An integrated computer-based dryer simulator

Computers chem. Engng, Vol. 18, Suppl., pp. S265-S269, 1994 Printed in Great Britain

0098-1354/94 $6.00+0.00 Pergamon Press Ltd

AN INTEGRATED COMPUTER-BASED DRYER SIMULATOR

C.T.KIRANOUDIS, Z.B.MAROULIS, D.MARINOS-KOURIS Department of Chemical Engineering, National Technical University, GR-15780, Athens, Greece

ABSTRACT A software package for steady-state simulation of drying processes is described. A brief review of the simulator structure, along with an outline of its operation is presented. User input to the simulator consists of three parts; the flowsheet description-ie, the process units and the streams connecting them; the design problem data-ie, the feed stream data, the specifications of unit design variables and major economic figures concerning cost; the convergence algorithm specifications-ie, the tear streams and an initial guess for their values. This information is passed to the simulator by means of an appropriate user interface developed in object-oriented Pascal code on an Apple Macintosh computer. This part of the program is then introduced into the execution part of the simulator, performing all the necessary flowsheet computations. A case study is also presented for the design of a dehydration plant for vegetable drying.

KEYWORDS Dryer Design; Conveyor Belt Dryer; Dryer Simulation; Dryer Flowsheet Structure.

INTRODUCTION Process flowsheeting and simulation has been in the focus of much research over a time span of more than three decades (Evans et al., 1968; Flower and Whitehead, 1973). During this time, a large number of process simulation products have been produced, both in the commercial and the academic field, basically confronting the simulation of processes involved in the conventional chemical plants. Their differences are mainly in the way that flowsheet equations are solved, and the facilities they provide (Rosen and Pauls, 1977; Pantelides, 1988). The need for detailed simulation of plants compriSing units and streams that greatly differ in structure from those of a conventional chemical plant, leads to the construction of specific simulators which treat the particular processes in a more dedicated way, aiming at producing more accurate and robust results within an environment which is tailored to the needs of each flowsheet (Bush and Silveston, 1978; Neville and Seider, 1980; Petrides et al., 1985; Spinos and Marinos-Kouns, 1992; Papafotiou et al., 1992). Although drying processes are included in the computational environment of many commercial process simulators, the mathematical models used are simple, and therefore inappropriate for more detailed simulation, while the database of thermophysical properties and transport phenomena constants is limited to some commercially popular solids. Furthermore, the peculiarities of each process unit and stream that make up a dehydration plant are not taken into consideration (Houska et al., 1988). In this study we present a computer-based tool that was built for simulation purposes of drying processes. The DryerDesigner program is a modular steady-state simulation program employing models for units and streams, which together complise the flowsheet of a dehydration pla....'lt.

STRUCTURE OF DryerDesigner SIMULATOR Simulation of a process flowsheet requires modeling of the unit operations in the process. These models have been coded into computer software (Kiranoudis, 1992). The users of the program are required to specify cenain information about each operation in the process. Based on the specifications, the model will calculate the performance of the unit. Any number of units can be connected to represent a panial or a complete flowsheet. The starting point for developing a process model is the process flowsheet (Fig. 1). From this point, the engineer prepares a block diagram of all unit and feed streams. The next step is to specify flowrates, specific design requirements, and operating conditions for each unit operation block. S265

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Table 1. Major subsystems of the DryerDesigner Simulator

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These steps define the problem to the simulator. The simulator then produces reports predicting the process performance. including the composition and properties of. all streams. and the size and performance of the individual process units. The simulator will also estimate the capital and operating cost of the plant (Fig. 1). The major subsystems of the DryerDesigner simulator are shown on Table 1. From this Table we can infer that many of the requirements for Simulating a dehydration process are the same as for simulating other chemical manufacturing processes. The differences observed are mainly due to the nature of the process streams, units. and products. compared to those of a conventional chemical plant.

Unit operations models are the core of a process simulator. The more rigorous and detailed these models are. the better job the engineer can do in modeling the processes in question. The total group of unit operation models available in Dryer Designer simulator is listed on Fig. 1. Flow informalion in a simulation run Table 2. In order to reduce the computational effort and enhance the robustness of performance. sequential algorithms were developed for almost all models. In all remaining cases. an iterative quasi-Newton algorithm was used in order to achieve convergence of the system of equations. An important part of the unit operations models is the group "Of steady-state conveyor belt drying chambers used for the dehydration of various agricultural products and solids. total annual cost

The physical property models and data system includes models for thermophysical properties of streams and transport coefficients for the drying phenomena involved. It comprises calculation of specific enthalpies. modeling of the phychrometric chart, models for thermodynamic equilibrium between gas and solid phases. and heat and mass transport coefficients for the drying process. The parameters of these models were estimated by regression of the particular models to experimental data for various agricultural products and solids participating in a dehydration plant. Evalurtion of the total annual cost is carried out by estimating its components which represent the capital and operating cost. respectively. The operating cost concerns thermal and electrical energy consumed at heat exchangers and funs. The capital cost is basically affected by the belt area of each drying chamber (a measure of construction expenses), the area of heat exchangers involved, and the drying air flowrates which determines the cost of fans. The execution system is an essential part of the simulator. Its tasks are the input and output control. the generation of the structured problem description, the automatic generation of the plant overall model for a specified simulation problem, and the control of the simulation convergence itself. There is some important information for the simulator. The process configuration, the initial product and air stream characteristics. and the design parameters of the various units participating in the total fiowsheet. The process configuration describes the kind of units that make up the plant. and their interconnections within the various process streams. The basic characteristics of input streams are their fiowrate. moisture content, and temperature. Finally, the design parameters concern operating conditions and construction characteristics for each unit. The simulator is based on the sequential modular approach. The calculation for each unit is organised into modules which are solved sequentially. Specification of variable values at certain points of the fiowsheet, as well a~ convergence of recycle streams is carried out by means of secondary units called stream convergers and controllers. The non-linear equations imposed by stream converger and controller units. were solved simultaneously. In this way, the problem of potential interactions between variables of the recycle streams as well as the controller variables was handled.

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OUTLINE OF DryerDesigner OPERATION The DryerDesigner simulation program was developed in object-oriented Pascal on an Apple Macintosh Computer (Kiranoudis, 1992). The basic data structures involved were polymorphic object classes which performed all necessary actions for user interfacing and flowsheet simulation. The abstract entities which served as the control center for managing the storage, maintenance, and display of the program data, were the documents of the program. Storage was managed by reading and saving data from the application file, while maintenance was achieved by updating the objects and keeping track of the events. The object classes used in the program involve both application dependent classes, such as unit and stream objects, as well as classes of the Macintosh interface, such as document windows, pull-down menus, buttons and dialogs. The document window of each application is shown in Fig. 2. It contains a palette of various graphical tasks as well as pull-down menus of various operational tasks. The palette is activated by the mouse by means of graphical cursors. Each cursor performs a different graphical task, such as creation of a new object, specification of its type, connection of unit objects by means of stream objects, erasing and displaying its parameter values by means of meaningful dialogs. Each pull-down menu performs a different operational task, such as keeping up with files, displaying, and converging the total flowsheet. In a Macintosh application, there is always a way to get user-generated events and dispatch them to the code which can handle them. When the user clicks the mouse in the menu bar, in a push button, in a document window, or a keyboard key is pressed, there is always an event-handler chain which assumes control of the information flow in the application.

DESIGN CASES The proposed simulator basically focuses on the examination of different design structures of process flowsheets for a dehydration-oriented plant. A typical flowsheet of such plants involves conveyor belt drying chamber units placed in series and grouped together into drying sections which are provided with a common conveyor belt. Each drying chamber is equiped with an individual heating utility and fans. Temperature and humidity of the drying air stream are controlled by an individual control system within each drying chamber. The objective of the design strategy can now be clearly stated. Given a specified product with known flowrate, to be dried from an initial to a desired final moisture content level, the following must be determined:

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(i) the flowsheet structure (evaluation of the number of drying sections and the number of chambers per section). (ii) the appropriate flowsheet construction characteristics (sizing of units) (iii) the operating conditions within each chamber (determination of the set points of controllers) The DryerDesigner simulator can be used for the determination of an appropriate flowsheet structure together with a simplified design procedure, by means of which various rival flowsheets were examined. The objective of this case was the determination of an appropriate flowsheet for the dehydration of 2400 tns/y of raw potato. The raw material which is cut in 10 mm cubes, enters the processing units at a rate of 200 kg/h db. Its initial

moisture content is 5 k.g/kg db and its desired level at the exit is 0.05 k.g/kg db. The product should not be heated to temperature levels exceeding 75°C, in order to prevent thermal degradation due to high temperatures. The operation horizon of the plant is 2000 h/y and its capital cost will be paid off within a period of 5 years. To begin with, a single-section dryer is examined involving equal sized drying chambers of 5 m' each. Temperature and humidity are kept uniform in all chambers. Temperature level is adjusted to its maximum level allowed. The desired material moisture content level at the exit of the dryers is achieved by regulating the drying air stream humidity which was kept uniform in all chambers. The total annual cost of the plant as a function of the number of drying chambers used in this single section dryer, is shown in Fig. 3a Oearly, an optimum was found for a total number of 16 chambers. The optimum annual cost was found to be $ 17
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Table 2. Unit Operation Models in !he DryerDesigner Simulator

Drying Chambers

Input, Output Units

Product Stream Feed, Air Stream Feed Product Stream Reject, Air Stream Reject

Mixer, Splitters and Separators Product Stream Mixer, Product Stream Splitter Air Stream Mixer, Air Stream Spliner

Flowsheet Convergence Air Stream Converger Controller

Units

Uncontrolled Conveyor Belt Temperature Controlled Conveyor Belt Humidity Controlled Conveyor Belt Temperature and Humidity Controlled Conveyor Belt Grouping Unit

Heat Exchangers, Heat Pumps General Process Heater/Cooler Air Cooler Output Temperature Controlled Plate Heat Exchanger Area Given Plate Heat Exchanger General Purpose Heat Pump

By means of the DryerDesigner simulator, parametric investigation of the flowshect can also be performed. At the single-section scheme of the 16 chambers, the way in which drying air humidity, which regulates the material moisture content at the exit of the section, varied when changing the flowrate of the input product, is shown in Fig. 3d. When the flowrate of the input product rises, the air stream humidity becomes smaller, that is to say, the drying conditions become more intense. In this way, the arnrnount of water evaporated is larger and the handling of bigger product input rate is achieved. Apart from the design aspects discussed so far, DryerDesigner simulator can solve various problems regarding the operation of an existing dehydration plant, such as the following: (i) Treatment of a different product to be dried. The user can take advantage of the already existing database which can be easily enriched, provided that all corresponding properties and transport coefficients are available. (ii) Change of plant load to increase productivity. (iii) Fluctuations of the initial moisture content of the product. (iv) Changes of operating conditions within each chamber.

CONCLUSION A modular-type steady-state simulator for drying processes, which was developed in object-oriented Pascal code on an Apple Macintosh computer, was presented. The main parts of its structure and operation were outlined. The way that a desired process flowsheet could be described, drawn, and solved, were described. The use of the proposed simulator in the design procedure of conveyor belt dryers was given by means of exploring various design configurations of a dehydration plant. REFERENCES Bush, M.1. and Silveston, P.L. (1978). Computer simulation of wastewater treatment plant, Cornpul. Chern. Engng, 2.143-151 Evans, L.B., Steward, D.G. and Sprague, C.R. (1968). Computer aided chemical process design, Chern Engng Prog, 64,39-46 Flower, l.R. and Whitehead, B.D. (1973). Computer aided design: a survey of flowsheeting programs, The Chern Engr, Part I: No 272, Part II: No 273 Houska, K., Valchar, J., and Viktorin, Z. (1988). Advances in drying (editor:A.S. Mujumdar): Computer-Aided design of dryers, Academic Press, NY Kiranoudis, C.T. (1992). Design of conveyor belt dryers for vegetable dehydration, PhD Thesis, National Technical University, Athens, Greece Neville, J.M. and Seider, W.D. (1980). Coal pretreatment extentions of FLOWTRAN to model Solids-handling equipment, Cornput. Chern. Engng,1,49-61 Pantelides, c.c. (1988). SPEEDUP-Recent advances in process simulation, Cornput. Chern. Engng, 12,745-755 Rosen E.M. and Pauls, A.C. (1977). Computer aided chemical process design: the FLOWTRAN system, Cornput. Chern. Engng,l,11 Papafotiou, K., Assimacopoulos, D., and Marinos-Kouris-D. (1992). Synthesis of a reverse-osmosis desalination plant. An object-oriented approach, Trans IChemE,1Q, in press. Petrides, D., Cooney, c.L., Evans, L.B., Field, R.P. and Snoswell, M. (1989). Bioprocess simulation: an integrated approach to process development, Cornpul. Chern. Engng, 13,553-561 Spinos, M. and Marinos-Kouris, D. (1992). Integrated computer aided process design of wate water treatment plants on a PC system, Wat. Sci. Tech.,2i,107-112