Si€ica-FreeBoiler Feed Water by Ion Exchange W. C. B.4UJIAAS,J. EICHHORN,
.AND
L. F. WIRTH
The Dow Chemical Company, aWdland, Mich.
I ci
R A W 'NATER
T h e complete removal of silica from water has been attained with ion exchange resins. Silica is converted to fluosilicic acid and removed as such b) an anion resin. Three methods of fluoride addition for silica renioial and s i x different anion resins were used in the stud). Alkali regeneration of exhausted anion beds must be preceded 1)) acid or salt treatment to a,oid precipitation of silica in the anion exchangers. Cost considerations show that this method of silica removal is best suited for silica conc-entrations beloir 10 parts per million.
I
I
(21 OF R E MAO C VI DASL
CATION
1
EXCHANGER
EXCHANGER/
EXCHANGER
1 4 111
(I) M E T A L L I C S A L T S ( N a C I , MgC12, elc.1 CONVERTED TO CORRESPONDING m o s , as-nci,n,so,, H ~ C O ~
Figure 2.
T
HE problem of siliceous deposits in boilers and turbines has increased in importance as boiler pressures and temperatures have continued to rise. Some success in minimizing siliceous scales in high pressure boilers has been reported (re),but such boiler treatments still may not prevent siliceous deposits in the turbines. Figure 1 emphasizes (13) the importance of silica reriioval froni boiler feed water used to generate steam for high pressure turbines. Froni studies on several 1200-pound-per-squareinch power plants, Straub and Grabonski found that silica concentrations above 0.1 part per million (p.p.ni.) in the steam would rause deposits in the low pressure portion of turbines. They reported that addition of sodium and potassium chlorides to the tmilcr watc'r reduced the amount of silica in the steam. Incrcasitig the p€I of the nater. hon-ever, was much mow effcctive.
R'-;;;o!si;&F;;D CONTAINING
SI02
510,
02
Typical Demineralization Unit
it is of prime importance to remove the last traces of silica in the demineralized boiler feed water. A simplified diagram of a typical demineralizing unit is shown in Figure 2. Such a unit cohsists of cation and anion exchangers placed in series. These dation resins are synthetic materials containing replaceable hydrogens as in carbosy, hydroxy, or sulfonic groups. .inion resins contain amine groups which form salts with acids. The cation exchangers are regenerated n-ith acid, whereas basic solutions are used for anion regeneration. I n passing through a demineralizing unit, the mineral salts in the incoming water are converted to the corresponding acids in the cation resin bed. These acids are then absorbed, by the anion resin. Typical equations for such a demineralizing unit are:
+ CaCl? --+ C'aRP + 2HC'l 2R'SH2 + 2HCI +2R'SHU.HC'l 2RH
(1) (2)
where I1 = cation resin R' = anion resin I n a misture of strong acids such as hydrochloric arid hult'uric, carbonic acid when present is not effectively removed in the anion bed. I t can he removed from the effluent, however, b y an open gravity degasifier. If oxygen-free 11-ateris desired, this degasifier may he replaced by a vacuum deaerator. K i t h present anion resins silica also is not removed efficiently by the st,andard demineralizing process ( 5 ) . Partial silica removal is obtained n-ith the 12 344.6.
GLASS ELECTRODE
I
I
596.2.
Figure 1
1he mo+t desirable method of reduciug siliceous scale Lvoultl tic t o eliminate tlie silica entirely. Several methods for reducing the silica concentration in boiler feed water \\-ith mctallic osidcs and hydroxides in various fornis have beeii auggestcd and used (8). At standard demineralization teniperaturcs of lcss than 100 F. none of these methods reduces the silica helo\\- about 2 p.p.m. When a demineralizing unit is being uscd w i t h such silica rc~nioval methods, 2 p.p.m. silica constitutes an appreciahlr amount of the total solids in tlie make-up watc~r. 1Iort. 5ilic.a is also plcsc.nt in the steam bcciusi~of the low boil(~rsalt contcLnt. C'onscqucntly, ?,
1453
I
I 0
IO
20
30
40
SO
60
10
60
I
1
90
100
PERCENT R E A G E N T FOR STOICHIOMETRIC NEUTRALIZATION
Figure 3
1454
Vol. 39, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
before the major portion of the run starts. A third method which has been utilized is given in Figure 6. Heie a relatively insoluble fluoride as CaFZ is intimately mixed Kith the cation resin bed. The solubility of calcium fluoride in water is so low that recirculation of acid cation exchange water through a calcium fluoride bed and back into the raw 13aater would require 1007, recycle to remove a few p.p.m. silica. By mixing the calcium fluoride with the cation bed, however, a much greater fluoride concentration is obtained for silica removal. The high fluoride concentration produced initially in this method is absorbed by the anion resin and remores silica later in the run. The initial reaction is:
HF
CONTAINING NO
SIO,
Figure 4. Demineralization Unit with Hydrogen Fluoride Addition for Silica Removal
CaFz (any relatively insol. fluoride)
+ 2RH+R2Ca + 2 HF (6)
The hydrogen fluoride iexnoves the silica as shoTvn in Equation 3. The excess l i j drogen fluoride reactions are:
NaF
I RAW M CONTI!
R’NH2
s102
6R’iVHz HF
+ H F +R’SH2.HF
+ SiOz+(R’?\TH2)2H S i F s + 4R’SH2+
(7)
HzO (8)
The removal of silica by the fluoride method seems to be an equilibrium phenomenon. Excess fluoride is required to remove the silica effectively from water. Six different anion resins were used. Four of these are commercially avaihble-Amberlite I R A (Resinous Products and Chemical Company), Deacidite (The Permutit Company), Duolite A-2 (Chemical Process Company), Ionac -4-293 (hmeiican Cyanamid and Chemical Corporation)-while the other two are experimental preparations. One of the experimental resins is a condensation product of phenol, formaldehyde, and tetraethj-lene pentamine prepared according to the method of Cheetham and Mvers (4). The other laboratory resin is a condensation product of phenol, formaldehyde, and ammonium sulfate. All six anion exchangers removed silica from water by the fluoride method. The optimum operating conditions for the resins, however, differed. Extensive work was done n i t h the pentamine laboratory preparation (designated 89
1‘7 1 1I lXGHANGERl EXCHANGER
‘ T i I S - I E D WATER C O N T A I N I N G NO
sloe
Figure 5 . Demineralization Unit with Soluble Fluoride Addition for .Silica Removal
more basic anion exchange resins in multibed operation, but for only a portion of the run (IO). The Dovv Chemical Company has been doing laboratory work on a process Khich converts silica to fluosilicic acid and removes it in the standard demineralizing process (3). The titration curves in Figure 3 show (6, 9) t h a t those acids below the curve for the anion exchange resin,,as fluosilicic, readily form stable salts of the anion exchange resin and are removed from solution. Silicic and other weak acids above the anion curve do not form stable salts of the resin and are incompletely removed from the water. I n demineralizing a natural water treated with fluoride for silica removal, the cation effluent contains carbonic, hydrofluoric, ’ hydrochloric, sulfuric, and fluosilicic acids. I n general the anion exchanger n ill absorb strong acids in preference to t e a k acids. Hence carbonic acid appears first in the anion effluent, folloned by hydrofluoric, hydrochloric, sulfuric, and fluosilicic acids as the run continues. Therefore silica as fluosilicic acid continues to be removed from the water after the conductivity of the anion effluent rises. Several methods can be used to react the fluoride with the silica. Figure 4 shows a scheme in which hydrofluoric acid is used. The reactions involved are:
+ SiOn+H2SiF6+ 2 H 2 0 2R’SH, + H&F6 -+- (R‘SH?),.H2SiFs 6HF
7 1 , rl, CONTAINING
1 1 CATION
EXCHANGER
I i i
u
AhlON
E XC H A h!G E R
FINISHED WATER C O N T A I Y I N G NO SI02
Figure 6. Demineralization Unit with Calcium Fluoride Addition for Silica Removal
I
(3) (4)
Another mrthod of fluoride addition is shonn in Figure 5 . Here a ivatci-soluble fluoride as S a F or S H 4 H F 2is fed to the incoming ivater before it enters the cation bed. I n this process the soluble fluoride salt is converted to hydrofluoric acid in the cation bed. The hydrofluoric acid then removes the silica as in Equations 3 and 4. The initial reaction is:
XaF (any sol. fluoride)
+ R H +R S a +HF
(5)
Best results were obtained by first feeding an excess of fluoiide to the water before adding the theoretical amount for silica removal. In this way some hydrofluoric acid is loaded on the anion resin
Figure 7 .
Glass Electrode Titration of Fluosilicic Acid
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1947
1455
fluoride 150-1755 of the theoretical fluoride is required to remove silica completely, when the fluoride is proportioned directly to the r a ~ vwater. Only 130c7, of the 1,heoretical fluoride is required n.hen the 30% excess fluoride is fed a t the start of the run and theoretical fluoride is proportioned to the raw water for the remainder of the run. Table I sho1T-s the tover data from such 3 run using T-B anion exchanger. Initial silica was 3.5 p.p.m., and the anion lied flow rate \vas 2 gallons per square foot per minute on a bcd depth of 30 inrhes.
'--, SiOe i n . 3pp.m.
n L .
~~
TABLEI. SODIL-11FLUORIDE ADDITIONFOR SILICARmrovaL, WITH 0.00376 CUBICFOOT ~ I O RESIS S T-U
cill,,'C u. I't. Anion
0
200
600
400
800
1000
200
1400
1600
I800
2000
GALLONS W A T E R P E R CUBIC F O O T A N I O N R E S I N
T-A in this paper), Duolite -4-2 (T-B), arid the ammonium sulfate resin (T-C). Regeneration of a silica-removal demineralizing unit differs from that for the st,andard units. The cation bed is still regenerated with acid, but the anion resin cannot be directly treated ivith a basic solution. Figure 7 s h o w (9) the complete titration curve for fluosilicic acid, \\-hich is stable only in the low pH portion of the curve. For p H values higher than about 4, fluosilicic acid decomposes t o silica. Hence, if the anion exchanger were treated direct,ly with a base, silica would be deposited in the bed. T o avoid this difficulty, the anion bed is first treated with a 5-107' solution of a strong acid. This acid treatment displaces the fluosilicic acid from the anion exchanger. The anion bed is then regenerated with a base as in the standard demineralizing units. Strong salt solutions \\-ill also remove the fluosilicic acid from the anion bed. The effluent from a cation exchanger being regenerated with acid can be used for this purpose. The reaction involved probably is:
+
2SaC1 2R'iSH2.HC1
+
SasSiFa
i46
1120 1305 1493 I080 2030 2240 24'3 2610
Figure 8
(R'NH2'L.H2SiFs
187 3i3
(P.p.m. silica = 0.0) ConductiT-ity, C;rains:Gal. as S a C l 1 0
0 3 0 0
0 0 0 0
0 0
0 3 0 3 0 3
1 5 3 0
...
6.7 5.6 5.3 5.0 4.7 4.6 4.1 3.8
This method of fluoriiic atlilirion \vas also used n 3 h a G-inch(liainetvi. glass c.olLunii corit:iining :ibout one half cubic foot anion resin 'I-Ci n about a 3&illch bcd dcpth. Figure 9 s h o m the 1abor:itoI.y setup for this unit. Pyrex columns \\-ere used frequently in thc experimental worlc and sho\ved o; detectable silica loss. The datii from a run of tliib unit is shuivn in Figure 10.
(9)
4 n excess amount of salt is required to remove the fluosilicic acid effectively from the anion bed. The salt t,reatment is follor\-ed by the usual base regeneration. The stability of the anion resin in alternate acid-base or saltbase regeneration d l decide which method can be used. A series of silica removal experiments were carried out using the three fluoride addition methods already described. Figure 8 shows the results obtained from a run in nhic~h 15% of the anion bed was convert,ed t o the hydrofluoride form by HF addition directly to the bed. The raw water to the exchangers was Midland city t a p water contairling 3 p.p,m. silica. The exhausted anion exchanger was treated with '"5 hydrochloric acid before regeneration. Silica removal from the anion bed was 98Y0 complete. Anion resin T--1 was used in a 1-inch-diameter Saran tower with 3 bed depth of 30 inches. The cation cxchanger used in all the silica tests was Dowes-30 (2). Runs w r e also made using a soluble fluoride, as KaF. Tower tests s h o w d that with sodium
PH 9.3 8.6
'
Figure 9.
Apparatus for Adding Fluoride
INDUSTRIAL AND ENGINEERING CHEMISTRY
1456
Vol. 39, No. 11
If/ .]
14
1.4
ANION R E S I N ( T - B ) Av. 5 1 0 2 : 4 . 3 p,p.m. C A vo.F F2 l: o w?40%The;:e;ico; R a t e . 5 Gel./Ft.+
_.
A n i o n B e d : .0369 Ft?
.
CO\OUCTIVITY p.p.m.os N o C I
1.0
.-
12
10
ANION R E S I N ( T - C )
S10, In 7 ~ p . m .
__
-~
--
-
, ~~~~
0
5
0
15
20
N o F 200% T h e o r e t i c a l Av.Flow R a t e 2 Gal / F t?/ M i n. Anion B e d 0 5 6 5 F t ? ___ii_-
30
25
45
40
35
50
0
200
HOURS RUN
400
Vigiire 10
70
60
50
40
30
COSTS INCLUDE FLUORIDE A N D
10
2
3
4
5
6
800
1000
1200
1400
Figure 11
SILICA REMOVAL COSTS USING FLUORIDE I N A DEMINERALIZING
20
600
GALLONS WATER P E R C U B I C F O O T A U I O N R E S I N
7
8
9
0
Figure 12
Somem-hat more fluoride was required for equivalent silica removal using this resin than resin T-R. Work was also carried out to determine if relatively insolublc~ fluorides as CaF? or MgF? could be used. Such fluorides can 1112 used by mixing them with the cation exchanger. Proper operation leaves the bottom portion of the bed relatively frec of tlica sparingly d u h l e fluoride while intimate mixing of the calcium fluoride and resin occurs above the slurry feed header. The acidic cation resin performs three functions: ( a ) removes metal ionfrom the incoming feed xater, ( b ) reacts with the calcium fluoride t o form hydrofluoric acid fQr silica removal, and ( e ) acts as a filtci, bed for retention of unconsumed calcium fluoridr particles. Figure 11 shows the results of a run using calcium fluorideab the fluoride source. . h i o n resin T-B was used in a 1.5-inch-dianieter glass t on-er. A11 silica ( 7 ) and fluoride ( 1 ) tests were madc colorimetrically. Salt concentrations were determined by a Sol U-Bridge and hardness by standard soap test. The experimental n-ork was done oii waters having a maximum silica concentration ~f ahout 7 p.p.ni, Rlidland city water is obtained from a river source. The average tap water analysis for the year 1942 is given in Table 11. Further v,-ork is planned to determine the optimum silica eoncentrations \$-hich can be treated by this method. A 200-gallonper-minute demineralizing unit using sodium fluoride for silica removal in boiler feed make-up water is scheduled t o go into operation i n 1947. h 3000-gallon-per-minute calcium fluoridcl unit has hcen debigned.
Ifi applications of this method for silica reniov:tl it should be renieniliered that the fluoride ion is one of the first t o break through. Great care must be taken in the opcxration of such units if fluoride is harmful in the effluent. Water used for drinking which contains over a few p,p.ni, fluoride causes mottled teeth. The c,ffect of fluorides in boilers has not as yet been definitely established. Of iiitcsrest t o prospective users is t,he cost of the fluoridt. silica removal method. Figure 12 shows cost curves for the removal of various silica concentrations with sodium fluoride, hydrogen fluoride (both 175% theoretical), and calcium fluoride (250'3, theoretical). For these calculations efficiencies for sulfuric acid cation regeneration of 35y0 and a sodium carbonate anion regeneration of 67% were used. re as of July 1946 ( 1 2 ) . ;In additional charge of tn-o dollars per ton was added to the calcium fluoride cost for grinding the commercial material through 300 Tyler mesh. The silica removal costs include only fluoride and regeneration costs, and do not allon- ior the additional wash water and equipment required. I t should be noted that viith hydrogen fluoride no additional cation volume is required. K i t h fluoride salts, hon-ever, a water of 200 p.p.m. as calcium carbonate and 1 p,p.m. silica nould require about 5s additional cation bed. The use of hydrogen fluoride introduce,. handling and maintenance problems n-hich offset its cost advantage. The calcium fluoride method is well suited t o large n.ator-treating units. Sodium fluoride, although the most expensive treatment, is easily handled and proportioned. I t is particularly ir-ell suited for small scale water treatment. The fluoride ion erchange treatment is the only one at present which n-ill removes silica in the cold to concentrations below 2 p.p.m. Concentrations t o 5 p.p.m. can be treated very economically. Fluoride treatment of silica higher than 10 p.p,m. is best preceded by an absoivtion method. The problcm of complete silica removal from tr-ater has long confronted t h r water profession. Now for the first time the last traces of silica ran be removed efficiently and also reononiically from n.ater.
~ I I C HTAP . WATER.-IT-ER.IGE .%SALYSIS TABLE11. ~IIL)IASI), FOR YEAR1942
90
20
9
55
5
2B
29
73
November 1947
I N D U S T R I A L A N D EN G I LITER4TURE CITED
Adolph, IT. H., J . Am. Chem. Soc., 37,2512 (1915). Bauman, W.C., IXD. ESG.CHEM.,38,46 (1946). Bauman, K. C., J . Am. R'ater F o r k s Assoc., 37,1211 (1945). Cheetharn, H. C., and Myers, R. J. (to Resinous Products & Chemical Co.), U. S.Patent 2,341,907 (Feh. 15, 1944). Embshoff, &A, C., Proc. 3rd Ann. r a t e r C o n f , Engrs.'Soc. Western Pa., 1942,110. Harman. R. IT.. J . Phus. Chem.. 31.619 (1927). Kahler, H. L., Im. E&. CHEM.'.A ~ A I ED., . . 13,530 (1941). Liebknecht, 0 . (to Perniutit c0.1,U.5.Patent 1,S60,781(1932) : Schwartz, 11.C . , J . Am. Water F o r k s Assoc.. 30, 669 (1938);
,RING CHEMISTRY
1457
Applebaum, S.B., I b i d . , 30,947 (1938); Behrman, S.,and Gustafson, H., ISD. Esr,. CHEM.,32,468 (1940); Tiger, H. L., Trans. Am. Soc. Mech. Engrs., 64,49 (1942). Malaprade, L., Ann. chim., [ l o ] 11, 133 (1929). Manring, IT,E., Proc. 6th Ann. Water Conf., Engrs.' Soc. Western Pa., 1945, 124. Oil, Paint and Drug Reportei , July 8, 1946. Parks. G. U..Trans. Am. SOC.Mech. Enqrs., 67, 335 (1945). Straub, F. G., and Grabomski, H. A,, Ibid., 67,309 (1945) R ~ C E I V ESeptember D 9, 1916.
Presented a t the 7th Annual Water Conference, Engineers' Society of Western Pennsylvania, Pittsburgh, Pa., January 19.47.
Paper Capacitors Containing Chlorinated Impregnants BENEFITS OF CONTROLLED OXIDATIOiN OF THE PAPER' D. -4.3ICLEbS Bell Telephone Laboratoriea, Inc., TlirrrciS Hill, \ . J .
Cuntrcr! to the usual belief, properl? controllecl oxidation of h a f t insulating paper can h a \ e niarLed beneficial effects on its electrical properties. The insulation resistance and power factor of kraft capacitor tisaue at ele>ated temperatures are impro\ed by oxidation. The effect of o\idation is permanent in the sense that it remains after the paper has been humidified and redried. These impro\ernents per-ist when the paper is impregnated with anthraquinone-stabilized chlorinated diphen?1. In addition, oxidation of the paper brings about a substantial improvement in the accelerated life performance of capacitors. rhe benefits of controlled oxidation h a l e been realized in the commercial production of capacitorc.
0
S I D A T I O S is knon-n to degrade the mechanical properties of paper. It has often been assumed that oxidation is also harmful to the electrical quality of insulating paper. The data presented here demonstrate that this is not aln.ays so; on the contrary, large benefits may be realized by subjecting insulating paper to controlled oxidation. These benefits have been demon5trated with respect to kraft capacitor tissue and to such tissue when impregnated n-ith pentachlorodiphenyl subsequent t o osidation. Effects of oxidation on other types of paper are unknown, but improvements in the cases studied are large enough to suggest extension of the study t o other types. Improvement of the electrical performancr of capacitor tissue by controlled oxidation is probably related io the stabilizing action of oxidizing organic compounds previously described ( 1 ) . In these experiments the samples were capacitor nindings comprising standard aluniiriuni capacitor foil interleaved with 0.0004inch kraft capacitor paper. I n some nindings tn-o layers of paper were used b e t w e n foils and in other cases thrce layers. These will be referred to as tn-o-layer and three-layrr samples. I n all cases ilie units n-ere of such size as to give a capacity of about 1 microfarad \{-hen impregnated with pentachlorodiphcnyl. T h e paper used represented the product of several suppliers. For the 1 T h e first three papers of this series appeared in .January 1945, l l a y 1946 ( I ) , and Sovember 1946 ( 3 ) .
impregrialit, ~~e1itaclilor~~diI,i,(.11?.1 ( . ~ l ' l J ~ ' l O l 1254 ' I st;tliilizcd n-ith l..jco anthraquinone was used. In some instances t h e uriits vxrc' assembled in c:ipcitor cans
with phenol plastic terminals, as shon-n iri Figure 1. Generally two units \yere assellibled in a can ant1 w r e coniiectctl i i i parallel to form a 2-microfarad test capacitor. -1small filling liol(, in the top [vas soldered shut aftel, pro( ing. In other instances pairs of units were held in steel clamps during processing and were mounted in glass tubes for measuring, as shoi\-n in Figure 2. In this case three lcads wcre brought out so that each 1-microfarad unit could be measured separaielj-. These variations i i i the method of mounting saniple did riot appear t o alter the nature of the results. 1Iany of t h e samples tested w ~ r evacuum-dried arid impregnated in the capacitor shop in ccininiercial equipinent. The shop process \vas designed to attain nioi.5tur.e (
