Desempenho global de fotobiorreatores híbridos

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(1)UNIVERSIDADE FEDERAL DE SANTA MARIA CENTRO DE CIÊNCIAS RURAIS PROGRAMA DE PÓS-GRADUAÇÃO EM CIÊNCIA E TECNOLOGIA DOS ALIMENTOS. Mariany Costa Deprá. DESEMPENHO GLOBAL DE FOTOBIORREATORES HÍBRIDOS. Santa Maria, RS 2018 1.

(2) Mariany Costa Deprá. DESEMPENHO GLOBAL DE FOTOBIORREATORES HÍBRIDOS. Dissertação apresentada ao Curso de Pós-Graduação em Ciência e Tecnologia dos Alimentos, da Universidade Federal de Santa Maria (UFSM, RS), como requisito parcial para obtenção do título de Mestre em Ciência e Tecnologia dos Alimentos.. Orientador: Prof. Dr. Eduardo Jacob Lopes. Santa Maria, RS 2018 2.

(3) Ficha catalográfica elaborada através do Programa de Geração Automática da Biblioteca Central da UFSM, com os dados fornecidos pelo(a) autor(a).. 3.

(4) Mariany Costa Deprá. DESEMPENHO GLOBAL DE FOTOBIORREATORES HÍBRIDOS. Dissertação apresentada ao Curso de Mestrado do Programa de Pós-Graduação em Ciência e Tecnologia dos Alimentos, Área de Concentração em Qualidade de Alimentos, da Universidade Federal de Santa Maria (UFSM, RS), como requisito parcial para obtenção do título de Mestre em Ciência e Tecnologia de Alimentos.. Aprovado em 02 de março de 2018:. _____________________________________________ Eduardo Jacob Lopes, Pr. Dr. (Presidente/Orientador). _____________________________________________ Alexandre Swarowsky, Prof. Dr. (UNIFRA). _____________________________________________ Roger Wagner, Prof. Dr. (UFSM). Santa Maria, RS 2018. 4.

(5) AGRADECIMENTOS. Ao meu orientador, Prof. Dr. Eduardo Jacob Lopes, por ser, para mim, o maior exemplo de profissional dentro do espaço acadêmico. Obrigada pelo incentivo diário, conselhos despendidos e oportunidades concedidas. À Profª. Dra. Leila Queiroz Zepka, pelo desafio inicial de orientar-me no auge da minha imaturidade científica. Agradeço pela oportunidade e o conhecimento compartilhado. Aos meus pais, Rudnei Deprá e Maristela Costa Deprá, pela paciência infinita, o amor depositado em mim e por compreender as ausências. Vocês são o meu estímulo para seguir em frente. À minha irmã, Rudiany Costa Deprá, por acreditar e torcer por mim em todos os momentos. Ao meu avô Hélio Godoy Costa, pelo incentivo aos estudos e por reconhecer e vibrar a cada pequena conquista. Aos colegas do laboratório 104 e 111, pelos ensinamentos compartilhados e, também, por partilharem das mesmas dúvidas e anseios. À Andrêssa Fernandes, pelo companheirismo no decorrer desta caminhada, uma vez que foi de extrema importância para que ao final “tudo desse certo”. Ao Vinícius Fernandes, por surgir ao final desse ciclo trazendo toda a calmaria necessária para atenuar as preocupações que a reta final proporciona. Obrigada pela paciência, carinho e apoio! Aos Professores membros da banca pela disponibilidade e contribuições. Ao órgão de fomento, CAPES. À Universidade Federal de Santa Maria, pela formação proporcionada. A Deus, por tudo que proporciona. Muito obrigada!. 5.

(6) Dia após dia, sem olhar para o além. (Autor desconhecido). 6.

(7) RESUMO DESEMPENHO GLOBAL DE FOTOBIORREATORES HÍBRIDOS AUTORA: Mariany Costa Deprá ORIENTADOR: Eduardo Jacob Lopes. Microalgas são microrganismos fotossintetizantes, os quais apresentam versatilidade de associar os processos de bioconversão de CO2 com a produção paralela de insumos com alto valor agregado. Contudo, o principal problema para a. consolidação. desses. microrganismos. está. na. indisponibilidade. de. configurações de fotobiorreatores que suportem altos volumes de trabalho, para atender as demandas industriais, sem interferir no desempenho metabólico das culturas. Em face disto, o trabalho teve por objetivos: (i) Caracterizar a performance hidrodinâmica em fotobiorreatores híbridos, (ii) avaliar o desempenho cinético do fotobiorreator híbrido, (ii) definir a capacidade de absorção de CO2 e produção de O2 em fotobiorreatores híbridos, (iv) determinar os perfis de concentração de carbono inorgânico e orgânico nas fases liquida e gasosa dos sistemas, (v) padronizar o método para cálculo de balanço de carbono em fotobiorreatores, (vi) estabelecer o balanço energético do fotobiorreator híbrido, (vii) mensurar a área superficial requerida pelo fotobiorreator híbrido. Os resultados obtidos demonstraram que o fotobiorreator híbrido apresentou desempenho hidrodinâmico adequado, com valores para tempo de mistura de 98 s, tempo de circulação de 39 s, número Reynolds de 2500 e EBRT de 60 s. A microalga Scenedesmus obliquus apresentou desempenho cinético satisfatório, com taxas máximas de consumo de dióxido de carbono e produtividade de biomassa de 76,27 kg/m³/d e 57,19 kg/m³/d, respectivamente. De acordo com a análise do balanço de carbono, os compostos orgânicos voláteis (82,75%) são os principais bioprodutos formados. Finalmente, os resultados evidenciaram o fotobiorreator híbrido como uma configuração potencial para aplicações ambientais e produção de commodities.. Palavras-chave:. Scenedesmus. obliquus,. fotobiorreator,. caracterização. hidrodinâmica, bioconversão de CO2, balanço de carbono.. 7.

(8) ABSTRACT GLOBAL PERFORMANCE OF HYBRID PHOTOBIORREATORS AUTHOR: Mariany Costa Deprá ADVISOR: Eduardo Jacob Lopes. Microalgae are photosynthetic microorganisms, which have the versatility of associating the processes of CO2 bioconversion with the parallel production of inputs with high added value. However, the main problem for the consolidation of these microorganisms is the unavailability of photobioreactor configurations that support high volumes of work, to meet the industrial demands, without interfering in the metabolic performance of the cultures. The objective of this study was to: (i) characterize hydrodynamic performance in hybrid photobioreactors, (ii) to evaluate the kinetic performance of the hybrid photobioreactor, (iii) to define the CO2 absorption capacity and O2 production in hybrid photobioreactors, (iv) to determine the inorganic and organic carbon concentration profiles in the liquid and gaseous phases of the systems, (v) to standardize the method for calculating carbon balance in photobioreactors, (vi) to establish the energy balance of the hybrid photobioreactor, (vii) measure the surface area required by the hybrid photobioreactor. The results showed that the hybrid photobioreactor presented adequate hydrodynamic performance, with values for mixing time of 98 s, circulation time of 39 s, Reynolds number of 2500 and EBRT of 60 s. The microalgae Scenedesmus obliquus presented satisfactory kinetic performance, with maximum rates of carbon dioxide consumption and biomass productivity of 76.27 kg/m³/d and 57.19 kg/m³/d, respectively. According to the carbon balance analysis, volatile organic compounds (82.75%) are the main bioproducts formed. Finally, the results evidenced the hybrid photobioreactor as a potential configuration for environmental applications and commodity production.. Keywords:. Scenedesmus. obliquus,. photobioreactor,. hydrodynamic. characterization, CO2 bioconversion, carbon balance.. 8.

(9) LISTA DE ILUSTRAÇÕES CAPÍTULO 2 Figure: 1 Photobioreactor diagram. 1: illumination platform; 2: bubble column reactor; 3: flow pumps; 4: air flow meter; 5: insertion CO2. All dimensions in cm. ......................................................................................................................... 30 Figure: 2 Carbon dioxide sequestration and oxygen release rates and relation with the and experimental value of the PQ ....................................................... 39 Figure: 3 Kinetic profile of carbon bioconversion .............................................. 42 CAPÍTULO 3 ANEXO A Figure: 1 Stages for conduction an LCA .......................................................... 59 ANEXO B Figure: 1 Driving steps for life cycle assessment.............................................. 65 Figure: 2 Flowchart for obtaining microalgal biodiesel in an integrated scenario ......................................................................................................................... 73 Box: 1 Example of quantification of inputs and outputs of the process of obtaining microalgal biodiesel .......................................................................................... 77. 9.

(10) LISTA DE TABELAS CAPÍTULO 2 Table 1: Parameters of hydrodynamic characterization of the hybrid photobioreactor ................................................................................................ 35 Table 2: Growth kinetics for Scenedesmus obliquus. ....................................... 38 Table 3: Carbon mass balance in the photobioreactor (hydraulic retention time of 72 h). ............................................................................................................... 41 Table 4: Requirements of area and energy by microalgae of the hybrid photobioreactor. ............................................................................................... 44 CAPÍTULO 3 ANEXO B Table 1: Studies of life cycle assessment in microalgae biofuels. .................... 71 Table 2: Bottlenecks of life cycle assessment implementation ......................... 74. 10.

(11) SUMÁRIO. INTRODUÇÃO ................................................................................................. 13 OBJETIVOS ..................................................................................................... 14 Objetivo geral ................................................................................................. 14 Objetivos específicos .................................................................................... 14 CAPÍTULO 1 .................................................................................................... 15 REVISÃO BIBLIOGRÁFICA ............................................................................ 15 1. Microalgas .................................................................................................... 16 2. Bioprocessos e bioprodutos baseados em microalgas ................................ 17 3. Sistemas de cultivo: biorreatores, operação e parâmetros de processo ...... 19 4. Balanço de carbono ..................................................................................... 21 REFERÊNCIAS ................................................................................................ 23 CAPÍTULO 2 .................................................................................................... 26 NOVEL HYBRID PHOTOBIOREACTOR FOR MICROALGAE CULTURE..... 26 Abstract ............................................................................................................ 27 1. Introduction .................................................................................................. 27 2. Material and Methods ................................................................................... 30 2.1 Microorganisms and culture media ............................................................. 30 2.2 Photobioreactor design............................................................................... 30 2.3 Obtaining the kinetic data in an experimental photobioreactor ................... 31 2.4 Kinetic parameters ...................................................................................... 32 2.5 Analytical methods ..................................................................................... 32 2.6 Hydrodynamic characterization .................................................................. 34 2.7 Mass balances............................................................................................ 35 2.8 Energy balances ......................................................................................... 35 2.9 Statistical analyses ..................................................................................... 35 3. Results and discussion ................................................................................ 35 3.1 Hydrodinamic characterization ................................................................... 35 3.2 Growth kinetics ........................................................................................... 39 3.3 Carbon dioxide removal kinetics and oxygen production ........................... 40 3.4 Carbon balance .......................................................................................... 42 3.5 Energy balance and optimization surface/volume ...................................... 44 4. Conclusions.................................................................................................. 47 5. References ................................................................................................... 47 11.

(12) CONCLUSÃO GERAL ..................................................................................... 57 CAPÍTULO 3 .................................................................................................... 58 ANEXO A - INTRODUCTORY CHAPTER: LIFE CYCLE ASSESSMENT AS A FUNDAMENTAL TOOL TO DEFINE THE BIOFUEL PERFORMANCE ......... 58 ANEXO B - LIFE CYCLE ASSESSMENT OF BIOFUELS FROM MICROALGAE ................................................................................................. 63. 12.

(13) INTRODUÇÃO. A versatilidade metabólica atribuída as microalgas proporcionam a produção de uma vasta gama de compostos como proteínas, carotenoides, lipídios e polissacarídeos. Assim, a demanda global por alimentos, a partir de matrizes microalgais, resultou na intensificação do consumo habitual, uma vez que cientificamente foram conferidos a estes compostos benefícios funcionais, além das considerações tradicionais de nutrição e saúde (WELLS et al., 2017). Em paralelo, a crise energética referente ao excesso de consumo dos combustíveis fósseis e seus impactos nocivos ao meio ambiente, impulsionaram seriamente o interesse mundial em utilizar as microalgas como fontes alternativas de energia renovável (RASTOGI et al., 2017). Contudo, tradicionalmente, a robustez da aplicação em escala industrial dos processos de engenharia ambiental e química fina, mediados por microalgas, ainda não foi consolidada. Esta problemática está diretamente relacionada a limitação dos sistemas de cultivo e seus parâmetros operacionais (MAEDA et al., 2018). Diante deste cenário, apesar da última geração de fotobiorreatores melhorarem substancialmente o modo de cultivo destes microrganismos fotossintetizantes, pesquisas vem sendo feitas para projetar e operar fotobiorreatores que atinjam elevadas eficiências de conversão de dióxido de carbono (CO2) através de configurações que permitam uma melhor absorção de luz, aumento do volume de trabalho e redução dos custos de instalações (JACOB-LOPES et al., 2014). Em face disto, a estratégia de suportar um processo de bioconversão de resíduos gasosos, com enfoque no desempenho hidrodinâmico e cinético do fotobiorreator híbrido, tornando-o viável para a utilização em larga escala e, por conseguinte, estabelecer o balanço global de carbono, poderá direcionar o bioproduto final, a fim de utilizá-lo como insumo em um vasto segmento industrial.. 13.

(14) OBJETIVOS Objetivo geral. Caracterizar a performance hidrodinâmica em fotobiorreatores híbridos.. Objetivos específicos. Avaliar o desempenho cinético do fotobiorreator híbrido. Definir a capacidade de absorção de CO2 e produção de O2 em fotobiorreatores híbridos. Determinar os perfis de concentração de carbono inorgânico e orgânico nas fases líquida e gasosa dos sistemas. Padronizar. método. para. cálculo. de. balanço. de. carbono. em. fotobiorreatores. Estabelecer o balanço energético do fotobiorreator híbrido. Mensurar a área superficial requerida pelo fotobiorreator híbrido.. 14.

(15) CAPÍTULO 1 REVISÃO BIBLIOGRÁFICA. 15.

(16) 1. Microalgas O termo microalga é completamente desprovido de valor taxonômico. São microrganismos diversificados, fotossintetizantes em sua grande maioria, e apresentam estrutura vegetativa conhecida como talo, cuja diferenciação celular é caracteristicamente pequena ou nula (LOURENÇO, 2006). As algas são muito distintas em termos evolutivos, por isso a necessidade de uma identificação taxonômica das estirpes de microalgas e cianobactérias é primordial para coleções de culturas, tanto para as indústrias quanto para as universidades, principalmente por se tratar de questões de propriedade intelectual e de biossegurança (EMAMI et al., 2015). Com mais de 40.000 espécies já identificadas as algas são classificadas em. vários. grandes. (Cyanophyceae),. agrupamentos,. algas. verdes. sendo. eles. as. cianobactérias. (Chlorophyceae),. diatomáceas. (Bacillariophyceae), algas verde-amarelo (Xanthophyceae), algas douradas (Chrysophyceae),. algas. vermelhas. (Rhodophyceae),. algas. castanhas. (Phaeophyceae), dinoflagelados (Dinophyceae) e picoplâncton (Prasinophyceae e Eustigmatophyceae) (SUGANYA et al., 2016). As microalgas podem também ser classificadas quanto a aquisição de carbono. Alguns desses microrganismos conseguem manter-se em condições heterotróficas, sendo necessárias fontes exógenas de carbono orgânico para sua manutenção celular. Todavia, habitualmente realizam a fotossíntese, sendo consideradas microrganismos autotróficos, uma vez que fazem uso de carbono inorgânico e energia luminosa para seu crescimento. Tanto o regime mixotrófico quanto o regime fotoheterotrófico consistem em regimes onde utiliza-se luz como fonte de energia e compostos orgânicos como fonte de carbono. A diferença consiste no fato das espécies fotoheterotróficas necessitarem de luz como fonte de energia para utilizarem os açúcares como fonte de carbono ao passo que as espécies mixotróficas podem alternar entre a utilização de luz e compostos orgânicos como fonte de energia (CHEN et al., 2011; SHARMA & SINGH, 2017). Pertencente à classe Chlorophyceae, a microalga Scenedesmus obliquus em termos morfológicos, possui células cilíndricas com 5-10 μm de diâmetro, 16.

(17) unicelular e forma colônias com quatro células. Quanto a atributos biotecnológicos, surge como um potencial de exploração, uma vez que se destaca pela sua elevada produtividade de biomassa, significativo teor lipídico e relevantes concentrações de pigmentos fotossintéticos (BECKER, 2007; KIM et al., 2016). Além disso, estudos relatam que esta espécie apresenta grande capacidade para biofixação de carbono, possibilitando a facilidade de cultivo em altas concentrações de dióxido de carbono (ZHANG et al., 2013; CHEAH et al., 2015).. 2. Bioprocessos e bioprodutos baseados em microalgas As microalgas possuem uma capacidade metabólica versátil, podendo ser transformadas em bioprodutos através de diversas rotas de processamento. Além da utilização em biocombustíveis e aplicações ambientais, possuem também moléculas bioquimicamente importantes para a nutrição animal e humana (HLAVOVA, TUROCZY e BISOVA, 2015; SUGANYA, 2016). Como. uma. alternativa. potencial. aos. recursos. tradicionais. de. combustíveis, as microalgas foram os microrganismos pioneiros utilizados para a produção de energia renovável, os chamados biocombustíveis de terceira geração, os quais incluem o biodiesel, bioetanol, biohidrogênio e entre outros (CHISTI, 2007; STEPHENS et al., 2010). Dentre as diversas matérias-primas utilizadas para a produção de biodiesel, a biomassa microalgal possui compostos poliinsaturados, o que leva à diminuição da estabilidade do biodiesel produzido, no entanto, devido à presença de ácidos graxos poliinsaturados, uma vantagem apresentada pelo biodiesel de microalgas é o alto rendimento em temperaturas baixas, característica que não é apresentada pelo biodiesel de oleaginosas convencionais, as quais apresentam pouco rendimento em temperaturas relativamente baixas (KOLLER et al, 2014). Dentre as biomoléculas bioquimicamente importantes, os extratos microalgais representam uma ampla e inexplorada fonte de compostos com atividade biológica de alto valor agregado. Diversas espécies de microalgas conseguem sintetizar lipídeos, os quais incluem ácidos graxos de cadeia longa ω-3 como os ácidos linolênico, eicosapentaenoico (EPA) e docosahexanoico 17.

(18) (DHA) e ω-6 como os ácidos linoleico, gama-linolênico (GLA) e araquidônico (ARA). Estes ácidos graxos, especialmente os ω-3 e ω-6 são determinantes para a integridade dos tecidos onde são incorporados. O GLA encontra aplicações terapêuticas e na formulação de cosméticos por revitalizar a pele e consequentemente retardar o envelhecimento. Os ácidos DHA e EPA estão associados à redução de problemas referentes a derrames cardiovasculares, a artrite e hipertensão, além de apresentarem importante atividade hipolipidêmica, através da redução dos triglicerídeos e aumento do colesterol HDL. O ácido DHA atua ainda no desenvolvimento e funcionamento do sistema nervoso. Os ácidos ARA e EPA apresentam ação agregativa e vasoconstritora em plaquetas e antiagregativa e vasodilatadora no endotélio, além de ação quimiotática nos neutrófilos (SPRAGUE, DICK e TOCHER, 2016). Outro aspecto de relevância na exploração de bioprodutos microalgais está associado aos pigmentos. Além da clorofila, as microalgas contêm pigmentos auxiliares como carotenóides e ficobiliproteínas. Estes extratos incluem. β-caroteno,. astaxantina. e. ficocianina,. que. quando. aplicado. industrialmente, são amplamente utilizados como potenciadores de cor em alimentos naturais e aplicações farmacológicas, uma vez que apresentam inúmeras atividades biológicas como, atividade nutracêutica, antimicrobiana, anti-inflamatória,. antienvelhecimento,. hipocolesterolêmica,. antioxidante,. agregativa. e. vasoconstritora,. imunossupressora,. fotoprotetora,. neurotransmissora entre outras (DEL CAMPO, GARCÍA-GONZÁLES e GUERRERO, 2007; VÍLCHEZ et al., 2011; RAPOSO et al., 2013). Nas aplicações ambientais, a utilização destes microrganismos para o tratamento de águas residuais é uma alternativa atraente frente às formas convencionais, devido sua eficácia em assimilar nutrientes como matéria orgânica, NH4+, NO3-, PO43-, CO2 e metais pesados, bem como a valorização dos resíduos por biotransformação em produtos de interesse desde que bem explorados. tecnologicamente. (MATA,. MARTINS. e. CAETANO,. 2010;. RODRIGUES et al., 2014). Além da assimilação de poluentes líquidos, nos últimos anos houve um exponencial interesse no uso de microalgas para o tratamento de poluentes gasosos. Um dos processos consiste na captura, biofixação e reutilização do 18.

(19) CO2, através da fotossíntese, visando a bioconversão subsequente de produtos com elevado potencial de aplicação (CHEAH et al., 2016). Adicionalmente, estes microrganismos tornam-se relevantes para bioprospecção e exploração comercial de diversas biomoléculas para novas fontes de produtos químicos incluindo metabólitos secundários como os compostos voláteis (BOROWITZKA, 2013; EMAMI et al., 2015). Apesar de ser alvo de muitos estudos as frações de metabólitos não voláteis, nota-se também que o rendimento em biomassa não corresponde completamente ao balanço de carbono total do sistema mostrando que parte do balanço está voltado para a produção de compostos orgânicos voláteis (JACOB-LOPES et al., 2010). Neste contexto, independente do bioproduto a ser explorado, suportar um processo de tratamento de resíduos industriais, bem como a produção de insumos que apresentam alto valor comercial, poderá contribuir efetivamente para a sustentabilidade das atividades de transformação industrial (QUEIROZ et al., 2013).. 3. Sistemas de cultivo: biorreatores, operação e parâmetros de processo Fotobiorreatores são equipamentos potenciais para conduzir processos industriais. No entanto, a necessidade de elevados níveis de produção de culturas fotossintetizantes levou ao aumento do desenvolvimento de diferentes configurações de fotobiorreatores que possuam características operacionais de interesse comercial (KUMAR et al., 2016). Dentre os parâmetros operacionais relevantes a serem considerados em cultivos de microalgas, incluem elevada eficiência de utilização da energia luminosa, adequado sistema de mistura, facilidade de controle das condições de cultivo como temperatura, quantidade de CO2, agitação, redução do estresse hidrodinâmico das células e facilidade para o aumento de escala (MUÑOZ & GUIEYSSE, 2006; PRUVOST et al., 2016). As configurações de fotobiorreatores, atualmente em operação, são extrapolações dos reatores químicos convencionais. Estes sistemas, bem como as suas variações foram desenvolvidos e adaptados para satisfazer as 19.

(20) peculiaridades de bioprocessos envolvendo microalgas. Em escala laboratorial e piloto, essas configurações têm de atender aos requisitos básicos dos processos fotossintéticos (JACOB-LOPES et al., 2014). Tradicionalmente, os cultivos de microalgas em larga escala, tiveram início antes da metade do século XX, desde então, uma ampla gama de sistemas de cultivo já foi relatada. A diferenciação destes sistemas de cultivo depende principalmente do custo, do tipo de produtos desejados, da fonte de nutrientes e da captura de CO2. Os sistemas de cultura são geralmente classificados de acordo com suas condições de projeto como sistemas abertos ou fechados (RAZZAK et al., 2013). Sistemas de cultivos abertos, como tanques abertos e raceways, são os mais simples e menos dispendiosos. Essas configurações consistem em um tanque raso (20-30 cm de profundidade), de geometria circular ou oval, dotado de sistemas de agitação mecânica, que expõem o meio de cultura ao ar por turbilhonamento. Infelizmente, esses fotobiorreatores permitem apenas um controle limitado das condições de operação. Além disso, a produtividade é baixa devido à baixa absorção de luz no fundo do tanque e pela maior probabilidade de contaminação. Outras limitações desse tipo de cultivo incluem a grande necessidade de espaço de terra para o cultivo, perdas por evaporação, alta temperatura e, consequente baixa eficiência de transferência de massa (VASUMACHI et al., 2012). Comparativamente, sistemas de cultivos fechados como fotobiorreatores possibilitam. uma. significativamente. grande o. variedade. desempenho. de. dos. configurações cultivos.. Dentre. e. incrementam as. principais. configurações que dominam os arranjos dos fotobiorreatores fechados estão os tubulares, airlift, coluna de bolhas e placas planas. Estes sistemas permitem um certo nível de controle sobre condições de funcionamento, caracterizados por elevada eficiência fotossintética, menor risco de contaminação, e minimização das perdas de água por evaporação (RAZZAK et al., 2013). Por outro lado, a construção dos sistemas fechados é mais onerosa, pois necessitam de material transparente,. sendo. mais. complexos. operacionalmente. e. de. difícil. escalonamento (LIAO, PONTRELLI e LUO, 2016).. 20.

(21) Neste contexto, reatores híbridos surgem como uma alternativa promissora na tentativa de compensar as desvantagens de um sistema e potencializar as virtudes do outro. Esses sistemas são baseados em uma proporção adequada de altura/diâmetro e permitem uma maior relação de superfície/volume, assim possibilitando um volume maior de trabalho, a fim de objetivar produtividades mais elevadas, redução do consumo energético, aumento da eficiência fotossintética e baixo custo (SINGH & SHARMA, 2012; JACOB-LOPES et al., 2014).. 4. Balanço de carbono O balanço de massa de carbono (BMC) é definido por tratar-se de um sistema trifásico com fases: sólida (biomassa), líquida (carbono orgânico e inorgânico dissolvidos) e gasosa (carbono orgânico e inorgânico voláteis). Entretanto, para estabelecer atualmente as quantificações de carbono, os principais tipos de análises quantitativas podem ser divididos em estequiometria, cinética e termodinâmica. Todavia são análises de caráter individual, dificultando assim a predição de valores globais de carbono (CLAASSENS et al., 2016). Quanto a fixação biológica de carbono, a maior parte dos microrganismos fotoautotróficos apresentam elevadas eficiências de conversão de uma fonte externa de carbono inorgânico em biomassa. Porém as microalgas sobressaem frente aos demais, uma vez que para cada 1 kg de biomassa são requeridos, em média, 1,83 kg de CO2 (MOREIRA & PIRES, 2016). Contudo, de acordo com Chen et al. (2013), a suplementação adequada de CO2 para o crescimento de microalgas é um dos fatores primários que influenciam a acumulação de carbono em biomassa. Neste contexto, estudos reportam que as análises de balanço de carbono indicam que apenas uma pequena fração (3,64%) do dióxido de carbono é fixada em forma de biomassa (JACOB-LOPES & FRANCO, 2013). Em contrapartida, na fase líquida do sistema, as microalgas caracterizamse por excretarem alguns compostos orgânicos solúveis, trazendo uma gama de consequências para as algas e seu ambiente. Estes compostos são conhecidos. 21.

(22) como exopolímeros, podendo ser encontrados como enzimas, tais como anidrase. carbônica,. aminoácido. oxidase. e. fosfatase. alcalina,. sendo. responsáveis assim, por uma determinada perda de carbono para a fase líquida (BOROWITZKA et al, 2016). Adicionalmente, a produção de compostos orgânicos voláteis (COVs) a partir de microalgas pode pertencer a diferentes classes de compostos, tais como ésteres, álcoois, hidrocarbonetos, cetonas, terpenos, ácidos carboxílicos e os compostos de enxofre (PAPALEO et al., 2013). Sua síntese pode ocorrer através do metabolismo secundário, com ocorrência de três vias de formação, sendo elas a dos terpenos, compostos nitrogenados ou acetil-CoA. Estes compostos são formados e desprendidos da fase líquida dos sistemas (EROGLU & MELIS, 2010). Por outro lado, existe uma necessidade de estudos prospectivos sobre a fração volátil de biorreatores com microalgas, bem como a elucidação das vias metabólicas de formação destes compostos (SANTOS et al., 2015). Contudo, a biossíntese de COVs depende da disponibilidade de compostos, tais como o carbono e de nitrogênio, bem como a energia fornecida pelo metabolismo primário. Neste contexto, a caracterização das três fases do balanço de carbono em biorreatores poderá contribuir, através da engenharia metabólica, para o estabelecimento de rotas de bioconversão de substratos e, permitir a identificação de potenciais aplicações dos bioprodutos formados, uma vez que será possível direcionar o processo para determinada aplicação comercial (JACOB-LOPES et al. 2010; JACOB-LOPES & FRANCO, 2013).. 22.

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(26) CAPÍTULO 2 NOVEL HYBRID PHOTOBIOREACTOR FOR MICROALGAE CULTURE O artigo será submetido para a revista Chemical Engineering Journal¹.. ___________________________ 1O. manuscrito foi formatado conforme as normas exigidas pela Revista. 26.

(27) NOVEL HYBRID PHOTOBIOREACTOR FOR MICROALGAE CULTURE. 1 2 3. Mariany Costa Deprá and Eduardo Jacob-Lopes*. 4. Food Science and Technology Department, Federal University of Santa Maria,. 5. UFSM, Av. Roraima 1000, 97105-900, Santa Maria, RS, Brazil, Tel. +55 55. 6. 3220 8822.. 7. E-mail: jacoblopes@pq.cnpq.br (E. Jacob-Lopes*). 8. Abstract: The objective of this study was to develop a new design of the. 9. photobioreactor. Hydrodynamic characterization, growth kinetics, removal of. 10. carbon dioxide and oxygen release rates, carbon and energy balances were. 11. determined. The results showed that the hybrid photobioreactor presented high. 12. hydrodynamic performance, with values of mixing time and Reynolds number in. 13. order of 39 s and 2500, respectively. The hybrid photobioreactor was tested for. 14. its kinetic performance, with the microalgae Scenedesmus obliquus, presenting. 15. growth rate of 0.96 d and the maximum cell concentration of 2.8 kg/m³. The. 16. energy requirements were 52 W/m³, being equivalent to the demand of the. 17. photobioreactor flat plate, however, they were superior to other configurations of. 18. commercial reactors since the required surface area of 22 m²/m³. Thus, the hybrid. 19. photobioreactor can be considered a feasible strategy to contribute to the. 20. technological advances in microalgae-based processes.. 21. Keywords: algae/chlorophyceae, carbon bioconversion, photobioreactors, mass. 22. balance, energy balance.. 23. 1.. Introduction. 24. Microalgae play an important role in today's world economy. The global. 25. market value of microalgae is estimated at about $ 6.5 billion, of which about $. 26. 2.5 billion is generated by the healthy food sector, $ 1.5 billion for the production. 27.

(28) 27. of DHA and $ 700 million for aquaculture (Mobin and Alam, 2017). This. 28. prominence is attributed to the high exploratory value of the microalgae biomass.. 29. Therefore, as the biotechnological interest of many of these autotrophic. 30. organisms was recognized, the subsequent challenge was to develop. 31. bioprocesses that linked scientific results to commercial needs (Reyna-Velarde et. 32. al., 2010). Thus, to achieve commercial viability, several innovative technological. 33. solutions are still needed to overcome their satisfactory performance, such as. 34. issues related to the development of microalgae strains, crop protection,. 35. harvesting, environmental processing, resource management and cropping. 36. systems (Brasil et al., 2017). In this context, the use of photobioreactors to perform. 37. microalgae-based processes boosted the development of different reactor. 38. configurations to achieve scale characteristics proportional to industrial demands.. 39. It is known that, currently, only two generalized systems of reactor. 40. configurations are in industrial operation. Raceways are the simplest and the least. 41. expensive, although they are low cost, they present disadvantages such as the. 42. need for large areas of land, problems with high evaporation rate and. 43. contamination, and low productivity (Vasumathi et al., 2012; De Andrade et al.,. 44. 2016). In contrast, the tubular photobioreactors emerged as promises of. 45. consolidation at the industrial level due to their high productivity and the low risk. 46. of contamination. Therefore, the construction of these closed systems is more. 47. expensive, since they require transparent material, being more complex in an. 48. operational way and difficult to scale, consequently not allowing the production of. 49. biomass for commercial purposes (Liao et al., 2016).. 28.

(29) 50. In contrast, new advances in reactor configurations, such as thin layers. 51. (Doucha and Lívanský, 2009; Apel et al., 2017) and biofilms (Blanken et al., 2017),. 52. are characterized by highly productive systems for the cultivation of several. 53. species of microalgae and reinforce their applications in commercial operations. 54. (Pruvost et al., 2017). However, they do not present significant volumetric. 55. performance and still require a large volume of area, making it impossible to. 56. consolidate them commercially at industrial scales. Thus, to date, the dynamics. 57. behind these promising observations is still not well understood, requiring a more. 58. in-depth investigation of the optimization and system update processes.. 59. A promising alternative in attempting to compensate for the disadvantages. 60. of one system and to enhance the virtues of the other, hybrid reactors arise. These. 61. systems are based on a suitable height-to-diameter ratio and allow a greater. 62. surface/volume ratio, allowing a greater volume of work, higher productivity,. 63. reduced energy consumption, increased photosynthetic efficiency and low cost. 64. (Singh and Sharma, 2012; Jacob-Lopes et al., 2014; Jacob-Lopes et al., 2016a;. 65. Jacob-Lopes et al., 2016b).. 66. Therefore, photobioreactor optimization projects, bases of microalgal. 67. biotechnology, remain a fundamental issue for the future impact of these. 68. microorganisms. Thus, to date, commercial production of microalgae has not. 69. become practical for large-scale, for products of high volume and lower value,. 70. because of the complexity and high operating costs of cultivation that hindered its. 71. development (Hu and Sato, 2016).. 72. In this sense, the aim of this work was to develop a new hybrid. 73. photobioreactor. The research focused on optimization and expansion in working. 29.

(30) 74. volume of photobioreactor and on the capacity of bioconversion of carbon dioxide. 75. by microalgae Scenedesmus obliquus CPCC05.. 76. 2.. Material and Methods. 77 78. 2.1 Microorganisms and culture media. 79. Axenic cultures of Scenedesmus obliquus (CPCC05) were obtained from. 80. the Canadian Phycological Culture Centre. Stock cultures were propagated and. 81. maintained in synthetic BG-11 medium (Braun-Grunow medium) (Rippka et al.,. 82. 1979). The incubation conditions used were 30 ºC, photon flux density of 30 µmol/. 83. m²/s and a photoperiod of 12 h.. 84. 2.2 Photobioreactor design. 85. The hybrid photobioreactor integrates two main reaction units: a bubble. 86. column reactor coupled to an illumination platform (Figure 1). The unit. 87. corresponding to the bubble column reactor consists of a closed non-transparent. 88. cylindrical structure, constructed of polyvinyl chloride (PVC) with a thickness of. 89. 0.3 cm, height of 22 cm and diameter of 15 cm, establishing a height-to-diameter. 90. ratio of 1.46. The gas dispersion system is formed by a 1.5 cm air diffuser in the. 91. center of the base column. Injection of airflow enriched with carbon dioxides is. 92. controlled and measured by a rotameter (KI-Key Instruments®, Trevose-PA,. 93. USA). The unit corresponding to the lighting platform is a variable set of at least. 94. eight open tubular structures, arranged linearly with a slope of 9º, constructed of. 95. PVC in a thickness of 0.1 cm, width of 0.75 cm, and internal radius of 3 cm with. 96. a total surface of 0.18 m2. The base of the structure along its entire surface has. 97. roughness of 0.5 cm of elevation/decline. This lighting platform was connected to. 98. two submersible centrifugal pumps located inside the bubble column reactor. The 30.

(31) 99. illumination arrangement is located above the platform, organizing 12 LED lamps. 100. (Osram Sylvania, São Paulo-SP, Brazil) with a capacity of 20W, distributed in. 101. order to provide the desired illumination intensity.. 102 103. Figure: 1 Photobioreactor diagram. 1: Illumination platform; 2: bubble column. 104. reactor; 3: flow pumps; 4: air flow meter; 5: Insertion CO2. All dimensions in cm.. 105. 2.3 Obtaining the kinetic data in an experimental photobioreactor. 106. The experiments were carried out in photobioreactor operating in batch. 107. mode, fed with 1.5 L synthetic BG-11 medium. Thus, the medium flow was. 108. adjusted at a rate of 0.65L/min being 0.30L in the illuminated volume (VI) and. 109. 1.20L in the dark volume (VD), making operationally the total volume (VT) of 1.5L.. 110. The experimental conditions were as follows: initial cell concentration of 100. 111. mg/L, isothermal reactor operating at 26 °C, photon flux density of 150 μmol/m/²/s. 112. and continuous aeration of 1VVM (volume of air per volume of culture per minute). 113. with the injection of air enriched with 15% carbon dioxide (Jacob-Lopes et al.,. 114. 2014).. 31.

(32) 115. Cell concentration and concentration of inorganic and organic carbon were. 116. monitored every 24 h during the growth phase of the microorganism. All. 117. experiments were conducted until the declining phase. The tests were carried out. 118. in duplicate and the kinetic data referred to the mean of four repetitions.. 119. 2.4 Kinetic parameters Biomass data were used to calculate the biomass productivity [P X = (Xi-Xi-. 120. – ti-1)-1, kg/m³/d], the maximum specific growth rate [ln(Xi/X0) = µmax.t, day-1],. 121. 1)(ti. 122. where Xi is biomass concentration at time ti (kg/m³) and Xi-1 is biomass. 123. concentration at time ti-1 (kg/m³), t is residence time (h). Residence time was. 124. defined as the elapsed time to reach maximum cell concentration.. 125. Carbon dioxide concentration data were used to calculate the elimination. 126. capacity [EC = (Co - Ci).Q.VR-1, kg/m³/d], where Co and Ci correspond to the inlet. 127. and outlet CO2 concentration, respectively, Q is the gas flow, and VR is the reactor. 128. volume.. 129. Oxygen data were determined using the photosynthetic quotient [PQ=. 130. (dO2/dt/dCO2/dt)], where dO2/dt and dCO2/dt corresponds to the variation of the. 131. O2 and CO2 concentration over time, respectively.. 132. 2.5 Analytical methods. 133. Cell concentration was evaluated gravimetrically by filtering a known. 134. volume of culture medium through a 0.45 µm filter (Millex FG, Billerica-MA, USA). 135. and drying it at 60 ºC for 24 h. Luminous intensity was determined by using a. 136. quantum sensor (Apogee Instruments, Logan-UT, USA), measuring the light. 137. incident on the external reactor surface. The temperature was controlled by using. 32.

(33) 138. thermostats. The flow rates of carbon dioxide, air, and CO 2 enriched air were. 139. determined with rotameters (AFSG 100 Key Instruments, Trevose-PA, USA).. 140. Gas chromatograph (GC) was used to determine the CO 2/O2. 141. concentrations of photobioreactor exhaust gases. The equipment used was a. 142. GC-Greenhouse (Shimadzu, Kyoto, Japan), equipped with six packed columns. 143. connected to the flame ionization detector (FID) and thermal conductivity detector. 144. (TCD), and helium as carrier gas. The amounts of removed carbon dioxide and. 145. produced oxygen have been determined from samples 50 µL, taken from the. 146. gaseous system phase (inlet and outlet) with a gas-tight microsyringe (Hamilton,. 147. Bellefonte-PA, USA). The areas were obtained using the integrator Software. 148. GCsolution, and they were compared according to reference curves to determine. 149. the CO2 and O2 concentrations.. 150. Measurements of total carbon in the liquid and gaseous phases were. 151. carried out on a carbon analyzer TOC-VCSN (Shimatzu, Kyoto, Japan) with a. 152. normal sensibility catalyst (platinum on 1/800 Alumina Pellets) to measure total. 153. carbon (TC) and inorganic carbon (IC). Organic carbon (OC) was calculated by. 154. the difference between TC and IC. For the liquid phase measurements, 20 µL of. 155. the medium was filtered in a 0.2 mm filter and used for the analysis. In the. 156. analysis of total carbon, 27 µL samples were carried to the combustion tube at. 157. 680 ºC, where the catalytic oxidization to CO2 occurred. For the analysis of. 158. inorganic carbon, samples of 27 µL reacted with hydrochloric acid 2 M to convert. 159. all the inorganic carbon into CO2. In both cases, the carbon dioxide was quantified. 160. by non-dispersive infrared absorption and the concentrations were calculated. 161. through analytical curves (peak area X concentration) previously constructed with. 162. standard solutions of potassium hydrogen phthalate for TC and sodium hydrogen 33.

(34) 163. carbonate for IC. For gaseous measurements, gas samples were collected in the. 164. entrance flow and in the exit flow of the photobioreactor. For measurements of. 165. total carbon, 50 µL samples were injected and carried over a flow provided by an. 166. analytical O2 cylinder into the combustion tube (680 ºC) and the samples were. 167. catalytic oxidized to CO2. Finally, to analyze inorganic carbon, 40 µL samples. 168. were injected and carried directly to the detector. They were quantified by non-. 169. dispersive infrared detector and the concentrations achieved with analytical. 170. curves of peak area x concentration were made using the same conditions, with. 171. pure CO2.. 172. 2.6 Hydrodynamic characterization. 173. The empty-bed residence time (EBRT) [EBRT=VL/FG] was calculated. 174. according to the relation of the contact time between the liquid volume and the. 175. gas inserted into the medium, where VL is the liquid volume and FG is the. 176. volumetric flowrate of the gas (Ko et al., 2000).. 177. The mixing time (TM) was determined by the acid tracer method [CT=[H+]. 178. =[H+]instantaneous - [H+]inicial/[H+]final - [H+]inicial] (Chisti, 1989). It was determined as the. 179. time required to attain a 5% deviation from complete homogeneity after the. 180. injection of a tracer pulse was injected into the reactor. Therefore, is a direct. 181. indicator of the mixing capacity of the reactor and allows the comparison with. 182. characteristic times of processes taking place inside the reactor.. 183 184. Circulation time (TC) was determined through the time interval that the entire volume of fluid passes through the system (Chisti, 1989).. 185. Reynolds number (Re) [Re = ρ.V.D/µ] was calculated to ascertain the. 186. movement of the fluid according to the different flow regimes, where ρ: specific 34.

(35) 187. mass of the fluid; V: is average fluid velocity; D: longitude characteristic of the. 188. flow; µ: dynamic viscosity of the fluid (Peiran and Shizhu, 1990).. 189. 2.7. Mass balances. 190. The carbon mass balance [Cin-Cout = VLMc (d[Cbiomass]/dt + d[DIC]/dt) +. 191. VgMc (d[Cheadspace]/dt)] was calculate from according to the inputs and outputs in. 192. the system, where Cin (kg/d): mass flow rate of carbon; Cout (kg/d): mass flow rate. 193. of carbon out of the PBR via venting; VL (m³): volume of the media; Vg (m³):. 194. volume of the headspace; Mc (kg/mol): atomic weight of carbon; (d[C biomass])/dt. 195. (kg/m³/d): rate of change in concentration of carbon in the biomass; (d[DIC])/dt. 196. (kg/m³/d): rate of change in dissolved inorganic carbon concentration;. 197. (d[Cheadspace])/dt (kg/m³/d): rate of change in carbon concentration in headspace. 198. (Lee et al., 2015).. 199. 2.8. Energy balances. 200. The net energy ratio (NER) [NER=∑Eout/∑Ein] of a system was defined as. 201. the ratio of the total energy produced (energy content of the residual biomass,. 202. MJ) over the energy required for hydrodynamic operating of the photobioreactor. 203. (MJ), where Eout is the renewable energy output and Ein is the fossil fuel energy. 204. input (Jorqueira et al., 2010).. 205. 2.9. Statistical analyses Descriptive analysis were performed with the Statistica 7.0 software. 206 207. (StatSoft, Tulsa-OK, USA).. 208. 3.. 209. 3.1 Hydrodinamic characterization. Results and discussion. 35.

(36) 210. In order to characterize the fluid behavior, the mixing performance and its. 211. light exposition, parameters of hydrodynamic characterization were evaluated. 212. and expressed in Table 1.. 213. Table 1: Parameters of hydrodynamic characterization of the hybrid. 214. photobioreactor. Hydrodynamics parameters. Value. TM (s). 98 ± 0.08. TC (s). 39 ± 0.03. Re EBRT (s). 2500 ± 0.00 60 ± 0.00. 215. TM: mixing time, TC: circulation time, Re: Reynolds; EBRT: empty bed residence. 216. time.. 217. The mixing time (tm) shows the time interval required to acquire a certain. 218. difference of the fully mixed state after the reactor input (Mirón et al., 2004). Thus,. 219. the photobioreactor presented the first change of inclination in 8 s, and soon after. 220. it remained constant in 106 s, thus verifying the total mixing time in the order of. 221. 98 s. Comparatively, studies by Hsieh and Wu, (2008) for a transparent. 222. rectangular chamber photobioreactor and Yang et al., (2016) for bubble column. 223. reactors reported mixing time values at 110 s and 104 s, respectively. This small. 224. difference between the values of tm is attributed to the variation of the reactor. 225. volume, the air flow and the height of the reactor, because these parameters. 226. determine the time needed to distribute the nutrients, facilitate the movement of. 227. the cells inside and outside the illuminated part of PBR (Ogbonna and Tanaka,. 228. 1996). However, an effective mixing time refers to the frequency of access to light. 36.

(37) 229. for microalgae cultivation (Richmond and Hu, 1996). In this sense, it is observed. 230. in the hybrid photobioreactor model that the retention time of the cells on the dark. 231. surface is 96% higher when compared to the retention time on the illuminated. 232. surface. Consequently, the permanent circulation of the liquid in the system gives. 233. the microalgal cells a constant contact in the light/dark cycles, avoiding the. 234. photoinhibition and photosaturation processes described by Molina Grima et al.. 235. (1996).. 236. photosynthetic activity and growth rates of microorganisms (Jacob-Lopes et al.,. 237. 2009; Zhou et al., 2015, Maroneze et al., 2016).. In. addition,. other. authors. report. that. photoperiods. increase. 238. In parallel, the mixing time is directly related to the circulation time (Tc). 239. (Kawase and Moo-Young, 1989). Therefore, the hybrid photobioreactor was. 240. established to values of 39 s. In comparison, Sadeghizadeh et al. (2017) obtained. 241. values of 12 s in air volumes of 0.002 VVM for airlift photobioreactor. On the other,. 242. flat plate photobioreactor was reported values in the order of 11 s (Changhai et. 243. al., 2005). However, although the values reported in the literature are lower, the. 244. values of circulation time of the hybrid photobioreactor did not interfere in the. 245. mixing time, keeping the value compatible with the other photobioreactor. 246. configurations. In addition, it should be considered that lower Tc values require. 247. higher gas velocity or higher input workforce, providing a costly energy demand.. 248. Yet, according to De Jesus et al. (2017), in airlift bioreactors, circulation and. 249. mixing time are relatively smaller than in bioreactors with a stirring system,. 250. because in these bioreactors air displacement is not strong enough to increase. 251. the gas residence time in the fluid as in bioreactors with mechanical stirring. 252. systems, resulting in a smaller gas holdup.. 37.

(38) 253. To evaluate the flow regime, the hybrid photobioreactor presented values. 254. in the order of 2500, is classified as transient flow regime, ensure certain. 255. turbulence so that all cells are frequently exposed to light (Ugwu et al., 2008). In. 256. comparison, studies with thin layer photobioreactors also presented a transition. 257. flow regime, with similar values of 2347 (Alpel et al., 2017). However, for the. 258. bubble column of photobioreactors, Kováts et al. (2017) presented values close. 259. to 6368. On the other hand, the raceway ponds have high Re values between. 260. 148900 and 814330, characteristic of the turbulent regime (Ali et al., 2017). The. 261. turbulent flow also causes a vertical mixing, which in turn may not have a. 262. sufficient mixture. In addition, vertical mixing in the raceways is very poor. 263. compared to the tubular photobioreactor (Kumar et al., 2015). Thus, high. 264. hydrodynamic forces may limit the performance of cyclic motions of the liquid. 265. between the bottom (dark zone) and the surface (light zone) of the raceways. 266. (Prussi et al., 2014).. 267. Finally, the EBRT can be a decisive factor when the maximum rates of. 268. elimination capacity are reached. The EBRT values were adjusted to 60 s.. 269. However, there are no data in the literature that report such relationship in. 270. microalgal photobioreactors, however, maximum values obtained for the. 271. elimination capacity were also reached for systems involving biological gas. 272. treatment (García-Pérez et al., 2018). Thus, it is possible to observe that the. 273. decrease of the EBRT provides the increase of the elimination capacity without a. 274. significant deterioration in the removal efficiency and the final concentration of. 275. biomass (Cheng et al., 2013). Therefore, it should be emphasized that the hybrid. 276. photobioreactor has values compatible with the hydrodynamic characteristics. 277. reported by commercial configurations of photobioreactors which present high 38.

(39) 278. hydrodynamic performance, however, they point severe limitations regarding. 279. their scale up potential due to the low workloads.. 280. 3.2 Growth kinetics. 281. The algae-based process dependent on its productivity. Thus, Table 2. 282. shows the kinetic parameters for the growth of Scenedesmus obliquus in hybrid. 283. photobioreactor.. 284. Table 2: Growth kinetics for Scenedesmus obliquus. Parameter. Value. pHmax. 8.2 ± 0.17. Xmax(kg/m³). 2.8 ± 0.07. µmax (d-1). 0.96 ± 0.00. Px (average) (kg/m³/d). 0.35 ± 0.00. Px (peak) (kg/m³/d). 0.57 ± 0.00. 285. Px: biomass productivity, Xmax: maximum cellular concentration, pHmax:. 286. maximum pH value obtained in cultivation.. 287. The microalgae Scenedesmus obliquus does not present a latency phase,. 288. assuming an exponential behavior soon after inoculation (data not show), getting. 289. a maximum cell concentration of 2.8 kg/m³, the growth rate of 0.96 d, maximum. 290. pH value of 8.2 and maximum biomass productivity of 0.57 kg/m³/d (HRT = 96h).. 291. Comparatively, Morales-Amaral et al (2015) showed that the biomass productivity. 292. of Scenedesmus sp. obtained was 1.15 kg/m³/d for thin-layer photobioreactors. 293. and 0.20 kg/m³/d for raceway ponds. However, thin-layer photobioreactors are. 294. highly exposed to light intensity and have reduced workloads, which can result in. 295. cellular photoinhibition. In contrast, raceways have low rates of cell growth,. 39.

(40) 296. because with increasing workload, the absorption of light intensity does not occur. 297. homogeneously, which inhibits the performance of photosynthesis. On the other. 298. hand, the data reported in this study were corroborated with Garcia-Cubero et al.,. 299. (2017), with biomass productivity of 0.49 kg/m³/d in bubble column. 300. photobioreactors, however, these photobioreactors have problems of stagger. 301. and zones dead in the photobioreactor, which represents operational difficulties.. 302. Thus, it is suggested that hybrid photobioreactors tend to have compatible cellular. 303. productivities with models already in use in the industrial follow-up, with additional. 304. advantages of increasing scale without interfering in their reactor performance.. 305. 3.3 Carbon dioxide removal kinetics and oxygen production. 306 307. The dynamics of CO2 sequestration and O2 release during the continuous alternation of the light/dark cycles of the experiment is shown in Figure 2.. 308. 40.

(41) 309. Figure 2: Carbon dioxide sequestration and oxygen release rates and relation. 310. with the and experimental value of the PQ.. 311. The microalgae Scenedesmus obliquus reaching its peak rates in 72 hours. 312. of culture (76.27 ± 0.3 kg/m³/dCO2 and 57.19 ± 0.7 kg/m³/dO2 for CO2. 313. sequestration and O2 release, respectively).. 314. Comparatively, Jacob-Lopes and Franco (2013), related CO2 conversion. 315. values 2.8 times lower (26.92 kg/m³/d) using the culture of Aphanothece. 316. microscopica Nägeli in a bubble column photobioreactor. In contrast, in open. 317. ponds the maximum rate of carbon sequestration was reported from 25.70. 318. kg/m³/d for microalgae Chlorella vulgaris (Eloka-Eboka and Inambao, 2017). It is. 319. known, that factors such as light intensity interfere directly in the process of CO2. 320. bioconversion and in its photosynthetic activity. However, the mechanism of. 321. carbon assimilation is the main factor of carbon absorption. Therefore, even in. 322. high concentrations of carbon inserted in the medium, the microalgal. 323. Scenedesmus obliquus assumes an advantage over the others, since the. 324. carbonic anhydrase is strengthened by the stress caused by the dark/light cycle. 325. and is not inhibited by the enzymes acetazolamide (AZA) and ethoxyzolamide. 326. (ETA), thus providing greater inorganic carbon access in the form of bicarbonate. 327. ions as well as internal carbonic anhydrase (Palmqvist et al., 1994).. 328. In addition, the photosynthetic quotient (PQ) for the microalga. 329. Scenedesmus obliquus was obtained values in the order of 1.3. Comparatively,. 330. the values of microalga Chlorella protothecoides were 1.02 (Santos et al., 2014).. 331. Yet, for planktonic diatom species, PQ values close of 1.2 were obtained (Spilling. 332. et al., 2015). However, according to Ferguson et al., (2017) it must be considered. 41.

(42) 333. that this relation experimental can vary between 1 and 1.3. Therefore, the. 334. response of photosynthetic efficiency is directly related to the metabolic versatility. 335. of assimilating carbon, as well as to a series of environmental parameters such. 336. as adequate exposure to light intensity, temperature, available nutrients and. 337. other combinations of these stressors (Tait et al., 2017).. 338. 3.4 Carbon balance. 339. Table 3 shows the determination of the carbon mass balance by the. 340. microalgae Scenedesmus obliquus in hybrid photobioreactor.. 341 342. Table 3: Carbon mass balance in the photobioreactor (hydraulic retention time of 72 h). Carbon balance. Mass (kg). Input carbon. 0.32 ± 0.02. Lost carbon. 0.22 ± 0.05. Converted carbon. 0.09 ± 0.01. Biomass. 1.28x10-3 ± 0.15. Exopolymers. 6x10-5 ± 0.02. Mineralization. 1.7x10-4 ± 0.07. Volatile organic compounds. 0.08 ± 0.5. 343 344. Through the injection of carbon dioxide, 0.32 kg of carbon were inserted. 345. into the photobioreactor and 0.22 kg of carbon were lost (69,37%). Thus, the. 346. microalgae was able to metabolize a total of 0.09 kg of carbon, representing. 347. 30.72% of the converted global carbon.. 348. It is known that the relation between carbon consumption and biomass. 349. production does not correspond to its total biotransformation (Jacob-Lopes et al., 42.

(43) 350. 2010; Jacob-Lopes and Franco, 2013). Since biofixation in biomass represented. 351. only a small fraction of 1.28% of converted carbon, it is notorious that the. 352. microalgae was using carbon dioxide to build other metabolites (Santos et al.,. 353. 2016). Thus, the conversion of CO2 into bioproducts followed the standard bell. 354. curve for the conditions evaluated (Figure 3).. 355 356. Figure 3: Kinetic profile of carbon bioconversion.. 357. The liquid fraction was investigated as to its potential excretion of. 358. expolymers and mineralization of inorganic carbon. In figure 3 it is possible to. 359. follow the behavior of the microorganisms during the 144 hours of culture. The. 360. microalgae Scenedesmus obliquus reached its maximum excretion point in 72. 361. hours at a concentration of 0.60 kg/m³. In parallel, studies reported by Mishra et. 362. al., (2009) and Mirsha et al., (2011), report finding average concentrations of. 43.

(44) 363. 0.056 kg/m³ in media with low salt concentration, whereas in highly saturated. 364. media, maximum concentrations can reach up to 0.94 kg/m³ for Dunaliella salina. 365. microalgae. However, these values are much lower when compared to. 366. cyanobacteria, which could be found values up to 22.34 kg/m³ for Cyanothece. 367. sp. (Chi et al., 2007).. 368. In contrast, the mineralization occurs once the CO2 inserted dissolves in. 369. aqueous medium forming the carbonic acid which is assimilated by the. 370. microalgae through ionic equilibrium forming the bicarbonate ion. Therefore, the. 371. microalgae Scenedesmus obliquus had maximum concentrations of mineralized. 372. carbon of 0.17 kg/m³. On the other hand, studies were done by Lee et al., (2015). 373. obtained average values of 0.42 kg/m³ when using 3% CO2 with Haematococcus. 374. pluvialis.. 375. The volatile fraction represents a total of 0.08 kg of converted carbon,. 376. these values represent a percentage of the 82.75%. This biotransformation is the. 377. most significant form of conversion during microbial culture in photobioreactors,. 378. especially when high rates of carbon dioxide are inserted in photobioreactor. 379. (Jacob-Lopes et al., 2010). In the same way as Jacob-Lopes and Franco (2013),. 380. which presented similar values of the order of 92%.. 381. 3.5 Energy balance and optimization surface/volume. 382. Although scalability and productivity rates are a more sensitive point of. 383. microalgae-based processes, the determinant factor for the commercial. 384. robustness of these still persists in the cost/benefit ratio caused by the high. 385. energy consumption for the production of microalgae biomass (Uduman et al.,. 44.

(45) 386. 2010). Therefore, Table 4 shows the requirements of surface area, the energy for. 387. hydrodynamic operating and energy return through the biomass.. 388 389. Table 4: Requirements of area and energy by microalgae of the hybrid photobioreactor. Parameter. Value. Surface area (m²/m³). 0.22. Volumetric productivity (kg/m³). 2.8. Energy consumed (W/m³). 52. Energy produced (MJ/m³). 64.4. Net Energy Ratio (NER). 0.34. 390 391. Hybrid photobioreactors presented energy demands in the order of 52. 392. W/m³ and reached return values in the order of 64.4 MJ, resulting in an NER of. 393. approximately 0.34. According to Jorqueira et al. (2010), raceway ponds. 394. presented reduced energy demands of about 3.72 W/m³, however, they present. 395. a substantial inferiority in volumetric productivity, with values of 0.35 kg/m³. 396. (Richmond and Cheng-Wu, 2001). Nonetheless, its return energy results in 8.05. 397. MJ, being possible to obtain NER values of 0.60. On the other hand, tubular. 398. photobioreactors despite having satisfactory productivity values (1.02 kg/m³),. 399. have high energy requirements, with values of 2500 W/m³ (Zittelli et al., 1999).. 400. Therefore, the return energy presents values of 23.46 MJ, resulting in a very. 401. unsatisfactory NER of 0.002. Comparatively, the energy values for flat plate. 402. photobioreactors are corroborated with the hybrid photobioreactors, which have. 403. slightly higher energy requirements, in the order of 53 W/m³. However, as its. 404. volumetric productivity is slightly lower (2.7 kg/m³), its return energy is 62.01 MJ,. 45.

(46) 405. resulting in an NER of 0.32 (Cheng-Wu et al., 2001). In this context, although. 406. hybrid photobioreactors require energy costs equivalent to flat plate. 407. photobioreactors, their superior volumetric productivity compensates for the. 408. energy demand, thus becoming the energy produced favorable to the viability of. 409. the reactor. Still, it should be considered that flat plate photobioreactors, while. 410. exhibiting energy balance and adequate kinetic performance, are costly and have. 411. many compartments and support devices that hamper magnification and. 412. operation.. 413. Finally, as an additional attribute for the mass production of microalgae,. 414. the influence of the required surface area is a determining factor for the. 415. commercial implementation and consolidation of the photobioreactors. Thus,. 416. hybrid photobioreactors presented requirement surface area of 0.22 m²/m³. In. 417. comparison, open systems were the first configurations to be implemented. 418. commercially, occupying an area of 3.33 m²/m³ (Weissman et al., 1988). In. 419. addition, tubular photobioreactors, have values generally of the order of 14.28. 420. m²/m³ (Pulz, 2001). However, capital costs make this configuration costly. Finally,. 421. the flat plate photobioreactors that were developed to produce high cell. 422. productivities has a surface area requirements of 14.42 m²/m³ (Sierra et al.,. 423. 2008). Thus, the increase in the required surface area represents, on a. 424. commercial level, leads to a significant increase in capital and consequently. 425. operating costs of the process. Given this scenario, the hybrid photobioreactors. 426. stand out for their capacity to considerably reduce the required surface area,. 427. ensure their adequate surface/volume ratio, thus reducing the costs of building. 428. systems, making possible the scale-up of the equipment for environmental. 429. applications and commodities production. 46.