Gas Film Mass Transfer Coefficient Research Articles

Simultaneous removal of carbon dioxide, sulfur dioxide and nitric oxide in a biofilter system: Optimization operating conditions, removal efficiency and bacterial community

Anthropogenic NOx, SO2 and CO2 emission from the fossil-fuel-fired power plants has aroused growing attention. This study investigated the removal performance of CO2, SO2 and NOx in flue gas as well as conversion efficiency of nitric- and sulfur-compounds in liquid phase in a biofilter. In order to develop the potential of the biofilter, simulative industry wastewater was employed as the spray solution. The satisfactory flue gas removal performance (75.23% CO2, 100% SO2 and 82.81% NO) were achieved under the optimal operating conditions of biofilter: initial solution pH of 9 and liquid-gas ratio (L/G) of 3. The gas film mass transfer coefficients (kGa) results showed that the resistance of gas mass transfer was decreased with increasing the pH value and L/G ratio, respectively. The final transformation product of NO was mostly N2 while about 78% SO2 was converted to elemental sulfur. The microbial community analysis results showed that the relative abundance of bacteria with denitrification capacity was increased by 3.05% which might have contributed to the conversion of NO intermediates products in present study. Collectively, this biofilter system achieve a better flue gas removal performance via the proper operation system, which provides an economic feasible strategy of flue gas purification and increases potential for industrial application.

Study of mass transfer correlations for rotating packed bed columns in the context of solvent-based carbon capture

The application of rotating packed beds (RPBs) in solvent-based carbon capture processes, will greatly reduce the physical footprint, capital and operating cost of the process. However, in designing RPBs, correlations for predicting mass transfer parameters are generally limited in literature and their prediction accuracies have not been demonstrated independently. In this paper, an RPB absorber model was developed in gPROMS ModelBuilder® and used to test and compare different correlations for predicting the effective interfacial area, liquid and gas film mass transfer coefficients. Our results showed that the modified packed column mass transfer correlations where the “g” term (i.e. gravitational acceleration) is replaced with “rw2” (i.e. centrifugal acceleration) commonly used in literature for RPBs generally give poor predictions compared to using correlations developed specifically for RPBs. Also, the Tung and Mah correlation has better predictive accuracy for the liquid film mass transfer coefficient in RPBs than more complex correlations. Finally, a set of new data for the gas film mass transfer coefficient for RPBs were also derived from overall volumetric mass transfer coefficient (KGa) experimental data from literature. This is the first report of gas film mass transfer data for RPBs. The results in this paper will guide researchers in selecting suitable correlations for predicting mass transfer parameters in RPBs.

Determination of gas film mass transfer coefficient in a packed bed reactor for the catalytic combustion of hydrogen

The overall kinetics of catalytic combustion of hydrogen in air is decided by the contribution of the different resistances offered in individual steps such as external diffusion, internal diffusion and surface reaction. To estimate the contribution of external mass transfer resistance, empirical correlations in terms of dimensionless numbers viz., Sherwood number, particle Reynold's number, and Schmidt number are used in general. The Schmidt number for hydrogen in air is approximately 0.2 and most of the mass transfer correlations in packed bed reactors are valid for Schmidt number higher than 0.6, which corresponds to that of water vapour in the air. The external mass transfer correlations applicable for hydrogen-air system are scarce. In the present study, the external mass transfer effects were decoupled from internal diffusion and intrinsic reaction by conducting experiments with different particle Reynolds numbers in a packed bed batch reactor with complete recycle. Further, the system is theoretically modeled and the external film mass transfer coefficients were estimated at different particle Reynold's numbers using experimental data. Based on the experimental data, a new mass transfer correlation has been proposed to determine the gas film mass transfer coefficient of hydrogen in the air in packed bed reactors, in terms of conventional dimensionless numbers i.e., Sherwood number, Schmidt number, and particle Reynolds number. Further, the contribution of external mass transfer resistance on the overall reaction kinetics was also estimated.

Mass Transfer Rate Parameters of Sulzer EX Laboratory Scale Gauze Structured Packing

Sulzer EX packing is widely used in lab-scale separation studies owing to its high efficiency, low theoretical stage height and small pressure drop. In this work the three fundamental characteristic mass transfer parameters of this packing, effective gas-liquid contact area (ae) gas-film and liquid-film mass transfer coefficient (kG) and (kL) are measured in a 0.19 mm ID counter-flow absorption column using aqueous liquids. With end effects removed, the results show that the effective gas liquid contact area is independent of gas and liquid loads and reaches about 90 % of the geometric area of the packing. Correlations of the dimensionless gas and liquid phase mass transfer parameters are determined as ??hG = 0.048 ????G1.0 ????G0.33 and ??h L = 0.050 ????L0.4????L0.5.

Liquid-Gas Mass Transfer of Volatile Substances in an Energy Dissipating Structure.

Mass transfer of a range of volatile substances was studied under highly turbulent conditions. The applied setup mimicked drop structures, where the release of volatile organic carbons likely occurs at a high rate. The experiments covered several substances in a range of resistances from residing entirely in the liquid film to being fully in the gas film. The two-film theory yielded a good prediction of the whole measured range. This allowed the experimental validation of a method where two reference substances are applied, to determine the transfer of any other substance - independently of where its resistance to mass transfer resides. One finding was that the range of dimensionless Henry's constants, where both films contributed by more than 5%, was 0.0027 to 1.05, which is over five times higher than the accepted rule of thumb (0.0005-0.18). Another finding was that the ratio between the liquid and the gas film mass transfer coefficients of the reference substances was similar for the two drop configurations studied. If this holds true over a wider range of configurations, such a ratio constitutes a valuable shortcut to the current practice of ignoring gas film resistance in the estimation of mass transfer rates.

Mass transfer inside a flux hood for the sampling of gaseous emissions from liquid surfaces – Experimental assessment and emission rate rescaling

This study assesses the mass transfer of compounds inside the US EPA flux hood, one of the enclosure devices most commonly employed for the direct measurement of atmospheric emissions from liquid surfaces in wastewater treatment plants (WWTPs). Experiments comprised the evaporation of water and the volatilisation of a range of volatile organic compounds (VOCs). Special attention was given to the evaluation of the mass transfer coefficients in the microenvironment created by the flux hood and the effects of concentration build up in the hood's headspace. The VOCs emission rates and the water evaporation rates generally increased with the sweep air flow rate, as did the mass transfer coefficients for all compounds. The emission of compounds whose volatilisation is significantly influenced by the gas phase was greatly affected by concentration build up, whereas this effect was not significant for liquid phase-controlled compounds. The gas-film mass transfer coefficient (kG) estimated inside the US EPA flux hood was of the same order as the respective kG reported in the literature for wind tunnel-type devices, but the emission rates measured by the flux hood can be expected to be lower, due to the concentration build-up. Compared against an emission model for the passive surfaces in WWTPs, the mass transfer of acetic acid (representing a gas phase-dominated compound) inside the US EPA flux hood was equivalent to conditions of wind speeds at 10 m height (U10) of 0.27, 0.51 and 0.99 m s−1, respectively, for sweep air flow rates of 2, 5 and 10 L min−1. On the other hand, for higher wind speeds, the emission rates of gas phase-controlled compounds obtained with the flux hood can be considerably underestimated: for instance, at U10 = 5 m s−1, the emission rates of acetic acid inside the flux hood would be approximately 23, 12 and 6 times lower than the emission rates in the field, for sweep air flow rates of 2, 5 and 10 L min−1, respectively. A procedure is presented in order to scale the emission rates of these compounds measured with the flux hood to field conditions of higher winds.

Mass Transfer Parameters for Packings: Effect of Viscosity

The effect of liquid viscosity (μL) on the effective mass transfer area (ae) and liquid film mass transfer coefficient (kL) of packing must be known to determine packing height. Most existing correlations are based on water and are not applicable to viscous systems. In this work, ae, kL, and gas film mass transfer coefficient (kG) were measured in a 0.43 m ID packed column with 0.5–3 m of packing. Liquid viscosity was varied from 0.8 to 70 mPa·s by adding glycerol to water. Liquid viscosity has an insignificant effect on ae over the range of μL investigated. The total dependence of kL on μL is −0.75, of which −0.35 is indirect influence through diffusivity, and −0.4 is direct influence through liquid turbulence. Universal mass transfer models for packing were developed as functions of packing geometry, operating condition, and fluid property.

Influence of the fetch parameter on results from empirical correlations for estimating odorous emissions at passive liquid surfaces.

Passive liquid surfaces in wastewater treatment plants may be potential sources of odorous emissions. This study investigates the occurrence and significance of deviations that may originate from the use of the effective diameter as fetch parameter in the empirical correlations utilised by the WATER9 model to estimate odorous emissions at passive liquid surfaces. A sensitivity analysis was performed using benzene as a model compound and considering representative conditions of wind speed and wind alignment. The gas-film mass transfer coefficient (kG) was found relatively in sensitive to the choice of the fetch parameter, deviating less than 15% for aspect rations up to 15. The calculation of the liquid-film mass transfer coefficient (kL) was much more sensitive (positive extreme of 126.98% and negative extreme of -54.80%), partially because of the use of different equations for different fetch-to-depth ratios. For more volatile compounds, such as benzene, these discrepancies will be significantly manifested in the estimated emission rate. When appropriate, the use of the actual fetch instead of the effective diameter is recommended.

Dimensionless Models for Predicting the Effective Area, Liquid-Film, and Gas-Film Mass-Transfer Coefficients of Packing

A total of 11 structured packings with surface areas (ap) from 125 to 500 m2/m3 and corrugation angles (θ) from 45 to 70° were studied to explore the effects of the packing geometry on mass transfer. A new concept, the mixing point density, was proposed to represent the effects of ap and θ. A generalized method to calculate the mixing point density from aP and θ was developed. Dimensionless mass-transfer models are developed to predict the effective area, liquid-film, and gas-film mass-transfer coefficients, with the mixing and Reynolds numbers imbedded into the correlations. Compared with literature models, the models developed in this work capture the effects of the packing geometry and predict the mass-transfer properties in aqueous systems.

Packing Characterization for Post Combustion CO2 Capture: Mass Transfer Model Development

Three mass transfer properties: the effective area (ae), liquid film and gas film mass transfer coefficients (kL and kG) are measured consistently for eleven packings with different surface area and corrugation angle. The effective area and liquid film mass transfer coefficient increases with liquid velocity while gas film mass transfer coefficient increases with gas velocity. The fractional effective area decreases with surface area and barely changes with corrugation angle. kL and kG increase as surface area increases or corrugation angle decreases. A new concept-mixing point density is proposed to represent the packing geometry influence on kL and kG. Correlations for ae, kL, and kG are developed including effects of gas and liquid rates and packing geometry.

Numerical Analysis for CO<sub>2</sub> Absorption and Regeneration Behavior in Porous Solid Sorbent by Modified Unreacted-Core Model

Lithium ortho-silicate (Li4SiO4) is a suitable solid sorbent for capturing CO2 from solid oxide fuel cells. CO2 absorption reactors packed with porous-solid spherical pellets of Li4SiO4 show unsteady temperature distribution and capture ratio behavior owing to the unsteady CO2 absorption rate and highly exothermic process. The CO2 absorption rate of this sorbent reportedly depends on temperature, CO2 concentration, and CO2 accumulation, expressed as the weight change of the sorbent. Nevertheless, discussions of detailed mechanisms of CO2 absorption by this sorbent are rare. In this study, the modified unreacted core model is proposed to explain the mechanism of CO2 absorption of a porous-solid spherical pellet, and numerical analysis was conducted to simulate the unsteady behavior of the sorbent. Important properties such as the reaction rate constant, the gas film mass transfer coefficient, and the coefficient for effective diffusion through the product layers were empirically derived using thermogravimetry and a diluted packed-bed reactor. Numerical analysis by applying these parameters to the modified unreacted core model adequately explained the complicated CO2 absorption and regeneration behaviors.

Effect of CO2 stripping on pH in open‐recirculating cooling water systems

A simplified mathematical model has been developed to evaluate the effect of CO2 stripping on pH of recirculating cooling water in power plant cooling systems. Experimental data from three pilot‐scale cooling towers using treated municipal wastewater as makeup water were used to determine the CO2 mass transfer coefficients in the gas phase kg and aqueous phase kw. The optimum values of the gas film mass transfer coefficient and water film mass transfer coefficient were determined by regression to be kg = 8.4 × 10−6 m/s and kw=8.4 × 10−8m/s, respectively. The results also indicate that the model is capable of predicting the pH of the cooling loop as a function of makeup water alkalinity and cooling loop alkalinity (in case of salt formation) as well as the operating parameters of the cooling system. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 275–282, 2014

Determination of Mass Transfer Coefficients for A Mixture of Compost, Sugarcane Bagasse, and Granulated Activated Carbon as Packing Medium in Biofilter

ABSTRACT A laboratory-scale biofilter unit packed with a mixture of compost, sugarcane bagasse, and granulated activated carbon (GAC) in the ratio of 55:30:15 by weight was used for a biofiltration study of air stream containing benzene, toluene, ethylbenzene, and o-xylene (BTEX). The effect of superficial velocity on mass transfer coefficient for the packing was studied by maintaining gas flow rates of 3, 4, 5, 6, and 8 L min−1 for inlet concentrations of 0.1, 0.4, and 0.8 g m−3 for each of benzene, toluene, ethylbenzene, and o-xylene. The maximum elimination capacity was found to be 20.92, 22.72, 20.73, and 18.94 g m−3 h−1 for BTEX, respectively, for stated flow rates. Removal efficiency of BTEX decreased from 99% to 71% for increasing inlet concentration from 0.1 to 0.8 g m−3. Gas film mass transfer coefficient predicted by modified Onda's equation was within ±10% of the experimental values.

Characterization of Novel Structured Packings for CO2 Capture

Structured packing is widely being considered for post-combustion CO2 capture because of its high mass transfer per unit of pressure drop. The hydraulic and mass transfer performances of structured packings with different corrugation angles were measured. The effective mass transfer area was measured by absorption of CO2 with 0.1 gmol/L NaOH. The liquid film mass transfer coefficient was measured by desorption of toluene into air. The gas film mass transfer coefficient was measured by absorption of SO2 from air into 0.1 gmol/L NaOH. The effective area and the liquid film mass transfer coefficient are dependent on liquid velocity while the gas film mass transfer coefficient is a function of gas velocity. Increasing the corrugation angle from 45° to 60° (Mellapak 250Y to Mellapak 250X) resulted in (1) a 54% decrease in pressure drop, (2) a 6% decrease in effective area, (3) a 17% decrease in kG, and (4) a 18% decrease in kL. Increasing the corrugation angle from 45° to 70°, a larger difference was found in pressure drop, kG and kL. However, the impact of corrugation angle on effective area is not significant. In conclusion, increasing the corrugation angle will have a benefit on pressure drop with a negative impact on mass transfer properties. Future work will be focused on economic analysis between operation costs (related to pressure drop) and capital costs (related to mass transfer properties) to determine the optimum corrugation angle for a particular separation task.

Packing Characterization: Mass Transfer Properties

Packing is widely used in post-combustion CO2 capture. This paper is focused on the measurement of three fundamental packing characteristics: effective gas-liquid contact area (ae), gas phase and liquid phase film mass transfer coefficient (kG) and (kL). Results show that contact area initially increases with liquid flow rate before it asymptotes to a value nearly equivalent to the nominal surface area. Initial attempts at constructing new mechanistic models have shown that existing kG and kL models can be improved. Both gas and liquid phase film mass transfer coefficients can be described as a power function of superficial gas or liquid velocity.

Determination of mass transfer coefficients for packing materials used in biofilters and biotrickling filters for air pollution control—2: Development of mass transfer coefficients correlations

Correlations that allow determination of gas film mass transfer coefficients ( k G a t , k G a w ) and liquid film mass transfer coefficients ( k L a w ) for packing materials used in biofilters and biotrickling filters for air pollution control are presented. Lava rock, polyurethane foam cubes (PUF), Pall rings, porous ceramic beads, porous ceramic Raschig rings, and various compost–woodchips mixtures were used as packing materials. The functionality of gas and liquid velocity on mass transfer coefficients ( k G a t , k G a w , k L a w ) obtained experimentally (see Part 1 of this paper) was correlated using modified Onda-type equations. The correlation equations helped to better understand the sensitivity of gas and liquid velocities on mass transfer, and the effects of packing wetting. Each packing had a different functionality with gas and liquid velocity and different wetting property, hence different correlation equations were needed for the different packing materials. Most of the fitted data fell within ± 20 % of the experimental values.

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 G a t , k G a w ) and liquid film mass transfer coefficients ( k L a 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 8000 m h - 1 and liquid velocities of 0.1 – 12 m h - 1 , i.e., a wide range that covers most biofilters and biotrickling filters. k G a t in biofilter packings ranged from about 500 to 2500 h - 1 , while k G a w and k L a w in biotrickling filters ranged from 100 to 8000 h - 1 , and 1 to 300 h - 1 , 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 G a 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 G a 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 L a 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.

Desulfurization in the gas-continuous impinging stream gas–liquid reactor

An investigation is made to evaluate the flue gas desulfurization (FGD) by absorption in a gas-continuous impinging stream gas–liquid reactor recently developed for systems involving fast reaction(s) in liquid. The mixture of air and SO 2 was used as the pseudo-flue gas and Ca(OH) 2-water suspension as the absorbent. By employing horizontal two-impinging streams, the reactor is simple in structure with few internal parts, while exhibits satisfied overall performance for FGD. Under moderate conditions, the content of SO 2 in the cleaned gas can achieve a level much lower than that permitted, while the pressure drop across the device is about 400 Pa only. The influences of some operating and structural parameters, such as V L / V G , Ca/S mole ratio, SO 2 concentration in flue gas, impinging distance S, and nozzle location, etc., are examined. The gas-film mass transfer coefficient, k G , is determined based on Sauter mean diameter of spray droplets. The results show that k G is essentially independent of concentration of SO 2 in flue gas, implying the process can be considered to be controlled by diffusion through gas film. The relationship between k G and impinging velocity, u 0 , is fitted to be k G = 2.9 × 10 - 4 u 0 1.75821 with the standard deviation of SD = 2.45 × 10 - 4 m s - 1 , suggesting u 0 is a strong effecting variable on mass transfer, and, consequentially, important operating variable. In the range of u 0 from 5.53 to 16.62 m s - 1 , the values determined for the volumetric mass transfer coefficient, k G a , are 0.577– 1.037 s - 1 , and those for k G are ranged from 0.00641 to 0.0416 m s - 1 .

Oxidation of zinc sulfide in a fluidized bed

Kinetics of oxidation of ZnS particles in a batch-type fluidized bed were studied at temperatures between 800 and 910°C. A two-phase model was employed for the fluidized bed, and the partial pressure of oxygen and the gas-film mass transfer coefficient on the particle surface were separately evaluated in gas bubbles and in the emulsion phase. The calculated fractional reaction coincided well with the experimental results. The difference in O2 partial pressure between gas bubbles and emulsion phase was found to be fairly large especially under the vigorous fluidizing condition. Furthermore, it was shown from the mathematical model that the reaction of ZnS particles in the gas bubbles is negligible because of the extremely low solid concentration and that the overall rate of reaction in the emulsion phase is virtually controlled by the rate of gas-film mass transfer at higher temperature. The resistance of interfacial reaction within the particle also becomes significant when the temperature is lowered.

An investigation of mass transfer in a countercurrent three-phase fluidized bed

The rate of absorption with chemical reaction has been measured in a bed of solid spheres fluidized by upward flowing gas and irrigated by downward flowing liquid for different reactant concentrations in the liquid phase and different values of the liquid superficial velocity. Values of the effective inerfacial area per unit volume of a static bed, a, and the gas-film mass-transfer coefficient, k g , are reported as functions of the liquid superficial velocity.