1、Technical Publication TP-10 Hydrate Decomposition Conditions in the System Hydrogen Sulfide- Methane, and Propane J. F? Schroeter Riki Kobayashi H. A. Hildebrand: Rice University Exxon Production Research Co.: Houston, Texas December, 1982 Gas Proces sors Association 1812 First Place Tulsa, Okia 741
2、03 Phone 918/582-5112 GPA TP-10 82 3824699 OOLLL78 7LT - FOREWORD Recently Professor Riki Kobayashi and his associates at Rice University obtained experimental hydrate decomposition data that we felt would be of value to the gas processing industry. Since their work was not supported by GPA, it coul
3、d not properly be published as a numbered Research Report (RR). To get information of this type to this industry, the numbered Technical Publication (TP) was developed. The data are being published and distributed as Technical Publication No. 10, entitled “Hydrate Decomposition Conditions in the Sys
4、tem Hy- drogen Sulfide, Methane, and Propane”. They should be valuable in predicting hydrate formation in acid gas systems. Thanks are extended to Professor Kobayashi and his Co-workers for obtaining the data and also to Karl Kilgren, chairman of the GPA Phase Equilibria Steering Committee, who edit
5、ed and prepared the final format for TP-10. casi Sutton, Secretary i GPA TP-LO E I 32Yb77 0011177 b5b Table of Contents Foreword . Table of Contents List of Tables G Figures . Introduction . Results . Discussion of Results Experimental Procedure References Page i ii iii 1 2 5 10 13 ii GPA TP-LO 82 H
6、 3824679 OOLLLBO 378 List of Tables Table I Hydrate Decomposition Conditions - 4.174% H2S, 7.172% C3H8 88.654% CH4 Mix. Hydrate Decomposition Conditions - 11.975% H2S, 7.016% C3H8 81.009% CH4 Mix. Hydrate Decomposition Conditions - 31.710% H2S, 7.402% C3H8, 60.888% CH4 Mix. II III Figure 1 List of F
7、igures Hydrate Formation Pressure Versus Temperature Data and Hydrate Program Predictions for 3 Sour Gas Compositions Hydrate Formation - Decomposition Hysteresis Curves (Actual Experiments) Hydrate Formation - Decomposition Hysteresis Curves (Idealized Case) Hydrate Formation Pressure Versus Mol Pe
8、rcent Propane in Gas Phase at -3OC (After Van der Waals E Platteuw, 1959) Page 3 3 3 Page 4 iii GPA TP-LO 82 = 3824699 OOLLLBL 204 INTRODUCTION Though the existence of hydrates was demonstrated by Davy in the early part of the nineteenth century, current interest dates from 1934, when Hammerschmidtl
9、 discovered that hydrates were responsible for plugging natural gas lines. This discovery stimulated a number of studies to determine hydrate decomposition conditions. derived equations for calculating the thermodynamic properties of gas hydrates based on a statistical thermodynamic model. This meth
10、od was used by Parrish and Prausnitz3 for calculating hydrate-gas equilibria in multi- component systems. Van der Waals and PlatteeuwL A computer model has been developed which combines the approach of 4 Parrish and Prausnitz with the BWRS equation of state phase behavior model. As in the work of Pa
11、rrish and Prausnitz, the Kihara potential function is used to represent the intermolecular forces between gas and water molecules. determined for each gas which forms hydrates. been determined, the accuracy of the computer model can be assessed by comparing its predicted hydrate decomposition condit
12、ions with observed data for systems containing more than one hydrate-forming component. This computer model was found to be very accurate for sweet systems, pre- dicting hydrate decomposition conditions to within 2 C, for gas, liquid, and two-phase hydrocarbon systems. used to adjust the Kihara para
13、meters in order to obtain comparable accuracy for sour gas systems. The Kihara potential has three parameters that must be Once these parameters have O Data from the present study have been Hydrates crystallize into two kinds of lattice structures: Structure I and Structure II. Pure H2S, as well as
14、gases composed of 1 GPA TP-10 82 El 3824699 OOLL182 140 only methane, CO2 and H S, form Structure I hydrates. sufficient amount of propane or isobutane are present, Structure II hydrates will be formed. Data on the system methane-hydrogen sulfide had been used to adjust Kihara potential parameters f
15、or H2S in hydrates of Structure I. Additional data were required to determine whether the However, if a 2 Kihara parameters for H S in 2 parameters for Structure II, decomposition condit ions for H S and forming Structure II 2 Structure I, when coupled with the cell would be adequate for predicting
16、hydrate systems containing significant amounts of hydrates. The present study was conducted to elucidate the hydrate decomposition properties of the Structure II forming systems containing H S and both methane and propane. 2 RE CULT S Experimental hydrate decomposition conditions are presented for t
17、hree O different H S-containing mixtures in the temperature region O C to 3OoC. The three mixtures investigated were 4% H S, 7% propane, 89% methane; 12% H2S, 7% propane, 81% methane; and 30% H S, 7% propane, 63% methane. Hydrate decomposition pressures and temperatures were obtained for each of the
18、se mixtures by observation of the pressure-temperature hysteresis curves associated with formation and decomposition of the hydrate crystals. A repeatable decomposition point was observed in every case, and this was identified as the hydrate point. used to adjust parameters in a computer model based
19、 on the Parrish and Prausnitz statistical thermodynamics method, coupled with the BWRS equation of state. After the parameter adjustment, the computer model predicted the behavior of the 12% H S and the 30% H S mixtures to within 2OC. 2 2 2 The results for the 4% H S mixture were 2 2 2 Experimental
20、data for the three mixtures are given in Tables I, II, III and plotted on Figure i. 2 GPA TP-10 82 3824699 OOLLL83 087 Temperature (Cl TABLE I Hydrate Decomposition Conditions For: Pressure (psial 2.8 81.4 4.6 102.4 11.0 205.8 14.2 293.5 18. O 488.3 4.174% H2S, 7.172% C3H8 and 88.654% CH4 TABLE II H
21、ydrate Decomposition Condit ions For: Temperature (Cl Pres sure (psial 2.7 49.2 10.4 118.5 19.5 408. O 11.975% H2S, 7.016% C3H8 81.009% CH4 TABLE III Hydrate Decomposition Conditions For: Temperature (Cl Pressurp (psia) 7.2 53.4 13.1 99.5 19.1 209.5 24.3 370.5 27.8 620. O 31.710% H2S, 7.402% C H 60.
22、888% CH4 3 8 3 io,ooo 1000 100 GPA TP-10 2 PI 3824699 OOLlL84 TL3 5 I I I 0 COMPOSITION A I I 0,88654 Ci / 0,07172 C3 0,04174 H2S COMPOSITION 8 COMPOSITION C P A/ /I 1 li i 0,07402 C3 0,31710 hgS IO 30 40 50 60 70 80 90 TEMPERATURE (OF) Fig. 1 Hydrate Formation Pressure Versus Temperature Data and H
23、ydrate Program Predictions for 3 Sour Gas Compositions 4 GPA TP-LO 82 I 3824699 001LL85 95T DISCUSSION OF RESULTS Figure 2 shows the results obtained from a typical experimental run As expected, the region near the hy- of the system H S-methane-propane. drate formation point shows a hysteresis curve
24、 in pressure versus temper- ature space. The hysteresis is a result of the metastability o hydrate forming compounds on the cooling (downwards) portion of the curve. this case, supercooling of as much as 5 C was observed on the initial downward pass. Due to the hysteresis, the location of the Ipoint
25、” of hydrate formation was found to be very much a function of its history. The decomposition conditions were much less ambiguous, but the crystals exhibited some resistance to total decomposition. divergence of the heating and cooling curves was observed to be repeatable over the complete range of
26、temperature drift rates and starting temper- atures. Additionally, this is the expected point of complete hydrate decomposition in an equilibriated system. We therefore take it as the hydrate point. The location of this point can be determined in our experiment with an accuracy of better than 0.1 C
27、in temperature and 0.1 psia in pressure. In the example of Figure 2, the hydrate point is determined to be T = 13.1C and P = 99.5 psia. 2 In O The upper point of O Note that the amount of supercooling varies from pass to pass in Figure 2. For example, the initial downward pass shown in that figure e
28、xhibits more supercooling than does the second pass. We have observed this phenomenon to depend upon the starting temperature, as shown ideally in Figure 3. In that figure, the three different cooling curves S and Sg correspond to three different starting temperatures. we have observed that the amou
29、nt of supercooling increases as the starting temperature increases. We speculate that this phenomenon is due to the 1, s2 In general, 5 GPA TP-LO 82 II 3824699 OOLLLb 896 II I I I 1 I O O - O0 o) a o) * o) O o) n O IL U t- h m c. a d E .- & a a x w af 3 iI ci v z m 1 O rn rn a Y .- 2 3 3 6 8 .,-I .-
30、 CI m O o c O .- ci a O k a CI -a 2 i? hl M z 6 GPA TP-LO 82 3824b99 OOLLL87 722 I GPA TP-LO 82 m 3824699 OOLLLBB bb9 m persistence of micro-crystals of hydrate above the decomposition point. These micro-crystals would act as nucleation sites for hydrate formation on the cooling cycle of the curve.
31、and higher starting temperatures, it is expected that fewer of the crystals survive. nucleating micro-crystals would exhibit greater amount of supercooling, thus accounting for the starting temperature dependence of the supercooling effects. As the system is cycled to higher We suspect that systems
32、containing a smaller number/size of Tables I - III give the results for the hydrate decomposition point for the three different gas concentrations. These data are plotted in Figure 1. Also plotted are the predictions of the computer model, with new Kihara potential parameters for H S in Structure II
33、 determined by the fit to the 4% H S data. The predictions are good for all three gas compositions, with a maximum temperature deviation from the data of approximately 2 C. 2 2 O There is one data set available in the open literature for Structure 9 II hydrates containing H2S . In that figure, the h
34、ydrate formation pressure in atmospheres is plotted versus C H - H S gas phase composition in mole percent. The data are shovm as points and the model predictions are shown as a solid line. Again, agreement between the predictions and data is good. predictions of hydrate formation conditions in sour
35、 systems using the computer model appear to be accurate to within 2 C, just as in sweet systems. The data are displayed in Figure 4. 38 2 Consequently, O GPA TP-10 82 H 3824699 OOLL187 5T5 m I STRUCTURESTRUCTUI Il I II e I I I I I I 4 -,-L. H9S 20 40 60 80 C3H8 L MOLE Yo IN GAS PHASE Eig. 4 Hydrate
36、Format ion Pressure Versus Plol Percent Propane in Gas Phase at -3 C (After Van der Waals E Platteuw, 1959) O 9 GPA TP-10 2 3824699 OOLLL70 217 EXPERIMENTAL PROCEDURE There are two popular experimental methods employed to determine the hydrate formation/decomposition point. At lower pressures, visua
37、l observation of the formation and decomposition of hydrate crystals in a windowed cell has been used successfully . servation of the stability of hydrate crystals, at constant temperature, for periods of six to eight hours. It is therefore somewhat time con- suming. Additionally, this method may on
38、ly be used unambiguously at temperatures where there is no danger of confusion between hydrate crystals and ice crystals - i.e. above the freezing point of water . 5 This method requires ob- 6 A less tedious, non-visual technique which can be used at high as well as low pressures has been developed
39、in our laboratory7*. method employs the pressure change due to hydrate formation or decomp- osition in an isochoric (constant volume) cell. Since gas is evolved by decomposing hydrate crystals, a pressure increase accompanies hydrate deccmposition. A similar drop in pressure signals the formation of
40、 the hydrates. This technique is more amenable to computer control than the visual method and offers the additional advantage of being applicable over the entire range of hydrate formation pressures and temperatures, except perhaps when nearly incompressible phases are involved. This technique was e
41、mployed in the present study. This A limitation of this technique is the rate at which pressure data can be collected. at a variety of different equilibrium temperatures. Since hydrates are slow to form near their decomposition point, relatively long periods of time are required or the pressure to c
42、ome to equilibrium after a temperature change . Generally, the cell is allowed to heat (or cool) at a rate which is much slower than the Inverse pressure equilibration time. In a practical experiment, the observer collects data 7 GPA TP-LO 82 111 3824699 OOLLL1 153 m Typically, this rate of change o
43、f temperature must be restricted to less than . 30C/hrg. It has frequently been demonstrated that hydrate forming compounds can exist in meta-stable states at temperatures substantially below the formation temperature. supercooling. Thus, a precise determination of the hydrate formation/ decompositi
44、on temperature requires an experiment spanning at least 10 C. If the initial guess of the location of the hydrate point is not correct, Often, systems may exhibit as much as 10C O an experiment can easily require 50 hours for even one pass through the hydrate formation/decomposition point. Typically
45、 at least three passes are necessary to establish the hydrate point. Such a long experiment, requiring that a data point be collected at approximately one-half hour intervals for fifty hours, cannot be con- ducted by a single experimenter. we have constructed a device which automatically controls th
46、e temperature, reads the pressure at specified intervals, and stores the data for later read-out. This frees the experimenter from the more tedious parts of the experiment and permits around-the-clock data acquisition. Using a Commodore PET microcomputer, To ensure equilibrium of tho phases present
47、in the cell, the ccll was rocked about its horizontal posit inn inside a temperature zxtnollnd bath using a gear motor and cam. Two 3/4 inch stainless steel ball bearings were placed inside the cell. Thus, the rocking cell acted as a ball mill in order to expose and convert liquid water which could
48、be trapped between the gas-hydrate interface and the cell wall. The agitation caused by the motion of the balls was observed to shorten the pressure equilibration time substantially. Temperature conrrol is provided by a refrigeration unit coupled with the computer controlled heater. The combined use
49、 of heating and refrigeration 11 GPA TP-LO 82 3824699 OOLLL92 09T = O provides temperature control of - t .O1 C. functions as a data collection and storage device, as well as a temper- ature controller. In this system, the computer The printer provides a hard copy data output. Before starting each run, the cell was removed from the bath and cleaned thoroughly with water, acetone, and ethanol. After cleaning, the cell was rinsed for approximately one half hour with triply distilled water in order to remove any possible contamination from the cleaning agent
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