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Evaluation methods for the scaling power of water
For decades, various attempts have been made to estimate the scaling power of natural waters and characterise the scaling formation mechanisms. We describe herein major different methods developed to date by emphasising whether they are representative with respect to a real-life scaling formation process.
These methods can be roughly divided into two categories: electrochemical methods and non-electrochemical methods. The first one is made up of chronoamperometry, chronoelectro -gravimetry and electrochemical impedance technique which are all based on the reduction of the oxygen dissolved in the test water by polarizing a metallic electrode at a sufficiently negative potential. Among the non-electrochemical methods, we can enumerate the critical pH method, two thermal methods, an evaporation method, the LCGE method, the rapid controlled precipitation method, a polymer scaling test and a continuous test on tubes.
(1) Electrochemical methods The first electrochemical method, chronoamperometry [7], has been proposed by Lédion. It is based on the electrochemical reduction of the oxygen dissolved in the test water by polarising a metallic electrode at a potential sufficiently negative according to: O2 + H2O + 4e Û 4OHThe generation of hydroxide ions in the vicinity of the electrode can increase the local pH for several pH units and forces calcium carbonate to precipitate in a solid crystalline phase through two steps: HCO3 + OHÛ CO3 2 + H2O
The limiting current intensity IL, which is proportional to the flow of oxygen moving by convective diffusion towards the electrode, decreases whereas the active surface is progressively blocked by the growth of scale. Finally, IL reaches a value close to zero when the surface is totally covered by the CaCO3 insulating layer.
The CaCO3 masse and the electrochemical impedance can also be measured with respect to time during scaling while the electrode was polarised at the limiting current for oxygen reduction. They are called the chronoelectrogravimetry and the electrochemical impedance technique respectively. A typical chronoamperometric curve is shown in Fig.1.2. It is characterised by a falling current shape with time whose slope is related to the scaling rate. Lédion defined the scaling time, ts, as the intersection of the tangent at the inflexion point of this curve and the time axis. It gives a rough estimate of the scaling potentiality of waters. The residual current ires is somewhat related to the deposit morphology: the more compact and insulating the scale, the lower the residual current.
Evaluation of scale inhibition performance by continuous tests
At the temperature of 40 °C, for 30 % Salvetat solution (Ca2+: 75.9 mg/L, HCO3 -: 246 mg/L), as shown in Fig.3.46, the ascending part of the curve corresponds to the first seven days (56 h) of tests with untreated water, the weight of scaling on tubes increases with time stably. From the 8th day, 0.2 mg/L (PASP+PESA) (1:1) was introduced into the solution, there was no weight change on tubes and the value has a bearing on the curve, that is, the scaling was totally inhibited. After the 13th day, no inhibitor was added to the solution, it was found that the increase of weight on tubes was very low for the first 2 days, which indicated that the remanent effect was remarkable. This is due to the fact that some of (PASP+PESA) (1:1) were adsorbed on the wall of the tank and pipes that connected the tubes. The normal increase of weight on tubes was recovered from the 3rd day after withdrawal of inhibitor. When the temperature rose to 50 °C, as seen in Fig.3.47, the total inhibition of scaling was always attained during the treatment phase with 0.2 mg/L (PASP+PESA) (1:1), however, the remanent effect didn’t exist any more, which indicated that the 6 7 amount of adsorbed inhibitor was insufficient. The results showed that the concentration of (PASP+PESA) (1:1) necessary for complete inhibition is even greater as the temperature rises. This is confirmed by the results shown in Fig.3.48. At the temperature of 60 °C, the weight of scaling on tubes increased slightly for the treated water adding 0.2 mg/L (PASP+PESA) (1:1), which meant that inhibition of scaling was not complete. In other words, for a total inhibition at this temperature, it would either increase the dosage of inhibitor or decrease water flow.
Comparison Between Inhibition Effectiveness of Zinc Ion and that of Copper Ion
By comparing the inhibition effectiveness of zinc ion and copper ion in the 300 mL Salvetat solution where the calcium ion concentration was 126.5 mg/L (Figs. 4.9 and 4.10), it could be found that zinc ion and copper ion showed different inhibitory strength on the scaling of CaCO3. For the zinc ions of different concentrations, their difference in inhibitory strength was relatively obvious, and the total efficiency of 100% had already reached for the zinc ion concentration of 0.4 mg/L; however, 0.9 mg/L copper ion was needed to totally inhibit the precipitation in 70 min (Table 4.3), and with the increase in 7 7 concentrations of copper ions, their inhibitory capacity displayed a slow growth rate, which indicated that the anti-scaling performance of zinc ions was superior to that of copper ions. As seen
Table of contents :
Chapter 1. Introduction
1.1 Scaling
1.2 Hazard of scale
1.3 Formation of CaCO3 scale
1.4 Inhibition of CaCO3 formation
1.5 Evaluation methods for the scaling power of water
1.6 Methods of scale inhibition
1.7 Scale inhibitor
1.8 Green scale inhibitor
1.9 Mechanism of scale inhibition
Chapter 2. Evaluation of anionic polymers as scale inhibitors on CaCO3 and Ca-phosphonate precipitates
2.1 Experimental
2.1.1 Main materials
2.1.2 Static tests for Ca-Phosphonate precipitation of various phosphonate inhibitors
2.1.3 Static tests for effects of polymeric inhibitors on Ca-Phosphonate formation
2.1.4 Static tests for inhibition efficiency of inhibitors on CaCO3 scale formation
2.2 Results and Discussion
2.2.1 Static tests for Ca-phosphonates precipitation
2.2.2 Inhibition of Ca-Phosphonate precipitation
2.2.3 Inhibition of CaCO3 scale formation by phosphonates and observed “threshold effect”
2.2.4 Inhibition of CaCO3 scale formation by terpolymer
2.3 Summary
Chapter 3. Performance of polyaspartic acid/ polyepoxysuccinic acid and their synergistic effect on inhibition of scaling
3.1 Experimental
3.1.1 Materials
3.1.1.1 PASP
3.1.1.2 PESA
3.1.1.3 Water studied
3.1.2 Methods
3.1.2.1 Static tests for scale inhibition efficiency
3.1.2.2 Rapid controlled precipitation (RCP) tests
3.1.2.3 Continuous tests
3.2 Results and discussion
3.2.1 Comparison of anti-scaling performance of PASP and PESA
3.2.1.1 Static tests of scale inhibition
3.2.1.1.1 Inhibition of CaCO3 scale
3.2.1.1.2 Inhibition of CaSO4·2H2O scale
3.2.1.1.3 Inhibition of BaSO4 scale
3.2.1.1.4 Inhibition of SrSO4 scale
3.2.1.2 RCP tests
3.2.1.2.1 Inhibition effect of PASP
3.2.1.2.2 Inhibition effect of PESA
3.2.1.2.3 Comparison of anti-scaling performance
3.2.1.3 Anti-scaling mechanism
3.2.1.3.1 SEM analysis
3.2.1.3.2 XRD analysis
3.2.2 Synergistic effect of PASP and PESA on inhibition of scaling
3.2.2.1 Evaluation of scale inhibition performance by static tests
3.2.2.1.1 Research of synergistic effects on inhibition of CaCO3
3.2.2.1.2 Research of synergistic effects on inhibition of CaSO4·2H2O
3.2.2.1.3 Research of synergistic effects on inhibition of BaSO4
3.2.2.2 Evaluation of scale inhibition performance by RCP tests
3.2.2.2.1 Effect of dosage of PASP combined with PESA on inhibition of CaCO3
3.2.2.2.2 Effect of Ca2+ concentration on inhibition of CaCO3
3.2.2.2.3 Effect of temperature on inhibition of CaCO3
3.2.2.3 Evaluation of scale inhibition performance by continuous tests
3.3 Summary
Chapter 4. Effects of Copper and Zinc Ion in Preventing Scaling of Drinking Water
4.1 Experimental
4.1.1 Materials
4.1.1.1 Reagents
4.1.1.2 Water studied
4.1.2 Method
4.2 Results and discussion
4.2.1 Anti-scaling Tests of Zinc Ions of Different Concentrations
4.2.2 Anti-scaling Tests of Copper Ions of Different Concentrations
4.2.3 Comparison Between Inhibition Effectiveness of Zinc Ion and that of Copper Ion
4.2.4 Inhibition Mechanism of Scaling by Copper and Zinc Ions
4.2.5 Effects of temperature and dissolved CO2 on the scaling of water in the presence of copper and zinc
4.2.5.1 Tests on scaling capacity of Salvetat water
4.2.5.1.1 The influence of temperature
4.2.5.1.2 The influence of dissolved CO2
4.2.5.2 Inhibition of scaling in the presence of copper
4.2.5.2.1 The influence of temperature
4.2.5.2.2 The influence of dissolved CO2
4.2.5.3 Inhibition of scaling in the presence of zinc
4.2.5.3.1 The influence of temperature
4.2.5.3.2 The influence of dissolved CO2
4.3 Summary
Chapter 5. Evaluation of a new “green” solid scale inhibitor with real “threshold effect”
5.1 Experimental
5.1.1 Materials
5.1.1.1 Test water
5.1.1.2 Synthesis of Solid inhibitor
5.1.2 Methods
5.1.2.1 RCP
5.1.2.2 Measurement of solubility
5.2 Results and discussion
5.3 Summary
Conclusions & Perspective
Conclusions
Perspective
References
