July 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
the flow rate. It is noteworthy that no visible precipitation occurred with 0.1 N sulfuric acid, and the capacity was nearly equal t o that attained after regeneration with hydrochloric acid of the same concentration. The first exhaustion with calcium chloride, which followed a 2 N sulfuric acid regeneration of resin in the sodium form, gave a capacity of 0.365; this confirmed the fact that the low capacities found are due t o incomplete regeneration and not to poor removal of calcium by the fully regenerated resin. It was assumed that the resin was more completely regenerated by sulfuric acid than the data indicate, but subsequently exhausted when the rinse water dissolved the precipitated calcium sulfate. A series of runs was made with 0.5 N sulfuric acid t o test this hypothesis. Regeneration was interrupted after 500
1409
ml. had been run, and the precipitated calcium sulfate washed out of the resin bed with 2000 ml. of water before regeneration was completed with the remaining 1500 ml. of acid. This raised the reproducible capacity to 0.203 equivalent, an indication that the assumption is correct. LITERATURE CITED
(1) Bauman, W. C., IND. ENG.CHEM.,38,46 (1946). (2) Bauman, W. C., and Eichhorn, J., J. Am. Chem. Soc., 69, 2830
(1947). (3) Kunin, Robert, IND.ENG.CHEM.,40,41 (1948). (4) Thompson, R. B., and Roberts, E. J., Chem. Eng. Progress, 43,97 (1947). RECEIVED April 27, 1948. This investigation was financed by the Hawaiian Sugar Planters' Association,
Maintenance of Heat Transfer bv Cooling Water Treatment J -
W. A. HALL AND ROY F. WESTON The Atlantic Refining Company, Philadelphia 1 , Pa.
A simple method is described by
'
which exchanger fouling can be evaluated in terms of throughput capacity. Results of a study of plant equipment are presented which demonstrate %he applicability and usefulness of the method. The fouling encountered was caused by slime deposits from the cooling water. Bacteriological, chemical, physical, and fouling data are given for two systems, both with and without water treatment. Water treatment consisted of phenolic-type compounds in one case and chlorination in the other.
will reproduce on synthetic culture media. These numbers indicate that probably 25% or more of the dry weight of the slime was bacterial cells. The remainder of the slime would consist of absorbed and adsorbed minerals and debris typical of the bacterial growth and the water supply. Table I1 gives typical chemical analyses of slimes. An accumulation of about 0.68gram of this dry slime per square foot of surface area caused approximately a 33% loss in heat transfer capacity for the design conditions of one condenser. The nature of the slimes indicated that chemical treatment of the cooling water could control exchanger fouling. Consequently,
D
URING the war, when maximum throughput rates were needed for hydrocarbon processing equipment which had
long on-stream time, difficulty was experienced with loss in heat transfer capacity due to the fouling of water-cooled heat exchangers. The main units involved were condensers in which light hydrocarbons were being condensed on the shell side and cooling water was passing through the tubes. Inspection of the units after only two months of operation showed slimy accumulainch thick, on the water side of the tubes. Some tions, 1/16 to tubes had become clogged with this slime. The procedure described for evaluating a change in heat transfer capacity due t o fouling of the transfer surface has been used to study the commercial units just mentioned; it employs data normally available, and does not require the elaborate instrumentation or study needed for determining the actual scale resistance t o heat transfer. Results correlated so well and the required calculations were so short that this procedure ha? been established as a routine weekly check on cooling tower treatment. Enough bacteriological, chemical, and physical data are given here to demonstrate the applicability of the evaluation procedure. A preliminary study of the slime deposits indicated that they were bacterial in origin. Capsulated slime-forming organisms similar t o those previously described (3-7) were isolated from the slime. Table I shows the bacterial content of typical slimes from the plant cooling systems, as estimated by cultural methods. The estimated bacterial contents of 1 to 5 billion organisms per gram of dry slime represent only those living organisms which
TABLE I. BACTERIOLOQICAL CHARACTERISTICS OB SLIME FROM CONDENSER TUBESOB UNTREATED SYSTEMS
Source of Slime 850 unit Refrigeration plant Crude laboratory
Total Bacteria Plate Count per G r a m of Dry Slime (Millions) Date Sabouraud's W a k e m p ' s Thioi n Nutrient dextrose fungi glycplate 1944 agar5 aparb agare mediumd 2/29 1300 ... 5/20 2930 426 40 5/27 5130 119 O.'i3 45
Solid culture medium of neutral reaction conducive t o the growth of a wide variety of heterotrophic bacteria. b Solid culture medium (PH about 5.5) conducive to the growth of yeast and molds. Solid culture medium (PH about 4.0) inhibitory t o most bacteria and conducive to mold growth. d Liquid medium for anaerobic and facultative anaerobic bacteria.
TABLE11. CHEMICAL CHARACTERISTICS OF SLIMEFROM CONDENSER TUBESOF UNTREATED SYSTEMS Compositiona, 9% 7 -
No.
Samples Source of Slime Avgd. 851 unit 3 1 850 unit Refrigeration plant 1 Crudelahoratory 1 3 farm units 16 Biological filter l b a b
Loss
on ignition 31.8 34.8 32.3 33.8 35.7 16.1
Oven-dried sample. Sulfur-oxidizing biological filter.
Insol. in HCI 30.8 25.6 36.3 35.6 34.9 60.5
RzOs CaO MgO CuO 29.4 30.9 30.3 27.4 17.4 16.3
1 . 1 0 . 7 0.0 1.7 1.0 2.2 0 . 6 1.3 . , 0 . 6 1.6 0.9 0 . 9 7:1! 1.2 1.7 ...
.
Loss and undtd. 6 2 3.8 -0.8 1.0 -.6
4.2
1410
INDUSTRIAL AND ENGINEERING CHEMISTRY
,
.0090I
The water flow was obtained by heat balance, from which overall heat transfer coefficient was calculated. The anticipated heat transfer coefficient for a clean unit was also calculated. The difference between the calculated clean coefficient and the actual coefficient gave an estimated dirt resistance. To obtain consistent dirt resistances, it was necessary to select condensers with suitable operating conditions as follows:
.0080
2 .0070 z t-
I? .0060
1. Over-all temperature difference of a t least 30" F. 2. Negligible correction for cross flow. 3 . Cooling water heated a t least 20" F. for reasonable ami-racy in calculating water flow. 4. Heat capacity of both streams linear with temperature. For example, condensers that desuperheat the vapor or subcoo) the liquid as well as condense the vapors are to be avoided because of the uncertainty of roper evaluation of At and the varying coefficient of heat transgr. I n the units tested the vapor entered the condenser at its dew point, and the condensate was collected in a drum a t vapor pressure. Thus the heat capacity w a p nearly linear with temperature and the normal At correction (2) applied. Impurities caused condensation over a temperature range.
P
+ E
.0050
10
20 TIME
- DAYS
40 50 A F T E R CLEANING
30
Figure 1. Time v s . Dirt Resistance for 850 Unit with No Chemical Treatment
a detailed study of tn-o plants was initiated to determine the efficacy of different treating procedures. Data were obtained on the bacterial and chemical charactcristics of the make-up and recirculated cooling waters, water treating procedures, the physical conditions of plant operation, and heat transfer capacity. Each plant had a forced-draft cooling tower system supplied with make-up water from the Schuylkill River; characteristics of this water are shown in Table 111. It is significant that the organic content of Schuylkill River water normally varies from 2 to 10% of that of an average municipal sewage and therefore provides ample food supply for bacterial growth. The quantity of make-up required to supply Jvindage and mechanical losses m.as of the order of 5% of the water circulation rate.
TABLE111. AVERAGE CHARACTERISTICS
OF SCHUYLKILL
MAKE-UPTT'ATER
Constituent, p.p.m. Hardness Calcium Magnesium Sodium Potassium Bicarbonate Sulfate Chloride Iron Silica Dissolved solids Suspended solids Ammonia nitrogen Phosphate phosphorus
PH
&day B.O.D. at 20' C . Total bacteria count per ml,
Vol. 41, No. 7
These dirt resistances were estimated from the time the unit was cleaned until it became quite dirty, while the mater was not being treated. The units were mechanically cleaned as part of regular maintenance. Such cleaning leaves the tubes visually free of deposits, but not polished or chemically olean as would be required for scale-free heat transfer coeffirients. A curve of dirt resistance for 850 unit with time after cleaning was obtained as shown on Figure 1, which indipales a typical rate of slime a r cumulation.
RIVER
85.4 32.2 12.6 22.2 3.3 63.3
101.8 18.6 0.22 8.2 254
'
60 ,0030
Figure 2.
.0040
I 1 .0060 .0060 .0070 D I R T RESlSTANCE
I
I
,0080
.0090
Calculated Percentage Through-
put vs. Dirt Resistance for Exchangers of 850
Unit
260
1.9
0.087 7.11 9.6
1,623,000
The cooling water system of one unit, known as 850 unit processing light hydrocarbons, was treated with phenolic-type compounds. The other unit was an alkylat,ion plant, known as 851 unit, on which chlorine was used. I n all the condensers studied the water velocity in the tubes ranged from 2 to 6 feet per second, which is high enough to keep solids from settling on the tube surface. HEATTRANSFERDATA
Heat transfer data were taken on both units t o determine the success of treatment. On 850 unit the transfer rates for three condensers were followed-namely, C-5 condensing isobutane, C-7 condensing normal butane, and C-18 condensing isopentane. The following data were taken on each condenser: hydrocarbon flow as measured by meter, temperature of hydrocarbon vapor to condenser and of condensate out, and water temperatures in and out.
This curve is the average of the dirt resistances calculated for the three condensers. Since these resistances were obtained by the difference betwccn an actual dirty cocfficicnt and an estimated clean coefficient, the absolute value is not significant and the accuracy for nearly clean tubes is poor. Owing to thc inaccuracy in determining the clean coefficients, the base value which represents a reasonably clean unit may come out either positive or negative. The negative values can be used just as well as the positive to determine trends on a comparative basis. However, the trends demonstrated by a number of points give a reliable indication of the condition of the units. T o show how this dirt resistance affects capacity, Figure 2 was prepared on the assumption that the water flow, hydrocarbon pressure, and hydrooarbon temperatures were constant, and that the hydrocarbon flow was reduced as the dirt accumulated to maintain these operating conditions. Figures 1 and 2 show that after one month of operation normal fouling would reduce the capacity by about 25% This prediction was borne out by actual plant experience. PHEIVOLTC TREATMENT
Similar data taken when phenolic-type treating compounds, dichromate, and hypochlorite xTere added successively to t,he
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
July 1949
i
1
140
I
I
I
II
I
I
a
I
Ioo,ooo,ooq
1411 I
I
c W
0
I
1
40,OOd
20
I
40
TIME -0AYS
y
,E;f Q M
k!
.00501
.
.0040. 1,
.0030 -
e
0
Figure 3.
I
I
20
40 T I M E - DAYS
8
..
.
8
I
60 AFTER
.
I
*
b
'-I
c-5
c-7 I
c-18
80 100 CLEANING
Figure 4.
-
, 120
Data for 850 Unit with Chemical Treatment
cooling tower are plotted on Figure 3. Although the points scatter, the level of these points is nearly that of a clean unit even after four months of operation. Inspection after operation at this level showed the tubes to be essentially free of deposit. Figure 3 shows the amount of chemical added and the residual concentration of chemical in the cooling tower for this period, The quantity of chemical is probably in excess of minimum requirements. Data not shown indicate that the heat transfer capacity of fouled surfaces may be improved somewhat by water treatment with phenolic-type compounds. Figure 4 gives typical bacteria counts in the cooling water system; the count a t times of no treatment was normally 2,000,000 to 6,000,000 per ml. At times of Santobrite or Nalco treatment the bacteria counts were somewhat higher, normally ranging from 15,000,000 to 50,000,000
60 AFTER
I
I
I
I
80 100 CLEANING
Time us. Bacteria Count for Circulating Water of 850 Unit
per ml., depending on the concentration of chemical. I n general, the higher counts were obtained with the higher chemical concentrations. For this type of treatment as many as 60% of the bacterial population were pigmented-Le., orange and yelloworganisms. Ordinarily pigmented organisms are present in very small numbers. Although crude cultures of bacteria developed slime in laboratory glassware a t Santobrite concentrations as high as 80 p.p.m., plant data indicate that both Santobrite and Nalco prevent excessive slime accumulations a t somewhat lower average concentrations. The average bacteria count of the cooling water appears to have no direct correlation with the effectiveness of control for these chemicals. This must mean that the mechanism of control is independent of bactericidal or bacteriostatic action in the water solution. CII LORINE TREATMENT
With 851 unit only one condenser, C-6 which handles isobutane, had suitable operating conditions for reliable heat transfer data. Typical data for periods with no treatment and with chlorine treatment are shown in Figure 5 . The unit was cleaned and run for 14 days without water treatment, and in this time the dirt resistance increased to approximately double the initial value. Figure 6, similar to Figure 2, indicates that this was equivalent to approximately a 20% reduction in capacity. Chlorination was then started and proceeded for 94 days. As Figure 5 shows, the resistance to heat transfer due t o dirt decreased, was nearly down to that of a clean unit after 50 days, and remained essentially constant during the remaining 44 days. Then treatment was stopped, and the dirt resistance again increased a t a rapid rate as in the first 14 days. These data show that it was not only possible to keep the unit essentially clean, but that once the unit had been fouled, water treatment gradually cleaned it. A gas IO0
I-
90
W
a
W
w
3
-I
.ooaoC
80 I-
a a 70
W
a 60
,0020
.0030
.0040 OlRT
TIME-DAYS
Figure 5.
AFTER
CLEANING
Data for 851 Unit, with and without Chlorination
IJ 120
.0050
.OOSO
.0070
.0080
RESISTANCE
Figure 6. C a l c u l a t e d P e r c e n t a g e Throughput us. Dirt Resistance for Exchanger C-6 of 851 Unit
1412
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE IV. CHEMICAL COSCENTRATIONS REQUIRED TO KILL” A PURECULTURE OF A LARGE SLIME-FORMING ORGANISM Concn., P.P.M., after Contact for 1 2 4 1.5 3.0 min. Chemical min. hr. hr. hr. Chlorine 11 7.5 6.0 3.5 2.5 Chloramine 29 20 13 8.7 5.9 >3.5 Silver ion Santobrite 4000 2350 1350 i96 460 a For comparison a reduction of 99.9% in bacteria count is used rather t h a n a complete kiil. 48-hour culture used in all cases. Initial bacteria counts raried from 31,000 to 310,000 per ml.
Vol. 41, No. P
I n both units water treatment made it possible to maintain high throughput rates for prolonged periods. Production and operating data confirmed the heat transfer data. Restricted production or operating difficulties were encountered a t the times relatively high dirt resistances were indicated, and high throughput rates were possible at the times low dirt resistances were indicated. Production expcriences, plant observations, and bacteriological data all indicate the practicability of using plant heat transfer data for evaluating the efficacy of slime control procedures. I
ACKNOWLEDGMENT
feed chlorinator was operated one hour out of every four, and data as shown in Table Is’ indicated that free available chlorine is probably essential to optimum slime control. Free available chlorine and chloramine chlorine were determined by the otolidine-arsenite method ( 1 ) . The bacteria count in the cooling water system was practically zero a t times of chlorinator operation. Although there mas practically complete loss of free chlorine and about one third or greater loss of chloramine chlorine in passing through the cooling tom-er, the bacteria count never increased to more than a few hundred per ml. a t times of adequate chlorine dosage, The mechanism of control here is definitely that of bactericidal action. Chlorine dosage was not optimum during a large part of the period shown on Figure 5 . I n this unit slime accumulations apparently sloughed off and did not reform a t times when the water temperature was about 130’ to 135’ F. or higher. This observation was confirmed by the inspection of condenser tubes at times of shutdowns.
The authors wish to acknolyledge the assistance of C. H. Welker, E. J. Rollins, and J. G. Demann in obtaining and organizing the basic data. LITERATURE C I T E D
(1) Am. Pub. Health Assoc., “Standard Methods for Examination of Water and Sewage,” 9th ed., 1946. ( 2 ) Bowman, R. A., Mueller, A. C., and S a g i e , K. M., Trans. Am. SOC. Mech. Engrs., 62, 283-94 (1940). (3) Holmes, J. A., Proc. 3rd Ann. Water Cons. Engrs. SOC.West Penna., 1942, 61-7. (4)Martin, R. B., Trans. Am. Soc. Mech. Engrs., 60,475-83 (1938). (5) Martin, R. B., and Dobson, J. G., Paper Trade J . , 121,No. 15, 39 (194.3. -_, (6) Sanborn, J. R., J.Bact,, 48, 211-17 (1944). (7) Wattic, Elsie, Sewage Works J.,15, No. 6, 476 (1943).
.-.
RECEIVED January 14, 1948. Presented as part of t h e 14th Annual Chemical Engineering Symposium of the Division of Industrial and Engineering Chemistry, AMERICAN CHEMICAL SOCIETY,Illinois Institute of Technology, Chicago., Ill., Other papers of this symposium appeared in June 1948.
of Soluble Organic ent Growths H. HEUKELEKIAN New Jersey Agricultural Experiment Station, Rutgers University, New Brunswick, AT. J .
A
process of aerating concentrated, soluble, nontoxic, organic wastes has been developed by returning to the raw waste the liquid i n which the biological growth is dispersed. Experiments, using the batch process, have been conducted to determine the importance of factors affecting the process and the extent of B.Q.D. reductions. The work was done with streptomycin and penicillin wastes.
HE activated sludge process with its modifications has become established as the main aeration method for biological treatment of sewage. It has been established that plain aeration of sewage does not bring about appreciable purification within practical detention periods. The tendency a t present is to pattern waste treatment closely after sewage treating processes. Among biological processes, activated sludge, trickling filters, and sand filtration have been successfully applied to diverse wastes. Certain wastes, however, are not amenable to treatment by such conventional processes because of the high concentration of organic matter. Other wastes may be more conveniently or economically treated by newer biological processes. Differences in chemical composition and concentration of certain wastes from sewage make the search for new biological methods imperative. Aeration in the absence of flocculent growths which develop characteristically from sewage, as a means of biological treatment of wastes, has not yet been studied.
Seeding is essential for high-rate biological activity. I n the activated sludge process this is acconiplished by returning a portion of the copious quantity of sludge formed. This procedure ensures a large number of organisms in a relatively small volume of seed material because the organisms are mainly aggregated in the sludge floc. It is conceivable, however, to treat certain wastes in which the growth is not so aggregated but is dispersed. The return of the liquor containing the dispersed growths may answer the purpose of seeding. Even if variable quantities of sludge are formed in some wastes, it may not be desirable to ieturn the settled sludge as seed. Either the settled liquor or the mixed liquor before settling may furnish the necessary amount of seed. Treatment by nonflocculent growths may be specially adapted to soluble organic wastes of high concentration. Such wastes would automatically produce much less sludge as result of aeration than would wastes of equivalent strength in colloidal OF settleable form. Furthermore, i t is difficult to produce flocculent growths from wastes containing high concentrations of soluble organic materials. Experiments are described to illustrate the importance of some factors and results to be expected when penicillin and streptomycin wastes are aerated in the absence of flocculent growths. Aeration was conducted in cylindrical glass tubes 2 5 inches in diameter and 24 inches long. Air was supplied through a diffuser bulb attached to the rubber stopper a t the bottom of the tube.
