Steam Savings in Multiple Effect Evaporators

51 = initial steam economy (i.e., lb water evaporated per lb of steam) 

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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. 

Multiple-effect evaporators are most often used in glycerine recovery plants of medium to large size, with the typical plant having a two-effect evaporator. The user must calculate and compare the savings in steam use for operating additional effects against the additional equipment and maintenance costs as well as space and operational complexity for additional effects. 

Optimum number of effects. The cost of each effect of an evaporator per square meter or square foot of surface is a function of its total area and decreases with area, approaching an asymptote for very large installations. Thus the investment required for an iV-effect evaporator is about N times that for a single-effect evaporator of the same capacity. The optimum number of effects must be found from an economic balance between the savings in steam obtained by multiple-effect operation and the added investment required. 

Low pressure steam, i.e. <4 bar, is normally used in evaporators, and frequently by-product steam ( 1.5-2 bar) from some other process is employed. Nevertheless, 1 kg of steam cannot evaporate more than 1 kg of water from a liquor, and for very high evaporation duties the use of process steam as the sole heat source can be very costly. However, if the vapour from one evaporator is passed into the steam chest of a second evaporator, a great saving can be achieved. This is the principle of the method of operation known as multiple-effect evaporation Figure 8.42). As many as six effects have been used in practice. 

Alternatively, this steam is used to mn a backpressure turbine coupled to the main air blower (which needs considerable power). The exhaust steam from the turbine can be used for process heating, e.g., evaporation of alum or phosphoric acid solutions or for obtaining potable water from sea water (in certain locations) by multiple-effect evaporators, thus saving heat energy required for such processes. This is cogeneration. 

Because each effect of an evaporator produces almost as much vapor as the amount it condenses, the total evaporation accomplished per unit of prime steam, or steam economy, increases in almost direct proportion to the number of effects used. The total heat load is also split up between the effects so that each effect has a much lower heat duty than a single effect for the same total evaporation load. However, the total available AT is also split up similarly so that each effect of a multiple effect requires about as much heating surface as a single effect operating over the same total temperature difference. Thus in selecting the number of effects to use in any installation, steam cost savings and capital cost of effects have to be balanced. Even before... 

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