The two most important olefins ethylene and propylene are co-produced in steam crackers and refineries. The production ratio is relative and is determined by feedstock used and severity of the 'cracking'. Gas crackers that process ethane-rich gas, produce more of ethylene than propylene, whilst processing of refinery streams by fluid catalytic cracking (FCC) can produce more of propylene than ethylene. Almost 70% of propylene produced globally comes from steam cracking, while FCC yields about 25%. The balance 5% comes from alternate or 'on-purpose' processes, which aim to selectively produce propylene, with the exclusion or minor production of by-products.
Projected annual demand growth for propylene at 4-5% is expected to outpace that of ethylene (3%). Globally, more than 25% of new crackers that started up in the 2003-2007 were based on ethane and produce little propylene. Ethylene growth will be largely satisfied by new crackers being built in the Middle East, using ethane as the key feedstock. This will make the crackers poor sources of propylene. Though propylene demand is only about half of ethylene, a shortage of propylene from conventional sources is estimated as steam cracker expansions/additions cannot keep pace with propylene demand growth.
This will drive investments in 'on-purpose' propylene production. The options include: propane dehydrogenation (PDH), metathesis, olefins cracking and methanol to propylene (MTP) or methanol to olefins (MTD) technologies.
PDH, which, as the name suggests involves catalytic removal of hydrogen from propane to yield the olefin, is clearly the most important 'on-purpose' route now. A small amount is also produced by olefin meta thesis, in the double bonds of olefins (say, C2 and C4 mixtures) are broken and different olefins (C3, in this case) are formed. An even smaller amount comes from cracking of C4/ C5 olefins, which technology is similar to metathesis in that low value hydrocarbon streams are converted to higher value olefins (however, their chemistry is a combination of olefin oligomerisation, cracking, disproportionation and hydrogen transfer).