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

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Article Summary

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 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 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 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 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 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 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 2017. 

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 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.

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