Otimização da secagem convectiva intermitente com foco na redução de consumo de energia e melhoria da qualidade - aplicação a beterrabas vermelhas
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Data
2022-03-07
Autores
Filippin, Ana Paula
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Universidade Estadual Paulista (Unesp)
Resumo
A secagem com intermitência térmica é uma alternativa para minimizar as desvantagens da secagem convencional. Consiste na aplicação de um primeiro estágio de secagem em uma temperatura alta, favorecendo a evaporação enquanto a superfície está saturada de água e, após determinado tempo, reduzi-la (segundo estágio). As vantagens dessa técnica dependem da determinação correta dos parâmetros de processo que devem ser estabelecidos considerando características do material. Este trabalho avalia a secagem convectiva intermitente sob o ponto de vista da economia de energia e da manutenção da qualidade de beterraba vermelha, fonte de pigmentos naturais (betalaínas). Para isso, foram feitas simulações de várias condições intermitentes, considerando 3 temperaturas para o primeiro estágio (80, 90 e 100 ºC) e 3 para o segundo (60, 70 e 80 ºC). A duração máxima do primeiro estágio foi determinada através da análise da evolução da temperatura das amostras e se baseou no ponto de elevação abrupta da temperatura, que foi 50, 60 e 70 minutos para temperaturas de 100, 90 e 80 ºC, respectivamente. Um modelo baseado na Lei de Fick, escrito em coordenadas lagrangeanas, foi integrado numericamente para descrever perfis de umidade durante a secagem de fatias de beterraba. O encolhimento, determinado experimentalmente durante secagens conduzidas em 60, 70, 80, 90 e 100 ºC, mostrou-se predominantemente longitudinal, linearmente dependente da umidade e independente da temperatura do ar de secagem. A difusividade mássica efetiva mostrou dependência com a temperatura, descrita pela equação de Arrhenius, e foi superestimada quando a retração foi desprezada, pois este caso considera um caminho de difusão maior do que o real ao longo da secagem. Quando o encolhimento não foi considerado, os perfis de umidade espaciais não corresponderam à realidade, o que gerou desvios perceptíveis entre valores preditos e observados. O modelo matemático que inclui o encolhimento foi validado experimentalmente com secagens contínuas e intermitentes, com parâmetros estatísticos satisfatórios. O consumo de energia, avaliado através de um balanço de entalpia e através de um medidor de grandezas elétricas, foi linearmente dependente do tempo de secagem e variou com a temperatura. Várias configurações de secagem intermitente mostraram economias de energia elétrica relevantes, chegando a 30,5% (90ºC/40min+70ºC/120min), com redução de 33% de tempo de secagem. A degradação de betalaínas foi avaliada em função do tempo de secagem (60, 70, 80, 90 e 100 ºC) sendo que as retenções tiveram decaimento no início das secagens e mantiveram-se estáveis e com valores superiores a 75% durante a maior parte do tempo, exceto quando as amostras atingiram temperaturas superiores a 70 °C. Os valores de retenção obtidos ao final de algumas secagens intermitentes corroboraram os resultados obtidos na cinética de degradação, levando à conclusão de que configurações que limitam a temperatura das beterrabas a 70 °C durante toda a secagem preservam as características relacionadas à qualidade do produto desidratado, além de reduzir o tempo total de processo. A modelagem e a simulação matemática foram ferramentas imprescindíveis para avaliar a secagem intermitente, que se mostrou uma técnica vantajosa sob o ponto de vista ambiental, econômico e nutricional.
Drying with thermal intermittent is an alternative to minimize the disadvantages of conventional drying. It consists of applying a first drying stage at a high temperature, favoring evaporation while the surface is saturated with water and, after a certain time, reducing it (second stage). The advantages of this technique depend on the correct determination of the process parameters that must be established considering material characteristics. This study evaluates intermittent convective drying from the point of view of energy saving and quality maintenance of red beet, source of natural pigments (betalains). For this, simulations of several intermittent conditions were performed, considering 3 temperatures for the first stage (80, 90 and 100 ºC) and 3 for the second (60, 70 and 80 ºC). The maximum duration of the first stage was determined by analyzing the temperature evolution of the samples and was based on the abrupt temperature rise point, which was 50, 60 and 70 minutes for temperatures of 100, 90 and 80 ºC, respectively. A model based on Fick's Law, written in Lagrangian coordinates, was numerically integrated to describe moisture profiles during the drying of beetroot slices. Shrinkage, experimentally determined during dryings carried out at 60, 70, 80, 90 and 100 ºC, showed to be predominantly longitudinal, linearly dependent on moisture and independent of the air-drying temperature. The effective mass diffusivity showed dependence on temperature, described by the Arrhenius equation, and was overestimated when shrinkage was neglected, as this case considers a diffusion path greater than the real one along the drying process. When shrinkage was not considered, the spatial moisture profiles did not correspond to reality, which generated noticeable deviations between predicted and observed values. The mathematical model that includes the shrinkage was experimentally validated with continuous and intermittent drying, with satisfactory statistical parameters. The energy consumption, evaluated through an enthalpy balance and through an electrical magnitude meter, was linearly dependent on the drying time and varied with temperature. Several intermittent drying configurations showed significant electrical energy savings, reaching 30.5% (90ºC/40min+70ºC/120min), with a 33% reduction in drying time. Betalains degradation was evaluated as a function of drying time (60, 70, 80, 90 and 100 ºC) and retentions had a decay at the beginning of drying and remained stable at and with values greater than 75% during most of the time, except when the samples reached temperatures above 70 °C. The retention values obtained at the end of some intermittent drying corroborated the results obtained in the degradation kinetics, leading to the conclusion that settings that limit the temperature of the beetroots to 70 °C during the entire drying process preserve the characteristics related to the quality of the dehydrated product, in addition to reducing the total process time. Modeling and mathematical simulation were essential tools to evaluate intermittent drying, which proved to be an advantageous technique from an environmental, economic and nutritional point of view.
Drying with thermal intermittent is an alternative to minimize the disadvantages of conventional drying. It consists of applying a first drying stage at a high temperature, favoring evaporation while the surface is saturated with water and, after a certain time, reducing it (second stage). The advantages of this technique depend on the correct determination of the process parameters that must be established considering material characteristics. This study evaluates intermittent convective drying from the point of view of energy saving and quality maintenance of red beet, source of natural pigments (betalains). For this, simulations of several intermittent conditions were performed, considering 3 temperatures for the first stage (80, 90 and 100 ºC) and 3 for the second (60, 70 and 80 ºC). The maximum duration of the first stage was determined by analyzing the temperature evolution of the samples and was based on the abrupt temperature rise point, which was 50, 60 and 70 minutes for temperatures of 100, 90 and 80 ºC, respectively. A model based on Fick's Law, written in Lagrangian coordinates, was numerically integrated to describe moisture profiles during the drying of beetroot slices. Shrinkage, experimentally determined during dryings carried out at 60, 70, 80, 90 and 100 ºC, showed to be predominantly longitudinal, linearly dependent on moisture and independent of the air-drying temperature. The effective mass diffusivity showed dependence on temperature, described by the Arrhenius equation, and was overestimated when shrinkage was neglected, as this case considers a diffusion path greater than the real one along the drying process. When shrinkage was not considered, the spatial moisture profiles did not correspond to reality, which generated noticeable deviations between predicted and observed values. The mathematical model that includes the shrinkage was experimentally validated with continuous and intermittent drying, with satisfactory statistical parameters. The energy consumption, evaluated through an enthalpy balance and through an electrical magnitude meter, was linearly dependent on the drying time and varied with temperature. Several intermittent drying configurations showed significant electrical energy savings, reaching 30.5% (90ºC/40min+70ºC/120min), with a 33% reduction in drying time. Betalains degradation was evaluated as a function of drying time (60, 70, 80, 90 and 100 ºC) and retentions had a decay at the beginning of drying and remained stable at and with values greater than 75% during most of the time, except when the samples reached temperatures above 70 °C. The retention values obtained at the end of some intermittent drying corroborated the results obtained in the degradation kinetics, leading to the conclusion that settings that limit the temperature of the beetroots to 70 °C during the entire drying process preserve the characteristics related to the quality of the dehydrated product, in addition to reducing the total process time. Modeling and mathematical simulation were essential tools to evaluate intermittent drying, which proved to be an advantageous technique from an environmental, economic and nutritional point of view.
Descrição
Palavras-chave
Simulação numérica, Economia de energia, Encolhimento, Secagem em dois estágios, Betalaínas, Numerical modeling, Energy saving, Shrinkage, Two stage drying, Betalains