CA1257614A

CA1257614A - Removal of oxidizable impurities from methane and ethane

Info

Publication number: CA1257614A
Application number: CA000496359A
Authority: CA
Inventors: Douglas B. Taggart; Paul J. Busse
Original assignee: Quantum Chemical Corp
Current assignee: Equistar Chemicals LP
Priority date: 1984-12-17
Filing date: 1985-11-27
Publication date: 1989-07-18
Also published as: FR2574678A1; GB8529167D0; BE903841A; DE3544163A1; AU5123885A; GB2168720A

Abstract

Abstract of the Disclosure A method is disclosed for the purification of methane and/or ethane which comprises oxidizing impurities in the gas with oxygen at 175°C to 375°C in the presence of a catalyst which contains silver as the active component.

Description

~i;7~

REMOVAL OF OXIDIZABLE I~PURITIES FROM METHANE AND ET~ANE
__ _ _ __ .
This invention describes a process for the removal or reduction in concentration of certain oxidizable impurities from methane and ethane. These impurities are typically found in methane and ethane that is derived from synthetic sources but may also be found in natural gas or in other gas mixtures.
These impurities may be carbon monoxide, high hydrocarbon impurities, including ethane (if pure methane is desired), propane, butane, and isobutane as well as higher hydrocarbons containing five or more carbon atoms, oxygenated hydrocarbons such as alcohols, aldehydes, ketones, or organic acids. They may also be heavy hydrocarbon oils, organo sulfur compounds such as mercaptan odorants in the gas, alkanolamines such as are used in drying natural gas, polyglycols such as are used for water removal from natural gas, or olefins such as may be present in methane derived from a refinery.
These organic oxidiizable impurities can be removed or reduced in concentration by a wide variety of chemicall or physical procedures. Physical procedures for removal from methane and ethane are represented by distillation and adsorption. Both methods are based on equilibrium processes which require multiple steps for each additional increment of impurity removal. In addition, the physical methods generally are operated under cryogenic conditions which are comparatively costly. As a consequence, physical methods of impurity separation are terminated at an economic barrier depending on the economic advantage of removing an additional increment of impurity or at the desired methane/ethane purity level which may be beyond the economic barrier. It is particularly costly to purify methane or ethane beyond the economic barrier~

An alternative to physical methods of impurity removal from methane is the use of catalytic reduction. In this process, excess hydrogen is mixed with the impure methane stream which is then fed to a catalytic reactor in which certain of these impurities, such as oleEins, may be converted to methane.
This procedure does not extract the value of the impurities but does reduce their concentration. However, hydrogen is a relatively expensive material and is not always conveniently availableO
In many cases the impurity level of methane and/or ethane may not affect its use and in some cases it may even be beneficial, such as in the case of odorantsO ~Iowever, in certain cases, it is important that the purity level of the methane and/or ethane be extremely high and the complete absence of these impurities is preferred. To accomplish this control of the impurity concentration via the extant physical or chemical procedures would be complicated and expensive or both. Thus it can be seen that there is a need for a less expensive method which is capable of reducing the concentration of these impurities in methane and/or ethane to very low levels.
The present invention provides a method oE purification of methane and/or ethane which is based on oxidation of the impurities in the methane and/or ethane. The oxidizing agent is oxygen and the process is conducted in a catalytic reactor operating in the temperature range of 175C to 375C. Catalysts effective for this process contain silver as the active component.
The process consists of mixing a sufficient quantity of oxygen with the methane and/or ethane stream to permit combustion of the contaminantsO This stream is then passed ~ ~7~ ~

through a catalyst bed providing the re~uired conditions of oxidation. The resulting products of the purification step are carbon dioxide and water. These may be removed via conventional processes or left in the methane and/or ethane stream as desired.
When an impure methane and/or ethane stream containing oxygen and optionally an inert gas such as nitrogen is passed over a catalyst as described below, combustion occurs predominantly among the contaminants before methane and/or ethane combustion occurs. With the catalysts described herein, the combustion process is normally very efficient providing carbon dioxide and water as the sole products. Control of the reaction can be achieved by controlling the variables of oxygen concentration, reactor temperature, and flow rate. By proper adjustment of these parameters, the impurities can be removed with little effect on the methane and/or ethane concentration.
If the oxygen concentration required for complete removal of the impurities is higher than the flammability limit of the mixture, then a multiple-step process may be conducted in which oxygen is added to the methane and/or ethane stream at a safe concentration in each step until the desirerd impurity level is reached.
When an impure methane stream containing oxygen and optionally an inert gas such as nitrogen is passed over a catalyst as described below, combustion occurs predominantly in the order C4 before C3 before C2 before Cl. With the catalysts described herein, the combustion process is normally very efficient providing carbon dioxide and water as the sole products. Control of the reaction can be achieved by controlling the variables of oxygen concentration, reactor ~7~

temperature, and flow rate. By proper adjustment of these parameters, C3+ hydrocarbons can be removed with little effect on ethane or methane concentration. The process can then be operated in a manner to significantly reduce the ethane concentration with relatively little reaction of methane. In some situations, the C2+ hydrocarbons can be removed in one oxidizing step.
It goes without saying that for economic reasons it would be preferred to remove all of the contaminants in a one-step process if that is possible. The process of the present invention can be performed in one step if the concentrations of the contaminants are such that the oxygen concentration required to oxidize them is below the flammability limit of the gas mixture so that the purification can take place without fear of an explosionO The methane and/or ethane stream and the oxygen are led to a catalytic reactor wherein the reaction temperature is controlled with the range of 175 to 375C. If the temperature is below 175C, then the reactivity is insufficient and if the temperature is above 375CC, then too much methane and/or ethane is oxidized. For carbon monoxide, it is preferred that the temperature range be from 200C to 375C. The oxygen concentration should be maintained at a level higher than but close to the stoichiometric concentration required to oxidize the contaminants. Preferably, the oxygen concentration range i5 from 0.5% to 1% above the stoichiometric concentration because more oxygen provides no benefit and would bring the mix closer to the flammability limit.
In situations where the concentration of the higher hydrocarbons requires an oxygen concentration which is above the flammability limit of the gas mixture, multiple oxidation steps ~7~

are required~ Preferably, two oxidations take place. In each oxidation step, the concentration of the oxygen is kept below the flammability limit because the oxygen concentration res~uires only that necessary to oxidize the higher hydrocarbons, C3~, in the first step and, iE it is to be removed, the ethane in the second step. This is possible because the oxidation occurs selectively with the higher hydrocarbons being oxidized before the lower hydrocarbons. The first oxidation removes almost all of the C3~ hydrocarbons and the second removes as much of the ethane as is possible although it is very diEficult to separate out all of the ethane without also oxidizing some methane.
The C3+ hydrocarbons are oxidized by passing the methane and/or ethane stream over a silver catalyst at a temperature in the range of 175C to 375C, preferably 230C to 320C. The oxygen concentration should be maintained close to the stoichimetric concentration required for removal of the amount of C3~ hydrocarbons present in the gas, preferably within 0.5 to 1~6 of this amount. The space velocity is also important here.
In order to obtain optimum results, the space velocity should be kept in the range of 50 to 1,000 hour~l, preferably for the most practical operation 100 to about 500 hour~l.
If purified methane is desired, the ethane can be oxidized by passing the gas stream over a silver catalyst at a temperature in the range of 175C to 375C, preferably 250C to 350C. The oxygen concentration should be maintained close to the stoichiometric concentration required for removal of the amount of ethane present in the methane, preferably within 0.5 to 1% of this amount. The space velocity is again important.
In order to obtain optimum results, the space velocity should be kept in the range of 50 to 1,000 hour~l, preferably for the most ~7~

practical operation 100 to about 500 hour~l.
The amount of time tha-t the gas mixture is in contact with the catalyst is also an important variable. The contact time can be increased by increasing the size of the reactor. It can also be increased by slowing down the flow rate of the gases through the reactor. The contact time is generally measured in terms of space velocity in units of hour~l. ThuS, it is preferred that the space velocity be in the range from about 50 to about 1,000 hour 1, preferably for the most practical operation 100 to about 500 hour~l, if the reaction takes place at atmospheric pressure because it allows maximum use of the reactorO Higher flow rates can be used if the temperature is increased.
In situations where the concentration or type of the impurities requires an oxygen concentration which is above the flammability limit of the gas mixture, multiple oxidation steps are required. Preferably, for safety and economic reasons, two oxidations take place. In each oxidation step, the concentration of the oxygen is kept below the flammability limit to prevent explosive combustion.
A wide variety of silver-based catalysts can be used to advantage in the present invention. These catalysts generally comprise a silver salt deposited on a porous support material such as alumina, silica or other inert refractory material. The catalyst might also include promoters such as alkali metals and alkaline earth metals. Commercially existing ethylene oxide silver-based catalysts generally provide acceptable performance in this process. Catalysts of this type are described in U.S.
Patent No. 3,725,307, issued April 3, 1973.
The process may be operated with inert gases such as nitrogen or argon in the methane and/or ethane stream. The combustion products of the oxidation reaction are carbon dioxide and water and may be removed by conventional purification techniques i~ their presence is not desired in the purified methane and/or ethane stream. For use in an ethylene oxide reactor, the presence of carbon dioxide or water in the methane and/or ethane stream should not be detrimental to the process as both components are normally present in the reactor. If oxygen is not totally consumed in the purification process, the unreacted oxygen may be used to supplement the oxygen feed to the reactor.
The process is capable of reducing contaminant concentrations to the 5 ppm level or less with oxygen selectivities of 90% or more. The oxygen selectivity is defined as the percentage of oxygen consumed for removal of contaminants divided by the total oxygen consumed.

Oxygen Selectivity = 100 x Oxyge_ Consum_d for Contaminants Total Oxygen Consumed This process is particularly suited for the removal of oxygenated hydrocarbons from methane, ethane and methane and ethane gas mixtures. Alcohols, aldehydes, ketones, organic acids, alkanolamines, polyglycols, etc. are all examples of oxygenated hydrocarbons which can be removed from methane and/or according to the present process. Methanol is an alcohol which is commonly used as an antifreeze agent in natural gas pipelines and thus there may be a need for its removal therefrom. Also, alkanolamines are used in the drying of natural gas and polyglycols are also used for water removal. It is quite possible that residual amounts of these materials may remain in natural gas and have to be removed.

In addi-tion to the oxygenated hydrocarbons, heavy hydrocarbon oils from pumping equipment may contaminate natural gas. Organo sulfur compounds used as odorants are also a source of contamination which it may be necessary to eliminate. These materials can also be removed according to this process.
The process is particularly suited for the removal of carbon monoxide from methane, ethane, and methane and ethane gas mixtures. Carbon monoxide may be present in methane or ethane which is derived from synthetic natu~al gas plants or from refineries.
_xample I
Methanol Rem_val From Methane Streams Methanol may be added to a methane stream to prevent icing which may occur in wet streams. The added methanol may be removed via selective oxidation to carbon dioxide and water in the presence of methane. The resulting inorganic impurities may be removed by conventional procedures if so desired.
Complete combustion of methanol to carbon dioxide and water requires 1.5 volumes of oxygen per volume of methanol.
For optimum removal of the methanol, a slight excess of oxygen may be added, from 0.5 to 1.00~. The oxygen concentration should not exceed the flammability limit for the mixture that is to be purified.
If the oxygen concentration requir~d for complete removal of the methanol is higher than the flammability limit of the mixture, then a multiple-step process may be conducted in which oxygen is added to the methane stream at a safe concentration.
This mixture is fed to the catalytic reactor where oxidation of part of the methanol is conducted. The methane stream containing a reduced quantity of methanol may again be treated ~j71~

with a second portion of oxygen and reacted againO This procedure may be repeated until the methanol concentration is reduced to the desired level.
~ methane stream containing 0.5% methanol as impurity is mixed with oxygen to give an oxygen concentration of 1.5~ in the gas mixture. The resulting mix is fed to the reactor containing the catalyst at a space velocity of 200 per hour and a temperature at 230-2~0C. The methanol concentration of the stream leaving the reactor is greatly reduced.
Example II
Ethanolamine Removal From Methane Streams Ethanolamine is used in the natural gas industry to remove acid gases from a methane and/or ethane stream. The resulting methane stream may be contaminated with small quantities of the ethanolamine. This residual ethanolamine may be removed by the process described herein in ~hich the impurity is oxidized to carbon dioxide and water.
Complete combustion of ethanolamine to carbon dioxide and water requires 3.25 volumes of oxygen per volume of ethanolamine. For optimum removal, from 0.5 to 1.0% excess oxygen may be added to the methane stream.
As indicated for methanol removal, the oxygen concentration should not exceed the flammability limit. In cases where high oxygen concentrations would be necessary for complete removal, a multiple-step treatment can be conducted as described for methanol removal from methane.
A methane stream containing 200 ppm of ethanolamine is mixed with oxygen to give an oxygen concentration of 0.6%. This mixture is fed to the catalyst at a preferred space velocity of 200 per hour and at a temperature of 230-240C. The ethanolamine concentration of the stream leaving the reactor is greatly reduced.
Exampl_ III
Methanol Rem_val From Ethane Streams Methanol may be added to an ethane stream to prevent icing which may occur in wet streams. The added methanol may be removed via selective oxidation to carbon dioxide and water in the presence of ethane. The resultiny inorganic impurities may be removed by conventional procedures if so desired.
Complete combustion of methanol to carbon dioxide and water requires 1.5 volumes of oxygen per volume of methanol.
For optimum removal of the methanol, a slight excess of oxygen may be added, from 0.5 to 1.0~. The oxygen concentration should not exceed the flammability limit for the mixture that is to be purified.
If the oxygen concentration required for complete removal of the methanol is higher than the flammability limit of the mixture, then a multiple step process may be conducted in which oxygen is added to the ethane stream at a safe concentration.
This mixture is fed to the catalytic reactor where oxidation of part of the methanol is conducted. The ethane stream containing a reduced quantity of methanol may again be treated with a second portion of oxygen and reacted again. This procedure may be repeated until the methanol concentration is reduced to the desired level.
An ethane stream containing 0.5% methanol as impurity is mixed with oxygen to give an oxygen concentration of 1.5~ in the gas mixture. The resulting mix is fed to the reactor containing the catalyst at a space velocity of 150 per hour and a temperature in the range o 220-230C. The methanol ~i7~

concentration of the stream leaving the reactor is greatly reduced.
Example IV
Removal of _cetylen_ and Carbon Monoxide from Methan_ Acetylene and carbon monoxide may be present in methane derived from an ethylene cracker. Both components should preferably be removed from the methane before it is used as ballast gas in an ethylene oxide reactor. Oxidative purification can be applied to the purification of the methane stream.
Complete combustion of acetylene to carbon dioxide and water requires 2.5 volumes of oxygen per volume oE acetylene whereas complete combustion of carbon monoxide to carbon dioxide requires 0.5 volumes of oxygen per volume of carbon monoxide.
For enhanced efficiency of contaminant removal, an excess of oxygen may be added to the methane stream with from 0.5 to 1%
normally being adequate for optimum removal.
If the concentration of contaminants requires an oxygen concentration greater than the flammability limits of the gas mixture, then a multiple pass process should be conducted in which oxygen is mixed with the methane stream to a concentration below the flammability limit. This mixture is passed through the catalytic reactor in which combus-tion of some of the impurities occurs. The resulting partially purified methane stream is again admixed with oxygen at a concentration below the flammability limit and reacted for a second time. This process is repeated until the concentration of impurities is down to the desired level.
A methane stream containing acetylene/ 1000 ppm, and carbon monoxide, 2000 ppm, is mixed with oxygen to give an ~57~
oxygen concentration of 0.75%. The oxygenated methane stream is fed to a reactor at a space velocity of 200 per hour and in the temperature range of 230-240C. The acetylene and carbon monoxide concentration of the outlet stream is greatly reduced.
Example _ Six samples of silver based catalysts were utilized for the evaluations reported herein. The test reactor in which the catalysts were evaluated consisted of 1/2 inch stainless steel tubes located in an oven wi~h a preheating zone for the gas feed. The catalysts were evaluated under the conditions indicated in the tables and the products were evaluated by gas chromatography. In all of the following experiments, the catalyst was placed in the reactor and used to oxidize the indicated hydrocarbons at atmospheric pressure under the specified conditions.
Catalyst F was a sample of ethylene oxide catalyst that had been used in the manufacture of ethylene oxide. Catalysts A
through E were prepared by impregnation with a mixture of a silver salt, one or more promoters as described below, lactic acid and water. The support used for all five catalysts was Norton SA-5202 which is a low surface area alumina support in the form of 1/4 inch spheres~ The impregnated support was dried at 60C, and then activated via procedures designated in the table.
_ataly_t Pr_paration a__ Activation SilverBarium Sodium Total Salt Carbonate Carbonate Volume Activation Catalyst gm. gm. gm. ml. Procedure A Carbonate .705 -- 100 l, 2, 3 16.2 B Oxide .704 .505 100 13.2 ~57~

C Carbonate .700 .506100 1, 2, 3 16.2 D Oxide .704 -- 1001, 2, 3 13.6 E Oxide .300 -- 1001,2 13.6 1 - under nitrogen for 5 to 6 hours.

2 - under 5% oxygen in nitrogen for 2 hours.

3 - under 5% hydrogen in nitrogen for 2 hours.
Table 1 C3+ C3t Oxygen Methane Temp. Conc. Conv. Sel. Sel. Flow Rate Catalyst C ppm ~ % % ml./min.
A 265 239 94.7 80.399.9 70 248 " 98.2 93.4100~0 55 243 " 100.0 92.399.9 40 B 284 " 67.4 59.399.9 55 280 " 70.1 68.299.9 40 C 284 " 80.9 84.599.9 55 280 " 87.7 84.899.9 40 D 277 102 100.0 47.099.5 70 E 277 " 85.4 45.799.7 70 F 277 " 74.6 47.399.8 70 While all the catalysts per~ormed properly, Catalysts A
and C were particularly good at selectively converting the C
hydrocarbons.
Table 2 C2 C2 Oxygen Methane Temp.Conc. Conv. Sel.Sel. Flow Catalyst C % % % % Rate A 302 .811 58.4 53.798.9 55 282 " 35.0 68.299.6 70 280 " 65.1 52.798.6 40 248 " 23.1 93.4100.0 55 243 " 27.1 92.399.9 40 B 302 " 8.0 48.299.8 55 298 " 9.6 50.699.8 40 C 302 " 17.8 73.899.8 55 298 " 20.6 75.299.8 40 ~5~

D 282 .787 35.4 42.2 99.3 70 ~ 304 .7~7 19.6 39.~ 9g.6 70 F 304 .787 18.4 40.7 99.6 70 All of the catalysts worked reasonably well, but Catalysts A and C were the best at converting ethane.
Tabl_ 3 Experiments with High Concentrations of C3~ Hydrocarbons.
Catalyst A

Oxygen C3+ C3+ OxygenMethane Conc. Temp. Conc. Conv. Sel. Sel. Flow % C ~ ~_ % % Rate 2 297 .765 76.1 96.0 99.9 70 3 297 .765 89.6 92.6 99.~ 70 Catalyst A removed a high percentage of C3~ hydrocarbonsO
Thus, a second pass could have easily removed the remaining hydrocarbons.
Example VI
Ethane ~ay contain propane and butane as impurities which may be removed by oxidative purification. The reacted propane and butane is converted to carbon dioxide and water. The inorganic combustion products may be removed by conventional procedures if so desired.
If the oxygen concentration required for removal of the C3~ hydrocarbons is higher than the flammability limit of the mixture, a two or more step process may be conducted in which oxygen is added to the ethane stream at a safe concentration.
This mixture is fed to the catalytic reactor where oxidation of part of the methanol is conducted. The partially purified ethane stream may again be treated with a second portion of oxygen and reacted again. This procedure may be repeated until the C3~ concentration is reduced to the desired level.

An ethane stream containing 1,000 ppm of propane and 200 ppm of butane is mixed with oxygen to give an oxygen concentration of 1.25~ in the gas mixture. The resulting mix is fed to the reactor containing the catalyst at a space velocity of 150 per hour and a temperature of 220-230~C. The C3+
concentration of the stream leaving the reactor is greatly reduced.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for removing from methane, ethane and mixtures thereof impurities selected from the group consisting of oxygenated hydrocarbons, heavy hydrocarbons oils, organo sulfur compounds and olefins, which comprises oxidizing the impurities in the gas with oxygen at 175°C to 375°C in the presence of a catalyst which contains silver as the active component wherein the concentration of the oxygen is below the flammability limit of the gas mixture.

2. The method of claim 1 wherein the oxygen concentration is slightly in excess of the stoichiometric concentration required to oxidize all of the impurities in the gas.

3. A method for removing carbon monoxide from methane, ethane and mixtures thereof which comprises oxidizing the impurities in the gas with oxygen at 200°C to 375°C in the presence of a catalyst which contains silver as the active component wherein the concentration of the oxygen is below the flammability limits of the gas mixture.

4. The method of claim 3 wherein the oxygen concentration is slightly in excess of the stoichiometric concentration required to oxidize all of the impurities in the methane.

5. A method for the purification of methane, ethane and mixtures thereof which comprises oxidizing the higher hydrocarbon impurities in the gas with oxygen at 175°C to 375°C
in the presence of a catalyst which contains silver as the active component wherein the concentration of the oxygen is below the flammability limit of the gas mixture.

6. The method of claim 5 wherein the oxygen concentration is slightly in excess of the stoichiometric concentration required to oxidize all of the hydrocarbon impurities in the gas.

7. A method for the purification of methane, ethane and mixtures thereof which comprises:
(a) first oxidizing the C3+ hydrocarbon impurities in the gas with oxygen at 175°C to 375°C in the presence of a catalyst which contains silver as the active component, and (b) then oxidizing the ethane in the gas with oxygen at 175°C to 375°C in the presence of a catalyst which contains silver as the active component.

8. The method of claim 7 wherein the oxygen concentration in each step is slightly in excess of the stoichiometric concentration required to oxidize all of the hydrocarbon impurities in the gas.