WO1998056740A1

WO1998056740A1 - Multi-reactor system for enhanced light olefin make

Info

Publication number: WO1998056740A1
Application number: PCT/US1998/011874
Authority: WO
Inventors: J. F. Carpency
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1997-06-10
Filing date: 1998-06-08
Publication date: 1998-12-17
Also published as: AU7956398A

Abstract

The invention provides a process for improving the conversion of a hydrocarbon feedstock to light olefins comprising the steps of thermally converting the hydrocarbon feedstock to produce an effluent; quenching the effluent to produce a quenched effluent; and contacting the quenched effluent with a light olefin-producing cracking catalyst. The trim thermal cracking is carried out between 700 °C to 1000 °C for 0.1 to 20 seconds. Preferably the light olefin-producing cracking catalyst is a zeolite catalyst free of added metal oxides with a hydrogenation/dehydrogenation function and is contacted at a temperature within the range of about 500 °C to about 750 °C. Feedstock flows are at a weight hourly space velocity in the range of about 0.1 Hr-1 WHSV to about 100 Hr-1 WHSV.

Description

MULTI-REACTOR SYSTEM FOR ENHANCED LIGHT OLEFIN MAKE

Field of the Invention

The invention provides a process for increasing yield of ethylene and propylene in a catalytic cracking process by use of a thermal cracking step before catalytic cracking.

Background of the Invention

Thermal and catalytic conversion of hydrocarbons to olefins is an important industrial process producing billions of pounds of olefins each year. Because of the large volume of production, small improvements in operating efficiency translate into significant profits. Catalysts play an important role in more selective conversion of hydrocarbons to olefins.

Particularly important catalysts are found among the natural and synthetic zeolites. Zeolites are complex crystalline aluminosilicates which form a network of AIO₄ and SiO₄ tetrahedra linked by shared oxygen atoms. The negative charge of the tetrahedra is balanced by the inclusion of protons or cations such as alkali or alkaline earth metal ions. The interstitial spaces or channels formed by the crystalline network enable zeolites to be used as molecular sieves in separation processes. The ability of zeolites to adsorb materials also enables them to be used in catalysis. There are a large number of both natural and synthetic zeolitic structures. The wide breadth of such numbers may be understood by considering the work "Atlas of Zeolite Structure Types" by W. M. Meier, D. H. Olson and C. Baerlocher (4th ed., Butterworths/lntl. Zeolite Assoc. [1996]). Catalysts containing zeolites have been found to be active in cracking light naphtha to ethylene and propylene, the prime olefins.

Of particular interest are the acidified zeolites effective for conversion of light hydrocarbons such as low boiling naphthas to the prime olefins. Typical catalysts include ZSM-5 zeolite described and claimed in U.S. Pat. No. 3,702,886, and ZSM-11 described in U.S. Pat. No. 3,709,979, and the numerous variations on these catalysts disclosed and claimed in later patents.

Previous uses of multiple reaction or temperature zones in prime olefin production have used hydrocracking or hydrogenolysis to produce ethane and propane with little production of prime olefins in the first stages. Franck et al., U.S. Patent No. 4,137,147 used multiple hydrogenolysis stages to which each stage operated at 5° to 25°C higher than the preceding stage. The light hydrocarbons up to C3 in the effluent from the hydrogenolysis stages were then steam cracked to prime olefins, while the C₄+ production was separated and at least part of it sent to further hydrogenolysis for additional ethane and propane production. The steam cracking unit was supplied a fraction consisting essentially of ethane and propane for conversion to ethylene. Lionetti et al., U.S. Patent No. 4,388,175, discloses a two stage system for production of aromatics from heavy oil. The second stage is operated at a higher temperature than the first to produce light naphtha, gasoline and needle coke. There was no indication of any application to prime olefins production. Tabak, U.S. Patent No. 4,487,985 and its divisional U.S. Patent No. 4,560,536 teaches oligomerization of lower olefins in a multistage series of reactors wherein catalyst partially inactivated in the primary stage is employed at a higher temperature in a secondary stage prior to catalyst regeneration. GB 2,105,362 teaches a two stage thermal cracking process in a catalyst free system wherein the first reaction zone heats the steam/feedstock from 800°C to 1000°C and then passes the feedstock to a second catalyst free zone where it is heated 850°C to 1150°C. Mauleon et al., U.S. Patent Nos. 5,506,365 and 5,264,115, teach a multiple zone process wherein hot catalyst is used in a mild steam thermal cracking process and reacted further downstream with additional catalyst at a lower temperature in a process aimed at gasoline production. Mao, U.S. Patent 4,732,881 teaches steam cracking of hydrocarbons (propane is exemplified) followed by contact with a multi-component zeolite- containing catalyst comprising a zeolite of the ZSM-5 type coupled with a metallic oxide having a hydrogenation/dehydrogenation function. The thermal cracking unit is operated at a higher temperature than the downstream catalytic cracker. Adams, U.S. Patent No. 3,360,587, also teaches a steam cracking step followed by a catalytic cracker, again the catalytic cracker is at a lower temperature than the upstream thermal cracker.

Heretofore the art has not recognized that thermally cracking a hydrocarbon mixture followed by passing the effluent stream over suitable zeolite catalysts free of added metal oxides having hydrogenation/dehydrogenation functions can result in significant increases in production of prime olefins without prior separation of components or removal of C-₄+ materials from the feed stream.

Summary of the Invention

The present invention provides a process for improving the conversion of a hydrocarbon feedstock to light olefins comprising a catalyst-free thermal cracking step followed by contacting the effluent from the thermal cracking step with a light olefin-producing cracking catalyst comprising a zeolite free of added metal oxides having hydrogenation/dehydrogenation functions, in a catalyst contact zone.

Detailed Description of the Invention

Definitions

"Light naphtha" means a hydrocarbon fraction that is predominantly C₅ to C₇ hydrocarbons.

"Virgin naphtha" means a hydrocarbon fraction obtained from crude oil or natural gas without additional conversion processing. "Cat naphtha" means a refinery distillate fraction obtained by catalytic cracking of a heavier hydrocarbon fraction.

"BTX" means a mixture containing benzene, toluene, and xylenes.

"Light olefins" or "prime olefins" means ethylene, propylene or mixtures thereof.

"Improved conversion" means producing an increase in production that is a greater yield within the precision of the measurement system over cracking the same feedstock in a thermal cracking step alone.

"Hydrocarbon feedstock" means a stream comprising one or more hydrocarbons to be broken into fragments by thermal, chemical or catalytic action, the fragments forming light olefins.

Reaction Conditions and Catalysts

Substantial amounts of ethylene and propylene can be produced by cracking refining or chemical feedstocks such as light cat naphtha (LCN) or light virgin naphtha (LVN) over catalysts; particularly zeolite containing catalysts such as those which contain ZSM-5. The present invention provides a method for enhancing ethylene and propylene yields which comprises thermal cracking a hydrocarbon feedstream followed by contacting the effluent from the thermal cracking step with an acidic zeolite catalyst free of added metal oxides having hydrogenation/dehydrogenation functions. The key to the invention is separating the ethylene generation reactor from a downstream catalytic reactor or reactors that takes the effluent of the first reactor and runs it at different conditions in order to enhance further overall light olefin (ethylene plus propylene) make. The downstream catalytic reaction also results in a significant reduction in the amount of acetylenes and diolefins produced in the overall reaction system, which has additional benefits in the downstream olefins recovery process. An example of the process is as follows where the thermal cracking is carried out in a conventional steam cracking furnace. A steam cracking hydrocarbon feedstock is fed to a steam cracking furnace in the presence of steam (usually between 0.2 and 0.5 wt steam/wt hydrocarbon feedstock), and is run to a coil outlet temperature of between 760°C and 860°C. Suitable hydrocarbon feedstocks for a steam cracking furnace when practicing the present invention would include any feedstock typically feeding steam crackers. Examples are ethane, propane, butane, naphthas, gasoils, Fischer- Tropsch liquids, raffinates, field natural gasolines, petrolatum, waxes, and vacuum gasoils. As in a conventional steam cracker, when applying the present invention the reactor effluent needs to be quickly quenched via quench hydrocarbon injection and/or heat removal via transfer line exchangers typical in a steam cracker to lower the temperature to about 600°C to 720°C in order to mitigate unwanted reactions that would result in lowering the ethylene yield. Any hydrocarbon may be used as a quench hydrocarbon to lower temperature, although the preferred quench hydrocarbon would come from the group of normal steam cracking feedstocks such as ethane, propane, butane, naphthas, gasoils, Fischer-Tropsch liquids, and streams containing olefins or diolefins such as butenes, butadiene, steam cracked naphtha, cat cracked naphtha, and coker naphtha. More preferred quench hydrocarbon materials are light cat naphtha (LCN), unhydrogenated C₄ or C₅ from normal steam cracker effluent, or portions of the effluent from the process of this invention as a recycle, or light virgin naphtha (LVN). The amount of quenching accomplished with direct injection of quench hydrocarbon versus the amount done with heat removal may be determined by one skilled in the art as a function of the amount of additional hydrogen required, which is further a function of the hydrocarbon feed to the first reactor and its operating conditions, as well as the desired temperature in the downstream reactor(s). A further consideration would be the availability of suitable quench hydrocarbon material. The effluent from the first reactor after quenching then enters the downstream catalytic reactor(s). The purpose of this reactor is to make additional ethylene and/or propylene out of the reactive species (namely C₂+ paraffins, C₂+ acetylenes, C₃+ diolefins, C₄+ olefins, and C₅+ naphthenes) remaining in the first reactor effluent, as well as the hydrocarbon material used as quench. The catalytic reactor or reactors can be fixed bed, moving bed, fluidized bed, such as a riser or dense fluid bed system or stationary fluid bed system, although the present invention is not limited to these types of reactors. Also, it is not necessary to have a 1 :1 correspondence between ethylene generation reactors such as steam cracking furnaces and the downstream catalytic reactor(s); for example, it is possible to an ethylene generation reactor feeding numerous downstream catalytic reactors. The temperature in this downstream catalytic reactor(s) is optimized to achieve different ratios of ethylene to propylene as desired, with a normal operating temperature range in the range of about 500°C to 750°C; more preferably in the range of 550°C to 725°C; most preferably in the range of 600°C to 700°C. Preferably the thermal step is carried out within the range of 700°C to 1000°C; more preferably in the range of 720°C to 900°C. In the thermal cracking step the residence time is maintained in the range of 0.02 to 20 seconds, preferably 0.05 to 5 seconds, and most preferably for 0.1 to 1 seconds. The catalyst contacting process is preferably carried out at a weight hourly space velocity (WHSV) in the range of about 0.1 Hr¹ WHSV to about 100 Hr¹ WHSV, more preferably in the range of about 1.0 Hr¹ WHSV to about 50 Hr¹ WHSV, and most preferably in the range of about 10 Hr¹ WHSV to about 40 Hr¹ WHSV.

Any cracking catalyst free of added metal oxides having hydrogenation/dehydrogenation functions that is operable to selectively produce prime olefins may be used in the catalytic cracking step after the thermal cracking step. Suitable zeolites for use as the cracking catalyst are typically the acid form of any of the naturally-occurring or synthetic crystalline zeolites, especially those having a silica to alumina molar ratio within the range of about 2.0:1 to 2000:1. By employing a simple bench test, one skilled in the art can determine quickly whether a catalyst displays improved conversion by staging catalytic with thermal conversion.

Examples of zeolites useful in the claimed process include gallium silicate zeolite, zeolite beta, zeolite rho, ZK5, titanosilicate zeolite, ferrosilicate zeolite, borosilicate zeolite, zeolites designated by the Linde Division of Union Carbide by the letters of X, Y, A, L (these zeolites are described in U.S. Pat. Nos. 2,882,244; 3,130,007; 3,882,243; and 3,216,789, respectively); naturally occurring crystalline zeolites such as faujasite, chabazite, erionite, mazzite, mordenite, offretite, gmelinite, analcite, etc., and ZSM-5, (described in U.S. Pat. No. 3,702,886).

Particularly suitable catalysts are found among the medium and small pore zeolites. Such medium pore zeolites are considered to have a Constraint Index from about 1 to about 12. The method by which Constraint Index is determined is described fully in U.S. Pat. No. 4,016,218. Zeolites which conform to the specified values of Constraint Index for medium pore zeolites include ZSM-5, ZSM-11 , ZSM-5/ZSM-11 intermediate, ZSM-12, ZSM- 21 , ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-50, MCM-22 and zeolite Beta which are described, for example, in U.S. Pat. Nos. 3,702,886 and Re. No. 29,949, 3,709,979, 3,832,449, 4,046,859, 4,556,447, 4,076,842, 4,016,245, 4,229,424, 4,397,827, 4,954,325, 3,308,069, Re. 28,341 and EP 127,399 to which reference is made for details of these catalysts. These zeolites may be produced with differing silica to alumina molar ratios ranging from 12:1 upwards. They have been, in fact, produced from reaction mixtures from which alumina is intentionally excluded, so as to produce materials having extremely high silica to alumina ratios which, in theory at least, may extend up to infinity. Preferred medium pore zeolites include ZSM-5, ZSM- 11 , ZSM-12, ZSM-35 and MCM-22. Particularly preferred is ZSM-5. Small pore zeolites, include such crystalline aluminosilicate zeolites as erionite, chabazite, ferrierite, heulandite, phillipsite, and such synthetic counterparts thereof as zeolites A and ZK5, as described in U.S. Pat. Nos. 2,882,243 and 3,247,195, respectively.

Preferably the zeolite catalyst is selected from the group consisting of faujasite, chabazite, erionite, mordenite, offretite, gmelinite, analcite, phillipsite, ZSM-5, ZSM-11 , ZSM-5/ZSM-11 intermediate, ZSM-12, ZSM-21 , ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-50, MCM-22, gallium silicate zeolite, zeolite Beta, zeolite rho, ZK5, titanosilicate, and zeolites having a silica to alumina ratio within the range of about 2.0:1 to 2000:1 ferrosilicate; borosilicate and zeolites designated by the Linde Division of Union Carbide by the letters of X, Y, and A. An especially favored zeolite is ZSM-5. Preparation of suitable zeolite containing catalysts may be carried out as described in the preceding references or purchased from commercial suppliers well known to those skilled in the art.

Advantages of the present invention are many. The use of multiple reactors allows the adjustment of operating conditions in each reactor to optimize the proportion of light olefin products from the overall reactor system as a function of current economics. Hence, ethylene-to-propylene ratios can be adjusted over a wider range than is possible in steam cracking alone. Further, heat addition in the thermal cracking step can be done by means of a fired reactor, for example, a steam cracker may be used as an initial reactor. Additionally, temperature control of the reactors is easy, is independent and can be altered by, for example, altering the amount of hydrocarbon quench or heat removed. The use of multiple reactors provides feed flexibility— for example, the first thermal reactor may run on normal saturated feedstocks, while the downstream catalytic reactor can have additional economically attractive olefinic (or diolefinic) streams introduced to it. Example 1

A blend of model compounds consisting of hydrogen, methane, ethylene, ethane, propylene, butenes, butadiene, hexane and benzene was used to simulate the effluent from a steam cracking step. A first run was conducted at 680°C, 24 Hr¹ WHSV over a fixed bed of 0.6 g ZCAT40, a ZSM-5 zeolite catalyst commercially available from Intercat. Inc., of Sea Girt, New Jersey. Prior to the cracking tests, ZCAT40 was steamed with 100% steam at 704°C and 1 atmosphere for 16 hours for the purpose of aging the catalyst.. A second run under the same conditions treated the same model compound blend spiked with additional hexane to simulate a hydrocarbon quenching stream. In test runs, steam to hydrocarbon weight ratio was 0.33. The effluent stream was analyzed by on-line gas chromatography. A column having a length of 60m packed with fused silica was used for the analysis. The GC used was a dual FID Hewlett-Packard Model 5880. The results are tabulated below.

Table 1

Catalyst Cracking of Simulated Steam Cracker Effluent

Stream Steam Effluent Product Change

Key Results, Wt. %

Ethylene 29.5 32.2 2.7

Propylene 21.0 24.9 3.9

Butenes 9.1 7.6 -1.5

Butadiene 9.8 0.0 -9.8

Aromatics 4.6 8.2 3.6

Light Saturates* 19.6 23.3 3.7

Hexane 6.4 1.9 -4.5

♦Hydrogen, Methane and Ethane

As shown in Table 1 , when simulated steam cracker effluent was further cracked over ZCAT40 at 680°C and 24 hr¹ WHSV, conversion was 76.4 Wt. %, while ethylene and propylene yields were 32.2 wt % and 24.9 wt %, respectively. These yields are 2.7 and 3.9 Wt. % higher than those typically observed with steam cracking alone.

Example 2

The preceding experiment was repeated except the model effluent was spiked with hexane to simulate the addition of a hydrocarbon quenching stream to the steam cracker effluent. The results are summarized in table 2.

Table 2

Effect of Hexane Quench

Feed Composition

Stream Base Added Hexane Product Change

Key Results, Wt. %

Ethylene 29.5 27.6 32.3 4.7

Propylene 21.0 19.7 25.5 5.8

Butenes 9.1 8.5 7.2 -1.3

Butadiene 9.8 8.5 0.0 -8.5

Aromatics 4.6 4.1 6.5 2.4

Light Saturates* 19.6 18.7 24.2 5.5

Hexane 6.4 12.9 2.8 -10.1

♦Hydrogen, Methane and Ethane

In Table 2, the simulated effluent from steam cracking spiked with an additional 6.5 Wt. % hexane was subjected to acid catalyst at 680°C and 24 hr^"1 WHSV. Ethylene yield increased 4.7 Wt. % to 32.3 Wt. % from 27.6 Wt. %. Propylene yield increase to 25.5 Wt. % from 19.7 Wt. %, a 5.8% gain.

The preceding examples are presented to illustrate the invention and not as limitations. There are many variations on the invention that will be apparent to those skilled in the art. The invention is defined and limited by the claims set out below.

Claims

CLAIMSWe claim:

1. A process for improving conversion of a hydrocarbon feedstock to light olefins comprising thermally converting the hydrocarbon feedstock to produce an effluent; and thereafter contacting the effluent with a light olefin-producing cracking catalyst free of added metal oxides having hydrogenation/dehydrogenation functions.

2. The process of claim 1 which further comprises the step of lowering the temperature of the effluent from the thermal cracking step prior to contacting the effluent with a cracking catalyst.

3. The process of claim 2, wherein the temperature lowering is carried out by heat removal or addition of a hydrocarbon.

4. The process of claim 1 or 2 wherein the effluent is quenched by contact with a quench hydrocarbon and thereafter the quenched effluent is contacted with a light olefin-producing cracking catalyst free of added metal oxides having hydrogenation/dehydrogenation functions.

5. The process of claim 4 wherein the quench hydrocarbon is selected from the group consisting of streams containing olefins or diolefins, butenes, butadiene, steam cracked naphtha, cat cracked naphtha, coker naphtha, light cat naphtha, unhydrogenated C₄ or C₅ from normal steam cracker effluent, or a portion of effluent from this process as a recycle.

6. The process of claim 4 wherein the quench hydrocarbon is selected from the group consisting of ethane, propane, butane, naphthas, gasoils, Fischer-Tropsch liquids, raffinates, field natural gasolines, petrolatum, waxes, and vacuum gasoils.

7. The process of claim 4, 5 or 6 wherein the quenched effluent flow is at a weight hourly space velocity in the range of 1.0 Hr¹ to 100 Hr¹.

8. The process of claim 4, 5 or 6 wherein the quenched effluent flow is at a weight hourly space velocity in the range of 1.0 Hr¹ to 50 Hr¹.

9. The process of claim 4, 5 or 6 wherein the quenched effluent flow is at a weight hourly space velocity in the range of 10.0 Hr¹ to 40.0 Hr¹.

10. In a process for producing ethylene and propylene by catalytic cracking wherein a hydrocarbon feedstock is contacted with an acidic zeolite cracking catalyst free of added metal oxides having hydrogenation/dehydrogenation functions in a catalyst cracking zone at the improvement which comprises providing a thermal cracking step upstream from the catalyst cracking zone.

11. The process of any of the preceding claims wherein the cracking catalyst comprises a zeolite.

12. The process of any of the preceding claims wherein the cracking catalyst comprises a zeolite having a silica to alumina ratio within the range of about 2.0:1 to 2000:1.

13. The process of any of the preceding claims wherein the zeolite is selected from the group consisting of ferrierite, heulandite, mazzite, phillipsite, faujasite, chabazite, erionite, mordenite, offretite, gmelinite, analcite, ZSM-5, ZSM-11 , ZSM-5/ZSM-11 intermediate, ZSM-12, ZSM- 21 , ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-50, MCM-22 and zeolite beta, gallium silicate zeolite, zeolite rho, ZK5, titanosilicate, ferrosilicate; borosilicate zeolites; and zeolites designated by the Linde

Division of Union Carbide by the letters of X, Y, and A.

14. The process of claims 1-12 wherein the zeolite comprises ZSM-5.

15. The process of any of the preceding claims wherein the hydrocarbon feedstock is selected from the group consisting of ethane, propane, butane, naphthas, gasoils, Fischer-Tropsch liquids, raffinates, field natural gasolines, petrolatum, waxes, vacuum gasoils and streams containing olefins or diolefins such as butenes, butadiene, steam cracked naphtha, cat cracked naphtha, coker naphtha, light cat naphtha, unhydrogenated C₄ or C₅ from normal steam cracker effluent or the effluent from this process as a recycle.

16. The process of any of the preceding claims wherein the catalyst is contacted at a temperature within the range of 500┬░C to 750┬░C.

17. The process of any of the preceding claims wherein the catalyst is contacted at a temperature within the range of 550┬░C to 725┬░C.

18. The process of any of the preceding claims wherein the catalyst is contacted at a temperature within the range of 600┬░C to 700┬░C.

19. The process of any of the preceding claims wherein the thermal cracking step is carried out at a temperature within the range of 720┬░C to 900┬░C.

20. The process of any of the preceding claims wherein the thermal cracking step is carried out at a temperature within the range of 720┬░C to 900┬░C.

21. The process of any of the preceding claims wherein the thermal cracking step is carried out with a residence time of 0.01 to 20 seconds.

22. The process of any of the preceding claims wherein the thermal cracking step is carried out with a residence time of 0.05 to 5 seconds.

23. The process of any of the preceding claims wherein the thermal cracking step is carried out with a residence time of 0.1 to 1 seconds.