Equilibrium catalyst beds

Catalytic reformers are normally designed to have a series of catalyst beds (typically three beds). The first bed usually contains less catalyst than the other beds. This arrangement is important because the dehydrogenation of naphthenes to aromatics can reach equilibrium faster than the other reforming reactions. Dehydrocyclization is a slower reaction and may only reach equilibrium at the exit of the third reactor. Isomerization and hydrocracking reactions are slow. They have low equilibrium constants and may not reach equilibrium before exiting the reactor. 

Kinetically Limited Process. Basically, this system limits the temperature rise of each adiabatically operated reactor to safe levels by using high enough space velocities to ensure only partial approach to equilibrium. The exit gases from each reactor are cooled in external waste heat boilers, then passed forward to the next reactor, and so forth. This resembles the equilibrium-limited reactor system as shown in Figure 8, except, of course, that the catalyst beds are much smaller. 

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. 

This refers to the total gas flow through a plane of catalyst where Nx is the mole fraction of X in the gas passing through the plane, NWeq is the mole fraction of X at equilibrium under conditions at this point in the catalyst bed, and dv is the incremental catalyst volume. 

The rate model contains four adjustable parameters, as the rate constant k and a term in the denominator, Xad, are written using the Arrhenius expression and so require a preexponential term and an activation energy. The equilibrium constant can be calculated from thermodynamic data. The constants depend on the catalyst employed, but some, such as the activation energy, are about the same for many commercial catalysts. Equation (57) is a steady-state model the low velocity of temperature fronts moving through catalyst beds often justifies its use for periodic flow reversal. 

A solution of concentration C0 is pumped at a velocity u through a catalyst bed in which the dispersion, coefficient is D and the rate equation is r = 0.001(C-Ce) where Ce is constant. For a boundary condition, note that C will remain constant as distance z = > a>. Find the reactor lengths z that will reduce the displacement of concentration from the equilibrium value by 50% under steady state conditions when (a) D = 0,2 and u=0.05 (b) D - 0.2 and u = 0 (c) D = 0 and u = 0.05. 

In ATR process, the feedstock is reacted predominantly with oxygen by the use of a burner and a fixed catalyst bed is used for attaining reaction equilibrium. 

The hydrocarbon feedstock is reacted with a mixture of oxygen or air and steam in a sub-stoichiometric flame. In the fixed catalyst bed the synthesis gas is further equilibrated. The composition of the product gas will be determined by the thermodynamic equilibrium at the exit pressure and temperature, which is determined through the adiabatic heat balance based on the composition and flows of the feed, steam and oxygen added to the reactor. The synthesis gas produced is completely soot-free [28]. 

Among the most effective of the modifications to Claus operating procedure is accurate temperature control of the catalyst beds. Gamson and Elkins (27) in the early 1950 s showed that equilibrium sulfur conversion efficiencies in the catalytic redox reaction rise dramatically as operating temperatures are lowered toward the dewpoint of sulfur. While some highly efficient subdewpoint Claus type processes are now in use the bulk of sulfur production from H2S still requires that the converters be operated above the dewpoint. Careful control of converter bed temperature has, however, contributed to improved efficiencies. This has in large part resulted from better instrumentation of the Claus train and effective information feed back systems. 

Tile partially purilied synthesis gas leaves the C02 absorber containing approximately 0.1% CO2 and 0.5% CO. This gas is preheated at the methanator inlet by heat exchange with the synthesis-gas compressor interstage cooler and the primary-shift converter effluent and reacted over a nickel oxide catalyst bed in the methanator. The methanation reactions are highly exothermic and are equilibrium favored by low temperatures and high pressures. 

Coking of Equilibrium Catalyst. A 200 g portion of equilibrium USY octane catalyst (Catalyst A) was precoked in a fluidized bed using a stream of isobutene diluted with nitrogen gas (400 ml/min) at 510°C. The catalyst was held in a 2-inch diameter quartz tube while 27.8 g of isobutene gas was passed upflow through it at atmospheric pressure over a period of 25 minutes. 

Improved selectivity in the liquid-phase oligomerization of i-butene by extraction of a primary product (i-octene C8) in a zeolite membrane reactor (acid resin catalyst bed located on the membrane tube side) with respect to a conventional fixed-bed reactor has been reported [35]. The MFI (silicalite) membrane selectively removes the C8 product from the reaction environment, thus reducing the formation of other unwanted byproducts. Another interesting example is the isobutane (iC4) dehydrogenation carried out in an extractor-type zeolite CMR (including a Pt-based fixed-bed catalyst) in which the removal of the hydrogen allows the equilibrium limitations to be overcome [36],... 

Still another multi-reactor approach is to divide the MTG reaction into two steps as shown in Figure 7. In the first step, methanol is partially dehydrated to form an equilibrium mixture of methanol, dimethyl ether and water over a dehydration catalyst. About 15% of the reaction heat is released in this first step. In the second step, this equilibrium mixture is converted to hydrocarbons and water over ZSM-5 catalyst with the concomitant release of about 85% of the reaction heat. Though this two step approach does not have any of the inherent complications of the previously mentioned multibed reaction systems, it leaves one with a substantial amount of the reaction heat (85%) still to be taken over one catalyst bed. This requires a fairly high recycle stream to moderate the temperature rise over the second reactor. Such a high recycle design would require careful engineering in order to transfer heat efficiently from the reactor effluent to the recycle gas and reactor feed. However, this two stage reactor system is the simplest of the fixed-bed systems to develop. 

The steam reforming of methane cycle suffers from the problem of coke deposition on the catalyst bed. The primary objective of this project was to study the stability of a commercial nickel oxide catalyst for the steam reforming of methane. The theoretical minimum ratios of steam to methane that are required to avoid deposition of coke on the catalyst at various temperatures were calculated, based on equilibrium considerations. Coking experiments were conducted in a tubular reactor at atmospheric pressure in the range of 740-915°C. 

---