Growth and Oil Extraction from Chlorella vulgaris: A Techno

Article pubs.acs.org/IECR

Growth and Oil Extraction from Chlorella vulgaris: A TechnoEconomic and Environmental Assessment Juan J. Jaramillo, Javier M. Naranjo, and Carlos A. Cardona* Instituto de Biotecnología y Agroindustria, Departamento de Ingeniería Química, Universidad Nacional de Colombia, Sede Manizales, Carrera 27 No. 64-60, Manizales, Colombia ABSTRACT: In this work, the technical, economic, and environmental viability of the growth, harvest, drying, and extraction of oil from Chlorella vulgaris was evaluated. A flue stream from a rice husk processing plant was taken as the substrate for microalgae growth and the production of 1 ton/h of microalgae oil. The mass and energy balances were calculated using Aspen Plus Software. The economic assessment was developed using Aspen Process Economic Analyzer Software. The environmental impact evaluation was carried out using the waste reduction algorithm (WAR). The yields of the process were 0.37 kg of oil/kg of dry microalgae and 0.63 kg of cake/kg of dry microalgae. The production costs were 0.56 USD/kg of oil and 0.33 USD/kg of cake. The potential environmental impact was 0.003 PEI/kg of product. The results indicate significant mitigation of smog formation potential because gases are used to generated value-added products.

1. INTRODUCTION Algae are considered renewable sources of vegetable oils and starch and at the same time a CO2 capturers.1 The biodiesel production from microalgae has been studied because of its high potential to obtain oils in high quantity. Some studies have reported oil yields between 2 and 550 times greater than conventional energy crops.2 In the past several years, biofuels production guaranteeing food security has been studied. Many works (e.g., refs 2−15) have addressed biofuels production from algae biomass. However, these assessments focused on a particular stage of the process (cultivation, extraction, and/or transformation) or specific separate aspects (e.g., energy and environmental performance).16 An overall technical and economic analysis has not been carried out for this kind of system, namely, industrial production of oil from microalgae. Although microalgae oil is present at low concentration in the complete broth medium and its separation costs are higher than that for oil production from traditional crops because of water removal,17 a techno-economic and environmental assessment permits an analysis of the viability of these processes for future improvements and the detection of opportunities for decreasing production costs. In this work, the microalga Chlorella vulgaris was used to produce oil and cake. The microalgae were grown in a tubular photobioreactor, and centrifugation was used as the oil extraction technology. The results indicate that industrial operation might be economically and environmentally sustainable.

utilized to calculate the activity coefficients in the liquid phase, and the Hayden−O’Connell equation of state was used to model the vapor phase. Other references and databases were used for the calculation of oil properties,20−27 algal material, and other compounds used in the simulation.28 Mass and energy balances were calculated by simulation. Economic evaluations were performed using Aspen Process Economic Analyzer in the Colombian context (with an annual interest rate of 17% and a tax rate of 33%). A straight-line depreciation method was used over a 12-year period of analysis. For feedstock prices, the international reports from ICIS pricing were employed; operating charges such as operator and supervisor labor costs were defined for Colombia at 2.14 and 4.29 USD/h, respectively. Electricity, potable water, low and high steam pressure costs were 0.0304 USD/kWh, 1.25 USD/ m3, and 8.18 USD/ton, respectively. The environmental impact was assessed with the waste reduction (WAR) algorithm (developed by the U.S. Environmental Protection Agency) to estimate the potential environmental impact (PEI) generated in the process considering eight environmental impact categories: human toxicity potential by ingestion (HTPI), human toxicity potential by dermal and inhalation exposure (HTPE), terrestrial toxicity potential (TTP), aquatic toxicity potential (ATP), global warming potential (GWP), ozone depletion potential (ODP), photochemical oxidation potential (PCOP), and acidification potential (AP). The mass flow rate of each component in the process streams was multiplied by its chemical potential to determine its contribution to the potential environmental impact categories.29

2. METHODOLOGY Aspen Plus software (AspenTech: Cambridge, MA) was used to simulate the global process. The physicochemical properties were obtained from the National Institute of Standards of Technology (NIST)18,19 and the group-contribution method developed by Ceriani et al.20 at three different levels. The nonrandom two-liquid (NRTL) thermodynamic model was © 2012 American Chemical Society

Received: Revised: Accepted: Published: 10503

January 23, 2012 April 10, 2012 June 20, 2012 June 20, 2012 dx.doi.org/10.1021/ie300207x | Ind. Eng. Chem. Res. 2012, 51, 10503−10508

Industrial & Engineering Chemistry Research

Article

Figure 1. Cultivation and extraction flowsheet of metabolites of microalga Chlorella vulgaris.

Table 1. Mass/Volume Fractions Used in the Model

a

stream mass flow rate (kg/h) volume flow rate (m3/h)

flue gas 8621 7560.142

water 9500 9.562

nutrient 108.301 70.516

air 100000 91885.195

water tetradecanoic acid triglyceride palmitic acid triglyceride heptadecanoic acid triglyceride 9,12-octadecadienoic acid triglyceride 9-octadecanoic acid triglyceride octadecanoic acid triglyceride 10-nonadecenoic acid triglyceride 11-eicosenoic acid triglyceride eicosanoic acid triglyceride carbon dioxide nitrogen oxygen carbon monoxide carbon source hydrogen source nitrogen source oxygen source n-hexane hydrogen paste algae mass flow (kg/h)

0.038 0 0 0 0 0 0 0 0 0 0.232 0.73 0 0 0 0 0 0 0 0 0

1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0.505 0 0.37 0 0 0 0 0.124 0

0 0 0 0 0 0 0 0 0 0 0 0.767 0.233 0 0 0 0 0 0 0 0

oil-solv: microalga oil;

b

air-m: gases emitted;

hexane 12312.618 18.383

oil-solva 12831.766 19.166

mass fraction 0 0 0 504 ppm 0 0.005 0 0.003 0 0.007 0 0.023 0 0.001 0 139 ppm 0 162 ppm 0 135 ppm 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0.96 0 0 0 0

air-mb 100752.39 96853.929

fg-rc 6403.134 280.57

liquid 10132.238 10.533

0.006 0 0 0 0 0 0 0 0 0 0 0.761 0.232 0 0 0 0 0 0 0 421.339

0.001 0 0 0 0 0 0 0 0 0 0.054 0.944 0 0 0 0 0 0 0 0 0

0.853 0 0 0 0 0 0 0 0 0 0 0.014 0.133 0 0 0 0 0 0 0 0

c

fg-r: humidified air.

3. RESULTS Rice husk is a residue produced in the rice industry. Because of its composition, it is possible to develop a processing plant for producing value-added products such as synthesis gas, energy, and ash (considered as a product because of its potential in the cement industry for use as an additive or base for composites). In the processing plant, rice husk undergoes a gasification process. Then, the synthesis gases produced are burned in a

boiler to generate both high-pressure steam and combustion gases. The thermal energy produced is enough for the selfsufficient support of the process and can provide energy to other coupled processes. (For 1 kW of energy demanded by the process, the rice husk processing plant generates 15 kW, with a yield of 975.6 kW per kg/s of rice husk.) These features make coupling between a rice husk processing plant and an algae system an interesting alternative for an integrated process. In 10504

dx.doi.org/10.1021/ie300207x | Ind. Eng. Chem. Res. 2012, 51, 10503−10508


Article

this work, the products considered from a rice husk processing plant (975.6 kJ of energy and 1.6 kg of CO2 per kilogram of rice husk) are used and integrated into algae processing technology. The flowsheet of the process proposed for the algae system is shown in Figure 1, and the mass/volume fractions used in the model are listed in Table 1. This model was designed based on the criteria proposed by Mahalec et al.30 A combustion gas stream from a rice husk facility was purified to enrich the CO2 and fed to a photoreactor for algae growth. A train of compression and cooling stages was proposed for conditioning the combustion gases at the proper pressure to absorb carbon dioxide in water. The inlet pressure of the combustion gases at the end of the train of intercooling was 20 bar, as suggested by Mahalec et al.30 High pressure ensures proper separation of CO2 from the combustion gases. The thermal energy produced in this area will be used later to supply the energy required in drying of algae biomass. The amount of CO2 produced from the rice husk facility permits the production of almost 1 ton/h of microalgae oil as a base for calculating the biodiesel production process. The flow of carbonated water from the absorption process was fed with other nutrients for growing microalga Chlorella vulgaris in a photoreactor. The growing of microalgae is described by the equation

Table 2. Mass and Energy Balances of the Cultivation of Microalga Chlorella vulgaris feedstock

flow rate (kg/h)

combustion gases water nutrients air hexane products

8621 9500 108.30 100000 12312.62 flow rate (kg/h)

microalgae oil microalgae cake humidified air solvent recuperated gases emitted cell-free broth energy

347.38 593.11 100752.39 12312.62 6403.13 10132.24 flow rate (MJ/h)

oil-solv paste air-m oil-solv fg-r liquid stream QT −

exchange energy energy demand

3479.8 3009.6

Table 3. Economic Evaluation of the Cultivation and Extraction of Chlorella vulgaris Metabolites cost (USD/kg)

sunlight + 6CO2 + 4.75H 2O + nutrients → algae (C6H11.5O2.9N) + 6.295CO2

stream flue gas water nutrients air hexane stream

category feedstock utilities operation and maintenance costs operational charges indirect costs general and administrative costs total cost

(1)

The residence time in the photoreactor was 24 h, which is the minimum recommended time for Chlorella vulgaris based on its generation time, as discussed elsewhere.2,31−34 The generation time is the time necessary for the duplication of biomass, and a residence time of 24 h guaranteed the duplication of biomass. A biomass recycle was used for both process intensification and mixing improvements. This avoided black areas in the photobioreactor. Mahalec et al. 30 recommended using a recycle ratio of 0.8−0.12 for residence times in the reactor of about 2−24 h. In this work, a biomass recycle ratio of 0.12 was used for a residence time of 24 h. The fractional conversion of CO2 in the photoreactor was 99.2% (molar), as recommended by Molina et al.17 The emerging stream was subjected to centrifugation with the purpose of separating the broth from the biomass and dried to reduce the moisture content of the biomass to about 12% by weight. The energy requirements of the drying tunnel with air were supplied completely by the exchange energy in the train of intercooling. The dry biomass was carried to the solvent extraction unit, where hexane was used to extract the microalgae oils and produce a paste residue rich in starch, protein, and fiber. The mass and energy balances of the process are listed in Table 2. The calculated process yields were 0.37 kg of oil and 0.63 kg of cake per kilogram of microalgae biomass on a dry basis, with a net energy demand (total energy necessary in the process) of 3.7 MJ per kilogram of dry algal biomass and an energy exchange (energy that can be integrated in a net exchange of heat) of 3.2 MJ per kilogram of dry microalgae. These results demonstrate that, if energy integration is performed, the process can supply 86% of the total energy demand. The production costs of cake and microalgae oil are listed in Table 3. The economic yields were found to be 0.56 USD/kg of microalgae oil on a dry basis and 0.33 USD/kg of microalgae

biomass production (microalgal biomass dry basis total)

oil (microalgae oil dry basis)

cake (microalgae cake dry basis)

0.10 0.03 0.03

0.27 0.08 0.08

0.16 0.05 0.05

0.004

0.03

0.006

0.02 0.02

0.05 0.05

0.03 0.03

0.21

0.56

0.33

cake on a dry basis. Figure 2 shows a diagram with the percentage economic distribution of production costs for the cultivation and extraction scheme proposed in this scenario. The raw material cost in the production of biomass microalgae depends on several factors as follow: pretreatment of gases (9.52%), formulated growth medium (33%), and light costs and maintenance (57.14%). In the pretreatment of gases, the main cost is due to the train of compression and cooling stages proposed for conditioning the combustion gases at the proper pressure to absorb carbon dioxide in water. In the formulated medium, the cost is due to the prices of salts used as nutrient sources for microalgae [KH2PO4, MgSO4, (NH4)2SO4, NaCl, CaCl2, and peptone water]. In this case, it is important to find inexpensive nutrient sources (e.g., wastewater) to decrease the raw material costs. The light cost is due to the use of artificial light for microalgae growing in the continuous photobioreactor; in this case, light-emitting diodes of 1000 lx were used for optimum microalgae growing (recommended by Chisti2,34). Figure 3 shows an analysis of the environmental impact of the cultivation of microalga Chlorella vulgaris, excluding products in the waste stream. The total PEI mitigated in the process is beneficial and equal to −0.55 PEI/kg of product. 10505



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Figure 2. Cost distribution of Chlorella vulgaris cultivation.

Figure 3. Environmental impact analysis for the cultivation of microalga Chlorella vulgaris.

4. DISCUSSION From the perspective of the culture of microalgae, Chlorella vulgaris provides a huge panorama for the production of biofuels and other metabolites. Its oil extraction yield and cake calculated in this scenario was extremely close to those obtained by other researchers.2,4,6,7,33 The performance of this microalgae oil is very high. However, the cake biomass can play an important role because its composition is rich in carbohydrates and protein. These components can be processed to obtain higher-value metabolites such as bioethanol and to produce energy by cogeneration processes. The cost of

microalgae oil associated with the growth of Chlorella vulgaris (0.504 USD/L) could be competitive in the future compared to other oils such as Jatropha oil (0.325 USD/L), palm oil (0.426 USD/L), sebum oil (0.186 USD/L), and waste cooking oil (0.139 USD/L). Microalgae oil is more expensive than other common oils for biodiesel production (0.532 times more expensive), but the volume of production makes that the effective cost lower and makes the process more efficient in the context of the manufacturing productivity of biofuels in Colombia (see Table 4). The oil price would be reduced by the intensification of the cultivation and growth of biomass, 10506



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Table 4. Comparison between Palm and Microalgae for Biodiesel Production in Colombiaa yield of oil (ton/Ha) required area (Ha) created jobs

palm

microalgae

2.1 403684 80737

67.5 12533 2507

Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +57 6 8879300, ext. 50417. Fax: +57 6 8879300, ext. 50199. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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a

Oil production in Colombia to supply the biodiesel demand of 846000 tons.35

ACKNOWLEDGMENTS The authors thank the Office of Research Direction of National University for the financial support given to this work.

because the algae have a high photosynthetic capacity and a high duplication rate of cells in a short residence time with lower cultivation area compared to conventional energy crops. The production cost is highly influenced by the pretreatment processes for biomass and the extraction of metabolites of interest. The cost of harvesting, drying, and solvent extraction turns out to be 2.7 times greater than the cost of producing raw biomass. This is a consequence of the energy demand for drying. However, energy integration through the use of intercooling areas in the process resolves around two-thirds of the total energy needs, and if the energy produced at the rice husk processing plant is exploited in this scenario, the costs associated with energy would decrease. Figure 2 shows the production costs associated with the cultivation of microalgae for required metabolite extraction.This cost is strongly affected by feedstock price (50% of the total cost) because of the amount of solvent used and the nutrients required by the culture medium and microalgae adaptation. Moreover, the substrate is obtained from a combustive gas stream, and its cost is the result of transportation. The environmental impact analysis (see Figure 3) shows a huge mitigation potential of greenhouse gases and smogformation (PCOP), because of the transformation (by algae) of a combustive gas stream with a negative potential load into metabolites with industrial value. However, the PEI/kg of output products is increased by CO2 waste streams and the residual broth. If these streams were discharged into the ecosystem, they would increase the potential for both acid rain and soil acidification and add to the global warming effect (AP and GWP) and possible damage to human health and animal poisoning (HTPI and TTP).

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REFERENCES

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5. CONCLUSIONS The cultivation of microalgae has a huge advantages over other crops given its ability to assimilate waste streams (greenhouse gases). The intensive cultivation of algae is an atractive alternative for fossil fuel substitution and production of valueadded products. Microalgae are characterized by a high productivity and yield per unit area of cultivation, as well as ecosystem adaptability with sufficient nutrients and water availability. However, technology is not sufficient to ensure low costs at the oil separation stages from enormous quantities of water. This last step requires more research developments with rapid introduction into industry. In the context of Colombia, the possible implementation of algae farms to replace most agro-industrial crops as feedstocks for biodiesel and bioethanol is an interesting possibility. When microalgae and rice husk facilities are integrated, the energy demand decreases, and the process could become even more profitable from the point of view of production costs of microalgae oil and cake. Although some authors36 indicate a lack of data, this work could be considered as a first approximation to industrial operation. 10507



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Growth and Oil Extraction from Chlorella vulgaris: A Techno

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