Distillate mild hydrocracking
Figure 1: In practice, no more than 5–10% of the XHGO can be routed to the DHT and the unit will be unable to achieve more than a 5–7°C T95 shift. Consequently, most of the XHGO and the LVGO have to be sent to the hydrocracker and/or fluidised catalytic cracking (FCC) unit, which constrains their intake.

A consequence of processing at higher-than-design crude rates is the generation of additional feed requiring significant upgrading to finished products. Two streams that are particularly challenging are the extra heavy gas oil (XHGO) and light vacuum gas oil (LVGO) fractions. In recent years, refiners have explored cost-effective ways of capturing maximum value from these feeds without affecting refinery economics. One approach that is generating a lot of interest – and substantial additional margin – is to revamp a distillate hydrotreater (DHT) into a distillate mild hydrocracker (DMHC).

There are several options for managing XHGO and LVGO streams. Blending directly into the fuel oil pool without upgrading clearly fails to capture maximum value. Routing to the hydrocracker potentially constrains the capacity of this critical unit for costly yield losses of both the normal and incremental feedstock. Lastly, processing in the DHT does not enable sufficient shift in the distillation tail to meet ultra-low-sulphur diesel specifications.

Figure 2: A DMHC can handle much greater quantities of XHGO, and LVGO
Figure 2: A DMHC can handle much greater quantities of XHGO, and LVGO can also be added (up to cold-flow property limits). Compared with a DHT, the T95 shift is much higher, and there is an improvement in other product properties. Furthermore, this helps to relieve the intake constraints on the hydrocracker and/or FCC.

To address the challenge, Criterion Catalysts & Technologies (Criterion) has been helping refiners around the world to develop an alternative solution: revamping a high-pressure DHT into a DMHC. In comparison with a conventional DHT, a DMHC delivers a step change in T90+ shift with benefits to density, cetane and distillate recovery. Mild hydrocracking is a low-cost solution to upgrading existing and incremental XHGO and LVGO material at a high margin without affecting hydrocracker profitability, as shown in figures 1 and 2.

Cracking the heavy tail in a DMHC requires a highly customised catalyst solution that promotes ring opening, an advanced chemical upgrading technique. Such a solution does not come off the shelf, however. As with an aromatic saturation solution, a mild hydrocracking solution is devised through close collaboration with the catalyst provider and the unit licensor to apply the right catalyst solution and enable reliable operation with full product recovery. Selecting mild hydrocracking requires an economic evaluation that takes into account the relative value of diesel and cracked naphtha in a refiner’s specific market.

Figure 3: The tailored catalyst solution delivered a substantial increase in T95 shift for Tüpraş.
Figure 3: The tailored catalyst solution delivered a substantial increase in T95 shift for Tüpraş.



The Turkish company Tüpraş, one of the world’s most competitive refiners, recently revamped an 80-bar dewaxing unit into a DMHC at its 5-Mt/y Kırıkkale refinery in order to process heavier feeds and increase diesel yield. This has had a major impact on the unit’s profitability, writes Ersen Ertaş, R&D Manager, Tüpraş.

“We were originally processing HGO and meeting the T95 ultra-low-sulphur-diesel specification of 360°C. To improve margins, we were keen to route XHGO to the unit, but needed a solution to achieve the required T95 shift.

“We worked with Shell Global Solutions and Criterion to evaluate revamping the unit into a DMHC. This work included dedicated pilot plant testing and thermal stability reviews.

“The solution we devised was a customised catalyst system. No hardware changes were necessary. It features a latest-generation, high-activity NiMo catalyst from Criterion and a zeolite cracking catalyst from Zeolyst International to crack the heavy tail. As the latter catalyst also has a tendency for dewaxing, we could still meet our winter diesel specifications.

“As a result, we achieved a significant T95 shift of base (the average of the 2008 cycle) plus 16°C (Figure 3). We have also benefited from a large density improvement, and therefore volume gain, and high diesel recovery. We estimate that this revamp has enhanced the unit’s profitability by between $6.5–9.1 million a year, depending on the product prices.”

Key Takeaways

  • XHGO and LVGO are high-margin but difficult-to-process feeds.
  • Processing XHGO and LVGO in a hydrocracker may not be the most economic solution, but the T90 or T95 specifications limit how much can be processed in a DHT.
  • A DHT can be revamped at low cost into a DMHC to process XHGO and LVGO. In this way, the T90 or T95 specifications can be met and substantial margin can be captured.

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