There have been ever increasing oil discoveries in Brazil that contain high concentrations of basic nitrogenous compounds, be they aromatic or polyaromatic, that may or may not be branched, with predominantly heterocyclic chemical structures, that are concentrated in high boiling point hydrocarbons fractions such as Heavy Gas Oil (HGO), Atmospheric Residue (AR), Vacuum Residue (VR), by-products of the distillation process, among others, such as Heavy Coker Gas Oil (HCGO), by-products of delayed coking units, and even deasphalted oil (DESO) a by-product of asphalt production units.
Basic nitrogenous compounds, when present in fluid catalytic cracking processing, feedstocks tend to promote deactivation of the catalyst acid sites and to increase the level of coke deposits on the catalyst, with the subsequent loss of product conversion and selectivity in the process.
Fluid catalytic cracking (FCC) is performed by the contact of hydrocarbons in a reaction zone with a catalyst made up of fine particulate matter. Feedstocks that are commonly submitted to FCC processing are, usually, petroleum refinery process streams that come from longitudinally segmented vacuum towers, called Heavy Vacuum Gas Oil (HVGO), streams coming from delayed coking units, Heavy Coker Gas Oil (HCGO) or, heavier that the former, coming from the bottom of atmospheric towers, Atmospheric Residue (AR), or even mixtures of these feedstocks.
These streams, that typically have a density in the range of 8 to 280 API, must be chemically processed using a process such as the catalytic cracking process, which fundamentally alters its composition, converting them into lighter, more valuable hydrocarbon streams.
During the cracking reaction, substantial portions of coke, a by-product of the reaction, are deposited on the catalyst. Coke is a material of high molecular weight, made up of hydrocarbons containing, typically, from between 4 and 9% of its compositional weight in hydrogen. The catalyst covered with coke, usually called “spent catalyst” by the specialists in the field, is continually removed from the reaction zone and is substituted with catalyst that is essentially free of coke from the regeneration zone.
In the regeneration zone and in a regenerator vessel kept at high temperature, the coke deposited on the surface and on the pores of the catalyst is burned off. Removing the coke through its combustion allows for a recovery of the catalyst activity and frees heat in sufficient amount to fulfill the thermal requirements for catalytic cracking reactions. The fluidization of the catalyst particles by gaseous feeds allows the catalyst to be transported between the reaction zone and the regeneration zone and vice-versa. The catalyst, aside from fulfilling its main function of expediting the catalyzation of chemical reactions, as well as providing a method for transporting heat from the regenerator to the reaction zone.
The technique contains many descriptions of hydrocarbon cracking processes in a fluidized catalyst feed, with catalyst transported between the reaction zone and the regeneration zone, and coke burning in the regenerator.
In spite of the long-time existence of the FCC process, techniques to improve the process have continually been sought to increase the production of derivatives of greater aggregate value, such as Naphtha and LPG. Generally speaking, it could be said that the main purpose of FCC processes is to maximize the production of these more valuable derivatives.
This maximization is basically obtained in two ways. First, by increasing the so-called “conversion” used to reduce the production of heavy products such as clarified oil and light recycled oil. And second, by reducing the production of coke and combustible gas, in other words, less “selectivity” towards these products.
A lower production of these last two products, besides expediting an increase in the production of gasoline and LPG, by increasing the process selectivity towards these derivatives, provides as a result the additional benefit of lower air blower and wet gas compressor use (machines with a high deadweight and large power consumption), which in turn usually cause a limitation of the FCCU capacity.
It is well known that an important aspect of the process and the initial contact of the catalyst with the, feedstock that exerts a decisive influence on the conversion and the selectivity of the process in generating noble products. In the FCC process, the feedstock of preheated hydrocarbons is injected next to the base of a conversion zone or riser, where it enters into contact with the flow of the regenerated catalyst, from which it receives sufficient heat to vaporize it and supply the demand of the endothermic reactions that dominate the process.
After the riser, (which is an elongated vertical pipe whose dimensions, in industrial units, are around 0.5 to 2.0 m in diameter by 25 to 40 m high, and is where chemical reactions occur) the spent catalyst, with coke still deposited on its surface and pores, is separated from the reaction products and is sent to the regenerator in order to burn off the coke so as to restore its activity and to generate the heat that, transferred by the catalyst to the riser, will be used by the process.
The conditions existing at point of the feedstock's entry into the riser are determined by how many products are formed in the reaction. In this area an initial mixture occurs of the feedstock with the regenerated catalyst, which has been heated to the boiling point of its components and to vaporization of the greater part of these components. The total residence time of the hydrocarbons in the riser is around 2 seconds. So that the catalytic cracking reactions may be processed, vaporization of the feedstock in the mixing area with the catalyst must occur rapidly, so that the vaporized hydrocarbon molecules may enter into contact with the catalyst particles—whose size is close to 60 microns—and permeate into its micro-pores, undergoing the effect provided by its acid sites in catalytic cracking. If this rapid vaporization is not achieved, thermal cracking will result of the feedstock's liquid fractions.
It is well known that thermal cracking leads to the formation of by-products such as coke and combustible gas, mainly in residual feedstock cracking. Coke, in addition to its low commercial value, obstructs the pores of the catalyst. Therefore, thermal cracking in the bed of the riser competes in an undesirable fashion with catalytic cracking, which is the purpose of the process.
Feedstock conversion optimization usually requires maximal removal of coke from the catalyst in the regenerator. Combustion of the coke may be obtained by partial combustion or total combustion. In partial combustion, the gases produced by combustion of the coke are principally made up of CO2, CO and H2O and the percentage of coke in the regenerated catalyst is on the order of between 0.1% a 0.2% by weight. In this case of total combustion (performed in the presence of a great excess of oxygen), practically all of the CO produced has already reacted and been converted to CO2. The oxidation reaction of CO to CO2 is strongly exothermic, so that when this total combustion happens it releases a great amount of heat, resulting in very elevated regeneration temperatures. However, total combustion produces catalyst containing less than 0.07% and, preferably, less than 0.05% in weight of coke, making this feature more advantageous than partial combustion, in addition to precluding the need to use a burdensome boiler to combust the CO afterwards.
The increase in coke on the spent catalyst results in an increase in the coke combustion in the regenerator per unit of mass of circulated catalyst. Heat is removed from the regenerator in conventional FCC units in the combustion gas and mainly along the regenerated hot catalyst stream. An increase in the percentage of coke on the spent catalyst increases the temperature of the regenerated catalyst and the difference between the temperatures between the regenerator and the reactor.
Meantime, a reduction in the output of regenerated catalyst towards the reactor, (usually called circulation of the catalyst), is necessary in order to fulfill the thermal demand of the reactor and to maintain the reaction at a constant temperature. However, the lower catalyst circulation rate demanded by the great difference in temperature between the regenerator and the reactor, which results in a decrease in the catalyst/oil ratio, which in turn lowers the conversion.
So, the circulation of the catalyst in the regenerator towards the reactor is defined by the thermal demand of the riser and by the temperature established in the regenerator, (a function of the production of coke). Since coke that is generated in the riser is affected by the circulation of the catalyst itself, a conclusion may be drawn that the catalytic cracking process works under a system of thermal balance. However, (for the indicated reasons), very elevated temperatures are undesirable in the regeneration operation.
Usually, with modern FCC catalysts, the temperatures of the regenerator and, consequently, that of the regenerated catalyst, are kept below 760° C., preferably under 732° C., since the loss of activity would be very severe above this number. A desirable operational range is between 685° C. and 710° C. The lower value is dictated, mainly, by the need to guarantee proper combustion of the coke.
With the ever increasing weight of feedstocks processed, there is a trend towards raising the production of coke and the total combustion operation requires catalyst coolers to be installed in order to keep the temperature of the regenerator at acceptable limits. The catalyst coolers usually remove heat from the catalyst stream coming from the regenerator, returning to this vessel a substantially cooled catalyst stream.
As regards the fluid-dynamic characteristics of the riser, where the catalytic cracking reactions are processed of the present invention, what is known is that solid catalyst particles are dragged, by the reaction itself, during contact with the feedstock and other vaporized materials.
These types of reactors usually have the shape of a pipe where, in order to reduce the production of by-products, it is necessary to operate within a hydrodynamic stream system, in such a way as to allow the surface velocity of the gas to be either high or sufficient enough to cause the catalyst to flow in the same direction as the feedstock and the other vapors there existing. In other words, the liquid and vaporized feedstock drags the catalyst particles with it through the input passageway in the pipe reactor.
These stream systems are known by technicians in the field as fast fluidized bed, riser systems, or more generically as transport systems, which are the preferred systems when dealing with reaction systems that require continuous flow reactors.
Usually, for any given area in the cross section of a pipe reactor (which is a function of the diameter of the reactor itself), the concentration of the catalyst, in the fluidized bed of a reactor, decreases with an increase in the surface velocity of the gas. The greater the surface velocity of the gas, the greater the height required by the reactor to allow a given quantity of the feedstock to be able to contact the required amount of catalyst. These greater surface velocities (of the gas) require a higher L/D (Length/Diameter) ratio, or “aspect ratio” in the reactor. This ratio is the ratio between the height of the reactor and its diameter.
Additionally, in many cases, it may be desirable to build fluidized bed reactors with large cross section areas so that considerable feedstock output can be achieved with a single reactor. However, when the diameter of the fluidized bed is increased, particularly in the transport system, the height of the reactor must be increased as well. This increase in height is necessary because a certain minimum height is required in the reactor (L/D ratio) in order to achieve a fully developed flow pattern that is closer to the behavior of a continuous flow reactor.
However, fluidized bed reactors with an elevated L/D ratio are expensive, difficult to build and maintain because they must have very large and heavy separating tanks in the top, containing, in their interior, equally heavy equipment, that are targeted at capturing and controlling the catalyst flow and the products in the reactor.
Finally, FCC Units with multiple risers, may have small diameter feedstock conversion zones precisely due to having a multiplicity of risers and therefore are able to maintain an adequate L/D ratio to promote the necessary fully developed stream systems, with a reasonable reactor height.
The increase in participation of domestic petroleum, originating from the oil fields of Campos Basin, on the coast of the State of Rio de Janeiro, presents some technical problems regarding the refining of hydrocarbon feedstocks derived from these oils, especially when the presence of basic organic nitrogenous compounds compromises the performance of the catalysts used in the fluid catalytic cracking (FCC) process, the major supplier of gasoline, diesel and liquefied petroleum gas (LPG) for domestic consumption.
Basic organic nitrogenous compounds, present in petroleum, are predominantly, made up of the quinoline, benzoquinoline, alkylpyridines, amides, alkyl and hydroquinolines, acridines and phenanthridines families. Structurally, they are aromatic and polyaromatic heterocyclic compounds, that may or may not be branched, that accumulate on the heaviest fractions of crude oil in separation processes, mentioned above. Heavy gas-oil originating from Cabiunas petroleum oil may present about 1000 parts basic nitrogen per million (ppm).
Any refinery or specialist in the field of hydrocarbon feedstock refining will recognize the problems arising from the presence of basic nitrogenous compounds in the refinery process, especially in the FCC process: Basic nitrogenous compounds are responsible to a large extent for deactivation of the cracking catalysts, an increase in the level of coke, and gum formation in gasoline. In summary, all this represents a loss of capacity in the catalytic cracking unit with consequent great financial damage to the refinery.
In the first attempt to resolve this problem, or at least to minimize it, several refineries resorted to changing the catalyst used in the catalytic cracking units, in the search for a catalyst that would be more resistant to contamination through basic nitrogenous compounds. As the catalytic cracking catalyst in use in the great majority of refineries is made up of an acidic crystalline aluminosilicate—a zeolite, dispersed in a clay matrix, the poisoning of the catalyst by the basic nitrogenous compounds occurs precisely by the neutralization of the zeolite's acid centers that are, in the last analysis, the active centers for cracking the hydrocarbon molecules of the feedstock.
To overcome the deactivation caused by the basic nitrogenous compounds, many manufacturers of catalytic cracking catalysts have offered catalysts with a greater number of acid centers to their clients that come from a higher percentage of zeolite in the catalyst or by the use of acid matrices. Said resource may work well when the percentage of basic nitrogenous compounds in the hydrocarbon feedstock is low, or, when the basic nitrogenous compounds present have a low molecular weight.
It has been verified, in this last instance, that the basic nitrogenous compounds of low molecular weight only “poison” (in a reversible way), the acid centers of the catalyst and that, after the catalytic cracking catalyst regeneration stage, the activity of the cracking catalyst is restored, momentarily, until the catalyst thus regenerated enters again into contact with the hydrocarbon feedstock containing basic nitrogenous compounds. However, when basic nitrogenous compounds are made up of aromatic or polyaromatic compounds that are of a higher molecular weight, as is the case with the basic nitrogenous compounds present, for example, in CGOs, the deposit of the molecules of these basic nitrogenous compounds onto the surface of the catalyst particles is irreversible because it neutralizes the acid centers and reduces the specific area of the catalyst, that loses activity and selectivity. This is not, therefore, a good solution for fluid catalytic cracking units that process heavy feedstocks of hydrocarbons with these basic nitrogen characteristics.
Experimental studies performed on a multipurpose pilot FCC Unit of the Applicant, to evaluate the proposal of multiple injections in riser described in the patent CN 1088246 A, of the Petrochemical Research Institute of China Petrochemical Corp., for feedstock rich in contaminants, disclosed that said proposal was very advantageous in industrial FCC Units that process Coker Gas Oil, having a level of aromatic, resins, and basic nitrogenous compounds on the order of between 800 and 1000 ppm.
Studies of catalyst sampling in the risers of an industrial FCC Unit, described by Waldir Martignoni et al, in work presented at the Encontro Sul Americano de Refino, held in Manaus, AM, Brazil, in April of 2000, proved that the catalyst presents greater activity in first the 15 meters of the riser, where most of the conversion occurs in the process.
The injection of feedstocks considered refractory to cracking, as for example, CGOs, in positions above the traditional nozzle, where most of the process conversion has already occurred, increases, significantly, the process conversion of noble products.
In parallel fashion, studies of acid treatments of CGOs, performed at the work bench level in the laboratories of the Applicant's Research Center, according to concepts contained in patent PI 9803585-1 A, also belonging to the Applicant, disclosed that the basic nitrogenous compounds present in this CGO present a much more marked reactivity than the existing compounds in direct distilled heavy gas oil (DDHGO), a fact that would favor a more marked deactivation of the catalyst.
It is also known that Heavy Coker Gas Oil is a refractory feedstock in cracking, considering that HCGOs are a fraction derived from a thermal process. The Applicant performed runs in a prototype FCC Unit, with the feedstock output of 200 kg/h, proving a decrease in the process conversion, when the CGO fraction is mixed with the process feedstock, as will be shown in Example 1 of the present report.
In function of the greatest reactivity with the basic nitrogenous compounds present in HCGOs and of the lower crackability of this type of feedstock, in comparison with vacuum gas oil, if there were the possibility of adjusting differentiated operational conditions for HCGOs (mainly through the temperature of the reaction), it would be a promising alternative for the optimization of the industrial unit. This temperature adjustment would only be possible, if this stream were processed in a secondary riser of a FCC unit with two or more different risers.
Another benefit additional to the above achieved by said feedstock segregation would be that the segregated DDHGO might have the possibility of entering into contact with a more active catalyst in the main riser, due to the fact that the basic nitrogen present in the most contaminated feedstocks would not neutralize the acid sites of this catalyst in this riser, because it would be being fed into another secondary riser. This effect would be more pronounced than the effect observed with multiple injection feedstocks in the same riser.
U.S. Pat. No. 6,156,189 describes a type of alternate injection, in rapid feed cycles, made in the risers of pilot FCC Units with one or more risers, that, similar to the present invention, is presented as an alternative to the processing feedstock mixtures with different properties, when feedstocks with different properties are injected in the same riser. It should be emphasized that, industrially speaking, this procedure is an extremely complicated job, due to the fact that the patent description suggests alternating feeds in intervals or cycles of between 20 seconds and 2 minutes to achieve an increase in conversion.
Different from the State of the Art, the process of the present invention involves simultaneous processing, in fluid catalytic cracking units with multiple risers, of feedstocks containing different percentages of contaminants, especially contamination with basic nitrogen, where said feedstocks contaminated with the catalyst damaging basic nitrogen compounds are segregated into a secondary riser. Additionally, the present invention even includes the use of cooling streams in the secondary riser, to adjust the catalyst/oil (CTO) ratio in the risers.
The proposal of the present invention also guarantees that the acid sites of the catalyst in the main riser shall remain more active along the length of the riser, extending the beneficial effect theoretically achieved by U.S. Pat. No. 6,156,189.
Beyond the additional cited benefit, the fact that if segregated feedstocks are processed, this will allow for more operational flexibility, once the reaction temperature may be altered in each riser and the thermal balance modified, increasing the CTO in the risers.
So, despite the proposal literature, the technology for fluid catalytic cracking process still needs to involve simultaneous processing, in fluid catalytic cracking units, in multiple risers, for feedstocks containing different percentages of contaminants, especially of basic nitrogen, where the most contaminated feedstocks are segregated into secondary risers, these risers which are still cooled by cooling (quenching) fluids. A fluid catalytic cracking process that presents such features is described and claimed in the present application.