Special Report. Shale gas drives new opportunities for US downstream

Transcription

1 Special Report LNG, NGL and Alternative Feedstocks A. MALLER, D. DHARIA and E. GBORDZOE, Technip Stone & Webster Process Technology, Houston, Texas; and N. LAMBERT, Axens, Rueil-Malmaison, France Shale gas drives new opportunities for US downstream The refining and petrochemical industries are facing a variety of opportunities and challenges due to the rise of shale gas. The availability of low-cost light feedstocks has provided a strong incentive for petrochemical companies to process ethane in steam crackers (SCs) to produce ethylene. This trend is expected to continue over the next decade, especially in the US. However, this shift to lighter feeds for the SC will reduce the future availability of other petrochemical feedstocks in particular, propylene. Historically, propylene supply has been met primarily by steam cracking; however, when cracking ethane, limited quantities of propylene are produced. Coupled with the anticipated increased global demand, the result is a future production gap that may create attractive margins for on-purpose propylene production. Refiners can fill the supply gap with fluidized catalytic cracking (FCC) technology, as part of an integrated refinery/petrochemical complex geared to light olefins production. Heart of the modern refinery. FCC units (FCCUs) have long been the heart of the modern oil refinery for their ability to upgrade low-value feedstocks to higher-value products. This technology can be tailored to process a variety of hydrocarbons. More importantly, the modern FCCU can be designed for onpurpose production of LPG and propylene at more than 40 wt% and 20 wt% yield on fresh feed basis, respectively. FCC technology for light olefins. The FCC process uses circulating fluidized catalyst to selectively crack heavy hydrocarbons into lighter, higher-value products. TABLE 1 summarizes the product yields of an FCCU geared to gasoline production. Also, TABLE 1 compares the typical product yields using three FCC designs focused for propylene production. Light olefins production requires higher reaction temperatures, and the addition of catalyst additives, such as ZSM-5, which selectively crack gasoline/naphtha to LPG olefins. Overall, depending on the feed and operating conditions, an FCCU with ZSM-5 additives can achieve 6 wt% 12 wt% propylene yields, as shown in TABLE 1. Deep catalytic cracking (DCC) technology was developed for maximum light olefin production. 1, a This technology, as firstly commercialized in China and then in Thailand and KSA, is achieving leading propylene yields in excess of 20 wt%. a The DCC process also uses proprietary catalysts optimized for propylene production without the disadvantage of dilution effects caused by mixing a base catalyst with ZSM-5. FIG. 1 illustrates the DCC process using a traditional riser coupled with a unique bubbling bed cracking type reactor. It operates at a low hydrocarbon partial pressure to increase propylene production. The design is optimized with the right balance between reactor operating pressure, hydrocarbon partial pressure and steam usage to reduce equipment cost. Production of light olefins from the DCC process can be enhanced further by recycling streams from the product- section to the reactor. A light-cat naphtha (LCN) cut (C F) is often recycled because it is rich in olefinic species, which are more likely to crack to propylene. Mixed C 4 s may also be recycled to boost propylene and ethylene production. The yields of a typical DCC unit operating with hydrotreated vacuum gasoil (VGO) feed are shown in TABLE 1. Depending on the feed quality, the unit can produce between 15 wt% 25 wt% propylene and 4 wt% 5 wt% ethylene (based on fresh feed). 2 Maximizing propylene production requires a light, hydrogen-rich feedstock that is low in sulfur and metals. However, the FCC process is well suited to upgrade the bottom of the barrel hydrocarbon streams. A new process technology can convert low-quality heavy resid materials to lighter olefins. b The resid to propylene (resid-to-c 3 ) process uses some common design features from the traditional resid FCC (RFCC) process, such as two-stage regeneration, but operates at higher severity. The TABLE 1. Typical reactor yields comparison for different FCC technologies Unit type FCC base FCC w/ ZSM-5 Resid-to-C 3 DCC Feedstock HT-VGO HT-VGO Resid HT-VGO Yields, wt% fresh feed: Dry gas Propane, C Propylene, C Butanes, C Butylenes, C Naphtha LCO Slurry Coke Hydrocarbon Processing JANUARY

2 FIG. 1. DCC reactor-regenerator design. Propylene C 4 LPG Distillate Fuel oil FCC fresh feed FCC unit Offgas C 3 and heavier Propane FIG. 3. FCC SC integrated flow diagram. addition of a second external riser is an option to maximize propylene production, as shown in FIG. 2. In the second external riser, less reactive recycle streams, for example LCN, are cracked at a higher cat-to-oil (C/O) ratio and temperature than what is achievable in the main feed riser. The higher severity in operating conditions increases propylene production without significantly increasing the total dry gas production. TABLE 1 includes the yields for a typical resid-to-c 3 unit with the second external riser, which is processing a resid feed. The process can produce propylene in excess of 12 wt% with heavy resid feedstocks with Conradson carbon from 2 wt% to 6 wt%. FCC integration with a steam cracker. A byproduct of the basic FCC operation is C 2 and lighter hydrocarbons. Often referred to as offgas, this material is typically used as fuel gas inside the refinery. An FCCU designed to produce on-purpose propylene will also produce a greater quantity of offgas containing approximately 40 wt% 50 wt% ethylene. The ethylene from this stream may be desirable, but it requires additional processing units that are traditionally associated with a 46 JANUARY 2015 HydrocarbonProcessing.com Steam cracker FIG. 2. Resid-to-C 3 unit with an external second riser. c Fuel gas Ethylene Tar/fuel oil SC fresh feed petrochemical facility. With the drive to larger, high-olefin producing FCCUs, the economics of the plant may dictate incorporating an ethylene unit (ERU). The level can vary in complexity depending on whether the ethylene-rich stream is sold over-the-fence or as polymer-grade ethylene (PGE) feedstock. The PGE production requires pretreating the offgas to remove moisture and contaminants. The product of the ERU is a stream that is about 99.9 vol% ethylene. Propylene and heavier hydrocarbons are recycled back to the FCCU s unsaturated gas plant (USGP) or propylene unit (PRU) to minimize the loss of valuable products. An FCCU located within the same petrochemical complex as an SC provides the opportunity to integrate the two product- systems, as shown in FIG. 3. The FCC offgas is sent to the SC purification and fractionation section. This eliminates the need for a stand-alone ERU dedicated to the FCCU. With this arrangement, the SC processes materials lighter than propylene and includes the C 2 -splitter used to produce PGE. Ethane in the FCC offgas is recovered and recycled to the SC. The FCCU processes materials heavier than ethane and includes the C 3 -splitter for the production of polymergrade propylene (PGP). This arrangement maximizes the efficiency of higher-valued products. Propylene, which is lost to the FCC offgas is recovered in the SC and returned to the USGP via the C 3 + stream, allowing for less stringent specifications as compared to a design in which the offgas is used as refinery fuel gas. One such integrated complex is operating successfully in Saudi Arabia. 3 In any of the schemes for integrating the FCCU with ethylene, contaminants present a unique challenge. The level of contaminant removal required for PGP and PGE production is very stringent. TABLE 2 lists PGP and PGE product specifications with regard to common contaminants. A combination of fractionation, amine/caustic wash, and regenerable and nonregenerable adsorbent beds are used to achieve the low contaminant levels required. TABLE 3 shows typical treatment methods for these contaminants. Unintended cross contamination when considering an integrated FCC and ERU should not be overlooked. For example, carbon monoxide (CO), which is formed in the FCC and ends up in the FCC offgas, is typically of no consequence. However, when integrated with an ERU, it is a major concern in the operation of the acetylene reactors, particularly if the concentration varies. Also, large quantities of dienes (butadiene and propadiene) produced in the SC furnace become a concern when processed through the FCC product- section. At the high temperatures found in the USGP tower reboilers, these species will polymerize and foul heat exchangers. Special consideration of the design and operation of these tower reboilers is necessary to minimize diene polymerization.

3 Fresh feed Fresh feed Fresh feed FCC FCC FCC Max gasoline mode Product Product C 4 s C 5 s/c 6 s Max distillate mode boiling-range oligomer recycle Max propylene mode Full oligomer recycle Product FIG. 4. flow schemes. Percent C 4 s C 5 s/c 6 s C 4 s C 5 s/c 6 s Distillate /distillate Max gasoline Max gasoline Max distillate Max propylene FCC alone FCC + oligomerization FCC + oligomerization FCC + oligomerization Dry gas LPG w/o C 3 C 3 Distillate DCO Coke G/D FIG. 5. Flexibility of integrated FCC and oligomerization unit G/D C 4 processing options. The FCC process produces a significant amount of C 4 olefins from the carbenium ion β-scission cracking mechanism. The olefinic C 4 s, along with isobutane, are valuable as feed to downstream units. Depending on the refinery configuration, this C 4 stream can boost propylene, gasoline or distillate production. Optimizing the flow scheme for the production of mixed C 4 s by the FCCU will maximize refining margins. A simple way to utilize C 4 s is recycling them back to the riser to boost propylene production. The mechanism for this conversion in the FCC riser involves limited oligomerization step preceding the catalytic cracking reactions. The conversion of mixed C 4 s can be increased by utilizing a dedicated external second riser that operates at high severity cracking conditions. c A common destination for C 4 s produced from the FCC process is the alkylation unit, where the olefinic C 4 species are reacted with isobutane. The C 4 feed to the alkylation unit is pretreated to remove contaminants. The clean alkylate has a high octane rating and low vapor pressure, making it ideal for gasoline blending. The alkylation process can be used to offset the naphtha loss associated with operating the FCCU to maximum propylene production. An alternative process for upgrading the FCC C 4 product stream is in an oligomerization unit. 4, 5, d In this unit, mixed butenes are transformed by a fixed-bed process into gasoline and middle distillates through oligomerization. The oligomerized product can be recovered as high-quality motor fuels, or recycled to the FCC in an integrated process with an oligomerization unit. e This process involves recycling the oligomer product to the FCCU for selective cracking to propylene. As compared to direct C 4 and LCN recycling, the selectivity of integrated FCC and oligomerization process to propylene conversion is increased, with low offgas and aromatics production. C 4 olefins produced by the FCCU can essentially be recycled to extinction to maximize propylene production. The oligomerization unit can be configured to produce either gasoline or middle distillates. 5 An FCCU in conjunction with TABLE 2. Typical PGP and PGE product specifications Analysis PGP PGE Acetylene, ppm < 2 < 1 Oxygen, ppmv < 4 < 2 Carbon monoxide, ppmv < 2 < 1 Carbon dioxide, ppmv < 2 < 2 Total sulfur (as S), ppmv < 2 < 2 Water, ppmv < 5 < 5 Methanol, ketone, ppmv < 5 < 5 COS, ppbv < 20 Nitrogen, ppmv < 100 < 100 Sulfur dioxide, ppmw < 0.5 Hydrogen sulfide, pmw < 0.5 Arsine, ppbw < 20 Ammonia, ppmw < 0.2 < 1 TABLE 3. Contaminants and treatment methods Contaminant Acetylene/MAPD Oxygen Carbon monoxide Carbon dioxide Total sulfur (as S) Water Methanol, ketone COS Nitrogen Sulfur dioxide Hydrogen sulfide Arsine Ammonia Mercury Treatment method Hydrogenation reaction Eliminated by scheme/fractionation Eliminated by scheme/fractionation Amine and/or caustic wash and adsorption Amine and/or caustic wash and adsorption Eliminated by scheme/fractionation Amine and/or caustic wash Amine and/or caustic wash Acidic water wash and adsorption Hydrocarbon Processing JANUARY

4 an oligomerization unit can be operated in maximum gasoline, maximum distillate, or maximum propylene modes. FIG. 4 illustrates the flow scheme for the different operating modes. FIG. 5 shows the relative yields for an RFCC unit processing a resid feed and operating in various production modes, in conjunction with an oligomerization unit designed for C 3 /C 4 feed. TABLE 4. Refinery A summary Unit type Key technology feature Fresh feed type DCC Integrated DCC + ethane cracker + alkylation unit Hydrotreated VGO Fresh feedrate, Mbpd 92 Fresh feed, SG Fresh feed, CCR, wt% 0.2 Fresh feed metals (Ni + V), ppm 0.3 Ex-unit yield, wt% FF Ethylene 4.5 Propylene 18.4 Butylene LCO 11.8 Ex-unit product flow, Mtpy PGE 1,350 PGP 950 TABLE 5. Refinery B summary Unit type DCC Key technology feature DCC + oligomerization + ERU Fresh feed type Hydrotreated resid Fresh feedrate, Mbpd 29 Fresh feed SG Fresh feed CCR, wt% 2.9 Fresh feed metals (Ni + V), ppm 2.5 Ex-unit yield, wt% FF Ethylene 5.3 Propylene 23.3 Butylene LCO 11.5 Ex-unit product flow, Mtpy PGE 73 PGP 324 Case history: Refinery A Integration of DCC, SC and alkylation units. Refinery A utilizes DCC technology, which is integrated with an ethane cracker (EC) unit and located within the same refinery/petrochemical complex. The unit, which started up in 2009, was designed to process 92 Mbpd of hydrotreated VGO feed. FIG. 6 is a simplified block flow diagram of the key processing units with the integration of the FCC and EC units. The combined production and of polymer feedstock from the integrated DCC and EC units are approximately 950 Mtpy of PGP and MMtpy of PGE, respectively. The complex maximizes propylene and ethylene production by efficiently utilizing both product systems. Propylene from the DCC USGP, along with offgas are processed in the EC section to minimize the cost of the propylene section. Heavy hydrocarbon materials, including propylene, are returned to the USGP via the C 3 + stream. Propane product from the bottom of the plitter tower is fed to the EC furnace to produce propylene. The mixed C 4 s produced by the DCC are selectively hydrogenated to reduce the concentration of butadienes, the majority of which are coming from the EC section. The treated C 4 product is fed to the alkylation unit to boost gasoline production. TABLE 4 summarizes the DCC operation including the total production rates from the integrated units. The efficient integration of DCC and EC process in a large-scale commercial operation is demonstrated by Refinery A. Such integrated complexes are attractive as they improve project economics and provide flexibility for processing multiple feedstocks. Case history: Refinery B Integration of DCC and ERU. Refinery B is designed to utilize DCC to convert low-value heavy hydrocarbon material into higher-value purified petrochemical feed. The light olefin yield is maximized by integrating FCC with an oligomerization unit, and selected recycles are processed in a second riser. e The net C 4 product production is low because the C 4 s are recycled to extinction. e Refinery B, scheduled to start up in 2016, processes 29 Mbpd of hydrotreated resid. f The scheme uses the oligomerization process with an external secondary riser. The external secondary riser also processes recycled mixed C 4 s and LCN from the DCC USGP to produce additional propylene at high severity operating conditions. c FIG. 7 shows the key processing units in the complex. The refinery/petrochemical unit has a dedicated ERU, which produces PGE from the DCC offgas. TABLE 5 summarizes the overall unit performance. VGO HDT Hydrotreated VGO feed Polypropylene unit Refinery A DCC reactor/ regenerator Reactor effluent Propylene vent LCN HCN LCO DCC unsaturated gas plant Clarified oil FIG. 6. Refinery A block flow diagram. C 3 + Cracked gases Mixed Fuel gas DCC offgas EC plitter system Pygas product Ethane recycle Ethane EC furnace Treated C 3 LPG offgas Offgas treatment PGE 1.35 MMtpy Fresh ethane PGP 950 Mtpy Mixed C 4 s 840 Mtpy 48 JANUARY 2015 HydrocarbonProcessing.com

5 Case history: Refinery C Integration of resid-to-c 3, C 4 block and ERU. Refinery C is the largest resid-to-c 3 unit designed to date and is scheduled to start up in early It is designed to process 127 Mbpd of unhydrotreated atmospheric resid feed with a high recycle rate of highly olefinic LCN to a dedicated external secondary riser operating at high temperature and C/O ratio. FIG. 8 shows a simplified block flow diagram of the resid-to-c 3 unit. The unit is designed to produce over 12 wt% propylene from low-quality resid feedstock. The external secondary riser is used to maximize propylene yield and produce large quantities of C 4 LPG and ethylene. The C 4 LPG is recovered in a unique C 4 processing block that will maximize the value of the mixed C 4 components. The full mixed-c 4 stream is first treated in the C 4 selective hydrogenation unit (C 4 SHU), where butadiene and sulfur species are removed. The 1-butene components in the mixed-c 4 stream are also isomerized to 2-butene. A portion of the treated mixed C 4 is fed to the deisobutanizer (DIB), which removes the isobutane and isobutylene. The raffinate from the DIB, which is primarily 2-butene, is taken as a product stream. The extract from the DIB (isobutane and isobutylene), along with the balance of mixed C 4 s from the C 4 SHU, are fed to a sulfuric acid alkylation unit. The produced alkylate will be used in gasoline blending to boost the total gasoline production from the resid-to-c 3 unit. Unreacted n-butane from the alkylation unit is fed to a butane ERU PGE ATB Residue hydroprocessing RHP DCC + USGP Oligomer recycle PRU PGP Hydrodesulfurization gasoline HDS-g g Light naphtha Heavy naphtha LCO Slurry FIG. 7. Refinery B block flow diagram. FG scrubber LPG treat ERU PRU PGE PGP C 4 SHI PRU C 4 Resid to C 3 / USGP Naphtha split Alky/SAR HPU But ISOM AR feed WCN HT LCO Slurry FIG. 8. Refinery C block flow diagram. 49

6 TABLE 6. Refinery C summary Unit type Resid-to-C 3 Key technology feature Resid-to-C 3 + ERU + C 4 block Fresh feed type Atmospheric residue Fresh feedrate, Mbpd 127 Fresh feed SG Fresh feed CCR, wt% 4.4 Fresh feed metals (Ni + V), ppm 17 Ex-unit yield, wt% FF Ethylene 2.0 Propylene 12.3 Butylene LCO 12.9 Ex-unit product flow, Mtpy PGE 128 PGP 803 isomerization unit (BUT ISOM) to convert it to isobutane, and then recycled to the alkylation unit. Ethylene from the FCC offgas is recovered in a stand-alone ERU to produce a PGE product stream. The PGE is combined with the 2-butene via a metathesis reaction in a downstream OSBL unit to produce additional propylene. TABLE 6 summarizes the total unit performance. The flow scheme of Refinery C is integrated beyond what typical refineries may consider. The resid-to-c 3 unit plays an important role in the overall refinery economics by converting the residue from the crude unit into high-value petrochemical and motor fuel products. Downstream processing options provide flexibility which can be adapted to meet refinery production goals. Conclusion. The refining and petrochemical industries are facing a new era, following the abundance of available shale gas, associated with new upstream production techniques. This cheap feedstock is driving the economics for numerous future SC projects, which is expected to create a production gap for other petrochemical base molecules The FCC process must be viewed as a key unit to address such imbalances, through deeper integration with petrochemical facilities. The inherent flexibility of the FCC to shift to changing market conditions is further enhanced through innovative downstream processing options. To offset reduced gasoline margins, the lighter fraction of FCC naphtha (150 F-) may be recycled for added propylene production. Not only is the production of propylene and other petrochemical feedstocks important but so is their. Smart and effective integration of traditional refinery units with petrochemical units inside a larger overall complex results in a production facility that can be extremely flexible to changing economic conditions. Such a facility can extract the maximum value from crude oils and other feeds with maximum efficiency. Larger and more complex integrated production facilities that are flexible to changing economic realities are the future. Recent advancements in both hardware and catalyst technology mean that the FCC will continue to serve as the key link between the bottom and the top of the barrel for years to come, as it always has been. ACKNOWLEDGMENTS The authors thank and acknowledge Steve Gim and Miguel Maldonado from Technip, and Mai Phuong Do from Axens. LITERATURE CITED 1 Dharia, D., W. Letzsch, H. Kim, D. McCue and L. Chapin, Increase light olefins production, Hydrocarbon Processing, April 2004, pp Fu, A., D. Hunt, J. Bonilla and A. Batachari, Deep catalytic cracking plant produces propylene in Thailand, Oil & Gas Journal, Jan. 12, Dharia, D., G. Keady, H. Kim, E. Hennings, S. Mahmood, A. Gazzaz and H. Doi, Petro Rabigh World s largest deep catalytic cracking unit, 17th Refinery Technology Meet, Bangalore, July Mai Phuong, D., Retrofitting your FCC into a diesel machine, 10th FCC Forum, San Francisco, May Gagniere, M., A. Pucci and E. Rousseau, Tackling the gasoline/middle distillate imbalance, Petroleum Technology Quarterly, 2Q Roux, R., Upgrading of Heavy Cuts into Max Olefins, 26th JPI Petroleum Refining Conference, October NOTES a Deep catalytic cracking (DCC) technology, licensed by Technip Stone & Webster Process Technology (Technip), was developed for maximum light olefin production. This technology was also developed by Sinopec and originally commercialized in China. b Technip and Axens license the resid-to-propylene (R2P) technology to produce maximum propylene from low-quality heavy resid feedstocks. c Resid-to-propylene unit with an external second riser, PetroRiser. d An alternative process for upgrading the FCC C 4 product stream is in a Polynaphtha unit or oligomerization unit licensed by Axens. e Licensed by Axens, the FlexEne process involves recycling the oligomer product to the FCCU for selective cracking to propylene. f Licensed by Axens, the Hyvahl process is for hydrotreating residue. g Prime-G+ technology is licensed by Axens and developed for selective desulfurization of cracked naphtha. EDTOR S NOTES Based on an earlier presentation at the American Fuels and Petrochemical Manufacturers (AFPM s) Annual Meeting, Orlando Florida, March 24 26, ALEXANDER MALLER serves as lead engineer specializing in the process design of FCC units. He has experience with residual, VGO, and high olefins catalytic cracking technology. His responsibilities include conceptual design of new process technologies, technical proposals, PDP/FEED design and plant technical services. He joined Technip S&W in Mr. Maller holds a BS degree in chemical engineering from Tulane University, New Orleans. DILIP DHARIA is a program director for High Olefin FCC Technology at Technip Stone & Webster Process Technology, Houston, Texas. He is responsible for all aspects of the program including technology and business development and licensing of technologies worldwide. Mr. Dharia has 39 years of experience in the areas of refining, petrochemical, gas processing and advanced energy conversion systems. He holds BS and MS degrees in chemical engineering and an MBA degree in administration. He is a member of AIChE and is a registered professional engineer. DR. EUSEBIUS GBORDZOE is the FCC process design manager as well as FCC technology development manager at Technip Stone & Webster Process Technology. He has more than 25 years of R&D experience. Dr. Gbordzoe is involved in all phases of FCC design including development, design, startup and troubleshooting. He holds several patents and has authored several technical papers. Dr. Gbordzoe holds a PhD in chemical engineering from the University of Western Ontario, Canada. NICOLAS LAMBERT is a technologist in Axens middle distillates and conversion business line, which focuses on FCC technology. He is a graduate of Arts & Métiers ParisTech. 50 JANUARY 2015 HydrocarbonProcessing.com