REVERSE OSMOSIS DESALINATION

Osmosis:

When fluid of different concentration in a vessel are separated by a membrane, the dilute solution will flow through the membrane into the concentrated solution.

Osmotic pressure:

The level of dilute solution drops and the level of concentrated solution rises until an equilibrium is reached. The pressure difference between the two levels is called osmotic pressure.

Reverse osmosis:

If a pressure excess to a osmotic pressure is applied to the concentrated solution, the flow is reversed from the concentrated solution to  the diluted solution.

Membranes used:

• Hollow fine fibre
• Spirally wound

For sea water application membranes used:

• Polyamide
• Polysulphonate

In reverse osmosis desalination:

Pic: Reverse osmosis desalination

• Sea water feed is pressurized using high pressure pump.
• The high pressure sea water is passed through a chamber containing a permeable membrane.
• The membrane stops the passage for salt and allows pure water to pass.
• The clean water is then sent to a potable water tank.

BASIC REFRIGERATION - Vapour compression, Vapour absorption

Definition: -

Refrigeration is defined as a process in which the temperature of the space or its content is reduced below its surrounding temperature.

Purpose of refrigeration on ships:

• To prolong the life of perishable food stuffs.
• Liquefaction of boil off on LNG carrier.
• To maintain sub zero temperature for certain chemicals.
• Removal of moisture from control air.
• Air conditioning for the comfort of personnel and prolonging life of electronic components.
• For cooling the drinking water.
• For making ice on passenger ships.
• For making interference fit of small components such as exhaust valve seat and valve guide etc.

Classification of refrigeration system:

Classification based on the uses

• Vapour compression
• Vapour absorption
• Steam ejection method
• Air refrigeration method
• Unconventional method

Classification as per the size of the plant

• Primary system
• Primary/secondary system

Classification based on the refrigerant 

• Freon-22
• R-134a
• R-404
• R-507
• Ammmonia

Different refrigeration system:

• Ice refrigeration.
• Vapour compression refrigeration system.
• Vapour absorption refrigeration system.
• Air refrigeration 
• Steam jet refrigeration system
• Spray refrigeration system
• Dry ice refrigeration
• Thermo-electric refrigeration

Vapour Compression Refrigeration system: -

Pic: Vapour compression refrigeration system

Components of vapour compression system:

• Evaporator
• Compressor
• Condenser
• Expansion valve

Operation: -

• In vapour compression system, the phase will change from liquid to vapour in evaporator and vapour to liquid in condenser.
• The refrigerant absorbs the latent heat from evaporator and reduces the temperature of the space from its surrounding thus change its phase from liquid to vapour.
• This high temperature refrigerant is then compressed in a compressor to make the pressure equal to the condensing pressure.
• In condenser, the refrigerant is cooled thus leaving the latent heat. The phase of refrigerant changes from vapour to liquid.
• This high pressure low temperature refrigerant is then passed through the expansion valve where its pressure is reduced and made equal to evaporating pressure.
• This refrigerant is thus again used for the refrigerant purpose.