Modeling and process simulation of hollow fiber membrane reactor systems for propane dehydrogenation

We report a detailed modeling analysis of membrane reactor systems for propane dehydrogenation (PDH), by integrating a two‐dimensional (2‐D) nonisothermal model of a packed bed membrane reactor (PBMR) with ASPEN process simulations for the overall PDH plant including downstream separations processes. PBMRs based on ceramic hollow fiber membranes—with catalyst placement on the shell side—are found to be a viable route, whereas conventional tubular membranes are prohibitively expensive. The overall impact of the PBMR on the PDH plant (e.g., required dimensions, catalyst amount, overall energy use in reaction and downstream separation) is determined. Large savings in overall energy use and catalyst amounts can be achieved with an appropriate configuration of PBMR stages and optimal sweep/feed ratio. Overall, this work determines a viable design of a membrane reactor‐based PDH plant and shows the potential for miniaturized hollow‐fiber membrane reactors to achieve substantial savings. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4519–4531, 2017     

A comparative simulation analysis of propane dehydrogenation in composite and microporous membrane reactors

Propane dehydrogenation has been simulated for a composite membrane reactor and a microporous membrane reactor using plug‐flow reactor models, in which both were packed with Pt/Al₂O₃ catalyst in the tube‐side. The reaction kinetics employed in the analysis were obtained from experimental data produced in an integral fixed bed reactor with the same catalyst. Comparative studies were carried out to analyse the performances of reactors containing the different membranes in terms of contact time, flow pattern and flow rate of sweep gas, and pressure. In general, the composite membrane reactors gave the better performance for all cases investigated. © 2002 Society of Chemical Industry     

Methylcyclohexane dehydrogenation for hydrogen production via a bimodal catalytic membrane reactor

The dehydrogenation of methylcyclohexane (MCH) to toluene (TOL) for hydrogen production was theoretically and experimentally investigated in a bimodal catalytic membrane reactor (CMR), that combined Pt/Al₂O₃ catalysts with a hydrogen‐selective organosilica membrane prepared via sol‐gel processing using bis(triethoxysilyl) ethane (BTESE). Effects of operating conditions on the membrane reactor performance were systematically investigated, and the experimental results were in good agreement with those calculated by a simulation model with a fitted catalyst loading. With H₂ extraction from the reaction stream to the permeate stream, MCH conversion at 250°C was significantly increased beyond the equilibrium conversion of 0.44–0.86. Because of the high H₂ selectivity and permeance of BTESE‐derived membranes, a H₂ flow with purity higher than 99.8% was obtained in the permeate stream, and the H₂ recovery ratio reached 0.99 in a pressurized reactor. A system that combined the CMR with a fixed‐bed prereactor was proposed for MCH dehydrogenation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1628–1638, 2015     

多孔膜反应器中丙烷催化脱氢制丙烯的模拟研究

针对丙烷高效脱氢制丙烯的多孔膜反应器构建了无量纲数学模型并进行了模拟研究,考察了催化剂活性、透氢膜性能、操作条件对多孔膜反应器中丙烷脱氢的转化率、丙烯收率、氢气收率和纯度的影响。结果表明,移走产物氢气可以有效提升膜反应器的性能,其性能的提升程度由不同温压条件下催化剂和透氢膜性能共同决定。高活性催化剂是丙烷高效转化的基础,催化剂活性越高,膜反应器内的产氢速率越快;其次,膜的选择性和渗透通量越高,氢气的移除效率越高,可在最大程度上打破热力学平衡的限制,使反应向生成丙烯的方向移动。当多孔透氢膜的氢气渗透率在10^-7~10^-6 mol·m^-2·s^-1·Pa^-1,H₂/C₃H₈选择性达到100时,其丙烷转化率可以与Pd膜反应器内的转化率相当,但分离的氢气纯度低于Pd膜反应器。与传统的固定床反应器相比,膜反应器由于促进了化学平衡的移动,可以在较低的反应温度下获得相当高的丙烷转化率,且丙烷转化率随着反应压力的增加呈现出一个最大值。该模拟研究可为实际生产过程中膜反应器用于PDH反应的高效强化提供有益的技术指导。     

Pure H2 production through hollow fiber hydrogen‐selective MFI zeolite membranes using steam as sweep gas

Hollow fiber MFI zeolite membranes were modified by catalytic cracking deposition of methyldiethoxysilane to enhance their H₂/CO₂ separation performance and further used in high temperature water gas shift membrane reactor. Steam was used as the sweep gas in the MR for the production of pure H₂. Extensive investigations were conducted on MR performance by variations of temperature, feed pressure, sweep steam flow rate, and steam‐to‐CO ratio. CO conversion was obviously enhanced in the MR as compared with conventional packed‐bed reactor (PBR) due to the coupled effects of H₂ removal as well as counter‐diffusion of sweep steam. Significant increment in CO conversion for MR vs. PBR was obtained at relatively low temperature and steam‐to‐CO ratio. A high H₂ permeate purity of 98.2% could be achieved in the MR swept by steam. Moreover, the MR exhibited an excellent long‐term operating stability for 100 h in despite of the membrane quality. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3459–3469, 2015     

Hydrogen separation at elevated temperatures using metallic nickel hollow fiber membranes

Nickel is a cheaper metallic material compared to palladium membranes for H₂ separation. In this work, metallic Ni hollow fiber membranes were fabricated by a combined phase inversion and atmospheric sintering method. The morphology and membrane thickness of the hollow fibers was tuned by varying the spinning parameters like bore liquid flow rate and air gap distance. H₂ permeation through the Ni hollow fibers with N₂ as the sweep gas was measured under various operating conditions. A rigorous model considering temperature profiles was developed to fit the experimental data. The results show that the hydrogen permeation flux can be well described by using the Sieverts’ equation, implying that the membrane bulk diffusion is still the rate‐limiting step. The hydrogen separation rate in the Ni hollow fiber module can be improved by 4–8% when switching the co‐current flow to the countercurrent flow operation. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3026–3034, 2017     

High H2 permeable SAPO-34 hollow fiber membrane for high temperature propane dehydrogenation application

SAPO-34 hollow fiber zeolite membranes are successfully synthesized on α-Al₂O₃ hollow fiber ceramic substrates by secondary growth method, and used to separate H₂ from a binary mixture (H₂, C₃H₈) or ternary mixture (H₂, C₃H₈, and C₃H₆) under a wide temperature range (25–600°C) with the aim of using them for propane dehydrogenation (PDH) reactions at high temperature. The results show excellent performance for H₂/C₃H₈ and H₂/ C₃H₈ & C₃H₆ separation, with high H₂ permeance of 3.1 × 10⁻⁷ mol/m²/s/Pa and H₂/C₃H₈ selectivity of 41 at 600°C. Additionally, the membrane shows stable performance for 140 hr of H₂/C₃H₈ separation test at 600°C. The high performance of this membrane is mainly attributed to the thin (∼2 μm) zeolite layer and asymmetric-wall of the hollow fiber support. So far, this membrane offers the highest hydrogen permeation and selectivity for H₂/C₃H₈ separation at high temperature (600°C) compared to those reported in literature.     

Deactivation of alkane oxidative dehydrogenation catalyst by deep reduction in periodic flow reactor

The activity loss of β‐NiMoO₄ catalyst has been studied in the oxidative dehydrogenation of propane with an operating periodic flow reactor. Under severe operating conditions (i.e., high temperature and long periods of the pulses), the lattice oxygen depletion is faster than the reoxidation one. After an induction period, carbon whiskers on the catalyst surface were detected together with noticeable amounts of cracking products in the gas phase as CH₄, C₂ hydrocarbons and H₂. Deep reduction dramatically changes the reaction path and irreversibly modifies the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modelling of packed bed membrane reactors for autothermal production of ultrapure hydrogen

The conceptual feasibility of a packed bed membrane reactor for the autothermal reforming (ATR) of methane for the production of ultrapure hydrogen was investigated. By integrating H₂ permselective Pd-based membranes under autothermal conditions, a high degree of process integration and intensification can be accomplished which is particularly interesting for small scale H₂ production units. A two-dimensional pseudo-homogeneous packed bed membrane reactor model was developed that solves the continuity and momentum equations and the component mass and energy balances. In adiabatic operation, autothermal operation can be achieved; however, large axial temperature excursions were seen at the reactor inlet, which are disadvantageous for membrane life and catalyst performance. Different operation modes, such as cooling the reactor wall with sweep gas or distributive feeding of O₂ along the reactor length to moderate the temperature profile, are evaluated. The concentration polarisation because of the selective hydrogen removal along the membrane length was found to become significant with increasing membrane permeability thereby constraining the reactor design. To decrease the negative effects of mass transfer limitations to the membrane wall, a small membrane tube diameter needs to be selected. For a relatively small ratio of the membrane tube diameter to the particle diameter, the porosity profile needs to be taken into account to prevent overestimation of the H₂ removal rate. It is concluded that autothermal production of H₂ in a PBMR is feasible, provided that the membranes are positioned outside the inlet region with large temperature gradients.     

Ammonia decomposition in catalytic membrane reactors: Simulation and experimental studies

Catalytic decomposition of NH₃ with H₂‐selective microporous silica membranes for CO_x‐free hydrogen production was studied theoretically and experimentally. The simulation study shows that NH₃ conversion, H₂ yield and H₂ purity increase with the Damköhler number (Da), and their improvement is affected by the effect of H₂ extraction as well as NH₃ and N₂ permeation through the membranes. The experimental study of NH₃ decomposition was carried out in a bimodal catalytic membrane reactor (BCMR), consisting of a bimodal catalytic support and a H₂‐selective silica layer. Catalytic membranes showed H₂ permeances of 6.2–9.8 × 10^?7 mol m^?2 s^?1 Pa^?1, with H₂/NH₃ and H₂/N₂ permeance ratios of 110–200 and 200–700, respectively, at 773 K. The effect of operating conditions on membrane reactor performance with respect to NH₃ conversion, H₂ yield and H₂ purity was investigated, and the results were in agreement with those calculated by the proposed simulation model. © 2012 American Institute of Chemical Engineers AIChE J, 59: 168–179, 2013     

Catalytic dense membranes of doped Bi4V2O11 (BIMEVOX) for selective partial oxidation: chemistry of defects versus catalysis

A catalytic dense membrane reactor (CDMR) is used to physically separate the reaction step from the reoxidation of the catalyst. By decoupling the redox mechanism prevailing in mild oxidation of hydrocarbons, the operating conditions may be optimized resulting in an increase of selectivity. The membranes are made up of BIMEVOX oxides, obtained by partial substitution of V in γ-Bi₄V₂O₁₁ by ME (Co, Cu, Ta). Experiments performed on BIMEVOX dense membranes using propene and propane are described in terms of, (i) active sites on polished or unpolished surfaces, (ii) operating conditions (T, pO₂ in the high oxygen partial pressure compartment), which determine the selectivity, either to mild oxidation products (acrolein, hexadiene, CO), or to partial oxidation products (CO, H₂), and, (iii) nature of ME cations and relative properties. The discussion deals with the respective role of electronic versus oxide ion conductivities which depend on defects in the structure as well as on the redox properties of cations.     

Theoretical investigation of a water‐gas‐shift catalytic membrane for diesel reformate purification

The novel application of a catalytic water‐gas‐shift membrane reactor for selective removal of CO from H₂‐rich reformate mixtures for achieving gas purification solely via manipulation of reaction and diffusion phenomena, assuming Knudsen diffusion regime and the absence of hydrogen permselective materials, is described. An isothermal, two‐dimensional model is developed to describe a tube‐and‐shell membrane reactor supplied with a typical reformate mixture (9% CO, 3% CO₂, 28% H₂, and 15% H₂O) to the retentate volume and steam supplied to the permeate volume such that the overall H₂O:CO ratio within the system is 9:1. Simulations indicate that apparent CO:H₂ selectivities of 90:1 to >200:1 at H₂ recoveries of 20% to upwards of 40% may be achieved through appropriate design of the catalytic membrane and selection of operating conditions. Under these conditions, simulations predict an apparent hydrogen permeability of 2.3 × 10^?10 mol m^?1 Pa, which compares favorably against that of competing hydrogen‐permselective membranes. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4334–4345, 2013     

Experimental and modeling studies on the low-temperature water-gas shift reaction in a dense Pd–Ag packed-bed membrane reactor

In this work, an experimental and modeling study is described, focusing on the performance of a Pd–Ag membrane reactor recently proposed and suitable for the production of ultra-pure hydrogen. A packed-bed membrane reactor (MR) with a “finger-like” membrane configuration has been used for carrying out the water-gas shift reaction (WGS) in the region of low temperature operation using a simulated reformate feed.The experiments were performed under a broad range of operating conditions of temperature (200–300 °C) and space velocity (1200–10,800 L_N kg_cat⁻¹ h⁻¹); the effect of feed pressure (1–2 bar) was also analyzed, as well as the operating mode at the permeate side: vacuum (30 mbar) or sweep gas (1.0 bar; nitrogen at 1 L_N min⁻¹). A one-dimensional, isothermal and steady-state model is proposed, which assumes axially dispersed plug flow pattern and pressure drop in the retentate side and plug flow with constant pressure in the permeate side. An innovative composed kinetic model was also used to describe the catalytic activity of the catalyst for the WGS reaction. In general, the simulation results showed a good agreement to the experimental data, in terms of carbon monoxide conversion and hydrogen recovery (and also outlet retentate composition) using only two fitting parameters related to the decline of H₂ permeability due to the presence of CO. Both simulation and experimental runs showed that the MR achieves high performances, for some operating conditions clearly above the maximum limit for conventional packed bed reactors. The performance reached is particularly relevant when hydrogen is recovered via sweep gas mode (a high sweep flow rate was employed), because a lower partial pressure could be reached than using vacuum pumping. In the first case, almost complete CO conversion and H₂ recovery could be reached.     

Overcoming mass‐transfer limitations in partial hydrogenation of soybean oil using metal‐decorated polymeric membranes

The conventional soybean oil hydrogenation process (metal catalyst on solid support particles slurried in oil, H₂ bubbled through the oil) is compared with metal‐decorated integral‐asymmetric polyetherimide (PEI) membranes, as far as changes in temperature and pressure are concerned. Using metal‐decorated polymeric membranes, H₂ is supplied to the catalytic sites by permeation from the membrane substructure. As opposed to the slurry process, metal‐decorated membranes show only slightly increased trans fatty acid (TFA) formation when the temperature is raised (50–90°C) to accelerate the process. This is likely due to the efficient and to some extent self‐regulating H₂ supply directly to the catalytic sites on the membrane skin. The hydrogenation rate and TFA formation of the metal‐decorated membrane process show a minor dependence on pressure. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Enhancing co‐production of H2 and syngas via water splitting and POM on surface‐modified oxygen permeable membranes

In this article, we report a detailed study on co‐production of H₂ and syngas on La_0.9Ca_0.1FeO_3?δ (LCF‐91) membranes via water splitting and partial oxidation of methane, respectively. A permeation model shows that the surface reaction on the sweep side is the rate limiting step for this process on a 0.9 mm‐thick dense membrane at 990°C. Hence, sweep side surface modifications such as adding a porous layer and nickel catalysts were applied; the hydrogen production rate from water thermolysis is enhanced by two orders of magnitude to 0.37 μmol/cm²?s compared with the results on the unmodified membrane. At the sweep side exit, syngas (H₂/CO = 2) is produced and negligible solid carbon is found. Yet near the membrane surface on the sweep side, methane can decompose into solid carbon and hydrogen at the surface, or it may be oxidized into CO and CO₂, depending on the oxygen permeation flux. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4427–4435, 2016     

Recent developments in fuel‐processing and proton‐exchange membranes for fuel cells

In recent years, great progress has been made in the development of proton‐exchange membrane fuel cells (PEMFCs) for both mobile and stationary applications. This review covers two types of new membranes: (1) carbon dioxide‐selective membranes for hydrogen purification and (2) proton‐exchange membranes; both of these are crucial to the widespread application of PEMFCs. On hydrogen purification for fuel cells, the new facilitated transport membranes synthesized from incorporating amino groups in polymer networks have shown high CO₂ permeability and selectivity versus H₂. The membranes can be used in fuel processing to produce high‐purity hydrogen (with less than 10 ppm CO and 10 ppb H₂S) for fuel cells. On proton‐exchange membranes, the new sulfonated polybenzimidazole copolymer‐based membranes can outperform Nafion^® under various conditions, particularly at high temperatures and low relative humidities. Copyright © 2010 Society of Chemical Industry     

CeO2‐CrOy/γ‐Al2O3 redox catalyst for the oxidative dehydrogenation of propane to propylene

CeO₂‐CrO_y loaded on γ‐Al₂O₃ was investigated in this work for the oxidative dehydrogenation (ODH) of propane under oxygen‐free conditions. The ODH experiments of propane were conducted in a fluidized bed at 500°C‐600°C under 0.1 Mpa. The prepared catalyst was characterized by N₂ adsorption‐desorption measurements, H₂‐temperature‐programmed reduction, O₂‐temperature‐programmed desorption, NH₃‐temperature‐programmed desorption, x‐ray photoelectron spectroscopy, and x‐ray diffraction. The change in the selectivity of propylene resulted from the thermal cracking of the propane and the competition for lattice oxygen in the catalyst between propylene formation and propane and propylene combustion. Therefore, to achieve higher propylene yield in the industry, the reaction temperature should be 550°C‐575°C for the 17.5Cr‐2Ce/Al catalyst. The results of H₂‐TPR (from 0.2218 mmol/g‐0.3208 mmol/g) revealed that the addition of CeO₂ can enhance the oxygen capacity of CrO_y. Compared with that for 17.5Cr/Al, the conversion can be enhanced from 22.4% to 28.5% and the selectivity of propylene can be improved from 72.2% to 75.9% for the 17.5Cr‐2Ce/Al catalyst. In addition, CeO₂ can inhibit the evolution of lattice oxygen (O^2?) to electrophilic oxygen species (O₂^?), causing the average CO_x (CO and CO₂) selectivity to decrease from 9.64% to 6.31%.     

Structure and catalytic consequence of Mg‐modified VOx/Al2O3 catalysts for propane dehydrogenation

Supported VO_x catalysts are promising nonoxidative propane dehydrogenation (PDH) materials for their commercially attractive activity and propylene selectivity. However, they frequently suffer from rapid deactivation caused by coke deposition. This article describes the promoting role of magnesium on the stability of VO_x/Al₂O₃ catalysts for PDH. A series of VO_x/Al₂O₃ and Mg‐modified VO_x/Al₂O₃ catalysts were synthesized by an incipient wetness impregnation method. The catalysts were carefully characterized by Raman spectra, UV‐Vis spectra, STEM, TGA and in situ DRIFTS. We showed that the stability of a 12V/Al₂O₃ catalyst was significantly improved on addition of small amounts of MgO. Experimental evidences indicate that V₂O₅ nanoparticles emerge in the 12V/Al₂O₃ samples, and appropriate Mg addition helps dispersing the V₂O₅ nanoparticles into 2D VO_x species thus decreasing coke formation and improving stability in nonoxidative dehydrogenation of propane. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

A CO2‐stable hollow‐fiber membrane with high hydrogen permeation flux

A Mo‐substituted lanthanum tungstate mixed proton‐electron conductor, La_5.5W_0.6Mo_0.4O_11.25?δ (LWM04), was synthesized using solid state reactions. Dense U‐shaped LWM04 hollow‐fiber membranes were successfully prepared using wet‐spinning phase‐inversion and sintering. The stability of LWM04 in a CO₂‐containing atmosphere and the permeation of hydrogen through the LWM04 hollow‐fiber membrane were investigated in detail. A high hydrogen permeation flux of 1.36 mL/min cm² was obtained for the U‐shaped LWM04 hollow‐fiber membranes at 975°C when a mixture of 80% H₂?20% He was used as the feed gas and the sweep side was humidified. Moreover, the hydrogen permeation flux did not significantly decrease over 70 h of operation when fed with a mixture containing 25% CO₂, 50% H₂, and 25% He, indicating that the LWM04 hollow‐fiber membrane has good stability under a CO₂‐containing atmosphere. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1997–2007, 2015     

Highly efficient recovery of propane by mixed‐matrix membrane via embedding functionalized graphene oxide nanosheets into polydimethylsiloxane

To construct rapid C₃H₈ transport pathways in polymer matrix, alkyl chain‐functionalized graphene oxide (GO) was prepared via grafting octadecylamine (ODA) molecules and then embedded into polydimethylsiloxane (PDMS) matrix to obtain high‐efficiency mixed matrix membranes (MMMs). The incorporation of alkyl chains contributes to lowering the surface energy of GO nanosheets and providing higher affinity with PDMS matrix. Additionally, the alkyl chains on the surface of ODA‐functionalized GO nanosheets (ODA‐GO) are in favor of C₃H₈ adsorption, thus conferring continuous and specific transport pathways for C₃H₈. The optimized membrane with ODA‐GO loading of 0.3 wt% exhibits the C₃H₈ permeance of 1897 GPU and the C₃H₈/N₂ ideal selectivity of 67, which are 50.2 and 72.5% higher than those of bare PDMS membrane, respectively. The simultaneous enhancement of C₃H₈ permeance and C₃H₈/N₂ ideal selectivity indicates that ODA‐GO is an effective filler applied in MMMs for C₃H₈ recovery. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3501–3510, 2017