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1 J Petrol Explor Prod Technol (2014) 4:201207 DOI 10.1007/s13202-013-0073-y ORIGINAL PAPER - PRODUCTION ENGINEERING A new approach of splitting C6+ composition of pipeline gas for hydrocarbon dew point determination Livinus Aniefiok Obah Boniface Received: 26 April 2013 / Accepted: 29 July 2013 / Published online: 18 August 2013 The Author(s) 2013. This article is published with open access at Springerlink.com Abstract Hydrocarbon dew point (HCDP) is a critical new approach and some common C6? composition splits consideration for pipeline operations. Equation of state is made. (EOS) method combined with Gas Chromatograph analysis is one of the methods used for HCDP determination. Most Keywords C6? splits Pipeline gas Hydrocarbon dew of the GCs on pipeline gas and in end-user installations are point generally C6? design and a few are C9? design. In applying a HCDP limit using C6? data, it is prudent to use an appropriate split of the C6? composition. Though Introduction several fixed ratios of C6? composition splits have been published in a Gas Processors Association standard and by Hydrocarbon dew point (HCDP) defines whether the nat- leading chromatograph manufacturers for application, but ural gas stream in a pipeline at a given pressure and tem- may not truly reflect the C6? composition of a particular perature consists of a single gas phase or two phases, gas gas. This work therefore presents a very simple procedure and liquid. Figure 1 shows a phase diagram depicting the for splitting C6? component into C6/C7/C8 for any par- HCDP for a typical natural gas. ticular pipeline gas provided that the specific gravity of the When natural gas is processed, the heavier compo- C6? data is known. The method involves; determination of nents are removed in order to supply high quality gas to the molecular weights of C6/C7/C8 using three well- the market and also to ensure the safety and reliability of established hydrocarbon physical properties correlations, the pipeline system. For example, gas turbine generator solution of the algebraic equations of the apparent molec- plants have a written requirement in their warrantees that ular weight of C6? using matrix notation, and application the fuel gas must be totally gaseous. In order to comply of logarithmic distribution to the calculated quasi-mole with this requirement, the gas is superheated to a mini- fraction of the individual C6? components. An applica- mum of 50 F/28 C above the highest dew point of the tion of this approach to a field C6? data is presented. In gas at the pressure regulators located ahead of the burner order to show the capabilities of the new approach, results section. comparison of calculated HCDP as well as cricondentherm The two methods currently in use for determining (using HYSYS with Peng Robinson EOS) between the HCDP are; the manual visual dewpoint approach and the equation of state (EOS) method using Gas Chromatograph (GC) analysis. The manual method was developed by the US Department of Interior, Bureau of Mines and has been L. Aniefiok (&) codified into a standard test method by the American Department of Chemical/Petroleum Engineering, University of Society of Testing and Materials (ASTM D 1142-95 1995). Uyo, Uyo, Akwa-Ibom State, Nigeria It uses a chilled mirror or dew point tester. This approach is e-mail: [email protected] labour intensive. Although automated continuous units are O. Boniface available, they are expensive and, unlike GCs, are currently Federal University of Technology, Owerri, Imo State, Nigeria not part of most existing gas transmission facilities 123

2 202 J Petrol Explor Prod Technol (2014) 4:201207 Fig. 1 A phase diagram for a typical natural gas (source: Shane 2000, Emerson Process Management, class # 5300) (NGC-GPA 2005). In addition, the determination of the standard (NGC-GPA 2005): GPA 60 % C6, 30 % C7, 10 % phase envelope or the cricondentherm would be difficult C8; Daniels 47 % C6, 36 % C7, 17 % C8; GPA 50 % C6, using this method since it is a periodic spot checking only. 25 % C7, 25 % C8. Application of any of these fixed ratios The indirect method uses a GC for compositional analysis for a particular delivery point of natural gas may not be a in conjunction with an EOS to estimate the dew point as truly representative of the measured gas because the specific well as the cricondentherm and phase envelope if desired. gravity of C6? generated with the fixed ratios may likely As the HCDP is the condition when the heavy components deviate from that of the measured gas. Even with the most begin to drop out into the liquid phase, the accurate mea- recent approach, used by gas processors and end-users to surement of the heavier components is critical for mean- determine the percentage characterization of C6? for a ingful determination of the HCDP. The composition of the given pipeline, which is by taken the weighted average pipeline gas for custody metering is determined by the use compositions of the regional supply on that pipeline, the of GCs, most of these analyzers are generally C6? and a ability of the average characterization to reflect the true few are C9? design (Jack 2010). The GC C6? design composition of a particular gas within a region greatly measures the individual hydrocarbons up to normal-pen- depends on the variance of the individual components of all tane using GPA 2261 (2000) procedure and then report the gases throughout the region (NGC-GPA 2005). Though, the heavier components as a combined C6? measurement. traditional C6? analysis provides insufficient data for a In the absence of detailed analytical data for hydrocar- valid HCDP calculation (Ernst and Pettigrew 2005), bon plus fractions in a hydrocarbon mixture, erroneous improving on the repeatability of the prediction capability predictions and conclusions can result if the plus fraction is of the C6? characterisation is essential. used directly as a single component in the mixture phase This work presents a very simple and practical proce- behaviour calculations. Numerous authors have indicated dure for extending the percentage distribution of C6? that these errors can be substantially reduced by splitting or composition of pipeline gas into C6/C7/C8 in order to breaking down the plus fraction into a manageable number improve HCDP determination. An illustrative example in of fractions for equation of state calculations. which the new procedure is applied to field C6? data is For energy calculation and HCDP calculation, many gas also presented. In order to show the capabilities of the new processors and end-users are now applying equation of approach, results comparison of calculated HCDP as well state, either a PengRobinson (PR) or SoaveRedlich as cricondentherm (using HYSYS with Peng Robinson Kwong (SRK) equation of state, to pipeline gas composi- EOS) between the new approach and some common C6? tion; the C6? component is split into a fixed ratio of n- composition splits is made. hexane, n-heptane and n-octane. Some commonly used Using C6? data for HCDP limit application on a par- percentage C6? composition splits, based on empirical ticular pipeline gas, it is prudent that an appropriate split be studies of most pipeline gases, are published in a GPA employed. If we consider the specific gravity, molecular 123

3 J Petrol Explor Prod Technol (2014) 4:201207 203 weight, mole fraction of C6? of a gas are known, the correlations (Katz and Firoozabadi 1978; Ahmed 1985; challenge facing the gas processor or end-user becomes Robinson and Peng 1978) have been chosen for the one of splitting the C6? fraction into C6/C7/C8 that can be determination of the molecular weights of C6/C7/C8; in so used to predict the HCDP and phase behaviour of the gas doing three forms of Eq. (3) are generated. Table 1 pre- by equation of state. Based on the observation reported by sents the approximate molecular weight values of C6/C7/ several researchers (Katz 1983; Lorenz et al. 1964; Pe- C8 from the chosen correlations. dersen et al. 1982; Ahmed et al. 1985) that lighter hydro- The three forms of Eq. (3) are thus; carbon systems exhibit exponential molar distribution, an 28:96cC6 84y6 96y7 107y8 4 equation is therefore formulated which could be used to appropriately split the C6? data after the calculation of the 28:96cC6 85y6 94y7 106y8 5 quasi-mole fraction of the individual C6? pseudocom- 28:96cC6 86y6 100y7 114y8 6 ponents from the application of three well-established hydrocarbon physical properties correlations. Note: Katz and Firoozabadi (1978) proposed a set of The procedure employed to generate an appropriate tabulated properties. Ahmed (1985) correlated Katz percentage distribution of C6? composition of a gas is Firoozabadi-tabulated physical properties with the summarized below: number of carbon atoms of the fraction. The generalized equation is of the form: The apparent molecular weight of the C6? composi- tion, if characterized into C6/C7/C8, is defined math- h a1 a2 n a3 n2 a4 n3 a5 =n 7 ematically by the following equation; where h any physical properties, but for this work, it represents the molecular weight; n = number of carbon X 8 Ma C6 yi M i 1 atoms, i.e. 6,7, , 45; a1-a5 = coefficients of the equa- i6 tion given Table 2. Robinson and Peng (1978) put forward a correlation for If the specific gravity of C6? (cC6 ) is considered, the determination of the molecular weight of the paraffinic Eq. (1) can then be expressed as: group of hydrocarbon system. The equation is of the form; Ma C6 X 8 For Paraffinic group : MW 14:026n 2:016 8 cC6 yi Mi Mair i6 MW = molecular weight; n = number of carbon atoms. Equation (4) through (6) can be written in matrix form X 8 cC6 Mair yi Mi 2 as follows; i6 2 32 3 2 3 84 96 107 y6 28:96cC6 Expanding the equation and inserting the value, 28.96, 4 85 94 106 54 y7 5 4 28:96cC6 5 9 of the apparent molecular weight of air (Mair) gives; 86 100 114 y8 28:96cC6 28:96cC6 y6 M6 y7 M7 y8 M8 3 To solve the matrix expression requires that the specific gravity of the C6? must be known. Mi ; molecular weight of the ith component in the C6?; The calculated quasi-mole fraction, that is c6 ; c7 ; c8 , yi ; quasi-mole fraction of the component i in the C6?. of the individual C6? pseudocomponents from the Though there are many correlations for estimating the application of three well-established hydrocarbon phys- molecular weight of petroleum fractions, most of which use ical properties correlations are then expressed in an specific gravity and boiling point as correlation parameters, exponential molar distribution percentage pattern. The three well-established hydrocarbon physical properties equation is of the form; Table 1 Approximate values of the molecular weight of C6/C7/C8 Katz and Firoozabadi (1978) Component C6 C7 C8 Molecular weight, M 84 96 107 Ahmed (1985) Component C6 C7 C8 Molecular weight, M 85 94 106 Robinson and Peng (1978) Component C6 C7 C8 Molecular weight, M 86 100 114 123

4 204 J Petrol Explor Prod Technol (2014) 4:201207 Table 2 Coefficients of Ahmed for calculating hydrocarbon molecular weight h a1 a2 a3 a4 a5 Molecular weight, M -131.11375 24.96156 -0.34079022 2.4941184 9 10-3 468.32575 Expyi c6 ; c7 ; c8 are 0.2133, 0.1919, -0.1706 respectively. To % mole distribution; z i P8 100 10 obtain the actual percentage molar distribution of the C6? i6 Exp(yi composition, the quasi-mole fraction are expressed in Equations (9) and (10) are therefore the proposed exponential pattern using Eq. (10); equations to be used for the splitting of C6? composition of any given pipeline gas provided that the C6? specific zi P8 Expyi 100 Exp(yi gravity is known. i6 P 8 Exp(yi Expy6 Expy7 Expy8 I6 Application to field C61 data Exp0:2133 Exp0:1919 Exp0:1706 1:2378 1:2115 0:8432 3:2925 The first example uses a gas sample from Queensland Gas Pipeline reported by Jemena Asset Management (Queens- Thus, the percentage split of the C6? components are; land Gas Pipeline Measurement Manual, GTS-199-PR- 1:2378 GM-001 2011). The gas sample was analyzed online by a z6 100 37:593 % 37:59 % 3:2925 C6? gas chromatography for its bulk molecular composi- tions. The sample information, gas molecular compositions 1:2115 z7 100 36:798 % 36:80 % and sample properties are presented in Table 3. The C6? 3:2925 has a 0.0304 mol%. If we assume the specific gravity of 0:8432 z8 100 25:609 % 25:61 % 0.6245 reported to be that of the C6? data, then the molar 3:2925 percentage distribution of the C6? data can be determined The percentage distribution of the C6? component of using Eqs. (9) and (10). Equation (9) therefore becomes; the gas sample from Queensland Gas Pipeline with specific 2 32 3 2 3 84 96 107 y6 28:96 0:625 gravity of 0.6245 is approximately; 37.59 % C6, 36.80 % 4 85 94 106 54 y7 5 4 28:96 0:625 5 C7, and 25.61 % C8. Thus, Table 4 illustrates the resulting 86 100 114 y8 28:96 0:625 distributions of the concentration of hexanes, heptanes, 2 32 3 2 3 octanes using common composition splits (of the C6? 84 96 107 y6 18:085 4 85 94 106 54 y7 5 4 18:085 5 percentage mole of 0.0304) similar to using a company, 86 100 114 y8 18:085 contract or historical characterization assumption, and the new method. Using MATLAB, the above matrix algebra can be For the C6? data compositions in Table 4, calculation solved. Thus, the results of the quasi-mole fraction, i.e. of the HCDPs as well as cricondentherm, using HYSYS with Peng Robinson EOS, were performed. (Note that: in Table 3 Queensland gas pipeline gas sample the case of C6? data with no split, the C6? is treated as a single cut; its normal boiling point of 311.98 F R was Components Composition, mol% estimated using the correlation by Riazi and Daubert N2 2.4910 (1987).The predicted HCDPs, at a pipeline pressure of CO2 0.9960 215 psia, for the five C6? data are presented in Table 5. A C1 89.9510 comparison of the HCDP results, at the pipeline operating C2 4.9800 pressure, shows a slight variance between the new method C3 0.9940 and the traditional C6? split ratios (Daniels 47/35/17, GPA i-C4 0.3080 50/25/25 and the GPA 60/30/10). The predicted HCDP n-C4 0.3090 ranges from as low as -40 F for the C6? data with no i-C5 0.1001 split ratio to 2.861 F for the new split method. The HCDP n-C5 0.1002 results for Daniels 47/35/17, GPA 50/25/25 and the new C6? 0.0304 split method are -2.151, 1.038 and 2.861 F respectively; Specific gravity (air = 1) 0.6245 this shows that the new split method has the maximum HCDP value. 123

5 J Petrol Explor Prod Technol (2014) 4:201207 205 Table 4 Illustration of the resulting distribution of C6/C7/C8 for various common splits Component C6? data with no C6? data with the new C6? data with 47/35/ C6? data with GPA 2261 C6? data with GPA 2261 split method split ratio 17 split ratio split ratio (50/25/25) split ratio (60/30/10) (composition, (composition, mol%) (composition, mol%) (composition, mol%) (composition, mol%) mol%) N2 2.4910 2.4910 2.4910 2.4910 2.4910 CO2 0.9960 0.9960 0.9960 0.9960 0.9960 C1 89.9510 89.9510 89.9510 89.9510 89.9510 C2 4.9800 4.9800 4.9800 4.9800 4.9800 C3 0.9940 0.9940 0.9940 0.9940 0.9940 i-C4 0.3080 0.3080 0.3080 0.3080 0.3080 n-C4 0.3090 0.3090 0.3090 0.3090 0.3090 i-C5 0.1001 0.1001 0.1001 0.1001 0.1001 n-C5 0.1002 0.1002 0.1002 0.1002 0.1002 C6? 0.0304 C6 0.01143 0.014432 0.0152 0.01824 C7 0.01130 0.010747 0.0076 0.00912 C8 0.00786 0.00522 0.0076 0.00304 Total 100 100 100 100 100 Table 5 Comparisons of hydrocarbon dew point predictions and cricondentherm along with cricondenbar C6? data C6? data with C6? data with the C6? data with GPA 2261 C6? data with GPA 2261 with no split 47/35/17 split ratio new method split split ratio (50/25/25) split ratio (60/30/10) Hydrocarbon dew point -40 -2.151 2.861 1.038 -8.0140 prediction at 215 psia (F) Cricondentherm (F) -22.78 4.627 9.204 7.334 -0.5866 Cricondenbar (psia) 1,047 1,104 1,114 1,107 1,091 Fig. 2 Different phase 1200 envelopes, dew point curve portions, using different C6? characterisation methods of a 1000 pipeline gas with specific gravity of 0.6245 800 Pressure, Psia 47/35/17 600 GPA 50/25/25 GPA 60/30/10 400 New Method C6+ with no split 200 0 -100 -80 -60 -40 -20 0 20 Temperature, F Also, comparison of the predicted cricondentherm using the predicted cricondentherm values. Figure 2 shows the the various C6? characterisation approaches shows sig- comparison of the phase envelopes (of the dew point curve nificant variation. Table 5 also presents the comparison of portions) for the various C6? data compositions. The 123

6 206 J Petrol Explor Prod Technol (2014) 4:201207 Table 6 Typical union gas sample with specific gravity of 0.58 is approximately; 37.3 % C6, Components Composition, mol% 36.6 % C7, and 26.1 % C8. Thus, Table 7 presents the resulting distributions of the concentration of hexanes, N2 2.4910 heptanes, octanes and nonanes using common composition CO2 0.9960 splits (of the C6? percentage mole of 0.010) similar to O2 0.02 using a company, contract or historical characterization C1 95.2 assumption, and the new method. C2 2.50 Figure 3 shows the comparison of the phase envelopes C3 0.20 (of the dew point curve portions) for the various split i-C4 0.03 methods of the C6? data compositions of the Union Gas n-C4 0.03 system. The phase envelope of the new split ratio gives the i-C5 0.01 maximum cricondentherm value. There is a slight differ- n-C5 0.01 ence in the phase envelope predicted using GPA 50/25/25, C6? 0.01 Daniels 47/35/17 and the new split ratio. However, for this Specific gravity (air = 1) 0.58 example, the GPA 50/25/25 dew point curve tends to be closer to that of the new split ratio than the dew point curve predicted by the Daniels 47/35/17 split ratio. The phase phase envelope of the new split ratio is above those of the envelope of the C6? data with no split appears to be far traditional C6? split ratios (Daniels 47/35/17, GPA 50/25/ away from that of the other split methods. 25, and GPA 60/30/10). The worst case seems to be the C6? with no split which is far below all split methods (Table 5). Conclusions Performing similar calculations, using Eqs. (9) and (10), for the typical pipeline gas on the Union Gas system given Monitoring of HCDP of a pipeline gas is very important in in Table 6 (Chemical Composition of Natural Gas (2005, order to ensure that high quality gas is supplied by gas http://www.uniongas.com) with specific gravity of 0.58 processor/pipeline operators to the end-users. It is prudent gives the following C6? percentage distributions; to use an appropriate split of the C6? composition when (37.303 % C6, 36.568 % C7, 26.130 % C8. The ratio of applying a HCDP limit using C6? data. Therefore, a very the split of the C6? component of the typical pipeline gas simple and practical procedure for the estimation of the Table 7 Illustration of the resulting distribution of C6/C7/C8 for various common splits Component C6? data with C6? data with 47/35/ C6? data with the new method C6? data with GPA 2261 C6? data with GPA 2261 no split 17 split ratio split ratio (37.3/36.6/26.1) split ratio (50/25/25) split ratio (60/30/10) (composition, (composition, mol%) (composition, mol%) (composition, mol%) (composition, mol%) mol%) N2 2.4910 1.3 1.3 1.3 1.3 CO2 0.9960 0.70 0.70 0.70 0.70 O2 0.02 0.02 0.02 0.02 0.02 C1 95.2 95.2 95.2 95.2 95.2 C2 2.50 2.50 2.50 2.50 2.50 C3 0.20 0.20 0.20 0.20 0.20 i-C4 0.03 0.03 0.03 0.03 0.03 n-C4 0.03 0.03 0.03 0.03 0.03 i-C5 0.01 0.01 0.01 0.01 0.01 n-C5 0.01 0.01 0.01 0.01 0.01 C6? 0.01 C6 0.004747 0.00373 0.005 0.006 C7 0.003535 0.00366 0.0025 0.003 C8 0.001717 0.00261 0.0025 0.001 Total 100 100 100 100 100 123

7 J Petrol Explor Prod Technol (2014) 4:201207 207 Fig. 3 Different phase 900 envelopes, dew point curve 800 portions, using different C6? characterisation of the union gas 700 47/35/17 Pressure, Psia system with specific gravity of 600 37.3/36.6/26.1 (New Split 0.58 500 ratio) GPA 50/25/25 400 300 GPA 60/30/10 200 C6+ with no split 100 0 -150 -100 -50 0 Temperature, F percentage distribution of C6? composition for any par- Chemical Composition of Natural Gas (2005) http://www.uniongas. ticular pipeline gas with a known C6? specific gravity in com Ernst K, Pettigrew D (2005) Hydrocarbon dew point monitoring of order to improve HCDP determination has been formu- natural gas using field-mounted on-line gas chromatographs. lated. The appropriate percentage distribution of C6? Pipeline Gas J pipeline gas composition may hovers around 3738 % C6, GPA 2261 (2000) Analysis for natural gas and similar gaseous 3637 % C7 and 2426 % C8. Based on the field C6? data mixtures by gas chromatography, 2000. Gas Processors Associ- ation, Tulsa examples, the new split method has the maximum HCDP Jack H (2010) Hydrocarbon dew point is a critical consideration for value when compared to the HCDP predictions of some pipeline operations. Pipeline Gas J 237(7) common C6? split ratios. Application of this new method Katz D et al. (1983) Overview of phase behaviour of oil and gas to split the C6? data may therefore represent and also production. JPT 12051214 Katz DL, Firoozabadi A (1978) Predicting phase behaviour of enhance the prediction of HCDP of any particular gas. condensate/crude-oil systems using methane interactions coeffi- cients. JPT 16491655 Open Access This article is distributed under the terms of the Lorenz J, Bray BG, Clark CR (1964) Calculating viscosities of Creative Commons Attribution License which permits any use, dis- reservoir fluids from their compositions. JPT Trans AIME tribution, and reproduction in any medium, provided the original 231:1171 author(s) and the source are credited. NGC-GPA (2005) White paper on liquid hydrocarbon dropout in natural gas infrastructure. Natural Gas Council for the Federal Energy Regulatory Commission, NGC? Liquid Dropout Task References Group (Feb) Pedersen K, Thomassen P, Fredenslud A (1982) Phase equilibria and separation processes. In: Report SEP 8207. Institute for Ahmed T (1985) Composition modelling of Tyler and mission canyon Kemiteknit, Denmark Tekniske Hojskole, Denmark formation oils with CO2 and lean gases, final report submitted to Queensland Gas Pipeline Measurement Manual (2011) Document the Montanas on a new track for science (MONTS) program. number: GTS-199-PR-GM-001 Montana National Science Foundation Grant Program Riazi MR, Daubert TE (1987) Characterization parameters for Ahmed T, Cady G, Story A (1985) A generalised correlation for petroleum fractions. Ind Eng Chem Res 26(24):755759 characterizing the hydrocarbon heavy fractions. In: SPE paper Robinson DB, Peng DY (1978) The characterization of the heptanes 14266 presented at the SPE 60th annual technical conference, and heavier fractions. In: Research report 28, GPA, Tulsa Las Vegas, 2225 September 1985 Shane H (2000) Determination of hydrocarbon dew point using a gas ASTM D 1142-95 (1995) Standard test method for water vapor chromatograph, class # 5300. Emerson Process Management, content of gaseous fuels by measurement of dew-point temper- Gas Chromatograph Division, Houston, USA ature. American society for testing and materials, Philadelphia 123

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