(519c) Theoretical Studies on Sorption-Enhanced Hydrogen Production

            Adsorptive
reactors represent an important class of multifunctional reactors, which provide
much potential for process intensification (Stankiewicz, 2003). Such hybrid
configurations may substantially improve reactant conversion or product
selectivity and, for reversible reactions, establish more favourable reaction
equilibrium than that could be achieved under conventional reactor operation.

This work deals with the theoretical study of an alternative process for
hydrogen production through steam methane reforming (SMR), based on the concept
of adsorption-enhanced reaction. . Unlike previous studies in this area (Hufton
et al., 1999; Stepanek et al., 1999; Ding and Alpay, 2000; Waldron et al.,
2001), the continuous flow of adsorbent within a packed or structured reactor
is proposed. Hence, this process can be regarded as the adsorptive reactor
equivalent of the fluid catalytic cracking (FCC) process, but in this case the
transported medium is the adsorbent. Similar to the FCC process, the benefits
of this process are expected to be significant, with the excellent control of
adsorbent residence time, the continuous supply of feed to a single unit, the
enhanced mass and heat transfer, and an integrated energy supply system. The newly proposed process consists of a reactor/adsorber unit and
regeneration (desorption) unit. The novelty of this approach is the use of a
stationary SMR catalyst phase, through which adsorbent flows for the inÐsitu
and selective removal of carbon dioxide. Such CO₂ removal results in
favourable shifts in the reaction equilibria of both the reforming and water-gas
shift reactions towards further carbon dioxide production. Furthermore, the
reaction can be carried out at a moderate temperature range of 400-500^oC,
which is considerably less than that of the conventional SMR process (>800^oC).
Adsorbent regeneration is carried out ex-situ, and hot regenerated adsorbent
passed back to the reactor unit. Thus, the reaction heat may also be supplied
in a direct manner. As a result, a continuous, energy-integrated process is
enabled, in which high purity hydrogen at the reactor pressure is produced.

A non- isothermal mathematical model, accounting for general reaction
kinetics, mass transfer limited adsorption kinetics and non-linear
(Langmuirian) adsorption equilibria, has been developed. Particular attention
has been given to the evaluation of effective gas-catalyst and gas-adsorbent
contact, and therefore effective sorption-enhanced reaction. As a result, the nature
of the stationary phase is of great importance. Specifically, packed bed and
monolith catalyst structures have been considered. The modelling studies are
supportive of the pilot-scale reactor experiments on gas-solid two-phase
mixture flow through such structures by our collaborators at the University of
Leeds (Wang et al., 2004; Ding et al., 2005). The work has also enabled the
evaluation of the feasibility of new adsorbent materials currently being
developed by our collaborators at the University of Bath. Particularly, two
types of CO₂ adsorbent have been considered, an hydrotalcite and a
lithium zirconate based one.

Simulation results indicate the feasibility of this process concept. The
results also provide evidence for the use of monolithic reactor as an
adsorptive reactor. The determination of key design and operating parameters,
such as reactor dimensions and temperature, superficial gas velocity and
adsorbent mass flux, has been enabled by adopting a model-based process
optimisation approach. This is essential in order to investigate the optimal
energy integration between reaction and regeneration stages of the process. As
a system with thermally coupled recycle, there is much potential for
interesting dynamic behaviour. The model is also used to explore this and
perform a stability analysis.

References

Y. Ding, E. Alpay, Chem. Eng. Sci. 55
(2000) 3929.

Y. Ding, Z. Wang, M. Ghadiri, D. Wen, Powder Technol. 153 (2005) 51.

J.R. Hufton, S. Mayorga, S.Sircar, A.I.Ch.E Journal 45 (1999) 248.

A. Stankiewicz, Chem.
Eng. Process. 42 (2003)
137.

F. Stepanek, M. Kubicek, M. Marek, P.M. Adler, Chem. Eng. Sci. 54 (1999) 1493.

W.E. Waldron, J.R. Hufton, S.Sircar, A.I.Ch.E Journal 47 (2001) 1477.

Z.L.
Wang, Y.L. Ding, M. Ghadiri, Chem. Eng. Sci. 59 (2004)
3071.