Non-oxidative dehydrogenation of light alkanes using ceramic membranes | A technological and techno-economic assessment
Jord Haven is a PhD student in the research group Catalytic Processes and Materials (CPM). (Co)Promotors are prof.dr.ir. L. Lefferts and prof.dr. J.A. Faria Albanese from the faculty of Science and Technology (S&T).
This dissertation presents a technological and techno-economic assessment of applying ceramic proton-conducting membranes in light alkane dehydrogenation processes. The corresponding light olefin (ethylene, propylene) products are widely used to produce e.g. plastics. Light olefins are traditionally obtained from carbon and energy intensive cracking processes. The non-oxidative dehydrogenation (NODH) of C2-C3 alkanes to olefins represents a more direct alternative production pathway. However, light alkane NODH is limited by thermodynamic equilibrium, requiring high temperatures (500-700 ᵒC) for restricted olefin yields (30-40%).
The alkane NODH equilibrium limitation can potentially be overcome by using ceramic hydrogen permeable membranes that shift the equilibrium towards olefins. The use of membranes could thereby substantially reduce the required energy input and carbon footprint of olefin production processes. Two different types of ceramic membranes are explored, namely (i) mixed proton-electron conducting (MPEC), and (ii) proton-conducting electrolysis cell (PCEC) membranes. While MPECs only offer the opportunity for shifting the alkane NODH equilibrium, the PCEC systems have the additional advantages of process electrification and tailoring the reaction thermodynamics and kinetics through the applied potential. The challenge of integrating ceramic membranes into large scale olefin production facilities is the delicate balance between the optimal operating conditions of the dehydrogenation catalyst and the proton-conducting membrane. This dissertation assesses the main technological hurdles and the potential techno-economic benefits and barriers of membrane-assisted light alkane NODH, as introduced in Chapter 1.
The alkane, olefin, and hydrogen concentrations will vary drastically inside membrane reactors. The influence of changing ethane, ethylene, and hydrogen concentrations on the kinetics and mechanism of the ethane NODH reaction over a traditional Pt-Sn dehydrogenation catalyst is investigated in Chapter 2. The results indicate that the ethane surface coverage is negligible under reaction conditions, whilst the surface occupancy of ethylene and hydrogen inhibit ethylene formation. The proposed Langmuir-Hinshelwood-Hougen-Watson (LHHW) reaction model comprises of four elementary steps: (i) dissociative ethane adsorption, (ii) surface hydrogen removal, (iii) ethylene desorption, and (iv) hydrogen desorption, where step (i) is identified as the rate-determining step. Moreover, ethylene and hydrogen adsorption are strongly coverage-dependent in this model.
Dense ceramic membranes for proton conduction function optimally under moistened atmospheres. Alkane dehydrogenation catalysts will, therefore, inevitably be exposed to steam rich conditions in ceramic membrane reactors. The influence of steam on the structure and performance of Pt-based catalysts in the ethane NODH reaction is explored in Chapter 3. There it is demonstrated that steam enhances the ethylene formation rate on Pt-Sn/ZnAl2O4 and Pt/ZnAl2O4. The tin-free Pt/ZnAl2O4 catalyst is not physicochemically modified by the steam. The corresponding increase in ethylene formation rate under wet conditions is, therefore, attributed to the surface cleaning role of steam in Pt/ZnAl2O4. By contrast, the Pt-Sn/ZnAl2O4 catalyst is physicochemically modified by steam, as XRD indicates PtSn dealloying in presence of steam and XPS shows a more oxidized Pt species in the Pt-Sn catalyst after steam treatment. Hydrocarbon attraction and ethane dissociation are possibly boosted on the more oxidized Pt species in steam treated Pt-Sn/ZnAl2O4, which increases the corresponding ethylene formation rate.
In established membrane reactor configurations, the membrane is usually physically separated from the catalyst. In an alternative configuration, the Pt catalyst could be deposited directly onto the ceramic membrane material to optimize the mass transfer from catalyst to membrane. To assess the potential of this strategy, Pt is deposited on promising MPEC (lanthanum tungstate, LWO) and PCEC (barium zirconium cerium yttrium oxide, BZCY) materials and the performance of the concerning catalysts is compared to the conventional Pt/ZnAl2O4 catalyst in Chapter 4. Relative to Pt/ZnAl2O4, a higher methane selectivity and a lower ethylene selectivity are observed for both proton-conducting supports. The enhanced methane formation for Pt/LWO and Pt/BZCY is ascribed to Lewis acid centers in LWO and BZCY. Additionally, the Pt/LWO and Pt/BZCY catalysts deactivate over time as compared to a stable Pt/ZnAl2O4 catalyst, caused by Pt sintering. Metal-support interactions are supposedly stronger in Pt/ZnAl2O4 than in Pt/LWO and Pt/BZCY. Nevertheless, cofeeding steam appears to facilitate Pt redispersion and thereby suppresses Pt sintering in Pt/LWO and Pt/BZCY.
The potential techno-economic benefits and challenges of the membrane-assisted alkane NODH concept are explored in two different process simulation studies in Chapter 5 and 6. In Chapter 5, it is investigated whether membrane-assisted propane NODH could be a feasible alternative to the already commercialized Honeywell/UOP Oleflex process for propane NODH. The results indicate that the MPEC-assisted propane NODH process is not an attractive alternative, as it is ca. 18 times more expensive and has a ca. 40% higher energy demand than the Oleflex process. This is attributed to high MPEC membrane reactor equipment costs and an enormous sweep gas heating demand, both related to the weak driving force for hydrogen permeation through MPECs. By contrast, the capital investment of the PCEC-assisted process is ca. 20% lower and the total energy demand ca. 30% lower as compared to Oleflex, making the PCEC process a suitable industrial alternative. These financial and energetic benefits are related to the reduction in process stream and unit operation sizes as a result of the higher single-pass propylene yield of the PCEC process (ca. 50%) relative to Oleflex (ca. 36%).
In Chapter 6, it is explored whether the use of PCEC membranes could brighten the industrial perspective of the non-commercialized ethane NODH process. To this end, the techno-economics of PCEC-assisted ethane NODH are compared to a conventional ethane steam cracking (SC) process for ethylene production. The results indicate that ethylene yields of ca. 25%, which have been achieved so far in experimental PCEC-assisted ethane NODH studies, are insufficient to financially and environmentally compete with ethane SC. Single-pass ethylene yields of ca. 50% would be required to possibly outcompete ethane SC in terms of carbon footprint, capital investment, and profitability.
The main research outcomes are summarized in Chapter 7. This chapter further provides suggestions for future research, related to e.g. improving the stability of PtSn catalysts under wet conditions and improving the stability and ethylene selectivity of Pt catalysts supported onto proton-conducting materials. Moreover, a list of industrial guidelines is provided in this chapter that need to be fulfilled to possibly make membrane-assisted alkane NODH a financially and ecologically attractive alternative to conventional olefin production routes.
