3. METHODS OF FEEDING OF EVAPORATORS

Evaporators are classified by the number of effects . In case of a single-effect evaporator, the vapor from the boiling liquor is condensed and the concentrated product is withdrawn from the bottom of the evaporator. Although the operation is simple, the device does not use steam efficiently. Typically 1.1 to 1.3 kg of steam is required to evaporate 1 kg of water.

The steam consumption per unit mass of water evaporated can be increased by putting more than one evaporator in series such that the vapor from one evaporator is used in the second evaporator for heating. The vapor from the second evaporator is condensed and the arrangement is called double-effect evaporators. The heat from the vapor generated in the first evaporator is used in the second evaporator. Evaporation of water is nearly doubled in double effect evaporation system compared to single effect per unit mass of steam used. Additional effects can be added in series in the same way to get a triple-effect evaporator, quadruple-effect evaporator and so on. There are several configurations based on feeding arrangement.

3.1.  Forward feed

The typical feeding method of multi-effect evaporators is forward. Both feed and steam are introduced in the first effect and the feed passed from effect to effect parallel to the vapor from the earlier effect. Concentration increases from the first effect to the last. Forward feeding operation is helpful when the concentrated product may degenerate if exposed to high temperature. The product is withdrawn from the last effect. It requires a pump for feeding of dilute solution to the first effect. A pump removes thick liquor from the last effect. The liquid from one effect to the next effect also can be transferred without a pump as the flow occurs in the direction of decreasing pressure. The arrangement of forward feeding is shown in Figure 3.7a .

3.2. Backward feed

In backward feed configuration, the feed enters at the last effect (coldest effect) and is pumped through the successive effects. The product is withdrawn from the first effect (hottest) where the steam is introduced (Figure 3.7b). This method of feeding requires a pump between each pair of effects to transfer liquid from lower pressure effects to higher pressure effects. It is advantageous when cold feed entering needs to be heated to a lower temperature than in forward feed operation. Backward feed is commonly used when products are viscous and exposure to higher temperature increases the rate of heat transfer due to reduction in viscosity of the liquid.

3.3. Mixed feed

In the mixed feed operation, the dilute feed liquid enters at an intermediate effect and flows in the next higher effect till it reaches the last effect of the series. In this section, liquid flows in the forward feed mode. Partly concentrated liquor is then pumped back to the effect before the one to which the fresh feed was introduced for further concentration as shown in Figure 3.7c. Mixed feed arrangement eliminates some of the pumps needed in backward configuration as flow occurs due to pressure difference whenever applicable.  

3.4. Parallel feed

The fresh feed is introduced to each effect and in this configuration the product is withdrawn of from the same effect in parallel feed operation (Figure 3.7d). In parallel feeding, there is no transfer of liquid from one effect to another effect. It is used primarily when the feed is saturated and the product is solid containing slurry. This is most common in crystallizing evaporators.