Number reactive trays

Example 11.8 With highly reactive absorbents, the mass transfer resistance in the gas phase can be controlling. Determine the number of trays needed to reduce the CO2 concentration in a methane stream from 5% to 100 ppm (by volume), assuming the liquid mass transfer and reaction steps are fast. A 0.9-m diameter column is to be operated at 8 atm and 50°C with a gas feed rate of 0.2m /s. The trays are bubble caps operated with a 0.1-m liquid level. Literature correlations suggest = 0.002 m/s and A, = 20m per square meter of tray area. 

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

Global Newton Naphtali and Sandholm (42) Holland (8) High number of trays, Tew components All type mixtures including nonideal Requires good starting values Chemical and reactive systems Two of condenser duty, re boiler duly, reflux, and boilup plus all side product flows, one purity allowed... 

In reactive distillation, chemical reactions are assumed to occur mainly in the liquid phase. Hence the liquid holdup on the trays, or the residence time, is an important design factor for these processes. Other column design considerations, such as number of trays, feed and product tray locations, can be of particular importance in reactive distillation columns. Moreover, since chemical reactions can be exothermic or endothermic, intercoolers or heaters may be required to maintain optimum stage temperatures. Column models of reactive distillation must include chemical reaction... 

The initial distillate cut is the lightest and, as the distillation progresses, the liquid remaining in the reboiler becomes continuously richer in the heavier components, and subsequent distillate cuts become increasingly heavier. The residue remaining in the reboiler after the last distillate cut is the heaviest cut. A multicomponent feed mixture may be separated in one batch distillation column into a number of products with specified purities. Given the required number of trays and reflux ratio, a batch distillation column could, in principle, separate a normal feed mixture (one that is not reactive or azeotrope forming) into its pure constituents. 

Fig. 5.9 shows the geometric representation of these balances in the McCabe-Thiele diagram. The distillate composition xP is located at the intersection of the condenser operating line and the operating line of the RD section. By specifying xP, the number of reactive trays can be estimated from the classical staircase construction. From the intersection of the operating line of the column section with that of the reboiler line, the bottom composition is determined. 

In tray columns the first mechanism is dominant. This can lead to a large number of different steady state solutions for a given set of operating conditions. If N is the number of steady states (typically an odd number). Then (N + l)/2 of these steady states are stable. This can lead to complex multi-stable dynamic behavior during column startup and set-point or load changes. These phenomena were observed for vanishing as well as for finite intra-particle mass transfer resistance. An example with a total number of six trays (two reactive and two non-reactive trays plus reboiler and condenser) is shown in Fig. 10.16 for the well-known MTBE process. In contrast to the previous section, the column is now operated in the kinetic... 

In conventional distillation design, tray holdup has no effect on steady-state compositions. In reactive distillation, tray holdup (or amount of catalyst) has a profound effect on conversion, product compositions, and column composition profiles. Therefore, in addition to the normal design parameters of reflux ratio, number of trays, feed tray location, and pressure, reactive distillation columns have the additional design parameter of tray holdup. 

Ciric and Gu (1994) present a MINLP-based approach for the design of RD columns for systems where multiple reactions take place and/or where reactive equilibrium or thermal neutrality caimot be assured. This method is based on the combination of a rigorous tray-by-tray model and kinetic-rate-based expressions to give basic constraints of an optimization model that minimizes the total annual cost. The major variables are the number of trays in the column, the feed tray location, the temperature and composition profiles within the column, the reflux ratio, the internal flows within the column and the column diameter. 

The results of this section show that a considerable number of bifurcation diagrams can be identified even for a simplified RD application. By addressing a single reactive tray we aim to explore exclusively the interaction between phase equilibrimn and chemical reaction. The possibilities to extend this approach to a more realistic and full-scale RD... 

Number of trays [2 recti ying 8 reactive 5 stripping]... 

Answer. Reduction in the number of reactive trays, more uneven distribution of catalyst and identification of suitable spots for side reactors. 

Reactive distillation is also different from conventional distillation in that there are both product compositions and reaction conversion specifications. The many design degrees of freedom in a reactive distillation column must be adjusted to achieve these specifications while optimizing some objective function such as total annual cost (TAC). These design degrees of freedom include pressure, reactive tray holdup, number of reactive trays, location of reactant feedstreams, number of stripping trays, number of rectifying trays, reflux ratio, and reboiler heat input. 

Another limitation for reactive distillation is the need for reasonably large specific reaction rates. If the reactions are very slow, the required tray holdups and number of reactive trays would be too large to be economically provided in a distillation column. 

With all feed conditions and the column configuration specified (number of trays in each section, tray holdup in the reactive section, feed tray locations, pressure, and desired conversion), there is only one remaining degree of freedom. The reflux flowrate is selected. It is manipulated by a distillate composition controller to drive the distillate composition to 95 mol% C. The vapor boilup is manipulated to control the liquid level in the base. Note that the distillate and bottoms flowrates are known and fixed as the dynamic model is converged to the steady state that gives a distillate composition of 95 mol% C. The composition of the bottoms will be forced by the overall component balance to be 95 mol% D. 

The previous section showed no surprises regarding the effect of tray holdup. In this section we look at changing the number of reactive trays. Intuition would lead us to think that the more trays the better. This is certainly the case in conventional distillation. However, as we will see, this is not the case with a steady-state reactive distillation column for this type of reaction (two reactants, two products). 

The situation is just the reverse with a large number of reactive trays. The concentrations of the reactants at the ends of the column where they are fed become large because much of the other reactant has been consumed on the many trays in the reactive zone before it reaches the opposite end. There is little B arriving near the bottom of the reactive zone, so the concentration of A is large near where it is fed. Likewise, there is little A arriving near the top of the reactive zone, so the concentration of B is large near where it is fed. 

Figure 2.6 Effect of number of reactive trays on temperature profile.

These curves indicate that low ptessuie and the resulting low temperatures would result in low reaction rate, which would require large holdups on reactive trays. Alternatively, for a fixed number of trays and a fixed holdup, the concentrations of the reactants in the reactive zone would have to be large at low temperatures. This would require a large vapor boilup and reflux flowrate to keep the reactants from escaping out of the top or bottom. 

We will return to this problem in Chapter 3 and show how the design of the reactive column must be altered to handle cases with temperature dependence of the relative volatilities. As expected, the optimum column pressure decreases and the number of reactive trays increase as the values of a39o decrease. Reactive distillation becomes economically unattractive for a39o values of less than 1.2. 

However, there are two parameters that give counterintuitive results. The most important of these is the number of reactive trays. More trays does not necessarily improve the steady-state economics of a reactive distillation column. However, the effect of more reactive trays on the dynamic performance of a reactive column is distinctly different, as we will demonstrate in Chapter 10. 

The number of reactive trays is varied over a wide range, and steps 4-14 are repeated for each value of Nrx, generating its corresponding TAC. 

Figure 3.9 demonstrates the effects of the number of sqjaration stages (Ns = Nr) on the economical steady-state design of a reactive distillation colmnn with three different operating pressures. The results are given for the base case (Ke( 266 = 2 with constant reactive trays Nrx = 9. The graph in the middle shows that the column cost increases as the number of... 

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