Fluid mechanics of microchannel reactors for synthesis gas production
Authors: Chen Junjie 
Published: January 2023

The present study is focused primarily upon the fluid mechanics of
microchannel reactors for synthesis gas production. The effect of
surface features on the reactor performance is explored for the steam
reforming reaction. The conversion rate is used to compare the reactor
performance of different configurations. For the purpose of comparison,
a baseline case is modeled which is a straight channel of the same
dimensions as those for the cases with surface features in terms of
channel length, channel width, and gap size. The reactor performance
with surface features is quantitatively measured using different
enhancement factors. The results indicate that the surface features are
preferably at oblique angles, neither parallel nor perpendicular to the
direction of net flow past a surface. Flow boiling can achieve very high
convective heat transfer coefficients, and that coupled with the
isothermal fluid allows the heat transfer wall to remain at
quasi-constant temperature along the flow direction. Due to the
existence of vapor slugs, severe flow and pressure oscillation may occur
in microchannel boiling. Critical heat flux occurs when the temperature
difference reaches a point where the heat transfer rate changes from
nucleate and bubbly flow to local dry out and gas phase resistance
starts to dominate heat transfer. As the momentum is increased at higher
Reynolds numbers, the relative vorticity or angular force to spin the
fluid also increases and thus the number of contacts or collisions with
or near the active surface feature walls is also increased. The
performance enhancement of the active surface features relative to a
corresponding featureless or flat or smooth wall is typically improved
as the residence time is decreased.

Fluid mechanics of microchannel reactors for synthesis gas
production

Junjie Chen

Department of Energy and Power Engineering, School of Mechanical and
Power Engineering, Henan Polytechnic University, Jiaozuo, Henan, 454000,
P.R. China

Corresponding author, E-mail address: cjjtpj@163.com

Abstract

The present study is focused primarily upon the fluid mechanics of
microchannel reactors for synthesis gas production. The effect of
surface features on the reactor performance is explored for the steam
reforming reaction. The conversion rate is used to compare the reactor
performance of different configurations. For the purpose of comparison,
a baseline case is modeled which is a straight channel of the same
dimensions as those for the cases with surface features in terms of
channel length, channel width, and gap size. The reactor performance
with surface features is quantitatively measured using different
enhancement factors. The results indicate that the surface features are
preferably at oblique angles, neither parallel nor perpendicular to the
direction of net flow past a surface. Flow boiling can achieve very high
convective heat transfer coefficients, and that coupled with the
isothermal fluid allows the heat transfer wall to remain at
quasi-constant temperature along the flow direction. Due to the
existence of vapor slugs, severe flow and pressure oscillation may occur
in microchannel boiling. Critical heat flux occurs when the temperature
difference reaches a point where the heat transfer rate changes from
nucleate and bubbly flow to local dry out and gas phase resistance
starts to dominate heat transfer. As the momentum is increased at higher
Reynolds numbers, the relative vorticity or angular force to spin the
fluid also increases and thus the number of contacts or collisions with
or near the active surface feature walls is also increased. The
performance enhancement of the active surface features relative to a
corresponding featureless or flat or smooth wall is typically improved
as the residence time is decreased.

Keywords: Fluid mechanics; Microchannel reactors; Surface
features; Synthesis gas; Catalyst deactivation; Energy efficiency

1. Introduction

A synthesis gas product is a product comprising primarily carbon
monoxide and hydrogen. Reformed hydrocarbons may be further reacted in
one or more shift reactors to form additional hydrogen in the process
stream and separated in a separation unit, such as a pressure swing
adsorption unit, to form a hydrogen product [1, 2]. Synthesis gas is
conventionally used to produce synthesis gas products such as synthetic
crude, or further upgraded to form intermediate or end products [3,
4]. The synthesis gas may also be used to produce one or more
oxygenates, for example, ethers and alcohols. Synthesis gas can be
produced from methane-containing feedstocks by any number of primary
synthesis gas generation reactors [5, 6]. For example, synthesis gas
can be produced in a steam methane reformer, an endothermic reactor
where reaction is carried out either in heat exchange reactors, or by
other means where substantial heat may be transferred to the reacting
fluid, such as in the case of autothermal reforming, where a portion of
the feedstock is combusted inside the reactor to provide heat for steam
reforming either subsequently or in the same location as the combustion
[7, 8]. Synthesis gas can also be produced from methane-containing
feedstocks by dry reforming, catalytic or thermal partial oxidation and
other processes.

Various feedstocks can be used to produce synthesis gas and industry
desires to process multiple feedstocks [9, 10]. Industry desires the
ability to change from one feedstock to another during operation without
shutting down the reactor [11, 12]. For example, a synthesis gas
producer may desire to use natural gas for six months, naphtha for three
months, and then a mixture of natural gas and naphtha for two months
[13, 14]. Industry desires to process different feedstocks at
optimal energy efficiency while avoiding carbon formation in the primary
synthesis gas reactor [15, 16]. In addition to being able to process
multiple feedstocks, industry desires to be able to process a feedstock
where the composition, particularly the light hydrocarbon concentration
in the feedstock, varies over time [17, 18]. For example, synthesis
gas may be produced from a refinery off-gas where the light hydrocarbon
concentration varies depending upon the refinery operation [19, 20].
If the feedstock contains higher hydrocarbons than methane, that is,
hydrocarbons having two or more carbon atoms are used in the steam
reforming process, the risk for catalyst deactivation by carbon
deposition in the primary synthesis gas generation reactor is increased
[21, 22]. Industry desires to avoid carbon formation in the
synthesis gas generation reactor.

In order to reduce the risk of carbon deposition in the primary
synthesis gas generation reactor, hydrogen and synthesis gas production
processes may employ at least one catalytic reactor prior to the primary
synthesis gas generation reactor where the catalytic reactor is operated
at conditions less prone to hydrocarbon cracking than the primary
synthesis gas generation reactor [23, 24]. These reactors positioned
before the primary synthesis gas generation reactors are referred to as
pre-reformers [25, 26]. Pre-reformers can be operated adiabatically
or convectively heated by indirect heat transfer with combustion
products gases from the primary synthesis gas generation reactor [27,
28]. The activity of the catalyst in the pre-reformer may degrade with
use. Industry desires to compensate for the degradation of the
pre-reforming catalyst through operational changes to avoid carbon
formation in the primary synthesis gas generation reactor while
maintaining optimal energy efficiency of the overall process [29,
30]. In hydrogen and synthesis gas production processes employing
pre-reformers and steam methane reformers, the hydrocarbon feedstock may
be mixed with hydrogen for a resultant stream having one to five percent
hydrogen by volume, and subsequently subjected to a
hydrodesulphurization pretreatment to remove Sulphur [31, 32]. The
hydrocarbon feedstock may also be treated to remove olefins in a
hydrogenation reactor. In case hydrogen is present in the feedstock,
additional hydrogen might not be added [33, 34]. For steam reforming
of heavy naphtha, hydrogen concentrations as high as about 50 percent by
volume of hydrogen are known where the mixture is subsequently
pretreated in a hydrodesulphurization unit and a hydrogenation reactor
[35, 36]. Even higher hydrogen concentrations are possible depending
on the feedstock provided.

The feedstock, after pretreating, is combined with superheated steam to
form mixed feed having a prescribed steam-to-carbon molar ratio [37,
38]. The steam-to-carbon molar ratio is the ratio of the molar flow
rate of steam in the mixed feed to the molar flow rate of
hydrocarbon-based carbon in the mixed feed. The steam-to-carbon molar
ratio for steam methane reforming of natural gas typically ranges from 2
to 5, but can be as low as 1.5. The steam-to-carbon molar ratio is
generally higher for steam methane reforming of feedstock containing a
greater number of higher hydrocarbons, for example, propane, butane,
propane and butane mixtures, and naphtha. Higher steam flow rates are
used to suppress carbon formation and enhance the steam reforming
reaction. However, higher steam-to-carbon molar ratios disadvantageously
decrease the energy efficiency of the reforming process. Industry
desires to improve the energy efficiency of steam-hydrocarbon reforming
systems [39, 40]. A significant disadvantage which inhibits the
wider use of fuel cells is the lack of a widespread hydrogen
infrastructure. Hydrogen has a relatively low volumetric efficiency and
is more difficult to store and transport than the hydrocarbon fuels
currently used in most power generation systems. One way to overcome
this difficulty is the use of reformers to convert the hydrocarbons to a
hydrogen-rich gas stream that can be used as a feed for fuel cells
[41, 42]. Fuel reforming processes, such as steam reforming, partial
oxidation, and autothermal reforming, can be used to convert hydrocarbon
fuels into a hydrogen rich gas. In addition to the desired product
hydrogen, undesirable byproduct compounds such as carbon dioxide and
carbon monoxide are found in the product gas. For many uses, such as
fuel for proton exchange membrane or alkaline fuel cells, these
contaminants reduce the value of the product gas in part due to the
sensitivity of proton exchange membrane fuel cells to carbon monoxide
and sulfur [43, 44]. In a conventional steam reforming process, a
hydrocarbon feed is vaporized, mixed with steam, and passed over a steam
reforming catalyst. The majority of the feed hydrocarbon is converted to
a mixture of hydrogen, carbon monoxide, and carbon dioxide. The
reforming product gas is typically fed to a water-gas shift bed in which
much of the carbon monoxide is reacted with steam to form carbon dioxide
and hydrogen [45, 46]. However, water-gas shift beds tend to be
large complex units that are typically sensitive to air, further
complicating their startup and operation.

The present study is focused primarily upon the method and apparatus for
steam reforming methanol. Carbon monoxide, carbon dioxide and mixtures
thereof, can be removed from the hydrogen-rich reformate by subjecting
the hydrogen-rich reformate to one or more of a water gas shift
reaction, methanation, and selective oxidation. The present design
provides a method of generating electricity comprising the steps of
reducing the sulfur content of the sulfur-containing hydrocarbon fuel,
catalytically converting the reduced-sulfur hydrocarbon fuel to
hydrocarbons, steam reforming the mixture of hydrocarbons at a steam
reforming temperature in a catalyst bed to produce a reformate
comprising hydrogen and carbon dioxide, fixing at least a portion of the
carbon dioxide in the reformate with a carbon dioxide fixing material in
the catalyst bed to produce a hydrogen-rich reformate, and feeding the
hydrogen-rich reformate to an anode of a fuel cell, wherein the fuel
cell consumes a portion of the hydrogen rich reformate and produces
electricity, an anode tail gas and a cathode tail gas. The method can
further include the step of feeding at least a portion of the tail gases
to a combustor or anode tail gas oxidizer to produce an exhaust gas for
use in the steam reforming of sulfur-containing hydrocarbon fuels.
Optionally, but preferably, the method further includes the step of
reducing the amount of carbon monoxide and carbon dioxide in the
hydrogen-rich reformate by subjecting the hydrogen-rich reformate to one
or more of a water gas shift reaction, methanation and selective
oxidation. An integrated system in which tail gas from the fuel cell and
hydrogen storage system is used to provide heat needed to reform the
feed fuel and regenerate the calcium oxide bed. This study aims to feed
the hydrogen-rich reformate to an anode of a fuel cell, wherein the fuel
cell consumes a portion of the hydrogen-rich reformate and produces
electricity, an anode tail gas, and a cathode tail gas. Particular
emphasis is placed upon a fuel cell configured to receive the
hydrogen-rich reformate from the fuel processor and wherein the fuel
cell consumes a portion of the hydrogen-rich reformate and produces
electricity, an anode tail gas, and a cathode tail gas.

2. Methods

The microchannel reactor is illustrated schematically in Figure 1 for
the steam reforming process. A catalytic reaction channel is a channel
containing a catalyst, where the catalyst may be heterogeneous or
homogeneous. A homogeneous catalyst may be co-flowing with the
reactants. Microchannel apparatus is similarly characterized, except
that a catalyst-containing reaction channel is not required. The sides
of a microchannel are defined by reaction channel walls. These walls are
preferably made of a hard material such as a ceramic, an iron-based
alloy such as steel, or a Ni-based, Co-based, or Fe-based superalloy
[47, 48]. They also may be made from plastic, glass, or other metal
such as copper, aluminum and the like. The choice of material for the
walls of the reaction channel may depend on the reaction for which the
reactor is intended. In some cases, reaction chamber walls are comprised
of a stainless steel or Inconel® which is durable and has good thermal
conductivity [49, 50]. The alloys should be low in sulfur, and in
some cases are subjected to a desulfurization treatment prior to
formation of an aluminide. Typically, reaction channel walls are formed
of the material that provides the primary structural support for the
microchannel apparatus. Microchannel apparatus can be made by known
methods, and in some cases are made by laminating interleaved plates,
and preferably where shims designed for reaction channels are
interleaved with shims designed for heat exchange. Some microchannel
apparatus includes at least 10 layers laminated in a device, where each
of these layers contain at least 10 channels; the device may contain
other layers with less channels.