Abstract:
Reduction of fouling in a thermal process for treating feed streams comprising naphtha, pyrolysis oils or a mixture thereof said feed stream having a combined olefinic content from 10 to 50 weight % and the balance inert hydrocarbons at a temperature from 100° C. to 300° C. which comprises decreasing the amount of carbon steel in the apparatus contacting said feed stream and increasing the amount of stainless steel.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to chemicals processing, more particularly chemical processing of hydrocarbon stream at elevated temperatures. In processing apparatus comprising a significant amount of carbon steel there is a tendency for the apparatus to become fouled. This fouling reduces the efficiency of such devices and leads to more frequent shut downs to clean the devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     A significant amount of information is available on the thermal stability of aviation fuels over various metal surfaces, however, most of the work done to date has been at temperatures where coking reactions predominate (&gt;350° C.), using feedstocks that have negligible olefin and diene content.  
         [0003]     L. E. Faith, G. H. Ackerman, H. T. Henderson, Heat Sink Capability of Jet A Fuel: Heat Transfer and Coking Studies,  NASA Report CR -72951, Shell Development Company on contract, Cleveland, Ohio, July, 1971 evaluated the thermal stability of Jet A fuel using an Alcor JFTOT at 210° C. Jet fuel is predominantly a mixture of aliphatic, naphthenic and aromatic species, with very low olefinic content, which reacts predominantly via autoxidation. This is in contrast to pyrolysis gasoline, which contains upwards of 50% reactive olefins such as styrene, indene, and derivatives, and reacts via thermally and catalytically induced free-radical polymerization.  
         [0004]     There have not been significant studies on the effect of the composition of steel on feed streams at temperatures below 300° C., typically from 100° C. to 300° C. preferably from 150° C. to 250° C. The present invention seeks to reduce fouling in the treatment of feedstocks having a significant olefin/diolefin content (e.g. from 10 to 50 weight %) at such temperatures.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a process to reduce fouling in a petrochemical thermal process for treating a feed stream comprising naphtha, pyrolysis oils or a mixture thereof said feed stream having a combined olefinic content from 10 to 50 weight % and the balance inert hydrocarbons at a temperature from 100° C. to 300° C. which comprises decreasing the amount of carbon steel in the apparatus contacting said feed stream and increasing the amount of stainless steel having an iron content from 60 to 75 weight %, a chrome content from 15 to 21 weight %, a nickel content from 1 to 15 weight %, a manganese content from 0 to 3 weight %, a molybdenum content from 0 to 3 weight % of and the balance comprising one or more elements selected from the group consisting of Si, Co, Cu, and other trace elements provided no single such element is present in an amount of greater than 1 weight %. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a schematic diagram of the Alcor hot liquid process simulator (HLPS) used in the examples.  
         [0007]      FIG. 2  shows the temperature profiles for heater tube for various alloys in the HPLS as a function of distance from the top of the heater tube.  
         [0008]      FIG. 3  shows the propensity for fouling of pyrolysis gasoline over various alloys of heater tube at 250° C. at a nitrogen pressure of 150 psig and a flow rate of 3.0 ml/min. 
     
    
     DETAILED DESCRIPTION  
       [0009]     Thermal process in accordance with the present invention involve process in which a feed stock is heated in an apparatus at temperatures below 300° C., typically from 100° C. to 300° C., preferably from 150° C. to 250° C. Such processes may be heat exchange processes, distillation, and the like. The pressures in the process may range from sub-atmospheric to several thousand pounds, typically from atmospheric to about 5,000 psig, preferably from about atmospheric to 5,000 psig, most preferably from about 15 psia to about 1,500 psig.  
         [0010]     The feedstock is typically naptha or pyrolysis oil or pyrolysis gasoline. Generally the feedstock has a combined olefinic content from 10 to 50 weight % and the balance inert hydrocarbons. Typically the olefinic content comprises styrene monomers which are unsubstituted or substituted by one or more C 1-4 , preferably C 1-2  alkyl radicals and C 9-12  bicyclic aromatic ring species which are unsubstituted or substituted by one or more C 1-4  alkyl radicals preferably C 1-2  alkyl radicals. Typically the bicyclic aromatic compounds are substituted indenes. The feedstock may have a content of styrene monomers which are unsubstituted or substituted by one or more C 1-4  alkyl radicals from 25 to 32 weight %. The feedstock may have a of C 9-12  bicyclic aromatic ring species which are unsubstituted or substituted by one or more C 1-4  alkyl radicals from 4 to 10 weight %. Preferably the combined content of styrene monomers which are unsubstituted or substituted by one or more C 1-4  alkyl radicals and C 9-12  bicyclic aromatic ring species which are unsubstituted or substituted by one or more C 1-4  alkyl radicals from 35 to 42 weight %.  
         [0011]     Typically the stainless steel useful in accordance with the present invention steel has an average surface roughness (R a ) of less than 25 μm most preferably less than 20 μm. Generally the stainless steel of the present invention has an iron content from 60 to 75, preferably less than 73 weight %, a chrome content from 15 to 21, preferably from 16 to 21, most preferably from 16 to 18 weight %, a nickel content from 1 to 15, preferably from 0.3 to 13, most preferably less than 6 weight %, a manganese content from 0 to 3, preferably from 0.5 to 2, most preferably less than 1.5 weight %, a molybdenum content from 0 to 3, preferably 0.1 to 2.5 weight % of and the balance comprising one or more trace elements typically selected from the group consisting of Si, Co, Cu, and other trace elements provided no single such element is present in an amount of greater than 1, typically in an amount not greater than 0.5 weight %. The sum of the elements in the steel should add up to 100 weight %.  
         [0012]     The present invention will now be illustrated, in a non-limiting manner by the following examples.  
         [0013]     The HLPS used in the experiments is shown in  FIG. 1 . The feedstock is kept in a feed tank or reservoir  1 . A feed line  2  passes from the feed tank  1  to a filter  3 . The line then passes into the heater tube  4  made from the various alloys tested heated in a shell. The heater tube had thermocouples  5  and  6 . Additionally, a temperature measuring device at thermocouple  7  was located internally in the heating tube. The fluid passed over the surface of the tube and then passed via line  8  through a filter  9 . A pressure drop module  10  was placed across the filter and measured the pressure drop across the filter with time as the fluid cycled through the tube. When the pressure drop reached 200 mm Hg the experiment stopped (the cut off point). The time to reach the cut off point was recorded. After flowing through the filter  9  the fluid flowed through line  11  to a pump  12  and back to the feed tank  1 . The feed tank was pressurized with nitrogen through line  13 . The nitrogen pressure used in the experiments was 150 psig.  
         [0014]     The feedstock for the experiments was pyrolysis gasoline obtained from NOVA Chemicals flexi-cracker at Corunna. The pyrolysis gasoline had an olefinic content shown in Table 1.  
                                         TABLE 1                                   Compound Type   Weight %                                        Styrene   17.4           Aromatic C 1  substituted styrene   17.9           Aromatic C 2  substituted styrene   1.4           Indene   5.3           Aromatic C 1  substituted indene   1.5           Aromatic C 2  substituted indene   Less than 1           Total styrenics   36.8           Total indenes   6.7           Inerts   55.7           Total   99.2                      
 
         [0015]     Note the inerts are predominantly non-olefinic hydrocarbons other than styrene and indene.  
         [0016]     The compositions of the various alloys used in the experiments are shown in Table 2.  
                                                                 TABLE 2                                   Element   1018CS   446SS   304SS   316SS                                        Al   0.4   0.0   0.0   0.0           Si   0.1   0.1   0.2   0.1           S   0.0   0.0   0.0   0.0           Cr   0.1   20.5   15.9   16.1           Mn   0.7   0.8   1.1   1.7           Fe   94.0   72.2   69.5   62.6           Co   0.0   0.0   0.2   0.1           Ni   0.1   0.4   8.9   12.4           Cu   0.1   0.1   0.3   0.3           Mo   0.0   0.1   0.3   2.5           Total weight %   95.4   94.3   96.3   95.8                      
 
         [0017]      FIG. 2  shows the temperature profiles for heater tube for various alloys in the HPLS as a function of distance from the top of the heater tube. The average surface temperature for the various tubes is set forth in Table 3.  
                                                             TABLE 3                                   316SS   304SS   446SS   1018CS                                        T surface, ave  (° C.)   199   195   195   193                        
         [0018]     The surface roughness for the various tubes was measured and the results are shown in Table 4.  
                                                           TABLE 4                           Surface Roughness of HLPS Heater Tubes                1018CS   304SS   316SS   446SS                        Average   0.27 ± 0.09   0.21 ± 0.03   0.23 ± 0.02   0.11 ± 0.02       Surface       Roughness       R a  (μm)                  
 
         [0019]     Tube surface roughness was analyzed using a Mitutoyo 402 Surftest Series 178 Instrument (Table 4). Surface roughness, measured as (R a ), is the arithmetical mean of the roughness profile. The data shows that surface roughness is similar for all heater tubes.  
         [0020]     The test procedure used was as follows:  
         [0021]     500 cc of pyrolysis gasoline was gravity filtered using 8 μm filter paper to remove pre-existing solids/gums, and was charged to the HLPS under 150 psig N 2 . Liquid flow was set to 3.0 ml/min, giving the liquid a residence time of ˜11 seconds in the heated section. Fouling was monitored as a function of differential pressure (dP) across a 17 μm filter at the outlet of the test section. The test was stopped when the dP reached 200 mm Hg.  
         [0022]     The fouling propensity for the various alloys is shown in  FIG. 3 . Relative to 316SS (relative rate=1.0), fouling rates over 1018CS, 304SS, and 446SS proceed according to the following scale:  
                                                               Alloy   1018CS   446SS   304SS   316SS                           Relative Initial   ˜1.70   ˜1.50   ˜1.47   1.00           Fouling Rate                      
 
         [0023]     In an earlier study by Faith and coworkers, the relative mass of deposits from Jet A fuel at 210° C. over the same alloys follows according to:  
                                                               Alloy   304CS   1015SS   316SS   446SS                           Relative Fouling   ˜70   ˜56.6   ˜3.00   1.00           Propensity                      
 
         [0024]     Results from the current work differ from work by Faith and coworkers because of the nature of the hydrocarbon fluid. Jet fuel is predominantly a mixture of aliphatic, naphthenic and aromatic species, with very low olefinic content, which reacts predominantly via autoxidation. This is in contrast to pyrolysis gasoline, which contains upwards of 50% reactive olefins such as styrene, indene, and derivatives, and reacts via thermally and catalytically induced free-radical polymerization.  
         [0025]     The experiments show that stainless steel offers advantages over carbon steel in fouling mitigation for streams high in olefin and diene content.