Original Research PaperCFD simulations of droplet and particle agglomeration in an industrial counter-current spray dryer
Graphical abstract
Introduction
The thermal dehydration of wet materials is widely used in industry in order to decrease product weight and to prevent materials from undergoing biological deterioration. The form of the received material strongly depends on the chosen drying method. Dehydration by spray drying is one of the most common thermal methods to obtain dry loose material from a solution, slurry, suspension or paste. The method involves the atomisation of a liquid feed in a stream of hot drying agent which leads to the rapid extension of the phase contact area and the significant acceleration of moisture evaporation rate [1]. The evaporation process runs at a relatively low droplet temperature inside the spray envelope (wet-bulb temperature), which prevents against overheating of the dried material. Furthermore, particle drying and residence time inside the drying chamber is short which minimises the probability of product thermal degradation. Thus, spray drying is used in many industries, from pharmaceutical to food and chemical, also for drying of heat-sensitive materials [2]. Because of different methods of phase introduction in the drying chamber the spray-drying process can be carried out in three ways: co-current, counter-current and mixed. Each method results in different physical and morphological properties of the produced powder.
In co-current spray-drying towers, due to parallel phase flow, air and particle recirculation hardly occurs which limits particle agglomeration and wall deposition [3].
Counter-current spray dryers are used mainly in the production of detergents and chemical fertilisers. In terms of flow dynamics, the process is more complex than the co-current because of the intensive mixing of the phases. There are just few pilot-scale or industrial scale counter-current spray drying systems that enable the measurement of flow dynamics of a continuous phase and changes in the morphology and properties of dried materials during the process [4], [5]. Currently, the most comprehensive source of experimental data for counter-current spray drying were obtained at the Faculty of Process and Environmental Engineering at the Lodz University of Technology, Poland [6].
In order to minimise operating costs, the counter-current spray drying systems are optimised in terms of energy consumption and required properties of the product. Industrial companies show growing interest in computer simulations of the spray-drying process in order to determine the impact operating parameters on energy consumption and product quality. This is simply because computer simulations are time and cost effective compared to experiments. Optimisation of the process based on computer simulations requires a detailed description of the parameters of heat and mass transfer between the phases and dryer geometry and operating parameters [7]. Additionally, in most models the effect of agglomeration process on drying kinetics is simplified or totally ignored [8], [9].
The coalescence of droplets and the agglomeration of particles are phenomena that occur in many industrial processes. During counter-current spray drying, due to an intensive mixing of phases, particles and droplets can collide and form bigger agglomerates. Particle agglomeration is difficult to control and its effect on the drying kinetics, the temperature distribution of the two phases, and the flow dynamics of drying agent in the tower is complex. With the current state of the art of agglomeration model we cannot predict the size of the resulting product or optimise the process without experimental methods.
Section snippets
Particle agglomeration in spray drying
As a result of atomisation, liquid is disintegrated into droplets, which upon drying change into particles. Droplet size depends on the type of atomiser and the physicochemical properties of the liquid, and may vary from a few to several hundred microns. As a consequence of air recirculation and rapid changes of flow trajectory, the droplets/particles collide. During the spray-drying process, we can distinguish three types of collisions: the collision of two droplets, droplet-particle collision
Particle agglomeration model
Commercially available software for CFD calculations will allow us to calculate the changes in the discrete phase on the basis of the PSI-Cell method [21]. The process of calculations is divided into two stages: in the first stage, the velocity distribution field inside the computational domain is calculated by solving the system of Navier-Stokes (N–S) equations. Then, following the changes of airflow velocity, the particle trajectories and their impact on the continuous phase parameters are
Drying kinetics model
The temperature of a dried particle changes due to convection and moisture evaporation:
The coefficient of heat transfer between the particle and air is described by the Ranz-Marshall equation:
In order to describe the drying kinetics, a characteristic drying curve (CDC) was used. In this approach, relative evaporation rate, f, is defined in terms of the average dry basis particle moisture content [23]:
Experiments
The object of simulation was the drying tower for the detergent, built at the beginning of the 70 s and modernized in 1993, operated in Raciborz, Poland. A schematic diagram of the drying tower is shown in Fig. 3. The total tower height is 37 m (33 m height of drying chamber), and the inner diameter of the cylindrical part is 6 m. At the top of the tower there is a system of bag filters, which purify the outlet air. The filter consists of 720 bags, each 2.6 m long and 0.15 m in diameter. The
CFD model set-up
Drying air is heated in a gas burner and supplied to the drying chamber through a distribution ring with a rectangular cross-section. Hot air from the distribution ring flows to the drying tower through 16 connection ducts perpendicular to the tower wall inclined at an angle of 45°. The inlets of the ducts are divided into three parts. In five connection ducts, the cross-sectional area was reduced locally in size to 2/3 (Fig. 3). The tangential supply of air and differences in the
CFD Simulation results
Experimental temperature distributions were compared with time-averaged results of the CFD simulation. The results are shown in Fig. 4. Comparison of the results confirms good agreement of temperature distributions obtained from simulations with temperature distributions of the real drying process on an industrial scale.
Fig. 5 shows not only changes in flow direction but also a change in the location of eddies in the horizontal axis at different heights of the drying tower. Swirl air motion can
Summary and conclusions
An original model of particle agglomeration during the spray-drying process was proposed and verified. Based on this model particle trajectories, changes of particle morphology (resulting from the drying process), as well as the effect of process parameters on the rate of agglomeration and particle size distribution (PSD) are calculated. The ANSYS program was used in the flow dynamics calculations of the continuous phase and as a solver to calculate differential equations in the DPM model which
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