Steam cracking coking deposition

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. 

In industrial practice, however, the most widespread technique consists in passmg a mixture of hydrocarbons and steam through tubes placed in a furnace. The hydrocarbons, which are raised to high temperature, are pyrolysed and the resulting products are separated after a rapid quench. Coke deposits are periodically removed by controlled combustion. This is the technology of steam cracking, which is the main focus of this chapter. 

In the second step, the dioxanes are vaporized, superheated, and then cracked on a solid catalyst (supported phosphoric acid) in the presence of steam. The endothermic reaction takes place a about 200 to 2S0°C and 0.1 to OJ. 10 Pa absolute. The heat required is supplied by the introduction of superheated steam, or by heating the support of the catalyst, which operates in a moving, fluidized or fixed bed, and, in this case, implies cyclic operation to remove the coke deposits formed. Isoprene selectivity is about SO to 90 mole per cent with once-through conversion of 50 to 60 per cent The 4-4 DMD produces the isoprene. The other dioxanes present are decomposed into isomers of isoprene (piperylene etc.), while the r-butyl alcohol, also present in small amounts, yields isobutene. A separation train, consisting of scrubbers, extractors and distillation columns, serves to recycle the unconverted DMD, isobutene and fonnol, and to produce isoprene to commercial specifications. 

Metal granules also have been found in cokes formed or deposited on iron, cobalt, and nickel foils in experiments using methane, propane, propylene, and butadiene (7-10). Platelet-type coke, whose properties match those of graphite also was produced in some cases. Lahaye et al. (11) investigated the steam cracking of cyclohexane, toluene, and n-hexane over quartz, electrode graphite, and refractory steel. They report that heavy hydrocarbon species form in the gas phase, condense into liquid droplets which then strike the solid surface, and finally react on the solid surfaces to produce carbonaceous products. The liquid droplets wet and spread out on certain surfaces better than on others. 

Finally, it is relevant to observe that this dissolution presents strong analogies with a condensation process discussed and stressed by several authors (Cai et al., 2002) as being responsible for coke formation/deposition in the TLE tube outlet section at operating temperatures of 350 450°C. Indeed this mechanism can be explained on the basis of the solubility of heavy species of the process fluid phase in the soft polymer. There has also been research into the computer generation of a network of elementary steps for coke formation during steam cracking process (Wauters and Marin, 2002). 

Carbonaceous deposition during steam cracking is the net result of steady state formation and removal processes. If the measured oxidation rates in water vapour did represent the removal of the deposit in situ, then this would be an extremely rapid process over the temperature range at which deposition on radiantly heated process tubes is most significant. Thus, 1 mm thickness of deposit would be oxidised by 362 mm Hg water partial pressure in 300 h at 800°C, 33 h at 900°C and 5 h at 1000°C. If a hydrocarbon, or its decomposition products, enhanced the oxidation rate these times could be decreased. Coke removal by thermal oxidation cannot be ignored, therefore, although its extent would depend on specific plant operating conditions. 

Gasification Kinetics of Coke Deposited on Silica-Alumina. Within the temperature range 1400 to 1600°F and in the presence of excess steam, the gasification reaction of coke deposited on the silica-alumina cracking catalyst closely followed first-order kinetics with respect to unreacted carbon (Figure 1). First-order rate constants were calculated from the slopes of these plots (Table III), and yielded an activation energy of 55.5 Kcal/mole. 

Description The DCC process overcame the limitations of conventional fluid catalytic cracking (FCC) processes. The propylene yield of DCC is 3-5 times that of conventional FCC processes. The processing scheme of DCC is similar to that of a conventional FCC unit consisting of reaction-regeneration, fractionation and gas concentration sections. The feedstock, dispersed with steam, is fed to the system and contacted with the hot regenerated catalyst either in a riser-plus fluidized dense-bed reactor (for DCC-I) or in a riser reactor (for DCC-II). The feed is catalytically cracked. Reactor effluent proceeds to the fractionation and gas concentration sections for stream separation and further recovery. The coke-deposited catalyst is stripped with steam and transferred to a regenerator where air is introduced and coke on the catalyst is removed by combustion. The hot regenerated catalyst is returned to the reactor at a controlled circulation rate to achieve the heat balance for the system. 

It has been shown [440] [518] for coke deposits on cracking catalysts that the reactivity after oxidation depends mainly on the surface area of the coke and that the rate quickly becomes diffusion limited with a risk of overheating the catalyst pellet. In practice, the bum-off of coke can easily be performed by adding a few percent of air to the steam flow at temperatures above approximately 450°C as illustrated in Figure 5.38 [388] [389]. 

In steam cracking processes, the amount of coke produced and deposited on the heated pipeline walls depends on the type of fuel employed, operation conditions and the metallurgic nature of pipelines. In addition, coke is also produced in heat exchangers (where temperatures ean be between 400 and 700°C)." Coke deposits with a thickness of some millimeters/centimeters make heat transfer difficult, so the temperature in the reactor must be increased which in turn leads to higher coke formation. Moreover, coke accumulation favors a pressure drop which results in reduced production of olefins. Over time, production must be frequently stopped to remove coke (decoking) from the reaction system. Decoking is carried out with a mixture of water and air to burn the coke. This process is undesirable as it results in a drop in the production of olefins, is expensive to maintain and reduces the longevity of the pipelines. 

---