I
ROBERT COLE and
H. L. SHULMAN
Department of Chemical Engineering, Clarkson College of Technology, Potsdam, N. Y.
Adsorbing Sulfur Dioxide on Dry Ion Exchange Resins
. . . for
Reducing Air Pollution
Because of their favorable adsorptive capacity-temperature characteristics, dry anion exchange resins in the chloride form have potential application for recovering sulfur dioxide from waste gas streams
c
ONSIDERABLE effort has been directed toward reducing or eliminating the direct discharge of waste sulfur dioxide to the atmosphere. This was prompted by the recognition that sulfur compounds are major atmospheric contaminants as well as by the reserve shortage of elemental sulfur which became acute toward the latter part of 1950. Ion exchange resins, because of their porous nature, might be used not for their ion exchange potential but as adsorbents in gas-solid systems for the removal of sulfur dioxide from waste gas streams. The work described here was undertaken to determine the adsorptive capacity-temperature characteristics of dry ion exchange resins in contact with air-sulfur dioxide mixtures. The data obtained are compared with those in the literature for silica gel, gas-adsorbent carbon, and molecular sieves.
Experimental Materials. The ion exchange resins employed were Dowex 1, 2. 3, and 21-K anion exchange and Dowex 50 cation exchange resins (Dow Chemical Co.) and IRA-400 and -410 anion exchange and IRC-50 cation exchange resins (Rohm & Haas Go.). Type 5-A Molecular Sieves in '/*-inch pellet form were also used (Linde CO.).
dioxide and a Pressovac pump. Air was dried by bubbling through concentrated sulfuric acid, metered using a calibrated rotameter, and mixed with sulfur dioxide previously metered using a directly calibrated rotameter. The known-concentration, air-sulfur dioxide mixture was passed through coils immersed in a constant temperature bath to a bank of immersed adsorption bulbs and thence discharged to the atmosphere. Procedure. The cation exchange resins and the chloride forms of anion exchange resins were dried in weighing bottles in an oven at 110' C. until constant weight was attained. These resins are reported to be stable up to 150 O C. Upon reaching constant weight, the resins were transferred to adsorption bulbs and sandwiched between two layers of glass wool. Dry resin weight was obtained by difference, and resins were not exposed to moist air until tests weie completed. Samples of Type 5-A Molecular Sieves were supplied with data for sulfur dioxide equilibrium at one temperature, but additional data were required for comparison. As none could be found in the literature. adsorption isotherms were obtained under the same conditions as for the resins. The loaded adsorption bulbs were placed in the constant temperature bath and brought into continuous contact with known-concentration air-sulfur dioxide mixtures. Constant weight at a given concentration was generally attained within 6 to 20 hours. Equilibrium data were obtained at 31°, 4 6 O , 60' C., and in some initial runs at 21" C. Starting at the lowest temperature and concentration, the adsorption bulbs were weighed at intervals until constant weight was reached, indicating that equilibrium had been attained. The sulfur dioxide concentration was increased and data recorded once more, thus determining the second equilibrium point. In this fashion the enriching curve was determined; reversal of the procedure provided the stripping curve.
Results and Discussion
Effect of Temperature on Resin Stability. Comparative adsorption data obtained for IRA-400 and -410 chloride form resins, Dowex 50 and IRC-50 hydrogen form resins, and Dowex 2 chloride and hydroxyl form resins indicated that adsorptive capacity depends upon the type of base group.
CC S O z / G R A M R E S I N
IRA-400 chloride form resin showed good absorptive capacity at both 25" and 100" C. when compared with commercial adsorbents A.
Silica gel
C.
6.
Amivated charcoal 250' C.
D.
. - - - -. 1 ooo c.
Molecular Sieve 5 - A IRA-400 C I -
Because exceeding the commercially recommended operating temperatures degrades the base groups (quaternary
Equipment. The component sources included a tank of compressed sulfur
Literature Background Subject Effect of temperature on resin stability is degradation of quaternary amines Heats of adsorption obtained from adsorptive capacity-temperature data by reference substance diagrams Experimental adsorption isotherms and techniques for adsorption of SO2 on silica gel Experimental adsorption isotherms for adsorption of SO2 on activated charcoal
Ref.
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Table 1. lsosteric Heats of Adsorption Were Obtained from Reference Substance Plots Capacity. Xar Cal./ Temp.. Grams/ Adsorbent Gram-Nole O C. Gram IRA-400 - 9,580 0.25 IRA-410 - 8,640 0.25 Dowex 1 - 8,610 0.25 Dowex 2 - 7,490 0.25 Dowex 21-K IRC 50 Silica gel Silica gel Activated charcoal
9,400 - 16,000 - 1 1 ,000 - 8,120
0.25 0.075 0.075 0.25
- 13,400
0.20
SOP at S.T.P., Cc. O n ad-
sorbent after ,Idsorbent
Molecular Sieves (Type 5 - 4 Activated charcoal Silica gel IRA-400 resin a
Adsorbed
deaorp-
tiori
Desorbed
96
70
26
57 32 98
4
43
2 30
30
Based on 1 grain of adsorbent.
Isotherms for the IR'4-400 chloride form resin, silica gel, activated charcoal, and Molecular Sieve type 5-A (see figure) were constructed either from reference substance plots or characteristic curves, except that for Type 5-14, Molecular Sieve a t 100' C. which was estimated. The temperatures chosen are ty-pical, in that adsorption might occur at room temperature, and desorption could be accomplished a t 100' C., by means of air, saturated steam: sulfur dioxide, or airsulfur dioxide mixtures. I n gas adsorption operations, the driving force for mass transfer is the pressure difference between the partial pressure of the adsorbate in the contacting phase and the equilibrium pressure of the adsorbate corresponding to the concentration adsorbed. I n general, the smaller the driving force, the lower the rate, even when desorbing from an adsorbate-free phase, so there exists a practical limit to which the adsorbent may be srripped. The molecular sieves, for example, mayadsorb 95 cc. of sulfur dioxide per gram at 25' C. from a stream containing sulfur dioxide a t 5 cm. of mercury partial pressure. Desorptionusing dryair at 100°C., for example, should remove all adsorbed sulfur dioxide. T h e equilibrium iso-
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The unabridged manuscript from which this condensed version was taken (see order coupon) contains, in addition to expanded text, the following tables and figures: TABLES Resin Characteristics lsosteric Heats of Adsorption Volume of Sulfur Dioxide Desorbed Tabulated Results FIGURES Schematic Diagram of Equipment Adsorption Isotherms, Dowex 2 1 -K, CI Adsorption Isotherms, Dowex 1, CI ~Adsorption Isotherms, Dowex 2, C1Adsorption Isotherms, IRA-400, CIAdsorption Isotherms, IRA-41 0, CICharacteristic Curves, Dowex 1, 2, 21 -K, CICharacteristic Curves, IRA-400, -4 10, CI Reference-Substance Diagram, Dowex 21 -K, CIAdsorption Isotherms, Dowex 50, H Adsorption Isotherms, IRC 50, H" Characteristic Curve, IRC 50, H f Adsorption Isotherms, Molecular Sieves, Type 5-A Comparison of Adsorption Isotherms at 25" C. Comparison of Adsorption Isotherms at
100"
c.
Hysteresis Effect, Dowex 2, CI-
therms. show, however, that it would be impractical to lower the concentration much below 70 cc. of sulfur dioxide per gram of adsorbent. For comparison, the volume of sulfur dioxide adsorbed and desorbed under identical conditions is tabulated for all four adsorbents in Table 11. The practical limit for desorption has been set as 2 cm. of mercury equilibrium pressure. From the standpoint of adsorption capacity and volume of sulfur dioxide desorbed, IRA-400 performed best. From the standpoint of ability to adsorb large quantities at extremely low concentrations. however. the molecular sieves appear to be far superior. The comparison is based on the assumption that a t 100" C. and low concentrations, the adsorption and desorption curves are identical. Literature Cited
For complete manuscript:
0
Adaorb-
mercury partial pressure, 25O C. Adsorbents stripped with dry air at 100' C . Limiting desorption equilibrium pressure set at 2 cm. of mercury.
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68
ents contacted withSO2-air mixtureat 5 cm.of
amines) rather than the copoiymer masuch higher operating temperatrix (a), tures can affect adsorptive capacity. Anion Exchange Resins, Chloride Form. The Dowex 21-K, a high porosity resin, exhibited the greatest adsorption capacity followed by IRA-400, Dowex 1, Dowex 2, and IRA-410, which are medium porosity resins. S o attempt was made to obtain quantitative adsorption rate data. Qualitatively, the resins ranked in the following order: IRA-400> Dowex 21-K Dowex 1: IRA410, and Dowex 2. The IRA-400, however, had by far the highest adsorption rate. Those resins exhibiting the highest adsorptive capacity also showed the highest adsorption rates. The.experimenta1 data were also used to determine reference substance plots for the resins. Aside from their use in interpolating and extrapolating experimental data, the plots can be used to obtain isosteric heats of adsorption, A, ( 3 ) (see Table I). Comparison of Adsorption Data. As one means of evaluating resin adsorption potential, adsorption equilibrium data were obtained from the literature for two common adsorbents : activated charcoal (7) and silica gel (2). Isotherms were obtained from reference substance diagrams and characteristic curves, and isosteric heats of adsorption were determined (Table I).
ORDER C O U P O N TO:
Table II. IRA-400 Showed the Highest Desorption of Sulfur Dioxidea
AND ENGINEERING CHEMISTRY
(1) Gregg, S. J., J. Chem. SOC. 1927, Pt. 2, p. 1494. (2) McGavack. .J., Patrick, N. A . , J. Am. Chem. SOC. 42, 946 (1920). (3) Othrner, D. F., Sawyer, F. G.. I K D . ENG.C H E M . 35, 1269 (1943). (4) Rohm & Haas Co., Amber-Hi-Lites No. 45 (May 1958).
RECEIVED for review September 16, 1959 ACCEPTED June 21, 1960 Division of Petroleum Chemistry, 138th Meeting, ACX, New York, September 1960.
