Apr-2016

Revamping for ULSD production

A sandwich catalyst system has given the additional activity needed to process difficult feed in a hydrotreater.

MIKE ROGERS, Criterion Catalysts & Technologies
KIRIT SANGHAVI, Alon USA Refining

Article Summary

Alon Big Springs refinery and Criterion Catalysts have worked for over 10 years to achieve an impressive track record of successes in their distillate hydrotreater, boosting diesel capacity and production by up to 20%, and greatly improving unit flexibility. With the current strong market for diesel, this gain has allowed them not only to track their product demands, but it has dramatically boosted profitability for their shareholders. The sustained collaboration between Alon and Criterion has been a crucial factor in the success of the refinery.

In the early 2000s, Criterion worked with Shell Global Solutions to find a cost-effective way to bring Alon’s distillate hydrotreater unit to ULSD production. Low operating pressure, difficult feed and limited hydrogen availability were serious challenges. The refinery chose to install new reactor vessels with reactor internals from Shell Global Solutions. The catalyst was Criterion’s first-generation Cobalt-Molybdenum ULSD product.

In the years following its successful transition to ULSD production, Alon continued to work closely with Criterion. The challenge was to extend the catalyst life cycle and increase straight run diesel (SRD) and light cycle oil (LCO) cut points for improved refinery economics. The refiner needed to push the cycle from about nine months to at least 12 months in order to synchronise with the semi-regen reformer regenerations. Increasing demand for diesel was putting pressure on the refinery to raise the cut points of their straight run and LCO to maximise diesel yields. In 2010, Alon loaded a first-generation Centera sandwich system, which gave a large boost in activity and enabled significant gains towards increasing the unit’s flexibility and profitability.

In 2014 Alon implemented a revamp in the crude distillation unit. One of the main objectives of that revamp was to boost the straight run diesel yield, which would increase available feed to the ULSD unit for higher diesel production. Again, the success of this project was crucial for the refinery to increase profitability in the face of shifting market demand. Criterion’s second-generation Centera sandwich system with DC-2635 CoMo and DN-3636 NiMo has given the additional activity needed to process the additional difficult feed and at the same time maintain catalyst cycle flexibility and once again contributing to Alon’s success.

US refiners shift towards increased diesel production
Most refineries in North America and other regions in the world were designed and built 40 or more years ago, and in order for them to survive they have had to adapt continually to shifting fuels markets. The major recurring themes in these shifts have been tighter environmental requirements (lower sulphur and so on) and a push towards higher energy efficiency, particularly in transportation fuels. In the Americas, the recent arrival of shale oil with higher yields of naphtha has also had an impact on the fuels market by contributing to gasoline oversupply.

Figure 1 shows average gasoline and diesel prices in the US since 1979. The diesel-gasoline price gap has been relatively stable over most of the period, with some notable exceptions. In the late 1970s/early 1980s, major improvements in automobile fuel efficiency pushed gasoline demand downwards, which had the effect of closing the long-standing gap between the two transportation fuels prior to that time. In 2006, the introduction of ULSD specifications for on-road vehicles marked the beginning of an upward trend in diesel pricing, but the major shift in the price gap which occurred in the 2000-2010 period was driven largely by European and other countries that recognised the higher energy efficiency of diesel over gasoline as a transportation fuel. European governments implemented various forms of vehicle taxation and fuel incentives that favoured diesel powered vehicles. The effect was dramatic. In 1990, diesel powered cars in Europe represented about 13.8% of demand, while by 2009 that proportion had increased to 46%. European refiners have been unable to keep up with the demand shift ever since, and as a result, exports of diesel to Europe as well as imports of gasoline from Europe have grown, thus further driving the price gap in the US. In recent years, the arrival of domestic shale oil production has pushed the price gap even further apart, due to the higher proportion of condensates and naphtha that are produced from those operations. Today, practically all North American refiners are pushing to maximise their diesel and kerosene production. The resulting increase in yields of diesel range material in the refinery has increased feed rates and feed difficulty in many ULSD units.
 
Maximising distillate production
Over the past 20 years or more, refiners have been making large efforts to maximise diesel production by optimising cut points, improving fractionation efficiencies and driving their conversion units to maximise diesel range material. In 2010, the US Energy Information Administration (EIA) released a paper1 on this subject which outlines a range of measures that refineries have implemented to follow this trend.

In a typical refinery, diesel and distillate products are made from blends of a range of refinery streams (see Figure 2). The distillate hydrotreater (ULSD unit) is an essential and critical element to diesel production since it has the role of transforming the lower quality diesel range material from the crude unit, the FCC and the coker into finished diesel components. As the production of diesel range material in those upstream units is increased, the DHT unit must have the ability to handle that additional load in terms of higher throughput and higher catalyst activity.
 
Fractionation improvements in the crude unit for higher diesel yield
The yield of diesel and kero range material from a crude distillation column is adjusted by changing the boiling range (cut points) of the draw stream. Increasing the cut point range affects properties of the stream such as the heaviness and the cold flow properties.

Lowering the ‘front end’ of the boiling range brings naphtha range material into the kero jet. This light material lowers the flash point of the kerosene, which is limited by the final product specifications. Raising the ‘back end’ of the distillation will bring vacuum gas oil range material into the diesel, affecting the cold flow properties and gravity as well as bringing in heavier sulphur and nitrogen species that are more difficult to treat.

In the FCC fractionation column, lowering the front end cut point of LCO will shift heavy gasoline into the LCO. This affects not only the flash point, but it also lowers the diesel cetane index. The heavy end of the LCO contains not only difficult sulphur and nitrogen, but also polyaromatic species, which have a large impact on the performance of the ULSD catalyst.

Poor or inefficient fractionation causes overlap of the distillation cuts and will negatively affect important properties such as flash point and cloud point. It limits the ability to expand the cut point range because the property constraints (flash, cold flow and so on) are reached sooner. Improving product separation is therefore an important aspect of maximising diesel yield.
 
Diesel/ VGO separation
Poor separation between the diesel and VGO cut will reduce diesel yield in two ways:
1. Diesel left in the VGO by inefficient separation will result in those molecules going with the VGO to FCC feed. They are subsequently cracked to light naphtha in the FCC reactor.
2. When VGO molecules flash and are recovered in the diesel cut (as ‘over-flash’) the cold flow properties are affected, and it is often necessary to reduce the cut point (by reducing the diesel draw rate) to correct for this effect.