Investigation of multiphase hydrogenation in a catalyst‐trap microreactor

BACKGROUND: Multiphase hydrogenation plays a critical role in the pharmaceutical industry. A significant portion of the reaction steps in a typical fine chemical synthesis are catalytic hydrogenations, generally limited by resistances to mass and heat transport. To this end, the small‐scale and large surface‐to‐volume ratios of microreactor technology would greatly benefit chemical processing in the pharmaceutical and other industries. A silicon microreactor has been developed to investigate mass transfer in a catalytic hydrogenation reaction. The reactor design is such that solid catalyst is suspended in the reaction channel by an arrangement of catalyst traps. The design supports the use of commercial catalyst and allows control of pressure drop across the bed by engineering the packing density. RESULTS: This paper discusses the design and operation of the reactor in the context of the liquid‐phase hydrogenation of o‐nitroanisole to o‐anisidine. A two‐phase ‘flow map’ is generated across a range of conditions depicting three flow regimes, termed gas‐dominated, liquid‐dominated, and transitional, all with distinctly different mass transfer behavior. Conversion is measured across the flow map and then reconciled against the mass transfer characteristics of the prevailing flow regime. The highest conversion is achieved in the transitional flow regime, where competition between phases induces the most favorable gas–liquid mass transfer. CONCLUSION: The results are used to associate a mass transfer coefficient with each flow regime to quantify differences in performance. This reactor architecture may be useful for catalyst evaluation through rapid screening, or in large numbers as an alternative to macro‐scale production reactors. Copyright © 2008 Society of Chemical Industry     

Single and Multiphase Catalytic Oxidation of Benzyl Alcohol by Tetrapropylammonium Perruthenate in a Mobile Microreactor System

A mobile microreactor system, with flow and temperature control for organic synthesis, is described. The system can be used anywhere a venting outlet is available. The microreactors can be operated in different flow patterns (continuous flow, stop‐flow, or programmed‐flow) providing reaction times from a few minutes to a few hours. The system was tested for the catalytic oxidation of benzyl alcohol to benzaldehyde by tetrapropylammonium perruthenate (TPAP) with N‐methyl‐morpholine‐N‐oxide in the liquid phase under stop‐flow mode and on supported TPAP with oxygen under continuous flow mode. The conversion of benzyl alcohol in the microreactor was close to that of a small batch reactor for the liquid phase reaction. For the multiphase reaction, a conversion of 30–40 % was obtained with residence times below 1 min.     

Catalytic Production of Liquid Fuels from Biomass‐Derived Oxygenated Hydrocarbons: Catalytic Coupling at Multiple Length Scales

The catalytic processing of biomass‐derived feedstocks to liquid fuels and chemical intermediates is complex and expensive. Therefore, conversion processes involving a limited number of reaction, separation, and purification steps are necessary. Coupling of catalytic processes has the potential to lead to the development of new processes, thereby improving the overall economics of biomass conversion. Functional coupling at the molecular scale has the potential to produce novel catalytic materials to replace homogeneous catalysts. Active site coupling of different sites within the same reactor can help reduce operating costs by combining sequential reactions in a single reactor. Chemical reaction coupling of heterogeneous and homogeneous reactions may lead to improvements in overall catalytic performance for liquid phase processes by enhancing surface reactions with liquid phase reactions. Finally, phase coupling leads to improvements in overall yield by improving the equilibrium conversion or by suppressing undesired side reactions.     

Harnstoffhydrolyse für die selektive katalytische Reduktion von NOx: Vergleich der Flüssig‐ und Gasphasenzersetzung

In order to reduce the NO_x concentration in car exhausts usually the selective catalytic reduction with ammonia is used. However, to avoid the transport of ammonia in vehicles urea is applied as NH₃ precursor. Controlled urea decomposition before the injection into the exhaust gas system itself may be accomplished by the use of a separate reactor. Urea decomposition to ammonia in the liquid phase under pressure in a heated reactor was compared to its decomposition in the gas phase. In the liquid phase, higher conversion rates relative to the reactor volume were realized than in the gas phase. Catalysts which showed high activity in the gas phase influenced the decomposition in the liquid phase only slightly.     

A Palladium Wall Coated Microcapillary Reactor for Use in Continuous Flow Transfer Hydrogenation

Herein we describe the preparation of a novel continuous flow multi‐channel microreactor in which the internal surface has been functionalised with a palladium coating, enabling its use in catalytic heterogeneous liquid‐phase reactions. Simple chemical deposition techniques were used to immobilise palladium(0) on the channel wall surface of a polymeric multi‐capillary extrudate made from ethylene‐vinyl alcohol copolymer. The Pd coating of the microcapillaries has been characterised by mass spectrometry and light and electron microscopy. The functional activity of the catalytic Pd layer was tested in a series of transfer hydrogenation reactions using triethylsilane as the hydrogen source.     

微型气液固三相等温微分浆态床反应器的优化

针对气液固三相浆态床催化反应中,传递、反应、催化剂的原位表征均比较复杂的问题,为了有利于气、固相均匀分散于液相和反应温度在反应器中实现等温,通过对气液固三相反应工艺特性和反应器性能要求的分析,对微型气液固三相浆态床反应器进行了优化。根据微型浆态床对气液固三相反应分析的要求,采用图像法研究了分布器为G1、G2、G3,砂板直径为2、2.5、3 cm反应器中的流体力学性能特征,考察了气体流速、温度、反应器直径及气体分布器对气含率、气泡尺寸、气泡上升速率以及气泡分布的影响,并进行流体动力学模拟计算,确定了微型浆态床反应器的直径为2 cm,气体分布器为G3砂板的反应器结构,该反应器可以应用于反应过程中间态及液体产物生成过程的测试。     

Concentration slip and its impact on heterogeneous combustion in a micro scale chemical reactor

The rarefied gas effect on concentration slip and on heterogeneous combustion in microscale chemical reactors was investigated. First, a concentration slip model to describe the rarefied gas effect on the species transport in microscale chemical reactors was derived from the approximate solution of the Boltzmann equation. Second, the model was verified using the direct Monte-Carlo method for the pure diffusion problems at different Knudsen numbers. The comparison showed that the present analytical model for the concentration slip boundary condition reasonably predicted the rarefied gas effect in the slip regime. Finally, the impact of the concentration slip on the coupling between the surface catalytic reactions and the homogeneous gas phase reactions in a microscale chemical reactor was examined using the one-step overall surface reaction model with a wide range of Knudsen and Damköhler numbers. It was shown that the rarefied gas effect significantly reduced the reaction rate of the surface catalytic oxidization for large Knudsen numbers. Furthermore, it was shown that the impact of slip effects on catalytic reactions strongly depends on the competition between the reaction rate and diffusion transport. It was found that the concentration slip causes a nonlinear reaction rate distribution at large Damköhler numbers. The results also showed that an accurate prediction of the rarefied gas effect on catalytic reactions in microscale reactors has to consider both the temperature slip and the concentration slip.     

Magnetic emulation of microgravity for earth‐bound multiphase catalytic reactor studies—Potentialities and limitations

A method is proposed to generate Earth‐bound artificial microgravity in a controlled facility capable of emulating lunar/Martian gravity or microgravity for experiments on passive/reactive catalytic multiphase flows. Its applicability was illustrated for trickle beds where flowing gas and liquid experience artificial microgravity inside the bore of a superconducting magnet generating large gradient magnetic fields to compensate for gravity. Artificial gravity is realized by commuting into apparent gravity acceleration the magnetization force at work on common “chemical engineering” non‐magnetic fluids. The scaling property to be matched and maintained invariant in multiphase systems to achieve magnetic mimicry is phasic mass magnetic susceptibility. Hydrodynamic (liquid holdup, wetting efficiency, pressure drop) as well as catalytic reaction (conversion and selectivity) measurements were obtained. The main finding is a proof that magnetic fields affect reactor outcomes exclusively via hydrodynamic phenomena making them appealing proxies for emulating non‐terrene reactor applications. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

The effect of system pressure on gas‐liquid slug flow in a microchannel

Characteristics of gas‐liquid two‐phase flow under elevated pressures up to 3.0 MPa in a microchannel are investigated to provide the guidance for microreactor designs relevant to industrial application. The results indicate that a strong leakage flow through the channel corners occurs although the gas bubbles block the channel. With a simplified estimation, the leakage flow is shown to increase with an increase in pressure, leading to a bubble formation shifting from transition regime to squeezing regime. During the formation process, the two‐phase dynamic interaction at the T‐junction entrance would have a significant influence on the flow in the main channel as the moving velocity of generated bubbles varies periodically with the formation cycle. Other characteristics such as bubble formation frequency, bubble and slug lengths, bubble velocities, gas hold‐up, and the specific surface area are also discussed under different system pressures. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1132–1142, 2014     

Study of Ethylene Polymerization Under Single Liquid Phase and Vapor‐Liquid Phase Conditions in a Continuous‐Flow Tubular Reactor

The feasibility of using continuous‐flow tubular reactors (CFTR) as an efficient research tool for polymerization reactions is investigated. This is a continuation of the extensive effort that had been made at Dow in recent years to set up and employ an electro‐thermal microreactor (an ohmically‐heated CFTR), which resulted in several internal and external publications and a US Patent. The main focus of this work is to investigate the effect of operating conditions and flow composition, mainly the number of existing phases, on the molecular weight of the polymer. A series of polymerization experiments were performed in single‐phase (liquid) and two‐phase (vapor‐liquid) flow regimes. In single‐phase polymerization, the ethylene concentration falls continuously along the length of the reactor. This will have a significant effect on the kinetics of polymerization, particularly the molecular weight of the produced polymer. A key advantage of operating in the two‐phase region is that an almost constant ethylene concentration is maintained along the length of the reactor. In effect, the vapor phase serves as a reservoir that replenishes the ethylene consumed in the liquid phase by polymerization. The molecular weight data show that this assumption is valid provided that the rate of mass transfer is significantly higher than the rate of the polymerization reaction.     

Prediction of the Induced Gas Flow Rate from a Self‐Inducing Impeller with CFD

The complex task of describing computationally two‐phase turbulent flows in aerated stirred‐tank reactors was overcome by proposing that the gas flow rate in the hollow impeller can be estimated from single‐phase flow simulations of the liquid phase in the reactor: the pressure at the impeller surface obtained from liquid phase simulations can be related to the gas induction rate. A commercial lab‐scale reactor with a radial six‐bladed hollow impeller was chosen for the study. To validate the presented methodology, the induced gas flow rate was measured experimentally from the tracking of the position of bubbles in a dynamic sequence of flow images. Notwithstanding the simplifications assumed in the presented CFD methodology, good agreement has been obtained between numerical results and experiments.     

An investigation of the influence of porosity variations on the performance of liquid phase packed bed chemical reactors at low Reynolds numbers

The influence of a fluid—solid interaction on the performance characteristics of a single liquid phase packed bed catalytic reactor at low Reynolds numbers has been investigated. A simple theoretical model is suggested which allows for the influence of fine scale dispersion as well as more gross mixing phenomena associated with packing irregularities. The validity of the concept was tested experimentally by measuring the conversion from a chemical reactor and the residence time distribution simultaneously. A comparison of the experimental and theoretical results for beds of various packing arrangements suggests that a fluid—solid interaction is present and that it can be described by the proposed model.     

装配水平筛板的气升式反应器中气液传质特性的数值模拟

利用Turbulent–Lehr组合模型对装配水平筛板的气升式反应器进行了计算流体力学(CFD)模拟，研究水平筛板对气含率、气泡直径、体积传质系数(kLa)和气液流速的影响。结果表明，筛板对气相的囤积作用和对液相的阻碍作用增加了反应器的整体气含率。筛板对气相的二次均布作用减弱了筛板和液面之间区域的气泡聚并过程，筛板筛孔对气泡的破碎作用产生了大量小于初始直径的气泡，增加了气泡比表面积(a)；筛板对液相的阻碍作用提高了筛板附近的气–液相流动速度差，从而提高了该区域的液膜传质系数(kL)，强化了反应器内的气液传质效果。     

A combined MAS nuclear magnetic resonance spectroscopy,in situ FT infrared spectroscopy and catalytic study of the conversion of allyl alcohol over zeolite catalysts

A combined study of allyl alcohol conversion over zeolite catalysts using catalytic measurements in a flow microreactor, in situ FTIR and MAS NMR spectroscopy is reported. Rate constants for the conversion in the flow reactor and the static in situ reactor used in the FTIR studies are in broad agreement, emphasising the viability of the experimental approach. In the flow microreactor allyl alcohol conversion over the zeolite catalyst is shown to form diallyl ether, hydrocarbons and acrolein. The in situ study successfully models the formation of diallyl ether and hydrocarbon as initial reaction products, but unfortunately acrolein is found to be rapidly converted to hydrocarbons under the condition used in the in situ cells. The studies are combined to provide a model for the reaction which involves two parallel pathways for the formation of the hydrocarbons and acrolein.     

A CFD study on the effect of the characteristic dimension of catalytic wall microreactors

A three‐dimensional computational fluid dynamics study of the steam methane reforming (SMR) in microreactors is presented. Emphasis has been made on investigating the effects of the characteristic dimension (d: 0.35, 0.70, 1.40, and 2.80 mm) on the performance of two microreactor geometries: square microchannels and microslits. Results have shown that for both geometries the SMR conversion decreases markedly as d increases. Conversely, the microchannels provide a methane conversion slightly higher than that of the microslits. The different performance of the microreactors is only partially due to the different surface‐to‐volume ratio. Pronounced transverse temperature and concentration gradients develop as the characteristic dimension increases especially for microslits in the first half of the reactor. Therefore, external transport limitations can affect the performance of microreactors for SMR, although the characteristic dimensions are of the order of very few millimeters. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

乙炔加氢制乙烯浆态床反应器的CFD模拟

对浆态床反应器中乙炔加氢制乙烯过程进行了模拟研究，采用TFM-PBM耦合方法描述浆态床内气相与浆态相的流动，并耦合乙炔加氢反应动力学建立流动-反应综合模型。通过小试实验对该模型进行验证，并将验证后的模型应用于浆态床中试装置中内构件作用机制与操作条件影响的模拟分析。结果表明，在浆态床反应器放大时，可通过设置竖管内构件，以破碎气泡，抑制气相径向运动，使乙炔加氢过程均匀、充分地进行。乙炔加氢制乙烯过程与气相停留时间和反应温度密切相关，在反应器放大中需严格控制温度，并可通过改变反应器内液位高度实现对气相停留时间的调控，从而可在保证乙炔充分转化的同时获得更高的乙烯选择性。     

Influence of operating conditions on the observed reaction rate in the single channel monolith reactor

The catalytic hydrogenation of nitrobenzoic acid (NBA) to the aminobenzoic acid was used as a model reaction for a quantitative study of influences of the operating conditions on the observed reaction rate in a single channel monolith reactor operated in Taylor flow regime. A simple mathematical model was derived and used for the analysis of hydrogenation experiments carried out in batch mode. Results showed that in the investigated concentration range of NBA, i.e. 0.0005–0.02 mol/l and under the hydrogen pressure of 1 bar, the observed reaction rate is considerably limited by mass transport. At higher concentrations of NBA, the reaction is controlled by the hydrogen mass transport while at lower concentrations the mass transport of NBA is dominant. The analysis of experimental results, which were obtained when the length of gas bubbles and liquid slugs were varied, showed that the reaction took place in the thin liquid film surrounding the gas bubble. The liquid slug serves as exchanger of reactants and reaction products between bulk liquid slug and liquid film surrounding the catalyst surface.     

Liquid–liquid two‐phase flow in ultrasonic microreactors: Cavitation,emulsification, and mass transfer enhancement

The effects of ultrasound on the hydrodynamic and mass transfer behaviors of immiscible liquid–liquid two‐phase flow was investigated in a domestic ultrasonic microreactor. Under ultrasonic irradiation, cavitation bubble was generated and underwent violent oscillation. Emulsification of immiscible phases was initiated by virtue of oscillating bubbles shuttling through the water/oil interface. The pressure drop was found to decrease with increasing ultrasound power, with a maximum decrement ratio of 12% obtained at power 30 W. The mass transfer behavior was characterized by extraction of Rhodamine B from water to 1‐octanol. An enhancement factor of 1.3–2.2 on the overall mass‐transfer coefficient was achieved under sonication. The mass transfer performance was comparable to passive microreactor at similar energy dissipation rate (61–184 W/kg). The extraction equilibrium was reached under a total flow velocity 0.01 m/s and input power 20 and 30 W, exhibiting its potential use in liquid‐liquid extraction process. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1412–1423, 2018     

In situ synthesis of SAPO-34 crystals grown onto α-Al2O3 sphere supports as the catalyst for the fluidized bed conversion of dimethyl ether to olefins

SAPO-34 is an excellent catalyst for the conversion of dimethyl ether (DME) to olefins, but because conventionally synthesized SAPO-34 crystals are too small to be used directly in a fluidized bed, they have to be used as, and have the disadvantages of, a spray-dried catalyst. In this study, SAPO-34 crystals were synthesized in situ to grow on the surface of small α-Al₂O₃ spheres to produce a zeolite catalyst for a fluidized bed reactor. The influences of the composition of the crystal gel and surface structure of the support were investigated. The catalytic performance of the zeolite crystals grown on the support (surface zeolite) for the conversion of DME to olefins was investigated in a fixed bed microreactor and a fluidized bed reactor. The experiments showed that these surface SAPO-34 crystals gave the same activity and product selectivity as conventionally synthesized free SAPO-34 crystals and a higher reaction rate (normalized to the weight of SAPO-34) than the spray-dried catalyst. In situ synthesis is a simple and effective way to produce a SAPO-34 catalyst for a fluidized bed reactor.