Method and system for removing light fractions and non-condensable substances to prevent accumulation in an olefin / paraffin membrane separation process

Abstract

The present invention provides systems and methods for preventing the accumulation of light fractions such as methane, ethylene, and ethane in an olefin / paraffin separation system that uses a combination of a membrane and a distillation column for the separation. In one embodiment, a small stripper column is provided downstream of a selective hydrogenation reactor. In another embodiment, a pressure regulating vessel with a receiver is added to the retentate stream of the membrane unit.

Description

Technical Field

[0001] The present disclosure generally relates to methods and systems for separating olefins and paraffins in a membrane system. More specifically, the present disclosure relates to methods for preventing the accumulation of light fractions in a propylene stream. Background Art

[0002] Methods incorporating membranes have been developed for purifying a stream of a mixture comprising propane and propylene. It has been found that the gas mixture going to these membranes contains light fractions including hydrogen, methane, ethane, and ethylene. These light fractions become a problem because they can accumulate in the system and degrade the performance of the upper compressor or the membrane without meeting the product specifications.

[0003] In a process for producing propylene, fluid catalytic cracking produces naphtha and crude oil liquid products as well as lighter hydrocarbons and hydrogen. The lighter hydrocarbons can be subjected to various treatments including removal of sulfur compounds by processes such as UOP's Merox process to oxidize mercaptan compounds and sent through a C3 / C4 separator to remove C4 hydrocarbons from the C3 / C4 stream as well as a C3 separator (a distillation tower to remove propane). In prior art processes, a deethanizer tower is used to remove and recycle C2.

[0004] In a typical prior art FCC process, after producing process products including a heavy naphtha product, a light cycle oil product, and a heavy oil product, there is a vapor stream containing some lighter hydrocarbons and unstable gasoline. The gasoline is removed for processing as fuel gas as is. There are C3 and C4 hydrocarbons which are sent for further processing including alkylation or polymerization and processes for removing mercaptans such as UOP's Merox process. After removing mercaptans, in a typical plant, the stream is sent to a C3 / C4 separator to recover C4 hydrocarbons and then C2 hydrocarbons are removed (as part of the tail gas stream) by a deethanizer and recycled to a gas concentration unit. The stream is then sent to a C3 separator to separate propylene and propane. Summary of the Invention

[0005] In one embodiment, the present invention is a method for removing hydrogen and lighter hydrocarbons from a hydrocarbon stream, the method comprising: conveying a gas stream comprising hydrogen and C1-C3 hydrocarbons to a stripper tower to produce a stripper tail gas stream from a top section of the stripper tower and a bottom stream comprising C3 hydrocarbons, conveying the bottom stream through a C3 separator to produce a second bottom stream comprising propane and a second top stream comprising propylene; sending the second top stream through a membrane unit to produce a permeate stream and a retentate stream, the permeate stream comprising a higher concentration of propylene than the second top stream and the retentate stream comprising a higher concentration of propane than the second top stream; and returning the retentate stream to the C3 separator.

[0006] Another embodiment of the present invention is a method for removing lighter hydrocarbons from a gas stream, the method comprising: sending a gas stream containing C1 to C3 hydrocarbons to a membrane unit to produce a propylene permeate stream and a retentate stream containing C1-C3 hydrocarbons; sending the retentate stream to a pressure regulating vessel to separate the retentate stream into a propane stream and a pressure regulating vessel tail gas stream containing C1 to C2 hydrocarbons; and sending the pressure regulating vessel tail gas stream to a gas concentration unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 A flow chart with an added stripper column is shown.

[0008] Figure 2 A flow chart with an added pressure regulating vessel is shown. DETAILED DESCRIPTION

[0009] In the present invention, there are two alternative methods for removing light fractions from a propylene stream. The first method is to remove them by providing a small stripper column after the selective hydrogenation reactor. The second method is to remove the light fractions leaving the retentate by providing a pressure regulating vessel with a packing section for efficient separation.

[0010] In existing plants, in the absence of an existing deethanizer that removes C2 and lighter components and returns them to the gas concentration unit, a small stripper column must be included to manage the light fractions and non-condensables leaving the KLP reactor effluent. The KLP process is a hydrogenation process for converting acetylene compounds to olefins. Light fractions (if not removed) can accumulate in the upper part of the C3 separator and can cause several problems: reducing compressor performance, increasing / accumulating concentration over time without an outlet (although light olefins, ethylene can permeate together with C3=), and reducing product purity. The stripper can operate at a pressure of 2170 - 3204 kPa (300 - 450 psig), or preferably 2377 - 2997 kPa (330 - 420 psig), and includes 10 - 30 trays, or preferably 15 - 25 trays. The stripper can also be a packed (with random / structured packing) column with high efficiency and low HETP (height equivalent to a theoretical plate), having 5 - 25 theoretical plates, or preferably 8 - 22 theoretical plates. At this operating pressure, the upper condensation temperature is 37.8 - 65.6 °C (100 - 150 °F), or preferably 43.3 - 60 °C (110 - 140 °F), which can be cooled with supplied air or cooling water. Hydrogen and methane can be completely removed in the upper part of the stripper, while C2 hydrocarbons can be removed up to 80 - 100%, or preferably 90 - 99%. The C2 concentration in the stripper bottom stream is 0 - 30 wt ppm, or preferably 2 - 25 wt ppm.

[0011] In the case of not using a stripper column, the second method for removing light fractions in the present invention is to install a pressure-regulating vessel with a receiver on the retentate stream of the membrane. Light fraction components such as methane and ethane cannot permeate but are retained together with propane. However, if there is no outlet, these components can easily accumulate in this stream due to the recirculation loop back to the column. Therefore, setting the temperature of the pressure-regulating vessel provides an outlet for these components and prevents their accumulation in the system. It should be noted that any C3 discharged together with C1 and C2 in the exhaust stream can be routed back to the gas concentration unit for further C3 recovery (the same as the treatment of deethanizer off-gas and stripper off-gas). The pressure-regulating vessel is a receiver that has a dehydration guide zone and a vertical discharge section at the top. The vertical discharge section is a packed section / chimney that has a height of 0.6 - 6.1 m (2 - 20 ft) or preferably 1.2 - 3.7 m (4 - 12 ft), and a diameter of 0.30 - 1.5 m (1 - 5 ft) or preferably 0.3 - 0.9 (1 - 3 ft). The top of the vertical section, 0.3 - 3.0 m (1 - 10 ft), or preferably 0.61 - 2.4 m (2 - 8 ft), is cooled with a refrigerant to maintain at 7.2 - 21.1 °C (45 - 70 °F), preferably 12.8 - 18.3 °C (55 - 65 °F). The refrigerant can be an external refrigerant or an internal process fluid (propylene product), which can be recycled and recompressed and cooled to meet the required cooling load. In the case of having a pressure-regulating vessel, the total ethane removal is 85 - 99%, preferably 90 - 98%. It should be noted that if the effluent of the acetylene hydrogenation reactor has more than 100 vol ppm of ethylene, the first configuration with a stripper is superior to the exhaust stripper due to the fact that ethylene can permeate through the membrane together with C3=. Even if the effluent of the acetylene hydrogenation reactor has up to 2800 mol ppm of ethane (or 0.28% molar C2 concentration), the exhaust stripper can effectively remove ethane. The exhaust stripper can also remove any water in the retentate and accumulate it in the guide section of the device. The off-gas from the pressure-regulating vessel has a C2 molar concentration of 0.1 - 20%, or preferably 0.5 - 10%, and can be routed to the gas concentration unit to recover C3 components. Those skilled in the art can change the sequence or apply the proposed configuration in various ways. A feasible combination will combine the two configurations with a stripper after the exhaust stripper on the KLP reactor and the retentate stream.

[0012] Figure 1 A solution is shown where the stripper 164 is shown after the selective hydrogenation reactor 158. In Figure 1In [the figure], a fresh hydrocarbon feed 126 is shown being sent to a fluid catalytic cracking reactor 120 (FCC) where it is processed according to methods well known in the art. A regenerator 114 is located downstream of the reactor 120, into which air 112 enters together with spent catalyst 122. The catalyst is regenerated and returned to the reactor 120 via line 124. A flue gas stream 116 is shown leaving the top of the regenerator 114. A stream 128 leaves the top of the reactor 120 and is sent to a main separation column 130 to be separated into a heavy recycle oil product 134, a light recycle oil product 136, and a heavy naphtha product 138. An upper stream 132 containing a mixture of hydrocarbons, hydrogen, and unstable gasoline leaves the top section of the main separation column 130 and is sent to a gas concentration unit 140. A gas stream 146, a debutanized gasoline stream 142, and a hydrocarbon stream 144 containing C1 to C4 hydrocarbons are sent to a unit 148 to remove mercaptans, thereby producing a stream 150, which is then shown being sent to a C3 / C4 separator 152, where a C4 bottoms stream 154 is sent to storage and a vapor top stream 156 is sent for further processing (shown as a processor 158, which includes a selective hydrogenation reactor in which acetylene reacts with hydrogen 159). A stream 160 is then sent to a stripper column 164 to produce a lighter tail gas stream 162 and a bottoms stream 166, which is then sent to a C3 separator 168 to produce a propane stream 170 that can be sent to storage (not shown) and a lighter stream 172 that is sent to a membrane unit 174. A retentate stream 176 is returned to the C3 separator 168, while a permeate stream 177 is sent to a compressor / dryer 180 to produce a propylene stream 182.

[0013] Figure 2 A solution is shown where a pressure regulating vessel 290 is shown treating the retentate 276 from the membrane unit 274. In Figure 2In [the figure], a fresh hydrocarbon feed 226 is shown being sent into a fluid catalytic cracking reactor 220 (FCC) where it is processed according to methods well-known in the art. A regenerator 214 is located after the reactor 220, into which air 212 enters together with spent catalyst 222. The catalyst is regenerated and returned to the reactor 220 via line 224. A flue gas stream 216 is shown leaving the top of the regenerator 214. A stream 228 leaves the top of the reactor 220 and is sent to a main separation column 230 to be separated into a heavy recycle oil product 234, a light recycle oil product 236, and a heavy naphtha product 238. An upper stream 232 containing a mixture of hydrocarbons, hydrogen, and unstable gasoline leaves the top section of the main separation column 230 and is sent to a gas concentration unit 240. A fuel gas stream 246, a debutanized gasoline stream 242, and a hydrocarbon stream 244 containing C1 to C4 hydrocarbons are sent to unit 248 to remove mercaptans, thereby producing a stream 250, which is then shown being sent to a C3 / C4 separator 252, where a C4 bottoms stream 254 is sent to a storage device and a vapor top stream 256 is sent for further processing (shown as a processor 258, which includes a selective hydrogenation reactor where acetylene reacts with hydrogen 259). Then a stream 260 is sent to a C3 separator 268 to produce a propane stream 270 that can be sent to a storage device (not shown) and a lighter stream 272 that is sent to a membrane unit 274. A hold-up stream 276 is returned to the C3 separator 268, while a permeate stream 277 is sent to a compressor / dryer 280 to produce a propylene stream 282. A pressure regulating vessel 290 through which the hold-up gas stream 276 is passed is shown. The hold-up gas stream is first cooled by a cooler 275, where most of the liquid is sent to the C3 separator 268 via line 294, water leaves at 296 and is optionally recycled to the lighter stream 272 to bring this stream to an appropriate humidity level for entering the membrane unit 274. An exhaust tail gas stream 262 is returned to the gas concentration unit 240. Also shown is an inlet 292 for a refrigerant stream such as propane refrigerant for entering the pressure regulating vessel 290, and a refrigerant stream 293 leaving the pressure regulating vessel 290.

[0014] Specific implementation examples

[0015] Although the following is described in connection with specific embodiments, it should be understood that the description is intended to illustrate and not limit the scope of the foregoing description and the appended claims.

[0016] A first embodiment of the present invention is a method for removing hydrogen and lighter hydrocarbons from a gas stream, the method comprising: conveying a gas stream containing hydrogen and C1-C3 hydrocarbons to a stripper column to produce a stripper offgas stream from the top section of the stripper column and a bottom stream containing C3 hydrocarbons, conveying the bottom stream through a C3 separator to produce a second bottom stream containing propane and a second top stream containing propylene; sending the second top stream through a membrane unit to produce a permeate stream and a retentate stream, the permeate stream containing a higher concentration of propylene than the second top stream and the retentate stream containing a higher concentration of propane than the second top stream; and returning the retentate stream to the C3 separator. One embodiment of the present invention is one, any or all of the first embodiment to the previous embodiments in this paragraph, wherein the stripper column operates at a pressure of 2170 - 3204 kPa (300 - 450) psig and preferably 2377 - 2997 kPa (330 - 420) psig. One embodiment of the present invention is one, any or all of the first embodiment to the previous embodiments in this paragraph, wherein the stripper column comprises 10 - 30 trays and preferably 15 - 25 trays. One embodiment of the present invention is one, any or all of the first embodiment to the previous embodiments in this paragraph, wherein the stripper column is packed with packing and comprises 5 - 25 theoretical plates, preferably 8 - 22 theoretical plates. One embodiment of the present invention is one, any or all of the first embodiment to the previous embodiments in this paragraph, wherein 100 wt% of hydrogen and methane and 80 - 100 wt% of C2 hydrocarbons are removed in the stripper column and sent to a gas concentration unit in the upper stream. One embodiment of the present invention is one, any or all of the first embodiment to the previous embodiments in this paragraph, wherein the stripper bottom stream contains 0 - 30 wt ppm of C2 hydrocarbons. One embodiment of the present invention is one, any or all of the first embodiment to the previous embodiments in this paragraph, wherein the hydrocarbon stream containing hydrogen and C1-C3 hydrocarbons contains 0 - 10 mol% hydrogen and preferably 0 - 2 mol% hydrogen. One embodiment of the present invention is one, any or all of the first embodiment to the previous embodiments in this paragraph, wherein the hydrocarbon stream containing hydrogen and C1-C3 hydrocarbons contains 0 - 10 mol% methane, preferably 0 - 2 mol% methane and 0 - 10 mol% C2 hydrocarbons, preferably 0 - 5 mol% C2 hydrocarbons.One embodiment of the present invention is one, any, or all of the first embodiment to the previous embodiments in this paragraph, wherein the hydrocarbon feed stream containing hydrogen and C1-C3 hydrocarbons is monitored by an analyzer and an on-line system to monitor and measure the H2, CH4, and C2 hydrocarbon concentrations. One embodiment of the present invention is one, any, or all of the first embodiment to the previous embodiments in this paragraph, wherein the stripper off-gas stream communicates with an upper condenser and a discharge condenser. One embodiment of the present invention is one, any, or all of the first embodiment to the previous embodiments in this paragraph, wherein the discharge condenser vapor outlet stream contains 0-10 mol% hydrogen, preferably 0-5 mol% hydrogen and 0-50 mol% C2 hydrocarbons or preferably 0-35 mol% hydrocarbons. One embodiment of the present invention is one, any, or all of the first embodiment to the previous embodiments in this paragraph, wherein the discharge condenser vapor is directed to a fuel gas system, discharged, or directed to a gas concentration unit where ethylene and propylene are recovered. One embodiment of the present invention is one, any, or all of the first embodiment to the previous embodiments in this paragraph, wherein the discharge condenser vapor is completely condensed, and no condensable gas is detected in the feed stream sent to the stripper column. One embodiment of the present invention is one, any, or all of the first embodiment to the previous embodiments in this paragraph, wherein the stripper column is operated to communicate directly with the C3 separator column by bypassing the feed stream to the stripper column.

[0017] A second embodiment of the present invention is a method for removing hydrogen and lighter hydrocarbons from a gas stream, the method comprising: sending a gas stream comprising C1 to C3 hydrocarbons to a membrane unit to produce a propylene permeate stream and a retentate stream comprising C1-C3 hydrocarbons; sending the retentate stream to a pressure regulating vessel to separate the retentate stream into a propane stream and a pressure regulating vessel tail gas stream comprising C1 to C2 hydrocarbons; and sending the pressure regulating vessel tail gas stream to a gas concentration unit. One embodiment of the present invention is one, any or all of the second embodiment in this paragraph to the previous embodiments in this paragraph, wherein the pressure regulating vessel is a receiver, the receiver having a dehydration guiding zone and a vertical discharge zone at the top section of the pressure regulating vessel. One embodiment of the present invention is one, any or all of the second embodiment in this paragraph to the previous embodiments in this paragraph, wherein the vertical discharge zone is a packing zone, the packing zone having a height of 0.6 to 6.1 m and preferably 0.3 to 0.9 m. One embodiment of the present invention is one, any or all of the second embodiment in this paragraph to the previous embodiments in this paragraph, wherein the top section of the pressure regulating vessel is cooled to 7.2-21.1 °C. One embodiment of the present invention is one, any or all of the second embodiment in this paragraph to the previous embodiments in this paragraph, wherein the pressure regulating vessel tail gas stream contains 0.5 to 10 mol% C2 hydrocarbons. One embodiment of the present invention is one, any or all of the second embodiment in this paragraph to the previous embodiments in this paragraph, wherein a total of 85 to 99 wt% of ethane is removed.

[0018] Although no further detailed description is provided, it is believed that those skilled in the art can make the most of the present invention by using the foregoing description and can easily determine the basic features of the present invention without departing from the essence and scope of the present invention to make various changes and modifications thereto and adapt it to various uses and conditions. Therefore, the foregoing preferred specific embodiments should be understood as being illustrative only and not in any way limiting the remainder of the disclosure, and are intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

[0019] In the foregoing, all temperatures are shown in degrees Celsius and all parts and percentages are by weight unless otherwise indicated.

Claims

1. A method for removing hydrogen and lighter hydrocarbons from a hydrocarbon feed stream, the method comprising: a. transferring a hydrocarbon feed stream containing hydrogen and C1-C3 hydrocarbons to a stripper column to produce a stripper offgas feed stream from the top section of the stripper column and a bottoms feed stream containing C3 hydrocarbons; b. transferring the bottoms feed stream through a C3 separator to produce a second bottoms feed stream containing propane and a second tops feed stream containing propylene; c. sending the second tops feed stream through a membrane unit to produce a permeate stream and a retentate stream, the permeate stream containing a higher concentration of propylene than the second tops feed stream and the retentate stream containing a higher concentration of propane than the second tops feed stream; d. sending the retentate stream to a pressure regulating vessel to separate the retentate stream into a propane stream, a water stream, and an offgas stream containing C1 to C2 hydrocarbons; and e. returning the propane stream to the C3 separator.

2. The method according to claim 1, wherein the stripper column operates at a pressure of 2170 - 3204 kPa, 300 - 450 psig.

3. The method according to claim 1, wherein 100 wt% of hydrogen and methane and 80 - 100 wt% of C2 hydrocarbons are removed in the stripper column and sent to a gas concentration unit in the upper feed stream.

4. The method according to claim 1, wherein the stripper bottoms feed stream contains 0 - 30 wt ppm of C2 hydrocarbons.

5. The method according to claim 1, wherein the hydrocarbon feed stream containing hydrogen and C1-C3 hydrocarbons contains 0 - 10 mol% methane and 0 - 10 mol% C2 hydrocarbons.

6. The method according to claim 1, wherein the hydrocarbon feed stream containing hydrogen and C1-C3 hydrocarbons is monitored by an analyzer and an on-line system to monitor and measure the H2, CH4, and C2 hydrocarbon concentrations.

7. The method according to claim 1, wherein the stripper offgas feed stream communicates with an upper condenser and a discharge condenser.

8. The method according to claim 7, wherein the discharge condenser vapor outlet feed stream contains 0 - 10 mol% hydrogen and 0 - 50 mol% C2 hydrocarbons.

9. The method according to claim 8, wherein the discharge condenser vapor is directed to a fuel gas system, discharged, or directed to a gas concentration unit where ethylene and propylene are recovered.

10. The method according to claim 9, wherein the discharge condenser vapor is completely condensed and no condensable gas is detected in the feed stream sent to the stripper column.

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