Plant multistage evaporating

With respect to the incrustation of heat transfer areas of multiple effect plants, multistage flash evaporators can be more favorable. In Fig. 7.6-2 such a unit is illustrated. As a rule 20 stages are installed. The seawater is preheated in the various stages and is fed into the first stage after the passage through a final heater. A certain part of the seawater is flash evaporated by depressurization and is con-... 

In case a given process-engineering solution turns out to require excessive investment or operating costs, the process engineering technology has to be optimized. The approach shall be explained with the dumbed down example of a multistage evaporating plant... 

For the multistage evaporating plant schematically shown in Figure 2.11, the optimum number of stages with regard to the minimization of the manufacturing costs shall be determined. 

Figure 2.11 Schematic of a multistage evaporation plant with n stages.

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. 

Fig. 4. The 341,000-m /d multistage flash (MSF) evaporation desalination plant A1 Taweelah B in Abu Dhabi, United Arab Emirates. Courtesy of Italimpianti SpA. It is a dual-purpose plant, composed of six identical power and desalination units. The desalination units at 56,800 m /d each are currently (1997) the largest ia the world. They have 17 recovery and 3 reject stages and a Performance Ratio of 8 1. The plant also produces 732 MWe of...

In the United States boric acid is produced by United States Borax Chemical Corp. in a 103,000 2 3 ric ton per year plant by reacting cmshed kernite ore with sulfuric acid. Coarse gangue is removed in rake classifiers and fine gangue is removed in thickeners. Boric acid is crystallised from strong hquor, nearly saturated in sodium sulfate, in continuous evaporative crystallizers, and the crystals are washed in a multistage countercurrent wash circuit. 

This is a process mainly used in power plants for separation of dissolved matters by evaporation of the water. Multistage flash distillation, multiple-effect vertical long-tube vertical evaporation, submerged tube evaporation, and vapor compression are effective process equipment. It may require pH adjustment. The process removal efficiency is about 100%. 

Figure 2. Typical process flow diagram for multistage flash evaporator sea water conversion plant...

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 progress made in membrane-based desalination has lead to the substitution of evaporation plants with RO systems in different part of the world. For example, the United States holds the second position in worldwide desalination capacity (15.2% of the world production) and the 78% of their production come from RO treatments. Mediterranean countries, including Spain, Malta, Cyprus, and Israel, have also reverted from traditional multistage flash (MSF) to RO during the past two decades. The water production by reverse osmosis desalination plants passed from 36% of the global desalting capacity in 1996 to 42%... 

The evaporation is carried out in multistage plants. Evaporated salt is very pure (> 99.95%, with ca. 100 ppm ofCa2+). 

The primary purpose of this study was to optimize the cost of water produced in a multistage flash evaporator supplied by brine heated in a solar heat collector. It was therefore not possible to select both plant capacity and the solar collector area and still permit optimization. In addition, the brine temperature limitation would be set by an economic evaluation of the multistage flash... 

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