Increase in Octane Rating

The response of the petroleum industry to an economically practicable catalytic reforming process has thus been enormous, since the particular forte of the processes is greatly to increase the octane rating of naphthas over that which can be secured readily and economically by other refinery processes. For a time, non-platinum catalyst reforming processes enjoyed popularity because of the cheaper catalyst they employ and the relative insensitivity to poisons. This is no longer an advantage because increased by-product hydrogen availability has encouraged refiners to install facilities to clean up contaminated charge stocks, and thus protect the platinum catalyst.

Most of the reforming processes other than Platforming employing platinum catalysts utilise regeneration in situ to prolong catalyst life. Because the catalyst does not require it, Platforming is unique in not employing a separate regeneration system. This is one of the reasons why Platforming requires in general less catalyst per barrel of daily charge capacity than do most of the other processes. But in doing so, however, it suffers no penalty in effective catalyst life, as measured in barrels of charge processed per pound of catalyst employed in the reactor system. Moreover, the higher initial cost and operating complications of regeneration are avoided.

Reactions in Reforming

The numerous reactions comprising catalytic reforming over platinum are complex, interdependent and proceed at different rates. Conditions of temperature, pressure, hydrogen recycle ratio and space velocity therefore are chosen to achieve the optimum overall equilibrium as indicated by the desired octane level (in the case of high octane fuels) or aromatisation (when running for aromatic hydrocarbon production) with a given charge stock.

The reactions may be summed up thus for simplicity:

The dehydrogenation reaction is particularly energy-consuming and is largely responsible for the decrease in temperature which characterises catalytic reforming. Since the temperature level affects the reaction kinetics and equilibria, heat must be supplied as the reactions proceed.

Because the writer is most familiar with it, the Platforming process is used here to illustrate the commercial catalytic reforming of petroleum naphthas by means of platinum.

The flow diagram above illustrates a typical Platforming process. The raw charge stock is a petroleum naphtha which is prefractionated to separate for the reactor charge a cut boiling roughly between 200 and 400°F (93 to 204°C) from lower and higher boiling hydrocarbons which may be present. The reactor charge is then mixed with hydrogen generated in the process and heated to the desired reaction temperature, ranging from 850 to 950°F (454 to 510°C). The hot charge is admitted to the first of four (sometimes three) reactors.

Effluent from the first reactor is considerably cooler than when it entered, as a consequence of the endothermic reactions which occur, and is reheated to the operating temperature before it enters the next reactor. This is repeated with the material entering the third and the fourth reactors.

The final effluent is usually heat-exchanged against incoming charge, then further cooled, and finally enters the products separator. Enough hydrogen for process requirements is compressed and recycled to the charge entering the first reactor, while the remainder is by-product hydrogen of high purity. The separator liquid is fractionated to the initial boiling point desired by the refiner, the light hydrocarbons so separated being available for other uses in the refinery.

The illustration on page 39 is an overall view of the Platforming unit installed in the Kent Refinery of The British Petroleum Company Ltd. This has a capacity of 6,000 barrels per stream day. A smaller Platforming unit installed in a refinery in Southwestern United States is shown below, while the figure overpage shows a close-up of the heart of a Platforming unit—the heater and reactor sections. The platinum catalyst is contained in the four cylindrical vessels in the centre of the picture.

Characteristics of Charge and Product

The table on page 43 shows the characteristics of the depentanised charge to the reactor section of a Platforming unit and the Platformate product. The reactor charge had an octane rating of 48 Research, which probably would have increased to 70 octane or so upon the addition of 3 ml. of tetraethyl lead per gallon. This would not be a suitable fuel for a modern automobile. After Platforming, however, the octane number was 93 Research, unleaded, and upon the addition of 3 ml. of tetraethyl lead per gallon was rated at over 100 octane. Moreover, the distillation range suits this material for use as an automotive fuel upon the addition of “outside” light ends to bring the initial boiling point to approximately 100°F and increase the vapour pressure to the range of 9 to 13 pounds. Another important point shown in these data is that a yield of 83.5 per cent C₅+ by volume was achieved, despite the very high octane rating secured in the final product and the inherent volume shrinkage which accompanies aromatisation.

Higher octane ratings than are illustrated in the table have been achieved by Platforming. Using the variation called Rexforming, which includes solvent extraction to make the desired product, octane ratings of up to nearly 105 Research, leaded, have been reported in commercial operations. Even higher octane ratings can be secured with this process.

World capacity of catalytic reforming units employing platinum-containing catalysts has climbed enormously from the modest 1,500 barrels per stream day represented by the first Platformer. Latest published reports (1, 2) indicate that there are now 1,100,000 barrels per day of platinum-catalyst reforming units in operation in the world, of which nearly 800,000 daily barrels of capacity is in United States refineries. These and other (3) reports show that another 775,100 daily barrels of capacity are either planned or under construction since January 1, 1956. A grand total of 1,885,100 barrels per day of catalytic reforming capacity is thus in sight.

It is a little difficult to estimate how much catalyst or what weight of platinum is represented in these units, since the platinum content of the various catalysts varies with the process and with the type of catalyst within processes. Moreover, there is no uniformity in either the weight of catalyst employed per daily barrel of charge capacity or in the bulk density of the catalyst. Jensen (4) reports that typical catalyst compositions range from 0.3 to 0.8 weight per cent of platinum, and that from three to five pounds of catalyst are required per daily barrel of capacity.

Variations in Practice

There are a number of reasons for these variations, among them respective catalyst manufacturing techniques, differences in composition intended to permit the catalyst to be used under various operating conditions, and different processing schemes. One of UOP’s catalysts, for instance, contains more platinum than any of this company’s other catalysts and is tailored particularly for the production of very high octane Platformates. In order to achieve continuity of operations, some regenerative-type processes employ a “swing reactor” which takes the place of that one of the other reactors undergoing regeneration of spent catalyst. Still other processes use more catalyst per barrel of rated capacity in order to extend the time between regenerations, but taking a broad view of the industry it can be said that the amount of platinum embodied in existing reforming installations is measured in hundreds of thousands of ounces, while installations now planned or under construction will absorb further correspondingly large amounts.

The continuing upward trend of octane number requirement for automobile engines shows no sign of ending; there is still a margin of virgin gasolines and naphthas available as reformer feed, while the possibility of widening the range of hydrocarbons economically suitable for feed stocks is being studied. Active expansion of catalytic reforming capacity is therefore probable for some time to come, but a “saturation point” must ultimately be reached after which the building of reforming facilities is likely to slow down and run parallel with the growth of general refining capacity.