Note: Descriptions are shown in the official language in which they were submitted.
S~'~
HOE 77/H 042
This invention relates to a process for recovering
phosphane, methylphosphane or a mixture o~ these two
phosphanes from a waste gas mixture containing one or
more phosphanes in admixture with hydrogen, nitrogen and/
B 5 or oneor more ~ ~ower hydrocarbonsO
Phosphanes are normally separated from gas mixtures,
especially from those which contain phosphanes in admixture
with hydrogen, by subjecting these gas mixtures to con-
densation. In the case o~ PH3, it is necessary to ef~ec-t
the condensation at temperatures of -120 to -130C, which
is a highly expensive procedure.
This is the reason why substantially no use has been
made of the condensation method just described ~or the
recovery o~ phosphanes ~rom gas mixtures o~ low phosphane
concentration, and i-t has indeed often been necessary for
these mixtures to be disposed of.
U.S. Patent 3 982 912 discloses a process, wherein a
silane/phosphane-mixture is separated into its components
with the aid of a zeolite. Use is more specifically made
in -this process of K-A grade zeolites that are made ~rom
standard A-grade zeolites, of which the exchangeable ions
are replaced to an extent o~ 33.3 to 83.3 % by potassium
ions and to an extent of approximately 17.7 to 66.7 ~ by
zinc ions.
Re~erence to the good adsorbing power of these K-A
grade zeolites prepared in the specific manner JUSt
described has been made in German Patent 2 208 Z14.
ii3~
These prior art processes are notl however, i.ree frc.m adverse
effects inasmuch as the zeolites used have to ~e prepared frcm co~mercially
available zeolites which are subjected to special pre-treatment. In acldition
to this, the particular zeolites employed are just suitable for use in the
separation of silane/phosphane-mixtures. As is known, the separating pc~er
of these zeolites is critically influenced by their composi-tion (K : Zn
-ratio~ so that they are not di.rectly of assistan oe in the t.reatment of other
phosphane/gas-mixtures.
The adsorption of methylphosphane in contact with a zeolite has not
been described heretofore in any literature reference of which we are aware.
PH3 which is produced continuously from yellow phosphorus and
sodium hydrc)xide solution is always obtained together with PH3 containing con-
siderable hydrogen, the latter being formed in a post-reaction during which
hypophosphite unde.rgoes disprc~portionation into PH3 and phosphite, and reac-
tion with water to phosphite and hydrogen.
Heretofore it has indeed been very difficult to recover concen-
trated and pure phosphane frc~ phosphane~hydrogen-mixtures which contain
relatively mmior proportions of PH3.
The present invention now unexpectedly provides a process for re-
covering pure phosphane and/or methylphosphane from a waste gas mixture con-
taining relatively minor proportions of these phosphanes in admixture with
hyclrogen, nitrogen and/or non-polar lower hydrocarbons, which comprises:
contacting the waste gas mixture at temperatures within the range -20 to
+30C, preferably at room temperature, with a zeolite having a pore size of
5 x 10 8 to 15 x 10 8 cm; separating the phosphanes from the remainder of the
waste gas mixture by adsorbing them on the zeolite, until-the la~ter is
saturated therewith; heating the zeolite to a temperature within the range
180 to 230C and collecting the desorbing phosphanes.
Preferred features of the present process provide:
a) for the waste gas mixture to contain 1 up to at most
70 volume % of the phosphanes speci~ied;
b) for the zeolite to comprise commercially available
zeolites, preferably Na-A grade zeolites, with a pore
size within the range 5 x 10 8 to 10 x 10 8 cm;
c) ~or the zeolite with the adsorbed phosphanes thereon
to be heated to 180 to 200C so as to desorb the phos
phanes;
$ 10 d) for the~ ~ lower hydrocarbons to comprise C1 - C4
hydrocarbons.
The fact that the present process can e~fecti~ely be
carried out with the use of commercially available zeolites,
which need no-t be pretreated, has also been an unexpected
result. The useful zeolites comprise more especially A~grade
zeolites which have a pore width of 5 ~, an Al:Si-ratio of
0.9 - 1.1:1 and a Na:Ca-ratio of 0c6 - 1.1:1. A-grade
zeolites with a pore width of 10 ~, an Al:Si-ratio o~
0.6 - 0.9:1 and a Na:Ca-ratio of 15 - 20:1 can also be used.
In contact with the zeolite, the starting gas mixture
is freed substantially completely ~rom the phosphanes; only
traces of PH3 and/or CH3PH2 were found to have been retained
in the remaining gas~ The steep blowout behaviour at
saturation is also an index of the good adsorptive capacity
25 ~or PH3 and CH3PH2. The load capacity is 130 g PH3/kg
- æeolite or 180 g CH3PHlk~ ~liteat -20C, and 100 g PH3/kg
zeolite or 155 g CH3PH2/kg zeolite at +20C~ irrespective
o~ the composition of the gas mixture. In those cases in
which the gas mixture to be separated contains phosphane
together with methylphosphane, the two gases are adsorbed
simultaneously. They are easy to separate ~rom one another
by desorbing and condensing the methylphosphane at tempe-
ratures within the range -30 to -80C or by adsorbing it
in concentrated hydrochloric acid.
3~
Steam which is present in the gas mixture in accor-
dance with the steam partial pressure prevailing is also
adsorbed substantially in the absence of any signi~icant
adverse effect on the load capacity for PH3 or CH3PH~
during an adsorption operation.
Immediately after the treatment of the zeolite with
PH3 and/or CH3PH2, it is possible to ef~ect the desorption
by heating the zeolites to temperatures within the range
180 to 230C which causes the adsorbed gas to be continuous-
ly set free. At temperatures higher than 200 C, -the zeolite
is substantially free from PH~ and CH3PH2, respectively.
After cooling down to room temperature, the zeolite can be
used again. Phosphane or methylphosphane which is recovered
in the manner described is very pure (99.0 volume ~0~,
irrespective of the composition of the starting gas mixtureO
In a series of adsorption and desorption tests, the
process o~ the pres~nt invention was found to be repro-
ducible in respect of capacity and separating ef~icienc~.
In a further long time test 9 the adsorbed phosphane was
found to remain stable over some prolonged period of time.
The following Examples illustrate the invention:
EXAMPLE 1:
390 g of a grade A zeolite (5 ~; bead~ 2mm; bulk
density 760 g/l; dried at 300~) was placed in a double-
jacketed glass column and cooled under nitrogen down to-20C (a methanol/dry ice-mixture was the cooling liquid
which was~ kept under circulation).
Next, the column was fed with PH3 until significant
proportions thereo~ were found to get into a burner down-
stream o~ the col D . The adsorption heat caused thetemperature in the adsorption zone to increase to +20 to
30C. The cooling liquid was removed and the zeolite was
heated to 200C by means of a circulating heating liquid.
PH3 was set free continuously. The adsorbed quantity of
PH~ was weighed by determining the difference in weight
of the steel bottle tank, the volume of desorbed gas was
determlned by means of a gas me-ter and, after conversion,
compared with the adsorbed quantity of PH3 . 90 to 95 ,~
of the adsorbed PH3 was ~ound to have been set free by
desorption at 200 C.
EXAMPLE 2:
Methylphosphane was adsorbed as described in Example 1
on 440 g of a grade A zeolite which had a pore size of
10 x 10 8 cm at -16C. 74 g o~ CH3PH2 was found to have been
adsorbed.
A gas mixture composed of 85 volume % H2 and ~ volume ~'
PH3 was separated into its components as described in
Example 1 by contacting it at room temperature with the
zeoli-te. The gas co~ing from the adsorption column packed
with the æeolite was composed of:
99.9 volume % H2
0.02 volume % PH3
The gas desorbed by heati.ng the zeolite was composed
as follows:
99.8 volume % PH3
0.2 volume % H2
E~AMPLE 4:
A gas mixture composed of 64 volume % CH4, 32 volume
PH3, and 4 volume % N2 was separated into its components
at room temperature as described in Example 1. The gas
coming from the adsorption column was composed of:
94.1 volume % CH4
5.8 volume ~ N2
30 0.02 volume ~o PH3
and the desorbed gas was composed of:
99.0 volume % PH3
0.04 volume % CH4.
