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Case Study:

Developing Self-Heating ?Smart? Packages

Smart packages are paving the way to a new era in the consumer-products industry. Packages with sensors and indicators that depict shelf life and temperature of products are already in use, and the food industry is building on this technology. This case study describes how Stress Engineering designed and analyzed a self-heating smart package for hot beverages. This package has the convenience of providing the consumer with a hot beverage anywhere and on-demand without any external source of energy. Heating is initiated by inducing an exothermic chemical reaction between water and hygroscopic salt placed around a beverage cup within the package.

Using predictive simulation methods to model beverage heating, Stress Engineering has developed a design-by-analysis approach for exploring various configurations to accelerate the heating process. These approaches make it possible to prototype and evaluate package performance in a virtual environment, prior to committing to expensive tooling or time-consuming trial-and-error development.

Package Design

The package consists of a 50-ml aluminum foil beverage cup placed inside an insulated plastic container. A double-wall arrangement is used to insulate the container. Anhydrous calcium chloride salt is placed in the chamber (Figure 2). The lower region of the cup contains a water reservoir. Heat is generated by reacting calcium chloride with water. To initiate the reaction, the package is first turned upside-down, and pushed on the pin which ruptures the calcium-chloride chamber. Water then flows into the calcium-chloride chamber and the resulting reaction generates heat. The chemical kinetics of this process are described by the following equation.

CaCl₂ + 6H₂O = CaCl₂.6H₂O + Heat

Anhydrous calcium chloride is hygroscopic in nature, and it readily absorbs water molecules to form a hydrogen bond between salt and water. This process is accompanied by a release of energy as heat. If excess water is present, then calcium chloride dissolves in water and dissociates to form ions. This process absorbs energy. However, the net result of the reaction between calcium chloride and water is a release of energy. The amount of energy released depends on the purity of the calcium chloride salt. For example, the energy released using regular calcium chloride (CaCl₂ 2H₂O) is about half the energy released when anhydrous calcium chloride (CaCl₂) is used. The actual heat released using a particular grade of calcium chloride can be measured using a table-top experiment.

The amount of calcium chloride and water required to induce a temperature rise of about 158?F (70?C) is estimated using a sizing calculation based on the chemical kinetics of the hydration reaction. This is then used to estimate the volume required for the calcium chloride chamber and water reservoir. The overall size and weight of the package is summarized in Figure 3. A light-weight (170 gm) package capable of heating the beverage to a temperature of 194?F (90?C) from a room temperature of 68?F (20?C) was designed.

Analyzing Package Design

Package performance is examined by studying the heating process. The hydration reaction is very fast compared to beverage heating. For this analysis it is assumed that heat is released from the hydrated salt over a period of 10 seconds. Simulation is used to examine the transient release of heat from the hydrated salt and heating of the beverage.

The package is placed upside-down and the thermal field at various times starting from the reaction initiation is examined. Thermal profiles in the package and the beverage at various times are depicted in Figure 4, showing a rapid heating of air in the calcium chloride chamber and water reservoir. Buoyancy-induced thermal currents result in appreciable heating of this region. However, buoyancy effects lead to thermal stratification in the beverage cup. As one might expect, cooler fluid is observed near the inverted lid of the cup and hotter fluid near the base of the cup (Figure 4). The flow field induced by buoyancy currents is shown in Figure 5. In this arrangement, buoyancy-induced currents in the cup lead to thermal stratification. The average temperature of the beverage in the cup is 124?F (51?C) after two minutes of initiation. This arrangement does not provide sufficiently rapid heating of the beverage in the cup.

An alternate arrangement, where the package is placed in the upright position and heated from below was also examined. The thermal profile of the package and the beverage is depicted at various time intervals in Figure 6. Thermal stratification of air in the calcium chloride and water reservoir chamber is observed. Appreciable thermal currents depicted by the velocity field are shown in Figure 7. In this arrangement, the heating and mixing of the beverage in the cup is enhanced due to the effect of buoyancy. As a result, temperature of the beverage in the cup is fairly uniform. Average temperature is 131?F (55?C) two minutes after initiation. It must be noted that in this configuration, it is necessary to add design features that hold the hydrated salt in position around the perimeter of the cup.

Heating can be enhanced by stirring the beverage or shaking the container. The effect of shaking the container on the beverage temperature (Figure 8) provides a fairly uniform temperature averaging 122?F (50?C) after about 80 sec. Results indicate that beverage heating can be accelerated by adequate stirring or shaking of the container.

Stress Engineering has demonstrated that a self-heating smart package using heat released by hydrating a hygroscopic salt can be designed by applying predictive simulation methods to model beverage heating. The design-by-analysis example illustrated here uses a light-weight package capable of heating the beverage to 194?F (90?C), starting from a room temperature of 68?F (20?C). Predictive simulation methods can also be applied to examine transient heating of the beverage. Simulation indicates that buoyancy-induced heating due to thermal currents is a relatively slow process, reaching an average beverage temperature of 124-131?F (51-55?C) in 2 minutes. However, if the container is shaken, an average temperature of 122?F (50?C) is attained in just 80 seconds.

For more information on this unique approach to self-heating package design,

Contact Stress Engineering in Cincinnati at 866-888-8333