Khalda Petroleum Company has mercury removal unit at its Salam gas processing plant in Egypt's Western Desert, which are intended to protect the downstream aluminum heat exchangers from mercury corrosion.
Last year, the mercury removal adsorbent was exposed to liquid entrainment due to liquid condensation upstream the mercury removal unit. The liquid entrainment caused (1) physical damage to the absorbent pellets which became one solid, cement-like block inside the vessel, and (2) higher levels of mercury in the treated gas. Therefore, the absorbent had to be offloaded and replaced to improve the unit performance.
This article will present the reasons of the liquid condensation upstream the mercury unit and show how (1) the problem was solved, (2) the absorbents were replaced, and (3) the liquid condensation was avoided.
Implications of the effect of mercury in natural gas were not reported until 1973, when a catastrophic failure of aluminum heat exchangers occurred at Skikda LNG plant in Algeria [Kinney 1975]. It was found that a combination of mercury and water temperatures around 0.0°C caused corrosion in the aluminum tubes (constructed of aluminum alloy 6061). This discovery prompted several research studies into this phenomenon.
After the Skikda failure, a study of the Groningen field in Holland revealed similar corrosion in the gas gathering system. CO2 was initially thought to be the cause [Leeper 1980], but later investigations pointed to mercury, with concentrations ranging from 0.001 to as high as 180 µg/std. m3.Although the concentration of mercury in a given natural gas may be considered extremely low, Audeh observed that its effect is "cumulative as it amalgamates" [Audeh 1988].
Elemental mercury forms an amalgam with the surface layer of the metal it contacts. With aluminum, the amalgam is much weaker than the metal itself [Phannenstiel 1976].The mercury-aluminum amalgam process removes the tightly adhering aluminum oxide layer, which then allows aluminum corrosion to occur.
To avoid mercury condensation under pipeline conditions, mercury should be removed from sales gases to less than 20 µg/ std. m3 [Mussig and Rothmann 1997].
Khalda Western Desert gas development project at Salam is 70 km from Matrouh. The plant processes gas condensate from Salam field, Qasr field, South Umbarka, and an oil plant's associated gas. The project produces about 200 MMscfd of export gas at an export pressure of 101 bara and 9,000 STB of condensate. The plant started up in July 1999.
The sales gas is designed to have a maximum CO2content of 3%, a maximum H2S content of 4 ppm (vol), a gross heating value greater than 1,040 btu/std. ft3, a water dewpoint of less than 0.0ºC. at 71 bara, and a cricondentherm of 5ºC. The condensate is designed to have a maximum of 11-psi RVP.
Fig. 1, a diagram of the Salam gas plant, shows gas from the wells flows into two parallel trains. First it enters a three-phase separator where the main water-condensate-gas separation takes place. Gas from the three-phase separator goes to the mercury-removal unit. Then it flows to the glycol contactors to remove water from the gas to avoid hydrate formation and to achieve water-content specifications.
Gas is then diverted to the dewpointing package, whose function is to separate entrained traces of condensate and heavier hydrocarbons that condense as liquids from the gas at lower temperatures. This step is to achieve hydrocarbon dewpoint specifications with the turbo-expanders.