Metal-contaminated feedstocks processing

Processing of metals-contaminated feedstocks has been practiced since 1961 by Phillips at Borger, Texas. By 1981, 24 other units were using heavy oils in their FCC operations (29-30). At the present, it is believed that by 1989, 15-20 resid units will be operational and that by 1995, 25-30% of the world s FCC units will be partially operating as resid crackers (2). Refiners without the capability of converting resids and less costly oils into transportation liquids will probably suffer in a most competitive energy market. 

In many of the other processes that use base metal catalysts, irreversible poisoning of the catalyst occurs as a result of deposition of metal contaminants from the process feedstock onto the catalyst surface. These catalysts are not considered to be regenerable by ordinary techniques. 

In 1973, the Arab oil embargo and the sudden escalation in crude oil prices (Figures 1,2) and availability placed refiners under great economic pressures to process more abundant, less expensive, metals-contaminated crude oils and residuum feedstocks. The need for metals-resistant FCC became apparent, and work on understanding metal-catalyst interactions became an area of intense research in industrial laboratories and in the academic community. 

Removal of the metal contaminants is not usually economical, or efficient, during rapid regeneration. In fact, the deposited metals are believed to form sulfates during removal of carbon and sulfur compounds by combustion that produce a permanent poisoning effect. Thus, if fixed-bed reactors are to be used for residuum or heavy oil hydrodesulfurization (in place of the more usual distillate hydro-desulfurization) it may be necessary to first process the heavier feedstocks to remove the metals (especially vanadium and nickel) and so decrease the extent of catalyst bed plugging. Precautions should also be taken to ensure that plugging of the bed does not lead to the formation of channels within the catalyst bed which will also reduce the efficiency of the process and may even lead to pressure variances within the reactor because of the distorted flow patterns with eventual damage. 

In summary, the hydrodesulfurization of the low-, middle-, and highboiling distillates can be achieved quite conveniently using a variety of processes. One major advantage of this type of feedstock is that the catalyst does not become poisoned by metal contaminants in the feedstock since only negligible amounts of these contaminants will be present. Thus, the catalyst may be regenerated several times and onstream times between catalyst regeneration (while varying with the process conditions and application) may be of the order of 3 to 4 years (Table 6-6). 

These processes are designed to remove sulfur, nitrogen, asphaltene, and metal contaminants from residua and are also capable of accepting whole crude oils or topped crude oils as feedstocks (Brossard, 1997 Hydrocarbon Processing, 1998). The major product of the processes is a low-sulfur fuel oil and the amount of gasoline and middle distillates is maintained at a minimum to conserve hydrogen. 

It is well known that the processing of heavier, more contaminated feedstocks (metals, asphaltenes) tends to increase the production of coke and gas and deactivates the catalyst. This is mainly the results of (8) ... 

The quality of feedstocks processed in FCC units continues to decline. Therefore more units will reach operating constraints due to larger amounts of coke precursors and metal contaminants. While special process and unit design modifications are developed to handle the processing of these heavier feedstocks highly metal tolerant catalysts are required. 

The fluid catalytic cracking unit (FCCU) is used for vacuum distillates and residues into olefinic gases. The great demand in processing heavy feedstocks and the high amounts of metals in Brazilian oils, forced to develop novel catalysts that are more resistant to metal contamination. 

These poisons are removed during regenerative burning but may cause problems by producing NOx in combustion effluents. It is better to remove them before the process. Hydrotreating removes nitrogen, and this is standard procedure with all catalytic cracking feedstocks. This treatment also removes sulfur and much of the heavy metal contamination. 

The AER process shows promise in reducing metal contamination in the feedstock. Significant enrichment of lead and zinc in the baghouse filter suggests that the AER process may also be useful in the recovery of inorganics in wastes that can volatilize at AER operating conditions. 

Metal contamination has detrimental effects on FCC catalyst performance and should be closely monitored. Sodium, which is present in fresh FCC catalyst, can also be present in the feedstock. Other metals, which include nickel, vanadium, iron, and copper, originate mostly from the heavy ends of the hydrocarbon feedstock. As catalyst comes in contact with feed in the unit, the metal components deposit on the catalyst and stay there. Sodium is known to neutralize the acidic sites and causes the zeolite structure to collapse. Nickel, vanadium, iron, and copper are promoters for dehydrogenation reactions. Vanadium is also known to destroy zeolite activity. FCC units, which process heavy feedstock, typically have higher metal contamination on the equilibrium catalyst. 

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