Spray drying: Influence of developing drop morphology on drying rates and retention of volatile substances. 1. Single-drop experiments

Precise experiments were performed in which single drops of coffee or sucrose solutions containing a trace quantity of SF6 were dried. Simultaneous measurements were made of the drying rate, the loss rate of SF6, the drop temperature, and the physical appearance of the drop. The effects of adding small amounts of carboxymethyl cellulose (CMC) and sodium dodecyl sulfate (SDS) were also explored. Mass-transfer rates of water and SF6 increased during morphological development, or puffing. The enhanced loss of water was caused by liquid-phase convection resulting from stretching of the drop during internal bubble growth and mixing during bubbling. The loss of SF6 during this period was by liquid-phase diffusion into rupturing bubbles. The cooling effect from enhanced water loss during droplet inflation was often sufficient to condense the water vapor in a bubble, thereby leading to inflation-deflation cycles that prevent rupture and remove water selectively.