December, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
chemical rarely makes up less than 5.0 per cent and usually is over 10.0 per cent of the dry weight. If such concentrations could be maintained under conditions conducive to decay, they would undoubtedly exert considerable preserving action. But leaching is usually a factor under such conditions, and all of the nitrogen compounds appear to be easily and completely removed from wood under conditions favoring leaching. By heat treatment it might be possible to fix a part of the urea so it would resist leaching, but i t has not been established that such action takes place under present kiln-drying practices. It is logical to conclude from these considerations that, when wood treated with these nitrogen compounds is used under conditions decay hazards, it be given a standard preservative treatment.
1515
Literature Cited pp. 20-21 (1) Aasoc. of 0 5 c i d Agr. Chem.. Methods of Analysis. . ._ (1930). (2) Berliner, J. F. T., Mech. Eng., 64 (3),181-6 (1942). (3) Champion, F.J., Am. Foreats, 42, 178-9 (1942). (4) Findlay, W. P.K., Ann. Botany, 48, 109-17 (1934). (5) Hart, E.B., Am. Miller, 70 (l),151-2 (1942). (6) Hill, D.F., and Mottet, A., Progress Rept., West Coast Lumbeman's Assoc., 1941. (7) Loughborough, W.K., Forest Products Lab. Ciro., 1938. Loughborough, W. K., Timberman, Feb., 1938. @) (9) Nelson, L. A., Zbid., July, 1938. (10) Schmitz, H., and Kaufert, F., Am. J. Botany, 23, 635-8 (1936). (11) Sinden, J. W., Penna. Agr. Expt. Strt., Bull. 365 (1938).
PAP^ 1998, Soientifio Journal Series, Minnesota Agrioultural Experiment Station.
Shattering and Cracking of Ice ROLE OF CARBON DIOXIDE PHILIP W. WEST Louisiana State University, Baton Rouge, La.
Waters having low total dissolved solids are capable of causing cracking problems when used for the manufacture of ice. In such cases the usual methods of eliminating cracking often fail. The effect of free carbon dioxide has been studied in these cases, and it has been found that carbonation of the waters being frozen will often eliminate cracking difficulties. In view of the present findings it is possible to reinterpret the results of earlier investigations. Where treated waters were alkaline and cracking resulted, it was probably caused by removal of the free carbon PROBLEM which has long confronted the ice industry is the tendency for certain manufactured ices to crack and shatter when frozen at low temperatures. Since
A
the capacity of any given plant is greatly increased by maintaining the lowest freezing temperatures practicable, it is of considerable economic interest that good ice be frozen at low temperatures. The production of manufactured ice is usually carried out by submerging cans, containing 300 pounds of water, in brines held a t 12' to 20" E'. The ultimate aim of each manufacturer is to use the lowest brine temperature consistent with the production of marketable ice. The desirability of low freezing temperatures is obvious from the relation between freezing time and temperature. Burks (4) gives this relation aa
dioxide. Where cracking tendencies were reduced by treatment with sulfuric acid or alum, the effect may be attributed to the resulting release of free carbon dioxide from the naturally occurring bicarbonates and carbonates in the water. No special equipment is needed for this new treatment. The cost per week for an average size ice plant will run about three dollars, using liquid carbonic as the source of carbon dioxide. Such treatment makes possible up to 30 per cent increases in production of ice by making feasible lower freezing temperatures. where F. T.
=
freeeing time, hours
6.25 = constant determined by experience 11 = can width at top, inoches
T = brine temperature, F.
Consequently, lowering the freezing time greatly increases the capacity of a plant of any given size. Typical plant data showing the effect of temperature on freezing time is shown in Figure 1. Unfortunately the use of low brine temperature leads to the formation of opaque ice, white butts, and other operational difficulties when the more highly mineralized waters are frozen. I n addition to the formation of opaque ice, decrease in brine temperatures increases the tendency of the frozen blocks of ice to crack or shatter upon their removal from the brine tank. I n general, the experienced chemist or engineer can predict from analyses of the water concerned whether or not transparent ice can be expected under normal operating conditions. When slight opacities are encountered,
INDUSTRIAL AND ENGINEERING CHEMISTRY
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they can often be corrected by lime softening, treatment with alum, conversion of bicarbonates to chlorides or sulfates by means of acids, or increasing the air pressure so as to ensure more complete agitation of the water being frozen.
K,
18 16 DEGREES
14
F.
Vol. 34, No. 12
ing results from these strains due to unequal expansion during temperature changes. Gibbs (7) points out that there are probably many causes of ice cracking. Among observed factors he lists the following: Neutralization of lime-treated waters by addition of aluminum sulfate reduces the amount of cracking, probably through conversion of bicarbonates and carbonates to the corresponding sulfates; addition of ammonium chloride reduces the ice-cracking properties of certain waters; limesoftened waters, where excess lime is present, tend to produce ice which cracks; waters containing carbonates will often produce ice which cracks; sodium-aluminate-treated waters often show increased tendencies to produce ice which cracks; alum, especially ammonium alum, sometimes increases icecracking tendencies.
12
FIGURE1. EFFECTOF BRINETEMPERATURE ON FREEZIKQ TIME 20
16
18
DEGREES
In the case of cracking and shattering, however, i t has often been impossible to predict the characteristics of ice frozen from different waters a t low temperatures. Furthermore, when cracking is encountered it is often extremely difficult to prevent. The effect of this situation upon the ice industry is of interest because of the importance of solid ice in the subsequent handling of the blocks after their removal from the freezing cans. Most plants must store their ice for some hours before it can be sold on the platform or route. During the handling of the blocks from the time they are removed from the cans, through storage, scoring, and delivery, they usually pass through many manually conducted operations. Handling of blocks which are weakened by cracks constitutes an industrial hazard to the workers. I n addition, cracked ice constitutes an economic loss because of the difficulty of scoring and reducing the size of the large blocks to the smaller sized cakes called for by the trade.
Work by Other Authors Behrman (1) calls attention to the importance of the composition of water as a factor influencing the cracking tendencies in manufactured ice. He points out that carbonates and bicarbonates are particularly troublesome, and that cracking difficulties can often be corrected by treatment of the raw water so as to convert them to the corresponding sulfates. Macintire (9) attributes cracking to the presence of bicarbonate ions, which he believes cause internal stresses during the freeze. Irwin and Puffet (8) state that water coagulated with ammonium alum influences the quality of ice produced by altering its mechanical properties. They propose the theory that the ammonium ions which are introduced into the water increase the solubility of calcium and magnesium carbonates and thus decrease the tendency for them to precipitate from solutions as the salts are concentrated in the core. Ormsby (IO) believes that temperature gradients within the brine tank cause strains during the freezing of ice. Crack-
14 F.
12
FIGURE 2. EFFECT OF CARBOK DIOXIDE ON THE CRACKIKG OF ICE The most thorough investigations of ice manufacture from the technological standpoint are those of Burks (2-6). Based on studies of the production of ice a t low brine temperatures (4), he concludes that the following factors are significant in preventing cracking: (a) Composition and concentration of solutions being frozen affect cracking tendencies; usually the greater the concentration of solids in the water being frozen, the greater is the cracking tendency. (b) The addition of ammonium ions usually decreases the amount of cracking. (c) pH is of minor importance when
TABLE I. ANALYSISOF WATER Hydrogen-ion oonoentration (pH) Odor
208
9.3
Slightly aromatic
+ PAOa)
0 . 8 ~p.. m. 8.0 181 .o 107.5 0.0 10.8 0.8 22.6
33.2
5.9 0.1 0.0 0.0
0.0 0.005 0.0
19.7 65.2
17.8 21.6 8.1
Hy othetioal combinations 8aco* Cas04 MgSO4 Na,SOc NaCl
44.5
52.4 29.2 7.2 32.5
December, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
1517
total solids, especially chlorides, are low; for more highly mineralized waters it is wise to keep the pH a t approximately 7.0. (d) Controlled annealing of ice may help prevent cracking. The work of previous investigators has a number of apparent contradictions. Some explanation may be had if we assume, logically, that there are probably a number of causes of cracking; therefore, a given treatment may eliminate trouble in one case and aggravate it in another. It should be noted, however, that where chemical composition of the “raw” water is c,onsidered as the source of cracking, carbonates and bicarbonates, especially those of calcium and magnesium, are usually blamed. Consequently, standard procedure for the elimination of cracking calls for softening of the water before freezing, conversion of carbonates to sulfates by alum or sulfuric acid, or for addition of various anticrack compounds of the ammonium chloride type. I n those cases where cracking occurs with waters which are only slightly mineralized, standard treatments usually fail. The following investigation deals with the causes and methods of preventing cracking in these latter cases.
Experimental Procedure PRELIMINARY EXPERIMENTS. An ice plant in southern Louisiana which had been confronted with cracking problems for a number of years was stu&ied. This plant had a rated daily capacity of about 42 tons. However, its actual production could not be increased beyond about 33 tons per day because of cracking which took place as production was stepped up through lowering the freezing temperatures. The water used in this plant was a municipal tap water (Table I) which had been taken originally from the Mississippi River. Treatment of the water consisted of ferrous sulfate coagulation, in the presence of sufficient lime to raise the pH to 9.0 or over, followed by filtration and chlorination. Although this water seemed ideal for use in ice manufacture as judged by present standards, it could not he frozen successfully a t low temperatures. Previous attempts to eliminate cracking had included lime softening, installation of all new cans, improving brine circulation through installation of new circulation propellers and a new bafRe system, and addition of various anticrack agents. An investigation of the physical characteristics of ice frozen in this plant was made. Using both petrographic microscopes and large sheets of Polaroid, numerous cakes of cracked and uncracked ice were examined for lines of strain which might be revealed in the form of interference figures under polarized light. Uncracked blocks showed no general areas of strain and cracked blocks showed strain patterns only along cracks. This indicated that general lines of strain due to temperature differentials during the freeze were absent and that cracking was due to induced strains after freezing. Further investigation showed that there were distinct differences in hardness (physical) between ices which were subject to cracking and those which were not. Noncracking ices were sometimes 30 to 40 per cent softer as indicated by penetration tests. An interesting observation was obtained from a near-by plant where cracking had just started after years of troublefree operation. I n this case water treatment had been
(Top) Opaque Block Caused by Insuscient Agitation of the Water during the Freese (Center) A Shattered Block of Ice (Bottom) A Strong Block of Ice; Opaque Core Formed When Air Agitation of Water Was Stopped
INDUSTRIAL AND ENGINEERING CHEMISTRY
1518
changed from alum coagulation to ferrous iron coagulation coincident with the onset of cracking. The effect of small amounts of aluminum and iron which might be left in the water after these respective treatments did not seem significant. The reactions taking place in each case did seem important: 3Ca(HCO& AMSO& 6,0 pH - 8.j * 3 ~ a ~ 0 4 +
+
2Al(OH)a
I
+ 6 COz
excess Ca(0H)Z (from raising PH)
(1)
' CaCOs
Reactions 1 and 2 represent coagulation as carried out by use of alum and iron, respectively. The general reactions are similar except that iron coagulation is accomplished at such a high pH that any free darbon dioxide liberated by the reaction is removed as the carbonate. Because free carbon dioxide is quite soluble in water (the solubility increasing with decrease in temperature) an experiment was made to see if it might not also be taken up in ice frozen from water containing carbon dioxide. The results of this experiment were as follows : Con in Water, P. P. M.
+
Vol. 34, No. 12
cracking of ice. The method of carbonation used was to feed liquid carbonic through a reducing valve directly into various parts of the system. The results of these studies are given in Table 11. The results obtained in the carbonation experiments indicated that the point of carbonation was probably unimportant. Since slightly better results were obtained when carbonation was carried out a t the storage tank, however, all subsequent feeding of carbon dioxide was carried out a t this point. The effect of free carbon dioxide on the cracking of ice frozen a t different temperatures was next considered. The results of this study are shown in Figure 2. 1320 Carbonation lowered the pH of the water to be frozen from an original of 9.3 to values from 7.2 to 8.5. To establish the fact that lowering thd pH was not the main factor reducing the amount of cracking, the pH was reduced with sulfuric acid in a few cans. No definite reduction in the percentage of cracked blocks mas obtained. As a further check a series of runs was made in which the pH was established a t various levels using sodium hydroxideboric acid buffers. The results are shown in Table 111. TABLE111. EFFECT OF pH
ON
CRACKING (AT 16" F.)
COe in Ice, P. P. h l .
%
Craoked Unbuffered Buffered with NaOH-HsBOa
The amount of cracking was reduced through the carbonation. Also, it was noted that the ice frozen from the carbonated water was distinctly Bofter (physically) than ice frozen from carbon-dioxide-free systems. OF CARBON DIOXIDEO N ICECRACKING TABLE11. EFFECT
Brine Temp., ' F. MidStart freeze Pull 16 16 16
%
Treatmenta Cracked Remarks Blocks tempered 15 28 min. 16 16 16 Cot fed a t dosing tank, 17 Cracks due largely t o bent drop blocks tempered 15 min. tubes 16 16 15 COe fed a t storage 3.5 Cracks a t drop tank blocks temtubes pered 15 min. 16 15 15 COz fed t1;rough air 4.0 Cracksduelargely t o bent drop line COe In air 0.5l.O%, blocks temtubes pered 15 min. a Where carbonation was used the concentration of COSin the water was adjusted between 15-38 p. p . m. ?or the start of the freeze.
.... ....
DATAAND OBSERVATIOKS. Based on the findings of the preliminary experiments, it was decided to install plant-scale carbonation and study the effect of carbon dioxide on the
Aclcnowledgment The author wishes to thank the Louisiana Public Utilities Company for their cooperation. He wishes especially to thank B. A. Vetter for his assistance and E. T. Love11 and S. IT.Elberson for helpful suggestions.
Literature Cited (1) Behrman, A . S., J. IND.EAG.CHEaf., 13, 237 (1921); Refrigera-
tion, 38, 56 (1926). (2) Burks, Dana, Jr., Ill. Eng. Espt. Sta., Bull. 219 (1930). (3) Ibid., 253 (1933). (4) Ibid., 254 (1933). (5) Burks, Dana, Jr., IND. ENG.CHEM.,24, 605 (1932). (6) Burks, Dana, Jr., Refrig. Eng., 22, 247 (1931). (7) Gibbs, R . E., Ice a n d Refrig., 83, 433 (1932). (8) Irwin, C. J., and Puffet, D. H., Ibid., 71, 221 (1926). (9) Macintire, H. J., Ibid., 63, 287 (1925). (10) Ormuby, E., Ibid., 70, 17 (1926). PRDSENTED before the Division of Water, Sewage, and Sanitation Chernistry a t the 103rd hleeting of the AXERICAXCIIBMICAL SOCIETY, BIemphis, Tenn. Louisiana State University, through the author, is making patent application t o cover the improved process as described here.
