Distillation & Separation Flashcards
Separation Processes
A process in which a mixture is separated into individual components or group of components
Why?
Separation Processes
- Product specification
- Recovery
- Purification
Methods
SEPARATION
DISTILLATION
SIEVE
ABSORPTION
EXTRACTION
ADSORBTION
CRYSTALLIZATION
Distillation
It involves a volatile vapor phase and a liquid phase that vaporizes
Absorption
A solute or solutes are absorbed from the gas phase into a liquid phase in absorption.
Adsorption
One or more liquid or gas stream are adsorbed on the surface or in the pores of a solid adsorbent.
Crystallization
Solute components soluble in a solution is removed from the solution by adjusting the conditions
Liquid-liquid Extraction
When a solute or solutes are removed from a liquid phase to another liquid phase
Leaching
Fluid is used to extract a solute from a solid.
Membrane separation
Separation of molecules by the use of membranes that control the rate of
movement of molecules between two phases.
Types of Mixture
Homogeneous
Create another phase
or Add another phase
Types of Mixture
Heterogeneous
Exploit the existing phase difference
Separation of Heterogeneous Mixtures
- Filtration
- Centrifuge
- Floatation by gas bubble
- Settling by Gravity
Phase creation
General Separation Technique
Separation by phase creation
- distillation
- crystallization
- evaporation
Addition
General Separation Technique
Separation by phase addition
e.g. absorption, extraction
barrier
General Separation Technique
Separation by barrier
e.g. reverse osmosis,
gas permeation
solid agent
General Separation Technique
Separation by solid agent.
e.g. adsorption, ion exchange
force field or gradient
General Separation Technique
Separation by force field or gradient.
e.g. electrolysis, centrifugation
Types of Separation Processes
- Equilibrium
Governed - Rate
Governed
Equilibrium
Governed
- Flash
Vaporization - Distillation
- Absorption
Rate
Governed
- Membrane
- Crystallization
- Electrolysis
Concept Of Distillation
a process of physically separating mixtures into two or more products that have different boiling points, by preferentially boiling the more volatile components out of the mixtures.
Why Distillation?
- Continuous
- Relatively cheap
- High capacity
- Efficient
- Easy to scale-up
- No moving parts
Distillation Not suitable for :
- Small volatility difference,
- Small quantity of high boiling component to be recovered from the feed,
- A compound is thermally unstable even under vacuum conditions,
- Mixture is extremely corrosive
Equilibrium Stage Concept
A distillation column may be
considered as a series of equilibrium flashes with two feeds and two products
Bubble Point Temperature
The temp. at which the first vapor bubble is formed when liquid is
heated slowly at constant pressure.
Dew Point Temperature
The temp. at which the first liquid droplets is formed when a vapor
is cooled slowly at constant pressure
Bubble Point Pressure
The pressure at which the first bubble is formed when a liquid is
expanded slowly at constant temp
Rectifying section
Section of tower above the feed point. Heavy components are condensed out of the vapor in
this section
Stripping section
Section of tower below the feed point. Light components are stripped out of the liquid in this section
Reflux
Liquid from the overhead condenser that is returned to the top of the tower. The reflux ratio is the reflux rate divided by the overhead product rate
Rules of Thumb
More Separation
- More Reflux
- More Trays
- Lower Pressure
Typical distillation contains several major components
– a vertical shell where the separation of liquid components is carried out
– column internals such as trays/plates and/or packings which are used to enhance component separations
– a reboiler to provide the necessary
vaporisation for the distillation process
– a condenser to cool and condense the vapour leaving the top of the column
– a reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column
FEED
– liquid mixture that is to be
processed.
– introduced usually somewhere near the middle of the column to a tray
known as the feed tray.
– feed tray divides the column
into a top (enriching or rectification) section and a bottom (stripping) section.
REBOILER
– Heat is supplied to the reboiler
to generate vapour.
– Source of heat input can be
any suitable fluid, although in
most chemical plants this is normally steam. In refineries, the heating source may be the output streams of other columns.
– Vapour raised in the reboiler is
re-introduced into the unit at the bottom of the column.
– Liquid removed from the reboiler is known as the bottoms product or simply, bottoms.
CONDENSER
– Vapour moves up the column,
and as it exits the top of the unit, it is cooled by a condenser.
– Condensed liquid is stored in a holding vessel known as the reflux drum.
– Some of this liquid is recycled back to the top of the column and this is called the reflux.
– The condensed liquid that is removed from the system is known as the distillate or top product
Draw Tray – Total Draw
– All liquid is drawn from the tray.
– No net liquid flow internally down to the section below.
– No vapor-liquid contact on the tray
Purpose
Pumpback
- Good control of desired product spec
- Keep the trays/packing in the pumpback section completely wet all the times
Pumparound
– Sub-cooled liquid recycle in a column
– Removes heat from the column
– Reduces vapour traffic
– Condenses liquid product
Advantages
Pumparound
- Efficient recovery of heat at high temperature (used for preheat, reboiling)
- Higher capacity or smaller tower diameter above p/a section
- Unload overhead condensing system
Multi-component Separation
The number and type of fractionators required depend on the number of products to be made and the feed composition
Naphtha minus separation
From Naphta minus, go through Debutanizer to produce LPG & Full Range Naphta. LPG go through De-Ethanizer to produce Gas & LPG. Full Range Naphta go through Naphta Splitter to produce Full Range Tops & Naphta
Function of process control systems
to maintainyields and product quality by holding certain operating variables at the desired values, or by returning to those values after an upset.
Common Problem Faced In Distillation
- Flooding
- Dry trays
- Foaming
- Mis-operation
- Controls
- Feed Quality Variation
- Utility Problem
- Non-condensibles
- Water in feed
Multi-Component Separation
The number and type of fractionators required
depend on
the number of products to be made and
the feed composition
Contacting Device Selection
- Reduce investment cost
- Debottleneck throughput or improve product specification
- Save energy via lower pressure drop or higher efficiency
- Improve flexibility or turndown
- Provide reliable construction / easy maintenance
