Scale inhibition with proprietary compounds is a commonly used method and is termed threshold treatment, as the quantity employed is much less than the stoichiometric amounts required to combine with the calcium and magnesium present in the feed. The principal proprietary compound is Hagevap which is a mixture of sodium polyphosphates, lignin sulphonic acid derivatives and various esters of polyalkylene glycols. The dosing quantities employed range from 2-5 p.p.m. which are effective up to temperatures of around 85°C (185°F). The mechanism by which Hagevap prevents scale deposition is not clearly understood. One explanation is that the polyphosphate chain enters the scale crystal lattice while it is at an embryo stage and prevents further bond formation which results in a finely dispersed micro-sludge.
As the temperature of the sea-water brine is increased, the polyphosphate molecules become hydrolysed and then combine with magnesium ions to form magnesium phosphate which deposits as a sludge on the heat exchange surface. This hydrolysis reaction places a temperature limitation on the use of polyphosphate compounds to less than 85°C (185°F). The use of Hagevap at a maximum temperature of 93°C (200°F) and a concentration factor of 1.5 in a 4 546 m3/day (1 m.g.d.) MSF plant is reported bv Mulford et al,  who observed the occurrence of light sludge deposits as evidenced by a rise in pressure and temperature of the steam supply to the brine heater and a fall in the plant
Hagevap + acid
20 40 60 80 100 120 140
Number of days
Fig. 3.4. Performance ratio variation on a polyphosphate-dosed plant.
performance ratio as shown in Fig. 3.4 from reference.  Any sludges which do form as a result of polyphosphate hydrolysis may be readily removed by acid slugging, i.e. a pulse of dilute acid is sent round the plant at intervals dictated by the reduction in plant performance. The rate of deposition may also be controlled by the water side velocity in MSF plants where a velocity of order 2 m/s (6.5 f/s) can maintain a limiting sludge deposit.
The design penalties of using polyphosphate treatment is a reduction in the overall heat transfer coefficient which in MSF plants can be of the order of 567 W/m2 °C/week (100 Btu/h ft2 °F/week) initially. However, if enough heat transfer surface is built into the plant, an equilibrium situation is reached and acid cleaning may only be required at roughly six-monthly intervals. Reported polyphosphate treatment costs for the state of Kuwait are approximately 2 per cent of product cost.  The use of the Taprogge system for maintaining heat transfer surface performance has already been mentioned and as reference to Fig. 3.3 shows, this method can reliably maintain plant output.
The use of threshold treatment for the prevention of Mg(OH)2 scale in the 85°-107°C (185°-225°F) range is reported by Elliot and Hodgson.  The principles are similar to the polyphosphate treatment described previously, viz. a delay in the onset of precipitation and a deformation of the crystal lattice so that the scale does not adhere. The distorted crystals form a soft porous deposit in contrast to the hard crystalline Mg(OH)2 scale. This deposit has a high heat transfer resistance but is very easily removed. Related work on theoretical mechanisms of magnesium scale formation has been reported by Harris and Finan.  The successful pilot plant tests using this new additive for Mg(OH)2 prevention mean that the flash range can be extended before acid treatment has to be adopted.
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