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Crystallisation
33. Crystallisation Object
The massecuite when discharged from the pan is at a high supersaturation. If it is allowed to stand, the sugar still contained in the mother liquor will continue to be deposited as crystals, but this massecuite is very dense and the mother liquor very viscous. Crystallisation will soon cease if the massecuite is left undisturbed, because the layer of mother liquor surrounding the crystals will be rapidly exhausted, and the viscosity of the mass will prevent the more distant molecules of sugar from circulating and coming in contact with the crystals. If we are to take advantage of the strong tendency of the massecuite to crystallise after boiling, it must be kept in motion in order to change constantly the relative positions of the particles of mother liquor and of crystals. In factory parlance this is termed "crystallisation"; strictly speaking, of course, the whole process in the pans constitutes crystallisation of the sugar, but in the factory the term "crystallisation" is used particularly for the crystallisation in motion after dropping the massecuite from the pan. Crystallisation then is a process which consists of mixing the massecuite for a certain time after dropping from the pans, and before passing to the centrifugals; and which aims at completing the formation of crystals and forcing further the exhaustion of the mother liquor. Proportions of different massecuites
We have already seen (p. 508) the usual relationship between the 3 massecuites. We repeat them in Table 110, adding the figures of other authors. TABLE 110 3-MASSECUITE SYSTEM! QUANTITIES OF MASSECUITES (cu.ft./t.C.)
Our figures A massecuite B massecuite C massecuite
Values often Tromp used (pp. 435-517)
3.50 1.75 1.40
3.50 2.10 1.40
3 1.60 1.40
6.65
7
6
Density of the massecuite
For hot massecuite we have used (cf. p. 465-475) a figure of 1.47. For temperatures between 30 and 40°C, (86-104°F) a value of 1.50 may be used. Dilution of the massecuite
When the massecuite purges badly at the centrifugals it is sometimes diluted with water, or with a suitable grade of molasses. This dilution, especially with water, impairs the exhaustion and should be avoided. If it proves necessary to dilute for fugalling, it should be done only a few hours before passing to the centrifugals.
530
CRYSTALLISATION
33
Flow of the massecuite
The minimum slope of chute to be provided for discharge of the massecuite leaving the pans is given by Tromp (p. 449) as 9° (16%). He recommends preferably 11° or 20%. However, he indicates elsewhere (p. 521) 5% or 3°, for the cold massecuite after crystallisation. This value is certainly a minimum and it would be advisable to keep well above it. Temperature of cooling of the massecuite
The massecuite on leaving the pans is at a temperature of 70-75°C (160-165°F). To what temperature is it advisable to cool it in the crystalliser? Low-grade massecuites. Noel Deerr (p. 404) considers that the best temperature to which to cool the low-grade massecuite is 41-43°C (105-110°F), and that if it is taken below that figure, the mother liquor becomes so viscous that the gain in sugar crystals is offset by the quantity of additional water needed for purging in the centrifugals. Jenkins (7.5.7., (1942) p. 123) suggests 38°C (100°F) as about the practical limit. High-grade massecuites. For massecuites of higher purity the limit of temperature would be lower, since the increase in viscosity with cooling would be less than with low grades. Jenkins recommends crystallising A and B massecuites as for C strikes, but emphasises the importance of efficient pan work; and has since expressed the view that with modern pans capable of pro ducing a massecuite of maximum crystal content, crystallisation of high-grades is less attractive. Few factories have considered it in their interests to practise crystallisation of high-grades, firstly on account of the danger of having the massecuite go solid in the crystalliser, and secondly on account of the space required for the extra equipment necessary. However, the operation would be interesting. Re-heating of the massecuite
If the massecuite is difficult to handle at the centrifugals, it may be re-heated before centrifuging in order to reduce its viscosity. We shall discuss this again in connection with centrifugals {cf. p. 590). Speed of crystallisation
The speed of crystallisation of a massecuite in motion is a function of the temperature and the supersaturation. If it is desired to maintain a constant rate of crystallisation, it is necessary to adjust the temperature as a function of the supersaturation or vice versa {cf Table 94, p. 464). Purity drop during crystallisation
Praeger claims (7.5.7., (1940) p. 287) that, in the 3-massecuite system, it would be possible to obtain by crystallisation an additional 6 points purity drop for each grade of massecuite. In Louisiana, Daubert claims {I.S.J., (1948) p. 159) that it should normally be possible to obtain, with a well controlled C massecuite, an apparent purity drop of 27 points between massecuite and molasses, e.g.: 15-16 points in the pan 11-12 points in the crystalliser. This is a maximum value, which we have rarely known to be achieved.
33
531
SUGAR RECOVERY
Sugar recovery
Problem. What are the proportions of sugar and molasses produced by a given massecuite? Solution. Let: Pm = purity of massecuite Ps = purity of sugar Pe = purity of the molasses Bm = the weight of dry substance % of massecuite Bs = weight of dry substance % of sugar Be = weight of dry substance % of molasses Qm = weight of massecuite Qs = weight of sugar produced Qe = weight of molasses obtained. By a reasoning identical with that of the problem on p. 522, we obtain: Qm
Bs
Ps — Pe
where the first term represents the weight of sugar % on weight of massecuite. Since the massecuite is generally reckoned by volume, we shall have: Qm = Vm ' dm
Vm = volume of massecuite in cu.ft. for example dm = density of massecuite in lb./cu.ft. for example. Hence: Qs
Pm — P e
Jim ' Um
Vm
Bs
(376)
Ps
We have, very closely: dm= 94 lb./cu.ft. Bs = 100
which gives: &=0.94Bm£^zlL Vm
rs
( 377)
re
where the first term represents the recovery R from 1 cu.ft. of massecuite, in pounds of sugar. Example. If we assume the figures of Table 110A: TABLE 110A YIELD OF SUGAR FROM MASSECUITES (NORMAL VALUES)
A massecuite B massecuite C massecuite
we would have theoretically:
Bm
Pm
95 96 97
82 70 60
Ps
99 98 96
60 50 40
532
CRYSTALLISATION
33
RA = 50 lb./cu.ft. RB = 38 lb./cu.ft. Re = 33 lb./cu.ft. On account of the dilutions carried out, these yields in practice would barely exceed: R
(378)
Installation of crystal Users
In most factories, the crystallisers are placed at ground level. This is a mistake which can do great harm to the recovery. In such a case, much trouble has been taken, particularly with the third strike, to obtain a very tight massecuite. Hence it is inevitably thick, stiff, dense (sometimes wrongly called viscous) and difficult to handle, especially when it is cold, after crystallisation. The pumps refuse to handle it, and the result practically always is that the operators responsible for feeding the low-grade centrifugals add molasses, sometimes even water, to dilute the mass so that it will be more readily accepted by the pump. Hence there is a complete destruction of all the work laboriously carried out, up to that stage, to obtain a dense massecuite; a destruc tion all the more complete because the operators have a tendency to abuse the dilution water valve. One palliative consists of having good pumps of Rota type, well located below the trough which supplies them, and fitted with short suction pipes of large diameter. Another palliative consists of reheating the massecuite in the delivery trough, by furnishing this trough with a double bottom which is heated by exhaust steam, but this again involves some risk of re-solution of crystal. However, there exists only one elegant method of really and completely solving the problem; that is, to design each installation in such a way as to avoid and completely banish the handling of the massecuite by a pump. For this it is necessary to locate the pans at a higher level, above the crystallisers, which consequently will be located on the first floor level, just below the pans; and the crystallisers in their turn should be above the distributer-mixer supplying the centrifu gals, which then will be located at floor level, and just below their mixers. This arrangement of the plant has an influence on the final recovery the importance of which can hardly be exaggerated. The modifications to the installation which it might involve in a badly laid out factory would probably be amply repaid in the first season. Ordinary crystal User
The ordinary or "kneading-trough" crystalliser (Fig. 295) is a simple steel vessel, of U-shaped cross-section, fitted with an agitator permitting it to maintain the mass in slow and continuous motion. Speed of rotation. Noel Deerr (p. 403) recommends a rotational speed of the screw of one turn in 1 min 45 sec. Tromp (p. 521) suggests | - f r.p.m. This speed is not of very great impor tance, the best values being the lowest (J r.p.m.). It has been found, by varying the speed, that it has hardly any perceptible effect as long as there is some movement.
33
ORDINARY CRYSTALLISER
533
Power. The power required for the crystalliser depends on this speed of rotation. Deerr (p. 404) estimates about 1 h.p. per 1,000 cu.ft. of massecuite. Tromp (p. 521) 1.5-3 h.p. per crystalliser, according to its volume (450-1,750 cu.ft.) or 300-600 cu.ft./h.p. At] 0.5 r.p.m., a figure of about 1.4 h.p./l,000 cu.ft. may be assumed, for crystallisers of 700-1,000 cu.ft. capacity.
Fig. 295. Crystalliser.
Time for crystallisation. Tromp (p. 517) gives the following times of crystallisation: A massecuite 12 h B massecuite 12 h C massecuite 72 h General practice is to keep the A massecuite a very short time in the crystalliser, to give a short period of crystallisation for B massecuite, and to keep as many crystallisers as possible for the C massecuite. It is essential to provide long and careful crystallisation for the final massecuite, but there would be considerable advantage in doing the same for the higher massecuites. When ordinary crystallisers are provided, it is not recommended to go below the following times: 12 h crystallisation for A massecuite 24 h crystallisation for B massecuite 72 h crystallisation for C massecuite Capacity. Noel Deerr estimates a total capacity for the battery of crystallisers of 168 cu.ft./ t.c.h. Tromp estimates the proportions of the respective massecuites as follows: A massecuite B massecuite C massecuite
50% = 3 cu.ft./t.c. 28% = 1.6 cu.ft./t.c. 22% = 1.4 cu.ft./t.c.
From these figures, and the crystallisation times just quoted, he calculates the required crystal liser capacities as: 3 x X~ + 1.6 x ^ + 1.4 x ^ = 1.5 + 0.8 + 4.2 24 24 24 or: 6.5 cu.ft./t.c./24 h = 160 cu.ft./t.c.h. This is a theoretical figure. However, taking into account loss of time and allowing a neces sary margin of safety, he finally quotes the following as practical figures:
534
33
CRYSTALLISATION
Cuba 180-240 cu.ft./t.c.h. Phillippines 266 cu.ft./t.c.h. We add some figures for crystalliser capacities in some other countries (see Table 111): TABLE 111 CAPACITY OF CRYSTALLISERS (cU.ft./t.C.h.)
Minimum Maximum Natal (F.A.S., (1933) p. 256) Porto Rico (T.S.J., (1950) p. 53)
135 61
Mean
620 208
118
In Cuba, (F.A.S., (April 1940) p. 31), requirements are estimated as follows: A massecuite B massecuite C massecuite or a total of
21.5 32.2 107.5 161.2
cu.ft./t.c.h. cu.ft./t.c.h. cu.ft./t.c.h. cu.ft./t.c.h.
Where figures are given for C massecuite only, which is the most important, we find (I.S.J. (1939) p. 425): Queensland 70 cu.ft./t.c.h. Hawai' / w a t e r " c o °l e d crystallisers\ 150 cu.ft./t.c.h. \ordinary crystallisers / 300 cu.ft./t.c.h. A further figure for C massecuite is given by Baikow (F.A.S., (July 1956) p. 56) for watercooled crystallisers of modern design of the Blanchard type; 72 cu.ft./t.c.h., plus 2 crystallisers, to allow for one filling and one emptying. In order to arrive at a figure among values varying so greatly, we would comment that the times of crystallisation which we have recommended above (12, 24 and 72 hours) give figures of: A mass. 12 24 72 3.50 x — + 1.75 x — + 1.40 x — = 24 24 24 or respectively: or a total of 184 cu.ft./t.c.h.
B mass.
1.75 -f 42
+
C mass.
1.75 + 4.2 cu.ft./t.c./24 h. 42
+ 100 cu.ft./t.c.h.
This constitutes a good average economic value; however, one cannot but gain by increasing to values approaching those used in Hawaii. Unit capacity. The unit capacity of the crystallisers, or capacity of each unit, should be fixed in proportion to the size of the pans. It is desirable to avoid mixing in the one crystalliser massecuite from two different pans. For preference, crystallisers will be chosen, the unit capacity of which is equal to that of the pans which they serve, increased by 20% (crystallisers of 6,000 gal., for example, for pans of 5,000 gal.). Alternatively, two crystallisers may be installed to serve one pan (crystallisers of 3,000 gal. for a pan of 5,000 gal., for example).
33
WATER-COOLED CRYSTALLISERS
535
TYPES OF CRYSTALLISER Crystal User with double helix
These are crystallisers which are widely used in certain British countries. They are fully analo gous to ordinary crystallisers, but carry two shafts and two helical stirrers rotating in opposite directions, being driven by two worm wheels from the same worm (Fig. 296). The upper helix is provided with a triple stirrer strip and one-third of its height is above the massecuite level.
-T' Fig. 296. Double helix crystalliser.
Since the crystalliser is narrower in relation to its volume than the ordinary type, it offers a relatively greater cooling surface; and the upper helix brings thin layers of massecuite into contact with the air and so gives a much more rapid cooling. Contrary to an idea which was widespread until recently, it has been found that this method of stirring the massecuite does not present any disadvantage, and that the crystalliser with a double helix allows of a cooling time shorter by half than that required for ordinary crystal lisers. Tromp (p. 525) indicates that a capacity of 133 cu.ft./t.ch. would be sufficient for final massecuite with this type of crystalliser. It is presumed that this capacity is given for a duty equivalent to that which would be obtained with the 266 cu.ft./t.ch. indicated above for all massecuites with ordinary crystallisers (cf. p. 534). Ragot crystalliser
The Ragot crystalliser is an ordinary crystalliser in which the stirring strip of the helix has been replaced by a coil carrying water. This will obviously allow of rapid cooling, and also presents the advantage that reheating of the massecuite can also be carried out before fugalling by replacing the cold water in this coil with hot water. Water-cooled crystallisers
We shall mention only for record purposes crystallisers fitted with a series of fixed tubes for water circulation, arranged in planes perpendicular to the shaft of the crystalliser. They are not recommended. Modern water-cooled crystallisers are practically always designed with cold water inlet and hot water outlet arranged in the shaft, which is then in the form of a central tube carrying the cold water, surrounded by an outer tube so that the hot water returns through the annular
536
CRYSTALLISATION
33
space between the inlet tube and this outer sleeve. The inlet and the outlet for the water are generally located on the one end of the crystalliser. The heat exchange surface is attached to the outer tube. The form of this surface varies from one designer to another/In one system widespread in the American hemisphere, it consists of two concentric helices. In the Fletcher-Blanchard (French licence Babcock and Wilcox), the heat exchange surface consists of tubes with closed ends, one series straight and one series curved, into which and from which the water flows during the rotation. In the Fives-Lille crystalliser, it is in the form of a hollow helical strip, which on the outside contributes to a lateral movement of the massecuite, and on the inside, like the preceding model, is traversed by the cooling water as the shaft rotates. All these crystallisers permit the same degree of cooling to be obtained as for the ordinary type of crystalliser, without any disadvantage and in a much shorter time, generally in 12-20 h, averaging say 16 h, as compared with 2-3 days. They thus afford a great economy in space required in the factory. Measurements have been made in Australia (I.S.J., (1957) p. 19) of the heat transfer coef ficient for Fletcher-Blanchard water-cooled crystallisers. Figures ranging from 4.6 to 4.9 B.Th.U./sq.ft./h/°F were obtained according to the quantity and the rate of circulation of cooling water. Werkspoor crystalliser
This crystalliser (Fig. 297) has an exterior form similar to that of the ordinary crystalliser.
Fig. 297. Werkspoor crystalliser-cooler (with re-heating in the last five compartments) showing massecuite outlet and water inlet (Fives-Lille.
33
537
WERKSPOOR CRYSTALLISER
,Water inlet
Water outlet
Fig. 298. Disc of Werkspoor crystalliser.
However, the shaft, instead of carrying a helix, is fitted with discs (Fig. 298) with a gap in the form of a 45° sector. Both shaft and discs are hollow and designed to permit circulation of water. The massecuite is introduced continuously at the end from which the shaft is driven,
Cold water
I Mass.
.a*— = m
Fig. 299. Graph of temperatures in Werkspoor crystalliser.
538
CRYSTALLISATION
33
and flows along the crystalliser by gravity, passing from one space between discs to the next by means of the open sector of the disc, and overflows at the opposite end of the crystalliser; the cooling water enters at the latter end, which is thus the outlet end for the massecuite, passes in succession through all the discs, and returns through the hollow shaft of the crystalliser to leave at the end at which it entered. This is therefore a counter current circulation. It presents the advantage that the hot masse cuite entering comes into contact only with water which has already been heated, and that at any point the temperature of the cooling water is progressively lower as the massecuite becomes cooled (Fig. 299). These conditions practically eliminate risk of false grain formation. Power required. This is about half of that indicated for the ordinary crystalliser (cf. p. 533). Honig (T.S.J., (Sept. 1952) p. 22) has observed that the power for Werkspoor crystallisers is also substantially lower than that required for crystallisers with the cooling water passing through tubes, and is of the order of | the requirements of the latter, whereas the heat trans mission is of the same order. This is an important advantage when very heavy massecuites are being treated. Quantity of water required. Let: 7o = temperature of the massecuite entering the crystalliser T = temperature of the massecuite leaving to = temperature of water entering t = temperature of water leaving. The quantity of water required would be theoretically: w
—
/ — to
]b. 0 f water per lb. of massecuite
(379)
c = specific heat of the massecuite = 0.44. In practice, we shall have: W=aVdc—
(380 t — to
W = total quantity of water required in lb./h a — coefficient taking into account the cooling of the molasses from the centrifugal which is added to the massecuite. A value of 1.15-1.20 is often taken V — volume of massecuite to be treated, in cu.ft./h d = density of the massecuite = 94 lb./cu.ft. It is not necessary to consider losses of heat during the cooling operation: (1) Because the water circuit is completely submerged in the massecuite, hence the efficiency is practically equal to unity. (2) Because there is an approximate compensation between two associated thermal phenome na, which are secondary and which we shall neglect for this reason, namely: (a) The massecuite is also cooled through the outer walls of the crystalliser, and through its surface exposed to the air. (b) On the other hand, the cooling water must absorb, in addition to the sensible heat of the massecuite, the heat of crystallisation of the sucrose which deposits on the crystals during the process. This heat of crystallisation generally represents from 8 to 10% of the sensible heat involved.
33
539
WERKSPOOR CRYSTALLISER
However, the favourable effect (a) is generally somewhat greater than the unfavourable effect (b), consequently the apparent efficiency is slightly greater than 1. In practice, it is found that the quantity of water used is of the order of: w = 0.75-0.80 lb./lb. of massecuite or approximately: w' = 1.2 cu.ft./cu.ft. of massecuite
(381)
Tromp (p. 518) gives: w = 0.8 lb./lb., and the figures which he quotes correspond to a coefficient a = 1.4. Cooling surface. The cooling surface should be proportional to the capacity of the crystalliser, or more precisely to the quantity of massecuite to be cooled per hour. It depends also on the massecuite temperature entering the crystalliser, the temperature of cooling water available, the degree of cooling required, etc. Tromp indicates as optimum the temperatures which we quote in Table 112 and which we have used in the graph of Fig. 299. TABLE 112 WATER-COOLED COUNTER-CURRENT CRYSTALLISERS. OPTIMUM TEMPERATURES (TROMP)
Massecuite entering Massecuite leaving Water entering Water leaving
°C
°F
68 34 30 54
154 93 86 129
Generally, the values for vacuum which we have recommended will give massecuites leaving the pans at 80-85°C (176-185°F), and which, even after a period in the storage mixer, will still be at 70-75°C (158-167°F). At the other end, cooling the massecuite below 40°C (104°F) is not attempted; and the temperature of cooling water is not a matter of choice. However, the cooling surface depends mainly on the massecuite to be treated. If we define as for vacuum pans (cf. eqn. 346, p. 499) the ratio: S _ Cooling surface of the crystalliser, in sq.ft. V Working capacity of the crystalliser, in cu.ft. we shall require, for first massecuites, ratios SjV of the order of 2-3 sq.ft./cu.ft. and, for final massecuites, ratios of the order of 0.3 sq.ft./cu.ft. This difference is due to the divergence between the viscosities of high purity and low purity massecuites, which necessitates a much slower and more gradual cooling for the latter. The true coefficient of heat transfer in a Werkspoor has been established by the experiment station of Java and reported by Pieter Honig (Ã.5./., (Oct. 1951) p. 11) who had participated in the determinations. It was of the order of: k = 7-10 B.Th.U./sq.ft./h/°F (383) on all massecuites with a new and clean crystalliser. A similar determination, made in Australia on C massecuites, with rather older crystallisers with some scaling (Q.S.S.C.T., (1956) p. 49), has given: k = 2.5 B.Th.U./sq.ft./h/°F
540
33
CRYSTALLISATION
However, taking into account the favourable factors indicated above, designers generally apply, for the simplified method of calculation corresponding to eqn. (380), the apparent coefficients given in Table 113. TABLE 113 APPARENT HEAT TRANSFER COEFFICIENT FOR WERKSPOOR CRYSTALLISERS
(B.Th.U./sq.ft./h/°F) For A massecuites For B massecuites For C massecuites
k = 15 k = 13 k= 5
We consider that it is prudent, when considering performance over a normal period of years to base designs on the figures given in Table 113A. TABLE 113A APPARENT HEAT TRANSFER COEFFICIENT FOR WERKSPOOR CRYSTALLISERS. RECOMMENDED VALUES FOR DESIGN (B.Th.U./sq.ft./h/°F)
For A massecuites For B massecuites For C massecuites
k = 12 k = 10 k= 4
Calculation of cooling surface. We have: _ Vdc To — T , o S = 60a — — In A: (7o — 0 — (×to)
To — / T—to
(384)
S = cooling surface of the Werkspoor, in sq.ft. a = factor taking into account the diluting molasses added V = volume of massecuite (before dilution), in cu.ft./h d = specific gravity of the massecuite = 1 . 5 c = specific heat of the massecuite = 0.44 l k = heat transfer coefficient, in B.Th.U./sq.ft./h/°F, given by Table 113 or 113A To — temperature of the massecuite entering the apparatus in °F T = temperature required for massecuite leaving, in °F to = inlet temperature of cooling water in °F / = outlet temperature of water, °F. We assume generally: To = 167°F T = 104°F / = 122°F
a n d a = 1.15
(385)
We would recall that: In x = 2.3 log x Cooling time. In the cane sugar factory, the cooling times allowed are generally as given in Table 114.
33
WERKSPOOR CRYSTALLISER
541
TABLE 114 COOLING TIMES GENERALLY ALLOWED IN WERKSPOORS
A massecuite (if boiling 3 massecuites) 1st massecuite (if boiling 2 massecuites) B massecuite (if boiling 3 massecuites) C or final massecuite
\\ h 2 h 2 h 12-15 h
Standard dimensions. The Werkspoor licence for France has been acquired by the Compagnie de Fives-Lille. This firm manufactures crystallisers for A massecuite of which the diameter D increases from 1.300 m (50 in.) to 3 m (10 ft.) in steps of 1 decimetre. The discs have an outside diameter D' about 20-30 mm (1 in.) less than the interior diameter D of the vessel (£>' = 2.780 m, for example, for a crystalliser of D = 2.800 m). The open sector of the discs is 45°, and their unit cooling surface s may be calculated approximately by: 5 = 1.2D'2
(386)
s = cooling surface of one disc, in sq.ft. D' = exterior diameter of a disc, in ft. Actually the coefficient 1.2 varies from 1.13 for D' = 1.280 m up to 1.27 for D' = 2.980 m and it would be more accurate to write: s
= (l + 0.03Z>)D/2 (Br. units)
(387)
or for D in metres: s= (l + - ^ - ) Z>'2 (m. units)
(387)
The height of massecuite level above the axis is about 1/20-1/10 of the diameter. However, the capacity of the crystalliser is generally estimated as if the tank contained no discs, shaft, or pipes and assuming that the massecuite level was limited to the axis (which amounts to assuming that the volume of massecuite above the axis compensates for the volume occupied by the shaft, the discs, etc.). For the working volume, so defined per unit length of the tank, we have therefore: nD2 u= - — = 0.3927Z)2 (388) 8 u = theoretical working volume per unit length of the crystalliser, in cu.ft./ft. D = diameter or interior width of the tank in ft. The tank of the crystalliser is generally proportioned so that the length is about 3-3.5 times its diameter, without allowing this secondary consideration to override the eventual require ments of space required and difficulties of installation. The space between discs increases from inlet to outlet of massecuite, but the mean value of this spacing should not fall below 20 cm (8 in.). It is determined by the formula: -
ß-W+P) \-p
e = average spacing between discs, in ft. L = total length of the crystalliser tank, in ft.
(389)
542
33
CRYSTALLISATION
p = number of intermediate bearings (the length between bearings should be at least 10 ft. and at most 20 ft.) n = number of discs (which should always be an odd number). Design of a Werkspoor crystalliser. Data. We shall assume we are dealing with a factory working at 59 t.c.h., employing a 3-massecuite system, and obtaining 3.59 cu.ft. of A massecuite per t.c. We require to calculate the dimensions of a Werkspoor crystalliser intended to treat the A massecuite. Volume and dimensions of the Werkspoor. The quantity of massecuite to be treated is: v = 3.59 cu.ft./t.c. or a total volume of: V= 3.59 x 59= 211.8cu.ft./h which will be increased by the addition of molasses for diluting to: V'= 211.8 x 1.20= 254.2 cu.ft./h. The time necessary for cooling will be 1 h 30 min according to Table 114. Hence the capacity of the Werkspoor, that is the quantity of massecuite which it must hold: C = 254.2 x 1.5= 381.2 cu.ft. We require then a Werkspoor of 381.2 cu.ft. If we allow for a length L of the vessel equal to 3.5 times the diameter Z), it will require: nD2 C= -—L=
nD2 —— x 3.5/)= 1.37D3= 381.2 cu.ft.
whence: 381.2 Z)3 = — — = 278.3 1.37
D = 6.5 ft.
We shall adopt therefore a diameter of 6.5 ft. For this diameter, the theoretical working volume is (eqn. 388): u = 0.3927D2 = 16.59 cu.ft./ft. hence the length of the tank:
£=
381 2
w
·
=23ft
·
Cooling surface. Tf the cooling water available is at 77°F and if we allow 122°F as temperature of outlet water, and a value of 12 B.Th.U./sq.ft./h/°F as heat transfer coefficient, we shall have (eqn. 384): r, .,* , «, 211.8 167—104 , 167—122 5 = 40 x 1.15 x ——— x —— — — —x In 12 (167—122) —(104 —77) 104 — 77
33
543
WERKSPOOR CRYSTALLISER
or: 5 - 812 x 3.5 x 0.51 = 1,449 sq.ft. ( l n ^ ~ „„ = I n 4 = 2.3 log4= 2.3 x 0.222 = 0.5l\ & \ 104 — 77 3 3 / The cooling surface of one disc has a value of (eqn. 387): 5
= 1.2 x 6.44 2 = 49.8 sq.ft.
hence the number of discs necessary: 49.8 Say 29 discs. We shall have for accurate value of the area: S= 29 x'49.8= 1,444 sq.ft. Let us now verify the average spacing between discs. With 1 intermediate bearing (eqn. 389): -
í ·29—1 ? + ,—! 1)
- 0 - 7 3 3 ft. = 8.8 in.
Quantity of water required. We have (eqn. 380): W= 1.15 x 211.8 x 1.5 x 0.44 x
167 — 104 — = 225 cu.ft./h 122 — 77
'
or approximately 0.4 imp. gal./sec. Comment. The use of a coefficient a = 1.20 in the calculation of volume and 1.15 in the calculation for cooling is intentional. The molasses from purging at the fugals is generally cooler by several degrees than the massecuite in the crystalliser, and its specific heat is slightly higher. Checking an existing Werkspoor. When a Werkspoor is installed and it is desired to know what temperatures of massecuite and water it would give for the quantities of massecuite to be cooled and with the available cooling water, we have: T—t
(
ks
kS
ë
To — t t—to To — T
a Vdc W
— r
(the symbols having the same values as in eqns. 380 and 384), hence we have: Ã = to + m(To — t) t = to + r(To — T) solving for Tand /:
544
33
CRYSTALLISATION
ô — /o(1i ~ m ) + 1 —
mTo(l — r)
(390)
imr
t =■ to + r(Tc >-T)
We may recall that: - X = In y = 2.3 log y. (1) lfy = e~* (2) Log e-* = —0.4343 x. (3) If a negative logarithm is found, for example: log a = — 0.372 we shall write: log a = Ú.628 Results. Table 115 gives some results which we have obtained, operating with 3 massecuites, with a Werkspoor handling the first 2 massecuites, A and B: TABLE 115 EXAMPLE OF RESULTS WITH A WERKSPOOR CRYSTALLISER
A massecuite B massecuite When dropped from pans Brix of massecuite Apparent purity of mass. Apparent purity of mother liquor Leaving the Werkspoor Purity of molasses (no washing) Total purity drop Purity drop of molasses Temp, of mass, entering crystalliser °F Temp, of mass, leaving crystalliser °F Temperature of cooling water °F Cooling time
94.4 80.2 61.5
95.6 65.5 50.1
56.7 23.5 4.8 163 115 75 1 h 40 min
46.2 19.3 3.9 162 118 81 1 h 45 min
In Java (I.S.J., (1940) p. 286) figures of the following order are reported: Cooling Purity drop of the mother liquor
25°C (45°F) in a period of 2 h 30 min 5°
Storage vessel. Since the Werkspoor operates continuously, and the pans batchwise, it is obviously necessary to provide an intermediate vessel to receive the massecuite between the pans and the Werkspoor. For this duty an ordinary mixer is used (or sometimes two). It is necessary that its capacity should be 1.5 times that of the largest pan discharging into it. Spacing between discs. We know that: (1) The speed of crystallisation is greater at higher temperatures. (2) This speed increases with the supersaturation, but in practice attains a maximum at a relatively low supersaturation. (3) Viscosity increases with supersaturation. It is therefore of advantage to work with a relatively low supersaturation and to cool as rapidly as possible the hot massecuite at the inlet, since it permits of a maximum rate of crystallisation. It is for this reason that the Werkspoor progressively increases the spacing between cooling discs from the massecuite entry to the massecuite outlet.
33
LAFEUILLE CRYSTALLISER
545
Reheating. Reheating of the massecuite before fugalling is often practised in order to reduce its viscosity {cf. p. 590). The Werkspoor lends itself particularly well to this operation; it is sufficient to provide a little extra length, so as to enable 3 or 4 reheating discs to be placed after the cooling discs. The calculation of this reheating portion is made in a similar manner to that of the cooling portion. However, the reheating should be rapid and of short duration, hence the reheating discs are placed close together. Use of the Werkspoor. Compared with ordinary crystallisers, the Werkspoor offers the ad vantage of taking up much less space and of improving the exhaustion. The latter advantage is perhaps most marked with A and B massecuites; with final massecuite, its use is somewhat more critical, particularly with massecuites of high density. However, it is finding increased use with low-grade massecuites, notably in Queensland, when used with efficient reheating arrangements. Lafeuille crystal User
This apparatus, also known as the crystalliser pan, serves either as pan or as crystalliser. It consists of a large cylinder, which rotates on its axis, being supported on rollers on which it rests per medium of 2 large rings (Fig. 300).
Fig. 300. Lafeuille crystalliser pan (U.C.M.A.S.).
Connections are provided for vacuum, for introduction of massecuite or syrups, for water and for vapour. All connections are made at the shaft: vacuum, massecuite and syrups at one end; steam, water and incondensable gas at the other. Inside the apparatus is a bundle of longitudinal tubes parallel to the axis. These tubes can be supplied with steam or with water. The apparatus is filled only to £ of the available interior volume. The rotational movement given to it then produces an effective agitation and mixing of the massecuite. Dimensions. The diameter is approximately 6.5 ft., the length variable and the capacity accordingly varies from 1,000 to 2,000 cu.ft. Speed of rotation. The speed of rotation is very low, of the order of 1 r.p.m. when used as a pan, and only \ of this when used as a crystalliser {I.S.J., (1934) p. 358).
546
CRYSTALLISATION
33
Power. The power required is similarly very low, of the order of 4-8 h.p. Heat transfer coeflBcient As for the Werkspoor, this has been determined by the experiment station in Java, and reported in an interesting article by Honig (T.S.J., (Oct. 1951) p. 41) giving values of: k = 30 to 45 kcal/m2/h/°C = 6.1 to 9.2 B.Th.U./sq.ft./h/°F
(391)
Cooling surface. The cooling surface per unit volume is approximately 2 sq.ft./cu.ft. Duration of the charge. This is approximately 6 h for an A massecuite, 12-18 h for lowgrades. Application. The Lafeuille is a relatively recent apparatus. It offers the advantage of taking up little space and of reducing the hydrostatic pressure to a minimum. It has given very satisfactory results in the Philippines (I.S.J., (1934) p. 358): (a) Very high brix of massecuite when discharged, being able to attain 98-100. (b) Very high purity dfcops» being able to attain 30-32 points. However, it is tricky to operate, particularly when used as a vacuum pan, and should not be left indiscriminately in the hands of the average operator.
The massecuite when discharged from the pan is at a high supersaturation. If it is allowed to stand, the sugar still contained in the mother liquor will continue to be deposited as crystals, but this massecuite is very dense and the mother liquor very viscous. Crystallisation will soon cease if the massecuite is left undisturbed, because the layer of mother liquor surrounding the crystals will be rapidly exhausted, and the viscosity of the mass will prevent the more distant molecules of sugar from circulating and coming in contact with the crystals. If we are to take advantage of the strong tendency of the massecuite to crystallise after boiling, it must be kept in motion in order to change constantly the relative positions of the particles of mother liquor and of crystals. In factory parlance this is termed "crystallisation"; strictly speaking, of course, the whole process in the pans constitutes crystallisation of the sugar, but in the factory the term "crystallisation" is used particularly for the crystallisation in motion after dropping the massecuite from the pan. Crystallisation then is a process which consists of mixing the massecuite for a certain time after dropping from the pans, and before passing to the centrifugals; and which aims at completing the formation of crystals and forcing further the exhaustion of the mother liquor. Proportions of different massecuites
We have already seen (p. 508) the usual relationship between the 3 massecuites. We repeat them in Table 110, adding the figures of other authors. TABLE 110 3-MASSECUITE SYSTEM! QUANTITIES OF MASSECUITES (cu.ft./t.C.)
Our figures A massecuite B massecuite C massecuite
Values often Tromp used (pp. 435-517)
3.50 1.75 1.40
3.50 2.10 1.40
3 1.60 1.40
6.65
7
6
Density of the massecuite
For hot massecuite we have used (cf. p. 465-475) a figure of 1.47. For temperatures between 30 and 40°C, (86-104°F) a value of 1.50 may be used. Dilution of the massecuite
When the massecuite purges badly at the centrifugals it is sometimes diluted with water, or with a suitable grade of molasses. This dilution, especially with water, impairs the exhaustion and should be avoided. If it proves necessary to dilute for fugalling, it should be done only a few hours before passing to the centrifugals.
530
CRYSTALLISATION
33
Flow of the massecuite
The minimum slope of chute to be provided for discharge of the massecuite leaving the pans is given by Tromp (p. 449) as 9° (16%). He recommends preferably 11° or 20%. However, he indicates elsewhere (p. 521) 5% or 3°, for the cold massecuite after crystallisation. This value is certainly a minimum and it would be advisable to keep well above it. Temperature of cooling of the massecuite
The massecuite on leaving the pans is at a temperature of 70-75°C (160-165°F). To what temperature is it advisable to cool it in the crystalliser? Low-grade massecuites. Noel Deerr (p. 404) considers that the best temperature to which to cool the low-grade massecuite is 41-43°C (105-110°F), and that if it is taken below that figure, the mother liquor becomes so viscous that the gain in sugar crystals is offset by the quantity of additional water needed for purging in the centrifugals. Jenkins (7.5.7., (1942) p. 123) suggests 38°C (100°F) as about the practical limit. High-grade massecuites. For massecuites of higher purity the limit of temperature would be lower, since the increase in viscosity with cooling would be less than with low grades. Jenkins recommends crystallising A and B massecuites as for C strikes, but emphasises the importance of efficient pan work; and has since expressed the view that with modern pans capable of pro ducing a massecuite of maximum crystal content, crystallisation of high-grades is less attractive. Few factories have considered it in their interests to practise crystallisation of high-grades, firstly on account of the danger of having the massecuite go solid in the crystalliser, and secondly on account of the space required for the extra equipment necessary. However, the operation would be interesting. Re-heating of the massecuite
If the massecuite is difficult to handle at the centrifugals, it may be re-heated before centrifuging in order to reduce its viscosity. We shall discuss this again in connection with centrifugals {cf. p. 590). Speed of crystallisation
The speed of crystallisation of a massecuite in motion is a function of the temperature and the supersaturation. If it is desired to maintain a constant rate of crystallisation, it is necessary to adjust the temperature as a function of the supersaturation or vice versa {cf Table 94, p. 464). Purity drop during crystallisation
Praeger claims (7.5.7., (1940) p. 287) that, in the 3-massecuite system, it would be possible to obtain by crystallisation an additional 6 points purity drop for each grade of massecuite. In Louisiana, Daubert claims {I.S.J., (1948) p. 159) that it should normally be possible to obtain, with a well controlled C massecuite, an apparent purity drop of 27 points between massecuite and molasses, e.g.: 15-16 points in the pan 11-12 points in the crystalliser. This is a maximum value, which we have rarely known to be achieved.
33
531
SUGAR RECOVERY
Sugar recovery
Problem. What are the proportions of sugar and molasses produced by a given massecuite? Solution. Let: Pm = purity of massecuite Ps = purity of sugar Pe = purity of the molasses Bm = the weight of dry substance % of massecuite Bs = weight of dry substance % of sugar Be = weight of dry substance % of molasses Qm = weight of massecuite Qs = weight of sugar produced Qe = weight of molasses obtained. By a reasoning identical with that of the problem on p. 522, we obtain: Qm
Bs
Ps — Pe
where the first term represents the weight of sugar % on weight of massecuite. Since the massecuite is generally reckoned by volume, we shall have: Qm = Vm ' dm
Vm = volume of massecuite in cu.ft. for example dm = density of massecuite in lb./cu.ft. for example. Hence: Qs
Pm — P e
Jim ' Um
Vm
Bs
(376)
Ps
We have, very closely: dm= 94 lb./cu.ft. Bs = 100
which gives: &=0.94Bm£^zlL Vm
rs
( 377)
re
where the first term represents the recovery R from 1 cu.ft. of massecuite, in pounds of sugar. Example. If we assume the figures of Table 110A: TABLE 110A YIELD OF SUGAR FROM MASSECUITES (NORMAL VALUES)
A massecuite B massecuite C massecuite
we would have theoretically:
Bm
Pm
95 96 97
82 70 60
Ps
99 98 96
60 50 40
532
CRYSTALLISATION
33
RA = 50 lb./cu.ft. RB = 38 lb./cu.ft. Re = 33 lb./cu.ft. On account of the dilutions carried out, these yields in practice would barely exceed: R
(378)
Installation of crystal Users
In most factories, the crystallisers are placed at ground level. This is a mistake which can do great harm to the recovery. In such a case, much trouble has been taken, particularly with the third strike, to obtain a very tight massecuite. Hence it is inevitably thick, stiff, dense (sometimes wrongly called viscous) and difficult to handle, especially when it is cold, after crystallisation. The pumps refuse to handle it, and the result practically always is that the operators responsible for feeding the low-grade centrifugals add molasses, sometimes even water, to dilute the mass so that it will be more readily accepted by the pump. Hence there is a complete destruction of all the work laboriously carried out, up to that stage, to obtain a dense massecuite; a destruc tion all the more complete because the operators have a tendency to abuse the dilution water valve. One palliative consists of having good pumps of Rota type, well located below the trough which supplies them, and fitted with short suction pipes of large diameter. Another palliative consists of reheating the massecuite in the delivery trough, by furnishing this trough with a double bottom which is heated by exhaust steam, but this again involves some risk of re-solution of crystal. However, there exists only one elegant method of really and completely solving the problem; that is, to design each installation in such a way as to avoid and completely banish the handling of the massecuite by a pump. For this it is necessary to locate the pans at a higher level, above the crystallisers, which consequently will be located on the first floor level, just below the pans; and the crystallisers in their turn should be above the distributer-mixer supplying the centrifu gals, which then will be located at floor level, and just below their mixers. This arrangement of the plant has an influence on the final recovery the importance of which can hardly be exaggerated. The modifications to the installation which it might involve in a badly laid out factory would probably be amply repaid in the first season. Ordinary crystal User
The ordinary or "kneading-trough" crystalliser (Fig. 295) is a simple steel vessel, of U-shaped cross-section, fitted with an agitator permitting it to maintain the mass in slow and continuous motion. Speed of rotation. Noel Deerr (p. 403) recommends a rotational speed of the screw of one turn in 1 min 45 sec. Tromp (p. 521) suggests | - f r.p.m. This speed is not of very great impor tance, the best values being the lowest (J r.p.m.). It has been found, by varying the speed, that it has hardly any perceptible effect as long as there is some movement.
33
ORDINARY CRYSTALLISER
533
Power. The power required for the crystalliser depends on this speed of rotation. Deerr (p. 404) estimates about 1 h.p. per 1,000 cu.ft. of massecuite. Tromp (p. 521) 1.5-3 h.p. per crystalliser, according to its volume (450-1,750 cu.ft.) or 300-600 cu.ft./h.p. At] 0.5 r.p.m., a figure of about 1.4 h.p./l,000 cu.ft. may be assumed, for crystallisers of 700-1,000 cu.ft. capacity.
Fig. 295. Crystalliser.
Time for crystallisation. Tromp (p. 517) gives the following times of crystallisation: A massecuite 12 h B massecuite 12 h C massecuite 72 h General practice is to keep the A massecuite a very short time in the crystalliser, to give a short period of crystallisation for B massecuite, and to keep as many crystallisers as possible for the C massecuite. It is essential to provide long and careful crystallisation for the final massecuite, but there would be considerable advantage in doing the same for the higher massecuites. When ordinary crystallisers are provided, it is not recommended to go below the following times: 12 h crystallisation for A massecuite 24 h crystallisation for B massecuite 72 h crystallisation for C massecuite Capacity. Noel Deerr estimates a total capacity for the battery of crystallisers of 168 cu.ft./ t.c.h. Tromp estimates the proportions of the respective massecuites as follows: A massecuite B massecuite C massecuite
50% = 3 cu.ft./t.c. 28% = 1.6 cu.ft./t.c. 22% = 1.4 cu.ft./t.c.
From these figures, and the crystallisation times just quoted, he calculates the required crystal liser capacities as: 3 x X~ + 1.6 x ^ + 1.4 x ^ = 1.5 + 0.8 + 4.2 24 24 24 or: 6.5 cu.ft./t.c./24 h = 160 cu.ft./t.c.h. This is a theoretical figure. However, taking into account loss of time and allowing a neces sary margin of safety, he finally quotes the following as practical figures:
534
33
CRYSTALLISATION
Cuba 180-240 cu.ft./t.c.h. Phillippines 266 cu.ft./t.c.h. We add some figures for crystalliser capacities in some other countries (see Table 111): TABLE 111 CAPACITY OF CRYSTALLISERS (cU.ft./t.C.h.)
Minimum Maximum Natal (F.A.S., (1933) p. 256) Porto Rico (T.S.J., (1950) p. 53)
135 61
Mean
620 208
118
In Cuba, (F.A.S., (April 1940) p. 31), requirements are estimated as follows: A massecuite B massecuite C massecuite or a total of
21.5 32.2 107.5 161.2
cu.ft./t.c.h. cu.ft./t.c.h. cu.ft./t.c.h. cu.ft./t.c.h.
Where figures are given for C massecuite only, which is the most important, we find (I.S.J. (1939) p. 425): Queensland 70 cu.ft./t.c.h. Hawai' / w a t e r " c o °l e d crystallisers\ 150 cu.ft./t.c.h. \ordinary crystallisers / 300 cu.ft./t.c.h. A further figure for C massecuite is given by Baikow (F.A.S., (July 1956) p. 56) for watercooled crystallisers of modern design of the Blanchard type; 72 cu.ft./t.c.h., plus 2 crystallisers, to allow for one filling and one emptying. In order to arrive at a figure among values varying so greatly, we would comment that the times of crystallisation which we have recommended above (12, 24 and 72 hours) give figures of: A mass. 12 24 72 3.50 x — + 1.75 x — + 1.40 x — = 24 24 24 or respectively: or a total of 184 cu.ft./t.c.h.
B mass.
1.75 -f 42
+
C mass.
1.75 + 4.2 cu.ft./t.c./24 h. 42
+ 100 cu.ft./t.c.h.
This constitutes a good average economic value; however, one cannot but gain by increasing to values approaching those used in Hawaii. Unit capacity. The unit capacity of the crystallisers, or capacity of each unit, should be fixed in proportion to the size of the pans. It is desirable to avoid mixing in the one crystalliser massecuite from two different pans. For preference, crystallisers will be chosen, the unit capacity of which is equal to that of the pans which they serve, increased by 20% (crystallisers of 6,000 gal., for example, for pans of 5,000 gal.). Alternatively, two crystallisers may be installed to serve one pan (crystallisers of 3,000 gal. for a pan of 5,000 gal., for example).
33
WATER-COOLED CRYSTALLISERS
535
TYPES OF CRYSTALLISER Crystal User with double helix
These are crystallisers which are widely used in certain British countries. They are fully analo gous to ordinary crystallisers, but carry two shafts and two helical stirrers rotating in opposite directions, being driven by two worm wheels from the same worm (Fig. 296). The upper helix is provided with a triple stirrer strip and one-third of its height is above the massecuite level.
-T' Fig. 296. Double helix crystalliser.
Since the crystalliser is narrower in relation to its volume than the ordinary type, it offers a relatively greater cooling surface; and the upper helix brings thin layers of massecuite into contact with the air and so gives a much more rapid cooling. Contrary to an idea which was widespread until recently, it has been found that this method of stirring the massecuite does not present any disadvantage, and that the crystalliser with a double helix allows of a cooling time shorter by half than that required for ordinary crystal lisers. Tromp (p. 525) indicates that a capacity of 133 cu.ft./t.ch. would be sufficient for final massecuite with this type of crystalliser. It is presumed that this capacity is given for a duty equivalent to that which would be obtained with the 266 cu.ft./t.ch. indicated above for all massecuites with ordinary crystallisers (cf. p. 534). Ragot crystalliser
The Ragot crystalliser is an ordinary crystalliser in which the stirring strip of the helix has been replaced by a coil carrying water. This will obviously allow of rapid cooling, and also presents the advantage that reheating of the massecuite can also be carried out before fugalling by replacing the cold water in this coil with hot water. Water-cooled crystallisers
We shall mention only for record purposes crystallisers fitted with a series of fixed tubes for water circulation, arranged in planes perpendicular to the shaft of the crystalliser. They are not recommended. Modern water-cooled crystallisers are practically always designed with cold water inlet and hot water outlet arranged in the shaft, which is then in the form of a central tube carrying the cold water, surrounded by an outer tube so that the hot water returns through the annular
536
CRYSTALLISATION
33
space between the inlet tube and this outer sleeve. The inlet and the outlet for the water are generally located on the one end of the crystalliser. The heat exchange surface is attached to the outer tube. The form of this surface varies from one designer to another/In one system widespread in the American hemisphere, it consists of two concentric helices. In the Fletcher-Blanchard (French licence Babcock and Wilcox), the heat exchange surface consists of tubes with closed ends, one series straight and one series curved, into which and from which the water flows during the rotation. In the Fives-Lille crystalliser, it is in the form of a hollow helical strip, which on the outside contributes to a lateral movement of the massecuite, and on the inside, like the preceding model, is traversed by the cooling water as the shaft rotates. All these crystallisers permit the same degree of cooling to be obtained as for the ordinary type of crystalliser, without any disadvantage and in a much shorter time, generally in 12-20 h, averaging say 16 h, as compared with 2-3 days. They thus afford a great economy in space required in the factory. Measurements have been made in Australia (I.S.J., (1957) p. 19) of the heat transfer coef ficient for Fletcher-Blanchard water-cooled crystallisers. Figures ranging from 4.6 to 4.9 B.Th.U./sq.ft./h/°F were obtained according to the quantity and the rate of circulation of cooling water. Werkspoor crystalliser
This crystalliser (Fig. 297) has an exterior form similar to that of the ordinary crystalliser.
Fig. 297. Werkspoor crystalliser-cooler (with re-heating in the last five compartments) showing massecuite outlet and water inlet (Fives-Lille.
33
537
WERKSPOOR CRYSTALLISER
,Water inlet
Water outlet
Fig. 298. Disc of Werkspoor crystalliser.
However, the shaft, instead of carrying a helix, is fitted with discs (Fig. 298) with a gap in the form of a 45° sector. Both shaft and discs are hollow and designed to permit circulation of water. The massecuite is introduced continuously at the end from which the shaft is driven,
Cold water
I Mass.
.a*— = m
Fig. 299. Graph of temperatures in Werkspoor crystalliser.
538
CRYSTALLISATION
33
and flows along the crystalliser by gravity, passing from one space between discs to the next by means of the open sector of the disc, and overflows at the opposite end of the crystalliser; the cooling water enters at the latter end, which is thus the outlet end for the massecuite, passes in succession through all the discs, and returns through the hollow shaft of the crystalliser to leave at the end at which it entered. This is therefore a counter current circulation. It presents the advantage that the hot masse cuite entering comes into contact only with water which has already been heated, and that at any point the temperature of the cooling water is progressively lower as the massecuite becomes cooled (Fig. 299). These conditions practically eliminate risk of false grain formation. Power required. This is about half of that indicated for the ordinary crystalliser (cf. p. 533). Honig (T.S.J., (Sept. 1952) p. 22) has observed that the power for Werkspoor crystallisers is also substantially lower than that required for crystallisers with the cooling water passing through tubes, and is of the order of | the requirements of the latter, whereas the heat trans mission is of the same order. This is an important advantage when very heavy massecuites are being treated. Quantity of water required. Let: 7o = temperature of the massecuite entering the crystalliser T = temperature of the massecuite leaving to = temperature of water entering t = temperature of water leaving. The quantity of water required would be theoretically: w
—
/ — to
]b. 0 f water per lb. of massecuite
(379)
c = specific heat of the massecuite = 0.44. In practice, we shall have: W=aVdc—
(380 t — to
W = total quantity of water required in lb./h a — coefficient taking into account the cooling of the molasses from the centrifugal which is added to the massecuite. A value of 1.15-1.20 is often taken V — volume of massecuite to be treated, in cu.ft./h d = density of the massecuite = 94 lb./cu.ft. It is not necessary to consider losses of heat during the cooling operation: (1) Because the water circuit is completely submerged in the massecuite, hence the efficiency is practically equal to unity. (2) Because there is an approximate compensation between two associated thermal phenome na, which are secondary and which we shall neglect for this reason, namely: (a) The massecuite is also cooled through the outer walls of the crystalliser, and through its surface exposed to the air. (b) On the other hand, the cooling water must absorb, in addition to the sensible heat of the massecuite, the heat of crystallisation of the sucrose which deposits on the crystals during the process. This heat of crystallisation generally represents from 8 to 10% of the sensible heat involved.
33
539
WERKSPOOR CRYSTALLISER
However, the favourable effect (a) is generally somewhat greater than the unfavourable effect (b), consequently the apparent efficiency is slightly greater than 1. In practice, it is found that the quantity of water used is of the order of: w = 0.75-0.80 lb./lb. of massecuite or approximately: w' = 1.2 cu.ft./cu.ft. of massecuite
(381)
Tromp (p. 518) gives: w = 0.8 lb./lb., and the figures which he quotes correspond to a coefficient a = 1.4. Cooling surface. The cooling surface should be proportional to the capacity of the crystalliser, or more precisely to the quantity of massecuite to be cooled per hour. It depends also on the massecuite temperature entering the crystalliser, the temperature of cooling water available, the degree of cooling required, etc. Tromp indicates as optimum the temperatures which we quote in Table 112 and which we have used in the graph of Fig. 299. TABLE 112 WATER-COOLED COUNTER-CURRENT CRYSTALLISERS. OPTIMUM TEMPERATURES (TROMP)
Massecuite entering Massecuite leaving Water entering Water leaving
°C
°F
68 34 30 54
154 93 86 129
Generally, the values for vacuum which we have recommended will give massecuites leaving the pans at 80-85°C (176-185°F), and which, even after a period in the storage mixer, will still be at 70-75°C (158-167°F). At the other end, cooling the massecuite below 40°C (104°F) is not attempted; and the temperature of cooling water is not a matter of choice. However, the cooling surface depends mainly on the massecuite to be treated. If we define as for vacuum pans (cf. eqn. 346, p. 499) the ratio: S _ Cooling surface of the crystalliser, in sq.ft. V Working capacity of the crystalliser, in cu.ft. we shall require, for first massecuites, ratios SjV of the order of 2-3 sq.ft./cu.ft. and, for final massecuites, ratios of the order of 0.3 sq.ft./cu.ft. This difference is due to the divergence between the viscosities of high purity and low purity massecuites, which necessitates a much slower and more gradual cooling for the latter. The true coefficient of heat transfer in a Werkspoor has been established by the experiment station of Java and reported by Pieter Honig (Ã.5./., (Oct. 1951) p. 11) who had participated in the determinations. It was of the order of: k = 7-10 B.Th.U./sq.ft./h/°F (383) on all massecuites with a new and clean crystalliser. A similar determination, made in Australia on C massecuites, with rather older crystallisers with some scaling (Q.S.S.C.T., (1956) p. 49), has given: k = 2.5 B.Th.U./sq.ft./h/°F
540
33
CRYSTALLISATION
However, taking into account the favourable factors indicated above, designers generally apply, for the simplified method of calculation corresponding to eqn. (380), the apparent coefficients given in Table 113. TABLE 113 APPARENT HEAT TRANSFER COEFFICIENT FOR WERKSPOOR CRYSTALLISERS
(B.Th.U./sq.ft./h/°F) For A massecuites For B massecuites For C massecuites
k = 15 k = 13 k= 5
We consider that it is prudent, when considering performance over a normal period of years to base designs on the figures given in Table 113A. TABLE 113A APPARENT HEAT TRANSFER COEFFICIENT FOR WERKSPOOR CRYSTALLISERS. RECOMMENDED VALUES FOR DESIGN (B.Th.U./sq.ft./h/°F)
For A massecuites For B massecuites For C massecuites
k = 12 k = 10 k= 4
Calculation of cooling surface. We have: _ Vdc To — T , o S = 60a — — In A: (7o — 0 — (×to)
To — / T—to
(384)
S = cooling surface of the Werkspoor, in sq.ft. a = factor taking into account the diluting molasses added V = volume of massecuite (before dilution), in cu.ft./h d = specific gravity of the massecuite = 1 . 5 c = specific heat of the massecuite = 0.44 l k = heat transfer coefficient, in B.Th.U./sq.ft./h/°F, given by Table 113 or 113A To — temperature of the massecuite entering the apparatus in °F T = temperature required for massecuite leaving, in °F to = inlet temperature of cooling water in °F / = outlet temperature of water, °F. We assume generally: To = 167°F T = 104°F / = 122°F
a n d a = 1.15
(385)
We would recall that: In x = 2.3 log x Cooling time. In the cane sugar factory, the cooling times allowed are generally as given in Table 114.
33
WERKSPOOR CRYSTALLISER
541
TABLE 114 COOLING TIMES GENERALLY ALLOWED IN WERKSPOORS
A massecuite (if boiling 3 massecuites) 1st massecuite (if boiling 2 massecuites) B massecuite (if boiling 3 massecuites) C or final massecuite
\\ h 2 h 2 h 12-15 h
Standard dimensions. The Werkspoor licence for France has been acquired by the Compagnie de Fives-Lille. This firm manufactures crystallisers for A massecuite of which the diameter D increases from 1.300 m (50 in.) to 3 m (10 ft.) in steps of 1 decimetre. The discs have an outside diameter D' about 20-30 mm (1 in.) less than the interior diameter D of the vessel (£>' = 2.780 m, for example, for a crystalliser of D = 2.800 m). The open sector of the discs is 45°, and their unit cooling surface s may be calculated approximately by: 5 = 1.2D'2
(386)
s = cooling surface of one disc, in sq.ft. D' = exterior diameter of a disc, in ft. Actually the coefficient 1.2 varies from 1.13 for D' = 1.280 m up to 1.27 for D' = 2.980 m and it would be more accurate to write: s
= (l + 0.03Z>)D/2 (Br. units)
(387)
or for D in metres: s= (l + - ^ - ) Z>'2 (m. units)
(387)
The height of massecuite level above the axis is about 1/20-1/10 of the diameter. However, the capacity of the crystalliser is generally estimated as if the tank contained no discs, shaft, or pipes and assuming that the massecuite level was limited to the axis (which amounts to assuming that the volume of massecuite above the axis compensates for the volume occupied by the shaft, the discs, etc.). For the working volume, so defined per unit length of the tank, we have therefore: nD2 u= - — = 0.3927Z)2 (388) 8 u = theoretical working volume per unit length of the crystalliser, in cu.ft./ft. D = diameter or interior width of the tank in ft. The tank of the crystalliser is generally proportioned so that the length is about 3-3.5 times its diameter, without allowing this secondary consideration to override the eventual require ments of space required and difficulties of installation. The space between discs increases from inlet to outlet of massecuite, but the mean value of this spacing should not fall below 20 cm (8 in.). It is determined by the formula: -
ß-W+P) \-p
e = average spacing between discs, in ft. L = total length of the crystalliser tank, in ft.
(389)
542
33
CRYSTALLISATION
p = number of intermediate bearings (the length between bearings should be at least 10 ft. and at most 20 ft.) n = number of discs (which should always be an odd number). Design of a Werkspoor crystalliser. Data. We shall assume we are dealing with a factory working at 59 t.c.h., employing a 3-massecuite system, and obtaining 3.59 cu.ft. of A massecuite per t.c. We require to calculate the dimensions of a Werkspoor crystalliser intended to treat the A massecuite. Volume and dimensions of the Werkspoor. The quantity of massecuite to be treated is: v = 3.59 cu.ft./t.c. or a total volume of: V= 3.59 x 59= 211.8cu.ft./h which will be increased by the addition of molasses for diluting to: V'= 211.8 x 1.20= 254.2 cu.ft./h. The time necessary for cooling will be 1 h 30 min according to Table 114. Hence the capacity of the Werkspoor, that is the quantity of massecuite which it must hold: C = 254.2 x 1.5= 381.2 cu.ft. We require then a Werkspoor of 381.2 cu.ft. If we allow for a length L of the vessel equal to 3.5 times the diameter Z), it will require: nD2 C= -—L=
nD2 —— x 3.5/)= 1.37D3= 381.2 cu.ft.
whence: 381.2 Z)3 = — — = 278.3 1.37
D = 6.5 ft.
We shall adopt therefore a diameter of 6.5 ft. For this diameter, the theoretical working volume is (eqn. 388): u = 0.3927D2 = 16.59 cu.ft./ft. hence the length of the tank:
£=
381 2
w
·
=23ft
·
Cooling surface. Tf the cooling water available is at 77°F and if we allow 122°F as temperature of outlet water, and a value of 12 B.Th.U./sq.ft./h/°F as heat transfer coefficient, we shall have (eqn. 384): r, .,* , «, 211.8 167—104 , 167—122 5 = 40 x 1.15 x ——— x —— — — —x In 12 (167—122) —(104 —77) 104 — 77
33
543
WERKSPOOR CRYSTALLISER
or: 5 - 812 x 3.5 x 0.51 = 1,449 sq.ft. ( l n ^ ~ „„ = I n 4 = 2.3 log4= 2.3 x 0.222 = 0.5l\ & \ 104 — 77 3 3 / The cooling surface of one disc has a value of (eqn. 387): 5
= 1.2 x 6.44 2 = 49.8 sq.ft.
hence the number of discs necessary: 49.8 Say 29 discs. We shall have for accurate value of the area: S= 29 x'49.8= 1,444 sq.ft. Let us now verify the average spacing between discs. With 1 intermediate bearing (eqn. 389): -
í ·29—1 ? + ,—! 1)
- 0 - 7 3 3 ft. = 8.8 in.
Quantity of water required. We have (eqn. 380): W= 1.15 x 211.8 x 1.5 x 0.44 x
167 — 104 — = 225 cu.ft./h 122 — 77
'
or approximately 0.4 imp. gal./sec. Comment. The use of a coefficient a = 1.20 in the calculation of volume and 1.15 in the calculation for cooling is intentional. The molasses from purging at the fugals is generally cooler by several degrees than the massecuite in the crystalliser, and its specific heat is slightly higher. Checking an existing Werkspoor. When a Werkspoor is installed and it is desired to know what temperatures of massecuite and water it would give for the quantities of massecuite to be cooled and with the available cooling water, we have: T—t
(
ks
kS
ë
To — t t—to To — T
a Vdc W
— r
(the symbols having the same values as in eqns. 380 and 384), hence we have: Ã = to + m(To — t) t = to + r(To — T) solving for Tand /:
544
33
CRYSTALLISATION
ô — /o(1i ~ m ) + 1 —
mTo(l — r)
(390)
imr
t =■ to + r(Tc >-T)
We may recall that: - X = In y = 2.3 log y. (1) lfy = e~* (2) Log e-* = —0.4343 x. (3) If a negative logarithm is found, for example: log a = — 0.372 we shall write: log a = Ú.628 Results. Table 115 gives some results which we have obtained, operating with 3 massecuites, with a Werkspoor handling the first 2 massecuites, A and B: TABLE 115 EXAMPLE OF RESULTS WITH A WERKSPOOR CRYSTALLISER
A massecuite B massecuite When dropped from pans Brix of massecuite Apparent purity of mass. Apparent purity of mother liquor Leaving the Werkspoor Purity of molasses (no washing) Total purity drop Purity drop of molasses Temp, of mass, entering crystalliser °F Temp, of mass, leaving crystalliser °F Temperature of cooling water °F Cooling time
94.4 80.2 61.5
95.6 65.5 50.1
56.7 23.5 4.8 163 115 75 1 h 40 min
46.2 19.3 3.9 162 118 81 1 h 45 min
In Java (I.S.J., (1940) p. 286) figures of the following order are reported: Cooling Purity drop of the mother liquor
25°C (45°F) in a period of 2 h 30 min 5°
Storage vessel. Since the Werkspoor operates continuously, and the pans batchwise, it is obviously necessary to provide an intermediate vessel to receive the massecuite between the pans and the Werkspoor. For this duty an ordinary mixer is used (or sometimes two). It is necessary that its capacity should be 1.5 times that of the largest pan discharging into it. Spacing between discs. We know that: (1) The speed of crystallisation is greater at higher temperatures. (2) This speed increases with the supersaturation, but in practice attains a maximum at a relatively low supersaturation. (3) Viscosity increases with supersaturation. It is therefore of advantage to work with a relatively low supersaturation and to cool as rapidly as possible the hot massecuite at the inlet, since it permits of a maximum rate of crystallisation. It is for this reason that the Werkspoor progressively increases the spacing between cooling discs from the massecuite entry to the massecuite outlet.
33
LAFEUILLE CRYSTALLISER
545
Reheating. Reheating of the massecuite before fugalling is often practised in order to reduce its viscosity {cf. p. 590). The Werkspoor lends itself particularly well to this operation; it is sufficient to provide a little extra length, so as to enable 3 or 4 reheating discs to be placed after the cooling discs. The calculation of this reheating portion is made in a similar manner to that of the cooling portion. However, the reheating should be rapid and of short duration, hence the reheating discs are placed close together. Use of the Werkspoor. Compared with ordinary crystallisers, the Werkspoor offers the ad vantage of taking up much less space and of improving the exhaustion. The latter advantage is perhaps most marked with A and B massecuites; with final massecuite, its use is somewhat more critical, particularly with massecuites of high density. However, it is finding increased use with low-grade massecuites, notably in Queensland, when used with efficient reheating arrangements. Lafeuille crystal User
This apparatus, also known as the crystalliser pan, serves either as pan or as crystalliser. It consists of a large cylinder, which rotates on its axis, being supported on rollers on which it rests per medium of 2 large rings (Fig. 300).
Fig. 300. Lafeuille crystalliser pan (U.C.M.A.S.).
Connections are provided for vacuum, for introduction of massecuite or syrups, for water and for vapour. All connections are made at the shaft: vacuum, massecuite and syrups at one end; steam, water and incondensable gas at the other. Inside the apparatus is a bundle of longitudinal tubes parallel to the axis. These tubes can be supplied with steam or with water. The apparatus is filled only to £ of the available interior volume. The rotational movement given to it then produces an effective agitation and mixing of the massecuite. Dimensions. The diameter is approximately 6.5 ft., the length variable and the capacity accordingly varies from 1,000 to 2,000 cu.ft. Speed of rotation. The speed of rotation is very low, of the order of 1 r.p.m. when used as a pan, and only \ of this when used as a crystalliser {I.S.J., (1934) p. 358).
546
CRYSTALLISATION
33
Power. The power required is similarly very low, of the order of 4-8 h.p. Heat transfer coeflBcient As for the Werkspoor, this has been determined by the experiment station in Java, and reported in an interesting article by Honig (T.S.J., (Oct. 1951) p. 41) giving values of: k = 30 to 45 kcal/m2/h/°C = 6.1 to 9.2 B.Th.U./sq.ft./h/°F
(391)
Cooling surface. The cooling surface per unit volume is approximately 2 sq.ft./cu.ft. Duration of the charge. This is approximately 6 h for an A massecuite, 12-18 h for lowgrades. Application. The Lafeuille is a relatively recent apparatus. It offers the advantage of taking up little space and of reducing the hydrostatic pressure to a minimum. It has given very satisfactory results in the Philippines (I.S.J., (1934) p. 358): (a) Very high brix of massecuite when discharged, being able to attain 98-100. (b) Very high purity dfcops» being able to attain 30-32 points. However, it is tricky to operate, particularly when used as a vacuum pan, and should not be left indiscriminately in the hands of the average operator.