Cracking, coking deposition

The visbreaking process thermally cracks atmospheric or vacuum residues. Conversion is limited by specifications for marine or Industrial fuel-oil stability and by the formation of coke deposits in equipment such as heaters and exchangers. 

The cracked products leave as overhead materials, and coke deposits form on the inner surface of the dmm. To provide continuous operation, two dmms are used while one dmm is on-stream, the one off-stream is being cleaned, steamed, water-cooled, and decoked in the same time interval. The temperature in the coke dmm is in the range of 415—450°C with pressures in the range of 103—621 kPa (15—90 psi). Overhead products go to the fractionator, where naphtha and heating oil fractions are recovered. The nonvolatile material is combined with preheated fresh feed and returned to the furnace. The coke dmm is usually on stream for about 24 hours before becoming filled with porous coke, after which the coke is removed hydraulically. 

Coke deposition is essentially independent of space velocity. These observations, which were developed from the study of amorphous catalysts during the early days of catalytic cracking (11), stiU characteri2e the coking of modem day 2eohte FCC catalysts over a wide range of hydrogen-transfer (H-transfer) capabihties. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

The specific rate is expected to have an Arrhenius dependence on temperature. Deactivation by coke deposition in cracking processes apparently has this kind of correlation. 

Figure 9.10. Scheme of an FCC Unit. Cracking ofthe heavy hydrocarbon feed occurs in an entrained bed, in which the catalyst spends only a few seconds and becomes largely deactivated by coke deposition. Coke combustion in the regenerator is an exothermic process that generates heat for the regeneration and for the endothermic cracking process. 

The two limiting cases for the distribution of deactivated catalyst sites are representative of some of the situations that can be encountered in industrial practice. The formation of coke deposits on some relatively inactive cracking catalysts would be expected to occur uniformly throughout the catalyst pore structure. In other situations the coke may deposit as a peripheral shell that thickens with time on-stream. Poisoning by trace constituents of the feed stream often falls in the pore-mouth category. 

After the cracking run is complete, the coked catalyst is regenerated by passing air saturated with water at room temperature over the catalyst at an elevated temperature (1250° F). The amount of coke deposited on the catalyst is determined by the difference in reactor weight before and after the regeneration. 

State of the art riser termination devices have significantly reduced the coke deposition in the reactor disengager vessel. These modifications have significantly reduced the hydrocarbon residence time and potential thermal cracking in the disengager. 

The technical development of petroleum coking has been inseparable from the development of thermal cracking. Untold millions have been spent on research and development trying to eliminate the formation of petroleum coke. Added millions have been spent learning how to prevent coke from forming in heating coils and making the coke deposit where it could be removed most conveniently (18). 

In operation, preheated feedstock meets a controlled stream of hot. regenerated catalyst. Vaporized oil and catalyst ascend in the riser, such that the catalyst particles are suspended in a dilute phase. Essentially all of the cracking occurs in the riser. The catalyst particles are separated from the cracked vapors at the end of the riser and the catalyst containing a coke deposit is relumed in the regenerator. The cracked vapors puss through one or more cyclones located in the upper portion of the reactor and proceed to Ihe fractionator (main column) thai produces the side streams indicated. 

The coke deposited on the catalyst is burned off in the regenerator along with the coke formed during the cracking of the gas oil fraction. If the feedstock contains high proportions of metals, control of the metals on the catalyst requires excessive amounts of catalyst withdrawal and fresh catalyst addition. This problem can be addressed by feedstock pretreatment. 

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