(417c) Improvement of Gas-Liquid Mass Transfer for Viscous Fluids in Batch Vacuum Operation

This study focuses on the removal of volatile residues from viscous fluids in a large-scale agitator reactor under vacuum conditions. The aim of this process is to accelerate chemical reactions by eliminating volatile residues or extracting such residues from polymers. The batch operation for the removal of volatile residues in a vacuum operation proceeds in three stages: a boiling phase, a transition phase, and a surface evaporation phase. During the final stage, surface evaporation occurs without the fluid boiling. Within this surface evaporation stage, the evaporation of volatile residues only occurs through the gas-liquid interface, thereby rendering the rate of mass transfer between gas and liquid crucial.

In order to augment the mass transfer at the gas-liquid interface and ensure adequate circulation of fluid within the agitator tank for sufficient renewal of fluid near the gas-liquid interface, we selected a large multi-stage impeller [1] developed by the authors. This impeller excels in vertical mixing even in medium-viscous fluids. For the purpose of promoting mass transfer at the gas-liquid interface, a large gate-type impeller has been positioned to span the gas-liquid interface. Based on insights gained from the improvement of mass transfer in the large multi-stage impeller, a large gate-type impeller has been positioned to span the gas-liquid interface on a pre-existing impeller (three-stage MIG impellers). This is advantageous for existing agitation equipment as it reduces design and equipment installation time and cuts equipment investment costs.

As a result, the installation of a large-scale multi-stage impeller equipped with an extended gate-type impeller that passes through the free surface inside the reactor, and the addition of a large gate-type impeller to the existing impeller (the MIG impellers), have led to an improvement in the rate of mass transfer at the gas-liquid interface.

The large multi-stage impeller, developed by the authors, is designed for processing polymer products with medium viscosities in the range of 0.1 to 10 Pa·s [1]. The impeller (Figure 1) comprises a rotating shaft and three vertically-oriented impellers: one two-bladed large paddle impeller and a pair of two-bladed large gate-type impellers. The shaft is vertically installed at the center of a cylindrical mixing tank. These three impellers are attached at the lower, middle, and upper stages on the rotating shaft, respectively. The paddle impeller, attached at the lower stage on the rotating shaft, is also arranged close to the bottom surface of the mixing tank. The pair of gate-type impellers are attached at the middle and upper stages on the rotating shaft, respectively. The one positioned at the middle stage is arranged at a crossing angle of 45 degrees ahead in the direction of rotation of the lower stage paddle impeller. Furthermore, it overlaps vertically with the paddle impeller. The other one, positioned at the upper stage, is arranged in the same way as the middle stage one. These three impellers have the same diameter. This impeller has the advantages of relatively low power consumption when mixing medium-viscous liquids, fewer stagnant areas within the agitator tank, and fewer vortices perpendicular to the rotation direction of the impeller, resulting in axial circulation flow.

To verify the effects of mass transfer in large gate-type impellers positioned at the top, an evaluation of the mass transfer coefficients was conducted for two types of large gate-type impeller: those submerged in liquid and those spanning the gas-liquid interface.

On the other hand, the existing three-stage MIG impellers are arranged in the lower, middle, and upper sections of the tank, respectively, each at a crossing angle of 90 degrees to the rotation direction of the impellers. Building on the knowledge gained from improving mass transfer with the large multi-stage impeller, a large gate-type impeller was installed to span the gas-liquid interface above the existing impeller.

The gas-liquid mass transfer during the surface evaporation step in a batch operation to remove volatile residues under vacuum conditions is a phenomenon where the volatile residues move from the liquid phase to the gas phase. Here, rather than evaporating the volatile substance, the volumetric mass transfer coefficient (kLa) was obtained from the pressure history of the gas phase by dissolving the volatile substance into the polymer liquid. Using the mass transfer coefficients at the gas-liquid interface for the two types of impellers - the large multi-stage impeller and the existing impeller with the additional gate type impeller - we evaluated the characteristics of mass transfer for each agitation impeller. Furthermore, we also examined the dimensionless correlation equations for each agitation impeller.

This study enabled the evaluation of mass transfer enhancement effects by various stirring impellers using kLa against the volumetric power consumption (Pv) and dimensionless correlation formulas. From the relationship between Pv and kLa in the multistage impeller equipped with the extended gate-type impeller and the multistage impeller submerged in liquid, it was found that the kLa of the multistage impeller equipped with the gate-type impeller was significantly improved. In addition, the dimensionless correlation of mass transfer was compared in the range from the laminar flow region to the turbulent flow region. Further, by separating cases by Froude number (Fr), it was evaluated using dimensionless numbers regardless of tank dimensions and fluid viscosity.

On the other hand, in the case where the gate-type impeller was added to the existing impeller, it was found that the kLa against Pv was significantly improved by placing this gate-type impeller. Furthermore, by using the relationship of Sherwood number (Sh), Reynolds number (Re), Schmidt number (Sc), and Froude number (Fr), it was also possible to estimate kLa for a commercial-scale stirred tank based on small-scale experimental data at various viscosities.

These correlations enabled the estimation of mass transfer coefficients in the final stage of volatile component removal from polymers. Based on these achievements, this method was applied in the introduction of new equipment and the improvement of existing production equipment in processes dealing with viscous fluids where mass transfer at the gas-liquid interface becomes rate-limiting.

As a result, this multi-stage impeller and the addition of the gate-type impeller have been adopted in many industrial reactors and mixing tanks in the KANEKA Group, contributing to the improvement of productivity in the functional polymers manufacturing process.

Reference

[1] Y.Sumi, M. Kaminwano: J. Chem. Eng. Japan, Vol34, No.4, 485-492(2001)