BACKGROUND OF THE INVENTION 
     The present invention relates to a process and system for the production of olefins and particularly to processing the charge gas feed to more effectively recover the product and process the by-products. 
     Ethylene, propylene and other valuable petrochemicals are produced by the thermal cracking of a variety of hydrocarbon feedstocks ranging from ethane to vacuum gas oils. In the thermal cracking of these feedstocks, a wide variety of products are produced ranging from hydrogen to pyrolysis fuel oil. The effluent from the cracking step, commonly called charge gas or cracked gas, is made up of this full range of materials which must then be separated (fractionated) into various product and by-product streams followed by reaction (hydrogenation) of at least some of the unsaturated by-products. 
     The typical charge gas stream, in addition to the desired products of ethylene and propylene, contains C 2  acetylenes, C 3  acetylenes and dienes and C 4  and heavier acetylenes, dienes and olefins as well as a significant quantity of hydrogen and methane. Aromatic as well as other ring compounds and saturated hydrocarbons are also present. 
     In U.S. Pat. No. 5,679,241 and U.S. patent application Ser. No. 10/202,702, filed Jul. 24, 2002, ethylene plant front-end catalytic distillation column systems are disclosed in which the highly unsaturated hydrocarbons, as acetylenes and dienes, are reacted with the contained hydrogen in the steam cracker charge gas compressor train to form olefins. In the process, it is desired to control the catalyst bed temperatures to as high a level as possible consistent with a low fouling rate. This maximum temperature minimizes the quantity of catalyst required. It can also increase overall selectivity to ethylene and propylene. The conditions that achieve the optimum catalytic distillation catalyst temperature can, however, result in a column bottoms temperature that is relatively high and can increase the fouling rate in the bottom of the column. While this fouling rate can be controlled by adding inhibitors, it is desirable to design the catalytic distillation hydrogenation system to achieve high catalyst bed temperatures while maintaining a low bottoms temperature and a low core fouling rate in the column system. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide and operate a front-end catalytic distillation hydrogenation system in an olefins plant to maximize the catalyst bed temperatures in the system while maintaining a low bottoms product temperature to reduce the fouling rate. The invention involves using two columns operating at different pressures. The catalytic reactor structures are in the first, high pressure column together with some fractionation zones. In the bottoms of the high pressure column, the temperature is regulated such that some lighter hydrocarbons remain. The bottoms from the high pressure column is sent to a second column which is a fractionator, operating at a lower pressure. The net bottoms product of this column is the net bottoms from the system. The temperature of this stream is low because total pressure of the column is low. The catalyst bed temperatures remain about the same as a single column, single pressure system but the bottoms temperatures in each of the columns is significantly lower. The net overhead of the low pressure column is totally condensed and sent back to the high pressure column. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow sheet for a front-end catalytic distillation hydrogenation system according to the prior art. 
         FIG. 2  is a flow sheet for a front-end catalytic distillation hydrogenation system according to the present invention illustrating two variations in the flow scheme. 
         FIG. 3  illustrates a portion of the system of  FIG. 2  but shows an alternate flow path for the return line from the low pressure column to the high pressure column. 
         FIG. 4  also illustrates a portion of the system of  FIG. 2  showing the addition of a heat recovery step in the overhead of the high pressure column. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For a better understanding of the present invention, a prior art front-end catalytic distillation hydrogenation system as represented by  FIG. 1  will be briefly described. As previously mentioned, such systems are disclosed and much more fully described in U.S. Pat. No. 5,679,241 and U.S. patent application Ser. No. 10/202,702. The objective of these systems is to remove a significant fraction of the hydrogen by hydrogenating the C 2  to C 5  diolefins and acetylenes without significant hydrogenation of the ethylene and propylene. In this system, the compressed charge gas  10 , which may be heated at  11 , is fed to the catalytic distillation hydrogenation column  12  which simultaneously carries out a catalytic reaction and distillation. The column  12  has a stripping section  14  below the feed  10  and a rectifying/reaction section  16  above the feed. Both of the sections contain distillation internals forming separation zones  18 ,  20  and  22  while the rectifying/reaction section  16  contains one or more catalyst beds forming a catalyst zone  24 . The column has a reboiler loop  26  and can also incorporate side condensers, interreboilers and pump-around either with or without heat exchange. None of these are shown but they are disclosed and shown in U.S. Pat. No. 5,679,241 and U.S. patent application Ser. No. 10/202,702 and can be utilized to enhance the performance of the dual pressure system in specific applications. 
     The bottoms liquid  28  when the column  12  is operated as a depentanizer contains the C 6  and heavier components and is usually used for gasoline processing. The column can also be operated as a debutanizer where the bottoms  28  is a C 5 + stream. The overhead vapor  30  from the column  12  passes through the condenser  32  and the partially condensed stream  34  is fed into the separation vessel  36 . Cooling water is the preferred cooling medium in condenser  32 . Vapor and liquid are separated and liquid reflux  38  is returned to the column  12 . The vapor  40  is further cooled at  42  and fed to the separation vessel  44  with the liquid  46  being recycled back and combined with the reflux liquid in the vessel  36 . The net vapor product  48  is then sent for further processing. 
     The prior art system of  FIG. 1  is a single pressure system in which the entire catalytic distillation hydrogenation operation is carried out in a narrow pressure range. The following Table 1 is an example of a material balance of such a prior art system operated as a depentanizer and lists the key operating parameters involved. In this table and the following tables and throughout this description, the pressures are given as the absolute pressures. In order to maintain a desirable catalyst bed temperature of approximately 125° C. when operating the column system at a single operating pressure of approximately 17 kg/cm 2 , with a feed gas obtained from cracking a typical naphtha at moderate cracking severity, it can be seen in Table 1 that the bottoms temperature is 203° C. At this temperature, it is possible that some fouling could occur without the use of inhibitors. If the system of  FIG. 1  were operated as a debutanizer, the specific data would vary but the bottoms temperature would still be high. This is because the debutanizer must operate at a higher pressure than a depentanizer for the column  12  overhead to be partially condensed by ambient temperature cooling medium. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Stream No. 
                 Stream 10 
                 Stream 48 
                 Stream 28 
                 Stream 38 
               
               
                   
               
             
             
               
                 Name 
                 Feed 
                 Net Overhead 
                 Bottoms 
                 Reflux 
               
               
                 Phase 
                 Vapor 
                 Vapor 
                 Liquid 
                 Liquid 
               
               
                 Fluid mol % 
               
               
                 Hydrogen 
                 16.8 
                 12.9 
                 0.0 
                 0.4 
               
               
                 Methane 
                 27.8 
                 29.6 
                 0.0 
                 2.1 
               
               
                 Acetylene 
                 0.5 
                 0.1 
                 0.0 
                 0.0 
               
               
                 Ethylene 
                 30.2 
                 32.3 
                 0.0 
                 9.1 
               
               
                 Ethane 
                 6.2 
                 6.9 
                 0.0 
                 2.7 
               
               
                 MAPD 
                 0.5 
                 0.2 
                 0.0 
                 0.4 
               
               
                 Propylene 
                 9.7 
                 10.6 
                 0.0 
                 11.1 
               
               
                 Propane 
                 0.3 
                 0.4 
                 0.0 
                 0.5 
               
               
                 Butadiene 
                 2.5 
                 0.3 
                 0.0 
                 1.2 
               
               
                 Butene 
                 2.4 
                 4.9 
                 0.0 
                 20.9 
               
               
                 Butane 
                 0.2 
                 0.2 
                 0.0 
                 0.9 
               
               
                 Pentadiene 
                 1.0 
                 0.0 
                 0.0 
                 0.6 
               
               
                 Pentene 
                 0.3 
                 1.3 
                 0.0 
                 29.8 
               
               
                 Pentane 
                 0.1 
                 0.2 
                 0.0 
                 5.9 
               
               
                 C 6 + 
                 1.5 
                 0.1 
                 100.0 
                 14.4 
               
               
                 Total Rate, 
                 763 
                 718 
                 10 
                 543 
               
               
                 kg-mol/hr 
               
               
                 Total Rate, kg/hr 
                 18787 
                 17977 
                 810 
                 31965 
               
               
                 Molecular Weight 
                 24.6 
                 25.0 
                 81.0 
                 58.9 
               
               
                 Temperature, ° C. 
                 41 
                 16 
                 203 
                 38 
               
               
                 Pressure, kg/cm 2   
                 17.6 
                 16.2 
                 17.7 
                 16.7 
               
               
                   
               
             
          
         
       
     
     The dual pressure catalytic distillation hydrogenation system of the present invention is shown in  FIG. 2 . The first column of this system is a high pressure column  50  which is operated generally at the same pressure as column  12  in  FIG. 1  which, in this example, is in a narrow range of around 17 kg/cm 2  pressure. The high pressure column pressure can range from 14 to 20 kg/cm 2  depending on the composition of the cracked gas and the temperature of the cooling medium. A typical pressure is 16 to 18 kg/cm 2 . The charge gas  52  from an intermediate or final stage of a charge gas compressor of an ethylene plant flows to this first, high pressure column  50  of the dual pressure column system. The charge gas feed  52  is preferably heated, although it can enter the high pressure column without preheating. Preferably, preheat temperatures range from 80 to 120° C. More preferably, the feed is preheated by heat exchange with the gross overhead of the low pressure column described later. Alternately, it can be preheated by the gross overhead of the high pressure column (not shown in  FIG. 2 ), before cooling in the reflux condenser  70 . The high pressure column  50  typically has two fractionation zones  54  and  56  in the rectifying/reaction section  58 , one above and one below the catalyst zone  60 . The catalyst zone  60  functions as a fractionation zone as well. Below the vapor charge gas feed  52  to the column  50 , it is possible to utilize only the separation provided by the reboiler  62 . Preferably, however, there is an additional fractionation zone  64  in this stripping section  66 . This fractionation zone  66 , when present, typically consists of very few theoretical separation stages. The fractionation zones in both this high pressure column  50  and the low pressure column to be described use standard industrially-available mass transfer contacting devices, including trays, such as valve trays, sieve trays, segmental trays, shed decks, or packing such as random packing, structured packing, etc. 
     The charge gas feed  52  travels upwards in the high pressure column  50  and is contacted by downflowing liquid. The charge gas feed enters the catalytic distillation hydrogenation zone  60  wherein contained hydrogen in the gas reacts with unsaturates, especially acetylenes and dienes, to preferably form the corresponding olefin compounds. Any oligomerization products formed are washed off the catalyst by the downflowing hydrocarbon liquid. Thus, these compounds are removed from the catalyst surface immediately as formed, limiting the fouling rate over the catalyst. The catalyst zone  60  contains hydrogenation catalyst, such as noble metal catalysts or mixtures thereof, such as palladium or silver. Alternately, they can contain non-noble metal hydrogenation catalysts, such as nickel. Further alternately, they can contain both non-noble and noble metal catalysts, either admixed or preferably layered. The catalyst bed temperature is in the range of 90 to 135° C. and is 125° C. in this example. 
     The catalyst in the catalytic hydrogenation zone can be bulk-loaded, made up of extrudates, pellets, balls, open ring-shapes, etc. More preferably, the catalyst is part of a structure, such as catalyst deposited on the surface of wire mesh or other types of gauzes or catalyst contained on the walls of a monolith structure. Most preferably, the catalyst is contained in specially-designed containers, as described in U.S. Pat. Nos. 6,000,685, 5,730,843, 5,189,001, and 4,215,011. 
     Exiting the catalyst bed, the upflowing gas, with the bulk of the acetylenes and dienes hydrogenated, enters the second fractionation zone  56 , where it is contacted with reflux. The overhead vapor  68  is partially condensed in the reflux condenser  70  against ambient temperature cooling, preferably cooling water. Vapor and liquid are separated at  72  and liquid reflux  74  is returned to the high pressure column  50 . The vapor  76  can be further cooled in the vent condenser  78 . If practiced, liquid  82  is separated at  84  from the vent condenser effluent and this liquid  82  is combined with the liquid from the main reflux condenser  70  and returned to the high pressure column as the reflux  74 . The net vapor product  86  is then further hydrogenated to remove any remaining concentration of acetylene (not shown). After this, the vapor product would flow either to the charge gas compressor or to the chilling train of the ethylene plant for separation of valuable hydrocarbons and hydrogen products from fuel products. The valuable hydrocarbons are subsequently processed to produce chemical-grade and/or polymer-grade ethylene and propylene products. 
     In the high pressure column  50 , there is preferably a small fractionation zone  64  below the vapor feed  52 . Liquid flowing downwards in the column below the vapor feed is stripped (stabilized) of most of the light, high vapor pressure components, such as ethane and lighter, by contact with upwardly flowing vapor from the reboiler  62 . Most of the C 3 &#39;s are also stripped. However, complete depentanization is not accomplished. Significant levels of C 4 &#39;s and C 5 &#39;s are allowed to leave in the bottoms  88  to keep the temperature low. Typically, the reboiler is heated by condensing steam. Alternately, waste heat from the ethylene plant, as quench oil can be utilized as the heating medium. The bottoms product  88  from the high pressure column  50  is low in light components and high in more mid-range components, especially in C 4  and C 5  hydrocarbons in addition to the C 6 + hydrocarbons. It is desirable to remove the lights from this stream so as to be able to totally condense the overhead stream in the low pressure column without using refrigeration. The temperature of the bottoms  88  from the high pressure column is less than 200° C. and preferably less than 160° C. 
     The bottoms product  88  from the high pressure column  50  is sent to the low pressure column  90 , preferably without cooling. The low pressure column  90  in this example is also a depentanizer, similar to the high pressure column  50 , and is operated at a pressure of about 6 kg/cm 2 . The pressure for this low pressure column can range from 4 to 10 kg/cm 2  depending on composition. A typical pressure is 4 to 8 kg/cm 2 . This low pressure column  90  contains separation zones  92  and  94  above and below the feed  88  respectively. The low pressure column preferentially has a few fractionating trays represented by  92  above the feed tray. Alternately, column  90  can be operated as a stripping column with feed  88  entering on the top tray or alternately directed into line  96  overhead of column  90 . The overhead product is C 5 &#39;s and lighter and the bottoms product is C 6 &#39;s and heavier. The gross overhead  96  is totally condensed partially by heat exchange at  98  with the charge gas feed and then in heat exchanger  100 . Cooling water is the preferred coolant in heat exchanger  100 . The totally condensed stream  102  is pumped at  103  and a portion  104  of the liquid stream is then returned as reflux. The reflux can be returned at the temperature leaving pump  103 , as shown in  FIG. 2 . Alternately, the reflux can be preheated against the gross overhead stream  96  in a separate exchanger service, or as an additional exchanger service in  110 , by using multi-pass platefin exchangers (not shown). The net overhead product liquid stream  106  from the column  90  is preheated at  110  with heat from the overhead  96  and then returned as stream  112  to the high pressure column  50 . This stream  112  is preferably partially vaporized before being returned to the high pressure column. Further preheating is accomplished in heat exchanger  114  by an external heat stream such as steam or some waste heat stream. The column entry point for this return heated stream  112  is typically below the catalyst bed  60 , for example, at the same entry point as the vapor feed  52  to the system. This further preheating in heat exchanger  114  will decrease the reboiler duty requirement in the high pressure column. As the return stream has a negligible concentration of potential oligomers, it is preferable to maximize the heat input to this stream as compared to heat inputted into the high pressure column reboiler. 
     The bottoms  116  of the low pressure column is the C 6 + hydrocarbon components. The C 5  content of this stream  116  is typically less than 1% and preferably less than 0.1%. The temperature of this bottoms stream  116  is also less than 200° C. and preferably less than 160° C. This material is typically combined with pyrolysis gasoline streams collected elsewhere in the ethylene plant and further hydrogenated to produce motor gasoline. Alternately, the stream can be further treated to recover aromatics, as benzene or toluene or xylenes. Table 2 is a material balance for a system of the present invention according to  FIG. 2  being operated as a depentanizer. This table lists the key operating parameters involved. 
     
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Stream No. 
                 Stream 52 
                 Stream 86 
                 Stream 116 
                 Stream 74 
                 Stream 112 
                 Stream 88 
                 Stream 96 
                 Stream 104 
               
               
                   
               
             
             
               
                 Name 
                 Feed 
                 HP Column 
                 LP Column 
                 HP Column 
                 LP Column 
                 HP Column 
                 LP Column 
                 LP Column 
               
               
                   
                   
                 Net 
                 Bottoms 
                 Reflux 
                 Return to 
                 Bottoms 
                 Overhead 
                 Reflux 
               
               
                   
                   
                 Overhead 
                   
                   
                 HP Column 
               
               
                 Phase 
                 Vapor 
                 Vapor 
                 Liquid 
                 Liquid 
                 Liquid 
                 Liquid 
                 Vapor 
                 Liquid 
               
               
                 Fluid mol % 
               
               
                 Hydrogen 
                 16.8 
                 12.9 
                 0.0 
                 0.4 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Methane 
                 27.8 
                 29.6 
                 0.0 
                 2.1 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 Acetylene 
                 0.5 
                 0.1 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                 Ethylene 
                 30.2 
                 32.3 
                 0.0 
                 9.1 
                 1.6 
                 1.5 
                 1.6 
                 1.6 
               
               
                 Ethane 
                 6.2 
                 6.9 
                 0.0 
                 2.7 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
               
               
                 MAPD 
                 0.5 
                 0.2 
                 0.0 
                 0.4 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 Propylene 
                 9.7 
                 10.6 
                 0.0 
                 11.1 
                 3.2 
                 3.1 
                 3.2 
                 3.2 
               
               
                 Propane 
                 0.3 
                 0.4 
                 0.0 
                 0.5 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 Butadiene 
                 2.5 
                 0.3 
                 0.0 
                 1.2 
                 0.3 
                 0.2 
                 0.3 
                 0.3 
               
               
                 Butene 
                 2.4 
                 4.9 
                 0.0 
                 20.9 
                 4.2 
                 4.0 
                 4.2 
                 4.2 
               
               
                 Butane 
                 0.2 
                 0.2 
                 0.0 
                 0.9 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
               
               
                 Pentadiene 
                 1.0 
                 0.0 
                 0.0 
                 0.6 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 Pentene 
                 0.3 
                 1.3 
                 0.0 
                 29.8 
                 4.0 
                 3.9 
                 4.0 
                 4.0 
               
               
                 Pentane 
                 0.1 
                 0.2 
                 0.0 
                 5.9 
                 0.8 
                 0.7 
                 0.8 
                 0.8 
               
               
                 C 6 + 
                 1.5 
                 0.1 
                 100.0 
                 14.4 
                 84.7 
                 85.4 
                 84.7 
                 84.7 
               
               
                 Total Rate, kg-mol/hr 
                 763 
                 718 
                 10 
                 477 
                 225 
                 235 
                 233 
                 9 
               
               
                 Total Rate, kg/hr 
                 18787 
                 17977 
                 810 
                 28035 
                 17495 
                 18305 
                 18185 
                 690 
               
               
                 Molecular Weight 
                 24.6 
                 25.0 
                 81.0 
                 58.8 
                 77.9 
                 78.0 
                 77.9 
                 77.9 
               
               
                 Temperature, ° C. 
                 41 
                 16 
                 147 
                 38 
                 124 
                 150 
                 127 
                 39 
               
               
                 Pressure, kg/cm 2   
                 17.6 
                 16.2 
                 6.5 
                 16.7 
                 17.5 
                 17.4 
                 6.0 
                 6.1 
               
               
                   
               
             
          
         
       
     
     The above description describes the invention operating as a depentanizer but the invention can also be practiced as a debutanizer. The flow scheme is similar but with higher operating pressures. The pressure is higher both in the high pressure column, to allow for reflux to be produced, and in the low pressure column to allow for the overhead to be cooled and totally condensed by ambient temperature media, preferably cooling water. The higher operating pressure in the high pressure column, with respect to a depentanizer operation, maintains the catalyst bed temperature at the desirable temperature range of 100 to 135° C., preferably 110–125° C. The flow scheme for operation as a debutanizer is similar to that of  FIG. 2  or  FIG. 3 . The high pressure column pressure will range from 28 to 43 kg/cm 2 , depending upon the composition of the cracked gas and the temperature of the cooling medium. Typical pressure is 34 to 39 kg/cm 2 . The pressure of the low pressure column varies between 5 to 14 kg/cm 2 . Typical pressure is 11 to 12 kg/cm 2 . When operated as a debutanizer, the C 5  content in stream  86  is typically less than 1% and preferably less than 0.1%. The C 4  content in stream  116  is typically less than 1% and preferably less than 0.1%. 
     A variation on the  FIG. 2  flow scheme when operated as a depentanizer is shown in  FIG. 3 . In this variation, the net overhead  112  from the low pressure depentanizer  90  may be sent through valve  118  to the top of the high pressure depentanizer at a point above the catalyst bed  60 . This is contrasted to  FIG. 2  where this net overhead stream  112  is sent to the high pressure depentanizer below the catalyst bed. This alternate return point varies the liquid flow rate and composition over the catalyst bed, as compared to the  FIG. 2  scheme, while achieving the same goal of reducing the maximum temperature in the system by approximately 50° C. The net column overhead from the low pressure column to the high pressure column can now be returned without preheating or with less preheating. Unlike the  FIG. 2  flow scheme, the return stream for the  FIG. 3  stream is not vaporized. It is also possible to send part of the net overhead from the low pressure depentanizer to the bottom of the high pressure depentanizer through valve  120  and the other part to the top of the catalyst bed. This provides additional flexibility in operation to vary catalyst bed conditions at the same overall maximum temperatures at the bottom of the two depentanizers. 
     The advantages of the present invention can be seen from a comparison of the key operating characteristics for the dual pressure system of the invention as shown in Table 2 versus a single pressure system of the prior art as shown in Table 1 where both are operating as depentanizers in front end catalytic distillation hydrogenation systems for a naphtha feedstock steam cracker. 
     Referring to Table 1, the operating pressures for the streams range from 16.2 to 17.7 kg/cm 2 . The average catalyst bed temperature is maintained at approximately 125° C. and the highest temperature is stream  28 , the net column bottoms, at 203° C. 
     Referring to Table 2, the high pressure column  50  of  FIG. 2  is operated at the same pressure as the column  12  of  FIG. 1 . The feed  52  to the system and the net overhead  86  from the system are the same, in terms of flow rate, composition and temperature as in Table 1. Stream  116 , which represents the net bottoms of the system, is the same in flow and composition as stream  28  in Table 1. However, in Table 2, the temperature of stream  116  is 147° C. versus the 203° C. temperature for stream  28  in Table 1. This lower temperature is because stream  116  in Table 2 is at 6.5 kg/cm 2  as compared to 17.7 kg/cm 2  in Table 1. 
     Stream  88  in Table 2 is the stream flowing from the first, high pressure depentanizer to the second, low pressure depentanizer. This stream pressure is 17.4 kg/cm 2  or approximately the same pressure as stream  28  in Table 1. However, this stream  88  temperature is 150° C. as compared to 203° C. for stream  28  of Table 1. This lower temperature is achieved by operating the high pressure depentanizer of  FIG. 2  so that there is approximately 15 mol % of the C 5 &#39;s and lighter hydrocarbon in the stream  88 , lowering the temperature of this stream. The average catalyst bed temperature is approximately 120° C. which is approximately the same average bed temperature of the prior art single pressure system configuration of  FIG. 1 . Thus, the  FIG. 2  flow scheme, as demonstrated in the Table 2 data referring to this flow scheme achieves the purpose of maintaining catalyst bed temperatures but at the same time decreasing the maximum temperature in the system by approximately 50° C. 
       FIG. 4  shows a variation of the  FIG. 2  process scheme. In this variation, the pumped reflux stream  74  is preheated in heat exchanger  122  by cooling and partially condensing a portion of the overhead  68  of the high pressure column. This preheat results in a higher temperature of the vapor leaving the top tray of the column  50 . All other process conditions in the column itself are essentially unchanged. The higher temperature leaving the column enables a greater degree of waste heat recovery as in exchanger  124 . This waste heat can be utilized for preheating, as for preheating feed  52  (not shown in  FIG. 4 ) or other services in the ethylene plant requiring low temperature heat. Alternately, waste heat can be recovered simply by adding a waste heat recovery exchanger  124  without preheating the reflux. This is simpler in design and operation; however, less waste heat can be recovered. 
     Modifications to the operating conditions of this dual pressure system can be made for lower or higher average catalyst temperatures while still maintaining lower maximum temperatures in the system than is achievable with the prior art single operating pressure catalytic distillation hydrogenation system.
The charge gas from the thermal cracking of a hydrocarbon feedstock is processed in a front-end catalytic distillation hydrogenation system of an olefins plant to more effectively recover ethylene and propylene product and to process the by-products. The rate of fouling in the system is reduced by employing two columns in the system with the first column operating at a higher pressure and the second column operating at a lower pressure. The hydrogenation as well as fractionation takes place in the first column while the second column is only a fractionator. The temperature of the bottoms from each column is maintained at a temperature less than 200deg C. To avoid fouling.