Coke forms

The vapor-phase conversion of aniline to DPA over a soHd catalyst has been extensively studied (18,22). In general, the catalyst used is pure aluminum oxide or titanium oxide, prepared under special conditions (18). Promoters, such as copper chromite, nickel chloride, phosphoric acid, and ammonium fluoride, have also been recommended. Reaction temperatures are usually from 400 to 500°C. Coke formed on the catalyst is removed occasionally by burning. In this way, conversions of about 35% and yields of 95% have been reported. Carba2ole is frequently a by-product. 

Further down, ca 75 cm below the electrode tips, the mix is hot enough (2200—2500°C) to allow the lime to melt. The coke does not melt and the hquid lime percolates downward through the relatively fixed bed of coke forming calcium carbide, which is Hquid at this temperature. Both Hquids erode coke particles as they flow downward. The weak carbide first formed is converted to richer material by continued contact and reaction with coke particles. The carbon monoxide gas produced in this area must be released by flowing back up through the charge. The process continues down to the taphole level. Material in this area consists of soHd coke wetted in a pool of Hquid lime and Hquid calcium carbide at the furnace bottom. 

Fig. 3. Weight of coke formed (AQ and coking rate (r) in ethane cracking as a function of time (51).

Coke formed on the catalyst surface is thought to he due to polycondensation of aromatic nuclei. The reaction can also occur through a car-honium ion intermediate of the benzene ring. The polynuclear aromatic structure has a high C/H ratio. 

The object is to indicate the relative coke forming tendency of... 

The Micro-method uses an analytical instrument to measure Conradson carbon in a small automated set. The Micro-method (ASTM D4530) gives test results that are equivalent to the Conradson carbon residue test (D189). The purpose of this test is to provide some indication of relative coke forming tendency of such mat al. 

Coke forms in the reactor and main column circuit because of ... 

Coke Factor is coke-forming characteristics of the equilibrium catalyst relative to coke-forming characteristics of a standard catalyst at the same conversion. 

Before MPW is fed into the process, a basic separation of the non-plastic fraction and size reduction is needed. This prepared feedstock is then introduced in the heated fluidised bed reactor which forms the core of the process. The reactor operates at approximately 500 °C in the absence of air. At this temperature, thermal cracking of the plastics occurs. The resulting hydrocarbons vapourise and leave the bed with the fluidising gas. Solid particles, mainly impurities formed from, e.g., stabilisers in plastics, as well as some coke formed in the process mainly accumulate in the bed. Another fraction is blown out with the hot gas and captured in a cyclone. 

The amount of coke formed as a function of the number of turnovers is shown in Fig. 2. The steeper slopes of these curves for Pt/y-Al203 and Pt/Ti02 indicate the higher selectivity of Pt/y-AI2O3 and Pt/TiOj to form coke than Pt/ZrOj Hydrogen chemisorption capacity decreased nwkedly after some time on stream (see Table 2), but could be completely restored by oxidative treatment. 

When the amount of coke formed as a function of time on stream is compared to the decrease in catalytic activity (see Fig. 3), two regimes of deactivation can be noticed for the strongly deactivating catalysts, i e, a slow initial deactivation which is followed by a rapid loss of activity This first phase is characteristic of a slow transformation of the reactive carbon into less reactive coke. The second phase is attributed to carbon formed on the support which accumulates there and rapidly covers the Pt particles when its amount reaches a critical value causing the sudden decay of catalytic activity. 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

In contrast to the Pt catalysts discussed above, Ni based catalysts (i.e., also when supported on ZrO usually form coke at such a rapid rate that most fixed bed reactors are completely blocked after a few minutes time on stream (see Fig. 8) [16], The coke formed with the Ni catalysts is filamentous. The Ni particle remaining at the tip of the filament hardly deactivates as the coke formed on its surface seems to be transported through the metal particle into the carbon fibre, but the drastic increase in volume causes reactor plugging and prevents use of the still active catalyst (see Fig. 8). The TEM photographs indicate that the carbon filaments have similar diameters to those of the Ni particles. 

For these types of reactor solids, the carbonaceous solids content varies usually from about 20 to 40%. The components of these solids are listed in Table VII. Optical examination of the solids has shown that they are primarily composed of mixtures of semi-cokes formed during liquefaction by retrogressive reactions with chars derived from macerals. Unreacted macerals comprise only a small fraction of these solids (65,74,75). 

Petroleum coke is the residue left by the destructive distillation (thermal cracking or coking) of petroleum residua. The coke formed in catalytic cracking operations is usually nonrecoverable because of adherence to the catalyst, as it is often employed as fuel for the process. The composition of coke varies with the source of the crude oil, but in general, is insoluble on organic solvents and has a honeycomb-type appearance. 

The feedstock to the TCC reactor varies in its coke-forming properties. This is an important source of disturbance in the kiln operation. Figure 23 shows the effect of a 20% change in coke. The temperature above the air inlet responds with a damped oscillation of the temperature before reaching a steady state 40°F (22°C) above the previous steady state. The temperature at the bottom of the lower zone rises monotonically 75°F (42°C) to the new steady-state value. This rise in catalyst temperature will influence the performance of the reactor, since the catalyst is returned to the top of the reactor after some heat losses from cooling coils in the bottom of the kiln and in the air lift and the separator. The change of temperature... 

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. 

Ash yields of the liquid products were determined by evaporating a sample (25 g) in a platinum crucible until coke formed. The ashing was completed in a muffle furnace at 850 C. 

It can also identify texture of the semi coke formed as illustrated in Figure 6. If a binder is used with a coal, the Plastofrost technique can determine the coal-binder interaction and the texture of coke formed from the binder phase. Although not considered in studies undertaken at Waterloo, the axial location of the thermocouples in the sample holder makes the Plastofrost procedure capable of measuring coal-coke conductivity as a function of coal, temperature and compaction pressure, with just a modest redesign of the heating slab. 

Figure 5 Amount of coke formed on the catalyst in function of...

In addition, hydrotreating converts polynuclear aromatic (PNA) compounds and other potential coke-forming material to more easily cracked naphthenes and paraffins. Like the removal of nitrogen compounds, the saturation of PNAs also requires more severe... 

Because of the high coke-forming tendencies of reduced crude, mainly due to a high concentration of Ramsbottom Carbon, the adverse effects of nickel appear to be somewhat moderated in the trip up the riser. Therefore, nickel does not appear to be quite as harmful as first anticipated. 

Another coke formed in a FCC unit is occluded or residual coke. In a commercial unit this coke corresponds to coke formed on catalyst porosity and its content depends on textural properties of the catalyst (pore volume and pore size distribution) and the stripping system capacity in the reaction section. Finally on the FCC catalyst rests some high-molecular weight of nonvaporized hydrocarbons. These molecules do not vaporize or react at the reactor conditions and accumulate in the catalyst pores like a soft carbonaceous residue with high hydrogen content. 

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