EXPERIMENTAL STUDY OF FLUIDIZED BED AGGLOMERATION OF ACEROLA POWDER

Transcription

1 Brazlan Journal of Chemcal Engneerng ISSN Prnted n Brazl Vol. 25, No. 1, pp , January - March, 28 EXPERIMENTAL STUDY OF FLUIDIZED BED AGGLOMERATION OF ACEROLA POWDER G. C. Dacanal and F. C. Menegall * Laboratory of Process Engneerng, LEP, School of Food Engneerng, State Unversty of Campnas, UNICAMP, Phone: +(55) (19) ; Fax: +(55) (19) Cx. P. 6121, CEP: , Campnas - São Paulo, Brazl. E-mal: fcm@fea.uncamp.br, E-mal: gdacanal@fea.uncamp.br (Receved: March 9, 26 ; Accepted. November 7, 27 Abstract - The am of ths work was to study the man effects of acerola powder on fludzed bed agglomeraton. A fractonal factorng desgn was used to evaluate the man operatng condtons (fludzng ar temperature, fludzng ar velocty, atomzng ar flow and heght of nozzle n the bed). The mechancal and physcochemcal product changes were determned by analyss of partcle dameter, mosture content, wettng tme and bed porosty. The partcle enlargement by agglomeraton occurred when the relatve humdty n the bed ncreased and, thus, the mosture of the product ncreased. However, the excessve ncrease n relatve humdty resulted n a decrease n yeld, caused by cakng and product ncrustaton. The consoldaton of small granules resulted n an ncrease n the nstant propertes, decreasng the wettng tme and ncreasng the solublty n a short perod of agtaton. Keywords: Agglomeraton; Granulaton; Flud bed; Acerola; Powder. INTRODUCTION Food powder agglomeraton s commonly used to mprove the nstant propertes of spray-dred products. However, ths process can also be used when t s desrable to mprove the flowablty or change the vsual propertes of the powder by enlargement of the partcles (Schubert, 1987). The man propertes of agglomerates, such as sze, porosty, solublty, wettablty, shape and densty, depend on the type of agglomeraton process and on the operatng condtons durng agglomeraton. Fludzed bed agglomeraton s one of the processes sutable for producng agglomerates wth hgh porosty and good mechancal resstance for handlng and packagng (Turchul et al., 25). The prncples of operaton of fludzed bed agglomeraton consst of fludzaton of partcles by a hot ar flow and surface partcle wettng by lqud bnder atomzaton. Collsons between wet partcles on the flud bed form lqud brdges and partcle coalescence. On dryng these partcles, the brdges soldfy resultng n a consoldaton of the agglomerates (Iveson et al., 21). MATERIAL AND METHODS The Acerola Powder Samples contanng.5 kg of spray-dred acerola powder (Malpgha ermagnata) were used as raw materal n the granulaton. The acerola powder was characterzed as a slghtly cohesve product (Geldart group A) wth a mean partcle dameter of 94.5 µm and a sold densty of 1126 kg/m³ and a bulk densty of 578 kg/m³. *To whom correspondence should be addressed

2 52 G. C. Dacanal and F. C. Menegall Equpment The equpment used was a batch-fludzed bed wth.5 kg of powder n each run. The flud bed chamber had a concal base wth a dameter of 75 mm and a heght of 15 mm and a cylndrcal body wth a dameter of 15 mm and a heght of 6 mm (Fgure 1). An on/off electrcal heater controlled the fludzng ar temperature. The fludzng ar flow was montored by a rotameter. The elutrated partcles were collected by a cyclone. The amount of acerola powder used n each batch was.5 kg. The lqud bnder used was water at 27 C wth a flow rate of 2 ml/mn. The atomzng ar flow at 1 bar of pressure was measured by a rotameter. The nozzle heght n the bed was also measured. Methods The agglomerated products and raw acerola powder were evaluated by the followng physcal propertes: mean partcle sze and dstrbuton, mosture content, wettng tme and bulk porosty. a) Partcle Sze Dstrbuton and Mean Partcle Dameter (DPM) Partcle sze dstrbuton and mean partcle dameter were measured by sevng. The mesh apertures were between 38 µm and 118 µm. Equaton 1 evaluates the mean partcle dameter based on the mass fracton of the samples retaned on the several mesh apertures. DPM = Σ x dp (1) where dp s the mesh aperture and x s the mass fracton of the sample. b) Mosture (MST) Product mosture was determned by dryng at 7 C n vacuum (pressure 1 mmhg) samples contanng 3 to 4 g of acerola powder. c) Wettng tme (WET) The tme necessary to complete wettng and mmerson of a powder sample on a lqud surface s called the wettng tme. Wettng tme s commonly used to evaluate the nstant propertes of food powders. The wettng tme devce s shown n the scheme n Fgure 2. Measurement of wettng tme started when the powder sample (5 g of powder or agglomerated product) and the testng lqud (8 ml of water at 27 C) were brought nto contact by quckly pullng the slder that separate the powder and lqud sectons. Outlet Ar Lqud Atomzaton Product (Powder/ Granules) Fludzng Ar Fgure 1: Fludzed bed agglomeraton scheme. Brazlan Journal of Chemcal Engneerng

3 Expermental Study of Fludzed Bed Agglomeraton of Acerola Powder 53 5mm Sample = 5g (Powder or granulated) Slder Dstlled water (8ml) Fgure 2: Wettng tme devce scheme (Hogekamp and Schubert, 23). d) Bulk Porosty (POR) Bulk porosty (ε ) was calculated by the rato of the sold to bulk denstes (Equaton 2). The bulk densty ( ρ b ) was determned wth a texture analyzer (model: TATXT2, Stable Mcro Systems, Surrey, England). About 2 g of sample was placed on a cylndrcal contaner wth an nner dameter of 4 mm and a surface contact sensor was used to determne the bulk sample heght and volume. The rato between the mass and the volume of the sample s the bulk densty. The sold denstes ( ρ s ) were measured by the toluene pcnometrc method. About 5 g of sample was placed n a 25 ml bottle. The resdual volume was flled wth toluene. Sold densty s the rato between the mass and the free vod sample volume. ρb ε= 1 (2) ρ s e) Cakng (CAK) and product ncrustaton (INC) The presence (+) or absence (-) of cakng and product ncrustaton was evaluated for each expermental run. The presence of one or both factors ndcates an unstable relaton between the amount of atomzed lqud and the dryng capacty. For agglomeraton knetcs, where there was (+) ncrustaton, the process yeld was also measured. Expermental Desgn A fractonal factoral desgn wth four central ponts was used to evaluate the man varables of the fludzed bed granulaton. The process tme or perod of lqud atomzaton was fxed at 5 mnutes. The ndependent varables studed n the expermental desgn were Fludzng ar temperature (TAR); Fludzng ar velocty (VAR); Atomzng ar flow (ATM); Heght of the nozzle (NOZ). Table 1 shows the 1, and +1 levels and ther relatve values. The agglomerated and raw powder propertes were statstcally evaluated by the followng tests: Mean partcle dameter (DPM); Mosture content (MST); Wettng tme (WET); Bulk porosty (POR). The software STATISTICA v5.5 was used to calculate the effects responses. The most sutable operatonal condtons were selected based on the effects responses and ther statstcal sgnfcance. Under these selected condtons, agglomeraton knetcs evaluates partcle growth and product transformatons over tme. Table1: Independent varables and levels n the study. Varable TAR ( C) VAR (m/s) ATM (l/h) NOZ (cm) Brazlan Journal of Chemcal Engneerng Vol. 25, No. 1, pp , January - March, 28

4 54 G. C. Dacanal and F. C. Menegall Table 2: Operatonal condtons of the process and average results obtaned wth the expermental desgn. Run TAR VAR ATM NOZ DPM MST WET POR ( C) (m/s) (l/h) (cm) (µm) (%) (s) (%) CAK INC ± ± ± Agglomeraton Knetc The duraton of the agglomeraton knetcs was ten mnutes and the followng operatonal condtons were used: TAR = 75 C; VAR =.45 m/s; ATM =.7 l/h; NOZ = 7 cm; m lq = 2 ml/mn. The agglomeraton knetcs was evaluated for ten dfferent batches, totalng ten runs for the ten mnutes of granulaton. RESULTS AND DISCUSSION Expermental Desgn The results obtaned from the expermental desgn runs and respectve operatonal condtons are shown n Table 2. The statstcal analyss evaluates the data at a confdence level of 95%. Fgures 3, 4, 5 and 6 show the results for raw and agglomerated acerola powder and the sgnfcant statstcal effects at fve mnutes of granulaton. a) Mean Partcle Dameter Only the varable VAR had a sgnfcant effect on the mean partcle dameter (DPM) (Fgure 3). The DPM decreased about µm when the fludzng ar velocty ncreased from.25 m/s to.45 m/s. However, the partcles of the agglomerated product were stll 24% larger than those of the raw acerola powder. The ncrease n fludzng ar velocty (VAR) wth the lqud flow fxed at 2 ml/mn resulted n a dluton of the nner ar humdty due to the decrease n the rato between atomzed lqud flow and fludzng ar flow, so the nner relatve ar humdty decreased too, causng a decrease n partcle growth rate. The nlet ar temperature (TAR) should also affect partcle growth, but at the levels of the present study, ths varable was not statstcally sgnfcant. b) Mosture The varables TAR and ATM had statstcally sgnfcant effects on the mosture response (Fgure 4). The results show that when TAR ncreased from 75 C to 9 C, the agglomerated powder mosture decreased by about.25%. The relatve ar humdty at the bed bottom decreased when TAR ncreased, and consequently, the agglomerated product became drer. Product mosture also decreased by about.37% when the atomzng ar flow ncreased from.5 l/h to.7 l/h. The average mosture of the agglomerated product was 7% hgher than that of the raw acerola powder. The hgher atomzng ar flow produced small drops that were easly elutrated; consequently, the agglomerated powder mosture became lower than that for the larger drops (lower atomzng ar flow). c) Wettng Tme The varables TAR and VAR had statstcally sgnfcant effects on the wettng tme response (Fgure 5). The results show that when TAR ncreased from 75 C to 9 C, the wettng tme ncreased by.9 s. When VAR ncreased from.25 m/s to.45 m/s the wettng tme also ncreased by 1.8 s. However, the average wettng tme of the agglomerated powder decreased around 55% n relaton to the raw acerola powder (14.3 s). Brazlan Journal of Chemcal Engneerng

5 Expermental Study of Fludzed Bed Agglomeraton of Acerola Powder 55 d) Bulk Porosty None of the varables had a statstcally sgnfcant effect on the bulk porosty response. However, the average bulk porosty of the agglomerates was 5.5% hgher than that of the raw acerola powder (Fgure 6). e) Cakng The cakng was qualtatvely evaluated. The varable CAK n Table 2 ndcates the presence (+) or absence (-) of large agglomerates after 5 mnutes of granulaton. The cakng occurred when the nozzle heght (NOZ) was at the lowest poston (5 cm). f) Product Incrustaton Product ncrustaton was also qualtatvely evaluated by the presence (+), absence (-) or partal ncrustaton (±) of the product on the nner wall of the chamber. Product ncrustaton was ntensfed when the fludzng ar velocty (VAR) was at the lowest or mddle levels (.25 m/s and.35 m/s). g) Sutable Operatonal Condtons Sutable operatonal condtons are those that result n a process wth a hgh yeld and low rates of cakng and product ncrustaton ( CAK and INC). However, t was also desrable that the agglomerated product have the followng characterstcs: Low wettng tme ( WET); Low mosture content ( MST); Hgh mean partcle dameter ( DPM); Hgh bulk porosty ( POR). Based on the statstcally sgnfcant effects of the expermental desgn, Table 3 shows values at the whch the man varables should be set to acheve the desred process. Table 3 ndcates that t was not possble to select and satsfy all desred condtons. The operatonal condtons opted for were those that gave a process wth a hgh yeld wth a low wettng tme and a low mosture content, so, the parameters selected were TAR: 75 C, VAR:.45 m/s, ATM:.7 l/h and NOZ: 7 cm. DPM (µm) µm Raw acerola powder µm Mean (Agglom.) µm VAR Fgure 3: Representaton of the mean partcle dameter for the raw/agglomerated acerola powder and sgnfcant effect at a confdence level of 95%. WET (s) s Raw acerola powder 6.45s Mean (Agglom.).9s TAR 1.8s VAR Fgure 5: Representaton of the wettng tme for the raw/agglomerated acerola powder and sgnfcant effects at a confdence level of 95%. MST (%) % Raw acerola powder 3.77% Mean (Agglom.) -.25% -.34% TAR Fgure 4: Representaton of the mosture content for the raw/agglomerated acerola powder and sgnfcant effects at a confdence level of 95%. POR (%) % 54.22% QB Raw acerola powder Mean (Agglomerated) Fgure 6: Representaton of the bulk porosty for the raw/agglomerated acerola powder. Brazlan Journal of Chemcal Engneerng Vol. 25, No. 1, pp , January - March, 28

6 56 G. C. Dacanal and F. C. Menegall Table 3: The operatonal condtons sutable for fludzed bed agglomeraton. CAK INC WET MST DPM TAR ( C) * 9 - VAR (m/s) -.45* ATM (l/h) * - NOZ (cm) 7* * Selected operatonal condtons. Agglomeraton Knetcs The agglomeraton knetcs used the operatonal condtons obtaned from the expermental desgn. a) Mean Partcle Dameter The behavor of the growth knetcs can be observed by the dsplacement of the partcle sze dstrbuton peak (Fgure 7). Fgure 8 shows how the mass fracton changed over tme for each partcle sze. The mass fracton of fnes (64 µm and 9.5 µm) decreased to form larger partcles (128 µm, 181 µm and 256 µm). Fgure 9 shows that the mean partcle dameter ncreased 3% wth agglomeraton. Durng the frst 5 mnutes the agglomeraton rate was lower. The hghest growng rate was seen n the perod between 5 and 7 mnutes. After that partcle consoldaton stopped and the mean partcle dameter remaned unchanged due to the consoldaton and breakage counterpose. b) Mosture Content Fgure 1 shows that the agglomerated powder mosture ncreased 8% n relaton to that of the raw acerola powder (MST = 2.2%). The product mosture ncreased untl the seventh mnute of the process. After that, absorpton of the mosture stopped and product mosture remaned at 4%. c) Bulk Porosty Fgure 11 shows that the bulk porosty ncreased from 35.5% to 48% n 8 mnutes of the process. d) Wettng Tme Fgure 12 shows that the wettng tme decreased from 14.2 s to 4.2 s n 7 mnutes of the process. The partcle growth resulted n an mprovement of the nstant propertes. The agglomerated powder had larger and more porous partcles than the raw acerola powder and penetrated the lqud surface more quckly due to the capllary forces. e) Product Incrustaton Fgure 13 shows the decrease n yeld due to product ncrustaton. The ntal amount of acerola powder was.5 kg. Durng agglomeraton, a porton of the product coalesced on the nner wall and formed an ncrustaton layer due to lqud atomzaton. The ncrustaton mass fracton at 1 mnutes of the process reached 7%. Based on ths result, a vbraton or scrapng system should be attached to equpment to mnmze product ncrustaton and make fludzed bed agglomeraton a vable process. 1 mn Mass fracton (%) 8 mn 6 mn 4 mn 2 mn Raw Matéra acerola Prma powder DPM (µm) Fgure 7: Peak dsplacement on partcle sze dstrbuton over agglomeraton tme. Mass fracton (%) dp (µm) Fgure 8: Evoluton of the mass fracton (x ) of partcle dameter (dp ) over agglomeraton tme. Brazlan Journal of Chemcal Engneerng

7 Expermental Study of Fludzed Bed Agglomeraton of Acerola Powder 57 Granule sze (µm) Fgure 9: Increase n mean partcle dameter over agglomeraton tme. Porosty (%) Fgure 11: Increase n bulk porosty over agglomeraton tme. Mosture (%) Fgure 1: Increase n product mosture over agglomeraton tme. Wettng tme (s) Fgure 12: Decrease n wettng tme over agglomeraton tme. Yeld (%) Fgure 13: Fracton of product loss by ncrustaton durng agglomeraton. CONCLUSIONS The expermental desgn and statstcal analyss evaluated how the operatonal condtons modfed the product propertes. The ncrease n fludzng ar temperature resulted n a lower ncrease n agglomerated mosture and a decrease n the wettng tme. The ncrease n fludzng ar velocty resulted n a decrease n product ncrustaton due to the decrease n the rato between atomzed lqud and fludzng ar flows. However, for ths stuaton, the nstant propertes and the partcle growth rate decreased. The ncrease n atomzng ar flow caused producton of small droplets and a decrease n the rate of mosture adsorpton by the product. The cakng decreased when the nozzle heght was at the hghest poston. Based on the results on the agglomeraton knetcs, t can be concluded that the partcle growth resulted n an ncrease n bulk porosty and an mprovement of the nstant propertes. The agglomeraton tme necessary to form the nstant acerola powder was establshed at 7 mnutes. NOMENCLATURE ATM Atomzng ar flow l/h CAK Cakng (-) DPM Mean partcle dameter µm dp Seve aperture µm INC Product ncrustaton (-) POR Bulk porosty % TAR Fludzng ar flow C TI Wettng tme s MST Mosture content % m lq Atomzng lqud flow ml/mn NOZ Nozzle heght m VAR Fludzng ar velocty m/s x Mass fracton % Brazlan Journal of Chemcal Engneerng Vol. 25, No. 1, pp , January - March, 28

8 58 G. C. Dacanal and F. C. Menegall ρ b Bulk densty kg/m³ ρ Sold densty kg/m³ s ACKNOWLEDGMENTS The authors would lke to thank Conselho Naconal de Desenvolvmento Centífco e Tecnológco CNPq Brazl for ther fnancal support. REFERENCES Hogekamp, S. and Schubert, H., Rehydraton of Food Powders, Food Scence and Technology Internatonal, no. 9, (23). Iveson, S. M., Ltster, J. D., Hapgood, K. and ENNIS, B. J., Nucleaton, Growth and Breakage Phenomena n Agtated Wet Granulaton Processes: A Revew, Powder Technology, no. 117, 3-39 (21). Schubert, H., Food Partcle Technology: Part I. Propertes of Partcles and Partculate Food Systems, Journal of Food Engneerng, no. 6, 1-32 (1987). Turchul, C., Elouala, Z., el Mansour, N. and Dumouln, E., Fludsed Bed Agglomeraton: Agglomerates Shape and End-use Propertes, Powder Technology, no. 157, (25). Brazlan Journal of Chemcal Engneerng