Hydrogen Exchange Resin for Steam Purity Testing R. W. LANE, T. E. LARSON,
AND
J. W. PANKEY
Illinois State Water Survey, Urbana, Ill.
I n the conductivity measurement of condensed steam for purity, inaccuracies result from the presence of carbon dioxide, ammonia, and/or volatile amines. Measurement of conductivity a t atmospheric boiling point eliminates major interference by carbon dioxide but only minor interference by ammonia and/or amines. If pretreated condensate is passed through a hydrogen exchange bed, ammonia, volatile amines, and other cations are replaced by hydrogen ion; subsequent reboiling eliminates the free carbon dioxide. The remaining anions are in the form of acids and are readily apparent by their conductance. This method provides a rapid, accurate test for steam purity to indicate deficiencies of feed water treatment, of boiler design, and of plant operation, and limits of safe feed water and boiler solids control. It also detects condenser leakage if ammonia or volatile amines are present in steam condensate.
S
TEAM produced by the modern steam power plant must be of high purity in order t o prevent damage t o superheaters, turbines, and other power equipment. High maintenance costs can be expected when impure steam resulting from carry-over of boiler water solids is produced. Also, impure steam when employed in processing may contribute to the contamination of manufactured goods. By way of definition, steam quality (a measure of B.t.u. content) refers t o moisture content as HzO, whereas steam purity refers t o ingredients other than HzO which might be present. Whether the particular ingredients are desirable, unimportant, or harmful need have no place in a basic definition. Likewise, causative factors need no place in the definition.
1000 Lbs. of St ea m/ hr.
From an academic standpoint, any divergence of conductivity from 0.054 micromho per cm. a t 25" C. should be an indication of the presence of impurities, whether they be gaseous or mineral. The object of all steam purity testing is t o observe or record deviations from a uniform recording of conductivity. The limits of impurities of various types have been neither completely identified nor quantitatively defined for various operating conditions. Pure steam or condensate or water meeting the above definition is not necessary and probably not desirable. Steam impurities may be of the volatile class which includes silica, carbon dioxide, sulfur dioxide, ammonium, the volatile and filming amines, and oil, or of the nonvolatile class which includes mineral ingredients obtained from mist, spray, prime, or other causes.
25
20
PLANT 24 - BOILER 8 6
Conductivity Micro m h os / c m. at 25°C.
4
2 I
0 IAM.
2
3
5
Figure 1. Conductivity Measurements Showing Effect of Varying Percentages of Make-up Water on Ammonia Content of Steam January 1955
5.0
I
6.0
7.0 pH (25" C.)
I
8.0
I
9.0
Figure 2. Equilibrium Constants for Carbon Dioxide, Ammonia, pH, and Conductivity Relationships
INDUSTRIAL AND ENGINEERING CHEMISTRY
47
The presence of volatile iiiipui,ities in steam and condensate is subject to the l a w of cheiiiicid equilibria and is dependent primarily on rating, pressure, and boiler water concentrations. The design of boiler drum interiials miiy also influence the resultant concentration of volatile compounds, particularly sulfur dioxide and silica, in steam to a greater or lesser degree. hlao, steady state conditions in tlie boiler are proportionally upset hy increasingly rapid changes in load. Sampling and testing procedures for the volatile coniponeiits are usually of more specific interwt a t the particular points of potential harm.
I
PLANT 21 - B O I L E E 2
8 30
Figure 3.
10 30
PM
230
vented chamber containing a cooling water coil. Sufficient cooling was provided t'o maintain a constant temperature of about 210" F. a t atmospheric pressure. A single point strip chart recorder of 0-40 and 0--1200 micromho ranges was employed to measure the conductivity of the condensed steam. This instrument, Ivhen employing an electrode n-ith a cell constant of 0.1, recorded the conductivity i n iiiicromhos per centimeter, corrected to a temperature ol 25" C. The advantages of this measurement of the conductivity a t 210' F. were the accuracy and the simplicity of tlie temperature control of the condensate near tlie atmospheric boiling point of water. This device, however? was found to be only partially efficient i n t,he removal of ammonia. -1pplying corrections for the ammonia and/or carbon dioxide coiitents was found Lo be tedious, and inexact, because of the onrying gas contents a,nd the lrequent analyses required. ,4 new graph (Figure 2 ) employing the latest equilibrium constants (6) TYas prepared for the carbon dioxide, ammonia, pH, and conductivity relationships, but this also did not serve the purposc of simple interpretation of result,s. The specific application of this chart is limited by the low confidence in the results of direct Kesalcrization for ammonia ( 1 0 . 1 p.p.m.) and by t'he lack of iriformation on the temperature coefficient of the individual ionic conductances a5 well as the equilibrium constants, unless the tests are made a t 25" C. Both of these limitations may be overcome in time.
' l
4 30
Vent
Conductivity RIeasurements at Plant 21 Condenser
The presence of nonvolatile impurities is not governed by chemical equilibria and may or may not be expected to be quantitatively dependent on rating, pressure, or boiler water concentration but to a large extent is dependent on design and water treatment. Deviation from a uniform conductivity may be considered as abnormal. A4ppropriate sampling and testing procedures for the abnormal presence of nonvolatile impurities (carry-over) are of primary importance. Steam purity measurements have, therefore, been fouiid to be important in the design of boiler internal baffling, of steam purifiers, and of feed water treatment, as well as in the control of plant operating variables, such as water level, boiler load. firing practices, and feed water treatment. During the past 30 years, measurements of steam purity have been made principally by calorimetric, gravimetric, and conductometric methods ( 8 , 4). The conductometric method, lion ever, is the most accepted method because it is the simplest. most sensitive, and speediest. Its main disadvantage has h w n the interference provided by gases, such as carbon dioxide, ammonia, and more recently, amines, n-hich have significant conductivities when condensed. This interference problem became acute in studies on steam purity at Illinois State Institutions supplementing the Feed Water Treatment Service providcd by the Illinois State Water Survey. I n the institution type plant, a variable percentage of make-up water is employed from hour to hour. This produces a widely varying ammonia content in the steam. When the ammonia recycles through the plant system, the interpretation of the recorded conductivity is further complicated. Such conductivity results a t two plants are shown in Figure 1 and Figure The early tests used a modified Straub degasifier or constant temperature device to provide a Eimple control of the conqtant temperature required for conductivity measurement. The steam wae injected in the degasifier and conden=cd in a 48
I
,Reboil S t e a m Chamber
Conductivity Cell
ti y d r o g e n Ex c h a qge Resin
Figure 4.
Steam I ' u r i ~ y .inalyzes
Equipuirn~
The futile attempts to apply corrections for ammonia and carbon dioxide interference led to a decision to remove the gas prior t o the measurement of conductivity by incorporation of a hydi oyen exchanger in the system, I-ollovring the boiling point condensation. Subsequent reboiling was provided to remove remaining caibon dioxide a t the more favorable low pH conditions resulting from the hydrogen exchange. Although it was anticipated that carry-over conductivity due t o hydroxide and carbonate would be lost by the ion exchange and reboiling processes, it was also recognized that the incr c a d conductivity of the resultant hydrochloric and d f u r i c acid a ould more than compensate for this loss and actually increase the bensitivity of the measurement. Caliwlations revealed that the
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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Ion Exchange conductivity of the acids was about 3.3 times the conductivity of the corresponding sodium salts, and therefore more than 50% of the boiler solids would have t o be of hydroxide and carbonate. salts before any decrease in conductivity would result from the ion exchange.
-' 7 11 P L A N T 21- BOILER
5.5
2
! --6.0
210' F. as in the original chamber. Sampling locations were provided for measuring the conductivity previous to and following the hydrogen exchange in order to determine the effectiveness of this process in removal of the interfering substances. Figure 4 shows the complete steam purity testing equipment. I n these tests, chemicals were fed with the boiler feed water direct t o the boiler by means of a chemical feed pump or a pot feeder connected to the boiler feed line. Considerable differences of opinion (1,3, 8) have been reported as t o proper sampling rates, so the authors chose a more or less average rate of 40 t o 60 pounds per hour, in order t o avoid liquid films and corrosive effects associated with t h e lower sampling rates and t o avoid the excessive loss of condensate associated with higher sampling rates.
2z
a
TEST RESULTS
As anticipated, continuous p H measurement following hydrogen exchange also indicated steam purity (Figure 5). 'The application of sodium sulfite t o the effluent from the boiling point condenser provided parallel indications of its presence when measurements were made of conductivity, pH, and redox potential (Figure 6). At plant 21, cyclohexylamine mias applied hourly with other
--6.5 Recorded
;AM
Figure 5.
ib
Conductivity
ii
%AM
(After
b
H
Exch.)
io
ii
Continuous pH Measurements Following Hydrogen Exchange 121
The procedure proved to be successful (Figure 3). Since the gas contaminants were reduced to a minimum, any increase in conductivity was now attributed with certainty to be mineral carryover ( 5 ) .
1
- I d
PLANT 2 1 - B O I L E R
2
"t
Peaks due to 0.024 Ib.
I
TEST METHODS
Saturated steam w u sampled from institutional power plant boilers producing steam at the rate of 20,000 to 50,000 pounds per hour at 125 to 250 pounds per square inch pressure. Steam sampling nozzles were installed according to the ASME specifications (1). One-quarter inch copper and stainless steel tubing carried the steam from the nozzle to the stainless steel or brass degasifiers. Following condensation in the constant temperature device, the condensate was passed through a 4-inch diameter bed of polystyrene resin (0.04 cu. ft.) in the hydrogen cycle, then through a reboil chamber, where the temperature was brought back t o
:---II
After Hydrogen Exchanger
OJ
2 EM.
3
5
4
6
Figure 7. Conductivity Measurements of Cyclohexylamine Treated Water before and after Hydrogen Exchange Treatment
pi
PLANT 24 Before H Exchanger---After H Exchanger0
-
BOILER 8
-
Reboil Reboil O f f w
Applied
* zug. I i! I ::
Morpholine
Micrornhos/crn. 0
.4
,8
1.0
p p m SO;-(est.) Figure 6. Conductivity, pH, and Redox Potential Measurements Following Application of Sodium Sulfite to Effluent from Vented Condenser January 1955
345 4 4 5 545 6 4 5
9 30
I1 30
Figure 8. Hydrogen Exchange Treatment Plus Reboil Eliminated Effects of Cyclohexylamine, Morpholine, and Ammonium Chloride Used for Condensate pH Control to Prevent Corrosion
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49
boiler feed water treatment by means of an electrically operated pump. Prior t o the application of the hydrogen exchange process, the interpretation of the conductivity data would have indicated a serious boiler water carry-over problem. However, the ion exchange process eliminated this interference and indicated that good steam purity was being provided (Figure 7 ) . At plant 24, application of cycloCondensate hexylamine, morpholine, and ammonium chloride (Q), chemicals normally employed for condensate p H control hydrogen for corrosion preExchange vention, revealed Resin that the hydrogen exchange process plus reboil eliminated the interference provided by these chemicals (Figure 8). Preliminary tests on the effect of oc9’ ‘Ondensate A1la’J’zer Equipment tadecylamine ( 7 ) , a filming amine employed t o prevent condensate corrosion, indicate that t,his amine, apparently due t o its low solubility and ionization, does not appreciably influence the conductivity results. When these data mere discussed with F. G. Straub, a t the University of Illinois, he designed a small unit without temperature control for testing condensate to determine condenser leak__I
age in power plants employing ammonia or amines for condensate p H control (Figure 9). It was estimated t h a t immediately after installation the results detected a 0.01% condenser leakage (Lake Michigan water). CONCLUSION s
By combining the procedure of temperature control at the atmospheric boiling point and the hydrogen exchange process with reboiling, an easy foolproof method of testing steam purity has been developed. This method minimizes interference from carbon dioxide, ammonia, cyclohexylamine, and morpholine; provides a simple temperature control; and detects boiler water carry-over by continuous conductivity, pH, or redox measurement. LITERATURE CITED
(1) Ani. Soc. hlech. Engrs., 29 W. 39th St., Kew York, Power Test Code, ”Detexmination of Quality of Steam, Part 11,” 1940. ( 2 ) Am. Soc. Mech. Engrs., 29 W. 39th St., New York, Steam Contamination Subcommittee of the Joint Research Com-
mittee on Boiler Feedwater Studies, “Bibliography on Steam Contamination.” (3) Am. Soc. Testing - Materials. Committee D 1 0 6 M 9 T . “Tentative 8Iethod of Sampling Steam,” 1949. (4) Am. Soc. Testing Materials, Proc., 41, 1261 1340 (1941). (5) Lane, R. W., Larson, T. E., and Pankey, J. W., Proc. I 4 f h Ann. Water Conf., Eirg7s. Soc. Wesf. P e n n , 119-39 (1953). ( G ) Latimer, W. AI., “Osidation Potentials,” Prentice Hall, New York. 1952. ( 7 ) Xlaguire, J. J., IND.ENG.CIimI., 46, 994 (1954). ( 8 ) Place, P. B., Comb,ustion, 2 3 , 4 9 4 3 (-4pril 1951). (9) Proc. 14th Ann. W a t e r Conf., Eilgrs. SOC.W e s t . P a n . , “Panel Discussion on Corrosion and Deposits in Condensate and Feedwater Systems” (1953). RECEIVED for review March 29, 1054.
BCCEPTED
September 23, 1954.
Electrodialysis of ater Using Multiple Membrane Cell A. G. WINGER, G. F. BODARIER, R. KUNIN, C. J. PRIZER, AKD G. W. IIA4RRION,Rohm & Haas Co., Philadelphia, P a . T h e electrochemical properties of recently developed synthetic ion exchanger membranes and those of electrolytic solutions have been utilized to predict the performance of a multiple membrane electrodialysis cell in deionizing sodium chloride solutions. Equations were derived for the energy requirement and flow capacity of a cell of n unit cells with series flow connection, as a function of concentration change, concentration range, voltage per unit cell, cell geometry, and membrane and solution properties. A 50-unit mul.tiple membrane cell was built with Amberplex A-1 and Amberplex C - l membranes a1ternately separating the chambers. Agreement of results on energy requirements per unit volume of depleted effluent is satisfactory, that on the capacity of the cell. much less so. The results are analyzed to discover the probable reasons for discrepancy between predicted and actual performance.
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N RIANY areas of the world, there exists so severe a shortage of potable water that means for recovering such water from brackish and sea water are being studied. The shorbages have become acute because of the incrcaFing population in cert,ain areas. increased demands of industry, poor conpervatmionpracticee. and pollution. Use of distillat,ion inet,hods. ion exchange 50
resins, and electrodialysis have received the major share of attention. The recent development of synthetic ion exchange membranes has greatly stimulated interest in the deionizat>ionof saline and brackich 11-atersby elcctrodial . Heretofore, thrpractical application of the well-lcno\l-nprii les of electroinigi.atiol1~t,i~i~ of ioiis had been greatly handicappctl by a lucli of suitable dia-
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Vol. 47, No. l
