EFFECT IN TOWERS AND COLUMNS

Towers or columns are the equipments which are used in the industrial operation for the separation and purification process. All most every chemical industries contains these columns varying in different sizes, and even in analytical operations, instruments such as HPLC (high performance liquid chromatography-columns )which are used for the separation of mixed components in the samples depends on the column design. Tray towers offer more pressure drop than packed towers. so, for simple separation tray towers which consists of trays or sieves are preferred than packed towers where a packing material is used instead of trays.

Overview of the Effects in a Column and Terminology :


All mechanical aspects which occur in a column are referred as Effects in Tower, these mechanical problems caused on the physical properties and the mechanism by which column is operated by the control valves and  inlet and outlet streams flow rates, even the structure and internal design also considered in this concept, we see some of the important and very well faced problem for every column, which are as show with comparison of ideal condition of sieve tray column:
diagram of ideal condition of the tray column operation
Ideal column operation
Blue colour indicates= liquid flow pattern
Green colour indicates= vapor flow pattern
Red colour indicates= forth

Flooding in a column

 It occur in a packed column due to high pressure drop. At the same gas flow rate, the pressure drop in a packed tower being irrigated with liquid is greater than dry packed tower. Operating velocity in a packed tower is usually equal to the flooding velocity. This effect can be well understood as simple as liquid filling up from the bottom of the column to the top and exhausted out from top inlet of the column.
The point at which this effect occurs the velocities of which a column is operated is called as flooding velocities. Downcomer and space between the trays are completely filled up by the liquid than the tower is said to be flooded, due to high pressure drop due to increased flow rates of the streams.
diagram of flooding condition in a tray column
Flooding condition of the sieve column
Effects due to flooding:
1. Tray efficiency falls
2. Liquid may force out of the exit pipe at the tower top
Overall tray efficiency is defined as the ratio of number of real trays required to the number of ideal trays required. Channeling is most severe in towers packed with stacked packing. Wetted wall tower experiment  are used to determine volumetric coefficient of two interacting phases.  
Priming in a distillation column is desirable from point efficiency considerations. Priming is an exaggerated condition of liquid entrainment. Packed column provides substantially smaller liquid hold-up as compared to plate column. Outlet weirs (provided on the plate in a plate column) maintain a desired liquid level on the plate. Inadequately large weir height may cause all of the foregoing; a common weir height for absorbers and strippers is 3 to 4 inch. Binary liquid-liquid system has two degrees of freedom.
diagram of priming condition in a plate and tray column
Priming condition of sieve column
 Due to high gas velocity, liquid from the bottom trays are carried away along with the vapor to the top trays.


Coning in a Tray Tower

occurs due to low liquids flow velocities when compared to gas which results in pushing of the liquid away from the tray openings.
diagram of coning effect condition in a plate and tray column
Coning condition in Sieve Tray Column


Weeping in a Sieve Tray Column


is due to at low gas velocity which is not equal to liquid flow velocity, and the liquid is not enough resisted to hold on the tray pass from the down comers, complete liquid will flow through the openings in the tray itself. so,weeping occurs when gas velocities (in a plate column) are too low. Most of the liquid  is rained down from tray openings and some through the down comer.
diagram of weeping effect condition in a plate and tray towers
Weeping condition of the Sieve Tray Column

In the event of severe weeping, no liquid reaches the downspouts. Complete  liquid drop down by the  tray opening only. This phenomenon is known as dumping.
dumping effect condition in plate and sieve tray columns diagram
Dumping condition in Sieve Tray Column
Gas hold up is defined as fraction of the liquid –gas mixture occupied by the gas.
Weber number is defined as ratio of shear forces to inertial forces and ratio of inertial forces to surface forces. Absorption factors is defined as mE/R.
Stripping factor is defined as R/mE.. Maragoni effect is also known as interfacial turbulence.
The "capillary number" is (K L/ g) (g/gC) where K: permeability; : liquid surface tension; L: liquid density. Large depths on trays (reasonable gas velocities) in a tray column lead to high pressure drop but high tray efficiencies. Recommended plate spacing for tower diameter of 12 to 24 ft is 36 inch.
A ternary liquid-liquid system has three degrees of freedom. In a counter current liquid-liquid extractor, slip velocity [Ud/ +UC/(1- )] where Ud and Uc are, respectively, dispersed and continuous phase superficial velocities and  is the fractional dispersed phase hold-up) is Us. In a packed countercurrent extractor, slip velocity is Us'. The relations is Us' / Us >1. In a binary distillation column if the feed contains 40 mol% vapors, the q line will have a slope of -1.5.