design and operation of a mixed sun-gas solar dryer … · most african countries are not...

18th International Drying Symposium (IDS 2012) Xiamen, China, 11-‐15 November 2012

DESIGN AND OPERATION OF A MIXED SUN-GAS SOLAR DRYER FOR MANGOES IN WEST AFRICA

C. Heilporn1, B. Haut1, A. Nonclerq2, L. Spreutels1

1Service TIPs, Université Libre de Bruxelles 50, Avenue F.D. Roosevelt, CP 165/67, 1050 Bruxelles, Belgium

Tel.:+32 2 6502918, E-mail: [email protected]

2Service LIST, Université Libre de Bruxelles 50, Avenue F.D. Roosevelt, CP 165/51, 1050 Bruxelles, Belgium

Tel.:+32 2 6503086, E-mail: [email protected] Abstract: Mango drying is used in West Africa as a preservation method. This paper presents a mixed solar-gas dryer for mangoes constructed in Mali. The gas is used as an additional feed when the solar flux is no more self-sufficient. This study presents the design of the dryer by balance equations, its operation and the analysis of different drying trials analyzed separately with quality and economic considerations. It shows that this dryer has a shorter drying time than the most popular gas dryer, Atesta. The dried mangoes quality is particularly improved in terms of taste and color. Keywords: solar drying, mango, design, food preservation, West Africa development

INTRODUCTION

In sub-Saharan Africa, 30% of the population regularly experiences chronic malnutrition (FAO, 2009). Most African countries are not self-sufficient in food and are dependent on imports to ensure the food security of their people, in spite of a farmer population often between 70 and 80% of the total population. Taking into account recent price spikes of foodstuffs on the world market, this situation is becoming intolerable for an increasingly large proportion of the world population (FAO, 2008).

One of the major causes of lack of food accessibility is the difficulty of conserving food, particularly in tropical regions where high temperatures and humidity accelerate deterioration. In Africa, almost 30% of all crops are lost during storage and it rises to about 50% for fruit and vegetables (FAO, 1989; Reusse, 2002). Improvement of food preservation can therefore play an important role in food security on the continent.

Food preservation methods, such as canning or freezing, practiced in more economically developed countries are inappropriate for sub-Saharan Africa, as they generally require high-energy consumption and advanced technology. Hence, other methods such as drying, salting or smoking are preferred. Drying is widely used in Africa because the temperature to which food has to be raised is quite low compared to that needed for canning. This allows to use sun as heat source and water as heat-carrying fluid that

requires simpler machinery and lower energy costs. Drying increase food shelf life (Ramaswamy and Marcotte, 2006) by slowing down the activity of the micro-organisms (Orsat et al., 2007).

Drying process development still represents a great scientific challenge in the food domain. One of the biggest limitations of drying is the high-energy consumption. Drying is responsible for between 10 and 20 % of the industrial energy consumption in the developed countries (Mujumdar, 2007). Moreover, fruits and vegetables drying is a complex operation to study mostly because coupled mass and heat transfers are implied inside a complex porous structure, which is changing during the drying process (Dissa, Desmorieux, & et al., 2008). Regarding food drying, not only the economical and energetic costs count; the final properties of the dried product must be taken into account because they are very sensitive to the operational conditions (temperature, humidity, gradients…).

In sub-Saharan Africa, drying is traditionally operated by direct exposure to the sun. The food is spread on the soil surface, which is usually covered in order to protect the food from contamination. Unfortunately, this method often causes degradation of the product. For instance, Trögera et al. (2007) found that traditionally dried onions and tomatoes were highly contaminated with microorganisms and sand (Trögera et al., 2007). Furthermore, the drying time is long and the final moisture content of the product is high. To prevent those issues, drying is


more and more operated indirectly by placing the product in an isolated chamber, protected from infection. The heat source can be gas, sun or sun and gas. In sub-Saharan Africa, most of the modern dryers are gas supplied (Rivier et al., 2009). The benefits of those dryers over solar ones are full year utilization and simpler machinery, hence lower purchase money. However, gas dryers miss considerable production savings, which can make solar dryers cheaper in the longer term.

In sub-Saharan Africa, mango is one of the most exported dried fruit. For instance, in Burkina Faso, the mango trees represent over 55 % of the fruit trees area. Around 50 % of the production is lost every year because market opportunities are missing (Rivier et al., 2009). Drying mangoes to conserve, transport and export them is a competitive solution compare to the loss of production observed.

The general objective of this paper is to present a dryer constructed in a cooperative in Koulikoro, Mali, with local materials and labor. This dryer can be used with solar energy only or with both solar and gas energies. It is used for mango drying and it aim to dry about 40 kg of fresh pulp per day.

This paper is composed of three main sections. Firstly, the design of the dryer by balance equations, the dryer itself and its operation are presented in the materials and methods section. This section is closed by the presentation of different drying trials that are used for the analysis of the performance of this dryer. Secondly, the different data collected during the drying trials are presented in the results section. Finally, the discussion section is devoted to the analysis of these drying trials but also to a quality and economic analysis of the dryer.

The global approach taken by our team and the work presented in this paper is in line with the new development strategies towards farming polices adopted by the World Bank as it is concerned with smallholders support, job generation in downstream farming activity and farmers organization reinforcement (World Bank, 2008).

MATERIALS AND METHODS

Presentation of the dryer

The dryer is divided in two coupled units: the heating and the drying units, presented in Figure 1.

The heating unit is composed of 15 solar collectors aimed to heat water. The materials used for the construction were chosen in terms of availability in Mali, utility and minimal cost. The external surface of the collectors is made of wood. A steel sheet is placed in the collectors, on the top of the wood and is painted in black to improve heat flow. The water-air exchanger is made of copper tubes and the window

used to close the solar collectors is chosen double-glazed, for developing a greenhouse effect in the system (Nonclerq et al., 2009).

The second unit is the drying unit, also called the drying chamber and is divided in three sections. A radiator is placed at the entrance of each of those sections. It is used to bring the air temperature in contact with the products as close as possible to a chosen reference drying temperature.

A 100 l tank is placed between the heating and the drying units. A pump is used to drive the water heated in the solar collectors to the tank and from the tank to the three radiators placed in the drying chamber. The tank provides a stock of heated water that is used to keep the drying chamber at a chosen drying temperature. Before its entrance in the radiators, the water can also be heated by a boiler, in case of low sunshine - e.g. at the end of the day or in case of bad weather.

Fig. 1. Schematic diagram of the mixed dryer

composed of 6 trays


Fig. 2. Dryer constructed in Koulikoro. Top side: drying chamber with different probes measuring temperatures and moisture contents. Bottom side:

solar collectors.

Several trays are placed in the drying chamber. They are used for the disposal of the mango slices. The trays structure is a wooden frame, which is woven with nylon coils, which are themselves covered by a thin net. Mango slices are placed on this net during the operation of the dryer.

Air is forced in the drying chamber by a fan placed at its top. The airflow is perpendicular to the trays where the mango slices are placed. The air exits the drying chamber loaded in moisture resulting from the evaporation of the water contained in the drying mango slices.

Three valves, placed at the entrance of each radiator, are used to regulate and modify the radiators water temperature in order to maintain the air temperature in contact with the food products very close to the chosen reference temperature.

Design through balance equations

In this section, three key inputs for the design of the dryer are presented. The inputs considered are the number of trays, the minimum airflow and the solar collectors surface.

Number of trays

The dryer needs to be able to dry a batch of maximum 40 kg of fresh mango pulp. The form of a tray is chosen to be square, with 0.9 m sides to ensure easy handling of the trays. From experimental tests, it is known that each tray can support a maximum of 5 kg of fresh pulp. Thus, 8 trays are necessary to dry the maximum amount of mango pulp placed in a dryer.

Minimum flow rate

The following mass balance equation on the dryer can be written:

(1

)

Where Tout is the temperature of the air leaving the drying unit (K), Qa,min is the minimum airflow rate on a dry basis that has to be used to evacuate the evaporated water from the mangoes (kg of dry air

s-1), Ysat(T) is the air moisture content at saturation at a temperature T (kg of water kg of dry air-1), Yatm is the atmospheric air moisture content (kg of water kg of dry air-1), Mma is the mass of fresh mango pulp introduced in the drying unit (kg of mango pulp), ts is the drying time (s), X0 is the initial moisture content of the mango pulp (kg of water kg of dry solid-1) and Xf is the final moisture content of the mango pulp (kg of water kg of dry solid-1).

The temperature of the air leaving the dryer, Tout, must be close to 60°C to obtain dried mangoes of a good quality (Desmorieux et al., 2008; Talla et al., 2001; Talla et al., 2005).

Ysat(Tout=60°C) is close to 150 g of water per kg of dry air. The drying time is limited to 8 hours, due to the local expectations.

If the initial moisture content of the mangoes is 5 kg of water per kg of dry solid and if their desired final moisture content is about 0.1 kg of water per kg of dry mass, Qa,min= 29 kg per hour is calculated with Equation 1 with Mma=40 kg and Yatm=0.010 kg of water per kg of dry air.

In order to ensure a good homogeneity of the drying, a security factor of 10 is taken on this calculated flow rate. Therefore, the airflow rate in the drying unit, Qa, is fixed to 290 kg of air per hour. Indeed, this ensures that the air exiting the drying unit has a moisture content that is lower than approximately 10% of its maximum value (as usually, Yatm is much smaller than Ysat(Tout=60°C)).

In conclusion, as the area of the trays is 0.81 m2, the air superficial velocity in the drying unit must be close to 10 centimeters per second in order to obtain good drying conditions. Solar collectors surface

An overall energy balance equation for the dryer is

(2)

Where cp,air is the heat capacity of air (Jkg-1K-1), Tatm is the ambient air temperature (K), Lk is the water latent heat of vaporization (J kg of water-1), Ωc is the solar collectors area (m2), Fs is the average solar flux (Wm-2) and η represents the efficiency of the solar collectors (-). Its value is estimated to 0.5.

During the mango season, the air ambient temperature is around 30°C and the average solar flux in Mali is about 600 W per m2. Height hours around zenith are considered for the evaluation of the average solar flow. The heat capacity of the air, cp,air, is about 1000 J per kg per K and the water latent heat of vaporization Lk is equal to 2550 kJ per kg of water. Therefore, Ωc= 18 m2 is calculated with Equation 2. 20 m2 of solar collectors are constructed.


Operation of the dryer

Several steps take place before the beginning of the dryer operation: -‐ evaluate the amount of mangoes required to

have the desired quantity of pulp -‐ peel the mangoes -‐ cut the mangoes in the desired shape -‐ weigh the mass of mango slices placed on each

tray before placing them in the drying chamber

During the drying process, the position of the radiators valves is manually adjusted, in order to maintain the temperature of the air leaving the radiators between 60 °C and 65 °C. For this purpose, digital temperature probes are used.

The drying is stopped is traditionally stopped after a good visual assessment of the mangoes quality.

Drying trials

In order to analyze the behavior of the dryer, three drying trials, with increasing amount of mango pulp placed in the drying unit, are realize. For each of these trials realized, a series of parameters are monitored during the drying process. The values of the following parameters are recorded: -‐ the mass of fresh mango pulp introduced in the

drying unit, Mma, measured with a standard balance (d=1g)

-‐ the final mass measured at the end of the drying process, measured with a standard balance (d=1g)

-‐ the drying unit inlet and outlet air temperatures, measured with a thermo-hygrometer probe Testo 400

-‐ the radiator temperature (the temperature of the air leaving the radiators) is measured with a Testo 905-T1 probe

-‐ the inlet (Yatm, kg of water kg of dry air-1) and outlet (Yout, kg of water kg of dry air-1) air moisture contents, measured with the thermo-hygrometer probe Testo 400. A drying trial is stopped when the inlet and outlet air moisture contents are measured equal to each other.

RESULTS

A summary of the trials is presented in Table 1, summarizing several key-drying parameters: Mma (kg) is the mass of fresh mango pulp introduced in the drying unit, Mf/s (kg) is the mass of dried mangoes weighted after the end of the drying trial, Mw (kg) is the mass of water that is evaporated from the mango slices during the drying process, Mass loss (-) is the mass of water evaporated during the drying process, expressed as a percentage of Mma, t (h) is the drying time experimentally measured, # of trays is the number of trays used, Trad (°C) is the average radiator temperature measured during the drying

process and Mp (kg) is the mass of fresh mango pulp spread on one tray.

Table 1: Summary of the drying trials

Drying trials 1 2 3 Mma (kg) 7.0 15.2 34.6 Mf/s (kg) 1.1 2.4 4.2 Mw (kg) 5.9 12.8 30.4 Mass loss (-) 0.84 0.84 0.88 t (h) 7.4 8.33 9.25 # of trays 3 6 8 Trad (°C) 65 60 65 Mp (kg) 2.3 2.5 4.3 Heat source S S S+G

The air superficial velocity u and the air flow rate in the drying chamber Qa are constant for all trials and are equal to 0.1 meter per second and 0.081 kg of air per second. The drying trials have been conducted with sun as heat source as much as possible.

In Figure 3, the drying rate defined as the mass of water evaporated from the mango slices in the drying unit per hour, divided by Mf/s, is presented for the three drying trials. This drying rate is calculated with the following equation:

(3)

Where Yatm and Yout and measured continuously during the drying trials.

Fig. 3. Evolution of the drying rates for different

initial masses of mango pulp is represented versus time

The drying rate observed for the three trials presented on Figure 3 is about one kg of water evaporated per kg of dry mass and per hour, which is a very good value. The maximum drying rate is observed for drying trial 3, which has the bigger initial mass of mango slices. It proves the good design of the dryer. The drying trials ends after one day of drying regardless the initial mass of mangoes introduced in the dryer.


DISCUSSION

Drying trials

Several drying trials have been performed in Koulikoro with an increasing mass of fresh mango pulp introduced in the dryer. The initial mass is increased progressively to reach the maximal quantity of mangoes that the dryer should be able to dry properly, 40 kg of mango pulp.

A first drying trial, drying trial 1, was performed with only a small amount of mango pulp, 7 kg, in order to make sure that the dryer worked properly. The radiator temperature reached 65°C, when at least 60°C are required, the mass loss was over 80% (expected for a good quality of the dried product) and the drying was completed in one day.

As it can be observed in Table 1, regardless the initial mass of mango pulp introduced in the dryer, the drying time never exceeded one day. The drying trial 3 has required an additional contribution of gas in addition to the solar energy because the drying time was longer than for the other trials. Furthermore, the large quantity of fresh pulp used in this trial (almost 35 kg) required a longer preparation time for peeling and slicing the mangoes and a slightly longer drying time. The beginning of the drying was slightly delayed in comparison with the other trials. For these reasons, some gas was used to complement the solar flux.

Good drying temperature and high mass losses, respectively 84, 84 and 88 %, were observed during the three trials. In addition, it is noted that all mangoes were dried homogeneously regardless the mass of fresh mango pulp introduced in the dryer.

Given the results of drying trial 3, which was performed with a mass of mango pulp close to the maximum quantity allowed by the dryer, it is shown that the dryer is designed properly.

Quality and economic analysis of the dryer

The aim of the developed dryer is to support agricultural development in sub-Saharan Africa by improving the processing chain. For that purpose, it is important to take care of the drying costs. The economical performances of the dryer presented in this article can be compared to those of an Atesta dryer the most popular mango dryer in sub-Saharan Africa (Rivier et al., 2009). This dryer works with natural convection and uses butane combustion as heating source. It can dry about 180 kg of mangoes using 12 liters of gas. In April 2010, the price of 12 liters of butane was 4000 CFA francs, which is the only operation cost. The specific cost is thus 22 CFA francs per kg of fresh mango pulp introduced in the dryer.

In the dryer constructed in Koulikoro, the quality of the dried mangoes is improved because the product is never placed in direct contact with the combustion gas. The taste and the color of the dried mangoes are significantly different than those obtained with an Atesta dryer, which contributes to increase the selling the prices.

In the case of the mixed sun-gas dryer presented in this paper, the operational costs are spread in electricity (260 W pump and 23 W fan) and in gas (3 last hours after sunshine, 1 liter) costs. In April 2010, the costs for those energies were 130 CFA francs per kWh and 2000 CFA francs per 6 liters of butane. If we consider a 10 h drying for 48 kg of mango pulp introduced in the dryer, the specific cost is 15 CFA francs per kg of fresh mango pulp introduced in the dryer. This is about 70 % of the Atesta drying cost. This difference is very interesting for sub-Saharan mango transformers and can be explained by the use of solar energy in an optimized way.

Furthermore, Nonclerq et al. showed that the emission of greenhouse gases is significantly reduced when using the solar dryer instead of a gas dryer. An equivalent of about 5000 liters of petrol is avoided each year. This also reduces the cost of the drying process.

CONCLUSION

In conclusion, this study proves that the dryer in Koulikoro was properly designed. The dryer gives very good drying conditions as long as the maximal quantity of mango pulp allowed, 40 kg, is not exceeded. An additional contribution of gas is sometimes needed when the drying does not start early enough in the day. It has also been evaluated that the use of the proposed dryer reduces significantly the operation costs, compared to the use of the Atesta dryer.

Moreover, the quality and taste of the mangoes is significantly improved in comparison with the dried mangoes produced in an Atesta dryer. In fact, the mangoes are not longer placed in direct contact with the combustion gases.

The emission of greenhouse gases is also significantly reduced which has an impact on the environment but also on the cost of the drying operation. The use of such a dryer could therefore increase the income received by the local cooperatives.

NOMENCLATURE cp,air heat capacity of air Jkg-1K-1 Fs average solar flux Wm2 Lk water latent heat of

vaporization J kg of water-1


Mass loss mass of water evaporated during the drying process

-

Mf/s mass of dried mangoes weighted after the end of the drying trial

kg

Mma mass of fresh mango pulp introduced in the drying unit

kg of mango pulp

Mp mass of fresh mango pulp spread on one tray

kg

Mw mass of water that evaporated from the mango slices during the drying process

kg

Qa airflow rate in the drying unit kg dry air s-1

Qa,min minimum airflow rate on a dry basis that has to be used to evacuate the evaporated water from the mangoes

kg of dry air s-1

t drying time experimentally measured

h

Tatm ambient air temperature K Tout temperature of the air leaving

the drying unit K

Trad average radiator temperature measured during the drying process

°C

ts drying time s u superficial velocity of the air ms-1 X0 initial moisture content of the

mango pulp kg of water kg of dry solid-1

Xf final moisture content of the mango pulp

kg of water kg of dry solid-1

Yatm atmospheric air moisture content

kg of water kg of dry air-1

Yout air moisture content after the last tray


Ysat(T) air moisture content at saturation at temperature T


Ωc solar collectors area m2

η efficiency of the solar collectors -

ACKNOWLEDGEMENTS

This study was led in close relationship with the Regional Chamber of Agriculture of Koulikoro,

Mali, in order to share knowledge from both countries. In this way, local partners have had the opportunity to take part in each phase of the project and are better able to use some techniques traditionally employed in more economically developed countries.

Laurent Spreutels and Caroline acknowledge financial support from the Fonds de la Recherche pour l’Industrie et l’Agriculture (FRIA, Belgium).

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design and operation of a mixed sun-gas solar dryer … · most african countries are not...

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