Natural Gas Hydrates - OnePetro

Natural Gas Hydrates

B'i DON B. CARSON* AND DONALD L. KATZ,t MEMBER A.I.M.E.

(Dallas Meeting, October 1941)

NATURAL gases under pressure form crystalline hydrates with water. Experimental data are reported on four-phase equilibrium for the methane-propane-water, methane-pentanewater, and methane-hexane-water systems. Temperatures and pressures for equilibrium between gas, water-rich liquid, hydrocarbonrich liquid, and hydrate were measured, as well as the percentages of methane and propane in the hydrate. The data indicate that natural gas hydrate behaves as a solid solution and that pentanes and heavier hydrocarbons do not entcr into the solid phase. Vapor-solid equilibrium constants are presented that permit the approximation of the conditions for hydrate formation, from the composition of a gas.

INTRODUCTION

Natural gas hydrates have been the object of considerable research in recent years, because of the trouble they have caused in the natural-gas and naturalgasoline industries. Natural gas hydrates are white crystalline compounds of water and gas, which, under pressure, exist at temperatures considerably above the freezing point of water. Because of the relatively high temperatures at which the hydrates exist, they become a nuisance in highpressure gas operations where water is present, since their formation causes partial or complete plugging of valves and pipes. From a practical standpoint, the trouble incident to hydrate formation has been solved by dehydration of the gas

Manuscript received at the office of the Institute July 16, 1941. Issued as T.P. 1371 in PETROLEUM TECHNOLOGY, September 1941.

• Graduate Student. University of Michigan Ann Arbor, Mich. '

1: Assistant Professor of Chemical Engineering University of Michigan. '

I References are at the cnd of the paper.

before it enters the plant or pipe line, or by other remedial measures.

Extensive work on gas hydrates in the latter part of the nineteenth century, by Villard1 and others, gave data on methane hydrate and on ethane hydrate. Schroeder summarized these theoretical studies.?: Hammerschmidt3 introduced the information on gas hydrates to the gas industry and Deaton and Frost 4-6 gathered a considerable number of data on the behavior of natural gas hydrates. Recent papers7,8 have extended both the data and the theory of these hydrates.

At present, data are available on temperatures and pressures of hydrate formation for pure methane, ethane, propane, n-butane, methane-ethane mixtures, methane-propane mixtures, methane-butane mixtures, and some IS natural gases, all in the presence of excess water, How¥ ever, up to the present time no method has been available for predicting the temperature at given pressures or .the pressure at given temperatures at which natural gases will form hydrates if saturated with water. This paper presents data on the methane-propane-water, methane-pentane-water and methane-hexane water systems in four phases, one of which is the hydrate, The application of the phase rule to the methane-propanewater data proves that hydrates behave as solid solutions. The data of this paper and those in the literature were used to develop vapor-solid equilibrium constants,8 which permit prediction of couditions of hydrate formation from the analysis of a natural gas.

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DON B. CARSON AND DONALD L. KATZ 151

EXPERLMENTAL 1iEAsUREMENTS

The experimental data are limited to measurement of temperatures and pressures for hydrate formation and phase

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cell. The presence of the hydrocarbon-rich liquid with two hydrocarbon components present, ensured a constant composition

vapor at given temperatures and pres-

BATH AND HYDRATE CELL

FIG. I.-ApPARATUS FOR FORMING HYDRATES.

compositions using three-component systems in four phases-gas, hydrocarbon-rich liquid, water-rich liquid and hydrate or solid phase.

The apparatus (Fig. I) consists mainly of a glass-windowed cell similar to that previously described8 but which is capable of rotation for agitation of the contents of the cell. The temperature of the cell is indicated by the glass thermometer in the constant-temperature water bath surrounding it. The pressures were measured by a calibrated Bourdon tube gauge.

The materials used consisted of methane from the Buttonwillow field in California, reported to contain 0.3 per cent carbon dioxide and 99.7 per cent methane. The propane used in most of the experiments was c.p. propane from Phillips Petroleum Co. with negligible impurities but in the last eight runs of methane-propane-water the propane used contained 0.9 per cent unsaturates and similar amounts of other impurities. The pentanes were a cut from a distillation analysis of a butane-free naphtha. The hexane was c.p. n-hexane from Phillips Petroleum Co.

The data obtained were the pressures, at defined temperatures under which hydrates formed, with four phases in the

sures, and the resulting hydrate was thus formed in equilibrium with constant composition vapor and liquids. The technique of obtaining hydrates with the two

TABLE 1.-Four-phase Equilibria Data for M ethane-propane-water System

EQ,ui- Mol Mol Tempera- libnum Per Cent Per Cent

ture, Pressure, Propane in Methane in Deg. F. Lb. per Sq. Hydrate. Hydrate.

In. Abs. Dry Basis Dry Basis

72·5 1052 39 61 69.7 774 53.6 46.4 65.5 521 58.8 41. 2 61.9 403 73.6 26.4 58.5 310 74·1 25·9 65.3 534 51.2 48 .8 51.2 177 86.5 13·5 64. 2 478 69.5 30·5 51.6 187 55. 1 230 61.9 408 64.9 508 69.8 832 68.8 764 48 .5 51.5 71.4 789 56.6 256 76.5 23·5 54·1 263

liquids and gas phase present was similar to that previously reported except that agitation was continuous. The data obtained are given in Table 1.

The composition of the hydrocarbons in the hydrate was measured for the methanepropane-water system. A large quantity of hydrate was formed in the presence of

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NATURAL GAS HYDRATES

four phases; the hydrocarbon-rich liquid phase disappeared as the end of the hydrate formation was reached. Mercury was used to displace the gas and as much of

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FIG. 2.-QUADRUPLE LOCUS FOR THF. METHANEPROPANE-WATER SYSTEM.

the water-rich phase as possible, leaving the crystals with some adherent liquid. The

TABLE 2.-Four-phase Equilibrium Data for M ethane-pentane-water and Methane-hexane-water Systems

E~uilibrium Hydrocarbon Temperature, ressure, Components Deg. F. Lb. per Sq.

In. Abs.

Methane, ,,-hexane ... 41.0 620 41.6 622

Methane, ,,-hexane ... 39.8 618 46 . 2 861

Methane, pentanes ... 47.6 9Il 36.6 487 38.2 519

hydrate was then decomposed and the gas evolved was analyzed for methane and propane by the gas-density method. The results are shown in Table I.

Mixtures of pentanes with methane and n-hexane with methane were also brought into the four-phase equilibria using technique similar to that for the methanepropane-water system. Table 2 gives the data obtained for these mixtures with water in excess.

PHASE RULE PROOF OF SOLID SOLUTION

The phase rule states that the number of components plus two and minus the number of phases equals the degrees of freedom:

The data on the methane-propane-water equilibria studied are plotted on Fig. 2

and show one degree of freedom. According to the phase rule, this system then comprises four phases, since there are three components. The gas, water-rich liquid and hydrocarbon-rich liquid phases were observed to be present in addition to the solid hydrates, which indicates that the solid hydrate is only one phase.

The hydrate-composition data are plotted on Fig. 3, giving the mol per cent propane on a water-free basis.

The continuous change in the concentration of the hydrocarbon constituents of the hydrate along the four-phase equilibrium line or quadruple locus indicates that the single-phase hydrate gradually changes in composition. This behavior is characteristic of solid solutions and indicates that natural gas hydrates are likely to be solid solutions.

COMPOSITION OF PHASES

Deaton and Frost6 presented hydrateformation conditions for various methanepropane mixtures in the presence of liquid water, These measurements were made with three phases at equilibriumgas, water-rich liquid and hydrate. This information and the data on the methanepropane system9 were used to construct Fig. 4, which shows the intersection of vapor compositions in equilibrium with

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hydrate and in equilibrium with hydrocarbon liquid. These intersections are the four-phase equilibria points if it is assumed that the effect of water on the methane-

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pure propane increases the vapor-liquid equilibrium pressure from 171 lb. only to 179 lb. per sq. in. abs. at 90°F. These two observations indicate that the pres-

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FIG. 3.-HYDROCAru30N-PHASE COMPOSITIONS FOR FOUR-PHASE EQUILIBRIUM.

propane vapor-liquid equilibrium reported by Sage, Lacey and Schaafsma9 at these temperatures and pressures is negligible. The quadruple locus of Fig. 2 has been

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ence of a water-rich liquid has little effect on the vapor-liquid equilibrium at the indicated temperatures. At least, the change in the ratio of the hydrocarbon

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"" f'--~". 40 t--t-t-3 VAPOR -WATER - HYDRATE EQU ILlBRrUM

VAPOR-WATER- LIQUID HYDROCARBON

EO~'LI~RIUr-L-1 I I I I J 20 40 60 60 Ie o MOL PERCENT PROPANE IN VAPOR

(WATER-FREE BAS IS 1 FIG. 4.-PREDICTION OF QUADRUPLE LOCUS FOR METHANE-PROPANE-WATER SYSTEM.

placed on Fig. 4, and gives a smooth extrapolation of the vapor-liquid equilibria curves. Chaddock reportslO that the presence of a water-rich liquid phase with

compositions because of the presence of the water may be slight. On this basis, the data on the liquid-vapor equilibrium9

have been placed on Fig. 3 and used as

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154 NATURAL GAS HYDRATES

the dry compositions of the vapor and hydrocarbon-rich liquid in the four-phase equilibria.

The scattering of the points on the hydrate compositions (Fig. 3) probably is due to incomplete separation of the phases when the gas and liquid phases were removed from the cell.

ABSENCE OF PENTANE AND HEXANE

HYDRATES

Since extrapolation of the trend of formation of ethane, propane, and n-butane hydrates indicates that the temperatures

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.GAS A (8) ~---.!h4'-:l'7L----I+GAS B (10)

-GAS C (10) '---::;P-H'-:7r~----i' GAS 0 (10) • aGAS L (8)

• C,-c..MIXTURE .. C,-C."MI X TURE

FIG. 5.-CONDITIONS FOR FORMATION OF NATURAL GAS HYDRATES.

at which pure pentane would be expected to form hydrate are below 32°F., no data on pentane or hexane hydrates are obtainable by the usual procedure. If pentane forms a hydrate, mixtures of methane and pentane should form hydrate at temperatures above 32°F. The addition of ethane, propane and the butanes to methane lowers the pressure for hydrate formation below that required for pure methane at the same temperature. This change in equilibrium pressure is accompanied by the change in hydrate composition from

pure methane and water to a solid solution of both hydrocarbons and water.

The data on methane-pentanes-water and methane-n-hexane-water of Table 2 when plotted fall along the curve for pure methane (Fig. 5). This indicates that paraffin hydrocarbons boiling above nbutane may not form hydrates. The data of Deaton and Frose on binary hydrocarbon mixtures have been plotted on Fig. 6 to show the effect of constituents other than methane. The hydrate-formation pressure at 35°F. for the mixtures is subtracted from 447 lb., the value for pure methane, and this difference is plotted as a function of the concentration of the other constituent. The deviation increases from ethane to propane to isobutane but decreases to n-butane. It should be noted that the equilibrium pressure for the 0.8 per cent n-butane concentration was reported to be the same as for pure methane. Since the concentration of isopentane or ncpentane, in the vapor never exceeded 1.5 per cent, complete proof of the inability of pentane to form hydrate at high pressures with higher concentrations is lacking. However, the trend from ethane to n-butane is such that there is a distinct possibility that pentane does not enter into the hydrate crystal structure.

EQuILmRIUM CONSTANTS TO PREDICT

HYDRATE FORMATION

The substantiation that natural-gas hydrates are solid solutions and the hydrate-composition data given justify an elaboration of the vapor-solid equilibrium constant concept previously proposed. 8

These constants permit computation of the hydrate-formation conditions from the analysis of the gas, which is the desired correlation. The vapor-solid equilibrium constant, K._. is defined by equation 2, and considers only the hydrocarbon portion of the vapor and solid phases. These vapor-solid equilibrium constants may be

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K v _, = ~ = z

mol fraction of a hydrocarbun ill vapur «(lry LJasis) m"ol fraction of a hydrocarhon in hydrate (ory haSis)

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used in a manner similar to that for vaporliquid equilibrium constants, computing

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In obtaining the vapor-solid equilibriumconstant chart for methane (Fig. 7), three sources of data were used. The points where K = I are from the hydrate-formation curve for pure methane. 6 •7 One experimental K was available for methane for

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ELl) _;NO ~Q9-PRESSURE DEVIATION FROM METHANE. LBS./SQ. IN

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FIG. 6.-EFFECT OF HEAVIER COMPONENT ON HYDRATE FORMATION AT 35°F.

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FIG. 7.-TENTATIVE KV-B FOR METHANE.

hydrate-formation temperature or pressures instead of dew-point temperatures and pressures, and so forth.

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FIG. 8.-TENTATIVE K._. FOR ETHANE.

each pressure from the present data (Fig. 3). Hammerschmidt3 reported the composition of a pipe-line gas and a gas

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resulting from decomposition of a hydrate formed at 600 lb. per sq. in., giving K for methane at this pressure. Using these points, the K-curve for methane at 600

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FIG. 9.-TENTATIVE KV-<I FOR PROPANE.

lb. per sq. in. was drawn, and curves for other pressures were drawn in symmetrically. By extrapolation, the range of pressures covered was expanded.

The equilibrium constants for ethane, propane, and isobutane next were calculated by using the binary hydrocarbonmixture data for methane and these several components. 6 The relationship

used in this calculation is ~ i must equal

unity at conditions of hydrate formation. The equilibrium constants shown in Figs. 8, 9 and 10 were obtained in this manner.

In the use of the equilibrium constants to calculate hydrate-formation conditions for some temperature, assumptions are

made as to the pressure until ~ i = I

for all the components. All components heavier than the butanes, including nitrogen, have a vapor-solid equilibrium con-

stant of infinity, since they are not known to form hydrates. Fig. 6 shows that the hydrate properties of n-butane in naturalgas mixtures are close to those of ethane,

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FIG. lo.-TENTATIVE K._ FOR ISOBUTANE.

and since there are few data on n-butanemethane mixtures, the equilibrium constant for n-butane is taken as equal to that for ethane. This assumption involves considerable error if the n-butane concentration in the gas is high. Carbon dioxide and hydrogen sulphide form hydrates with water in the pure state and when present in a gaseous mixture may be expected to enter the solid solution. Examination of data on the pure cop.stituents indicates carbon dioxide equilibrium constants might lie halfway between ethane and methane while hydrogen sulphide may be similar to isobutane in its tendency to form hydrates. In the absence of data, the use of such equivalents is recommended for low concentrations.

It is appreciated that the assumptions and extrapolations used to prepare the equilibrium-constant charts might leave

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DISCUSSION 157

considerable doubt as to their reliability. The use of these charts to predict the hydrate-formation conditions of natural gases and comparison with experimental conditions will evaluate their accuracy and utility. Table 3 gives the comparison of computed and experimental pressures for IO natural gases from the literature.

TABLE 3.-Comparison of Calculated with Experimental Conditions for Hydrate

Formation

Pressure at Which

Tem-Hydrate Formed, Lb. per Sq. In.

Gas Investiga- Ref-pera- Abs. ture, tor erence Deg.

F. Calcu- Experi-lated mental

- -- --

I-B Wilcox 8 40 156 156 50 309 296 60 677 600 67 1,273 1,090 75 3,620 3,000

2-C Wilcox 8 40 242 231 50 482 448 60 1,017 935 70 2,4' 0 2,430

Wilcox 74 over 4,000 4,000

3-D • 40 186 193 50 369 388 60 796 815 70 1,915 2,170 74 3,380 3,580

4-B Deaton and • 40 lOS II7 Frost 50 223 240

5-C Deaton and , 40 ISO '40

Frost 50 300 280 6-D Deaton and ,

40 168 166 Frost 50 343 325

7-E Deaton and , 35 126 125

Frost 45 242 248 55 507 505

8-F Deaton and , 40 180 182

Frost 50 367 365 9-G Deaton and 3 40 181 210

Frost 50 365 415 (Hammer-schmidt)

lO-I Deaton and • 40 250 295 Frost 50 495 590

60 1,053 I,rSa

Table 4 gives a typical calculation of the pressure at which hydrate forms when the given gas is in equilibrium with liquid water at 50°F.

The discrepancy between computed and experimental pressures in Table 3 may be caused by incorrect equilibrium constants or by inaccurate analyses of the gas. The calculation shows a high sensitivity to the propane and butane concentration. The presence of plus and minus

errors indicates that the constants are a fair representation of the data available.

TABLE 4.-Calculation of Pressure for Hydrate Formation at 50°F.

Mol Kat Kat Com- Frac- 300 Lb. y/K 350 Lb. y/K ponent tion per per

in Gas Sq. In. Sq. In.

----- ------Methane .. 0.784 2.04 0.384 1.90 0.412 Ethane .... 0.060 0·79 0.076 0.63 0.0953 Propane ... 0.036 0.113 0·318 0.09 0.400 i-butane ... 0.005 0.0725 0.069 0.06 0.0834 n-butane .. 0.019 0·79 0.024 0.63 0.0302 Nitrogen .. 0.094 infinity 0.000 infinity 0.0000 Carbondi-

oxide ... 0.002 I.4IQ 0.0014 1.26 /0.0016 -----------1.000 0.8724 1.0225

I

Interpolating linearly, y/K = 1.0 at 343 lb. per sq. in. absolute.

The experimentally observed hydrate-formation pressure at 50°F. was 325 lb. per sq. in. abs.

a Taken as average of methane and ethane.

ACKNOWLEDGMENTS

The Southern California Gas Co. furnished the methane and Phillips Petroleum Co. furnished the propane and hexane used. Mr. Richard Rosencranz assisted in construction of the apparatus and took some of the preliminary measurements.

REFERENCES

I. Villard: Campt. rend. (1887) 106. 602; (1890) III 302-5; (1888) 107.395.

2. W. Schroeder: "History of Gas Hydrates," Ahren's "Sammlung Chemischer und Chemischtechnischer Vortrage," 29, pps. 1-97 (192 6-7).

3. E. G. Hammerschmidt: Western Gas (1933) 9 (12). Also, Ind. and Eng. Chern. (1934) 26. 851-55.

4. W. M. Deaton and E. M. Frost: Amer. Gas. Jnl., (1937) 146 (6), 17-21; Amer. Gas Assn. Monthly (1937) 19.211)-22.

5. W. M. Deaton and E. M. Frost: Gas Age (1938) 81 (I 1),33-34; Amer. Gas Assn. Nat. Gas Dept. Proe. (May 1938) II2.

6. W. M. Deaton -and E. M. Frost: Amer. Gas Assn. Nat. Gas Dept. Proe. (May 1940) 122.

7. Roberts, Brownscombe and Howe: Oil and Gas Jnl. (1940) 39 (30),37.

8. Wilcox, Carson and Katz: Ind. and Eng. Chern. (1941) 33. 662.

9. Sage, Lacey and Schaafsma: Ind. and Eng. Chern. (1934) 26. 214-271.

10. Chaddock: Ph.D. dissertation, University of Michigan (1941).

DISCUSSION CR. O. Garrett presiding)

E. C. HAMMERSCHMIDT, * Fritch, Texas.The authors point out that the use of the equilibria constant charts to predict the

* Texoma Natural Gas Co.

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hydrate-formation conditions of natural gas in comparison with experimental conditions will evaluate their accuracy and utility. Additional data on four different gases are presented in Table 5 as a supplement to Table 3. The data

TABLE 5.-Data on Four Different Gases

Gas .............. . K L M H

----------------Carbon dioxide. . . . . . . o. 8 Methane ................. 84.5277.5339.8367.4 Ethane........ 4.30 5.9118.07 3·7 Propane..... 3.33 4.8530.15 1.9 iso-butene...... 0.47 0.46 3.60 0·5 n-butene ......... 0.69 1.02 8.27 0.7 Pentane plus........... 0.08 0.62 0.08 Nitrogen............... 6.61 9.61 25.0 British thermal units. . . . .. 1,047 1,076 1,879 827 Experimental hydrate-for·

mation pressure at 40oP., lb. per sq. in ........... .

Calculated hydrate-formation pressure at 4oo P., lb. per sq. in ...... ....... .

Experimental hydrate-formation pressure at 50°F .. lb. per sq. in ........... .

Calculated hydrate-formation pressure at 50°F., lb. per sq. in . ............ .

180

173

390

358

170 110 230

148 <100 218

350

317

on gas H were taken from reference 5. The hydrate-formation experimental data of gas M were obtained from gasoline-plant vent vapors on Jan. 9, 1939, whereas the sample for the analysis of the vent vapors was not taken until May 31, 1939. For this reason, the data on gas M might be somewhat questionable, although the operating conditions of the gasoline plant were the same on both dates.

Gas L is a gasoline-plant absorber-inlet gas, sampled on Oct. 25, 1938, and gas K is a gasoline-plant absorber-inlet gas sampled on Dec. 5, 1939·

Excluding gas M, the average deviation of the calculated hydrate-formation pressure from the experimental data is 7.2 per cent. This average includes the 30 points in Table 3 together with the six additional points of Table 5. Of the 36 points, the maximum devia· tion is 20.7 per cent and the minimum deviation is zero. Comparing the hydrate-formation pressures of the richest gas in reference 5 (gas C, Kettleman Hills, Calif.) with the leanest gas (gas I, Cayuga, Tex.), the deviation of gas C from gas I is approximately 50 per cent. The deviation between pure methane and pure ethane is even greater, so that an attempt to use hydrate-formation-pressure curves of some particular gas for a gas whose hydrate-formation pressure is unknown could lead to much greater errors than the average percentage deviation that is shown by the calculated hydrate-formation pressures. It appears therefore that the authors have provided a practical and relatively simple method of estimating the hydrate-forming pressures. However, the large deviation between the experimental and calculated data of gas M indicates that calculated hydrate-formation pressures should be used cautiously in connection with a rich gas of this type until more data become available.

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Natural Gas Hydrates - OnePetro

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