Patent Publication Number: US-8535610-B2

Title: Apparatus for regenerating catalyst

Description:
BACKGROUND OF THE INVENTION 
     The field of the invention is the distribution of catalyst in a catalyst regenerator vessel. 
     Fluid catalytic cracking (FCC) is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of highly carbonaceous material referred to as coke is deposited on the catalyst. A high temperature regeneration operation within a regenerator zone combusts coke from the catalyst. Catalyst containing coke, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone. 
     A common objective of these configurations is maximizing product yield from the reactor while minimizing operating and equipment costs. Optimization of feedstock conversion ordinarily requires essentially complete removal of coke from the catalyst. This essentially complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 wt-% coke. In order to obtain complete regeneration, the catalyst has to be in contact with oxygen for sufficient residence time to permit thorough combustion. 
     Conventional regenerators typically include a vessel having a spent catalyst inlet, a regenerated catalyst outlet and a combustion gas distributor for supplying air or other oxygen-containing gas to the bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the flue gas before the gas exits the regenerator vessel. 
     Complete catalyst regeneration can be performed in a dilute phase fast fluidized combustion regenerator. Spent catalyst is added to a lower chamber and is transported upwardly by air under fast fluidized flow conditions while completely regenerating the catalyst. The regenerated catalyst is separated from the flue gas by a primary separator upon entering into an upper chamber in which regenerated catalyst and flue gas are further separated. Regenerated catalyst from the upper chamber is transported to the lower chamber by a recycled catalyst conduit to assist in heating the spent catalyst. 
     Oxides of nitrogen (NO X ) are usually present in regenerator flue gases but should be minimized because of environmental concerns. Production of NO X  is undesirable because it reacts with volatile organic chemicals and sunlight to form ozone. Regulated NO X  emissions generally include nitric oxide (NO) and nitrogen dioxide (NO 2 ), but the FCC process can also produce N 2 O. In an FCC regenerator, NO X  is produced almost entirely by oxidation of nitrogen compounds originating in the FCC feedstock and accumulating in the spent catalyst. At FCC regenerator operating conditions, there is negligible NO X  production associated with oxidation of N 2  from the combustion air. Low excess air in the regenerator is often used by refiners to keep NO X  emissions low. 
     After burn is a phenomenon that occurs when hot flue gas that has been separated from regenerated catalyst contains carbon monoxide that combusts to carbon dioxide. The catalyst that serves as a heat sink no longer can absorb the heat thus subjecting surrounding equipment to higher temperatures and perhaps creating an atmosphere conducive to the generation of nitrous oxides. Incomplete combustion to carbon dioxide can result from poor fluidization or aeration of the spent catalyst in the regenerator vessel or poor distribution of spent catalyst into the regenerator vessel. 
     To avoid after burn, many refiners have carbon monoxide promoter (CO promoter) metal such as costly platinum added to the FCC catalyst to promote the complete combustion to carbon dioxide before separation of the flue gas from the catalyst at the low excess oxygen required to control NO X  at low levels. While low excess oxygen reduces NO X , the simultaneous use of CO promoter often needed for after burn control can more than offset the advantage of low excess oxygen. The CO promoter decreases CO emissions but increases NO X  emissions in the regenerator flue gas. 
     On the other hand, many refiners use high levels of CO promoter and high levels of excess oxygen to accelerate combustion and reduce afterburning in the regenerator, especially when operating at high throughputs. These practices may increase NO X  by up to 10-fold from the 10-30 ppm possible when no platinum CO promoter is used and excess O 2  is controlled below 0.5 vol-%. 
     When catalyst is not thoroughly, evenly distributed in the regenerator, high temperature differentials can occur across the cross section of the regenerator vessel. Hot spots can occur in some sections of the vessel which can cause damage to nearby equipment. The cooler sections of catalyst may avoid combustion at the same rate as other sections which may allow excess oxygen to travel to the upper vessel to raise the risk of afterburn as well as insufficiently regenerated catalyst. 
     Improved methods are sought for preventing after burn and generation of nitrous oxides. Even distribution of catalyst and combustion gas in a regenerator promotes more uniform temperatures and catalyst activity fostering more efficient combustion of coke from catalyst. 
     SUMMARY OF THE INVENTION 
     In an apparatus embodiment, the subject invention comprises a catalyst regenerator comprising a first chamber. A gas distributor and a cup are in the first chamber. A catalyst conduit extends to the cup. 
     In an additional apparatus embodiment, the subject invention comprises a catalyst regenerator comprising a first chamber. A gas distributor is in the first chamber. A cup is also in the first chamber. The cup has a closed upper end and an open lower end. A recycled catalyst conduit and a spent catalyst conduit extend to the cup. 
     In a further apparatus embodiment, the subject invention comprises a catalyst regenerator comprising a first chamber. A gas distributor is in the first chamber which has a cap that is impermeable to gas. A cup in the first chamber has a closed end and an open end and a catalyst conduit extending to the cup. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, elevational view of an FCC unit incorporating the present invention. 
         FIG. 2  is a sectional view taken along segment  2 - 2  in  FIG. 1 . 
         FIG. 3  is a schematic, elevational view of an FCC unit incorporating an alternative embodiment of the present invention. 
     
    
    
     DEFINITIONS 
     The term “communication” means that material flow is operatively permitted between enumerated components. 
     The term “downstream communication” means that at least a portion of material flowing to the subject in downstream communication may operatively flow from the object with which it communicates. 
     The term “upstream communication” means that at least a portion of the material flowing from the subject in upstream communication may operatively flow to the object with which it communicates. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The subject invention is an apparatus and process for mixing spent and recycled regenerated catalyst in a confined volume in the regenerator vessel. By mixing hotter recycled catalyst with cooler spent catalyst in the confined volume of a mixer, uniform catalyst temperature is maintained through the regenerator. 
     Although other uses are contemplated, the process and apparatus of the present invention may be embodied in an FCC unit.  FIG. 1  shows an FCC unit that includes a reactor section  10  and a regenerator vessel  50 . A regenerated catalyst conduit  12  transfers regenerated catalyst from the regenerator vessel  50  at a rate regulated by a control valve  14  to a riser  20  of the reactor section  10 . A fluidization inert gaseous medium such as steam from a nozzle  16  transports regenerated catalyst upwardly through the riser  20  at a relatively high density until a plurality of feed injection nozzles  18  inject hydrocarbon feed across the flowing stream of catalyst particles. The catalyst contacts the hydrocarbon feed cracking it to produce smaller, cracked hydrocarbon products while depositing coke on the catalyst to produce spent catalyst. 
     A conventional FCC feedstock or higher boiling hydrocarbon feedstock are suitable feeds. The most common of such conventional feedstocks is a “vacuum gas oil” (VGO), which is typically a hydrocarbon material having a boiling range of from 343 to 552° C. (650 to 1025° F.) prepared by vacuum fractionation of atmospheric residue. Such a fraction is generally low in coke precursors and heavy metal contamination which can serve to contaminate catalyst. Heavy hydrocarbon feedstocks to which this invention may be applied include heavy bottoms from crude oil, heavy bitumen crude oil, shale oil, tar sand extract, deasphalted residue, products from coal liquefaction, atmospheric and vacuum reduced crudes. Heavy feedstocks for this invention also include mixtures of the above hydrocarbons and the foregoing list is not comprehensive. 
     The resulting mixture continues upwardly through the riser  20  to a top at which a plurality of disengaging arms  22  tangentially and horizontally discharge the mixture of gas and catalyst from a top of the riser  20  through ports  24  into a disengaging vessel  26  that effect separation of gases from the catalyst. A transport conduit  28  carries the hydrocarbon vapors, including stripped hydrocarbons, stripping media and entrained catalyst to one or more cyclones  30  in a reactor vessel  32  which separate spent catalyst from the hydrocarbon vapor stream. The reactor vessel  32  may at least partially contain the disengaging vessel  26  and the disengaging vessel  26  is considered part of the reactor vessel  32 . A collection chamber  34  in the reactor vessel  32  gathers the separated hydrocarbon vapor streams from the cyclones  30  for passage to an outlet nozzle  36  and eventually into a fractionation recovery zone (not shown). Diplegs  38  discharge catalyst from the cyclones  30  into a lower portion of the reactor vessel  32  that eventually passes the catalyst and adsorbed or entrained hydrocarbons into a stripping section  40  of the reactor vessel  32  across ports  42  defined in a wall of the disengaging vessel  26 . Catalyst separated in the disengaging vessel  26  passes directly into the stripping section  40 . The stripping section  40  contains baffles  43 ,  44  or other equipment to promote mixing between a stripping gas and the catalyst. The stripping gas enters a lower portion of the stripping section  40  through a conduit to one or more distributors  46 . The spent catalyst leaves the stripping section  40  of the reactor vessel  32  through a reactor catalyst conduit  48  which delivers it to the regenerator vessel  50  at a rate regulated by a control valve  52 . The spent catalyst from the reactor vessel  32  usually contains carbon in an amount of from 0.2 to 2 wt-%, which is present in the form of coke. Although coke is primarily composed of carbon, it may contain from 3 to 12 wt-% hydrogen as well as sulfur and other materials. The reactor catalyst conduit  48  with an inlet  48   a  in downstream communication with the reactor vessel  32  may deliver spent catalyst to the regenerator vessel  50 . 
       FIG. 1  shows an embodiment of a regenerator vessel  50  that is a combustor type of regenerator, which may use hybrid turbulent bed-fast fluidized conditions in a high-efficiency regenerator vessel  50  for completely regenerating spent catalyst. However, other catalyst regenerators may be suitable applications for the present invention. Spent catalyst regulated by control valve  52  descends in the reactor catalyst conduit  48  and enters a lower or first chamber  54  of the combustor regenerator vessel  50  through catalyst inlet  56 . Reactor catalyst conduit  48  delivers spent catalyst to a catalyst mixer  60 . 
     Oxygen-containing combustion gas, typically air, from combustion gas line  55  is primarily delivered to the regenerator vessel  50  by a combustion gas distributor  80  below the catalyst mixer  60 . In an embodiment, combustion gas distributor  80  distributes most of the combustion gas to the regenerator vessel  50  and is fed by a distributor gas line  55   a  from combustion gas line  55 . 
     A sectional view of the lower chamber  54  is shown in  FIG. 2  taken along segment  2 - 2  in  FIG. 1 . The reactor catalyst conduit  48 , the mixer  60  and recycle catalyst conduit  108  obscure the combustion gas distributor  80 , so parts of it are shown in phantom. The distributor includes a riser  82  shown in phantom in  FIG. 2  which receives combustion gas from line  55   a  as shown in  FIG. 1 . The riser has an upper end that terminates in a cap  84  that is typically impermeable to gas flow. Branches  86  radiating from the riser  82  carry combustion gas to a plurality of flutes  88 . Apertures (not shown) in the flutes  88  are arranged to emit combustion gas equally to the entire cross section of the regenerator vessel  50 . Consequently, the combustion gas distributor typically distributes no combustion gas to the central region of the lower chamber  54 . 
     Turning back to  FIG. 1 , the combustion gas distributor  80  distributes gas from distributor gas line  55   a  to the lower chamber  54  of the regenerator vessel. The oxygen in the combustion gas contacts the spent catalyst and combusts carbonaceous deposits from the catalyst to regenerate the catalyst and generate flue gas. The rising combustion gas and produced flue gas lifts the catalyst at a superficial velocity of combustion gas in the lower chamber  54  of at least 1.1 m/s (3.5 ft/s) under fast fluidized flow conditions. In an embodiment, flow conditions in the lower chamber  54  will include a catalyst density of from 48 to 320 kg/m 3  (3 to 20 lb/ft 3 ) and a superficial gas velocity of 1.1 to 2.2 m/s (3.5 to 7 ft/s). 
     If air is the combustion gas, typically 13-15 kg (lbs) of air is required per kilogram (pound) of coke fed on catalyst to the regenerator. The temperature of the regenerator vessel  50  is about 500 to 900° C. (932° to 1652° F.) and usually about 600 to 750° C. (1112° to 1382° F.). Pressure in the regenerator vessel  50  is preferably 173 to 414 kPa (gauge) (25 to 60 psig). The superficial velocity of the combustion gas is typically less than 1.7 m/s (5.5 ft/s) and the density of the dense bed is typically greater than 320 kg/m 3  (20 lb/ft 3 ) depending on the characteristics of the catalyst. 
     During combustion, the mixture of catalyst and gas in the lower chamber  54  ascend through a frustoconical transition section  116  to the transport, riser section  118  of the lower chamber  54 . The riser section defines a tube and extends upwardly from the lower chamber  54 . The mixture of catalyst and gas travels at a higher superficial gas velocity than in the lower chamber  54  due to the reduced cross-sectional area of the riser section  118  relative to the cross-sectional area of the lower chamber  54  below the transition section  116 . Hence, the superficial gas velocity will usually exceed about 2.2 m/s (7 ft/s). The riser section  118  will have a relatively lower catalyst density of less than about 80 kg/m 3  (5 lb/ft 3 ). 
     The regenerated catalyst and flue gas are transported from the first or lower chamber  54  into a second or upper chamber  104 . The mixture of catalyst particles and flue gas is discharged from an upper portion of the riser section  118  into the upper chamber  104  which is in downstream communication with the lower chamber  54 . Substantially completely regenerated catalyst may exit the top of the riser section  118 , but arrangements in which partially regenerated catalyst exits from the lower chamber  54  are also contemplated. Discharge is effected through a disengaging device  120  that separates a majority of the regenerated catalyst from the flue gas upon entry into the second chamber  104 . Initial separation of catalyst upon exiting the riser section  118  minimizes the catalyst loading on cyclone separators  122 ,  124  or other downstream devices used for the essentially complete removal of catalyst particles from the flue gas, thereby reducing overall equipment costs. In an embodiment, catalyst and gas flowing up the riser section  118  impact a top elliptical cap  126  of the riser section  118  and reverse flow. The catalyst and gas then exit through downwardly directed openings in radial disengaging arms  128  of the disengaging device  120 . The sudden loss of momentum and downward flow reversal cause at least about 70 wt-% of the heavier catalyst to fall to the dense catalyst bed  92  and the lighter flue gas and a minor portion of the catalyst still entrained therein to ascend upwardly in the upper or second chamber  104 . Downwardly falling, disengaged catalyst collects in the dense catalyst bed  92 . Catalyst densities in the dense catalyst bed  92  are typically kept within a range of from about 640 to about 960 kg/m 3  (40 to 60 lb/ft 3 ). 
     A fluidizing gas line  55   b  delivers fluidizing gas to the dense catalyst bed  92  through a fluidizing distributor  131 . Fluidizing gas may be combustion gas, typically air, and may branch from combustion gas line  55 . In combustor regenerator vessel  50 , in which full combustion of coke is effected in the lower chamber  54 , approximately no more than 2 wt-% of the total gas requirements within the process enters the dense catalyst bed  92  through the fluidizing distributor  131  with the remainder being added to the lower chamber  54 . In this embodiment, gas is added to the upper chamber  104  not for combustion purposes, but only for fluidizing purposes, so the catalyst will fluidly exit through the catalyst conduits  108  and  12 . 
     If air is the combustion gas, typically 13 to 15 kg (lbs) of air is required per kilogram (pound) of coke fed on catalyst to the regenerator. The combustor regenerator vessel  50  typically has a temperature of about 593° to about 704° C. (1100 to 1300° F.) in the lower chamber  54  and about 649 to about 760° C. (1200 to 1400° F.) in the upper chamber  104 . Pressure may be between 173 and 414 kPa (gauge) (25 to 60 psig) in both chambers. 
     The combined flue and fluidizing gas and entrained particles of catalyst enter one or more separation means, such as the cyclone separators  122 ,  124 , which separates catalyst fines from the gas. Flue gas, relatively free of catalyst is collected in a collector  90  and is withdrawn from the combustor regenerator vessel  50  through an exit conduit  130  while recovered catalyst is returned to the dense catalyst bed  92  through respective diplegs  132 ,  134 . Catalyst from the dense catalyst bed  92  is transferred through the regenerated catalyst conduit  12  back to the reactor section  10  where it again contacts feed as the FCC process continues. 
     In an embodiment, to accelerate combustion of the coke in the lower chamber  54 , hot regenerated catalyst from a dense catalyst bed  92  in an upper or second chamber  104  may be recycled into the lower chamber  54  via an external recycle catalyst conduit  108  regulated by a control valve  106 . Hot regenerated catalyst enters an inlet  108   a  of recycle catalyst conduit  108  which is in downstream communication with the upper chamber  104 . Recycled regenerated catalyst enters the lower chamber  54  through regenerated catalyst inlet  76 . 
     The reactor catalyst conduit  48  may extend to and communicate with the catalyst mixer  60 . The outlet end of the reactor catalyst conduit  48  may be in upstream communication with a catalyst mixer  60  in the lower chamber  54 . A predominant portion of the reactor catalyst conduit  48  is disposed above the catalyst mixer  60 . A segment  48   b  in the reactor catalyst conduit  48  may protrude through a vertical wall  50   a  of the regenerator vessel  50 . Spent catalyst enters the catalyst mixer  60  through an entrance  64  in downstream communication with the catalyst inlet  56 . The entrance  64  may simply be an opening in the side of the cup  62  which may correspond to the inner diameter of the respective conduits  48 . 
     The recycle catalyst conduit  108  may extend to the catalyst mixer  60 . The outlet end of the recycle catalyst conduit may be in upstream communication with a catalyst mixer  60  in the lower chamber  54 . A predominant portion of the recycle catalyst conduit  108  is disposed above the catalyst mixer  60 . A segment  108   b  in the reactor catalyst conduit  48  may protrude through a vertical wall  50   a  of the regenerator vessel  50 . Recycled, regenerated catalyst enters the catalyst mixer  60  through an entrance  66  in downstream communication with a catalyst inlet  76 . The entrance  66  may simply be an opening in the side of the cup  62  which may correspond to the inner diameter of the respective conduits  108 . 
     The mixer  60  defines a confined volume  61  in which spent catalyst having coke deposits and regenerated catalyst are mixed in the regenerator vessel  50  to raise the temperature of the spent catalyst. In an aspect, mixed catalyst is allowed to exit downwardly from the confined volume  61 . In an aspect, the mixer  60  is located in the lower chamber  54  of the regenerator. The mixer  60  may comprise a cup  62  defining the confined volume  61 . The cup  62  may be inverted to have a closed upper end and an open lower end and may define a cylinder. The cup  62  may have a cylindrical wall. The closed upper end may be a cap or a dome enclosing the cylindrical wall while the open lower end may be an opening defined by the cylindrical wall. A catalyst conduit may extend to the cup  62  to deliver catalyst to the cup. In an aspect, the reactor catalyst conduit  48  extends to the cup  62  to deliver spent catalyst to the cup  62 . In a further aspect, the recycle catalyst conduit  108  may extend to the cup to deliver recycled regenerated catalyst to the cup  62 . Spent catalyst and recycled regenerated catalyst may be mixed in the cup. The entrance  64  may be diametrically opposed to the entrance  66 . One or both of the entrances  64  and  66  may be tangentially oriented with respect to the cylinder defined by the cup to generate a mixing vortex upon entry into the mixing volume. The bottom of the cup may have a serrated or dentated lower edge  68  to break gas bubbles that may descend and rise around the lower edge to prevent uneven distribution. 
     In the mixer  60 , spent catalyst from reactor catalyst conduit  48  mixes with regenerated catalyst recycled to the mixer  60  from recycle catalyst conduit  108 . Mixing hot regenerated catalyst from recycle catalyst conduit  108  with relatively cool spent catalyst from the reactor catalyst conduit  48  entering the lower chamber  54  raises the overall temperature of the catalyst and gas mixture in the lower chamber  54 . By mixing the recycled and spent catalyst in a confined volume  61  in mixer  60 , the temperature differential of catalyst in the regenerator is diminished because catalyst temperature is maintained at a uniform temperature throughout the lower chamber  54 . The catalyst mixer  60  distributes the mixed catalyst to the lower chamber  54  of combustor regenerator vessel  50 . 
     In an aspect, the cup may be disposed above the combustion gas distributor  80 , and the open end of the cup  62  may face the combustion gas distributor  80 . Oxygen may be distributed to the regenerator vessel below the cup  62 . The premixing of the spent and regenerated catalyst provides a uniform mixture of spent and regenerated catalyst and therefore a uniform temperature profile throughout the cross section of the lower chamber  54 . 
     The mixed regenerated and spent catalyst exit from the cup. In an aspect, the mixed catalyst is allowed to exit from the cup  62  downwardly. The combustion gas from distributor  80  contacts the mixed catalyst descending from catalyst mixer  60  and combusts coke deposits from the spent catalyst to produce regenerated catalyst and flue gas. 
       FIG. 3  shows an embodiment of a regenerator vessel  50 ′ which may be more ideal for a revamp situation than the embodiment in  FIG. 1 . Elements in  FIG. 3  with the same configuration as in  FIG. 1  will have the same reference numeral as in  FIG. 1 . Elements in  FIG. 3  which have a different configuration as the corresponding element in  FIG. 1  will have the same reference numeral but designated with a prime symbol (′). The configuration and operation of the embodiment of  FIG. 3  is essentially the same as in  FIG. 1 . 
     In  FIG. 3 , the segment  48   b ′ may bend from the rest of the conduit  48 ′ and may extend horizontally or at a smaller angle relative to horizontal than the rest of the conduit  48 ′ as it protrudes through the vertical wall  50   a ′ and extends to the entrance  64  to the cup  62 . Similarly, the segment  108   b ′ may bend from the rest of the conduit  108 ′ and may extend horizontally or at a smaller angle relative to horizontal than the rest of the conduit  108 ′ as it protrudes through the vertical wall  50   a ′ and extends to the entrance  66  to the cup  62 . In an aspect, the reactor catalyst conduit  48 ′ may deliver spent catalyst to the mixer  60  through the segment  48   b ′ through the entrance  64  to the cup. In an aspect, the regenerator catalyst conduit  108 ′ may deliver hot regenerated catalyst to the mixer  60  through segment  108   b ′ through entrance  66  to the cup. Entrances  64  and  66  may simply be openings in the side of the cup  62  which may correspond to the inner diameter of the respective conduits  48 ′ and  108 ′. 
     A transport gas which may be a combustion gas is provided by fluidizing gas line  55   c  originating from combustion gas supply  55  to propel the spent catalyst through segment  48   b ′ to the catalyst mixer  60 , specifically to the confined volume  61 . A nozzle  57   c  directs the transport gas through the segment  48   b ′ of the reactor catalyst conduit  48 . A transport gas which may be a combustion gas is provided by transport gas line  55   d  to propel the recycle catalyst through segment  108   b ′ to the catalyst mixer  60 , specifically to the confined volume  61 . A nozzle  57   d  directs the transport gas through the segment  108   b  of the reactor catalyst conduit  48 . Combustion gas added via gas lines  55   c  and  55   d  may account for about 10-20 wt-% of the gas to the combustor regenerator vessel  50 . 
     As explained with respect to  FIG. 2 , the distributor  80  does not distribute combustion gas to the central region of the lower chamber  54  above the cap  84  of the combustion gas riser  82 . The open end of the cup  62  may be vertically aligned with the cap  84 . By using combustion gas to urge catalyst through segments  48   b ′ and  108   b ′, combustion of spent catalyst may begin in conduit  48 ′ and combustion gas exiting from the bottom of the mixer  60  with the mixed catalyst enters into the central region to provide combustion gas to catalyst located in the central region. 
     Indeed, fluidizing gas may be used in the reactor catalyst conduit  48  and recycle catalyst conduit  108  of  FIG. 1 . If the fluidizing gas is a combustion gas it may provide the same distribution of combustion gas to the central region as above explained with respect to  FIG. 3 . 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. 
     In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.