The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

S Malevu

________________________________________________________________________ 

Page 113 

J ACID PLANT CAPACITY INCREASE 

Siyabonga Malevu 

Chemical Initiatives Umbogitwini 

 

1. INTRODUCTION

Chemical Initiatives’ Umbogintwini plant is the production facility of Chemical 

Initiatives’ business. Umbogintwini’s J Plant manufactures sulphuric acid. The plant was 

designed and built by Simon Carves, and was commissioned in 1974. 

The plant was designed to produce 500 tons/day of Sulphuric acid (H2SO4) at maximum 

rates. The Sulphuric acid manufacturing is through the famous contact process whereby 

liquid Sulphur is injected through dried air in a burner operating at 1000 o

C to form 

sulphur dioxide (10.5% SO2). The SO2 gas is then cooled in waste heat boiler no.1 before 

being fed to a four pass converter which provides 99.5% SO2 conversion to sulphur 

trioxide (SO3). The SO3 gas is the absorbed by 98.5% H2SO4 through the inter-pass & 

final tower to make more concentrated acid and dilute it with water back to 98.5% H2SO4

in acid pump tanks and dispatched to storage tanks. 

The manufactured Sulphuric acid is supplied to a variety of industrial manufacturing 

customers includes Pulp & Paper industry, Mining industry, Chemical manufacturing 

industry etc. 

During the 1st quarter of the year 2007, Chemical Initiatives undertook a study to up rate 

the sulphuric acid plant output by 10% (i.e. from 500 tpd to 550 tpd). 

This was to be achieved by the replacement of the converter fourth pass Monsanto catalyst (LP 

110) with Haldo-topsoe cesium based catalyst (VK 69). The simulated analysis yielded an 

increase from 500 tpd to 547 tpd by the addition of 30,000 litres of HT VK69 in the fourth pass 

as well as increasing the volume of LP 110 from 20,000 litres to 25,000 litres in the third pass. 

The simulation study revealed that this setup had a potential to increase the SO2 conversion 

from 99.5% to 99.79% and therefore providing an extra 47tons/day of Sulphuric acid. 

The project proposal included: 

• Replacement of the LP110 catalyst with VK69 TOPSOE catalyst from the 4th

pass of the converter. 

• Increasing the 4th pass VK69 TOPSOE catalyst loading by 26% (from 22 

000litres to 30 000 litres) 

• Increasing the 3rd pass LP110 catalyst loading by 18.6% (from 20 350 to 25 

000litres) 

• Running the air blower to maximum rate of 47 500m3

/hr (from 45 000m3/hr. 

Other process parameters that needed to change in order to achieve desired plant 

throughput included: 

• Increasing sulphur feed rate from 6.8 to 7.4 tons/hr. 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

S Malevu

_____________________________________________________________________ 

Page 114 

• Operating SO2 burner at 1025 oC generating 11% SO2 (from previous 10.5% 

SO2), 

• Maintaining gas inlet to the fourth pass of the converter below 405 o

C (from 

below 435 o

C). 

2. ADVANTAGES OF SWITCHING TO VK69

VK69 is designed specifically for the final pass(es) of double absorption plants, and the 

unique VK69 formulation offers unmatched high activity throughout the entire operating 

temperature range. 

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. 

Benefits 

The VK69 offers a number of benefits 

• 50% reduction in SO2 emission from existing double-absorption plants 

• possibility for significantly increased production without increasing SO2 

emissions 

• low ignition temperature for faster and smoother start-ups 

• long lifetime and low screening losses 

3. IMPACT ON MAIN PLANT EQUIPMENT 

3.1 AIR BLOWER 

• The design maximum capacity of the blower is 47 500m3

/hr at a discharge 

pressure of 47kPa. 

• The blower is currently running at discharge pressure of 42kPa, providing 

a gas flow of 43 000m3

/hr 

From the blower curves, it was established that the blower would be able to provide 47 

500m3

/hr at 46.5 kPa. This could be done by increasing the blower vanes angles to 

maximum (i.e. 10o

). The previous setting of the blower vanes angle was 50o

(approximately 50% opening). 

3.2 SULPHUR BURNER 

Sulphur consumption would have to increase from 6.8tons/hr to 7.4tons/hr and the air 

flow would also increase as specified above. The burner temperature would also increase 

from the current 1000o

C (10.5% SO2 content) of 1025OC, generating 11% of SO2 content. 

SULPHUR Burner Capacity: GLTA Engineering Company, the original designers of 

the acid plant was consulted regarding the capacity of the Sulphur burner and it was 

reported that “the internal volume of the burner is large enough for the acid plant to be 

increased by 20%. 

Sulphur Gun: The current sulphur gun is an atomising type of spray gun which operates 

with sulphur pressure of 400 – 700kPa. The possibility of sulphur carry over from the 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

S Malevu

_____________________________________________________________________ 

Page 115 

burner to the air inlet duct of the drying tower could arise when increasing the sulphur 

inlet flow however this could be minimized/eliminated if the burner temp is accurately 

controlled at permissible level (i.e. 1025 OC) 

3.3 WASTE HEAT BOILER NO.1 

The simulated 1st pass converter inlet temp was 430o

C and prior to the upgrade the 1st

passing was operated at 433o

C. However there is a flexibility of shutting off the boiler by 

pass completely to enable maximum SO2 gas cooling from WHB #1. The boiler bypass 

prior to the upgrade was 25% open. 

Due to the burner temp increase from 1000 to 1025 OC, the amount of steam generated 

from the WHB #1 would increase by 1.8tons/hr. 

3.4 WASTE HEAT BOILER NO.2 

The simulated 3rd pass converter inlet temp was 445o

C and prior to the upgrade it was 

running at 461o

C. The boiler bypass which used to be fully opened would have to be 

partially shut to enable the boiler to remove the additional extra heat load. 

3.5 HOT & COLD HEAT EXCHANGERS 

The heat exchangers were redesigned for extra heat load and a lower pressure drop during 

the year 1998. Because of that modification, there’s approximately 2.2kPa allowance in 

additional pressure drop. 

3.6 CONVERTER 

With the proposed mod of changing the 4th pass type catalyst from LP110 to VK69 and 

increasing the catalyst loading from a 3rd and a 4th pass, a 99.79% conversion shall be 

achieved. The previous conversion was 99.5%. 

 Converter capacity: 

As per GLTA Engineering Company, the 4th pass capacity could accommodate a 

maximum amount of 30 300litres. As part of the performance upgrade, the catalyst 

volume of the 4th pass would be increased from 22 000 to 30 000litres. 

 

The 3rd pass catalyst volume would be increased from 20 350 to 25 000 liters and the 

maximum capacity is 25 100litres. 

Pressure drop: The predicted/simulated 4th pass pressure drop was 0.79kPa and the preupgrade pressure drop was 0.5kPa. 

Thermocouples: The thermocouples would also be repositioned in the converter shell to 

accommodate the increased in catalyst depth. 

3.7 DRYING, INTERPASS AND FINAL TOWERS 

GLTA Engineering Company also declared the towers capacity as sufficient to handle a 

10% increase in gas flowrate. 
The Southern African Institute of Mining and Metallurgy 

Sulphur and Sulphuric Acid Conference 2009 

S Malevu

_____________________________________________________________________ 

Page 116 

3.8 STRONG ACID COOLERS 

The acid temperature to the absorption coolers is allowed to rise to 80o

c and prior to the 

upgrade the acid temperature was running at 71o

c, well below the maximum design temp. 

4. ECONOMIC BENEFIT

The capital expenditure plant upgrade was R1.735m and the main benefits from the 

project are the additional 47 ton/ day and 43.2 ton/ day of sulphuric acid and steam 

production, respectively 

The payback period for the project was calculated at 2.8 months. 

5. RESULTS

Production trends (Aug 07-Dec 08) 

450

470

490

510

530

550

570

Aug-07Sep-07

Oct-07Nov-07

Dec-07

Jan-08

Feb-08Mar-08

Apr-08

May-08

Jun-08

Jul-08

Aug-08Sep-08

Oct-08Nov-08

Time (months)

H2SO4 (tpd)

The modifications in the converter catalyst were done during the August 07 turnaround 

shutdown and prior to the upgrade the production rate was averaging 477 tpd. After the 

upgrade, the plant production rate increased to an average of 533 tpd. The reason for not 

constantly producing at 547 tpd is the failure of maintaining the 4th pass inlet temperature 

below 405 0

C without compromising the desired export steam temperature at the boiler 

station. Plans are in place to reduce heat losses on the export steam line. 

6. CONCLUSION

Average production prior to implementation 

: 477 tpd 

Target production after implementation 

 : 547 tpd 

Average production achieved after implementation 

 : 533 tpd