Biodiesel From Algae Feedstocks 2

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Determining the Energy Requirement for Drying and Optimizing the Wet

Biomass Volume to Solvent Ratio in One Step Transesterification

Mallory Ho, Probir Das and Jeff Obbard

National University of Singapore

ABSTRACT

Biodiesel has become more attractive recently because of its environmental benefits and the fact

that it is made from renewable resources. Microalgae are considered as the sustainable source ofbiodiesel. The energy used in drying the wet biomass after harvesting from the culture, however, is

a hurdle to commercialization of the product. We have studied the optimum energy requirement in

converting wet biomass into biodiesel. While drying the wet algae biomass, the thickness of the

algae paste should be minimum. For higher thickness heat cannot reach the interior of the paste and

heat is wasted. The most commonly used method is transesterification. Since drying of wet algaeconsumes more energy, a method of one step transesterification process was developed to convert

wet biomass into biodiesel. In this study, the optimum biomass to solvent ratio for the one steptransesterification was determined. The optimum biomass to solvent ratio for the one step

transesterification was found as 1:20.

INTRODUCTION

With the rapidly depleting supply of fossil fuel, increasing oil prices and growing environmentalconcerns, there has been an increased interest in biodiesel. Biodiesel, an alternative diesel fuel, is

made from renewable biological sources. It is biodegradable and non-toxic, has low emission

profiles and so, is environmentally beneficial. Studies have shown, microalgae have the potential to

produce fuel at the lowest footprint. There are mainly three primary ways to make biodiesel;microemulsions, thermal cracking (pyrolysis) and transesterification However, producing biodiesel

from microalgae in an energy and cost effective way is still a major challenge. After harvesting themicroalgae, the wet biomass needs to be dried for further processing. The most common method of

drying is by supplying heat. The required heat energy used in drying the biomass can be much

higher compared to the final energy produce by the biodiesel extracted from the biomass. The mostcommon method of extraction of the biodiesel is transesterification. The transesterification reaction

requires a catalyst in order to obtain reasonable conversion rates. It is also necessary to determine

the optimum conditions for the transesterification process.

Objectives:

1. Determine the heating requirement, i.e., the thickness of the paste and time, for drying the wetalgae2. The maximum biomass to solvent ratio for the one step transesterification process.


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MATERIALS AND METHODS

Algae species and cultivation process

In this study a locally isolated marine yellowish green microalgae, Nannochloropsis sp. was

used. Under optimum conditions, the strain can double its biomass in less than a day. Although thestrain accumulates low level of neutral lipid in phototrophic growth mode, the lipid content and

classes can be enhanced by adding appropriate organic substrate which makes the strain an ideal

candidate for biodiesel feedstock production. Seawater used to grow the algae was first filteredthrough progressively lower pore size filters and finally sterilized using UV light by a TMI filtration

system. 10 liter sterile culture ofNannochloropsis sp. was transferred into 200 liter plastic bags.

When the culture in the plastic bag was sufficiently dense (optical density @ 680nm wavelengthwas 1.2), 20 liter of culture was transferred into an outdoor raceway pond (20 cm depth and 250liter

capacity). Sunlight was the only light source for the photosynthesis that occurred in the raceway

ponds. The depth of the culture, as increased by rainfall, was not adjusted later. Guillard f/2-Si was

used as the nutrients medium for the culture. A submergible water pump was used to provide a

20cm/s liquid flow in the raceway pond. No organic was supplemented throughout the entire cultureperiod. Once the culture reached in its stationery phase (after 12 days), it was kept there for 3

additional days to accumulate more lipid. Finally the culture density reached about 0.52gm/liter asdry weight.

Harvesting & biomass processing techniqueThe algae biomass was harvested using ferric chloride as coagulant and air sparging assisted

coagulation flocculation (ASACF) process, a process developed in our lab, which is a very simple

and robust technique consuming less than 20 times energy of any reported value. Details of this

process can be found elsewhere. After decanting the top layer (95% by volume), the bottom partwas centrifuged to reduce water content further. At this stage, the biomass was in paste form.

Drying the biomass in the ovenIn order to estimate the heat necessary to dry the wet algae, the algae paste was kept in the oven

at 800C and the time required to obtain complete dryness was recorded. The algae paste was spread

uniformly on pre-weighted aluminum cups to different thickness (5, 10 and 15 mm). Such thicknesswas chosen to spread known amount of algae paste (5, 10 and 15 gm) (W1). At different time

intervals (30, 60, 90, 120 and 150 minutes), aluminum cups were taken out from the oven and

placed in a desicator until it cooled to room temperature. Next the total weight (dried algae and thealuminum cup) was taken immediately (W2).


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At each measured time, loss in moisture content of the biomass was calculated by the following

equation:

% loss in moisture content =(1)

Where,

IMC = initial moisture content.

The initial moisture content (IMC) was calculated by keeping a known amount of thin algae pasteon an aluminum cup in oven at 80

0C and allowing it to complete dryness. At the same time the loss

in algae paste weight was calculated before & after of the freeze drying.

Quantifying the iron content in the biomass

Details of the iron extraction technique from algae biomass was reported elsewhere. 2gm of

algae paste was taken into 50ml of centrifuge tube and a 32.5% concentrated HNO 3(HNO3:H2O=1:1) solution was added into the tubes. Next the mixture was homogenized for 2

minutes using Hiedolf homogenizer. After centrifuging the solution top layer was collected and

filtered using a 0.22m filter. The filtrate was then diluted and analyzed for iron using Optima DV-

3000 Inductively Coupled Plasma Optical Emission Spectrophotometer (ICP-OES). Finally theamount of iron was expressed in terms of percentage of dry weight.

Processing biomass for FAME synthesisTotal lipid was extracted using chloroform and methanol (2:1) solvent mixture according to

Folch method. Biomass harvested using centrifuge was used for total lipid quantification. For

FAME synthesis and analysis the following One Step Transesterification was used.

Approximatelty 100mg of biomass was added into the vial. Transesterification reaction solution(TRS) was prepared using solvent(s) and catalyst. A transesterification reaction solution was

prepared by mixing H2SO4 in methanol at a ratio 1:10. 4 ml of freshly prepared transesterificationreaction solution was added into the vial and was immediately sealed with a septum and aluminum

cap. Next the vials were kept in ultrasonic bath (100 watt, 300C) for 10 minutes. The vials were then

transferred into oven and kept there at 800C. After 2 hours the vials are removed from the oven and

allowed to cool at room temperature. Next the entire solution was transferred into a centrifuge tubeand immediately 1 ml of water was added. Next 3ml hexane was added in the tube to extract the

FAME. The tube was kept in a vortex and later centrifuged at 4000 rpm for 5 minutes and the top

layer was collected. The process was repeated one more time. Thus obtained extracted solution wasanalyzed for FAMEs using Agilent7900 Gas Chromatography fitted with Flame Ionization Detector

(GC-FID).


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RESULTS AND DISCUSSION

Energy requirement in dryingFrom the Figure 1, it can be concluded that lower the thickness of the algae paste, faster the

drying process. It took almost 40 minutes to dry a 5mm thickness algae paste (the paste contained

80% moisture which was calculated later). However, during the drying it was observed that, linearincrease in thickness didnt result in linear time requirement for drying. In fact, the time

requirement for complete dryness was even higher. The heat flux, which is responsible for heat

transfer and eventual drying, will face incremental loss as it goes deeper. A lesser thickness(


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Table 1: Heat and Surface area requirement for processing 1 ton of dry biomass

Thickness(mm)

Time taken to dry(>99% moisture)

(Minutes)

Surface area requirement toobtain 1 ton of dry biomass

per batch (m2)

5 40 200

10 100 100

15 >150 67

Optimizing the wet biomass volume to solvent ratio

Ma et al. (1999) reported that maximum conversion efficiency of oil to FAME depends on the

molar ratios of alcohol to oil and catalyst to alcohol. Theoretically for 1M FAME yield 1M alcohol

is necessary. Since the transesterification reaction is reversible, additional alcohol is used to forcethe reaction forward. For base catalyst reaction the molar ratio of 6:1 (alcohol to oil) is often used;

however for acid catalyzed reaction the ratio is higher i.e., 30:1 (Ma et al. 1999). In OSTP, such

ratios cannot be used for wet algae biomass. Additional solvent, i.e., methanol, is required to extractthe intracellular lipid from the biomass. While extracting the maximum lipid from the dried biomass

the ratio of biomass weight to solvent volume can be as high as 10mg: 8 ml (Folch 1959, Lews et

al, 2004). The ratio of the wet biomass to the volume of solvent varied from 50 mg: 4 ml to1000mg: 4ml. In this study the wet biomass contained 90% water (w/w). Hence the ratio of dry

biomass to volume of the solvent varied from 5mg: 4 ml to 100mg: 4 ml. Considering a 20% lipid

content, on a weight basis the ratio of alcohol to oil varied from 4000:1 to 4000:20 which is muchhigher than 30:1 used in acid catalysis. FAME yields for different biomass weight to solvent

volume ratios are given in Figure 2. Maximum FAME yield of (19% of dry biomass) was achievedfor biomass weight to solvent volume ratios up to 20mg: 4ml. For increasing the biomass content

(>20mg) to solvent volume (4ml), FAME conversion yield reduced significantly.

The results can be explained by the following: (1) Under the experimental conditions, the fixed

amount of solvent, i.e., 4 ml, can extract maximum amount of lipid up to biomass content of 20 mg.Prior to heating at 100

0C, the glass vials, containing the wet biomass and the alcohol, were kept in

sonication bath to extract the lipid. Sonication inside the glassware could have resulted in better

lipid extraction for higher biomass content (>20 mg) and thus provide higher FAME yield; (2) for afixed amount of solvent, higher amount of wet biomass will produce higher amount of moisture

content. As the catalyst content was also fixed, i.e., 0.4ml per 4 ml of solvent, higher moisture

content diluted the acid and could have reduced the catalytic activity. Moreover the algae biomasscontains proteins, carbohydrates and cell metabolites other than lipid. A portion of the catalystcould have reacted with these non-lipid fractions of the biomass. The higher these non-lipid

fractions in the system, more of the catalyst will be used up leaving less catalyst for FAME yield;

(3) On a weight basis 10% iron was attached in the biomass which could have competed for the


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catalyst. Thus more biomass introduced more iron in the system for a fixed amount of catalyst

which also could have reduced the FAME conversion yield for biomass content higher than 20 mg.

Figure 2: Effect of biomass to solvent ratio of FAME yield.

CONCLUSION

From the experiments, the more thinly the biomass is spread, the more effective the method of

drying by supplying heat. Thus by spreading it thinly, sun drying becomes a considerable option asit would not consume any energy input. The ratio of the wet biomass to the volume of solvent

varied from 50 mg: 4 ml to 1000mg: 4ml. In this study the wet biomass contained 90% water

(w/w). Hence the ratio of dry biomass to volume of the solvent varied from 5mg: 4 ml to 100mg: 4ml. The reasons for the wide range in ratios were also discussed above and if more studies were

carried out, taking into account these observations, perhaps a more specific ratio can be determined.


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Biodiesel From Algae Feedstocks 2