Detail of MSF System

Detail of MSF system

The efficiency and performance of MSF plants very much depend on the formation of scales in heat transfer areas. As soon as the scale deposition starts the heat exchange rate begins reducing demanding more heat to achieve the desired temperature or otherwise reduce the production also the rate of condensation reduced due to less cooling of the heat exchange surfaces

The scale mainly consists of the constituents present in the sea water that is calcium, magnesium, bicarbonate, and sulfates. When water temperature increase bicarbonates convert into carbonate and combined with calcium to form calcium carbonate and precipitated when the concentration exceeds the saturation limits. Magnesium hydroxide (alkaline scale) also forms during this process which can be controlled by reduction of pH decomposing some bicarbonates reducing the saturation limits of calcium carbonate to reduce its precipitation. This precipitation can also be controlled by some special chemicals the threshold agents    

MSF plants when operating at TBT < 120 C⁰ maintaining lower limits of precipitation, but when salt concentration raised the chances increased for the formation of CaSO4. Monitoring of seawater pH is very important to inhibit scale formation and corrosion protection in the plant

So different chemical treatment methods are used to inhibit scale formation in MSF plants as under:

1-    Acid treatment method: Acid (mostly H₂SO₄) being used to decompose carbonates present in seawater into H2O and CO2 just to keep concentration lower than the precipitation limits. This is an old method being used in earlier MSF desalination plants.

2-    Chemical additive treatment method: The available scale inhibitors are being used to control scale formation. These inhibitors, when dosed into sea water at the rate of few ppm or mg/liter, suppressed or delayed the scale formation. Another effect of such additives is to keep precipitated particles in disperse and suspended and preventing adhesion of particles to the particle on the internal tube surface. The process also supported by mechanical means that is the ball cleaning to keep internal tube surface free of deposits which certainly helps in heat exchange and condensation of vapors increasing the efficiency of the plant.

3-    Hybrid treatment method: Both of the above treatment methods are being used means acid for partial decomposition of carbonate and scale inhibitors at reduced dosing rate.

4-    Here I would like to mention my own experience regarding the “Onload acid cleaning”  at Bomba Power and Desalination Plant Libya under the guidance of Dr. Boom. (A desalination expert from Amsterdam) I used to do this cleaning while the plant is in operation. The first thing is to increase the dosing of antiscalant and anticorrosion about 10% and gradually increased up to 25%. All Parameter are noted in the control room. After about one hour of running the plant on high dosing, this is the time to increase sulphuric dosing gradually keeping an eye on the brine pH which also gradually decrease. When the pH comes down up to 5 very slight effects noticed, but when it goes down further around 4.5 pH, this is the time to dissolve Magnesium hydroxide (In solubility chart, magnesium hydroxide start dissolving at pH while Calcium carbonate start about 3.5 pH.) as the acid now is being consumed for dissolving magnesium hydroxide the pH starts to increase at the same rate of dosing. Now have increase acid dosing to bring back the pH to 4.5 or even less until the pH do not increase, this means almost all Magnesium hydroxide scale is removed. Now start increasing gradually the acid dosing further showing the pH indicator going down and when reaching to about less than 4 pH and increasing acid dosing, a point will come when even by increasing acid pH not decreasing but increasing. This is the indication that the carbonate scale is being dissolved. Try to keep this pH position by increasing acid dosing slowly and be careful at this stage to control acid dosing which may any time jumped down because as soon as the carbonate scale completely removed the acid will be free to decrease the pH quickly. Now is the time to reduce acid dosing slowly not suddenly coming back to normal dosing, because the carbonates are still in water in dissolved condition, So better to come back on the normal position within one or two hours and keeping the other dosing on higher rates until the acid dosing back to normal.

           During this cleaning there are significant changes found in the control room as under:

§  Increase in production

§  Decrease in steam consumption

§  Boiler fuel reduced

§  Production quality improved (Reduced TDS)

During this cleaning have check copper contents in brine, certainly it will be increased

Maybe about 10 ppm but it just for few hours the acid reaction on the cleaned surfaces of the tubes.

Important Note: Full attention has to pay about acid dosing and pH and do not leave the process unattended while acid cleaning is going on.   

     

⁎ Corresponding author. Fax: +966 3 3431615.

E-mail address: rdc@swcc.gov.sa (O.A. Hamed). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.01.004

     

Brine Recycle

The fresh sea water is not directly going to plant but some part of hot brine from blow down Is mixed with the fresh sea water to maintain the initial seawater temperature say 25 C^o

This mixture of seawater and brine mixed in brine recycle pump and let it flow to the tube bundle box to enter into the tube. This mixture keeps flowing from stage one to the end of stages just to condensate hot vapors inside the chamber and being collected in the separate trays as distillate. During this condensation process the seawater inside the tube is gaining the

temperature and up to the end, it approximately reaches to ¾ of the designed temperature.

Brine Heater

This water from brine recycles enters into the brine heater where the steam ( from the boiler or from the exhaust of turbogenerator in case of a plant designed as power and water desalination plant) further heat it up to achieve the designed temperature say 110C^o and enter into the first stage for flashing. In the brine heater, the steam is supposed to pass through the tubes and water to be heated is passing from outside.

Flashing stages:

The hot water from brine heater enter to the first stage floor for evaporation and these vapors fly up passing through the demisters then contact with the bundle of tubes in which the cold sea water is passing to keep it cool for condensing vapors and then drip down in the separate distillate trays. The bundle of tubes, the distillate trays are connected and passing through all along the stage and finally, a distillate pump collects the distillate and passing through the post-treatment system to adjust its quality and then send to the product tank.

The hot sea water on the floor is connected to the next stage and an adjustable plate is fixed in between the two-stage to keep a certain level of the water in each stage.

The pressure regulation is very important using the principle of reducing the ambient pressure in the various stages to boil the water at lower than 100C⁰.in each stage with supplying extra heat to the sea water. For this purpose, a vacuum is being created by ejector using high-pressure steam and each stage on the top connected with this vacuum system. The negative pressure is not the same in each stage but it is relatively increased as the water temperature decreasing in consequent stages to get the maximum vapors produces in each stage and this is economy of this process. The water cycle in the plant can be seen in the diagrams as shown below.