Dynamics And Assessment of Some Control Strategies of a Simulated Industrial Rotary Dryer

Rotary dryers are widely used for the continuous drying of minerals and chemicals on a large scale. Hot gases are passed parallel to the flowing solid to achieve the desired product moisture content. Because these dryers are energy intensive, it is mandatory to operate them as efficiently as possible to respond to economic pressures. Using a dynamic rotary dryer simulator for mineral concentrate, five control strategies are evaluated and compared. Two control strategies are based on PI controllers and the others use neural network models. Results clearly show that a feedforward action, in conjunction with a PI controller or incorporated within the structure of a neural network model, led to the best performances provided an accurate measurement of the feed moisture content is available.     

I.D.S'90, PRAGUE,CZECHOSLOVAKIA

ABSTRACT

Fluidized bed dryers are utilised in almost every area of drying applications and therefore improved control strategies are always of great interest. The nonlinear character of the process, exhibited in the mathematical model and the open loop analysis, implies that a fuzzy logic controller is appropriate because, in contrast with conventional control schemes, fuzzy control inherently compensates for-process nonlinearities and exhibits more robust behaviour. In this study, a fuzzy logic controller is proposed; its design is based on a heuristic approach and its performance is compared against a conventional PI controller for a variety of responses. It is shown that the fuzzy controller exhibits a remarkable dynamic behaviour, equivalent if not better than the PI controller, for a wide range of disturbances. In addition, the proposed fuzzy controller seems to be less sensitive to the nonlinearities of the process, achieves energy savings and enables MIMO control.     

Evaluation of Simple Control Strategies for Rotary Dryers

Abstract

Rotary dryers are very versatile process equipment as it can accommodate a large variety of products over a wide range of flow rates. To obtain a uniform product quality and to operate the dryer economically with minimal environmental impact, it is necessary to use efficient control strategies. In this article, two simple control strategies are proposed to improve the feedback-only controller that is normally used. One strategy is a partial feedforward control loop addition to the feedback loop and the second is a moisture predictor that allows taking control actions on the estimates of the discharge moisture content rather than waiting to measure its value at the discharge. The performance of these two strategies was compared with the performance of a feedback-only controller and an ideal feedback-feedforward controller. It is shown that the two proposed control strategies significantly improve the performance of a feedback-only control strategy without having to install new sensors.     

HYBRID NEURAL APPROACHES FOR MODELLING DRYING PROCESSES FOR PARTICULATE SOLIDS

ABSTRACT

This work presents methods for synthesizing drying process models for particulate solids that combine prior knowledge with artificial neural networks. The inclusion of prior knowledge is investigated by developing two applications with the data from two indirect rotary steam dryers. The first application consisted in the modelling of the drying process of soya meal in a batch indirect rotary dryer, The external and internal mass transfer resistances were associated in the hidden layer of the network to linear and sigmoidal nodes, respectively. The second application consisted in the modelling of the drying process of soya meal in a continuos indirect rotary dryer. The model was constructed using the Semi-parametric Design Approach. The model predicts the evolution of solid moisture content and temperature as a function of the solid position in the dryer. The results show that the hybrid model performs better than the pure “ black box” neural network and default models. They also shows that prior knowledge enhances the extrapolation capabilities of a neural network model,     

AUTOMATIC CONTROL OF COMMERCIAL CROSSFLOW GRAIN DRYERS

ABSTRACT

An unsteady state model of crossflow grain drying has been developed. The model has been used to develop several empirical models as process models for a feedforward dryer control system.

The control system consists of an empirical drying model, an on-line moisture meter, a tachometer, data acquisition software, a microcomputer, and the feedforward/feedback control software. It has been tested on several commercial crossflow maize (corn) dryers. With the inlet grain moisture content varying between 16.1% to 34.3% (w.b.), the new control system controlled the outlet grain moisture content to + 0.6% of the set point.     

CONVENTIONAL BASIC DESIGN FOR CONVECTION OR CONDUCTION DRYERS

Abstract

The items to be considered prior to selection of dryers are explained, and a simple method for a rough estimation of dryer sizes was proposed based on data obtained from operating industrial dryers.

The equations of basic design for batch or continuous type dryers were derived. The heat was supplied to materials by convection and/or conduction. The equations were simplified to the case when the falling rate of drying is proportional to the moisture content of materials under the constant drying conditions. The heat transfer coefficient used in the equations can be determined based on the calculations or the data obtained from the experimental or industrial dryers. The equations are useful for estimating the scale-up effect of dryers.     

COMPARISON OF TIMBER DRYING USING SOLAR ENERGY,ELECTRICAL HEATING AND DEHUMIDIFIER.

ABSTRACT

The performance of three different types of dryers for the hot air drying of sawn-limber planks are compared. These were the electric resistance dryer, solar dryer, and the dehumidifier dryer. Whilst the electric and solar dryers depended only upon hot air for drying, the dehumidifier dryer relied on hot dehumidified air. The results of investigations carried out on timber drying employing these three types of dryers in the Engineering Faculty are compiled and compared here in this paper. The results showed that the electric dryer produced the fastest drying lime and lowest moisture content, followed by dehumidifier drying. The solar dryer achieved a lower moisture content and a faster drying rate compared to natural drying, although the difference in drying times was marginal.     

AMARANTH GRAIN DRYING IN A FLUIDIZED BED

ABSTRACT

An empirical equation as a function of drying time and temperature was developed to calculate the moisture ratio required Tor balch fluidized bed dryers design for amaranth grain drying

The relative deviations of the moisture ratio values calculated with the proposed equation with regard to the experimental ones ranged between 0.014 and 0.095 for a drying air temperature between 60 and 100 ° C, and a grain initial moisture content between 23.5 and 16.6 % wb.     

Drying Events in Lódz,Poland

ABSTRACT

Printing and heavier grades paper are dried over steam heated cylinder dryers, a mature process which is characterized by low drying rates and lack of ability for control of moisture uniformity across the sheet. The present study examines factors relating to the applicability of through air drying for such grades of paper. This drying process yields much higher drying rates but now is restricted to high permeability, light weight products such as tissue and toweling as the cost of providing through flow across the sheet is the key economic factor in this technique. With over 400 through drying experiments the parameters characterizing the drying rate curves were determined for 10 quite different types of uncalendered paper produced on a variety of papermachines.

The relation between drying conditions and two characteristics, the moisture content at the end of increasing rate period and the constant drying rate, did not vary significantly between these types of paper. However, the extent of the constant rate and falling rate drying periods varied substantially as the critical moisture content is specific to the type of paper.     

MATHEMATICAL MODEL DEVELOPMENT AND SIMULATION OF HEAT PUMP FRUIT DRYER

ABSTRACT

A mathematical model of a heat pump fruit dryer was developed to study the performance of heat pump dryers. Using the moisture content of papaya glace' drying, the refrigerant temperature at the evaporator and condenser and the performance, was verified. It was found that the simulated results using closed loop heat pump dryer were close to the experimental results. The criteria for evaluating the performance were specific moisture extraction rate and drying rate. The results showed that ambient conditions affected significantly on the performance of the open loop dryer and the partially closed loop dryer. Also, the fraction of evaporator bypass air affected markedly on the performance of all heat pump dryers. In addition, it was found that specific air flow rate and drying air temperature affected significantly the performance of all heat pump dryers.     

A Method to Measure Emissions from Dryers with Diffuse Leakages,Using Evaporated Water as a Tracer

Abstract

This paper describes a novel method to measure emission from dryers. The method resolves the known difficulties caused by diffuse emissions, and also solves the problems associated with high moisture content of the drying medium. The basic idea is to use water vapor to determine the exhaust flow, while a dry ice trap is used both to preconcentrate emitted VOCs and to determine the moisture content of the drying medium.     

Drying of Carrots in a Fluidized Bed. II. Design of a Model Based on a Modular Neural Network Approach

Abstract

The present paper deals with the design of a neural network type model for drying of carrots, which includes the associated transport mechanisms of the process. The model uses the operational variables and the time as input parameters. Two sub-layers of linear and sigmoidal nodes make up the hidden layer, to represent the external and internal resistances to the diffusion of water vapors during the drying process. The single output node weights the contribution of each mechanism of the drying process to predict the exit moisture content of the product. This model was used to predict the drying of carrot particles in a mechanically fluidized bed dryer reported in a previous paper [Reyes, A.; Alvarez, P.; Marquardt, F. Drying of carrots in a fluidized bed: I.- effects of drying conditions and modeling. Drying Technology 2002, 20 (7), 1463–1483.]. Simulated drying curves obtained with this model fits adequately the curves determined experimentally for the most operation conditions, which would indicate that this model is appropriate to be used for rough estimations in the design, the selection of optimal operational conditions, and the scaling up of dryers.     

A New Algorithm for Dryer Selection

ABSTRACT

Selection of industrial dryers is one of the most complex and poorly understood areas in drying technology. A new procedure has been developed by SPS to assist the choice between the many types of dryers and ancillaries currently available. Dryers are classified by various criteria and this information is used to construct an equipment me. Likewise, information is collected on the material to be dried. including the results of bench-scale tests, and basic flowsheet parameters such as solids throughput and required reduction in moisture content. A five-step algorithm is then used to analyse the problem; at each step, mles based on the material and flowsheer considerations are used to test the different dryers in the equipment tree. Each individual dryer is awarded a merit score and may be eliminated if clearly unacceptable; the merit scores for each remaining dryer can be compared at the end of the procedure. Rough sizing and costing calculations are also included. Two worked examples are provided.     

SCALE-UP OF CONTACT DRYERS

ABSTRACT

The scale-up of contact dryers is still based on experimental drying curves. In order to keep the effort to a minimum the drying curve is determined using a small laboratory or pilot dryer of similar geometry to the production dryer.

This paper introduces a new scale -up method for contact dryers. The new scale-up method is based on the assumption that heat transfer is the controlling mechanism. The scale-up method is derived from the material balance, the energy balance, the kinetic equation of heat transfer and thermodynamic equilibrium. The scale up method can be used to convert the drying time required to achieve a certain residual moisture content from the laboratory or pilot dryer to the production dryer and/or different drying conditions.

The scale-up method was verified by drying test with four different products in conical mixer dryers of 1, 60, 250, 1000 I volume. Two products were free flowing and two products were non free flowing in the wet state. The products can be considered non-hygroscopic in the moisture range investigated.     

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. Reliable transfer functions for each process module were obtained by fitting several transfer function models on the simulated data using a least-squares approach. It was found that when input material moisture content could be instantly measured, the system responded slowly enough so that excellent control could be achieved for material moisture content at the exit of each chamber. In this case a Pi-feedback cascade temperature controller was used. When a 15 sec delay measuring sensor was introduced, poor performance was observed. A simplified lead-lag feedforward controller, added to the system, in conjunction with the primary Pi-feedback cascade controller, resulted in good control performance of the delay sensor system.     

ARTIFICIAL NEURAL NETWORKS: PRINCIPLE AND APPLICATION TO MODEL BASED CONTROL OF DRYING SYSTEMS - A REVIEW

Abstract

This paper reviews the developments in the model based control of drying systems using Artificial Neural Networks (ANNs). Survey of current research works reveals the growing interest in the application of ANN in modeling and control of non-linear, dynamic and time-variant systems. Over 115 articles published in this area are reviewed. All landmark papers are systematically classified in chronological order, in three distinct categories; namely, conventional feedback controllers, model based controllers using conventional methods and model based controllers using ANN for drying process. The principles of ANN are presented in detail. The problems and issues of the drying system and the features of various ANN models are dealt with upto-date. ANN based controllers lead to smoother controller outputs, which would increase actuator life. The paper concludes with suggestions for improving the existing modeling techniques as applied to predicting the performance characteristics of dryers. The hybridization techniques, namely, neural with fuzzy logic and genetic algorithms, presented, provide, directions for pursuing further research for the implementation of appropriate control strategies. The authors opine that the information presented here would be highly beneficial for pursuing research in modeling and control of drying process using ANN     

Control of the Moisture Content of Milk Powder Produced in a Spouted Bed Dryer Using a Grey-Box Inferential Controller

Although the moisture content of dried products is an important variable in industrial dryers, it is often not measured directly for control purposes. Alternative and simpler meters might provide information to be used in a physical-mathematical model to estimate the moisture content. When this procedure is applied to a control strategy, an inferential controller is developed. In this article, a physical-mathematical model was used to infer the moisture content of milk powder produced in a spouted bed dryer. Afterwards, simulations of an inferential proportional-integral controller were carried out using the inlet air heating rate as the manipulated variable. The physical-mathematical model used in the procedure was a hybrid model, which considers mass and energy balances and one term which is estimated by an artificial neural network. The controller parameters (controller gain and integral time) were tuned by trial and error. Even though the procedure was quite simple, it was proven to be effective in yielding a stable closed-loop response for both servo and regulatory control of the (inferred) powder moisture content.     

SIMULATION OF DEEP BED DRYING OF HAZELNUTS

Abstract

The deep bed drying modelling of hazelnut (Corylus avellana L.) have been studied by considering the deep bed as a series of thin layers. A partial differential equations model has been developed to simulate heat and mass transfer in fixed deep bed hazelnut dryers. The computer program developed permits the calculation of temperature and moisture content profiles along the dryer. As a process of validation of the model, the predicted and experimental results of average moisture content of the whole bed were compared. It has been observed that the simulation results agreed quite well with experimental data.     

SPRAY DRYING DYNAMIC MODELING WITH A MECHANISTIC MODEL

ABSTRACT

In this work we suggest the dynamic modeling of a spray dryer considered as a series of well-stirred dryers. That is, a series of dryers in which the output variables are equal to the state variables. The state equations were obtained from the heat and water mass balances in product and air. Additionally, heat and water mass balances in interface jointly with water equilibrium relation between product and air were considered. A pilot spray dryer was modeled assuming one, two, five and 20 well stirred steps. Low-fat milk with 10–20% of solids was dried at different inlet air temperatures (120–160°C), air flow rate of 0.19 kg dry air s^?1 and different feed rates (1.4 ? 4.2 × 10^?4 kg dry solids s^?1). Stationary result showed that the model predicts the experimental air outlet temperature, at different inlet conditions with a maximum deviation of 6°C. The dynamic simulation reproduce the experimental one with moderate accuracy. Experimental dynamic showed that the pilot plant spray dryer has a well-stirred process behavior. The model represents a method for estimate outlet product moisture as function of the outlet air temperature. This has application for automatic control because there is not an easy way to measure on-line measure the outlet product moisture content.     

RELATIONSHIP OF PROCESS PARAMETERS AND STACK WIDTH IN TIMBER DRYING

ABSTRACT

Drying schedules applied in kiln drying, especially for hardwood species should be supplemented with values of air velocity. When dryers have no air velocity control their drying schedule can be suitably corrected. The correction should take into account factors related to dried material, i.e. wood moisture content, timber thickness, dried timber volume and primary width of a stack. Drying efficiency may be the criterion of modifications. The applied procedure of drying efficiency calculations lets to determine quantitative relationships between process and material factors.