Oxygen Mass Transfer Limitations

It is clear from the previous section that the gas flow, temperature, and pressure are bound both by the chemical reaction and by safety considerations. As was already noted, the optimized reaction temperature for selective oxidation of furanic compounds is considerably lower than that for para-xylene (see Tables 19.2 and 19.3). Operating at lower temperature will lower the vapor pressure of the solvent (acetic acid) and thus also lower overall reactor pressure. Consequently, the partial oxygen pressure is also reduced, which can be detrimental to the performance of the oxidation process as the oxygen partial pressure is the driving force to transfer the oxygen from the gas to the liquid phase (NjJ following Eq. (19.1)  

To compensate for a reduced gas- to liquid phase oxygen transfer driving force that results from lower temperatures, technical solutions to improve the oxygen mass transfer from gas to liquid will have to be used (e.g., efficient stirrer designs). 

After the established industrial use of the Co/Mn/Br catalysis system for para-xylene oxidation and its anticipated industrial use for oxidation of HMF and derivatives, we can ask the question whether this catalyst and process can also be applied to other bio-based target molecules. Surprisingly, few Co/Mn/Br oxidations to other bio-based targets have been described so far. Only LA oxidation to SA [41] and oxidative treatment of lignin have been reported to date [42]. 

Another aspect to consider is that the selectivity in the TA and FDCA systems in part may also be due to the insolubility ofthe diacids formed. Thus, the product is removed before it can be oxidized further. 

The Co/Mn/Br system that was originally developed and further optimized by Amoco, Mid-Century, BP, Eastman Chemicals, and others for the oxidation of para-xylene to TA is a very powerful oxidation tool, but its implementation is not straightforward and restricted by the operational window, whereas its application is limited to specific feedstock that allow sufficient selectivity in the radical chain mechanism. There is an opportunity for the highly selective HMF and derivatives oxidation to FDCA and there may be a potential use in lignin oxidation. 

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The selective oxidation of lactose to lactobionate with air on palladium-bismuth catalysts was first reported in patents [47]. Hendriks et al. [16] studied the oxidation of a 0.5 mol solution of lactose as a function of pH, temperature and Pd/Bi ratios of promoted Pd/C catalysts. Sodium lactobionate was obtained with 100% selectivity up to 95% conversion on Pd-Bi/C catalysts (Bi/Pd = 0.5) at 333 K and pH 9. Oxygen mass-transfer limited the maximum initial reaction rate (0.47 mol kg s ). The catalyst was recycled 15 times without any significant loss of activity and selectivity. 

Much as bacteria catalyze the anode reaction in an MFC, bacteria can also catalyze the terminal reduction reaction at the cathode (Rabaey et al., 2003). Biocathodes are both low-cost and sustainable making them an attractive research area. However, biocathodes have not made much of an impact in the MFC field yet as power densities to date have been quite low. These low power densities appear to be mainly a product of oxygen mass transfer limitations associated with diffusion through a biofilm (Clauwaert et al., 2007). Measured current densities for biocathodes were significantly lower than their Pt counterparts but may have other advantages such as being able to reduce several different oxidized contaminates, such as Cr(IV) and Cr(OH)3 (Clauwaert et al., 2007 Ter Heijne, 2010). 

The oxygen consumption by the oxidation of the acetic acid was maximal at a solids temperature of about 440 °C, while the maximal mass transport over the gas film at this temperature was a factor of 10 higher. Hence, no oxygen mass transfer limitation occurred for the kinetics and mass transfer enhancement assumed. 

Reactor Configuration. The horizontal cross-sectional area of a reactor is a critical parameter with respect to oxygen mass-transfer effects in LPO since it influences the degree of interaction of the two types of zones. Reactions with high intrinsic rates, such as aldehyde oxidations, are largely mass-transfer rate-limited under common operating conditions. Such reactions can be conducted effectively in reactors with small horizontal cross sections. Slower reactions, however, may require larger horizontal cross sections for stable operation. 

The minimum oxygen utihsation rate is xjjbnvJY0l. If the system is mass-transfer limited, C, approaches zero. Then the amount of oxygen absorbed is exactly equal to the amount of oxygen consumed. Equation (3.11.8) leads to the following ... 

Checking the absence of internal mass transfer limitations is a more difficult task. A procedure that can be applied in the case of catalyst electrode films is the measurement of the open circuit potential of the catalyst relative to a reference electrode under fixed gas phase atmosphere (e.g. oxygen in helium) and for different thickness of the catalyst film. Changing of the catalyst potential above a certain thickness of the catalyst film implies the onset of the appearance of internal mass transfer limitations. Such checking procedures applied in previous electrochemical promotion studies allow one to safely assume that porous catalyst films (porosity above 20-30%) with thickness not exceeding 10pm are not expected to exhibit internal mass transfer limitations. The absence of internal mass transfer limitations can also be checked by application of the Weisz-Prater criterion (see, for example ref. 33), provided that one has reliable values for the diffusion coefficient within the catalyst film. 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

The shell progressive model in Example 11.15, part (b) envisions a mass transfer limitation. Is the limitation more likely to be based on oxygen diffusing in or on the combustion products diffusing out ... 

Monitoring of the oxygen pressure during reaction indicated that the rate of conversion of glycerol to glyceric acid under basic conditions (see section 3.3) was limited by oxygen mass transfer. All other reactions were free from gas-liquid diffusion control but this does not exclude the possible limitation by intra-porous diffusion. 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

In 1976 he was appointed to Associate Professor for Technical Chemistry at the University Hannover. His research group experimentally investigated the interrelation of adsorption, transfer processes and chemical reaction in bubble columns by means of various model reactions a) the formation of tertiary-butanol from isobutene in the presence of sulphuric acid as a catalyst b) the absorption and interphase mass transfer of CO2 in the presence and absence of the enzyme carboanhydrase c) chlorination of toluene d) Fischer-Tropsch synthesis. Based on these data, the processes were mathematically modelled Fluid dynamic properties in Fischer-Tropsch Slurry Reactors were evaluated and mass transfer limitation of the process was proved. In addition, the solubiHties of oxygen and CO2 in various aqueous solutions and those of chlorine in benzene and toluene were determined. Within the framework of development of a process for reconditioning of nuclear fuel wastes the kinetics of the denitration of efQuents with formic acid was investigated. 

Using 324 measuring points taken at temperatures between 35 and 75 °C, hydrogen concentrations between 1.6 10-3 and 11.0 10-3 mol NdnT3 and oxygen concentrations between 1.7 10 3 and 7.3 10 3 mol Ndm-3, a kinetic expression for the reaction was determined on the basis of a Langmuir-Hinshelwood model (Figure 2.30). The Mears criterion was applied to verify that no mass transfer limitation was to be expected for the system from the gas phase to the non-porous catalyst ... 

To increase the efficiency of the electrochemical wastewater treatment process with conventional anodic materials, the mediated oxidation method has been proposed. This method avoids the production of oxygen, thanks to the generation of precursors that are successively transformed to active oxidants. When the BDD anodes are used, a positive contribution of the generated active oxidants can also be foreseen, but only in the previously defined region IV of the treatment. The production of strong oxidants in this region avoids the mass-transfer limitation and treatment efficiency is recovered. 

The mass transfer equation is written in terms of the usual assumptions. However, it must be considered that because the concentration of the more abundant species in the flowing gas mixture (air), as well as its temperature, are constant, all the physical properties may be considered constant. The only species that changes its concentration along the reactor in measurable values is PCE. Therefore, the radial diffusion can be calculated as that of PCE in a more concentrated component, the air. This will be the governing mass transfer mechanism of PCE from the bulk of the gas stream to the catalytic boundaries and of the reaction products in the opposite direction. Since the concentrations of nitrogen and oxygen are in large excess they will not be subjected to mass transfer limitations. The reaction is assumed to occur at the catalytic wall with no contributions from the bulk of the system. Then the mass balance at any point of the reactor is... 

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