4 Mar, 2016
What is the Impact of Light Hydrocarbons on the Natural Gas Hydrate Formation Conditions?
The December 2012  and January 2016  Tips of the Month (TOTM) discussed the hydrate phase behavior of natural gas mixtures containing high content hydrogen sulfide, carbon dioxide, or nitrogen. Specifically, it showed nitrogen and carbon dioxide inhibit the hydrate formation slightly while hydrogen sulfide enhances hydrate formation considerably. This tip will extend the previous studies on the natural gas hydrate formation phase behavior. Specifically, it will study the impact of light hydrocarbons on the formation of hydrate in a natural gas mixture.
The hydrate formation temperature of a gas depends on the system pressure and composition. There are several methods of calculating the hydrate formation conditions of natural gases [3-6]. References [3-4] present rigorous methods while [5-6] present the shortcut methods suitable for hand calculations. This study uses a rigorous method using the Soave-Redlich-Kwong (SRK) equation of state  in ProMax  software.
Table 1 presents the compositions (mol %) of the gas mixtures studied. Notice that for non-hydrocarbons (gases B, C, and D) about 18 mol % of methane is replaced with about 20 mol % of either nitrogen, carbon dioxide or hydrogen sulfide. These compositions are for a gas stream leaving a separator at 100 °F and 1000 psia (37.8 °C and 6900 kPaa) saturated with water.
Table 1. Water-saturated compositions (mol %) of gas mixtures studied
Figure 1 presents the calculated hydrate formation and the dew point portion of the phase envelope (continuous curves) of a sweet natural gas (gas E of Table 1) containing 0 mol % C2H6. Figure 1 also presents the dew point and hydrate formation (broken curves) for gas F of Table 1 containing 17.8 mol % C2H6.
Figure 1. The impact of C2H6 on the hydrocarbon dew point and hydrate formation curves
Figure 1 indicates that the presence of 17.8 mol % C2H6 has a negligible effect on the hydrate formation curve. Note that the points to the left and above the hydrate curves represent the hydrate formation region. From an operational point of view, this region should be avoided. This figure also indicates that the presence of C2H6 decreases the cricondenbar pressure and the cricondentherm temperature; therefore, the two-phase (gas + liquid) region within the envelope shrinks.
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Figure 2 presents the calculated hydrate formation and the dew point portion of the phase envelope (continuous curves) of a sweet natural gas (gas G of Table 1) containing 0 mol % C3H8. Figure 2 also presents the dew point and hydrate formation curves (broken curves) for gas H of Table 1 containing 12.7 mol % C3H8. Figure 2 indicates that the presence of 12.7 mol % C3H8 shifts the hydrate formation curve to the right promoting the hydrate formation condition. This figure also indicates that the presence of C3H8 decreases the cricondenbar pressure while having little effect on the cricondentherm temperature; the two-phase (gas + liquid) region within the envelope shrinks.
Similarly, Figure 3 presents the impact of 12.7 mol % iC4H10 on the dew point and hydrate formation curves for gases I and J of Table 1. This figure indicates that iC4H10 like C3H8 is a hydrate promotor and shifts the hydrate curve to the right.
Figure 2. The impact of C3H8 on the hydrocarbon dew point and hydrate formation curves
Figure 3. The impact of iC4H10 on the hydrocarbon dew point and hydrate formation curves
Similarly, Figure 4 presents the impact of 11.4 mol % nC4H10 on the dew point and hydrate formation curves for gases K and L of Table 1. This figure indicates that contrary to iC4H10, nC4H10 is a hydrate inhibitor and shifts the hydrate curve to the left. Both iC4H10 and nC4H10 lower the cricondentherm temperature and increase the cricondenbar pressure.
Figure 4. The impact of nC4H10 on the hydrocarbon dew point and hydrate formation curves.
Figure 5 presents a summary of the calculated hydrate formation curves for sweet gas A of Table 1 (Continuous curve), and gases B (20 mol % H2S), gas C (20 mol % CO2), gas D (20 mol % N2), gas F (17.8 mol % C2H6), gas H (12.7 mol % C3H8), gas J (12.7 mol % iC4H10), gas L (11.4 mol % nC4H10) (broken curves). For the cases studied, this figure clearly indicates that the impact of N2 is much less than of H2S and slightly less than of CO2. Nitrogen, carbon dioxide, and nC4H10, depress the hydrate formation condition (shift the hydrate curves to the left). Between these three components, nC4H10 has the larger depression effect even though its mol % is smaller. While C2H6 has the same effect as CH4 on the hydrate formation condition (no shift on the hydrate formation curve), C3H8, iC4H10, and H2S promotes hydrate formation condition. Among these hydrate promotors, H2S has the largest contribution even for only 10 mol %. Note that “Sweet Gas” refers to gas A in Table 1.
By: Dr. Mahmood Moshfeghian
For training courses pertaining to this month's topic, view our Gas Processing and Production & Completions Engineering disciplines.
- Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2012/12/sour-gas-hydrate-formation-phase-behavior/
- Moshfeghian, M., http://www.jmcampbell.com/tip-of-the-month/2016/01/what-is-the-impact-of-nitrogen-on-the-natural-gas-hydrate-formation-conditions/
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