310204g Distillation And Fractionation Flashcards
What is distillation?
Distillation is a broad term used to describe a group of methods that use heat to separate the components of a liquid mixture
The differing vapour pressures of the components facilitates the separation
Vapour from a boiling mixture contains higher concentrations of the lower boiling point components
The condensation of the vapour produces a liquid that is enriched with the lower boiling point components
What are the 2 types of distillation?
- Fractional
- Simple
What is the difference between fractional distillation and simple distillation?
Fractional distillation is used when the boiling points of components in a mixture are close to each other
Simple distillation is used when the boiling points are significantly different.
Define vapour pressure.
Vapor pressure is the pressure of a vapor in contact with its liquid form
Define liquid vapor pressure.
Liquid vapour pressure is the pressure at equilibrium
At equilibrium, the vapour space is saturated or is holding as much vapoir as possible.
Define boiling point.
Boiling point is the temperature at which a liquid boils
A liquid boils when its vapor pressure begins to exceed the pressure surrounding it.
Boiling is the formation of vapor bubbles within the liquid.
True or false
Liquids that have high vapour pressures do not need as much heat to boil as liquids that have low vapour pressures.
True
Define bubble point
Bubble point refers to mixtures of materials that have different vapour pressures
Bubble point is the lowest temperature at which vapour bubbles form
What is the difference between boiling point and bubble point?
Boiling point refers to peer materials whereas bubble point refers to mixtures of materials that have different vapour pressures
Define volatility.
Volatility is indicates the tendency of a liquid to change into a vapour state
A liquid that has a high vapour pressure with respect to its surrounding pressure is volatile
With equal pressure, do low volatility solutions have higher or lower boiling points.?
With equal pressure, low volatility solutions have higher boiling points.
With equal pressure, do high volatility solutions have lower or higher boiling points
With equal pressure, high volatility solutions have lower boiling points
Increasing the temperature of a liquid increases or decreases the volatility of the liqui
Increasing the temperature of a liquid increases its volatility
Decreasing the pressure of a liquid increases or decreases the volatility of the liquid?
Decreasing the pressure of the liquid increases its volatility
What is relative volatility?
Relative volatility describes the difference in volatility of components within a mixture.
It is an indication of how much effort is needed to separate the 2 components.
Distillation equipment separates materials based on what?
Distillation equipment separates materials based on their differing vapour pressures
What are the 4 main components of a basic distillation unit?
- The distillation tower or column
- The reboiler
- The condenser
- The reflux drum
What are the advantages and disadvantages of using tower reflux?
Advantages:
1. Read flux flow reduces the number of trays required in a tower
Disadvantages:
1. Reduces distillate output
2. Increases the mole fraction of distillate in the tower bottoms
What are the 3 common styles of reboilers?
- Kettle reboiler
- Thermo syphon reboiler
- Internal reboiler
What is the function of the tower reboiler?
The tower reboiler generates the upward moving vapor stream
The fuel can be of any type, but most used steam
What are the pros and cons of a kettle reboiler?
Pros:
1. Dependable
2. Stable
3. Heat exchanger tubes are covered, which reduced deposit buildup
Cons:
1. Cost
2. High liquid hold up; if the tower goes off specification, there is a significant amount of material in the reboiler that must be reprocessed
What are the pros and cons of a thermo syphon reboiler
Pros:
1. Less expensive than a kettle style reboiler
2. Less liquid hold up than a kettle style reboiler if it goes off specification
Cons:
1. Subject to surging, which can cause column upsets
2. The tubes are more susceptible to fouling; the accumulation of unwanted material that is a detriment to the function of the reboiler
What are the pros and cons of an internal reboiler?
Pros:
1. Compact
2. Easy to operate
Cons:
1. Limited capacity of the contact surface area results in limited energy injection
2. The tower design must include additional large flanging to support the large tube bundle
3. When the reboiler requires servicing, the tower must be shut down
What are some common problems that could cause a column upset in a distillation tower?
- Changing feed composition; changes in the mole fraction of key product and the feed also changes the purity of the distillate.
- Flooded trays; too much reflux running down the tower can flood a tray to the point where excessive liquid builds up, causing a vapour lock to occur underneath the tray. The vapour pressure increases until it pushes through causing the tower to eject liquid out the top, this is sometimes referred to as puking the tower
- Dry trays; an excess of heat at the bottom of the tower increases the velocity of the upward traveling vapours. At a specific threshold, the velocity is high enough to drive the liquid off of a tray, which makes it ineffective. The downcomer from the tray above thr dry tray no longer has a liquid seal, so the vapour finds its way up and circumvents the tray above. This results in the column effectively losing 2 stages for each dry tray.
In a fractionation tower, the smallest molecules with the lowest boiling points come off the top or bottom of the tower?
The smallest molecules with the lowest boiling points come off the top of each fractionation tower
What happens to the molecules in the fractionation tower as they travel down the tower?
As the molecules travel down the tower, they gradually get larger with higher boiling points, higher Ignition temperatures and have greater viscosity or resistance to flow
True or false
Fractionsion can occur as different steps in a single tower or in successive distillation units.
True
What is steam distillation or steam stripping?
Is a type of distillation used for temperature sensitive materials
The process relies on the principle that a mixture of components that are immiscible (do not mix well together), often have lower boiling point temperatures then the pure components of the mixture
Steam is immiscible with hydrocarbons and the introduction of superheated steam into a hydrocarbon mixture either increases the vapour pressure of the hydrocarbons or reduces the boiling point.
A separation now takes place at a lower temperature
This effect can be further amplified by pulling a vacuum on this mixture.
The volatile components leave with the steam and separate.After flowing through the condenser
The bottoms have been stripped of the lighter components
The columns can be either the tray type or packed columns
If component A has a lower vapour pressure than component B what can be said about their boiling points?
Component A has a higher boiling point
Liquids that have high vapour pressures do not need as much heat to boil as liquids that have low vapour pressures.
Distillation uses what property of the mixture components to facilitate a separation?
Distillation uses vapour pressures to facilitate a separation
Explain the purpose of distillation tower pressure control
The purpose of tower pressure control is to provide the necessary temperature ranges for boiling to occur
Give 2 advantages to tower reflux
- Controls the purity of the overhead product
- Reduces the number of trays required in a tower
What would be observed in the temperature profile of a tower if it had a dry tray?
There would be a temperature increase around the dry tray
How do liquids enter and leave distillation tower trays?
Liquids enter and leave the tower trays through the downcomers
What is the difference between a packed column and a trayed distillation column?
Packed columns use packing material to achieve liquid vapour contact and have a continuous enrichment profile along the tower
Trayed columns use towertrays and have a stepwise enrichment profile.
Explain the difference between fractionation and distillation
Fractionation refers to breaking a composition into multiple groups
Distillation refers to separating or purifying a single component from a group
Tower tops undergo ________ while the tower bottoms undergo ________
Tower tops undergo rectification, while the tower bottoms undergo stripping.
What effect does steam have on hydrocarbons in a stripping tower?
Steam lowers the boiling point
