The submerged coil evaporator

A single-effect submerged coil evaporator as shown in Fig. 2.8 (p. 23) and consists of a shell, a steam-heating coil, a condenser cooled by cold sea water, part of which is bled off for the evaporator feed. Provision is made for distillate removal and brine blowdown to ensure that the evaporation brine concentration does not rise unduly. In operation, the heating steam caused evaporation to take place from the brine surface. The vapour, which is pure water, condenses on the condenser tubes...

Heat transfer

Heat transfer occurs because of a temperature difference. In distillation plants there is the requirement to evaporate from a liquid phase and condense the vapour to form the distillate. The condensation is often achieved by enclosing the cool sea-water feed to the evaporation section in a tube envelope and recovering the latent heat of vaporisation, thereby partially heating the feed-water. Thus heat recovery is obtained, the heat transfer taking place from the vapour space through the tube...

Cu

Plate Distillation

Now assumptions have to be made about the bubble population which relates S to L. The net result is that 4.16 may be integrated to yield Where ne is a lumped parameter termed the evaporation index and is constant for any given set of flow conditions. The model predicts that flashing is an exponential decay process and therefore that the attainment of a high fraction of equilibration is very dependent on chamber length. The model has been tested on the data of Richardson Westgarth 6 for flash...

Calcium sulphate scaling

Solubility Curve For Calcium Sulphate

Calcium sulphate scaling occurs as a result of its reverse solubility in sea water, i.e. as sea-water brine temperatures and concentrations increase, the CaS04 solubility in sea water decreases. As calcium sulphate is not at saturation level in normal sea water, it is possible to prevent this scale forming if the distillation process is operated below temperatures and concentrations at which it occurs. As this scale cannot be readily removed by acid cleaning, its deposition must be prevented....

Composition and properties of typical raw feedwater

The composition and properties of the raw feed are extremely important in several respects. The principal ones are the treatment necessary to prevent scale and the determination of the economic boundaries which makes one purification process more economical to run than another. As far as distillation processes are concerned, the raw feed-water can vary within quite wide limits as the cost of the product is not nearly so dependent on the salinity of the feed or the product purity as in other...

Flash range

The flash range of a MSF plant is the temperature range over which the plant operates. For any thermodynamic device which utilises a source at temperature rmax and a sink at temperature rmin the maximum possible temperature range over which the device can operate is Tmax - Tmin. For a distillation plant, Tm-in the sink temperature is fixed by location, e.g. the sea and in practice from second-law considerations heat rejection is always at a temperature above Tmin thus the discharge temperature...

The second law of thermodynamics

This law specifies the direction in which physical processes proceed. Its usual statements go along the lines that heat transfer takes place from a hot body to a cooler body or 'It is impossible to construct a system which will operate in a cycle, extract energy from a reservoir and do an equivalent amount of work on the surroundings'. Note that the opposite process, heat flow from a cold body to a hot body the hot body becoming warmer the cold body becoming cooler does not violate the first...

Multiple Effect Distillation Performance Ratio

Chapter 1 A very general introduction 1 The first and second laws of thermodynamics 2 Desalination processes and their applicability 8 Heat transfer surface and performance ratio 18 The second law of thermodynamics 23 Chapter 3 The control of scale and corrosion 25 Composition and properties of typical raw feed-water 25 Impingement and cavitation corrosion 40 Corrosion caused by polluted sea water 40 Chapter 4 Multi-stage flash distillation 50 The submerged coil evaporator 52 Multi-stage flash...

Boiling point elevation

Boiling Point Seawater

The commonest was of explaining boiling point elevation is to imagine two insulated containers A and B as shown in Fig. 2.6. A contains sea water and B pure water. The sea water temperature is iA, and The pure water temperature fB, the vapour temperatures are equal and the vapour space between the two connected by a compressor. For equal temperatures in both A and B the vapour pressure in B is greater than that of A, i.e. PB gt PA . Thus in order to prevent vapour transport from B to A the...

Hydrostatic head effects

Temperature losses have so far been discussed in the context of MSF. However, in one form of ME distillation a major thermodynamic loss is introduced by hydrostatic head effects. Consider the single-effect submerged coil evaporator shown in Fig. 2.8. Heating steam is sent through the coil and causes evaporation to take place. However, the steam temperature required for evaporation must be such that not only are the temperature losses of both boiling point elevation and pressure drop overcome...

Electrodialysis

Electrodialysis involves the passage of electric current through brackish or low salinity water in a chamber in which many closely spaced ion selective membranes are placed, thus dividing the chamber into compartments. The electric current causes the salts to be concentrated in alternate compartments with reduced salt content in the remainder. The constraints of electrodialysis are such that feed-waters up to 2 000 p.p.m. may be processed to give a product with 300 p.pjn. total dissolved...

Distillation

Distillation involves the boiling or evaporation of sea water to form water vapour which is condensed to yield a salt-free stream. Energy requirements are virtually independent of the feed-water salinity and product purity of less than 50 p.p.m. can readily be achieved. The principal choices of plant are Multiple Effect evaporation ME and Multi-Stage Flash distillation MSF . The inclusion of 'multi' means that a cascading effect is employed whereby multiple re-use of the energy content of the...

Reverse osmosis

Reverse osmosis uses the reverse application of osmotic pressure. When salt water and fresh water are separated by a semi-permeable membrane, osmotic pressure causes the fresh water to flow through the membrane to dilute the saline water until osmotic equilibrium is established. Applying this in reverse, if a greater pressure is applied to the salt water then relatively pure water will pass through the membrane leaving a concentrated brine to be disposed of. The process technology is intimately...

Water supply and demand

As the demand for water increases so does the cost. In the non-arid zones increasing the supply can take the shape of more catchment schemes, aquifer abstraction, river abstraction and, of course, re-use. Figure 1.2 shows that the projected UK water demands will double in 25 years. Some sources put the doubling time at 15 years. Yet the UK has a relatively stable population whose doubling time is roughly 110 years. This tells us that per capita consumption is growing. The demands of industry...