how quickly the sulphur particles sink to adjust the rate of surfactant addition. ORP measurements indicate whether the catalyst solution is properly regenerated in the oxidiser, a key parameter for determining catalyst activity. As shown in Figures 4 and 5 , ORP typically stayed within the optimum operating range. If the catalyst is over regenerated (for example, data points above the optimum range), chemical usage is higher. Under regenerating the catalyst (for example, data points below the optimum range) reduces the active catalyst available for sulphur reaction and may cause off- specification treating in the absorber section. It appears this was the case early in the operation of Navy 1, which may have caused some of the high H₂S in the vent gas noted in Figure 2 . During the early years of Navy 1 unit operations, the importance of alkalinity as an operating variable rather than pH was discovered. Where most Lo-Cat units monitor pH only, that is not the case when treating streams with high concentrations of CO₂. Large amounts of acidic CO₂ in solution reach equilibrium with the basic Lo-Cat solution and serve as a buffer, stabilising the pH. This makes
pH measurement a less responsive indicator of the solution’s H₂S solubility. In addition, raising the alkalinity is known to reduce the solubility of CO₂ in the solution. In December 2000, Navy 1 data (see Figure 4 ) indicated that solution alkalinity was at an all-time low. This corresponded to the highest treated gas H₂S content of 90 ppm shown in Figure 2 . The low alkalinity caused the solubility of the H₂S in the solution to become so low that significant amounts of H₂S were measured in the vent gas. The alkalinity and ORP in Navy 210 were more tightly controlled over Navy 1 due to the lessons learned during early operations. Sulphur product The two outputs from the Lo-Cat unit are the vent gas (discussed previously) and the sulphur cake. The sulphur cake is approximately 65% sulphur and 35% moisture. Washing the slurry during the drying process minimises the loss of chemicals (and operating cost). The elemental sulphur produced by the unit is sold to Hondo Incorporated in lieu of going to landfill. Hondo blends the sulphur with gypsum and sells it as a
ORP
Alkalinity
Minimum alkalinity
December 2000
Figure 4 Solution analyses for Navy 1 unit
www.decarbonisationtechnology.com
94
Powered by FlippingBook