Distillation model development

For a given Rexf, the vapour rate profile is averaged to obtain an average Vexp to be used in the batch distillation model developed by Greaves et al.. Figure 3.12... 

This matrix will contain information regarding loading characteristics such as flooding hmits, exchanger areas, pump curves, reactor volumes, and the like. While this matrix may be adjusted during the course of model development, it is a boundary on any possible interpretation of the measurements. For example, distillation-column performance markedly deteriorates as flood is approached. Flooding represents a boundary. These boundaries and nonlinearities in equipment performance must be accounted for. 

Verneuil et al. (Verneuil, V.S., P. Yan, and F. Madron, Banish Bad Plant Data, Chemical Engineering Progress, October 1992, 45-51) emphasize the importance of proper model development. Systematic errors result not only from the measurements but also from the model used to analyze the measurements. Advanced methods of measurement processing will not substitute for accurate measurements. If highly nonlinear models (e.g., Cropley s kinetic model or typical distillation models) are used to analyze unit measurements and estimate parameters, the Hkelihood for arriving at erroneous models increases. Consequently, resultant models should be treated as approximations. 

Many industrial processes involve mass transfer processes between a gas/vapour and a liquid. Usually, these transfer processes are described on the basis of Pick s law, but the Maxwell-Stefan theory finds increasing application. Especially for reactive distillation it can be anticipated that the Maxwell-Stefan theory should be used for describing the mass transfer processes. Moreover, with reactive distillation there is a need to take heat transfer and chemical reaction into account. The model developed in this study will be formulated on a generalized basis and as a consequence it can be used for many other gas-liquid and vapour-liquid transfer processes. However, reactive distillation has recently received considerable attention in literature. With reactive distillation reaction and separation are carried out simultaneously in one apparatus, usually a distillation column. This kind of processing can be advantageous for equilibrium reactions. By removing one of the products from the reactive zone by evaporation, the equilibrium is shifted to the product side and consequently higher conversions can be obtained. Commercial applications of reactive distillation are the production of methyl-... 

Rigorous and stiff batch distillation models considering mass and energy balances, column holdup and physical properties result in a coupled system of DAEs. Solution of such model equations without any reformulation was developed by Gear (1971) and Hindmarsh (1980) based on Backward Differentiation Formula (BDF). BDF methods are basically predictor-corrector methods. At each step a prediction is made of the differential variable at the next point in time. A correction procedure corrects the prediction. If the difference between the predicted and corrected states is less than the required local error, the step is accepted. Otherwise the step length is reduced and another attempt is made. The step length may also be increased if possible and the order of prediction is changed when this seems useful. 

Bosley and Edgar (1994a,b) developed techniques to calculate directly dynamic distillation (DDD) derivatives (non iterative), which were then employed to integrate rigorous DAEs (index 2) based batch distillation model. 

Hybrid Model Development for Pilot Batch Distillation Column... 

Nonequilibrium methods (Sec. 4,2.13) tend to be global Newton methods extended to solve mass-transfer-inhibited systems. Nonequilibrium methods are not yet completely extended to more common systems, but these methods should see the greatest amount of development in distillation modeling. 

Equations 7.3.11 and 7.3.14 are used in the development of expressions for modeling mass transfer in multicomponent distillation, a topic we consider in Chapter 12. The addition of resistances concept has seen use in distillation models by Krishna et al. (1981a), Burghardt et al. (1983, 1984) and by Gorak and Vogelpohl (1985). 

Consider the model for an ideal binary distillation column developed in Example 4.13. We have ... 

The mass balances (Eqs. (10.3) and (10.4)) assume plug-flow behavior for both the vapor and the liquid phase. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the column performance, for example via backmixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for catalytic distillation processes, we performed experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK [77]. 

Much work on liquid metal velocity has been done with sodium and serves to illustrate its influence on corrosion. Bagnall and Jacobs [21] have attempted to unify the available data in the literature and correlate corrosion rate with temperature. Sodium velocity and oxygen were the two major variables taken into consideration. With oxygen interpreted on the vanadium wire equilibration scale, it was shown statistically that corrosion rate, R, was independent of velocity above about 3 m/s and directly proportional to oxygen concentration. An example of this correlation is shown in Fig. 4 where the variation of R with oxygen is plotted for type 316 stainless steel at 700°C. Data obtained at very low oxygen levels (<0.5 ppm by vanadium wire) deviate horn the predictive curve. At these low concentrations, mass loss appears to approach the model developed by Weeks and Isaacs [4]. Such behavior is not unreasonable since it is clear that mass loss will, in 2my event, not drop to zero at zero oxygen. Also, an approximate correlation between the vanadium wire scale and vacuum distillation values is shown on the abscissa. Note that the latter scale is not linear and that the vacuum distillation analytical method becomes insensitive below about 5 ppm. 

The four models used to teach process troubleshooting are the distillation model, the reaction and separation model, the absorption and stripping model, and the combination model. Each model has a complete set of process control instrumentation and equipment arrangements. Various troubleshooting methodologies are applied to these four models. A complete range of troubleshooting scenarios has been developed and is typically included with these models. 

Venimadhavan et. al. (1999) developed batch reactive distillation model for production of butyl acetate in presence of sulfuric acid catalyst. They also studied the kinetics of esterification of acetic acid with butanol and calculated the thermodynamic equilibrium constant in a temperature range of 373 K- 393 K. They found that the equilibrium constant did not vary strongly with temperature. 

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