-. ~ AMERICAN INSTITUTE OF MISISG ASD METALLURGICAL ENGINEERS Technical Publication No. 1748 (CLASS.- G. PETROLEUM ~ ~. -- DIVISION. NO. 210) -, DISCUSSION OF THIS PAPER IS INVITED. Discussion in writing (2 copies) may be sent to the Secretary. American Institute of Mining and Metallurgical Engineers 29 West 39th Street. New York 18. N. Y. Unless special arrangement is made. discussion of this paper will c~dse Sept. 30, 1944. Any discussion offered thereafter should preferably be in the form of a new paper. Prediction of Conditions for Hydrate Formation in Natural Gases ABSTRACT CHARTS for predicting the pressure to which natural gases may be expanded without hydrate formation have been prepared for BY DONALD L. KATZ,* MEMBER A.I.M.E. (New York Meeting. February 1944) PREDICTION OF CONDITIONS HYDRATE FORMATION IN NATURAL GASES Each natural gas under a given pressure will form solid hydrates at a corresponding TABL' I.-TYP~C~ ComP~~iti~ns of temperature provided sufficient water is Gases Corresponding lo Gas Gravities The temperature of natural Calculated gas gases below about 5000 lb. per sq. in. degravity..... 0.603 0.704 0.803 0.906 1.023 creases when the gases are expanded freely.@s7 This decrease in temperature may Constituent Mol Fractions cause the expanding gas to enter the Methane..... region of temperature and pressure at 0.9267 0.8605 0.7350 0.6198 0.5471 Ethane... 0.0529 0.0606 0.1340 0.1777 0.1745 which hydrates will form. The final pres- Propane.. 0.0138 0.0339 0.0690 0.1118 0.1330 i-butane.... o.0018~ 0.0084 0.0080 0.0150 0.0210 sure to which a natural gas may be exn-butane..... 0.00338 0.0136 0.0240 0.0414 0.0640 Pentanes PIUS... 0.0014 0.0230 0.0300 0.0343 0.0604 panded without hydrate formation de- Pressure of hydrate forma- pends upon the initial temperature and tionat so F., 1 '' / pressure and the gas composition. lb. per sq. in. abs.: Calculated.... This paper presents charts that give the 462 323 254 I91 Smooth curve. final pressures to which gases of gravity Fig. I..... I I IS9 0.6 to 1.0 at given initial temperatures and pressures may be expanded without forgases of even' gravity. Pressure-temperature mation of hydrate. curves for hydrate formation were established for gases having gravities from 0.6 to 1.0. HYDRATE-FORM~C TE~ERATURES AND These curves and the thermal behavior of the PRESSURES AS A FUNCTION gases during free and adiabatic expansion were OF GAS GRAVITY used to prepare the charts for estimating the permissible expansion of natural gases without Experimental data of pressure versus hydrate formation. temperature at which solid hydrate will The ~roblem was solved by L. F. Albright, form, provided sufficient water is present, W. T. Boyd, J. J. McKetta, G. Martin, F. are available for a series of gases.2-j A Poettman, and A. P. Snyder, who are first- method of predicting the pressure-tern- Year graduate students in the department of perature curve for a given gas composition Chemical and Metallurgical Engineering at has been reported.l The solution to this the University of Michigan. This paper is essentially a summary of their results. problem requires that these curves relating -- pressure to temperature be established for Manuscript received at the office of the Institute Feb. of given gravities. 15. ~944. Professor Chemical Engineering. Univer- - - sity of Michigan, Ann Arbor, Michigan. 1 References are at the end of the paper. Copyright. 1944. by the American Institute of Mining and Metallurgical Engineers, Inc, PBraoLpm TRCHNOL~GY, July 1944. Printed in U, S, A, FOR
GAS GRAVITY -HYDRATE PRESSURE-TEMPERATURE RELATIONS AS A FUNCTION OF GAS G RAVITY FIG. 2.-PRESSURE-TEMPERATURE I I I I I 50 60 70 80 90 TEMPERATURE -OF. CURVES FOR PREDICTING HYDRATE FORMATION. 2
-INTERSECTION OF FREE EXPANSION CURVES WITH HYDRATE FORMATION GRAVITY GASES. FIG. 4.-PERMISSIBLE FINAL PRESSURE - PSIA. EXPANSION OF METHANE WITHOUT HYDRATE FORMATION. 3
4 PREDICTION OF CONDITIONS FOR HYDRATE FORMATION IN NATURAL GASES Compositions of typical natural gases were selected to cover the gravity range from 0.6 to 1.0. The pressures at which hydrate would form at 40, so0, 60, and straight-line section at the high pressure. The curves were terminated at 40m lb. per sq. in., since the nature of the curves at higher pressures is not known. 70 F. were computed, using vapor-solid equilibrium constants.' The results of these calculations, along with the experimental data,%aa were plotted on Fig. I, using gas gravity to represent compositi~n.~s@ From the smooth curves on Fig. I, the curves of Fig. 2 were drawn, using the experimental curves2 as a guide for the As there was considerable variation bctween the points and the curves drawn on Fig. I, the compositions of some natural gases represented by the hydrate curves on Fig. 2 for a given gravity are tabulated in Table I. Only natural gases having compositions similar to these arc represented by the curves of Fig. 2.
DONALD L. KATZ 5 CHARTS FOR PREDICTISG PERMISSLBLE until this expansion curve reaches the EXPANSION OF NATURAL GASES hydrate-formation curve of Fig. 2. WITHOUT HYDRATE FORMATION The enthalpy-entropy charts described The curves of Fig. 2 are the minimum by G. G. Brown6v7 provide the relationship temperatures at corresponding pressures of temperature versus pressure for gases lo IATION. to which a given gas may be cooled by ex- of even gravities by following constant pansion, or otherwise, without hydrate enthalpy lines for free expansion. Fig. 3 formation. A gas under specified initial gives the pressure-temperature curves temperature and pressure may be ex- when expanding a 0.6 gravity gas from panded freely, decreasing temperatures 2000 lb. per sq. in. and 110 F. (A) and corresponding to decreasing pressures, from 1900 lb. per sq. in. and 120 F. (B).
The intersection of these curves with tionships for pure methane while Figs. 5 the hydrate-formation curve taken from through 9 are for the natural gases of Fig. 2 is the final pressure to which the given gravity. gases may be expanded without hydrate The enthalpy-entropy information for formation. methane was limited to 3000 1b.,l0 hence CATION. Using the enthalpy-entropy charts7 and the curves from Fig. 2, charts were constructed giving a complete range of initial pressures and temperatures with corresponding final pressures to which the gases may be expanded freely without hydrate formation. Fig. 4 gives the rela- this chart,does not go to the IO,WO-lb. initial pressure used for the natural gases. Errors in constructing the charts were located by cross-plotting the final pressures to which the gases could be expanded as a function of gas gravity for a series of initial pressures. The cross plot for a
DONALD L. KATZ 7 2000-lb. initial pressure is given by factor with temperature is positive, the Fig. 10. gas decreases in temperature upon expan- The reversal in the curves at high final sion, and where the rate of change is FINAL PRESSURE - PSIA FIG. 8.-PER~SSIBLE EXPANSION OF A 0.9 CR.~VITY NATVRAL GAS WITHOUT HYDRATE FORMATION pressures on Figs. 5 through 9 occurs because the temperature of natural gases increases on free expansion from above 5000 to 6000 lb. per sq. in. and then decreases after reaching pressures in this range. This pressure at which gases decrease in temperature depends upon the compressibility-factor curves. Where the rate of change of the compressibility negative, the gas increases in temperature upon free expansion. The slight curvature of the initial temperature curves at low final pressures is due to the exit of the free expansion CUNe from the hydrate-formation region as illustrated by curve B of Fig. 3. When two alternative final pressures are given for an initial pressure and temperature, the
final pressure after expansion without hydrate formation may be the higher pres- answer, from Fig. 5, is 1050 lb. per sq. in. absolute. sure as a minimum and the lower pressure as a maximum. Fig. 4 through 9 may be used to solve two types of problems, with examples as follows: 1. How far may a '0.6 gravity gas at zoo0 lb. pressure and loo F. be expanded without danger of hydrate formation? The 2. How far may a 0.6 gravity gas at zoo0 lb. and 140 F. be expanded without hydrate formation? The answer, from Fig. 5, is that hydrates will not form upon expansion to atmospheric pressure. 3. A 0.6 gravity gas is to be expanded from'15oo Ib. to 500 lb. What is the minimum initial temperature that will permit the expansion without danger of hydrates? The answer, from Fig. 5, is an initial temperature of 99 F. or above.
DONALD L. KATZ 9 4. A 0.6 gravity gas at 10,ooo lb. and A word of caution should be given that 140 F. is expanded freely and adiabatically: the charts are valid only for gases having a. At what lower pressure will the tem- similar compositions for the gravities corperature become 140 F.? responding to those of Table I. The ex- TE INITIAL b. At what pressure would hydrates be likely to appear if sufficient water is present? The answer, from Fig. 5, is (a) 4300 Ib. and (b) I 150 pounds. Similar problems may be solved for gases of other gravities with interpolation between charts when necessary. pansions described are always free and adiabatic. Hydrates will form only when sufficient (usually liquid) water is present, and the charts do not apply to anhydrous natural gases. ACKNOWLEDGMENT The cooperation of the Graduate Students in the preparation of this paper and
10 PRISDICTION OF CONDITIONS FOR IIYDHATI.; I.'ORhIATION IS SATURAL GASES in the construction of the figures is 5. Brownscombe and Howe: Oil and Gas Jnl. (1940) 39 (30). 37. appreciated. 6. Brown: Plot. Nat. Gasoline Assn. of Amer. (May 1040) 54. REFERENCES 7. ~rown; A ' ~&es of Enthalpy-entropy I. Carson and Katz: Trans. A.I.M.E. (1~41). -. Charts for Natural Gases. New York 146, 150. Meeting, A.I.M.E.. Feb. 1944. 2. Wilcox, Carson and Katz: Ind. and Eng., 8. Katz: Plot. Nat. Gasoline Assn. of Amer. Chem. (1941) 33, 662. (1942) 41; Refiner (1942) 21 (6). 64. 3. Deaton and Frost: Amer. Gas Assn. Nat. 9. Katz: Amer. Petr. Inst. Drill. and Prod. Gas Dept. Plot. (May 1940) 122. Practice (194a) 137. I. Hammerschmidt: Ind. and Rna. Chem. I ro. Sage, Olds, and Lacey: Amer. Petr. Inst. Drill. and Prod. Practice (1942) 162.