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FIGURE 9.13 Concept of multiscale modeling of bubble column reactors (from Bauer and Eigenberger, 1999).

Aeration gas

FIGURE 9.14 Iterative process used in multiscale modeling (from Bauer and Eigenberger, 1999).

Aeration gas

FIGURE 9.14 Iterative process used in multiscale modeling (from Bauer and Eigenberger, 1999).

In this approach, a simplified reactor model acts as a hub. Instead of invoking conventional simplifying assumptions to develop the model, this approach uses information obtained from detailed fluid dynamic models and detailed bubble-bubble interaction (population balance) models. Such a basic reactor model provides the local mean bubble diameter and the interfacial mass flux to the hydrodynamic model. The details of modeling approaches to simulate the fluid dynamics of bubble column reactors are discussed in Chapter 11. We restrict the scope here to discussing the overall approach. The whole reactor behavior is simulated by employing an iterative procedure over these different modeling layers as shown in Fig. 9.14. Some results obtained by Bauer and Eigenberger (1999) using this approach to simulate a pseudofirst-order reaction in a two-dimensional bubble column are shown in Figs 9.15 to 9.17. Four iterations were required to obtain the converged results. Gas sparged at the bottom disappears rapidly due to the reactive consumption and does not extend over the entire column. This changes the flow field of the liquid phase, and mixing significantly. The influence of changes in mixing in the reactor can be clearly seen from the mass fraction profiles within the reactor (Fig. 9.17). The approach can be extended to simulate more complex industrial bubble column reactors.

Recently Ranade (2000) used a similar multiscale approach to simulate a complex industrial loop reactor. The objective of the project was to develop a comprehensive understanding of the fluid dynamics of the operating loop reactor and to develop appropriate scale-up guidelines based on such an understanding. The schematic of the industrial loop reactor considered is shown in Fig. 9.18 (in 2D). The reactor was designed to carry out a pre-polymerization (condensation polymerization) reaction. The low molecular weight products of the condensation reaction and solvent are vaporized in the heater section. These vapors lead to gas-lift action and generate the circulation within the loop reactor, which ensures the desired mixing in the reactor. Generally, the circulation rate is orders of magnitude greater than the net flow through the reactor. The vapors generated are removed from the top after separating from the liquid in the vapor separator. Vapor bubbles erupting at the gas-liquid interface throw some liquid droplets into the vapor space and a fraction of these liquid droplets may be carried over with the removed vapor. Such a carry-over of liquid often imposes limits on enhancing reactor capacity. One of the objectives of the project was, therefore,

mass transfer, (b) accounting for mass transfer.
mass transfer, (b) accounting for mass transfer.
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