gas absorption

Question 1

what is absorption

Answer 1

-separation process
-component A is removed from a gas stream (inert and non-condensable ) using a liquid solvent, which A dissolves in to
-Takes place in a packed column

Question 2

What are the main applications of gas absorption

Answer 2

-Air purification (removal of environmental pollutants)
-Manufacture of ammonia (NH3 is removed from N2/H2 mixture by dissolving into water)
-Removal/recovery of any compounds in a gas stream

Question 3

Key physical elements of an absorption column

Answer 3

-Long cylinder, high length/diameter ratio
-Filled with packing material to maximise gas/liquid contact, surface area and time (and therefore removal)
-Gas enters at bottom, liquid solvent enters at top
-A distributor is used at the top and bottom to ensure evenly distributed fluid flow across the diameter of the column
-There are hold up areas at the top and bottom to give steady state flow during operation

Question 4

What is channeling and why is it an issue?

Answer 4

-main operating challenge, particularly for large columns
-occurs when films of falling liquid grow thicker, eventually forming channels that flow in localised paths
-Results in reduced gas/liquid contact area and poor mass transfer

Question 5

How can channeling be reduced?

Answer 5

-Using a suitable column/packing diameter ratio
-Distributors every 5-10m to direct and regulate flow within the column

Question 6

What is the loading point?

Answer 6

-When the downward liquid flow is held up in the column by the upward flowing gas
-Causes high gas/liquid interaction and an increased pressure drop

Question 7

What is the flooding point?

Answer 7

-When the liquid fills up the column, and forms one continuous phase
-Column gets ‘flooded’

Question 8

What are the conditions of loading?

Answer 8

-Falling liquid is delayed by rising gas -> longer G/L contact times
-High G/L interaction -> better mass transfer of solute from G to L

Question 9

What are the conditions of flooding?

Answer 9

-Liquid fills up column forming a continuous liquid phase filling all the voids
-Column is ‘flooded’ by liquid which ‘chokes up’ the gas
-Must be avoided

Question 10

What is the operating line

Answer 10

-Describes the change in conc. of the solute in the gas,y, as a function of its conc. in the liquid, x, along the length of the absorption column
-L/V is its gradient

Question 11

What does a larger L value mean for the column

Answer 11

-Large column diameter
-Expensive distillation process to recover solute

Question 12

What does a smaller L value mean for the column

Answer 12

-Generally means a taller column is needed
-Therefore more expensive to build