Kỹ thuật sấy nông sản thực phẩm - pdf

Gio n K thut sy nng sn thc phm

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CHNG I: I TNG SY V MI QUAN H CA CHNG1.1 NGUYN VT LIU MTheo quan im ho l, vt m l mt h lin kt phn tn gia pha phn tn v mi trng phn tn. Pha phn tn l mt cht c cu trc mng hay khung khng gian t cht rn phn u trong mi trng phn tn ( l mt cht khc). Da theo tnh cht l hc, ngi ta c th chia vt m ra thnh ba loi: - Vt liu keo: l vt c tnh do do c cu trc ht. Nc hoc m dng lin kt hp th v thm thu. Cc vt keo c c im chung l khi sy b co ngt kh nhiu, nhng vn gi c tnh do. V d: gelatin, cc sn phm t bt nho, tinh bt... - Vt liu xp mao dn: nc hoc m dng lin kt c hc do p lc mao qun hay cn gi l lc mao dn. Vt liu ny thng dn hu nh khng co li v d dng lm nh (v vn) sau khi lm kh. V d: ng tinh th, mui n v.v... - Vt liu keo xp mao dn: bao gm tnh cht ca hai nhm trn. V cu trc cc vt ny thuc xp mao dn, nhng v bn cht l cc vt keo, c ngha l thnh mao dn ca chng c tnh do, khi ht m cc mao dn ca chng trng ln, khi sy kh th co li. Loi vt liu ny chim phn ln cc vt liu sy. V d: ng cc, cc ht h u, bnh m, rau, qu v.v... 1.1.2 CC DNG LIN KT TRONG VT LIU M: Cc lin kt gia m vi vt kh c nh hng rt ln n qu trnh sy. N s chi phi din bin ca qu trnh sy. Vt m thng l tp hp ca ba pha: rn, lng v kh (hi). Cc vt rn em i sy thng l cc vt xp mao dn hoc keo xp mao dn. Trong cc mao dn c cha m lng cng vi hn hp hi kh c th tch rt ln (th tch xp) nhng t l khi lng ca n so vi phn rn v phn m lng c th b qua. Do vy trong k thut sy thng coi vt th ch gm phn rn kh v cht lng. C nhiu cch phn loi cc dng lin kt m. Trong ph bin nht l cch phn loi theo bn cht hnh thnh lin kt ca P.H. Robinde (Hong Vn Chc, 1999). Theo cch ny, tt c cc dng ln kt m c chia thnh ba nhm chnh: lin kt ho hc, lin kt ho l v lin kt c l.

1.1.1 PHN LOI CC NGUYN VT LIU M

Gio n K thut sy nng sn thc phm 1.1.2.1Lin kt ho hc

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Lin kt ho hc gia m v vt kh rt bn vng trong , cc phn t nc tr thnh mt b phn trong thnh phn ho hc ca phn t vt m. Loi m ny ch c th tch ra khi c phn ng ho hc v thng phi nung nng n nhit cao. Sau khi tch m tnh cht ho l ca vt thay i. m ny c th tn ti dng lin kt phn t nh trong mui hydrat MgCl2.6H2O hoc dng lin kt ion nh Ca(OH)2. Trong qu trnh sy khng t vn tch m dng lin kt ho hc. 1.1.2.2Lin kt ho l Lin kt ho l khng i hi nghim ngt v t l thnh phn lin kt. C hai loi: lin kt hp ph (hp th) v lin kt thm thu. Lin kt hp ph ca nc c gn lin vi cc hin tng xy ra trn b mt gii hn ca cc pha (rn hoc lng). Cc vt m thng l nhng vt keo, c ct to ht. Bn knh tng ng ca ht t 10-9 - 10-7 m. Do cu to ht nn vt keo c b mt bn trong rt ln. V vy n c nng lng b mt t do ng k. Khi tip xc vi khng kh m hay trc tip vi m, m s xm nhp vo cc b mt t do ny to thnh lin kt hp ph gia m v b mt. Lin kt thm thu l s lin kt ho l gia nc v vt rn khi c s chnh lch nng cc cht ho tan trong v ngoi t bo. Khi nc b mt vt th bay hi th nng ca dung dch tng ln v nc su bn trong s thm ra ngoi. Ngc li, khi ta t vt th vo trong nc th nc s thm vo trong. 1.1.2.3 Lin kt c l y l dng lin kt gia m v vt liu c to thnh do sc cng b mt ca m trong cc mao dn hay trn b mt ngoi ca vt. Lin kt c hc bao gm lin kt cu trc, lin kt mao dn v lin kt dnh t. - Lin kt cu trc: l lin kt gia m v vt liu hnh thnh trong qu trnh hnh thnh vt. V d: nc trong cc t bo ng vt, do vt ng c khi n c cha sn nc. tch m trong trng hp lin kt cu trc ta c th lm cho m bay hi, nn p vt hoc ph v cu trc vt... Sau khi tch m, vt b bin dng nhiu, c th thay i tnh cht v thm ch thay i c trng thi pha.


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- Lin kt mao dn: nhiu vt m c cu to mao qun. Trong cc vt th ny c v s cc mao qun. Cc vt th ny khi trong nc, nc s theo cc mao qun xm nhp vo vt th. Khi vt th ny trong mi trng khng kh m th hi nc s ngng t trn b mt mao qun v theo cc mao qun xm nhp vo trong vt th. - Lin kt dnh t: l lin kt do nc bm dnh vo b mt vt. m lin kt dnh t d tch khi vt bng phng php bay hi ng thi c th tch ra bng cc phng php c hc nh: lau, thm, thi, vt ly tm... 1.1.3 CC C TRNG TRNG THI M CA VT LIU M Nhng vt em i sy thng cha mt lng m nht nh. Trong qu trnh sy m, cht lng bay hi, m ca n gim i. Trng thi ca vt liu m c xc nh bi m v nhit ca n. 1.1.3.1 m tuyt i B qua khi lng kh v hi khng ng k, ngi ta c th coi vt liu m l hn hp c hc gia cht kh tuyt i v m. m = mo + W y: m: khi lng nguyn vt liu m mo: khi lng cht kh tuyt i W (hoc mn): khi lng m m tuyt i: l t s gia khi lng m W v khi lng cht kh tuyt i mo ca nguyn vt liu:X= W * 100(%) mo

X thay i t 0 n h. Gia khi lng cht kh m0, khi lng chung m v m tuyt i X c mi quan h:m0 = m 1+ x

1.1.3.2 m tng i: l t s gia khi lng m W trn khi lng chung ca nguyn vt liu:w= W W = 100(% ) m m0 + W

w m tng i ca nguyn liu m thay i t 0 n 1. Vi w = 0 ngha l vt liu kh tuyt i; vi m0=0, ngha l ch c m th w=1.


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Gia cht kh tuyt i v m tng i ca nguyn liu c mi quan h: mo = m.(1 - w) Gia m tng i v m tuyt i ca nguyn liu c mi quan h:x= w 1 w w= x 1+ x

Trc khi sy khi lng ca nguyn liu m l m1 v m tng i l w1, sau khi sy l m2 v w2. Bit rng trong khi sy khi lng cht kh mo khng thay i nn ta c: mo = m1(1-w1) = m2(1-w2) T ta c:m2 1 w1 = m1 1 w 2

Trong biu thc trn c 4 i lng l khi lng v m tng i ca nguyn liu trc v sau khi sy, nhng nu 3 i lng bit th t ta c th tnh c i lng th t. Nng sut ca mt my sy c th xc nh theo khi lng m (W tch ra t nguyn vt liu trong qu trnh sy: W = m1 - m2W = m1 W = m1 w w2 w1 w 2 = m2 1 1 w1 1 w2 x x2 x1 x 2 = m2 1 1 + x1 1+ x2

Mun quan st qu trnh sy bng ng cong sy mt cch r rng (to thnh im un gia hai on sy) ngi ta thng s dng m tuyt i X, cn vi m tng i w thng biu th trng thi m ca nguyn vt liu.

1.2

TC NHN SYTc nhn sy l nhng cht dng chuyn ch lng m tch ra t vt sy.

Trong qu trnh sy, mi trng bung sy lun lun c b sung m thot ra t vt sy. Nu lng m ny khng c mang i th m tng i trong bung sy tng ln, n mt lc no s t c s cn bng gia vt sy v mi trng trong bung sy v qu trnh thot m t vt sy s ngng li. Do vy, cng vi vic cung cp nhit


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cho vt ho hi m lng, ng thi phi ti m thot ra khi vt ra khi bung sy. Ngi ta s dng tc nhn sy lm nhim v ny. Cc tc nhn sy thng l cc cht kh nh khng kh, khi, hi qu nhit. Cht lng cng c s dng lm tc nhn sy nh cc loi du, mt s loi mui nng chy v.v...Trong a s qu trnh sy, tc nhn sy cn lm nhim v gia nhit cho vt liu sy, va lm nhim v ti m. mt s qu trnh nh sy bc x, tc nhn sy cn c nhim v bo v sn phm sy khi b qu nhit. Sau y, chng ta s nghin cu hai loi tc nhn sy thng dng l khng kh v khi. 1.2.1 KHNG KH M: CC THNG S C TRNG Khng kh l loi tc nhn sy c sn trong t nhin, khng gy c hi v khng gy bn sn phm sy. Khng kh l hn hp ca nhiu cht kh khc nhau. Thnh phn ca khng kh bao gm cc cht, ch yu l N2, O2, hi nc, ngoi ra cn c 1 s cht kh khc nh: CO2, kh tr, H2, O3...Khng kh l mt kh thc, nhng thc t khng kh s dng sy thng p sut thp (p sut kh quyn) v nhit khng cao (t hng chc n di vi trm ). V vy, khi s dng c th coi khng kh l kh l tng, mc d trong khng kh c cha hi nc, nhng p sut ring phn ca n khng ln. Trong cc iu kin nh trn, khi coi khng kh l kh l tng th sai s gp phi l chp nhn c ( t2) v nhu cu nhit q cng phi ln hn tc l on thng 1-3. 2.3.3 My sy nhiu cp

i(5)

2 t2(2)

(4)

(3) (2) (4)

sn phm

(5) (3)

t3

3

(1)

Q

(1)

Qbs

X1

X5, X3

X

Hnh 2.3 : S my sy nhiu cp v biu din ca qu trnh sy trn th i-X


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Nhu cu nhit ring s c tnh theo cng thc : i i i' i q= 5 1 = 3 1 ' x 5 x1 x 3 x1

(2.5)

So snh vi my sy mt cp : - Nhu cu nhit bng nhau nu m ban cui ca khng kh sy ging nhau, trong khi nhit un nng khng kh thp hn nhiu. - Nhu cu nhit nh hn nu nhit un nng khng kh sy ging nhau, trong s thay i di/dx xy ra tng t nh my sy mt cp. 2.3.4 My sy tun hon imth (4) (3) (2) (3) (1) sn phm (3) x = const (4) =1

(2)

Q

(2) (1) mKKm

X1

X5, X3

X

Hnh 2.4 : S my sy tun hon v biu din qu trnh sy trn th i-X - My sy tun hon c s dng vi nhng sn phm nhy cm vi nhit sy v khng kh sy i vo cn c gi trng thi t b thay i, tc l t ph thuc vo iu kin thi tit bn ngoi, khi khng kh mi (khng kh bn ngoi) c hn hp vi mt phn khng kh sy i ra khi my sy - Cn bng hn hp : mKKm + mth = mKK (2.6) i vi lng khng kh sy : i vi hm m : mKKm.X1 + mth.X4 = mKK.X2 (2.7) i vi nng lng : mKKm.i1 + mth.i4 = mKKm.X2 (2.8) - Cn bng nhit lng : mKKm.i1 + Q = (mKK - mth).i4 = mKKm.i4 Q = mKKm.(i4 - i1)

Gio n K thut sy nng sn thc phmi i Q Q m KKm =q= = l.(i 4 i1 ) = 4 1 . mW m KKm m W x 4 x1

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(2.9)

2.3.5 My sy thc t Q m Q Q = q = l.(i 3 i1 ) bs + G (i Gr i Gv ) i Wv + tt mW mW mW mW

(2.10)

hoc q = l.(i3-i1) - qbs + qG - iWv + qtt (2.11) i vi my sy l thuyt ta c : q = l.(i3-i1), tc l qbs + iWv = qG + qtt I II Nu b qua nhit lin kt ca nc trong sn phm sy : Nu tn tht nhit qtt v tn tht nhit do sn phm trang b vn chuyn mang ra cn bng vi nhit ung nng b sung, qu trnh sy xy ra theo ng hm nhit khng i ngha l I = II Nu qbs=0, th III).

2.4

CHUYN NG M TRONG SN PHM SY

Qu trnh chuyn m trong vt liu sy bao gm : chuyn di m t bn trong vt liu m ti b mt ca n, m bay hi b mt, chuyn di m dng hi t b mt vt liu n lung khng kh sy bao quanh vt liu sy. m chuyn di t b mt vt liu sy ra mi trng sy chung quanh, cn c n b bng cch chuyn m t bn trong vt liu sy ra n b mt ca n. Lng m bay hi v chuyn t b mt vt liu ra mi trng xung quanh c th tnh theo phng trnh : (kg) (2.14) Wbh = r.(PM - PB).F.T Trong : PM : p sut ring phn ca hi nc trn b mt vt liu sy (N/m2) PB : p sut ring phn ca hi nc trong khng kh (N/m2) T : thi gian sy (s;h) r : h s bc hi (kg/N.s hoc kg/m2.h) dn m : l qu trnh chuyn di m bn trong sn phm sy do s chnh lch m gia cc lp b mt v cc lp bn trong ca vt liu sy, c thc hin nh lc khuch tn, thm thu, lc mao qun... Do c dn m m m chuyn di th lng khi m ln hoc th hi khi m b, theo hng t trung tm ra n b mt ca n. Trong giai on vn tc sy khng i, m ch bc hi b mt vt liu sy.


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Sau im ti hn th I, qu trnh bc m xut hin bn trong cc mao qun. Trong giai on vn tc sy gim, m c chuyn t bn trong vt liu ra n b mt thng th hi, m hi ny c to ra "lp bay hi" hay cn gi l "mng sy" su trong vt liu v km theo s khuch tn th lng. Sau im ti hn th II s chuyn di m trong sn phm sy hu nh ch th hi. Lng m chuyn di do m dn m qua b mt F, sau thi gian T, t mt im ca vt liu c m W1 n im khc c m W2 (nu W1>W2), c th xc nh theo biu thc sau : w w (2.15) m w = K w .F.T. 1 2 (kg) b Trong : Kw : h s dn m, ph thuc vo lc lin kt m trong vt liu sy v tnh cht ca vt liu (m2/s). W : m ca vt liu sy kg/kg cht kh. b : khong cch gia hai im c nng m khc nhau (m) Ngoi ra, m cn c th chuyn di nh hin tng dn nhit m. Qu trnh ny c thc hin di tc dng ca nhit khuch tn v s co dn ca khng kh trong cc mao qun, nhit chuyn di theo hng t ni c nhit cao n ni c nhit thp hn, ngha l t b mt nng nht pha ngoi vo su trong vt liu (t ngoi vo trong) v km theo m. Lng m chuyn di qua b mt F v sau thi gian T t im c nhit t1 n im c nhit t2 (vi t1>t2) c th xc nh theo biu thc sau : t t (2.16) m t = K t .F. 1 2 .T (kg) b Trong , Kt : h s dn nhit m [kg/m.s.] b : khong cch gia 2 im trong vt liu c nhit khc nhau t1 v t2 [m] Hin tng dn nhit m lm cn tr chuyn ng ca m t bn trong ra n b mt vt liu sy, r nht l bt u giai on tch m lin kt hp ph v thm thu. Tng kt qa chuyn di m trong qu trnh sy s l (2.17) mw = mw - mt

2.5

VN TC SY

2.5.1 Khi nim v vn tc sy dW u= (kg/m 2 .h) F.dT Trong : W : lng m bay hi trong thi gian sy (kg/h)

(2.12)


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F : tng b mt bay hi ca sn phm sy (m2) T : thi gian sy (h) Nu vn tc sy khng i, khi bit vn tc sy, thi gian sy c th c tnh theo cng thc : G .(w w 2 ) T= k 1 (h ) (2.13) u.F Trong : - Gk : khi lng vt liu sy tnh theo khi lng kh tuyt i (kg/h) - W1, W2 : m ban u v ban cui ca sn phm sy tnh bng kg/kg sn phm kh tuyt i. Cc yu t nh hng n thi gian sy : Bn cht ca sn phm sy : cu trc, thnh phn ho hc, c tnh ca lin kt m... Hnh dng v trng thi ca sn phm sy m ban u, ban cui v m ti hn ca sn phm sy. Nhit , m v vn tc ca tc nhn sy. Chnh lch nhit ban u v ban cui ca tc nhn sy. Cu to ca my sy, phng thc sy v ch sy. 2.5.2 Cc giai on vn tc sy : ng cong vn tc sy : biu th quan h gia vn tc sy v m ca sn phm sy, c xc nh bng thc nghim. W (%) A B dW FdT K1 B

K1 C T (h) Hnh 2.5 : ng cong sy W=f(T) Wcb Wth A W (%) Hnh 2.6 : ng cong vn tc sy

Qu trnh sy n m cn bng gm cc giai on chnh : - Giai on t nng sn phm sy, tng ng vi on AB. - Giai on vn tc sy khng i (ng tc), on BK1 . - Giai on vn tc sy gim dn, tng ng vi on K1C.


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- im K1 gi l im ti hn, tng ng vi m ti hn Wth, ti xut hin m t do. Vic xc nh hai giai on sy c ngha quan trng thit lp ch sy ph hp vi tng giai on sy v tng loi sn phm sy. 2.5.3 Tnh ton vn tc sy 2.5.3.1 Giai on vn tc sy khng i m c tch ra ch yu l do bc hi t b mt ca sn phm sy, do : dW = b.dQ, trong b : h s t l. Mt khc, dQ = a.F.(tK - ts).dT Trong : a : h s cp nhit (kcal/m2.) F : b mt trao i nhit (m2) tK : nhit ca khng kh sy (oC) ts : nhit ca sn phm sy (oC) T : thi gian sy trong giai on vn tc sy khng i (s) dW = b.a.F. (tK - ts).dT dW = kt.F. (tK - ts).dT (2.18) 2 Trong : kt [kg/m .s.] = b.a, l h s chuyn khi ph thuc vo nhit . Phng trnh tnh vn tc sy trong giai on ng tc s l : dW u1 = = k t .(t K t s ) [kg/m2.s] (2.19) F.dT ng lc ca qu trnh sy khng ch c biu th bng s chnh lch m, m cn bng s chnh lch nhit gia tc nhn sy v b mt sn phm sy. Ngoi ra n cn c biu din bng hiu s p sut ring phn ca hi nc bo ho ca khng kh Pbh tng ng vi nhit bay hi b mt sn phm sy v p sut ring phn trong khng kh Ph, hoc bng hiu s ca hm m khng kh trn b mt vt liu sy Xbh (c th coi nh hm m ny tng ng vi trng thi bo ho) v hm m ca khng kh sy Xh. 2.5.3.2 Giai on vn tc sy gim dn (thay i) Qu trnh sy xy ra l phc tp. ng cong sy c th cong u hoc c im un. n gin ho v vi mc gn ng, ngi ta c th coi nh vn tc sy gim theo ng thng. ng lc ca qu trnh sy l hiu s gia m ca sn phm sy v m cn bng ca n v phng trnh c dng : dW2 u2 = = k w .(W Wcb ) (kg/m2.h) (2.20) F.dt Trong : W : m ca sn phm sy (kg/kg cht kh) Wcb : m cn bng ca SP sy (kg/kg cht kh)

Gio n K thut sy nng sn thc phm kw : h s chuyn khi (kg/m2.h)

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2.5.4 Tnh ton thi gian sy Thi gian sy l mt thng s c bit quan trng c s dng trong tnh ton thit k v vn hnh thit b sy. Thi gian sy ph thuc vo nhiu yu t nh loi vt liu sy, hnh dng, kch thc hnh hc ca vt liu, m u v cui ca vt liu, loi thit b sy, phng php cp nhit, ch sy. Do vic xc nh thi gian sy bng gii tch gp nhiu kh khn. V vy trong tnh ton thc t cc thit b sy thi gian c xc nh theo thc nghim v c theo kinh nghim vn hnh. Tuy nhin trong nghin cu cc thit b sy mi v sy cc vt liu khi cha c kinh nghim ngi ta phi da vo l thuyt gii tch hoc na gii tch na thc nghim tnh ton thi gian sy. Nguyn tc xc nh thi gian sy bng gii tch : 1- Xy dng m hnh vt l ph hp vi vt liu cn sy v vi mt thit b sy no ph hp vi phng php cp nhit v ch sy. 2- T m hnh vt l thit lp m hnh ton hc ca bi ton truyn nhit truyn cht, ngha l vit h phng trnh truyn nhit truyn cht cng vi cc iu kin n tr tng ng. Trong h phng trnh truyn nhit truyn cht phi th hin m hnh vt l mt cch ton din, chnh xc nhng cng lc b nhng nhn t ph m hnh ton hc n gin v c th gii c. 3- Gii m hnh ton hc xc nh thi gian sy Do trong m hnh vt l v c m hnh ton hc c b i mt s nhng yu t v vy thi gian sy xc nh bng gii tch s sai khc vi thc t, cho nn cn phi tri qua thc nghim chnh l cho ph hp. 2.6 PHNG PHP V THIT B SY Sy c th c chia ra hai loi : sy t nhin v sy bng thit b (sy nhn to). Sy t nhin : qu trnh phi vt liu ngoi tri, khng c s dng thit b. Cc phng php sy nhn to thc hin trong cc thit b sy. C nhiu phng php sy nhn to khc nhau. Cn c vo phng php cung cp nhit c th chia ra cc loi : sy i lu, sy bc x, sy tip xc, sy thng hoa, sy bng in trng dng cao tn, sy in tr... 2.6.1 Phi v sy bng nng lng mt tri Sy bng cch phi nng (khng c s dng thit b sy) c s dng rng ri nht trong ch bin nng sn.


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Trong cc phng php phc tp hn (sy bng nng lng mt tri), nng lng mt tri c thu nhn lm nng khng kh. Sau khng kh nng c s dng sy. Thit b sy bng nng lng mt tri c th phn ra cc loi sau : + thit b sy trc tip c tun hon kh t nhin (gm thit b thu nng lng kt hp vi bung sy). + thit b sy trc tip c b phn thu nng lng ring bit. + thit b sy gin tip c dn nhit cng bc (thit b thu nng lng v bung sy ring bit). 2.6.1.1 u im cng ngh n gin, chi ph u t v vn hnh thp. khng i hi cung cp nng lng ln v nhn cng lnh ngh c th sy lng ln v ma vi chi ph thp. 2.6.1.2 Nhc im kim sot iu kin sy rt km tc sy chm hn so vi vi sy bng thit b, do cht lng sn phm cng km v dao ng hn. qu trnh sy ph thuc vo thi tit v thi gian trong ngy. Hnh 2.7. : S TB sy bng nng i hi nhiu nhn cng. lng mt tri 2.6.1.3 Thit b C nhiu kiu thit k thit b sy bng nng lng mt tri khc nhau. Nhng thit b nh : thng c cng sut nh, tc sy v cht lng ci tin khng ng k so vi phng php sy phi (c m bo v sinh), do t c s dng.

Hnh 2.8 : Thit b sy bng nng lng mt tri c k nguyn liu


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Hnh 2.9 : S h thng sy bng nng lng mt tri c tr nhit Nhng thit b ln hn, c s dng qut chy bng nng lng mt tri vi cng sut 200-400 kg m ang c s dng nhiu cc nc vng a trung hi sn xut tri cy sy xut khu cho th trng chu u. Nng lng mt tri cn c th c dng lm nng trc khng kh cc thit b sy vn hnh bng nhin liu tit kim mt phn nng lng. 2.6.2 Sy i lu 2.6.2.1 Nguyn l hot ng Khng kh nng hoc khi l c dng lm tc nhn sy c nhit , m, tc ph hp, chuyn ng chy trm ln vt sy lm cho m trong vt sy bay hi ri i theo tc nhn sy. Khng kh c th chuyn ng cng chiu, ngc chiu hoc ct ngang dng chuyn ng ca sn phm. Bng 2.1 so snh cc phng php chuyn ng khc nhau ca tc nhn sy. Sy i lu c th thc hin theo m (gin on) hay lin tc. Trn hnh v di l s nguyn l sy i lu bng khng kh nng.

Hnh 2.10 :


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Sn phm sy c th ly ra khi bung sy theo m hoc lin tc tng ng vi np vo. Caloriphe 2 t nng khng kh c th l loi caloriphe in, caloriphe hi nc v.v... Kt cu thc ca h thng rt a dng, ph thuc vo nhiu yu t nh : ch lm vic, dng vt sy, p sut lm vic, cch nung nng khng kh, chuyn ng ca tc nhn sy, s lm vic, cu trc bung sy... Bng 2.1 :So snh cc hnh thc chuyn ng khc nhau ca tc nhn sy. Hng chuyn u im Nhc im ng TNS Cng chiu Tc sy ban u cao, t b co kh t c m cui thp ngt, t trng thp, sn phm t v khng kh ngui v m h hng, t nguy c h hng do thi qua sn phm sy. VSV Ngc chiu Nng lng c s dng kinh t Sn phm d b co ngt, h hn, m cui cng thp hn. hng do nhit. C nguy c h hng VSV do khng kh m, m gp nguyn liu t. Dng kh thot Kt hp u im ca sy cng Phc tp v t tin hn so trung tm chiu v ngc chiu nhng vi sy mt chiu. khng bng sy bng dng kh thi ct ngang. Dng kh thi Kim sot iu kin sy linh hot u t trang b, vn hnh v ct ngang bng cc vng nhit c kim bo dng thit b phc tp sot ring bit; tc sy cao v t tin. i vi qu trnh sy chi ph nng lng l yu t chnh nh hng n hiu qu kinh t sn xut, v vy khi thit k, cn ch n cc bin php lm gim s tht thot nhit, tit kim nng lng. V d : cch nhit bung sy v h thng ng dn. tun hon kh thi qua bung sy s dng thit b trao i nhit thu hi nhit t khng kh thot ra ung nng khng kh hoc nguyn liu vo. s dng nhit trc tip t la t kh t nhin v t cc l t c c cu lm gim nng kh oxit nit. sy thnh nhiu giai on (v d : kt hp sy tng si vi sy thng hoc sy phun kt hp vi sy tng si). c c trc nguyn liu lng n nng cht rn cao nht c th.

Gio n K thut sy nng sn thc phm kim sot t ng m khng kh bng my tnh. Cc tiu chun chn la thit b sy c m t trong bng 2.2.

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2.6.2.2 Thit b sy i lu Thng sy (bin dryer) Cu to : l mt thng cha hnh tr hoc hnh hp c y dng li. Khng kh nng thi ln t pha y ca nguyn liu vi vn tc tng i thp (v d : 0,5 m/s). ng dng : - do c sc cha ln, gi thnh v chi ph hot ng thp chng c s dng ch yu sy kt thc sau khi sn phm c sy trc bng cc thit b sy khc. Chng c th c dng cn bng m sn phm sau khi sy. Yu cu i vi nguyn liu : do thit b sy c th cao vi mt, yu cu nguyn liu phi cng c hc chng li sc p, duy tr khong trng gia cc ht, gip khng kh nng c th xuyn qua c. Bung sy : Cu to : gm c mt bung cch nhit vi cc khay li hoc t l, mi khay cha mt lp mng nguyn liu (dy 2-6cm). Khng kh nng thi vo vi tc 0,5-5 m/s qua h thng ng dn v van i hng cung cp khng kh ng nht qua cc khay. Cc thit b un nng ph tr c th c t thm pha trn hoc dc bn cc khay tng tc sy. ng dng : - dng trong sn xut nh (1-20 tn/ngy) hoc trong th nghim. Chng c gi thnh, chi ph bo dng thp v c th s dng linh hot sy cc loi nguyn liu khc nhau. Tuy nhin, iu kin sy tng i kh kim sot v cht lng sn phm dao ng do s phn phi nhit n nguyn liu khng ng u. L sy y l nhng to nh 2 tng trong sn nh c git g mng c t pha trn l t. Khng kh nng v sn phm chy t l t xuyn qua lp nguyn liu c dy n 20 cm. Chng c s dng theo truyn thng sy to M hoc hoa hp-lng chu u, tuy nhin vic kim sot iu kin sy rt kh khn v thi gian sy tng i lu. Do yu cu phi o sn phm thng xuyn, vic cht nguyn liu v tho d sn phm c thc hin bng th cng nn chi ph nhn cng cao. Tuy vy, chng c u im l sc cha ln, d xy dng v bo dng vi chi ph thp.


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Bng 2.2 : Cc phng php sy v tiu chun chn la.Phng php sy Thp sy Bung sy Bng chuyn Tang trng Thm bt Tng si M/ lin tc B B C C C B/C Rn/ Lng S S S S L S c tnh nguyn liu m Nhy du cm nhit Thp Tr.bnh Tr.bnh Tr.bnh Tr.bnh Kch thc TrL TrL TrL Nh Nh Cng c hc C Tc sy Chm Tr.bnh Tr.bnh Tr.bnh Nhanh Tr.bnh iu kin sy m cui Nng sut bay hi ti a tiu biu Thp Tr.bnh 55-75 Tr.bnh 1820 Tr.bnh Thp 410 910 V d sn phm

C

C

L sy Sy in trng dng cao tng Kh ng Bc x Thng quay Spin flash Sy phun Nng lng mt tri Hm Bng chuyn/ k chn khng

B B/C

S S

Tr.bnh Thp

TrL Nh

Chm Nhanh

Tr.bnh Thp

-

Rau c Rau c, tri cy ng cc, tri cy, bnh ko, bch quy, rau c, ht u dch st, xi r ng, khoai ty n lin, gelatin nc p tri cy u, rau c dng mng hoc dng cc, ht la, bt hoc sn phm p n, tri cy, du kh, gia v to dng lt, dng nhn, hp lng cc loi bnh t bt m

C C B/C C C B C C

S S S L S S S L

Thp Thp Tr.bnh Tr.bnh Tr.bnh Tr.bnh Tr.bnh

C C C

Nh Nh Nh TrL/Nh TrL TrL TrL

C C

Nhanh Nhanh Tr.bnh Nhanh Nhanh Chm Tr.bnh Chm

Thp Tr.bnh Thp Tr.bnh Tr.bnh Tr.bnh Thp

15900 1820-5450 7800 15900 18200

tinh bt, bt sp, cho khoai ty cc loi bnh t bt m ht cacao, lc, ng cc chn, pasta, c ph ho tan, sa bt tri cy, rau c tri cy, rau c nc p tri cy, chit xt tht, mu s c la


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Hm sy Cu to : cc khay cha nguyn liu c cht ln cc xe gong, c lp trnh chuyn ng qua hm cch nhit c tc nhn sy chuyn ng theo mt hoc nhiu hng khc nhau nh m t trong bng 2.1. Sn phm sau khi ra khi hm c th c sy kt thc trong cc thng sy. Mt hm sy tiu biu di 20 m c 12-15 xe gong vi tng sc cha 5000 kg nguyn liu. ng dng : do kh nng sy lng ln nguyn liu trong mt thi gian tng i ngn, chng c s dng rng ri. Tuy nhin, phng php ny hin b thay th bng phng php sy bng chuyn v sy tng si do hiu sut nng lng ca sy hm thp hn, chi ph lao ng cao hn v cht lng sn phm khng tt bng hai phng php nu sau. Sy bng chuyn Cu to : l thit b lm vic lin tc, c th di n 20m, rng 3m. Nguyn liu c t trn mt bng chuyn li c y su 5-15 cm. Dng kh lc u c hng t di ln qua y ca nguyn liu v cc giai Hnh 2.11 : Cc hnh thc chuyn ng ca on sau c hng xung tc nhn sy trong hm sy. di sn phm khi b thi ra a) cng chiu b) ngc chiu c) kt hp cng - ngc chiu d) ct ngang khi bng chuyn. cc thit b sy 2 hoc 3 giai on nguyn liu sau khi c sy mt phn s c xo trn v cht ng li vo cc bng chuyn k tip su hn (n 15-25 cm hoc 250-900 cm cc my sy 3 giai on), nh ci tin c tnh ng nht ca qu trnh sy v tit kim c khng gian. Sn phm thng c sy n m 10-15 % v sau c sy kt thc thng sy. Thit b sy c th c cc khu vc sy c lp vi nhau c kim sot bng my tnh v h thng t ng np nguyn liu v tho sn phm gim chi ph nhn cng.


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ng dng : - do iu kin sy c kim sot tt v nng sut cao nn thng c dng sy sn phm quy m ln (v d : n 5,5 tn/h). - ng dng trong phng php sy thm bt : nguyn liu dng lng c chuyn thnh dng bt bn bng cch cho thm tc nhn to bt v c ho kh nit hoc khng kh. Nguyn liu dng bt c tri ln bng chuyn c l n dy 2-3 mm v c sy nhanh trong 2 giai on bng dng kh thi cng chiu v sau ngc chiu. Sy thm bt nhanh khong gp 3 ln so vi sy cht lng c dy tng t. Lp thm xp mng ca sn phm sy sau c nghin thnh bt d chy, c tnh hi nguyn rt tt. Qu trnh sy nhanh v nhit sn phm thp gip cho cht lng ca sn phm tt, nhng khi sn xut vi nng sut cao i hi din tch b mt ln, chi ph do cng cao. My sy thng quay (rotary dryer) Cu to : mt thng cha kim loi hnh tr hi nghing (khong 5o) quay trn quanh trc c gn vi cc cnh hng bn trong nguyn liu xung xuyn qua dng kh chuyn ng cng chiu hoc ngc chiu qua my sy. Din tch b mt ca nguyn liu c phi by ti a trong khng kh nn tc sy cao v cht lng sn phm sy ng u. ng dng : - c bit thch hp cc loi nguyn liu c khuynh hng b ri hoc dnh vo nhau trong bng chuyn hoc khay Tuy nhin, do s h hi do va p, c xt trong my, chng ch ch s dng cho tng i t loi phm (v d : sy ht u, ht cacao ...). cho

sy. hn sn Hnh 2.12 : S h thng sy thng quay

Sy tng si (fluidized bed dryer) Cu to : my sy tng si gm mt thit b phn phi khng kh ng u quanh phn y ca nguyn liu; mt bung thng gi vo to ra mt vng kh ng nht, ngn nga tc cao cc b; v mt vng thot kh pha trn tng si nhng phn t b gi cun ln thot ra. Khng kh thot ra t tng si thng c thi vo cc xiclon tch nhng phn t mn, sau chng c a tr li vo sn phm hoc c lm kt cc. Pha trn ca h thng phn phi kh, cc khay li c cha lp nguyn liu dng ht c b dy n 15 cm. Khng kh nng thi xuyn qua lp nguyn liu lm chng l lng v rung ng mnh, phi by ti a din tch b mt nguyn liu (hnh v ). Nhng my sy kiu ny gn v kim sot tt iu kin sy v tc sy cao.


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Hnh 2.13 : Thit b sy tng si dng trong sy ng

ng dng : Cc thit b sy tng si lm vic theo m c s dng trong sn xut nh, trong , sn phm c xo trn mnh nn m ca sn phm ng nht. Thit b sy lin tc c cc khay rung chuyn sn phm t khay ny sang khay k tip di tc dng ca trng trng. m ca sn phm sy dao ng trong phm vi rng hn v thng c kt hp vi thng sy. Cc p dng chnh bao gm sy cc loi nguyn liu dng ht nh c kh nng si m khng b h hi c hc qu mc nh men kh, da kh, thc la,

gia v, c ph ho tan, ng, tr... Ngoi ra, cn c nhiu kiu thit k khc ca phng php sy tng si nh : - my sy Torbed, trong lp nguyn liu si xung quanh mt bung sy "hnh chn ct" do khng kh nng thi trc tip t bung t ln (hnh v ). My sy c tc truyn khi v truyn nhit rt cao, thi gian sy rt ngn mt cch ng k. Thit b c b vi x l kim sot qu trnh sy v chng c th c s dng nng, rang nguyn liu dng ht. - my sy spin-flash, trong bung sy c gn vi rotor y. Khng kh nng c thi vo theo phng tip tuyn vo bung sy v di tc ng ca rto khng kh chuyn ng xoy ln trn. Cc mu nguyn liu, nh l tht cua nho, bnh c ca hoc ko gm, khi vo bung sy s b bt sy kh dnh vo. Nhng Hnh 2.14 : My sy tng si cc ny ri xung y chng b si do Torbed khng kh thi v b quay bi r to. Khi kh, nhng cc ny v ra thnh bt v c mang ln trn thnh bung sy v ly ra qua cc l phn loi c kch c thay i c. - my sy tng si ly tm


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My sy kh ng Nguyn l hot ng : bt m hoc nguyn liu dng ht, thng thng m di 40 % v kch thc ht trong phm vi 10-500 m, c np vo h thng ng dn bng kim loi v b Hnh 2.15 : My sy spin-flash thi l lng trong khng kh nng. cc my sy thng ng, dng kh c iu chnh sao cho cc ht nh v nh hn, tc l s kh nhanh hn, c a n thit b tch xiclon trc, cn nhng ht m v nng hn phi lu li trong ng sy trng thi l lng lu hn t n m yu cu. i vi nhng sn phm i hi thi gian lu lu, h thng ng sy c dng vng (my sy kh ng dng vng) v nguyn liu c quay vng cho n khi chng kh. Nhng my sy dng vng c nhit sy cao, thi gian sy ngn c s dng lm n tinh bt trong khoai ty, c rt, to

Hnh 2.16 : H thng sy kh ng dng ng ra cu trc xp v rn gip ci tin c tc sy cho cc phng php sy thng thng khc giai on sy sau v gip qu trnh hi nguyn nhanh. Qu trnh sy ko di 2-10 giy, v th chng thch hp cho cc sn phm mt m nhanh trn b mt. Qu trnh lm lnh do bay hi ca ht ngn nga s h hi v nhit, gip sn phm c cht lng cao. ng dng : my sy kh ng c vn v chi ph bo dng tng i thp, tc sy cao v iu kin sy c kim sot cht ch. Chng thch hp vi cc nguyn liu nhy cm vi nhit. Nng sut u ra

Hnh 2.17 : S h thng sy kh ng dng vng


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ca sn phm khong t 10kg/h n 25 tn/h. Chng thng c dng sau khi sy phun to ra nhng sn phm c m thp hn so vi bnh thng (v d : sa c bit, bt trng, khoai ty dng cc). My sy phun Mt h phn tn mn ca nguyn liu c c c trc (40-60 % m) c phun hnh thnh nhng git mn, ri vo trong dng kh nng cng chiu hoc ngc chiu nhit 150-300 oC trong mt bung sy ln. Cc kiu vi phun sau c s dng : - Vi phun ly tm : cht lng c np vo gia tm ca chn hoc a quay c vn tc ngoi vi 90-200 ms-1. Cc git lng ng knh 50-60 m c bn xung t mp ra to thnh lp git lng u. - Vi phun p sut : cht lng b cng bc di p sut cao (700-2000 kPa) qua mt k h nh to nn nhng git lng c kch thc 180-250 (m. - Vi phun kh ng : khng kh c s dng phun dung dch. Trc ht khng kh qua ng phun tng tc ri thi ra ming phun, cht lng c a n ming vi bng bm. Khng kh c tc cao s thi dung dch vng ra thnh ht nh. Vi phun l d b nght bi cc ht nguyn liu v nguyn liu cng mi mn dn dn cc khe l lm rng l ra do tng kch thc trung bnh ca cc git lng. Qu trnh sy xy ra nhanh (1-10 giy) do din tch b mt ca nhng git lng rt ln. Tc np liu c kim sot sao cho nhit khng kh ra 90-100 oC, tng ng vi nhit bu t (v nhit sn phm) 40-50 oC sn phm t b h hi. Bt kh thu c y thit b sy v c ly i bng vt ti C nhiu kiu thit k khc nhau v vi phun, bung sy, h thng t nng khng kh v h thng thu hi bt, xut pht t yu cu ca rt nhiu loi nguyn liu sy phun khc nhau (v d : sa, trng, c ph, ca cao, ch, khoai ty, hn hp kem , bt yaourt, pho mt, tc nhn lm trng c ph, nc p tri cy, gia v ng gi v tinh bt ng, la m). Cc thit b sy phun cng c th c gn vi thit b sy tng si sy kt thc sn phm thu c t bung sy. Thit b sy phun khc nhau v kch c t cc thit k mc th nghim sy nhng sn phm khi lng nh, gi tr cao nh enzym, gia v cho n cc thit k sn xut quy m ln dng trong thng mi vi nng sut n 10.000 kg sa sy/gi. u im ln l qu trnh sy nhanh, sn xut lin tc quy m ln, chi ph nhn cng thp, vn hnh v bo dng tng i n gin. Hn ch chnh l chi ph u t cao, yu cu m ban u cao bo m nguyn liu c th bm n thit b to git lng. iu ny dn n chi ph nng lng cao hn ( tch m) v tht thot cc cht d bay hi cao hn. Thit b sy bng chuyn v sy tng si ang bt u thay ch sy phun do chng gn hn v c hiu qa s dng nng lng tt hn.


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2.6.3 My sy tip xc (contact dryer) nhit c cung cp bng dn nhit u im chnh so vi sy i lu : + khng cn thit phi un nng lng ln khng kh trc khi sy do hiu qu nhit cao hn. + qu trnh sy c th thc hin khng cn s c mt ca oxy nn cc thnh phn d b oxy ho ca nguyn liu c bo v. Nhu cu nhit ring thng thng l 2000-3000 kJ/kg nc bay hi so vi 4000-10.000 kJ/kg nc bay hi ca my sy i lu. Tuy nhin, thc phm c dn nhit thp, tr thnh kh hn nn kh dn nhit hn trong qu trnh sy, v vy cn phi sy lp mng nhit dn nhanh, trnh gy h hi cho sn phm. Sy tang trng (sy trc ln) (roller dryer)

Hnh 2.18 : S thit b sy tang trng trc n

Hnh 2.19 : S thit b sy tang trng trc kp

Cc trc rng bng thp quay chm c un nng bn trong bng hi nc p sut cao n 120-170 oC. Mt lp mng nguyn liu c tri u ln b mt bn ngoi bng phng php nhng, phun, tri hoc bng cc trc ln np liu ph. Trc khi trc ln hon thnh 1 vng quay (khong 20 giy n 3 pht) sn phm sy c co ra bng li dao tip xc u vi mt trc theo chiu di ca n. Thit b sy c th c 1 trc, 2 trc hoc trc kp. Thit b n trc c s dng rng ri, v chng linh ng, t l din tch b mt trc s dng sy ln, d dng tip cn bo dng v khng c nguy c b h hi do kim loi ri vo gia hai trc. Thit b sy trc c tc sy cao, hiu qu nng lng cao, chng thch hp vi nguyn liu dng st c kch thc cc cu t ln qu mc c th sy phun c. Sy trc ln dng trong sn xut khoai ty dng mnh (flake), ng cc nu sn, mt ng, xp bt, pu r tri cy v sa tch kem (whey). Tuy nhin, do gi thnh trc ln cao v thnh phn nguyn liu nhy cm nhit d b h hi, nn trong sn xut ln chng b thay th bng phng php sy phun.


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Sy bng qu cu nng Bung sy c gn vi vt ti quay chm c cha cc qu cu bng s, c lm nng bng khng kh nng, thi vo bung sy. Nguyn liu dng vin, cc c sy ch yu do s dn nhit do s tip xc vi nhng qu cu nng v c dch chuyn qua bung sy bng vt ti, thot ra y. Thi gian sy c kim sot bng tc vt ti v nhit ca nhng qu cu nng. My sy bng chuyn chn khng v k sy chn khng (vacuum band dryer, vacuum shelf dryer)

Hnh 2.20 : S thit b sy bng chuyn chn khng Nguyn liu di dng st c tri hoc phun ln 1 bng chuyn thp chy qua 2 trc ln rng trong 1 bung chn khng c p sut 1-70 mmHg. Lc u nguyn liu c sy bng trc ln c lm nng bng hi nc v sau bng ng xon trao i nhit c hi nc lm nng hoc cc thit b cp nhit bc x t pha trn cc bng chuyn. Sn phm sy c lm ngui bng trc ln th 2 c nc lnh trong v c tch ra bng li dao. K sy chn khng gm cc k t trong 1 bung chn khng vi p st 1-70 mmHg. Nguyn liu c t thnh 1 lp mng trn cc khay thp phng c lm cn thn m bo s tip xc tt vi cc k. Hi nc hoc nc nng chy qua cc k cp nhit cho qu trnh sy. Qu trnh sy nhanh v s h hi do nhit n sn phm c hn ch gip cho 2 phng php ny thch hp vi cc nguyn liu nhy cm vi nhit. Tuy nhin, cn cn thn trnh sn phm khi b chy trn cc khay trong cc k sy chn khng v s co ngt lm gim s tip xc gia nguyn liu vi b mt nng c 2 thit b. Chng c gi thnh cao, chi ph vn hnh cao v nng sut thp, c dng ch yu sn xut cc sn phm sy phng (puff dried).


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Sy n phng (explosion puff drying) Bao gm sy s b nguyn liu n m trung bnh, sau ng kn n trong bung p sut. Nhit v p sut trong bung tng ln n mc no th c "gii phng" tc th. Do p sut gim nhanh, sn phm n ra, to mt cu trc xp mn. iu ny gip cho qu trnh sy kt thc sau v qu trnh hi nguyn ca sn phm xy ra nhanh hn. Gi tr dinh dng v cm quan cng c lu gi tt. Phng php ny lc u ch c p dng i vi loi sn phm ng cc n phng dng cho im tm, nhng dn nay chng c p dng cho mt lot sn phm rau v tri cy sy. 2.6.4 Sy thng hoa Sy thng hoa l qu trnh tch m ra khi vt sy bng s thng hoa ca nc. Qu trnh thng hoa l qu trnh chuyn trc tip t th rn sang th hi. iu kin bnh thng, m trong thc phm dng lng, nn thng hoa chng cn c chuyn sang th rn bng phng php lnh ng. Chnh v vy nn cn gi l phng php Sy lnh ng (Freeze Drying hay Lyophillisation). Qu trnh sy thng hoa bao gm hai giai on : lm lnh ng v tip theo sy kh bng chn khng thp. C hai h thng u hot ng rt tn km v khi thit b sy thng hoa hot ng theo m, chi ph vn hnh cng tng cao. Hin nay c cc thit b lm vic lin tc, nhng chi ph u t rt cao. V vy phng php sy thng hoa ch hn ch s dng i vi cc sn phm t tin, nhng sn phm m khng th sy c bng cc phng php khc. Bn cnh khng phi bt k nguyn liu no cng c th sy bng phng php lnh ng. i vi nhng nguyn liu c cu trc d b h hi trong qu trnh lnh ng th sn phm sy thng hoa khi hi nguyn s c kt cu ti. Sy thng hoa, nht l phng php sy nhanh (AFD : accelerated freeze drying) c p dng rng ri M sy cc loi nguyn liu t tin nh tht gia sc, gia cm ... Ngoi ra n cn c s dng sy cc sn phm khc nh : c ph, gia v, trong dc phm v.v... 2.6.4.1 Cc giai on ca sy thng hoa Giai on lm lnh ng Giai on u tin ca qu trnh sy thng hoa l lm lnh ng sn phm. Qu trnh lm lnh ng bng thc hin bng hai cch. Cch th nht trong thit b lm lnh ng thng thng hoc nit lng lm lnh ng sn phm bn ngoi bung sy thng hoa. Cch th hai l vt sy t lnh ng ngay trong bung sy thng hoa khi bung sy c ht chn khng. Sn phm cn c lm lnh ng rt nhanh hnh thnh cc tinh th bng nh t gy h hi n cu trc t bo ca sn phm. i vi sn phm dng lng, phng lm


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lm lnh ng chm c s dng bng to thnh tng lp, cc lp ny to nn cc knh gip cho hi nc dch chuyn d dng. Giai on thng hoa Giai on k tip l tch nc trong sut qu trnh sy tip theo lm kh sn phm. Nu p sut hi nc c gi di 4,58 mmHg (610,5 Pa) v nc dng bng, khi sn phm c cung cp nhit, th bng rn s thng hoa trc tip thnh hi m khng b tan chy (hnh v : th 3 pha ca nc). Hi nc tip tc c tch ra khi sn phm bng cch gi cho p sut trong bung sy thng hoa thp hn p sut hi nc trn b mt ca bng, ng thi tch hi nc bng my bm chn khng v ngng t n dt bng cc ng xon rut g lnh, cc bn lnh hoc bng ho cht. Khi qu trnh sy Nhit tip din, b mt thng hoa di chuyn vo bn trong sn phm ng lnh, lm Hnh 2.21 : S 3 pha ca nc sn phm c sy kh. Nhit lng cn thit dch chuyn b mt thng hoa (n nhit thng hoa) c truyn n sn phm do s dn nhit hoc do vi sng cung cp. Hi nc di chuyn ra khi sn phm qua cc knh c hnh thnh do bng thng hoa v c ly i. Nh vy, nu khng tnh qu trnh mt m trong phng php vt m t lnh ng trong bung sy khi ht chn khng th sn phm c sy trong hai giai on : trc tin do qu trnh thng hoa xung khong 15 % m v sau do bay hi ca phn nc khng ng bng n 2% m bng qu trnh nh m ng nhit. Qu trnh nh m ng nhit (desorption) t c bng cch nng nhit my sy ln gn nhit mi trng xung quanh trong khi vn gi p sut thp ging nh qu trnh sy cc thit b sy chn khng thng thng. ng cong sy p sut


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Hnh v 2.21 l ng cong sy v ng cong nhit t, W ca vt sy trong qu trnh sy thng hoa trong vt sy t A lnh ng trong bung sy. Khi ht chn khng, p sut trong bung sy gim xung, m t do t bay hi mnh lm gim nhanh nhit ca n xung n nhit ng bng tB ca m C D (ng A-B). Qu trnh ng W tB bng ca m c to nhit nn B nhit ca vt sy tng ln mt cht (B-C). Qu trnh Hnh 2.22 : ng cong sy v nhit sy thng hoa m din ra khc vi trong sy thng hoa. qu trnh sy th nht (tc khng i) trong sy i lu l nhit tng ln mt t theo thi gian sy (on C-D dc ln). iu c gii thch l lp su bn trong vt sy cn c m ang ng bng. Giai on sy tip theo l giai on bay hi m lin kt, nhit ca vt sy tng nhanh. Trong mt s sn phm (v d nc p tri cy, dch chit c ph c c), s hnh thnh nn trng thi thu tinh trong qu trnh ng bng gy ra nhiu kh khn cho vic di chuyn hi nc. V vy, cht lng cn c ng bng dng bt (phng php sy thng hoa bt : vacuum puff freeze drying), hoc l nc p tri cy sy cng vi phn tht (ci). C hai phng php u to nn cc knh nh hi nc c th thot i c. phng php th ba, nc tri cy sau khi ng bng c nghin thnh cc, nh sy nhanh hn v cho php kim sot kch c ca ht bt tt hn. Tc sy ph thuc phn ln vo tnh cn tr nhit ca sn phm v mc thp hn vo cn tr dng hi (dch chuyn khi) ra khi b mt thng hoa. 2.6.4.2 Tc truyn nhit C ba phng php truyn nhit n b mt thng hoa. Nhit truyn xuyn qua cc lp ng bng. Tc truyn nhit ph thuc vo dy v dn nhit ca lp bng. Khi qu trnh sy xy ra, chiu dy ca lp bng gim xung v tc truyn nhit tng ln. Nhit b mt ca thit b cp nhit c gii hn trnh lm tan bng. Nhit truyn qua lp kh. Tc truyn nhit n b mt thng hoa ph thuc vo chiu dy v din tch b mt ca sn phm, dn nhit ca lp kh v chnh lch nhit gia b mt ca sn phm v b mt bng. Khi p sut bung sy khng i, nhit ca b mt bng duy tr


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khng i. Lp kh ca sn phm c dn nhit rt thp (tng t nh vt liu cch nhit) v v th gy ra s cn tr ln vi dng nhit. Khi qu trnh sy tip din, lp ny tr nn dy hn v s cn tr nhit tng ln. Lm gim kch thc nguyn liu v tng chnh lch nhit s lm tng tc truyn nhit. Tuy nhin, sy thng hoa, nhit b mt b gii hn n 40-65 oC trnh s bin tnh protein v cc thay i ho hc khc, c th lm gim cht lng ca sn phm. Truyn nhit bng vi sng. Nhit c to ra trn b mt bng v tc truyn nhit khng b nh hng bi dn nhit ca bng v cht kh hay dy ca lp kh. Tuy nhin, nhit vi sng kh kim sot v c nguy c b tnh trng qa nng cc b dn n s tan chy bng. 2.6.4.3 Tc truyn khi Khi nhit truyn ti b mt thng hoa, nhit v p sut hi nc ca bng c tng ln. Hi nc di chuyn xuyn qua cht kh n vng c p sut hi thp trong bung sy. p sut 67 Pa, 1g bng hnh thnh 2 m3 hi v do , my sy thng hoa cn phi ly i hng trm mt khi hi trong 1 giy qua cc l hng ca cht kh. Cc yu t kim sot chnh lch p sut hi nc l : - p sut trong bung sy - nhit ca thit b ngng t hi, c hai cn thp n mc chi ph cho php. - nhit ca bng b mt thng hoa, cn cng cao cng tt nhng khng tan chy. Trong thc t m bo tnh kinh t, p sut bung sy thp nht vo khong 13 Pa v nhit thit b ngng t thp nht l khong -35 oC. V l thuyt, nhit ca bng cn nng ln mc ch va di im ng bng. Tuy nhin, trn mt nhit ti hn nht nh, gi l nhit sp (collapse temperature), cu trc sn phm s b ph hu ngay lp tc. Trong thc t, v th tn ti nhit bng ti a, nhit ngng t ti thiu v p sut bung sy ti thiu v nhng thng s ny kim sot tc chuyn khi. Trong qu trnh sy, m h xung t mc ban u rt cao trong vng lnh ng n mc thp hn lp kh, ph thuc vo p sut hi nc trong bung sy. Khi nhit chuyn qua lp kh, quan h gia p sut trong bung sy v p sut trn b mt bng l : k Pi = Ps + d ( t s t i ) (2.21) b. s Trong , Pi (Pa) l p sut ring phn ca hi nc b mt thng hoa, Ps (Pa) p sut ring phn ca hi nc b mt, kd (Wm-1K-1) : dn nhit ca lp kh, b (kg.s-1.m-1) thm ca ca lp kh, s (J.kg-1) : n nhit thng hoa, ts (oC) : nhit b mt v ti (oC) nhit ti b mt thng hoa. Thi gian sy c th c tnh bng cng thc sau :


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td =

x 2 (M 1 M 2 ) s 8k d ( t s t i )

(2.22)

trong : td (giy) l thi gian sy, x (m) : chiu dy ca sn phm, ( (kg.m-3) : t trng ca cht kh, M1 : m ban u v M2 : m cui cng. Ch rng : thi gian sy t l vi bnh phng dy ca sn phm : do gp i chiu dy sn phm s ko di thi gian sy gp 4 ln. 2.6.4.4 Thit b sy thng hoa Cc my sy thng hoa bao gm mt bung chn khng c cha cc khay ng sn phm v thit b un nng cp n nhit thng hoa. Cc ng xon rut g lnh hoc cc bn dt lnh c s dng ngng t hi nc trc tip thnh bng. Chng c gn vi thit b t ng lm tan bng gi cho b mt ca cc dy xon rut g c trng ti a cho vic ngng t hi nc. iu ny l cn thit bi v phn ln nng lng u vo c dng lm lnh ng cc thit b ngng t v v th tnh kinh t ca sy thng hoa c xc nh bi hiu sut ca thit b ngng t : Nhit thng hoa Hiu sut = (2.23) Nhit tc nhn lm lnh thit b ngng t Bm chn khng tch i cc thnh phn hi khng ngng t.

Hnh 2.23 : S cu to hm sy thng hoa 2.6.4.5 nh hng qu trnh sy thng hoa n cht lng sn phm Sn phm sy thng hoa lu li rt tt cc c tnh cm quan v cht lng dinh dng v thi gian bo qun di khi c bao gi ng cch. Cc cht d bay hi khng


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b cun vo hi nc sinh ra trong qu trnh thng hoa m b mc li trong khung sn phm. Kt qu l 80-90 % mi c gi li. Kt cu ca sn phm c tt : t b co ngt v khng b hin tng cng v. Cu trc xp cho php qu trnh lm t tr li nhanh chng v hon ton, nhng n d v v cn bo v trnh b h hi c hc. Ch c nhng thay i nh v cht lng protein, tinh bt v cc hydrocacbon khc. Tuy nhin cu trc xp ca sn phm c th cho oxy xm nhp v gy oxy ho lipit. V vy, sn phm c bao gi trong kh tr. Nhng thay i ca thiamin v axit ascorbic trong qu trnh sy thng hoa mc va phi v s tht thot ca cc vitamin khc khng ng k (xem bng). Tuy nhin, s tht thot cc cht dinh dng do cc qu trnh chun b trc khi sy, c bit l chn hp rau c th nh hng ng k n cht lng dinh dng ca sn phm sy thng hoa. Bng 2.3 : Tn tht vitamin trong qu trnh sy thng hoa Tht thot (%) Thc phm Vitamin Vitamin Thiamin Riboflavin Axit Niacin Axit C A folic Pantothenic u xanh 26-60 0-24 0 10 u H lan 8-30 5 0 0 10 Nc cam 3 3-5 Tht b 2 0 + 0 13 Tht heo 80 oC, lnh t

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