US3927978A - Electrolytic stripping cell and method - Google Patents

Abstract

A cell useful, for example, in analyzing sea water for the presence of dissolved hydrocarbons, electrolytically generates hydrogen gas bubbles which strip hydrocarbons from the sea water. The hydrocarbon concentration in the head space is analyzed by gas chromatography, and knowing the volume of the sea water and the amount of hydrogen generated, the hydrocarbon concentration in the sea water and the partition coefficient may be calculated.

Description

United States Patent 91 Wasik 1 Dec. 23, 1975 1 ELECTROLYTIC STRIPPING CELL AND METHOD [75] Inventor: Stanley P. Wasik, Rockville,

[73] Assignee: The United States of America as represented by the Secretary of Commerce, Washington, DC.

[22] Filed: Nov. 20, 1974 [21] Appl. No.: 525,430

3,224,837 12/1965 Moyat 23/230 R 3,401,099 9/1968 McEvoy..... 204/129 3,582,475 6/1971 Prestorius et a1. 204/1 T 3,725,236 4/1973 Johnson Jr. 204/195 R 3,836,449 Lovelock 204/129 X Primary ExaminerRobert M. Reese Attorney, Agent, or Firm-David Robbins; Alvin Englert [57] ABSTRACT A cell useful, for example, in analyzing sea water for the presence of dissolved hydrocarbons, electrolytically "generates hydrogen gas bubbles which strip bydrocarbons from the sea water. The hydrocarbon concentration in the head space is analyzed by gas chromatography, and knowing the volume of the sea water and the amount of hydrogen generated, the hydrocarbon concentration in the sea water and the partition coefficient may be calculated.

7 Claims, 1 Drawing Figure TO GAS CHROMATOGRAPHY [5 6] References Cited UNITED STATES PATENTS 3,109,788 11/1963 Miller et a1. 204/129 X CONSTANT CURRENT sounce 29 U.S. Patent Dec. 23, 1975 3,927,978

\To GAS CHROMATOGRAPHYJ CONSTANT CURRENT 3 SOURCE 29 ELECTROLYTIC STRIPPING CELL AND METHOD BACKGROUND OF THE INVENTION The present invention relates to an analytical apparatus and method and more particularlyto apparatus and a corresponding method for determining the amount of hydrocarbons in various aqueous media.

Although not limited thereto, the invention has particular utility in analyzing for the amount of hydrocarbons dissolved in a body of water, for example, sea water. The invention is also applicable to hydrocarbons in other aqueous media such as fresh water, subsurface brines, and biological fluids.

The measurement of widely varying concentrations of hydrocarbons dissolved in water is useful in many fields such as pollution control, petroleum exploration, and biochemical research.

Such aqueous solutions can be characterized by determining the concentration of a particular hydrocarbon in both the solution and in the vapor above the solution. The ratio of its concentration in the two phases is an equilibrium constant called the partition coefficient. The solubility of a hydrocarbon is the maximum concentration it can have at equilibrium in the solvent at a given temperature and can be determined from the values of the partition coefficient and the vapor pressure of the pure liquid hydrocarbon at that temperature.

Various approaches to the measurement of hydrocarbon solubilities in water are known in the art. One known procedure is referred to as head space analysis which requires analysis of only the vapor phase in equilibrium with the liquid phase. One such method described by McAuliffe in"Chemical Technology, Vol. I, page 46, January 1971, employs successive gas chromatographic analyses after successive phase equilibriums are achieved by passing helium through an aqueous sample containing the dissolved hydrocarbons. Another process employing head gas analysis is described by Wasik, S. and Brown, R. L., Proc. Joint Conference, Prevention and Control of Oil Spills, p. 223, March 13-15, 1973, Washington, DC. This process, which also employs helium as the stripping gas, involves after initially making an analysis, discarding a known portion of the gas phase, replacing it with pure helium, and making further analysis.

It isrecognized that there is a need for further improvements in analyzing the amount of hydrocarbons dissolved in aqueous fluids. In particular the increasing concern over pollution of sea water by petroleum oil and products has generated a need for improved methods for analyzing sea water for dissolved hydrocarbons. The complexity of crude oil and oil products, and the low solubility of hydrocarbons makes the analysis of hydrocarbons in sea water difficult.

SUMMARY OF THE INVENTION It is a primary object of the present invention to provide a novel method and apparatus for measuring concentrations of hydrocarbons dissolved in aqueous fluids. Another principal object of the invention is to provide a continuous process for producing a head space vapor phase which may be analyzed-to permit determination of the presence and amount of hydrocarbons in the gaseous phase and in the liquid phase. Still another object of the invention is to generate bubbles of a stripping gas in situ by electrolysis to facilitate the Briefly, the present invention utilizes an electrolytic,

stripping cell which contains a known amount of the hydrocarbon containing liquid phase, for example, hydrocarbons dissolved insea water. One electrode is disposed at the bottom of the. cell to generate small bubbles of hydrogen so that dissolved hydrocarbons are equilibrated with the hydrogen bubbles rising into the head space above the liquid phase. The second electrode is disposed externally of the liquid phase in another electrolyte and is separated therefrom in a manner to-prevent fluid intermingling while allowing electric current flow. Preferably a constant .current device provides the current to the cell so that the amount of hydrogen generated in situ can be determined.

The gas phase in the head space is connected to a gas sampling valve so that the hydrocarbon concentration in the head space may be analyzed by gas chromatography. Knowing the amount of hydrogen bubbled through the stripping cell, the hydrocarbon concentration in the'sea water may be determined from the volume of the liquid phase and the hydrocarbon concentration in the head space. The partition coefficient is the ratio of the concentrations in the two phases.

The prior art methods referred to previously are limited to employing a small volume of the liquid phase because of the difficulty of equilibrating the small volume of gas with a large volume of liquid. In the analysis of hydrocarbonsin sea water in the parts per billion (ppb) level it is desirable to extract the hydrocarbons from large volumes of sea water in order to achieve the necessary sensitivity. The present process which is a continuous process has the further advantage that hydrocarbons can be extracted from large volumes of sea water by increasing the size of the cell and the amount of liquid phase.

Other advantages of the present invention are that no impurities are added to the liquid phase by the extracting solvent (hydrogen); and the analysis may be made automatic since valves controlling the various operations can be selectively energized by a suitable timer.

The present invention is particularly useful in making analyses of aromatic hydrocarbons where aliphatic hydrocarbons are also present. It has been found that aliphatic hydrocarbons including olefins are eluted more rapidly than aromatic hydrocarbons leaving behind a large fraction of the aromatic hydrocarbons in solution, thereby simplifying the analysis.

The above and other objects, features and advantages of the invention will become more apparent as this description proceeds.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is an elevational view of an electrolytic stripping cell in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION An electrolytic stripping cell generally designated by reference numeral 10 has two compartments, a large compartment within a vertical cylinder 12, and a smaller compartment within a branch tube 14 which is connected to the side of cylinder 12. In the end of a horizontal extension 16 of tube 14 there is a coarse porosity glass frit plug 18. A silicic acid plug 20 is disposed on the side of frit plug 18 facing the smaller compartment.. These plugs prevent the flow of liquid between compartments while providing only a low resistance to the flow of electrical current between the compartments.

A pair of gold electrodes 22, 24 are disposed in cylinder l2 and tube 14 respectively and are held in place by means of a septum located in fittings 26 and 28 respectively. Preferably electrode 22 has a spiral configuration to provide a larger surface area.

The measured amount of liquid sample to be analyzed, for example, sea water containing hydrocarbons, is placed in the large compartment. The smaller compartment contains a suitable electrolyte such as dilute sulfuric acid. In practice, the cell is maintained at a constant temperature by placing it in a water bath maintained at, say, 20 C plus or minus 0.05 C. A constant current is supplied to the cell by any suitable source 29. The total volume, V, of hydrogen evolved is then given by V it/F.22,400/2.T,/273.l where i is the constant current in amperes, t is the time that the current flowed in seconds, F is the Faraday constant and T, the temperature of the cell in C.

The hydrogen gas flows upwardly from cathode 22 in the form of minute bubbles stripping out a fraction of the hydrocarbons. The hydrogen and the stripped hydrocarbons enter the head space 30 in the upper part of the cell. From head space 30 the gaseous phase is passed through fitting 32 in the top of the cell to a gas sampling valve (not shown) and'to a gas chromatographic unit. As will be appreciated by those skilled in the art, the gas sampling valve and tubing leading from the cell to the valve may be held at a desired temperature, for example, 100 C by a heating tape or the like. Also, while not illustrated, it will be understood that the chromatographic column may be of known design. For example, it might be a /4 inch by 12 foot length of stainless steel tubing packed with 6080 mesh glass beads that had been coated with 1% w/w silver nitrite. A hydrogen flame detector may be employed to monitor the effluent from the'column, and an electronic integrator may be used to measure peak areas.

Oxygen is generated at the anode 24 in the smaller chamber, and the oxygen bubbles upwardly through the electrolyte and is then exhausted through the reduced diameter tube 29 to the atmosphere.

While not wishing to be bound thereby, it is presently believed that the following is thetheoretical basis for the present invention. Assuming that the concentration of a particular hydrocarbon in the hydrogen leaving the cell is proportional to its concentration in the sea water at that moment, then C (V) S,cv)K where C,(V) is the hydrocarbon concentration in the hydrogen, S,( V) is its concentration in the sea water, and UK, is the proportionality factor. If there was equilibrium, K would be equal to the partition coefficient. If V is the volume of hydrogen which has passed through the sea water, the ratio of C,(V) to its value, c,(O), at zero hydrogen volume is given by the expression IVLKI where V is the volume of the sea water. A plot of lnC,(V) vs. V/ V,, or V should be linear if K is constant, and the slope will give its value.

Equilibrium of the hydrocarbons in the seawater with the hydrogen bubbles depends on the size of the bubbles. By making the electrode surface area large, e.g. 12 cm, very small bubbles 0.l mm in diameter) are evolved at the gold electrode. Under these conditions equilibrium would be expected and the measured K should be the partition coefficient.

The rising bubble technique provides a convenient means for determining partition coefficients for slightly soluble compounds in aqueous salt solutions. In particular, the method is capable of measuring partition coefficients of compounds with very low Ks 0.l Prior to the present invention there was no good method for these measurements.

The invention has been found quite useful in the analysis of aromatic hydrocarbons in aqueous medium. The chromatographic analysis of aromatic hydrocarbons in a sea water extract of petroleum or petroleum products is complicated by the large number of different aliphatic hydrocarbons that are eluted along with the aromatic hydrocarbons. The usual procedure is to separate the hydrocarbons into aromatic and aliphatic hydrocarbon groups by liquid chromatography. Each group is then analyzed separately by gas chromatography. This is a lengthy procedure and invariably some loss of hydrocarbons takes place due to irreversible adsorption and evaporation. Use of the present stripping cell to extract and separate the hydrocarbons into groups avoids these pitfalls.

From equation I one obtains:

where S,(O) is the concentration of a particular hydrocarbon in the original sea water and S (V) is its concentration after a volume V of hydrogen has bubbled through the cell. The fraction of the total amount of a particular hydrocarbon, S,(V)/S,(O), extracted for a given V/V depends upon the magnitude of K In the case of gasoline dissolved in sea water at 20C it was observed that of the aliphatic (including olefinic) hydrocarbons and only 0.04% of the aromatic hydrocarbons are extracted when V/V is equal to 0.046. Thus the aliphatic hydrocarbons are quickly eluted for low values of V/V with a large fraction of the aromatic hydrocarbons remaining in solution. After the aliphatic hydrocarbons are eluted the concentration of the different aromatic hydrocarbons are measured for different V values. From the lnC,(V) vs. V/ V,, plots the slopes UK, and intercepts lnC,(O) are obtained. The hydrocarbon concentrations in the original sea water are calculated from the expression S (0) K,C,(O). A chromatogram taken during the initial elution in which the aliphatic hydrocarbons predominate would be complicated by the large number of aliphatic hydrocarbon peaks eluted at the same time as the aromatic hydrocarbon peaks. However, a subsequent chromatogram taken thereafter would be less complex and the peaks corresponding to the aromatic hydrocarbons would be readily identifible.

In the analyses described above the hydrocarbon concentrations in the sea water extract were in the ppm range. The volume of head-space sampled for chromatographic analysis was one ml. For the analyses of hydrocarbons in the ppb range a much larger headspace volume should be sampled and V should be made larger in order to obtain the necessary sensitivity. The hydrocarbons in the sample volume should then be concentrated by some suitable means before being injected into the analytical column for analysis. This may be accomplished by solvent extraction or column concentration.

The analyses of hydrocarbons in sea water using larger sample volumes does not invalidate the method dV 6. (1 AT V+AVI2 c. (V) S (4) Substituting C,(V) into equation 4 and integrating Substituting C,(V) into equation 5 and taking the logarithm of both sides From the slopes, UK, and intercept, (3(0), of the lnC (V) vs. V/V plots the hydrocarbon concentration in the original sea water may be obtained from the expression 8 (0) C|(O)K| x/sinh x For aromatic hydrocarbons, the correction factor, x/sinh x, is small for large values of AV due to the relatively large value of K At C K is approximately equal to 4. For the aliphatic hydrocarbons where K is approximately equal to 0.01 the correction factor may be large.

In order to obtain the necessary sensitivity at the ppb range there are experimental parameters that must be considered. Substituting 8 (0) into equation 7 and multiplying by AV gives The quantity AVEXV) is the number of moles of a particular hydrocarbon in the sample volume AV. This quantity may be increased by decreasing K and increasing V and AV. The quantity K may be made smaller by increasing the temperature of the stripping cell. The temperature dependency of K for the light aromatic hydrocarbons is approximately 4%/C. Lowering K increases the pre-exponential term and lessens the exponential term. Increasing V counteracts the effect of lowering K in the exponential term.

In the analysis of aromatic hydrocarbons in a sea water extract of gasoline described above, V and AV were large enough to insure the desired sensitivity since the concentration of the hydrocarbons, 8 (0), were at the ppm level. The same sensitivity, AVC,(V), could be obtained at the ppb level by increasing V to 2800 ml, AV to 250 ml, and raising the stripping cell temperature to 60C (K E 0.8).

A presently preferred embodiment of the invention has been described with particularity. Since various changes may be made without departing from the scope of the invention herein, it is intended that all matter contained in the description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense. It is intended to encompass all changes and modifications that fall within the scope and spirit of the claims.

I claim:

1. A method for stripping hydrocarbons from an aqueous liquid phase, comprising passing hydrogen bubbles upwardly through the liquid phase, collecting the hydrogen and stripped hydrocarbons in a head space region above the liquid phase, and analyzing a portion of the gas phase to determine the hydrocarbon concentration therein.

2. A method according to claim 1, further comprising generating the hydrogen gas electrolytically within said liquid phase.

3. A method according to claim 1, wherein said liquid phase is sea water.

4. A method according to claim 1, further comprising analyzing a sample of the gas phase by gas chromatography.

5. The method according to claim 1, further comprising determining the concentration of a particular hydrocarbon in said liquid as well as in said gas phase, and from these concentrations determining the partition coefficient.

6. An electrolytic stripping cell comprising a first compartment for reception of an, aqueous hydrocarbon containing liquid phase, a second compartment, means for separating said first and second compartments to prevent liquid flow while permitting the flow of electricity therebetween, an electrolyte distinct from said liquid phase disposed within said second compartment, first electrode means disposed in said first compartment to upon energization generate hydrogen bubbles within said liquid phase, second electrode means disposed in said second compartment to upon energization generate oxygen within said electrolyte, means defining an upper portion of, said first compartment and constituting head space region, for receiving hydrogen and hydrocarbons stripped from said liquid phase by the generated hydrogen whereby a gas phase is established within said head space region, and means including chromatographic means for analyzing a portion of said gas phase.

7. Apparatus according to claim 6, wherein said means for separating said first and second compartments comprises a layer of a porous glass frit and a silicic acid plug on the side facing said second compartment.

Claims (7)

1. A METHOD FOR STRIPPING HYDROCARBONS FROM AN AQUEOUS LIQUID PHASE, COMPRISING PASSING HYDROGEN BUBBLES UPWARDLY THROUGH THE LIQUID PHASE, COLLECTING THE HYDROGEN AND STRIPPED HYDROCARBONS IN A HEAD SPACE REGION ABOVE THE LIQUID PHASE,

2. A method according to claim 1, further comprising generating the hydrogen gas electrolytically within said liquid phase.

3. A method according to claim 1, wherein said liquid phase is sea water.

4. A method according to claim 1, further comprising analyzing a sample of the gas phase by gas chromatography.

5. The method according to claim 1, fuRther comprising determining the concentration of a particular hydrocarbon in said liquid as well as in said gas phase, and from these concentrations determining the partition coefficient.

6. An electrolytic stripping cell comprising a first compartment for reception of an aqueous hydrocarbon containing liquid phase, a second compartment, means for separating said first and second compartments to prevent liquid flow while permitting the flow of electricity therebetween, an electrolyte distinct from said liquid phase disposed within said second compartment, first electrode means disposed in said first compartment to upon energization generate hydrogen bubbles within said liquid phase, second electrode means disposed in said second compartment to upon energization generate oxygen within said electrolyte, means defining an upper portion of said first compartment and constituting head space region for receiving hydrogen and hydrocarbons stripped from said liquid phase by the generated hydrogen whereby a gas phase is established within said head space region, and means including chromatographic means for analyzing a portion of said gas phase.

7. Apparatus according to claim 6, wherein said means for separating said first and second compartments comprises a layer of a porous glass frit and a silicic acid plug on the side facing said second compartment.

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