1. Introduction
Endothermic
catalytic reaction apparatus, for converting hydrocarbon feedstock to
hydrogen-rich gases, has attracted increasing attention in recent years
[1, 2]. Commercial production of hydrogen is commonly achieved by a
process known as steam reforming [3], that involves the endothermic
reaction between a mixture of hydrocarbon feedstock and steam passed
through a catalyst filled reactor tubing that is heated [4]. In
commercial steam reformers for large-scale production of hydrogen from
hydrocarbon feeds, endothermic heat is commonly supplied by the
combustion of carbonaceous fuel and oxidant in a diffusion or turbulent
flame burner that radiates to the refractory walls of a combustion
chamber [5], thereby heating them to incandescence, and providing a
radiant source for heat transfer to a tubular reaction chamber [6].
Uniform radiation to the surfaces of the tubular reaction chamber is
essential since excessive local overheating of the tube surface can
result in mechanical failure [6]. In large-scale commercial steam
reformers, mal-distribution of heat within the furnace chamber is
minimized by providing large spacing between the individual reactor
tubes, the furnace walls, and the burner flames [7, 8]. However, for
small-scale catalytic reaction apparatus that is uniquely compact, such
as for the production of hydrogen for small fuel cell applications,
special design features are needed to prevent tube overheating.
A compact reformer comprises an annular reaction chamber concentrically
disposed around an internal burner chamber containing a vertically
disposed cylindrical radiant burner that uniformly radiates in the
radial direction [5, 6]. A uniform radiation pattern to a
concentrically disposed annular reaction chamber that surrounds the
radiant burner, is provided, thereby avoiding the problems with flame
impingement and local overheating of tube surfaces that are associated
with the use of diffusion or turbulent flame burners in compact reformer
apparatus [7, 8]. However, there are practical limitations regarding
the use of an annular reaction chamber for small-scale reformers having
hydrogen production rates of less than about 1500 standard cubic foot
per hour. The heat transfer coefficient of gaseous reactants contained
within an annular reaction chamber is directly related to the velocity
of the gaseous reactants within the annular space [9, 10]. In order
to limit the reaction chamber wall temperature, the velocity of gaseous
reactants within the annular space must be sufficiently high to absorb
the radiant heat flux that impinges on the reaction chamber tube walls
[11, 12]. However, for very small-scale reformers, this requires
that the width of the annular reaction chamber space be small [13,
14]. It is common practice to limit the maximum diameter of the
catalyst particles packed within an annular space to less than 20
percent of the width of the annular space [15, 16] in order to
ensure that the catalyst is evenly distributed within the reaction
chamber and to prevent gas channeling along the walls of the reaction
chamber [17, 18]. However, for an annulus having a small width
dimension, this requires use of catalyst particles of particularly small
diameters thereby resulting in an undesirably high pressure drop through
the catalyst bed.
The benefits of a flameless radiant burner for use in compact catalytic
reaction apparatus of annular reaction chamber geometry are known [19,
20]. For small-scale reformer applications, a tubular reaction chamber
geometry is preferred over annular reaction chamber geometry in order to
simultaneously achieve high heat transfer coefficients and low pressure
drops within the reaction chamber [21, 22]. There is need for a
compact endothermic catalytic reaction apparatus to achieve the objects
of compact design, while avoiding the problems of flame impingement,
excessive reaction chamber wall temperatures, and excessive reaction
chamber pressure drop by application of a tubular reaction chamber that
is heated by the radiant burner [23, 24]. The tubular endothermic
reaction chamber may employ a combination of catalyst particle sizes and
reactant mass velocities to control the reactor pressure drop and the
maximum reaction chamber tube wall temperature within certain needed
limits [25, 26]; and the radiant burner is operated at specific
ranges of combustion intensity and excess air to control surface
temperature of the radiant burner within certain needed limits [27,
28]. The design extends the practical range of tubular endothermic
reaction chamber geometry that can be used in combination with radiant
burners for converting hydrocarbon feedstock to useful industrial gases.
Many chemical processes utilize catalysts to enhance chemical conversion
behavior. A catalyst promotes the rate of chemical conversion but does
not affect the energy transformations which occur during the reaction.
The present study is focused primarily upon the performance and
efficiency analysis of steam-methanol reforming processes in combined
parallel plate heat exchanger-reactors. The steady-state continuity,
momentum, energy, and species conservation equations are solved in the
fluid phase and the heat equation is solved in the solid phase using a
finite volume approach. An adaptive meshing scheme is used for the
discretization of the differential equations. Computational fluid
dynamics simulations are carried out over a wide range of material
conductivities. Continuity in temperature and heat flux is applied at
the fluid-solid interfaces. Neither heat-transfer nor mass-transfer
correlations are employed. Parallel processing employing a message
passing interface is used to speed up the most demanding calculations.
The present study aims to explore how to effectively enhance chemical
conversion behavior by utilizing catalysts. Particular emphasis is
placed upon the effect of wall thermal conductivity on the performance
and efficiency of steam-methanol reforming processes in combined
parallel plate heat exchanger-reactors.