Distillation Flashcards
When to use:
Flash Distillation
Have a large difference in boiling point
When to use:
Steam Distillation
*Avoid thermal decomposition of a component.
*When one component to be distilled has an unusually high normal boiling temperature.
*High boiling component immiscible in water
When to use:
Differential / Rayleigh Distillation
Experimentation on relative volatilities
When to use:
Simple Continuous Distillation
*Components have a low relative volatility.
*No reflux used
When to use:
Fractional Distillation
*Components have a low relative volatility.
Condenser
used to extract heat from the vapor leaving the topmost stage to produce the low-boiling enriched distillate.
Reboiler
inject heat to the liquid leaving the most bottom stage to produce the low-boiling stripped bottoms product.
Factors Affecting Distillation
- Method used
- Feed conditions (q, x, y, relative volatility)
- Reflux Ratio, R
- Number of Stages, N
When is dew point and bubble point the same?
Dew point and bubble point are the same for pure substances
How the following affects Distillation:
1. Method used
2. Feed conditions (q, x, y, relative volatility)
3. Reflux Ratio, R
4. Number of Stages, N
- Method used
- Feed conditions (q, x, y, relative volatility)
q =
x = - Reflux Ratio, R
higher R, lower N - Number of Stages, N
Lower N needs higher R
Data Needed for:
Flash Distillation
*Feed:
Flowrate
Composition
Temperature
Pressure
*P,drum < crit P,mix
Flash Distillation
*Pump - Heater - Valve - Flash Drum
*The fluid is pressurized and heated and is then passed through a throttling valve or nozzle into the flash drum
Distillation used in petroleum refining
Flash distillation
Demister
Entrainment Eliminator
* Employed to prevent liquid droplets from being entrained in the vapor
Ponchon - Savarit Method
Distillation graphical method
* Heat effects and heats of solution is not negligible anymore.
Constant molar overflow
For each stage
*vapor in = liquid out
Gilliland equation
Estimate the number of equilibrium stages in a distillation column.
Fenske equation
Used to calculate the N,min for separation of binary feed stream by a fractionation column at R,max
Fenske-Underwood-Gilliland Method
Multicomponent shortcut methods for design approximation of simple distillation
*Combines Fenske, Underwood and Gilliland equations
Underwood Equation
Apply when some of the components do not appear in either the distillate or the bottom products at minimum reflux
Differentiate Fenske vs Underwood vs Gilliland Equations
Fenske - N,min
Underwood - R,min
Gilliland - Actual R and N
Enriching Section
Upper section of distillation column
Striping Section
Lower section of distillation column
q
fraction of saturated liquid in feed
f
fraction of saturated vapor in feed
Pinch point
Intersection of feed line and equilibrium curve
When N = infinite,and R,min
How __ Affects Distillation:
Pressure
High P:
* decreases relative volatility - More stages
* increase viscosity = decrease/increase diameter of distillation column @ low/high pressure distillation
* decreases flooding velocity = increase distillation column area required
High P allows separation of
How __ Affects Distillation:
Feed Temperature
Higher T = lower reboiler power consumption = higher condenser power consumption
How __ Affects Distillation:
Vacuum
Lower P = Lower T required
*Used to separate high boiling mixtures
*Thermal sensitive feed
Extractive Distillation
- 3rd Component: solvent (has low volatility)
*more liquid product than azeotropic
*introduce a solvent to relative volatilities by changing the intermolecular interactions of the components within the mixture.
Azeotropic Distillation
- 3rd Component: Entrainer (Mass separating agent) (forms low boiling azeotrope.)
*Forms azeotrope AFTER adding 3rd component
*More vapor than extractive - More complicated distillation
Azeotropic distillation is employed to separate ___
constant boiling mixture.
Constant Molar Overflow
- To achieve 4 eq, 4 unknowns
*Molar flow rates of the vapor and liquid phase within a column section are presumed not to change from one stage to another.
V,n = V,n+1
L,n-1 = L,n
When to use:
Open Steam Distillation
*Used when one of the components is water.
*Instead of a reboiler, steam is used to sustain the countercurrent effect.
Flooding
Excessive accumulation of liquid inside the column
Jet Flooding
Froth of liquid-vapour mixture touches the next above tray
Downcomer Flooding
Liquid level in downcomer on any tray rises above the weir
Causes of Flooding
- Clogged tray perforation
- High vapor velocity = High vaporization rate of reboiler
Preventing Flooding
1.Decrease feed rate
2. Decrease reflux rate
3. Decrease reboiler duty
Foaming
Expansion of liquid due to passage of vapour or gas.
Weeping / Dumping
*Caused by low vapour flow
*Liquid in tray WEEPS down the packing
*Pressure exerted by the vapour is insufficient to hold up the liquid on the tray
Priming
*Foam height on plate reaches next plate
*Reduces separation efficacy
Why N,min is impossible?
N,min needs very large condenser (infinite reflux)
Why R,min is impossible?
R,min needs infinite stages
Feed Factors Affecting Distillation
CLASH
*Close boiling liquids
*Low boiling liquids
*Azeotropic mixtures
*Scaling and Fouling
*Heat sensitive liquids
DISTILLATION Design Considerations
*Tray Performance
*Jet Flooding
*Operating pressure
*Mass Loading
COLUMN TYPES:
Packed Columns
ADVANTAGES
*When high number of theoretical stages are required
*When Small diameter is required (liquid distribution concerns)
*Vacuum distillation
*Lower cost and pressure drop
*For foaming, corrosive, heat sensitive and high viscosity
*Large surface area
DISADVANTAGES
*Does not high turnover ratio
*Can’t remove side products
COLUMN TYPES:
Plate / Tray Columns
ADVANTAGES
*Wide range of gas and liquid flow rates
*Can have side products
*High solids content / fouling while HIGH DIAMETER
DISADVANTAGES
*Easier fouling and solids cleanup
*More expensive
Reflux Ratio
Reflux Amount / Distillate Amount
Why is there __ in Distillation?
*Reflux
*Boil up
*Reflux
-to provide a steady stream of liquid to contact with vapor
-to maintain temperature and pressure profile accross the column
*Boiler
–to provide a steady stream of vapor to contact with liquid
Why azeotropes are hard to distill?
Even how much you boil the liquid mixture, its composition is equal to its vapor.
Separation becomes impossible
Total reflux in a distillation operation requires minimum ____
number of plates
In distillation, overhead product contains ____
any number of components
With increase in pressure, the relative volatility for a binary system ____
decreases.
McCabe Thiele vs Ponchon Savarit
*McCabe Thiele
-Ideal binary
*Ponchon-Savarit
-Non ideal binary mixture
-Appreciable heat of solution and mixture
Packed towers are preferred over plate towers because it gives ___ pressure drop and ___ hold up
low pressure drop and low hold up.
Weeping in a distillation column results due to ____
results due to very low gas velocity.
Plate towers are preferred to packed towers, when ____ because ____
*WHEN large temperature changes are involved
*BECAUSE packing may be crushed due to thermal expansion/ contraction of the components of the equipment
Channeling
*Due to low liquid flowrate, and no liquid distributor
*Liquid tends to flow in column wall (if not distributed well)
Channeling in a packed tower results from the ___
*maldistribution of liquid
*non-uniformity of packing
Inside the distillation column, pressure drop is the ___
driving force for the vapour flow
Operating velocity in a packed tower is usually __________ the flooding velocity.
half
Solvent used in extractive distillation
____
forms a low boiling azeotrope.
Channeling is prevented by ____
liquid distributor
Temperature from of distillation column from bottom to top ___
decreases
Azeotropes obey Raoult’s Law at ____
at all temperatures
___ vapor fraction of feed ____ reflux ratio
increases ; increases
Channeling is caused by __
low tray efficiency
Petroleum Fractions
LiGa Na KaDa FueL
1. LPG
2. Gasoline
3. Naphtha
4. Kerosene
5. Diesel
6. Fuel Oil
7. Lubricating Oil
When to Use:
Random or Structured Packing
*Random: high liquid loading > 49 m3/hr/m2
*Structured: low liquid loading < 49 m3/hr/m2
When is LLE used over Distillation?
*Heat sensitive feed
*High latent heat of vaporization
*Separation applications that would not be cost-effective, or even possible, with distillation
Distillation to obtain methanol
Destructive Distillation of coal
Sweetening and its Importance
*Remove dihydrogen sulfide and mercaptans
*Improve color, odor and oxygen stability
Slope of rectifying line is unity if the reflux ratio is ___
Infinite
DECREASING the distillation tower temperature ___ top product purity and ___ bottom product purity
Decrease T
= Decrease top and bottom purity
Increasing condenser and reboiler duty will ____ top purity and ____ bottom purity, respectively
Increased duty =
increased top and bottom purity
Flooding can be recognized by ____
significant reduction of product quality
Dew and Bubble Point Pressures
*Dew Point
Pdew = sum (xi * Psat)
*Bubble Point
Pbbl = 1/[sum (yi/Psat)]
Assumptions of McCabe-Thiele Method
- dH,vaporization equal for all components
- dH,mix and sensible enthalpy negligible
- Equal pressure throughout column
- No heat loss
Distillation Limiting Conditions
*R,min –> N = infinite
*N,min –> R = infinite or D = 0 and V=L
*Relative Volatility, a = infinite –> N = 1
When to use LLE over Distillation?
- High latent heat of vaporization
- Forms azeotrope
- Close-boiling and Close-melting mixture
- Heat sensitive
- Recovery of small concentrations of desired
Advantage / Disadvantage:
* Bubble-Cap
* Valve Tray
* Sieve Tray
- Sieve Tray
+ Cheapest
+ Fouling liquids
+ Easiest to clean - Very low turndown
- Valve Tray
+Higher turndown than sieve tray
+Most economical for fluids with solids - Bubble-Cap
+Suited for either low or high flow rates.
-Fouling and polymer-forming liquds
-Expensive
Packing
*Increase surface area for contact between liquid and vapor phases
*Tortuous path to increase residence time
*Made of metal, ceramics, plastics or glass
Turndown Ratio
[Max vapor velocity] / [Vapor velocity on offset of weeping]
When to use:
* Plate Columns
* Packed Columns
- Plate Columns
- Large diameter
- Side products required
- Solid content and fouling
- High turndown ratio is required
- Packed Columns
- Corrosive, fouling and foaming liquids
- Small diameters
- Low liquid flowrates
