major pollution sources. As a pollution2free and en2 ergy2saving power supply for electric vehicles, the fuel cell is the best candidate because of its high en2.
第 23 卷 第 6 期 Vol. 23 No. 6
催 化 学 报
2002 年 11 月
November 2002
Chinese Journal of Catalysis
Article ID : 025329837 (2002) 0620491202
Microchannel Reactor f or Methanol Autothermal Ref orming CHEN Guangwe n , YUAN Quan , L I Shulian ( Dalian Instit ute of Chem ical Physics , The Chi nese A cadem y of Sciences , Dalian 116023 , L iaoni ng , Chi na)
Key words : microchannel reactor , met hanol , hydrogen , proton exchange membrane , fuel cell , autot hermal reforming CLC number : O643/ TQ03 Document code : A
Automotive exhaust is currently one of t he major pollution sources. As a pollution2free and en2 ergy2saving power supply for electric vehicles , t he fuel cell is t he best candidate because of its high en2 ergy conversion efficiency ( 50 % ～ 70 %) and zero or nearly zero emission. Hydrogen is t he fuel for t he proton exchange membrane fuel cell ( PEM FC ) . The on2board generation of hydrogen using liquid al2 cohols and hydrocarbons is t he most practical way for PEM FC vehicles. Nowadays , PEM FC technolo2 gy has been well developed , and is gradually in t he stage of commercial application , while t he hydro2 gen2generating technology has become t he bottle2 neck for t he practical utilization of fuel cells. The miniaturization of t he hydrogen source is prerequi2 site for its practical application. [ 1 ,2 ] . Microchemical technology ( MCT) is a new di2 rection originating in t he early 1990s. This technol2 ogy focuses on t he study of chemical engineering process properties and principles of micro devices and systems whose dimensions are less t han hun2 dreds of microns. Owing to t he small dimensions , t he specific area increases , t he surface effect is en2 hanced , and t he entrance effect of transport process2 es (flow , heat and mass transfer ) leads to a remark2 able increase of transfer rates , which exceed t hose of conventional2sized devices by 2～3 orders of magni2 tude. The application of MCT can improve greatly t he efficiency of systems and diminish t heir volume and weight . The integration of a microchannel reac2 tor system and a heat exchanger system is t he main technique of t he MCT. Furt her study of it will pro2 vide important t heoretical directions for t he minia2 turization of a fuel cell hydrogen source to accelerate its commercialization [ 3～5 ] . An on2board fuel processor will require novel
catalysts and reactor configurations. This paper mainly focuses on t he performance of t he met hanol autot hermal reforming ( MA TR ) in microchannel reactors. MA TR , in principle , is a combination of partial oxidation and steam reforming on t he same catalyst particles , t he overall reaction rate is t hus quite fast and t he catalyst bed can be small. This combination has a net reaction ent halpy change of zero , t hus t he reactor for t his process does not re2 quire any extra external heat transfer after having reached t he reaction temperature. The molar ratio of H2 to MeO H , β, varies wit h t he reaction temper2 atures. It can be seen in Table 1 , t hat β increases wit h t he reaction temperatures under stoichiometric ratio and adiabatic conditions. CH3 OH + ( 1 - 2β) H2 O + βO2 CO2 + ( 3 - 2β) H2 Table 1 Effect of reaction temperature on β
θ/ ℃ β
300
400
500
600
700
0. 123
0. 129
0. 133
0. 137
0. 140
Fig 1 shows a chip of t he microchannel reactor made by a chemical etching met hod. The chip is made of stainless steel. Table 2 shows its structure parameters.
Fig 1 Chip of microchannel reactor
The catalyst on t he chips was prepared as fol2 lows. First , t he metal substrates were cleaned by Na2 SiO3 solution to remove any oily substances , t hen rinsed wit h distilled water , and dried at 110 ℃
Received date : 2002208227. First author : CHEN Guangwen , male , born in 1967 , doctor , associate professor. Corresponding author : CHEN Guangwen. Tel : (0411) 4379031 ; E2mail : gwchen @dicp . ac. cn. Foundation item : Supported by NNSF of China (20176057) and Innovative Fund of DICP , CAS ( K2001 E3) .
© 1994-2010 China Academic Journal Electronic Publishing House. All rights reserved.
http://www.cnki.net
催 化 学 报
492
Table 2 Parameters of the structure of the chip h
L
wc
ww
W
n1
n2
V / ml
0. 17
30
0. 5
0. 5
45
48
5
1. 1
H
0. 34
Thickness of chip , h Dept h of channel , L Lengt h of channel , w c Widt h of channel , w w Wall t hickness between channels , W Widt h of total chan2 nels , n 1 Number of channel per chip , n 2 Number of chip , V Volume of microreactor The unit of H , h , L , W , w c and w w is mm. H
for 2 h. Second , a wash2coat on t he stainless steel was made by a CeO22ZrO 2 solid solution [ 6 ] . Finally , t he main active component , Pt , was added by Pt ( N H3 ) 4 ・( O H) 2 solution ( 011 g/ ml ) via dip2coat2 ing , and dried at 120 ℃, t hen calcined at 400 ℃ for 3 h , and reduced wit h 10 %H2290 %N 2 at 400 ℃ for 2 h. The molar ratios of H2 O and O 2 to MeO H in t he feed were 112 and 013 , respectively , and air was used as oxidant . Met hanol and water were pre2 vaporized at 150 ℃, t hen mixed wit h air and en2 tered t he microreactor which was placed in an oven. Water and met hanol were removed from t he exit gas by a cold trap , while t he dry gas entered a GC and soap bubble flowmeter for composition analysis and flow2rate measurement respectively. The reaction was carried out at 450 ℃. From Fig 2 , it is clear t hat t he met hanol con2 version in t he monolit h reactor is rapidly reduced to lower t han 80 % at 20 000 h - 1 , while it is still higher t han 93 % in t he microchannel reactor even at 186 000 h - 1 , which is about 10 times higher t han t hat in t he monolit h reactor. The reason is t hat t he intrinsic rate of MA TR is very high , so t he ap2 parent reaction rate is heavily limited by mass and heat transport . While t he reaction rate increased re2 markably and t he conversion is still high even at high GHSV in t he microchannel reactor , owing to
第 23 卷
its rapid heat and mass transfer wit h its small di2 mensions. Fig 3 shows t he composition of t he dry product gas of MA TR. H2 has a high value of 43 % and CH4 is lower t han 015 % , while CO is higher t han 15 % under t he reaction condition. CO is t he main poison of t he electrode catalyst ( Pt ) of t he PEM FC , so it must be reduced to less t han 1 ×10 - 5 . To attain t hat , supplementary reactions , such as gas2water shift reaction and preferential oxidation , have to be applied. Meanwhile , novel catalyst for MA TR wit h high hydrogen selectivity should be developed to reach t his low CO concentration level.
Fig 3 Product composition vs GHSV( MeOH) in microchannel reactor (1) H2 , (2) CO , (3) CO 2 , (4) CH4 ( The reaction condition is t he same as in Fig 2)
In conclusion , owing to t he small characteristic dimensions of t he microchannel reactor , t he effect of heat and mass transport is increased remarkably. The reaction rate increases and t he met hanol con2 version remains high at very high GHSV. The miniaturization of t he hydrogen generation system can be achieved wit h a microchannel reactor. References
Fig 2 MeOH conversion vs GHSV( MeOH) in different reactors (1) Microchannel reactor , (2) Ceramic monolit h reactor ( Reaction condition : n (O2) / n (MeOH) = 0. 304 , θ= 450 ℃)
1 Choi Y , Stenger H G. A ppl Catal B , 2002 , 38 ( 4) : 259 2 Tonkovich A Y , Zilka J L , LaMont M J , Wang Y , We2 geng R S. Chem Eng Sci , 1999 , 54 ( 13/ 14) : 2947 3 Service R F. Science , 1998 , 282 ( 5388) : 400 4 Jensen K F. A ICh E J , 1999 , 45 ( 10) : 2051 5 Wengen R S , Drost M K , Brenchley D L . In : Ehrfeld W ed. Proceedings of t he 3rd International Conference on Microreaction Technology. Frankfurt , 1999. 1 6 Li Sh L , Chen G W , Sun J L , Li H Q. Cuihua X uebao ( Chi n J Catal ) , 2002 , 23 ( 4) : 341
© 1994-2010 China Academic Journal Electronic Publishing House. All rights reserved.
( Ed W GZh) http://www.cnki.net
