Molecular sieve abstract
The present invention provides a method of removing impurities
from a stream of silane, SiH.sub.4. Most notably, the present invention
provides a method of removing ethylene from a silane stream by converting
the ethylene to ethylsilane in the presence of a molecular sieve
and distilling the desired silane from the ethylsilane contaminant.
Molecular sieve claims
What is claimed is:
1. A process for removing impurities from a stream of silane comprising:
a) passing a stream of silane, said stream of silane containing
heavier impurities, lighter impurities and ethylene, through a first
distillation column at a first temperature and pressure sufficient
to remove said lighter impurities, leaving a second stream;
b) passing said second stream through a second distillation column
at a second temperature and pressure sufficient to remove said heavier
impurities, leaving a third stream;
c) passing said third stream through a molecular sieve, said molecular
sieve having a porosity of about 4 angstroms, at a third temperature
and pressure sufficient to absorb the majority of the ethylene in
said third stream onto said molecular sieve and convert the remainder
of the ethylene in said third stream to ethylsilane, leaving a fourth
stream containing ethylsilane; and
d) passing said fourth stream containing ethylsilane through a
third distillation column at a fourth temperature and pressure sufficient
to produce a fifth stream containing a reduced concentration of
ethylsilane.
2. The process of claim 1 in which said reduced concentration of
ethylsilane is a concentration of ethylsilane below about 0.01 ppm.
3. The process of claim 1 in which said first temperature is in
the range of from about -83.degree. F. to about 20.degree. F.
4. The process of claim 1 in which said first temperature is in
the range of from about -65.degree. F. to about -11.degree. F.
5. The process of claim 1 in which said first temperature is in
the range of from about -31.degree. F. to about -28.degree. F.
6. The process of claim 1 in which said first pressure is in the
range of from about 120 psig to about 585 psig.
7. The process of claim 1 in which said first pressure is in the
range of from about 170 psig to about 400 psig.
8. The process of claim 1 in which said first pressure is in the
range of from about 300 psig to about 315 psig.
9. The process of claim 1 in which said second temperature is in
the range of from about -87.degree. F. to about 19.degree. F.
10. The process of claim 1 in which said second temperature is
in the range of from about -68.degree. F. to about -13.degree. F.
11. The process of claim 1 in which said second temperature is
in the range of from about -34.degree. F. to about -31.degree. F.
12. The process of claim 1 in which said second pressure is in
the range of from about 110 psig to about 575 psig.
13. The process of claim 1 in which said second pressure is in
the range of from about 160 psig to about 390 psig.
14. The process of claim 1 in which said second pressure is in
the range of from about 285 psig to about 300 psig.
15. The process of claim 1 in which said third temperature is in
the range of from about -92.degree. F. to about 75.degree. F.
16. The process of claim 1 in which said third temperature is in
the range of from about -75.degree. F. to about 30.degree. F.
17. The process of claim 1 in which said third temperature is in
the range of from about -65.degree. F. to about 0.degree. F.
18. The process of claim 1 in which said third pressure is in the
range of from about 100 psig to about 565 psig.
19. The process of claim 1 in which said third pressure is in the
range of from about 140 psig to about 380 psig.
20. The process of claim 1 in which said third pressure is in the
range of from about 160 psig to about 285 psig.
21. The process of claim 1 in which said fourth temperature is
in the range of from about -92.degree. F. to about 18.degree. F.
22. The process of claim 1 in which said fourth temperature is
in the range of from about -72.degree. F. and about -15.degree.
F.
23. The process of claim 1 in which said fourth temperature is
in the range of from about -38.degree. F. to about -34.degree. F.
24. The process of claim 1 in which said fourth pressure is in
the range of from about 100 psig to about 565 psig.
25. The process of claim 1 in which said fourth pressure is in
the range of from about 150 psig to about 380 psig.
26. The process of claim 1 in which said fourth pressure is in
the range of from about 270 psig to about 285 psig.
27. The process of claim 1 in which said molecular sieve having
a porosity of about 4 angstroms is a zeolite A molecular sieve.
28. The process of claim 27 in which said zeolite A molecular sieve
is a zeolite of sodium cation form.
29. A process for removing impurities from a stream of silane,
said impurities comprising ethylene, heavier impurities and lighter
impurities, comprising:
a) passing said stream of silane through a first distillation column
at a first temperature and pressure sufficient to separate said
lighter impurities from said stream of silane to produce a second
stream;
b) passing said second stream through a molecular sieve, said molecular
sieve having a porosity of about 4 angstroms, at a second temperature
and pressure such that substantially all of the ethylene is converted
to ethylsilane to produce a third stream; and
c) passing said third stream through a second distillation column
at a third temperature and pressure sufficient to separate said
silane from said heavier impurities and said ethylsilane to produce
a final stream of purified silane.
30. The process of claim 29 in which said molecular sieve having
a porosity of about 4 angstroms is a zeolite A molecular sieve.
31. The process of claim 30 in which said zeolite A molecular sieve
is a zeolite of the sodium cation form.
32. The process of claim 29 in which said first temperature is
in the range of from about -83.degree. F. to about 20.degree. F.
33. The process of claim 29 in which said first temperature is
in the range of from about -65.degree. F. to about -11.degree. F.
34. The process of claim 29 in which said first temperature is
in the range of from about -31.degree. F. to about -28.degree. F.
35. The process of claim 29 in which said first pressure is in
the range of from about 120 psig to about 585 psig.
36. The process of claim 29 in which said first pressure is in
the range of from about 170 psig to about 400 psig.
37. The process of claim 29 in which said first pressure is in
the range of from about 300 psig to about 315 psig.
38. The process of claim 29 in which said second temperature is
in the range of from about 75.degree. F. to about 356.degree. F.
39. The process of claim 29 in which said second temperature is
in the range of from about 210.degree. F. to about 356.degree. F.
40. The process of claim 29 in which said second pressure is in
the range of from about 110 psig to about 575 psig.
41. The process of claim 29 in which said second pressure is in
the range of from about 285 psig to about 300 psig.
42. The process of claim 29 in which said third temperature is
in the range of from about -92.degree. F. to about 18.degree. F.
43. The process of claim 29 in which said third temperature is
in the range of from about -72.degree. F. to about -15.degree. F.
44. The process of claim 29 in which said third temperature is
in the range of from about -38.degree. F. to about -34.degree. F.
45. The process of claim 29 in which said third pressure is in
the range of from about 100 psig to about 565 psig.
46. The process of claim 29 in which said third pressure is in
the range of from about 150 psig to about 380 psig.
47. The process of claim 29 in which said third pressure is in
the range of from about 270 psig to about 285 psig.
48. The process of claim 29 further comprising heating said second
stream to a temperature in the range of from about 75.degree. to
about 210.degree. F. prior to passing said second stream through
said molecular sieve.
49. The process of claim 29 further comprising heating said second
stream to a temperature in the range of from about 190.degree. F.
to about 210.degree. F. prior to passing said second stream through
said molecular sieve.
50. The process of claim 29 further comprising cooling said third
stream to a temperature in the range of from about -25.degree. F.
to about -35.degree. F. prior to passing said third stream through
said second distillation column.
Molecular sieve description
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4554141 Scull et al., demonstrates a process for
the removal of ethylene from silane. According to Scull et al. ethylene
is removed from a silane stream by passing the contaminated silane
through a column of zeolite, preferably zeolite A having a 4 Angstrom
micropore. This 4 Angstrom zeolite A has a high capacity for ethylene
and is readily regenerated. Since this work was conducted, however,
it has been found that this method removes most ethylene, but converts
a significant amount into another silane contaminant, ethylsilane.
U.S. Pat. No. 4632816 (Marlett) describes a process for the production
of high purity silane by reacting silicon tetrafluoride exclusively
with sodium aluminum tetrahydride, potassium aluminum tetrahydride,
or a mixture of the two, preferably in an inert liquid reaction
medium comprising an ether.
U.S. Pat. No. 5075092 (Boone et al.) teaches a process for continuously
preparing silane and a coproduct by reacting a metal hydride such
as NaAlH.sub.4 with a silicon halide, such as SiF.sub.4 by utilizing
equipment which includes, in series, a primary reactor, a secondary
reactor and a separation zone.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the more preferred embodiment of the present
invention where the light and heavy impurities are distilled from
the ethylene/silane mixture before the purification using the molecular
sieve.
FIG. 2 illustrates a preferred embodiment of the present invention
where only the light impurities are distilled from the ethylene/silane
mixture before the purification using the molecular sieve.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention concerns, in general, a method of producing
silane of acceptable purity for the production of high-purity polysilicon.
More particularly, the present invention concerns a method of removing
ethylene from a stream of silane.
Generally, silane (SiH.sub.4) is produced by the reaction of sodium
aluminum tetrahydride and silicon tetrafluoride in the reaction:
A more thorough description of known processes for producing silane
can be seen in the above-mentioned U.S. Pat. Nos. 4632816 and
5075092 both of which are incorporated herein by reference.
Unfortunately, a known impurity, sodium aluminum diethyl hydride,
is often present in the NaAlH.sub.4 used in this reaction. This
impurity also reacts With the silicon tetrafluoride to produce silane
and a number of unwanted impurities. Among these are heavier impurities
such as ethane, ethylene, ethylsilane and diethylsilane and lighter
impurities such as methane and hydrogen. For the purposes of this
application, it should be understood that the term "heavier
impurities" refers to those compounds or materials which may,
because of their molecular weights or physical qualities, be distilled
at a temperature greater than the boiling point of silane. Similarly,
it is understood that the term "lighter impurities" refers
to those compounds or materials which, because of their molecular
weights or physical qualities, can be distilled at a temperature
lower than the boiling point of silane.
The concentration of these impurities in crude silane has been
found to be, by weight, about five percent lighter impurities, about
one percent heavier impurities and about 40 ppm ethylene. Through
a series of distillations, it has been found relatively easy to
remove the majority of these impurities. A number of these impurities
boil at temperatures significantly different from the boiling point
of silane(-111.8.degree. C.). For example, methane boils at -161.49.degree.
C. and ethane boils at -88.63.degree. C.
A first distillation column operated at about 315 psig and about
-30.degree. F. was found to remove the lighter impurities. A second
distillation column operated at about 300 psig and about -35.degree.
F. was found to remove the silane and ethylene from the heavier
impurities. These conditions may be subject to alteration, and such
would be understood by one skilled in the art.
The remaining ethylene presents a problem in that it is not readily
distilled from the desired silane. Ethylene boils at about -104.degree.
C. While this temperature differs from the boiling point of silane,
vapor liquid equilibrium data and distillation data indicate that
at low ethylene concentrations, ethylene in silane exhibits azeotropic
behavior which makes separation by distillation impractical. Further,
the existing ethylene may function as a carbon impurity in later
uses of the silane, most notably the production of high purity polysilicon.
It has been known that various molecular sieves function well in
removing ethylene from a stream of silane, most notably zeolite
A with a 4 Angstrom micropore. Such processes for removing ethylene
from silane are disclosed in the above-mentioned U.S. Pat. No. 4554141
which is incorporated herein by reference.
However, the mere process of removing ethylene with a molecular
sieve has been found to create problems not noted in the prior art.
When an ethylene-contaminated volume of silane is passed through
such a molecular sieve, the majority of ethylene may be absorbed
onto the sieve. Previously unknown is the fact that this process
causes the formation of an additional contaminant, ethylsilane.
This new contaminant still provides a possible source of carbon
contamination when the silane is used for the production of high
purity polysilicon. Even trace amounts of carbon contaminants have
been found to be undesirable in silane used for such purposes.
It has, therefore, become a primary object of the present invention
to provide a method of removing ethylene from silane which does
not leave additional contaminants in the desired silane. It has
been found desirable to absorb the ethylene onto a means which removes
it from the silane stream. It is even more desirable to convert
the ethylene to ethylsilane, as ethylsilane and silane may be readily
separated by distillation.
A preferred embodiment of the present invention involves distilling
the already existing impurities and removing the ethylene by a combination
of absorption and conversion of ethylene to ethylsilane, which may
be distilled from the silane. In this preferred embodiment, the
process comprises a) passing the silane which contains lighter impurities,
heavier impurities and ethylene through a first distillation column
at a temperature and pressure sufficient to remove the lighter impurities
from the mixture, b) passing the silane and the remaining heavier
impurities and ethylene through a second distillation column at
a temperature and pressure sufficient to remove the silane and ethylene
from the heavier impurities, c) passing the ethylene-contaminated
silane through a molecular sieve, such as those mentioned in U.S.
Pat. No. 4554141 to absorb most of the ethylene and convert the
remainder to ethylsilane, and d) passing the now ethylsilane-contaminated
silane through a third distillation column at a temperature and
pressure sufficient to remove the desired silane from the ethylsilane
contaminant. In the descriptions provided in the present application,
the term distillation column is used to describe any conventional
means of distillation and is not intended to be limiting in any
way. The present invention has been successful in removing substantial
amounts of ethylsilane from the silane stream. Ethylsilane concentrations
in the silane stream exiting the molecular sieve beds have been
noted to be at or in excess of 0.1 ppm. The present invention has
been seen to be successful in producing a silane stream with an
ethylsilane concentration of less than about 0.01 ppm.
In a more preferred embodiment of the present invention, the silane
stream containing the ethylene contaminant would be passed over
a reactor, such as the above-mentioned molecular sieve bed, that
would convert substantially all of the ethylene present to ethylsilane.
In this more preferred embodiment, the resulting ethylsilane could
be removed along with the other heavier impurities. Not only does
this eliminate the need for a third means of distillation, but also
replaces an absorber, which requires regular maintenance or replacement,
with a more financially preferred reactor.
In this preferred embodiment of the present invention, the silane
which contains heavier impurities, lighter impurities and ethylene
is a) passed through a first distillation column at a sufficient
temperature and pressure to remove the lighter impurities present,
b) passed through a molecular sieve bed, such as those mentioned
in U.S. Pat. No. 4554141 at a temperature and pressure sufficient
to convert substantially all of the ethylene present to ethylsilane,
and c)passing the silane, ethylsilane and other heavy impurities
through a second distillation column at a temperature and pressure
to remove a purified silane from the contaminants present.
In understanding and describing the present invention, it is beneficial
to refer to the two drawings provided. Each gives an organizational
basis for describing the present invention, but neither is intended
to be limiting to the legal scope of the present invention.
The illustration labelled FIG. No. 1 demonstrates the more preferred
embodiment of the present invention. In this embodiment, the silane
mixture which contains lighter and heavier impurities, including
ethylene, is treated in a first distillation column (I) to remove
the lighter impurities. This removal is conducted at a temperature
in the range of from about -83.degree. F. to about 20.degree. F.,
preferably in the range of from about -65.degree. F. to about -11.degree.
F., most preferably in the range of from about -31.degree. F. to
about -28.degree. F., and at a pressure in the range of from about
120 psig to about 585 psig, preferably in the range of from about
170 psig to about 400 psig, and most preferably in the range of
from about 300 psig to about 315 psig.
Once the lighter impurities are removed, the silane mixture containing
the heavier impurities, including ethylene, is treated in a second
distillation column (II) to remove the silane and ethylene from
the remaining heavier impurities. This is conducted at a temperature
in the range of from about -87.degree. F. to about 19.degree. F.,
preferably in the range of from about -68.degree. F. to about -13.degree.
F., most preferably in the range of from about -34.degree. F. to
about -31 F, and at a pressure in the range of from about 110 psig
to about 575 psig, preferably in the range of from about 160 psig
to about 390 psig, and most preferably between about 285 psig and
about 300 psig.
The remaining silane and ethylene contaminant are then passed through
a molecular sieve (III), such as those disclosed in U.S. Pat. No.
4554141 preferably a zeolite A with a porosity of 4 Angstroms.
This silane mixture is passed over the molecular sieve at a temperature
in the range of from about -92.degree. F. to about 75.degree. F.,
preferably in the range of from about -75.degree. F. to about 30.degree.
F., most preferably in the range of from about -65.degree. F. to
about 0.degree. F., and at a pressure of from about 100 psig to
about 565 psig, preferably in the range of from about 140 psig to
about 380 psig, most preferably in the range of from about 160 psig
to about 285 psig. Under this temperature and pressure range, the
majority of the ethylene present is absorbed onto the structure
of the molecular sieve, thus removing it from the stream of silane.
The remaining ethylene is converted to ethylsilane, from which the
silane can be removed in a third distillation column (IV). The illustration
of Appendix No. 2 illustrates the stream of silane and ethylsilane
passing from the molecular sieve bed (III) into a filter (V). This
filter is designed to collect any particulate matter from the molecular
sieve bed which may become borne by the silane stream. Any conventional
filtering means which will function under the existing conditions
to collect such particulate matter, without creating an additional
source of contamination to the silane stream, may be used. It has
been found that a sintered metal filter works well for this purpose.
Finally, the ethylsilane-containing stream of silane is passed
through a third distillation column (IV) to remove the desired silane
product from the ethylsilane contaminant. This is conducted at a
temperature in the range of from about -92.degree. F. to about 18.degree.
F., preferably in the range of from about -72.degree. F. to about
31 15.degree. F., and most preferably in a range of from about -38.degree.
F. to about -34.degree. F., and at a pressure in the range of from
about 100 psig to about 565 psig, preferably in the range of from
about 150 psig to about 380 psig and most preferably in the range
of from about 270 psig to about 285 psig. The resulting silane stream
is then of sufficient purity to be utilized in the production of
high purity grade polysilicon.
The drawing labelled FIG. No. 2 provides an illustration of a preferred
embodiment of the present invention. In this embodiment, the original
silane, which contains lighter and heavier impurities, including
ethylene, is treated in a first distillation column (1) at a temperature
and pressure sufficient to remove the lighter impurities from the
silane mixture. While the removal of such types of impurities is
well known in the art, it is preferred that this be carried out
at a temperature in the range of from about -83.degree. F. to about
20.degree. F. and at a pressure of from about 120 psig to about
585 psig, preferably in the range of from about -65.degree. F. and
about -11.degree. F. and from about 170 psig and about 400 psig,
most preferably in the range of between about -31.degree. F. and
about -28.degree. F. and from about 300 psig to about 315 psig.
The silane mixture, minus the lighter impurities, is then passed
through a conventional interchanger (6) and heater (5) to raise
the temperature of the mixture to in the range of from about 75.degree.
F. to about 210.degree. F, preferably in the range of from about
190.degree. F. to 210.degree. F. This heated mixture is then passed
over the molecular sieve, preferably a zeolite A sieve with a 4
Angstrom pore at a temperature in the range of from about 75.degree.
F. to about 356.degree. F., more preferably in the range of from
about 210.degree. F. to about 356.degree. F., and a pressure in
the range of from about 110 psig to about 575 psig, more preferably
in the range of about 285 psig to about 300 psig. Under these conditions,
substantially all of the ethylene present is converted to ethylsilane,
which is much more susceptible to removal from silane by distillation.
This new mixture of silane and heavy impurities, including the
newly produced ethylsilane, is then passed through the interchanger
(6) and a cooling means (4) to reduce its temperature to the range
of from about -25.degree. F. to about -35.degree. F. prior to entering
the second distillation column (3). In this second distillation
column, the silane is removed from the heavier impurities at a temperature
in the range of from about -92.degree. F. to about 18.degree. F.,
preferably in the range of from about -72.degree. F. to about -15.degree.
F., most preferably in the range of from about -38.degree. F. to
about -34.degree. F., and a pressure in the range of from about
100 psig to about 565 psig, preferably in the range of from about
150 psig to about 380 psig, most preferably between about 270 psig
and about 285 psig. |