A COMPARISON OF SCALE POTENTIAL INDICES WITH TREATMENT PROGRAM RESULTS IN OZONATED SYSTEMS

R.J. Ferguson
French Creek Software, Inc.
Kimberton & Hares Hill Roads, Box 684
Kimberton, PA 19442

A.J. Freedman
Arthur Freedman Associates
390 Penn Estates
East Stroudsburg, PA 18301-9045
(717) 420-1525

Presented at CORROSION '94

ABSTRACT

Ion association model saturation levels were calculated for ozonated cooling systems. This paper compares treatment program results, including deposits present, to indices calculated for calcium carbonate and other scale forming species typically encountered in cooling systems.

Keywords: cooling water, scale, indices, ozone

INTRODUCTION

Water chemistry studies reported for ozonated cooling systems have historically evaluated scale potential in terms of the simple calcium carbonate scale potential indices: the Langelier Saturation Index, the Ryznar Stability Index, and the Practical Scaling Index. These indices can provide reliable predictions of calcium carbonates scale potential in low dissolved solids, low sulfate water. But the indices do not account for "common ion effects" such as the apparent increase in calcium carbonate solubility in high sulfate waters. Their usefulness is limited to ionic strengths of systems operating at a total dissolved solids well below that of the super cycled zero blow down systems. And perhaps as importantly, these indices are limited to calcium carbonate scale potential. They predict nothing of other scale forming species such as silica, magnesium silicate as a co-precipitate of magnesium hydroxide and silica, or calcium sulfate.

This study evaluated the water chemistry of ozonated cooling systems using an ion association model to predict the scale potential for calcium carbonate, magnesium hydroxide, amorphous silica, and calcium sulfate.

The ion association model used is applicable to the ionic strength range of the highest concentration ratio ozonated systems. The model accounts for the common ion effects through the use of ion pairing and the calculation of indices based upon free, uncombined ions concentrations rather than analytical values. Alkalinity is corrected for non-carbonate contributions by the model so that calcium carbonate scale potential indices calculated are based upon the most accurate estimate of the carbonate species distribution. The results are reliable even in the presence of free hydroxide alkalinity and high silica levels.¹

Scale potentials are reported as saturation level (degree of supersaturation). This index describes the ratio of the observed water chemistry to the water chemistry at equilibrium, for the reactants involved.

For example for calcium carbonate:

• • _________________(Ca)(CO₃)
    Saturation Level = ^{_______________}
    _________________K_{sp CaCO3}
    
    where

• (Ca) is the free ion activity of calcium in the water.
• (CO₃) is the free ion activity of carbonate in the water.
• Ksp is the solubility product for calcium carbonate such that
  (Ca)(CO₃) = Ksp at equilibrium.

Saturation ratios can also be calculated for other scale forming species.

• • __________________(Ca)(SO₄)
    Saturation Level = ^{________________ ______________________}for calcium sulfate
    ________________ _K_{sp CaSO4}

    _________________(Mg)(OH)²
    Saturation Level = ^{______________ _______________}for magnesium hydroxide
    ________________K_{sp Mg(OH)2}

    ______________________H₄SiO₄
    Saturation Level = ^{_____________________} __________for amorphous silica
    _________________(H₂O)² . K_{sp SiO2}

These indices can be loosely interpreted as follows:

• • If the saturation level is less than 1.0, a water is undersaturated with this scale forming specie and will tend to dissolve the scale if present in a solid form.
    
  • If the saturation level is 1.0, the water will not tend to form or dissolve the scale. The water is at equilibrium with respect to the scale forming specie.
    
  • As the saturation level rises above 1.0, the driving force for this scale to form increases.

Saturation level calculations can be applied to estimating the driving force for scale formation for common scale forming species.

Saturation levels were calculated in this manner for:

• • Calcium carbonate as the calcite polymorph.
  • Silica as amorphous silica.
  • Magnesium hydroxide as brucite.
  • Calcium sulfate as gypsum.

METHOD

Each system was evaluated by:

1. Comparing actual makeup and recirculating water chemistry, and operating parameters to estimate the actual concentration ratio of the systems.
2. The theoretical concentration of the recirculating water chemistry was calculated based upon the makeup water and apparent, calculated concentration ratio.
3. The Theoretical and Actual ion concentrations were compared to determine precipitation of major species.
4. The driving force for precipitation for the major scale forming species were calculated for the Actual and Theoretical recirculating cooling water chemistry.
5. Differences in the Theoretical to Actual chemistry were compared to the cleanliness of the cooling systems with respect to heat transfer surface scale, scale formation in valves and non heat transfer surfaces, and precipitate buildup in the tower fill and basin.

SYSTEM OPERATING PARAMETERS

The cooling systems included in this evaluation were comfort heating and cooling systems, where the cooling water serviced freon exchangers, or light industrial systems. The heat exchangers serviced by the cooling systems can be characterized as low heat load, high flow velocity with tube side cooling water.

OZONE APPLICATION

All of the cooling water systems described in this paper were treated solely with ozone. No other chemical treatment compounds, including sulfuric acid, were used during the course of this work. Ozone was added to these systems at dosages ranging from 0.05 to 0.2 ppm in the circulating pump suction pit. The intent, in each case, was to apply ozone continuously to each system. Because of the various mechanical problems and changes in operation required by the tower owners, the ozone application varied substantially in most cases. Unfortunately, accurate records of ozone application over time for each system is not available for inclusion in this paper.

WATER CHEMISTRY RANGE

Three categories of systems were encountered in the evaluation:

• CATEGORY 1: Those where the theoretical recirculating water chemistry is not scale forming when cycled;

  CATEGORY 2: Those where the recirculating water would have a moderate to high calcium carbonate scale forming tendency when cycled. Recirculating chemistry for these systems is similar to that encountered in conventional alkaline cooling water programs;

  CATEGORY 3: Those with an extraordinarily high scale potential for calcium carbonate and magnesium hydroxide when cycled. These systems operated with a recirculating water chemistry more like that of a softener than of a conventional cooling system.

RESULTS VERSUS WATER CHEMISTRY

Table 1 outlines the theoretical versus actual water chemistry for the thirteen (13) systems evaluated. Table 2 presents saturation levels for the theoretical and actual recirculating water chemistries.

• Category 1 - Recirculating Water Not Scale Forming:
  In the case of category 1 systems, no scale formation was observed, as expected.

  Category 2 - Conventional Alkaline Control Range:
  Scale formation was observed when category 2 water chemistry was present in eight of nine systems evaluated. In these cases, the driving force for scale formation was in the range where a traditional scale control treatment program could prevent scale deposition on heat transfer surfaces. It was not above the level where inhibitors such as phosphonates lose control.

  In one case, scale potential was low (calcite saturation level less than 40), and holding time index relatively short (12 hours half life). No scale was observed under these conditions.

  Category 3 - The Cooling Tower as a Softener:
  Deposit formation on heat transfer surfaces was not observed in most of these systems. In one case, deposits were observed in a valve.

  The driving force for scale formation was so high in these systems that gross precipitation of calcium carbonate, magnesium hydroxide, and in some cases, silica would be expected. Yet no deposits were found on heat transfer surfaces.

RATIONALE FOR RESULTS

Category 2 systems fall into the general operating range for conventional alkaline cooling water systems. Typical calcite saturation ratios for such systems fall into the range of 20 to 150 ([Ca][CO₃]/Ksp). In the absence of chemical treatment, scale formation in these systems normally occurs by growth on existing active sites. Because of the inverse solubility of calcite with temperature, most accumulation is on heat transfer surfaces.

Even though clearly visible scale formation occurred in the Category 2 systems listed in Tables 1 and 2, the saturation ratios are low. That is, since the circulating water is only slightly supersaturated with calcite, only a small amount of the total reactive material in the solution precipitates.

By comparison, the saturation ratios in Category 3 systems listed in Tables 1 and 2 range well above 1,000. These levels of supersaturation are typical of calcium carbonate softening processes. Under these conditions, calcite tends to precipitate as finely divided particles in the bulk solution, rather than by growth on specific sites as described above.

Once bulk precipitation begins, calcite formation on metal surfaces is greatly reduced because of the greater surface area of the suspended calcite crystals. The flow velocity of water through the condenser tubes tends to keep the crystals in suspension, so that calcite does not accumulate in the condenser. In these cases, calcite accumulates in the basin, and sometimes on the tower deck and fill. This explains why Category 3 systems, with much higher saturation ratios than Category 2 systems, show less tendency to form calcium carbonate deposits on heat transfer surfaces.

THE ROLE OF OZONE IN CALCIUM CARBONATE SCALE FORMATION

The data in this paper do not show that ozone played any significant role in calcite precipitation or scale formation on heat transfer surfaces. Although there were variations in the dosage, frequency and continuity of ozone feed to these cooling systems, the overall feed rates were in the 0.05 to 0.2 ppm range, as explained above. There does not seem to be any real correlation between ozone dosage and feeding method with calcium carbonate scale formation in the system. Conventional water treatment technology recognizes two different approaches to controlling calcium carbonate scale formation. These are sometimes described as "stabilization chemistry" and "precipitation chemistry." Stabilization chemistry refers to the use of specific chemicals to "stabilize" supersaturated solutions of calcium carbonate, with saturation ratios typical of the Category 2 systems in this paper. The chemicals stabilize the calcium carbonate so that it will not precipitate during the residence time in the system. Precipitation chemistry, on the other hand, refers to the use of dispersants and biocides to maintain system cleanliness and keep precipitating calcium carbonate in suspension and prevent deposition on heat transfer surfaces. The performance of ozone as a biocide in cooling water systems is well known. It may be that the role of ozone in preventing scale formation is to keep both the internal system surfaces and the bulk water clean and free of biofouling that can attract and "glue" small crystals to surfaces.

CONCLUSIONS

Analysis of the water chemistry data from a number of recirculating cooling systems treated solely with ozone show that calcium carbonate (calcite) scale forms most readily on heat transfer surfaces in systems operating in a calcite saturation ratio in the range of about 20 to 150. This range is typical of conventional alkaline cooling water operations. At much higher saturation ratios, above 1,000, calcite precipitates in the bulk water. Because of the very high surface area of the precipitating crystals compared to the metal surface in the system, continuing precipitation leads to crystal growth in the bulk water rather than scale formation on heat transfer surfaces. The presence of ozone in these systems does not appear to have influenced calcite precipitation and/or scale formation. Although not conclusively proven, it seems that the main role of ozone may be to keep the system clean and free of biofouling that can encourage scale formation.

REFERENCES

1 R.J. Ferguson, Computerized Ion Association Model Profiles Complete Range of Cooling System Parameters, International Water Conference, 52nd Annual Meeting, Pittsburgh, Pennsylvania, IWC-91-47

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