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Abstract(s)
O objectivo desta tese é dimensionar um secador em leito fluidizado para
secagem de cereais, nomeadamente, secagem de sementes de trigo. Inicialmente
determinaram-se as condições de hidrodinâmica (velocidade de fluidização, TDH,
condições mínimas de “slugging”, expansão do leito, dimensionamento do distribuidor
e queda de pressão). Com as condições de hidrodinâmica definidas, foi possível
estimar as dimensões físicas do secador. Neste ponto, foram realizados estudos
relativamente à cinética da secagem e à própria secagem. Foi também estudado o
transporte pneumático das sementes. Deste modo, determinaram-se as velocidades
necessárias ao transporte pneumático e respectivas quedas de pressão. Por fim, foi
realizada uma análise custos para que se soubesse o custo deste sistema de
secagem.
O estudo da secagem foi feito para uma temperatura de operação de 50ºC,
tendo a ressalva que no limite se poderia trabalhar com 60ºC. A velocidade de
operação é de 2,43 m/s, a altura do leito fixo é de 0,4 m, a qual sofre uma expansão
durante a fluidização, assumindo o valor de 0,79 m. O valor do TDH obtido foi de 1,97
m, que somado à expansão do leito permite obter uma altura total da coluna de 2,76
m. A altura do leito fixo permite retirar o valor do diâmetro que é de 0,52 m. Verifica-se
que a altura do leito expandido é inferior à altura mínima de “slugging” (1,20 m), no
entanto, a velocidade de operação é superior à velocidade mínima de “slugging” (1,13
m/s). Como só uma das condições mínimas é cumprida, existe a possibilidade da
ocorrência de “slugging”. Finalmente, foi necessário dimensionar o distribuidor, que
com o diâmetro de orifício de 3 mm, valor inferior ao da partícula (3,48 mm), permite a
distruibuição do fluido de secagem na coluna através dos seus 3061 orifícios.
O inicio do estudo da secagem centrou-se na determinação do tempo de
secagem. Além das duas temperaturas atrás referidas, foram igualmente consideradas
duas humidades iniciais para os cereais (21,33% e 18,91%). Temperaturas superiores
traduzem-se em tempos de secagem inferiores, paralelamente, teores de humidade
inicial inferiores indicam tempos menores. Para a temperatura de 50ºC, os tempos de
secagem assumiram os valores de 2,8 horas para a 21,33% de humidade e 2,7 horas
para 18,91% de humidade. Foram também tidas em conta três alturas do ano para a
captação do ar de secagem, Verão e Inverno representando os extremos, e a Meia-
Estação. Para estes três casos, foi possível verificar que a humidade específica do ar
não apresenta alterações significativas entre a entrada no secador e a corrente de
saída do mesmo equipamento, do mesmo modo que a temperatura de saída pouco difere da de entrada. Este desvio de cerca de 1% para as humidades e para as
temperaturas é explicado pela ausência de humidade externa nas sementes e na
pouca quantidade de humidade interna. Desta forma, estes desvios de 1% permitem a
utilização de uma razão de reciclagem na ordem dos 100% sem que o comportamento
da secagem se altere significativamente. O uso de 100% de reciclagem permite uma
poupança energética de cerca de 98% no Inverno e na Meia-Estação e de cerca de
93% no Verão. Caso não fosse realizada reciclagem, seria necessário fornecer à
corrente de ar cerca de 18,81 kW para elevar a sua temperatura de 20ºC para 50ºC
(Meia-Estação), cerca de 24,67 kW para elevar a sua temperatura de 10ºC para 50ºC
(Inverno) e na ordem dos 8,90 kW para elevar a sua temperatura dos 35ºC para 50ºC
(Verão).
No caso do transporte pneumático, existem duas linhas, uma horizontal e uma
vertical, logo foi necessário estimar o valor da velocidade das partículas para estes
dois casos. Na linha vertical, a velocidade da partícula é cerca de 25,03 m/s e cerca de
35,95 m/s na linha horizontal. O menor valor para a linha vertical prende-se com o
facto de nesta zona ter que se vencer a força gravítica. Em ambos os circuitos a
velocidade do fluido é cerca de 47,17 m/s. No interior da coluna, a velocidade do fluido
tem o valor de 10,90 m/s e a velocidade das partículas é de 1,04 m/s. A queda de
pressão total no sistema é cerca de 2408 Pa.
A análise de custos ao sistema de secagem indicou que este sistema irá
acarretar um custo total (fabrico mais transporte) de cerca de 153035€. Este sistema
necessita de electricidade para funcionar, e esta irá acarretar um custo anual de cerca
de 7951,4€.
Embora este sistema de secagem apresente a possibilidade de se realizar uma
razão de reciclagem na ordem dos 100% e também seja possível adaptar o mesmo
para diferentes tipos de cereais, e até outros tipos de materiais, desde que possam ser
fluidizados, o seu custo impede que a realização deste investimento não seja atractiva,
especialmente tendo em consideração que se trata de uma instalação à escala piloto
com uma capacidade de 45 kgs.
The objective of this thesis is to “design” a drying system, utilizing a fluidized bed dryer to dry wheat. The first step is to determine the hydrodynamic conditions, such as TDH, pressure loss and the minimum conditions for slugging, etc. After determining these conditions, the next step is to size the actual dryer. After this, the drying and its kinetics were studied. To extract the dried seeds from the dryer, a pneumatic transport system was devised. The last step was to estimate the fabrication, the transport and the operational cost of this installation. The chosen temperature for the operation was 50 ºC, however a temperature of 60 ºC can also be utilized, bearing in mind that this is an extreme case. The operational velocity is 2,43 m/s. The height of the packed bed is 0.4 m, which expands during the fluidization process, reaching a height of 0.79 m. The TDH parameter has the value of 1.97 m, which, added to the expanded bed allows for a total column height of 2.76 m. The height of the packed bed allows the determination of the column’s diameter, which is 0.52 m. The minimum conditions for slugging must also be verified. It was noted that the expanded bed height is less than the minimum height for slugging (1,20 m). However, the operating speed is higher than the minimum slugging velocity, which is 1,13 m/s. Therefore, although only one of the minimum requirements is met, there is a small possibility that slugging may occur. Last but not least, the distributor was sized, using a hole diameter of 3 mm, which is smaller than the particle size (3,48 mm), which forces the distributor to have 3061 holes, a rather significant number. The first step of the drying stage was to determine the required time to reach the final humidity. The aim of this drying is to reach 13% humidity, starting with either 21,33% or 18,91%. For higher temperatures or for lower cereal humidity levels, less time is needed. 2,8 hours is the time required to dry the seeds if the starting humidity is 21.33%, and 2,7 hours for a humidity level of 18.91%. Taking into consideration that the air will captured during three different seasons (Summer and Winter representing the extremes, and mid-season), it was observed that regardless of the season, the specific humidity of the air shows virtually no alteration between the entrance and the exit of the dryer, with changes of approximately 1%. The same can be said for the air’s temperature. These small changes are explained by the low internal humidity content of the seeds and the absence of external humidity. This way, 100% recycling can be used, which is quite advantageous in terms of energy saving, translating into 98% consumption reduction in the winter and 93% in the summer. In the absence of recycling, 24.67 kW are required to heat the air from 10 ºC to 50 ºC (Winter), 18.81 kW to heat the air from 20 ºC to 50 ºC (Mid-Season) and 8.90 kW to heat the air from 35 ºC to 50 ºC (Summer). The pneumatic system will have two orientations, a vertical line and a horizontal one. Each one has an associated solid velocity. The vertical line has the lesser value (25,03 m/s), explained by the fact that in this line, the air must overcome the force of gravity to move the seeds. The horizontal line has a velocity of 35,95 m/s. The fluid velocity in both lines has the value of 47,17 m/s. Regarding the column, the fluid velocity has the value of 10,90 m/s and the particle velocity is 1,04 m/s. The total pressure loss suffered by the system while the pneumatic transport is occurring is 2408 Pa. Last but not least, this system will have a total cost of 153 035€, being estimated that it will require an annual electricity cost of 7951,4€. Although this drying system shows the possibility of using 100% recycling and has the chances of adapting it for another kind of cereals or even other kinds of materials, as long as they can be fluidized, its inherent cost prevents its use, especially if you take into consideration that this system was designed in a pilot-scale with a capacity of 45 kgs.
The objective of this thesis is to “design” a drying system, utilizing a fluidized bed dryer to dry wheat. The first step is to determine the hydrodynamic conditions, such as TDH, pressure loss and the minimum conditions for slugging, etc. After determining these conditions, the next step is to size the actual dryer. After this, the drying and its kinetics were studied. To extract the dried seeds from the dryer, a pneumatic transport system was devised. The last step was to estimate the fabrication, the transport and the operational cost of this installation. The chosen temperature for the operation was 50 ºC, however a temperature of 60 ºC can also be utilized, bearing in mind that this is an extreme case. The operational velocity is 2,43 m/s. The height of the packed bed is 0.4 m, which expands during the fluidization process, reaching a height of 0.79 m. The TDH parameter has the value of 1.97 m, which, added to the expanded bed allows for a total column height of 2.76 m. The height of the packed bed allows the determination of the column’s diameter, which is 0.52 m. The minimum conditions for slugging must also be verified. It was noted that the expanded bed height is less than the minimum height for slugging (1,20 m). However, the operating speed is higher than the minimum slugging velocity, which is 1,13 m/s. Therefore, although only one of the minimum requirements is met, there is a small possibility that slugging may occur. Last but not least, the distributor was sized, using a hole diameter of 3 mm, which is smaller than the particle size (3,48 mm), which forces the distributor to have 3061 holes, a rather significant number. The first step of the drying stage was to determine the required time to reach the final humidity. The aim of this drying is to reach 13% humidity, starting with either 21,33% or 18,91%. For higher temperatures or for lower cereal humidity levels, less time is needed. 2,8 hours is the time required to dry the seeds if the starting humidity is 21.33%, and 2,7 hours for a humidity level of 18.91%. Taking into consideration that the air will captured during three different seasons (Summer and Winter representing the extremes, and mid-season), it was observed that regardless of the season, the specific humidity of the air shows virtually no alteration between the entrance and the exit of the dryer, with changes of approximately 1%. The same can be said for the air’s temperature. These small changes are explained by the low internal humidity content of the seeds and the absence of external humidity. This way, 100% recycling can be used, which is quite advantageous in terms of energy saving, translating into 98% consumption reduction in the winter and 93% in the summer. In the absence of recycling, 24.67 kW are required to heat the air from 10 ºC to 50 ºC (Winter), 18.81 kW to heat the air from 20 ºC to 50 ºC (Mid-Season) and 8.90 kW to heat the air from 35 ºC to 50 ºC (Summer). The pneumatic system will have two orientations, a vertical line and a horizontal one. Each one has an associated solid velocity. The vertical line has the lesser value (25,03 m/s), explained by the fact that in this line, the air must overcome the force of gravity to move the seeds. The horizontal line has a velocity of 35,95 m/s. The fluid velocity in both lines has the value of 47,17 m/s. Regarding the column, the fluid velocity has the value of 10,90 m/s and the particle velocity is 1,04 m/s. The total pressure loss suffered by the system while the pneumatic transport is occurring is 2408 Pa. Last but not least, this system will have a total cost of 153 035€, being estimated that it will require an annual electricity cost of 7951,4€. Although this drying system shows the possibility of using 100% recycling and has the chances of adapting it for another kind of cereals or even other kinds of materials, as long as they can be fluidized, its inherent cost prevents its use, especially if you take into consideration that this system was designed in a pilot-scale with a capacity of 45 kgs.
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Instituto Politécnico do Porto. Instituto Superior de Engenharia do Porto