Performance of titania based tail gas catalyst at start-up
Titania supported tail gas catalyst offers a number of operational benefits, particularly for TGTU installations operatingwith steam reheaters at low temperature
BART HEREIJGERS Euro Support
I n this article, the ease and robustness of the in-situ start-up procedure when using a new generation titania based tail gas cat- alyst is compared with that of tradi- tional alumina based counterparts. In 2017, Euro Support introduced its titania supported tail gas cata- lyst and highlighted the number of operational benefits on offer, espe - cially for TGTU installations oper- ated with steam reheaters at low temperature. 1 Since then, the com- pany has received increasing feed- back from users of low temperature tail gas treating units (LT-TGTU) loaded with traditional alumina based LT catalysts whose in-service operations are often fraught with hiccups. In particular, serious chal- lenges exist to maintain the required low emissions over time. When considering the operation of an LT-TGTU installation, special attention to the catalyst start-up procedure is justified. This involves a complex and time-consuming pro- cedure whereby pristine catalyst is exposed to hydrogen and hydro- gen sulphide containing gas to be transformed into the active sulphide phase. 2 It has been shown multiple times in literature that a successful sulphidation is a measure of strict temperature control. For the high temperature application, the exo- thermic reaction should be curbed to prevent sintering of the cata- lyst, whilst under low temperature start-up conditions the main chal- lenge is to make sure the exothermic reaction heats up a sufficiently large portion of the catalyst bed to allow for deep sulphidation of the metals. Either way, the procedure requires tight process control and specific
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Figure 1 Claus 4 SCOT DC-1903 temperature profile during in-situ sulphidation procedure. Catalyst temperature at several depths in the hydrogenation reactor of an LT-TGTU during start-up of TG-107. T2 is top bed, T7 is bottom bed temperature Image courtesy of Roisin et al. 3
gas feed conditions, and during this period the tail gas is routed to the incinerator, leading to increased emissions. The big advantage of a high temperature system here is that a failed sulphidation can be restored by heating the bed in H 2 S/ H 2 to a temperature above 300°C. In the case of a low temperature sys- tem, the inlet temperature is simply limited to 240°C and, once the cat- alyst is partially sulphided, an exo- thermic reaction is circumvented. In a publication by Roisin et. al. 3 from 2009, the actual temperature wave in an industrial reactor loaded with low temperature tail gas cata- lyst TG-107 was monitored in detail, as shown in Figure 1 . The conclusion was that about one-third of the cat- alyst bed can reach a temperature above 300°C for a short time, whilst the remaining two-thirds reached
a maximum temperature around 260°C, which could be sustained for 1-2 hours. This means that in prac- tice, even when a perfect start-up procedure is performed, a maxi- mum one-third of the loaded catalyst will be reasonably sulphided. Shell reported that such a catalyst sul- phided at 260°C can reach between 66-83% of the activity compared to the activity reached after start-up at 300°C and 10% hydrogen concen- tration. 4 It should be stated, though, that sulphidation of an alumina based tail gas catalyst at 300°C is not as effective as a real high temper - ature sulphidation. 1 In reality, the reduced activity that can be expected may cause a slip of sulphur com- pounds through the reactor. The nature of the different compounds can cause different operational prob - lems for the TGTU as a whole; from
Catalysis 2022 35
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