微通道内气-液弹状流动及传质特性研究进展 (Review on flow and mass transfer characteristics of gas-liquid slug flow in microchannels)

Rotating foam reactors: Mass transfer and reaction rate

Enhanced production of ethyl pyruvate using gas–liquid slug flow in microchannel

The near-wall transport characteristics, inclusive of mass transfer coefficient and wall shear stress, which have a great effect on gas–liquid two-phase flow induced internal corrosion of low alloy pipelines in vertical upward oil and gas mixing transport, have been both mechanistically and experimentally investigated in this paper. Based on the analyses on the hydrodynamic characteristics of an upward slug unit, the mass transfer in the near wall can be divided into four zones, Taylor bubble nose zone, falling liquid film zone, Taylor bubble wake zone and the remaining liquid slug zone; the wall shear stress can be divided into two zones, the positive wall shear stress zone associated with the falling liquid film and the negative wall shear stress zone associated with the liquid slug. Based on the conventional mass transfer and wall shear stress characteristics formulas of single phase liquid full-pipe turbulent flow, corrected normalized mass transfer coefficient formula and wall shear stress formula are proposed. The calculated results are in good agreement with the experimental data. The shear stress and the mass transfer coefficient in the near wall zone are increased with the increase of superficial gas velocity and decreased with the increase of superficial liquid velocity. The mass transfer coefficients in the falling liquid film zone and the wake zone of leading Taylor bubble are lager than those in the Taylor bubble nose zone and the remaining liquid slug zone, and the wall shear stress associated falling liquid film is larger than that associated the liquid slug. The mass transfer coefficient is within 10−3 m/s, and the wall shear stress below 103 Pa. It can be concluded that the alternate wall shear stress due to upward gas–liquid slug flow is considered to be the major cause of the corrosion production film fatigue cracking.

Successful high-rate treatment of wastewaters in bioreactors is largely dependent on effective sludge retention. Despite the availability of sludge granulation techniques, physical retention by membranes remains a good option, especially when good sludge granulation cannot be guaranteed. The granulation of anaerobic sludge is, for example, impeded by the effects of sodium on sludge properties, such as a weakened granule strength, which might be attributed to disruption of bivalent cation linkages between extracellular polymeric substances (EPSs) that play a key role in granular sludge stability. Under such conditions, the use of membranes ensures full sludge retention, providing a suspended solids-free effluent. However, the feasibility of using membranes in wastewater treatment, especially under anaerobic conditions, requires major improvements in attainable membrane fluxes. This study has therefore investigated methods to increase the membrane flux of anaerobic membrane bioreactors that are operated under saline process conditions. Two methods for increasing membrane flux have been tested. The first method involved increasing the shear stress at the surface of the tubular membrane employed, in order to enhance the back transport of foulants from the membrane surface to the bulk solution; slug bubbles and inserts were used to increase the shear stress. The second method involved decreasing the concentration of foulants in the bulk solution through the addition of adsorbents and the use of coagulation. Coagulation was induced by the sodium ions naturally present in saline wastewater and through the direct addition of an aluminum-based coagulant. The applied gas slug appeared to be unable to adequately control fouling, resulting in rapidly increasing trans-membrane pressures (TMP) when operating at a flux in excess of 16 L/m.h, as described in Chapter 2. However, the chemical oxygen demand (COD) removal efficiency did not show any significant deterioration, whereas the specific methanogenic activity (SMA) increased from 0.16 to 0.41 g COD per g volatile suspended solid (VSS) per day. The tubular membrane was subsequently equipped with inert inserts in order to produce locally increased shear stress for enhanced fouling control. Results showed that, following the mounting of the inserts in the membrane tube, the membrane flux increased from 16 L/m.h to 34 L/m.h. However, the pressure drop along the membrane was also greatly increased and it was therefore concluded that the gas slugs were insufficient to increase the membrane flux and the inserts did not offer a practical solution. In order to understand why the bubbles did not effectively increase the membrane flux, the mass transfer by the bubbles was quantified through computational fluid dynamics modeling. The model and its results are presented in Chapter 3. The modeling indicated that the mass transfer capacity at the membrane surface was higher at the noses of gas bubbles than at their tails, which is in contrast to the results when water was used instead of sludge. The filterability of the sludge at a given mass transfer rate was found to have a strong influence on the TMP, at a steady flux. The model also showed that the shear stress within the internal space of the tubular membrane was mainly around 20 Pa, but could be as high as about 40 Pa due to gas bubble movements. Nevertheless, a stable particle size distribution (PSD) for sludge particles was found at these shear stresses. It was, therefore, hypothesized that a high flux would be possible by applying biogas bubbles induced slug flow conditions in

Critical review of vertical gas-liquid slug flow: An insight to better understand flow hydrodynamics' effect on heat and mass transfer characteristics

Mass Transfer during gas absorption from a linear cluster of slugs in the presence of inert gases

Estimation of gas and liquid slug lengths for T-shaped microreactors

transportation facilities. In an offshore oil and gas production facility, pipeline-riser systems are required to transport two-phase hydrocarbons from subsurface oil and gas wells to a central production platform. Severe slugs reaching several thousands pipe diameters may occur when transporting gas and liquid in these pipeline-riser systems. Severe slugging creates potential problems in the platform facilities, e.g. separators, pumps, and compressors. Severe slugging may cause flooding and overpressurization of the separator, rupture of the pipe, and an increased back pressure at the wellhead. All of these might lead to the complete shutdown of the production facility. Therefore, the accurate predictions of severe slugging characteristics, e.g. slug length, oscillatory period, are essential for the proper design and operation of two-phase flow in the pipeline-riser systems. Pipelines used for the transportation of hydrocarbons in an offshore production facility, are laid out over the seafloor. The uneven seafloor topography forms different pipeline-riser configurations. In this dissertation, we described the severe slugging characteristics in a long downward inclined pipeline-riser system. We carried out experiments in a relatively long pipeline-riser configuration, and also performed numerical simulations using a one-dimensional two-fluid model. It was found experimentally, as also reproduced numerically, that transient slugs were generated in the pipeline upstream of the riser base. These transient slugs effectively contributed to the initial blockage of the riser base. Furthermore, an existing analytical model for the prediction of the flow behaviour in the pipeline-riser system was modified. The modified model, which was tested against our experimental results, showed a better performance than previously published models. We developed a transient drift flux model to simulate the severe slugging characteristics in a pipeline-riser system. The model was tested against experimental data and interestingly, could predict the occurrence of severe slugging in a horizontal pipeline-riser system, which is a subject of debate in the open literature. That motivated us to conduct experiments in a horizontal pipeline-riser configuration. It was observed that severe slugging can develop even in the horizontal pipeline-riser configuration. Moreover, a new class of severe slugging was found and referred to as dual-frequency severe slugging, which corresponds to dual-frequency pressure and flow rate fluctuations. It was found that dual-frequency severe slugging evolves when the pipeline length exceeds a certain threshold. In this dissertation, we also described the severe slugging characteristics in a hilly-terrain pipeline-riser configuration. A hilly-terrain pipeline consists of interconnected horizontal, downhill, and uphill sections. It was observed that, the existence of a hilly-terrain unit in a pipeline-riser system induces a more severe type of slugging, which exhibits longer slugs than that of a horizontal pipeline-riser system. So far we have summarized our work on the characteristics of severe slugging in a pipeline-riser system. In this dissertation, we also discuss the occurrence of severe slugging in an extended reach well. In response to meet the world energy demand, the oil and gas industry has also moved towards development of resources in scattered, isolated oil and gas pockets. Snake wells and fish-hook wells are extended reach wells, which have been used to develop these small hydrocarbon deposits more efficiently than conventional vertical or horizontal wells. The extended reach well resembles the pipeline-riser configuration. The flow conditions, e.g. pressure, and the pipe specifications, e.g. diameter, at the bottom of a well are generally different than the pipeline laid out over the seafloor. It is expected that severe slugging at the bottom of the well is less likely to occur. In this dissertation, we performed numerical simulations to study the possible formation of severe slugging at the bottom of an extended reach well. It was found that severe slugs were initiated at the bottom of the extended reach well. This teaches one to study the well hydrodynamics more carefully when designing an extended reach well.

Gas–liquid slug flow is characterized by complex and intermittent hydrodynamic features that offer an efficient alternative to promote biofilm control. In the present work, the mechanism of transferring a gaseous solute into a co-current liquid in a micro-scale slug flow system was inspected in detail. Specifically, the gas–liquid mass transfer from an individual Taylor bubble filled with oxygen was numerically studied using CFD techniques. To accurately describe the referred phenomenon, the hydrodynamic and concentration fields were simultaneously solved. Furthermore, the interface capturing based on the VOF methodology was also coupled to this solution approach. Three sub-categories within slug flow pattern were identified based on the flow behavior in the liquid phase: no liquid in recirculation (Case A); closed wake below the bubble tail (Case B); and recirculation ahead and below bubble (Case C). Regarding the solute distribution, in Case A the solute is dispersed only backwards, it accumulates in the closed wake structure in Case B, and it reaches the wall within the film region in Case C. Local and average mass transfer coefficients were also estimated for the different cases. The influence of the two most relevant dimensionless groups (Reynolds and Capillary numbers) was also briefly analyzed. Global mass transfer coefficients results confirmed that the penetration theory can provide reasonable estimations for systems like Case C.

Oxidation of 5-hydroxymethylfurfural (HMF) using air or pure oxygen was performed in polytetrafluoroethylene capillary microreactors under gas–liquid slug flow, with Co/Mn/Br as the homogeneous catalyst in the acetic acid solvent. The temperature was varied from 90 to 165 °C at a pressure of 1 or 5 bar. At atmospheric pressure conditions (and 90 °C), acetaldehyde was further added as a co-oxidant to accelerate the reaction. At 150 °C, 5 bar oxygen and a residence time of 2.73 min, an HMF conversion of 99.2% was obtained, with the yields of 2,5-diformylfuran (DFF), 5-formylfurancarboxylic acid (FFCA) and 2,5-furandicarboxylic acid (FDCA) being 22.9%, 46.7%, and 23.8%, respectively. By operation under wetted slug flows and elevated partial oxygen pressures, mass transfer limitations and oxygen depletion in the microreactor could be eliminated. This allowed to run the microreactor under kinetically controlled conditions, where both the HMF consumption and DFF formation were found zero order in partial oxygen pressure and roughly first order in HMF. The total selectivity towards DFF/FFCA/FDCA was ca. 40% at low partial oxygen pressures due to the dominant occurrence of side reactions. By using pure oxygen at 5 bar the total selectivity was improved to 60–94%. The space time yields of DFF and FFCA in the microreactor exceeded those obtained in conventional (semi-)batch reactors at slightly elevated temperatures and pressures, due to the superior mass transfer and higher initial HMF concentrations in the microreactor. For highly efficient FDCA synthesis, more dedicated microreactor operations are needed to tackle its precipitation.

Design and operation of gas–liquid slug flow in miniaturized channels for rapid mass transfer

Combined mass and heat transfer during nonisothermal absorption in gas-liquid slug flow

Mass transfer and modeling of deformed bubbles in square microchannel

Process intensification is a vibrating topic in the field of chemical engineering. Its aim is to develop processes and methods with increased performance for a given chemical transformation at decreased energy consumption and waste production. Among all methods of process intensification, the microreactors are very promising (chapter I). In the case of fast gas-liquid reactions, microreactors allow to speed up the physical processes. Due to high surface to volume ratio, high mass and heat transfer performances can be achieved compared to conventional processes, leading to process intensification. In the present work (chapter III), the flow patterns and the mass transfer were studied in glass microreactors for CO2-water based systems. Volumetric mass transfer coefficients up to 9 s-1 were measured for slug flow regime in microreactors with hydraulic diameters of 400 µm. These values are at least one order of magnitude higher than for conventional contactors. Based on the experimental results, empirical correlations were developed to predict the flow regimes and the mass transfer performance. The obtained results enable a rational design of microreactors for fast gas-liquid transformations. In the case of slow chemical reactions, high pressure and temperature (high PT) can be used to speed up the intrinsic kinetics. The microreactors offer the opportunity to perform reactions under harsh conditions due to excellent control of process parameters. The potential of unusual operating conditions (high PT) was investigated to reduce the reaction time and increase the specific productivity of the aqueous Kolbe-Schmitt synthesis of beta-resorcylic acid (chapter IV). Based on a kinetic and thermodynamic model, the optimal operating window for the reaction was defined. The model prediction was successfully validated with a new micro-plant operated under high pressure and temperature. Moreover, a scale-up strategy was proposed using Sulzer milli static mixers. The new process allows to synthesise beta-resorcylic acid with 100% selectivity, to increase the performance by 2 orders of magnitude as compared to conventional batch process and to reduce by a factor of 4.2 the consumption of the costly reagent KHCO3. The same strategy was further applied to intensify the ethoxylation of dodecanol, an important industrial reaction (chapter V). As ethylene oxide processes have inherent safety vulnerability, an extensive calorimetric study was carried out under high PT For the first time, solubility and kinetic models were established up to 523K and 50 bar. Simulations were performed to predict the productivity of a new milli-plant based on Sulzer milli static mixers. It was demonstrated that, using a multi-injection reactor, the specific productivity can be increased 6 times as compared to conventional semi-batch reactors.

Experimental study of vertical co-current slug flow in terms of flow regime transition in relatively small diameter tubes