In this summary of the subject Separation Process Design which is taught at Wageningen University, all topics covered during the lectures are discussed. The reading material from the reader is also included in this summary. In short: a more complete summary for this subject cannot be found!
Separation Process Design – Summary lectures + reader
Lecture 1 - Introduction
Separation process -> always consists of multiple separation steps (unit operations) -> never one
separation step!
Two existing types of separations:
1. Phase/particle separation: 2 or more phases (solid, liquid, gas) are separated from each other.
2. Molecule separation: molecules of different compounds are separated from each other.
Phase separations are harder than molecule separations in general!
Designing process -> use:
1. Experience
2. General scheme for separation processes:
3. Some general rules:
- Remove the most plentiful and/or easiest to remove impurities first
- Make the most difficult and expensive separations last
- Use the greatest differences in the physico-chemical properties between the product and the
impurities
,- Select and sequence processes that exploit different driving forces for separation
- Minimize costs and use of energy and additional materials
- Maximize efficiency, selectivity, concentration factor and purity of the product
- Use mathematical models and experiments
Driving forces for biotechnological separation processes are differences in:
1. Volatility
2. Solubility/polarity
3. Density
4. Size
5. Electrical charge
When modelling a separation unit use:
- Mass balances
- Energy balances
- Force balances
- Equilibrium-based approach (thermodynamic approach) or rate-based approach
Symbols + units used in this course:
Phase/Particle separations Molecule separations
Quantity Symbol Unit Symbol Unit
Stream flow rate φ m3 s-1 F mole s-1
Volumetric or ε m3 m-3 x or y mole mole-1
mole fraction (volumetric (mole fraction)
fraction)
Distribution m mole mole-1
coefficient
(equilibrium
constant)
Density ρ kg m-3
Total cT moles m-3
concentration
Velocity v m s-1
Superficial u m s-1 u m s-1
velocity
Area A m2
Pressure Δp Pa
difference
Flux J mole m-2 s-1
Diffusion D m s-2
coefficient
Boundary layer δ m
thickness
Distance z m
perpendicular to
interface
Transfer k m s-1
coefficient
Specific interface a m2 m-3
area
, Volume V m3 V m3
Heat input NQ W NQ W
Electrical power P W P W
input
Separator Yi Yi
efficiency for
component i
Separator Φij Φij
selectivity for
components i + j
Concentration CFi CFi
factor for
component i
A good separation process has a:
1. High efficiency -> much product is recovered and not much product is lost (economically good)
2. High concentration factor -> much higher concentration of product in the outgoing stream than in
the ingoing stream of the separator (facilitates further processing)
3. High selectivity -> much higher product-solvent ratio in the outgoing stream than in the ingoing
stream of the separator
A single separator seldom scores well on all goals (high efficiency, high concentration factor, high
selectivity)!
Efficiency whole process = efficiencies single steps of process multiplied
Concentration factor whole process = concentration factors single steps of process multiplied
Lecture 2 – Mass balances, energy balances, thermodynamic equilibrium
General mass balance process:
Accumulation = in – out + production – consumption
No reaction occurs in a separation process so production = 0 and consumption = 0
General mass balance separation process:
Accumulation = in – out
, Mass balance for phase/particle separator:
Accumulation = in - out
Include density (ρ) in mass balance!
No accumulation (steady state)? -> divide mass balance by density (ρ) -> result = volumetric balance
Mass balance for molecule separator:
Accumulation = in – out
Sum of mole fractions (x or y) = 1
Sum of volumetric fractions (ε) = 1
For 1 unit operation you have:
- Multiple mass balances -> # = amount of components you have in your unit operation -> total mass
balance is NOT an extra equation, you can use it instead of one of the component balances, but you
cannot use it together with all the component balances
- 1 energy balance
Sorts of energy (all units are J mole-1):
1. Kinetic energy (ek): the energy which leads to motion of objects/particles.
2. Potential energy (ep): the energy an object/particle has due to its position in a force field.
- Electrical energy
- Gravitational energy
3. Internal energy (eI): the energy of a mass which is determined by random motions of the
molecules of that mass so which is determined by the temperature.
General energy balance process:
Accumulation = in – out + production – consumption
Energy can’t be produced or consumed so production = 0 and consumption = 0
General mass balance (separation) process:
Accumulation = in – out
Energy balance molecule separator in steady state:
0 = Σ Fi (ek,i + ep,i + eI,i) – Σ Fo (ek,o + ep,o + eI,o) + Σ pi Fi VM,I – Σ po Fo VM,o + P + NQ
Σ Fi (ek,i + ep,i + eI,i) -> input of energy via molecules which enter the separator
Σ Fo (ek,o + ep,o + eI,o) -> output of energy via molecules which leave the separator
Σ pi Fi VM,I -> input of energy by doing work to let the molecules enter the separator
Σ po Fo VM,o -> output of energy by doing work to let the molecules leave the separator
P -> other work done which increases (+P) or decreases (-P) the energy of the separator
NQ -> added (+NQ) of substracted (NQ) heat from the separator
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