Introduction to Desalination. Fuad Nesf Alasfour

kinetic and potential energy terms have been neglected from energy balance (first law of thermodynamics)? Explain.

      3 Explain the process associated with change of water droplet phase that surrounds the exterior pipe wall. What is the type of evaporation process?

      4 Based on real numbers, compare the value of h1 against h2. Explain the physical meaning of such difference.

      5 Sketch T–v, P–v, and T–s diagrams showing boiling process.

      Example 1.2 Flashing Phenomenon

      Adiabatic rigid tank (control mass) is divided into two nonequal volumes: A and B by a flexible membrane. Part A contains H₂O liquid at 7 kg, 220 kPa, , and at 25 °C, and part B is at sub‐atmospheric pressure under vacuum state condition with no mass and has a volume of 2 .

      If membrane is ruptured by making a hole such that two systems reach a final equilibrium state at 25 °C:

       Find:

      1 What is the type of water evaporation process?

      2 Final specific volume.

      3 Final pressure.

      4 Equations that represent exergy destruction (irreversibility).

      Solution

      1 Flashing process, where sudden drop in pressure causes water to evaporate via flashing, it is always associated with lightning and heat release.

      2 Hence

      3 P2 = 3.17 kPa

      4 Entropy generation

      Exergy destruction

       Extra activity:

      Student can perform the following:

      1 Describe the flashing process in MSF system.

      2 Sketch T–v, P–v, and T–s diagrams showing flashing process.

      3 Perform a parametric study to investigate the effect of initial water temperature on final phase. Sketch and explain.

      4 Perform a parametric study to investigate the effect of VB on final phase. Sketch and explain.

      5 Calculate the amount of heat release from flashing process, and explain physically what is the source and cause of such heat.

      6 Perform a parametric study to investigate the effect of ratio on irreversibility.

      Example 1.3 Feedwater Heat Exchanger

      An adiabatic heat exchanger is used to heat feedwater (H₂O) from 40 to 120 °C.

       Find:

      1  ratio.

      2 Heat transfer rate from thermal load (steam) to feedwater.

      3 Exergy destruction (irreversibility) per kg of feedwater.

      Solution

      1 Mass balance: Energy balance (first law of thermodynamics):or .

      2 

      3 Hence

       Extra activity:

Student can perform the following:

      1 Perform parametric study to investigate the effect of varying Tfeed during summer to winter seasons on heat transfer performance. Plot and explain. Take Tsummer = 35 °C.

      2 Calculate exergy flow rate at all four states. Explain.

      3 Resolve example using feedwater as seawater with salinity of 40 000 ppm. Refer to Appendix A for thermo‐physical properties.

      1.4.2 Membrane Desalination System

      There are several types of membrane technologies that are used today in the market, and the most predominant one is RO; the separation process using RO depends on the process of pressurizing feed fluid against semipermeable membrane, and such pressure causes only water molecules to cross (pass, migrate) through membrane, where salts and unwanted constituents such as minerals are rejected in a form of brine or concentrate. In RO process there is no heat addition during desalination process, hence no feed phase change occurred during separation process; besides RO technology, there are other membrane desalination systems used at small and limited scale such as nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF).

The RO desalination system is characterized with low amount of energy consumption compared with conventional thermal types and with high water recovery, but membrane lifetime is short due to fouling and scaling; unlike thermal system RO can only treat feed salinity from 50 to 46 000 ppm, and the value of SEC varies from 1 to 6 kWh/m³. As per electrical demand an example of 50 mG/d capacity RO plant requires 20–35 MW in case of using feed seawater (sea water reverse osmosis [SWRO] system) and 8–20 MW in case of using brackish feedwater (brackish water reverse osmosis [BWRO] system).

      The following are advantages and disadvantages of RO system:

       Advantages of RO:Low energy consumption compared with thermal desalination system.Small in size and compact.Low value in investment.

       Disadvantages of RO:Membrane has certain thermal stability limit.Life of a membrane is limited and short.Low feedwater can treat only salinity compared with thermal system.Costly in pretreatment.Use electrical power to drive system.Costly in maintenance.

      Example 1.4 Reverse Osmosis

      1 A simple one‐stage RO desalination system operates under steady flow; brackish water with feed capacity of 200 m3/h and 3000 ppm salinity needs to be desalted to 150 ppm as a potable water.Find:Concentrate flowrateConcentrate salinityRecovery ratioSolutionMass balance: Assume constant density (incompressible)Then Salinity balance: Recovery ratio: = 70%Extra activity:Student can perform the following:Derive RR relation as a function of three stream salinities.Select one practical operational parameter and examine its effect on RR. Plot and explain.Calculate salt rejection (SR), where SR is defined asSR = 100 − SPSP is salt passage across membrane SP = xp/xf, then perform a proper parametric study. Plot and explain.Perform parametric study to investigate the effect of feed salinity on concentrate salinity. Plot and explain.

1.5 Which Desalination System Is the Best?

      Recall that feed type, characteristics, and quality play a major role in selecting desalination system. In addition potable water purity, type of energy consumption, water production cost, and emissions are also factors that play a role during desalination technology selecting; engineers must evaluate all the mentioned parameters to optimize his or her selection process.

For example, if we take a case of 45 000 ppm feed seawater, thermal system can be used, while in other cases such as brackish water with 8000 ppm salinity, membrane system is recommended. In certain