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 contain 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 by 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 is as shown with comparison of ideal condition of sieve tray column:

Ideal column operation

Blue colour indicates= liquid flow pattern
Green colour indicates= vapor flow pattern
Red colour indicates= forth

Flooding in a column

 It occurs 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 the 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 the 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.

Flooding condition of the sieve column

Effects due to flooding:
1. Tray efficiency falls
2. The liquid may force out of the exit pipe at the tower top

Overall tray efficiency is defined as the ratio of a number of real trays required to the number of ideal trays required. Channelling is most severe in towers packed with stacked packing. Wetted wall tower experiment are used to determine the 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. The 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. The binary liquid-liquid system has two degrees of freedom.

Priming condition of sieve column

 Due to high gas velocity, liquid from the bottom trays is carried away along with the vapor to the top trays.

Coning is 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.

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 downcomers, the 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 downcomer.

The weeping condition of the Sieve Tray Column

In the event of severe weeping, no liquid reaches the downspouts. Complete liquid drops down by the tray opening only. This phenomenon is known as dumping.

Dumping condition in Sieve Tray Column

The gas hold up is defined as the 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 countercurrent 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% vapours, the q line will have a slope of -1.5.