Calcium sulphate scaling

Calcium sulphate scaling occurs as a result of its reverse solubility in sea water, i.e. as sea-water brine temperatures and concentrations increase, the CaS04 solubility in sea water decreases. As calcium sulphate is not at saturation level in normal sea water, it is possible to prevent this scale forming if the distillation process is operated below temperatures and concentrations at which it occurs. As this scale cannot be readily removed by acid cleaning, its deposition must be prevented.

Figure 3.5 shows the theoretical scaling limits of calcium sulphate. It is seen that three forms of deposition can occur, each being operative over a given temperature range. These three forms and their chemical compositions are:

(ii) hemi-hydrate, CaS04 .\H20

(iii) anhydrite, CaS04

Each form has its respective solubility range, the anhydrite being the least soluble and therefore the one which would be deposited first. However, for scaling to occur, three requirements must be met. These are, availability of nucleation sites, supersaturation of the scaling compound, adequate residence or nucleation time.

The first and third conditions are readily fulfilled in most distillation plants. Thus the prevention of calcium sulphate scaling is done by the non-fulfilment of the second condition with the added proviso that the anhydrite form requires a long residence time for its occurrence. It is thus possible to run a distillation plant to the right of the anhydrite line in Fig. 3.5, but to the left of the hemi-hydrate solubility line. This permits higher operating temperatures at a given brine concentration. This method is commonly practiced in acid-dosed MSF plants as the flashing brine spends a relatively short time at the high temperature end of the plant. Should any calcium sulphate scale nuclei be formed, they either redissolve or

y2H2o

Solubility Curve For Calcium Sulphate

Anhydrite

----CaS04 boundary from MSF plant tests y2H2o

Anhydrite

----CaS04 boundary from MSF plant tests

Temperature, F

Temperature, C

Fig. 3.5. Solubility of CaS04 for varying sea-water concentration and temperature.

do not crystallise when the brine is cooled below the point of supersaturation when it reaches the low temperature end of the plant.

It should be noted that in the boundary layer adjacent to the tube wall, the retention time may be such that anhydrite scaling can occur. This is governed by the combination of residence time at the wall and the wall temperature which at

Gypsum Solubility Curve Temperature

Chlorinity

Fig. 3.6. Solubility produce of CaS04 in sea-water concentrate.

Solubility product [Ca] [S04] in (g.mols)2 (kg)"2 Chlorinity in g./1000g.

Chlorinity

Fig. 3.6. Solubility produce of CaS04 in sea-water concentrate.

Solubility product [Ca] [S04] in (g.mols)2 (kg)"2 Chlorinity in g./1000g.

high heat flux operation can be several degrees warmer than the bulk fluid temperature as discussed in [12]. Generally, operation above the anhydrite solubility limits is possible and avoidance of the gypsum and hemi-hydrate solubility limits is required. Much experimental work has been done in this field [3-5, 13, 14] and a useful result is Fig. 3.6 from the work of Simpson and Hutchinson [14] which shows the variation in solubility product with chlorinity

(or concentration) of the sea-water brine with temperature as a parameter. The solubility product K is defined as the product of the molar concentration of ions present, i.e.

at fixed temperature while the [ ] symbol denotes molar concentration.

As the solubility of a compound is governed by the solubility product, it is a relatively easy matter to calculate the maximum temperature to which sea-water brine may be heated for any given concentration. Thus for a concentration factor of 2, i.e. a chlorinity value of 40 g/1 000 g and from Table 3.2

[COi-] =2.(0.028 8) gmol/1 Thus the product of the molar ionic concentrations is

Referring to Fig. 3.6 on the hemi-hydrate curves at a chlorinity of 40 and log10 solubility product of —2.9 gives a maximum temperature of 126°C (260°F) to which sea-water brine of concentration factor 2 can be heated without risk of hemi-hydrate deposition.

At the other extreme, sea water of concentration factor 1, i.e. log10 solubility product = logio 3.523 can be heated to a theoretical maximum temperature of 160°C (320°F) without risk of hemi-hydrate formation. This theroretical maximum must be treated with caution as actual MSF plant maximum operating temperatures at a concentration factor of 1 have been nearer 143°C (290°F). [15]

It is seen that calcium sulphate scaling considerations currently fixes the top temperature at which distillation plants can operate at any fixed concentration factor. The incentives for increasing operating temperature may be readily grasped from the following simplified example.

Consider an MSF plant using Hagevap feed treatment with a flash range of 50°C (90°F), i.e. a top temperature of 88°C (190°F) which uses steam near atmospheric pressure (10s Pa) in the brine heater; and another MSF plant using an acid-dosed feed with a flash range of 89°C (160°F), i.e. top temperature 127°C (260°F), bottom temperature 38°C (100°F) using steam near 3.10s Pa (30p.s.i.g.) in the brine heater. Then, all other things being equal, the 89°C (160°F) flash range plant will produce approximately 1.75 times the output of the 50°C (90°F) flash range plant, as output is proportional to flash range if the brine recirculation quantities on each plant are equal.

The latter plant gives a much greater output than the former for much the same energy input though the 3.10s Pa (30p.s.i.g.) steam may be more expensive to purchase than the atmospheric pressure steam. There are clear incentives for increasing the top temperatures in MSF plants as generally the smaller the flash range the greater is the capital cost for a given output and much research is being devoted to increasing the top temperature limit. One proposal is the multi-effect multi-stage plant (MEMS) which uses brine with a concentration factor near unity in a high temperature flashing effect consisting of several flashing stages before cascading the brine to another lower temperature effect where it acts as the feed for a brine stream of greater concentration factor.

In this way the flashing range may be considerably extended albeit at the price of plant complexity. A proposal to operate a single-effect multi-stage (SEMS) plant at a top temperature of 149°C (300°F) has also been reported. [16]

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