Raw juice extraction from sugar cane and sugar beet

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laboratorium voor Chemische Technologie

Verslag behorende

bij het fabrieksvoorontwerp

van ~AILHE C.J.R. BLANCHARD J.P. ... -

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Oudraadtweg 31 26 1 2 SL DELFT ,

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CONTENTS Abstract Introduction PART ONE I MILLING

SUGAR CANE PLANT

I-I Combination of cane preparators

1-2 Extraction efficiency of milling

1-3 Pre-milling operations I .• _\, · ' p 2 p 3 p 4 p 6 1-4 1-5 1-6

power requirements of mills p 8

Total power necessary for milling'process '" p 12 Electric and steam drive of mills

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DIFFUSION

11-1 Diffusion processes

11-2 Choice of a diffusion process 11-3 DdS diffusion mass balance

11-4 Choice between diffusion and milling

111 STEAM PRODUCTION AND USAGE

IlI-1 Composition of bagasse IIl-2 Calorific value of bagasse IlI-3 Steam turbine

1II-4 Bagasse furnace

IIl-5 Calculation of the superheater III-6 Boiler

IlI-7 Economiser and air-heater IlI-8 Calculation of hea t- exchange

surfaces of economiser and air-heater IlI-9 Draught. Farts power requirements lIl-JO Boiler feed water pump

lIl-I I Steam-reducing valve and de-superheater IlI-12 Energy requirements of the plant

p 13 p 16 p 23 p 24 p 27 p 28 p 29 p 34 p 38 p 39 p 40 p 41 p 43 p 46 p 47 p 48 p 50

IV UTILIZATION OF BAGASSE AS A BY-PRODUCT

IV-I What we can do with the surplus bagasse

IV-2 Production of electricity from surplus bagasse (

IV-3 Manufacture of briquettes

V COST EVALUATION

C _{V-I Investment cast}

V-2 Process dependent casts V-3 Production cast

(

Conclusion

Flow sheet

(

Mass and heat balances

PART TIJO SUGAR BEET PLANT

VI Caracteristics of beet, wet pulp, dry pulp

VII Diffuser. Mass and heat balances

VIIISteam production and usage

VIII-I Calorific value of fuel oil

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_{VIII-2 Steam turbine}

VIII-3 Calculation of the superheater VIII-4 Boiler

VIII-5 Calculation of the economiser

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_{VIII-6 Calculation of the air-heater}

VIII-7 Draught. Power requirements of fans VIII-8 Feed water boiler pump

VIII-9 Power requirements of the plant

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_{VIII-IO Steamrreducing valve and de-superheater}

IX PRESSING AND DRYING

p 51 p 53 p :::58 p 62 p 63 p 64 p 66 p 67 p 68 p 72 p 74 p 76 p 81 p 83 p 84

P

85 p 85 p 86 p 87 p 87

P

88

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IX-I Drier p 90

IX-2 Power requirement for pressing p 90

IX-3 Calculation of the drier p 92

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IX-4 Power consumption p 95

IX-S Fuel oil consurnption p 95

IX-6 Capital cost p 96

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IX-7 Production cost of pressing and drying p 97

X ALTERNATIVE FUELS

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X-I The wet pulp as a fuel p 98

X-2 The dried pulp as a fuel p 102

XI COST EVALUATION

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Investrnent cost _P 103

XI-2 Process dependent costs p 104

XI-3 Production cost p 105

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XI-4 Sale of dried pulp p lOS

Conclusion

Flow sheet p 107

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_{Mass and heat balances p 108}

RE FE REN CES p 113

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LIST OF SYMBOLS c o Q Yo

sugar concentration in raw juice

quantity of raw juice relative to unit of cane

diffuser efficiency _Î

sugar concentration ln m~~se at diffuser output sugar concentration ln juice at diffuser output

J quantity of juice in cane relative to 1 unit of cane L lenght of the diffuser

S sugar concentration ln cane ( .... d ,.,

/-S1 sugar concentration ln juice at 1

st mill output w/w w/w w/w m w/w w/w

CT coefficient taking into account differences between S and S1 z

o sugar concentration in bagasse at diffuser input w/w G quantity of bagasse relative to 1 unit of cane at input

g

IJ

CSM P. l C. l c

sugar concentration ln bagasse at diffuser input diffusion coefficient

mean specific heat

mass of gas in flue gases relative to 1 kg of fuel mean specific heat of gas l

mean specific heat of water or steam

kg

kcal/kg/OC kcal/kg/oC

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(I -1-ABSTRACT

This study contains two malns parts:

a) Raw JUlce extraction from sugar cane,

b) Raw JUlce extraction fr om sugar beet. The first part is divided in:

(i) Energy requirement of the plant

(ii) Energy production for the plant

(iii) By-products manufacture. The second part is divided in:

(i) Energy requirement oÎ the plant (ii) Energy production for the plant (iii) Pressing and drying of wet pulp.

From sugar cane, raw JUlce can be obtained either by milling or diffusion. Energy considerations and total investment studies have given diffusion to be the most economical process. From a plant capacity of 100 t.c.h., we ob-tain 11G tons of raw juice per hour with a percentage by weight of 12.5%. Starting fr om cane with a fibre content of 0.125, 25 tons of bagasse

are produced per hour from which 10 tons only are necessary for the total energy supply of the plant. The remaining 15 tons per hour are used for ma-nufacturing briquettes.

From sugar beet, raw juice can only be obtained by diffusion. For a plant capacity of 100 t.c.h. we obtain 119 tons per hour with the same

percen-tage as for sugar cane. Unfortunately, beet pulp cannot be burnt as such

in the boiler furnaces to supply the necessary energy for the process. The-refore we have to rely on fuel. Au economical study has pointed out that the lowest product ion cost for dried pulp is obtained bycombining high presslng efficiency and drying (hence by using three presses), instead of a lower

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' ) -2-INTRODUCTION

In November 1975, Brazil IS National Alcohol Pro gramme ' (pnA), llilder decree 76.593, was legislated for the product ion of anhydridous alcohol (anidrol) initially as a blending component in gasolines, and at a later stage as a feedstock for chemical manufacture. The present aim of the programme is to save U.S.

S

500 million per year in foreign exchange credits on the im-portation of petroleum. This expresses the response of the brazilian go-vernment to the 1973 oil crisis and shows the willof people to get out of the energy dependance problematic.

At present, serious consideration is being glven to sugar cane and sugar beet for use in distilleries. However other crops are also being eonside-red: manloc, a root erop, babassu, a nut, Jerusalem artiehoke in the Freneh Carburol programme, and sorgho, the sweet steem, similar to sugar eane but with a thrice as large growth rate and a much more resistant erop.

However, at the present moment, in the optie of the "Fabrieksvoorontwerp"

programme at the Delft Institute of Teehnology, we are exclusively interes-ted in alcohol production from sugar eane and beet. Two other fellow stu-dents, ~tr van Baalen and Hogendoorn were assigned the task of calculating the distillation proeess starting from raw juiee (the extraction product of eane and beet). Ours is to supply this particular starting material and the neeessary energy for the entire plant. An economie study follows,

whi-eh is determinant in the ehoiee of the adopted extraction process and the

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PART ONE:

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THE SUGAR CANE EXTRACTING UNIT

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-3-COMBINATION OF CANE PREPARATORS

There are varlOUS types of equipment which are placed ahead of the mills with the object of preparing the cane so that the pressure applied at the mills will yield a maximum extract ion . :

a) The knives b) The crushers

c) The shredders

Hugot (1, p.

74)

recommands for a capacity of 100 tch

Two sets of knives, the first placed at the bottom of the inclined portion

of the carrier, the second just above, with minimum clearance.

Then, a hatlL'ller-type shredder which achieve a complete preparation. Af ter the shredder, a mill-crusher.

Then, four other mills·.

This gives us a tandem of 15 rollers. From (1, fig. 21.3 ), by extrapo-lation, we realise that the same extraction will be obtained from a 12

rollers battery with an imbibition of ~= 3.2 than from a 15 rollers

battery with an imbibition of À= ': a common value fOr imbibition ).

In a normal sugar factory, the imbibition water must be calculated to mini-mlse the losses during crystallisation;

In our plant it is necessary to dilute the raw JUlce to 12.5

%

ln sugar. So, we are free to use a high quantity of imbibition water.

Then, by using a 12 rollers battery we will minimise the power consumption

and capital co st of the milling unit.

>< :> Cl) c 70 60 ~ 50 Il> "0 ~ 40 Q a. Il> - -

.

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\ , - -I I I ~ 30 -"0 c o c ~ 20~---~----+---~---t----~----t=~ ~ 101----~--+_--~--~---t---4----[_1 L~L.--.-JL-L-.;L--L----J2r----''---'-:~1I.1.

o Imb,bi·t,on "0 de ligneux = À FI{;. ~\.J. _ Variation du jus I>crdu ~~ de ligncux c? fo~ction de

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4-EXTRACTION EFFICIENCY OF MILLING

In this chapter we will calculate the extraction efficiency which can

be obtained with a 12 rollers battery and a high imbibition water.

By extrapolation from (1, fi~. 21.3 ) , we find that for a 12 rollers

battery with an irnbibition ~

=

3.2 , the losses in the bagasse will be 33 % undil uted j ui ce ~~ fi bre .

If we assurne a quantity of cane equal to 1,

The fiber lS 0.125

The total sugar content ln cane 0.15 The water content of cane 0.725

Hence, we loose in bagasse

0.33 x O. 125

=

0.041 non diluted JUlce.

The concentration of non dÏluted JUlce

sugar ln cane

sugar ln cane + water ln cane

=

O. 15

=

0.1714

0. 15 + 0.725

lS

80, the arnount of sugar lost ln bagasse will be 0.1714 x 0.041

=

0.0070

and in

%

bagasse

0.0070 x 100 0.25

= 2.8

%

bagasse

Hence, the extraction efficiency will be

e

=

=

sugar in cane sugar lost ln bagasse x 100 sugar in cane

_0_. -'15=----_0 _. 0_0-,-7_0---,x:..:..-.l_0_0

=

95 %

0.15

Dilution of raw juice.

The definition of imbibition lS

~

=

w

f

w quantity'of irnbibition water

f fiber content of cane

80, w

=

À x f

=

3.2 x 0.125

=

0.40 The arnount of 8ugar obtained ln cane 18

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-5-cane sugar content - sugar lost ln bagasse

=

0.15 - 0.007()

=

0.1430

The weight of raw juice obtained will be :

water in cane - water ln bagasse + imbibition water + sugar ln raw juice

0.725 - 0.25 x 0.48 + 0.40 + 0.143

=

1.15

So, the concentration of raw JUlce will be

0.143

1. 15

=

12.5

%

by weight;

Conclusion

With a 12 rollers battery and a high imbibition factor ( À

=

3.2 ) we

can obtain a good extract ion ( 95

% )

and a raw juice at the required

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-6-PRE-MILLING OPERATIONS

Various types of equipment are placed ahead of the mills with the object

of preparing the cane. These are: (i) The cane carrler

(ii) The lmi yes (iii) The shredder

The cane carrler

The cane carrler lS the movlng apron which conveys the cane into the fac-tory, and which assures the feed to the mills by transporting the cane from the yard to the crusher.

Roughly, we may reekon, as a first approximation for the power consumed by cane carriers:

3z + A

p'=

20

12.5 h·r· ( 1, p. 25 )

z = total length of the carrler, ln meters A

=

crushing rate of the mills, in t .c.h.

The installed power should be appreciably higher, say:

P.

=

l Intermediate carrlers 3z + A 10

=

150 + 100 = 10 25 h.p.

=

lb.4 kW

The intermediate carrlers are the conveyors which move the bagasse from one mill to the feed of the next. The mean power consumption of the ln-termediate carrier lS generally scarcely a matter of concern, since this power is furnished by the mill itself, and ln a way forms an integral part of the power required to operate the mill (1, p.80).

Bagasse conveyor

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-7-the boiler furnaces. It is picked up by a bagasse elevator, which drops

it into a horizontal conveyor; this distributes it along the length of

the boiler station to the furnaces.

Roughly speaking, we may take 1.10 kW for every 10m total lengh of the

bagasse conveyor ( i.e., about twice the actual lengh of the carrier

=

upper run + lower run ). If we choose 40m, we obtain:

p

=

4x1. 10 = 4.40 kW 1, p. 96)

Knives

As one set of knives is insufficient to assure satisfactory slicing up

of the cane (1, p. 74), we shall commence with two sets, the first

pla-ced at the bottom of the inclined portion of the carrier, the second

just above, with minimum clearance

The average power absorbed by a set of knives depends on:

1) The tonnage of cane

2) The fibre in cane

3)

The nature of the fibre, whether more or less resistant.

4) The proportion of cane actually cut

5) The number of blades

The speed of rotation

The radius of the cutting circle

6)

7)

8)

Diverse variabIe factors: friction, lubrication, knives more or less worn.

The following figures are quoted by Hugot (1, p.42)

TABLE I

Java Australia Queensland South Africa 1st set 1.5- 2 2nd set 2.5

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3 both 4 2 2 values ln h.p·1 t.c.h.

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-8-For mean power consumption, we may estimate in general, for knives,

ap-proximately 0.74 - 1.47 kW per t.c.h. On account of the large variations

experieneed in the feeding of the knives, the power to be instalIed for the drive mbto~ should be substantially higher than the mean power

esti-':

\.1-'

mated.

\'

From Hugot f1, p.44) it is advisable to install a motor of 25 h.p. per t.,f.h. for factories working at more than

6

t.f.h. Thus, for our case

',---~

with 12.5 t.f.h. we then install 230 kW.

The shredder

The object of the shredder lS to complete the preparation and desinte-gration of the cane, so as to facilitate the extraction of the juice

by the mills.

It is generally assumed that shredders of the Searby type consume on an

average about 1.47 kW per t.c.h. However, ln order to allow for momen-tary peak loads, the motors instalIed have a nominal power of 1.84-2.21

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kW per t.e.h. We choose 210 kW per t.e.fti

POWER REQUIREMENTS OF MILLS

The determination of the power required by a mill lS rather complex be-cause a number of factors enter into it. To begin with, this power may be split up into

6

different terms:

a) Power consumption by compression of bagasse

b) Power consumed ln friction between the bearings of the rollers and the shafts

c) Power consumed by friction between bagasse and trash plate

d) Power consumed by friction of scrapers and toe of the trash plate against the rollers, to which should be added the work of dislodging the

bagasse at these points

e) Power consumed ln driving the intermediate carrlers f) Power absorbed ln the gearing

Furthermore, these components of the power themselves depend on certain factors rather difficult to measure or estimate, such as: variety of cane, state of the friction surfaces, quality and regularity of lubri-cation, adjustment of the setting and of the trash plate, etc ... Owing

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-9-to the impossibility of taking into account all these factors, the

va-lues found in practice may differ appreciably from the mean power

figu-res which we shall derive. This difference may be as much as 20 or even 25% of the normal value furnished by the formula.

Summarizing Hugot,Swork (1, ch.14), and assembling the mean terms for

the powers quoted above, we can write for the total power consumption

of one mill:

p =

'

!Y

[F(OA'

,_

6r

-

5 _ • \/;:

+

O.OH)

+

4L,1

P \'r(1 1- \/r - l )

P

=

total power consumed by a three cylinder mill

n

=

rotation velocity of the cylinders, r.p.m.

D = cylinder diameter, m L

=

cylinder width, m

p

= efficiency of the gearlngs

(1, eq. 14.27)

F

=

total hydrolic pressure exerted on upper cylinder, tons r = reabsorption factor

C

=

specific opening between cylinders

a

N.B. In the case of the first mill, the power required for breaking

C' up the structure of the cane, even when prepared by knives, is

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substancially higher than that absorbed by compression of

baga-sse. For this reason, it is recommanded( 1, p.232) to replace for

the first mill the coefficient 0.40 by 0.45.

In order to calculate the power consumed in a given mill, we must first

dimensionate that mill, that is, find the values of the different terms

in formula

(3)

Dimensioning of the mills

From table(tI) , (l,p. 181) we can derive the number of mills necessary

for a capacity of 100 t.c.h. ,tne cylinders diameter and length, the

num-ber of rollers. (For more details on choice of number of mills, see

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-10-TABLEt II) C' c / = 0,125 0" L" 42" 40 3~ H4 36 7X 34 7X 34 72 32 72 32 /: 66 30 x 66 30 x 60 21> ~, 60 26 x 54 20 x 36

CAPACITÉ DES 1\10ULlNS 181

I\lur t(lute cnmbinaisoll d'engins corn~spondant it Uil coenicient C' =F c, multiplier par

TABLEAU 11.3

lil!'(lcilé dl!s hOllrries de IllOulim, elI I c Ir

C ul1I!'u.\jr;oll de la hClfll'l'j(' '\'''II/hr<, de cdil/dr('s i\' [)""" L'''''' LI)! I 065 llllll 2 134 Illm 2.420 m' 1016 <)15 813 76U 760 71U 710 660 660 61U 2 \34 2 1.14 I 9HO .: I ~.1() ;.. I ~30 x I 675 :< 1675 x I 525 x I 525 ., I 37U ., I 37U x I 22U A I 220 61U ' - X 1065 560 ;, I 065 51U - X 'lIS D

=

760 mm L

=

1675mm N

=

12 2, I <)9 1.9H7 1.7H7 1.65ti 1,47H 1,.166 1,210 1,IU7 0.%7 U.XHI U,769 U.691 0,597 0,53 I 0,454 0,396 0,334 0,231l 1/ = 5 tOUfJ.'/Il1/ D ,. JA! 4,\/ [) -1. 4M 5M 0 ~ 5M 6'\( D ., 6M 7Af 11 12 3.32 3.46 21<) 203 1117 172 160 145 134 121 111 99 90 80 n 64 57 49 43 37 221l 212 195 179 166 152 140 127 116 UQ1,1 94 84 75 66 59 51 45 38 28 14 .1.74 246 229 211 19~ IXO 164 151 \37 125 112 102 91 81 72 64 55 4~ 42 30 15 3.87 255 237 219 201 186 16~ 157 142 130 116 105 94 84 74 66 57 50 43 3 I 17 III 4.12 4.24 271 252 233 214 In 18U 167 151 138 123 112 IUO 90 79 70 61 53 46 33 279 259 204 186 172 155 142 127 115 IU3 lil 72 63 55 47 34 - - -- - _ ...

_-

-.. _ - --- . 20 4.47 294 252 232 215 196 11>1 164 150 IJJ 122 108 97 86 76 66 58 50 36 21 4,58 302 280 259 238 220 201 185 1611 153 137 125 111 10U 88 78 68 59 51 37

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-11-The roller speed lS then glven by formula 4 , (l,P.IC~)

I

A

where:

A

=

battery capacity (100t.c.h.)

f = percentage bone dry fibre (0.125)

c

=

coefficient relative to preparing machine( 1.25) see (1. p184)

n

=

rotation velocity of cylinders, in r.p.m. By replacing letters by ciphersJ we find n

=

4.77 r.p.m.

Calculation of cylinder openlng

Rearranging formula (1, p. 191) we get:

t

a

A.f

= --3-3-0-n-c_D2 Lf~

l

values for

e,

are tabulated on p. 12

a'

values for f! are found in ( 1, p.196)

l

Calculation of the reabsorption factor r

r_i = 0.75 + 0.0 17v + 0 . 65

Ti

v =TTnD

=

11.39m/mn Cp A.f I i = 60lTnDLe . al (1,p.153) (1,p.131)

Knowing e . for each mill, we can calculate r .. Values for r. are

ta-al l l

bulated on p. 12

Calculation of the hydraulic pressure F

The only unknown here lS ~i

and V.V.E.

=

-d. xl.01 JO (1, eqn. 10.90) - ( _ _ 1_.-,-20,-- _ 0.86) f' d jox 1.01

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-12-We have already fixed f' for each mill. Hence

lst mill 2nd mill 3rdmill 4th mill

d.

₌

1.04 1.03 1.02 1. 01

JO

From this we can calculate (V.V.E.), which glves ~i and consequently

F .•

l

Table 111 concentrates the results of the preceding section and glves

besides the calculated powers of the four mills from formula 14.21

Table III Mill l f. e _E.Q,~

~i

d. F. P. l al r. "l JO l l 102 3 3 _{tons kW} mm kg/m kg/dm 0.33 18.91 2.49 0.648 1. 36 1.04 258 173 2 0.42 14.85 1.96 0.824 1. 4t) 1.03 307 187 3 0.47 13.28 1. 75 0.922 1. 54 1.02 370 219 4 0.50 12.47 1.64 0.982 1. 58 1. 01 390 229

TOTAL POWER NECESSARY FOR THE MILLING PROCESS

Now, we can calculate the total power necessary for the milling battery,

tnat lS, for the conveying of the cane from the yard through the knives,

shredder, mills, and the final distribution of bagasse to the boiler fur-naces. Cane carrler 9.2 kW Knives 176.4 kW Shredder 147 .0 kW First mill 173.0 kW Sêeond ".miiL 187.0 kW Third mill 219.0 kW Fourth mill 229.0 kW Bagasse conveyor 4.4 kW Total lUIS kW

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-13-ELECTRIC ~ND STEAM DRIVE OF MILLS

In this chapter, we will summarlze the advantages and disadvantages of electric drive and steam drive of mills.

A. Advantages of electric drive

a. Neatness and cleanliness

An electric mill house is much neater than one where the mills are driven by steam engines, and even somewhat neater than one with turbine drive: there are no steam joints leaking or dripping, no oil splashing from the crank oiler or dripping from the lubrificator, no cumbersome steam pipes.

b. More complete and definite control

Each mill is driven by aseparatemotor, Slnce electric drive lends itself much better to individual drive than the steam engine and at least as well as the steam turbine. The power consumed byeach mill is ascertained at any moment by the simple reading of an ammeter; This is an important point in favour of electric drive, and lS quickly translated into improved ex-traction.

c. Ready general regulation of speed.

The speed of the whole mill tandem is controlled, very conveniently, from the power house.

d. Ease of starting and stopping.

The mills are started by a push-button control~ Stopping the mills is equally simple, also their reversal, as required in the case of a choke.

e. Lower operating and maintenance cost.

Costs of lubrification for electric motors are much lower than those in-volved for steam engines. In the same way costs of maintenance are much lower.

f. Accidents fewer.

No fear of water-hammer, or of fracture of a crank-pin.

g. Safeguard against passage of large pleces of tramp iron.

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-14-immediately; with a steam turbine the inertia of the flywheel compels

the foreign body to pass, at the expense of the roller grooving.

B. Disadvantages of electria drive

a. Higher first co st of installation

The combination : high pressure boiler, turbo-alternator, switchboards,

motors and cables, and supplementary stage of gearing, costs more than the combination : low pressure boiler, steam pipes and steam englnes;

b. Additional double transformation of energy.

With an electrical installation, in addition to the analogous

transfor-mation effected in the turbo set, the energy must also undergo :

the transformation of movement into electric energy ln the alternator;

the transformation of electric energy into movement ln the mills motors.

Each of these transformation involves a loss of efficiency.

c. Extra stage of reduction gearlng.

The speed of electric motors necessitates the interposition of an extra

stage of speed reduction between motor and mill, taking up additionnal

space and involving further loss of power.

d. Less complete speed control.

Turbines and particularly steam engines maintain their power bet ter at

low speeds, and are more flexible.The steam range obtainable with a steam

engine is much greater and more complete.

e. Accidents more serlOUS.

The accidents liable to occur with electric drive are more serlOUS, and

necessitate a judicious provision of spare parts.

f. More specialised personnel.

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-15-D.L. Hughes, (1, p.240) gave figures for the relative performance of a

turbine and that of an a.c.-d.c. cascade system (see table IV). He

consi-dered that these two systems are those which, with modern technique, would

give the most complete answer to the requirement of mill drive.

TABLE IV

COII/flaraiso/l eli/re IlIrhi/le ei \'Opellr el cascade allemalij:co/lli/lll f(Jill' á l;golilé de plliss(J/lce /lolI/i/loll'

COllple disponiblc il :

Vitcsse -, 100 o~

XO 60

-Consommation dt! vapeur pour couplc I 00 ~ ~ : Vilesse 1000

80 '~ 60

-Couple de surchargc sur 2". ~o du couple de pleinc charge :'

Vilesse -100 0/

ö() '~

60 -Puissance à ino;tallcr à capacité é~,d~ :

Turbil/e à ra!,ellr 100 0 118 -1.15 -100 ~ 0 X7 -74 -100 118 -1.15 -100 o~ Cascade "lteru.·culll;Jlll 100°/ 125 --166 -100 0.' 80 60 -125 ~~ 156 -207

-Since we know now that Hughes was speaking in favour of electric drive,

we must admit that, if this table was given in absolute values, instead

of 1n values relative to those corresponding to full speed as a basis,

there would be several items where the advantage would lie with the

tur-bine.

However, having balanced the var10US advantages and desavantages of both

techniques, the final choice lS more a matter of economics and personal taste. If we agree that the company may afford a higher investment cost

by purchasing mills for substantial added returns due to the much lower

operating and maintenance costs, then electric drive may be a very good

( ( ( ( ( ( (

c

'

o

-16-DIFFUSION Description

There are two ways of extracting sucrose fr om cane by diffusion. the

first lS pure diffusion or diffusion of cane. It consists of operating ln manner completely analogous to that of beet; the cane is prepared

and broken into small pleces by means of knives, shredder or desinte-grator, then is sent to the diffuser. The prepared cane retains its full weight and still contains all its sugar. If the factory is treat-ing 100 t.c.h. at 15% sugar content, the 100 tons of cane and the 15% sugar pass through the diffuser.

The second type of diffusion is called diffusion af ter mills or

bagas-se diffusion. The cane, prepared as for milling, goes first into a mill which extracts as much juice as possible, say 65 - 70% of the sugar in

the cane. It is the bagasse from this mill which lS sent to the

diffu-ser; the diffuser thus recelves only 30 -35% of the sugar ln the cane,

and the weight of bagasse is perhaps 40 tons for 100 tons of cane. This method of bagasse diffusion is based on the idea that, while it lS very expensive to employ 4 or 5 mills to extracts 94 - 96% of the sugar ln

the cane, it lS a much more payable proposition to employ one mill to extract 70%; The extraction of 70% by one mill lS three or four times

superior to that of 16 - 23% for each millof the train.

For the following reason we prefer the bagasse diffusion process: a) Passage of the cane through a first mill with coarse

groo-ving canveniently completes the preparation of the cane for diffusion

b) Losses by inversion and fermentation during diffusion no longer act_on the whole of the sugar content of the cane but only on 30% of it

c) The diffuser can be greatly shortened, since the material

o

treated na langer contains 14% of sugar but only 4%, that

I

I

I

o

10

is, 30% of the original sugar within the cane.

DIFFUSION PROCESSES

The diffusion processes which can claim at present to have a place ln the world sugar industry are the following:

I

I

( ( ( ( ( ( (

o

o

o

-17-D.d.S. ( Danish) de Smet (Belgian) B.M.A. t German) Silver (Ameriean ) THE D.d.S. EROCESS

D.d.S. are the initialof" De Danske Sukkerfabrikker", the organisation whieh produces most of the sugar made ln Denmark. The D.d.S. diffusion

is the only continuous diffusion process in the strict use of the term. It is simple, not expensive, and the space required is relatively small. For the description of the diffuser, we refer to (1, P.338). The follo-wing seheme

~

100(86) ~ " ~

O<i!

~,/

00

_I

(35) Jus Ier M

't

(68) ~ (103) J M

will give an idea of the process.

Jus de d,ffusion' Eau 1(30) Megasse

l

~~ \.\':01 Q~n Bagasse

Ou uO

fmale

Petlt JUs (71) (32) Petlt jus du

ADM DM

FtG. 24.7. - Difrusion D.d.S. Bilan matièrcs.

lOM = dernier moulin; ADM -~ avant-dernier moulin).

Steam consumption: about 25 kg/t.e., say 2500 kg/h for a capacity of 100t.c.h.

Power consumption:

Cane preparation: The best eane preparation for the D.d.S. proeess is that obtained by knives, and it lS desirable

to have two sets. The preparation may also be completed I

I

( ( ( ( ( ( ( ( ,

o

o

o

-18-by a shredder, but the bagasse from a shredder lS less permeable than

that fr om knives, and there is a risk that the gain in extract ion ob-tained by this bet ter preparation may be lost or offset due to the con-sequent difficulties in regulation and operation. Power of the set of knives:176.4kH(seep.3 ).

Mills: The diffuser is preceded by one mill which achieves a complete preparation of the cane entering the diffuser. In any diffusion process the megasse cannot be left as such; the cane sugar factory requires fuel

and it is quite impossible to burn megasse, on account of its high

wa-ter content. It is therefore necessary to press this megasse in order

to remove the excess juice. Two mills are generally used to reduce the

moisture content from

85%

to

48%,

in other words to convert megasse

into ordinary bagassewhich can be burnt in the boiler furnaces. Hence,

one mill upstream from the diffuser, Two mills downstream, with an appro-ximate power of 191. 1 . kH

Diffuser: Power requirement is relatively low. Each helix lS driven by

a d.c. motor through reduction gearing and cardan shafts. The direct

current is given by a set consisting of an asynchronous motor, driving

an independently exited generator. Including the discharge wheel and the

conveyors, the total power required for diffusion is approximately 1.5 kW

per t.c., that lS 150 kW for a capacity of 100 t.c.h.

From there we can estimate the total power required by the d.D.S. process

2 sets of knives 176.4 kH

3 mills

diffuser

THE de SMET PROCESS

Total

573.3 k\{

147 kVT

896.7 kW

The de Smet diffusion process differs from the d.D.S. diffusion process as well as in energy consumption as in steam consumption. It offers also

(

-19

-(

a) Good visibility of what is taking plaee inside the diffuser b) Possibility of regulating the height of the megasse layer as

( well as its speed of movement

( ( (

c

o

o

o

e) Opportunity of sampling the juiee from eaeh tray, over the full

length of the diffuser.

Fig. 24.16 show the essential of the proeess.

300100100 100100100 100100100 ~

FI(;. 24. I 6. - Oilru,ioll UI! Sml!!. Ililan-ma!ièrl!s.

JtU(;CII. -- 1.1.1 .llllrCr;" dj' lal/nel.

1 J

Power eonsumption:

The eonsumed power is about 1.03 kW per t.e.h.; the installed power is around 1.2 kW per t.e.h., say then 120 kW for a eapaeity of 100 t.e.h. For the preparation of the eane, two ordinary sets of knives may be used, plus one mill. The megasse eompresslon to bagasse requires two mills downstream to the diffuser. From there the total power requirement ean be ealeulated:

2 sets of knives _176.4 kl-l

3 mills 573.3 kW

diffuser _{( 1, p. 352)} 120 kW total

869·1

kl"r

( ( ( ( ( (

c

o

o

o

-20 -Steam consumption:

The quantity of steam consumed varles between 80 and 85 kg per t .c.h.; say e500 kg/h for a capacity of 100 t .c.h.

B.M.A. DIFFUSION PROCESS

The de S!!:et diffuser is the standard type of equipment and process from which several other diffusers have been developed, in particular the B.M.A. in Egypt and theSilver Procesán the U.S.A. The B.M.A

diffu-ser is very similar to the de Smet diffuser and is distinguished from

it only by details. One of the main difference lies in the method of moving the megasse, which obtained by scrapers pushing the material on

fixed perforated plates, with conical holes of one cm in diameter. On account of the rine bagasse passing through these holes with the juice

the juice is handled by an unchokeable pump which returns it to the pre

-ceding compartment

B.M.A. advices against a shredder and recornmand preparation by two sets

of knives and one mill. Shredded cane gives lower speeds of percolation

(see fig. 24.21 B.M.A. diffuser)

68,5 Eau 2'1,5 I ' _ . . - - - ; L-70

!~

( ( ( (. ( ( ( ( [)

c

o

-21-Power eonsumption:

We may reekon approximately 1.00 kW per t .e.h. for the B.M.A. diffuser.

For the preparation of the eane, power ealeulation is similar to pre~

VlOUS di ffusion proeesses( 1, ~. 366) 2 sets of knives 2 mills + diffuser roller total 176.4 kW 573.31 kW 100 kvl 849.7 kl-l

The steam eonsumption lS approximately 120 kg/t.e.h., henee 12000kg/h

for a eapaeity of 100 t.e.h.

THE SILVER DIFFUSION PROCESS

The Silver diffusion proeess is also derived from the de Smet type.

This proeess, however displays its originality not so mueh in the dif -fuser as in the efforts made to break as eompletely as possible with

the elassieal methods of extraction, namely knives and mills. The power eonsumption ean be ealeulate as sueh (1 , p.372)

knives 0.51 kW

_/

t . e.h.

Cane buster 3.45

_"

fiberizer 3.01 "

diffuser: rotation 0.04

_"

Serews 0.51 ft,'

horizontal bagasse eonveyor 0.04

"

~ ,

Freneh press 4.48

_"

total 12.04 kW / t.e.h.

For a eapaeity of 100 t.e.h .• this beeomes 1204kW.

The steam eonsumption for the heaters and steam injectors lS of the

l ( ( ( ( ( (

c

o

o

0

-23-CHOICE OF A DIFFUSION PROCESS

Now we have glven the characteristics and power requirements of the

aval-lable diffusion processes, we hav.e to make a choice amoung them.

Therefo-re, it is necessary to estimate the total investment cost for each process

and deduce the product ion cost of raw juice in the case a diffuser only

would be sufficient. Knowing the price of a B.M.A. diffuser, 3 000 000 f

(delivered but not installed), we shall assign it a coefficient 1 and make

a rough estimate of the other diffusers'costs by comparing their volumes

(masses), material used for construct ion , power consumption, degree of

au-tomatisation, complexi ty of construct ion , ,space required by the wü t ,etc ...

Consequently, the value found for these complexity factors are tabelled

on p. 23

For th,= prlce.lsed for electricity:0.07 f/kwn), see bagasse manuf2~t'''::''3

For the prlce of steam, we used the following calculation:

Energy obtainable from L.P .. steam:

[ ( 1)j. 3 0 C, 3b ar) - ( 900 C, 1 b ar)] x 4. 1 8

=

Corresponding :)ower: .l,,\\\:" "', " 2236 kJ/kg '\~ / " ~~<

(

/

(625 -

90)

~

/3600

=

0.6212

k

~

kg/h

Let us apply a conversion factor of 0.3 to obtain equivalent electric power

The converSlon factor becomes: 0.6212XO.3

=

O. 1~64

TABLE V

Cornl. fact. Compl. Fact. Del., Cost IilSt. Cost l Energ. Cost

rel. to D.d.S. rel. to B.!![.A. ~ electr. ) D.d.S.

1

,

1.4 de Smet 0.76 2 280 000 ~ 275 400 301 258 1.08 3 240 000 1 1 761 200

23G

507 B.M.A. 1.3 ,1.00 3 000 000 1) _{890 000 285 499} Silver 1.5 1. 15 3 450 000 1~ 523 500 404 544 , / \ ~i'Î..1~· ~ I .::r's l'l/f5C

- - - -

-

-

--

-

. ---

-

-

- -

.. .

--

---_

/-

___

--J

,

'---'

7

~

!:)l~OJU

3~

I03

COST'

I

<>

I

Eq. Pow. of Steam Cost

i Steam. (kW)

,

I 466 156 576

\~

L~(lÀ~~l,.

1584.4 532 258 ê 'l, • -~ J 2~36.8 751 565 ',y-281 837

Incl. ST & C

y~

,\V Excl. St E:((cl. C.F. 2 195'878 3 288 617 _ 3 323 964 3 _{(9)1 081} 'J 2 2 2 2 039 302 756 359 572 404 909 244 3 105 665 3 32i 964 2 973 281

r r [

o

o

o

-24-D.d.S. DIFFUSION. MASS BALM{CE.

Our choice goes then to the D.d.S. diffuser. For this type of process, we are now gOlng to calculate the extraction efficiency and the amount of sugar left in the baeasse.For this calculations, we will make use of the formulae developed by Hugot, chapter 24. These equations are:

(1,24.24; 24.26; 24.27; 24.28; 20.4)

Since the simultaneous resolution of these equations glves rlse to 3 nd

and 4th order equation systems, we will proceed by iteration.

We start from

1

of cane.Let suppose a bagasse with

2%

sugar. Hence the quantity of sugar in the bagasse will be:

0.02

X

0

.25

=

0.005

Gazing at figure

24

.11

we deduce that:

c . Q +

0.005

=

0.15

o

slnce the sugar content ln cane lS

15

%

.

From there:

0.15-0.005

₌

1.16 Q

=

_0.125

c

= 0.125

is our desired final concentration of raw JUlce.

o

From formula

(24.24)

we can now come to know

Q = 1.

16

Q - J - 2J + 1) ~ ex:) Q J

=

0.15

+

0.725

=

0.875

l!.

L

=

p. t

=

o.

13

x

30

= 3.90

11 L( 1 - ~) Q

t

=

residence time of juice ln the diffuser p

=

factor given by Hugot

(1,

p.

348)

Hence,

Eo=

0.4183

+ J - 1

l. ( ( ( ( ( (

o

o

o

-25-From eq. (24.26) we can calculate y :

o JS - ( J - S) Yo = _Q - J + G + _{( 1} Sl

-

J)l.

( 1, eqn. 2)+. 26 ) S = Su~ar

%

of cane = 0.15 = 0.1714

o.

15 + 0.725 , ') \

-

\ -. \ -v-, v- / Sl =<rS = 1.03 x 0.1714 = 0.1843 \ '., ' \ c". \ I . ,- I .. ~ '\JA. .1 l", , L",-,,\·tJ ( see eq. 20.4) f'

=

f G = f( 1 - f')

₌

0.2538 'f + G f'

since f'

=

0.33 for the first mill

f

=

0.125 for cane

Knowing all terms in eq. (24.26) we can find y

o

y

=

0.0600

o

Out of eq. (24.27) flows Z

o Z

=

~G~.~g __ +~(J~-~G~)~SJ1 __ __ o J and g = 0.15 - (J -G)Sl_ = 0.1399 Hence, G From Z

=

0.0832 o Yo c o

=

(J - G) Sl + Q

S>

~L

= 0.0251

Q - (J - G) y o = O. 1239

The quantity of sugar ln the bagasse becomes:

(1 - J)zL

=

0.125 x 0.0251

=

0.00341

The percentage of sugar in the cane is then:

c .Q + 0.000341

=

0.1472

o

which is almost 15%.

The extract ion efficiency is:

e

=

sugar ln cane - sugar ln bagasse

=

sugar in cane

0.15 - 0.00341

r -26 -EalJ 0 -2J '100

I

2 J 100

iJ

+

"

rr

~

,

; ,

O

~

0"'""'

-;:;.;;.-

z J G 0 ij GI (Yu) YJ ZL

~---=::~=---~YL

... ~ x L x 0 . . . . . S EflicacllC . . . " 11 - O,lluslon D.d . . /-rt,. _.... . . L 100 J

o

D

o

-

2

7-CHOICE BETWEEN DIFFUSION AND HILLING

In the great majority of cane sugar factories throughout the world, extraction of sugar from cane is effected by means of mills. Today, many manufacturers of cane sugar return to the other method of extraction, namely, diffusion. There are sound reasons which have prompted them :

The mills consume considerable power, out of proportion to the result obtained.

The mills are very heavy and very expensive, both the purchase price and ~n _{cost of operation and maintenance.}

Whatever the power expended, it is acknoledged that a certain proportion of the juice contained in the cane cannot be

extrac-ted by pressure.

Diffusion permits gains ~n _{extraction, but ~s more critical}

to control than milling, on account of the risks of invers ion and deterioration which are involved. The initial cost is sli-ghUy higher for diffusion : _{it is appropriate to compare the}

normal tandem with its cane preparators and its four mills, with the same tandem with the diffuser, only three mills and

the preparatory plant. Now, the cost of the diffuser is close

tothat of

14

mill. 1he initial cost

~s

_{thus roughly}

equiva-lent, or slightly higher for the diffusion plant.

However, apart from the ga~n _{in extraction, diffusion presents}

a~ _{othèradvantage ;}

The maintenance of the diffuser is less costly, on account. of the cost of rollers, trash plates and scrapers.

Taking into acount on one hand the gain ~n recovery and these economies, on the other hand the slightly higher investment cost, our final preference will go to diffusion.

l (

c

( ( (

c

o

-28-STEAM PRODUCTION AND USAGE

The fibre in the cane lS generally sufficient to enable the bagasse

pro-duced by the mills to supply all the steam necessary for power

produc-tion and for manufacture, when utilized as fuel in the boiler furnace.

The excess bagasse can be used 1n the cane sugar by-products industry,

and we will see some applications in a later study. We shall study successively:

a) the composition of bagasse

b) the combustion of bagasse

c) the boiler and accessor1es d) the steamturbine

THE COMPOSITION OF BAGASSE

Final bagasse or simply bagasse is the solid material which leaves the delivery openine of the last millof the tandem, af ter extract ion of the

juice. It is the residue from the milling of the cane.

Physical composition

In spite of the diversity of milling plants and machines employed, the

physical composition of bagasse varies between rather narrow limits. lts

most important property, from the point of view of steam production, lS

its moisture content. The most frequent values goes from w

=

45%

to

50%.

(w is the moisture content of the bagasse ) Generally we shall not

in-volve any great error in adopting for practically all cases the standard

value: w

=

48%.

In addition to water bagasse contains:

a) insoluble matter consisting mainly of cellulose, and comprising

o

the fibre content of bagasse

o

b) substances in solution in the water, consisting of sugar and

im-purities. They represent between 2 to 4% of the weight of bagasse.

It remalns for the fibres:

f' =

100 -

48 -

2

=

50%

( ( ( ( (

c

o

o

o

-29-Quantity of bagasse

We have fixed for the fibre content of the cane, f

=

0.125 (mean value

given by Hugot (1) ). We obtain the quantity of bagasse produced from

100 tons of cane per hour by equating the weight of fibre entering the

mills to that leaving:

lOOf = B.f' hence

B = 100 x O. 125

=

25 t/h

0.5

n

Calorific value of bagasse

The calorific value (or C.V.) is the quantity of heat which will be re-leased by combustion of unit weight of the fuel under consideration. We have

two different calorific values:

a) The gross calorific value (G.C.V.): this is the heat liberated by thecombustion of 1 kg of the fuel, taken at OOC and 760 mm Hg,

all the products of combustion being reduced to the same conditions. The

water present in the fuel, as well as the water formed by combustion of

the hydrogen entering into its composition, is consequently condensed.

b) . The Nett Calorific Value (N.C.V.), which assuroes on the contrary

that the water formed by combustion, as also the water of constitution

of the fuel, remalns ln the vapour state.

But since ln industrial practice, it has not yet been found practicable

to reduce the temperature of the combustion products below the dew point,

the N.C.V. gives a more accurate indication of the heat practically ob-tainable.

The N.C.V. of a fuel lS therefore glven by the formula:

N.C.V. = G.C.V. - 600E

E = weight of vapour present in the gases produced by the combustion of

kg fuel, expressed in kg

600 = vaporisation enthalpie of water ln kcal/kg

I

( ( ( ( ( ( ( (

o

o

-30-Gross Calorific Value of dry bagasse

In spite of considerable differences ln appear3.11Ce between different

va-varieties of cane, the G.C.V. of dry bagasse is remarkably constant in all

~

countries and for all varieties of cane. G.C.V.

=

4600 kcal/kg

Calorific Value of wet bagasse

The G.C.V. and N.C.V. of wet bagasse lS glven by the formula(l, p. 813) G.C.V

=

4600( 1 - w) - 1200s

<I

N.C.V.

=

4250 - 4850w - 1200s

..

.\ ":1 s

=

sugar

%

of bagasse w

=

moisture

%

of bagasse

We have found (see p.28); s

=

0.02 and w

=

0.48, hence

G.C.V. ~ 2368 kcal/kg

N.C.V.

=

189b kcal/kg

Quantity of steam obtainable

We may now calculate the quantity of steam which we can obtain from 1 kg

of bagasse. The losses of heat in the furnace and at the boiler consist

of the following:

a) Latent heat of the water formed by combustion of hydrogen ln the

bagasse

b) Latent heat of the water contained in the bagasse

c) Sensible heat of the flue gases leaving the boiler

o

o

o

-

31-e) Losses by radiation from the furnace and especially from the boiler f) Losses due to bad combustion of carbon giving CO instead of CO

2.

Nov, the use of the N.C.V. has already taken into account losses a) and

b) . The loss c) is given by the formula ( 1, p.824)

q = [( 1 - v) ( 1. 4m - O. 1 3) +

o.

5

1

t

q = sensible heat loss 1n flue gases, 1n kcal/kg t

₌

temperature of flue gas, 1n °c

v

₌

moisture content of bagasse

m = ratio of air weight used for combustion to that theoretically ne-cessary

For m

=

1.4, v

=

0.48 and t

=

180°C, we find

,-"

q

=

260 kcal/kg bagasse

The three other losses are taken into account by means of coefficients applied to the total quant i ty of heat which is still available after the first three losses:

c( = coefficient taking into account losses due to unburnt solids; 0<. lS

of the order of 0.98 for ordinary furnaces

~

=

coefficient taking into account losses due to radiation; for a well-lagged boiler a value of 0.975 may be used

~

=

coefficient taking into account losses due to incomplete combustion For a vell-conducted combustion, we may use a value of 0.96

the quantity of heat rema1n1ng to be transferred to the steam l S

there-fore given by the expression:

Hence,

Mv

=

(4250 - 1200s -4850w - q) d\, ~

M

=

1503 kcal/kg v

c

c

( (

c

o

0

0

- 32 -0 4 2

The enthalpy of high pressure steam at 400 e and 0 kg/cm lS 767 kcal/kg

The enthalpy of feed water at 90 oe to the economiser is 90 kcal/kg.

Hence, the energy necessary to supply 1 kg high pressure steam will be

767 - 90

=

677 kcal/kg.

Sa, the amount of high pressure steam produced by one kg bagasse will be :

( for choice of values T

=

400 Oe

~nd

P

=

40

kE/cm~

see p. 34 ) .

L'~

l2QL

= 2.22 kg a

~-0,

LV--

.~~~.,,,,,-

," 677

eomposition of flue gases

We know the total weight of flue gases per kg bagasse burnt

P :

=

5.76 (1 - w) m + 1 g (1, eqn. 42.29) w

=

0.48 m = 1.4 Hence, P

=

5.193 kg g

The weight of each individual gas lS

a. Nitrogen N 2 : N 2

=

4.43 ( 1 - w) m

=

3.225 kg b. Oxygen O 2 O 2

=

1.33 (1 w) (m - 1)

=

0.277 c. Water H 20 H 20

=

0.585 (1 - w) + W

=

0.784 d. Carbon dioxide CO 2 : e0 2= 1.72 (1 - w)

=

0.894

Specific heat of flue gases

(cf. 1, p. 8n) 62.3

%

5.3

%

15. 1 % 17.3 % 100 % v

"-Mean specific heat of flue gases between 0 (or 30°C) and T (1, p. 822)

N 2 : CSM

=

0.246 + 0.000020 T O 2 : CSM

=

0.214 + 0.000018 T H 20 CSM

=

0.468 + 0.000156 T e0 2 CSM

=

0.199·+0.000082 T

L

( (

c

( ( (

c

o

o

o

(1 CSM

=

l:

x.

l CSM. l

-33-X.

=

wei8ht of 8as i ln flue 8ases l

CSM.

=

specific heat of gas l.

l Hence,

CSM

=

0.270 + 0.000051 T

Calculation of combustion temperature

The combustion temperature prevaling in the bagasse furnace is readily

calculated from the fact that the heat produced during the combustion lS received in the gases passing from the furnace to the boiler :

t = t + ~~ N. l 0

_L

E. C.

\!

.L J, I.-

!

!.

t

P.. C. = P

.

CSM

"

"

..

g t

₌

103°C 0 do = 0.98

~

=

0.975 N. _l = 1898 kcal/kg p

₌

_{5. 193 kg} g CSM

=

0.270 + 0.000051.T

We have to calculate T by trial and error T

=

1100 °c

--->

CSM

=

0.270 + 0.000051 x 1100

=

0.325 kcal/kg

oe

Hence, t

=

1182

oe

( ( ( ( ( ( (

o

o

o

o

-34-STEM! TURBINE

The steam turbine will produce electricity from hif,h pressure steam coming

from the boiler.

For complete details about the calculation of the steam turbine,refer to (1, p. 987,1007).

Data

Pressure at entry

Temperature of superheated steam at en try Back-pressure at exhaust t a 2 38 kg/cm eH. 390 °C. 2 3 kg/cm eH. I Power required at the alternator

terminals 1991 kW.

~'-~. \.. ~.'. ,~ ~ ... 1

Rotational speed of the turbine n

=

9000 rpm. Calculations

If we assume an alternator of 1500 rpm, we shall need a speed reducer of 6 to 1.

Heat drop. The Mollier diagram gives

Aa 760 kcal/kg.

Ab

625 kcal/kg. Aa - ' \ = 135 kcal/kg. Diameter Since U

= ~

60 3. 14 • D • 9000 Let us take 200 60 Hence, D 0.425 m Number of wheels. .

<r

2 x. 04

=

~8_2 0_5_._1_3-;;:5_----'J'--_ 2002 27.7 •

'3

2

( ( ( ( ( ( (

o

o

-

35-Let us look for the best solution amonr; the different corresponding

values.

For x

=

3

~

=

0.331

x

=

4

cs=

0.382

x 5

3=

0.427

x

=

6

3=

0.470

x = 6

g~ves

us

a~v

e

r

y

close to 0.45 and so the best efficiency. If on the contrary ,.Je recyuired simple and cheap turbine, we could come down to x = 3 •

But we shall reject these t\.JO extreme solutions and take an intermediate

solution wi th x = 4 and wi th

~

= O. 3é:l2

Scale of pressure.

We have then

~

= 0.382 Hence

,

v

= u 'ff6:@ •. "

=

524

mis.

0.382 , ) On the other hand :

v

=

91.5. 0.94. ql

First wheel. ,Hence

2

91.5 .0.94

34.8 kcal/kg.

On the I~llier diagram, we look for point I on the vertical of A, such ;

Point " _a - a ~I = 760 corresponds to Other wheels. 34.8

=

725.2 kcal/kg. 2 . . -29 kg/cm abs: and 315 °C·.

We have just found the heat drop for the first wheel, q]. We have now to release:

( ( ( ( ( ( ( C

o

o

-

36

-AI - ' \

= 725.2 - 625 = 100.2 kcal/kg.

and to use three wheels. Consequently each of them will have to work

under a heat drop of : 100.2

qn = 3 33.4 kcal/kg.

And we ean check that we have ~n fact

~=~

_V 200

0.382 91.5 ~ 0.98 ~ 33.4

with the help of the Mollier diagram, we can now establish the scale

of pressure stages : ( see p. 37 )

Total heat (kcal/kg) Boiler 767 Admission 760 st wheel 725.2 2 nd wheel 69 1.8 3 rd wheel 658.4 4 th wheel 625 Steam consumption. Temperature 400 390 315 240 170 143 Abs. Pres. 2 p (kg/ cm ) 40 39 29 16 8 4 EH. Pres. 2 p' (kg/cm) 39 38 28 15 7 3

The turbine will drive the alternator. Then, the steam consumption per

kWh at the terminals of the switch board, is given by :

P

I

VJ=O,72

'W

~(

( I ,

~.

r"

r~

,

I

table!44-1) (I, eqn. 44-42) 'Iv

~u

{ ( ( ( ( ( ( C:

o

o

o

-37

-ft'

0.98 ( I , p. 919)

ft

= 0.93 ( I , p. 919) Hence, Q 860 9.71 kg/kWh. 135

.

0.72

.

0.98

.

0.93

In order to obtain the actual steam consumption, we shall have to add to the above value

0 :

For losses through condensation 3 % (I, p. 919) For losses by leaks 2 % (I, p. 919)

So, Q = 9.71.1.05 10.2 kg/khTh .

The quantity of steam necessary to provide 1991 kW will be 1991 • 10.2

=

20306 kg/ho

a

1

~ ~h~~7~

t

-i

l

o

,

_

-

₁₁

° °-- __ °_.

-J-~~:4~

;9.

~~

~~

~

;zt;

-;é~~Z36~

~ -~I

-t

-

,

> -

r

-Orlrnlr od"tlbofI(IU~ ti","' Irvwril ~"frr/~u"

3'~

...vkl

(J'Of-drJ..,,!Jc&n~) - °

6,OUj-- -- r

ti, ,

4-

w

kl

1,7

FIG. 41: . .5. - Di"crOlmmc de Mollier.

100 Enlrop/~ 1,8 - - - - •. - - - - -

-..

,. 0

"

r. ~ ë z

"

'" r-> '" > ;:: c: ,. 00 o '"

l ( (

c

( ( (

c

o

o

o

o

-3/) -Bagasse furnace

There are four types of bagasse furnace

a. The step-grate furnace

b. The co ok or horsehoe furnace

c. The ward furnace

The spreader-stoker furnace.

a. The step-grate furnace

The step-grate furnace, the oldest one ~s now no longer produced.

b. The horsehoe furnace, more recent permits of very high combustion rates and gives

ex-cellent efficiency.

c. The ward furnace, of amer~can origin, closely ressembles the horsehoe furnace.

It permits a great saving in the total floor space necessary for furnace and boiler,

and a substantial savine in refractories. It has given excellentresult and high

effi-ciency.

d. The spreader stoker furnace

This is the most recent type of furnace.

The quantity of unburnt ' solids remaining ~n the ash pit ~s greater (c( =0.975).

But it is considered that the spreader stoker furnace permits of reducing the normal

excess air to 40

%

and consequently of improvine the efficiency consequently.

Moreover, this type of furnace permits a combustion rate very superior to that of other

types ( 35 to 40 kg of steam per hour and m2 of heating surface)

Finally, the spreader stoker furnace makes ash removal easy, is easy to clean and

economical in brickwork.

Because of all these advantages, a spreader stoker furnace will be chosen.

Combustion chamber volume

The volume of combustion chamber should be proportioned to the volume of gases necessary for combustion.

The volume of the combustion chamber ~s th en given by

v

v

B • N· _~

200000

(I, eqn. 42.52)

combustion chamber volume in m3

( ( (

o

o

o

-3 9-N.

=

NCV of bagasse ~ 1503 kcal/kg l Hence, V = 101 m 3

Calculation of the superheater

We have two principal equations : t/)

~ M = ('j.PC ( Tl - T 2) = p ( ( 1 - X ) r + c ( T - t ) ) ( 1, eqn. 42.60) M = ~= P = C = T = 1 T

=

2 p = X =

quantity of heat transmitted to the steam, ln kcal/ho

coefficient of efficiency, generally 0.90

,,.

weight of gas passlng over the superheater, ln kg/h

specific heat of these gases, ln kcal/kg °c

temperature of gases at entry to the superheater, ln °C.

temperature of gases leaving the superheater, in °c

weight of steam to be superheated, in kg/ho

dryness fraction of saturated steam, between t and T

t

=

temperature of saturated steam, at the boiler pressure.

T

=

temperature of superheat desired.

T + t

( 1, eqn. 42.61)

2 2

k = coefficient of heat transfert, in kcal/m2/h/ °c

Generally a value of 55 may be used (1, p.84~)

. . 2

S

=

heatlng surface of the superheater, ln m

Heat available per kg bagasse burnt : 1503 kcal

Weight of steam supplied per kg bagasse burnt 2.22 kg

lr- \ fI • .:,<

Hence, total weight of bagasse to be burnt to produce 29967 kg/h steam

B = 29967 = 13498 kg/h

2.22

Weight of gases produced . We have (1, eqn. 42.29)

P

=

5.76 ( 1 - w ) m + 1

=

5.193 kg

g

<-( ( ( ( (

o

o

o

-40-Temperature of gases leaving the superheater •

p (\. PC

a.

=

0.90 (I, p. 844) p

=

29967 kg/h T =1182

°

C 1 P

=

70055 kg/h ( (I - x) • r + c • (T - t) )

C = 0.333 kcal/kg oe (bet'.Jeeri T 1 and T 2 )

x 0.98 (I, p.845)

r 411 kcal/kg (I, table 42-1)

c = 0.G14 kcal/kg.

°c

(I, eqn. 42-f)4)

T t

=

249 oe (I, table 42-1) (I, eqn. 42-60) 29967 • ( 0.02 ~ 411 + 0.614 ~ (400-249) ) Hence, T 2 =1182 -0.90 70055. 0.333

The heating surface of the superheater instalIed lS thus

p.(I-x).r + pc(T - t)

s

k=55 (l,p.844) T + t ) 2 Hence S 29967.(1 - .0.98).411 + 29967. 0.614. (lLOO-249) 55 ( H8~+ ilG3~ 2 400 + 249 ) 2

( ( ( ( ( (

o

o

o

o

-4 1-BOILER

Quantity of heat transmitted to steam or water

M = ~ PC T o - Tl ) = P ( X r + c (T -

to ))

(1.= p

=

T

=

o C

=

P

=

0.90 ( 1 , p. 70055 kg /h 1038 0 C 0.339 kcal/kg 29967 kg/h 844 ) o C _{( between T and Tl)} 0

X

= 0.98 (dryness fraction

of steam at boiler output, 1 p. 845 )

r

= 411 kcal/kg

(1 , table 42.1

c = 1 kcal/kg

T = 249 0 C (1 , table 42. 1 ~

t

=

161 0 C (water temperature at economlser output) o