1. Introduction
Chemical
reactors are devices or vessels within which chemical processes are
carried out for experimental or manufacturing purposes [1, 2]. The
preparation of many industrially important chemicals by reversible
reactions, for example, the production of hydrogen by steam reforming,
is limited by reaction rates that are relatively low under conditions of
favorable equilibrium [3, 4]. A number of measures have been devised
for the purpose of creating more favorable equilibrium conditions, in
order to increase reaction rates and hence productivity for a particular
reaction [5, 6]. In the case of the production of hydrogen by steam
reforming, for example, the disadvantage of the relatively low
equilibrium constant at normal pressures, for the temperatures at which
the reaction takes place, may be overcome by the use of increased
pressure to the steam-hydrogen equilibrium, resulting in increased
productivity of hydrogen [7, 8]. The presence of a catalyst in other
cases, for example, in the production of sulfur trioxide from sulfur
dioxide and oxygen, permits the use of a lower, more advantageous
temperature by shortening the time needed to establish equilibrium
consequently resulting in shorter reaction times and increased
productivity.
For every equilibrium reaction in which the reverse reaction is favored
by high temperatures, there is some intermediate temperature at which
the forward production rate and equilibrium reconversion can be balanced
to give an optimum design for the forward reaction in an isothermal
reactor system [9, 10]. If temperature is controlled to vary with
time, an optimum heat-up and cool-down cycle for this system exists
[11, 12]. For such a cycle, heat is supplied or removed uniformly to
or from the reactor contents, so that the temperature change of the
reactor contents is progressively isothermal, or in other words, is
substantially uniform throughout the reactor at a given time [13,
14]. This is conventionally accomplished by heating the reactor and
contents uniformly, or by heating the reactor and stirring the contents
thereof [15, 16]. The temperature differential or gradient
consequently imposed, is in respect of time only, referred to as a
time-temperature gradient. The optimum time-temperature relation for a
slug of gas in a well-designed sulfur dioxide converter, for example,
calls for an increase in temperature in the early stage, to maximize the
reaction rate, followed by a decrease in temperature in the final stage,
to take advantage of more favorable equilibrium conversions at the lower
temperatures [17, 18]. When pressure has an influence on the ratio
of the fugacity coefficients for the components of an equilibrium
reaction, or when there is a change in the total number of moles of
products as compared with the total number of moles of reactants; it can
be expected that the production capacity of a reactor unit will vary
with the pressure [19, 20]. The usual way of coping with the
situation is to predetermine and fix the pressure of such a reaction at
a practical and advantageous level and carry out the reaction according
to the optimum time-temperature cycle, which can be determined by
standard techniques.
The production capacities of such systems can be increases considerably
and in some cases by as much as 100 percent over those capacities
obtained by using the optimum time-temperature cycle, by heating the
reactor and contents to reaction temperatures and above, non-uniformly,
so that two or more zones of unequal temperatures are created throughout
the reactor [21, 22]. The temperature differential or gradient
consequently imposed is in respect of position in the reactor, as well
as time, referred to as a time-position-temperature gradient. The cooler
portion or portions of the reactor prevent dissociation of the product
and excessive pressure build-up, while permitting a higher reaction rate
in the warmer portion or portions of the reactor than would otherwise be
obtainable [23, 24]. Moreover, the increased density of the gaseous
components within the cooler portion or zones of the reactor permits
larger charges of reactants to be made and stored within the reactor,
without the accompanying disadvantage of over-pressurization [25,
26]. The over-all result is an increase in productivity of the
reactor, or in other words, more pounds per hour of sought-for product
[27, 28]. It is accordingly necessary to provide a means for
increasing productivity of chemical reactors in equilibrium reactions
wherein the forward reaction is favored by conditions of high pressure.
It is further necessary to provide a means for increasing the
productivity of a chemical reactor in equilibrium reactions wherein the
total number of moles of product components is less than the total
number of moles of reactant components. It is also necessary to provide
a means for increasing the productivity of a high-pressure chemical
reactor in equilibrium reactions wherein the compressibility of the
products is greater than the compressibility of the reactants. It is
very necessary to provide a means for increasing the productivity of a
chemical reactor used for producing hydrogen by steam reforming.
The present study is focused primarily upon the heat management of
thermally coupled reactors for conducting simultaneous endothermic and
exothermic reactions. Computational fluid dynamics simulations are
carried out to better understand how to manage thermally coupled
reactors for conducting simultaneous endothermic and exothermic
reactions. Thermally coupled reactors are used for conducting
simultaneous endothermic and exothermic chemical reactions. The
endothermic reaction may be conducted in the one or more process layers
and may comprise a steam reforming reaction. The exothermic reaction may
be conducted in the one or more heat exchange layers and may comprise a
combustion reaction or a partial oxidation reaction. Exothermic heat may
transfer from the one or more heat exchange layers to the one or more
process layers. When more than one process layer and more than one heat
exchange layer are used, they may be aligned in alternating sequence, or
two or more process layers and two or more heat exchange layers may be
positioned adjacent to each other. The present study aims to provide a
fundamental understanding of the mechanisms of how to manage thermally
coupled reactors for conducting simultaneous endothermic and exothermic
reactions. Particular emphasis is placed upon the mechanisms involved in
the heat transfer processes in thermally coupled reactors for hydrogen
production by steam reforming.