One major operation in the modem refinery is the process of catalytic cracking. In this process, some of the heavier oils (often called"gas oils") produced upon fractionation of whole crude oil are decomposed or "cracked"using fluidized zeolite-containing catalysts.

As the supply of light, sweet crude oils has dwindled during past years, catalytic cracking has become increasingly important in maintaining a supply of hydrocarbons suitable for use in various fuels such as gasoline. A problem that has occurred because of the increasing use of heavier, more sour crudes is that the heavier crudes contain substantially more organic metal compounds, such as vanadium and nickel porphyrins. These metals cause many undesirable reactions in heavy oil cracking in that the metals, specifically nickel and vandium, are quite harmful to the fluidized cracking catalysts used. These metals, present in the high-boiling fractions, deposit on cracking catalyst and accumulate with time. They act as poisons and have the resulting effect of increasing undesirable hydrogen and coke yields, decreasing the selectivity of the catalyst in making liquid products. It is also established that vanadium also attacks the zeolite itself, the high activity component of a catalytic cracking catalyst.

While the mitigation of Ni has been reasonably successful by using group V element additives, the solution for vanadium has remained elusive. The vast majority of prior art has dealt with the additive approach, the addition of so- called"trapping agent"into FCC system either with the catalyst or with hydrocarbon feed. This passivation procedure may be seen in, e. g., U. S. Patents 4,377, 494 (barium); 4,451, 355 (calcium); 4,473, 463 (barium); and 4,824, 815 (strontium). In addition to its obvious drawback of diluting catalyst activity, this approach of using additive trapping particles has suffered from lackluster V trapping performance. The reason is that the multitude of reactive conditions in FCC system interferes with the V trapping chemistry of most V traps. For example, it has been recognized that the presence of SOx in regenerator interferes with V-trapping chemistry. U. S. 5,324, 416 teaches that V-trapping is more effective with the simultaneous use of SOx transfer agents. This invention has a number of advantages over prior art: (1) no catalyst activity dilution ; (2) it isolates V trapping from SOx and reducing environments, so that some effective V trap materials (Those that are also susceptible to SOx) such as the divalent metal oxides can be used; (3) V trapping is done under conditions that can be tuned to optimize V transfer rate while keeping hydrothermal deactivation of zeolite catalyst in the vessel at minimum; (4) because the vessel may be simultaneously used as a cat cooler, it may allow handling of more con-carbon in hydrocarbon feed; (5) depending on the concentration of V gathered by the trap, the V laden trap might be of value as source of vanadium metal.

Other V mitigation strategies include the use of V-trapping agent in isolated environment. U. S. 5,286, 691 (Mobil) teaches the use of large V-trap particles which remain in the FCC regenerator (and is exposed to SOx environment), and U. S. 5,260, 240 (Chevron) teaches the use of V-traps in a external reactor. The V-ladden FCC catalyst is charged into the reactor and the operation is non-continuous.

SUMMARY OF THE INVENTION The present invention is a fluidized catalytic cracking system which includes a fluidized catalytic cracking reactor, a conventional regenerator, and an outboard vessel containing a vanadium trapping material. Vanadium contaminated catalyst is circulated from regenerator to outboard vessel where the catalyst is fluidized with the V trap and vanadium is removed from the catalyst.

Optionally the vessel may have features of a cat cooler to help lower the regenerator temperature.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic diagram of the present invention.

Figure 2 shows a schematic diagram of a sub-assembly of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT This invention teaches the concept and process scheme of a means to remove contaminant vanadium in FCC system. It is specifically concerned with the use of an outboard vessel that is adjoined to the FCC regenerator. Such a vessel sets up a circulation of regenerated FCC catalyst from and back to the regenerator. It contains millimeter size, properly formulated inorganic oxide particles (V trap) that are capable of trapping vanadium (forming stable metal vanadate). Under specific operating conditions these particles are fluidized with the FCC cat particles but, because of their much larger size, do not circulate with them. For conditions required for this segregated fluidization of particles of different sizes, see"Fluidization Engineering"by Kunii and Levenspiel, 1977, Robert E. Krieger Publishing, Huntington, N. Y., or"Fluidization and Fluid Particle Systems"by Zenz and Othmer, 1960, Reinhold Publishing, New York.

During contact in the vessel, vanadium on catalyst is transferred to the trap particles. The fluidizing gases (air and steam) for the vessel make SOx partial pressure in the vessel much lower than in the regenerator, reducing the interfering effect of SOx with V trapping to a minimum. The spent V trap particles can be withdrawn from the vessel and fresh V trap can be added. The net result is the removal of vanadium from FCC system. The outboard vessel may also serve as a heat exchanger, allowing the temperature of the regenerator to be better controlled and more concarbon to be handled.

Examples of an on-board heat exchanger (cat cooler) can be found in U. S. 4,923, 834, U. S. 4,439, 533, and U. S. 4,396, 531. These heat exchangers are vessels adjoined to FCC regenerator in much the same manner as the V trapping vessel. They allow catalyst to flow through the shell side of the heat exchanger and a cooling medium through the tube side. Similar heat exchanger tubes may be installed inside the outboard vanadium trapping vessel of the invention and cooling of catalyst may be effected by passing a cooling medium, such as water, through the tube side.

Figure 1 shows a schematic diagram of the main structures of the FCC system of the present invention. The system includes a reactor (3), a regenerator (1) and an outboard vanadium removing vessel (2).

Figure 2 shows a schematic diagram of the outboard vanadium removing vessel that is a subassembly of one embodiment of the present invention. The invention involves the use of a vessel with which vanadium on FCC catalyst is continuously removed as part of normal FCC operation. The vessel is connected to a conventional FCC regenerator. A non-limiting illustration of how the vessel works in a typical operation is as follows: a slip stream of the regenerated catalyst in the regenerator (1) is continuously fed into the vanadium removing vessel (2) through an angled transfer line with slide valves (3). Air and steam are fed to the vessel from the bottom of the vessel through a distribution grid (4) to fluidize a bed of FCC catalyst and V trap particles which are much larger than FCC particles. The velocity of the fluidization gases is such that vigorous fluidization of all particles is achieved and yet only the catalyst particles, due to their smaller size, are carried to the upper zone of the vessel and they are captured by a cyclone (5) and returned to the lower zone of the vessel. An optional fanning-out feature in the upper portion of the vessel, for reducing the linear velocity of the fluidized particles, may also be included. So, in the vessel, a segregated fluidization zone is attained with FCC catalyst on top and V-trap particle-rich phase at the bottom of the bed.

An overflow well may be used to control the inventory. A J-bend transfer line (6) with a slide valve takes the catalyst with reduced V level back to the regenerator. After a certain period of operation when the V trap materials reach their trapping capacity or their V trapping activity needs to be regenerated, the trap material may be withdrawn from the vessel through elutriator-like withdrawal system (7). Regenerated or fresh V trap materials can be charged into the vessel through a side opening on the vessel (8).

In order to ensure that SOx partial pressure in the vessel is at a minimum, a"bubble-up"purge with steam and/or air on the angled transfer line (3) may be optionally included.

Example 1 - Preparation of V Loaded FCC Catalyst 500 g of an Orion series commercial Fluid Catalytic Cracking catalyst was wetted with a solution of 83. 3 g vanadyl naphthenate (3.2% V) in 175 g toluene to achieve uniform incipient wetness. The mixture was dried in a vacuum oven at 70°C and then was calcined at 600°C for six hours to bum off carbon residue. Vanadium on the support analyzed by elemental analysis: 5000 PPM.

Example 2 - Preparation of V Trap Materials Metal (Mg, Ca, Sr, Ba, La) nitrate or carbonate was first dissolved in water or dilute nitric acid. The salt solutions was used to wet a commercially available macroporous silica support which was in the form of one millimeter cross section granules and had a macroporous structures with pore size distribution around 150A. The desired level of metal loading was achieved by varying the salt solution concentration. The metal salt loaded materials were dried in a vacuum oven at 70°C, and then calcined at 200°C for 30 minutes and at 500°C for 2 hours consecutively. The exact loading of metal on the silica support was determined by elemental analysis.

Example 3 - Fluidization of V Loaded Catalyst and Metal Oxide Loaded Silica - The Effect of Steam Partial Pressure A 1 : 1 mixture of V loaded FCC catalyst as prepared in Example 1 with SrO loaded silica as prepared in Example 2 (3. 51% Sr) was fluidized in a tubular reactor at 700°C for four hours. Fluidization was effected by passing a stream of air and steam with varied steam partial pressure. The total pressure was one atmosphere and the superficial velocity of the stream was sufficient to cause vigorous fluidization of both the catalyst and the much larger trap particles. After the cofluidization the reactor was cooled and the solids discharged and separated by sieving. The V level on the trap was analyzed by elemental analysis. The results showing the effect of temperature and steam partial pressure are as follows: Steam partial pressure, % of 1 atm 10 20 30 40 V on SrO/Silica, PPM 515 848 1190 1570 Average Trapping Rate, %V/hour 3.33 4.25 5.95 7.85 Example 4 - Fluidization of V Loaded Catalyst and Metal Oxide Loaded Silica - The Effect of Temperature A 1: 1 mixture of V loaded FCC catalyst as prepared in Example 1 with SrO loaded silica as prepared in Example 2 (3. 51% Sr) was co-fluidized in a tubular reactor at predetermined temperatures for two hours. The fluidization was effected by passing a stream of air and steam (70: 30 mole ratio), under one atm total pressure. The superficial velocity was sufficient to cause vigorous fluidization of both the catalyst and the much larger trap particles. After the cofluidization the reactor was cooled and the solids discharged. The catalyst and trap were separated by sieving and the V level on the trap was analyzed by elemental analysis. The results showing the effect of temperature are as follows: Temperature, °C 650 675 700 725 V on Sr0/Silica, PPM 560 686 750 905 Average Trapping Rate, %V/hour 5.60 6.86 7.50 9.05 This example illustrates that under conditions comparable to those found in the regenerator (typically 700°C and steam partial pressure of 15% of 2 atr : l total pressure) a vanadium trapping rate of 300-450 PPM/hour, or 6-9% V on catalyst/hour, is attainable in the vessel of this invention with appropriate designed, millimeter size V trap materials.

Example 5 - Process Conditions In order for the outboard vessel to effectively trap vanadium and to be economically feasible, a preferred embodiment for the vessel size and the process condition are defined as the following: Size of vessel 1/100 - 1/5 of the regenerator size % of cat inventory in the vessel 0. 5 - 20 Cat flow rate through the vessel, tons/day 300 - 50, 000 Residence time in the vessel, minute per pass 0.01 - 30 Tons of trap particles in vessel 1-50 Fluidizing gas for the vessel 5 - 100% steam, make up air Pressure in the vessel 0 - SO psig Temperature, °F 1200 - 1600 The following example shows a typical operation and its effectiveness for vanadium removal: Assuming the following FCC operation conditions: total cat inventory, tons 450 barrel of feed per day 50,000 and conditions in the outboard vessel: % cat inventory in vessel 5 cat residence time per pass, minutes 1 - 10 tons of trap in vessel 22.5 therefore the following numbers: tons of cat in vessel 22. 5 flow rate through vessel, tons/day 300 - 50, 000 cat residence time in vessel per day, min. 72 Using vanadium trapping rate of 5%/hour, one obtains 6% vanadium removal on total cat inventory. If one further assumes that equilibrium catalyst has 5000 PPM V, 6% vanadium represents 0.135 ton vanadium removal per day with the vessel (450 tons x 5000 x 10% x 6% = 0.135 ton). This translates into an additional 18 PPM vanadium from the feed (0.135/ (50,000 bl x 300/2000) = 18 PPM).