阅读说明：本技术 脱除八氟环丁烷中氟碳烯烃的方法 (Method for removing fluorocarbon olefin from octafluorocyclobutane ) 是由 张红敏 尚杨 潘海涛 齐相前 孙猛 金向华 于 2021-09-06 设计创作，主要内容包括：本发明公开了一种脱除八氟环丁烷中氟碳烯烃的方法,包括将原料气通过氧化柱进行氧化,氧化柱包括负载有氧化剂的载体,载体形成有多孔结构,并且载体在原料气流动方向形成有供原料气流通的通路。本发明方案设备投入少,耗能少,经济行好,工艺操作简单,产品纯度高。(The invention discloses a method for removing fluorocarbon olefin from octafluorocyclobutane, which comprises the step of oxidizing a feed gas by an oxidation column, wherein the oxidation column comprises a carrier loaded with an oxidant, the carrier is provided with a porous structure, and the carrier is provided with a passage for the feed gas to flow in the flow direction of the feed gas. The technical scheme of the invention has the advantages of low equipment investment, low energy consumption, good economy, simple process operation and high product purity.)

1. A method for removing fluorocarbon olefin from octafluorocyclobutane comprises the steps of oxidizing a feed gas by an oxidation column, wherein the oxidation column comprises a carrier loaded with an oxidant, the oxidant is loaded on the surface of the carrier and is obtained by spraying an aqueous solution, the concentration of the aqueous solution of the oxidant is 0.0001-0.0005g/ml, the volume ratio of the spraying amount of the aqueous solution of the oxidant to the air inflow amount of the feed gas is 1 (5-10) during working, the carrier is formed with a porous structure and is provided with a passage for the feed gas to flow in the flow direction of the feed gas, the carrier is pretreated by putting 4.5-5 kg of newly ground graphite micropowder with the grain size of 1-10 micrometers into a reactor, adding the carrier with the volume 2-3 times that of the graphite micropowder, and sequentially adding nitric acid accounting for 0.2-0.32 percent of the weight of the graphite micropowder, mixing for 10-30min, adding 2-3 volume times of 2-3 wt% ammonia water and 0.1-0.2M alkali solution, stirring at 90-100 deg.C, soaking for 2-3 hr, taking out, and drying.

2. The method for removing fluorocarbon olefins from octafluorocyclobutane of claim 1, wherein the oxidation conditions are: the temperature is room temperature and the pressure is 0.15-0.25 MPa.

3. The method for removing fluorocarbon olefins from octafluorocyclobutane of claim 2 wherein the temperature during said oxidation is from 20 ℃ to 40 ℃.

4. The method for removing fluorocarbon olefin from octafluorocyclobutane as claimed in claim 1, wherein the space velocity of the raw material gas during the oxidation is 800-^-1。

5. The method for removing fluorocarbon olefins from octafluorocyclobutane of claim 1, wherein the oxidizing agent comprises potassium permanganate or potassium dichromate.

6. The method for removing fluorocarbon olefins from octafluorocyclobutane of claim 1 wherein the oxidizing agent is supported on the surface of the porous structure of the support.

7. The method for removing fluorocarbon olefins from octafluorocyclobutane of claim 1 wherein the support is zeolite or porous alumina or porous magnesia or porous ceramic.

8. The method for removing fluorocarbon olefins from octafluorocyclobutane of claim 7 wherein the specific surface area of the carrier is 1 to 5m²/g。

9. The method for removing fluorocarbon olefins from octafluorocyclobutane of claim 8 wherein the carrier has a mesh morphology formed by a braid.

Technical Field

The invention relates to the technical field of gas in fluorine chemical industry and electronic industry, in particular to a method for removing fluorocarbon olefin from octafluorocyclobutane.

Background

The octafluorocyclobutane is a green environment-friendly special gas with stable chemical property, good insulation property, low potential value of greenhouse effect and zero ozone consumption index value.

Octafluorocyclobutane has a wide range of applications, and has been widely used in recent years as a refrigerant to replace a prohibited chlorofluorocarbon compound, and also as a heat pump working fluid, a large-scale circuit etchant, high-voltage insulation, a spray, an aerosol, and the like; high purity octafluorocyclobutane (not less than 5N) is mainly used as etchant and cleaning agent for large scale integrated circuit, and even very small amount of impurities in the application of integrated circuit etchant may increase the defect rate of high density integrated circuit; the octafluorocyclobutane produced by thermal cracking of tetrafluoroethylene is analyzed by common rectification of fluorocarbon impurities, most of the impurities can be removed, the purity can reach 3-3.5N, and the analysis result of the octafluorocyclobutane shows that the remaining impurities mainly comprise substances azeotropic with the octafluorocyclobutane and with similar boiling points and isomers of the octafluorocyclobutane, and most of the impurities are fluorocarbon olefin impurities.

The fluorocarbon impurities in octafluorocyclobutane are one or more of octafluoroisobutylene, octafluoro-1-butene, octafluoro-2-butene, hexafluoropropylene, hexafluorocyclobutene, octafluorocyclopentene, decafluorobutane and tetrafluoroethylene, and octafluoroisobutylene CF₂＝C(CF₃)₂Octafluoro-1-butene has the formula CF₃CF₂CF＝CF₂The cis-or trans-form of octafluoro-2-butene is CF₃CF＝CFCF₃Hexafluoropropylene of the formula CF₃CF＝CF₂Decafluorobutane C₄F₁₀Tetrafluoroethylene has the chemical formula CF₂＝CF₂. The impurities in the octafluorocyclobutane are various, impurities with boiling points close to or azeotropic with the octafluorocyclobutane often exist, particularly fluorocarbon impurities, and the impurities are difficult to remove by a single adsorption method or a single rectification method to meet the purification requirement.

Patent CN110845299A provides a method for removing perfluoroolefin in octafluorocyclobutane, but this method uses strong acid hydrochloric acid, sulfuric acid, nitric acid, etc., and has strong corrosivity, high operation temperature in a reaction kettle, large equipment investment in industrial production, high cost, and relatively low safety factor.

The information disclosed in this background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

Disclosure of Invention

The invention aims to provide a method for removing fluorocarbon olefin from octafluorocyclobutane, which obtains a product with the purity of more than 99.999 percent by adopting a method combining adsorption, oxidation and rectification and has the advantages of high safety, low equipment investment, low energy consumption, low cost, simple process operation, high product purity and the like.

In order to achieve the above object, an embodiment of the present invention provides a method for removing fluorocarbon olefin from octafluorocyclobutane, which includes oxidizing a raw material gas through an oxidation column, wherein the oxidation column includes a carrier loaded with an oxidant, the carrier is formed with a porous structure, and the carrier is formed with a passage for the raw material gas to flow through in a flowing direction of the raw material gas. The oxidized feed gas can complete the purification work of the octafluorocyclobutane after passing through an adsorption tower. The flow direction here refers to the feasible flow path from an upstream point to a downstream point in the flow path.

In one or more embodiments of the invention, in order to enhance the oxidation activity in the oxidation process and fully exert the activity and efficiency of oxygen radicals, the carrier is fully activated and pretreated, wherein the pretreatment comprises the steps of putting 4.5-5 kg of newly ground graphite micropowder with the particle size of 1-10 microns into a reactor, adding 2-3 times of the volume of the carrier of the graphite micropowder, sequentially adding 0.2-0.32% of nitric acid in percentage by weight of the graphite, fully mixing for 10-30min, then adding 2-3 times of 2-3% by volume of ammonia water and 0.1-0.2M alkaline solution (preferably sodium hydroxide solution) into the mixture, fully stirring the mixture at 90-100 ℃, soaking for 2-3 hours, and taking out and drying the mixture. The activation of the carrier is provided with a suitable oxidizing environment, and the medium is subjected to preliminary oxidative activation so as to realize the preliminary activation of the medium. Then, under the condition of low-concentration alkali, the residual acid centers are further eliminated, so that the formation of active particles containing elements such as aluminum, magnesium, calcium and the like is promoted in the later-stage activation process of the carrier, and the formation of oxidation active centers is realized. The realization of the scheme also reduces the consumption of liquid phase medium, effectively improves the environmental friendliness of working medium, is beneficial to the pollution treatment in the later period, and controls the cost. And the realization of more active centers further improves the efficiency of oxidation.

In one or more embodiments of the invention, the conditions of the oxidation are: the temperature is room temperature and the pressure is 0.15-0.25 MPa. The room temperature is defined as a general environment in a room including a factory building without additionally using temperature adjusting equipment such as an air conditioner, a heater, and water cooling equipment. Preferably, the temperature is 20-40 ℃.

In one or more embodiments of the invention, the space velocity of the raw material gas during oxidation is 800-^-1. Wherein the space velocity is: amount of gas treated per unit volume of oxidant per unit time in m³/(m³Catalyst h) to h)^-1。

In one or more embodiments of the invention, the oxidizing agent comprises potassium permanganate or potassium dichromate.

In one or more embodiments of the invention, the oxidizing agent is supported on the surface of the porous structure of the support.

In one or more embodiments of the present invention, the loading amount of the oxidizing agent on the surface of the support is obtained by spraying an aqueous oxidizing agent solution. The concentration of the oxidant aqueous solution is 0.0001-0.0005 g/ml. When the device works, the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1 (5-10).

In one or more embodiments of the invention, the support is a zeolite or porous alumina or porous magnesia or porous ceramic.

In one or more embodiments of the invention, the support has a specific surface area of 1 to 5m²/g。

In one or more embodiments of the invention, the carrier has a mesh morphology formed by a braid.

In one or more embodiments of the present invention, the woven body has a porous structure having micropores at least on the surface thereof.

Compared with the prior art, octafluorocyclobutane containing fluorocarbon olefin impurities is oxidized into corresponding carboxylic acid, carbon dioxide and hydrogen fluoride through a solid material loaded with an oxidant potassium permanganate, the octafluorocyclobutane serving as alkane does not react with the oxidant, and the reacted gas is deacidified, dehydrated and decarbonized through an adsorbent to obtain the octafluorocyclobutane with the fluorocarbon olefin impurities less than 1 ppm.

Drawings

FIG. 1 is a flow diagram according to an embodiment of the present invention;

FIG. 2 is a partial form schematic of a carrier according to an embodiment of the present invention.

Detailed Description

The following detailed description of the present invention is provided in conjunction with the accompanying drawings, but it should be understood that the scope of the present invention is not limited to the specific embodiments.

Throughout the specification and claims, unless explicitly stated otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element or component but not the exclusion of any other element or component.

As shown in fig. 1 to 2, a method for removing fluorocarbon olefin from octafluorocyclobutane according to a preferred embodiment of the present invention.

Example 1

The method for removing the fluorocarbon olefin from the octafluorocyclobutane comprises the step of oxidizing a feed gas of the octafluorocyclobutane containing the fluorocarbon olefin of tetrafluoroethylene and hexafluoropropylene by an oxidation column at the temperature of 20 ℃ and the pressure of 0.15MPa at the airspeed of 800h^-1Then adsorbing with an adsorbent to perform deacidification, dehydration and decarbonation. Wherein the oxidation column comprises a supported oxygenCarrier of agent potassium permanganate with specific surface area of 1m²A porous ceramic carrier, and the carrier is formed with a passage through which a raw material gas flows in a flow direction of the raw material gas. Wherein the loading of the oxidant on the carrier is realized in the form of oxidant solution spraying, the concentration of the oxidant aqueous solution is 0.0001g/ml, and the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1: 5.

As shown in FIG. 2, the carrier 01 has a mesh form formed by the braid 011 (mesh 010 is formed as a raw material gas flow path), and the braid has a porous structure having micropores at least on the surface thereof, and the maximum diameter of the mesh is 5 mm.

The sample obtained in the embodiment is sampled by 10ml for detecting the purity, the sampling is continuously carried out for 10 times at intervals of 1 hour, and the concentration of the residual fluorocarbon olefin is less than 1 ppm.

Example 2

The method for removing the fluorocarbon olefin from the octafluorocyclobutane comprises the step of oxidizing a feed gas of the octafluorocyclobutane containing the fluorocarbon olefin of tetrafluoroethylene and hexafluoropropylene by an oxidation column at the temperature of 30 ℃ and the pressure of 0.2MPa at the airspeed of 900h^-1Then adsorbing with an adsorbent to perform deacidification, dehydration and decarbonation. Wherein the oxidation column comprises a carrier loaded with an oxidant potassium permanganate and has a specific surface area of 2m²A zeolite per gram, and the carrier is formed with a passage in the flow direction of the raw material gas through which the raw material gas flows. Wherein the loading of the oxidant on the carrier is realized in the form of oxidant solution spraying, the concentration of the oxidant aqueous solution is 0.0002g/ml, and the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1: 6.

The sample obtained in the embodiment is sampled by 10ml for detecting the purity, the sampling is continuously carried out for 10 times at intervals of 1 hour, and the concentration of the residual fluorocarbon olefin is less than 1 ppm.

Example 3

The method for removing the fluorocarbon olefin from the octafluorocyclobutane comprises the step of oxidizing a feed gas of the octafluorocyclobutane containing the fluorocarbon olefin of tetrafluoroethylene and hexafluoropropylene by an oxidation column at the temperature of 40 ℃ and the pressure of 0.25MPa at the space velocity of 1200h^-1Then is adsorbedDeacidifying, dehydrating and decarbonizing by using an additive. Wherein the oxidation column comprises a carrier loaded with an oxidant potassium permanganate and has a specific surface area of 3m²The porous alumina, and the carrier is provided with a passage for the raw material gas to flow through in the flowing direction of the raw material gas. Wherein the loading of the oxidant on the carrier is realized in the form of oxidant solution spraying, the concentration of the oxidant aqueous solution is 0.0003g/ml, and the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1: 7.

As shown in FIG. 2, the carrier 01 has a mesh form formed by the braid 011 (mesh 010 is formed as a raw material gas flow path), and the braid has a porous structure having micropores at least on the surface thereof, and the maximum diameter of the mesh is 3 mm.

The sample obtained in the embodiment is sampled by 10ml for detecting the purity, the sampling is continuously carried out for 10 times at intervals of 1 hour, and the concentration of the residual fluorocarbon olefin is less than 1 ppm.

Example 4

The method for removing the fluorocarbon olefin from the octafluorocyclobutane comprises the step of oxidizing a feed gas of the octafluorocyclobutane containing the fluorocarbon olefin of tetrafluoroethylene and hexafluoropropylene by an oxidation column at the temperature of 25 ℃ and the pressure of 0.23MPa at the space velocity of 1000h^-1Then adsorbing with an adsorbent to perform deacidification, dehydration and decarbonation. Wherein the oxidation column comprises a carrier loaded with an oxidant potassium dichromate, and the specific surface area is 4m²A porous magnesia in a volume of one gram per gram, and the carrier is formed with passages in the flow direction of the raw material gas through which the raw material gas flows. Wherein the loading of the oxidant on the carrier is realized in the form of oxidant solution spraying, the concentration of the oxidant aqueous solution is 0.0004g/ml, and the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1: 9.

The carrier is formed by stacking particles with the particle size of 3-5 mm.

The sample obtained in the embodiment is sampled by 10ml for detecting the purity, the sampling is continuously carried out for 10 times at intervals of 1 hour, and the concentration of the residual fluorocarbon olefin is less than 1 ppm.

Example 5

The method for removing the fluorocarbon olefin from the octafluorocyclobutane comprises the step of removing the fluorocarbon olefin containing fluorocarbon olefinOxidizing octafluorocyclobutane raw material gas with tetrafluoroethylene and hexafluoropropylene as hydrocarbon at 35 deg.c and 0.17MPa in an oxidizing column at 1100 hr^-1Then adsorbing with an adsorbent to perform deacidification, dehydration and decarbonation. Wherein the oxidation column comprises a carrier loaded with an oxidant potassium dichromate, and the specific surface area is 5m²A porous ceramic carrier, and the carrier is formed with a passage through which a raw material gas flows in a flow direction of the raw material gas. Wherein the loading of the oxidant on the carrier is realized in the form of oxidant solution spraying, the concentration of the oxidant aqueous solution is 0.0005g/ml, and the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1: 10.

The carrier is formed by stacking columns of 3-5 cm.

The sample obtained in the embodiment is sampled by 10ml for detecting the purity, the sampling is continuously carried out for 10 times at intervals of 1 hour, and the concentration of the residual fluorocarbon olefin is less than 1 ppm.

Example 6

The method for removing the fluorocarbon olefin from the octafluorocyclobutane comprises the step of oxidizing a feed gas of the octafluorocyclobutane containing the fluorocarbon olefin of tetrafluoroethylene and hexafluoropropylene by an oxidation column at the temperature of 20 ℃ and the pressure of 0.15MPa at the airspeed of 800h^-1Then adsorbing with an adsorbent to perform deacidification, dehydration and decarbonation. Wherein the oxidation column comprises a carrier loaded with an oxidant potassium permanganate and has a specific surface area of 1m²A porous ceramic carrier, and the carrier is formed with a passage through which a raw material gas flows in a flow direction of the raw material gas. Wherein the loading of the oxidant on the carrier is realized in the form of oxidant solution spraying, the concentration of the oxidant aqueous solution is 0.0001g/ml, and the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1: 5.

As shown in fig. 2, the carrier has a mesh form formed of a woven body, and the woven body has a porous structure having micropores at least on the surface thereof and is pretreated.

The pretreatment of the carrier can be to take 4.5kg of newly ground graphite micro powder with the particle size of 10 microns and put into a reactor, then add the carrier with the volume of 2.5 times of the graphite micro powder, sequentially add nitric acid accounting for 0.3 percent of the weight of the graphite, fully mix for 30min, then mix again, add 2 volume times of 2.5 weight percent of ammonia water and 0.1M sodium hydroxide solution, fully stir at 110 ℃, soak for 2 hours, then take out and dry.

The sample obtained in this example is sampled for 10ml each time, the sample is continuously sampled for 10 times at an interval of 1 hour each time, and the average value is taken to obtain a product with purity of more than 99.99999% after purification.

Example 7

The method for removing the fluorocarbon olefin from the octafluorocyclobutane comprises the step of oxidizing a feed gas of the octafluorocyclobutane containing the fluorocarbon olefin of tetrafluoroethylene and hexafluoropropylene by an oxidation column at the temperature of 30 ℃ and the pressure of 0.2MPa at the airspeed of 900h^-1Then adsorbing with an adsorbent to perform deacidification, dehydration and decarbonation. Wherein the oxidation column comprises a carrier loaded with an oxidant potassium permanganate and has a specific surface area of 2m²A zeolite per gram, and the carrier is formed with a passage in the flow direction of the raw material gas through which the raw material gas flows. Wherein the loading of the oxidant on the carrier is realized in the form of oxidant solution spraying, the concentration of the oxidant aqueous solution is 0.0002g/ml, and the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1: 6.

The pretreatment of the carrier can be to take 5kg of newly ground graphite micro powder with the particle size of 1 micron to be placed into a reactor, then add the carrier with the volume of 3 times of the graphite micro powder, sequentially add nitric acid accounting for 0.32 percent of the weight of the graphite, fully mix for 20min, then add 3 volume times of 3 weight percent ammonia water and 0.15M sodium hydroxide solution after mixing, fully stir and soak for 3 hours at 95 ℃, then take out and dry.

The sample obtained in this example is sampled for 10ml each time, the sample is continuously sampled for 10 times at an interval of 1 hour each time, and the average value is taken to obtain a product with purity of more than 99.99999% after purification.

Example 8

The method for removing the fluorocarbon olefin from the octafluorocyclobutane comprises the step of oxidizing a feed gas of the octafluorocyclobutane containing the fluorocarbon olefin of tetrafluoroethylene and hexafluoropropylene by an oxidation column at the temperature of 30 ℃ and the pressure of 0.25MPa at the space velocity of 1200h^-1Then, thenAdsorbing with adsorbent to deacidify, dehydrate and decarbonize. Wherein the oxidation column comprises a carrier loaded with an oxidant potassium permanganate and has a specific surface area of 3m²The porous alumina, and the carrier is provided with a passage for the raw material gas to flow through in the flowing direction of the raw material gas. Wherein the loading of the oxidant on the carrier is realized in the form of oxidant solution spraying, the concentration of the oxidant aqueous solution is 0.0003/ml, and the volume ratio of the spraying amount of the oxidant aqueous solution to the air input amount of the raw material gas is 1: 7.

As shown in FIG. 2, the carrier has a mesh form formed by a woven body, and the woven body has a porous structure at least the surface of which has micropores, the porous structure of the micropores is formed by water drop-shaped protruding tips protruding from the surface of the woven body, the minimum distance between two adjacent water drop-shaped protruding tips is 0.3-0.5mm, and the carrier is pretreated. The projecting tips herein constitute a state of a distribution center of partial agglomerates on the surface, the number of the projecting tips of each agglomerate portion is 6 to 10, and the minimum pitch of the projecting tips of two adjacent drops is 0.3 to 0.5mm for each agglomerate portion, preferably, the pitch of two adjacent agglomerate portions should not be less than 1cm and should not be more than 3cm at the same time in order to reduce the resistance to the gas flow. By the aid of the water drop-shaped tip structure, additional surface area is increased, disturbance and contact are enhanced by gaps and structures among the tip structures, the degree of gas reaction at the interface position can be adjusted by the circulation state in and among the agglomeration part while full reaction is achieved, disturbance and mixing efficiency of gas flow at the interface is enhanced, gas flow stratification is reduced, and the efficiency of gas exchange between the gas flow close to the interface of the interface and the outer layer in the flow is reduced due to the interface state and the stratification phenomenon in the gas flow process, so that the reaction efficiency of the gas is reduced.

The pretreatment of the carrier can be to take 4.75kg of newly ground graphite micro powder with the particle size of 5 microns and put into a reactor, then add the carrier with the volume of 2 times of the graphite micro powder, sequentially add nitric acid accounting for 0.2 percent of the weight of the graphite, fully mix for 10min, then mix again, add 2.5 volume times of 2wt percent ammonia water and 0.2M sodium hydroxide solution, fully stir at 100 ℃, soak for 2.5 hours, then take out and dry.

The sample obtained in the embodiment is subjected to single sampling of 10ml for detecting purity, the sampling is continuously carried out for 10 times at the interval of 1 hour every time, and the product with the purity of more than 99.999995% after purification is obtained by taking the average value.

The verification proves that when the protruding tips are arranged on the surface completely or only a small amount of protruding tips are arranged, the impurity removal is not improved, the two effects are basically consistent, and the purity of the obtained product is about 1 ppm.

Example 9

Opening a steel bottle of octafluorocyclobutane sample containing fluorocarbon olefin such as tetrafluoroethylene and hexafluoropropylene, controlling the flow rate by a flowmeter to be 160ml/min, introducing into an oxidation column filled with a solid material loaded with potassium permanganate, and keeping the airspeed at 800h^-1At normal temperature and 0.2MPa, directly introducing part of gas (after adsorption and impurity removal) after passing through the oxidation column into an analyzer for analyzing and detecting tetrafluoroethylene and hexafluoropropylene fluorocarbon impurities, and collecting the other part of gas into a gas buffer tank; the concentration of the fluorocarbon olefin in the octafluorocyclobutane obtained after passing through the oxidation column is less than 1 ppm.

Example 10

Opening a steel bottle of octafluorocyclobutane sample containing fluorocarbon olefin of octafluoroisobutylene, octafluoro-2-butene and hexafluorocyclobutene, controlling the flow rate by a flowmeter to be 160ml/min, introducing into an oxidation column filled with a solid material loaded with potassium permanganate, and keeping the airspeed at 800h^-1At normal temperature and 0.2MPa, directly introducing a part of gas passing through an oxidation column (after adsorption and impurity removal) into an analyzer for analyzing and detecting the fluorine-carbon impurities of octafluoroisobutylene, octafluoro-2-butene and hexafluoro-cyclobutene, and collecting the other part of gas into a gas buffer tank; the concentration of the fluorocarbon olefin in the octafluorocyclobutane obtained after passing through the oxidation column is less than 1 ppm.

The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and its practical application to enable one skilled in the art to make and use various exemplary embodiments of the invention and various alternatives and modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalents.