circulation in vacuum p ans

8/17/2019 Circulation in Vacuum P Ans

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PRO-56-1

C ircula tion in V acuu m Pan sBy ALFRED L. WEBRE,1 BURLINGTON, N. J.

Th is paper has for i ts object an expos i t ion and discuss ion

o f n e w fa c t s b r o u gh t o u t i n c o n s e q u e n t i a l t e s t s an d s tu d i e s o f th e d e s i g n a n d o p e r a t io n o f v a c u u m p a n s i n th e s u g a r

i n d u s tr y, a n d th e l o g ic a l c o n s e q u e n c e o f th e s e d a ta i n th e

readoption o f the ra ther o ld an d genera l ly d iscarded

principle o f mech anica l c ircula t ion.

I t i s d e m o n s tr a te d b y a c tu a l o b s e r v a t i o n s th a t th e

h e a t i n g o f th e h e a vy a n d v i s c o u s m a s s e -c u i t e c i r c u l a t i n g

th r o u g h th e tu b e s o f th e p a n s is n o t a t a l l u n i fo r m , a s

has genera l ly been assum ed. Du ring cr i t ica l per iods o f

operat ion, s tream l ine f low ob ta ins an d the liquid i s

heated only a t the skin . Recorded dif ferences o f tem

perature in the m asse-c ui te show observable varia t ions o f

fu l ly 30 F, w i th a l l indica t ions o f unobservable d i f ferences

o f a s m u c h a s 5 0 F. W i th m e c h a n i c a l c i r c u l a t io n , a s p r o

p o s e d b y th e a u th o r , th e s e v a r i a t i o n s a r e c u t d o wn to a b o u t o n e - fo u r th o f th e i r i n te n s i ty .

This radica l ly d i f ferent operat ion makes poss ib le new

a n d i n te r e s t i n g r e s u l t s , a m o n g wh i c h m a y b e m e n t i o n e d

u n i n te r r u p te d g r o wth o f s u g a r c r y s ta ls , u n i fo r m i ty o f

p r o d u c t s , g r e a t ly d e c r e a s ed fo r m a t i o n o f c o n g l o m e r a te s ,

absen ce o f fa l se gra in , fas ter operat ion, low er-s team

pressures.

O n e o f th e v er y i n te r e s t i n g d e v e l o p m e n ts a l so b r o u g h t

o u t i n th e p a p e r i s a m e th o d o f c o o l in g l o w-g r a d e s t r ik e s

i n th e p a n s th e m s e l v e s in th e s h o r t p e r i od o f fo u r h o u r s,

th u s e l i m i n a t i n g th e t i m e -h o n o r e d c r y s ta l l i z e r f r o m th e

raw-sugar industry .

F i g . 1 C a l c u l a t e d S p e e d o f C i r c u l a t i o n f o b “ C ” S t r i k e

THE subject of circulation in vacuum pans has

been bad ly neglected and th e explana tions offered by the average su gar boiler reveal th is fa ct. He will

assure you that this or tha t pan has good or bad circulation, and makes good grain or bad grain, as the casemay be. He will also tell you tha t good circulation is

indispensable, that you cannot work without it, and soon. Then you ask him how fast mu st the circulation beto give good results. This is anothe r story. I t mu st befast, good, uniform, active, etc., but how fast he doesno t know. His chief does not know. The man who

taug ht his chief does not know. As a matte r of fact, no body knows.

Why not? If circulation is as importan t as it is sup

posed to be, somebody should be able to tel l how fas tit is. There is nothing in the literature about it. There

are many data relating to almost everything except circulation, which is always described in terms of glittering

generalities. And yet, every one agrees th at it is theone most important requirement of any pan. The truthof it is th at it is almost impossible to measure the speedof circulation, and that statement applies to evaporatorsas well as to vacuum pans.

D e t e r m i n i n g t h e S p e e d o f C i r c u l a t i o n

In attemp ting to determine the speed of circulation some

thorough tests were made on a good standard calandria pan, inwhich all determ inable facts were recorded. One of these was the

rate of evaporation in pounds per hour observed at ten-minuteintervals. Since the circulation is not measurable, it was decided to estimate it as follows:

1Engineer, U. S. Pipe & Foun dry Co., Burlington, N. J. Mem.A.S.M.E. Mr. Webre has been identified for the pa st thir ty yearswith the sugar industry as designer of evaporators, vacuum pa ns, andheaters. He is the autho r of a textbook entitled “Ev apo ration,” aswell as of many papers and bulletins on related subjects, presented before groups of su ga r techno log ist s.

Contributed by the Process Division and presented at the AnnualMeeting, December 3 to 7, 1934, of T h e A m e r i c a n S o c i e t y of M e -c h a n i c a l E n g i n e e r s .

Discussion of this paper should be addressed to the Secretary,A.S.M.E., 29 West 39th Street, New York, N. Y., and will be accepteduntil February 11, 1935, for publication in a later issue of Transac

tions. N o t e : Statements and opinions advanced in papers are to beunderstood as individual expressions of their authors, and not thoseof the Society.

It was assumed tha t on the average masse-cuite is heated in thetubes, goes up to the boiling surface, releases its heat by flashing,

and returns below the calandria throug h the downtake for a newcircuit. The temperature at the boiling surface and the temperature as the masse-cuite left the tube s were known. The difference between the two gave the temp erature rise. The specificheat of masse-cuite and the r ate of evaporation, which is the same

as the rate at which heat is absorbed by the masse-cuite as it passes throug h the tube s, were also known. I t was thereforeeasy enough by simple arithme tic to arrive at th e weight of masse-cuite per un it of time passing through the tubes, and from th at,the volume, since the specific gravity was known. Hav ing the

total cross-section of the tubes, the speed of circulation in feet

per second was eas ily calculated, n ot wi th absolute precision, butcertain ly closely enough. The results are shown in Fig. 1.

This chart was an eye opener! I t shows th at at its best, the

circulation in vacuum pan s is very poor. At its worst, it is notcirculation at all but stagnation, in which there are all sorts of

undesirable heating effects.

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TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS

F i g . 2 M o u n t i n g o f E x p e r i m e n t a l T h e r m o m e t e r

urth er evidence appears in a paper by J. C. Keane and E. K.

ntre presented before the International Society of Sugar Canehnologists at San Jua n, Puerto Rico, in 1932. Mr. Keanend that on low grades, sugars obtained from the pan when

, but before final concentration, showed 100 per cent higher

erability than sugars obtained after final concentration, during

ch time the circulation is poorest, and the heat injury, there

, greatest.Is t h e H e a t U n i f o r m ?

here is another point about heating viscous masses moving

wly through large-diameter short tubes such as exist in vacm pans. How can it conceivably be assumed tha t this heat

is uniform throug hou t the masse? There is no justification

atever for any such assumption, for, in the first place, thet conductivity of the material must be very low due to itsosity, and, furthermore, at these low speeds, streamline flowt exist, in which case there would be very little mixing.

T e m p e r a t u r e C o n d i t i o n s

was decided, therefore, to embark upon a special investiga to determine as far as possible just what temperaturesld be found in the tubes of these pans during operation.

After much thought, it wasdecided to confine observationsto one tube in the pan. The

scheme finally adopted was to

attach a ferrule to a tube near

est the shell of the pan, thisferrule extending perhaps 6 in.

above the upper tube sheet.See Fig. 2. The n a very ac

curate and sensitive thermometer was mounted in a proof

stick and lined up centrally

with the tube in question andas close to th e tube sheet as

practicable. An opening was

made in the side of the ferrule

pe rm itt ing the free passage of

the thermo meter and its mounting. With it the temperatureof the masse-cuite as it leftthe tube and in any part of

the stream could be observed.

„ O b s e r v a t i o n P r o c e d u r eo n V a c u u m P a n s

After the usual preliminary

runs, it was decided to observe only three temperature positions,

namely: (1) just inside the tube to show the skin-heating effect,(2) at the center of the tube to show the probable minimumtemperature, and (3) outside the tube in the body of the masse-

cuite. The in strum ent has its limitations, since the mercury

bu lb is ab ou t 7/s in- long an d, therefore, an y readin g shows theaverage for this length and not that at the local points to which

par tic ul ar inte re st is at tach ed . I t is, therefore, evident th a t in

every case neither the m axima nor the m inima could be obtained.In addition, when the thermometer is thrust to mid-position 2,

there is bound to be a w arping of the streamlines whose influenceon the readings m ust be appreciable. Moreover, while the reaction of the mercury was rapid, it was far from being instantaneous, and, for that reason, a certain lag was inevitable.

A standard procedure was used to compensate as much as possible for th e forem entioned inadequacies. Th ree positionswere accurately fixed, and templets were provided by which any

one could be spotted instantly . In all cases the instrumen t wasallowed to remain at each station for a period of two minutes,

with readings every ten seconds, after which a quick shift wasmade to the next, and so on, rotating definitely to the three positions for the entire duration of the strike.

F i g . 3 T e m p e r a tu r e O b s e r v a t i o n


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masse. It is easy enough to estimate the value of this force as amaximum.

If the foregoing premise is true, then the maximum circulation

would be at the end of the strike when the levels are high, andit is known that this is not so. Be tha t as it may, taking the panwith the masse-cuite six feet above the upper tube sheet, as anexample, then consider the following:

The pan thermometer during the test already quoted read 59

C (138.2 F) at six feet. It is also known th at the minimum tem pe rature at the boiling surface was ab ou t 124.0 F, since thevacuum was 27.3 in. when this observation was made. The

minimum temperature in the downtake could not be less thanthis. The difference, therefore, is 14.2 F.

Let us further assume that the change of temperature in thetubes was gradual thro ugh out their length. We can, therefore,

say that we have two columns of masse-cuite nominally 8 fthigh. The expansion due to this difference of temp erature isabou t 0.83 per cent. In o ther words, the difference of head between the two columns would be 0.83 per cent of 8 ft, which is

0.0664 ft, or 0.7968 in., as an absolu te maximum. Actually,through the tubes themselves it would certainly not be one-fourth of that figure.

How any such difference of pressure could produce an appreciable motion of thick, heavy masse-cuite through the tubes of a

vacuum pan is inconceivable, and this hypothesis must be abandoned.

T h e R e a l M e c h a n i s m o f N a t u r a l C i r c u l a t i o n

It has been shown tha t masse-cuite as it leaves the tub es of the pa n on its way up is not unifo rmly he ate d, but , on th e contra ry,contains many spots at different temperatures, the general average of which is represented by the readings of the pan thermometer. As the masse proceeds upward , even in the tubes them

selves, when these spots reach a level where the local vacuumcorresponds to th eir boiling temp erature, a flash takes place withthe release of a vapor bubble whose size corresponds to the mag

nitude of the ho t spot. This volume displacement causes asudden readjustment of position, permitting the vapor so liber

ated to come into contact with some of the cooler masse-cuite,when immediate condensation takes place.

This procedure is repeated on the way up, gradually lowering

the temperature of the hot spots by raising the temperature ofthe cold spots through these little explosions and condensations,ultimately attaining the general average temperature indicated

by the pan thermo meter. Th is average exists in unifo rmity atthe level of uninterrupted ebullition, probably limited to a zone

extending 12 in. below the surface of the masse-cuite in the pan.There is no doubt in the author’s mind that this is the main

motive force causing circulation, for whereas the individual dis placements are small in volume and du rat ion , theyare of infinite number, and extend from the tubes

themselves to the very top of the masse. As fastas one flash bubble condenses, another takes its

place near-by. I t can be easi ly seen, therefore,that the weight of this agitated masse-cuite in theouter periphery of the pan is inevitably lightened

as compared with the downward moving centralstream which is free from flash spots.

T h e D a n g e r s I n v o l v e d

Incidentally, it might be noted that these intense volumetric disturbances caused by these so-called flash explosions, within reasonable limits, inno way injure the operation of the pan ; quite the

contrary, they cause a continual rapid change of position of the sugar cry stal s in thei r su rroundingmolasses or syrup, thus permitting a more com

ple te exhaustion .There are two dange rs involved, however. One

is the fact that if local temperatures are too high,

there is likelihood of a considerable color increment. The other is th at undoubtedly a point ofunsaturation is reached, wherein sugar actually

redissolves into the molasses or syrup. This is

not only a destruction of work already done, but,in addition, is likely to bring about the formation of conglomerates and possibly false grain.

There are well-defined limits to all of this.

C o n d u c t i v i t y o f M a s s e C u i t e

The probable low heat conductivity of masse-cuite has been

mentioned. The autho r is opposed to laboratory experiments

in these investigations, as the change of proportions involvesa change of the problem. W ith the experimental thermometer,therefore, an interesting test was made tending to shed light on

this subject, as follows: A stand ard “C ” strike was boiled untilthe level of the masse was about 4 ft above the top of the uppertube sheet. Then the steam was shut off, the vacuum dropped,

and readings of the experimental thermometer were begun, butthis time, at stations 1 and 2 only. The idea was to find ou t howlong it would take for the temperature at the inner periphery ofthe tube to equalize with tha t at the center. As in the former

case, the thermometer was kept at each station for two minutes,

and readings were taken every ten seconds. Fig. 4 shows theresults, ind icating tha t it would require a t least one hour or morefor these temp eratures to stabilize.

W hat then is the significance of this new fact? Evidently,the penetration of heat into the masse is very shallow indeed,

and there mu st be purely a skin-heating effect. If such is thecase, it is possible to reconcile the data of Fig. 3.

The skin-heating effect brings out anoth er point. Wh at re

tard s circulation is the viscosity of the masse. Viscosity is al

ways an inverse function of temp erature. If the skin of thestream flowing through the tube has a greatly reduced viscosity,

F i g . 4 T e s t S h o w i n g E f f e c t o f S t o p p in g P a n


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then we have the equivalent of a lubricant at this point, and thehigher the temperature, the greater the lubrication. In theabsence of boiling in the tube, then this masse-cuite would passthrough as an oiled sausage. This is perhaps an exaggeration,

bu t it gives a bet te r picture of the conditions as they are.

P e e d D i s t r i b u t i o n

Let us now consider the usual ma nner in which feed is admittedto the pan. This feed is of relatively low density , probably 65

brix , whereas the strike is at 90 brix or be tte r. Syrup is inv ari ably admitted through one large pipe at one point, an d it is verydifficult to get a complete and rapid mixture. This is one of theoutstanding defects, but pa rticularly on low grades. In theabsence of an immediate thorough mixture, it is easily possiblefor some of this light liquid to float at once to the to p, thus bringing about a poor operating condition, as the two liquids are certainly not easily miscible.

A much more desirable method would be to introduce the feedthrough a perforated coil located under the bottom tube sheetso as to compensate as much as possible for the difficulty pointedout, and thus minimize the undesirable results.

B a s i s o f P a n O p e r a t i o n

In view of these new field results, it is now advisable to review carefully the basis on which these pans are operated, so as

be tter to appre cia te thei r true significance, and it would do noharm to start at the beginning.

TEMPERATURE DEGREES"C"

The vacuum pan works under constantly changing conditions.The first step is to concentrate the syrup in limited quantitiesuntil grain is obtained, then to make these individual crystalsgrow in size, but n ot in num ber, un til the volume of the full strikehas been reached. The fact must be emphasized tha t the crystals must grow uniformly without the formation of new andsmaller ones. It is essentially a batch operation which starts with

the liquor level just above the upper tube sheet, and finishes

F i g . 5 Su p e r s a t u r a t i o n C u r v e s f o r Su c r o s e B a s e d o n H e r z -

f e l d ’ s So l u b i l i t y T a b l e s

F i g . 6 B o i l i n g - P o i n t - R i s e , Su g a r So l u t i o n s

with a strike or a skip whose volume has grown perhaps six orseven feet above the original starting point.

Unlike most soluble salts, sugar crystals do not form in thesolution as soon as the saturation poin t is reached. Indeed,

grain will not form until the concentration has been pushed quitea bit farther. Even after the crystals are formed they will notgrow rapidly unless a certain am ount of supersa turation is maintained. The man ipulation of densities eviden tly requires skilland knowledge, for if too high concentrations are maintained,new small crystals will form and these will have to be destroyed

by bringing the syrup below saturatio n, which involves not onlyloss of time, but also the heat expense of evaporating the waterused for dilution. The growing densities and crystal-forming

densities must be carefully controlled for satisfactory operation.

Sa t u r a t i o n , S t t p e r s a t t j r a t i o n , a n d U n s a t u r a t i o n

This is all delicate work, totally impossible in inexperiencedhands, and must be carefully analyzed to be properly understood.

The solubility of sucrose in water is the foundation. For this,tables by Herzfeld are used. Fig. 5 has been plot ted from thistable. In it ordinates represent the solubility of sucrose in parts


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918 TRANSACTIONS OF TH E AM ERICAN SOCIETY OF MECHANICAL ENGINEERS

pe r hund red par ts of wate r. Th e abscissas give the tempera

tures within our range. The heavy line is the solubility or saturation . Above it are lighter lines showing the different levels ofsuper saturation, and below, similar lines of unsa turation. The

effect of a change of temperature on a solution of any concentratio n can be easily and quickly read. The c hart is for pure sucrose solutions, and would have to be modified if impurities were

presen t in mate ria l proportions.

B o i l i n g - P o i n t R i s e

Fig. 6 shows the rise in boiling points of sucrose solutions abovethe boiling points of water. This is usually called the “boiling-

F i g . 7 B o i l i n g T e m p e r a t u r e s i n P a n s f o r D i f f e r e n t V a c u a

a n d D e p t h s — S a t u r a t i o n 1 . 20 , P u r i t y 1 0 0 P e r C e n t

po int rise,” ab brev iated B.P.R. Th e data were taken fromClaassen.

P a n T e m p e r a t u r e s

Figs. 5 and 6, supplemented by the steam tables, make it pos

sible to plot the very interesting diagram of Fig. 7, showing themaximum temperatures that can be attained in the boiling masse-cuite in a vacuum pan while it is in operation, for any vacuum,and a t any dep th below the level of the liquid.

Fig. 7 is based on a supersaturation of 1.20, as recommended by bo th Claassen an d Theim e, who are recogn ized au thor itieson the subject. This 1.20 point is subje ct to some variations.It may drop as low as 1.15 for high-purity syrups while makingfine granulated sugar. It may also reach 1.30 for low-gradecrystallizer strikes. For the present, it is assumed th at the 1.20

suffices as fairly representative of average operating conditions.The ordinates represent temperatures, but in this instance

they are plotted in the reverse order for convenience. The

abscissas give the absolute pressures and vacua under which the pan is opera ting. Th e to p line shows the vacuum tem pe rature . Next comes the boiling temperatur e a t the surface o f the masse-cuite, which was obtained by adding the B.P.R. to the vaportemperature for a supersaturation of 1.20 at the particular tem

pe rature . I t mus t be no ted th at the B.P.R. is not th e same for

different vacua, even though the supersaturation is constant, because the solubility of sucrose varies with the temperature andthe same supersaturation at various temperatures involves dif

ferent concentrations, and hence different B.P.R’s.The n ext line below shows the tem perature required for ebulli

tion at a dep th of one foot below the surface. This is arrived at by correctin g th e vacuum in th e pa n for th e superposed hydro static head, and then adding the same B.P.R. as before for the1.20 supersa turatio n. The succeeding lines give this boiling

po int for de pths of 2 ft, 3 ft , an d so on, to 14 ft.The dotted line is significant, showing the depth at which

ebullition temperatures have brought the saturation down to

unity. The modified Herzfeld chart makes it possible to plotthis curve. Above this critical line is a condition of supersatu

ration in which the sugar crystals will grow, and below it, a condition of unsaturation in which the sugar crystals will dissolve.In the latter event, n ot only will the size of the crystals decrease,involving a destruction of work done, but at the same time theconcentration of the syrup or molasses will increase, and this, ifcarried too far, can easily bring about the formation of “smear,”false grain, or conglomerates when the masse reaches the boilingsurface, as at this point the supersaturation will be suddenly in

creased beyond the safe limit. Very high strikes always involvethis danger, as can be readily seen.

V a c u u m V a r i a t i o n s

Pan operators always complain vigorously when there areappreciable changes of vacuum during the operation. A study

of Fig. 7 will show the reason for this. Assume, for example, a

drop of vacuum from 26 in. to 24 in., and after a period of time,a return to the original 26 in. The boiling temp erature chart,

Fig. 7, shows that the boiling points at 1.20 supersaturation are140.0 and 157.5 F. Going back now to the Herzfeld diagram,

Fig. 5, the first change lowers the supersaturation from 1.20 to

1.075. Nothing happ ens except that the pan slows down alittle until the standard supersaturation at the new vacuum is

attained. The second change, however, when the vacuum issuddenly brought back to 26 in., boosts the supersaturation to

1.325 and, with this, the formation of false grain is inevitable.All necessary precautions should be provided to avoid vacuum

variations during the making of strikes in vacuum pans. Itwill pay big dividends in improved qua lity of sugar, and in steam

saving as well.

S u m m i n g U p t h e D a t a

The experimental and theoretical studies have shown up manyweak points in no uncertain manner, and we are now ready to

draw conclusions:(1) The so-called good circulation was no t so good after all.

At best, it was poor, and at worst, it was stagnation.(2) Local overheating attains unbelievable proportions tha t

bring the syrup or molasses in th e masse-cu ite in the tub es much below satu ra tio n, pe rm itt ing red issolu tion of cry sta ls with destruction of work done. The consequent additional danger of

color increment is also present. With the increased concentra

tion of syrup above normal, there is also the imminent probabilityof false grain and conglomerates.


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F i g . 9 T e m p e r a t u r e O b s e e v a

fact that the temperature changes noted by means of the experimental thermometer have been cut down to about one-fourth oftheir original intensity. It is too much to expect complete elimination of variations, which cannot be achieved except at abnor

mal expense altogether unjustified.

E v a p o r a t i n g R a t e s

At the beginning of the strike, when the level is low, evaporating rates, with a nd w ithout mechanical circulation, are abou t thesame, but as soon as the level passes 3 ft above th e top tube sheet,there is a decided advan tage in favor of the circulator. This advantage gradually increases as the strike goes up, until, towardthe end of the operation, the rate of evaporation with the circu

lator is abou t five times as fast as without it. Another pointworthy of comment is that these new pans, since the circulationis independent of the steam pressure, can operate at any heightabove the top tube sheet. Also they can run with much lowersteam pressures than before. Actually, time and again, “ C ”

strikes have been made with steam at 5 in. vacuum and the levelshave been carried as high as 8 ft above the upper tube sheet.In some of the refineries the strike level has been carried 12 ftabove the heating surface, with perfectly good results.

P o w e r R e q u i r e m e n t s

The power required for driving the mechanical circulator ismuch more than that required by the former design, because ithas greater capacity, but it does an incomparable job in terms ofthe old units. However, the power capacity is not a t all excessive. The load is relatively small until final concentration takes place, at which there is qu ite a peak, the du ratio n being perhap s15 min for low grades and only about 3 min for refinery work.

The u nit has to be designed for maximum conditions and hence

it is of rugged construction. Whe n the cooling of low grades isundertaken, the power increases greatly owing to the higherviscosity of the masse, and when such an operation is contem

plate d, ei ther the power mus t be increased or th e speed reduced.

B a s i c C o n d i t i o n s

In th e m aking of a strike there are two factors, one being theconcentration of the sucrose solution by evaporation and theother the absorption of this sucrose by the sugar crystals kept insuspension by the circulation within the pan. Evap oration will

be as fast as the heat ing surface under the existing tempe ratureconditions will permit, but must not be faster than the crystalswill absorb the sucrose thus made available, otherwise the supersatura tion will increase until it passes the critical limit an d false

grain will begin to appear.The question then is, How fast will the crystals assimilate

this sucrose? It has been established by investigators th at withhigh-purity solutions, this rate is very fast indeed, probablyfaster than it is possible to evaporate. Wh at factors affect therate of sucrose absorption by the crystals?

S p e e d L i m i t s o f C r y s t a l G r o w t h

It is known that this rate is directly proportional to the sumof the surfaces of the crystals . Fo r any solid structu re of definiteformation, it is a fact that the weight varies with the cube of itsdimensions and the surface with the square of its dimensions.For a given weight of sugar crystals, therefore, the sucrose-absorbing surface will be indirectly proportional to their individualsizes, and this means th at the smaller the crystals, the greater therate at which they can absorb sucrose from a solution of a givenconcen tration. In othe r words, the finer the grain formation,the faster the strike can be made without danger of bringing outfalse grain. False grain never is found in a fine granu lated strike.If confectioner’s “A” is being made, however, the problem is

entire ly different. Since the crystals are very large, the sum oftheir surfaces is very small, and the operation is difficult becauseof the inability of these crystals to absorb the sucrose from thesyrup as fast as evaporation makes it available, even at slowrates. In the ordinary pan, under these conditions, evaporationwould have to be slowed down so much that it would kill thecirculation and the crystals would drop out of suspension. The

situation must be remedied by feeding water to evaporate intothe pan to maintain circulation fast enough to keep the bigcrystals moving. This is paid for in terms of the steam requiredto evaporate this water. With the mechanical circulation pans,since the circulation is independent of the rate of evaporation,it is believed tha t it m ay be possible to reduce the amou nt of this

boiling w ater, if not elim inate it altogether.

C o n d i t i o n s a t t h e B e g i n n i n g o f a S t r i k e

An interesting phase of the same condition is the period atwhich grain is made in the pan, and the immediate subsequentoperations . As has been said before, the problem consists in themaking of a limited number of crystals and the n causing them toincrease in size and n ot in number.

It is not within the scope of this discussion to cover the entiretechnology of the making of strikes. I t will suffice to state th atthe first step is to concentrate the syrup to a point of excessivelyhigh supersaturation, called the “graining point,” at whichcrystals begin to form. This can be done in a number of ways,the simplest being to let the concentration proceed until smallcrystals begin to appear and to allow those to increase in numberun til a sufficient qu an tity is presen t. After this stage is reached,the formation of new crystals mus t be arrested. This is known


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PROCESS INDUSTRIES PRO-56-1 921

as “hardening the grain.” Knowingly or not, this is accom plished by lowering th e su pe rsatu ra tio n from its gra in forming

1.30-1.40 to 1.20 or less, at which new crystals will not form.Time must be allowed for the fine crystals to grow in size until

they have sufficient surface to absorb sucrose as fast as it is released by evaporation, without crowding the supersaturation to

the poin t where new crystals will again form.At this point, mechanical circulation has been found to be

valuable, for intense movement can be maintained withoutevaporation, and hence, the rapid growth of the infinitesimalcrystals can be effected without additional concentration whichmight bring out new grain. It is a critical condition whichrequires very careful attention in the ordinary pan. As the sizeand, therefore, the sucrose-absorbing surface of these crystals,increases, the evaporation is speeded up correspondingly, untilsoon the danger is past, inasmuch as the possible rate of sucroseabsorption is greater than the rate at which evaporation makes itavailable, and it is no longer possible to p ush th e su persaturation

to the point of spontaneous crystallization from the syrup. Asthe crystals increase in size, their sucrose-absorbing capacity becomes greater and g rea ter, and all imminent danger has passed.This does not mean that it is not possible to bring out falsegrain by sudden tempera ture changes. This can always bedone.

The sum of the crystal surfaces at the end of the strike ismany times greater than it is at earlier stages, and this is whatmakes possible the rapid final excessive concentration withoutdanger of bringing o ut false grain.

The reaction is a balance between the rate of evaporationand the speed of crystal growth, governed by the sup ersaturation,

which adju sts itself to these two factors.

P u r i t y

Pu rity has a great influence on pan performance. If the purit y is low, then it is necessary to change the film surro undingeach crystal as fast as the available sucrose in this film is absorbed, so as to permit new richer molasses to come in contact

with the grain. It is because of this effect tha t low-purity strikeshave to be made more slowly than high -purity strikes. Natu rehelps here, for low-purity syrups are of higher viscosity and theevaporation is slower with the same steam-temperature condi

tions.

M o v e m e n t

Evidently, to effect rapid changes of the molasses film surrounding the crystal, motion and agitation are required, and are

obtained by good circulation. This is another factor in operations and probably the most imp ortant. W ith fast mechanical

circulation, co nstantly maintained, irrespective of steam pressure,rate of evaporation, height of strike, purity of masse-cuite, or

anything else, better results than before should be obtainable.

N e c e s s a r y P r e c a u t i o n s

It would be incorrect to say that no precautions are necessary

in the operation of mechanical-circulation pans. One in particular should be mentioned. I t was stated previously tha t atthe latter stages of the strike these pans are about five times as

fast as pans witho ut mechanical circulation. It has been observed that at the very end of low-grade strikes, when the usualfinal conc entratio n is given, a sme ar sometimes appears. Thisis due to the fact that the increase of supersaturation is too fast

for the proper absorption by the sugar crystals. Another factorusually aggravates this condition. When the final concentration is given, the ra te of evaporation decreases due to the increased

viscosity. If the pan has a fixed rate of injection on its individualcondenser, and this injection is not reduced, the vacuum will

rise perhaps one inch, which will reduce the temperature, thusincreasing the supe rsaturation beyo nd normal. These two

effects, namely, added concentration and simultaneous decreaseof temperature, push the supersaturation beyond the criticallimit, resulting in the smear. The remedy is obvious, and with

a little intelligent attention, there should be no trouble from thissource.

C r y s t a i .l i z e r E l i m i n a t i o n

After the details of the mechanical-circulation pan had all been worked ou t, it developed th a t since the forma tion andgrowth of sugar crystals took place und er radically altered conditions, it mig ht be possible to do the work of the crystallizer in thenew pan.

Obviously, for proper exhaustion of molasses in the low-gradestrikes after the normal work of the pan has been accomplished,

certain basic reactions are necessary. The masse-cuite must becooled gradually and uniformly at a rate no faster than thesugar crystals can absorb the sucrose made available by the decreased solubility due to reduced temp erature. The speed at

which these crystals can absorb sucrose from the molasses inturn depends upon how fast and how thoroughly the exhaustedfilm surrounding them is moved away, as mentioned previously,and that in turn depends upon the mechanical agitation of the

masse, and the tenacity of the film, which is a function of itsviscosity. It is known th at the viscosity is greater as the temperature is reduced, but this temperature reduction squeezes out sucrose from molasses, since its solubility curve is direct, and resu lts

without cooling cannot be hoped for.The re are thus conflicting tendencies in the cooling of the masse-


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922 TRANSACTIONS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS

F i g . 10 T y p i c a l C r y s t a l l i z e r E l i m i n a t i n g O p e r a t i o n

cuite. One is to make more sucrose availab le for crystallizationdue to reduced solubility, and the other is to make it moredifficult for the crystals to absorb the sucrose due to increased

viscosity. A compromise po int is necessary. In practise it isfound tha t it does not pay to carry the cooling too far and manyauthorities agree that 100 to 110 F is about the economic limit.

The task then is to cool the masse-cuite from its normaltemperature at the end of the strike down to 110 F, at thesame time building up the liberated sucrose upon the surfacesof the existing sugar crystals. Three ways of doing this are

available.In the first place, cold water could be passed through the

calandria of the pan with the circulator in motion. This is notvery attractive, because it is impossible properly to distributethe cooling water throughout the calandria, and unequal cooling

would result, too much at the water inlet and too little at theoutlet. If there is unequal cooling, trouble would result immediately because cooling increases the viscosity of the masse,

and the tubes having cool masse-cuite would stagnate, while thecirculation would move rapidly through the tubes having nocooling. This, in turn , would bring abo ut ab rup t changes in

local temperatu res and inevitably false grain. This method has been tried with ind ifferen t success and does not offer likely possi

bilit ies.The other method is to cool by raising a high vacuum on the

strike while being circulated, thus lowering the temperature byflashing. This, of course, would be perfectly uniform andgradual, and could be controlled by regulating the vacuum. Theflashing involves evaporation and evaporation brings about in

creased concentration and, therefore, increased supersaturation.It is necessary to dilute the strike during this procedure both toavoid false grain and to keep down the horsepower required forthe circulator drive. It has been found tha t there are too many

adjustments involved and in the end the results wanted are notobtained.

However, flashing may be used in another way. It is understood that there must be no temperature shock in this cooling,which is necessarily gradual at a rate commensurate with theability of the crystals to absorb th e liberated sucrose. Themethod is to fill the calandria with hot water at a temperatureapproximating that of the strike, which involves no shock.Then, raising vacuum in the calandria only, the point is reached

at which this water begins to boil. The heat of the masse-cuitecirculating on the inside of the tubes is transmitted into thewater, and is thus dissipated gradually, uniformly, and withoutshock, at a rate which can be controlled, since it is possible toregu late the vacuum. A 6-in. pipe was installed in the calandria

extending up on the inside within 4 in. of the upper tube sheetand this pipe was connected to an auxiliary condenser. In thespace of four hours, the cooling of the strike can be completed,and th e final molasses made ab out the same as would be obtainedin three days in the crystallizers.

R e h e a t i n g

This operation was supplemented with one of reheating inthe pan before dropping to the mixer. Wa ter was dumped out

of the calandria and steam was turned on at a vacuum of about18 in. The tem perature rose in about a half hour to 125 F.The p urging of the low grades at this temp erature is much quickerand more thorough due to the reduced viscosity of the molasses.

There is no redissolution of sucrose, since the concentration of

the molasses is no t lowered below satu ratio n. One raw-sugarmill has used this method exclusively for the last three or fouryears. Fig. 10 shows the record taken from this operation and

indicates all the temperature changes, as well as the gradual exhaustion of the molasses. The recording amm eter gives the corresponding changes of viscosity brought about by these changes

of temperature.

F i n a l , C o n c l u s i o n s

Mechanical circulation, properly designed and properly ap plied, is now an accomplished fac t. Insta lla tio ns have been in

operation long enough to establish the following possible per

formances:

(1) Thorough and uniform distribution of feed as it entersthe circulating masse, avoiding local dilution or segregation by

differences of gravity.(2) Accurate control of operating densities made possible by

the use of a recording ammeter giving the power required for

the drive, and hence indicating the viscosity.(3) Adequ ate rapid movement of the masse at all times

depending only on controlled mechanical equipment.

(4) Minimum local variatio ns of temperature.(5) An effective and reliable method of cooling masse-cuite

in the pans themselves, avoiding the necessity of crystallizers.(6) All the consequential improvements brought about by

these accomplishments.

circulation in vacuum p ans

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