Abstract:
A method for upgrading a hydrocarbon using active hydrogen, includes the steps of: feeding a hydrocarbon to a two-compartment cell having a first compartment, a second compartment and a membrane dividing the compartments, wherein the hydrocarbon is fed to the first compartment which functions as a chemical compartment; feeding a source of hydrogen to the second compartment which functions as an electrochemical compartment; and pulsing an electric current or cathodic current across the second compartment.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Environmental concerns have driven the need to remove sulfur-containing compounds from hydrocarbons and hydrocarbon fuels because they are known to produce precursors to acid rain and airborne particulate material. 
         [0002]    Moreover, nitrogen-containing compounds are responsible for fuel instability due to chemical reactions during storage. 
         [0003]    Industrial processes that enhance petroleum feedstock refining and fuel treatment are focused on physical and chemical methods that remove some contaminants. 
         [0004]    Hydrotreating has been advantageously used in large integrated refineries because it is very effective, and relatively inexpensive. 
         [0005]    Despite being a primary process for the oil refining industry, some aspects still remain as drawbacks. Hydrotreating processes are usually carried out under severe conditions. 
         [0006]    Special catalyst formulation and high hydrogen pressures and temperatures are needed. 
         [0007]    An electrochemically assisted hydrocarbon feed upgrading process is known which uses active or atomic hydrogen, and this process is an assisted electrochemical reaction that uses atomic hydrogen produced from water electrolysis. 
         [0008]    Substrate conversion values can be dramatically increased with such processes. However, the need remains for even more effective and efficient processes. 
       SUMMARY OF THE INVENTION 
       [0009]    According to the invention, it has been found that electrochemically assisted hydrocarbon upgrading processes can be enhanced by pulsing current applied to the apparatus. This decreases the time during which the current density is applied, thereby conserving energy, and also reduces the atomic hydrogen excess on the palladium surface. 
         [0010]    The excess hydrogen released in this manner promotes a thermodynamic and kinetic favored concomitant reaction. 
         [0011]    A methodology to improve electrochemically assisted hydrocarbon feed upgrading by using active hydrocarbon has therefore been provided. 
         [0012]    This methodology can be applicable to any process based on electrochemically assisted reactions by using active hydrogen including but not limited to those applied in fuel or oil upgrading. 
         [0013]    Thus, in accordance with the present invention, a method is provided for upgrading a hydrocarbon using active hydrogen, comprising the steps of: feeding a hydrocarbon to a two-compartment cell having a first compartment, a second compartment and a membrane dividing the compartments, wherein the hydrocarbon is fed to the first compartment; feeding a source of hydrogen to the second compartment; and pulsing an electric current across the second compartment. 
         [0014]    Other details and advantages of the present invention will appear herein below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A detailed description of preferred embodiments of the invention follows, with reference to the attached drawings, wherein: 
           [0016]      FIG. 1  illustrates an apparatus for carrying out the process of the present invention; 
           [0017]      FIG. 2  shows three different current density pulsing regimes which were tested in accordance with the invention; 
           [0018]      FIGS. 3 a  and 3 b    show conversion rate of thiophene using a pulsing method with different ON periods, and illustrate conversion as a function of time and applied charge density, respectively; 
           [0019]      FIGS. 4 a  and 4 b    show conversion rate of 4, 6-DMDBTP using 10 minute ON pulses as a function of time and applied charge density, respectively; and 
           [0020]      FIGS. 5 a  and 5 b    show conversion rate of quinolone (QUI) using 10 minute ON pulses as a function of time and applied charge density, respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The invention relates to electrochemically assisted hydrocarbon upgrading using atomic hydrogen produced from electrolysis. The reaction is performed in a reactor that comprises two compartments, one electrochemical in which an electrolyte or hydrogen source is present, and the other chemical through which a feed to be upgraded is passed. The chambers are separated by a palladium foil or membrane. According to the invention, current density is pulsed across the electrochemical chamber such that atomic hydrogen is effectively conveyed into contact with the feed such that undesirable substrates in the feed are converted into more easily removed materials. 
         [0022]      FIG. 1  shows a schematic system for carrying out the method of the present invention, and shows a reactor  10  having two compartments  12 ,  14  separated by a membrane  16 . Compartment  12  is a reaction compartment wherein the feed with undesirable substrates is exposed to atomic hydrogen under conditions which favor conversion of the substrate into more readily removed specie. Thus, compartment  12  is considered a chemical compartment. Compartment  14  contains an electrolyte or hydrogen source, and is configured to receive a current density, for example between palladium foil on membrane  16  acting as a cathode and a platinum mesh electrode  18  within chamber  14  and acting as an anode. Components such as an amperimeter  20  and galvanostat  22  can be connected in a circuit including membrane  16  and electrode  18  for applying current density pulses across chamber  14  as desired. A temperature controller  24  can also be communicated with chamber  14  as shown. Temperature can preferably be maintained in the cell between about 20 and about 40° C. Membrane or foil  16  can preferably have a thickness of between 0.125 and 0.300 mm. 
         [0023]    The electrolyte or hydrogen source can advantageously be an alkaline solution such as NaOH, KOH or the like, preferably NaOH. This can advantageously be provided in solutions having a concentration of between about 0.1 and 1M. 
         [0024]      FIG. 1  also schematically illustrates a source  26  of feed and an outlet to a pump  28 . Product from chamber  12  can be passed through pump  28  to any other process and/or storage (not shown here) as desired. 
         [0025]    According to the invention, instead of application of a steady current density across chamber  14  for the duration of the process, it has been found that pulsing of the current density, that is, successive periods of ON current density followed by OFF current density, can produce atomic hydrogen in chamber  12  in a far more effective manner than can be accomplished using a steady constantly ON current density. Thus, the method of the present invention provides a significant increase in the conversion rates of undesirable substrates such as thiophene (TP), 4, 6 dimethyldibenzothiophene (4, 6 DMDBTP) and quinolone (QUI). 
         [0026]    It has been shown that substrate conversion by using active or atomic hydrogen is favored at the following conditions: 
         [0027]    (i) application of a current density of between about −0.5 and about −10 mA cm −2 , more preferably between −1 and −3 mA cm −2  in the electrochemical compartment to produce atomic hydrogen; 
         [0028]    (ii) high surface area palladium or palladium black exposed to the chemical compartment, wherein the membrane has a surface roughness factor S preferably in the range of between 100-200; and 
         [0029]    (iii) mild temperature (in the range of 20-40° C., preferably about 25° C.) 
         [0030]    The amount of active or atomic hydrogen available to react may be directly related to the charge applied in the electrochemical compartment. It was shown that −1.92 mA cm −2  is an excellent current density value to be applied in the electrochemical compartment, due to high permeation yields (0.96). Moreover, palladium foil microstructure is not negatively affected. α-PdH reversible phase formation is favored. 
         [0031]    The method disclosed herein involves the application of pulses of current density, for example at −1.92 mA cm −2  or a cathodic current of 1.92 mA cm −2 . 
         [0032]    In other words, a current density pulse is applied during a period of time (ON time), and then the current density is stopped (OFF time). This ON and OFF operation will be referred to herein as pulsed current density. Based on desired conversion values (preferably 100%), the duration of current density ON pulses (and resting OFF time) are set up. The pulse current density regime of the present invention leads to the following advantages: 
         [0033]    (i) spontaneous releasing of accumulated atomic hydrogen in the palladium foil; 
         [0034]    (ii) decreasing energy consumption, less applied charge means less energy consumption and expenses; and 
         [0035]    (iii) evaluated substrates are still totally converted. 
         [0036]    Thiophene (TP) is one common substrate to be converted, and a conversion value of 92% was achieved using the pulsed current density method of the present invention. For comparison, the conversion value was only 60% after steady application of current density for a period of 50 hours. 
         [0037]    Moreover, the 92% conversion was established considering the total reaction time; which includes the time of the OFF pulses. During the OFF pulses, time is provided to allow the spontaneous release of accumulated atomic hydrogen in the palladium foil of the membrane. Based on the conversion values obtained for TP and 4, 6-DMDBTP, the application of −1.92 mA cm −2  ON pulses for a duration of 10 min was chosen for QUI conversion tests as well. 
         [0038]    The resting time or OFF periods allow spontaneous releasing of accumulated atomic hydrogen from the palladium foil, which keeps the substrate conversion active during the OFF pulse, and also keeps the atomic hydrogen in quantities that favor substrate conversion over other reactions. The OFF periods can preferably last between about 1 and 10 minutes, and a particularly suitable time for the OFF pulses is about 5 minutes. This value was selected taking into account that substrate conversion does not vary during this period of time. 
         [0039]    In order to evaluate the durations of ON pulses in the method of the present invention, a series of test processes were conducted using the ON and OFF pulse regimes as illustrated in  FIG. 2 .  FIG. 2  shows that with resting times of 5 minutes, ON pulses of 5, 10 and 20 minutes were evaluated. These pulse times were used in a system such as is illustrated in  FIG. 1 , with the following conditions: Feed: 900 ppm TP+n-C 7 H 16 ; reaction temperature of 25 C; electrolytic medium in chamber  14  was 0.1 M NaOH; cathode in the electrochemical compartment is palladium foil; anode in the electrochemical compartment is platinum mesh; platinum foil thickness is 0.125 mm; applied current density is −1.92 mA cm −2 ; ON pulses of 5, 10 and 20 minutes; OFF times of 5 minutes; and palladium black surface roughness factor on the side exposed to the chemical compartment was 136. 
         [0040]      FIGS. 3 a  and 3 b    show the conversion rates obtained for the different ON periods in terms of conversion rate over time ( FIG. 3 a   ) and conversion rate per applied charge density ( FIG. 3 b   ). As shown, the greatest slope line corresponding to the highest and fastest conversion rate was at ON pulses of 10 minutes. Table 1 below also sets forth the conversion rates obtained. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 TP conversion rates obtained by using current density 
               
               
                 pulse regimes 
               
             
          
           
               
                   
                 Conversion rate, 
                 Conversion rate, mol 
               
               
                 t ajc , min 
                 mol min −1   
                 cm 2  C −1   
               
               
                   
               
             
          
           
               
                 5 
                 7.5 × 10 −6   
                 6.5 × 10 −5   
               
               
                 10 
                 1.7 × 10 −5   
                 1.5 × 10 −4   
               
               
                 20 
                 1.4 × 10 −5   
                 1.2 × 10 −4   
               
               
                   
               
             
          
         
       
     
         [0041]    Without being bound by a particular theory, it is believed that the low conversion rate obtained for 5 min ON pulses can be explained considering that generated atomic hydrogen is the limiting reagent. Thus, the amount of atomic hydrogen is not high enough to convert the TP. For the 20 min ON pulses, TP conversion is lower than that obtained for 10 min. At this condition, hydrogen evolution may become more favored (high atomic hydrogen surface coverage) than TP conversion. Thus, while conversion is effective for ON pulses of between 5 and 20 minutes, 10 minutes appears to be the most favorable duration of the ON pulses. By applying −1.92 mA cm −2  during 10 minute ON pulses (6 pulses), the TP conversion was 92% in 60 min. In absence of the current density pulses, TP conversion is only 60% in 3,000 min. 
         [0042]    For the same system as discussed above, a feed of 900 ppm 4, 6-DMDBTP+n-C 7 H 16  was evaluated using pulses of 10 minutes and all other factors as discussed above.  FIGS. 4 a  and 4 b    show the effect of current density pulses in the electrochemically assisted conversion of 4,6-DMDBTP. A conversion rate of 94% is achieved by applying −1.92 mA cm −2  during 10 minute ON pulses (28 pulses, reaction time of 280 min). In a conventional process applying steady current density, 4,6-DMDBTP conversion is only 40% over a 3,000 minute test. 
         [0043]    Next, the method of the present invention was evaluated for effectiveness in conversion of QUI. In this evaluation, the feed was 800 ppm QUI+CH 2 Cl 2 , with all other factors being maintained the same as above.  FIGS. 5 a  and 5 b    show the results in terms of QUI conversion, and excellent results are obtained. According to the invention, a 94% conversion rate is achieved using 9 pulses of 10 minutes for a total reaction time of 90 minutes. In contrast, when using a steady current density application, QUI conversion was only 30% even after 300 minutes. 
         [0044]    Finally, the method of the invention was evaluated using diesel as a feed. All other factors were maintained the same as above, and the results were compared to a steady current density application. The desulfurization percentage in each case is comparable (18-20%). Nonetheless, the time needed to achieve these values was dramatically different. With the current density pulses regime of the present invention, pulses of −1.92 mA cm −2  ON for 10 min, and OFF for 5 minutes, with 20 pulses, a desulfurization of 18% is achieved in 5 hours. In absence of current density pulse regime, for a steady applied current density of −1.92 mA cm −2  for 24 hours, a desulfurization of 20% is achieved. From an electrochemical point of view, charge q (j c  t) is the key variable to consider, since it is directly correlated to the economy of the process. The less charge applied, the less cost is the cost of the process. Further, reaching 18% desulfurization in 5 hours as opposed to 20% desulfurization in 24 hours is also a cost advantage in that additional feed or other processes can be carried out during the saved time. 
         [0045]    One or more embodiments of the present disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.