EXPERIMENTAL DRYING OF TOBACCO LEAVES

Transcription

1 6 TH INTERNATIONAL MULTIDISCIPLINARY CONFERENCE EXPERIMENTAL DRYING OF TOBACCO LEAVES Bndk Krks and Tamás Antal Collg of Nyírgyháza, Faculty of Enginring and Agricultur, H-441 Nyírgyháza, Hungary, ABSTRACT: Th purpos of th invstigation was to dtrmin th moistur rlationship xprimntally in th drying of tobacco lavs. A simpl xprimntal modl was applid for calculating th moistur ratio. Th masurmnts carrid out in a climat box, studying diffrnt tobacco lavs. Two masuring cycls wr carrid out during th latst rsarch work. In th first th tobacco lavs wr hung up in thir natural condition, but in th scond trial th midrib was rmovd from th tobacco lavs. As w xpctd, th dhydration of th mutilatd tobacco laf with th midrib rmovd - was much quickr than drying of th whol laf. Th dhydration of th midrib itslf was considrably slowr than that of th lamina. An important rsult was that th Walton xponntial function was found to b suitabl for dscribing dhumidification. Th final conclusion is that th drying cofficint incrasd gratly whn th drying tmpratur was raisd. INTRODUCTION During th dhydration, th tobacco laf is vry snsitiv to changs in th tchnological paramtrs. Th rlationship btwn th tobacco laf and its nvironmnt is not asily dfind. Thrfor, a way was found to dtrmin of th most important thrmo-physical and transport proprtis of th laf,.g., th variation of surfac tmpratur, and th thrmal diffusivity, with spcial rgard to th binding and diffusion nrgis of watr in tobacco or othr agricultural products (Lngyl t al., 23). Thr ar diffrnt applid thortical and xprimntal mthods xisting to dtrmin for dtrmination of th basic charactristics of th drying tobacco lavs, so w had svral suitabl systms to us during our rsarch work. On of th succssful ways was a numrical mthod which controlld th drying procss of th basic parts of tobacco lavs mathmatically. Th continuous chang in th dhydration and surfac tmpratur of th whol laf could b monitord by a computr program. This program was suitabl for stimating th modl paramtrs, using our masurmnts (Fnyvsi t al., 1998). Evaporation and hat dvlopmnt, th most important surfac changs, may b fully charactrisd by th xamination of mass transport. Th analytical modl, which dscribs th hat dvlopmnt on th laf surfac, provids an opportunity to stimat svral typical

2 paramtrs (α, λ, ρ, c). Th modl rquirs a mthod of masurmnt which involvs a controlld nvironmntal box whos paramtrs may b fixd. Th accuracy of th thrmal matrial charactristics which w hav dtrmind with th numrical modl may b improvd by incrasing th numbr of masurmnts. Th conditions undr which th masurmnts ar takn should match thos typically mployd during actual tobacco curing tchnology. This was don for som masurmnts of th thrmal proprtis of th tobacco lavs during xprimntal curing. Surfac tmpratur changs wr monitord with an infrard camra (Krks t al., 1998). Th sris of th picturs showd diffrncs in diffrnt parts of th laf. It was concludd, that th tmpratur of th laf surfac was only homognous at th nd of th drying. Th tmpratur was lowr at th midrib. Prsumably, th proprtis of th matrial changd for th diffrnt componnts of th tobacco laf. THEORY Th purpos of th rsarch work was to find mathmatical modls and xprimntally dtrmin thir applicability to th moistur contnt of th drid tobacco lamina and midrib. Th applid mathmatical modl for th rat of avrag moistur contnt of th lamina as a function of tim is xprssd as follows (Walton t al., 1976): 2 2 D λ n t Θ(t) = (1) n= whr: Θ = moistur ratio L = half thicknss of th lamina (m) λ = n ( 2n + 1) π 1 2L (m ) ( λ L) n =, 1, 2,.. D = diffusion cofficint basd on th mass of watr pr unit mass of 2 1 solid (m s ) t = tim (s) It is wll known that th sorption isothrms chang almost linarly with th following quation: n 2 whr: dm dt ( M M M = moistur contnt at tim t, dry basis, prcnt M = quilibrium moistur contnt, prcnt k = drying cofficint (h -1 ) t = tim of drying (h) = k ) (2)

3 Th solution of th mntiond diffrntial quation is: M M M M whr: M = initial moistur contnt, prcnt Th valu of k can b dtrmind by a rgrssion approach on th basis of th moistur ratio. Th moistur ratio can b calculatd by using th rsults of th xprimntation (4): = kt (3) M M Θ = (4) M M Svral assumptions wr mad during th dtrmination of th drying and mass diffusion cofficints: - ach componnt is a homognous matrial; - th moistur contnt of th tobacco is a linar function of th laf tmpratur and vapour concntration in th por spacs; - th diffusion cofficint is a constant for a givn st of nvironmntal conditions; - th moistur in th por spac of th laf is in th vapour phas. A suitabl mthod was dvlopd to dtrmin th diffusion cofficint charactristic of matrial transport using som wll known mathmatical xprssions for th moistur contnt of th tobacco laf. Th most suitabl formula was producd by adjusting th practical drying paramtrs and priodical masurmnt of th watr contnt, considring th thicknss of tobacco laf. EXPERIMENTATION Th masurmnts wr don in a climat box, studying diffrnt tobacco lavs. Th tst paramtrs wr adjustd according to th suggstd tchnological paramtrs of drying air. Th xaminations wr compltd aftr thy had rachd an quilibrium (constant) moistur contnt. Th whol drying procss was monitord to attain isothrmal conditions (rading of th instrumnts vry 4-8 hours). Two masuring trials wr conductd during th latst pic of rsarch work. In th first th tobacco lavs wr hung up in thir natural condition, but in th scond th midrib was rmovd from th tobacco laf, so w could xamin th pur lamina and midrib sparatly. At th nd of th two cycls th tobacco lavs wr xsiccatd at 15 o C. Th masurd mass was considrd to b th dry mass of th tobacco. Th spcific dry masss wr calculatd from th rats of th masss. Figur 1 shows th xprimntal drying of th maturd tobacco lavs.

4 Figur 1 Th normal tobacco lavs in th dryr Drying priod, t (h) Tabl 1. Conditions of th masurmnts Rlativ humidity, φ (%) Drying tmpratur, T ( o C) Tmpratur, T ( C), Rl. humidity, φ (%) Drying priod, t (h) Drying tmpratur Rlativ humidity Figur 2 Tst paramtrs for th xprimntal drying

5 RESULT AND DISCUSSION Tabl 2 contains th rsults of th xprimntal curing (drying), with spcial rgard to th changs of th mass (Δm) and th tim (t), (m = mass at th nd of a givn drying priod). Tabl 2. Th avrag mass chang during dhydration Rlativ mass chang, (Δm/m) x 1, % Tim, Normal (whol) t (h) lamina midrib tobacco laf rlativ mass chang, Δm/m (%) Drying priod, t (h) Normal (whol) laf Lamina Midrib Figur 3 Th rlativ mass chang of diffrnt tobacco sampls Aftr drying diffrnt tobacco lavs w dtrmind th quilibrium moistur contnt (M ) to calculat th moistur ratio (Θ). In this way w could stimat th drying cofficints (k), according to Tabl 3. Th rliability of th xprimntal data originating from th tsts which wr rpatd four tims was ovr 9 %. Th rror in th xprimntal data was about 8 %.

6 Tabl 3. Calculatd valus of th drying cofficint Drying cofficint, k (h -1 ) Drying tmpratur ( normal (whol) C) Lamina midrib laf 38,49,454,394 4,673,628,651 44,1286,1547,13 47,1338,1666,157 48,562,7589, ,775,8334, ,677 1,765, ,879 2,265, Drying cofficint, k (h -1 ) 2,5 2 1,5 1,5 y lam =,25x 2 -,158x + 2,1449 R 2 =,9542 y norm =,19x 2 -,164x + 1,2671 R 2 =,9294 y mid =,8x 2 -,498x +,6926 R 2 =, Tmpratur, T ( C) Normal (whol) laf Lamina Midrib Polinom. (Normal (whol) laf) Polinom. (Lamina) Polinom. (Midrib) Figur 4 Chang of th drying cofficints To dtrmin of th mass diffusion cofficints, w applid th prviously mntiond Walton formula. On th basis of th chang of mass w calculatd th moistur contnt against th tmpratur and rlativ humidity for th lamina part of th lavs. Applying th formula th diffusion cofficints (D) wr stimatd progrssivly as a function of tim, tmpratur and rlativ humidity. W attmptd to fit th quation to th moistur contnt data by minimizing th sum of squars of th diffrncs btwn th obsrvd and prdictd valus of th moistur ratio (Θ), through an itrativ procss. Th computr paramtr was th mass diffusion cofficint which gav th bst fit of th quation to th xprimntal data. Th stimatd mass diffusion cofficints ar shown in Tabl 4. At

7 ths tims w xamind th sam sampls, not nw ons. So th tobacco was gradually drid throughout th procss. Tabl 4. Avrag mass diffusion cofficints (D) as a function of tim (t), tmpratur (T) and rlativ humidity (φ) t (h) T ( o C) φ (%) D (m 2 s -1 )x1 8 (lamina, sorption) , , , , , , , ,419 CONCLUSIONS Th main conclusion was that th initial rlativ humidity of th drying cycl was of dcisiv importanc for th whol curing procss. Th bst drying procss occurrd whn th rlativ humidity was about 8% in th climatic chambr whn ovr-ripnd (yllowd) tobacco lavs wr usd. As xpctd th dhydration of th mutilatd tobacco laf without th midrib was much quickr than that of th whol laf. Th dhydration of th midrib diffrd considrably from th bhaviour of th lamina or th complt laf. Th rsults confirmd that th applid modl was sufficintly accurat in dscribing th xprimntal moistur curvs. Th modl dscribs th natur of th moistur transfr procss of th lamina wll. Th rang of th calculatd mass diffusion cofficints corrsponds to th data dscribd prviously in diffrnt tobacco rports. Th mass diffusion cofficints of th lamina and midrib ar analogous to th vapour diffusion cofficint of packd flu-curd lavs as dtrmind by Stinson t al., Th moistur contnt chang of th midrib as a function of tim is diffrnt from th moistur contnt chang of th lamina, so it is ncssary to apply a modifid mathmatical modl to dscrib th natur of th midrib. An important rsult is that a suitabl mathmatical modl for th drying cofficint is a simpl xponntial quation which includs th drying cofficint but dos not charactris th chang in diffusion (Walton l al., 1976). W finally conclud that th drying cofficint incrasd substantially by raising th drying tmpratur.

8 Latin Symbols NOTATION D diffusion cofficint m 2 s -1 K drying cofficint h -1 L thicknss of th lamina m M moistur contnt % M quilibrium moistur contnt % M initial moistur contnt % t tim of drying h Grk Symbols λ tobacco laf cofficint m -1 Θ moistur ratio Subscripts n o numbr of itration initial infinit LITERATURE [1] Krks, B., Lngyl, A., Sikolya, L. (1998), Thrmo-physical and Transport Proprtis in Curing of Tobacco. IDS 98, DRYING 98. Volum A, pp [2] Lngyl, A., Bodnár, G. (23), Dtrmination of drying charactristics of sour chrry, Procdings of th 5 th Hungarian Drying Symposium, pp [3] Stinson, J.B., Young, J.H. and Whitakr, T.B. (1974), Evaluation of th vapour diffusivity for curd tobacco, Papr No , ASAE [4] Walton, L. R., Hnry, Z. A., Hnson, W. H. (1976), Moistur Diffusion in th Curd Burly Tobacco Laf. TRANSACTIONS of th ASAE, 19 (4), p