Coke yield

The MTO process employs a turbulent fluid-bed reactor system and typical conversions exceed 99.9%. The coked catalyst is continuously withdrawn from the reactor and burned in a regenerator. Coke yield and catalyst circulation are an order of magnitude lower than in fluid catalytic cracking (FCC). The MTO process was first scaled up in a 0.64 m /d (4 bbl/d) pilot plant and a successfiil 15.9 m /d (100 bbl/d) demonstration plant was operated in Germany with U.S. and German government support. 

In the CIS pitch coke is made by carbonizing a hard coke-oven pitch in modified coke ovens. The hard pitch has an R-and-B softening point of 140—150°C and is made by air-blowing a mixture of medium-soft pitch and recycled coking oils. This feedstock is charged in the molten state over a period of 5 h and coked for 17—18 h at 1250—1300°C. The coke yield is 70%. Oils, which are recycled, amount to 20% by weight of the pitch fed. The gas yield... 

If FCCU operations are not changed to accommodate changes ia feed or catalyst quaUty, then the amount of heat required to satisfy the heat balance essentially does not change. Thus the amount of coke burned ia the regenerator expressed as a percent of feed does not change. The consistency of the coke yield, arising from its dependence on the FCCU heat balance, has been classified as the second law of catalytic cracking (7). 

Coke on the catalyst is often referred to as delta coke (AC), the coke content of the spent catalyst minus the coke content of the regenerated catalyst. Delta coke directly influences the regenerator temperature and controls the catalyst circulation rate in the FCCU, thereby controlling the ratio of catalyst hydrocarbon feed (cat-to-od ratio, or C/O). The coke yield as a fraction of feed Cpis related to delta coke through the C/O ratio as ... 

Equation 6 relates the catalytic coke yield (as a fraction of the feed) to the delta coke, to the conversion, and to the catalyst residence time. 

It has been shown that coke yield as a fraction of feed does give a linear relationship with second-order conversion (13) indicating a positive coke yield at 2ero conversion. This coke yield at 2ero conversion is the additive coke contribution to the total coke yield and is related to feed properties, particularly Conradson carbon content. The amount of this additive coke is significantly less than the Conradson carbon value of the feed (14), probably in the range of 50% of the Conradson carbon. 

Additive inhibitors have been developed to reduce the contaminant coke produced through nickel-cataly2ed reactions. These inhibitors are injected into the feed stream going to the catalytic cracker. The additive forms a nickel complex that deposits the nickel on the catalyst in a less catalyticaHy active state. The first such additive was an antimony compound developed and first used in 1976 by Phillips Petroleum. The use of the antimony additive reportedly reduced coke yields by 15% in a commercial trial (17). 

Thus decreasing the specific heat of combustion results in an increase in catalyst circulation rate. Because of this relationship to coke yield (eq. 9), the increase in the catalyst circulation rate results in a decrease in regenerator temperature. 

All subsequent green coke operations were made in a second coker, which was fashioned from steel pipe approximately 18 cm in diameter and 25 cm in length. A metal plate was welded to one end and a metal collar was welded to the other end such that a steel lid could be bolted to the system. Typically, about 250 to 500 g of pitch were sealed imder nitrogen in the coker reactor and the system placed in a large temperature-programmable furnace. The heat treatment process was as follows. The temperature was raised 5°C/min to 350 °C and then l°C/min to 425°C and the temperature held at 425°C for 90 minutes. Finally the temperature was raised further at 3°C/min to between 500 and 600°C, and held there for 3 hours. The coker was cooled to room temperature and the material recovered to determine green coke yield. 

The green cokes were calcined by placing a weighed amount of green coke into an alumina tube. The tube was fitted with end caps to allow for a constant purge of nitrogen. The alumina tube was then inserted into a high-temperature furnace and the temperature raised to about 1000°C for a period between 30 and 60 minutes. The furnace was turned off, cooled to room temperature, and the product recovered to determine the calcined coke yield. 

Table 17. Effect of hydrogenation on green coke yields...

Table 18. Effect of blending hydrogenated coal-dcrived pitch and coal extract on green coke yields, WVGS 13421...

Blending ratio Green coke yield, wt% TGA yield, u[Pg.225]

A good catalyst is also stable. It must not deactivate at the high temperature levels (1300 to 1400°F) experienced in regenerators. It must also be resistant to contamination. While all catalysts are subject to contamination by certain metals, such as nickel, vanadium, and iron in extremely minute amounts, some are affected much more than others. While metal contaminants deactivate the catalyst slightly, this is not serious. The really important effect of the metals is that they destroy a catalyst s selectivity. The hydrogen and coke yields go up very rapidly, and the gasoline yield goes down. While Zeolite catalysts are not as sensitive to metals as 3A catalysts, they are more sensitive to the carbon level on the catalyst than 3A. Since all commercial catalysts are contaminated to some extent, it has been necessary to set up a measure that will reflect just how badly they are contaminated. 

Narrow range of coke yields unless some heat removal system is incorporated... 

In most units, the increase in hydrogen make does not increase coke yield the coke yield in a cat cracker is constant (Chapter 5). The coke yield does not go up because other unit constraints, such as the regenerator temperature and/or wet gas compressor, force the operator to reduce charge or severity. High hydrogen yield also affects the recovery of Cj-H components in the gas plant. Hydrogen works as an inert and changes the liquid-vapor ratio in the absorbers. 

For example, a catalyst with a MAT number of 70 vol% and a 3.0 wt% coke yield will have a dynamic activity of 0.78. However, another catalyst with a MAT conversion of 68 vol% and 2.5 wt% coke yield will have a dynamic activity of 0.85. This could indicate that in a commercial unit the 68 MAT catalyst could outperform the 70 MAT catalyst, due to its higher dynamic activity. Some catalyst vendors ha% c begun reporting dynamic activity data as part of their E-cat inspection reports. The reported dynamic activity data can vary significantly from one test to another, mainly due to the differences in feedstock quality between MAT and actual commercial application. In addition, the coke yield, as calculated by the MAT procedure, is not very accurate and small changes in this calculation can affect the dynamic activity appreciably. 

For a given catalyst and feedstock, catalytic coke yield is a direct function of conversion. However, an optimum riser temperature will minimize coke yield. For a typical cat cracker, this temperature is... 

It is apparent that the type and magnitude of these reactions have an impact on the heat balance of the unit. For example, a catalyst with less hydrogen transfer characteristics will cause the net heat of reaction to be more endothennic. Consequently this will require a higher catalyst circulation and, possibly, a higher coke yield to maintain the heat balance,... 

The material balance around the riser requires the reactor effluent composition. Two techniques are used to obtain this composition. Both techniques require that the coke yield be calculated. 

The coke yield should be calculated using air rate and flue ga.s composition. 

As discussed in Chapter 1, a portion of the feed is converted to coke in the reactor. This coke is carried into the regenerator with the spent catalyst. The combustion of the coke produces H2O, CO, CO, SO2, and traces of NOx. To determine coke yield, the amount of dry air to the regenerator and the analysis of flue gas are needed. It is essential to have an accurate analysis of the flue gas. The hydrogen content of coke relates to the amount of hydrocarbon vapors carried over with the spent catalyst into the regenerator, and is an indication of the rcactor-stripper performance. Example 5-1 shows a step-by-step cal culation of the coke yield. 

The coke yield of a given cat cracker is essentially constant. The FCC produces enough coke to satisfy the heat balance. However, a more important term is delta coke. Delta coke is the difference between the coke on the spent catalyst and the coke on the regenerated catalyst. At a given reactor temperature and constant CO2/CO ratio, delta coke controls the regenerator temperature. 

Reducing dry gas and coke yields, therefore, increasing total liquid products... 

Higher delta coke and coke yield, which are associated with residue feedstocks, will result in elevated regenerator temperature and higher combustion air requirements. 

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