Physical Gas Absorption Research Articles

The enhancement of the physical absorption of gases in aqueous activated carbon slurries

The enhancement of the physical absorption of gases in aqueous activated carbon slurries

Gas-liquid mass transfer in a slurry reactor operating under olefinic polymerization process conditions

The equilibrium solubilities, C ∗ , and the volumetric liquid-side mass transfer coefficients, k L a, of propylene, ethylene and hydrogen in liquid n-hexane containing up to 30 wt% solid polypropylene powder were obtained in a 4−1 agitated batch reactor operated in surface-aeration mode. The data were collected under pressures between 2 and 55 bar, temperatures from 313 to 353 K, mixing speeds from 13.3 to 20.0 Hz and solid concentrations between 0 and 30 wt%. The gas solubilities were calculated using a modified Peng-Robinson equation of state (PR-EOS) and the mass transfer coefficients were determined using the transient physical gas absorption technique. The solubilities of hydrogen, ethylene and propylene in n-hexane were found to obey Henry's law and the values were not affected by the presence of solids. The gas with the closest solubility parameter to that of liquid n-hexane appeared to have the highest solubility. As expected, the mass transfer coefficients of the three gases in liquid n-hexane with and without solids increased with increasing mixing speed. The k L a values of hydrogen in n-hexane and slurries were found to slightly increase whereas those of ethylene and propylene slightly decrease with increasing the mean partial pressure of the gas component. The temperature appeared to have no effect on k L a values of hydrogen in n-hexane with and without solids while those of ethylene and propylene were slightly increased with temperature. The k L a values for the three gases increased at low solid concentration (10 wt%) and decreased at high solid concentration (30 wt%). A dramatic decrease of k L a values for ethylene and propylene in liquid n-hexane was observed at particular operating conditions ( T = 353 K, N = 13.3 Hz, W s = 30 wt% and P 1, m ≥5 bar). This behavior was attributed to the high slurry viscosity prevailed under these particular conditions. The k L a values of the three gases used in n-hexane were correlated with operating variables using empirical correlations.

The enhancement of the physical absorption of gases in aqueous activated carbon slurries

The enhancement of the gas-liquid mass transfer rates in aqueous slurries containing small activated carbon particles was studied in a semi-batchwise operated stirred cell absorber with a plane interface. The maximum observed enhancement factors for absorption of propane, ethene and hydrogen in the aqueous slurries were 3.6, 3.3 and 2.0, respectively. It was shown that for our results and those reported in the literature the maximum enhancement factor, E max, decreases with increasing liquid side gas-liquid mass transfer coefficient, k l . From all the experimental data the following relationship is found: (E max−1) = 0.0019 ( k t [m s −1])−0.71 although different types of activated carbon particles and differences in sizes were used by the various research groups. To describe these results a simple theory is presented.

A new technique for the determination of mass transfer coefficients in packed columns for physical gas absorption systems

A mass transfer model has been developed to describe the physical absorption of gases in packed columns. The model is based on recycle systems, and yields a new technique for the determination of mass transfer coeffifients. Experimental data were collected, using a carbon dioxide-water system in a column packed with Raschig rings. It was found that the liquid film mass transfer coefficients calculated with the present technique agree well with the literature.

Physical absorption of CO2 and propene into toluene/water emulsions

AbstractThe physical absorption of CO2 and propene into toluene/water emulsions is studied in a stirred cell and laminar film absorber. Experimentally observed masstransfer rates are compared to an absorption model, based on Higbie's penetration theory describing physical gas absorption into an emulsion. For all absorption experiments in a stirred cell absorber (toluene fractions and stirring rates), experimentally observed mass‐transfer rates are considerably higher than the rates predicted by the absorption model. Moreover, the absorption rate decreases with increasing stirring rate, whereas no influence of the stirring rate is predicted by the absorption model. In contradiction to the absorption experiments in a stirred cell absorber, the observed mass‐transfer rates in the laminar film absorber agree with the model simulations. Probable existence of a very thin toluene layer is observed on top of the emulsion for the stirred cell experiments, likely due to minor phase separation. Since in the laminar film absorber gas‐liquid interface and the gravity force are parallel, this phenomenon does not probably occur significantly for absorption experiments in this absorber. The observed mass‐transfer phenomena can be explained, at least qualitatively, from the occurrence of a thin toluene layer.

Mass transfer characteristics of gases in methanol and ethanol under elevated pressures and temperatures

Abstract The effect of pressure, temperature and mixing speed on the solubilities C*) and mass transfer coefficients KLa for different gases (H2, N2, CO, CH4 and CO2) in two liquids (methanol and ethanol) were investigated in a 4-liter gas-inducing agitated autoclave. The C* and kLa values were obtained using a modified Peng-Robinson equation of state and the transient physical gas absorption technique respectively. The C* values for all gases studied in methanol and ethanol were found to increase with the solute gas partial pressure P1,F. In the range of operating conditions used, the solubilities of the gases in both liquids behaved as (C*)CO2 > (C*)CH4 > (C*)CO > (C*)N2 > (C*)H2. At 328 K and 378 K the C* values for H2, N2, CO and CH4 in ethanol were slightly higher and those for CO2 were lower than those in methanol. At 428 K, however, the C* values for H2, N2, CO and CH4 were slightly higher and those for CO2 were lower than those in ethanol. kLa values for all gases in methanol and ethanol appeared to be a strong function of mixing speed and increased slightly with the mean partial pressure of the gas component P1,m. kLa values for H2 in methanol and ethanol were the highest, those for CO2 were the lowest, and kLa values for N2 CO and CH4 in both liquids were not significantly different. An empirical correlation was proposed to predict kLa values fo H2, N2, CO, CH4 and CO2 in methanol and ethanol with a reasonable accuracy.

Solubilities and mass transfer coefficients of gases in liquid propylene in a surface-aeration agitated reactor

The solubility C*, and volumetric mass transfer coefficient, k La , values for hydrogen and ethylene in liquid propylene were obtained in a 41 surface-aeration agitated reactor operating under pressures between 11 and 55 bar, temperatures from 297 to 333 K, and mixing speeds of 13.3–20.0 Hz. The pressure—time profile of the hydrogen—propylene system exhibited an anomalous behavior due to the vaporization of liquid propylene into the gas phase. The equilibrium gas solubilities were calculated using a modified Peng—Robinson equation of state (PREOS) and the mass transfer coefficients were determined using the transient physical gas absorption technique. A calculation procedure for determining the equilibrium composition and mass transfer coefficients for hydrogen in liquid propylene which accounted for the anomalous behavior of this system was developed. The equilibrium vapor—liquid mole fractions obtained using this procedure compared favorably with available literature values. The C* values were found to increase with the partial pressure of the solute gas. The k La values increased with mixing speed for both gases. The solubilities of ethylene in liquid propylene were found to be higher than those of hydrogen, whereas the mass transfer coefficients for hydrogen were appreciably higher than those of ethylene. An empirical correlation which predicted k La values for hydrogen and ehtylene gases in liquid propylene in a surface-aeration reactor with an accuracy of ± 30% was developed.

The enhancement of the gas-absorption rate in agitated slurry reactors due to the adhesion of gas-adsorbing particles to gas bubbles

The effect of small gas-adsorbing particles on the quasi steady-state absorption rate of a gas into a degassed slurry is investigated in an agitated slurry reactor in which no chemical reaction occurs. This investigation shows that, in a slurry reactor, gas-adsorbing particles can adhere to gas bubbles so that during steady-state operation a fraction ζ of the gas-liquid interface is covered by adhering particles, resulting in an enhancement of the gas-absorption rate with respect to the particle-free situation. A model is derived to calculate the enhancement of the physical gas-absorption rate as a function of the fraction of bubble-surface coverage ζ at different particle concentrations in the slurry. This Enhanced Gas-Absorption model is experimentally verified by flotation experiments and by hydrogen absorption into aqueous solutions containing different concentrations of small catalyst particles.

Mass transfer in a three-phase reactor operating at elevated pressures and temperatures

The solubilities ( C*), mass transfer coefficients ( k La ), and volumetric mixing power input ( P*/ V L ) were measured for methane in n-hexane/coal slurries at various pressures (1–50 bar), temperatures (328–378 K), mixing speeds (16.7–20.0 Hz) and solid concentrations (0–15 wt%) in 41 gas-inducing mechanically agitated autoclave. Upper Freeport and Illinois # 6 coals at 200 mesh × 0 and 28 mesh × 0 were used. The C* values were calculated using the Peng—Robinson equation of state. The k La data were determined using the transient physical gas absorption technique and P*/ V L values were measured using a torquemeter. The calculated C* values for methane in n-hexane were found to increase with pressure and decrease with temperature. The k La values increased with mixing speed and pressure but an unclear dependency on the temperature was noticed. The k La values decreased with solid particle loading in n-hexane. No significant difference between k La values, however, was obtained for both coals at 28 mesh × 0 and 200 mesh × 0 particle sizes. An empirical correlation was proposed to predict k La values for methane in n-hexane/coal slurries.

Mass transfer characteristics of gases in n-hexane at elevated pressures and temperatures in agitated reactors

The solubilities C*, mass transfer coefficients k L a, and volumetric mixing power input P*/ V L were measured for H 2, N 2 and CH 4 in n-hexane at various pressures (1–50 bar), temperatures (328–378 K), and mixing speeds (800–1200 rpm) in a four-liter gas-inducing type of agitated autoclave. The C* values were calculated using the Peng-Robinson equation of state. The k L a data were determined using the transient physical gas absorption technique and a rigorous calculation method considering the change of the liquid-phase volume was introduced. The P* values were obtained using a torquemeter. All operating variables, including the decline of system pressure as a function of time, were recorded using an on-line computer and data acquisition system. Analysis of the data showed that C* for the three gases increased with gas partial pressure. C* values for H 2 and N 2 increased whereas those for CH 4 decreased with temperature. In the ranges of operating conditions used, the C* values followed the order ▪ The experimental C* values were modeled using quadratic relationships. k L a values for all the gas—liquid systems increased with mixing speed. k L a values for H 2 decreased whereas those for N 2 and CH 4 increased with pressure. No particular trend for the effect of temperature on k L a was observed. In the ranges of operating conditions used, k L a values followed the order ▪ k L a values were found to increase with P*/ V L at constant pressure and temperature. An empirical correlation to predict k L a for H 2, N 2, and CH 4 in n-hexane as a function of dimensionless numbers was proposed.

Mass transfer characteristics of gases in aqueous and organic liquids at elevated pressures and temperatures in agitated reactors

The solubilities ( C * ), mass transfer coefficients ( k La ) and volumetric mixing power input ( P * / V L ) were measured for N 2 and CH 4 in n-hexane and in water pressures (1–50 bar), temperatures (328-378 K) and mixing speeds (13.3-20.0 Hz) in a four-liter agitated autoclave. The C * -values were calculated using the Peng-Robinson equation of state. The k La data were determined using the transient physical gas-absorption technique and a calculation method considering the change of the liquid-phase volume was introduced. The P * / V L values were obtained using a torque meter. All operating variables including the decline of systems pressure as a function of time were recorded using an on-line computer and data acquisition system. Analysis of the data showed that C * for the two gases increased with gas partial pressure in both n-hexane and water. C * -values for N 2 increased with temperature in both liquids, while C * -values for CH 4 decreased with temperature. In the ranges of operating conditions used, the C * -values followed the order: ( C * CH4 > ( C * N2 in both n-hexane and water. Also, for both gases, the solubilities in n-hexane were much higher than those in water. The experimental C * -values for both gases in the two liquids were modeled using quadratic relationships. k La values for all gas/liquid systems studied increased with mixing speed. k La values for N 2 and CH 4 increased with system mean pressure in n-hexane but were independent of system mean pressure in water. No particular trend for the effect of temperature on k La values for N 2 and CH 4 in both liquids was observed. In the ranges of operating conditions used, k La values followed the order: ( k La CH4 < ( k La ) N2 in both n-hexane and water. k La values increased with P * / V L at constant pressure and temperature. An empirical correlation to predict k La for CH 4 and N 2 in water and n-hexane as a function of dimensionless numbers was proposed.

The influence of adsorption capacity on enhanced gas absorption in activated carbon slurries

The enhanced absorption of gases in aqueous activated carbbon slurries of fine particles is studied with a non-steady-state absorption model, taking into account the finite adsorption capacity of the carbon particles. It has been found that, for the different gas/activated carbon slurry systems studied in the literature, a remarkable similarity exists in the maximum achievable enhancement factor and the minimum solids concentration needed to reach the maximum enhancement. The enhance physical absorption of gases via absorption on the carbon particles, extensively investigated by Alper, is simulated with a penetration-type model. For the O 2 and CO 2 absorption it is demonstrated that, at the experimental bulk solids concentrations, no enhanced absorption could occur, because the adsorption capacities of these gases on activated carbon is too small. At these low solids concentrations, the enhancement is limited due to a rapid build-up of a layer of saturated particles close to the interface. High interfacial particle concentrations are necessary for any enhancement. It is therefore proposed, also on the basis of the observations in the literature, that the interfacial carbon concentration is much higher than the bulk concentration of the activated carbon particles. It is demonstrated that the non-steady-state absorption model can also be used for modelling enhanced gas absorption (adsorption followed by surface reaction). The absorption of O 2 in aqueous Na 2S activated carbon slurries is taken as an example.

Gas absorption with wetted-wick column

The newly designed wetted-wick column was constructed by using fiber glass and wire mesh and studied to investigate its feasibility as an alternative to the conventional packed tower in gas purification processes. Absorption of carbon dioxide in water for physical absorption and in aqueous mono-ethanolamine (MEA) solution for chemical absorption were employed for this investigation. A comparison with the packed tower, based on overall mass transfer coefficients (KOGa) and height of transfer units (HTUOG) at the same range of operating conditions, feed gas compositions, and gas retention time was made. High mass transfer coefficients (KOGa) and low heights of transfer unit (HTUOG) were obtained for the wetted-wick column. The high efficiency of mass transfer was caused by a number of factors: the separate passage of liquid and gas flow, an increased wetting surface, a longer period of residence time of the liquid and gas stream, and an increased interfacial area. The present study shows that the wetted-wick column is a highly recommendable device as an alternative to the packed tower in gas purification processes, either for physical gas absorption or for chemical gas absorption.

Influence of hydrodynamics on physical and chemical gas absorption in packed columns

Presentation d'un modele de simulation pour traiter de l'influence de l'hydrodynamique sur le transfert de masse dans les deux cas suivants: absorption pour le systeme CO 2 -eau et absorption avec reaction pour le systeme CO 2 -H 2 S-MDEA

An experimental dynamic method of investigation of counter-current absorption of oxygen in water in a packed bed column

An experimental method and technique are described in the paper of simultaneous detection of the transfer functions outlet-gas-stream-to-inlet-gas-stream and outlet-liquid-stream-to-inlet-gas-stream for the absorption of oxygen into water in a counter-current packed bed column. Both transfer functions were simultaneously monitored by means of three oxygen electrodes operating on the polarographic principle. The signals of these electrodes were processed in three steps to yield parameters of the model of physical absorption of gas. The first step was on-line evaluation of the Fourier coefficients of the principal harmonic component in all three monitored streams. The second step was the calculation of the frequency characteristics of both transfer functions while the third step yielded parameters of the model by optimization in the frequency domain. The method permits simultaneous evaluation of the parameters of the flow of both phases in the column and the interfacial transfer of oxygen.

Competitive physical absorption of gases into water

An experimental set-up is presented allowing the volume change due to mass transfer of a gas in contact with a finite amount of liquid to be observed. By means of an expansion tube the pressure in the gas phase was kept constant. At the beginning of each run the gas phase was a pure gas while the liquid (water) had been either totally degassed or saturated with another gas.The experiments with water, which had been degassed initially, allow the mass transfer coefficients necessary to predict the absorption rate in previously saturated water to be determined. In the latter case desorption also occurs and, depending on the ratio of the diffusion coefficients of both gases in water, the degree of saturation of the water changes during the diffusion process, e.g. the water may get oversaturated. (The degree of saturation of water is expressed by the sum of the partial pressures of the water vapour and of the gases dissolved in the water referred to the total pressure in the gas phase. The water is considered to be oversaturated, saturated, or partially saturated depending on whether this ratio is larger, equal or smaller than unity, respectively.) Furthermore, the diffusion coefficients can also influence the final volume of the gas phase.These different effects are demonstrated using the gases O2, N2, He and Ne.

Design of packed, adiabatic absorbers: physical absorption of acid gases in methanol

An algorithm was developed to calculate concentration and temperature profiles for an adiabatic, packed absorber. Calculations performed with this algorithm compared favorably with data obtained on a pilot scale absorber having a diameter of 0.127 m (5 in.) and packed with 6.25-mm (0.25 in.) ceramic Intalox saddles. The data were obtained for absorption of acid gases from a stream produced from cylinder gases and by coal gasification; refrigerated methyl alcohol was the solvent used in the absorber. The systems examined considered both single-component transfer and multicomponent absorption. The data obtained in this study are thought to be the only data available in the literature for multicomponent, adiabatic packed column absorption for which reasonable mass and energy balance closures were obtained. Several correlations for mass transfer coefficient-interfacial area estimation were tested against experimental data.

Temperature-dependent physical properties in physical gas absorption

A solution for interface temperature rise in physical gas absorption due to heat of solution was obtained by Danckwerts [1,2] with the limitation that the pertinent physical properties—diffusion coefficient, solubility of the gas and thermal conductivity of the liquid remained constant. This paper complements Danckwerts' contribution by considering the pertinent physical properties as temperature-dependent, the dependence being of the form: Q Qi = exp [γ Q ( T − T i )]. Wagner [3] has obtained solutions for unsteady-state diffusion when the diffusion coefficient is similarly dependent on concentration. Wagner's solutions are adapted to provide a solution for the present problem. Two approximate solutions, one valid at high Lewis Number and the other when Lewis Number approaches unity are also presented. For the normal values of activation energies, Danckwerts' model is shown to be accurate when interfacial temperature rise is less than one percent of the absolute temperature of bulk liquid. The extent of departure from the Danckwerts' model is shown to depend on combinations of activation energies of the pertinent physical properties.

Gas absorption mechanism in catalytic slurry reactors

Several aspects of the gas absorption mechanism in catalytic slurry reactors are considered both theoretically and experimentally. The results of Lee and Tsao (1972) - i.e. oxygen absorption into glucose solutions containing platinium deposited on powdered activated carbon - are analysed critically. The experiments which were carried out in stirred cells with unbroken - hence known - interfaces with two different systems (i.e. oxygen absorption into glucose solutions containing Pt/C and carbondioxide absorption into carbonate buffer solutions in the presence of immobilized carbonic anhydrase) showed that the absorption rate can be enhanced because of the catalytic reaction. During the course of this study, it has also been found that finely powdered activated carbon increases, i.e. facilitates the physical absorption of gases significantly.

Review of Physical Absorption of Gases

Review of Physical Absorption of Gases