Mechanical Vapour Compression Desalination System

Mechanical Vapour Compression Desalination System

1  Introduction:

The Vapor compression distillation process is generally used for small to medium scale desalting units. The heat for evaporating the water comes from the compression of vapor rather than the direct exchange of heat from steam produced in a boiler. Method use to condense vapor to produce enough heat to evaporate the incoming seawater is mechanical compressor.

2  Mechanical and Thermal  vapor compressor systems :

The mechanical compressor creates a vacuum in the vessel and then compresses the vapor taken from the vessel and condenses it inside a tube bundle. Seawater is sprayed on the outside of the heated tube bundle where it boils and partially evaporates, producing more water.

The mechanical vapor compressor (MVC) system is most attractive as it is compact and does not require an external heating source, but it does need highly skilled /trained operators and also required periodic
maintenance.

The operation of the system is at low temperature ranges from 60 to 70⁰ C. At this low temperature the scale and the corrosion are reduced. Another advantage is at lower temperature the losses are minimized and requirement for thermal isolation is also minimize.

The MVC system has many advantages. Some of these are:

•          Flexible operation, i.e. load adjustment through temperature variations
•          Simplicity of pre-treatment
•          Good product quality
•          High reliability and long lifetime

Some of the disadvantages of system are:

•       Limited capacity of the units
•       High maintenance and spare parts requirements of the mechanical “ Compressor “ parts
•       Highly skilled operators for the operation of the unit
•       System required large heat transfer area, which increase capital cost

   Description of the Mechanical Vapor Compression process :

It is a seawater distillation system with a horizontal spray film design. Feed water is taken through two plate-type heat exchangers in parallel, each one of them heated by the generated product water and reject brine water, respectively.

Both the product and the reject brine are discharged from the evaporator. Then the seawater is mixed with the re-circulated brine and pumped to the evaporator through spray nozzles, which spread the brine over the tubes of the evaporator and are collected in the bottom of the vessel. A sufficient amount of the seawater re-circulated in order to get thin continuous film over the tubes of the evaporator. Thus process increases the rate of heat transfer.

The starting and make up steam is given through external electrical boiler. Vapor is generated from the re-circulating brine as it is sprayed over the heat transfer surface of the tube bundle. The vapor is drawn through demister to the centrifugal compressor, which increases the pressure and temperature of the vapor by compression. The vapor discharged in to the inside of the tube bundle where it condenses in to distillate. Also the vapor compressor produces a vacuum when suctioning in such a way that pressure is lower than that of equilibrium at feed temperature, and there for part of seawater evaporates.

See the figure schematic representation of the spray film vapor compression system

Flow Diagram

A vacuum pump is charged with the system to produce the start-up vacuum and extract the non-condensable gases from the system to attain a good heat transfer process during the operation of the unit.

The vapor compressor is a radial centrifugal type; casing and impeller are from aluminum alloy. The evaporator material is also of Cu/Ni-steel alloy. The design operating temperature of the unit is 70⁰ C.

The most significant parameters considered for the design are as under:

·         Feed water flow rate

·         Feed water salinity, % TDS

·         Feed water temperature, ⁰ C

·         Product water flow

·         Product salinity, ppm

·         Brine discharge flow rate

·         Brine salinity, % TDS

·         Evaporation temperature

·         Operating pressure

·         Pre-treatment and antiscale dosing, ppm

·         Compression ratio

     Evaporator assembly :

• The shell is of cylindrical shaped insulated from the outside that is used to house the major items of the evaporator assembly mounted on the skid. The shell is constructed of 316L and externally reinforced with steel
• The tube bundles heat the re-circulated feed water by condensing the compressed steam. The upper tubes are made of titanium alloys while the below tubes are of zinc alloys to protect them from the scale.
• The boiler provides the small quantity of starting and make-up heat required for the operation of the unit depending on the feed water temperature and compressor load.
• The vapor separator (demister) is provided to remove any entrained raw water droplets from the vapor before entering the compressor.
• With  spray nozzle pipe assembly the re-circulated seawater sprayed over the tube bundle.

In addition to the above items,  parts are included in the evaporator assembly are,  glass                       observation windows for viewing the interior of the evaporator shell, liquid level glasses, drains, vent and safety valves.

    Heat Exchanger :

A plate-type heat exchanger with a natural rubber gasket is provided. The exchanger is used to preheat the incoming feed water and cool the distillate to about ambient temperature. Thus the exchangers handles the three streams.

    Centrifugal Compressor ;

The compressor is of high speed centrifugal type with an enclosed impeller to provide maximum strength and efficiency. The compressor is designed for easy maintenance and can be disassembled without the use of special tools. All moving parts are well enclosed to protect operating personnel.

     Pumps :

Centrifugal-type distillate and re-circulation pumps are provided. The pumps are constructed of steel casting with mechanical seals for continuous operation.

     Post-Treatment for Potable water :

NSG gas having high concentration of carbon dioxide removed from the seawater passed in to product water converting in to carbonic acid. The carbonized water is passed through the lime stone tower to increase the desirable TDS level.

Note: For better understanding schematic diagram is taken from the IDE presentation.