WATER TREATMENT
Parameter Utility A Utility B Utility C Calcium( mg / l as Ca ² +) 91.2 83.2 170.0 Magnesium( mg / l as Mg ² +) 42.9 42.2 NR Bicarbonate( mg / l as HCO₃ ² −) 301.5( total as CaCO₃ 86.9) 255.0( total as CaCO₃)— Carbonate( mg / l as CO₃ ² −)— 100.0— Sulfate( mg / l as SO₄ ² −) 0.0 166.9 NR Chloride( mg / l as Cl−) NR 147.2 NR Phosphate( mg / l as PO₄ ³ −) 0.0 1.0 0.0 Final Solution pH 9.0 – 9.2 8.8 – 8.9 9.2 – 9.3 Temperature(° C) 50 ° C 71 ° C 50 ° C Calcite Saturation 94.4 × 43.5 × 140.3 × Langelier Saturation Index 2.5 2.0 2.7 Calcite Momentary Excess( mg / l) 46.1 19.2 35.4 Treatments
Blank / 500 ppb PBTC / 500 ppb PEPMA / HPSP
Blank / 500 ppb HEDP / 500 ppb PEPMA / HPSP
Blank / 500 ppb HEDP / 500 ppb PEPMA / HPSP
Table 1- Synthetic water conditions frees up the polymer or phosphonate molecules to interact with other forming crystal lattices, supporting the sub-stoichiometric relationship.
Distorted crystals tend to be more easily stabilised( colloidal) and dispersed as particulates. The modified crystals themselves are much less likely to form permanent attachments to surfaces. The comparative modification can be readily shown in a comparison of untreated calcite versus a sample precipitated in the presence of the PEMPA( Figure 1). 2
In once-through cooling applications, the functionalities of polymers and phosphonates do not all necessarily come in to play, due to the short retention time in the condenser, high water flow velocities and ultra-low additive dosages. For example, at ppb dosage levels and high velocities, they would probably not provide much functionality for dispersion or stabilisation.
Evaluation & methodology
In once-through cooling applications, threshold inhibition is the primary mechanism of either polymers or phosphonates. They are typically used at ppb range levels, due to the short retention time of the water through the utility condensers. Therefore, the quantification of induction time versus additive dosage is a valuable tool for comparing additives.
There are several ways to compare additives using induction time experiments. Measuring the pH inflection of CaCO 3 is one tool to indicate the onset of precipitation. As CaCO 3 forms in waters that are not highly buffered, a decrease in pH occurs shortly after the precipitation, due to the removal of alkalinity when it forms.
As insoluble CaCO 3 forms, an increase in turbidity is observed which indicates precipitation. However, both of these methods have limitations of sensitivity and show a lag between actual precipitation and observance of a pH inflection.
A third possible method is to examine mass balance changes in calcium and / or alkalinity via water analysis but this 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 to use calcium ionspecific electrodes. However, my own experience has been that that they lack stability and accuracy. They are also a lagging indicator of the onset of precipitation.
QCM option
The use of a quartz crystal microbalance( QCM, pictured) is a low-cost, 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. QCM data collection software continuously records and reports the data.
The QCM resonator can detect changes in frequency due to the deposition of foreign matter onto the surface QCM crystal at a level down to μg / cm 2
. These can be measured in real time and observed within the residence time of typical condenser operations. The QCM is an extremely valuable tool for measuring real-time changes and observing the onset of scale formation and deposition.
A Stanford Research Systems( SRS) QCM-200 instrument was used with a gold-plated quartz crystal wafer. Separate solutions of the desired anionic components and cationic components of the water chemistry were prepared and heated to the indicated temperature. After stabilisation of the baseline QCM frequency, the cation solution was treated with inhibitor and mixed at 30 seconds( Time 0) with the anion solution. Frequency changes were observed and recorded versus time to compare inhibitors.
Figure 2 shows an example of an untreated blank experiment. In this experiment, the cation solution was prepared with 83 ppm calcium( as ion) and 42 ppm magnesium( as ion) and heated to 71 º C. Separately, an anion solution was prepared with 87 ppm bicarbonate, 100 ppm carbonate, 167
46 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981