Cracking products separation

The Coastal process uses steam pyrolysis of isobutane to produce propylene and isobutylene (as weH as other cracked products). It has been suggested that the reaction be carried out at high pressure, >1480 kPa ( 15 atm), to facHitate product separation. This process was commercialized in the late 1960s at Coastal s Corpus Christi refinery. 

In the moving-bed process, oil is heated to up to 1,300"F and is passed under pressure through the reactor where it comes into contact with a catalyst flow in the form of beads or pellets. The cracked products then flow to a fractionating tower where the various compounds are separated and collected. The catalyst is regenerated in a continuous process where deposits of coke on the catalyst are burned off. 

In addition to the distillation of crude oil coming into the refinery, stills of various designs are used in other types of service throughout the refinery. Cracked products are separated in distillation equipment which is very similar to an atmospheric crude pipe still. The principal difference is that these products are hot from the cracking operation, so that a fired heater is not required. 

Product separation for main fractionators is also often called black oil separation. Main fractionators are typically used for such operations as preflash separation, atmospheric crude, gas oil crude, vacuum preflash crude, vacuum crude, visbreaking, coking, and fluid catalytic cracking. In all these services the object is to recover clean, boiling range components from a black multicomponent mixture. But main fractionators are also used in hydrocracker downstream processing. This operation has a clean feed. Nevertheless, whenever you hear the term black oil, understand that what is really meant is main fractionator processing. 

The cracked products pass out through two stages of cyclones which collect entrained catalyst and return it to the dense bed. Velocities at the outlet of the dense bed are normally 2.0-3.0 ft./sec. Upon leaving the cyclones, the vapors go to the primary fractionator which separates the heavy products from the gasoline and lighter components. The light products go on to the light ends recovery unit. The heavy material is separated and either recycled to the reactor or withdrawn from the system. 

In most of today s FCC operations, the desired reactions take place in the riser. In recent years, a number of refiners have modified the FCC unit to eliminate, or severely reduce, post-riser cracking. Quick separation of catalyst from the hydrocarbon vapors at the end of the riser is extremely important in increasing the yield of the desired product. The post-riser reactions produce more gas and coke versus less gasoline and distillate. Presently, there are a number of commercially proven riser disengaging systems offered by the FCC licenser designed to minimize the post-riser cracking of the hydrocarbon vapors. 

In an olefins plant, the feed is subjected to very high temperatures in cracking furnaces for a few moments and then cooled rapidly to stop the cracking. Elaborate separation facilities are necessary to separate the olefins. from the by-products of the cracking process. 

Displacement desorption Good for strongly held species avoids risk of cracking reactions during regeneration avoids thermal aging of adsorbent Product separation and recovery needed (choice of desorbent is crucial)... 

C-2 C-3 UPPER PHASE OFF GAS COOLER h2so4 post-crack product cooler R-5 UPPER PHASE POST-DECOMPOSITION GAS-LIQUID SEPARATOR... 

There, the depolymerizate is hydrogenated under high pressure (about 10 MPa) at some 400-450°C, using a liquid phase reactor without internals. Separation yields a synthetic crude oil, which may be processed in any oil refinery. Light cracking products end up in the off-gas and are sent to a treatment section, for removal of ammonia and hydrogen sulphide. A hydrogenated bituminous residue comprises heavy hydrocarbons, still contaminated with ashes, metals and salts. It is blended with coal for coke production (2 wt%). 

This stream cools the vapors and scrubs the remaining catalyst out of the cracked products. Most of the slurry of catalyst in heavy oil withdrawn from the bottom of the tower is recirculated to the top of the disc-and-donut section, while a small portion is withdrawn for recovery of catalyst. The latter stream (slurry return) usually amounts to 3 to 10% of the volume of fresh feed to the reactor. Catalyst concentration in the slurry can be decreased by increasing the rate of withdrawal, and is usually maintained below 0.5 lb./gallon to avoid erosion of slurry pumps and valves. The slurry-return stream may be pumped to a separate settler (e.g., a Dorr thickener or a simple cone-bottom tank) or the settler may be incorporated in the bottom of the fractionating tower (25). About 70% of the heavy oil is removed from the settler as a clarified oil containing less than 0.01 lb. catalyst/gallon. The sludge is diluted with fresh feed and pumped to the reactor to return the catalyst to the system. 

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