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    WATER TREATMENT PLANT


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Figure 1. Example of water treatment plant.


Thermal power plants such as nuclear plants; combined cycle; conventional plants of coal, oil or gas; or solar thermal plants require high purity water for steam turbine. Because of that water treatment plants are needed to obtain demineralized water appropriate to its consumption in the boiler and in the steam water cycle. Obtaining desmineralized water is carried out in two steps:
1)    Softening or desalination. This phase will eliminate most of the salts in the water. If the original water source is a river or a fresh water stream, the process is called softening because water hardness is removed. If it is sea water, the process is called desalination.
2)    Refining. This is the second phase in which demineralization is refined, eliminating to a large extent of the salts that may be contained in the softened or desalinated water, that at the most it should have concentrations of 10 ppm or less if possible.
The processes used for the purification and adaptations of supply water to water suitable for being used are:

1. DESALINATION

Softening process is less demanding than desalination; because fresh water is used and it has much less dissolved salts than salt water. So we will focus on desalination, because it is the same as the softening but with much harder conditions owing to the fact of salty water has more concentration of dissolved elements.

Although there are several criteria to classify desalination processes, in general there are two kind processes: the ones that require a phase change and the ones processes that do not.
Among the processes involving a phase change can be mentioned:

  • Multiple-effect distillation.
  • Multiple-effect Flashing.
  • Cooling.

Processes that do not involve a change of phases are:

  • Reverse osmosis.
  • Electro dialysis.

The installation efficiency is given by the output factor, which measures the energy consumed per kg of produced water. Clearly, a process will be more efficient process as higher is its output factor.

1.1 DESALINATION BY DISTILLATION AND FLASHING ON MULTIPLE EFFECT

                          
Figure 2. Desalination by distillation.

Desalting and flashing in multiple effect are known as MED (Multi Effect Distillation) and MSF (Multi Stage Flash). With both processes, high quality distilled water is obtained from saltwater.
By distillation will reduce the typical salinity of water to one ten-thousandth. If the salinity of seawater is 35,000 ppm, the distillate is about 10 ppm or less.
The result obtained by distillation is the reduction of water typical salinity to one ten-thousandth. If sea water salinity is 35.000 ppm, the distillate water is about 10 ppm or less.


In order to obtain higher FR values, several simple distillers are coupled in series making up plants called multiple effect distillation (MED), being FR higher when higher is the number of effects (also called stages or cells.) Because of economic reasons, the number of effects is usually more than 14. Each stage can be compared to a simple distiller in which the thermal energy that evaporator requires, it is provided by steam condensation produced in previous stage.
In order to eliminate as many deposits formations and inlays inside the cells, working temperatures are around 70ºC.To produce evaporation and condensation at these temperatures, there must be certain emptiness in the cells, thereby evaporation temperature falls to the desired valued.
Desalination plants in multiple-effect flashing (MSF) have strong similarities with the MED plants, although with some differences:
- Water evaporation in each effect is not produced by the contribution of thermal energy in a heat exchanger, but by flashing (brusque expansion of pressurized hot water to a pressure below saturation). This eliminates a heat exchanger at each stage.
- The working temperature in a MSF plant is in the range of 115-120 º C, while in a MED plant is about 70 ° C. The existence of higher temperatures in an MSF plant requires a pretreatment that based on, acidification, degassing and neutralization, so the costs are higher.


- In an MSF plant, the amount of seawater introduced into the process should be between 5 to 10 times higher than the distilled one. If it is compared with MED, the seawater amount is only double of the produced.


1.2 REVERSE OSMOSIS

Osmosis is a physical-chemical phenomenon that happens when two aqueous solutions of different concentration are in contact through a semi-permeable membrane. This membrane only allows water to pass. Thus, water tends to pass through the membrane in the direction from lower to higher concentration, to level both.
But if the aqueous solutions are in different pressure, the water flow may change. In this way, if the pressure on the side that has higher salt concentration is greater than the lower concentration side, the water will run through it, losing its saltiness, which will stay in the more concentrated side. In the reverse osmosis desalters, water at high pressure is driven against the "membrane racks." The pressure that makes this phenomenon take place is called osmotic pressure.



Figure 3. Reverse osmosis.
 
The main obstacles found years ago that have been solved are mainly:
  • Greater production per unit membrane area and per unit volume, spiral configuration.
  • Low tolerance to the presence of free chlorine in tap water used for purifying.
  • Low resistance to microbial attack.

1.2.1 Conventional reverse osmosis plant consist of these main elements:

  • Water intake pumps.
  • Pre-treatment, acid injection.
  • Filters.
  • High pressure pumps with a recovery turbine.
  • Backwash tank.
  • Final chemical treatment.

Seawater pretreatment is used to ensure the optimum conditions of the water to feed the reverse osmosis modules, both from the standpoint of physical and chemical properties. In a reverse osmosis plant is fundamental an appropriate pretreatment of raw water to achieve a satisfactory facility operation , thereby eliminating any substances that damage the membrane. Pretreatment consists of several stages, which try to eliminate biological activity existence and organic and inorganic colloidal materials in water, because these would reduce considerably the good performance of reverse osmosis modules. Pretreatment includes a water acidification to prevent calcium carbonate precipitation on the modules. Also usually a removal of chlorine that can hold water can be executed, because it affects the semi-permeable membrane life.

After pretreatment, filtration is carried out to remove suspension particles that may exist in the water that would reduce the efficiency of reverse osmosis membranes. Once water has been pre-treated and filtered, it passes to the high-pressure motopumps that inject it into the reverse osmosis modules at the required pressure to pass it off for them. Not all the water is injected into the reverse osmosis modules, to pass through them and to be is desalted. Some of water is rejected as brine; this brine is usually passed through a recovery turbine to take advantage of its mechanical energy. Turbine axis is connected directly to the motopump axis.

ELECTRODIALYSIS

Electrodialysis is another of the processes that desalinate sea water without a stage change. These kind of plants are based on the fact that if it is made a continuous current flow in a ionic solution, positively charged ions (cations) move towards the negative electrode or cathode. In the same way, negatively charged ions (anions) move toward the positive electrode or anode. Therefore, if between the anode and cathode a pair of semi-permeable membranes are placed, one of them is permeable to cations and other anions, gradually will be formed a zone of low salinity between the two membranes.

As in reverse osmosis plants, electrodialysis plants require careful water pretreatment to avoid damaging the membranes.



Figure 4. Electrodialys.

1.4 COOLING

Another method used to obtain desalinated water is cooling, which is based on chilling water until it freezes. The freezing process makes the salt water sinks because weigh more and does not freeze. To freeze salt water, lower temperature is needed. When freezing process is finished, the ice on the surface has a lower salt concentration, so it is removed and then fused to obtain fresh water.



Figure 5. Desalt water production by cooling.

2. REFIN

The water obtained in the previous process can be stored as desalted water or softened water, or go directly to the next process without intermediate storage. The refining is the final process to adjust the supply water quality to the boiler. It eliminates the salts that might be still in the water. The process uses ion exchange resins. It can be done in two phases. One using cationic and anionic resins separately, or the other one just in one step, making pass the water to be treated by a only deposit where are the mixed cationic and anionic resins. These deposits are called mixed beds.

Once water have gone through these beds, the water must have the necessary chemical characteristics for its consumption in the boiler. The demineralized water is usually stored in a buffer tank, from where water is pumped to the point of water-steam cycle in which it is added to the circuit, usually to the condenser or feedwater tank. Before joining it, certain chemicals are added mainly, to adjust the pH and dissolved oxygen content.


Figure 6. Refining by ion exchange resins, equipment. Water purification domestic equipment.

3. DEMINERALIZATION PROCESS. BLOCK DIAGRAM

Here can be seen a diagram of inlets and outlets to a treatment plant with sea water evaporation desalter.


Figure 7. Water treatment diagram.

The main entrance to the plant will be the supply water, either of sea water or tap water.

Main outlet will have only treated or demineralized water. As secondary outlets we will have:

  • Oil lubrication.
  • Potabilizer, in some cases.
  • Anti-inlays.
  • Coagulant.
  • Sulfuric acid, sulfamic acid and sodium hydroxide to neutralize spillages.
  • Air.
  • Electricity.

To see more clearly the processes involved in a desalination plant through evaporation and resin-bed demineralization, we can be guided by the following block diagram.



Figure 8. Block diagram of a water treatment plant, evaporation desalination and demineralization.

1) Filtration. This process often use sand filters. These filters have sand of different types and particle size, and make a first filtering the water. To keep particles, a coagulant is added. Also at this stage and to have a thinner filtering, it is made water from sand filters pass for a filter cartridge.

2) Desalination. Basically, electricity is used for heaters, pumps, etc., And an anti-inlay product is added to the water.

3) Storage of desalted water. It exists in many plants, but it is not indispensable. This storage provides a buffer tank that allows the production of demineralized water without the necessity of the continuously desalination plant operation.

4) Demineralization. Here, as we have said, the separation of the minerals that water has, is done. This process is based on some beds with cationic and anionic resins. These beds must be regenerated with the addition of sulfuric acid and soda.

5) Storage of demineralized water.

6) Distribution of demineralized water consumers, with the help of electric pumps.




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