HOT-AIR CONVECTIVE DRYING OF POTATOES ESP2109 Design Project: Part B Group 2B Franco Lim Fang Jeng • Steven Zhang Shihua • Kriti Kakria

HOT-AIR CONVECTIVE DRYING OF POTATOES ESP2109 Design Project: Part B Group 2B Lim Fang Jeng, Zhang Shi Hua, Kriti Kakria National University of Singapore Engineering Science Programme

ABSTRACT In ESP2109 Design Project Part B, a potato dryer of the most feasible and economical way will be built to be able to 1000 kg of potatoes per day. Several designs have been revised, studied and modeled in COMSOL Multiphysics, followed by building of prototypes and theoretical and experimental results are obtained. With these results and improvement of our designs, the best design which has the highest feasibility and most cost-efficient in terms of material, time and labour was selected. This report describes the above-mentioned design process and presents the best result in detail. It starts off by introducing the background of this project and with a brief marketing research and quality evaluation. Subsequently theoretical analysis is presented with discussions of COMSOL models. After the modeling section, a scaled-down dryer is constructed which gives us experimental results to obtain diffusion coefficient and drying time is then estimated. 3 dryer models are discussed and evaluated. Finally, a guideline is provided for building an actual dryer in an industrial plant. Most importantly, the drying time and desired moisture content for potato to be in the best condition for storage are also being discussed.

INTRODUCTION Potatoes are popular agricultural products in every part of the world. They can be consumed as food or used as important industrial material. In 2007, a total of 321.69 million tons of potatoes are produced worldwide (Food and Agricultural Centre of United States, 2008). Hence, the industry of potato production is crucial and many researches such as designing a best dryer for potatoes in terms of quality is currently being extensively studied. As such, the dehydrated potatoes are often being sent for storage and for other commercial purposes. As a result, potatoes are often dried for storage and transportation purposes and subsequently made into other products. According to United States Department of Agriculture (United States Department of Agriculture, 2006), 2 commonly seen products are frozen French fries, potato chips and potato flour. Therefore, study of dried potatoes and design of an efficient dryer is crucial to this industry.

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OBJECTIVES As requested by investors, a potato dryer that dries up to 1000 kg potatoes per day is designed. In order to ensure a good market value, the dryer should be cost efficient, labour efficient and the product of drying should meet certain standards. In order to build such a dryer, modeling of designs is to be done by utilizing sophisticated computer modeling software such as

COMSOL Multiphysics. A scaled down version is then built to acquire experimental data and evaluate the performance. Finally, experimental results are presented along with guidelines to build the actual potato dryer. BACKGROUND As mentioned previously, potato is widely recognized as a remarkable industry that can penetrate into a new global market currently in the world. Hence, in this project, a dryer that fits into both global market as well as niche markets is to be built. In order to find an industry which needs dried potatoes more than any other industry, extensive research is conducted. In addition, marketing surveys and the quality criteria for dried potato samples such as nutrition values, moisture levels, etc are to be found out. Presently, a major portion of fresh potatoes are sent for storage and for later uses. However, the problem that we faced in storage of fresh potatoes is that the potatoes tend to accumulate reducing sugars, which will eventually cause degradation in the quality. According to Karuna D. Kulkarni et al, dehydrating of potatoes could help overcome the problem of sugar accumulation and reducing bulk for storage and transportation without much physiological and biochemical changes. Hence, dehydration of the potatoes is crucial and it is being done in this dryer design project. MATERIALS & PREPARATION The basic materials used in building the dryer consist of three types of material: mesh wire, aluminium foil and polycarbonate sheet. As for energy source, hot air blower will be the heat source for the dryer. Mesh wires are cut into small sheets and bent into the desired shape, in particular, cuboidal shape and cylindrical shape. Aluminium foil can be folded into a connector that integrates the air blower and the dryer. In addition, it can also act as the second insulating layout which encapsulates the mesh wire structures. Polycarbonate is used to construct the walls for dryer because its specific heat capacity (1.2 kJ/kg K). Compared with the specific heat capacity of aluminium (0.897 kJ/kg K), polycarbonate can withstand more heat, which indicates that polycarbonate is a better insulator than aluminium. However, polycarbonate sheets could not be bent into different shapes with the tools provided. As a result, it will be only be used to build an insulating with rectangular shape. To start designing a suitable dryer that fulfills the purpose of investors, a marketing survey is conducted to study the current markets, types of potatoes available in the market and most active potato industry that is present. MARKETING SURVEY AND QUALITY EVALUATION

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Current Market With the increase in global wheat and rice prices in the current market, potatoes are rediscovered as a nutritious crop (Reuters, 2008) and its market is recognized to be lucrative and demanding. With the nickname of “hidden treasure” by United Nations, potato is already the third most important food crop after the global food commodity, i.e. rice and wheat in the world.

Potato is also known for its great source of complex carbohydrates, a compound which acts as main energy provider in human bodies. According to Reuters (2008), the sales of bread in Baltic country of Latvia has dropped by 10-15% in January and February, 2008. At the same time the sales of potatoes increased by 20%. As a result, it can be observed that some consumers are beginning to shift their eating habits from rice or wheat to potatoes. According to statistics provided by Food and Agricultural Centre of United States(2008), Asia is the region at which potatoes were produced at the highest rate, as shown in the chart below.

Potato Production of the World (2007)

Millions Tonnes

Quantity Production 140 120 100 80 60 40 20 0

Chart 1- Potato Production of the World (2007)

Two commonly seen products in the currently markets are frozen French fries and potato chips are discussed in this section. Potato Chips Statistics show that in the U.S, potato chip industry is the current popular market up to year 2006 (United States Department of Agriculture, 2006). In Singapore, potato chips form the largest product category under snacks, which consists of 62% of the total snack market share. Today, more than 3.8 billion potatoes are used by Frito-Lay 1 to produce chips per annum. According to Food Innovation Online, the global market for potato chips has generated total revenue of 16.4 billion dollars in 2005. This generation of revenue had accounted for 35.5% of the total savory snacks market in that particular year (46.1 billion dollars).

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Frozen French Fries With fast food chains like McDonald's and Burger King being extremely popular with the present generation, frozen French fries are in great demand. Hence, this demand serves as the motivation for designing potato dryers in this project. In 2006, 129,469.00 tons of potatoes were used to make frozen French fries in the US. After studying today‟s market, a particular type of potatoes is to be chosen. Among the 1

A division of PepsiCo, Inc. which manufactures, markets and sells a variety of corn chips, potato chips and other snack foods.

varieties of potato which can be found in the market such as Rooster potato, King Edward Potato, Yukon gold potato, Russet potato, etc, potatoes with comparatively low moisture levels will be chosen as it can be used in the industry of frozen French fries and potato chips industry (British Potato Council). Due to the availability of potatoes in Singapore, Russet potatoes are chosen to carry out the experiments. Properties of Potato Chosen The quality of the dried product has to be taken into consideration. As a result, several qualitative evaluation and properties of the potato such as nutrition content, shrinkage of sample, effect of drying on its structure are investigated and observed before and after the drying process. Maintaining the quality of the sample after drying process enables us to secure the economical value of our dryer and our products. With this information, we can evaluate the quality of our dryers. 

Nutrition (Vitamin C)

The dried potatoes are not desirable to have so much loss in its nutritional value. Once the nutritional value decreases, the economic value of the potatoes will inevitably fall. A rough qualitative estimation of the Vitamin C content in the potato can be measure by using C-strips to monitor the nutrition content of the potatoes before and after drying. C-STRIPS are chemically treated test papers (2.5 cm x 0.75 cm) which is used to measure the Vitamin C content of substances. The color of the paper will be changed from blue to white in the presence of Vitamin C in the sample. However, due to the unavailability of C-strips, this technique was not put into practice after the equipment is available. 

Shrinkage

Compromising on the physical aspect of the potato is undesirable. After doing a set of experiments, it was discovered that extremely thin wafers tend to shrink considerably. To avoid this, thicker potato slabs were chosen to be the raw material for the dryer.

After Drying



Taste

As far as drying is concerned, one of the most important aspects which should be take note is the taste of the final product. Manufacturers are concerned about the taste of the potatoes when they are being sold off to consumers as it will be related to their profit and gain. It can be found that the taste of the potatoes will become qualitatively not good when too high temperature is used. This aspect will be studied more extensively in the future if time allows.

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

Loss of natural pigments:

After drying of the potatoes, natural pigment will lose due to the expossure to air and high temperature. The major carotenoids, i.e., carotenes and oxycarotenoids are present in food which controls the pigment of the potatoes. As a countermeasure, chemical treatments should be

done in order to preserve the quality and more extensive researches should be carried out under this area. 

Microbiological Aspects

Removal of water increases the solute concentration of the potato system and thus reduces the availability of water for microorganisms to grow. Drying is an effective way to preserve the growth of microorganisms in the food, even though their numbers may be reduced and a proportion will be sublethally damaged (Jayaram, 1995). As a result, drying to a certain safe moisture level is needed in order to prevent the activity of microorganisms prevail in our dried samples. GOVERNING EQUATIONS AND MATHEMATICAL MODELS Heat conductivity is studied to monitor the drying processes in order to obtain the drying kinetics of the potatoes. Below is the equations used in analysis of drying action of potatoes and designing of dryers. Fourier’s Law of Heat Conduction q = −k

dT dx

Followed by the one-dimensional mathematical model, the following equation is obtained: dT d2 T = dt dx 2 The equation above has great significance for the research since it allows the analysis of the heat required to heat up the potato slices and to study the temperature distribution along the length of the potato sample. Heat Conduction Along a Hollow Cylindrical Wall The heat transfer inside the dryer also can be modeled with Fourier‟s Law of heat conduction. In the final design, a cylindrical wall is used. Hence, a study of the behavior of temperature difference in a cylindrical dryer is to be carried out. Consider a cylinder with outer radius b, inner radius a and length H, and with no generation of energy, the following equation is being utilized: 1d dT r r r dr 𝑑𝑟

=0

T r = C1 ln 𝑟 + 𝐶2

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If T1 and T2 are the temperature at both ends where T1 >T2, heat is released and the temperature will reach equilibrium in a steady state, which is given by the equation below: Q = T1 − T2

2πkH b ln a

where Ti (i=1,2)= Temperature at both ends DESIGN AND MODELLING Designs and models of dryers are done with COMSOL Multiphysics by inputting designs into the computer and producing a theoretical result. The process of designing is an iterative process. Initially a design is conceived and is built into a prototype. From the result of the first design, a second one is modeled and a prototype is constructed accordingly. By refining our design multiple times, the best design is achieved and a scaled down version is constructed according to the final design. When every new design proposed, a model of the design in COMSOL Multiphysics is done to find out the pros and cons of the design and how it can be improved. There are total of 3 designs considered, as shown below:   

Design 1 : Horizontal open ended cuboidal tube dryer Design 2 : Vertically upward open ended cylindrical tube dryer Design 3 : Encapsulate cuboidal shape polycarbonate sheet outside the dryer

Detailed discussions of each design are done below: (1) Design 1 : Horizontal open ended cuboidal tube dryer

Figure 1- Design 1: Horizontal Cuboidal Dryer

First of all, a design with controlled environment is to be constructed. Controlled environment is a crucial condition in designing a dryer as it decides how the end product will be. The interior of the dryer is shown in the Figure 2, a wire mesh is being put at the same height of the position of the mouth of the hot air blower as a platform for potato samples. The reason for having this setting is because the air flow is the most uniform in the front section of the hot air blower. By doing so, we can make the drying of the sample to be in maximum performance. This can be justified by the model done in COMSOL Multiphysics, with inlet velocity 10 ms-1. The surface plot and graph of velocity distribution along the surface of the platform is as shown below:

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Figure 2- Internal section of dryer

F i g u r e 3

Figure 4- COMSOL Modeling of Horizontal Dryer ( v=10 ms-1)

It can be observed, in Figure 4, that the air velocity distribution is uniform along the platform, with the average of 3.05 m s-1. As a result, one would normally presume that the longer the dryer, the more potato can be dried as it can be placed along the platform since due to the uniformity of airflow. However, this way is not feasible and does not give a reproducible result as the samples at the inlet will be dried faster than the sample further from the blower, as observed in the following model, the air velocity is high only at the front piece of sample and blocked the samples at the back will be blocked by the first sample which is the nearest to the blower.

Figure 5- Simulations with Potato Samples

Furthermore, the temperature distribution is not uniform along the platform. This will cause inconsistency of dried product and hence causing low reproducibility of experimental data. Hence, another design was proposed in order to overcome the problem arose.

With the principle of decreasing of the air density in high temperature and non-uniformity in drying action, the horizontal model is turned out to be not as good as it is expected. Hence, instead using horizontal setting, a vertical setting of 6cm x 6cm x 25cm is now considered. As shown in Figure 6, the cross section is changed from rectangular to circular. This change has been done based on the fact that the temperature distribution is faster for circular cross section. As shown in the Graph 1, we can conclude that the temperature of the drying vessel will reach the ambient temperature faster than the one with rectangular cross section with the same cross section area.

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(2) Design 2 : Vertically upward Open ended cylindrical tube dryer

Figure 6Design 2 prototype

Graph 1 – Temperature distribution of different shapes of cross section

To investigate the effect of the dryer to the sample after the changes were made, modeling by COMSOL Multiphysics was done to obtain the simulation of the airflow in the chamber, as shown in Figure 7. It can be observed that, from COMSOL, the air flow is uniform but it will only start to go uniform when it is approaching the end and the velocity is low when approaching the end of the dryer, i.e. 2.43 m s-1 if the initial velocity is 20 m s-1. As shown in Graph 2 below,

Figure 7COMSOL simulation of Cylindrical dryer

Graph 2-Velocity distribution along cross sections of dryer at different heights for Figure 6

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This shows that the platform for potato samples can only be built at a certain height where the velocity distribution is uniform so that the sample can be dried uniformly. This is a great waste of material and energy in a large scale setting. Hence, instead of controlling the height one way to resolve this problem is to add honeycomb near the blower mouth of the dryer. The purpose for adding the honeycomb is to distribute the velocity more uniformly throughout the vessel so that the design will not be too tall in height and the velocity can be controlled easily (Figure 8).

Figure 8- Addition of honeycomb at the bottom of dryer

Furthermore, the new design is easier to build in a large scale setting. It might show little

difference in the scaled down model, but it does provide a significant difference when the small scale design is being extended to large scale. (3) Design 3: Encapsulation of polycarbonate sheet outside the dryer but retaining the interior cylindrical surface of the dryer. After realizing the limitations, several improvements are done, third improved design is again built using different materials. Polycarbonate sheet is used to add to the outside of the cylindrical tube of the dryer as an insulating wall. This design is done since polycarbonate is a better insulator than aluminium foil. Aluminium sheet will cause some amount of heat loss resulting in lowering the efficiency of the dryer. Three holes on each side are created at 20 cm height of the polycarbonate instead of making a large outlet at the top of the drying tube. The purpose of changing the outlet style is based on the reason that the airflow will be distributed more evenly when air escape from the exhausts, which means that the air flow can be controlled by the position where the outlet is situated at. With other airflow regulator construction such as addition of honeycomb, the height of the dryer need not to be constructed too high, which will further decrease the material cost for large scale construction. The illustration is shown in the diagram below,

Figure 9Design 3: Polycarbonate Encapsulation

Same level but 2nd design gives much better velocity distribution and take shorter height to reach relatively lower velocity.

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Figure 10- COMSOL Comparison of improved interior dryer design

Furthermore, a more improved version of this design is to be done by constructing an inclined plane from the dryer to the wall of the dryer. By doing this, the velocity from the blower can be reduced based on the law of conservation of mass, under the assumption that air is treated

as an incompressible fluid. Q = ρ1 A1 v1 = ρ2 A2 v2 = ⋯

A2 = 2A1 , 1 v2 = v1 2

A1 , v1

As a result, in order to reduce the velocity of the air blown from the bottom by the factor of , cross section area is enlarged by 2 times. Consequently, the velocity which exits from the 2 larger cross section will be reduced. This design was not implemented in the prototype built. However, if more extensive research and time are given, a design of changing cross section area will be built. 1

EXPRIMENTS AND PROCEDURES Apart from COMSOL modeling and prototype designing, the effect of the dryer to the sample and estimation of certain parameters are important aspects which should not be ignored. Therefore, experiments were set up to put the designed dryer into practice. Several types of experiments were carried out to study the behaviors of potato samples inside the designed dryer. First of all, drying experiments were carried out to obtain dry mass, followed by the approximation of the temperature dependent diffusion coefficient and estimation of drying time. The parameters of the experiments are as stated below: (1) (2) (3) (4)

Sample size (Potato) : 2 x 2 x 0.2 cm3 Drying temperature: 50oC ~ 75oC Surrounding temperature: 19oC~22oC Initial velocity of hot air blower: 20 ms-1

Drying Experiment Procedures and Dry mass

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First of all, the potato samples were put at the centre of the cross section of inside the designed dryer at the height of 18 cm and drying experiments were carried out. Mesh wires were used to fix the position of the potato samples to prevent them from being blown away by the hot air blower and thus causes inaccuracy of the results. The sample was taken out and weighed in 5 minutes interval. The experiment procedure was repeated until the mass reaches a constant value. After the experiments were done, dried samples are put into the oven and dried in the atmosphere of 150oC for 10~20 minutes until a constant mass is reached. The experiments were done in triplicate. The dry mass is then obtained for that particular group of samples. The definition of dry mass is the mass of an object without the presence of water. To obtain the dry mass, samples should be dried under a certain level of temperature without any destruction of the internal structure of the sample. The dry mass is to be obtained so that the dry base moisture level, X of the sample can be obtained from the relation: Xdb =

mwet − mdry mdry

Through the moisture content obtained, drying curves can be obtained. With this, one can estimate the diffusion coefficient, D by utilizing the Arrhenius equation with the assumption that the diffusion coefficient is dependent only on temperature. Temperature Effect of temperature with the drying coefficient was studied and experiments were carried out. Different sets of value of temperatures (50oC to 75oC) were carried out in order to find the value of initial diffusion coefficient, D0, and constant, D1. With different sets of temperatures, different drying curves will be obtained. Hence, to obtain the constants stated above, one could utilize numerical methods, computer simulating software, etc. In this report, the value of the constants is obtained using COMSOL Multiphysics. Pressure The effect of vapor pressure towards the drying rate is desired to be studied in the experiments. The study begins with the hypothesis „The drying rate will be increasing in the presence of low pressure‟. In design 3, the pressure of the drying chamber is lowered when the honeycomb is added in. Hence, the two designs are to be put into drying experiments and the sample will be weighed in every 5 minutes. To investigate this effect, experimental conditions of constant surrounding temperature and samples with same size, same weight are to be used. Sample Size

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Effect of sample size to the drying kinetics is also required to be studied. This experiment is to be done to investigate what is the optimum size for potato drying which will give the best results and yet not degrading the quality of the final product. Different shapes and size of potatoes can be cut and put into the drying experiments. Mass of sample is to be weighed in every 5 minutes until it reaches a constant value. The hypothesis for this experiment is „The larger the surface area of the sample in contact with the air, the greater drying rate will be. However, this experiment was not carried out so it has only been discussed qualitatively.

RESULTS AND DISCUSSIONS Effect of Temperature to Sample After the experiments, the following graph is obtained.

Relative Moisture Content, Xavg/X0 (db)

1

Relative Moisture Content vs. Time in various temperatures

0.9 0.8

T=50C

0.7

T=60.7C

0.6

T=72.9C

0.5

Theoretical (T=52.9C)

0.4

Theoretical (T=60.7C)

0.3

Theoretical (T=72.9C)

0.2 0.1 0 0

10

20

30

40

50

60

70

80

Time (min)

Graph 3- Drying curves of various temperatures

It can be observed that the sample with higher temperature (T=72.9oC) reaches constant value at a higher rate. This is due to the phenomenon of dependent of diffusion coefficient with temperature. However, it can be observed in Table 1 that the final moisture content of the samples increases with the increase in temperature. This can be explained by the phenomenon of forming of a dry layer at the surface of the sample which blocks the drying action in the interior of the sample. Hence, it has come to conclusion that drying temperature of too high will not be used to dry the potatoes in the large scale dryer as it will affect the drying quality of the potatoes. Hence, it has come to agreement that the optimum temperature that should be dried is 70 oC because it will give a good result and the internal structure of food sample (potato) is not destroyed. Time (min) 65 70 75

Relative moisture content (Xavg/X0) T=50.3oC T=60.7oC T=72.9oC 0.040404 0.051724 0.090909 0.040404 0.043103 0.090909 0.040404 0.043103 0.090909

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Table 1-Final relative moisture moisture in different temperatures

Arrhenius Equation of Diffusion Coefficient For each set of data, one-dimensional diffusion models were done by COMSOL Multiphysics to obtain the theoretical result and compared to the experimental result. The

diffusion coefficient, D at temperature T is recorded. The relationship between D and T is obtained through the following methodology: D1

D = D0 e− T 𝐷 𝐷1 ln =− 𝐷0 𝑇

ln 𝐷 = ln 𝐷𝑜 −

𝐷1 𝑇

After that, graphs were of ln(D) versus 1/T was plotted and linear regression analysis was used to analyze the data. T D 1/T ln(D) 72.9 1.97E-09 0.00289 -20.0452 60.7 1.47E-09 0.002995 -20.338 50.3 1.22E-09 0.003092 -20.5244 -20 0.00285 -20.1

0.0029

0.00295

0.003

0.00305

0.0031

0.00315

ln(D) vs. 1/T

ln(D)

-20.2 ln(D) -20.3

Linear (ln(D))

-20.4 -20.5

y = -2379.6x - 13.182 R² = 0.9899

-20.6

1/T (K-1)

Graph 4- ln(D) vs. 1/T

From the linear regression from the data in Graph 4, the following constants are obtained: D0 = e−13.025 = 2.20452 x 10−6 [m2s-1]

D1 = 2435.8 [K-1]

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Prediction of Drying Time and Desired Moisture Content After obtaining the diffusion coefficient and its relationship with temperature, drying time and moisture content are decided in order to achieve the best and optimum product from the samples. When deciding the moisture content, a safety limit is to be considered. This is because if the final moisture content is too high, potato diseases caused by bacterial infection such as Potato brown disease caused by Ralstonia solanacearum, Pink rot disease caused by Phlyphthora erythroseptica, etc (Kaur & Mukerji, 2006) will infect the potatoes causing wide range of diseases, which will lead to the degradation of potato during storage. However, if the final moisture content is too low, the structure of the potato will be destroyed. According to

Pulley (1974), the best moisture level for good storage ability which will not cause discoloration and loss in vitamin C is of 7%. Hence, the final moisture level of the sample will be 7% in the designed dryer. Apart from moisture level, the drying temperature is also a crucial parameter. If the drying temperature is too low, the diffusion coefficient for the certain temperature will be small and hence will take a longer time to dry to the desired moisture level. However, the other extreme of temperature will cause the drying of surface faster than the interior of the sample. This will cause non-uniform drying, in other words, the sample will have been cooked at the surface before it is getting dried uniformly. In conclusion, the final moisture is chosen to be 7% and the drying temperature is 70oC.

Graph 5- Drying curves for 70oC of sample thickness 2 mm

Pressure

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According to Kurt M., the pressure of the drying chamber will affect drying properties of the food sample. Particularly, the drying rate of the food sample will increase in the presence of low pressure. Hence, from the results by Kurt, the dryer design with honeycomb is to be used because the pressure in the drying chamber is lower (compared to the same design but without honeycomb. The simulation is shown in the figure below:

Figure 11- Pressure Surface plot of Final dryer design

GUIDELINES FOR BUILDING A POTATO DRYING PLANT A day: 24 Hours, 10 Batches According to the experiment, it takes 100 minutes (1.67 hours) for a potato sheet of 2mm thickness to be dehydrated to 3% moisture under 70℃. A good estimation for the drying time of large scale dryer will be two hours per batch. In 24 hours, it’s possible to dry 10 batches of potatoes taking into account the interval between batches. Dimensions: 2.4 m (L) by 2.4 m (W) by 10 m(H)

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The ideal dimension for the large scale dryer would be 2.4 m (L) by 2.4 m (W) by 10 m(H), namely 40 times larger than the scaled down model. The dryer needs to be large enough to process 1000KG per day yet it needs to be as small as possible to save material cost and simplify the process of construction.

Figure 12- Dimesion of Large Scale dryer

To calculate the size of the dryer, some approximations and estimations are made. Typically a 5cm by 5cm potato sheet of 2mm thickness weighs 15g. In the experiment such a

sheet is dried in 100 minutes. Thus it is assumed the dryer runs at 2hrs/batch or more than 10 batches per day with 100KG per batch. 100KG is about 6600 sheets of potatoes. To accommodate all 6600 sheets, there are two ways: one is to build a huge dryer and the other is to have a few smaller ones. Having 3 dryers with the dimensions of 2.4m by 2.4m by 10m is ideal for several reasons. First of all, it is able to accommodate (2.4m * 100cm/m / 5cm)^2 = 2304 potato sheets, or more than 7000 sheets with 3 dryers. Secondly, the height of the dryer is roughly the height of a twostorey building -- any dryer higher than that will be costly to build. Having multiple dryers also helps saving energy which will be elaborated later in the report. Slider Mechanism In the scaled down version, potatoes inside the dryer are handled by removing the top lid of the dryer. When it comes to large scale dryer, lifting a giant lid will be very inconvenience. Consequently a slider mechanism is come up so that inserting and removing potato trays are made easier. When fresh potatoes are sliced, aligned on a tray and needs to be dried, simply slide the door open and place the tray inside the dryer. When potatoes are dehydrated, slide the door open, remove the tray, put in a new tray of potatoes and slide the door back closed. This mechanism significantly shortens the time needed to change potatoes when there are 10 batches per day. Pre-processing of Potatoes: Cutting of potatoes Potatoes need to be sliced into sheets before they are dried. The shape and thickness depends on the customer‟s requirement: French fry makers generally want their potatoes to be stick-shaped while chip manufacturers require thin potato sheets. There are a lot of commercial potato slicers and peelers available, but the prices of these machines are not available. Post processing of Potatoes: Packaging and Delivery

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Tight integration is crucial to food industries. In a warm and humid environment like the climate in Singapore, agricultural products, including dried potatoes, usually go Figure 13- Typical Industrial Potato slicer2 bad within a week. Since dehydrated potatoes are raw materials for making consumer products like potato chips, the best solution is to have the drying plant geographically integrated with chip makers, or have them directly delivered to French fry makers.

2

Picture Source: http://www.michigan.gov/mda/0,1607,7-125-1566_1733_22582-53809--,00.html

DESIGN HIGHLIGHTS Vertical Structure: Space Efficient It can be seen that our design is being changed from horizontal to vertical. The reason for this change is that potatoes can be dried more uniformly and in a larger region. Unlike horizontal setting, the potato will dry faster to the most exposed region which is the one nearest to the hot air blower whilst the other potatoes which is situated at the back of that potato slices will not be dried at the same rate as the first one. This causes inefficiency of the design and it will not be feasible in a large scale dryer. Vertical dryers have smaller base areas and require less space compared to horizontal ones. Usually the land used by a long horizontal dryer can accommodate more than three vertical ones.

Figure 14- Space Efficient Design

Uniform Air Velocity Distribution: Consistent Product According to the COMSOL design, the air velocity distribution in the vertical dryer appears to become uniform when the air reaches closer to the outlet of the drying chamber. As a result, potato slices will be placed at that respective height (0.18*40 m) to achieve a full crosssectional drying action. A simple illustration is shown in Figure 13:

Flexibility of Multiple Dryers: Save Energy, Save Cost, Prevent Disasters Figure 15- Illustration of Consistent Product

In this age of energy crisis, saving energy has become the priority for many design processes. In fact, the cost of energy for drying 1000kg potatoes can be more expensive than the potatoes themselves. Here‟s the calculation behind this statement:

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The power of the hot air blower used for the scaled version is rated at 1600W. Since the actual dryer will be 40X larger, it can be assumed the power consumption is 40 times as much, i.e. 1600W x 40 = 64kW. Using the prevailing electricity price in Singapore (S$0.3 per 1kWH), a single dryer consumes up to S$460. In contrast, 1000KG rooster potatoes cost around S$700 according to The Irish Farmers' Association (2007). Although the assumption made above assumes linearity in cost vs. size, it is sufficient to show that the cost of energy is significant compared to the cost of raw material (potatoes). Sometimes the potato supply fluctuates. If the dryer‟s maximum capacity is 1000KG, typically it will be supplied with 900KG per day. If, there is a slight shortage of potato supplies and only 700KG is available, running a 1000KG dryer will clearly be inefficient and will waste a lot of energy.

By having multiple dryers (3 in this particular case), energy can be used more efficiently. When there are only 700KG potatoes, one out of three dryers can be shut down which translate into 30% less energy consumption. In addition, having multiple dryers ensure the uptime of the drying plant: when one dryer breaks down, it‟s possible to repair/replace this particular dryer while keep the other dryers running. In the case of having only one dryer in a plant and the dryer breaks down, the consequence will be disastrous: if the dryer is down for a single day, 2000KG fresh potatoes are waiting to be processed and 1000KG of them are not dried in time which increases the chances for potatoes to go bad. Simple Structure and Simple Maintenance There are no complex components such as rotating mechanical parts or delicate electrical circuits used inside the dryer. This means the dryer can be easily constructed, especially in rural areas where industrial materials are not readily available and workers do not have much engineering knowledge. A simple design also reduces the difficulty in maintenance. CONCLUSIONS At the beginning of this design project, various studies of potatoes were conducted and market survey was carried out. It was found out that the potato market was gigantic and extensive demand had been increasing in the past few decades. Huge potential for the market justifies the need for a dedicated potato dryer that dries up to 1000KG a day. In order to design such a dryer, many experiments were performed to obtain parameters of potato, the air flow, the temperature for controlled environment, etc. These parameters serve as input for subsequent COMSOL Multiphysics modeling and simulation which is an important step in this design project. Designing a dryer is an iteration process. An initial design is proposed and is modeled in COMSOL. From the simulation, part of the design is either refined or improved resulting in a second design. The second design in turn evolves into a third one, and so on. With COMSOL, it‟s possible to visualize the air velocity, pressure field, etc. which greatly facilitates the design process.

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Eventually, the best design is achieved. According to the design, a scaled down version of the dryer is constructed and guidelines for building and operating the actual large scale dryer is also provided in this report. Last but not least, this report discusses the highlight of the design for drying potatoes.

REFERENCES British Potato Council. (n.d.). Retrieved October 12, 2008, from Potato Varieties: http://www.britishpotatoes.co.uk/potato-varieties Food and Agricultural Centre of United States. (2008). Retrieved October 27, 2008, from Potato world: http://www.potato2008.org/en/world/index.html Food Innovation Online. (n.d.). Retrieved October 23, 2008, from Savory Snacks: Global Industry Guide: http://www.potatopro.com/Pr/E-shot/Savory%20Snacks%20Global%20Industry%20Guide.aspx Jayaram, D. (1995). Drying of Fruits and Vegetables. In A. Majumdar, Handbook of Industrial Drying Volume 1 (pp. 669-686). New York: Marcel Dekker, Inc. Karuna D. Kulkarni, N. G. (n.d.). Production and use of raw potato flour in Mauritian traditional foods. Retrieved October 23, 2008, from United Nations Univeristy: http://www.unu.edu/Unupress/food/8F172e/8F172E0d.htm Kaur, S., & Mukerji, K. (2006). Potato Diseases and their Management. In C. Chatterjee, & B. Dube, Fruit and Vegetable Diseases (Volume 1). Springer Netherlands. Kurt M., W. Analysis of Mass Transfer Mechanisms during Drying of Extruded Durum Semolina. West Lafayette, USA. Pulley. (1974). Patent No. 3800047. United StTates. Reuters. (2008, April 15). As other staples soar, potatoes break new ground. Retrieved October 17, 2008, from Reuters: http://www.reuters.com/article/newsOne/idUSN0830529220080415?sp=true The Irish Farmers' Association. (2007, September 24). Potato Market Prices Reported to IFA. Retrieved November 3, 2008, from Potato Market Prices Reported to IFA: http://www.ifa.ie/

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United States Department of Agriculture. (2006). Utilization of U.S. potatoes, 1959-2006. Retrieved from United States Department of Agriculture: http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1235