Steam multistage evaporation

From the process viewpoint, the two parameters that should be regulated are the concentration and flow rate of the bottoms product. If the composition of the feed stream is constant, good control of the feed rate and the evaporation rate will give the desired concentrated product at the proper production rate (see Fig. 1). Of course, the method of control can depend upon the evaporator type and method of operation. When evaporation rate is to be maintained at a constant rate, a steam flow controller is generally used. Steam flow control usually is accomplished by throttling the steam which results in a loss of temperature difference. Steam may, therefore, be uncontrolled to achieve maximum capacity. Steam pressure controllers may be used to protect the equipment or to assure substantially constant temperatures in the front end of a multistage evaporation system. Constant temperatures in the later effects of the evaporator can be controlled with a pressure controller on the last effect. 

Estimated data of specific steam consumption of single or multistage evaporation of... 

Fig. 7-13. Steam rate of a multistage evaporation unit. EV Evaporator PH Preheater...

This means that the live steam demand of a multistage evaporation plant is inversely proportional to the number of stages. Thus, taking only the costs of the live steam into consideration, a plant with an infinite number of stages would be optimal. The fact that this result is contradictory to the investment is obvious. 

As the feed-to-steam ratio is increased in the flow sheet of Fig. 11-125 7, a point is reached where all the vapor is needed to preheat the feed and none is available for the evaporator tubes. This limiting case is the multistage flash evaporator, shown in its simplest form in Fig. 11-125 7. Seawater is treated as before and then pumped through a number of feed heaters in series. It is given a final boost in temperature with prime steam in a brine heater before it is flashed down in series to provide the vapor needed by the feed heaters. The amount of steam required depends on the approach-temperature difference in the feed heaters and the flash range per stage. Condensate from the feed heaters is flashed down in the same manner as the brine. 

Carver-Greenfield multistage, oil dehydration, steam evaporator system (fermentation waste) ... 

The optimization of the large-capacity multistage flash evaporator was based on the consumption of the 370 thermal megawatts of energy available from the nuclear steam generator. It was necessary to determine the capital cost for various assumed terminal temperature differences and numbers of stages. Added to the amortized capital cost were all other costs necessary for operation of a complete plant, such as steam, labor, utilities, materials, and overhead. 

Table I shows a detailed breakdown of the operating cost for this plant. The cost of steam represents about half of the water cost for the optimum plant. The capital charges for the evaporator plant, which includes amortization, interest on working capital, and real estate, represent about 30%. The remaining 15 to 20% is equally divided between the cost of chemicals for scale control and all the other costs. The converted water is estimated to cost approximately 42 cents per thousand gallons. This water cost represents a realistic figure for a large-capacity multistage flash evaporator that could be built today when the energy in the form of steam costs between 35 and 40 cents per million B.t.u.

The heat recovery evaporator is a multistage, multi-effect evaporator which is different from conventional multiple effect evaporator or multistage flash evaporator. Asahi Chemical s heat recovery evaporator can concentrate the catholyte from 21% to about 40% without steam by utilizing heat generated during electrolysis. To obtain product caustic soda of 50% concentration, a small amount of steam is supplied to the finishing evaporator. 

A combination of flash evaporators, direct-contact condensers, and liquid-liquid exchangers has been described by Othmer (01). In this process water vapors, produced in a multistage flash distillation of heated sea water at successively reducing pressures, are condensed by direct contact with a recycle steam of product water. The heat is from the hot product water and is recovered by an immiscible petroleum oil in one spray column and transferred to the incoming sea water feed in a second spray column. 

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