# Oil Sludge Deposition in Storage Tanks: A Case Study for Russian Crude Oil in Mo-he Station
###### Abstract
In that case the wax forms a uniform ring on the pipe wall [**?**]. However, for the settlement in the tank, the fluid in the tank is basically static under the storage conditions. [PERSON] [**? **] et al. proposed that the wax crystal settlement is mainly caused by Brownian motion and gravity. The wax crystals collide with each other, gather and adhere to the fast settled particles, forming large particles and settling under the action of gravity. Gradually, the concentration of wax or particles in the upper layer is lower than that in the lower layer, resulting in a possible back flux of the small particles. Therefore, after a certain degree of sedimentation, the settlement reaches an equilibrium state, and the sedimentary layer is no longer further thickened, which is called the settlement equilibrium state (see Figure **?**). He also pointed out that the Peclet number (Pe) could be used as a characteristic number to describe the gravity sedimentation and Brownian motion intensity in the simulation. The precipitation process of wax crystal was monitored by a self-  
Figure 1: Schematic diagram of particle settlement. and analyzed by Pe. Their results show that the gravity and diffusion force almost cancel each other in the absence of wax crystal aggregation. Therefore, [PERSON] et al. believes that the aggregation of particles is the start of the sedimentation process. The deposition process of asphaltene is mainly described by two thermodynamic models, the solution model, or the colloidal model. The solution model mainly considers that crude oil and asphaltene are a kind of homogeneous binary mixture, in which asphaltene is a solid solute and crude oil is a solvent. In this model, the amount of asphaltene dissolved in crude oil is determined by the solubility of asphaltene in crude oil, and whether asphaltene precipitates in crude oil is determined by whether the content of asphaltene dissolved in crude oil reaches the saturation state [?? ]. In the 1980s, [PERSON] and [PERSON] [?? ] proposed a colloidal model. The model considers that asphaltene in crude oil is the core part, and the colloid adsorbed on asphaltene constitutes the dispersed phase, while other components constitute the continuous phase. In this model, it is a dynamically stable colloidal system under normal conditions. At this time, the colloid is non-polar, but the dynamic equilibrium of the colloidal system may be broken due to the factors such as temperature, pressure, crude oil composition, precipitant, etc., and the colloidal molecules outside the asphaltene are destroyed, which makes the colloidal particles with polarity. These particles attract each other and collide with each other, and then they settle under the action of gravity. For the interaction between the two kinds of particles, there is no unified conclusion. Chang [?? ] and other coupled Langevin type equations were used to determine the collision trajectories of particles. Considering the net gravity, van der Waals force, electric repulsion force and Brownian diffusion force between two particles, the effects of particle size and reduced density ratio on particles were studied, and the adsorption efficiency formula between particles was proposed. For the interaction between wax and asphaltene, [PERSON] [?? ] believed that when the asphalt concentration was 0.01%, it would flocculate on the wax crystal structure, while when the asphalt concentration was greater than or equal to 0.05%, it would form a flocculent, which provided conditions for wax crystallization. [PERSON] [?? ] et al. suggested that asphaltene would lead to the precipitation of amorphous spherical wax particles. Some scholars believe that the interaction between wax and asphaltene is relatively small, but asphaltene may be surrounded by wax crystals. [PERSON] also suggests that asphaltene is the main reason for the formation of the sedimentary layer, because when the density is lower than the crude oil, the wax will precipitate to the bottom since the resin and asphaltene settle together with the wax crystal.