Abstract:
The present invention relates to a method of production of light olefins, with the objective of maximizing the production of propylene and in particular ethylene by the use of a special catalyst containing high-silica zeolite, whose composition also includes a dehydrogenating metal, so as to generate light olefins and appreciable deposition of coke on the catalyst. Gains in selectivity for light olefins are observed, and at the same time the energy deficiency of catalytic cracking in petrochemical operations with light hydrocarbons is minimized, avoiding problems due to the need to burn heating oil in the catalyst regenerating section to make up for the energy deficit of the converter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of processes for the production of light olefins, more particularly ethylene and propylene, in circulating fluidized-bed reactors by means of catalytic conversion with solid acids, being applicable to a feed comprising light hydrocarbons. The method described in the present invention teaches the use of a special catalyst for producing light olefins and depositing an appreciable amount of coke on the catalyst. Besides increasing the selectivity for light olefins and maximizing the production of propylene and, in particular, ethylene, at the same time use of the method minimizes the energy deficiency of catalytic cracking in petrochemical operations with light hydrocarbons. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the fluidized-bed catalytic cracking process, the cracking reactions of hydrocarbons take place by contact of a feed with a catalyst in conditions of dynamic flow, in a tubular reactor in ascending flow, also known as riser, or in descending flow, also known as downflow, converting the feed into streams of lighter hydrocarbons of greater economic value. Thus, streams of hydrocarbons in petroleum refining with boiling points between 350° C. and 550° C. are converted to lighter hydrocarbons, predominantly constituted of gasoline with a distillation range between 35° C. and 220° C. The typical catalyst of the process has zeolite Y as the principal active component and the reaction temperatures, in the reactor, are around 540° C. at reactor outlet. 
         [0003]    In catalytic cracking managed for production of petrochemicals, the main objective of the process is the production of hydrocarbons of molecular weights even lower than those found in gasoline, mainly light olefins with two to four carbon atoms (C 2   =  to C 4   = ). To achieve this objective, the catalytic system is modified using a special component that is able to convert olefins of five to ten carbon atoms to lower olefins. The presence of this specific component, for example zeolite ZSM-5, in itself only increases the yields of light olefins, but results in much less deposition of coke than is produced with zeolite Y. Optionally, it is also possible to increase the reaction temperature, to a value that may exceed 600° C. at reactor outlet, to increase the yields of light olefins. 
         [0004]    The cracking reactions for production of light olefins are highly endothermic, drastically increasing the thermal demand in the reactor, which makes it difficult to meet the energy demand. In conventional catalytic cracking, this thermal demand is supplied by burning the coke that was deposited on the catalyst in the reaction section. The catalyst is burnt with air in a regenerating section at temperatures around 700° C. In this way, its catalytic activity is restored and the heated catalyst can be returned to the reaction section, to supply the heat necessary for the endothermic reactions. The problem of meeting the thermal demand is aggravated if the streams used as feed are formed by light hydrocarbons, such as diesel or naphtha, which usually deposit smaller amounts of coke on the catalyst compared with the heavier feeds such as vacuum gas oils or atmospheric residues. Nevertheless, besides the increase in the heat of reaction, the thermal demand is not met for three main reasons:
       1) the actual feed used is not a good precursor for formation of coke, having low Ramsbottom carbon residue,   2) light hydrocarbons are more refractory to cracking, requiring higher reaction temperatures;   3) conventional zeolites ZSM-5 do not produce much deposition of coke on the catalyst and the amount that must be used in the catalytic system is very large.       
 
         [0008]    The energy deficiency in catalytic cracking directed towards light olefins is normally supplied by burning torch oil, also known as heating oil, in the regenerator. With this approach, the regenerator is transformed to a combustor, where burning of the oil generates sufficient heat for heating the catalyst. The regenerator bed must be heated to temperatures around 700° C. Temperatures lower than 680° C. make it difficult to burn the heating oil in the regenerating bed and cause uncontrolled circulation of catalyst in the direction of the cracking reactor. The heat generated is transported to the reaction section, generally a riser, by the catalyst itself. The burning of oil in the regenerating bed promotes the development of various problems in operation of the regenerator. For example, the heating oil to be used must be selected carefully, as heating oils with very low distillation point can cause afterburning, i.e., combustion outside of the bed. The temperature differential between the bed and the combustion gases may reach 300° C., generating high temperatures in the cyclones and in other equipment inside and outside of the regenerator. Another problem that may arise is wear of the atomizers for introducing the heating oil into the regenerator. There may also be premature deactivation of the catalyst, owing to the generation of points of high temperature in the combustion bed. 
         [0009]    The patent literature discloses various methods for the production of light olefins by means of the use of solid catalysts, including those described in U.S. Pat. No. 4,980,053 and U.S. Pat. No. 6,210,562 and in the publication EP 0922744. However, none of these relates to solution of the problem of energy deficiency of the converter, since they use heavy hydrocarbons, generally atmospheric residues or heavy vacuum gas oils of paraffinic origin, as process feed. There is no reference to one that offers a method capable of solving the problem arising from the energy deficiency resulting from operations with light hydrocarbons. 
         [0010]    As described below, the present invention advantageously presents gains in selectivity for the production of light olefins, propylene and in particular ethylene, while at the same time minimizing the problems caused by the energy deficiency arising in the process. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a method of production of light olefins through the catalytic cracking of light hydrocarbons, for production of propylene and ethylene at high reaction temperature and with high catalyst/oil ratio by using a catalyst containing a solid acid, preferably a high-silica zeolite, whose composition also includes a dehydrogenating metal. More preferably, said solid acid is a zeolite in the pentasil family selected from the group ZSM-5, ZSM-8, and ZSM-11. 
         [0012]    In the present invention, light olefins are produced, such as ethylene and propylene, and moreover an appreciable amount of coke is deposited on the catalyst. Gains are observed in selectivity for light olefins, and at the same time the energy deficiency of catalytic cracking in petrochemical operations with light hydrocarbons is minimized, avoiding the problems caused by the burning of heating oil in the regenerating section of the catalyst to make up for the energy deficiency of the converter. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    In accordance with the method of the present invention, the reactions of catalytic cracking take place in a tubular reactor, with ascending or descending flow, where the catalyst in the form of solid particles is entrained by the vapours produced and by other auxiliary vapours introduced into the process, without the addition of hydrogen. The velocity of the vapours must be sufficient to ensure stable flow of the catalyst, performing injection of auxiliary vapour, called carrier vapour, below the point of feed injection, to convey the catalyst as far as the feed injection nozzles. The liquid feed, injected near the reactor bottom, evaporates and the subsequent chemical reactions form products that contribute to entrainment of the particles of catalyst that pass through the tubular reactor. A rapid cooling (quench) can be introduced in the intermediate section of the reactor if necessary. A series of cyclones separates the catalyst from the reaction products. After passing through the reactor, firstly, the catalyst is rectified by the injection of vapour, i.e. the more volatile hydrocarbons that were entrained by the catalyst are separated. Next, the coke deposited on the surface of the catalyst is burnt in the regenerator. Thus, the regenerated catalyst is obtained, and is returned to the beginning of the reactor at an elevated temperature. A new cycle of reactions begins in the process, when the regenerated catalyst comes in contact with a new feed introduced into the reactor. 
         [0014]    The method of the present invention not only maximizes the production of light olefins (propylene and in particular ethylene), but also generates a significant amount of coke. It comprises the following stages:
       1) supply of a feed constituted of hydrocarbons as the feed of a tubular reactor;   2) supply of a heated stream of special catalyst, the composition of which contains at least one dehydrogenating metal, M;   3) promotion of contact between the stream of hydrocarbons and the stream of special catalyst and making it possible for reactions to occur in the vapour phase, to obtain a mixture of hydrocarbons and coked catalyst;   4) separation of the catalyst from the products discharged from the reactor;   5) rectification of the coked catalyst with steam;   6) regeneration of the coked catalyst with air, directing it to a heating section, to permit combustion of the coke deposited on the catalyst and the burning of torch oil, with a stream of air, and to obtain a heated catalyst capable of supplying the process with the necessary energy;   7) returning the regenerated and heated catalyst to the tubular reactor to begin the process again;   8) directing the hydrocarbon stream generated to a separating and fractionating section, for separating the products obtained.       
 
         [0023]    The process feed can be constituted of streams from petroleum refining containing light hydrocarbons with boiling points above 150° C. The vapour-phase reactions must take place at temperatures between 550° C. and 750° C. and with catalyst/feed ratio between 10 and 50. 
         [0024]    The special catalyst of the present invention has a triple function; 
         [0025]    1) to convert paraffins to olefins by reactions of dehydrogenation, promoted by the dehydrogenating metal present in the special catalyst; 
         [0026]    2) to convert the olefins thus generated and other olefins present to lower olefins, with two, to four carbon atoms, increasing the yield of light olefins to the detriment of the yield of gasoline; 
         [0027]    3) to promote the formation of an amount of coke on the catalyst sufficient to supply the energy deficiency. 
         [0028]    Special catalysts are used in the present invention, for example catalysts with zeolites of type ZSM, such as ZSM-5, which have a pore size between 6 Å and 7 Å, or other zeolites of the pentasil family, such as ZSM-8 and ZSM-11. 
         [0029]    Moreover, any zeolites are used with high silica/alumina ratio, such as zeolites ZSM-12, ZSM-35, beta and mordenite. The special catalyst can be prepared by any method traditionally used for incorporation of metals, such as ion exchange, coprecipitation, impregnation on the zeolite before it is processed to the microsphere format, as well as deposition of metals during or after formation of microspheres. 
         [0030]    There are also various methods for incorporation of zeolites that are selective for olefins, in various matrices, forming microspheres suitable for the FCC process. These methods can be used for incorporating a zeolite ZSM-5 or a zeolite M/ZSM-5 (ZSM-5 modified with one or more than one dehydrogenating metal, M). 
         [0031]    M is selected from metals with high dehydrogenating power, preferably nickel, iron, manganese, cobalt, molybdenum and gallium. The amount of M varies between 1.0% and 15%, calculated as the percentage by weight of metal relative to the weight of catalyst. 
         [0032]    Maximization of the production of light olefins is confirmed by the higher yields of products leaving the reactor, by at least 10% for ethylene and 15% for coke, compared with the yields that are obtained by other methods, without the use of said special catalyst. 
         [0033]    The ethylene is separated from the stream of fuel gas (FG) and the propylene is separated from the stream of liquefied petroleum gas (LPG), thus identified in the tables of test results in the examples, providing evidence of the gains in selectivity obtained by the method taught in the present invention. 
         [0034]    Thus, the present invention relates to a method for fluidized-bed catalytic cracking of a stream of light hydrocarbons, which maximizes the production of light olefins, chiefly of propylene C 3   =  and in particular ethylene C 2   = , and at the same time produces an appreciable deposition of coke on the catalyst, lessens the energy deficiency of the converter and thereby reduces the need to burn heating oil in the regenerator. 
         [0035]    The use of a special catalyst, with high content of dehydrogenating metal, minimizes or even eliminates the need to burn heating oil in the regenerator. In addition, the special catalyst aids in the conversion of paraffins to olefins, which are promptly cracked in the pores of the catalyst and give rise to olefins of lower molecular weight. 
         [0036]    The reactions of dehydrogenation are decisive for the conversion of saturated hydrocarbons to olefins with more than five carbon atoms, precursors of light olefins, besides directly converting small saturated compounds to light olefins of low molecular weight. The gains achieved by application of the method of the present invention can be seen from the results obtained and presented in the following examples. The examples are only illustrative and do not constitute a restriction of the scope of this invention. 
       EXAMPLES 
       [0037]    First, two suspensions of modified zeolite were prepared. In the first, 1 kg of zeolite type ZSM-5 was added to 2.4 litres of 0.10-molar aqueous solution of chloride or nitrate of a dehydrogenating metal M. The suspension was held at 80° C. for 2 h, stirring slowly to prevent sedimentation (solutions with higher concentrations of M are used for altering this operation and generating modified zeolites with higher contents of M). This suspension of M/ZSM-5 was used for preparing samples of special catalyst A, B, and C. A suspension with the same zeolite ZSM-5, but without modification with metal M, was used for the preparation of two reference catalysts, R1 and R2. 
         [0038]    A special catalyst D was prepared using a second suspension of M/ZSM-5 obtained by an alternative method, which comprises another embodiment of the present invention. The suspension of M/ZSM-5 obtained previously was filtered, washed, and dried at 120° C. for 16 h. Then it was calcined at 500° C. for 1 h, obtaining M/ZSM-5 in powder form. This powder was suspended in water again, for preparing the special catalyst, D. In this way, suspensions with more suitable solids contents of M/ZSM-5 can make its use feasible in methods such as those mentioned previously. 
         [0039]    The special catalysts were prepared from a hydrosol containing a mixture of colloidal silica and colloidal alumina. The suspension of zeolite ZSM-5 or, as taught in the present invention, a suspension of M/ZSM-5, was added to the suspension of colloidal particles at a temperature below 50° C. Then a suspension of kaolin with solids content of 30% and a 30% w/w solution of phosphoric acid were added to the mixture. The final mixture was dried in a spray dryer. 
         [0040]    Samples of the catalysts prepared underwent hydrothermal treatment with 100% steam, at 788° C., for 5 hours in a fixed bed before each catalytic test. For carrying out the catalytic tests, an equilibrium catalyst obtained from an industrial unit was mixed with each catalyst sample prepared, and an ACE laboratory unit (made by Kayser Technology) was used. Maximization of the production of light olefins (C 2   =  and C 3   = ) by the catalytic cracking of a stream of gas oil was observed, and the process variables remained controlled. 
         [0041]    Table 1 shows the properties of the feeds used in the examples. 
         [0042]    Table 2 shows some characteristics of the catalysts investigated. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Characteristics of the feeds used  
               
               
                 in the catalytic tests 
               
             
          
           
               
                 Feed 
                 C1 
                 C2 
               
               
                   
               
             
          
           
               
                 °API 
                 16.9 
                 26.7 
               
               
                 Density 
                 0.9497 
                 0.8899 
               
               
                 Aniline Point (° C.) 
                 75.2 
                 65.2 
               
               
                 Total Sulphur (% w/w) 
                 0.79 
                 0.552 
               
               
                 Total Nitrogen (ppm) 
                 3185 
                 1870 
               
               
                 Basic Nitrogen (ppm) 
                 1254 
                 892 
               
               
                 Ramsbottom carbon residue (%) 
                 0.7 
                 — 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Characteristics of the catalysts prepared 
               
             
          
           
               
                 Catalyst 
                 R1 
                 R2 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
          
           
               
                 Chemical composition (%): 
                   
                   
                   
                   
                   
                   
               
               
                 SiO 2   
                 63.0 
                 62.3 
                 62.0 
                 60.5 
                 57.20 
                 64.0 
               
               
                 Na 2 O 
                 0.23 
                 0.38 
                 0.04 
                 0.00 
                 0.01 
                 0.03 
               
               
                 Al 2 O 3   
                 22.5 
                 23.4 
                 21.6 
                 20.3 
                 18.5 
                 20.5 
               
               
                 TiO 2   
                 0.72 
                 0.97 
                 0.62 
                 0.82 
                 0.68 
                 0.48 
               
               
                 Fe 2 O 3   
                 1.09 
                 0.83 
                 1.10 
                 1.13 
                 0.97 
                 0.96 
               
               
                 P 2 O 5   
                 1.21 
                 11.5 
                 13.92 
                 12.28 
                 11.89 
                 12.57 
               
               
                 NiO 
                 0.00 
                 0.00 
                 0.55 
                 5.35 
                 11.58 
                 1.46 
               
               
                 Textural properties: 
               
               
                 Micropore volume (mL/g) 
                 0.033 
                 0.030 
                 0.031 
                 0.046 
                 0.050 
                 0.030 
               
               
                 Area of mesopores (m 2 /g) 
                 19 
                 18 
                 16 
                 24 
                 51 
                 21 
               
               
                 Surface area (m 2 /g) 
                 89 
                 81 
                 82 
                 123 
                 159 
                 84 
               
               
                 Physical properties: 
               
               
                 d50% (μm) 
                 85 
                 — 
                 80 
                 75 
                 90 
                 76 
               
               
                   
               
             
          
         
       
     
       Example 1 
       [0043]    After the hydrothermal treatment, an amount of equilibrium catalyst E1, obtained from a commercial unit, equivalent to a weight ratio of 8% to 92% of E1, was mixed with each sample investigated. Catalytic tests were carried out using feed C1 at a temperature of 535° C. 
         [0044]    Table 3 presents the most important results of the catalytic tests. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Catalytic tests performed in the ACE unit with feed C1 
               
             
          
           
               
                   
                 Condition (mixture 92% E1 + 8% catalyst) 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
               
             
          
           
               
                   
                 Catalyst 
               
             
          
           
               
                   
                 R1 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
          
           
               
                 Conditions: 
                   
                   
                   
                   
                   
               
               
                 Reaction temperature (° C.) 
                 535 
                 535 
                 535 
                 535 
                 535 
               
               
                 Catalyst/feed ratio (w/w) 
                 5.0 
                 5.0 
                 5.0 
                 5.0 
                 5.0 
               
               
                 Balance relative to the feed: 
                   
                   
                   
                   
                   
               
               
                 FG—Fuel gas (% w/w) 
                 3.0 
                 3.1 
                 3.7 
                 3.4 
                 3.4 
               
               
                 LPG—Liquefied gas (% w/w) 
                 19.2 
                 19.6 
                 19.5 
                 19.3 
                 18.7 
               
               
                 Gasoline: C5 −220° C. (% w/w) 
                 33.8 
                 32.6 
                 32.6 
                 32.5 
                 33.1 
               
               
                 +220° C. (% w/w) 
                 39.8 
                 39.6 
                 38.6 
                 38.8 
                 39.7 
               
               
                 Coke (% w/w) 
                 4.2 
                 5.1 
                 5.6 
                 6.0 
                 5.1 
               
               
                 Coke relative to catalyst R1 
                 — 
                 +22% 
                 +32%  
                 +43% 
                 +22% 
               
               
                 Total (% w/w) 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                 Light olefins relative to the 
                   
                   
                   
                   
                   
               
               
                 feed: 
                   
                   
                   
                   
                   
               
               
                 Propylene (% w/w) 
                 7.6 
                 7.9 
                 7.9 
                 7.7 
                 7.7 
               
               
                 Ethylene (% w/w) 
                 1.14 
                 1.31 
                 1.6 
                 1.3 
                 1.41 
               
               
                 Ethylene relative to catalyst R1 
                 — 
                 +15% 
                 +40% 
                 +17% 
                 +24% 
               
               
                   
               
             
          
         
       
     
         [0045]    Comparing condition 1 with the other conditions 2 to 5, it can be seen that the method taught in the present invention (using modified zeolite) provides an increase in the yield of ethylene between 15% and 40%. Furthermore, there is an increase in the yield of coke between 22% and 43%, providing an increase in coke deposited on the catalyst. Therefore the method described provides gains in selectivity and in conversion in the production of light olefins, mainly propylene and in particular ethylene, by reactions of catalytic cracking, as well as offering additional gains in the energy balance of the unit, by generating extra coke. 
       Example 2 
       [0046]    After the hydrothermal treatment, an amount of equilibrium catalyst E2, obtained from a commercial unit, equivalent to a weight ratio of 80% to 20% of E2, was mixed with the sample investigated. It should be pointed out that in this example catalytic tests were carried out at 600° C., a higher reaction temperature within the range of reaction temperature employed for production of light olefins, and feed C2, typical of middle distillates with low coke forming potential. 
         [0047]    The results are shown in Table 4. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Catalytic tests performed in the ACE  
               
               
                 unit with feed C2 
               
             
          
           
               
                   
                 Condition  
               
               
                   
                 (mixture 80% catalyst + 20% E2) 
               
             
          
           
               
                   
                 (6) 
                 (7) 
               
             
          
           
               
                   
                 Catalyst 
               
             
          
           
               
                   
                 R2 
                 D 
               
               
                   
               
             
          
           
               
                 Conditions: 
                   
                   
               
               
                 Reaction temperature (° C.) 
                 600 
                 600 
               
               
                 Catalyst/feed ratio (w/w) 
                 5.0 
                 5.0 
               
               
                 Balance relative to the feed: 
                   
                   
               
               
                 FG—Fuel gas (% w/w) 
                 8.40 
                 11.24 
               
               
                 LPG—liquefied gas (% w/w) 
                 22.7 
                 17.4 
               
               
                 Gasoline: C5 −220° C. (% w/w) 
                 25.6 
                 28.0 
               
               
                 &gt;220° C. (% w/w) 
                 39.5 
                 39.0 
               
               
                 Coke (% w/w) 
                 3.77 
                 4.36 
               
               
                 Coke relative to catalyst R2 
                 — 
                 +16% 
               
               
                 Total (% w/w) 
                 100.0 
                 100.0 
               
               
                 Light olefins relative to the feed: 
                   
                   
               
               
                 Ethylene (% w/w) 
                 5.29 
                 6.01 
               
               
                 Ethylene relative to catalyst R2 
                 — 
                 +15% 
               
               
                   
               
             
          
         
       
     
         [0048]    Comparing condition (7) with condition (6), it can be seen that the method taught in the present invention (using modified zeolite) provided an increase of 15% in the yield of ethylene. Furthermore, there was an increase of 16% in the yield of coke, giving an increase in the coke deposited on the catalyst.