Mass transfer coefficients types

There are a number of different types of experimental laboratory units that could be used to develop design data for chemically reacting systems. Charpentier [ACS Symp. Sen, 72, 223-261 (1978)] has summarized the state of the art with respect to methods of scaUng up lab-oratoiy data and tabulated typical values of the mass-transfer coefficients, interfacial areas, and contact times to be found in various commercial gas absorbers as well as in currently available laboratoiy units. 

Not only is the type of flow related to the impeller Reynolds number, but also such process performance characteristics as mixing time, impeller pumping rate, impeller power consumption, and heat- and mass-transfer coefficients can be correlated with this dimensionless group. 

Three criteria for scale-up are that the laboratory and industrial units have the same mass-transfer coefficients /cg and E/cl and the same ratio of the specific interfacial surface and liquid holdup Tables 23-9 and 23-10 give order-of-magnitude values of some parameters that may be expected in common types of liquid/gas contactors. 

The other major type is gas absorption of inorganic components in aqueous solutions. For this type design one uses mass transfer coefficients. Packed towers are used so often for this type that its discussion is often included under sections on packed towers. However, in this book it is included here. 

Calculate mass transfer coefficient in a 60 m3 fermenter with a gas and liquid interfacial area of a = 0.3 m2-m 3, given pbroth = 1200kg m-3. The small reactor has working volume of 0.18m3, 1 vvm aeration rate. Oxygen transfer rate (OTR) is 0.25kmol in 3 h 3. There are two sets of impellers, and flat-blade turbine types of impeller were used, HL= 1.2/),. Find the exact specifications of a large fermenter. 

The results are of interest partly with respect to the design of certain types of trickle-flow operation and partly because they demonstrate that higher mass-transfer coefficients may be obtained for cocurrent than for countercurrent operation. 

Lowe and Christie 17 used a 1.3 m square experimental column fitted with a number of different types of packing and measured heat and mass transfer coefficients and pressure... 

Laboratory reactors for studying gas-liquid processes can be classified as (1) reactors for which the hydrodynamics is well known or can easily be determined, i.e. reactors for which the interfacial area, a, and mass-transfer coefficients, ki and kc, are known (e.g. the laminar jet reactor, wetted wall-column, and rotating drum, see Fig. 5.4-21), and (2) those with a well-defined interfacial area and ill-determined hydrodynamics (e.g. the stirred-cell reactor, see Fig. 5.4-22). Reactors of these two types can be successfully used for studying intrinsic kinetics of gas-liquid processes. They can also be used for studying liquid-liquid and liquid-solid processes. 

Mass-transfer coefficients for a single newtonian component in various module types and flow regimes can be correlated by Eq. (20-65) with values for the constants in Table 20-18 ... 

Eqn. 3.86 shows the linear relationship between ip and the original analyte concentration C, and also the influence of the mass transfer coefficient, i.e. its type of motion (stirring or rotation). However, basically eqn. 3.85 indicates that ip is directly proportional to ALCa = VCa, i.e. the mass of metal dissolved in the ultra-thin mercury this relationship was also confirmed by Roe and Toni105 in their approximate equation... 

Design of this type absorber quite often involves a system whose major parameters are well defined such as system film control, mass transfer coefficient equations, etc. Reference 1 gives design data for certain well known systems such as... 

The constants characterizing the electrode reaction can be found from this type of polarization curve in the following manner. The quantity k"e is determined directly from the half-wave potential value (Eq. 5.4.27) if E0r is known and the mass transfer coefficient kQx is determined from the limiting current density (Eq. 5.4.20). The charge transfer coefficient oc is determined from the slope of the dependence of In [(yd —/)//] on E. 

Numerous turbulent mass-transfer relationships are given in Eqs. (39)-(50), Table VII. Although the most important ones in practical applications are those for channels and tubes, several other configurations also have been investigated because of their hydrodynamic interest. Generally, it is not possible to predict mass-transfer rates quantitatively by recourse to turbulent flow theory. An exception to this is for the region of developing mass transfer, where a Leveque-type correlation between the mass-transfer coefficient and friction coefficient/can be established ... 

Whereas the fugacity approach was used by Mackay for the computation of mass flows and the concentration levels, the SimpleBox adopt the concentration-based piston velocity type mass transfer coefficients (ms-1). This is, mainly, because most scientific papers express the mass transfer in these terms, rather than in terms of the fugacity-based conductivity type coefficients (mol h 1 Pa-1). Furthermore, the transfer and transformation phenomena are treated as simple pseudo first-order processes, similar to Mackay models. 

For diffusion in the soil air-pores, a molecular diffusivity of 0.02 m2/h is reduced to an effective diffusivity using a Millington-Quirk type of relationship by a factor of about 20 to 10-3 m2/h. Combining this with a path length of 0.05 m gives an effective air-to-soil mass transfer coefficient kSA of 0.02 m/h, which is designated as U5. 

Table 9.3 gives data of common types of L-L contactors. Since the given range of kLa is more than 100/1, this information is not of direct value for sizing equipment. The efficiencies of various kinds of small liquid-liquid contactors for physical extraction are summarized on Figure 8.1. Larger units may have efficiencies less than half of these values. In some cases, however, enhancement of the L-L mass transfer coefficient by reaction may be as appreciable as in some gas-liquid cases. 

Ideal reactors can be classified in various ways, but for our purposes the most convenient method uses the mathematical description of the reactor, as listed in Table 14.1. Each of the reactor types in Table 14.1 can be expressed in terms of integral equations, differential equations, or difference equations. Not all real reactors can fit neatly into the classification in Table 14.1, however. The accuracy and precision of the mathematical description rest not only on the character of the mixing and the heat and mass transfer coefficients in the reactor, but also on the validity and analysis of the experimental data used to model the chemical reactions involved. 

The Brunauer type I is the characteristic shape that arises from uniform micro-porous sorbents such as zeolite molecular sieves. It must be admitted though that there are indeed some deviations from pure Brunauer type I behavior in zeoHtes. From this we derive the concept of the favorable versus an unfavorable isotherm for adsorption. The computation of mass transfer coefficients can be accompHshed through the construction of a multiple mass transfer resistance model. Resistance modehng utilizes the analogy between electrical current flow and transport of molecular species. In electrical current flow voltage difference represents the driving force and current flow represents the transport In mass transport the driving force is typically concentration difference and the flux of the species into the sorbent is resisted by various mechanisms. 

If data are available we can calculate whether film resistance to heat transfer is important by the estimate of Eq. 36., and whether film resistance to mass transport is important by comparing the observed first-order rate constant based on the volume of particle with the mass transfer coefficient for that type of flow. 

Shah (1979) evaluation of the mass transfer coefficients in various types of equipment. Also experimental devices for finding these coefficients. 

A list of mass transfer coefficient relations provided in Table 4.2 illustrates the types of experimentally determined relations that exist. They are typically of the form of equation (E4.5.4), although some deviations occur due to a theoretical analysis of mass transfer. Theoretical and experimental analyses have shown that Sh for an interface that acts like a fluid and Sh for an interface that acts like a solid. 

The annual production of HT materials depends on the type of FIT technologies. Taking into account the highest input —> output mass transfer coefficient of each process, the following masses of HT materials would theoretically be produced in Switzerland ... 

---