Background of the Invention Petrochemical offgases, such as refinery offgases from Fluid Catalytic Cracker Units (FCCU) or Coker units, generally comprise an offgas mixture which comprises: hydrogen; nitrogen; carbon monoxide; ethane; ethylene; argon; propylene; as well as, butane and pentanes. In particular, the refinery offgasses from a FCCU contain olefin components, up to about 20 percent by volume ethylene and up to about 11 percent by volume propylene, which components normally are not recovered from the offgases, but which components may have value to warrant recovery and use in other petrochemical processes or uses in downstream processing. The typical range of components is listed in Table 1.

Normally, such refinery offgases are burnt and used as fuel. When the refinery FCCU is located near an ethylene or propylene plant, then recovery and downstream use of the recovered ethylene and propylene or propylene is economical.

In such recovery processes, ethylene and propylene are recovered together and then directly or separately recovered from the mixture.

In the publication, Chemical Engineering, May 1999, pages 30-33,"Refiners Get Cracking on Petrochemicals"hereby incorporated by reference, the attributes of Fluid Catalytic Cracker (FCC) technology are explained. Propylene is increasing in demand, since it is a feedstock for polypropylene. About 30 percent (30%) of the global demand is a byproduct of FCC operations for producing gasoline. Over the next five years, the growth in demand is anticipated at

over 5 percent (5%) per year. In response to this demand, FCC process technology is being modified to enhance olefin's production. Thus, the current yield of 5 percent (5%) of the feedstock converted to propylene is being increased to about 35 percent (35%), while the ethylene yield is increased from about 1 to 4 percent (1-4%).

Another option is the reaction of ethylene with 2-butene to produce propylene, termed metathesis. Thus, the recovery of these components from a FCC provides the opportunity for further propylene yields.

U. S. Patent No. 5,546,764, issued August 20,1996, and incorporated herein by reference, discloses an absorption process for recovering ethylene from a feed gas stream in an absorber stripper employing a heavy hydrocarbon absorption solvent, the absorber stripper bottoms stream is then fractionated to produce an overhead, ethylene product stream.

The recovery of valued olefin components from offgasses may be accomplished by partial condensation from the offgas feed stream; however, such recovery requires low temperatures, e. g., about-150 F or lower. These low temperatures require refrigeration supplied by turboexpander processes and heat exchangers with multi-pass plate fin heat exchangers. This type of equipment is not used extensively in oil refineries.

Furthermore, the use of such low temperatures increases the possibility of gum formation by the reaction of NOX compounds with butadiene, which gums may be explosive when the plant is shut down and warmed up for restart.

U. S. Patent No. 5,345,772, issued September 13,1994, shows a single column process with Natural Gas Liquid (NGL) recycled to the upper zone of the column. This process is applied to the recovery of the paraffinic components of propane or ethane from gas streams with a high CO2 content.

These gases are encountered in enhanced oil recovery projects where the CO2 content of the gas stream may be 83 percent (83%), as shown in the patent example.

It is desirable to provide a system and process for the recovery of olefins from offgases at warmer temperatures, such as propane refrigeration levels, to avoid the disadvantages of prior art recovery processes and systems and to provide other economic and process efficiency advantages.

Summary of the Invention The invention relates to a system and process for the recovery of olefins from petrochemical offgases. In particular, the invention concerns the recovery of ethylene and propylene or propylene from refinery offgases from a Fluid Catalytic Cracker Unit (FCCU) of a refinery at propane refrigeration temperatures.

The process provides for maintaining the overhead condenser temperature by the use of the liquid bottoms recycle stream from the single column. Generally, the overhead condenser is maintained at above-140°F, but depending on the olefin component to be recovered, the overhead condenser temperature may range, for example, from above about-114°F for ethylene recovery or above about-35 to-40°F for propylene recovery, or within a range of about-20 F to-114 F for a mixture of ethylene and propylene.

The system and process generally comprises a single distillation column to receive an olefin-containing, hydrocarbon gas stream and maintaining the overhead condensation temperature at a warmer temperature than-150°F, such as, at a refrigeration temperature level of above -140°F, by recycling a heavy C3+ bottoms stream from the single column, to the overhead condenser or upper section of the distillation column, and withdrawing a liquid bottoms stream ; a vapor side stream rich in the selected olefin; and an overhead vapor stream lean in the olefin components.

The system and process of the invention comprises a system and process for the recovery of olefin components, such as: ethylene; propylene; or combinations thereof from a refinery offgas, such as: a FCCU; a reformer; a Coker offgas;

or other olefin gas source, which process comprises: introducing the gas feed stream into a recovery distillation column with an overhead condenser; withdrawing a liquid bottoms product stream from the recovery column; recycling at least a portion of the liquid or lower section bottoms product stream into the overhead condenser or into an upper tray section, e. g., top 5 to 10 trays of the recovery column, to warm the overhead condenser; maintaining with the recycled product stream, the overhead condenser at selected propane, propylene, or ethylene refrigeration levels of above-140°F and generally above-114; withdrawing a heavy liquid product stream from the recovery column; withdrawing a vapor phase side downstream from the column, which side downstream is rich in the olefin components to be recovered; and condensing and recovering the selected olefin components from the vapor phase side downstream, for downstream processing use; and withdrawing an olefin lean, overhead vapor stream from the overhead condenser or an upper section of the recovery column; and optionally, employing the lean overhead vapor stream as regenerated gas in the dehydration of the feed gas stream, fuel use, or other use.

Typically, and optionally, the refinery offgas feed stream process includes: pretreating steps of water washing to remove impurities; removing acid gases, like carbon dioxide and hydrogen sulfide; and dehydrating the offgas feed stream.

The process may be directed to the recovery of propylene or ethylene-propylene, generally, for downstream processing use, such as, where an ethylene-propylene petrochemical plant is located near the offgas source.

The system and process provide multiple economic and process advantages over prior art systems and processes to include, but not be limited to: eliminating the requirement of a separate column to produce the C+ recycle stream, while only propane level refrigeration is required in some instances for the system and process. The system and process eliminate

the formation of dangerous and explosive prior art gum formation, because of the relatively warm refrigeration level temperatures employed.

When propane refrigeration alone is used, the process permits the use of a carbon steel column and carbon steel shell and tube heat exchangers, due to the propane or higher temperature refrigeration levels and no expensive turboexpanders are employed. While in one embodiment illustrated, acid gas removal occurs in the offgas feed stream, the acid gas need not be removed upstream of the recovery column, because of potential freezing in the recovery column as in prior art cryogenic systems, but may be removed downstream of the recovery column. In the system and process, less severe dehydration of the offgas feed stream is required than for a low temperature cryogenic system, so that glycol- based dehydration may be used, e. g., the DRIZOe process (DRIZO is a registered trademark of OPC Design, Inc. for a gas dehydration system). The system and process provide for high olefin recovery, for example, from a FCCU offgas, about 98% + for propylene, with low C2/C3 ratios of less than about 0.01, and about 90% + for ethylene.

The process and system can be used for the recovery of ethylene and ethane for the offgases ; however, the utility consumption will be higher, and the C2 components of the offgas now become the side downstream along with the C3 components.

The system and process shall be described for the purposes of illustration only in connection with certain embodiments; however, it is recognized that various changes, additions, improvements and modifications to the illustrated embodiments may be made by those persons skilled in the art, all falling within the spirit and scope of the invention.

Brief Description of the Drawing The drawing is a schematic illustration of the process and system for the recovery of propylene from a Coke or FCCU offgas source.

Description of the Embodiments The same general system and process configuration in the drawing can be employed for either propylene (and heavier components) recovery or ethylene (and heavier components), including propylene recovery.

In the drawing, process steps are designated by the 10, 11,12,13 series. Process equipment within step 13 is designated by the 20,21.... series. Process streams within step 13, designated by the 1,2.... series.

The refinery offgases considered in this described embodiment are the combined FCCU and Coker gases. The gases are at low pressure, near atmospheric pressure, they are compressed to about 270 psig in compressor 10, cooled in exchanger 11 to 100 F, and then processed in stages in a pretreatment step 12. These stages may be comprised of a waterwash; an amine contactor column for H2S removal or other acid gas removal; and a dehydration stage for water vapor removal. The treated gas stream now enters the single column process 13.

The following is a description of the single column process for propylene recovery. The feed vapor 1 enters column 20 at tray 16. Tray designations are theoretical as encountered in the computer simulations. Tray 1 is the top tray, in this case, the reflux condenser 21. The two phase liquid-vapor stream from the overhead reflux condenser 21 is directed to a reflux drum 22 for reflux and separation. The separated, reflux liquid stream from reflux drum 22 is returned to the upper tray section of the column 20. The separated, overhead vapor stream 6 is withdrawn as an olefin- lean product stream.

The column has a side reboiler 23, for heat conservation purposes, at tray 33. A side vapor draw, stream 8, is extracted from the column 20 at this stage. The use and positioning of this side reboiler is important in concentrating the side draw in the desired propylene to be recovered and minimizing the ethylene component. The side reboiler 23 employs liquid from an intermediate tray of the column, below the point of introduction of the feed stream, and then after reboiling, returning the reboiled liquid to the tray below the tray from which the liquid is withdrawn. The use of a column intermediate side reboiler 23 enhances the concentration of the olefin component in the vapor side draw stream 8. The vapor side draw stream 8 is withdrawn from between the two intermediate trays used for the side reboiler 23.

The bottom reboiler 25 is tray 43. The bottoms liquid, stream 9, is pumped by 27, then cooled in 26 and split into two streams 4 and 5. Stream 5 is cooled and recycled to the condenser 21, joining the column overhead vapor stream at the condenser inlet. This stream is partially condensed by refrigerant in the reflux condenser 21. The vapor stream 6, and liquid stream 7, are separated in the reflux drum 22. The reflux is returned to the column 20. The vapor is reheated against the recycled stream 5 in the exchanger 27, simultaneously chilling this stream to-20°F.

The vapor phase side draw, stream 8, is cooled and condensed in exchanger 24; stream 3 is then the light hydrocarbon product. The split off stream 4 is exported as the heavy liquid hydrocarbon product. The reheated stream 2 goes to the refinery fuel gas stream.

The operating conditions for the column are listed in Table 2, and the overall material balance and the recycle stream flow and composition is given in Table 3.

The following features of this column simulation are important for the following reasons:

1) The column is operated at relatively low pressure, 250 psig.

2) The column's reflux condenser operates at-35 F.

This level of refrigeration can be supplied by propane or propylene. The materials of construction can be carbon steel for the entire system.

3) All the propylene recovered is in the light liquid hydrocarbon stream, and 98 percent (98%) of the propylene in the feedstock is recovered in this stream. The C2 and lighter components are rejected to the fuel gas. The C2 specification for stream 8 is: 4) The heavy liquid stream contains C, + components, and it is predominately C6+ and has a molecular weight of 79.4.

These components are available in the feed gas. The composition of this recycled stream and net heavy liquid product is dependent on their relative quantities in the feed.

The recycle to feed ratio is 0.18 moles/mole.

5) The coldest temperature in the tower, i. e.,-35°F, is well above any potential gum deposition temperature, i. e., -150 F.

The single column, ethylene recovery process flow scheme is the same as shown in the drawing. The column operating conditions are altered to achieve the high ethylene recovery.

The column conditions are listed in Table 4. The overall material balance and recycle stream flow and composition are listed in Table 5.

The features of this operation are: 1) The column operates at 250 psig, which is a considerably lower pressure than that disclosed in U. S. Patent No. 5,546,764, issued August 20,1996, in which the absorber is operated at 550 psig, which requires higher power consumption for the feed gas compressors.

2) The overhead temperature is-114 F. A cascade refrigeration system is required for this system, and stainless steel materials are required where the lower temperatures are encountered.

3) The ethylene recovery is about 90 percent (90%).

The CH4 ratio = 0.0025 C2H6 + C, H, in the recovered light liquid stream. This temperature can be set to eliminate the need for downstream demethanization of the purified ethylene.

4) The recycle stream is comprised of C3+ components, which are available in the feed. A net purge, stream 4, maintains the material balance for these components. The recycle to feed ratio is 0.47 moles/mole.

5) The lowest temperature in the system is above the gum formation and deposit temperature.

The cited specific examples (supra) for propylene and ethylene recovery supply the details of stream compositions and flows and the column conditions. It is recognized that the system and process may be employed to any olefin recovery design process requirement, as appropriate.

The range of conditions for which the column may be designed is as follows: Pressure, psig 150 to 550 Condenser temperature, °F-140 to 0 Recycle stream composition C3+, C, +, C5+ or C6+ Recycle/feed ratio, mole/mole 0.03 to 1.0 Olefin components recovered C, H6+ or C2H, +

Another embodiment of the process and system of the invention is the recovery of ethylene from synthesis gas produced by the steam cracking of hydrocarbons in an ethylene production facility. A typical gas composition contains: Typical Ethylene Plant Syngas Gas Percentage/volume H2 23 Cl 19 C2H432 C2H6 13 C3H67 C3H81 C, + 3 The ethylene plant syngas is similar to the component range shown for refinery offgases and is within the capability of the process and system of the invention.

TABLE 1 FCC and Coker Offgas Component Concentration Ranges: Component Percentage Volume H2 7 to 22 CO2 0 to 2 H2S 2 to 11 N2 1 to 17 CH4 25 to 45 C2H6 12 to 16 C2H4 2 to 20 C3He 0.5 to 8 C3H6 2 to 11 i, nC4HlO 1 to 2 Butenes 0.4 to 4 C5 1 to 3 CH4

TABLE 2 Single Column Operation Propylene Recovery TRAY HEATER/PRESSURE TEMPERATURE DUTY COOLER PSIG F MMBTU/HR 1 CONDENSER 250-35 5.1 16 FEED 257 50 33 SIDE REBOIL 258 175 4.9 VAPOR DRAW 43 REBOILER 260 345 12.3

TABLE 3 Single Column Material Balance Propylene Recovery 12345Strea@ID NaeFeed Fuel Gas Light Liquid Heavy Liquid Recycle PhaseMixedDryVaporDryLiquidDryLiquidDryLiquid @luid Rates, lb/mol/hr @ H2O # 0000.0000 2 H2S. 0295. 0288 6.4850E-04 7. 6394E-10 1.0955E-08 3 N2 101.0916 101.0917 4 CO 15. 3521 15.3521 14.745214.74522.3822E-061.1595E-141.6628E-135CO2 287.0592287.0594.0000.0000.00006H2 7 C1 1108.8446 1108. 8454 8.5987E-10 4. 6351E-20 6.6471E-19 8 Ethylene 302.8797 302.8713 8. 6336E-03 5.9396E-10 8.5179E-09 9 C2 486. 4302 484.5020 1.9285 1. 1477E-06 1.6459E-05 10 Propylene 254.2555 5.1181 5000 138.7629.4241138.2981.0402.576911C3 12 Isobutene 18. 1834.3218 17.3523.5092 7. 3024 13 @Butene 25.3941 . 4805 24.1390.7745 11.1070 14 T2Butene 17-9134.4505 16.4713.9915 14.2194 15 C2Butene 12.9536.3417 11.7580.8538 12.2442 16 13Butd. 3810 7.2268E-03.3614.0124.1782 17 IC, 31.2459.4924 30.1260.6275 8.9985 18 NC4 23. 5549.7016 21.6213 1. 2319 17.6671 19 3M1Butene .3733 .0124 . 2993.0616.8838 20 lPentene 6.1519.1679 4. 6490 1.3349 19. 1431 21 2M1Butene 3. 1259.0817 2.3595.6846 9. 8181 22 .11754.08641.574822.58435.7787 23 T2Pentene 5.0993 . 1166 3.7056 1. 2771 18.3152 24 C2Pentene 2.9111.0658 2.1088.7365 10.5618 25 ICs 13.2697 3. 5967 51.5801 26 NC5 8.2941.2025 6.0681 2. 0235 29.0189 22.9652.249713.01439.7010139.1198271Hexene 28 NC6 14.8743.1235 7.9553 6. 7954 97.4522 5.6047.01522.00503.584551.404729NC7 30 NC8 1.1669 9107 13.0599 31 NC9 .1702 1321 32 NC10 . 0463 1.4052E-06 3.2544E-03.0431.6178 Total Rate, lb-@ol/hr 2933.0148 2324. 4973 570.9684 37. 5492 538.4857 Temperature, #F 100. 0 81.0 100.0 100. 0 100.0 Pressure, psig 260 246 254.7 252 300 26.266119.730249.380779.394479.3944MolecularWeight

TABLE 4 Single Column Operation Ethylene Recovery TRAY HEATER/PRESSURE TEMPERATURE DUTY COOLER PSIG F MMBTU/HR 1 CONDENSER 250-114 2.8 17 FEED 257 17 27 SIDE REBOIL 258 155 13.9 VAPOR DRAW 44 REBOILER 260 337 20.1

TABLE 5 Single Column Material Balance Ethylene Recovery Strea@23451 NameNameFeed Fuel LiquidHeavyLiquidRecycleLight Phase VaporMixedDryLiquidDryLiquidDry Fluid Rates. lb/ol/hr .0000.0000.0000.0000.00001H2O .02951.4165E-08.02956.3460E-108.7321E-082H2S 3 N2 101. 0916 101.0918 8.3535E-06. 0000 # 0000 4 CO 15. 3521 15.3521 0000 S CO2 14.7453 8.9805 5. 7650 5.3078E-12 7. 3035E-10 6 H2 287.0592 287.0596 1108.84461106.95241.89372.6010E-163.5790E-147C1 302.879731.7627271.12542.8096E-083.8660E-068Ethylene 9 C2 486.4302.0118 486.4334 2. 6369E-06 3. 6284E-04 10 Propylene 254.2555.0605 254.1671.0358 4. 9273 11 C3 138. 7629.0910 3670 12 Isobutene 18.1834.4567 17.8763.2600 35.7821 13 1Butene 25. 3941.0635 24.9574.3725 51.2536 14 T2Butene 17.9134.0380 17.5477.3269 44.9778 15 C2Butene 12.9536.0238 12.6742.2549 35.0752 16 13Butd. 3810 8303E-03.8023 17 IC, 31.2459.1028 30.7498. 3930 54. 0781 23.5549.058823.0823.412856.803418NC4 19 3M1Butene # 3733 4.9747E-04.3606.0121 1.6673 20 lPentene 6.1519 5.3771E-03 5.8889.2567 35.3160 21 2MlButene 3.1259 2. 5028E-03 2.9901.1328 18.2728 22 2H2Butene 5. 7787 2.9723E-03 5.4626. 3120 42. 9258 23 T2Pentene 5.0993 03.2742E-03 4.8462.2489 34.2422 24 C2Pentene 2.9111 1.7979E-03 2.7650. 1438 19. 7891 25 IC5 17.3760 .0169 16.6683.6883 94.7113 26 NC5 8. 2941 6.1164E-03 9813 27 lHexene 22.9652 5.2600E-03 20. 7222 2.2284 306. 6281 28 NC6 14.8743 2.5264E-03 13.2102 1. 6546 227. 6777 29 NCv 5. 6047 1.9756E-04 4.37241. 2264168.7467 30 NC8 1.1669 7.1470E-06.7216.4426 60.8989 31NC.,. 1702 1.2484E-07.0722.0972 13.3781 32 NC10 # 0463 3.0275E-09.0118.0342 4.7007 Total Rate, lb-@ol/hr 2933.0149 1551. 7443 1371.2705 10. 0000 1376. 0029 Temperature,' ? 100.0 85.5 100.0 100. 0 100. 0 Pressure, psi@ 270. 5 246.0 253.9 252 300 Molecular Weight 26. 2661 14.7674 38.8876 79. 6262 79.6262