Mar., 1918
T H E J O U R N A L OF I i Y D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
THEORY AND PRACTICE IN THE DESIGN OF MULTIPLE EVAPORATORS FOR SUGAR FACTORIES B y A. ..I WEBRE Received February 7. 1918
The purpose of this article is t o give a brief outline of the d a t a used in t h e proper design of a multiple effect evaporator for sugar factories, and its application in practice. We shall assume a general knowledge of multiple evaporation and, therefore, will not touch upon elementary considerations. I n order t o study in detail we will subdivide our discussion as follows: A-Heat transmission considered from the steam side of t h e surface. B-Heat transmission considered from the juice or liquor side. C--4 typical problem. D-Distribution of temperature drops in the various bodies. E-Heat balance showing flow of heat and liquor for the problem under consideration. F-Distribution of heating surface arrived a t from the heat balance. %Considerations necessary in t h e proper design of a juice heater. H-Proportioning of the bodies and vapor pipes. I-The entrainment problem, its cause, provisions required against it. A-HEAT
TRANSHISSION
CONSIDERED F R O M T H E STEAM
S I D E OF THE S U R F A C E
The transmission of heat from steam through the surface of an evaporator is very similar t o the operation of a surface condenser, with t h e exception of t h e fact that. t h e temperatures of t h e steam being condensed are higher and, therefore, t h e specific volume per unit
of weight much smaller. Professor E. W. Kerr in his excellent papers on the subject has shown t h a t t h e coefficient of heat transmission varies quite considerably with t h e temperature, being much higher as t h e temperature increases, and we reproduce a curve herewith showing this variation (Fig. I ) . We also find t h a t t h e presence of air or non-condensable gas mixed with steam or vapor has a marked re-
191
tarding effect. This is particularly true if a condition of approximate quiescence obtains within the steam space. It is, therefore, very evident t h a t in order t o obtain good results proper provisions should be made t o overcome this difficulty. There are designs on the market to-day in which this feature has been carefully
.......
.- ..
Lr
*5.'
_----
-Tic
3r.r'
Plc-
I
i
Typicol
- L a y o u t o f Tube. Sheetsr
-The
a
W e b r e Emperafar ond abora
-Soma
RIG+ Fed
snon
'5
--Grod~mlly Desreos
-Wedge
-
-
-3 S e c t en-
Shaped
-
FIG. 2
studied out, giving a rapid, uniform agitation on t h e steam side. It is t o be noted t h a t particularly under vacuum this difficulty will be more noticeable, for not only does a given amount of these non-condensable gases occupy a larger volume by virtue of the reduced pressure, but from the very fact t h a t t h e equipment is under a pressure lower than t h e atmosphere, whatever leakage in joints, sight glasses and fittings may take place, this leakage occurs from the outside inward, and becomes mixed with t h e vapor which eventually reaches the heating surface of t h e succeeding body, resulting in a n accentuated difficulty a t this point, so t h a t the provisions relating t o this trouble cannot ,be too thorough. Fig. 2 shows t h e writer's arrangement for this purpose. I n this connection i t can readily be understood t h a t i t is against good logic t o vent the steam space of each effect into its vapor space, for in t h a t way t h e noncondensable gases removed from the steam space of t h e first effect are in turn put into t h e steam space of the second, and so on, thus accumulating t h e undesirable results. This, of course, is doubly true if these gases have a tendency t o attack t h e surfaces. The best plan, and t h e one in general practice, is t o vent each steam belt into a large header connecting t o the vapor space of t h e last body. There are two details in connection with this header t h a t are well worth mentioning. The first is t h a t the header should be in a horizontal position with preferably a slight fall towards the last effect, and t h a t i t should enter t h e vapor belt without rising, for there is always a certain amount of vapor condensing in this pipe, and if the discharge
192
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
end is higher than the other, condensed water will accumulate in the lowest point, partially choking the pipe, or a t least offering a considerable amount of friction. Under these conditions the steam belt of the first effect which needs least venting will vent most freely, and t h a t of the latter effects which need the most venting will be prevented from doing so, due t o the increased pressure a t the point of attachment t o the header. The other point is t h a t the size of the pipe should be amply large so t h a t the discharge from the vent of the first effect will not prevent the last ones from discharging freely. I n fact, the writer has obtained best results on multiple effects having a large number of bodies by having an independent vent for the steam belt of the last body, and having the others connected t o the manifold referred t o above. It is useless t o state t h a t the steam compartments of the various bodies should be thoroughly drained and water gauge glasses should be installed thereon t o make sure t h a t such drainage is complete and thorough, otherwise, of course, water accumulates, blanking off the heating surface and decreasing the work t o that extent. I n brief, the above gives a fair idea of the problems t o be taken care of on the steam side of the heating surface. We now come t o a consideration of the liquor side. I-HEAT
TRANSMISSION
Vol.
IO,
No. 3
be seen under the surface of the water clinging t o t h e inside surface of the tubes. Finally the evaporator began t o boil and increased very quickly t o rapid agitation and circulation. When this took place three things happened which give a very good insight into the real operation of a n equipment of this sort: I-The pressure dropped very quickly from 11 lbs. t o about 5 lbs. 11-The level as indicated by the water gauge glass dropped t o a point about two-thirds the height of the tubes. 111-The level of the liquid inside showed intense agitation, and the mixture of steam bubbles and water rose to a point about 1 2 in. above the top tube sheet.
CONSIDERED FROM THE JUICE
OR LIQUOR SIDE
Liquid in ebullition in a n evaporator corresponds t o the cooling water in a surface condenser, but the problems involved and the general behavior of the apparatus in operation are quite different. Perhaps the best way t o give a good idea of these is t o describe an experiment which was made for the purpose. A small evaporator of the vertical tube type having a central downtake was used. The tubes were of copper 2 in. in diameter by 48 in. in length. This equipment is shown in Fig. 3. The steam side was baffled according t o a design originated by the writer and referred to above, and all necessary provisions were made in order t o obtain as nearly ideal conditions as possible. For simplicity's sake the evaporator was operated atmospherically, i. e., with the top off, so t h a t a good and uninterrupted observation could be made. The test was run with water, so as t o eliminate the density and boiling temperature loss (discussed later). The evaporator was filled until the level of the water was flush with the top tube sheet. The water was cold, about 60' F. The steam valve from a constant high pressure main was opened t o a fixed point and allowed t o remain thus, so t h a t the amount of steam flowing through was fixed. The air vent was opened t o such a point as to make sure that there were practically no dead gases in any part of the heating surface. A t first with the water still cold, the pressure in the steam space was j lbs., and as the temperature increased, this gradually rose until finally just before ebullition started it was 1 1 lbs. By looking down from the top a large quantity of small bubbles could
L
-Exper/hedu/ E_vC-ora+cr
/c*
FIG. 3
Let us consider carefully each of the above developments in detail with a view to logical interpretation. I-Inasmuch as the pressure dropped very materially, it follows t h a t the coefficient of heat transmission must have increased in proportion. This increase was due t o two causes: First, the bubbles of steam which clung t o the heating surface before ebullition started were liberated and the surface blanked off by these bubbles was now exposed t o contact with water. This is merely a duplicate of the oft-mentioned experiment of making a teakettle heat faster by stirring the water with a spoon and liberating the bubbles on the bottom. Second, before ebullition started there was a quiescent
Mar., 1918
T B E J O U R N A L O F I N D U S T R I A L A N D ENGINEERING CEIEMISTRY
condition on the inside of the tubes. Now instead we had a rapid circulation. This undoubtedly had a marked beneficial effect on the coefficient of heat transmission. It will be remembered t h a t Mr. Orrok i n his experiments on surface condensers found t h a t within certain limits the coefficient of heat transmission varied with the square root of the velocity. Estimating as near as possible, the velocity in this case was about 1 2 f t . per second. 11-The fact t h a t the water went u p on the inside of t h e evaporator and t h a t the water in the gauge glass went down is explained in this way: The gauge glass shows t h e static pressure a t the bottom; the water on the top is in a dynamic condition, for we must assume t h a t the velocity of the water coming out of each tube must be nearly equal t o the velocity of the vapor, and as stated above, this was 1 2 f t . per second. If this is so, the water left t o itself would jump about two and one-quarter feet above the tube sheet. Each tube then is spouting up a t this rate and a blanket of water is practically kept in suspension by impingement from below, and this impingement from the tubes near the downtake prevents t h e free flow of water from the outer diameter of the evaporator towards the center; in other words, interferes with circulation, the result being t h a t a large amount of water is kept constantly above the tube sheet. Now the net area of the diameter of the evaporator being much larger than the net area of the tubes, the original water which was contained in the tubes, now being held in the evaporator above the tubes, does not correspond t o as much height in the water gauge glass. I n addition, a good part of this water above the tube sheet, is in the form of a spray in mid-air, we might say, and this does not show on the water gauge glass. This lowering of the static pressure as shown by t h e gauge glass lowers the pressure on the bottom of the heating surface, and consequently tends to cause a n increase in the coefficient by giving a larger net temperature drop a t this point. To make i t plainer, before ebullition begins, the hydrostatic pressure a t the lower part of the tube is 4 f t . of water (the tubes being 4 f t . long), which corresponds roughly t o two points. The boiling temperature a t t h a t pressure is about 218' F. as against 212' a t the top of t h e tube. If we had 5 lbs. pressure on the steam side a t this point we would have had 2 2 7 ' steam, with a net drop of go, as compared with I 5 a t the top. Now with the lower static head, the boiling temperature a t the lowest part of the heating surface is reduced t o about 2 1 6 O , and, therefore, we can evaporate more inasmuch as the net temperature difference has been increased, which is the same thing as saying t h a t the coefficient of heat transmission as a whole has been increased. As a supplement t o our experiment, the evaporator was again filled t o the top tube sheet, brought t o ebullition as above, and the steam suddenly shut off, when the level in the water gauge glass went back t o exactly the starting point. Another experiment conducted with the same apparatus was t o carry a uniform pressure of 5 lbs. and maintaining the levels a t different points t o determine
I93
what would be the best working level, as shown by the water gauge glass. I n this case also there were several surprises. The method of operation was t o make time runs in which either the feed or the condensate were measured carefully and the coefficient of heat transmission determined from this. Fig. 4 represents t h e results obtained. Two things are apparent from this curve. The first is t h a t u n d e r our conditions of operation the best level was from one-fourth t o one-third the height of the tube; the second was t h a t the slowingup effect of carrying the level too high was much more than expected. It is easy t o estimate theoretically what this should be. It is represented by the dotted line above the graph. The other things t h a t were brought out by this test were t h a t contrary t o the common impression, circulation understood as the rapid traveling of the liquor from the bottom of the evaporator t o the top and back again has a negligible effect on heat transmission. Test runs were made a t the following levels above the bottom tube sheet: 6 in., 1 2 in., 18 in., 24 in., 30 in., 36 in., 4 2 in., and 48 in. At 6 in. and 1 2 in. the tops of the tubes were perfectly dry, and a
IO
30
40
FIG.4
thermometer placed immediately above the tube sheet, showed a superheat of a few degrees. The highest coefficient was obtained when tops of the tubes were just wet. I n this case there was no liquor above the tube sheet, a t all, only a little spray out of each tube, and this spray did not reach the down tube. Therefore, we can say t h a t there was no circulation in t h e proper sense of the word, inasmuch as none of the water returned t o the bottom of the evaporator v i a the central pipe. Of course, there was agitation and velocity of travel caused by the rapid vapor currents coming out of the tube, but there was no circulation. As soon as the level was carried beyond this point t o such a height t h a t water was going down the center pipe, the coefficient of heat transmission began t o decrease, the result being more and more marked, until when water showed level with the tube sheet in the gauge glass, the rate of evaporation was two-thirds of the maximum recorded. We can then safely make the statement t h a t we found the critical point of maximum work t o be t h e level required t o keep all parts of the surface wet. The function of circulation in the proper sense of
= 94
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
the term is evidently a thorough mixing of the solution in order t o prevent the material from concentrating locally, resulting i n excessive densities in certain parts of the apparatus and low densities in other parts. Under those conditions there should be selected for the running point a sufficiently high level so t h a t this mixing will take place. This is only slightly above the critical maximum point referred t o in the discussion above. Any additional liquor beyond this is a material detriment t o the successful and efficient operation of the equipment and should be carefully avoided. I n order t o accomplish this result, provision should be made t o insure positively against improper operation by using automatic level regulators, the selection of the design depending upon the character of t h e work. For instance, in non-crystallizing solutions in which the concentration is carried on continuously, a n overflow control is advisable, whereas in crystallizing solutions, or solutions concentrated in batches, a properly designed float control is necessary. It will be understood the above-mentioned working part of one-third the length of the tube only refers t o the particular equipment in question under t h e conditions of the experiment. Changing the proportions of the tubes, the temperature of ebullition, the surface tension of the material being concentrated, t h e percentage of solids contained, the viscosity, or the rate of evaporation, all affect the behavior of t h e equipment, and the best point should be determined in each case, and the control set so t h a t the level will be high enough t o secure proper circulation under all possible conditions of operation for the particular apparatus in question. Another factor which greatly affects the performance of any equipment is the accumulation of scale, dirt or incrustations on the liquor side of the tubes. It is, therefore, most essential for uninterrupted operation t o remove as far as possible all suspended matter and scale-forming elements before the material enters the evaporator. 1.t is beyond t h e scope of this paper t o discuss in detail the character of each, or the most advisable means for their removal, except t o state t h a t in the sugar business the usual method is t o clean out a t reguiar intervals, possibly once a week, by boiling out first with a weak caustic soda solution and then with muriatic acid. If the character of the scale is very obdurate i t may be necessary t o supplement this treatment by mechanically scraping the tubes. It is far better in extreme cases t o clean a t shorter intervals if the deposit is found t o accumulate very rapidly, as it is much easier t o remove two thin layers of scale t h a n one thick layer. I n this connection i t might be well t o state t h a t whereas rapid circulation has a tendency t o reduce the incrustations and change their character, i t is far from being a n infallible remedy. Especially is this true in the case of calcium sulfate, and calcium and magnesium carbonates. When the incrustations consist of mechanically suspended matter, on the other hand, rapid circulation greatly minimizes troubles from this source.
C-A
Vol.
IO,
No. 3
TYPICAL PROBLEM
Having briefly discussed general considerations, we shall now proceed more into detail by designing a quadruple effect in accordance with standard practice. We shall assume t h e following conditions:
...............
Capacity of factory per 24 hours.. 1,250 short tons Weight of juice, 120 per cent of wt. of c a n e . . 1,500 short tons Weight of juice per hour.. 62.5 short tons Weight of juice per hour.. 125,000 Ibs. Evaporation per hour required, 75 per c e n t . . 93,750 Ibs. Initial temperature at juice heater.. ,;. 75" F. Hot juice Ieaving heater.. 205' F. Hot iuice enterinrt evaoorator from defecator. . . . . . . 180' F. Init& density of-juic; entering evaporator. 13.7Brix Final density of juice leaving evaporator.. : 55 Brix Exhaust pressure available first body. 5 lbs. Vacuum obtainable.. 26 in.
...... ...................... ...................... ...... ..... ...... .......................
....... ... .... ............ ...........................
D-DISTRIBUTION
OF
TEMPERATURE
DROPS
IN
THE
VARIOUS BODIES
We will select in this case a quadruple effect with juice heating b y vapors from first body. The first item t o consider in our design is the drop of temperatures and vacua from one body t o the next, and the logical distribution of these drops.
FIG.5
The temperature of steam a t 5 lbs. is 2 2 7 ' I?., and Our total range of t h a t a t 2 6 in. vacuum is 125'. temperature, therefore, is 1 0 2 O , t o be divided u p among the four bodies of the evaporator. This is not equally divided, being much more on the last body t h a n on the first, for a number of reasons: I-The coefficient of heat transmission is not t h e same in all t h e bodies, but is much more in the first than in the last body. As stated before, therefore, i t follows t h a t we should divide up the temperature fall in each according t o t h e coefficient for t h e particular temperature of steam in the heating compartment. This is true even if we had purely surface condenser conditions, but we have no such conditions, and, besides, there are other factors t o consider. (Refer t o Fig. I.) 2-The coefficient referred t o in the above paragraph relates t o the actual temperature difference between liquor on one side and steam on the other. We find,
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Mar., 1918
however, t h a t as concentration goes on, and t h e percentage of solids increases, the boiling point rises materially above t h a t of water, which would be the same as t h a t of the vapor leaving the liquid. A curve is shown in Fig. 5 which gives this increase for normal cane sugar juice. We must then deduct from our total temperature fall the summation of these differences for each body. 3-It is true t h a t the pressure a t which ebullition takes place is in general represented by the vacuum gauge on each body, which also represents the pressure of the vapor going from there t o t h e succeeding effect, and, therefore, its temperature, but we must recall t h a t this is the pressure a t the surface of the liquid. Now, inasmuch as ebullition is going on from the top t o the bottom of the tube it is fair and logical
centration increases also seems t o use u p temperature drop. The proper determination of this loss is not yet thoroughly defined, but careful experiments by Professor Kerr seem t o indicate t h a t for practical purposes we can assume t h a t in cane sugar juices under average conditions it is substantially equal t o the boiling temperature loss. Without going into the detailed calculations, below is a table showing the temperature losses in each body and the distribution of t h e “working drop” taking into account the available coefficient of heat transmission as affected b y steam temperature, and finally the steam and vapor temperatures with their corresponding pressures and vacua (Fig. 6). BODIES Boilingjtemperature loss. Static head loss.. Viscosity loss.. w - a - +.Totalltemperature losses. Working,drops
.. ........ .......... . ...........
E-HEAT
.Om
1 1.5 1.0 1.5
2
3
2.0
3.0 3.0 3.0
2.0
-
2.0 -
4.0 11 0
6.0 12.0
__
............ .. .....
15.0 227-212 5 Ibs. 0 lbs.
BALANCE
SHOWING
Tota1:drops.. Steam and;vapor temp.. Pressure and vacua..
I
I95
4 9.0 8.0 9.0
-
9.0 14.0 -
26.0 20.0 -
18.0 194.0 9 in.
23.0 171 .O 17 in.
46.0 125..0 26 in.
FLOW
OF
HEAT
AND
LIQUOR F O R THE PROBLEM U N D E R CONSIDERATION
I Dro P ,dorktnd
I
clap
Fro. 6
t o say t h a t we should consider the average pressure throughout the length of the tube as representing fairly correct conditions, and not the pressure on t h e surface represented by the vacuum gauge. There will, of course, be a difference between the two which repres e n t s a net loss in the temperature fall, and should be taken into account in our calculations. I n this instance we will assume, in conformity with accepted practice, t h a t the tubes are 2 in. in diameter and 5 f t . long and t h a t the level is maintained a t one-third the length. Our static pressure a t the bottom of the tube will then be 2 0 in. of liquid, and the arithmetical mean will be I O in. I n converting this back t o mercury pressure, we must take into account t h e density of the liquid which is always higher t h a n t h a t of water. 4-The increased viscosity of the juices as the con-
Having studied t h e proper and logical distribution of temperature differences, we are now in a position t o make up a heat balance, showing the flow and distribution of steam, vapor, condensate and juices. I n making up this heat balance, in order t o avoid complications, we have assumed t h a t the specific heat of sugar solutions will be unity in‘ all cases. Of course, this is not so, b u t the error introduced is so small t h a t i t can be neglected. Also we have assumed t h a t t h e condensate coming out of each steam compartment will leave a t steam temperature. AS a matter of fact this condensate is always slightly cooler but the error introduced is very small. The cycle used is as follows: I-Juice at 180’ F. is fed t o the first effect, then to the second, t o the third, and t o the fourth, whence i t is removed in a concentrated condition by the syrup pump. 2-Steam is admitted into t h e first effect a t 5 Ibs. pressure. The vapor from here goes partly t o the heater, where i t warms the juice from 7 5 O t o 2 0 5 O , and the remainder goes t o the second effect; the vapors from the second t o the third, and from the third t o the fourth, and from the fourth t o the condenser. 3-The condensate from the first effect goes back t o the boilers, t h a t from the heater is wasted, t h a t from the second steam chest is passed t o t h e third, and from the third t o the fourth, whence it is removed by a pump. The heat balance and diagram (Fig. 7 ) represent theoretically what happens in the evaporator. T h e steam consumption, however, will be slightly greater, due t o radiation losses. These are comparatively small amounting t o about 21/2 per cent in a well-insulated equipment. It is advisable in basing calculations t o allow 5 per cent, so t h a t our consumption under these conditions should be 40,500 lbs. per hour.
T H E J O U R N A L OF I N D U S T l U A L A N D EhTGINEERING C H E M I S T R Y HBATR o w HEAT BALANCS~ FIRSTBODY Steam 38 250 lbs. at 5 lbs. = 38250 X 961 = 36,700,000 Deduct fdr heating 125,000 X (213.5 180 = 33.5) = 4,190,000
.................. =... ..................... 33,500 Transferred to No. 2 . .......... 91,500 SECOND BODY Vapor from No. 1 . . ....................... 32 5 10 000 Liquor flash 91,500 X (213.5 - 196 = 17.5) = 1:600:000 Heat available. ........................... 34,110,000 Deduct for juice heater 125,000 X (205 - 75 = 130) = .............................. 16,250,000 Available for evaporation.. ................. 17,860,000 17,860,000
L a t 194' = 981,
E
=
T~= .......................
18,200
......... 73,300 ~
Transferred to No. 3 . . THIRDBODY Vapor from No. 2 . . 17,860,000 Liquor flash 73,300 X (196 174 = 22) =.. 1,610,000 Condensate flash 16,750 X (212 194 = 18) = 302,000
....................... -
G-CONSIDERATIONS
........................
Transferred t o No. 4 .
- - - - -_ --
19,850
-
.......... 53,450
.......................==....
Vapor from No. 3 . . 19,772,000 2,135,000 Liquor flash53450 X (174-134 = 40) Condensateflas)h34.950 X (194- 1 7 1 = 23)805.000
..................
-
Available for evaporation. 22,712,000 22 712 000 I 1021
L a t 125O a 1021, E
....................... .......
22,200
-
Concentrated liquor out. 31,250 NOTE-The above figures are correct t o slide rule accuracy. All quantities in pounds. 1 Representing hourly work.
F-DISTRIBUTION
O F HEATING
NECESSARY I N THE PROPER
DE-
SIGN O P A JUICE HEATER
.................19,772,000
V n n-. a v a Rnnv --
I n this connection it might be well to correct a common mistake which consists of giving t o the first effect only as much more surface as is contained in the juice heater. This is entirely in error, for the evaporator is working with a temperature drop of only I 5 O , whereas.the heater has a mean temperature difference of practically 72O. Furthermore, the two units being of entirely different design and performing a n entirely different class of work, we can say t h a t their coefficients of heat transmission have no relation one t o the other except t h a t both transmit heat into juice through copper tubes.
-
Available for evaporation.. 19 772 000 L a t 171°= 995, E = = 995
No. 3
First effect.. Second effect.. Third effect ............ Fourth effect..
125,000
Available for evaporation.. 32,510,000 32 510 000 I, a t 212O = 970, E = u 970 -
IO,
........... 6800 sq. ft. ......... 3400 3400 sq. ft. ft. ......... 3400 sq. sq. ft.
LIQUORFLOW
................................
Vol.
S U R F A C E ARRIVED
AT
FROM THE HEAT BALANCE
The next step is t o determine the heating surface required and its proper distribution. The total amount of surface will depend upon the design of the evaporator and a unit basis must be taken which has been proved out in practice. In our case, we shall assume that we are contemplating a standard effect and we shall take an evaporation of 5l/* lbs. per sq. f t . per hour as a fair basis for a quadruple under the assumed conditions. Our total evaporation being 93,750 lbs. per hour, the surface required will, therefore, be 17,000 sq. f t . The actual heat flowing through the surface of each body per hour is as follows:
................ B. T. U. .............. 36,700,000 16,260,000 B. T. U. ................ 18 162 000 B T U .............. 20:577:000 B: T: U:
First effect., Second effect.. Third effect.. Fourth effect..
It is t o be noted that all the heat given up by the steam in the first body must be transmitted through the surface. It is likewise t o be noted t h a t the heat represented by the liquor flash in the other bodies does not have t o be transmitted. These facts have been given due consideration in t h e above. If we proportioned the surface in each effect in accordance t o the above, we should have four different-sized units. We, therefore, make a comparison by making the first effect of one size, and the other three of another size which will be an approximate mean of their individual requirements. By referring t o the figures, it is evident that practically we can do this, giving the first body twice as much surface as the others. Our distribution then will be as follows:
And while we are on the subject of the heater let say t h a t i t is a very material advantage t o bring the juice while passing through t o as near steam temperature as possible, for if supplemental heating i s t o be done i t must be accomplished by the use of expensive single-stage heat. This being true, the surface should be ample for the work. The steam side should be designed so as t o give good circulation with thorough removal of condensed vapor and noncondensable gas. The liquor side must be so proportioned as t o give rapid flow of juice through t h e tubes, thereby minimizing fouling. This, of course, is done by means of cells or divisions in the heads giving many passes from the front to the back. The writer has found t h a t with a high juice velocity excellent results are obtainable; indeed the juice can be heated t o steam temperature if t h e unit is properly cleaned a t regular intervals. With a good design, as outlined above, a coefficient of 2 j0 can be obtained; therefore, the surface requjred would be 1000 sq. f t . There should be two such heaters, as they have t o be cleaned frequently, the juice going through them not having been defecated. US
H-PROPORTIONING
OF
THE BODIES AND VAPOR P I P E S
Now going back t o the design of the evaporator, i t is merely a question of laying out the tube sheet to get the proper diameter. Downtakes or circulating tubes should be provided, so as t o get a complete mixing of the liquor being concentrated. The height above t h e tube sheet should be ample, not less than 8 t o I O f t . The steam pipes must be large enough t o take care of the vapors without undue friction. We find that a velocity of IOO f t . per sec. is good practice for exhaust pipes and vapor pipes except in the case of the last effect, when, due t o the low density of the steam, it is permissible t o increase this t o zoo f t . per sec. On this basis the following sizes are advisable:
............... 20 in. ............ 20 in. ............ 15 in. ............ 15 in. ............ 18 in. ............ 24 in. ............ 32 in.
Exhaust pipe.. Vapor ex. No. 1 . . Vapor to heater.. Vapor ex. No. 2 . . Vapor ex. No. 2 . . Vapor ex. No. 3 . . Vapor ex. No. 4 . .
By maintaining t h e velocities and sizes as above, t h e friction losses will be practically negligible. There are, of course, other small details of design which it is not in the scope of this paper t o discuss as we are
Mar., IgsS
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
197
2 200.
confining ourselves t o the important items. Such, for instance, are the size and arrangement of liquor pipes, drain pipes, air pipes, etc. I-THE
ENTRAINMENT
PROBLEM, ITS CAUSE, PROVISIONS
REQUIRED AGAINST I T
We now pass t o another phase of our consideration. This is entrainment. The loss of liquor from this source can be divided up generally into two classes. The first is foaming or frothing and occurs in the sugar industry only very rarely, in cases where the juices have been frozen or derived from burnt cane and fermentation has taken place. I t is very difficult t o overcome, perhaps the best method being t o boil a t a high vacuum. There are a number of other expedients, such as floating a small quantity of grease or tallow on the surface of the liquid, carrying the level very low, etc. This occurs so rarely, however, t h a t a brief mention of i t is sufficient. The other phase of the problem is loss by spraying or spouting of the tubes, the liquor entering the vapor pipes, The explanation of this is that,while evaporation is going on, theoretically a t least, some of the liquor leaving the top of t h e tubes must travel as fast as the vapor driving it out. It is easy t o estimate what this is, for obviously all of the vapor generated i n each tube must pass through its upper end. Knowing the surface of the tubes, the rate of evaporation per tuba and the pressure or vacuum in each, we can
readily determine, not only the velocity, but the maximum height t o which drops will be projected in this way. Below is a table giving this information for each effect. s p . VOl. Steam Vol. per Evaporation Speed Mean Height Cu. Ft. Tube per Hr. per Sec. of Pro’ection per Tube@) BODIESLbs. Der Hr. per Lb. Cu. Ft. Ft. 26.8 346 First effect.. 5.01 0.390 12.9 Second effect 524 7.60 0.896 14.0 37.4 Third effect 896 13.00 2.625 58.6 15.3 176.7 3020 43.80 29.800 Fourth effect. 17.1 ( a ) Tubes are 2 in. 0.D.. No. 16,5 ft. 0 in. long as stated previously.
.. ........
A.
It is therefore apparent t h a t under normal conditions, spray in the first and second effects is negligible. This is not true of the third effect and very far from being so in the last. I t must be remembered t h a t not only is this spray projected very high in the last body, but in the very act is broken up into a fine mist, which is floated along by the upward vapor currents. Therefore, one cannot be too careful in providing against loss of liquor from this source. I n addition t o the regular separators or catchalls commonly used, the writer places baffles in the vapor space where by shifting the direction of vapor currents a t a low velocity, it is possible t o secure a n excellent preliminary separation before reaching the catchall, for i t must be remembered t h a t once this spray enters the separator the velocities obtained are so high t h a t the particles of liquor break into fine ones by impingement against the baffles, resulting in a very fine mist which floats along with the vapor t o the condenser, with resultant loss.
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Along this same topic, it is a very wise precaution to use great care in admitting the liquor from each body to the succeeding one. If it is simply allowed t o enter into the bottom below the tube sheet, the “flash” will be local, causing the tubes in the immediate vicinity to spout violently, projecting liquor t o the dome. The proper remedy is t o distribute this feed in the bottom by means of a perforated coil or manifold, or if this is not used, t o provide a flush pot or recipient on the outside of the evaporator, with the upper part connecting with the vapor belt and the lower part with the bottom. Still another way is to use a spray pipe above the tube sheet, and still another is t o feed from above with the pipe extending t o the center, the opening facing downward. Many have the tendency to make small of this problem, but when it is recalled that enormous quantities are treated in a given time, it will be found that the game is well worth the candle. For instance, a loss of ‘/4 per cent in the evaporator contemplated above would amount t o about Boo lbs. of sugar per day, worth, on a six cent basis, $48, and in a campaign of one hundred and twenty working days, this would be $5760, which justifies almost any kind of provision to recover it. And yet there are many evaporators which lose more than l/4 per cent, but the man who owns it does not know, for evidently the loss is greatest in the last body when the vapor goes into the condenser, and in so doing is diluted about 30 to I . E. B. BADGER & SONSCOMPANY BOSTON,MASS.
NOTES ON THE ANALYSIS OF MOLASSES By HERBERTS. WALKER Received January 11, 1918
I n comparing the results of a large number of determinations of sucrose in final molasses analyzed by students a t the College of Hawaii and by myself, I have noticed that the same sample of molasses appears to contain from 0.5 per cent t o 1.0 per cent less sucrose if clarified with dry lead subacetate than if the lead subacetate solution is used. These discrepancies were a t first attributed t o personal errors, but as the differences invariably persisted in the same direction, an attempt was made t o trace out their causes and ascertain which, if either, of the two methods of clarification could be relied upon. The method of clarification by lead subacetate solution used in this laboratory is that prescribed by the Hawaiian Chemists’ Association. 3 5.75 g. molasses are dissolved in water, clarified with 40 cc. of a solution of basic lead acetate of 54’ Brix, made up to 2 j 0 cc. with water and filtered. 50 cc. of the filtrate are treated with I cc. of a saturated solution of aluminum sulfate, made up to 5 5 cc. with water and filtered. Reading (in a 2 0 0 mm. tube) multiplied by 2 gives the direct polarization. 7 5 cc. of the original filtrate are inverted by the Herzfeld method and made up t o I I O cc. Reading multiplied by 8/3 is the invert polarization. The factor used is 1 4 2 - o.5t. For clarification with dry lead subacetate a method derived from that proposed by Cross and Taggartl 1
Louisiana Bulletin 136.
Vol.
IO,
No. 3
has been tried. 35.75 g. molasses were dissolved in water and made up t o 250 cc., then clarified with 1 2 t o 1 5 g. dry basic lead acetate and filtered. About 5 0 cc. of the filtrate were de-leaded and made slightly acid by the addition of 0.3 g. dry powdered sodium bisulfite and filtered for direct polarization. 7 5 cc. of the original filtrate were inverted and made up as in the previous method for invert polarization. Since. the same concentrations of molasses and of lead subacetate were used in both methods, the direct polarizations were both made in a slightly acid solution and the inversion procedure was identical, i t follows t h a t the difference in results must have been due either t o the volume occupied by the lead precipitate causing too high results in the ‘(wet” method, or t o the dilution in the “dry” method produced by an excess of lead going into solution over that required t o precipitate impurities, which would tend t o yield too low figures. VOLUME OCCUPIED B Y THE LEAD PRECIPITATE
35.7 5 g. of a waste molasses were dissolved in water, clarified with 40 cc. lead subacetate solution and the precipitate washed by decantation during a period of several days until the clear decantate from four consecutive washings showed no polarization in a 400 mm. tube. This sugar-free lead precipitate was transferred t o a 2 5 0 cc. flask together with 2 2 g. granulated sugar, made up t o the mark with water and polarized in a 400 mm. tube, giving R = 68.36. 2 2 g. of the same sugar made up with water alone in the same flask read 67.46. The difference of 0.90 or 1.33 per cent of the total polarization could have been caused only by t h e volume occupied by the lead precipitate. The volume left in the flask for the solution in this case must have
X 67‘46 -~ - 2 4 6 . 7 2 cc. The 68.36 precipitate itself then occupied 3 . 2 8 cc. The sugarfree lead precipitate from another 35.75 g. sample of this same molasses was placed in a 2 5 0 cc. flask with 3 2 . 5 0 g. granulated sugar, made up to the mark with water, filtered and polarized in a 400 mm. tube, giving R = 101.20. The same weight of sugar dissolved in The presence of the 2 5 0 cc. water alone read 99.80. lead precipitate caused an increase of 1.40 per cent of the total polarization. If the molasses from which this precipitate was made contained say 35 per cent sucrose, its apparent value would be increased by 35 X 0.014, = 0.49 per cent sucrose. The washed lead precipitate from 35.75 g. of molasses from another plantation was still more voluminous. Duplicate tests on it were as follows:
been not
250
cc., but
250
READING 99.74 99.83 32.50 g. sugar made up to 250 cc. with water alone.. 32.50 g . sugar made up to 250 cc. with lead precipitate and water .. . 101.63 101.87
. .... .. ............. . ... .. .. ...... .. .. ........ . . . . -2 . OB Increase due to lead precipitate.. , . . . . . , . . . , . . . . . . . . . . . 1.89
..................
AVERAGE
1.96
If this molasses contained 3 5 per cent sucrose it would appear t o contain 35.69 per cent if analyzed by t h e H. C. A. method, providing there were no other errors in the method.
