Apr-2010

Catalytic reforming options and practices

Design and practice in catalytic reforming is evolving to meet refinery challenges, including lower gasoline pool benzene content and increased demand for hydrogen

Tom Zhou Fluor Enterprises
Frederik Baars Fluor BV

Article Summary

Refiners throughout the world utilise catalytic reforming to produce high-octane reformate for gasoline blending and high-value aromatics (benzene, toluene and xylene, BTX) for petrochemical use. Reforming is also a major source of refinery-based hydrogen.

Reforming operations continue to be challenged in the context of lowering gasoline pool aromatic/benzene content; however, the catalytic reforming unit is still a mainstay of refinery operations. The recent upward trend in hydrotreatment needs has put even more emphasis on reformer hydrogen production. The main differences in technology among the various reforming processes are discussed in this article, and special attention is given to chloride control and corrosion management.

Feedstock
The standard feed to a catalytic reforming unit (CRU) is hydrotreated straight-run naphtha (SRN), typically containing C6 through C11 paraffins, naphthenes and aromatics. Naphtha from different sources varies greatly in its ease of reforming. Most naphthenes react rapidly and efficiently to form aromatics. This is the basic reaction of reforming. Paraffins are the most difficult compounds to convert. A rich naphtha (lower paraffin, higher naphthene content) makes the operation much easier and more efficient. The types of naphtha used as feed to the CRU can impact the operation of the unit, activity of the catalyst and product properties. When catalytic reforming is used mainly for BTX production, a C6-C8 cut (initial and final boiling points IBP-FBP 60–140°C), rich in C6, is usually employed. For production of a high-octane gasoline pool component, a C7-C9 cut (IBP-FBP 90–160°C) is the preferred choice.¹

Reformate benzene content can be reduced by minimising the amount of benzene and benzene precursors (cyclohexane and methylcyclopentane) in the reformer feed via prefractionation. Alternatively, the benzene can be reduced by post-fractionation of the reformate and further processing of the light reformate.

In a refinery where maximisation of middle distillate production is a priority, the heavier portion of the naphtha that is traditionally routed to a catalytic reformer unit may instead be sent to the kerosene or diesel pool, within flash point specification limits. In most cases, a lighter CRU feed will result in an increased cycle length for a semi-regenerative (SR) unit due to decreased coke make.

Non-straight-run naphthas (for instance, fluid catalytic cracking (FCC) naphtha or visbreaker/coker naphtha) can also be processed in a CRU, but only after severe hydrotreatment involving (di)-olefin saturation, in addition to the basic naphtha hydrotreater functionality of removing heterogeneous atoms (sulphur and nitrogen). Their higher endpoint and/or higher paraffin content results in a higher coke laydown. Cyclic and continuous catalyst regeneration (CCR) reformers are generally able to process FCC naphtha with a higher feed endpoint as long as regenerator capacity exists to burn the additional coke that is produced.² The reprocessing of FCC naphtha is typically restricted to the lower octane middle cut. If desulphurisation only is required, processing the FCC naphtha in a selective hydrotreating unit is the more straightforward solution.

Fixed-bed units vs CCR reformers
The conventional CRU type is the SR fixed-bed reforming unit, which is used for limited octane improvement. The unit is operated at high pressure to mitigate carbon formation. As carbon laydown increases, reactor temperatures are raised to achieve the target octane at the expense of reformate yield. A cyclic regenerative process with a swing reactor system is used for higher severity and octane operation. With CCR reforming (see Figures 1 and 2)^3,4, extremely high severities are obtainable without frequent shutdowns due to catalyst deactivation. The units operate at a low pressure with the associated yield benefits of higher reformate and hydrogen yields.

The decision to convert high- pressure SR catalytic reformers to CCR-type units hinges entirely on economics.5 Some reforming licensors have developed a hybrid unit, by adding a CCR reactor and regenerator to an original SR reforming unit.^4,6,7,8 Typical examples are shown in Figures 3 and 4. The conversion could cost less than half that of a new CCR and increases throughput and/or cycle length.⁴        

To some refiners, a complete conversion to CCR remains economically attractive relative to a hybrid unit, due to the higher on-stream factor, lower operating pressure, and higher yields of hydrogen and naphtha.9 Virtually all new reforming units are of the CCR design.

Reactor design
There are three types of reactors predominantly in use in the reforming process. These are spherical, downflow and radial. As catalyst improved over the years, the reactor pressure could be reduced to take advantage of the increased C5+ and hydrogen yields at lower operating pressure. At lower pressure, the pressure drop through the reactor becomes an important consideration; therefore, more modern designs of reforming units employ reactors that are radial flow in design and combine good flow distribution with low pressure drop.

The combined feed is directed from the reactor inlet nozzle into so-called scallops, which are long, vertical channels positioned along the entire circumference of the reactor. The scallops have holes or, more commonly these days, profile wire screens along the entire length, through which gas passes radially into the annular catalyst bed and inwards to a centre pipe that collects the reactor products and directs them to the reactor outlet. Low flow should be avoided, as it will result in accelerated coke laydown.   

Reactor metallurgy
Reactor vessels in a SR CRU service are standalone items and can be either hot or cold shell, depending on design preference. In cold-shell designs, an internal refractory lining protects the vessel wall from exposure to the process temperature. In CCR service, the reactors are invariably of the hot-shell design and can be either individually positioned or stacked to form a compartmented single vessel.8 In a SR CRU, a cold wall (carbon steel with refractory lining) with an inner stainless steel liner is the norm.