Sulphur passivated reforming

J.R. Rostrup-Nielsen, Sulphur-Passivated Nickel Catalysts for Carbon-Free Steam Reforming of Methane , J. Catal., 85 31-43 (1984). 

The most efficient examples are those with highest potential for carbon formation as illustrated in Figure 2.18 (refer to Section 5.2). This is handled by the use of noble metal catalysts or by the sulphur passivated reforming process (SPARG) (refer to Section 5.4). 

This requires that the steam reformer is operating at a ratio (H20+C02)/CH4 elose to one, whieh means high risk of carbon formation (refer to Section 5.2). One solution is the use of sulphur passivated reforming allowing both schemes in Figure 2.19 to operate close to or beyond thermod5mamic carbon limits (refer to Section 5.5). 

Ensemble control is also involved in carbon-fiee steam reforming on a sulphur passivated catalyst [390] (Section 5.5). Ensemble control was also reported for the addition of Bi [500] or B [526] to nickel and for bimetallic catalysts such as Ni,Au [50], Pt,Re [367], Pt,Sn [474], and Ni,Sn [217] [247] [431] [456] [545]. Alloying nickel with copper [49] [16] can also decrease the rate of carbon formation, but it is not possible to achieve die same high surface coverage with copper as with sulphur and gold, because copper and nickel forms a bulk alloy with a fixed surface concentration of copper over a wide range as alloy composition. 

In principle, there are two situations for sulphur passivated reforming as illustrated in Figure 5.48 (in accordance with the trend in Figure 5.15) ... 

Figure 5.48 Carbon limits and sulphur passivated reformer. The principle of equilibrated gas predicts an upper carbon hmit for gases with high H/C and a lower carbon limit for...

In the case of steam reforming (the second situation for sulphur passivated reforming), it is not possible to operate under conditions where the principle of equilibrated gas shows potential for carbon because the centre of the pellet will have equilibrated gas (refer to Example 5.3). However, with the sulphur passivation it is possible to operate at conditions under which the principle of actual gas predicts carbon because of the high (-AGc) overpotential required for nucleation of carbon (Figure 5.47). 

The sulphur passivated reforming was demonstrated in a series of tests in a full-size monotube reformer in Figure 3.6 [152]. Some of the tests are referred to in Figure 5.49 and Table 2.6 (Section 2.4.2). A t3T>ical temperature profile is shown in Figure 5.49 with a fast heat up of the gas to the reaction temperature for the sulphur passivated catalyst. 

Figure 5.49 Sulphur passivated reforming. Dibbem et al. [152], Axial temperature from full-size monotube pilot plant H2O/NG=0.96 mol/carbon. CO2/NG=0.64 mol/C-atom, P=7 bar abs. Reproduced with the permission of Gulf Publishing.

Limit C is a result of controlled sulphur passivation of the reforming catalyst as practiced in the SPARG process (see section 2.6.2). 

The retarding effect of sulphur is a dynamic phenomenon. This means that carbon may be formed at certain conditions in spite of sulphur passivation - although at strongly reduced rates and by a mechanism different from the formation of whisker carbon. Therefore, it was important to develop design criteria to make sure that the kinetic balance is in favour of no carbon formation at all positions in the reformer tube. This work was carried out mainly in the full-size monotube process demonstration plant (Dibbern et al., 1986). The influence of various process parameters (pressure, heat flux, sulphur content in feed, etc) was studied. It was demonstrated that the impact of variations in sulphur content in the feedstream on tube wall temperature and exit gas composition was completely reversible. 

It was possible to operate at heat fluxes close to 80.000 kcal/m /h in spite of the sulphur passivation. The catalyst temperature increases quickly to above 750-800 C, at which the reforming rates are sufficient for conversion of methane even at the high load. The high reaction temperature means that higher hydrocarbons must be converted in an adiabatic pre-reformer to eliminate the risk for carbon formation by thermal cracking (reaction (10)). 

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