Molecular Dynamics Study On Nucleation Behavior Of Calcium Carbonate For Liquid And Metal Wall

Fouling will not only increase the thermal resistance and reduce the heat exchanger efficiency,but also cause the deterioration of heat transfer and damage heat exchange devices.Excessive fouling deposited inside the flow channel can also increase the flow resistance and cause additional losses.In order to solve the fouling problems in the industry and improve the economic benefits of heat exchangers,numerous studies on anti-fouling methods have been performed in recent decades.Meanwhile,calcium carbonate（CaCO₃）,a common fouling existed in the industry,has also become a main research object.Although a variety of anti-fouling methods have been explored,there is still a lack of understanding of the specific formation process of CaCO₃ fouling,which also makes it difficult to improve a targeted anti-fouling method.Therefore,this article uses the molecular dynamics method to simulate the nucleation of supersaturated CaCO₃ solution and the adsorption of CaCO₃ to the metal surface,so as to observe the evolution of CaCO₃ fouling in the early process of formation.In the simulation of homogeneous nucleation,it can be found that the nucleation of CaCO₃can be divided into three stages:pre-nucleation clusters（PNCs）spontaneous growth,solubility equilibrium of stable PNCs and aggregation of PNCs inducing nucleation.The PNC belongs to a chain-like structure which Ca-C coordination number approximately equal to 2.When different PNCs meet each other and aggregate during thermal motion in the solution,the structure of the aggregate changes,the Ca-C coordination number increases and the hydration number decreases.The variation in CaCO₃ clusters caused by aggregation may induce the generation of nuclei.Meanwhile,this article also discusses the variation of coordination number in clusters versus concentration and temperature during nucleation.As the initial concentration of supersaturated CaCO₃ solution increases from 0.48 mol/L to 0.96 mol/L,the Ca-C coordination number of the aggregate decreases from 2.62 to 2.53 and the hydration number increases from 4.86 to 5.01.When the temperature increases from 300 K to 360 K,the Ca-C coordination number of the aggregate increases from 2.18 to 2.62 and the hydration number decreases from 5.38 to 4.92.It can be found that the Ca-C coordination number of the aggregate increases with the decrease of the concentration and the increase of the temperature,which is similar to the variation of anhydrous CaCO₃ crystals.The aragonite with higher coordination number is more likely to form at low concentration and high temperature,while calcite or vaterite is usually found at high concentration and low temperature.Therefore,the species of CaCO₃ crystals may be related to the nucleation process.The formation of CaCO₃ fouling includes not only the reaction in the solution,but also the adsorption of CaCO₃ to the wall.Therefore,on the basis of the homogeneous supersaturated solution model,this article also established a model containing copper flat to simulate the adsorption of CaCO₃ to understand the early process of fouling formation.It can be found that the adsorption process of CaCO₃ to the surface includes both the diffusion of smaller ions,ion pairs or chain-like CaCO₃ molecules,as well as the adsorption of clusters.In order to be able to observe the variation in the structure of CaCO₃ during dehydration,in this simulation,higher temperatures were set for the metal flat to remove the water inside CaCO₃.According to the radial distribution functions and hydration number,it can be deduced that the dehydration product generated in the simulation is still amorphous CaCO₃.In addition,it can be found that the Ca-O coordination number in the CaCO₃ is approximately 6 at a high temperature of 700 K,which is equivalent to vaterite and calcite,indicates that the CaCO₃ has a tendency to transform into ordered crystals during dehydration.