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Mass and energy balances are not just for process and equipment design. They can also be used to improve dryer operations by answering various "what-if " questions about temperatures, flowrates, and moisture contents.
A spray dryer is a vessel into which a liquid feed is sprayed as droplets into a hot gas. The moisture (usually water) evaporates into the hot gas, leaving dry particles that are separated from the cool, moist gas using cyclones, filters and/or electrostatic precipitators. The process yields a solid dried product from a liquid solution in a single step, which reduces the amount of material that needs to be handled. This feature has considerable appeal in the production of food, such as dairy powders e.g., skim milk, whole milk, lactose, whey, casein), and consumer products (e.g., detergents).
However, spray dryers tend to produce amorphous and unstable solid products. The instability is based on the tendency of amorphous solids to spontaneously adsorb moisture from the atmosphere and then crystallize after spray drying. Sometimes this is a desirable attribute, such as when the powder will be used as a sorbent.
Generally, evaporating moisture is an energy-intensive process. Preconcentrating the liquid feed that goes into a spray dryer is a good way to reduce overall energy requirements, since preconcentration reduces the evaporative load on the dryer and, thus, the amount of water required for evaporation.
Figure 1 shows a stainless steel pilot-scale dryer that is used for research and development on the spray drying of foodstuffs. Figure 2 is a schematic diagram of the energy flows around a spray dryer.
Performing mass and energy balances enables engineers to use information on one side of the balance to determine unknown quantities on the other side of the balance. References 1 and 2 provide an overview of how to apply mass and energy balances to dryers, applying fundamental concepts to examples that illustrate how to determine flowrates, temperatures, compositions, etc.
Because of the well-mixed nature of many spray dryers (I), the outlet conditions of the gas and the solids (e.g., temperatures, moisture contents and humidities) are the same as those of the particles inside the equipment. Thus, material properties can be predicted based on the particles' outlet temperatures and moisture contents calculated by mass and...