WATER TREATMENT
There are several ways to compare additives using induction time experiments . For CaCO 3
, the measurement of pH inflection is one tool that indicates the onset of precipitation . As it forms in waters that are not highly buffered , a decrease in pH can be observed shortly after precipitation , due to the removal of alkalinity from the water via
CaCO 3 formation .
This is one of the simplest ways to screen additives in a lab environment but with the limitations of sensitivity and the lag between actual precipitation and observance of a pH inflection . Another tool is turbidity . As insoluble CaCO 3 forms , an increase in turbidity is observed , which indicates precipitation . This method also has limitations of sensitivity and being a lagging indicator of precipitation .
A third possible method would be to examine mass balance changes in calcium and / or alkalinity via water analysis . This method is least preferred due to the need for collection , stabilisation and rapid analysis of samples within the short time frame in once-through cooling applications .
One potential way to overcome these issues is with the use of ionspecific electrodes . However , my experience is that these lack stability and accuracy . Furthermore , as with pH electrodes , they are a lagging indicator of the onset of precipitation .
The use of a quartz crystal microbalance ( QCM ) is a low-cost , somewhat novel , effective and highly sensitive tool for the purposes of evaluating ultra-low dosages of additives . 3 In this , a quartz crystal is set to oscillate at a specific high frequency . Data collection software continuously records and reports the data .
The QCM resonator can detect changes in frequency due to deposition of foreign matter onto the surface QCM crystal at a level down to µ g / cm 2
. These levels can be measured in real time and observed within the residence time of typical condenser operations . Where the other aforementioned techniques are lagging indicators of scale formation , QCM can measure real-time changes and observie the onset of scale formation and deposition .
For the experiment , a Stanford Research Systems QCM-200 instrument was used with a goldplated quartz crystal wafer . Separate solutions of the desired cationic and anionic components of the water chemistry were prepared and heated to the indicated temperature . The former was prepared with 83 ppm calcium ( as ion ), 42 ppm magnesium ( as ion ) and heated to 71 º C . The latter was prepared with 87 ppm bicarbonate , 100 ppm carbonate , 167 ppm sulfate , 147 ppm chloride , and 1 ppm orthophosphate and heated to 71 º C .
After stabilisation of the baseline QCM frequency , the cation solution was treated with inhibitor and then mixed at 30 seconds ( Time 0 ) with the anion solution to begin the experiment . Frequency changes were then observed and recorded versus time to compare inhibitors . The resulting pH from the combined solutions was 8.8-8.9 . The leading industry software for scaling indices for once-through waters shows this water to have a calcite saturation of about 43.5 times , a Langelier saturation index of 2.01 and a momentary excess for calcite of about 19.18 mg / l .
This index is a good indication of the driving force for precipitation , while the calculation of momentary excess provides an understanding of how much calcite will precipitate ( untreated ) before the water returns to equilibrium , defined as the condition in which precipitation and dissolution rates are equal , resulting in net zero precipitation of CaCO 3
.
In this and all of the experiments presented in this paper , the QCM probe was allowed to stabilise in the cation solution until a steady baseline frequency was observed . The experiment was then started and data recording initiated . The anion solution was then added to the cation solution at 30 seconds , which becomes Time 0 , since this is the first opportunity for precipitation and potential deposition .
The untreated blank , an inflection or reduction in frequency of about 10 hz , is recorded within three seconds of the solutions being mixed ( Figure 2 ). This is indicative of the onset of precipitation and actual deposition onto the QCM probe . This is the time in which induction of CaCO 3 precipitation is observed and recorded .
While the initial reduction of vibrational frequency was observed at just three seconds , the test is typically continued until a full inflection is observed and no further decrease in frequency is measured . In the untreated blank test , this occurred at about 50-55 seconds . At this point , the overall change in delta frequency ( ∆F ) over the duration of the test provides an indication of the relative deposition quantity .
Using the same methodology described above , the QCM instrument was utilised to compare the efficacy of a PEPMA / HPSP blend to phosphonate and an untreated blank in synthetic solutions , all at 50 ppb , replicating specific water and operating conditions of three oncethrough utility applications .
Table 1 shows the water conditions . The final pH is that of the solution once the cation and anion solutions were combined and treated . Reporting of alkalinity is based upon customer-provided detail and was replicated in the synthetic solutions .
Figure 3 shows the comparative QCM data . In each synthetic utility water tested , the untreated blank exhibited the highest amount of deposition , as would be expected . Both PBTC and HEDP treatments at 500 ppb indicated an improvement in deposition amounts but did not necessarily delay the onset of precipitation .
In all cases , PEPMA / HPSP significantly limited the amount of total deposition observed and substantially increased the induction time where the onset of CaCO 3
NOV / DEC 2023 SPECCHEMONLINE . COM
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