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