Mass Transfer Intensification Research Articles

A new trend in combustion engine’s deep waste heat recovery by application of condensing economizers in exhaust boilers

Waste heat recovery technologies to remove heat and harmful pollution from exhaust gas–vapor mixtures remain a general trend in enhancing the efficiency of power plants. The exhaust gases from combustion engines when water-fuel emulsion (WFE) combustion contain a large amount of solid particles, deposition of which on the heating surfaces of the exhaust gas boilers (EGB) can degrade their heat transfer performance and shorten the service life. The processes of pollution deposition from exhaust gases on the economizer condensing heat exchange surface (CHES) of the EGB were studied, and their effect on the CHES heat and mass transfer characteristics was assessed. It is found that increasing the water content in the WFE to 30 % enhances the intensity of acid mass transfer due to decreasing the low-temperature corrosion rate with reaching the maximum-intensity at wall temperatures of 110…120 °C against 140…150 °C in conventional practice, which allows deeper exhaust heat utilization and increasing boiler heat productivity by about 30 %. At a water content of 30 % the sulfuric acid concentration was 57 %, which confirms the hypothesis about the passage of the nitrous mechanism of sulfuric acid formation in the condensate on CHES. The optimized CHES cleaning periodicity values corresponding to the smallest fouling coefficient and the highest heat transfer were determined. The correlations for determining the fouling, heat transfer and thermal efficiency coefficients for the CHES of the EGB when WFE combustion are provided. They are helpful for designing the economizer CHES of EGB.

Effect of Volume Condensation on the Intensity of Heat and Mass Transfer

Effect of Volume Condensation on the Intensity of Heat and Mass Transfer

Experimental investigation on product and temperature distribution in a continuous flow packed-bed reactor for dodecahydro-N-ethylcarbazole dehydrogenation

The dehydrogenation of dodecahydro-N-ethylcarbazole (H12-NEC) generates enormous gas accompanied by intense heat absorption, affecting heat and mass transfer in porous catalysts. A packed-bed reactor equipped with a temperature measurement device was designed to investigate the continuous flow dehydrogenation of H12-NEC. The effects of weight hourly space velocity (WHSV), reaction temperature, catalyst particle size, and pressure were evaluated. The results reveal that the volume flow rate of hydrogen will increase with the increase of WHSV. And the reduction of gas–liquid ratio and catalyst particle size facilitates heat transfer causing a more uniform axial temperature distribution within the reactor. As the temperature rises, the hydrogen yield increases along with more by-products. 456 K and 2 mm of catalyst particle size are determined to be the optimal reaction conditions in the study, achieving the hydrogen production rate of 9.61 molH2·gPd-1·h-1. With pressure increasing, the hydrogen yield gradually decreases, but the decreased evaporation of H12-NEC results in a smaller temperature difference between the reactor and heating wall. A comparison with the thermodynamic equilibrium model reveals that the effect of pressure on chemical equilibrium may be more significant than the temperature.

Insights into turbulent airflow structures in blind headings under different ventilation duct distances

The paper explores the 3D stationary vortex structure of turbulent airflow near the dead-end face of a blind heading, ventilated via a forcing ventilation system. Despite its significance in blind heading ventilation, previous studies primarily focused on temporal dynamics of harmful impurities, overlooking flow structure details crucial for mass transfer processes. Our study delves into the ventilation flow structure across diverse parameters of the auxiliary ventilation system. Alongside standard flow visualization tools, we introduce three dimensionless indicators to comprehensively characterize flow structure, facilitating analysis of its variations with parameter changes and quantitative evaluation of system efficiency. Analysis revealed the formation of a single large-scale vortex within the entire range of considered ventilation system parameters in the heading. This vortex induces a significant increase in air circulation, approximately 2.5–3.5 times greater than airflow emerging from the ventilation duct’s end, thus intensifying mass transfer processes within the heading. We found that ventilation efficiency of the dead-end face zone in a blind heading with a 29.2 m² cross-section decreases linearly with increasing distance between the ventilation duct’s end and the dead-end face. However, compensating for this distance by increasing duct velocity is feasible, bearing significant implications for mine ventilation safety, particularly in ventilating blind headings with distant ducts from the dead-end face.

Ventilation of large section blind headings under conditions of changing ore load size

Relevance. The need to ensure safe working conditions in the working areas of underground mines. Stagnant zones with low air velocities can be formed in large section blind heading behind the ore bulk when ore is shipped by mining machines. There is a possibility of the formation of local gas accumulations and their abrupt removal later in such stagnant zones due to the unsteadiness of accumulation and transfer of gas impurities in a blind heading. A sharp removal of gas will lead to contamination of the load-haul-dump operators’ workplace. Aim. To identify removal of exhaust gases in large section blind headings in conditions of changing ore load size. Objects. Large section blind headings with of complex geometry. Methods. Three-dimensional numerical simulation of unsteady turbulent flow of a gas-air mixture in the Ansys CFX, visualization and analysis of simulation data in the Wolfram Mathematica. Results. The paper presents the results of mathematical modeling of ventilation of large section blind headings of complex geometry under conditions of changing ore load size formed during mining operations by the load-haul-dump machines with an internal combustion engine. It is shown that either a single vortex is formed that ventilates the blind headings space, or a stagnant zone behind the bulk ore, which is characterized by a relatively low intensity of mass transfer. Vortex formation depends on the height of the ore bulk. The authors have obtained the dependences of the change in the average concentration of exhaust gases at the outlet of the blind headings space on the operating time of the machine. They determined the dependence of the correction volume coefficient on the geometric parameters of the blind headings. An analytical model of the removal of gases from the blind headings space under conditions of changing bulk ore volume is obtained. The expression of increment in gas concentration on an operator’s workplace makes it possible to derive formula to find maximum time of load-haul dump operation in a stope such that gas concentration is never higher than maximum allowable concentration.

Starch hydration properties in relation to kinetic modelling of mass transfer and properties of deep-frying batter

The hydration properties of starch were examined in relation to mass transfer and the characteristics of deep-frying batter. With extended deep-frying, the oil content in all samples increased, while the moisture content exhibited an inverse trend. Analyses of zeta potential, disulfide bonds, secondary structure, particle size of the proteins, thermal properties, and water status indicated that prolonged deep-frying time facilitated protein aggregation and led to a decrease in both gelatinization enthalpy (ΔH) and the percentage of bound water. Additionally, deep-frying batter containing starch with medium hydration properties (carboxymethyl freeze-thawed tapioca starch), accelerated protein aggregation, delayed the dissipation of bound water, and significantly reduced both gelatinization and ΔH. This ultimately weakened the intensity of mass transfer, thereby diminishing oil distribution. The Pearson’s correlation test confirmed a negative correlation between mass transfer intensity and starch hydration properties, indicating that both parameters significantly influenced the characteristics of the deep-frying batter.

Intensify Mass Transfer and Molecular Oxygen Activation by Defect‐Bridged Asymmetric Catalytic Sites Toward Efficient Membrane‐Based Nanoconfined Catalysis (Adv. Funct. Mater. 37/2024)

Intensify Mass Transfer and Molecular Oxygen Activation by Defect‐Bridged Asymmetric Catalytic Sites Toward Efficient Membrane‐Based Nanoconfined Catalysis (Adv. Funct. Mater. 37/2024)

Study of the Dynamics of a Single Bubble

The behaviour of bubbles in cavitation and boiling processes is determined by the thermodynamic parameters of the two-phase medium and the intensity of heat and mass transfer, which affect the final dynamic effects. In this review, we analyse the influences of these factors on bubble behaviour, as described in existing mathematical models. In particular, we analyse the physical processes that govern bubble behaviour, the influence of mass transfer, vapor and liquid temperature, vapour, and liquid pressure on the inertial and dynamic stages of development. In conclusion, we summarize the problems associated with modelling, the accuracy of numerical predictions, and propose directions for further research.

Evaporation characteristics of single free-falling liquefied natural gas droplet under different ambient conditions based on the VOF method

The evaporation characteristics of droplets is a fundamental issue for accurate depictions of moving liquefied natural gas (LNG) droplets and constantly changing ambient environment owing to intense heat, mass transfer, and momentum exchanges. In this study, a modified droplet evaporation model is developed to investigate the evaporation behavior of a single free-falling LNG droplet under various ambient conditions. The proposed model captures the droplet interface and calculates the mass transfer rate through the vapor concentration gradient using the volume of fluid method integrated with a smooth function. Evaporation experiments were conducted on a single free-falling LNG droplet to capture the evaporation process. The results indicate that it is acceptable to ignore the effect of the Stefan flow of a single free-falling LNG droplet. This study also examined the impact of different ambient conditions on evaporation characteristics. The results indicate that the evolution of the droplet diameter is mainly influenced by the ambient temperature because it affects the thermal properties of the surrounding gas and the distribution of the boundary layer. The temporal characteristics of the Nu and Sh numbers generally stabilize after a period of rapid decline. This study provides a theoretical basis for understanding the evaporation mechanisms of LNG droplets.

Conceptual study on the intensification of gas–liquid mass transfer in trickle bed reactors by the application of strut-based and novel sheet-based periodic open cellular structures (POCS)

Rapid progress in the development of additive manufacturing technology enables the production of structured reactor internals of complex geometry. To address mass transport limitations structured internals can be used in trickle bed reactors (TBRs) as the most frequently used reactor for heterogeneously catalyzed multiphase reactions. However, reactions in TBRs are frequently limited by gas–liquid mass transfer.As periodic open cellular structures (POCS) as reactor internals were not addressed regarding g–l mass transfer yet, this contribution provides first data on gas–liquid mass transfer for different types of POCS. Therefore, desorption of dissolved oxygen in water with nitrogen gas and the two-phase pressure drop were measured. Strut-based Kelvin and diamond unit cell POCS were compared to a sphere random packed bed as a benchmark. A novel sheet-based unit cell was developed realizing meandering channels demonstrating a fivefold increase in volumetric mass transfer coefficient compared to the benchmark. To compare the performance of the setup with literature data, state of the art neural network correlations were used for comparison. This proof of concept highlights the potential of additively manufactured POCS for intensified processes in trickle bed reactors, and demonstrates their versatility in application.

Use of process intensification concepts for targeted delivery of inhaled aerosolized medicines

Process intensification (PI) concepts have potential applications beyond the classic industrial problems solved by chemical engineering. This paper discusses the flow and mass transfer intensification using acoustic waves to enhance drug delivery to the nasal cavity. The efficiency of drug delivery to the nose still needs improvement, and the mechanisms related to using acoustic waves to increase it have yet to be studied. The influence of pressure pulsations induced by an acoustic wave (∼100 Hz) on the concentration, droplet size, and flow structure of the mists delivered from medical nebulizers was studied. The direct visualization showed that pulsations intensify aerosol deposition inside narrow channels due to particle displacement perpendicularly to the main flow direction. This motion also allows for particle penetration via narrow openings. UV-assisted observation of the intensified aerosol deposition inside the anatomical cast of the nasal cavity confirmed the importance of acoustic pulsations for intranasal drug delivery, also helping aerosol penetrate hard-to-reach areas (including the paranasal sinuses). A new idea regarding appropriate time control of acoustic pulsations has been proposed to improve the aerosol drug mass transfer in the nose. The study shows that PI has great potential to expand into new and interdisciplinary areas.

Evaluation of Effective Interfacial Area in a Rotating Packed Bed Equipped with Dual Gas Inlets

This study investigates the effective interfacial area in a novel rotating packed bed (RPB) equipped with dual gas inlets instead of the conventional single-gas-inlet RPB. The aim is to enhance the mass transfer efficiency of gas-liquid contacting processes in RPBs by increasing the number of gas inlets to improve the spread of gas supply into the packing. The RPB is a promising gas-liquid contactor configuration known for its intensified mass transfer characteristics. However, the impact of additional gas inlets on the effective interfacial area of the packing remains unexplored. An experimental method assessed the interfacial area under varying operational conditions which include a liquid flow rate of 0.30-0.60 m3/h, a gas flow rate of 100-300 Nm3/h, and a rotation speed of 600-1000 rpm. At operating conditions covering the maximum rotation speed of 1400 rpm, gas flow and liquid flow rates of 300 Nm3/h and 0.60 m3/h respectively, the results showed that on average, 55 to 97% of the 2400m2/m3 specific packing area can be effectively utilized for gas-liquid mass transfer during separation operations using the RPB. Compared to results reported for single-gas-inlet RPBs using similar packings, the RPB with double gas inlet proved to provide higher utilization of the packing. By simply doubling the number of gas inlets, the findings provide valuable insights into optimizing RPB designs and operations which could enhance mass transfer efficiency for various chemical and environmental applications.

Numerical Investigation of the Reactive Impinging Jet Cooling – the role of vortices in heat and mass transfer intensification

While fundamental mechanisms of non-reactive jet impingement have been extensively researched, the reactive jet impingement is yet unexplored although it offers the potential to increase further the heat transfer. The present work fills this knowledge gap via high-fidelity numerical simulation of reactive jet impingement invoking the finite rate one-step chemistry. Endo-/exothermic reactions are found to have very limited influence on time-averaged fluid flow, while they induce substantial modifications in heat transfer characteristics. This influence is manifested as discernible changes in the momentary or instantaneous fluid dynamics. Specifically, the primary vortices are responsible for downwashing cold fluid (mostly N2O4) towards the hot plate, posing low local convective heat transfer. The secondary vortices are responsible for extracting heat from the hot plate via the dissociation reaction N2O4→2NO2, followed by transporting NO2 away from the hot plate and towards the upwashing side of the primary vortex. At the upside of the primary vortex, hot NO2 is cooled by the environment, therefore reforming N2O4 to give out energy. This intense and long-lasting heat absorption and heat release process explains the outstanding performance of reactive jet impingement.

Drop Dissolution Intensified by Acoustic Levitation.

Acoustic levitation can provide significant benefits for many fundamental research questions. However, it is important to consider that the acoustic field influences the measurement environment. This work focuses on the dissolution of immobilised drops using acoustic levitation in liquid-liquid systems. Previous work demonstrated that the acoustic field of standing waves impacts mass transfer by affecting the spread of dissolved substances in the continuous phase in two distinct ways: (I) solutes may either pass through nodal planes of the standing waves or (II) not pass. The binary systems examined for case (I) are 1-hexanol-water and 1-butanol-water, and for case (II), n-butyl acetate-water and toluene-water. This work quantifies the intensification effect of acoustic levitation on dissolution for the two types of behaviour, by comparing them with reference measurements of mechanically attached dissolving drops. The system was designed to ensure minimal intensification. The minimum intensification of mass transfer for levitating drops in the used setup of case (I) was 25%, and for case (II), it was 65%, both increasing with decreasing surface-equivalent diameter. With this understanding, acoustic levitation can be used more accurately in the field of mass transfer studies.

Gas–Liquid Mass Transfer Intensification for Selective Alkyne Semi-Hydrogenation with an Advanced Elastic Catalytic Foam-Bed Reactor

The Elastic Catalytic Foam-bed Reactor (EcFR) technology was used to enhance a model catalytic hydrogenation reaction by improving gas–liquid mass transfer. This advanced technology is based on a column packed with a commercial elastomeric polyurethane open-cell foam, which also acts as a catalyst support. A simple and efficient crankshaft-inspired system applied in situ compression/relaxation movements to the foam bed. For the first time, the catalytic support parameters (i.e., porosity, tortuosity, characteristic length, etc.) underwent cyclic and controlled changes over time. These dynamic cycles have made it possible to intensify the transfer of gas to liquid at a constant energy level. The application chosen was the selective hydrogenation of phenylacetylene to styrene in an alcoholic solution using a palladium-based catalyst under hydrogen bubble conditions. The conversion observed with this EcFR at 1 Hz as cycle frequency was compared with that observed with a conventional Fixed Catalytic Foam-bed Reactor (FcFR).

Intensify Mass Transfer and Molecular Oxygen Activation by Defect‐Bridged Asymmetric Catalytic Sites Toward Efficient Membrane‐Based Nanoconfined Catalysis

AbstractEfficient spontaneous molecular oxygen (O2) activation is expected in advanced oxidation processes. However, it remains a great challenge to promote the reactants adsorption and accelerate the interfacial electron transfer to boost the activation kinetic of O2. Herein, defect‐rich N‐doped reduced graphene oxide/CoFe2O4 (NGCF‐OV) membrane containing asymmetric Co‐OV‐Fe sites is prepared for O2 activation. The intrinsic catalytic activity is that the asymmetric Co‐OV‐Fe sites regulate the O─O bond length, promoting more and faster electron transfer to O2 for selectively producing 1O2. Meanwhile, the adjacent graphitic N sites help confine organics to the surface and thus greatly shorten the reaction distance of 1O2 and improve its utilization efficiency. The NGCF‐OV membrane demonstrates complete degradation of bisphenol A within a retention time of 86 ms, achieving a k‐value of 0.047 ms−1, which exceeds the performance of most Fenton‐like systems. This work provides new horizons for designing an efficient and stable catalytic membrane, enriching the domain of advanced wastewater treatment strategies.

Nucleation, growth and bubble detachment in liquid-vapor phase change on structured surfaces

Comprehensive grasp of heat and mass transfer, particularly in applications involving liquid-vapor phase change, hinges on management of nucleation, growth, and detachment of vapor bubbles. Various parameters influence the dynamics of phase-change heat and mass transfer and thus dictate the interactions between the surface, the liquid, and the vapor, profoundly impacting the underlying processes. To tailor these phenomena and harness them for technical applications involving high heat flux densities and intense mass transfer, such as boiling and electrolysis, surface functionalization is under intense development. By designing structured surfaces and creating preferential nucleation sites that promote heterogeneous nucleation, it is possible to exert control over the location and density of active nucleation sites on the surface. This, in turn, enables the regulation of bubble growth and detachment from the surface. With the aim of identifying optimal surface treatments for functionalizing surfaces and enhancing their performance in phase-change applications, we evaluated the nucleation, growth and detachment of a single bubble in a liquid-vapor phase change on untextured and laser-textured surfaces during water electrolysis. Platinum was chosen as the preferential material due to its favourable electrochemical properties for the hydrogen evolution reaction in acidic media. Electrolysis was performed at various voltages and the bubble dynamics were evaluated through high-speed imaging to investigate the intricacies of bubble growth and their mutual interactions. At 3.5 V using the laser textured surface, hydrogen bubbles detaching from the electrode surface had on average 40% smaller diameter, while the frequency of their detachment was 2,5-times higher compared to the untextured surface. This opens the possibility for further research that could lead to improving the efficiency of electrolysis.

Experimental study of the dynamics and mass transfer of single CO2 bubble accompanied by reactions in Hele-Shaw cell

The reduction of carbon emissions has become a critical global issue, and the use of monoethanolamine (MEA) solution for CO2 absorption is prevalent in industry. To elucidate the mass transfer mechanisms in reactive multiphase flow, we employed high-speed photography and digital image processing to examine the dynamics and mass transfer behavior of CO2 bubbles in a Hele-Shaw cell. The results indicate that as the MEA solution concentration increases, oscillations during bubble ascent diminish, and the terminal velocity decreases. Based on changes in the mass transfer coefficient, the reaction process can be segmented into a phase of intensified mass transfer, marked by a rapid decrease in bubble equivalent diameter, and a phase of deteriorating mass transfer, where the diameter stabilizes. Additionally, we introduced a dimensionless mathematical model for the Sherwood number based on experimental findings, and its reliability was confirmed.

Gas-liquid-soild triphasic continuous flow microreactor for improving homogeneous distribution of solid composites in heterogeneous photocatalytic degradation progress

The homogeneous distribution of solid composites in continuous flow photocatalytic progress is difficult due to the settle and clog in capillary. To overcome this challenge, a gas–liquid-soild triphasic continuous flow photoreactor was designed and used for heterogeneous photocatalytic degradation of antibiotics wastewater. In this work, take Bi2WO6 as photocatalyst, photocatalytic degradation of ofloxacin wastewater was studied systematically under different operational parameters and flow characteristics. The photocatalytic degradation performance of flow photoreactor was superior to batch counterpart, especially in later stage of reaction. The apparent reaction rate constants (k) of flow photoreactor was about 1.6 times that of batch counterpart. The gas–liquid mass transfer coefficient of flow photoreactor was more than 25 times that of batch photoreactor. Moreover, the influence mechanism of gas fraction, flow rate and tube diameter on photocatalytic degradation performance was investigated under various continuous flow conditions. Higher gas fraction, faster flow rate and smaller tube diameter were beneficial to improve photocatalytic degradation performance due to the enhancement of irradiation transmission and intensification of mass transfer. Therefore, the results of this work provided a guideline for the application of gas–liquid-soild triphasic continuous flow photocatalytic degradation technology in the future.

STUDY OF THE PROCESS OF PLANT RAW MATERIAL EXTRACTION IN A STATIC-TYPE CAVITATION INLINE MIXER

Abstract. The flowering heads of calendula have a significant content of biologically active substances (BAS) with a wide range of pharmacological activities. Traditional methods for obtaining their aqueous extracts include maceration or percolation. However, these methods have significant drawbacks. The rational choice of equipment and the study of the influence of thermophysical conditions on the extraction process of specific plant raw materials to intensify mass transfer processes are relevant. The aim of the study is to investigate the transition of solid substances of calendula flowers into a soluble state in a flow cavitation mixer of a static type to intensify mass transfer processes in obtaining aqueous extracts. For the research, previously ground dry calendula flower heads, swollen in distilled water at a hydro module of 1:15 and reduced to 0,1-2 mm, were used. The work was carried out on an experimental cavitation mixer of the Venturi tube type with a variable cavitation reactor with a nozzle throat diameter ranging from 0,006 m to 0,12 m. The efficiency of extraction was determined by the amount of water-soluble substances, and the cavitation phenomenon was characterized by the cavitation number. Results: The overall design of the cavitation apparatus, the profile and geometric parameters of the nozzle, the flow parameters, as well as the properties of the plant material, significantly affect the hydrodynamic conditions and the development of cavitation effects during processing. The Venturi-type flow cavitation mixer used in the research showed high efficiency in processing mass transfer processes during the extraction of a water suspension of calendula flower heads. The results obtained indicate that the most significant impact of cavitation effects is observed at nozzle throat diameters of 0,08 m and 0,10 m, with cavitation numbers χ = 0,53 and χ = 0,84, respectively. In this case, the process duration can be limited to 60 seconds. The obtained conclusions are confirmed by microstructure studies.