Chemistry Reference
In-Depth Information
vibrated beds and pulsed beds [2]. The general trend
today is to produce simple, low-cost cells that offer
flexibility in materials and operation.
Three-dimensional electrodes can be operated
either in a flow-through or flow-by configuration
where the electrolyte and overall current flow are
parallel or perpendicular, respectively. The latter
offers the means of independently varying the length
of the electrodes in the direction of fluid flow and
the thickness of the electrode in the direction of
current flow. Therefore, in the case of limiting
current operation, the design in principle can ensure
a uniform current, subject to restrictions and limita-
tions in fluid flow and mass transport.
Reactor design for multiphase reactions
In some applications the reactor must contend with
more than one fluid phase. As a multiphase reactor,
the electrochemical unit must couple together
an interphase mass-transfer process between, for
example, a liquid and a gas. Probably the simplest of
procedures for carrying out electrochemical reac-
tions involving gaseous reactants is to feed the gas
directly into the cell as a dispersed two-phase
mixture or to sparge the gas directly into the cell. The
presence of gas reduces the effective conductivity of
the electrolyte and thus optimisation of performance
is required. Several processes have considered using
the packed bed as a two-phase electrochemical
reactor in which the packing is the active electrode
material. Examples of this include the removal of
chlorine by reduction, the oxidation of SO
2
directly
on a carbon bed electrode and the production of
hydrogen peroxide [8]. In the latter case, peroxide is
produced by oxygen reduction on carbon electrodes
and the cell is operated in a trickle flow regime (see
Fig. 19.6).
Fig. 19.6
Electrochemical cell for the production of hydrogen
peroxide.
reuse include the production of methanol, either
catalytically from carbon dioxide and hydrogen
and formic acid, catalytically or electrochemically
from carbon dioxide and water. Thus 'energy cycles'
are envisaged in which hydrocarbon-based fuels
(methanol, formic acid) are used to produce, for
example, electric power by fuel cells and the carbon
dioxide generated is reconverted back to fuel.
These cycles are net energy inefficient and thus are
not sustainable unless the fuel is produced by, for
example, fermentation, e.g. ethanol or wood alcohol
(methanol). Thus a potential fuel cycle based on
methanol could be developed in which, for example,
methanol is used directly in the fuel cell.
A similar concept called 'hydrogen economy' was
put forward in the 1970s, where hydrogen is used as
the major energy vector. In practice this could mean
that water is used to generate hydrogen and oxygen,
by electrolysis, which are used in fuel cells to gener-
ate power. Thus we have an energy system based
on water or a 'water energy economy' (Fig. 19.7).
Although the majority of electricity is generated
by fossil fuel combustion, the future generation
would need to be based on 'renewable' sources
4 Electrochemistry and
Energy Sustainability
Human demand for energy puts increasing pressure
on the world carbon sources. Although the trend,
historically, has been to use fuels with lower carbon
content, e.g. natural gas, rather than those with
higher carbon content, e.g. coal, this cannot be sus-
tained indefinitely unless there is developed effective
technology to recover and reuse carbon dioxide, the
combustion product. Methods proposed for this
