microalgal biomass drying by a simple solar device

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MICROALGAL BIOMASS DRYING BY A SIMPLE SOLAR DEVICE∗ a

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J. PRAKASH , B. PUSHPARAJ , P. CARLOZZI , G. TORZILLO , E. MONTAINI & R. MATERASSI a

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Department of Physics, Ramjas College, University of Delhi, Delhi, 110009, India

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Centro di Studio dei Microrganismi Autotrofi C. N. R. Piazzale delle Cascine-27, Firenze, 50144, Italy Published online: 25 Apr 2007.

To cite this article: J. PRAKASH , B. PUSHPARAJ , P. CARLOZZI , G. TORZILLO , E. MONTAINI & R. MATERASSI (1997): MICROALGAL BIOMASS DRYING BY A SIMPLE SOLAR DEVICE∗, International Journal of Solar Energy, 18:4, 303-311 To link to this article: http://dx.doi.org/10.1080/01425919708914325

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Inr. l Solor Energy, 1991. Vol 18. pp 303-311 Reprink smilablc dirrcrly from the publisher Photocopyingpermittcd by lcenreonly

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MICROALGAL BIOMASS DRYING BY A SIMPLE SOLAR DEVICE* J. P R A K A S H ~ ! B. ~ , PUSHPARAJ~,P. CARLOZZI, G . TORZILLO~,E. MONTAINI and R. MATERASSI~ aDepartrnent of Physics, Rarnjas College, University of Delhi, Delhi-110009, India, bCentro di Studio dei Microrganisrni Autotrofi C. A! R. Piazzaledelle Cascine-27 50144-Firenze, Italy (Received19 Ocrober 1995; Infinalform 26 January 1996) An inexpensive and simple solar device was constructed and monitored for drying microalgal biomass with 90% moisture content. The drier is capable of producing about 140 g dry biomass per sq.m ofcollector area in 3-5 hours.The dried biomass contains less than 10% moisture and is biologically of good quality. The experiments are done with two microalgae - Spirulina and

Scenedesmus. Keywords: Solar drier; Solar energy; Spirulina; Scenedesmus; Microalgae; Algal biomass

INTRODUCTION It is well known that the efficiency of bioconversion of solar energy in microalgal cultivation is at least an order higher as compared to that in conventional agriculture. But the economic viability of microalgal cultivation for harnessing solar energy under the present state ofart is not without doubt. The major factor influencing the cost of marketable algal biomass are low productivity rate, the high costs of input nutrients, and the cost of maintaining optimal ambient conditions.The economics of biomass utilization is made even more unfavourable by the high cost of harvesting. It is estimated (Becker, 1980; Richmond, The work was supported by a grant from ICTP Programme forTraining and Research in Italian Laboratories - 1991,Trieste, Italy. t Corresponding author.


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1987) that centrifugation and drying alone account for 25-30% of the total production cost. However, there seems to be a possibility of reducing drying cost if a suitable technology is employed. There have been five main techniques in use for algal biomass drying. These are: (i) spray fed electrically heated single drum drying; (ii) steam heated double drum drying; (iii) low pressure shelf drying; (iv) spray drying; and (v) sun drying. Most of the workers employ drum drying due to convenience and dependability in spite of its high capital and operational cost. Sun dryingof algae has been practised by local populations of African countries for making dry cakes called "Dihe" (Ciferri, 1983) and by Maya tribes in Mexico for making "Tecuitlatl" (Furst, 1981) from ancient times. In the recent past some workers (Lincoln, 1980; Venkataraman et al., 1980) have used sun drying to varying degree of success. It is difficult to maintain the quality of the end product suitable for human consumption with traditional open sun drying methods. Besides other factors, the slow drying rate due to low temperature is the main cause of biomass degradation and consequential rise in bacterial count. But in a closed solar device high temperature and thereby high rate of drying may produce a good quality dry biomass. The cost of drying algal biomass by solar energy is much less as compared to any of the other methods. In the present paper we report the performance studies of a simple solar biomass drier constructed by us for drying algal and cyanobacterial biomass. The studies were also made to select the material of the containers used for drying the slurry in the solar device. This design of the solar drier has the characteristics oflow capital and operational costs, easy fabrication, high chamber air temperature and good ventilation for moisture removal.

DESCRIPTION OF THE SOLAR DRIER The device is broadly based on the cabinet type design for solar drying and is schematically shown in Fig. 1. A general view of the drying device is also shown in Fig. 2. It uses both direct drying as well as indirect drying. The device consists of a 1.8 m2 flat d a t e collector area. This collector is assembled in the laboratory using a 2cm thick polyurethane with its top coated with a black sheet (commercially called Stiferite in Italy and is available at 5-6 US $ per sq.rn). The top black sheet serves as a black absorbing surface for solar radiation while the rest of the polyurethane material serves as an insulator to reduce thermal losses from the bottom. The polyurethane is covered by a two layer plexiglass panel (called Alveoler sheet and is available at 15 US $ per sq.m) having channels (1.2cm x 3cm) along the length for air flow. A

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MICROALGAL BIOMASS DRYING

FIGURE 1 A schematic representationof thesolar drier.

2cm spacing between the black absorbing surface of the thermocole and the glazing is maintained for air flow. The air passes through the channels in the two layers of the glazing and between the black surface and the glazing. The flat plate collector thus acts like a two pass solar air heater. The cold air enters at the inlet as shown in the figure and passes through the solar panel collecting thermal energy both from the black absorbing surface and the lower plexiglass layer of the glazing and this hot air enters at the bottom of a drying chamber. The flat plate collector is oriented towards south and is inclined at an angle of 15" from the horizontal. This inclination is slightly less than the optimal value for the maximum solar gain in the summer months. This slight variation from the optimum slope does not affect the solar radiation input in summer months as the geometric factor (the ratio of beam radiation on the tilted surface to that on a horizontal surface) for inclinations within 10" of the optimal value at any time of the day during summer is nearly equal to one. However, this inclination is sufficient to create a pressure head good enough for the swift movement of air from the collector input to collector output. The drying chamber is like a cabinet (1 m x 0.5 m x 1m) and its structure is made of 2.5 cm thickcompressed particle board. The south wall and the top are made from 5 mm thick glass to receive direct solar radiation. The walls on the east and the west are provided with doors for loading and unloading drier with biomass. The chamber has 6 shelves for keeping biomass slurry. These are made of aluminium L-sections and T-sections. The shelves




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are separated from each other by 10 cm distance. The shelves are covered with black plastic nets (pore size 1x 2 mm) for uniform distribution ofhot air inside the drying chamber.The channels in the shelves are designed to hold the trays carrying biomass slurry. On the top is provided a 30cm high chimney of 10 x 10 cm cross-section. A 10 cm diameter exhaust fan is fitted in the chimney. The aim of fixing the exhaust fan on the outlet side is to create a low pressure zone inside the cabinet for avoiding leakage of hot air from the structure of collector or that of the cabinet.The air leak points serve only as inlets for cold air and not as outlets for hot air. In the design both thermosyphon mode and forced convection mode were used for the movement of air by alternately operating the fan for few minutes and then switching it off for 15-20 minutes so as to keep sufficiently high temperature and low humidity in the drying chamber.

SELECTION OF DRYING CONTAINERS The choice of material oftray and the thickness ofthe slurry layer in these trays pose an interesting problem.This material should have the following qualities: (1) the dried biomass should be easily detachable from it; (2) it should not react chemically with the biomass; (3) it should not degrade at temperatures of 7080°C; (4) it should not emit fumes; (5) it should be cheap and light weight. We tried trays made of several materials like stainless steel, aluminium, plastic, glass, polyethelene, etc. We found that the biomass slurry sticks to the material of the tray after drying and needs a mechanical device to detach it from most of the trays except those of polyethelene. The process of collection of dried biomass thus becomes cumbersome and also the trays get scratches. The polyethelene trays also have the characteristics of low cost, light weight, no chemical reaction with the biomass and stable at drying temperature. These trays are commonly used for keeping and conserving eatables and are also reusable.

RESULTAND DISCUSSION The solar drier was monitored for several weeks in summer months at Florence in Central Italy type climate for drying biomasses of two micro algae Spirulina and Scenedesmus. Figure 3 shows the drier temperature as a function of time under unloaded conditions for a typical clear day in summer. The drier temperature reaches 7 5 - 8 0 " ~on a clear day around 2 PM even though the ambient temperature at this hour is around 35°C.


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Time of the day (hr) FIGURE 3 Drier temperature under unloaded conditions as a function of time. -Drying chamber temperature, - - - Ambient temperature, -. -incident solar radiation.

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FIGURE 4 Mo~sturccontent of Splrullne b~omasskept In three d~ffercntshelves as a Funct~on ofdrymg tlme 0 bottom shelf, A mlddle shelf, 0 top shelf, Solar rad~at~on, 7 dr~cr temperature, ambient temperature.

Figure 4 shows the change in moisture content of Spirulina biomass kept in three different shelves.(top,middle and bottom) of the drier as a function of drying time. Figure 5 shows the same for Scen'edesmus biomass. The drying process was started at about I1 AM with biomass having about 90% moisture.



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Time (hr) FIGURE 5 Moisture content of Scenedesmus biomass kept in three different shelves as a hmction of drying time. 0 bottom shelf, A middle shelf, top shelf, W Solar radiation, +drier temperature, ambient temperature.

On a clear summer day the dried biomass with less than 10% moisture content was collected in 3-5 hours depending on the placement of the biomass in the drier.The drying chamber temperature was maintained around 60°C by regulating the ventilation. (The solar drier is capable of reaching a maximum chamber temperature of 80°C on a sunny day with low ventilation.) This produces a sufficiently high drying temperature and causes a quick removal of humidity from the chamber. The figures also show the solar radiation and the ambient temperature for the period of drying. It can be noticed that the biomass dried faster in the trays kept in the upper shelves.This can be attributed to a high contribution of direct sun drying and a relatively high temperature in the upper zone of the chamber. The pattern of drying process for both the algal biomass is similar. Further one may notice that the dryingcurves are similar to the typical curves (Prakash et al., 1988; Garg et al., 1991) obtained for solar drying of crops, timber, etc. Another problem observed was that once the top layer of the slurry is dried it forms a boundary almost impermeable for the water vapour from the lower layers to escape. In this way even though the temperature of the slurry was high enough for fast evaporation yet the moisture content of the biomass did not decrease but instead it degraded. We tried trays with slurry layer thicknesses varying from 1mm to 6 mm. We observed that the biomass in the trays carrying upto 3 mm thickslurry driedwell. In these trays the toplayer after drying cracked and slowly the thin sheet of dried biomass detached from the tray


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at the cracks, thereby facilitating the moisture from the lower layers to leave. In this way the moisture content from the bottom layers could also decrease leaving a thin sheet ofdried biomass that can be detached easily from the tray without the need of scrubbing. In the trays containing more than 3 mm thick biomass slurry the top layer after drying did not crack and the lower layers retained moisture enough to degrade the biomass at high drier temperature. Similar observations were also made by Venkataraman et al. (1980). We also observed that Scenedesmus poses less problems in drying as compared to Spirulina. However, Spirulina prewashed with deionized water'produced better quality dried biomass. Spirulinaslurry dried producing biomass in the form of thin small flakes whereas Scenedesmus slurry on drying produced large sized flakes. The average output of the drier was about 250-300 g per day. This corresponds to about 140glday-m2 of total collector area. This dry biomass production rate is in the same range as reported by Lincoln etal. (1980); McGarry etal. (1971) and Richmond et al. (1987). However, in our case the drying time was only about 3-5 hours and not the full day We further observed that the dried biomass did not emit unpleasant odour generally associated with sun drying process as the drying rate was accelerated so as to achieve the dried product in 3-5 hours at a medium temperature of 60°C. Acknowledgements

The authors acknowledge the technical assistance given by Messers Sacchi Angelo and Eduardo Pinzani in the construction of the solar drier. J. Prakash is indebted to Prof. G. Furlan, Head, ICTP Programme for Training and Research in Italian Laboratories for providing the financial support and to Prof. R. Materassi, Director, Centro Studio dei Microrganismi Autotrofi C. N. R., for providing the working facilities. This research was partially supported by National Research Council of Italy through special project RAISA, sub-project-4, paper no. 2618. References E.W. Becker and L.V.Venkataraman, "Production and processing of algae in pilot plant scale experiences of Indo-German project". Algae Biomass, Eds. G . Shelef and C.J. Soeder, Elsevier/North Holland Biomedical Press, p. 35 (1980). 0.Ciferri, "Spirulina, the Edible Microorganism". MicrobiologicalReviews 47, 551 (1983). P.P.T. Furst, "Alga Espirulina Alimento Prehispanico". Revisfa de la Subdireccion Cultural, No. 4,6-10 (1981) (in Spanish).



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H.P. Garg, J. Prakash and D.S. Hrishikesan, "Theoretical analysis of a solar timber drying system". ISESCongress Denver, Colorado, USA (1991). E.P. Lincoln and D.T. Hill, "An integrated microalgae system". AlgaeBiomass, Eds. G. Shelef and C.J. Soeder, Elsevier/North Holland Biomedical Press, p. 229 (1980) M.G. McGarry and C. Tongasame, "Water reclamation and algae harvesting". Journalof Water Pollurion ConrrolFederarion 43, 824 (197 1) . J. Prakash, H.P. Garg, G. Datta and A. Ray, "Performance prediction of a cabinet type solar drier". Solarand Wind Technology 5 , 289 (1988). A.Richmond, "Spirulina, Microalgal Biotechnology", Eds. M.A. Borowitzka and L.J. Borowitzka. Cambridge University Press. New York, p. 85 (1987). A. Richmond and W.C. Becker, "Technological aspects of mass cultivation - A general outline" in CRCHandbook of Microalgal Mass Culture, Ed. A. Richmond, CRC Press Inc. Florida (1986). L.V. Venkataraman, B.P. Nigam and P.K. Ramanathan, "Rural Oriented Fresh Water Cultivation and Production of Algae in India". Algae Biomass, Eds. G . Shelef and C.J. Soeder, Elsevier/North Holland Biomedical Press, p. 81 (1980).