Specific energy consumption and particle attrition in pneumatic conveying

" ~ ~ ~

ELSEV|ER

Powder Teclmology 95 ( 1998 } I-6

Specific energy consumption and particle attrition in pneumatic conveying T. Taylor Unilever Researt'h tilld/:,'ll,gill~.'erill,~,Porl Slmli,k,hl Ltlbortllol3'. ()llarry Road/:.tl.w. Bebin,ghm, Wirrul L63 3JW. UK

Received 3{) August 1996: revised 31) April 1997

Abstract The selection and design of pneumatic conveying systems inwdve key considerations of energy consumption and efl'ecl,s upon l,he product. Pneumatic conveying must, compel,e with other met,hods such as mechanical or hydraulic conveying in l,e|'ms of capital and operating costs and in the abilil,y l,o operal,e wil,h minimal effects upon the environment and l,he particles. One of the most, auracl,ive opt,ions for friable panicles is dense phase pneumatic conveying. Measurernenl,s were made of specilic energy consumpl,ion and panicle al,l,ril,ionfi~r a limited range of paniculal,e during dense phase pneurnatic conveying. The results have been compared with published co|'relal,ions and the implications discussed for energy consumption, panicle and bulk properties. Specific energy consurnpl,ion correlated best wil,h solids/air mass ratio but particle a!l.l,ril,iotlwas xery low because of l,he relatively low conveying velocity involved. ~0 1998 Elsevier Science S.A. K,,yw.rd,w I'neumalic conveying: Conveying: Allt'itio,: Specilic energy

I. Introduction Pneumatic conveying is bein~ increasingly used in Ihe chemical process iaduslries hec:mse of its Ilexihility. simplicity ar,.I environmental corupatihility. The systems can ol'len be Ilexibly routed and |'etro-litted in complex chemical plants. They olin be easily operated and programmed Ibr batch or COtltirluous Ol:)eratior|. They function without dust emission, spillage and material hosses. However, other key factors need to be borne in mind wllen selecting a conveying method. These include capital and operating cost. The capital cost per metre :rod the openttir, g cost per tonne of n'|aterial conveyed can tiler, be compared on the same basis. An important determinant of operating cost is the energy consumption in pneumatic conveying. This is determined by the specific energy

consumption which Ilas tile units of kJ kg ~o1' nmteri:d conveyed. It varies for a given pneumatic conveying system depending upon the material properties, for example, the partide size. density, lluidisation behaviour and air permeability. Unilever currently uses pneumatic conveying for an enormous r:l,nge of materials ranging in size lhm~ sub-nficron aluminosilicates to complete packaged materials such as 'instant meals'. The conveyability of these different materials is of prime c o n c e r n since selection of conveying method will be an important factor in the determirmtion of manufacturing cost. Work has been done to compare the pneumatic convey0032-5910/98/$19.00 © 1998 Elsevier Science S.A. All rights re.served PIIS0032-5910( 97 )03309-3

ing behaviour of some of these different materials, in particuhu" the relative specific energy consunlption. Tilts has allowed the comparison of conveying melhods on a ration.'d basis. This paper presents the results of exl'~erimental work to compare the specific energy con.,,t|mpliorl of different raw n'mterials varying, g,reatly in size and conveyiu~ behaviour. Trials are described of dense phase lmeur|'mlic conveyin~ of materials between 5 and 20 tonne h ~over it dislance of 35 orgl m and using an air injection line. 'lT,e materials ranged in size from 5 to I0 000 p.m in size. The conveying rates and specific energy consumption are tabulated fbr the materials. A correlation is proposed between specific energy consumption :rod solids/air mass ratio. The specific energy consumplion in pneumatic conveying is then compared with other conveying methods. Finally. attrition is discussed from tile results obtained and attempts to relate them to particle properties :rod process variables.

2. Experimental The materials conveyed covered a wide range of properties. The trials used an air addition line and a blow tank system under a variety of pressures.

2

1~ l"ayh,rl Pou'&'r I'eclm,dogy t)5 l lt)t),Sj !--¢~

2.1. Materials Four materials were examined varying t~om 5 to 10 (~0 I.tm in size and in bulk density from 440 to 800 kg m - ~. The materials covered a range of over 1000 in size and 2 in bulk density and represented a reasonable selection of the different materials handled by Unilever.

2.2. The Imeumatic com'o'ing equipment The system comprised a hopper feeding a blow tank. Details are given in Fig. I. The blow tank led a pneumatic conveying pipe loop of either 35 or 91 m in length and litted with an air addition line, which was introduced into the pipe at a number of points along its length by means of an external pipe delivering approximately 5~;~volume fraction of the total air flow. The air flow at each point could be adjusted by means of a needle valve. The positions of these valves were not changed for the trials. A check valve was also included in the external air pipe to prevent back flow of air and particles At each point the air was introduced through an annulus feeding eight holes and a flexible gasket. Pipe internal diameter was constant at 73.8 ram. The conveying air was lirst compressed to 6.8 ba G, dried and then fed to a reservoir of 761X) i capacity, providing ample air lor each run at 15 °C. This reservoir then fed the blow tank and the air addition line. The air to the blow taltk was introduced at the top and through seven air.jets in the cone in order to aid discharge of particle,, OI

o ~ o o o q b 0 o 0

t,%o[

| |

,,,,,o

DQI~ pipe 8olkle

into the conveying pipe. A sight glass just alter the blow tank discharge allowed visual observation of the flow. Alter the sight glass the second bend was of rubber to provide a flexible connection to the plant. The conveyed material was collected in a gravity separating chamber which allowed observation of flow and recycling of material, if required, to the feed hopper. The hopper and dust extraction unit were exhausted to a separate dust filter unit for analysis and recycling of the dust if required.

2.3. The nperating comlitions Approximately 500 ! of material were gravity led into the feed hopper above the blow tank. The blow tank discharge valve was closed and then the inlet valve was opened in order to feed the material from the hopper above. The valves were then closed and the blow tank wlts then pressurised with air fi'om tile compressed air reservoir. The air from the reservoir was reduced in pressure to approximately 2.5 bar,, before distribution, The blow tank discharge valve was then opened and the material pneumatically conveyed. The material flow prolile could be observed using the sight glass and at the gravity separation chamber. The air Ilow into the bypass could be adjusted using needle valves and these were prese~ before the trials. The conveyed material wlts separated in the settling chamber and weighed. The time ol"conveying wlts noted, as were the initial and linal pressures in the reservoir. This allowed calculation of tile average i'nass Ilow rate ol" material. tile total volume and mass of air used. tile average air velocity and the specilic energy conSUml~tion, The pressures were measured using Conlnlercial i~t'essure gauges which were accurate 1o ±().()1 haG, Weighings could he verilied In ± I kg, Times of c.nveying could be measured to within :]: I s, For each material, plvliminary trials identilied sleady Mate I{;w throughput conveying conditions where flow was established rapidly. Trials were then repeated where possible at increasing conveying pressure and throughput. At each stage, the particle flow was recorded following sight glass inspection.

p~ne

L.,Wl~tm; ~s sl mmcle

InlmtlOls Ilmtatl

¢~l~med

elf S,lIM~;

2.4. Text method.~',l'm"Imrtic'h's

s o

1P 11

. vll~

v~ l,Slmll N~ale

Fig, I,Pueumalic couxeyiug l~,p mid hlo~ tallk.

1"he particle size distribution was measured before and alier conveying for materials B and C by sieve tests using an Alpine Air:tel sieve. "File friability of these particles was measured by the British Standard Institution Method ISO/TC 47/ WG I I, Both methods give results accurate to within +_2c;~,. Bulk density wits measured by ~veighing a known volume of particles. Unconlined yield strength wlts measured at a consolidation stress of 1.5 kPa using a standard cylindrical test cell, Compressibility was measured using a cylindrical test cell and calculated its the ~ change ol" particle bed volume under it consolidation stress of 100 kPa. Particle density was measured using an air penneametry method.

T. 71tyh,r / Powth'r 7i'HIm,h,g~ 95 ¢1998b !-6

3. Results

The results below cover the material characterisation, the throughput, pressure drops and the specific energy consumption. 3. !. Material characteristics The materials characterisation is reported in Table !. The materials covered a range of size, density and flow behaviour. Material A possessed poor flow properties on account of its low mean size and the consequent high interparticle forces. Materials B and C had much better flow properties and were much less cohesive. Material D consisted of very large particulates which were plastic under load. 3.2. Pnelonatic torn'eying rate alui pressitre dr~q,

The results for material throughput and p r e s s u r e drop are shown in Table 2. Material A was observed by the sight glass to be conveyed in plug flow with a solids/air mass ratio of 14. Material B was conveyed at higher ratio. Plug flow was observed via the sight glass at a solids/air mass ratio of

3

approximately 20 and dune llow at a ratio of 29. Material C was conveyed in plug Ilow at a solids/air mass ratio of 17.536.5 and with dune flow at a ratio of 15. Distinct plugs of particles could be observed with both these materials also at the settling chamber. Material D was difficult to convey. owing to its very high mean size and then only conveyed with relatively high air usage and low pressure.

3.3. Particle attrition

Results for particle attrition were limited to materials B and C only. Representative results are shown for particle size distribution belbre and after conveying in Table 3. They showed little attrition ( only i-2.5",4 increase in particles less than 180 ~m in size) in conveying owing to the relatively high solids/air mass ratio and low particle velocities. expected to be of the order of 4 - 6 m s Materials B and C are known to ~ive greater attrition at higher conveying velocity, e.g. 20 m s t. The order of magnitude is 5-15'~ increase in particles less than 180 I.tm.

Table I Physical properties of materials Property

Material A

Material B

Malerial C

Material !)

Bulk tlensit.~' I kg m ' I'arlich' density I kg m '1 Mean particle size d,,,,, t laln) Ihti'tick' ~izc I'atlge (lath) I.!u¢olflim:d .~'ield ~ll'¢llglh (kN m ('OUll~re~sihilil) 1'; I

.151} I I11O 5 I-5() 3,3 35

81111 1301) 30() I1)-(~1)1) 11,3 5

441) qlIl) 5811 I11- I1)11() 0,3 23

5811 111511 II11)1)11 811111b-15111111 2,1) If)

'l'al~Ic2 'l°hl't~ii~lh1111!i|lld air ¢t,l~tllllplloll II~lll}/dl'll~C phi|~e pl|¢unll|li¢ conveying

Material

A B

(,

D

Trial

I I 2 3 4 5 0 7 8 I 2 3 4 I 2 3 4 5

Convc),i,g tlisli|llee I m)

No. of air I~Oillls

91 91 91 91 91 35 35 35 91 t) I 91 35 35 35 35 35 35 35

12 12 12 12 12 7 7 7 12 12 12 7 7 7 7 7 7 7

Bh~w ta,k i|irl~oints

11 2

2 2 5 5 3 7 7

7 7

Air ~,ch,city

Air

St,lids

ill'¢s,~Ul'~2

tlnilgt'

l'll|t'

( I~i|1' )

( I i i ~,

I Ill ~ h

( h11111L' Jl

2.117 1.73 2.42 3.80 2.4 1.7 3.8 2.4 2.2 2.28 3.11 1.73 2.42

29.4 8.9 14.1 29.11 "14

452 137 217 447 -218 152 no5 128 188 184 373

7.8 3,2 5.32 9.3 5.3 9.73 21,2 111.28 4,13 3.S7 h.h4 8.(~ II ,25 1.4 113 .t.57 2,53 II .(11)

131ow tank [ii'¢ s,~tll'e (bar)

II)pa.,,,,,

113) 1.38 2.21 3.45 2.21 1.52 3.1 2.21 2.17 2,116 2.41 1.37 1.86 11.86 I.II3 O,68 1.0 I.II3

1.114 1.52 2.7¢, 1.38 2.42

I)

39.3 8.3 12.2 12.11 24,3 12.8 28.0 I0.2 46.7 27.~ 111.1 40.u

i'J("

43 I 157 719 421 156 h31)

Solid/air I'ii1 hi t)

I kt: k~ ') 14.3 I~J.4 211.4 11,3 2O 53.4 29.2 h7.() 18.3 17.5 14X ~¢~.5 21.S 7.h 13.¢, 9. I 13.5 14.h

/,¢" ,,t

J

,.,.

/,,

7: 7i!vh,'/Powder 7",clmolo,~y 95 f l~,~) I ~)

4

Table 3 Particle size distribution of particles before and after torn'eying C u m u l a t i v e ~,I.G greater lhan size ( ~nz )

Material

B run 7 before B run 7 after C run I befiwe C run ! after

5(111

355

251)

! 8(1

125

I (lO

63

7.5 7.5 3 !.8

30. I 29.3 55.4 54.1

48.5 40.5 69.9 67.3

81.4 79.2 80.0 77.3

92.0 91.8 90.9 84.6

93.9 93.7 92.9 88.8

96.5 96.3 94. I 93.3

30.9

Table 4 Power and ~pecilic energy ¢on~unlption ill den~,¢ phase plleulnilti¢ conveying Material

Trk, l

Air ma.~, Ilo~ 1";.11t2 ( kg h ~ )

Blow tank pres.,,ure t bar abn. )

A

I

523

B

I

165

2 3 4 5 6 7

2(~I 535 262 IS2 725 154

2 " ~8 _.. 3.21 4.45 3.21 2.52 4.' 3.21

..6

3,17

9.73

1211

10.4

I 2 3

221 44,~ 23~ 51 ,~

3.2,~ 4. I I 2.7~ ,L42

3.87 1~.(~ I 1.3

I 1.2 15.8 4.(~

8.(~

12. I 2t). I Ill.9 29.3

I ~tl

I ,.~1~

1.4

5.4

~t,5 5(15 Is7

2.o3 I r('~ 2()

7(~5

2,11~

13.6 ~.7 ().4 ~.5 ~, I

("

4

I)

I 2 -~. 4 5

Solid,, t h r o u g h p u t ~hmne h ' )

Power ( kW )

Specific e n e r g y c o n s u m p t i o n ( MJ lollne ~ )

7.8

I (~.6

3.2

(~.6

5.32 t).3 5.3 4.1 i(1.28 21.2

12.0 36.7 14.0 7.7 4711 8.25

7.7 7.4 8. I 14.2 t).5 2.85 81) 2.9

I 1,7

t).4

28. I I~_.ll (~.(I

4,(t 2.5 I I,I)

24. t/

3,4, I ) r m o ' . m l ~lr'c ilh' um'rtlv ('r,ll,~llllllffiotl

Powder throughput tonne/hour

The theoretical po~,~,erattd ~pe¢itic ¢net'~y cot|stln|ptiot| for ¢[l¢11 t)f II1¢ trials at'¢ ~iv¢ll ill Table 4, I]lo~v tank pressul'¢ in giv¢ll ill bar absohll¢; air discharge pressure was alttlml)heri¢, Po¢,,¢r arid swciii¢ ettet'B~' were calctih|lctl tisi.g the cqult~ rio. l I I:

2s

Pov,:r = 2m,,R7" hi( Pl/P, ) I~W ¢¢her¢ m,, R 1' Ih I ),

air nuts.,, th)~v rate ( kg s ~ ) ultiversal ~ItS ¢OlL',,lltlll (().287 kJ kg ' K ~) i|bsoltll¢ telltperittur¢ ( "D88 K ) ah" inlet pressure ( bar al'.,olute ) air oull¢! pl'csMIl'C ( I.O bilr ab,,,ohtl¢ )

The speciti¢ energy collsumption ~'aricd ~vidcl.~ a.d ,,,e~eral po,,,,,,ible correlation.,, ,acre tried to relate it to ,~,'stcm ehara~.'teristics a.,,di,,,cus,,,cd bclo~,.

4, Di~usslon 4, I. Spet'{li(" e.et;ey (,ott,~m.l)tio.

The results for pneumatic conveying over 35 m are showo in Fig. 2. Materials B a,d C could be readily conveyed over

! Com~nente B

'°

I

10

Component C D

, / ~ / ~ ~ ~ C . 1:3 1:3

(b k l W ~ ~ k l ~ / I

0

mtlot ~glO

O.OS 0.1 0.1S 0.2 0.2S Air mass flow rate kg/sec

I:ig, 2, I)~.'lls~.' phase I~nettlllltlh. ' CttlD, e~,itl[2 tt~,k'l ~.~.~ III.

this distance vdlh solids/air mass ratio 20 Io 30: material D with the largesl size mid highest prrmeametry was dil'lieult to convey above solids/air mass ratio I0 ;uld required relalively high air pressure and volume. The behaviour is si,nilar to thai previously reported 121 rot coal and suB:.'. Results of conveying over 91 m arc shown in Fig. 3. Materials A. B and C could be easily conveyed over this distance with :l solids/ air mass ratio o f a p p r o x i m a t e l y

20 and exhibiting plug or

T 7 i n ' h , r / l~oudcr

Ii'~'hm,loev 95 11t~9,~'~ I I~ Table 5 l;llCl'g~, coll.~tlnll'~lion ill nolid con~ e 3 m g

Powder throughput tonn~dl~our

10F

5

..... Component A

('OIIX

Spccilic cnerg.~ cotl~umptitm

t2\or

MJ tom~c

Component B Pneumatic donne phanc Bucket clcx itlor

Component C

Belt

2

0

|

*

~

I

I

•

I

•

I

,

I

2--1

0.2-11.4 O. I ---11.3

4.2. Particle attrition

g-10 1

')

In discussion of particle attrition, the most important process variable has been pointed out to be conveying velocity

I

0,04 0.08 0.12 0.02 0.06 0.1 0.14 Air mass flow rate kg/sec.

141. This is shown also by Ghadiri 151 where work on the impact of single particles showed that the Ii"actional loss of material was .~iVell by the equation"

Fig. 3. i)lletlnla|i¢ t.'OllV~2}illg [)~rei° t)l III. 16

~= (r( pt':lH ) / ( K,',-d) : )

14

where ~ ~o •~

P

6

I" •

2 0

I 20

I 40

! H

•

I

60

80

Solk~air ratio(kg/l~)

Fig. 4..~pecJlJ¢ t'llCt'o~..

COllnUlllpliOll. I)llel.lllliltJc ¢OlD, t'%Jll~.. o~er .~. alld

~) I m.

dune Ilow. "File relatively small particle,, and lower air permeability contribute to this and tile result,~ bear COl))l)arist)I) with tho,,,e reported r,'eviou.sly 121 for Baryle.s, cement and i~uh,ez'i,~ed Ilue ash, The spec'ilic energy con~uml~litm for all pneunmtic conveying trial,5 is .~hown in Fig. 4. The bc~t cOt'l'elatit)n It) col|lparc the re stlll,~i,~that ol'specilic energy vet'su.~ .~olid.~/air mass r.'ltio. It decrea,~e.~ al~pn)ximalely expot)entially with increa,~e of the .~olids/air ratio. This ,~hows that at high ~olid,~/air ratio, the energy is being more el'liciently used than at hlw ratio, From thi.~ cur~e approximate ligures of the specilic energy required ci|n be calculated for a given solids/air mass ratio. This, of course, is valid only for the pneumatic conveying phmt showll in Fig. I. If the results are further analysed, the specilic energy con.~un)ption incre~ses, its expected, with distance conveyed. For both materials B and C, the specilic energy doubles its the distance increases Ii'om 35 to 91 m. It is interesting to conlparc the specilic energy consumption for i~neumatic COllveying with that for alternative method,~ ,~tlch as bucket elevators or belt conveying 131. At apl)roxinmtely the same throughput, 15 tonne I1 ~. ,'rod distance per unit of elevation (e.g. 33 m distance and 5 m elevation) the specilic energy consumption is much higher for pneumatic conveying than Ibr the other methods as .~hown in Table 5. For longer distances than considered in this paper ( 91 m I the specilic energy consumption for pneumatic conveying would probably be even higher.

fractional weight loss in impact prol')ortionality t'actor particle density ( kg rn ~) particle impact velocity (m ~ ~) particle linear dimension ( m ) particle hardness (Pa)

K,

fracture toug111Tess ( N in "-" )

d,~

constraint factor

This .show.,, that the attrition of senfi-b1"ittle particle.,, is p1"ol)ortional to the .square of the in11")ac'tvelocity. The mechanism is by a chil'~ping proces,, where nmterial relnoV~d i.,, t'roln the comers and edge.,, of i'mrticle,s through the pn)l'm~ation of .,,ub,,,url'ace lateral crack.,,. The relatively low attrition observed in our trials illUnl be I~ecau,,,c of the relatively Io~' velocity u.,,ed. Indeed. if ~,e .,,ub,stitute .,,omc e,,tiluated ralLieS ill the abo~.c equation, the fr:tctional attrition can be estimated. For examl'fle, for material B, the variation of fractional attrition with particle velocity i.,, estimated a,s ,shown in Fig 5. 'l'hi.s was calculated with the l'mrameter.s in the above equation as follow.,,: (~ l.)rol'~ortionality factor ( 0.87 ) p l')arlicle density ( 1300 k~ m ~ ) ~. l:)article impact velocity ( variable ) 1111..,, I ) i particle linear dimension (().0()03 m) H particle hardl)es.s 10.2 x I()" Pat K,. fracltlt'etOtl~hnes.s ( 0.4 X I()" N m ' : ) t/~ constritjnt factor( I, il,s,sutned) The qualitative aoreement in good. For materials such a.,, B. and the h)w particle velocities expected in I'meumatic conw.ying with average air velocities up to 5 m .~ ~. the fractional at~¢rition is predicted t()be low. much le.,,s than 5'~. For higher particle velocities of the order of 20 m s ~ much higher atlritio11 is predicted and tilts is home OUt in aCltlal practice. N,~te that particle velocities cannot be higher than the average all" veh)city in pneumatic conveying and are visuall.~ observed, e.~p .ciall);n dense ph:lse Ilow to he much lower.

1" 1".vh,"/ Po~v(h'r 7"¢dmology 95 ( 1998j l..t~ 0.5

..z.

0,4

i

i

i

i

i

iii

the implications discussed for energy consumption, particle

iii

and bulk properties.

f'

0.3

6. List o f s y m b o l s d3t,.~

E H

0.1 0 :, - , : - - - 0

t $

I

I0

15

20

Parlk:~veloc~ Fig. 5. Fnictional attrition vs. panicle veh~'ity.

/¢ / l~la lh

Interestingly, Ghadiri also shows that there is a critical load to cause lateral fracture and that this can be tnmslated to a critical tnmsition velocity below which only plastic deformation is found. This velocity is given by U,h = (K,/tt)~(F-p

~'21 2tH~/2)

E

R T Ut'h

1'

fracture toughness ( N m - 3/_, ) particle linear dimension ( m ) air mass flow rate ( kg s " ' )

air inlet pressure ( bar absolute ) air outlet pressure ( bar absolute ) universal gas constant tl).287 kJ k g ' K ') absolute temperature (K) critical velocity I m s ' ) particle impact velocity (m s ' )

G r e e k letlcrs

where p

P2

Rosin-Rammler characteristic particle size (IJUm) Young's modulus (Pa) particle hardness (Pa)

Young's nlodulus (Pa) panicle density ( kg m ~) At much higher velocity, particle fragmentation takes over

as the mechanism fronl chipping. With materials B and C ill

(.v .~ p d~

proportionality factor fractional weight loss In impact particle density (kg m ~) constraint factor

our trials, the plastic deformation region probably lies below particle velocity 5 m s ' and the fragmentation region greater

than 31) m s ',

~, Condu~lons The ,,¢l¢¢lion and deqgn of pneumatic ¢otlv¢~,.in~ .,,y,,lcni., iti~oiv¢k¢~ ¢otisid~i'ation~oi'en~i.~}~¢on~tlml.lthm mid eff'e¢l,, upon the product. |~li~.'lIIiI~||i~' ¢OIIV~,'ili~IIILI~!~'OIlip~l~'with other methods such {is nl¢chanical or hydnadic ~:onveyi.~ in temls of capital and o~raling costs and in the ability to ol~nll¢ with minimal effects (ll~.) the ¢l~vimnmenl al):! the particle~. One of lh~ most attn~!ive options for friable partides is dense phase plleumatic conveying, Measurementn were made ol'sl~cili¢ energy ¢onsunlption fora linlitcd ranBe of pmliculalcs during d¢llSe phase plleUlll~,lti~ ¢Ollveyillg, "J'h¢ results have ~en compared ~,vilh published correlations and

Acknowledgements The author thanks Ir, l .... Melles. Ibrn)erly of I.Jrlilever Research Vlaardingen. for his ¢onlril'..llion to tile trials arid in their iuilial reporlin~.

Rd'~rences I II I). Mill~. Pnctlulalic ('o,~c~ing IX',,i~n (iuide, lhttler~orlh, l.~,ndon, I~)tlll, p, X~,

121 I), ,Mill,,. l't|~.'tllilath: (,'OIl~'~ill~ I)e',igl| (;tlitiL'. I|tltt~r~,~,'tlrtli. la~ndoli, I ~J~}I. p. 4r~2.

I :LI k.ll. Ik'rr~. (.'hcn~ical I(IDgiucci's' lhmdl~L~ok. McCh'a~-Ilill. Ne~ YOrk. It173. pp. 7- Ill. 7° 13. 141 I), Mills. l"riholog.~ m Parliculat¢ 'l'cchlmlo~. Adam I|ilger. llrislol. 1~7, pp, 3q11--~1115, 151 M, (ihadiri. Pl'¢~c, A.I, ('hL'nl. I':, I~al'li~:l~' Tcchnoh~g.~ F,u'um. I)eil~¢l'. ('(1. USA, Aug IrIs}4. Parl 2, Pp. 247 251,