Water-Gas shift reaction

Water-Gas Shift Reaction(WGS)Group B

Water-Gas shift reactionGroup WGS B

David Chen, Carl-Robert Florn, Richard Johansson and Fredrik Edman2012-10-19

IntroductionBecause of todays increased electricity usages the importance for conversion of chemical energy into electrical energy has grown. Through fuel cells, the free energy of a chemical reaction is converted into electrical energy. This has led to increased interest of high purity H2 in the exhaust from a combustion engine, because it is used to power the fuel cells. H2 can be obtained from synthesis gas that mostly consists of H2, CO, CO2 and H2O. There is a requirement for the fuel cells that the concentration of CO must be held below 100 ppm or else its electrodes will be poisoned. This project is involving Water-Gas Shift (WGS) reaction which is one way to keep a low CO concentration. It is also possible to increase the quantity of H2 by changing the composition of the synthesis gas by using WGS as a method. The WGS reaction reacts in multiple stages, but the overall reaction can be described as seen below:

where its equilibrium constant, Keq is highly affected by the temperature. The conversion of CO is generally equilibrium limited during characteristic reaction conditions within industrial processes. Catalysts based on copper and zinc or iron is often used in WGS but their efficiency is reduced because of air exposure. Therefore new catalysts less sensitive to air have been developed consisting of noble metals such as platinum, ruthenium or rhodium supported on oxides like Al2O3, CeO2 and ZrO2.

The used catalyst in this project consists of 1.4 weight% Pt and 8.3 weight% CeO2 on an Al2O3 layer. The aim is to design a WGS reactor system reducing the CO concentration in a gas mixture below 1 mole%. A monolith reactor will be used with the catalyst as a washcoat and it is important that the total reactor volume and catalyst load becomes as small as possible.

To optimize the reactor volume, different values of the diameter, length, inflow temperature and catalyst load will be simulated. This is to find working conditions so that the outflow reaches a maximum concentration of CO at 1 mol%.

MethodologyThe simulations were run in MATLAB as a single-channel model where all channels are considered identical and radial variation in the monolith reactor is neglected. The cross-sectional of the monolith for the single channel is illustrated in Figure 1: dw is the catalyst thickness, dH is the channel dimension and w/2 is the thickness of the monolith wall. The monolith is considered to operate adiabatically and the reactor contains 400 channels per square inch. Each monolith channel is modeled as a tanks-in-series of 15 tanks where the catalyst is washcoated onto the channel walls in 5 layers. By discretizing the catalyst in layers, pore mass transport resistance in the washcoat is accounted for. To account for the radial gas phase temperature and concentration gradients in the channels, a film models is introduced in the simulation, as well as the heat transport in monolith and washcoat are taken into account.

Figure 1: The cross-sectional of the monolith for a single channelThe tanks-in-series is modeled by the following mass and heat balances for component i in tank k:

where is the lumped mass transfer coefficient

Washcoat mole balance for component i in tank k and washcoat layer n:

with

For the bottom washcoat layer:

where the axial heat flux for a given tank k is:

Radiative heat transfer for the first and last tanks (axial heat flux) is accounted for as: and

The heat and mass transport coefficients and are estimated from the equations above through correlations with Sherwood and Nusselt number as an asymptotic value [R.D. Hawthorn, AIChE Symp. Ser. 70 (1974) 428].The effective diffusivities in the washcoat are estimated from the Bosanquet formula [G.F. Froment, K.B. Bischoff, J. De Wilde, Chemical Reactor Analysis and Design, third ed., John Wiley & Sons, New York, 2010, p. 175]:

where fD is the ratio of internal void fraction and tortuosity factor and is set to 0.1.Originally the generic program simulated another type of catalytic reaction, but used the same type of monolith reactor. Therefore changes had to be made in order to simulate the water-gas shift reaction. First the reaction taking place in WGS is a reversible reaction. Due to that the MATLAB program requires two reactions the reversible reaction is divided into one forward (r1) and one backward (r2) reaction. The necessary properties of the catalyst that were not given had to be calculated. However, the catalyst heat capacity and heat conductivity was assumed to be the same since the values are hard to find for porous materials. In order to find accurate data Tref was changed to 293.15K from where the properties could be found. The total pressure was set to 1 atm and assumed to be constant over the reactor. Due to a change in reference temperature new kref had to be calculated from the Arrhenius equation:

The equilibrium relationship [A.A. Phatak et al., Catalysis Today 123 (2007) 224234] that relating the forward and the backward reaction is expressed as:

The diffusivities are taken at STP from [Atmospheric Environment, 32A 1111-1127. 1998. Massman, WJ] and calculated for our temperature and pressure assuming

All the necessary data that had to be changed in MATLAB is summarized in Table 1.The dimensionless numbers Sherwood (Sh), Nusselt (Nu), Reynolds (Re), Schmidt (Sc) and Prandtl (Pr) are assumed constant throughout the reactor. They are assumed to be the same as from the generic program. The mass and heat transfer coefficients are calculated in the program from correlations using these dimensionless numbers.

Table 1: The input data for the simulation of the monolith reactor for water-gas shift processCatalyst properties

Channel dimension [m]1.1176e-3

Wall thickness of monolith [m]1.524e-4

Catalyst layer thickness [m]25e-6

Density of catalyst [kg/m3]1100

Bulk density [kg/m3]74.5155

Catalyst surface are per monolith volume [m2/m3]2647.7

Fraction of open area to total cross sectional area0.7067

Catalyst mean pore diameter [m]10e-9

Kinetic properties [1]

kref, forward [mol/kg/s]6.1598e-10

kref, backward [mol/kg/s]6.1598e-10

Ea (forward) [J/mol] [1]86000

Ea (backward) [J/mol] [1]86000

A (pre-exp. factor)2.9245e-25

Standard enthalpies of formation [2]COH2OH2CO2CH4 inertN2 inert

Hf [kJ/mol]-110.54-241.8180-393.42200

Physical propertiesCOH2OH2CO2CH4 inertN2 inert

Diffusivity [m2/s] [3]1.856e-52.4752e-56.1e-51.51e-52.168e-52.032e-5

Molar mass [kg/mol]28e-318e-32.02e-344e-316.04e-328.01e-3

Cp = A+B*T+C*T2+D*T3+E*T4A

B

C

D

E

CO29.56-6.58e-32.01e-5-1.22e-82.26e-12

H2O33.93-8.42e-32.99e-5-1.78e-83.69e-12

H225.402.02e-2-3.85e-53.19e-8-8.76e-12

CO227.444.23e-2-1.96e-54.00e-9-2.99e-13

CH434.94-4.00e-21.92e-4-1.53e-73.93e-11

N229.34-3.55e-31.01e-5-4.31e-92.59e-13

Results and DiscussionThe totals of 2 series monolith reactors are required as to reduce the CO mole fraction lower than 1 mol%. The temperature of the outlet gases from the first reactor will increase due the exothermic catalytic reaction. Hence, proper cooling system should be installed at the outlet of the first reactor in order to cool down the gases before feeding into the second reactor. Both of the reactors are designed in such a way that they will have the same dimensions. Different combinations of the designs (dimension, catalyst mass and operating temperature) for the both of the monolith reactors are shown in Table 2 below. The optimization is performed as to minimize the volume of reactor and the total catalyst mass required for the WGS reaction. Table 2: Different settings are set and tested to optimize the design of the monolith reactor in converting carbon monoxide to the level lower than 1 mol % for the water gas shift (WGS) processBasic Settingsthe 1st reactor1234567

Catalyst thickness (x10-6 m)25252525355075

Length (x 10-2 m)6666669

Diameter (x 10-2 m) 20202020181410

Volume (x10-6 m3)18851885188518851524924707

Mass of catalyst (kg)0.14050.14050.14050.14050.15780.13450.1502

Inlet temperature (oC)300310290310310310310

Outlet mole fraction of CO (mole %)1.531.532.001.531.531.772.05

The 2nd reactor

Catalyst thickness (x10-6 m)25252525355075

Length (x 10-2 m)6666669

Diameter (x 10-2 m) 20202020181410

Volume (x10-6 m3)18851885188518851524924707

Mass of catalyst (kg)0.14050.14050.14050.14050.15780.13450.1502

Inlet temperature (oC)300310290290290290290

Outlet mole fraction of CO (mole %)0.700.740.830.690.680.810.95

First optimization is performed to obtain the best combination of temperatures to operate the series monolith reactors. (Run 1- 4). The Run 2 and 3 have shown that it is not favorable to operate both of the reactors at either higher or lower than 300 oC as it will result in lower conversion. However, if the operating temperatures for the reactor 1 and reactor 2 are set at 310 oC and 290 oC respectively as in the run 4, the mole fraction of CO could be reduced to 0.69 mol% for the same reactor dimensions of: length= 6 cm, diameter= 20 cm and volume= 1885 cm3 with the total catalyst mass of 0.1405 kg. The aforementioned phenomena could be explained by the water-gas shift equilibrium behavior. The forward reaction of the water-gas shift process is an exothermic reaction while the backward reaction is an endothermic reaction. Although high temperature will enhance the catalytic activity of the catalyst, the rate of the forward reaction will be decreased as the equilibrium will be actually shifted back to the left under these high temperature conditions. The rate of endothermic backward reaction will increase with temperature and reverse the produced CO2 back to CO and therefore it is not recommended to run the reactor at high temperature. It is important to operate the reactor at the temperature where the forward reaction rate is always higher than the backward reaction rate to enhance the production of CO2.

Figure 2: The effectiveness factor changes along the first monolith reactor (Run 4)

Figure 3: The effectiveness factor changes along the second monolith reactor (Run 4)

Figure 4: The effectiveness factor changes along the first monolith reactor (Run 7)

Figure 5: The effectiveness factor changes along the second monolith reactor (Run 7)As shown in Figure 2 and 3, the effectiveness factors obtained from the Run 4 for both of the reactors are close to unity which means that the internal mass transfer resistance within the catalyst layer is not significant. In other words, all the catalysts coated on the monolith inner wall layer are almost fully utilized for the water-gas reactions. Thereby, the catalyst thickness could be increased further while the volume of the reactor could be reduced as to have more catalyst mass per volume. Series of optimization (Run 5-7) are performed as to have more compact design for both of the monolith reactors by increasing the catalyst thickness and reducing the volume of the reactor (diameter and length) in the same time while maintaining the low catalyst load. In order to have low catalyst load, the effectiveness factor need to be maintained at the value close to unity along the reactor length as to ensure that the coated catalyst is fully utilized for reaction and not wasted. The total volume of the reactor had been reduced to 707 cm3 each through the optimization process (Run 5-7). The total catalyst mass coated on the monolith for both of the reactor is 0.1502 kg each and the outlet CO mole fraction becomes slightly higher to 0.95 mole% but still below the desired value of 1 mole%. The values of the effectiveness factor obtained for the forward reaction for both of the first and second reactor are close to unity as illustrated in Figure 4 and 5. Moreover, the values only drop slightly approximately by 0.10 and 0.02 for first and second reactor respectively if comparing the Run 4 to the Run 7. It can be noticed that the values of effectiveness factor for the backward reaction for all the runs are higher than unity and this reflects that the rate of reaction occurred within the catalyst layer is higher than the rate of reaction on the catalyst surface. This can be explained by the endothermic reaction characteristic of the backward reaction. During the water- gas shift reaction, the heat is accumulated within the catalyst pore due to the highly exothermic forward reaction and this will result in high temperature conditions. The backward reaction will be favored at high temperature conditions. Therefore, it is also crucial to ensure that the rate of the backward reaction is always lower than the forward reaction along the reactor to have better conversion of CO. In order to achieve the aforementioned condition, the dramatically temperatures change along the reactor is not recommended. Thereby, good heat transfer is required to remove the accumulated heat within the catalyst layer.

Figure 6: The average of the forward and backward reaction along the first monolith reactor (Run 7)

Figure 7: The average of the forward and backward reaction along second monolith reactor (Run 7)As illustrated in Figure 6 and 7, the average of forward reaction rate is always higher than the average of the backward reaction rate and the equilibrium is not achieved along the first and second reactor for the Run 7. By comparing Figure 6 to Figure 8 or Figure 7 to Figure 9, it can be seen that the rates of reaction for both backward and forward reaction are increasing with temperature along the reactor. However, it is also noted that the backward rate increases much faster than the forward rate due to the backward reaction is endothermic and the increasing concentration of CO2 along the reactor, this further drives the speed of the backwards reaction.

Figure 8: The temperature profiles of gas and catalyst surface along the first reactor (Run 7). No significant difference for the temperatures of gas and catalyst surface.

Figure 9: The temperature profiles of gas and catalyst surface along the second reactor (Run 7). No significant difference for the temperatures of gas and catalyst surface.Therefore, the overall rate of reaction (rate of forward reaction-rate of backward reaction) will actually decrease when moving from the inlet to the outlet for both of the reactor. It is not recommended to have extreme low overall rate of reaction along the reactor. The average overall reaction rates in the Run 7 are reasonable high.

Figure 10: The concentration profile of CO changes along the first reactor (Run 7)

Figure 11: The concentration profile of CO changes along the second reactor (Run 7)From the Figure 8 and 9, it can be seen that there is not significant difference between the gas temperature and the temperature at the catalyst surface. This means that the heat transfer occurred between the gas and the solid catalyst at the catalyst surface is excellent with monolith design done in the Run 7. The heat transfer resistance is relatively low. The concentration surface plot for the first reactor and second reactor are shown in Figure 10 and 11 respectively. From the plot, it can be noticed that the difference between the surface concentration (x=0) and the concentration within the catalyst layer (0