1、Technical Publication TP=26 Mutual Solubility in Water/Methanol Hydrocarbon Systems DB Robinson Research Ltd. Norsk Hydro May, 2000 Gas Processors Association 6526 East 60th Street Tulsa, Oklahoma 74145 Phone: 9181493-3872 * FAX: 9181493-3875 FOREWORD Methanol is commonly used as hydrate inhibitor a
2、nd for hydrate melting in gas production and processing facilities. GPA has funded several projects investigating the hydrate inhibition effects of methanol. Additionally it is important to have a good understanding of the methanol distribution in the hydrocarbon and aqueous phases, when designing t
3、hese systems. The available experimental data for methanol distribution is limited. This Technical Publication (TP) is a summary of work done by Norsk Hydro. The data was compiled by DBR and Associates, the principal investigators for this work. The GPA wishes to thank Norsk Hydro for their willingn
4、ess to share this data with the industry and save the GPA from using their limited research budget for remeasuring similar data. - Subgroup 2 David Bergman/ Chairman Technical Section F “Copyright 2000 by Gas Processors Association. All rights reserved. No part of this Publication may be reproduced
5、without the written consent of the Gas Processors Association.” 1 GPA DISCLAIMER GPA publications necessarily address problems of a general nature and may be used by anyone desiring to do so. Every effort has been made by GPA to assure accuracy and reliability of the information contained in its pub
6、lications. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. It is not the intent of GPA to assume the duties of employers, manufacturers, or suppliers to warn and properly train employees, or others exposed, concerning health and safety ris
7、ks or precautions. GPA makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any federal, state, or municipal regulation with which this publ
8、ication may conflict, or for any infringement of letters of patent regarding apparatus, equipment, or method so covered. 11 MUTUAL SOLUBILITY IN WATEWMETHANOL HYDROCARBON SOLUTIONS 1 .O INTRODUCTION In the course of carrying out engineering calculations involving the use of methanol as a dehydrating
9、 agent or as a hydrate depressant, one frequently encounters the problem of knowing the distribution of methanol between the aqueous liquid phase and the hydrocarbon liquid or gaseous phases. Although limited data are available on the mutual solubility of methanol and liquid hydrocarbons or related
10、materials such as hydrogen sulfide and carbon dioxide, little or no data seem to exist on the distribution of the methanol in such a system if it equilibrated with water. In practice, this lack of information creates a problem in calculating the methanol requirements to achieve a specified depressio
11、n. For example, the specified methanol concentration in the aqueous liquid may be 20 wi. percent, but the total methanol that must be added to the flowing system containing liquid condensate to achieve this concentration cannot be reliably calculated. These matters are of obvious concern to Companie
12、s such as Norsk Hydro which have extensive oil and gas operations in the North Sea where all of the conditions necessary for hydrate formation are normally present. In order to advance the technology in this area, Norsk Hydro undertook a systematic investigation of the distribution of water and meth
13、anol between hydrocarbon and aqueous phases. The first part of the study included measurements on simple well defined systems containing the classes of compounds of interest, namely paraffinic, aromatic and naphthenic hydrocarbons. The information on these systems can be used for testing existing mo
14、dels and for model development. The final stage of the study used actual North Sea condensate fluids to provide data on the actual systems of interest to Norsk Hydro and to provide information to evaluate the proposed models. 2.0 SCOPE Part I covered measurements on three hydrocarbon mixtures contai
15、ning constant methane and propane content and varying amounts of toluene, n-heptane and methyl cyclohexane. One mixture was rich in n-heptane, one in toluene and one in methylcyclohexane. They had the following compositions: Mole Fraction Component Mixture I Mixture II Mixture III CH4 C3H8 nC7 O .63
16、 0.07 0.1 6 0.63 0.07 0.07 Methylcyclohexane 0.07 0.16 Toluene 0.07 0.07 0.63 0.07 0.07 0.07 0.16 It was originally planned to study each mixture at temperatures of -100, 200 and 5OoC, at a pressure of 100 bars and at methanol concentrations of nominally 20, 40 and 60 percent by weight. However, the
17、 20 and 40 wi.% methanol mixtures could not be studied at -lOC because of hydrate formation. Consequently, measurements were made about 1C above the hydrate formation temperature. This was 12.30C for the 20 wt.% methanol solution and -1.70C for the 40 wt.% solution. At each experimental temperature/
18、pressure/concentration condition, the composition of the equilibrium hydrocarbon liquid, the aqueous liquid and the vapor phase was determined. Part II covered measurements on three hydrocarbon mixtures. The first mixture contained methane, propane, toluene, n-heptane and methyl cyclohexane; the sec
19、ond mixture contained methane and benzene; and the third mixture contained methane and 1,3,5 trimethyl benzene. These mixtures had the following compositions: 1 Mole Fraction Mixture i Mixture I! Mixture iiJ Component CH4 C3H8 nC7 0.6405 0.6 0.0679 0.0972 Met hy I Cyclohexane 0.0972 Toluene 0.0972 B
20、enzene 0.4 0.6 Mesitylene 0.4 The experimental conditions for mixture I were pressures of 50, 1 O0 and 150 bars, at a temperature of 20C and at methanol concentrations of nominally 20, 40 and 60 percent by weight at each pressure condition. This experiment was to investigate the effect of pressure o
21、n relative phase compositions for each component. It was originally planned to study mixtures II and III at temperatures of -loo, 200 and 50C, at a pressure of 100 bars and at methanol concentrations of nominally 20, 40 and 60 percent by weight. In addition, the amount of methanol-water solution cha
22、rged into the cell for the test was to be kept to a minimum. However, the lowest temperature that could be used for mixture II was OC, in order to avoid hydrate formation and/or freezing of benzene. Similarly, the 20 and 40 wt Y methanol mixtures could not be studied at -10C for mixture III because
23、of hydrate formation. Consequently, a temperature of 3C was used instead. At each experimental temperature/pressure/concentration condition, the composition of the equilibrium hydrocarbon liquid, the aqueous liquid and the vapour phase were determined. However, only the methanol and water concentrat
24、ions in the aqueous phase were determined for the studies on mixture II and 111. This was because the amount of aqueous liquid present in the system was too small for a reliable determination of the hydrocarbons solubility. Part III covered measurements on a stabilized condensate when mixed with two
25、 prepared gas mixtyes. The compositions of the condensate and the two prepared gas mixtures are given in Table 1. The experimental conditions for this study were at pressures of 1 O0 and 200 atm and at temperatures of O“, 20“ and 5OoC in the presence of 40, 60 and 80 wi % methanol solutions. A minim
26、um amount of the methanol solution was used in each experiment. It was in the order of 0.5 cm4. At each experimental temperaturelpressurekoncentration conditions, the composition on the equilibrium hydrocarbon liquid and the vapor was determined. However, only the methanol and water concentrations i
27、n the aqueous phase were determined. This was because the amount of aqueous liquid present in the system was too small for a reliable determination of the hydrocarbons solubility. 3.0 METHODOLOGY 3.1 Experimental Equipment The experiments were carried out by confining the prepared mixtures in a wind
28、owed cell fabricated from stainless steel and tempered pyrex glass that permitted through - observation of the contents. The cell had a working volume of approximately 80 cm and was operated in the variable volume mode. The cell contents were confined over mercury which could be added or withdrawn u
29、sing a high pressure pump to vary the volume and hence the pressure. The cell was mounted inside a temperature controlled liquid bath. The entire cell-bath assembly was mounted on trunnions so that it could be rocked back and forth about a horizontal axis in order to hasten the attainment of equilib
30、rium. The pressure of the system was measured with a calibrated 0-200 bar Heise gauge. Temperatures were measured using calibrated copper-constantan thermocouples with a digital readout so that temperatures were known to within kO.1oC. 2 3.2 SamDle Preparation The hydrocarbon samples were prepared i
31、n a stainless steel pressure container that had been thoroughly cleaned and evacuated. Based on the known composition of the desired mixture, the required amount of each component was successively added to the container until the required composition was achieved. The amount of each component added
32、was obtained gravimetrically by weight difference on a balance reliable to - +0.01 g. Following this preparation, the contents were pressurized to approximately 275 bars and homogenized by rocking overnight to ensure that a single phase existed at room temperature. The mixture was sampled and analyz
33、ed chromatographically to confirm that no errors had been made during the preparation. 3.2.1 Stock Tank Condensate One liter of stock tank condensate was supplied by Norsk Hydro. It was transferred into a high pressure sample cylinder and was then compressed into a single phase by injecting mercury
34、into the cylinder. A portion of the sample was then withdrawn for composition analysis and density determination of room temperature. The composition of the stock tank condensate is given in Table 1. 3.2.2 Gas Mixtures The gas mixtures with specified compositions were prepared in a stainless steel p
35、ressure container which had been thoroughly cleaned and evacuated. Based on the known composition of the desired mixture, the required amount of each component was successively added to the container until the required composition was achieved. The amount of each component added was obtained gravime
36、trically by weight difference on a balance reliable to +0.01 g. Following this preparation, the contents were pressurized to approximately 275 bars and homogenized by rocking overnight to ensure that a single phase existed at room temperature. The mixture was sampled and analyzed chromatographically
37、 to confirm that no errors had been made during the preparation. 3.3 Equilibration Procedure Part I KVL2, the ratio in the vapor phase relative to the aqueous liquid phase; and KLlL2, the ratio in the hydrocarbon liquid phase relative to the aqueous liquid phase. 4 Part II The data for the nine expe
38、rimental pressure-temperature-concentration conditions for mixture I are given in Tables 8, 9 and 10. The data for the nine experimental conditions for the methane-benzene studies and for the nine experimental conditions for the methane-mesitylene studies are presented in Tables 11 and 12 respective
39、ly. Equilibrium ratios were calculated for each component in the system for each pair of phases. These are given in Tables 13, 14 and 15 and include KVL, the ratio for the concentration in the vapour phase relative to the concentration in the hydrocarbon liquid; KVL2, the ratio in the vapour relativ
40、e to the aqueous phase; and KLIL2, the ratio in the hydrocarbon liquid phase relative to the aqueous phase. Part 111 The data for thirteen experimental pressure-temperature-concentration conditions for hydrocarbon mixtures of Gas 1 and stabilized condensate in the presence of three methanol solution
41、s are given in Tables 16, 17 and 18. The data for five experimental pressure-temperature-concentration conditions for hydrocarbon mixtures of Gas 2 and stabilized condensate in the presence of three methanol solutions are given in Table 19. The detailed equilibrium phase compositions of the individu
42、al hydrocarbon for Tables 16, 17, 18 and 19 are presented in Tables 20 through 26. 5.0 DISCUSSION OF RESULTS, PART I The data set obtained in this work provides information on the mutual solubility of five hydrocarbons in methanol-water solutions at 27 different composition and temperature condition
43、s and a pressure of 1 O0 bars. In alt, 81 separate equilibrium phase analyses were carried out. 5.1 Data Consistency The equilibrium ratios KVL2 and KLIL2 for methanol show that with few exceptions the data on the distribution of methanol is consistent and within the accuracy possible when dealing w
44、ith these difficult mixtures. The equilibrium ratios for the hydrocarbons in the system was not a primary objective of the study, the data shown on the graphs does illustrate the remarkable consistency achieved throughout the work. The range of numerical values for the equilibrium ratios covers six
45、orders of magnitude. The solubility of all hydrocarbon components increased sharply as the methanol concentration increased from 20 to 60 wl. -percent. The XLI/XL2 equilibrium ratios decrease as the temperature increases. This reflects the fact that the concentration of the hydrocarbons in the aqueo
46、us liquid phase increases as the temperature increases, hence decreasing the value of the equilibrium ratio where appears in the denominator. 5.2 Influence of Aromatic and Naphthenic Compounds The replacement of part of the normal C7 hydrocarbon in the system with the aromatic toluene caused a measu
47、rable increase in the amount of methanol that dissolved in the hydrocarbon liquid phase. On the other hand, replacement of the same amount of the normal hydrocarbon with the naphthenic methyl-cyclohexane did not cause a measurable difference. Any measured differences between the solubility of methan
48、ol in the paraffinic and naphthenic C7 compounds are thought to be within the tolerance of the experimental measurements. 5 5.3 Water Distribution The concentration of water in both the vapor and hydrocarbon liquid phases was measured at each of the 27 experimental conditions. However, there was too
49、 much scatter in this data to justify any graphical representation. The inherent difficulties in dealing with very low water concentrations coupled with the need to use TCD instead of FID undoubtedly contributed to the loss of accuracy. It was not felt that water distribution was an important part of the overall study, so no major effort was expended to try to resolve the problem. 6.0 DISCUSSION OF RESULTS, PART II The data set for mixture I obtained in this work provides information on the mutual solubility of a five-component hydrocarbon mixture in methanol-water solutions at 20C and
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