Oligomerization of ethylene to liquid transportation fuels with post synthesis treated ZSM-5 catalyst

Abstract

A process for converting ethylene to liquid fuel products utilizing a ZSM-5 catalyst that is essentially free of catalyst metals other than silica and alumina, where the catalyst has been subjected to a post-synthesis treatment comprising contacting the catalyst with a solution comprising at least one of an acid or a base to increase the resistance of the catalyst to accumulation of coke in the zeolite crystallite and extend catalyst life. The base treatment is provided by a weak sodium hydroxide of less than 1 Molar. The acid treatment comprises a hydrogen chloride solution exceeding 2 Molar.

Description

OLIGOMERIZATION OF ETHYLENE TO LIQUID TRANSPORTATION FUELS WITH POST SYNTHESIS TREATED ZSM-5 CATALYST

[0001] This application claims priority to U.S. Utility Application Ser. No. 15 / 216,330 filed July 21, 2016 and titled "Oligomerization of Ethylene to Liquid Transportation Fuels with Post Synthesis Treated ZSM-5 Catalyst", U.S. Utility Application Ser. No. 15 / 216,362 filed July 21, 2016 and titled "Oligomerization of Ethylene to Liquid Transportation Fuels with Post Synthesis Treated ZSM-5 Catalyst", and U.S. Application Serial No. 15 / 216,382 filed July 21, 2016 and titled "Oligomerization of Ethylene to Liquid Transportation Fuels with Post Synthesis Treated ZSM-5 Catalyst". FIELD OF THE INVENTION

[0002] This invention relates to converting light alkanes to fuel and especially to improved catalysts for the economic and practical implementation of a commercial conversion process. BACKGROUND

[0003] The US shale gas boom has resulted in a significant increase in natural gas production, as well as a significant increase in the production of natural gas liquids. One of the main components of the natural gas liquids produced with natural gas is ethane. Ethane is most commonly used as petrochemical feedstock such as for the production of ethylene. Ethylene is a feedstock for many, many high volume chemical based products such as polyethylene and styrene plastics, etc. However, there are no other sizable consumption markets for ethane. US ethane supplies currently exceed demand by about 300,000 barrels per day causing depressed prices for ethane and attracting considerable investment into new ethane to ethylene production facilities. Most supply / demand estimates indicate that ethane will remain in surplus for many years and these predictions take in to account the new ethane to ethylene conversion capacity being built. Therefore, new markets for ethane and new technologies for converting ethane to products that have large existing or substantially growing demand would be very attractive in light of the projected low prices for ethane for many years. One of the largest end use markets is liquid transportation fuel and a simple conversion technology to any transportation fuel could prove to be quite profitable.

[0004] With the expectation that ethane will be plentiful and cheap for the foreseeable future, established technologies are being reconsidered that use ethane as a feedstock. One conventional technology involves conversion of ethane to liquid transportation fuels, such as gasoline and / or diesel. However, known chemical processes for converting ethane to gasoline and / or diesel have yet to be enabled at commercial scale. Thus, new, commercially-viable processes need to be developed that can convert the excess ethane currently being produced to liquid transportation fuels. BRIEF SUMMARY OF THE DISCLOSURE

[0005] The invention more particularly relates to oligomerizing ethylene to transportation fuel products in a reactor with a fixed bed of ZSM-5 catalyst that is essentially free of catalyst metals other than silica and alumina. The ZSM-5 catalyst is provided with post-synthesis acid treatment to resist coke formation in the zeolite crystallites and extend catalyst life. The oligomerizing is conducted at a pressure between 0 psig and 800 psig, a temperature of between 260°C and 420°C, and a gas hourly space velocity of between 1000 and 5000 inverse hours wherein at least 85% of the ethylene is converted. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

[0007] Figure 1 is a schematic drawing of a reactor for the present invention;

[0008] Figure 2 is a chart showing catalyst life for ethylene conversion in the schematic process according to Figure 1 for various catalysts; and

[0009] Figure 3 is a chart showing improved catalyst life over and above that shown in Figure 2 obtained by the catalyst treatment described in the present invention. DETAILED DESCRIPTION

[0010] Turning now to the detailed description of the preferred arrangement or arrangements of the present invention. It should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

[0011] Ethane may be converted to liquid transportation fuel in a process 10 shown schematically in Figure 1. Ethane from an ethane stream 12 is fed directly into a cracking furnace 14. The process of cracking ethane is typically an uncatalyzed process relying on pressure and temperature in a furnace, such as a cracking furnace 14. However, there are catalytic processes for cracking ethane that are acceptable for the present invention. The cracking furnace 14 produces a number of products primarily including hydrogen, ethylene, water, methane, and unconverted ethane, but also including small amounts of propylene, acetylene and butadiene, along with trace amounts of other hydrocarbons. All of these products are suitable for conversion in an ethylene oligomerizer in accordance with the present invention.

[0012] The products from the cracking furnace 14 are fed directly to a quench tower 16 via a furnace conduit 15 to stop further thermal reactions. A liquid product is taken from the bottom of the quench tower 16 via drain 18 comprising gasoline and fuel oil density materials that may be fed to a refinery. The bulk of the products are vaporous under the conditions in the quench tower 16 and exit the top via overhead conduit 17. The bulk of the products comprise raw ethylene along with the other lighter products described above.

[0013] The raw ethylene stream is fed into a catalytic oligomerization reactor 20. The catalytic oligomerization reactor 20 includes a fixed bed of catalyst to convert the raw ethylene stream into a number of products primarily including a gasoline product having an octane rating of about 88. The products from the catalytic oligomerization reactor 20 are conveyed via an outlet conduit to a product separator 22 to separate the products into at least three streams or cuts. A bottom cut comprising a stream of C4 to C15 hydrocarbon molecules that exits the bottom via product conduit 23. These are the valuable products that may be blended into current liquid transportation fuels such as gasoline and perhaps diesel or jet fuel.

[0014] The top cut comprises light gases such as hydrogen and C1 to C3 hydrocarbons that are conveyed back to the furnace 14 via light gas conduit 24. These light gases are burned in the furnace 14 to generate heat or supplemental heat for cracking the ethane. A middle cut comprising light hydrocarbons having a C3 to C5 chain length exits middle cut conduit 26. The light hydrocarbons may be recycled to the catalytic oligomerization reactor 20 via recycle conduit 27 or may be purged from the system 10 via purge line 28. If the middle cut is to be purged, it is preferably directed into a refinery stream or to a NGL fractionator for capture and sale.

[0015] Several components of the process are conventional and well-characterized in the field, including the furnace 14, and the second unit (i.e., oligomerization reactor 20). Interestingly, our studies show that although there are other products in the feed stream to the catalytic oligomerization reactor 20, about 98% of the ethylene was converted and over 75% of the products from the ethylene converted to C5+ materials (Table 1). This is very exciting. With ethane prices depressed and gasoline prices being among the highest priced non-specialty refinery products this conversion technology could prove to be quite profitable.

[0016] The operating conditions may be in a range where the oligomerizing is conducted at a pressure of between 0 psig and 800 psig, the temperature is maintained in a range of between 260°C and 420°C, and the feed rate measured as a gas hourly space velocity is in a range of between 1000 and 5000 inverse hours. While higher productivity is desired, ideally at least 85% of the ethylene is converted.

[0017] So, while this kind of system is exciting, there is a down side. The down side is that although there are quite a number of known catalysts for the catalytic process of oligomerizing ethylene, the usable lifespan of these catalysts is too short to make a commercial system financially viable. The best results so far have been accomplished with ZSM-5 catalysts, but the lifespan of ZSM-5 catalysts typically measured in hours and has been far-shorter than economically-viable for a commercial application. The productivity and catalyst life of commercially available ZSM-5 catalysts varies substantially. The basic formulation and sieve structure is the same in these catalysts, but variables include: the range of crystallite sizes, the range of catalyst particle sizes and the sizes of micropores and mesopores. There may be many other differences from one manufacturer to another even though the sieve size is quite standardized as set by the crystal structure of alumina and silica and there are no other catalyst metals or materials otherwise added to the ZSM-5 catalyst.

[0018] Basically, we theorize that coke quickly accumulates on the catalysts, and although the catalysts may be regenerated through conventional oxidation by burning the coke off the catalyst, the short catalyst life between regeneration cycles unduly increases operational expense. The consequent increase in operating costs could substantially reduce or eliminate the profitability of such a system. [Image disponible dans le document PDF, Image available in the PDF document]

[0019] Focusing on catalyst lifespan, the invention may be illustrated by showing the side-by-side testing of two commercially available ZSM-5 catalysts, arbitrarity designated Catalysts A and B. These catalysts were tested in a bench-scale reactor maintained at 50 psig and 310°C. High conversion of the ethylene was monitored, as shown in Figure 2. Catalyst A immediately lost substantial conversion activity, while Catalyst B maintained high conversion activity. Even so, a high conversion activity for only 100 hours remains insufficient for a prospective commercial operation.

[0020] Focusing on avoiding coke formation to increase catalyst life, it is believed that the process of forming coke in this reaction system is actually a multi-step reaction process that begins with oligomerization, then proceeds to cyclization, then to poly-nuclear cyclization and then finally to coke formation on the catalyst.

[0021] This present invention comprises post-synthesis treatments for ZSM-5 catalysts to make them more resistant to coke formation while converting ethylene to liquid hydrocarbon products. So, the ZSM-5 has already been produced by the manufacturer and the inventive process relates to altering the catalyst post-synthesis to minimize the production of coke.

[0022] The inventive process comprises a one-step or two-step process. One embodiment of a one-step process is an acid treatment. Catalyst B performed better than Catalyst A in initial activity studies (see Figure 2), so Catalyst B was utilized to assess post-synthesis treatments. A sample of Catalyst B was subjected to a 4 Molar HCl (hydrogen chloride) solution for 2 hours at 80°C. The sample was then thoroughly washed again in distilled water and dried at 120°C for 10 hours. Finally, the sample of Catalyst B (herein after Acid Treated Catalyst B) was calcined at 550°C for 6 hours. The Acid Treated Catalyst B was tested in a lab scale ethylene conversion process shown in Figure 1 and the conversion and time on stream are shown in Figure 3.

[0023] Another embodiment of a one-step process is a post-synthesis base treatment. A sample of Catalyst B was exposed to a 0.01 Molar concentration of NaOH (sodium hydroxide) at 40°C for 0.5 hours. The temperature was raised after the initial 30 minutes to 60°C to continue for an additional 60 minutes at the same NaOH concentration. The sample was washed in distilled water and dried at 120°C for 10 hours. Finally, the sample of Catalyst B (herein after Base Treated Catalyst B) is calcined at 550°C for 6 hours. Base Treated Catalyst B was tested in a lab scale ethylene conversion process shown in Figure 1 and the conversion and time on stream are shown in Figure 3.

[0024] The last process is a two-step process comprising both an acid treatment and a base treatment. A sample of Catalyst B is exposed to a 0.01 Molar concentration of NaOH (sodium hydroxide) at 40°C for 0.5 hours. The temperature is raised after the initial 30 minutes to 60°C to continue for an additional 60 minutes at the same NaOH concentration. The sample is washed in distilled water and then subjected to an acid wash for 2 hours at 80°C in a 4 Molar concentration HCl (hydrogen chloride) solution. The sample of Catalyst B (hereinafter Base-Acid Treated Catalyst B) is thoroughly washed again in distilled water and dried at 120°C for 10 hours and then calcined at 550°C for 6 hours. The two-step process may be undertaken in either order, but the better results were obtained with the Base treatment being taken first with the Acid treatment second. The Base-Acid Treated Catalyst B was tested in a lab scale ethylene conversion process shown in Figure 1 and the conversion and time on stream are shown in Figure 3.

[0025] One potentially harmful effect of these post-synthesis treatments was damage to the underlying framework Si / Al ratio. To assess whether the catalyst was being altered in a manner that would reduce the number of regeneration cycles the catalyst could endure, the framework Si / Al ratios were determined by solid-state NMR for each sample. Referring to Table 2 below, the Si / Al ratios are reported for Catalyst B without post treatment, Acid Treated Catalyst B, Base Treated Catalyst B and the Base-Acid Treated Catalyst B. These results suggest that none of the acid treatment, the base treatment or the base-acid treatments have much impact on ZSM-5 framework Si / Al ratios. X-ray diffraction patterns also indicate that the treatments did not destroy the ZSM-5 structure in Catalyst B. Both of these results are unexpected findings, and we hypothesize may be due to the presence of an alumina binder in Catalyst B. [Image disponible dans le document PDF, Image available in the PDF document]

[0026] Referring to Figure 3, it should be appreciated that each of the post-synthesis treatments provided improved catalyst life over Catalyst B that was not subjected to post- synthesis treatment using the same conditions that produced the data shown in Figure 2. The comparison of the untreated catalyst lifespan and catalyst lifespan data shown in Figure 3 demonstrates a profound improvement catalyst lifespan following post-synthesis treatment.

[0027] The tests were performed under the reaction conditions shown in Table 1 with a representative product slate. The conditions were adequate to compare the stability of the catalysts. For a commercially viable fixed-bed operation (with swing-reactor regeneration), it is desirable to have about 10 days of catalyst life before it becomes necessary to regenerate the catalyst, and at least a year before replacement of the catalyst becomes necessary (assuming that the catalyst will have undergone many cycles of online production and offline regeneration). The base-acid treated Catalyst B approached 10 days online before its ethylene conversion dropped below 90%. This suggests that the catalyst life approached the minimum duration for viable commercial operation.

[0028] It should be recognized that a wide variety of acid solutions and basic solutions may be used other than HCL and NaOH. Further, the conditions for contacting the catalyst is not intended to be limited to what is described in the above examples. For example, the base treatment of sodium hydroxide may be in a range of between about 0.001 and about 0.5 Molar concentration, optionally in a range of between about 0.005 and about 0.25 Molar concentration, optionally, between about 0.0075 and about 0.15 Molar concentration. Similarly, the acid treatment of hydrogen chloride may be in a range from 1 to 10 Molar concentration, optionally, in a range from 2 to 7 Molar concentration, optionally in a range from 3 to 5 Molar concentration. As noted, it is preferred that the catalyst is dried and calcined after the post-synthesis treatment is completed. Drying preferably is performed at a temperature above 105°C, and calcining is preferably performed at a temperature above 400°C. The post-synthesis treatment and / or contacting may optionally be performed at elevated temperatures of between about 35°C and 90°C.

[0029] Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein.

Claims

1. An oligomerization process to produce transportation fuels, the process comprising directing a feed stream comprising ethylene to transportation fuel products in a reactor that contains a bed of ZSM-5 catalyst that is essentially free of catalyst metals other than silica and alumina, wherein the ZSM-5 catalyst is supported by an alumina binder, wherein prior to the oligomerization the ZSM-5 catalyst is provided with a post-synthesis treatment comprising an acid wash and a base wash in series to resist coke formation in the zeolite crystallites and extend catalyst life, wherein the oligomerizing is conducted at a pressure between 1 bar and 56 bar, a temperature between 260°C and 420°C, and a gas hourly space velocity between 1000 and 5000 inverse hours, wherein at least 85% of the ethylene is converted.

2. The process according to claim 1, wherein the acid wash comprises an acidic solution.

3. The process according to claim 2, wherein the acidic solution comprises a hydrogen chloride solution ranging from 1 M to 10 M.

4. The process according to claim 3, wherein the acidic solution comprises a hydrogen chloride solution ranging from 2 M to 7 M.

5. The process according to claim 3, wherein the acidic solution comprises a hydrogen chloride solution ranging from 3 M to 5 M.

6. The process according to claim 1, wherein the base wash comprises a basic solution.

7. The process according to claim 6, wherein the basic solution comprises a sodium hydroxide solution ranging from 0.001 M to 0.5 M.

8. The process according to claim 7, wherein the basic solution comprises a sodium hydroxide solution ranging from 0.005 M to 0.25 M.

9. The process according to claim 7, wherein the basic solution comprises a sodium hydroxide solution ranging from 0.0075 M to 0.15 M.

10. The process according to claim 1, wherein the post-synthesis treatment comprises contacting the catalyst with a basic solution, followed by washing the catalyst with water, then contacting the catalyst with an acidic solution.

11. The process according to claim 10, wherein the basic solution comprises a sodium hydroxide solution ranging from 0.001 M to 0.5 M, and the acidic solution comprises a hydrogen chloride solution ranging from 1 M to 10 M.

12. The process according to claim 11, wherein the basic solution comprises a sodium hydroxide solution ranging from 0.005 M to 0.25 M, and the acidic solution comprises a hydrogen chloride solution ranging from 2 M to 7 M.

13. The process according to claim 11, wherein the basic solution comprises a sodium hydroxide solution ranging from 0.0075 M to 0.15 M, and the acidic solution comprises a hydrogen chloride solution ranging from 3 M to 5 M.

14. The process according to claim 10, wherein the post-synthesis treatment is performed at a temperature ranging from 35°C to 90°C.

15. The process according to claim 1, wherein the post-synthesis treatment is performed at a temperature ranging from 35°C to 90°C.

16. The process according to claim 14 or claim 15, wherein the post- synthesis treatment is performed at a first temperature in the range from 35°C to 90°C for at least 30 min then at a second temperature in the range from 35°C to 90°C, wherein the second temperature is at least 20°C higher than the first temperature.

17. The process according to claim 1, 6 or 10, wherein the post-synthesis treatment further comprises the steps of drying the catalyst and calcining the catalyst.

18. The process according to claim 17, wherein the drying is performed at a temperature above 105°C and the calcining is performed at a temperature above 400°C.