Process for reduction of content of sulphur compounds and polyaromatic hydrocarbons in distillate fuels

Abstract

A process for reducing content of sulphur compounds and polyaromatic hydrocarbons in a hydrocarbon feed having a boiling range between 120° C. and 450° C., which process comprises in combination contacting the feed and hydrogen over a hydrotreating catalyst and hydrotreating feed at hydrotreating conditions, cooling the effluent consisting of hydrocarbon, polyaromatic hydrocarbons, hydrogen sulphide and hydrogen-rich gas from the hydrotreating reactor, contacting said effluent and hydrogen gas over a hydrotreating catalyst in a post treat reactor at a temperature appropriate to lower the polyaromatic hydrocarbon content.

Description

[0001] The present invention is directed towards the improvement of distillate fuels. More particularly, the invention relates a process for reducing concentration of sulphur and polyaromatic compounds in those fuels.[0002] Many countries are in the process of tightening specifications for the sulphur content of diesel fuels. For example, current legislation specifies a maximum sulphur content in diesel in the European Union of 50 wppm from the year 2005, and in California 15 wppm from 2004. Current specifications for low sulphur diesel fuel in many cases also include limits for the maximum content of polyaromatic hydrocarbons, for the maximum density (or specific gravity) and for the minimum cetane number. It is expected that the specification for all three properties will be tightened further in the near future to meet requirements for reduced emissions from diesel engines.DESCRIPTION OF PRIOR ART[0003] Sulphur can be removed by means of hydrotreating. The diesel fuel is passed ov...

AI Technical Summary

Benefits of technology

[0010] The present invention is a process for achieving a low PAH content in distillate streams with only slight additional investment in reactor volume and without reduction in run length. The essence of the inventive process consists of cooling the effluent exiting the hydrotreating reactor and passing the cooled product through a small post treat reactor containing a suitable catalyst. The PAH content of the product exiting the hydrotreating reactor is reduced in the post treat reactor owing to the more favourable equilibrium conditions at the lower temperature. As a consequence, the end of run temperature in the main hydrotreating reactor is not limited by the PAH content of the product exiting the main reactor, and a lower overall reactor volume (main hydrotreater plus post treat reactor) is required for a given run length. In another embodiment of the process the final bed of the main hydrotreating reactor is operated at lower temperature instead of using a post treat reactor. The process can be used to lower the density and raise the cetane number of the diesel product. Since the densities of PAH compounds are in general higher than the corresponding monoaromatic compounds, lowering the PAH content of the product also lowers the density of the product. In the same way, the cetane number and cetane index of PAH compounds are lower than the corresponding monoaromatic compounds, and reducing PAH content results in an increase in cetane number and cetane index.

Problems solved by technology

A shorter run length means high costs due to a higher rate of catalyst replacement, and relatively more downtime (i.e. time off-stream) for catalyst change-out with a resultant loss of revenue due to reduced diesel fuel production.

The reactions are reversible and at high reaction temperatures and low hydrogen pressure the conversion of the PAH compounds is thermodynamically limited by equilibrium.

This is clearly unacceptable and would necessitate considerable investment in extra reactor volume or construction of a new unit at higher hydrogen pressure in order to maintain run length.

Thus, increasing pressure or reducing LHSV can extend the run length, but both measures require costly investments.

Since the densities of PAH compounds are in general higher than the corresponding monoaromatic compounds, lowering the PAH content of the product also lowers the density of the product.

During operation, the catalyst must be in a sulphided condition and removal of H.sub.2S from the effluent by phase separation between the two reactors is not desired.

This will result at some point in an increase in the PAH content in the effluent of the hydrotreater reactor due to equilibrium limitations.