Ethylene and propylene, in that order, are the two most important olefins, finding use as building blocks for chemicals and polymers. The two are co-produced in steam crackers and refineries, though their relative ratio is determined by feedstock used and severity of the 'cracking'. Gas crackers that process ethane-rich gas, produce more of ethylene than propylene, although processing of refinery streams by fluid catalytic cracking (FCC) can produce more of propylene than ethylene.

These two conventional propylene processes account for a bulk of the propylene produced globally: about 70% 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.

The need for alternate routes

In the Asia-Pacific, where refineries still continue to be built, FCC propylene represents a growing and significant chunk of total propylene capacity. In Europe and North America, although there is significant production of propylene from refineries, the lack of investments in new refineries implies there is little scope for increasing output from this source. In the Middle East, in contrast, the development of petrochemicals has hinged around massive ethane-crackers, to the neglect of propylene and its derivatives. Globally, more than 25% of new crackers that started up in the 2003-2007 timeframe were based on ethane and produce little propylene.

While the projected annual demand growth for ethylene is at about 3%, propylene is expected to see faster growth at 4-5%, driven, in particular, by strong demand for polypropylene (PP) in emerging markets. Going forward, it is clear that steam cracker expansions and/ or additions cannot keep pace with propylene demand growth. While ethylene growth will be largely satisfied by new 'mega-crackers' being built in the Middle East, their feedstock (primarily ethane), will again make them poor sources of propylene.

In short, although propylene demand is only about half of ethylene, the world is heading to a shortage of propylene from conventional sources.

The current options for 'on-purpose' propylene

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).

The process economics

Installed PDH capacity has increased from about 2-mtpa in year 2000 to close to 6-mtpa in 2007, although not at all of the capacity is continuously operational. An important factor that decides the fate of PDH units is the 'delta' (price difference) between the price of propane and propylene. At a 'delta' below $200 per ton over a long term, PDH economics turn out to be difficult to continue operating plants and operators typically resort to shutdowns at these times. One aspect of the process, which can work both as an advantage and a disadvantage, is that it produces little of co-products. The simple PDH unit also enjoys lower capital investment, as compared to a steam cracker or FCC splitter.

In recent years, the economics of PDH have shown an improving trend. In W. Europe, for instance, PDH units have historically been run only intermittently in periods of high demand, but with little investment in European refineries or steam crackers expected, the proportion of propylene supply from PDH is expected to increase to balance demand.

Similarly, 'on-purpose' propylene is taking an increasing share of investments in the Middle East. PDH is clearly the first option, but metathesis plants are also making their appearance. The economics of the latter is determined by the relative prices of ethylene, propylene and butene (low butene prices or P/E price > 1.2) and by availability of surplus ethylene.