Strategies for H2 Production By Steam Reforming of Ethanol with Pressure Swing Adsorptive Reactor

Strategies for H₂ Production by Ethanol Steam Reforming
with Pressure Swing Adsorptive Reactor

Yi-Jiang Wu¹, Ping Li¹, Jian-Guo Yu¹ and Alirio E. Rodrigues²*

1 School of Chemical Engineering, East China University of Science and Technology,

Shanghai 200237, China

2 Laboratory of Separation and Reaction Engineering, Associated Laboratory LSRE/LCM,
Department of Chemical Engineering, Faculty of Engineering, University of Porto,

Rua Dr. Roberto Frias s/n, Porto 4200-465, Portugal

Tel. 0086-021-64252171           Email: wuyijiang@ecust.edu.cn

Ethanol, which can be produced from renewable biomass resources, is an ideal feedstock for H₂ production, and the overall reaction of ethanol steam reforming can be described as:

CH₃CH₂OH(g)+3H₂O(g)↔6H₂(g)+2CO₂(g) (¦¤H⁰_298K = +173.3 kJ∙mol^-1)   (1)

However, the product stream always contains a large amount of CO₂, and other undesired products from side reactions. By using an adsorptive reactor, where steam reforming reaction with in-situ CO₂ adsorption taking place, the thermodynamic equilibrium can be shifted towards the product side, which is known as sorption-enhanced reaction process (SERP) (Hufton et al., 1999). Most researches have focused on the reaction stage where SERP was performed with different catalysts and adsorbents, while the cyclic operation process for H₂ production from SERP with ethanol as feedstock is yet to be developed and improved.

In this work, a two-dimensional reactor model developed in our previous work (Wu et al., 2014a) has been employed, since the temperature gradient in radial direction is found able to affect the H₂ production performance comparing with the one-dimensional reactor model (Wu et al., 2014b) where the effect of radial temperature difference is ignored. A model considering multi-compound and overall mass balance, Ergun relation for pressure drop, energy balance for the bed-volume element, and nonlinear adsorption equilibrium isotherm coupled with reactions to describe coupled mass, momentum and heat transport phenomena within the adsorptive reactor. Numerical solution of model equations for the cyclic process was obtained by orthogonal collocation with finite elements method. Besides, Ni-based hydrotalcite (Wu et al., 2013b) has been used as the reforming catalyst and the high temperature CO₂ adsorbent employed is the K-promoted hydrotalcite (Wu et al., 2013a), as shown in Fig.1.

Figure 1 2D reactor model used in this work

The effect of different operating strategies on the performance of cyclic pressure swing sorption-enhanced ethanol steam reforming for high-purity H₂ production has been investigated. The feasibility and effectiveness of conventional pressure swing SERP operating procedure and the use of reactive regeneration (with 10% of H₂ in the feed during the regeneration step) instead of direct steam purge developed by (Xiu et al., 2002) have been investigated. A schematic diagram is illustrated in the following Figure 2.

  

Figure 2 Schematic diagram of the cyclic operation for sorption-enhanced ethanol steam reforming

The simulation of a cyclic pressure swing process has been constructed accordingly with the following steps:

l  Reaction (co-currently to feed). Sorption-enhanced reaction process at p_H;

l  Depressurization (counter-currently to feed).  Pressure of the column is reduced to atmospheric pressure (p_L);

l  Regeneration (counter-currently to feed). Regenerating the CO₂ sorbent by steam (y_H2O = 100%) or steam with hydrogen (y_H2O = 90% and y_H2 = 10%) at p_L;

l  Purge (counter-currently to feed). Purging the column with H₂ and steam gas-mixture (y_H2 = y_H2O = 50%) with a pressure increase to p_H before the next cycle.

Finally, the effects of operating conditions (reaction temperatures, pressures, length of each step, feeding flow rate as well as the used of reactive regeneration) on the hydrogen purity, productivity and energy efficiency have been investigated by numerical simulation. High purity H₂ product (> 99 mol%, dry basis) with traces of CO content (< 30 ppm) can be produced directly from the pressure swing adsorptive reactor.

References

Hufton, J.R., Mayorga, S., Sircar, S., 1999. Sorption-enhanced reaction process for hydrogen production. AIChE J. 45, 248-256.

Rohland, B., Plzak, V., 1999. The PEMFC-integrated CO oxidation °ª a novel method of simplifying the fuel cell plant. J. Power Sources 84, 183-186.

Wu, Y.-J., Li, P., Yu, J.-G., Cunha, A.F., Rodrigues, A.E., 2013a. K-Promoted Hydrotalcites for CO2 Capture in Sorption Enhanced Reactions. Chem. Eng. Technol. 36, 567¨C574.

Wu, Y.-J., Li, P., Yu, J.-G., Cunha, A.F., Rodrigues, A.E., 2013b. Sorption-enhanced steam reforming of ethanol on NiMgAl multifunctional materials: experimental and numerical investigation. Chem. Eng. J. 231, 36-48.

Wu, Y.-J., Li, P., Yu, J.-G., Cunha, A.F., Rodrigues, A.E., 2014a. Sorption-enhanced steam reforming of ethanol For Continuous High-Purity Hydrogen Production: 2D Adsorptive Reactor Dynamics and Process Design. Chem. Eng. Sci. 118, 83-93.

Wu, Y.-J., Li, P., Yu, J., Cunha, A.F., Rodrigues, A.E., 2014b. High-Purity Hydrogen Production by Sorption-Enhanced Steam Reforming of Ethanol: A Cyclic Operation Simulation Study. Ind. Eng. Chem. Res. 53, 8515¨C8527.

Xiu, G.-h., Li, P., E. Rodrigues, A., 2002. Sorption-enhanced reaction process with reactive regeneration. Chem. Eng. Sci. 57, 3893-3908.