HOLD – UP STUDIES (FLOODING VELOCITY)

Importance :

Usually low holdup is desired by reasonable holdup is necessary for efficient column operation. The weight of liquid held in the following must be considered when determining the supports loads at the bottom of the packing as well as the column itself. The higher the holdup for any particular packing the greater will be the gas pressure drop and the longer the column drainage time when shut down. Smaller size packing tends to have greater holdup than larger packing. Further the prediction of liquid holdup is of interest to the designer is estimating the response of the system to variations in operating conditions.
Hold-up:

Holdup refers to the liquid retained in the packed column. It may be due to wetting the packing by liquid and as pools caught in the crevices between packing material.


There are three different types of liquid holdup :

1. Total holdup (ht)

2. Static holdup (hs)

3. Operating holdup (ho)



All the three are expressed as cubic meters of liquid per cubic meter of packing.

1. The total holdup ht, defined as the total liquid in the packing under operating conditions.

2. The static holdup hs, defined as the liquid in the packing which does not drain from the packing when the liquid supply to the column is discontinued.

3. The operating holdup ho, defined as the liquid which continuously moves through
the packing and replaced regularly and rapidly by new liquid, represents the liquid
which will drain from the packing when the water flow is stopped.

The relation between the three holdups is given by:

ht = hs +ho


Static holdup depends upon the balance between surface tension forces tending to hold liquid in the bed and gravity or other forces that tend to displace the liquid out of the bed.
Estimate of static holdup may be made from the following relationship of Shulman et al.

        C1 µ1c2 σc3
hs = --------------
          ρ10.37


Where µ1 = Liquid viscosity, Kg/m-s

σ = Surface tension, N/m

ρ1 = Liquid density, Kg/m3

Operating holdup contributes effectively to mass transfer rate, since it provides residence time for phase contact and surface regeneration via agglomeration and dispersion. Static holdup is limited in its contribution to mass transfer rates. In laminar regions holdup in general has a negative effect on the efficiency of separation.

Experimental setup

  • The experimental set up (fig) consists of a glass column 50 mm I.D. and 600 mm in length which is mounted vertically on a stand using support plates.
  • The material of construction of all lines and pipe fittings is galvanized iron of . The valves and pressure tapings are of brass.
  • At the bottom of column, a water seal is provided to prevent the leakage of air.
  • A rotameter is installed to measure flow rate of water, which can measure flow rate.
  • The air inlet is connected to balance designed to give steady flow rate of air. An orifice meter is connected to calibrated mercury U Tube Manometer to measure flow rate of air.
  • A lever arrangement is provided which consists of two vessels. One at top of column to drain and other at bottom of column to collect operating holdup. Here both the vessels move simultaneously cutting of inlet water and facilitating the collection of operating holdup in bottom vessel. 
  •  Packing material is Raschig ring of Outer dia 12 mm and Inner dia of 6 mm.



Measurement of Operating liquid holdup

Ensuring the obtainability of reproducible data, the equipment is made ready for operation. The dry packing was thoroughly wetted by setting the water rate upto 120000 Kg/hrm2 for around ten minutes. Then the water rate was set to the desired value and the air rate was also set at approximately 100 Kg/hrm2. The water and air flow rates were kept constant for ten minutes before the operating holdup was measured. This ensured a steady flow rate. The time for collecting hold up was fixed at five minutes as it ensured complete drainage. Simultaneously the manometer readings were recorded and the water rate was increased to the next higher value.

The different sets of packings of series of runs were made in the similar fashion with an without air flows with increasing water rates. The water flow rate ranged from 30000 to 110000 Kg/hrm2. Lower than 30000 Kg/hrm2 water rate was not employed as it was felt that good liquid distribution could not be ensured at lower rates.

AIR WATER SYSTEM
Berl Saddle – 12mm
Density of water at 28˚C = 996.26 Kg/m3 – ρw
Volumetric flow rate of water ( R) = 2 lit/min
Mass flow rate of water (L0) = 2 x 10-3 x 996.26 x 60 = 119.55 Kg/hr
Π (Dt)2 Area of packed column =

     Π (67 x 10-3 )2
= --------------------
          4
At = 3.52 x 10-3 m2
        119.55 ho
L = -------------- --------
     3.526 x 10-3 At

= 33905.27 Kg/hr-m2

Mass flow rate of air (G)

                4.74 x 10-4 x 1.0593 x 3600
(G) = ------------------------------------------
                         3.526 x 10-3

= 516.61 Kg/hr-m2

Operating Hold up

               Volume of water held up
ho = -----------------------------------
             Volume of packed column

         250 x 10-6
= ----------------------------------
      3137.8 x 10-6 m3

= 0.0796 m3 / m3

Total holdup (ht) = ho + hg + hs + hst
Static holdup (hst), Solid holdp (hs) are assumed to be constant for a particular packing
hs = 0.390 m3 / m3
hst = 0.239 m3 / m3

Therefore,

1 = 0.0796 + hg + 0.390 + 0.239

hg = 0.2914 m3 / m3