Abstract:
This invention relates to a new two-stage riser catalytic cracking process, particularly to an improvement of the conventional riser reactor and reaction-regeneration system by application of a two-stage riser reactor to fulfill the aims of the concatenation of oil-vapor, catalyst in relays as result in shortening reaction time and increasing average performance of the catalyst.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable  
         STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    This invention relates to a new two-stage riser catalytic cracking process, particularly to an improvement of the conventional riser reactor and reaction-regeneration system. It uses a two-stage riser reactor to fulfill the aims of the concatenation of oil-vapor, catalyst in relays as result in shortening reaction time and increasing average performance of the catalyst.  
           [0005]    2. Background Art  
           [0006]    In the petroleum processing industry, the catalytic cracking technology is one of the most important processes. In the prior arts of current catalytic cracking, till today, it follows the early riser reactor and reaction-regeneration system. All the riser reactors are very long, mostly 30˜36 meters. And some of them are even more than 40 m. In the riser reaction-regeneration system, preheated feedstock enters a riser reactor through the feed nozzle. It comes into contact with the high-temperature catalyst coming out of a regenerator and was vaporized. Then reactions happen. The oil-vapor carrying catalyst flows along the riser upwards at an average linear velocity of about 10 m/s. It reacts while it flows, which takes about 3 seconds. During the reaction procedure, coke is generated and deposits on the surface and the active center of catalyst so that the activity and selectivity of catalyst drop rapidly. For this reason, the coked catalyst must be separated from the oil-vapor in time. And it should enter the regenerator to be regenerated for recycling application, which forms a circuit of catalyst. The oil-vapor enters a distillation system to separate into products. The slurry oil, which is not converted into light fractions (generally called recycle oil) after the reaction, enters the riser reactor again. It is the basic process of the catalytic cracking reaction-regeneration system. Due to the especial characteristics of heavy oil, varieties of difficulties are brought into the catalytic cracking processing. In recent years, the development of the catalytic cracking technology was focused mainly on the RFCC technology. In the prior art, lots of revamps have been made before or after the riser reactor and achieved certain positive effects. The following are some examples of main new technologies and their functions:  
           [0007]    One example is the atomization technology of heavy feed (nozzle). It improves the contact state between feedstock and catalyst and to enhance the yield of light oil.  
           [0008]    A second example is the gas-solid high-speed separation technology at the outlet of risers to separate the gas-solid quickly to reduce the over cracking reaction.  
           [0009]    A third example is the high-efficiency multi-stage stripping technology of spent catalyst. This technology increases stripping effects, reduce the yield of coke and increase the yield of light oil.  
           [0010]    A fourth example is the two-stage high-efficiency regeneration technology. The technology enhances the burning of coke, reduce coke content on regenerated catalyst and maintain the activity of catalyst.  
           [0011]    A fifth example is the reaction termination technology. This technology shortens the reaction time, reduce the harmful secondary reactions and enhance the yield of light oil.  
           [0012]    A sixth example is the multi-position of the feedstock injection technology. It treats different feedstock with different characteristics and optimizes the reaction process.  
           [0013]    A seventh example is the millisecond catalytic cracking technology. It also shortens the reaction time and decreases the detrimental secondary reactions. (But it is still in R &amp; D phase.)  
           [0014]    An eighth example is the down flowed riser technology which is now still in R &amp; D phase.  
           [0015]    In the prior arts , except that the last two items involved the changing of the riser reactor, those improvements didn&#39;t change the general structure of the current riser reactor.  
           [0016]    The most major disadvantage of the current riser reactor is that the riser is too long, which leads to an overlong reaction time of about 3 seconds. It has been proved that the current catalyst activity at the outlet of the riser is only about ⅓ of the initial activity. After only one second&#39;s the reaction, the catalyst activity declines by 50%. Obviously, the activity and selectivity of catalyst has dropped to a fairly low level in the second half of the riser reactor so that catalysis is weakened and the thermal cracking reactions and detrimental secondary reactions are promoted. This kind of situation not only limits the enhancement of the single-pass conversion of the feedstock, but also makes the content of olefin in FCC gasoline very high (45˜60%) which cannot meet the new requirements of gasoline standard.  
         BRIEF SUMMARY OF THE INVENTION  
         [0017]    The aim of this invention is to avoid the shortcomings of the above-mentioned technologies and to provide a new two-stage riser catalytic cracking process. It adopts a two-stage riser reactor and a double-loop reaction-regeneration system to realize the concatenation of oil-vapor, the relay of catalyst, shortened reaction time and increased average performance of catalyst. The invention is characterized by application of a two-stage riser reactor and a double-loop catalyst regeneration system. It makes the process a brand new one with the concatenation of oil-vapor in two-stage riser and the relay of catalyst along two routes.  
           [0018]    In order to realize the above-mentioned aims of this invention, the inventors carry out the brand new reaction technology in the following steps: The high-temperature catalyst coming from the regenerator ( 2 ) enters firstly into the bottom of the first stage of riser ( 6 ), and contacts with the feed stock ( 15 ) which is vaporized immediately. After about one second&#39;s reaction, the mixture of oil-vapor and catalyst enter an intermediate separator ( 4 ) to separate the partially coked catalyst. After being stripped in the stripping section ( 5 ), the partially coked catalyst flows into the regenerator ( 2 ) to complete the first cycle. The oil-vapor coming from the intermediate separator ( 4 ) enters the second stage riser ( 7 ) to contact with the hot catalyst from the external heat absorber ( 3 ). The oil-vapor carrying the catalyst flows upwards and performs catalytic cracking reactions continuously. After reaction, the mixture of oil-vapor and catalyst enter the settler ( 1 ) to separate. The oil-vapor flows out into the fractionation system while the catalyst returns to the regenerator ( 2 ) after it is stripped, which completes the second cycle of catalyst. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
       [0019]    [0019]FIG. 1 is a schematic process flow chart of the present process, wherein the reference numerals in the appended drawing are described as follows:  1 =Reaction settler;  2 =Regenerator;  3 =External heat absorber;  4 =Intermediate separator;  5 =Stripping section;  6 =First stage riser;  7 =Second stage riser;  8 =Outlet of the oil-vapor;  9 =Outlet of the flue gas;  10 =Pre-lifting gas;  11 =Lifting medium of the catalyst;  12 =Air for fluidizing the catalyst;  13  =demineralized water;  14 =Saturated vapor;  15 =Feedstock;  16 =Air. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    In the reaction-regeneration system of two-stage riser catalytic cracking process, the high-temperature catalyst coming from the regenerator entered the bottom of the first stage riser ( 6 ) to contact with the feedstock, which was vaporized and then began to reaction. After about one second&#39;s reaction, the mixture of oil-vapor and catalyst entered the intermediate separator ( 4 ) to separate. After being stripped in the striping section ( 5 ), the partially coked catalyst returned to the regenerator ( 2 ) to complete the first cycle. The oil-vapor coming from the intermediate separator ( 4 ) entered the second stage riser ( 7 ) to contact with the hot catalyst coming from the external heat absorber ( 3 ). The oil-vapor carrying the catalyst flows upwards and performed catalytic cracking reactions continuously. After reaction, it entered the settler ( 1 ) to separate with the coked catalyst. The oil-vapor flew out into the fractionating system. At the same time, the spent catalyst returned to the regenerator ( 2 ) after it was stripped, which completed the second cycle.  
         [0021]    In the present invention, the changes brought by the two-stage riser reactor and the new reaction-regeneration system result in the replacement of the partially coked catalyst, which is separated from the reaction system between the two stages of the riser reactor, by new regenerated catalyst during the second half reaction. It fulfills the two-route cycle of the catalyst, concatenation of the oil-vapor, catalyst in relays, and short reaction time (the total reaction time being only ½˜⅔ of the conventional catalytic cracking one). So it enhances the average performance of the catalyst in the riser reactor (average activity and selectivity of the catalyst enhanced), strengthens and improves the catalytic cracking reaction process.  
         [0022]    The operating parameters of the two-stage riser FCC technology are similar to those of the conventional one. The reaction temperature is 470˜540 degree centigrade; the ratio of catalyst/oil is 4 to 9, and the total resident time of oil-vapor is 1˜2 seconds. According to the experimental data comparison between the single stage riser reactor and a two-stage one, some conclusions can be obtained as follows.  
         [0023]    Firstly, the single-pass feedstock conversion is increased by 8˜10 percents under the identical yield of light oil.  
         [0024]    Secondly, the yield of light oil is increased by 1˜2 percents, the yield of liquid is increased by 6˜8 percents, the dry gas is reduced by somewhat, and the coke is increased by 2 percents when the feedstock conversion is 8˜10 percents higher than that of the single stage riser reactor.  
         [0025]    Thirdly the quality of products is improved distinctly, and the content of olefin in catalytic gasoline is reduced greatly. Under proper conditions, it can be reduced by more than 15 percents (if it is compared with the data of the fluorescence method, it would be reduced by more, and the content of olefin is also decreased by more.). The content of isoalkane in gasoline can be increased by 6˜7 percents (with the analytical data of the chromatography). The content of aromatic hydrocarbon is increased by 5 percents. And the octane number of the FCC gasoline is increased by 1˜2 units.  
         [0026]    In comparison with the prior arts, the process of the present invention is able to strengthen and improve the reaction process of catalytic cracking of heavy oil, and also can increase the single-pass feedstock conversion greatly. It can obtain a better product distribution and enhance the yield of light oil and improve the quality of FCC gasoline distinctly as evidenced by both reducing the content of olefin and increasing the contents of isomeric hydrocarbon and aromatic hydrocarbon. The present process can also enhance the octane number. Thus, the new two-stage riser catalytic cracking process described in this invention has a wide range of application and introduction prospect.