adsorption

Question 1

Features of physisoprtion

Answer 1

-adsorbate attached to adsorbent by weak van de waals
-easily reversible, can adsorb and desorb with ease
-reversibility due to low adsorption energy, bonds are easily broken

Question 2

Features of chemisorption

Answer 2

-adsorbate attached to adsorbent with stronger ionic/covalent bonds
-These are a higher adsorption energy so are harder to break
-Mainly irreversible process

Question 3

example applications of adsorption

Answer 3

-Purification of streams e.g waste treatment
-Recovery of materials e.g. proteins or precious metals
-Carbon caption
-Creating heterogeneous catalysts

Question 4

how does adsorption take place

Answer 4

-Solute moves in bulk till it reaches boundary layer

-solute molecules transfer through boundary layer by diffusion onto the adsorbent solid surface, driven by conc. difference

-from adsorbent surface solute transfers by two types of diffusion onto adsorption sites
-pore diffusion (through fluid filled pores)
-solid diffusion (using surface hopping mechanisms)

-at adsorbent site molecules are rapidly taken up and bonded to the site

Question 5

What properties are desirable for adsorbent materials

Answer 5

-High selectivity to target compound, by having ‘correct’ functional groups
-High capacity, provided by large SA and no. of adsorption sites
-High mechanical strength
-Thermal stability
-Chemical durability, not reactive with materials
-Affordable, if expensive it must be regenerable to be used again

Question 6

What are the main differences in natural vs synthetic adsorbents

Answer 6

natural are lower cost

synthetic are more expensive but have a designed/controlled pore structure

Question 7

What are the features of batch adsorption

Answer 7

-takes place in a stirred container
-takes place at a rapid rate due to thorough mixing which increases turbulence and reduces thickness of boundary layer around adsorbent
-low mass transfer resistance
-long residence time
-contain baffles to avoid formation of vortexes and improve mixing

Question 8

What are the typical adsorbents used in pressure swing adsorption and why?

Answer 8

-activated carbon, silica gel, zeolite
-high selectivity, high porosity, large specific areas, and molecular sieves (carbon and zeolite)

Question 9

How does pressure swing adsorption work?

Answer 9

At high pressure gases tendency to be adsorbed increases

At lower pressure the gas is released/desorbed

Question 10

How does temperature affect adsorption?

Answer 10

-high temp, adsorption decreases -hot purge stream will remove desorbed components
-lower temp, high adsorption