Gas-liquid transfer

A second-order reaction takes place in a two-phase batch system. Reactant A is supplied by gas-liquid transfer and reactant B supplied by liquid feed. The model equations are... 

As shown below, the influence of three quite distinct dynamic processes play a role in the overall measured oxygen concentration response curve. These are the processes of the dilution of nitrogen gas with air, the gas-liquid transfer, and the electrode response characteristic, respectively. Whether all of these processes need to be taken into account when calculating KLa can be seen by examining the mathematical model and making simulations. 

For the well-mixed continuous-flow liquid phase shown in Fig. 1, the balance equations for oxygen and substrate must account for the supply of each component both by convective flow and by gas-liquid transfer, as well as by the diffusion rate into the biofilm. 

At the chosen acetophenone concentration (0.3mol.l ), the reaction is not limited by gas-liquid transfer for a stirring speed comprised Detween 1500 and 2000 rpm and the reaction rate is independent of the catalyst weight (0.1[Pg.246]

The overall rate of reaction calculated for the three-phase fluidised-bed reactor above is approximately one tenth of the rate calculated for the agitated tank slurry reactor in Example 4.6. The main reasons are the very poor effectiveness factor and the relatively smaller external surface area for mass transfer caused by using the larger particles. Even the gas-liquid transfer resistance is greater for the three-phase fluidised-bed, in spite of the larger particles being able to produce relatively small bubbles these bubbles are not however as small as can be produced... 

In deriving the material balance equations, the dispersed plug flow model will first be used to obtain the general form but, in the numerical calculations, the dispersion term will be omitted for simplicity. As used previously throughout, the basis for the material balances will be unit volume of the whole reactor space, i.e. gas plus liquid plus solids. Thus in the equations below, for the transfer of reactant A kLa is the volumetric mass transfer coefficient for gas-liquid transfer, and k,as is the volumetric mass transfer coefficient for liquid-solid transfer. 

If we consider a gas-liquid transfer for the species i in a hollow-fiber module with the liquid phase in the shell side and the gas phase in the lumen side of hydrophobic membranes, the interface is established at the outer diameter of the fibers and the overall mass-transfer coefficient can be calculated by [1] ... 

When one or more reactants are fed as a gas, the possible effects of the transfer from gas to liquid have also to be considered, of course. Here, too, correlations allowing the calculation of the corresponding transfer coefficients are available. Gas-liquid transfer effects can be easily diagnosed experimentally, however. The absence of gas-liquid transfer effects can be assumed if the observed specific rate is independent of the stirring frequency and /or of the catalyst concentration over the range of conditions investigated. 

Environmental monitoring has also taken advantage of acoustic levitation for the investigation of physico-chemical processes relevant to the troposphere — mainly at temperatures below 0°C. Gas-liquid transfer of H2O2 from the gas phase to the levitated droplet was studied from in situ chemiluminescence measurements. Also, freezing of stably positioned droplets was observed by means of a microscope and a video camera, and the usefulness of this technique for simulation and investigation of cloud processes thus demonstrated. Ex situ microanalysis of sub-microlitre droplets by the use of an optical fibre luminometer also proved an effective means for investigating important physicochemical processes at the micro scale [100]. 

The optimal configuration depends on numerous factors, including the required gas transfer and mixing rates, and an acceptable range of shear rates. Some factors may be mutually contradictory as in a crystallizer where the required gas-liquid transfer rate is best achieved by increasing turbulence and shear yet the crystals are shear sensitive. In these cases, successful design carefully balances the contradictory factors. 

The spectrophotometric method for the determination of ozone in ozonized air current by measurement of its corresponding iodide-starch complex at 580 nm using a FIA system with chemical gas-liquid transfer microreactor has been developed [1]. 

Another procedure for the determination of TOC and its fractions in industrial effluent samples has been recently introduced [129]. A flow injection system using a gas-liquid transfer microreactor is developed, and adapted to a turbidimetric spectrophotometer. Samples are decomposed into glass vials in a microwave oven and a fraction of the CO2 is injected into a carrier gas and pumped to a glass microreactor. With minor modifications, the system allows the determination of different carbon fractions. The advantages of the proposed procedure are simplicity, low volume of samples and reagents, high frequency of determinations, and low cost. The dynamic range is 20-800 mg C/L, and the calculated LOD is 17 mg C/L. 

J. Paniz, E. Flores, V. Dressier, and A. Martins. Flow injection turbidimetric determination of total organic carbon with a gas-liquid transfer microreactor. Anal. Chim. Acta, 445 139-144, 2001. 

We know moreover that surfactant substances influence mass transfers, even gas-liquid transfers, as a result of modifications of the interfacial area and of the transfer coefficient (among others (J)). 

Keller, K. H., and Shultis, K. L. (1979). Oxygen permeability in ultrathin and mictoporous membranes during gas-liquid transfer. Trans. ASAIO 25, 469. 

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