Conveyor-belt Dryer Research Articles

A Dynamic Two-Dimensional Model for Deep-Bed Drying of Mate Leaves (Ilex paraguariensis) in a Single-Pass/Single-Zone Conveyor-Belt Dryer

A fundamental model for deep-bed drying of mate leaves in a cross-flow, single-pass/single-zone conveyor-belt dryer is suggested. It involves a set of four partial differential equations obtained by solute mass and energy balances in the solid and fluid phase of the dryer. A set of experimental results of moisture content, absolute humidity, air temperature, and solid temperature available in the literature for deep-bed drying of corn and rough rice in batch dryers was preliminarily used to validate the model. To confirm the reliability of the model for current purposes, drying experiments were carried out at steady-state conditions by feeding 0.05 m, 0.10 m, and 0.15 m deep beds of mate leaves in a bench-scale conveyor-belt dryer at inlet average drying temperatures close to 39°C, 47°C, and 53°C. Air and solid temperature at the top of the beds and average moisture content were well-captured by the model. A good agreement between experimental and calculated open-loop transient responses of average moisture of mate leaves in the discharge of the dryer, in the presence of imposed disturbances in the speed of the conveyor, was also observed. In all cases, the dynamic model was solved numerically by the method of lines without involving any adjustable parameter.

Автоматизированное управление процессом термообработки какао-бобов с использованием ПИД-регулятора

Thermal treatment processes such as roasting and drying are very energy intensive and considering the ever-increasing cost of energy, there is the need for continuous process improvement in order to reduce operational cost, while maintaining or improving the quality of the end cocoa products. This work describes the design and simulation of an automated control system (ACS) based on a PID controller algorithm for the control of the thermal treatment of cocoa beans in a conveyor belt dryer. The thermal treatment processes is modelled as a continuous, spatially distributed single input single output system (SISO), where the control variable is the temperature of the inlet drying air and the controlled variable, the product temperature in the drying chamber. Software and hardware requirement of the ACS were also presented and discuss. Numerical simulations of the ACS were carried out using Matlab and Simulink. Overall, the numerical simulation results show that the PID controller is stable and robust in terms of input disturbance rejection. The system provides relatively fast response in terms of eliminating offset and steady state error in the thermal treatment process.

Thin-Layer Drying of Mate Leaves (Ilex paraguariensis) in a Conveyor-Belt Dryer: A Semi-Automatic Control Strategy Based on a Dynamic Model

A feedback strategy of drying control of mate leaves in a thin-layer conveyor-belt dryer was experimentally evaluated. Moisture content in the discharge of the continuous dryer was controlled by manually adjusting the speed of the moving belt between 3.7 × 10−4 and 15.2 × 10−4 m s−1 for approximately 7200 s in 120 s time steps. The sets of PID controller parameters and manipulated conveyor velocities were computed with a dynamic drying model at conditions identical to those found in the closed-loop experiments. The model is represented by a system of two partial differential equations built by energy and solute mass balances in the solid phase of the dryer. A large set of experimental drying curves and temperature of mate leaves as a function of drying time, in the temperature range from 55 to 130°C, confirmed the reliability of the considered model. Experimental closed-loop responses of discharge moisture content in the presence of disturbances in the feed moisture content (≈ 0.5 − 1.7 dry basis) and variations in set-point (≈ 0.1 − 1.0 dry basis) validated the suggested control scheme.

Performance of a convective, infrared and combined infrared- convective heated conveyor-belt dryer.

A conveyor-belt dryer was developed using a combined infrared and hot air heating system that can be used in the drying of fruits and vegetables. The drying system having two chambers was fitted with infrared radiation heaters and through-flow hot air was provided from a convective heating system. The system was designed to operate under either infrared radiation and cold air (IR-CA) settings of 2000W/m(2) with forced ambient air at 30°C and air flow of 0.6m/s or combined infrared and hot air convection (IR-HA) dryer setting with infrared intensity set at 2000W/m(2) and hot at 60°C being blown through the dryer at a velocity of 0.6m/s or hot air convection (HA) at an air temperature of 60°C and air flow velocity 0.6m/s but without infrared heating. Apple slices dried under the different dryer settings were evaluated for quality and energy requirements. It was found that drying of apple (Golden Delicious) slices took place in the falling rate drying period and no constant rate period of drying was observed under any of the test conditions. The IR-HA setting was 57.5 and 39.1% faster than IR-CA and HA setting, respectively. Specific energy consumption was lower and thermal efficiency was higher for the IR-HA setting when compared to both IR-CA and HA settings. The rehydration ratio, shrinkage and colour properties of apples dried under IR-HA conditions were better than for either IR-CA or HA.

Soft-sensor model design for control of a virtual conveyor-belt dryer of mate leaves ( Ilex paraguariensis)

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

A study on the exergetic analysis of continuous textile dryers

This study deals with the exergetic analyses of the continuous textile dryers consisting of a stenter and a conveyor belt dryer. The exergy destructions in each of the components of the overall system are determined for average values of experimentally measured parameters. Exergy efficiencies of the system components are calculated to assess their performance. The effects of varying exhaust air humidity ratio and dead state temperature are also investigated. The total exergy efficiency values for the stenter and conveyor belt dryer are found to be 13.7% and 14.3% for the exhaust air humidity ratio of 0.1 kgair/kgwater, respectively.

A study on the exergetic analysis of continuous textile dryers

This study deals with the exergetic analyses of the continuous textile dryers consisting of a stenter and a conveyor belt dryer. The exergy destructions in each of the components of the overall system are determined for average values of experimentally measured parameters. Exergy efficiencies of the system components are calculated to assess their performance. The effects of varying exhaust air humidity ratio and dead state temperature are also investigated. The total exergy efficiency values for the stenter and conveyor belt dryer are found to be 13.7% and 14.3% for the exhaust air humidity ratio of 0.1 kgair/kgwater, respectively.

From Laboratory Experiments to Design of a Conveyor-Belt Dryer via Mathematical Modeling

A conveyor-belt dryer for picrite has been modeled mathematically in this work. The necessary parameters for the system of equations were obtained from regression analysis of thin-layer drying data. The convective drying experiments were carried out at temperatures of 40, 60, 80, and 100°C and air velocities of 0.5 and 1.5 m/sec. To analyze the drying behavior, the drying curves were fitted to different semi-theoretical drying kinetics models such as those of Lewis, Page, Henderson and Pabis, Wang and Singh, and the decay models. The decay function (for second order reactions) gives better results and describes the thin layer drying curves quite well. The effective diffusivity was also determined from the integrated Fick's second law equation and correlated with temperature using an Arrhenius-type model. External heat and mass transfer coefficients were refitted to the empirical correlation using dimensionless numbers (J h , J D = m · Re n ) and their new coefficients were optimized as a function of temperature. The internal mass transfer coefficient was also correlated as a function of moisture content, air temperature, and velocity.

ANALYSIS OF THE DRYING OF RESIDUAL SLUDGE: FROM THE EXPERIMENT TO THE SIMULATION OF A BELT DRYER

ABSTRACT The drying of residual sludge is a current environmental problem not sufficiently described in the literature, hence research investigations on this dewatering process are required. This paper proposes a modelling of a conveyor dryer belt. However the study of residual sludges dewatering in order to design or simulate drying equipment requires some experimental investigations. An analysis in terms of drying kinetics is proposed in this paper. Specific experiments were also needed in order to better describe some particular aspects such as crusting, shrinkage, and particle size impact. According to these data some analytical expressions have been derived and integrated in a belt dryer model. The set of air operating conditions (T, RH, v) was reduced to a single parameter, the drying potential. The results of the simulations show that a conveyor belt dryer is not well adapted to activated sludge. This is essentially due to a crust phenomenon. For a PVC industrial sludge this kind of dryer is more efficient and allows drying with reasonable residence times. Several simulations were run in order to point out the relevant parameters of the process.

Pareto design of conveyor-belt dryers

Most research efforts in the field of conveyor-belt dryer design focus on the evaluation of the appropriate structural and operational process variables so that total annual plant cost involved is optimized. However, the increasing need for dehydrated products of the highest quality imposes the development of new criteria that together with cost components, determine the design rules for any drying process. Quality of dehydrated products is a complex resultant of properties characterizing the final products, where the most important one is color. Color is determined as a three-parameter resultant, whose values for products, which have undergone drying should deviate from the corresponding ones of natural products, as little as possible. In this case, conveyor-belt dryer design is a complex multi-objective optimization problem, involving the color deviation and the unit cost of final product as an objective vector and as constraints the ones deriving from the process mathematical model. The mathematical model of the dryer was developed and the fundamental color deterioration laws and critical financial parameters were determined for the conveyor-belt drying process. Non-preference multi-criteria optimization methods were used and the Pareto optimal set of efficient solutions was evaluated. An example covering the drying of sliced potato was included to demonstrate the performance of the design procedure, as well as the effectiveness of the proposed approach.

PRODUCT QUALITY MULTI-OBJECTIVE DRYER DESIGN

ABSTRACT Design of conveyor-belt dryers constitutes a mathematical programming problem involving the evaluation of appropriate structural and operational process variables so that total annual plant cost involved is optimized. The increasing need for dehydrated products of the highest quality, imposes the development of new criteria that, together with cost, determine the design rules for drying processes. Quality of dehydrated products is a complex resultant of properties characterizing the final products, where the most important one is color. Color is determined as a three-parameter resultant, whose values for products undergone drying should deviate from the corresponding ones of natural products, as little as possible. In this case, product quality dryer design is a complex multi-objective optimization problem, involving the color deviation vector as an objective function and as constraints the ones deriving from the process mathematical model. The mathematical model of the dryer was developed and the fundamental color deterioration laws were determined for the drying process. Non-preference multi-criteria optimization methods were used and the Pareto-optimal set of efficient solutions was evaluated. An example covering the drying of sliced potato was included to demonstrate the performance of the design procedure, as well as the effectiveness of the proposed approach.

Design and operational performance of conveyor-belt drying structures

The design and operational characteristics of conveyor-belt dryers constitute an important field of chemical engineering, which is still governed by empiricism. In this work, both aspects were studied in a straightforward way based on mathematical reasoning. A mathematical model describing the convective drying process was developed. Design procedures aimed at the determination of optimum equipment arrangement, size and operational characteristics for conveyor-belt dryers were carried out by optimizing the total annual cost of each equipment arrangement for a given production capacity. All dryer arrangements were compared by evaluating optimum configurations for a wide range of production capacity values. Once the dryer configuration was specified, its operational performance was evaluated by comparing the optimum operational cost versus production capacity for a specific optimum designed structure. An example covering the drying of sliced potato is included to demonstrate the performance of each design case, as well as the effectiveness of the proposed approach.

Designing Dryers Using Heat and Mass Exchange Networks: An Application to Conveyor Belt Dryers

Classical 1-D numerical simulations of dryers involve the solution of non linear differential relations which include balance equations and transfer functions; namely, the drying rate equation and the heat transfer equation. These relations are integrated by means of the network theory and a new approach—the Number of Transfer Units method. This process leads to a non-linear algebraic system used to develop computer simulation programs. In this paper, the applications are based on the study of multiple-conveyor dryers. This type of dryer is used for many chemical engineering industrial applications to dry coarse particles (i.e., over 5 to 10mm) or for food engineering. Numerical simulations are performed to analyse the drying efficiency of these conveyor dryers. Due to the heat and mass numbers of transfer units and to the network idea, the computer programs run faster than classical ones. This program is involved in the expertise and study of the effectiveness ofmultiple-conveyor dryers. Some rules of behaviour of conveyor dryers are discussed and may be used in order to improve their design.

Drying of solids: Selection of some continuous operation dryer types

Design and study of operational performance of convective industrial dryers are a strong indicator in the last stages of the selection procedure. This problem is addressed in a straightforward way for the three most popular types; conveyor-belt, fluidized bed and rotary dryers. Design was carried out by appropriately optimizing the total annual cost of each configuration for a given production capacity. All dryer types were compared explicitely by evaluating optimal configurations for a wide range of production capacity values. Once the dryer configuration is specified, its optimal operational performance was evaluated by comparing the optimum operational cost versus production capacity for predefined optimal designed structures. Rotary dryers turned to be rather expensive compared to fluidized bed dryers, regarding design. On operational grounds, however, it is the other way around due to the understandably favored heat transfer achieved in rotary dryers. Conveyor-belt dryers, when utilized, lie somewhere in between producing satisfactory results both on design and on operational grounds. A characteristic example of food dehydration process is included, to demonstrate the effectiveness of the proposed approach.

Design and production planning for multiproduct dehydration plants

Drying operations constitute an important field of chemical engineering, which is still governed by empiricism. This paper deals with two important aspects related to the construction and operation of dehydration plants. The design problem involves the determination of process flowsheet structure, when a nominal production level is specified for all products processed in the plant. The production planning problem describes the production policy of the plant within a long-range planning horizon, under time-varying product demand and market prices of raw materials and saleable products. The policy adopted assigns production levels and duration of production runs for each product processed, in each one of the plant processors, at a certain time period, within the planning horizon. The objective in each approach is to optimize the total annual profit resulting from the construction of a new plant or the operation of an existing one. The process was described by deducing the mathematical model of conveyor-belt dryers. For the forementioned problems, appropriate formulations were developed and studied. The most general model presented involves numerous integer and continuous decision variables and a large number of space variables and constraints, resulting in cumbersome calculations and tremendous computational load. For the reduction of the computational effort, a shortcut modeling of total annual plant cost was proposed and evaluated from the operational data of a large number of possible flowsheet structures, through a simple analytical equation. The parameters of the proposed shortcut equation were estimated by nonlinear regression over an extensive number of computed points; each one of them was determined by solving an NLP optimization problem. The design and production problems were formulated as MINLP problems in which use of the shortcut cost equation reduced drastically the computational effort involved. Related problems (i.e. process modification and design under production planning criteria) were also taken into consideration. Characteristic examples were presented in order to demonstrate the effectiveness of each proposed approach.

MIMO Control of Conveyor-Belt Drying Chambers

ABSTRACT The development of MlMO control systems for individual conveyor-belt drying chambers was studied for the case of industrial units used for the moisture removal from wet raisins. The process was considered to be a 2×2 open-loop system in which material moisture content and temperature at the exit of the drying chamber were to be controlled. Its dynamic behavior was investigated via digital simulation of the corresponding process mathematical model which involves 6 state variables. The effect of several manipulated and load variables on process outputs was explored by examining the corresponding responces obtained when the input variables were step forced into the non-linear dryer model simulator around the operational point studied. Reliable transfer functions for each interaction module were produced, based on simulated data and process basic constants. The best control configuration was selected bv deriving the RGA values of each one that was based on its frequency information. The propsed schem...

Production planning for multiproduct dehydration plants

A nonlinear programming (NLP) approach to short- and long-term production planning for a multiproduct dehydration plant involving continuous conveyor belt dryers, utilized for the dehydration of agricultural products over a finite time horizon, is presented in this paper. Limitations imposed by storage capacity and customer orders, as well as constraints dictated by supply and demand patterns of raw materials and final products, respectively, are considered. The two problems dealt with are expressed in NLP formulations, which include in their objective function an operational cost component, inferred by deducing the mathematical model for all units participating in the plant. Characteristic examples are presented to demonstrate the effectiveness of the proposed approach.

Modelling and design of conveyor belt dryers

A mathematical model suitable for the design of conveyor belt dryers was developed. The objective of design was the evaluation of optimum flowsheet structure, construction characteristics, and operational conditions. The methodology followed was based on constructing a superstructure which involves a large number of minor structures, and optimizing it by means of non-linear mathematical programming techniques. The optimization problem can be stated in an equivalent way so that the computational effort involved in its solution is greatly reduced. This decomposition strategy transforms the original problem into an optimization problem of a major and a minor stage.

DYNAMIC SIMULATION AND CONTROL OF CONVEYOR-BELT DRYERS

Abstract The dynamic behavior of conveyor-belt dryers involving externally controlled heat and mass transfer phenomena has been studied via digital simulation. The investigation concerned an industrial dryer used for the moisture removal from wet raisins. The dryer consisted of three drying chambers and a cooling section, all involving the same conveyor belt. For each chamber, perfect temperature control was assumed for the drying air temperature, while its humidity was left uncontrolled. The effect of material temperature and moisture content at the entrance of the dryer and the drying air temperature on material temperature and moisture content at the exit of the dryer and the corresponding drying air humidity, have been explored by step forcing the disturbance and manipulated variables in the non-linear dryer model simulator. Results showed that material moisture content at the exit of the dryer is greatly affected by material moisture content at the entrance as well as by the drying air temperature. R...

MODELING AND OPTIMIZATION FOR DESIGN UNDER PRODUCTION PLANNING CRITERIA IN MULTIPRODUCT DEHYDRATION PLANTS

ABSTRACT The design problem of a multiproduct dehydration plant involving continuous conveyor-belt dryers utilised for the dehvdration of agricultural products over a fmite tim horizon, is presented. The peculiarities of the problem under demand-varying market patterns and time-variable market prices of raw materials are underlined. Limitations imposed by storage capacity, custormer orders, and constraints dictated by the process itself, are also taken into consideration. The overall problem is cast into a MINLP formulation involving the total annual cost of the plant as its objective function. This was infered by deducing the mathematical model of conveyor-belt dryers. Repeated evaluation of the total annual cost for various structures and operating points, allowed the determination of overall performance characteristic operational and structural curves. approximated by suitable mathematical expressions. A test case is presented in order to demonstrate the effectiveness of the proposed approach.