Structured reactors as alternative to pellets catalyst for propane oxidative steam reforming

Abstract

The performance of a Pt/CeO₂ catalyst as packed bed, coated on monolith and as self-structured bed has been evaluated during C₃H₈ oxidative steam reforming. Structured bed, prepared by a new aqueous tape casting method, combining high total porosity (80%) with a self-supported channel structure, offers a better and more efficient control of heat and mass transfer along the catalytic bed, showing, especially at high gas hourly space velocity (30 × 10⁴ h⁻¹), better performance in terms of fuel conversion, hydrogen production and low by-products formation coupled with an economy of the catalyst of about to 43% with respect to the traditional packed bed system.

Introduction

Proton exchange membrane fuel cells (PEMFCs), recently, have received much attention as clean and efficient sources of electrical power for transportation as well as for both stationary and portable devices [1], [2], [3], [4], [5], [6]. PEMFCs systems require a high purity hydrogen for operation. Hence, their commercialization requires a readily available hydrogen source, which is either used directly or is produced in an fuel processor [7]. In absence of a hydrogen refuelling infrastructure, the fuel reforming technology is a practical short-term alternative to supply hydrogen. Hydrogen can be produced by hydrocarbons and/or alcohols reforming. Natural gas, liquefied petroleum gas, gasoline and diesel appear as attractive hydrocarbon sources for fuel processors, due to their existing distribution and supply infrastructure [8], [9], [10], [11]. Fuel processing development represents a significant challenge to the commercialization of PEMFCs. Key requirements for a fuel processor include small size and weight, simple design, low cost, high reliability and safety associated with rapid start-up, good dynamic-response to change in hydrogen demand, high fuel conversion, stable performance for repeated start-up and shutdown cycles and maximum thermal integration [12], [13]. Steam reforming (SR), partial oxidation (POX) and autothermal reforming (ATR) are the primary methods used in the hydrocarbons reforming to produce hydrogen [14], [15]. Catalytic steam reforming, C_nH_m + nH₂O → nCO + (n + m/2)H₂, involving the reaction of steam with fuel is an endothermic process that requires a large energy input, considered to be the major drawback for the process [16]. Catalytic partial oxidation, C_nH_m + n/2O₂ → nCO + m/2H₂, is a more rapid process with a much higher reaction rate than steam reforming, with a lower H₂ yield/C fuel hydrocarbons [17], [18], [19]. The autothermal reforming process, C_nH_m + xO₂ + (n − 2x)H₂O → nCO + (n − 2x + m/2)H₂, that combines partial oxidation and steam reforming reaction can be considered a better option for converting hydrocarbons to H₂ [20], [21], [22], [23], [24], [25]. The oxygen (or air) to fuel molar ratio controls the overall heat balance of the reaction, the water required to convert the C present in the fuel into CO₂ and the maximum H₂ yield achievable in the products.

ATR catalysts must be active for both steam reforming and partial oxidation reactions; various supported transition metals (Ni, Co, Fe) or noble metals (Pt, Rh, Ru, Pd) are the standard catalyst formulations for autothermal reforming of hydrocarbons [26], [27], [28], [29]. Bimetallic Pt–Ni/δ-Al₂O₃, catalysts with superior performance characteristics compared to monometallic catalysts, has been reported by Çağlayan et al. [30], [31]. Supports as ceria, cerium–zirconium or lanthanum oxide, are attractive because of their redox capacity. Ceria and doped ceria (ceria–gadolinium, ceria–samarium) have been suggested as promising supports for oxidation and reforming catalysts [8]. Moreover, the oxygen storage ability of ceria has been indicated as an important feature, for the related catalysts, to improve the carbon resistance [32]. Ceria supported platinum catalysts have demonstrated high performance in hydrocarbon reforming [17], [33], [34], [35], [36]. In previous papers Ni/CeO₂ and Pt/CeO₂ catalysts have shown high activity in C₃H₈ oxidative steam reforming (OSR) [37], [38].

In addition to new catalysts, new reactor engineering approach may be required to achieve the performance targets needed for small-scale applications for distributed or mobile hydrogen production. In literature different configurations (foam, honeycombs, gauze) were proposed as alternative to traditional packed bed reactors [39], [40], [41], [42], [43]. Catalysts performance in pellets form are limited by mass-heat-transport phenomena especially at high flow rate [44], to increase the total H₂ production, volume and weight of reactors must be dramatically increased together with the catalyst cost. The large surface-to-volume ratios realized by structured reactor systems leads to a good heat and mass transfer properties, moreover, the reactors are small and compact and thus interesting for small-scale applications [45], [46].

In this paper, self-structured catalyst with circular concentric design was prepared by an aqueous tape casting method. The catalytic activity of this structured catalyst, compared with the same catalyst in pellets form and coated on cordierite monolith has been evaluated in propane oxidative steam reforming regime. Investigations on propane conversion and hydrogen yield as a function of catalyst configuration, space velocity and catalyst amount have been performed. Supported 1.13 wt% Pt/CeO₂ sample, that has showed good catalytic activity during the propane oxidative steam reforming [38] has been used as catalyst.