Analysis and Removal of Arsenic From Natural Gas Using Potassium Peroxydisulfate and Polysulfide Absorbents

Abstract

Natural gas from the large Abo gas field in southeastern New Mexico contains arsenic (0.2 to 2.5 μg/L) in the form of trialkylarsines. A method of analysis for arsenic which is based on the extraction of the alkylarsines into a saturated solution of potassium peroxydisulfate (PPDS) followed by determination of total arsenic, as AsO₄ ⁻³, by hydride generation- d.-c. helium plasma emission spectrometry (HG-DCHPES) is described. The extraction of the alkylarsines was carried out by either shaking 1L gas samples with PPDS or sparging the gas in PPDS solutions. Analytical results of acceptable precision can be obtained using either procedure. Temperature (24°—45°C) or pH (1.10-4.10) had little effect on the As concentrations obtained by the sparging method. The measured arsenic concentrations of natural gas samples stored in Cr-Mo steel cylinders decrease with time of storage (-30% in the first 4 weeks). A standard blend of trimethylarsine in CH₄ (1 mole TMA in 10⁶ moles of CH₄), stored in nickel-containing steel cylinder, showed a similar decrease in As concentration with time. The measured arsenic concentrations increase when the temperature of a cylinder of natural gas is heated indicating that part of the alkylarsines may be present in a condensed or physically adsorbed form on the inner walls of the cylinder. Scanning electron microscopy with X-ray microanalysis of the metal powders (Ni, Cr, Mo, Fe) contacted with TMA indicate that arsenic-rich solids also containing C and O are formed in the case of Ni and Cr. This phenomenon may represent an additional factor in the observed decrease in As concentration with time of storage. Polysulfides in solution form (Na₂S_x) or solid (Na₂S₄) can quantitatively remove the trialkylarsines from natural gas.

Key Words:

• Arsenic
• natural gas
• trialkylarsines
• removal