# Patent

## Title
Selective CO oxidation for acetylene converter feed CO control

## Abstract

A system and process for acetylene selective hydrogenation of an ethylene rich gas stream. An ethylene rich gas supply comprising at least H2S, CO2, CO, and acetylene is directed to a first treatment unit for removing H2S and optionally CO2 from the gas stream. A CO oxidation reactor is used to convert CO to CO2 and form a CO-depleted gas stream. A second treatment unit removes the CO2 from the CO-depleted gas stream and an acetylene selective hydrogenation treats the CO-depleted gas stream.

## Background
FIELD OF THE INVENTION
The invention relates to a system for reducing CO concentration in an ethylene rich stream.
DESCRIPTION OF RELATED ART
Industrial processes for producing ethylene include catalytic and thermal cracking of hydrocarbon feedstocks. In at least some cases, the cracking process effluent contains carbon monoxide. For example, certain product separation and recovery systems produce a vapor stream rich in ethylene and containing hydrogen, methane, acetylene, ethane and other contaminants such as CO, CO2, and H2S that must be removed in order to produce a high purity ethylene product. Acetylene in polymer grade ethylene is typically limited to a maximum of 5 vol ppm. A typical polymer grade ethylene specification is shown in Table 1.







TABLE 1





Typical Polymer Grade Ethylene Specifications



















Ethylene
99.90
vol % min



Methane plus ethane
1000
vol ppm max



Ethane
500
vol ppm max



Acetylene
5 
vol ppm max



C3 and heavier
10
vol ppm max



CO
2 
vol ppm max



CO2
5
vol ppm max



Sulfur
2 
wt ppm max










Acetylene removal is typically effected by acetylene conversion to ethylene via selective hydrogenation. Carbon monoxide (CO) attenuates the activity of the commonly used acetylene selective hydrogenation catalysts and thus excessive CO concentration can be problematic.
Hence it would be beneficial to be able to control the amount of CO that enters the acetylene conversion unit.
SUMMARY OF THE INVENTION
The present invention relates to controlling CO concentration in a stream prior to subjecting the stream to an acetylene selective hydrogen catalyst.
One embodiment of the invention is directed to a system for acetylene selective hydrogenation of an ethylene rich gas stream comprising: (a) an ethylene rich gas supply comprising at least H2S, CO2, CO, and acetylene; (b) a first treatment unit for removing H2S and, optionally, CO2 from the gas stream; (c) a CO oxidation reactor to convert CO to CO2 and forming a CO-depleted gas stream; (d) a second treatment unit for removing the CO2 from the CO-depleted gas stream; and (e) an acetylene selective hydrogenation downstream of the CO oxidation reactor.
Another embodiment of the invention is directed to a process for acetylene selective hydrogenation of an ethylene rich gas stream comprising: (a) supplying an ethylene rich gas comprising at least H2S, CO2, CO, and acetylene to a first treatment unit and removing H2S and, optionally, CO2 from the gas stream; (b) supplying the H2S and CO2 free gas stream to an CO oxidation reactor and converting CO to CO2 to form a CO-depleted gas stream; (c) supplying the CO-depleted gas stream to a second treatment unit to remove the CO2 from the CO-depleted gas stream; and (d) treating the CO-depleted or CO-depleted gas stream to an acetylene selective hydrogenation unit to convert the acetylene to ethylene.
These and other embodiments relating to the present invention are apparent from the following Detailed Description.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of part of a PetroFCCTM product treating and recovery system.
FIG. 2 is a schematic of an ethylene rich lean gas preferential CO oxidation reactor system in accordance with one embodiment of the invention.
FIG. 3 is a schematic of ethylene rich lean gas preferential CO oxidation reactor system in accordance with another embodiment of the invention.



The same reference numbers are used to illustrate the same or similar features throughout the drawings. The drawings are to be understood to present an illustration of the invention and/or principles involved.
DETAILED DESCRIPTION
FIG. 1 is a schematic of a separation scheme for recovering ethylene and propylene. A vapor stream (2) comprised of ethylene and propylene is compressed (4) to produce a propylene rich liquid stream (6) and an ethylene rich vapor stream (7). The ethylene vapor stream (7) is treated and concentrated in a primary absorber (8) and a sponge absorber (10) to form an ethylene rich lean sponge gas (12). The lean sponge gas (12) includes other light hydrocarbons, primarily hydrogen, methane, acetylene, ethane and other contaminants such as CO, CO2, and H2S that must be removed in order to produce a high purity ethylene product.
The ethylene and propylene stream may be obtained from any industrial process for producing ethylene including catalytic and thermal cracking of hydrocarbon feedstock product streams. For example, US20080078692 discloses a hydrocarbon cracking process and subsequent treatment of the effluent streams. US 20080078692 discusses various conventional terms and process steps used in processes for recovering ethylene and propylene after a hydrocarbon cracking process, see especially paragraphs 0012-0018, 0034-0041, 0045-0055, and is hereby incorporated by reference in its entirety,
The ethylene purification scheme shown in FIG. 1 includes an amine treatment unit (14) to remove H2S and CO2 from the ethylene rich lean sponge gas (12) forming stream (22). Treatment in the amine treatment unit reduces the H2S to less than about 0.1 ppm and CO2 to less than about 50 ppm. The stream (22) is then fed to an acetylene selective hydrogenation unit (16) to hydrogenate the acetylene into ethylene.
In the scheme shown in FIG. 1, acetylene is hydrogenated upstream of the demethanizer (18) and ethane-ethylene splitter fractionators (20). For this example, stream (22) includes sufficient hydrogen for hydrogenating the acetylene in the gas to ethylene. Hence, no additional hydrogen is required to be added to the feed stream into the acetylene selective hydrogenation unit (16). Additionally, the acetylene selective hydrogenation unit (16) normally operates above ambient temperature while the demethanizer (18) and ethane-ethylene splitter (20) typically operate sub-ambient. Positioning the CO oxidation reactor and acetylene conversion reactor upstream of the demethanizer lessens feed heating and effluent cooling duty compared to an arrangement that includes CO oxidation and acetylene conversion in the sub-ambient section of the process.
The concentration of CO in stream (22) is variable, generally in a range of 0 to 6 vol %. It is desirable to maintain the CO concentration of the stream (22) (the acetylene selective hydrogenation reactor feed stream) within a certain operating range, typically about 1 to 0.2 vol %. In general, as the CO concentration of the acetylene selective hydrogenation reactor feed stream increases, the operating window of the acetylene selective hydrogenation reactor system and the time between catalyst regenerations decreases.
The operating window is the set of operating conditions that enables selective and stable performance. Specifically, allowing complete hydrogenation of acetylene while minimizing hydrogenation of ethylene to ethane. The operating window is affected by process conditions including reactor inlet temperature, feed acetylene, hydrogen, and CO concentrations, space velocity, and catalyst type.
Thus, as discussed above, the feed stream (22) entering the acetylene selective hydrogenation unit (16) often contains unacceptably high carbon monoxide (CO) concentrations. The present invention is directed to a process of controlling or reducing the amount of CO in feed stream (22) entering an acetylene selective hydrogenation unit (16).
The feed stream (12) from the sponge absorber contains unacceptably high levels of CO. An oxidation reactor will oxidize CO in the feed stream using elemental oxygen as an oxidant: “CO+0.5 O2→CO2”. The CO to CO2 conversion selectivity depends on the catalyst choice and composition of the feed stream. However, the feed stream (12) from the sponge absorber contains H2S which is a catalyst poison for oxidation and must be removed from the feed stream prior to entering the oxidation reactor.
It was discovered that placing a CO oxidation reactor downstream of the amine treatment unit (14) enables control of the CO concentration in the feed to the acetylene selective hydrogenation unit (16). As shown in FIG. 2, the ethylene rich stream (12) from the sponge absorber (not shown) flows to a first amine treatment unit (14). For this illustration, it is assumed that the first amine treatment unit (14) removes both H2S and CO2 even though CO2 removal upstream of the oxidation reactor is not required. Thus, the process does not require CO2 removal at this stage.
The ethylene rich stream from the amine treatment unit (14), essentially H2S and CO2 free, is combined with a stream (32) that provides a source of elemental oxygen, for example, air or oxygen enriched air. The combined gases A (H2S and CO2-depleted stream) flow to the CO oxidation reactor (30). After CO conversion to CO2, the effluent stream B (CO-depleted stream) continues to a second amine treatment unit (34) downstream of CO oxidation reactor (30). This second amine treatment unit (34) removes CO2 from effluent stream B. The CO2-depleted effluent then continues to the acetylene selective hydrogenation unit (16).
As also shown in FIG. 2, the amine treating arrangement uses a common amine regenerator (36) to regenerate rich amine from both the first and second amine treatment units (14) and (34). In doing so, amine treating equipment is minimized. The combination of the preferential CO oxidation reactor (30) and amine treatment unit (14) to remove H2S enables control of the CO concentration within a suitable range for subsequent acetylene conversion via conventional selective hydrogenation technology.
A sensor (38) may be placed in the effluent B stream after the CO oxidation reactor (30) to detect the amount of CO in the stream. The sensor may be placed at any position subsequent to the CO oxidation reactor where CO is present in detectable levels. The sensor may signal whether the amount of oxygen or air supplied by line (32) should be modified. The effluent stream B ideally comprises less than about 50 ppm-vol CO.
The oxidation temperature in the CO oxidation reactor (30) is typically between about 70° C. and about 160° C.
Suitable catalysts for selectively oxidizing CO using air or oxygen enriched air include, but are not limited to ruthenium metal disposed on an alumina carrier, such as those described in U.S. Pat. No. 6,299,995, hereby incorporated by reference in its entirety. The ruthenium metal comprises well dispersed ruthenium crystals having an average crystal size less than or equal to about 40 angstroms. Other suitable catalysts utilize platinum and copper.
Other treatments may be used instead of amine treatment units. Alternative treatment units include absorbers with amine or solvent flow arranged in a cascading relationship. As shown in FIG. 3, an ethylene rich feed gas (12) flows into a first absorber (40) wherein H2S and CO2 are removed by absorption. The ethylene rich stream from the first absorber (40), essentially H2S and CO2 free, is combined with a stream (47) that provides a source of elemental oxygen. The combined gases A (H2S and CO2-depleted stream) flow to the CO oxidation reactor (42). After CO conversion to CO2, the effluent stream B (CO-depleted stream) continues to a second absorber (44) downstream of CO oxidation reactor (42). This second absorber (44) removes CO2 from effluent stream B. The CO2-depleted effluent then continues to an acetylene selective hydrogenation unit (not shown). The CO2 rich amine from the second absorber (44) flows to first absorber (40). The CO2 and H2S rich amine from the first absorber (40) flows to an amine regenerator (46). The lean amine from the amine regenerator (46) then flows into the second absorber (44). A CO sensor (not shown) may be placed downstream of CO oxidation reactor (42) similar to the system shown in FIG. 2 in order to control the amount of air or oxygen added to combined gases A.
In the amine treatment unit (14) shown in FIG. 1, (14) and (34) shown in FIG. 2, (40) and (44) shown in FIG. 3 selective removal of H2S and CO2 can be achieved using amine-containing chemical solvents. For example, UOP AMINE GUARDTM FS may be used to remove the H2S and CO2. Such solvents provide selective removal of H2S via amine selection. Other treatment units may use other chemical solvents. Chemical solvents are used to remove the acid gases by a reversible chemical reaction of the acid gases with an aqueous solution of various alkanolamines or alkaline salts in water.
Other treatment units may utilize physical solvents. With a physical solvent, the acid gas loading in the solvent is proportional to the acid gas partial pressure. For example, the UOP SELEXOLTM process may be used which uses a physical solvent made of dimethyl ether of polyethylene glycol. Chemical solvents are generally more suitable than physical or hybrid solvents for applications at lower operating pressures.
As discussed above, in accordance with the present invention, a CO oxidation reactor is placed upstream of the acetylene selective hydrogenation unit to enable control of the CO concentration within a suitable range for the acetylene selective hydrogenation reaction occurring in the acetylene selective hydrogenation unit. Further aspects of the invention are therefore directed to a method for controlling the CO concentration in an acetylene selective hydrogenation unit feed stream by preferential CO combustion (i.e. oxidation) with air or oxygen enriched air providing the oxygen.
EXAMPLES
The following examples and tables summarize the expected performance of the preferential CO oxidation reactor processing a typical ethylene-rich lean gas as shown in FIG. 2. Stream “A” is oxidation reactor feed and “B” is oxidation reactor effluent. The examples assume selectivity based on a ruthenium on alumina catalyst.
Example 1
The lean gas (i.e. H2S and CO2 removed) from the amine treatment unit is mixed with air. The oxygen available for oxidizing CO is controlled to limit the CO conversion to  ̃50%. As shown in Table 2, the CO concentration is reduced from  ̃2600 ppm to  ̃1300 ppm.
Specifically, stream A is introduced into a CO oxidation reactor and stream B exits the reactor under the following conditions:
















Inlet
Outlet









Reactor Temperature (° F.)
194  
221



Reactor Pressure (psia)
246.7




Air to Reactor (lbmol/hr)
66 


















TABLE 2







Stream “A”
Stream “B”













Mole
Mole

Mole
Mole



Fraction
%

Fraction
%





H2O
0.003869
 0.387
H2O
0.005189
 0.519


Oxygen
0.001314
 0.131
Oxygen
0.000000
 0.000


Nitrogen
0.068311
 6.831
Nitrogen
0.068401
 6.840


Hydrogen
0.106621
10.662
Hydrogen
0.105446
10.545


CO
0.002615
 0.262
CO
0.001303
 0.130


CO2
0.000005
 0.001
CO2
0.001321
 0.132


Methane
0.248449
24.845
Methane
0.248775
24.878


Acetylene
0.000503
 0.050
Acetylene
0.000504
 0.050


Ethylene
0.485833
48.583
Ethylene
0.486472
48.647


Ethane
0.076446
 7.645
Ethane
0.076546
 7.655


Propylene
0.006035
 0.604
Propylene
0.006043
 0.604









Example 2
The lean gas (i.e. H2S and CO2 removed) from the amine treatment unit is mixed with air. The oxygen available for oxidizing CO is controlled to limit the CO conversion to  ̃75%. The CO concentration is reduced from  ̃2600 ppm to  ̃600 ppm, see Table 3. Undesirable side reactions include “H2+0.5 O2→H2O”, as well potential oxidation of light hydrocarbons including olefin products. Assuming sufficient reactant, the CO oxidation reactor essentially completely removes CO.
Stream A is introduced into a CO oxidation reactor and stream B exits the reactor under the following conditions:













Inlet 
Outlet

















Reactor Temperature (° F.)
194  
234


Reactor Pressure (psia)
246.7



Air to PreFOX Reactor (lbmol/hr)
99 

















TABLE 3







Stream “A”
Stream “B”













Mole
Mole

Mole
Mole



Fraction
%

Fraction
%















H2O
0.003857
 0.386
H2O
0.005833
 0.583


Oxygen
0.001965
 0.196
Oxygen
0.000000
 0.000


Nitrogen
0.070561
 7.056
Nitrogen
0.070700
 7.070


Hydrogen
0.106289
10.629
Hydrogen
0.104530
10.453


CO
0.002607
 0.261
CO
0.000644
 0.064


CO2
0.000005
 0.001
CO2
0.001973
 0.197


Methane
0.247674
24.767
Methane
0.248161
24.816


Acetylene
0.000501
 0.050
Acetylene
0.000502
 0.050


Ethylene
0.484318
48.432
Ethylene
0.485271
48.527


Ethane
0.076207
 7.621
Ethane
0.076357
 7.636


Propylene
0.006016
 0.602
Propylene
0.006028
 0.603









In view of the present disclosure, it will be appreciated that other advantageous results may be obtained. Those having skill in the art, with the knowledge gained from the present disclosure, will recognize that various changes can be made in the above apparatuses and methods without departing from the scope of the present disclosure. Mechanisms used to explain theoretical or observed phenomena or results, shall be interpreted as illustrative only and not limiting in any way the scope of the appended claims.

## Claims

1. A process for acetylene selective hydrogenation of an ethylene rich gas stream comprising:
(a) supplying an ethylene rich gas comprising at least H2S, CO2, CO, and acetylene to a first treatment unit and removing H2S from the gas stream;
(b) supplying the H2S depleted gas stream to an CO oxidation reactor and converting CO to CO2 to form a CO-depleted gas stream;
(c) supplying the CO-depleted gas stream to a second treatment unit to remove the CO2 from the CO-depleted gas stream to form a CO2-depleted gas; and
(d) treating the CO-depleted or CO2-depleted gas stream in an acetylene selective hydrogenation unit to convert the acetylene to ethylene.

2. The process of claim 1 further comprising supplying oxygen or air to the H2S depleted gas stream prior to the CO oxidation reactor.

3. The process of claim 2 further comprising detecting an amount of CO in the CO-depleted gas stream with a sensor positioned downstream of the CO oxidation reactor and controlling the supply of oxygen or air to the H2S depleted gas stream based on the amount of CO detected.

4. The process of claim 1 wherein the CO oxidation reactor comprises a catalyst.

5. The process of claim 4 wherein the catalyst comprises an active material selected from the group consisting of ruthenium, copper, and platinum on a substrate selected from the group consisting of alumina, titania, and silica.

6. The process of claim 5 wherein the catalyst is ruthenium on alumina.

7. The process of claim 1 further comprising utilizing a solvent in the first treatment unit, the second treatment unit, or both.

8. The process of claim 7 further comprising regenerating the solvent.

9. The process of claim 8 further comprising regenerating solvents from the first treatment unit and the second treatment unit in single regenerator.

10. The process of claim 8 further comprising regenerating a flow of CO2 and H2S rich solvent stream from the first treatment unit and flowing the regenerated stream to the second treatment unit.