Multiple-effect evaporators Staging

Low pressure vapours are evolved from evaporators, but they cany lot of heat as latent heat which should be recovered. This is not possible always hence, as much portion of vapour as possible from either a single- or multiple-effect evaporator stage is mixed with (by sucking out and then mixing) higher pressure steam to yield steam at an intermediate temperature and pressure. 

A thermodynamic analysis of the energy requirements of desalting processes is presented, to clarify the conditions under which such calculations are valid. The effects of departure from isothermal operation, finite product recovery, differential as opposed to single-stage operation, and salt concentration in the feed are examined. A comparison shows that there is essentially no difference between the energy requirements for a distillation and a freezing process. The minimum heat consumption and maximum number of efFects for a multiple-effect evaporation plant are calculated. The above analysis leads to the conclusion that efficiencies in the range 10 to 20% will be very difficult to achieve. 

An alternative scheme is to split the evaporation process into stages (in different vessels), commonly referred to as effects, where the flow of liquor may be in the same direction, the backward direction, or normal to the flow of vapor. In such a scheme, the vapor generated in a given effect is used to boil the liquid in a different effect where a proper temperature driving force exists. In multiple-effect evaporation, one exchanges savings in steam costs for increased capital investment in equipment. 

Equation (1) holds for heat transfer in each stage of a multiple-effect evaporator (Fig. 3). Eor the first stage (1) then... 

Tall oil, derived from the Swedish tallolja meaning pine oil, is recovered from the black liquor of softwood pulping. It is taken out at an intermediate stage of the multiple-effect evaporation when the liquor contains about 30% total solids, after it is allowed to stand [21]. The soaps (sodium salts of fatty acids present) are insoluble, cream to the top of the vessel, and are skimmed off. The residual black liquor is returned to the evaporators to continue chemical recovery. The soap yield, which can range from 10 to 200 kg/tonne of pulp (or even higher for pine), is then acidified and the free fatty acids and resin acids obtained are separated by distillation. The fatty acids recovered consist mainly of oleic and linoleic acids and are employed in soap manufacture and as the drying oil components of paints and varnishes [22] (Chap. 19). Resin acids consist of terpene acids such as abietic acid and its positional and reductive variants, and are mainly employed in paper sizing. 

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. 

Fig. 7-11. Optimal number of stages of a multiple effect evaporation unit.

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