Investigation on the role of drying air humidity in shaping the conditions of spray drying using model feed materials

This reserach was divided into three stages: Stage I: spray drying/evaporation of distilled water at variable feed rate (FR), inlet air humidity (IAH) and inlet air temperature (IAT). The process was analyzed based on a comparison of the inlet and outlet air parameters (temperature, relative and absolute humidity). Specific energy consumption (Es) and drying air consumption (s) were calculated and presented on graphs. Tested variables included: 0.16, 0.21, 0.38, 0.58, 0.67, 0.83 mL/s feed rates; 0.1-0.3 g/m3, 1.1-1.3 g/m3 drying air humidity; 80, 100, 120 ℃ inlet air temperatures. Stage II: spray drying of 30% (solids, w/w) MD solutions at the same levels of independent variables and procedure as in stage I. Powder recovery (Rp) , moisture content (MC), water activity (aw), loose bulk and tapped densities (dL and dT), flowability (HR) were determined. The morphology of selected variants was analyzed as well (L, M, H signifies the low, medium or high level of drying air humidity; 80, 100, 120 signifies the inlet air temperature; 0.16, 0.58, 0.67, 0.83 signifies the feed rate). Results were presented in tables and graphs. Additionally, PCA and HCA were performed and presented as a graph and a dendrogram. Tested variables included: 0.16, 0.21, 0.38, 0.58, 0.67, 0.83 mL/s feed rates; 0.1-0.3 g/m3, 1.1-1.3 g/m3 drying air humidity; 80, 100, 120 ℃ inlet air temperatures. Stage III: optimization of MD solutions spray drying process variables (inlet air temperature – IAT, inlet air humidity – IAH, feed solution concentration – FC) for maximum powder recovery and powders of low moisture content and activity using response surface methodology (RSM) with random run order and center points runs added 19 combinations of variables were considered: 8 factorial points in 2 replications and 3 center points runs placed in the beginning, middle and end of the experiment. Particle morphology and particle size (D50), loose bulk and tapped densities (dL and dT), flowability (HR) of obtained powders were determined. Tested variables included: 0.1-0.3 g/m3, 1.1-1.3 g/m3 drying air humidity, 10, 20, 30% feed solution concentration and 80, 100, 120 ℃ inlet air temperatures. Results were presented as RSM graphs, a table with results sorted into homogenous groups, and PCA and HCA graphs. This research was funded by the National Science Centre (in Polish: NCN—Narodowe Centrum Nauki), program: Preludium, grant number 2021/41/N/ST8/01742 (“Investigation on the role of drying air humidity in shaping the conditions of spray drying”).