Efficiency, tray diffusivities

Determine the effects of the physical properties of the system on column efficiency. Tray efficiency is a function of (1) physical properties of the system, such as viscosity, surface tension, relative volatility, and diffusivity (2) tray hydraulics, such as liquid height, hole size, fraction of tray area open, length of liquid flow path, and weir configuration and (3) degree of separation of the liquid and vapor streams leaving the tray. Overall column efficiency is based on the same factors, but will ordinarily be less than individual-tray efficiency. 

TCE), vinyl chloride, aldicarb (pesticide), and VOCs are listed in Table 9. The removal performances of a diffused aeration system for several organics (VOCs and carbofuran) are listed in Table 10. PTAs and multiple-tray aerators are effective in removing hydrogen sulfides and carbon dioxide in the range of 90-95%. Comparison of removal efficiencies for PCA (PTA) and diffused aeration is shown in Table 3. As this shown in this table, PCA is more efficient than diffused aeration, but diffused aeration can achieve a comparable removal of trans-1, 2 dichloroethylene and TCE. 

In practice, RSPs rarely operate at thermodynamic equilibrium. Therefore, some correlation parameters, such as tray efficiencies or HETS values, have been introduced to adjust the equilibrium-based theoretical description to reality. For multicomponent mixtures, however, this concept often fails, since diffusion interactions of several components result in unusual phenomena such as osmotic or reverse... 

In order to convert point efficiencies to Murphree tray efficiencies, the Chan and Fair correlation uses the same general mixing model as the AIChE model (125). This model uses Lewis case 1 (Sec. 7.1.3), i.e., mixed vapor and plug flow of liquid. In addition, some liquid back-mixing is assumed and correlated via an eddy diffusion coefficient. The model gives... 

Tray efficiencies are usually assumed to have the same value for each component in the separation. However, actual separating efficiency for several components may vary because of differing molecular properties, such as diffusivity. 

An important reference often used for design is O Connell s correlation of tray efficiencies. This work is an improvement of an earlier publication by Drickamer and Bradford. In O Connell s correlation (Fig. 10), it is assumed that mass and heat transfer depend on diffusivities and liquid viscosity, and that both effects can be correlated by using the liquid viscosity and the component relative volatility for the separation. 

In fact, through use of matrix models of mass transfer in multicomponent systems (as opposed to effective diffusivity methods) it is possible to develop methods for estimating point and tray efficiencies in multicomponent systems that, when combined with an equilibrium stage model, overcome some of the limitations of conventional design methods. The purpose of this chapter is to develop these methods. We look briefly at ways of solving the set of equations that model an entire distillation column and close with a review of experimental and simulation studies that have been carried out with a view to testing multicomponent efficiency models. 

Edmistei equations for absorption and stripping factors, 463 group method, 480-490 Efficiency stage, 55,394 of trays, 335, 394,509-514,521-522 Electrodialysis, 20, 23 Electrolysis, 20,23 EMD diffusion, 629... 

The Murphree efficiency is a convenient concept for correlating experimental results on the mass transfer on trays, but a more fundamental way to do this is to express the results in terms of mass transfer coefficients and effective diffusivities. 

For stage or tray efficiencies to be high the time of contact should be long to permit the diffusion to occur, the interfacial surface between phases must be made large, and a relatively high intensity of turbulence is required to obtain... 

A feature of CMT is able to predict the liquid turbulent mass diffusivity D, which is commonly regarded as representing the extent of back-mixing (non-ideal flow), and thus, it is an influential factor to the tray efficiency. Figure 4.8 displays the distribution of Dli on tray number 8 at the depth z of 50 and 100 mm, respectively, apart from tray floor. As shown from the figure, the >Lt is non-uniformly distributed, which reflexes the effectiveness or efficiency of mass transfer is varying with position on a tray. 

---