Fluid coke, formation

Deoxygenation reactions are catalyzed by acids and the most studied are solid acids such as zeolites and days. Atutxa et al. [61] used a conical spouted bed reactor containing HZSM-5 and Lapas et al. [62] used ZSM-5 and USY zeolites in a circulating fluid bed to study catalytic pyrolysis (400-500 °C). They both observed excessive coke formation on the catalyst, and, compared with non-catalytic pyrolysis, a substantial increase in gaseous products (mainly C02 and CO) and water and a corresponding decrease in the organic liquid and char yield. The obtained liquid product was less corrosive and more stable than pyrolysis oil. 

The SEM images indicate that the structure of the larger coke beans is consistent with a formation mechanism involving the agglomeration of 150 pm-fluid coke particles. The growth or agglomeration events, as indicated by the particle diameter, appear to be terminated... 

The technique of extracting virtually nonvolatile substances is particularly useful for materials that decompose before reaching boiling point and is therefore well suited to the extraction of the liquids formed when coal is heated to about 400°C (750°F). Thus, supercritical gas or fluid extraction affords a means of recovering the liquid products when they are first formed, avoiding undesirable secondary reactions (such as coke formation), and yields of extract up to 25 or 30% have been recorded. 

Cracking is carried out in a fluid bed process as shown in Fig. 7.9. Catalyst particles are mixed with feed and fluidized with steam up-flow in a riser reactor where the reactions occur at around 500°C. The active life of the catalyst is only a few seconds because of deactivation caused by coke formation. The deactivated catalyst particles are separated from the product in a cyclone separator and injected into a separate reactor where they are regenerated with a limited amount of injected air. The regenerated catalyst is mixed with the incoming feed which is preheated by the heat of combustion of the coke. 

The catalyst activity is determined by the coke extraction rate relative to the coke formation rate. The coke extraction rate is determined by the solubility of the coke compounds in the reaction mixture and the diffusivity of the extracted compounds through the porous catalyst. The coke formation on the catalyst occurs from the hexenes, and more significantly, from hexene oligomers formed in the fluid phase catalyzed by traces of peroxide impurities [3]. A detailed mathematical model is presented here to interpret the results presented in our earlier paper [2] and to develop a better understanding of the underlying physicochemical processes. 

The coke formation leads to catalyst fouling. This is solved in the UOP Process by continuously removing a portion of the catalyst and passing this to a separate regenerator. After regeneration by combustion of the coke in air, the catalyst is sent back to the main reactor. In concept this is similar to fluid-cat cracking of refinery stocks. The process layout is illustrated in the Figure 11.7. 

Fluid catalytic cracking (FCC) (Fig. 13.5) was first introduced in 1942 and uses a fluidized bed of catalyst with continuous feedstock flow. The catalyst is usually a synthetic alumina or zeolite used as a catalyst. Compared to thermal cracking, the catalytic cracking process (1) uses a lower temperature, (2) uses a lower pressure, (3) is more flexible, (4) and the reaction mechanism is controlled by the catalysts. Feedstocks for catalytic cracking include straight-run gas oil, vacuum gas oil, atmospheric residuum, deasphalted oil, and vacuum residuum. Coke inevitably builds up on the catalyst over time and the issue can be circumvented by continuous replacement of the catalyst or the feedstock pretreated before it is used by deasphalting (removes coke precursors), demetallation (removes nickel and vanadium and prevents catalyst deactivation), or by feedstock hydrotreating (that also prevents excessive coke formation). 

Using the resin, asphalt (R+AT) and aromatics (AR) separated from an atmospheric rcsid oil (ARO) as fc stocks, we have investigated the effects of catalytic coke additive coke (Cgdd) on the cracking activity of a commercial FCC catalyst in a fixed bed (FB) and a rixed fluid bed (FFB) pilot units. Correlations between catalyst activity (a) and coke on catalyst (Q.) have been developed. A catalyst deactivation model, which is useful in modeling of cracking reaction kinetics, has been derived through rate equations of coke formation. 

The catalysts used in Fluid Catalytic Cracking (FCC) are reversibly deactivated by the deposition of coke. Results obtained in a laboratory scale entrained flow reactor with a hydrowax feedstock show that coke formation mainly takes place within a time frame of milliseconds. In the same time interval conversions of 30-50% are found. After this initial coke formation, only at higher catalyst-to-oil ratios some additional coke formation was observed. In order to model the whole process properly, the coke deposition and catalyst deactivation have to be divided in an initial process (typically within 0.15 s) and a process at a larger time scale. When the initial effects were excluded from the modeling, the measured data could be described satisfactory with a constant catalytic activity. 

Fluid Catalytic Cracking (FCC) is one of the most important process in oil refining. The evaluation of the catalysts in the laboratory scale is often carried out in a micro-reactor, the so called micro-activity test [1-3] (MAT). Coke formation plays an important role in the deactivation of FCC catalysts, which can be deactivated either permanently (loss of surface area, zeolite collapse, metals) or temporarily deactivated (coke). 

Minicucci, D. Zou, X.-Y. Shaw, J.M. The impact of liquid-liquid-vapour phase behaviour on coke formation from model coke precursors. Fluid Phase Equilibria 2002, 194-197, 353-360. 

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). 

Yen eta/ [1988] Fluid catalytic cracking Coke formation is kinetics control with fourth order reaction None considered (1) Complex - many parameters (2) Tested against actual coke data from both pilot and commercial fluid catalytic cracking units... 

More successful was the extension of fluidized-bed technology to fluid coking, a process to upgrade heavy petroleum residues. Fluid coking presented many chemical engineering problems. Coke is laid down in layers on existing coke particles which thus tend to grow. Therefore, it is necessary to provide small seed particles. There is no natural seed formation as in crystallizers, so seed particles must be... 

Den Hollander, M. A. Makkee, M. Moulijn, J. A. Coke Formation in Fluid Catalytic Cracking Studied With the Microriser. Catal. Today 1998 46, 27. 

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