Introduction:
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.
Drying is one of the most commonly used unit operation, industries ranging from chemical, food, pharmaceutical, biotechnology, and pulp and paper. Drying consists of heating up a solid, semi-solid, or liquid to evaporate a liquid into the vapor phase by convection, conduction, or radiation. The vast majority of dryers used in industry are convective, using hot air or steam. In most cases water is the liquid that is evaporated. While drying processes are of the utmost important to industry, their energy consumption makes drying one of, if not the most energy-intensive unit operation due to water’s high latent heat of vaporization but also due to the very low efficiency of these systems.
A tunnel dryer uses steam flowing through heat exchangers to transfer heat to air being pumped through the machine by a blower. The hot air comes to contact with the surface of the object in order to evaporate the liquid. The pressure and recycle conditions of the tunnel dryer can be manipulated in order to optimize both energy consumption and system efficiency. A few variables are to be manipulated during this study to achieve our goal. Some of these variables are tunnel temperature, steam pressure, air velocity, relative humidity, and time. We will be varying some of these variables using the pressure valves, dampers and vents to adjust steam flow in the tunnel dryer. Thermocouples, humidity probe, and flow meters measured these variables.
In this study, sliced “Granny Smith” apples were placed in a tunnel dryer, in order to determine the optimal time and temperature that it takes to reduce the moisture content to the desired value of 20% for a counter-current semi-continuous process. The energy consumed per mass of apple will also be determined in this study, while our ultimate goal is to minimize the energy use and throughput mass per unit time.
Theory:
Drying involves, transient transfer of both mass and heat, making this process difficult to model and scale up, by using fundamental principles and assumptions, we can properly model it. In order to dry 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 within 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. From our mass transfer knowledge we know that 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:
N=-MdsAd(X-X*)dt( SEQ eq * MERGEFORMAT 1)
Assuming consistent drying conditions, t is time, Mds is the mass of the dry solid, A is the evaporation area, X is the dry moisture content of the solid, and X* is the equilibrium moisture content in other words the point at which the solid stops drying. The flux N, has units of kg /m2 h. When the area is unknown, then N is considered 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
Thickness, shape and arrangement of the apple slices
Relative humidity of the air; this is important to determine the amount of moisture in the drying air
Air temperature
Air velocity (constant in our case)
Case hardening, while essentially greatly slows down the drying process because at high temperatures and low humidity, water is removed from the surface faster than it can diffuse from within the apple slice causing a hard layer on the surface and preventing the drying of the material’s inside CITATION Wil04 l 1033 (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 changes in color and texture of the apple slice. By plotting N vs. X we get the drying-rate curve. The time required for drying to reduce the solid to desired moisture content is calculated using equation 2:
t=-X1X2MdsAdXN( SEQ eq * MERGEFORMAT 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 Fresh slice-Mass of Dried sliceMass of Fresh slice*100%( SEQ eq * MERGEFORMAT 3)
Hot air drying is energy-hungry because heat transfer from air to the apple slices is experimentally inefficient and a large portion of the energy is lost to the surroundings. So, a proper assessment of the dryer performance, by calculating energy consumption, is important since convective dryers account for 85% of all industrial dryers CITATION TKu12 l 1033 (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:
Q=N?Hv( SEQ eq * MERGEFORMAT 4)
where ?Hv is the latent heat of vaporization for water, and N is the evaporation rate of water out of the apple slice.
Equally as important, is to calculate the energy consumption of our system. The energy balance on the heat exchangers can be represented by:
Q=W?Hs( SEQ eq * MERGEFORMAT 5)
where ?Hv is the latent heat of steam and W is the mass flow rate of steam through the heat exchangers, which can be calculated by:
W=?V( SEQ eq * MERGEFORMAT 6)
Where ? is the density of water, and V is is the volumetric flow rate of the condensate.
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 V we will collect the volume of water condensate from the heat exchanger(s) and record the collection time.
