Condition of amorphous particle deposits and products at different drying rates in a spray dryer

摘要

Deposition of amorphous particles is a prevalent problem particularly in the spray drying of fruit and vegetable juices due to the low molecular weight sugars encountered. The deposition mechanism of these amorphous particles depends strongly on the condition of the particles upon collision with the wall. This paper attempted to investigate the condition of the amorphous particles impacting the wall at different drying rates with the aim of giving more insights on the deposition mechanism and physical phenomena in the chamber. A model sucrose-maltodextrin solution was used representing the low molecular weight sugars. Deposits were collected using an internal rig constructed and analyses were undertaken on the moisture content, particle rigidity (SEM) and size distribution. Taking the moisture content as the general indication of stickiness, product particles exhibit relatively higher moisture when compared to the deposits. Moisture profile in the chamber shows that apart from deposits at the atomizer region, which is relatively wet, deposits at other regions exhibit consistent and lower moisture content. At the highest temperature, particles became rubbery indicating dominance of the solid liquid bridge formation as the dominant deposition mechanism. Further analysis on the particles size distribution confirms the particle segregation mechanism as predicted in past CFD simulation. Two problem faced in the analyses of such particles were highlighted to be addressed by future workers; moisture determination of small samples and particle size analysis for sintered particles. The finding from this work will form the basis for our future modelling work on wall deposition of amorphous particles in spray drying using CFD.