Air-side mass transfer

This states that the concentration of the lower chlorinated PCDD/Fs in vegetation is the time integral of the product of the air-side mass transfer coefficient (the inverse of the air-side resistance), the specific surface area of the vegetation and the gaseous air concentration. 

In this expression, Area is the horizontal area of contact between two compartments, m Ca is the bulk contaminant concentration in the air compartment, mol m and Cs is the bulk contaminant concentration in the surface soil compartment, mol m . Table 8.3 provides definitions of the fugacity capacities Zair, Zs, Za, and Zap. Table 8.2 provides suggested values for Va, PC, pap, and rain. Chapter 6 gives more details on obtaining values for these deposition parameters. But the key parameters in this expression are the air-side mass-transfer coefficient (MTC) Ua and the soil-side MTC f/s (m d ) as given below ... 

Measurement and Prediction of Air-Side Mass Transfer Coefficients..234... 

MEASUREMENT AND PREDICTION OF AIR-SIDE MASS TRANSFER COEFFICIENTS... 

Table 19.2 summarizes air-side MTCs obtained from various literature sources. The MTCs increase as a function of wind speed windy-daytime represented by 5-10 mile h (8-16 km h ) winds and calm nighttime represented by 0.5-3.0 mile h (0.8-5 kmh ) winds with average MTCs of 500 and 100 cmh", respectively. This is equivalent to air-side resistances (i.e., I/ a) of 0.002-0.01 hern", respectively. Therefore, as with other interfacial exchanges, the air-side mass transfer controls the kinetics of chemical sorption to the surface film for lower vapor pressure SOCs whereas the reverse is true for higher vapor pressure volatile organic compounds... 

Values of the Air-Side Mass Transfer Coefficients for SOC for Wind Speeds <20 mph ... 

Air is commonly run with tube-side feed. The permeate is run countercurrent with the separating sldn in contact with the permeate. (The feed gas is in contact with the macroporous back side of the membrane.) This configuration has proven to be superior, since the permeate-side mass-transfer problem is reduced to a minimum, and the feed-side mass-transfer problem is not limiting. 

Now we need the air-side and the water-side mass transfer velocities ... 

The second generation of nonporous membranes was silicon based which displayed increased CO2 permeabilities. In 1965, Bramson et al. commercialized the first nonporous membrane BO [18]. Since the diffusion coefficient of oxygen and carbon dioxide in air is about four orders of magnitude higher than in blood, the gas side mass-transfer resistance was negligible. The major resistance to respiratory gas transfer was due to the membrane and the liquid side concentration boundary layer [19]. Though nonporous membrane BOs reduced blood damage, up to 5.5 m membrane surface area was often required to ensure adequate gas transfer rates. 

The literature on measurement of mass transfer in vertical tubular reactors is very sparse. Kasturi and Stepanek (K3, K4) have presented data for a, ki a, and kca measured under identical conditions in the case of annular flow, annular spray flow, and slug flow. For the aqueous systems used (COj, air, NaOH) they have proposed the following correlation for the interfacial area fl = 0.23[(l - a)/QJ(AP/Z)i( whereQt is incm /sec and AP/Z is in N/m . Correlations for true liquid-side and gas-side mass-transfer coefficients by the same authors are difficult to generalize, as viscosity and surface tension were not varied. 

The rise velocity in water of an air bubble 0.004 m in diameter is about 0.2 m/s. Estimate the liquid-side mass transfer coefficient k, for oxygen transfer at 23 °C,... 

There is considerable information available in the hterature on the design of ejectors (steam jet ejectors, water jet pumps, air injectors, etc.) supported by extensive experimental data. Most of this information deals with its use as an evacuator and the focus is on ejector optimization for maximizing the gas pumping efficiency. The major advantage of the venturi loop reactor is its relatively very high mass transfer coefficient due to the excellent gas-liquid contact achieved in the ejector section. Therefore, the ejector section needs careful consideration to achieve this aim. The major mass transfer parameter is the volumetric liquid side mass transfer coefficient, k a. The variables that decide k a are (i) the effective gas-hquid interfacial area, a, that is related to the gas holdup, e. The gas induction rate and the shear field generated in the ejector determine the vine of and, consequently, the value of a. (ii) the trae liquid side mass transfer coefficient, k. The mass ratio of the secondary to primary fluid in turn decides both k and a. For the venturi loop reactor the volumetric induction efficiency parameter is more relevant. This definition has a built in energy... 

The scant information regarding the influence of internal draft tubes on the liquid side mass transfer coefficient is of conflicting nature [67,70]. The following correlation, due to Bello et al. (71 ], may be used to estimate the volumetric mass transfer coefficient in air lift reactors with a downcomer and riser ... 

At a sufficiently high temperature, the evaporation rate from a wetted surface into its pure superheated vapor is higher than into perfectly dry air. The temperature at which this transition rate occurs is called the inversion value. Experimental values for this temperature range between 160 [103] and 230°C [104] in comparing steam with air. The inversion occurs because of the difference in properties between steam and air (which have different temperature coefficients) and the absence of a gas side mass transfer resistance when... 

For a given values of the gas and liquid volume flow rates, the film thickness 5 and the mean film flow rate v can be found with eqs. (4.67) and (4.68). If we assume that there are no ripples, we find that for an air-water system the effect of the gas flow on the film Aicbiess and film velocity may be considerable when the gas flow rate is on the order of 30 m/s or higher. When the film flow is laminar, the liquid side mass transfer coefficient k can be estimated by... 

Example 8.5-1 Oxygen mass transfer Use Equation 8.5-9 to estimate the overall liquid-side mass transfer coefficient at 25 °C for oxygen from water into air. In this estimate,... 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

In a packed column, operating at approximately atmospheric pressure and 295 K, a 10% ammonia-air mixture is scrubbed with water and the concentration of ammonia is reduced to 0.1%. If the whole of the resistance to mass transfer may be regarded as lying within a thin laminar film on the gas side of the gas-liquid interface, derive from first principles an expression for the rate of absorption at any position in the column. At some intermediate point where the ammonia concentration in the gas phase has been reduced to 5%. the partial pressure of ammonia in equilibrium with the aqueous solution is 660 N/nr and the transfer rate is ]0 3 kmol/m2s. What is the thickness of the hypothetical gas film if the diffusivity of ammonia in air is 0.24 cm2/s ... 

---