Organic Research Laboratory, T h e Doiv Chemical Company, Midland, Mich.
Laboratory tests hawe shown that 30 Lo 50% sodium hydroxide solutions react with calcium sulfate scale more completely than do lower concentrations of sodium hydroxide. The oplimum temperatures are above 1 7 P F. Commercial heat exchange units have been treated with 30 to 50y0 sodium hydroxide solutions to remove the calcium sulfate scale. Aidctailed description is included here for the treatment of a 50,000gallon brine cvaporalor from which 100,000 pounds of calcium sulfate scale have been rcmoved.. A piece of scale chipped from the evaporator is shown i n the photograph at the left.
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w
rewive trcatcd ~ r a t c ra t all tirrics. Tlicrefoi~e,any boiler that uses an untreated sulfate water nil1 accumiila,tc H calcium sulfate I t induslry is also bothered with scale on the water side. T h the accumulation of calcium sulfate scale. Alany of the brinw contain a sinal1 percentage of sulfate, and, on continued ovaporation of water from the brine in the concentrators or evaporators, a scale of calcium sulfate deposits on the tubes, hinders heat, transfer, and lowers the efficiency of the evaporator. Alkaline solutions have been and are being used in removing heales from boilers. The early patents (1, 2, 4)tcll of many alkaline solutions n-hich have been patented as boiler scale rcniovers. Calcium sulfate Rill react Tvith alkalics (5)according t o tho following equations:
OST boilers and other heat exchange equipnierit IJoconxt A Fouled during operation as a rewlt of the deposition o n the heat exchange surfaces of various paits which are present in thc n:at,er., This fouling may be minimized by the treatment of water 1.0 reduce the total solids content of the incoming water and by blowing down boilers to reduce the total solids of water in t,he boiler. With all of these precautions, some sludge 01' scale accumulates in boilers. AIechanical moans were and arc still aiied t o rid the boilers of this ncc1m~ulation. A turhine tool is ii-c:d on water tube boilers; fire iulii, tioilers are dismantled, and t lie scale and awumulation are liainnieiwl off the outside of the 1 ubcs. \lore recently, chemical meail; of tloscnliiig heat-exchangers havc been introduccd. This r n t : i l i t ~ 1 1 conzists of introducing into the equipment an aqueous solution of a chemical Jvhieh \vi11 remt v i t h the deposits t o form soluble reaction products. The common mineral acids, such as hydrochloric, sulfuric and phosbccnuse of their good performphoric, are the most widely u ance, ease of inhibition, availa t.y, and low cost. Hydrocliloric acid will react v i t h many of tht: scxles relatively insoluble in water and change thcm over to rcndily soluble compounds: iron oxides arc converted t o iron chlorides, c.nlcium hydroxy phosphate t o calciuni chloride and phosphoric acid, calcium carbonate t o c:ilcium chloride and carbon dioxide. Among the compounds frequently found in heat, exchange equipment is calcium sulfate. This compound is only slightly soluble in dilute mineral acid solutions and does not lend itself very well to chemical treatment. Calcium sulfate is rommonly found in boilers and heat exchange equipment that are forced t o use untreated or only partly treated water. Boilers in schools, theaters, and laundries often use untreated water because these units are too small to mpport a water treating system. Since railroad boilers and marine boilers are part of a mobile unit, they cannot
CaSOa
+ 2NaOH
&(OH):!
+ n'n2SOa
The above reactions, hovever, when made n-ith dilute solutions ( l - l O % ) of sodium hydroxide or sodium carbonate are only surface reactions. After the scale becomcs coated with calcium hydroxide or calcium carbonate, the reaction stops. Then the scale must receive an acid treatment, as hydrochloric acid, to remove these reaction products and cxposc another layer of calcium sulfate scale. Then the alkaline treatment followcd by the acid treatment is repeated as many times as necessary to remove the scale completely. Laboratory tests on calcium s u l ~ i ~scale t e actually taken from a boiler show that more concentrated solutions of sodium hydroxide, 30-507,, have a much greater reaction rate and capacity than the lower concentrations. Temperatures of 175" F. or over are necessary for the reaction. The rate of reaction is directly proportional to the rise in temperature.
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April, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
395
SCALE TESTS
Most of the laboratory work was carried out on a scale turbined fr?m a raw water boiler of a large power plant in the eastern states. X-ray analysis showed it to be lo070 talcium sulfate. The chemical analysis showed:
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P
6 7 . 1 8 904 95.2 Cas04 0 . 2 Fe = 0 . 3 Fez08 3 . 3 SiOz &SSiO~ Total
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98.8%
The silica may have been present as a silicate such as serpentine or analcite, in which case the total would have been higher. The scale was inch thick and broke into flakes about inch in diameter. The color ranged from white to a pink tan. The same procedure was used for all reaction tests on this scale. Thus, comparable data were obtained as to the effcct of changes Figure 1. Reaction of Calcium Figure 2. Reaction of Calcium in time, temperature, or conSulfate Scale with a 50% Sulfate Scale with a 30% centration of sodium hydroxSodium Hydroxide Solution Sodium Hydroxide Solution ide solutions on the extent of the reaction. The time of the was washed three or four times with 100-cc. portions.of watcr. tests varied from to 24 hours, temperature from 75" to 230' The water was decanted in each cayc with extreme caution to reF., and sodium hydroxide concentration from 5 to 50%. For tain all of the scale. each test 100 ml. of sodium hydroxide solution (regardless of During the reaction of caustic solutions on calcium sullatc, concentration) were used. It was contained in an 8-ounce widethe calcium hydroxide and sodium sulfate formed were present mouth bottle and preheated a t the desired temperature for 30 as a finely divided precipitate, most of which was washed from minutes; 2.0 * 0.05 grams of calcium sulfate scale were then the scale with water. Then to make certain that all of the rcacadded to the sodium hydroxide solution. Only actual pieces of tion products were separated from the unreacted scale, a quick scale were used; any powdery scale was discarded. 1% hydrochloric acid rinse was given the scale followed by anAfter the duration of the test a t the desired temperature, other water wash. The scale was transferred to a watch glass, most of the solution was dqcanted from the scale, and the scale dried a t 225 O F. for an hour, and then rhweighed. In a few cases the finely divided precipitate which was washed away from the scale was analyced by x-ray diffraction to deterTABLEI. PERCENTAGE REACTION OF CALCIUM SULFATE SCALE mine whether any unreacted calcium sulfate was being washed away. The analyses revealed that the precipitates consisted WITH SODIUM HYDROXIDE SOLUTIONS mostly of calcium hydroxide, some sodium sulfate, and a triple ,-Time, Hours % NaOH 1/1 1 2 4 6 16 24 salt of sodium hydroxide, calcium sulfate, and sodium sulfate. No calcium sulfate was detected, The tests indicated that no Temperature, 70' F. 10 7 . 30 5 10 12 unreacted calcium sulfate scale was decanted from the bottles. 50 .. 5 2 5 2 10 .. The results of the laboratory tests (Table I) indicate the followTemperature, 150" F. ing conclusions: (a) At 70" F. the amount of reaction with 30 5 .. .. 58 60 *. .... or 50%sodium hydroxide is very low regardless of the time of the .. .. .. 60 10 65 .. .. .. 80 80 20 test. ( b ) At 150' F. 30% sodium hydroxide seems t'o be the 25 35 60 io 100 30 .. most active, while 50% has very little effect on calcium sulfate. 35 45 .. .. .. .. . . 40 .. .. 40 .. ,. .. (c) At 150" F. the percentage reaction with 30% sodium hy45 .. .. .. .. 4! 5 2 50 7 '5 droxide is directly proportional to the time. (a!) At 175" F. the Temperature, 175' F. amount of reaction obtained with 5 and 10% sodium hydroxide 5 60 .. 60 is about the same as a t 150 a F., but the percentage reaction with 10 .. 60 52 20-5001, sodium hydroxide shows a definite increase. The 50% .. 20 70 85 52 35 G i Qi 100 30 .. caustic which has been quite unreactive a t lower temperatures .. .. 70 35 .. 70 40 .. .. .. .. ,. has become reactive a t 175" F. ( e ) At 200" F. 50% sodium 75 .. .. 45 .. io 10 40 100 96 50 .. hydroxide gives the greatest increase in percentage reaction. (f) At 230" F.,10and 20% sodium hydroxide show the same reTemperature, 200' F. action as at 200 a F.,whereas 30% caustic gives an increase. 10 *. 55 55 60 .. 70 .. 20 .. 40 40 60 . 95 .. The data of Table I were plotted to point out certain charac30 .. .. 65 90 96 .. .. 50 100 100 100 . , 100 .. .. teristics of the reaction between calcium sulfate and sodium hydroxide. Figure 1 presents the importance of temperature when Temperature, 230' F. 10 . . 45 55 ,, .. .. .. reacting calcium sulfate with 50% caustic. At 150"F. all of thc 20 .. ,. 100 40 50 ., .. .. tests show a low percentage reaction; a t 200" F. all of the tests 30 90 .. ... . .* .. show complete reaction. ~~~~~~
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396
Vol. 38, No. 4
Sketch of a Typical Cone-Type Brine Evaporator
Figure 3.
S, steam inlet; V , vapor; C, condenaate
Figure 2 presents data on 307, caustic. The reaction rate is directly proportional to rise in temperature and increase in time of test. SOLTBILITY OF REACTION PRODUCTS IN CAUSTIC SOLUTIONS
The reaction products of calcium sulfate with sodium hydroxide are calcium hydroxide and sodium sulfate. Only a small portion of the reaction products are soluble in the sodium hydroxide solutions. When 2 grams of calcium sulfate scale are reacted with 100 cc. of sodium hydroxide solution for 16 hours a t 200" F., the percentage of calciuin remains constarit as the caustic is varied; the sulfate content decreases definitely as the caustic is increased from 10 to 50%: . 70S a O H 10
30 50
Figure 4.
Close-up \'iew of Tubes i n Brine Eiaporator
Top, hefore treatment; c e n t e r , after firs^ stage of 4 5 4 0 % sodium hydroxide treatm e n t ; bottom, after second stage of 4.5-90 % sodium hydroxide treatment.
% Ca 0.003 0.002 0.002
70SO4 0.93
0.27 0.04
-4fter this comparative data on the one scale had been accumulated, tests were made on scales containing calcium sulfate from other boilers and from a brine evaporator. Most of the scales were considerably thicker than inch and contained other ingredients besides calcium sulfate. The laboratory tests on these other scales showed the folloTving results: (a) Thicker scales need more severe conditions of treatment-Le., higher temperatures and longer times. ( b ) Porosity affects the rate of reaction. The more porous the scale is, the more easily it will react. (c) It+ moving scale Ivhich is in place on a tube is more difficult than completely reacting a loose piece of a similar scale. ( d ) Scales containing as low as 50y0 calcium sulfate were disintegrated completely by
April, 1946
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
their reaction with 30-50y0 sodium hydroxide solution. The remainder of the scales consisted of calcium carbonate and magnesium hydroxide. These laboratory data led to the actual treatments of the following nine heat exchange units with sodium hydroxide solutions in order to remove the scale high in ca!cium sulfate: one 1500-gallon locomotive boiler for a steel company, one 500-gdlon crane boiler for a steel company, one 6000-gallon vertical tube boiler for a tool company, three 6000-gallon Wickes -4-type boilers for a sugar refinery, two 6000-gallon boilers for a coal tar refinery, and one $0,000-gallon brine evaporator for a chemical company. $11 the units except the brine evaporator were first treated with 30-50% sodium hydroxide and then treated with approximately 5% hydrochloric acid to help dissolve any of the reaction products. All the treatments were 85-100% successful; that is, 85-100% of the scale in the boiler was removed. Considerable quantities of the scale disintegrated and fell t o the bottom of the units, however, and, although not completely reacted, was readily removed. TREATMENT OF BRINE EVAPORATOR
The 50,000-gallon brine evaporator was a cone-bottom evaporator (Figure 3) consisting of a steel shell and containing 4100 IO-foot copper tubes (2 inches i.d.) rolled into steel tube sheets (Figure 4). Its capacity was 7000 cubic feet or approximately 50,000 gallons. Analysis of four samples of scale showed 100% calcium sulfate, from to inch thick. The amount of scale was estimated to be 100,000 pounds. Turbining this evaporator had never been very satisfactory. The hard scale and the relatively soft copper tubes were not the best combination. The scale had a tendency t o divert the turbine tool off its course through the side of the tube. METHODOF TREATIXG. The evaporator was filled t o its operating level with 48-50% sodium hydroxide and kept a t 230" F.,(llOo C.) for a total of 5 days with constant circulation. The first stage of the treatment lasted 3 days. During this time the caustic concentration dropped from 49.6 to 47.6%. The sodium hydroxide was drained from the evaporator and pumpecl into settling tanks. The evaporator was then filled with water,
397
brough6 to a boil, and washed thoroughly. Inspection showed that thescale was approximately half gone (Figure 4). The evaporator was refilled with 46.0% sodium hydroxide and kept at 230' F. for another two days. During this stage the caustic concentration had dropped to 45.4%; it was then increased to 48.2% by evaporating under vacuum some of the water from the .solution and replenishing with 50% sodium hydroxide. The 48.2% caustic dropped t o 46.9% a t the end of the treatment. The second stage was stopped 6 hours before its scheduled 2clay run because of the following observations: A large increase occurred in the amount of sludge in the samples. The amperage .on the circulating pump increased from 170 to 270 amperes due to the heavier liquid being pumped. A temperature difference was noticed above and below the tubes. This difference was due to poor circulation. On inspection the evaporator was pronounced 90% clean (Figure 4). During the first stage the solids in the sodium hydroxide solution had increased from 0 t o 15.1% by weight. During the second stage they had increased from 6.9 to 20.9%, or 14.0%. The scale from the evaporator a t the end of first stage was 100% calcium sulfate; at the end of the second stage, it was made up of 85% calcium hydroxide, 10% calcium carbonate, and 5% sodium sulfate. CALCULATIONS. The amount of solids in the drain solutions was 98,500 pounds in the first stage and 92,000 in the second, or a total of 190,500 pounds of solids formed. Since calcium sulfate formed about twice its weight of insoluble reaction products, about 95,000 pounds of calcium sulfate were removed from this evaporator. Therefore it was found that the efficiency of the evaporator increabed 4045%, and the rate of steam condensation increased from 55,000-60,000 to 85,000 pounds of rondensate per hour. LITERATURE CITED
(1) Brock, James, U. S. Patent 89,121 (April 20, 1869). (2) Burgess, Hugh,Ibid., 168,222 (Sept. 28, 1875). (3) Hers, W., 2.anorg. Chem., 71, 206-8 (1911). (4) Riley, James, U. S. Patent 182,774 (Oct. 3, 1876). PRESENTED on the program of the Division of Industrial and Engineering CHEMICAL SOCIETY. Chemistry of the 1945 Meeting-in-Print, AMERICAN
Multicomponent Tray Calculations Based on Equilibrium Curve of Key Components EDWARD G . SCHEIBEL' Polytechnic I n s t i t u t e of Brooklyn, N . Y .
PREVIOUS paper (IO)presented an equation for calculating the minimum reflux ratio in multicomponent distillation. In the design of fractionating towers to effect a given separation, it IS then necessary t o carry out a set of tray calculations to determine the total trays and the feed tray location a t a reflux ratio greater than the minimum. The first rigorous method of multicomponent tray calculations involved a matching of the components a t the feed tray by a trial-and-error process. Jenny (7') proposed a method whereby this trial and error can be eliminated. The method consists of estimating a feed tray temperature and determining the liquid and vapor compositions a t this temperature. The calculations can then be made down the column until the bottom product 1
Present address, Hoffmann-La Roche, Ino., Nutley, N. J.
composition is matched and up the column until the overhead composition is obtained. Thjs method is more direct in that all the heavier components become negligible several trays above the feed and all the lighter components become negligible several trays below the feed. Thus, the triakand-error method of metching is eliminated. The uncertainty in the method lies in the choice of the feed tray temperature since, if the temperature is too high or too low, a larger number of trays will be required. At reflux ratios close to the minimum, a feed tray temperature must be estimated very accurately, but at reflux ratios generally used in practice, the effect of feed tray temperature on the number of trays is quite small, and little difficulty will be encountered if the temperature is estimated according to the empirical method described by Jenny (7).
