Steam cracker unit

Since CPP cracking effluent is molecularly similar to that of heavy distillate cracking it will be logical to construct integrated CPP-steam cracker units, or even to add a CPP reactor-regenerator as a revamp side-cracker expansion feature. Various plans are under review for such prospective projects. 

The following example illustrates the dimensions of a commercial steam cracker unit. At the same time it introduces the reader to the global mass balancing of chemical plants, an important technique for dimensioning unit operations, for example, for a given production capacity. 

The capacity of typical steam cracker units in industry is around 1 Mio tons per year. Such world-scale capacities are beneficial due to the reduction of specific investment costs with increasing plant size. Note that this effect applies not only for the steam cracker itself but also for all the downstream production plants running on the diverse products from the cracker. 

Steam cracker units operating on naphtha produce a wide range of products. Their complete utilization is crucial to realize competitive operation of the cracker. As a consequence, steam cracker units are typically operated in networks "Verbund ) of other plants that consume all steam cracker products on the same production site. 

Sud-Chemie, Selective Hydrogenation Catalysts in Steam Cracker Units (1997). 

More than 90% of today s petrochemicals are produced from refineiy products. Most are based on the use of C2-C4 olefins and aromatics finm hydrocarbon steam cracking units, which are even more closely linked to refineries. In North America, the feedstock for steam cracker units have generally been ethane, propane, or LPG. As a result, most of the propylene and aromatics have been provided by FCC units and catalytic reformers. In maity other parts of the world where naphtha feed has been more readily available, suppUes of propylene and aromatics have been produced directly by steam cracking. When necessary, the catalytic dehydrogenation of paraffins or dealkylation of toluene can balance the supply of olefins or benzene. In Table 7.2 some of the catalytic processes that convert olefins and benzene from a steam cracker into basic petrochemicals for the modem chemical industry are shown. 

Propylene has many commercial and potential uses. The actual utilisation of a particular propylene supply depends not only on the relative economics of the petrochemicals and the value of propylene in various uses, but also on the location of the supply and the form in which the propylene is available. Eor example, economics dictate that recovery of high purity propylene for polymerisation from a smaH-volume, dilute off-gas stream is not feasible, whereas polymer-grade propylene is routinely recovered from large refineries and olefins steam crackers. A synthetic fuels project located in the western United States might use propylene as fuel rather than recover it for petrochemical use a plant on the Gulf Coast would recover it (see Euels, synthetic). 

Until 1960, coal was the source material for almost all benzene produced in Europe. Petroleum benzene was first produced in Europe by the United Kingdom in 1952, by Erance in 1958, by the Eederal Republic of Germany in 1961, and by Italy in 1962. Coal has continued to decline as a benzene source in Europe, and this is evident with the closure of coke ovens in Germany (73). Most of the benzene produced in Europe is now derived from petroleum or pyrolysis gasoline. In Europe, pyrolysis gasoline is a popular source of benzene because European steam crackers mn on heavier feedstocks than those in the United States (73). 

Table 6 compares the total production of butylenes in the United States, Western Europe, andjapan. Included in this table are relative amounts of productions from different processes. In the United States, about 92% of the butylene production comes from refinery sources, whereas only about 45% in Western Europe andjapan are from this source. This difference arises because the latter cracks mostiy petroleum distillates in the steam crackers that produce larger amounts of butylenes than the light feedstocks cracked in the United States. 

The various sources of isobutylene are C streams from fluid catalytic crackers, olefin steam crackers, isobutane dehydrogenation units, and isobutylene produced by Arco as a coproduct with propylene oxide. Isobutylene concentrations (weight basis) are 12 to 15% from fluid catalytic crackers, 45% from olefin steam crackers, 45 to 55% from isobutane dehydrogenation, and high purity isobutylene coproduced with propylene oxide. The etherification unit should be designed for the specific feedstock that will be processed. 

For comparative purposes the typical weight percentage yields for a DCC unit, an FCC unit and a steam cracker are shown in Table 8.1. Propylene yields from the DCC unit are considerably higher than those from an FCC nnit. The DCC mixed C4s stream also contains increased amounts of bntylenes and iso-C4s as compared to an FCC. These high olefin yields are achieved by deeper cracking into the aliphatic components of the initially prodnced naphtha and life cycle oil (LCO). 

A major current refinery-petrochemical project is under construction for Saudi Aramco-Sumitomo Chemical at their Rabigh, KSA site. In conjunction with our JGC partner we have linked the upstream refinery expansion to a combined mega-DCC unit and mega-ethane steam cracker. Corresponding production rates are 1500 kta ethylene/950 kta propylene. The corresponding integrated layout is shown below in Figure 8.2. 

Larger scale DCC units (and associated FCC units) offer greater product recovery options. These are particularly attractive when linked to either an existing or new steam cracker. In such schemes several of the individual unit operations can be combined or substituted by a more efficient system. 

A plant contains all production units required to produce a certain intermediate or finished product. The relationship between a site s plants varies between two extremes. At integrated sites (Verbundstandorte) the plants cover certain steps of the overall production process and are closely linked by material flows. This can be seen as a "plant within a plant" concept based on a process focus. Integrated sites are especially common in production of commodity chemicals. For example, the new integrated site built by BASF in Nanjing, China, consists of a steam cracker producing among others ethylene and propylene. Nine other plants further process the substances. The overall investment to build the site was U.S. 2.9 billion.10... 

Priority action was required in petrochemicals, in the large thermoplastics, in fertilizers, and in synthetic fibers where the most serious investment mistakes had been made. The hardest cases were those of petrochemicals and thermoplastics. For one thing, a steam cracker cannot technically operate under 60 percent of its capacity. For another, the products that emerge are linked to one another in almost invariable proportions. Finally, a polymerization unit cannot have its pace slowed down without this affecting the upstream monomer unit to the same extent. 

The 1983 agreement between ENI and Montedison put some order in Italy s chemical industry, as ENI took over the PVC and polyethylene operations of Montedison. Previously in France, Rhone-Poulenc had sold its petrochemicals division and its thermoplastics to the Elf Aquitaine group. At the same time, steam crackers were being shut down in Feyzin and Lavera, and a vinyl chloride unit in Jarrie. The association between BP Chimie and Atochem in polypropylene and the exchange of Atochem s Chocques unit for ICI s Rozenburg polyethylene unit were other instances of rationalization. 

A number of American companies became involved in restructuring. Union Carbide sold its Antwerp site to BP Chemicals Monsanto, its Seal Sands acrylonitrile unit to BASF Esso, its Stenungsund steam cracker to Statoil while Hercules joined up with Montedison to set up the Himont company, which accounted for 20 percent of the world polypropylene market. 

The scale effect, which until then was assumed to be cost-saving, showed its weaknesses as the giant steam crackers proved more expensive to run at low capacity than smaller units already written off and working at full capacity. 

ElfAtochem. At this stage anyway and for historical reasons the approach of TotalFinaElf can be considered unconventional when we compare it with that of the oil companies. With improving crude oil prices and better capacity utilization in their refinery units, the oil companies are concentrating their major reinvestments on what they consider to be their core business and limiting their production of chemicals to the base commodities derived from the olefins of the steam cracker or other feedstocks. 

In addition, the propylene growth rate is faster than ethylene s and the typical C3=/C2= ratio from the naphtha steam cracker is about 0.43. Because the European and Asian demand, Figure 5.20, is for a C3=/C2= ratio of about 0.70, it is inevitable that propylene will have to be produced in greater quantity by the FCC unit to supply the chemical and plastic industries. 

Feeds C4 cuts from steam cracker and FCC units with isobutene contents ranging from 12% to 30%. 

The process is designed to utilize olefinic feedstocks from steam crackers, refinery FCC and coker units, and MTO units, with typical C4 to C8 olefin and paraffin compositions. The catalyst exhibits little sensitivity to common impurities such as dienes, oxygenates, sulfur compounds and nitrogen compounds. 

The C5s are recycled to the steam cracker or an isomerization unit. Sulfur and nitrogen compounds are removed in the second stage (3) hydrogenation units. The BTX cut is ideal for processing in an aromatics complex. 

IFP/Chinese Petroleum Corp. Propylene FCC and steam-cracker C4cuts Meta-4 upgrades pyrolysis C4 cuts to propylene has attractive ROI when combined with IFP selective hydroisomerization unit 1 NA... 

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