Elsevier

Powder Technology

Volume 381, March 2021, Pages 313-323
Powder Technology

Experimental and numerical investigations on heterogeneous condensation of insoluble particles in the fluid catalytic cracking unit

https://doi.org/10.1016/j.powtec.2020.11.031Get rights and content

Highlights

  • The process of heterogeneous condensation of FCC particles was investigated by experiment and simulation.

  • The interactions of gas, liquid film and solid particles were simulated by Euler-Euler-Lagrange method.

  • Based on nucleation and growth theory, the particle size and temperature were calculated.

Abstract

In the fluid catalytic cracking (FCC) unit, cyclones possess very less efficiency in the separation of particles smaller than 5 μm. Heterogeneous condensation, one of the most promising preconditioning technologies, can be applied in the FCC unit to improve the particle removal efficiency of cyclones. In this paper, the lab-scale equipment on heterogenous condensation was established and tested. Based on experimental investigation, a novel numerical method was presented to simulate the process of movement and growth of particles in the growth tube. The effects of critical operational parameters, such as the wall film temperature, gas inlet temperature, gas flow rate, and particle number concentration, were studied. The results show that when the gas flow rate is 2 lit·min−1, the average particle size at the outlet increased to 6.67 μm, and the number of particles smaller than 4 μm decreased to about 5%. Overall, this study guides for practical engineering applications.

Introduction

Heterogeneous condensation is an essential phenomenon in many processes, such as cloud physics, particle counter, particle deposition in the human lung, and particle abatement [1]. In order to decrease the emission of fine particulate matters from industry, the most promising pretreatment of particle abatement by heterogeneous condensation has drawn the attention of researchers worldwide. Therefore, it is necessary to study the heterogeneous condensation process more accurately [[2], [3], [4]].

The feasibility of heterogeneous condensation at relatively low supersaturation makes it interesting for lots of industries, including chemical, cement, and metallurgy production systems and combustion facilities, all of which are interested in the removal of fine particles from waste gases [5]. Many experimental investigations have already been reported on the heterogeneous condensation. De Joannon et al. studied the heterogeneous nucleation rate and particle activation probability by experiments. And it found that the theoretical prediction of the activation probability was not valid in predicting the occurrence of the vapor condensation on particles at lower vapor concentration [6]. Mavliev et al. realized the heterogeneous nucleation activation of NaCl, KCl, AgCl, and Ag particles at a low concentration of vapor [7]. Hering et al., at first, presented the method for the enlargement of particles through the growth tube where the saturation profiles were calculated using a two-dimensional model of convective and diffusive heat and mass transfer [8]. Based on this method, many experiments have been conducted to study the process of enlargement. The influence of operating parameters and the addition of wetting agents on the condensation of vapor for coal-fired fine particles was tested in a one-section growth tube [9]. At higher particle concentration (>106 cm−3), the performance of condensation enlargement for SiO2 particles was also investigated [10]. In addition, for effectively enhancing particle enlargement, the studies of particle growth by heterogeneous condensation in a two-section growth tube and partial gas circulation growth tube were presented [11,12]. Besides, the effects of the strength of the magnetic field on the particle growth in the one-section growth tube have been explored. The results revealed that the strength of the magnetic field ameliorates the wettability of particle surface and facilitates the process of particle growth [13].

In 1926, Volmer et al. first proposed the spontaneous nucleation theory [14]. Based on the hypothesis of Volmer et al., Farkas, Becker and Doring, Frenkel and Zeldovich determined the classical homogeneous nucleation theory which well explains the sudden appearance of spontaneous condensation [15]. Then to get more accurate results, many researchers have developed and revised the homogeneous nucleation theory [[16], [17], [18], [19], [20], [21]]. However, vapor nucleation on particle depends on both environmental parameters and particle characteristics. Temperature, pressure, vapor concentration, fluid-dynamic field, as well as particle composition, dimension, and morphology, all can affect heterogeneous condensation process, thus there are few theoretical studies on heterogeneous condensation of particles. Fletcher at first proposed the classical heterogeneous nucleation theory of insoluble particles in 1958 [22]. Following this, some researchers developed the classical heterogeneous nucleation theory [[23], [24], [25], [26], [27]]. After heterogeneous nucleation, the particles grow rapidly owing to the condensation of vapor on the particle surfaces [28]. Fuchs et al. systematically analyzed the growth of droplets by considering the simultaneous heat and mass transfer based on Maxwellian expressions in 1960 [29]. Gyarmathy developed a new mathematical model that can predict the growth rate, the surface temperature, and the rate changes of the droplets in 1982 [30]. Subsequently, Young derived the formula of droplet growth rate for condensation flow with any Knudsen number of steams containing inert gas in 1993 [31]. And an improved analytical expression for the growth of a single droplet in the transition and continuum regime was proposed by Kulmala in 1993 [32,33]. Park et al. used the analytical expression proposed by Kulmala to solve the condensation problem of polydisperse aerosols [34].

In this study, the condensation growth experiments were carried out using fine particles from cyclone separator in the FCC unit. Also, a novel gas-liquid-solid three phases coupling numerical method based on the classical heterogeneous nucleation theory and Kulmala growth theory was presented to simulate the process in the growth tube, providing theoretical guidance for industrial applications.

Section snippets

Numerical model

In this study, the calculation process solved by the method of finite element analysis are displayed in Fig. 1. The gas flow and evaporation of liquid film are predicted by the Eulerian approach, and the particle movement and growth are simulated by the Lagrangian approach. The numerical geometry model was constructed based on the experimental equipment and was discretized using structured mesh. When the cell number is greater than 301,210, the calculation results tend to be stable. Therefore,

Experimental apparatus

The experimental system is shown in Fig. 3. The gas and particles were cooled to approach their dew point temperature in the cooling unit. The gas temperature and relative humidity at the inlet of the growth tube were measured. Air and particles were fed into the growth tube with a 6 mm injection tube, which weakened the deposition of particles towards the tube walls due to diffusion. The major part of the growth tube has an internal diameter of 15 mm and a length of 377 mm, as shown in Fig. 4.

Effects of the temperature of liquid film

Fig. 7a, b provides the distribution of supersaturation at different temperatures of the wall film. It can be found that supersaturation at first increases and reaches a stable state in the axial direction, and the distribution of supersaturation in the radial direction changes from “m-shape” to “n-shape” with an increase in the axial height. When the air is near the inlet, the vapor is difficult to diffuse from walls to the center of tube due to the short residence time. Therefore, the H2O

Conclusions

In the FCC unit, the catalyst particles are separated by the cyclone, which mainly is efficient for particles with sizes above 5 μm. For fine particles, generally, the removal efficiency of a cyclone based on inertial impact is below 50%. The heterogeneous condensation on the FCC particles can be used as a technique of preconditioning to increase the particle size for enhancing the particulate abatement.

For this purpose, experiments were conducted to investigate the characteristics of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 51676146).

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    These authors contributed equally to this work and should be considered co-first authors.

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