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53 NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIALS PERFORMANCE APRIL 2017 ef f i ci enc y. O n se veral o c ca sions, maint enance shut-down re vealed salt deposition and corrosion dam- age on the wire mesh of the demister pads in gas scrubbers upstream of glycol absorption. Gas-gas shell-and- tube heat exchanger leakages were o b s e r v e d t h a t w o u l d n e g a t iv e ly impact the operational efficiency of the glycol absorption system. • The foaming observed in the absorp- tion tower could have resulted from accidental inclusion of oxygen at the operating temperature that causes degradation of TEG, and produces acid s and oth er compound s. De- graded TEG does not possess the equivalent moisture removal capabil- ity of pure glycol. • The presence of excessive levels of aci d gase s (H 2 S and C O 2 ) in th e incoming gas reduces the moisture absorption capacity of TEG. 7 • Under the operating conditions for inlet gas (pressure, gas flow rate, and moisture content), the temperature of the glycol absorption column and TEG purity may not be optimum for an appropriate degree of dehydra- tion. 8 For example, the TEG concen- tration needs to be maintained at 98.5% to achieve a dew point of 0 °C at the operating temperature of 45 °C. TEG concentrations of <98.5% are almost always a possibility in the operating scenarios. Figure 2 shows a photograph of an ex- port pipeline black powder deposit. Table 3 documents the physical characteristics and chemical analyses of the water- and acid- soluble compounds present in the black FIGURE 2 An export pipeline black powder deposit. FIGURE 3 Particle size distribution of an export pipeline black powder deposit. powder deposits found in a glycol tower demister pad and the after-scrubber. The x-ray diffraction analysis of black powder deposits shows they are comprised of vary- ing amounts of iron sulfides classified as pyrite, greigite, mackinawite, marcasite, as well as iron sulfate (melanterite). Figure 3 shows the particle size distribution of the black powder deposit in the export line. The total suspended fine particles in the gas stream is estimated to have a solid con- centration of 8.4 ppmw. Improving the Efficiency of the Glycol Absorber The effective removal of H 2 S and CO 2 in a gas stream in the sweetening plant prior to entering the glycol absorber would en- hance the absorber moisture removal effi- ciency. Maintaining quality control of fresh TEG is mandatory, so periodic testing of the TEG from the absorber and regenera- tion system would ensure on-stream qual- ity control. The absorber sample testing would indicate whether the system is oper- ating at the required TEG concentration, and the moisture content tests of the TEG sampl e from th e regen eration sy st em would ensure that the regenerated TEG meets the quality requirements for the ab- sorber. A results-oriented process optimi- zation, based on the quality of the dehy- d ra t e d ga s , w o u l d i m p r o v e a b s o rb e r efficiency. Analysis of the dehydrated gas quality could be based on moisture content tests or dew point analysis. The design pa- ram et ers (t emp erature, pressure, f low rates, etc.) could be continually optimized on-stream if the situation demands it. Prevention and Mitigation of Black Powder Formation In oil and gas production operations, materials engineering is an important component of corrosion and cost control. A general rule of thumb is that the use of bare CS with 3 mm (0.125 in) corrosion allow- ance would be adequate and cost effective in situations where the pipeline is not in- ternally exposed to very corrosive condi- tions (e.g., the corrosion rate is assessed to be <0.12 mm/y [5 mpy]). In environments where CS corrodes at ~0.12 to 0.24 mm/y (10 mpy), CS is used with 6 mm (0.25 in) corrosion allowance. However, for those locations where the corrosion rate is as- sessed to be >0.24 mm/y, corrosion resis- tant alloys (CRAs) are used without any corrosion allowance. For the internal side of CS tanks and vessels, in situ coatings are used in conjunction with sacrificial anodes and no internal corrosion allowance, the same as CRAs. The last recommendation is germane for the prevention of black Black Powder in Export Gas Lines