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We have NHTU section preceding our Platformer in the CCR unit. Initially we were loading NHTU with 60% low sulphur SRN and 40% high sulphur FCC gasoline (from gasoline splitter section). SRN had feed sulphur of 100-150 ppm and FCC gasoline sulphur was around 1200 ppm. However because of some issues we stopped taking FCC gasoline and the plant was taken on 100% SRN throughput. This resulted in higher NHTU furnace load ( as exotherm in reactor and preheat across CFE reduced tremendously ). However the NHTU r/d sulphur which in case of FCC gasoline operation was 0.3-0.5 ppm increased to around 1 ppm in the latter case ( 100% SRN throughput ). Why has r/d sulphur increased if feed sulphur has decreased (all other paramters are constant)? P=45 kg/cm2 and RIT =280 deg cel are the same at in both the cases.
Secondly, is there a minimum partial pressure of H2S in the hydrotreater recycle gas needed to maintain and ensure that the reactor catalyst remains in the sulphided state and does not go into its reduced form?
 
Answers
24/01/2013 A: Mahendra Desu, HPCL, mahen.desu@gmail.com
Are you taking SRN directly from units or from storage. Please check preheat exchanger inlet temperature and maximize hot feed to the unit if it is within the limits of pump mechanical design temperature (check pump seal design temperature also).
Reasons for high R/D sulphur : Either due to low RIT or heavier organic sulphur components. Check the distillation of SRN. RIT of 280 °C appears to be low.
H2S in the recycle gas can be maintained in the range of 0.05 to 0.1 vol%. This can be reduced further if your feed stock sulphur does not go to very lower levels.
05/01/2013 A: Amarjit Bakshi, Refining Hydrocarbon Technologies LLC, abakshi@rhtgulfcoast.com
I agree with the both the comments earlier your avg reactor temperature and ROT both hvae gone down which will affect the SRN outlet sulfur. The only thing is that you can raise the inlet temperature and hope that the existing Feed reactor effluent exchanger will take care of it due higher LMTD with ROT being higher other wise revamp of the exchanger system will be requited. Essentially your reactor Avg temperature and ROT are the cause for not meeeting the requirement. Desulfiding of the catalyst is not expected if small amount of H2S is in the recycle treat gas.
09/11/2012 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
RIT is the usual control temperature, but your average reactor temperature has probably decreased for the reasons you gave. That would lead to a higher sulfur level in the reactor outlet. You have ample sulfur in the system for the catalyst to remain sulfided. If the heater is at maximum capacity, try reducing feed rate and increasing RIT as a short test run. Ultimately, a revamp of adding reactor effluent to feed heat exchange may be advisable.
09/11/2012 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
Presumably, NHTU only has one catalyst bed and space velocity LHSV is not conservative. If so, then I suspect higher residual organic sulphur is because the weighted average bed temperature is lower for all straight-run heavy naphtha versus mix of straight-run heavy naphtha and whatever FCC gasoline fraction was being co-processed. There are substantial amounts of linear olefins and cyclo-olefins and alkene/alkyl benzenes in FCC gasoline fractions as typically considered for further processing via catalytic reforming. Exotherms associated with hydrosaturation [HDBr#] of these quickly raises the bulk processing temperature early in the NHTU reactor and thus raises the RxT,outlet temperature. RxT,outlet temperature controls residual sulphur levels as long as absolute value is less than about 345C and outlet H2S is less than about 0.5 vol%. Combination of organic sulphur in heavy straight-run naphtha and modest amounts of H2S in recycle gas should keep catalyst fully sulfided.