Designing flow and temperature uniformities in parallel microchannels reactor

A design methodology is proposed to maintain gas and liquid flow nonuniformities below an acceptable limit in a parallel micro/millichannels reactor by determining the maximum allowed temperature deviation in each part of the reactor. The effect of temperature deviation on flow distribution was quantified using a hydraulic resistive network model. The effect of flow rate on temperature deviation was demonstrated using a one‐dimensional energy balance model. Experiments were conducted using the barrier‐based micro/millichannels reactor (BMMR). Flow distribution in the BMMR is based on placing hydraulic resistances (barrier channels) in the gas and liquid manifolds to regulate the flows. Temperature deviation in the barrier channels affects flow nonuniformity by 10 times more than in the reaction channels. Above a certain critical liquid residence time, flow rate has no significant effect on the temperature deviation which depends on the liquid used, reactor material of construction, and its geometrical dimensions. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1941–1952, 2014     

Flow distribution and mass transfer in a parallel microchannel contactor integrated with constructal distributors

Flow distribution and mass transfer characteristics during CO₂‐water flow through a parallel microchannel contactor integrated with two constructal distributors have been investigated numerically and experimentally. Each distributor comprises a dichotomic tree structure that feeds 16 microchannels with hydraulic diameters of 667 μm. It was found that constructal distributors could ensure a nearly uniform gas–liquid distribution at high gas flow rates where the ideal flow pattern was slug‐annular flow. Nevertheless, at small gas flow rates where the ideal flow pattern was slug flow, a significant flow maldistribution occurred primarily due to the lack of large pressure barrier inside each distributor, indicating that dynamic pressure fluctuation in parallel microchannels greatly disturbed an otherwise good flow distribution therein. It was further shown that the present parallel microchannel contactor could realize the desired mass transfer performance previously achieved in one single microchannel under relatively wide operational ranges due to the integration of constructal distributors. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Multiphase hydrodynamics and distribution characteristics in a monolith bed measured by optical fiber probe

The optical fiber probe has been for the first time applied to investigate the hydrodynamics and gas‐phase distribution at high gas/liquid ratios in a two‐phase flow monolith bed with 0.048 m diameter and 400 cpsi. Local hydrodynamic parameters including gas holdup, bubble frequency, bubble velocity, and bubble length in single channels were measured by 16 inserted single‐point optical fiber probes within the bed under a nozzle as the liquid distributor. The following findings are reported. (1) The optical fiber probe can be used as an efficient and convenient technique for measuring local hydrodynamic parameters inside the channels of a monolith bed; (2) within the range of high gas/liquid ratios under which experiments were conducted, churn flow regime occurred. In this regime, the monolith bed radial distribution of gas holdup, bubble frequency, bubble velocity, and bubble length is nonuniform in nature. © 2013 American Institute of Chemical Engineers AIChE J 60: 740–748, 2014     

Experimental investigation of gas‐liquid flow in monolith channels using monofiber optical probes

Two‐phase hydrodynamics has been experimentally investigated using optical fibre probes in individual channels of a laboratory scale monolith bed. Experimental investigations were carried out to validate the optical probe measurements in a single capillary. Optical probes were positioned at selected single channels of a monolith block, and the signals were processed to assess the local hydrodynamics under cocurrent gas‐liquid downflow configuration, using air and water as fluids. The investigations were performed for three different distributors, viz. single pipe, multipipe, and packed bed distributor configurations. The different distributor configurations were evaluated on the basis of void fraction and bubble frequency for a wide range of flow velocities. The specific novelty aspect of this study comes from the fact that we have undertaken channel scale investigations in monoliths under conditions where we have also reported the global gas‐liquid distribution. Thus, one can readily correlate the bed‐scale hydrodynamics with the local channel‐scale hydrodynamics. © 2016 American Institute of Chemical Engineers AIChE J, 63: 327–336, 2017     

Study of Hydrodynamics,Mixing and Gas‐Liquid Mass Transfer in a Split‐Rectangular Airlift Reactor

Global hydrodynamic characteristics, liquid mixing and gas‐liquid mass transfer for a 63 L split‐rectangular airlift reactor were studied. Correlations for gas holdup and overall liquid circulation velocity were derived for the air‐water system as a function of the specific power input; these were compared to data and correlations for reactor volumes between 4.7 L and 4600 L. A partial recirculation of small bubbles in the riser was observed when U_gr > 0.03 m/s, which was attributed to the use of a single‐orifice nozzle as the gas phase distributor. The dimensionless mixing time and the overall axial dispersion coefficient were nearly constant for the range of gas flow rates studied. However, values of K_L/d_B were greater than those reported in previous studies and this is caused by the partial recirculation of the gas phase in the riser. While scale effects remain slight, the use of a gas distributor favouring this partial recirculation seems adequate for mass transfer in split‐rectangular airlift reactors.     

Experimental Studies on Suspension of Solid Particles in a Low‐Shear Stirred Vessel

A low‐shear stirred vessel was explored. Experimental studies on the suspension of solid particles in solid‐liquid and gas‐solid‐liquid systems were conducted to examine the performance of this new reactor. The method based on the power number curve was modified to determine the critical impeller speeds required for just complete off‐bottom suspension of solids under non‐gassed (N_js) and gassed conditions (N_jsg) in this reactor, and a PC‐6A fiber‐optic probe for the measurement of solid distribution was used to complementarily validate this method. A more homogeneous flow field was gained with a draft tube installed, so that the standard deviations of average shear rate and maximal shear rate are reduced. The modified power consumption method can determine N_js and N_jsg, and the values of N_js with a draft tube are much lower than those without it. N_jsg increases slightly with increasing gas flow rate, and N_jsg with a higher solid weight fraction is larger in this lower‐shear reactor.     

Design method of flow distribution in nuclear reactor systems

Flow distribution is encountered widely in nuclear reactor systems including distribution of liquid coolant, feedwater or steam sparger, and high energy fluid into condenser of steam turbine. Distributors with parallel ports or openings are common designs which may be very sensitive to variations of structure and dimensions of a manifold and its ports. Non-uniformity of flow distributions causes largely potential malfunction and poor efficiency, even failures. In this paper, a theoretical model was developed for a tapered distributor. Analytical solutions were obtained for evaluation of influence of operating conditions and structures and dimensions of distributor and ports on flow distributions. Structural parameters were identified and hydraulic characteristics were analysed. The orifices, slope and tube protrusion are useful for improvement of the flow distributions. The uniformity of the flow distributions can be improved about 10–60% by fitting orifice rings or tube protrution with a range of the minor loss coefficients: 0.5–3. Finally design criteria were suggested for industrial applications. This provides a simple and easy-to-use tool for design of the flow distribution in nuclear reactors and power boilers.     

Hydrodynamic feedback on bubble breakup at a T‐junction within an asymmetric loop

Bubble breakup at a microfluidic T‐junction by taking into consideration the hydrodynamic feedback at the downstream channels is presented. Experiments are conducted in square microchannels with 400 μm in width. The splitting ratio of the bubble size in the bifurcations varies nonmonotonically with the flow rate ratio of gas/liquid phases, and it is also affected by the liquid viscosity. A critical size of the mother bubble determines the variation trend of the splitting ratio of bubble size with flow rates of both phases and the liquid viscosity, which is related to the different breakup mechanisms for long and short bubbles at the junction and the different additional resistances induced by long and short bubbles in downstream channels. A theoretical model is proposed to predict the tailoring size of bubbles at the T‐junction by taking into account of the additional resistance in the presence of bubbles in downstream channels. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1920–1929, 2014     

Flow Manifold Optimization for a Uniform Velocity Distribution in a Laminated Microreactor with Micro‐Pin‐Fin Arrays

Velocity uniformity in reaction channels has a significant effect on the performance of laminated microreactors with micro‐pin‐fin arrays (MPFAR) for hydrogen production. The hydrogen production efficiency can be improved by optimizing the structure of flow manifolds. The relationship between the flow manifold structure and the velocity distribution in the reaction channel with MPFAR is established by an equivalent electrical resistance network model validated via simulation. The effects of the flow manifold structure on the velocity distribution are investigated. The results show that the velocity distribution can be improved by increasing the y‐direction coordinate Y_pi of the flow manifold inlet tube. The flow manifold structure is optimized for better velocity distribution in reaction channels with different MPFAR widths.     

一种新型板翅式换热器气液分配器分配特性的敏感性分析

两相流动分配不均是影响板翅式换热器换热效率的主要因素。传统的"先混合,后分配"方法不能解决在导流翅片中流向突变时气液分离引起的气液两相流体分配不均问题,因此采用"先分配,后混合"的理念提出了一种新型的气液分配器,气体和液体分别从各自的通道进入分配器,在分配器内均匀混合后进入换热器的翅片换热通道进行换热。通过对分配器内部流场的数值模拟,发现:分配器的气液分配不均匀度随流量的增加而增加,且不均匀度受液相流量的影响比气相大。该气液分配器的气液分配不均匀度相比传统封头结构降低了一个数量级,能够有效改善板翅式换热器层间通道的气液分配特性,提高板翅式换热器的换热效率。     

Mass Transfer Characteristics in a Rotor‐Stator Reactor

Mass transfer characteristics in a rotor‐stator reactor in terms of the overall volumetric mass‐transfer coefficient (K_xa) using the N₂‐H₂O‐O₂ system were investigated. The effects of various operating parameters including rotation speed, liquid volumetric flow rate, and gas volumetric flow rate on K_xa were systematically examined, and a gas‐liquid mass transfer model was established to predict K_xa. Results reveal that K_xa increased with higher rotation speed, liquid volumetric flow rate, and gas volumetric flow rate. The results also confirm that the predicted values of K_xa were in agreement with the experimental values with deviation within 15 %. The results contribute to a better understanding of mass transfer characteristics in rotor‐stator reactors.     

Influence of geometry on gas holdups in a reversed flow jet loop reactor

The effects of ratio of draft tube to reactor diameter (D_d/D), liquid nozzle diameter (d_N), aeration tube diameter (d_G) and immersion height of the two-fluid nozzle into the draft tube (H_N) on overall and annulus gas holdups for the air-water system were evaluated experimentally in a reversed flow jet loop reactor over wide ranges of gas and liquid flow rates. Both the gas holdups increased with increasing gas and liquid flow rates and with decreasing d_N and H_N. The influence of d_G on gas holdups is found to vary with gas flow rates. Correlations are proposed to predict gas holdups.     

Bubble shape,gas flow and gas–liquid mass transfer in pulp fibre suspensions

Gas–liquid mass transfer in pulp fibre suspensions in a batch‐operated bubble column is explained by observations of bubble size and shape made in a 2D column. Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates. For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical‐capped/dimpled‐elliptical bubbles. At relatively low mass concentrations (C_m = 2–3% for the softwood and C_m ? 7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension. Instead, a network of channels formed through which gas flowed. In the bubble column, the volumetric gas–liquid mass transfer rate, k_La, decreased with increasing suspension concentration. From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column. A minimum in k_La occurred between C_m = 2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed.     

高气液化下分布器设计对结构化床层气液分布的影响(英文)

Experiments were carried out to investigate the liquid flow distribution at high gas/liquid ratios in a cold model monolith bed of a 0.048 m diameter with 62 cells per cm2.Three types of distributor for the liquid distribu-tion were used to evaluate their distribution performance.Local liquid saturation in individual channels was meas-ured using 16 single-point optical fiber probes mounted inside the channels.The results indicate that 1) The optical fiber probe technique can measure phase distribution in the monolith bed;2) Liquid saturation distribution along the radial direction of the monolith bed is not uniform and the extent of non-uniformity depends on the distributor de-sign and phase velocities;and 3) The tube array distributor provides superior liquid distribution performance over the showerhead and nozzle distributors.     

Influence of hydrodynamics on liquid mixing during Taylor flow in a microchannel

The miscible liquid‐liquid two phases based on Taylor flow in microchannels was investigated by high‐speed imaging techniques and Villermaux/Dushman reaction. The mixing based on Taylor flow was much better compared with that without introducing gas in microchannels, even the ideal micromixing performance could be obtained under optimized superficial gas and liquid velocities. In the mixing process based on Taylor flow, the superficial gas and liquid velocities affected the lengths and the velocities of Taylor bubble and liquid slug, and finally the micromixing performance. The formation process of Taylor flow in the inlets, the initial uniform distribution of reactants and the internal circulations in the liquid slug, and the thin liquid films all improved the mixing performance. Furthermore, a modified Peclet number that represented the relative importance of diffusion and convection in the mixing process was proposed for explaining and anticipating micromixing efficiency. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1660–1670, 2012     

A computational fluid dynamics-based method to investigate and optimize novel liquid distributor

Liquid distributors have an important influence on packed towers' hydrodynamics and mass transfer performances. This work has designed a narrow-trough liquid distributor with stepped baffle plates to regulate liquid flow. The liquid mainstream is diverted layer by stepped baffles to realize the uniform distribution of liquid. The relationship between liquid flow and the baffle plates arrangement is studied by computational fluid dynamics (CFD) simulation. Furthermore, we put forward a CFD-based structural optimization scheme to arrange baffle plates in an arc shape, which leads to a uniform and stable flow of each distribution orifice in the range of liquid spray density of 5–120 m³·(m²·h)⁻¹. The simulation results agree with experiments, which proved that the novel liquid distributor has excellent performance. Compared with the traditional trough liquid distributor, the novel liquid distributor can provide more liquid drip points, more gas-phase channels, higher operating flexibility, and take up less space.     

Comparison of hydrodynamic and mass transfer performances of an emulsion loop-venturi reactor in cocurrent downflow and upflow configurations

Hydrodynamic parameters (gas-induced flow rate and gas hold-up) and mass transfer characteristics (k_La, k_L and a) have been investigated in a gas–liquid reactor denoted “Emulsair” in which the distributor is an emulsion-venturi and the gas phase is self-aspired by action of the kinetic energy of the liquid phase at the venturi throat. Two configurations, respectively cocurrent downflow and cocurrent upflow were compared. A chemical method involving the dispersion of a CO₂–air mixture in a monoethanolamine (MEA) aqueous solution was used to measure mass transfer parameters. Experimental results showed that only the homogeneous bubbling regime prevailed in the upward configuration, while an annular regime could also be observed for cocurrent downflow at low liquid flow rate. Gas-induced flow rate and gas hold-up were usually smaller for cocurrent upflow, both at constant liquid flow rate and specific power input. The same stood for mass transfer properties. Conversely, specific power requirements were lower at constant liquid flow rate and mass transfer characteristics were enhanced at constant gas-induced flow rate for cocurrent upflow. A comparison with other gas–liquid contacting devices showed that the Emulsair reactor is a versatile tool avoiding the presence of mechanically moving parts when high and quickly adaptable dissolved gas supply is required. The cocurrent upflow configuration can be preferred when high gas flow rates are desired because the evolutions of gas-induced flow rate and mass transfer characteristics exhibit a stronger dependence on specific power input in the homogeneous bubbling regime for this configuration.     

Modeling and parametric analysis of hollow fiber membrane system for carbon capture from multicomponent flue gas

The modeling and optimal design/operation of gas membranes for postcombustion carbon capture (PCC) is presented. A systematic methodology is presented for analysis of membrane systems considering multicomponent flue gas with CO₂ as target component. Simplifying assumptions is avoided by namely multicomponent flue gas represented by CO₂/N₂ binary mixture or considering the co/countercurrent flow pattern of hollow‐fiber membrane system as mixed flow. Optimal regions of flue gas pressures and membrane area were found within which a technoeconomical process system design could be carried out. High selectivity was found to not necessarily have notable impact on PCC membrane performance, rather, a medium selectivity combined with medium or high permeance could be more advantageous. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Radial Gas Mixing in a Fluidized Bed with a Multi‐Horizontal Nozzle Distributor using Response Surface Methodology

The gas mixing in the radial direction within a fluidized bed equipped with a multi‐horizontal nozzle distributor was studied using response surface methodology (RSM), which enables the examination of parameters with a moderate number of experiments. All experiments were carried out in a circular fluidized bed of 0.29 m I.D. cold model fluidized bed. The distributor is placed beside twenty‐two horizontal nozzles that are arranged in three concentric circles with all existing discharge directed clockwise. The tracer gas (CO₂) was discharged into the bed as a tracer gas and the analysis was performed with a gas chromatograph. In order to compare the different internal circulations, the tracer gas was discharged in the center area or annular area of the bed. In RSM, the static bed height, superficial velocity and the open area ratio of the distributor are chosen as the research variables, and the standard deviation of the time averaged radial tracer concentration is used as the objection function. A mathematical model for the gas mixing as a function of the operating parameters was empirically proposed. The results show that the standard deviation of time averaged radial tracer concentration is well correlated with the operating and geometry parameters, (U – U_mf)/U_mf, H_s/D and ψ_d, and that the tracer gas injected to the center position has a better dispersion than when injected to the annular position. This model can be used for optimizing the design of fluidized bed reactors at a required performance level.     

Experimental studies of liquid flow maldistribution in a random packed column

Liquid flow distribution has been a major concern when scaling up random packed columns. This study concerns the measurements of liquid flow distribution in a large scale column packed with 25.4 mm stainless steel Pall rings. The liquid flow distribution was studied with packed bed height from 0.9 to 3.5 m, liquid flow rate from 2.91 to 6.66 kg/m²·s, and gas flow rate from 0 to 3.0 kg/m²·s. In addition, three systems, water/air, aqueous detergent solution/air and Isopar/air, were used to study the effect of liquid physical properties on liquid flow distribution, and two different liquid distributors were employed to test the effect of liquid distributor design. It was found that liquid flow distribution was strongly influenced by liquid distributor design, packed bed height, gas flow rate and liquid viscosity, slightly influenced by liquid flow rate, but not by liquid surface tension.