Heavier feeds

As feedstocks progress from ethane to heavier fractions with lower H/C ratios, the yield of ethylene decreases, and the feed per pound ethylene product ratio increases markedly. Table 3-15 shows yields from steam cracking of different feedstocks, and how the liquid by-products and BTX aromatics increase dramatically with heavier feeds. 

Reducing delta coke will lower the regenerator temperature. Many benefits are associated with a lower regenerator temperature. The resulting higher cat/oil ratio improves product selectivity and/or provides the flexibility to process heavier feeds. 

Feedstock quality. The quality of the FCC feedstock impacts the concentration of coke on the catalyst entering the regenerator. A heavier feed containing a higher concentration of coker gas oil will directionally increase the delta coke as compared with a lighter, resid-free feedstock. 

If the object of debottlenecking is to run heavier feeds, multiple test runs may be needed with heavy feed added in stages. 

The feed pump will be re-rated for the new conditions. With higher viscosity and higher gravity, the pump driver may need work. If the system is not adequate, heavier feed can be piped through a separate circuit in parallel with the existing circuit, preferably on flow ratio control. 

Referring to the hardware in Figure 5—4, there are much larger facilities required for heavier liquids cracking than for ethane or propane. As you saw in Table 5—1, the yield of ethylene from the heavier feeds is much lower than from ethane. That means that to produce the same amount of ethylene on a daily basis, the gas-oil furnaces have to handle nearly five times as much feed as ethane furnaces. As the design engineer scales up these volumes, he or she has to worry about the size of the cubes necessary to heat up that much feed, the residence times best for each kind of feed, and the best pressure/temper-ature/steam mixture conditions. 

Finally, PILC, REY-PILC, and a commercial equilibrium catalyst were evaluated at near constant conversion using a heavier feed, hydrotreated resid. The product yields are shown in Table III. Steam deactivated (D), REY-PILC, produced the same gasoline selectivity, LCO/HCO ratio, and coke yield as calcined PILC. The equilibrium catalyst which represents a more severely deactivated (E) sample had higher gasoline selectivity, lower coke yield, and lower HCO/LCO ratio. The higher coke yield of REY-PILC could be due to occlusion of high molecular weight hydrocarbons in the microstructure of the pillared clay. 

The importance of diffusion enhancement to heavy oil cracking is further illustrated by the alumina-montmorillonite complexes which crack heavier feeds, i.e. Wilmington fraction No. 6, more effectively than REY. When used as matrices for REY, the alumina-montmorillonites results in considerably more active catalysts, at the same zeolite content, compared with a catalyst having a kaolin-binder matrix, while the selectivity properties differs very little between the two types of catalysts (Sterte, 3. Otterstedt, 3-E. Submitted to Appl.Catal.). 

However, the naphtha yield for the somewhat heavier feed C was lower than the naphtha yield of feed B, especially at the naphtha maximum around 75 wt% conversion, see Figure 3.23. 

Both feeds had similar LCO and HCO yields at conversions around their naphtha maxima but at lower conversions the heavier feed C gave lower LCO yield and higher HCO yield than the lighter feed B, see Figures 3.24 and 3.25. 

The light hydrocarbons produce only minor amounts of by-products, while naphtha and heavier feeds produce substantial quantities of propylene, butadiene, and aromatics. Thus, while in the United States these products are obtained generally from other routes at present, in Europe and Japan ethylene production serves as a major source of these chemicals. As discussed in greater detail later, by-product outlet considerations can play an important role in feedstock selection, and by-product realizations can have a major effect on the ethylene production economics. 

As for the other by-products produced via cracking heavier feeds, the general trend is toward reduced off-gas production and increased pro-... 

At current price levels, heavier feeds in the United States are not competitive with light hydrocarbon feeds. With U.S. naphtha at 1.6 /lb (10 /gal), the ethylene production costs from this feed ranges about 40-70% higher than costs associated with lighter feeds, assuming premium by-product values. The differences are even greater with fuel byproduct values prevailing. 

The current unattractiveness of heavier feeds in the United States notwithstanding, ethylene can be made more cheaply from 1.1 /lb heavy gas oil than from 1.6 /lb naphtha even though a gas oil plant is more expensive and requires more feed. This applies for both premium and fuel by-product cases. Even so, ethylene production costs with gas oil feed are about 25-50% higher than costs with light hydrocarbon feeds. 

Under the assumptions of naphtha price and aromatics value stated above, naphtha pyrolysis clearly would be superior to light hydrocarbon pyrolysis at their current feed prices. A similar analysis can probably be made also for gas oil. Thus, if the possible developments discussed above do materialize, the heavier feeds could probably dominate almost all new U.S. ethylene plant construction in the future. 

If widely predicted future price increases for the natural gas liquid feeds materialize, naphtha and gas on feedstocks will become more competitive in the United States even at current prices for the heavier feeds. 

Naber et al (9) have demonstrated that FCC still has a considerable potential to remain the (resid) conversion "workhorse" of the oil industry. At present about 45% of the world s crude can be envisioned to be within the frontiers of Resid FCC (figure 1). Apart from the importance of FCC feed pretreatment and FCC unit design, also the impact of FCC catalyst performance is crucial to allow the processing of heavier feeds. 

Table I illustrates typical products obtained on pyrolyz-ing the relatively light feedstocks from ethane through butane, but significant variations occur because of the design and operating conditions employed with each light paraffin. The compositions of products obtained from naphthas, gas oils, and even heavier feedstocks differ to an even greater extent the compositions of these heavier feeds vary over wide ranges. Tables II and III report typical...

Khouw et al [20] report that catalysts contaminated to high Vanadium levels are stilt capable of converting light feeds, but not heavier feeds. This is illustrated in the following table. 

In principle, the SBA/Topspe process consists in mixing the burner exit gases with steam and sending the mixture to a fixed-bed reactor on a nickel base catalyst, at about 2.10 Pa absolute and about 950 C This technique, sometimes called partial catalytic oxidatioEu is only applied to the conversion of natural gas, LPG and naphthas, particularly because, if heavier feeds are used, problems arise in the prior separation of sulfur derivatives that cannot be t[Pg.42]

Most hydrodeaikylatioQ processes can be adapted to produce xyienes or naphthalene from heavier feeds. For example, two Hydeai units exist producing naphthalene. Two other specific processes for naphthalene manufacture can be added to the list of techniques already mentioned, the catalytic Union Oil of California Unidak process and the Standard Oil of Indiana thermal process. 

Naphtha cracking provides about 4.3 million tonnes of propylene per year, which is out of a total demand for propylene in excess of 5.3 million tonnes per year. The difference (about 20%) is made up by propylene extracted from refinery off-gases, particularly FCC operations (used to produce gasoUne from heavier feed stocks such as heavy gas-oil or residua). 

The second group comprise LPG feedstocks made from crude oil. These are products of refinery and petrochemical operations processing heavier feeds such as gas oil and vacuum gas oil and residual fuel oils. These LPG streams contain materials of direct interest to petrochemical operations for further processing to other chemicals. With suitable treatment (hydrogenation) they can be used as cracker feedstock or sold to other users as an energy fuel. 

The inhibition effect is only weakly dependent on feed properties. It appears that heavier feeds, with higher carbon to hydrogen ratios, exhibit larger inhibition in their rate constants. 

The location of the minimum indicates the most economic steam dilution. Higher cracking severity and heavier feed stock shifts the optimum steam dilution to higher values. Existing naphtha furnaces operate mostly with steam dilution between 0.5 and 0.6. An example of the reduction of steam dilution for existing furnaces is discussed at the end of this chapter. 

With heavier feeds, potassium is added to poison acid sites and inhibit coking. Phosphorous is also used, presumably to inhibit acidity but also to promote dispersion of the molybdenum compounds. ... 

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