Experimental Study Of The External Conditions On Crystallization Of Calcium Carbonate Fouling

Complex energy, mass and momentum transfer occurs in the formation of the fouling of heat exchangers, accompanied with a variety of physical and chemical processes. It is well known that fouling has caused great economic loss in the design and operation of heat exchange equipments. To date, it is one of the main unsolved problems in the heat transfer field. According to the statistical data, calcium carbonate crystallization fouling dominates the types of fouling collected in almost all the regions of our country. So from a point of view of Chinese conditions, this research focuses on calcium carbonate fouling. Considering the complexity and the uncertainty, this work mainly focuses on the effect of different external conditions on the formation, types and properties of fouling by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and other means. This thesis has discussed the logic and the experimental procedures in detail. In summary, the main points have been shown as follow:(1) In literatures, researchers always use artificial hard water to accelerate the formation of fouling in the study of fouling formation under lab conditions. But little has been known whether the fouling obtained with this method is the same with the actual cooling water, that is, whether they are interchangeable in the controlled laboratory conditions. To clarify this question, this research studied the effect of different reaction on the calcium carbonate crystallization fouling type and microstructure, comparing several kinds of reaction methods. Experimental results indicate that calcite dominates the phases of the precipitated fouling both with the current various chemical reaction method and with the artificial hard water method. That is, if it is only used to study the formation and microstructure of the calcite fouling, the two methods can reach the same goal. But comparing with calcium carbonate crystallization fouling prepared by tap water boiling precipitation, there is somewhat different. A mass of petal-shaped aragonite has been obtained in boiling tap water, because tap water contains magnesium and other metal ions. It has been found that the grain size of crystallization fouling is increased with the increase of ions concentration in solution. The change of the lattice constant of the precipitation fouling will be little when the experimental variables such as room temperature or heated to boiling and other reaction conditions alter. But the existence of magnesium ion in tap water will reduce the lattice constant of the precipitation fouling substantially.(2) To date, the study has mainly focused on the fouling formed by boiling at high temperature, but the reports focusing on low temperature precipitated fouling are limited. In this study, fouling precipitated in the low-temperature heat transfer process has been investigated by using low-temperature boiling device. The effect of different temperature on the calcium carbonate crystallization fouling type and microstructure is studied. Results show that the temperature affects not only the type of precipitated calcium carbonate fouling, but also the lattice constant of precipitates and precipitated particle size. The lattice constant and the grain size tend to decrease as the temperature increases, which will also affect the strength of the fouling scale. Fouling generated at low temperature is mainly the phase of calcite, and the aragonite increased gradually as the temperature increases. Meanwhile, low-boiling perturbation can reduce the aggregate size of calcium carbonate fouling particles availably.(3) The effect of different metal ions on the fouling type and microstructure of calcium carbonate crystallization has been studies by adding different amounts of metal ions. Experimental results show that sodium ions have no significant effect on the type of calcium carbonate crystallization fouling. Only the calcite crystalline phase of calcium carbonate fouling has been obtained, but the calcite lattice constant varies with the increase of sodium ions concentration. However, magnesium ions and aluminum ions can promote the formation of aragonite phase significantly. The further calculation by using first principles indicate that the doped magnesium and aluminum ions in calcite cell can improve the overall energy of the structure cell, therefore the stability of cell is decreased, while the doped sodium ion has little effect on the cell. As the concentration of magnesium and aluminum ions increases, it tends to increase the final energy of calcium carbonate cell, resulting in lattice instability, which can further promote the transformation of calcite to aragonite.