Safe adsorption process for the separation of hydrocarbons from oxygen containing gas

In a pressure swing adsorption process conducted in a plurality of cyclically interchangeable adsorbers, for recovering a hydrocarbon, e.g., ethylene, from a gaseous feed stream containing hydrocarbons and less than 15% by volume of oxygen, the steps of: PA1 (a) selectively adsorbing the hydrocarbon to be recovered during an adsorption phase conducted under elevated pressure; PA1 (b) during the adsorption phase and during at least one cocurrent expansion phase following the adsorption phase, withdrawing a gaseous stream, at the outlet end of an adsorber, which stream is depleted in the hydrocarbon to be recovered; PA1 (c) during a subsequent countercurrent expansion phase of desorption, withdrawing a stream enriched in the desorbed hydrocarbon to be recovered from the inlet end of the adsorber; and PA1 (d) after the desorption in step (c), conducting a pressure buildup phase by repressurizing the adsorber to the adsorption pressure with a gas containing less than 15% by volume oxygen, preferably the feed gas; and as required, withdrawing gas before the repressurizing is completed so as to prevent localized increased concentrations of oxygen, and then repeating the cycle.

BACKGROUND OF THE INVENTION 
This invention relates to a pressure swing adsorption process for obtaining 
a hydrocarbon from a gaseous stream containing hydrocarbons and a small 
amount of oxygen. 
DOS No. 3,035,255 discloses a process for obtaining or recovering 
hydrocarbons from a gaseous stream containing 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 is 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 purpose contains, as an essential process step, 
an air displacement step with readily adsorbable components, especially 
with the methane to be obtained as the product, subsequently to an 
adsorption phase. Such a displacement step, effected to adsorption 
pressure, i.e. the highest process pressure, is disadvantageous because 
recompression of the methane to adsorption pressure is required for this 
purpose. 
The hydrocarbon concentration in the applications contemplated by DOS No. 
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 hydrocarbonoxygen mixture into the explosive range. 
SUMMARY 
An object of the present invention is to provide a process of the 
aforementioned type wherein individual hydrocarbons can be safely removed 
from a hydrocarbon mixture contaminated by small amounts of oxygen, e.g., 
not more than about 5% by volume of oxygen, generally in the range of 1 to 
10%. 
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 obtained; and after desorption, 
repressurizing the adsorber to the adsorption pressure with a gas low in 
oxygen. 
In the process of this invention, various streams of gaseous mixtures are 
encountered; and in all such streams the oxygen content of these mixtures 
is kept so low that there is no danger of explosion. The limitation of the 
oxygen concentration required for this purpose depends, in an individual 
case, on the respective gas composition as 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 oxygen 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%. In any case, the concentration of oxygen should be 
not more than 90%, especially not more than 80%, of 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 the 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 process 
utilizes in an energy efficient manner the pressure of the expansion gases 
and moreover leads to an increased yield of the component to be obtained, 
such component being usually the unadsorbed process stream. Processes of 
this type are disclosed, for example, in German Pat. No. 1,769,936; DOS 
No. 2,840,357; or DOS No. 2,916,585. 
It has now been found that it is impossible in many cases to apply these 
actually well-proved methods to the separation of individual hydrocarbons 
from hydrocarbon mixtures containing oxygen, because unduly high oxygen 
concentrations are encountered in this procedure. Consequently, in the 
process of this invention, at least one hydrocarbon is selectively 
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 gas with a comparable composition 
likewise exits from the adsorber. An oxygen content which lies below the 
explosive limit must also still be maintained in these enriched gases 
wherein the entire oxygen content of the raw gas is recovered. Moreover, 
the 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 local oxygen concentration is built up when 
introducing this gas into an adsorber to be repressurized, which 
concentration leads easily to exceeding the maximally permissible oxygen 
concentration. This occurs because uniform oxygen distribution does 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 regenerated adsorber 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 from the 
adsorber end. Optionally, pressurizing can, however, also be effected with 
other suitable and available gaseous streams, for example methane. The use 
of foreign gases free of oxygen, such as methane, is advantageous, in 
particular, if a relatively high oxygen concentration prevails initially 
in the gaseous stream. 
A nonuniform distribution of the oxygen concentration is also produced in 
the adsorber if the adsorbers are pressurized with a gas of low oxygen 
content, as represented by the gaseous mixture to be separated. From the 
raw 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 adsorber. However, by the 
retention of individual components, there is thus a rise in oxygen 
concentration toward 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 
separating a gaseous stream containing predominantly methane and ethylene, 
and with the use of activated carbon as the adsorbent, the coadsorption of 
methane, besides the desired adsorption of ethylene, is relatively 
extensive, whereby a higher oxygen concentration occurs at the outlet end 
of the adsorber than, for example, with the use of silica gel as the 
adsorbent. 
In a further development of the process of this invention, for avoiding 
undesirably or dangerously high oxygen concentrations at the outlet end of 
the adsorber during a pressurizing phase, the gas is withdrawn via the 
outlet end of the adsorber before termination of the pressure buildup 
phase. 
This can be effected by control means generally known in the art, for 
example by a flow controller. 
By means of the above described step, dangerous oxygen peak concentrations 
are prevented from forming. When obtaining ethylene from gaseous streams 
containing 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 concentration 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 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. If the concentration of the desired product component in this 
gaseous stream is still inadequate, then this stream of desorbate can be 
separated, in a further separator, into a highly enriched stream of the 
hydrocarbon to be obtained and into a residual gas stream. Suitable 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 actual adsorber 
beds and can be arranged in a common housing together with the primary 
adsorbers. An especially suitable 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, preferably at 
least four adsorbers, are particularly suitable for the process of this 
invention.

DETAILED DESCRIPTION 
The pressure swing adsorption installation shown in FIG. 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 vanes 12, 22, 32, and 42 to a residual gas conduit 6 through which the 
unadsorbed components of the raw gas are withdrawn. Prior to discharge of 
the residual 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 by way of this conduit. 
Valves 14, 24, 34 and 44, respectively, are provided on the 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 introduced into tank 50 by way of 
conduit 51. 
Finally, valves 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. 
FIG. 2 illustrates the time schedule for the operation of the installation. 
The four adsorbers pass through identical cycles chronologically displaced 
with respect 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 
E1 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. This gas passes via the 
opened valves 13 and 43 to the outlet end of the adsorber 4 and flows 
through the adsorber 4, which is in a scavenging phase S, before it is 
conducted 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 of desorbate 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 subjected to a scavenging 
phase S. For this purpose, expansion gas from the cocurrent expansion 
phase E2 of the adsorber 2 is conducted via the opened valves 23 and 13 
countercurrently to the adsorption direction through the adsorber 1. The 
scavenging gas effects displacement of the adsorbed product component, 
which latter exits, in a still high concentration, from the inlet end of 
the adsorber. The scavenging phase is kept relatively short to avoid 
unnecessary dilution of the product gas conducted to the buffer tank 50 
with scavenging 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 thus-attained improvement in 
product quality, however, is achieved at the cost of 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 adsorber 1 via 
the opened 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 FIG. 2. 
The process of this invention is especially suitable, for example, for the 
separation of waste gases from plants for ethylene oxide production; these 
waste gases can contain essentially valuable ethylene, less valuable 
methane, small amounts of oxygen, as well as in some cases additionally 
inert gases, such as argon, nitrogen, 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 in the range of 10-40%, e.g., about 25 mol-%, while the oxygen 
content ranges generally between about 1 and 7 mol-%, especially, in case 
ethylene oxide is produced with oxygen, in a range of about 5-7 mol-%. 
Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiment is, therefore, 
to be construed as merely illustrative, and not limitative of the 
remainder of the disclosure in any way whatsoever. In the following 
example, all temperatures are set forth uncorrected in degrees Celsius; 
unless otherwise indicated, all parts and percentages are by volume. 
Ethylene is converted in an ethylene oxide plant with oxygen to produce 
ethylene oxide. A purge gas is withdrawn from the plant in an amount of 
500 Nm.sup.3 /h at a pressure of 10 bar and a temperature of 40.degree. C. 
This gas has the following composition: 
______________________________________ 
Methane 50% 
Ethylene 28% 
Ethane 1% 
Oxygen 5% 
Carbon Dioxide 6% 
Inert Gases 10%. 
______________________________________ 
For recovering the ethylene content of this gas, it is conducted to an 
adsorption plant as described with reference to the drawing. Silicagel is 
used as adsorbent and the plant operates with a cycle time of 16 minutes, 
the duration of each adsorption phase being 4 minutes. From the outlet end 
of the adsorption plant, a residual gas with reduced ethylene content is 
withdrawn in an amount of 222 Nm.sup.3 /h at a pressure of 6 bar and at a 
temperature of 40.degree. C. 
The composition of the residual 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.sup.3 /h at a pressure of 1.5 bar and a 
temperature of 40.degree. C. The 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%. 
______________________________________ 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art 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.