Activation Energy Variation for Catalytic Oxidation of Aqueos SO2.W. Pasiuk-Bronikowska; A. SokoÅ‚Owski.1983

7/25/2019 Activation Energy Variation for Catalytic Oxidation of Aqueos SO2.W. Pasiuk-Bronikowska; A. SokoOwski.1983

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ACTIVATION ENERGY VARIATION FOR CATALYTIC

OXIDATION OF AQUEOUS SO t

W. PASIUK-BRONIKOWSKA*

and A. SOKOEOWSKI

Institute of Physical Chem istry, Polish Academy of Sciences, 01-224 Warszawa, Kasprzaka44/52. Poland

(Received 12 May 1982; accepted 16 August 1982)

Abstrart-SO, oxidation in aqueous solutions catalysed with mang anous sulphate was studied to determine

temperature dependencies of the reaction rate. The process was carried out at relatively h igh sulphuric acid

(reaction prod uct) concentration s with regard to its application for SO2 removal from waste gases. Variation o f the

apparent act ivation energy h as been linked with alteration of reaction rate determining steps.

INTRODUCTION

The catalytic influence of some transition metal ions on

oxidation of SO, absorbed in aqueous solutions has been

known since the 19th century. However, there arc at

least two important reasons why research on this process

is still carried on. One is its complexity involving

laborious studies on the process mechanism and another

one is its practical importance for SO1 emission control.

To develop such a technology sufficiently safe data are

needed, particularly the optimum catalyst, its concen-

tration, relative concentrations of reagents, reaction

temperature etc.

In this work we attempted to determine experimentally

the activation energy of oxidation of SO, absorbed into

the MnSO. aqueous solution hoping to gain additional

information as to the reliability of the reaction model

proposed prcviously[l]. Our intention was also to

explain discrepancies in the scarce literature on the

intlucncc of temperature upon the process. Some authors

reported relatively high values of activation energy

(Hoathcr et 01. 2]-27.3 kcallmol? 4 , Huss[3]--19.8 +

0.7 kcallmol), whereas others indicated very low ones

(Tarbutton et a/.[41 did not observe any measurable

effect of temperature. the value evaluated from the data

of Copson et al. [5] is lower than 2 kcal/mol).

Apparatus

EXPERIMENTAL

Experiments on SO2 oxidation were conducted in a

semibatch foam reactor with constant flow of gaseous

reactants, previously passed through a mixer and a

humidifier. We have chosen this type of gas-liquid con-

tact as it is directly transferable to industrial scale. A

vertical Pyrex tube was used as the reactor (Fig. I),

supplied at the bottom with the fine glass-frit as a gas

distributor and, at the top with the PTFE lid with

through-pipes. The pipes were coupled with a reflux

corklenscr discharging after-reaction gases, tube for

liquid sampling, separatory funnel applied for introduc-

ing initial or excessive (taken through the sampling tube)

Author to whom correspondence should be addressed.

tPart of this work was

presented at the European Conference

of the Federation of European Chemical Societies: Chemical

Pathways i n the Environment Palaiseau, France 1980.

solution into the reactor, and a Pyrex jacketed ther-

mocouple for indicating temperature of the reacting

mixture. Both the reactor and the gas conditioning sys-

tem were immersed in a water bath and thermostatted

within f O.lC.

iki aten als

Sulphur dioxide.

The supply of sulphur dioxide was

from a technical cylinder placed outside through an

intermediate steel bottle (2 I.) placed in the vicinity of the

reactor and periodically loaded by distillation of liquid

SO* from the outer cylinder. The gas was also cleaned by

passing it through a silica gel column and then through a

fine porous glass plate.

Air. Molecular oxygen from air was used as an oxidiz-

ing agent. Air was sucked from the outside of the

laboratory with a diaphragm pump and next passed

through a silica gel column, cloth titer and fine porous

plate.

Manganous s hate. The analytical grade reagent was

from PPH POCH, Gliwicc. It was applied without any

further purification.

Water.

All solutions were prepared with redistilled

water.

Procedure

As soon as the bath temperature was fixed at the

desired level the flow of air was turned on and then the

reactor filled with the catalyst aqueous solution of known

volume and Mn concentration. Simultaneously with the

solution the proper stream of SO, was introduced into

the flowing air and hence the oxidation was under way.

From the start of supplying SOI a run was timed and

successive liquid samples were withdrawn. Only 1 ml

liquid portions

were

needed for analysis and the excess

of the withdrawn liquid was returned to the reactor.

The temperature of the reflux condenser was adjusted

according to the programme worked out on the basis of

preliminary experiments so as to keep the possibly con-

stant volume of the reacting liquid, independently of the

increasing H2S04 concentration with the progress of

oxidation.

Samples were analysed for H2S04 to get information

on the extent of reaction and for Mn to allow correction

for changes in the reacting solution volume. Alkalimetric


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122

W. PASIUK BRONIKOWSKA and

A.

SOKOLOWSKI

Fig. 1.

Schematic drawing

of experimental apparatus: 1, pyrex tube 0.07m i.d.; 2, gas distributor/glass-friitl; 3,

PTFE lid: 4, reflux condenser: 5, separatory funnel: 6, sampling tube; 7, thermocouple; 8, humidifier; 9, mixer of

gaseous reagents; 10, rotameter; 11, Ueometer; 12, fine porous plate; 13, cloth filter; 14, silica gel column; 15,

diaphragm pump.

titration with metal masking nd calorimetric deter-

mination in the presence of formaldoxime were

employed respectively.

Below fundamental parameters of experiments are

specified:

MuSO concentration 1.2

x

lo--0.18 mol/dm3

SO? concentration 0.5-3 vol.

flow of gaseous mixture 0.1 l-O.44 dm/s

liquid volume 0.134.25 dm

temperature 14.2-43.3C.

RILWLTSANDDIS USSION

The rate of sulphuric

acid production which is equal to

the rate of SOz oxidation can be obtained from the

equation

r = (dnJdt)/

V =

dc,/dt - (dcJdt)(cJcM)

(1)

where n. = n.(t) and

V= V t ) were

not measured while

c, = c,(t) and CM = c&t) were experimentally deter-

mined concentrations of sulphuric acid and manganese,

respectively.

The data points were expressed as higher order poly-

nomial functions of time using the least squares method.

Exemplary

fitted curves

are shown in Pi. 2.

In view of previous reports[l-3.51 as well as of

observations made in this work sulphuric acid causes

significant retarding effect on the rate of its production.

Therefore to examine the reaction sensitivity to tem-

perature one should compare reaction rates at diRerent

temperatures but alike acid concentrations or reaction

extents when starting with an aqueous solution contain-

ing no acid. Temperature dependencies for SO2 oxidation

at MnSOa concentration 5 x lo- mol/dm3 and various

5 10 15 2 25 3

tno T S

Fii.

2. Sulpburicacid and manganese

concentration va time;

A,

14.2C; 0, 25.1C and Cl, 34.8C.

reaction extents are plotted in Fig. 3. It shows results of

experiments given

as

In r vs l/T instead of

the

typical

Arrhenius plot, as the process is complex and the form

of its rate equation may he still in question. Under the

condition that experiments were performed at constant

concentrations of both substrates the value of the ap-

parent activation energy found from Fig. 3 did not differ

from that determined from the classic Arrhenius equa-

tion.

Data points reported in Fig. 3 are arranged in two

regions.

one of variable activation energy which


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Activation energy variation for catalytic oxidation of aqueous SOI

123

n

-t

6~ lo- mol/dm whereas experiments in Fii. 3

were at C~ = 5 x lo-mol/dm3. Hence it can be con-

cluded that the unusual effect of temperature observed in

Fig. 3 reveals mainly the complex reaction mechanism.

As the plot is curved downward this is not the case of

parallel pathways but of reaction successive steps.

According to Pasiuk-Bronikowska et a/.[11 who pro-

posed the reaction model for oxidation of SOI catalysed

with MnSOd the theoretical rate equation for the set of

parameters applied in this work may be expressed as:

where k, = kc0 and K, = K (co/cM2).

As results from the equations contribution of the

oxygen solubility temperature effect to the empirical

value of the apparent activation energy should be con-

sidered. This effect is expected to be excluded when

K,c, B K. In this case

W, = (kJK)cM.

(6)

Plotting experimental data as rc, against c, the ap-

proached values of (t/K9cM2 = const could be found

for several temperatures in the lower range and hence

the single apparent activation energy as given in Fig. 6 (a

and b-l).

Values of the coefficient ( ,/K)c, were also cal-

culated from eqn (4) takine k K as the initial rate of

oxidation at c. = 0 (see b-2 in Fig. 7). Thus obtained

values for the activation energy are 20.2 -(b-l) and

23.4 kcal/mol (b-2) indicating discrepancy caused by the

way of data treatment. The results bear a slight compen-

sating effect with respect to the second term of

denominator in eqn (4) as co decreases with the rise of c..

Huss[31 who supplied the best evidence for his

experiments gave the value of activation energy found

at c0=7.S x lo-*mol/dm3, c, = 0 and c, = 3.31 x

lo- mol/dm3 for temperatures between 25 and 38C. In

view of previous considerations on reaction mechanism[l]

the conditions were fulfilled for the kinetics described by:

r = kocM2.

(7)

Therefore values of activation energy found in this work

and reported by Huss should not necessarily be equal.

CONCLUSIONS

Variation of activation energy for the reaction of SO?

oxidation catalysed with Mn has been proved experi-

mentally. To cbstinguish between the tirst order reaction

and diiusion controlled kinetics with respect to oxygen

the influence of other reagents (MnS04 and H2S04) in

the complex reaction was successfully examined.

Attempts were made to link such a behaviour with the

reaction mechanism, which allowed to settle that SO*

oxidation kinetics may or may not significantly depend on

temperature according to the reaction order with respect

to oxygen. With the order increasing (from 0 up to 1) the

value of apparent activation energy diminishes from

19.8 kO.7 kcallmol in the absence of HJ O, or 24


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Activation energy variation for catalytic oxidation of aqueous SO*

a

Fig. 6. Determin ation of apparent activation energy basing on eqn (6) (a and b-l) or eqn (4) (b-2).

24kcal/mol at relatively high concentrations of H2S 04

to about 1 kcal/mol. The latter is attained in foam reac-

tors at high Mn concentrations encountered in practice to

overcome the energy-related problem of environment

acid contamination.

NOTATION

specific interfacial area with respect to

gas-liquid volume, m-

kinetic

const nts

in experimental rate

equations, mol s/dm and s respec-

tively (eqns 3 and 4)

kinetic constant in experimental rate

equations (3 and 4), s

manganous sulphate

concentration,

molldm3

oxygen concentration (in liquid),

mol/dm3

sulphuric acid concentration, mol/dm

apparent activation energy of the reac-

tion (process), kcallmol

intercept for I/r = f(c.) (oxygen inde-

pendent region), dm3 s/m01

liquid side mass transfer coefficient, m/s

complex rate constant

complex equilibrium constants

moles of sulphuric acid, mol

rate of the reaction, molldm s

slope for i/r = f(c,), s/mol* dm6

time, s

temperature of the reaction (process),

C

absolute temperature, K

volume of reacting liquid, dm3

gas flow-rate, mls

Cl1

Dl

E:;

P

W

REFERENCF.S

Pasiuk-Brouikowsks W. and Bronikowski T.. C /rem. Ensare

- _

Sci. 1981 36 215.

Ho her R . C. and Goodeve C. F., Trans. Far aday Sot. 1934

30 1149.

Huss A. Jr., Ph.D. Thesis. Urbana. Illinois 1978.

Tarbutton G., Driskell I. C., Jones T. hf., Gray F. J. and

Smith C. M., bd. Engng

Chem 1957 9 392

Copson R. L. and Payne J. W., Ind Engng Chem 1933 25

9m

__.

G rich W., Esenwein H. and Kmuss W..

ht. Chem. Engng

1978 18

38

Documents

Activation Energy Variation for Catalytic Oxidation of Aqueos SO2.W. Pasiuk-Bronikowska; A. SokoÅ‚Owski.1983