Abstract:
Processes are disclosed for increasing the condensed phase production of BCl 3  comprising less than about 10 ppm phosgene, less than 10 ppm chlorine, and less than 10 ppm HCl. In one embodiment the process comprises injecting an inert gas into a container having condensed BCl 3  therein, the condensed BCl 3  having therein a minor portion of phosgene impurity. A major portion of the phosgene in the condensed BCl 3  is decomposed to carbon monoxide and chlorine by increasing temperature to produce a phosgene deficient stream. The temperature of the phosgene deficient stream is then decreased, and contacted with an adsorbent to remove the chlorine in the stream by adsorption to form a chlorine and phosgene free condensed stream. The chlorine and phosgene free stream is stripped using an inert gas to form a BCl 3  product condensed stream, and an inert gas is used to pump the BCl 3  product condensed stream to a product receiver.

Description:
BACKGROUND OF THE INVENTION  
         [1]    1. 1. Field of the Invention  
           [2]    2. The invention relates to processes and systems for purifying boron trichloride. In particular, the invention relates to processes and systems or apparatus which remove several critical impurities of boron trichloride to produce a highly purified final product required for some of its more stringent applications.  
           [3]    3. 2. Related Art  
           [4]    4. Boron trichloride (also referred to herein as “BCl 3 ”) is a highly reactive compound packaged as a liquid under its own vapor pressure of 1.3 bar (130 kPa) absolute at 21° C. that has numerous diverse applications. It is used predominantly as a source of boron in a variety of manufacturing processes. For example, in the manufacturing of structural materials, boron trichloride is the precursor for chemical vapor deposition (“CVD”) of boron filaments used to reinforce high performance composite materials. BCl 3  is also used as a CVD precursor in the boron doping of optical fibers, scratch resistant coatings, and semiconductors. Some of the non-CVD applications of BCl 3  are reactive ion etching of semiconductor integrated circuits and refining of metal alloys. In metallurgical applications, it is used to remove oxides, carbides, and nitrides from molten metals. In particular, BCl 3  is used to refine aluminum and its alloys to improve tensile strength.  
           [5]    5. Two of the most stringent applications for high purity BCL 3  involve semiconductor and optical fiber manufacturing. In these industries the specified impurity levels in BCl 3  must be of the order of 1 ppm or less in order to maintain product quality. In fact, the impurities in most commercially available BCl 3  are often present at levels over two orders of magnitude beyond acceptable levels for these processes such as, for example, air, CO 2 , HCl, Cl 2 , and COCl 2  (“phosgene”). Furthermore, in these particular applications, any oxygen or oxygen containing impurities (such as phosgene) in the BCl 3  are especially detrimental to the manufacturing process due to the formation of certain oxide compounds. Another class of detrimental impurities in BCl 3  for these processes are metal containing impurities.  
           [6]    6. Geographically, BCl 3  is produced almost entirely in the United States. As of 1995, as much as 220 metric tons has been consumed in the United States where about 30% has gone into the production of boron reinforcement filaments, the remaining split primarily among semiconductor etching, Friedel-Crafts catalysis reactions, and intermediate use in pharmaceuticals. In comparison, Japan consumes 70 metric tons which was all imported from the United States. In Japan, BCl 3  is used primarily in semiconductor etching and manufacture of crucibles for silicon ingots. Western European countries consumed only about 5 metric tons. (Chemical Economics Handbook, October, 1996.)  
           [7]    7. The source cost of BCl 3  varies considerably per pound depending upon purity grade and supplier. There is a strong incentive to purchase BCl 3  domestically at a low cost and purify the material to stringent semiconductor purity requirements of technically 1 ppm or less for the light impurities.  
           [8]    8. After extensively searching the literature and patents, there appears to be no production process technology to have been described or patented regarding how to efficiently remove various impurities from boron trichloride by an integrated purification process technology comprising several different functional chemical processes which are connected sequentially and various impurities associated with boron trichloride are removed sequentially and continuously.  
           [9]    9. The removal of some impuritites in BCl 3  has been disclosed previously. In particular, most publications have focused on how to remove phosgene from boron trichloride. This is because phosgene has similar vapor pressure to BCl 3  and hence becomes difficult to remove by simple distillation. The previous methods for phosgene removal from BCl 3  include electrical discharge, laser pyrolysis, fractional distillation, UV photolysis, and redox chemistry.  
           [10]    10. Although the individual methods aforementioned had indicated to be able to reduce phosgene content in boron trichloride to a certain degree, these methods do have their drawbacks. For instance, the use of electrical discharge and laser pyrolysis is difficult to implement on a larger industrial scale without extensive equipment and capital costs, and therefore, the economics are not feasible. UV photolysis lacks effectiveness for phosgene removal to very low ppm levels. Further, the similarity of physical properties of phosgene and boron trichloride makes phase separation by distillation and differential surface adsorption difficult to implement in a practical manner. It is also known to use selective chemistry to remove phosgene from BCl 3 . In these methods phosgene in the BCl 3  is allowed to oxidize molten metals such as mercury, copper, and titanium to form the corresponding metal chlorides and carbon monoxide. Although effective in removing phosgene, this approach presents problems with metal contamination, which is particularly difficult due to the volatility of metal chlorides.  
           [11]    11. In view of all the drawbacks aforementioned, the preferred process of removing phosgene is by thermal decomposition via a catalyst with a specified elevated temperature. For example, the phosgene decomposition on a preferably metal free carbonaceous catalyst was described by two earlier publications. However, in each of these two cases, other troublesome impurities were generated (chlorine in one case, and hydrogen chloride in the other) which require independent purification steps.  
           [12]    12. Another problem with known BCl 3  purification methods is the need to resort to vacuum generating devices or thermal heating of source material and associated handling systems to improve the rate of vapor transport through packed beds of adsorbents or catalytic materials. In known BCl 3  purification methods using packed beds such as the case of carbonaceous catalysts, there are significant pressure drops associated with packed beds when high volumetric flow rates are employed and good surface contact required. For many gases, this is not a problem. But, when it comes to BCl 3 , material transport through such pressure drops becomes significantly hindered due to the BCl 3  liquid having only a 1.3 bar vapor pressure at ambient temperature. Thus, maintaining reasonable flow rates through such devices requires some auxiliary means of promoting flow. Conventionally, flow throughput can be advanced by either increasing upstream pressure or decreasing downstream pressure. Increasing upstream pressure can be done using commonly known techniques of gravimetric feeding, mechanical pumping, or thermal heating of source material. However, in the specific case of producing high purity corrosive gases like BCl 3 , the reactive nature of BCl 3  makes the mechanical devices undesirable requiring high maintenance and excessive costs while providing low reliability and the increased likelihood of contamination of the BCl 3  by metallic impurities. Gravimetric feeding (in other words, elevating source material relative to the rest of the system) effectively promotes flow as only 2 meter height provides almost 1 bar additional upstream pressure. However, this approach still suffers from the intolerable feature of requiring material transport through the system as entirely liquid phase instead of vapor phase. As a consequence of liquid phase present in the system, excessive contamination of BCl 3  by metallic impurities can occur from enhanced liquid phase corrosion mechanisms thereby degrading product purity with detrimental metallic impurities.  
           [13]    13. One known method of increasing upstream pressure with vapor condensation downstream is to heat the source material and all associated gas handling components to an isothermal temperature. The method is feasible but requires careful temperature control to assure uniform temperature throughout the system. Although feasible, this technique becomes difficult to implement in practice especially for high capacity industrial production.  
           [14]    14. Resorting to decreasing downstream pressure has its difficulties also. The simplest approach of mechanical pumping suffers from the same problems as in the upstream case. The use of simple low temperature condensation of BCl 3  downstream prevents the problems of mechanical pumping but will lead to accumulation of metallic impurities in the final product collected hence degrading purity.  
         SUMMARY OF THE INVENTION  
         [15]    15. In the processes of the present invention, phosgene removal is performed by the preferred thermal decomposition route in a manner in which the decomposition impurities are preferably continuously removed. In accordance with the present invention, low temperature condensation is utilized along with secondary inert gas stream such as He, N 2 , or Ar. In this technique, as disclosed in further detail herein below, the BCl 3  material is carried through the defined purification system alone with a secondary inert gas stream. The presence of such a gas stream having higher vapor pressure allows the overall system to be operated at higher pressures than that provided from BCl 3  vapor pressure alone. This is preferably performed most simply by bubbling the inert gas through the liquid BCl 3  and flowing the mixed gas stream through the system, after which the inert gas is easily separated from the purified BCl 3  product collected.  
           [16]    16. A first aspect of the invention is a process of producing a BCl 3  vapor stream containing an inert gas selected from the group consisting of helium, argon, krypton, neon, xenon, or mixtures of one or more of these, from a lower purity BCl 3  source, the BCl 3 /inert gas vapor stream having less than 10 ppm chlorine, less than 10 ppm phosgene, and less than 10 ppm each of light impurities including, but not limited to, nitrogen, oxygen, carbon dioxide, carbon monoxide, and hydrocarbons such as methane, and less than 10 ppm of nonvolatile metal containing species. In one embodiment, using helium as the inert gas, the process comprises injecting helium into a container of a lower purity BCl 3  source having phosgene impurity to produce a vapor stream comprising BCl 3 , helium, and phosgene; decomposing a major portion of the phosgene in the BCl 3 , helium, phosgene vapor stream by heating the vapor stream to a first temperature, in the presence of a first material, to decompose substantially all the phosgene to carbon monoxide and chlorine, to form a first intermediate vapor stream comprising BCl 3 , helium, carbon monoxide, and less than 10 ppm phosgene; and adsorbing a major portion of the chlorine in the first intermediate vapor stream at a temperature lower than the first temperature using a second material, thereby producing the BCl 3 /helium vapor stream having less than less than 10 ppm chlorine, less than 10 ppm phosgene, and less than 10 ppm each of the light impurities. In preferred processes of the invention, the first and second materials are substantially the same.  
           [17]    17. A preferred process embodiment in accordance with this aspect of the invention is wherein the heating step comprises preheating the vapor stream comprising BCl 3 , helium, and phosgene prior to the vapor stream comprising BCl 3 , helium, phosgene contacting the first material, which promotes phosgene decomposition.  
           [18]    18. A particularly preferred process embodiment in accordance with this aspect of the invention is wherein the preheating comprises heat exchanging the first intermediate vapor stream with the vapor stream comprising BCl 3 , helium, and phosgene.  
           [19]    19. Preferably, the phosgene decomposition step occurs in the presence of a catalyst, the catalyst comprising materials selected from the group consisting of carbon-based materials, alumina-based materials, silica-based matrials, and mixtures thereof. Preferably, if carbon is used, it is selected from the group consisting of naturally occurring carbon, carbon molecular sieve, or other synthetic carbonaceous material. Alternatively, phosgene decomposition can be implemented in the processes of the invention with other reactive elements such as boron, silicon, and various metals such as titanium or zinc, as described in U.S. Pat. Nos. 3,037,337; 3,043,665; and 3,207,581; however, such elements are not catalytic as they are consumed in the process, and are thus subject to depletion, thus they are not therefore the preferred materials for the phosgene decomposition step.  
           [20]    20. In accordance with this aspect of the invention, the inert gas functions to increase pressure of the vapor stream comprising BCl 3 , inert gas, and phosgene to a pressure substantially higher than the vapor pressure of the lower purity BCl 3 .  
           [21]    21. Preferably, the phosgene decomposition step occurs at a temperature greater than about 200° C., and the adsorption of chlorine step preferably occurs at a temperature lower than about 50° C., although some chlorine will be adsorbed on the first material at a higher temperature in the phosgene decomposition step.  
           [22]    22. Furthermore, the chlorine adsorption step preferably comprises using a bed of adsorbent until loaded, removing the bed of adsorbent, heating the removed bed of adsorbent, and reinstalling the bed. More preferably, a second chlorine adsorption bed of same or different adsorbent could be utilized while the first is regenerating, in order to maintain continuity of the process. Alternatively, but less preferable, is the use of one bed of chlorine adsorbent with the appropriate valve configuration to allow isolation from the process and conduit connection to a regeneration system, be it via heated purge or vacuum induced desorption.  
           [23]    23. A second aspect in accordance with the invention is a process for producing an ultra-pure BCl 3  condensed phase from a vapor phase comprising impure BCl 3 . The process comprises condensing a first vapor stream in a condenser, the first vapor comprising a major portion of BCl 3  and a minor portion of HCl, light impurities, and a first inert gas selected from the group consisting of helium, argon, krypton, neon, xenon, and mixtures thereof, to form a first condensed phase comprising BCl 3  and a second vapor comprising the first inert gas, BCl 3 , and light impurities; routing the second vapor stream to a secondary condenser, at a lower temperature, thus forming a gaseous stream containing HCl, light impurities, and the first inert gas and a second condensed phase comprising BCl 3 ; and routing the first condensed phase to a stripper, or using the condenser itself at a more optimal temperature, wherein a second inert gas (the same as or different from the first) is used to strip molecules having vapor pressure greater than BCl 3  from the first condensed phase to produce a higher purity first condensed phase having less than 50 ppm hydrogen chloride, preferably less than 1 ppm hydrogen chloride, and a stripped vapor phase.  
           [24]    24. Preferably, the stripping step includes the step of allowing the first condensed phase to come to room temperature, and then contacting it with helium at a pressure ranging from about 20 psig to about 30 psig [from about 240 kPa to about 440 kPa].  
           [25]    25. Also, preferred are processes in accordance with this aspect wherein the stripped vapor phase is routed to the secondary condenser to recover residual BCl 3 , and processes wherein the stream containing only traces of BCl 3  from the secondary condenser is routed to a scrubber to remove residual traces of BCl 3 , along with HCl impurity and introduce a gaseous stream containing the inert gas and light impurities which are discharged to the atmosphere.  
           [26]    26. Further preferred processes in accordance with this aspect are those wherein the higher purity first condensed phase is transferred to a product container using ultra-high purity inert gas, preferably helium and without any other pumping or vacuum means.  
           [27]    27. A third aspect of the invention is a process for producing ultra-high purity boron trichloride in condensed phase from a lower purity boron trichloride condensed phase having phosgene impurity, the process comprising injecting an inert gas, preferably helium, into a container of lower purity BCl 3  liquid having phosgene impurity to produce a vapor stream comprising BCl 3 , inert gas, and phosgene; decomposing a major portion of the phosgene in the BCl 3 , inert gas, phosgene vapor stream by heating to a first temperature to form a first intermediate vapor stream comprising BCl 3 , inert gas, carbon monoxide, chlorine and less than 10 ppm phosgene; adsorbing a major portion of the chlorine in the first intermediate vapor stream at a temperature lower than the first temperature using a solid adsorbent material, thereby producing the BCl 3 /inert gas vapor stream having less than 10 ppm phosgene and less than 10 ppm Cl 2 ; routing said BCl 3 /inert gas vapor stream having less than about 10 ppm phosgene and less than 10 ppm Cl 2  to a condenser; condensing a first vapor stream in the condenser, the first vapor comprising a major portion of BCl 3  and a minor portion of HCl, inert gas, and light impurities to form a first condensed phase comprising BCl 3  and a second vapor comprising the inert gas, residual BCl 3 , and light impurities; routing the second vapor stream to a secondary condenser, thus forming a gaseous stream containing only traces of (preferably less than about 10 ppm) BCl 3  and a second condensed phase comprising BCl 3 ; and routing the first condensed phase to a stripper (or using the secondary condenser itself at a more optimal temperature) wherein inert gas (preferably ultra-pure helium) is used to strip molecules having a vapor pressure greater than BCl 3  from the first condensed phase to produce a higher purity first condensed phase having less than 50 ppm HCl, preferably less than 1 ppm HCl, and a stripped vapor phase.  
           [28]    28. A fourth aspect of the invention is a process for increasing the condensed phase production of BCl 3  having less than about 10 ppm phosgene, less than about 10 ppm chlorine, less than about 10 ppm each of light impurities, and less than about 10 ppm HCl, the process comprising the steps of: introducing an inert gas selected from the group consisting of helium, argon, neon, xenon, krypton, and mixtures thereof into a container having condensed BCl 3  therein, the condensed BCl 3  having therein a minor portion of phosgene impurity; converting a major portion of the phosgene in the condensed BCl 3  to carbon monoxide and chlorine by increasing temperature of the condensed BCl 3 ; decreasing the temperature of the stream and removing the chlorine by adsorption and the carbon monoxide by stripping with an inert gas selected from the group consisting of helium, argon, xenon, krypton, neon, and mixtures thereof (preferably helium); and using the inert gas to transfer the BCl 3  product to a product container.  
           [29]    29. In accordance with the present invention, several of the problems encountered in the prior art methods are overcome in the processes and apparatus of the present invention. By use of the inventive purification process technology, all significant impurities of interest in BCl 3  for such high purity applications as semiconductor and fiber optic manufacturing are removed in the inventive processes such that a low purity boron trichloride now can be purified into an ultra-pure product with a purity of 99.9995% or higher (on a helium-free basis), or higher required for certain semiconductor and fiber optic manufacturing. The inventive processes and apparatus are preferably designed so as to minimize capital investment costs and to improve reliability. In addition, environmental emission is minimal, thereby reducing exhaust abatement requirements and increasing product yield. The inventive chemical process technology is composed of several different functional chemical processes or operating units as listed in the following:  
           [30]    30. Injecting an inert gas, preferably helium, into a source container of lower purity BCl 3  liquid and extract the vapor out the container;  
           [31]    31. Using a functional catalyst such as activated carbon to thermally decompose phosgene at elevated temperature;  
           [32]    32. Using an adsorbent such as activated carbon to remove remaining chlorine at 50° C. or lower;  
           [33]    33. Condensing BCl 3  vapor which has substantially phosgene and chlorine than the source BCl 3 ;  
           [34]    34. Using an inert gas to strip the BCl 3  liquid to remove carbon monoxide, carbon dioxide, hydrogen chloride, nitrogen, oxygen and other lighter gas impurities that may be associated with lower purity BCl 3  at the beginning, and/or generated during phosgene and chlorine removing processes upstream.  
           [35]    35. Transfilling the final BCl 3  product from the inventive system into the product storage container using inert gas pressure and no other pumping or vacuum means.  
           [36]    36. It has been demonstrated that the inventive process technology is fully capable of producing an ultra-pure BCl 3  product due to the following important new features.  
           [37]    37. Activated carbon is a particularly preferred material for the catalytic and adsorption steps, used both at high and low temperatures in such a way as to decompose phosgene and adsorb chlorine byproduct, respectively. One aspect that is surprising and unexpected in the present invention is that the carbon monoxide and chlorine byproducts of phosgene decomposition can be introduced into a lower temperature carbon bed without reformation of phosgene under the process conditions presented. The preferred activated carbon material was found to be fully regenerable to chlorine adsorption without degradation inactivity from BCl 3 . The preferred activated carbon catalyst which decomposes phosgene has shown the function of a catalyst at the elevated temperature, and therefore, the carbon can be continuously used without addressing the concern of saturation and regeneration.  
           [38]    38. An ultra-dry inert gas such as helium is employed in the inventive process technology which overcomes the problem of BCl 3 &#39;s low vapor pressure, and the inert gas can drag BCl 3  vapor out of the low purity container and carry the vapor through different purification process units. As a result, this process totally eliminates the requirement of heating the lower purity BCl 3  liquid in order to provide enough vapor pressure penetrating each production process unit and of maintaining an isothermal operating condition in order to avoid the vapor condensation where the recondensation is not desired.  
           [39]    39. Further, the BCl 3  purification processes and systems of the present invention do not require any mechanical devices either to transfer the low purity BCl 3  into the purification system, or to transfill the final high purity product BCl 3  from the inventive system into a storage container. The potential contamination on the final high purity product BCl 3  by mechanical transfer means is therefore preferably eliminated, and consequently, the inventive processes and systems also operate more dependably and reliably because no mechanical component is involved in the transfer process.  
           [40]    40. In addition, the inventive processes and systems are able to run the vapor condensation and the liquid stripping separately, or simultaneously. Each chemical process unit operation of the inventive processes is preferably connected sequentially and the impurities removal operating is preferably continuously. The operating process minimizes potential air contamination and effects thereof because the entire process can be done without breaking down the system except changing the low purity and product containers. Besides, the production processes of the invention are very economical due to the product recovery from the process being 99.99% or higher within the secondary condenser, and consequently, this process technology is environmentally nonintrusive because the product is almost totally recovered with remaining trace BCl 3  and HCl impurity easily removed by conventional scrubber technology.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [41]    41.FIG. 1 represents in schematic format an apparatus and process in accordance with the present invention; and  
         [42]    42.FIG. 2 represents in schematic format the apparatus and process of FIG. 1, emphasizing certain details of the inventive apparatus.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [43]    43. Referring to FIG. 1, a preferred system  1  of the invention includes a low purity BCl 3  source container  2  and first and second valves  6  and  8  which together form a dual valve T assembly which is sealed into source container  2 , as further described in the examples. A tube  5  extends from the bottom of valve  6  into source container  2 ; an exterior port of valve  6  is connected to a valve  10 . Valve  10  in turn is connected to a conduit  12  leading to a source  20  of inert gas, for example helium. A second valve  14  and another conduit  16  also connect to the helium source  20  as well as a third conduit  18  which leads to conduit  22  and other parts of the apparatus. A connection off conduit  24  leads to a scrubber unit, while conduit  24  itself leads to a valve  26  and conduit  28  which itself leads to a heat exchanger  27 . Heat exchanger  27  represents a positive heat flow (preferably from heat exchange with flow of hot vapor exiting a reactor  30 ) into a low purity BCl 3 /helium mixture emanating from low purity BCl 3  source container  2 . An electrically heated furnace surrounding the reactor supplies supplemental heat input into reactor  30  as required. The low purity BCl 3 /helium mixture passes through conduit  28  and heat exchanger  27  and enters phosgene decomposition reactor  30  preferably from the bottom, although this is not necessary. The substantially “phosgene free” boron trichloride vapor having helium therein is directed through a conduit  32 , valve  36 , conduit  38 , and another heat exchanger  37  which removes heat from the substantially phosgene free mixture before flowing into a chlorine adsorption unit  40 , where an effective amount of an adsorbent is packed for chlorine removal. As with heat exchanger  27 , heat exchanger  37  can be any type of a variety of heat exchanger designs, such as shell and tube, tube and tube, cooling fins attached outside of conduit  38 , or even spiral wound heat exchangers. In any case, chlorine adsorption unit  40  is plumbed through a conduit  44  and a valve  46 , a conduit  64 , and a valve  72 , into a condenser  50 . A valve  34  and a conduit  42  are provided for bypassing of chlorine adsorption unit  40  if it is not needed as further explained herein. A valve  48  allows for introduction of additional helium pressure flow into the system. A conduit  52 , a valve  54 , and a conduit  56  may be used to take a product stream from the system of the invention. A valve  58  and another conduit  62  preferably lead to the analytical systems such as FTIR and UV analyzers.  
         [44]    44. Condenser  50  is fitted with a dual valve T formed from valves  72  and  74 , valve  72  having a dip tube  55  extending into condenser  50 , preferably as illustrated in FIG. 1. After a substantial portion of the boron trichloride vapor is liquefied in condenser  50 , the gas in line  76  may still contain boron trichloride vapor. This vapor is routed to a secondary condenser  60  through valve  82  to dip tube  65 . Valves  82  and  84  form another dual valve T assembly. A conduit  78  and a valve  86  form a bypass around secondary condenser  60 . Any non-condensed BCl 3 , in the flow exiting condenser  60 , is routed to a conduit  88 , a conduit  94 , and a valve  92  followed by to a scrubbing unit. A valve  90  allows helium from source  20  and conduit  18  to force vapor through the scrubber.  
         [45]    45. Referring now to FIG. 2, some details of one preferred apparatus are explained in further detail. Where numerals appear as first indicated in FIG. 1, those numerals are equivalent to those in FIG. 2. Thus, FIG. 2 illustrates phosgene decomposition reactor  30 , and chlorine adsorption unit  40 . Conduit  44  leading out of chlorine adsorption unit is shown in this figure to lead to a filter  63  which removes particles which may have been carried over from the phosgene decomposition reactor  30  and/or the chlorine adsorption unit  40 . Filter  63  is connected to a conduit  64 , valve  72 , and dip tube  55 , and into condenser  50 . Condenser  50  is vertically positioned in a vacuum jacketed top sealed container  100 , and is typically and preferably surrounded by a liquid nitrogen cooling coil  102 . Both condenser  50  and cooling coil  102  are immersed in a heat transfer medium  104 , such as an alcohol liquid bath. Liquid nitrogen enters the cooling coil through conduit  126  to exchange heat with the liquid bath and container  100 . Gaseous nitrogen or a mixture of gaseous nitrogen and liquid nitrogen exits through conduit  128 . As will be apparent to the skilled artisan, other low temperature fluids may serve this purpose as well, such as liquid argon.  
         [46]    46. Referring again to FIG. 2, illustrated is a conduit  106 , exiting from container  100 , leading to a stripper column  120 . Stripper column  120  has a source of helium, typically entering at the lower end of column  120  through a conduit  108 . This helium flows up the stripper column, and exits with some trace level BCl 3  vapor and other impurities through valve  101  and conduit  103 , and leads preferably to another vacuum jacketed top sealed container  110  having therein secondary condenser  60 . Secondary condenser  60  is surrounded with a liquid nitrogen cooling coil where liquid nitrogen enters through a conduit  130  and either a gaseous nitrogen, or a combination of liquid and gaseous nitrogen exits. Vacuum jacketed and top sealed container  110  contains a heat transfer bath  112  and both coil  114  and secondary condenser  60  are immersed in the heat transfer fluid  112  contained in container  110 .  
         [47]    47. Both the vacuum jacketed and top sealed container  100  and  110  have vent systems. As depicted in FIG. 2, container  100  has a vent conduit and valve  71  and  73  leading to a scrubber, while container  110  has a vent conduit  81  and valve  83  also leading to a scrubbing unit. Stripped product is removed from stripper  120  via conduit  116  and valve  118 . The operation of the various inventive apparatus depicted in FIGS.  1  and  2  are now explained in further operational detail using helium as the inert gas.  
         [48]    48. Helium with a pressure ranging from about 150 to about 250 psig (about 1130 to about 1820 kPa) from source  20  has been previously directed into a molecular sieve bed (not illustrated) for trace moisture removal. Hence source  20  is a supply of ultra-dry helium (simply referred to as helium hereinafter). The ultra-dry helium stream is then preferably branched to one or more different processing operations with an individually specified pressure. Helium from source  20  has also passed through a gas filter (not illustrated) where particles with a size of 0.003 μm or larger were removed.  
         [49]    49. One helium flow, with a pressure ranging from about 20 to about 30 psig (about 240 to about 310 kPa), is directed via dip tube  5  into the low purity boron trichloride liquid container  2  and bubbles through the low purity BCl 3  liquid where a mixture of the helium and BCl 3  vapor is generated. This mixture is carried into the phosgene decomposition reactor  30  in which an effective amount of catalyst, preferably activated carbon, is packed. The phosgene, as one of the impurities associated with low purity boron trichloride, is decomposed into CO and Cl 2  with the help of the catalyst at an operating temperature ranging from about 480 to about 700° F. (250 to 370° C.). Reactor  30  is heated by an electric furnace surrounding the reactor. Within reactor  30 , an elevated phosgene concentration of 500 ppm or higher in the low purity BCl 3  can be reduced to less than 0.1 ppm. In a laboratory setting, the superficial residence time was about 1 second in reactor  30 . Due to the fact that the activated carbon functions as a catalyst, saturation of the activated carbon is not a concern in this technology.  
         [50]    50. Then “phosgene free” boron trichloride vapor mixed with the helium is decreased in temperature to between 50 to 80° F. (10 to 26° C.) by heat exchange with air. The cooled gas is then directed into adsorption unit  40  where an effective amount of activated carbon is packed mainly for the purpose of chlorine removal. Since CO and Cl 2  can reform into phosgene at slightly elevated temperature, it is imperative to reduce the temperature to less than about 80° F. (26° C.) prior to directing the phosgene free BCl 3  into the second low temperature adsorbent unit  40  and maintain this low temperature in order to prevent reformation of phosgene. Further since both Cl 2  adsorption and reformation of phosgene are exothermic reactions, adsorption unit  40  is preferably configured to prevent substantial temperature build-up in adsorption unit  40 . By experiments, the preferred catalyst, activated carbon, used in adsorption unit  40  has chlorine adsorption capacity of 20%. Adsorption capacity less than 20% is considered within the invention, but it should be at least 10% to be practical. In other words, one pound (454 grams) of the preferred activated carbon can preferably retain 0.2 pound (91 grams) of chlorine. By this unit operation, the generated chlorine can be reduced to 1 ppm or less in the BCl 3  stream. The preferred activated carbon can be regenerated by heating the bed for a time sufficient to drive off the adsorbed chlorine.  
         [51]    51. Either the phosgene decomposition reactor  30  or chlorine adsorption unit  40  may contribute particles into the boron trichloride stream due to the fact that both are packed preferably with a granular material. Therefore, the flow stream exiting chlorine adsorber unit  40  preferably passes through a filter  63  in which particles having a size of 0.003 μm or larger will be retained.  
         [52]    52. After the particles are removed, the stream is then passed into a condenser  50  through a dip tube  55 . The temperature of condenser  50  is controlled between −80 and −100° F. (−62 and −73° C.) thus causing the majority of the boron trichloride vapor to be liquefied and stored. Condenser  50  is preferably vertically positioned in a vacuum jacketed top sealed container  100  (more fully described in reference to FIG. 2) in which condenser  50  is surrounded by a liquid nitrogen cooling coil  102 . Both condenser  50  and cooling coil  102  are immersed in a heat transfer medium  104  such as an alcohol liquid bath.  
         [53]    53. The alcohol liquid bath  104  is refrigerated and maintained at a designated condensation operating temperature by liquid nitrogen passing through coil  102 . After the boron trichloride vapor is liquified in condenser  50 , the helium flow exiting from condenser  50  in line  76  may still contain between 0.5 and 1.5% of boron trichloride vapor, the actual amount depending upon operating parameters typically used by skilled artisans. This vapor is routed to a secondary condenser  60  through valve  82  and dip tube  65  for further boron trichloride vapor collection where the operating temperature is preferably controlled at between −120 and −125° F. (−84 and −87° C.). The configuration and arrangement of secondary condenser  60  are similar to condenser  50  except for the lower operating temperature. Secondary condenser  60  is cooled by cooling coil  114 . Both coil  114  and secondary condenser  60  are immersed in a heat transfer bath  112  contained in vacuum jacketed, top sealed container  110 . The BCl 3  concentration in the effluent from secondary condenser  60  through valve  84  and conduit  88  is less than 100 ppm. This effluent is directed to a scrubber through valve  92  and conduit  94 . Once the BCl 3  liquid level inside condenser  50  reaches the designated holding capacity, the cold liquid BCl 3  then is preferably totally transferred via line  106  into the stripper column  120  by the helium for further impurities removal.  
         [54]    54. After the BCl 3  liquid in stripper  120  has warmed up to room temperature, the BCl 3  liquid is stripped by the helium entering at conduit  108  at an operating pressure ranging from about 20 to about 30 psig (about 240 to about 310 kPa) to strip the gas impurities out of the BCl 3  liquid. The stripped-out flow stream in line  103  is comprised of carbon monoxide, carbon dioxide, nitrogen, oxygen, hydrogen chloride, and other light gas impurities along with BCl 3  entrained in helium. The stripped-out flow containing BCl 3  vapor is directed into secondary condenser  60  for further BCl 3  vapor recovery by opening valve  101 . The effluent stream from secondary condenser  60  in conduit  94  and valve  92  is neutralized by a wet chemical scrubber (not shown) to remove trace BCl 3  vapor and other acid components such as HCl before final discharge to atmosphere.  
         [55]    55. The stripping operation in stripper  120  is continued for a length of time depending upon the starting impurity concentration and the final product specification requirements. This process can reduce the concentrations of carbon monoxide, carbon dioxide, nitrogen, and oxygen to less than 0.1 ppm in gas phase. One more important accomplishment is that this process is able to reduce hydrogen chloride to 1 ppm or lower in gas phase.  
         [56]    56. Once the concentrations of the impurities meet the final product specifications, the product is pushed out from the purification system via conduits  116  and  122  and valves  118  and  124  into a product container (not shown) by helium. Stripper  120  is then ready for another stripping operation while the vapor condensation is continued in condenser  50 .  
       EXAMPLES  
     Example 1  
       [57]    57. In this example, the BCl 3  source container  2  was an approximately 50 liter carbon steel storage vessel that was equipped with a “dual valve tee” at one end. “Dual valve tee” refers to two valves connected to a tee union whereby the base of one valve has a dip tube extending into the vessel  
         [58]    58. The dual valve tee design was used in order to introduce He (at a few guage pressure) into the liquid port valve  6  and withdraw resultant He and BCl 3  vapor mixture from the vapor port valve  8 . In this way He, in effect, bubbled directly through the liquid phase of BCl 3  carrying primarily BCl 3  vapor into the purification system. When using He in this manner no recondensation of BCl3 was observed inside the processing or analytical systems even though ambient temperature vapor pressure is only 1.3 bar.  
         [59]    59. High purity He and N 2  were used for inert gas purging where needed. The inlet to the exhaust scrubber system was a water venturi drawing a vacuum of about 20 inches Hg (50 cm Hg) (gauge pressure). This vacuum source was also available at various points along the purification train to allow removing of BCl 3  vapor from the conduits. As a precautionary measure, the He line had a molecular sieve drier placed upstream to prevent any moisture contamination from the He source. Such moisture would react with BCl 3  to form boric acid (a solid) and HCl. The drier turned out to be highly preferred because in one set of tests moisture contamination was present in some of the helium delivery lines. The resultant moisture contamination in this case lead to formation of HCl at high ppm levels; the additional HCl formation was eliminated upon installation of the drier.  
         [60]    60. After the He/BCl 3  vapor mixture left the source container  2 , it entered a phosgene decomposition reactor  30 , which decomposed the COCl 2  impurity. This tubular reactor was arranged vertically in a clam shell furnace with flow entering the bottom of the reactor. The temperature of reactor  30  was controlled at 350° C. by means of an external electrical heater. The reactor  30  contained 8.5 lbs. (about 4.2 kg) of BPL 4×6 granular activated carbon from Calgon. The reactor had dimensions of 4 inches (10 cm) in diameter and 36 inches (about 90 cm) in length. Prior to use, the activated carbon was extensively dried by a heated N 2  purge for several weeks.  
         [61]    61. After passing through the phosgene decomposition reactor  30 , the He/BCl 3  mixture with some CO and Cl 2  passed through some intermediate 0.5 inch (1.27 cm) stainless steel tubing wrapped with thin metal heat transfer fins and a tube-in-tube heat exchanger before entering the chlorine adsorption unit  40 . The fins and heat exchanger were needed for two purposes, to reduce the temperature of the He/BCl 3 /Cl 2 /CO gas stream exiting reactor  30  so valves in the system were not destroyed by the high heat, and to prevent heating of the chlorine adsorption unit  40 , which can lead to reformation of COCl 2 . The unit  40  was much smaller in size than reactor  30  and was oriented horizontally. It contained approximately 0.2 lbs. (0.1 kg) of the same activated carbon as reactor  30 . The unit  40  was used to remove any chlorine generated and then released from reactor  30 . In performing Cl 2  analysis after the carbon beds  30  and  40 , it was observed that initially all the Cl 2  was absorbed by reactor  30  alone. Eventually, when reactor  30  became saturated with Cl 2 , breakthrough occurred. The released Cl 2  was then removed by adsorption unit  40 .  
         [62]    62. After passing through adsorption unit  40 , the BCl 3  was transferred towards two low temperature condensers  50  and  60  maintained at two differing sub-ambient temperatures. Condensers  50  and  60  were equivalent in size to BCl 3  source vessel  2 . Both condensers had dual valve tees and were plumbed in series, with gas entering the inner tube of the first condenser  50  and exiting to the inner tube of the second condenser  60 . The first condenser  50  was contained in a dewar  100  with a glycol solution cooled by a refrigeration unit. The temperature of the cylinder was controlled from −11 to 40° C. During purification runs, the glycol solution was typically at about −5° C. The second condenser  60  was also contained in a dewar  110  which was packed in dry ice (about −78° C.).  
         [63]    63. FTIR and UV analyzers were installed to allow sampling of gas from many points in the purification system. Sampling of source BCl 3  was done by directly connecting BCl 3  source container  2  to the FTIR/UV analytical system. Gas flow exited the analytical system directly to the scrubber (not shown).  
         [64]    64. Design of the scrubber proved to be a fairly daunting task because of the properties of BCl 3 . Its relatively low vapor pressure at room temperature (about 1.3 bar, or about 130 kPa) causes it to vaporize very slowly. This combined with the fact it forms a solid (boric acid) upon contact with moisture caused a lot of problems with clogging of the scrubber lines. The original scrubber system used for this study was a conventional wet scrubber for acid gases. The input lines had a water venturi system with a flow rate of about 4 gallons/min (about 17.6 liters/min) which recirculated from scrubber to venturi. The venturi created a vacuum of about 20 inches Hg (about 51 cm Hg). This set-up was especially effective for hydroscopic gases like HBr or HCl that readily dissolve in water. BCl 3 , however, forms solid boric acid on contact with water. This lead to plugging problems and the scrubber design had to be slightly modified.  
         [65]    65. Modification of the scrubber was made in order to alleviate such problems described above, and is covered by applicant&#39;s copending Ser. No. 09/ ——— , filed Sep. — , 1999, and incorporated by reference herein. In order to allow the BCl 3  to dissolve in the water yet avoid contact with moisture vapor in the sampling lines, a two liquid phase system involving a halocarbon oil and sodium hydroxide solution was used. The halocarbon oil, having a density greater than water, settles on the bottom of the scrubber container. The gas stream to be treated is then directed to the bottom of the oil layer after which it bubbles up to an aqueous sodium hydroxide layer and reacts. The aqueous sodium hydroxide layer is typically a 3-6% by weight solution of NaOH. In one case experiment, this halocarbon-aqueous scrubber was placed just prior to the venturi inlet of the conventional acid scrubber unit. The vacuum created by the venturi was reduced in order to prevent any rapid evaporation of the NaOH solution from the two-phase unit. The use of the halocarbon-aqueous scrubber greatly reduced plugging of the conventional acid scrubber system.  
         [66]    66. All of the conduits used in the purification system were made of 0.25 inch (0.635 cm) and 0.5 inch (1.27 cm) diameter 316L SS electropolished tubing while some of the FTIR sampling lines were 0.125 inch (0.317 cm) 316L SS. Actual flow rates were determined by tracking weight loss of the source container  2  and the weight increase of the collection cylinders (not shown) over time.  
       Analysis and Calibrations  
       [67]    67. The FTIR used was a Midac FTIR configured to operate at 2 cm −1  resolution with a MCT detector. It had an Axiom folded path gas cell with an effective path length of 4 meters. Prior to this study, calibration of the FTIR was done for COCl 2 , HCl, and CO.  
                                         TABLE 1                                                Peak                       Cell Pressure   Location   Peak Height   Concentration   Detection       Impurity   (psig)   (cm-1)   (Abs units)   (ppm)   Limit (ppm)               COCl 2     Near ambient   851   0.422   23   ˜0.1       (bal N 2 )   pressure       HCl   5   3014, 2998   0.038, 0.051   50   ˜0.5       (bal N 2 )       CO   5   2172   0.044   50   ˜0.5       (bal N 2 )                  
 
         [68]    68. For HCl, the peaks analyzed were at 2998 cm −1  and 3014 cm −1 . These peaks were chosen since they did not interfere with the large BCl 3  peaks located within the HCl band. The estimated noise level provided detection limits of approximately 0.5 ppm under these experimental conditions.  
         [69]    69. For CO analysis, the peak at 2172 cm −1  was chosen. There is an interference with BCl 3  throughout the entire CO band. However, this was not a problem for the analysis of CO since the line width of the BCl 3  peak is much broader than the line width of the CO peaks. A simple sparging with He effectively reduced the CO below the detection limit of 0.5 ppm under these experimental conditions.  
         [70]    70. For Cl 2  analysis, a UV/VIS spectrometer (Ocean Optics) with a fiber-coupled one-meter gas cell was utilized. The purpose of this analysis was to make sure no Cl 2  from COCl 2  pyrolysis remained in the purified product. Calibration of this instrument was performed using Cl 2 /N 2  mixtures. No Cl 2  was seen in the purified product during these initial purification runs even though Cl 2  was formed from the phosgene decomposition. This is believed to be due to the high adsorption efficiency of the carbon used in the set up.  
         [71]    71. During analysis with FTIR or UV/VIS, the concentration of BCl 3  in the He/BCl 3  mixture varied from day to day somewhat due to resulting temperature of source BCl 3 . This was due to variations of both ambient temperature (changing the vapor pressure of BCl 3 ) and the flow rate of helium (helium flow rate is not controlled only helium pressure). In order to determine the BCl 3  concentration when helium was present, a weak BCl 3  band at 2139 cm −1  was measured. By monitoring this peak and comparing to that from 100% BCl 3 , a determination of the BCl 3  concentration was estimated. Typically, BCl 3  level was around 60-70%.  
       Preparation of System  
       [72]    72. The activated carbon beds were dried down with a N 2  purge at the operating temperature of 350° C., and above, for several weeks prior to their first exposure to BCl 3 . At no time during the pilot scale trials were the carbon beds purged with either helium or nitrogen. BCl 3  is left stagnant in the trap between purification runs. This is basically keeping the system free of outside impurities, particularly trace moisture, that will exist in the purge gas at low levels. It also minimized the loss of any BCl 3  during purification. After more than six months of operation, the same carbon was still being used in reactor  30  without any noticeable degradation in performance.  
       Example 2  
       [73]    73. In this case, the source container was replaced with a larger unit containing approximately 1200 lbs. (600 kg) of BCl 3 . This container was positioned horizontally offering larger liquid-vapor interface areaand in this example the inner tube of the container had a dip tube that allowed He to flow directly through the liquid BCl 3  and out a second valve of the vapor phase portion of the container into the purification train. In this modification of the system, the process procedure was the same as in Example 1 except additional helium was injected into the low temperature condensers by feeding He in just after the second (low temperature) carbon bed and thus having it flow through the two condensers and out the scrubber like a normal purification run. This additional injection of helium lowered the CO and HCl impurities down to detection limits of 1 ppm or less.  
         [74]    74. Subsequent gas chromatography analysis indicated no light impurities were present in the purified BCl 3  above a detection limit of 100 ppb from current or previous purification work.  
         [75]    75. Based on the current limited sampling results available today, the concentration level of metals falls within the range of that measured from a competitive high purity BCl 3  supplier even though the inventive system did not have any secondary vaporization process specifically for removing metals. Even so, typically, the level of metals (whether from samples produced by inventive system or the competitive high purity BCl 3  sample) fall around a few to tens of ppb level for most elements. Very often the most abundant impurity elements found in BCl 3  from either the inventive system or the competitive high purity BCl 3  sample are Fe, Ca, and Si. These analysis results are taken with liquid phase sampling followed by residue analysis.  
         [76]    76. Overall, the BCl 3  purification process and system of the present invention was a success and high purity BCl 3  required for existing semiconductor manufacturers is obtained from low purity BCl 3 . The main goal of this invention was to take low purity BCl 3  with ˜100 ppm of COCl 2  and produce pure product meeting today&#39;s typical semiconductor specifications.  
         [77]    77. While reference has been made to specific embodiments, these are only meant to be illustrative and those possessed of ordinary skill in the art may alter such embodiments without departing from the scope of the appended claims.