U.S. patent application number 11/433809 was filed with the patent office on 2007-11-15 for enhanced process for the purification of anhydrous hydrogen chloride gas.
Invention is credited to Eric F. Boonstra, Kaspar Hallenberger, Anke Hielscher, John M. Teepe, Renae M. Vandekamp.
Application Number | 20070261437 11/433809 |
Document ID | / |
Family ID | 38683841 |
Filed Date | 2007-11-15 |
United States Patent
Application |
20070261437 |
Kind Code |
A1 |
Boonstra; Eric F. ; et
al. |
November 15, 2007 |
Enhanced process for the purification of anhydrous hydrogen
chloride gas
Abstract
The present invention relates to a process for purifying
anhydrous hydrogen chloride gas ("aHCl"), and preferably the
anhydrous hydrogen chloride gas recovered from an isocyanate
production process. In the process of the present invention, the
content of chlorinated organics may be reduced from up to 1000 ppm
by volume to below 10 ppb by volume levels. Generally, the process
of the invention allows for chlorinated organic levels to be
reduced to from 1 to 100 ppb, rendering the treated hydrogen
chloride gas usable in a catalytic oxychlorination process or a
Deacon process. The treated gas is also suitable for absorption in
water or dilute hydrochloric acid.
Inventors: |
Boonstra; Eric F.; (Baytown,
TX) ; Teepe; John M.; (Baytown, TX) ;
Vandekamp; Renae M.; (Beaumont, TX) ; Hielscher;
Anke; (Leverkusen, DE) ; Hallenberger; Kaspar;
(Leverkusen, DE) |
Correspondence
Address: |
BAYER MATERIAL SCIENCE LLC
100 BAYER ROAD
PITTSBURGH
PA
15205
US
|
Family ID: |
38683841 |
Appl. No.: |
11/433809 |
Filed: |
May 12, 2006 |
Current U.S.
Class: |
62/617 |
Current CPC
Class: |
C01B 7/0712
20130101 |
Class at
Publication: |
062/617 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Claims
1. A cooling and distillation process to remove contaminants having
boiling points higher than hydrogen chloride from a hydrogen
chloride-containing gas comprising: a) compressing said hydrogen
chloride-containing gas, b) cooling the resultant compressed gas in
a first heat exchanger resulting in a first condensate stream and a
first gas stream, wherein said compressed gas is cooled to a
temperature low enough to partially condense said contaminants and
at a rate sufficiently low that fog formation is prevented, c)
feeding said first gas stream from said first heat exchanger to a
distillation column having a top portion and a bottom portion to a
point between said top portion and said bottom portion, to cause
mass transfer between liquid and gas and to thereby concentrate the
contaminants in the bottom portion of said column and hydrogen
chloride gas in the top portion of said column, d) feeding said
hydrogen chloride gas from said top portion to a second heat
exchanger whereby the hydrogen chloride gas is partially condensed
to form a second condensate stream and a second gas stream, e)
feeding said second condensate stream to said top portion of said
column to provide reflux to said column, f) feeding said first
condensate stream to said distillation column below the point where
said first gas stream is fed, g) feeding said second gas stream
from step d) to said first heat exchanger as cooling medium, h)
recovering purified hydrogen chloride gas from said first heat
exchanger, and i) feeding said contaminants from the bottom potion
of said column to a collection vessel.
2. The process of claim 1, wherein the contaminants contained in
the gas stream are chlorinated aromatic compounds.
3. The process of claim 2, wherein the gas stream also contains a
contaminant with an intermediate boiling range between the hydrogen
chloride boiling point and the chlorinated aromatic compound
boiling point.
4. The process of claim 3 wherein said intermediate is phosgene and
said intermediate is removed from said distillation column.
5. The process of claim 1 wherein the temperature of the compressed
gas is reduced to a temperature of from +10 to -25.degree. C. in
said first heat exchanger.
6. The process of claim 1 wherein in step a), the gas is compressed
to a pressure of from 5 to 30 bars absolute.
7. The process of claim 1, wherein step f) comprises f1) feeding
said first condensate to a separation vessel (or vessels) to trap
solids and wherein a solids stream and an overflow condensate
stream are formed, f2) feeding said overflow condensate stream to
said distillation column at a point below the point where the first
gas stream is fed.
8. The process of claim 1, wherein step e) comprises: e1) feeding
said second condensate stream to one or more separation vessels
used to trap any solids present and to form a solids stream and a
third condensate stream, e2) feeding said third condensate stream
to said top portion of said column to provide reflux to said
column.
9. The process of claim 1 wherein step i) comprises i1) feeding
liquid from the bottom portion of said column to a reboiler to
generate stripping vapors for the bottom portion of the column, and
wherein the reboiler heats the said liquid at low heat flux so as
to prevent foaming action and i2) removing any remaining liquid
from the reboiler to a collection vessel for disposal.
10. The process of claim 9, wherein the reboiler is designed to
prevent the formation of foam and has a heat flux of from 500 to
20,000 BTU/hr/ft.sup.2 as a lower limit and from 3,000 to 30,000
BTU/hr/ft.sup.2 as a higher limit.
11. The process of claim 9, wherein a portion of the liquid removed
from the reboiler comprises hydrogen chloride and contaminants and
is sprayed into the gas stream being fed to the first heat
exchanger.
12. The process of claim 9, wherein from 5 to 95% by weight the
liquid fed to said reboiler is evaporated.
13. The process of claim 1, wherein the purified hydrogen chloride
gas from the first heat exchanger is further purified by treatment
with activated charcoal.
14. A cooling and distillation process to remove contaminants
having boiling points higher than hydrogen chloride from a hydrogen
chloride-containing gas comprising: a) compressing said hydrogen
chloride-containing gas, b) cooling the resultant compressed gas in
a first heat exchanger resulting in a first condensate stream and a
first gas stream, wherein said compressed gas is cooled to a
temperature low enough to partially condense said contaminants and
at a rate sufficiently low that fog formation is prevented, c)
feeding said first gas stream from said first heat exchanger to a
distillation column having a top portion and a bottom portion to a
point between said top portion and said bottom portion, to cause
mass transfer between liquid and gas and to thereby concentrate the
contaminants in the bottom portion of said column and hydrogen
chloride gas in the top portion of said column, d) feeding said
hydrogen chloride gas from said top portion to one side of a third
heat exchanger flasher and feeding said contaminants from the
bottom portion of said column to the other side of said third heat
exchanger to flash against and cool the hydrogen chloride gas
passing through said third heat exchanger, whereby the following
streams are formed: 1) a second gas stream containing contaminants,
2) a contaminant stream, 3) a third cooled gas stream, and 4) a
second condensate stream, e) feeding said third gas stream to a
second heat exchanger whereby the hydrogen chloride gas is
partially condensed to form a third condensate stream and a fourth
gas stream, f) combining said second condensation stream and said
third condensation stream and feeding the resulting combined stream
to said top portion of said column to provide reflux to said
column, g) feeding said first condensate stream to said
distillation column below the point where said first gas stream is
fed, h) feeding said fourth gas stream from step d) to said first
heat exchanger as cooling medium, and i) recovering purified
hydrogen chloride gas from said first heat exchanger.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a process for purifying
anhydrous hydrogen chloride gas ("aHCl"), and preferably the
anhydrous hydrogen chloride gas recovered from an isocyanate
production process. In the process of the present invention, the
content of chlorinated organics may be reduced from up to 1000 ppm
by volume to below 10 ppb by volume levels. Generally, the process
of the invention allows for chlorinated organic levels to be
reduced to from 1 to 100 ppb, rendering the treated hydrogen
chloride gas usable in a catalytic oxychlorination process or a
Deacon process. The treated gas is also suitable for absorption in
water or dilute hydrochloric acid.
[0002] A number of important chemical processes generate aHCl as a
byproduct. Examples of such processes include chlorination
processes, silane production processes and phosgenation processes.
Because large amounts of aHCl can not be disposed of, one of the
challenges encountered with each of these processes is purification
of the aHCl generated to obtain a usable technical product or raw
material for other processes. Several processes for purifying aHCl
generated during production processes have been proposed. Thermal
treatment of the aHCl at temperatures of up to 800 to 1600.degree.
C. is disclosed in U.S. Pat. No. 5,126,119. Full condensation and
distillation under elevated pressure is disclosed in U.S. Pat. No.
4,935,220. The processes disclosed in these patents require high
amounts of energy and expensive equipment.
[0003] Treatment of aHCl at pressures of 5 to 20 bar absolute and
final temperatures below -20.degree. C. is disclosed in U.S. Pat.
No. 6,719,957. The process disclosed in the '957 patent results in
contaminant levels occasionally unacceptable for use in vinyl
chloride production. The contaminant level achieved is always
unacceptable for use in Deacon processes.
[0004] In the commercial phosgenation processes for the production
of isocyanates such as TDI (toluene diisocyanate, MDI
(diphenylmethane diisocyantes) and HDI (hexamethylen
diiscocyanate), two moles of aHCl are formed per isocyanate group
produced. This large quantity of by-product must be used in a
secondary process.
[0005] One such secondary process is the production of muriatic
acid. However, the volume of HCl byproduct produced often exceeds
the market demand. Another alternative is to use the aHCl in a
catalytic oxychlorination process with ethylene to produce ethylene
dichloride and finally vinyl chloride as the commercial product.
This catalytic process is very sensitive to traces of organic
compounds, particularly (chloro-) aromatic compounds which can
deactivate the catalyst employed.
[0006] Another secondary process is the Deacon process, which
produces chlorine and water by passing gaseous HCl and oxygen over
a transition metal catalyst. This process is very sensitive to
traces of some contaminants, such as sulfur and some organic
compounds, which over time can lead to catalyst deactivation and/or
plugging of reactors, which in turn can lead to unwanted by-product
formation.
[0007] The most commonly used solvents in isocyanate production are
chlorobenzene and dichlorobenzene (See G. Oertel, Polyurethane
Handbook, page 66 (Carl Hanser Verlag, Munich (1985)). The aHCl
recovered from the phosgenation process is saturated with these
chloro-aromatics. Deep chilling of the aHCl gas can reduce the
chloro-aromatic content, but not to the necessary level. Another
complicating factor is the high melting point of dichlorobenzene
(o-isomer: -17.5.degree. C., p-isomer: +52.8.degree. C.), which
limits the usefulness of this approach. Low pressure phosgenation
processes such as those described in G. Oertel, Polyurethane
Handbook, p. 66 (Carl Hanser Verlag, Munich (1985)), which yield
aHCl gas at pressure ranging from atmospheric to below 5 bar, will,
even with deep chilling, contain chloro-aromatics in a
concentrations of from several hundred ppm to 1000 ppm.
[0008] The present invention has several objects: i) a process for
the removal of one or more contaminants from hydrogen chloride gas,
ii) a process for separating small quantities of high boiling
material, e.g., (chloro) aromatic compounds from large volumes of
anhydrous HCl gas; and, iii) a process for reducing the
concentration of contaminants such as (chloro)aromatic compounds in
anhydrous HCl gas to <100 ppb.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 schematically illustrates a flow diagram for the
present invention.
[0010] FIG. 2 schematically illustrates a second embodiment of the
present invention.
[0011] FIG. 3 schematically illustrates a preferred embodiment of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The present invention is broadly directed to a cooling and
distillation process to remove contaminants having boiling points
higher than hydrogen chloride from a hydrogen chloride-containing
gas comprising: [0013] a) compressing said hydrogen
chloride-containing gas, [0014] b) cooling the resultant compressed
gas in a first heat exchanger resulting in a first condensate
stream and a first gas stream, wherein said compressed gas is
cooled to a temperature low enough to partially condense said
contaminants and at a rate sufficiently low that fog formation is
prevented, [0015] c) feeding said first gas stream from said first
heat exchanger to a distillation column having a top portion and a
bottom portion to a point between said top portion and said bottom
portion, to cause mass transfer between liquid and gas and to
thereby concentrate the contaminants in the bottom portion of said
column and hydrogen chloride gas in the top portion of said column,
[0016] d) feeding said hydrogen chloride gas from said top portion
to a second heat exchanger whereby the hydrogen chloride gas is
partially condensed to form a second condensate stream and a second
gas stream, [0017] e) feeding said second condensate stream to said
top portion of said column to provide reflux to said column, [0018]
f) feeding said first condensate stream to said distillation column
below the point where said first gas stream is fed, [0019] g)
feeding said second gas stream from step d) to said first heat
exchanger as cooling medium, [0020] h) recovering purified hydrogen
chloride gas from said first heat exchanger, and [0021] i) feeding
said contaminants from the bottom portion of said column to a
collection vessel.
[0022] In the compression step (step a)), the gas is preferably
compressed to a pressure of from 5 to 30 bars absolute.
[0023] The contaminants contained in the gas stream are preferably
chlorinated aromatic compounds. In one preferred embodiment, the
gas stream also contains a contaminant with an intermediate boiling
range between the hydrogen chloride boiling point and the
chlorinated aromatic compound boiling point. The intermediate
contaminant is removed from the distillation column, is
subsequently depressurized, and is discarded. In one especially
preferred embodiment, the intermediate contaminant is phosgene.
[0024] In the cooling step (step b)), the incoming contaminated gas
is cooled slowly. The temperature difference between the cooling
wall of the first heat exchanger and the inlet gas temperature is
preferably between 0.5 and 40.degree. C., and most preferably in
the range of from 5 to 25.degree. C. The temperature of the
compressed gas is preferably reduced to a temperature of from +10
to -25.degree. C. in the cooling step (step b)).
[0025] In one embodiment of the invention, in step f), the
condensate stream (of step b)) is fed to a separation vessel (or
vessels) used to trap solids. If multiple vessels are used, one
vessel can be used to collect solids while overflowing condensate
from the vessel being fed to the distillation column at a point
below the point where the first gas stream is fed, while the other
vessel is depressurized to enable collected solids to be purged to
waste.
[0026] In another preferred embodiment, step e) comprises: [0027]
e1) feeding said second condensate stream to one or more separation
vessels used to trap any solids present and to form a solids stream
and a third condensate stream, [0028] e2) feeding said third
condensate stream to said top portion of said column to provide
reflux to said column. The solids collected in the solids stream
can be purged to waste.
[0029] In an other preferred embodiment, step i) comprises i1)
feeding liquid from the bottom portion of said column to a reboiler
to generate stripping vapors for the bottom portion of the column,
and wherein the reboiler heats the said liquid at low heat flux so
as to prevent foaming action and i2) removing any remaining liquid
from the reboiler to a collection vessel for disposal. Preferably,
from 5% to 95% of the liquid reaching the reboiler is evaporated.
Most preferably, the reboiler design prevents the formation of
foams and has a heat flux of from 500 to 20,000 BTU/hr/ft.sup.2 as
a lower limit and from 3,000 to 30,000 BTU/hr/ft.sup.2 as a higher
limit. In any even more preferred embodiment, a portion of the
liquid removed from the reboiler comprises hydrogen chloride and
contaminants and is sprayed into the gas stream being fed to the
first heat exchanger, most preferably in amount of from 1 to 25% by
weight of the weight of the incoming gas stream.
[0030] The temperature of the gas being fed into the distillation
column is preferably reduced to a temperature of from 0 to
-35.degree. C. during the distillation step.
[0031] In another preferred embodiment, the purified hydrogen
chloride gas from the first heat exchanger is further purified by
treatment with activated charcoal.
[0032] In a second broad embodiment, the invention comprises [0033]
a) compressing said hydrogen chloride-containing gas, [0034] b)
cooling the resultant compressed gas in a first heat exchanger
resulting in a first condensate stream and a first gas stream,
wherein said compressed gas is cooled to a temperature low enough
to partially condense said contaminants and at a rate sufficiently
low that fog formation is prevented, [0035] c) feeding said first
gas stream from said first heat exchanger to a distillation column
having a top portion and a bottom portion to a point between said
top portion and said bottom portion, to cause mass transfer between
liquid and gas and to thereby concentrate the contaminants in the
bottom portion of said column and hydrogen chloride gas in the top
portion of said column, [0036] d) feeding said hydrogen chloride
gas from said top portion to one side of a third heat exchanger and
feeding said contaminants from the bottom portion of said column to
the other side of said third heat exchanger to flash against and
cool the hydrogen chloride gas passing through said third heat
exchanger, whereby the following streams are formed: [0037] 1) a
second gas stream containing contaminants, [0038] 2) a contaminant
stream, [0039] 3) a third cooled gas stream, and [0040] 4) a second
condensate stream, [0041] e) feeding said third gas stream to a
second heat exchanger whereby the hydrogen chloride gas is
partially condensed to form a third condensate stream and a fourth
gas stream, [0042] f) combining said second condensation stream and
said third condensation stream and feeding the resulting combined
stream to said top portion of said column to provide reflux to said
column, [0043] g) feeding said first condensate stream to said
distillation column below the point where said first gas stream is
fed, [0044] h) feeding said fourth gas stream from step d) to said
first heat exchanger as cooling medium, and [0045] i) recovering
purified hydrogen chloride gas from said first heat exchanger.
[0046] In addition, any of the various preferred parameters and
embodiments described above can be used with this second broad
embodiment. For example, in step g), the condensate stream (of step
b)) can be fed to a separation vessel (or vessels) used to trap
solids. If multiple vessels are used, one vessel can be used to
collect solids while overflowing condensate from the vessel being
fed to the distillation column at a point below the point where the
first gas stream is fed, while the other vessel is depressurized to
enable collected solids to be purged to waste. Additionally, step
f) can comprise: [0047] f1) combining said second condensation
stream and said third condensation stream, [0048] f2) feeding the
combined condensate stream to one or more separation vessels used
to trap any solids present and to form a solids stream and a fourth
condensate stream, and [0049] f3) feeding said fourth condensate
stream to said top portion of said column to provide reflux to said
column. The solids collected in the solids stream can be purged to
waste. Similarly, the column can be provided with a reboiler, with
the liquid from the reboiler being fed to the flasher. A portion of
the liquid removed from the reboiler which contains hydrogen
chloride and contaminants can be sprayed into the gas stream being
fed to the first heat exchanger.
[0050] By following the present invention, fog (or aerosol)
formation is avoided by controlling the cooling rate of the
incoming gas, and by the use of a condensate spray to promote more
homogeneous cooling.
[0051] The present invention provides an enhanced method of
purifying a contaminated hydrogen chloride stream by using a
modified cooling and distillation process. Small quantities of high
boiling contaminants, e.g. chlorinated aromatic hydrocarbons, can
be removed down to a concentration of 10 ppb in the purified gas.
In particular, this process works well for purifying byproduct
streams created by isocyanate production processes, which coproduce
with the isocyanate, large volumes of anhydrous hydrogen chloride
gas with contaminants including monochlorobenzene and
dichlorobenzenes (ortho, meta and para isomers).
[0052] The process of the invention will now be further described
with reference to the drawings. Numerals and letters in the
drawings refer to the same devices and streams.
[0053] As shown in FIG. 1, the contaminated hydrogen chloride gas
(shown as stream A) enters compressor 1, exits the compressor and
enters the first heat exchanger 2. As it passes through the first
heat exchanger, the compressed gas is cooled to a temperature low
enough to partially condense the contaminants and at a rate
sufficiently low that fog formation is prevented. Two streams flow
from the first heat exchanger a first condensate stream C and a
first gas stream B. The first gas stream B is fed to a distillation
column 3 at a point between the top and bottom of the column. In
the distillation column, mass transfer occurs between liquid and
gas, with the contaminants being concentrated in the bottom portion
of the column, and hydrogen chloride gas being concentrated in the
lower portion of the column. The hydrogen chloride gas is fed
(stream D) from the top portion of the column to a second heat
exchanger 4 (that is provided with an appropriate coolant via
stream shown in the figure as arrows entering one side of the
exchanger and exiting on the other side) wherein the gas is
partially condensed to form a second condensate stream E and a
second gas stream F. The second condensate stream E is fed back to
the top portion of the column to provide reflux to the column. The
first condensate stream C is fed to the column at a point below the
first gas stream B is fed. The second gas stream F is fed back to
the first heat exchanger as cooling medium. Purified hydrogen
chloride gas is recovered via stream G from the first heat
exchanger and the liquid bottoms (that contain concentrated
contaminants) of the column are fed via stream H to a collection
vessel (not shown) for subsequent disposal.
[0054] The configuration in FIG. 2 is similar to that in FIG. 1
with several added improvements shown. FIG. 2 illustrates the
process as if all the improvements were used. Of course, the
artisan will recognize that not all the improvements must be used.
As shown, the inlet gas stream A enters compressor 1 exits the
compressor and enters the first heat exchanger 2. As it passes
through the first heat exchanger, the compressed gas is cooled to a
temperature low enough to partially condense the contaminants and
at a rate sufficiently low that fog formation is prevented. Two
streams flow from the first heat exchanger a first condensate
stream C and a first gas stream B. The first gas stream B is fed to
a distillation column 3 at a point between the top and bottom of
the column. In the distillation column, mass transfer occurs
between liquid and gas, with the contaminants being concentrated in
the bottom portion of the column, and hydrogen chloride gas being
concentrated in the lower portion of the column. The condensate
from the first heat exchanger flows (stream C') into a solids
trapping vessel 5. If more than one vessel is used, they may be
used interchangeably. In the separation vessel two streams result,
a solids stream C'' that can be collected and purged to waste and a
third condensate stream C. The third condensate stream (without
solids) C is fed as a liquid feed to the distillation column at a
point below the point where the first gas stream is fed.
[0055] The concentrated hydrogen chloride gas from the top portion
of the column is fed via stream D to a third heat exchanger 9. The
bottoms stream H from the distillation column can be split into two
streams, H' and H'' which contain concentrated amounts of the
contaminants. The H'' stream can be pumped via pump 10 back into
the inlet of the HCl gas entering the first heat exchanger. The H'
stream can be flashed against the concentrated hydrogen chloride
gas entering the third heat exchanger. This step can result in
several streams--i) a gas stream J containing hydrogen chloride (at
a lower pressure) and, in the case of a starting gas from an
isocyanate production facility, phosgene (this stream can be
collected and used again in another appropriate process), ii) a
stream J' containing mainly organic contaminants (that are then
collected and disposed), iii) a gas stream D' that is fed to the
second heat exchanger 4 and iv) a condensate stream E''. Stream E''
can be combined with the second condensate stream E and fed to the
top portion of the column 3 to provide reflux to the column.
Alternatively, stream E'' can be combined with the second
condensate stream E and fed to a collection vessel 6 where solids
are collected and discarded via stream E''' with the overflow
condensate from the collection vessel being fed via stream E' back
to the top portion of the column 3 to provide reflux to the column.
The second gas stream F is fed back to the first heat exchanger as
cooling medium.
[0056] FIG. 2 also shows a side draw-off stream I from the
distillation column to remove intermediate boiling contaminants.
This stream can be fed to an activated charcoal bed 8 to remove
organics, resulting in stream I' which is led off to disposal.
[0057] FIG. 2 also shows purified gas stream G being fed to an
activated charcoal bed 7, resulting in a stream G' of purified
hydrogen chloride gas.
[0058] The embodiment shown in FIG. 3 is identical to that shown in
FIG. 2, except that the bottom liquids of the distillation column
are fed via stream H to a reboiler 11 (that is provided with an
appropriate coolant via stream shown in the figure as arrows
entering one side of the exchanger and exiting on the other side)
to generate stripping vapors that are fed via stream K to the
bottom portion of the column. The reboiler heats the bottoms liquid
at low heat flux to prevent foaming action. The condensate stream
H''' from the reboiler can either be collected and discarded or can
be sent to either the third heat exchanger (stream H') or back to
the inlet of the first heat exchanger (stream H'').
[0059] The compressor A can be of any kind of equipment capable of
increasing the pressure to from about of 5 to 30 bar absolute and
preferably above 12 bar absolute. Preferred compressors include
piston compressors, screw compressors, optionally with oil
injection, and centrifugal compressors. The final pressure of the
gas must be adjusted so as to overcome the pressure drop in overall
system.
[0060] Once compressed, the gas enters the first heat exchanger at
which point gas condensate spray mixes with the incoming gas in the
amount of five to twenty-five weight percent of the total amount of
incoming gas. Some condensation from this first heat exchanger can
flow into solids collection vessel(s) to catch solids. The liquid
condensate from these vessel(s) overflows and is fed to the
distillation column.
[0061] The heat exchangers used in the present invention can be of
any type. Shell and tube heat exchangers are preferred.
[0062] HCl offgas from isocyanate units containing
monochlorobenzene, dichloro-benzene, and chlorinated methanes
impurities are preferably used as the initial gas. The gas is
compressed to a pressure of 8 to 20 bar, preferably 12 bar. The
resulting compressed gas is fed to the first heat exchanger for
cooling to between -5 and -20.degree. C., preferably -10.degree.
C., to partially condense impurities. The condensed impurities are
preferably first passed through a solids collection vessel to
remove any solids and then led to the distillation column as liquid
feed. The gas stream from the first heat exchanger is fed to the
distillation column as gaseous feed. Overhead vapors from the
distillation column pass through a second heat exchanger and are
cooled to between -18 and -30.degree. C. (preferably -25.degree.
C.) to partially condense between 0.01 and 25%, preferably between
2 to 5%, of the inlet vapor stream to provide liquid reflux for the
distillation column. The purified HCl gas from the second heat
exchanger are pumped back to the inlet gas stream to be injected
like a spray to promote condensation in the inlet gas and to
prevent the formation of an aerosol or fog. The partially purified
gas now has a concentration of from 0.1 to 100, and preferably 1 to
10 ppm organic impurities.
[0063] The partially purified HCl gas can then be fed to an
activated charcoal adsorption column for final purification to
reach a final organic impurities level of from 10 to 1000 ppb,
preferably from 50 to 100 ppb.
[0064] The bottom stream of the distillation column contains most
of the impurities and is allowed to flash to a lower pressure of
between 1 bar and 10 bar, preferably 1.05 bar where much of the
remaining HCl flashes off and is led off to an absorption step or
to waste. The remaining residue, containing most of the impurities,
is led off to incineration or other waste treatment. The cooling
effect of the bottoms stream flash can help to cool the vapors
leaving from the top of the column.
[0065] If desired, a reboiler (preferable operation between -10 and
+8.degree. C.) is provided to return most of the condensed HCl as
vapor back to the column as stripping gas. The design of the
reboiler preferably uses a heat flux of between 1000 and 12000
BTU/hr/ft.sup.2 and preferably between 2500 and 4000
BTU/hr/ft.sup.2.
[0066] Although the invention has been described in detail in the
foregoing for the purpose of illustration, it is to be understood
that such detail is solely for that purpose and that variations can
be made therein without departing from the spirit and scope of the
invention except as it may be limited by the claims.
* * * * *