MARINE ENGINEERING SYSTEMS
Distillation and Freshwater Generation
The distillation and generation of freshwater is immensely important onboard a ship. Not only does the crew need water to remain hydrated to perform their functions, or for other domestic uses, but freshwater is also used as a cooling medium for the ship’s propulsion system, for washing down the ship superstructure and decks, and for creating steam in the ship’s boilers as a source of heat to be used in heat exchangers.
The freshwater distillation process involves the evaporation of seawater, using an evaporator, and then re-condensing the vapour by making use of a condenser. To this extent, a boiling or flash process can be used to create the water vapour for distillation. Another method of generating freshwater is by making use of a Reverse-Osmosis Plant. Domestic fresh water, as it is known on-board a ship, is usually generated from a Reverse-Osmosis Plant (RO Plant).
As stated previously, the boiling of seawater is one method that can be used to distil fresh water. The simplified boiling process, shown in the diagram below, for this distillation method is as follows:
- Seawater at the bottom of the distillation plant is boiled by means of a heater nest or coil fed by either hot jacket cooling water from the main propulsion engines, steam or electricity. This is referred to as the evaporator. On big ships, the heat produced for boiling the seawater is usually provided by steam originating from a boiler.
- Air suction is provided on the plant which reduces the pressure by means of creating a vacuum, which in turn lowers the boiling point of the water.
- Steam produced during the boiling process rises and passes through a water separator/demister.
- A condenser nest or coil, supplied by cold seawater as the cooling medium, causes the steam to condense and drop into a catch where it is drawn up by a distillate pump and fed to a freshwater storage tank.
- Finally, brine which results as a byproduct from the boiling process of seawater, which you do not want to contaminate your distilled freshwater, is able to drain from the plant via an overflow weir.
The Flash Process is yet another method used to distil fresh water from seawater. The flash process involves a method of distillation through several (multi-stage) chambers as can be seen in the diagram below. This process is also referred to as a Multi-Stage Flash (MSF) Process. An example of a Wartsila designed and manufactured MSF process plant is shown below.
The fundamentals regarding the operation of such an MSF process is as follows:
- In the MSF process, each successive stage of the plant operates at progressively lower pressures.
- The feed water is first heated under high pressure, and is led into the first “flash chamber” where the pressure is released causing the water to boil rapidly resulting in sudden evaporation.
- This ‘flashing’ of a portion of the feed continues in each successive stage, because the pressure at each stage is lower than in the previous stage.
- The vapour generated by the flashing process is converted into fresh water by being condensed on heat exchanger tubing that runs through each stage, cooled by sea water. The condensed water from each stage collects in a catch and drains to a distillate pump which removes the fresh water and pumps it into a fresh water tank.
- Generally, only a small percentage of the feed water is converted into vapour and condensed during this process.
- Highly concentrated brine is discharge from the system in the last stage
Another method of freshwater generation from seawater, which is mostly used on-board ships for the generation of drinking and domestic freshwater, is Reverse-Osmosis. Reverse-Osmosis is a membrane process and does not rely on a boiling or flash process to generate water vapour. A simplified schematic of a RO plant is depicted below. An example of a RO Plant is also shown next (note the RO membrane tubes). The RO process, with reference to the below, is as follows:
- A seawater pump draws up seawater which feeds the RO plant.
- 2 & 3. The seawater passes through various filters to remove any large particulates or contaminants in the seawater.
- 4A, 4B, 5 & 6. Throughout the RO system, various pipe equipment is installed to regulated, monitor and control pressure.
- A high-pressure pump increases the line pressure to force the filtered seawater through the RO membranes which are usually encapsulated in tubes.
- In Osmosis, a solvent moves through a membrane from an area of low solute concentration i.e. a high-water potential, to an area with high solute concentration i.e. low water potential. This occurs naturally, but by applying an external pressure that overcomes the osmotic pressure we can reverse the Osmosis process, providing us with a solvent of low solute concentration and high-water potential i.e. drinkable water. Thus, the semi-permeable membranes allow the removal of ions and molecules which would generally make the water unfit for human consumption.
- 12A, B, C. Once the seawater has passed through the RO membrane, brine water is removed from the plant and discharged overboard; water that has a TDS (Total Dissolved Solids) of more than 500ppm, as detected by the TDS meter, is also discharged overboard while drinking water is pumped to the freshwater permeate tank for consumption.
Reverse Osmosis Plant