Pygas upgrading for European steam crackers
Upgrading secondary by-products from naphtha-based steam crackers is imperative to remain profitable in a competitive petrochemical field. This article presents available technology options for BTX recovery, styrene extraction, C5 olefin upgrading and more
Joseph C Gentry and Meijuan Zeng
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In order to maintain competitiveness in the ethylene business for European steam crackers, more emphasis must be placed on upgrading all of the by-products that are generated by liquid crackers. Producers who do not upgrade these by-products will face increasing pressure on plant margins due to competition from the low-cost regions of the world.
Ethylene is one of the world’s most important chemical feedstocks. Together with propylene, it accounts for 50–60% of all organic petrochemical production. Most ethylene is produced via the steam cracking of hydrocarbon feedstocks such as ethane, propane, butane, naphtha or gas oil. Steam cracking of saturated hydrocarbons has been the dominant technology for several decades. It involves thermal pyrolysis in the presence of steam to dehydrogenate the saturated hydrocarbon. Ethane crackers primarily produce ethylene and only small amounts of co-products, whereas liquid crackers produce a range of olefins and aromatics, including butadiene, propylene and benzene.
West European ethylene production is approximately 24 MM tpa, which represents a 21% market share of worldwide production. Naphtha and condensates together provide 74% of the feed to the European crackers; 13% of the feedstock is from NGL (ethane, propane and butane), with the balance being gas oil. On average, from 2008–2011, ethylene capacity is expected to increase by 3%.
In a relative sense, European steam crackers use more liquids in the feed slate (eg, naphtha or gas oil) than Middle Eastern producers, which use cost-advantaged NGLs. NGL-fed plants or gas crackers are less complicated because they require only a water quench system and fewer process units for the by-products. The main attraction for gas crackers is the lower feedstock cost, which is often subsidised by national governments and is lowest in the Middle East.
Downstream processing of cracked hydrocarbons from liquid feedstocks (naphtha and gas oil) is somewhat more complex than the processing of gaseous feedstocks because heavier components are present. In this case, a primary fractionator is installed upstream of the compressor to remove fuel oil, which is produced in considerable amounts when cracking naphtha or gas oil. This fuel is partly used in the direct quench operation, while the remainder may be sold or used as fuel for steam generation. The primary fractionator also produces a gasoline stream, called pyrolysis gasoline or pygas, which is rich in aromatics (benzene, toluene and C8 aromatics). This pygas portion of the cracker products contains a host of valuable petrochemicals that can be recovered if they are available in sufficientquantity and a suitable recovery method is chosen.
Figure 1 provides a guideline of which products may be economically recovered as a function of ethylene rate. With today’s typical increased cracker size, many of these products are economical to produce. When the ethylene rate exceeds 400 KTA, butadiene, benzene and toluene production become economically feasible. At 600 KTA and above, styrene extraction, production of C9 resins, xylenes, benzene derivatives and C4 derivatives become economic. Isoprene, dicyclopentadiene (DCPD) and piperylene recovery will be economical at 800 KTA ethylene feed. The steam cracker C5s have a variety of end uses. These are often overlooked or considered as orphan streams due to a lack of knowledge about end markets and processing methods.
Polyisoprene is a substitute for natural rubber, which is being constrained by land use in Southeast Asia that has been set aside for palm oil plantations. The supply/demand balance for isoprene has tightened in recent years, sometimes resulting in sharp upward shifts in pricing. Piperylenes (cis and trans 1,3 pentadiene) are used as a reactive monomer in the manufacture of plastics, adhesives and resins. DCPD is used for printing inks and as a precursor to unsaturated polyester resins.
After ethylene and propylene, benzene is the next most widely produced steam cracker petrochemical. Benzene has been denigrated as a cancer-causing dangerous product. Government legislators have deemed that the benzene content in motor gasoline should generally be lower than 1 wt% worldwide and 0.62% in the US to protect the citizens in these areas. While this has dampened the spirit for aromatics recovery, benzene, toluene and xylene (BTX) production is very much alive and needed, and somewhat encouraged by the need to remove the components from gasoline. In order to maintain competitiveness in the ethylene business, more emphasis must be placed on upgrading all of the by-products that are available in the value chain of liquid crackers.
Figure 1 is not exact for every producer, but is a useful guideline to highlight what typical commodity products could be recovered. Individual plant sites will have different economies based on local supply/demand, logistics of product movements and construction costs. The first priority for component recovery from the pygas is BTX. Most plants already have aromatics extraction units or sell the BTX cut to other units that recover the aromatics. Two technologies are used for BTX recovery:
— Liquid-liquid extraction (LLE)
— Extractive distillation (ED).
The schematics of these two processes are shown in Figure 2. In the last few years, nearly all new units have been designed using ED technology, which offers a simpler design and better process performance.
GTC offers the proprietary GT-BTX process for aromatic recovery by ED. The working principle of ED is the alteration of the relative volatility of components in the presence of a highly selective solvent. In a mixture containing aromatics and non-aromatics, the relative volatility of the non-aromatic components is enhanced over that of the aromatic components in the presence of a solvent. This enhancement allows the non-aromatics to be distilled overhead in a conventional distillation column, while the aromatics are recovered in the column bottoms. The solvent used in the GT-BTX process is the proprietary blend, Techtiv-100, which makes it possible to achieve the process performance with BT- or BTX-containing feedstocks. Techtiv-100 can process wide-cut feeds instead of benzene-only in the ED configuration. Previous generations of solvents could be used only with fractionated narrow-cut feeds or with very high aromatic content to avoid the situation of three-phase distillation. The primary properties of the solvent Techtiv-100 blend are shown in Table 1.
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