Residue catalysts conradson carbon

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). 

The third type is the additional coke related with the feedstock quality. FCC feedstock contains a dissolved carbon, polynuclear aromatic compounds, called Conradson carbon residue (CCR ASTM D-189). It is deposited over the catalyst surface during cracking reactions. In the FCC unit, this material is part of the coke remaining in the catalyst. Some researchers have investigated cracking of heavy feedstock and observed that, in particular cases, the amount of Conradson carbon is linearly related with the carbon-hydrogen ratio of the feedstock [3]. 

Before TPO analysis it was necessary to Soxhlet extract the FCC samples to remove nonvaporized hydrocarbons and avoid their accumulation in the pores of the catalyst as carbonaceous residue with high hydrogen content. As a result, the interference during TPO analysis caused by the desorption and decomposition of these compounds at high temperatures was eliminated. In this study it was observed that this type of coke is directly related to the Conradson carbon content of the feedstock. 

The test conditions for this Microscale Simulation Test (MST) correspond to the low vapor contact times as applied in today s FCC riser technology. An effective feed preheat and feed dispersion is ensured, while the isothermal reactor bed is set to the dominating kinetic temperature in the riser, being approximately the feed catalyst mix temperature. The MST conditions enable the testing of high Conradson Carbon residue feedstocks. 

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. 

Cracking experiments have been performed using the microriser, an industrial equilibrium catalyst, and a commercially available hydrowax feedstock. This feedstock was selected for its low aromatics content, low Conradson Carbon Residue (CCR), and very low sulfur or metal concentrations. The characteristics of the feedstock are given in table 2. 

The INT-R1 catalyst has been tested with extremely difficult feedstocks regarding their metal, Conradson Carbon and asphaltene content. Pentane and hexane deasphalted oils from extra heavy crude oils, and atmospheric residue of these crudes were among the feedstocks tested [6,7]. 

The INT-RI catalyst can be used in fixed bed hydrotreating units to improve the residue quality, from a high sulfur residual to a low sulfur fuel oil [6]. Tests were performed to demonstrate the technical feasibility of directly processing these residues, using atmospheric residue of Cerro Negro and Iranian Gach Saran crude oils as feedstocks. These residues are characterized by a higher content of asphaltenes and Conradson Carbon than the deasphalted oils [5,6]. Therefore, the effect of these two variables on the performance of the catalyst, can be evaluated. 

The catalyst used in these tests was a commercial GY-15 (containing 15 wt% of rare earth exchanged 2eolite Y) with an initial microactivity (MA) of 66 (according to ASTM 3907 method). The feedstocks were a vacuum gas oil (VGO) and an ARO with a Conradson carbon residue (Ccr) of 4.92. The reactor system and test results are shown in Fig, 2. 

The catalyst used was Topsoe TK-751. It is a general purpose HDS catalyst for residual feedstocks. Their functions are desulphurization, demetallation and asphaltenes and Conradson carbon reduction. It is recommended for HDS of residua with moderate metals content and for 2 stage catalyst in composite fillings. It has good HDS activity, good HDM selectivity and capacity for metals uptake. It is Ni/Mo type. 

When I had instructed the crude unit operators to reduce vacuum tower wash oil by 50%, the entrainment of resid or tar into the gas-oil FCCU feed had substantially increased. As shown in the above tabulation of laboratory data, the entrainment had caused an increase in conradson carbon residue of the vacuum gas oil. This was of no consequence. The extra coke made due to the higher conradson carbon was compensated for by cutting the FCCU feed preheat temperature to hold the regenerator in heat balance. Of greater importance was the increase in nickel content from 0.5 ppm in vacuum gas oil to 2.0 ppm. This fourfold multiplication in nickel content was reflected by a concurrent increase of nickel accumulation of the circulating catalyst. As the nickel content of the catalyst increased. 

Change in Variables Riser Outlet Temp., °F Regenerator Bed Temp., °F Catalyst-to-Oil Ratio, Wt/Wt (TF) Catalyst Microactivity (MAT) Carbon on Regenerated Catalyst, Wt % Hydrocarbon Partial Pressure at Feed Injection Point, psia Feed Conradson Carbon Residue, Wt % Riser Outlet Temp. Regen. Bed Temp. Cat/Oil-Ratio/ TF Activity Carbon on Regen. Catalyst Hydrocarbon Partial Pressure Feed Con Carbon Residue ... 

Consequently, there are significant differences in FCC unit operation when residue is added to normal feed. Conversion falls and less gasoline is produced, as shown in Table 5.2, and the catalyst-to-oil ratio must rise as coke yields increase. The coke also has a different composition relative to that produced from normal feed not only because of the higher Conradson carbon levels and high-boiling compounds, which are absoibed by the catalyst particles, but also from the dehydrogenation activity of the metal impurities, which leads to polymerization reactions and contaminant coke formation. 

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. 

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