Report on Desalination Process

Contents: 1. Mechanical Vapor Compression 2. Reverse Osmosis 3. A Comparative Analysis of other Desalination Processes 4. 1. A brief description of various Desalination Processes 4. 2. 1. Distillation Processes 4. 2. 2. 1. Multistage Flash Distillation 4. 2. 2. 2. Multieffect Distillation 4. 2. 2. 3. Vapor Compression 4. 2. 2. Membrane Processes 4. 2. 3. 4. Electrodialysis 4. 2. 3. 5. Reverse Osmosis 4. 2. 3. Solar Distillation/Humidification 4. 2. Process Selection 4. A Derivation of the Rate of depletion of Limestone used to make water portable 5. 3. Derivation 5. 4. Sample Calculation 5. 5. Graph Chapter 3 A Comparative Analysis of other Desalination Processes In the following chapter we compare and contrast various types of Desalination Processes. There are many methods of desalination currently in practice today. They vary in efficiency, cost of installment, purity of the product, geographical requirements and a variety of other factors which will be further highlighted in the following pages. 3. 1. A brief description of various Desalination Processes: The following section gives an introduction as well as the principle, orking, chemical treatments required and advantages & disadvantages of the various desalination processes. 3. 1. 1. Distillation Processes: 3. 1. 1. 1. Multistage Flash Distillation Introduction: Single stage flash evaporators have been used since the early nineteen hundreds when they were used in the Alberger salt process to obtain salt from brine.

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Multistage flash evaporators however, were first installed about 55 years ago and were usually of small capacity with low thermal efficiencies. In 1956, a four stage, four unit, 9460 m3/d flash plant was installed in Kuwait. By avoiding separate shells for each stage, a great improvement was made in the economics of evaporators. Today, Multi-stage flash distillation plants produce over 85 percent of all desalinated water in the world, despite the fact that Reverse Osmosis plants are the more numerous. The patent for the MSF process was filed by Mr. Silver. It was described as a plant employing flash distillation in which the number of stages is more than twice the performance ratio (pounds of distillate produced/1,000 BTU of heat input) which in some cases was about 3 times the actual value. MSF solved some of the basic problems of the desalination process such as scale formation, which when combined with the ability of these plants to be built in large capacities resulted in these plants being the largest source of desalinated water in the world. By 1984, 67. 6% of all desalination plants (6,075,000 m3/day) were operating on the MSF principle. The unit size has also increased as much as 100 fold since 1972. Principle: Multistage flash processes work on the concept that vapors can be produced from any liquid which is at its boiling point by lowering the pressure. This is due to the fact that reducing the pressure decreases the boiling point of water. Working: The plant has a series of spaces called stages, each containing a heat exchanger and a condensate collector.

The series has one cold end and one hot end while the stages in between have intermediate temperatures. The pressure in each stage corresponds to the pressure required to boil water at the respective temperatures.

Beyond the hot end there exists a container called the brine heater. When the plant is operating in steady state, feed water at the cold inlet temperature flows through the heat exchangers in the stages and is heated. When it reaches the brine heater it already has nearly the maximum temperature. In the heater, an amount of additional heat is added. After the heater, the water flows through valves back into the stages which have ever lower pressure and temperature. As it flows back through the stages the water is now called brine, to distinguish it from the inlet water. In each stage, as the brine enters, its temperature is above the boiling point at the pressure of the stage, and a small fraction of the brine water boils or flashes to steam thereby reducing the temperature until equilibrium is reached. The resulting steam is a little hotter than the feed water in the heat exchanger. The steam cools and condenses against the heat exchanger tubes, thereby heating the feed water as described earlier.

The total evaporation in each stage is about 15% of the water flowing through the system, depending on the range of temperatures used. When temperature is further increased, there are growing difficulties of scale formation and corrosion. 120°C has been the maximum thus far, although scale avoidance may require temperatures below 70°C. The feed water carries away the latent heat of the condensed steam, maintaining the low temperature of the stage. The pressure in the chamber remains constant as equal amounts of steam is formed when new warm brine enters the stage and steam is removed as it condenses on the tubes of the heat exchanger. The equilibrium is stable, because if at some point more vapor forms, the pressure increases and that reduces evaporation and increases condensation. In the final stage the brine and the condensate has a temperature near the inlet temperature. Then the brine and condensate are pumped out from the low pressure in the stage to the ambient pressure. The brine and condensate still carry a small amount of heat that is lost from the system when they are discharged.

The heat that was added in the heater makes up for this loss. The heat added in the brine heater usually comes in the form of hot steam from an industrial process co-located with the desalination plant. The steam is allowed to condense against tubes carrying the brine (similar to what happens in the stages). Schematic of a multi-stage flash desalinator A – Steam in B – Seawater in C – Potable water out D – Waste out E – Steam out F – Heat exchange G – Condensation collection H – Brine heater Chemical Treatment: Pretreatment: MSF plants require sea water with a temperature range of 20 -35°C and a salinity of 42,000 ppm as the primary feed. This water undergoes pretreatment consisting of filtration, chlorination, antiscale chemical dosing, de-aeration/de-carbonation before being processed by the plant.

Without this treatment there would be frequent interruptions to the plant operation. Scale prevention: By increasing the flash or temperature range of the plant with the same surface area, the performance ratio is increased however the limitations are, that on the cold side the minimum achievable temperature is the temperature of sea water and on the hot side the maximum achievable temperature is limited due to scale formation. Some of the methods to prevent this are: a) Polyphosphates: treatment with polyphosphates causes the formation of sludge as opposed to scales in the condenser tubes. maximum achievable temperature is 91°C) b) Acid treatment: Addition of HCl or H2SO4 in fixed quantities reduces alkalinity and prevents scale formation. (maximum achievable temperature is 121°C) c) High temperature additives: HTA’s such as Belgard EV prevent scale formation and produce crystal distortion. (maximum achievable temperature is 112°C) Advantages & Disadvantages: Advantages| Disadvantages| Can be constructed in very large capacities. | Performance ratio is limited since the upper temperature is limited to 121°C| Boiling does not take place on the tube surface therefore there is less susceptibility to fouling. Low heat transfer coefficients which require a greater surface area when compared to MED. | Scale prevention is less hazardous because threshold chemicals are extensively used (acid treatment is not preferred) therefore there is less likelihood of corrosion due to overdosing. | Often operates well below the design capacity and in some cases as much as 60% below said capacity. | Low cost steam can be used. | Large capital costs with large intake structures. | Water is very pure. | Large amount of seawater required compared to production which requires a large amount of pumping power. Considerable operating experience is available. | Long term effects of the additives are unknown. | Economies of scale work well. | Improper material selection and noise pollution have caused problems in the past. | Table 3. 1 (Desalination Processes and MSF distillation practice by Arshad Hassan Khan) 3. 1. 1. 2. Multi Effect Distillation: Introduction: The MED process also has a rather long history. Many such plants were build by chemical industries over the last thousand years although their primary purpose was the recovery of brine. This process however was also among the first used to produce significant amounts of water from the sea and although it has been replaced by MSF as the leading source of desalinated water it still accounts for a significant portion of the water produced by such plants today. To put it numerically ME plants produce about 492,636m3/day and at present the largest ME unit produces 20,000m3/day.

Principle: In this process, vapors are produced by two means. The first is by pressure reduction (flashing) and the second by heat input (boiling). Working: In ME distillation, 2 or more effects are present. Each operates at a lower temperature and pressure than the previous.

The first effect is heated by low pressure steam and vapors are generated from the feed water in the effect tubes. These vapors are then directed, through a demister, to tubes in the second effect. Since this effect is at a lower temperature, the vapors can be used to evaporate the brine. This occurs when the vapors condense on the inner side of the tubes and release heat to the next effect.

Some of the vapors produced in each effect are sent to the associated preheater, where they heat incoming feed and are condensed. All the brine which does not evaporate is then sent to the next effect for further vaporization. The process can be repeated as long as the temperature difference is high enough to act as the driving force. Brine from the last effect is rejected as blow down and vapors from this effect are condensed in a final condenser where feed water serves as the coolant. Most of the feed water Is rejected after passing through the condenser. Schematic of a multiple effect desalination plant.

The first stage is at the top. Pink areas are vapor; lighter blue areas are liquid feed water. Stronger turquoise is condensate. F – Feed water in S – Heating steam in C – Heating steam out W – Fresh water (condensate) out R – Brine out O – Coolant in P – Coolant out VC – Last-stage cooler Chemical Treatment: Pretreatment costs are very low. Since when feed is at high temperature it is at its lowest concentration the risk of scaling is reduced. Standard antiscaling procedure is followed.

Advantages & Disadvantages: The advantages and disadvantages are similar to those seen in multistage flashing. It does however have an additional disadvantage in that it cannot be used on the same scale as MSF. On the other hand it has a considerably lower construction cost when compared to MSF therefore this method of desalination is still widely used today in many forms. Overall for relatively small units (less than one mgd (22. 8245m3/s)) this method is often preferred over MSF however for larger units MSF is more cost effective. 3. 1. 1. 3. Vapour Compression: As this process has already been described in detail, only the advantages and disadvantages are listed here. Advantages| Disadvantages| Simplicity & compact construction| The reliability of the unit is directly dependant on the compressor which is likely to fail| Operation is stable and can be done with no recirculation and low labor costs| At lower temperatures the vapor specific volume increases causing the compressor load to increase| Low pumping power required| At higher temperatures scaling occurs| No cooling water required| Fluctuations have a greater tendency to effect productivity or destabilize the plant. Low capital cost | When increasing the number of effects, energy consumption of the compressor increases causing a reduction in performance ratio| High performance ratio/unit of heat transfer surface area| As in the case of MEDs this process is only practical on a relatively small scale| Table 3. 2 (Desalination Processes and MSF distillation practice by Arshad Hassan Khan) 3. 1. 2. Membrane Processes: 3. 1. 2. 1. Electrodialysis: Introduction: This process is particularly useful is desalting brackish waters. The principle of this process has been known since the early 1900s although the first unit was put into service only in the year 1954 in the Middle East. Units are medium sized (100-400m3/day range) and can be used to desalinate water with extremely high salinity. These plants have been used to bring water with mineral content of several thousand ppm down to 500ppm.

These plants produce over 450,000m3/day which is about 4. 7% of the world’s total produce. Principle: The ED method is based on the principle that the dissolved salts are ionic in nature and hence when subjected to an electric field the cations will travel to one end while the anions travel to the other.

Also certain selective membranes are used that allow only positive charged particles to pass through on one side and negative particles on the other (anion and cation permeable membranes) Working: As explained earlier, selective membranes are placed alternately between the cathode and the anode. When a DC current is passed though the liquid, anions like SO42- and Cl- move towards the positive end while the cations like Na+ and Ca2+ move towards the negative end. They each pass through their respective membranes and enter a cell which is a space made up of one ionic membrane and one cationic membrane. The anions while be separated from the positive end by a cation membrane and the cations from the negative end by a cation membrane. The result is that alternate cells become concentrated and the intermediate cells become dilute which is the product. The rate of salt removal is controlled by Faraday’s laws and practical demineralization rates are in the 75 to 99% range.

Pretreated feed water is passed along parallel paths which ensure a continuous flow of the rejected brine and the product water streams. At the electrodes oxidation and reduction reactions take place. The cathode system is treated with acid and recycled while the anode system is usually sent to the drain. Figure 3. 3 (www. fumatech. com) During this process it is necessary add acid to presoften the feedwater to achieve stable operation. Non mineral substances spoil the membrane surfaces and units show marked deterioration with time To combat this, the direction of the DC current is nowadays reversed every 20 minutes or so. This process has therefore been called EDR or Electrodialysis reversal. Chemical Treatment: Acid or polyphosphate is added to the brine system to prevent deposition of salts. If scaling becomes excessive at any point, cleaning is done either mechanically after disassembling the stack or during the process itself. Advantages & Disadvantages: Advantages| Disadvantages| Can be used for water with extremely high salinity| Polarization takes place within the membranes which requires an increase in the DC voltage. Low cost of construction and labor| Membrane fouling due to non mineral substances| Extremely low pumping requirements| Leaks in the membrane occur causing contamination of product| Maintenance is relatively simple| Degradation of electrodes occurs with time. | Table 3. 3 (Desalination Processes and MSF distillation practice by Arshad Hassan Khan) 3. 1. 2. 2. Reverse Osmosis: As this process has already been described in detail, only the advantages and disadvantages are listed here.

Advantages| Disadvantages| Easy and simple operation| Ratio of product to input is low and therefore can only be used in places where the source of water is the sea (not a limited source)| Rapid delivery and installation| Variety of materials required for pretreatment| Easy expansion due to modular concept| Rate of production is slow| Low energy consumption and maintenance| Cost effectiveness declines as many modules are setup| Operation at low temperatures lowers corrosion and allows cheaper materials to be used| Input water cannot be highly concentrated| Modular concept allows for replacement of defective parts easily| Product water has a comparatively high chloride content| Table 3. (Desalination Processes and MSF distillation practice by Arshad Hassan Khan) 3. 1. 3. Solar Distillation/Humidification: Introduction: Although it was first developed over a century ago in Chile when it produced 27m3/day, serious research was not conducted till much later. The increase in the cost of fossil fuels was what spurred research in this field. Principle: The basic principle is the greenhouse effect and the fact that water evaporates below its boiling point. Working: Solar Energy is used for the distillation of salt waters in a solar still which acts as an absorber for solar energy.

Water is heated due to greenhouse effect to a temperature of 50 – 60°C. The rate does of course depend on the intensity of solar radiation. The evaporated water is drained and collected. Figure 3. 4 (www. dr1. com) Chemical Treatment: Chemical treatment in this process is very low. Advantages & Disadvantages: Advantages| Disadvantages| Ecofriendly| Can only be used on a very small scale| Very low maintenance and construction cost| Product is variable as the rate of production is based on incident sunlight| No electricity required| Can only be constructed in certain geographic zones| No labor required| Cost effectiveness reduces when its constructed in large numbers| Table 3. 5 3. 2. Process Selection Important Factors: * Product water quality and quantity Feedwater quantity & characteristics * Energy availability * Location * Economic Constraints Based on these factors the processes are selected. When product water quality requirements are high, MSF or MVC are used while when it is lower, whereas when requirements are comparatively lower, RO and Electrodialysis can be used.

When product water quantity needs to be high MSF is most preferred process while solar distillation can be used if quantity requirements are lower. Energy Availability is required for processes like ED, MSF, etc while processes like solar distillation require almost no continuous energy.

Location plays an important role with regard to pumping power. Also availability of direct sunlight is required for solar distillation. Economic Constraints also play a role. When the budget is low processes like EDR and RO are used assuming quantity of water required is low while if required water quantity of water is high, MSF is the most cost effective method. Chapter 4 Derivation of a formula to calculate rate of consumption(R) of CaCO3 (Limestone) 4. 1. Derivation: Information required: * Temperature of product water (T (°C)) * Conductivity of water (before being passed through the limestone but after dosing of NaOCl) (G1 (µS/cm)) * Conductivity of water (after being passed through the limestone) (G2 (µS/cm))) * Mass flow rate (V (tones/hr)) Assumptions: CaCO3 is the only compound that dissolves in water while it is passed through the limestone (other compounds like sulfates dissolve in negligible proportions) * The temperature coefficient of conductivity (? ) of CaCO3 is . 02/°C Procedure & Theory of Calculation: Step 1: The difference in conductivity is first measured (Gt = G2 – G1). This gives us the change in conductivity due to CaCO3. Step 2: Based on this value we can calculate change in conductivity at 25°C (G) using the formula: G = Gt / [1+ {? ? (T-25)}] (? = . 02) The value of conductivity at 25 °C for various concentrations of CaCo3 (in parts per million) were obtained from www. omega. om and verified by calculation.

These values are as follows: Conductivity (G) (µS/cm)| ppm CaCO3| 1020| 425. 0| 415| 170. 0| 315| 127. 5| 210| 85. 0| 105| 42. 5| 42. 7| 17. 0| 32. 1| 12. 7| 21. 4| 8. 5| 10. 8| 4. 25| 4. 35| 1. 70| 0. 055| none| Table 4. 1 (www. omega. com) Using the above values a graph was plotted (G versus ppm). As can be seen, the graph is more or less a straight line (curves at higher concentrations) for our area of observation. Step 3: With the help of the graph, the ppm of CaCO3 which corresponds to the conductivity G (at 25°C) is found. Step 4: Finally to calculate R we use the formula: R (g/hr) = ppm (CaCO3) ? V (tones/hr) To find total mass of CaCO3 dissolved in time t (hrs) we can multiply t with the average value of R over the given time. 4. 2. Sample Calculation for Average Values Information required: * Temperature of product water (T (°C)) = 29°C (Design input) + 2. 5 = 31. 5°C * Conductivity of water (before being passed through the limestone but after dosing of NaOCl) (G1 (µS/cm)) = 31. 5 ((18 + 45)/2) * Conductivity of water (after being passed through the limestone) (G2 (µS/cm))) = 500 * Mass flow rate will remain as a variable (V (tones/hr)) Procedure & Theory of Calculation: Step 1: The difference in conductivity is first measured (Gt = G2 – G1). This gives us the change in conductivity due to CaCO3. Gt = 500 – 31. 5 = 469. 5 Step 2: Based on this value we can calculate change in conductivity at 25°C (G) using the formula: G = Gt / [1+ {? ? (T-25)}] (? = . 02) G=469. 5/[1+{. 02? (31. 5-25)}] = 415. 5 Step 3: With the help of the graph, the ppm of CaCO3 which corresponds to the conductivity G (at 25°C) is found. ppm of CaCO3 = 170 (approx) Step 4: Finally to calculate R we use the formula: R (g/hr) = ppm (CaCO3) ? V (tones/hr) R = 170 ? V = 170V Hence rate of consumption of CaCO3 is 170V g/hr (where V is in tones/hr) 4. 3 Graph Figure 4. 1 (Drawn by project trainee based on information from www. omega. com)

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