Water-side mass transfer

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

Measurement and Prediction of Water-Side Mass Transfer Coefficients. ... 220... 

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

Chemical transport across the water-sediment interface takes place through numerous chemical, biological, and physical mechanisms (Reible et al., 1991 Thibodeaux and Mackay, 2007). These reflect in large part the characteristics of the particular aquatic system, which include flowing freshwater streams, lakes, estuaries, and the marine, both near shore and beyond. This chapter is focused on so-called diffusive type processes on either side of the sediment-water interface. Specifically, it covers the water-side mass transfer coefficient in the fluid boundary layer above the bed and diffusion within the interparticle pore spaces in the near-surface bed sediment layers. 

FIG U RE 12.1 Product of the mass transfer Stanton number (St = k/ut) and Schmidt number (Sc = v/D) vs plate Reynolds number (Re = u LIv) with Schmidt number as parameter (Modified from the original Higashino, M. and M.G. Stefan. 2004. Water Environmental Research 76, 292-300.) kc is the water-side mass transfer coefficient at the sediment-water interface (cmh ), V is the kinematic viscosity of water (cm h ), Z) is the diffusivity of gypsum in water (cm h ), is the friction velocity at the sediment-water interface (cmh ), L is the gypsum plate length (cm). 

Absorption of hypochlorous acid into water, a liquid-side mass transfer-limited process, showed HTU values as low as 4 cm, with a strong dependence on liquid-flow rate. Heat of absorption removal was identified as a potential issue with absorption in rotating beds (9). 

Experiments of physical absorption of C02 diluted with N2 into pure water allow to measure gas side mass transfer coefficients. In this case, we measure the total mass transfer resistance (liquid+gas), using the method for the kLa determination ( 3.1). Knowing the liquid resistance, we can then calculate the gas side one. The pressure of interest in this case is the partial pressure of C02, which is obtained by simultaneous measurements of total pressure and molar fraction of C02 versus time. 

Recently, Mathur and Wellek33 measured the liquid-side mass-transfer coefficients for the absorption of C02 in water from the mixture of C02 and... 

For a bubble-free ozonation of water, W.L. Gore Associates (Elkton, Maryland) commercializes the DISSO3LVE module made of ozone-resistant hbers in PTEE. A helix arrangement of the hbers leads to higher shell side mass-transfer coefficients than the parallel conhguration. 

Feed-side and strip-side concentration polarization result in a reduction in the driving force for mass transfer. There is a decrease in water activity at the feed-membrane interface and an increase at the strip-membrane interface. This results in a reduction in the water vapor pressure gradient across the membrane. The feed side and strip side mass transfer co-efficients, Kf and K, respectively, can be expressed in terms of the solute diffusion co-efficient in the boundary layer, D, ... 

The liquid-side mass-transfer coefficient, ki a (s ), is related to the superficial gas velocity, Ejc (m/s), and the gassed power per unit volume of liquid, Pq/V, (kW/m ). The viscosity term j, / j, accounts for the effect of process viscosity on the mass-transfer coefQcient relative to standard conditions, typically water at 20°C ... 

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,... 

Flgvire 16.Liquld>side mass transfer aoeffldent in the stirred cell ( Carbcndixide- water system at 25 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 IMW scale DWT steam generator test model has been operated since 1S>91. So far, good thermal hydraulic performance was validated. Four of 10 heat transfer tubes were plugged to obtain thermal hydraulic performance at higher water side mass velocity. 

Here, P is the saturation pressure of the solvent (water) at the surface of the particles, is the respective molar fraction in the gas phase, and 3 is the gas-side mass transfer coefficient. 

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 6.3.12 Consider selective pervaporative transport of a VOC species i through a silicone rubber membrane in preference to water. The separation conditions are such that Ei is 1, of the order of 1000 further, the permeation rates are quite low with respect to the feed side mass-transfer coefficient (i.e. Vz ku). In this pervaporation process, it is known that the membrane resistance is quite low but the boundary layer resistance is quite high for the VOCs. On the other hand, for water, it may be safely assumed that the membrane resistance controls the water transport. Determine an expression for the separation factor, and find the selectivity of the membrane for VOC over water when the permeate pressure is negligible. Assume that the VOC concentration in water is low for this pervaporation process. 

The best type of processes which can be used for determination of the gas-side controlled mass transfer coefficient is nonequilibrium absorption accompanied with instotaneous chemical reaction. Such processes are, for example, the absorption of NH3 in water solution of strong acids, or absorption of SO2 in NaOH or KOH. To be sure that the liquid-side mass transfer is completely eliminated, the concentrations in the gas phase have to be as low as possible and the concentration of the reagent in tite liquid phase has to be higher. Taking into account the above mentioned and for reduction of the inaccuracy from analytical point of view, the following concentrations can be recommended ... 

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,... 

Example 8.5-4 Overall mass transfer coefficients in a packed tower We are studying gas absorption into water at 2.2 atm total pressure in a packed tower containing Berl saddles. From earlier experiments with ammonia and methane, we believe that for both gases the mass transfer coefficient times the packing area per tower volume is 18 Ibmol/hrft for the gas side and 530 Ibmol/hrft for the liquid side. The values for these two gases may be similar because methane and ammonia have similar molecular weights. However, their Henry s law constants are different 75 atm for ammonia and 41,000 atm for methane. What is the overall gas-side mass transfer coefficient for each gas ... 

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