Technology - WSA
February 19, 2003

Introduction

Associated Links

---

Introduction

The Wet Sulphuric Acid (WSA) process was developed by Haldor Topsøe A/S in the mid 1980’s.  The process can be used to treat a variety of gas streams containing sulphur compounds such as SO₂, H₂S, CS₂ and COS generated in the petrochemical, cellulose, metallurgical, power generation and mineral industries.  These streams typically contain or result in a gas containing relatively low concentrations of SO₂ which would be difficult to treat in a conventional contact sulphuric acid plant.

The main features of the process are:

• 95 to 99% removal and recovery of the sulphur content
• Commercial grade sulphuric acid is produced
• Recovery of process heat
• Low consumption of utilities such as cooling water
• No waste products of waste water
• Simple layout
• Wide turndown capability

The WSA process treats the waste gas without the requirement to remove the water from the gas as in a conventional contact sulphuric acid plant.  Therefore, no cooling/condensation step is required nor is there a requirement for a sulphuric acid drying tower.

If the feed gas contains H₂S, CS₂, COS, etc. it is first incinerated to convert all sulphur compounds to SO₂.  The gas is then heated or cooled as the case may be to the required inlet temperature of the converter.  The converter contains Haldor Topsøe’s VK-WSA catalyst which has been specially developed for this application.  The SO₂ undergoes conversion to SO₃ in the presence of the catalyst.  Single or multiple beds are used depending on the SO₂ concentration and the degree of conversion required.  In a multi-bed arrangement inter bed cooling can be done in a number of different ways depending on the heat balance of the plant and the requirement to recovery energy from the process.

At the exit of the converter the gas is cooled which allows the SO₃ formed to react with the water vapour to form sulphuric acid in the gas phase.

SO_{3 (g)} + H₂O _(g) -> H₂SO_{4 (g)} + 101 kJ/Mol

The cooled gas enters the WSA condenser which condenses the sulphuric acid gas to form the liquid product.  The WSA condenser is a vertical shell and tube falling film condenser/concentrator with tubes made of boronsilicate acid and shock resistant glass.  The process gas flows up the tubes and is cooled by ambient air circulating on the outside of the tubes.  Sulphuric acid condenses in the tubes and flow downward counter-current to the rising hot process gas.   This contact with the hot process gas concentrates the acid to the desired product acid concentration.

The acid collects in the brick lined lower section of the WSA Condenser where it is pumped out and cooled before delivery to storage.

The process is easily adapted to handling gases containing impurities such as NOx.  A Selective Catalytic Reduction (SCR) reactor can be located prior to the SO₂ converter to treat the NOx.  Ammonia is introduced into the gas stream upstream of the SCR reactor in a stoichiometric amount to the NOx in the gas.  The NOx is converted to nitrogen and water according to the reaction:

NO + NH₃ + ¼ O₂ -> N₂ + 3/2 H₂O + 410 kJ/Mol