Chemical kinetics liquid-vapor equilibrium

There are many reactors in industry that use evaporative cooling. The liquid in the reactor boils to remove reaction heat. The vapor leaving the reactor is condensed in an external heat exchanger, and the liquid is returned to the reactor. Clearly the vapor phase is important in these autorefrigerated reactors. In addition to chemical kinetics, the vapor-liquid equilibrium properties influence the design of the reactor-condenser system. 

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. 

Droplet vaporization is a phenomenon occurring in a gas-liquid system, although only recently have serious efforts been made towards understanding the various hquid-phase processes and their influence on the overall behavior. The problem is a complex, yet interesting and important one. Fundamental research in the interdisciplinary areas of fluid mechanics, chemical kinetics, phase equilibrium analysis, and heat and mass transfer are required to achieve a good understanding of the problem. The following discussions may substantiate this point and stimulate future research efforts. 

The approaches for calculating equilibrium gas-phase composition in a two-phase system containing aqueous solution of HP. air, and JPF are also suggested. The further step in evaluating the performance of the dual-fuel, air-breathing PDE [13] is to incorporate chemical kinetics of HP decomposition and JPF oxidation. Preliminary results on simulation of JPF (or HP) liquid drop ignition and combustion in air with HP (or JPF) vapor have been reported [16]. 

Experimental studies were carried out to derive correlations for mass-transfer coefficients, reaction kinetics, liquid holdup and pressure drop for the new catalytic packing MULTIPAK (see [9,10]). Suitable correlations for ROMBOPAK 6M were taken from [70] and [92], The vapor-liquid equilibrium is calculated using the modification of the Wilson method [9]. For the vapor phase, the dimerization of acetic acid is taken into account using the chemical theory to correct vapor-phase fugacity coefficients [93]. Binary diffusion coefficients for the vapor phase and for the liquid phase are estimated via the method purposed by Fuller et al. and Tyn and Calus, respectively (see [94]). Physical properties like densities, viscosities and thermal conductivities are calculated from the methods given in [94]. 

Contrast this with what can be done in a conventional multiunit flowsheet. The reactors can be operated at their optimum pressures and temperatures that are selected to be the most favorable for their given chemical kinetics. The distillation columns can be operated at their optimum pressures and temperatures that are selected to be the most favorable for their vapor-liquid equilibrium properties. 

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. 

Figure 7 further shows that, as gaseous C02 moves up the absorber, phase equilibrium is achieved at the vapor-liquid interface. C02 then diffuses through the liquid film while reacting with the amines before it reaches the bulk liquid. Each reaction is constrained by chemical equilibrium but does not necessarily reach chemical equilibrium, depending primarily on the residence time (or liquid film thickness and liquid holdup for the bulk liquid) and temperature. Certainly kinetic rate expressions and the kinetic parameters need to be established for the kinetics-controlled reactions. While concentration-based kinetic rate expressions are often reported in the literature, activity-based kinetic rate expressions should be used in order to guarantee model consistency with the chemical equilibrium model for the aqueous phase solution chemistry. 

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. 

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. 

The situation becomes more complicated when the reaction is kinetically controlled and does not come to complete-chemical equilibrium under the conditions of temperature, liquid holdup, and rate of vaporization in the column reactor. Venimadhavan et al. [AIChE J., 40, 1814 (1994)] and Rev [Ind. Eng. Chem. Res., 33, 2174 (1994)] show that the existence and location of reactive azeotropes is a function of approach to equilibrium as well as the evaporation rate. 

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. 

To calculate residue curve maps for the synthesis of TAME one has to proceed in the same manner as the MTBE example and calculate phase equilibria bet veen liquid and vapor phases, chemical equilibrium constants in the liquid phase, and kinetics of the chemical reactions. 

For high Da the column is dose to chemical equilibrium and behaves very similar to a non-RD column with n -n -l components. This is due to the fact that the chemical equilibrium conditions reduce the dynamic degrees of freedom by tip the number of reversible reactions in chemical equilibrium. In fact, a rigorous analysis [52] for a column model assuming an ideal mixture, chemical equilibrium and kinetically controlled mass transfer with a diagonal matrix of transport coefficients shows that there are n -rip- 1 constant pattern fronts connecting two pinches in the space of transformed coordinates [108]. The propagation velocity is computed as in the case of non-reactive systems if the physical concentrations are replaced by the transformed concentrations. In contrast to non-RD, the wave type will depend on the properties of the vapor-liquid and the reaction equilibrium as well as of the mass transfer law. 

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 design equations would include, in addition to the usual heat and mass balances and vapor-liquid equilibria, equations for chemical equilibria and/or reaction kinetics. The occurrence of a chemical reaction can severely restrict the allowable ranges of temperatures and phase compositions by virtue of the additional equations for chemical equilibrium/kinetics. This effect can be quantitatively analyzed by constructing a residue curve map (RCM). It explicitly shows the shifting of distillation boundaries in the presence of reaction and defines the limits of feasible distillation column operation. We illustrate this (Venimadhavan et al., 1994) by considering the reaction... 

Based on solute solubility considerations, a chemical reaction in a SILP catalyst system can be assessed. Thus, a higher substrate solubility directly leads to a faster reaction rate and therefore to a faster conversion. Figure 9.11 shows the results of propene hydrogenation in four different SILP catalyst systems at 303 K. The conversion X (r = ) is plotted over the reaction time. The data points describe the experimentally measured conversions from batch experiments, and the drawn lines characterize the calculated conversion. For the calculated conversions, the vapor/liquid equilibrium (Eq. (9.1)) of all substrates and the product and the kinetics Eq. (9.13) are solved simultaneously. Reasonably, the partial reaction orders and... 

FIGURE 13 Examples of kinetic processes classified by types of phases involved, (a) Gas-gas reaction equilibrium between hydrogen gas, iodine gas, and hydrogen iodide gas. (f>) Gas-Uquid evaporation of liquid water from a glass, (c) Liquid-Liquid gradual separation of an oil-water mixture, (d) Gas-solid chemical vapor deposition of a thin Si film, (e) Liquid-solid corrosion of Cu metal in seawater, (f) Solid-solid precipitation of CuAlj particles from a copper-aluminum alloy during a heat treatment process. 

For the development of the process model various physical and chemical phenomena occurring in each process unit has to be known. Most important phenomena in chemical production plants are reaction kinetics, heat and mass transfer, phase equilibrium and hydraulics. Reactors require kinetic and mass transfer models that describe how local concentrations and temperature affect the reaction rates. Separation processes require knowledge of phase equilibria (vapor/liquid, liquid/liquid) and knowledge of heat and mass transfer. Very often some specific knowledge of the equipment are also required like capacities and efficiencies of distillation trays. 

---