Catalytic reforming is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas and straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed. Reforming can be defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and cracking reactions which produce gas. Many of these reactions also result in coke being deposited on the catalyst.
In a reforming operation, one or a series of reactors, providing a series of reaction zones, are employed. Typically, a series of reactors are employed, e.g., three or four reactors, these constituting the heart of the reforming unit. Each reforming reactor is generally provided with a fixed bed, or beds, of catalyst, typically a platinum-containing catalyst or a platinum/promoter metal catalyst, which receive downflow feed. Each reactor is provided with a preheater, or interstage heater, because the net effect of the reactions which take place is typically endothermic. A naphtha feed, with hydrogen, and/or hydrogen-containing recycle gas, is passed through the preheat furnace then to the reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction and a vaporous fraction, the former usually being recovered as a C.sub.5 + liquid product. The latter is rich in hydrogen, usually contains small amounts of normally gaseous hydrocarbons, and is recycled to the process to minimize coke production.
The sum total of the reforming reactions occurs as a continuum between the first and last reactor of the series, i.e., as the feed enters and passes over the first fixed catalyst bed of the first reactor and exits from the last fixed catalyst bed of the last reactor of the series. During an on-oil run, the activity of the catalyst gradually declines due to build-up of coke on the catalyst. During operation, the temperature of the process is gradually raised to compensate for the activity loss caused by coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processing of this type, the catalyst must necessarily be periodically regenerated by burning off the coke at controlled conditions.
Several major types of reforming are practiced in multi-reactor reforming process units. All require periodic reactivation of the catalyst. The initial sequence requires burning the coke from the catalyst followed by steps wherein agglomerated metal components are atomically redispersed. Major types of reforming include semi-regenerative, cyclic and semi-cyclic. In semi-regenerative reforming, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst, which is decreased by the coke deposition. The entire process unit is finally shutdown for regeneration, and reactivation, of the catalyst when the activity of the catalyst drops to a predetermined level.
Cyclic and serhi-cyclic reforming units consist of three or four reactor vessels in series, plus an additional "swing" reactor vessel. The swing reactor, by means of a valve manifold arrangement, can be placed on-stream in substitution for any one of the other reactors. This arrangement enables any one of the reactors to be removed from service and connected to a separate system of lines and valves for catalyst regeneration. After the catalyst has undergone coke burn-off and other desirable regeneration steps (for example, metals redispersion, reduction, sulfiding, etc.), the reactivated catalyst is returned to on-stream reforming service and another reactor is removed from service for catalyst regeneration. Cyclic reforming offers advantages in that the catalyst can be regenerated, and reactivated, without shutting down the unit. Moreover, because of this advantage, the unit can be operated at higher severities to produce higher C.sub.5 + liquid yields than semi-regenerative reforming units. Unfortunately, "swinging" a regenerated reactor back on oil can be very disruptive to the unit. For example, reactor swings are typically characterized by a significant increase in total gas make and about 2 to 10 vol. % loss in recycle gas hydrogen purity. These transient effects, which may last for up to a day after the reactor swing, affects not only the operation of the reformer's furnace, reactor, and recycle gas sections, liquid stabilizing and gas handling systems, but also other process units found within the refinery. The sharp loss of hydrogen purity is a particularly drastic effect, causing disruptions to downstream hydroprocessing units which use hydrogen generated in the reformer. Also, reformer C.sub.5 + liquid yield is depressed as a result of the increased fraction of naphtha feed which is cracked to C.sub.4 - light gases.
One of the distinguishing differences between semi-regenerative reforming and cyclic reforming is that during catalyst regeneration in a semi-regenerative reforming unit, the hydrogen levels are typically greater than 20 vol. % and sometimes greater than 70 vol. %. Hydrogen levels are generally not higher than only 4 to 8 vol. % during cyclic catalyst regeneration until after catalyst regeneration is complete and the reactor is pressured up with hydrogen recycle gas when ready to "swing".
Attempts have been made in the past for shortening, or otherwise decreasing the severity of reforming unit restartup or swing upsets. For example, U.S. Pat. No. 3,507,781 teaches a start-up procedure wherein a naphtha feed is contacted with a catalyst (platinum/iridium on alumina) in the presence of an inert gas, such as nitrogen. The patent teaches that by use of such a procedure, the pressure in the reforming zone should be about 200 psig and the catalyst temperature about 343.degree. C., when the feed is first contacted with the catalyst at a space velocity of about 1 volume/volume/hour. Thereafter, the temperature is increased to about 480.degree. C. over a 2 to 3 hour period while building up autogenous pressure of produced hydrogen.
Another method is described in U.S. Pat. No. 4,148,758, wherein excessive hydrocracking or hydrogenolysis of a sulfur sensitive reforming catalyst is suppressed by incorporating within the reforming catalyst a sulfurous acid or sulfuric acid component.
Also, U.S. Pat. No. 4,261,810 teaches a start-up procedure of a reforming unit for preventing temperature runaways and over-cracking. The catalyst, during initial use or in a freshly-regenerated state, is contacted prior to contact with the chargestock, with a reformate characterized by an octane number (R+O) between about 90 and about 100. The reformate has an aromatics content within the range of 40 to 50 mole percent for a specified period of time at a temperature between about 315.degree. C. and 400.degree. C. The procedure taught in this '810 patent cannot be practiced in a conventional cyclic reforming unit in which reactor vessels are regenerated one at a time while the unit continues to maintain on-stream production.
While various attempts have been made to mitigate the disruptions of reactor swings after regeneration of catalyst, there still remains a need in the art for improved procedures for accomplishing same.
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