Science Behind Technology

sulph 04 11 2020

Technologies for Sulphuric Acid Production

Desmet Ballestra plants are based on different technologies:

  • DuPont MECS® Single Contact Single Absorption or Double Contact Double Absorption leading technology for large integrated H2SO4 plants
  • Proprietary know-how (which uses DuPont MECS® catalyst and key components) Single Contact Single Absorption process for low capacity plants (up to about 200 TPD)

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Sulphuric acid is widely used in different business areas such as:

  •  Fertilizers
  •  Mining industry
  •  Waste gas treatment


The processes that Desmet Ballestra makes available can produce sulphuric acid from several sources:

  • Elemental sulphur, based on dry air combustion (conventional process)
  • Exhaust SO2 gas, coming from roasting/smelting of pyrites, copper, zinc, lead, nickel ores and similar
  • Spent acid or sludge obtained from alkylation processes
  • H2S and SO2 off gas from various other chemical processes

Possibility to produce high quality H2SO4 for special applications like battery grade, analytical grade, electronics and others.

Option for production of Oleum as well as liquid SO2 and liquid SO3.

Waste heat recovery by steam production, with steam turbine power generation systems to increase the overall plant efficiency and boost the return on investment.

DuPont MECS® HRSTM technology to maximize the heat recovery from the plant.

Wide range of production capacities, from small size for local very specific applications to world scale production units.

Compact plant layout, for investment cost optimization (e.g. piping and duct routing), taking into account safety/maintenance/operation principles and according to customer site requirements.

Air pollution control system, to contain the plant emissions to the minimum level required by the most stringent laws and standards.

High quality construction materials and use of acid resistant special alloys.

High yield of conversion granted by the best available technology together with DuPont MECS® catalysts allowing for an extended lifetime, low pressure drop and low screening losses.

Conventional Process (Dry route) Principle

Process technology is based on the production of sulphur dioxide (SO2) by sulphur burning using dry air, followed by catalytic conversion to produce sulphur trioxide (SO3) which is finally absorbed in water (H2O) to obtain sulphuric acid (H2SO4)

All the above reactions are extremely exothermic at high temperatures and therefore the recovery of the heat generated during the process is highly valuable.

The typical converter configuration is based on 4 stages of catalytic conversion with the possibility to increase by 1 stage and/or to use a combination of vanadium/cesium based catalyst when lowest emissions are required.

Single or Double absorption

According to the requested production capacity, the selected conversion yield, the specific plant requirements, flexibility, startup time, the plant can be designed for:

  • Single Contact Single Absorption (SCSA) - Ballestra technology
  • Single Contact Single Absorption (SCSA) - DuPont MECS® technology
  • Double Contact Double Absorption (DCDA) - DuPont MECS® technology

The conversion factor for a Ballestra SCSA plant is 98.5%, typically requiring a tail gas scrubber to control the SO2 stack emissions. This plant is specifically designed to achieve a very fast startup, thus granting a high operational flexibility.

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DuPont MECS® DCDA are designed for higher conversion factor, typically >99.8%, thus granting SO2 emissions at stack within the limit of 280-ppmV without the need for a tail gas scrubber.

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Minimization of gaseous emissions

Improvement of SO2 stack emissions can be achieved by a combination of the following:

  • Cesium-based catalyst instead of Vanadium-based ones.
  • 5 stages catalytic conversion.
  • Dedicated Acid Tank and cooler for Final Absorption Tower.

The solutions above allow to have SO2 emissions at stack within the limit of 100 ppmV (equivalent to a conversion factor over 99.92%) without the use of a tail gas scrubber.

Emissions control: DuPont MECS® SolvRTM

SolvRTM is a proprietary DuPont MECS® technology for selective removal of SO2 from exhaust gases that can achieve nearly zero emissions, with a regenerative SO2 recovery.

The system is composed of four different sections:

  1. Dynawave® Humidifying Tower to quench the tail gas and remove H2SO4 and HCl
  2. SO2 absorbing tower to lower gas SO2 content down to 20 ppmv by solvent extraction
  3. SO2 stripping tower to recover dry SO2 from the solvent
  4. solvent regeneration to remove sulphates and residual acidity. Solvent is recycled back to process

The SolvRTM unit can be retrofitted into existing plants, due to its modular design. Plot space is limited and compact, solvent losses are minimal due to low vapour pressure and regeneration performed at atmospheric pressure.

Waste Heat Recovery

Energy recovery is extremely important in the economics of new sulphuric acid plants.

Desmet Ballestra can offer sulphuric acid plants based on the conventional heat recovery systems as well as on the DuPont MECS® HRSTM system.

The heat recovery system of a conventional sulphuric acid plant recovers most of the heat produced during the sulphur combustion and the SO2 → SO3 conversion, producing 1.2 ÷ 1.3+ tons of MP superheated steam (at 25 ÷ 42 bara and about 400°C) per ton of sulphuric acid.

The steam produced can feed a turbogenerator for electric power production or it can be delivered at unit battery limits, according to the plant requirements.

Electric Power Production

Turbogenerator units can be condensing steam turbines, to maximize electric power generation, or backpressure steam turbines, to still have exhaust steam available as utility at unit battery limits.

Intermediate pressure level injections and extractions are possible to satisfy specific requirements.

DuPont MECS® HRSTM

DuPont MECS® HRSTM is a system designed to enhance the sulphuric acid plant performances in terms of waste heat recovery.

The system includes the HRS tower and its ancillaries that replace the interpass absorbing tower and allow the recovery of the heat generated during the interpass absorption thus reducing the heat that would have been lost to cooling water. As a result, additional LP saturated steam is produced and, as a collateral benefit, size of cooling water system (e.g. cooling towers) is reduced together with relevant consumptions.

With DuPont MECS® HRSTM system, the heat recovered can be increased up to 90% of the total reaction heat produced in the Sulphuric Acid Unit.

 Conventional plantHRS plant
P steam ton / H2SO4 ton 1.3+ 1.3+
HRS steam ton / H2SO4 ton 0 Up to 0.40 ÷ 0.48
Total Heat Recovered 70% approx Up to 90% approx


Oleum and SO3 Production

The Oleum and the SO3 production are strictly connected to the H2SO4 production technology.

Liquid SO3 is produced by evaporation of Oleum and subsequent cooling and condensation.

Oleum can be produced by absorbing part of the SO3 leaving the sulphuric acid converter in a circulation of Oleum at the required strength and maintaining a steady concentration by adding sulphuric acid at 98.5% w.

This absorption is carried out in a dedicated Oleum tower installed immediately upstream the absorption tower.

Typical grade of Oleum is at 20÷35% or 65% free SO3.


Liquid SO2

Desmet Ballestra preferred design for liquid SO2 is based on the cryogenic condensation process.

The feed is an SO2 rich gaseous stream produced in a sulphur furnace that is fed to an absorption tower for SO3 entrainments removal. Then the gas is sent to a chilling group for SO2 condensation. The condensed fraction flows to battery limit, while the uncondensed gas stream is sent to a SO2 converter for H2SO4 production.
The cryogenic condensation process is strictly connected to the H2SO4 production, hence the unit can be a stand-alone plant or a secondary product package unit installed within a large scale H2SO4 production plant.

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Wet gas technologies

The MECS® SULFOXTM wet gas technology is an alternative process to the conventional dry sulphuric acid route.

This process is designed to produce H2SO4 acid from SO2 or H2S off-gas, spent acids, sulphates regeneration feedstocks or organic wastes bearing sulphur.

The SULFOXTM technology can effectively process the wet gas without gas drying upstream of the reaction section.

The core process consists of a reaction section that converts the wet stream of SO2 into a wet stream of SO3, followed by a column that condenses H2SO4. A final mist precipitator grants low acid mists emissions at the stack.

The sulphuric acid concentration from a SULFOXTM plant depends on the feedstock composition. Typically, 96%÷98% concentration is achievable.

Sulphuric Acid from off-gas

Off-gas is produced from essentially any metallurgical process; it contains SO2 gas that can be transformed into sulphuric acid, reducing emission levels as required, but also cutting energy and water use, enhancing plant economics.

The technology can handle off-gas with high levels of impurity including arsenic, fluorides and mercury and/or high SO2 concentrations.

A first stage includes gas cleaning sections that remove both liquid and particulate matters to prevent corrosion and fouling of the acid plant. The gas is then dried and compressed, heading to the SO2-SO3 conversion system.

When off-gas is rich in SO2 a pre-treatment is included, converting a portion of the high strength gas to SO3 ahead of the main converter, thus reducing CAPEX while maximizing energy recovery.

The solutions that Desmet Ballestra can propose are based on DuPont MECS® technology, including proprietary equipment (e.g. DynaWave®) and the worldwide experience and references on the specific application.

Tail Gas Emission Control

Desmet Ballestra plants based on DuPont MECS® double absorption technology fully comply with the most stringent international standards and laws and therefore, in principle, do not need additional gas pollution control systems. In case of special requirements, or when the single absorption technology is foreseen, a tail gas scrubbing system is provided.

Using a tail gas scrubber SO2 stack emissions within the limit of 10 ppmV can be achieved.

Desmet Ballestra can supply different scrubbing technologies, including the DuPont MECS® DynaWaveTM system, based on different reagent media:

  • Sodium Hydroxide NaOH
  • Hydrogen Peroxide H2O2
  • Calcium Hydroxide Ca(OH)2
  • Calcium Carbonate CaCO3
  • Magnesium Hydroxide Mg(OH)2
  • Ammonia Solution NH4OH
  • others