I had some concentrated sulfuric acids of the flask. The distillation process is very difficult. Because of the high pressure, it took me 8 hours. Even if I use a saturated potassium, Hydroxide solution to prevent sulfuric acid vapor from fucking the vacuum. Sulfuric acid is slightly boiling. But vapor hardly goes through the condenser. So I want to give up. I open the valve and let air to flow to the flask.