Jan-2010

Choosing quench interbed technology

A design for hydroprocessing interbed internals favours separate mixing of gas and liquid phases before contacting of the two phases occurs

Ed Schouten, Jan Stolwijk and Ed Ouwerkerk, Shell Global Solutions International
Sujatha Degaleesan, Wenhua Yang and Sandro Mazzini, Shell Global Solutions

Article Summary

The performance of a hydroprocessing reactor is determined not only by the loaded catalyst but also by the design of its internals. The internals positioned at the interbed have the combined function of:
• Remixing reaction fluids in order to eliminate radial maldistribution
• In-mixing of liquid and/or gas quench streams
• Redistribution of gas and liquid over the catalyst bed below.

A wide range of different types and designs of equipment has been developed. Assessment of the validity of the different designs against the functionalities listed above has been hampered by insufficient understanding of the mixing process and consequential uncertainty in scale-up from small-scale laboratory experiments to industrial operation with reactor dimensions over 5m in diameter. Furthermore, the feedback loop from the implementation of 
these designs to actual practice is often long.

The Ultra-Flat Quench (UFQ) technology is based on separate mixing of gas and liquid phases. This technology combines the advantage of centrifugal mixing 
for gas with the efficiency of impingement for liquid mixing. It provides a superior and well-described mixing process that is robust in a wide range of operating conditions and dimensions.

The Shell Group has developed, tested and optimised this technology as the owner/operator of hydro-processing units and as a leading licensor of these technologies over 
a significant number of years. Multiple case studies show the actual performance of the technology. Further illustration of the advantages of UFQ designs is offered by advanced computational flow dynamics (CFD) in industrial conditions. CFD is also used to optimise quenching and mixing between catalyst beds, with opti-mum reduction in temperatures.

Catalytic hydroprocessing
Catalytic hydroprocessing is a technology that has been applied for more than 50 years to upgrade hydrocarbon streams. The reactions are typically carried out in a co-current, adiabatic, fixed-bed reactor. Recent advances in catalyst technology, including the Centera catalysts supplied by Criterion, have made it possible to optimise the operation of these reactors and to meet tight new specifications.

The performance of such reactors is determined not only by the loaded catalyst, but also, to a large extent, by the design of their internals. In the last 15 years, considerable attention has been given to the issue of maldistribution in gas/liquid 
(G/L) applications: insufficient distribution of gas and liquid 
inside the reactor leads to, for instance, under-utilisation of the catalyst and the formation of local hotspots. This has detrimental effects on catalyst cycle length, product quality, unit reliability and process safety. In a number of articles,1-5, 13 successful revamps have been presented, showing how the installation of state-of-the-art internals has had positive effects 
on these parameters, as well as providing an economic way of debottlenecking units for refiners, including Naftan, Repsol, Preem, Norco, North-Atlantic and Petrobras.

The focus of these articles has often been on the distribution tray at the top of the catalyst bed. Shell Global Solutions’ HD trays have been able to improve the performance of units by effective distribution of gases and liquids over the catalyst bed. The robustness of these distribution trays against a wide range of operating conditions, combined with a high tolerance against tray tilt and a boltless, weldless and ergonomic design, has made these trays a desirable choice for refiners in revamps and for new units, with more than 1500 trays supplied to customers in the last 
ten years.

Besides the Shell Global Solutions HD tray (marked “0” in Figure 1), 
a number of other internals contribute to the success of these revamps and grass-root units (see Figure 1).

This article focuses on the quench internals for fixed-bed reactors to:
• Show the importance of well-functioning quench internals
• Provide insights into the quench mixing process obtained from laboratory work, advanced CFD techniques and field experience
• Formulate the main criteria for quench equipment and assess different quench concepts against them
• Demonstrate the superior performance of UFQ technology in multiple case studies.

Quench interbed internals
The catalyst in hydroprocessing units can be separated into multiple catalyst beds to limit temperature rises and limit the total bed length. Typically, the temperature increase per bed is limited to 30–40°C, and the bed length is 3–6m for hydrocrackers and 12m for hydrotreating units.7 Quench zones are positioned between beds, facilitating the addition of quench gas and/or liquid to the reaction medium. These quench flows 
play a key role in ensuring product quality. They contribute to process safety and provide a built-in capability to handle a variety of feeds so that the feedstock slate can be adjusted in response to market conditions.

Conventionally, cold quench hydrogen is introduced to reduce the reaction temperature, improve product quality and reduce catalyst deactivation. Increasingly, cold liquid quenches are applied that have higher heat capacities and so more easily lower the reactor temperature, while not increasing the (gas) compression cost. How-ever, this is at the expense of an added quench oil pump. The choice between gas and liquid quench is mainly dictated by the availability of the quench stream, the overall economics of the process, and product quality and production requirements. In a few cases, a combination of both a gas quench and a liquid quench can be applied.

In all cases, high-performance internals are required in the interbed in order to:
• Thoroughly mix a hot process stream with a cold quench stream
• Remove any radial temperature and concentration maldistributions in the liquid and gas entering from above
• Distribute the gas and liquid streams evenly over the catalyst bed below.