17th general meeting of the American Society of Sugar Beet Technologists

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17th General \Iccting of the American So~icty of Sugar Beet Technologists, PHOENIX, Arizona, U.S.A.

Februarv 2i . March 2, 1972

ON

THE NEW R. T. CO~TINUOUS SACCHARATE PROCESS

by R. VA:\DEWIJER, J. JACQUES and R. PIECK

RAFFINERIE TIRLEMONTOISE, S. A.

ON

TH

E

SUMMARY

R.T.

CO

[\l

TIMUOUS

PHOC

ESS

by

R. VANDEWIJER. J. JACQUES AND R. PIECK RAFf-lNERIE TIRLEMONTOISE, S.A.

TIENEN (BELGIUM)

The R,T. continuous saccharate process is described. I t is characterized by the following particulars :

Low investments costs for the equipment. For a daily capaci-ty of 100 T molasses, the Tirlemon1: suear factory hc1.s inst.al· led a U shaped reactor of 12 mJ, a small settling tank of

5

mJ and one continuous belt filter of 7~5 m2 filtering area This equipment is completed by the usual accessories :for a

saccharate plant, a mill for the lime, a cooling installa-tion, etc.

- The molasses solution for feeding the plant must no longer

' .

be diluted to 6 ~ sugar, but may be introduced into the reactor at 12 ~ sugar concentr~tion.

- Formation of a saccharate cake with excellent filterability

(6

to 10 times better than in the bat.ch process), which enables high purities to be reached in the saccharate (93.5 as average on the 1971 campaign).

- Ver~ high efficiency of the powdered lime, Co.O

tion amounts between 90 to 110

%

on sugar.

whose con.sump- .

All these advantages enable a most economical recovery of the molasses sugar.

*

*

JTavirig run t.J1i~, Stc:f:fen l1ousc ;fo:r over 20 years , ',, • lart;c cxpcrionce on th<?. saccharatc proce~:c.:; has b ecrn gainc::Li..

Many laboratory trials were made :for the impr

ove-ment of the process. They l ed to the development o:f the

R.T. continuous cold saccharatc proc~ss

(

*

)

which, after

having been tried out in a pilot plant, has now been run with great success on industrial scale :for 2 years.

h1Lile developing this continuous process, three

main items were aimed at

lowest requirements on lime powder per unit of sucrose ;

- minimal suc:co se cunteut in the cold filtrate;

optimal f i l terabtlity of the saccharatc cake, which

r

.

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__ ,.. _____________ M •·--- -·---·----A·--·•••·-- ---·•••

--by tJ~c quality o.r the Lime and by the mo~·in ,·:r :i. t i :, r:.::oL ·.

in contact with the J.iqui<.l phase.

Within th0 lirni ts of' the present p;-,.:;:-·-2r, KC' c.:-,111::,-:

go into a detailed discussion of l:he compl0~.;. chem~~.3Tl'~- c :..' the cold saccharate precipitation. We only reca l l Cc:tt ·,::c-2·,

adding finely powder ed quick' l ime to a cold di lu tr~c. sol

1...:.-tion of' sucrose, the alkalinity and the suc;o.r cont C'l1 t 01.~

the liquid phase change as represented by figure 1 .

The f ir s t fraction of l ime - up to about 2 moles

CaO per mole of sugar - is fully dissolved and no snccha

ra-te i s precipitated durinc; this stage.

Further additions of lime give r i se to a rapid de

-crease of· both sucrose concentration and aJ.1~al i n i ty of' the

f i l t rate. The slope of this curve, as well as the a1110un.:: cf

unprecipitated sugar r emaining in the mother liquor , are

highly dependant upon a number of' exper imcntal ·f aci:ors ,

such as the t emperature of the solution, the grannluE1etry

of the ground l ime powder, the W"'Y the lj111e has been burnt,

the method of addition, and- so on.

The amount of lime require~: for the desugarizing

goes always largely beyond the stocchlomctric ratio cor

-responding to the build.:i.ng of t r i :...-:n.c:chara te. Such a qum~ t

i-ta t i ve reaction can only be approached by adding d:r.'op,,·i :3e

NaOH into a sucrose solntion in ,_,h:ich the r equired amount

of CaC1

... -.- .-,. ..

-

...

-

.. ,

__

_

r~naLt:S !:~tJ;3Ji~F1

r caclic·1·1 a,·,d r: cc,1;11>J c l c: c)c::.-lJ{_::ir i:,jT1;".' c,1ll 1.il' ,ii> h:: ii•c:r! 1: [ . ,. somc·,,:hnt m(;rc· t-L~\JJ 6 f'qt1-.i v,::.l cn t,, (Y:aOH + C:,CJ.,) ) pr:r Jf;l•.1,, ,_ ( ,··0rr t·'-·0_{,LlJ1d·'LJ''"} ·t.(J _{,.1.l_)OiJ.t} ,.,_ • . . ~ ~) J • - I t."I '.>0 p.-1 J:i s of' CaO p<.:r 1 (J(_i parts of· <'l' ,.,,.., ., • \ .:-> 'U'-"- / •

In this rc,act:i.0~1 the precipj tn lion J_s necc,;;-;;ar:i Jy

realized throuch Cn. ( OTJ) ;?, wh:i. ch s ecrns snrpri sing in r c,1_?;;1.cd

to the of t en adv~nccd theory of sacchar8te being acl:,orbcd

upon the surf'acc of' colloi dal unhydrat cd l il))e .

We believe that the chemical mechani sm of the pre -cipitntion of the saccharate may pass through a stage of'

Ca(OH)

2 in "statu na scendi" whereby this very rea ctive com-

-pound could bind sucrose. Tn favourof' this hypothesis we

may cite the follov.ring arguments

As wel l kno,,rn, saccharat e cannot be precipitated wltli

hydrat ed lime. Fresh qu ickl:ime is to be us eel. The above

mentioned building of saccharate by usinc;_ CaCJ.

2 and NaOH

shows however that Ca(OH)

2 may act as precipitati11g agent,

provided its formation takes place in the medium ,~~ich

contains the sucrose. I t therefore looks acceptabl.e that

the reaction with CaO proceeds through an identical

reac-tion mechanism.

Laboratory trials were made in which we used dime

thylf'or-mamide (D~fl~) instead of water as dissolvine agent for the saccharose. By adding J.ime powder to this solution, no

sacchara te could be precipitated. A precipitate orily

up-pear-3 when the solution contains at least

75

~b

wat er

(and

25

~~ DJ,U•'). This seems to indicate that the r eact i on

isn1_{t just a combination between CaO and .sucrose, but that.}

the water plays an important part, probably for hydra t ine:

good rc·sldt s. 'J'!d:j h itJi ~;pccif':ic ::;l,r1';:icc promotes an :ii;

-s tan. tr1rLcous col] i_ sion b0. h>'L'C'll tln' hy d:ca t cd Ji.i:tc p~i.r t i c J_

c:,

,

and the u.iss0lved sucroE,c~ molecll1C:3 1:hich C:811 then e;_,td.ly

be boe11d t o tl1e nascent 1irne be:Corc: i t has l ost i ts h:i c_J1

energetic state.

The l ime powder must be composed of' easily disposable

lime , whi ch means i t may neither be overburnt , nor may i t s

surface be coated by a thin layer 6~ carbonate. Such con

-ditions slow down the rate of slaking of the lime and in-hibit a possibl e thermnl "explosion" of the lime part i cle

into very fine fragments , each of ,d1ich may then r eact

with sucrose.

When adding the quick l ime to the sucrose solution, one

must cr eate a maximal speed of dispersion of the limf~. In

doing so, the chances of collision between the nascent

Ca(OH)

2 particles and the sucrose mol ecules are again e n-hanced.

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FigL1.::.'e 2 g:i. ve ,_; a schcm<1. tic d r~,,·:Lng of' tll e i:r.s tal L :-

-tion as nov set up in our Tirlcrnont plant (daily capacit :.·

of' 100 T molasses).

The molasses is continuously diluted. in tank (1 ) to

about 12

5

0

sugar content by addition of' water, which has

previously been cooled to 8 to

10°

C

(50

°

F) . As wjll be ex

-plained later, the lime slurry collected in the settler (6),

is al so broueht into tank (1). From here the diluted molass0s

solution f'lows into the reactor (2) which consists of a ho

--rizontal U shaped trough equipped with an agitator. The

vo-lume of the reactor is calculated as to ensure about 15

mi-nutes reaction time.

Two recirculations are provided for the reactor

the first (about

10/1

volumes) is extracted from about

the middle of the reactor and reintroduced at

1/6

of the

total length. This flow is used :for the dispersion of'

the lime powder. The qui cl\ lime from the grinder is

weighed batchwise. From the vessel of the weighing

appa-ratus

(J),

a screw conveyor

(4)

brings a continuous flu~

of lime upon a

800

mm wide overflow of' saccharate

solu-tion. Due to the high rate of recirculation i t is

instan-taneously mixed with the liquid phase and finely

dis-persed through the centrifugal pump . (5) bef'ore being re-introduced into the reactor.

- the second recirculation (about

J/1

volumes) leaves t~c

reactor at the bottom of the last part and is reintrodu

-ced very close to the poj_nt where the fresh diJ.uted

mo-lasses solution enters. This flow is sent tllrou~h an

Alfa-Laval heat exchanger where i t is cooled to about

SETTLER

MILK OF .

--ti-SACGHARm

i> WASTE WATER

This f'J ow cooJs the r c:act inG :nc:·d_i liil! a11d en,11Jlcs tlic· rc·

-ci.rcuL·,.ti on o:f a l n.rc-e nrnount of prccipita·tccJ. nuclcj_

o

r

sacch;-irate . Al tJ1ouGh the crystalline naturG of' the

saccharate i s most unlikely, th0s~~ nuclei play an i mpur

-tant port in the growth of the particles ,,,J.ich are Lull

::-up into l arge conglomerat es insterld of rema ining in c1

finely divided state as is met in a batch Steffen process.

Due to the described recirculation of mother l iquor

and saccharate, the fresh molasses s6lution, although ini

-tially only diluted to 12

1

b

polarisation, is immediately dis -persed in a medium with low sucrose concentrati on. This hcls

proven to be a very important factor for the success of the

operation. Actually, sugar concentrati on of the surrounding

liquor changes only within narrow limits between the entry

and the outlet of the reactor. Temperature in the reactor ne -ver exceeds 11 to 12° C (52° F) .

After an average retention time of 15 minutes, the

saccharate leaves the reactor and is brought into a smal l settling tank

(6)

where i t stays for about

5

more minutes .

This settler, the use of which has been patented separately,

plays a very important part which will be detailed some,,ha t

more.

We have mentioned already that, even with ideal

con-ditions for all experimental criteria, as lime quality, t efil

-perature, lime addition, etc . • • . , the ~equirements of l ime

per unit of sucrose exceed largely the stoichiomet r ic data.

The difference will be as greater as more imperfections occur

in the practical process (unsufficient grinding, overcalci

-ned lime, bad dispersion, etc . • • . ). Close examination of the

outflowing mixture from the reactor showed the presence of

signi~icant amounts of unslaked lime particles which escaped

"

and the J lr.1c con$l.llllption ,.;ouJd be j ncr cascd hy tlte c,.,:i·_:.'._

ponding n.moun-L. These lirnc p;:1.rticJ('S proved to seLi..i(' v c .. '

-quickly, for which renson the settler has been pro¥id~d i~

the operat i onal scheme . Hy sendin& the slurry from th0 .==Cl' L

-ler uack into the diluted molasses (Tank 1), the sJo.1-::.inf,·

tendency i s largely enhanced. due t o the prDsencc oi' i'ree

sugar ,,·hich catalyzes the dissolution.

I t may, moreover, be r ecalled that - as seen in figure 1

the first fraction of' lime

(2

moles/mole of sugar) i s in

any case dissolved and may thus be added as Ca(OH)

2 • This

means that the recirculated _slurry of lime, even when pa

rt-ly already hydrated, yet plays a i'ull part as react ing ,~gen 1:,

as long as the ratio of 2 moles/mole of sugar is not excee

-ded.

The outflow from the settling tank is brought to a

continuous

7.5

m2 belt filter

(7),

where the mother liquor

is removed and a thorough washing of the saccharate cake is

realized. The saccharate slurry is brought to the sugar

fac-tory, whi_le the waste water may eventually be sent to a hot

, - - - -

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IN 'l"IJ1: H.T. CO?\TJ.XUOtiS

I

EXPEHD!J~j'~T:\T, rmSUL'l.::;' HEAClfFD

..._ _ _ _ _ _ __ _ _ __ _ _ s_A_c_c_.1_11_\_r~_11_1_'1_·~_1

1

_1{_0~-E-s_~_ ___ _ _

J

We l ist hereai'tcr the figures obtained during the

1971 campaign with the equipment instqllcd at the Tirlemont

plant. We believed i t to be interesting to present three

serials of data.

The first vertical colunun r eproduces the average

figures collected during the whole campaign. They include

results obtajncd during periods where the equipment was

in-tentionnally run above its normal capacity. This was parti

-cularly true for the bel t f i l ter . . Its 7.5 m2 surf'ace should

correspond at a maximal daily throughput of' about 100 T.

The second and third columm relate to periods with

lower capacity during 1rl1ich the lime consumption was redu

-ced even as low as 85 parts per 100 parts of sugar.

TABLE 1 T Molasses processed/24 h Diluted Molasses Brix Polarization Apparent Purity Lime po1.,rder used Total Cao Active Cao

%

particles ) 74 ,l[m Temperature

g Cao used% sugar in molasses

Filtrate: Polarization Saccharate cake: Purity

±

100 T 75 T 80 T 20, l~ 12,0 12,0 12,0 58,6 90

~b

~from 77 to 92) 78

~

lo

from 70 to 81) 1 5, L~ 40° C 118,7 105 85,0 0,85 0,71 1 , 4 5 93,5 93,3 93,5 ( +'-~· .. r,,., r:' .,__

_9~

_,o)

.l...L V U l _{y,::.,' -'} l , U

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[ '0''('11

1'"

T()

>TC:.::

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, l\ . , _, :"") t - ,!.'\,.,

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--

----

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-The mi1J1er-i.cal dnta l i s ted in Tab1c 1 cl earJy sJ,ow

the importa.nt advantages o:f the new !'38ccharate process ,

Due to the effici ent way of introducing the pow

-dered quick lime into the solution and to the r ecovery of

any unreacted Cao by means of- the sett l ing t ank, the lime

consumption can be reduced in a very significant way,

The continuity of the process, as wel l as the pre

-cipitation in a medium with low sugar content and in the

presence of many nuclei of saccharate , enable the building

of large particles which filter very easily, Microscopic

analyses of the saccharate showed that the particles i ssued

from the continuous process easily r each

JO

to

l.~o

~

ter, compared to 1 to 2

f'-

m for the batch process.

Filterability was measured and proved to be 6 to 10

better than in the batch process.

m

diame-ti.mes

Such large particles are characterized by a low

specific surface, which r educes considerably the adsorption

of impure mother liquor, Moreover, the good filterability

enables a thorough washing , Both factors explain the high

figures obtained for the purity of' the cake, I t is evident

that with such high value on the purity, much less non-sugar s

are recirculated to the sugar house whereby the suppl(!me

n-tary charge of the saccharate process on the boiling scheme

gets much smaller,

Besides the above mentioned advantages, i t should

be underlined that the continuous R.T. desugarization calls

for much less· equipment in comparison to the batch process.

"

For a dnily ca.p8.city o:f 100 'i' 1110Jnsses, on0 11;::;

inst~lled nt Tirlcmont : Batch Process

J

coolers of 8 mJ each

J

rotary f i lters of JO m2 each

--,

Continuous Process

I

- - -·

---1 r eactor of 12 mJ 1 settling ta.nk of

5

mJ 1 belt filter of

7

.

5

m2

Fina lly, i t should be underlined that the molasses

solution is diluted to only 12

%

sugar (instead of 6

16

in the usual process). The volume to be cooled is thus reduced

to the half and when the waste waters have to be concentra -ted, the steam requirements for the evaporation are nmch l ess.

We believe tha.t all these advantages make the R,T.

continuous saccharate process very attractive since i t

ena-bles a recovery of the molasses sugar in the most economic