Abstract:
Methods and systems are provided for separating a selected xylene isomer. The method includes separating a feed stream including a plurality of aromatic hydrocarbons into a first stream including toluene and isomers of xylene, and a second stream including isomers of xylene. The method further includes separating the first stream into a third stream including toluene and a fourth stream including isomers of xylene. The method further includes combining the second stream and the third stream in an adsorptive separation unit including an adsorbent configured to adsorb the selected xylene isomer from the second stream. The third stream desorbs the selected xylene isomer to produce a fifth stream including the selected xylene isomer and toluene and a sixth stream including non-selected xylene isomers and toluene. Still further, the method includes separating the sixth stream into a seventh stream including the non-selected xylene isomers and the third stream including toluene.

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to systems and methods for separating aromatic hydrocarbons, and more particularly relates to systems and methods for separating isomers of xylene using selective adsorption. 
     BACKGROUND 
     Xylene isomers are important intermediates in chemical syntheses, and specific xylene isomers are desired for different processes. Paraxylene is a feedstock for terephthalic acid, and terephthalic acid is used in the manufacture of synthetic fibers and resins. Metaxylene is used in the manufacture of certain plasticizers, azo dyes, and wood preservatives. Orthoxylene is a feedstock for phthalic anhydride production, and phthalic anhydride is used in the manufacture of certain plasticizers, dyes, and pharmaceutical products. 
     Xylene isomers typically are separated from mixed xylene streams by using an adsorbent selective to the desired isomer. The desired isomer is adsorbed, and the remaining isomers are discharged in a mixed raffinate stream. A desorbent is then used to desorb the desired xylene isomer, and the desorbent and desired xylene isomer are collected and separated by distillation (also referred to as fractionation). The desorbents are typically referred to as either heavy or light, where a heavy desorbent has a higher molecular weight and a higher boiling point than xylene and a light desorbent has a lower molecular weight and a lower boiling point than xylene. Xylene isomer recovery systems with heavy desorbents tend to use less energy than systems with light desorbents because the desorbent does not need to be repeatedly evaporated and lifted in the fractionation step. However, heavy desorbent systems require stringent feed purity to control the accumulation of undesired compounds in the recycled desorbent. The undesired compounds are impurities that reduce the desorbent effectiveness and product purity. Additional equipment is needed to maintain the heavy desorbent purity during the desorbent recycling process. The distillation columns in heavy desorbent systems have higher reboiler temperatures, which leads to higher operating pressures. These higher operating pressures require higher pressure ratings for the equipment involved, which increase the equipment capital costs and maintenance expenses. 
     A light desorbent system allows a relaxed feed specification relative to a heavy desorbent system. This helps to offset the increased energy costs associated with recovering light desorbent as a distillation column overhead stream. The light desorbent systems also provide substantial savings in the total equipment count for xylene recovery systems, because the additional equipment for desorbent storage and recovery is not needed. The light desorbent xylene recovery systems also have lower distillation column operating pressures, so thinner shell thicknesses and lower pressure ratings can be used to further reduce capital costs for installing new systems. Toluene is one example of a light desorbent that can be used, and toluene is less expensive than many of the heavy desorbents available. 
     As such, in the prior art, a trade-off exists between systems that use heavy desorbents and systems that use light desorbents. In situations where the feed specification is not able to be tightly controlled, it may be desirable to use a light desorbent system as opposed to a heavy desorbent system for the reasons discussed above. However, the prior art fails to disclose any systems that address the undesirable increase in energy use associated with such light desorbent systems. 
     Accordingly, it is desirable to develop methods and systems for producing selected xylene isomers from mixed xylene streams using light desorbents in a manner that reduces overall energy consumption. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background. 
     BRIEF SUMMARY 
     Methods and systems are provided for separating a selected xylene isomer. In one exemplary embodiment, a method for separating a selected xylene isomer includes the steps of separating a feed stream including a plurality of aromatic hydrocarbons into a first stream including toluene and isomers of xylene, and a second stream including isomers of xylene. The method further includes separating, in a first zone of a multi-zone separation apparatus, the first stream into a third stream including toluene and a fourth stream including isomers of xylene. The method further includes combining the second stream and the third stream in an adsorptive separation unit including an adsorbent configured to adsorb the selected xylene isomer from the second stream. The third stream desorbs the selected xylene isomer from the adsorbent to produce a fifth stream including the selected xylene isomer and toluene and a sixth stream including non-selected xylene isomers and toluene. Still further, the method includes separating, in a second zone of the multi-zone separation apparatus, the sixth stream into a seventh stream including the non-selected xylene isomers and the third stream including toluene. 
     In another exemplary embodiment, a system for separating a selected xylene isomer includes a first hydrocarbon separation apparatus configured to separate a feed stream including a plurality of aromatic hydrocarbons into a first stream including toluene and isomers of xylene, and a second stream including isomers of xylene. The system further includes a multi-zone separation apparatus including a first separation zone configured to separate the first stream into a third stream including toluene and a fourth stream including isomers of xylene. Still further, the system includes an adsorptive separation unit including an adsorbent configured to adsorb the selected xylene isomer from the second stream. The third stream desorbs the selected xylene isomer from the adsorbent to produce a fifth stream including the selected xylene isomer and toluene and a sixth stream including non-selected xylene isomers and toluene. Additionally, the multi-zone separation apparatus includes a second separation zone configured to separate the sixth stream into a seventh stream including the non-selected xylene isomers and the third stream including toluene. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The present embodiments will hereinafter be described in conjunction with the following drawing FIGURE, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a process flow diagram illustrating a method implemented on a xylene isomer separation system in accordance with various embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the application and uses of the embodiment described. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
     The various embodiments described herein relate to systems and methods for separating a selected xylene isomer from a mixed xylene feedstock using adsorptive separation with a light desorbent. In the disclosed systems, certain separation apparatus, in particular distillation columns, are combined in a novel manner to reduce the overall equipment count needed to implement the system, thereby lowering the overall energy costs, which, as noted above, tend to be higher in light desorbent systems. As such, the present disclosure provides separation systems and methods that improve over the prior art by allowing for a relaxed-specification (i.e., less tightly controlled specification) feed stock, while at the same time reducing overall energy costs. 
     Reference is now made to  FIG. 1 , which provides a process flow diagram illustrating a method implemented on a xylene isomer separation system  10  in accordance with various embodiments of the present disclosure. As shown therein, a feed stream  11  is provided to system  10 . Suitable feed streams  11  for separating a selected xylene isomer are available from many sources. For example, a fluid catalytic cracking (FCC) unit and fractionator, when run in high severity mode, can produce a fraction with hydrocarbons having 7 to 10 carbon atoms (C7-C10), where about 60 mass percent of the hydrocarbons are aromatic. Certain coal liquefaction processes produce hydrocarbon streams rich in aromatic compounds, and these hydrocarbon streams are suitable for use as the feed stream  11 . Other possible sources include various petroleum refining processes, thermal or catalytic cracking of hydrocarbons, or petrochemical conversion processes, including hydrocarbon streams processed in a reformer using a catalyst designed to produce aromatic compounds, such as reformed naphthas. 
     In one particular embodiment, the feed stream  11  is a naphtha stream. Naphtha feedstocks include aromatics, paraffins, and naphthenes, and may include small amounts of olefins. Feedstocks which may be utilized include straight-run naphthas, natural gasoline, synthetic naphthas, thermal gasoline, catalytically cracked gasoline, and in particular reformed naphthas. The feedstock may be encompassed by a full-range naphtha as defined by boiling points, or from about 0° to about 230° C., although naphthas having a greater percentage (such as greater than about 50%, greater than about 70%, etc.) of aromatic hydrocarbons are preferred. 
     As shown in  FIG. 1 , the feed stream  11 , particularly in embodiments where the feed stream  11  is a reformed naptha stream, is fed to a reformate splitter distillation column  12 . The reformate splitter distillation column  12  functions to separate or “split” by distilling the feed stream  11  into a lower boiling stream as an overhead stream  13  and a higher boiling stream as a bottom stream  14 . The reformate splitter distillation column may be configured such that, for example, the overhead stream  13  may include primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) hydrocarbon molecules having seven or fewer carbon atoms (C7−). The bottom stream  14  may thus include primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) hydrocarbon molecules having eight or more carbon atoms (C8+). 
     The bottom stream  14  may thereafter be passed to a clay treater  15  for the removal of any alkylates and olefins that may be present in the stream  14 , as is known in the art. The clay treater  15  may be configured in any known manner suitable for this purpose. Stream  16  leaving the clay treater  15  may thus include primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) C8+ hydrocarbons with alkylate and olefin compounds substantially (such as greater than about 90%) removed therefrom. 
     C8+ hydrocarbons stream  16  is thereafter passed to a “stripper” distillation column  17  for separating the stream  16  into various fractions. The different fractions (such as C7−, C8, and C9+) are separated based on the relative boiling points of the compounds present. As shown in  FIG. 1 , stripper distillation column  17  includes a “stabilizer” distillation column associated therewith. Stabilizer distillation column  18 , in one embodiment, is integrated within an overhead portion of the stripper distillation column  17 , as illustrated. By integrating the stabilizer distillation column  18  within the stripper distillation column  19 , capital costs can be reduced by eliminating a dedicated stabilizer vessel. In this embodiment, vessel partitions provide isolation from the main stripper tray section so that a portion of the overhead liquid can be stabilized. In another embodiment, however, the stabilizer  18  may be a separate unit. Regardless of the particular configuration, the stripper distillation column  17  produces an overhead stream  19  that may include primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) hydrocarbon molecules having four or fewer carbon atoms (C4−). The column  17  further produces a mixed xylene stream  20  as a “side draw” product that may include primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) C8+ hydrocarbons. Still further, the column  17  produces, as a bottom stream  21 , a stream that primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) includes hydrocarbon molecules having nine or more carbon atoms (C9+). The stabilizer  18 , which as noted above is associated with an overhead portion of the stripper distillation column  17 , receives a portion of the hydrocarbons in the overhead portion of the stripper distillation column  17  and produces a liquid product stream  22  including primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) hydrocarbons having between 5 and 7 carbons atoms (C5-C7) and a gas product stream, which includes primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) C4-hydrocarbons and is joined with the overhead stream  19  in the stripper distillation column. 
     C9+ stream  21  is thereafter passed to a further “heavy aromatics” distillation column  23  for further separation of the stream  21 . The heavy aromatics distillation column  23  produces as an overhead stream  24  a stream that includes primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) hydrocarbons having either nine or ten carbon atoms (C9-C10). The heavy aromatics distillation column  23  further produces a bottom stream  25  that includes primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) hydrocarbons having eleven or more carbon atoms (C11+). The overhead stream  24  is passed to a transalkylation unit (unit  39  in  FIG. 1 ), as will be described in greater detail below, for producing additional C8 aromatic hydrocarbons. The bottom stream  25  is removed from the system  10  and may be used as fuel, as input material for other processes, or otherwise utilized. As an additional matter, C4− stream  19  from the stripper distillation column  17  (and also from the stabilizer distillation column  18 ) is also removed from the system  10  and may be used as fuel, as input material for other processes, or otherwise utilized. 
     C5-C7 stream  22  may be joined with C7− reformate splitter distillation column overhead stream  13 . This combined stream  26 , including primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) C7− hydrocarbons, is thereafter passed to an extractive distillation process unit  27  for removing non-aromatic compounds from the stream  26 . In one particular embodiment, extractive distillation process unit  27  may employ a sulfolane solvent to separate aromatic compounds from non-aromatic compounds, as is known in the art. Other extraction methods, such as liquid-liquid solvent extraction are also known in the art and practiced for separation of non-aromatic compounds from aromatic compounds, and their use in place of or in addition to unit  27  is within the scope of the present disclosure. Extractive distillation unit  27  produces a first stream  28  that includes primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) C7− non-aromatic hydrocarbons and a second stream  29  that includes primarily (such as greater than about 80%, greater than about 90%, or greater than about 95%) benzene and toluene. The second stream  29  may further be passed to a clay treater  30  for increasing the purity of the aromatic compounds in such stream, for example by removing any alkylates or olefins that may be present therein in a manner as described above with regard to clay treater  15 , thus producing a treated benzene and toluene stream  31 . 
     The treated benzene and toluene stream  31  is thereafter passed to a further distillation column  32  for the separation of the benzene from the toluene in the stream  31 . The benzene, having a lower boiling point than toluene, is removed from column  32  as an overhead product  33 , and the toluene, having a higher boiling point than benzene, is removed from column  32  as a bottom product  34 . Bottom product  34 , which includes primarily (such as greater than about 40%, greater than about 60%, or greater than about 70%) toluene, but may also include some percentage of heavier aromatic hydrocarbons such as various xylene isomers, is thereafter passed to a multi-zone separation apparatus, such as a split shell distillation column  35 , for further purification of the toluene. Split shell distillation column  35  includes a first zone or “side”  35   a  and a second zone or “side”  35   b , which are separated from one another in the mid and lower portions of the column by baffle  35   c , but which share a common upper (overhead) portion, as shown in  FIG. 1 . Toluene stream  34  is provided to the first side  35   a , wherein any heavier aromatic compounds are removed as a bottom product of the first side  35   a  in stream  36  (which may then be recycled back to the stripper distillation column  17  as shown in  FIG. 1 ), and wherein the purified toluene is removed as an overhead product in stream  37 . 
     The purified toluene stream  37  is thereafter provided as a “light” desorbent (stream  37   a ) for the separation of selected xylene isomers in adsorptive separation unit  38  and also as a feed material (stream  37   b ) for the previously-alluded-to transalkylation unit  39 . First, regarding the transalkylation unit  39 , the toluene stream  37   b  is provided to the unit  39  wherein it is reacted with the C9-C10 stream  24  from the heavy aromatics distillation column  23 . As is known in the art, transalkylation is a process that converts toluene and heavier (i.e., C9-C10) aromatics into mixed xylenes. Hydrogen gas is provided as a further feed material for the transalkylation reactions via stream  40 . Transalkylation processes may employ, for example, silica-alumina and zeolites such as dealuminated mordenite, ultra stable Y-zeolite (USY), and ZSM-12 as catalysts for the transalkylation reactions. During the process of xylene production, a number of reactions such as disproportionation, transalkylation, and dealkylation take place. The methyl groups are shifted from one phenyl group to another via disproportionation and transalkylation to produce mixed xylenes. During dealkylation, the ethyl, propyl, and butyl groups attached to phenyl groups are removed to produce benzene and toluene. The transalkylation unit  39  produces a byproduct stream  41  that includes hydrocarbon gasses such as butane, propane, etc., and a stream  42  that includes primarily (such as greater than about 40%, greater than about 60%, or greater than about 70%) xylenes and toluene. 
     The xylene/toluene stream  42  is thereafter passed to a further stripper distillation column  43  for purifying the xylene/toluene stream  42 , for example, by separating out any lighter aromatics (benzene, toluene) that may be present in the xylene/toluene stream  42 . Any benzene or toluene present is removed from the stripper distillation column  43  via stream  44 , which is then recycled back to join with benzene and toluene stream  31  prior to its entry into the distillation column  32 . The xylene product, which is removed from the distillation column  43  via stream  45 , is then recycled back to join with hydrocarbons stream  16  prior to its entry into the stripper distillation column  17 . (Note that although illustrated in “reverse” for ease of illustration, stream  44  is the “overhead” stream and stream  45  is the “bottoms” stream of the distillation column  43 .) 
     Turning now to the adsorptive separation unit  38 , as shown in  FIG. 1 , stream  20 , which as noted above is the mixed xylene stream removed as a side-draw from the stripper distillation column  17 , provides the xylene feed product to the adsorptive separation unit  38 . The mixed xylene stream  20  is introduced to the unit  38  to absorb a selected xylene isomer. The selected xylene isomer is paraxylene in many embodiments, but the selected xylene isomer may also be metaxylene in other embodiments. The separation unit  38  includes a selective adsorbent that preferentially adsorbs the selected xylene isomer over the other xylene isomers. In an exemplary embodiment, the selective adsorbent may be crystalline alumino-silicate, such as type X or type Y crystalline aluminosilicate zeolites. The selective adsorbent contains exchangeable cationic sites with one or more metal cations, where the metal cations can be one or more of lithium, potassium, beryllium, magnesium, calcium, strontium, barium, nickel, copper, silver, manganese, and cadmium. Adsorption conditions vary, but typically range from about 35° C. to about 200° C. (about 100° F. to about 400° F.) and from about 100 kPa to about 3,500 kPa (about 14 PSIG to about 500 PSIG). 
     The mixed xylene stream  20  is separated into a mixed raffinate stream  46  and an extract stream in the adsorptive separation unit  38 . An extract column (not shown) is used to separate the toluene desorbent from the selected xylene isomer (such as paraxylene) in the mixed extract stream, thereby producing paraxylene product stream  47 . The selective adsorbent preferentially adsorbs the selected xylene isomer, and the remaining raffinate xylene isomers are discharged with excess desorbent in the mixed raffinate stream  46 . Desorbent is charged into the unit  38  by the toluene “light” desorbent stream  37   a  to desorb the selected xylene isomer. The desorbent is then separated from the selected isomer by distillation that occurs within the unit  38 , and the selected xylene isomer is discharged as product stream  47 , which is removed from the system  10  as the selected xylene product. Several different embodiments of the adsorptive separation unit  38  are possible, such as a single bed operated in batch fashion, where the mixed raffinate stream  46  is collected before the desired xylene isomer is desorbed, and the extract stream is collected after desorbing. In another embodiment, a plurality of adsorbent beds are used, and the introduction point of the mixed xylene stream  20  and the desorbent stream  37   a  are gradually moved through the different adsorbent beds. The discharge points of the extract stream and the mixed raffinate stream  46  are also gradually moved through the different adsorbent beds, so each individual adsorbent bed is used in a semi-batch mode and the combination simulates a continuous operation. As noted above, the desorbent has a lower molecular weight than xylene, and a desorbent boiling point lower than the selected xylene isomer boiling point or the raffinate xylene isomer(s) boiling point. 
     The mixed raffinate stream  46  produced from adsorptive separation unit  38  is thereafter passed to the second side  35   b  of the split shell distillation column  35 . In the second side  35   b , toluene desorbent in the mixed raffinate stream  46  is separated by boiling point distillation from the non-selected xylene isomers. The toluene, as noted above, is removed from the split-shell distillation column  35  in the combined overhead region via stream  37  for recycling back to streams  37   a  and  37   b , and the non-selected xylene isomers are removed as a bottom product from the second side  35   b  via stream  48 . The use of the split-shell configuration for the distillation column  35  saves capital cost as compared to a system that uses two separate distillation columns. As the overhead product for the distillation occurring in side  35   a  and side  35   b  is the same (i.e., toluene), comingling the overhead product while maintaining separate mid and bottom regions allows for two separate distillation processes to be integrated into a single distillation column, thereby requiring only one distillation column and common overhead system equipment. 
     The non-selected xylene isomer stream  48  is thereafter passed to an isomerization unit  49  wherein the raffinate xylene isomers, which are the xylene isomers other than the selected xylene isomer, are isomerized to produce more of the selected xylene isomer. The selected xylene isomer was removed in the adsorptive separation unit  38 , and the removal of one isomer shifts the isomer composition away from equilibrium. As such, the selected xylene isomer, which is the isomer primarily absent from the stream  48 , is produced in the isomerization unit  49  to bring the mixture closer to an equilibrium ratio. The equilibrium ratio is about 20 to about 25 percent orthoxylene, about 20 to about 30 percent paraxylene, and about 50 to about 60 percent metaxylene at about 250° C., and this equilibrium ratio varies with temperature and other conditions. 
     In an exemplary embodiment, the isomerization unit  49  includes an isomerization catalyst, and operates at suitable isomerization conditions. Suitable isomerization conditions include a temperature from about 100° C. to about 500° C. (about 200° F. to about 900° F.), or from about 200° C. to about 400° C. (about 400° F. to about 800° F.), and a pressure from about 500 kPa to about 5,000 kPa (about 70 PSIA to about 700 PSIA). The isomerization unit includes a sufficient volume of isomerization catalyst to provide a liquid hourly space velocity, with respect to the stream  48 , from about 0.5 to about 50 hr −1 , or from about 0.5 to about 20 hr −1 . Hydrogen may be provided to the isomerization unit via stream  50  at up to about 15 moles of hydrogen per mole of xylene, but in some embodiments hydrogen is essentially absent from the isomerization unit  49 . The isomerization unit  49  may include one, two, or more reactors, where suitable means are employed to ensure a suitable isomerization temperature at the entrance to each reactor. The xylenes are contacted with the isomerization catalyst in any suitable manner, including upward flow, downward flow, or radial flow. 
     In one embodiment, the isomerization catalyst includes a zeolitic aluminosilicate with a Si:Al 2  ratio greater than about 10/1, or greater than about 20/1 in some embodiments, and a pore diameter of about 5 to about 8 angstroms. Some examples of suitable zeolites include, but are not limited to, MFI, MEL, EUO, FER, MFS, MTT, MTW, TON, MOR, and FAU, and gallium may be present as a component of the crystal structure. In some embodiments, the Si:Ga 2  mole ratio is less than 500/1, or less than 100/1 in other embodiments. The proportion of zeolite in the catalyst is generally from about 1 to about 99 weight percent, or from about 25 to about 75 weight percent. In some embodiments, the isomerization catalyst includes about 0.01 to about 2 weight percent of one or more of ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), Iridium (Ir), and platinum (Pt), but in other embodiments the isomerization catalyst is substantially absent of any metallic compound, where substantial absence is less than about 0.01 weight percent. The balance of the isomerization catalyst is an inorganic oxide binder, such as alumina, and a wide variety of catalyst shapes can be used, including spherical or cylindrical. 
     An isomerized xylene stream  52  exits the isomerization unit  49  and returns to the stripper distillation column  17  after joining with first side bottom product stream  36 . A byproduct stream  51  that include light gasses such as butane, propane, etc., is also removed from the isomerization unit  49 . The isomerized xylene stream  52  includes more of the selected xylene isomers than in the isomerization raffinate stream  46 , so more of the selected xylene isomer is available for recovery. In this manner, the total amount of the selected xylene isomer recovered may exceed the equilibrium value. 
     As such, described herein are various exemplary systems and methods for separating a selected xylene isomer from a mixed xylene feedstock using adsorptive separation with a light desorbent. In the disclosed systems, certain separation apparatus, for example distillation columns  17  and  18 , and  35   a  and  35   b  (as split-shell column  35 ), are combined in a novel manner to reduce the overall equipment count needed to implement the system, thereby lowering the overall production costs, which, as noted above, tend to be higher in light desorbent systems. As such, the present disclosure provides separation systems and methods that improve over the prior art by allowing for a relaxed-specification (i.e., less tightly controlled specification) feed stock typical of light desorbent systems (which results in energy savings), while at the same time reducing overall capital costs as compared to systems known in the prior art. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the application in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing one or more embodiments, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope, as set forth in the appended claims.