Demineralizing Solutions by a Two-step Ion Exchange Process HOWARD L. TIGER AND SIDNEY SUSSMAN The Permutit Company, New York, N. Y.
With the advent of the acid-regenerated organic cation exchangers (so-called hydrogen zeolites), it became possible to replace all metallic cations with hydrogen ions, leaving only the corresponding acids in solution. The operating principles of these hydrogen cation exchangers i n the field of water treatment have been discussed in previous papers. Various classes of organic cation exchangers have been developed : coal derivatives, lignite derivatives, materials made from paper waste and acid sludge, and resins suchas tannin-formaldehyde and other polyhydric phenol-formaldehyde derivatives. After it became practical to replace the metallic cations in solution by hydrogen ion, i t was clear that, if the resultant free acids conld be absorbed by some insoluble substance which could be regen-
erated and operated indefinitely in repeated cycles like the cation exchangers, it would be possible to remove both cations and anions by a practical process. Various materials for absorbing these acids appeared in rapid succession and the principles of operation are discussed. Anion exchangers have been developed that eliminate troubles experienced with earlier materials and, in some respects, excel the well-established cation exchangers, especially in capacity and regenerating efficiency. The principles of operation with respect to electrolyte contents of waters that can be economically treated by this process are discussed, as well as the influence of type of anion and operating cost. The method of operation to be follow-ed to obtain a properly demineralized final effluent is described.
+ +
+
CaC12 HnZ+ CaZ 2HCI HC1 R& + R8N.HC1
F
OR almost a hundred years, it has been known that salts like calcium and magnesium bicarbonate, which are frequently present in water supplies, can be removed to a substantial extent by precipitation with lime. By this means their concentration can be reduced to about 15-50 p. p. m., depending upon the temperature and excess of reagents. With the advent of the acid-regenerated organic cation exchangers (so-called hydrogen zeolites), it became possible to replace all metallic cations with hydrogen ion, leaving only the corresponding acids in solution. It was clear that if only the resultant acids could be removed, it would be possible for the first time to reduce the total dissolved solids chemically rather than by distillation in waters containing chlorides, sulfates, or sodium bicarbonate. The first actual attainment of this objective occurred with waters containing sodium carbonate or bicarbonate. The volatile carbonic acid formed by reaction of these salts with the acid-regenerated cation exchanger was readily removed by degasification (5). Removal of the nonvolatile acids and production of an effluent comparable in quality to distilled water depended upon the development of an insoluble material that would absorb acids and that could be regenerated and operated indefinitely in repeated cycles, like the cation exchangers. Various acid absorption materials soon appeared in rapid succession, and the ion-exchange process of removing salts From solutions (commonly called “demineralizing”) was launched commercially.
where Z
RsN
= =
(1) (2)
cation exchanger anion exchanger
Any chlorides and sulfates present in the original solution will be transformed by this treatment into the corresponding hydrochloric and sulfuric acids. Bicarbonates are transformed into carbonic acid. I n step 2, the anion exchanger (called “De-Acidite” in the Permutit process) absorbs the strong acids but does not take up the weaker carbonic acid, which is readily decomposed into water and carbon dioxide in the degasifier that follows the two ion exchange units in series. Thus, a final effluent which approaches distilled water in quality is obtained. The precise composition of this demineralized effluent depends to some extent upon the quality of the raw water, but in general the following average analysis represents its composition: 0-2
1-6 0-4 0-3
5-10
Below 5
The demineralizing process does not remove silica from water. If the influent contains a sufficiently high silica concentration to be harmful in the particular application under consideration, as ia frequently the case in the treatment of high-pressure boiler feed water, then silica removal treatment must precede the demineralizing treatment. Practical processes and equipment for effective silica removal are now available (17), and several large-scale installations are in operation.
Principles of Two-step Demineralizing I n principle the process is a two-step series operation as illustrated in Figure 1. The reactions involved in these steps may be represented as follows, using calcium chloride as a typical salt: 186
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
February, 1943
187
RAW
I NF L U E N T
1 E-ACIDITE
ZEO-KARB H
ALKALI REGENERATING SOLUTION
1
EGASlFlER
I rI
STEP N 0 . I R E P L A C E M E N T O F METALLIC C A T I O N S BY HYOROGEN (H ION) CONVERTS SALTS P R E S E N T I N R A W W A T E R I N T O CORRESPONDING
TO SERVICE
DE-MINERALIZED WATER
STEP N 0 . 2 REMOVAL OF A C I D S F O R M E D I N S T E P NO. I. THE HCL AND I i ~ S O ARE ~ ABSORBED IN T H E D E - A C I DIT€ U N I T AND THE HzCOj DECOMPOSES I N T O H 2 0 A N D C O s r T H I S CO1 ESCAPING TO THE A T M O S P H E R E I N THE D E G A S l F l E R
ACIOSOIIZ.HCL,H~SO~.H~CO~)
FIGURE 1. TWO-STEPPROCESS FOR DEMINERALIZING WATER
In the acid regeneration, the cation exchanger is reconverted to its original condition as follows: CaZ
+ HzSOa e H2Z + Cas04
(3)
Although De-Acidite and similar materials are usually called “anion exchangers”, the indications are that they may take up all the free acid molecules in much the same way that ammonia reacts with hydrochloric acid (14). I n the case of these anion exchangers, the acid absorption is accomplished by amino groups which form a part of large insoluble organic molecules. When the capacity for absorbing acids has been exhausted, the material is regenerated with an alkali, such as sodium carbonate, according to the following reactions:
+ NazCOa
2R3N.HC1
2RsN
+ 2NaC1 +
H20
+ COz
(4)
The sodium chloride, sodium sulfate, and carbon dioxide formed in this regeneration pass to waste in the rinse. Of the many factors influencing the demineralizing of a particular water supply, the most obvious are the nature and performance of the cation exchanger and acid absorber used.
results were obtained under identical experimental conditions and are expressed in milliequivalents (meq;) per liter of original bed volume. They can be conveniently converted to the usual form of expression, “kilograins per cubic foot expressed as CaCOi’ by dividing by 45.8. With certain ion exchangers there is a gradual loss of material as the result of solution or attrition. Since the total capacity of the installation may be dropping while the materials show a n apparently constant capacity per unit volume of ion exchanger when measured a t the same time the capacity of the installation is measured, data in Figures 3 and 4 are given in terms of the initial bed volume of ion exchanger. This is done since, in commercial installations, knowledge of the capacity of the entire installation is of greater value than the capacity per unit volume of exchanger. These capacity data were obtained by a tube testing method which provides a means for simulating large-scale operation in the laboratory (16). A water or solution of known composition flows through a laboratory tube containing a definite volume of the granular ion exchanger until the predetermined end point is reached. The capacity is calculated from the volume of water treated, the amounts of cations or anions removed from solution, and the volume of ion exchanger employed. Except when a series of compara-
Cation Exchangers Various classes of organic cation exchangers have been developed: sulfonated coal derivatives, caustic-treated lignite derivatives, materials from paper waste and acid sludge, tannin-formaldehyde and other polyhydric phenol-formaldehyde resins, and modifications of these produced by supplementary treatments. The coal derivatives, such as Zeo-Karb, were the earliest materials developed and have been most extensively applied in practice. Zeo-Karb installations have been operating for a number of years and for thousands of cycles without deteriorating physically, dropping in capacity, or developing colored effluents. It appears that although some resins are slightly higher in capacity, a good sulfonated coal is more resistant and has greater stability. Figure 2 presents typical results on organic cation exchangers which indicate their long-term stability. These
TABLE I. EFFECT OF TYPEOF ANIONSON CATIONEXCHANGER CAPACITY (TUBETESTS) (Influent solution oonoentration, S meq. er liter: flow rate, 50 ml. er minute per liter
in a 2.3-om. diameter tube; regeneratezby 605 meg. of 0.4 N H& of bed.)
Salt NaCl CaCli MgClr
Capacity (Meq./L. of Cation Exchanger) Pol hydrio pzenolSulfonated formaldehyde coal resin 19s 268 224 318a, 342 b) 266 3231332 b) 303
NaHCOa Ca(HC0dr Mg(HC0a)n
492 296(S0Sa) 444
Initial run on calcium influent. 605 meq. of 0.4 N HC1 per liter of bed. b Regenerant
5
496 296
...
188
INDUSTRIAL AND ENGINEERING CHEMISTRY
tive tests is made under a fixed set of identical experimental conditions, it is necessary when presenting capacity data to specify the amount of regenerant used per unit volume of ion exchanger, as well as other conditions which affect the capacity. The operation of the hydrogen cation exchangers in the field of water treatment has been fully discussed in earlier papers (1-6, 8, 9, 10, 12, 16, 19).
Vol. 35, No. 2
tion 6). With strong acids, such as hydrochloric, sulfuric, etc., the ionization equilibrium shifts predominantly to the lefthand side of Equation 7, and introduces a common ion effect on the right-hand side of Equation 5 which tends to drive this equilibrium to the left and consequently reduces the cation exchange capacity:
+ HzZ S M2Z + 2H+ + HCOs-=HzCOa + C1- = HCI
2M+
(5)
H+ H'
(6) (7)
where pYI+ = metal ion
Tables I and I1 also show that with salts of strong acids higher capacities are obtained with divalent cations than with monovalent cations. This is in agreement with known ion-exchange theory and performance (18). The N U M B E R OF CYCLES fact that the bicarbonates of both mono- and divalent metals give approximately the same capacities in flow and equilibTUBETESTCAPACITY DETERXINATIONS FIGURE 2. LONG-TERM rium experiments may be ascribed to the fact that the ON ORGANIC CATIONEXCHASGERS presence of this anion overshadows the effect produced by differences in valence of the cations. The quantity of cation exchanger necessary for deminer.--alizing a particular water has been found t o depend partly upon I the nature of the anions present. Experiments performed in --0 this laboratory indicate that the capacity of a cation exw 1000. --\ m changer operating on the hydrogen cycle is profoundly affected by the type of anions present. Table I illustrates this phenomenon with data obtained by tube testing; the results are the averages of three to six runs. Table I1 presents similar data obtained by a simplified capacity test. This test is carried out by shaking solutions of a t least two known concentrations with weighed amounts of the ion exchanger until equilibrium is reached. The residual concentrations of ions in solution are determined, and the quantities of the ions removed from solution per unit weight of exchanger are then calculated. The logarithms of amount of a given ion absorbed per unit weight of exchanger are plotted against the logarithms of the concentrations of the 0 50 100 150 200 250 corresponding ions in the final solution. The resulting N U M B E R O F CYCLES Freundlich isotherms are extrapolated t o any desired concentration. As a rule, extrapolation to final concentrations FIGURE 3. LOSG-TERM TUBE TESTCAP.4CITY DETERMINATIONS ON ANION EXCHASGERS of 1 meq. per liter under certain conditions gives results that are comparable to those obtained by the tube testing method. This method is useful for the rapid estimation of The differences between the capacities obtainedTwith ion exchanger capacities on very small samples of material. calcium salts and those with magnesium salts were apparently caused by the deposition of insoluble calcium sulfate on the cation exchanger during regeneration. This precipitated TABLE 11. EFFECT OF TYPE OF AXIONSON CATIONEXCHANGER material apparently blocked access to a part of the cation exC A P A C I T Y (EQUILIBRIUM DETERMINATIONS) changer surface, thereby reducing the capacity. ConfirmaCapacity (Meq./L. of Cation Exchanger) tion of this hypothesis was obtained by determining capaciPolyhydric ties on fresh cation exchanger beds and on beds regenerated phenolSulfonated formaldehyde by hydrochloric acid. In runs on fresh cation exchanger Salt coal resin beds (results marked a in Table I) , the capacities agreed with NaC1 53 73 those obtained when using the corresponding magnesium 209 167 CaClz 209 185 MgClz salts. Subsequent runs on relatively high calcium influents 53 NalSOl 73 with the same cation exchangers resulted in reduced capaci20Q 185 MgSOa ties. Runs made when using hydrochloric acid as regener281 NaHCOs 270 ant (results marked in Table I) gave calcium capacities 316 324 Mg(HC0a)t in agreement with magnesium capacities obtained when using either regenerant. I n practice, this calcium sulfate deposition occurs only The general improvement in cation exchanger capacity rarely, and in these cases, in view of the higher cost of hydroobserved when the influent water contains appreciable concenchloric acid and the difficult corrosion problem introduced trations of anions of weak acids is caused by a shift to the right by its use, it is cheaper and more expedient to use a larger in the cation exchanger-salt equilibrium (Equation 5 below) volume of cation exchanger than to attempt to obtain maximum cation-exchange capacity by regenerating with this as a result of the removal of hydrogen ions from the solution by the formation of the weakly ionized carbonic acid (Equaacid.
.'..
--.,I
\
