Jul-1998

Catalytic distillation to enhance gasoline quality: Part II

European regulations covering gasoline quality standards have moved on since Part I of this article. With further reductions in sulphur levels now likely, a comparison of catalytic distillation and conventional technology in benzene reduction is discussed

R M Foley, K L Rock, A Bakshi, W Groten, G Gildert, D Weidert and T McGuirk, CDTech

Article Summary

In February the European Parliament voted to support mandatory improvements in fuel quality levels in the European Union. Final fuel specifications are still subject to a two month conciliation, at the time of writing. However, gasoline specifications will include significant reductions in olefins, aromatics, benzene, and sulphur relative to both current typical European gasoline properties and the original European Auto-Oil Programme (EPEFE) proposals (Table 1).

These additional reductions in gasoline sulphur specifications increase the need for the FCC gasoline desulphurisation process discussed in the previous issue. Pool gasoline sulphur levels between 40 and 50ppm can be achieved by fully utilising that catalytic distillation (CD) process to reduce full range FCC gasoline sulphur levels to 100ppm, or less. Olefin, aromatics and benzene reduction processes utilising CD are discussed below. Refinery capital investments to meet these new specifications can be minimised using the CD technologies.

European Union-supported diesel fuel specification changes are included here for reference, and these include significant reductions in sulphur and polycyclic aromatics relative to the EPEFE proposal (Table 2). CD processes for sulphur and aromatics reduction in diesel range products are under development and will be discussed in the future.

Benzene reduction
The principal source of benzene in gasoline is reformate which accounts for 50  to 90 per cent of total benzene at various refineries. The secondary source is normally light straight run gasoline (LSR). In most cases, benzene reduction is achieved by dealing only with the reformate, although depending on the specific refinery benzene balance, treating the LSR may be necessary to meet benzene level specifications. With the European Union restrictions, treating reformate alone may not be sufficient to meet the 1 per cent benzene specification. There are several options for reducing benzene in reformate.

Precursor fractionation
This is the least costly approach and will be the primary solution for refiners who only need a small benzene reduction. Different potential solutions are:
Existing naphtha fractionator. Change tower operation to take more of the feed as distillate. This reduces the C6 content of the reformer feed, thus reducing benzene in the reformate. However, some benzene is still produced from dealkylation of heavier aromatics in the reformer. Also, any benzene contained in the LSR passes into the gasoline pool unless the LSR is isomerised. So overall total benzene reduction is limited.

When the C6 fraction is taken overhead, some C7 goes with it because the naphtha fractionator is not normally designed to make a sharp split. As a result, less aromatics are produced and additional octane is lost. Hydrogen production is also reduced by this method.

Modified or new naphtha fractionator. With new tower internals, it may be possible to upgrade the separation capability of the existing tower. This modified tower, or a new naphtha fractionator designed for a sharper split, can reject the C6 fraction with minimal carryover of C7. This approach will reduce the octane and hydrogen loss of the previous option but at increased capital cost.

Isomerisation of LSR. If the LSR is sent to an isomerisation (Isom) unit, changing the naphtha fractionator split affects the operation of the unit. The higher feed rate to the Isom increases space velocity, thus reducing conversion and lowering the isomerate octane. Increased content of benzene in the Isom feed results in hydrogenation of the benzene and releases a significant heat of reaction.

The resulting temperature rise in the Isom reactor reduces the extent of the isomerisation reaction and causes a loss of octane in the isomerate. In addition, the light C7s in the LSR are branched chains and tend to crack in the Isom reactor. As a result, the C7s are converted to fuel gas rather than gasoline. Higher temperature in the Isom reactor also ages the catalyst faster and reduces catalyst life.

The cyclohexane contained in the C6 fraction inhibits the paraffin isomerisation in the Isom reactor, resulting in a decrease of the isomerate octane number.

Recovering benzene for petrochemical feedstock
In order to preserve hydrogen, some refiners may fractionate the reformate to produce a C6 heartcut which contains most of the benzene. The benzene can then be removed in a benzene extraction unit and the C6 raffinate returned to the gasoline pool. However, the market has limited potential to absorb additional benzene.

If much benzene is extracted from gasoline, the result could be a considerable reduction in benzene value, making this option undesirable.

New reformate splitter. Unless an existing reformate splitter was designed to produce a C6 heartcut, it is unlikely to make the required split. The impacts of this operation are reduced gasoline volume and the associated loss of octane barrels.

Existing benzene extraction. The availability of an existing benzene extraction unit, either within the refinery or located close by, presents the refiner with a relatively low cost option for recovering benzene and returning the remaining C6s to the gasoline pool. Of course, this assumes there is spare capacity in the extraction unit.

Significant capital cost may be required to expand capacity. If the extraction unit is not located in the immediate vicinity, restrictions on shipping the benzene heartcut may increase the cost of transportation and make this option unattractive. The shipping cost for the return of the C6 raffinate must also be considered.