scale formation in thermally-driven seawater …iwtc.info/wp-content/uploads/2017/05/51.pdf ·...

Twentieth International Water Technology Conference, IWTC20 Hurghada, 18-20 May 2017

412

SCALE FORMATION IN THERMALLY-DRIVEN SEAWATER DESALINATION:

EVALUATION OF TETRAPOLYMER ANTISCALANT AT HIGHER

TEMPERATURE OPERATION

Y.Singh

1, M. Shahzad

2 and K. NG*

3

1.2.3

King Abdullah University of Science and Technology (KAUST), Water Desalination and Reuse Center

(WDRC), Biological & Environmental Science & Engineering Division (BESE), Thuwal 23955-6900,

Saudi Arabia, [email protected]

[email protected]

[email protected]

ABSTRACT

Deposition of scales is quite common and prevalent in thermally driven desalination processes such as

Multi-effect distillation (MED) and Multi-stage flash distillation (MSF). With temperature, various salts /

ions (Ca2+

, Mg2+

, CO32-

, SO42-

etc.) dissolved in the sea/brackish water, start precipitating and depositing;

impeding the heat transfer necessary for evaporation. This limitation poses tremendous challenges; tends

to restrict the efficiency and economics of thermal desalination. This study is about optimizing the dosage

and scale inhibitory action of newly developed tetra polymer based antiscalant at 70°C and 98°C with

Red Seawater at concentration factor (CF) of 1.5 and 2.5 with 1ppm, 2ppm and 4 ppm dosing (as product)

of antiscalant under reflux condition for seven days along with metallic coupons. To understand the role

of antiscalant and onset of scale deposition, the inhibition action was evaluated by monitoring pH,

turbidity, and alkalinity. It was observed that addition of antiscalant displayed its inhibitory action by

altering alkalinity resulting in either delay or completely suppressing the scale deposition depending upon

dosage of antiscalant. It was also learnt that scale deposition and inhibition were closely related to

changes in pH and alkalinity which were also reflected in turbidity measurements. Scales deposited on the

coupons were also analyzed for morphology and constituent elements by SEM/EDAX which further

supported the inhibitory action of newly developed tetrapolymer antiscalant at a dose of ~2-4 ppm

depending upon seawater CF and temperature.

Keywords: Scales, Thermal Desalination, Antiscalant, Alkalinity

1 INTRODUCTION

The deposition of the scales in water related industries is widely anticipated issue. This is one of the

major problem faced by most of the seawater distillation plants. Most of the scales deposited are from the

sparingly soluble ions present in water viz. calcium, magnesium, carbonate, sulfate, silica etc. Scaling

happens when these sparingly soluble ions are concentrated beyond their solubility limits (Stumm, 1995,

Hamrouni and Dhahbi, 2001, Amjad, 2015) either by water evaporation (cooling tower, MED, MSF,

MD)(Shams El Din et al., 2005, El-Dessouky et al., 2004, Hillier, 1952, Hoang, 2015, Langelier, 1954,

Mubarak, 1998, Schwarzer and Bart, 2016) or by water separation (SWRO)(Antony et al., 2011). The

deposition of scales can cost the efficiency of the desalination process (Schwarzer and Bart, 2016) by loss

of heat transfer occurring on the surface in thermally driven processes or by decline in the permeation

through membranes (Antony et al., 2011). As a consequence of scaling, the top brine temperature (TBT)

in thermal desalination processes is restricted to 70°C, especially in multi-effect distillation and

membrane distillation. Most of the scales deposited in thermal processes are of calcium carbonate and

magnesium hydroxide but it can also lead to deposition of harder scales of calcium sulfate if temperature

mailto:[email protected]




413

and concentration of seawater are increased further (Alahmad, 2008, Dooly and Glater, 1972, El Din et

al., 2005, Fellows and Al-Hamzah, 2015, Shams El Din et al., 2005).

There are various theories and mechanisms reported about scale formation in seawater and about their

relation to pH, temperature and ionic strength of carbonic acid species (bicarbonate/carbonate ions,

dissolved carbon dioxide) (Amjad, 2015, Amjad and Koutsoukos, 2014, Alahmad, 2008, Antony et al.,

2011, Fellows and Al-Hamzah, 2015, Langelier, 1954, Rahman and Amjad, 2010, Shams El Din et al.,

2005). According to Langelier (Langelier et al., 1950), scale formation starts with thermal decomposition

of bicarbonate ions to produce carbonate ions as illustrated in eq. 1 and this combines with calcium to

form calcium carbonate (eq. 2). This is followed by hydrolysis of carbonate ions (eq. 3) to generate

hydroxide ions initiating magnesium hydroxide scale formation (eq. 4). In another theory, the thermal

decomposition of bicarbonate first produces hydroxide ions (Dooly and Glater, 1972) which further

results in the formation of carbonate ions as explained in equation 6.

2HCO31−

Δ CO3

2− + CO2 ↑ +H2O 1

Ca2+ + CO3

2− CaCO3 2

CO32− + H2O

Δ CO2 ↑ + 2OH− 3

Mg2+ + OH− Mg(OH)2 4

HCO31−

Δ OH− + CO2 ↑ 5

HCO31− + OH−

CO32− + H2O 6

There are also different models in literature which support carbonate scale formation mechanism at

lower temperature (< 80° C) whereas at higher temperature these favor formation of magnesium

hydroxide (García et al., 2006, Hasson et al., 1978, Mubarak, 1998, Pitzer, 1986, York and Schorle,

1966). Despite these all, It is still quite ambiguous about the mechanism for alkaline scale formation with

respect to competitiveness towards either calcium carbonate or magnesium hydroxide (García et al., 2006,

Hasson et al., 1978, Mubarak, 1998, Pitzer, 1986, Schwarzer and Bart, 2016, Shams El Din et al., 2002,

Shams El Din et al., 2005, Shams El Din and Mohammed, 1988, York and Schorle, 1966),

To increase the TBT for thermal desalination, though different approaches have been exploited viz.

acid addition, nano-filtration, thermal shock etc.(Abd-Eltwab et al., 2016, Al-Sofi et al., 1998, Ayoub et

al., 2014, Chernozubov et al., 1966, Tabata, 1974) but the use of antiscalants seem most convenient and

effective way to tackle scale formation. There are various reports and studies for the development and

evaluation of ecofriendly, stable, efficient and economic antiscalants (Al-Rawajfeh et al., 2015, Budhiraja

and Fares, 2008, Chauhan et al., 2012, Hamed et al., 1999, Hoang, 2015) and still lots of developments

are going on in this regard. The main objective of this study is to evaluate the performance of newly

developed tetrapolymer based antiscalant with real water (Red Seawater) at higher temperature range (up

to 98°C) to prevent scale formation and to understand the mechanism of scale formation by monitoring

pH, turbidity, and alkalinity. The morphological changes and elemental composition of the scales were

also examined by using SEM/EDAX.

2 EXPERIMENTAL DETAILS

2.1 MATERIALS

Fresh Red seawater was used to perform the scaling experiment. Sea water was collected and

concentrated under vacuum (20 mbar) at 30°C in rotatory evaporator. The concentration of seawater was

maintained to 1.5 and 2.5 factor by measuring conductivity at room temperature and adding fresh Red

seawater. The pH and turbidity of the experimental solutions were measured prior to each experiment.


414

Titration method was used to measure the alkalinity by adding phenolphthalein and mixed indicator

(Bromocresol Green/Methyl Red). 0.002N HNO3 solution was used as titrant. Stock solution of tetra

polymer antiscalant (400 ppm) was prepared in distilled water. The scaling experiments were carried out

with 400 ml seawater of mentioned concentration factors (CFs) at controlled temperature of 70°C and

90°C under reflux in three necked round bottle flask with stirring at 450rpm, using oil baths to maintain

uniform temperature. The stock solution of antiscalant was added by maintaining volume to 400ml to

obtain antiscalant dose of 0ppm, 1ppm, 2ppm and 4ppm. Small coupons were used to examine the scale

deposition (figure 1) by Scanning Electron Microscope/ Energy Dispersive X-Ray Analysis

(SEM/EDAX).

Figure 1. Experimental set up for seawater scaling study

2.2 METHODS

The aliquots were refluxed at assorted concentration and temperature with defined dose of antiscalant

for seven days. The solutions were pipetted out immediate after experimental temperature was achieved

and at defined time intervals. Immediate turbidity was measured followed by cooling down the solution at

room temperature. Prior to measuring the pH and alkalinity of the samples, these were centrifuged at 9000

rpm for 10 minutes to settle down all the precipitates. 5ml of the supernatant was pipetted out into conical

flask and titrated with 0.002 N HNO3. The coupons were removed from the flask and dried for

morphological and elemental analysis with SEM/EDAX.

3 RESULT DISCUSSION

The findings of the scaling experiments with assorted dose of tetra polymer based antiscalant at two

different concentration factors of 1.5 & 2.5 and at two different temperatures of 70°C and 98°C with Red

Seawater are presented and discussed in the following.


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3.1 SCALE FORMATION AT 70°C

Figure 2 Variation in pH, Alkalinity (Left plot) and Turbidity (Right Plot) for seawater with CF 1.5 at 70°C

under reflux condition

Figures 2 & 3 depict the effect of tetra polymer antiscalant on pH, alkalinity and turbidity at 70°C for

seawater with concentration factor of 1.5 and 2.5 respectively. It was observed that the pH of baseline

condition for both CFs started declining immediately after it reached 70°C followed by gradual increment

again in pH value. Although, the change in the pH was quite substantial and faster (just after 10 hours) for

seawater with 2.5 CF than with 1.5 CF. The initial drop in pH value is anticipated due to formation of

calcium carbonate which aligns with Langelier theory (Langelier, 1954, Langelier et al., 1950) of

bicarbonate decomposition as shown in equation 1 & 2. According to him, heating of the seawater results

in thermal decomposition of bicarbonate ions shifting equation 1 in forward direction and results in

formation of carbonate ions. As concentration of bicarbonate ions started declining with time, the

decomposition of bicarbonate ions seemed to favor hydroxyl ions formation as explained in equations 5 &

6 (Dooly and Glater, 1972, Fellows and Al-Hamzah, 2015, Hoang, 2015, Langelier, 1954, Langelier et

al., 1950), contributing towards gradual increment in pH value after initial decline and it was more intense

in seawater with higher concentration.

It is quite evident from figures 2 & 3 that overall alkalinity of the seawater solution also declined with

time. As most of the carbonic species (bicarbonate and carbonate ions) participated in scale formation, so

it was reflected in the subsequent drop in alkalinity. As seen from the figures, turbidity values also

increased immediately which was contemporaneous to decline in alkalinity for base line run and

formation of crystals, though the value of turbidly was substantially higher for seawater with higher

concentration. The immediate increase in the turbidity can be attributed to agglomeration and crystal

formations which later induced nucleation on the surface of the round bottom flask and coupons, causing

the deposition of crystals of CaCO3 and their growth. The growth of the crystals initiated immediate

decline in turbidity which started nearly after 2 hours of heating till it stabilized. At higher CF (2.5),

turbidity for baseline was much higher due to higher concentration of carbonic species ions and it soared

abruptly after 120 hours, which is suspected to be due to modification or weakening of carbonate scales

by incorporation of magnesium hydroxide formed due to decomposition of carbonate ions into hydroxyl

ions with time.


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Figure 3. Variation in pH, Alkalinity (Left plot) and Turbidity (Right Plot) for seawater with CF 2.5 at 70°C


Figure 4. SEM/EDAX micrographs showing the effect of antiscalant on morphological changes and elemental

composition of the scales at 70°C on seawater with CF 1.5

Addition of tetra polymer antiscalant seems to induce some physio-chemical changes affecting the

scaling mechanism. As depicted in the figures 2 & 3, it was observed that pH of seawater with antiscalant

seemed to resist any changes initially and then started increasing gradually followed by its sharp decline

depending on the concentration of antiscalant. Similar trends were observed for change in alkalinity for

seawater with antiscalant. It is believed that addition of antiscalant interfered in scale formation by

shielding the calcium ions responsible for carbonate scales as expected from equation 1 & 2. This lead to

further decomposition of bicarbonate and bicarbonate ions to form hydroxyl ion as explained earlier,

contributing towards pH increase of the solution. Therefore, carbonate scale formation was delayed and

this delay in scale formation was found to be related to the concentration of antiscalant as evident form

the figures 2 &3.

The increment in the pH continued till the concentration of hydroxyl ion exceeded solubility product

of magnesium ion and hydroxide ions which subsequently initiated magnesium hydroxide scale

formation. Formation of magnesium hydroxide scales further decreased the pH of solution due to removal

of hydroxyl ion and hence, shifted the decomposition of the bicarbonate. This was also reflected in the

turbidity of the solution. As soon as scale formation was initiated, turbidity of the seawater solution also

started increasing. The change in turbidity was also found to be related to the concentration of antiscalant

which governed scale formation. At low concentration of antiscalant, calcium carbonate was the major

contributor in scale formation, hence the value of turbidity stabilized after certain time period due to

deposition of the crystals. But at higher concentration of antiscalant, formation of hydroxides and

antiscalant affinity towards calcium suppressed carbonate scale formation and lead to formation and


417

incorporation of magnesium hydroxide which were not adhered strong enough to the surface than calcium

carbonate. Therefore, increase in the turbidly of the solutions observed due to dispersion of the scales.



The modification of the scales either by incorporation of magnesium or preference to from magnesium

hydroxide was also supported by SEM/EDAX analysis from the crystals deposited on the coupons.

Figures 4 & 5 depict the SEM/EDAX analysis of the scales deposited on the coupons at 70°C. It was

observed that calcium (Ca) element was the major constituent of the scales formed without use of

antiscalant, though traces of magnesium (Mg) element were also observed which was expected and

reflected from pH and alkalinity analysis.

The extent of scale deposition on the coupons at lower concentration of seawater was dependent on the

concentration of antiscalant as reflected in SEM micrographs in figure 4. It is clear from EDAX spectra

that the use of tetra polymer antiscalant suppressed the participation of calcium (Ca) in scale formation

and favored magnesium hydroxide scale formation along with silica which was quite surprising.

Similarly, at higher concentration of seawater, most of the scales formed were from calcium carbonate

with traces of magnesium for base line conditions as depicted in figure 5. Therefore, the use of antiscalant

enhanced the incorporation of magnesium and silica as observed from SEM/EDAX analysis and nearly

restricted calcium ions to participate in scale formation at higher dose (4ppm).

3.2 SCALE FORMATION AT 98° C

As discussed earlier, the composition and mechanism of the scale formation differ slightly at higher

temperature (>80C). At this temperature range, most of the bicarbonate ions thermally decompose to

evolve carbon dioxide and hydroxide ion (Dooly and Glater, 1972). The scale formed at higher

temperature are mostly made up of calcium sulfate which become prominent with higher seawater

concentration. Figures 6-7 depict the inhibitory effect of tetra polymer based antiscalant reflected in the

analysis of various parameters viz. pH, alkalinity and turbidly at 98°C which is further supported by the

analysis of crystals/ deposits by SEM/EDAX technique for their compositional and morphological

changes.


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Figures 6 & 7 illustrate the effect of addition of tetra polymer antiscalant on the value of pH, turbidity

and alkalinity with time at seawater concentration factor of 1.5 and 2.5 respectively. At lower seawater

CF, it was observed that pH declined immediately after reaching the experimental temperature (98°C)

which resulted in immediate decline in alkalinity followed by subsequent increase in turbidity. These

changes were ascribed to calcium carbonate agglomeration favored by higher temperature. Meanwhile,

carbonic species present in water started to dissociate to form hydroxyl ions and resulted in gradual

increase of pH.


under reflux

As explained earlier that formation of hydroxyl ions favored the formation of magnesium scales and at

around 80 hours, the concentration of hydroxyl ions reached beyond solubility limit to induce

precipitation of magnesium hydroxide scale. Therefore, the value of the turbidity also changed abruptly at

the same time. As evident from the alkalinity analysis that the use of antiscalant inhibited calcium ions to

combine with carbonate ions to form carbonic scales and hence slowed down the alkalinity drop. The pH

of the solution was raised up slightly before transforming to hydroxyl scales as expected from

decomposition of bicarbonates at higher temperature. It is quite evident form the figures 6&7 that the

changes in the value of pH and delay in the decline in alkalinity were directly related to concentration of

antiscalant. More the concentration of antiscalant, more it delayed/resisted the changes in pH, alkalinity

and turbidity.

Similarly for higher concentration (2.5 CF) of seawater, deviations were more vigorous for alkalinity

drop and pH which could be attributed to higher concentration of seawater. At this CF and temperature, it

is believed that sulfate ions exceeded the solubility product for calcium sulfate scale formation. The pH of

the solution also stabilized after initial abrupt fall because of counter effect of hydroxyl ion produced


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from the decomposition of carbonic species ions till hydroxyl ions concentration exceeded to the level to

induce magnesium hydroxide scale formation. Addition of antiscalant again interacted in such a way to

retain the pH and alkalinity of seawater depending upon the concentration of tetratpolymer antiscalant.

Similarly, the value of turbidity (figures 6&7) observed for this concentration factors were exceptionally

higher than seawater with 1.5 CF due to higher concentration of ions and their participation in scale

formation.





SEM/EDAX analysis as depicted in figures 8 & 9 also support the formation of sulfate, magnesium

and silica scales than carbonate at higher temperature. At lower concentration of seawater as depicted in

figure 8, no traces of calcium were observed from EDAX analysis. At higher CF (2.5), calcium ions

combined with sulfate ions to form calcium sulfate crystals along with magnesium hydroxide displaying

prominent peaks of S and Ca elements and feeble peaks of Mg and Si for base line. But addition of the

antiscalant suppressed the calcium participation in scale formation which was also supported by SEM

micrographs and EDAX spectrum as discussed earlier during the analysis of pH and alkalinity variations.

Therefore, the peak of calcium element faded with the concentration of antiscalant and peaks of

magnesium and silica became more intense and prominent.


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Figure 10. Effect of the concentration of tetrapolymer antiscalant on the amount of scale formation at 70°C

and 98°C from seawater with Concentration factor of 1.5 and 2.5

Figure 10 illustrates the overall inhibitory effect of assorted concentrations of antiscalant on total

reduction in scale formation by weight at different concentration of seawater and temperature. The

amount of scale formed at 1.5CF of seawater reduced by ~75 % at 4ppm dose of tetrapolymer antiscalant

whereas it reduced to ~10 % at 2.5 CF of seawater with same dose of antiscalant irrespective of the

temperatures. Hence, the addition of tetrapolymer antiscalant effectively minimized the overall scale

formation by restricting and shielding the calcium ions resulting in either delay or by shifting the

equilibrium towards magnesium scale formation.

4 CONCLUSION

It is concluded that seawater concentration factor and temperature are very crucial and govern the scale

formation. Monitoring the values of pH, alkalinity and turbidity are important; these can suggest the onset

point of scale formation. Addition of newly developed tetrapolymer antiscalant is very effective in

preventing calcium ions to participate in scales formation but it’s not efficient enough to prevent scales

from magnesium and silica. It is also concluded that amount of antiscalant needed to prevent the scales is

dependent on the duration of experiment or residence time for seawater while heating. Longer the

residence time, more will be scale formation and higher dose of antiscalant will be needed. The optimum

dose of tetrapolymer antiscalant needed is found to vary from 2-4 ppm which is directly related to

seawater concentration (up to 2.5 CF), temperature and duration of experiment/residence time under

reflux condition for seven days or less.

ACKNOWLEDGEMENTS

The authors would like to acknowledge ‘NALCO Water’, Saudi Arabia for providing newly

developed tetrapolymer based antiscalant.

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