Systems with Interfacial Mass-Transfer Resistances

Experimental study of more systems with interphase mass transfer, with the aim of correlating interfacial resistance with other physical properties. 

Immobilization onto a solid support, either by surface attachment or lattice entrapment, is the more widely used approach to overcome enzyme inactivation, particularly interfacial inactivation. The support provides a protective microenvironment which often increases biocatalyst stability, although a decrease in biocata-lytic activity may occur, particularly when immobilization is by covalent bonding. Nevertheless, this approach presents drawbacks, since the complexity (and cost) of the system is increased, and mass transfer resistances and partition effects are enhanced [24]. For those applications where enzyme immobilization is not an option, wrapping up the enzyme with a protective cover has proved promising [21]. 

In the group with positive spreading coefficients (e.g., toluene-in-water and oleic acid-in-water emulsions), the values ofkj a in both stirred tanks and bubble columns decrease upon the addition of a very small amount of oil, and then increase with increasing oil fraction. In such systems, the oils tend to spread over the gas-liquid interface as thin films, providing additional mass transfer resistance and consequently lower k values. Any increase in value upon the further addition of oils could be explained by an increased specific interfacial area a due to a lowered surface tension and consequent smaller bubble sizes. 

With a finely divided solid catalyst as typically used In the Flscher-Tropsch synthesis In slurry reactors It Is generally agreed that the major mass-transfer resistance, If It occurs, does so at the gas-liquid Interface. There are considerable disagreements about the magnitude of this resistance that stem from uncertainties about certain physical parameters, notably interfacial area, but also the solubility and mass transfer coefficients for H2 and CO that apply to this system. However when this resistance Is significant, the concentrations of Hg and CO in the liquid in contact with the solid catalyst become less than they would be otherwise, which not only reduces the observed rate of reaction but can also affect the product selectivity and the rate of formation of free carbon. 

PTC incorporated with other methods usually greatly enhances the reaction rate. Mass transfer of the catalyst or the complex between different phases is an important effect that influences the reaction rate. If the mass transfer resistance cannot be neglected, an improvement in the mass transfer rate will benefit the overall reaction rate. The application of ultrasound to these types of reactions can be very effective. Entezari and Keshavarzi [12] presented the utilization of ultrasound to cause efficient mixing of the liquid-liquid phases for the saponification of castor oil. They used cetyltrimethylammo-nium bromide (CTAB), benzyltriethylammonium chloride (BTEAC), and tetrabutylammonium bromide (TBAB) as the catalysts in aqueous alkaline solution. The more suitable PT catalyst CTAB can accumulate more at the liquid-liquid interface and produces an emulsion with smaller droplet size this phenomenon makes the system have a high interfacial surface area, but the degradation of CTAB is more severe than that of BTEAC or TBAB because of more accumulation at the interface of the cavity under ultrasound. 

Effect of gas-liquid mass transfer Different opinions on the importance of mass transfer limitations have been uttered by Satterfield and Huff (81,82,94) and Zaldl et al. (54) and Deckwer et al. (87). Satterfield and Huff (81) assxuned a bubble diameter of eUaout 2 mm and concluded that the FTS in slurry phase may be significantly limited by gas-liquid mass transfer. New experimental results do, however, confirm that in the molten paraffin system bubble diameters are less than 1 mm (53-55). With this low value and the high gas holdup observed in FT liquid f ase, i.e., eq. (8), high interfacial areas are obtained. Therefore slgniflccuit mass transfer can be excluded and for the catalyst systems studied until now the FT process in slurry phase is mainly reaction controlled (54,87). In addition, it follows from the reactor model used by Satterfield and Huff (81) that the relative mass transfer resistance is given by (95)... 

Later publications have been concerned with mass transfer in systems containing no suspended solids. Calderbank measured and correlated gas-liquid interfacial areas (Cl), and evaluated the gas and liquid mass-transfer coefficients for gas-liquid contacting equipment with and without mechanical agitation (C2). It was found that gas film resistance was negligible compared to liquid film resistance, and that the latter was largely independent of bubble size and bubble velocity. He concluded that the effect of mechanical agitation on absorber performance is due to an increase of interfacial gas-liquid area corresponding to a decrease of bubble size. 

The majority of RDC studies have concentrated on the measurement of solute transfer resistances, in particular, focusing on their relevance as model systems for drug transfer across skin [14,39-41]. In these studies, isopropyl myristate is commonly used as a solvent, since it is considered to serve as a model compound for skin lipids. However, it has since been reported that the true interfacial kinetics cannot be resolved with the RDC due to the severe mass transport limitations inherent in the technique [15]. The RDC has also been used to study more complicated interfacial processes such as kinetics in a microemulsion system [42], where one of the compartments contains an emulsion. 

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

Where and LH are the corresponding activation energy and enthalpy of phase transition and the coefficient defines the maximum probability that molecules will cross the interface between the liquid and SCF (vapor) phases. This simple relationship can explain the behavior of the mass transfer coefficient in Figure 15 when it is dominated by the interfacial resistance. Indeed, increases with temperature T according to Eq. (49) also, both parameters E and A// should decrease with increase of pressure, since the structure and composition of the liquid and vapor phases become very similar to each other around the mixture critical point. The decrease of A/f with pressure for the ethanol-C02 system has been confirmed by interferometric studies of jet mixing described in Section 3.2 and also by calorimetric measurements described by Cordray et al. (68). According to Eq. (43) the diffusion mass transfer coefficient may also increase in parallel with ki as a result of more intensive convection within the diffusion boundary layer. 

It seems that the role of micelles was first considered by Goldberg et al., ° who stndied transport of micelle-solnbilized drugs from the aqueous environment to an oil phase dispersed as drops. They measured mass transfer rates in two systems where solnbility of the oil in the aqneons phase was negligible. Their model included partitioning between the bnlk aqneons phase and micelles and both diffnsional and interfacial resistances to mass transfer. Agreement with experimental data was best when interfacial resistance to mass transfer was limiting. 

Although the phenomena of transport of various species to (or from) the interface and the simultaneous reaction can be accounted for by the theories of gas-liquid systems, the third phenomenon crossing of the interface may require a different treatment. Firstly, in cases of heavily contaminated liquid-liquid systems, interfacial resistance may no longer be negligible. Secondly interfacial turbulence, which is produced by the interaction of mass transfer with interfacial tension, is in many cases very important. Indeed, there is very limited information in the role of this and certain other secondary phenomena in extraction with reaction (9). 

In this system, the reaction of hydrolyzed CO2 with hydroxyl ions is moderately rapid, and the partial pressure of CO2 in equilibrium with the bulk liquid is zero for practical purposes. The effect of the liquid-phase reaction is to reduce substantially the liquid-film resistance, because the distance the absorbed solute must diffuse is only about 10% of that customary for a simple physical absorption. The liquid-film mass transfer coefficient is a function of liquid flow rate [4]. The liquid-film mass transfer coefficient is constant at any fixed percentage carbonation and almost independent of gas flow rate below the loading region. This system, therefore, is used to measure the relative interfacial area for different packings as influenced by the liquid flow rate. 

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