Multiple effect evaporation steam consumption

Monitor performance of multiple effect evaporators steam consumption, concentration of incoming dilute feed and outgoing concentrated solutions... 

The filtrate is then concentrated to 60% of dry substance in multiple-effect evaporators economical in energy consumption ( 1 lb of water evaporated by 1/n lb of steam, where n is the number of effects). However, the temperature to which the liquor is exposed in the multiple-effect evaporators is much higher than in the single-effect, vacuum evaporators that are used in the final evaporation of refined liquor to afford finished-syrup concentration this exposure to high temperature causes, in the syrup liquor, formation of color and off-fla-... 

There are several ways to increase the steam economy, or to get more evaporation with less steam input, for certain types of evaporation applications. The use of multiple-effect configurations or compression evaporation can be considered for large flow rates of relatively dilute aqueous solutions. Both multiple-effect and compression evaporation systems require a sizable incremental capital investment over single-effect evaporators, and these systems are larger and more complex than the simpler one-stage evaporators. Like the multiple-effect evaporators described above, compression evaporation systems can only be justified by a reduced level of steam consumption. 

In any real multiple effect evaporation unit, the above mentioned conditions are not fulfilled however, the reduction of the energy consumption is substantial. In industrial plants the specific steam consumptions according to Table 7.2-1 are obtained. 

In multiple effect evaporation nnits the steam consumption ) and the liquid flow rates between the various effects can be calculated by means of energy and material balances of the first effect or any other effect. The steam consumption b (here only the heat of condensation r is utihzed) of a forward-feed operation unit according to Fig. 7.2-2 is given by... 

Monitoring performance of steam consuming units such as process reactors, multiple effect evaporators (measuring their steam consumption and condensate recovery)-,... 

The above evaporators do not recover the latent heat of vaporization while a multiple effect evaporator can recover this latent heat. This can reduce the steam consumption for the same evaporation duty for concentrating the dilute solution. 

It should be borne in mind that a multiple effect, where evaporators are connected in series, has the same capacity as a single effect, provided the initial steam pressure and vacuum are the same. However, the steam consumption is reduced in proportion to the number of effects installed and this increased economy is the only reason for installation of multiple effects, which are naturally much more expensive than single- effect evaporators. In some cases, evaporators are arranged parallel with individual steam connections and condensers, and it is evident that the capacity will increase with the number of evaporators, but the steam consumption will also rise in the same proportion In the first case, the total temperature fall T has been divided into a number of smaller temperature falls, Ti, T2, Tz, etc., and the evaporating capacity of the multiple effect will be Q = i o (Ti + 2 + Tz, etc.) = KoTj which is also the capacity of a single effect. In the second case, the capacity of each evaporator, Qn KoTn and therefore the total output of all the evaporators equals the sum of all Q =KoT... 

The principal measures of the performance of a steam-heated tubular evaporator are the capacity and the economy. Capacity is defined as the number of kilograms of water vaporized per hour. Economy is the number of kilograms vaporized per kilogram of steam fed to the unit. In a single-effect evaporator the economy is nearly always less than 1, but in multiple-effect equipment it may be considerably greater. The steam consumption, in kilograms per hour, is also important. It equals the capacity divided by the economy. 

Evaporation can be multiple-effect, to conserve energy, and some of the vapor generated can be used to heat the salt slurry before dissolving. In either approach, the steam consumption is less than 1 ton per ton salt (say, 0.7-1.0). 

Recompression evaporation is an alternative to the multiple-effect approach that reduces steam consumption. Section 7.1.5.2B treated both mechanical and thermal recompression. The power consumption or the motive steam usage depends on the increase in pressure that is necessary to achieve the desired condensing temperature. 

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