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 operating with steam reheaters at low temperature.
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In this article, the ease and robustness of the in-situ start-up procedure when using a new generation titania based tail gas catalyst is compared with that of traditional alumina based counterparts.
In 2017, Euro Support introduced its titania supported tail gas catalyst and highlighted the number of operational benefits on offer, especially for TGTU installations operated with steam reheaters at low temperature.1 Since then, the company has received increasing feedback 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 challenges 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 procedure whereby pristine catalyst is exposed to hydrogen and hydrogen 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 exothermic reaction should be curbed to prevent sintering of the catalyst, whilst under low temperature start-up conditions the main challenge 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 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 H2S/H2 to a temperature above 300°C. In the case of a low temperature system, the inlet temperature is simply limited to 240°C and, once the catalyst is partially sulphided, an exothermic 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 catalyst TG-107 was monitored in detail, as shown in Figure 1. The conclusion was that about one-third of the catalyst 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 practice, even when a perfect start-up procedure is performed, a maximum one-third of the loaded catalyst will be reasonably sulphided. Shell reported that such a catalyst sulphided 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 concentration.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 temperature sulphidation.1 In reality, the reduced activity that can be expected may cause a slip of sulphur compounds through the reactor. The nature of the different compounds can cause different operational problems for the TGTU as a whole; from increased SO2 emissions by COS and CS2 slip to the incinerator to sulphur fouling of the quench water and increased amine degradation by S8 and SO2 slip.
All catalyst tests reported were performed in the test laboratories at Euro Support Manufacturing Czechia s.r.o. in a dedicated glass bench-scale test reactor of 30 mm I.D. loaded with 70 mL shaped catalyst. The reactor is controlled on inlet temperature and the temperature profile in the reactor is monitored by a ten-point thermocouple. The glass reactor and preheater are inert for hydrogenation and shift reaction under the applied conditions. The test unit is fully automated and can operate autonomous without interruption.
The input gas composition is controlled by mass flow controllers. Water is added through a HPLC pump and evaporated in the preheater. Both input and output gas compositions are analysed by an online gas chromatograph. Prior to analysis, any elemental sulphur vapour is removed and the gas is dried.
In-situ sulphiding conditions:
H2S = 1.5 mol%, H2 = 6.0 mol%, H2O = 6.7 mol%. GHSV = 450 h-1 STP.
Claus tail gas composition:
H2S = 1.0 mol%, SO2 = 0.5 mol%, CO = 1.1 mol%, H2 = 1.5 mol%, COS and CS2 = 250 ppm, CO2 = 16.7 mol% and H2O = 22 mol%. GHSV = 1500 h-1 STP.
Results and discussion
Figure 2 displays the total concentration of sulphur compounds (SO2, CS2, COS, methyl mercaptan, and elemental sulphur, excluding H2S) found in the reactor outlet of the TGTU catalyst as function of the reactor inlet temperature and maximum reached in-situ sulphidation temperature. This represents the total sulphur load that was not converted to H2S. The largest absolute contribution to the slip is in the form of elemental sulphur.
In Figure 2, the performance of Euro Support’s new titania based catalyst is compared to alumina based LT-TGTU and the first generation TiO2/CoMo TGTU catalysts. Performance is plotted for two conditions where the ΔT 240°C and ΔT 300°C indicate the maximum temperature reached by the heat wave from the exothermic reaction during in-situ sulfidation. These represent ‘worst-case’ and ‘perfect start-up’ procedures, respectively. The shaded area in between these lines represents the activity that can realistically be expected from the LT-TGTU catalyst. The graph for the alumina based LT-TGTU catalysts shows that after perfect in-situ sulphidation an operation temperature of above 230°C is required to prevent slip of sulphur compounds already at start of run. The data suggests that a worst-case start-up makes it virtually impossible to operate at full conversion. In practice, the activity will fall in between in the shaded area, but in any event leaves limited room for the inevitable catalyst deactivation over time.
Although the first generation of TiO2-CoMo catalyst readily outperforms an alumina based catalyst under similar sulphidation conditions, the possibility to operate at start of run in the Tin < 230°C range still depends largely on the success of the sulphidation treatment, as indicated by the grey area in Figure 2.
The new TiO2-CoMo catalysts show a completely different picture. At inlet temperatures above 210°C, both catalysts sulphided at 240°C or 300°C perform equally well and no slip in unconverted sulphur compounds was observed. This means that if the heatwave during in-situ start-up is lower than expected, because of not having the optimal conditions in the reactor, the catalyst will still be sulphided almost to its full potential, with no significant effect on catalyst activity.
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