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Distillation

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EXPERIMENT 3
GROUP B
AIM:
To find the overall efficiency of plate column operating under total reflux, for a binary system (iso-propanol & n- propanol).
APPARATUS:
* Distillation Column with reboiler * Automatic digital refractometer * Two fluids with different volatilities.
PROCEDURE:
* Distillation column was filled with the two fluids. * Then we switched on the heater and set the temperature to 126.6 ̊C * And then we allow the system to reach the equilibrium. * The equilibrium was tested by taking out the samples (from reboiler & distillate) at the interval of 10min such that the refractive index hence concentration was made constant.
THEORY:
Distillation is one of the most common separation techniques used in the chemical and petroleum industries. It is defined as “a process in which a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat. Distillation provides the basis for separating numerous substances into valuable components. The components of a mixture are separated based on relative volatilities, which depend on the differences in boiling temperatures of the components making up the mixture. The separation becomes easier as the difference between the relative volatilities increases. The component with the lowest boiling point flows to the top of the column as a vapour, condensed and exits the column as the distillate. A percentage of the condensate is returned to the column as the reflux. The component with the higher boiling point flows down the column and exits as the liquid bottoms product.
Distillation is based on the principle of counter-diffusion (NA= - x*NB). In this process the fluid with higher volatility goes from liquid to vapor while the fluid with lesser volatility is being transferred from vapor to liquid stream. Hence the vapor become rich in more volatile component and liquid state become rich in less volatile component. The amount of separation also depends upon the reflux ratio i.e. the ratio of amount of distillate put back into the system and the amount of distillate taken as product. Maximum separation occurs for total reflux condition where we put back all the distillate into the system and nothing is withdrawn as product.
A typical distillation column with total reflux condition:

Important processes involved in distillation: 1. Enriching: Vapor rising is washed with the liquid that is being recycled. The liquid coming from the condenser is leaner, less volatile as compared to the vapor rising. This causes mass transfer of less volatile component from the vapor to the liquid phase and vapor phase becomes richer in more volatile component.

2. Stripping:
The liquid is being stripped off of more volatile component by the vapor produced at the bottom. Liquid (coming from condenser) has more volatile component and hence is stripped off by the vapor. Thus liquid now will contain less volatile component in more quantity. 3. Reflux Ratio:
Reflux ratio is the ratio of the amount of the condensed vapor which is recycled back to the column to the amount of distillate withdrawn. The reflux ratio decides how far or close the operating line is from the equilibrium curve. As the reflux ratio increases, the operating line moves away from the equilibrium curve and number of stages decreases. a. Minimum Reflux Ratio:
The minimum reflux ratio corresponds to the pinch point, i.e., the operating line and equilibrium curve cut each other. At minimum reflux, the operating line should be touching equilibrium line. The number of stages is the maximum (theoretically infinite) when reflux is minimum. b. Total Reflux Ratio:
At total reflux, minimum number of trays is required. This happens when the entire condensate is recycled back to the system and the complete liquid is reboiled (no product withdrawn). Minimum number of stages results when the operating line is at 45̊ diagonal line and in this case reflux ratio is infinite.
The total numbers of theoretical plates were calculated by following procedures: 1. We got the plot of equilibrium line by the help of equilibrium data available. 2. Then for operating line (total reflux) we put y=x diagonal line. 3. For drawing the number of stages we started from distillate point then moved vertically and touched the equilibrium curve and then moved horizontally to touch the diagonal line. 4. We repeated the same steps as we reached the residue point.

For calculation of overall efficiency:
Overall Efficiency = Number of theoretical stages Number of actual stagesX 100
OBSERVATIONS:
Temp of reboiler = 95.7 ̊ C
Temp of ditillate = 89. 8 ̊ C Refractive Index of Distillate = 1.3754
Refractive Index of Residue = 1.3782
CALCULATION:
For distillate we have µ=1.3754 and coresseponding to this we get y (mole fraction of more volatile component) = 0.657
For residue µ=1.3782
Hence, x (mole fraction of more volatile component in residue) = 0.257
Hence, total number of theoretical stages ≈ 3.03
Number of actual stages = 3 + 1 (reboiler as a stage) = 4
Overall Efficiency = Number of theoretical stages Number of actual stagesX 100 = 3.034X 100 = 75.75%
RESULT:
Total number of theoretical stage was found to be 3.03 and the overall efficiency was 75.75%.
CONCLUSION:
From the calculated number of theoretical stages we can see that total number of theoretical stages is less than actual number of actual stages as in real life we can’t reach to the equilibrium curve means equilibrium is not attained in the process. This is mainly because the interaction of liquid and vapor on the plates were not sufficient and hence the transfer of components from liquid to vapor and vapor to liquid was not efficient and as a result we got only partial transfer of components. While for calculation we assumed that for every stage we reached equilibrium.
The temperature difference between top (distillate) of the column and bottom (reboiler) of the column is the key thing that we could see at the time of experiment as a measure of separation. As this difference here was 5.9 °C and we got significant separation.

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