Naphtha catalytic cracking for propylene production
Ethylene is produced through steam cracking of hydrocarbon feedstock (for
example, ethane, naphtha and gas oils) derived during conventional and unconven-tional 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 naph-thas (SRN) from refinery crude units and naphtha derived from condensates during natu-ral gas production. While ethylene is the worlds primary petrochemical building block, propylene is second in impor-tance 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 gaso-line-producing refinery fluid catalytic cracking (FCC) processes.
Global ethylene and propyl-ene demand has recovered
Investment in on-purpose propylene production technology based on naphtha-based feedstock is taking on various process configurations
CHRISTOPHER DEANHigh Olefins FCC Technology Services
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from the 2008-2009 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 annu-ally 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 deter-mines 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 CMAIs 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 primar-ily ethane-based feedstock for producing ethylene. The North American units are emerging due to access to cheap shale-based ethane feedstock (less than 22.50 cents per gallon as of mid-January 2013), and several will come on line after 2016.1 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
North America 10%
Middle East 21%
Indian subcontinent 14%
Northeast Asia excl. China 3%
Southeast Asia 8%
Figure 1 2012-2016 ethylene capacity additions
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with the North American propylene market should all the announced ethane-based steam crackers (approximately seven) go online by 2017.
As ethane cracking capacity increases, propylene production decreases significantly and is reflected in the increased pric-ing 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 tech-nology will therefore be developed.
High-severity FCC process-ing (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 petro-chemical 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 differ-entiate those FCC processes
feedstocks of naphtha and gas oils produces about 60% of the global propylene demand, while 30% comes from tradi-tional FCC units that produce gasoline. High propylene yields from steam cracking are ultimately produced through various recycling and operat-ing severities of these heavy feeds or non-ethane-based feedstocks. Steam cracking produces more pounds of ethylene to pounds of propyl-ene on a weight basis. Table 1 is based on general industry knowledge and shows the typi-cal 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 produc-tion propylene processes are needed to meet this demand.
Increasing P/E ratios beyond 1.0The P/E ratios of 0.65 and 0.53 for gas oil and naphtha, respec-tively, 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 0.85. 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 naph-tha 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 multi-ple 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 ethyl-ene 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
Feedstock Ethylene, wt% Propylene, wt% P/E Ethane 80 3 0.04 (0.0375)Propane 44 15 0.34Naphtha 30 16 0.53Gas oil 23 15 0.65
Typical light olefin yields for steam cracking
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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, includ-ing 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 petro-chemical 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 exist-ing FCC processes primarily designed for producing gasoline and other fuels. These processes have been modified by operat-ing 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
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demand with an increase in diesel demand, which reduces the FCC units effectiveness for maximising propylene produc-tion. 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 petro-chemicals. 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 hea