Patent Publication Number: US-2020277530-A1

Title: Catalyst column and thermal cracking system

Description:
RELATED APPLICATIONS 
     This application is a national stage filing under section 371 of International Application No. PCT/CN2017/070398 filed on Jan. 6, 2017, which claims priority to U.S. provisional application No. 62/275,420 filed on Jan. 6, 2016, which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to a thermal cracking system. More particularly, the present invention relates to a thermal cracking system with catalyst tower. 
     Description of Related Art 
     In the below description, the meaning of the term “prior technique” is the same as “traditional technique” and is equivalent to the term “prior art”, “conventional technology”, or “conventional technique”, and so on. 
     Thermal cracking, also known as pyrolysis, has been deemed as a potential alternative way of disposal by the waste management industry. Processing hydrocarbon wastes with a thermal cracking reaction can be beneficial in that (a) it does not generate toxic pollutions as incineration and landfilling do, and (b) it produces valuable commodities such as liquid fuel, combustible gas and carbon black. A major bottleneck in thermal cracking of waste materials has been that the process is usually unable to deliver a desired liquid fuel quality within an acceptable production cycle time. A quality liquid fuel product requires adequate degree of reaction of the waste material. This generally results in a long production cycle which boosts the production cost. Several methods have been tried to solve this problem, whose pros and cons are summarized in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Prior art 
                 Pros 
                 Cons 
               
               
                   
                   
               
             
            
               
                   
                 (1) Subjecting 
                 Most ensured 
                 Highest cost due 
               
               
                   
                 premature liquid 
                 quality for 
                 to extra equipment 
               
               
                   
                 fuel to secondary 
                 the liquid 
                 and long overall 
               
               
                   
                 treatment such as 
                 fuel product 
                 production time 
               
               
                   
                 reactive distillation 
               
               
                   
                 (2) Modifying the 
                 Lowest extra 
                 Technically 
               
               
                   
                 catalyst 
                 cost 
                 difficult 
               
               
                   
                 formulation for 
               
               
                   
                 thermal cracking 
               
               
                   
                 reaction 
               
               
                   
                 (3) Integrating 
                 Successful 
                 Technically 
               
               
                   
                 a catalyst tower 
                 integration 
                 difficult 
               
               
                   
                 into the system 
                 is equivalent 
               
               
                   
                 to further 
                 to having a 
               
               
                   
                 treatment such 
                 built-in 
               
               
                   
                 as reformation 
                 reactive 
               
               
                   
                 or purification 
                 distillation 
               
               
                   
                 of gas molecules 
                 with lower 
               
               
                   
                 before they are 
                 cost than 
               
               
                   
                 collected as 
                 method (1). 
               
               
                   
                 liquid fuel 
               
               
                   
                   
               
            
           
         
       
     
     Theoretically, method (3) in Table 1 allows a solution that best balances technical difficulty and the resulting production cost. However, adding the catalyst tower to a thermal cracking process brings its own safety issues. When oil gas produced from the thermal cracking reaction passes the catalyst tower, the packed catalyst allows only a fraction of the gas stream to pass. This slows down the gas flow, thereby causing pressure buildup in the piping upstream of the catalyst tower as a result of oil gas accumulating at the entrance of catalyst bed. Several methods have been tried to combat this problem, but none has been successful, as summarized in Table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Prior Art 
                 Problem 
               
               
                   
                   
               
             
            
               
                   
                 Using gas extraction 
                 Costs incurred by extra 
               
               
                   
                 means (e.g. vacuum 
                 equipment; safety concerns 
               
               
                   
                 pump) to increase 
                 (e.g. mechanical failure 
               
               
                   
                 gas flow through 
                 in the extraction means 
               
               
                   
                 the catalyst tower 
                 during reaction) 
               
               
                   
                 Using a relief 
                 Frequent release of 
               
               
                   
                 valve to pass gas 
                 combustible oil gas may 
               
               
                   
                 to the open air in 
                 bring environmental as well 
               
               
                   
                 case of system 
                 as safety threats to the 
               
               
                   
                 overpressure. 
                 neighborhood. 
               
               
                   
                   
               
            
           
         
       
     
     Another bottleneck facing the commercialization of waste disposal with thermal cracking technology is the frequent maintenance downtime. For a thermal cracking system, when maintenance of individual components is needed, the whole system typically needs to be shut down in light of the risky coexistence of high temperature heat (for providing energy to the reactant) and combustible hot oil gas on site during operation. 
     Reference is now made to  FIG. 1 , which is a schematic diagram of a prior art thermal cracking system  100 . As shown in  FIG. 1 , in the thermal cracking system  100 , a catalyst tower  102  is set between a reactor  101  and a condenser assembly  103  which comprises sub-condensers  103 A,  103 B, and  103 C working in series. A thermal cracking reaction takes place inside the reactor  101  and produces oil gas. Oil gas from the reactor  101  enters the catalyst tower  102  and passes through a packed catalyst bed  105  where it gets reformed with the aid of catalyst in the catalyst bed  105 . The reformed oil gas then leaves for the sub-condensers  103 A,  103 B, and  103 C, where it gets condensed into a liquid fuel. The liquid fuel is then collected and stored in a storage vessel  106 . The catalyst bed  105  is held on a catalyst holding plate  104 . The catalyst holding plate  106  comprises a plurality of openings set thereon for oil gas to pass through. The catalyst tower  102  is typically tall and thin in shape. 
     The black arrow in  FIG. 1  depicts the piping network in the thermal cracking system  100 . As shown, the piping comprises three portions, with the first portion being between the reactor  101  and the catalyst tower  102 , the second portion being between the catalyst tower  102  and the condenser assembly  103  (including those being between individual sub-condenser  103 A,  1038  and  103 C), and the third portion being between the condenser assembly  103  and the storage vessel  104 . For convenience, the piping connecting the reactor  101  to the catalyst tower  102  will be referred to as the pre-catalyst tower pipping, and that connecting the catalyst tower  102  to the condenser assembly  103  will be referred to as the post-catalyst tower piping hereinafter, respectively. 
     The white arrows in the catalyst tower  102 , on the other hand, represents the direction of gas flow inside the catalyst tower  102 . 
     It should be noted that individual components of thermal cracking system  100  are not depicted in actual relations to one another in terms of size. For example, the catalyst tower  102  is not typically larger than the reactor  101 . Also, the piping in  FIG. 1  is represented by a thin line, but as will be appreciated by those skilled in the art, the thickness of the line does not imply the pipe size relative to other components of the thermal cracking system  100 , such as the reactor  101 , the catalyst tower  102  or the condenser  103 , in real practice. 
     It should also be noted that the pre-catalyst tower piping in  FIG. 1  is depicted to connect to the catalyst tower  102  from the side, but it is also a common practice that the connection is made at the bottom of the catalyst tower  102 . 
     SUMMARY 
     The present invention provides a catalyst tower which comprises a pressure buffer which is set upstream of the packed catalyst bed. The pressure buffer exploits one of the behaviors of a gaseous matter that the pressure it exerts typically drops when it expands in volume. The present invention also provides a thermal cracking system which implements this self-pressure buffering catalyst tower. The present invention enhances operational safety for the thermal cracking process. 
     According to one aspect of the invention, a thermal cracking system is provided, which comprises a reactor, a catalyst tower, a condenser, a set of pre-catalyst tower piping, and a set of post-catalyst tower piping. A catalyst holding plate is set in the catalyst tower. One end of the pre-catalyst tower piping is connected to the catalyst tower, while the other end to the reactor. One end of the post-catalyst tower piping is connected to the catalyst tower, while the other end to the condenser. The distance between a gas inlet of the catalyst tower and the catalyst holding plate is directly proportional to the diameter difference between the catalyst tower and a portion of the pre-catalyst tower piping which is in direct connection with the catalyst tower. 
     According to another aspect of the invention, a catalyst tower is provided, which comprises a gas inlet and a catalyst holding plate set therein. The gas inlet is the opening where the catalyst tower and the upstream piping connects with one another. The distance between the gas inlet and the catalyst holding plate is directly proportional to the difference in diameter between the catalyst tower and the upstream piping. 
     According to another aspect of the invention, a thermal cracking system is provided, which comprises a reactor assembly comprising at least two sub-reactors, a catalyst tower, a condenser, at least two sets of pre-catalyst tower piping, and a set of post-catalyst tower piping. For each set of the pre-catalyst tower piping, one end thereof is connected to the catalyst tower, and the other end is connected to one corresponding sub-reactor. On the other hand, one end of the post-catalyst tower piping is connected to the catalyst tower, and the other end to the condenser. 
     According to another aspect of the invention, a thermal cracking system is provided which comprises a reactor, a catalyst tower assembly comprising at least two sub-catalyst towers, a condenser, at least two sets of pre-catalyst tower piping, and at least two sets of post-catalyst tower piping. Each sub-catalyst tower is connected to one corresponding set of pre-catalyst piping at one end, and one corresponding set of post-catalyst piping at the other end. The pre-catalyst and post-catalyst tower piping are connected to the reactor and the condenser at the other end, respectively. 
     According to another aspect of the invention, a thermal cracking system is provided, which comprises a reactor, a catalyst tower, a condenser assembly comprising at least two sub-condensers, a set of pre-catalyst tower piping, and at least two sets of post-catalyst tower piping. The pre-catalyst tower piping is connected to the catalyst tower at one end and the reactor at the other end. For each set of the post-catalyst tower piping, one end thereof is connected to the catalyst tower, and the other end is connected to one corresponding sub-condenser. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows. 
         FIG. 1  is a schematic diagram of a prior art thermal cracking system. 
         FIG. 2A  is a schematic diagram of a thermal cracking system according to one embodiment of the invention. 
         FIG. 2B  is a schematic diagram of a thermal cracking system according to another embodiment of the invention. 
         FIG. 3A  is a schematic diagram of a thermal cracking system according to one embodiment of the invention. 
         FIG. 3B  is a schematic diagram of the sub-catalyst tower assembly of  FIG. 3A . 
         FIG. 3C  is a schematic diagram of a thermal cracking system according to another embodiment of the invention. 
         FIG. 3D  is a schematic diagram of a thermal cracking system according to yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The technical means adopted in the present invention for achieving intended purposes of the invention are further described below with accompanying drawings and specific embodiments. Those skilled in the related art can understand that the directional terms provided in the specific embodiments, such as up, down, left, right, front, or back etc., are used for elaboration with reference to the directions of the drawings only, but are not intended to limit the present invention. In addition, many variations and modifications can be made by those skilled in the related art without departing from the spirit and scope of the invention, and the practical examples derived therefrom are also within the scope of the invention. 
     The inventor has observed that for most prior art catalyst towers implemented in thermal cracking-based waste disposal processes, the problem of upstream pressure buildup results from a relatively small cross-sectional area selected for the catalyst tower. Rather than being designed from scratch, most conventional catalyst towers used in the thermal cracking of wastes are nothing more than a direct transplant from the designs used in the petrochemical industry where the catalyst tower has been deemed as a “mature” technology. This saves the R&amp;D costs, but also introduces side effects. The petrochemical industry typically is able to afford high-level pressure equipment which is a luxury the waste management industry in general does not have. Further, the chemical nature of both processes is different. For example, in thermal cracking of wastes, the composition and condition of feedstock is usually not well controlled. The reaction behavior thus varies from batch to batch, causing wide-ranging fluctuations in operating conditions including the system pressure. These fluctuations should be addressed by the design either on a component level or for the system as a whole. The catalyst tower is one of those components that should be designed with these factors in mind. 
     The inventor has discovered that one potential solution to the problem of upstream pressure buildup is to employ an increased cross-sectional area (i.e. an increased diameter) for the catalyst tower, so that the incoming oil gas, once inside the catalyst tower, expands in volume thereby rendering a decrease in its pressure. However, when a gas expands in volume, its temperature tends to drop, which may lead to premature condensation of the oil gas inside the catalyst tower, thereby hurting the system throughput. Further, a large catalyst tower usually means higher capital and operational costs. Therefore, a balance must be found between the effectiveness of the system design in pressure buffering and system economics. 
     Theoretical as well as empirical findings from the inventor that form the foundation of the present invention are described as follows. 
     In general, a catalyst tower of a smaller diameter has a holding plate with smaller area. When the same amount of catalyst is put on a smaller holding plate, a higher packed bed is resulted. This means more time is needed for the gas to pass through the bed, which is favorable for the occurrence of upstream pressure buildup. Therefore, a greater diameter may be used for the catalyst tower to lower the height of packed bed when the same amount of catalyst is used, and thereby help to combat the upstream pressure buildup. 
     An elevated location of the holding plate in the catalyst tower also helps in buffering upstream pressure, as it allows more space for the incoming gas to expand into. 
     Therefore, a thermal cracking system is provided. The system includes a reactor, a catalyst tower, and a condenser. A catalyst holding plate is set in the catalyst tower. A pre-catalyst tower piping connects the catalyst tower and the reactor, and a post-catalyst tower piping connects the catalyst tower and the condenser. The distance between a gas inlet of the catalyst tower and the catalyst holding plate is directly proportional to the diameter difference between the catalyst tower and a portion of the pre-catalyst tower piping which is in direct connection with the catalyst tower. 
     The diameter of catalyst tower in the prior art thermal cracking systems used for waste disposal is typically larger than that of the pre-catalyst tower piping, for example, mostly between 2 to 4 times of the diameter of pre-catalyst tower piping. Such diameter difference allows the incoming gas to expand to a certain degree when it enters the catalyst tower, but in general is not enabling sufficient expansion of gas to overcome the pressure buildup. 
     It should be noted that, the diameter may not be constant throughout the entire catalyst tower. For example, as shown in  FIG. 1 , the catalyst tower  102  contracts in cross-sectional area toward the end. For convenience, the diameter of a catalyst tower will be referred to hereinafter as the one belonging to a portion of the catalyst tower that accounts for the most of its body. Similarly, the diameter of the pre-catalyst tower piping may not be constant. For example, the end of the pre-catalyst tower piping connected with the reactor may be of one diameter, while the other end connected with the catalyst tower be of another. Thus, for convenience, the diameter of the pre-catalyst tower piping will be referred to hereinafter as the one of the opening where the pre-catalyst piping and the catalyst tower connects with each other, i.e. the gas inlet of the catalyst tower. 
     Other observations made by the inventor are described in Table 3 in conjunction with  FIGS. 2A and 2B   
     Reference is now made to  FIG. 2A  and  FIG. 2B , wherein  FIG. 2A  is a schematic diagram of a thermal cracking system  300  according to one embodiment of the invention, and  FIG. 2B  is a schematic of a thermal cracking system  309  according to another embodiment of the invention. It can be seen from  FIG. 2A  and  FIG. 2B  that the only difference between thermal cracking system  300  and  309  is the location of the connection point of the pre-catalyst tower piping ( 304 ) to the catalyst tower ( 302 ). In thermal cracking system  300  in  FIG. 2A , the connection is made at the bottom of the catalyst tower  302 , while in thermal cracking system  309 , the connection is made to the side of the catalyst tower  302 . It should be noted that, as can be appreciated by those skilled in the art, the pre-catalyst tower piping  304  may also be connected to the catalyst tower  302  at both locations at the same time without deviating from the concept of the invention. 
     As shown, both thermal cracking system  300  and  309  comprise a reactor  301 , a catalyst tower  302 , a condenser  303 , a set of pre-catalyst tower piping  304  with one end connected to the reactor  302  and the other end to the catalyst tower  302 , and a set of post-catalyst tower piping  305  with one end connected to the catalyst tower  302  and the other end to the condenser  305 . 
     The reactor  301  has a diameter  3011  and a length  3012 . The catalyst tower  302  has a diameter  3021  and a height  3025 . A catalyst holding plate  3022  is set in the catalyst tower  302 , wherein the diameter of the catalyst holding plate  3022  is substantially equal to that of the catalyst tower  302 . The catalyst holding plate  3022  is apart from the top of the catalyst tower  302  by a distance  3023 , and is apart from the connection point of the pre-catalyst tower piping  304  to the catalyst tower  302  by a distance  3024 . In other words, the height  3025  of the catalyst tower  302  is substantially the sum of distance  3023  and  3024 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Design 
                   
               
               
                   
                 Parameter 
                 Inventor&#39;s Findings 
               
               
                   
                   
               
             
            
               
                   
                 1. Capacity 
                 The larger capacity of reactor 301, 
               
               
                   
                 of reactor 
                 the greater rate at which gas is 
               
               
                   
                 301 
                 generated therefrom. Thus, a larger 
               
               
                   
                   
                 capacity for catalyst tower 302 is 
               
               
                   
                   
                 needed to ensure sufficient 
               
               
                   
                   
                 pressure buffer. 
               
               
                   
                 2. Capacity 
                 A large catalyst tower capacity 
               
               
                   
                 of catalyst 
                 generally provides more space 
               
               
                   
                 tower 302 
                 for gas to expand into, thereby 
               
               
                   
                   
                 allowing greater buffering 
               
               
                   
                   
                 effect. However, the cost of 
               
               
                   
                   
                 catalyst tower 302 increases 
               
               
                   
                   
                 exponentially with its size. 
               
               
                   
                   
                 Further, excessive expansion may 
               
               
                   
                   
                 lead to premature condensation 
               
               
                   
                   
                 of gas due to the dropping 
               
               
                   
                   
                 temperature of gas at expansion. 
               
               
                   
                   
                 For example, for gas with certain 
               
               
                   
                   
                 compositions, condensation occurs 
               
               
                   
                   
                 when its temperature drops below 
               
               
                   
                   
                 250° C. This could hurt the system 
               
               
                   
                   
                 yield. 
               
               
                   
                 3. Diameter 
                 A larger catalyst tower diameter 
               
               
                   
                 3021 of 
                 3021 generally results in more 
               
               
                   
                 catalyst 
                 space for gas to expand into, 
               
               
                   
                 tower 302 
                 thereby allowing greater buffering 
               
               
                   
                   
                 effect. However, when diameter 
               
               
                   
                   
                 3021 increases, the height 3025 of 
               
               
                   
                   
                 catalyst tower also needs to 
               
               
                   
                   
                 increase (see point 4), resulting 
               
               
                   
                   
                 in a higher cost as described in 
               
               
                   
                   
                 point 2 above. 
               
               
                   
                 4. 3024 
                 The larger diameter of catalyst 
               
               
                   
                 vs. 3021 
                 tower 3021, the longer distance 
               
               
                   
                   
                 3024 should be used for enabling 
               
               
                   
                   
                 gas to fully expand prior to 
               
               
                   
                   
                 reaching the holding plate 3022, so 
               
               
                   
                   
                 that it gets to contact the catalyst 
               
               
                   
                   
                 at the center as well as the outer 
               
               
                   
                   
                 portion of holding plate 3022. This 
               
               
                   
                   
                 ensures all catalyst in the packed 
               
               
                   
                   
                 bed is made use of, thereby 
               
               
                   
                   
                 contributing to the system economics. 
               
               
                   
                   
                 A tradeoff exists though, that the 
               
               
                   
                   
                 catalyst tower height 3025 increases 
               
               
                   
                   
                 with an increasing distance 3024, 
               
               
                   
                   
                 resulting in a larger catalyst tower 
               
               
                   
                   
                 302 thus a higher cost as described 
               
               
                   
                   
                 in point 2. 
               
               
                   
                   
               
            
           
         
       
     
     Based on these observations, catalyst towers embodying the principles of the invention are provided with the objective of rendering a balance between the effectiveness of system in pressure buffering and system economics in mind. These embodiments and implementation thereof in thermal cracking systems are described as below. 
     In one embodiment of the invention, the diameter  3021  of the catalyst tower  302  is a function of that of the pre-catalyst tower piping  304 . As mentioned above, the diameter difference between the catalyst tower  302  and pre-catalyst tower piping  305  is generally not large enough to enable sufficient expansion of gas to overcome the pressure buildup. In one preferred example of this embodiment, the diameter  3021  of catalyst tower is equal to or greater than 4.5 times that of the pre-catalyst tower piping  304 . For instance, a diameter of 8 inches (approx. 0.2 meters) is one of the common pipe sizes used for pre-catalyst tower piping  304  in the prior art thermal cracking systems for waste disposal. In such case, the catalyst tower according to this embodiment comprises a diameter equal to or greater than 9 meters. 
     In one embodiment of the invention, the diameter  3021  of the catalyst tower  302  is determined based upon a predetermined ratio of the capacity of catalyst tower  302  to reactor  301 . The general rule is that the ratio of diameter of catalyst tower  302  to reactor  301  (i.e.  3021 / 3011 ) increases with a decreasing ratio of capacity of catalyst tower  302  to reactor  301 . In other words, the more the reactor  301  oversizes the catalyst tower  302  in capacity, the larger diameter is employed for the catalyst tower  302 . 
     In one example of this embodiment, the capacity of catalyst tower  302  is selected to be 0.07˜0.13 ( 1/15˜⅛) that of reactor  301 . And this may be optionally implemented together with a corresponding value of  3021 / 3011  being 1˜0.33 (1˜⅓). More preferably, the capacity of catalyst tower is selected to be 0.09˜0.11 ( 1/11 to 1/9) that of reactor  301 , which may also be optionally implemented together with a corresponding value of  3021 / 3011  being 1˜0.5 (1˜½). 
     In one embodiment of the invention, the reactor  301  is a rotary kiln reactor with a diameter of 2.8 meters and a length of 6.6 meters (resulting a capacity of approximately 40 cubic meters). The feedstock is selected from one of the following, or combinations thereof: plastics, tire and waste oils. The reactor  301  is selected to operate between room temperature and 420° C. A matrix of design parameter combinations and associated findings are presented in Table 4. Parameters investigated include reactor temperature, diameter  3021  of catalyst tower  302 , ratio of  3024  to  3021  (i.e.  3024 / 3021 ) and the capacity of catalyst tower  302 . 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                 Capacity of 
                   
               
               
                 Temperature 
                   
                   
                 catalyst 
                 Pressure at 
               
               
                 of reactor 
                 3021 
                   
                 tower 302 
                 the gas 
               
               
                 301 
                 (m) 
                 3024/3021 
                 (m 3 ) 
                 inlet (MPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 300~360° C. 
                 2.8 
                 2.8/2.8 = 1 
                 23.70 
                 &lt;0.01 
               
               
                 300~360° C. 
                 2.52 
                 2.52/2.8 = 0.9 
                 17.28 
                 &lt;0.01 
               
               
                 300~360° C. 
                 2.1 
                 2.1/2.8 = 0.75 
                 13.33 
                 &lt;0.01 
               
               
                 300~360° C. 
                 1.8 
                 1.8/2.8 = 0.64 
                 6.30 
                 &lt;0.01 
               
               
                 300~360° C. 
                 1.6 
                 1.6/2.8 = 0.57 
                 4.42 
                 &lt;0.01 
               
               
                 300~360° C. 
                 1.4 
                 1.4/2.8 = 0.5 
                 2.96 
                 &lt;0.01 
               
               
                 300~360° C. 
                 1.2 
                 1.2/2.8 = 0.43 
                 1.87 
                 &lt;0.01 
               
               
                 300~360° C. 
                 0.93 
                 0.93/2.8 = 0.33 
                 0.88 
                 &lt;0.01 
               
               
                 300~360° C. 
                 0.8 
                 0.8/2.8 = 0.29 
                 0.55 
                 &gt;0.01 
               
               
                 300~360° C. 
                 0.7 
                 0.7/2.8 = 0.25 
                 0.37 
                 &gt;0.01 
               
               
                   
               
            
           
         
       
     
     As shown in Table.  4 , with the tried parameter combinations, the pressure at gas inlet of the catalyst tower  302  (which substantially equals to the pressure in the pre-catalyst tower piping  304 ) is below 0.01 Mpa except for the last two data sets where  3021  is less than ⅓ of the reactor diameter. 
     As mentioned in point 4 of Table 3, a longer distance  3024  should be employed for catalyst tower  302  in the case of a larger diameter  3021  thereof. This can be seen from Table 4, where the pressure at gas inlet is consistently kept low when the value of  3024 / 3021  is selected to increase with an increasing  3021 . A few embodiments of the invention are therefore developed based upon the design parameter of  3024 / 3021 . In one embodiment, the value of  3024 / 3021  is equal to or greater than 0.4. More preferably, the value of  3024 / 3021  is equal to or greater than 0.5. This design parameter can also be implemented together with other design parameters to provide a method for use in designing the overall system. For example, in one embodiment, a thermal cracking system is provided, which comprises a reactor, a catalyst tower with a catalyst holding plate set therein, and a condenser. The system further comprises a set of pre-catalyst tower piping and a set of post-catalyst tower piping, where one end of the pre-catalyst tower piping is connected to the reactor and the other end to the catalyst tower, while one end of the post-catalyst tower piping is connected to the catalyst tower and the other end to the condenser. The thermal cracking system is characterized in that the distance between the holding plate and the connection point of the pre-catalyst tower piping to the catalyst tower (i.e.  3024 ) is directly proportional to the difference in diameter between the pre-catalyst tower piping and the catalyst tower. 
     Reference is now made to  FIG. 3A , which is a schematic diagram of a thermal cracking system  310  according to one embodiment of the invention. In this embodiment, thermal cracking system  310  is based on those systems described in conjunction with  FIG. 2A  and  FIG. 2B . As shown in  FIG. 3A , in thermal cracking system  310 , the catalyst tower  302  is an assembly which comprises at least one sub-catalyst tower, namely  302 A,  302 B, etc. It should be noted that, in  FIG. 3A  three sub-catalyst towers are depicted, but as will be appreciated by those skilled in the art, the number of the sub-catalyst tower is not limited to three, other number can be implemented without deviating from the invention concept. 
     In this embodiment, the design capacity of catalyst tower assembly  302  is equivalent to the sum of that of individual sub-catalyst towers  302 A,  302 B and  302 C. The capacity of individual sub-catalyst towers may be identical or not. Also in this embodiment, the pre-catalyst tower piping  304  refers to the piping network which connects the reactor  301  to the catalyst tower assembly  302 , and the post-catalyst tower piping  305  refers the piping network which connects the catalyst tower assembly  302  to the condenser  303 . The sub-catalyst towers  302 A,  302 B and  302 C work in parallel: oil gas coming from the reactor  301  is free to enter any of the sub-catalyst towers simultaneously. Then, gas streams from individual sub-catalyst towers join into one single stream in the post-catalyst tower piping  305  and continue to flow towards the condenser  305 . 
     In one example of this embodiment, each of the diameter  3021 A of the sub-catalyst tower  302 A,  3021 B of the sub-catalyst tower  302 B, and  3021 C of the sub-catalyst tower  302 C, is equal to or greater than ⅓ of the diameter  3021  in  FIGS. 2A and 2B . 
     In one example of this embodiment, the diameter of the pre-catalyst tower piping  304  is equal to that of the pre-catalyst tower piping  304  in  FIGS. 2A and 2B . 
     Reference is now made to  FIG. 3B , which is a schematic diagram of the catalyst tower assembly  302  in  FIG. 3A . Individual sub-catalyst towers  302 A,  302 B and  302 C comprises a catalyst holding plate set therein, namely  3022 A,  3022 B and  3022 C, respectively. As shown in  FIG. 3B , in the sub-catalyst tower  302 A, the distance between the catalyst holding plate  3022 A and the gas inlet (i.e. the connection point of the sub-catalyst tower  302 A with the pre-catalyst tower piping  304 ) is  3024 A. Similarly, the distance between the catalyst holding plate  3022 B and the gas inlet (i.e. the connection point of the sub-catalyst tower  302 B with the pre-catalyst tower piping  304 ) is  3024 B, and the distance between the catalyst holding plate  3022 C and the gas inlet (i.e. the connection point of the sub-catalyst tower  302 C with the pre-catalyst tower piping  304 ) is  3024 C. In one example of this embodiment, the value of  3024 A/ 3021 A,  3024 B/ 3021 B and  3024 C/ 3021 C is equal or greater than 0.4. More preferably, the value of  3024 A/ 3021 A,  3024 B/ 3021 B and  3024 C/ 3021 C is equal or greater than 0.5. In this embodiment, if the diameter of the pre-catalyst tower piping  304  is selected to be equal to that of the pre-catalyst tower piping  304  in  FIGS. 2A and 2B , the optimal value of  3024 A/ 3021 A,  3024 B/ 3021 B and  3024 C/ 3021 C may be less than that of  3024 / 3021  in  FIGS. 2A and 2B , because in such case the difference in diameter of sub-catalyst tower  302 A,  302 B and  302 C with the pre-catalyst tower  304  is smaller as compared to that in  FIGS. 2A and 2B . 
     The thermal cracking system  310  is characterized in that when any of the sub-catalyst tower  302 A,  302 B and  302 C needs to be serviced or have a catalyst change, the system does not need to be shut down completely. As shown in  FIG. 3A , the pre-catalyst tower piping  304  may further comprise sub-control valves  3042 A,  3042 B and  3042 C, with each being set on the piping leading to the sub-catalyst tower  302 A,  302 B and  302 C, respectively. When maintenance or a catalyst change is to be performed to the sub-catalyst tower  302 A, the sub-control valve  3042 A is closed and  3042 B and  3042 C are opened, such that oil gas only enters the sub-catalyst tower  302 B and  302 C. As a result, production may continue by employing sub-catalyst towers  302 B and  302 C when the sub-catalyst tower  302 A is being serviced. Similarly, production may continue by employing sub-catalyst towers  302 A and  302 C when the sub-catalyst tower  302 B is being serviced, and may continue by employing sub-catalyst towers  302 A and  302 B when the sub-catalyst tower  302 C is being serviced. 
     The benefits of the thermal cracking system  310  include cost saving allowed by the light and modular sub-catalyst towers  302 A,  302 B and  302 C. When it is required to increase the overall capacity of the system, additional sub-catalyst towers  302 A,  320 B and  302 C can be added to the original system instead of replacing the single large catalyst tower with a larger one. Similarly, a reduced overall capacity can be rendered by simply shut one or more of the sub-control valves thereby disabling one or more of the sub-catalyst towers. 
     Reference is now made to  FIG. 3C , which is a schematic diagram of a thermal cracking system  320  according to another embodiment of the invention. Thermal cracking system  320  is a variant of thermal cracking system  310  in  FIG. 3A . As shown in  FIG. 3C , in thermal cracking system  320 , the condenser  303  is an assembly which comprises at least one sub-condenser, namely  303 A,  303 B, etc. It should be noted that, in  FIG. 3C  three sub-condensers are depicted, but as will be appreciated by those skilled in the art, the number of the sub-condenser is not limited to three, other number can be implemented without deviating from the invention concept. In this embodiment, the design capacity of condenser assembly  303  is equivalent to the sum of that of individual sub-condensers  303 A,  303 B and  303 C. The capacity of individual sub-condensers may be identical or not. In one example of this embodiment, each of sub-condenser has a capacity equal to ⅓ of the design capacity of the condenser assembly  303 . 
     Also in this embodiment, the pre-catalyst tower piping  304  refers to the piping network which connects the reactor  301  to the catalyst tower assembly  302 , and the post-catalyst tower piping  305  refers to the piping network which connects the catalyst tower assembly  302  to the condenser assembly  303 . As shown in  FIG. 3C , in this embodiment, each of the sub-catalyst tower  302 A,  302 B and  302 C is connected to a corresponding sub-condenser  303 A,  303 B and  303 C through the post-catalyst tower piping  305 . The white two-way arrows in the post-catalyst tower piping  305  indicate that oil gas is allowed to move freely in both directions in the pipe. In other words, the sub-condensers  303 A,  303 B and  303 C work in parallel. If control valves are set at the location of the white arrows to block gas flow, each of the sub-catalyst tower/sub-condenser pair  302 A/ 303 A,  302 B/ 303 B, and  302 C/ 303 C becomes independent of one another. 
     The thermal cracking system  320  is characterized in that when any of the sub-condenser  303 A,  303 B and  303 C needs to be serviced, the system does not need to be shut down completely. As shown in  FIG. 3C , the post-catalyst tower piping  305  may further comprise sub-control valves  3052 A,  3052 B and  3052 C, with each being set on the piping leading to the sub-condenser  303 A,  303 B and  303 C, respectively. When maintenance is to be performed to the sub-condenser  303 A, the sub-control valve  3052 A is closed and  3052 B and  3052 C are opened, such that oil gas only enters the sub-condenser  303 B and  303 C. As a result, production may continue by employing sub-condensers  303 B and  303 C when the sub-condenser  303 A is being serviced. Similarly, production may continue by employing sub-condensers  303 A and  303 C when the sub-condenser  303 B is being serviced, and may continue by employing sub-condensers  303 A and  303 B when the sub-condenser  303 C is being serviced. 
     Reference is now made to  FIG. 3D , which is a schematic diagram of a thermal cracking system  330  according to yet another embodiment of the invention. Thermal cracking system  330  is a variant of thermal cracking system  320  in  FIG. 3C . As shown in  FIG. 3D , in thermal cracking system  330 , the reactor  301  is an assembly which comprises at least one sub-reactor, namely  301 A,  301 B, etc. It should be noted that, in  FIG. 3D  three sub-reactors are depicted, but as will be appreciated by those skilled in the art, the number of the sub-reactor is not limited to three, other number can be implemented without deviating from the invention concept. In this embodiment, the design capacity of reactor assembly  301  is equivalent to the sum of that of individual sub-reactors  301 A,  301 B and  301 C. The capacity of individual sub-reactors may be identical or not. In one example of this embodiment, each sub-reactor has a capacity equal to ⅓ of the design capacity of the reactor assembly  301 . Also in this embodiment, the pre-catalyst tower piping  304  refers to the piping network which connects the reactor assembly  301  to the catalyst tower assembly  302 , and the post-catalyst tower piping  305  refers to the piping network which connects the catalyst tower assembly  302  to the condenser assembly  303 . 
     The thermal cracking system  330  is characterized in that when each of the sub-reactor  301 A,  301 B and  301 C needs to be serviced, the system does not need to be shut down completely. As shown in  FIG. 3D , the pre-catalyst tower piping  304  may further comprise sub-control valves  3042 A,  3042 B and  3042 C, with each being set on the piping leading to the sub-catalyst tower  302 A,  302 B and  302 C, respectively. When maintenance is to be performed to the sub-reactor  301 A, the sub-control valve  3042 A is closed and  3042 B and  3042 C are opened, such that oil gas only enters the sub-catalyst tower  302 B and  302 C. As a result, production may continue by employing sub-reactors  301 B and  301 C when the sub-reactor  301 A is being serviced. Similarly, production may continue by employing sub-reactors  301 A and  301 C when the sub-reactor  301 B is being serviced, and may continue by employing sub-reactors  301 A and  301 B when the sub-reactor  301 C is being serviced. 
     One of the benefits provided by thermal cracking system  330  is flexibility in use. In  FIG. 3D , the white two-way arrows in the pre-catalyst tower piping  304  and post-catalyst tower piping  305  indicate that oil gas is allowed to move freely in both directions in the pipe. In other words, the sub-catalyst tower  302 A,  302 B and  302 C work in parallel, and so do the sub-condensers  303 A,  303 B and  303 C. If control valves  3042 A and  3042 B are closed, the resulting configuration of thermal cracking system  330  is substantially equivalent to the thermal cracking system  320  described in conjunction with  FIG. 3C . If, in addition to control valves  3042 A and  3042 B,  3052 A and  3052 B are also closed, the resulting configuration of thermal cracking system  330  is substantially equivalent to the thermal cracking system  310  described in conjunction with  FIG. 3A . 
     The embodiments in  FIG. 2  and  FIG. 3  can be combined. For example, if the embodiment in  FIG. 3A  is to be integrated into that in  FIG. 3D , then individual sub-catalyst tower  302 A,  302 B or  30 C in thermal cracking system  330  in  FIG. 3D  would become an assembly which comprises at least one secondary sub-catalyst tower e.g.  302 AA, etc. 
     It should be noted that, although in the embodiments and examples mentioned above the reactor and catalyst tower are described in the shape of cylinders, other shapes may be implemented without deviating from the invention concept. For example, if the reactor and catalyst tower are implemented in the shape of cuboid, then the dimensions of a cuboid that has similar significance to those of cylinder may be used as design parameters. For example, the length of cuboid may be used in place of the length of cylinder, and the face diagonal of cuboid may be used in place of the diameter of cylinder. 
     While the invention has been described in various embodiments in the above, they are not intended to limit the invention. Any changes and modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. In view of foregoing, it is intended that the scope of the invention is defined by the following claims.