Preliminary evaluation of adsorbent-based mercury removal systems for gas condensate

Abstract

Mercury can be present in gas condensate in its metallic form and/or as organometallic compounds with boiling points comparable to that of the range of the condensate. The damage caused to industrial plants, particularly petrochemical plants, by the presence of certain mercury species can be financially crippling especially when unscheduled shut-downs are forced. Speciation of the mercury content of gas condensates is vital in the evaluation of an adsorbent system’s efficiency and for the development, improvement and monitoring of the performance of newly developed or commercially available mercury removal systems. However, with the mercury species content being at such low levels, great demands are made of an analytical technique. Three commercially available mercury removal systems designated AA, BB, and CC were tested under pilot plant conditions to evaluate their performance. System AA was a two-stage process involving a species conversion step prior to trapping of mercury on an alumina-adsorbent impregnated with a metal sulphide. Systems BB and CC were single-stage trapping processes using sulphide impregnated carbon and molecular sieves, respectively. Both real and substitute condensate sample i.e. hexane containing dimethyl mercury (DMM), diethyl mercury (DEM) and dibutyl mercury (DBM) species, were used in the trials. The determination of total mercury was carried out by a proven method which involved the direct vaporisation with subsequent adsorption of mercury species onto a gold-coated silica trap maintained at 200°C. To release metallic mercury, the trap was heated at 900°C and the analyte determined by atomic fluorescence spectrometry (AFS). The determination of mercury species in the samples was carried out by direct injection using gas chromatography coupled, via a pyrolysis unit, with atomic fluorescence detection. All three mercury-removal systems AA, BB and CC showed a reduction in the mercury content of the final condensate streams. In the two-stage system, AA, the hydrogenolysis reactor converted some of the organomercury present in the gas condensate feed to its elemental form. However, the elemental mercury measured in the liquid product from this first reactor was only about 30% of the total mercury content. Incomplete conversion of the organomercury species to mercury metal by this reactor may be due to competition between the organomercury species and the unsaturated compounds in the matrix during the hydrogenolysis reaction. The second reactor, for mercury trapping, was able to adsorb efficiently elemental mercury present in the liquid stream (the product from the first reactor) but was unable to remove the organomercury content from the condensate stream. For the single-stage adsorbent system BB, the efficiency in removing the species DMM, DEM and DBM from n-hexane hydrocarbon samples was 100% with no indication of mercury present in the product stream over an 8 h continuous run. For the adsorbent system CC, the efficiency of removal for different mercury species in n-hexane was variable. While DMM and DEM showed a consistent removal range of 50–80%, the DBM was efficiently adsorbed, but only for a short period (>80% at 2 h) and it was rapidly released back into the product stream after 4 h.