Interfacial Area In Gas Research Articles

On the measurement of local gas hold-up and interfacial area in gas–liquid contactors via light sheet and image analysis

Particle image velocimetry techniques coupled with advanced image processing tools are receiving an increasing interest for measuring flow quantities and local bubble-size distributions in gas–liquid contactors. Aim of this work is that of providing a numerical procedure able to reconstruct local gas hold-up and specific interfacial area from images obtained by laser sheet illumination. A correction for measured quantities accounting for laser sheet thickness is proposed and tested by means of Monte Carlo simulations. The algorithms proposed are robust and independent of any measured parameters of the actual bubble size distribution.

Effectiveness of interfacial area for mass transfer in two-phase flow in microreactors

A new criterion for the effectiveness of the interfacial area in gas–liquid microreactors is proposed. The criterion should be met in order to avoid saturation of the liquid film covering the channel walls in the case of physical absorption, or liquid reagent exhaustion in this film for absorption with chemical reaction. The criterion takes the form: LD u δ 2 ⪡ 1 . For slow reaction catalysed by a solid catalyst deposited on the channel walls the reverse criterion should be met.

Mass Transfer Characteristics in Structured Packing for CO2 Emission Reduction Processes

Acid gas treating and CO2 capture from flue gas by absorption have gained wide importance over the past few decades. With the implementation of more stringent environmental regulations and the awareness of the greenhouse effect, the need for efficient removal of acid gases such as CO2 (carbon dioxide) has increased significantly. Therefore, additional effort for research in this field is inevitable. For flue gas processes the ratio of absorption solvent to gas throughput is very different compared to acid gas treating processes owing to the atmospheric pressures and the dilution effect of combustion air. Moreover, in flue gas applications pressure drop is a very important process parameter. Packing types are required that allow for low pressure drop in combination with high interfacial areas at low liquid loading per square meter. The determination of interfacial areas in gas−liquid contactors by means of the chemical method (Danckwerts, P. V. Gas-liquid reactions; McGraw-Hill: London, 1970) has been very frequently applied. Unfortunately, many of the model systems proposed in the literature are reversible and therefore this condition possibly is not met. Versteeg et al. (Versteeg, G. F.; Kuipers, J. A. M.; Beckum, F. P. H.; van Swaaij, W. P. M. Chem. Eng. Sci. 1989, 44, 2292) have demonstrated that for reversible reactions the conditions for the determination of the interfacial area by means of the chemical method are much more severe. In a study by Raynal et al. (Raynal, L.; Ballaguet, J. P.; Berrere-Tricca, C. Chem. Eng. Sci. 2004, 59, 5395), it has been shown that there is a dependency of the interfacial area on the packing height. Unfortunately, most model systems used, e.g., CO2−caustic soda (as used by Raynal et al.), are much more complex and consist of (a set of) reversible reaction(s). The natures of these systems make the conditions at which the interfacial area can be determined much more severe and put more limitations on the process conditions and experimental equipment than a priori can be expected. Therefore, an extended absorption model is required to determine the conditions at which the interfacial area can be measured without detailed knowledge of the values of the liquid-side mass transfer coefficient, kl, beforehand.

Structural identifiability analysis of the dynamic gas–liquid film model

AbstractA structural identification analysis of the dynamic gas–liquid film model is applied to identify its theoretically accessible parameters. The analysis was performed considering the different experimental situations found in nonstationary bubble‐column reactors. The system has been modeled considering both the liquid and the gas phases in a well‐mixed flow regime with a global second‐order irreversible chemical reaction. To describe the mass‐transfer phenomenon at the gas–liquid interface level, chemical reactions and diffusive mass transport were considered simultaneously in the liquid film. The identifiability analysis shows that the simultaneous measurement of concentrations in the liquid and the gas phases is required to determine the parameters. Furthermore, model parameters such as the diffusion coefficients, the specific interfacial area, or the film thickness cannot be uncoupled from other parameters, although the kinetic rate constants could be identified in the fast or slow chemical regimes. It is demonstrated that the specific interfacial area cannot be measured from chemical data coming from a nonstationary bubble‐column reactor. This result is discussed with respect to the classical Danckwerts' method for determination of the interfacial area in gas–liquid equipment. © American Institute of Chemical Engineers AIChE J, 2006

Local bubble size distributions, gas–liquid interfacial areas and gas holdups in a stirred vessel with particle image velocimetry

Abstract Particle image velocimetry (PIV) was used to measure local bubble size distributions (BSD), gas–liquid interfacial areas, gas holdups and flow velocities simultaneously from a flat-blade turbine agitated 14 dm3 vessel. Air–water and CO2–n-butanol systems were investigated at several agitation conditions in order to find out the effect of physical properties on the vessel hydrodynamics. Dispersion was illuminated with a laser light sheet to minimise the blurriness in the images. The depth of field (DOF), needed in the calculation of local gas–liquid interfacial areas and gas holdups was obtained from the calibration experiments with a bubble gel. A simple method was developed to correct some bias errors of the measurement technique. Calibration experiments verified the need and the relevance of the developed correction. The measured BSDs varied reasonably with the measurement point in both air–water and CO2–n-butanol systems and were in agreement with the well-known correlations of Calderbank [P.H. Calderbank, Physical rate processes in industrial fermentation. Part 1. The interfacial area in gas–liquid contacting with mechanical agitation, Trans. Inst. Chem. Engrs. 36 (1958) 443–463]. The results show that local hydrodynamic quantities can be measured simultaneously with the PIV from a stirred vessel. This is a benefit, since more consistent experimental information is obtained for the validation of gas–liquid stirred tank simulation tools.

Interfacial area in gas–liquid Couette–Taylor flow reactor

In this study the specific interfacial area (gas–liquid) generated within a stirred tank and Couette–Taylor–Poiseuille flow (CTPF) reactor was determined for a range rotational speeds, and gas and liquid volumetric flow rates. The specific interfacial area was obtained experimentally using the oxidation sodium dithionite in alkaline solution. The specific interfacial area for a CTPF was found to be in range 700–1200 m 2 m −3 over the range of operating rotational speed (0–3000 rpm), liquid flow rate (1 dm 3 min −1) and gas/liquid volumetric flow rates (0.5–2.0). Typically, the specific interfacial area obtained for the CTPF reactor was an order of magnitude higher than those obtained (in this study) for a stirred tank reactor. It was also found that the liquid phase mass transfer coefficient, calculated from the measured a L and using previously obtained k L a values, was of the order 10 −4 m s −1, which was consistent for 2.5 mm diameter bubbles as calculated in this study.

Horizontal Couette–Taylor flow in a two-phase gas–liquid system: flow patterns

Favorable feature of the Couette–Taylor (CT) flow recently resulted in application it to two-phase systems. High value of mass transfer coefficients and high value of interfacial area in gas–liquid flow was obtained in [Chem. Eng. Sci. 54 (1999) 2963; 56 (2001) 1131; Dłuska et al., 6th World Congress of Chemical Engineering, Melbourne, Australia, 23–27 September 2001]. However, so far very little is known about hydrodynamics of such a flow. In this work typical flow structure for gas–liquid horizontal CT flow was described. The maps of the flow regime were presented. The observed regime was plotted as a function of power input (connected with the rotor rotation) and modified liquid mass flux. The most interesting for practical application was the “ring flow” regime. Such a flow was observed where liquid vortices are separated by the gas rings.

Measurements of local interfacial area: application of bi-optical fibre technique

Local mean interfacial area in gas–liquid bubbly flow is experimentally investigated; data is obtained from the experiments carried out with a double optical sensor intrusive probe and analysed by statistical methods. Local variables are measured in the riser of an external loop airlift reactor in air/water medium. The results are presented in the form of the radial profiles of void fraction, bubble diameter and local average interfacial area at superficial gas velocities ranging from 0.003 to 0.11 m/ s . The axial variations of the same parameters are also investigated. The values of local interfacial area obtained from the classical equation ( a i t=6ε g/ d 32 ) and from bubble velocity statistical treatment methods agree reasonably well. This shows that the double optical probe is a reliable method for local measurements of interfacial area. Eventually, the maximum angle α 0 between the probe vertical direction and bubble velocity vector at the piercing point is evaluated and the influence of gas superficial velocity on α 0 is demonstrated.

Some issues related to the modeling of interfacial areas in gas–liquid flows I. The conceptual issues

Two-phase flow modeling has been under constant development for the past forty years. Actually there exists a hierarchy of models which extends from the homogeneous model valid for two-phase flows where the phases are strongly coupled to the two-fluid model valid for two-phase flows where the phases are a priori weakly coupled. However the latter model has been used extensively in computer codes because of its potential in handling many different physical situations. The two-fluid model is based on the balance equations for mass, momentum and energy, averaged in a certain sense and expressed for each phase and for the interface between the phases. The difficulty in using the two-fluid model stems from the closure relations needed to arrive at a complete set of partial differential equations describing the flow. These closure relations should supply the information lost during the averaging of the balance equations and should specify in particular the interactions of mass, momentum and energy between the phases. Another requirement for the interaction terms is that they should satisfy the interfacial balance equations. Some of these terms such as the added mass term or the lift force term do not depend on the interfacial area but some others do, such as the mass transfer term, the drag term or the heat flux term. It is then necessary to model the interfacial area in order to evaluate the corresponding fluxes. Another benefit resulting from the modeling of the interfacial area would be to replace the usual static flow pattern maps which specify the flow configuration by a dynamic follow-up of the flow pattern. All these reasons explain why so much effort has been put during the past twenty years on the modeling and measurement of the interfacial area in two-phase flows. This article contains two parts. The first one deals with the conceptual issues and has the following objectives: 1. to give precise definitions of the interfacial area concentrations; 2. to explain the origin of the interfacial area concentration transport equation suggested by M. Ishii in 1975; 3. to explain some paradoxical behaviors encountered when calculating the interfacial area concentration transport velocity.

The kinetics of the reaction between CO 2 and diethanolamine in aqueous ethyleneglycol at 298 K: a viscous gas—liquid reaction system for the determination of interfacial areas in gas—liquid contactors

The reaction between CO 2 and diethanolamine (DEA) in aqueous ethyleneglycol (ETG) at 298 K has been studied over the complete composition range. The application of this reaction as a viscous gas—liquid system for the determination of interfacial areas in gas—liquid contactors by the chemical method is discussed. The reaction kinetics have been determined by mass transfer experiments of CO 2 into solutions of DEA in aqueous ETG. To this end laboratory-scale stirred cell reactors with a flat surface have been used. In accordance with the same reaction in water at 298 K the reaction between CO 2 and DEA in aqueous ETG at 298 K can be described by the zwitterion mechanism of Caplow. Special attention has been paid to the reversibility of the reaction between CO 2 and DEA. Calculation show that the influence of the reversibility on the mass transfer rate can be neglected for partial pressures of CO 2 below 3 kPa. It is demonstrated that the reaction between CO 2 and DEA in aqueous ETG can be used for the determination of interfacial areas in gas—liquid contactors at higher viscosities.

Mass transfer phenomena and hydrodynamics in agitated gas—liquid reactors and bubble columns at elevated pressures: State of the art

All important studies on the influence of pressure on mass transfer phenomena in gas—liquid systems and reactors are reviewed critically. Points of agreement and conflict are indicated and discussed. It is concluded that: (1) the initial bubble size at a single orifice decreases with increasing pressure; (2) the gas-phase mass transfer coefficient k G is inversely proportional to the pressure to the power n, where n depends on the mass transfer mechanism; (3) the liquid-phase mass transfer coefficient k L is not influenced by pressure; (4) the gas hold-up ε G in bubble columns increases with increasing pressure. However, insufficient data on the influence of the operating pressure on the interfacial areas in gas—liquid contactors are available.

The use of the chemical method for the determination of interfacial areas in gas—liquid contactors

The interfacial area a chem in a gas—liquid contactor as determined by the chemical method deviates from the true geometrical interfacial area a geo, because the overall conversion of the gas phase reactant represents an incorrect average if bubble sizes and residence times are not uniform. The deviations of a chem from a geo become larger the broader the distribution τ b/ d b and the higher the overall conversion Ω A of the reactant in the gas phase. Model calculations, which take into account both the effect of gas phase backmixing as well as the effect of bubble coalescence on the deviation of a chem from a geo, are performed for a mechanically agitated gas—liquid reactor and a bubble column at practical micro- and macromixing conditions. For a gas—liquid model reaction, which is first-order in the gas phase reactant, it is found that: (1) for a mechanically agitated reactor the error in a chem will always be smaller than 10% if Ω A is lower than 0.99, and (2) for a bubble column the error in a chem will be smaller than 20% for most practical applications if Ω A is lower than 0.99. Gas—liquid model reaction systems with absorption of CO 2 in alkanolamine solutions are recommended for the determination of interfacial area in gas—liquid contactors.

New chemical systems for the determination of liquid—side mass transfer coefficient and effective interfacial area in gas—liquid contactors

Two new reactive systems have been used to determine liquid—side mass transfer coefficients and effective interfacial area in gas—liquid contactors

Interfacial areas in gas—liquid stirred vessels

The light transmission technique is used to measure interfacial areas in stirred gas—liquid reactors under various conditions of pressure and temperature. Interfacial area increases as pressure increases and in certain cases this increase can be as much as 75% above that obtained under similar operating conditions but at atmospheric pressure. Gas density will increase with pressure and if the gas enters saturated with solvent there will be a further increase in density due to this factor. This increase in gas density increases the kinetic energy of the gas at the inlet sparger. The increase in interfacial area can be correlated with this kinetic energy. Calderbank's correlation for predicting interfacial area has been independently verified. A modification of this correlation is proposed to account for the effects of pressure and temperature. The modified correlation has a wide range of applicability. The data presented has potential use in establishing better design criteria for gas—liquid contacting in pressurized stirred vessels.

Desorption of isoamylene from loaded acid liquors

The desorption of 2-methyl-2-butene (isoamylene) from loaded sulphuric acid solutions, having an acid strength of about 60 per cent (w/w) and loading upto 0·45 g mole of isoamylene per g mole of H 2SO 4, was found to be preceded by a fast reaction, which occurred in the film adjacent to the interface. The specific rates of desorption of isoamylene into inert hydrocarbons— n-heptane and toluene and an inert gas—nitrogen, were found to be proportional to the isoamylene concentration j, expressed as g mole of isoamylene per g mole of H 2SO 4, and agreed among themselves at the same value of j. The technique of desorption preceded by a fast reaction was employed for the measurement of effective interfacial area in liquid—liquid and gas—liquid agitated contactors. The absorption of isobutylene into fresh and loaded solutions of sulphuric acid was also used for the measurement of effective interfacial area in gas—liquid agitated contactors for comparative purposes. The values of effective interfacial area for the gas—liquid system obtained by the desorption technique were found to be comparable with those obtained from the absorption of isobutylene in fresh and loaded solutions of sulphuric acid, under otherwise comparable conditions.

Chemical methods of the determination of liquid—side mass transfer coefficient and effective interfacial area in gas—liquid contactors

Chemical methods of the determination of liquid—side mass transfer coefficient and effective interfacial area in gas—liquid contactors