Residue catalysts feed coke

Feed residue coke is the small portion of the (non-residue) feed that is directly deposited on the catalyst. This coke comes from the very heavy fraction of the feed and its yield is predicted by the Conradson or Ramsbottom carbon tests. 

The surface area of the catalyst as well as the pore size distribution can easily be measured, and the zeolite and matrix surface areas of the catalyst can be determined by the t-plot method. The different FCC yields can then be plotted as a function of the ZSA/MSA ratio, zeolite surface area or matrix surface area, and valuable information can be obtained [9], The original recommendation was that a residue catalyst should have a large active matrix surface area and a moderate zeolite surface area [10,11]. This recommendation should be compared with the corresponding recommendation for a VGO catalyst a VGO catalyst should have a low-matrix surface area in order to improve the coke selectivity and allow efficient stripping of the carbons from the catalyst [12], Besides precracking the large molecules in the feed, the matrix also must maintain the metal resistance of the catalyst. 

In the past, ORC experienced plugging from exactly this type of phenomenon. When processing residue-containing feedstocks, coke would build up just above the feed injection nozzle causing the flow to the riser reactor to become restricted. As a result, runs would have to be ended prematurely. The coke build-up tended to be the worst at low catalyst-to-oil ratios when catalyst flow rates were also low. 

Feed properties and operation conditions determine catalyst life in the residue hydrodesulfurization. In a high conversion operation of vacuum residue, catalyst deactivation due to coke is as important as the one due to metals. Though many researchers have worked on understanding and modelling deactivation of residue hydrodesulfurization catalysts, there has still been a controversy in a coke deactivation mechanism [2, 3]. Very few publications are available discussing an effect of a bed temperature profile on catalyst deactivation in large scale adiabatic commercial reactors. Most of the studies on deactivation of residue hydrodesulfiirization catalysts have been done with small-scale isothermal reactors [2,3,4,5]. The activity tests of the used catalysts were conducted to study the catalyst deactivation in the commercial reactors. This paper also describes an effect of a bed temperature profile on coke deactivation, which was tested in the commercial reactors. 

About 1.2-1.4% Acoke forms on residue catalysts compared with about 0.7- 0.8% on gasoline catalysts. The distribution of Acoke for both types of feed is shown in Table 5.12. Most of the increase is associated with contaminant and feed coke. 

The FCC process is used worldwide in more than 300 installations, of which about 175 are in North America and 70 in Europe. Figure 9.10 shows the principle of an FCC unit. The preheated heavy feed (flash distillate and residue) is injected at the bottom of the riser reactor and mixed with the catalyst, which comes from the regeneration section. Table 9.5 gives a typical product distribution for the FCC process. Cracking occurs in the entrained-flow riser reactor, where hydrocarbons and catalyst have a typical residence time of a few seconds only. This, however, is long enough for the catalyst to become entirely covered by coke. While the products leave the reactor at the top, the catalyst flows into the regeneration section, where the coke is burned off in air at 1000 K. 

The zeolite to matrix surface area ratio can be used for optimization of catalysts for catalytic cracking of atmospheric residues. For North Sea long residues this ratio should be as large as possible, but the ratio should not exceed an upper limit. For the main catalyst type (A) used in this investigation the upper limit of the ZSA/ MSA ratio was around 3.5. There is also a lower limit for the matrix surface area. If the matrix surface area is lower than this limit, the catalyst will not be able to crack all the heavy components in the residue feed, and the coke on the matrix will increase dramatically. This will prevent the catalyst from working properly. Different type of catalysts must be optimized individually, as well as different type of long residues. 

These deposits responsible for fouling can block out the reactants and prevent them from reaching the active sites, or even block the internal pores of the catalyst. Hydrocarbons and aromatics are usually the cause of coking. The chemical nature of the carbonaceous deposits relies on many parameters temperature, pressure, feed composition, nature of products, and catalyst age share the responsibility of the residue formation on catalysts. 

Pilot plants are often used for studying the FCC process. In order for results to be meaningful, it is necessary that pilot plant operation be consistent with that of commercial units. When processing feedstocks containing residue, this becomes even more important. Failure to pay attention to details, such as feed/catalyst contacting, can lead to problems with data integrity and with coke buildup in the equipment. 

Carbon and metal sulfide deposits are the two main causes of deactivation of residue hydrodemetallization (HDM) catalysts. During a catalytic test, the metals contained in the feed (Ni, V) are slowly deposited on the catalyst surface leading to the build up of large particles of metal sulfides which ultimately plug the catalyst pores. Carbon, on the other hand, is known to accumulate quickly on the catalyst surface within the first days of a run until a steady state is reached (1-20). At the beginning of a run, a strong deactivation of the residue HDM catalyst rapidly occurs to which both types of deposits may contribute. However at the present time it is not clear whether this initial deactivation is mainly due to coke or metal sulfide deposits. 

In fluid coking (Figure 2.2), the residium feed is injected into the reactor, where it is cracked thermally in a fluidized bed catalyst. Products other than coke leave the top of the reactor and are quenched in a scrubber, where residual coke is removed. The coke fines and some of the heavy fractions are recycled to the reactor. The lighter fractions are fed to conventional fractionating equipment. 

While originally designed for cracking the overhead stream from vacuum distillation units, known as vacuum gas oil (4), most FCC units currently operate with some higher boiling vacuum distillation bottoms (Resid) in the feed. Table 5.1 illustrates the difficult challenges faced by refiners, process licensors and FCC catalysts producers the resid feeds are heavier (lower API gravity), contain many more metals like Ni and V as well as more polyaromatic hydrocarbons prone to form coke on the catalysts (Conradson Carbon Residue, or CCR). 

Excessive heat generation in the regenerator is a particular problem when using residual feed when coke formation is higher. Residual fuel FCC operations generally have additional heat removal mechanisms in the regenerator. This can be steam raising coils or external catalyst coolers. 

In resid cracking the high feed metals and Conradson Carbon Residue (CCR) require careful consideration when assessing both catalyst design and performance evaluation. This paper addresses the issues of the latter with respect to coke, delta coke and catalyst deactivation. 

Waste plastics potentially can also be processed in hydrocracking process as an additional feed stream in mixture with vacuum gas oil or crude oil residues. Careful plastic segregation is then necessary since inorganic additives and impurities of plastics can foul the hydrocracking catalyst. Noncatalytic high-temperature olefin pyrolysis (700-800°C) and coking are insensitive to fouling. 

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