Propylene needs
Propylene is one of the fastest growing petrochemicals, driven primarily by the high growth rate of polypropylene (Figure 1). Polypropylene demand currently is growing in the U.S. at 6 %/yr, and in some regions of the world the growth rate is considerably higher.
While steam cracking continues to supply most of the world's propylene, there is an increasing need for propylene from other sources. The growth in steam cracker capacity is driven by the need for ethylene, and co-product propylene production is not keeping up with propylene demand growth. Furthermore, co-product propylene production from steam cracking is determined largely by the feedslate and much of the new steam cracking capacity is based on ethane feed, which produces little propylene.
Propylene production from FCC units is the second most important source of worldwide petrochemical propylene supply (Figure 2). As demand has increased, Refiners have been able to increase propylene production in FCC's by optimizing catalyst and operating conditions. In particular, use of ZSM-5 catalyst additive is increasing as Refiners find it profitable to boost FCC propylene production. In the U.S., FCC propylene has in fact surpassed steam
However, the potential for production of propylene in existing Refinery FCC's is limited (by the capacity of the units and the cost to debottleneck to accommodate increased volumes of gas). New on-purpose propylene technologies will be required to provide the additional supplies of propylene needed to meet the growth projections. Several on-purpose propylene technologies are available, such as propane dehydrogenation and metathesis, but have seen only limited applicability. Propane dehydro requires high investment and both technologies require opportunistic feedstock economics. A newly emerging technology involving catalytic cracking of olefinic naphthas and/or C4's can now be considered as well.
Olefin cracking for Propylene
Catalytic cracking of olefinic streams to produce primarily propylene along with ethylene and butylenes has been described by several companies in recent years, including Mobil (MOISM), Kellogg Brown & Root (SUPERFLEXSM) and Lurgi (PROPYLUR). These processes are characterized by the use of ZSM-5 catalysts to convert higher molecular weight olefins and paraffins to lighter olefins.
ZSM-5 is an aluminosilicate zeolite having a pore size of 5.1 to 5.6 Angstroms. This pore size provides shape selectivity by limiting access to the interior of the catalyst to mostly linear (non-branched or mono-methyl) paraffin and olefin molecules. The active sites in the zeolite catalyst promote cracking. The resulting distribution of C3 and higher olefins approaches equilibrium, with propylene being the olefin product having highest yield (see Figure 3). Ethylene also is produced, but its yield is largely dependent on reaction conditions. Reaction chemistry and the use of ZSM-5 catalyst favor conversion of the cracker propylene as the largest
source of U.S. petrochemical propylene supply.olefinic molecules, but depending on severity more or less paraffin conversion may occur also.