CPE Adsorption (12)

Describe the kinetics of adsorption.

the adsorptive diffused from the donor film to the boundary film around the adsorbent

the adsorptive diffuses through the boundary layer and into the pore system of the adsorbent

the diffused adsorptive accumulates on the inner pore surface

Name the stages of adsorption.

external mass transfer

boundary film/external diffusion

pore diffusion

actual adsorption

What step determines the overall speed of adsorption?

the slowest step

What is the velocity-determining step in the case of high in-flow velocity?

pore diffusion

What is the velocity-determining step in the case of low incident flow velocity?

boundary film diffusion

What are the applications of adsorption?

purification of gases or liquids from small amounts of foreign matter

separation of gas mixtures

fractionated recovery of substances from solutions

How can the component be bound to the solid surface?

physically or chemically

Characteristics of physisorption

arises from van der Waals forces

reversible under normal conditions (low adsorption/desorption energies)

in many cases selective

substances accumulate at the surface

Characteristics of chemisorption

chemical bond

usually not reversible

What are typical adsorbents?

silica gel, zeolites, alumina

activated C

highly porous solids

What is desorption?

release of an adsorbed substance from the surface

What is a hysteresis and why does it occur?

adsorption and desorption curves are different

at the same pressure amount adsorbed is different depending on whether adsorption or desorption is taking place

due to capillary condensation in mesopores (liquid condenses in pores at certain pressure during adsorption but evaporates at lower pressure during desorption, creating a hysteresis loop)

Explain capillary condensation in mesopores

gas can condense into a liquid-like phase inside the mesopores at pressures lower than saturation vapor pressure

Name different isotherms

Freundlich

Langmuir

BET

What is the mass transfer zone?

the region of the adsorption column where the solute is actively being transferred from the fluid phase to the adsorbent

Define adsorption

complete or partial accumulation of a substance (adsorptive) from a fluid phase onto the surface of a solid (absorbent) phase

What is the name of the adsorbed substances?

adsorbate

What does adsorption equilibrium mean in Langmuir?

adsorption rate at free binding sites equals the rate of desorption

What case of equilibrium is described by adsorption isotherms?

gas/solid equilibrium

What does an equilibrium isotherm describe?

the loading X of an adsorbent with an adsorptive in dependence on the concentration of adsorptive in the fluid at constant temperature

What are the assumptions of Langmuir isotherm?

monolayer adsorption

adsorption enthalpy is the same for all adsorption sites and is not dependent on degree of coverage

no interactions between adsorbed particles

probability of particle adsorption/desorption does not depend on whether the neighbouring adsorption sites are occupied or not

Describe the characteristics of Type I (Langmuir) isotherm

reversible

microporous substances with relatively small external SA (activated C, zeolites)

at low and medium pressure, micropores are filled

at high pressure (saturated vapor pressure), only monolayer adsorption occurs due to limited external surface available

Describe the characteristics of Type II (BET) isotherm

reversible

non-porous materials or materials with macropores

rapid increase of loading until monolayer adsorption is complete

slow increase of loading until pressure is sufficient to adsorb further layers

Describe the characteristics of Type III isotherm

reversible

exponential shape

no identifiable monolayer

at high pressures resembles Type II

can be observed with water on hydrophobic substances (non-porous)

Describe the characteristics of Type IV isotherm

adsorption hysteresis following monolayer adsorption caused by capillary condensation in mesopores (2-50 nm)

common for mesoporous technical absorbents (silica gel, alumina)

first part corresponds to Type II with identifiable end of monolayer adsorption

Describe the characteristics of Type V isotherm

like type III but for porous materials

observed mainly in adsorption of polar components on hydrophobic surfaces (water on activated C)

at low pressure, few molecules adsorb at the surface

further adsorption is promoted by adsorbate-adsorbate interactions (cooperative adsorption) —> rapid increase in adsoprtion

Describe the characteristics of Type VI isotherm

stepwise multilayer adsorption on a uniform non-porous surface

e.g. nitrogen or krypton on graphitized carbon black

Properties of adsorbents depend strongly on

their surface and pore structure

What has an effect on adsorbent performance?

high loading capacity

specific surface area

accessibility of surface

sieving effect of micropores (size/shape selectivity)

How are adsorbent properties determined?

by physisorption experiments

Requirements for technical adsorbents

selectivity

high loading

easy desorbability

chemical resistance

no undesired reations

no decrease in loading capacity with repeated use

sufficient mechanical strength and thermal resistance

How can desorption of physisorbed particles be achieved?

small T increase

How can desorption of chemisorbed particles be achieved?

activation energy must be applied

What does a breakthrough curve show?

how solute concentration in the effluent of an adsorption column changes over time

What is a breakthrough point?

point where the effluent concentration reaches a specified fraction of inlet concentration

What does a breakthrough point indicate?

that adsorbent is no longer effectively removing the solute

What are the characteristics of discontinous operation?

simple, robust

low maintenance costs

high regeneration costs

parallel operation

sensitive to contamination/dust

high personnel cost

What are the characteristics of continuous operation?

smaller equipment

lower investment costs

less effort for control

lower regeneration costs

less sensitive to contamination/dust

problems due to abrasion

Counter-current continuous activated coke adsorber

activated coke inlet is at the top, exhaust gas enters from the bottom and travels through adsorber bed

gas outlet is located between the activated coal supply and adsorbed bed

Cross flow continuous activated coke adsorber

exhaust gas enters from the side at the top and travels through the layer of activated coke horizontally

gas outlet is at the bottom

What are some safety considerations for adsorption processes?

corrosion

ignition risk (activated C)

hot spots, oxygen during regeneration

possibility of explosive mixtures

adsorbent poisoning