Patent Publication Number: US-2007095724-A1

Title: FCC process for the maximization of medium distillates

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is based upon, claims the benefit of, priority of, and incorporates by reference, the contents of Brazilian Patent Application No. PI 0504854-0 filed Oct. 31, 2005.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention concerns a FCC (fluid catalytic cracking) process of low severity that maximizes medium distillates. More specifically, it relates to FCC processes for heavy or blended feedstocks that results in the maximization of medium distillates containing. low levels of aromatic compounds while, at the same time, maintaining the bottoms generation (cut at about 340° C. + ).  
      2. Description of the Prior Art and Background of the Invention  
      Fluid catalytic cracking (FCC) is carried out within a tubular reactor zone, or riser reactor, by the contact between hydrocarbons and a catalyst composed of fine particulate material. Most of the feedstocks submitted to the FCC process are refinery streams from side cuts of a vacuum tower, referred to as heavy gasoleum of vacuum (GOPDD), or even heavier streams from the bottom of an atmospheric tower, referred as atmospheric residue (RAT), or a blend of such streams. These streams, with typical densities within the range of 8° to 28° API, usually undergo a chemical process, catalytic cracking for instance, in order to alter their composition, therefore being converted to more profitable lighter hydrocarbons streams. The starting of primary reactions occurs in the base of the riser reactor. The large molecules of the feedstock, containing 40 or more carbons, crack to form olefins of lower molecular weight. As the reactions proceed, the olefins either crack or undergo other reaction types in the formation of cyclic compounds and isomers. In a secondary reaction type, the cyclic compounds transfer hydrogen atoms to the olefins and form aromatic compounds and paraffin. Thermodynamics favors the aromatic compounds formation while time and high temperature contribute in approaching the thermodynamic equilibrium of the product profile, that is, the maximization of the aromatic compounds. The cracking yields coke deposit on the surface and pores of the catalyst, referred as waste catalyst, which burns in a regenerator zone in order to restore the original activity. The coke combustion releases the heat necessary for the cracking reactions. Besides catalyzing the chemical reactions, the catalyst particles fluidized in the gas stream convey heat between the regeneration and reaction zones.  
      An important aspect is the influence of the early contact between the catalyst and the feedstock on the conversion and the process selectivity to yield prime products. In the FCC process, one introduces the pre-heated hydrocarbons feedstock near the base of the riser, a conversion zone where it contacts the catalyst flow, from the regenerator, and exchanges enough heat not only to vaporize the hydrocarbons but also to supply energy to the endothermic reactions, which predominate in the process. The feedstock vaporization must be complete and fast otherwise thermal cracking occurs in the liquid fraction of the feedstock. Thermal cracking favors undesired coke and fuel gas formation, mainly when a residue is the feedstock being processed. Coke not only poisons acidic sites but also blocks catalyst pores. The quick feedstock vaporization allows the hydrocarbons to adsorb on the surface of the catalyst particles, which size is about 60 micra, and afterwards diffuse into micropores of the particle core, up to reaching the acidic sites where the catalytic reactions take place. The riser reactor is a vertical tube of about 0.5 m to 2 m of diameter per 25 m to 40 m of height. The hydrocarbons residence time inside the reactor is about 2 seconds. The cycle closes when one separates the products after the riser and the catalyst returns to the regenerator. Therefore, the conditions in the region where the feedstock meets the catalyst are intimately associated to the process yields profile. Moreover, optimized conversion usually depends on the efficiency of the coke removal in the regenerator. The coke burning proceeds at partial or total combustion. Increasing amounts of coke in the waste catalyst results in more coke burned in the regenerator per mass unit of circulated catalyst. In conventional FCC units, the combustion gas stream and mainly the hot regenerated catalyst remove heat from regenerator. The higher the coke contents in the waste catalyst, the higher both the temperature of the regenerated catalyst and the difference between regenerator and reactor temperatures. The regenerated catalyst feed flow to the reactor, usually referred as catalyst circulation rate, can regulate the reactor energy claim and can keep constant temperature. Notwithstanding, using lower catalyst circulation rates, in order to drop the temperature difference between regenerator and reactor, results in reduced catalyst-to-oil ratio (CTO) as well as reduced conversion. Therefore, the catalyst circulation intensity must provide the reactor thermal claim and must keep an appropriate regenerator temperature, which is a function of the coke made. As the catalyst circulation itself affects the coke yield, one can establish that the catalytic cracking process runs in a thermal balance regime, being undesirable operations at much too high regeneration temperatures. Nowadays, in operations with modern FCC catalysts, the regenerator temperatures, that is, the regenerated catalyst temperatures practiced are below 760° C., preferably below 732° C., inasmuch as the activity loss should be excessive for higher temperatures. A desirable operational range is between 685° C. and 710° C. The lowest temperature limit depends on the necessity of ensuring proper coke combustion.  
      Previously, the FCC process was developed in order to produce gasoline of high-octane number (high aromatic compounds content). The light cycle oil (LCO), an FCC by-product, is about 15% to 25% of the total yield and consists of a typical distillation range between 220° C. and 340° C. The LCO usually contains high concentrations of aromatic compounds, sometimes up to more than 80% of the total fraction. When somehow one wants to blend LCO to a pool of diesel oil, it is challenging to change the FCC operation parameters in order to maximize the LCO streams. However, the high aromatic compounds concentration in LCO brings about increased density and worse detonation quality for diesel engines (low cetane number). The high aromatic compounds content also make difficult the improvement of LCO properties through a hydrotreating process. Most of the operations carried out to maximize medium distillates in a FCC process reduce the reaction temperature to extremely low values (between 450° C. and 500° C.), reduce the catalyst circulation, and employ low activity catalysts. This leads to both increased yield and improved LCO quality (lower aromatic compounds). The problem is that such operations also lead to increased FCC residual fraction (cut at 340° C. + ), as often as not directed to fuel oil of low market value.  
      Another aspect to be considered is the energy balance of the FCC process. The energy consumed in the riser reactor is supplied by the regenerator and conveyed by the catalyst circulation. Lowering the temperature implies shorter amount of catalyst in the riser and lower conversion. When one applies a higher reaction temperature, the unit automatically increases the catalyst circulation. On one hand, it is harmful whether the target is lowering the aromatic compounds in the LCO fraction. On the other hand, it is useful to improve bottoms conversion. As the claim for high quality medium distillates has increased, mainly in Brazil and Europe, specialists in the area have discussed changes in the operation mode of FCC units with the aim of increasing LCO yield. Many articles report modifications in the catalytic system and process variables in order to diminish the process severity, aiming both to increase mediums fraction yield and to decrease the aromatic compounds content in such fraction. Most report modifications to achieve reaction temperature reduction, catalyst-to-oil ratio reduction and catalytic activity reduction. All modifications lead to conversion reduction and, therefore, enhanced bottoms yield. The following references approach this subject: 1) Peterman, R. W.  Distillate yield from the FCC process and catalyst changes for maximization of LCO , Catalysts Courier; 2)  Advanced hydrotreating and hydrocarbon technology to produce ultra 2- clean diesel fuel , Hydrocarbon Publishing Company, 2004; 3)  Studies on maximizing diesel oil production from FCC , Fifth International Symposium on the Advances in Fluid Catalytic Cracking, 218 th  National Meeting, American Chemical Society, 1999; and 4)  New development boosts production of middle distillate from FCC , Oil and Gas Journal, August, 1970.  
      Concerning the patents literature, one finds several works aiming at diverse targets. They propose the injection of an auxiliary stream above the feedstock inlet point. Thus, water or any other oil fraction can be used in order to promote mixture temperature increase by the zone of feedstock introduction, aiming to increase the amount of vaporized residual feeding portions without changing the riser outlet temperature. This approach is reported in U.S. Pat. No. 4,818,372, which teaches an apparatus for FCC with temperature control that consists of one ascendant or descendant reactor, one arrangement to introduce the hydrocarbons feedstock under pressure in contact with the waste catalyst and at least one arrangement to inject an auxiliary stream of fluid after the reactor zone where the catalyst meets the feedstock, as it is desired to reach higher temperature in the zone of mixture between feedstock and catalyst. The patent also teaches using an external inert fluid to chill the injection zone of auxiliary stream, with both temperature control and catalyst circulation increase.  
      According to the teaching of U.S. Pat. No. 4,818,372, the injection of an external stream distinctly in an upward point of the riser reactor is carried out in order to control the riser reactor temperature profile, and, in such a way, one keeps the riser initial segment at a relatively higher temperature, without changing the temperature in the top of the riser reactor (temperature of reaction or TRX). Hence, it follows that it provokes a thermal chock, at high temperatures, in the base of the riser reactor, which promotes the cracking of asphaltenes and residual fractions, as well as improves the feedstock vaporization, achieved by partial pressure reduction. There is no concern about the impact on the LCO quality. In the present invention, adversely, there is no interest for cracking asphaltenes and it is recommended to avoid it, inasmuch as the fraction of aromatic compounds in LCO tends to increase by this proceeding. It is possible that asphaltenes are aggregated to the coke yielded by keeping the conditions in the base of the riser reactor likewise in the UFCC conventional operation, which aims at gasoline maximization. Besides, according to the reference cited, one must introduce the chilled stream near the reactor base in order to reach a better feedstock atomization by reducing the vapor pressure, what does not lead to the desired results of low aromatic compounds in the LCO without increasing the bottoms conversion, which is an objective of the present invention.  
      This control can also be carried out through a recycle of heavy naphtha, such as taught in the U.S. Pat. No. 5,087,349, which recommends specifically the recycle of naphtha with a boiling point range of 165-221° C. as coolant. The recycle of such a fraction is not adequate to meet the objective of the invention, seeing that this heavy naphtha is blended in the diesel fraction and its recycle would increase the content of aromatic compounds, lowering its quality. In an aspect of the present invention, the proposed hydrocarbons recycle is driven to the light naphtha, having a boiling point range of about 15-150° C., evidently in order to avoid contamination of the additional aromatic compounds formed in the recycle in the LCO (higher boiling point range, &gt;150° C.). An advantage of recycling light naphtha, regarding the invention objective, is that it is rich in olefins (contrary to heavy naphtha) and it is able to continue reacting to yield LPG and propylene.  
      With similar objectives, U.S. Pat. No. 5,389,232 teaches a heavy naphtha recycle in upward points of the riser reactor.  
      In addition, U.S. Pat. No. 4,764,268 suggests the introduction of a LCO stream in the top of the riser reactor with the aim to minimize naphtha over-cracking reactions.  
      A similar alternative, taught in the U.S. Pat. No. 5,954,942, aims at a conversion increase through a quench with an auxiliary stream of vapor in the upside zone of the riser reactor.  
      Publication WO 93/22400 mentions the possibility of introducing a cracking product along the riser reactor, such as LCO, with the aim to carry out a riser reactor cooling and, consequently, not only to promote an increase of the catalyst circulation, but also to improve the performance of ZSM-5-based additives.  
      U.S. Pat. No. 6,416,656 teaches a process to increase simultaneously the yields of diesel and LPG. In such a process, gasoline is cracked to LPG yield increase, being injected in a point lower than the feedstock inlet mouth. The process feedstock is injected through multiple points along the riser reactor, reducing the contact time and hence increasing the LCO yield.  
      Thus, one notices that despite the development of techniques aiming to maximize medium distillates through a FCC process, it is still necessary to discover a process which combines the maximization of LCO with low content of aromatic compounds and the minimization of bottoms conversion. By way of the present invention, it has been discovered that this combination can be achieved by ensuring a high starting temperature in the first meters of the riser reactor, where the primary reactions occur, and employing a coolant forward, inhibiting the secondary reactions and creating an additional energy claim applied to increase the catalyst circulation, and, in such a way, the fast chilling of the reaction mixture environment after the starting reactions does inhibit the secondary reactions, avoiding the formation of aromatic compounds.  
     SUMMARY OF THE INVENTION  
      In broad terms, the present invention relates to a FCC process to obtain medium distillates through the maximization of LCO with low aromatic compounds content and the minimization of the bottoms conversion. This beneficial combination is achieved by ensuring a high starting temperature in the first meters of the riser reactor where the primary reactions occur and using a coolant forward in such a way that the lower half of the riser reactor operates conventionally, while the upper half operates at low severity. Therefore, the invention provides a FCC process to obtain medium distillates where the process conditions lead to the production of LCO with low content of aromatic compounds while at the same time minimizing the bottoms conversion. The invention also provides a FCC process to obtain medium distillates where the coolant is injected from an inlet point located above ¼ of the riser reactor height. The invention still provides a FCC process to obtain medium distillates where the coolant injection, through an inlet point located above ¼ of the riser reactor height, has, as a consequence, the inhibition of secondary reactions and the creation of an additional energy claim applied to increase the catalyst circulation, in such a way that the fast cooling of the reaction mixture environment, just after the starting reactions, inhibits the secondary reactions, avoiding the formation of aromatic compounds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  discloses equipment for a FCC process where the coolant is injected at a point along the height of the riser reactor, in accordance with an aspect of the present invention. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION  
      The present invention concerns a fluid catalytic cracking (FCC) process of mixed hydrocarbons feedstocks from different sources with the objective of maximizing the medium distillates fraction.  
      The present invention is an FCC process which makes feasible the production of medium distillates with low aromatic compounds content while minimizing bottoms conversion, as it occurs in low severity FCC operations to maximize mediums.  
      In the FCC unit operation in low severity mode, it allows an improvement of the quality of the medium distillate produced, the direct consequence is a considerable increase of the decanted oil yield, by virtue of the conversion drop. In the present invention, it is able to reach the improvements, in terms of medium distillates quality, by means of low severity operation, keeping low bottoms yield.  
      This effect is achieved through the fast cooling of the reaction environment, just above the feedstock introduction point, employing a stream of water and/or liquid hydrocarbons, which removes heat from the system during vaporization. By way of this inventive process, compared to the coolant injection processes found in the prior art, it is possible to produce medium distillates with lower content of aromatic compounds and lower bottoms conversion than when the injection of said coolant is carried out in the riser base inlet.  
      In preferred embodiments, useful feedstocks for the FCC process of the invention, for obtaining medium distillates, consist of isolated feedstocks or mixed hydrocarbons feedstocks with an °API inferior to 20, as for instance, vacuum gasoleum.  
      Useful catalysts for the process preferably include zeolitic catalysts, such as ZSM-5, exchanged with rare earths.  
      Proper coolants preferably include water or water vapor and light naphtha. Typical light naphtha contains between 20w % to 25w % of aromatic compounds and a final boiling point around 200° C. to 205° C.  
      The proportion of coolant or quench in relation to the feedstock is preferably between about 200 and about 50 kg/m 3 , preferably between about 100 kg/m 3  and about 50 kg/m 3  of feedstock.  
      The reaction temperature is preferably about 480° C. to about 520° C., depending on the feedstock quality, and the catalyst-to-oil ratio is preferably between about 4 and about 10.  
      According to the process, in the mixture zone the temperature preferably reaches about 670° C. to about 710° C., typical of low severity operations, with a reaction temperature within the preferred range of about 480° C. to about 520° C.  
      Concerning the catalyst type, one notices that though experimental data have been obtained with conventional catalysts, the present process preferably employs catalysts used for mild fluid catalytic cracking (MFCC), which are known as high matrix catalysts. In this case, the use of coolant helps even more, since the high matrix catalysts increase the delta coke of the unit and warm the regenerator and, therefore, reduce the catalyst circulation. The employment of coolant in this case would restore the circulation, due to regenerator cooling.  
      Thus, the preferred catalyst for the present process consists of low acidity cracking catalysts and reduced hydrogen transfer, in order to yield an aromatic compounds content as low as possible. The employment of such a catalyst type achieves good LCO selectivity with moderated catalyst-to-oil ratios, preferably from about 4 to about 7, being not necessary to reduce the reaction temperature below about 520° C. This aspect is important for the processing of feedstocks with characteristics of naphthalene-aromatics, inasmuch as it avoids the excessive production of decanted oil.  
      The invention will be depicted as follows by reference to the  FIG. 1 . The catalyst regenerated and warmed from regenerator ( 6 ) is accelerated with vapor in the lift zone ( 1 ). The feedstock is introduced in the position ( 2 ), meets the warmed catalyst, vaporizes and the catalytic cracking reactions start to take place. In the zone ( 3 ) located at from 25% to 50% of the riser&#39;s height above the feedstock inlet mouths, a quench fluid is injected which vaporizes and provokes the quick cooling of the reaction mixture environment. During the cracking reactions, the catalyst deactivation proceeds by the deposition of coke, a cracking by-product. The mixture catalyst/products proceed forward along the riser being separated in the cyclones located in the reactor vessel ( 4 ), the products are collected over line ( 5 ) being directed to the fractionation and lights recovering and the catalyst is directed to the regenerator ( 6 ) and rectified with vapor. In the regenerator, the coke of the catalyst is burned with air ( 8 ), cleaning the catalyst and generating most of the necessary energy for both the feedstock vaporization and for the cracking reactions.  
      The invention will be depicted by the following examples, which are not intended to limit the invention.  
     EXAMPLES  
      One series of tests is carried out in a FCC circulation prototype unit, for a feed rate 200 kg/h, catalyst inventory 200 kg, and working in thermal balance. Gasoleum of vacuum is used as feedstock from “Bacia de Campos”, which contains 18.7° API and 42% of aromatic compounds. The experiments are accomplished in the presence of an equilibrium catalyst containing 1.78% of rare earths, 1,779 ppm of vanadium, 2,481 ppm of nickel, about 1% of ZSM-5 crystal and 142 m 2 /g of specific area.  
      The prototype unit riser has height 18 m with a feedstock axial injection mouth in the base. An auxiliary line, also in the riser base, and a second injection point located at 3 m from the base can be used optionally for introducing water or naphtha. The naphtha used as coolant contains 22% of aromatic compounds and a final distillation boiling point of 205° C.  
      All analyses of the aromatic compounds content are carried out employing the Super Critical Chromatography technique of the total liquid produced.  
      During the experiments, one varied the reaction temperature, the flow rate, the injection point and the coolant type.  
      In the base case (A), in which a typical operation for gasoline maximization is configured, the reaction temperature is fixed at 540° C. and no coolant is injected.  
      The reaction temperature is then reduced to 495° C., to simulate an operation of maximum LCO, and the water flow rate, as coolant, is varied between 0 and 20 kg/h, being introduced through the 3 m injector mouth, cases (B), (C) and (D).  
      Afterwards, the temperature is set to 505° C. and again the water flow rate is varied through the 3 m injector mouth, cases (E), (F) and (G).  
      It follows that in case (H) water is injected at 6 kg/h in the base of the riser.  
      After completing the experiments involving water, the coolant is switched to light naphtha, which is introduced in the base of the riser and in the 3 m injector mouth, cases (I) and (J), respectively.  
      In the cases where the coolant is introduced through the 3 m injector mouth, two effects take place: the catalyst circulation is increased between 5 to 20% and the temperature in the starting segment of the riser increased between 10 and 30° C., depending on both the cooling flow rate and the temperature at the end of the reactor.  
      In the cases where the coolant is introduced in the base of the riser, the only effect observed is the catalyst circulation increase.  
      In all cases where coolant is injected, there is a coke yield enhancement, what already was expected due to the additional energy claim (Table 1). When the temperature is dropped from 540° C. to 505° C. and afterwards 495° C., the aromatic compounds content diminished to 57.2%, going to 50.2% and finally to 45.7%, but, the bottoms yield enhances from 15.1% to 29.4% and 43.2% respectively, confirming the sacrifice of the bottoms conversion expected in the traditional operation of medium maximization.  
      The coolant introduction through the 3 m injector mouth is able to restore the bottoms conversion at a smallest increase of the aromatic compounds content, as it displays the comparison between the cases (B) and (C) as well as (D) (E) and (F) (G) in Table 1.  
      The coolant introduction through the base of the riser leads to lower bottoms conversion and higher aromatic compounds content than the coolant introduction through the 3 m injector mouth, comparisons between (H) (F) and (I) (J).  
      The switching of gasoline to water as a coolant option showed the same effects observed previously for water. The recycled gasoline reacted to produce more LPG and less naphtha (despite the recycle) than the case without coolant and even in the case where the coolant used was water, comparison between the cases (E), (F) and (J), pointing out an interesting option for restoring the LPG production, which was lost due to the riser temperature reduction.  
      The results obtained are summarized in Table 1 as follows.  
                                                           TABLE 1                                   A   B   C   D   E   F   G   H   I   J                                                                                T reaction (° C.)   540   495   495   495   505   505   505   505   505   505       Flow rate, riser base   0   0   0   0   0   0   0   6   6   0       inlet (kg/h)       Flow rate, 3m   0   0   6   20   0   6   20   0   0   6       injector mouth (kg/h)       Coolant   —   —   water   water   —   water   water   water   naphtha   naphtha       T initial (° C.)   555   507   519   536   519   525   544   518   518   524       Conversion (w %)   69.8   41.9   46.0   57.1   53.8   56.9   64.1   54.7   55.5   56.1       Fuel gas (w %)   11.6   9.1   9.1   10.3   8.4   9.6   7.8   8.0   10.4   10.5       LPG (w %)   17.4   9.6   1.1   14.2   12.8   14.9   12.5   12.9   14.0   15.2       Naphtha (w %)   35.7   19.2   21.5   28.1   28.0   28.4   33.5   29.2   26.8   26.2       LCO (w %)   15.2   15.0   14.9   14.8   16.9   15.8   15.5   17.0   15.3   15.2       Bottoms (w %)   15.1   43.2   39.1   28.2   29.4   27.3   20.3   28.3   29.2   28.7       Coke (w %)   5.0   4.0   4.4   4.9   4.3   4.5   5.2   4.5   4.5   4.5       SFC (100-Aro) (w %)   42.8   54.3   54.3   52.2   49.8   50.2   48.0   49.6   49.6   51.1       SFC Mono-Aro (w %)   23.5   18.3   18.1   18.6   20.1   20.2   21.4   20.5   21.0   56.7       SFC Di-Aro (w %)   23.1   18.5   18.1   19.4   19.8   19.5   20.3   20.0   19.5   18.9       SFC Tri-Aro (w %)   7.5   6.4   6.4   6.5   6.8   6.7   6.8   6.5   6.4   6.2       SFC Poly-Aro (w %)   3.2   2.4   3.1   3.3   3.4   3.3   3.5   3.3   3.4   3.3                 SFC = Supercritical Fluid Chromatography.             
 
      The foregoing comments and examples prove, therefore, that according to the invention, the introduction of a coolant stream between 25% (¼) and 50% (½) of the riser height, and preferably with a proper flow rate, ensures higher conversion of residual oil without increasing the LCO aromatic compounds content, in comparison to other prior art processes.