Heavy liquid cracker

Describe how youd use the following processes to handle the C4 stream in the heavy liquids cracker in Figure 5-3 ... 

The hydrocarbon mixture at the furnace outlet is quenched rapidly in the transfer line exchangers (2) (TLE or SLE), generating high-pressure steam. In liquid crackers, cracked gas flows to a primary fractionator (3) after direct quench with oil, where fuel oil is separated from gasoline and lighter components, and then to a quench water tower (4) for water recovery (to be used as dilution steam) and heavy gasoline production (end-point control). 

The term hydrotreating tends to be used to cover all refinery hydrogenation processes. These may range from hydrogenation of olefmic mixtures (liquid cracker products for example) under mild conditions, to the hydrodesulphurization of heavy fractions and the hydrogenation of (poly-) aromatic components. 

Processing of metals-contaminated feedstocks has been practiced since 1961 by Phillips at Borger, Texas. By 1981, 24 other units were using heavy oils in their FCC operations (29-30). At the present, it is believed that by 1989, 15-20 resid units will be operational and that by 1995, 25-30% of the world s FCC units will be partially operating as resid crackers (2). Refiners without the capability of converting resids and less costly oils into transportation liquids will probably suffer in a most competitive energy market. 

Of the many factors which influence product yields in a fluid catalytic cracker, the feed stock quality and the catalyst composition are of particular interest as they can be controlled only to a limited extent by the refiner. In the past decade there has been a trend towards using heavier feedstocks in the FCC-unit. This trend is expected to continue in the foreseeable future. It is therefore important to study how molecular types, characteristic not only of heavy petroleum oil but also of e.g. coal liquid, shale oil and biomass oil, respond to cracking over catalysts of different compositions. 

The performance analysis and product analysis results confirm previous findings (3,8) that hydrotreating improves the quality of catalytic cracker feedstock and the resultant products. In addition, it was shown that the quality of the liquid products and the yields of the coke and the heavy cycle oil (HCO) from cracking of the severely hydrotreated feedstock (WM-2-9) were independent of the conditions of the cracking process. These results imply that there exists a degree of pretreatment hydrotreating above which... 

In the cracker, heavy oil cracking and the steam-iron reaction take place simultaneously under conditions similar to thermal cracking. Any unconverted feedstock is recycled to the cracker from the bottom of the scrubber. The scrubber effluent is separated into hydrogen gas, liquefied petroleum gas (LPG), and liquid products that can be upgraded by conventional technologies to priority products. 

The removal of all mercury species from liquid hydrocarbon feedstocks (raw condensates, crude oil, condensate cuts) is difficult since the majority of the mercury in these feeds is in organometallic form. Most of the time the presence of mercury is associated with the injection of natural gas condensates into the steam cracker. These heavy condensates can contain very high levels of mercury compounds and these mercury species are spread over all the cuts according the following distribution 0-5 % C2, 30-35 % C3, 55-65% C4, 5-10% others cuts. 

First, the vacuum residue enters (after some preheating) the coker distillation tower, where the products of the cracker are separated into different fractions (gases, gasoline, diesel oil, etc.). The bottom product of the distillation column consists of the vacuum residue (without some lighter compounds, which may still have been present in the residue and which are now separated by distillation) and the heavy recycle oil (about 20% of the cracker products), which also remains liquid in the bottom of the column. This combined liquid heavy oil phase is then pumped through a fired heater to bring the mixture from about 350 °C to the desired heater outlet temperature of about 500 °C. 

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