ethanol-water mixtures at atmospheric pressure in a column 305 mm diameter operating at total reflux.

Values of HTU in terms of flowrates and physical properties

The proposals made for calculating transfer coefficients from physical data of the system and the liquid and vapour rates are all related to conditions existing in a simpler unit in the form of a wetted-wall column. In the wetted-wall column, discussed in Chapter 12, vapour rising from the boiler passes up the column which is lagged to prevent heat loss. The liquid flows down the walls, and it thus provides the simplest form of equipment giving countercurrent flow. The mass transfer in the unit may be expressed by means of the j-factor of Chilton and Colburn which is discussed in Volume 1, Chapter 10. Thus:

G' \p£/v V ^ Jv where the flowrate and physical properties refer to the vapour. This type of unit has been studied by Gilliland and Sherwood(73), Chari and Storrow(77), Surowiec and Furnas(78) and others.

For a wetted-wall column:

area of interface volume of column G

k' a where the linear characteristic length is taken as the diameter of the column dc. Rev and Scv are the Reynolds and Schmidt numbers with respect to the vapour. Surowiec and Furnas were able to express their results, obtained with alcohol and water, in this form. For transfer through the liquid film, an expression was derived based on the analysis of heat transfer from a tube to a liquid flowing under viscous conditions down the inside of the tube.

The equation was presented as:

where: B' is a constant,

Mm is the mean molecular weight of the liquid,

M is the point value of the molecular weight,

Z is the height of the tube, and

Re; and Sc; are the Reynolds and Schmidt numbers with respect to the liquid.

It has been suggested, however, that for mass transfer, the transfer in the liquid phase is from a vapour-liquid interface where the liquid velocity is a maximum to the wall where it is zero. With a liquid flowing inside the tube the heat transfer is from a layer of zero velocity at the wall to the fluid all the way to the centre of the tube where it is moving with a maximum velocity. Hatta(79) based his analysis on the more closely related process of diffusion of a gas into a liquid, and obtained the expression:

It may be seen that, despite the difference in the arguments, the two equations are really of a similar nature.

The application of the ideas for wetted-wall columns to the more complex case of packed columns requires the assumptions: (a) that the mechanism is unchanged and (b) that the expressions are valid over the much wider ranges of flowrates used in packed columns. This has been attempted by Sawistowski and Smith(76) and Pratt(80). Pratt started from the basic equation:

and suggested, from the examination of the available data, that the importance of the degree of wetting may be taken into account by writing this as:

where: G' |
is |
the mass velocity (mass rate per unit area), |

de |
is |
the hydraulic mean diameter for the packing, |

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