Evaporator performance

performance of evaporator

1. EVAPORATOR PERFORMANCE Mamta Sahurkar

2. Since evaporators dealing with boiling solutions, and in particular with solutions with non-volatile solutes, any calculations must account for the effect of boiling point elevation. The vapor pressure of an aqueous solution is less than that of pure water at the same temperature; so the boiling point of the solution will be higher than that of the water. This is called Boiling Point Elevation (BPE) or vapor pressure lowering. The boiling point of a solution is a colligative property -- it depends on the concentration of solute in the solution, but not on what the solute and solvent are. Boiling Point Elevation

3. T he equilibrium vapor rising from a solution exhibiting boiling point elevation will exist at a temperature and pressure such that it is superheated with respect to pure vapor. The vapor rises at the solution boiling point, elevated with respect to the pure component boiling point. The vapor, however, is solute free, so it won't condense until the extra heat corresponding to the elevation is removed, thus it is superheated. When working problems involving heat transfer to or from boiling solutions, it is necessary to adjust the temperature difference driving force for the boiling point elevation.

4. For strong solutions, one can take advantage of  Duhring's Rule. The boiling point of a given solution is a linear function of the boiling point of water at the same temperature. This lets us plot T BP solution  against T BP water  and get a straight line for each concentration. Another way of thinking of these plots -- they plot the temperature where the vapor pressure of the solution is equal to some fixed value against the temperature where the vapor pressure of water equals the same value. For Duhring Plots to be valid, the range of boiling points must be relatively narrow and the solution must obey Raoult's Law.   Duhring's Rule

5. To use a Duhring plot: For a particular system pressure, determine the boiling temperature of pure water. This can be done from a vapor pressure equation or steam table. Enter the plot from the bottom (the water boiling point), trace up to the diagonal line representing the NaOH fraction, then trace left to read the solution boiling point from the vertical axis. The boiling point elevation is the difference between the two temperatures .

6. There are three main measures of evaporator performance: Capacity (kg vaporized / time) Economy (kg vaporized / kg steam input) Steam Consumption (kg / hr.) The performance of a steam-heated evaporator is measured in terms of its capacity and economy. Performance Measures

7. Capacity of evaporator Capacity is defined as the number of kilogram of water vaporized per hour.

8. Capacity of evaporator is defined as the number of kilograms of water  vaporized/evaporator  per hour. The rate of heat transfer Q through the heating surface of evaporator is the product of  heat transfer coefficient , heat transfer surface area and the overall temperature drop.   The capacity of an evaporator depends upon the temperature of the feed solution. If the feed solution is at the  boiling temperature corresponding to the pressure in vapor space of an evaporator, all the heat supplied will be utilized for evaporation, thus increasing the capacity of evaporator. Q = U×A× T Where Q = Rate of heat transfer A = area of the heat transfer surface T = overall temperature drop

9. Economy of evaporator Economy (or steam economy) is the number kilogram of water vaporized from all the effects per kilogram of steam used.

10. Economy is the number of kg of water vaporized per kg of steam fed to the unit . The rate of heat transfer Q through the heating surface of an evaporator, by the definition of overall heat transfer coefficient, is product of three factors. The area of heat transfer surface A The overall heat transfer coefficient U The overall temperature drop ΔT Q = U * A * ΔT

11. Economy calculations are determined using enthalpy balances . The key factor in determining the economy of an evaporator is the number of effects . The economy of a single effect evaporator is always less than 1.0 .   Multiple effect evaporators have higher economy but lower capacity than single effect . The thermal condition of the evaporator feed has an important impact on economy and performance.  If the feed is not already at its boiling point, heat effects must be considered.  If the feed is cold (below boiling) some of the heat going into the evaporator must be used to raise the feed to boiling before evaporation can begin; this reduces the capacity .

12. Steam consumption Steam consumption is very important to know, and can be estimated by the ratio of capacity divided by the economy .   That is the steam consumption (in kg/h) is: Consumption = Capacity/Economy.

13. For single effect evaporator, the steam economy is about 0.8 (<1). The capacity is about  n  -times that of a single effect evaporator and the economy is about 0.8  n  for a  n  -effect evaporators. However, pumps, interconnecting pipes and valves are required for transfer of liquid from one effect to another effect that increases both equipment and operating costs.

14. The rate equation for heat transfer takes the form: Q = U * A * ΔT where:  Q is the heat transferred per unit time U is the overall coefficient of heat transfer A is the heat transfer surface T is the temperature difference between the two streams .   Heat transfer in evaporators

15. In applying this equation to evaporators, there may be some difficulty in deciding the correct value for the temperature difference because of what is known as the boiling point rise (BPR) or boiling point elevation (BPE ) If water is boiled in an evaporator under a given pressure, then the temperature of the liquor may be determined from steam tables and the temperature difference is readily calculated.  At the same pressure, a solution has a boiling point greater than that of water, and the difference between its boiling point and that of water is the BPR or BPE .

16. For example, at atmospheric pressure (101.3 kN/m2 ), a 25 per cent solution of sodium chloride boils at 381 K and shows a BPR of 8 deg K. If steam at 389 K were used to concentrate the salt solution, the overall temperature difference would not be (389 − 373) = 16 deg K, but (389 − 381) = 8 deg K. Such solutions usually require more heat to vaporize unit mass of water, so that the reduction in capacity of a unit may be considerable. 

17. The value of the BPR cannot be calculated from physical data of the liquor, though Duhring’s rule is often used to find the change in BPR with pressure .   Duhring’s rule states that the boiling point of given solution is a linear function of the boiling point of pure water at the same pressure . Thus, if the boiling point of the solution is plotted against that of water at the same pressure, then a straight line is obtained .

18. Thus , if the pressure is fixed, the boiling point of water is found from steam tables, and the boiling point of the solution from Duhring’s plot. Different lines are obtained for different concentrations. The boiling point rise is much greater with strong electrolytes, such as salt and caustic soda.

19. Problems Technical problems can arise during evaporation, especially when the process is applied to the food industry. Some evaporators are sensitive to differences in viscosity and consistency of the dilute solution. These evaporators could work inefficiently because of a loss of circulation. The pump of an evaporator may need to be changed if the evaporator needs to be used to concentrate a highly viscous solution .

20. Fouling O ccurs when hard deposits form on the surfaces of the heating mediums in the evaporators. In foods, proteins and polysaccharides can create such deposits that reduce the efficiency of heat transfer. Foaming can also create a problem since dealing with the excess foam can be costly in time and efficiency. Antifoam agents are to be used, but only a few can be used when food is being processed. Corrosion  O ccur when acidic solutions such as citrus juices are concentrated. The surface damage caused can shorten the long-life of evaporators. Quality and flavor of food can also suffer during evaporation. Overall, when choosing an evaporator, the qualities of the product solution need to be taken into careful consideration.

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