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DEGASSING PROCESS AND APPARATUS FOR REMOVAL OF OXYGEN
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to processes involving the removal of oxygen from fluid streams and to apparatuses used for this purpose.
2. Description of the Prior Art
Selective removal of a gas from a fluid stream is a common problem in many areas of chemistry. Examples include separating aliphatically-unsaturated hydrocarbons from mixtures containing the same (e.g. in the preparation of ethylene), recovering helium from natural gas, separating hydrogen from a petroleum cracking product, and the like. Among the methods developed for gas separation are those involving transport of gases through a membrane that retards the passage of some or all of the remaining fluid components. Examples of these processes can be found in the following U.S. Patents: recovery of helium from natural gas, U.S. Pat. No. 3,246,449; recovery of hydrogen from cracked petroleum, U.S. Pat. No. 3,246,450; removal of gas bubbles from analyte streams, U.S. Pat. No. 3,463,615; removing carbon dioxide from blood, U.S. Pat. No. 3,651,616; removing gases from liquid streams, U.S. Pat. No. 3,751,879; and separating olefins from other hydrocarbons, U.S. Pat. No. 4,239,506. Various apparatuses for carrying out these separations are disclosed in these patents and in U.S. Pat. No. 4,336,138, which discloses a permeation separation apparatus.
Typical of these disclosures is U.S. Pat. No. 3,751,879 which indicates that gases which pass through the membrane are removed by a vacuum pump, by venting to the atmosphere, by collection in an evacuated and sealed chamber, or by physical absorption or adsorption. Thus two general methods of gas removal have taken place in the prior am first, removal of gas from the vicinity of the membrane either by a vacuum pump or by passive diffusion into the atmosphere, and second, collection of the gas near the membrane in a vacuum chamber or physical absorbant. Both these methods suffer from disadvantages when applied to oxygen removal from fluid streams, the first requiring an expensive vacuum pump (since passive diffusion into oxygencontaining air is clearly inappropriate) and the second having a limited capacity and being difficult to monitor for loss of absorbing ability.
One area in which oxygen removal from fluid streams is very important is the field of automated luminescence measurement of analytes. Many organic compounds fluoresce or phosphoresce, and these properties are widely used for analysis. Molecules are generally excited by the absorption of ultraviolet radiation to a higher electronic state to produce measurable luminescent emission. Excited molecules rapidly lose excess energy by a variety of nonradiative de-excitation steps to the lowest excited singlet or triplet state, at which point the molecule can return to -the ground state by emission of a photon. Various nonradiative de-excitation processes compete with and often greatly reduce the measurable luminescence. Of these processes, quenching has the most pronounced effects. Quenching is defined as any proces that results in a decrease in the true fluorescence or phosphorescence efficiency of a molecule. Quenching processes divert the absorbed
energy of a molecule into channels other than fluorescence or phosphorescence.
The presence of molecular oxygen contributes significantly to quenching because most organic molecules in
5 an excited state will nonradiative] y deactivate after one or two collisions with molecular oxygen. Quenching is often a serious problem for phosphorescence since the longer lifetimes of the excited state allow more opportunities for collisions to occur. The effect of oxygen
10 quenching on fluorescence is pronounced for solutions of many polynuclear aromatic compounds, but the fluorescence of virtually all organic compounds is quenched, at least slightly, by oxygen. Thus, the presence of oxygen decreases the luminescence efficiency of
15 a sample.
Several methods of deoxygenation are currently used for preparation of fluorescent samples. These methods include nitrogen purging, freeze-thaw techniques, and preparations of samples within a vacuum. However,
20 these methods have varying degrees of effectiveness. Furthermore, they are time-consuming and rather tedious. Thus, sample deoxygenation is usually not carried out for routine fluorometric testing despite the obvious advantages relating to fluorescence efficiency
25 which could be obtained by deoxygenarion. Accordingly, a routine and easily carried out process for the removal of oxygen from a sample being measured in a fluorescence or phosphorescence spectrophotometer is greatly needed as is a method for removing oxygen
30 from fluid streams in general.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a process for removing oxygen from a fluid 35 stream.
It is a further object of this invention to provide an apparatus for carrying out the process described herein.
These and other objects of the invention as will hereinafter become more readily apparent have been accom
40 plished by providing a method for decreasing the oxygen content of a fluid stream, which comprises contacting said fluid with one side of an interface capable of passing oxygen and retarding the passage of said fluid wherein the second side of said interface is in contact
45 with a chemically deoxygenating environment and said contacting takes place in a container permeable to oxygen only through said interface.
The invention further comprises an apparatus capable of decreasing the oxygen content of a fluid stream. The
50 apparatus includes an interface capable of passing oxygen and retarding the passage of said fluid; a first container for maintaining the fluid in contact with a side of the interface, the first container being impermeable to gas except through the interface and having an inlet and
55 outlet through which the fluid is conducted into and out of the first container; and a second container for maintaining a deoxygenating environment in contact with the second side of the interface.
60 BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the invention becomes better understood by reference to the following detailed description 65 when considered in connection with the accompanying drawings, wherein:
FIG. la schematically shows a flow path of liquid through an apparatus of the invention included as part
of a larger apparatus which measures the luminescence of an analyte;
FIG. lb shows a perspective drawing of an actual apparatus of the type shown schematically in FIG. la;
FIG. 2 shows in schematic form the processes which 5 occur at the interface;
FIG. 3 shows a plot of enhancement factor versus degassing time in a static experiment; and
FIG. 4 shows a plot of enhancement factor versus degassing time in an isochronal experiment. 10
DESCRIPTION OF THE PREFERRED
The present invention provides a simple and effective alternative to current methods of sample degassing of 15 fluorescent and phosphorescent samples and is applicable to removing oxygen from fluid streams in general. Generally, the method of the invention comprises contacting a fluid, preferaby a liquid, from which oxygen is to be removed with one side of an interface capable of 20 passing oxygen and retarding the passage of the fluid. This contacting takes place in a container permeable to oxygen only through the interface. The second side of the interface is in contact with a chemically deoxygenating environment. Oxygen passes through the 25 interface and is removed by chemical reaction, thereby greatly reducing the oxygen concentration on the second side of the interface, a process which greatly increases the rate of diffusion through the interface. One useful aspect of this invention is that the effective re- 30 moval of oxygen is independent of the beginning oxygen content of the fluid being deoxygenated, unlike some prior art processes. After oxygen has been removed from the fluid, the fluid is conducted from the container through a gas impermeable conduit and is 35 utilized for whatever purpose is desired. For example, an analyte-containing liquid could be conducted to a gas impermeable fluorescence or phosphorescence chamber where the fluorescence or phosphorescence of the material contained in the liquid would be measured. 40
One key aspect of the present invention is the use of an interface permeable to oxygen which retards the fluid from which oxygen is being removed. The term "interface" is used in this application to include all materials through which this process can occur. Generally, 45 the interface is in a form of a "membrane"; i.e., a thin, pliable layer of natural or synthetic material that is permeable to oxygen. At least rwo general types of membranes are possible. Membranes in which the passage of a gas takes place by dissolution on the face of the 50 membrane which is situated on the side on which the concentration of oxygen is higher, followed by diffusion of oxygen through the membrane and desorption on the second face of the membrane, are known as "permeable" or "semi-permeable" membranes. Oxygen 55 permeable membranes, both flat and tubular, are wellknown. Examples include polytetrafluoroethylene, polysilicone, polypropylene, and polyethylene. These materials are particularly suitable for use with caustic or orherwise chemically reactive chemical deoxygenating 60 reagents. Polyethylene tubing is particularly suitable when oxygen is being removed from an organic solvent since it has a high permeability to oxygen and is compatible with many organic solvents.
In addition to the permeable membranes discussed 65 above, other oxygen passing interfaces, such as porous membranes, are also suitable for the practice of the present invention. A "porous membrane" is one in
which a gas passes through the membrane merely by following the path of the pores and not by diffusion through the material of the membrane as in the case of permeable membranes. Typically, a porous membrane has pores with an apparent diameter of about 0.01-3 microns, preferably 0.05-1.5, microns and having a critical surface tension of less than 40 dynes/cm at 20° C. Suitable materials are described in, for example, Blanchard et al, U.S. Pat. No. 3,651,616, which is herein incorporated by reference. Of course it is also possible to use membranes which operate by both mechanisms or to use two membranes, each of which operates by one of these mechanisms, at the same time.
A second essential feature of the method of the present invention is the use of a chemically deoxygenating environment in contact with the second side of the interface. Generally, this environment will comprise a gas, liquid or solution capable of reacting with gaseous oxygen and converting the oxygen by chemical means to an inert product. Electrochemical removal of oxygen by means of an electrode is also encompassed by this invention. Any chemical system capable of maintaining a partial pressure of oxygen of less than 1 X 10~4 mm of mercury when maintained at room temperature (25° C.) is preferred for use in the practice of this invention.
Typical chemical systems capable of producing rhe desired deoxygenated environment include the following (comments on preparation and use are given where appropriate):
A. Solutions of chromous (Cr2+) ions. A typical solution is made by adding 13.5 g ... to 100 ml H2O (about 0.4 M Cr2+) after which about 25 ml concentrated HC1 is added. CrCh and CrSo4 may also be used, but the latter is not as satisfactory. Amalgamated zinc (about 33 g/100 ml solution) is then added. The zinc is prepared by washing for 30 sec with 3 M HC1, amalgamating with HgCh for 3 min (10 g HgC^/lOO ml H2O), and washing the amalgamated zinc product thoroughly with distilled water. The resulting solution of chromous ions is dark blue initially and turns light blue to green when it is inactivated after reacting with sufficient oxygen.
B. A mixture of 0.5 g lithium aluminum hydride and 10 g benzopinacolone (Ph3CCOPh) in 50 ml pyridine is useful in applications where water vapor is also not desirable. The solution is effective as long as it remains red.
C. A strongly alkaline aqueous solution of pyrogallol (1,2,3-trihydroxybenzene) is effective in removing oxygen rapidly. However, it is difficult to detect visibly when the solution is no longer active.
D. Fieser's solution is prepared by adding 2 g sodium hydrosulfite ... also known as sodium dithionite) to 100 ml water containing 20 g KOH. The solution is kept warm until a blood-red solution is obtained. The solution is used after cooling. The cool solution is effective until the color turns to brown or brown-red or until a precipirate appears. One disadvantage of this chemically deoxygenating solution is that hydrogen sulfide gas may be produced under some circumstances. However, H2S may be removed from the fluid stream by many known methods, for example by contacting the fluid stream with a second interface which is permeable to H2S and which contains a solution of saturated aqueous lead acetate on the second side of the interface.
E. A simplified version of Fieser's solution is a 10% aqueous sodium dithionite solution. However, the solution is unstable and must be used while fresh.
F. Benzophenone ketyl-containing organic solvents are effective for removing oxygen and water vapor from non-aqueous fluids. An organic solvent is predried and any acids present are removed by stirring the warm solvent over NaOH or KOH. The filtered solvent is 5 then refluxed over metallic sodium for at least an hour, after which solid benzophenone is added to give a deep blue-purple color. The ketyl mixture is effective in removing oxygen and water vapor from non-reactive gases until the color fades or turns pale. 10
G. BTS Catalyst, supplied by BASF Co., 866 Third Avenue, New York, NY 10022, is effective in removing oxygen, as well as H2, HiS, CO, COS and vinyl chloride, from both gases and liquids. The catalyst is supplied in the oxidized state and must be reduced at '5 120°-200° C. with H2 or CO before it can be used for removing oxygen.
Other methods of oxygen removal also exist and can be used in the practice of this invention. See, for example, The Manipulation of Air-Sensitive Compounds, by 20 D. F. Shriver (McGraw-Hill, New York, 1969), which is herein incorporated by reference. Although many deoxygenating methods are readily adopted to the present invention, some require complicated techniques and ^ are therefore not preferred. For example, a nitrogen or argon stream on the second side of the interface may be passed through copper filings heated at 500°-600° C. The nitrogen (argon) can then be recycled to pick up additional oxygen. Such a technique, however, requires 3Q elaborate heating, cooling and circulating devices and loses many of the advantages of simplicity afforded by other means of forming a chemically deoxygenating environment, such as those discussed above in detail. Solutions containing chemical species which react with 35 oxygen, such as those discussed above as well as solutions of sodium sulfite ... hydrazine and vanadous sulfate, are preferred deoxygenating environments. Although many of these chemically deoxygenating environments had previously been used to remove 40 oxygen from a gas stream bubbled therethrough, there was no indication prior to the present invention that efficient deoxygenation would take place in a liquid or gaseous fluid stream that was separated from the deoxygenating environment by an interface as described 45 herein.
One particularly useful embodiment in the invention employs Cr2+ ions to consume the oxygen. Thus, as oxygen diffuses into the deoxygenating environment, the oxygen is immediately consumed and reduced to 50 water in the presence of Cr2+ and H+. The net result is that the oxygen concentration on the second side of the interface does not reach a level sufficient to eliminate the concentration gradient that drives difussion toward the second side of the interface. Consequently, oxygen 55 continues to diffuse across the interface until the oxygen content of the fluid is in equilibrium with the oxygen concentration in the deoxygenating environment (i.e., essentially oxygen free).
Although satisfactory deoxygenation can occur in a 60 simple Cr2+ solution until the Cr2+ ions are consumed, a more preferred embodiment uses a second material to reduce the Cr3+ ions, formed by reaction with oxygen, to Cr2+ ions. A particularly preferred embodiment uses amalgamated zinc as the reducing agent. The solution 65 containing the Cr2+ ions is maintained in contact with amalgamated zinc. The zinc effectively reduces the Cr3+ ions to Cr2+ ions which can then recombine with
oxygen. These two reactions are summarized by the following equations:
Zn(Hg) + 2Cr3+^Zn2 + + 2Cr2 + + Hg
4Cr2+ +02+4H30+^4Cr3+ +6H20
Various methods and apparatuses for conducting the deoxygenating process can be used. However, it is essential that the container which maintains the fluid in contact with the interface, as well as the conduit which conducts the fluid from the container to the point of ultimate use, must be oxygen impermeable in order to prevent the diffusion of oxygen back into the fluid after it has been deoxygenated. Typically, the container, conduit, and other non-oxygen-permeable parts of the apparatus walls are constructed of a material, such as glass, metal and non-permeable plastics and polymers, through which oxygen cannot pass from the air.
The shape and construction of the devices used in carrying out this method are not limited other than by the limitations specifically set forth in this application. The combinarion of flow rate, fluid thickness in contact with the interface and contact surface area (the three principal factors which determine the effectiveness of oxygen diffusion out of the fluid being deoxygenated) can easily be selected to accommodate the different effectiveness of the various chemically deoxygenating environments and the amount of oxygen in the fluid being deoxygenated. Slower flow rates, thinner fluid thicknesses and larger surface areas all increase the effectiveness of oxygen diffusion out of the fluid. These factors can be adjusted by simple experimentation by testing the oxygen content of the fluid exiting the contacting chamber. Suitable testing methods include oxygen sensitive electrodes as well as contacting with a small volume of any of the chemical deoxygenating methods discussed above, such as pyrogallate solution, which produces a color change on reacting with oxygen.
In a particularly preferred embodiment of the present invention, the interface through which diffusion occurs is in the form of tubing through which the fluid is conducted. When constructed in this manner, the interface tubing itself forms the first containing means. Tubing is preferred since the fluid stream can readily pass into and out of the tubing from conduits of the same diameter without significant additional mixing, an important factor when analytes carried in a fluid stream, such as in an automated analyzer, are being detected or quantified. The tubing is immersed in a liquid in which the chemically deoxygenating environment is maintained. When a sample to be deoxygenated, such as a liquid containing a fluorescent or phosphorescent material, is conducted into the tubing (first containing means) from a sampling device or other source, oxygen passes through the walls of the tubing into the solution where it reacts with the deoxygenating chemical present in the fluid. The interface tubing then connects with a conduit, also generally a tube, made of a second material which is impervious to oxygen. The deoxygenated fluid is transported through the conduit until it reaches the point at which it is used or analyzed. If the fluid contains a luminescent material for analysis, for example, the fluid is conducted to a gas impermeable luminescence chamber, typically in a fluorescence or phosphorescence spectrophotometer of orherwise standard design.
The method of this invention of removing oxygen from a fluid stream as well as the apparatus for carrying out this method can easily be adapted for use in any of the many processes which would benefit from oxygen removal. For example, many analytical techniques in- 5 eluding chromatography and electrochemistry can be benefited by oxygen removal. Manufacture of oxygen sensitive chemicals, such as in the photographic chemicals industry, could be greatly aided by the present invention, since no contact between the fluid being 10 deoxygenated and the deoxygenating chemical itself is necessary. Air sensitive chemicals could be stored for longer periods of time if the oxygen introduced during the manufacturing process from solvents and the like were eliminated using this method. 15
A particularly preferred embodiment of the present invention involves the removal of oxygen from fluid streams containing analytes, such as in an automated analyzer, which are to be measured by phosphorescence or fluorescence. Existing spectrophtometers can 20 easily be adapted to the practice of the present invention by incorporating a deoxygenating appartus into the conduit that is conducting the fluid being analyzed into the luminescence chamber. No further modifications should be necessary since the luminescent material 25 being measured is not contacted directly with the deoxygenating chemical.
As used herein, a "luminescent" material is one which fluoresces or phosphoresces after its electrons have been raised to a higher electronic state, whether this 30 occurs because of electromagnetic or chemical energy being absorbed by the luminescent material. Typically, electromagnetic energy, usually in the ultraviolet range, is used to generate the excited state. The fluorescence or phosphorescence step is not itself part of the present 35 invention, which is related to deoxygenation of the sample containing the luminescent material prior to the excitation and luminescence events.
The method of the invention can be applied to any method which involves the measurement of lumines- 40 cence. Examples include direct analysis for the presence of a fluorescent substance and the use of fluorescent molecules to label antibodies which are specific for a particular analyte. Other examples include improved detection in liquid chromatography and electrochemi- 45 cal measurements by deoxygenation. Preferred methods involving the measurement of luminescence which can be used with the present deoxygenating method include measurement of room temperature phosphorescence by micellular enhancement as well as automated fluores- 50 cence and phosphorescence measurements.
A more complete understanding of the invention is afforded by reference to the accompanying drawings in connection with the following description. FIG. 1 shows a schematic diagram of an apparatus of the inven- 55 tion and illustrates a method of practicing the invention in tne context of a complete system for analysis of a sample to be deoxygenated. Referring to FIG. 1, sampling container 1 takes in a sample to be analyzed and passes the sample through conducting conduit 2 under 60 the influence of pump 3. Although the pump is shown at a given point in FIG. 1, those skilled in the art will readily recognize that the pump may be used at any point in the fluid stream. Neither conducting conduit 2 which leads from sampling container 1 to pump 3 nor 65 conducting conduit 4 which leads from pump 3 to the deoxygenating apparatus 5 needs to be gas impermeable although gas impermeability for these conducting ele
ments is preferred. Deoxygenating apparatus 5 by itself contains the essential parts of the apparatus of the invention.
In the deoxygenating portion of the apparatus, the sample passes into a first container 6 where it contacts interface 7. In the embodiment shown in FIG. lb, container 6 and interface 7 are the same and consist of a tubing made of oxygen permeable material, and the pump 3 is a peristaltic pump. Nitrogen is shown being pumped into deoxygenating apparatus 5 in order to exclude air therefrom. However, nitrogen is not required if apparatus 5 is airtight. The container 6 is immersed within the liquid containing area of deoxygenating environment container 8. Container 8 contains deoxygenating environment 9 which in FIG. lb comprises a solution containing a chemical capable of reacting with oxygen and removing oxygen from solution 9. The continued pumping action of pump 3 forces liquid through container 6, thereby degassing the sample. The sample then passes to conducting conduit 10, which is impermeable to oxygen, and is transported through this conducting conduit to the fluorescent or phosphorescent chamber 11 of a fluorescence or phosphorescence spectrophotometer 12. After the fluorescence or phosphorescence is measured in chamber 11, the sample passes out of spectrophotometer 12 through conducting conduit 13 for disposal or recovery at a disposal or recovery station 14, which is flask 14 in FIG. lb.
FIG. 2 shows an enlarged view of a portion of the deoxygenating apparatus 5 showing only the first container 6 in the form of a tube and deoxygenating solution 9 in order to illustrate the processes occurring at the interface. The fluid being degassed is passed through container/interface 6-7. During this passage, oxygen present in the sample solution migrates through interface 7 by passive diffusion. When the oxygen enters solution 9, it reacts with a deoxygenating chemical present in the solution as illustrated by the reaction with Cr2+ and H+ shown in FIG. 2. This reaction prevents oxygen from building up in the deoxygenating solution and thereby prevents the buildup of any partial pressure due to oxygen in the solution on the deoxygenating side of the interface.
As is clear from the description given above, the shape of the interface is not essential. For example, the interface may consist of tubing which passes through the liquid containing area of liquid container such as is illustrated in FIG. lb. However, other types of interfaces and containing means are also possible. For example, multiple parallel tubes, such as are commonly found in kidney dialysis machines, may be used in place of the single convulated tubing shown in FIG. 1. It is also possible to use flat membranes, for example, a single flat membrane which divides a chamber into two compartments. The first compartment, which should be relatively thin in order to allow easy diffusion of oxygen out of the sample, contains the sample to be deoxygenated. The second compartment contains the deoxygenating solution. Multiple flat interfaces arranged so that alternating layers of sample and deoxygenating solution flow past each other are also suitable as are any other variations which produce a deoxygenating interface as described herein. For example, any of the vacuum degassing apparatuses shown in the U.S. Patents cited in the section of this application entitled Description of the Prior Art (which are herein incorporated by reference) can be easily adapted to produce an apparatus of this invention, now that the invention has been described.