Historically, the main product of the hydrocracker was middle distillates, and the main liquid feed for the ethylene cracker was naphtha. However, the picture today is far more complex. In recent years, several operators have transformed the mode of operation of these two critical units: essentially, they run them as a single, optimised operation and vary the feeds and product slates according to market dynamics. They have reported substantially enhancing their margins through this integrated approach, so we review some of the key opportunities in this article.
1. Adapt existing assets to exploit enhanced economics in petrochemicals
In a conventional refinery configuration, the hydrocracker typically converts vacuum gas oil (VGO) to middle distillates, and the hydrocracker bottoms (hydrowax) are recycled to maximise feed conversion.
In some markets, however, the economics have become more attractive for petrochemicals than for fuels. In this case, there is an opportunity for businesses with a single-stage hydrocracker with a recycle, or a two-stage hydrocracker, to revamp it to a single-stage, once-through operation. The uncracked hydrowax provides a high-quality ethylene cracker feedstock and the hydrocracker’s capacity can be substantially increased for a moderate investment.
Such a change can help to maximise the value to the overall refinery–petrochemicals enterprise rather than just to an individual unit. For more on this, see, Proof point: Revamping a hydrocracker for petrochemicals production.
At its own facilities around the world, Shell has revamped many hydrocracking units and integrated them with ethylene crackers, including at the Shell Eastern Petrochemicals Complex in Singapore, Rheinland in Germany, Pernis in the Netherlands and Norco and Deer Park in the USA.
2. Reduce ethylene cracker feedstock costs
Although cracking ethane produces the highest ethylene yield, its supply is limited and plants have had to expand their feedstock range. Consequently, they have also used liquefied petroleum gas (LPG), naphtha, hydrotreated VGO and, recently, hydrowax. Each time the feedstock gets heavier, the yield of ethylene falls and the amount of undesirable residue increases, but, of course, the feedstock price is lower.
As in refining, the petrochemicals sector experiences its own economic cycles, so, when operating an integrated refinery-petrochemicals business, the key to long-term success is feedstock flexibility.
Hydrocracking has an important advantage here. Because it can put hydrogen selectively into the bottoms product, it can pull back some of the yield loss. The higher the hydrogen content, the higher the ethylene yield and the higher the value of this feedstock. Consequently, many modern ethylene crackers are being designed to process a wide variety of liquid feedstocks, from LPG to naphtha to hydrowax. The quality of the hydrowax is key; for more on this, see : How hydrowax quality affects ethylene cracker performance.
Revamping a hydrocracker for petrochemicals production
Increasing demand for petrochemicals and an enhanced petrochemical margin provided the triggers that led to a Shell refinery changing the mode of operation of a full-conversion, two-stage hydrocracker that was originally geared towards middle distillate production.
The unit, designed in the 1980s, had increased its original capacity by 67% through a series of incremental improvements.
Shell intended to build an ethylene cracker in the adjacent petrochemicals plant to increase its petrochemicals output, so commissioned Shell Global Solutions to help plan the investment and to understand and evaluate its technical and operational options.
It was during this review that Shell Global Solutions’ strategic planners identified that the economics of the refinery and the petrochemicals site could be much better by adapting the hydrocracker to produce large amounts of hydrowax, which could be used as ethylene cracker feedstock.
Working with the refinery’s management, Shell Global Solutions devised a plan to revamp the hydrocracker into parallel, single-stage reactors operating at reduced conversion, but with an increased fresh feed capacity of some 300% of the original design capacity and a catalyst cycle length of two years.
The modifications required, which did not require major capital expenditure, have had a major impact on the economics of the wider enterprise.