Optimising Sulphur Plant Performance to Ensure Reliability and Avoid Emission Violations


products and sulcrete can plug up the seal leg and therefore frequent observation into the ‘look-box’ should be performed to ensure a steady flow of the produced liquid sulphur. Any abnormalities in the sulphur flow may be attributable to partial plugging of the seal leg, failure in part of the jacketing and/or a tubesheet leak in the upstream condenser that causes sulphur to solidify in the outlet section.


Direct acid-gas- or fuel-gas-fired re-heaters have the potential to cause catalyst deactivation in the downstream converter. Direct acid gas re-heaters should be operated from 60 to 75 % burn stoichiometry (not 33 % as with the reaction furnace) and direct fuel


The most important instrument in the sulphur recovery unit is the tail gas analyser or the air demand analyser.


gas re-heaters should be operated from 95 to 99 % stoichiometry. As with the main burner, it is very important to ensure that the burners are not operated below minimum flow rates to protect the burner tip from flame impingement. Indirect re-heaters must be protected from thermal shock to ensure the integrity of the tubes and tubesheet.


Catalytic converters contain alumina and/or titanium catalyst that must be protected from contaminants/poisons, thermal shock (sulphur fires), amine, condensed water, liquid sulphur and soot. Catalyst can be permanently poisoned by benzene, toluene, ethylbenzene and xylene (BTEX), heavy hydrocarbons, amine and methanol. Thermal shock from sulphur fires as well as condensed water and amine can destroy the integrity of the catalyst, while liquid sulphur and soot only plug the catalyst pores, which can be reversed.


Instrumentation plays a critical role in the reliability of the SRU. The most important instrument in the SRU is the tail gas analyser or the air demand analyser. This instrument measures the


concentration of H2S and SO2 in the tail gas stream and relays this information (i.e. ‘feedback’) to the combustion air control system, normally the trim air valve. This feedback signal allows the process


chemistry to be maintained with an H2S:SO2 ratio of 2:1 for optimal Claus conversion. Any significant deviation from this ratio will result in a loss in the overall recovery efficiency of the SRU. Regular PMs of this instrument, including calibration, must be conducted. Plugging of the sample line can be a result of inadequate heat tracing of the sample line and/or liquid sulphur entrainment from the final (or upstream) condenser.


Other instrumentation that should be maintained on a regular basis includes the acid gas and air flow meters, reaction furnace optical pyrometers and ceramic thermocouples, igniter operation, flame scanners, direct acid-gas- or fuel-gas-fired re-heater meters, level controllers and high-high level shut-downs.


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Good operations are key to the reliability of the SRU. The SRU is unforgiving in the sense that, when things go wrong, they do so very quickly and with serious consequences. It is therefore very important that a facility has experienced operators and an excellent training course is made available for new or inexperienced operators. The training programme should include hands-on control room/board training and classroom training. The operator’s rounds/checks schedule should include everything that has been mentioned thus far and will include varying frequencies laid out by process management.


Regular third-party on-site SRU performance testing should be conducted at least once per year. Quarterly or semi-annual testing has been proved to be very effective for SRU operators, in that problem areas are detected early and can be resolved with far lesser consequences than is the case if problems are only discovered later, when an emergency or unscheduled shut-down must be performed. Emergency shut-downs are thermally very stressful on the equipment in the SRU, are potentially unsafe and can limit the facility’s production rates.


SRU start-ups and shut-downs are difficult and dangerous procedures that have the potential to cause serious damage to the SRU if not conducted properly. It is very important to have clear and concise standard operating procedures in place so that all personnel are clear on what is to be accomplished and how it can


The sulphur recovery unit is unforgiving in the sense that, when things go wrong, they do so very quickly and with serious consequences.


be performed safely and efficiently. Shut-downs are particularly difficult owing to the threat of serious sulphur fires in the catalytic converters. Any ‘free oxygen’ that is allowed to reach a converter during the early stages of a shut-down will result in a sulphur fire which, in the worst case, can destroy the catalyst and the internals of the bed. Burning fuel gas in the reaction furnace must be conducted carefully and will include good control of the air-to-fuel gas flow ratios, inert gas for tempering the reaction furnace flame and combustion gas analysis that will allow for the evaluation of the fuel gas burn stoichiometry at any given time. Third-party assistance is highly recommended owing to the experience factor and the feedback which gas composition analysis provides.


Management must be aware of all of the above issues so that appropriate measures are taken to ensure that optimum overall recovery efficiencies are being achieved on a day-to-day basis and that reliability continues to improve. The operating budget for an SRU must take all of this into account; the consequences of not doing so can be extremely dire for personnel, the environment and the gross revenue of the facility. n


HYDROCARBON WORLD – VOLUME 6 ISSUE 2


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