Clean gasoline
17 Nov 2000
BP claims to have developed a breakthrough gasoline desulphurisation technology that has the potential to significantly reduce the cost of producing low sulphur gasolines to meet current and future environmental legislation.
The process, named the OATS process (or Olefinic Alkylation of Thiophenic Sulphur) produces gasoline blendstocks containing less than 10 parts per million (ppm) of sulphur, significantly below the levels required by current, and most anticipated future, legislation.
The process, developed at a cost of some $10 million primarily at BP's research centre at Naperville, Illinois, USA, will soon be tested at a commercial scale.
Small-scale trials are under way at BP's Texas City refinery in the USA and a large-scale, 6,000 barrels a day trial has been carried out at the Bayern Oil refinery in Bavaria, Germany, a joint venture refinery between BP, Agip and Veba.
If the trials continue to prove successful, up to seven units using OATS could be installed in BP's refineries before 2003.
The OATS process reduces sulphur levels in gasoline without the disadvantages associated with many alternative technologies. It is able to cut sulphur content to very low levels, to below 10 ppm, while consuming only low amounts of expensive hydrogen and without significantly reducing the octane rating, the most important influence on the performance of a gasoline engine.
'The ability to reduce sulphur in gasoline is vital to producing cleaner burning fuels,' says Ptoshia Burnett, BP's OATS project team leader. 'Sulphur in gasoline causes pollution itself, but it also reduces the effectiveness of catalytic converters and so leads to increases in other vehicle tailpipe emissions – carbon monoxide, nitrogen oxides, and unburnt hydrocarbons.'
Graham Butler, BP's manager of new refining technology, adds: 'Although there is always a cost associated with the removal of sulphur from gasoline, we believe that OATS offers the lowest margin loss of all the processes developed to date. It also has several key advantages in its simplicity and flexibility and is easy to retrofit to existing refineries.'
Current typical gasoline sulphur concentrations around the world are in the order of several hundred parts per million. Federal requirements in the USA dictate that average gasoline sulphur levels must be lower than 120ppm and mostly below 30ppm by 2006. In Europe, EU legislation will reduce gasoline sulphur levels to below 50ppm by 2005. The UK is already introducing tax incentives for 50ppm sulphur gasoline and Germany intends to introduce incentives for 10ppm sulphur gasoline in 2003.
Typically, 90 per cent of the sulphur found in gasoline comes from the naphtha stream produced by fluid catalytic crackers at refineries. Over 90 per cent of the sulphur in this stream is contained in compounds called thiophenes. Thiophenes are ring-structure organic sulphur compounds that have boiling points around 85°C. The basis of the OATS process is the conversion of these thiophenes to higher boiling compounds that can be readily removed from the gasoline stream.
The conventional method of removing thiophenes from the stream is hydrotreatment, reacting with hydrogen, which both consumes expensive hydrogen and also results in considerable loss of octane rating.
The OATS process instead facilitates the separation of the thiophenes by catalytically causing them to react with olefins present in the naphtha to produce heavier compounds with boiling points above 200°C.
The higher boiling sulphur fraction is readily removed by fractionation and added to the diesel refinery stream where the sulphur can then be removed by conventional hydrotreatment as gasoline octane rating is then no longer a consideration. In total between one and four per cent of the OATS feed is separated to join the diesel stream, the rest becoming desulphurised gasoline.
This basic OATS process, together with some other refinements, removes 99.5 per cent of sulphur compounds from the gasoline stream with an octane rating loss of only 0 to 2 octane numbers, compared with a typical loss of 6 to 10 numbers through hydrotreatment.
More at www.bp.com
