Measurement and modeling of hydrocarbon dew points for five synthetic natural gas mixtures

doi:10.1016/j.fluid.2005.11.010

Fluid Phase Equilibria

Volume 239, Issue 2, 31 January 2006, Pages 138-145

https://doi.org/10.1016/j.fluid.2005.11.010 Get rights and content

Abstract

The dew points of five synthetic natural gas (SNG) mixtures were measured using a custom made chilled mirror apparatus. The chilled mirror apparatus was designed to detect hydrocarbon dew points from low pressures up to the cricondenbar. The experimental temperature range was from 235 to 280 K and the pressure range from 0.3 to 10 MPa. The synthetic natural gases were comprised of methane and gravimetrically prepared fractions of ethane, propane, i-butane, n-butane and n-pentane. The experimental data were compared to calculations with the Soave–Redlich–Kwong (SRK) equation of state with classical mixing rule. However, considerable and increasing deviations between calculated and experimental dew points were observed as the pressure approached the cricondenbar. Therefore, a model was utilized based on the Redlich–Kwong (RK) equation of state. The Mathias and Copeman (MC) function was used to express the temperature dependence of the attractive term for all components. For methane, different sets of MC coefficients were used below and above the critical point. An optimization procedure was employed to fit the coefficients of supercritical methane to both pure component fugacity and experimental dew points. There is good agreement between experimental data and modeling results. For pressures higher than the pressure corresponding to the cricondentherm, the proposed model is better than the standard SRK equation of state. Good predictions with the model were obtained when comparing to bubble and dew points data from literature.

Introduction

There is always a risk of hydrocarbon condensation in natural gas transmission pipelines. Hydrocarbon liquid from condensation will increase the pressure drop and introduce operational problems resulting from two-phase flow. It is important to prevent condensation by keeping the natural gas temperature and pressure in the single-phase region. Optimal control of the hydrocarbon dew point is therefore important for economical, operational and safety reasons.

Natural gas from the Norwegian continental shelf is transported to the European market through multi-phase, rich and dry gas pipelines. The gas is transported from offshore production installations as partly processed gas and condensate (multi-phase) or rich gas (single-phase) to onshore processing plants where hydrocarbon liquid is extracted, and finally as dry gas to the European market. Rich gas is partially processed natural gas transported in the dense phase region, where the capacity of the pipelines is limited by the lowest possible arrival pressure (cricondenbar). Dry gas transport is the transport of fully processed natural gas. The lowest acceptable temperature is limited by the hydrocarbon dew point specification on sales gas (cricondentherm) and the requirement of no liquid hydrocarbon formation in the export pipelines. To utilize the natural gas production and transmission system optimal it is important to be able to predict the phase behavior of natural gases both near the cricondentherm and cricondenbar.

The dew points of natural gas mixtures can be predicted using traditional equations of state; however, the predictions are often inaccurate [1], especially for pressures higher than the pressure corresponding to the cricondentherm [2].

Using a chilled mirror apparatus, Avila et al. [2] and Jarne et al. [3] measured the dew points of synthetic natural gases with compositions similar to the natural gases supplied through the Spanish gas distribution network. They also modeled their measurements using an equation-of-state approach. Blanco et al. [4] studied the phase behavior of a synthetic natural gas similar to Algerian natural gas. All these studies indicate that the average absolute error in predicting dew point temperatures could be as large as 3.7 K for simple synthetic gas mixtures. The demand for more accurate calculations of the natural gas dew points was apparent. It was therefore decided to perform a systematic study of hydrocarbon dew points of synthetic natural gases.

The main components of a dry natural gas are hydrocarbons (80–100 mol%), nitrogen (0–15 mol%), and carbon dioxide (0–3 mol%). To simplify, and isolate the effect of hydrocarbons only, the focus of the experimental measurements in this work was hydrocarbon gasses without nitrogen and carbon dioxide. The dew points of five synthetic natural gas mixtures comprised of light hydrocarbons (methane to n-pentane) were measured. The obtained experimental data was then simulated using an equation-of-state approach. Nasrifar et al. [1] compared the accuracy of 15 equations of state for predicting the dew points of synthetic and real natural gas mixtures. The comparisons revealed that the Soave–Redlich–Kwong (SRK) [5] equation of state, or one of its variants, can reasonably predict the dew points of synthetic natural gases, especially near cricondentherm. Thus, the Redlich–Kwong (RK) equation of state [6] with the Mathias and Copeman temperature dependent term [7] were used to improve the accuracy of the calculations. For methane, different sets of MC coefficients were used below and above the critical point. The coefficients of supercritical methane were fitted to both pure component fugacity and the experimental dew points measured in this work. The results using the RK equation of state with the Mathias and Copeman temperature dependent term (RKMC) and the modeling show improved accuracy compared to using the SRK equation of state.

Section snippets

Experimental equipment

A custom made apparatus for measurement of hydrocarbon dew points of dry- and rich natural gasses has been built up in the Statoil R&D laboratories in Trondheim, Norway. The apparatus was used without any modification during the measurements of this work. The experimental equipment consists of a piston circulating the sample back and forth between two equally large chambers. As the gas is circulated in a closed loop, it passes a mirror whose temperature is controlled by fitting a cooled copper

Experimental procedure

The experimental apparatus and all external piping were vacuumed at a controlled temperature of 320 K for a minimum of 12 h. Then, the system was filled with the gas sample to the highest possible pressure (gas bottle pressure was typically 10–12 MPa). The system was stabilized by circulating the gas back and forth between the two chambers for half an hour. The mirror was cooled steadily and slowly while the gas was circulating at 400 accm/h until a visually observable amount of hydrocarbon

Experimental results

The compositions and code names for the five SNG mixtures are given in Table 1. One mixture (SNG2) contains less than 85% methane while the other four mixtures contain at least 93% methane. In addition to methane, the gases consisted of ethane, propane, i-butane, n-butane and n-pentane. All gases were supplied by Yara, Norway [11]. The claimed compositions for the gases by the supplier were validated in the Statoil R&D laboratory using gas chromatography. There were close agreement between the

Modeling with equation of state

In order to describe the phase envelopes for the synthetic natural gas mixtures, the RK equation of state [6] with Mathias and Copeman (MC) temperature dependent term [7] are used (RKMC). The pressure–volume–temperature relationship for the RKMC equation of state may be expressed by: $P = \frac{R T}{v - b} - \frac{a_{C} α (T_{r})}{v (v + b)}$ with $b = 0.08664 \frac{R T_{C}}{P_{C}}$ $a_{C} = 0.42748 \frac{R^{2} T_{C}^{2}}{P_{C}}$ While the temperature dependent attractive term for the SRK equation of state [5] is expressed by: $α (T_{r}) = {[1 + (0.48 + 1.574 ω - 0.176 ω^{2}) (1 - \sqrt{T_{r}})]}^{2}$ the temperature

Results and discussion

The measured dew point pressures and temperatures for the five synthetic natural gas mixtures are plotted in Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7. Also shown in Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 are the modeling results. For the sake of comparison, the dew point conditions predicted by the SRK equation of state [5] and the conventional RKMC equation of state are also shown. Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 show that the predictions adequately describe the experimental values from

Conclusion

The dew point temperatures and pressures of five synthetic natural gas mixtures were measured using a chilled mirror apparatus. The experimental data were compared to the predictions with the original SRK equation of state and to the predictions with the RK equation of state and the Mathias and Copeman temperature dependent term. For methane at supercritical temperatures, a temperature dependency similar in form to the Mathias and Copeman function at subcritical temperatures was proposed. The

Acknowledgements

Statoil is an integrated oil and gas company with substantial international activities and the operator of a large part of Norwegian oil and gas production. Gassco is operator for transporting Norwegian gas to continental Europe and the UK through a 6600 km network of pipelines. This R&D work has jointly been financed by Gassco and Statoil.

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Measurement and modeling of hydrocarbon dew points for five synthetic natural gas mixtures

Abstract

Introduction

Section snippets

Experimental equipment

Experimental procedure

Experimental results

Modeling with equation of state

Results and discussion

Conclusion

Acknowledgements

Fluid Phase Equilib.

Chem. Eng. Sci.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

Energy Fuels

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

Chem. Rev.