CHART ESTIMATES WATER CONTENT OF SOUR NATURAL GAS

Gordon C. Wichert
University of Calgary

Edward Wichert
Gascan Resources Ltd.
Calgary

A simplified chart method to estimate the equilibrium water-vapor content of sour natural-gas mixtures in SI units has been developed.

The method requires use of a chart for the water-vapor content of sweet natural gas and a chart showing the ratio of the water-vapor content of sour gas compared with that of sweet gas.

The following are conditions limiting the method:

• Pressure to 70 MPa

• Temperature to 175 C.

• Equivalent H2S content to 55 mol %.

EXTENSIONS

The sour gas/sweet gas water-vapor-content ratio chart was developed on the basis of published experimental water-vapor-content data for natural gas containing either or both H2S and CO2.

The chart was expanded to 70 MPa pressure on the basis of data points generated with a computer program based on the Soave-Redlich-Kwong (SRK) equation of state developed by Robinson.1

Fig. 1 is the chart for determining the equilibrium water-vapor content of sweet natural gas in SI units. A major portion of the chart is based on the well known McKetta and Wehe chart,2 which has been in use since 1958 and is in Imperial units.

Experimental sweet gas water-vapor-content data in the high-temperature region published by Olds3 were used to extend the range of Fig. 1 to 200 C. (393 F.).

The computer simulation program HYSIM4 was also used to generate water-vapor-content data in the high temperature, high-pressure region; these data compared favorably with the experimental data of Olds.

In light of these results, the program was used to generate a 100 MPa (abs) (14,500 psia) isobar extending the pressure range of the sweet-gas chart.

Also included in Fig. 1 are other features of the McKetta and Wehe chart, such as corrections for gas relative density and salinity, as well as comments regarding the hydrate line and water-vapor content in the sub-zero temperature region.

BASED ON PUBLISHED DATA

Fig. 2a gives the ratio of the water-vapor content of sour natural gas as compared to the water-vapor content of sweet natural gas at the same pressure and temperature.

This chart was developed on the basis of published experimental water-content data5-7 of natural gas containing either or both H2S and CO2 up to a pressure of about 17 MPa. The Robinson model extended the chart to 70 MPa.

As with the Robinson method, it is necessary to calculate an equivalent H2S content for the sour-gas mixture by adding 75% of the CO2 content to the H2S content of the sour natural gas. The experimental data used included 11 gas mixtures with 55 data points containing H2S up to 29 mol % and CO2 up to 60 mol %.

Temperatures varied from 37.8 C. to 176.7 C. and pressures ranged from 1.48 MPa (abs) to 17.31 MPa (abs). With the use of the ratio chart, 37 calculated points were within 5% of the experimental data; another 14 points were within 10%.

Of the four points exceeding 10%, a maximum deviation of 15% occurred for a gas mixture containing 10 mol % H2S and 60 mol % CO2 at 11.8 MPa (abs) and 176.7 C. Another point at the same temperature for the same mixture and a pressure of 17.3 MPa (abs) showed a deviation of 2%.

A version of the water-content ratio chart in Imperial units was also developed and is presented in Fig. 2b. The Imperial units ratio chart is intended for use with the McKetta and Wehe sweet gas water-content chart (Fig. 20-3 GPSA Engineering Data Book, Vol. 28).

SAMPLE PROBLEM

The following example problem illustrates the use of this new method for estimating in SI units the water-vapor content of sour natural gas:

• Problem:

  What is the equilibrium water-vapor content of a sour gas containing 30 mol % CH4, 10 mol % H2S, and 60 mol % CO2 at 107 C. and 8.36 MPa (abs)?

• Solution:
  1. Determine the equilibrium water-vapor content of sweet gas at the same conditions using the chart in Fig. 1 (approximately 14.2 kg/1,000 cu M).

  2. Calculate the equivalent H2S content of the mixture by adding the mol % H2S to 75% of the mol % CO2 (that is, 10 mol % + [0.75 x 60 mol %] = 55 mol % equivalent H2S).

  3. Enter the SI water-content-ratio chart (Fig. 2a) at 107 C. and move to the right to the 55% equivalent H2S line.

     From this point, move vertically to the absolute pressure of 8.36 MPa. Move to the left side of the chart and obtain the corresponding water-vapor content ratio (approximately 1.20).

  4. To obtain the equilibrium water-vapor content of the sour-gas mixture, multiply the water-vapor-content ratio by the equilibrium water-vapor content of sweet gas obtained in Step 1 (14-20 kg/1,000 cu m x 1.20 = 17.04 kg/1,000 cu m).

The actual published experimental water-content value for this gas mixture at these conditions is 17.14 kg/1,000 cu m.

The method was compared against published experimental data, as well as the method published by Robinson. A range of selected data points is shown in Table 1.

REFERENCES

1. Robinson, J.N., Wichert, E., Moore, R.G., and Heideman, R.A., "Charts help estimate H2O content of sour gases," OGJ, Feb. 6, 1978, pp. 76-78.

2. McKetta, J.J., and Wehe, A.H., "Use This Chart for Water Content of Natural Gases," Petroleum Refiner, August 1958, pp. 153-154.

3. Olds, R.H., Sage, B.H., and Lacey, W.N., "Composition of the Dew-Point Gas of the Methane-Water System," Industrial and Engineering Chemistry, 1943, pp. 1223-1227.

4. Hyprotech Ltd., Calgary, Alta.

5. Sharma, S.C., "Equilibrium Water Content of Gaseous Mixtures," unpublished doctoral thesis, University of Oklahoma, 1969.

6. Lukacs, J., "Water Content of Hydrocarbon-Hydrogen Sulphide Gases," unpublished masters thesis, University of Alberta, 1962.

7. Maddox, R.N., Lilly, L.L., Moshfeghian, M., and Elizondo, E., "Estimating Water Content of Sour Natural Gas Mixtures," presented to the Laurence Reid Gas Conditioning Conference, Norman, Okla., Mar. 8, 1988.

8. Engineering Data Book, 10th ed. (1987), Vol. 2, Gas Processors Suppliers Association, Tulsa, pp. 20-3.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.