Legislation enforcing emission control of toxic substances has been passed worldwide. An important subgroup is comprised of the sulfur-based compounds; sulfur recovery units are a critical component of the processes of oil refining and LNG. Fast, accurate, and continuous analytical techniques are necessary to comply with these regulations, and to improve efficiency in sulfur recovery processes.
A variety of recovery processes are normally operated at a single plant; using a single type of analyzer for a majority of applications guarantees better technical support and a higher level of expertise in plant operators.
This paper reviews the utilization of spectroscopy in step with diode arrays, a time-tested, industry-accepted technology for sulfur recovery applications.
One analyzer with different sampling systems can be used for a variety of applications. Common applications include Hydrogen sulfide (H2S), Tail Gas, Natural Gas and Mercaptans.
Petroleum (oil), gas and coal all contain sulfur to some extent, and need to be purified prior to being distributed to the consumer. Table I shows sulfur recovery applications utilizing dispersive Ultra Violet (UV) Absorption techniques. Table II shows Sulfur Dioxide (SO2) emissions from Natural Gas, Oil, and Coal.
Raw natural gas contains, among other impurities, hydrogen sulfide. Gas containing H2S above a certain concentration limit (1%) is known as ‘sour gas.’ The process for removing hydrogen sulfide from sour gas is referred to as 'sweetening' the gas. Gas sweetening commonly utilizes the amine process, in which amine solution absorbs the sulfur from the natural gas. The amine solution is then regenerated by heating and degassing the H2S out, allowing the solution to be reused in the amine absorber. The sour gas coming out of the recycled amine unit contains high amounts of sulfur and requires further processing. This gas is fed into a Claus process resulting in oxidation to elemental sulfur. The sweet clean gas, distributed to the end user – known as ‘sales gas’ – is restricted to an H2S content of 4 PPM.
The Tier 2 emission standard for cars under the Clean Air Act requires that the average concentration of sulfur in gasoline be lower then 30 PPM (the previous limit was 500 PPM). In 2006 refiners were required to produce ultra-low sulfur fuel (ULSD), where the highway diesel fuel limit is lower then 15 PPM.
The global increase in energy demand often requires the industry to exploit resources that contain higher amounts of sulfur. This, in addition to the strict regulations, causes an increase in sulfur recovery processing. The petroleum refiners and natural gas plants need to further optimize processing in a cost-effective way.
Process analysers provide critical information both for process control and for verification of compliance, and are instrumental in this optimization process.
This paper was first presented at the ISA - Analysis Division Symposium in April 2007, by Aaron Rollo, Applied Analytics. Copy right the Instrumentation, Systems and Automation Society. Select this link for the complete paper.
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