Drying woody biomass holds the potential to improve the energy efficiency of certain processes, such as in CHP plants. Drying can also be a necessary unit process in several energy conversion ...processes (e.g. in biomass gasification). Belt dryers are typically used for drying when low temperature air (
100-110 °C) is used. This article aims to produce new knowledge about the influence of the main design parameters on the drying costs of a low temperature belt dryer when three different materials (forest residue, bark, as well as sawdust and soot sludge mixture) are dried using it. The influence is analyzed by changing the following parameters: bed height, air temperature, air velocity and initial/final moisture contents of the material. The study aims to evaluate which of these parameters has an actual effect on drying costs. Results indicate that the lowest costs are achieved with the highest air temperature if the heat price is the same for every air temperature level. However, an optimal bed height depends on the material. Increasing the air velocity does not necessarily decrease the costs. In the sensitivity analysis, to factor in the influence of the temperature on the heat price, the price was changed for every drying air temperature (1, 5, 10 and 15 €/MWh). This analysis showed that the lowest drying costs are achieved by the lowest air temperature in all cases, thus indicating that the price of the heat has a remarkable influence on the economics of drying. Furthermore, the results support the use of low temperature heat sources in drying if they are clearly less expensive than higher temperature heat sources. However, if the prices for lower and higher air temperatures are of the same magnitude, the higher air temperatures are preferable. In general, this paper shows that it is important to pay attention to the main design parameters to optimize total drying costs. For example, if an overly low bed height is used in woodchips or bark drying, the total drying costs might be dozens of per cent higher than in the most economic case.
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•We added the viscous dissipation term in the energy equation.•We used the porous medium model and the evaporation model in the calculation process.•When material thickness increases, the “ripple” ...phenomenon disappears.•Material thickness has the largest influence on moisture uniformity, followed by velocity.•Wing defector reduces areca-nut moisture content and improves moisture content uniformity.
In order to improve internal air flow field of belt dryer and increase moisture standard deviation of drying material in dryer, we used FLUENT software to study influences of material thickness, air velocity, air temperature and air relative humidity on material moisture content and uniformity in dryer on the basis of computational fluid mechanics and heat and mass transfer theory. Viscous dissipation was added to the energy equation. Porous medium model and evaporation model were used to improve result accuracy. A comparison between the moisture content of material inside the dryer was made when no deflector, a common deflector and a wing deflector are applied. Results show that air velocity in each measuring point is consistent with the numerical simulation. Material thickness had the largest influence on moisture content uniformity among four factors, followed by drying air velocity. The result of variance of material moisture content indicates that the thicker material thickness was, the better moisture content uniformity would be. When test conditions were shown as follows: 80mm thickness; 343K inlet air temperature; 1.5m/s air velocity; 0.24 air initial humidity, moisture standard deviation of material was optimal. The wing deflector reduced material moisture content as a whole and moisture content uniformity was improved.
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13.
Improving the efficiency of corn drying in a conveyor belt dryer Permyakov, Valeri; Ganeev, Ildar; Akhmetyanov, Ilshat ...
Journal of environmental science and health. Part B, Pesticides, food contaminants, and agricultural wastes,
2021, Volume:
56, Issue:
10
Journal Article
Peer reviewed
The paper deals with the theoretical background of drying corn kernels in the moving bed. The authors present a system of the drying medium (drying air) uniform distribution in the dryer. They ...proposed a system that can improve the performance of the conveyor dryer. A rationale for the essential geometry features of the system is provided. The system implies air supply through two rows of multidirectional nozzles. The proposed layout increases the flow velocity at the nozzle outlet by 35-45%. The study determined the optimum values of the four-stage process for drying corn kernels in a conveyor dryer: for the drying medium, the average temperature was tdm =35 ... 50 °C, the velocity at leaving the moving bed was Vdm = 1...1.5 m/s.
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The mathematical investigation presented in this paper concerns the conveyor-belt dryer with tangential flow operating in co-current. This dryer is bigger than the continuous through-circulation ...conveyor dryer but has the advantage of better preserving the organoleptic and nutritional qualities of the dried product. In a previous work a mathematical modeling of the conveyor-belt dryer with tangential flow was carried out to offer guidelines for its optimized design. The last of those design guidelines indicated the need for an optimized adjustment of the dryer to ensure the constant maintenance of the final moisture content of the product. The fast and very precise measurement of the moisture content as the first step in the feedback chain was therefore necessary. Considering the difficulty of this type of measurement, two specific ordinary differential equations (ODEs) were obtained with the mathematical investigation of this work. Their solution became a relationship between the final moisture content of the product, the outlet air temperature, and other quantities that could be easily implemented in an automatic dryer control system. Therefore, the fast and accurate and much less expensive measurement of the temperature of the air leaving the dryer, owing to the relationship found, replaces the measurement of moisture content for the adjustment system. The experimental verification of the relationship highlighted the need to introduce a modification by which the relationship was finally validated.
This work presents the mathematical modeling of the conveyor-belt dryer with tangential flow operating in co-current, which has the advantage of improving the preservation of the organoleptic and ...nutritional qualities of the dried food. On the one hand, it is a more cumbersome dryer than the perforated cross flow belt dryer but, on the other hand, it has a low air temperature in the final section where the product has a low moisture content and, therefore, it is more heat sensitive. The results of the mathematical modeling allowed a series of guidelines to be developed for a rational design of the conveyor-belt dryer with tangential flow for the specific case of the moisture content of the final product XF lower than the critical one XC (XF < XC). In fact, this work follows a precedent in which a mathematical model was developed through the differentiation of the drying rate equation along the dryer belt with the hypothesis that the final moisture content XF of the product was higher than the critical one XC. The relationships between the extensive quantities (air flow rate and product flow rate), the intensive quantities (temperatures, moisture content and enthalpies) and the dimensional ones (length and width of the belt) were then obtained. Finally, based on these relationships, the rules for an optimized design for XF < XC were obtained.
The mathematical modeling presented in this work concerns the conveyor-belt dryer with the tangential flow of air with respect to food. This dryer, if operating in co-current, has the advantage of ...well preserving the organoleptic and nutritional qualities of the dried product. In fact, it has a low air temperature in the final stretch where the product has low moisture content and is therefore more temperature sensitive. It is a bulkier dryer than the continuous through-circulation conveyor dryer with a perforated belt. The latter is therefore more frequently used and has received greater study attention from researchers and designers of the industry. With the aim to propose guidelines for a rational design of the conveyor-belt dryer with tangential flow, a mathematical model was developed here through the differentiation of the drying rate equation followed by its integration performed along the dryer belt. Consequently, and with the assumption that the final moisture content XF of the product is higher than the critical moisture content XC, the relationships between the intensive quantities (temperatures, humidity and enthalpies), the extensive quantities (air and product flow rates) and the dimensional ones (length and width of the belt), were obtained. Finally, on the basis of these relationships, the rules for an optimized design for XF > XC were obtained and experimentally evaluated.
Abstract
The popularity of heat drying of wastewater sludge has increased over the past several years because it can reduce sludge mass and volume, and hence disposal costs. However, drying sludge ...using conventional combustion-heated dryers is energy-intensive. Heat pump dryers can be efficient and offer significant energy savings by recycling the drying heat. This paper describes a heat pump dryer designed for continuous drying of industrial wastewater sludge. The dryer constructed was essentially a closed-loop air system. The air used for drying is dehumidified to recover the latent heat of vaporization, re-heated using the recovered heat, and recirculated in a closed environment. The closed-loop layout eliminates emissions of dust, malodorous gases, and volatile compounds, obviating the need for exhaust treatment otherwise required to meet environmental regulations. Data on the moisture extraction rate, specific moisture extraction rate, and specific energy consumption are presented and discussed.
This paper presents the mathematical model and numerical analysis of the convective drying process of thick slices of colloidal capillary-porous materials slowly moving through conveyor-belt dryer. A ...flow of hot moist air was used as drying agent. The drying process has been analyzed in the form of a 2-D mathematical model, in two directions: along the conveyor and perpendicular on it. The mathematical model consists of two non-linear differential equations and one equation with a transcendent character and it is based on the mathematical model developed for drying process in a form of a 1-D thin layer. The appropriate boundary conditions were introduced. The presented model is suitable for the automated control of conveyor-belt dryers. The obtained results with analysis could be useful in predicting the drying kinetics of potato slices and similar natural products.
nema
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This paper presents a mathematical model and numerical analysis of the
convective drying process of small particles of potatoes slowly moving
through the flow of a drying agent - hot moist air. The ...drying process was
analyzed in the form of a one-dimensional thin layer. The mathematical model
of the drying process is a system of two ordinary nonlinear differential
equations with constant coefficients and an equation with a transcendent
character. The appropriate boundary conditions of the mathematical model were
given. The presented model is suitable in the automated control. The
presented system of differential equations was solved numerically. The
analysis presented here and the obtained results could be useful in
predicting the drying kinetics of potatoes and similar natural products in a
conveyor-belt dryer.
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The paper presents an empirical soft-sensor model to control the moisture content of mate leaves in the discharge of a virtual conveyor-belt dryer. The empirical model is applied to infer the ...unmeasured control variable based on virtual transient readings of temperature in the solid phase of the dryer. The inferential control uses a proportional–integral–derivative (PID) controller, which manipulates the conveyor speed to keep the outlet moisture content of mate leaves between 0.024 and 0.034 (dry basis). The parameters of the soft-sensor model were tuned on a set of around 2000 experimental data of surface temperature and moisture content of mate leaves. These results emerged from batch drying experiments at between 70 and 120 °C for up to 9600 s using leaves with initial moisture content varying from 0.14 to 1.7 (dry basis). Open- and closed-loop responses of moisture content and temperature of the mate leaves were obtained by simulation with a dynamic model. It was represented by a system of two partial differential equations derived from energy and mass balance for water in the solid phase of the dryer. Closed-loop simulations in the presence of arbitrary process disturbances confirmed the reliability of the inferential designed controller.
►Dynamic process model simulates real-size drying plant for mate leaves. ►Inferential control keeps outlet moisture content between 2.4 and 3.4% (d.b.). ►The velocity of the conveyor-belt dryer is the manipulated variable. ►Soft-sensor model infers the control variable based on leaf temperature
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