CHBE_221_Lecture_2_-_Intro_to_Adsorption
Page 1: Introduction to Adsorption
Lecture 2 - Adsorption (Detour)
Notable Quote: "It is a scientific fact that your body will not absorb fat if you take it from another person’s plate" - Dave Barry
Page 2: Topics to Cover
Key Topics:
Adsorption and related applications
Adsorption mechanisms
Langmuir, BET, and Freundlich isotherms for gas-solid and liquid-solid adsorption
Graphical representation of isotherms to determine model parameters
Page 3: Understanding Adsorption
Definition:
Adsorption involves the adhesion of liquid or gas molecules (adsorbate) to the surface of a solid (adsorbent).
Absorption:
Distinct from absorption, where the absorbate penetrates the absorbent's volume.
Page 4: Importance of Adsorption
Natural Phenomenon:
Found in many chemical and biological processes.
Applications:
Purification processes (e.g., pharmaceutical separation)
Analytical processes (e.g., ion exchange chromatography)
Pollution control (e.g., CO2 capture, water treatment)
Heterogeneous catalysis systems
Desorption:
Opposite of adsorption; involves detachment of molecules from the surface.
Kinetics Factors:
Influence includes system state (pressure, temperature, pH, concentration, surface area).
Page 5: Adsorption Process Flow
Process Steps:
Feed/sample
Adsorption
Wash
Elute
Time Depiction: Timeline visualizing the adsorption process.
Page 6: CO₂ Capture Example
Process Description: CO₂ capture for sequestration involves:
Absorbing CO₂ from a process stream.
Removing other gases followed by desorption via decreased pressure/increased temperature.
Collecting CO₂-rich gas in various applications (e.g., pipelines, power plants).
Page 7: Adsorbent Structure
Adsorption Column:
Contains a porous resin bead (with or without ligand) designed for optimal adsorption.
Open pore structure with sizes ranging from 10 to 150 μm.
Page 8: Antibody Screening and Purification
Mechanism:
Antibodies selectively bind to specific molecules (antigens).
Used to screen for/purify antibodies against target antigens through adsorption.
Allows for purification of desired proteins using antibodies as adsorbents.
Page 9: Adsorption Mechanisms
Binding Types:
Physisorption: Physical binding via weak intermolecular forces (e.g., van der Waals).
Chemisorption: Stronger binding via covalent or ionic bonds from chemical reactions.
Thermodynamics: Inquiry if adsorption is typically exothermic or endothermic and the implications for engineers.
Page 10: Requirements for Adsorption
Necessary Conditions:
Availability of free adsorption sites.
Contact of adsorbate with the adsorption site.
Energy level for binding (activation energy).
Energy dissipation upon binding is essential.
Dynamic Equilibrium:
Occurs when the rate of adsorption equals the rate of desorption.
Page 11: Surface Coverage
Extent of Surface Coverage (θ):
Defined as the ratio between occupied and available adsorption sites.
Can be measured or estimated through various methods.
Page 12: Adsorption Isotherms
Basic Representation:
Adsorption of substance A on adsorbent M can be modeled as:
A + M(surface) ⇌ AM
Rates depend on concentration or partial pressure.
Isotherm Models:
Graphically represent adsorbate quantity at equilibrium as a function of concentration or pressure at constant temperature.
Page 13: Langmuir Isotherm Assumptions
Key Assumptions:
Monolayer coverage is the maximum limit for adsorption.
Adsorbate occupies vacant sites only.
All adsorption sites are equivalent.
No interactions between adsorbed molecules.
Page 14: Adsorption Kinetics (Langmuir)
Kinetic Modeling:
Rate of adsorption is proportional to adsorbate collisions and vacant sites.
Expressed as: ( \frac{d\theta}{dt} = k_a p N (1 - \theta) )
Page 15: Desorption Dynamics
Desorption Equation:
Rate of change in θ due to desorption: ( \frac{d\theta}{dt} = -k_d N \theta )
Equilibrium Condition:
( k_a p N (1 - \theta) - k_d N \theta = 0 )
Page 16: Langmuir Isotherm Equation
Final Form:
Rearranging yields: ( \theta = \frac{\alpha p}{1 + \alpha p} )
Where p is the adsorbate's partial pressure.
Page 17: Gas Langmuir Representation
Substitution:
Use of volume ratios to express θ as ( \theta = \frac{V}{V_\infty} )
Rearranges to define behavior & represent within a linear framework.
Page 18: Plot Interpretation
Graph Analysis:
Producing a linear graph of p/V vs p helps determine suitability of Langmuir model for data evaluation.
Page 19: Langmuir Isotherm Example
Data: Collected p(kPa) and corresponding p/V values analyzed.
Confirmed linear trend, indicating Langmuir as an appropriate model with calculated values: ( V_\infty = 111 ; cm^3 ) and ( \alpha = 7.51 \times 10^{-3} ; kPa^{-1} )
Page 20: Trends in Langmuir Isotherm
Graphical Results: Adsorption isotherms for varying α values; trends indicative of model behaviors.
Page 21: Langmuir Isotherm for Liquids
Same principles apply but focus shifts to concentration (C) instead of partial pressure.
Rearrangement enables plotting to determine suitable model parameters.
Page 22: BET Isotherm Introduction
Need for BET: Developed for systems beyond monolayer limits, allowing multiple adsorption layers.
Gas Adsorption Equation:( \frac{V}{V_{mon}} = \frac{cz}{1 - z} [1 - \frac{1 - c}{c}] )
Page 23: BET Behavior Analysis
Observation of Trends: Theoretical understanding of BET behaviors with increasing adsorption concentrations.
Constants defined for both layers in BET model.
Page 24: BET Isotherm Rearrangement
Rearranging leads to linear expression: ( y = mx + b )
System modeled based on straight-line criteria demonstrating adherence to BET.
Page 25: Example - BET Isotherm
Data Evaluation: Analyzed nitrogen adsorption data at specified conditions; determined that c = 298 & Vmon = 816 mm3.
Page 26: Temkin and Freundlich Isotherms
Addressing Limitations:
Temkin and Freundlich consider variability in energy during adsorption.
Formulations provided for gas and liquid systems.
Page 27: Model Assessment with Freundlich
Linear Transformations:
Organizing Freundlich equations into a linear form facilitates assessment of model suitability.
Page 28: Practical Application in Labs
Upcoming Lab: Investigating adsorption of methylene blue on activated carbon; measuring dye concentration over time, determining kinetics and isotherm suitability.
Importance: Accurate modeling critical for catalysis and designing separation processes.
Page 29: Design Considerations
Design Query: Essential factors for designing an adsorption column to treat organic pollutants.
Page 30: Future Studies
Course Continuation: Next lectures will pivot to cell biology basics and evolutionary considerations in bioengineering.