超臨界二酸化炭素によるポリ酢酸ビニルおよびポリスチレン中からのベンゼンの抽出

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超臨界二酸化炭素を抽剤とし, ポリ酢酸ビニルおよびポリスチレンの二種類のポリマー中からのベンゼンの分離実験を行い, 超臨界流体抽出法の適用性を検討した。従来から行われている代表的な分離法である真空ストリッピング法に比べて, 超臨界流体抽出法では, 比較的低い操作温度下で短時間に低濃度まで, ポリマー中よりベンゼンを除去することが可能であった。また, この抽出実験データを単純な物質移動モデルに基づいて整理したところ, ベンゼンの物質移動速度の増進の原因は, ポリマー中へ二酸化炭素が溶解し, ポリマー中のベンゼンの拡散係数が増大したことによるものであることがわかった。In order to test the applicability of the supercritical fluid extraction technique to the separation of impurities in polymers, separation of benzene from two polymers of poly(vinyl acetate) and polystyrene was carried out using supercritical carbon dioxide.Figure 1 shows a schematic diagram of the supercritical fluid extraction apparatus. It consists of the following sections: (1) compression of carbon dioxide, (2) extraction, and (3) control and measurement of carbon dioxide flow rates. A sample polymer disk of known dimensions which had dissolved a known quantity of benzene, was placed in the extraction cell. The concentration of benzene in the sample polymer before and after extraction was determined using a gas chromatograph. Table 1 gives the conditions used in the experiments.Vacuum stripping (a conventional separation method) was performed along with the supercritical fluid extraction method for the benzene+poly(vinyl acetate) system. It appeared that the supercritical fluid extraction method was able at near room temperature to remove benzene more rapidly and to lower concentrations than vacuum stripping, as shown in Figures 2 and 3.The flow rate of carbon dioxide had but slight influence, under the conditions used, on the experimental results. Hence, if the mass transfer resistance of benzene is separated into two parts, i.e., one in polymer phase and the other in the supercritical carbon dioxide phase, it is evident that the mass transfer of benzene is controlled by the polymer phase (Figs. 4, 5).Extraction experiments were carried out varying initial concentration of benzene in poly(vinyl acetate); the results seemed but little affected by varying the initial concentration (Fig. 6).Extraction experiments were also carried out with samples of different thicknesses in the poly(vinyl acetate) system. After 55 minutes the yield of extraction increased up to 99.8% at 313K and 7.95MPa when the sample thickness was 0.5mm. However, the yield decreased with increasing sample thickness (Figs. 7, 8).The experimental results after 1h at temperature range of 300-373K and pressures of 7.95MPa and 14.8MPa are shown in Figure 9. The rate of extraction of poly(vinyl acetate) system was much greater than that of the polystyrene system. As for the effect of pressure, extraction at 14.8MPa could be performed with a greater rate than at 7.95MPa; on the other hand, the effect of temperature seemed rather complex. Yields of extraction of the poly(vinyl acetate) system at 7.95MPa and 14.8MPa increased with increasing temperature. The experimental results of the polystyrene system at 14.8MPa showed a tendency similar to that of the poly(vinyl acetate) system. However, the experimental results of the polystyrene system exhibited a minimum at 343K and at 7.95MPa.The diffusion coefficient of benzene in polymers in the presence of supercritical carbon dioxide was evaluated from the experimental data by using a simple mass transfer model (Figs. 10, 12). The calculated diffusion coefficient of benzene in poly(vinyl acetate) was found to be about midway between the value in supercritical carbon dioxide and that in poly(vinyl acetate) in the absence of supercritical carbon dioxide (Fig. 11). The calculated diffusion coefficient was more than 6 orders larger than that in poly(vinyl acetate) in the absence of supercritical carbon dioxide. This was the consequence of dissolution of supercritical carbon dioxide into the polymer.
石油学会の論文
1989-03-01

超臨界二酸化炭素によるポリ酢酸ビニルおよびポリスチレン中からのベンゼンの抽出

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