Gas-phase mass transfer, rate

As not all of the parameters needed can be calculated in advance (kg can only be approximated, and a must be determined experimentally), the gas-phase mass transfer rate for a given scrubber design must be determined experimentally by operating the scrubber imder conditions where y approaches zero. Under these conditions, Eq. (12) can be integrated to give ... 

For GL reactions, whether the reaction is controlled by gas phase mass transfer, rate of mass transfer through the liquid film resistance at the surface or the reaction rate affects the configuration we select for the reactor. Two parameters that show where the reaction occurs are the Hatta number, Ha, and the dimensionless bulk/film volume ratio (ratio of the total liquid volume to the film volume),... 

From the experimental results obtained in the trickle flow reactor and model predictions, it could be concluded that for the H2S—O2 reaction, no mass transfer limitations are to be expected below about 200°C (Figure 22.14). However, at about 300°C, both external mass transfer and diffusion within the dense solids suspension are likely to offer substantial resistance to the reaction. Ultimately, at higher temperatures, the conversion rate can be determined by gas-phase mass transfer rate only, similar to the much faster reaction of H2S with SO2 (Figure 22.14). 

The gas-phase mass-transfer rate is much greater than the liquid-phase mass-transfer rate. Thus, the surface concentration (at z = 0) is constant at surf be relaxed in Section 15.5.31. 

Another interesting selectivity problem arises when there are two different reactants in the supply phase, say A and C, that both react with the liquid phase reactant B, forming P and Q respectively, where the formation of Q is undesired. An example of practical importance is the selective absorption of hydrogen sulfide from an inert gas containing also carbon dioxide, in an alkaline solution (containing, e.g., alkanol amines). Conditions can be such that carbon dioxide (C) reacts rapidly with the alkanol amine, whereas hydrogen sulfide (A) reacts instantaneously. The consequence is that the absorption rate of hydrogen sulfide is practically determined by the gas phase mass transfer rate, and Ae rate of carbon... 

It should be noted that the highest possible absorption rates will occur under conditions in which the hquid-phase resistance is negligible and the equilibrium back pressure of the gas over the solvent is zero. Such situations would exist, for instance, for NH3 absorption into an acid solution, for SO9 absorption into an alkali solution, for vaporization of water into air, and for H9S absorption from a dilute-gas stream into a strong alkali solution, provided there is a large excess of reagent in solution to consume all the dissolved gas. This is known as the gas-phase mass-transfer limited condition, wrien both the hquid-phase resistance and the back pressure of the gas equal zero. Even when the reaction is sufficiently reversible to allow a small back pres-... 

If the liqmd-phase reaction is extremely fast and irreversible, the rate of absorption may in some cases be completely governed by the gas-phase resistance. For practical design purposes one may assume (for example) that this gas-phase mass-transfer limited condition will exist when the ratio yj/y is less than 0.05 everywhere in the apparatus. 

Whenever these conditions on the ratio yjy apply, the design can be based upon the physical rate coefficient /cg or upon the height of one gas-phase mass-transfer unit He- The gas-phase mass-transtor hmited condition is approximately vahd, for instance, in the following systems absorption oi NH3 into water or acidic solutions, vaporization of water into air, absorption of H9O into concentrated sulfuric acid solutions, absorption of SO9 into alkali solutions, absorption of H9S from a dllute-... 

The net reaction is the transfer of C02 at a rate close to 1 mole per 2 Faradays, and the production of water. The process is quite complex the detailed analysis showed that cathode-side, gas-phase mass transfer of C02 was totally controlling only at the lowest C02 levels and high current densities. At other conditions chemical reaction rates and transport through the membrane became important representative results... 

JA = flux rate of component A [moles (total moles)-1 s-1 m-2] k1A = gas phase mass transfer coefficient (s-1 m-2) k2A = water phase mass transfer coefficient (s-1 m-2)... 

Furthermore, a liquid-phase distributor is used on the top of the bed and the overall gas-phase mass transfer coefficient was experimentally measured as 0.153 s 1 for liquid flow rate equal to 14 X 10 5 m3/s. Under these conditions, the experimental value of sulfur dioxide conversion was approximately 18%. 

SER is the specific emission rate, D is the diffusion coefficient, 8is the thickness of the boundary layer, Cs is the concentration of the target VOC at the source surface and Q is the concentration of the target VOC in the air and fcg is the gas-phase mass transfer coefficient. 

Sparks, L.E., Tichenor, B.A., Chang, J. and Guo, Z. (1996) Gas-phase mass transfer model for predicting volatile organic compound (VOC) emission rates from indoor pollutant sources. Indoor Air, 6, 31 40. 

Gas phase mass transfer fluxes (Stefan flux in the gas phase is negligible as long as the vapor phase mole fractions are below say 20 %, which means either moderate temperatures and/or high sweep gas flow rates) are ... 

Figure 4.24 shows the reactive arheotrope trajectories according to Eq. (83) for various amounts of the liquid phase mass transfer resistance - that is, for various values of Kiiq and a low sweep gas flow rate G (at large NTt/ -values). As a result, the reactive arheotropic composition X, 02 is shifted to larger values as the liquid phase mass transfer resistance becomes more important - that is, as the value of Kuq decreases. Note that the interface liquid concentrations are in equilibrium with the vapor phase bulk concentrations. Therefore, gas phase mass transfer resistances cannot have any influence on the position of the reactive arheotrope compositions. On the other hand, liquid phase mass transfer resistances do have an effect, though the value of all binary hiq have been set equal. Again, this effect results from the competition between the diffusion fluxes and the Stefan flux in the liquid phase. 

The chemical reactions considered in the model are listed in Table I, together with the appropriate constants used in the calculation. The rate expression for gas-phase mass transfer is given by... 

Effects of Temperature on kG and k, The Stanton-number relationship for gas-phase mass transfer in packed beds, Eq. (5-301), indicates that for a given system geometry the rate coefficient kG depends only on the Reynolds number and the Schmidt number. Since the Schmidt number for a gas is approximately independent of temperature, the principal effect of temperature upon kG arises from changes in the gas viscosity with changes in temperature. For normally encountered temperature ranges, these effects will be small owing to the fractional powers involved in Reynolds-number terms (see Tables 5-17 to 5-24). It thus can be concluded that for all... 

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