Naphtha catalytic cracking for propylene production

Transcription

1 Naphtha catalytic cracking for propylene production Investment in on-purpose propylene production technology based on naphtha-based feedstock is taking on various process configurations Christopher Dean High Olefins FCC Technology Services Ethylene is produced through steam cracking of hydrocarbon feedstock (for example, ethane, naphtha and gas oils) derived during conventional and unconventional natural gas production and from refinery crude oil processing. Ethane and natural gas liquids (NGLs) are derived from natural gas and heavy liquid feedstocks of naphtha, and gas oils from crude. Naphtha is the predominant feedstock on a global basis, including straight-run naphthas (SRN) from refinery crude units and naphtha derived from condensates during natural gas production. While ethylene is the world s primary petrochemical building block, propylene is second in importance only to ethylene as a raw material building block. Traditionally, propylene has been obtained as a byproduct from steam cracking naphtha and gas oils to produce ethylene, and from gasoline-producing refinery fluid catalytic cracking (FCC) processes. Global ethylene and propylene demand has recovered China 34% Other 10% Southeast Asia 8% North America 10% Middle East 21% Indian subcontinent 14% Northeast Asia excl. China 3% Figure ethylene capacity additions from the recession, and longer term demand expansion is expected. Propylene demand was increasing faster than ethylene demand before the recession, which is not the case today. Up to 2007, global propylene demand was increasing annually at 6.0%, while today it is expected to increase by 3.6% on average for the next several years, according to analysis from Nexant ChemSystems. Even at these lower demand predictions, there is expected to be shortages in propylene supply. Nonetheless, future global ethylene demand still determines steam cracking capacity and is expected to be met in each region, as shown in the graphic developed by CMAI in Figure 1. Figure 1 reflects ethylene capacity additions according to CMAI s research. Asia is the fastest-growing light olefin market and uses naphtha as its feedstock. Existing Middle Eastern steam crackers as well as those being built use primarily ethane-based feedstock for producing ethylene. The North American units are emerging due to access to cheap shalebased ethane feedstock (less than cents per gallon as of mid-january 2013), and several will come on line after These bargain prices for US-based ethane and natural gas feedstock are expected to be sustained beyond 2016 relative to similar feedstocks in Asia and elsewhere. However, in spite of these competitive prices, it is well known that ethane-based steam crackers produce very little propylene relative to naphtha and gas oil-based steam crackers, which is why the onus is on investing in on-purpose propylene production (OPP) technology. Steam cracking heavy Processing Shale Feedstocks

2 Feedstock Ethylene, wt% Propylene, wt% p/e Ethane (0.0375) Propane Naphtha Gas oil Table 1 Typical light olefin yields for steam cracking feedstocks of naphtha and gas oils produces about 60% of the global propylene demand, while 30% comes from traditional FCC units that produce gasoline. High propylene yields from steam cracking are ultimately produced through various recycling and operating severities of these heavy feeds or non-ethane-based feedstocks. Steam cracking produces more pounds of ethylene to pounds of propylene on a weight basis. Table 1 is based on general industry knowledge and shows the typical ethylene and propylene yield in weight percentage for a pound of feed as it varies for a particular feedstock. The propylene/ethylene (P/E) ratio indicates the selectivity of the cracking conditions to produce propylene. The P/E ratio is one way of tracking global propylene demand in relationship to ethylene demand. This ratio also indicates which production propylene processes are needed to meet this demand. Increasing P/E ratios beyond 1.0 The P/E ratios of 0.65 and 0.53 for gas oil and naphtha, respectively, indicate that heavier feeds produce a higher ratio of propylene to ethylene. It is important to note that globally gas oil steam cracking is being reduced due to these heavier feedstocks being diverted to meet higher product demand for diesel and other fuels. In the pre-recession period up to 2007, it was estimated that the global propylene demand required a P/E ratio of greater than Today, this demand is still expected to be higher than those P/E ratios produced by cracking naphtha and even gas oil feedstocks. Therefore, in order for OPP proposals to materialise, they have to be better than the P/E ratios of 0.53 to 0.65 for naphtha and gas oil steam cracking, respectively. Existing FCC and the new high olefin FCC (HOFCC) process will produce P/E ratios from 1.0 to greater than 2.0 to meet this propylene demand. The well-documented shift in ethane production from multiple shale plays in North America has placed steam crackers utilising heavy feeds of naphtha and gas oil at a competitive disadvantage in spite of their high propylene production capacity (P/E between 0.53 to 0.65) relative to ethane-based steam crackers (P/E only 0.04). Since steam cracking is determined by ethylene demand and the shift to gas feedstocks from liquids, global propylene demand cannot be met from the expected increase in steam cracking production. This is particularly the case with the North American propylene market should all the announced ethane-based steam crackers (approximately seven) go online by As ethane cracking capacity increases, propylene production decreases significantly and is reflected in the increased pricing ratio of propylene to ethylene. Besides, CMAI, Nexant and others predict that propylene pricing will remain higher than ethylene pricing. This is especially true in the US, where there was historically an abundant propylene supply due to significant refining capacity. However, refining capacity is decreasing and what remains is shifting from gasoline to more diesel production, which reduces propylene production. Since ethylene demand is expanding proportionally faster to propylene, naphtha steam crackers cannot meet the expected incremental demand for propylene. Other OPP technology will therefore be developed. High-severity FCC processing (HS-FCC) produces high yields of light olefins and reduces liquid fuels. Existing FCC units can operate at more severe conditions that will increase light olefin yields but still produce significant amounts of fuels (gasoline). The HS-FCC term is somewhat confusing due to licensing issues and for processes that specifically produce petrochemical feedstocks. These FCC processes produce light olefins and highly aromatic content liquid products that are used for petrochemical unit feedstocks. To clarify, the term HOFCC will be used to differentiate those FCC processes 2 Processing Shale Feedstocks

3 that are only petrochemical product based. In this instance, HS-FCC refers to HOFCC-type processes. CMAI research indicates OPP capacity will increase from 13% to 20% over the next several years and future demand for non-steam cracking propylene sources will continue. Propane dehydrogenation (PDH) processes currently show the largest increase for meeting this propylene demand. Most of these processes besides the HOFCC are being installed by chemical companies and not refiners to meet their propylene feedstock requirements. Other OPP technology for propylene production, including metathesis of ethylene and butylenes, and olefinic naphtha cracking, require integration with a steam cracker or other processes that produce olefins as byproducts. In addition, these processes cannot produce significant propylene yields at cost advantages compared to HOFCC processes. These processes also cannot produce the additional byproduct petrochemical feedstocks of butylenes and aromatics as those from the HOFCCs. As mentioned previously, the second significant source of propylene production is in the form of a byproduct from existing FCC processes primarily designed for producing gasoline and other fuels. These processes have been modified by operating at higher severities and different catalysts to produce high levels of propylene and other light olefins and aromatics at the expense of gasoline and other liquid fuels. In the US, there is a current and expected future slump in gasoline demand with an increase in diesel demand, which reduces the FCC unit s effectiveness for maximising propylene production. As a result, significant propylene production increases from these modified FCC units will not meet the expected propylene demand. Catalytically cracking naphtha The HOFCC processes and related technologies will be the future OPP drivers for petrochemicals. Future incremental propylene supply will come from these enhanced FCC processes that target light olefin production and heavier petrochemical feedstocks, such as aromatics, instead of the traditional gasoline product. These future processes will not just be heavy oil feedstock, but will also include FCC processes to catalytically crack naphtha. As previously inferred, HOFCC is a group of proprietary FCC processes targeting light olefin production instead of gasoline product from traditional FCC process technology. These HOFCC processes utilise traditional FCC technology with some modifications. Operating severities and different catalysts are used for producing and maximising propylene and other light olefin products. These HOFCC processes use heavier treated feedstocks from crude oil, gas oils or resids, similar to gasoline FCC feedstocks. These units produce more light products and will minimise gasoline produced for the gasoline fuel blending pool, which is why HOFCC units are essentially petrochemical feedstock units. They are also being integrated into refining and petrochemical complexes, resulting in configurations on a world-class scale. Traditional FCC units were designed to meet gasoline demand by cracking heavy gas oils or resids that generally produce propylene yields from 8.0 wt% to 12 wt%, which is why the major FCC licensors have focused on HOFCC technology to increase propylene yields by 15 wt% to 25 wt% (and higher). However, existing gasoline FCC units cannot be easily or cost-effectively modified or revamped to produce and recover significantly higher propylene quantities above original design. Several significant modifications to FCC units have been designed and commercialised. These modifications or enhancements include adding a second reactor in the form of a riser reactor or a downflow reactor for recycle of a cracked product; or as a separate feedstock reactor to generate more propylene and lighter products. In addition, significant changes to the gas and lighter product recovery sections are necessary. Modifying existing units in certain marketing regions may increase propylene yields, but not significantly enough to meet the expected propylene demand. These designs have been mentioned in numerous articles and are fairly well known in the industry. The engineering companies are the primary HOFCC third-party licensors, including Technip Stone & Webster, Honeywell UOP, Lummus Technology, KBR and Axens. These HOFCC processes will provide significant OPP capac- Processing Shale Feedstocks

4 ity. However, feedstock to these units is derived from crude oil feedstock. Hence, not enough of these units may become available due to feedstock competition and economics favouring fuel production (primarily diesel). The increasing naphtha and lighter liquid hydrocarbons developed from crude and gas production are becoming more attractive for producing propylene. Therefore, next-generation HOFCCs are being designed to crack naphtha as the primary feedstock for propylene production. Naphtha supply Naphtha supply is increasing globally. Naphtha availability from crude oil refining of lighter crudes, and naphtha derived as byproduct condensate (and some NGLs) in natural gas production, are the sources of this increasing global supply. Increased global condensate production, reduced gasoline demand in the US and other developed countries, switching to diesel, cleaner and more efficient fuels, and declining naphtha steam cracker capacity all impact the naphtha balance, producing higher naphtha inventories. These factors will continue to exert pressure on naphtha balances, driving lower naphtha prices and a larger differential between crude oil and naphtha pricing. A final conclusion on future naphtha pricing is somewhat decoupling of naphtha from crude pricing. Higher propylene prices are expected to continue, with propylene pricing spiking due Recycle C 4 -C 6 non aromatics Straight run naphtha Proprietary KBR FCC reactor features Propylene/ethylene (P/E) product ratio 1 Proprietary catalyst from SK Corporation All proven hardware and processes Robust and flexible, compared to other processes Figure 2 Advanced Catalytic Olefins to lower supply. With expected lower naphtha pricing and more ethane being used in ethylene production, the expected worldwide propylene demand will be met by catalytically cracking paraffinic naphtha from a refinery or naphtha derived from natural gas production. Naphtha will be the feedstock of choice for future HOFCCs. There are commercially viable catalytic cracking naphtha processes available for Figures 3a and b Two Axens and Stone & Webster process technology concepts of using the downflow reactor as an enhancement to an existing resid feedstock-based FCC and as a standalone unit propylene production, while others are in the final stages of development. One that is licensed by KBR is Advanced Catalytic Olefins (ACO), as shown in Figure 2. This process is a dual riser system utilising refinery straight-run naphtha and recycle material as feedstock. The ACO produces ethylene, propylene and high BTX content in the gasoline product. The process produces a P/E ratio of approximately 1.0 that is significantly better than the P/E ratio of 0.55 from naphtha steam cracking. Another naphtha cracking process is based on the HS-FCC process technology that is in semi-commercial operation in Japan and licensed by Axens and Technip Stone & Webster. This naphtha cracking process is in the final development stages and can be used as an enhancement (modification) to existing FCC units, utilising a downflow reactor or downer as a second reaction zone for cracking refinery light straightrun or other paraffinic naphthas. This downflow reactor process has shown propylene yields of 16.8 wt%, with ethylene yields of 7.1 wt% for a P/E ratio of 2.4. The standalone unit in Figure 3b shows the downflow reactor system and indicates the locations for feed injection, the downflow reactor section and the important rapid product separator that separates the reaction products from the catalyst. The catalyst from the bottom of the product separator empties into a catalyst stripper, where additional products are steam stripped to remove any remaining 4 Processing Shale Feedstocks

5 hydrocarbon products before the catalyst is regenerated. The downflow reactor enhancement can be utilised by any existing FCC process technology, as shown in Figure 3a. This shows that the reaction system and catalyst stripper are located at similar elevations. The downflow reactor allows for higher operating severities with higher catalyst-to-oil ratios and higher reactor temperatures than in riser or up-flow reactor FCC unit designs. To prevent undesirable side reactions from occurring, short contact time in the reactor cracking zone or low residence time at these high reactor conditions is provided. Figure 4 shows the expected yields from catalytic cracking full-range naphtha from crude oil with the yields from steam cracking light and full-range naphtha. From catalytic cracking, there is a significant advantage in P/E ratio over steam cracking of 2.4 to 0.55, respectively. There are fewer byproducts of gas and more aromatic gasoline than in steam cracking. Clearly, there are viable processes available that can utilise naphtha to produce propylene from standalone units such as the ACO process, and one using a downflow reactor. In addition, adding a Yield, % Catalytic cracking downer 16.8% 48.5% Full range P/E % 7.1% 8.6% second riser or downflow reactor as an enhancement to existing FCC units to produce additional propylene from naphtha is being commercialised. Conclusion The HOFCC process technology will utilise naphtha derived from natural gas production and any excess from crude oil refining to meet global propylene demand. With the increase in naphtha and NGL supply from US shale gas production, utilising catalytic naphtha cracking has significant merit to meet future propylene demand. In the US, areas within close proximity to existing refineries near shale oil and gas plays, and in other 14.5% 28.1% 15.5% 4.8% 7.4% 24.2% Full range P/E 0.55 Steam cracking furnace 16.5% 31.1% 17.1% 5.3% 5.6% 17.4% Figure 4 Downflow vs steamcracking yield summary Light P/E 0.55 C 1 C 2 C 3 C 4 C 4 Gasoline areas with high concentrations of integrated refinery and petrochemical complexes, catalytic naphtha cracking has significant potential to meet propylene demand and increase profitability. Refineries can produce significant propylene and aromatics as well as maintain fuel production to address the needs of both the petrochemical and fuels markets for increased profitability and operating flexibility. References 1 Christopher F Dean is an independent consultant with over 35 years experience. He is the founder of High Olefins FCC Technology Services LLC ( Processing Shale Feedstocks