SIMULTANEOUS HEAT AND MASS TRANSFER IN AGITATED DRY- ER

Transcription

1 G-2010-Proess Tehnoloies SIMULTANEOUS HEAT AND MASS TRANSFER IN AGITATED DRY- ER Mária Örvös Assoiate Professor Tibor Poós Ph.D. student Abstrat: Dru dryers are often used for oisture redution of ranulated aterial and wet slude fro uniipal or industry waste water. Moisture redution and theral treatent an be done in aitated dru dryers where onvetive heat transfer is helped by ontat heatin jaket and soeties by heated aitators as well. For the desription of siultaneous heat and ass transfer in the dryer, atheatial odel was reated to investiate the influene of the operatin paraeters. The odel ontains voluetri heat and ass transfer oeffiients that are funtions of ontatin wet aterial and heated surfae, ontatin wet aterial and hot air, and ontatin air and heated surfae. Measureent and evaluation ethods have been developed for deterinin the voluetri heat and ass transfer oeffiients of ranulated aterials. Relations between the voluetri transfer oeffiients were deterined, with diensionless Nu -Re nubers. The atheatial odel and the transfer oeffiients ake possible to alulate the ain dru dryer paraeters. Keywords: diret-indiret dryer, aitated dryer, heat and ass transfer. 1. INTRODUCTION Dru dryin is urrently used in the heial and proess industries to dry heavy pastes, thik liquids and partiulate solids. The ixin of the aterial to ensure a better heat transfer an be realized with rotatin dru or with aitator. For the investiation the dryin proess of wet ranular solid an aitated diret (dryin as), indiret (heatin wall) heated dryer was onstruted. Heat is also transferred to the aterial fro the dryin as and the overed wall surfae via onvetion and ondution. However, in appliations involvin relatively low teperatures (as and wall teperature of approxiate 100 C), the wall-to-aterial and the dryin as-to-aterial heat transfer beoes the doinant ehanis [2]. So the effet of wall-todryin as heat transfer is neliible. In a work Din et al. [3] opared the epirial expressed Nusselt nuber (at the dryin as-aterial) with experiental results of Tshen and Watkinson [4]. The differene between the plotted urves is sinifiant, espeially at low Reynolds nubers. Ohori et al. studied the effet of the learane between heatin wall and aitated aterial to the heat transfer [5]. In our researh this effet was neleted. 1 / 8

2 G-2010-Proess Tehnoloies 2. MATHEMATICAL MODELL For odellin the proess of siultaneous heat and ass transfer in aitated dryer [1] desribed a ethod that takes into onsideration the heat transitted fro the as as well over the wall to the aterial to be dried. Fi. 1. shows the odes of heat transfer in a short setion of the dryer. Fi. 1. Heat transfer in short setion of the aitated dryer With heat and ass balane for the differential setion dh, the teperature and huidity of the dryin as and aterial an be dedued. The variation of as huidity alon the lenth of the dru: dy a Aq (YF Y ) (1) dh The variation of the as teperature alon the lenth of the dru: dt Aq a T T ) kva f ( T T ) dh pwg ( T T ) dy ( F dh The variation of the oisture ontent of the dryin aterial alon the dru lenth: dx dy (3) dh dh The variation of the teperature of the dryin aterial alon the dru lenth: dt A q rf dx k t a (T T ) a(t T ) (4) dh dh Transfer oeffiients in Eq. (1) (4) - fro the heat flow between the as and the aterial: dq (T T ) da (5) - fro the heat flow between the wall and the as: dq k ( T T ) da (6) f v - fro the heat flow between the wall and the aterial: dq k ( T T ) da (7) - fro Lewis analoy: for the syste air-water vapour: t Le z f (2) (8) z Le 1 (9) 2 / 8

3 G-2010-Proess Tehnoloies Voluetri ontat area - between the as and the aterial: - between the wall and the as: - between the wall and the aterial: da a (10) A dh q daf a f (11) A dh q daf a (12) A dh Overall onvetive heat transfer oeffiient between the wall and the as: R f k a a a v f f in those ase f an be alulated approxiately kva f f a f. f q f (13) Overall ontat heat transfer oeffiient between the wall and the aterial: R k a a a t (14) in those ase f an be alulated as kta a. For the alulation the dru lenth by Eq. (1)-(4) voluetri heat and ass transfer oeffiients a ; f a f ; f a f ; a are required. 3. DETERMINATION OF THE VOLUMETRIC HEAT AND MASS TRANSFER COEFFICIENTS 3.1. Measureent desription The dryin experients were onduted in a pilot plant unit shown in Fiure 2. On this shee all easured and ontrol variables are indiated with instruent sybols. The instruentation of the pilot plant dryer are built up by standard sensors. Central part of the ehanial syste is a dru dryer (1). It is an 850 lon aitated bath dryer onsistin of a 250 wide U-for dryin spae overed by a flat plate. The feed (pasty or ranular aterial) is aitated by an eletrially driven srapin-aitator set (2). The aitator speed is adjustable by the aid of a variable frequeny drive (SIC-10). Dryin as is fed axially at one end and let out at other end at the top. The onstrution of the dryer akes it possible to arry out experients either with ondutive or onvetive heatin or applyin both. On the ylindrial botto part of the dryin spae a ontrolled eletri heater (TIC-6) is built on. The dryin dru, the eletri enine and the earbox drivin the aitator are built on a frae that is hun on a well-onstruted weihin syste with load ell (WI-11). Capaitive huidity sensors are used for relative huidity easureent (XI-3, 8). Differential pressure transitter is used by easureent of as flow. The pressure differene before and after the orifie plate is used to alulate the flow veloity (FI-2). Theroouples (TI-4, 7, 9) and an infrared sensor (TI-5) are used for teperature easureent. 3 / 8

4 G-2010-Proess Tehnoloies Fi. 2. Pilot-plant aitated dru dryer equipent Dryin air is heated by eletri heater (3). The teperature of the dryin air is ontrolled with teperature ontrol (TIC-1). The voluetri flow rate of dryin as is varied by a variable frequeny driven blower (4). At the start of the experients, the dryer was set to the required dryin onditions, i.e., the teperature, huidity and veloity of the inlet dryin air were onstant. The easured teperatures, huidity, ass and veloity were reorded in every inute by usin a data loer. The position and the ain easures of the aitator and the ross-setion of dru dryer an be seen in the Fiure 3. Fi. 3. The aitator 3.2. Evaluation ethod In aitated dryer the ontatin surfaes for heat transfer both fro the as side and heated wall side annot be easured exatly. For this ase the voluetri heat transfer oeffiients are defined. For the evaluation of the heat and ass transfer oeffiient between the as and the aterial the dryin proess was arried out only with hot air. In the onstant dryin rate period the heat transferred fro the as to the aterial is onsued by evaporation: T T )A N ra r (Y Y ) A (15) ( onst surfae Dryin rate in the onstant dryin period: N onst A t (Y surfae Y ) (16) 4 / 8

5 G-2010-Proess Tehnoloies Aordin to Eq. (10) A a A H a V (17) q Fro Eq. (15)-(17) the voluetri heat transfer oeffiient between the as and the aterial for the onstant dryin rate period, where T=onstant: r a (18) t T V Siilarly to the previous ase, for the onstant dryin rate period when heat is transferred only fro the wall toward the aterial the voluetri heat transfer oeffiient between the wall and the aterial: r a (19) t( T T ) V 3.3. Results of the easureent f Table 1. Measured and evaluated quantities lo Material d Tf T,in n l a-. a- a f-. af- [] [ C] [ C] [1/s] [1] [ 3 /s] [W/ 3 K] [W/ 3 K] aize 0,0060 no 100 0,633 0,2277 0, ,7 aize 0,0060 no 100 0,950 0,2277 0, ,7 aize 0,0060 no 100 1,267 0,2192 0, ,6 teaar 0,0010 no 90 0,475 0,2530 0, ,0 teaar 0,0010 no 93 0,475 0,2530 0, ,0 teaar 0,0010 no 81 0,475 0,2530 0, ,0 illet 0,0015 no 100 0,633 0,1605 0, ,5 illet 0,0015 no 100 0,333 0,1300 0, ,9 illet 0,0015 no 100 0,500 0,1300 0, ,8 illet 0,0015 no 100 0,667 0,1300 0, ,6 illet 0,0015 no 100 0,833 0,1300 0, ,8 illet 0,0015 no 100 1,000 0,1300 0, ,7 illet 0,0015 no 100 0,333 0,2000 0, ,1 illet 0,0015 no 100 0,500 0,2000 0, ,0 illet 0,0015 no 100 0,667 0,2000 0, ,4 illet 0,0015 no 100 0,833 0,2000 0, ,7 illet 0,0015 no 100 1,000 0,2000 0, ,7 illet 0,0015 no 100 0,500 0,2500 0, ,3 illet 0,0015 no 100 0,667 0,2500 0, ,2 illet 0,0015 no 100 0,833 0,2500 0, ,8 illet 0,0015 no 100 1,000 0,2500 0, ,4 aize 0, ,950 0,2382 0, ,3 819,1 aize 0, ,267 0,2382 0, ,4 1163,9 aize 0, ,633 0,1726 0, ,1 882,3 illet 0, ,633 0,1605 0, ,5 1238,7 ornris 0, ,633 0,2256 0, ,4 1450,0 ornris 0, ,633 0,2256 0, ,1 1300,0 ornris 0, ,633 0,2256 0, ,0 1270,0 DRYING GAS (diret) DRYING GAS + HEATING WALL (diret+indiret) 5 / 8

6 G-2010-Proess Tehnoloies 3.4. Correlations with diensionless nubers Fro the voluetri heat transfer oeffiient a odified Nu -nuber was reated, both for the as-aterial and wall-aterial heat transfer: 2 2 ( a ) d ( a ) d Nu Nuf (20)-(21) Suppose that a partile is rotatin and ovin alon the dru lenth. The averae veloity fro the respetively partile and as veloities vetor: 2 2 ax w w ir w (22) The odified Reynolds-nuber with the iruferential and the axial veloities: w d Re (23) The dru loadin fator: V V l (24) Vd Aq H Fro the experients listed above in Table 1. the odified Nu - Re orrelation are illustrated in Fi. 4. for onvetive heat transfer between the as and the dryin aterial. Fi. 5. shows the orrelation between Nu f- Re for ontat heat transfer between the wall and the dryin aterial. The results in Fi. 4. and Fi. 5. are valid in the intervals: Fi. 4. Modified Nusselt and Reynolds nuber between the dryin as and the aterial 6 / 8

7 G-2010-Proess Tehnoloies Fi. 5. Modified Nusselt and Reynolds nuber between the heated wall and the aterial 4. CONCLUSION Measureents were arried out with different test aterials (aize, illet, teaar and ornris). The oisture ontent of the aterials varied fro the free surfae oisture ontent near to the equilibriu oisture ontent. With the heat and ass balane of the dryer, and fro the ass redution of the aterial on the onstant dryin rate period the transfer oeffiients an be deterined. The heat and ass transfer oeffiients serve as input paraeter for the atheatial odel. Relations between the diensionless Nu -Re nubers for airaterial and wall-aterial are deonstrated. Experients will ontinue to deterine exat equations between Nu Re relation. 7 / 8

8 G-2010-Proess Tehnoloies 5. NOMENCLATURE Roan letters Greek letters Sybols Denoination Units Sybols Denoination Units a voluetri surfae area 2 / 3 heat transfer oeffiient W/ 2 K A heat transfer area 2 theral ondutivity W/K Aq ross setional area 2 ass transfer oeffiient k/ 2 s speifi heat J/kK kineati visosity 2 /s d diaeter of partile H lenth of dru dryer Subsripts k overall heat transfer oeffiient W/ 2 K Sybols Denoination l dru loadin fator 1 odified Le Lewis nuber 1 ax axial ass k ondense ass flow k/s ir iruferential n rotation 1/s t ontat N dryin rate k/ 2 s v onvetive Nu Nusselt nuber 1 d dru q heat flux W/ 2 f inner surfae of the dru r heat of evaporation J/k F surfae R resistane 3 K/W f- wall aterial Re Reynolds nuber 1 f- wall dryin as t tie s dryin as T teperature C - dryin as - aterial flow rate 3 /s in inlet V volue 3 aterial w air veloity /s v vapour X oisture ontent of the aterial k/k WG water vapour Y absolute huidity of the as k/k z onstant REFERENCES Journals: [1] Balázs T., Örvös M., Töösy L.: Heat and ass transfer in an aitated ontatonvetive heated dryer, Food and Bioproduts Proessin Volue: 85 Issue: C3 Paes: Published: Sep 2007 [2] Charles A. Cook, Vi A. Cundy: Heat transfer between a rotatin ylinder and a oist ranular bed, International Journal of Heat and Mass Transfer, Volue 38, Issue 3, February 1995, Paes [3] Y.L. Din; R.N. Forster; J.P.K. Seville; D.J. Parker: Soe aspets of heat transfer in rollin ode rotatin drus operated at low to ediu teperatures, Powder Tehnoloy 121 (2001.) pp [4] S. H. Tshen, A. P. Watkinson: Convetive heat transfer in a rotary kiln, Can. J. Che. En. 57 (1979) [5] T. Ohori, M. Okazaki, R. Toei: Heat transfer oeffiient in stationary heatin-plane type of indiret heat aitated dryer, Journal of Cheial Enineerin of Japan (1986.) pp / 8