Steam Reformers Pressure Drop

Considering methane steam reforming [see Eq. (2.12)] as a large-scale process, Xu and Froment found that only the outer 2 mm of the catalyst pellets actually participates in the reaction [41], thus theoretically allowing for a two orders of magnitude reduction in catalyst volume. However, the well-known pressure drop limitations have prevented practical applications in the industrial field so far. 

The steam requirements in an ammonia unit can be reduced by lowering the steam-to-carbon ratio to the primary reformer. However a number of drawbacks can exist downstream in the I I I S and LTS reactors. The drawbacks include By-product formation in the HTS, Pressure drop buildup in the HTS, Reversible poisoning of the LTS catalyst, and Higher CO equilibrium concentrations exiting the HTS and LTS reactors. 

Gas Replacement of packed bed, integration of heat exchange and reaction Small-scale H2 production Methane steam reforming Low-pressure drop, fast response Pt, Rh monolith (136)... 

There are additional reasons for applying a higher S/C ratio. First, it prevents carbon deposition on the catalyst, which may not only increase the pressure drop but also reduce the catalyst activity. As the rate of the endothermic reforming reaction is lowerded this way, it can result in local overheating of the reformer tubes (hot bands) and premature failure of the tube walls. Second it provides necessary steam for the shift conversion (Section 4.2). Third, it reduces the risk of carburization of tube material. 

A fixed-bed reactor often suffers from a substantially small effectiveness factor (e.g., 10 to 10 for a fixed-bed steam reformer according to Soliman et al. [1988]) due to severe diffusional limitations unless very small particles are used. The associated high pressure drop with the use of small particles can be prohibitive. A feasible alternative is to employ a fluidized bed of catalyst powders. The effectiveness factor in the fluidized bed configuration approaches unity. The fluidization system also provides a thermally stable operation without localized hot spots. The large solid (catalyst) surface area for gas contact promotes effective catalytic reactions. For certain reactions such as ethylbenzene dehydrogenation, however, a fluidized bed operation may not be superior to a fixed bed operation. To further improve the efficiency and compactness of a fluidized-bed reactor, a permselective membrane has been introduced by Adris et al. [1991] for steam reforming of methane and Abdalla and Elnashaie [1995] for catalytic dehydrogenation of ethylbenzene to styrene. 

Residual methane is present at the exit of the combustion zone. In the catalytic bed, the methane steam-reforming and the water shift reactions take place. The gas leaving the ATR reactor is in chemical equilibrium. Normally, the exit temperature is above 900-1100°C. The catalyst must withstand very severe conditions when exposed to very high temperatures and steam partial pressures. One example of an ATR catalyst is nickel supported by magnesium aluminum spinel. For compact design, the catalyst size and shape is optimized for a low pressure drop and high activity. 

Let us demonstrate Fig. 1.8 with an example. A process for steam reforming naphtha is carried out at HjO/HC - 10, SOO C, and 40 atm total pressure. A Ni/AIjOj catalyst, diameter 4 mm, 0 0.6, was tested in a pilot unit and gave 75% conversion at a GHSV of 5000 hr. For pressure drop reasons, the process designer wants to double the size of the particle but keep all conditions the same. Will this change the conversion ... 

Build up of carbon on the surface can necessitate replacement of the catalyst. Crystalline carbon encapsulates nickel and deactivates the catalyst. However, carbon may also dissolve in nickel and reprecipitate at a grain boundary, resulting in a nickel particle being raised on a column of carbon [3,4]. The characteristic whisker carbon produced in this way (Figure 5) blocks the reactor and causes high pressure drop without, necessarily, affecting catalytic activity. This results from the fact that the nickel particle on the tip of the carbon whisker remains accessible to the gas and continues to promote steam reforming. 

The steam reforming catalyst is normally based on nickel. The properties are dictated by the severe operating conditions. The activity depends on the nickel surface area and particle size. The shape should be optimized to achieve maximum activity with minimum increase in pressure drop. 

Carbon formation on steam reforming catalysts takes place in three different forms whisker-like carbon, encapsulated carbon, and pyrolytic carbon as described in Table 2.2 [1]. Whisker-like carbon grows as a fiber from the catalyst surface with a pear-shaped nickel crystal on the end. Strong fibers can even break down catalyst particles increasing the pressure drop across the reformer tubes [4], The carbon for whisker formation is formed by the reaction of hydrocarbons as well as CO over transition metal catalysts [1], The whisker growth is a result of diffusion through the catalyst and nucleation to form a long carbonaceous fiber. 

Pyrolytic carbon is formed mainly by three different reactions, namely, the reversible decomposition of methane (Reaction 2.5), the irreversible cracking of higher hydrocarbons (Reaction 2.6), and/or coke formation (Reaction 2.7). The formation of these carbon deposits leads to the breakdown of the catalyst and hot spots in the reactor. Pyrolytic carbon is usually found as dense shales on the reformer wall or encapsulating the catalyst particles. The process leads to the deactivation of the catalyst and increase of pressure drop across the reformer tubes. The thermal cracking of hydrocarbon occurs at high temperatures and at low steam to hydrocarbon ratios. 

From the above discussion it can be concluded that the close approach to the maximum rate point on the rate dependence curve at the entrance of the reformer tube gives higher initial rate of reaction but at the same time gives faster approach to the positive order region where the order of reaction drops sharply. This means that there is always an optimum steam feed partial pressure that gives maximum reactor performance (expressed in terms of methane conversion) and this optimization problem is a result of the non-monotonic dependence of the rate of reaction upon steam partial pressure. 

Figure 4 shows operation of the fuel processor subsystem on iso-octane for 11 days, after which the fuel was switched to a simulated gasoline of 74% iso-octane, 20% xylene, 5% methylcyclohexane and 1% 1-pentene. After only two days of operation on this simulated gasoline, the pressure drop between the ATR and HTS increased due to carbon formation. Post characterization of the carbon formed showed that a high concentration of solidified hydrocarbons were present in the carbon (30% by weight). To prevent carbon formation between these reactor sections, the HTS water injection was moved from the inlet to the HTS to the outlet of the steam reformer section. 

The use of small catalyst particles in regions where heat transfer matters and larger particles in other zones to limit the pressure drop, as in primary steam reformers. 

---