The origin of seawater desalination is to be found in the Navy.
Steam ships generally need much greater quantities of fresh water
than they can actually carry, and have always had recourse to distillation
to meet their needs. Although rather rudimentary at first, the technique was later
developed and then adapted to terrestrial needs with the
emergence of the OPEC countries which, despite not having sufficient
water (resources lower than 1,200 m3/ day), enjoyed the
considerable advantage of having a very cheap source of energy at their
disposal.
MULTI-STAGE
FLASH DISTILLATION
Multi-Stage Flash Distillation (MSF) was
developed in the Gulf States in the 1970’s and still represents 90% of
the desalination units which have been set up in the region.
The MSF technique consists in applying the following distillation
principle:
The system merely reproduces the same natural
phenomenon of rain by concentrating it in time and space.
In a closed environment, two phases (here, water and its vapour)
find themselves in equilibrium at certain pressure and temperature levels.
For example, at atmospheric pressure (1 bar), water at 100 0
C is in equilibrium with its vapour, but below a pressure level of 0.1
bar, the boiling temperature is lower, at around 45 0 C, thus,
when superheated seawater is introduced into a vacuum tank, the water
cools down spontaneously, releasing vapour to be in equilibrium with the
pressure. When it comes into
contact with a cold surface in the vessel, the water vapour condenses and
produces water which is free of all dissolved salts.
The name given to the remaining water – which has a high
concentration of salts – is brine.
Cleveland Bridge & Engineering Middle East
(Pvt) Ltd., through its strategic location in Dubai is ideally positioned
to service the regional desalination market.
Details of a recently completed desalination plant at Ghazlan for
the Saudi Arabian utility SCECO are as follows:
Multistage Flash Evaporator
Design and constituent
| § |
3 no. evaporator units, each of the Multi-Stage Flash
(MSF) type and cross flow tube design. |
| § |
Each evaporator unit consists of 20 stages. 17 heat
recovery stage and 3 heat rejection stages. |
| § |
The stage chamber including condenser tube bundle,
weir, splash plate, demister, distillate orifice, distillation tray,
manhole, water box etc. |
| § |
The evaporator was fabricated separately into 10
blocks and shipped to site for final assembly and commissioning. |
| § |
Water boxes, Brine inlet boxes and Brine hotwells were
also manufactured for each unit. |
| § |
The evaporator bears an ASME Code Stamp. |
