Reforming
The demand of today’s automobiles for high-octane gasolines has stimulated the use of catalytic reforming. In catalytic reforming, the change in the boiling point of the stock passed through the unit is relatively small as the hydrocarbon molecular structures are rearranged to form higher-octane aromatics with only a minor amount of cracking. Thus catalytic reforming primarily increases the octane of motor gasoline rather than increasing its yield; in fact, there is a decrease in yield because of hydrocracking reactions which take place in the reforming operation.
The typical feedstocks to catalytic reformers are heavy straight-run (HSR) gasolines and naphthas and heavy hydrocracker naphthas. These are composed of the four major hydrocarbon groups: paraffins, olefins, naphthenes, and aromatics.
NoDoC designed the cost simulation for four major reactions that take place during reforming. They are:
(1) dehydrogenation of naphthenes to aromatics,
(2) dehydrocyclization of paraffins to aromatics,
(3) isomerization, and
(4) hydrocracking.
NoDoC defines reforming processes as continuous, cyclic, or semiregenerative depending upon the frequency of catalyst regeneration.
NoDoC considers the the continuous process to permit the removal and replacement of catalyst during normal operation as described below:
As a result, the catalyst can be regenerated continuously and maintained at a high activity. The semiregenerative unit is at the other end of the spectrum and has the advantage of minimum capital costs. Regeneration requires the unit to be takenoff-stream. Depending upon severity of operation, regeneration is required atintervals of 3 to 24 months. High hydrogen recycle rates and operating pressures are utilized to minimize coke laydown and consequent loss of catalyst activity. The cyclic process is a compromise between these extremes and is characterized by having a swing reactor in addition to those on-stream in which the catalyst can be regenerated without shutting the unit down. When the activity of the catalyst in one of the on-stream reactors drops below the desired level, this reactor is isolated from the system and replaced by the swing reactor. The catalyst in the replaced reactor is then regenerated by admitting hot air into the reactor to burn the carbon off the catalyst. After regeneration it is used to replace the next reactor needing regeneration. The pretreated feed and recycle hydrogen are heated to 925 to 975°F (498–524°C) before entering the first reactor. In the first reactor, the major reaction is the dehydrogenation of naphthenes to aromatics and, as this is strongly endothermic, a large drop in temperature occurs. To maintain the reaction rate, the gases are reheated before being passed over the catalyst in the second reactor. As the charge proceeds through the reactors, the reaction rates decrease and the reactors become larger, and the reheat needed becomes less. Usually three or four reactors are sufficient to provide the desired degree of reaction and heaters are needed before each reactor to bring the mixture up to reaction temperature. In practice, either separate heaters can be used or one heater can contain several separate coils. The reaction mixture from the last reactor is cooled and the liquid products condensed. The hydrogen-rich gases are separated from the liquid phase in a drum separator, and the liquid from the separator is sent to a fractionator to be debutanized.
The hydrogen-rich gas stream is split into a hydrogen recycle stream and a net hydrogen by-product which is used in hydrotreating or hydrocracking operations or as fuel.
The typical feedstocks to catalytic reformers are heavy straight-run (HSR) gasolines and naphthas and heavy hydrocracker naphthas. These are composed of the four major hydrocarbon groups: paraffins, olefins, naphthenes, and aromatics.
NoDoC designed the cost simulation for four major reactions that take place during reforming. They are:
(1) dehydrogenation of naphthenes to aromatics,
(2) dehydrocyclization of paraffins to aromatics,
(3) isomerization, and
(4) hydrocracking.
NoDoC defines reforming processes as continuous, cyclic, or semiregenerative depending upon the frequency of catalyst regeneration.
NoDoC considers the the continuous process to permit the removal and replacement of catalyst during normal operation as described below:
As a result, the catalyst can be regenerated continuously and maintained at a high activity. The semiregenerative unit is at the other end of the spectrum and has the advantage of minimum capital costs. Regeneration requires the unit to be takenoff-stream. Depending upon severity of operation, regeneration is required atintervals of 3 to 24 months. High hydrogen recycle rates and operating pressures are utilized to minimize coke laydown and consequent loss of catalyst activity. The cyclic process is a compromise between these extremes and is characterized by having a swing reactor in addition to those on-stream in which the catalyst can be regenerated without shutting the unit down. When the activity of the catalyst in one of the on-stream reactors drops below the desired level, this reactor is isolated from the system and replaced by the swing reactor. The catalyst in the replaced reactor is then regenerated by admitting hot air into the reactor to burn the carbon off the catalyst. After regeneration it is used to replace the next reactor needing regeneration. The pretreated feed and recycle hydrogen are heated to 925 to 975°F (498–524°C) before entering the first reactor. In the first reactor, the major reaction is the dehydrogenation of naphthenes to aromatics and, as this is strongly endothermic, a large drop in temperature occurs. To maintain the reaction rate, the gases are reheated before being passed over the catalyst in the second reactor. As the charge proceeds through the reactors, the reaction rates decrease and the reactors become larger, and the reheat needed becomes less. Usually three or four reactors are sufficient to provide the desired degree of reaction and heaters are needed before each reactor to bring the mixture up to reaction temperature. In practice, either separate heaters can be used or one heater can contain several separate coils. The reaction mixture from the last reactor is cooled and the liquid products condensed. The hydrogen-rich gases are separated from the liquid phase in a drum separator, and the liquid from the separator is sent to a fractionator to be debutanized.
The hydrogen-rich gas stream is split into a hydrogen recycle stream and a net hydrogen by-product which is used in hydrotreating or hydrocracking operations or as fuel.
