COMPARISION OF SINGLE AND MULTIPLE EFFECT EVAPORATOR

Comparison of single  and multiple effect evaporator

Single-effect evaporators 

• Single-effect evaporators are used 

1. when the throughput is low, 
2. when a cheap supply of steam is available, 
3. when expensive materials of construction must be used as is the case with corrosive feedstocks 
4. when the vapour is so contaminated so that it cannot be reused. 

• The feed and saturated steam with temperature at TF and TS respectively enter the heat- exchange section 
• Condensed steam leaves as condensate or drips.
• The solution in the evaporator is assumed to be completely mixed.
• Hence, the concentrated product and the solution in the evaporator have the same composition.
• Temperature T1 is the boiling point of the solution.
• The temperature of the vapor is also T1, since it is in equilibrium with the boiling solution.
• The pressure is P1, which is the vapor pressure of the solution at T1
• If the solution to be evaporated is assumed to be dilute and like water, then 1 kg of steam condensing will evaporate approximately 1 kg of vapor (if the feed entering has TF near the boiling point) 
• Single-effect evaporators are often used when the required capacity of operation is relatively small and/or the cost of steam is relatively cheap compared to the evaporator cost.
• However, for large-capacity operation, using more than one effect will markedly reduce steam costs
• The heat requirements of single-effect continuous evaporators may be obtained from mass and energy balances. 

Multiple - effect evaporator

• The single effect evaporator uses rather more than 1 kg of steam to evaporate 1 kg of water. 
• The latent heat of the vapor leaving in single effect evaporator is not used but is discarded.
• Much of this latent heat, however, can be recovered and reused by employing a multiple - effect evaporator, that is, vapor from one effect serves as the heating medium for the next.
• The economy of the system, measured by the kilograms of water vaporized per kilogram of steam condensed, increases with the number of effects.
• in multiple effect evaporator, the pressure in each effect is lower than that of the effect to which it receives steam and higher than that of the effect to which it supplies vapors
• Each effect, in itself, act as a single effect evaporator, and each has a temperature drop across its heating surface corresponding to the pressure drop in that effect.

Forward-feed multiple - effect evaporator 

• A simplified diagram of a forward-feed triple- effect evaporation system is shown in Fig. 

• If the feed to the first effect is near the boiling point at the pressure in the first effect, 1 kg of steam will evaporate almost 1 kg of water.
• The first effect operates at a temperature that is high enough that the evaporated water serves as the heating medium to the second effect.
• Here, again, almost another kg of water is evaporated, which can then be used as the heating medium to the third effect.
• As a very rough approximation, almost 3 kg of water will be evaporated for 1 kg of steam in a three-effect evaporator.
• Hence, the steam economy, which is kg vapor evaporated/kg steam used, is increased.
• This also holds approximately more than three effects.
• However, the increased steam economy of a multiple-effect evaporator is gained at the expense of the original first cost of these evaporators
• In forward-feed operation as shown in Fig. fresh feed is added to the first effect and flows to the next in the same direction as the vapor flow.
• This method of operation is used when the feed is hot or when the final concentrated product might be damaged at high temperatures.
• The boiling temperatures decrease from effect to effect. This means that if the first effect is at P1 = 1 atm abs pressure, the last effect will be under vacuum at a pressure P3.
• The concentration of the liquid increases from the first effect to the last effects
• This pattern of liquid flow is the simplest
• It requires a pump for feeding dilute solution to the first effect, since this effect is often at about atmospheric pressure, and a pump to remove thick liquor from the last effect.
• The transfer from effect to effect, however, can be done without pumps, since the flow in the direction of decreasing pressure, and control valves in the transfer line all that is required.

Backward-feed multiple - effect evaporator 

• In the backward-feed operation shown in Fig. for a triple-effect evaporator, the fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect. 

• This method of reverse feed is advantageous when the fresh feed is cold since a smaller amount of liquid must be heated to the higher temperatures in the second and first effects.
• However, liquid pumps must be used in each effect, since the flow is from low to high pressure.
• This reverse-feed method is also used when the concentrated product is highly viscous.
• The high temperatures in the early effects reduce the viscosity and give reasonable heat-transfer coefficients.
• Backward feed often gives a higher capacity than forward feed when the thick liquor is viscous, but it may give a lower economy than forward feed when the feed liquor is cold.

Parallel-feed multiple-effect evaporators

• Parallel-feed in multiple-effect evaporators involves the adding of fresh feed and withdrawal of the concentrated products from each effect.
• The vapor from each effect is still used to heat the next effect.
• This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt