Delta coke

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. 

Using the operating data from the case study. Example 5-5 shows heat balance calculations around the stripper-regenerator. The results are used to determine the catalyst circulation rate and the delta coke. Delta coke is the difference between coke on the spent catalyst and coke on the regenerated catalyst. 

The heat balance exercise provides a tool for in-depth analysis of the unit operation. Heat balance surveys determine catalyst circulation rate, delta coke, and heat of reaction. The procedures described in this chapter can be easily programmed into a spreadsheet program to calculate the balances on a routine basis. 

Reducing delta coke will lower the regenerator temperature. Many benefits are associated with a lower regenerator temperature. The resulting higher cat/oil ratio improves product selectivity and/or provides the flexibility to process heavier feeds. 

Many factors influence delta coke, including quality of the FCC feedstock, design of the feed/catalyst injection system, riser design, operating conditions, and catalyst type. The following is a brief discussion of these factors ... 

Feedstock quality. The quality of the FCC feedstock impacts the concentration of coke on the catalyst entering the regenerator. A heavier feed containing a higher concentration of coker gas oil will directionally increase the delta coke as compared with a lighter, resid-free feedstock. 

Feed/catalyst injection. A well-designed injection system provides a rapid and uniform vaporization of the liquid feed. This will lower delta coke by minimizing non-catalytic coke deposition as well as reducing the deposits of heavy material on the catalyst. 

Riser design. A properly designed riser will help reduce delta coke by reducing the back-mixing of already coked-up catalyst with fresh feed. The back-mixing causes unwanted secondary reactions. 

Cat/oil ratio. An increase in the cat/oil ratio reduces delta coke by spreading out some coke-producing feed components over more catalyst particles and, thus, lowering the concentration of coke on each particle. 

Reactor temperature. An increase in the reactor temperature will also reduce delta coke by favoring cracking reactions over hydrogen transfer reactions. Hydrogen transfer reactions produce more coke than cracking reactions. 

Reactor stripper. Operational and hardware changes to the stripper improve its performance by removing the entrained and adsorbed hydrocarbons. The benefits are lower delta coke and more 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. 

An active matrix and a low hydrogen transfer activity to convert the bottoms and minimize delta coke... 

Delta Coke is the difference between the coke content of the spent catalyst and the coke content of the regenerated catalyst. Numerical value of delta coke is calculated from ... 

Delta coke = coke yield (wt%/catalyst-to-oil ratio)... 

Delivery receipt, 25 330 Delocalized cationic azo dyes, 9 423 Delta-3-carene (A-3-carene), 24 499 8-alumina, 2 404, 406t Delta coke, 22 704 8-damascone, 24 570... 

The benefits of an improved feed injection system and riser can be lower delta coke, lower regenerator temperature, higher conversion, higher gasoline yield, and increased liquid yield. More feed can be processed and an air blower or wet gas compressor limitation can be relieved. 

An improved riser terminator can lower delta coke, reduce dry gas up to 40%, lower regenerator temperature and allow more feed or higher conversion. Adding post-riser vapor quench can independently reduce dry gas, increase gasoline and reduce diolefins in the alkylation stream and the FCC gasoline. 

Post-riser quench can be used if a reactor vessel has a metallurgical limit and a higher riser outlet temperature is desired. Higher octanes and more alkylation feed may be the result. Improved vaporization of the feed could lower delta coke. 

Add packing to top of stripper to insert oil to increase delta coke when processing very low delta coke feeds... 

Dispersion steam should be adjusted to see if further improvement in the operation is possible. More steam may rednce dry gas and delta coke. If resid is added to the feed or the amount of resid in the feed is increased, the feed dispersion steam is also increased to improve atomization and vaporization. 

Jawad, Z. S. FCC Fundamentals Coke Yield/Delta Coke. PTQ, Q2, 2007. 

Catalyst mass flowrates exceeding about 1600 Ib/ft -min (7800kg/m -min) results in poor steam/catalyst contacting, flooded trays, insufficient catalyst residence time, and increased steam entrainment to the spent catalyst standpipe. This is reflected by the stripper efficiency and catalyst density shown in Figure 7.10. The primary concern is hydrocarbon entrainment to the regenerator leading to loss of product, increased catalyst deactivation, increased delta coke and potential loss of conversion and total liquid yield, and feed rate limitation. A rapid decrease in stripper bed density is an indication that... 

Maximize feed preheat and increase delta coke... 

M. Denmar, A. Triki, J.P. Franck, "Advanced Analyses Improve Delta Coke Prediction for Resids", OGJ, Sept. 15, 1986. 

Developments in the zeolite field in terms of non-framework alumina control, leading to super low delta coke zeolites and the advent of enhanced surface activity zeolites, seem promising for the resid cracking field (3). 

High nitrogen resistant catalysts and/or nitrogen traps are available nowadays unfortunately, no real breakthroughs have been made in reducing the effect of polycyclic aromatics (Conradson Carbon Catchers ). At present super low delta coke catalysts are produced to allow for the additional coke produced by these polycyclic aromatics. 

Catalyst priority 1. Super low delta coke 1. Metal resistance... 

For a commercial unit this implies that we need to enhance the meso pore activity while avoiding a drop in CTO due to (for instance) a higher delta coke of the catalyst. 

It is clear that the zeolite delta coke will have a strong effect on the regenerator temperature and hence on the catalyst deactivation. Depending on the trend in FCC regenerator temperatures, the aspect of hydrothermal stability might become of greater importance. 

It has been observed that, the indirect effect of delta coke on catalyst deactivation, and the direct effect of delta coke on the blocking of acid sites (early) in the riser seems to be a prime factor, which dominates conversion and selectivity effects in a resid type of operation. 

A poor coke selectivity resulting in a high delta coke (or low cat-to-oil ratio) will aggravate the poisoning effect of the fraction of the Conradson Carbon Residue, which is converted to coke. 

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. 

In fixed-bed catalytic cracking tests the proper decreasing delta coke response as catalyst-to-oil is increased is possible if a constant catalyst load and a constant feed injection rate are maintained. As CCR increases above 4 wt%, however, fixed-bed cracking methods are suspect because the mass balance drops significantly and the cracking performance can be measured better using other techniques (e.g.s., circulating pilot plants or fluidized-bed reactors). 

Contrary to commercial performance, this results in an increase in delta coke versus cat/oil ratio. Catalytic coke yield (% on feed) is expected to be proportional with cat/oil ratio. Consequently that delta coke should remain constant or drop ... 

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