Process for nitrogen trifluoride synthesis

14 Pages·2013·0.99 MB·English

USO05637285A
United States Patent [19]
[111 Patent Number: 5,637,285
Coronell et al. [45] Date of Patent: Jun. 10, 1997
[54] PROCESS FOR NITROGEN TRIFLUORIDE Jones, A.G., “Liquid Circulation in a Draft-Tube Bubble
SYNTHESIS Column”, Chemical Engineering Science, vol. 40, No. 3 p.
449, 1985.
[75] Inventors: Daniel G. Coronell, Allentown;
Thomas H.-L. Hsiung, Emmaus;
Anderson, et al., USAF Propellant Handbooks, vol. 111, Part
Howard P. Withers, Jr., Breinigsville;
A, Nitrogen Tri?uoride, Systems Design Criteria, Jan. 1978.
Andrew J. Woytek, Allentown, all of
Pa. John H. Perry’s Chemical Engineers’ Handbook Fourth
Edition (1963), McGraw-Hill Book C0., pp. 19-5 through
[73] Assignee: Air Products and Chemicals, Inc.,
Allentown, Pa. 19-16.
[21] Appl. No.: 593,779
Primary Examiner-Wayne Langel
[22] Filed: Jan. 30, 1996 Altomey, Agent, or Fi rm-Geo?iey L. Chase
[51] Int. Cl.6 ................................................... .. C01B 21/06 [57] ABSTRACT
[52] US. Cl. 423/406
[58] Field of Search ............................................. .. 423/406 The present invention is directed to a method for the
synthesis of nitrogen tri?uoride from elemental ?uorine gas
[56] References Cited
and a source of armnonia in a gas-liquid phase reaction
U.S. PATENT DOCUMENTS comprising;
4,091,081 5/1978 Woytek et a1. ....................... .. 423/406
4,543,242 9/1985 Aramaki et a1. ...................... .. 423/406
FOREIGN PATENT DOCUMENTS
2255513 10/1990 Japan ........................... .. C01B 00/21
2255511 10/1990 Japan C01B 00/21 wherein the melt ratio HF/NH3 represented by x in the above
225512 10/1990 Japan C02B 00/21
formula is at least 2.55 and the reaction liquid is agitated or
170307 7/1991 Japan ................................... .. 423/406
mixed with a mixing apparatus with power input to the
3232710 10/1991 Japan .
mixing apparatus at a high level equivalent to or greater than
OTHER PUBLICATIONS
1000 watts per cubic meter. This improved synthesis method
Bakker, et al., “How to Disperse Gases in Liquids”, Chemi allows for enhanced nitrogen tri?uoride yields of 70% or
cal Engineering, Dec. 1994, pp. 98-104. greater.
“Chemical Reactor Development”, D. Thoenes, Kluwer
Academeic Publishers,Dordrecht, The Netherlands (1994)
pp. 98-114. 24 Claims, 6 Drawing Sheets
100 -
90 -
80 -
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2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3
HF Content (melt ratio)
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5,637,285
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PROCESS FOR NITROGEN TRIFLUORIDE and capital costs of such larger production plants, it is
SYNTHESIS increasingly important to achieve the highest yields possible
from the reaction synthesizing nitrogen tri?uoride. The
TECHNICAL FIELD OF THE INVENTION present invention achieves these high yields as will be set
forth in greater detail below.
The present invention is directed to the ?eld of nitrogen
tri?uoride synthesis from elemental ?uorine and ammonium BRIEF SUMMARY OF THE INVENTION
acid ?uoride. More speci?cally, the present invention is
The present invention is a method for synthesis of NF3 by
directed to the use of a combination of higher I-IFzNH3 melt
ratios and greater synthesis reactor agitation to dramatically the reaction of a ?uorine reactant and an ammonium com
increase the yield of nitrogen tri?uoride. plex reactant selected from the group consisting of NH4H
(x_1)Fx, (NH4)yMF:.nHF and mixtures thereof where x is
BACKGROUND OF THE INVENTION equal to or greater than 2.55, y is l-4, z is 2-8, 11 is su?icient
at reaction conditions to maintain said ammonium complex
There is currently a large and growing requirement of NF3
reactant as substantially as a liquid and M is selected from
for use in semiconductor manufacturing. Methods of pro 15 the group consisting of elements from Group IAthrough VA,
ducing NF3, however. are not nearly as ef?cient as theoreti Group IB through VllB and Group VI[[ of the Periodic Table
cally possible which is shown by current syntheses’ poor
of the Elements and mixtures thereof, and the reactants are
ef?ciency of utilizing expensive F2. The two principle
mixed at a rate to maintain a percent NF3 yield of at least 65
methods. electrochemical ?uorination (ECF) and direct ?uo percent.
rination (DF), have yields less than 50% of theory and most
Preferably, the reactants are mixed at a rate to maintain a
typically in the range of 30-45%. These yields are based on
percent NF3 yield of at least 80 percent.
the equivalents of F2 used in the process. In the direct
?uorination of NH3 or NIL,+ salts to produce NF3, there are More preferably, the reactants are mixed at a rate to
maintain a percent NF3 yield of at least 90 percent.
competing reactions as follows:
25 Preferably, the reaction is conducted in a stirred reactor.
Preferably, the reaction is conducted in a bubble column
reactor.
Preferably, the reaction is performed in a continuous
manner with ?uorine and an ammonia source continuously
(3) 30 added to said reaction.
Preferably, ammonia source is selected from the group
The most favored reaction according to thermodynamic
consisting of NH3, NH4F, NH4HF2, NH4Cl, NH4Br, NH4l,
calculations is (2), which produces only undesirable N2 and
NH4NO3, (N'H4)2SO4 and (NH4)2CO3 and mixtures thereof.
HF. The prior art has attempted to enhance reaction (1) to
produce NF3 and minimize the extent of reactions (2) and 35 Preferably, x is greater than or equal to 2.65.
(3). Previous attempts to produce NF3 by the direct ?uori More preferably, x is greater than or equal to 2.85.
nation of NH3 in liquid ammonium acid ?uoride resulted in Preferably, the temperature of said reaction is in the range
yields of 30 to 63%. of 93° C. to 204° C.
Currently the most e?icient process for producing NF3 is More preferably, the temperature of said reaction is in the
US. Pat. No. 4,091,081. It involves the direct ?uorination of range of 121° C. to 160° C.
ammonium ions by F2 whereby gaseous F2 is contacted with
Preferably, the pressure of the reaction is maintained in
liquid (molten) ammonium acid ?uoride (AAF) while gas
the range of 0 to 400 psig.
eous NH3 is separately contacted with the liquid AAF to
Preferably, the pressure of the reaction is maintained in
generate ammonium ions. This process typically gives NF3
the range of O to 50 psig.
yields of 40-50%. It is operated to maintain a molar ratio of 45
The present invention in an alternative embodiment is a
by-product HF to ammonia of 2.0 to 2.5 (melt ratio) in the
method for synthesis of NF3 by the reaction of a ?uorine
reaction liquid and at temperatures above the melting point
reactant and an NH4H(x_1)F, reactant, where x is equal to or
of ammonium bifluoride, NH4HF2, which is 127° C. The
greater than 2.55, and the reactants are mixed using a mixing
contacting of F2 withAAF is done using a specially designed
means at a mixing level achieved by input of power to said
sparger having a plurality of small holes. The most signi? 50
mixing means of greater than or equal to 1000 watts per
cant shortcoming of this process is the low selectivity and
cubic meter of reactor volume based upon mixing means
yield of NFa.
comprising a ?at blade turbine.
US. Pat. No. 4,543,242 discloses a synthesis of NF, using
gaseous ?uorine and solid (NH4)3AlF6. The yield of NF3 Preferably, the power is greater than or equal to 5000
was in the range of 65-78% based upon ?uorine. 55 watts per cubic meter.
Japanese Patent Kokai 03-232710 discloses the synthesis More preferably, the power is greater than or equal to
of NF3 from a metal ?uoride, an inorganic ammonium salt 35,000 watts per cubic meter.
and elemental ?uorine. The present invention is further a method for synthesis of
Gas phase reactions of ammonia and ?uorine are NF3 by the reaction of a ?uorine reactant and an ammonium
described in Japanese Patent Kokais 02-255511; 02-255512 complex reactant selected from the group consisting of
and 02-255513. In the former Kokai, an NF3 yield of 43.7% NH4H(x_1)F,,(NH4)yMFz.nHF and mixtures thereof where x
is reported. is equal to or greater than 2.55, y is 1-4, 2 is 2-8, 11 is
The prior art has failed to produce nitrogen tri?uoride in su?icient at reaction conditions to maintain said ammonium
yields in the range achieved by the present invention. With complex reactant substantially as a liquid and M is selected
the increased demand for nitrogen tri?uoride from the 65 from the group consisting of elements from Group IA
electronics industry. a need has arisen to utilize larger through VA, Group IB through V1113 and Group VIII of the
nitrogen tri?uoride production plants. At the higher energy Periodic Table of the Elements and mixtures thereof, and the
5,637,285
3 4
reactants are mixed at a rate to maintain a percent NF3 aluminum, gallium, indium, thallium, silicon, germanium,
selectivity of greater than or equal to 50 percent. tin, lead, nitrogen, phosphorus, arsenic, antimony, bismuth,
Preferably, the percent NF3 selectivity is greater than or copper, silver, gold, zinc, cadmium, mercury, scandium.
yttrium, lanthanum, actiniurn, titanium, zirconium, hafnium,
equal to 60 percent.
cerium, vanadium, niobium, tantalum, praseodymium,
More preferably, the percent NF3 selectivity is greater
chromium, molybdenum, tungsten, manganese, rhenium,
than or equal to 80 percent.
iron, ruthenium, osmium, cobalt, rhodium. iridium, nickel,
palladium and platinum, y is 1-4, 2 is 2-8. In other words.
BRIEF DESCRIPTION OF THE DRAWING
‘54” in the formula is equal to the ratio of HP to NH3 in the
FIG. 1 is a graph of % NF3 Yield versus HF content reaction liquid and is also called the melt ratio. Preferably,
(HF/NH3 melt ratio) for a preferred embodiment of the the melt ratio is greater than or equal to approximately 2.65,
present invention for nitrogen tri?uoride synthesis wherein most preferably, greater than or equal to approximately 2.85.
the melt ratio was varied from 2.28 to 2.85 per Example 1, Operation at these high melt ratios results in unexpectedly
below. Enhanced NF3 yields were obtained at high melt high NF3 yields of 65%, preferably 80% and up to 90%,
ratios. The power was 35,000 Watts/cu. meter in a stirred 15 when using an elevated mixing rate as in a stirred reactor.
reactor. Also at melt ratios above or equal to approximately 2.55,
FIG. 2 is a graph of % NF3 selectivity versus HF content lower reaction temperatures can be used because the melting
(HF/NH3 melt ratio) for the same data represented in FIG. point of the ammonium complex reactant liquid decreases
1 and Example 1. with higher melt ratio. Lower reaction temperatures favor
FIG. 3 is a graph of % NF3 Yield versus power/volume reaction (1) over reaction (2).
(watts/cu. meter) for two preferred embodiments of the For the purpose of the present invention, the % yield of
present invention at high melt ratio (MR=2.65, 2.85) com NF3 shall be on a molar basis determined as follows:
pared to an example at low melt ratio (MR=2.3) in nitrogen [3(moles of NF3 produced)/moles of F2 fed] 100. The %
tri?uoride synthesis. per Example 2. Higher NF3 yields at selectivity to NF3 shall be on a molar basis determined as
the high melt ratios of the present invention are sensitive to 25 follows: [3(moles of NF3 produced)/moles of F2 reacted]
power/volume. while the low melt ratios typical of the prior 100. The % F2 conversion shall be on a molar basis
art are insensitive to power/volume. determined as follows: [moles of F2 reacted/moles of F2
FIG. 4 is a graph of % F2 Conversion versus power/ fed] 100. The melt ratio shall be the molar ratio of HF to NH3
in the reaction liquid.
volume (watts/cu. meter) for the same experimental data
reported in Example 2 and FIG. 3. Again high melt ratios are In practicing the method of the present invention in a
more sensitive to power/volume than low melt ratios in preferred embodiment, a stirred reactor vessel contains
nitrogen tri?uoride synthesis. ammonium bi?uoride and HF in such a ratio to give ammo
nium poly(hydrogen ?uoride) complexes, NH4H(,,_1)F,,.
FIG. 5 is a graph of % NF3 Yield versus versus Tempera
where x§~2.55. While rapidly stirring this liquid, gaseous
ture (°C.) for the embodiments of Example 3 demonstrating
that high melt ratios (MR) are less temperature sensitive at 35 F2 is passed into the liquid through a sparger. The F2 reacts
low temperatures than low melt ratios in nitrogen tri?uoride with the ammonium poly(hydrogen ?uoride) complex to
synthesis. form NF3 very selectively along with by-product HF and
small amounts of N2. Separately, gaseous NH3 is passed into
FIG. 6 is a graph of % NF3 Yield versus HF content
the liquid at the same time through a sparger to react with the
(HF/NH3 melt ratio) for: (a) the stirred reactor embodiment
HP to generate more ammonium poly(hydrogen ?uoride) for
of the present invention for nitrogen tri?uoride synthesis
reaction with F2. The rate of NH3 addition is maintained at
wherein the melt ratio was varied from 2.28 to 2.85 per
a level such that the melt ratio remains Z~2.55. The high
Example 4, below (O); (b) the prior art technique of U.S.
NF3 yields are observed with or without gaseous NH3
Pat. No. 4,091,081 (A); and (c) an unstirred bubble column
cofeed, however, a continuous process is desirable and thus
reactor at high melt ratio (III). The stirred reactor of the
45 NH3 cofeed is advantageous. The reactions are outlined as
present invention. at low melt ratios, gave results similar to
follows, where xé~2.55:
the prior art and the bubble column at high melt ratios. The
stirred reactor at the high melt ratios of the present invention
was superior to the prior art and the bubble column at high
melt ratios in % NF3 yield.
(5)
DETAILED DESCRIPTION OF THE
INVENTION (6)
The method of the present invention involves contacting Any source of ammonia or ammonium ion could be used
gaseous F2 as the ?uorine reactant with a vigorously mixed 55 such that its combination with the proper amount of HF
ammonium complex reactant comprising ammonium acid generates substantially a liquid within the operating range.
?uoride liquid (NH4H(,,_1)FX) or (NH4)yMF=.nHF or mix For example. sources such as ammonia or ammonium salts
tures thereof in which the ratio of equivalents of HF to NH3 including NH3, NH4F. NH4HF2, NH4Cl, NH4Br. NH4l,
in this ammonium acid ?uoride complex reactant is greater NH4NO3, (NH4)2SO4, and (NH,,,)2CO3 could be used. Also
than or equal to approximately 2.55 (i.e., X§~2.55), n is complex ammonium ?uorometallate poly(hydrogen
suf?cient to maintain the ammonium complex reactant at ?uorides) generated by the reaction of ammonium ?uoro
reaction conditions substantially as a liquid (at least 50% by metallates with the proper amount of HF to give substan
volume). M is selected from the group consisting of ele tially a liquid could be used. This general class of ammo
ments from Group IA through VA, Group IB through VIIB nium complexes can be described by (NH4)yMF:.nHF,
and Group VIII of the Periodic Table of the Elements, 65 where M is a metal selected from the group consisting of
namely: lithium, sodium. potassium, rubidium, cesium. elements from Group IA through VA, Group IB through
beryllium, magnesium. calcium, strontium. barium, boron. VIIB and Group V111 of the Periodic Table of the Elements
5,637,285
5 6
or mixtures thereof; y=1—4; z=2-8 and n is su?icient to cal Engineering, Dec. 1994, pp 98—104, summarizes the
maintain the ammonium complex as a liquid in the existing design criteria and standard correlations for optimizing
reaction conditions. This class of compounds could also be gas-liquid mixing and refers to other articles that discuss the
generated by the reaction between ammonium ?uoride, HF state of the art for gas-liquid mixing in more detail. It is this
and metal ?uorides or the reaction between ammonium article, with the commonly accepted theory and correlations,
poly(hydrogen ?uoride) and metal ?uorides. that forms the basis for the following explanation of how to
The present inventors have found unexpectedly that high quantify the enhanced mixing or agitation required in the
melt ratios lead to higher selectivities of NF3. Therefore, present invention.
another aspect of the present invention is conducting the It is common in gas-liquid reactors to quantify the inten
reaction at melt ratios at or above approximately 2.55 and at sity of mixing or degree of gas dispersion by determining the
NF3 selectivities of at or above 50%, preferably at or above power dissipation per unit volume of liquid. It follows then
60%, most preferably at or above 80%. that the gas-liquid mass transfer rate and the contact time are
dependent on the power-input per unit volume, Pg/Vb as
TABLE 1 discussed in the following. Gas holdup, or, is de?ned as:
15
Operating ranges and limits for the present invention are:
Operation Preferred oi=(zg-z,,)/zg (7)
HFINI'I3 ratio é~2.55 ;~2.65—3.0 where Z8 is the volume of gas-liquid mixture in the reactor
Temperature 93-204" C. 121-160’ C. during the gas dispersion process and Z,‘ is the volume of
Pressure 0-400 psig 0-50 psig liquid in the reactor without gas being dispersed into the
Fzcontacttime 1secto5min 5secto2min
liquid. The contact time, 'c is calculated as:
NH; flow rate su?cient to maintain proper melt ratio
F2 concentration 10—100% 100%
NH; concentration 10—100% 80-100%
Power per unit volume:
25
stirred reactor. E1000 watts/m3 E5000 watts/m3
where V g is the volume of gas dispersed in the liquid and Qg
bubble column: E100 watts/m3 2.500 watts/m3
is the volumetric ?ow rate of gas being dispersed into the
liquid. Contact time is related to gas holdup through Vg as
In the present invention, it has been found that the follows:
unexpectedly high yield of nitrogen tri?uoride and high
conversion of ?uorine are achieved when the combination of
high melt ratio and enhanced mixing or agitation of the
reactants occurs. The mixing or agitation is typically
substituting for V8 in Equation (8) gives,
achieved by a stirrer or turbine. The extent of this stirring or
agitation can be expressed as power per unit volume of 35
reactor based upon a given stirrer or turbine geometry. In the
1J=0tZ‘/Qx (10)
present invention in a stirred reactor, it has been found that
improved results of NF3 yield are achieved at or above a
Thus increases in the contact time can be obtained by
power of 1000 watts per cubic meter, preferably at or above increasing the gas holdup.
5000 watts per cubic meter, most preferably at or above 40 For a stirred reactor the gas holdup can be increased by
35.000 watts per cubic meter. These power values are based increasing the power-input per volume according to the
upon a stirrer or agitator of a ?at blade turbine design and the following relationship:
corresponding or equivalent values for other stirrer or agi
tator designs or geometries can be calculated based upon the
correlations found in the literature discussed below. 45
In this new process, enhanced mixing or agitation is a
necessary requirement to achieve high F2 conversion. At the where Pg/V, is the power-input per unit volume, v is the
state of enhanced mixing or agitation, the reactant gas, F2, super?cial gas velocity and Cw A and B are constants. Also
is well dispersed in the liquid ammonia acid ?uoride (AAF). for a stirred reactor the gas-liquid mass transfer rate is
Increasing the degree of dispersion enhances the contact of increased through an increase in the power-input per unit
the F2 gas with the liquidAAF thus increasing the rate of the volume according to this correlation, where kLa is the
reaction. The degree of gas dispersion also has an effect on volumetric mass transfer coe?icient:
the amount of time that the gas remains in contact with the
liquid. This is known as the residence or contact time.
Usually, higher gas dispersion results in higher gas hold-up 55 kLa=Cna(P,/V1)“\"’ (12)
(void fraction of gas in the dispersed liquid) and hence
Cua, a_ and b are different constants.
increases the residence time of the gas in the liquid. Longer
For a bubble column reactor, the power input, Pg, derives
residence times allow more time for the reactants to react
from the gas ?owrate, as described by A. G. Jones in “Liquid
and results in higher conversion. Thus in the present inven
Circulation In A Draft-Tube Bubble Column”, Chemical
tion of operating at a high melt ratio it is required that there
Engineering Science, Vol. 40, No. 3, p. 449 (1985). It can be
be a high degree of dispersion so that both the gas-liquid
approximated by the isothermal work done by the gas
mass transfer rate and the contact time are high enough to
bubbles expanding through the liquid, i.e.,
give improved yields over the prior art
Theory and models for understanding gas-liquid mixing
and their use for reactor design are well established and 65
accepted by the engineering community. Arecent article by
Bakker. et. al., “How to Disperse Gases in Liquids”, Chemi or in terms of power-input per unit volume,