Determination of Trace Quantities of Water in Hydrocarbons Application of the Calcium Carbide-Gas Chromatographic Method to Streams Containing Methanol ALAN GOLDUP and M. 1. WESTAWAY The British Petroleum Company Itd., BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex, England NITROCE N CYL I N DER
b The calcium carbide-gas chromatographic method introduced b y Knight and W eiss for determining trace quantities of water in hydrocarbons has been investigated. Optimum conditions for converting the water to acetylene in a gas phase reactor have been determined and enabled equilibration times of the order of one hour to be achieved. The temperature of operation of the bed has been found to be critical, low acetylene values being obtained at temperatures above 40" C. Methanol has been shown not to interfere even when present at concentration levels as high as 3.5 parts per thousand. The technique has been applied to determine the water content of propane and n-butane at -45" and - 6.5" C., respectively, equilibrated in the liquid phase with a range of aqueous methanol solutions. Standard deviations for consecutive deterrninations were 10 per cent at 2 v.p.m. and 1 per cent at 250 v.p.m.
D
URING
A
RECEST
INVESTIGATION
of the water contents of liquefied petroleum gases equilibrated with aqueous methanol, a method was required for measuring trace quantities of water in the presence of up to 3500 v.p.m. (parts per million by volume) of this alcohol. Known methods for trace water determination are either inapplicable in the presence of alcohols or too insensitive. It was therefore necessary to develop a suitable analytical technique. The calcium carbide-gas chromatographic method introduced by Knight and Weiss ( 8 ) seemed to offer the most promising approach since the authors claim "traces of alcohol react slowly or not a t all and do not interfere." Unfortunately the authors give no indication of the acceptable alcohol concentrations but preliminary investigations in our laboratories showed that methanol concentrations as high as a few parts per thousand could in fact be tolerated. The analytical technique was developed with two objects in mind. First
t SA SIEVE 50 paig ( I f t x 2 i n ID)
GAUGE
DUPLICATE SYSTEM CAP1LLARY RESTRICTION
RE ORDER
FLAME
AMPLIFIER
$
$
~
~
~
~
DIAPHRAGM
N
VALVE
VAPOR DIFFUSION CELL L - J THERMOSTAT
Figure 1.
Flow system
to improve the calcium carbide-gas chromatographic method for trace water determination and second to establish the conditions under which methanol would not interfere. A vapor-flow calcium carbide reactor was used throughout the investigation since this technique is experimentally more attractive than the pressurized liquid-flow reactor used by Knight and Weiss for most of their determinations. It is intriguing that although a number of workers have used vapor-flow calcium carbide reactors for analytical purposes (3-6, 8, 9) neither the optimum conditions for carrying out the conversion of water to acetylene nor the stoichiometry of the reaction a t low water concentrations appears to have been established. Some attention therefore has been given to both these aspects using for the first time a diffusion dilution cell ( 2 ) to prepare gas streams containing precisely known trace quantities of water.
gas sampling valve and gas chromatographic column were inserted so that the effluent gases from the carbide bed could be analyzed (chromatographic monitoring system). This latter arrangement was used to investigate the effect of methanol and later to determine the water and methanol contents of hydrocarbons that had been equilibrated with aqueous methanol. DIFFUSIONDILUTIONCELL. Figure 2 shows a diagram of the diffusion cell used. Capillary tubes of diameters 0.8 to 2.5 mm. constructed from precision bore glass tubing (Veridia Ltd.) gave water concentrations of 2 to 400 v.p.m. at nitrogen flow rates between 5 and 80 ml. minute-'. Tubes were thermo~ stated a t 30" or 40" C. to d ~ 0 . 0 5C. and calibrated by measuring the depth I of the liquid meniscus below the open end of the capillary a t various time intervals t over a period of several days. Linear plots of la vs. t were obtained and from the slope X the rate of diffusion at a given value of I could be calculated using the relationship
EXPERIMENTAL
s =Xp-4y
Apparatus. GENERAL ARRANGEThe optimum conditions for carrying out the water to acetylene conversion were determined using the flow system shown in Figure 1 (continuous monitoring system). Later a
MENT.
where p is the density of the liquid in the capillary and A the cross sectional area ( 2 ) . Values of X and S for the capillaries used together with the concentration of water C obtained at 1 =
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NO. 12, NOVEMBER 1966
1657
THE GRAPH INDICATES THE REPRODUCIBILITY OF N I N E CARBIDE BEDS, DESIGNATED BY THE DIFFERENT SYMBOLS X I3 0
A 0 BEDS 230crnX 3rnrn.id
a
12.5cm.x B m m i d I2.5cm.X 5 mm. i d
A v
SINTER
12.5 crn.X 3 m m id 23.0crnX 2rnm.id
800
-L
--
CAPILLARY
THERMOMETER
WATER OR ALCOHOL
WATER
k
--
-300 d
- 200
THERMOSTAT
- 100
Figure 2.
Diffusion dilution cell
7.5 cm. and a nitrogen flow rate of 20 ml. minute-' are given in Table I. The diffusion dilution cell was also used for preparing nitrogen streams containing known quantities of alcohols. Values of X and S at 1 = 7.5 cm. for methanol are also included in Table I. GAS CHROMATOGRAPH. The chromatograph was constructed in the laboratoIy and incorporated a modified Perkin-Elmer gas sampling valve of 0.3ml. capacity and a flame ionization detector similar to that described in a previous publication ( 2 ) . Hydrogen and air flow rates were 20 and 600 ml. minute-', respectively. The ion current obtained under an applied potential of 120 volts was amplified (Electronic Instruments Ltd., Model 37B) and fed t o a potentiometric recorder (Sunvic RSP2) operated at 2.5-mv. sensitivity. The same detector was used for the continuous monitoring system. Polar columns similar to those used by Knight and Weiss elute acetylene after ethane and were found unsatisfactory for precise acetylene determinations on hydrocarbon streams containing ethane as a major impurity. A %-foot x 1/8-inch i.d. column containing 25 per cent squalane on 80-100 mesh silanized Chromosorb W (Johns Manville) was therefore preferred on which acetylene was eluted prior to all major impurities, including ethane,
Table 1.
Capillary
radius, mm. 0.404 0.796 1.23 0.796
1658
0
2
4
6
8 IO I2 14 SIGNAL (omp. x 10-11)
16
18
20
Figure 3. Signal as a function of the rate of diffusion of vapor into a nitrogen stream
present in commercial propane and butane. The elution time for acetylene was approximately 6 minutes a t nitrogen flow rates of 15 ml. minute-'. Methanol determinations were made using a 19-inch x '/*-inch i.d. column packed with 25 per cent w/w polyethylene glycol 400 on the same support. Elution required 12 minutes at a flow rate of 50 ml. minute-'. Both columns were used a t room temperature. To avoid delays caused by the excessive tailing of the major hydrocarbon the squalane column was prepared in duplicate so that once the acetylene had been eluted the column could be replaced by a second column while the original was purged on a separate gas line. CALCIUM CARBIDE BEDS. Lump calcium carbide (Hopkins and Williams Ltd., London) was ground with a mortar and pestle, sieved to 20-30 mesh and packed in glass U-tubes. A range of different bed lengths (2.5 to 23 cm.) and internal diameters (2 to 5 mm.) was used. The beds were conditioned by heating to 140' C. in a stream of dry nitrogen (20 ml. minute-') and maintaining them at this temperature for 15 to 30 minutes, and cooling
to the working temperature and drawing through them approximately 50 ml. of laboratory air. By this means fresh beds could be equilibrated with gas streams containing trace quantities of water in 2 to 3 hours compared to about a day when used directly. For precise water determinations bed temperatures were controlled to within =t0.l0 c. Rocedure. WATERTO ACETYLENE CONVERSION.The operating variables studied were bed length and diameter, bed temperature, and flowrate. Unless otherwise stated the continuous monitoring system was used. To determine approximate reactor dimensions giving reproducible water to acetylene conversions, nitrogen a t 20 ml. minute-' containing 2 to 40 v.p.m. water was passed over a wide range of beds at 40' C. Results together with the dimensions of the beds are given in Figure 3 and Table 11. Equilibration times for beds of different dimensions maintained a t 40' =t 0.1' C. were determined by changing the water concentration of the nitrogen feed (20 ml. minute-') from ca. 10 to ca. 20 v.p.m. and vice versa in a stepwise
Characteristics of Capillary Tubes Used to Produce Known Vapor Concentrations in Nitrogen Streams Cell temperature, 40' C. Cell temperature 30' C. x 106 s x 109 x 106 s x 100 Vapor (em.* sec.-l) (9.sec.-l) C v.p.m. (cm.2 sec.-l) (9.set.-*) C. v.p.m. 1.63 5.62 23.9 2.91 10.1 41.2 Hz0 1.65 21.7 88.5 2.92 39.0 159 HzO 1.60 52.1 212 2.96 93.7 381 Hz0 14.06 146.8 336 CHsOH
ANALYTICAL CHEMISTRY
x
x
(DOTTED CURVE INDICATES INCOMPLETE CONVERSION) BED DIAMETER 2 mm. FLOW RATE 20 ml. min.-l
-t
IO
20
30
50
40
70
60
90
80
100
110
120
T I M E (MINUTES)
Figure 4.
Table II.
Effect of Bed Diameter and Length on Conversion of Water to Acetylene
Bed length, cm. 3 4.5 7 23 6 12 15 24 43 2.5 4.5 10.5 17.5
Effect of bed length on equilibration time
Bed diameter, mm 3 3
.
Water content, v.p.m.
Observed signal X 100 Calculated signala
10 10 10 10 40 40 40 40 40 16 16
49 93 100 99 52 94 100 100 95 37 63 108 92
3 2 2 2 2
16
16
From regression line in Figure 3.
manner and recording the effluent signal. The concentration changes were effected by rapidly increasing or decreasing the temperature of the diffusion cell. The results obtained are shown in Figvres 4 and 5. Equilibration times were found to be independent of the direction of the change. To investigate the effect of temperature, nitrogen containing 10 v.p.m. water was passed over a 12.5-cm. X 2-inm. i.d. bed a t temperatures ranging from 18" to 70" C. The signal decreased from 6.8 X 10-l' to 6.4 X lo-'' ampere on raising the temperature from 18" to 40' C. Above this teniperature the signal decreased rapidly reaching a value of only 4.7 x 10-11 ampere at 70" C. The effect of flow rate was examined first using a 23-cm. X 3-mm. i.d. bed in series with a diffusion cell from which the mass flow rate was 5.2 x gram second-'. Peak areas obtained using the chromatographic monitoring system were found to be inversely proportional to flow rate over In the range 7 to 40 ml. minute-'. later experiments using the continuous monitoring system nitrogen from a cyl-
inder containing ca. 10 v.p.m. water was passed over a 23-cm. X 2-mm. i.d. bed a t 15 to 25 ml. minute-'. The signal was found to be proportional t o the flow rate. Similar experiments with a short 3-cm. X 3-mm. i.d. carhide bed, however, gave higher conversions a t lower flow rates. With all the beds equilibrium was attained more rapidly as the flow rate was increased. EFFECTOF ALCOHOL VAPOR. The temperature at which methanol reacted with calcium carbide was determined by passing a 20 ml. minute-' nitrogen stream containing ca. 2500 v.12.m. methanol over a 23-cm. X 3-min. i.d. carbide bed maintained at 40", 60°, 70", loo", and 120" C. Analysis of the effluent showed that acetylene was produced only a t 100' and 120" C. To established that methanol did not interfere with the water to acetylene conversion, peak areas were compared when nitrogen containing 6.9 v.p.m. water was passed over the same bed a t 40" C. with and without the addition of 700 v.p.m. methanol. No change was observed. While calibrating the apparatus for methanol determinations, concentrations as high as 3500 v.p.m. were passed over the bed a t this temperature and no acetylene peaks were obtained. Experiments with ethanol and isopropanol a t 600 v.p.m. in nitrogen showed that these alcohols also were inert a t 40" C. DETERMINATION OF TRACC WATER IN HYDROCARBONS CONTAINING XETHANOL. Liquid propane and butane were equilibrated with a range of aqueous methanol solutions a t -45" and -6.5' C., respectively, in a suitahle cell. After phase separation was complete a sample of the hydrocarbon layer was displaced continuously via a sample tube and vaporizer, t o the calcium carbide bed (23-cm. X 3-mm. i.d.) and gas chromatograph. The flow rate
2.c
BE0
LENGTH 1 2 . 5 ~ ~ .
a 0 X
ci E l
p/ -I
4
0
0
IO
20
Figure 5.
30
40
60 70 TIME (MINUTES)
50
80
90
100
I10
123
3
Effect of bed diameter on equilibration time VOL. 30,
NO. 12, NOVEMBER 1966
1659
Table 111.
Water and Methanol Contents of Liquid Butane and Propane in Equilibrium with Aqueous Methanol Liquid butane a t -6.5" C.
yo Weight
methanol in aqueous phase
Water content, v.p.m.
Standard deviation
42 41 43 41 49 53 61 71 150 261 267
10
15 20 ~. 30 40 50 60 70 80
Methanol content v.p.m.
Standard deviation
182 320
4 13
745 1280 2010 2130 3055
7 29 6 4 13
925 930 1260 2540 3580
12 25 14 21 55
3 1
Liouid DroDane a t -45.0' C, I
I
1
2.0 3.0 4.6 7.5 9.5
50 55 60 70 75
0.2 0.2 0.3 0.2 0.5
through the bed was 20 ml. minute-' and the bed temperature maintained at 40" 0.1' C. The apparatus was calibrated using the diffusion dilution cell at concentration levels up to 381 v.p.m. water and 3500 v.p.m. methanol. Frequent check calibrations over a period of three months using the same carbide bed showed no change. Table I11 shows the standard deviations obtained for 10 consecutive samples taken from each mixture over a period of approximately five hours. Methanol concentrations determined using the short polyethylene glycol column are also recorded. RESULTS A N D DISCUSSION
CONVERSION OF WATER TO ACETYLEPI'E. Little is known about the reaction between water and calcium carbide under the conditions pertinent to this investigation, Knight and Weiss state that the predominant reaction is CaCz 2HL0+ Ca(0H)z CZHZ (1) However, it is known that when calcium carbide is present in excess, a further slow reaction takes place between calcium carbide and calcium hydroxide yielding another molecule of acetylene (7')-i.e. , CaC2 Ca(OH)z+ 2Ca0 CZHZ (2) so that the overall reaction is
+
+
+
+
+
+
CzHz (3) CaC2 HzO + CaO This investigation has shown that the acetylene content of the effluent gas is linearly related to the water concentration (Figure 3). If it is assumed that Reaction 1 predominates, the molar response factor ( 2 ) calculated from the slope of the regression line is 0.82 coulomb. Csing the same detector a n d - a standard blend (British Oxygen Co.) of acetylene in nitrogen the molar response factor for acetylene WRS found to be 0.44 coulomb. Reaction 3 rather than Reac1660
0
ANALYTICAL CHEMISTRY
tion 1 therefore governs the conversion of water to acetylene under the conditions used. The regression line in Figure 3 has the form signal (ampere x lo-") = 0.0228 rate of diffusion (grams sec.-l x 10-l1) 0.929
+
the correlation coefficient between signal and rate of diffusion being 0.997. This equation predicts the signal strength with a standard error of 4 x lo-'* ampere. It is interesting to note that the blank water content of the "dried" nitrogen stream calculated from the constant in the above equation is 2.1 v.p.m. and agrees with the value quoted by Walker and Campion (12) for gases dried over molecular sieve. Investigations of bed dimensions (Table 11) show there is a minimum length below which conversion is incomplete. This minimum depends upon both the water concentration and the bed diameter. Thus with the 3-mm. diameter bed the length necessary for complete conversion is 6 em. at 10 v.p.m. and 14 em. at 40 v.p.m. Assuming the same approximate concentration -length relationship holds for smaller diameter columns, a length of 8 em. would be required for determining water concentrations a t 10 v.p.m. using a 2mm. i.d. bed. For routine analytical work at the 10 v.p.m. concentration level, beds 10 to 20 per cent longer than these are recommended, the additional length allowing for the gradual deactivation that takes place, particularly if opened to atmosphere. At higher concentration levels larger beds should be used, (cf. determination of equilibrium data on the hydrocarbon/aqueous methanol systems). The most inconvenient feature of the
calcium carbide conversion method is the long equilibration time especially a t low concentration levels. The results shown in Figures 4 and 5 illustrate that equilibration times increase in proportion to bed length and decrease with decreasing diameter. Flow rate investigations showed that equilibrium times could be reduced by increasing the flow but high flow rates conversion is incomplete. The optimum is about 20 ml. minute-l for the beds recommended above giving equilibrium times of the order of one hour. The brief investigation of bed temperature showed that the acetylene content of the effluent decreased with increase in temperature, the change being particularly marked at temperatures above 40' C. A decrease in acetylene concentration with rise in temperature has also been reported by Hersch and Deuringer (6). This limited study has shown that operating temperatures of 40' C. or below are satisfactory and in anticipation that the conversion would be more rapid a t the higher temperature, 40' C. was chosen for the reactor studies. Early investigations using unthermostated beds at room temperature showed variations in ambient temperature produced small changes in acetylene concentration. This could be avoided by thermostating the bed to within 0.1' C. and this procedure is recommended where high precision or maximum sensitivity is required. EFFECTOF ALCOHOLS.Liquid methanol and ethanol are known to react with calcium carbide to produce calcium methoxide or ethoxide, respectively, and acetylene (IO). Villelume (11) has shown that in the vapor state both alcohols react to give a wide range of products. This investigation has shown, however, that methanol a t concentration levels up to 3500 v.p.m. does not interfere even over long periods a t bed temperatures of 40" C. No detailed investigations were made with ethanol and isopropanol but a t 600 v.p.m. in nitrogen these also did not interfere. The method therefore appears to be generally applicable to the determination of trace quantities of water in the presence of small amounts of alcohols. APPLICATION TO HYDROCARBOS STREAMS. Standard deviations for water determinations on the propane and butane streams were 10 per cent at 2 v.p.m. and 1 per cent a t 250 v.p.m. (Table 111). These results compare favorably with those obtained by Knight and Weiss. No attempts were made to determine water contents below 2 v.p.m. and the exact lower limit of the method cannot be stated. For the maximum sensitivity in the gas chromatographic analysis, it is important that the acetylene is eluted prior to any major component. Application of the method to ethane, ethylene,
or methane streams could be particularly difficult and even small amounts of these gases in higher boiling hydrocarbons can seriously complicate the analysis. For example, the presence of only 0.9 per cent ethane in the propane used for the solubility determinations made it necessary t o use a 36-foot squalane column to obtain an adequate acetylene/ethane ssparation. With the apparatus described acetylene contents down to ca 0.1 v.p.m. could be detected. In the absence of interfering hydrocarbons acetylene concentrations of the order 0.01 v.p.m. or less could be measured by improvements in the design of gas chromatographic apparatus (6). It is doubtful, however, if this would lead to a significant improvement in the overall sensitivity of the method since quantitative water to acetylene conversion
would be difficult to achieve a t concentration levels much below 0.1 v.p.m. Although the main objective in this investigation was to develop a method for determining trace amounts of water in propane and butane streams containing methanol, the technique is of course applicable to all gases that are inert toward calcium carbide. It may therefore be applied to a very wide range of problems. LITERATURE CITED
S. T., Smith, V. N., ANAL. CHEM.34, 1129 (1962). (2) Desty, D. H., Geach, C. J., Goldup, A., “Gas Chromatography 1960” p, 46, R. P. W. Scott, ed., Butterworths, London, 1960. (3) Duswalt, A. A,, Brandt, W. W., ANAL.CHEM.32, 272 (1960). (4) Forbes, J. W., Ibid., 34, 1125 (1962). (5) Hersch, P. A., Deuringer, R., Pitts(1) Abrams,
burgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1965. (6) Huytens, F. Hii Rijnders, G. W. rl., Beersum, W. V., Gas Chromatography 1962” p. 235, M. van Swaay, e d , Butterworths, London, 1962. (7) Kirk-Othmer, “Encyclopedia of Chemical Technology,” J’ol. 2, p. 837, Interscience Encyclopedia Inc., New York, 1948. (8) Knight, H. S., Weiss, F. T., ANAL. CHEM.34, 749 (1962). (9) Kyryacos, G., Boord, C. E., 131st Meeting ACS, &$ami, April 1957. (10) hiiller, S. A., Acetylene-Its Chemistrv and Uses.” Vol. 1. Ernest Benri Ltd:, London, 1965. (11) S‘illelume, J. de, Ann. C h i m (Paris) 7, 265 (1952). (12) Walker, J. A. J., Campion, P., Analyst 90, 199 (1965). RECEIVED for review February 21, 1966. Accepted July 27, 1966. Permission t o publish this paper has been given by The British Petroleum Co., Ltd.
Solution Thermodynamics from Gas-Liquid Chromatography R. E. PECSAR’ and J. J. MARTIN University o f Michigan, Ann Arbor, Mich.
b Gas-liquid chromatography has been employed to study the thermodynamic solution properties of twocomponent volatile nonelectrolyte solutions at infinite dilution. The expressions relating chromatography to thermodynamics have been developed and the theory of hydrogen bonding has been used to order liquids into distinct classes and predict solution deviations from ideality. Homologous series of alkanes, chloromethanes, alkyl formates, aldehydes, amines, and alcohols were studied in water, 2-pentanone, and 2,3,4-trimethylpentane from 20’40” C. The resulting solution thermodynamics agreed quite well with the anticipated behavior as well as existing literature results. The data in general were successfully correlated with a linear relation between the partial molar excess free energy and the hydrocarbon chain length of the solute for a homologous series in a given solvent.
T
HE INITIALwork
deriving thermcdynamic solution properties from chromatographic measurements was that of Littlewood, Phillips, and Price ( 2 1 ) in 1955. The authors defined the concept of the specific retention volume which allowed a comparison of data between different investigators inde1 Present address, The Corp., Van Nuys, Calif.
Marquardt
pendent of chromatographic variables. Subsequent to this work, numerous thermodynamic studies have been made utilizing gas-liquid chromatography and the properties obtained by this technique have been adequately validated by measurements made employing static techniques on the same systems. Porter, Deal and Stross (16) demonstrated that the properties obtained represented true thermodynamic equilibrium and were constant for wide ranges of the chromatographic parameters. They also ran static equilibrium studies verifying the data obtained by gasliquid chromatography (GLC) . As the solvent in GLC is dispersed on a solid support, the role of this support in the thermodynamic equilibration process has been closely scrutinized. Adlard, Khan, and Whitham (1) studied benzene in dinonyl phthalate spread on a solid support in sufficient quantity to ensure complete surface coverage. Later these same authors (8) coated dinonyl phthalate in a capillary column and again studied benzene and other solutes. The agreement obtained by the two different techniques suggests that with a carefully coated inert support material the data obtained represent true vaporliquid equilibrium between the solute and the solvent. I n the present study a basic desire was to extend the technique of GLC to include studies of systems with a volatile solvent. I n all of the work just cited,
the solvent was a large organic molecule which had an exceedingly low vapor pressure a t the temperature of interest. By employing a volatile solvent the more common binary solutions may be studied, not merely those peculiar to GLC. A few attempts at studying such systems had previously been made. Purnell and Spencer (16) conducted a preliminary investigation on the separation of chlorinated methanes utilizing water as a solvent and obtained a boiling point separation. Pollard and Hardy (14) in a more thorough study determined that as the solute concentration decreased the elution order changed from that based on boiling points t o one dependent on the solubilities of the chloromethanes in water. The latter authors also considered the methanolwater system. Both of the above investigations were hampered by the nonstationary nature of the stationary phase. In order to reduce this problem Kwantes and Rijnders (10) first utilized a forecolumn or presaturator. In the presaturator, the carrier gas is saturated with solvent, either in bulk form or coated on a solid support, a t a given temperature and pressure. While this reduces bleeding in the main column to a large extent, because of expansion of the gas phase during transit through the system the problem is never completely alleviated. The two-component volatile study of Kwantes and Rijnders (10) was limited to normal paraffins in their VOL. 38, NO. 12, NOVEMBER 1966
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