Agitated Gas Liquid Contactors Research Articles

Prediction of volumetric mass transfer coefficient in mechanically and pneumatically agitated contactors

The volumetric mass transfer coefficient (kLa) plays a crucial role in industrial design in the case of the process controlled by gas–liquid mass transfer. Prediction of kLa is nowadays mostly based on literature correlations. Our research goal is to establish suitable kLa correlations for different types of devices that would be based on the experimental dataset. This article aims at the description of one universal correlation that would be viable for mechanically agitated gas–liquid contactors and also for pneumatically agitated gas–liquid contactors such as airlift reactor.

Mass transfer characteristics of multiple-impeller fermenters for their design and scale-up

Mechanically agitated gas–liquid contactors are frequently used in the chemical, food and biochemical industries as fermenters and as hydrogenation or chlorination reactors. However wide the usage of such vessels is, their design is not based on chemical engineering data, but is still rather empirical. Thus, it is highly desirable to have a tool for the rational design of agitated gas–liquid contactors that is based on fundamental chemical engineering parameters that are transferable to other systems and operating conditions. Focusing on liquid film-controlled processes and using the data from fermenters of different scales, we develop kLa correlations that are suitable for scale-up.First, we discuss the way how to determine the proper experimental kLa values, which are not distorted by other equipment parameters as the gas residence time is. We demonstrate the possible kLa distortion on the pilot-plant experimental data by the comparison of the results obtained by two different experimental techniques. Further, we present physically correct kLa data for coalescent (pure water) and fully non-coalescent (sodium sulphate solution) batches. The data are presented both for laboratory and for pilot-plant fermenters. We identify the process parameters, the values of which are dependent on the vessel scale when operated under the same power input per unit volume, and, using these parameters, we develop common kLa correlations suitable to describe the data for various scales of the vessel.The correlations developed reduce the uncertainty in predicting the volume of industrial scale fermenters from almost 1/2 to 1/4 of their total volume, thereby enabling significant reductions in both the initial costs and operating costs.

Multiplicity phenomena in mechanically agitated gas–liquid contactors

For a gas–liquid contactor equipped with two Rushton disk turbines (DT), where the upper DT is located very near the static liquid surface, we have determined experimentally its volumetric mass-transfer parameter kLa as a function of the power input, P/V. It is interestingly found that in such a contactor multiple steady states occur, i.e., for a given P/V value, two kLa values exist or vice versa in the region of low P/V, leading to two (lower and upper) branches in the kLa vs. P/V plot. Detailed investigations indicate that the lower branch represents the mass transfer behavior of typical gas–liquid contactors with two DTs in the literature, while the upper branch results from a different mass transfer mechanism. We believe that presence of the upper branch is because of the small distance of the upper DT to the static liquid surface, which in the range of low P/V values enhances the surface gas entrainment, leading to substantial difference in the local gas holdup between the upper and lower DTs. It is this inhomogeneity that generates the multiplicity phenomenon.

CFD simulation of mixing in tall gas–liquid stirred vessel: Role of local flow patterns

In this work, we have used the computational fluid dynamics (CFD)-based models to investigate the gas–liquid flows generated by three down-pumping pitched blade turbines. A two-fluid model along with the standard k – ε turbulence model was used to simulate the dispersed gas–liquid flow in a stirred vessel. Appropriate drag corrections to account for bulk turbulence [Khopkar and Ranade, 2005. CFD simulation of gas–liquid flow in a stirred vessel: VC, S33 and L33 flow regimes. A.I.Ch.E. Journal, accepted for publication] were developed to correctly simulate different flow regimes. The computational snapshot approach was used to simulate impeller rotation and was implemented in the commercial CFD code, FLUENT4.5 (of Fluent. Inc., USA). The computational model has successfully captured the flow regimes as observed during experiments. The particle trajectory simulations were then carried out to examine the influence of the different flow regimes on the circulation time distribution. The model predictions were verified by comparing the predicted results with the experimental data of [Shewale and Pandit, 2006. Studies in multiple impeller agitated gas–liquid contactors. Chemical Engineering Science 61, 489–504]. The computational model and results discussed in this study would be useful for explaining the implications local flow patterns on the mixing process and extending the applications of CFD models for simulating large multiphase stirred reactors.

Studies in multiple impeller agitated gas–liquid contactors

Experiments have been performed to study the effect of the density and the volume of the tracer pulse on the mixing time for two impeller combinations in the presence of gas in a 0.3 m diameter and 1 m tall cylindrical acrylic vessel. The tall multi-impeller aerobic fermenters, which require periodic dosing of nutrients that are in the form of aqueous solution, is a classic case under consideration. Conductivity measuring method was used to measure the mixing time. Two triple impeller combinations; one containing two pitched blade downflow turbines as upper impellers and disc turbine as the lowermost impeller (2 PBTD–DT) and another containing all pitched blade downflow turbines (3 PBTD) have been used. Other variables covered during experiments were the density and the amount of the tracer pulse, the impeller rotational speed and the gas superficial velocity. Fractional gas hold-up, Power consumption and mass transfer coefficient have also been measured for both the impeller combinations. Influence of aeration and impeller speed on the mixing time has been explained by the interaction of air induced and impeller generated liquid flows. Three different flow regimes have been distinguished to explain the hydrodynamics of the overall vessel (i.e., multiple impeller system). A compartment model with the number of compartments varying with the flow regimes have been used to model liquid phase mixing in these flow regimes. A correlation for the prediction of the dimensionless mixing time in the loading regime has been proposed in order to account the effect of the density and the amount of the tracer pulse on the mixing time. Correlations have also been proposed to predict fractional gas hold-up and k L a .

Regional Mean Bubble Diameters and Gas Holdup in Agitated Gas-Liquid Contactors

Gas-liquid contacting in mechanically agitated vessels is widely used in the process industry. Bubble size measurements at different flow regions in the vessel provide useful information for the mechanisms of gas dispersion and gas-liquid flow. In this work, bubble size distributions and distributions of Sauter mean bubble diameters at four different regions in air-water and air-NaCl solution systems agitated by a six-blade disk turbine are measured by using the photographic method. The effects of gassing rate, impeller speed and electrolyte presence in the system have been examined.

Regional Mean Bubble Diameters and Gas Holdup in Agitated Gas-Liquid Contactors

Regional Mean Bubble Diameters and Gas Holdup in Agitated Gas-Liquid Contactors

Dispersed phase hold-up in mechanically agitated gas-liquid contactors

In the present study, in mechanically agitated gas–liquid contactors (MAC), a generalized correlation for the direct estimation of dispersed gas phase hold-up is proposed. This present proposed new correlation in terms of fundamental operating variables, involving gas velocity, impeller speed in terms of Froude number, vessel geometry and the physical properties, is more accurate than those previously reported and also simpler to use. © 1998 SCI

Dispersed phase hold‐up in mechanically agitated gas–liquid contactors

In the present study, in mechanically agitated gas–liquid contactors (MAC), a generalized correlation for the direct estimation of dispersed gas phase hold-up is proposed. This present proposed new correlation in terms of fundamental operating variables, involving gas velocity, impeller speed in terms of Froude number, vessel geometry and the physical properties, is more accurate than those previously reported and also simpler to use. © 1998 SCI

Interfacial areas in agitated gas-liquid contactors

Chemical reactions with known chemical and physical properties were used for the determination of integral values of the interfacial areas created with an agitator in a gas-liquid contactor. The reactions between oxygen and a sulphite solution and between carbon dioxide and sodium hydroxide solutions were chosen. For the first system the chemical reaction in the liquid boundary layer is proved to be rate determining under our experimental conditions. The existence of two regions is demonstrated: 1. (a) A region without agitation effect. At agitation rates below a minimum agitation rate n 0 D interfacial areas are not affected by the stirring; they only depend on the gas load v s and the type of gas sparger. 2. (b) A region with agitation effect. At agitation rates above n 0 D the interfacial areas are linearly dependent on the agitation rate and independent of the gas load and type of gas inlet. The value of n 0 D depends on the tank to agitator diameter ratio and probably is independent of the liquid viscosity. Generally valid correlations for the minimum agitation rates and the interfacial area under fully baffled conditions could be derived for a great number of geometrically similar impellers of different sizes in vessels with diameters ranging from 0.14 to 0.90 m. Although vessels with a diameter larger than 1 m were not investigated the method of correlation seems to give good results for scaling-up. The influence of the physical properties of the liquid remains uncertain.

Mean residence time of gas in mechanically agitated gas–liquid contactors

AbstractIn the present study of gas–liquid contactors, mean residence/contact time was calculated from knowledge of superficial velocity and the gas phase hold‐up, for various gas rates and impeller geometry and speeds, and compared with values obtained from RTD measurements. A new correlation, involving Flow Number, Froude Number, system geometry and the physical properties, is proposed. This uses the authors data and those available in literature.

Mass transfer in mechanically agitated gas—liquid contactors

The theory of absorption with pseudo mth order irreversible reaction was used to obtain effective interfacial area, a, in mechanically agitated vessels. The liquid-side mass transfer coefficient, k L a was found by using the theory of absorption with slow chemical reaction. Eleven different systems were used to obtain a and four different systems were used to obtain k L a. The diameter of the contactor was varied from 12.5 cm to 70 cm. The effect of the following variables on a and K L a was investigated: speed, type and diameter of the agitator; tank diameter, height of the agitator and height of the clear liquid from the base of the contactor; ionic strength, nature of ions, viscosity and surface tension of the liquid. It appears that values of a and k L a cannot be predicted a priori, as system properties have a profound effect on a and k L a. However, experiments can be done in a small agitated contactor (say, 20 cm dia. contactor) and scale-up can be done on the basis of equal tip speed divided by the square root of the contactor diameter, possibly with a fair degree of confidence.

Gas Absorption in Agitated Gas-Liquid Contactors

Gas Absorption in Agitated Gas-Liquid Contactors

Design of agitators for gas-liquid contacting

The influence of the various relevant process and design variables on the interfacial area in agitated gas-liquid contactors is discussed. An optimum agitator to vessel diameter ratio ( D/ T) opt exists where the total interfacial area is a maximum for a chosen power input into the liquid. The determination of ( D/ T) opt and corresponding values of power input, agitation rate and shaft rotation speed is demonstrated. As a general rule it can be said that high speed, small diameter impellers have to be used for an efficient use of power. Furthermore it is shown that agitator power input or agitator Reynolds number are not adequate variables for correlating interfacial areas. Recommendations are given for the design of agitated gas-liquid contactors.

Interfacial areas in agitated gas-liquid contactors

Chemical reactions with known chemical and physical properties were used for the determination of integral values of the interfacial areas created with an agitator in a gas-liquid contactor. The reactions between oxygen and a sulphite solution and between carbon dioxide and sodium hydroxide solutions were chosen. For the first system the chemical reaction in the liquid boundary layer is proved to be rate determining under our experimental conditions. The existence of two regions is demonstrated: (a) A region without agitation effect. At agitation rates below a minimum agitation rate n 0 D interfacial areas are not affected by the stirring; they only depend on the gas load v s and the type of gas sparger. (b) A region with agitation effect. At agitation rates above n 0 D the interfacial areas are linearly dependent on the agitation rate and independent of the gas load and type of gas inlet. The value of n 0 D depends on the tank to agitator diameter ratio and probably is independent of the liquid viscosity. Generally valid correlations for the minimum agitation rates and the interfacial area under fully baffled conditions could be derived for a great number of geometrically similar impellers of different sizes in vessels with diameters ranging from 0·14 to 0·90 m. Although vessels with a daimeter larger than 1 m were not investigated the method of correlation seems to give good results for scaling-up. The influence of the physical properties of the liquid remains uncertain.

Correction - Performance of Agitated Gas-Liquid Contactors

Correction - Performance of Agitated Gas-Liquid Contactors

Performance of Agitated Gas-Liquid Contactors

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPerformance of Agitated Gas-Liquid ContactorsC. M. Cooper, G. A. Fernstrom, and S. A. MillerCite this: Ind. Eng. Chem. 1944, 36, 6, 504–509Publication Date (Print):June 1, 1944Publication History Published online1 May 2002Published inissue 1 June 1944https://doi.org/10.1021/ie50414a005RIGHTS & PERMISSIONSArticle Views1234Altmetric-Citations502LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1012 KB) Get e-Alerts