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Mountain high, ocean deep: part two

Hydrocarbon Engineering,

Read part one of this article here.

Removing sulfur within the refinery gates

Shipping operators essentially have two options when looking at methods to reduce harmful emissions. The first is the adoption of lower sulfur content fuel where the focus is very much on technology to desulfurise the very heavy and sulfur laden crude at the refinery.

Almost every oilfield 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 sulfur 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 desulfurise crude. However, the more stringent the desulfurisation process becomes, increasing Claus plant loadings with hydrogen sulfide and ammonia, the more frequently bottlenecks in the production process also become. Claus plants operating in refineries process concentrated hydrogen sulfide fractions, converting them into elemental sulfur. The technology is also able to destroy pollutants, particularly ammonia.

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

Oxygen enrichment of the combustion air significantly increases sulfur 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 approximately 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 sulfur recovery unit. The maximum oxygen enrichment level, which 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 sulfur plant will require no equipment modification other than the provision of a tie in point for oxygen injection into the combustion airline. 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 sulfur plants and air only burners are unsuitable for use with highly oxygenated air.

Addressing this challenge, a new type of burner, SURE™, has been specifically designed by Linde Gas 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 fired and tangential fired furnace designs. The burner achieves excellent mixing of hydrogen sulfide and oxygen enriched air over a wide load range.

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

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

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 excellent contaminant destruction efficiencies.

There is no sulfur condenser between the first waste heat boiler (WHB) and the second reaction furnace. In addition, 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 autoignition temperature of hydrogen sulfide and sulfur vapour, under all normal and turn down operation conditions. This system allows for low pressure drop, which is easy to control and easy to install.

The result of this type of control is a temperature profile ideally suited to the Claus process. Operating temperatures in the first reaction furnace are high enough to destroy ammonia and hydrocarbons, but remain well below refractory limitations. KOA Oil in Japan has successfully harnessed Linde’s double combustion process since 1990.

A novel approach has used the benefits of a multi pass WHB for plants with restricted footprint. The zone between the first and second passes of the boiler is utilised as the second reaction furnace of the double combustion process. In this situation, lances are installed in the channel head connecting the first and second pass of the WHB tube sheets (where the remaining oxygen can be added). For the optimum design and location of the SURE burner and oxygen lances, Linde uses a validated CFD model. This particular approach has been operational at API Falconara, Italy, since 1996 and at Shell, Puget Sounds, and General Chemicals Anacortes.

The change out of the WHB can improve energy efficiency at a plant through the generation of valuable high pressure steam. Other energy efficiency benefits arise from the much reduced process gas flow through the plant. This reduces the converter reheat and incinerator fuel gas requirements to a minimum, and reduced energy requirements mean significantly reduced carbon dioxide emissions.

Read part three of this article here.

Written by Stephen Harrison and Ismail Erilhan, Linde Gases. This is an abridged version of an article taken from Hydrocarbon Engineering’s August 2015 issue.

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