Elsevier

Powder Technology

Volume 297, September 2016, Pages 89-105
Powder Technology

Simulation of effect of internals on particulate mixing and heat transfer in downer reactor using discrete element method

https://doi.org/10.1016/j.powtec.2016.04.018Get rights and content

Highlights

  • Particulate heat transfer model used in pyrolysis downer reactor

  • Two analysis method for mixing degree of binary granular materials

  • Effect of internals on particulate mixing and heat transfer in downer reactor

  • Pyrolysis terminal temperature and increasing rate of particle temperature

  • Contribution of three forms of heat transfer on particle temperature increasing

Abstract

To investigate the effect of internals in downer reactor on the particulate mixing and heat transfer, a model of heat transfer based on discrete element method (DEM) has been developed and validated by experiments. The agreement between the simulated results and the experiment phenomena is able to prove the reasonability and correctness of the model mechanism of the particulate heat transfer using DEM. The effect of internals in downer reactor on the temperature increasing rate of particles has been predicted by the particulate heat transfer model based on DEM, which is important for the fast pyrolysis process. Two types of internals, the tube group internals and the baffle internals, have been designed to improve the mixing and heat transfer between fuel particles and solid heat carriers in downer reactor. The internals not only increase the particle residence time in downer reactor, but also enhance the mixing degree of binary particulate materials. Most importantly, the internals affect the temperature increasing rate of fuel particles controlled by the mixing degree of fuel particles and heat carriers. The terminal temperature fuel particle at the exit of downer reactor is determined by the average temperature increasing rate and the mean residence time of the fuel particle. As shown in the simulated results, the mixing degree of binary particulate materials in the baffle type downer reactor is remarkably better than that in the tube type downer reactor. At the same time, the temperature increasing rate of fuel particles in the baffle type downer reactor is also larger than that in the tube type downer reactor. It can be concluded that the baffle internals not only restrict the increase of particle residence time, but also mix the fuel particles and solid heat carriers sufficiently, which make the fuel particles heated rapidly. A shorter resistance time, a perfect mixing degree of two types of granular materials and a higher temperature increasing rate can confirm that the internal of baffle group is an excellent choice for the fast pyrolysis process in downer reactor.

Introduction

Downer reactor already has been used in the process of hydrocarbon plasma pyrolysis for acetylene products since 1939. By the early 1960s, downer reactor has been used to implement the process of pulverized coal plasma pyrolysis to produce acetylene. Co-current downer reactor has been mentioned in the generalized fluidization theory in the year of 1966, and has not attracted the attention from academia and industry until the late 1970s [1], [2]. The characteristics of downer reactor can be summarized as follows [2], [3]: 1) a higher solid flow rate; 2) a uniform distribution of solid concentration along the radial direction and a negligible back mixing along the axial direction; 3) no restriction in mixing ratio of solid phase and gas phase especially for high load of solid particles; 4) a lower energy consumption in pneumatic conveying; and 5) a shorter mean residence time (MRT) and a narrow residence time distribution (RTD) for solid particles. Thus, the downer reactor is known as a new and efficient chemical reactor in the 21st century, and has a very broad application prospect in process engineering, such as coal or biomass fast pyrolysis in downer reactor [4], [5], for its advantages mentioned above.

The downer reactor coupling with the fluidized bed reactor [6], [7], [8], [9] is applied in the coal pyrolysis process, which can be illustrated in Fig. 1. The pulverized coal powders charged from a port at the top of the downer reactor are mixed with the hot ash or silica sands (heat carriers) from the fluidized bed reactor, and heated rapidly to release the gas volatile matter. The pyrolysis products including the gaseous components (volatile matter) and solid phase (char particles) will be separated by a gas–solid separator quickly. The gas components will be cleaned and quickly cooled down by circulating cooling water system to obtain the liquid products and the coal gas with medium calorific value. The pyrolysis solid products and the cool ash or the silica sand will be sent back to the bottom of the fluidized bed reactor. The char particles, as the main solid pyrolysis product, will be burnt and heat the cool ash or silica sand in the fluidized bed reactor. The amount of hot ash or silica sands (heat carriers) separated from the fluidized bed reactor can be changed by the butterfly valve on the top of the charging port of coal powders, which will control the reaction condition and processing capacity of coal pyrolysis process.

To realize the cogeneration of heat and power and obtain a higher yield of light liquid products and fine chemicals, a lot of attention should be paid to how to achieve rapid heating of the coal particles, rapid separation of the pyrolysis products and rapid cooling of the pyrolysis oil and gas. It is not difficult to known that the rapid heating of coal particles is the most important in the coal pyrolysis process, which is decided by the rapid mixing between the fuel particles and heat carriers. Therefore, some internals can be designed and installed in the downer reactor to strengthen the mixing and heat transfer [10], [11], [12], [13] between the pulverized coal and circulating hot ash. The internals cannot only increase the particle mean residence time in downer reactor, but also enhance the mixing degree of coal and hot ash. Most importantly, the internals can affect the temperature increasing rate of fuel particles controlled by the mixing degree of fuel particles and heat carriers.

To investigate the mechanism of particulate mixing and heat transfer in the downer reactor, discrete element method (DEM) [14] coupling with the model of particulate heat transfer, widely used in exploring the microscope mechanism of particulate system [15], [16], has been developed and validated. The effect of internals in downer reactor on the particulate mean residence time (MRT), the particulate residence time distribution (RTD), the mixing degree of coal and hot ash, and the profile of the particulate temperature and its increasing rate along the height of downer reactor has been discussed based on the developed model of particulate heat transfer using DEM in this research.

Section snippets

Mathematical modeling

In the basic hypothesis of DEM, the particle entity is illustrated as geometry element of sphere. When collision or compression happens, a certain overlap area between two particles is allowed. The size of the overlap area is very tiny compared with particle surface area. The foundational principle of DEM can be illustrated in Fig. 2.

As shown in Fig. 2, DEM, as a Lagrange method for modeling the individual trajectory of each particle in granular system, can be described as a type of time-driven

Validation

The developed model of particulate heat transfer based on DEM can be validated by the laboratory scale experiments of a rotary calciner equipment built by Chaudhuri [25], as shown as in Fig. 5. Two types of spherical granular materials, alumina particles and copper alloy particles, have been adopted in the experiments, whose parameter values in mechanic model [26] and thermodynamic model [25] are summarized in Table 3.

About half of the calciner was filled with the spherical alumina particles

Simulation conditions

To investigate the effect of internals installed in downer reactor on the granular behavior of mixing and heat transfer, the simulation of particulate system in downer reactor has been established by the developed DEM software package coupling the particulate heat transfer model. The shape and size of the downer reactor with different types of internals are illustrated in Fig. 9, where the front view and side view are presented.

As shown in Fig. 9, the downer reactor is mainly composed of two

Residence time distribution

The granular flow behavior of coal and sand affected by internals in three types of downer reactor has been simulated by the developed DEM software package. The profile of particle velocity along the height of downer reactor at 0.2 s has been shown in Fig. 11.

The mass flow rate at the entrance and exit of the downer reactor can be considered as the signals of excitation and response. The curve of residence time distribution (RTD) can be obtained directly. However, the granular materials can be

Conclusions

The effect of internals on the particulate mixing and heat transfer in downer reactor has been predicted by the developed model of particulate heat transfer based on DEM, which has been validated by the experiments. Two types of internals, the tube group internals and the baffle internals, have been designed to improve the mixing and heat transfer between fuel particles and solid heat carriers in downer reactor.

The internals not only increase the mean residence time of particles in downer

Scalar

    Agap

    area of tiny gas interval between two contact surface, m2;

    cp

    specific heat capacity, J kg 1 K 1;

    dgap

    width of tiny gas interval between two contact surface, m;

    di

    diameter of particle i, m;

    dij

    distance of two particles, m;

    Ii

    rotational inertia of particle i, kg m2;

    E

    elastic modulus, Pa;

    e

    coefficient of restitution, −;

    mi

    mass of particle i, kg;

    m*

    reduced mass of two particles, kg;

    Nu

    Nusselt number;

    kg

    thermal conduction coefficient of gas phase, W m 1 K 1;

    kn

    normal stiffness, N m 3/2;

    kt

    tangential stiffness, N m 1

References (29)

Cited by (12)

  • High-temperature fast pyrolysis of coal: An applied basic research using thermal gravimetric analyzer and the downer reactor

    2021, Energy
    Citation Excerpt :

    Especially for reactions with high-load particle, the downer reactor is more advantageous. Thirdly, the high heat and mass transfer of downer reactor also played an important role [9,19]. The downer reactor not only maintains the high gas-solid contact efficiency and high solid circulation, but also reduces the momentum loss caused by the gas flow transportation [20].

  • Liquid oils produced from pyrolysis of plastic wastes with heat carrier in rotary kiln

    2020, Fuel Processing Technology
    Citation Excerpt :

    In view of this, the dynamic simulation of the heat carriers was modeled by the discrete element method (DEM), and the simulated dynamic process was used to evaluate the motion of sand particles in the rotary kiln. The discrete element method (DEM) is a reliable and efficient approach for modeling a granular system, due to the benefits of no oversimplifying assumptions [41,42]. The analysis and simulation of heat carrier at different filling ratios (5%, 10%, 15%, and 20%) in the rotary kiln reactor by using EDEM® software were presented in Fig. 2.

  • Development of novel heat conduction interaction model for solid body thermal contact in CFD based particle flow simulations

    2018, Chemical Engineering Science
    Citation Excerpt :

    This work presents a technique to simulate the conductive thermal interaction between solid bodies without contact between the computational representations of the particles or other objects, allowing for the simulation of locally resolved thermal conductive interactions. A sample of the diverse set of complex processes and phenomena that stem from thermally dependent phenomena in particle laden flows includes chemically reacting flows, ceramics processing, powder metallurgy, packed bed reactors, blast furnace operations, propellant burn, flow path ablation, turbine blade wear, coal pyrolysis, additive manufacturing, fouling of flow paths, magneto-hydrodynamics applications, and novel solar collectors (Das et al., 2017; Haddad et al., 2016; Kosinski et al., 2013, 2014; Lattanzi and Hrenya, 2016; Liu and Li, 2016; Liu et al., 2013; Morris et al., 2016a, 2016b; Oschmann et al., 2016; Vargas and McCarthy, 2002; Yang et al., 2015). With the wide ranging applications, advancement in the body of knowledge of underlying physics of these processes and phenomena is needed.

  • Granular flow characteristics and heat generation mechanisms in an agitating drum with sphere particles: Numerical modeling and experiments

    2018, Powder Technology
    Citation Excerpt :

    Nguyen [22] modeled the heat flow generated by friction and its transfer by conductance during the discharge of a silo. DEM and DEM-CFD numerical approaches have been widely applied in the simulation of particle flow field, such as mixers [23–26], heaters [27, 28], particle conveying facilities [29, 30], reactors [31, 32], transmission pipelines [33] and ball mills [34]. The DEM-CFD numerical method was firstly applied to calculate heat transfer in gas-solid flows by Li et al. [35].

  • The effect of the number of conveyor belt carrying idlers on the failure of an impact place: A failure analysis

    2017, Engineering Failure Analysis
    Citation Excerpt :

    Bierwisch et al. [23] used DEM to study the rapid flow of particles moving from a container and formation of the angle of repose. Liu and Li [24] applied DEM for study of the effect of internals in downer reactor on the particulate mixing with the inclusion of heat transfer. The results were validated based on laboratory experiments.

View all citing articles on Scopus
View full text