Zeolite membrane reactor for high-temperature isobutane dehydrogenation reaction: Experimental and modeling studies
Graphical abstract
Introduction
Isobutylene is an important intermediate for synthetic rubber, plastics, and various chemical and petrochemical products such as methyl tert-butyl ether, alkylate gasoline, and butyl rubber [1]. The most popular industrial technique to synthesize isobutylene is catalytic dehydrogenation of isobutane [2]. A dehydrogenation reaction requires relatively high operating temperature for achieving high yield in a conventional reactor. Rigorous operating conditions results in inevitable catalyst deactivation because of coke formation [[3], [4], [5], [6], [7], [8]]. However, in packed bed membrane reactors (PBMR), the membrane assists in moving equilibrium towards the forward reaction due to selective H2 removal across the membrane (Eq. 1). For this reason, the PBMR is capable of exceeding the equilibrium limitations, which would not be possible without the addition of H2-selective membrane.
In recent years, isobutane dehydrogenation (IBDH) has received plenty of attention as an industrial process due to versatile use of isobutylene [[9], [10], [11], [12], [13]]. Takeshi et al. [14] used a palladium membrane reactor (MR) to conduct IBDH reaction with Pt-Al2O3 and Cr2O3-Al2O3 catalyst. Pt-Al2O3 catalyst exhibited lower isobutylene yield in comparison to Cr2O3-Al2O3 catalyst, but in both cases isobutylene yield was higher than the thermodynamic limit. Johan et al. [15] studied IBDH in a DD3R zeolite MR at 439 °C and 489 °C. Isobutane was used as the incoming feed stream with nitrogen as sweep gas. At 500 °C, the DD3R zeolite membrane exhibited an exceptional H2/isobutane permselectivity of 520 as well as a moderate H2 permeance of approximately 4.5 × 10−8 mol m-2 s-1 Pa-1. The isobutylene yield in a MR was 41%, where the equilibrium yield was 28% at 489 °C. The increased performance can be attributed to the removal of almost 85% of H2 from the reaction side at lower space velocity. Removing H2 increased coke formation, suppressed hydrogenolysis reaction, and decreased the catalyst activity [[16], [17], [18]]. Ciavarella et al. [19] researched IBDH reaction in a MR, integrated with a bimetallic Pt-In-zeolite fixed catalyst bed with a microporous MFI zeolite tubular membrane. The effect sweep gas flow rate had on membrane performance was investigated with both counter-current and co-current mode. Isobutylene yield of PBMR was four times larger than that of the packed bed reactor (PBR). Weiqiang et al. [20] studied IBDH both experimentally and by modelling for a PBMR and PBR using a Pt/alumina catalyst. Comparative tests showed that a PBMR had higher isobutylene yield and higher selectivity. The simulations showed good agreement with experimental results but slightly over predicted values. Casanave et al. [21] studied IBDH in a zeolite PBMR in combination with Pt-In catalyst. Increased isobutylene yield was achieved in a PBMR by separation of H2 from the reaction side. Both counter and co-current methods were operated and it was found that H2 selectivity and reaction performance was higher in counter-current mode.
Van dyk et al. [22] investigated both Pd and MFI catalytic membrane reactors (CMRs). Both CMRs included Pt catalyst with the membranes for IBDH and showed higher isobutylene yield than PBR. Although both membranes showed different separation properties, the two CMRs showed similar isobutylene yield of 24%. This can be attributed to the fact that the whole process was kinetically limited and thus any increase in separation properties could not increase isobutylene yield. Loannides et al. [23] studied IBDH using a commercial chromia-alumina catalyst for both PBR and PBMR using a dense silica membrane. Impact of temperature and feed composition on reactor performance was studied. A decrease in catalyst activity was observed in the initial 2–3 h and this lessening of catalyst activity escalated with temperature but was reduced by H2 addition in the feed. Membrane permeance was found to be 8.1 × 10−7 mol m-2 s-1 Pa-1 and the H2/hydrocarbon permselectivity was 80 - 300. At all operating conditions, a PBMR showed better isobutylene yield and selectivity than a PBR. Farsi et al. [2] studied modeling for IBDH reaction in PBRs that operated in radial flow. It was simulated heterogeneously based upon the laws of energy and mass conservation. In the model, IBDH was treated as the main reaction and propane dehydrogenation, coke formation, and hydrogenolysis were considered as side reactions. The isobutylene yield and conversion of isobutane at optimum conditions were 91% and 40%, respectively. Kobayashi et al. [1] studied the impact iron oxide had on IBDH over a Pt/Fe2O3-Al2O3 catalyst and found that catalyst activity, selectivity, and stability were greatly improved after a small amount of Fe2O3 was incorporated to the catalyst. Analysis of the adsorbed carbon monoxide by FT-IR revealed that the electron density of the Pt was enhanced by the formation of bimetallic particles.
The MFI-zeolite membranes have been reported to be one of the promising candidates for high temperature reaction and separation [[24], [25], [26], [27], [28]]. However, there are very few reports on high temperature IBDH PBMR. We used MFI-type zeolite membranes in order to investigate the impact of operating conditions on the IBDH reaction at 500–650 °C. For further investigation of the zeolite PBMR, a one dimensional (1D) model of a plug flow reactor (PFR) was built and utilized to check for accuracy as well as to study the effect of operating conditions upon the performance of IBDH PBMR.
Section snippets
Membrane synthesis
Secondary growth method was used to synthesize the MFI zeolite membrane on the surface of a seeded α-alumina disk [25]. To prepare the membrane, macroporous α-alumina disks with a thickness of 1 mm, porosity of 25%, and diameter of 1 inch (Coorstek) were used as the supporting structures. In order for proper membrane growth to occur, it was necessary for the surface of the α-alumina disks to be smooth therefore a sandpaper polishing device was used to polish disks [24,29]. After the discs were
Membrane separation properties
In Fig. 2, SEM images revealed that the MFI-type zeolite membrane is ˜12 μm thick. Fig. 3 shows the binary gas permeation characteristics for H2/i-C4H10 and H2/i-C4H8 mixture from room temperature to 600 °C. At room temperature, membrane was more selective to i-C4H10 and i-C4H8 as the separation was adsorption dominant and H2/i-C4H10 separation factor was less than 1. On increasing the temperature, separation becomes diffusion dominant and H2 being smaller molecule diffuses faster and therefor H
Conclusions
Isobutane dehydrogenation (IBDH) reaction was performed by experiments and by modeling calculation with microporous MFI-type zeolite membranes. It was found that packed bed membrane reactors (PBMR) successfully exceeded the equilibrium limit and enhanced the i-C4H10 conversion due to H2 permeation across the membrane. The IBDH PBMR exhibited higher conversion, selectivity, and yield than the packed bed reactor (PBR). The impact of operating specifications upon H2 recovery (RH2) was studied and R
Acknowledgements
The authors thankfully acknowledge funding from Oklahoma State University. This research was partially supported by ICT & Future Planning(#NRF-2017R1C1B1002851), Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science.
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