construction-working-types of evaportor-open kettle or pan-falling film-forced circulating evaporator-multiple effect evaporator-feed mode arrangement-foward feed-backward feed-mixed feed

 

Table of content:

1.1          Evaporator and Why evaporation is a cooling process?

1.2          Construction of evaporator:

1.3          Working of the evaporator:

1.4          Is there any difference between evaporator and condenser?

1.5          Types of evaporators:

1.6          Open kettle or pan evaporator:

1.7          Falling film evaporator:

1.8          Forced circulating evaporator:

1.9          Multiple effect evaporator:

1.10        Feed mode arrangements:

1.11        Why backward feed mode is more give more economy than forward feed mode?

what is evaporators and why evaporation is a cooling process?

As we all know, the evaporator is a device that turns liquid into a gaseous state and causes cooling.

Evaporation is a process which turns liquid into vapours. 

 Why does evaporation cause cooling?

 Evaporation is a cooling process because it absorbs energy (which is in the form of heat) from surrounding to make the evaporation process happens. In the evaporation phenomenon, the energy or heat loses to change the phase of liquid into vapors. The loss of heat leads to down temperature and a fall in temperature causes cooling. That’s why evaporation causes cooling.

Evaporation cause cooling effect.

Construction of evaporator

If we search for the construction of an evaporator, then unfortunately we get very complex data which makes us confused about equipment but fortunately its construction is so simple.

construction of evaporator.

Each evaporator is constructed of four parts which are.

1              A heat exchanger

2              A vapor separator

3              A Condenser

4              A Vacuum pump

In evaporators, these all parts perform their specific work.

•             Heat exchanger provides sensible heat (to raise the liquid to its boiling point) and latent heat (to convert the phase of liquid into vapors).

•             Vapor separator to separate vapors from concentrate.

•             Condenser to remove vapors as condensate.

•             Vacuum pump to reduce the pressure of evaporator. It causes a lower boiling point of solvent which leads to faster evaporation.

Working of evaporators:

An evaporator absorbs energy from the surrounding or from heating media in evaporators and provides it to the liquor in the form of sensible and latent heat and converts liquid into vapor form causing cooling.

latent heat absorbs from surrounding leads to phase change of liquid into vapors.

Is there any difference between evaporator and condenser?

The working principle of the evaporator is opposite to the condenser. Both evaporator and condenser have a property to call them heat exchangers. As the evaporator absorbs heat and the condenser release heat.

evaporators absorb heat while condensers release heat.

Types of evaporators:

Evaporators are divided into many different types based on the process of heat exchange and their different design. Some types are the following.

1              Open kettle or pan evaporator:

The simplest type of evaporator consists

•             A pan or kettle (in which liquid boils)

•             A steam jacket outside the pan (to provide heat to liquid) or sometimes the pan is directly fired instead of a steam jacket.

Open kettle or pan evaporator.

For which purpose is an open kettle or pan evaporator used?

•             It is used to concentrate jams.

•             Used for preparation of jellies

•             Used for preparation of pharmaceutical products.

2              Falling film evaporator:

A falling film evaporator is a tubular heat exchanger in which vertical tubes are arranged in a shell of an evaporator. Its design relies on gravity, Feed flow through tubes downward like falling in tubes.

Falling film evaporator.

For which is purpose falling film evaporator use?

•             It is used to concentrate the material which is heat sensitive.

•             When thermal and flow properties are poor.

•             When feed has high contents of solids and has high viscosity.

•             It is also used to recover solvent and to produce a high-quality product that’s why it is also known as solvent recovery.

3              Forced circulating evaporators:

It is a kind of tubular heat exchanger. It consists of a heat exchanger (to provide heat), a separator (to separate vapors from liquid), and a circulating pump (to pump liquid portion and resent to the heat exchanger).

Forced circulating evaporator.

For which purpose forced circulating evaporator use?

•             It is used for the liquids having a fouling tendency.

•             It is used for viscous liquids.

4              Multiple effect evaporators:

The effects of the multiple effect evaporator are arranged in series.

What is the effect of an evaporator?

The effect of an evaporator consists of three components.

A Heat exchanger,

A vapor-liquid separator, and

A re-circulation pipes

The combination of all three is known as an effect.

In multiple effect evaporators, the feed enters the first effect, and the concentrated liquor is entered to the second effect then the third, and so on. Finally, we get our required product from the last effect.

Similarly, we have the movement of vapors. Where steam enters the first effect and vapors generated in the first effect are enter the second effect as heating media for the second effect and vapors generated from the second effect enter the third effect as the heating media for the third effect and so on. These vapors enter to condenser from the last effect to condensate these vapors. The whole assembly is connected to a vacuum pump to reduce the pressure of effects.

Multiple effect evaporator.

In a multiple effect evaporator, the temperature of the first effect is T1 so the temperature of both vapors and concentrated liquor is similar. Then what is the driving force in the second effect to cause evaporation without temperature difference?

The driving force is maintained in the second and further effects by reducing the pressure of these effects with help of a vacuum pump.

Let’s consider, that the pressure of the first effect is P1 and so on.

So,

P1 > P2 > P3

The pressure of second effect P2 is less than the pressure of the first effect and the pressure of third effect P3 is less than second effect P2.

When the pressure of the second effect suddenly falls from P1 to P2 then flashing occurs inside the effect. Due to flashing the liquid will try to occupy the pressure P2 and it will come with a low temperature because with a reduction in pressure the boiling point is also reduced.

So whatever temperature the liquid entering the second effect will automatically occupy T2. And because of flashing (T1-T2) whatever heat is lost converts some amount of liquid into vapors.

These effects are arranged to reduce the pressure of the effect to lower its boiling point and temperature. The lowering of feed temperature increases the heating area required and decreases the economy.

T1 > T2 >T3

In how many manners we can provide feed in multiple effect evaporators?

In multiple effect evaporators, the feed is provided in 3 manners mostly.

1.            Forward feed mode

2.            Backward feed mode

3.            Mixed or parallel feed mode

• Forward feed mode:

In forward feed, the flow of solution to be concentrated is parallel to the stream flow. (Co current)

In forward feed mode, the steam and feed enter the first evaporator, and partially concentrated liquid is sent to the second effect, and so on. Feed flows from the higher pressure to lower pressure.

Does not need a pump for moving the solution from effect to effect as a vacuum is maintained in the last effect.

In forward feed, the maintenance charges and power costs are less.

The forward feed arrangement is less effective thermally.

 

Forward feed multiple effect evaporator.

• Backward feed mode:

In backward feed, the flow of solution to be concentrated is in the opposite direction of steam. (Counter current)

In backward feed mode, the feed is introduced in the last effect, and steam is introduced in the first effect.

In backward feed mode, the transfer between successive effects is done by the pump.

In backward feed, the maintenance charges and power costs are more for the same duty.

The backward feed arrangement is more effective thermally.

Backward feed mode is better in a multiple-effect evaporator because it gives higher economy than forward feed and tackles the problems of crystallization on the increase of concentration of the feed.

Backward feed multiple effect evaporator.

1. Why backward feed mode gives more economy than forward feed mode?

In the backward feed, the solution is heated in each effect which results in a better economy than that of the forward feed in which heating of feed is just done in the first effect.

• Mixed feed mode:

In mixed feed mode, the feed is introduced to an intermediate effect and flows to the first effect from where it is fed to the last effect for final concentrate, and steam is introduced to the first effect.

Mixed feed multiple effect evaporator.

This arrangement is the combination of the forward and backward feed to achieve the best overall performance.