Multiple effect evaporator

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In single effect evaporator steam is used to heat the liquids which provides the latent heat of vaporization. The vapors are condensed in the condenser where the latent is given up to the cooling water and it goes to waste.

To avoid this wastage two evaporators are connected together with the piping arrangement so that the vapor from the calandria of the first effect is used to heat the calandria of the second effect . this means that the calandria of the second effect is used as a condenser for the first time. So that the latent heat of vaporization is used to evaporate more quantity of the liquid instead of its going as waste. The vapor from the second effect then taken to a condenser and converted in to the liquids

In general not more than two or three effects are combined together to have economical and efficient evaporation of liquids

construction, working, applications

CONSTRUCTION

The construction of the multiple effect evaporator uses three evaporators so it is called as triple evaporators. The vapor from the first evaporator serves as heating medium for the second evaporator. Similarly vapor from the second evaporator serves as a heating medium fro the third evaporator. Last evaporator is connected to a vacuum pump.

WORKING

Parallel feed arrangement is used in this example.

Parallel feed

In this method a hot saturated solution of the fed is directly fed to ach of the three effects in parallel without transferring the materials from the one effect to the other. The parallel feed arrangement is commonly used in the concentration of the salts solution where the solute crystallizes on concentration without increasing the viscosity.

Operations

In the beginning the equipment is at room temperature and the atmospheric pressure. The liquids feed is introduced to all the three evaporators up to the level of the upper tube sheets.

1. The vent valves V1, V2 and V3 are kept open and all other valves are closed.

2. Now a high vacuum is created in the liquids chambers of evaporators.

3. The steam valves S1 and condensate valves are opened. Steam is supplied. Steam first replaces cold air in the steam space of 1st evaporator. When all the cold air is removed the valves V1 is closed.

4. The supply of steam is continued until the desired pressure P0 is created in the steam space of 1st evaporator. At this pressure the temperature of the steam is t0.

5. Steam gives its temperature to the liquids feed in the 1st evaporators and gets condensed. Condensate is removed through the valve C1.

6. Due to the heat transfer the liquids temperature increases and reaches the boiling point. During this process vapor will be generated from the liquid feed.

7. So formed vapor displaces air in the upper part of 1st evaporator. Move rover the vapor also displaces the air in the steam space of the 2nd evaporator.

8. After complete displacement of air by vapor in the steam compartments of 2nd evaporator the valves V2 is closed.

9. The vapor of 1st evaporator transmit its heat to its liquids of the 2nd evaporators and gets condensed, condensate is removed through the valve C2. These steps continue in the 3rd evaporator also.

As the liquid in the 1st evaporator gains temperature the difference in the temperatures between the liquids and the steam decreases hence therate of the condensate decreases. As the result the pressure in the vapor space of the 1st evaporators gradually increases to P1 by the increasing temperature to t1 which is the boiling point of the liquids in the 1st

evaporator and decreasing the temperature difference (t0 - t1).

A similar change takes place in the 2nd place evaporators and the liquids reaches the boiling point. Similarly the process will be repeated in 3rd evaporators. Finally three evaporators (or effects as they are called) come to steady state with the liquid boiling in all the three bodies.

As boiling proceeds liquid level in 1st evaporator comes down. Feed is introduced through the feed valve to maintain the liquid level constant. Similarly evaporation of liquids takes place in 2nd and 3rd evaporators. To maintain liquids level constant, feed valves F2 AND F3 are used for 2nd and 3rd evaporators, respectively. This process is continued until the evaporators reaches the desired viscosity.

Now the products valves are opened to collect the thick liquid. Thus in this evaporator there is a continuous supply of feed continuous supply of steam and continuous withdrawal of liquid from all the three evaporators. Hence the evaporators works continuously with all the temperature and pressure in balance.

The evaporators can be fed by forward feed method, backward fed method and mixed feed method. In the forward feed method the mother liquor is introduced in to 1st then transferred to 2nd and then to 3rd. In the backward fed method the mother liquior is introduced in to the 3rd evaporator, then transferred to 2nd and them transferred to 2nd and then transferred to 1st. ln the mixed feed method, the mother liquor is introduced in to 2nd evaporators then transferred to 3rd evaporators and then transferred to 1st evaporator.

PHARMACEUTICAL APPLICATIONS

Vertical tube evaporator or short tube evaporator is used in the manufacture of the cascara extract.

Vertical tube evaporator or short tube evaporator is used in the manufacture of salts and caustic soda.

Vertical tube evaporator or short tube evaporator is used in the manufacture of salts.

ADVANTAGES

1. It is suitable large scale and continuous operation.

2. It is highly economical when compared with single effect.

3. About 5 evaporators can be attached.

REFERENCES

1. Mehta RM, Pharmaceutics-I: heat processes - evaporation (1996), Page no: 169.

2. C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices - evaporation, 2001, page no. 356 - 359.

About the Author

Saraswathi.B's picture
Author: Saraswathi.B

Assistant Professor in St. johns college of pharmacy

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