2. The efficient regeneration technology of waste solvent containing VOC by “vacuum evaporation with air flow” method

As a conventional method of the evaporation of IPA from the waste solvent, the vacuum evaporation and the distillation with heating were generally used. However, the distillation with heating has a problem which consumes a lot of energy and cost, and it cannot be applicable for flammable solvents. The usual vacuum evaporation has a problem of a small treatment amount.

In this study, we have developed “vacuum evaporation with air flow” method to solve these problems. This method is leaking air into the vacuum vessel and efficiently releases IPA which is evaporated from waste solvent. This method can perform higher evaporation of IPA from waste solvent, lighter equipment and cost reduction. Because this method need not operate at high vacuum condition compared with the usual vacuum evaporation.

The removal solvent (TPGME) containing IPA was sprayed to vacuum vessel at flow rate of 0.4L/min, and then IPA was rapidly evaporated from TPGME. The recovery efficiency of IPA from TPGME was 94.2% at 44.6°C of evaporation temperature. It was found that regeneration of TPGME in real time can be achieved by “vacuum evaporation with air flow” method. For the cold condensation of IPA which was released from the vacuum pump, the exhaust gas was introduced into the cold trap which was kept at a temperature of -60°C. 99.0% of IPA in the exhaust gas was condensed and recovered. The concentration of IPA at the outlet of the cold trap was 101ppm, so that less than 200ppm of the acceptable concentration of IPA could be confirmed.

TEGDME was the best removal solvent for VOCC such as DCM and TCE due to its high solubility. The regeneration experiment of TEGDME as a removal solvent containing DCM and TCE was carried out. The recovery efficiencies of DCM from TEGDME were 80.2% at $ 40.8^{\circ} $ C and 64.5% at $ 24.6^{\circ} $ C and that of TCE from TEGDME were 74.6% at $ 58.5^{\circ} $ C and 50.3% at $ 26.9^{\circ} $ C in the case of spraying flow rate of 0.4L/min. The regeneration experiment of TPGME which is one quarter the price instead of TEGDME containing DCM and TCE was also carried out. The recovery efficiencies of DCM from TPGME was 75.3% at $ 25.3^{\circ} $ C and that of TCE from TPGME were 51.4% at $ 25.3^{\circ} $ C in the case of spraying flow rate of 0.4L/min.

For the cold condensation of DCM and TCE which was released from the vacuum pump, the exhaust gas was introduced into the cold trap which was kept at a temperature of $ -97.3^{\circ}C $ . More than 99% of DCM and TCE in the exhaust gas were condensed and recovered. However, concentrations of DCM and TCE at the outlet of the cold trap were 75ppm and 18ppm, respectively. These concentrations exceeded the acceptable concentration of 50 ppm for TCM and 10ppm for TCE. In order to achieve the acceptable concentrations of DCM and TCE, the hybrid type of cooling tube was developed. The developed cooling tube was the usual cooling tube of copper coil which was connected to the straight tube packed with cupper wool as a condensation nucleus. The concentrations of DCM and TCE at the outlet of the hybrid type of cooling tube at the temperature of $ -97.2^{\circ}C $ were 25ppm and 2ppm, respectively. The cold condensation of DCM and TCE by the hybrid type of cooling tube could decrease lower than their acceptable concentrations.

Finally, on the basis of the above research results, the efficient regeneration equipment of waste solvent containing VOC and the recovery equipment for the evaporated VOC from waste solvent by cold condensation were produced for the development of the real product by cooperative research with ANEST IWATA corp. and TOSEI corp. The developed equipment can regenerate waste solvent containing VOC with treatment ability of 120L/h and recovery the evaporated VOC.