Degasification of hydrogen sulfide from sour San Andres source water in West Texas has increased the availability of water for a flood project in Midland County. The plant is treating water for Mobil Oil Co.'s flood project in the Pennsylvanian beds of the Pegasus field.
About three years ago, it became apparent that supplies of water to be used in secondary recovery in the Permian Basin would soon become critically short and that use of available sour waters would be necessary in many floods yet to be started. It became further apparent that this sour water would have to be treated for removal of hydrogen sulfide before it could be used economically.
Several methods of stripping H2S from water which were considered of limited potential success included: aeration with forced draft aerators; stripping the H2S with engine exhaust in a packed column; and stripping the H2S with exhaust gas produced by a submerged burner in a packed column.
Finally, after studying all available methods, the following method was decided upon: Stripping the H2S with exhaust gas generated in an external vessel under carefully controlled conditions to produce an oxygen-free gas, using a bubble cap column of sufficient height to remove hydrogen sulfide to an acceptable residual, arbitrarily chosen as 1.5 ppm. However, this method had several drawbacks:
The method had not been tried, even on a pilot scale.
No equipment had been designed for this purpose.
Design efficiencies had not been established.
Time did not permit pilot plant operation.
The bubble cap column was chosen since it is less subject to fouling than a packed column and, if fouled, is more readily cleaned. Some data were established by the author in a similar column using natural gas to strip oxygen from water. That column has operated successfully for approximately six years under operating conditions which place extreme design restrictions on the column, with resulting low efficiencies.
Design of the column is based on Henry's law, which states that "the solubility of a gas in a liquid is directly proportional to the partial pressure [in atmospheres] of that gas in the surrounding atmosphere at a given temperature." Thus, if the partial pressure of H2S in the atmosphere surrounding the water approaches zero and if, at the same time, the H2S is given sufficient opportunity to escape from the water, the dissolved H2S will also approach zero. It is obvious that countercurrent stripping is ideal for obtaining these conditions.
Design of the column trays was made using published data and the following criteria:
Capacity = 13,000 BWPD
Riser velocity = 10 ft/sec
Tower diameter = 4 ft 1D
Weir length = 3 ft
Weir height = 4 in.
Available gas pressure = 5 psig
Since no samples of water from supply wells were available for the design, the following assumptions were made:
Minimum operating temperature, 68 F
Operating pressure, Atmospheric
H2S content of water, 170 ppm
Tray efficiency, 7 per cent
pH of water, 6.0
Sulfides which are H2S, 86 per cent
Gas/water ratio, 66.
7 scf/bbl
