Jan-2009

Clean diesel hydrotreating

Design considerations for clean diesel hydrotreating. Critical issues are discussed when designing a hydrotreating facility to produce diesel fuel with very low levels of total sulphur

Ed Palmer, Stan Polcar and Anne Wong
Mustang Engineering

Article Summary

For over three decades, refiners worldwide have been implementing various projects in their facilities to accommodate a variety of regulations to improve the quality of transportation fuels in order to reduce vehicle emissions. One of the key areas of interest has been the reduction of sulphur in diesel fuel to very low levels. For example, since mid-2006, the maximum sulphur content of on-road diesel fuel in the US has been limited to 15 wppm. Similar regulations are in place or are in various phases of implementation in many countries.

Design considerations
Middle distillates contain various types of sulphur species, including mercaptans, sulphides, thiophenes and aromatic sulphur compounds. Sterically hindered dibenzothiophenes are a group of aromatic sulphur compounds that are among the most difficult to remove when hydrotreating to very low sulphur levels. This is particularly true for diesel fuels that contain significant quantities of cracked stocks, such as FCC light cycle oil (LCO), which contains a large concentration of aromatic sulphur compounds. The effective removal of these species requires tailored catalysts and process conditions, as well as consideration of other factors such as feed nitrogen content and aromatics equilibrium.

There are numerous issues to be addressed in the design of a hydrotreater, including:
—  Feed characteristics and variability
—  Other product quality requirements, especially cetane index
—  Catalysts selection
—  Optimisation of reactor process variables
—  Equipment design requirements
—  Reliability
—  Minimising product contamination
—  Handling of off-spec diesel product.

All of these factors should be carefully considered during the front-end process design.

Process flow
Figure 1 shows a simplified process flow diagram for a diesel hydrotreater. Fresh feed from the surge drum is heated with stripper bottoms, then mixed with recycle hydrogen. The combined feed is further heated by reactor effluent, then brought to reactor inlet temperature in the charge heater. Reactor inter bed quench may be required in one or more locations, depending on the volume of cracked stocks (FCC LCO, light coker gas oil) in the feed. This flow diagram shows recycle gas being used as quench. Reactor effluent exchanges heat with the combined feed and flows to the hot, high-pressure separator (HHPS). Vapours from the HHPS are used to heat recycle gas and stripper charge before being cooled in the reactor effluent air cooler (REAC) and entering the cold, high-pressure separator (CHPS). Wash water is injected upstream of the REAC to remove ammonium bisulphide. HHPS liquid is combined with heated CHPS liquid and flow to the product stripper.

Vapours from the CHPS are contacted with amine in a scrubber for hydrogen sulphide removal and flow to the recycle compressor suction drum. Make-up hydrogen is compressed and combines with the recycle gas in the suction drum. Some of the recycle gas may be purged from the compressor suction drum to improve the purity of the recycle gas hydrogen. If needed, the quantity will depend on the reactor hydrogen partial pressure requirements and the purity of the make-up hydrogen.

In the product stripper, superheated steam is introduced into the bottom of the tower to effect the removal of hydrogen sulphide. Stripper overhead vapours are condensed and flow to the stripper accumulator. Accumulator vapour and liquid (wild naphtha) are processed in other off-site facilities. The stripped diesel product is used to heat the feed, it is cooled, then it flows to drying facilities (not shown). This could be a coalescer/salt dryer or a vacuum drying system.

Feed and product characteristics
Sulphur, nitrogen and aromatics content are the most important feed characteristics that impact the process design for diesel hydrotreating facilities. The nitrogen content of the feed has a significant impact on the required operating pressure for a new design. Nitrogen has to be removed to essentially the same level as sulphur to reach the ultra-low target. This means the catalyst employed and the hydrogen partial pressure selected must be consistent with a high nitrogen removal operation. Normally, the bulk of the feed nitrogen is contained in light coker gas oil and FCC LCO. The aromatics content of the feed will govern the chemical hydrogen consumption at the low space velocities and high hydrogen partial pressures required for very low sulphur diesel production.

Generally, cracked stocks can be included in the feed up to the level limited by the product cetane index or gravity without having a significant impact on hydrotreater design. There is a small increase in the gravity and cetane index during the hydrotreating reaction. If a significant improvement in cetane (say, three to five units or more) or gravity is required, a multi-stage design using aromatics saturation catalysts in the second stage may be the more economical option. The final choice will be driven by the magnitude of the improvement in gravity and cetane required.

Obviously, the design product sulphur target is a key issue, not only from a process design standpoint, but also for off-site storage and transfer. For the ultra-low sulphur diesel (ULSD) programme in the US, most new and revamp facilities have been designed for a product sulphur content of 8–10 wppm to ensure a final product sulphur content of 15 wppm at the point of sale.

Pilot testing of the feed is practically mandatory to confirm reaction process conditions. Testing for variations in feed characteristics, especially FCC LCO and coker light gas oil back-end distillation, should also be considered, because the separation achieved in the products fractionators from these facilities is notoriously poor. This can result in a temporary spike in the content of the most difficult-to-treat sulphur compounds in the hydrotreater feed and requires an increase in reactor temperature. This will increase the catalyst deactivation rate and, hence, directionally reduce the cycle length.