The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

Chris Winkler

Page 117 

MECS CATALYST PRODUCTS 

& TECHNICAL SERVICES UPDATE 

Chris D. Winkler 

MECS, Inc. 

St. Louis, Missouri, U.S.A. 

Abstract 

Vanadium-based sulphuric acid catalyst has been utilized to oxidize SO2 to SO3 since 

the early 1900’s. This versatile product displaced the expensive and easily-poisoned 

platinum-based catalyst in nearly all applications by the 1930’s. MECS has been 

manufacturing the vanadium-based catalyst since 1925 and today is the leading supplier 

in the world of sulphuric acid catalysts in a variety of forms. Currently, MECS has 

world-wide customer/technical support for the catalyst as well as a dedicated catalyst 

manufacturing plant in Martinez, California. MECS also has a strong technical and 

research and development program dedicated to creating new and improved products 

and services. This paper will describe some of the technical details of vanadium-based 

sulphuric acid catalysts as well as offering a unique look into how caesium-promoted 

MECS catalysts can be used in single and double absorption acid plants, and off-gas 

plants running on irregular gas feeds, to reduce emissions and pre-heater run times. An 

update on MECS technical services extended for use in characterizing converter 

performance is also presented. 

Standard Catalyst Composition

Sulphuric acid catalyst is composed of potassium (K) and vanadium (V) salts supported 

on a silica (SiO2) carrier. The silica support is diatomaceous earth (DE) which consists 

of skeletons of diatoms (microscopic sea creatures); the DE provides the ideal 

properties for the sulphuric acid catalyst at an acceptable cost. The 

potassium/vanadium salt mixture actually liquefies under reaction conditions (> 350°C) 

and forms a molten salt catalyst. The salt formation reaction can be shown as follows: 

K2S2O7 + V2O5 -----> KwVxSyOz (molten salt compound(s)) 

The actual composition of the critical molten salt is still widely disputed; hence, the 

generic labelling of the salt compound with w, x, y, and z values. 

MECS Standard Catalyst Products

MECS is proud to offer a wide variety of standard potassium-promoted catalysts 

suitable for every application: 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

Chris Winkler

Page 118 

The “XLP” six-lobed, ribbed-ring line of catalysts offer extended surface area and 

lowest pressure drop and highest conversion performance in all beds of the converter. 

• XLP-220 (Beds 1 & 2) 

• XLP-110 (Beds 2, 3, 4 & 5) 

The “LP” raschig-ring line of catalysts offer low pressure drop, high activity and low 

fouling rates. LP Catalysts have been a standard in the sulphuric acid industry for many 

years. 

• LP-120 (Beds 1 & 2) 

• LP-220 (Beds 1 & 2) 

• LP-110 (Beds 2, 3, 4 & 5) 

The “T” pellet lines of catalysts are for low gas velocity converter designs and where 

maximum durability is required for lower screening losses. 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

Chris Winkler

Page 119 

• T-210 (Beds 1 & 2) 

• T-11 (Beds 2, 3, 4 & 5) 

Factors Affecting Catalyst Life

There are a number of factors which can impact the life of the sulphuric acid catalyst. 

The catalyst life can be shortened through the following mechanisms: 

Vanadium Loss: dust accumulation in the bed; iron oxide corrosion 

products; chlorides in the gas stream; and acid/moisture 

contact with catalyst. 

Moisture Contact: leaching of the active salts; decreased catalyst hardness 

Poison: arsenic (oxide coating of the catalyst) 

Carrier Degradation: fluorine attack (forms volatile SiF4); thermal cycling. 

Temperature “Memory” Effects in Catalyst

Over the years, this "philosophy" of catalyst performance has limited the capabilities of 

many sulphuric acid plants to optimize the converter performance. Plant operators have 

hesitated to raise catalyst bed inlet temperatures (with the potential result of increasing 

conversion) because of fear of "damaging" the catalyst with respect to lower 

temperature operation. In general, within a catalyst bed, catalyst "damage" will only 

occur if the bed temperature has been at least 100o

C higher than the initial operating 

temperature for an extended time period (> 7 days). The damage that can occur is more 

physical than chemical in nature. At the high temperatures, the structure of the "catalyst 

support" can change with subsequent decrease in surface area. This lower surface area 

will directly result in reduced catalyst activity at a lower operating temperature. The 

reaction rate at the higher temperatures is so large that there will be little effect on the 

overall achieved conversion. Hence, a catalyst that has been operating at 530o

C for a 

long time period (for example in the middle of the first catalyst bed) will not perform as 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

Chris Winkler

Page 120 

well as fresh catalyst when operated at 430o

C due to this structural change. Because of 

this phenomenon, it is therefore suggested that plant operators never move catalyst from 

the bottom of Bed # 1 to the cooler top of the first bed. The same recommendation 

applies to Bed # 2. Catalyst within Beds # 3 and # 4 can be freely rearranged 

depending on the convenience of the plant personnel. However, it is recommended 

practice to always place any fresh catalyst on the top of any bed during normal plant 

maintenance. 

The active ingredients within a sulphuric acid catalyst will vary from bed to bed as the 

level of SO3 (a major component of the active molten salt) in the gas stream is 

dependent on the overall conversion. Also, the SO2/SO3 ratio in the gas phase has a 

large effect on the composition of the active catalyst phase. Because of these chemical 

"circumstances", it is recommended that, if necessary, plant operators should always 

move catalyst in the lower beds to the upper beds (for example, from Bed # 4 to Bed # 

3; never moving catalyst from Bed # 3 to Bed # 4). The more highly sulphated lower 

bed catalyst will always perform very well in the upper beds, but both temperature and 

chemical effects on the upper bed catalyst prevent it from operating properly in the 

lower beds. 

Caesium-promoted Catalyst Composition

The caesium-promoted sulphuric acid catalyst is actually very similar to the standard 

catalyst supplied by MECS. The Caesium Catalyst is based on a standard potassiumpromoted vanadium formulation in which some of the potassium promoter has been 

replaced with an equi-molar amount of caesium (Cs) compounds. The caesium helps to 

stabilize the vanadium in the molten salt and prevents the precipitation of the vanadium 

below (410°C) as is observed for the conventional sulphuric catalysts. This 

precipitation results in catalyst deactivation and very low activity at low inlet 

temperatures. It is important to note that the caesium-promoted catalyst is still a 

vanadium-containing product just as in the case of the standard sulphuric acid catalysts; 

thus enabling the Caesium Catalyst to be handled in a manner identical to the standard 

sulphuric acid catalysts. 

MECS Caesium Catalyst Products

MECS is proud to offer a wide variety of caesium-promoted catalysts for use in 

achieving low plant stack emissions and lower ignition temperatures to affect faster 

plant start-ups. 

• XCs-120 (six-lobed, ribbed-ring; All Beds) 

• Cs-120 (raschig ring; Beds 1 & 2) 

• Cs-110 (raschig ring; Beds 2, 3, 4 & 5) 

• SCX-2000 (“super-Cs”, six-lobed, ribbed-ring; Beds 4 & 5) 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

Chris Winkler

Page 121 

Caesium Catalyst Applications & Benefits

The applications and advantages of the Caesium Catalyst are varied and numerous. The 

following is a description of some of the applications for this product which would be 

of interest to sulphuric acid catalyst customers: 

(1) Reduced First Bed Inlet Temperatures: In all first bed installations of the MECS 

Caesium Catalyst, the required inlet temperature has been significantly reduced relative 

to the temperature required for standard catalyst. In some cases, first bed inlet 

temperatures as low as 360oC (although optimal performance is around 390°C or 

greater) have been realized for feed gas containing high SO2 and high O2

concentrations. For first bed applications, a “cap” of the Caesium Catalyst is loaded on 

top of the bottom layer of the standard potassium-promoted catalyst (typical 

recommended Caesium Catalyst loading is 30-50% of the first bed volume depending 

upon the operating inlet temperature). The temperature of the gas exiting the Caesium 

Catalyst layer is above the ignition temperature of the standard catalyst layer which can 

then “complete” the conversion operation within the first bed at a higher level than if a 

caesium cap was not introduced (Figure 1). It should be noted that once the gas enters 

the typical ignition temperature zone of standard catalysts (415 - 420°C), the caesium 

catalyst provides no additional activity benefit. 

Figure 1: Bed # 1 Conversion Performance with Caesium Cap 

(2) Plant Re-Start Following Short Shutdowns: When some sulphuric acid plants must 

shutdown for short time periods, it is often the case that a pre-heater must be used to re0

20

40

60

80

100

120

380 430 480 530 580 630 680

TEMPERATURE (°C)

% SO2 CONVERSION

Equilibrium Curve

LP-120

XCs-120/LP-120

XLP-220 

XCS-120/XLP-220 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

Chris Winkler

Page 122 

heat the catalyst beds prior to starting up the plant. With Caesium Catalyst loaded in 

the first and last pass of the converter, it may be possible to re-start the plant without 

using the pre-heater, saving both time and fuel costs. Plant start-ups after a cold 

shutdown are also facilitated by the Caesium Catalyst in both reduced fuel consumption 

and short start-up time. 

(3) Reduction of SO2 Emissions (Double Absorption Plant): Through the use of the 

MECS “Super” Caesium (SCX-2000) Catalyst, it is possible to significantly increase 

the SO2 conversion through a double absorption plant and hence reduce the SO2 stack 

emissions. Stack SO2 concentrations well below 100 ppm have been realized through 

the use of the SCX-2000 Catalyst in the final beds of double absorption plants 

(Figure 2). Final bed inlet temperatures within the range of 390-410oC permit greater 

conversion due to the shift in the allowable thermodynamic conversion limits. The 

high conversion levels possible with the Caesium Catalyst are either unattainable with 

conventional sulphuric acid catalysts or would require massive volumes of the standard 

catalyst. There are a number of examples of significant emissions reductions in double 

absorption sulphur burning, spent acid, and metallurgical plants. 

Figure 2: Use of SCX-2000 in a Double Absorption Plant to Reduce SO2 Emissions 

(4) Reduction of SO2 Emissions (Single Absorption Plant): The use of the MECS 

Caesium Catalyst in single absorption plant applications can also significantly reduce 

the SO2 concentration in the stack gas. In cases where post-converter scrubbing of the 

SO2 is used to minimize emissions, the use of the Caesium Catalyst can significantly 

reduce the amount of salts or weak acid produced in the scrubber and save on raw 

material and waste elimination costs. 

(5) Control of First Bed Outlet Temperatures: The use of the MECS XCs-120 or 

Cs-120 caesium-promoted catalysts as a 30-50% “cap” on the first catalyst bed has been 

very effective in a number of installations. With this type of application, the Caesium 

Catalyst generates sufficient heat (even at low inlet temperatures near 360-380oC to 

130

55 0

50

100

150

SO2 

Emissions 

(ppm)

XLP-110

(420°C)

SCX-2000

(390°C)

4th Bed Catalyst

Metallurgical Plant; 

10.5% SO2, 11.5% O2

Same 4th Bed Loading for Each Catalyst
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

Chris Winkler

Page 123 

“ignite” the standard catalyst layer below. In this case, the customer takes advantage of 

the low temperature properties of the Caesium Catalyst as well as benefiting from the 

excellent activity of the standard MECS XLP-220 ribbed-rings at the higher 

temperatures in the bottom layer. An example of this application is found for a large 

metallurgical plant that has very high SO2 and O2 levels in the feed gas. Using 

conventional XLP-220 ring catalyst in the first pass with a normal inlet temperature of 

415 – 420oC, the first bed outlet temperature would be in excess of 650oC, which is 

unacceptable for long term operations. With a top layer of XCs-120 ribbed-rings, the 

first pass inlet temperature for this plant can be set at 390oC with a manageable outlet 

temperature now less than 640oC. 

MECS Technical Services

Most catalyst suppliers offer various technical services to support their customers. 

These services include catalyst sample activity and hardness analyses and some 

computer simulation studies. MECS extends these services as well as complete worldclass sulphuric acid process support. Additionally, for nearly two decades now, MECS 

has offered the Portable Gas Analysis System (PeGASyS) service for an extensive 

evaluation of the customer’s plant operations. This gas chromatography-based system 

allows for evaluation the SO2 and O2 levels in any accessible gas stream. Uniquely 

designed gas sampling techniques provide the analytical sample to be free of SO3 which 

would damage the equipment. Utilizing this analytical data along with the computer 

simulation program (SO2OPT), the operation of the plant can be fully characterized 

with the results appearing in a detailed report supplied to the customer. There are 

countless examples of where the PeGASyS service has solved conversion problems, 

identified heat exchanger leaks, and increased the productivity of sulphuric acid plants; 

all without having to take the plant down from valuable production. 

Summary

This paper has described some of the fundamentals of the standard potassium and 

caesium-promoted sulphuric acid catalyst formulations along with details of the various 

factors that can impact catalyst performance. A variety of applications for Caesium 

Catalyst were presented with the corresponding impacts on performance that can be 

realized within the sulphuric acid plant. Specific examples were given to assist in 

quantifying the benefits associated with the use of Caesium Catalyst. Lastly, an update 

on the MECS technical services extended today for use in characterizing converter 

performance was also provided. 

Acknowledgments

The author offers thanks to the MECS, Inc. Research and Development Team for its 

dedication and efforts towards continually improving MECS catalyst products and 

services. 

Also, high praises go to the MECS, Inc. Catalyst Manufacturing Team for its 

contribution to the production of the high quality catalyst products which include the 

caesium-promoted catalysts. 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

Chris Winkler

Page 124 

The Author 

Christopher D. Winkler, MECS, Inc., Business Manager 

Mr. Winkler has been with MECS, Inc. since 2000, spending the early parts of his 

career as a Catalyst Technical Engineer in which his primary responsibilities were in 

the catalyst engineering field (i.e. design, troubleshooting, customer relations) as well 

as testing sulphuric acid plants to assist in the characterizing of the overall performance 

of the plant. Mr. Winkler has also been involved in designing heat exchangers for use 

in MECS sulphuric acid plants. Today, Christopher is the Business Manager of the 

Catalyst and Heat Exchanger Business Units within MECS, Inc.