Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 366-371
366
Thermal Deactivation of Strong-Acid Ion-Exchange Resins in Water Leonardus Petrus,' Eire J. Stamhuis, and Geerl E. Department
H. Joosten
of Chemical Engineering, University of Groningen, NJenborgh 76, 9747 AG Groningen, The Netherlands
The thermal deactivation in water of several poly(styrene-divinylbenzene-sulfonic acid) resins in the hydrogen-ion form has been measured in a fixed-bed flow reactor at temperatures between 420 and 480 K. The deactivation is caused by the removal of sulfonic acid groups from the polymer matrix, giving sulfuric acid. The removal of sulfonic acid groups is found to be catalyzed by hydrogen ions, which in the present case originate from the acid groups in the resin. Two types of -S03H groups are present in the resins, showing widely differing rates of decomposition. The reaction by which the more stable -S03H groups (about 90% of the initial resin capacity) decompose has an activation energy of approximately 120 kJ/mol for most resins tested. The resin XE-307 was the most stable resin tested at the highest temperatures in the range studied. In the lower range the resin Dowex HCR-W X8 is the most stable one.
Introduction Since their development, synthetic organic ion-exchange resins are mainly used for water treatment, for instance in the preparation of demineralized water (Helfferich, 1962; Kunin, 1972). Usually they are used at room temperature in these applications. In recent years ion exchangers have been receiving more and more interest as basic or acidic catalysts, having the advantages of a heterogeneous system (Polyanskii and Sapozhnikov, 1977). To obtain acceptable reaction rates it often is necessary to carry out the reactions at elevated temperatures. However, a hitherto experienced disadvantage for the industrial application of ion-exchange resins as heterogeneous catalysts at elevated temperatures is the increasing thermal instability at higher temperatures. For instance, cation exchangers are now being used as acid catalyst in the commercial hydration reaction of propylene at temperatures up to 430 K (Neier and Wollner, 1975), the activity loss of the catalyst being one of the main problems. In general, activity loss can be ascribed to poisoning, blocking, or hydrolysis of active groups. In the hydration of propylene the hydrolysis of sulfonic acid groups is the dominating deactivation process. The aim of this experimental study, as a part of our research into the hydration of lower olefins catalyzed by acid ion-exchange resins, is to determine the kinetics of the thermal degradation of strong-acid cation exchangers in the hydrogen form in water. Literature Survey A literature search showed that nearly all thermal desulfonation experiments in water are performed in sealed glass ampules (Hall et al., 1970; Marinsky and Potter, 1954; Minto et al., 1972; Polyanskii and Tulupov 1971; Tulupov, 1971; Tulupov et al., 1972, 1973; Tulupov and Greben, 1972). The temperature range in these experiments was about 420-480 K, the main or only decompositionproduct being sulfuric acid. In one contribution the main reaction is said to be the formation of sulfurous acid, leaving weakly acidic phenolic groups in the resin (Hall et al., 1970). This assertion is ambiguous, however, since the mass balance does not tally. It is reported that the decomposition rate is not the same for all active groups. Several authors mention that a fraction of the active groups (about 5-20%) decomposes fast (Marinsky and Potter, 1954; Minto et al., 1972; Tulupov, 1971; Tulupov et al., 1972, 1973; Tulupov and Greben, 1972). It has been suggested that these groups are sulfonic acid groups attached to divinylbenzene units 0196-4321/81/1220-0366$01.25/0
in the matrix (Znamenskii et al., 1976). Based on batch experiments various kinetic schemes are reported for the decomposition of strongly acidic cation exchangers. They range from a single first-order decomposition (Hall et al., 1970), via the sum of a first- and a second-order reaction (Tulupov, 1971), to a combination of two or three overall second-order reactions (Tulupov and Greven, 1972). A recent paper suggests a single secondorder reaction for the decomposition of Amberlyst 15 (Bothe et al., 1979). Many authors report that resins in the salt form have a higher thermal stability than in the H+-form (Hall et al., 1970; Marinsky and Potter, 1954; Minto et al., 1972; Polyanskii and Tulupov, 1971). Treatment of the resins with aqueous sulfuric acid solutions up to about 15 N instead of pure water at 448K leads to an increase of the rate of decomposition, suggesting acid catalysis. The rate of decomposition levels off at high concentrations of sulfuric acid as resulfonation of the unsubstituted phenyl groups takes place (Tulupov et al., 1973). The effect of sulfuric acid, formed by the decomposition reactions, on the rates of the decompositions can be eliminated by continuously removing the acid with a flow of water. The few flow experiments published (Fejes, 1969) show an initial fast decomposition followed by a fmt-order reaction, yielding sulfuric acid as the only decomposition product. The temperature range in these experiments was 380-410 K. The above-mentioned contradictory results do not permit one to predict the decomposition behavior of an ion-exchange resin in an actual reactor. This was the reason for carrying out the experiments described in this contribution. Resins Tested A survey of the strongly acidic ion exchange resins tested for their thermal stability in water is given in Table I. In the following sections abbreviated resin names are used. Experimental Section (1) Equipment. The rates of decomposition of the resins were obtained from experiments carried out in a 316 stainless steel vertical tubular reactor (l), 1.26 m long, 8 mm i.d. and Teflon lined (see Figure 1). The resin sample (2), about 5 mL, was contained in the lower section heated by an oil jacket; the upper part was cooled with water. Temperatures on both sides of the resin bed were measured by copper-constantan thermocouples inserted in axially mounted thermowells, protected against corrosion 63 1981 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 387 Table I. Resin Properties
manufacturer
resin Amberlite 252
XE 307 Imac C 16 P
Rohm and Haas Co., Philadelphia, PA Akzo Chemie Nederland BV, Amsterdam Noord
Imac C 201 P Dowex HCR-W X8
moisture” content of wet resin, wt %
resin type
capacity“ of dry resin equivlkg
?
macroporous
4.8
52
?
macroporous macroporous
3.9 5.0
51 46
macroporous gel
4.6
52
5.2
50
macroporous
4.7
50
wt % DVB
16
Dow Chemical Co., Midland. MI
Dowex M S C l
Capacity and moisture content of H+-form resin, particle size range 0.71-1.0 mm in wet Na+-form. A
Figure 1. Resin test apparatus.
by sealed Pyrex glass tubes. The resin sample was fixed between quartz wool and Teflon plugs. The whole packet was kept in place by the thermowells. Twice-distilled demineralized water was used. It moved in upflow through the resin bed. The water was pumped by a reciprocating pump equipped with pulse dampers from a polyethylene container (3) through a preheating section (4) to the reactor. The liquid flow leaving the reactor was cooled ( 5 ) , filtered (6) and passed through a back-pressure rgulator (7). All tubes in the pressurized section were made of 316 stainless steel, using Swagelok fittings. The pH of the exit stream was measured and the acid liberated from the resin was neutralized continuously by the addition of 1 N NaOH solution; the amount of hydroxide added was recorded. The liquid flow rate could be measured (9) before the liquid entered the collecting container (10). The preheating section consisted of a 2 mm i.d. tube 1 m long placed in a thermostat bath (4a) and a tube 0.5 m long (4b), heated by the silicone oil circulating through the reactor oil jacket. Electrical heating tape (8) served to avoid heat losses from the reactor section containing the sample to the preheating and cooling sections. These heaters were adjusted so as to keep the temperatures on both sides of the bed at the same value as the temperature of the circulating oil. All heated parts were insulated with glass wool. The temperature difference between the thermocouple readings on both sides of the bed never exceeded 1 K. The pressure in the reactor was 1-2 MPa to prevent vapor formation. Reactor temperatures up to 463 K could be maintained. In the higher temperature range (up to 480 K), a completely electrically heated reactor was also used, giving the same results as obtained in the oil heated one. (2) Pretreatment of the Resins. The resin pretreatment started by sieving out a narrow particle size fraction
from the wetted Na+-form resin using tap water and standard ASTM screens. This fraction was then cycled at least three times (until free of “fines”) between the Na+ and H+-form, using 2 N NaOH and 2 N HC1 solutions as regenerants. The H+-formresin was centrifuged for 3 min at about 300 g, and approximately 5 g was accurately weighed and quickly transferred into the reactor. Another weighed portion was used for the determination of the initial exchange capacity. (3) Capacity and Moisture Content. The strong-acid capacity was determined by passing an excess of 2 M NaCl solution over the resin sample and titrating the effluent with standard NaOH solution. The weak-acid capacity was measured using the same sample by treating it with a known amount of 0.1 N NaOH, washing with water, and back-titrating the effluent with 0.1 N HCl solution. The moisture content of the resin was obtained by drying a weighed portion under vacuum (2 kPa) for 4 h at a temperature of 368 K. At the end of an experiment the resin was quantitatively collected from the reactor. Strong and weak-acid capacities were determined again. Results Preliminary experiments showed that the decomposition reaction takes place at virtually the same rate throughout the bed. Particle size effects in the measured range (0.6-1 mm) were absent. The main reaction is RS03H H 2 0 RH + H2S04 (R = resin matrix) (1) The amount of acid liberated matches the decrease in exchange capacity within a few percent. Weakly acidic phenolic groups, formed according to the reaction RS03H + H2O ROH + HzSO3 (2) were never detected. During all experiments the effluent water was somewhat colored, indicating some matrix degradation. The pH value of the effluent water was never lower than 2. In nearly all experiments the residual number of Ht equivalents in the resin ( N ) was less than 50% of the initial number (No). Figure 2 gives a representative example of the rate of decomposition as measured in our experiments. In this figure the ratio of the residual to initial number of H+ equivalents in the resin sample (N/No),as calculated from the amount of acid liberated, is plotted against time. The figure clearly shows the fast decomposition at the beginning of an experiment. Attempts to fit all experimental rates of decomposition to a single first-, second-, or third-order reaction failed. Assuming that the decomposition of -S03H groups is catalyzed by resin H+ions, the total rate of decomposition
+
-
-
368 Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981
-
-
-
__
-
~
_
-
.
L
L” -i
~
Figure 2. Typical example of the ratio N/N,vs. time. The dotted line represents the slowly decomposing groups.
i
-
-
_ _ L- i _ i _
53
+
l
b
Figure 3. Typical example of N o / N vs. time.
can be represented by two parallel overall second-order reactions according to the differential equations (3)
(4)
In these equations Cf and C, are the concentrations of the fast and slowly decomposing groups at time t. Normalizing these equations with respect to the total initial concentration of -S03H groups (= Co), and assuming the volume of the resin to be constant during the decomposition reaction gives (5)
Table 11. Measured Kinetic resin s ’ type T, K __ -~ Amb 252 470 18 458 17 4 54 15 44 8 8.3 44 1 5.8 C16P 473 23 453 11 446 8.3 5.0 434 C 201 P 477 30 462 16 12 456 8.3 446 43 9 4.4 3.3 428 HCR-W 464 13 44 8 8.9 44 2 4.4 44 2 5.8 437 4.7 4 26 3.3 XE-307 477 33 470 13 458 20 44 5 7.8 44 1 6.9 439 4.2 MSC-1 465 19 457 9.7 456 12 44 7 11 44 2 7.5 5.6 437 4.2 427
Data ~~~
XfO
161 55.8 50.8 35.0 19.9 187 54.7 26.3 12.6 333 99.4 10.3 33.6 19.3 10.1 81.9 22.5 16.5 13.8 11.6 4.97 65.6 41.1 34.4 11.4 9.83 6.78 143 57.8 59.4 33.6 29.2 20.0 10.6
0.063 0.053 0.093 0.059 0.0 53 0.092 0.048 0.069 0.060 (0.201) 0.099 0.126 0.088 0.095 0.057 0.053 0.047 0.027 0.034 0.036 0.028 0.091 0.084 0.117 0.078 0.063 0.079 0.176 0.162 0.139 0.167 0.091 0.090 0.073
k2 as intercept and slope, respectively. The value of xf at time t can be calculated by substracting the value of x, (calculated using eq 8) from the measured value of N / N o . To obtain an initial guess of kl and xm, it is assumed that the value of x, is constant during the fast decomposition reaction (x,N xd), and furthermore that xf
