Note: Descriptions are shown in the official language in which they were submitted.
1--
SAFE ~DSORPT~ON PROCESS FOR THE SEPARATION
OF HYDROCARBONS F~OM OXYGEN CONTAINING 5AS
Background of the Invention
This invention relates to a pressure swinq adsorption
process for obtaining a hydrocarbon from a gaseous stream
containing hydrocarbons and a small amount of oxygen~
DOS 3,035,255 discloses a process for obtaining or
recovering hydrocarbons from a gaseous stream con~aining
hydrocarbons and a carrier gas, wherein methane in particular
is separated in a pressure swing adsorption installation
from a methane-air mixture. In this system, essentially
methane-free air is discharged from the outlet end of the
adsorbers while enriched methane is obtained as a stream
of desorbate during the regenerating phase of the adsorbers~
This method i~ oriented toward the complete separation
of all hydrocarbons, especially toward the complete
separation of methane from a methane-air mixture. The
adsorption process proposed for this eurposc contains,
as an essential process step, an air di`splacement step with
~0 -readily adsorbable components, especially with;khe methane to be
obtained as the ~roduct, s~lbsequently to an adsorption
phase. Such a displacement step, effected to adsorption pressure,
i.e. the highest process pressure, is disadvantageous be~ause
recompression of the methane to adsorption pressure is
required for this purpose~
--2
The hydrocarbon concen-tration in the applications
contemplated by DOS 3,035,255 is relatively low; methane
concentrations are cited of between 1 and 40 vol-% in the raw
gas. Since the additional component is to be air, oxygen
concentrations of about 12-20 vol-% are encountered in the raw
gas. With such a high oxygen content, however, there is very
great danger of explosion of the mixture so that operation of
such a plant appears to be hazardous.
Also when processing raw gases of a low oxygen
content, there is the danger that localized oxygen
concentrations may occur in the adsorption plant shifting the
hydrocarbon-oxygen mixture into the explosive range.
SUMMARY
According to the present invention a process is
provided, wherein individual hydrocarbons can be safely removed
from a hydrocarbon mixture contaminated by small amounts of
oxygen, e.g., not more than about 15% by volume of oxygen,
preférrably less than 10% by volurne.
Upon further study of the specification and appended
claims, further objects and advantages of this invention will
become apparent to those skilled in the art.
The process of this invention comprises the steps
of: selectively adsorbing the hydrocarbon to be obtained
during an adsorption phase under elevated pressure; during the
adsorption phase and during at least one cocurrent expansion
phase following the adsorption phase, withdrawing from the
outlet end of an adsorber a stream depleted in the hydrocarbon
to be obtained; withdrawing from the inlet end of the adsorber
during a subsequent countercurrent expansion, a stream of
desorbate (desorbed product) enriched in the hydrocarbon to be
obtalned; and after desorption, repressuri7ing the adsorber to
the adsorption pressure with a gas low in oxygen.
-3-
In the process of this invention, ~arious streams
of gaseous mixtures are encountered; and in all such streams
the oxygen content of these mixtures is k~pt so low that there
is no danger of explosion. The limitation of ~he oxygen
~oncentration required for ~his purpose depends, in an
individual case, on the respective gas composition a~ well
as on the process conditions under which the gas is produced
and processed. For example, the explosive limit for gases
having a predominant proportion of methane and ethylene
lies, with a pressure of 10 bar, at an oxy~en concentration
of about 19%, while such limit, with a pressure of 20 bar,
is an oxygen concentration of about 16%. In general, for
-- a safe operation, the oxygen content should be maintained
below 15~, preferably below 12~, in particular below 10%.
lS In any case, the concentration of oxygen should be not
more than 90%, especially not more than 80%, o~ the
explosive limit calculated for every stream.
During a pressure swing adsorption process, individual
components are retained in the adsorbers, so that there
results a local as well as chronological variation in the
gas composition. In this connection, care must be taken,
to ensure safe operation of the process, that the maximally
permissible oxygen concentration is not exceeded at any
time and at any location.
For conducting the pressure swing adsorption, heretofore,
processes proved to be especially advantageous wherein,
for pressurizing a regenerated adsorber, there was used;
at least partially, gas withdrawn during an expansion phase
from the adsorber, following thc adsorption phase. This
is usually an expansion gas withdrawn from the adsorber
cocurrently with the adsorption direction, the composition
of this expansion gas corresponding substantially to the
composition of the unadsorbed gaseous stream withdrawn
during the adsorption. Such a mode of operating the proce~s
utilizes in an energy efficient manner the pressure of
the expansion gases and moreover leads to an increased
yield o~ the component to be obtainedJ such component being
~4
- usually the unadsorbed proces~ ~treamO Processe~ of this
typ~ are disclosed, for example, in German Patent 1,769,936;
DOS 2,840,357, or DOS 2,916,585.
It has now been found tha~ it is impossible in many
cases to apply these actually well-proven methods to the
separation of individual hydrocarbons from hydrocarbon
mixtures containing oxygcn, because unduly high oxygen
concentrations are encountered in this procedure.
Consequently, in the process of this invention, at lea~t
one hydrocarbon is sclectively adsorbed, while other
hydrocarbons, oxygen, and any further components, which
may be contained in the gas, flow through the adsorber
- and exit in an enriched state from the adsorber. During
a cocurrent expansion following the adsorption phase, a
lS gas with a comparable composition likewise exits from the
adsorber~ An oxygen content which lies below the explosive
limit must also still be maintained i~ these enriched gases
wherein the entire oxygen content of the raw ga~ is recovered.
Moreover, ~he essential aspect of the invention is that
this gas, even then, must not be employed for the pressure
buildup of a regenerated adsorber. For it was discovered
that a lo~al oxygen concentration is built up when
introducing this gas into an adsorber to be repressurized,-
which concentration leads easily to exceeding the maximally
pcrmissible oxygen concentra~ion. This occurs because
uniform oxyg~n distribution docs not take place in the
adsorber to be pressurized; rather, there is an oxygen
concentration which increases in the direction toward the
outlet end of the adsorber.
For this reason, the pressure buildup of a regen~rated
adsorb~r is carried out with a gas low in oxygen in the
process of this invention. Suitable as such a gaseous
stream is in many cases the gaseous stream to be separated~
the oxygen content of which is lower than that of the gas
exiting rom the adsorbex end. Optionally, pressurizing
can, however~ also be effected with other suitable and
available gaseou~ streams, for example purified hydrocarbon
gases or mixtures thereo-f which,-if necessary, may be com-
pressed. The use
of gases free of oxygen, such as methane, is advantageous,
in particular, if a relatively high oxygen concentration pre-
vails initially in the gaseous stream because an intolerable
high concen-trati.on of oxygen during the repressurization of
a regenerated adsorber is safely preven.ted thereby.
A nonuniform distribution of the oxygen concentration
is also produccd in the adsorber if ~.he adsorbers are
pressuriz~d with a ga~ of low oxygen content, as represented
by the gaseous mixture to be s~parated. From the r~w gas
utilized for pressurizing, the hydrocarbons to be obtained
are selectively separated with the formation of an adsorption
front moving toward the outlet end, while the unadsorbed
components can immediately advance up to the closed outlet
, end of the adsorher. However, by the retention of individual`~ 15 components, there i5 thus a risc in oxygcn concentration
tow~rd the outlet end of the adsorber. The increase in
oxygen concentration depends, inter alia, also on the
selectivity of the adsorbent for the hydrocarbon to be
obtained. Thus, for example, when ~eparating a gaseou~
stream containing predominantly methane and ethylene, and
with the use of activated carb.on as the adsorbent, the
coadsorption of methane, besides the desired adsorption
of ethylene, is relatively extensive, whereby a higher
oxygen concentration occurs at the outl~t end of the
adsorber than, for example, with the use of silica gel
as the adsorhentO
: . In a further development of the process of thls
invention, for avoiding undesirably or dangerously high
oxygen concentrations at the outlet end of the adsorber
during a pressurizing phase, the qas is withdrawn via the
outlct end of the adsorber before termination of the pressure
buildup phase.
This can be effected by control means generally
know~ ln th~ art~ for example by a flow controller.
By means o~ the abo~e described step, danqerou~ oxygen
peak concentrations are prevented from forming. When
obtaining ethylene ~rom gaseous stre~ms containin~ methane
.. ~
and ethylene, this mode of operation is especially expedient
if activated carbon is utilized as the adsorbent. When
using silica gel as the adsorbent, this version of the
process, effected for safety reasons, becomes unnecessary.
Following an adsorption phase, a cocurrent expansion
is conducted in the process of this invention, during which
a cocurrent expansion gas is discharged essentially devoid
of the component to be adsorbed, which gas escapes from
the voids in the adsorbent packing during cocurrent expansion.
The adsorption phase is interrupted at a point in time
when the adsorption front has not as yet reached the outlet
end of the adsorber so that, during the cocurrent expansion
-- phase, the components to be obtained, still contained in
the gaseous mixture filling the voids, are separated while
the adsorption front advances further toward the outlet
end of the adsorber.
Although the gaseous mixture obtained during the
cocurrent expansion phase and exhibiting an oxygen content
higher than that of the raw gas must not be utilized for
pressurizing a regenerated adsorber, such mixture can be
utilized, due to its low concent:ration of the component
to be obtained, for scavenging an adsorber prior to the
pressurizing phase. Such a scavenging step can follow
the countercurrent expansion of the adsorber, during which
the actual product fraction is withdrawn~ so that optionally
a residual load on the adsorber is reduced. However,
scavenging of the adsorber is not required in each and
every case.
The stream of desorbate produced during countercurrent
3n expansion contains the hydrocarbon to be obtained in a
concentration depending on the concentration in the raw
gas as well as on the selectivity of the adsorbent. I
the concentration of the desired product component in thi~
gaseous stream i~ still inadequate, then this stream o
desorbate can be separated, in a further separator, into
a highly enriched stream o~ the hydrocarbon to be obtained
and into a residual ~as stream. Suitahle for such a
subsequent separation is, for example, another pressure
swing adsorption installation.
It is possible in some cases for the raw gas stream
to be separated to contain components more readily
adsorbable than the hydrocarbon to be recovered. In such
a case, in a further embodiment of the process of this
invention, the more readily adsorbable components are
separated in adsorbers installed upstream, optionally using
a different adsorbent. Such upstream adsorbers can be
arranged in a separate adsorber station or also can be
designed as preliminary beds in connection with the ac~ual
adsorber beds and can be arranged in a common housing together
- with the primary adsorbers. An especially suita~le
scavenging gas for regenerating such upstream adsorbers
is the gaseous stream exiting from the outlet end of the
primary adsorbers. This mode of operation is advantageous,
in particular, if the unadsorbed gas is obtained as a low-
quality residual gas which can be used only for heating
purposes, for example.
Adsorption installations having at least three adsorbers,
pre~erably at least four adsorbers, are particularly suitable
for the process of this invention.
Brief Description of Drawings
Additional details of the invention will be described
2S below with reference to a preferred comprehensive embodiment
schematically illustrated in the figures.
In the drawings:
FIGURE 1 is an installation for conducting the process
o~ this invention; and
FIGURE 2 is a time schedule flow chart for the operation
of the plant according to FIGURE 1.
Detailed Description
The pressure swing adsorption installation shown in
~IGURE 1 comprises four adsorbers 1, 2, 3, and 4. The
adsorbers are connected on the inlet side via valves 11,
21, 31 and 41 to a raw gas conduit 5 and on the outlet
side via valves 12, 22, 32, and 42 to a residual gas cond~it
6 through which the unadsorbed components of the raw gas
~9~
--8
are withdrawn. Prior to discharge sf the xesidual gas,
the pressure is reduced in the controllable expansion valve
7. Furthermore~ the outlet ends of the adsorbers are
connected via the valves 13, 23, 33 and 43, respectively,
to a conduit 8 in communication with the residual gas conduit
via the valve 9. Expansion gases produced during a first
expansion phase and rich in unadsorbed components are
discharged into the residual gas hy way of this conduit.
Valves 14, 24, 34 and 44, respectively, are provided
on ~he inlet side of the adsorbers, connecting the inlet
end of the adsorbers with a conduit 51 leading to the buffer
or surge tank 50. Tank 50 is connected to the product
conduit 53 by way of a control valve 52. The product gas
obtained during a countercurrent expansion, as well as
scavenging gas and desorbate produced during a scavenging
phase are introd~ced into tank 50 by way of conduit 51.
V R, V ~S
!~ Finally, ~lavc3 15, 25, 35, and 45 are arranged on
the inlet side of the adsorbers, these valves being connected
via conduit 54 and control valve 55 with the raw gas conduit
5. The pressure buildup of the scavenged adsorbers by
means of raw gas is effected via this conduit.
FIGURE 2 illustrates the time schedule for the operation
of the installation. The four adsorbers pass through
identical cycles chronologically displaced with resp et
to one another so that one adsorber is always in the
adsorption phase, thus ensuring continuous operation. The
adsorption phase ADS, conducted under constant pressure,
is followed by a first expansion El in cocurrent mode.
The thus-formed expansion gas is discharged into the residual
gas~ In a second cocurrent expansion phase E2, further
gas is withdrawn via the outlet end of the adsorber. Thi~
gas passes via the opened valves 13 and 4~ to the outlet
end of the adsorber 4 and flows through the adsorber 4,
which is in a scavenging phase S, before it is condueted
via valve 44 and conduit 51 into the buffer tank 50
Following the second cocurrent expansion, a countercurrent
expansion phase E3 is conducted, during which a stream
&~8~
g
of desorba~e is conducted via the opened valve 14 and conduit
51 to the buffer tank 50. After termination of the counter-
current expansion E3 the adsorber is subjecte~ to a scavenging
phase S. For this purpose~ expansion ga~ from the cocurrent
expansion phase E2 of the adsorber 2 is conducted via the
opened valves 23 and 13 countercurrently to the adsorption
dircctlon through the adsorber 1.. The scavcnging gas cffect~
displaccmcnt of the adsorbcd product componcnt, which l~tte~
exits, in a still high concentration, from the inlet end
of the adsorbcr. The scavenging phase is kept relatively
short to avoid unnecessary dilution of the product gas
~ conducted to the buffer tank 50 with scave~ging gas. In
- another version of the process, it is also possible to
conduct, instead of the scavenging step S, a desorption
with the use of subatmospheric pressure. The ~hus-attained
improvement in product quality~ however, is achieved at
the cost o~ increased energy expenditure for a vacuum pump.
After completion of the scavenging phase S, the adsorber
can be repressurized to the adsorption pressure. This
is done during the pressure buildup phase B, during which
raw gas is conducted into the aclsorber 1 via the open~d
valves 55 and 15. After the raw gas pressure has been
build up in adsorber 1, the latter has completed a whole
cycle. The remaining adsorbers pass through the same cycle
but on a displaced time schedule, as illustrated in FIGURE 2.
Thc proccss of this invention i~ espccially suitablc,
for exmaple, for the scparation of waste gascs from plants
for ethylene oxidc production; these waste gases can contain
essentially valuable ethylene, less valu~ble methane, small
amounts of oxygen, as well as in some cases additionally
incrt ~ases, such as argon~ nitroqen, or carbon dioxide,
and other lighter hydrocarbons~ The amount of ethylene
contained in such a gaseous stream depends on the method
used for the production of ethylene oxide and is typically
3S in the range of 10-75~o~ e.g., about 25 mol-~, while the
oxygen content ranges generally b~ween about 1 and 8 mol-% 9
especially, in case ethylen~ oxide is produced with oxygen,
in a range o~ abou~ 2-8 mol-~.
3~
9A-
Without further elaboration, it is believed that one
~killed in the art can, using the preceding description,
utilize the present invention to lts fullest extent. The
following preferred specific e~bodiment is , therefore,
to be construed as merely illustrative, and not limitative
of the remainder of the disclosure in any way whatsoever.
In the ~ollowing example , all temperatures are set forth
uncorrected in degrees Celsius; unless otherwise indicated,
all parts and percentages are by volume.
Ethylene ls converted ln an ethylene oxide plant
wlth oxygen to produce ethylene oxlde. A purge gas is
withdrawn from the plant ln an amount of 500 Nm~/h at
a pxessure of 10 bar and a temperature of 40 C. Thls
gas has the following compositlon:
lS Methane 50 ~
Ethylene 28 %
Ethane 1 %
Oxygen 5 %
Carbon Dioxlfla 6 %
Inert Gases 10%.
For recoverlng the ethylen~ content of thl~ gas, lt ls
conducted to an adsorption plant as descrlbed wlth reference
to the drawing. Sllicagel is used as adsorbent and the
plant operates with a cycle tlme of 16 minutes, the duratlon
of each adsorptlon phase heing 4 minutes. From the outlet
end of the adsorption plan~, a residual gas with reduced
ethylene content ls withdrawn ln an amount of 222 Nm' /h at
a pressure of 6 bar and at a temperature o 40 C.
~9~
-9B-
The.composltlon of the resldual gas is as follows:
Methane 69.9 %
Ethylene 1.7 %
Ethane 1.4
Oxygen 9.0 ~
Inert Gases 18.0 %.
During the regeneration of previously loaded adsorbers
an ethylene rich gas is obtained in an amount of 278 Nm'/h
at a pressure of 1.5 bar and a temperature of 40 C. ~he
composition of this gas is as follows:
Methane 34.1
Ethylene 49.0
Ethane 0.7 ~
Oxygen 1.8 %
Carbon Dioxide 10.8 ~
Inert Gases 3.6 ~,
~: 25
:: :
~ 5~
~: ~,,.;
8~3~
--10--
The preceding examples can be repeated with s'milar
success by substituting the generically or specifically
described reactants and/or operating conditions of this
invention for those used in the preceding examples.
S From the foregoing description, one skilled in the
axt can easily ascertain the essential characteristics
of this invention, and without departing from the spirit
and scope thereof, can make various changes and modifications
of the invention to adapt it to various usages and conditions.
