Sulphuric Acid

Technologies for Sulphuric Acid Production

Desmet plants are based on different technologies.

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

Several fields of application

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

•  Fertilizers
•  Mining industry
•  Waste gas treatment

Sulphur Sources

The processes that Desmet 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 applicationslike 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 generationsystems to increase theoverall plant efficiency and boost the return on investment.

DuPont^TM MECS^® HRSTM technology to maximize theheat recovery from the plant.

Wide range of production capacities, 2 from small size of 8 TPD for local or very specific applications to large industrial production units up to more than 2,000 TPD.

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 specialalloys.

High yield of conversion, granted by the best available technology together with DuPont^TM MECS^® catalysts allowing for an extended life time, 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.

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) – Desmet technology.
• Single Contact Single Absorption (SCSA) - DuPontTM MECS^® technology.
• Double Contact Double Absorption (DCDA) - DuPontTM 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.

DuPont^TM 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.

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.

Waste Heat Recovery

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

Desmet can offer sulphuric acid plants based on the conventional heat recovery systems as well as on the DuPont^TMMECS^® HRSTMsystem.

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 can be delivered at unit battery limits, according to the plant requirements.

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.

DuPont^TM MECS^® HRSTM

DuPont^TM MECS^® HRS^TM is a system designed to enhance the Sulphuric Acid Plant performances in terms of waste heat recovery.

The system consists of the HRS tower and its ancillaries that replace the interpass absorbing tower and allow the recovery of the heat generated during the interpass absorption reducing thus 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^TM MECS® HRS^TM system, the heat recovered can be increased to 90% of the total reaction heat produced in the Sulphuric Acid Unit.

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 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.

Wet gas technologies
The MECS^® Sulfox 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 Sulfox 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.

When the feedstock is H₂S rich gas, a waste liquid or a spent acid, a preliminary oxidation section (both thermal or catalytic) is added to obtain the SO₂ inlet stream.

The Sulphuric acid concentration from a Sulfox plant depends on the feedstock composition. Typically 96%÷98% concentration is achievable.

MECS^® Sulfox process is an effective alternative to “dry” processes for specific concentration of the gas feedstocks, based on the conditions detailed below.