(25b) Membrane Reactors for Multiphase Hydrotreating of Biomass: Overcoming Gas-Liquid Mass Transfer Limitations

An important step in the realization of large scale biorefineries is the efficient conversion of a diverse range of biomass feedstock into not only biofuels, but also an array of chemical products.  However, due to the physical properties of the substrate, hydrotreating biomass with conventional methods is not easily achieved.  Biomass intermediates are not easily vaporized and must be processed in the liquid phase, requiring extreme hydrogen pressures to ensure sufficient hydrogen coverage at the catalyst.  In this work membrane contactors have been evaluated for the selective hydrotreating of model compounds with hydrogen pressure as low as 1/50^ththe pressure required by traditional reactors.

The membrane contactors considered are hollow fiber pervaporation membranes, in bench-scale modules and coated in-situwith various noble catalysts.  Hydrogen was supplied from the shell side of the membrane fiber and permeated from the shell side to the bore side, where it adsorbed directly onto the metal surface.  Liquid reactant is circulated through the bore side, allowing the liquid to come into direct contact with the metal coated surface of the membrane where the hydrotreatment occurred.  Our membrane contact reactor approach replaces the traditional three phase batch slurry reactor.  These traditional reactors possess inherent mass transfer limitations due to low hydrogen solubility in liquid and slow diffusion to the catalyst surface. This causes hydrogen starvation at the catalyst surface, resulting in undesirable side reactions.        

Here we report successful hydrodeoxygenation (HDO) of bio-oil using a catalytic membrane reactor.  Fast pyrolysis of lignocellulosic biomass produces bio-oil and other by-products.  The bio-oil produced has a high oxygen content and has limited fuel value in this form; further processing is required to obtain a useful fuel.  Bio-oil upgrading can be achieved through heterogeneous three-phase catalytic HDO; however, state of the art HDO requires severe conditions (130 atmospheres and 250-400°C).  We were able to successfully HDO bio-oil at relatively mild conditions: 3 atm H₂ and 90°C. 

Another reaction under study is the hydrogenolysis of lignin model compounds; lignin being a waste product of cellulosic ethanol production.  Lignin is an organic polymer comprised of functionalized benzene rings as the monomers.  The monomers are held together through various C-C and C-O linkages; by breaking these linkages it is possible to produce functionalized benzene molecules that resemble the original monomers.  These functionalized benzene rings can be further hydrotreated to produce the valuable platform chemicals benzene, toluene, and xylene (BTX).  Similar to other biomass substrates, three phase heterogeneous catalysis suffers from low hydrogen solubility in the liquid phase.  Consequently, hydrogenolysis of lignin and lignin model compounds requires hydrogen pressure of 100 atm or more.  Membrane reactors are being developed to perform lignin hydrogenolysis at much lower pressures.

Extending this technology from the bench scale to commercial use requires a more complete understanding of how process conditions and reactor design influences system performance.  Scale up questions were investigated for membrane reactors in the partial hydrogenation of vegetable oil with low trans-fats; previous bench scale studies in our lab have shown that using membrane reactors reduce the trans-fat produced by over 70%.  It was found that the ratio of membrane surface area to volume of oil reacted has a significant impact on the reaction rate. Additionally, the system is shown to be nearly zero order in hydrogen, indicating that the catalyst surface maintains high hydrogen coverage throughout the experiments.  Implications for system design and operation will be discussed.

This presentation will include an introduction to the novel membrane contactor technology, a discussion of the use of this technology in multiple applications, and information on system scale up.