Continuous Distillation Column 2016 (Updated/Modified)

Summary

This script explains the distillation process in a tower, detailing how feed is heated in a preheater, boiled under reduced pressure, and separated into vapors and liquid. It describes the use of trays or packing for heat transfer, the importance of reflux and reboiling for purity, and the critical role of temperature and pressure control. The script also discusses the impact of differential pressure on system efficiency and the use of pump-arounds for temperature regulation.

Takeaways

• 🔁 The distillation process involves moving feed through a series of steps including preheating, boiling, and condensation to separate components based on their boiling points.
• 💧 The feed is preheated under pressure to just below its boiling point before entering the distillation tower where lower pressure causes boiling and separation of lighter components.
• 🌡 The tower's operation relies on temperature control at various points, including the top, bottom, and feed point, to ensure effective separation of components.
• 📉 The temperature gradient, the difference in temperature from the bottom to the top of the tower, is crucial for the distillation process and must be carefully managed.
• 💨 The vapors from the boiling liquid rise up the tower while the heavier components move down and are collected at the bottom, some of which are drawn off as the bottom product.
• 🔄 Reboilers and condensers play a key role in the distillation process, with reboilers vaporizing the remaining lighter components at the bottom of the tower and condensers cooling and condensing the vapors into liquid.
• 🔄 Refluxing is a method used to increase product purity by reintroducing cooled liquid back into the top of the tower, promoting the condensation of heavier fractions.
• 🔩 Different types of distillation towers use various equipment like sieve trays, bubble caps, and packing to facilitate the separation of vapors and liquid and maximize heat transfer.
• 🌀 The differential pressure, the difference in pressure between the bottom and top of the tower, is an important indicator of the tower's performance and can signal operational issues.
• 🚰 The operation of the distillation tower is influenced by factors such as feed rate, boil up rate, and condenser efficiency, which can affect the tower's differential pressure and overall efficiency.
• ⚙️ Controlling the tower's pressure, often through a control valve on the overhead receiver or a vacuum system, is essential for maintaining the desired separation of components.

Q & A

What is the primary function of the preheater in a distillation system?

The preheater is used to heat the feed mixture under pressure to just below its boiling point before it enters the distillation tower.

Why does the feed start to boil when it enters the tower?

The feed starts to boil because the pressure in the tower is lower than the pressure in the preheater, causing the liquid to boil as it expands.

What happens to the vapors that rise in the distillation tower?

The vapors, which primarily contain the lighter components of the feed, rise in the tower and are eventually routed to a condenser to be cooled and condensed back into liquid form.

What is the role of the reboiler in a distillation tower?

The reboiler is a heat exchanger that vaporizes the lighter components remaining in the liquid from the bottom of the tower, and the vapors or a mixture of vapors and liquid is reintroduced into the tower.

How do sieve trays function in a distillation tower?

Sieve trays have many openings that allow vapors to rise through them, and they are designed to hold liquid, with dams or weirs that allow the liquid to overflow into downcomers, facilitating the separation of vapors and liquid.

What is the purpose of the condenser in a distillation system?

The condenser's purpose is to cool and condense the vapors from the tower back into liquid, which then flows into a receiver or accumulator.

What is refluxing in the context of distillation?

Refluxing is a method used to maximize product purity by returning part of the condensed liquid from the receiver back into the top of the tower, cooling the top of the tower and promoting the condensation of heavier fractions.

How does a reboiler affect the distillation process?

A reboiler heats the bottoms liquid, creating a mixture of vapors and liquid that is reintroduced into the tower, vaporizing any lighter fractions in the liquid at the bottom and reducing their amount in the bottoms product.

What is the significance of the temperature gradient in a distillation tower?

The temperature gradient, which is the difference in temperature from the bottom to the top of the tower, is crucial for the separation process, ensuring that lighter components vaporize and rise while heavier components condense and move down.

Why is pressure control important in a distillation system?

Pressure control is important because it affects the boiling temperature of the liquid and influences the efficiency and separation process within the tower. Incorrect pressure can lead to decreased product purity.

What is the role of differential pressure in a distillation tower?

Differential pressure, the difference in pressure between the bottom and top of the tower, is indicative of vapor flow and can signal operational issues such as an overload of feed or problems with the condenser or reboiler.

Outlines

00:00

🔬 Distillation Tower Operation

This paragraph describes the process of distillation within a tower. The feed is stored in a tank and pumped to a preheater where it is heated under pressure. The mixture then enters the tower where it starts to boil due to the lower pressure, causing lighter components to rise as vapors and heavier components to move down. The liquid at the bottom is either drawn off as product or sent to a reboiler to vaporize remaining light components. The vapors, often called boil-up, are directed to a condenser to cool and condense back into liquid, which is collected and can be recycled or drawn off as product. The tower uses sieve trays to separate vapors and liquid, with openings allowing vapors to rise and dams or weirs allowing liquid to overflow into downcomers, facilitating heat transfer and separation.

05:00

🌡 Controlling Distillation Tower Temperatures

The second paragraph focuses on the importance of temperature control in a distillation tower. It discusses how the feed mixture is preheated to regulate the temperature at the feed point, and how the reboiler adds heat to control the bottom temperature. The reflux rate, which is the cooled liquid pumped back into the tower, affects the top temperature. The paragraph also explains the concept of refluxing, where part of the condensed vapors is reintroduced into the tower to increase product purity by condensing heavier fractions. Reboiling is another method mentioned, where the bottom liquid is heated to form vapors that help reduce lighter fractions in the bottoms product. Temperature gradients and the effects of incorrect tower pressures on product purity are also highlighted.

10:03

🛠 Distillation Tower Pressure and Efficiency

The final paragraph delves into the role of pressure in distillation, emphasizing its impact on boiling temperatures and product purity. It explains how pressure is controlled through valves and vacuum systems, and how differential pressure, the difference between bottom and top pressures, is crucial for vapor flow and tower operation. The paragraph discusses how changes in differential pressure can indicate issues such as high feed rates, high boil-up rates, or condenser inefficiencies. It suggests that monitoring and adjusting these parameters are essential for maintaining efficient and effective distillation processes.

Keywords

💡Feed

In the context of the video, 'feed' refers to the mixture that is being processed in a distillation tower. It is the substance that is heated and separated into its components based on their boiling points. The script mentions that the feed is stored in a tank and then pumped to a preheater, indicating its importance in the initial stages of the distillation process.

💡Preheater

A 'preheater' is a device used to heat the feed mixture to a specific temperature before it enters the distillation tower. The script describes how the feed is heated 'just below its boiling point' in the preheater, which is crucial for initiating the boiling process once the feed enters the tower where the pressure is lower.

💡Boiling Point

The 'boiling point' is the temperature at which a liquid turns into vapor. In the script, it is mentioned that the feed is heated to just below its boiling point in the preheater, and the pressure difference between the preheater and the tower causes the feed to start boiling, which is the fundamental principle of separating components in a distillation process.

💡Vapor

Vapors are the gaseous form of the lighter components in the feed that rise within the distillation tower. The script explains how these vapors, primarily containing lighter components, rise up the tower, which is a key part of the separation process where lighter components are separated from heavier ones.

💡Tray

In distillation towers, 'trays' or 'sieve trays' are used to separate vapors and liquid. The script provides a detailed description of how trays have openings that allow vapors to rise and how they are designed to hold liquid, facilitating the separation process through contact between vapors and liquid.

💡Reboiler

A 'reboiler' is a device connected to the bottom of the distillation tower that vaporizes the lighter components remaining in the liquid from the bottom of the tower. The script mentions that the reboiler is usually a heat exchanger, emphasizing its role in providing the necessary heat for the distillation process.

💡Condenser

A 'condenser' is used to cool and condense the vapors into liquid after they have risen up the tower. The script describes the condenser's purpose and how the liquid from the condenser flows into a receiver or accumulator, which is part of the process of collecting and managing the distillation products.

💡Reflux

Refluxing is a method used in distillation to increase the purity of the products. The script explains that part of the liquid from the receiver is pumped back into the top of the tower as 'external reflux,' which cools the top of the tower and aids in the separation of heavier fractions.

💡Packing

'Packing' refers to various devices used in distillation towers instead of trays to increase the surface area for contact between vapors and liquid, thus enhancing heat transfer. The script describes different types of packing, such as 'rash rings' and 'burl saddles,' and how they break up the liquid to improve the efficiency of the distillation process.

💡Temperature Gradient

The 'temperature gradient' is the decrease in temperature from the bottom to the top of the distillation tower. The script mentions that this gradient is important for the separation process, as it affects the boiling and condensing of different components within the tower.

💡Differential Pressure

Differential pressure is the difference in pressure between the bottom and the top of the distillation tower. The script explains that changes in differential pressure can indicate problems with the feed rate, boil up rate, or condenser efficiency, and it is a critical parameter for monitoring the operation of the distillation process.

Highlights

Feed is stored in a tank and moved to a preheater using a pump during operation.

Mixture in the preheater is heated under pressure to just below boiling point.

Feed boiling in the tower due to lower pressure compared to the preheater.

Vapors from boiling liquid, containing lighter components, rise in the tower.

Heavier components in the feed move down the tower and collect at the bottom.

Some liquid is drawn off as the bottom product, and some is routed to a reboiler.

Reboiler is a heat exchanger designed to vaporize lighter components from the bottom liquid.

Vapors from the reboiler rise in the tower, aiding in the distillation process.

Vapors are routed to a condenser to cool and condense into liquid.

Liquid from the condenser flows into a receiver or accumulator.

Part of the receiver's liquid is pumped back into the tower, and part is drawn off as the overhead product.

Distillation Tower uses sieve trays to separate vapors and liquid.

Trays have many openings allowing vapors to rise and liquid to build up, facilitating heat transfer.

Bubble caps disperse rising vapors through the liquid for maximum heat transfer.

PAC towers contain packing instead of trays, increasing surface area for heat transfer.

Packing materials must be compatible with the liquid and operating conditions.

Refluxing is a method to increase product purity by condensing and reintroducing vapors.

Reboiler heats the bottom liquid to form a mixture of vapors and liquid, reducing lighter fractions in the bottoms product.

Temperature control at the top and bottom of the tower is crucial for product purity.

Feed point temperature should be within the boiling range of the mixture.

Temperature gradient from bottom to top of the tower is essential for separation.

Pressure affects boiling temperature and is an important factor in distillation.

Differential pressure indicates the flow of vapors and can signal potential problems.

Pump arounds help control internal reflux temperature by cooling and reintroducing liquid.

Transcripts

in this example the feed is stored in a

tank during operation a pump is used to

move the feed from the tank to a

preheater

in the preheater the mixture is heated

under pressure to just below its boiling

point the pressure in the tower is lower

than the pressure in the preheater so

when the feed enters the tower it starts

to boil the vapors from the boiling

liquid which primarily contained the

lighter components in the feed rise in

the tower the remaining liquid which

consists primarily of the heavier

components in the feed moves down the

tower and collects at the bottom some of

this liquid is drawn off as the bottoms

product and some of it's routed to a

device called a reboiler which is

connected to the bottom of the tower the

reboiler is usually a heat exchanger

that's designed to vaporize the lighter

components that remain in the liquid

from the bottom of the tower vapours

from the reboiler or in some cases a

mixture of vapors and liquid re-enter

the tower the vapors then rise up in the

tower these vapors and the heat they

contain are often referred to as boil up

the hot boiler provides heat that's

needed for the distillation process to

take place in the tower the vapors that

rise up in the tower are routed to a

condenser the purpose of the condenser

is to cool and condense the vapors into

liquid from the condenser the liquid

flows into a receiver or accumulator the

receiver provides a reservoir for the

liquid part of the liquid from the

receiver is pumped back into the top of

the tower and part of it is drawn off as

the towers overhead product

this is a simplified illustration of the

inside of one type of distillation Tower

this tower uses trays called sieve trays

to separate vapors and liquid the trays

are spaced throughout the tower

they're called sieve trays because they

have many openings in them like a sieve

here's a closer view of some of the

trays we've exaggerated the size of the

openings to make them easier to see the

openings in the trays allow vapors to

rise through the trays on their way up

the tower each tray is also designed to

hold liquid dams or Weir's on each tray

allow liquid to build up on the tray

liquid that overflows the Weir's flows

into down comers the downcomers channel

the liquid from tray to tre down the

tower when the upward moving vapors and

the downward flowing liquid come in

contact on each tray the vapors transfer

some of their heat to the liquid two

things happen the heavier components of

the vapors become cooler and condense

into liquid and the lighter components

of a liquid boil the vapors then rise

toward the next tray as this process

continues the rising vapors contain a

higher concentration of lighter

components and a lower concentration of

heavier component let's start with a

tower that uses trays with bubble caps

the holes in each tray are covered with

caps called bubble caps the slots in

these bubble caps disperse the rising

vapors through the liquid on the tray

each bubble cap has many slots and each

tray has many bubble caps to spread out

the vapors this ensures a maximum

contact between vapors and liquid so

that maximum heat transfer can take

place

another type of tower called a PAC tower

contains layers of devices called

packing instead of trays with bubble

caps there are many different types of

packing some towers have sections that

are filled with cylindrical rings like

the one Illustrated here it's known as a

rash agreeing another type of packing is

known as a burl saddle the packing

breaks up the liquid so that it flows

over a large amount of surface area this

exposes more of the liquid

the vapors and increases heat transfer

from the vapors to the liquid packing

can be made for many different materials

including porcelain copper aluminum and

iron the main requirement is that the

material must be compatible with the

liquid in the tower and the conditions

under which the tower is operated this

is a section of another type of packing

from a distillation tower it's called a

packing grid each layer in the grid has

spaces for vapors to rise through the

packing on their way up the tower liquid

flows over these surfaces which channel

the liquid as it flows down the tower

the packing grid provides a great deal

of surface area where contact between

the vapors and the liquid can take place

so heat transfer is maximized

distillation systems use several methods

to help maximize the purity of the

products one of these methods is called

refluxing the vapors coming off the top

of the tower are condensed in a

condenser and then collected in a

receiver part of the liquid from the

receiver is sent to storage or to other

units in the plant as the towers

overhead product the rest of the liquid

is pumped back into the top of the tower

the liquid that's reintroduced into the

tower is called external reflux because

it consists of liquid that was cooled in

the condenser the external reflux is

cooler than the liquid in the top of the

tower as the external reflux cools the

top of the tower vapors made of heavier

fractions condense the liquid made of

heavier fractions flows down the tower

and is referred to as internal reflux

meanwhile the top of the tower is still

hot enough to keep the lighter fractions

in vapor form the vapors are drawn off

the top of the tower and into the

condenser refluxing increases the purity

of the overhead product because

condensing the vapors made of heavier

fractions keeps them out of the stream

of vapors that leaves the top of the

tower another method used to maximize

product purity is called reboiler the

bottoms liquid that's drawn off from the

tower is sent to a heater called a

reboiler the rest of the bottoms liquid

is sent to storage or to other units in

the plan

as the towers bottoms product the

reboiler heats the liquid it receives so

that a mixture of vapors and liquid is

formed depending on the system either

vapors or the mixture of vapors and

liquid is then reintroduced into the

tower the hot vapors cause any lighter

fractions in the liquid at the bottom to

vaporize and move up the tower this

reduces the amount of lighter fractions

in the bottoms product the reboiler

provides the major portion of the heat

that's required to make the distillation

process work we'll begin with the

temperature at the top of the tower the

temperature at the top of the tower

should be at or slightly above the

boiling temperature of the desired

overhead product at the operating

pressure of the tower if the temperature

at the top of the tower is higher than

it should be more of the heavier

components will vaporize and become part

of the overhead product instead of

flowing down the tower as a liquid on

the other hand if the temperature at the

top of the tower is lower than it should

be less of the lighter components will

vaporize some of the lighter components

will remain as a liquid and flow down

the tower the temperature at the bottom

of the tower is also important the

temperature at the bottom of the tower

is usually slightly below the boiling

point of the heavier component if the

temperature at the bottom of the tower

is too high more of the heavier

components will vaporize and move up the

tower instead of remaining as a liquid

if the temperature at the bottom of the

tower is too low less of the lighter

components will vaporize and move up the

tower another place where temperature

control is important is at the feed

point the temperature at the feed point

should be within the boiling range of

the mixture the temperature at the feed

point should be close to the temperature

of the feed tray the temperature of the

feed tray depends on its physical

location in the tower for example the

lower the feed tray is in the tower the

higher its temperature will be if the

temperature at the feed point is higher

than it should be more of the heavier

components will vaporize and move up the

tower instead of moving down the tower

as a liquid on the other hand if the

feed point temperature is too low less

of the lighter components

vaporize and more lighter components

will end up in the bottom of the tower

the temperature decreases as the

material moves higher in the tower the

gradual decrease in temperature from the

bottom of a distillation tower to the

top is called the temperature gradient

the temperature gradient is measured in

terms of the difference between the

temperature at the bottom of the tower

and the temperature at the top of the

tower

you

one way is to control the temperature of

the feed mixture by using a preheater

this regulates the temperature at the

feed point at the bottom of the tower

temperature is controlled by the amount

of heat that is added by the reboiler

this added heat is referred to as boil

up the temperature at the top of the

tower is controlled by the amount or the

temperature of the cool liquid that's

pumped back into the tower from the

overhead receiver this is called the

reflux rate increasing the reflux rate

decreases the temperature at the top of

the tower some distillation systems

contain equipment known as pump arounds

the purpose of a pump around is to

remove hot liquid from the tower and

pump it through a cooler the cooled

liquid is then reintroduced at a higher

level in the tower a pump around helps

control the temperature of the internal

reflux since pressure affects the

boiling temperature of a liquid it's an

important factor in the distillation

process if precious in a distillation

system are not correct product purity

could decrease tower pressure is often

controlled by a pressure control valve

located on the overhead receiver this

valve controls pressure by releasing

vapors and any non condensable gases

that have collected in the receiver in

some cases a vacuum system draws gases

from the receiver to control tower

pressure another pressure that's

important to an operator is the towers

differential pressure the towers

differential pressure is the difference

between the pressure at the bottom of

the tower and the pressure at the top of

the tower this difference in pressure is

caused by the flow of vapors in the

tower without vapor flow there is no

differential pressure generally if the

rate at which the vapors move up the

tower decreases the differential

pressure will also decrease and if the

rate at which the vapors move up the

tower increases the differential

pressure will increase changes in the

differential pressure may indicate that

a problem exists for example an increase

in differential pressure could be an

indication that the feed rate is too

high too much feed entering the tower

overload it if this happens the

differential pressure will increase and

the tower will be unable to make the

desired separation in this case it might

be necessary to decrease the feed rate

now changes in the towers differential

pressure may be caused by other problems

for example if the differential pressure

increases the boil up rate may be too

high in other words the reboiler may be

returning too much vapor or vapor liquid

mixture to the tower this problem can be

corrected by reducing the boil up rate

another problem that may affect the

towers differential pressure is a

decrease in condenser efficiency if the

condensers tubes are plugged or there's

not enough cooling water flowing through

the condenser the condensers pressure

will increase as a result the flow of

vapors from the tower to the condenser

will decrease and so will the vapor flow

up the tower this means that the tower

top pressure will increase and the

differential pressure will decrease if

the condenser problem is suspected the

condenser should be checked and

corrective actions should be taken if

necessary

you