The paper presents a review of research works, experiments and simulations on spray drying in the industrial scale. The research area in the industrial spray drying process covers control of powder properties (bulk density, morphology), particles agglomeration (final PSD), residence time, wall deposition and safety of the process and the product. Limited number of literature references on spray drying in the industrial scale is due to high costs of running experiments and validation of theoretical models. Recent works on hydrodynamics in spray towers showed transient, oscillating, 3D flow of dispersed and gas phase which results from construction of inlet and exhaust ducts and swirl decay due to wall roughness (deposits). Applications of the most advanced and accurate empirical methods to calculate spray drying kinetics in industrial scale; the Characteristic Drying Curves (CDC) and the Reaction Engineering Approach (REA) were presented. The main obstacles denying confident scaling up of the spray drying process; lack of dynamic similarity between small and large units, gas turbulence modelling, determination of drying kinetics and neglecting of agglomeration process are highlighted. Methods of modelling of the agglomeration process in spray drying (transition functions, full scale agglomeration) are described. Examples of modelling and measurements of hydrodynamics, wall deposition, agglomeration process and drying kinetics in the industrial towers are presented and discussed.

An intensive deposition of powder on the walls in the counter-current spray towers develops the risk of deposits burning in the vicinity of hot air inlets. Cooling of the hot air inlets area could reduce wall temperature to avoid deposit burning. CFD calculations were performed to estimate the effect and to optimize the position of cold air inlets on cooling efficiency of the hot air inflow areas in industrial spray tower. To keep the same evaporation capacity of the tower when cold air was introduced, the heat balance of the dryer was recalculated. Experimental analysis of the wall temperature distribution near air inlets enabled validation of the CFD model. Analysis of the results shows a difference in circumferential cool air distribution in all analyzed configurations due to the upward and downward flows of drying gas near the inlet ring. The wall can be cooled also by the downward flow of a drying agent temporarily induced by the instable flow pattern in the whole tower. The simulation methodology developed in this work allows to select the optimal cooling air inlets construction and location.

We investigate the development of stresses in a drying process of paste-like materials theoretically and experimentally. An invasion percolation model on a nonlinear elastic lattice can describe cracking processes resulting from drying contraction as well as dry processes in the bulk of the system and gives a non-dimensional parameter to determine the conditions at which cracking precedes drying. This model also predicts that horizontal tensions increase isotropically with drying. However, paste often exhibits anisotropic crack formation due to plasticity, as is known as the memory effect. We measured stresses in a drying process of calcium carbonate (CaCO₃) paste and confirmed that anisotropy in residual stress increases with uniform drying until crack formation when shaking is applied to a paste layer for a short time initially.

Present paper proposes a new method to evaluate the aggregate extent of dried nano-size particles using measurement of photoluminescence spectra. When a high sensitivity photoluminescence spectrometer is used, photoluminescence spectra can be recorded from varieties of dried aggregated-particles. Confinement of electrons in nano-sized particles can affect the wavelength of the photoluminescence, and the aggregation of the particles also can exhibit the influence thereon. In this work, colloidal silica are dried to make films of particle aggregate, and a photoluminescence spectrometer equipped with a pulsed UV laser of 248 nm wave length was used to obtain photoluminescence spectra from this particle aggregate. Here, the drying condition was varied to observe the influence of the microscopic structure of the particle-aggregate on the photoluminescence spectra. The volumes of mesopores and micropores analyzed by N₂ adsorption were used to evaluate the microscopic structure of the particle-aggregates. As a result, it was observed that a peak in the photoluminescence spectra was shifted to longer wavelength when the number of the contact points between each particle was speculated to be larger. It is considered that the method using the photoluminescence spectrometer may be a simple and non-contacting method to quickly evaluate the structure of the particle-aggregates.

Deformation is a phenomenon that is common during drying. Generally, deformation is evaluated and reported in terms of volumetric shrinkage. However, volumetric shrinkage cannot be used to describe non-uniform deformation, which commonly takes place during drying of shrinkable materials such as biopolymers, fruits and vegetables. In the present study, algorithms and software were developed to first reconstruct 3D images from 2D images, and then to monitor non-uniform deformation of such images. Agar gel with different sugar contents were used as the test materials and were allowed to undergo drying at different rates. The developed image reconstruction algorithms were successfully validated with standard geometrical shapes. Various image-based parameters, both in two dimensions, i.e. projected area, perimeter, major axis length, minor axis length, equivalent diameter, extent and fractal dimension; and in three dimensions, i.e. volume, sphericity, Wadell’s sphericity, Wadell’s roundness, radius ratio and Hoffmann shape entropy, were then calculated. The results showed that extent and fractal dimension could be used to identify the onset and to monitor non-uniform deformation in two dimensions, while sphericity could be used for such tasks in three dimensions.

The present article presents the energy efficient concept of biomass indirect drying. The options for the integration of indirect biomass drying into the cycles of steam power plants and hot water heating plants were analyzed. New concepts of wet biomass utilization in energy systems with an integrated indirect biomass dryer and the utilization of latent heat of waste vapor released during drying were proposed. Prototypes of the dryer and the waste vapor condenser were designed and the feasibility of the proposed solution was experimentally verified. Drying characteristics and energy consumption were determined. The potential for recuperating the waste vapor heat was analyzed. In comparison with other drying methods, the designed solutions are energy efficient and effectively utilize the waste steam heat. The profitability of the proposed solution is apparent from the increase in the cycle efficiency in comparison with the case without drying. Integration of drying into energy cycles makes it possible to achieve higher energy efficiency of the systems and significant fuel savings, and to replace high quality biomass with low-grade moist forms.

Encapsulation efficiency is affected by several parameters such as oil-droplet diameter, solid and oil contents, as well as by the processing conditions of spray drying. The surface-oil ratio is the most important parameter that estimates the shelf life of fish oil in spray-dried powders. In the present study, sodium caseinate (3 wt%), maltodextrin (DE=8, 11, 19, 25, and 40) as the wall material, and fish oil as the core material were used to form spray-dried powders with 30, 40, 50 and 60 wt% of oil load of the total solid content. The oil load is expressed as a weight percentage of additional oil to the total solid content. Feed emulsions were prepared using a rotor-stator homogenizer at 8000 rpm and/or a high-pressure homogenizer at 20 or 100 MPa. The effects of oil-droplet diameter and particle diameter on the surface-oil ratio and the encapsulation efficiency of emulsified spray-dried fish oil were investigated. The surface-oil ratio was remarkably increased when the ratio E=(d_e/d_p) of the average reconstituted oil-droplet diameter (d_e) to the average particle diameter (d_p) was higher than 0.01. The surface-oil ratio was defined as the weight ratio of surface oil to total oil in the powder. The encapsulation efficiency y was related to E by the equation y=(1–2E)³. The oil-droplet diameter might affect the stability of fish oil in spray-dried powders.