VK series

Sulphuric acid catalysts 
Scanning Electron Micrograph of a typical diatom skeleton.

Selected diatomaceous earths are used as essential raw material for the sulphuric acid catalyst carrier and are crucial for the fi nal catalyst properties.
VK series

– catalysts for today and for the future

The catalytic SO2 converter is the heart of the sulphuric acid 

plant and the quality and characteristics of the selected catalysts 

are crucial to a reliable and energy-efficient operation.

The catalyst properties are influenced by the chemical 

composition, the physical properties including the nature of the 

support material and the manufacturing process. Topsøe VK 

catalysts are uniquely balanced to combine high and stable 

activity, robustness, low pressure drop and a long service life.

Continual improvement of the VK catalysts is achieved through 

close cooperation between Topsøe’s Catalyst Group, R&D and 

the production facilities. Over the years this cooperation has 

resulted in major product breakthroughs, providing the industry 

with new catalysts for more energy-efficient operation, lower 

SO2

 emissions and higher production rates.

Topsøe’s VK series comprises four different formulations in a 

variety of sizes and shapes, effectively covering all operating 

conditions in any sulphuric acid plant.

By combining outstanding activity with low pressure drop and 

exceptionally high mechanical strength, the VK catalysts ensure 

cost-effective plant operation by providing:

- high SO2 conversion efficiency 

- improved energy efficiency

- enhanced operating flexibility

- low screening losses

- long service life

Product range

VK38

The VK38 formulation provides excellent activity over a wide 

range of operating conditions. VK38 is the only catalyst on 

the market that can be used effectively in all beds of any SO2

converter. 

For continuous operation VK38 offers a broad temperature 

range of 400-630°C (750-1170°F) and can withstand repeated 

temperature spikes up to 650°C (1200°F). 

VK38 is designed to initiate the reaction at a temperature as low 

as 360°C (680°F), ensuring a smooth plant start-up, minimising 

pre-heat time due to its superior catalyst activity. 

VK48

VK48 is a high-vanadium version of the standard all-round VK38 

catalyst.

The catalyst composition is specifically formulated for lower 

pass service, particularly where the process gas contains 

large amounts of SO3

. The conversion ratio of SO2 affects the 

balance between the active vanadium species in the catalyst. 

An optimised ratio of the alkali-metal promoters enables an 

increased vanadium content in VK48, resulting in a considerably 

enhanced activity.

In high SO3

 gas environments, such as the lower passes of 

single absorption plants, or the third pass of a 3:1 double 

absorption plant, VK48 offers a significant performance 

advantage.
Caesium catalysts

Incorporating caesium as an additional catalyst promoter 

enhances the action of the vanadium and activates the catalyst 

at a much lower temperature than conventional non-caesium 

catalysts. 

Exceptionally high activity

Topsøe’s caesium-promoted VK catalysts are up to three times 

more active than non-caesium catalysts, depending on the 

operating conditions. Topsøe’s unique production techniques 

result in an optimised silica pore system that combines two 

important features: a very uniform distribution of the catalytically 

active melt within the pore system and better access for the 

process gas to the internal surfaces of the catalyst. This greatly 

improves the activity of the caesium-promoted VK catalysts.

Low ignition temperature

VK59 and VK69 have an exceptionally low ignition or “strike” 

temperatures of 320-330°C (610-625°F) that offer new 

possibilities for faster and cleaner start-ups. Autothermal restarts 

can be accomplished after a signifi cantly longer idle time without 

the use of pre-heat.

VK59

VK59 is a caesium-promoted catalyst optimised for medium 

to high strength SO2

 gasses with continuous operation down 

to 370°C (700°F). A top layer of VK59 in the fi rst pass makes it 

possible to accommodate high strength SO2

 feed gas without 

exceeding the maximum outlet temperature of 630°C (1165°F) 

and still maintain high bed conversion effi ciency. 

VK59 installed in the lower pass(es) of single absorption plants 

allows for signifi cant conversion improvement. 

Benefi ts

- accommodates strong SO2 feed gas without excessive fi rst 

pass exit temperatures

- greatly improved operating fl exibility for rapidly changing 

feed gas composition

- low ignition temperature for faster and cleaner start-ups

- signifi cantly extended idle time for autothermal restarts

- improved overall conversion in single absorption plants

VK69

VK69 is designed specifi cally for the fi nal pass(es) of double 

absorption plants. In the lean SO2

 gas environment after the 

inter-pass absorption tower, the unique VK69 formulation 

offers unmatched high activity throughout the entire operating 

temperature range. This affords existing plants the opportunity 

to greatly reduce SO2

 emissions and/or increase production 

rates. For new or revamped plants, SO2

 emissions of under 

50 ppm are possible.

Using caesium as a promoter, VK69’s extremely low ignition or 

“strike” temperature will signifi cantly reduce pre-heat time and 

improve start-up performance. 

VK69 is manufactured in a unique 9 mm Daisy-shape. The 

high surface area contributes to the high activity in this gas 

environment and the high void-fraction ensures a low pressure 

drop and good dust tolerance.

Benefi ts

- possibility for over 50% reduction in SO2 emission from 

existing double absorption plants

- possibility for signifi cantly increased production without 

increasing SO2

 emissions

- less than 50 ppm of SO2 emissions possible from new or 

revamped plants

- low ignition temperature for faster and cleaner start-ups

- long lifetime and low screening losses industrially proven 

since 1996.

1944 1970 1984 1987 1988

VK 38 First with rings First with Dasiy High vanadium VK48 First with caesium VK58
Sizes and shapes

25 mm Daisy - dust protection catalyst

For catalyst beds suffering from rapid pressure drop increase 

due to dust in the feed gas, a top layer of Topsøe’s 25 mm 

Daisy-shape catalyst can provide a cost effective remedy. The 

extra void fraction and a lower specifi c surface area allow a 

signifi cantly higher capacity for dust, distributing it throughout 

a larger volume of the catalyst bed. The result is a lower rate of 

pressure drop increase providing improved energy effi ciency and 

longer intervals between catalyst screenings.

Benefi ts

- 30-50% longer on-stream time compared to existing ringshaped dust-protection catalysts

- 100% longer on-stream time compared to 12 mm Daisy

- maintenance cost savings due to less frequent catalyst 

screening

- energy cost savings due to lower plant pressure drop

- production increase due to longer time at maximum rate 

before pressure drop forces production delay

12 mm Daisy

The 12 mm Daisy has become the most widely used catalyst 

shape on the market since introduced by Topsøe in 1984. 

The high void fraction results in improved energy effi ciency 

throughout the operating cycle due to low initial pressure drop 

and increased capacity for dust. 

Topsøe’s 12 mm Daisy-shape has proven to be robust as well, 

with reported screening losses typically well below 10%.

Topsøe’s VK38, VK48, and VK59 formulations are all available in 

the 12 mm Daisy-shape.

Benefi ts

- low initial pressure drop

- high dust capacity yielding in a lower rate of pressure 

drop increase

- high activity

- low screening losses

9 mm Daisy 

VK69 is produced in the unique 9 mm Daisy-shape. In the low 

SO2

 environment, after the intermediate absorption tower, the 

9 mm size and Daisy-shape combination gives 30% extra 

activity compared to a 12 mm Daisy-shape, in part due to the 

higher surface area. Also, the Daisy-shape ensures remarkably 

low pressure drop.

10 mm rings and 6 mm cylinders

VK catalysts in the form of 10 mm rings and 6 mm cylinders 

remain available for plants that desire or require their continued 

use.

1996 1998 2007

2nd caesium generation

VK69 VK59 25 mm Daisy

0

 0

100

200

300

400

500

6 12 18 24 30

Operating time (months)

No dust guard

10 cm top layer of 20 mm rings

10 cm top layer of 25 mm Daisy

Relative pressure drop, %

Pressure drop development across various catalyst loadings in a dust laden feed gas. 

Installation of a 10-15 cm (4-6 inches) top layer of the 25 mm Daisy results in 

30-35% longer production campaigns compared to the 20 mm rings or a doubling 

when compared to the 12 mm Daisy.
ISO

Catalyst manufacturing facilities are located in Denmark and 

in Houston, Texas, USA. Both plants are certified to the ISO 

9001: 2008 and ISO 14001: 2004 standards. Topsøe’s quality 

management system includes continuous improvement to 

achieve the highest quality. 

Topsøe recognises its responsibility towards the local and 

global community in all its activities. We protect the external 

environment through responsible behaviour and are dedicated 

to a continual improvement of personal safety and internal work 

processes. 

Packaging and storage

The standard packagings for VK catalysts are supersacks and 

200 litre fibre drums. A polyethylene liner protects the catalyst 

from moisture. VK catalysts are very durable and can be stored 

for years without loss in activity or strength when kept dry and in 

the original packaging.

Single absorption Plant

VK59 - ignition layer VK38 

VK38 

VK48 

VK48 VK69 

VK38 

VK38 

VK48 

VK59 - for high conversion - VK38 

and/or 

increased production rates

Double absorption plant

3+1 layout

VK38 - dust protection layer

Optional

VK38 VK48

Type Potassium-promoted Potassium-promoted

Ignition temperature 360°C/680°F 360°C/680°F

Continuous operating 

temperature range

400-630°C 

750-1165°F

410-550°C 

770-1020°F

Thermostability 650°C/1200°F 650°C/1200°F

Size and shape 12 mm Daisy

25 mm Daisy

10 mm rings

 6 mm cylinders

12 mm Daisy

10 mm rings

 6 mm cylinders

Opportunities Excellent activity in the full range of operating conditions.

Suitable for use in all passes of all converters.

High-vanadium catalyst with enhanced activity for lower pass 

service, particularly where the process gas contains significant 

amounts of SO3

.

Tailored catalyst loadings

Topsøe’s range of VK catalyst formulations and shapes allow tailored loadings for each plant’s requirements. 

Topsøe’s catalyst experts will assist in selecting the optimum catalyst type and shape for each pass in any 

converter design, keeping the client’s objectives in mind.
Technical service

Based on experience over more than half a century, Topsøe has 

established an extensive technical service programme available 

to our VK catalyst users. In addition to frequent contact, the 

service programme includes activity testing of catalyst samples, 

evaluations of catalyst performance, plant optimisation, trouble 

shooting and management of catalyst replacement.

Activity testing of catalyst samples represents a valuable source 

of information, which combined with computer evaluations 

of the plant performance enables Topsøe to optimise future 

catalyst management.

Topsøe’s Portable Gas Analysis Unit, TOPGUN enables 

a thorough analysis of a sulphuric acid converter system. 

TOPGUN is a fully portable, infrared gas-analysis unit for 

SO2

 and O2 analyses. The gas analyses are used as input to 

Topsøe’s proprietary computer programs, which can accurately 

simulate the converter performance. TOPGUN has proven a 

very efficient tool for troubleshooting and optimising the plant 

performance.

VK69

VK38 Low temperature

caesium catalysts

open a larger

equilibrium window

Temperature

Converter temperature - conversion profile

SO2 conversion

Equilibrium conversion

after interpass absorber

Equilibrium conversion

VK59 VK69

Type Caesium-promoted Caesium-promoted

Ignition temperature 320°C/610°F 320°C/610°F

Continuous operating 

temperature range

370-500°C 

700-930°F

370-500°C

700-930°F

Thermostability 650°C/1200°F 650°C/1200°F

Size and shape 12 mm Daisy 9 mm Daisy

Opportunities Optimised for medium to high-strength SO2

 gases. 

Use as an ignition layer in any bed. Improved 

conversion in single absorption plants.

Double absorption plants only - optimised for use in the pass(es) 

after the intermediate absorption tower. Extremely high activity 

throughout the entire temperature range.

 
Haldor Topsøe A/S - Nymøllevej 55 - 2800 Kgs. Lyngby - Denmark

Tel. +45 4527 2000 - Fax. +45 4527 2999 - www.topsoe.com

The information and recommendations have been prepared by Topsøe specialists having a 

thorough knowledge of the catalysts. However, any operation instructions should be considered to 

be of a general nature and we cannot assume any liability for upsets or damage of the customer’s 

plants or personnel. Nothing herein is to be construed as recommending any practice or any 

product in violation of any patent, law or regulation.

Corporate PR 09.2008.2/ R05.2011