ABSTRACT
Black powder is a generic term used to describe entrained corrosion products that can accumulate in sales gas pipelines, potentially damaging process equipment. Black powder mitigation is a challenge in the gas industry. Many factors govern the formation of black powder, including gas composition (particularly CO2, H2S, and O2 impurities), condensation rate, relative humidity, and the existence of hygroscopic salts on the steel surface. The use of volatile corrosion inhibitors (VCIs) could constitute an economic and effective black powder mitigation method. However, little is known about the applicability of VCIs in sales gas environments. Therefore, it is important to study the effect of the key operating parameters on the efficiency of VCIs. The main goal of this study was to evaluate the efficiency of model inhibitor compounds with different functional groups (amine, thiol) and commercial inhibitors as candidates for black powder mitigation. The inhibition efficiencies of these VCIs were determined in dewing and hygroscopic conditions in both sweet (CO2) and sour (CO2/H2S) environments. Weight loss methods were employed to evaluate each VCI. Steel specimens were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and high resolution optical profilometry. Based on the results of laboratory tests, it is found that volatile corrosion inhibitors can be used to prevent corrosion in a sales gas system with carbon dioxide and hydrogen sulfide. The tested amine (morpholine) seemed to affect the pH of the condensed water but did not have ”filming” properties. The obtained data show that VCIs reduced the mass of corrosion product and, therefore, the amount of black powder that potentially could be formed in CO2/H2S environments. Based on the measured corrosion rates and surface analysis of specimens, commercial inhibitor CI1 was shown to be most effective in inhibiting formation of black powder.
INTRODUCTION
Black powder causes problems in the gas industry by, for example, clogging and eroding valves and contaminating the customer's sales gas. As described in the surveyed literature, ”black powder” consists of various forms of iron sulfide, iron oxide and iron carbonate, mechanically mixed or chemically combined with any number of contaminants such as salts, sand, hydrocarbons, and metal debris.1-4 Black powder can form due to the simultaneous presence of corrosive gases (CO2 and H2S) and condensable moisture (H2O) in pipelines; the presence of oxygen (O2) can also lead to its formation.5-8 If dew point temperatures are reached, water vapor would condense on the inner walls of pipelines. Water can also enter the pipeline through periodic upsets that causes moisture carryover into the line.1 The main strategies for controlling black powder formation in sales gas lines are gas dehydration and mechanical removal (i.e., pigging) of any solids formed.10 In addition, the use of chemical inhibitors, which are common practice in multiphase oil and gas transport pipelines, could constitute an additional mitigation technique.
