Critical steam-to-carbon ratio

At a given temperature and for a given hydrocarbon feed, carbon will be formed below a critical steam to carbon ratio (15), the carbon limit A in Fig. 5. It can be shown that this critical steam to carbon ratio increases with temperature. By promotion of the catalyst, it is possible to push this limit to the thermodynamic carbon limit B reflecting the principle of equilibrated gas (4,15) ... 

The catalyst parameters influencing the critical steam to carbon ratio can be analyzed by the simplified sequence in Fig. 4 whieh leads to the following expression for the steady state activity of carbon ... 

A no potential for carbon in actual gas. A critical steam-to-carbon ratio (carbon formation on Ni). B thermodynamic limit potential for carbon in equilibrated gas. 

Figure 5.22 Actual critical steam-to-carbon ratios in NH3plant steam reformer [386].

The critical steam-to-carbon ratio depends on feedstock composition. As shown in Figure 5.4 (Section 5.2.1), the contents of aromatics and olefins may be harmful. The feedstock will rarely contain olefins, but even traces of ethylene may be critical. Ethylene may be formed by thermal cracking in preheaters, from oxidative coupling of methane if oxygen (air) is added to the feed or from dehydration of ethanol if added to the feed. 

Apart from the catalyst type and its modification, the steam to carbon ratio has the largest influence on coke formation. Formation may be expected below a certain, critical, steam to hydrocarbon ration. The critical ratio was found to increase rapidly with temperature and to be influenced by the type of hydrocarbon and by catalyst [6],... 

The effect of suJfur poisoning on the coking tendency of alumina-supported ruthenium SNG catalyst has been studied. The clean KU/AI2O3 catalyst has exceptional coking resistance, and at 490 C and 25 atm, can tolerate steam to carbon ratios below stoichiometric (steam/carbori=0.6) with light naphtha before a continuous accumulation of carbon will occur. However, at this temperature (appropriate for SNG production), sulfur can adsorb on the active metal surfaces to a level which will cause a slow but steady accumulation of less reactive carbon. The critical sulfur coverage that adversely affected the steam to carbon ratio necessary to prevent continuous coking appears to fall just above one-half the maximum capacity of the catalyst. 

Carbon limits depend on deviation from graphite thermod5mamics, meaning that a eatalyst with small nickel crystals can operate at more critical conditions. This is illustrated in Figure 5.16 for conditions in a prereformer [415] [451]. The upper carbon limit temperature depends on the steam-to-carbon ratio and the nickel crystal size of the catalyst. 

The risk of carbon formation may be assessed by the critical steam-to-hydrocarbon ratio [384] [389]. This decreases with temperature and depends on the t5q)e of hydrocarbon and the type of catalyst. 

Operation of the reformer at a reduced steam to carbon ratio requires modification of the downstream shift (if the steam to carbon ratio goes below the critical level for by-product formation) and carbon dioxide removal sections in order to fully obtain the potential energy savings. The reduced steam to carbon ratio will also require a highly active reforming catalyst in order to eliminate the risk of carbon formation and resulting hot bands on the reformer tubes. 

When the steam to carbon ratio in the reforming section is not reduced below the critical limit for carbide formation, it is possible, by adding an extra shift catalyst bed after the conventional shift unit, to reduce the carbon monoxide leakage to around 0.05 dry vol%. This means that less hydrogen will be lost by reaction with carbon monoxide in the methanator, and that tht synthesis loop will become more efficient due to the lower inert content in the make-up gas. 

It is critical to determine and control the steam-to-carbon (S/C) and/or oxygen-tointernal reforming) to avoid carbon deposition. Thermodynamic analysis is commonly used to estimate the minimum ratios. For example. Figure 33.18 shows the equilibrium number of moles of carbon per mole of methane introduced into an ATR as a function of S/C and O/C at two reformer inlet temperatures of 150 and 400 °C [8]. It can be seen that for aU values of O/C between 0 and 1.5, carbon deposition should not be a concern if an S/C > 1.2 is maintained in the fuel gas mixture entering the ATR (fiiUy mixed inlet stream). It should be noted that many thermodynamic calculations (as in this example) assume adiabatic equilibrium reactions and do not take into account reaction kinetic effects. The inclusion of reaction kinetics in the analysis may lead to different results. 

Carbon formation is to be expected if the actual steam-to-hydrocarbon ratio is lower than the critical ratio as illustrated in Figure 5.21 [384] [389]. 

Extrapolated thermodynamic measurements for the sulfur-poisoned Hu/Al2°3 catalyst show that, given a sufficiently long exposure time, feedstock sulfur levels of 1 ppb will be enough to reach this threshold of sulfur coverage. With sulfur contamination above this level (conventional zinc oxide guard beds typically reduce sulfur levels to 0,1 ppm)t steam/carbon ratios of greater than or equal to 3.D and/or more H2 arc needed to prevent CCD in the critical inlet portions of the catalyst bed. Virtually complete sulfur removal is required to avoid c t Ly t coking and deactivation ... 

From consideration of the thermodynamics of sulfur chemisorption on ruthenium (ref. 6), the gas phase sulfur activities (llgS/l ) of the lightly and moderately sulfur-poisoned Ru catalysts in equilibrium with the adsorbed sulfur at the process temperature (190 C), were approximately 0.02 and 1 ppb+ respectively. On the basis of these results, the equivalent partial pressure ratio for critical sulfur coverage is about 1 ppb at 490 C. This level is well below that attainable by conventional sulfur removal methods. Thus our result confirms the need for high performance desulfurization technology (ref 3) that can reduce sulfur contaminants in feedstocks to a sufficiently low sulfur level to avoid carbon fouling cf Ru/A Og steam reforming catalysts. 

---