Determination of light Hydrocarbons in Water Carbon Dioxide Stripping Method L. A. WEBBER
AND
C. E. BURKS, Phillips Chemical Co., Borger, Tex.
In plants processing light hydrocarbons it is desirable to evaluate hydrocarbon losses in process cooling water and steam condensate. The conventional analytical procedures for oil in water fail to recover the light hydrocarbon fractions. A n accurate carbon dioxide stripping method has been developedfor determining butadiene in water, and may be used also for other light hydrocarbons not recovered by the conventional procedures. The procedure consists of stripping the water sample with gaseous carbon dioxide and collecting the hydrocarbon vapor in a buret over concentrated potassium hydroxide solution. The composition of the gaseous hydrocarbons may be established by Orsat, fractional distillation, or spectrophotometry.
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N T H E manufacture of butadiene from butane at the Plains
butadiene plant near Borger, Tex., it has been desirable to evaluate the process losses of butadiene and other Cahydrocarbons resulting from cooling by direct water quenching, and t o determine the quantity and composition of light hydrocarbons in the condensate returned aa boiler feed water. The butadiene losses are important not only because of the greater value of butadiene, but because of the greater solubility of butadiene in water than the more saturated C, hydrocarbons. A review of the literature fails to reveal any satisfactory method of determining Cq and lighter hydrocarbons in water. There arc several methods for determining higher boiling hydrocarbons in water, developed in conjunction n-ith studies of oils in boiler feed water and later adapted to the petroIeum industry program of stream pollution prevention. In the standard method the mater sample is evaporated to dryness, the residue taken up with petroleum ether, and the solvent evaporated in a tared flask. With the wet extraction methods the water sample is shaken in a separatory funnel with chloroform, ether, or benzene, and the solvent phase is evaporated to dryness in a weighed flask. The No11 method uses a ferric hydroxide precipitate to absorb the oil from the sample and the oil is extracted from the dried floc. Serious evaporation losses prevent these methods from being of value for hydrocarbons more volatile than lubricating oil Musante ( 2 ) appreciably improves the arcuracy of the wet extraction methods by substituting the use of distillation for evaporation in separating the oil extract from the solvent. While Musante's modification extends the use of wet extraction methods to hydrocarbons boiling as low as 225" F., the hydrocarbon recovery decreases as the boiling range of the hydrocarbon approaches the boiling point of the solvent. Xone of these methods is of value for light hydrocarbons and gases boiling below the solvent. A carbide dioxide stripping method which is reliable for determining Cc and lighter hydrocarbons in water consists of desorbing the hydrocarbons from the water by passing a continuous stream of pure carbon dioxide through the sample, and passing the gas stream through an absorber and buret filled with potassium hydroxide solution. The carbon dioxide is removed by absorption in the potassium hydroxide solution. The volume of light hydrocarbons is measured in the buret as a vapor. If an analysis is desired, the composition can be established by Orsat, fractional d i s tillation, and/or spectrophotometry. The principle of the method is the same as that used by Hinckley (1, 3 ) for determining the purity of polymerization grade butadiene, and adapted by Webber ( 4 )for measuring residual C4's in furfural and absorption oil. APPARATUS
The apparatus consists of a carbon dioxide generator, sample stripper, carbon dioxide absorber, and gas buret. The assembled apparatus is shown in Figure 1.
Carbon Dioxide Generator. Pure carbon dioxide is supplied by a 1-liter filtering flask filled with crushed dry ice. A glass tubing outlet from the flask is immersed in a mercury-filled tube, which serves as a means of controlling the pressure necessary to force the carbon dioxide through the apparatus. A water layer prevents mercury from being blown out of the pressure regulator. The rubber stopper of the generator is v-ired or chained to the filtering flask. A more permanent supply of carbon dioxide may be obtained from commercial liquid cylinders or by packing a pressure cylinder with dry ice ( 1 , 3 ) . Sample Stripper. The sample stripper, which is made from a filtering tube with a medium porosity sintered-glass disk, is fitted at the top Rith the male member of a standard-taper joint. The female member of the joint is provided with a stopcock, graduated filling tube, and outlet tube. Carbon dioxide enters through the bottom of the stripper and leaves at the top. A sample stripper requiring less glass fabrication can be made from a 2000-ml. graduated cylinder and a medium porosity aerator stone. A three-hole stopper in the top of the graduated
SAMPLE STRIPPER CAPACl TY 2000 ML
COS ABSOReKR map( ~ O M L
Figure 1. Apparatus for Determining Light Hydrocarbons in Water
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2
1087
cylinder carries a carbon dioxide inlet tube connected to the aerator stone at the bottom of the cylinder, the filling tube, and the outlet connection to the absorber. Carbon Dioxide Absorber. The construction details of the absorber are shown in Figure 1. The absorber is filled with potassium hydroxide solution, and is identical to the one used in the Koppers-Hinckley method ( 1 , 3 ) for the determination of butadiene purity. Equipment of this type is stocked by several laboratory supply houses. Gas Buret. The gas buret is water-jacketed and graduated in 0.1 nil. at the top. REAGENTS
Potassium Hydroxide Solution. Prepare a solution of approximately 46% by weight of potassium hydroxide. The potassium hydroxide solution may be used for approximately 2 hours of continuous stripping. Carbon Dioxide. The carbon dioxide should be of high purity. The criterion of adequate carbon dioxide purity, as well as of complete removal of air from the apparatus, is that, in 15 minutes of carbon dioxide flow through the apparatus, not more than 0.1 ml. of gas be collected in the buret.
Table I. Hydrocarbon Butadiene Butadiene Butadiene Butadiene Butadiene Methane
Synthetic Sample Analyses of Light Hydrocarbons i n Water Present, P.P.M. 582 640 571 640 27 19
Found, P.P.M. 561 622 554
622 26
18
Recovery. % 96 97 97 97 93 95
PROCEDURE
Lubricate the tapered joint of the sample stripper and all stopcocks of the apparatus with a water-insoluble lubricant. Silicone grease is ideal for the pur ose. Fasten the two members of the tapered joint securely withielical springs or strong rubber bands. Introduce sufficient mercury through the absorber stopcock to counterbalance the head of potassium hydroxide solution to be added to the absorber and buret. The mercury leg acts as a seal to prevent the potassium hydroxide solution from reaching the absorber stopcock. The absorber and buret are filled with potassium hydroxide solution by leaving the buret open as the reagent is added through the leveling bulb. Pass the stream of carbon dioxide through the sample stripper and out the absorber stopcock to the air. To save time in flushing, increase the flow from the generator. Flush the sample inlet line with carbon dioxide by opening the stripper stopcock. Do not allow the carbon dioxide to pass into the sample stripper xith the outlet stopcocks closed, because the pressure may be sufficient to part the tapered joint. After the purging of the apparatus is complete, pour the water
sample into the graduated filling tube of the sample strip er Stop the carbon dioxide flow and open the absorber stopcocg to the air. Immediately introduce a sample of the desired size into the scrubber through the stripper stopcock. A practical volume of water sample is 1000 ml., but a greater volume may be desired for increased accuracy and the recovery of sufficient hydrocarbon for further analysis. After closing the sample inlet stopcock following the introduction of water sample, turn the absorber stopcock to connect the stripper to the absorber. Start the flow of carbon dioxide and continue until all the hydrocarbon has been stripped from the sample. The removal of all hydrocarbon may be considered complete when successive readings a t 15-minute intervals show not more than 0.1-ml. increase in the gas volume. When the stripping is complete, stop the flow of carbon dioxide. adjust the leveling bulb, and determine the volume of gas collected in the buret. In calculating the weight of light hydrocarbon present, make appropriate corrections for water vapor and the impurity from the carbon dioxide. Normally the probable composition of the gas collected will be known, and the conversion from gas volume to weight can be made. When this is not the case, sufficient gas may be collected in a freeze-down bulb or bomb through the gas buret stopcock to establish its composition by Orsat, fractional distillation, spectrometry, or a specific gravity measurement. The results are reported in parts per million from the following equation : Light hydrocarbon (in p,p.m.) =
weight of light hydrocarbon X 1 0 6 weight of sample
RESULTS AND DJSCUSSION
Table I shoirs analytical data for synthetic samples of butadiene in water and methane in water. The data indicate good recovery of butadiene and methane. An accuracy approaching 1 p.p.m. is indicated a t low concentrations of hydrocarbon gases. While the work of this laboratory has been confined to C, and lighter hydrocarbons, it is believed the method could be modified to determine heavier hydrocarbons in water. It would be necessary to measure both the vapor and liquid phase of the separated hydrocarbon in the gas buret, and it might be desirable to heat the sample to speed the stripping of the heavier hydrocarbons. LITERATURE CITED (1) Am.
doc. Testing Materials, “Standards on Petroleum Products and Lubricants,” Part 5, D 973-481‘, 1949. (2) hlusante, A. F. S., ANAL.CHEM.,2 3 , 1 3 7 4 (1961). ( 3 ) Office of Rubber Reserve, Reconstruction Finance Corp., Butadiene Analytical Committee, “Butadiene Laboratory Manual,” Method L.M. 2 . 1 . 1 . 1 0 , 1946. (4) Webber, L. A., Proc. Am. Petroleum Inst., 111,28th Annual Meetr ing, 28, 19 (1948). RECEIVED for review March 17, 1952.
Accepted June 5 , 1952.
A Differential Refractometer for Process Control E. C. MILLER, F. W. CRAWFORD, AND B. J. SIMMONS Research Division, Phillips Petroleum Co., Bartlesville, Okla.
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N MODERN petroleum plants many processes require continuous analytical data for the proper adjustment of the process controls. The rapid response of continuous process systems to operating variables, the large throughput capacities of these systems, the ever-increasing product quality specifications, and the economy of operating a t a uniform quality level require that these analytical data be applied to process control with minimum time lag. These requirements are being met by automatic analyzer-controllers. In any particular application satisfactory control depends not only upon the sensitivity of the analyzer as compared to allowable fluctuations of the measured variable but also upon the reliability
and safety of the device under the conditions prevailing at the installation site. Several continuously recording refractometers have been described in the literature. Barstow ( 1 ) has described a recording refractometer of the dipping type, which is now commercially available. Several recording refractometers (2, 3) are based upon the variation of reflectivity with index of refraction at a glassliquid interface. These devices all compare the index of refraction of a liquid to that of glass. Thomas ( 4 ) and Zaukelies ( 5 ) have described laboratory-type continuously recording differential refractometers which compare the index of refraction of a fluid to that of a standard fluid without a direct comparison to glaes. To
