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Jan-2012

Oxygen enrichment in desulphurisation

Oxygen enrichment technology is a viable and a cost-effective solution for significantly increasing sulphur treatment capacity

Shivan Ahamparam and Stephen Harrison
Linde

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Article Summary

As a primary source of sulphur emissions, the 
refining segment of the petrochemical industry has found itself juggling fears around energy insecurity with concerns about climate change. Sulphur dioxide (SO2) is one of the major air pollutants that impact our climate and is a key focus for the United Nations and environmental activists. It can be harmful to health, as it is a potent asthma trigger and can cause other potentially damaging respiratory health effects. When sulphur combines with oxygen to create SO2, it is defined as a critical air pollutant by the US Environmental Protection Agency (EPA) and can form dangerous sulphates, which can be breathed deep into the lungs. Once oxidised by air, it also forms sulphuric acid, the major component of acid rain. Acid rain harms fish, damages forests and plants, and can erode buildings.

SO2 is formed when sulphur-containing fuels, such as coal and oil, are burned. The primary sources of SO2 emissions are power plants, refineries and smelting facilities. SO2 is also found in the exhaust of diesel fuel and gasoline. Despite technological advances, controlling sulphur remains a technical challenge for the petrochemical industry, as the sulphur content of the world’s dwindling crude oil resources is increasing.

Almost every oil field produces crude with a unique mixture of characteristics, which presents distinct challenges to oil companies involved in separating crude into different products. In addition to sulphur content, refineries are being challenged to manage increased levels of acid gas or sour water stripper gas and the occasional lean acid gas feed.

For refiners, throughput can be limited by the speed at which plants can desulphurise crude. However, the more stringent the desulphurisation process becomes, increasing Claus plant loadings with hydrogen sulphide (H2S) and ammonia, the more frequently bottlenecks in the production process also become. Claus plants operating in refineries process concentrated H2S fractions, converting them into elemental sulphur. The technology is also able to destroy pollutants, particularly ammonia.

Increased capacity
Although not new, oxygen enrichment technology has now come to the fore as a viable and cost-effective solution for significantly increasing a plant’s sulphur handling capacity, as well as addressing problems associated with contaminants such as ammonia and hydrocarbons.

Oxygen enrichment of the combustion air significantly increases sulphur handling capacity. Associated benefits include increased productivity achieved without changing the pressure drop, more effective treatment of ammonia-containing feeds and less effort required for tail gas purification (reduced nitrogen flow). Oxygen enrichment is also a highly customisable approach to improving Claus plant yield, with options varying from low-level oxygen enrichment to employing advanced proprietary technology to bring about capacity increases of up to around 150%.

In practical terms, this means that refineries can delay new Claus investment decisions as they can extend their existing Claus plant capacity. This is a particular advantage to those refineries whose plant footprints cannot accommodate the introduction of additional Claus plants.

Low-level enrichment is achieved by injecting oxygen via a diffuser into the process air to the sulphur recovery unit. The maximum oxygen enrichment level that can be accommodated via this method is 28% and provides a capacity increase of approximately 30% when processing acid gas rich in H2S, as is the case in most oil refineries.

Generally, the sulphur plant will require no equipment modification other than the provision of a tie-in point for oxygen injection into the combustion airline.1 However, when even greater capacity is needed and increased levels of oxygen beyond 28% are required, it is necessary to introduce the oxygen into the reaction furnace separately from the air supply, as the combustion air piping in conventional sulphur plants and air-only burners is unsuitable for use with highly oxygenated air.

Self-cooled burner
Addressing this challenge, a new type of burner, Sure, developed by Linde Gas, has been specifically designed for this purpose — a self-cooled tip-mix burner with separate ports for acid gas, oxygen and air supply. The burner can be used in both end- and tangential-fired furnace designs. The burner achieves effective mixing of H2S and oxygen-enriched air over a wide load range.

The intensive mixing characteristics of these burners have been developed through test work at Linde’s own pilot plant, a commercial-scale sulphur recovery unit, harnessing computational fluid dynamics (CFD) modelling to achieve effective contaminant destruction and significantly increased tonnage output.

For operation with high levels of oxygen enrichment, greater than 45%, methods must be employed to mitigate a high flame temperature in the reaction furnace. The Sure double combustion process provides full capability at up to 100% oxygen in an uncomplicated process that is easy to install, operate and maintain.

Double combustion
Double combustion, as the name implies, splits the heat release into two separate reaction furnaces with cooling between. In the first reaction furnace, all amine gas, sour water stripper gas and, if required, air are fed to the Sure burner together with the supplied oxygen, the level of which depends on plant throughput. The tip-mix burner allows for thorough mixing, giving effective contaminant destruction efficiencies.

There is no sulphur condenser between the first waste heat boiler (WHB) and the second reaction furnace. Also, there is no burner in the second reaction furnace. By design, the gases exiting the first WHB and entering the second reaction furnace are substantially above the auto-ignition temperature of H2S and sulphur vapour, under all normal and turn-down operation conditions. This system allows for a low pressure drop, which is easy to control and easy to install.


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