The vertical tube evaporator discussed earlier is a single effect evaporator (Figure). Such evaporators are connected in several ways to achieve large-scale evaporation as well as a greater economy. Although multiple effect evaporators are not used in the pharmaceutical industry, the principles are of interest and should be understood. This is illustrated using an example of a triple effect evaporator with a parallel feed mechanism.
Advantages of Multiple Effect Evaporator
- It is suitable for large-scale and continuous operations.
- It is highly economical when compared with a single effect.
- About 5 evaporators can be attached.
Construction of Multiple Effect Evaporator
The construction of a multiple-effect evaporator is shown in Figures using 3 evaporators, i.e. triple effect evaporator. The other aspects of the construction of the vertical tube evaporator remain the same as mentioned earlier. A perusal to Figure indicates that the vapor from the first evaporator serves as a heating medium for the 2nd evaporator. Similarly, vapor from the 2nd evaporator serves as a heating medium for the 3rd evaporator. vacuum pump. The last evaporator is connected to a vacuum pump.
Working of Multiple Effect Evaporator
A parallel feed arrangement is used in this example.
Parallel feed: In this method, a hot saturated solution of the feed is directly fed to each of the three effects (evaporation) in parallel without transferring the material from one effect to the other. The parallel feed arrangement is commonly used in the concentration of salt solution s. where the solute crystallizes on concentration without increasing the viscosity.
Operations: In the beginning, the equipment is at room temperature and atmospheric pressure. The liquid feed is introduced to all three evaporators up to the level of the upper tube sheets. The following operations are attempted to achieve the effects as specified below.
- The vent valves V1 V2 and V3 are kept open and all other valves are closed (not shown in Figure).
- Now a high vacuum is created in the liquid chambers of evaporators.
- The steam valve S₁ and condensate valve C₁ 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 valve, Vi is closed.
- 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
- Steam gives its temperature to the liquid feed in the 1st evaporator and gets condensed. Condensate is removed through the valve C₁.
- Due to heat transfer, the liquid temperature increases and reaches the boiling point. During this process, vapor will be generated from the liquid feed.
- So formed vapor displaces air in the upper part of the 1st evaporator. Moreover, the vapor also displaces the air in the steam space of the 2nd evaporator.
- After the complete displacement of air by a vapor in the steam compartment of the 2nd evaporator, valve V2 is closed.
- The vapor of 1st evaporator transmits its heat to the liquid of 2nd evaporator and gets condensed. Condensate is removed through valve C2. These steps continue in the 3rd evaporator also.
As the liquid in the 1st evaporator gains temperature, the difference in temperatures between the liquid and steam decreases, hence, the rate of condensation decreases. As a result, the pressure in the vapor space of the 1st evaporator gradually increases to P₁ by increasing the temperature to which is the boiling point of the liquid in the 1st evaporator and decreasing the temperature difference (t0-t1).
A similar change takes place in the 2nd evaporator and the liquid reaches the boiling point. Similarly, the process will be repeated in 3rd evaporator. Finally, three evaporators (or effects as they are called) come to a steady-state with the liquid boiling in all three bodies.
As boiling proceeds, the liquid level in 1st evaporator comes down. Feed is introduced through the feed valve to maintain the liquid level constant Similarly evaporation of liquid takes place in 2nd and 3rd evaporators. To maintain liquid levels constant, feed valves F2 and Fj are used for 2nd and 3rd evaporators, respectively. This process is continued until the liquid in all the evaporators reaches the desired viscosity.
Now the product valves are opened to collect the thick liquid. Thus in this evaporator, there is a continuous supply of feed, a continuous supply of steam, and a continuous withdrawal of liquid from all three evaporators. Hence, the evaporator works continuously with all the temperatures and pressures in balance.
The evaporator can also be fed by the forwarding feed method, backward feed method, and mixed feed method. In the forward feed method, the mother liquor is introduced into 1st then transferred to 2nd, and then to 3rd. In the backward feed method, the mother liquor is introduced into the 3rd evaporator, then transferred to 2nd, and then transferred to 1st. In the mixed feed method, the mother liquor is introduced into the 2nd evaporator then transferred to the 3rd evaporator, and then transferred to the 1st evaporator.
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