of a burner for surface combustion, as a substrate and a Cu-based catalyst for reforming reaction was applied on the substrate.

Compositions	Iron	Chromium	Aluminium	Yttrium	Silicon
wt.%	Balance	20.0	5.0	∼ 0.1	0.3

Table 1 Chemical compositions of metal fiber mat used in this work

Experimental

A base catalyst employed in this study for methanol steam reforming was a Cu(33%)/Zn(34%)/Al2O3 catalyst supplied from ICI (Imperial Chemical Industries, ICI 53-1).  The fresh catalyst with outer diameter of 5.3 mm and length of 3.6 mm was crushed into fine particles of below 10 □m size by ball-milling to prepare a coating slurry.  For coating with the catalyst, metal fiber mat used as a substrate was supplied from Bekaert Co.  The composition of the mat is listed in Table 1.  A mat module (30 x 70 mm) in PPR was prepared by spray coating of the catalyst on the metal fiber mat.  Five-sheet modules were employed to accomplish a reactor frame.  A schematic diagram of PPR system for the reforming reaction is shown in Figure 1.  Prior to the catalytic performance test, the coated catalyst was pre-treated at 350 oC for 2 h under hydrogen flow of 1,000 cc/min.  Methanol-steam mixture (CH3OH/H2O = 1/1.5) was sprayed into the upper side of the reactor through a pre-heating zone by a digital liquid pump (Waters, Model 590).  The gas composition produced from the reforming reaction was determined by on-line gas chromatography (HP-5890 Series II) using a TCD detector and a carbosphere 80/100, 10x1/8 SS column.  Especially, the reaction conversion of methanol is corrected by the following equations calculated from mass balance of the previous reaction equations, Eq. (1) and Eq. (4), where R1 and R2 are the molar extent of the respective reactions, Qdry is the total flow rate of gas produced from the methanol reforming reaction, and the subscript is CO:

- Qdry + 4 R1 - R2 = 0	(5)

- XCOdryQdry + R2 = 0	(6)

Therefore, the methanol conversion is able to be determined from the measurement of the CO mol fraction and the total flow rate.  The reforming was carried out at the reaction temperature of 250 ∼ 300 oC and the reactor volume of 100 ml under the atmospheric pressure.

Fig. 1 Schematic diagram of parallel passage reactor (PPR) system

for methanol steam reforming reaction.

Results and discussion

Fig. 2	SEM images of metal fiber mat: (a)	fresh (x20), (b) fresh (x2000),	(c) thermal treat-ment (x2000), and	(d) coated with ICI  catalyst (x2000).

As presented in Fig. 2 (a) ∼ (d), this study employed a metal fiber mat as a catalytic substrate for methanol PPR.  The knitted mat was prepared by the spinning  with a yarn made from metal fibers which were composed with chromium, aluminum, and iron in balance, as listed in Table 1.  Due to its flexibility and easy processing, the metal mat is able to provide a reformer with various shapes (cylindrical or flat) and high surface area for coating the reforming catalyst.  Furthermore, the mat is very stable up to 1,000 ℃ enough for the present reaction system of about 300 ℃ temperature.  As depicted in Fig. 2, the width of the fiber was about 50 □m.  For sufficient coating of the reforming catalyst on the mat surface, the particle size of the catalyst needs to be below 10 □m size to get a well dispersed coating.  In this work, ball milling process was carried out sufficiently to obtain a fine particle of the catalyst in proper size at ambient condition over 50 h.  Prior to the application as a substrate, the mat was calcined at 1,000 ℃ for 5 h to develop the roughness on the surface of the fiber, as shown in Fig. 1(c), to increase the coating efficiency of the catalyst.  While the width of the fiber was reduced to about 40 ∼ 45 □m size by the thermal treatment, the mechanical property of the flexible mat maintained in the fresh conditions.  Upon the surface of the thermally treated mat, the reforming catalyst was well dispersed with catalyst loading of about 0.015 g/cm2-mat as shown in Fig. 2 (d).

Figure 3 shows the dry-basis product distribution from the methanol reforming reaction over the PPR with respect to the reaction temperature.  Over the PPR as shown in Fig. 1, hydrogen and carbon dioxide compositions in product gas remained almost constant with respect to the reaction temperature.  Although the CO was relatively a slight amount of 1.5 ∼ 2%, however, its composition increased with the temperature.  Since CO in the reformate should be below 10 ppm for PEMFC, preferential oxidation (PROX) process is required additionally after the reforming system.  The present work provided total downstream flow rate of about 1.0 liter/min at reaction temperature of 300 ℃ under the feed rate of 1.0 cc/min and water to methanol mol ratio of 1.5.

Fig. 3 Product distribution with respect to the	reaction temperature for PPR coated	with ICI catalyst: H2O/MeOH mol	ratio = 1.5/1.0, feed rate= 1.0 cc/min,	and coated weight of catalyst = 1.4	g (●: H2, ■: CO2, and ▲: CO).

References

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