SECAGEM DE GRÃOS EM ALTAS TEMPERATURAS Prof. Daniel Marçal de Queiroz Viçosa, MG Agosto/2002 _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Classificação dos secadores quanto ao fluxo de produto leito fixo fluxos cruzados fluxos concorrentes fluxos contracorrentes fluxos mistos estacionários contínuos intermitentes _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secador de leito fixo ou de camada estacionária Constituição: sistema de aquecimento de ar ventilador câmara de distribuição de ar ou plenum câmara de secagem Características: configuração simples baixo custo _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secador de Camada Fixa Modelo UFV _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Modelo UFV

Secador de leito fixo ou de camada estacionária Manejo do secador de leito fixo variáveis de projeto do secador: potência do sistema de movimentação de ar área da câmara de secagem altura máxima da camada variáveis relacionadas com o produto e o ambiente: tipo de produto altitude do local temperatura e umidade relativa ambiente temperatura e umidade inicial do produto _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secador de leito fixo ou de camada estacionária Manejo do secador de leito fixo variáveis de controle: espessura da camada de grãos temperatura de secagem revolvimento do produto repouso do produto resultado: resistência ao fluxo de ar gradiente de umidade na camada de grãos consumo de energia qualidade do produto final _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secador de leito fixo ou de camada estacionária - Top Dry _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Secagem em leito fixo no topo do silo, resfriamento na parte inferior.

Secador de leito fixo ou de camada estacionária - com sistema de revolvimento Componentes: Silo metálico Piso plano perfurado Ventilador Aquecedor Revolvedor de grãos Espalhador Sistema de descarga

Secador de leito fixo ou de camada estacionária - com sistema de revolvimento Secagem em silos com revolvimento da massa de grãos: alturas de camada de até 5,50 m, temperaturas de secagem de até 60o C. Fluxo de ar aumenta em até 33% com o uso do revolvedor. _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secador de leito fixo ou de camada estacionária - com sistema de revolvimento Detalhe do sistema de revolvimento

Secador de leito fixo ou de camada estacionária Sistema utilizado no Brasil para secagem de amendoim. _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secador de leito fixo ou de camada estacionária Problema: gradiente de umidade na camada, para contornar o problema utilizar temperaturas de secagem entre 35 a 40o C. _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Convecção Natural

Secadores em cascata ou de calhas Câmaras de secagem e resfriamento constituídas por uma série de calhas invertidas Fileiras de calhas espaçadas de 20 a 30 cm Grãos movimentam-se por gravidade Ar de secagem ou resfriamento entra por uma linha de calhas e sai pelas linhas imediatamente superior e inferior Ora o fluxo de ar é concorrente ora é contracorrente ao fluxo de produto Primeiros secadores eram dotados de ventiladores centrífugos, hoje quase todos os fabricantes utilizam ventiladores axiais _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores em cascata ou de calhas _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores em cascata ou de calhas Produto a ser seco deve apresentar baixo teor de impurezas caso contrário o fluxo de produto pode tornar-se desuniforme Existe um limite abaixo do qual o fluxo de produto não pode ser reduzido À medida em que se reduz a velocidade do produto dentro do secador pode ocorrer desuniformidade do fluxo de produto Secador pode ser operado em regime contínuo apenas quando a umidade inicial é igual ou inferior a 18% Para melhorar a eficiência energética os secadores passar a serem fabricados com opção de reaproveitamento do ar de resfriamento e de exaustão _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Secadores de Calhas

com reproveitamento do ar de resfriamento e sistema de coleta de Secador tipo cascata com reproveitamento do ar de resfriamento e sistema de coleta de partículas _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores em cascata ou de calhas Temperaturas de secagem Milho e soja para consumo: 100o C Trigo para consumo: 70o a 80o C Arroz: 70o a 80o C Sementes: 50o a 60o C (não permitir os grãos atinjam temperaturas superiores a 43o C _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

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Secadores de fluxos cruzados Secador mais usado no mundo Configuração, construção e operação simples Passagem do ar perpendicular à passagem dos grãos que se movem entre duas chapas perfuradas Espessura da camada de grãos: 20 a 30 cm _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores de fluxos cruzados Secador convencional: problemas com gradiente de umidade e elevado consumo de energia Para melhorar desempenho: reversão do fluxo de ar e reversão da camada de grãos para reduzir o gradiente de umidade na camada reaproveitamento do ar de exaustão para reduzir o consumo de energia _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secador de fluxos cruzados Esquema básico _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores de fluxos cruzados _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores de fluxos cruzados _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ Secador é modular permitindo diferentes configurações para obtenção de diferentes capacidades de secagem.

Secadores de fluxos cruzados Eclusas próximas à entrada de ar descarregam o produto mais rápido para reduzir o gradiente de umidade na camada. _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Recirculação do ar _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores de fluxos concorrentes Ar e grãos fluem no mesmo sentido Altas taxas de evaporação ocorrem na parte superior da camada, uma vez que o ar mais quente encontra o grão mais úmido Trocas intensas de energia e massa ocorrem quando o ar encontra o produto Temperatura do produto permanece consideravelmente abaixo da temperatura do ar de secagem Resfriamento é geralmente feito em fluxos contracorrentes _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores de fluxos concorrentes _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores de fluxos concorrentes _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores de fluxos concorrentes Vantagens: Melhor qualidade final do produto Maior capacidade de secagem. Alta eficiência energética. Baixo custo de instalação e manutenção _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores de fluxos contracorrentes O ar de secagem e grãos fluem em sentido contrário. A frente de secagem permanece sempre próxima ao fundo perfurado. À medida que o produto desce, sua temperatura aumenta, ficando próximas à temperatura do ar de secagem. A temperatura de secagem não deve ultrapassar 70oC. _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores intermitentes Produto ao passar pelo secador retorna à parte superior do secador Quando dotados de câmaras de repouso geralmente o produto permanece 10% na câmara de secagem e o restante do tempo na câmara repouso Câmaras de repouso: melhor eficiência energética e melhor qualidade do produto final _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secadores intermitentes _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Secagem em combinação Secagem em altas temperaturas até o produto atingir 16% a 18% de umidade Secagem em baixas temperaturas no final do processo de secagem Melhor eficiência energética e melhor qualidade do produto final _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Seca-aeração _______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Aumento da capacidade de secagem do secador: 50% a 75% Seca-Aeração Aumento da capacidade de secagem do secador: 50% a 75% Redução do consumo de energia: 20% a 40% Melhoria da qualidade do produto final ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________