IN AN ISOMERIZATION ZONE

FIELD OF THE INVENTION

[OOOl] This invention generally relates to a process for controlling a temperature in an isomerization zone.

DESCRIPTION OF THE RELATED ART

[0002] The performance of a light naphtha isomerization unit can deteriorate from a start- up of the unit to an end of a run prior to catalyst replacement. Often, isomerization units can include multiple isomerization reactors. Catalyst in the first isomerization reactor may be permanently deactivated by catalyst poisons during the run. Catalyst deactivation may result in a reduced product quality by the failure to adjust process conditions to correspond to catalyst activity. Many refiners need to produce a high octane product throughout the entire run, and thus, there is often a desire to minimize the reduction in product octane as the end of the run is approached.

SUMMARY OF THE INVENTION

[0003] One exemplary embodiment can be a process for controlling a temperature in an isomerization zone. The process can include passing respective feeds through a first, a second, and a third isomerization reactors. Usually, a portion of an effluent from the first and second isomerization reactors is passed through respective exchangers to control the temperature in downstream reactors.

[0004] The embodiments disclosed herein can distribute the catalyst across multiple reactors that can allow more active catalyst to be available at the end of the run. In order to take advantage of all of the active catalyst, heat exchangers can be provided between all reactors to allow each reactor to be operated at the optimum temperature for the specific feed composition and percent of active catalyst in the reactor.

DEFINITIONS

[0005] As used herein, the term "stream" can include various hydrocarbon molecules, such as straight-chain, branched, or cyclic alkanes, alkenes, alkadienes, and alkynes, and optionally other substances, such as gases, e.g., hydrogen, or impurities, such as heavy metals, and sulfur and nitrogen compounds. The stream can also include aromatic and non- aromatic hydrocarbons. Moreover, the hydrocarbon molecules may be abbreviated CI , C2, C3...Cn where "n" represents the number of carbon atoms in the one or more hydrocarbon molecules. Furthermore, a superscript "+" or "-" may be used with an abbreviated one or more hydrocarbons notation, e.g., C3 ⁺ or C3 ^~ , which is inclusive of the abbreviated one or more hydrocarbons. As an example, the abbreviation "C3 ⁺ " means one or more hydrocarbon molecules of three carbon atoms and/or more.

[0006] As used herein, the term "zone" can refer to an area including one or more equipment items and/or one or more sub-zones. Equipment items can include one or more reactors or reactor vessels, heaters, exchangers, pipes, pumps, compressors, and controllers. Additionally, an equipment item, such as a reactor, dryer, or vessel, can further include one or more zones or sub-zones.

[0007] As used herein, the term "rich" can mean an amount of at least generally about 50%, and preferably about 70%, by mole, of a compound or class of compounds in a stream.

[0008] As depicted, process flow lines in the figures can be referred to interchangeably as, e.g., lines, pipes, feeds, products, portions, or streams.

[0009] The term "kilopascal" may be abbreviated " Pa" and all pressures may be absolute.

BRIEF DESCRIPTION OF THE DRAWING

[0010] The FIGURE is a schematic depiction of an exemplary isomerization zone.

DETAILED DESCRIPTION

[0011] Referring to the FIGURE, an exemplary isomerization zone 100 can include a first exchanger or cold combined feed-effluent exchanger 120, a second exchanger or mid- combined feed-effluent exchanger 140, a third exchanger or hot combined feed-effluent exchanger 160, a fourth exchanger or charge heater 180, a first isomerization reactor 200, one or more downstream reactors 230, and valves 108, 188, 212, 220, 252, 260, 296, and 304. Typically, the valves 188, 212, 220, 252, 260, 296, and 304 are control valves. Moreover, one or more downstream reactors 230 may include a second isomerization reactor 240 and a third isomerization reactor 280. Exemplary feeds, hydrogen, catalyst, isomerization reactors, and other equipment may be disclosed in, e.g., US 7,223,898.

[0012] A feed 1 10 can be provided to the isomerization zone 100 and can include hydrocarbon fractions rich in one or more C4-C6 hydrocarbons, preferably C5-C6 hydrocarbons, such as normal alkanes. One exemplary feedstock is substantially pure normal alkane streams having one or more C4-C6 hydrocarbons. Other useful feedstocks may include light natural gasoline, light straight run naphtha, gas oil condensate, light raffinates, light reformate, light hydrocarbons, field butanes, and straight run distillates having distillation end points of about 77° C and containing substantial quantities of C4-C6 alkanes. The feed stream may also contain low concentrations of unsaturated hydrocarbons and hydrocarbons having more than 6 carbon atoms.

[0013J Hydrogen can be admixed with the feed 1 10 in an amount that will provide a hydrogen to hydrocarbon molar ratio of about 0.01 : 1.0 - about 10: 1.0 in an effluent 312 from the isomerization zone 100. Additionally, make-up gas can be provided through a line 104 by opening a valve 108, as required.

[0014] The feed 110 can pass on the feed side of the exchangers 120, 140, 160, and 180 before being provided to the first isomerization reactor 200. Generally, the feed 1 10 is heated through the exchangers 120, 140, and 160 prior to reaching the fourth exchanger or charge heater 180. Steam may be provided via a line 184 to properly heat the feed 1 10.

Furthermore, a first temperature indicator controller 192 may measure a feed temperature prior to entering the first isomerization reactor 200, and compare with a setpoint to regulate the positioning of the control valve 188. Afterwards, the feed 1 10 can pass to the first isomerization reactor 200.

[0015] The first isomerization reactor 200 can operate at any suitable temperature, such as a temperature of about 90 - about 235° C, preferably about 1 10 - about 205° C, and the pressure can be about 700 - about 7,000 KPa. The liquid hourly space velocities may range from about 0.5 - about 12 hr ^{" 1} . The catalyst used in the first isomerization reactor 200 may include a strong acid catalyst, such as at least one of a chlorided platinum alumina, a crystalline aluminosilicate or zeolite, a sulfated zirconia, and a modified sulfated zirconia, preferably at least one of a chlorided platinum alumina and a sulfated zirconia.

[0016] As a class, the crystalline aluminosilicate or crystalline zeolite catalyst may include a crystalline zeolitic molecular sieve having an apparent pore diameter large enough to adsorb neopentane. Generally, the catalyst may have a silica alumina molar ratio

Si0 ₂ :Al ₂ 0 ₃ of greater than about 3: 1 and less than about 60: 1 , and preferably about 15: 1 - about 30: 1. Catalysts of this type for isomerization and methods for preparation are disclosed in, e.g., US 7,223,898.

[0017] An effluent 204 can exit the first isomerization reactor 200. The effluent 204 can be split into a first portion 208 and a second portion 216. The relative amounts of the portions 208 and 216 can be determined by a setpoint in a second temperature indicator controller 224, which in turn can regulate the valves 212 and 220 to position these valves to control the amounts of the portions 208 and 216. Generally, the first portion 208 passes through the first valve 212, and then passes through the hot combined feed-effluent exchanger 160, and thereby be cooled. The second portion 216 can pass through the second valve 220 prior to being rejoined with the first portion 208 to form a second feed 234. The second feed 234 may be provided to the downstream reactors 230, particularly the second isomerization reactor 240.

[0018] The second isomerization reactor 240 can include, independently, the catalyst and operate similarly as the first isomerization reactor 200 discussed above. Preferably, the second isomerization reactor 240 may operate at a temperature of about 90 - about 180° C, preferably about 104 - about 175° C.

[0019] A second effluent 244 from the second isomerization reactor 240 can be split into a third portion 248 and a fourth portion 256. A third temperature indicator controller 264 can measure the temperature of a third feed 284 and compare to a setpoint. The third temperature indicator controller 264 can provide signals to the third valve 252 and the fourth valve 260 to control the positioning of the valves and regulate the relative amounts of the third portion 248 and the fourth portion 256. The third portion 248 can pass through the mid-combined feed- effluent exchanger 140 to be cooled prior to being combined with the fourth portion 256 passing through the fourth valve 260. The third portion 248 and the fourth portion 256 may be combined to form the third feed 284.

[0020] The third feed 284 can be provided to the third isomerization reactor 280. The third isomerization reactor 280 can include, independently, the catalyst and operate similarly as the first isomerization reactor 200 discussed above. Preferably, the third isomerization reactor 280 may operate at a temperature of about 90 - about 160° C. [0021] The third effluent 288 can be split into a fifth portion 292 and a sixth portion 300. A hand indicator controller 308 can be used to regulate the positioning of the fifth valve 296 and the sixth valve 304 to control the relative amounts of the fifth portion 292 and the sixth portion 300. Generally, the positioning of the valves 296 and 304 can be determined by the desired amount of heating desired for the feed 1 10 in the cold combined feed-effluent exchanger 120. The fifth portion 292 can be provided to the cold combined feed-effluent exchanger 120 prior to being combined with sixth portion 300 passing through the sixth valve 304. Combining the fifth portion 292 and sixth portion 300 may form the effluent 312.

[0022] Usually, the effluent 312 is processed to separate the desired isomerized products from hydrogen, light ends, lower octane isomerized products, and cyclohexane plus heavy hydrocarbons having 7 or more carbon atoms.

[0023] Often, a stabilizer column is employed to remove light gases and butane from the products stream. An exemplary stabilizer column is disclosed in, e.g., US 5,453,552. At least a portion of the effluent 312 separated by the stabilizer column may be passed to a gasoline pool for blending to produce a motor fuel.

[0024] During operations, the second temperature indicator controller 224 and the third temperature indicator controller 264 can send signals controlling, respectively, the first valve 212 and the second valve 220, and the third valve 252 and the fourth valve 260. The set points for the controllers 224 and 264 can be set depending on the condition of the catalyst and the composition of the feed 1 10. Often, catalyst in first isomerization reactor 200 can be poisoned and otherwise deactivated over time. As such, the temperature in the first isomerization reactor 200 can be raised by sending at least one portion 208 and/or 248 of one or more effluents 204 and 244 from the isomerization reactors 200 and 240 through the exchangers 140 and 160. As an example, the temperatures of the first isomerization reactor 200 can range from about 120 - about 180° C, the second isomerization reactor 240 can range from about 100 - about 160° C, and the third isomerization reactor 280 can range from about 100 - about 160° C after initially starting-up the isomerization zone 100 after, e.g., a unit shutdown. At the end of a run before a unit shutdown to, e.g., replace catalyst, the temperatures of at least some of the isomerization reactors can be increased. As an example, the temperatures of the first isomerization reactor 200 can range from about 160 - about 210° C, the second isomerization reactor 240 can range from about 120 - about 180° C, and the third isomerization reactor 280 can range from about 100 - about 160° C. [0025] Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0026] In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

[0027] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.