Technology for Porous Ammonium Nitrate Production: Analysis of Drying Machines’ Operating Modes

. The article is devoted to the development of the technological principlesregarding thefinal drying process of the porous ammonium nitrate (PAN) granules in multistage gravitational shelf dryers. The data on the dryer’s optimal technological operatingmodes are obt ained. PAN samples are studied; the regularity of the change in the porous structure of the granule depending on the hydrodynamic and thermodynamic conditions of the dryer is established. Experimental data obtained during the research will be used to create a methodology for the engineering calculation of gravitational shelf dryers.The data for calculating the residence time of particles in the dryer under various hydrodynamic operating modes are obtained. Moreover, the data about the optimal operating conditions of the drying machines at the final drying stage will be used to improve the technology to form porous granules from ordinary ammonium nitrate.


Introduction
Porous ammonium nitrate (PAN) is a necessary component of the industrial explosive ANFO [1]. The ammonium nitrate is an effective substitute for TNT-containing industrial explosives in a mixture with distillate diesel fuel. PAN can successfully absorb and retain diesel fuel distillate due to the network of pores with different configurations and sizes on the granule surface. It is possible to obtain such a system of pores in different ways . Based on the analysis of works [24][25][26] and the author's research [27][28][29], this article proposes a method to obtain PAN, consisting of two stages. At the first stage, ordinary ammonium nitrate is moistened with water or a particular solution in the vortex granulator's workspace or outside it. At the second stage, the moistened granules are dried in a flow of hot heat transfer agent. The second stage requires special attention in ensuring the required drying time while maintaining the granule's strength properties. In the study [30], it is proposed to carry out the drying process in two stages using two types of devices with a directed motion of the fluidized bed to maintain the strength of the core of the granule. , Fig. 1. A unit to produce PAN granules by using a vortex granulator and a gravitational shelf dryer [28] (top) with process schematic diagram [30](bottom).
The final drying process is carried out in a multistage dryer. At its every stage, various hydrodynamic conditions for the motion of granules can be created [31,32]. The aim of the article is to study and describe the main operating modes of a gravitational shelf dryer and to assess the effect made by the operating mode of the device on the PAN granules structure. A scheme regarding the use of a vortex granulator as the primary device and a shelf dryer at the final drying stage is taken as a fundamental technological scheme to obtain PAN ( fig. 1) [33,34]. The applicability to use such a scheme is justified in the works [30,35].

A physical model of the dispersed particles motion hydrodynamics
The model for determining the trajectory and residence time of a particle in a shelf dryer is based on the following principles. The flow of a continuous phase, modeled by the system of Navier-Stokes equations and the flow continuity equation, gives part of the momentum to the discrete particles [36][37][38].
When the dispersed particles appear in the workspace of the device, they (except for the forward motion caused by an inclined shelf) are drawn into the vertical motion due to the gas flow energy. The introduction of the dispersed phase will cause a significant change in the value of the circular component of the gas flow velocityV  .The momentum of the gas phase (density g  ), which have a vertical velocity V if there is no dispersed phase in the workspace (linear dimensionthe length of the shelf l) is as follows: The momentum of the gas phase after interaction with the dispersed phase in the workspace is where ' V -the gas flow velocity after interaction with the dispersed phase.
The momentum of the dispersed phase motion (mass m d , motion velocity W), obtained after interaction with a gas flow is: Considering the ratios of dispersed phase flows rate and gas Q d Q g , the equation (3) is as According to 27 we write the equality: In this case, the gas flow velocity after interaction with the dispersed phase is

Operating modes of the gravitational shelf dryer
The experimental unit of the multistage shelf dryer is shown in fig. 2. During experimental studies of the dispersed material motion in a shelf dryer, it becomes possible to identify the effect made by the particle packages motion on each other (zones of packages collision, vortex formation, dispersed material motion with greater or lesser intensity, etc.). Model material: 75% of polypropylene (d = 3-3.5 mm, x = 1.7%) and 25% of ammonium nitrate (d = 2-2.5 mm, x = 0.2%).
The main modes of the dispersed material motion, defined by the experimental studies include: 1. The gravitational falling layer mode ( fig. 3). This mode was investigated at the drying agent's flow rate of 24.6 m 3 / h and dispersed material of 12 kg / h. The dispersed material moves along the surface of the shelf due to the inertia force, caused by the transmission of momentum when loading from the pipe or when moving from the previous shelf, and force of rolling on the tilted surface. The drying agent's ascending flow force does not significantly affect the dispersed material motion mode. In this mode, the gas flow velocity is less than the first critical gas flow rate, which corresponds to the weighing mode.      The data analysis from the studies on the shelf device's operating modes allows offering to narrow operating modes of the device to the first three modes. In the fluidized bed mode of the dispersed material, small particles (due to the difference in critical velocities) leave the workspace of the device with a high probability. It should be noted that it is important to identify (except for the gas flow rate) the residence time of the dispersed material on the shelf of the device in each dryer's operating mode.
The range of stable operation of a gravitational shelf dryer is determined by two critical velocities of the gas flow: 1. The first critical velocity, the beginning of dispersed material fluidization 1 V cr .
2. The second critical velocity, the material ablation from the dryer's workspace affected by the ascending gas flow 1 V cr .
The value of this velocity depends on the size (mass of the particles). An increase in the size (mass of particles), as well as an increase in the constraint degree of the flow, leads to an increase in the values of critical velocities. One should note that the falling gravity layer mode is successfully realized at  . The rest of the modes can be investigated in the range ; The data of experimental studies enable to establish the range of relative velocities of the gas flow, related to 1 V cr and 2 V cr ( fig. 8) as well as the relative change of the first and second critical velocities depending on the constraint degree of the flow ( fig. 9).  In each mode, the nature of the change in residence time has its own increased intensity. Analysis of fig. 10 shows that it is possible to provide a longer residence time of the dispersed phase in the device in the transitional mode and at the beginning of the ablation mode. However, according to further studies of the crystal structure in the PAN granules, the excessive residence time in the workspace of the device (excessive contact with hot heat transfer agent) leads to the granule strength reduction.

Structure of the PAN granule surface
The crystal structure of the ammonium nitrate granules surface at different stages of PAN production is studied to confirm the selected operating modes of the shelf dryer. Figures 11-16 show microscopy images of the granules surface. The analysis of the figures shows that the nature, structure and number of pores on the surface of the granule depend on the heat treatment time and the contact mode between the drying agent and the granule. Table 1 presents the results of the research. Fig. 11. Crystal structure of the ordinary ammonium nitrate surface.
Crystal structure of PAN surface after humidification and heat treatment in a vortex.

Conclusions
The results from investigations of the multistage shelf devices' operating modes and the crystal structure of PAN granules enable to establish the optimal range of dryer's operation at the final drying stage. The perforated shelf contacts construction for each stage of the dryer is chosen through the calculation of the required residence time of the granule at the drying stage [39]. Thus it is necessary to provide the given porous structure quality indicators of granules [40]. The main condition for selection: the residence time of the granules in the vortex granulator and the shelf dryer should not be less than the estimated drying time. Based on this condition, there is an algorithm for the optimization constructive calculation of the drying unit: selection of the number of steps, the length of the shelf, its perforation degree, the tilt angle to the horizon, etc.