Activation of sulfur dioxide oxidation monolithic catalyst by flue gas treatment

Applied Catalysis B: Environmental 41 (2003) 53–60

Activation of sulfur dioxide oxidation monolithic catalystby flue gas treatment

Esperanza Alvarez, Jesús Blanco∗Instituto de Catálisis y Petroleoquı́mica, CSIC, Cantoblanco 28049, Madrid, Spain

Received 10 December 2001; received in revised form 5 April 2002; accepted 6 April 2002

Abstract

Low sulfur oxides concentration have been used to activate monolithic catalysts at different activation temperatures andvarious SO2/SO3 inlet ratios in order to evaluate the feasibility of their activation with flue gas.

In these conditions, the activation of the catalyst does not depend on the SO2/SO3 feed ratio; however high temperaturesand activation times over 50 h are required.

The activation of the catalyst carried out at low SOx concentrations slightly improve the final activity as compared withthose obtained by the conventional activation step where higher SOx and O2 concentrations are used.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Sulfur dioxide oxidation; Flue gas treatment; Catalyst activation process

1. Introduction

The SO2 control in flue gas from industrial pro-cesses is usually carried out by wet and dry scrubbingtechnologies where the disposal of the waste productsis giving rise to major problems due to the lack ofsuitable deposits. Latterly, the catalytic oxidation toSO3 has emerged as an alternative to those processes.In these catalytic systems, a useful SO3 by-productis obtained, from which sulfuric acid as a reagent orintermediate for manufacture of other chemicals maybe produced. The catalysts used for sulfuric acid man-ufacture are described, at the operating conditions,as a molten salt comprising M2S2O7/V2O5/SO2/SO3species, where M= Na, K, Cs or a mixture of these[1].

∗ Corresponding author. Tel.:+34-91-585-4802;fax: +34-91-585-4789.E-mail address:[email protected] (J. Blanco).

Commercial sulfuric acid catalysts present a widevariety of pellet shapes and sizes, depending on theapplication and industrial processes[2]. However,pressure drop through the catalytic bed is an econom-ical factor to be considered, especially when highvolumes of dusty flow have to be treated. Moreover,the catalytic molten salts can migrate into the dustgiving rise to a loss of active-phase from the catalyst.The selection of a honeycomb-type catalytic structure,which operate at laminar flow, would minimize thecontact between the dust and catalyst external surfaceand allow an improvement of the overall process.Several works have been carried out using monolithiccatalysts[3,4] for the SO2 oxidation from dusty fluegas streams, prepared by impregnation and thereforesubject to deactivation by abrasion. In this study anincorporated honeycomb catalyst has been preparedand used to minimize pressure drop and deactivationby abrasion.

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54 E. Alvarez, J. Blanco / Applied Catalysis B: Environmental 41 (2003) 53–60

The basic formulation of these catalysts comprisesvanadium, alkali and sulfate-type compounds as theactive-phase and diatomaceous earth as the support.Although several suppliers have tried some other al-ternatives such as the incorporation of sulfated com-pounds as raw materials[5,6], in our manufacturingprocess, the final step corresponds to the activationof the green catalyst by calcination in SO2/SO3 andO2 atmosphere. This step allows sulfating the V-alkalispecies that give rise to the catalytic pyrosulfate-typecompounds.

The influence of the activation step on the chemicaland physical characteristics of the final catalyst, espe-cially when is going to be used for SO2 oxidation influe gas conditions might be of special relevance. Theeffective thickness of the molten active-phase, whichis distributed on the porous structure of the support,depends on the chemical and physical properties of thegreen catalyst, but also might depend on the mecha-nism of the active-phase formation.

In the manufacture of this type of catalyst the ac-tivation is carried out at high concentrations of oxy-gen and sulfur oxides (4–10 vol.%) and temperaturesabove 420◦C [7,8]. The activation of the catalystsat low sulfur oxides concentrations may affect theprocesses involved in the active-phase formation andtherefore the performance of the final catalysts.

This work has the objective to study the feasi-bility of the activation of these catalysts at flue gasconditions in order to facilitate their industrial im-plementation. The activation of the catalysts at lowsulfur oxides concentrations may affect the processesinvolved in the active-phase formation and thereforethe performance of the final catalysts, especially forflue gas applications. In this study, low sulfur oxidesconcentration has been used to activate the monolithiccatalyst at different activation temperatures and alsoat various SO2/SO3 inlet ratios.

2. Experimental

2.1. Catalyst shape and composition

The preparation of the catalyst used in this studyhas been previously reported[9]. It has a square-cellstructure and, after activation, a cell density of 7.7 cellscm−2, pitch of 0.36 cm, wall thickness of 0.09 cm,

geometric surface area of 833 m2 m−3, bulk den-sity of 0.7 g cm−3 and a crushing strength of around47 MPa. The V/K atomic ratio was 3.5, with 7.4 wt.%V2O5 content. The catalyst before treatment had thefollowing weight composition: [diatomaceous earth]= 0.937; [K2SO4] = 0.394 and [NH4VO3] = 0.139.

2.2. Activation tests

The activation of the green catalyst was carriedout using a five-channel monolith of 18 cm length,operating with an inlet feed composition of: [SO2] =630 ppm; [O2] = 5 vol.% and [N2] balance at a totalpressure of 120 kPa, gas hourly space velocity (GHSV,at normal conditions) of 4850 h−1 and linear velocity(LV) of 0.45 N m s−1. The temperature was increasedat a rate of 13◦C min−1 from room temperature tothe designed activation temperatures of 360, 430 and470◦C. The SO2 concentration was measured by UVfluorescence analyzer and SO3 collection followingthe EPA Method 8 and titration technique for analysis.

In a second set of experiments, the activation tem-perature was fixed at 470◦C, and mixture of SO2 andSO3 were used instead of pure SO2 in the compositionof the inlet gas: [SO2] = 365 ppm+[SO3] = 275 ppm.

In all experiments, it has been considered that theactivation process was completed when the massbalance between the inlet and outlet sulfur oxides(SO2 + SO3) concentrations was closed.

2.3. Characterization techniques

Laser Raman spectroscopy was used to analyzethe surface composition of the used monolith wall.The Raman spectra were obtained using a RenishawMicro-Raman System 1000 equipped with a CCDdetector at−73◦C. The spectra acquisition consistedof five accumulations of 60 s for each sample and thetemperature maintained at 430◦C.

3. Results and discussion

In a previous work carried out with the samecatalyst at micro-reactor conditions[9], several stepsof the activation pattern were observed when risingfrom room temperature to 470◦C: the SO2 physicaladsorption at low temperature (20–100◦C), the exit

E. Alvarez, J. Blanco / Applied Catalysis B: Environmental 41 (2003) 53–60 55

Fig. 1. Outlet SO2 during the first 3 h of activation at different activation temperatures. Feed composition: [SO2] = 630 ppm; [O2] = 5 vol.%;[N2] = balance. Total pressure= 120 kPa; GHSV= 4850 h−1; LV = 0.45 N m s−1.

of NH3 from ammonium vanadate decomposition(170–220◦C), and the formation of potassium pyrosul-fate (250–360◦C). Above 330◦C, the K2S2O7-V2O5eutectic would start to be formed[10,11]and fusionwould take place, initiating the formation of the liquidactive-phase.

A first visual inspection informed about the randomcolor distribution on the surface of the green catalyst.The background corresponded to a greenish crystallinesolid. Orange and brown/blue colors were spatteredon this background, indicating that the mixture of theprecursors was not homogeneously distributed. More-over, the heterogeneous distribution might be indica-tive of a variety of V/K ratios on the surface, whichwould give rise to different active species after the ac-tivation step.

3.1. Activation at different temperatures

The mechanism of the liquid active-phase forma-tion comprises several steps, which can be summa-rized as the sorption of the SO2 and O2 by the VKsalts giving rise to VK-pyrosulfate-type compounds.When the operating temperature is high enough,the already formed species are melted. This viscousliquid is in motion on the support, leaving more cat-alyst precursor species subject to further transforma-tion into the liquid active-phase. The rate of eachphysical and chemical phenomenon would mainly

depend on the operation temperature discussed in thefollowing paragraphs and shown inFig. 1.

3.1.1. Activation at low temperature (360◦C)During the first 70 min of operation the amount of

SO2 collected by the green catalyst decreases as thepotassium pyrosulfate is formed (Fig. 1). Afterwards,the eutectics start to be formed giving rise to an in-crease in the SO2 uptake. However, as the activationtemperature is close to the active-phase melting point,the melting process takes time and the slow-motion ofthe liquid active-phase, due to its high viscosity, wouldallow small portions of underlying green catalyst to beexposed. Consequently, the time required for the ac-tivation process become too long and the steady stateis not reached after 50 h in operation (Fig. 2).

3.1.2. Activation at medium temperature (430◦C)When the activation temperature was increased

from 360 to 430◦C, the pyrosulfate formation andthe simultaneous melting and movement of theactive-phase should take place at higher rates. There-fore, the active-phase dispersed on the support is morerapidly formed and melted and the activation step wascompleted after 30–35 h in operation (Fig. 2).

3.1.3. Activation at high temperature (470◦C)When the activation temperature was increased to

470◦C, the activation process was slower later than


Fig. 2. SO2 removal during activation. Test carried out at three activation temperatures. Feed composition: [SO2] = 630 ppm; [O2] = 5 vol.%;[N2] = balance. Total pressure= 120 kPa; GHSV= 4850 h−1; LV = 0.45 N m s−1.

at 430◦C, being 45–50 h the time needed to reachthe SOx mass balance. This behavior could be ex-plained by the effect of the temperature on SO2 ab-sorption in the gas–liquid system, because at highertemperature there is lower gaseous reactant solubility[12,13].

The activity of the monolithic catalysts, which havebeen activated at 430 and 470◦C, was tested in the

Fig. 3. Evolution of SO2 conversion with reaction temperature for the monolithic catalyst activated at (a) 430◦C and (b) 470◦C.[SO2] = 630 ppm; [O2] = 5 vol.%; [N2] = balance. Total pressure 120 kPa; GHSV= 4850 h−1; LV = 0.45 N m s−1.

range of 330 and 470◦C, using the same fluid dy-namic parameters and inlet feed concentration men-tioned above. The experimental results are shown inFig. 3, where the SO2 conversion at the steady-stateconditions is plotted versus reaction temperature. Theresults showed that higher activation temperature gaverise to better catalytic performance in the studied re-action temperature range.


3.2. Activation in the presence of SO3

At the activation temperature of 470◦C, the inletsulfur oxides concentration was set at 355 ppm of SO2and 275 ppm of SO3, maintaining 5 vol.% of O2 andN2 balance. The activation pattern and activation timeneeded was similar to those obtained with pure SO2at the inlet feed. The activity tests of this catalyst acti-vated in the presence of SO3 gave also similar results.

However, the activity of the catalyst activated with10 vol.% of both (SO2 + SO3) and O2, was slightlylower than that activated with the above flue gas typeconditions. This high oxidizing atmosphere may sta-bilize the V2O5 and part of this compound can reactwith SO2 to form vanadyl sulfate with loss of catalyticactivity [9].

3.3. Performance of the activated catalyst

3.3.1. Effect of high reaction temperatures oncatalytic activity

The loss of catalytic activity at temperatures above410◦C shown by the catalyst activated at 470◦C(Fig. 3) has been observed before at temperaturesaround 500◦C and even higher[14,15]. It has beenpointed out that the thickness of the liquid active-phaseand the porous structure of the support which de-termine the accessibility of the gaseous reactants tothe molten catalyst, are factors to be accounted forin the overall reaction rate[16–21]. Besides[2], thetransport of species through gas and especially liquidlayers should be considered to control the rate limit-ing step. However, no data are available in spite ofefforts for determining the gas solubility and gas–liquid diffusivity of SO2, SO3 and O2 [22].

The variation of the above-mentioned parameters attemperatures above 410◦C give rise to the decrease ofthe observed reaction rate. The assumption of the de-crease of the liquid-surface-to-liquid-volume ratio pa-rameter with temperature might explain this catalyticbehavior. If liquid viscosity decreased with tempera-ture, the mobility of the liquid would cover the narrowpores, lowering the gas–liquid interface. The interfacecould also decrease when liquid blocks interparticulatespace. Therefore, the liquid-surface-to-liquid-volumeratio would decrease at higher temperature and it mightbe a factor accounting for the loss of activity at highreaction temperatures.

3.4. Heterogeneous active-phase distributionthrough the monolithic catalyst

After activation at 470◦C and cooling the reactorin the presence of gaseous reactant mixture, differenttexture and color were observed inside the monolith.The entrance of the monolith presented a dark fluffyloosely held solid on the surface, whereas a compactgreenish surface was observed at the bottom of themonolith.

In order to study the influence on the catalytic ac-tivity of the different texture and color observed at thetop and bottom of the monolith a series of experimentswere carried out using different lengths of the catalystcut from the top and bottom of the monolithic struc-ture. In these experiments five-channel monolithic cat-alysts with square open cells of 2.8 and 1.2 mm wallthickness were used. The temperature was held at430◦C and the total pressure at 120 kPa. The linearvelocity was in the range at 0.45–0.85 N m s−1 and theinlet feed composition was: [SO2] = 700 ppm, [O2] =5 vol.% and [N2] = balance. A blank experiment wascarried out using the empty reactor, being 0.8% theobtained conversion.

The results of experiments, carried out using cata-lyst cut from the top, medium and bottom of the previ-ously activated monolith, are shown inFig. 4. It is clearthat no differences in activity are observed. Moreover,the data were well adjusted to a pseudo-first-orderequation with respect to SO2 as it is shown inFig. 5.A similar kinetic approach has been carried out whenusing very low SO2 concentrations[22].

The inner surface from both end-points of themonolith was analyzed by Raman spectroscopy. Theseanalyses only confirmed the difference in chemicalcomposition after activation and no identification ofthe active species could be done because samples werenot representative of the real catalyst performance.The Raman spectra for the inlet (top) and outlet (bot-tom) of the monolith are plotted inFig. 6and the moresignificant peaks identified. The Raman spectra for themonolith outlet show S2O7 characteristic bands 1080–730 cm−1 [9] partially masked by the wide bandscentered at 780 and 990 cm−1, which were assignedto the interaction between vanadium and support. Thebands assigned to S2O7 were not clearly present inthe Raman spectra for the monolith inlet. Both sam-ples show a band near 1015 cm−1 generally assigned


Fig. 4. Variation of SO2 conversion with the space velocity at 430◦C. Experimental data carried out with catalyst cut at medium (�),top (�) and bottom (�), previously activated at 470◦C. Feed composition: [SO2] = 700 ppm; [O2] = 5 vol.%; [N2] = balance. Totalpressure= 120 kPa; LV= 0.45–0.85 N m s−1.

to surface vanadyl groups[9]. The bands at 1297and 1124 cm−1 from the monolith outlet spectra wereassigned to sulfate-type species. These compoundswere not observed in the inlet sample, i.e. where theSO3 concentration is lower. In neither samples strongbands for K2SO4, KHSO4 and V2O5 were detected.

Fig. 5. Correspondence of the experimental data to a pseudo-first-order reaction.

From the activity results obtained it seems thatthe catalytic behavior of the activated catalyst in thepresence of the gases, at the working conditions,is quite homogeneous from the top to the bottomand does not depend on the differences found in thetexture, color or Raman spectra.


Fig. 6. Raman spectra of the monolithic catalyst after cooling from the inlet and outlet end-points.

4. Conclusions

• The influence of the activation process, carriedout at low SOx concentrations, on the perfor-mance of V-K-based monolithic catalysts forthe oxidation of SO2 from exhaust gas has beenstudied.

• High activation temperatures (∼470◦C) improvethe performance of the final catalyst.

• The activation of the catalyst does not depend onthe SO2/SO3 feed ratio.

• Data obtained at 430◦C with a monolithic catalystpreviously activated at 470◦C, are well adjusted toa pseudo-first-order equation.

• No matter the differences found in the color, tex-ture and Raman spectra on the top and the bottomof the activated monolithic catalyst, their catalyticbehavior is quite similar.

• It is feasible to activate these catalysts with flue gas;the activation of the catalyst carried out at low SOx

concentrations slightly improve its final activitycompared with those obtained by the conventionalactivation step where higher SOx and O2 concen-trations are used. The best results were obtainedat an activation temperature of 470◦C for at least40 h.

Acknowledgements

The authors are thankful to the ECSC for their finan-cial support (7220-ED/093) and to Dr. M.A. Bañaresfor his contribution to the Raman analysis.

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Activation of sulfur dioxide oxidation monolithic catalyst by flue gas treatment