H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions Studies in Surface Science and Catalysis, Vol. 98 �9 1995 Elsevier Science B.V. All rights reserved.

213

EXPERIMENTAL. AND THEORETICAL. STUDIES OF WATER AND

SULFUR DIOXIDE SELECTIVE ADSORPTION IN 3A ZEOL.ITES

I

Kathryn M. Shaw, Mladen Eic and Rajendra Desai

Department of Chemical Engineering, University of New Brunswick, P.O. Box 4400,

Fredericton, N.B., Canada E3B 5A3

INTRODUCTION. Fragile environmental conditions require accurate monitoring of

sulfur dioxide content in flue gas from fossil fuel combustion processes. The current

methods of on-line analyzing require that the sample of flue gas have the water vapor

content removed. The present study investigated an alternative way making use of zeolite 3A to selectively remove water without adversely affecting SO 2 composition in

a carrier gas. Three different samples of zeolite A were used to measure breakthrough curves of SO 2 and water vapor in N 2 as a carrier gas. Limited information using

synthetic flue gas (15 vol% CO2) was also obtained.

EXPERIMENTAL.. An ion exchange solution consisting of KCI, KOH and EDTA with

pH of 9 was used to convert binderless Grace Davison 5A sieve to binderless 3A. A

procedure involved continuous flow of the solution through a column packed with original sample at 70-80~ About 80% of ion exchange was achieved. Another

modification involved addition of boric acid in Linde 3A zeolites according to the

procedure given by Vansant 1.

Breakthrough measurements were carried out with the treated samples described

above and samples of Linde 3A. All samples were crushed to 8-16 mesh and packed

in 10 cm column (ID = 2.54 cm). The carrier gas was saturated with water (5 vol%).

Sulfur dioxide was fed by using mass flow controller at a flow rate corresponding to

about 2,000 ppmv in the carrier gas stream. The breakthrough experiments were run with a column temperature of approximately 60~

RESULTS AND .DISCUSSION. Breakthrough measurements using binderless 3A and modified 3A samples showed virtually no SO 2 adsorption. Water vapor

adsorption varied greatly depending on the regeneration conditions used. Effects of

regeneration on the adsorptive properties of water in these samples are still under

investigation.

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Breakthrough curves for sulfur dioxide on Linde 3A zeolite (about 20% binder) in the presence and in absence of water vapor showed almost instantaneous break of SO 2

followed by spreading of the curves. The results were interpreted using a model

based on the second moment analysis for a biporous adsorbent. 2 Assuming that

spreading of the sulfur dioxide mass transfer zone is only attributed to the axial

dispersion in the packed bed, one can extract the experimental value of the axial

dispersion using the model (all other mass transfer resistances are neglected). This

can be compared with the predicted value of axial dispersion based on the semi-

empirical correlation developed by Edwards and Richardson. 3 The close agreement

of both experimental and predicted values proved that the spreading of mass transfer zone for SO2 on 3A zeolite was entirely due to the axial dispersion. 4

Breakthrough curves for water vapor adsorption can be analyzed using linear driving

force model for a non-isothermal and axially dispersed plug flow 5. In addition

Langmuir model for adsorption equilibrium was employed. A comparison of the

breakthrough curves obtained experimentally to those obtained with the model

showed very good agreement. A set of breakthrough curves were also developed

using synthetic flue gas as a carrier gas. Results of these experiments showed no

significant differences with those obtained using nitrogen as a carrier gas. 4

Linde 3A proved to be selective for SO2/H20 adsorption. Further studies are required

to address the problem of SO2 delayed response that is associated with the spreading

of its mass transfer zone.

REFERENCES

o

.

.

4. 5.

E.F. Vansant, Pore Size Engineering in Zeolites, Chapter 2, J. Wiley and Sons, New York, 1990. J. K~rger and D. M. Ruthven, Difffusion in Zeolites and Other Microporous Solids, p. 299, J. Wiley and Sons, New York, 1992. M. F. Edwards and J. F. Richardson, Chem. Eng. Sci. 23, 109 (1968). K. M. Shaw, M. Sc. Thesis, University of New Brunswick, Canada, 1993. D. M. Ruthven, Principles of Adsorption and Adsorption Processes, pages 270- 270, J. Wiley and Sons, New York, 1984.