Showing papers on "Mass transfer coefficient published in 1977"

Turbulent mass transfer rates to a wall for large Schmidt numbers

Abstract: New measurements are presented on the influence of Schmidt number on the rate of mass transfer between a turbulent fluid and a pipe wall. It is found that for large Schmidt numbers the fully developed mass transfer coefficient is related to the friction velocity and the Schmidt number by the equation The experiments are accurate enough to rule out the Sc−2/3 or the Sc−2/3 relations commonly used, deduced from plausible limiting expressions for the eddy diffusivity close to a wall. It is argued that these expressions arevalid only over a vanishingly small portion of the concentration field as Sc → ∞.

Kinetic models of diagenesis in disturbed sediments: Part 1. mass transfer properties and silica diagenesis

Abstract: The results of chemical analysis of the interstitial water of several samples of sediments from a large muddy zone along the Belgian North Sea coast are reported. When special care is taken to collect the cores without disturbing the water-sediment interface, the vertical concentration profiles display typical patterns that cannot be explained by constant diffusivity models and suggest the existence of two distinct sedimentary layers with different mass transfer properties. A two-layer model is proposed to describe the vertical silica profiles. It assumes that the mass transfer coefficient in the first 3.5 cm of the sediment upper layer is 100 times higher than in the compacted lower layer. This large increase is mainly due to turbulent processes induced by the movement of the overlying water. From this model, the flux of dissolved silica across the water-sediment interface is calculated.

Exact solution of combined heat- and mass-transfer problem during film absorption

Abstract: Exact solutions of the system of equations of heat and mass transfer accompanying absorption of vapor by a liquid film are obtained. Expressions for the main characteristics of heat and mass transfer are obtained.

Mass transfer and hydrodynamic characteristics of gas inducing type of agitated contactors

Abstract: Power consumption, rate of gas induction (Q), fractional gas hold-up and mass transfer characteristics of 0.41, 0.57 and 1.0 m i.d. gas inducing type of agitated contactors were studied. The power consumption was measured by a torque table. The values of effective interfacial area (a), and liquid side mass transfer coefficient (kLa) were obtained by using chemical methods. Two types of impellers (pipe and flattened cylindrical) were employed. The impeller speed was varied from 3 to 11.7 r/s. The effects of impeller diameter, baffle position, orifice area on the impeller, number of impeller blades, position of impeller from the bottom and liquid submergence on the values of Q and a were investigated. Further, the effect of liquid viscosity and surface tension on the values of Q was studied. A satisfactory agreement was found between the observed and the predicted value of the minimum impeller speed at which gas induction occurs. The performance of the gas inducing type of agitated contactor has been compared with the conventional mechanically agitated contactor.

Combined heat and mass transfer during absorption in drops and films

Abstract: Combined heat and mass transfer during absorption in films and drops is discussed. Using an approximate approach, simple analytic relations are obtained for the basic characteristics of the process.

Liquid-phase volumetric and mass-transfer coefficient, and boundary of hydrodynamic flow region in packed column with cocurrent downward flow.

Abstract: Liquid-phase volumetric coefficients and Peclet numbers in liquid mixing were measured in packed columns with cocurrent downwardflow. The empirical equations of liquid-phase volumetric coefficient are distinctly different in spray, pulse and dispersed bubble flow regions. The boundaries for the respective flow regions, obtained by combining two of these equations of volumetric coefficient, are in good agreement with the boundaries which have previously been given from the equations of interfacial area in the same fashion. The foam flow region, which gives the maximumvalue of liquid-phase mass-transfer coefficient, was found at higher gas Reynolds number in comparison with pulse and dispersed bubble flow regions. Taking the ratio of packing to column diameter dp/T and surface shape factor of packing into consideration, the empirical equation of mass-transfer coefficient is presented in respective flow regions as

Dynamic measurement of the volumetric mass transfer coefficient in agitated vessels: Effect of the start‐up period on the response of an oxygen electrode

Abstract: The response of a polarographic oxygen electrode to a step change and to an exponential change in bulk oxygen concentration was studied theoretically and experimentally for the case where there is a significant liquid film resistance at the outerside of the membrane‐covered electrode. The probe response has been described considering the start‐up period of the concentration changes (the period of time that will elapse before the new concentration level is established and/or before the volumetric mass transfer coefficient kLa regains its steady‐state value after the gas supply is opened to the fermentor). A linear change of the pertinent characteristics is assumed during this start‐up period. It is shown that a substantial error could be introduced by neglecting the start‐up period for cases frequently occurring in practice. In addition, the dependences of the probe response on the direct contact of bubbles with an electrode and on the fluid flow field around it were discussed.

Influence of drag reducing polymers on turbulent mass transfer to a pipe wall

Abstract: Completely developed and entry region mass transfer rates in turbulent pipe flow of drag reducing polymer solutions were studied experimentally using electrochemical techniques. The percent change in the fully developed mass transfer rate at a given volumetric flow was found to be greater than the percent change in the pressure gradient. The data are interpreted by using the law of the wall, valid for Newtonian fluids at large Schmidt numbers, whereby K+ is related to Sc−0.74. The proportionality constant is correlated with percent drag reduction and is found to decrease with increasing drag reduction. The use of an analogy between momentum and mass transfer predicts a somewhat greater mass transfer reduction than was observed.

The tubular loop fermentor: oxygen transfer, growth kinetics, and design.

Abstract: Oxygen transfer measurements using a dynamic method and evaluated with an appropriate mathematical model have been made on a tubular loop bioreactor, Correlations of the type used in tank systmes are used to describe the influence of power and aeration rate on the mass transfer coefficient. Yeast cultures grown on hydrocarbon and glucose substrates show growth characteristics similar to conventional tank results. Model considerations for large-scale tubular fermentors allow for the prediction of the steady-state oxygen profiles and maximum reactor length. Combination with two-phases flow and oxygen transfer correlations yields a design procedure for commercial scale tubular loop fermentors.

Turbulent transfer at the ground: On verification of a simple predictive model

Abstract: Observations on heat transfer from ground-based plates and evaporation from free water surfaces in the laboratory and in the field are compared with predictions from a simple model. The model relates the convective transfer coefficient (or boundary-layer resistance) at any point on a surface to the momentum transfer (friction velocity) in the boundary layer immediately above it and should be applicable to practically any soil surface, open or vegetated. Heat-transfer data showed a standard deviation of 25%; between predictions and observations. Evaporation data provided only order-of-magnitude confirmation of the model because of uncertainty in effective water vapor density above small free-water surfaces.

Dispersed phase mass transfer during drop formation under jetting conditions

Abstract: A recent design procedure for perforated plate extraction columns requires extension to include jetting conditions at each perforation. For this purpose, correlations are obtained for jet length, jet contraction, drop size, and mass transfer coefficients in disperse phase controlled liquid-liquid systems. Experimental variables include nozzle size, flow velocity, and physical properties.

Oxygen transfer and axial dispersion in an aeration tower containing static mixers

Abstract: Oxygen transfer from gas to liquid under steady-state cocurrent flow conditions was modeled using the dispersion model, and the oxygen transfer coefficients were estimated from available data for a column with Koch motionless mixers. The dispersion in the column was estimated for several different gas and liquid flow rates using steady-state tracer experiments. The estimated oxygen transfer coefficients were compared with those estimated using complete mixing and plug flow models. The results indicate that the dispersion model is the most appropriate model for estimating the mass transfer coefficient from the available data.

On the reaction of solids with mixed gases

Abstract: A mathematical formulation is presented, describing the rate of reaction of porous solids with mixed gases in regimes when the overall rate is controlled by pore diffusion or by external mass transfer. The reduction of metal oxides with a hydro gen-carbon monoxide mixture is a typical example of such systems.

Influence of surface tension-driven flow on the kinetics of oxygen absorption in molten copper

Abstract: Absorption measurements have revealed that surface tension-driven flow, the Marangoni effect, can enhance the liquid phase mass transfer coefficient by at least an order of magnitude during the top jetting of molten copper with oxygen. This factor has been established by comparing the mass transfer coefficient for copper to the value measured for molten silver which does not display the surface phenomenon under similar jetting conditions. The spontaneous motion on the copper surface was found to arise under conditions of starvation oxygen transfer in the gas phase in which copper oxide forms in a localized region beneath the lance tip and spreads radially outward at about 30 cm s-1. The mass transfer coefficient was correlated satisfactorily to unsteady state diffusion theory using measured spreading velocities. Sulfur initially present in the copper bath at concentrations of 0.01 to 0.5 pct and silicon at 20 ppm had no effect on the mass transfer coefficient under the spreading conditions. By virtue of the size of the enhancement factor and the lack of influence of dissolved sulfur and silicon on mass transfer, it is suggested that the Marangoni effect may play an important role in the transfer of oxygen into process baths.

Electrochemical mass transfer in solutions containing drag-reducing polymers

Abstract: Electrochemical mass transfer was studied in the presence of polyethylene oxide as a drag-reducing agent using the cathodic reduction of K3Fe(CN)6 at a rotating cylinder electrode over a range of Reynolds numbers from 4100—41 000 Solutions containing polymer showed a lowered mass transfer coefficient than that without polymer Mass transfer data in solutions containing polymers was found to fit the correlation: (St) = 0475 (Re)−03(Sc)−0644 A comparison was made between the reduction in friction and the rate of mass transfer; it was found that at relatively low (Re) values, the reduction in the rate of mass transfer is higher than the reduction in friction, whilst at relatively high (Re) values, the reverse is true

Heat and Mass Transfer Coefficients for Water and Air in Aspen Excelsior Pads

Abstract: HEAT and mass transfer coefficients were determined for 51-mm thick aspen excelsior pads. The 594-mm square pads were vertically and horizontally oriented in wind tunnels, and measurements were made of flow rates and entering and exiting temperatures. Using these data plus results from previous work, the following correlation equations were obtained. where hLaH is the product of the liquid film heat transfer coefficient, hL(kW/m2C), and the area available for heat exchange per volume of pad, aH(*n2/m3). Ga is the airflow rate (kg/s m2 of air face area) and GL is the water flow rate (kg/s m2 of air face area). The correlation applied to both vertical and horizontal pads. For horizontal pads where ha is the gas film heat transfer coefficient. The flow rates ranged from 0.5 to 1.3 for air and from 0.045 to 1.0 for water. The mass transfer coefficient could be determined from the latter equations using the Lewis relation.