Haldor Topsøe A/S - Nymøllevej 55 

2800 Kgs. Lyngby - Denmark 

 

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F:\CAT\SEK\WINWORD\HES\2011\VK\VK-701LEAP5 PAPER.DOC 

Information contained herein is confidential; it may not be used for any

purpose other than for which it has been issued, and may not be used by 

or disclosed to third parties without written approval of Haldor Topsøe A/S.

Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 

By Kurt Christensen 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 2 / 13 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 3 / 13 

Meeting future SO2 emission challenges with Topsøe’s new 

VK-701 LEAP5TM sulphuric acid catalyst 

by 

Kurt Christensen 

Haldor Topsøe A/S 

Nymøllevej 55, DK-2800 Kgs.Lyngby, Denmark 

Introduction 

Caesium-promoted vanadium catalysts, which have been available since the late 

1980ies, have proved very efficient in reducing SO2 emissions from sulphuric acid 

plants by providing high activity at low temperatures. 

In 1996, Topsøe introduced VK69 designed for operation in the final passes of doubleabsorption plants [1, 2]. This catalyst offers a very significant activity advantage compared to potassium-promoted catalysts and earlier caesium-promoted catalysts. By 

installing the VK69 in the final pass of existing 3:1 double-absorption plants, the SO2

emission can be cut in half, or the acid production rate can be boosted by 15-20% 

without increasing SO2 emissions. SO2 emissions below 100 ppm became possible in 

3:1 double-absorption plants with the VK69. 

However, still more stringent regulations have placed new demands on the sulphuric 

acid industry to lower SO2 emissions and achieve conversions typically in the range of 

99.8-99.95% (0.3-1.3 kg SO2/MT H2SO4, 0.7-2.6 lbs/ST) for double-absorption plants 

and more than 98-99% (6.5-13 kg SO2/MT H2SO4, 13-26 lbs/ST) for single-absorption 

plants. For many existing plants, the lower emission limits are a challenge at their current or future production rate. 

Topsøe has responded to the new market demands by developing a novel catalyst, 

VK-701 LEAP5TM, designed for operation at low temperatures in converted strong 

gases. By offering exceptionally high activity, this novel catalyst pushes the limits for 

industrially achievable SO2 conversion in both single- and double-absorption plants. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 4 / 13 

Background 

Commercial sulphuric acid catalysts are based on vanadium oxides promoted with alkali-metal sulphates on an inactive porous silica support. The catalysts are so-called 

supported liquid phase (SLP) catalysts, where the oxidation of SO2 takes place as a 

homogeneous reaction in a liquid film covering the internal surface of the support material as illustrated in Figure 1. 

The alkali promoters are mainly potassium and sodium but caesium is also used in 

some catalysts, because it provides excellent low-temperature activity. In spite of numerous investigations and publications on the coordination chemistry of the catalytic 

melt and the reaction mechanism, the details of the mechanism are still not entirely 

known. However, there is substantial evidence that only oxidation state V5+ is active in 

the catalytic cycle in the form of a dimeric vanadium oxosulphato complex [3]. 

At steady-state, the distribution of vanadium species in the catalytic melt are in equilibrium with the surrounding gas causing certain amounts of V5+, V4+ and V3+. However, 

V4+ and V3+ compounds do not contribute to the catalytic activity. The degree of reduction increases at low temperatures and high SO2 partial pressure, and in addition some 

V4+ compound precipitates at temperatures below 460°C (860°F) and gradually depletes the melt of V5+ if the temperature is further lowered. This reduction and partial 

solidification eventually cause the activity to drop to practically zero at some minimum 

temperature of about 360°C (680°F) for a standard potassium-promoted vanadium 

catalyst. 

Figure 1. The active phase in a VK catalyst is a liquid film covering the internal surface of the silica support. To the right, a scanning electron micrograph shows the interior of a sulphuric acid catalyst at room 

temperature, where diatom skeletal fractions more or less covered with solidified catalytic melt are discernible. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 5 / 13 

The reaction rate for a catalyst may be limited by external mass and heat transfer to 

the pellet surface, by internal mass transfer in the porous support or by the intrinsic 

reaction rate itself. For sulphuric acid catalysts operating at a low temperature in high 

gas strength, the main limitation is the intrinsic reaction rate. The intrinsic reaction rate depends on the chemistry in the catalytic melt, but to a great extent, also on the distribution of 

the catalytic melt on the support and gas solubility and transport through the melt. At 

steady-state, the physical distribution of the melt is in equilibrium with the surrounding gas, 

and the distribution depends on the nano-scale surface properties of the silica support, e.g. 

the wettablility and surface tension. For existing commercial sulphuric acid catalysts, 

activity is seriously hampered by deep pools of catalytic melt restricting gas transport in 

the active phase. 

Topsøe has now developed a completely new catalyst called VK-701 based on our 

novel LEAP5TM technology which overcomes these limitations and provides outstanding activity. The improvements, which include a very high V5+ content, have been 

accomplished by: 

 Changing the intrinsic morphology and surface properties of the carrier. 

 An optimised active phase for high SO3 concentration. 

The first ideas for the new catalyst were tested in lab-scale, and, based on confirmed 

high activity of the new formulation, development was continued with pilot-scale production and tests. Extensive testing again showed excellent activity and stability of the 

product with mechanical strength and pressure drop on target. We installed and optimised a new unique production line necessary for the manufacture of our new 

LEAP5TM technology catalyst. Full-scale production was carried out and all performance parameters of the VK-701 product were confirmed. 

Properties and performance of VK-701 LEAP5TM 

The new VK-701 LEAP5TM is designed for operation at a low temperature in converted 

strong gases. A photo of the catalyst is shown in Figure 2 and the catalyst characteristics are summarised below: 

Catalyst name: VK-701 LEAP5TM 

Size and shape: 12 mm Daisy 

Application : Lower passes of single-absorption plants 

Third pass of 3:1 and 3:2 double-absorption plants 

Operating temperature: 400-500°C (750-930°F) 

Ignition temperature: 310°C (590°F) 

Thermostability: 650°C (1200°F) 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 6 / 13 

The choice of pellet size is a trade-off between activity and pressure drop. Pore diffusion limitations are modest in converted strong gases, and consequently VK-701 is 

produced in a standard size of 12 mm Daisy which provides lower pressure drop than 

the 9 mm Daisy. The pressure drop per bed height is equal to that of the well-known 

12 mm Daisy products (VK38, VK48 and VK59). 

Because the catalyst composition of VK-701 is based on well-known sulphuric acid 

catalyst materials, no additional efforts are required for disposal of the spent catalyst. 

Figure 2. The new VK-701 LEAP5TM catalyst in 12 mm Daisy shape. 

The activity of VK-701 operating at high conversion in a feed gas with 10% SO2 and 

10% O2 is compared to the traditional Topsøe products, VK48 and VK59, as a function 

of temperature in Figure 3. Compared to standard potassium-promoted VK48, the 

VK59 provides significantly higher activity at temperatures below 430°C (805°F) due to 

its content of caesium. The new VK-701, however, exceeds the VK59 activity by about 

a factor two in the entire temperature range. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 7 / 13 

Figure 3. Activity of the new VK-701 catalyst in comparison with VK48 and VK59. 

High levels of V5+

A major leap forward for the LEAP5TM technology is the high amount of vanadium in 

oxidation state V5+ in the catalytic melt. The VK-701 is produced with fairly high vanadium content but not so much higher than for other commercial catalysts. Once in operation in the plant, however, the chemical composition of the active melt will adjust to 

the conditions of the surrounding gas. In order to study this equilibration in greater 

depth, we have measured the contents of different vanadium oxidation states in our 

laboratory. Catalyst samples were crushed and screened to a size fraction of 1-2 mm 

particles. The particles were loaded in an isothermal glass reactor and operated in 93% 

preconverted feed gas containing 10% SO2 and 10% O2. After one day of operation, 

the samples were quickly quenched to room temperature in nitrogen and subsequently 

analysed for their contents of V5+, V4+ and V3+ by a three-step redox titration method. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 8 / 13 

Figure 4. Content of different vanadium oxidation states in VK-701 (red curves), VK48 

(green curves) and VK59 (blue curves). 

Analysis results show that all three oxidation states are present under these conditions, 

although the V3+ content is very low. VK48 standard catalyst contains some active V5+

at 380-440°C, but most of the vanadium is actually in the inactive V4+ and V3+ forms as 

shown in Figure 4. The caesium-promoted VK59 catalyst contains less reduced vanadium but about the same amount of V5+ as the VK48. At 380-400°C the V5+ content is 

even lower for all three catalysts because V5+ is reduced to V3+ and V4+ by SO2. 

The situation is markedly different for the VK-701 LEAP5TM. This catalyst contains 

much more active vanadium V5+ corresponding to about 70% of the total vanadium 

content at 400-440°C. The VK-701 contains two to three times more active vanadium 

than the commercial catalysts available on the market today. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 9 / 13 

Case studies 

Two examples of VK-701 LEAP5TM applications are given in the following: 

Example 1: Reduced emissions from a single-absorption plant 

The SO2 emissions from an existing single-absorption plant can be reduced significantly by loading VK-701 LEAP5TM as a replacement for conventional catalysts in the 

final pass. 

Layout: 4-pass single-absorption plant with indirect cooling 

SO2 source: Metallurgical off-gas 

Feed gas: 8.5% SO2, 14.7% O2

Catalysts in beds 1/2/3: VK38 / VK38 / VK48 

Conversion outlet bed 3: 97.5% 

Table 1 shows the performance of the new VK-701 catalyst in the fourth pass compared to standard potassium-promoted VK48 and caesium-promoted VK59. The total 

conversion of 98.57% achieved with VK48 can be increased to 98.74% corresponding 

to 12% lower SO2 emission by replacing the VK48 with VK59 and lowering the temperature by 10°C. 

If the catalyst in bed 4 is replaced with an equal volume of VK-701 and the inlet temperature is optimised to 407°C (765°F), the conversion can be further increased to 

99.00%. This corresponds to a reduction of the stack gas SO2 level by 20% compared 

to VK59 or 30% compared to VK48. 

Bed 4 

Catalyst VK48 VK59 VK-701 LEAP5TM 

Inlet temperature, °C (°F) 425 (797) 415 (779) 407 (765) 

Outlet temperature, °C (°F) 428 (802) 418 (784) 411 (772) 

Overall conversion, % 98.57 98.74 99.00 

SO2 in the stack, ppm 1390 1220 970 

Table 1. Performance of VK-701 LEAP5TM in the final pass of a single-absorption plant when replacing 

VK48 or VK59 with an equal volume of VK-701. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 10 / 13 

Example 2: Reduced emissions from a double-absorption plant 

The SO2 emissions from an existing double-absorption plant can be reduced beyond 

the level of VK69 by also loading VK-701 LEAP5TM in the third pass. 

Layout: 3:1 double-absorption plant 

SO2 source: Burning of elemental sulphur 

Feed gas: 11% SO2, 10% O2

Catalysts in beds 1/2: VK38 / VK38 

Conversion outlet bed 2: 88.5% 

The performance of VK-701 and VK69 is compared to standard catalysts in Table 2. 

With standard potassium-promoted VK48 in bed 3 and VK38 in bed 4, the overall conversion of the plant is 99.85% corresponding to 200 ppm SO2 in the stack gas. The 

feed gas to the fourth pass contains about 0.6% SO2 in this case. If higher conversion 

is required, the first choice is to replace the VK38 in bed 4 with caesium-promoted 

VK69, which is optimised for the final pass of double-absorption plants, and reduce the 

inlet temperature to 395°C (743°F). This cuts the SO2 emission to 100 ppm. 

Bed 3 

Catalyst VK48 VK48 VK-701 LEAP5TM

Inlet temperature, °C (°F) 440 (824) 440 (824) 423 (793) 

Outlet temperature, °C (°F) 461 (862) 461 (862) 447 (837) 

Conversion outlet bed 3, % 95.45 95.45 96.43 

Bed 4 

Catalyst VK38 VK69 VK69 

Inlet temperature, °C (°F) 425 (797) 395 (743) 395 (743) 

Outlet temperature, °C (°F) 442 (828) 413 (775) 409 (768) 

Overall conversion, % 99.85 99.92 99.95 

SO2 in the stack, ppm 200 100 64 

Table 2. Performance of VK-701 LEAP5TM in the third pass of a 3:1 double-absorption plant. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 11 / 13 

A further reduction in SO2 emission can be accomplished by replacing the VK48 in bed 

3 with an equal amount of VK-701 LEAP5TM and operating the bed at an inlet temperature of 423°C (793°F). Due to the higher activity of the VK-701, the SO2 content in the 

feed gas to bed 4 is reduced to 0.47%, and the overall conversion is increased to 

99.95%. The 64 ppm SO2 in the stack achieved with a combined VK-701/VK69 loading 

corresponds to a 36% reduction compared to the VK48/VK69 loading and as much as 

68% reduction compared to a loading of conventional VK48/VK38 catalyst in beds 3 

and 4. 

For existing plants the reduced SO2 emissions achievable with VK-701 provide an attractive alternative to investing in a caustic or hydrogen peroxide scrubber. Even for 

plants already equipped with tail-gas scrubbing, the VK-701 may be a cost-efficient 

way of reducing chemical consumption for the scrubbing. 

VK-701 LEAP5TM operating experience 

The first industrial installation of the VK-701 is in a single-absorption plant burning elemental sulphur located in the Western world. The SO2 converter contains five catalyst 

beds with indirect cooling between beds 1 and 2 and quench air cooling after beds 2 

and 3. No cooling is applied between beds 4 and 5. The feed gas to the converter contains 8.8% SO2 and 12% O2. 

During a shutdown in 2010, the VK59 and VK48 catalysts in the final passes were replaced by equal volumes of VK-701 in order to cut SO2 emissions and increase production capacity. Prior to installation of VK-701, the plant had been running at 245 MTPD 

(270 STPD) with about 420°C (788°F) inlet bed 4 and an overall conversion of 98.77% 

corresponding to about 1000 ppm SO2 in the stack gas. 

After start-up a test run was carried out during which analyses of SO2 and O2 were 

made and the temperature was measured at bed inlets and outlets. Multiple-point gas 

and temperature measurements revealed incomplete mixing of the quench air inlet bed 

4, compromising performance significantly. However, similar measurements for the 

fifth bed indicated a more homogeneous gas composition. 

The data obtained are reported in Table 3 alongside data from a comparable situation 

prior to the installation of VK-701 LEAP5TM. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 12 / 13 

Before installation of 

VK-701 LEAP5TM 

After installation of 

VK-701 LEAP5TM 

Catalyst loading in beds 4 and 5 12.0 m3 VK59 

13.2 m3

 VK48 

13.4 m3

 VK-701 

13.0 m3

 VK-701 

Production rate, MTPD (STPD) 245 (270) 266 (293) 

Inlet temperature, bed 4, °C (°F) 420 (788) 404 (760) 

Overall conversion, % 98.77 99.02 

SO2 in the stack, ppm 1005 720 

Table 3. Improvement of conversion efficiency observed in a single-absorption plant by installing VK-701 

LEAP5TM in the final passes. 

After the installation of 26.4 m3

 VK-701 in the final passes and reduction of the inlet 

temperature to 404°C (760°F), the SO2 concentration in the stack gas has been reduced to approximately 720 ppm at an increased production rate of 266 MTPD (293 

STPD). Hence, by replacing VK59 and VK48 with the new VK-701 it is possible to reduce SO2 emissions by 20% in terms of kg SO2 per MT H2SO4 produced even at 9% 

higher production capacity. The observed activity of VK-701 is the double of new VK59 

even with the mixing problems in bed 4. 

Conclusion 

Topsøe has responded to the demands placed on the sulphuric acid industry to lower 

SO2 emissions by developing a new sulphuric acid catalyst designated VK-701 LEAP5TM. 

The catalyst is based on our novel LEAP5TM technology which circumvents the transport 

deficiencies in the molten active phase of existing commercial sulphuric acid catalysts. 

The VK-701 contains two to three times as much active vanadium in the V5+ form compared to other commercial catalysts and, as a result, it offers exceptionally high activity 

at low temperature in converted strong gases. 

SO2 emissions from a typical 4-bed single-absorption sulphuric acid plant can be reduced by 20-30% by loading VK-701 in the final bed compared to a caesium-promoted 

catalyst or by 30-40% compared to a standard potassium-promoted catalyst. Emissions 

from the most efficient double-absorption plants can be further reduced by up to 40% 

when loading VK-701 in the third pass of a 3:1 plant and achieve down to 50 ppm SO2

emission depending on feed gas and plant layout. Operating experience from a sulphur-burning single-absorption plant confirms the high activity of VK-701 LEAP5TM. 
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 13 / 13 

Combining the high activities offered by VK-701 and VK69 presents opportunities for 

designing new 3:1 plants with as little as 20-50 ppm SO2 in the stack gas (0.1-0.25 kg 

SO2/MT H2SO4, 0.2-0.5 lbs/ST). For existing plants the reduced SO2 emissions achievable with VK-701 provide an attractive alternative to investing in a caustic or hydrogen 

peroxide scrubber. Even for plants already equipped with tail-gas scrubbing, the 

VK-701 may be a cost-efficient way of reducing chemical consumption for the scrubbing. 

References 

1. Jensen-Holm, H. (1996). New Catalyst Options for Improved Performance of 

Sulphuric Acid Plants. Sulphur ’96, pp 235-49, British Sulphur Publishing, London. 

2. Jensen-Holm, H. and Hansen, L. (1997). Demonstrated Performance Improvements in Sulphuric Acid Plants Using VK69 Catalyst. Sulphur ’97, pp 193-206, 

British Sulphur Publishing, London. 

3. Lapina, O.B., Bal’zhinimaev, B.S., Boghosian, S., Eriksen, K. M. and Fehrmann, 

R. (1999). Catalysis Today, 51, 469-479. 

www.topsoe.com
Meeting future SO2 emission challenges with Topsøe’s 

new VK-701 LEAP5TM sulphuric acid catalyst 14 / 13