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Monolithic Substrate Support Catalyst Design Considerations for Steam Methane Reforming Operation

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Monolithic Substrate Support Catalyst Design Considerations for Steam Methane Reforming Operation Rev Chem Eng 2017; aop Luqmanulhakim Baharudin and Matthew James Watson* Monolithic substrate support catalyst design considerations for steam methane reforming operation DOI 10.1515/revce-2016-0048 as compared to other processes discussed by Baharudin Received October 16, 2016; accepted March 10, 2017 and Watson (2017). In this review paper, a monolithic structured packing for steam methane reforming (SMR) Abstract: This paper reviews the research undertaken to reactor will be studied. This novel reactor packing offers study the design criteria that address the monolithic sup- the benefits of overcoming the heat conduction limitation port structure requirements in steam reforming opera- in the current industrial-scale reformers, process intensi- tion for the effective mass transfer of process gases to the fication for an overall plant optimization, and materializ- active sites and effective conductive heat transfer through ing the concept of compact reformer. SMR is expressed by tube wall to the active catalytic areas, as well as low pres- (Saito et al. 2015). sure drop operation. Design considerations include selec- tion of substrate materials that possess good mechanical CH42+↔HO CO + 3 H2 (1) strength to withstand the severe reaction conditions and prevent catalyst crushing that would lead to carbon for- In a nutshell, a monolithic structure is a single structure mation and catalyst deactivation, and excessive heating consisting of thin-walled narrow channels that are paral- of the tube that results in hot spots which is fatal to tube lel to each other (Heck et al. 2001, Roy et al. 2004, Zamani- lifetime. The support’s mechanical properties are listed yan et al. 2011) for ease of reactants flow (Ryu et al. 2007). for the purpose of providing guidelines on verifying the Due to the channels, the structure offers a pressure drop structure durability. The practical aspect of packaging that is one to two orders of magnitude smaller than that and stacking the monolith structures in the reformer tube of randomly packed pellets or beads in a packed bed (Ryu for ease of loading and discharge is discussed to under- et al. 2007, Zamaniyan et al. 2011). In general, the mono- stand its readiness in industrial application. lithic catalytic system enhances the mass transfer effec- tiveness due to increased specific surface area and evenly Keywords: design considerations; monolithic catalyst; distributed flow, and most importantly higher effective- monolithic structure; monolithic support; steam methane ness of heat transfer for heterogeneous gas-solid cata- reforming. lytic reactions. These advantages can potentially result in lower investment and higher productivity for the produc- tion operations. In addition, a higher mechanical strength 1 Introduction of the monolithic structure (Zamaniyan et al. 2011) has the potential of bringing down the operating costs through Steam reforming of natural gas (methane) is a significant avoidance of frequent reformer shutdown and start-up for process in petrochemical plants and petroleum refineries. catalyst recharge due to breakage. It is the most common pathway for producing hydrogen The main focus of this paper is to review the aspects of and synthesis gas (syngas, i.e. hydrogen + carbon monox- a SMR reactor where a monolithic catalytic system might ide) due to its cost effectiveness (Mohammadzadeh and be useful. The development of monolithic catalyst in SMR Zamaniyan 2002, Zamaniyan et al. 2011, Saito et al. 2015) is an initiative to overcome the challenges faced by indus- trial-scale operation where performance gets affected due to the difficulty in achieving effective heat trans- *Corresponding author: Matthew James Watson, Department fer from the external heat supply to the catalyst for this of Chemical and Process Engineering, College of Engineering, highly endothermic reaction. For the purpose of discus- University of Canterbury, Private Bag 4800, Christchurch 8140, sion throughout this paper, the heat transfer effectiveness New Zealand, e-mail: [email protected] Luqmanulhakim Baharudin: Department of Chemical and Process is defined as the measure of effective utilization of the Engineering, College of Engineering, University of Canterbury, external heat supply by the catalytic system of the steam Private Bag 4800, Christchurch 8140, New Zealand reformer, for better thermal efficiency of the system, and Brought to you by | University of Canterbury Authenticated Download Date | 8/28/17 6:01 AM 2 L. Baharudin and M.J. Watson: Monolithic support catalyst design considerations the potential for intensifying the process by increasing the increased throughputs, which corresponds to reduced extent of reactant conversion per unit volume of reactor. investment costs. In the current design, the reformer A lot of interest has been shown in the area of process tube diameter is limited due to the ineffective reaction intensification (Giroux et al. 2005), and the applica- heat removal by convection of the process gas from the tion of monolithic catalytic system can be one of the pelletized catalysts to the reformer tube walls. The tube means towards achieving it. In process intensification, diameter is limited as a design safety feature to eliminate the process volumes are reduced. There are two ways by the risks of temperature runaway, as a certain minimum which the process intensification initiative can be intro- process gas flow rate is typically required to prevent hot duced. One, multiple unit operations are consolidated as spots (Tronconi et al. 2014). A monolithic support also a single unit. Two, a process unit size is reduced such as offers the benefits of attaining a minimum pressure drop in the case of the monolithic reactor. Both initiatives offer at high throughput (Ryu et al. 2007) as well as achieving the benefits of cost and weight savings. Additionally, the nearer approach to equilibrium of the reforming reaction latter also offers a better thermal response to a transient at the reformer exit, allowing close to complete methane behavior as a result of a smaller energy amount required conversion (Mohammadzadeh and Zamaniyan 2002). to heat the smaller reactor (Giroux et al. 2005). An example of an SMR process intensification study on multiple unit operations in a combined unit was con- ducted by Mei et al. (2007). In their study, a jacket metal 2 Intensification of hydrogen monolith reactor containing two parts that couple an manufacturing processes and exothermic oxidation process with the endothermic SMR reaction was simulated. The first part is an inner pipe challenges in industrial steam where the exothermic methane combustion took place methane reforming operation over Pd/Al2O3. The second is an annular part between the outer and the inner pipes where the endothermic SMR In this section, the requirements in developing intensi- took place over Ni/Al2O3, utilizing the heat generated from fied hydrogen manufacturing processes with regards to the exothermic combustion and thus avoiding the require- compact steam reforming unit for hydrogen fuel stations ment for an external heat supply as in the conventional are reviewed. It is followed by a brief discussion on chal- SMR reactor. lenges faced in industrial SMR operations. The discussion This review also places a focus on developing a introduces monolithic substrate support structures and compact reformer as a hydrogen supply distribution reviews the monolith’s design considerations investigated network infrastructure (Giroux et al. 2005), which can by various researchers around the world. be made possible by process intensification through the application of a monolithic structured SMR with a reduced tube length. Commercialized automobiles utilizing hydro- 2.1 Small scale hydrogen manufacturing gen fuel cells would find convenience in a well-distributed for fuelling stations hydrogen system. On the other hand, hydrogen fuelling stations with the infrastructure where hydrogen can be With the dawning of fuel cell technologies, a compact generated on site would be convenient for the hydrogen reformer design is useful, and to materialize this, inten- producers. The design of small-scale hydrogen fuelling sification of the hydrogen manufacturing process is station with on-site SMR applying the monolithic catalytic required. In order to achieve high throughput, the desired support system was developed and presented by Roh et al. gas hourly space velocity (GHSV) in a compact reactor is (2010). made possible with the employment of catalysts of a par- Based on the design presented by Roh et al. (2010), ticular shape that can keep pressure drop at a minimum the monolithic catalytic system is found to be suitable for level. SMR over Ni catalyst supported on porous alumina a compact reformer due to the prospect of a heat transfer pellets is limited in the transport of heat. This pelletized enhancement across the catalyst bed and an improvement catalyst system is less efficient as it requires large reformer of the reactant accessibility to the active catalyst site, for tube areas and thereby a big catalyst volume in order to better reaction performance and hydrogen production receive the large amount of heat from the external supply efficiency. For the industrial-scale reformers, the signifi- at high temperatures (Ryu et al. 2007). cant improvement in the radial heat transfer would intro- A large radial temperature gradient, whereby heat is duce a possibility of enlarged reformer tube diameters for flowing from the reformer tube
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