Wetted Wall Column Equipment

Wetted wall column, mostly an experimental equipment used for determination of mass transfer coefficient during a mass transfer operation between a liquid and gas phase. It helps to find out individual mass transfer coefficients, used to derive the rate of mass transfer between real liquid-gas selected systems. The experimentation on the wetted wall column helps to understand the behaviour of the liquid and gas phase when they are made to contact naturally. It can be viewed with perfect imagination, considering a known dimension glass cylinder. So that its internal surface can be calculated with the formula “2π R.L” where ‘R’ is the inner radius and ‘L’ is the length of the column. The advantage of the column is that the liquid allowed to flow in such way that it adheres to column inner surface with a constant film thickness from the column wall. We can observe a thin layer of fluid curtain flows from top to bottom wetting the column inner surface. That is why the equipment is named as wetted wall column.

Now an idea of allowing gas inside the hollow space of the cylinder will make a perfect contact with the liquid phase flow on its surface. Due to the design advantage of the continuous flowing arrangement of both phases. Wetted wall column equipment can run continuously until the experiment is completed.  A counterflow of liquid and gas phase exhibit a standard concentration difference at each location along the length of the column. In commercial use, wetted wall column equipment did not gain much attention in large-scale operation but it became a starting column to test the separation operation in laboratory experiments.

This phenomenon did not only belong to mass transfer but also to the areas like heat transfer, transport phenomena and fluid mechanics. Lots of literature data established for the two-phase system using wetted wall column experiments. Behaviour in each set of a variable condition is determined.  Due to the chemical reaction between components like a neutralization system, enormous data on the operation condition of the chemical reaction are found with help of wetted wall continuous operation. Designing the separation equipments like distillation and absorption column depend on this type of data. Even heat exchangers called wet condensers are designed based on wetted wall column experiments. Wet condensers are those in which the flows of streams are parallel.

Generally, chemical and mechanical engineers are much familiar with wetted wall column equipment operation and techniques to gain out the useful data to design a unit processor operation.

Wetted wall column equipment, industrial application:

In thermal phosphoric acid production technology. Its crucial combustion chamber designed and built by the wetted wall column equipment model. Phosphoric acid is made in the column attached to the combustion chamber.  It operates, allowing the combusted phosphorus and air to flow from the top of the column and water flow on its wall. Phosphorus pentoxide reacts with the water that flows as a film on the surface of the wall forming H3PO4.

H3PO4 plants wetted wall column equipment model drawing Hydration reaction of phosphorus pentoxide:  P2O5+3H2O  → 2H3PO

Lab analysis application:

Orsat apparatus working is based on the absorption of the flue gas components in their respective solvents. Its absorption mechanism is an analogy to wetted wall column operation.

How Wetted Wall Column Equipment Studies Used For Packing Column And Packing Materials Designing:

Wetted wall column studies determine the condition of liquid to gas phase’s contact and their behaviour at different conditions. The data obtained is used by the simulation software to find out the operating condition of the mass transfer equipment like distillation and absorption systems. A vapour and liquid system is a two-phase involving process. For example, absorption of ammonia present in air by water is the chemistry obtained from the chemist laboratory.

Well in plant scale, the operation should handle larger quantities as per the time and production rate of an industry. Therefore, the chemistry data alone cannot help to design the equipment. The physical and rate behaviour are the important factor affecting the desired operation. Based on this concept a column in a cylindrical shape is used for support of liquid film formation and the area itself give out the known area of liquid that will be in contact with air or vapour that is allowed to flow from inside the cylinder.

This physical set up of wetted wall column just minimized to the level of packing material like raschig rings and saddles. The surface of the raschig rings does the same function of wetted wall column and so the rate of mass transfer can be easily calculated by summation of number or quantity of the packing used.  A packing column design directly depends on the overall mass transfer factors obtained from a wetted wall column experiment conducted for a particular system.

Micro-level packing material can be developed based on the data obtained from wetted wall column experiments because the packing material surface just resembles its cylinder. Engineering models of falling film reactors of micro size use the same technique and calculation of a wetted wall column equipment. The inner surface of the column is coated with a catalyst so that the reactants participate on the surface reaction on the surface of the active site of the wall to generate products.

Global rate constant that includes mass and heat transfer, derived from the falling film reactor equipment helps in designing the packing material dimensions in a fixed bed catalytic reactor. For example, cylindrical raschig rings when coated with catalyst powder will enhance physical and chemical reaction operations in one equipment itself, and replace regular spherical catalyst and its supports.