BY: TIMOTHY R. FELTHOUSE, Ph.D; MARIO P. DIGIOVANNI, P.E.; JOHN R. HORNE 

 AND SARAH A. RICHARDSON 

PRESENTED AT: THE AIChE CLEARWATER CONVENTION 2011 

PHOSPHATE FERTILIZER & SULFURIC ACID TECHNOLOGY CONFERENCE 

CLEARWATER BEACH, FLORIDA 

JUNE 10-11, 2011 

 IMPROVING SULFURIC ACID 

 PLANT PERFORMANCE 

 THROUGH NEW SHAPE & HIGHER ACTIVITY 

 CATALYSTS 
2

TABLE OF CONTENTS 

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

CATALYST PROTOTYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

GEARTM CATALYST FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

LOWER SO2 EMISSIONS AND INCREASED ACID PRODUCTION . . . . . . . . . . . 10 

ENERGY SAVINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

MECS CATALYST PRODUCT PORTFOLIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 
3

LIST OF FIGURES 

Figure Page 

1. MECS Sulfuric Acid Catalyst Shapes Range from (left to right) New 

 13-mm GEARTM Ribbed Ring, 12-mm XLP Ribbed Ring, 10-mm Ring, 

 and 6-mm Pellet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

2. Predicted Pressure Drop Build-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

3. SO2 Concentration Gradients for GEARTM Catalyst Cross-Section at 475oC 

 and 11.0% SO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

4. GEARTM Catalyst Requires Less Catalyst than XLP Catalyst for the Same 

Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

5. GEARTM Catalyst Reduces SO2 Emissions Compared to Same Volume 

 of XLP Catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

6. Random Catalyst Packing for GR-330 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

7. Relative Pressure Drop for LP-110, LP-120, XLP, and GR-330 Catalysts . . . . 13 

LIST OF TABLES 

Table Page 

1. Operating Conditions for the Catalyst GR-330 Prototype . . . . . . . . . . . . . . . . . 6 

2. Pass 1 Catalyst Sleeve Operating Conditions within a Converter . . . . . . . . . . . 8 

3. GEARTM Catalyst Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

4. Emissions Comparison for XLP and GEARTM Catalysts . . . . . . . . . . . . . . . . . . 12 

5. Estimated Savings for a Full Converter of GEARTM Catalyst in a 3300 

 STPD 3X1 IPA Acid Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

6. MECS New Catalyst Product Portfolio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 
4

ABSTRACT 

 A new line of sulfuric acid catalysts called GEARTM has been developed by 

MECS, Inc. that offers improved performance to the sulfuric acid industry. These 

catalysts combine a new low-pressure drop ribbed-ring shape with higher activity for a 

combination of increased energy savings and lower overall plant SO2 emissions. Pass 1 

plant tests with a prototype of our new shape ran for 27 months during which the pressure 

drop rose from 4 to 12 in W. C., less than half of the typical pressure drop rise. 

PeGASyS tests of the plant confirmed conversion performance of the catalyst in pass 1. 

This paper presents several cases to demonstrate the advantages of these new catalysts. 
5

INTRODUCTION 

 On December 31st of 2010, MECS, Inc. became a wholly owned subsidiary of 

DuPont and part of the company’s Sustainable Solutions business. The combined 

resources of MECS and DuPont create an expanded portfolio to enhance the safety, 

reliability, and environmental sustainability of customers’ facilities. 

 

 MECS has been in the sulfuric acid catalyst business since the 1920’s. Over the 

past 90 years, catalyst has evolved from pellets to energy-saving rings to low-emission 

cesium-promoted catalyst. As energy savings and environmental concerns create new 

operational and design challenges for sulfuric acid plants, innovations in catalyst 

technology can provide the resolution. Over the past few years, MECS has designed, 

developed, and tested in laboratory and acid plant operations several new catalyst shapes 

and formulations. Results from these catalyst evaluations led to significant 

improvements in shape, dust capacity, pressure drop, and activity. As a result of these 

catalyst performance improvements, MECS introduces in this paper their next generation 

of sulfuric acid catalysts known as GEARTM catalysts. 

 This paper presents the findings of acid plant and laboratory evaluations leading 

to the new line of GEARTM catalysts. Based on comparisons with MECS XLP-220 and 

XLP-110 catalysts, advantages of these GEARTM catalysts will be demonstrated. Figure 

1 depicts some of the shapes in the new MECS sulfuric acid catalyst portfolio including 

the new GEARTM catalyst (13-mm GR-330). 

 Figure 1. MECS Sulfuric Acid Catalyst Shapes 

 Range from (left to right) New 

 13-mm GEARTM Ribbed Ring, 12-mm 

 XLP Ribbed Ring, 10-mm Ring, and 

 6-mm Pellet. 
6

RESULTS 

Catalyst Prototypes 

As is often the case in scientific discovery, it takes a few trials to reach the desired 

results. The development of the GEARTM catalyst products took several years and a few 

different prototypes to produce a catalyst family with the desired characteristics. 

 A plant trial provided valuable information regarding pressure drop and dust 

handling capability of improved MECS catalyst. A bed of 13-mm catalyst prototype was 

placed in the first pass of a small sulfur-burning sulfuric acid plant and run for 27 months 

at the operating conditions shown in Table 1. 

Table 1. Operating Conditions for the Catalyst GR-330 Prototype. 

Converter Diameter, ft. 9.5

Production Rate, STPD 130

Pass 1 GR-330 Prototype Volume, L 4200

Gas Strength, %-SO2 8.9 to 9.5

Inlet Temperature, o

C ( oF) 411 (772)

PeGASyS testing that was done when the catalyst was first charged and at the conclusion 

of the pass 1 trial confirmed the catalyst performance. Clean bed pressure drop was 

measured at 4 in W. C. After 27 months of operation, the pressure drop in pass 1 was 12 

in W. C. The pressure drop build-up was fit to MECS’ proprietary ash pressure drop 

build-up model showing a result comparable to 15-ppm ash content in the sulfur feed. In 

contrast, the pressure drop of XLP-220 catalyst in pass 1 of this plant started out at 

7 in W. C. and reached 25 in W. C. in 18 months. Figure 2 shows these pressure drop 

build-up curves as a percent of clean bed pressure drop. The dust model results show that 

the larger shape, the GR-330 prototype, extends the operating time by at least 8 months, 

which is 40% longer run time. 
7

0%

100%

200%

300%

400%

500%

600%

700%

800%

900%

1000%

0 3 6 9 12 15 18 21 24 27

Operating Time, Months

%-Relative Pressure Drop

XLP-220

GR-330 Prototype

Figure 2. Predicted Pressure Drop Build-up. 

 A more active “GR” formulation is another feature developed for the GEARTM

catalysts. To evaluate this formulation with respect to catalyst “stickiness” and activity 

sustainability, a two year plant trial was employed. Four 36-inch deep cylindrical sleeves 

were filled with XLP-shaped (12-mm ribbed ring) catalyst containing this new, more 

active formulation. The sleeves were placed in the four quadrants of pass 1 of a large 

sulfur-burning sulfuric acid plant and operated under the conditions shown in Table 2. 

Two additional sleeves of standard production XLP-220 catalyst were also placed in 

pass 1. 
8

Table 2. Pass 1 Catalyst Sleeve Operating Conditions within a Converter.

Converter Diameter, ft. 42

Typical Plant Production Rate, STPD 3600

Gas Strength, %-SO2 11.5

Inlet Temperature, o

C ( oF) 428 (802)

Sleeve Diameter, ft. 0.67

Sleeve Length, ft. 3.0

Pass 1 GR-Formulation-Prototype Sleeve Volumes, L 30

Sleeve Distances From Bed Center, ft. 21

Sleeve Operating Time, months 24

 After 24 months of operation, the sleeves were recovered, inspected for relative 

“stickiness” with respect to the entrained dust, and sieved to separate the dust from the 

catalyst. Four zones consisting of different bed depths of catalyst were evaluated from 

each sleeve to determine where the dust accumulated. As expected, the “GR” 

formulation catalyst and the XLP-220 catalyst had comparable amounts of dust at the 

different sleeve depths. The more active “GR” formulation catalyst, along with the XLP220 catalyst, was free from any “crusting” from the dust. 

 Catalyst activity of the “GR” formulation catalyst sleeve samples was determined 

for each of the zones. As expected, the high dust zones at the top of the sleeves showed 

the greatest reduction in activity after two years of operation as measured by the 

differential conversion of SO2. In the lower three zones of the four sleeves the “GR” 

formulation catalyst showed a higher activity level relative to traditional MECS catalysts 

following two years of operation and was superior to XLP-220 catalyst in maintaining its 

overall activity level. 

GEARTM Catalyst Features 

The GEARTM catalyst shape evolved from both computational and comparative 

small pilot plant scale results on five different ribbed ring catalysts shapes. The 

computational results provided relative catalyst effectiveness as a function of each of 

these ribbed catalyst shapes. Figure 3 depicts SO2 concentration gradients (Comsol 

Multiphysics Finite Element Modeling) for the GEARTM shape indicating good gas 

penetration into the core of the catalyst from the outer and inner surfaces. 

 
9

Figure 3. SO2 Concentration Gradients for GEARTM Catalyst 

 Cross-Section at 475oC and 11.0% SO2. 

Pilot plant scale (20 to 30 L) catalyst volumes afforded comparative pressure drops and 

kinetic reaction rates for each of these five catalyst shapes. From these comparative tests, 

the GEARTM shape was chosen because it showed the best set of attributes. Table 3 

highlights the features of MECS’ new GEARTM catalyst. Note that the GEARTM catalyst 

comes in two sizes, providing the best balance of pressure drop and activity for various 

plant operating conditions. 

 

Table 3. GEARTM Catalyst Features. 

Catalyst GR-330 GR-310

Nominal Diameter, mm 13 11

Formulation Advanced Advanced

Ignition Temperature Range, o

C ( oF) 350-360 (662-680) 350-360 (662-680)

%-Lower Pressure Drop than XLP 30 15

 The GEARTM family of catalyst products can improve sulfuric acid plant 

performance by lowering SO2 emissions, increasing acid production, extending operating 
10

time, and saving energy. GEARTM is an acronym for the following catalyst features: 

G = Geometrically Optimized 

E = Enhanced Surface Area 

A = Activity Improvement 

R = Reduced Pressure Drop 

Advantages of the GEARTM catalysts are demonstrated in the next two sections. Several 

examples compare the performance of GEARTM catalyst to XLP catalyst.

Lower SO2 Emissions and Increased Acid Production 

The combination of enhanced surface area and activity improvement conferred by 

the advanced formulation and shape reduces the required catalyst volume. The bar graph 

in Figure 4 compares the relative volumes of XLP-220/XLP-110 catalyst and GR330/GR-310 catalyst for the same conversion. If gas strength remains constant and 

GEARTM catalyst volume is adjusted to meet the same emissions as XLP catalyst, then 

the GEARTM catalyst will require 5 to 15% lower catalyst volume. The trend in Figure 4 

shows that GEARTM is more effective at higher gas strengths than XLP catalyst. 

80%

90%

100%

110%

120%

130%

140%

10.0 10.5 11.0 11.5

Percent SO2

Relative Percent Catalyst Loading .

GEAR(TM) Catalyst

XLP Catalyst

Figure 4. GEARTM Catalyst Requires Less Catalyst than 

 XLP Catalyst for the Same Conversion. 
11

 If the volume of GEARTM catalyst is kept the same as XLP catalyst, then the 

increased catalyst activity of GEARTM catalyst results in 10 to 30% lower emissions for a 

given gas strength, depending upon the target emissions level. These relationships are 

shown in Figure 5. Alternatively, for a given catalyst volume, using GEARTM catalyst 

and increasing the gas strength can facilitate 5 to 6% higher plant capacity with the same 

SO2 emissions. For the lowest overall emissions, use of SCX-2000 in the final pass or 

passes is recommended. 

0

50

100

150

200

250

300

9.5 10.0 10.5 11.0 11.5 12.0

Inlet %-SO2 Gas Strength

SO2 Emissions, ppmv

XLP Catalyst

GR Catalyst

Figure 5. GEARTM Catalyst Reduces SO2 Emissions Compared 

to Same Volume of XLP Catalyst. 

 

 To illustrate further the SO2 emissions reduction afforded by the GEARTM

catalyst, four catalyst design cases were considered for a new 3X1 IPA sulfur burning 

plant. Table 4 summarizes the four cases that vary by catalyst type and pass 4 inlet 

temperature. Catalyst loading remained constant for all of the cases in this comparison. 

Compared to XLP catalyst, use of GEARTM catalyst offered 20% lower SO2 emissions. 

The plant SO2 emissions for the design with GEARTM catalyst and SCX-2000 were 65% 

lower than the XLP catalyst design. For ultra-low emissions of 65 ppm, SCX-2000 was 

used in both the third and fourth passes with GEARTM catalyst in the upper passes. 
12

Table 4. Emissions Comparison for XLP and GEARTM Catalysts. 

Case 1 Case 2 Case 3 Case 4

Pass 1 XLP-220 GR-330 GR-330 GR-330

Pass 2 XLP-110 GR-310 GR-310 GR-310

Pass 3 XLP-110 GR-310 GR-310 SCX-2000

Pass 4 XLP-110 GR-310 SCX-2000 SCX-2000

Pass 4 Inlet Temperature, o

C (oF) 425 (795) 420 (790) 390 (735) 390 (735)

Overall Conversion, % 99.812 99.849 99.935 99.953

SO2 in Stack, ppmv 260 210 90 65

%-Reduction 0% 20% 65% 75%

Basis: 11.5% SO2 3X1 IPA Sulfur Burning Plant 

Energy Savings 

 From the GR-330 prototype dust build-up results above (Figure 2), the pressure 

drop difference between GR-330 and XLP-220 is likely to widen as dust accumulates on 

the catalysts over time. These results indicate GR-330 should provide a greater energy 

savings over time in both pressure drop and extended operating time. Placing either GR330 or GR-310 catalysts in the lower passes will increase further the annual savings from 

the pressure drop advantage of these catalysts. The GEARTM catalyst geometry improves 

catalyst spacing in the converter. Evidence for this improved spacing comes from 

comparative pressure drop curves for the GEARTM catalysts compared to other MECS 

catalyst shapes. The random packing of the GEARTM catalyst is shown in Figure 6. 

Figure 6. Random Catalyst Packing for GR-330. 
13

 The optimization of the GEARTM catalyst shape resulted in a lower pressure drop 

catalyst. One at a time, each of the catalysts in MECS’ portfolio were packed into a 6-

inch vessel and the pressure drop was measured as a function of volumetric flow rate 

over the entire range of gas flow spanned by the Ergun equation (Ergun, 1952). With 

temperature and pressure recorded over the entire flow range (0 to 1000 SLPM), the 

Ergun k1 and k2 coefficients were fit by regression of the pressure drop values. Figure 7 

displays the relative pressure drop against the %-design gas flow rate for LP-110, LP120, XLP, and GR-330 catalysts. 

0%

50%

100%

150%

200%

250%

50% 60% 70% 80% 90% 100% 110% 120% 130% 140% 150%

Percent of Design Gas Flow Rate

Percent of XLP Design Pressure Drop

LP-110

XLP-110

LP-120

GR-330

Figure 7. Relative Pressure Drop for LP-110, LP-120, 

 XLP, and GR-330 Catalysts. 

 The pressure drop advantage of GEARTM catalyst can be more clearly shown by 

calculating the energy savings. A 3300 STPD sulfur burning 3X1 IPA acid plant with 

11.5% SO2 gas strength to pass 1 was used for this example. Table 5 shows the pressure 

drop savings achieved using GEARTM catalyst compared to XLP. With Florida 

electricity costs estimated for industrial use at $0.10/kWh (U. S. Energy Information 

Administration, 2010), the resulting savings from utilizing GEARTM catalyst equates to 

$20,000 per inch of pressure drop saved. 
14

Table 5. Estimated Savings for a Full Converter of GEARTM Catalyst 

 in a 3300 STPD 3X1 IPA Acid Plant. 

Production Rate, STPD 3300

Pass 1 Gas Strength, %-SO2 11.5

Gas Velocity to Pass 1, SLFM 100

GR-330/GR-310 Catalyst Pressure Drop Savings vs. XLP 15%

GR-330 Catalyst Pressure Drop Savings vs. XLP 30%

Estimated Electricity Cost, $/kWh $ 0.10 

Estimated Annual Savings, $/In W. C. 20,000 $ 

 

MECS Catalyst Product Portfolio 

As shown in Table 3, two new GEARTM shapes have been added to the MECS 

catalyst product portfolio. Catalyst screening trial tests of the GR-330 and GR-310 

catalysts showed these products to have the same durability and low screening losses as 

XLP-220 and XLP-110. 

 Additionally, the LP-110 formulation has been enhanced to create a new LP-310 

catalyst product with the same ring size and shape as LP-110 but in a more active 

formulation. The LP-310 catalyst can reduce SO2 emissions or increase acid production 

compared to LP-120, LP-220, and LP-110 in lower velocity converters. 

 

 MECS’ new product portfolio is shown in Table 6. The new products are 

highlighted in red. 

Table 6. MECS New Catalyst Product Portfolio. 

Ribbed Rings Smooth Rings Pellets Cesium

GR-330 LP-310 T-11 SCX-2000

GR-310 LP-220 XCs-120

XLP-220 LP-120 Cs-110

XLP-110 LP-110
15

CONCLUSIONS 

The GR-330, GR-310 and LP-310 catalysts will be available from MECS starting 

in the third quarter of 2011. As the data in this paper have shown, GEARTM catalysts 

provide a range of benefits for improved acid plant performance: lower SO2 emissions, 

increased acid production, extended operating time, and energy savings. Plants with 

lower velocity converters similarly can benefit from increased acid production or lower 

emissions through use of the LP-310 product. 

 Improved GEARTM catalysts are the latest edition to MECS’ portfolio of products 

for the sulfuric acid industry. MECS has technologies in this industry that include 

sulfuric acid plant processes, heat recovery systems for sulfuric acid plant processes, wet 

gas scrubbing, mist elimination, air preheating, corrosion resistant metals for sulfuric acid 

service, engineering and consulting services, and PeGASyS plant analysis. In 2011 

MECS now combines these products and services with the world-class sulfuric acid plant 

knowledge, experience, and process safety management of DuPont. MECS is ready to 

meet customer and technology challenges in the twenty-first century. 
16

REFERENCES 

Ergun, S, 1952. Fluid Flow through Packed Columns. Chem. Eng. Progr. 48(2): 89-94. 

U. S. Energy Information Administration, 2010. Average Retail Price of Electricity to 

Ultimate Customers by End-Use Sector, by State, published online at: 

http://www.eia.doe.gov/cneaf/electricity/epm/epmxlfile5_6_a.xls , January 2011 data 

released 2011 March 11. 

©MECS, Inc. 2011. Information herein is confidential and may not be used by, reproduced for, or revealed 

to third parties, except in accordance with contract or other written permission by MECS, Inc. 

 

The DuPont oval logo and the miracles of science(TM) are trademarks or registered trademarks of DuPont 

or its affiliates.