Ethylene/Propylene Yield naphtha

In a single stage, without liquid recycle, the conversion can be optimized between 60 and 90%. The very paraffinic residue is used to make lubricant oil bases of high viscosity index in the range of 150 N to 350 N the residue can also be used as feedstock to steam cracking plants providing ethylene and propylene yields equal to those from paraffinic naphthas, or as additional feedstock to catalytic cracking units. 

Significant products from a typical steam cracker are ethylene, propylene, butadiene, and pyrolysis gasoline. Typical wt % yields for butylenes from a steam cracker for different feedstocks are ethane, 0.3 propane, 1.2 50% ethane/50% propane mixture, 0.8 butane, 2.8 hill-range naphtha, 7.3 light gas oil, 4.3. A typical steam cracking plant cracks a mixture of feedstocks that results in butylenes yields of about 1% to 4%. These yields can be increased by almost 50% if cracking severity is lowered to maximize propylene production instead of ethylene. 

Table 6 shows the effect of varying coil oudet pressure and steam-to-oil ratio for a typical naphtha feed on the product distribution. Although in these tables, the severity is defined as maximum, in a reaUstic sense they are not maximum. It is theoretically possible that one can further increase the severity and thus increase the ethylene yield. Based on experience, however, increasing the severity above these practical values produces significantly more fuel oil and methane with a severe reduction in propylene yield. The mn length of the heater is also significantly reduced. Therefore, this is an arbitrary maximum, and if economic conditions justify, one can operate the commercial coils above the so-called maximum severity. However, after a certain severity level, the ethylene yield drops further, and it is not advisable to operate near or beyond this point because of extremely severe coking. 

Figure 5.3 shows light olefin yields of DCC process in four refineries with different feedstocks at reaction temperatures of 545-565°C. The propylene yield can reach 23 wt% with paraffinic feed, and about 18-19 wt% with intermediate-based feed. The propylene/ethylene ratio is about 3.5-6.2, much higher than that of steam cracking. The DCC operation can be modified to further increase the yield of propylene. For example, recycling a part of DCC cracked naphtha to the reactor resulted in a propylene yield increment of 3.5 wt % in Jinan Refinery [16]. 

Economics Ultimate range of ethylene yields vary from 83% (ethane) to around 25% (vacuum gas oils), 35% for the intermediate full-range naphtha. These correspond to the respective total olefins yields (ethylene propylene) from 84% (ethane) to 38% (vacuum gas oils), and 49% for an intermediate full-range naphtha. The specific energy consumption range is 3,100 kcal/kg ethylene (ethane) to 5,500 kcal/kg ethylene (gas oil), and 4,700 kcal/kg ethylene for an intermediate full-range naphtha. 

Based on the foregoing considerations, it can therefore be infen that a decrease in the tube diameter, which causes a reduction in residence ime, results in a higher ethylene yield (diagram a in Fig. 2.12) in industrial naphtha steam cracking conditions. Simultaneously, a drop in the propylene yield (diagram b in Fig. 2.12) is observed in the normal... 

The statisties for a naphtha cracker integrated with polymer and BTX production are illustrated in Table 9.2. The complex is based on the CLOSED case and only ethylene, propylene and pyrolysis gasoline pass to downstream processing. For brevity it is assumed the polymers are produced at 100% yield and require no other feedstock. The pyrolysis gasoline is passed to an aromatics extraction plant and produces 298 kt/y benzene, 149 kt/y toluene and 52 kt/y xylene. The rest of the pyrolysis gasoline (246 kt/y) produees a raffinate, whieh is sold as a gasoline. 

The reaction temperature in DCC is higher than that of conventional FCC but much lower than that of steam cracking. Propylene yields over 20 wt% are achievable with paraffinic feeds. Ethylene yield is much higher than the conventional FCC process. The DCC-mixed C s stream also contains increased amounts of butylenes and iso-C s as compared to an FCC. The high olefin yields are achieved by deeper cracking into the aliphatic components of the naphtha and ECO. The dry gas produced from the DCC process contains approximately 50% ethylene. The cracking reactions are endothermic, and compared to FCC, a higher coke make is required to satisfy the heat balance. Table 1 summarizes typical olefins yields for DCC with FCC. 

Description The most predominant feed used to produce ethylene today is naphtha, as more than half of the world s ethylene is currently derived from cracking naphtha feed. The Advanced Catalytic Olefins (ACO) process is an alternative process that catalytically converts naphtha feed and is thus able to produce higher ultimate yields of light olefins (propylene plus ethylene) and at a higher P/E production ratio relative to steam cracking, typically about 1 1. 

Figure 1. Each sloped line represents the loci of all possible combinations of average residence times and hydrocarbon partial pressures which are consistent with a fixed pyrolysis yield pattern, i.e., constant pyrolysis selectivity lines. For liquid feedstocks, the methane-to-ethylene ratio found in the pyrolysis reactor effluent has been used as a good overall indicator of pyrolysis reactor selectivity. Low methane-to-ethylene ratios correspond to a high total yield of ethylene, propylene, butadiene and butylenes. Consequently, the yields of methane, ethane, aromatics and fuel oil are reduced. TL refore, each constant pyrolysis selectivity line shown in Figure 1 is identified with a fixed methane-to-ethylene ratio. This specific selectivity chart applies to a Kuwait heavy naphtha which is pyrolyzed to achieve a constant degree of feedstock dehydrogenation, i.e., a constant hydrogen content in the effluent liquid products, which in this case corresponds to the limiting cracking severity.

Figure 4, Change of ethylene yield (a) and propylene yield (b) of naphtha cracking with degree of decomposition...

Today more butadiene is produced from butene (another C4) through steam cracking of naphtha gas oil from ethylene/propylene production. Through extractive distillation of this C4 cracker stream, the butadiene is obtained. Commonly the yield achieved for BD is dependent on the quality of the feedstocks used for ethylene production. Usually, the heavier the feedstock, the greater the BD production. Reportedly, the lighr feedstock only yields about one-fifth the yield of butadiene compared to the heavy feedstock. 

The base-load supply of butadiene is from olefins plants simply because butadiene is coproduced with the other olefins. There s not much decision on whether or not to produce it. It just comes out, but in a small ratio compared CO ethylene and propylene. Cracking ethane yields one pound of butadiene for every 45 pounds of ethylene cracldng the heavy liquids, naphtha or gas oil, produces one pound of butadiene for every seven pounds of ethylene. Because of the increase in heavy liquids cracldng, about 75% of the butadiene produced in the United States is coproduced in olefin plants. 

Lower molecular weight feedstocks, such as ethane and propane, give a high percentage of ethylene higher molecular weight feedstocks, such as naphtha and gas oil, are used if propylene demand is up. The following table summarizes the typical yields of olefins obtained from various feeds. 

Yield Pattern. Table XI presents a feed/product summary for a naphtha based billion lb/yr ethylene plant at various severities of 23, 25, and 27 wt % ethylene (once-through basis). The naphtha feed is the same one as referred to earlier (see Table III). It is immediately apparent that feed requirements are increased at lower severities for a given ethylene production rate. Also, production of olefin by-products increases as severity decreases. Note especially the 36% increase in propylene production as severity is dropped from 27% ethylene to 23% ethylene. Butadiene production goes up somewhat, while butylenes production jumps by over 100% going from 27 to 23% ethylene. 

Yields Product yields are dependent on feedstock composition. The process provides propylene/ethylene production at ratios of nearly 4 1. Case studies of olefin cracking integration with naphtha crackers have shown 30% higher propylene production compared to conventional naphtha-cracker processing. 

Yields of pyrolysis products also depend ou the chemical composition of the naphtha feedstock. The thermal stability of hydrocarbons increases in the follov. ing order paraffins, naphthenes, aromatics. It decreases as the chain becomes longer. Thus, it is usually obser ed that the ethylene yield, as well as that of propylene, is higher if the naphtha feedstock is rich in paraffms. 

The plant locations and capacity are listed in Table 1.7. Most of the Chinese plants are old, with capacities below 200,000 t/y. Many of these plants were designed to use gas-oil and naphtha as feedstock. This takes advantage of some of China s indigenous crude oil, which have high levels of paraffin wax in the gas-oil fractions. In steam cracking, such gas-oils give high yields of ethylene and propylene. Newer plants... 

Light naphtha can produce over 30% ethylene with about half this yield of propylene. Methane yield is also high at over 17% with produetion of pyrolysis gasoline lower than the heavier liquids in the region of 14%. This is considerably more than the yields of pyrolysis gasoline (Cs+ aliphatie moleeules plus BTX) from gaseous feed stocks discussed above. 

Full range naphtha produees less ethylene but relatively more propylene. There is a high yield of pyrolysis gasoUne. 

Gas oil produees similar yields of ethylene and propylene to fun range naphtha but there is a large inerease in the production of pyrolysis fuel oil (b.p. >200°C). 

The effect on full-range naphtha is illustrated by the data in Table 2.4. This shows that increasing severity increases the ethylene (and methane) yield at the expense of propylene and heavier products. 

Recently a new FCC catalytic system has been proposed which will generate ethylene and propylene from low value olefin rich naphtha feedstock. Typical yields are given in Table 10.4. 

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