New Mass-Transfer Model for Simulating Industrial Nylon-6 ...

5072 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004Figure 10. Bubble-gas kettle train predictions vs plant data for extrusion FAV, W&D FAV, and water extractables as a function of thescaled production rate (equilibrium; no parameter adjustments made).Figure 11. Bubble-gas kettle train prediction errors for extrusion FAV, W&D FAV, and water extractables as a function of the scaledproduction rate (equilibrium; no parameter adjustments made).4.2. Bubble-Gas Kettle Process. Section 3 showsus how to predict, a priori, mass-transfer coefficientsfor forced bubbles in reactors. After our analysis insection 4.1, we can now also predict, a priori, the boilingrate as a function of the temperature, pressure, andcomposition. We therefore analyze the bubble-gas kettleprocess in a truly predictive fashion (i.e., no parameteradjustments are made) and compare how the modelperforms when we assume three different devolatilizationcases: (1) diffusion only, (2) phase equilibrium only,and (3) diffusion and bubble nucleation.As in the direct-melt process, we specify and solve theequations that describe the kettles in Figure 1. Wemodel each kettle as a CSTR, with equations consistingof a mass balance, phase equilibrium in monomerrecoveryunits, a devolatilization model, a reactionkinetics model, and a liquid molar volume model. Wepredict both extrusion and W&D FAV and water extractables.Amine end groups are not measured becausethey can be computed from the W&D FAV as thepolymer is unterminated.Figure 8 shows how well the model predicts the plantdata, assuming that melts devolatilize through diffusiononly. Figure 9 shows the prediction errors.The diffusion-only model seriously underpredicts therate at which water volatilizes, as evidenced by the poorFAV predictions (-54.3% and -56.2% for extrusion FAVand W&D FAV, respectively). The diffusion model does,however, reasonably describe the rate at which caprolactamvolatilizes, as evidenced by an underpredictionof water extractables by only 14.3%. From this, weconclude that Woo et al.’s model for bubble diffusion 7-9alone is inadequate for simulating molecular-weightgrowth in industrial nylon-6 bubble-gas kettle processes.Figure 10 shows how well the model predicts plantdata, assuming no mass-transfer limitations, i.e., phaseequilibrium only. Figure 11 shows the correspondingprediction errors.