High Mass Transfer Rates Research Articles

EFFECT OF PRE-STEAMING ON MASS TRANSFER PROPERTIES OF FIR WOOD (ABIES ALBA L.); A GYMNOSPERM SPECIES WITH TORUS MARGO PIT MEMBRANE

In this research, the effect of pre-steaming on mass transfer properties, including air permeability and water vapor diffusivity of fir wood (Abies alba L.), a gymnosperm species with torus margo pit membrane, was evaluated. The pre-steaming was performed at temperatures of 120, 140, and 160°C for 1 hour under a pressure of 2-3 bars. Then, the pre-steamed specimens were conventionally dried at a constant temperature of 160°C and a relative humidity of 50% to the final moisture content of 10%. Subsequently, the mass transfer properties of the dried specimens were measured in longitudinal and radial directions. Overall, the pre-steaming was found to be an effective modification method to improve the mass transfer properties of Abies alba L. The improvement was more remarkable for the air permeability as well as through the radial direction. The specimens steamed at the temperature of 160°C had higher mass transfer rates than those steamed at the temperatures of 120 and 140°C. Results of chemical analyses, FT-IR spectroscopy, and SEM imaging provide some explanations for the effects of pre-steaming.

Mass transfer modeling of a consecutive reaction in rotating foam stirrer reactors: Selective hydrogenation of a functionalized alkyne

The external and internal mass transfer for a rotating foam stirrer reactor has been studied using the hydrogenation of 3-methyl-1-pentyn-3-ol as a model reaction. Catalytic foam stirrers with different number of pores per linear inch (ppi) and washcoating thicknesses are used and their performance are described in terms of activity and selectivity. The liquid–solid mass transfer does not limit the overall reaction rate when a 20ppi foam is used. For this catalytic foam stirrer, a kLSaLS value of 0.2s−1 is estimated at 300rpm. The use of finer pore foam structures is not convenient due to a drastic decrease of catalyst surface refreshment. As a result, the lower kLS value negates the benefit of a larger area (aLS) resulting in a lower liquid–solid mass transfer. The catalyst accessibility is a controlling factor in this fast reaction. The optimal washcoat thickness to avoid diffusion limitations is 5μm. Gas–liquid mass transfer rate is not a limiting factor. A kGLaGL of 0.6s−1 is measured at 300rpm by the physical absorption method. When compared to the conventional slurry reactor, the liquid–solid and the gas–liquid mass transfer rates are 17 and 6 times higher, respectively. However, the intrinsic activity of the foam catalyst is 4.6 times less than the slurry catalyst prepared with the same palladium loading. This is explained by a difference in palladium dispersion caused by a non-homogenous concentration of active sites along the washcoat layer. On the other hand, the less active foam catalyst leads to higher selectivity. The production rate is then increased by mounting more active blades on the stirrer shaft, thus creating more surface area for catalyst deposition while still keeping the high accessibility of a 4μm washcoat layer. Hence, rotating foam reactors have promising applications for multiphase processes. High mass transfer rates can be easily achieved, catalyst filtering is not longer needed and reusability of the foam catalyst is possible.

Energy from hydrogen. Hydrogen from renewable fuels for portable applications

Molecular hydrogen is an environmentally clean source of energy, but it is not available on Earth. Steam reforming of bio-derived compounds represents a valuable route for the generation of molecular hydrogen and has the advantage that it is CO2-neutral and it requires a limited amount of additional infrastructure for implementation. At present, suitable catalysts for selective bio-alcohol and dimethyl ether reforming into hydrogen and carbon dioxide are being developed, but their use on structured wall reactors for practical application is still under way. Among them, aerogel-based coated structures appear very promising due to their very high mass transfer rates and their ability to disperse highly active metal nanoparticles. The performance of these systems improves considerably by using microreaction technologies. Microreactors based on silicon micromonoliths together with integrated downstream carbon monoxide selective oxidation hold a promising futurefor the effective on-site and on-demand generation of hydrogen from renewable fuels in portable fuel cell applications.

Bionic system for countercurrent multi-stage micro-extraction

A bionic system, analogous to the human cardiovascular system, is developed to realize countercurrent, multi-stage micro-extraction. High mass transfer rates and high recovery efficiency were achieved. The system can be applied to any type of micro-extractor and may promote the application of micro-extraction in a wide variety of fields.

Ammonia removal by air stripping in a semi-batch jet loop reactor

Ammonia is very toxic chemical and it can be removed by air stripping at high pH. JLRs have found applications in wastewater treatment processes due to their high mass transfer rates. In JLRs, intrinsic high turbulence result in a very large air-liquid surface area for greater mass transfer. Therefore, in this study, ammonia removal by air stripping from synthetically prepared ammonia solution at the high pH in a semi-batch JLR due to its high mass transfer capabilities have been investigated. Investigated parameters in a JLR were initial ammonia concentration (10–500 mg/L), temperature (20–50 °C), air flow rate (5–50 L/min) and liquid circulation rate (35–50 L/min). While it was demonstrated that temperature and air flow rate have a significant effect on the ammonia removal, it was determined that initial ammonia concentration and liquid circulation rate have no significant effect on the ammonia removal. The overall volumetric mass transfer coefficients ( K L a) have been calculated from obtained model and it was determined that increasing temperature and air flow rate have a very significant effect on K L a. It was concluded that JLR provides higher mass transfer capabilities than other type of reactors even if less air is given.

Modeling of Chemical Vapor Deposition of Pyrolytic Carbon in a Gas-Spouted Bed Reactor

Spouted beds are used in chemical vapor deposition as they provide good solid mixing, thereby enhancing high mass and heat transfer rates. In the present study, investigations on the kinetics of a coating process by chemical vapor deposition were carried out in a spouted-bed reactor. Pyrolytic carbon coatings were studied by thermal cracking of acetylene mixed with carrier gas argon at 1350 °C. A theoretical model was proposed based on the mass balance of the precursor in the spout and annulus zones at isothermal condition, and the model equations were solved using MATLAB (version 7.1) software. The effect of the operating gas velocity, initial static bed height, and acetylene concentration on the coating rate was examined experimentally and also by simulation results. The simulated results agreed well with the experimental results. Model equations used for simulation were utilized to evaluate the reaction rate constant and radial gas diffusion coefficient and then to study the effect of spout diameter, operating gas velocity, and precursor concentration on the overall conversion of the precursor.

Local heat-transfer coefficient of immersed cylindrical surface in fluidized and vibrated fluidized beds

Due to the good air–particle mixing couple with the high heat and mass transfer rates, fluidized and vibrated fluidized beds of particles have been widely used for many chemical engineering processes involving particulate systems. On the other hand, in practice, for using such beds in the treatment of heat-sensitive materials (i.e., polymer, food products) the installation of heat-exchange surface within the bed are needed to provide indirect heat as well as prevent thermal degradation. Therefore, this paper presents an investigation to determine the local heat-transfer coefficient in fluidized and vibrated fluidized beds (by expressing Nu θ vs. Re) operated with glass ballotini particles ranging from 500 to 1100 μm, in diameter. The data show that, at a given air velocity, the local heat-transfer coefficient obtained in the vibrated fluidized beds is significantly higher as compared to those of fluidized beds. In addition, vibrated fluidized beds can achieve higher local heat-transfer coefficients as the particle diameter is reduced from 1100 to 500 μm and the vibration dimensionless ( Γ) is increased from 1 to 3.

Hydrodynamics and mass transfer studies on a plate microreactor

Prototype of structured microreactor plates with 300 μm deep flow channels was developed. Two plates with different structural elements, square and triangular, were used in the study. Flow behavior was investigated and the results showed excellent mixing behavior of gas and liquid phases. Hydrodynamic parameters and volumetric gas–liquid mass transfer coefficients were determined at various flow conditions. Gas holdup and the gas–liquid interfacial area were determined by an optical method using image processing software. Volumetric mass transfer coefficients were measured using nitrogen to strip oxygen from deionized water. The experimental results indicate a high mass transfer rate. Gas holdup and the total gas–liquid interfacial area were in the range of 20–45% and 2600–5600 m 2 m −3, respectively. Volumetric mass transfer coefficients were as high as 1.1 s −1. An empirical correlation for the mass transfer coefficient was derived.

Ultra-soft Sources as Type Ia Supernovae Progenitors

AbstractMissing from the usual considerations of nuclear burning white dwarfs as Type Ia supernovae progenitors are systems with very higher mass transfer rates, where more material than is needed for steady burning accretes on the white dwarf. This will expand the photosphere of the white dwarf, causing it to emit at longer wavelengths. Thus, we propose the name ultra-soft source (USS) for these objects.We present a VLT/FLAMES survey looking for USSs in the SMC, selected to be bright in the far UV and with blue far UV-V colors. While we find some unusual objects, and recover known planetary nebulae and WR stars, we detect no objects with strong He II lines, which should be a signature of USSs. This null result either puts an upper limit on the number of USSs in the SMC, or shows that we do not understand what the optical spectra of such objects will look like.We also discuss the unusual LMC [WN] planetary nebula LMC N66 as a possible example of a USS. It has a luminosity consistent with that expected, and its spectra show incompletely CNO-processed material — strong helium lines, some hydrogen, enhanced nitrogen and depleted carbon. It also shows periodic outbursts. USSs may resemble N66 in quiescence. However, it lacks a FUV excess, contrary to our predictions.

Marginally low mass ratio close binary system V1191 Cyg

In this study, we present photometric and spectroscopic variations of the extremely small mass ratio ( q ≃ 0.1) late-type contact binary system V1191 Cyg. The parameters for the hot and cooler companions have been determined as M h = 0.13 (1) M ⊙, M c = 1.29 (8) M ⊙, R h = 0.52 (15) R ⊙, R c = 1.31 (18) R ⊙, L h = 0.46 (25) L ⊙, L c = 2.71 (80) L ⊙, the separation of the components is a = 2.20 (8) R ⊙ and the distance of the system is estimated as 278 (31) pc. Analyses of the times of minima indicates a period increase of dP dt = 1.3 ( 1 ) × 10 - 6 days / yr that reveals a very high mass transfer rate of dM dt = 2.0 ( 4 ) × 10 - 7 M ⊙ / yr from the less massive component to the more massive one. New observations show that the depths of the minima of the light curve have been interchanged.

Hierarchical porous carbons: design, preparation, and performance in energy storage

Hierarchical porous carbons (HPCs) possess a multimodal pore size distribution of micro-, meso-, and/or macropores, and thus show high electrochemically accessible surface area, short diffusion distance, and high mass transfer rate when used as electrode materials in energy storage devices. Because of this, HPCs show excellent rate capabilities and have attracted considerable attention in recent years. Normally, HPCs are prepared by templating methods or combined templating/activation methods, in which carbons are inverse replicas of the template and the pore structure parameters of HPCs can be easily tuned. Recently, simpler template-free methods have been developed and show a promising future. In this review, we describe the recent advances in design, preparation, and electrochemical performance of HPCs, with particular attention to the main achievements using HPCs developed in our groups since 2008. Finally we give a brief outlook.

Design of Microscale Heat and Mass Exchangers for Absorption Space Conditioning Applications

This paper presents the development of a miniaturization technology for heat and mass exchangers used in absorption heat pumps. The exchanger consists of an array of parallel, aligned alternating shims with integral microscale features, enclosed between cover plates. These microscale features facilitate the flow of the various fluid streams and the associated heat and mass transfer. In an absorber application, effective vapor and solution contact and microscale features for the flow of both the solution and the coolant induce high heat and mass transfer rates without any active or passive surface enhancement. The geometry ensures even flow distribution with minimal overall pressure drops. A model of the coupled heat and mass transfer process for ammonia-water absorbers using this configuration under typical operating conditions demonstrates the potential for extremely small absorption components. The proposed concept is compact, modular, versatile, and in an eventual implementation, can be mass produced. Additionally, the same concept can be extended to the other absorption heat pump components as well as for several other industries involved in multicomponent fluid processes.

Mesoporous silica iron-doped as stable and efficient heterogeneous catalyst for the degradation of C.I. Acid Orange 7 using sono–photo-Fenton process

Iron-containing mesostructured silica (Fe 2O 3/SBA-15) was prepared and used as heterogeneous catalyst in the integrated sono–photo-Fenton process. The effect of hydrogen peroxide concentration, initial pH, Fe 2O 3/SBA-15 loading, ultrasonic power, and initial solution concentration on the decolorization of C.I. Acid Orange 7 (AO7) by the integrated heterogeneous sono–photo-Fenton process was investigated in order to optimize process efficiency. The results showed that the decolorization efficiency increased with the increase of hydrogen peroxide concentration, ultrasonic power, Fe 2O 3/SBA-15 loading, but decreased with the increase of initial pH and initial dye concentration. Catalyst stability was evaluated by measuring iron leaching in solution. The particles size distribution of spent catalyst evidenced a remarkable size reduction during reaction, leading to more reactive surface and higher mass transfer rate. The main reaction intermediates were separated and identified using GC–MS and a possible degradation pathway was proposed.

Electrochemical Removal of Copper Ions from Aqueous Solutions Using a Modulated Current Method

The use of porous electrodes to remove toxic metals from industrial effluents has been recognized over the years due to its high mass transfer rates resulting in high current efficiencies (CE) and low energy consumption (EC), even at very low metal concentrations. This work addresses the effects of flow velocity and electric current on current efficiency and energy consumption of copper electrodeposition using reticulated vitreous carbon, aiming to optimize the CE and EC by using a modulated current method. First, it was found that a flow velocity of 0.246 m s−1 maximizes the mass transfer coefficient (km). Applying this flow velocity and a modulated current, the electrodeposition process was investigated and compared with that carried out under galvanostatic mode. The results showed that using a current control there is a reduction of average EC due to the improvement of the average CE, but the operational time also increases when compared to that obtained using the galvanostatic mode.

Multistage rotor‐stator spinning disc reactor

AbstractThe scale up of a rotor‐stator spinning disc reactor by stacking single stage rotor‐stator units in series is demonstrated. The gas‐liquid mass transfer per stage is equal to the mass transfer in a single stage spinning disc reactor. The pressure drop per stage increases with increasing rotational disc speed and liquid flow rate. The pressure drop is more than a factor 2 higher for gas‐liquid flow than for liquid flow only, and is up to 0.64 bar at 459 rad s−1. The high mass and heat transfer coefficients in the (multistage) rotor‐stator spinning disc reactor make it especially suitable for reactions with dangerous reactants, highly exothermic reactions and reactions where selectivity issues can be solved by high mass transfer rates. Additionally, the multistage rotor‐stator spinning disc reactor mimics plug flow behavior, which is beneficial for most processes. © 2011 American Institute of Chemical Engineers AIChE J, 2012

Investigating dissolved lead at the tap using various sampling protocols

The authors investigated factors influencing the occurrence of dissolved lead in tap water using different sampling protocols. The principal factor affecting the concentration of dissolved lead at the distribution system taps was the length of lead service lines (LSLs). However, dissolved lead levels in first‐litre samples were also associated with lead particles being trapped in the aerator. Collecting the first‐litre sample after 30 min of stagnation provided a good estimate of lead concentration in premise plumbing and LSLs, even though it could sometimes underestimate peak lead concentrations in the LSLs. Also it gives mean exposure estimates close to that obtained using random daytime sampling. Lead levels remained relatively high in flushed samples despite short (26‐s) contact time between the water and lead pipe, illustrating high rates of mass transfer.

Mass Transfer Behavior of Annular Ducts under Conditions with Superimposed Pulsating Flow

AbstractRates of mass transfer were measured at the inner surface of an annular duct by an electrochemical technique under developing flow conditions with superimposed pulsating flow. Variables like the frequency and amplitude of the pulsating flow, steady flow velocity, and annulus height were studied. The pulsating flow was found to enhance the rate of mass transfer by a factor ranging from 1.2 to 5.5 compared to the steady laminar flow value. The superimposed pulsating flow had only a minor effect on the rate of mass transfer under steady turbulent flow conditions. The pulsating flow reactor offers the advantage over steady flow annular reactor in that it can be operated at low flow rates to increase the residence time. The high residence time and high rate of mass transfer resulting from pulsation together increase the degree of conversion per pass as well as the reactor productivity.

Tomography measurements of gas holdup in rotating foam reactors with Newtonian, non-Newtonian and foaming liquids

Rotating solid foam reactors have already proven to show high mass transfer rates and to be a potential alternative to slurry reactors. The rotation of a foam block stirrer results in a high mass transfer and in the development of different reactor sections showing specific hydrodynamics and gas holdup distributions. In order to optimize the reactor system the hydrodynamics in a lab scale reactor are studied using γ -ray tomography, a powerful method to measure the gas holdup in three-phase reactors. The influence of liquid properties, such as viscosity and surface tension, and the rotational speed on the gas/liquid distribution in the different reactor sections is investigated. Especially the viscosity has a strong effect on the entrapment of gas bubbles in the foam block structure, while the surface tension is the dominant parameter in the outer reactor section. The influence of these parameters on the inset of foaming and the collapse of the gas/liquid dispersion is investigated. Conclusions on the mass transfer performance are drawn and recommendations for further optimizations of the reactor design and the operational conditions depending on the liquid properties are developed.

EVALUATION OF MOLECULAR WEIGHT OF CHITOSAN IN THIN‐LAYER AND SPOUTED BED DRYING

ABSTRACT Chitosan is chitin in deacetylated form and is the main constituent of crustacean exoskeletons. Commercially, chitosan is dried in tray driers, and during the operation, polymerization may occur as the chitosan is composed of carbohydrates. The aim of this work was to analyze chitosan in spouted bed and thin‐layer drying, considering viscosity average molecular weight of the chitosan samples in the process. Results showed that spouted bed‐dried chitosan presented a molecular weight value similar (160 kDa) to that of the raw one (150 kDa). However, when dried on tray dryers, the molecular weight was 300 kDa, indicating that molecule polymerization occurred. PRACTICAL APPLICATIONSChitosan, a deacetylated form of chitin, is considered a functional food. The use of chitosan in various areas is directly related to the molecular weight and degree of deacetylation, which depends on the conditions of chitin deacetylation and chitosan drying. One of the main steps of the chitosan production process is drying. Industrial chitosan is dried in tray dryers; however, it is composed mainly of carbohydrate monomer units capable of undergoing polymerization during the drying operation. The spouted bed dryer is a method that promotes direct particle–fluid contact, with high rates of heat and mass transfer. Also, it presents an alternative to the spray‐dryer in an attempt to obtain dry products with the same quality, at low costs. This work presents the study of chitosan drying in spouted bed and thin layer, in paste form, considering the viscosity average molecular weight of the chitosan samples in the operation.

Decision support towards agile eco-design of microreaction processes by accompanying (simplified) life cycle assessment

Continuously running syntheses in microstructured reactors offers novel ways to intensify conventional chemical processes. An outstanding advantage of microreaction technology is the high surface-to-volume-ratio which enables intensive mixing phenomena as well as high mass and heat transfer rates. Thus, microstructured reactors may be a suitable means to improve multiphase reactions by increasing the interfacial area and the intensification of internal mixing. This improvement in reaction performances may lead to reduced environmental burdens of the process under consideration. The method of simplified life cycle assessment (SLCA) is a suitable tool to evaluate the environmental burdens caused by chemical processes. It has been applied already in research and development to identify the key parameters for a deliberate green process design of two biphasic reactions, the esterification of phenol and benzoyl chloride resulting in phenyl benzoate and the synthesis of one of the corresponding phase transfer catalysts, [BMIM]Cl. Further, SLCA is complemented by a simple cost estimation to investigate the main cost drivers relevant for possible industrial application of the syntheses investigated.