Prcceeunos of the 7th Inierruuumai Worhng Conference 011 Stored-product Protection. - Volume 2 Mathematical simulation of grain drying through ventilation and the test findings Yang Guofeng, Yang J m and Wan Zhongmmg! Abstract Based on the published mathematical models for gram drying, the authors of this paper have designed a simulation program for gram drymg through ventilation Through tills program, we can predict drying time for maize and paddy under different drying conditions, calculate the moisture and temperature changes, dunng the drying process, at different points along the movmg air currents m the gram stack, and conduct comprehensive analytic studies on the performances of gram drying process through mechamcal ventilation The results of the simulation can be pnnted out m two forms table or curve To test the Simulated program's rehabihty , we conducted ventilation drying tests on maize and paddy under certain drymg conditions by using the gram ventilation unit, which was assembled by ourselves The result of the tests shows that the amount of drymg time and the moisture and temperature changes at different points m the gram stack agree fundamentally With those calculated from the Simulation Preface The technology of gram drymg m the bm through ventilation (low temperature drying}, as an auxihary means of the mechamcal drying (high temperature drymg), IS being gradually applied m Chma Gram drying through ventilation refers to the techmque of sendmg natural atr or slightly heated air mto gram stacks through forceful means, forcmg It to exchange heat and moisture With the gram, so as to reduce the gram's moisture level This techmque IS charactenzed by the followmg advantages small mvestment, low energy consumption, convemence of operation, and easmess of techmcal extension However, compared With vanous mechamcal, high temperature drymg methods, the process and mechamsm of gram ventilation IS even more complicated m that the condition of air, the atr velOCity, the kinds of gram, their property and mOIsture contents, the heIght of the gram stack, the formatIOn of ventilation system. and the management and operatIOn 1 Department of Gram Engmeenng, Nanjlng Umverslty of Econoll11Cs method. WIll all affect the performances of ventilation drymg Besides, ventilation drymg ISa slow drying process, usually requmng scores or even hundreds of hours All these characteristics cause considerable difficulties m the research of ventilation drying The usual ventilation drying test conducted m big bms IS time-consurrung and labor- consummg, and unavoidably affected by vanous lmutations and ImpreCISIOnThus the true nature and complete picture of ventilation drying process can't be easily obtamed On the contrary, the computer Simulation technology, which has been developed m the past two or three decades, IS well SUIted for the comprehensive study of gram ventilatton drymg process After working out a calculation program, which IS based on certam mathematical models of ventilation drymg process, we can carry out, on the computer, Simulated calculations of the drymg process WIth vanous parameters and at different levels, analyze drying performances, and aid the design of drying systems By usmg a srrnphfied mathematical model of gram drying. the authors of this paper have worked out a calculation program of ventilation drymg , and have conducted ventilation drymg tests on maize and paddy by usmg the ventilation drying umt which was designed and assembled by ourselves The test results agree fundamentally With the Simulated calculations A Review of the Simulation Technology for Grain Drying As early as m 1940's, there appeared the mathematical model for the prediction of thermal medium's conductivity dunng the drying process A good mstance IS W V Hwkill's loganthnuc model, which IS simple and fast m calculation, but IS hable to errors In the 60's, With the development and diffusion of computer technology, there came some more complicated math models of gram drying, which used the computer to make calculations for results Most of the ongmal math models of this kmd were semi-empmcal, such as those developed by D S Boyce, J M Henderson and T L Thompson etc Especially, the eqUIlIbnum model advanced by T L Thompson assumed that gram and aIr could remam m a state of balance WIthma penod of time, resultmg m the SimplIfied calculation ThiS model IS well SUIted for the simulatIOns of many kmds of dryers, and ISa frequently used Simulation method 1040
Transcript
Prcceeunos of the 7th Inierruuumai Worhng Conference 011 Stored-product Protection. - Volume 2
Mathematical simulation of grain drying through ventilationand the test findings
Yang Guofeng, Yang Jm and Wan Zhongmmg!
Abstract
Based on the published mathematical models for gramdrying, the authors of this paper have designed a simulationprogram for gram drymg through ventilation Through tillsprogram, we can predict drying time for maize and paddyunder different drying conditions, calculate the moisture andtemperature changes, dunng the drying process, atdifferent points along the movmg air currents m the gramstack, and conduct comprehensive analytic studies on theperformances of gram drying process through mechamcalventilation The results of the simulation can be pnnted outm two forms table or curve To test the Simulatedprogram's rehabihty , we conducted ventilation drying testson maize and paddy under certain drymg conditions by usingthe gram ventilation unit, which was assembled byourselves The result of the tests shows that the amount ofdrymg time and the moisture and temperature changes atdifferent points m the gram stack agree fundamentally Withthose calculated from the Simulation
Preface
The technology of gram drymg m the bm through ventilation(low temperature drying}, as an auxihary means of themechamcal drying (high temperature drymg), IS beinggradually applied m Chma Gram drying through ventilationrefers to the techmque of sendmg natural atr or slightlyheated air mto gram stacks through forceful means, forcmgIt to exchange heat and moisture With the gram, so as toreduce the gram's moisture level This techmque IScharactenzed by the followmg advantages smallmvestment, low energy consumption, convemence ofoperation, and easmess of techmcal extension However,compared With vanous mechamcal, high temperature drymgmethods, the process and mechamsm of gram ventilation ISeven more complicated m that the condition of air, the atrvelOCity, the kinds of gram, their property and mOIsturecontents, the heIght of the gram stack, the formatIOn ofventilation system. and the management and operatIOn
1Department of Gram Engmeenng, Nanjlng Umverslty of Econoll11Cs
method. WIll all affect the performances of ventilationdrymg Besides, ventilation drymg ISa slow drying process,usually requmng scores or even hundreds of hours All thesecharacteristics cause considerable difficulties m the researchof ventilation drying The usual ventilation drying testconducted m big bms IS time-consurrung and labor-consummg, and unavoidably affected by vanous lmutationsand ImpreCISIOnThus the true nature and complete pictureof ventilation drying process can't be easily obtamed On thecontrary, the computer Simulation technology, which hasbeen developed m the past two or three decades, IS wellSUIted for the comprehensive study of gram ventilattondrymg process After working out a calculation program,which ISbased on certam mathematical models of ventilationdrymg process, we can carry out, on the computer,Simulated calculations of the drymg process WIth vanousparameters and at different levels, analyze dryingperformances, and aid the design of drying systems Byusmg a srrnphfied mathematical model of gram drying. theauthors of this paper have worked out a calculation programof ventilation drymg , and have conducted ventilation drymgtests on maize and paddy by usmg the ventilation drying umtwhich was designed and assembled by ourselves The testresults agree fundamentally With the Simulated calculations
A Review of the SimulationTechnology for Grain Drying
As early as m 1940's, there appeared the mathematicalmodel for the prediction of thermal medium's conductivitydunng the drying process A good mstance ISW V Hwkill'sloganthnuc model, which IS simple and fast m calculation,but IShable to errorsIn the 60's, With the development and diffusion of
computer technology, there came some more complicatedmath models of gram drying, which used the computer tomake calculations for results Most of the ongmal mathmodels of this kmd were semi-empmcal, such as thosedeveloped by D S Boyce, J M Henderson and T LThompson etc Especially, the eqUIlIbnummodel advancedby T L Thompson assumed that gram and aIr could remamm a state of balance WIthma penod of time, resultmg m theSimplIfied calculation ThiS model IS well SUIted for thesimulatIOnsof many kmds of dryers, and ISa frequently usedSimulationmethod
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Proceedmgs of the 7th Internatwnal Workmg Conference all,Stored-product Protecnon - Volume 2
Since the 70's, F W Bakker-Arkema, D B Brooker,O'Callaghan, M E Nelhst and others have developed a moresophisticated and more precise partial differential equationmodel (p d e model) This model uses four partialdifferential equations, I e moisture equihbnum equation,thermal equihbnum equation, thermal transfer equation anddrying rate equation, to descnbe the transfer of thermalmedium and the drying rate To solve the above equations,numencal calculation IS usually adopted, and the largecomputer IS needed for the calculationWith regard to the low temperature, small quantity air
drying, P D Bloome , T L Thompson etc have put forwarda simphfied version of p d e model, which can greatlyreduce the time of calculation C A Flood and othersdeveloped a ventilation drying model, m which thepossibihty of maize's gomg moldy IS taken mtoconsiderationBesides, there are some Simple gram drying models,
which only uses a calculator to get the approximate dryingtime under low temperature condition, but the results arenot so accurateIn Chma the research m this area started m the 1980' s
Researchers from Beijing Agncultural EngmeenngUmversrty, Northeastern Agncultural Institute and othermstitutions have done substantial work m the developmentof the thm layer drying model, and the deep-bed dryingmodel, m the study of gram's thermal properties, and theSimulation of different drying processes and types of dryersThe Simulation program of gram drying designed by BeijingAgncultural University can carry out the Simulation studiesof gram drymg on 4 types of dryers and 5 kinds of gramResearchers from Northeastern Agncultural Instituteoptimized the calculations for the drying technology ofmaize, wheat and paddy, and have obtamed bettertechnological parameters, which can gUIde the drymgoperation Many agncultural and gram departments m Chmahave also done some research on the computer Simulation ofgram drymg
The Mathematical Modeland Program Design
The mathematical model
The mathematical model of the deSigned program IS asfollowsThe thm layer drymg equatwllfor maize t = A /, In (MI) + B (lnMr)2 (0for paddy t = en~~)(Y /60 (2)
m which A = -1 706 + 0 00878TB = 148 67 exp ( - 0 0594 T)Mr-mOlsture ratIo
Mr= M-MeMo-Me
M-gram moisture c«, dry baSIS)Me-gram equihbnum moisture (%, d bMo-gram nutial moisture (%, db)T -r-temperature of heated air CC )t-drymg time (h)X = 0 01579+0 0001746T-0 01413rhY = 0 654 + 0 0024 T + 0 078671'h
The moisture eqiulibruun equatwn
[-In(l-rh) J1I2
for maize Me= 6 88/,10-5/,(T+45 56) (3)
[-In(l- I'll) ]112445
for paddy Me = 1 9187/,10-5 X (T + 51 16)
(4)m which Me-eqUlllbnum moisture (%, db)
T-temperature of the heated air CC)I'll-relative humidity (fraction)
o 68 exp (~O 114M)]m which L-vaponzatlOn heat (ljlkg)
M-gram moisture (%, d b)T-gram temperature CC)
Gram specifu. heat equationfor maize C = 0 452 + 0 011Mfor paddy C = 0 392 + 0 01076Mm which C-gram specific heat (kWlkg °C)
M -gram moisture (%, db)Gm11l umt wezght equation
1
(5)
(6)
(7)(8)
(9)
(10)for maize V = 609 /, 1 - 0 01Mfor paddy V = 499 5 + 8 325Mm which V-umt weight (kg/m")
M-gram mOisture (%, wet baSIS)The baSICprmclple of our Simulation IS to regard the gram
stack as several discrete layers piled together along the routeof air current, and the thickness of each layer IS ~x,Meantime, the drYI~g~tIme IS also separated mto severalpenods, and each penod IS ~t The conditIon of the airbefore entenng the ftrst layer, m terms of temperature andmOIsture, IS known, and the temperature and mOisturecontent of each gram layer are also known The process ofthe aIr current movmg through the gram stack under certamair conditions can be regarded as ItS movmg through eachdiscrete layer m succeSSIOn Suppose the gram stack ISseparated mto N layers, and the air condition before passmgthrough layer Z IS that temperature IS Tz and humiditycontent IS Hz, Suppose the mOIsture content of thiS layer'sgram IS Mz, and the temperature IS TGz After the drymgpenod ::'t, due to the transfer of heat and mOIsture betweenthe air and the gram, we can use the thermal medIUmeqUlllbnum equatIOn to calculate the layer's change oftemperature and mOIsture content after drymg as well as the
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Proceedmq: of the 7th lnternauonal Workmg Conference an Stored-product Protectwn - Volume 2
changed atr parameters, namely the layer's moisturechanges to Mf, temperature TGf, air temperature afterpassmg through the layer Tf , humidity Hf Then, we canregard the changed air ctt . Hf) as the entenng arr of thenext layer (1 e L + 1 layer) Repeat the above process untilthe Nth layer At this time, the moisture content andtemperature of each layer has also changed, compared withthose before b.t We now mcrease the time mterval tst; andstart a new round of simulation calculation from the firstlayer After several times of simulated calculations withseveral time mtervals of b.t, we can work out the ultimatemoisture content or temperature of each layer, we can alsofirst provide the required final moisture , and then work outthe required drying time
Program dessign
Based the above method, we have designed a gramventilation drying program This program adopts thestructural design technique often used m engmeenng, andmamly mcludes three modules, 1 e one for ongmal datainput, the second for data processmg, and the third for dataoutput Basic ISused and the program can run on computer286 or above The three modules are descnbed as follows1 Data input moduleIts function IS to mput the ongmal data of sImulated
calculations, whIch mclude aIr temperature, humIdIty,volume, the gram's mltIal mOlsture content, ItStemperature, heIght of the stack, and the fmal mOlstureafter drymg etc ThISmodule has a fnendly mterface so thatthe user can easIly enter or modIfy the data2 Data processmg moduleThISmodule ISthe core of the program, and Its function IS
to process the mput ongmal data, and send the calculatedresults to the relevant array ThIS module consIsts of twosubprograms, one for malZe drymg, and the other forpaddy3 Data output moduleMter the mput data are processed, the sImulation results
are stored m the relevant array, whIch must allow the userto manoeuvre freely ThIS module allows the output of notonly data, but also curves and drawmgs, so the user canobserve the whole drymg process VISibly The outputmcludes the followmg(l) The curve of the average mOlsture and temperature
changes, Wlth time, of the whole stack m the gram bm(2) The curve of the mOlsture and temperature changes,
With time, of each layer(3) The table of the mOlsture and temperature changes,
With time, of each layerIn deslgnmg the program, we have conSIdered the
possible occurrences of some speClal problems m gramventl1atIon drymg1 Mter calculation of each layer accordmg to the thermal
medium equihbnum equation, the air condition changes,1 e , temperature from Ti to T[', moisture from HL toHf When the layer IS dried, the evaporated moistureenters the atr , and thus Hf is higher than HL, Tf lowerthan Ti , which means the relative humidity of the airrh , after passing through the layer, mcreases Themaximum value of rh should be 100% But from thesimulated calculation, It IS possible for rh to be greaterthan 100%, which of course goes agamst the actualsituauon Therefore, we must work out Tf and HL againto make their rh not exceed 100% To solve thisproblem, we have set a Hf subprogram to work out Hi',and let the extra moisture to be condensed and returnedto the layer Followmg the calculation of each layer, thissubprogram IS always taken out to evaluate the air'srelative humidity and calculate Tf and Hf
2 Compared WIth the high temperature drying, gamventilation drying IS featured by low air temperature,small air quantity, unck gram layers When the airmoves through the layers, the temperature keeps fallingand the moisture content increases Thus, If the gram'sequihbnum moisture (which nses as the air temperaturefalls and air moisture mcreases ) IS greater than thegram's actual mOlsture content. the gram particles willmcrease theIr mOlsture, a phenomenon whIch oftenoccurs m the layers near the air current's end dunng thebegmnmg of the drymg process In order to stmulate thISphenomenon, we must have sUltable equation for themOlsture mcrease of the thm layer From the vanousacademIC sources. we can fmd the mOlsture mcreaseequation for maize, but no mOlsture mcrease equatton forthm layers of maIze and paddy Therefore, whendes1gnmg our program, we still used the thm layerdrymg equation to calculate the mOlsture mcrease, mother words. we assumed the rate of mOlsture mcrease ofthese two kmds of gram exhIbits the same tendency asthe drymg rate It must be pomted out that, by domg so,certam errors may occurFor the dIagram of the program, see fIgure 1
The Testing Unit ofVentilation Drying
To test and venfy the rehabthty of our calculation program,we utIhzed the self-made ventilation drymg testmg umt toconduct expenments on maIze and paddy For the structureof the umt, see fIgure 2 Atr blower veloClty 2850r/mmvolume range 0 - 480m3/h parameters of the drwmg motoro 37 kW/380V
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Proceedmqs of the 7th International Workmg Conference on Stored-product Protection. - Volume 2
Enter Parameters
Drvmg TIme Is Zero
Increase A Drying
Penod (ll t)
Calculate The MOIsture And
Temperature Of The Gram At
Each I -ver One B) One
1he Average
Moisture ::::;Pre-
N
,<.:1 Value
y
Enter Calculated
Results
Fig. 1. Program Diagram
Electric heater there are three heatmg tubes m theheatmg section of the ptpe , each WIth power capacityo 7kW, voltage 220V, which are coordmated With thetemperature controller The tubes can be switched onsimultaneously or separately so as to obtam different amountof heat When all the three are turned on, the total amountof heat is 7600kjlhHorizontal pipe total length (mcludmg the heatmg
section) 25m, cross section is circular, 130 mm m
diameter The pipe is made up of two galvanized ironsheets To reduce the heat loss, pearlite 15 mm thickness isstuffed between the two sheets as thermal insulationmatenal
FIg.2. Diogrom of the Vennotion Drying Test Unil1 Air blower, 2 Air odjustmg volve, 3 Electneheater, 4 Hon-zontol pipe, 5 Temperoturecontrollmg probe, 6 Fromenork, 7 Somplmg bloes,8 Thermometers, 9 Drvmg cylinder,10 Temperature controller
Gram drying cylinder total length of the drymg section is2 rn , which is divided mto two parts, each 1 m, Similar tothe honzontal PiP~ The drymg cylmder is made up of twoconcentnc cylmders, the mner cylinder is 300 mm across,and the outer one, 380 mm m diameter, m between is filledWith thermal msulation cotton At one side of the heatingcylmder, there are 10 holes for temperature measuring,while at the other side, there are 5 holes for takmg out thegram for samplmg When testing, thermometers areinserted through each temperature testmg hole mto thegram stack, for the purpose of measunng the gram'stemperature change at different heights at fixed time The 5holes at the other side are stopped With rubber corks Theyare opened only when taking samples. and each time thesampler takes out about 5 grams of gram The mtervalbetween two holes on the thermal testmg side is 20 mm,whereas the mterval of two holes on the samplmg side is 40mm
Ventilation Drying Test
Testmg Equipment and Material
Testmg Equipment ventilation drying urut, WMZK-08type thermostat controllmg mstrument (temperature controlrange 0 - 100°C), HlVI3type electnc ventilation dry and wetbulb temperature mstrument, FSF type gram gnnder ,
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Proceedmgs of the 7th Internatwnal Worhng Conference em Stored-product Protectto» - Volume 2
tunnel-style moisture tester (moisture testmg error IS lessthan 0 5%), DFA-II type anemometer (aIr velocitymeasurement range 1-10 m/s) , thermometer (measunngrange 0 - 100"(; ), samplerTestmg Matenals maize and paddy of the year, taken
from Shibuzhan Gram Facihtres, Qrxia Distnct, NanjmgCIty The moisture content of the maize for test IS 17 4%(wet baSIS) As the paddy moisture content IS less than 14%(w b ), the method of moisture mcrease mentioned mreference source < 1> IS adopted to mcrease the paddy'smoisture content to 18 4% (w b ) for test
Testing Method
As there IS only one drying umt at our disposal, we didthe drying test on the maize and paddy separately Theprocedure of the maize test IS as follows FIrst use themoisture tester to test the uutial moisture content of themaize, then fill the drying cylmder WIth the maize, use theanemometer to determme the wmd velocity at the surface ofthe maize When measurmg, fIX the anemometer at theupper end of the exhaust openmg of the drying cylmder andalso make sure It IS hermetically sealed Start the air-blower(switch off the heater) Measure the wmd velocity, and onthe baSIS of the ratio between the diameter of theanemometer and the mternal diameter of the mner cylmder ,convert It mto the wmd velocity at the surface of the maizeThen, accordmg to the total volume of the drying cylmder ,figure out umt air volume q (rrr' 1m3 mm) SWItch off theair-blower, take down the anemometer After tlus, msertthermometers through each hole mto the maize stack, todetermme the mitial temperature of the maize, msert theprobe of thermal control mstrument mto the thermal testmghole (the one near the bottom of the stack) and fix It firmlyStart the air-blower and turn on the electrical heater
(connect the three tubes at the same ume ) , tune thetemperature controller to the desired air temperature Fromthe electric ventilation dry and wet bulb temperature, readout the dry bulb temperature and the wet bulb temperatureof the environmental air Afterwards, every 3 hours readout gram temperature (at 10 pomts) , take out sample (at 5pomts) to determme the mOlsture content MeanwhIle, readout the dry and wet bulb temperatures of the air mdoors andwnte them down When the average mOlsture of the malZem the drymg cylmder has fallen to the pre-set values, stopthe test ImmediatelyHavmg flmshed thIS test, pour out thee maIze, f1l1m the
paddy, and repeat the above procedure for the testmg ofpaddy Dunng the testmg, keep the Wlndows open to let outthe exhaust aIr For the test results, see Tables 1and 2
Analysis of the Results
For the SImulated calculatlon from the ventllatlon drymg
program, see Tables 3 and 4 FIg 3 shows the cbanges ofthe maize's and paddy's average moisture content andtemperature From the figure, we can see the expenmentcurve fundamentally agrees with the simulation curveFIg 4 and FIg 5 show the moisture and temperature
changes of the maize and paddy at the upper, middle, andlower levels The two temperature curves fundamentallyagree with each other, while for the two moisture curves,there do exist some discrepancies The reason for thesedisagreements IS as follows
~Dlstu'e(%1lU ..... ..;...;...---.;...-....;,
18-1.,[,nq- Simylotlon
Dry,nq t'me(h), a)Mo,,,
FIg. 3. MOIze and Paddy Average MOisture & Temperature
Fig. 4. MOIze and Paddy Moisture Vanotions at the TopMIddle and Bottom Layers
Tcmocroturc('C) [cl1Gcrolure('C)
Fig. 5
6 9 2 1Orylnq Ilme(h)
[b )Poddy
Maize and Paddy Temperature VanatlOilS at theTop MIddle and Bottom Layers
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Proceeduuie of the 7th Iniernaiumai Workmg Conference an Stored-product Protection - Volume 2
Table 1. Test Resultsof Maize Drying Jan 5 - Jan 6 1998
Moisture/% , w b ) Temperature of maize °CTemperature
Temperature of drymg arr 32"C, MOisture content 0 006kglkg dry arr , fur volume 425m3 1m3 mm
1 The mathematical model that our simulation program isbased on is an approximation calculation model formulatedon the basis of certain assumptions This model hashigher precision for the high temperature, fast drying,whereas, for the low temperature, slow drymg , there doexist some errors The reason that we used this modelfor programmmg is that it is Simpler to program andfaster to run, compared With the p d e model
2 During low temperature, slow drymg , there may occurmoisture condensation or mcrease at the upper part of thegram stack To descnbe this physical phenomenon therelevant moisture increase equation should be used Thisis what mainly distmguishes low temperature, slowdrying from high temperature fast drying As we lack themoisture increase equations for maize and paddy, wehave the drying equations in their place, which may haveresulted m some errors We can see this quite clearlyfrom drawing 4 The test curve and the simulation curveof the gram moisture do not agree so well m the upper asm the lower layers
3 Some errors exist because fast moisture testmg method isusedWith all these shortcommgs, the ventilation drying
program, mtroduced by this paper, and which is used tocalculate the gram ventilation drying time, to predict themoisture and temperature changes at different points of thestack, and to keep control off ventilation drymg, is still aquite effective and good method
References Sources
C ai Xue-jian 1983 A Study of the Law of Gram's MOistureIncrease Gram Drying Technology 161-166
Cao Chong-wen 1984 Digital Simulation of Gram DryingJournal of Beijmg Institute of Agricultural Mechamzation,3, 79-98
Flood, C A , Sabbah, M A , Meeker, Duane and Peart,R M 1972 Simulation of a Natural-au Corn DryingSystem Transactions of the ASAE, 15(1),156- t59
Sharp, J R 1982 A Review of Low Temperature DryingSimulation Model Journal of Agricultural EngmeenngResearch, 27, 169-180
Thompson, T L, and Peart, R M MathematicalSimulation of Corn Drying - a New Model Transactions ofthe ASAE, 11(4),582- 586
