Continuous Contact Equipment | Mass Transfer - Chemical Engineering PDF Download

4.8. Continuous contact equipment 
The liquid and gas compositions in a packed tower changes continuously throughout the height. The composition of solute from entry to exit of the packed tower is represented by operating line and every point indicates some location in the packed tower. On the other hand, in tray tower, few points (number of trays) in the operating line represents the conditions in the trays.

Height Equivalent to Theoretical Plate (HETP) 
For a specified separation job, in packed tower, the height of packing is to be determined and in tray tower, numbers of ideal trays are determined. The ratio between packing height to number of trays required for the same separation is called height equivalent to theoretical plate

HETP varies with size and type of packing, flow rate of gas and liquid, concentration of solute, physical and transport properties as well as equilibrium relationship and uniformity of liquid and gas distribution. The concept of HETP is presented in Figure 4.10.

Figure 4.10: Model diagram of HETP.

          (4.38)
The mass transfer rate for h height can be expressed as
                                     (4.39)

                                                                                    (4.40)
where,
                      (4.41)
as (y-y*)av is taken as log mean gas phase gas phase driving force from A to B across nth tray.
It is observed from Figure 4.10 that:
       (4.42)
                     (4.43)
From Equations (4.41- 4.43) we have,
                      (4.44)            
 From Equation (4.40) we can get the value of h as:
             (4.45)
Hence, the same separation is achieved for h height in packed tower and in the nth tray which is =HETP.
                                                  (4.46)
where, H_toG is height of overall gas phase transfer unit.   is stripping factor  and m is Henry’s law constant. HETP is used to characterize packing. A good packing has small HETP.