Chemically enhanced mass transfer

Selectivity problems in situations with chemically enhanced mass transfer are treated in section S.4.2.2. 

The prediction of the effects of axial mixing in fluidized beds is very difficult. In practical models both residence time distribution, chemically enhanced mass transfer from bubbles to dense phase, and intraparticle diffusion are taken into account. 

We find that until a conversion of 0.8 the first plot yields a straight line, and the other one does not. For the last 10% of the conversion the second plot yields a straigh line, with an inclination of 1/3. These results indicate that in the first part of the process eq. (S.25) applies, and consequently, that the reaction in die bulk is rate determining. The reason that the kinetics change towards the end of the process can only be explained by the fact that die surface area of the solid has become a rate determining factor. Probably chemically enhanced mass transfer takes place here, see section 5.4.2.1, age 153. A correlation of the type of (5.12) applies. By extrapolation of the second line, we find that the apparent dissolution time of the second process is 25 minutes. 

As a first example, consider the case where eq. (5.12) applies (surface reaction, or chemically enhanced mass transfer). We find for the degree of conversion of the solid reactant B, in the exit flow of one CSTR, as a function of the ratio of the... 

An important mixing operation involves bringing different molecular species together to obtain a chemical reaction. The components may be miscible liquids, immiscible liquids, solid particles and a liquid, a gas and a liquid, a gas and solid particles, or two gases. In some cases, temperature differences exist between an equipment surface and the bulk fluid, or between the suspended particles and the continuous phase fluid. The same mechanisms that enhance mass transfer by reducing the film thickness are used to promote heat transfer by increasing the temperature gradient in the film. These mechanisms are bulk flow, eddy diffusion, and molecular diffusion. The performance of equipment in which heat transfer occurs is expressed in terms of forced convective heat transfer coefficients. 

From the above it can be concluded that only the reaction with component B may enhance mass transfer of ozone substantially. And only if the Hatta number HaB is much higher than 1. Therefore it can be expected that whenever we have to deal with an enhancement of mass transfer due to chemical reactions, this influences the selectivity of the oxidation process in a negative way. The factor which has to be considered in this respect is the Hatta number for the reaction of ozone with component B (equation 29). HaB increases with increasing value of kB and CBb and with decreasing value of the mass transfer coefficient for ozone, kHq,... 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

Every ozonation process where gaseous ozone is transferred into the liquid phase and where it subsequently reacts, involves physical and chemical processes which need to be considered in modeling. Physical processes include mass transfer and hydrodynamic properties of the reaction system, e. g. gas- and liquid-phase mixing. Chemical processes include, ideally, all direct and/or indirect reactions of ozone with water constituents. Of course these processes cannot be seen independently. For example, fast reactions can enhance mass transfer. 

Enhanced mass transfer performance results from the initial contact of the liquid feed with the rotor. Studies at the Higravitec Center of Beijing University of Chemical Technology (HCBUCT), using a video camera attached to the rotor, revealed a breakup of the liquid feed into smaller droplets that filled the void spaces... 

Absorption of a component of a gas stream into a liquid is a common practice in the chemical industry to affect cleanup of vent gases, conduct chemical reactions, purify products, or to recover products from process streams. The enhanced mass transfer capability of RPBs provides the opportunity to perform absorption processes in smaller equipment, to lower inventories, to shorten startup and shutdown times, and to lower pressure drop (48). Figure 8 provides a visual comparison of the size of a conventional absorber tower next to three RPBs that handle the equivalent gas and liquid flows (9). 

Sieve tray extractors are popular in the chemical and petrochemical industries. The trays minimize axial mixing, which results in good scale-up from laboratory data. The dispersed phase drops re-form at the each perforation, rise (or fall) near their terminal velocity, and then coalesce underneath (or above) the tray, as shown in Figure 14.14(d). The coalesced layer is important to prevent axial mixing of the continuous phase and to allow re-formation of the drops, which enhances mass transfer. The continuous phase passes... 

Gallagher PM, Coffey MP, Krukonis VJ, Klasutis N. Gas anti-solvent recrystallization new process to recrystallize compounds insoluble in supercritical fluids. In Johnston KP, Penniger JML, eds. Supercritical Fluid Science and Technology. Washington, DC American Chemical Society, 1989 334-354. Dixon D, Johnston KP, Bodmeier R. Polymeric materials formed by precipitation with a compressed fluid antisolvent. AIChE J 1993 39 127-136. Chattopadhyay P, Gupta RB. Production of griseofulvin nanoparticles using supercritical CO2 antisolvent with enhanced mass transfer. Int J Pharm 2001 228 19-31. 

Two aspects of velocity that need to be kept in mind in a consideration of the effects of velocity in connection with chemical reaction fouling are the effects on mass transfer and heat transfer. The transfer of mass and heat will be increased with increased turbulence. If the fouling deposition is solely controlled by the reaction rate then enhanced mass transfer to the surface will not change the situation. On the other hand if some of the products of reaction or intermediates even, remain in the reaction zone then the rate of production of deposits could be reduced. Increased turbulence (i.e. increased flow rate for a given geometry) will assist not only the rate of arrival of reactants but also the removal of products of reaction from the reaction zone. It is possible that in part, this may account for some of the contradictions apparent in the literature in respect of flow rate. 

In order to achieve a separation of chemical species, a potential must exist for the different species to partition between the two phases to different extents. This potential is governed by equilibrium thermodynamics, and the rate of approach to the equilibrium composition is controlled by interphase mass transfer. By intimately mixing the two phases, we enhance mass transfer rates, and the maximum degree of partitioning is more quickly approached. After sufficient phase contact, the separation operation is completed by employing gravity and/or a mechanical technique to disengage the two phases. 

Concentration Step. The concentfation step technique is a rarely used technique that deoxygenates the liquid phase by the addition of a small amount of a chemical compound like sulfite without interrupting aeration. This technique should not be confused with the chemical sorption methods as only a small amount of the compound is added with the intent of causing a dissolved gas concentration step change. For this method to work properly, the system must be very well mixed to ensure uniform dissolved gas concentrations. Also, care must be taken to ensure that the chemical compound being added does not alter the hydrodynamics or enhance mass transfer rates. 

Schonfeld, H., Hunger, K., Cecilia, R., and Kunz, U. (2004), Enhanced mass transfer using a novel polymer/carrier microreactor, Chemical Engineering Journal, 101(1-3) 455-463. 

If a substance is transferred from the gas phase into the liquid phase and bonded chemically, the mass transfer rate increases. The influence of the reaction on the mass transfer is defined by an enhancement factor E given in Eq. (3-33). In the case of a chemical reaction superimposed on the mass transfer is... 

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