CATALYST: The core of the sulfuric acid manufacturing plants.

In the sulfuric acid process, the converter is one of the main equipment (Figure1) in where sulfur dioxide (SO2) gas is converted into sulfur trioxide (SO3) via the below reaction: SO2 + ?O2? --? SO3 +??Heat

The specific conditions and applications in which the catalyst is used can have a significant impact on its longevity and effectiveness. Here are some factors that can affect the life of vanadium or cesium catalysts:

-??????? Moisture contact: can damage the active salts and decrease the hardiness. Humidity can be generated from non-efficient drying atmosphere air in the drying tower, or due to internal leakage in steam circuit (boiler, economizers, or superheaters).

-??????? Dust : can be accumulated in the catalyst due to non-efficiency of air filters, or when the quality of sulfur requires filtration before entering the furnace. This dust has the potential to plug the catalyst pores leading to a reduction in activity and conversion rate SO2/SO3.

-??????? Acid mist contact: impacts negatively the catalyst active sites, affects the integrity due to corrosion by sulfates or vanadium oxides formation.

-??????? Arsenic: can contaminate and poison the catalyst surface by absorbing it in the active sites.

-??????? Operating Conditions: Deviations from the design operating parameters (mainly high temperature) can lead to degradation of catalyst supports (alumina or silica) and affecting the overall structure.

How to raise the conversion yield of your sulfuric acid plant catalyst?

According to the equilibrium curve in figure 2, as the temperature increases, the equilibrium shifts toward the production of more SO?.

To achieve the highest conversion rate, it is necessary for the process to cool down the gas at the inlet of each converter bed, aiming for the maximum deltaT.

Some steps or tricks to consider for achieving the highest conversion yield of SO2 to SO3:

1-???? Catalyst conditions:

-??????? The catalyst must be in excellent condition concerning its shelf life, form, and reactivity.

-??????? A good-quality catalyst exhibits a yellow/gold color. Any alteration in color to green, blue, or dark blue signals the need for replacement. (figure 3 & 4)

-??????? After screening during Turnarounds, introduce a new catalyst if there is a need for make-up due to losses.

Generally, the screening cycle of each bed:

• 1st bed: every turnaround,
• 2nd bed : usually every 1 or 2 turnarounds (or in case of unexpected issue)
• 3rd bed: usually every 2 turnarounds (or in case of unexpected issue)
• 4th bed: usually every 2 turnarounds (or in case of unexpected issue or damage due to acid mist carryover from the intermediate absorption tower)

-??????? Prevent the presence of moisture within the gas circuit by optimizing the efficiency of the drying tower and avoiding internal water leaks in steam exchange equipment. (boiler, economizers, superheaters)

-??????? Ensure the upstream liquid sulfur filtration, before the combustion process in the furnace. In certain instances, it may be advisable to install a gas filter ahead of the converter.

?2-???? Heat exchange efficiency:

-??????? Ensure optimal efficiency in heat exchange for all heat exchange equipment, (boilers, economizers, superheaters, and gas heat exchangers)

-??????? Chemical cleaning can be performed to get rid of internal dusts or sulfates. But it can not be performed every TA due to negative impact on the metallurgical integrity. So Careful selection of cleaning agents and controlled application conditions are crucial to minimize the risk of corrosion during the cleaning process.

-??????? Minimize heat wastage from radiation or convection by insulating all gas equipment as per the design specification.

-??????? Typically, introduce the cesium catalyst in the 4th bed, either entirely or partially in the upper layer, at approximately 20%.

Prepared by : Oussama CHEDAD.