Abstract:
Methods for improving the efficiency of a catalytic reforming recycle gas compressor by combining a high molecular weight light hydrocarbon process stream from a unit operation associated with the catalytic reforming process with a lower molecular weight reactor off gas recycle stream to form a combined recycle gas stream.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    (1) Field of the Invention 
         [0002]    This invention concerns methods for improving the efficiency of a catalytic reforming recycle gas compressor by recycling a light hydrocarbon process stream from a unit operation associated with the catalytic reforming process into the recycle gas stream. 
         [0003]    (2) Description of the Art 
         [0004]    There is an increasing demand for clean fuels. This demand has led to a desire to increase the amount of hydrogen produced in catalytic reforming processes. One way to increase catalytic reforming hydrogen production is to lower to pressure of the catalytic reforming reactor(s). One consequence of increasing hydrogen produced in a catalytic reformer is an increase in the amount of hydrogen (hydrogen purity) in the recycle gas stream which translates into a reduction in the average molecular weight of the recycle gas stream. Thus, reducing reactor pressure increases the pressure rise across the catalytic reforming recycle compressor and it makes the recycle gas more difficult to compress because of its lower average molecular weight. 
         [0005]    In existing catalytic reforming units, reducing reactor pressure to improve hydrogen production increases the compressor head required to maintain the reactor hydrogen/hydrocarbon ratios. In some cases, reducing the reactor pressure is not feasible because the existing recycle compressor is not designed to handle the modified recycle gas and pressure drop conditions. Moreover, replacing a recycle gas compressor is costly. 
         [0006]    U.S. Pat. No. 5,015,587 discloses a method for optimizing recycle compressor operation by injecting a non-reactive gas such as nitrogen, ethane and propane into the process from an external source. The non-reactive gas increases the average molecular weight of the recycle gas stream and improves compressor efficiency. This solution is also problematic. Because the non-reactive gas comes from an external source, its availability and cost may be an issue. There remains a need therefore, for more reliable and cost effective methods of improving catalytic reformer hydrogen production without replacing or revamping the recycle gas compressor. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention is directed to methods for improving catalytic reforming hydrogen production using an existing recycle gas compressor by recycling a light gas stream from a unit operation associated with the catalytic reforming unit to either the recycle gas compressor inlet or outlet. 
         [0008]    One aspect of this invention are methods for increasing hydrogen flow through a recycle gas compressor associated with a catalytic reforming process comprising the steps of: operating the recycle gas compressor having an inlet and an outlet in a catalytic reforming process including at least one catalytic reforming reactor to increase the pressure of a combined recycle gas stream directed to an inlet of the at least one catalytic reforming reactor; directing a recycle gas stream including hydrogen from an outlet of the catalytic reforming reactor to the compressor inlet; combining a gaseous product stream from a unit operation associated with the catalytic reforming process with the recycle gas stream to form a combined recycle stream wherein the combined recycle stream molecular weight is greater than the recycle gas stream molecular weight; and directing the combined recycle stream into the inlet of the at least one catalytic reforming reactor. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0009]      FIG. 1  is a flow diagram showing one embodiment of a method of this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    The present invention relates to methods for reducing the head requirement of a recycle gas compressor associated with a catalytic reforming process. In particular, the methods of this invention improve the head developed by the recycle gas compressors operating at higher pressure differentials and/or with is higher purity hydrogen make than the compressors were originally designed for. 
         [0011]    Catalytic reforming is a preferred process for reforming naphtha to produce aromatics for petrochemical complexes, high octane gasoline, and hydrogen. A typical catalytic reforming unit may have three or more fixed catalyst bed reactors. Some catalytic reforming units have an extra “swing” reactor so that each reactor can be individually isolated so that any one reactor can be undergoing in situ regeneration while the other reactors are in operation. Still other catalytic reforming units have moving catalyst bed reactors. Such units are characterized by continuous in-situ regeneration of part of the catalyst in a special regenerator, and by continuous addition of the regenerated catalyst to the operating reactors. 
         [0012]    Catalytic reforming reactors can operate at pressures ranging from about 3 to about 45 atmospheres. The first reactor inlet temperature will generally range from about 495 to 555° C.—a temperature at which all reactants are gasses or vapors. As the vaporized reactants flow through the fixed bed of catalyst in the reactor, the major reaction is the dehydrogenation of naphthenes to aromatics which is highly endothermic and results in a large temperature decrease between the inlet and outlet of the reactor. To maintain the required reaction temperature and the rate of reaction, the vaporized stream is typically reheated in a second fired heater before it flows through the second reactor. The temperature again decreases across the second reactor and the vaporized stream must again be reheated in the third fired heater before it flows through a third reactor. As the vaporized stream proceeds through the three reactors, the reaction rates decrease and the reactors therefore become larger. At the same time, the amount of reheat required between the reactors becomes smaller. 
         [0013]    This invention will be described generally with respect to  FIG. 1 .  FIG. 1  is a schematic of a catalytic reforming unit that falls within the scope of the present invention. The catalytic reforming includes a reactor  10 . Reactor  10  in  FIG. 1  can represent any type of catalytic reforming reactor scheme such as a three reactor scheme, a three reactor plus swing reactor scheme and a moving catalyst bed reactor scheme. Reactor  10  includes an inlet  12  and an outlet  14 . The reactor product stream  16  is cooled before it is directed into separator  18  where it is separated into a separator liquid stream  19  and a separator gas stream  20 . 
         [0014]    Separator gas stream  20  which includes hydrogen is then further divided into a net gas stream  21  and a recycle gas stream  22 . At this point, net gas stream  21 , which is hydrogen rich, may be removed from the process as a net gas product for use in other the other refinery processes that consume hydrogen such as hydrodesulfurization units and/or hydrocracking units. Alternatively, the hydrogen can be recovered from net gas stream  21  and used as clean fuel for fuel cells and the like. 
         [0015]    In one embodiment of the present invention, net gas stream  21  is directed to compressor  23  which increases the pressure of net gas stream  21 . At the same time, pump  24  increases the pressure of separator liquid stream  19  after which the compressed net gas stream  21 ′ and the pumped separator liquid stream  19 ′ are combined to form a recontacted product stream  25 . Recontacted product stream  25  is directed into a separator  26  which separates recontacted product stream  25  into a final net gas stream  27  and a second separator liquid stream  28 . The second separator liquid product steam  28  is directed into a unit operation  29  which further separates the second separator liquid stream into useful product fractions. Unit operation  29  may be selected from any unit operation(s) capable of separating the second separator liquids stream  28  into useful fractions wherein at least one fraction is a gaseous product stream  32 . Non-limiting examples of useful unit operations include depropanizers, debutanizers, depentanizers or any of a depropanizer, debutanizer and depentanizer followed by a deethanizer. In one preferred embodiment, unit operation  29  is a debutanizer. 
         [0016]    Unit operation  29  will produce at least two or more product streams. At minimum, unit operation  29  will produce at least one liquid product stream  30  and at least one gaseous product stream  32 . In the embodiment shown in  FIG. 1 , unit operation  29  produces liquid product stream  30 , a gaseous product stream  32  and a side steam  31 . In the method embodiment shown in  FIG. 1 , either or both of side stream  31  and gaseous product stream may be directed in part or wholly as gaseous product stream  36  and combined with recycle gas stream  22  via either line  33  before the combined recycle stream enters the inlet of recycle gas compressor  34 , or via line  33 ′ after recycle gas stream  22  is compressed in recycle gas compressor  34  or via both line  33  and  33 ′. Combined recycled gas stream  35  which will typically be 70-85 mole % hydrogen is then combined with a vaporized reforming reactor feed stream  36  and directed into reactor  10  through inlet  12 . 
         [0017]    Gaseous product stream  36  from unit operation  29  may initially be a gas or it may initially be a liquid product stream that is flashed across a valve  37  to form a gas stream  36 ′. It is important that gaseous product stream  36  from unit operation  29  has a molecular weight that is greater than the molecular weight of recycle gas stream  22 . It is preferred that gaseous product stream  36  from unit operation  29  has a molecular weight that is about 3 to 6 times greater than the molecular weight of recycle gas stream  22 . That way, the combined cycle stream  35  will have a molecular weight that is from 25 to 50% greater than the molecular weight of recycle gas stream  22 . 
         [0018]    In one preferred embodiment, gaseous product stream  36  is an LPG stream. The term LPG steam is used herein to refer broadly to a stream that is primary butanes and propanes with small amount of ethanes and pentanes. 
         [0019]    The recycle compressor duty of a catalytic reforming process that includes a debutanizer and the processing scheme set forth in  FIG. 1  was modeled below and compared to a conventional catalytic reforming process. The base case with no LPG recycle requires a recycle gas rate of 13250 lbmole/hr and a recycle gas compressor suction pressure of 50 psia. The recycle gas molecular weight is 9.1. The discharge pressure of the recycle gas compressor is 120 psia. A head requirement of 94500 ft is required for this operation. In one embodiment of the flow scheme of  FIG. 1 , unit operation  29  is a debutanizer and LPG is recycled from the debutanizer overhead as gas or liquid. Thus any product stream from unit operation  29  that meets this criteria, such as gaseous product stream  32  or side stream  31  may be combined with recycle gas stream  22 . In this embodiment, 90% of the LPG from the debutanizer overhead is recycled back to the recycle gas compressor suction. To maintain about the same H2/reformer feed ratio as the base case, the combined recycle gas rate increases to 14270 lbmole/hr. The molecular weight of the combined flow is at 12.2. With the same suction pressure of 50 psia and discharge pressure of 120 psia, the head requirement drops to 69500 feet. With no equipment changes and the constant H 2 /Feed ratio with LPG recycle, the recycle compressor discharge pressure can be increased to 128 psia at a required head of 75300 feet. This example illustrates the decreases of recycle gas compressor head requirement by the LPG recycle. 
         [0020]    The use of an internal recycle stream to increase the molecular weight of the recycle gas stream avoids potential contamination and reduces the amount absolute LPG requirement if outside source is used. The amount of the LPG recycle can be changed and the recycle rate depends upon the performance of the existing compressor. 
         [0021]    The methods of this invention can be further improved with net gas recontact with the separator liquid as illustrated in  FIG. 1 . The recontact section allows the LPG to accumulate in the reactor and debutanizer loop and allow the process flow scheme of  FIG. 1  to operate in catalytic reformers with low LPG yields.