Carbon laydown

The chapter by Gruber deals with thermodynamic equilibrium considerations. It develops a graphical method for presenting these results, and touches on the potentially important problem of carbon-laydown on the catalyst. 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

Catalyst Poisons. Hausberger, Atwood, and Knight (33) reported that nickel catalysts are extremely sensitive to sulfides and chlorides. If all materials which adversely affect the performance of a catalyst were classified as poisons, then carbon laydown and, under extreme conditions, water vapor would be included as nickel methanation catalyst poisons. 

Carbon Laydown. The potential for carbon laydown is readily estimated from the thermodynamics of Reactions 4 and 5. The areas where carbon laydown, according to these reactions, is thermodynamically possible were developed by Gruber (36). It is readily seen that carbon laydown via Reaction 4 is thermodynamically favorable at the reactor inlet for practically any commercially conceivable feed gas composition. As noted by Gruber (36), carbon laydown is thermodynamically unfavorable at the reactor outlet for practically all commercially conceivable methanator conditions. The methanation reactor will therefore, in practice, have two zones—the first is a finite zone between the inlet and some way down the catalyst bed where carbon laydown is thermodynamically possible, and the second zone is the balance of the reactor. 

Carbon laydown cannot be tolerated in a commercial methanation process since it could lead to rapid plugging and shutdown of the catalyst bed. Fortunately there are catalysts which avoid laydown in the first zone (33, 37), as opposed to catalysts which do not (21, 22, 23, 24). 

As is indicated in Figure 1, the heat liberated in the conversion of carbon monoxide to methane is 52,730 cal/mole CO under expected reaction conditions. Also, the heat liberated in the conversion of carbon dioxide is 43,680 cal/mole C02. Such high heat releases strongly affect the process design of the methanation plant since it is necessary to prevent excessively high temperatures in order to avoid catalyst deactivation and carbon laydown. Several approaches have been proposed. 

Steam-Moderated Process. The basic idea behind this approach is to limit the extent of conversion of the methanation reaction, Reaction 1, by adding steam to the feed gases. This process simultaneously provides for (46) elimination of the CO shift, Reaction 2, to get a 3 1 H2 CO ratio from the make-up gas ratio of about 1.5 1 and avoidance of carbon laydown by operation under conditions in which carbon is not a thermodynamically stable phase (see Chemistry and Thermodynamics section above). 

The possible advantages of this system over the equilibrium-limited reactor system are smaller catalyst beds, lower gas recycle requirements, and lower capital requirements. The possible disadvantages of this system are (a) practically no turn-down since any turn-down would be equivalent to decreased space velocities, closer approach to equilibrium, and higher temperature rises (b) maldistribution of gases across the bed would give rise to excessive temperature rises in zones of low flow and (c) considerably shortened catalyst life because of possible high local or zonal temperature and, concurrently, greater chances for carbon laydown. 

L. Seglin I would like to take exception to that. If you look at the composition, not the equilibrium composition, but the composition of the feed gas in practically any of the methanation schemes I have seen around, there is enough CO to lay down carbon. You have a situation where at the feed point you can potentially lay down carbon. At the exit you are outside carbon laydown. So, some place in-between, for some significant space, there is sufficient carbon monoxide to form... 

Dr. Woodward May I ask a question of some of our colleagues here as a point of information You use the phrase "irreversible carbon laydown, yet the equation for carbon plus steam has arrows going in both directions. I wonder if anybody has any comments on whether it is irreversible or, if you inadvertently start to get some carbon down, is it reversible Can you steam it out ... 

Anonymous We have heard of carbon laydown as being a problem. Is the formation of metal carbides of concern, and can it be reversed ... 

A more steadily performing catalyst, requiring less attention and less frequent replacement, could permit a reduction in the semivariable costs for manning and maintenance stores. In the case of a catalyst system requiring frequent regeneration by burning off, a decrease in the carbon laydown (and consequent decrease in necessary burn-off frequency) may both increase throughput and reduce conversion costs. 

The aromatic and hydroaromatic components of coal-derived feedstocks are potential sources of carbon laydown, and steam would be needed to reduce reactor fouling. 

Intermediate Duty catalysts are for feeds with a significant content of components from ethanes up to liquid petroleum gas (LPG). The heavier feedstock increases the tendency for catalyst deactivation through carbon laydown and requires a special catalyst in the top 30% to 50% of the reformer tubes. This tendency also occurs when light feeds are run at low steam-to-carbon ratios and/or at a high heat flux. 

Whilst the ability of platinum-based catalysts to effect the dehydrogenation of alkanes to the corresponding alkenes is well established [1-4], carbon laydown and consequential deactivation of the catalyst during the dehydrogenation reactions is a well known phenomenon... 

Typical results for the activity, expressed as the percentage conversion of the propane feed, the selectivity, expressed as the ((propene yield)/(propane reacted)%), and the carbon laydown are shown in Table 1 for a reaction carried out under continuous flow conditions at 873 K and a GHSV of 3000 hr 1. 

Activity, selectivity and extent of carbon laydown during propane dehydrogenation at 873 K... 

From these results it can be seen that there is a rapid deactivation of the freshly reduced catalyst, accompanied by extensive carbon laydown in the veiy early stages of the reaction. Analysis after 10 sec. on stream showed that there was a carbon laydown of 48.4% of the propane feed and the only product in the reactor eluant was methane. In all subsequent analyses, methane, ethane, ethene and dihydrogen were detected along with propene and unreacted propane, the catalyst approaching a steady state with respect to conversion and selectivity after ca. 30 min. on stream. 

Only a relatively small fraction of the carbon laydown on the surface can be removed by high temperature dioxygen treatment. After regeneration carbon continues to build up on the catalyst surface in subsequent propane dehydrogenation reactions. 

Pretreatment of the catalyst with carbon monoxide or toluene at the reaction temperature results in carbon laydown on the catalyst, which dramatically reduces the amount of carbon deposition and increases the selectivity in subsequent propane dehydrogenation reactions. However, the carbon deposited during the pretreatment is different from that formed during propane dehydrogenation. 

In the reformer, multiple reactions occur simultaneously. This process is endothermic and is subject to carbon laydown thus, refiners must regenerate reforming catalysts. Several catalyst-regenerating approaches are possible. Semi-regenerative processes use... 

To study the influence of hydrogen with respect to carbon laydown and to determine whether isotope exchange occurred, a catalyst was reduced in [ H]H2 and aliquots of a 1 1 [ HJCsHg/ [ H]C3Hg passed over it at 873 K at 0.25 h intervals. The only gas phase product de-... 

Attempts to improve vapor distribution to the desuperheating packing or trays have usually been unsuccessful because of carbon laydown on the baffles. 

For lowest capital cost, the preheat furnace is usually a dry operation. Sometimes, however, two or three percent weight of superheated steam is added to the feed to give a higher internal velocity, thus reducing the rate of carbon laydown inside the tubes. This steam is termed velocity steam to differentiate... 

Despite the fact that hydrogen is an important product of reforming, it is nevertheless added in the process primarily to combat deactivation by coking, and is a key process variable. Rohrer,20 and Franck,24 have demonstrated the volcano effect of a reduction in hydrogen pressure on the rate of dehydrocyclisation. The progressive reduction in rate at pressures less than 87 psig (0.7 mPa) is a result of the increased rate of deactivation by carbon-laydown as hydrogen pressure is reduced. 

At start-up the catalyst is reduced and then carefully conditioned on-line, for example by sulphidation of the some part of the metal components.40 Vapourised naphtha feed, together with a source of hydrogen, is then passed into the process, and potentially all the reactions listed earlier start up. The exothermic reactions of hydrocracking and hydrogenolysis must be properly controlled, but carbon laydown is inevitable from the start, and the control of its early formation is crucial to the evolution of the reforming cycle. 

As in many hydrocarbon processes, the normal process of deactivation is by carbon laydown, and is inevitable in catalyst systems which are not perfectly selective significant quantities of carbon, say 10-20 wt% on the... 

Carbon laydown increases as the hydrogen-to-hydrocarbon ratio (HHCR) decreases.47 The variation of ageing rate with HHCR is very steep for HHCR < 10 molar (Fig. 6.5).27 Lowering the Weight-Hourly Space Velocity (WHSV) at constant HHCR increases carbon laydown.47... 

Fig. 6.6). The first effect is clearly thermodynamic and related to total pressure the second is the effect of reduced availability of hydrogen on the rate of carbon-laydown, and is represented more conveniently by HHCR. 

The effect of increasing the temperature of reaction is shown in Table 1 and, as might be expected, the activity of the catalyst is seen to increase with temperature but decrease with time. At 673 K the equilibrium conversion of butane to butenes is 6 %, at 773 K 23 % and at 873 K 58 %. Therefore from Table 1 the system is at equilibrium at 673 K and 773 K but not at 873 K. The reason for this is likely to be two-fold. Firstly the reaction is endothermic and as the conversion increases the extent of the temperature decrease increases, therefore the temperature may be lower in the catalyst bed than that detected in the thermocouple pocket. Secondly there is far more extensive carbon laydown, so that the catalyst will have suffered a far more rapid deactivation than at the lower temperatures. 

In Table 2 it can be seen that, as expected, an increase in space velocity results in a decrease in the overall conversion. However the yield of butenes remains approximately constant at the equilibrium conversion. Therefore the main reaction affected by the increase in space velocity is not the dehydrogenation reaction but the carbon deposition reactions. As carbon laydown is one of the major other reactions occurring we analysed... 

It can be clearly seen that the 1-butene values are decreasing with temperature while the cis- and trans-2-butene values are increasing with temperature. Hence it is unlikely that the carbon laydown reaction from 1-butene occurs on the same site as that for cis- and trans-2-butene. 

In contrast the reaction of a 1 1 molar mixture of i50-butanal and n-butanal ran successfully for 20 hours with no gross carbon laydown on the discharged catalyst (3.1 wt%). The products formed were 2-ethyl hexenal as expected from the self-condensation reaction of n-butanal and 2-ethyl-4-methyl-pent-2-enal, the dehydrated product from the crossed aldol reaction of n and /50-butanal (Table 6). The overall selectivity of this reaction to aldol products is lower than the selectivties achieved with crushed 4 wt %... 

A correlation between hydrogenation rate and carbon laydown of this kind would emphasize the role of chemisorbed ethylene in the hydrogenation reaction and rule out the mechanism of gaseous ethylene molecules striking chemisorbed hydrogen. 

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