ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT

Descaling of Evaporator Heating Surfaces in Cane Sugar Factories N. 0. SCHMIDT AND L. F. WIGGINS Deparfmenf of Sugar Chemisfry and Technology, The lmperial Co l l ege of Tropical Agriculfure, Trinidad, E . W e / ,

CALISG of heat transfer equipment in a cane sugar factory s and particularly, scaling oE the evaporator tubes is one of the major problems of the sugar technologist. Despite consid- erable effort, in many parts of the world, to eliminate or alleviate evaporator scaling, little success has been achieved. Extrava- gant claims have been made by makers of various electrical de- vices purporting to affect scale forming salts in sugar-cane juice so that evaporator scaling is prevented or a t least minimized, but when these devices are subjected io critical study, con- siderable doubt is cast upon their efficacy. In fact, none has been proved to be effective in preventing the scaling of sugar factory evaporator heating surfaces. The prevention of evap- orator scale by the introduction of very small quantities of certain compounds-for example, tetraphosphoglucosatc-to the juice entering the evaporator has, in general, been unsuccessful in cane sugar factories.

This lack of success in preventing scale formation on heating surfaces in a sugar factory leaves the sugar technologist with the problem of removing the scale after it has formed. Of the two pieces of equipment most prone to scaling, the juice heat- ers and the evaporators, the descaling of evaporators is the most difficult problem. Scale readily forms on the tubes in the juice heaters, but its removal rarely presents any serious problems. Very different circumstances obtain in the case of the evapora- tors. The scale which forms on the surfaces of the tubes is often very hard and even glasslike, and it is sometimes extremely diffi- cult to remove. Sugar factories in the British Caribbean area very often accumulate scale on the evaporator tubes, 1 mm. in thickness, in the course of 1 week, and since such a scale can reduce the heat transfer very considerably the problem involved is a serious one. The thermal conductivity of the evaporator heating surface has sometimes been impaired so drastically as to force reduction of the grinding rate of the mill or the extraction efficiency by lessening the amount of inbibition water used. I t is essential to keep the evaporator tubes as clean as possible.

In the Caribbean area cleaning is done during the wcelc-end shutdown. The methods of tube cleaning employed vary widely. Sometimes hand or mechanically operated brushing or cutting tools are passed several times down the tubes and sometimes chemical cleaning is employed; most often a combination of these tn-o methods is used. When chemical cleaning is carried out it is usual to spray the hot solution of the chemical into the evap- orators, with circulation of the solution through the evaporator and the supply tank. Caustic soda or soda ash alone or followed by dilute hydrochloric acid are usually used, depending on the nature of the scale. Sometimes ammonium fluoride is employed. Both the chemical and physical nature of the evaporator scale and the amount of it are highly variable. The amount of scale and its chemical character certainly varies from crop to crop and sometimes varies markedly within a single crop period in any particular factory. Sometimes the scale is friable and is easily removed by brushing and scraping, while a t other times a smooth hard scale is formed which is hardly affected by the brushing and

scraping tools. Considerable difficulties are often experienced in removing scale even by means of a combination of mechanical and chemical cleaning, and because there is a limited time during which to complete the cleaning operation it is sometimes neces- sary to commence work in the factory with imperfectly cleaned evaporator tubes. This inevitably results in inefficient working in so far as it becomes necessary to cut down the amount of evap- oration to be carried out by reducing throughput of cane or amount of imbibition water. Scaling is occasionally so serious that it is impossible to clean the evaporator tubes adequately within the period of the week-end shutdown. This happened on one occasion in a Trinidad factory during the 1953 crop, and start-up was delayed for 8 hours, which was very costly. If nuclei of scale are left on the tubes, scaling may take place to a grea'ter extent than would be the case if the tubes were perfectly clean.

This paper describes experiments on the use of the sodium salt of ethylenediaminetetraacetic arid (3) for the cleaningof sugar factory evaporator heating surfaces which were carried out during processing of the 1953 crop in Trinidad. The idea of using this material arose from its use in analytical chemistry where it haa come into prominence as the Schwarzenbach method (3) for determining hardness in water. The procedure has also been adopted to determine calcium and magnesium in sugar house products (1 , d) . The underlying principle is that metallic ions such as calcium and magnesium form water-soluble complexes with sodium cthylencdiaminetetraacetate which are dissociated to such a small extent that not even oxalate ions will precipitate calcium from the complex, Since this is the case, the converse must also be true-namely, that insoluble calcium and magnesium salts such as calcium oxalate, sulfate, phosphate, and silicate arid magnesium carbonate or calcium-magnesium aconitate will be dissolved by the sodium cthylenediaminetetraacetate. The reagent used for the analytical determination is a very expensive one, but a tcchnical grade has appeared on the market at a price (62 cents per pound) which makes possible cxperimcntation on scale removal with this chemical.

The trade name of the sodium salt of ethylenediaminetetra- acetic acid is Versene. I t is a white crystalline compound and solutions of it are highly alkaline but are stable to boiling. One pound of the pure material is capable, theoretically, of combining with 0.172 pound of calcium oxide or 0.416 pound of calcium sulfate, etc.

YaOOCCH2\ ,CH2C00>a

SaOOCCH2/ \CH2COOSa N-CHa-CHZ-X

Sodium Salt of Ethylenediaminetetraacetic Acid Dissolves Scale in Laboratory Tests

In the laboratory it was readily shown that pulverized evap- orator scale from the 1953 crop was rapidly dissolved to the extent of more than 90% by 5% Versene solutions. Howevrr, the sur-

May 1954 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 867

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT

face area of the pulverized scale i r as very large, and subsequent experiments showed that longer times are required when dealing with scale as deposited on the tubes of an evaporator. For es- ample, a small picce of sc:ile required boiling for 45 minutes with a 0.5% Versene solut,ion in order to dissolve it completely. Sub- sequent laboratory tests mere performed on an evaporator tube from the last vessel of a quadruple effect evaporator in a com- mercial sugar factory. The adhering scale was about 1 mm. thick and had not received any previous treatment. Analysis of the scale shelved it to be mainly calcium sulfatc n-ith somc silica (Table I).

Table I , Analysis of Evaporator Scale Composition,

i+-t. % Moisture (air dry) Loss by ignition

Calcium (Ca( hlaenesium (AIoO'

Oven Dry Scale

3 G 32.2

67.8 81.7 12 .6 25 .9 0.3 2 . 4 0 . 5

2 5 1 2 0

The evaporator tube was cut into 3-inch lengths and tlic picces boiled with various solutions of sodium ethylenediaminetetra- acetate. Table I1 summarizes the results.

Table II. Scale Boiled with Sodium Ethylenediaminetetra- acetate

Tube Piece No. 1 2 3 4 5

Sodium ethylenediaminetetraacetate Grams 5'! 50 20 Concentration, 70 0 5 2 . .

Sodium hydroxide Grams . . 2 0 . . 2 0 . . Concentration, % 2 2

Total scale on tube, grains l i : 9 17.3 I c :6 17 .6 l 6 : 6

. ' 2:

Scale removed, %, a h 1 hour 2 hmlm 2 . 5 hours 4 5 hours

left, grams Sodium ethylenediaminetetraacetate

77 100 . . . . 18

74 35 0 83

. . 61 21

. . 57' 19.

. . 1 . 3 . .

70

72 72

. .

0.4

The following conclusions may be drawn from these experi- ments :

The scale can be removed completely from the tube if a suf- ficient quantity of sodium ethylenediaminetetraacetate is used. This type of scale required about 2 parts of Versene to dissolve 1 part of scale. Sections of a scaled tube and of a 17crsene cleaned iube are shown in Figure 1.

The time required to dissolve the scale is very much affected bv the concentration of the Versene; a higher concentration is Gore effective.

Addition of sodium hydroxide to the Versene solution does not seem to have any beneficial effect.

A considerable portion of the scale fell off the tube in large and small flakes before it was actually dissolved. This flaking was more pronounced with increasing strength of the Versene solution.

The oxide film on the brass evaporator tube was dissolved, leav- ing the metal perfectly clean and bright. Moreover, the metal remained ckan and bright for several weeks. I n this respect the Versene differs from acid cleaning of tubes where an oxide film re-covers the metal very rapidly.

Experimental Sugar Factory Trial indicates Highly Efficient Cleaning Process

The laboratory experiments provided satisfactory evidence that sodium ethylenedianiinetetraacetate was of possible use in clean- ing sugar factory evaporator tubes and larger scale trials were

conducted. These were carried out in the Experimental Sugar Factory of the Imperial College of Tropical Agriculture. The evaporator station consisted of a triple cff& evaporat,or possess- ing 700 square feet of heating surface distributed equally among the three vessels, each having 233 square feet.

The first trial was performed on the first veesel of this evapora- tor. One hundred gallons of dilute solut'ion (0.5%) of Versene was poured into the first vessel and boiled for 1 hour. The solu- tion was then run off and the tubes examined. It was evident that the treatment had removed such a considerable portion of the scale from the tubes that any further cleaning treatment was unnecessary although, even on factory standards, they could not be termed "perfectly" clean.

A second trial was carried out a t the end of crop on t,he last vessel of the triple effect evaporator. This vessel normally suffers most from scaling. At the bime of the trial it had de- posits of scale from previous operations which adhered so strongly to the tubes that the weekly t'reatment by mechanical brushing had never rendered them clean. Thie vessel was treated with 100 gallons of 2.575 Vcrscnc, solution ( 2 5 pounds of solid Versene) by boiling the solution under partial vacuum (15 inches). Bfter 15 minutes of boiling, pieces of scale m r e observed in the liquid and after 60 minutes the solution as reddish-brown in color and titration with calcium ions showed that all the reagent had been consumed. The solution mas run off and the inside of the wsscl examined. It vas observed that the reagent had removed a major part of the scale on thc tubes, although patches still re- mained. Howevcr, most of the tube surface nom appeared brightly polished. Thr effect of the Versene was strikingly notice- able in the dovmtake where large a,mounts of scale had bceri deposited and \There, after the treatrncnt, the copper color of the metal had become visible. Tliere were sharp demarcation lines on the tubes betxeen places where scale had and Elad not been removed, which demonstrated that, the scale had flaked 'off before being completely dissolved.

The amount of Versenc uscd, however, TYas far in excess of that expected to be sufficient to dissolve t,he scale on the tubes. Examination of the vessel showed that the reason for this was t'hat the reagent had also reacted with and dissol id or partly dissolved the deposits found on the top and bottom of the tube plate, t,he donmtake, and the Fides and bottom of t,he vessel.

,Commercial Sugar Factory Evaporator Is Used to Study Factory Conditions

During week-end stoppage (Nay 2, 1953) an attempt was made to clean with Versene the last vessel of the evaporator a t a factory in Trinidad. The normal cleaning procedure at this factory is to boil the triple effect evaporator for 2 hours with caustic soda solution, with 30, 60, and 130 pounds of caustic soda in the first, second, and third vessels, respectively. .4fter boiling, Ivhieh is normally finished between 9 and 10 P.M., the solutions are left to soak overnight and early the next morning the tubes in the first t,wo vessels arc cleaned with rotary brushes while in the last vessel a rotary cutting tool is used. At present the heating surface of the evaporator totals 6266 square feet, n-ith 1950 square fcet in each of the first two vessels and 2366 square feet in the last one. The evaporation rate is 9 to 9.5 pounds pcr square foot per hour.

During the 1953 crop the mill was grinding about 34 tons of cane per hour and in the week in question a total of about, 4800 tons of cane vere ground. It was reported that normally tlie scale in the evaporator was easily removed by the rotary machines but that during the previous 3 weeks a hard scale had developed which adhered very st,rongly to the tubes in the last body. S o trouble was experienced in cleaning the first two vessels, but difiiculties in removing the scale from the last vessel persisted, so that grind- ing start'ed on April 26 with an imperfectly cleaned third ves- sel, neceseitating, for the first time during crop, reduction of

868 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 Vol. 46, No. 5

the imbibition water during the week from about 27y0 on cane to about 24%.

After the liquidation of the evaporator on May 2, the third vessel was washed with a hose and inspected through the manhole door. The scale on the tubes was about 0.5 mm. thick a t the top of the tubes, The inside of the vessel, however, had heavy incrus- tations not less than 3 mm. in thickness. The tube plate was covered with scale t o the ends of the tubes and the downtake also carried a very thick coating of scale.

Because of the large amounts of scale deposited in other places besides the tubes themselves, 500 pounds of Versene was used in the trial. The last vessel was filled with water (about 1000 gallons) and the 500 pounds of Versene (5% concentration) was shoveled through the manhole door. The other two vessels received normal quantities of caustic soda, and boiling under 25 inches of vacuum was commenced.

,Ifter 1 hour of boiling the bottom sight glass was cleaned completely. A sample of the solution showed that about 100 pounds of Versene was still intact, but this quantity was reduced to about 25 pounds by an additional hour of boiling. At this stage the manhole door was opened and the vessel inspected. I t appeared that the tubes, so far as could be seen, were clean, and the scale on the tube plate had disappeared. The liquid was left overnight and after the vessel was washed with water it was inspected thoroughly from top and bottom.

The tubes were clean for a distance of 12 t o 24 inches from the top; below this there was still a layer of scale. Although the tubes were not perfectly clean, they were considered clean enough for the next week of operation. The bottom of the vessel and the bottom tube plate were still covered with an extremely thick coat of scale which was estimated to be about 10 mm. thick in the bottom of the downtake.

The Versene treatment cleaned the last vessel at least as well as the routine method of cleaning. Nevertheless, as time per- mitted, the rotary cutting tool was also used and more scale was removed. Even then the bottom of the tubes contained some scale which was very hard.

During the week following the Versene treatment of the third vessel the evaporator worked very well with less steam than was used in the week immediately preceding the trial and with the full amount of imbibition.

The trial clearly demonstrated that the Versene treatment effectively cleaned a badly scaled vessel. * An unsatisfactory factor was that such large amounts of the reagent were needed to effect this cleaning. On the assumption that a high rate of scaling of 2 pounds of scale per 100 tons of cane ground obtained, it would be expected that 100 pounds of scale viould be deposited on the tubes and that 200 pounds of Versene would be needed to remove it. However, 500 pounds of Versene was used and even then some scale remained on the tubes. This observation dem- onstrates that in sugar factory practice the amount of scale de- posited, during the crop period, on parts of the evaporator vessels other than the tubes must be taken into account when chemical cleaning is considered.

Convenience, Speed, and Prevention of Scale Formation Contribute to Practicability of Process

These experiments established that evaporator scale can be removed completely by sodium ethylenediaminetetraacetate solution. The scales so far examined have been composed mainly of calcium salts, but even scales with larger contents of other ma- terials would be removed as the scale is built up of the different components in a lattice and so long as the Versene can remove some of the constituents of this the residual skeleton will fall apart when deprived of its reinforcement.

Sodium ethylenediaminetetraacetate is a chemical which will dissolve evaporator scale in an alkaline medium which is not harm- ful to the materials of construction of vessel. It is well known

that hydrochloric acid in sufficient amounts and in a fairly high concentration can completely remove the scale from evaporator tubes, but unless a satisfactory deterrent t o its corrosive action on the metals forming the body of the vessel is found, complete scale removal by this chemical could not be safely practiced. Even if the corrosive action of hydrochloric acid, so far as the vessel is concerned, could be inhibited, the vapors evolved by boiling the solution would corrode the pipes, etc., leading from the vessel.

ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT

May 1954 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 869

Figure 1 . Evaporator Tube after 1 Week of Sugar Cane Processing (Left) and after Cleaning with Sodium Ethylene-

diaminetetraacetate (Right)

Although the use of sodium ethylencdiaminetetraacetate pro- vides an answer to the problem of rapidly cleaning raw sugar evaporators, the economics of the process seem a t present to make its immediate adoption doubtful. The results of these ex- periments indicate that the weight of T'ersene necessary to com- pletely dissolve the scale is about tnice the weight of the scale. If an evaporator is producing scale a t the rate of 3 pounds per 100 tons of cane ground, then 6 pounds of Versene would be needed per 100 tons of cane milled. Assuming the price of the Versene to be about 62 cents per pound, the cost of the evaporator clean- ing would be 3.7 cents per ton of cane. Thus, the cost of cleaning by this method is greater than the amount of money normally spent on the weekly cleaning of the evaporators (about 1.8 cents per ton of cane), but indirect benefits such as convenience, ra- pidity of cleaning, and removal of the adverse effects of mechanical cleaning might lessen the apparent difference in cost. 'How- ever, the cost quoted is a minimum one and removal of the scale and rust deposits in the body of the vessel as well as on the tubes by the Versene further increase the cost. However, it is believed that once the complete vessel has been thoroughly cleaned the quantities of chemical used for maintaining the evaporator in a clean condition will be of the order quoted.

The experiments showed that the oxidized layer on the brass tube is dissolved by the Versene solution and that the surface so obtained remains clean and bright for a considerable period of exposure to the atmosphere. The surface of tubes cleaned with acid rapidly becomes oxidized. The removal of the oxide layer from the tubes may affect the subsequent deposition of scale.

The experiments also shoITed that the Versene solution has a loosening effect on the scale, causing large pieces to fall off. If it were possible to separate such scale pieces from the treating solution, the amount of reagent used could be greatly reduced, and in fact, the spraying system in general use in sugar factories seems to offer such a possibility if provision is made to strain off pieces of scale from the circulating system. If a sugar factory evaporator is cleaned thoroughly before crop processing and can

ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT

be kept completely clean by the use of Versene the efficiency of the factory will increase.

Efficient removal of scale from the tubes and all other surfaces of an evaporator might hare an important bearing on the more fundamental problem of scale prevention. Although the amount and character of evaporator scales vary to an extraordinary de- gree, it is a certaint,y that scale in sugar factory evaporators is caused by the presence of calcium ions in the juice. There is no doubt that if calcium ions could be removed from the juice then no scale problem would exist'. It is, of course, possible to remove all ions from cane juice by the ion exchange process, but the economics of the process a t present, make it unworkable. The judicious use of phosphoric acid, phosphates, or other chem- icals giving rise to insoluble calcium salts at the clarification station might also help to diminish the extent, of evaporator scaling. Sodium ethylenediaminetetraacct'ate offers a ne!y ap- proach to the problem of scale prevention. If an evaporator is cleaned thoroughly before thc beginning of crop by mechanical cleaning, followed by 1-crseiie t,reatment, i t niay be possible to keep tubes clean by one of the following processes:

Weekly cleaning with Tlersene Adding Versene to the juicc line from time to time

Resting one of the evaporator effects for 1 or 2 hours during normal factory operation by replacing the evaporating juice with Versene solution during this interval.

The last two possiblities are only tentative suggestioni but they are attractire because t'hey would allow cont'inuous processing of the crop. If 1-ersene were added to t'he juice it would be necessary to add sufficient cheniical t o complex not only the cal- cium and magnesium salts in the scale: but also all those posiible of complex formarion in the juice.

Acknowledgment

The authors are grateful to ?he Bersa-orth Chciiiral C o ~ p . of Framingham foi a generous gift of T'c,rsenr.

Literature Cited

(1) Honig, P., West Indies Sugar Corp., S e w York, Rep t . , 2 (1952). ( 2 ) Schmidt, N. O., and Wiggins, L. F., unpublished results. (3) Schwarzenbach, G., Biedermann, W,, and Bangerter, F., Helv.

Chim. Beta. , 29,'811 (1946); Chimia (SwLtz.), 2, 56 (1948).

RECEIVED for review August 24, 1953. ACCIIPTI;D October 10 , 1953

INFLUENCE OF SHAPE ON EVAPORATION OF DROP§ OF n-HEPTANE

N. T. HSU, K. SATO, AND B. H. SAGE California lnstitote of Jechnology, Pasadena, Calif.

ITTLE informat'ion i j available coiiccrnirig the effect3 of the shape of a drop or the level of turbulence of the approaching

stream on the rate of evaporation from drops (7 , PO, 22) . Maisel and Sherwood ( 3 2 ) ieviei\-etl available dat'rt concerning the evap- oration of drops in t,urbulent 5treams and presciited seine new information on material t'raiisfer from spheres. These results did not yield a simple analogy n-ith available corrclations of thermal transfer (14 , 21). PoTvell ( 2 7 ) , Friissliiig (6 , 7 ) , Raiiz :uicl Marsh& (28), Takahashi (S9), and Kliitman (40) iiive~tigatcd the be- havior, of drops in turbulent air Ptreams. Such data served as a limited basis for predicting material t#rander from qherical drops (35) . Since a more extensive revievi of the experimental hack- ground is available (SS), no furt'her discussion of earlicr work is required.

As a result of the absence of data coiiceriiing the cffwt of the shape of the drop, ai1 expciimeiital investigation T T ~ P made of drops of iz-heptane for a numhcr of configuration.. Pi,iniarg- em- phasis was placed on inracuimiciit of t,hc rate of material tmns- port rather t,han on the interrelation of theriiial and maierial t'ransfer. The esperiinentnl information obtained \vill, neverthe- less, permit, a limited coiiipnrison of the nioc*hxiiism ol the t,wo transport processes to bc m n t l e .

Measurementms of the rate of evaporation of drop3 of n-liept,ane in an air jet at) a free stream trniperature of 100' I;. x w e carried out for nominal velocities of 2; 4, 6, and 8 feet per ,second. At each air velocit,g measiiremeiit,s of the dimemions of the drop and its temperature mere made for a series of different rates of evapora- tion. For each rate of evaporation and air velocity, the dimen- sions of three independently formed drops were cstabliahed (9). These independent measurements on three different drops aided in establishing the reproducibility of the investigation.

The analysis of the evaporation of drops has been considered by

inang invesiigators. Hughes and Gilliland ( I O ) revicxwc:d t!he mechanics of drops. Langmuir (18) set forth the basic, rclation- ships for the evaporation of spherical drops into quicxcciit ail. whereas Fuchs ( 8 ) extended the ~ o r k to include t'he behavior of much smaller drops. Frossling (6) developed from cstablishcd simple boundary la jw theory that' the weight rate of evaporation of a drop in a turbulent stream could be established.

0

mk = 4 ? i ~ d D U , i i ( ~ k , i - u k . J ( l + K S C ~ / ~ R C ~ : ? ) (I)

Ingebo (11, lb) followed Frossling's analysis and developed an empirical expression of the same general form. More recontly, Ran2 and BIarehall (28) eonaidered the evaporat'ion of drops and confirmed the earlier work of Bedingficld and Drew ( 2 ) concc:rniilg the relative independence of the w t bulb teniperat'ure with re- spect t o the Reynolds number of the f l o i ~ around the drop. -111 earlier work appears to consider the gas phase as a perfect, gas. A somewhat' more complete review of the analysis of the evapoi'a- t,ion of drops is availahle (53) n-hich considers the gas phaac to be an ideal solution. In order to take into account deviations Ei,om

the peri'ect gas law, fugacity vias considered as the potential. Cnder these circumstances the i\€axmell hypothesis may lic \vi it,- ten for spherical coordinates in the following i\-ay (93) :

-

Equation 2 niay be combined ni th the normal tlcfinitioii oC the Maxwell diffusion coefficient to yield

Equation 3 describes the w i g h t rate of transport per unit area

870 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 Vol. 46, No. 5