Coke formation from ethylene

According to different mechanisms of coke formation, we have observed different products of polycondensation using chromatographic, luminiscent, and UV-spectroscopic methods. For example, in the case of decomposition of benzene on different catalysts only products of the dehydrocondensation of benzene with preservation of nuclei were observed (biphenyl, biphenylbenzenes, triphenylene, products of condensation of more than four benzene nuclei, etc.) and such products as naphthalene, anthracene, and phenanthrene were not observed. In tar and coke formation from ethylene on silica gel and aluminosilicates the formation of naphthalene, chrysene, 1,2-dibenzanthracene, fluorene, its derivatives, and others, takes place and if the process is carried out on alumina at a temperature lower than 500°, mainly anthracene, phenanthrene, pyrene, and coronene are formed, but aliphatic hydrocarbons, etc., do not appear. 

Chen and co-workers have studied the role of coke deposition in the conversion of methanol to olefins over SAPO-34 [111]. They found that the coke formed from oxygenates promoted olefin formation while the coke formed from olefins had only a deactivating effect The yield of olefins during the MTO reaction was found to go through a maximum as a function of both time and amount of coke. Coke was found to reduce the DME dilfusivity, which enhances the formation of olefins, particularly ethylene. The ethylene to propylene ratio increased with intracrystal-line coke content, regardless of the nature of the coke. 

Figure 7.13 presents a simplified flowsheet, which concentrates the essential features the balanced VCM technology, as conceptually developed in the previous sections, but this time with the three plants and recycles in place chlorination of ethylene (Rl), thermal cracking of EDC (R2) and oxyclorinahon of ethylene (R3). As mentioned in Section 7.3, from plantwide control three impurities are of particular interest (I]) chloroprene (nbp 332.5 K), (12) trichloroethylene (nbp 359.9K), and (13) tetrachloromethane (nbp 349.8). I, and 12 are bad , since the first can polymerize and plug the equipment, while the second favors the coke formation by EDC pyrolysis. On the contrary, I3 has a catalytic effect on the VCM formation, in some patents being introduced deliberately. 

Catalyst deactivation due to coke formation is relatively speedy for a reactions such as alkylation of benzene to ethylbenzene over zeolite, particularly when the benzene to ethylene ratio is low. Another problem of this reaction is the formation of xylenes, the major byproducts. Though their total amount produced in the process is very limited, they are harmful to the process because of the difficulty to remove them from the desired product ethylbenzene. Therefore, investigating the mechanism of catalyst coking is of practical significance for finding the potential ways for prolonging the reactor runtime and decreasing the xylenes selectivity. 

Table I presents results for six comparable pyrolysis runs made by using five laboratory reactors all runs were made with approximately 50% steam as diluent in the ethane feed. Conversions at the exit end of the reactor varied from 59% to 65%. Also, results reported for a commercial unit (11) are shown. Ethylene yields varied from about 78% to 89% in all cases except for run D44 made in the stainless steel 304 reactor. In that run, the ethylene yields were very low but production of CO, GOo, and net coke were much higher. Ethylene yields were highest in the run made in the Vycor glass reactor. In this run, coke formation was least of all runs, and no CO or C02 was detected in the product stream.

Tsai and Zhou have presented a somewhat simplified mechanistic model for ethane and propane cocracking, as shown in Table 1. Their investigation indicated the positive effects relative to ethylene and propylene production for this cocracking, savings in the separation of ethane and propane from LPG mixtures, and feedstock reduction for constant olefins production due to higher olefin selectivity. Their model, however, does not include any surface reactions that may be important plus reactions causing coke formation. 

The kinetic aspect common to all the topics discussed in this chapter is the pyrolysis reactions. The same kinetic approach and similar lumping techniques are conveniently applied moving from the simpler system of ethane dehydrogenation to produce ethylene, up to the coke formation in delayed coking processes or to soot formation in combustion environments. The principles of reliable kinetic models are then presented to simulate pyrolysis of hydrocarbon mixtures in gas and condensed phase. The thermal degradation of plastics is a further example of these kinetic schemes. Furthermore, mechanistic models are also available for the formation and progressive evolution of both carbon deposits in pyrolysis units and soot particles in diffusion flames. 

The orders of the rate of coke formation at 700 and 900°C were estimated from the measured buildup of coke on the metal coupons relative to the inlet ethylene partial pressure by the folowing equation ... 

Anthony and Singh concluded from a kinetic analysis of the methanol conversion to low molecular weight olefins on chabazite that propylene, methane, and propane are produced by primary reactions and do not participate in any secondary reactions, whereas dimethylether, carbon monoxide, and ethane do. Ethylene and carbon dioxide appear to be produced by secondary reactions. It was also shown that the product selectivities could be correlated to the methanol conversion even though the selectivity and the conversion changed with increasing time on stream due to deactivation by coke formation. 

The formation of hydrogenating sites is therefore favored, resulting in an increase in the selectivity to ethylene. The coke formation has a beneficial effect on the selectivity to ethylene, due to the fact that the hydrogenol54ic sites are quickly deactivated. From this point on, the coke formed on the hydrogenating sites has no significant effect on the activity but leads to a moderate increase in selectivity. 

Deactivation of HDC catalysts generally has been ascribed to either interactions between hydrogen chloride (HCl) and the catalyst [7,8] or to coke formation [1-4,9]. To understand the cause(s) of deactivation for this specific reaction and catalyst, the various products observed in the effluent were hydrodechlorinated at 523 K. Results from this study showed that the Pt/ri-alumina catalyst deactivated rapidly during the HDC of 111 TCA and 11 DCA (saturated chlorocarbons containing multiple Cl atoms) and remained stable for the HDC of 11 DCE (unsaturated chlorocarbon), chloroethane (saturated chlorocarbon containing only one Cl atom), and ethylene (unsaturated hydrocarbon). Large quantities of coke were observed on the Pt/rj-alumina after the HDC of 111 TCA, while very little coke was observed on the used catalyst after the HDC of any of the other compoimds. From these experiments, a conceptual model was developed to explain the causes of deactivation, and the reaction sequences that take place with different reactants and catalysts [3,4]. The deactivation of... 

Structure promoters can act in various ways. In the aromatization of alkanes on Pt catalysts, nonselective dissociative reaction paths that lead to gas and coke formation can be suppressed by alloying with tin. This is attributed to the ensemble effect, which is also responsible for the action of alkali and alkaline earth metal hydroxides on Rh catalysts in the synthesis of methanol from CO/H2 and the hydroformylation of ethylene. It was found that by means of the ensemble effect the promoters block active sites and thus suppress the dissociation of CO. Both reactions require small surface ensembles. As a result, methanol production and insertion of CO into the al-kene are both positively influenced. 

The endotherm observed at temperatures lower than 125 °C represents the loss of adsorbed water associated with low molecular mass volatile gas. The second broad endotherm at 300-465 °C is due to the formation of a metaplastic system. The large exotherm at 465-510 °C is attributed to a primary carbonization stage as a result of the formation of semi-coke. In the pyrolyzed gas evolved in the primary carbonization stage, Hj, CH4 and CjCyC, HjO, etc, are detected on the gas chromatogram. The second carbonization stage at about 510-750 °C is a transient process from semi-coke to coke (C J, ethylene C3, propylene C3, propane). 

Klouz investigated ethanol steam reforming and autothermal reforming over a nickel/copper catalyst on a silica carrier in the temperature range between 300 and 600 °C [197]. While the catalyst suffered from coke formation under steam reforming conditions, the addition of oxygen to the feed reduced both coke formation and carbon monoxide selectivity. By-products such as ethylene and acetaldehyde were not reported by these workers. 

Another route towards coke formation originates from the polymerisation of unsaturated hydrocarbons such as ethylene being present as surface intermediates on the catalyst. These species may then loose more and more hydrogen by formation... 

Aicher et al. [72] developed an autothermal reformer for diesel fuel dedicated to supplying a molten carbonate fuel cell system from Ansaldo Fuel Cells S.p.A., Italy. The diesel fuel (which contained less than 10 ppm sulfur for the pilot plant application) was injected into the steam and air flows, which were pre-heated by a diesel burner to 3 50 °C. The reactor itself was operated at 4 bar, a S/C ratio of 1.5 and high O/C ratio of 0.98, which makes the reactor into a steam supported partial oxidation device. Consequently, the dry hydrogen content of the reformate was rather low with less than 35 vol.%. The operating temperature of the honeycomb had to be kept well above 800 °C to prevent coke formation and the presence of light hydrocarbons such as ethylene and propylene in the reformate. The reactor was operated for 300 h, which led to a slight deterioration in the catalyst performance. 

Methanol to olefins (MTO), which provides a new route to produce light olefins such as ethylene and propylene from abundant natural materials (e.g., coal, natural gas or biomass), has been recently industrialized by the Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences. In this contribution, the process development of MTO is introduced, which emphasizes the importance of mesoscale studies and focuses on three aspects a mesoscale modeling approach for MTO catalyst pellet, coke formation and control in MTO reactor, and scaling up of the microscale-MTO fluidized bed reactor to pilot-scale fluidized bed reactor. The challenges and future directions in MTO process development are also briefed. 

Two hypotheses have been proposed to explain this behavior the first is that the accumulation of coke suppresses the free space in the cavities of zeolites thus limits the formation of methyl benzenes with 5—6 methyl groups, which thereby favors the ethylene formation. The second is that the diffusion of large product molecules from the cavities is hindered by partial blockage of pores and opening windows access to the cavities due to coke formation. Only smaller molecules such as ethylene can freely pass through... 

Ethylene, chlorine and oxygen feeds are supplied from headers and/or supply tanks. Therefore, no design constraint is required to be set for the production rate control. In terms of the relationships between the reactor conditions and the production rate, the pyrolysis has the most influence on the production rate through the reaction conversion by manipulating the reaction temperature. However, this manipulation needs great attention due to the trade-off between the reaction conversion and coke formation and by-product production. 

---