Sea water desalination by Multi-Stage Flash System

Sea Water Desalination by Multi-Stage Flash System

In this process, sea water is heated to the boiling point produce the maximum amount of water vapors which is being condensed to get fresh water. This is the simple basic principle of the distillation process.

In the big plants the huge amount of energy required to heat up the big amount of seawater, so there are several ways to economize the heat consumption to achieved maximum production of water.

The process can be designed as “Once through” consist of heat rejection and brine heater.

The second one is where the brine is partially circulated. This process consists of 3 sections, one is heat rejection section, second is heat recovery section and the brine heater.                                                                                                                                               

     

The brine recirculation MSF process is illustrated in Figure 1

In this process, seawater is taken into the plant and fed through the heat rejection section. This water passes through a series of heat exchangers, raising its temperature. The heated sea water is further heated in brine heater up to boiling point and then enters into the first stage that is the first recovery stage through an orifice and in so doing undergoes a decompression to a pressure below its saturation pressure. As the water was already at the saturation temperature for a higher pressure, it becomes superheated and has to give off vapor to become saturated again at the lower pressure. This is known as ‘flashing’. This process of decompression, flashing and condensation is then repeated all the way down the plant by both the brine and distillate streams as they flow down through the subsequent stages which are at successively lower pressures.

The heat recovery and rejection sections are made up of a series of stages. Each stage consists of a flash chamber where the water boils to produce vapors, The vapor produced passes through a wire mesh (demister) to remove any entrained brine droplets which could contaminate condensed /distilled water in the distillate channel. and then into a heat exchanger where it condenses, giving up its energy to heat up the upcoming brine flow.                 

The condenser (heat exchanger) is a bundle of several hundred or thousands tubes about 1 cm dia. made of coppero-nickel or coppero titanium a noncorrosive and a good heat exchanger. The water passing through is the fresh sea water plus enough quality of hot brine coming out from the flashing chamber just to maintain the designed temperature of feed water which also help to economize heat energy. This feed water enters into the bundle of tubes from flash chamber stage one up to the last stage of the recovery section and gain heat up to around 70 to 80 C^o and enters into the brine heater to heat up to the designed temperature (maybe 110 C⁰ or more enter into the flashing chamber one and so on. After chamber one or two when the temperature going down to below 100 C⁰  Then reduction of pressure (by creating a vacuum) just to reduce the boiling point of water in the remaining stages to get the full and maximum evaporation to flash off as vapors

passing through the demisters to condensed by the condensers (bundle of tubes) falling into the distillate tray throughout all the stages giving the maximum production.

This process going on throughout all stages to the bottom stage up to rejection section, where part of it is rejected as blow down and the rest is mixed with incoming feed (sea water)to utilize some heat to recycle via brine recycle pump to go into the condenser (bundle of tubes)

Important features in Multi-Stage Flash system

1.    Performance ratio:   Before designing a MSF plant performance ratio (PR: quantity of distillate produced per unit mass of steam consumed) is an important parameter to calculate the performance of the plant. In a good and efficient plant, the performance ratio of 12 is the upper limit of such plants.

2.    In MSF system the number of stages could be 4 up to 40. By increasing the number of stages we can reduce the heat required and that means reducing the capital cost which compensate the capital cost of extra stages. Besides the initial investment cost an optimizing calculations required to estimate capital cost versus operating cost to determine recovery capital cost in how many years of operation.

3.    In MSF plant the operating temperature usually between 90 – 120 C^O which also require proper chemical treatment of feed water. The higher than 120 C^O temperature is not advisable because of the problem of scale deposition in brine heater.

4.    The brine recirculation of seawater mixed with about one-third of hot brine passes through de-aerator reducing the non-condensable gases and lowering the corrosive nature of brine achieving higher efficiency of the stages because of the heat exchange and less deposition in the tubes

5.    MSF plants are easy to operate the only and very important thing is to protect the system from scaling and corrosion of the plant material. The very special attention is required for the chemical dosing to maintain pH by sulfuric acid and Belgard BVN, Belite-M8 for the protection from scales and corrosion. Some time antifoam is also required to reduce foaming in the brine when the concentration ratio exceeds resulting in the higher TDS of product water. Usually, concentration ratio can be calculated by checking the sea water TDS and the brine TDS getting from blow down. Generally, have to maintain up to 1.5 ratios.

6.    Even maintaining the chemical dosing in feed water as mentioned above, after a long interval of plant shutdowns, the plant requires some maintenance and chemical cleaning of the whole system including tube bundles, demisters, and brine heater just to get the excellent performance of the plant quality and quantity wise both. The life up to 40 years of such MSF plant is predicted in the Arabian Gulf area. In 1997 I have personally visited a 50 years old MSF plant in Kazakhstan (Atrao name of the city) still in operation.

Multi-Stage Flash distillation process played a major role in providing fresh water in many areas particularly in the Middle East and 75% of the global capacity produced in Gulf countries and the remaining 25% in Europe mainly in Spain and Italy. This MSF process has grown considerably over the last 25 years and plant installed capacity beginning from 5000 m³/d to 60,000 m3/d at present.

Manufacturers:

The Japanese and Korean manufacturers produce 45% of the world production of MSF plants.

Europeans produce 43% and USA 8% of the total production of the world.  

Economics:

The capital cost of an MSF plant depends mainly on the thermal energy to be produced by the boilers where diesel or crude oil (as per design) is the main consumption. Electricity demand only to use auxiliary equipment like pumps, chemical dosing pumps, vacuum ejectors etc.                                   It can be calculated as if a plant operating at performance ratio of 8, the thermal energy consumption

will be around 290 kJ/kg of water production while the electricity consumption can be in the range of 4 to 6 kWh/m³  

The detail of chemical dosing, offload and on load chemical cleaning will be explained in detail later.

General discussion about MSF plant operation.  

Seawater intake

For a medium size of the plant, the seawater intake usually was chosen a deeper point and around 100 to 500 meter in the sea. Pipes normally being used of GRP pipes (Glass Reinforced Polymer Composite Pipes meet the demanding needs of industry to transport corrosive and non-corrosive fluids for various applications including Oil and Gas, Petrochemical, Power Generation, Desalination, Potable Water, Municipal and General Industries.)

At the end of the pipe where sea water to be taken some screens of suitable size fixed (optional) with the small pipe for chlorine injection to divert marine life to go away from the pipe. Normally about 5 ppm to 20 ppm of chlorine is being injected depending about the weather situation and mod of the sea. The other end of GRP pipe opens into the settling tank which is deeper than the other end in the sea so that the sea water flows by itself with out any pump to fill up the settling tank. This settling tank is again separated by the different screens to separate the seaweeds or debris’s ("Seaweed" is the common name for countless species of marine plants and algae that grow in the ocean). At other end of the tank a little deeper portion built for pumps to supply sea water to the plant settling tank which also fixed with some fine screens to further filter the sea water.

·         Chemical Dosing

Now before entering the sea water into the plant some chemicals (Belgard EV or EVN, Belite M-8, and sulphuric acid) are being injected for safety from scaling, corrosion and PH adjustment to reduce the chances of caco₃ to be precipitated inside the system. That is what we call as scales So the deposition of Calcium Carbonate (CaCO₃), Magnesium hydroxide (Mg (OH)₂) is called as Alkaline Scale, and Calcium Sulfate (CaSO₄) called as non-alkaline scale

            Both of this scale can be described by the equation as under:

            2HCO₃   ←Heat→ CO₃^2- + CO₂ + H₂O

            CO₃^2- + H₂O ←Heat→  2OH^- + CO2

When calcium and magnesium ions present in the sea water come in contact with carbonate and hydroxyl ions formed by the decomposition of bicarbonate ions. See the reaction

2OH⁻ +Mg²⁺→ Mg(OH)2

The formation depend on the concentration of bicarbonate, temperature, partial pressure of CO₂, and the time of resistance.

The reaction required enough time, temperature when CO₂ release and 2-CO3 hydrolyze to form OH- which precipitate as Mg(OH)₂. At about 100C⁰ the rate of formation of CO₃ is higher than the formation of OH^- and as CaCO₃ is less soluble than MgCO₃, so calcium carbonate will first precipitate. Calcium carbonate and magnesium hydroxide have inversely solubility characteristics that are the solubility decreases with increasing temperature.

When chemical dosing is not properly monitoring particularly anti-scalant, the formation of magnesium hydroxide or calcium sulfate could be possible in the high-temperature stages which could cause the clogging in the tubes and is hard to remove even by chemical cleaning only mechanical cleaning need to be carried out. So it is very important to properly monitor the chemical dosing in the feed.

Antifoam Dosing

Foam developed in the system usually considered because of some organic compound concentration increased in brine which strengthening surface film reducing the formation of vapors as well as the problem of brine carried over into the distillate resulting the high TDS and some production losses. This means that when foaming occurs production reduces and contaminated.

There is one more reason of high TDS which is actually the design error meaning to say that the demisters close to the brine surface having the high vapor velocity resulting in the contamination as well as reducing the overall evaporator efficiency and product.       

Formation of scale on the heat transfer surface is the basic problem in thermal desalination process which can’t be totally diminished but can only be minimized by dosing anti sealants mostly available as polyphosphates, polyelectrolyte, and organo-phosphonates. Also available maleic acid based copolymer anti sealants #1 and #2 and a phosphonate based anti sealants #3

For each of the three anti scalants a comparative study performed in a pilot plant operated at top brine temperature (TBT) 119C⁰ with brine recycle concentration factor 1.9 for one month each at the dosing rate of anti-scalant 1ppm and the baseline test also performed without using anti scalant. A very effective results achieved in suppressing the scale formation. So it is recommended that in MSF plants scale can be satisfactorily controlled by anti-scalant with optimizing dosing rate and brine concentration.