Transport enhancement

Thus far only cases in which interaction between kinetic and transport factors has led to a limitation of the reaction have been presented. We now take up the case where the rate of transport is itself increased by the reaction taking place. 

The same method presented in Fig. 4.30 for solving the problem can also be used here. The only difference is that the effectiveness factor is referred to the 

Two extreme cases exist, as shown in Fig. 4.29 by concentration profiles c (for = 1) and e (for 1). As in all other cases, the relative magnitudes of the rate constants define the kinetic and diffusion-controlled regimes. It is, however, customary to describe the relative k values with a dimensionless number, Ha. For G L reactions with /cl ( li) 

The approximate solution is in error relative to the exact solution by only + 6%. The general solution is therefore also given in the form of a function of Ha 

The graphical representation of this solution is shown in Fig. 4.46. The region Ha 0.3 is the transport-limited regime, where the effective process rate is determined by physical adsorption. The reaction is slow here relative to the rate of transport It occurs completely in the liquid phase (bulk), and the reaction therefore has no influence on the transport rate. In the same regime, deviations due to different HI values of G L reactors (cf. Equ. 3.59) may occur, giving the curves shown in Fig. 4.56 (for smaller HI values). 

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A. A. Gregoli and A. M. Olah. Low-temperature pipeline emulsion transportation enhancement. Patent US 5156652,1992. 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Ultrasound frequency has revealed as the most important operational variable. Low frequency (20-60 kHz) has been most used to obtain mechanical effects such mass transport enhancement, shock waves, microjetting and surface vibration, especially used in the nanostructure preparation. It has been reported [118] that... 

Burns, M., Gaynor, K., Olm, V., Mercken, M., LaFrancois, J., Wang, L., Mathews, P.M., Noble, W., Matsuoka, Y., and Duff, K., Presenilin redistribution associated with aberrant cholesterol transport enhances beta-amyloid production in vivo, /. Neurosci., 23,5645,2003. 

In many lymphatic absorption studies a complex lipidic vehicle has been used, and the role of an individual component in the increased lymphatic transport is difficult to evaluate. However, some guidance regarding the nature of the lipid preferable for lymphatic transport enhancement can be drawn. 

Vanbever, R., M.R. Prausnitz, and V. Preat. 1997. Macromolecules as novel transdermal transport enhancers for skin electroporation. Pharm Res 14 638. 

Duo J, Fletcher H, Stenken JA. Natural and synthetic affinity agents as microdialysis sampling mass transport enhancers current progress and future perspectives. Biosensors Bioelectronics 2006, 22, 449-457. 

Choo EF, Leake B, Wandel C, et al. Pharmacological inhibition of P-glycoprotein transport enhances the distribution of HIV-1 protease inhibitors into brain and testes. Drug Metab Dispos 2000 28 655-660. 

The transport-enhancing effect of Zot was shown to be reversible and nontoxic (Fasano et al. 1991 Cox et al. 2002). More recently a smaller fragment of Zot in the size of 12 kDa referred to as /1G was identified (Di Pierro et al. 2001). AG displayed significant potential as permeation enhancer. In vitro studies showed that it is capable of significantly increasing the apparent permeability coefficients for a wide variety of drugs across Caco-2 monolayer (Salama et al. 2003, 2004). In the presence of peptidase inhibitors AG improved the bioavailability of mannitol, inulin and PEG 4000 after intraduodenal administration to rats (Salama et al. 2003, 2004). In another in vivo study the oral bioavailability of cyclosporin A was increased up to 50-fold due to the co-administration of AG when metabolic protection was provided (Salama et al. 2005). Results of this study are illustrated in Fig. 5.2. 

The effective diffusivity Dn decreases rapidly as carbon number increases. The readsorption rate constant kr n depends on the intrinsic chemistry of the catalytic site and on experimental conditions but not on chain size. The rest of the equation contains only structural catalyst properties pellet size (L), porosity (e), active site density (0), and pore radius (Rp). High values of the Damkohler number lead to transport-enhanced a-olefin readsorption and chain initiation. The structural parameters in the Damkohler number account for two phenomena that control the extent of an intrapellet secondary reaction the intrapellet residence time of a-olefins and the number of readsorption sites (0) that they encounter as they diffuse through a catalyst particle. For example, high site densities can compensate for low catalyst surface areas, small pellets, and large pores by increasing the probability of readsorption even at short residence times. This is the case, for example, for unsupported Ru, Co, and Fe powders. 

Paracellular transport Enhancement is achieved by modulating the tight junctions between the cells. Chelating agents such as ethylenediamine tetraacetic acid can chelate calcium ions and transiently open the tight junctions for drug... 

Ultrasound may enhance transdermal transport by inducing skin alteration and active transport (forced convention) in the skin. Various other means of transport enhancement, including chemicals, iontophoresis and electroporation, may enhance transport synergis-tically with US. Thus, the evaluation of the synergistic effect of low-frequency US with chemical enhancers and surfactants for permeation of mannitol revealed that application of US or sodium lauryl sulfate (SLS) alone, both for 90 min, increased skin permeability about 8 and 3 times, respectively. However, the combined use of US and a 1% SLS solution increased the skin permeability 200 times to mannitol [129]. 

Kim, Y. H., Ghanem, A. H., Mahmoud, H. and Higuchi, W. I. (1992a). Short-chain alkanols as transport enhancers for lipophilic and polar/ionic permeants in hairless mouse skin Mechanism(s) of action. nt. J. Pharm. 80 17-31. 

Mitragotri, S. Blankschtein, D. Langer, R. An explanation for the variation of the sonophoretic transdermal transport enhancement from drug to drug. J. Pharm. Sci. 1997, 86, 1190-1192. 

If the chemical reaction—we consider a water film—is fast compared to tlie transport time, the conditions used in defining equation 34 are no longer valid. In this case, we may assume immediate equilibrium between the species linkcxl by the fast reaction. The degree of mass transport enhancement caused by chemical reactions may be quantified by adding a chemical enhancement factor, a, to the term of liquid film resistance. 

An interesting example taking the opposite approach was presented by Wieczorek et al. [24]. To separate a mixture of dipeptide diastereoisomers and their phosphonic analogues, achiral crown ethers were used as transport enhancers. In this case, the diastereomeric complex is formed between the chiral transported molecule and not the optically active carrier. The observed stereoselectivity depends on the peptide structure and was independent of the presence of carrier, but the application of carrier increased the transport rate of both diastereoisomers. 

The use of anionic polymers coupled to catalytic enzymes or to catalytic chelating agents [105, 106], degradation-controUed drug elution, provides a complex system that behaves in catalytic conversion, selective transport enhancement and enrichment of products in simUar fashion to membrane reactors. Articles, deal with WSP hquid membrane reactors, were not found, nevertheless this direction is very outlook in the application of the BAHLM systems. 

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