Absorption

Question 1

What is absorption

Answer 1

The removal of selected components from a mixture of gases into a suitable liquid

Question 2

Name two types of absorption

Answer 2

• Those which are solely a physical process

- Those where a chemical reaction occurs

Question 3

Describe the mechanism of absorption

Answer 3

The gas is introduced at the bottom of the column and the liquid at the top, giving a counter current operation. For an absorption column to operate efficiently, the gas and liquid phases should be brought in close contact with each other

Question 4

What is the equation for Henry’s Law

Answer 4

Pa = H Ca

Question 5

Different types of configurations can be used in absorption. These configurations have what three things in common?

Answer 5

• The component removed from the feed mixture is in the gas phase
• The liquid is charged at the top of the column and recovered at the bottom with the desired component
• Some sort of packing is used to increase the surface area between the liquid and gas phase

Question 6

How is the solvent chosen?

Answer 6

• Gas solubility (should be high, increasing the rate of absorption and decreasing the quantity of solvent required)
• Volatility (should have low vapour pressure)
• Corrosiveness (liquid should be non-corrosive)
• Cost (solvent should be inexpensive, so losses are not costly and solvent is readily available)
• Viscosity (liquid with low viscosity - rapid absorption rates, low pressure drops on pumping, good heat transfer characteristics)
• Other (nontoxic, nonflammable, chemically stable, low freezing point)

Question 7

The extent to which a gas is absorbed by a liquid is determined by what?

Answer 7

The partial pressure at a given temperature and concentration

Question 7

With an increase in temperature, what happens to the solubility

Answer 7

Solubility decreases

Question 9

What liquids are packed columns the best for?

Answer 9

Liquids that foam excessively, very corrosive materials and for columns where only very low pressure drops are allowed

Question 10

How is the type of packing determined?

Answer 10

By its mechanical strength, resistance to corrosion, capacity for handling required flows, mass-transfer efficiency and cost

Question 11

Plate columns are used for…

Answer 11

Large scale operations and when the liquid flow rate is too low to wet the packing material sufficiently. They are also used when the gas velocity is low

Question 12

What design parameters need to be kept in mind

Answer 12

• Column diameter and pressure drop

- Determination of tower height

Question 13

Explain the operation of a packed column

Answer 13

Packed columns are used to bring two phases in close contact with each other. The liquid wets the packing preferentially and will flow as a film over its surface. The gas rises through the column making close contact with the liquid flowing down. The packing applied is selected to increase the interfacial area between the two phases and to give a high degree of turbulence in the fluids

Question 14

Packing can be classified into what four types?

Answer 14

• broken solids
• shaped packing
• grids
• structured packing

Question 15

What does the size of packing influence

Answer 15

• The height and diameter of the column
• The pressure drop
• Cost of packing

Question 16

Explain the operation of a plate column

Answer 16

Plate columns consist of two main parts - the shell and the plates. The operation is similar to a packed column where the gas flows upward and the liquid flows down.

Question 17

Explain the operation of a centrifugal absorber

Answer 17

The centrifugal absorber uses the benefit of repeated spray formations. The unit consists of a set of stationary concentric rings inter meshed with a second set of stationary concentric rings attached to a rotating plate. The liquid is fed to the center of the plate. It is carried up the first ring, splashes over the baffle and falls into and through between the rings. It then runs up the second ring and passes in a similar way. The gas can be introduced either at the top (co-current flow) or the bottom (counter current flow)