Measurement of the Hydrocarbon Dew Point of Real and Synthetic

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Energy & Fuels 2009, 23, 1640–1650

Measurement of the Hydrocarbon Dew Point of Real and Synthetic Natural Gas Mixtures by Direct and Indirect Methods Andrew S. Brown,*,† Martin J. T. Milton,† Gergely M. Vargha,† Richard Mounce,‡ Chris J. Cowper,‡ Andrew M. V. Stokes,§ Andy J. Benton,§ Dave F. Lander,| Andy Ridge,⊥ and Andrew P. Laughton† Analytical Science Team, National Physical Laboratory, Hampton Road, Teddington, U.K. TW11 0LW, EffecTech Ltd., DoVe Fields, Uttoxeter, U.K. ST14 8HU, Michell Instruments Ltd., Lancaster Way Business Park, Ely, U.K. CB6 3NW, National Grid, National Grid House, Warwick, U.K. CV34 6DA, Orbital Ltd., Cold Meece, Swynnerton, U.K. ST15 0QN, and AdVantica Ltd., Ashby Road, Loughborough, U.K. LE11 3GR ReceiVed October 29, 2008. ReVised Manuscript ReceiVed December 22, 2008

A detailed comparison of the performance of direct and indirect methods for measuring the hydrocarbon dew point (i.e., the temperature at which the condensation of hydrocarbons in a gas mixture first occurs) of a range of real natural gases and synthetic natural gas mixtures is presented. Good agreement is found between the results of the different methods, although a difference is observed between those from the synthetic gas mixtures and the real natural gases, where the relative order of the dew points determined by direct and indirect instruments is reversed. The relationship between measured hydrocarbon dew point and the condensation rate of the gases is investigated. Using an automatic chilled mirror instrument to measure the dew point of gases with low condensation rates can result in a discrepancy of up to 2 K when compared to the measurement of fast-condensing gases. The limitations of the indirect gas chromatography methods, particularly the difficulties in measuring hydrocarbon species containing 10 or more carbon atoms, have been modeled and can also result in a discrepancy of up to 2 K when compared to the measurement of gases containing no such C10+ species. The magnitude of these discrepancies is close to the uncertainty of the method.

Introduction Accurate measurement of the highest temperature at which hydrocarbons in natural gas condense (known as the “hydrocarbon dew point”1) is essential to ensure that it can be transported safely through pipelines. All natural gases must comply with specifications for hydrocarbon dew point that are set in order to prevent the formation of hazardous liquid condensate in pipelines. Processed natural gas distributed through transmission pipelines is a complex mixture comprising methane (typically at an amount fraction greater than 600 mmol/mol) together with a wide range of other hydrocarbon species (extending to at least C10 and often beyond C15), nitrogen, carbon dioxide, and other trace species. The amount fraction of each hydrocarbon typically decreases with increasing carbon number, ethane and propane are often present at levels above 10 mmol/mol, whereas the total hydrocarbon amount fractions for C11 and higher carbon numbers are typically less than 1 µmol/mol. The hydrocarbon dew point of natural gas is highly sensitive to the composition of the gas, particularly to the amount fraction of components with six or more carbon atoms. This is demonstrated by Figure * Corresponding author. E-mail: [email protected]; phone: +44 20 8943 6831; fax: +44 20 8614 0448. † National Physical Laboratory. ‡ EffecTech Ltd. § Michell Instruments Ltd. | National Grid. ⊥ Orbital Ltd. † Advantica Ltd. (1) ISO Technical Report Natural Gas s Hydrocarbon Dew Point and Hydrocarbon Content; International Organization for Standardization; ISO/ TR 11150:2007, 2007.

Figure 1. Calculated hydrocarbon dew point of a series of single straight-chain hydrocarbon isomers (C2H6 to n-C20H42) in methane at a pressure of 27 bar at amount fractions of 10 µmol/mol (filled circles), 1 µmol/mol (circles), 100 nmol/mol (filled triangles), 10 nmol/mol (triangles), and 1 nmol/mol (filled squares).

1, which shows the calculated hydrocarbon dew point (at 27 bar) of a series of mixtures containing between 1 nmol/mol and 10 µmol/mol of single straight-chain hydrocarbons in methane. (The hydrocarbon dew points have been calculated as described in the Experimental Section.) The plot shows that the calculated value of the hydrocarbon dew point for the 10 µmol/mol mixtures increases significantly for species containing six or more carbon atoms, and mixtures of the highest carbon number species have high dew points, even at low nmol/mol amount fractions. A further complication to the accurate measurement of hydrocarbon dew point is caused by the nonideal (“retrograde”) behavior of the plot of hydrocarbon dew point against pressure

10.1021/ef8009469 CCC: $40.75  2009 American Chemical Society Published on Web 01/30/2009

Hydrocarbon Dew Point of Natural Gas Mixtures

(i.e., as pressure increases, the dew point temperature typically first increases to a maximum (the cricondentherm) before decreasing at higher pressures). This places great importance upon the pressure at which any measurement of hydrocarbon dew point is made. In this work we compare the performance of direct and indirect methods for measuring hydrocarbon dew point. Direct methods are those that depend on the observation of the formation of condensate, whereas indirect methods calculate the conditions under which a condensate will form using data from other measurements, such as those of the composition of the mixture. These methods have different strengths and limitations, which we quantify. Direct methods for the measurement of hydrocarbon dew point rely upon the detection of the formation of a condensate film on the surface of a mirror as the temperature is reduced. Various types of manual chilled mirror instrument (MCMI)2 have been in widespread use since the 1940s.3 These rely on a skilled operator observing the formation of a film of liquid condensate on the surface of the mirror while the temperature of the system is decreased. The automatic chilled mirror instrument (ACMI)4 used in this study measures the light scattered from a light-emitting diode to detect the formation of a condensate film on the surface of a concave mirror. It has the advantage over the MCMI that the measurement does not rely upon the judgment of an observer. Although all chilled mirror instruments detect the onset of the condensation process directly, they depend on the availability of sufficient material in the vapor phase to form a detectable liquid film. This is quantified by the “condensation rate” of the mixture, which is the amount of condensate in milligrams of liquid per cubic meter of gas (i.e., the “potential hydrocarbon liquid content” (PHLC)5) formed per Kelvin of temperature change below the hydrocarbon dew point. (All calculations of condensation rate throughout this work were undertaken using temperature and pressure conditions of 15 °C and 1 atm.) In this work, we demonstrate for the first time that the calculated condensation rate can be used to explain the behavior of the direct method. The basis of all indirect methods is the use of a gas chromatograph (GC) to determine the composition of the mixture and then the use of a thermodynamic equation of state to calculate the condensation curve. Although the composition of the gas can be determined with very high accuracy,6-8 the overall accuracy of the method also depends on the validity of the equation of state used. (For highly complex mixtures such as natural gases, equations of state are often used to model the properties of components and amount fractions outside of those for which they were originally developed; this may lead to errors in the calculations.) Additionally, since the hydrocarbon dew point is highly sensitive to the presence of heavier (C6+) (2) Starling, K. E. Analysis of Processes Occurring in Manual Chilled Mirror Hydrocarbon Dew Point Equipment, American Gas Association Operations Conference, Phoenix, United States, 2004. (3) Handbook of Natural Gas Engineering; Katz D. L. Ed.,; McGraw Hill: New York, United States, 1959; p 194. (4) Benton, A. J. Instruments for Hydrocarbon Dew Point Measurement in Natural Gas; Natural Gas Quality Conference, Loughborough, United Kingdom, 2002. (5) Natural gas - Vocabulary; International Organization for Standardization; ISO 14532:2005, 2005. (6) Brown, A. S.; Milton, M. J. T.; Cowper, C. J.; Squire, G. D.; Bremser, W.; Branch, R. W. J. Chromatogr. A. 2004, 1040, 215–225. (7) Vargha, G.; Milton, M. J. T.; Cox, M.; Kamvissis, S. J. Chromatogr. A. 2005, 1040, 239–245. (8) Milton, M. J. T.; Harris, P. M.; Brown, A. S.; Cowper, C. J. Meas. Sci. Tech. 2009, 20, 025101. (9pp).

Energy & Fuels, Vol. 23, 2009 1641

hydrocarbon components, which may be present at levels that are below the limit of detection of the GC, there is a significant risk that a discrepancy may arise from the use of these methods. However, an advantage of indirect methods is that since they use an equation of state, the hydrocarbon dew point can be calculated at any pressure, thus enabling the development of a full condensation curve. A further advantage is that the results should not be influenced by the condensation rate of the gas being analyzed. A number of previous studies have compared a limited set of the available methods for measuring the hydrocarbon dew point of real natural gases. Recent examples of these include the comparison of: • Three ACMIs.9 This study also compared the performance of the instruments with simple, binary synthetic gas mixtures of propane in nitrogen and propane in methane. • Three ACMIs, a MCMI, and a GC.10 This study also examined the use of different commercially available software packages for the calculation of hydrocarbon dew point, and also different approaches for “lumping” together the GC responses from species containing the same number of carbon atoms. • An ACMI, a MCMI, and a GC.11 • An ACMI and a GC.12 • GCs and MCMIs13 in a proficiency testing scheme using synthetic gases and real gas samples obtained from a processing plant. Other instruments reported in the literature include a custommade MCMI,14 which was used to determine the dew point of five synthetic natural gas mixtures of limited composition, and an alternative direct method,15 which uses gravimetry to determine the mass of condensate formed at a given temperature.16 In this paper, we present a detailed comparison of the performance of different direct and indirect (chilled mirror and GC) instruments. To quantify the limitations imposed on direct methods by the condensation rate of the gas being analyzed, and those on indirect methods by the presence of high molecular weight components, we have analyzed a range of real natural gases and complex synthetic natural gas mixtures. Five synthetic natural gas mixtures (containing straight chain hydrocarbon species up to decane) have been used that cover a range of condensation rates. Five real natural gases of widely varying compositions have been used to cover the range of extended compositions found in gas pipelines. (9) Dusart, O.; Desenfant, P.; Henault, J.-M.; Meunier, S.; Ryszfeld C.; Le Bail, M. A Method to EValuate Automatic Hydrocarbon Dew Point Analyzers; American Gas Association Operations Conference, Phoenix, United States, 2004. (10) Yackow, A.; Laughton, A.; Gronemann, U.; Benito, A.; Lindgren, T.; Kukova, E.; Kaesler, H.; van Caneghem, P.; Solvang, S.; Panneman, H.-J.; Piron, A.; Papworth, D.; Maghini, F.; Viglietti, B.; Solbraa E.; Pospiech, C. GERG-Project 1.52 - Comparing and Defining a Relation between Experimental and Calculating Techniques for Hydrocarbon Dewpoint, International Gas Union Research Conference, Paris, France, 2008. (11) Dusart, O.; Cowper, C. Comparison of measured and calculated hydrocarbon dew point, Third Gas Analysis Symposium and Exhibition, Amsterdam, The Netherlands, 2004. (12) Avila, S.; Benito, A.; Berro, C.; Blanco, S. T.; Otı´n, S.; Velasco, I. Ind. Eng. Chem. Res. 2006, 45, 5179–5184. (13) Warner, H. R.; Leamer, E. E.; Spence, A. P.; Bone, R. L.; Hubbard, R. A.; Bernos J.; Kriel, W. A. Hydrocarbon Dew Point Determination of Lean Natural Gases, Report P2001.24; Gas Processors Association: Tulsa, United States, 2001. (14) Mørch, Ø.; Nasrifar, Kh.; Bolland, O.; Solbraa, E.; Fredheim, A. O.; Gjertsen, L. H. Fluid Phase Equilib. 2006, 239, 138–145. (15) Panneman, H.-J. A Traceable Calibration Procedure for Hydrocarbon Dewpoint Meters; American Gas Association Operations Conference, Orlando, United States, 2005. (16) Natural gas - Determination of potential hydrocarbon liquid content - GraVimetric methods; International Standard ISO 6570:2001.

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Brown et al. Table 1. GC Method Parametersa

system instrument column 1 column 2 column 3

GC1 Agilent 7890A GC Poropak R 100/120 mesh packed. 8.8 m × 0.75 mm Poropak R 100/120 mesh packed.

GC2

GC2

4.4 m × 0.75 mm HayeSep PS packed. 4.4 m × 0.75 mm

Danalyser 500 GC HP 4890 GC OPN/Porasil C packed. CP-Sil 5 capillary. 0.3 m × 1 mm 30 m × 0.53 mm × 2 µm 20% SF-96 on Chrom W packed. 2.1 m × 1 mm HayeSep N packed. 2.1 m × 1 mm

190 °C (isothermal)

80 °C (isothermal)

FID (300 °C) TCD (250 °C) helium (12 bar) 100 µL

TCD (80 °C)

column 4 column temperature (all columns) detector(s) and temperature(s) carrier gas sample volume a

helium (4 bar) 200 µL

GC3 Danalyser 700 GC OPN/Porasil C packed. 0.3 m × 1 mm 20% SF-96 on Chrom W packed. 2.1 m × 1 mm HayeSep N packed. 2.1 m × 1 mm CP-Sil 5 capillary. 15 m × 0.53 mm × 2 µm 85 °C (isothermal)

(1) 35 °C for 3.5 min (2) ramp to 200 °C at 6 °C.min-1 FID (225 °C) TCD (80 °C) FID (140 °C) helium (0.25 bar) hydrogen (2.5 bar) 1 mL (split 1:3) 200 or 500 µL

All column dimensions are stated as length × internal diameter × film thickness (for capillary columns).

The work focuses on comparing the results obtained from the ACMI and GC systems and discusses the differences between the results from the real and synthetic gases. A limited comparison of the performance of two direct methods (ACMI and MCMI) on five real natural gases is also discussed, as are the results from three GC systems for the analysis of two other real gases with very low hydrocarbon dew points. Experimental Section Synthetic Natural Gas Mixtures and Real Natural Gases. The synthetic natural gas mixtures were prepared and validated at the National Physical Laboratory (NPL). They were prepared gravimetrically using a method based on that in ISO 614217 and validated using gas chromatography against an independently prepared suite of national primary reference gas mixtures. The preparation of synthetic natural gas mixtures is a highly complex procedure where the minimum and maximum attainable amount fractions of each species and the maximum preparation pressure of the mixture are all dependent upon one or more of the following: the filling procedure, the gravimetric procedure, the vapor pressure of each “pure” component, and the dew point of the target mixture. The real natural gases were sampled from different entry points to the UK network using the method described in ISO 1071518 and stored in cylinders with suitable internal passivation. Each real sample was assigned an identifying letter (Gas A to Gas E for the five gases that form the focus of the discussion in this paper, and Gas X and Gas Y for two other gases with very low hydrocarbon dew points). The composition of all the real and synthetic mixtures was determined by one of the GC systems described below at the beginning and the end of the study in order to confirm that no change in their composition had occurred through either contamination or decay. Analysis Methods. Samples were analyzed by three GC systems (referred to as GC1, GC2, and GC3), one automatic chilled mirror instrument (ACMI), and one manual chilled mirror instrument (MCMI). The setup and operation of each of these GC systems is described below, and full experimental details are given in Table 1. Note that of the three GC systems, GC1 was a “laboratory” instrument; GC3 was a “process” instrument (an instrument designed for field-based analyses); and GC2 was a system combining a laboratory and a process instrument. Indirect Measurement (Gas Chromatography) Systems. The GC1 system was a single laboratory instrument, an Agilent 7890A GC (Agilent Technologies, Wokingham, United Kingdom) with a TCD and a FID, that measured all components up to n-pentane19 (17) Gas analysis - Preparation of calibration gas mixtures - GraVimetric method; International Standard ISO 6142:2006. (18) Natural gas - Sampling guidelines; International Standard ISO 10715:1997.

and also toluene individually. The C6 components were not measured individually, rather a “total C6” amount fraction was determined, and the amount fraction of each C6 species then estimated based on the relative amount fractions determined by GC2 (below); this approach was necessary as the GC was operated using a standard method resulting in little separation of the C6 species. All components containing seven or more carbon atoms were grouped by carbon number fraction (as described for GC2) and assigned the boiling point determined by the GC2 analysis and a specific gravity identical to that of the relevant straight chain alkane. GC2 consisted of two instruments: (a) a Danalyser model 500 GC (Emerson Process Management, Stockport, United Kingdom) configured as described in ISO 6974-5,20 which used a thermal conductivity detector (TCD) for the measurement of nitrogen, carbon dioxide, and C1-C5 hydrocarbons; and (b) a HewlettPackard model 4890 GC (Agilent Technologies) configured as described in ISO 23874,21 which used a flame ionization detector (FID) for the measurement of C5-C12 hydrocarbons. All components up to n-hexane, and also benzene, toluene, cyclohexane, and methylcyclohexane were measured and reported individually, whereas the total amount fraction of all other C7 components was reported as a group. All higher molecular mass species were reported as fractions grouped by carbon number (e.g., total C8 fraction, total C9 fraction, etc.) with the assumption that all isomers had the same response factor as the straight chain alkane present in the calibration gases used. Each component peak within these fractions was allocated a boiling point calculated on the basis of retention time, and the average boiling point of each hydrocarbon fraction was determined by weighting each measured quantity by its calculated boiling point, as described in ref 20. All hydrocarbon fractions were allocated a specific gravity identical to that of the relevant straight chain alkane. GC3 was a single instrument, a Danalyser model 700 “process” GC (Emerson Process Management), with a TCD and an FID. The components were measured in a similar manner as for GC2 with the exception that the range of measurable species was restricted to C9 compounds due to the use of an isothermal temperature program. Analysis and data processing were carried out in the same manner as for GC2. Direct Measurement (Chilled Mirror) Instruments. The MCMI used in this study was a “Dewscope” instrument (Chandler Engineering, Broken Arrow, United States). The rate of cooling of the mirror was varied by the operator and was (19) The nomenclature used throughout this paper is generally consistent with that used in the natural gas industry. For example, we refer to neopentane, i-pentane, and n-pentane, for which the IUPAC-recommended names are, respectively: 2,2-dimethylpropane, 2-methylbutane, and pentane. Other alkanes are treated similarly. (20) Natural gas - Determination of composition with defined uncertainty by gas chromatography - Part 5: Determination of nitrogen, carbon dioxide and C1 to C5 and C6+ hydrocarbons for a laboratory and on-line process application using three columns; International Standard ISO 6974-5:2000.


Figure 2. Relationship between the trigger level and mirror temperature of the ACMI for each of the five synthetic gas mixtures. The filled circles indicate measured values and the connecting lines are a guide to the eye.

typically 1.0-1.5 °C.min-1 at temperatures around the hydrocarbon dew point. A visual assessment of whether a film had formed was carried out at 0.5 °C temperature intervals, the judgment of the operator being validated by independent assessment of photographs of the formation of the condensate film; these images are provided separately.22 The temperature of the mirror was recorded by a calibrated alcohol in glass thermometer with a range of -100 to +10 °C with 1 °C graduations. It is important to note that for the MCMI used in this study, a film (rather than droplets) of condensate was formed on the surface of the chilled mirror, so the measured hydrocarbon dew points are directly comparable to those recorded by the ACMI. The ACMI was a Michell Instruments Condumax II instrument (Michell Instruments, Ely, United Kingdom), which detected the presence of a condensate film on an acid-etched semimatt stainless steel chilled mirror with a conical-shaped depression by using an optical detection method.23 Measurements were made on a fixed volume of sample enclosed within the sensor cell. The optical method used by the ACMI operates on the principle that when no condensate is present on the mirror, the incident beam of collimated visible red light is dispersed by the matt surface, thus providing a base signal to the detector. When condensate is present, the surface becomes reflective (due to the low surface tension of the condensate), and an annular ring of reflected light starts to form around the detector, resulting in a dramatic reduction in scattered light intensity reaching the detector. When this intensity falls below a set threshold (the “trigger level”) the temperature of the optical surface is taken to be the measured hydrocarbon dew point. It is therefore necessary to set a trigger level (which defines the sensitivity of the instrument) to a practical detection limit. In this study, the ACMI was operated at the manufacturer’s recommended trigger level of 275 mV. This trigger level has been shown to provide measurements of hydrocarbon dew point representative of a PHLC in the range 20 to 50 mg · m-3 15 and is used widely in order to maintain consistency with results for the MCMI. The relationship between the trigger level of the ACMI and the temperature of its mirror for each of the five synthetic gas mixtures is shown in Figure 2. This shows that the trigger level of 275 mV used for these experiments is on a point on the curves that is relatively insensitive since it meets a “plateau” in the response. For general use in field applications, the instrument is used at a single pressure of 27 bar; however, for the experiments reported here, the ACMI was operated at a range of pressures. The operation (21) Natural gas - Gas chromatographic requirements for hydrocarbon dewpoint calculation; International Standard ISO 23874:2006. (22) National Physical Laboratory; www.npl.co.uk/environment/hydrocarbondewpoint.html (accessed 1/09).

Energy & Fuels, Vol. 23, 2009 1643 of the ACMI and its temperature sensor were validated through the measurement of a 100 mmol/mol n-butane in nitrogen gas standard. Dew Point Calculations. All calculations of hydrocarbon dew point were carried out using the Excel add-in version 2.0 of the GasVLe software package (Advantica, Loughborough, United Kingdom).24 The RKS (Redlich-Kwong-Soave) equation of state,25 which has been demonstrated to be suitable for calculating the condensation properties of natural gas mixtures,26 was used for all calculations. The RKS equation is a modification of the van der Waals cubic equation of state, with the introduction of a temperature-dependent attractive term (to enable the accurate calculation of pure component vapor pressures) and binary interaction parameters (to give accurate phase behavior for multicomponent mixtures of nonpolar compounds). The use of alternative equations (for example the Peng-Robinson equation27) can yield results that are significantly (more than 2 K) different to those from the RKS equation.10,11 Although these cubic equations of state have been validated against measurements of simple gas mixtures, their validity when applied to real natural gases is less certain. A detailed discussion on the use of equations of state and associated parameters to model the behavior of natural gases is available in ref 28.

Results and Discussion Analysis of Synthetic Natural Gas Mixtures. The synthetic natural gas mixtures were prepared for this study using the method described above. Although the composition of these mixtures (which contained species up to C10 (n-decane)) were chosen to be as representative of natural gases as possible, they do not include the entire range of components found in real samples of natural gas. The role of these synthetic gas mixtures in this work was to establish a “baseline” for the comparison of the instrument by using samples of known composition. The composition of the synthetic gas mixtures was also chosen to encompass the range of hydrocarbon condensation rates observed in gases transported from United Kingdom gas fields, including those of the most extreme compositions. (As defined in the Introduction, “condensation rate” is the amount of condensate in milligrams of liquid per cubic meter of gas (i.e., the PHLC) formed per Kelvin of temperature change below the hydrocarbon dew point.) The full composition of the synthetic mixtures is given in Table 2, and the calculated condensation rates of the five mixtures are shown in Figure 3. One mixture was prepared with a nominally “high” condensation rate (mixture H1), two with nominally “medium” condensation rates (mixtures M1 and M2), and two with nominally “low” condensation rates (L1 and L2). Each of the five synthetic mixtures was analyzed using the ACMI and GC2. The other GC systems were not used, as the synthetic gas mixtures were not expected to provide a stern analytical challenge for these systems, and the main focus of these tests was to compare the GC results with those from the ACMI. (It should be noted that the GC analysis of the synthetic mixtures also provided an independent validation of the gravimetric composition of the mixtures.) (23) Bannell J. L. K.; Dixon A. G.; Davies T. P. The Monitoring of Hydrocarbon Dew Point; International Congress of Gas Quality: Groningen, The Netherlands, 1986. (24) Advantica; www.advanticagroup.com/gasvle (accessed 1/09). (25) Soave, G. Chem. Eng. Sci. 1972, 27, 1197. (26) Nasrifar, Kh.; Bolland, O.; Moshfeghian, M. Energy Fuels 2005, 19, 561–572. (27) Peng, D. Y.; Robinson, D. B. Ind. Eng. Chem. Fund. 1976, 15, 59–64. (28) George, D. L. Development of accurate methods for predicting hydrocarbon dew points; US Minerals Management Service, Herndon, United States, 2007. (Available from www.mms.gov/tarprojects/534.htm).


Brown et al.

Table 2. Composition of the five synthetic gas mixturesa gas mixture and amount fraction (µmol/mol) component

H1

M1

M2

L1

L2

methane 902309 893411 922325 986879 973065 ethane 68677 77122 63712 7059 16081 propane 17107 22104 10492 4597 8781 n-butane 8480 5428 1582 782.1 904.7 n-pentane 2653 1326 946.3 358.2 533.9 n-hexane 514.2 319.6 440.4 99.11 294.9 benzene 61.03 41.90 139.8 35.36 97.86 toluene 24.63 32.25 85.88 30.05 58.53 cyclohexane 45.23 55.79 59.19 31.60 50.10 n-heptane 93.83 119.7 157.5 59.23 72.74 n-octane 12.17 13.61 32.20 34.86 24.90 n-nonane 0.03 10.18 15.85 21.75 17.63 n-decane 0.01 0.03 0.05 9.84 11.75 a The figures in italics are those components where the pure material was not added to the mixtures; the content is made up entirely of impurities from other pure materials. The expanded uncertainty of the amount fraction of each component varies but is typically 0.01% relative for methane, 0.3% relative for n-hexane, and 0.6% relative for n-decane.

Figure 3. Condensation rates of the five synthetic natural gas mixtures (at the cricondentherm pressure) calculated using the gravimetric composition data (see Table 2).

Figure 4 shows the condensation curves for all five synthetic natural gases as: • calculated from the gravimetric compositions; • calculated from the measured compositions (determined by GC2); • as measured directly by the ACMI. These results show that for all gases, the gravimetric and GC2 curves agree very well: at the cricondentherm (the point of the condensation curve of maximum temperature) the difference between the determined dew points is less than 0.5 K for all the gases except the one with the lowest condensation rate (L1), a difference that is within the uncertainty of the measurement. (It has been estimated that the uncertainty in dew point at the cricondentherm that results from the uncertainty in each measured amount fraction alone is approximately 0.5 K.) The expanded gravimetric uncertainty in the amount fraction of the components in the mixtures vary from approximately 0.001% relative (for methane) to approximately 0.7% relative (for n-decane).) For these synthetic mixtures, the dew point calculated from the gravimetric data provides the “reference” value for the dew point since the composition of the mixture is well defined (although this does of course assume that the equation of state used is valid; see the Experimental Section and ref 27 for a detailed discussion of this issue). The good agreement observed between the GC2 and gravimetric data confirm that the GC method provides measurements of the composition that are sufficiently accurate to calculate the dew point of these mixtures. The data obtained from the ACMI displays a number of characteristics worthy of discussion. First, the hydrocarbon dew

point measured at the cricondentherm by the ACMI is in all cases lower than that calculated from the gravimetric and GC2 data; the average difference between the two sets of data at the cricondentherm being 1.9 K. A similar effect was reported in ref 14, which also used the RKS equation of state to determine the condensation curve of the synthetic mixtures. The principal reason for this difference is the fact that direct and indirect methods use fundamentally different approaches for dew point measurement; GC2 utilizes an equation of state to determine the dew point at which the first molecule of condensate theoretically appears, whereas the dynamic nature of the ACMI requires a real condensate film to be formed. As described in the Experimental Section, the trigger level of the ACMI was chosen to replicate the operation of a manual chilled mirror instrument, that is, to detect the point at which the PHLC is between 20 and 50 mg · m-3. The temperature at which two values of PHLC within this range (25 and 50 mg · m-3) are formed at the cricondentherm have been calculated for each mixture and are presented, along with the dew points measured by the ACMI, in Table 3, where it should be noted that the ACMI result has been estimated by interpolation between the nearest two data points (the additional uncertainty due to this step is negligible). Both sets of results in Table 3 exhibit closer agreement than the data shown in Figure 4: For a PHLC of 50 mg · m-3, all the dew points agree with the measured ACMI values to within 1.4 K, and the average difference between the two sets of data is only 0.4 K. For a PHLC of 25 mg · m-3, the maximum difference is 1.7 K, and the average difference is 1.1 K. This confirms the tendency for ACMI measurements to be influenced by the condensation rate of the gas being analyzed. The data calculated for a PHLC of 25 mg · m-3 also reveal that the two mixtures for which the closest agreement is obtained with the ACMI data (mixtures H1 and M1) are those with the highest condensation rates. For the remaining three mixtures (M2, L1, and L2), the dew points measured by the ACMI are all lower than those calculated for a PHLC of 25 mg · m-3. These observations support the expectation that direct methods are most accurate when determining the dew points of gases with high condensation rates, as the condensate film will form more readily for these gases. The accuracy of the ACMI when analyzing fast condensing gases is further demonstrated by the data calculated for a PHLC of 50 mg · m-3, where the calculated dew points of gases H1 and M1 also show good agreement with the ACMI data. For this value of PHLC, close agreement with the ACMI data is also observed for the two slowest condensing gases (L1 and L2). This may be an effect of the dynamics of condensation film formation on the surface of the mirror during the measurement cycle of the ACMI. The slower condensation rates of gases L1 and L2 mean that a longer period of time is required before the condensate is detected, thus resulting in a lower recorded dew point during the cooling cycle of the instrument. This has little effect on the measurements of the three other, faster condensing gases (H1, M1 and M2), whose calculated dew points at PHLCs of 25 and 50 mg · m-3 are very similar. From Figure 4 it can also be observed that for two of the mixtures (H1 and M1) the dew points measured by the ACMI at pressures above that at which the cricondentherm occurs show unexpected nonretrograde behavior (i.e., the data do not follow a trend of decreasing dew point at pressures above the cricondentherm). This is a consequence of the fact that the ACMI was operated at a pressure of 27 bar and with a trigger level chosen to replicate the sensitivity of an MCMI. Measurements at these higher pressures are outside the usual operating



Figure 4. Hydrocarbon condensation curves for the five synthetic natural gas mixtures (names shown in the top right-hand corner of each plot). Each plot shows the hydrocarbon dew point as calculated from the gravimetric composition (circles, solid line), as calculated from the composition determined by GC2 (triangles, long dashed line) and as measured by the ACMI (squares, short dashed line). Note that for the ACMI, each square indicates a direct measurement of the hydrocarbon dew point, so the short dashed line is a guide to the eye only. Table 3. Comparison of the Hydrocarbon Dew Points Measured by the ACMI with Those Calculated for PHLCs of 25 and 50 mg · m-3 at the Cricondentherm Pressure Indicateda hydrocarbon dew point/°C mixture H1 M1 M2 L1 L2

calculated pressure calculated (bar) ACMI (25 mg · m-3 PHLC) (50 mg · m-3 PHLC) 41 35 32 25 26

-6.5 -9.2 -6.9 -6.8 -4.1

-6.6 -8.5 -5.2 -5.3 -2.8

-6.7 -8.8 -5.5 -6.7 -4.0

a Note that ACMI results are an interpolation between the two nearest direct readings.

scope of the instrument. This apparent nonretrograde behavior can be overcome by varying the sensitivity of the ACMI linearly with pressure above the cricondentherm. This partially corrects for the fact that there is a larger quantity of gas in the cell at higher pressures; hence, a thicker film of condensate is required before triggering the instrument, unless the sensitivity is increased. The use of this “adaptive trigger point” approach to

reproduce the retrograde behavior of a hydrocarbon condensation curve has been demonstrated experimentally.15 Analysis of Real Natural Gases. Of the five real natural gases that form the focus of this discussion (Gases A-E), all were analyzed by the MCMI, the ACMI, GC2, and GC3. Gas A and Gas C were also analyzed by GC1. It should be noted that GC1 and GC2 are designed for laboratory use, so measurements using this system were therefore expected to provide more accurate analyses than GC3, which is a process instrument and is therefore designed for continuous use in a field environment. The GC1 analyses were undertaken at a somewhat later date than those with the other GC systems. The MCMI results are only of supplementary interest to this study and are not presented at this stage, but will be discussed at the end of this section. Two other real natural gases with very low hydrocarbon dew points, Gas X and Gas Y, were also analyzed using the MCMI, GC2, GC3, and (for Gas Y only) GC1. Because the hydrocarbon dew point of these mixtures fell below the lowest temperature achievable by the ACMI, no results are reported for this instrument, and these gases are discussed briefly later.


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Table 4. Composition of the Seven Real Natural Gases As Determined by GC2a amount fraction (µmol/mol) component(s) helium hydrogen oxygen + argon nitrogen carbon dioxide methane ethane propane i-butane n-butane neo-pentane i-pentane n-pentane 2,2-dimethylbutane 2,3-dimethylbutane 2-methylpentane 3-methylpentane n-hexane benzene cyclohexane toluene methylcyclohexane other C7 species total C8 fraction total C9 fraction total C10 fraction total C11 fraction total C12 fraction

Gas A 319 78 269 61120 569 835100 57600 23990 4863 8909 94 2816 2314 85 121 546 309 327 2.5 171 0.31 56 339 22 2.6 0.03 s s

Gas B 470 7.0 40 41880 10020 892900 39110 9790 1597 2048 49 565 528 42 34 124 63 161 250 72 32 56 148 35 12 1.2 0.08 s

Gas C 420 8.0 60 24600 4775 933100 28140 5024 874 1113 31 356 331 39 29 100 57 144 226 75 53 77 200 88 42 4.8 0.31 0.002

Gas D 329 100 160 71560 4713 859500 43580 10650 2253 3273 101 1240 1040 105 66 307 173 399 3.9 77 0.8 46 304 40 5.9 0.35 0.02 s

Gas E 80 11 50 8111 21730 880600 62940 20370 1801 3120 s 420 430 5.8 23 57 27 69 28 23 6.9 10 49 4.9 1.4 0.09 s s

Gas X 30 62 30 4450 17290 883200 73150 17830 1625 1968 s 216 143 1.5 5.5 11 5.3 10 3.4 1.9 0.51 0.61 4.9 0.17 0.03 s s s

Gas Y 120 4.0 40 2600 6730 986900 3360 130 30 27 s 7.5 7.3 1.8 3.4 2.2 1.2 3.9 0.21 32 0.15 3.0 6.7 3.1 2.0 0.83 0.30 0.13

a Note that the C and higher hydrocarbon species are grouped into total carbon number fractions, and the amount fractions of oxygen and argon are 8 reported as a sum because they were not separated by the GC. A dash indicates that the component (or group of components) was not detected.

species is key to the accurate determination of hydrocarbon dew point as even their presence at sub-µmol/mol amount fractions can have a large effect on this property. This is discussed in more detail below. In general, the three different GC systems show an acceptable agreement with the hydrocarbon dew points determined at the cricondentherm, as all values agree within 2 K with only one exception (Gas C). Differences of this magnitude are within the expected uncertainty of the measurement for mixtures of this complexity.

Figure 5. Condensation rates of the seven real natural gases (at the cricondentherm pressure) calculated using the data obtained by GC2 (see Table 4).

The composition of each gas (as determined by GC2) is given in Table 4, and the condensation rates calculated from these data are shown in Figure 5. Comparison with Figure 3 shows that the range of condensation rates is similar to those of the synthetic gas mixtures. Figure 6 shows the hydrocarbon condensation curves obtained from the analysis of Gases A-E by the ACMI and the GC methods. Analysis of Gases A-E: Comparison of GC Systems. We first compare the results obtained from the three GC systems. The dew point of each of the real gases determined at a point at or near to the cricondentherm is shown in Table 5, where it can be seen that the dew points determined from the three GC systems are always in the order GC1 > GC2 > GC3. This observation is consistent across all gases (and is therefore largely independent of the composition of the gas) and illustrates the difference in performance of each GC to heavier hydrocarbon species. For example, for Gas C, the total amount fraction of all C11 species measured was, respectively, 1.03 µmol/mol for GC1, 0.31 µmol/mol for GC2, and zero (not detected) for the process instrument GC3. The measurement of these heavier

Analysis of Gases A-E: Comparison of Direct and Indirect Methods. Comparison of the results from the GCs and the ACMI for Gas A to Gas E reveals a different trend to that observed with the synthetic gas mixtures. For all five gases, the dew points measured by the ACMI are greater than those determined by the GC systems, whereas the opposite effect is observed for the synthetic gas mixtures, (i.e., the dew points measured by the ACMI are lower than those determined by the GCs). This difference becomes further exaggerated when the theoretical dew point for the formation of a PHLC in the range 20-50 mg · m-3 (i.e., the range of PHLCs that the ACMI is designed to detect) is calculated. This phenomenon has not been reported in previous studies, and two hypotheses may be presented to explain it. It should also be noted that, unlike the case of the synthetic mixtures, there is no “reference” dew point for these real gases, so any difference in the data may be due to errors in either or both of the GC and ACMI data. The first hypothesis is that the difference is due to the difficulty of using a GC to accurately measure the heavier hydrocarbons contained within the real gases. Analysis of these species is also highly problematic because of the low signalto-noise ratios of the chromatographic peaks, which is caused in part by the hydrocarbon fractions consisting of numerous species in a range of isomeric forms. Additional uncertainty is also introduced by the method used to sum all the measured



Figure 6. Hydrocarbon condensation curves for five real natural gases (Gases A-E). The plots indicate the hydrocarbon dew points calculated from the composition determined by GC1 (crosses, long dashed line), GC2 (triangles, long dashed line), GC3 (diamonds, long dashed line), and those measured by the ACMI (squares, short dashed line). Note that for the ACMI, the squares indicate a direct measurement of the hydrocarbon dew point, so the short dashed line is a guide to the eye only. Table 5. Comparison of the Hydrocarbon Dew Point of Each Real Gas As Determined by the Three GC Instruments at the Indicated Cricondentherm Pressurea hydrocarbon dew point (°C) gas

pressure (bar)

A B C D E X Y

50 35 30 40 35 45 15

a

GC1 7.6 n/a 1.5 n/a n/a n/a -20.8

GC2

GC3

7.4 -8.4 -1.1 -3.7 -23.7 -37.6 -26.4

6.8 -10.0 -3.1 -5.7 -24.7 -37.8 -63.5

Measured to the nearest 5 bar. n/a indicates not analyzed by GC1.

species containing n carbon atoms into a single “Cn” faction. As discussed above, for one of the gas mixtures (Gas C), the amount fraction of all C11 species measured by the three GC systems varied from 1.03 µmol/mol to zero (not detected); the effect of not detecting C11 and C12 species during the analysis can be further demonstrated by considering Figure 7. Here, the results obtained from the GC2 analysis of Gas E and Gas A (where no C11 or C12 species were detected) have been used as a basis, and the change in dew point at the cricondentherm has been modeled for theoretical C11 and C12 amount fractions of

up to 500 and 100 nmol/mol, respectively. For Gas E, a large difference in dew point (approximately 2 K) is observed when either 500 nmol/mol of C11 species or 100 nmol/mol of C12 species are included in the composition of the mixture. The difference for Gas A (0.15 K for these same amount fractions of C11 or C12) is much less. The large difference observed for Gas E is greater than the variation in dew point determined by all the analytical methods studied and thus demonstrates the importance of accurate measurement of these species. This problem becomes more exaggerated at even higher hydrocarbon numbers (C13, C14, etc.), as these species have a greater influence on the hydrocarbon dew point than a similar amount fraction of lower molecular weight hydrocarbons. In this study, no species with more than 12 carbon atoms were measured, so the presence of any such compounds in the gases would result in an underestimation of the dew point. A second hypothesis to explain the difference is that the gases may contain “unidentified” components that the GC systems have not been configured to analyze. One example of such species are glycols that are added to natural gas to prevent the formation of highly hazardous methane clathrates (methane hydrates) by depressing the temperature at which the clathrates are formed. The high boiling points and strongly polar nature


Brown et al.

Figure 8. Calculated hydrocarbon condensation curves of Gas E with the addition of varying amount fractions of TEG as indicated below the curves.

Figure 7. Calculated relationship between the cricondentherm hydrocarbon dew point and amount fraction of C11 and C12 species for Gas E (Figure 7a) and Gas A (Figure 7b). The dark diagonal lines are of equal cricondentherm hydrocarbon dew point of the temperatures shown.

of glycols means that they would not be eluted from the chromatography columns used here and would therefore not be detected by the GC methods in this study. In the real natural gas samples used in this study (which were taken at various entry points in the United Kingdom’s National Transmission System (NTS)), the only glycol that could be present is triethylene glycol (TEG), which has a boiling point of 285 °C and is used by producers as a drying agent for the gas before it enters the NTS. Using data from the entry point where Gas E was sampled, addition of TEG at a quantity sufficient to reach vapor-liquid equilibrium at the typical temperatures and pressures employed by the producers would result in an amount fraction range of TEG of 4-150 nmol/ mol. (In some parts of the UK distribution system, monoethylene glycol (MEG) is also added to the gas in order to minimize leakages from old lead-yarn joints; the typical amount fraction of MEG in the gas is then estimated by the distributors of the gas to be between 3-70 µmol/molsamount fractions of the same order as results recently reported for the solubility of MEG in methane.29 However, the gas samples used in this study were taken at entry points to the UK NTS, and so will have had no MEG added artificially. The dramatic effect on the condensation properties of the gas due to these quantities of TEG can be seen in Figure 8, where condensation curves have been calculated for Gas E with the addition of a range of amount fractions of TEG between 1 and 150 nmol/mol. The hydrocarbon dew point increases markedly with even very low nmol/mol levels of TEG (for example, by 4.7 K for 1 nmol/mol and 10.0 K for 4 nmol/mol). The data in (29) Folas, G. K.; Berg, O. J.; Solbraa, E.; Fredhiem, A. O.; Kontogeorgis, G. M.; Michelsen, M. L.; Stenby, E. H. Fluid Phase Equilib. 2007, 251, 52–58.

Figure 8 is, however, likely to be very much a worse case scenario as, in reality, some of the TEG added to the gas would have condensed out during its transportation process, so the amount fraction of glycol present in the gases used in this study is likely to be less than that at the point where the glycol was added. In addition, the very low amount fractions at which TEG is present may not form condensate of sufficient volume to produce a film thick enough to be detected by the chilled mirror instruments. The analysis of glycols by GC at these nmol/mol levels would be highly challenging and would require a method such as direct on-column injection in order to avoid degradation of TEG occurring in the injection port of the GC. An inert sampling system would also be required to minimize loss of the TEG, and the very low levels of the analyte are likely to necessitate the use of preconcentration on a nonreactive substrate (such as glass beads) before analysis. Such an analysis was outside the scope of this study. An additional factor that may contribute to the differences in the data discussed above is the uncertainty inherent in the equation of state used to derive the calculated hydrocarbon dew points for the indirect methods. Although the RKS equation of state used throughout this study is used widely, it is difficult to perform an accurate and independent verification of its capability to accurately model the properties of complex natural gases. Analysis of Gases X and Y: Comparison of GC Systems. Although Gases X and Y are unusual in the fact that have very low hydrocarbon dew points, which are of little practical interest, the results obtained from their analysis by the GC systems (Figure 9) still merit a brief discussion. For Gas X, the two GC systems used give very similar measurements of hydrocarbon dew point, with the dew point determined by GC2 being greater than that determined by GC 3, thus demonstrating the same trend as discussed for the other five real natural gases. Gas Y, however, gives very inconsistent results; in fact, a difference of over 40 K is observed between the dew points determined by the different GC systems (which again are in the order GC1 > GC2 > GC3). This can be attributed to the very unusual composition of the gas, which includes a very high methane content (987 mmol/mol) combined with a very long hydrocarbon tail (C11 and C12 hydrocarbon fractions with amount fractions of greater than 0.1 µmol/mol), which makes accurate composition determination highly challenging. Analysis of Gases A-E: MCMI Results. We now discuss the results obtained from a limited series of measurements using the MCMI; these are compared against those from the ACMI in Table 6. Since the MCMI was operated at a single pressure for each gas (the pressure at which the cricondentherm was



Figure 9. Hydrocarbon condensation curves for the two real natural gases with very low hydrocarbon dew points (Gases X and Y). The plots indicate the hydrocarbon dew points calculated from the composition determined by GC1 (crosses, long dashed line), GC2 (triangles, long dashed line), and GC3 (diamonds, long dashed line). Note the larger x-axis scale used for Gas Y. Table 6. Comparison of the Hydrocarbon Dew Point of Each Real Gas As Measured by the Two Chilled Mirror Instruments at the Indicated Cricondentherm Pressurea hydrocarbon dew point (°C) gas

pressure (bar)

ACMI

MCMI

A B C D E X Y

50 34 32 39 38 46 13

8.2 -6.5 2.3 -1.8 -19.8 * *

5.0 -12.5 -4.0 -6.0 -27.5 -41.0 -31.5

a Note that ACMI results are an interpolation between the two nearest direct readings. An asterisk (*) indicates that the dew points of Gases X and Y were below the lowest temperature achievable by the ACMI.

predicted to occur), the comparable ACMI result has been estimated by interpolation between the nearest two data points. For all gases, the dew points measured by the ACMI are greater than those measured by the MCMI, the difference being between 3.2 and 7.7 K. These differences are larger than expected considering that both methods are direct, but are not dissimilar to those reported in ref 11, where the dew points of a real natural gas measured by an ACMI were approximately 1-5 K greater than the MCMI. The discrepancy in the chilled mirror results was investigated by comparing the dew point measured by the two instruments of a 100 mmol/mol n-butane in nitrogen binary gas mixture; such a mixture is straightforward to measure because of the propensity of the mixture to rapidly form a large volume of condensate just below the dew point. The dew points recorded by the MCMI were typically 2 K lower than those from the equivalent automatic instrument. This discrepancy only accounts for part of the difference in dew point recorded for the real gases and reduces the magnitude of the discrepancy to very similar to that reported previously.11 The independent observation of this magnitude of discrepancy in the results obtained from manual chilled mirror and automatic chilled mirror instruments deserves further investigation. Possible explanations for this discrepancy include differences in the cooling rate and sensitivity of the two instruments. The “True” Value of Hydrocarbon Dew Point. The current definition of hydrocarbon dew point agreed upon by the International Organization for Standardization (ISO) Standard is the: “Temperature above which no condensation of hydrocarbons occurs at a specified pressure”.5 (This definition is appended by two notes: (1) At a given dew point temperature there is a pressure range within which retrograde condensation can occur. The cricondentherm defines the maximum temper-

ature at which this condensation can occur. (2) The dew point line is the locus of points for pressure and temperature that separates the single phase gas from the biphasic gas-liquid region.) This definition can be considered to be equivalent to the temperature at which more than one molecule of liquid condensate forms; such an occurrence would be impossible to detect experimentally. The direct methods used in this work measure hydrocarbon dew point directly according to this definition. However, they require more than a single molecule of condensate to be detected, and therefore must determine a value that is lower than the true value. An alternative approach to the definition of hydrocarbon dew point would be to develop a new definition of a “measurable” dew point, in order to aid convergence of the values determined by different methods of measurement. One suggestion is that the determined hydrocarbon dew point should be reported with the physical measurement conditions under which is obtained, such as the method used or the value of PHLC that corresponds to the measurement. In the absence of a precise definition of hydrocarbon dew point, it is sufficient to observe that the results of direct and indirect methods are not exactly equivalent, because to some extent they are not measuring the same phenomenon. Conclusions A detailed comparison of the performance of different direct and indirect instruments to measure the hydrocarbon dew point of a range of real natural gases and complex synthetic natural gas mixtures has been presented. Analysis of five synthetic natural gas standards containing species up to n-decane has shown good agreement between the hydrocarbon dew points obtained from a GC system, those measured by an automatic chilled mirror instrument, and the “reference” dew points determined from the gravimetric data and an equation of state. However, the hydrocarbon dew points obtained by the ACMI are consistently slightly lower than the “reference” values (an average difference of approximately 2 K for the five synthetic mixtures). This difference is much reduced (to 0.4 and 1.1 K) when the dew point is determined for PHLCs of 50 and 25 mg · m-3, respectively, and is especially dependent on the condensation rate of the gas; using the recommended trigger level of the ACMI, the gases studied here exhibit a change in hydrocarbon dew point of up to 2 K that can be attributed to condensation rate. These results demonstrate that the use of direct methods is to some extent limited by the condensation characteristics of the gas analyzed. For the real natural gas mixtures, good agreement was again generally obtained between the hydrocarbon dew points deter-


mined by the different methods, although the direct systems reported dew points consistently higher than the indirect methods. These results demonstrate that the use of indirect methods is limited by difficulties in determining accurately the composition of the gas (for example, problems with measuring hydrocarbon isomers with amount fractions below the limit of detection of the method, and detecting unexpected species in the highly complex natural gas mixtures). We have demonstrated that the possible discrepancy in the hydrocarbon dew point attributable to not detecting, for example, 500 nmol/mol of C11 components or 100 nmol/mol of C12 components in one of the real natural gas samples would be up to 2 K. A much lower amount fraction of triethylene glycol can have a dramatic effect on the calculated dew point (e.g., 1 nmol/mol of triethylene glycol would increase the dew point of one gas by 4.7 K). It is therefore concluded that, although both direct and indirect methods have been demonstrated to measure the hydrocar-

Brown et al.

bon dew point of a range of natural gas mixtures with good accuracy, an understanding of the limitations discussed above is essential in order to select the most appropriate method for a high-accuracy analysis and to define a suitable calibration regime. The uncertainties inherent in the use of cubic equations of state to model natural gas mixtures have been discussed briefly, as has the need to recognize the practical difficulties associated with the definition of hydrocarbon dew point. The full raw data set from all the tests carried out for this study can be downloaded from the Web site of the National Physical Laboratory.21 Acknowledgment. The authors would like to acknowledge the funding of this work by the UK Department of Innovation Universities and Skills’ “Measurement For Innovators” and “Chemical and Biological Knowledge Base” Programmes. EF8009469

Measurement of the Hydrocarbon Dew Point of Real and Synthetic

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