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.

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: [email protected]

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

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.

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.