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.

Proof Point

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.

How hydrowax quality affects ethylene cracker performance

How hydrowax quality affects ethylene cracker performance

The quality of the hydrowax that a hydrocracker supplies, especially its hydrogen content, can have a major impact on the performance of an ethylene cracker. It affects the ethylene and pitch yields, furnace run length and coke make. Taking an integrated approach to the operation of these two units means, therefore, rigorously evaluating how any changes to the hydrocracker’s operation, such as operating conditions or the catalyst system, affect the downstream unit’s performance.

Produce a wider range of products from the ethylene cracker
Figure 1: Typical ultimate product yields for a steam cracker.

3. Produce a wider range of products from the ethylene cracker

As shown in Figure 1, the ethylene cracker’s product yield varies enormously according to the feed, but this can provide a competitive advantage: the more variables the operator has to play with, the greater the opportunity to tune their operations to provide the most profitable product mix.

The flows across the hydrocracker–ethylene cracker interface.
Figure 2: The flows across the hydrocracker–ethylene cracker interface.

4. Respond to feed price variations on a daily basis

There is huge potential value in optimising the flows between the hydrocracker and the steam cracker, which are shown in Figure 2.

By way of an example, consider LPG, a by-product of hydrocracking (and other refinery processes). The refinery could sell it, send it to the petrochemical plant as an ethylene cracker feedstock or burn it as refinery fuel. When the LPG price is high compared with its fuel value, it should be sold. When its value as a cracking feedstock is high compared with its fuel value, it should be sent to the ethylene cracker. However, when there is little difference between the two, then it should be burnt as a refinery fuel.

LPG disposition values as a percentage of their fuel value.
Figure 3: LPG disposition values as a percentage of their fuel value.

However, as shown in Figure 3, the associated economics are highly volatile. To capture this value, a business must monitor this variation and be able to allow for such a three-way disposition. Moreover, the ethylene cracker has to be able to swing between using LPG and other feedstocks.

5. Invest in a grassroots unit to benefit from efficient VGO upgrading

For those considering building a grassroots unit and integrating it with an ethylene cracker, once-through hydrocracking offers the most cost-effective hydrocracking process option on a dollar-per-barrel investment cost basis.

In addition to a lower capital cost, the operator also benefits from the price differential between VGO and naphtha, while consuming only a moderate amount of potentially expensive hydrogen.

Moreover, this configuration provides significant flexibility, as the conversion level can be adjusted during the catalyst cycle to move closer to the desired product slate, although the minimum conversion level depends on the diesel specification that has to be met. For example, facilities that need to achieve Euro V diesel specifications (<10-ppm sulphur content) would typically operate at 50–70% conversion, whereas those targeting Euro IV diesel specifications (<50-ppm sulphur content) could operate as low as 30% conversion. In addition, more significant changes to yield pattern can be achieved by changing the catalyst system in scheduled turnarounds.

Key Takeaways

  • Hydrocracker–ethylene cracker integration is proving to be an important differentiator for the competitiveness of refineries.
  • It also provides significant value to the chemicals business through secure, competitive supply of quality feedstock.
  • Because both the refining and petrochemical sectors are prone to market volatility, the ability to optimise routing and maximise the yield of whichever products are achieving the highest margins at a given time can be a key differentiator.

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