Final exam

Question 1

Name 4 types of adsorbents

Answer 1

Silica based
Alumina based
Activated carbon
Zeolites
Metal-organic frameworks

Question 2

How are adsorbents similar to solvents? (5 things)

Answer 2

Adding reagent
Used in flow separations
Equilibrium separation
Additional separation is required
Want to use distillation instead if possible

Question 3

Name 3 process configurations that are used by solid adsorbents to carry out separations?

Answer 3

Batch
Continuous flow system
PSA (pressure swing adsorption)
TSA (temperature swing adsorption)
Bed swapping

Question 4

How is the Langmuir isotherm related to the BET isotherm?

Answer 4

It is the 1st step of the BET

Question 5

Under what conditions does an adsorption isotherm follow a linear relationship (like Henry’s law)?

Answer 5

When the partial pressure is low

Question 6

What is the range of surface area of a good adsorbent?

Answer 6

100 - 3000 m2/g

Question 7

What adsorbent property is commonly measured with a BET isotherm?

Answer 7

Surface area

Question 8

What are two primary methods for regenerating adsorbents?

Answer 8

Pressure swing
Temperature swing

Question 9

What disposition of adsorbent does not require regeneration?

Answer 9

Throwing it away

Question 10

What are the four transport mechanisms in fixed bed adsorption?

Answer 10

Convection and dispersion
Boundary layer transport
Pore diffusion
Sorption/reaction

Question 11

Why is the limiting mass transfer resistance in fixed bed adsorption frequently pore diffusion rather than bulk diffusion to the surface of the particle?

Answer 11

Particle not 100% pore
Tortuosity

Question 12

What is the limiting condition in fixed bed adsorption defined by infinite mass transfer?

Answer 12

Equilibrium ideal breakthrough

Question 13

Name the limiting operating parameter/condition(s) for each of the five separation methods that we have studied this semester.
Absorption
Stripping
Distillation
Liquid/liquid extraction
Adsorption

Answer 13

Absorption: Minimum solvent rate
Stripping: Minimum vapor rate
Distillation: Minimum reflux ratio & minimum number of stages
Liquid/liquid extraction: Minimum solvent rate
Adsorption: Equilibrium ideal breakthrough

Question 14

What thermodynamic value determines the T dependence of adsorption?

Answer 14

Enthalpy, different types

Question 15

Of the separation methods that we have studied this semester (absorption, stripping, distillation, liquid/liquid extraction, adsorption) which one does not require using an active material or chemical that is not in the feed we want to separate?

Answer 15

Distillation

Question 16

Why is the computational design of packed bed adsorption more difficult than distillation or membranes?

Answer 16

Non-steady state

Question 17

What is THE major difference in the mechanism of separation between distillation and membranes? What are two other differences between membranes and distillation?

Answer 17

THE major difference is that membranes are rate-based vs equilibrium for distillation.
Membrane based on pressure difference. Distillation uses heat. Separations use work.
It’s hard to get high purity with membranes. Distillation can get high purity.
Membrane development in membrane vs none in distillation.

Question 18

What form of energy usually provides the separation energy for membranes?

Answer 18

Work

Question 19

What are the two products from a membrane separation called? What is a sweep?

Answer 19

Retentate and Permeate. Sweep is additional gas used to reduce potential pressure in permeate.

Question 20

Name the terms in this rate equation for membranes:
Ni= (PMi/lM) (driving force )). Based on each of the four terms in this equation, what can be done to increase the rate (mole/s) of mass transfer in a membrane?

Answer 20

P_M: permeability. l_M: thickness of membranes. Ratio: permeance. N: flux (mol/s).
Increase of rate: Increase permeability by using a different membrane material or decrease the thickness of the membrane.

Question 21

Define permeance and permeability? How are they analogous to diffusivity and the mass transfer coefficient?

Answer 21

Permeability - intrinsic property, Pmi * N*LM. Analogous to diffusivity.
Permeance - flux through the membrane, N/Pmi * Lm. Analogous to the mass transfer coefficient.

Question 22

Why don’t membrane systems have limiting behavior like Nmin and Rmin in distillation.

Answer 22

Because they are not equilibrium phenomena.

Question 23

What are two mechanisms for transport in membranes?

Answer 23

Pore diffusion
Solution

Question 24

What are the natural units of 1 barrer? How did we end up with such units? Why are “Barrer”s used to represent permeability?

Answer 24

1 barrer = 10^-10 cm3STPcm / (cm2 s*cmHg)
Scientists used to use mercury manometers to measure gas flows and pressure, so those were the easy units.

Question 25

Give two reasons why it makes sense to use membranes to separate hydrogen

Answer 25

Hard to separate with distillation
Small, so able to do it in membranes

Question 26

Name four commercial uses of membranes.

Answer 26

Reverse osmosis for water desalination
Hydrogen Separation
Air Separation
Removal of CO2 from natural gas

Question 27

What do membranes and LLE have in common? (1 thing)

Answer 27

They are both introducing a new material

Question 28

What properties make a good membrane material? (4 things)

Answer 28

Good selectivity for separation of interest
Good Permeability
Cheap
Durable

Question 29

What are the functions of the membrane contactor?

Answer 29

To provide membrane area
Mass transfer from bulk fluid to membrane module

Question 30

Why is the computational design of packed bed adsorption more difficult than distillation or membranes?

Answer 30

Because it’s a non-steady state problem

Question 31

Name 4 polymers that are used to make separation membranes?

Answer 31

Polyimide
Polystyrene
Teflon
Polycarbonate