Azeotropic distillation of isopropanol column
Thread Content
The company’s production process generates the following boiling water composition: 13.5% isopropyl alcohol, 82.5% water, and 4% other components. It definitely cannot be released due to environmental protection requirements. So, it is planned to use it for processing his equipment, while recovering isopropyl alcohol at the same time. The design team provided us with a equipment flow diagram and simple operating procedures. Azeotropic distillation was carried out by adding benzene as an azeotrope. The feeding method is intermittent feeding at the bottom of the tower. After separation in the phase separator, the overhead distillate flows back into the tower through the top, while the heavy phase is discharged. My understanding is that an azeotropic system is formed using benzene and water; water and benzene are carried out together via azeotropy, separated in a phase separator, with the benzene returning to the tower while water is continuously removed. After all the moisture in the bottom of the tower has been removed, isopropyl alcohol is distilled off and recovered. However, when I simulated this process in the laboratory, I found the following: the temperature at the bottom of the tower was 103°C, and the temperature at the top of the tower was 71.6°C. The liquid distilled from the top of the tower only shows slight stratification at the beginning. There are no more layers afterward. It is a homogeneous, transparent, and clear liquid. As I understand it, it should be benzene. What about water? I’m using a flask with a capacity of 1000, and there is also 700 worth of wastewater inside the flask. But only 5 ml of the heavy phase was obtained through fractional distillation. What went wrong?
This mixture is first distilled to obtain an azeotrope of isopropyl alcohol and water, and then toluene is added to remove water from this azeotrope
The isopropanol, water, and benzene system can form various binary azeotropes; their boiling points at atmospheric pressure (101.3 kPa) are as follows: benzene-water 69.3℃； Benzene-isopropanol 71.9℃ ； Isopropanol-water: 80.4°C. The azeotempers of the first two substances are very close to each other; therefore, if the number of theoretical plates in the distillation apparatus used in the laboratory for simulating this process is not very high, then both the benzene-water azeotrope and the benzene-isopropanol azeotrope will vaporize simultaneously from the top of the tower. As a result, the initial vapor stream may contain a higher proportion of the benzene-water azeotrope, leading to stratification. Furthermore, the isopropanol, water, and toluene system can also form various binary azeotropes; their boiling points at atmospheric pressure (101.3 kPaA) are as follows: toluene-water 84.1℃ ； Toluene-isopropanol 81.3℃ ； Isopropanol-water at 80.4°C; therefore, neither benzene nor toluene is suitable as an azeotrope component for the isopropanol-water system. The wastewater to be treated contains a relatively low amount of isopropyl alcohol; the energy consumption for distilling off the azeotropic mixture is high. It is best to leave the water at the bottom of the tower and use an azeotrope to evaporate the isopropyl alcohol from the top of the tower. It is recommended to use n-hexane as the azeotrope-forming agent, as the azeotopic point of n-hexane and isopropanol is 62.7°C.
It is necessary to conduct experimental tests to determine whether the n-hexane-isopropanol mixture will separate into layers; if it does not separate, then n-hexane cannot be used as an azeotrope-forming agent. It seems that it is difficult to choose an azeotrope, and other methods such as liquid-phase extraction need to be considered.