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Oct-1998

Sulphuric acid recovery from sulphurous waste streams

The WSA process is a catalytic system for producing concentrated sulphuric acid from any sulphur-containing offgas, without the use of chemicals

Ole Rud Bendixen, Haldor Tops√łe
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Article Summary
Sulphur recovery has for many years been an integral part of a refinery or a petrochemical plant, and is in most cases based on production of elemental sulphur from sulphur-containing gas. Depending on quality of the sulphur produced, the major part is in turn converted into sulphuric acid, which is the largest bulk chemical in the world with a production of more than 150 million tons per year.

Of this, about 60 per cent is used for production of phosphoric acid and phosphate fertilisers, another 10 per cent for production of TiO2 pigments, and the remaining 30 per cent for numerous purposes, such as the metal processing industry, leaching of non-ferrous ores and the production of some textiles and chemical fibres. A well-known example within refineries is the use of sulphuric acid for alkylation and purification of certain distillates.

The WSA (Wet gas Sulphuric Acid)  process, developed by Topsøe in the 1960s, is for the treatment of hydrogen sulphide-containing offgases under direct production of concentrated sulphuric acid. The original focus was dilute streams, which were otherwise difficult to treat effectively by sulphur-producing plants.

In the 1960s and 1970s many of the dilute streams were incinerated and emitted without further treatment. However, as the environmental requirements have gradually been tightened, in the mid-1980s the company introduced a novel version of the WSA technology that made it possible to treat practically any sulphurous offgas or spent sulphuric acid under production of a concentrated sulphuric acid (93–-98 wt%).

As described above, the acid can be utilised within the refinery or petrochemical plant or sold directly or via acid brokers to other industrial sulphuric acid users. With today’s surplus of sulphur, sulphuric acid may be a more valuable by product than elemental sulphur.

There are currently a total of 32 commercially licensed WSA units, among others, for the treatment of offgas from hydrotreaters, gasifiers and Claus tail gas as well as for onsite regeneration of spent sulphuric acid.

The WSA process is a catalytic process producing 93–98 wt% acid from a sulphur-containing offgas or from spent sulphuric acid without initial or intermediate drying of the gas. All compounds such as hydrogen sulphide, carbonyl sulphide, mercaptans, carbon disulphide or spent sulphuric acid are first oxidised into sulphur dioxide by thermal or catalytic combustion, followed by oxidation into sulphur trioxide. The SO3 is recovered as sulphuric acid in a condenser/concentrator.

This article focuses on applications and examples within the refining and petrochemical industry, such as for treatment of H2S-containing offgas from a hydrotreater or a gasifier, treatment of spent sulphuric acid from alkylation or desulphurisation of an FCC offgas.

Chemical reactions
The chemical reactions taking place in a WSA plant are shown in Table 1, on previous page. Notably all reactions are exothermic except for the decomposition of spent acid into sulphur dioxide and water, which requires energy supplied by combustion of support fuel as LPG, oil or possibly hydrogen sulphide.

The SO2 oxidation reaction is an equilibrium reaction, and in order to reach conversion of SO2 into SO3 of up to 99.3 per cent, it is necessary to divide the catalytic bed into more adiabatic steps with cooling in between. The principle is shown in Figure 1, on previous page. The hydration reaction is also an equilibrium reaction; however, it is very fast and no catalyst is needed. The hydration increases with decreasing temperature, and takes place at 450 to 200°C.

Selective condensation of concentrated acid takes place by cooling of the process gas to approximately 100°C in the WSA condenser (described later).
 
Process layout
Typical cases from licensed units for the refining and petrochemical industry are presented in the following:
Concentrated H2S gas from a hydrotreater in Russia.
Lean H2S gas from a BTX refinery in Germany.
Spent acid from an alkylation plant in Taiwan.
Lean H2S and SO2 gas from a Claus tail gas in North America.
Lean H2S gas from a gasifier in Sweden.
Waste gas from an RFCC regenerator in Korea.

The process layout for the above applications may be grouped into three:
- High concentration H2S gas and spent acid
- Lean H2S gas
- Flue gas

High concentration H2S gas and spent acid
The process comprises a number of process steps, as shown in Figure 2.
All sulphurous compounds, such as hydrogen sulphide or spent sulphuric acid, are first combusted into sulphur dioxide in an incinerator at about 1000°C. Due to the SO2/SO3 equilibrium, about 0.4 per cent of the SO2 will be oxidised into SO3. The hot incineration gas is cooled by generation of steam, typically at 40 bar, in a steam boiler and then passed into the catalytic SO2 reactor at a temperature of around 410°C.
The oxidation of SO2 to SO3 takes place in a converter, utilising Topsøe catalysts, and up to 99.3 per cent of the SO2 is oxidised into SO3 and recovered as acid. In case an even lower emission is required, an H2O2, caustic or ammonia tail end scrubber is introduced whereby the total sulphur recovery may reach 99.95 per cent.
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