Hydrodynamic and mass transfer efficiency of ceramic foam packing applied to distillation

In addition to a high void volume and specific area, solid foams possess other properties (low density, good thermal, mechanical, electrical, and acoustical behaviour) that make them attractive for applications such as heat exchangers and reformers. Applications using foams as catalysts or structured catalyst supports have demonstrated higher performance than classical catalysts. Several studies have explored the hydrodynamic behaviour of foams in monophasic and countercurrent systems and have reported very low pressure drops. This paper describes the application of ceramic foam to distillation. The β-SiC foam contains 5 pores per inch (PPI) with a 91% void volume and a surface area of 640 m²/m³. Performance parameters including pressure drop for the dry and wet packing, flooding behaviour, and dynamic liquid hold-up were measured in a column of 150 mm internal diameter. The mass transfer efficiency in terms of the height equivalent to theoretical plate (HETP) was determined by total reflux experiments using a mixture of n-heptane and cyclohexane at atmospheric pressure. The experimental results were used to develop a set of correlations describing pressure drop and liquid hold-up in terms of a dimensionless number. The hydrodynamic performance and mass transfer efficiency were compared with classical packing materials used in distillation.     

Axial dispersion in metal foams and streamwise-periodic porous media

We present a study on the axial dispersion in metal foams and laser sintered reactors. Commercially available metal foams of 20 and 30 ppi are compared to a designed streamwise-periodic structure in terms of axial dispersion coefficients and pressure drops. Therefore tracer pulse experiments were performed and post processed by means of a deconvolution method. The Peclet number Pe_p based on the pore size is ranging from 5×10⁴ to 8×10⁵ which is attributed to the increased velocities due to the high porosity of the material compared to fixed bed reactors. The attained dispersion coefficients ranging from 1.3×10⁻⁴ to demonstrate the trend of packed beds and common packing materials and increase monotone with the Peclet number Pe_p. The pressure drop versus the interstitial bulk velocity follows the Forchheimer equation and can be described by the conventional Ergun model for all investigated porous media. The parameters obtained correspond to values found in literature. The results of this study show the high potential of foam reactors for catalyst driven reactions. They provide the same or even a higher surface area per volume of catalyst bed while inducing a much smaller pressure drop than corresponding fixed beds.     

A general correlation for mass transfer in isotropic and anisotropic solid foams

Many open-cell foams exhibit ellipsoidal rather than spherical cells. The extent of the anisotropy of the structure typically increases with decreasing cell density. In dimensionless transport correlations, an anisotropy factor has to be introduced in order to lend them general applicability. This paper reports on such a correlation for mass transfer which has been proven to be useful for ceramic and metallic foams with pore counts between 10 and 45 ppi and porosities between 75% and 95%. Based on this novel correlation it was also possible to apply successfully the Lévêque equation which enables the prediction of mass transfer coefficients from pressure drop data.     

Hydrodynamics and mass transfer characteristics in gas-liquid flow through a rectangular microchannel

Researches on two-phase transfer and reaction processes in microchannnels are important to the design of multiphase microchemical systems. In the present work, hydrodynamics and mass transfer characteristics in cocurrent gas-liquid flow through a horizontal rectangular microchannel with a hydraulic diameter of have been investigated experimentally. Liquid side volumetric mass transfer coefficients were measured by absorbing pure CO₂ into water and a 0.3 M NaHCO₃ / 0.3 M Na₂CO₃ buffer solution. Interfacial areas were determined by absorbing pure CO₂ into a 1 M NaOH solution. Two-phase flow patterns and pressure drop data were also obtained and analyzed. This paper shows that two-phase frictional pressure drop in the microchannel can be well predicted by the Lockhart-Martinelli method if we use a new correlation of C value in the Chisholm's equation. Liquid side volumetric mass transfer coefficient and interfacial area as high as about and , respectively, can be achieved in the microchannel. Generally, liquid side volumetric mass transfer coefficient increases with the increasing superficial liquid or gas velocity, which can be described satisfactorily by the developed empirical correlations. A comparison of mass transfer performance among different gas-liquid contactors reveals that the gas-liquid microchannel contactor of this study can provide at least one or two orders of magnitude higher liquid side volumetric mass transfer coefficients and interfacial areas than the others.     

孔密度对泡沫金属内湿空气的换热与压降特性影响分析

通过实验研究,得到不同孔密度的泡沫金属内湿空气的换热和压降特性,并对泡沫金属换热器综合性能进行了分析。测试样件为泡沫铜,孔密度为5~40PPI(pores per inch),孔隙率为95%。研究结果表明,由于凝结水的存在,泡沫金属内的湿空气传热系数随着孔密度的增大先增大后减小,孔密度为15PPI时达到最大值;压降随着孔密度的增大而增大,且大于20 PPI时压降增大更明显。综合考虑传热系数与压降因素,泡沫金属孔密度为15PPI时综合性能最佳。     

Modeling the principle physical parameters of graphite carbon foam

Graphite carbon foam, a mesophase, pitch-based material, portrays highly ordered topology structures which exhibit superior mechanical and thermal properties. Typical graphite carbon foam with dimensions 5 cm³, can have a surface area greater than 11 m², making it an excellent candidate for heat transfer applications. Accurate three dimensional modeling of carbon foams is necessary to study and predict their properties in simulation. This paper describes a computer algorithm for modeling POCO Foam® and similar carbon foams. The algorithm, written in MATLAB, captures the principle physical parameters of the carbon foam including bubble and pore diameter ranges and overall foam void percentage while retaining the random dispersal of spherical bubbles found in manufactured foams.     

An experimental study of flow and heat transfer of supercritical carbon dioxide in multi-port mini channels under cooling conditions

This paper presents the fluid flow and heat transfer characteristics of supercritical CO₂ in a horizontal multi-port extruded aluminum test section consisting of 10 circular channels with an inner diameter of 1.31 mm. Both local and average pressure drop and heat transfer coefficients were measured as CO₂ was cooled in the multi-port circular channels with pressures ranging from 7.4 to 8.5 MPa, inlet fluid temperatures ranging from 22 to , and mass velocity ranging from 113.7 to 418.6 kg/m² s. The results indicate that the operating pressure, the mass velocity and the temperature of CO₂ had significant effects on fluid flow and heat transfer characteristics. The pressure drop and the average heat transfer coefficient increased greatly with increasing the average temperatures of CO₂ in the near-critical region; the average heat transfer coefficient attained a peak value near the corresponding pseudocritical temperature; and the maximum heat transfer coefficient decreased as the pressure increased. Both the pressure drop and the heat transfer coefficient increased with the mass velocity, but decreased with the operating pressure. The measured average heat transfer coefficients were compared with the experimental data reported in the literatures and a large discrepancy was observed. Based on the experimental data collected in the present work, a new correlation was developed for forced convection of supercritical CO₂ in horizontal multi-port mini channels under cooling conditions.     

Wall-to-liquid mass transfer in fixed beds at low flow rates

The wall-to-fluid mass transfer coefficient was obtained from coated wall dissolution experiments, for water flow through fixed beds of spheres with tube-to-particle diameter ratios of 2.9-11.6, and for particle Reynolds number (Re) in the range 3-200. The coefficients, in the form of dimensionless Sherwood numbers (Sh_wf), were shown to approach a nonzero limit as Re→0. The data were well-represented by the equation

     

Determination of mass transfer coefficients for packing materials used in biofilters and biotrickling filters for air pollution control. 1. Experimental results

Gas film mass transfer coefficients (k_Ga_t,k_Ga_w) and liquid film mass transfer coefficients (k_La_w) for packing materials used in biofilters and biotrickling filters for air pollution control were determined experimentally. Lava rock, polyurethane foam cube (PUF), Pall ring, porous ceramic beads, porous ceramic Raschig rings and compost-woodchips mixtures were investigated. The experiments were performed at gas velocities ranging from 100 to and liquid velocities of , i.e., a wide range that covers most biofilters and biotrickling filters. k_Ga_t in biofilter packings ranged from about 500 to , while k_Ga_w and k_La_w in biotrickling filters ranged from 100 to , and 1 to , respectively, depending on the packings and the conditions. This is markedly lower than mass transfer coefficients usually observed for conventional wet scrubbing. The gas film mass transfer coefficient (k_Ga_t) of 50:50% vol compost-woodchips mixture, a common biofilter packing, was greater than this of a 20% vol compost and 80% woodchips mixture, though the mass transfer was not increased by increasing further the volume fraction of compost. All compost mixtures exhibited a greater gas film mass transfer coefficient than lava rock or other synthetic materials. The mass transfer coefficients of compost mixtures was also influenced by packing method and it was directly proportional to the surface area of the bulking agents added. The gas film mass transfer coefficient (k_Ga_w) of five biotrickling filter packing materials increased linearly with gas velocity. The effect of liquid on the gas film mass transfer coefficient was not significant. Of all the biotrickling filter packings, the porous ceramic beads had the highest gas and liquid film mass transfer coefficients followed by lava rock, porous ceramic rings, 1 in Pall ring and PUF cubes. The liquid film mass transfer coefficient (k_La_w) was directly proportional to liquid velocity and the effect of gas velocity was negligible. Several correlations allowing prediction of mass transfer coefficients are presented in Part 2 of this paper.     

Determination of effective diffusion coefficient and interfacial mass transfer coefficient of bovine serum albumin (BSA) adsorption into porous polyethylene membrane by microscope FTIR-mapping study

Here, a new method for simultaneous determination of diffusion coefficient D and interfacial mass transfer coefficient (or convective mass transfer coefficient) k was proposed for bovine serum albumin (BSA) adsorption into porous polymeric membranes. The experimental data for BSA concentration at different membrane depth and different time were determined from FTIR-mapping measurements. Then the diffusion coefficient D and interfacial mass transfer coefficient k were estimated from the calculated dimensionless concentration data at different time and membrane depth by a trial-and-error method based on the diffusion equation initiated in this paper. The diffusion coefficient D and interfacial mass transfer coefficient k evaluated in this manner are respectively: and . The theoretical concentration values calculated from the determined parameters were compared with experimental reading from FTIR mappings, which showed a good agreement between them, especially for the case of a relatively long-time adsorption.     

Simulations of chemical absorption in pilot-scale and industrial-scale packed columns by computational mass transfer

A complex computational mass transfer model (CMT) is proposed for modeling the chemical absorption process with heat effect in packed columns. The feature of the proposed model is able to predict the concentration and temperature as well as the velocity distributions at once along the column without assuming the turbulent Schmidt number, or using the experimentally measured turbulent mass transfer diffusivity. The present model consists of the differential mass transfer equation with its auxiliary closing equations and the accompanied formulations of computational fluid dynamics (CFD) and computational heat transfer (CHT). In the mathematical expression for the accompanied CFD and CHT, the conventional methods of k-ε and are used for closing the momentum and heat transfer equations. While for the mass transfer equation, the recently developed concentration variance and its dissipation rate ε_c equations (Liu, 2003) are adopted for its closure. To test the validity of the present model, simulations were made for a pilot-scale randomly packed chemical absorption column of 0.1 m ID and 7 m high, packed with 1/2^″ ceramic Berl saddles for CO₂ removal from gas mixture by aqueous monoethanolamine (MEA) solutions (Tontiwachwuthikul et al., 1992 ) and an industrial-scale randomly packed chemical absorption column of 1.9 m ID and 26.6 m high, packed with 2^″ stainless steel Pall rings for CO₂ removal from natural gas by aqueous MEA solutions (Pintola et al., 1993). The simulated results were compared with the published experimental data and satisfactory agreement was found between them in both concentration and temperature distributions. Furthermore, the result of computation also reveals that the turbulent mass transfer diffusivity D_tvaries along axial and radial directions. Thus the common viewpoint of assuming constant D_t throughout the whole column is questionable, even for the small size packed column. Finally, the analogy between mass transfer and heat transfer in chemical absorption is demonstrated by the similarity of their diffusivity profiles.     

Towards a more realistic modeling of solid foam: Use of the pentagonal dodecahedron geometry

The geometric structure of a solid foam is approached using a pentagonal dodecahedron as the unit cell. Important properties of the foam (specific surface area, pressure drop, external fluid-solid mass transfer) are well represented and successfully compared with experiments. The approach enables accounting for triangular or cylindrical struts and solid accumulation at their meeting points. Knowledge of the void fraction and mean strut diameter (or pore diameter) are sufficient for estimating any geometric characteristics. The model has been tested with foams of porosity ranging from 0.75 to 0.98.     

Mass transfer around freely moving active particles in the dense phase of a gas fluidized bed of inert particles

The mass transfer coefficient around freely moving active particles under bubbling/slugging fluidized bed conditions was measured in a lab-scale reactor. The technique used for the measurements consisted in the oxidation reaction of carbon monoxide at over one or few Pt catalyst spheres immersed in an inert bed of sand. It was shown that this technique is simple and accurate, and allows to overcome most of the difficulties and uncertainties associated with other available techniques. The experimental campaign was carried out by varying the fluidization velocity (0.15-0.90 m/s), the active particle size (1.0-10.0 mm) and the inert particle size (0.1-1.4 mm). Results were analyzed in terms of the particle Sherwood number. Experimental data showed that Sh is not influenced by the fluidization velocity and by a change of regime from bubbling to slugging, whereas it increases with a square root dependence with the minimum fluidization velocity and with the active particle size. These results strongly suggest that the active particles only reside in the dense phase and never enter the bubble/slug phase. Data were excellently fitted by a Frössling-type correlation:

Sh=2.0·ε_mf+K·(Re_mf/ε_mf)^1/2·Sc^1/3     

Mass transfer kinetics on the heterogeneous binding sites of molecularly imprinted polymers

The mass transfer kinetics of the L- and D-Fmoc-Tryptophan (Fmoc-Trp) enantiomers on Fmoc-L-Trp imprinted polymer (MIP) and on its reference polymer (NIP), were measured using their elution peak profiles and the breakthrough curves recorded in frontal analysis for the determination of their equilibrium isotherms, at temperatures of 40, 50, 60, and . At all temperatures, the isotherm data of the Fmoc-Trp enantiomers on the MIP were best accounted for by the Tri-Langmuir isotherm model, while the isotherm data of Fmoc-Trp on the NIP were best accounted for by the Bi-Langmuir isotherm model. The profiles of the elution bands of various amounts of each enantiomer were acquired in the concentration range from 0.1 to 40 mM. These experimental profiles were compared with those calculated using the best values estimated for the isotherm parameters and the lumped pore diffusion model (POR), which made possible to calculate the intraparticle diffusion coefficients for each system. The results show that surface diffusion contributes predominantly to the overall mass transfer kinetics on both the MIP and the NIP, compared to external mass transfer and pore diffusion. The surface diffusion coefficients (D_s) of Fmoc-L-Trp on the NIP does not depend on the amount bound (q) while the values of D_s for the two Fmoc-Trp enantiomers on the MIP increase with increasing q at all temperatures. These positive dependencies of D_s on q for Fmoc-Trp on the MIP were fairly well modeled by indirectly incorporating surface heterogeneity into the surface diffusion coefficient. The results obtained show that the mass transfer kinetics of the enantiomers on the imprinted polymers depend strongly on the surface heterogeneity.     

Effect of surfactants on liquid-side mass transfer coefficients in gas-liquid systems: A first step to modeling

This paper focuses on the effect of surfactants on the mass transfer parameters (volumetric mass transfer coefficient k_La and liquid-side mass transfer coefficient k_L). Tap water and aqueous solutions with surfactants (anionic, cationic and non-ionic at concentrations up to are used as liquid phases. The bubbles are generated into a small-scale bubble column having an elastic membrane with a single orifice as gas sparger. To understand the effects of the surfactants on the mass transfer, not only the static surface tension is used, but also the characteristic adsorption parameters like the surface coverage ratio at equilibrium Se. The liquid-side mass transfer coefficient is obtained from the ratio of the volumetric mass transfer coefficient (measured by a chemical method) and the specific interfacial area. These two parameters are obtained simultaneously. The methods used to obtain these parameters are described in Painmanakul et al. [2005. Effects of surfactants on liquid-side mass transfer coefficients. Chemical Engineering Science 60, 6480-6491].Whatever the liquid phase, three zones are found on the liquid-side mass transfer coefficient variation with the bubble diameter. For bubble diameters less than 1.5 mm, whatever the liquid phases, the k_L values are roughly constant at . For bubble diameters greater than 3.5 mm, the k_L values do not vary much with the bubble diameter, but depend on the surfactant concentration. For bubble diameters between 1.5 and 3.5 mm, the k_L values increase from to the value reached at 3.5 mm. This increase depends on the surfactants. Higbie's model does not represent the k_L values for bubble diameters greater than 3.5 mm, even though there is a small amount of surfactant in the liquid phase. Thus, a model is proposed for each zone described above. Explanations are also proposed for the effect of the surfactant on the k_L values for each of the above zones.     

Static mixers with a gas continuous phase

The aim of this work was to characterise hydrodynamics and mass transfer in a gas-liquid contactor containing static mixers (SMs). The originality of this study lies in the fact that these mixing organs are used with a gas continuous phase. Two types of SM were implemented in co-current flows, Statiflo and Lightnin. The pressure drop ΔP, the volumic interfacial area a and the volumic mass transfer coefficient k_La were measured in several configurations: horizontal flow, vertical up-flow and vertical down-flow. The influences of position and flow rates were studied in order to understand the behaviour of these contactors, and to optimise the operating conditions. As expected, the pressure drop was found to increase mainly with gas velocity but also with liquid velocity, and to reach 3300 Pa in the range of velocities studied (the gas flow rate varied between 4 and and the liquid flow rate between 0 and 100 L/h), far less than Sülzer SM. The volumic interfacial area and the volumic mass transfer coefficient showed the same changes, a varying between 100 and , and k_La reaching 0.07 L/s. This is interesting compared with other classical absorption processes: indeed, even if packing towers can provide the same range of values, the operating conditions are more drastic or the dimensions of the apparatuses are far larger than SM ones. The position was also found to have an influence on the hydrodynamic and mass transfer parameters (ΔP, a and k_La).