Before the advent of modern technology such as mechanical refrigeration and oven dryers, men had to preserve food during bountiful times in order to survive the leaner times. Food preservation techniques widely varied based on the climate. In latitudes where freezing temperatures were common, people used their cold environment to preserve meat and fish; the Inuit are a prime example of that using the ice to freeze seal meat.
In more clement climates, people favored a different preservation method: drying. Evidence suggests that around the world after the annual fruit harvest, people needed to preserve the food since fresh fruit wouldn’t last more than a few weeks. So, fruits, herbs, and vegetables would be left out in the sun to dry. Once dried the food would last for months, and we know that as far back as 12,000 B.C. people were already drying food to preserve it. As the Romans conquered colder territories, their fondness for dried fruit drove them to build “still house” an archaic version of an oven dryer where they used a fire to heat up the chamber in order to mimic the natural sun drying process.
Nowadays, we have a variety of options when it comes to conserving food. Drying and freezing are the two processes that are most frequently used to conserve food. But freezing has its limitations, while it does preserve flavor better than drying, it is more expensive and requires maintaining the product at a low temperature from the plant to the retailer, and eventually to the customer. Dried foods on the other hand, don’t require a low temperature to keep well, also the equipment necessary to oven dry food is more affordable than freezing equipment, making it more popular than freezing.
The process of drying is a common unit operation in a wide variety of industries including chemical, pharmaceutical, biotechnology, mineral processing, and pulp and paper, all of which heavily utilize the expertise of chemical engineers. Drying is defined as heat being applied to a solid, semi-solid, or liquid feed to evaporate a liquid into the vapor phase by convection, conduction, or radiation. About 85% of dryers used in industry are convective, using hot air or steam. Over 99% of drying applications involve the removal of water. Drying processes are vital to industry, but are one of the most energy-intensive unit operations due to water having a high latent heat of vaporization and due to the inefficiency of using hot air, heated by steam, as the drying medium.
A tunnel tray dryer uses pressurized steam flowing through heat exchangers to transfer heat to air being pumped through the machine by a blower. The hot air is then contacted with the surface of the object to evaporate the liquid. The pressure and recycle conditions of the tunnel dryer can be manipulated in order to compare energy usage and efficiency of the system. Variables that can impact the product of this study include things such the chamber temperature, pressure, air velocity, relative humidity, time, and energy usage. These variables can be controlled by using instrumentation including valves, dampers and vents to adjust steam flow in the tunnel dryer. Thermocouples, humidity probes, and flow meters measured these variables.
In this study, sliced “Granny Smith” apples were placed in a tunnel tray dryer, in order to determine the optimal time and temperature that it takes to reduce them to the desired moisture content of 20% for a counter-current semi-continuous process. The energy consumed per mass of apple was also determined in this study to optimize the energy use and throughput mass per unit time. The goal of this study is to minimize both energy consumption and throughput time to reach the specified 20% moisture content.
Drying involves both, transient transfer of mass and heat, which can make scale up of a drying process very complicated. However, these processes can be modeled using fundamental principles and assumptions. In the process of drying apple, heat transfer occurs when the heated air is applied to the surface of the apple slice. There is also heat transfer taking place within the apple slice, which is made up of mostly water, sugars (mostly fructose), air and other compounds. As these components are heated, the water undergoes liquid diffusion (due to the gradient) through the apple due to convection. A phase change occurs as the liquid begins to evaporate into the vapor phase and leaves the surface of the apple, entering the air. The rate at which the water evaporates is the same as the drying rate of the apple slice, and is represented by N, which can be denoted as:
Under constant drying conditions, A is the evaporation area, t is time, Mds is the mass of the dry solid, X is the dry moisture content of the solid, and X* is the equilibrium moisture content or the point at which the solid stops drying. The flux N, has units of kg /m2 h. If the area is unknown, then N is the drying rate in units of kg/h.
The following factors greatly influence the drying rate:
Physical and chemical composition of the material, such as moisture content
Size, shape and arrangement of the pieces
Relative humidity of the air; this is important to determine the amount of moisture in the drying air
Air velocity (constant in our case)
Case hardening, this is important because at high temperatures and low humidity moisture is removed from the surface faster than it can diffuse from within the material causing a hard layer on the surface and preventing the drying of the material’s inside (Wilhelm, Suter, ; Brusewitz, 2004).
The dry moisture content, X, decreases linearly with time at the beginning of evaporation, but then decreases non-linearly with time until it reaches its equilibrium moisture content. Both physical and chemical transformations are taking place and these can be observed by shrinkage, changes in color and texture of the apple slice. Plotting N vs. X will result in a drying-rate curve. The time required for drying to reduce the solid to desired moisture content is calculated using equation 2:
where X1 is the initial moisture content, and X2 is the desired moisture content.
The moisture content of an apple slice can be calculated by:
X=(Mass of thefresh slice)?(Mass of Dried slice)(Mass ofthe fresh slice)?100%
Hot air drying demands a high-energy input because heat transfer from air to material is experimentally inefficient and a substantial portion of the energy is lost in the exhaust air. Therefore, assessing the dryer performance, by calculating energy consumption, is important since convective dryers account for 85% of all industrial dryers (Kudra, 2012).
The energy balance describing the heat transfer between the hot air passing over the slice and the water in the apple is represented by:
is the latent heat of vaporization for water and N is the evaporation rate of water out of the apple slice.
It is also important to calculate the energy consumption of our system. The energy balance on the heat exchangers can be represented by:
is the latent heat of steam and W is the mass flow rate of steam through the heat exchangers, which can be calculated by:
is the density of water, and V is is the volumetric flow rate of the con densate.
For our experiment, we will be using equation (5) to determine the heat duty and subsequently the cost of drying. Since the mass flow rate W is unknown, we will use the volumetric flow rate V to determine the mass flow rate equation (6). To determine W we will collect the volume of water condensate from the heat exchanger(s) and record the collection time.