Scale Formation on Laborator Evaporator F. 31. HILDEBRANDT AND IC. H. TC'_.IRREN 1'. S . Industrial Chemicals, Inc., Baltimore, Md.
It should be taken apart easily for ohsrivation or nieaiurement of the scale on t h e tube surface. Xeasurenient of scale formation by change in evaporation rate should be made easily. DESCRIPTIO\
O F APPARATUS
;1photograph of the appaiatub is shown in Fiyurc I
Figure 1.
Laboratory Evaporator Arranged for Natural Circulation
K E of the most serious problems to be solved in the evaporation of many materials is the drop in the coefficient of heat transfer because of the fouling of heat,ing surfaces by scale and occluded matter. Webre and Robinson ( I ? ) give data which show that in a n extreme case the over-all coefficient dropped, owing t o scale formaeion, from 1550 to 900 in less t h m 2 hours and then less rapidly to about 700. Other dat,a showing t h e heat transfer t o boiling liquids under varied conditions may be found in the literature ( 7 , 9, 10, 12, 16). The majority of these studies have been confined to plant scale equipment,. Laboratory data are difficult t o find arid are, in general, limited t o t'he investigation of a specific factor such as the film coefficient. This is due in part t,o the fact t h a t small scale equipment for evaporation studies is specialized for t,he study of these specific factors and in part to t,he feeling that laboratory apparatus is too far removed in size and operating conditions t,o give data valid for t,he larger evaporators. To experiment with full scale evaporators, however, is costly. illso such eyuiprrienl lacks flexibility, the machine is difficult to examine after use, and experimentation under production conditions is hard to interpret, a,s the uncontrollable factors of t h e larger operation affect the results erratically. JVith those things in mind attention vas turned t o the setting up of a small evaporator for laboratory studies of scale formatmion. Certain requirements for such an apparalus have been suggested by other workers ( 2 , 3,11, 18, 1 4 ) among which are the following: The apparatus should be designed t o evaporate small amounts of material under controlled conditions with t'he heat transfer tube constructed so that t,he scale forms on the outside where it can be observed readily.
il is t h e body of the evaporator constructed froin a 500-ml. Kjeldahl flask by adding a section to the neck and providing an outlet in the bulb and another outlet in thc neck about 1 inch from the bottom. B is another 500-ml. Kjeldahl with a tangential inlet, two outlets, one foi a thermonietei and the other near the top for vapor. The nccli of this flaqk is shortened and drawn down t o a reduced diameter for attachment of rubber tubing. C is a glass manifold with four outlets, interchangeably usrd for incoming feed, E , down-leg connection from B , or draining the apparatus. A , B, and C are connected by rubber tubing. G is a graduated receiver for water from R. condensed by bulb condenser, D. F is a graduated separatory funriel lor holding the feed and maintaining a supply at constant level in pump reservoir iV. Steam a t constant pressure enters the apparatus through regulator H . T h e gage, I , gives the pressure on the heater tube, and the condensate is removed by steam trap, K . The temperature of the steam is measured by a Weston thermometer, L. J1 is a braided metallic hose which admits steam t o the 0.5-inch brass evaporator tube, T . X is a rubber stopper fitted over tube T and into the neck of A . 0 is a steam trap connected t o a 0.125-inrh copper capillary tube within tube T, and drained by condensate discharge line P. Details of the oonstruction of the heater are shown in Figure 2. The construction and operation of feed pump R arc described in a previous publication (6). It consists of a rubber tube provided with stainless steel ball check valves. Pumping action is obtained by compressing the tube with a bar moved by means of ek motor-driven cam.
OPERATION O F EVAPORATOR
In actual operation, the evaporator is filled to the desired levd and the steam inlet valve opened. I n about 1 minute the liquid boils and circulation starts through the apparatus. From B, the catchall, the vapor goes to the condenser, D,and the oondensate is caught in a 100-ml. graduated cylinder. I n these experiments readings of the amount of condensate were made a t 5-minute intervals and the data obtained were used for plotting curves shown later in this paper. The liquid level is maintained relatively constant in the catchall (within about I inch)
simple and easily constructed laboratory apparatus designed for the study of evaporator scale formation is described. Results are presented graphically to show the decrease in the rate and amount of water cvaporated under controlled conditions as a coating of scale is formed on the heater tube, Experiments with calcium sulfate solutions and molasses stillage are described, and some of the more important factors affecting scale formation are emphasized. The apparatus is used to produce small amounts of evaporated material experimentally in the laboratory, in addition to its principal use as a tool for the investigation of the effect of various factors on scale fortnation.
April 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
755
OPERATION OF EQUIVMEVT ~‘O.D.COPPERTUBNG
RUBBER STOPPER BRASS HEX. NUT BRAZED
T O B R A S S TUBE
COPPER CAPILL BRAZED TO BUS
OSE
This type of evaporator has a number of advantages. It is constructed of standard material readily obtainable by any laboratory. T h e glass body permits visual observation of t h e tube with lightly colored solutions and scale formation can be observed a t t h e moment deposition starts. T h e evaporator can be readily taken apart for cleaning and inspection of the tube surface. With minor changes, it c a n be converted t o the forced circulation type. The apparatus is sinal1 enough t o be supported on a rack on the laboratory table. T h e feed may be introduced a t any one of several places t o give thorough mixing of the cold feed before i t reaches the tube surface. K i t h slight modifications it lends itself t o orthodox methods of heat transfer study. A further advantage is its use for evaporating small amounts of solutions experimentally under conditions which make for a minimum of heat injury. T h e unit can also be used for continuous evaporation. DETERMINATION O F EVAPORATION CAPACITY OF CLEAN TUBE
/ BRA%
TEE
CONDENSATE T O SEWER
Figure 2. Heater Cross Section Showing Tube Assembly and Steam Connections
by regulating the output of the feed pump. Saturated plant steam previously reduced to 15 pounds is fed to the regulating valve and a temperature of 238” F. is maintained on the steam side of the heater tube by adjustment of t h e steam pressure regulator. Steam enters the tube at the bottom apd the condensate leaves the system through t h e copper capillary tube shown in Figure 2 connected with a steam trap. During t h e course of the runs, as scale forms, the amount of water discharged from the condenser decreases. The most marked drop i n evaporation rate occurs when scale can be seen crystallizing on the tube surface. At the end of each run the evaporating tube is removed by disconnecting the rubber tubing from the outlets of the evaporator body A , and unscrewing the tube from the tee in which it is held. hleasurements of scale formed and observation at close range with the naked eye or under a stereoscopic microscope can be made readily.
r
,
I
I
I
As a preliminary t o the study of evaporator performance, tests were made on the clean tube with water. Other investigators have shown that it is difficult t o clean a scaled tube surface so as t o bring its heat transfer capacity back t o the original value (16). Tubes for the experiments described here were cleaned by several methods such as bufing with a scratch wheel and hand polishing with emery cloth (size 3/0). The latter method gave results which checked closest when water was evaporated and this was chosen as a standard procedure. JVith water the rates of evaporation of the tubes used in these experiments varied between 16.2 and 17.6 ml. of water per minute under the test conditions.
Figure 4. Typical R u n with Calcium Sulfate Solution Showing Formation of Scale on Brass Heater Tube
PRELIMINARY STUDIES OB CALCIUM SULFATE
.
.
RUN Nl NG
Figure 3. Left. Typical Curve Obtained by Evaporation of Calcium Sulfate Solutions. Right. Decrease in Initial Evaporation Rate with Ethyl Stillage from High Ash Molasses Stillage from fermentation maah : 1, without acid and backslopping; 2, with eiilfiiric acid and n o hackalopping; 3, with hnckslopping but no acid; 4, duplicate of 3 with 100 p.p.m. tetrnphoaphoqlucosate ndded to stillage
T o throw light on the basic problems of scale formation and prevention, experiments were first carried out with calcium SUI-( fate solutions, this salt being chosen because it is one of t h e prime scale formers in the evaporation of many solutions. T h e solution also permits visual observation of the tube surface and the low solubility of the salt minimizes the effect of viscosity. Likewise, calcium sulfate has a n inverted solubility curve which is a n essential requirement for scale formatiol! ( 2 , 4 ) . Solutions were made by dissolving 2 grams of desiccated calcium sulfate in 1 liter of water at room temperature, shaking the flasks about every E, minutes for half a n hour, and then filtering. The evaporator was started on water, and calcium sulfate solution was then fed a t such a rate that a constant level in t h e catchall was maintained, keeping steam pressure and temperature constant. Figure 3 (left) is a graphic representation of a typical run. When 1000 ml. of calcium sulfate solution had been fed, scale could be seen forming on the tube, this point corresponding t o 60 minutes of running time. A sharp drop in evaporation rate occurs a t this time and continues for another 60 minutes, a t which point the rate continues to fall off more slowly. In such
.
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
756
Figure 5 .
Tatal Condensate Collected 4-Ilour Kunning Time
O + ~ I
Aboae. 1, Cas04 solution untreated: 2, C a s 0 1feed with 100 p.p.m. cornstarch; 3, CaSOa feed with 100 p.p.m. Calgon; 4 CaSOa feed, tubc coated with a 2, resin. Below. 1, ' CaSOa solution untreated; Cas04 feed treated 100 p . p . m . Tetrafosforg; 3. Cas04 feed treated with 100 p.p.m. tetraphosphoglucosate; 4, CaSOi feed, tube precoated with randelilla wax and feed treated with 100 p.p.m. tetraphosphogluooaate
Vol. 41, No. 4
on the heater is maintained constant. This gives curves in which the control and experimental values lie close together, By raising the steam pressure or using a tube of larger area the points can be separated arid coildensate values obtained which correlate with the observed scale formation on the tube surface. A second method of expressing results, based on the water evaporated, s h o m the rate a t which evaporation falls off. 111 this case, t.he amounts of condensate collected in successivc. d a,s percentage of initial evaporation rate et'hod takes into account the initial condition of the tube and shows the progress of scale forniation during the course of t'he run. This type of graph is shown in Figure 3 (left), C,urve 1, Figure 6 (above) is characteristic of thc cvaporatiori rate of an untreated calcium sulfate solution, while the bent?ficial results of adding starch or Calgon are illustrated by curves 2 and 3. Thcre is a slight decrease in the evaporation shown t)y n 60 and 90 minutes of running time which 00incided n-ith heavy scale forrriat,ioii, but the period betrvertn 90 arid 120 minutes shows an increase in rate owing to a desc:diirg effect of the starch (8). While it vias hoped that the tube wvoiald become completely descaled, t,his did not occur and simultarirou$ scaling and descaling continued with less descaling as the run progressed. The data for curve 4 were obt,ained from a tube coated with a resin. This tube showed a scaling and descaling effect as when starch was present and the run vas contiiiued for 10 hours, during which time the rate of evaporation would first fall off as scale formed and then rise again as descaling took place (results from only the first 4 hours arc shown on the graph). Thrce coats of Bakelite varnish were used t o coat this tube and consequently its rate of evaporation was less than half of a normal average. As the scale was sloughed off, the tube maintained its rate of evaporation. Curve 1 of Figure 5 (below) shows the condensate produced during a 4-hour run from a calciuni sulfate solution. Curves 2 arid 3, Figure 5 (below) show results of adding commercial scale preventives to calcium sulfate solutioris. Curve 4 shows a remarkable increase in the amount' of water evaporated by coating the tube with wax before introducing liquid and then adding 100 p.p,m, of tetraphosphoglucosate to the calcium sulfate. h preliminary experiment with wax alone showed little or no improvenfeiit except t o give a n easily reniovable scale. Scale observed visually on t,he tube a t the end of r u n after removal from t8he npparatus showed that starch and c~aritlelillawax coatings with or without tetraphosphoglucosate (6) gave a nonadherent scale easily removed by light fingor pressure. Results from t,he other tests gave a n adherent scale which had t o be sanded off with emery cloth. R7hi1r visu:ti
experiments, the tube removed a t the end of 4 iiouiq iilvaliabl) shows a heavy coating of scale, ab can be seen by Figure 4. Figure 3 (left) shows the result graphically in terms of evaporation rate. In this curve evaporation rate is exprewed as percentage of the initial rate on water.
-
METHOD
Ok
EXI'HESSING TEST RESULTS
Results have been expressed graphically in two J I L ~ J Yusing data obtained from the amount of water evaporated over 5minute intervals, Since the principal criterion for evaporator performance is the amount of water evaporated from a given solution in a definite period ( I ) , the cumulative quantity for e:tch half-hour or hour has been plotted against running time. This type of graph has been used in Figure 5 (above). Ordinarily this method gives an easily interpreted graph. Hon-ever, with some solutions containing large amounts of dissolved solids, increase in viscosity and rise in boiling point, as the solution concentrates, lower the amount of Rater evaporated, if steam pressure
Figure 6. Comparison of Clean Tube with Lightly Scaled Tube after 4-Hour Run w-ith High Ash Stillage from Fermentation with N o 4cid arid No Bacldopping
April 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
757
EFFECT O F ADDING SOLUBLE CALCIUM SALT
Figure 7. R u n with High Ash Stillage Where Sulfuric Acid Had Been Used i n Wash Make-up
h sample of stillage found in plant operation t o be nonscaling was evaporated in the apparatus and no scale was formed after 4 hours of operation. Calcium chloride was added and the experiment repeated. I n both cases there was a gradual decrease in rate which was due t o the increase in viscosity of the solution. However, t h e heater tube remained clean in both experiments. Potassium chloride was then added t o t h e same stillage and again the tube remained clean. These experiments, which were repeated several times, showed t h a t t h e soluble calcium salt did not increase scaling and also t h a t an increase in the potassium ion was without effect. ADDITION OF SULFATES
Figure 8. Comparison of Clean Tube with Tube Scaled Heavily by Evaporation of High 4sh Stillage from Fermentation Where Stillage Had Been Used as Part of Mash Make-Up
Stillage giving no scale in plant operation was saturated with potassium sulfate at 25" C. Evaporation of the untreated stillage showed no scale formation, but t h e same material containing potassium sulfate gave scaling. I n this case the scale was light and occurred in patches. A sample was prepared by saturation with potassium sulfate at 70' C. Evaporation of this sample resulted in a heavy coating of scale evenly distributed over the entire length of the heater tube. -4second lot of stillage which was nonscaling was treated with sulfuric acid. Here again, scaling was heavy as compared with the control. It appears from these experiments t h a t sulfate added either as potassium sulfate or sulfuric acid aggravates the scaling tendency. B 4CK S LOPPING
observation of the amount and thickness of the scale coating tallied with the results as expressed graphically, t h e weight of the scale could not be correlated with these results. This is no doubt a result in part of the type of crystals formed. Examination of t h e coatings under a stereoscopic microscope showed that various treatments changed the crystal size and shape and also altered the amount of unscaled tube area between crystals. EVAPORATION O F STILLAGE FROM. YEAST FERMENTATION OF BLACKSTRAP MOLASSES
The evaporation of stillage from dealcoholized fermentation solutions presents many problems, and the hope of casting some light on these was one of the principal reasons this work was undertaken. Variation in rate of evaporation, not explainable by any obvious condition in the evaporators, is a common experience in distilleries. When malasses is the raw material, very serious operational difficulties arise with certain shipments. .4t such times frequent cleaning is necessary, which is expensive and results in loss of operation time. I n extreme cases the evaporator heads have t o be removed and the hard scale drilled out of tubes in t h e heater section of the evaporator. Experimentation on the factors responsible for scale formation could be oriented t o some extent by drawing on experience with plant evaporators. It was known t h a t the scale is primarily a calqium sulfate deposit, sometimes also a mixed calcium-potassium salt; t h a t the amount deposited is affected by the ash content of the molasses, being greater when the ash content is high; and i t was ,suspected t h a t there might be some aggravation of scale formation b y physical conditions in the heater section of the evaporator. Experiments were set up, therefore, t o answer the following questions on the effect of chemical and physical conditions on scale formation. 1. .Could scaling be produced by adding a soluble calcium salt to a nonscaling stillage? 2. Does the sulfate radical affect scaling? 3. Does scaling increase when stillage is used as a mash ingredient? (This is the common practice of backslopping.) 4. Does forced circulation decrease scaling? 5. Is foam in t h e heater section related t o scale formation?
A sample of stillage resulting from the fermentation o f a molasses high in ash which had given heavy scale in plant equipment was evaporated in the apparatus, This stillage could be evaporated in the laboratory evaporator without scale formation over a 4 h o u r run. However, when sulfuric acid was added, especially heavy scaling took place, either with or without backslopping. Backslopping alone gave moderate scale formation. I n general, the scaling was heavier with this sample than with stillage which gave no plant difficulties, although additions as noted above were necessary t o bring out the scaling tendency in the 4-hOur laboratory runs. It was evident from t h e appearance and quantity of scale formed after the treatments noted that a high ash molasses is more likely t o scale than one low in ash, and under such conditions, the use of stillage as a mash ingredient (backslopping) may aggravate the conditions sufficiently t o bring about serious scaling. With a molasses low in ash, the same rate of backslopping would not cause difficulty. Typical results are shown in Figure 3 (right), where rates are plotted as percentages of the initial rate on water. Curve 1 shows ethyl stillage from a fermentation mash make-up using high ash molasses. Examination of t h e tube after this run showed it to'be nearly free of scale (Figure 6). Compare this with curve 2 where sulfuric acid was added t o the mash make-up. This gave a heavy coating of scale (Figure 7) and repeated experiments showed this heavy scale t o be formed when acid was added either with or without backslopping. A comparison should also be made between curves 1 and 3, which shows the effect of backslopping which, by adding t o the salt content of the mash, aggravates the scale-forming tendency. The use of tetraphosphoglucosate retards scale formation as shown by a comparison of curves 3 and 4. Figure 8 further illustrates the effect of backslopping, and Figure 9 shows a lighter scale formation when tetraphosphoglucosate is used. RUNS MADE WITH FORCED CIRCULATION
Many commercial evaporators operate with forced circulation, a high capacity centrifugal pump being used in each effect t o bring the rate of flow of the solution into the heater up t o 5 t u
758
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 9. Comparison of Clean Tube with Tube Scaled Lightly after $-Hour Run with High Ash Stillage Treated w-ith 100 P.P.M. Tetraphosphoglucosate Fermentation mash make-up included backslop but n o acid used
Vol. 41, No. 4
lage produced no scale if the foam was absent, as is t,he case when natural circulation was used. Thus, it was shown tha,t where the solution t,o be cvaporated has scaling potentialit icx, Foaming produces a n initial rise in evaporation rate follon-cti h\heavy scaling. The most significant differences between natural and t'orccd circulation were the creation of more foam in the case of the latter and an increase in the rate of evaporation a t the start of the run. Arrangements of the apparatus shown in Figures 10 and 11 wew used t o c o n h u e the investjigation of t,he effect of foam on seal
