Drying and Degradation Kinetics of Heat Sensitive Products in Spray Drying

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Author Last Name: 
C Strumillo, I Zbicinski, M Kwapinska, M Piatkowski
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Research Area: 
Particle Formation
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The main aim of the project carried out in the years-1997-2003 was to deliver complex and profound, experimental and theoretical, description of co- and counter-current spray drying process including determination of drying and degradation kinetics, determination of final product properties and elaboration of our own CFD code for reliable scaling up of spray drying process. One of the most significant outcomes of the project was designing and building a 9 m long, 0.5 m in diameter spray drying tower. The tower capacity enabled identification of the effect of various drying process parameters, like the effect of feed properties, feed rate and feed temperature, drying agent temperature and air flow rate on drying and degradation kinetics, particle residence time, particle morphology, etc. The column was equipped with a 72 kW heating system, waste air cooling system, dust collecting system and optical glass windows to perform measurements using laser techniques (LDA, PDA). Feed was delivered from a stainless steel tank (equipped with a cooling/heating steam/water jacket, 150 liter volume) to the nozzle by a progressive-cavity pump. Two steam generators were used to deliver heating agent to the jacket to maintain the required temperature of the drying material. Each pipe and the nozzle were equipped with a water heated jacket. The construction of the tower enabled taking samples at subsequent time intervals and making laboratory analysis of moisture content, size distribution, etc., as well as the quality index specific for a given product at a different distance from the atomizer. To determine the flowfield in the spray tower and structure of the spray in the cross-sectional area and along the length of the tower, the laser technique was employed. The tower walls contain numerous portholes to allow laser measurements at different vertical locations. A transport system of laser device comprising hoists and pulleys enables the laser unit to move easily between levels and carry out measurements at an arbitrary height of the column and in selected points in a given cross section. First three years of the project were devoted to development and improvement of experimental equipment to perform in situ measurements of drying process parameters using Particle Dynamic Analysis (PDA) to determine the structure of spray and microseparator to find drying agent and product temperatures and humidities. This is definitely, the most sophisticated spray drying system ever developed for research of this process. Microseparator technique, significantly modified during the project realization can be easily used by industry people to find accurate temperature of a drying agent undisturbed by the presence of product particles moving in the air. Cocurrent spray drying operation mode was subjected to exhaustive experimental analysis in first three years of project realization. All relationships between initial drying process parameters and behaviors of continuous and discrete phase were determined. The results obtained throw a new light on the mechanism of the process which takes place inside the spray tower during drying of various materials. Most of the results were presented for the first time in literature. One of the most important findings of this work, in our opinion, was lack of aerodynamic segregation of particles in the drying chamber. The analysis of the results shows that in each point along the spray axis there is practically an identical particle size distribution. Spray is mixed very well from the very beginning of the process. This finding is of great importance for understanding the phenomenon of simultaneous momentum, heat and mass transfer during spray drying. One of the challenges of the project was to find drying and degradation kinetics in spray drying process. Substantial degradation of the products was found for most of the trials. Experiments showed a rapid decrease of baker’s yeast activity in the vicinity of the atomizer, the finer atomization the highest degradation of the product. Spray drying of heat sensitive products requires careful selection and control of process parameters.

To obtain a full picture of the mechanism of spray drying process, extensive experimental trials were extended to counter-current spray drying process. Opposite to the co-current spray drying process, due to complex hydrodynamics of continuous phase, an aerodynamic segregation of particles (more bigger particles close to the column wall) was found. Analysis of the results confirms literature suggestions about strong couplings of momentum transfer between continuous and discrete phases which cannot be neglected in modeling of counter-current spray drying process. An increase of mean particle size with the distance from the nozzle caused by agglomeration process in recirculation zones in the column was observed. The results proved high sensitivity of counter-current spray drying process to initial drying and atomization parameters and a position of the nozzle in the dryer. Generally, in relation to practical applications, we can conclude that a performance of the counter-current spray drying process is stable in a narrow range of process and atomization parameters which makes such a system difficult to control. Summarizing, we could conclude that complexity of spray drying process is an outcome of three factors: parameters of a drying agent (temperature, flow rate), atomization parameters and particle residence time in the column. Drying kinetics of spray drying process comes from difficult to predict influence of the above mentioned parameters. One of the leading ideas in the project extension was to develop a CFD model of spray drying process to point out why the existing codes, in a certain range of process parameters and given geometries of the spray drying chamber fail to predict the spray drying process. A spray drying column developed at Lodz Technical University, was employed to collect a database to verify a CFD model of spray drying process. A comparison of experimental and theoretical results of CFD modeling enabled us to formulate the conclusions which might be important for industry people to scale up or check the performance of spray drying process under different operating conditions.

Every CFD model of spray drying process which can be found in the literature or developed individually enables a relatively correct determination of the continuous phase parameters (e.g. distributions of drying air temperature and humidity) regardless a number of simplified assumption in initial and boundary conditions assuming that heat losses to environment and effect of atomizing air were taken into account and proper model of flow turbulence was selected. However, to obtain reliable results of CFD simulations concerning also the discrete phase parameters, it is necessary to introduce to the model real initial particle size distributions and mass flow rates of the disperse phase and real evaporation kinetics. This data are often difficult to obtain in industrial conditions. One of the main successes of the project was to construct and test a device for determination of disperse drying kinetics on a laboratory scale. Our main idea was to elaborate a system where we could reach evaporation rates (and drying times) similar to those obtained in a spray drying column. A laboratory drying tunnel equipped with effective heating system and stabile weight measuring system has been designed and built. The tests on drying of maltodextrin solutions proved repeatability and correctness of this method. In the developed unit, we finally gained evaporation rates and drying times similar to the conditions obtained in a drying column. An effect of the initial moisture content on the critical moisture content was observed which is related to a decrease of the equilibrium vapor pressure over the solution and a decrease of the driving force of evaporation and drying rate of the process. Results of the experiments proved that the generalized drying curves determined in the lab scale could be used in scaling up of spray drying process if the critical moisture content of the material is known. Outcome of this part of the work offers to practitioners a cheap, fast and precise technique to determine realistic spray drying kinetics from small scale experiments.

The last part of the project which delivered a complementary picture of the mechanism of the process consisted of complex experimental studies on the effect of drying and atomization parameters on the properties of selected materials during spray drying process. Materials from two groups were chosen for investigations, skin-forming (maltodextrin) and agglomerate-like (detergent and cacao), that are most often subjected to spray drying in industrial conditions. In the study, 352 experimental tests of spray drying of maltodextrin, detergent and cacao powder as a function of temperatures and air flow rate, atomization conditions as well as temperature and dry matter content in the feed were carried out (in the last two years). We found that for all powders the mean particle diameter was smaller than the mean droplet diameter in the spray which was caused by shrinkage of the particles during moisture evaporation. We also proved that, depending on drying and atomization conditions, each of the tested materials could be cohesive, slightly cohesive or loose. We determined quantitative relationships and explained effect of the initial distribution of particle diameters and their morphology on product bulk density. Determination of these conditions in the frame of this work has an important practical meaning. The work delivers a profound and complementary picture of the mechanism of the process which takes place inside the spray tower during drying of various materials. The behavior of dispersed phase, drying and degradation kinetics, quantitative relations between initial parameters of drying and atomization and physical properties of spray-dried products are presented for the first time in literature. Determination of the local particle size and velocity distribution, detection of the uniformity of spray structure in the dryer, elaboration of quantitative relations describing drying and degradation kinetics are among the most important outcomes of the project.