WO2011087478A1

WO2011087478A1 - Reducing impurities in ethylene

Info

Publication number: WO2011087478A1
Application number: PCT/US2010/003118
Authority: WO
Inventors: Isa K. Mbaraka; Duncan Coffey
Original assignee: Dow Global Technologies Inc.
Priority date: 2009-12-22
Filing date: 2010-12-08
Publication date: 2011-07-21

Abstract

Methods for reducing impurities in a crude ethylene stream produced by dehydrating ethanol, the crude ethylene stream including ethylene, carbon monoxide, and hydrogen. The method includes selectively oxidizing a portion of the carbon monoxide and hydrogen to, respectively, carbon dioxide and water using at least one metal-containing catalyst. A metal in each of the metal-containing catalysts is selected from a group consisting of platinum-group metal elements and group 11 elements of the periodic table. The ethylene stream subsequent to the selectively oxidizing has a lower carbon monoxide content and a lower hydrogen content than the crude ethylene stream.

Description

REDUCING IMPURITIES IN ETHYLENE

[001 ] The present disclosure relates to methods for reducing impurities in ethylene, and in particular for reducing impurities in a crude ethylene stream produced by dehydrating ethanol (EtOH).

[002] Ethylene used for production of polyethylene requires a high purity specification, e.g., a 98.0 or higher weight percentage (wt%) of pure ethylene, a 2.0 or lower wt% of total impurities, and less than 0.001 wt% of oxygen-containing compounds. Carbon monoxide (CO) is an oxygen-containing compound created during production of ethylene. CO is an undesired byproduct, e.g., impurity, that reduces catalytic activity of expensive Ziegler-Natta catalysts used in the

polymerization of ethylene into various polyethylene products.

[Q03] Cryogenic distillation is a separation process performed to attain a desired purity specification for efficient operation of a polyethylene production process. Cryogenic distillation removes methane (CH₄) and hydrogen (H₂) gases along with CO gas, which are all unwanted impurities in polyethylene production. For instance, a hydrocarbon-based steam cracking reactor's effluent contains, along with ethylene, concentrations of CFL) gas, e.g., within a range of from 50,000 parts per million (ppm) to 300,000 ppm, and H₂ gas, e.g., within a range of from 100,000 ppm to 400,000 ppm, that are higher than a concentration of CO gas, e.g., within a range of from 1000 ppm to 3000 ppm. That is, the hydrocarbon-based steam cracking reactor's effluent contains a wt% of CH₄ gas within a range of from 5 to 30, a wt% of H₂ gas within a range of from 10 to 40, and a wt% of CO gas within a range of from 0.1 to 0.3. Impurities, e.g., the CO, CH₄, and H₂ gases, are removed because the impurities interfere with polymerization of ethylene by poisoning the catalyst and/or by becoming a reactant in the polymerization reaction that inappropriately terminates the polymerization.

[004] Dehydrating EtOH is an alternative process for producing ethylene.

Concentrations of CH₄ and H₂ in an EtOH dehydration reactor's crude ethylene stream effluent are low relative to the hydrocarbon-based steam cracking reactor's effluent and are below the specification for polyethylene production. Dehydrating EtOH into a crude ethylene stream often produces a lower concentration of CO relative to that present in the hydrocarbon-based steam cracking reactor's effluent; however, the concentration is often higher than the ethylene purity specification for oxygen-containing compounds mentioned above.

[005] Cryogenic separation processes designed to remove CH₄ and H₂ present at the high concentrations in the hydrocarbon-based steam cracking reactor's effluent, among other impurities such as CO, require capital investment for purchase of equipment and result in operating costs for consumables, labor, and energy. The costs of cryogenic distillation appear to be particularly high, and even cost- prohibitive, e.g., as determined on a net present value ( PV) basis, when the cryogenic distillation processes are utilized merely for removal of the relatively low concentration of CO produced by dehydrating EtOH, especially when the

concentrations of CH and H₂ are already below the specification for ethylene.

[006] As described in the present disclosure, reducing the cost of ethylene production obtained from EtOH dehydration is accomplished by using a separation process to, in particular, remove CO from the crude ethylene stream. The separation process selectively oxidizes CO in the crude ethylene stream to carbon dioxide (C0₂) such that a final output of CO is at or below the oxygen-containing compound level of the ethylene purity specification mentioned above, in contrast to removing CO and other impurities via cryogenic distillation. In addition, H₂ is also selectively oxidized and converted to water (H₂0), which further reduces the amount of H₂ in the crude ethylene stream.

[007] The separation process for purification of crude ethylene produced from dehydrating EtOH includes selectively oxidizing a portion of the CO and H₂ to, respectively, C0₂ and H₂0 using at least one metal-containing catalyst. A metal in each of the metal-containing catalysts, e.g., at least one metal atom in each molecule of the catalyst, is selected from a group consisting of platinum-group metal (PGM) elements and group 1 1 elements of the periodic table. The PGM elements, which include ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt), catalyze oxidative reactions. The group 1 1 elements, which include copper (Cu), silver (Ag), and gold (Au), also catalyze oxidative reactions. The crude ethylene steam and/or a modified ethylene stream, e.g., with some of the CO and H₂ previously oxidized, is exposed to the at least one metal-containing catalyst in one or more reaction zones. The ethylene stream subsequent to the selective oxidization by the at least one metal-containing catalyst has a lower CO content and a lower H₂ content than the crude ethylene stream produced from dehydrating the EtOH. In a preferred embodiment, the CO content subsequent to the selective oxidation is at or lower than the specification for ethylene, e.g. , as used for polymerization of polyethylene.

[008] The present disclosure describes a step a) of selectively oxidizing a portion of the CO and H₂ using a Pd-containing and/or a Pt-containing catalyst and a step b) of selectively oxidizing a remaining portion of the CO and the H₂ using a Cu- containing, a Ag-containing, and/or a Au-containing catalyst, the ethylene stream subsequent to steps a) and b) having the lower CO content and the lower H₂ content than the crude ethylene stream. The present disclosure also includes step a) using the Pd-containing catalyst and step b) using the Cu-containing catalyst.

[009] In a preferred embodiment, step a) precedes step b). The preferred embodiment combines two discrete catalytic oxidation steps to remove CO and H₂ from the ethylene stream produced by dehydrating EtOH. Members of the PGM elements have high oxidative catalytic capacity relative to a number of other metals that also serve as oxidative catalysts, e.g., group 1 1 elements such as Cu, Ag, and Au. In a first reaction zone, CO and H₂ are selectively oxidized to C0₂ and H₂0 using purified oxygen (0₂) over a catalyst bed, where the catalyst contains Pd and/or Pt. In a more preferred embodiment, the catalyst in the first reaction zone contains Pd.

[010] In a second step of the preferred embodiment, the ethylene and remaining impurities, e.g. , unreacted CO, H₂, and excess 0₂, in the stream are transferred into a separate second reaction zone. In the second zone, levels of the unreacted CO, H₂, and excess 0₂ are further reduced by an oxidation reaction with a group 1 1 element-containing catalyst, e.g., the Cu-containing, the Ag-containing, and/or the Au-containing catalyst. In the more preferred embodiment, the catalyst in the second reaction zone contains Cu. In the preferred and most preferred embodiments, the ethylene stream subsequent to steps a) and b) has the lower CO content and the lower H₂ content than the crude ethylene stream. In the preferred embodiments, the CO content subsequent to the selective oxidation in the second zone is at or lower than the specification for ethylene, e.g., as used for polymerization of polyethylene.

[01 1 ] In a preferred embodiment, when the crude ethylene stream is fed into the reaction zone, e.g. , for step a) the first reaction zone, 0₂ is present in a substantially stoichiometric amount, e.g., by adding a measured volume of purified 0₂ gas with respect to CO and H₂ in the stream. "Stoichiometric" means that a number of individual oxygen atoms introduced with the crude ethylene stream, e.g., which is twice the number of 0₂ molecules, is equivalent to the number of oxygen atoms needed to oxidize essentially all of the CO and H₂ molecules in the stream into C0₂ and H₂0, respectively, e.g., in the first reaction zone and the second reaction zone. "Substantially" means that the number of oxygen atoms included in the crude ethylene stream is equivalent to that needed to oxidize essentially all of the CO and H₂ molecules within a predetermined margin of error based upon a determination of the concentrations of CO and H₂ present in the crude ethylene stream.

[012] In a preferred embodiment, 0₂ is not present above the substantially stoichiometric amount with respect to the CO and H₂ in the stream because unused 0₂ remains combined with the oxidative catalysts, e.g., in particular the group 1 1 elements. The substantially stoichiometric amount of 0₂ reduces an amount of 0₂ that remains combined with the oxidative catalysts after selectively oxidizing the CO and H₂ in the stream. Higher than the substantially stoichiometric amount of 0₂ reduces the ability of the oxidative catalyst to combine with substantially all of the 0₂ molecules, hence resulting in undesired output of excess 0₂ in a final ethylene product. Lower than the substantially stoichiometric amount of 0₂ reduces the ability of the oxidative catalyst to oxidize substantially all of the CO and H₂, hence resulting in undesired output of these impurities in the final ethylene product. The substantially stoichiometric amount of 0₂ is determined by measuring concentrations of CO and H₂ in the crude ethylene stream with gas chromatography at various time intervals during the process of dehydrating the EtOH and calculating the substantially stoichiometric amount of 0₂.

[013] The crude ethylene stream or a modified ethylene stream, e.g. , after a portion of which is oxidized in step a), along with the impurities within the crude or modified ethylene stream, is in a vapor state while being selectively oxidized using the at least one metal-containing catalyst. The crude ethylene stream prior to being selectively oxidized using the at least one metal-containing catalyst has a CO content of less than or equal to 1000 ppm and a hydrogen content of less than or equal to 250 ppm based upon a volume of the crude ethylene stream. The preferred concentrations for CO are less than or equal to 500 ppm and less than or equal to 125 ppm for H₂, with the most preferred concentrations of CO being less than or equal to 250 ppm and of H₂ being less than or equal to 100 ppm.

[014] The maximum and preferred impurity concentrations are determined by the oxidative capacity of PGM-containing catalysts, e.g., the Pd-containing catalyst, to enable lowering of the CO concentration of the ethylene stream to less than or equal to 10 ppm before the modified ethylene stream is transferred from the first reaction zone to the second reaction zone. A CO concentration of the modified ethylene stream that is less than or equal to 10 ppm enables the oxidative capacity of the group 1 1 element-containing catalyst, e.g., the Cu-containing catalyst, in the second reaction zone to further reduce the CO concentration of the ethylene stream to an even lower level, e.g., a concentration approaching zero, using the remaining 0₂ in the substantially stoichiometric amount originally present in the first reaction^'zone.

[ 5] Dehydrating EtOH is readily capable of yielding a crude ethylene stream with a maximum CO content of less than or equal to 1000 ppm and a maximum H₂ content of less than or equal to 250 ppm. Dehydrating EtOH is also capable of yielding a crude ethylene stream with concentrations of CO less than or equal to 250 ppm and of H being less than or equal to 100 ppm. Additionally, dehydrating EtOH is capable of yielding concentrations of CH less than or equal to 20 ppm and is even capable of yielding concentrations of CH less than or equal to 5 ppm, which are lower than the specification for ethylene. In contrast, a hydrocarbon- based steam cracking reactor's effluent often contains markedly higher concentrations of CO, H2, and/or CH₄, which would prevent such an effluent from being used in the described oxidative catalysis method.

[016] Because the PGM-containing catalysts, e.g., the Pd-containing and/or the Pt-containing catalysts, have a greater oxidative capacity relative to the group 1 1 element-containing catalysts, e.g., the Cu-containing, the Ag-containing, and/or the Au-containing catalysts, the PGM-containing catalyst is utilized for "bulk" oxidation of the CO and H₂ impurities in the crude ethylene stream and the group 1 1 element- containing catalyst is utilized for oxidizing, or "cleaning up", the remaining CO and H₂ impurities prior to output of the ethylene stream, e.g., for polymerization into polyethylene. Hence, in the preferred and the most preferred embodiments, step a) precedes step b) and step a) occurs in the first reaction zone and step b) occurs in the separate second reaction zone. [017] As an example of catalyst loading in a reaction zone, in the most preferred embodiment the first reaction zone utilizes a bed of a Pd-containing catalyst. The Pd-containing catalyst is an alumina carrier, e.g., AI2O3, impregnated with Pd in a range of from 0.10 wt% Pd to 0.80 wt% Pd based on the total weight of the dry catalyst. An example of a suitable Pd-containing catalyst is PuriStar RO-20/47 from BASF (although other Pd-containing catalysts are substitutable). In the most preferred embodiment, the separate second reaction zone utilizes a bed of a Cu- containing catalyst. The Cu-containing catalyst is an alumina carrier, e.g., AI2O3, impregnated with Cu in a range of from 20.0 wt% Cu to 80.0 wt% Cu based on the total weight of the dry catalyst. An example of a suitable Cu-containing catalyst is PuriStar R3-16 from BASF (although other Cu-containing catalysts are substitutable). Recommended volumes and reaction times for the crude ethylene stream containing the impurities to be oxidized in the presence of particular catalysts are obtainable from, or can be calculated from, information provided by the manufacturer and/or provider of the particular catalysts. Flow rates for the ethylene stream are calculated using such information. The manufacturer and/or provider of the particular catalysts also provide the recommended operating temperature of the particular catalysts.

[01 8] The present disclosure describes removing at least a portion of the C0₂ and the ¾0 resulting from the CO and H₂ being selectively oxidized using the at least one metal-containing catalyst. That is, the CO2 and FbO are optionally removed downstream using at least one of suitable separation processes after being generated by selectively oxidizing a portion of the CO and ¾ using at least one metal- containing catalysts selected from the group of PGM elements and/or group 1 elements or the two catalytic oxidation steps a) and b). These processes are considerably less expensive than cryogenic distillation. Remove the CO2 and H2O after step a) and/or step b), depending upon choice, e.g., of a plant operator or designer, among others.

[019] The present disclosure also describes removing at least a portion of hydrocarbon heavies, e.g., undesired ethane and hydrocarbons having three or more carbon atoms, resulting from dehydrating the EtOH to produce the crude ethylene stream. The heavies are undesired products that result from combination of one- carbon and two-carbon molecules resulting from dehydrating the EtOH. The heavies are removed downstream using a suitable separation process before and/or after selectively oxidizing a portion of the CO and H₂ using at least one metal-containing catalyst selected from the group of PGM elements and/or group 1 1 elements or before and/or after the two catalytic oxidation steps a) and b). That is, remove the heavies before step a) or after step a) and/or step b), depending upon choice, e.g., of the plant operator or designer, among others.

Comparative

[020] As determined on the NPV basis, a capital cost intensity for CO removal by cryogenic distillation is 0.59 United States cents per pound (fi/lb) of ethylene produced. In addition, an operating cost intensity, as determined on the NPV basis, for CO removal by cryogenic distillation is 0.07 eVlb.

Example

[021 ] In contrast to the cryogenic distillation, reduced expenditures for equipment to selectively oxidize the CO yields a capital cost intensity of 0.08 eVlb, thereby resulting in a savings of 0.51 eVlb of ethylene produced relative to the cryogenic distillation. Reduced expenditures for consumables, e.g. , catalysts, 0₂, etc., along with labor and energy input utilized in selectively oxidizing the CO yields an operating cost intensity of 0.05 /lb, thereby resulting in a savings of 0.02 eVlb of ethylene produced relative to the cryogenic distillation. The energy saving intensity is convertible to 40 British thermal units per pound (BTU/lb) of ethylene produced.

[022] Hence, replacement of the cryogenic distillation process with the described catalytic oxidation method reduces the capital and operating cost associated with separation of CO by approximately 80-90% and 30-40%, respectively.

[023] The method for removing impurities from a crude ethylene stream resulting from dehydrating EtOH, e.g., to be at or below a polyethylene production specification, is an improvement over pre-existing processes. Improvement results from replacing the cryogenic distillation process used to remove tail gases from the crude ethylene stream produced from dehydrating EtOH. Selectively oxidizing the CO and H₂ impurities: requires less input of capital, consumables, labor, and/or energy to remove impurities than the cryogenic process; can use two catalytic oxidation stages to ensure that an ethylene specification is met; and/or simplifies the process and increases the process utilization efficiency.

Claims

Claims What is claimed:

1 . A method for purifying a crude ethylene stream comprising dehydrating ethanol to produce the crude ethylene stream comprising ethylene, carbon monoxide, and hydrogen, selectively oxidizing a portion of the carbon monoxide and hydrogen to, respectively, carbon dioxide and water using at least one metal-containing catalyst, wherein a metal in each of the metal-containing catalysts is selected from a group consisting of platinum-group metal elements and group 1 1 elements of the periodic table, and the ethylene stream subsequent to the selectively oxidizing having a lower carbon monoxide content and a lower hydrogen content than the crude ethylene stream.

2. The method of Claim 1 , further comprising step a) selectively oxidizing a portion of the carbon monoxide and hydrogen using a palladium-containing and/or a platinum-containing catalyst, and step b) selectively oxidizing a remaining portion of the carbori monoxide and the hydrogen using a copper-containing, a silver-containing, and/or a gold-containing catalyst, the ethylene stream subsequent to steps a) and b) having the lower carbon monoxide content and the lower hydrogen content than the crude ethylene stream.

3. The method of Claim 2, wherein step a) uses the palladium-containing catalyst and step b) uses the copper-containing catalyst.

4. The method of Claim 2 or Claim 3, wherein step a) precedes step b).

5. The method of any one of Claims 2 through 4, wherein step a) occurs in a first reaction zone and step b) occurs in a second reaction zone.

6. The method of any one of the preceding claims, wherein oxygen is present in a substantially stoichiometric amount, with respect to carbon monoxide and hydrogen in the crude ethylene stream.

7. The method of any one of the preceding claims, wherein the crude ethylene stream or a modified ethylene stream is in a vapor state while being selectively oxidized using the at least one metal-containing catalyst.

8. The method of any one of the preceding claims, wherein the crude ethylene stream prior to being selectively oxidized using the at least one metal-containing catalyst has a carbon monoxide content of less than or equal to 1 000 parts per million and a hydrogen content of less than or equal to 250 parts per million based upon a volume of the crude ethylene stream.

9. The method of any one of the preceding claims, further comprising removing at least a portion of the carbon dioxide and the water resulting from the carbon monoxide and hydrogen being selectively oxidized using the at least one metal- containing catalyst.

10. The method of any one of the preceding claims, further comprising removing at least a portion of hydrocarbon heavies resulting from dehydrating the ethanol to produce the crude ethylene stream.