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


These two conventional propylene processes account for abulk 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 orminor production of by-products.


The need for alternate routes


In the Asia-Pacific, where refineries still continue to bebuilt, FCC propylene represents a growing and significant chunk of totalpropylene capacity. In Europe and North America, although there is significant productionof propylene from refineries, the lack of investments in new refineries impliesthere is little scope for increasing output from this source. In the MiddleEast, in contrast, the development of petrochemicals has hinged around massiveethane-crackers, to the neglect of propylene and its derivatives. Globally,more than 25% of new crackers that started up in the 2003-2007 timeframe werebased on ethane and produce little propylene.


While the projected annual demand growth for ethylene is atabout 3%, propylene is expected to see faster growth at 4-5%, driven, inparticular, by strong demand for polypropylene (PP) in emerging markets. Going forward,it is clear that steam cracker expansions and/ or additions cannot keep pacewith propylene demand growth. While ethylene growth will be largely satisfiedby 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 ofethylene, the world is heading to a shortage of propylene from conventionalsources.


The current options for 'on-purpose' propylene


This will drive investments in 'on-purpose' propyleneproduction. 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 removalof 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 doublebonds 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 ofC4/ C5 olefins, which technology is similar to metathesis in that low valuehydrocarbon streams are converted to higher value olefins (however, their chemistryis a combination of olefin oligomerisation, cracking, disproportionation andhydrogen transfer).


The process economics


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


In recent years, the economics of PDH have shown animproving trend. In W. Europe, for instance, PDH units have historically beenrun only intermittently in periods of high demand, but with little investmentin European refineries or steam crackers expected, the proportion of propylenesupply from PDH is expected to increase to balance demand.


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

 

Methanol based options


MTO and MTP technologies are not yet commercial, but are on the verge of becoming so. There is increasing interest in these technologies, especially in China, which is looking to leverage its coal reserves for chemicals production through methanol. The world's first MTO plant looks likely to be at Shenhua (China), with a 2011 start-up targeted. The project will include a 1.8-mtpa coal-based methanol plant, using Davy Process Technology and a 600,000-tpa MTO plant, using Chinese technology. There have been some moves to set up plants in Nigeria and Iran, using gas as a raw material for methanol, but both projects are currently stalled.


Similarly, Lurgi is known to have a pilot MTP plant in operation in Norway. Whilst a 400,000-tpa Iranian MTP project now appears to have stalled, two MTP plants utilizing Lurgi technology to produce PP are under construction in China and due to start up in 2009 and 2010.


The Chinese coal-to-olefins (CTO) cases show a decided advantage at large scales, as compared to conventional steam cracking, and justify the recent burst of activity in these projects. Two factors drive the cost advantage: low coal costs and relatively high propylene by-product credit due to unusually high propylene prices prevalent when the projects were announced in the boom years for petrochemicals. However, the erosion in propylene prices could act as a dampener for the proliferation of MTO/MTP plants in China. The key for the technology is low delivered methanol prices.


'Green' propylene


Further down the horizon are the so-called 'green' propylene technologies, which aim at producing the olefin using sustainable feedstock such as biomass (through gasification), via ethanol or butanol (produced by fermentation) or by the cracking of vegetable oils. These routes all vary with respect to feedstock (food crop versus bio-waste), process complexity (number of steps), and experience (commercially used process steps versus steps still needing development).


Production economics for many of the alternate processes based on renewable raw materials render them outside the realm of possibility for the moment.


But they are a pointer that propylene for the chemical industry of the future could well come from sources other than a barrel of crude.


Source: Chemical Weekly