Concept and Design of a Novel Compact Reactor for Autothermal

4624

Ind. Eng. Chem. Res. 2004, 43, 4624-4634

Concept and Design of a Novel Compact Reactor for Autothermal Steam Reforming with Integrated Evaporation and CO Cleanup Arı´stides Morillo, Andreas Freund, and Clemens Merten* Institute for Chemical Process Engineering (ICVT), University of Stuttgart, Bo¨ blinger Strasse 78, D-70199 Stuttgart, Germany

The design, development, and implementation of a novel autothermal reactor concept for decentral hydrogen generation by steam reforming of methanol was achieved by employing a systematic simultaneous design procedure. To overcome practical heat-transfer limitations, the reformer concept was designed following the operating principles of a plate heat exchanger. The design procedure considered numerical simulations and experimental verification, as well as simultaneous study of the mechanical stability of the reactor under realistic thermal and mechanical loads. The major optimization criteria for developing the reformer concept were maximum methanol conversion and minimum CO production. This design sequence allowed the development and final construction of a reformer prototype capable of producing hydrogen with an equivalent thermal output of 10 kWth that integrates feed evaporation/overheating, steam reforming, and the water-gas shift reaction in a single apparatus. 1. Introduction Given that direct hydrogen storage still encounters technical limitations regarding safety and handling, onboard hydrogen production for mobile applications (e.g., for fuel cells) has been actively investigated in the past several years. Among the available technologies for hydrogen production for mobile applications, the steam reforming of methanol (SRM) holds great promise because of its mild operating conditions (temperatures around 250-300 °C) and the ease of handling, distributing, and storing methanol. The SRM is represented by an overall endothermic reaction system that principally yields hydrogen (∼75 vol %) and carbon dioxide (∼24 vol %). Carbon monoxide (CO) is also produced in low concentrations (∼1.5 vol %). For a long time, multitubular catalytic fixed-bed reactors filled with catalyst pellets were the state of the art for hydrogen generation via the SRM.1 Now, the major problem is faced when attempting to implement this principle in on-board production processes. This kind of reactor system features severe limitations regarding heat integration. The required heat transferred from the heat carrier (e.g., thermo oil, electrical heating, etc.) directly to the catalyst bulk cannot match the duty imposed by the endothermic steam reforming reaction. Because of the low thermal conductivities within the catalyst bed (around 1 W‚m-1‚K-1) and poor heat transfer through the reactor wall, radial temperature gradients between the heating medium and bulk center are commonly present. Furthermore, because of this limited heat transfer, reactions in packed-bed tubular reactors tend to generate cold spots, resulting in inefficient catalyst utilization. Hence, reactor performance is strongly dependent on the surface area available for heat transfer. For this reason, parallel-plate-based apparatuses with associated large volume-related heat-exchange areas * To whom correspondence should be addressed. Tel.: +49 (0)711 6412204. Fax: +49 (0)711 6412242. E-mail: [email protected]. Internet: http://www. icvt.uni-stuttgart.de.

have been intensively investigated. Parallel-plate arrays have been employed in photocatalytical processes,2,3 reactive ion etching,4 and classical plate heat exchange. Several authors5,6 have performed detailed calculations on plate apparatuses for hydrogen production by methane steam reforming, pointing out the benefits of the reactor principle regarding heat transfer. By employing a systematic design procedure as presented in this work (see Figure 1), a novel foldedsheet reactor concept was successfully developed and evaluated. The path direction from the product specifications toward the final implementation of the aimed product is framed at this design modality in two great analysis fields: process design and mechanical design. Process design encompasses not only detailed simulation studies on chemical reaction behavior and apparatus dimensioning but also corresponding experimental verification. Mechanical design comprises simulation studies on the mechanical stability of the considered prototype under thermal and mechanical loads at realistic operating conditions. Two folded-sheet reformer prototypes (1 kWth and scaled-up 10 kWth) were successfully constructed and tested. Several facts regarding minimized heat-transfer resistance, ease of adjustment of operating conditions, well-controlled dynamic reactor behavior, and overall process integration are presented in the following discussion. Reaction System. SRM, commonly catalyzed by a ZnO/CuO-based catalyst (BASF K3-1107), is represented by the following overall endothermic reaction system

Methanol-steam reaction CH3OH + H2O S CO2 + 3H2 ∆H0R ) +49.53 kJ‚mol-1 (1) Decomposition reaction CH3OH S CO + 2H2

∆H0R ) +90.69 kJ‚mol-1 (2)

10.1021/ie0341449 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/28/2004

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4625

Figure 1. Schematic representation of the simultaneous mechanical and process design procedure applied.

Water-gas shift reaction CO + H2O S CO2 + H2

∆H0R ) -41.16 kJ‚mol-1 (3)

Hydrogen production is favored at increased operating temperatures, according to equilibrium reactions 1 and 2. Moreover, undesired CO (a poison for the platinumbased fuel-cell anode catalyst) is formed at high temperatures by methanol decomposition (eq 2) and the exothermic equilibrium-limited water-gas shift (WGS) reaction (eq 3). Therefore, the reaction system demands a sensitive adjustment of the operating temperature toward a desired performance.8 Additionally, in the design of steam reformers, severe temperature gradients and local temperature peaks, which also favor CO formation by methanol decomposition (eq 2), should be avoided. CO formation is commonly suppressed by operating in an excess of steam to favor the steam reforming reaction (eq 1) and to shift the equilibrium of the WGS reaction toward the right side (eq 3). This equilibrium displacement can also be thermodynamically affected by setting the reacting temperature to a low value (decreasing temperature profile). Reaction Power Output. Among the standard units used to characterize the reactor capacity, one of the most accepted and employed is the “thermal power rate” (in kJth‚s-1or simply kWth), which refers to the equivalent reaction heat generated by the total combustion of the produced hydrogen. 2. Folded-Sheet Reactor Concept The direct and efficient coupling of the endothermic steam reforming reaction with an external heat source is one of the principal concerns of this work. A novel reactor concept that follows the same operating principles as a plate apparatus has been developed at the Institute for Chemical Process Engineering (ICVT, University of Stuttgart, Stuttgart, Germany, ). The socalled “folded-sheet reformer concept”9 allows for the direct coupling of endothermic hydrogen production (on the steam reforming side, or simply the reforming side) with heat release (on the combustion side), resulting in an autothermal operation path (Figure 2). This concept differs from known plate apparatuses in that the resultant accordion-form structure is obtained from

folding a “single” metallic sheet. The folded sheet generates two independent, narrow reaction chambers separated only by thin metal walls. The resulting multiple-channel geometry offers a large specific surface area for heat transfer between the two reaction chambers. The use of corrugated structures (spacers) is a further important improvement included in this work. The spacers are inserted into the reaction chambers with their “fishbone” pattern lines oriented in the flow direction (see Figure 2). The spacers are either impregnated with catalyst or simply uncoated, serving not only as catalyst carrier but also as heat-exchange fins between the reactor walls. The final prototype should fulfill the following demands: on the reforming side, a proper channel structuring (spacer/catalyst) should result in an axial distribution of the desired reaction/process zones (evaporation, overheating, SRM, WGS). Liquid methanol and water have to be fed at the bottom of the reactor. Evaporation and overheating should be followed by steam reforming and the water-gas shift reaction. On the combustion side, the heat required for the endothermic steps should be supplied by the catalytic oxidation of hydrogen, according to the reaction

H2 + 1/2 O2 S H2O

∆H0R) -241.83 kJ‚mol-1 (4)

Combustion air meets the apparatus countercurrently from the reactor top. Hydrogen should also be fed to the main air stream at several axial positions so that the combustion heat can be locally generated and consequently transferred only where it is needed. Countercurrent flow should be implemented to achieve optimal heat recovery from the process streams, so that the process heat is retained only in the reformer body and the process streams enter and leave the apparatus “cold”. The first reformer prototype, conceived for producing 1-kWth, consists basically of five parallel reforming channels embedded by six respective combustion channels. The reforming side is divided into two reaction sections: one for the steam reforming of methanol and one for the water-gas shift reaction. In contrast to the 10-kWth reformer, the reactant mixture is evaporated and overheated externally. On the combustion side, two catalytic reaction zones are installed along the reform-

4626 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

Figure 2. (Left) Separating wall of a folded-sheet reactor and resulting reaction chambers. (Right) Axial structuring of the combustion and reforming side for the 10-kWth reformer.

ing length. Hydrogen combustion takes primarily place when the first catalytic zone is reached by the main air stream, which already contains initial amounts of hydrogen. Secondary hydrogen combustion occurs on the next catalyst layer where additional hydrogen is fed to the main air stream (excess oxygen) via a gas sideinjection. Similarly to the first prototype, the scaled-up 10-kWth reformer consists of 16 reforming channels and 17 combustion channels. On the reforming side, evaporation and overheating of the reacting mixture are followed by SRM and WGS. In the combustion channels, five reaction zones are axially placed: one for the evaporation section, one for the overheating section, and three for the reforming section (Figure 2). Therefore, hydrogen is injected at five side ports and burned in the respective catalytic zones with the hydrogen-free oxygen from the feed air-stream. 3. Simultaneous Design Procedure The procedure applied to the design of the foldedsheet reactor is schematized sequentially in Figure 1. Starting from the process specifications (i.e., on-board hydrogen generation for fuel-cell applications, overcoming of usual heat-transport limitations, optimal coupling of endo- and exothermic stages, reactor scale-up, etc.), predesign and conceptualization stages must be performed initially, so that the process requirements will, in principle, be fulfilled. Hence, the methanol steam reformer has been conceived following the operating principles of a parallel-plate apparatus, which offers close thermal interactions between the endo- and exo-

thermic chambers in a compact structure. Hydrogen side injections are implemented on the combustion side to ensure a well-distributed and controlled heat production and transfer from the exothermic to the endothermic side. This reactor concept allows for the study of the reaction performance (axial temperature distribution) and the verification of design parameters according to technical data sheet specifications for apparatus construction. Simultaneous mechanical and process design studies are carried out to determine optimum process loadings and reactor/reaction behavior under operating conditions, as well as to identify practical problems regarding mechanical stability. First results from the mechanical and process design assist in the performance of prototype experiments, which are designed to verify initial design assumptions. Intermediate conclusions lead to an optimized design and provide several oriented corrective changes to attain the next reactor prototype. This sequence aims constantly at the process requirements. Next, scale-up strategies can be applied to the optimized reactor prototype to reach the aimed reactor power output. In the next sections, process and mechanical design criteria, process integration, and reformer scale-up are explicitly described. 3.1. Process Design. Simulation studies of the reformer geometry and reaction performance, followed by experimental verification of the reactor behavior at the simulated reaction conditions, are the main focuses of the present process design. 3.1.1. Reaction Engineering Simulations. The simulated reactor concept was the folded-sheet reformer,


from which both reaction chambers were modeled: combustion and reforming. The reaction chambers were considered to be filled with either catalytically coated or uncoated metallic spacers. On the combustion side, additional fuel-gas (hydrogen) injections were also modeled to reproduce the local heat generation and transfer through the reactor wall toward the reforming side. Kinetic Model. The comprehensive kinetic model (Peppley et al.10) considers the methanol-steam reaction, the methanol decomposition reaction, and the water-gas shift reaction, according to eqs 1-3. Reaction rate equations consider limitations due to chemical equilibrium by introducing an equilibrium factor. This effect has a major impact on the equilibrium of the WGS reaction, which strongly depends on the operating reaction conditions.12 Equations describing the reaction (in mol‚s-1‚m-2), in terms of the overall rates, rsfc i surface area are as follows

rsfc 1 ) / (1) k1KCH 3O

(

/ (1) 1 + KCH 3O

pCH3OH

xpH

( )( pCH3OH

xpH2

CST1 CST1a

pH2O 2

(

( )( pCH3OH

xpH

pCH3OH

xpH

1-

2

( x )(

1-

pH2

xpH

pH2O

xpH

pCOpH2O

pCH3OH

Keq 3 pCH3OH

/ (2) + KOH

2

/ (1) 1 + KCH 3O

pH22pCO

2

)

)

(1 + xKH(1a)pH2)

xpH

/ (1) k3KOH

(

Keq 1 pCH3OHpH2O

2

/ (2) 1 + KCH 3O

rsfc 3 )

)

pH23pCO2

/ / (1)p + KHCOO CO2xpH2 + KOH(1)

/ (2) k2KCH 3O

rsfc 2 )

1-

(5)

)

CST2 CST2a

(6)

(1 + xKH(2a)pH2)

pH2pCO2

)

Keq 3 pCOpH2O

2

CST1

/ / (1)p + KCOOH CO2xpH2 + KOH(1)

pH2O

xpH

2

2

)

2

(7) where pj represents the partial pressure of the species j, ki is the rate constant of reaction i, and Keq i is the equilibrium constant of the reaction i, and K* is the equilibrium constant of the intermediate reaction steps. CSTi defines the total surface concentration of site i. A more detailed description of the parameters is reported in the related literature.10 The kinetic model for the catalytic combustion of hydrogen according to eq 4 on noble metal catalysts was taken from Kolios13 and can be described by the following power-law expression assuming hydrogen-controlled (excess oxygen) mass-transport limitations

r4 )

S kH2pH 2

S pO 2

)

g βH2acH 2

(8)

with 0 exp KH2 ) kH 2

[ (

1 E 1 R T0 TS

)]

(9)

0 The reaction rate coefficient (kH ) was adjusted ac2 cording to kinetic measurements to a value of 100 mol‚m-2BET‚bar-2‚s-1, and the activation energy was set to E/R ) 5000 K at T 0) 300 K.13

Model Assumptions. For modeling the axial temperature and concentration profiles, the following assumptions were made: (1) The bulk phase is perfectly mixed, i.e., reaction properties and parameters vary only in the axial direction (1-dimensional model in space). (2) Five phases are modeled: folded wall, gas, and catalyst on the combustion side; gas and catalyst on the reforming side. (3) The mass balance in the gas phase is assumed to be quasi-stationary. (4) Axial mass and heat transport in the gas phase results from convective and dispersive effects. In the solid phase, only heat conduction is considered. (5) Hydrogen combustion is limited by hydrogen mass transfer, and the steam-methanol reaction is kinetically limited. (6) The pressure drop along the reaction channels is neglected. (7) The gases are assumed to behave ideally. (8) The solid properties are assumed to be constant. (9) The reactor is assumed to be adiabatic. Model Equations. The modeling of the system in eqs 1-3, represented by two linearly independent reactions, includes the kinetic models for all participating reactions (eqs 5-7). Simulation studies of the steam reforming of methanol coupled with hydrogen combustion were performed using a detailed heterogeneous 1D model. A coupled system of parabolic partial differential equations (PDEs) is stated for (1) the mass balances of the key components on the reforming side, namely, methanol and carbon dioxide; (2) the mass balance of hydrogen on the combustion side, considering side injection streams; and (3) the energy balances of the separating wall (folded sheet), the reforming gas, the combustion gas, the reforming catalyst, and the combustion catalyst. The general form of the PDE system, considering convective and dispersive transport mechanisms, is

B

∂y b ∂y b ∂ ∂y b ) -C + D +Q B ∂t ∂z ∂z ∂z

(

)

(10)

where y represents the vector of the simulated state variables; z is the axial reaction coordinate; and B, C, D, and Q B are the accumulation, convection, and diffusion matrices and the source/sink term, respectively. Appropriate initial and boundary conditions have to be defined. Boundary conditions are of the general Danckwert type

Ry1 + β

|

∂y )γ ∂z 1

(11)

The coefficients R, β, and γ can be functions of both the time and the state variables (wMeOH, wCO2, Tw, Tref, Tfg, TSref, and TSfg). The concentrations of the remaining components of the reaction mixture are calculated from global balances using linear dependencies arising from the stoichiometry of eqs 1-3. The concentration of the inert components (e.g., N2 from the air on the combustion side) are calculated by using closure conditions. The influence of temperature and composition changes during the extent of the reaction on the specific heat


Figure 3. Simulation studies. (Left) Influence of the axial position of the combustion catalyst on reaction temperature and methanol conversion. (Right) Improved temperature control via a side feed injection plus a second oxidation catalyst section at different positions (top) and its influence on CO production at 99% methanol conversion (bottom).

capacity of the reaction mixture was considered according to the expression14 J

C h gp(z,t) )

wj(z,t) Cp,j(T g) ∑ j)1

(12)

Upon substitution of the “effective” Pe´clet number

Peax )

u0dh,spacer Dax

(13)

and the “molecular” Pe´clet number

Pe0 )

u0dh,spacer D

(14)

the axial dispersion coefficient, Dax, of the reacting components considering mixing effects of the fluid phase in the channels can be calculated from

Dbed/D 1 1 ) + Peax Pe0 Kax

(15)

with u0 as the gas velocity, dh,spacer as the hydraulic diameter of the reactor channel, D as the molecular diffusion coefficient, Dbed as the bed diffusion coefficient on the solid phase (no flow), and Kax ) 2.0 as a dimensionless calculation constant. The ratio Dbed/D depends directly on the porosity.15

Numerical Simulation Tool. The PDEXPACK16 simulation package was employed for the numerical solution of the given equation system, based on PDEX algorithms17 and developed at ZIB (Konrad-Zuse-Zentrum, Berlin, Germany). It is a fully adaptive code in space and time that uses a method-of-lines scheme. The main advantage of PDEXPACK consists of the errororiented spatial regridding and time step-size control, which allows for an accurate resolution of steep gradients and fast dynamics. Simulation Results and Discussion. The methanol reformer was modeled considering one reforming channel (completely catalytic, four spacer structures per channel) divided by a metal wall (folded sheet) from one contiguous combustion channel (one or two reaction zones of 0.01-m width). A reactor length of 0.6 m was preset. Simulations on the placement of the combustion zone along the combustion side were carried out. The position of the first reaction zone was varied as shown in Figure 3 (left). The resulting temperature and methanol conversion profiles are plotted over the reactor axial coordinate. They clearly indicate that the axial position of fuel injection directly influences the methanol conversion. With a single fuel-gas injection, the temperature profile displays a steep hot spot downstream from the injection point in the combustion channel. A similar but less pronounced effect is also observed on the reforming side. In general, the gas temperature levels in the reforming and combustion channels are quite similar, demonstrating the excellent heat transfer of the proposed design. With a single fuel injection, a


Figure 4. Schematic flow sheet of the experimental setup for the 1-kWth folded-sheet reformer.

methanol conversion of around 98% and an average CO outlet concentration of about 1.0-1.8 vol % in the reformate are predicted. Multiple fuel-gas injections were also simulated. Even a better effect is observed for distributed hydrogen combustion. Figure 3 (right) shows the simulation results for a split combustion zone: a portion of the hydrogen enters with the combustion air and is completely converted at the first catalytic zone, while the rest of the hydrogen is side-fed to the second catalyst slice located at a different position. In this scenario, the hot spot observed with only one reaction zone in the combustion channel can be reduced from about 60 K (Figure 3, left) to 30 K. In addition, 99% methanol conversion with 1.5 vol % CO can be obtained (profile a in Figure 3, right). In general, temperature peaks were only observed in the hydrogen combustion zones. Simulation studies show that the catalytic combustion has to be performed near the entrance of the methanol/water feed, so that most of the methanol conversion takes place in the first one-third of the reactor and the rest can be used for the water-gas shift reaction. It is also obvious that CO purification by water-gas shift reaction is limited to CO levels of about 1.1-1.5 vol % because of equilibrium constraints. These results led to a plausible quantification of the reforming catalyst utilization in the designed 1-kWth folded-sheet reformer. The optimization of the placement of either single or double reaction zones on the combustion side is suggested to maximize the methanol conversion with a minimum in CO formation. More details and further simulation studies on the geometrical dimensioning of the folded-sheet reformer are explicitly reported by Becker.18 3.1.2. Experimental Work. A folded-sheet reactor prototype for producing 1 kWth of hydrogen was successfully constructed and integrated into a laboratoryscale plant for experimental tests. This evaluation provided information regarding the proof of the concept and helped to verify simulation-based design specifications. Subsequently, a scaled-up 10-kWth reformer prototype with integrated feed evaporation and a watergas shift stage was also constructed and experimentally tested.

Experimental Setup. The constructed 1-kWth foldedsheet reformer was assembled in the laboratory plant, as shown in Figure 4. To carry out the experiments, liquid methanol and water were provided by two helium pressurized tanks (5.75 × 10-3 m3, p ) 0.4 MPa) and evaporated by independent electrically heated evaporators (ICVT patent19) by using liquid flow controllers (Bronkhorst Hi-Tec, type Liqui-Flow). The vapor mixture was fed to the reactor inlet using a single tempered line. Mass flow controllers (MKS, type 1179) were used to feed the main air/hydrogen mixture countercurrently to the combustion side, as well as pure hydrogen at the injection port. Unreacted methanol and water from the product stream were removed using a condensate splitter. The dry gas flow rate was measured with a mass flowmeter (MKS, type 1179). Afterward, this stream was either conducted to the analysis section or catalytically burned at the exit. The gas composition of the product stream was determined using an analytical device (Hartmann & Braun, type caldos/uras) employing thermal conductivity detection (for hydrogen) and nondispersive infrared analysis (for CO2, CO, and CH4). Many changes were made in the experimental setup for testing of the scaled-up 10-kWth folded-sheet reformer. Because this unit already contained an integrated evaporation stage, the external liquid evaporators were removed. On the combustion side, hydrogen was added only via side injection to the main air stream. Because of the large amount of hydrogen produced, the off-gas was burned by means of a flame burner. The flow sheet of the laboratory setup employed to perform the experiments with the 10-kWth reformer is shown in Figure 5. Experimental Results and Discussion. To produce an equivalent of 1 kWth of hydrogen, a methanol/water mixture at a molar steam-to-methanol ratio of 1.3 was fed at a rate of 170 g‚h-1 (7.1 mol‚h-1). On the combustion side, the hydrogen required to produce the necessary heat for the steam reforming reaction was axially distributed between the two reaction zones. The air flow rate was adjusted to achieve a combustion-to-reforming heat capacity flux ratio of 1. As shown in Figure 6 by two representative reforming gas temperature profiles, different methanol conversions and outlet carbon monoxide concentrations were obtained by adjusting the amount of dosed hydrogen on the combustion side (0.70


Figure 5. Schematic flow sheet of the experimental setup for the 10-kWth folded-sheet reformer.

Figure 6. Measured axial temperatures in the 1-kWth folded-sheet reformer for variations of the hydrogen flow rate on the combustion + fg side; influence on methanol conversion and CO production (outlet values). Tgas,ref ) 170 °C, mz,ref ) 0.16 kg‚m-2‚s-1, S/M ) 1.3, VH ) ([) 2 1.40 and (9) 1.60 mol‚h-1.

vs 0.80 mol‚h-1/combustion zone). Temperature peaks were observed where the hydrogen combustion occurred. Reaction temperature levels between 280 and 300 °C led to increased methanol conversions. Nevertheless, overall increased operating temperatures also led to elevated outlet CO concentrations. To quantify the effect of the decreasing temperature profile on the CO selectivity, gas samples were taken from the middle point of the reforming length. The samples were analyzed and compared with the outlet gas composition. In both presented cases (Figure 6), the intermediate CO concentration after the first half of the reformer averaged around 7.5 vol % while the CO outlet concentration was 2.4 and 2.5 vol %, thereby revealing the positive effect of decreasing temperature profile (WGS reaction) on the overall reaction selectivity. Even more than elevated temperatures, steep temperature “peaks” along the axial coordinate should be avoided. Instead, a well-distributed, flat temperature profile leads to high methanol conversions and diminished CO concentrations. This fact had to be strongly considered during the scale-up to the 10-kWth reformer. In the scaled-up unit, this task was accomplished by

inserting thin aluminum slices on both sides of the combustion catalyst to increase the axial heat conduction and make the form of the axial temperature profile more even. Conclusions. The presented experimental evaluation led to a better understanding of the reformer and its behavior under typical operating conditions. Reaction performance on the reforming side can be directly influenced by adjusting the reaction conditions on the combustion side. Higher methanol conversion can be achieved by setting a higher overall temperature level. Nevertheless, the CO levels consequently tends to increase. This behavior is to be expected when a steam reforming catalyst with a low activity is employed. Rather than steep temperature peaks, flatter axial temperature profiles on the reforming side should be configured to achieve complete methanol conversion at CO concentration levels below 1.5 vol %. The form of the temperature profile observed in the experimental evaluation could be reproduced in the simulation. The results obtained from both experimental and simulation studies were the starting point for the scale-up of the reformer prototype.


Figure 7. Symmetric geometric model of the finite-element simulations.

Figure 8. Axial temperature distribution in a reforming channel. Comparison between simulation and experiment.

3.2. Mechanical Design. The described folded-sheet reactor concept is the result of a simultaneous process and mechanical design. In this section, the main aspects in the investigation of the mechanical design are explained. For determining the dimension of the thinner wall of the folded-sheet reactor under operating conditions, i.e., pressure of 0.3 MPa and local temperatures up to 350 °C, no conventional design rules, as reported in technical design codes,20,21 exist. Therefore, both the mechanical and thermal stresses had to be considered. This problem was solved by using a detailed numerical simulation tool for the apparatus by means of the finite-element software package ANSYS.22 The precise determination of the loads, especially the spatial temperature distribution, is a crucial prerequisite for representative results in finite-element simulations. Therefore, a flow simulation was performed simultaneously using an integrated CFD simulation tool.23 Verification of the model with available experimental data, prediction of the mechanical behavior of the apparatus for scale-up purposes, and estimation of overall heat losses are the main aims of such a mechanical analysis. Finite-Element (FE) Model. For the FE simulations, a simplified three-dimensional parametric model of the 1-kWth reformer was used.24 Whereas the prototype reactor consists of six channels for the combustion reaction and five channels for the steam reforming reaction, the reduced finite-element model includes

three combustion channels and two reforming channels covered by the reactor case and insulation (Figure 7). The wall thickness of the reactor case was 1 mm, and that of the folded sheet was 0.2 mm. The depth of the channels is defined to 40 mm. For the prototype, operation at atmospheric pressure was studied, and the fluid inlets were neglected. For the flow simulation, the corrugated spacer structures in the channels were not considered. Instead, link elements (compression-only spar elements) were implemented for the structural analysis to reproduce the self-stabilization effects of the corrugated spacers. The geometric data from the reaction engineering simulations previously described are employed. The heat of reaction in each channel is represented by local, volume-related heat sources and sinks. Flow Simulation. In the first step, the flow simulation results for the 1-kWth reformer are analyzed to compare the spatial temperature distribution with experimental data. Figure 8 shows a comparison of the axial gas temperature distribution in the middle of a reforming channel. The results are in good agreement with the experiments. Thus, it is possible to predict the thermal behavior of this complex reactor with a simplified three-dimensional model containing only the main structures. Following the described method, a realistic temperature distribution that can be applied in a structural analysis is obtained. After this verification, the same procedure is applied for scale-up to the 10kWth reformer.


Figure 9. Typical deformations of the folded sheet (maximum displacement DMX ) 0.002 475 m) and equivalent stresses (SEQV) of the reactor. Cross section in the combustion region. (SMN, SMX, and SMXB ) minimum and maximum values and maximum bounded value, respectively, of plotted item, based on error estimate.)

Scale-up. The geometric changes for scale-up are as follows: (1) The number of combustion channels was increased from 6 to 17. (2) The number of reforming channels was increased from 5 to 16. (3) The wall thickness of the case was increased from 1 to 3.5 mm because of a design pressure of 0.3 MPa in the reforming channels. (4) The thickness of the folded sheet was decreased from 0.5 to 0.2 mm. (5) The channel depth was increased from 40 to 130 mm. The simulation of the 10-times-larger apparatus showed that the required heat of combustion had to be increased only by a factor of 7 to obtain the desired temperature distribution shown in Figure 8. Because of the simplified FE model, this result was expected because the reduction of the surface-to-volume ratio and the increased case wall thickness led to reduced heat losses so that a lower amount of combustion heat was required. In addition, the thinner folded sheet resulted in better heat integration. Structural Analysis. High operating pressures had to be considered as an important part of the process specifications. Subsequently, the simulation of the mechanical stress was performed. In a coupled-field analysis, the temperature pattern taken from the flow simulation and a mechanical load of 0.3 MPa internal pressure were applied. The deformation behavior of the reactor components was simulated according to the elastic material law. The nonlinear link elements describing the corrugated structures in the channels necessitated a nonlinear structural analysis of the whole model. Because of the complex geometry of the reactor, a complex three-dimensional stress behavior was obtained. In Figure 9, typical deformations and equivalent stresses of the reformer are demonstrated at a cross section of the combustion region. The deformation of the folded sheet primarily corresponds to the stress of the internal pressure. The deformed reactor is shown on the left side of Figure 9. As a result of the internal pressure, the folded sheet is bulged, but the corrugated spacers stabilize the whole structure. On the right side of Figure

9, the equivalent stresses of the reactor are depicted. Detailed analyses of the results25 indicate that the components of the reactor fulfill the specifications from technical codes.20 The stresses are limited by the 1% yield strength at elevated material temperatures. Consequently, the FE simulations indicate safe operation for the scaled-up 10-kWth reformer. Conclusion. An important conclusion from the mechanical simulations is that the thermal stresses in the areas with high temperature gradients are in the same order as the mechanical stresses. Therefore, it is crucial to determine all relevant loads, especially the temperature loads, as precisely as possible. As a consequence of the finite-element simulations performed, an important design criterion for the scaleup of the reactor was determined to be use of an adequate case/wall bracing to avoid pressure and temperature deformations. Likewise, for good heat integration, appropriate reactor insulation is required because each channel is in direct contact with the reactor case. 3.3. Process Integration/Scale-Up. The 10-kWth folded-sheet reformer was designed for the same crosssection-related loading as the 1-kWth prototype. To obtain better reactor compactness, a quadratic crosssection layout was preset. This resulted in an array of 16 reforming and 17 combustion chambers with a channel depth of 130 mm. The resulting prototype was successfully constructed and experimentally evaluated. Steady-state and dynamic experiments were carried out to evaluate the reactor performance. As an example, the obtained dynamical product-gas flow rates for several reactor power outputs are plotted as a function of time in Figure 10. The change of the reactor power output was evaluated at both increasing and decreasing power demand; reaction performance is reported only for increasing steps. On the combustion side, hydrogen injection was sensitively adjusted to achieve higher methanol conversions on the reforming side. Additionally, because axial temperature measurements were not implemented for the bench-scale reformer, only the external wall temperatures were monitored so that the overall temperature did not exceed the predefined limit level of 300 °C. The experimental results are summarized in Table 1. These results demonstrate the direct relationship


Figure 10. Experimental dynamic behavior of the 10-kWth reformer at different reforming loads, with successive adjustment of heat generated on the combustion side. Table 1. Results of Experimental Evaluation of the 10-kWth Folded-Sheet Reformer under Dynamic Conditions

1 2 3 4

thermal power output (kW)

yH2

yCO2

yCO

methanol conversion (%)

5 6 7 10

0.731 0.732 0.733 0.721

0.340 0.296 0.309 0.310

0.016 0.040 0.041 0.043

70 85 84 90

reformate molar fractions (dry gas)

between the methanol conversion and the CO production. As expected, higher methanol conversions lead to higher CO outlet concentrations. Total methanol conversion (100%) was not achieved. Additionally, elevated CO concentrations were observed. This fact implies the presence of a reforming catalyst with a low activity, so that a higher temperature is necessary to achieve maximum methanol conversion. As a result, CO production is highly favored. Adequate catalyst activity and fine control of reaction selectivity are crucial for satisfactory performance. In general, the scaled-up folded-sheet reformer demonstrated a rapid output response to changes in the feed load. In summary, the operating temperature can easily be adjusted by setting appropriate reaction conditions on the combustion side, demonstrating that the overall heat transfer is no longer the limiting step. This benefit can be successfully transferred and implemented for liquid evaporation and overheating (even at unsteady operating conditions), thereby achieving a useful integrated reactor design for hydrogen generation. 4. Conclusions and Overview A systematic method for the development of compact chemical process equipment that involves the integration of all concerned disciplines in a simultaneous engineering process was successfully demonstrated. The method was applied to the design of a novel autothermal methanol reformer resulting in an optimized, scaled-up prototype with experimentally verified performance in good agreement with the process specifications. The numerical design procedure was successfully evaluated and verified. Further investigations will be focused on the development and implementation of catalysts with higher

activities and selectivities to guarantee maximum methanol conversions at minimum CO production rates. An additional CO purification stage such as preferential oxidation (PrOx) should be incorporated to reach the CO concentration levels (

Concept and Design of a Novel Compact Reactor for Autothermal

Recommend Documents