The present work shows a planetary vision of gas soaking up in aqueous solutions with Tween80. The survey includes a word picture of the soaking up procedure based on hydrokineticss and a different one based on mass transportation. The consequence caused by surfactant concentration upon gas hold-up and gas-liquid interfacial country has been analysed. On the other manus, the analysis of the influence of the liquid-phase taint on the soaking up procedure has been carried out on the footing of the liquid-film mass-transfer coefficient ( kL ) , taking the consequence caused by the presence of wetting agent and the gas flow rate on the interfacial country ( a ) and, thereby, on the volumetric mass-transfer coefficient ( kLA·a ) .
Keywords: soaking up, wetting agent, interfacial country, mass transportation coefficient.
Several industrial procedures are based on the usage of gas-liquid equipment and for this ground, a suited gas-liquid mass transportation rate is of import to different chemical and biochemical procedures. The gas stage is placed in the contactor by agencies of little bubbles in order to provide a big interfacial country and an efficient mass transportation between the gas and liquid stages. The liquid stages normally found in the chemical industry are really complex, due to the presence of several compounds and to the operating conditions. Consequently, an of import research surveies have been performed to understand the influence of the liquid stage belongingss on the bubble formation phenomenon. Nevertheless, whilst the influence of the liquid denseness and viscousness has been widely studied, the liquid surface tenseness and its effects are mostly an unknown factor. Taken into history the consequences and decisions refering the consequence of the liquid surface tenseness, its influence is non truly separated from the consequence caused by denseness and viscousness [ 1, 2 ] .
In the last few old ages, several surveies have increased the cognition about the soaking up processes in systems that involve the presence of surface active substances. It is by and large recognized that little sums of surface-active additives or contaminations can markedly impact the mass transportation parametric quantities kLA·a and kL [ 3, 4 ] . These surveies have deepened on the influence of different operation variables upon the planetary mass transportation and, more specifically, upon the gas hold-up [ 5 ] , the bubbles diameter and gas-liquid interfacial country [ 6 ] , the interfacial turbulency [ 7 ] and the mass transportation coefficient [ 8 ] . Other research workers have shown some involvement in analyzing the influence of the wetting agents nature, taking into history the molecules size [ 9 ] every bit good as their ionic character [ 8 ] .
Presents, the presence of surface agents in gas-liquid systems has reached great importance, chiefly due to the presence of this sort of substances in bioreactors: ( I ) as a merchandise of the bioreaction ( production of biosurfactants ) [ 10 ] , and besides ( two ) as a stabilizer substance in the two-phase divider bioreactors ( TPPB ) to keep the emulsion formed in the liquid stage [ 11 ] .
The purpose of the present work is increasing the cognition of the soaking up procedure behavior in complex systems, being involved the usage of a surfactant commonly used in legion procedures. The system employed in this survey has been analysed taking into history hydrodynamic parametric quantities ( such as gas hold-up, bubble diameter and interfacial country ) and mass transportation based on the analysis of mass transportation coefficient.
2. Material and methods
Tween80 was supplied by Sigma-Aldrich ( CAS figure: 9005-65-6 ) . Commercial class C dioxide of 99.998 % pureness, supplied by Carburos Metalicos ( Spain ) , was besides used in this work. The solutions were prepared by mass utilizing a balance with a preciseness of A±10-7 kilogram, and bi-distilled H2O has been employed to fix the absorbent stages.
The surveies of C dioxide mass transportation to liquid stages were carried out utilizing a bubble column contactor similar to other employed in former surveies related to the soaking up processes [ 12 ] . The gas/liquid contactor used in these surveies has been a square bubble column ( side length = 6 centimeter ; height = 114 centimeter ) , made in methacrylate and working with a 3-litre liquid volume. The gas sparger has been a glass capillary with merely one opening to bring forth a little figure of bubbles, which allowed us a careful analysis of the operation conditions influence on the bubbles size.
The gas to be absorbed, pure C dioxide, was passed through two “ humidifiers ” at 25°C to fix the gas stage. This process removed the opposition to the mass transportation in the gas stage, and it merely allowed us the rating of the liquid stage opposition to the gas transportation. The gas flow-rate was measured and controlled with two mass flow accountants ( 5850 Brooks Instruments ) . The mass flow accountants employed in the present survey for the gas flow-rate and the force per unit areas were calibrated by the provider.
The force per unit area bead was measured between the column ‘s recess and mercantile establishment, utilizing a Testo 512 digital manometer. The operational government was uninterrupted in relation to the gas stage and batch as respects the absorbent liquid.
In this work we have analysed the consequence caused by the typical operational variables used in the present contact device ( gas flow-rate and liquid stage composing ) upon hydrodynamic parametric quantities ( gas hold-up and interfacial country ) , every bit good as the soaking up dynamicss of the C dioxide mass-transfer procedure to liquid stages. The liquid stages chosen for the present paper have been aqueous solutions of Tween80, utilizing different surfactant concentrations. Three gas flow-rates were employed ( 18, 30 and 40 LA·h-1 ) to analyze the influence of this variable upon the gas/liquid soaking up procedure.
The bubble diameter was measured utilizing a photographic method based on images of the bubbles taken along the tallness of the column, from underside to exceed. A Sony ( DCR-PC330E ) picture camera was used to obtain the images. A minimal figure of 100 chiseled bubbles along the bubble column were used to measure the size distribution of the bubbles in the liquid stage employed, and for each gas flow-rate that has been used. The Image Tool v3.0 package was used to transport out the necessary measurings of the bubbles geometric features.
The overall gas hold-up is an of import parametric quantity to find the gas-liquid interfacial country and it was measured utilizing the volume enlargement method. The computation of the volume alteration in the bubble column was based on the alteration observed on the liquid degree and on the addition of this value after gassing utilizing the cylindrical bubble column ( see equation 1 ) .
( 1 )
where VL is the ungassed liquid volume and DV is the volume enlargement after gas scattering, calculated from the liquid degree alteration and the cross sectional country. The alteration in the bubble column volume was calculated based on the alteration observed on the liquid degree and on the addition of this value after gassing.
The images of the bubbles we obtained in the liquid phases employed show an spheroidal form. For this ground, major ( E ) and minor ( vitamin E ) axes of the projected ellipsoid ( in two dimensions ) were determined. The diameter of the equivalent sphere ( equation 2 ) was taken as the representative bubble dimension.
( 2 )
Different writers recommend utilizing the Sauter mean diameter ( d32 ) , which can be determined utilizing the information calculated for the tantamount diameter:
( 3 )
where Ni is the figure of bubbles that have an tantamount diameter ( di ) .
The Sauter mean diameter and the gas hold-up values allow us the computation of the specific interfacial country utilizing equation 4.
( 4 )
3. Consequences and treatment
The present work has been focused on the consequence caused by the presence of different concentrations of a wetting agent in aqueous solution upon the soaking up procedure. The surfactant employed, Tween80, is commonly present in different procedures that imply gas-liquid mass transportation processes [ 13 ] . One of the surveies developed in this work is related to the hydrodynamic behavior, based on the analysis of the gas hold-up and the gas-liquid interfacial country produced in the contactor. In relation to the first parametric quantity ( gas hold-up ) , the behavior observed for this experimental system is shown in figure 1. The influence of the gas flow-rate ( or superficial gas speed ) upon the gas hold-up is shown in this figure, where we can detect that an addition in the gas flow-rate produces an addition in the gas hold-up value. This sort of behavior shown in figure 1 indicates that a alteration in the bubbling government is non produced in the studied scope. The bubbling government is pseudo-homogeneous in all instances and, so, the coalescency procedure is non observed. The experimental informations shown in figure 1 corresponds to pure H2O ( without Tween80 add-on ) but the experiments developed with different surfactant concentrations do non demo important alterations with respect to the values obtained for C dioxide – H2O system. This behavior is in understanding with old surveies that analyse the influence of wetting agents upon this variable [ 14, 15 ] .
Besides, the hydrodynamic word picture of the gas-liquid systems employed in present work includes the analysis of the bubbles size distribution produced in the bubble column. The bubble size distribution and gas hold-up could be used to gas-liquid interfacial country finding. An illustration of bubble size distribution determined in present work is shown in figure 2. This figure allows analyse the influence of surfactant concentration upon bubble size distribution. The experimental consequences indicate that a higher value in surfactant concentration produces a lessening in the bubble size. This behavior is in understanding with the consequences obtained for systems with little concatenation length surfactant [ 6, 9 ] . Besides, figure 2 shows that an addition in surfactant concentration produces a narrower size distribution.
This behavior is related to the influence caused by the surfactant presence upon the surface tenseness value. This sort of substances produces an of import lessening in the value of the surface tenseness, and this behavior has a high influence upon the bubble size produced in the contactor. Figure 2 shows the lessening observed in the bubbles size distribution produced by the wetting agent.
Using the bubble size distribution for each experimental conditions and the gas hold-up produced in the bubble column, the gas-liquid interfacial country was calculated utilizing equation 3. Figure 3 summarized the determined informations for the interfacial country under the experimental conditions analysed in this work. Sing the influence of the surfactant concentration upon the interfacial country, an addition in this parametric quantity was observed when the wetting agent concentration besides increases in the liquid stage. This behavior is due to the presence of this solute, that produces a lessening in the bubble size value ( vide supra ) with no influence upon the gas hold-up.
On the footing of experimental informations, we conclude that the largest alteration in the value of interfacial country is caused by the add-on of little measures of wetting agent. Higher values than 0.03 % of surfactant concentration produce practically changeless interfacial country values with little alterations, in malice of the fact that of import measures of wetting agent were added to the liquid stage. A similar behavior has been observed for other systems that have employed wetting agents in aqueous solution [ 8, 16 ] .
This behavior has been observed for all the gas flow-rates employed in this work. In relation to the influence of this operation variable, an addition in this value produces an addition in the interfacial country, although the ground is different to the one antecedently commented, when the surfactant concentration was varied. Sing the gas flow rate consequence, an addition in this variable does non bring forth important alterations in the bubble size, but on the other manus, this variable produces an addition in the gas hold-up ( see figure 1 ) .
The experimental informations obtained for the gas-liquid interfacial country has been fitted utilizing an equation based on different operation variables. The correlativity used is shown in equation 5, and similar equations based on possible effects of variables have been proposed by other writers [ 17, 18 ] .
( 5 )
where Cs is the surfactant concentration, cmc is the critical micelle concentration of Tween80 and Qg is the gas flow-rate.
Equation 5 includes the surfactant concentration and the gas flow-rate as of import operation variables, but this equation besides includes the value of the critical micelle concentration. This parametric quantity provides information about the concatenation length and other belongingss related to hydrofobicity. Figure 4 shows a comparing between the experimental values and the deliberate 1s utilizing the simple correlativity ( equation 5 ) . Merely one wetting agent has been employed in this work and so the value of the critical micelle concentration is changeless, but the look used in equation 5 includes this value to continue the equation general preparation. The usage of the same equation allows us to compare the value of the fit parametric quantities with old and future surveies that use other wetting agents. Figure 4 shows a comparing between experimental and calculated gas-liquid interfacial country under different experimental conditions, detecting a good understanding between the experimental values and the corresponding 1s calculated utilizing equation 5.
The present work besides includes gas-liquid mass transportation surveies matching to the soaking up of C dioxide in Tween80 aqueous solutions. The operation government was semicontinuous and so, the liquid stage was placed into the contactor and the gas stage was fed continuously to the bubble column. The soaking up dynamicss was obtained and this experimental information has been employed to cipher the volumetric mass transportation coefficient. The mass transportation coefficient ( kL ) was calculated taking into history the values of the volumetric mass transportation coefficient and the gas-liquid interfacial country. This manner, our purpose in this work is analyzing the influence of surfactant presence and concentration upon the mass transportation coefficient and the gas-liquid interfacial country, every bit good as analyzing the influence upon each parametric quantity separately.
Figure 5 shows an illustration of the development of the C dioxide absorbed concentration in the liquid stage. Bing more specific, two experiments are compared in figure 5 utilizing the same surfactant concentration and different gas flow-rate values. The experimental informations indicate that an addition in the value of the gas flow-rate Federal to bubble column produces a higher addition in the C dioxide concentration. This behavior indicates that the mass transportation rate additions with the gas flow-rate. This behavior is complemented with the fact antecedently commented: that an addition in the gas flow-rate produces an addition in the interfacial country that, at the same clip, produces an addition in the mass transportation rate.
The C dioxide concentration in the liquid stage has been calculated throughout clip, based on the experimental consequences obtained for the soaking up rate. In this sort of soaking up procedures, working in a semi-continuous government and utilizing a pure gas stage, equation 6 is used to find the volumetric mass transportation coefficient obtained, utilizing a gas stage mass balance.
( 6 )
where KLA·a is the volumetric mass transportation coefficient, and C* and C are the solubility and C dioxide concentration, severally. In the present work, the C dioxide solubility value [ 19 ] has been considered equal to the value matching to pure H2O, due the low surfactant concentration employed in the liquid stage. Under the experimental conditions ( without gas stage opposition to mass transportation ) employed, the single mass transportation coefficient of the liquid stage is considered equal to the planetary mass transportation coefficient.
The same experimental process has been carried out for the different experiments performed in the present work, and so the volumetric mass transportation coefficient has been calculated for each experimental status. Figure 6 shows the obtained behaviour and the influence of different operation variables, such as gas flow-rate and surfactant concentration. In relation to the influence of surfactant concentration upon the mass transportation coefficient value, an of import lessening in this parametric quantity was produced when little measures of Tween80 were added to the liquid stage, making a changeless value of mass transportation coefficient and non dependent of surfactant concentration. This behaviour is opposite to the old one commented about the influence of surfactant concentration upon the value of gas-liquid interfacial country ( the addition of surfactant concentration produced an addition in interfacial country until a changeless value ) . The influence of surfactant concentration upon the interfacial country must be an addition in the value of the volumetric mass transportation coefficient ; nevertheless, the obtained behaviour is the opposite. Taking this fact into history, a priori decision is that the presence of wetting agent in the liquid stage has a really of import negative consequence upon the mass transportation coefficient, and this of import consequence is higher that the positive influence caused upon the interfacial country.
On the other manus, in relation to the influence of gas flow-rate Federal to the bubble contactor, an addition in the value of the volumetric mass transportation coefficient has been observed when the gas flow-rate additions. Taking into history the antecedently analysed influence of the gas flow-rate upon the interfacial country and the consequences for volumetric mass transportation coefficient, we can reason that the gas flow-rate affects positively upon both parametric quantities ( mass transportation coefficient and interfacial country ) .
Using the experimental values of volumetric mass transportation coefficient and the antecedently determined interfacial country, we can cipher the mass transportation coefficient values for each experimental status by agencies of equation 7.
( 7 )
The experimental informations shown in figure 7 indicates a similar behavior to the old one obtained for the volumetric mass transportation coefficient. A lessening in the mass transportation coefficient value is observed when low add-ons of wetting agent are added to the liquid stage. Therefore, the decisions proposed a priori are certain. The consequence of the gas flow-rate upon the mass transportation coefficient ( shown in figure 7 ) indicates that this variable loses importance, and the mass transportation coefficient values for the different gas flow-rates are similar. This behavior is in understanding with old surveies that conclude the non- influence of the gas flow-rate upon the mass transportation in this sort of contactors [ 3 ] .
Previous surveies using similar systems indicate that the presence of wetting agent in the liquid stage reduces the mass transportation coefficient until a tableland for higher concentrations than the critical micelle concentration [ 3 ] . This decrease has been assigned to different grounds in relation to the increase in the conveyance opposition caused by the presence of surfactant molecules. These molecules produce a decrease in the liquid elements renewal near the interface. Then, it produces a lessening in the drive force that is straight related to the gas mass transportation rate to the liquid stage.
Other surveies have concluded that a low surfactant concentration produces an sweetening of mass transportation that produces an addition in the value of the mass transportation coefficient [ 20, 21 ] . In this work, this addition or sweetening is non observed and this behavior is assigned to the size of surfactant molecule, compared to the experimental systems that show the enhancement behavior [ 22 ] .
The lessening in the mass transportation coefficient by the presence of surfactant molecules in the liquid stage is assigned to different alterations caused by the accretion of surfactant molecules at the gas-liquid interface. This accretion causes a decrease in the reclamation of the liquid elements and so, a lessening in the value of the driving force. These effects produce a lessening in the mass transportation rate. Different surveies [ 3 ] have concluded that this decrease in the mass transportation rate is observed until the surfactant concentration reaches the value matching to the critical micelle concentration. When this concentration is reached, it is impossible to increase the surfactant concentration in a gas-liquid interface because the micelle formation has already been produced.
For gas-liquid systems affecting the presence of different measures of wetting agents in the liquid stage, a old work has developed a theoretical account that shows a good behaviour in the mass transportation coefficient finding and, taking into history the particular alterations produced by this sort of substance, upon the kineticss of gas-liquid systems as good [ 8 ] . This theoretical account predicts, under the experimental conditions employed in the present work, that the mass transportation coefficient must be included between two bounds called: : mass transportation coefficient for free surface ( Se = 0 ) and: mass transportation coefficient for a concentrated surface ( Se = 1 ) . On the footing of these conditions, this theoretical account for mass transportation coefficient appraisal has the look shown in equation 8.
( 8 )
Determining under present experimental conditions could be carried out utilizing Higbie ‘s equation [ 23 ] . On the other manus, the matching value to a concentrated interface, , has extra troubles since there are non theoretical accounts in the literature that allow us the computation of this coefficient. Frossling equation in peculiar can non be used straight for this computation. This value depends on the surfactant nature, so Sardeing et al [ 8 ] suggest the usage of equation 9 for this mass transportation coefficient.
( 9 )
where K is the surface assimilation equilibrium invariable. This changeless is really of import because high values imply that the surfactant molecules reach the gas-liquid interface rapidly. Then, the clean bubbles that are fed to the bubble column accumulate taint in the surface in a low operation clip, bring forthing a lessening in the mass transportation. Equation 9 besides includes the mass transportation coefficient determined by the Frossling theoretical account [ 24 ] .
Figure 8 shows the obtained behavior for the Higbie, Frossling and Sardeing et Al ( utilizing equation 9 ) theoretical accounts. The comparing with the experimental values of mass transportation coefficient ( see figure 8 ) indicates that the Higbie ‘s theoretical account overestimate the values of mass transportation coefficient except for the system in the absence of wetting agent. On the other manus, Frossling ‘s theoretical account takes lower values than the experimental 1s. At a high wetting agent concentration, the experimental values are closer to the corresponding 1s to Frossling ‘s theoretical account, due to the addition in the surfactant concentration at the gas-liquid interface. The last theoretical account, developed by Sardeing et Al, allows the computation of the mass transportation coefficient with better consequences when it is compared with the experimental information. But the values contributed by this last theoretical account when the surfactant concentration additions are really near to Frossling ‘s theoretical account, and the experimental information shows a tableland with a changeless value of mass transportation coefficient higher than the value calculated utilizing Frossling equation. Due to the behavior of Sardeing ‘s theoretical account, a alteration of this theoretical account has been performed in this work, by agencies of altering the changeless ( 1.744 ) of equation 9, since this value is related to the surfactant nature [ 8 ] . This invariable has been determined in the present survey utilizing the experimental information of mass transportation coefficient because the wetting agent we have employed is really different ( in molecular weight and size ) to the substances used in old plants that had used this theoretical account. Then, equation 9 has been modified, obtaining the look shown in equation 10. This alteration in the Sardeing et al theoretical account allows adjustment, with better consequences, the influence of Tween80 concentration upon the experimental values of mass transportation coefficient, in comparing with the other theoretical accounts analysed ( see figure 8 ) .
( 10 )
The present work has analyzed the consequence caused by the presence of Tween80 upon different parametric quantities ( gas hold-up, bubble size distribution, interfacial country and mass transportation coefficient ) related with mass transportation rate in a bubble column contactor. The presence of this substance produces an of import addition in gas-liquid interfacial country produced in the bubble column, and caused by an of import lessening in the value of bubble diameter, because non-influence of Tween80 upon gas hold-up was detected. On the other manus, an addition in the gas flow-rate produces besides an addition in interfacial country due to an addition in gas hold-up.
The presence of Tween80 produces the opposite behaviour upon mass transportation coefficient bring forthing a high lessening with the presence of low surfactant concentration. The consequence of gas flow-rate upon mass transportation coefficient was considered negligible.
A alteration of Sardeing theoretical account allows fit the experimental information taken into history the values matching to mobile a stiff bubbles, and the particular features ( in relation with its surface activity ) of Tween80.