Reactive vapor-liquid equilibria

In the following discussion, we will focus on reactive vapor liquid equilibria. Measurements of reactive liquid—liquid equilibria can be done in standard batch cells operated as mixer-setders using appropriate analytical methods, as long as only data on the fully established equilibrium is needed. Practically no data seems to be available in the literature on liquid-liquid equilibria in mixtures that have not reached chemical equilibrium. 

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This brief survey shows that there are many options for measuring phase equilibria in reacting systems, which allow to carry out such studies for a wide range of systems and conditions. The main limitation for experimental investigations of reactive vapor-liquid equilibria is related to the velocity of the reaction itself if phase equilibrium measurements of solutions are needed, which are not in chemical equilibrium, the reaction must be considerably slower than the characteristic time constant of the phase equilibrium experiment. Apparatus are available, where that time constant is distinctly below one minute. For systems with reactions too fast to be studied in such apparatuses, it should in many cases be possible to treat the reaction as an equilibrium reaction, so that the information on the phase equilibrium in mixtures, which are not chemically equilibrated is not needed. 

Combining chemical reaction and separation in a reactive-distillation device, can lead to significant economic advantage in term of investment and operation costs. Because the chemical driving force can be lowered by vapor-liquid equilibrium, this should be compensated by a more active catalyst. 

Summing up, the influence of the kinetics of a chemical reaction on the vapor-liquid equilibrium is very complex. Physical distillation boundaries may disappear, while new kinetic stable and unstable nodes may appear. As result, the residue curve map with chemical reaction could look very different from the physical plots. As a consequence, evaluating the kinetic effects on residue curve maps is of great importance for conceptual design of reactive distillation systems. However, if the reaction rate is high enough such that the chemical equilibrium is reached quickly, the RCM simplifies considerably. But even in this case the analysis may be complicated by the occurrence of reactive azeotropes. 

Analogously to batch distillation and the RCM, the simplest means of reactive distillation occurs in a still where reaction and phase separation simultaneously take place in the same unit. Additionally, we can choose to add a mixing stream to this still, and the overall process thus consists of three different phenomena chemical reaction, vapor liquid equilibrium, and mixing. Such a system is referred to as a simple reactive distillation setup. This setup is shown in Figure 8.1 where a stream of flowrate F and composition Xp enters a continuously stirred tank reactor (CSTR) in which one or more chemical reaction(s) take place in the liquid phase with a certain reaction rate r =f(kf, x, v) where v represents the stoichiometric coefficients of the reaction. Reactants generally have negative stoichiometric coefficients, while products have positive coefficients. For example, the reaction 2A + B 3C can... 

Knowledge of the equilibrium is a fundamental prerequisite for the design of non-reactive as well as reactive distillation processes. However, the equilibrium in reactive distillation systems is more complex since the chemical equilibrium is superimposed on the vapor-liquid equilibrium. Surprisingly, the combination of reaction and distillation might lead to the formation of reactive azeotropes. This phenomenon has been described theoretically [2] and experimentally [3] and adds new considerations to feasibility analysis in RD [4]. Such reactive azeotropes cause the same difficulties and limitations in reactive distillation as azeotropes do in conventional distillation. On the basis of thermodynamic methods it is well known that feasibility should be assessed at the limit of established physical and chemical equilibrium. Unfortunately, we mostly deal with systems in the kinetic regime caused by finite reaction rates, mass transfer limitations and/or slow side-reactions. This might lead to different column structures depending on the severity of the kinetic limitations [5], However, feasibility studies should identify new column sequences, for example fully reactive columns, non-reactive columns, and/or hybrid columns, that deserve more detailed evaluation. 

Fig. 10.21 Concentration wave fronts in a reactive terna separation after a step change in reflux rate. Ideal vapor-liquid equilibrium, kinetically controlled mass transfer, reversible chemical reaction dose to chemical equilibrium...

The value of p can be calculated by solving the mass transfer equation in the gas phase and the value of p can be calc lated from vapor-liquid equilibrium data. It is important to realize that, in order to calculate ip, one does not need to know the value of Henry s law constant in the reactive medium. 

A rigorous simulation and optimization of reactive distillation processes usually is based on nonlinear fimctions for a realistic description of the reaction kinetics and the vapor-liquid-equilibrium. Within GAMS models, this description leads to very complex models that often face convergence problems. By using the new so-called external functions, the situation can be improved by transferring calculation procedures to an external module. 

As demonstrated by former publications, the GAMS modeling system has been successfully used for the MINLP-optimization of single reactive distillation columns (Poth et al., 2001 Jackson and Grossmann 2001). The strong nonlinear functions required for a realistic description of the reaction kinetics and the vapor-liquid-equilibrium in these cases lead to very complex GAMS models that may face convergence problems. 

For calculation of the equilibrium compositions of the liquid phase either the equilibrium constants of the dissociation and polycondensation reactions have to be known or they can be computed by methods which use the approach of minimizing Gibbs free energy [200-202]. In addition, ab initio modeling techniques such as density functional theory (DFT) in combination with reactive molecular dynamic (MD) simulations could be used. Once the liquid phase system is modeled, there are in principle two options to describe the vapor-liquid equilibrium. Either equations of state (EOS) or excess Gibbs free energy models (g -models) may be used to describe the thermodynamics of the liquid... 

In addition, the excess of ammonia reduces the required synthesis pressure, which is related to the strongly nonideal behavior of the vapor-liquid equilibrium. At a NH3 CO2 ratio around 3, a pressure minimum azeotrope can be observed for the reactive system [13]. Starting from the composition at the pressure minimum, an increase of the NH3 CO2 ratio results in a lower pressure rise than a decrease. This contributes to the fact that ammonia is better soluble in the liquid water/urea phase than carbon dioxide [13]. For this reason, in most of the industrial urea processes, the molar NH3 CO2 ratio in the reactor is adjusted to be around 3 1 or higher. 

Although reactive distillation sounds like a great idea, its area of application is fairly restricted. Both the chemistry and the vapor-liquid equilibrium phase equilibrium must be suitable. 

In the next three chapters we will explore various aspects of the ideal quaternary chemical system introduced in Chapter 1. This system has four components two reactants and two products. The effects of a number of kinetic, vapor-liquid equilibrium, and design parameters on steady-state design are explored in Chapter 2. Detailed economic comparisons of reactive distillation with conventional multiunit processes over a range of chemical equilibrium constants and relative volatilities are covered in Chapter 3. An economic comparison of neat versus excess-reactant reactive distillation designs is discussed in Chapter 4. 

The effects of several important design and chemical parameters on the steady-state design of the ideal chemical system with four components are considered in this chapter. The impact of some parameters is similar to that experienced in conventional distillation. However, in some cases the effects are counterintuitive and unique to reactive distillation. The approach is to see the effect of changing one parameter at a time, while holding all other parameters at their base case values. The base case values of kinetic and vapor-liquid equilibrium parameters are given in Table 2.1. Table 2.2 gives design parameters and steady-state values of process variables for the base case. 

However, in reactive distillation, pressure affects both chemical kinetics and vapor-liquid equilibrium. Therefore, the optimum pressure may not correspond to the minimum attainable while stiU using cooling water in the condenser. For example, the optimum pressure for the base case is 8 bar, as we will demonstrate. The corresponding reflux-drum temperature is 353 K (80 °C, 176 °F), which is well above the temperature that could be achieved using 305 K (32 °C, 90 °F) cooling water. 

Reactive Distillation. Figure 3.18 and Table 3.8 give optimum design results for the reactive distillation process for a range of temperature-dependent relative volatilities. As the a39o parameter decreases, the optimum pressure decreases. This occurs because lower pressure helps the vapor-liquid equilibrium because it lowers temperatures and hence increases relative volatilities. However, a lower temperature is unfavorable for reaction because the reaction rates are too small. The result is a rapid increase in the required number of reactive trays. 

The economics of reactive distillation have been quantitatively compared with those of conventional multiunit processes with separate reaction and separation sections. With favorable chemistry and relative volatilities, reactive distillation is less expensive than a conventional process. However, if a mismatch occurs in the temperatures conducive for good reaction kinetics and the temperatures conducive for good vapor-liquid equilibrium, reactive distillation is not an attractive alternative. 

Although there are only three components involved in the reaction, in many of the A -h B C systems there are more than three components in the column because the feed-streams contain other components. These components are inert from the standpoint of the reaction, but they are not inert from the standpoint of their effect on the vapor-liquid equilibrium in the column. These inert components are present in the olefin feedstreams that contain the reactive iso C4 and C5 olefins in the examples cited. The reason for their presence is the great difficulty in separating the desired iso-olefin from the other components. For example, in the MTBE and ETBE cases, isobutene is produced in a catalytic cracker in a refinery along with a number of other C4 components (isobutane, w-butane, and n-butene). 

Figure 6.16fc provides the projections of liquid (X/) and corresponding equilibrium vapor (17) compositions for the transformed variables. The singular point with X, = T, at chemical equifibrium is the reactive azeotrope, which is shown in Figure 6.16c. The feature of this point is that the direction of the reaction is the same as the direction of the material balance line from a tray-by-tray calculation at an infinite reflux ratio. A more familiar expression for the reactive azeotrope is the maximum (or minimum) value on a T-X-Y diagram, except that X and Y are the transformed composition, as shown in Figure 6.l6d. Therefore, given a chemical reaction system and corresponding vapor-liquid equilibrium, we can compute the reactive azeotrope numerically with simple transformation.

The TAME reactive distillation system with a two-column methanol recovery system was successfully simulated in Aspen Dynamics. The system features two recycles (methanol and water) and three feedstreams (C5, methanol, and water). The system is essentially a ternary system with inerts, but the complex vapor-liquid equilibrium results in the formation of azeotropes that result in losses of methanol out of the top of the reactive column with the inerts. Therefore, a methanol recovery system must be included in the plant design and control. 

One of the limitations for making reactive distillation economically attractive is that the temperature range suitable for reasonable chemical reaction kinetics must match the temperature range suitable for vapor-liquid equilibrium because both separation and reaction occur in a single vessel. We alluded to this limitation in several previous chapters. In particular, we examined it quantitatively in Chapter 3 where we demonstrated how the economics of reactive distillation are adversely affected when a mismatch occurs. 

Both of these configurations have better economics than a reactive distillation column when there is a mismatch between favorable reaction temperatures and favorable vapor-liquid equilibrium temperatures. 

Another issue is that the effect of pressure on the design is not explored here because we are assuming constant relative volatility systems. The column pressure is fixed at 8 bar in this work. Pressure is very important in reactive distillation because of the effect of temperature on both vapor-liquid equilibrium and reaction kinetics. For exothermic reactions, the optimum column pressure is affected by the competing effects of temperature on the specific reaction rates and the chemical equilibrium constant. 

Only one specific example has been explored thus far. It is interesting to see whether the results can be extended to different cases (e.g., different relative volatilities) and how this vapor-liquid equilibrium change will impact the location of optimal feed trays and the percentage of energy savings. Note that for every case studied, the column is redesigned using the procedure in Section 18.1. This means the columns may have different Nr, Ns, and Nrx and the location of the feed trays are described in terms of their relative position in the reactive zone. 

Section 4.2 is focused on phase equilibrium-controlled vapor-liquid systems with kinetically or equihbrium-controlled chemical reactions. The feasible products are kinetic azeotropes or reactive azeotropes, respectively. 

Section 4.3 elucidates the role of vapor-liquid mass transfer resistances on the feasible products of nonreactive or reactive separation processes. The latter are considered under chemical equilibrium conditions (i.e., they are very fast reactions). The feasible products are denoted as arheotropes. 

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