Page 283 - Lignocellulosic Biomass to Liquid Biofuels

Fischer Tropsch synthesis of syngas to liquid hydrocarbons 237

production could benefit from compact and modular conversion tech-
nology. Space and weight as well as demanding for greatly combined,
solid, light-weight, and nontoxic approach represent the substantial lim-
itations [18,28,53,138 140]. Monolithic/Microstructured reactors were
considered to meet such requirements and comprise the advantages of
FBR and SPR reactor. Monolithic reactors have numerous promising
benefits in respect to traditional FT reactors, such as higher gas liquid
mass transfer rates, short diffusion distance, plug flow characteristics,
low pressure drip, wax-catalyst separation not necessary, and precise
temperature control by direct cooling of catalyst with the liquid
medium and external heat removal [59,141,142]. However, the
low thermal conductivity of the ceramic monoliths and small-scale pro-
duction capacity are the major drawbacks of the monolithic reactor
[57,84].
Over the past decade, CNFs and carbon nanotubes (CNTs) have been
reflected as a potential support material for active metals and oxides cata-
lysts. This specific interest is attributed to their wide properties, such as
relatively high surface area, high purity, chemical resistance to acids and
bases, and good electric and thermal conductivity [9,57,59,143]. The
CNFs showed an enhanced catalytic activity and high C 51 selectivity
compared to conventional support materials, such as alumina, silica, or
activated carbon [22,127,144 152]. Yu et al. [153] reported that CNF-
supported cobalt catalyst demonstrated high activity and high selectivity to
C 51 hydrocarbons compared with conventional cobalt catalysts supported
on alumina for FT synthesis. However, CNF-supported Co catalysts deac-
tivate quickly. Their activity could be improved using hierarchically struc-
tured CNF/CF composites as compact FT reactors [154]. Hierarchically
structured CNFs/CF composites in the FT synthesis showed several
advantages, such as improved dispersion and reducibility of cobalt nano-
particles, which results in high activities and selectivity [155,156].
Although industrial applications of CNTs and CNFs in powder form have
several difficulties, such as difficult to handling and transportation prob-
lem, quick pressure drop for gas phase, and accumulation with time on
stream [144]. Although the health risk and safety concerns associated to
the large-scale airborne presence of CNTs/CNFs, these aspects limit the
applicability of such catalysts for industrialization. To overcome these
limitations, several researchers observed the growth of CNTs/CNFs on
structured supports, where macroscopic shape offers easy handling and
transportation.