1. Introduction
Among
the various chemical reactions occurring in industrial reactors, the use
of catalytic gas-solid reactions is widespread. A packed bed reactor is
commonly used for these types of reactions. Conventional packed bed
reactors are associated with various difficulties and disadvantages,
including pressure drop, intra particle diffusion limitations, flow
channeling, and heat transfer limitations [1, 2]. Structured
catalyst reactors are frequently used to address these challenges. These
structured catalyst reactors are commonly utilized when there is a need
for controlled endothermic or exothermic reactions. However, existing
structured catalyst reactors, while demonstrating higher performance in
comparison to the packed bed reactors, still have a number of drawbacks,
including high cost, weight, thermal resistance, and heat management,
among others [3, 4]. Important products of the Fischer-Tropsch
reaction include gaseous hydrocarbons, such as lower olefins, paraffins,
or alcohols, and liquid hydrocarbons, such as higher olefins, paraffins
and alcohols [5, 6]. The Fischer-Tropsch reaction is highly
exothermic and therefore effective heat transfer and temperature
controls are important prerequisites for the successful operation.
Furthermore, the reaction operability range, with pressures between 1-30
bar and temperatures ranging from 200 °C to 350 °C, requires an
additional control to prevent formation of local hot spots responsible
for the deterioration of the catalyst [7, 8]. Thus, there is a need
for very efficient heat transfer during the reaction to prevent metallic
catalyst deactivation and formation of undesirable products.
Accordingly, there remains a need for a reactor that provides an
efficient control over endothermic and exothermic chemical reactions,
such as those carried out in the presence of a catalyst. For example,
there is a need for a reactor for the Fischer-Tropsch catalytic
processing of the synthesis gas that ensures very efficient heat
transfer during the catalytic reaction.
Natural gas and methane, a major constituent of natural gas, are
difficult to economically transport and are not easily converted into
liquid fuels or chemicals that are more readily contained and
transported. To facilitate transport, methane is typically converted to
synthesis gas which is an intermediate in the conversion of methane to
liquid fuels, methanol or other chemicals [9, 10]. Synthesis gas is
a mixture of hydrogen and carbon monoxide with a hydrogen to carbon
monoxide molar ratio of from about 0.6 to about 6. One chemical reaction
effective to convert methane to synthesis gas is steam reforming.
Methane is reacted with steam and endothermically converted to a mixture
of hydrogen and carbon monoxide [11, 12]. The heat energy to sustain
the endothermic reaction is generated by the external combustion of
fuel. A second chemical reaction effective to convert methane to
synthesis gas is partial oxidation. Methane is reacted with oxygen in an
exothermic reaction [13, 14]. Synthesis gas can be produced by
combined partial oxidation and steam reforming. The synthesis gas is
then converted to liquids by the Fischer-Tropsch process or can be
converted to methanol by commercial processes [15, 16]. In the
primary steam reforming of fluid hydrocarbons, such as natural gas or
methane, the feed material and steam are passed through
catalyst-containing vertically hanging reformer tubes maintained at an
elevated temperature by radiant heat transfer and by contact with
combustion gases in the furnace portion of the tubular reactor. The hot
reformer tube effluent may be passed to a waste heat recovery zone for
the generation of steam that can be used in the steam reforming
operations. Conventional primary steam reforming operations are commonly
carried out at temperatures of from about 750 °C to about 850 °C or
above [17, 18]. The primary steam reforming is a highly endothermic
reaction, and the large amounts of required heat are typically provided
by combusting external fuel at close to atmospheric pressures in the
reforming furnace. Consequently, the reformer tubes are generally made
of high alloy, expensive materials having a limited operating life under
such extreme conditions [19, 20]. The reaction temperatures existing
inside the reformer tubes are generally lower than about 850 °C so that
the effluent gas recovered from the primary reformer typically contains
2-6 percent methane [21, 22]. The effluent from primary reforming is
sometimes passed to a secondary reforming zone in which unconverted
methane present in the reformed gas mixture is catalytically reacted
with air, oxygen or other suitable oxygen-containing gas.
Large quantities of hydrogen, or of an ammonia syngas mixture of
hydrogen and nitrogen, are produced either by such steam reforming
operations or by partial oxidation reactions. Partial oxidation, like
secondary reforming, is an exothermic, autothermal, internal combustion
process [23, 24]. While secondary reforming is also a catalytic
process, however, the various known partial oxidation processes employ
non-catalytic reactions, and thus operate at higher reaction
temperatures on the order of about 1300 °C. The significant advantages
obtainable by the use of secondary reforming, or by the use of partial
oxidation processing, are off-set to some extent by the need to compress
the oxygen-containing gas to the desired reaction pressure or higher
[25, 26]. Another disadvantage of secondary reforming and of partial
oxidation processing is that part of the feed gas is combusted to carbon
dioxide and water instead of to desired product. As a result, more
natural gas or other feed gas is required to produce a given amount of
hydrogen or synthesis gas, although the autothermic processes do not
require any fuel. By contrast, the fuel consumption rate for primary
reforming is typically between 30 percent and 50 percent of the feed
rate [27, 28]. Despite such efforts to improve steam reforming
operations, it will be appreciated that there remains a desire to
achieve lower steam and fuel requirements and higher thermal
efficiencies in such operations [29, 30]. In addition, improved
mechanical designs are also desired to reduce the size of the overall
reforming systems employed and to achieve other useful purposes, such as
a reduction for the thermal stresses to which the primary reformer tubes
are subjected [31, 32]. It is also desired to carry out steam
reforming operations at higher pressures, as in the range of 20-100 Bar.
Such desired improvements also relate to the integration of primary and
secondary reforming operations, so as to obtain the benefits of
secondary reforming while achieving a more efficient overall reforming
operation than has heretofore been possible.
Currently, endothermic reactions performed in microreactors are driven
using heat from an external source, such as the effluent from an
external combustor. In doing so, the temperature of the gas stream
providing the heat is limited by constraints imposed by the materials of
construction. Practically, this means that tile effluent from an
external combustor must be diluted with cool gas to bring the gas
temperature down to meet material temperature constraints. This
increases the total gas flow rate, raising blower and compressor costs.
Moreover, heating the gas stream externally introduces heat losses and
expensive high temperature materials to connect the combustor to the
microreactor. On the other hand, an integrated combustor can produce
heat for the reaction in close proximity to the reaction zone, thus
reducing heat losses and increasing efficiency. Because traditional
combustion catalysts are riot stable at high temperatures due to noble
metal sintering, the integrated combustor must transfer heat at a rate
sufficient to keep local temperatures at the catalyst surface below this
level or risk rapid catalyst deactivation. The present study is focused
primarily upon the designs and operations of heat integrated reactors
for thermochemically producing hydrogen from methanol by steam
reforming. A symmetry boundary condition is used to model half of each
system where symmetry exists. Computations are performed using grids
with varying nodal densities to determine the optimum node spacing and
density that would give the desired accuracy and minimize computation
time. The final grid density is determined when the centerline profiles
of temperature and species concentration do not show obvious difference.
The second-order upwind scheme is used to discretize the mathematical
model, and the semi-implicit method for pressure-linked equations
algorithm is employed to solve for the pressure and velocity fields. The
simulation convergence is judged upon the residuals of all governing
equations. The present study aims to provide a fundamental understanding
of the designs and operations of heat integrated reactors for
thermochemically producing hydrogen from methanol by steam reforming.
Particular emphasis is placed upon the effect of various factors on the
thermochemical steam reforming processes in heat integrated reactors.