Oct-2001

Getting the maximum from hydroprocessing reactors

Recent advances in hydrotreater catalyst formulations mean that, with careful attention to distributor design and other mass transfer improvements, it is possible to effectively overcome reactor volume limitations.

Justin Swain, Criterion Catalyst and Technology Company
Marja Zonnevylle, Shell Global Solutions International
Dieter Pohl and Franz Geerdes, SRS Salzbergen

Article Summary

The majority of hydroprocessing reactors in operation today were designed and constructed in the early to mid 70s. As product yields and qualities have changed, many refiners have leveraged advances in catalysis to avoid major plant investments. Admittedly, the step changes in environmental legislation or in market demands for speciality products such as lubricants and white oils has required refiners to make capital investments in major process equipment, thus upgrading the units to operate at higher capacity and greater efficiency.

In many such upgrading projects, the reactor vessel, which is the heart of the hydroprocessing unit, is left undisturbed because of the general view that costs for modifications or reactor replacement become prohibitive. The ultimate goal must be to identify immediate and potential future bottlenecks that can be easily addressed.

Since the 1970s there have been tremendous advances in reactor design, internal distribution systems, catalyst loading and grading techniques. These advances are designed to increase the catalytic reactor’s capacity within limits of the existing shell. However, a detailed evaluation of the performance and design of the existing reactor system, coupled with a thorough review of reactor upgrading options, is required to cost effectively realise the system’s full potential.

The documented experience that is discussed further into this article shows how a detailed evaluation performed by a facility’s engineers revealed an under-performing hydroprocessing reactor. Working closely with facility staff members, existing reactor design and operating constraints were identified. This promoted a detailed desk study leading to identification of several possible solutions and associated benefits. Careful and deliberate selection and implementation of the most cost effective options resulted in a significant increase in unit capacity and flexibility.

Objectives
Over the years, the services that refiners have outsourced have ranged widely, from advice on top bed catalyst grading strategies – to alleviate pressure drop limitations – through to licensing and design of new process technologies, to produce products such as less than 5 vol% total aromatic diesel.

The general view towards upgrading hydroprocessing units seems to be that additional reactor volume is the solution to activity-limited units. In addition to their significant expense, there are extremely long lead-times for new reactor vessels, requiring refiners to incur considerable delays before the benefits are attained. However, it has been demonstrated on numerous occasions that another alternative can be available for activity-limited hydro-processing units.

Preferably, an alternative to an expensive investment in a new reactor should permit substantial increases in unit capacity and operational flexibility at a fraction of the investment costs and with implementation within a matter of months (typically during the next planned shutdown). This alternative option revolves around serious consideration of revamp opportunities within the existing reactor shell, specifically regarding the choice of catalysts and reactor internals design.

With this approach, the performance of an existing reactor was increased by over 50 per cent, with the refiner’s demands being fully satisfied within the existing reactor shell and avoiding the previously determined need for integrating an additional reactor vessel. This performance improvement warrants a more detailed investigation, including:
- Presentation of the latest advances in reactor and internals design and implementation philosophy, and the benefits these bring to the refining industry, the primary focus being on distribution tray technology
- Description of Criterion’s approach to hydroprocessing reactor-improvement opportunities (with direct reference to the experiences of a recent project at the SRS Salzbergen refining location, in Germany)
- Information resources necessary to facilitate benchmarking existing reactors.

Benchmarking performance
Once the hydroprocessing reactors with potential for significantly improved performance have been identified, a small team of experts from the refinery, and appropriate catalyst and reactor engineering experts, continue to prove most effective in determining the best revamp option for the reactor to achieve the optimum performance.
In such studies, installation of new reactor internals can be considered as a cost effective means to improve unit performance by:
- Allowing more catalyst to be loaded into the existing vessel – new reactor internals are more compact, or more effectively located
- Increasing utilisation of catalyst inventory by ensuring more uniform gas/liquid distribution, reducing risk of thermal maldistribution, and by reducing impact of fouling on cycle length.

Maximising catalyst volume and maximising catalyst utilisation by means of improving reactor internals are considered separately in the following two sections. The primary focus is on the distribution tray technology.

Maximising catalyst volume
Each and every hydroprocessing reactor vessel must give up a certain percentage of the available reactor volume to reactor internals at the expense of the primary catalyst. Proper selection and design of reactor internals will maximise the mixing of the reactants (gases and liquids) and enable efficient use of the primary catalyst.

Poor selection and design of reactor internals can lead to not only a significant and unnecessary loss of reactor volume but also to significant under-utilisation of the remaining catalyst volume. There are many types of reactor internals that can be applied in trickle flow reactors, such as:
- Inlet device; fouling collection tray;  primary liquid and gas distribution tray
- Catalyst support beams and screens; interbed quench/mixing tray(s) plus re-distribution tray(s)
- Grading materials for pressure drop control; grading materials for liquid re-distribution
- Catalyst support balls; outlet device.