Catalytic reforming system and process

Details Catalytic reforming system and process Catalytic reforming system and process

2001-12-11

2004-08-10

Griffin, Walter D. (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

Reforming

C208S135000, C208S137000, C208S138000, C208S139000, C208S136000

Reexamination Certificate

active

06773580

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of catalytic reforming of petroleum distillates such as naphtha.
BACKGROUND OF THE INVENTION
Catalytic reforming of naphtha (petroleum distillate with a boiling point of about 70-200° C.) is a process used in refineries to upgrade naphtha to high-octane gasoline and to generate BTX-aromatics (i.e., benzene, toluene, xylene) which is a valuable feedstock for the petrochemical industry. The composition of raw naphtha typically includes alkanes and cycloalkanes together with smaller amounts of aromatics.
Naphtha reforming can be carried out by semi-regenerative (SR) catalytic reforming technology. The use of one or more fixed-bed, adiabatic reactors with interstage fired heating is the most common design, as shown in FIG.
1
. Generally, the size of the reactors increases in the downstream direction. The main reforming reactions are highly endothermic, and interstage reheat is typically needed to maintain reactor inlet temperatures. During the reforming process, naphtha in the gas phase is contacted with catalyst contained in the adiabatic reactor vessels. Typically, catalyst is present as a solid bed of pellets through which the naphtha gas passes. A large excess of hydrogen gas is generally recycled in the process to reduce deactivation of the catalyst by coke laydown. Typically, the molar ratio of recycle gas to fresh naphtha feed ranges from about 3 to about 8.
In the reforming process there are four main classes of reactions, each of which is characterized by a different reaction rate. In addition to the main reactions, significant coke formation usually occurs, resulting in deactivation of the catalyst due to masking of the catalytically active sites. The amount of coke on the catalyst increases in the downstream direction and representative coke loadings of 5 wt %, 10 wt % and 15 wt % carbon in the first, second and third reactor of a 3-reactor unit are quite common at the end of a cycle. Table 1 lists the aforementioned reactions together with literature data for their relative reaction rates (Farrauto et al.,
Fundamentals of Industrial Catalytic Processes
, Chapman & Hall, London (1997) and Ramage, et al.,
Advances in Chemical Engineering
, Academic Press, Inc., vol. 13, pp. 193-267 (1987)).
TABLE 1
Reaction Classes in Naphtha Reforming
Relative
Reaction
Reaction Class
Rate
Conditions
Desired/undesired
(1) Dehydrogenation
180
Favored by high
Desired (aromat-
T and low p
ics are formed)
(reaches
equilibrium)
(2) Isomerization
   7-12
Favored kineti-
Desired (e.g. n- to
cally by high T
iso-paraffins)
and p (reaches
equilibrium)
(3) Dehydrocyclization
0.04-1
Favored by high
Desired (paraffins
T and low p (does
to naphthenes)
not reach
equilibrium)
(4) Hydrocracking
 0.1-4
Favored kineti-
Not desired (C5+
cally by high T
yield loss to
and p (does not
lights)
reach
equilibrium)
(5) Coking
slow
Favored by high
Undesired (cata-
T and low p
lyst deactivation,
occurs as result of
dehydro-genation
and polymeri-
zation)
From Table 1, it can be seen that favorable operating conditions generally occur at low pressure and high temperature. In semi-regenerative units, the operating pressures are usually in the range of about 17 to about 45 bar, and the temperatures at the reactor inlet are typically set to about 500 to about 550° C. To maintain the desired temperature, the process stream is reheated in between the reactors. Lower pressures, although desirable with respect to aromatics formation and gasoline octane, are generally avoided since they often result in significant increases in catalyst deactivation rate due to the formation of coke.
In order to operate at an optimum reactor pressure, a flat pressure profile is desired. Therefore, the pressure drop across the reactors should be as low as possible. This flat pressure profile is also of importance with respect to compressor loads and related operating expense since a large amount of hydrogen-rich gas is recycled. In practice, a small pressure drop is realized by using shallow, axial flow beds as were used mainly in older units, or radial flow reactors which are the dominating technology today. An example of a radial flow design is reported, for example, in U.S. Pat. No. 5,885,442.
FIG. 2
shows a schematic of a radial flow reactor typically used in semi-regenerative naphtha reformers. To account for settling of the catalyst pellets during reactor heat-up and thermal expansion of the reactor vessel, and to avoid potential bypass of the reactant gases through the top section of the catalyst bed, additional catalyst is added beyond that needed in the direct flow path. This catalyst, called “slump & seal”, is not used effectively for the main reactions, and often becomes highly coked due to the low flow rates present in that section of the reactor. In addition, the radial flow design includes a centerpipe and a secondary containment wall at the perimeter that contains the pellet catalyst in-between, further contributing to wasted space and reduction of the overall volumetric productivity of the reactor.
Due to the high cost of precious metal catalyst and the loss of production associated with frequent catalyst changeouts, semi-regenerative reforming catalysts are regenerated in the reactor on a regular basis. Since the main deactivation mechanism is due to coke deposition, this regeneration can be done by a controlled bum-off using dilute oxygen gas. In a subsequent step, the precious metals are redispersed on the catalyst support. During regeneration the unit is taken off-line and the regeneration gases are fed through the reactors. To control coke burning rate, the oxygen content and operating temperature are carefully controlled. If the oxygen concentration in the reactor inlet or the temperature are not controlled well, a hot spot can develop that can permanently damage the catalyst or cause severe mechanical damage to the reactor internals including the centerpipe, the scallop containment walls, or the reactor shell. After regeneration the catalyst is brought back to service. Due to some losses in activity caused by the regeneration process, the number of times catalyst can be regenerated is limited. A common problem with the regeneration procedure is that the flow conditions in the unused top space of the catalyst bed are badly defined (in standard operation, the desire is to have little or no gas flow in this section). It is not uncommon that in this section of the bed the oxidation is less controlled. In some cases, this results in temperatures above the limits of the catalyst and the reactor metallurgy, causing damage to the catalyst as well as the centerpipe, the reactor shell and other reactor internals. The economic impact due to replacement and downtime can be large.
The economic importance of catalytic reforming and other petroleum refining processes has led to the investigation and development of numerous catalyst materials and structures. For example, in Parmaliana, et al.,
Catalysis
, 1987, 43-50, it is reported that crushed honeycomb catalyst shows activity with respect to certain reforming reactions. U.S. Pat. Nos. 6,177,002; 5,958,217; 5,958,216; and 5,562,817 pertain to catalytic reforming processes employing various catalyst staging strategies in which reactant stream is passed over varying catalyst compositions to improve selectivity and/or product yields. International patent application publication WO 99/22864 reports a homogenous catalyst bed containing catalyst particles with a concentration or species profile. WO 97/26078 reports the optimization of pressure drop in an axial flow fixed-bed reactor by grading catalyst particles of different sizes. These graded catalyst beds are said to be useful in processes such as hydrotreating, naphtha reforming, hydrocracking and hydroisomerization.
Catalyst structures having multiple channels in such a geometry that promotes heat exchange with the reactor walls for use in steam reforming processes are reported in U.S. Pat. No. Re. 32,044. Rigid honeycomb catalyst structu

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