NLN •132-11/2016 "15
^MO HiNH TOAN HOC MO TA QUA TRJNH NHIET PHAN THAN GUI G 5
Mathematical Model of wood charcoal pyroiysis process ThS. Dgng The Hung''; TS. Ding Tran Thg^; PGS.TS Pham Hoang Lirong'
'- DH Slf Phgm Ky thuit Vinh; ^^- DH Bich khoa Hi Ndi
Bii tiio trinh biy kit aui nghien ciru xay dyng md hinh toan hoc md ta qua trinh nhigt phan than cu/ go. Cic yiu to anh hirang cua qui trinh nhtr. thdng so nhiet vat ly, eiu tnJc cua v$tligu, mdi tnrdng nhigt phin, giin do nhiet phan... da dirge ke den. Chit lirgng sin pham cua qui trinh nhigt phan than cui go phy thugc rat ldn vao cac btong so inh hirdng niy. Vi vgy, viec xic dfnh dwgfc moi quan hg toin hgc niy cd ich rat Idn cho viec nghien cu-u, tinh tinh toin va vgn hinh cic thiet bj nhigt phan de thu dirgv than ciJi cd chat Itrgng tot.
Tir khda: Nhifit phdn. than cui gd, Nhifit phan go, Md hinh nhifit phdn Bang 1. Ky hieu va y nghia cac dai Iwgng C4c d^i tirqng
TIKI qlw/m']
Q[W]
p[kg/m^
MW/mK]
R[J/mol K) Als'l
y nghia Nh>$t dO M d$ dfing nhi$t D nhiat K li/ong neng H s6 d^n nhifit H s6 khi, 8314 hs6m0 2-5><10"
Cic d^i tw^g t[s]
mikg]
ilkJ/kgl C[kJ/kg K) E[J/moJ/K]
Y[-l Mm]
Y nghia Th<ln gian Kh6i ti/tyng entanpi N dung nfing N Li«yng H ding H s6 s ti/cng char B^ r$ng kh6i gb
K. hi^u chin w f c
V
a P
Y nghia Wood (g6) Final (cu60 Char coal (than) Volatiles (ch/b6c) Active(hoat d ^ g ) Ding dp suit.
1.Gl6riTHl|U.
G i rdn Id nguydn lifiu diln hinh, dung de san xult than. G i cd ciu tgo tir cdc hpp ehlt cao phdn tir ty nhifin polime. Trong dd, cellulose chilm 50%, hemleellulose 25% vd lignin ddng gdp 25 % [1]. Qud trinh phdn ly cdc thdnh phan trong g i dien ra d cdc giai nhifit dfi khde nhau, eg thd. cellulose phdn li d viing nhifit dfi tir (240 - 350 "C) trong khi hemicellulose bj tdch ra d viing (200-260 °C) vd lignin Id (280-500 °C). G i Id mpt trong s l cac nguin nguydn lifiu diing d l san xult nhien lipu nhu than ctJi (charcoal), diu sinh hpc hofic Id khi nhidn lifiu.
San phim cua qud trinh nhifit phdn g l gim cdc thdnh phin ca ban nhu ehlt bie vd cdc ton c l djnh hay cdn gpi Id than (charcoal). Ddy Id mpt qua trinh phirc tgp, bao gim su thay dil ca v l hda hpe vd pha v$t ly, Id qud trinh mfit chieu khdng thd dao ngupc Tuy vdo mgc dich san XUIL cdng nghfi nhifit phdn dutpx; dp dyng phij hpp nhdm tgo ra san phim cd ndng suit cao vd chit lupng tit. Nhifit phdn go vdi mgc dich tgo ra chanxial Id qud trinh nhifit phdn chfim, bao gim qua trinh nhifit vfit ly vd qua trinh chuyin doi cde thanh phin hda hpc cd trong g i dd tgo thdnh nhifin lifiu. Qud trinh ndy phy vdo rit n h l ^ yiu t l nhu. ede thdng s l nhifit vfit ly, cau tnic vfit lifiu. mfii tardng nhifit phdn vd gian do nhifit phdn. VI vgy, tim ra cde md hinh nhifit phdn tii uu, phgc vg mgc dich sdn xult nhien lifiu Id vific cln thilt. dupc nhilu nhd khoa hpc dd vd dang quan tam nghien ciru
Trong nghidn ciru ndy. nhdm tdc gia di sdu tim mli lidn hfi vd ddnh gia sy anh hudng cua cdc
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cdc hipn tupng vfit ly diln tra trong qud trinh nhifit phdn. Xdc (5jnh cdc phuang trinh cin bdng ndng lupng, phuang trinh bao todn khii lupng, nhifit dp nhifit phdn t i i uu, tu- dd xde djnh mii lifin hfi giira cdc thdng s i vfit ly trong qud trinh nhifit phdn la nhipm vg ddt ra cho nhdm tde gia.
2. MO HiNH T O A N Q U A TRJNH NHIET PHAN.
2.1. Cac gia t h i l t 2.1.1. Cache hda hgc.
Cde ea chl hda hpe dd dupc phdt triln rfing rdi nhfim md ta cdc qud trinh phdn htjy dien ra t«>ng nhipt phan chit rdn. Tu- yiu t l tie dp, ca chl dfing hda hpc dupc phdn thdnh hai dgng, ca chl dfing hda h(?e cd tic dfi phdn ly hu'u hgn vd ca chl dfing hda hpc cd tic dd phdn ly khdng gidi hgn Ca chl dfing hoc - ty Ifi huTj hgn thudng dupc st> dgng rfing rdi cho cdc md hinh nhifit p^dn todn difin. Diln hinh eho ca chl ndy la sa do cua Arrhenius [1], cae san phim phdn ly hodn toan dupe tgo ra chi qua mfit phan img nhipt phdn goi la phan t>ng bfic 1. Ca chl dfing hoc - tic dfi phdn ll khdng gidi hgn dupc i>ng dgng cho cdc md hinh md ta ve su phdn ly tirng budc vdi gia thiet qud trinh nhifit p^dn dien ra d mfit nhifit dfi c i djnh.
Sy dan gian hda ndy giup logi bd cdc phan t>ng hda hpc phirc tgp. ddng thdi chi de cfip den cdc hifin tupng nhifit vfit ly trong qud trinh nhifit phdn.
^Trong nghien cuu ndy. vdi cdc ea c h l hda hpc dien ra trong qud trinh nhiet phdn, nhdm tde gia sir dgng md hinh dpng hpc - toe dfi h&u han vdi ddc diem san phim dupe tgo ra qua mfit phan irng ting hpp. Hinh 1 md ta sa do phan irng ting iirtV ' « • " ''"r BA^. U,na K^i 'HAi
16'NLN-132* 11/2016
thd vdi ty Ifi san lupng giOa charcoal (than thu
dupe) vd cdc chat b6c dupc xdc ^nh trudc.
^iT)=lc^iT)dT^C^iT-T„)
Chdt rdn ban ddu y Chat boc + (l-y) Than Htnh 1. Sc iS6 phin ung long thi cua Arrhenius 2.1.2. Cac gia thiit vgtiy.
Khao sat khii g i dfit trong mdi trudng ed nhifit dfi To, t}l ngodi ciJa khdi go tilp xue vdi mdt mfit dfi ddng nhifit lidn tgc, I n djnh. Hinh 2 md ta cdc vln d l vfit ly diln ra trong qud trinh nhiet phdn vdi din nhifit mpt chilu. O l dam bao tinh ddi xirng vd todn khii gd tilp xue vdi ngufin nhifit khdng dii, md hinh dupc gia djnh ring ddng nhifit dupc dp dfit trfin mpt phia cua tim go. phia kia la Idp cdch nhiet Khi nhipt dfi trdn t)l mdt khli g i diJ ldn. hifin tupng nhifit phdn bit diu xult hifin trdn khdp b l mfit Tilp dd. tgi cdc vj tri cd nhifit dfi nhifit phdn se hinh thdnh vung nhifit phdn vd vijng ndy cd xu hudng di chuyin v l phia phin each nhifit cua khli g i , ding thdi d l Igi phia sau dd Idp than (charcoal). Cdc hifin tupng xay ra trong viing nhifit phdn ndy dupe bilu thj trong hinh chu- nhfit eiia hinh 2. Cdc chit bic dupc tgo ra trong qud trinh nhifit phdn di chuyin tdi t i l mfit ctJa khii g i thdng qua ma trfin l i xdp trong ldp than tgo thdnh. Cde gia thilt dfit ra d ddy Id: (1), Cdc chit bic vd than hlnh thdnh d trgng thdi cdn bing vd go nguydn sinh khfing hip thu I m trong hon hpp cdc ehlt bic. (2) Thi tieh cua khoi khi trudc vd sau qud trinh than hda Id khdng d i i . ti>c khdng cd hifin tupng eo rtit cua than so vdi g i ban diu sau nhifit phdn. (3) Dfic tinh nhifit thay dii theo ty trpng nhung dupc tinh trung binh so vdi nhifit dfi vd dupc eho bdi phuang trinh 1.1
(1.1) 2.2. Md htnh d0ng hpc nhipt phdn vd^ t i c d$
hiru h^n.
Trong suit qud trlnh nhifit phdn, vdi mfi hlnh dfing hpc t i c dp hu-u hgn. khii lupng rieng g l rdn trong vimg nhifit phdn thay d i i lidn U^c tir khii lupng rifing ban d i u eua khli g i p„ d i n khli lupng ridng cijii cimg eua charcoal Pf. Vi vfiy, tgi bat ky thdi dilm ndo mfit phdn t i tgi viing nhifit phan cd the eoi Id h i n hp-p cua ca than hinh thdnh va g i chua nhifit phdn. Vdi gia thilt khdng cd sy CO ngdt cua than, tit ca khii lypng ridng diu dya tren t h i tich ban d i u cua khli g i rdn. Ta cd khii lupng ridng tire thdi ps cua khli g i dupc tlnh theo cdng thirc (2.1):
pSx,f) = pA^.l) + p,ixj), (2.1)
O o iigii>'^i bill
., - 0 ^
•A
^ 4
•a
.^
/•\S
i^V'
^
^ - ^ - > ^
i * r .-i-(».o Cllucoal
1
1
.T = Z Char
Trong 66: p, vi p^ An \u<fl IA kh6i ll/p'ng ri6ng cua khoi g5 vi than hlnh th^nh t$i vung nhi^t phan. Trong viing nhi$t phin, t^i thb'i <Iilm t = 0, p , ( i , / ) = 0 vi p,(.x.l) = p„(x,l) = p^vi t^i thtti ailm t = I, (cu6i qu^ trinh nhi^t phan), p,(x,l) = 0 Trong (56: ;- = . ^ ^ , la ty 1$ san li/o'ng than viii Pf
P..
luo'ng g5 ban Sku.
^ I ITloilg tUQng lit
i "* Nluet lot, mil
-Ax--
Hinh 2 Cic cc chi v^t ly dien ra Irong qua trlnh nhiit pttin
«*«« ...itn, JCjj niim 60 aHm 7 r „ » , „ , 'Dm lior 'Biirli «*•< " '
NLN'132-11/2016 • 17
Theo Atreya, A {3]. khii lupng g i rdn de san xult than vd chit bic cd t h i dup^c md hinh hda chinh xdc qua ting hprp phan img bfic 1 vdi t i c dfi phan img huru hgn vd su dgng ndng suit ehinh xac cua charcoal. Tuy nhidn. dilu ndy chi dupc chi ra bai cdc thi nghifim hofic Id md hinh trong dd ed phan ung dupc ting hpp gim nhidu budc song song. Do vfiy, vdi mge dich nghidn ciru aJa qud trinh, too dfi phdn ly dupc md ta bdi t i c dfi phan ung Arrhenius Pr bfic 1 vdi mfit nfing suit charcoal quy djnh. y = —^:
PM,
Vdi -^ = -kp^,-^=Ykp.a6ayk = Ae~^
dt dt
[1], tir ddy ta cd phuang trinh tiao todn khii lupng nhu sau:
3 f t + v . ™ ; = - ^ =
Bl dl Q-r)kp,, (2.2)
Trong d6, ml la thfing iuo-ng kh6i lifo'ng cua ch^tb6c v a - ^ = 0
5;
Tir day, ta cO phutTng trinh can bang nang iu^ng;
( P , C . + f t C , ) ^ - ^ C > : , V ^ = V(/lV7•)fe(7•), (2.3) Trong 06,
QiT) = [(C. -yC, - (I - r)C^)(T - r j - &i,]kp„
ia ngu6n nhi^t sinh ra vi Aioia iu'O'ng nhi$t cdn thilt cung d p cho qua trinh nhi$t phan d^ phdn iy mOt doTi vj khii iuvng g5 ran, a nhi#t 66 To. Trong phirorng trinh (2.3) hi s6 dSn nhi$t A cua khii ran ia ting h(?p tuyin tinh cua h$ s i din nhi^t phSn g i chira nhi$t phan va charcoal, dwyc xac djnh theo phirong trinh (2.4) [4]
P« Pf
(2.4) Vdi din nhifit mfit chilu, cdc trudng hpp khii g i cd d u tgo Id phiin, khii tn.i hay khii d u . phuang trinh (2.3). Arpc thu gpn 1^ nhu phuang trinh (2.5) sau.
( A C + p . C . ) ^ + C > : . ^ = l | - ( x u 5 ) + ( ? ( r ) . et "^ dx X ax Ox
(2.5) Trong dd. n Id thdng s i hlnh dgng cua khii g i . Vdi phiin g i , n = 0. frg g i n =1 vd khoi d u g i n = 2- Ding thdi, thdng lupng khdi luvng ctJa chit tlic cho todn bfi qud trinh dupc xdc djnh tiieo phuang trinh
n:(x.t)=li\-r)kpAjy^. (2-6) 2.3. Xac dinh nhi$t d$ nhigt phin.
Hinh 2 eho thiy cdc qud trinh diln ra trong vung nhifit phan. Cdc thfing s i trudc, trong vd sau qud trinh nhidt phdn dupc bilu thi theo chilu ddy ctja khii g i rdn. Vj tri cua viing trudc nhifit phdn duac xde djnh thdng qua thdng s i nhifit dp Tp vd nd cd t h i khdng ndm d gitra viing nhifit phin nhu phac do hiln thj. Phin tu- kilm sodt Ax dupc chpn du ddy de cd the tiao g i m ca md hinh t i e dfi gidi hgn vd md hinh trudc nhifit phdn. Khi viing trudc nhifit phdn di qua phdn ti> Ax, tgi thdi diem ban ddu t, nd se dupc chuyin d i i thdnh charcoal tgi thdi dilm t + At vdi gia thilt khfing cd sy eo ngdt.
De xdc i^nh Tp, d n luu •y khli tupng ty dfing cdn bdng tir p^, to pc vdi y khdng dii.
Phuang trinh cdn bdng ndng lupng dupc ting hgrp tir ca hai md hinh, md hlnh tie dfi tdi hgn vd md hinh tnjdc nhifit phdn. C i n luu ring khdng cd sy khac bifit v l nhifit lupng cdn Igi eua than vd lupng nhifit tnjyln cho cdc ehlt dd bay hai d ca hai mfi hlnh.
Ty lfi ndng lirpng thu dugc tir khi nhifit phdn theo md hinh dfing hpc toe dfi gidi hgn Id [5]:
(1-y) I kp^(^,t)c^(T{4,t)-T,)ti^.riT dd.
khdng cd sy hinh thdnh cdc chit d l bay tiai ngodi Ax vd ty Ifi ndy trd thdnh:
Q-r) \kp„(xJ)C^(Tixj)-T,)dx.
Cdc xdc nhfin ttrang irng vdi md hlnh nhifit phdn tnrdc id:
/>.(!-/) J S(.i-4(.t))^C^(.m,l)-T,)di . Ma rfing si/ thing nhit tr4n toan bfi miin khfing gian va su dyng tlnh chit cua ham delta [6], ty i#
naytrdthanh: p,(l-r)CJM)-T„) '^AO „.„
dl Biiu thirc nay biiu thj sy phg thufic vao thfifi gian cua nhi^t dfi nhi^t phan Tp, diiu nSy khfing Ihvc sv c i n thiit va h&u ich. Nhu' v^y, ting ho'p qua toSn bfi th«
gian cua qua trlnh than hfia, tim thiy nhi^t dfi nhifit phan t i l iru cho qua trlnh nhi$t phan nhi/ sau:
T^=\]nx,l)kp„dxdl. (2.7)
^ 2.4. Khao sat s y anh hu^ng cua cac t h i n g sfi vat ly.
'I AO iiUm Qrtrifng 'Zfc,; A „ , 'Bat4* Um« 3K.> W d /
18'NLN-132'11/2016
2.4.1. Anh huong cua hinh dgng go.
Chdnh Ifich khii lupng g i trudc vd sau qiJd trlnh nhtfit phdn cung ehinh la lupng chit bic thodt ra ttr khii g i , dupc md ta qua phuang trinh 2.6. 0" cung mpt nhifit dp nhifit phan, sy thay ddi ndy phg thufic vao hinh dang ciJa go. Vdi cdc gia trj n = 0. 1, 2 la hfi s i tuang irng vdi ciu tgo ciJa khii gd : Thanh gd, tn^ gd va khoi g i hinh d u . Phuang trinh 2.6 dupc cu thd theo hinh dgng khdi go va tic dfi thay doi khdi lupng 6iFiyc md ta d cdc hinh: hinh 3; hinh 4 vd hinh 5 dudi ddy.
— A Mo liiiili uluet pluD toe do phaa li sioi tiau ' - B1 Mo lunb nhiet phiu \'oi loc do phan li
kboust S30I hui Ci'u Antianus [1 ]
Hlnh 3. T6c Og thay doi khoi Iwgng trong thanh go.
/»l(i,t) = Y = —!— \{p^ + p^)dx
A Mo liiiili ulii^ pbau toe dp phan b aoi han Bl Mo hiidi iiluet phan \'oi toe do pbiii b
Idious 100) han Ci'ia Antamis [1]
•h
>
35 30
I S 10 05 00
" • - - B 1 A
k
:i
1
j
]\
'
150 200 250 300Tlioiaan {s)
Hlnh 4. Toe dp thay doi khoi lu^g trong trit go.
ml(x,t) = }• = f(p + p )xdx P,l- :.
2.4.2. Anh hirdng cua khoi lirgng rieng go din nhigt dg nhigt phan.
A Mo hiuh abet pbw loc do pbin li laoi bfD - B1 Mo liuifa oia^phau\<n toe dp phau b
7 0
60 A 50
s
2 40
^ 3 0 2 0
10
]di6ua. am hfsi Ci'u Anfaauus [1]
- :r\
1 \>
- 1 V ' Jl . . . \
Hinh 5. T6c dp thay doi khoi iu^g trong thanh go.
m,(x,l) = y: = TrhP.+Pjx'dx P..-L i
Tir phu'ong trinh 2.7, ta c6 du-iyc do thi hlnh 6 mfi ta 81/ anh hu'o'ng cua khoi Iu'O'ng rifing g i din nhi^t dfi dfi nhi^t phan. Khii iiro'ng rifing cua gS ifi'n hen se iam tang tai nhifit, giam ty Ifi dit nOng diiu nSy din din sir giam nhifit dfi nhifit phan va kit qua la qua trinh nhifit phan dien ra chjm hon. TCr d i thj mfi ta, Tp giam tif 646,8 K din 628,8 K khi khii luvng rifing p„ cua g i giam tir 400kg / m' din 800kg / m'
Hinh 6, Anh huxmg cua khoi It/qmg rieng cua go ti/i nhl^t di nhiit phin.
I 'Jriiiliui 1>tii lint 'liiirii Uitm *".« 'J^-.'
NLN* 132-11/2016'19
2.4.3. Anh bircng cua Tp den khoi Iwgng rieng cua than tao thanh.
Vdi sy thay dii ndng suit than dinh tn/dc theo md hinh nhtfit phdn tie dfi tdi hgn, tuang ung rnfit sy thay dii lupng than tgo ra, dieu ndy lam thay ddi nhifit dfi nhifit phdn. Hinh 7 md ta sy phg thufic ndy. Anh huoDg cua su thay dii trong ndng suit than din ten thay ddi tic dfi dit ndng. Nhifit dp nhifit phdn thay ddi tir 632K - 637K trdn dai ndng suit than 7,4 % - 59.2 %, tuang img vdi khii lupng rieng than thanh phdm 50 kg/m^ - 400 kg/m^
Khn kmia « f en SiMi (kym^l
Hlnh 7. Anh hu^g khil lugng than tdi nhl^t dg nhi^tphin.
cimg khli luong. Trong khi dd, t i c dd phdn ly Igi giam ddn khi ttiay d i i hinh dgng cua khii g i theo thif tu tir khii d u , khii trg d i n thanh g l ,
Gd cang dfic, tire khii lupng rifing cua g i cdng Idn se Idm tdng tai nhiet, dteu nay d i n tdi sy giam nhifit dd nhifit phdn va keo theo sy tdng thdi gian nhiet phan.
Slf thay dfii nhifit phdn eung tgo ndn sy bien d i i khii lupng than hinh thdnh, cy t h i : Khii lupng ridng cua than hinh thdnh se tdng len khi tdng nhiet dp nhiet phan va ngupe Igi.
4. K t T LUAN
Qud trinh nhiet phan chit rdn md diln hinh Id gd la qud trinh phifc tgp. Id t i hpp cac biln d i i hda hpc. vfit ly vd nhifit hpc. Vific xdc djnh tie dp than hda. nhifit dp nhifit phan cua qud trinh, tgo dieu kifin ddnh gia chit lupng, san lupng cua than sau nhifit phdn. Trong khudn khi de tai, nhdm tdc gta da vgn dgng kit qua cua cdc nhd khoa hpc di trudc trong vific thira nhfin cde qua trinh hda hpc xdy ra trong nhifit phdn ehlt rdn sinh hpe ndi chung, cg t h i hda eho g l ndi rifing. Tir dd tim mii lifin hfi cdc thdng s i vfit ly diln ra trong qud trinh nhifit phdn thdng qua vific xde djnh nhifit dp nhifit phdn, ding thdi ddnh gid, so sdnh vdi mdt s i kit qua dd cdng b l .
o l dam bao k i t qua t i t nhit, cln chi tilt hda sy anh hudng cua tat ca cde yiu t i n|f tgi nhu true vat lifiu, ede dfic tlnh cua vfit lifiu cOng nhu sy anh hydng ciia cdc yiu t i bdn ngodi nhu nhifit dfi mdi trudng, nhifit Iupng cung cip, sy thit thodt nhifit ra mdi trudng trong qud trinh nhifit phdn...
Cdc yeu t i nay se dupc d l cfip trong ede nghidn ciru tllp theo.
3. K^T QUA VA T H A O L U A N . Kit qua nghifin cuu cho thiy:
0 cung nhifit dfi nhifit phdn, hlnh dgng g i anh hudng den tie dfi phdn li vd thdi gian nhifit phdn g i , cg t h i : Vdi khoi g i ed dgng thanh chG" nhfit, thdi gian nhifit phdn diln ra nhanh han so vdi trg g i trdn, nhung Igi chfim han so vdi khii d u d
ABSTRACT
7?j»s attKle present the results ofreseansh using maUis modelling to describe the pyroiysis of charcoal. The fecfcre affecting tfiis pyro/ys/s process include the physKal parameters of pyroiysis. the structure of material, the envkonment and the c^art of pyroiysis. The process of wood pyroiysis in making charcoal depends on these theirnal physical parameters. Therefore, a successful modelling of wood pyn^s in making charcoal can assist in adjusting these thermal physk:al parameters to produce charcoal of good quality-
Key vi/ord: Pyroiysis, Charcoal, Wood charcoal pyroiysis, Pyroiysis model.
TAI LI$U THAM KHAO
[1], Arrtwnius. Atreya, A. (1983). Pyroiysis. ignition and fire spread on horizontal surfaces of wood Camtiridge. MA. Harvard Unrversity.
[2]. Atreya, A & Baum H R (2002) A model of opposed-flow flame spread over charrir>g matenals, Sapporo, Japan, Combustion Instilirte
[3] A t a y a , A . Wooldndge M. & Schwank, J (2003) 'Development of a Uierettcal model to enable design and control of a biomass harvester for field use Vnal thennoctiemically converts biomass to liquid bio-fuel for H^ production' Preapplication for DOE program.
[4) Baum, H.R & Alreya, A (2005). Transport of Gases in Chamng Sdlids Joint Meeting of U S Section Combustion Institute Phaadelphia.PA.USA
[5] Baum. H R & Atreya, A (2007) 'A model of transport of fuel gases in a chamng s d i d and its application to opposed-Row ftame spread" Proceedings of the Combustion Insttute 31 (2) 2633-2641
[6). GS TSKH Ding OuAc Ptiu. PGS TS. Ptiam Lft 0 4 n (2002) C a s e ky t h u j t nhj^t. nhS xuSt b i n gido dye
Phan bien: GS. TSKH. Nguyin ST Mdo
II 60 nam 'Jriirliig ^Oui U-te 'Bati, Ulintt JCa <Hii