U.S. patent application number 09/725965 was filed with the patent office on 2001-04-05 for processes and systems for purification of boron trichloride.
Invention is credited to Borzio, John P., Jursich, Gregory M., Kebbekus, Earle R., Uner, Jason R., Zhou, Derong.
Application Number | 20010000141 09/725965 |
Document ID | / |
Family ID | 23540195 |
Filed Date | 2001-04-05 |
United States Patent
Application |
20010000141 |
Kind Code |
A1 |
Zhou, Derong ; et
al. |
April 5, 2001 |
Processes and systems for purification of boron trichloride
Abstract
Processes are disclosed for increasing the condensed phase
production of BCl.sub.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.sub.3 therein, the condensed
BCl.sub.3 having therein a minor portion of phosgene impurity. A
major portion of the phosgene in the condensed BCl.sub.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.sub.3 product condensed stream, and an inert gas
is used to pump the BCl.sub.3 product condensed stream to a product
receiver.
Inventors: |
Zhou, Derong; (Doylestown,
PA) ; Jursich, Gregory M.; (Clarendon Hill, IL)
; Kebbekus, Earle R.; (Princeton, NJ) ; Borzio,
John P.; (Robbinsville, NJ) ; Uner, Jason R.;
(Chicago, IL) |
Correspondence
Address: |
Jeffrey L. Wendt
Intellectual Property Law Department
Air Liquide
2700 Post Oak Boulevard, Suite 1800
Houston
TX
77056
US
|
Family ID: |
23540195 |
Appl. No.: |
09/725965 |
Filed: |
November 29, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09725965 |
Nov 29, 2000 |
|
|
|
09389895 |
Sep 3, 1999 |
|
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|
Current U.S.
Class: |
423/292 ;
422/211; 423/241 |
Current CPC
Class: |
C01B 35/061 20130101;
F17C 2270/0518 20130101; F17C 2227/0302 20130101 |
Class at
Publication: |
423/292 ;
422/211; 423/241 |
International
Class: |
C01B 035/06 |
Claims
What is claimed is:
1. A process of producing a BCl.sub.3/inert gas vapor stream from a
low purity BCl.sub.3 source, the BCl.sub.3/inert gas vapor stream
having less than 10 ppm chlorine and less than 10 ppm phosgene
impurities, the process comprising: a) injecting an inert gas into
a container comprising low purity BCl.sub.3 having phosgene
impurity to produce a vapor stream comprising BCl.sub.3, inert gas,
and phosgene; b) decomposing a major portion of the phosgene in the
vapor stream comprising BCl.sub.3, inert gas, and phosgene by
heating to a first temperature to form a first intermediate vapor
stream comprising BCl.sub.3, inert gas, carbon monoxide, chlorine
and less than 10 ppm phosgene; and c) 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 said BCl.sub.3, inert gas
vapor stream having less than 10 ppm chlorine and 10 ppm
phosgene.
2. A process in accordance with claim 1 wherein said heating step
comprises preheating the vapor stream comprising BCl.sub.3, inert
gas, and phosgene prior to the vapor stream comprising BCl.sub.3,
inert gas, and phosgene contacting a catalytic material.
3. A process in accordance with claim 2 wherein said preheating
comprises heat exchanging said first intermediate vapor stream with
said vapor stream comprising BCl.sub.3, inert gas, and
phosgene.
4. A process in accordance with claim 1 wherein said phosgene
decomposition step occurs in the presence of a catalyst.
5. A process in accordance with claim 4 wherein said catalyst is
selected from the group consisting of carbon-based material,
alumina-based material, silica-based material, or mixtures
thereof.
6. A process in accordance with claim 5 wherein said carbon-based
material is selected from the group consisting of naturally
occurring carbon, carbon molecular sieve, synthetic carboneous
material which is not molecular sieve, and combinations
thereof.
7. A process in accordance with claim 1 wherein said inert gas
increases pressure of the vapor stream comprising BCl.sub.3, inert
gas, and phosgene to a pressure substantially higher than a vapor
pressure of the low purity BCl.sub.3.
8. A process in accordance with claim 1 wherein the phosgene
decomposition occurs at a temperature greater than about
200.degree. C., and the adsorption of the chlorine occurs at a
temperature lower than about 30.degree. C.
9. A process in accordance with claim 1 wherein said adsorption
step comprises using a bed of adsorbent until loaded, isolating the
unit from the first intermediate vapor stream, heating the isolated
unit for a time and at a temperature effective to desorb
substantially all chlorine, and reusing the unit.
10. A process in accordance with claim 1 wherein said adsorbent is
selected from the group consisting of carbon-based material,
alumina-based material, silica-based material, and mixtures
thereof.
11. A process in accordance with claim 10 wherein said carbon-based
material is selected from the group consisting of naturally
occurring carbon, carbon molecular sieve, synthetic carboneous
material which is not molecular sieve, and combinations
thereof.
12. A process in accordance with claim 1 wherein the catalyst and
the adsorbent each comprise carbonaceous material.
13. A process for producing an ultra pure BCl.sub.3 condensed phase
from a vapor phase comprising BCl.sub.3, the process comprising: a)
condensing a first vapor stream in a condenser, the first vapor
stream comprising a major portion of BCl.sub.3 and a minor portion
of HCl and an inert gas to form a first condensed phase comprising
BCl.sub.3, and a second vapor stream comprising the inert gas,
BCl.sub.3, and light impurities; b) routing the second vapor stream
to a secondary condenser, thus forming a gaseous stream containing
only trace amounts of BCl.sub.3 and a second condensed phase
comprising BCl.sub.3; and c) routing the first condensed phase to a
stripper wherein an inert gas is used to strip molecules having
specific gravity less than BCl.sub.3 from the first condensed phase
to produce a third condensed phase having less than 50 ppm hydrogen
chloride, and a stripped vapor phase.
14. A process in accordance with claim 13 wherein step (c) includes
the step of allowing the first condensed phase to come to room
temperature, and then contacting the first condensed phase with
inert gas at a pressure ranging from about 20 psig to about 30 psig
(from about 240 kPa to about 440 kPa).
15. A process in accordance with claim 13 wherein the stripped
vapor phase is routed to the secondary condenser to recover
residual BCl.sub.3.
16. A process in accordance with claim 13 wherein the gaseous
stream containing less than 10 ppm BCl.sub.3 from the secondary
condenser is routed to a scrubber to neutralize residual BCl.sub.3
in a liquid phase, and produce a gaseous stream containing nitrogen
and oxygen.
17. A process in accordance with claim 13 wherein the third
condensed phase is transferred to a product cylinder using ultra
high purity inert gas and without any other pumping means.
18. A process for producing ultra-high purity BCl.sub.3 in
condensed phase and containing less than 10 ppm phosgene from a low
purity BCl.sub.3 condensed phase having phosgene impurity, the
process comprising: a) injecting an inert gas through a low purity
BCl.sub.3 having phosgene impurity to produce a vapor stream
comprising BCl.sub.3, inert gas, and phosgene; b) decomposing a
major portion of the phosgene in the vapor stream comprising
BCl.sub.3, inert gas, and phosgene by heating to a first
temperature to form a first intermediate vapor stream comprising
BCl.sub.3, inert gas, carbon monoxide, chlorine, HCl and less than
10 ppm phosgene; c) 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 a BCl.sub.3/inert gas vapor stream comprising less than
about 10 ppm phosgene and some HCl; d) routing said BCl.sub.3/inert
gas vapor stream comprising less than about 10 ppm phosgene and
some HCl to a condenser; e) condensing at least a portion of
BCl.sub.3 in the BCl.sub.3/inert gas vapor stream in said
condenser, to form a first condensed phase comprising BCl.sub.3,
and HCl and inert gas and a second vapor stream comprising inert
gas, BCl.sub.3, oxygen, and nitrogen; f) routing the second vapor
stream to a secondary condenser, thus forming a gaseous stream
comprising less than about 10 ppm BCl.sub.3 and a second condensed
phase comprising BCl.sub.3 and HCl; and g) routing the first
condensed phase to a stripper wherein inert gas is used to strip
molecules having specific gravity less than BCl.sub.3 from the
first condensed phase to produce a third condensed phase comprising
less than 50 ppm HCl, and a stripped vapor phase.
19. A process for increasing the condensed phase production of
BCl.sub.3 comprising less than about 10 ppm phosgene, less than 10
ppm chlorine, and less than 10 ppm HCl, the process comprising the
steps of: a) introducing an inert gas into a container having
condensed BCl.sub.3 therein, said condensed BCl.sub.3 having
therein a minor portion of phosgene impurity; b) converting a major
portion of the phosgene in the condensed BCl.sub.3 to carbon
monoxide and chlorine by increasing temperature to produce a
phosgene deficient stream; c) decreasing the temperature of the
phosgene deficient stream and contacting it with an adsorbent to
remove the chlorine in the stream by adsorption to form a chlorine
and phosgene free condensed stream; d) stripping the chlorine and
phosgene free stream using helium to form BCl.sub.3 product
condensed stream; and e) using inert gas to transfer the BCl.sub.3
product condensed stream to a product receiver.
20. A system for producing ultra-high purity BCl.sub.3 in condensed
phase having less than 10 ppm phosgene from a low purity BCl.sub.3
condensed phase having phosgene impurity, the system comprising: a)
injecting inert gas through a low purity BCl.sub.3 having phosgene
impurity to produce a vapor stream comprising BCl.sub.3, helium,
and phosgene; b) decomposing a major portion of the phosgene in the
vapor stream comprising BCl.sub.3, inert gas, and phosgene by
heating to a first temperature to form a first intermediate vapor
stream comprising BCl.sub.3, inert gas, carbon monoxide, chlorine,
HCl and less than 10 ppm phosgene; c) 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 a BCl.sub.3/inert gas vapor
stream comprising less than about 10 ppm phosgene and some HCl; d)
routing said BCl.sub.3/inert gas vapor stream comprising less than
about 10 ppm phosgene and some HCl to a condenser; e) condensing at
least a portion of BCl.sub.3 in the BCl.sub.3/inert gas vapor
stream in said condenser, to form a first condensed phase
comprising BCl.sub.3, and HCl and inert gas and a second vapor
stream comprising inert gas, BCl.sub.3, and light impurities; f)
routing the second vapor stream to a secondary condenser, thus
forming a gaseous stream comprising less than about 10 ppm
BCl.sub.3 and a second condensed phase comprising BCl.sub.3 and
HCl; and g) routing the first condensed phase to a stripper wherein
inert gas is used to strip molecules having specific gravity less
than BCl.sub.3 from the first condensed phase to produce a third
condensed phase comprising less than 50 ppm HCl, and a stripped
vapor phase.
Description
BACKGROUND OF THE INVENTION
1. 1. Field of the Invention
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. 2. Related Art
4. Boron trichloride (also referred to herein as "BCl.sub.3") is a
highly reactive compound packaged as a liquid under its own vapor
pressure of 1.3 bar (130 kPa) absolute at 21.degree. 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.sub.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.sub.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.sub.3 is used
to refine aluminum and its alloys to improve tensile strength.
5. Two of the most stringent applications for high purity BCL.sub.3
involve semiconductor and optical fiber manufacturing. In these
industries the specified impurity levels in BCl.sub.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.sub.3 are
often present at levels over two orders of magnitude beyond
acceptable levels for these processes such as, for example, air,
CO.sub.2, HCl, Cl.sub.2, and COCl.sub.2 ("phosgene"). Furthermore,
in these particular applications, any oxygen or oxygen containing
impurities (such as phosgene) in the BCl.sub.3 are especially
detrimental to the manufacturing process due to the formation of
certain oxide compounds. Another class of detrimental impurities in
BCl.sub.3 for these processes are metal containing impurities.
6. Geographically, BCl.sub.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.sub.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. The source cost of BCl.sub.3 varies considerably per pound
depending upon purity grade and supplier. There is a strong
incentive to purchase BCl.sub.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. 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. The removal of some impuritites in BCl.sub.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.sub.3 and hence becomes difficult
to remove by simple distillation. The previous methods for phosgene
removal from BCl.sub.3 include electrical discharge, laser
pyrolysis, fractional distillation, UV photolysis, and redox
chemistry.
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.sub.3. In these methods
phosgene in the BCl.sub.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. 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. Another problem with known BCl.sub.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.sub.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.sub.3, material transport through such pressure drops
becomes significantly hindered due to the BCl.sub.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.sub.3, the reactive nature of BCl.sub.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.sub.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.sub.3 by
metallic impurities can occur from enhanced liquid phase corrosion
mechanisms thereby degrading product purity with detrimental
metallic impurities.
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. 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.sub.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. 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.sub.2, or Ar. In this technique, as disclosed in further
detail herein below, the BCl.sub.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.sub.3 vapor pressure alone.
This is preferably performed most simply by bubbling the inert gas
through the liquid BCl.sub.3 and flowing the mixed gas stream
through the system, after which the inert gas is easily separated
from the purified BCl.sub.3 product collected.
16. A first aspect of the invention is a process of producing a
BCl.sub.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.sub.3
source, the BCl.sub.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.sub.3 source having phosgene impurity to produce a vapor
stream comprising BCl.sub.3, helium, and phosgene; decomposing a
major portion of the phosgene in the BCl.sub.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.sub.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.sub.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. A preferred process embodiment in accordance with this aspect
of the invention is wherein the heating step comprises preheating
the vapor stream comprising BCl.sub.3, helium, and phosgene prior
to the vapor stream comprising BCl.sub.3, helium, phosgene
contacting the first material, which promotes phosgene
decomposition.
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.sub.3, helium, and phosgene.
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. In accordance with this aspect of the invention, the inert gas
functions to increase pressure of the vapor stream comprising
BCl.sub.3, inert gas, and phosgene to a pressure substantially
higher than the vapor pressure of the lower purity BCl.sub.3.
21. Preferably, the phosgene decomposition step occurs at a
temperature greater than about 200.degree. C., and the adsorption
of chlorine step preferably occurs at a temperature lower than
about 50.degree. C., although some chlorine will be adsorbed on the
first material at a higher temperature in the phosgene
decomposition step.
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. A second aspect in accordance with the invention is a process
for producing an ultra-pure BCl.sub.3 condensed phase from a vapor
phase comprising impure BCl.sub.3. The process comprises condensing
a first vapor stream in a condenser, the first vapor comprising a
major portion of BCl.sub.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.sub.3 and a
second vapor comprising the first inert gas, BCl.sub.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.sub.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.sub.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. 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. Also, preferred are processes in accordance with this aspect
wherein the stripped vapor phase is routed to the secondary
condenser to recover residual BCl.sub.3, and processes wherein the
stream containing only traces of BCl.sub.3 from the secondary
condenser is routed to a scrubber to remove residual traces of
BCl.sub.3, along with HCl impurity and introduce a gaseous stream
containing the inert gas and light impurities which are discharged
to the atmosphere.
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. 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.sub.3 liquid having phosgene
impurity to produce a vapor stream comprising BCl.sub.3, inert gas,
and phosgene; decomposing a major portion of the phosgene in the
BCl.sub.3, inert gas, phosgene vapor stream by heating to a first
temperature to form a first intermediate vapor stream comprising
BCl.sub.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.sub.3/inert gas vapor stream having less than 10
ppm phosgene and less than 10 ppm Cl.sub.2; routing said
BCl.sub.3/inert gas vapor stream having less than about 10 ppm
phosgene and less than 10 ppm Cl.sub.2 to a condenser; condensing a
first vapor stream in the condenser, the first vapor comprising a
major portion of BCl.sub.3 and a minor portion of HCl, inert gas,
and light impurities to form a first condensed phase comprising
BCl.sub.3 and a second vapor comprising the inert gas, residual
BCl.sub.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.sub.3 and a
second condensed phase comprising BCl.sub.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.sub.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. A fourth aspect of the invention is a process for increasing
the condensed phase production of BCl.sub.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.sub.3 therein, the condensed BCl.sub.3 having therein a minor
portion of phosgene impurity; converting a major portion of the
phosgene in the condensed BCl.sub.3 to carbon monoxide and chlorine
by increasing temperature of the condensed BCl.sub.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.sub.3 product to a product
container.
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.sub.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. Injecting an inert gas, preferably helium, into a source
container of lower purity BCl.sub.3 liquid and extract the vapor
out the container;
31. Using a functional catalyst such as activated carbon to
thermally decompose phosgene at elevated temperature;
32. Using an adsorbent such as activated carbon to remove remaining
chlorine at 50.degree. C. or lower;
33. Condensing BCl.sub.3 vapor which has substantially phosgene and
chlorine than the source BCl.sub.3;
34. Using an inert gas to strip the BCl.sub.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.sub.3 at the beginning, and/or generated during
phosgene and chlorine removing processes upstream.
35. Transfilling the final BCl.sub.3 product from the inventive
system into the product storage container using inert gas pressure
and no other pumping or vacuum means.
36. It has been demonstrated that the inventive process technology
is fully capable of producing an ultra-pure BCl.sub.3 product due
to the following important new features.
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.sub.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. An ultra-dry inert gas such as helium is employed in the
inventive process technology which overcomes the problem of
BCl.sub.3's low vapor pressure, and the inert gas can drag
BCl.sub.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.sub.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. Further, the BCl.sub.3 purification processes and systems of
the present invention do not require any mechanical devices either
to transfer the low purity BCl.sub.3 into the purification system,
or to transfill the final high purity product BCl.sub.3 from the
inventive system into a storage container. The potential
contamination on the final high purity product BCl.sub.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. 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.sub.3
and HCl impurity easily removed by conventional scrubber
technology.
BRIEF DESCRIPTION OF THE DRAWINGS
41. FIG. 1 represents in schematic format an apparatus and process
in accordance with the present invention; and
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. Referring to FIG. 1, a preferred system 1 of the invention
includes a low purity BCl.sub.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.sub.3/helium
mixture emanating from low purity BCl.sub.3 source container 2. An
electrically heated furnace surrounding the reactor supplies
supplemental heat input into reactor 30 as required. The low purity
BCl.sub.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. 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.sub.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. 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. 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.sub.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. 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. 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 .mu.m or larger
were removed.
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.sub.3 liquid where a mixture of
the helium and BCl.sub.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.sub.2 with
the help of the catalyst at an operating temperature ranging from
about 480 to about 700.degree. F. (250 to 370.degree. 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.sub.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. Then "phosgene free" boron trichloride vapor mixed with the
helium is decreased in temperature to between 50 to 80.degree. F.
(10 to 26.degree. 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.sub.2 can reform into phosgene at slightly
elevated temperature, it is imperative to reduce the temperature to
less than about 80.degree. F. (26.degree. C.) prior to directing
the phosgene free BCl.sub.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.sub.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.sub.3 stream. The
preferred activated carbon can be regenerated by heating the bed
for a time sufficient to drive off the adsorbed chlorine.
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 .mu.m or larger will be
retained.
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.degree. F. (-62 and
-73.degree. 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. 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.degree. F. (-84 and -87.degree. 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.sub.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.sub.3 liquid level inside condenser 50 reaches the designated
holding capacity, the cold liquid BCl.sub.3 then is preferably
totally transferred via line 106 into the stripper column 120 by
the helium for further impurities removal.
54. After the BCl.sub.3 liquid in stripper 120 has warmed up to
room temperature, the BCl.sub.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.sub.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.sub.3 entrained in helium. The stripped-out flow
containing BCl.sub.3 vapor is directed into secondary condenser 60
for further BCl.sub.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.sub.3 vapor and other acid components such as HCl before
final discharge to atmosphere.
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. 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. In this example, the BCl.sub.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. 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.sub.3 vapor mixture from the vapor port valve
8. In this way He, in effect, bubbled directly through the liquid
phase of BCl.sub.3 carrying primarily BCl.sub.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. High purity He and N.sub.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.sub.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.sub.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. After the He/BCl.sub.3 vapor mixture left the source container
2, it entered a phosgene decomposition reactor 30, which decomposed
the COCl.sub.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.degree. C. by means of an external electrical heater. The
reactor 30 contained 8.5 lbs. (about 4.2 kg) of BPL 4.times.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.sub.2 purge for several weeks.
61. After passing through the phosgene decomposition reactor 30,
the He/BCl.sub.3 mixture with some CO and Cl.sub.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.sub.3/Cl.sub.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.sub.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.sub.2
analysis after the carbon beds 30 and 40, it was observed that
initially all the Cl.sub.2 was absorbed by reactor 30 alone.
Eventually, when reactor 30 became saturated with Cl.sub.2,
breakthrough occurred. The released Cl.sub.2 was then removed by
adsorption unit 40.
62. After passing through adsorption unit 40, the BCl.sub.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.sub.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.degree. C. During purification runs, the glycol
solution was typically at about -5.degree. C. The second condenser
60 was also contained in a dewar 110 which was packed in dry ice
(about -78.degree. C.).
63. FTIR and UV analyzers were installed to allow sampling of gas
from many points in the purification system. Sampling of source
BCl.sub.3 was done by directly connecting BCl.sub.3 source
container 2 to the FTIR/UV analytical system. Gas flow exited the
analytical system directly to the scrubber (not shown).
64. Design of the scrubber proved to be a fairly daunting task
because of the properties of BCl.sub.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.sub.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. Modification of the scrubber was made in order to alleviate
such problems described above, and is covered by applicant's
copending Ser. No. 09/.sub.------, filed Sep..sub.--, 1999, and
incorporated by reference herein. In order to allow the BCl.sub.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. 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. The FTIR used was a Midac FTIR configured to operate at 2
cm.sup.-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.sub.2, HCl,
and CO.
1TABLE 1 Peak Cell Pressure Location Peak Height Concentration
Detection Impurity (psig) (cm-1) (Abs units) (ppm) Limit (ppm)
COCl.sub.2 Near ambient 851 0.422 23 .about.0.1 (bal N.sub.2)
pressure HCl 5 3014, 2998 0.038, 0.051 50 .about.0.5 (bal N.sub.2)
CO 5 2172 0.044 50 .about.0.5 (bal N.sub.2)
68. For HCl, the peaks analyzed were at 2998 cm.sup.-1 and 3014
cm.sup.-1. These peaks were chosen since they did not interfere
with the large BCl.sub.3 peaks located within the HCl band. The
estimated noise level provided detection limits of approximately
0.5 ppm under these experimental conditions.
69. For CO analysis, the peak at 2172 cm.sup.-1 was chosen. There
is an interference with BCl.sub.3 throughout the entire CO band.
However, this was not a problem for the analysis of CO since the
line width of the BCl.sub.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. For Cl.sub.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.sub.2 from COCl.sub.2
pyrolysis remained in the purified product. Calibration of this
instrument was performed using Cl.sub.2/N.sub.2 mixtures. No
Cl.sub.2 was seen in the purified product during these initial
purification runs even though Cl.sub.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. During analysis with FTIR or UV/VIS, the concentration of
BCl.sub.3 in the He/BCl.sub.3 mixture varied from day to day
somewhat due to resulting temperature of source BCl.sub.3. This was
due to variations of both ambient temperature (changing the vapor
pressure of BCl.sub.3) and the flow rate of helium (helium flow
rate is not controlled only helium pressure). In order to determine
the BCl.sub.3 concentration when helium was present, a weak
BCl.sub.3 band at 2139 cm.sup.-1 was measured. By monitoring this
peak and comparing to that from 100% BCl.sub.3, a determination of
the BCl.sub.3 concentration was estimated. Typically, BCl.sub.3
level was around 60-70%.
Preparation of System
72. The activated carbon beds were dried down with a N.sub.2 purge
at the operating temperature of 350.degree. C., and above, for
several weeks prior to their first exposure to BCl.sub.3. At no
time during the pilot scale trials were the carbon beds purged with
either helium or nitrogen. BCl.sub.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.sub.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. In this case, the source container was replaced with a larger
unit containing approximately 1200 lbs. (600 kg) of BCl.sub.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.sub.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. Subsequent gas chromatography analysis indicated no light
impurities were present in the purified BCl.sub.3 above a detection
limit of 100 ppb from current or previous purification work.
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.sub.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.sub.3 sample) fall around a few
to tens of ppb level for most elements. Very often the most
abundant impurity elements found in BCl.sub.3 from either the
inventive system or the competitive high purity BCl.sub.3 sample
are Fe, Ca, and Si. These analysis results are taken with liquid
phase sampling followed by residue analysis.
76. Overall, the BCl.sub.3 purification process and system of the
present invention was a success and high purity BCl.sub.3 required
for existing semiconductor manufacturers is obtained from low
purity BCl.sub.3. The main goal of this invention was to take low
purity BCl.sub.3 with .about.100 ppm of COCl.sub.2 and produce pure
product meeting today's typical semiconductor specifications.
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.
* * * * *