Growth and Oil Extraction from Chlorella vulgaris: A Techno-Economic and Environmental AssessmentJuan 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 ofoil from Chlorella vulgaris was evaluated. A flue stream from a rice husk processing plant was taken as the substrate for microalgaegrowth and the production of 1 ton/h of microalgae oil. The mass and energy balances were calculated using Aspen PlusSoftware. The economic assessment was developed using Aspen Process Economic Analyzer Software. The environmentalimpact evaluation was carried out using the waste reduction algorithm (WAR). The yields of the process were 0.37 kg of oil/kg ofdry 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 ofcake. The potential environmental impact was 0.003 PEI/kg of product. The results indicate significant mitigation of smogformation potential because gases are used to generated value-added products.

1. INTRODUCTION

Algae are considered renewable sources of vegetable oils andstarch and at the same time a CO2 capturers.

1 The biodieselproduction from microalgae has been studied because of itshigh potential to obtain oils in high quantity. Some studies havereported oil yields between 2 and 550 times greater thanconventional energy crops.2

In the past several years, biofuels production guaranteeingfood 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 ofthe process (cultivation, extraction, and/or transformation) orspecific separate aspects (e.g., energy and environmentalperformance).16 An overall technical and economic analysishas 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 higherthan that for oil production from traditional crops because ofwater removal,17 a techno-economic and environmentalassessment permits an analysis of the viability of these processesfor future improvements and the detection of opportunities fordecreasing production costs.In this work, the microalga Chlorella vulgaris was used to

produce oil and cake. The microalgae were grown in a tubularphotobioreactor, and centrifugation was used as the oilextraction technology. The results indicate that industrialoperation might be economically and environmentally sustain-able.

2. METHODOLOGY

Aspen Plus software (AspenTech: Cambridge, MA) was usedto simulate the global process. The physicochemical propertieswere obtained from the National Institute of Standards ofTechnology (NIST)18,19 and the group-contribution methoddeveloped by Ceriani et al.20 at three different levels. Thenonrandom two-liquid (NRTL) thermodynamic model was

utilized to calculate the activity coefficients in the liquid phase,and the Hayden−O’Connell equation of state was used tomodel the vapor phase. Other references and databases wereused 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 ProcessEconomic Analyzer in the Colombian context (with an annualinterest rate of 17% and a tax rate of 33%). A straight-linedepreciation method was used over a 12-year period of analysis.For feedstock prices, the international reports from ICISpricing were employed; operating charges such as operator andsupervisor labor costs were defined for Colombia at 2.14 and4.29 USD/h, respectively. Electricity, potable water, low andhigh 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. Environ-mental Protection Agency) to estimate the potential environ-mental impact (PEI) generated in the process considering eightenvironmental impact categories: human toxicity potential byingestion (HTPI), human toxicity potential by dermal andinhalation exposure (HTPE), terrestrial toxicity potential(TTP), aquatic toxicity potential (ATP), global warmingpotential (GWP), ozone depletion potential (ODP), photo-chemical oxidation potential (PCOP), and acidificationpotential (AP). The mass flow rate of each component in theprocess streams was multiplied by its chemical potential todetermine its contribution to the potential environmentalimpact categories.29

Received: January 23, 2012Revised: April 10, 2012Accepted: June 20, 2012Published: June 20, 2012

Article

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

3. RESULTS

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

boiler to generate both high-pressure steam and combustiongases. The thermal energy produced is enough for the self-sufficient support of the process and can provide energy toother coupled processes. (For 1 kW of energy demanded by theprocess, the rice husk processing plant generates 15 kW, with ayield of 975.6 kW per kg/s of rice husk.) These features makecoupling between a rice husk processing plant and an algaesystem an interesting alternative for an integrated process. In

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

Table 1. Mass/Volume Fractions Used in the Model

streammass flow rate (kg/h)

volume flow rate (m3/h)

flue gas8621

7560.142

water95009.562

nutrient108.30170.516

air100000

91885.195

hexane12312.61818.383

oil-solva

12831.76619.166

air-mb

100752.3996853.929

fg-rc

6403.134280.57

liquid10132.23810.533

mass fractionwater 0.038 1 0 0 0 0 0.006 0.001 0.853tetradecanoic acid triglyceride 0 0 0 0 0 504 ppm 0 0 0palmitic acid triglyceride 0 0 0 0 0 0.005 0 0 0heptadecanoic acid triglyceride 0 0 0 0 0 0.003 0 0 09,12-octadecadienoic acid triglyceride 0 0 0 0 0 0.007 0 0 09-octadecanoic acid triglyceride 0 0 0 0 0 0.023 0 0 0octadecanoic acid triglyceride 0 0 0 0 0 0.001 0 0 010-nonadecenoic acid triglyceride 0 0 0 0 0 139 ppm 0 0 011-eicosenoic acid triglyceride 0 0 0 0 0 162 ppm 0 0 0eicosanoic acid triglyceride 0 0 0 0 0 135 ppm 0 0 0carbon dioxide 0.232 0 0 0 0 0 0 0.054 0nitrogen 0.73 0 0 0.767 0 0 0.761 0.944 0.014oxygen 0 0 0.505 0.233 0 0 0.232 0 0.133carbon monoxide 0 0 0 0 0 0 0 0 0carbon source 0 0 0.37 0 0 0 0 0 0hydrogen source 0 0 0 0 0 0 0 0 0nitrogen source 0 0 0 0 0 0 0 0 0oxygen source 0 0 0 0 0 0 0 0 0n-hexane 0 0 0 0 1 0.96 0 0 0hydrogen 0 0 0.124 0 0 0 0 0 0paste algae mass flow (kg/h) 0 0 0 0 0 0 421.339 0 0

aoil-solv: microalga oil; bair-m: gases emitted; cfg-r: humidified air.

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this work, the products considered from a rice husk processingplant (975.6 kJ of energy and 1.6 kg of CO2 per kilogram of ricehusk) 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 themodel are listed in Table 1. This model was designed based onthe criteria proposed by Mahalec et al.30 A combustion gasstream from a rice husk facility was purified to enrich the CO2and fed to a photoreactor for algae growth. A train ofcompression and cooling stages was proposed for conditioningthe combustion gases at the proper pressure to absorb carbondioxide in water. The inlet pressure of the combustion gases atthe end of the train of intercooling was 20 bar, as suggested byMahalec et al.30 High pressure ensures proper separation ofCO2 from the combustion gases. The thermal energy producedin this area will be used later to supply the energy required indrying of algae biomass. The amount of CO2 produced fromthe rice husk facility permits the production of almost 1 ton/hof microalgae oil as a base for calculating the biodieselproduction process.The flow of carbonated water from the absorption process

was fed with other nutrients for growing microalga Chlorellavulgaris in a photoreactor. The growing of microalgae isdescribed by the equation

+ + +

→ +

sunlight 6CO 4.75H O nutrients

algae (C H O N) 6.295CO2 2

6 11.5 2.9 2 (1)

The residence time in the photoreactor was 24 h, which isthe minimum recommended time for Chlorella vulgaris basedon its generation time, as discussed elsewhere.2,31−34 Thegeneration time is the time necessary for the duplication ofbiomass, and a residence time of 24 h guaranteed theduplication of biomass. A biomass recycle was used for bothprocess intensification and mixing improvements. This avoidedblack areas in the photobioreactor. Mahalec et al.30

recommended using a recycle ratio of 0.8−0.12 for residencetimes in the reactor of about 2−24 h. In this work, a biomassrecycle ratio of 0.12 was used for a residence time of 24 h. Thefractional conversion of CO2 in the photoreactor was 99.2%(molar), as recommended by Molina et al.17

The emerging stream was subjected to centrifugation withthe purpose of separating the broth from the biomass and driedto reduce the moisture content of the biomass to about 12% byweight. The energy requirements of the drying tunnel with airwere supplied completely by the exchange energy in the train ofintercooling. The dry biomass was carried to the solventextraction unit, where hexane was used to extract themicroalgae 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 and0.63 kg of cake per kilogram of microalgae biomass on a drybasis, with a net energy demand (total energy necessary in theprocess) of 3.7 MJ per kilogram of dry algal biomass and anenergy exchange (energy that can be integrated in a netexchange of heat) of 3.2 MJ per kilogram of dry microalgae.These results demonstrate that, if energy integration isperformed, the process can supply 86% of the total energydemand.The production costs of cake and microalgae oil are listed in

Table 3. The economic yields were found to be 0.56 USD/kg ofmicroalgae oil on a dry basis and 0.33 USD/kg of microalgae

cake on a dry basis. Figure 2 shows a diagram with thepercentage economic distribution of production costs for thecultivation and extraction scheme proposed in this scenario.The raw material cost in the production of biomass

microalgae depends on several factors as follow: pretreatmentof gases (9.52%), formulated growth medium (33%), and lightcosts and maintenance (57.14%). In the pretreatment of gases,the main cost is due to the train of compression and coolingstages proposed for conditioning the combustion gases at theproper pressure to absorb carbon dioxide in water. In theformulated medium, the cost is due to the prices of salts used asnutrient sources for microalgae [KH2PO4, MgSO4, (NH4)2SO4,NaCl, CaCl2, and peptone water]. In this case, it is important tofind inexpensive nutrient sources (e.g., wastewater) to decreasethe raw material costs. The light cost is due to the use ofartificial light for microalgae growing in the continuousphotobioreactor; in this case, light-emitting diodes of 1000 lxwere used for optimum microalgae growing (recommended byChisti2,34).Figure 3 shows an analysis of the environmental impact of

the cultivation of microalga Chlorella vulgaris, excludingproducts in the waste stream. The total PEI mitigated in theprocess is beneficial and equal to −0.55 PEI/kg of product.

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

stream feedstock flow rate (kg/h)

flue gas combustion gases 8621water water 9500nutrients nutrients 108.30air air 100000hexane hexane 12312.62stream products flow rate (kg/h)

oil-solv microalgae oil 347.38paste microalgae cake 593.11air-m humidified air 100752.39oil-solv solvent recuperated 12312.62fg-r gases emitted 6403.13liquid cell-free broth 10132.24stream energy flow rate (MJ/h)

QT exchange energy 3479.8− energy demand 3009.6

Table 3. Economic Evaluation of the Cultivation andExtraction of Chlorella vulgaris Metabolites

cost (USD/kg)

category

biomass production(microalgal biomass dry

basis total)

oil(microalgaeoil dry basis)

cake(microalgae

cake dry basis)

feedstock 0.10 0.27 0.16utilities 0.03 0.08 0.05operation andmaintenancecosts

0.03 0.08 0.05

operationalcharges

0.004 0.03 0.006

indirect costs 0.02 0.05 0.03general andadministrativecosts

0.02 0.05 0.03

total cost 0.21 0.56 0.33

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4. DISCUSSION

From the perspective of the culture of microalgae, Chlorellavulgaris provides a huge panorama for the production ofbiofuels and other metabolites. Its oil extraction yield and cakecalculated in this scenario was extremely close to thoseobtained by other researchers.2,4,6,7,33 The performance ofthis microalgae oil is very high. However, the cake biomass canplay an important role because its composition is rich incarbohydrates and protein. These components can beprocessed to obtain higher-value metabolites such as bioethanoland 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 toother oils such as Jatropha oil (0.325 USD/L), palm oil (0.426USD/L), sebum oil (0.186 USD/L), and waste cooking oil(0.139 USD/L). Microalgae oil is more expensive than othercommon oils for biodiesel production (0.532 times moreexpensive), but the volume of production makes that theeffective cost lower and makes the process more efficient in thecontext of the manufacturing productivity of biofuels inColombia (see Table 4). The oil price would be reduced bythe intensification of the cultivation and growth of biomass,

Figure 2. Cost distribution of Chlorella vulgaris cultivation.

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

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because the algae have a high photosynthetic capacity and ahigh duplication rate of cells in a short residence time withlower cultivation area compared to conventional energy crops.The production cost is highly influenced by the pretreatment

processes for biomass and the extraction of metabolites ofinterest. The cost of harvesting, drying, and solvent extractionturns out to be 2.7 times greater than the cost of producing rawbiomass. This is a consequence of the energy demand fordrying. However, energy integration through the use ofintercooling areas in the process resolves around two-thirdsof the total energy needs, and if the energy produced at the ricehusk processing plant is exploited in this scenario, the costsassociated 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 thetotal cost) because of the amount of solvent used and thenutrients required by the culture medium and microalgaeadaptation. Moreover, the substrate is obtained from acombustive gas stream, and its cost is the result oftransportation.The environmental impact analysis (see Figure 3) shows a

huge mitigation potential of greenhouse gases and smog-formation (PCOP), because of the transformation (by algae) ofa combustive gas stream with a negative potential load intometabolites with industrial value. However, the PEI/kg ofoutput products is increased by CO2 waste streams and theresidual broth. If these streams were discharged into theecosystem, they would increase the potential for both acid rainand soil acidification and add to the global warming effect (APand GWP) and possible damage to human health and animalpoisoning (HTPI and TTP).

5. CONCLUSIONSThe cultivation of microalgae has a huge advantages over othercrops given its ability to assimilate waste streams (greenhousegases). The intensive cultivation of algae is an atractivealternative for fossil fuel substitution and production of value-added products. Microalgae are characterized by a highproductivity and yield per unit area of cultivation, as well asecosystem adaptability with sufficient nutrients and wateravailability. However, technology is not sufficient to ensure lowcosts at the oil separation stages from enormous quantities ofwater. This last step requires more research developments withrapid introduction into industry. In the context of Colombia,the possible implementation of algae farms to replace mostagro-industrial crops as feedstocks for biodiesel and bioethanolis an interesting possibility. When microalgae and rice huskfacilities are integrated, the energy demand decreases, and theprocess could become even more profitable from the point ofview of production costs of microalgae oil and cake. Althoughsome authors36 indicate a lack of data, this work could beconsidered as a first approximation to industrial operation.

■ AUTHOR INFORMATION

Corresponding Author*Tel.: +57 6 8879300, ext. 50417. Fax: +57 6 8879300, ext.50199. E-mail: ccardonaal@unal.edu.co.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

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Table 4. Comparison between Palm and Microalgae forBiodiesel Production in Colombiaa

palm microalgae

yield of oil (ton/Ha) 2.1 67.5required area (Ha) 403684 12533created jobs 80737 2507

aOil production in Colombia to supply the biodiesel demand of846000 tons.35

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