Reactor equilibrium limited

A promoted nickel type catalyst contained in the reactor tubes is used at temperature and pressure ranges of 700-800°C and 30-50 atmospheres, respectively. The reforming reaction is equilibrium limited. It is favored at high temperatures, low pressures, and a high steam to carbon ratio. These conditions minimize methane slip at the reformer outlet and yield an equilibrium mixture that is rich in hydrogen. ... 

Equilibrium-Limited Reactor. This system is the basis for most practical, commercial methanation plants. It is safe from the standpoint... 

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. 

In chemical processing the most fundamental constraint is that of the thermodynamics of the system. This constraint defines both the heat balance of the process and whether or not the processes in the reactor will be equilibrium limited. These constraints will limit the range of chemical engineering solutions to the problems of designing an economically viable process that can be found. 

The novel approach finally taken was to conduct the reaction and purification steps in a reactor-distillation column in which methyl acetate could be made with no additional purification steps and with no unconverted reactant streams. Since the reaction is reversible and equilibrium-limited, high conversion of one reactant can be achieved only with a large excess of the other. However, if the reacting mixture is allowed to flash, the conversion is increased by removal of the methyl acetate from the liquid phase. With the reactants flowing countercurrently in a sequence of... 

The finishing reactors used for PET and other equilibrium-limited polymerizations pose a classic scaleup problem. Small amounts of the condensation product are removed using devolatilizers (rotating-disk reactors) that create surface area mechanically. They scale as... 

Membranes in catalysis can be used to improve selectivity and conversion of a chemical reaction, improve stability and lifetime of the catalyst, and improve the safety of operation. The most well-known example is in situ removal of products of an equilibrium-limited reaction. However, many more ways of application of a membrane can be thought of [1-3], such as using the membrane as a reactant distributor to control the reactant concentration levels in the reactor, or performing catalysis inside the membrane and having control over reactant feed and product removal. 

Membranes can be applied to catalysis in different ways. In most of the literature reports, the membrane is used on the reactor level (centimeter to meter scale) enclosing the reaction mixture (Figure 10.3). In most cases, the membrane is used as an inert permselective barrier in an equilibrium-limited reaction where at least one of the desired products is removed in situ to shift the extent of the reaction past the thermodynamic equilibrium. 

To develop the rate equations suitable for process modeling and reactor design, experimental data have been analyzed on the basis of the postulated reaction mechanism [2] given in Table 1. Here the formation of polymer is excluded because it is not detected under our experimental conditions. All of the reactions are equilibrium-limited and the net rates for the formation of each component with some assumptions [3] are given as follows ... 

Membrane reactors are known on the macro scale for combining reaction and separation, with additional profits for the whole process as compared with the same separate functions. Microstructured reactors with permeable membranes are used in the same way, e.g. to increase conversion above the equilibrium limit of sole reaction [8, 10, 11, 83]. One way to achieve this is by preparing thin membranes over the pores of a mesh, e.g. by thin-fihn deposition techniques, separating reactant and product streams [11]. 

It is useful to combine reaction and separation for equilibrium-limited reactions and also for consecutive reactions, particularly when the desired intermediate products undergo faster undesirable reactions. The concept of extractive reactions for equilibrium-limited and consecutive reactions has been covered in Section 4.2.1. Distillation column reactors provide yet another strategy. 

Table VIII demonstrates the inverse relationship of conversion to S02 concentration in the feed that is a consequence of applying flow reversal to S02 oxidation using a single reactor. As the S02 concentration in the table moves from 0.8 to over 8 vol%, the conversion drops from 96-97% down to 85%. At the same time, the maximum bed temperature changes from 450 to 610°C. For an equilibrium-limited, exothermic reaction, this behavior is explained by variation of the equilibrium conversion with temperature.

Recently, Falk and Seidel-Morgenstern [143] performed a detailed comparison between fixed-bed reactors and fixed-bed chromatographic reactors. The reaction studied was an equilibrium limited hydrolysis of methyl formate into formic acid and methanol using an ion-exchange resin as both the catalyst and the adsorbent. The analysis was based on a mathematical model, which was experimentally verified. The comparison was based on the following four assumptions ... 

These reactors operate near equilibrium, and therefore the first reactor must be heated to high temperatures because this reaction is endothermic, and the second must be cooled to fairly low temperatures. The kinetics of these reactions are very important if one is designing a reactor in detail, but the major features of the process are governed by equilibrium limitations and heat effects. 

In fact, we usually want to operate exothemic reactions nonisothermally to take advantage of the heat release in the reaction to heat the reactor to a temperature where the rates are higher and reactor volumes can be smaller. However, if the temperature is too high, equilibrium limitations can limit the conversion, as we saw previously for NH3 and CH3OH synthesis reactions. 

These data are plotted in Figure 5-1 1. Thus we have a very serious problem if this reaction is reversible because the adiabatic reactor trajectory intersects the equilibrium curve at a low conversion. For these kinetics, equilibrium limits the process to a very low conversion at high temperatures. 

In any polymerization process one must be concerned with removal of the coproduct (typically H2O or HCl) so that equilibrium limitations do not limit the polymer size. The removal of the product in condensation polymerization to attain higher polymer lengths is a major consideration in polymerization reactor design. This can be done by withdrawing water vapor or by using two phases so that the water and polymer migrate to different phases. 

Combining chromatography with a chemical reactor can be used to achieve reaction and separation within the same reactor, and this can be used to generate products beyond the normal thermodynamic equilibrium limitation. 

Membrane reactors can offer an improvement in performance over conventional reactor configurations for many types of reactions. Heterogeneous catalytic reactions in membrane reactors [1] and the membranes used in them [2,3] have been reviewed recently. One well studied application in this area is to remove a product from the reaction zone of an equilibrium limited reaction to obtain an increase in conversion [4-10]. The present study involves heterogeneous... 

Ca (continuous line) and Cb 0dotted line) in a batch reactor for the equilibrium limited reaction. Initial conditions are Cao = 1 molm-3 and Cbo = 0 molm-3... 

The process is subjected to a number of disturbances, and the control structure handles all of them quite effectively. Dynamic responses to changes in the setpoint of the temperature controller in the first reactor are shown in Figure 6.109. At 0.1 h, the setpoint is increased from 245 to 255°C. At 3 h, it is decreased to 235°C. Decreasing the temperature in the first reactor results in an increase in throughput. The synthesis gas feedrate, the product rate, and the vent rate all increase. The opposite occurs when the temperature is increased. This indicates that the reaction is equilibrium-limited, not kinetically limited. Decreasing temperature increases the equilibrium constant of exothermic reactions. 

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