Basic Concepts of Separation

Separation

• Processes that change the relative amounts of substances in a mixture. 
• Homogeneous mixture & Heterogeneous mixture
• Some particles are either partially or totally removed from the sample

Reasons for separation

• Isolating a substance
• Removing a substance
• Alter the composition of a sample

Classification of Separation

• Based on the quantity of material to be processed
• Small amount of sample
• Large amount of sample
• Based on the physical or chemical phenomena 
• Rate phenomena
• Phase equilibria phenomena

Separations based on phase equilibria

• Gas – liquid
• Gas – solid
• Liquid – solid
• Liquid – liquid
• Supercritical fluid – solid
• Supercritical fluid – liquid

Separations based on rate phenomena

• Barrier separations
• Field separations

Based on phase equilibria

• Distribution coefficient

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• K = Concn of solute in one phase/ Concn of solute in another phase

Problem 1

• When a solution of 1.00 g of X in 100 cm³ of water was shaken with 10 cm³ of ether, 0.80 g of X was transferred to the ether layer. Calculate the distribution coefficient of X between ether and water.

Solution

• Concentration of X in ether = 0.80/10 g cm^-3
• Concentration of X in water = 0.20/100 g cm^-3

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• K = ?

Separation factor

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• α = K₂/K₁

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• K₁ = distribution coeff of component 1
• K₂ = distribution coeff of component 2

Based on rate

• Separation factor

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• α = V₂/V₁

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• V₁ = migration velocity of component 1
• V₂ = migration velocity of component 2

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Characterization of Separation Process

• The separation of solutions and mixtures into their single components is an operation of great importance for the chemical, petrochemical, and oil industries.
• Almost all chemical processes need preliminary raw material purification or the separation of primary from secondary products.

Characterization of Separation Process

• For instance, salt mixed in water dissolves to give a homogeneous solution and the separation of the components in the mixture requires what?
• What are the methods we can follow to separate salt and water?

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Characterization of Separation Process

• By heating the solution to make the water evaporate, and subsequently condensing it at a lower temperature.
• By cooling the solution in order to separate the water in the form of ice.
• By exploiting the selective properties of a membrane; water passes through this membrane more easily than salt.

Classification of separation processes

Classification of separation processes

Factors influence the selection of feasible process

• Feed conditions
• Product conditions
• Property differences
• Characteristics of separation operations

Ease of scale up and staging

Strengths of distillation

• Small equipment requirement
• Easy staging
• Economics of scale
• Energy costs
• Design and scale-up reliability

Cross Flow Filtration

• Also known as tangential flow filtration
• Crossflow filtration is different from dead-end filtration 
• Majority of the feed flow travels tangentially across the surface of the filter
• The filter cake (which can blind the filter) is substantially washed away during the filtration process

Dead end filtration

Cross flow filtration

CFF

Cross flow filtration

• Used for continuous process
• Selected for feeds containing a high proportion of small particle size solids
• Driving force - transmembrane pressure 
• Transmembrane pressure might decrease due to an increase of permeate viscosity
• Filtration efficiency decreases and can be time-consuming for large-scale processes

Cross flow filtration

• Prevented by diluting permeate or increasing flow rate of the system

Operation

• The feed is passed across the filter membrane (tangentially) at positive pressure relative to the permeate side
• Material which is smaller than the membrane pore size passes through the membrane (Permeate / filtrate)

Benefits over conventional filtration�

• A higher overall liquid removal rate is achieved by the prevention of filter cake formation
• Process feed remains in the form of a mobile slurry, suitable for further processing
• Filter cake washed away during filtration
• It is possible to fractionate particles by size
• Addition of filter aid is not required

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Industrial applications�

• The principles of cross-flow filtration are used in reverse osmosis , nano, ultra and micro filtration. 
• Water purification - very cost-effective comparison to the traditional evaporation
• Protein purification
• Extraction of soluble antibiotics from fermentation liquors

Techniques to improve performance�

• Backwashing
• Alternating tangential flow
• Clean-in-place
• Concentration
• Diafiltration
• Process flow disruption

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CF Filtration

• The flow rate in cross-flow filtration systems is given by the equation

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• J- liquid flux
• ΔP - transmembrane pressure
• R_m - Resistance of the membrane
• R_c - Resistance of the cake
• µ - liquid viscosity

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Electrofiltration

• Method that combines membrane filtration and electrophoresis in a dead-end process.
• Electrofiltration is based on overlaying electric field on a standard dead-end filtration.
• Created polarity facilitates electrophoretic force which is opposite to the resistance force of the filtrate flow

Electrofiltration

Electrofiltration

• Decreases the film formation on the micro- or ultra-filtration membranes
• Reducing the filtration time
• Compared with CFF, electrofiltration exhibits increased permeate flow and also reduces shear force stress 

Applications

• Dewatering
• Treatment of colloidal suspensions 
• Sludges in effluent and waste streams

CF – Electro filtration

• It is a multifunctional separation process
• Combines electrophoretic migration force present in EF and radial migration force present in CFF
• Reduces the formation of filter cake
• The presence of electroosmatic flux in the membrane and in any particulate enhance the filtration rate

CF – Electro filtration

• Theory
• Superimposed electric effects
• Electroosmosis
• Electrophoresis

CFF-EF

CFF-EF

CFF-EF

• Membrane resistance
• J_om = ΔP/R_om ---- (1)
• J_m = ΔP/R_m ---- (2)
• J_om – flux through membrane in absence of electric field
• J_m – flux through membrane in presence of electric field
• R_om – membrane resistance in absence of electric field
• R_m – membrane resistance in presence of electric field

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CFF-EF

• When electroosmotic effects do occur
• J_m = J_om + K_m E ---- (3)
• K_m – electroosmotic coefficient of membrane
• E – applied electric field strength

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CFF-EF

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• J_oc – flux through cake in absence of electric field
• R_oc – cake resistance in absence of electric field
• K_C – electroosmotic coefficient of cake

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CFF-EF

• Regimes of operation
• Three regimes – magnitude of electric field wrt critical voltage (E_C)
• E = E_C
• E < E_C
• E > E_C

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CFF-EF

• Advantages
• No filter cake formation
• Disadvantages
• Electrodeposition of particles at the electrode

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Dual Functional Filter

• Uni-flow filter
• Dewatering of slurries of micrometer sized, gelatinuous materials

Concept

• Vertical, collapsible, porous flow hoses as filter medium
• Nylon, polypropylene, cotton
• Cyclic in nature

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Dual Functional Filter

• Feed slurry enters at the top of the hose
• The bottom dumping valve being closed
• Filtrate is removed through the porous walls of the hose
• Filter cake builds up inside
• Filter process is terminated before solids fills the hose

Dual Functional Filter

Dual Functional Filter

• After the predetermined cycle time, the feed valve is closed
• Simultaneously dumping valve opens
• Flexible hose rejects the filter cake and unfiltered feed slurry into settling receiver
• Unfiltered slurry is recycled
• Rapid cycling – high filtration rates

Dual Functional Filter

• Advantages

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• High filtration rate
• High degrees of sludge dewatering
• No mechanical device
• No filter aid requirement