METHOD AND APPARATUS FOR MINIMIZING RECYCLING IN AN UNSATURATED GAS PLANT

Abstract: An unsaturated gas plant includes a first unit which receives a low pressure hydrocarbon gas input and provides a liquid output and a vapor output, an absorber which receives unstabilized gasoline and lean oil, a stripper, and a low temperature separator which provides a vapor feed to the absorber and a liquid feed to the stripper. A high temperature separator receives liquid and vapor from the first unit, bottoms liquid from the absorber and overhead vapor from the stripper and which provides a hot liquid feed to an upper section of the stripper and a vapor to the low temperature separator, after passing through a condenser. Some unstabilized gasoline is diverted from the absorber to the high temperature separator. The diverted unstabilized gasoline can be taken directly from a main column fluid catalytic conversion system fractionator.

Description: The present invention relates to unsaturated gas plants for use downstream of fluid catalytic cracking (FCC) or Thermofor catalytic cracking (TCC) units. Catalytic cracking units generate a lot of light olefins or unsaturated gas. These light olefins are usually recovered in an unsaturated gas plant. In conventional unsaturated gas plants, the compressor aftercooler acts like a partial condenser in the stripper. This causes excessive recycle between the low temperature separator and the stripper. Also, because all unstabilized gasoline enters the absorber, excessive light ends recycling occurs between the low temperature separator and the absorber. A conventional unsaturated gas plant is shown in Fig. 1. Low pressure gas rich in light olefins from, e.g., a FCC main column overhead receiver is fed to a first stage compressor 1. Unstabilized gasoline, the liquid phase from the main column overhead receiver is fed to primary absorber 3. The compressed gas from compressor 1 is fed to interstage cooler 5 which cools this gas and condenses some liquid. The gas going to second stage compressor 9 is cooled which increases energy efficiency. The cooled gas and condensed liquid from cooler 5 are sent to interstage receiver/separator 7. A gas phase is sent to compressor 9 and a liquid phase removed via line 11. Line 11 also contains water wash to the unsaturated gas plant. Compressed gas from second stage compressor 9 is combined with bottoms product from primary absorber 3, stripper overhead from stripper 13 and liquid from separator 7 to form a gas/liquid mixture in line 25 which is fed to aftercooler 17. The cooled mixture from aftercooler 17 enters low temperature-high pressure separator 15 where it is flashed and water is separated from the hydrocarbons. The liquid hydrocarbon phase from separator 15 is fed to stripper 13. The vapor phase from separator 15 is fed to primary absorber 3. Bottoms product from stripper 13 is passed to a debutanizer, not shown, while stripper 13 overhead vapor is sent via line 19 to mix with lines 11, 21 and 23 prior to being fed to aftercooler 17. The Fig. 1 prior art system is not as energy-efficient as desired due to mixing of the hot gas from compressor 9 and stripper 13 with cool liquid from separator 7 and absorber 3. After mixing, the mixture is sent through aftercooler 17 to three-phase separator 15. Line 29 carries a mixed stream at relatively low temperature into separator 15 . The low temperature liquid in line 29 absorbs a large amount of light ends. Thus, the hydrocarbon liquid phase from separator 15 contains a relatively large amount of light ends. Stripper 13 and its reboiler 31 must be oversized to reject light ends from stripper 13 via line 19. Phrased another way, stripper 13 removes light hydrocarbons via line 19, but much of this material is absorbed (in the hydrocarbon liquid in line 29 and separator 15) and recycled back to stripper 13. Although this process works, it would be beneficial if a more energy efficient system was available. Accordingly, the present invention provides an unsaturated gas plant apparatus, comprising a low pressure separator 7 for recovering a low pressure gas from a liquid, an absorber 3 for receiving an unstabilized gasoline feed and a lean absorber oil which produces a rich absorber oil as a bottoms product, a stripper 13, a low temperature separator 15 discharging an overhead vapor to the absorber 3 and liquid to the stripper 13, characterized by a high temperature separator 33 for separating a vapor/liquid mixture comprising the low temperature separator 15 liquid and the low pressure separator 7 gas, rich absorber oil from the absorber 3 and stripper 13 overhead vapor which provides a high temperature liquid hydrocarbon feed to the stripper 13 and a high temperature vapor phase which is cooled and discharged to the low temperature separator 15. Fig. 1 shows a prior art unsaturated gas plant. Fig. 2 shows an unsaturated gas plant of the present invention. Fig. 3 shows additional features of an unsaturated gas plant of the present invention. The unsaturated gas plant of the present invention provides increased energy efficiency by recovering thermal energy which is wasted in the prior art system shown in Fig. 1. The invention separates hot liquid hydrocarbons from the aftercooler feed. As shown in Figs. 2 and 3, hot liquid hydrocarbons from high temperature separator 33 enter stripper 13 after mixing with the low temperature separator 15 liquid hydrocarbons. The stripper feed is hotter, e.g., about 24°C (40°F) than in the Fig. 1 system. Feed to stripper 13 is decreased, decreasing recycle in stripper 13. These factors reduce the stripper 13 reboiler 51 duty. Figs. 2 and 3 show a high temperature separator 33 which receives gas from compressor 9 and stripper 13 overhead and liquid from absorber 3 bottoms and separator 7, via line 35. This corresponds to line 25 in the Fig. 1 system, which carries this mixed stream directly to condenser 17. Significant energy savings are achieved by pumping hot liquid from separator 33 via line 41 to stripper 13 to increase the feed temperature and feed molecular weight. This reduces the reboiler duty in the stripper 13 reboiler. Separator 33 overhead vapor in line 37 contains less heavy ends so the bottoms product from separator 15 contains relatively less light ends. Moreover, the amount of bottoms product from separator 15 is much less than the amount of bottoms in line 41, from separator 33. Recycling of light ends between stripper 13 and separator 15 is reduced compared to the system of Fig. 1. Further, in the Figs. 2 and 3 systems, aftercooler 17 has a smaller duty. Fig 3 differs from Fig. 2 in that a portion of the unstabilized gasoline feed in line 43 is diverted via line 47 to separator 33. Line 47 can connect with line 35 as shown, or to any of lines 11, 19, 21 or 23. Adding unstabilized gasoline via line 47 decreases the primary absorber liquid load and the total recycle of light components in and out of the primary absorber. Because part of the unstabilized gasoline is bypassed to separator 33 and because the debutanized gasoline is slightly increased to maintain the same liquid petroleum gas recovery, the liquid load of absorber 3 is decreased in addition to decreasing the recycle between absorber 3 and separator 15. Liquid from separator 33 can be fed via line 42 directly into stripper 13 at a tray somewhat below the line 43 feed point. Line 44 diverts cool liquid from line 21 to line 41 to provide temperature control of hot liquid from separator 33. The embodiments of Figs. 2 and 3 with separator 33, do not increase the wash water requirement as compared to a conventional system, e.g., Fig. 1, which uses only a low temperature separator 15. The water wash system can remain the same, except that wash water enters separator 33 before entering aftercooler 17. A pump may be necessary to pump wash water from high temperature separator 33 to aftercooler 17. The present invention is also applicable to an unsaturated gas plant with a one-tower de-ethanizer-absorber system. The efficiency benefits will probably not be as great in a single-tower type system, as compared to a Fig.1-type unsaturated gas plant. In one-tower de-ethanizer-absorber systems, the stripper overhead and absorber bottoms are not cooled with the compressor discharge and interstage liquid, as is done in a Fig. 1-type unsaturated gas plant. Therefore, the internal recycle and energy requirements in single-tower de-ethanizer-absorber systems is less than in Fig.1-type unsaturated gas plants. However, when the embodiments of Figs. 2 and 3 are applied to a Fig.1-type unsaturated gas plant, higher operational stability is provided particularly because buildup of water recycled throughout the system is prevented. Tables 1-3 below show a study of the Fig. 1 system as compared to the present invention. The study was based on a gasoline mode FCC, at 0.101m³/sec (55,000 barrels per stream day, BPSD) with 100% Beryl vacuum gas oil feed. The lean oil rate was varied to maintain a constant propane recovery of 92%, excluding the sponge absorber recovery. The C₂ content of the liquid petroleum gas product was set constant at 0.083 volume %. The sponge absorber, the debutanizer and their downstream equipment were not included in the computer simulation model. As shown in Table 2, Case C is an improvement over Case B, which itself is an improvement over Case A. The most important advantage of Case B over Case A is an 3.22 megawatts (11 MMBTU/hr) savings in stripper reboiler duty. The main advantages of Case C over Case B are in the H₂S content of the LPG product and in unloading the primary absorber. Diversion of unstabilized gasoline separator 33 provides an excellent means to control the corrosive components recycled throughout the system. H₂S recycle can be reduced by 61%, compared to Case A, if all the unstabilized gasoline is fed to separator 33. This increases the lean oil circulation and increases in the stripper liquid loading by 13%, eliminating savings on stripper reboiler duty compared to Case A. Case C represents a 33% split fraction (not optimized). This fraction can be optimized on a case-by-case basis. Both Case D and Case E correspond to preheating the stripper feed to 82°C (180°F). 11.7 megawatts (40 MMBTU/hr) of external heat is required to preheat the stripper feed in Case E while in Case D only 3.8 megawatts (13 MMBTU/hr) is needed. The aftercooler duty for Case E is six times that in Case D. The H₂S recycle and H₂S content of LPG in Case E are 2.33 and 1.28 times that in Case D. These differences increase as the feed preheat temperature increases. One effective method for reducing H₂S recycle in conventional unsaturated gas plants, such as that shown in Fig. 1, is to recontact only the absorber bottoms and not the overhead stripper. This is represented in Case F. In such case, stripper overhead is not combined with lines 11, 21 and 23 of Fig. 1. Comparison of Case C and Case F reveals that Case C not only reduces the H₂S recycle much more effectively than Case F, but is more efficient in all aspects of unsaturated gas plant operation than is Case F. The Figs. 2 and 3 embodiments increase the solubility of water in the stripper feed. Almost all of the additional water leaves the stripper with stripper overhead vapor, which is condensed in separator 33 and low temperature separator 15. Therefore, this should not be a disadvantage in the gas plant operation. Table 3 shows the effect of an interstage amine absorber. The present invention is applicable to an unsaturated gas plant with or without an interstage amine absorber. However, there will not be as much need for installation of an expensive interstage amine absorber if the Figs. 2 and 3 low H₂S recycle systems are implemented. In Fig. 3, hot unstabilized gasoline can be fed directly into separator 33 from a main column fractionator via line 61. Line 61 may also be connected to any of lines 11, 19, 21, 23 or 47. Feeding hot unstabilized gasoline from a main column saves energy which would otherwise be wasted in the main column overhead condenser. However, the wet gas compressor power requirement will slightly increase. Unstabilized gasoline can be diverted and recontacted with the first stage compressor discharge in a high temperature flash. The vapor will be cooled in the compressor aftercooler and then flashed in a low temperature separator. The liquids from the low temperature separator and the high temperature separator are then pumped to the high temperature separator of the unsaturated gas plant at a higher temperature than otherwise. This may provide additional energy savings.

Claims: An unsaturated gas plant apparatus, comprising a first stage compressor 1 for receiving a low pressure gas, an interstage cooler 5 connecting the first stage compressor with a low pressure separator 7, a second stage compressor 9 which compresses the vapor phase from low pressure separator 7, an absorber 3 for receiving an unstabilized gasoline feed and a lean absorber oil which produces a rich absorber oil as a bottoms product, a stripper 13, a low temperature separator 15 discharging an overhead vapor to the absorber 3 and liquid to the stripper 13, characterized by a high temperature separator 33 for separating a vapor/liquid mixture comprising the low pressure separator 7 liquid and the second stage compressor 9 gas, rich absorber oil from the absorber 3 and stripper 13 overhead vapor which provides a high temperature liquid hydrocarbon feed to the stripper 13 and a high temperature vapor phase which is cooled and discharged to the low temperature separator 15. The apparatus of Claim 1, further characterized by a diverter which sends a portion of the unstabilized gasoline feed to an inlet of the high temperature separator 33 to mix this gasoline with the vapor/liquid mixture upstream of the high temperature separator. A process for separating unsaturated gas from unstabilized gasoline characterized by changing unsaturated gas and unstabilized gasoline to the apparatus of any of Claims 1 to 2.

Inventors: HARANDI, MOSHEN NADIMI
Applicants: MOBIL OIL CORPORATION
Application year: 1985
Main classification: C10G7
IPC codes: C10G11, C10G7, C10G5