Wood Deterioration in Cooling U
Towers CHANGES IN CHEMICAL COMPOSITION DURING DETERIORATION B. L. BROWNING AND L. 0. BUBLITZ T h e Institute of Paper Chemistry, Appleton, Wis.
T
HE extensive use of wood in the construction of cooling
towers involves the durability of the wood as a major consideration in the economics of cooling toxer maintenance. Thus, any contribut,ion to knowledge of the nature of wood deterioration during use in cooling towers is of possible value to the overall problem of wood utilization in this important type of application. I n general, the deterioration of wood in cooling towers is attributed to chemical action, biological action, mechanical removal and erosion, or physical solution of wood components. In practice, all four possible causes may be operative at the same time, the relative contribution of each being dependent upon the preThe primary reasons for vailing conditions of operation. deterioration and the relat,ive importance of the various possible contributing factors in a specific sitmuationare not a h a y s well understood or generally agreed upon in the cooling tower industry. Much of the wood deterioration in cooling towers has been termed delignification-Iq-ithout respect to the causative agent’ or t o removal of any specific component from t’hewood. For example, Blohm and Frazier ( 5 ) applied t,he term to “the removal of alkali-orbisulfite-soluble matter.” Baker. (3)and others have used the term rather generally to describe deterioration in which fibers are loosened a t the surface of the wood. However, in many instances of deterioration which are called delignification, it has not been shown conclusively that selective removal of lignin actually occurs. The chemical substances which may be responsible for cleterioration of wood either remove certain compoiients or so alter the components as to affect the mechanical properties of the wood. The most important chemical agentjs are thought t o be chlorine, which is added to control biological grov-th, and alkalinity, which results from softening of the water or from alkali added to minimize corrosion of equipment. There is no doubt that chlorine is capable of removing lignin from wood, as in higher concentrations it is widely used both commercially and in the laboratory for this purpose. Whether in cooling tower use the amount of chloriiie customarily applied is capable of producing serious delignification has not been clearly shown. Baker (5)stated that deligiiification is caused by sodium carbonate in the water, in agreement with the conclusion originally reached by Moberg (9). Blohm and Frazier (5) also attributed deterioration to sodium carbonate. They found that the yeight loss from yellow pine, cypress, and redwood, after steeping for 60 days in sodium carbonate solutions, was proportional t o the concentration of sodium carbonate. Baechler and Richards (2) exposed redwood sect,ions t o sodium carbonate solut’ionsa t boiling temperatures for periods up to 27 days and found that delignification occurred under these conditions, in that the ratio of cellulose to lignin in the residual wood R-as increased. They found that redwood removed from cooling towers was less resistant to fungal decay than unused redwood and that the cross sections exposed to sodium carbonate or sodium hypochlorite were less resistant than those exposed to Tyater.
Baker ( 4 )has stated that delignification may occur m-hen either carbonate or bicarbonate is present. However, for the p H range usually exist’ing in cooling towers (pH 8 t’o 9), calculation shows that practically all the carbonic acid salt,s would be in t,he form of bicarbonate ions. The argument that sodium carbonate produces delignification in cooling tower wood, by analogy with the use of alkalies for delignification in commercial pulping for the production of wood pulps, is not necessarily convincing. Sodium carbonate is not an effective pulping agent in the industry, and in pulping with other alkalies temperatures considerably in excess of 100’ C. are used. Xevertheless, it seems to be common opinion that sodium carbonate does contribute in some manner to wood deterioration. Chemical changes may result from bacterial or fungal iirtioii. I t is not always easy to ascertain that deterioration has becw caused by biological action alone. However, there is no doubt that conditions of t,emperature and nioieture favorable to biological growth occur in many cooling towers over considerable periods ol time. Baker (3) cited analyses of wood attacked by white and brown rot, and in these the ratio of cellulose (Cross and Bevan) to lignin was 1.04 to 1.32, as compared with a value of 1.42 for unused redwood. In thew instances of biological attack, the celluloPe had been preferentially removed and delignification had not occurred. The present work was defiigned t o st,udy the changes in chemic:al composition of wood exhibiting some common types of deterioration. The objective has been to relate chemical change to the type of deterioration. The type has been judged largely on the basis of physical appearance and the location in which the deterioration occurred. S.44IPLES FOR ANALYSIS
All the specimens were redwood and those exposed were taken from cooling t,owers in the Texas-Louisiana area. The identifying descriptions of each specimen are as follows:
SANPLE 1. Unused redwood, taken from a freshly milled misteliminator slat. SAMPLE 2. Wood which had been exposed for 8 years, but appeared to be completely sound. Superficial deposits were present. The piece mas a mistreliminator slat. The average water pH during usc was 8.2 and the chlorine treatment was 0.7 p.p.m. SAMPLE 3. The surface of the specimen \vas ridged and fuzzy, with the typical appearance shown in Figure I . The springwood was eroded more than the summerwood. The piece was a misteliminator slat exposed for 10 pears. The general pH during USR was 9.0 and chlorine was used for algae control. SAMPLE 4. Att,ack occurred in the interior of the mood, as shown in Figure 2. The interior was soft and punky. with apparent removal of much of the wood substance. Microscopic exaIninat,ion of stained sections showed the presence of fungal hyphae in the tracheids and ray cells of this specimen. Because of the large numbers of fungal hyphae, i t is reasonable to expect that the attack was largely microbiological in nature. The piece represented a mist-eliminator slat after 7 years’ servim. The general pH was 7.4, and the chlorine treatment was 0.7 p.p.m. 1516
July 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
SAMPLE5. The surface of the wood was soft and brown. It had a checked appearance, similar to t h a t resulting from brown cubical rot (Figure 3, A ) . Microscopic examination of this specimen also showed the presence of fungal hyphae in the tracheids and ray cells. The piece was taken from a fill section after 10 years, and had been much reduced in cross section. The attack appeared to be limited t o the exterior portion, and below the deteriorated surface layer the wood appeared $0 be relatively sound. The recorded water p H was 8.7.
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method. The chlorite holocellulose was isolated by the method of Wise et al. (11). All analyses were made in duplicate. For each deteriorated specimen, portions representative of deterioration were used. These were outside Portions except for sample 2, from which superficial deposits were discarded, and sample 4, in part was used for which the inner The ples were prepared in a wiley to pass a 4 0 - ~ ~ ~ h sieve. ANALYTICAL RESULTS
Figure 1. Redwood Sample 3
SAMPLE 6. The wood was dark brown and was characterized by deep, roughly parallel fissures (Figure 3, B ) . This may represent a n advanced type of deterioration represented by sample 5, with a checked surface. The specimen was taken from a tower inner casing after 9 years. The only reported water p H was 8.8. SAMPLE7 . This specimen, shown in Figure 4, A , was taken from a tower louvre slat after 10 years. It had been exposed to air-borne fumes from a n acid plant, and the surface of the wood had been broken down into long reddish-brown threads. SAMPLE 8. This wood differed from the others examined, in t h a t the outer portion was white (Figure 4, B ) and individual fibers were easily separated. The piece was taken from a tower fill after 12 years. The water treatment had been variable, but the p H had been held a t approximately 8.5. CI
ANALYTICAL METHODS
I
The choice of analyses requires some consideration of wood composition. The major components of woods are extractives, lignin, and polysaccharides. The relationship of the components of extractive-free wood ha3 been shown graphically by Wise and Ratliff ( 1 2 ) . -4general outline of wood composition based on their chart is given in Figure 5 . It was desired t o characterize the wood specimens with a minimum number of analyses. The extractives and ash were determined as extraneous components. Lignin was determined as a major component. After delignification of the wood with acidified sodium chlorite to yield chlorite holocellulose, the a-cellulose content of the holocellulose was determined as a reasonably approximate measure of the cellulose content. Pentosans were determined as one of the major components of the hemicelluloses. The analytical methods used, unless otherwise stated, were those of the Technical Association of the Pulp and Paper Industry (IO). The alcohol and hot water extractions were performed successively. Pentosans were determined by the phloroglucinol
The results of analysis on the various prepared specimens of wood are given in Table I. It is evident t h a t most of the woods have picked up considerable inorganic material, as shown by the enhanced ash content. This is not significant, except in so far as the salts present may contribute t o chemical deterioration or cause physical disintegration of the wood b y mechanical action. The very appreciable ash content does mean t h a t appropriate correction must be made for other analyses because of the ash present. It is evident also t h a t all the woods exposed to water have lost a large portion of the alcohol-soluble extractives originally present. Other work by the authors, Baechler and Richards (2), and others has shown that the major part of the extractives must be lost during the first few months of exposure. I n the samples examined, it is not certain what portion of the extractives found was originally present in the wood, because a part may have been contributed by substances deposited from the water. Nitrogen was determined in samples in which microbiological action was suspected. The quantity of protein equivalent to the nitrogen found was by no means negligible, although no attempt was made to correct for it in other analyses. The analytical values for cellwall components can be compared best if the analyses are calculated to the ash-free, extractivefree basis. This basis is used in reporting analyses for lignin, acellulose, methoxyl, and pentosans in Table I. On the basis of lignin contents, the woods can be divided into three groups: (1)samples 2,3, and 7 have lignin contents not greatly different from that of unused redwood, (2) samples 4, 5, and 6 show greatly enhanced lignin contents, and (3) only sample 8 has a markedly reduced lignin content. Evidently, only in the last case has marked delignification occurred. The methoxyl contents of all specimens except sample 8 are nearly the same as t h a t of unexposed redwood. In woods having Figure 2. Redwood very high lignin contents, the Samole . * -4 methoxyl content has not increased proportionately, but this is evidently due t o the low methoxyl content of the lignin itself. It seems probable that demethoxylation of the lignin has occurred in these instances. The a-cellulose content of the wood has been drastically reduced in those cases of deterioration where lignin enrichment occurred. I n sample 7 , the low a-cellulose is attributed to the hydrolytic action of acid on the polysaccharides. The hydrolysis has reduced the molecular weight t o such an extent that the products are partially alkali-soluble. I n sample 8, the removal of lignin results in an enhanced a-cellulose content.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Vol. 45, No. 7
the composition of the deteriorated mood was originally the same as that of the unused redwood tested, which is true only as an approximation. Another is that the sample analyzed is identical with that on which the density measurement is made and, for practical reasons, this could not be done. Finally, it is assumed that no change in gross wood volume occurs during deterioration, because gross removal of portions of wood n-ould remove them from measurement. Despite the limitations of the treatment, it is believed that uscful comparisons result. Because of the irregular. shape and surface of the specimens, the wood densities in the present ivork were determined by measuring the buoyancy when the specimen waq immersed in mercury. I n 80 far as possible, the portion of the wood represented in the density measurement was limited to the deteriorated portion similar to that used for analysis. The values for extractives, lignin, a-celluloee, and pentosans, calculated on the basis of wood originally present, are given in Table 11, and those together with the values for ash are given in Figurc 6
Figure 3.
Redwood Samples A. B.
Sample5 Sample6
I n Table I, the ratio of lignin to a-cellulose in the various specimens shows clearly those examples in which lignin enrichment occurs and the one example in which delignification has taken place. The sum of a-cellulose and lignin, deducted from 100, is an approximate measure of the hemicellulose contents of the woods. If hydrolysis or oxidation of the cellulose has produced shortchain fragments, these also appear in the hemicellulose fraction soluble in alkali. The “hemicellulose” fraction was notably small in sample 4. I n samples 7 and 8 ) it is probable that degradation products of cellulose were included. The pentosans have proved markedly resistant. Even when deterioration had proceeded so far that the cellulose !vas removed, or altered so as t o be appreciably soluble in the a-cellulose test, the pentosans were not greatly affected. This also has been observed in studying the action of brown-rot fungi on wood ( 1 ) . The loss or apparent gain of individual wood constituents can be observed most effectively if the composition is calculated in each case on the basis of the wood originally present. For this purpose the apparent density of each specimen is determined. T h e density ratio is then calculated-that is, the apparent density of the specimen divided by that of unused redwood. If each analysis (on the same basis as that represented by the density specimen-Le., oven-dry unextracted) is multiplied by the density ratio, one obtains a figure for the constituent which is comparable through a series, regardless of what may have happened t o the other constituents. I n effect, the analysis is reported in terms of weight per unit volume. This procedure requires a number of assumptions. One is t h a t
Figure 4.
SAXPLES F R O U TABLE I. ANALYSISO F REDWOOD
Sample
So1,uble In AshQ, Alcohola,
%
%
0.1 1.2 8.2 1.6 9.6 21.2 35.5 18.6
10.8 1.6 0.6 3.1 2.8 1.8 8.QC 1.8
’
Soluble in Hot Water“,
%
1.4 2.8 1.4 4.6 2.8 3.1 4.60 2.4
Sitrogena,
%
0.p B
0.34 0.95
1.:4 e
Ligninbvc, % (.I) 35.6 39.5 36.6 58.2 65.5 60. 5 37.5 10.5
Calculated as % of oven-dry wood. Calculated as % on basis of ash-free, extractive-free wood. c Corrected for ash. d On basis of ash-free lignin. e N o t determined.
a b
Redwood Samples
A. U.
aC e l l u l o s e ~ ~ C , Ratio,
% (B)
A/B
42.1 38.4 42.9 33.6 21.6 15.4 22.9 56.2
0.85 1.03 0.85 1.73 3.03 3.93 1.64 0.19
CoOLIZrG
Sum A B,
+70
77.7 77.9 79.5 91.8 87.1 78.9 60.4 66.7
Sample 7 Sample 8
TOWERS
+% -B),
100
(A
22.3 22.1 20.5 8.2 12.9 24.1 39.6 33.3
Methoxyl. %b Inisolated In lignind woodb 13.8 13.4 13.8 10.0 9.0 9.6 13.4 9.8
6.37 5.99 5.72 6.40 6.48 6.84 6.63 1.96
Pentosansb
%
Qi 7 e
11.1 8.2 8.4 11.6 8.1
July 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
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attack. Areport byApenitiseta1. (1) on the attack of four brown-rotting fungi on sprucewood has shown that the compositions of the attacked Svb,ionc.a solubl. m ~ . ~ t !o rdp m , c ~ D l v . n i ~ .............................................. wood result in enhanced extractives ------_ and lignin contents and lowered Cross and Bevan cellulose content. The present work has given similar results in the case of samples 4,5, and 6, except that the wood exposed in cooling towers has no opportunity t o accumulate water-soluble extractives. The relative resistance of pentosans is noted consistently. Even the presence of so-called "white rot" is not necessarily evidence of delignification. Although the lignin may be preferentially removed under certain conditions (cf. 7 ) , both lignin and cellulose may be attacked rather uniformly ( 8 ) , or the attacked wood may even show a higher proportion of lignin than the original ( 6 ) . The nature of possible chemical Figure 5. General Composition of Wood attack by the action of chlorine and of alkalinity under conditions of cooling (the pentosans are included as part of the alkali-soluble polysactower operation is far from clear. Other work by the present charides). authors on redwood blocks exposed in a cooling tower with The extensive removal of extractives from all the specimens chlorine residuals up to 3 p.p.m. and pH's up t o 9.0 showed a exposed to water is clearly evident. It is also very clear that loss of extractives but no selective removal of either lignin or extensive removal of lignin has occurred in only two instances, polysaccharides after 8 months of exposure. samples 4 and 8. I n sample 4,the loss of lignin has been accomIt is not possible on the basis of composition of deteriorated panied by an even greater loss of a-cellulose, so t h a t the residual woods t o distinguish with assurance between chemical and biowood appears to have been enriched in lignin. I n sample 8, true logical attack. However, chemical analyses will give reliable delignification appears t o have occurred, although a portion of the information regarding the removal of specific constituents and a a-cellulose has also been lost. few well-chosen analyses serve t o establish the general change of composition upon deterioration.
TABLE 11.
DENSITY AND CALCULATED COMPOSITION BASISOF WOODORIGINALLY PRESENT
-4PPARENT ON
Total Apparent ExtraoaDensity, Density tives, Lignin, Cellulose, Pentosans, Sample G./Co. Ratioa % % % % 1 0.408 1.000 12.2 31.2 37.0 8.6 2 0.358 0,877 3.9 32.7 31.8 3 0.302 0.740 1.5 24.4 28.6 b 4 0.130 0.319 2.5 16.8 9.7 3.2 5 0.250 0.613 3.4 34.5 11.3 4.3 6 0.240 0.588 2.9 26.3 6.7 3.6 7 0.596 1.461 19.7 28.1 17.1 8.6 8 0.266 0.682 2.7 5.5 29.3 4.2 a Apparent density divided by apparent density of unused redwood (0.408). b Not determined.
Sample 2, representing used wood which is apparently sound, showed a slight loss of or-cellulose with little change in lignin, and no major changes in composition seem to have occurred except for loss of extractives. I n sample 3, the various components of the ash-free, extractive-free wood have been removed in approximately equal proportions. I n samples 5 and 6, severe losses of polysaccharides have occurred. The quantity of lignin does not seem t o have been changed greatly, so that apparent enrichment in lignin has resulted. Of the six types of deterioration examined, only one (sample 8) clearly represents delignification. It may be fair t o question whether some examples which have been attributed to delignification in the past may have, in fact, been of other types. The observations, in the present work, of lignin enrichment rather than delignification suggest the prevalance of biological
LITERATURE CITED
(1) Apenitis, A , , Erdtman, H., and Leopold, B., Svensk Kern. Tid., 63, NO.9,195-207 (1951). (2) Baechler, R. H.,and Richards, C. A.. Trans. Am. SOC.Mech. Engrs., 73, 1021-5 (1951).
0Ash
Lignin
Alpha- C ~ I I U I O S *
100
Sample I
T N . ~ 88
Simple 2
TOIOI
Sample
Toto1
14
3
Told
Sample
32
4
Told
61
59
Sample 6
TOIDI 146
Simple 7
TOIOI
Sample 8
Total
65
I
0
I
I
I
I
I
I
I
20
40
60
80
100
120
140
Sample 5
Par Cl"l
Figure 6.
Composition on Basis of Wood Originally Present
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Baker, D. R., Industry and Power, 56, No. 3, 94-7; No. 4 , 86-8 (September and October 1950). (4) Baker, D. R., paper presented before 1951 Midwest Power Conference, April 1951. (5) Blohm, C. L., and Frazier, H. D., Oil Gas J . , 48, No. 45, 72-4 (3)
Vol. 45, No. 7
Technical Association of Pulp and Paper Industry, Xew York, "TAPPI Standards." (11) Wise, L. E., Murphy, M., and D'Addieco, A. A , , Paper Trade J . , (10)
122, NO. 2 , 35-43 (1946). (12)
Wise, L. E., and Ratliff, E. K., Anal. Chem., 1 9 , 4 6 9 - 6 2 (1947).
(1950).
(6) HBgglund, E., and Lange, P. W., Makromol. Chent., 6 , Staudinger
Festband, 280-91 (March 1951). (7) Heuser, E., Shema, B. F., Shockley, W., b p l i n g , J. W., and McCoy, J. F., Arch. Biochem., 21, NO. 2 , 3 4 3 - 5 0 (April 1949). (8) Luthardt, W., Holzforschung, 3, No. 4, 117-21 (1949). (9) Moberg, A. R., Calif. Oil World und Petroleum I n d . , second issue, 34, No. 14 (July 1941).
f o r review July 3, 1962. RECEIVED ACCEPTED March 9, 1953. Presented before the Division of Water, Sewage, and Sanitation Chemistry, SyrnDosium on K a t e r Conditioning for Use i n Cooling Towers, a t the 121st Meeting of the AMERICAN CHEMICAL SOCIETY,Milwaukee, Wis. This wurk represents in part the results of a cooperat.ive project of the California Redwood Association kt The Instirute of Paper Chemistrx, and is published with the approval of the association.
Self-Heating of Hydrogen Peroxide Storage Vessels E. S. SHANLEY Buffalo Electro-Chemical Co., Inc., Division of Food Machinery 6% Chemical Corp., Buffalo 7 , N. Y.
0
VER the course of many years, hydrogen peroxide has been
stored in hundreds of locations in quantities from a few to many thousands of gallons and at concentrations up to 90%. This experience has been almost free from untoward incident. However, hydrogen peroxide is an energy-rich material, and the constantly increasing interest in stronger solutions makes it advisable to gather all pertinent information about the behavior of this chemical in storage vessels. The first part of this paper is a theoretical study of the heat balance in hydrogen peroxide storage vessels; the latter part contains practical recommendations on the handling and storage of this material. The methods used for handling the heat balance calculations should be equally valid for predicting the storage behavior of other energy-rich materials. BEHAVIOR OF HYDROGEN PEROXIDE SOLUTIONS DURING STORAGE
All hydrogen peroxide solutions decompose a t a finite rate, and this decomposition releases a relatively large amount of heat (ca. 1200 B.t.u. for each pound of hydrogen peroxide consumed). Consequently, every hydrogen peroxide storage vessel must continually transfer heat to its surroundings, a corollary lbeing t h a t such storage vessels are always warmer than the surroundings. The magnitude of this temperature difference is ,established by the balance between heat released by decomposition and heat transferred t o the atmosphere. The actual mechanism, of course, involves a gradual temperature rise in the contents of the vessel until the rate of heat transfer t o the surroundings becomes equal t o the rate of heat liberation by decomposition. However, the rate of heat transfer t o the atmosphere increases only linearly with temperature, while the rate of decomposition increases exponentially. As a consequence, for any particular storage vessel there exists a critical decomposition rate beyond which the rate of heat liberation will always exceed the rate a t which heat can be transferred to the surroundings. Once a storage vessel passes the critical condition, a self-accelerating decomposition will set in which, unless checked, may reach a very high rate. As hydrogen peroxide solutions are nearly impossible to detonate (3) and vapor explosions are possible only over very strong solutions ( I ) , the primary hazard is due only to pressure rise in and possible rupture of the container. T h e actual outcome of sustained self-heating will depend in large measure upon the original concentration of the peroxide solution. Any solution stronger than about 12% hydrogen peroxide decomposing adiabatically in an open vessel can heat itself
t o about 100" C. Solutions weaker than 65% mossess insufficient energy t o evaporate all of the water present and formed by decomposition, so t h a t the temperature cannot esceed the boiling point. Adiabatic decomposition of stronger solutions leads to far higher temperatures-for example, 750" C. in the case of a 90% hydrogen peroxide solution. TlXiams, Satterfield, and Isbin ( 5 ) have recently published detailed calculations on these adiabatic decomposition temperatures. The possible behavior of hydrogen peroxide in a storage vessel can be visualized as follows. Entering storage a t or below ambient temperature, the solution will spontaneously heat itself until the rate of heat transfer to the surroundings equals the rate of heat liberation by decomposition. I n normal circumstances, this rise will be very small, of the order of l o C. or less. However, the tank contents may become contaminated so that the decomposition rate is increased. Depending upon the circumstances, a new equilibrium temperature may be reached, or the temperature and decomposition rate may increase a t an accelerating rate. At first, only oxygen is released, but approaching 100" C. the heat energy is available to vaporize w t e r , so that the reaction climaxeis with the production of large volumes of steam and oxygen. At t h e same time, any vents in the container are apt to become choked with liquid thrown up by the boiling action. At this point, the container is likely to fail, resulting in all of the effects to be expected upon rupture of a pressure vessel. FACTORS IZTVOLVED IN SELF-HEATING OF STOR4GE TANKS
The self-heating rate of a peroxide solution under any given circumstances will be governed by the balance betyeen heat liberated by decomposition and heat lost to the surroundings. Quantitative estimates of the magnitude of these factors are presented below. The heat of deconiposition of h j drogen HEATLIBERATIOX. peroxide has been taken as 1230 B.t.u. per pound. The decomposition rate, hence the rate of heat liberation, for any given peroxide solution in a given state of purity or contamination is primarily a function of the temperature. The studies of Schumb ( 2 ) as well as studies in this laboratory indicate that over a range of temperatures, concentrations, and decomposition rates, the rate increases about 2.2- to 2.4-fold for each 10" C. rise in temperature. Accordingly, the average value of 2.3 has been used to calculate decomposition rates a t various temperatures for various assumed rates a t 25" C. The assumed 25" C. rates range from 1 yo loss per year, acceptable in commercial practice, to 50 and 100% loss per year which might be observed a t 25" C. in the case of
