Distillation Column Lab Experiment 5g2o58

Thomas Kolaczkowski Experiment 1: Distillation Column Introduction: Distillation columns are one of the main units used for separation processes in industry. The basic theory behind them is very simple and relies on a mixture being made of components of different boiling points. As a feed enters the column and some fractions may vaporise and rise up the tower. The vapour components will condense and leave the column at different levels as the temperature decreases up the tower. Based on a binary mixture, the more volatile component (mvc) will come out at the top of the tower, and the less volatile component (lvc) will leave at the bottom as a liquid. The lvc will have a higher boiling point so it will be a liquid in the column. This report will investigate the pressure drop over the distillation column for various boil-up rates and the amount of theoretical plates and overall column efficiency will be determined. Background Theory: Distillation columns are usually made up of a vertical tower containing a series of plates. As liquid runs down the tower vapour will be rising. The plates allow for an increase in the vapour/liquid interface, which leads to better separation. In this experiment the distillation column will be used to separate a binary mixture. In most cases in industry distillation columns are set to run continuously and at steady state so new feedstock is added as product is removed at the same rate. The column in this experiment will be set to run at total reflux. This means that all the top products are will be condensed and fed back into the top of the tower and allowed to flow down the column, so overall no top product is taken out of the system. Reflux is the term used to express the amount of top product, which is put back into the tower. The reflux ratio is the ratio of the amount of moles returned as refluxed liquid to the fractionating column and the amount of moles of final product, both per unit time. If the system is running at total reflux and the relative volatility is constant, the theoretical number of plates may be calculated using Fenske’s formula:

The relative volatility of a binary system compares the vapour pressures of the two components and can be written as follows:

The average relative volatility, which is used in the Fenske’s formula, can be found by using:

In an ideal column the number of plates needed to separate the binary system is found by using the Fenske’s formula, however, in practise more plates would be required and the efficiency of a column can be calculated by using the following relationship:

Method: An 8-plate distillation column was used to carry out the separation process of the ethanol/water mixture. The column was set up by the demonstrator and was left to warm up. The reflux pump was set so the column would run at total reflux. A digital wattmeter was used to control the amount of power went to heating up the tower. To warm the tower up initially the power was set to 1.5kW, once a high temperature was reached the power was reduced to 0.75kW and the experiment could be conducted. A manometer was used to measure the pressure drop from the top to the bottom of the tower. This was a vertical manometer part filled with water. The boil-up rate was taken by recording the time taken for the distillate flow to fill a beaker of known volume. The distillate flow could be tapped from a valve on the stream. Samples of the distillate and the distillation bottoms were obtained from valves and were analysed using a GC/ Refractometer to find the refractive index of the mixture. From this, the volume percent of ethanol in the mixture could be calculated. This method was used to obtain results for when the distillation column was running at three different powers. Results: From the experiment itself the following data was found: Power / kW 0.75 1.00 1.25

Volume from distillate stream / cm3 5 20 24

Time taken to acquire volume / s 17 6 8

Refractive Index of distillate 1.36658 1.36395 1.36493

Refractive Index of bottoms 1.34748 1.34732 1.34765

Pressure drop / cmH2O 6.6 18.4 11.5

The refractive index value can be converted into the ethanol volume percentage by using the following: Ethanol vol% = (Refractive Index – 1.3353) / 0.0005 For example the ethanol vol% obtained from the distillate at power 0.75 kW would be: (1.36658 – 1.3353) / 0.0005 = 62.56 vol% The boil-up / lhr-1 rate can be found as shown in the example below for a power of 0.75 kW: Boil-up rate = Vol / time = (5 / 17) x 3.6 = 1.06 lhr-1 The pressure drop was recorded in cmH2O. This can be converted using the equation: Pressure = density x height x gravity For example, using the results for power setting 0.75 kW: Pressure drop = 1000 x 0.066 x 9.81 = 647.5 Pa This data is tabulated below:

Power / kW 0.75 1.00 1.25

Pressure drop / Pa 647.5 1805 1128

Ethanol vol% distillate 62.56 57.30 59.26

Ethanol vol% bottoms 24.36 24.24 24.70

Boil up rate / lhr-1 1.06 12 10.8

The log/log graph is plotted below showing the pressure drop versus the boil-up rate.

log Pre ssure drop / cm wate r

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

log Boil up rate (litre per hour)

The results from the Refractometer can be converted to from vol% to mol fraction using the method shown below in an example for the distillate at the power 0.75 kW: 62.56 vol% ethanol, 37.44 vol% water. Assume % are volumes 62.56 ml E and 37.44 ml W The mass of each component is equal to its volume times its density (m = DV). Where the density of ethanol is 0.79 gml-1 and water is 1 gml-1. So mass of ethanol is 0.79 x 62.56 = 49.42g and the mass of water = 37.44g % by Mass = ( g ethanol / g ethanol + g water ) x 100% = 56.90 mass % ethanol and therefore, 43.10 mass % water. Assume mass % are masses. Now the conversion from mass % to mole fraction using the molar masses. Molar mass of ethanol is 46 gmol-1 and water is 18 gmol-1. Using the equation moles = mass / molar mass: 56.90 / 46 = 1.24 mol of ethanol 43.10 / 18 = 2.40 mol of water mole fraction of ethanol = 1.24 / (1.24 + 2.40) = 0.34 This method was used to convert all the vol% to mole fractions:

Power / kW 0.75 1.00 1.25

Ethanol vol% distillate 62.56 57.30 59.26

Ethanol mole fraction distillate 0.34 0.29 0.31

Ethanol vol% bottoms 24.36 24.24 24.70

Ethanol mole fraction bottoms 0.09 0.09 0.09

Water mole fraction bottoms 0.91 0.91 0.91

Ethanol mole fraction bottoms 0.09 0.09 0.09

Below is a table showing the mole fractions: Power / kW 0.75 1.00 1.25

Water mole fraction distillate 0.66 0.71 0.69

Ethanol mole fraction distillate 0.34 0.29 0.31

Using a vapour liquid diagram the values for xA, xB, yA, yB can be obtained. Where x and y represent the mole fraction in liquid and gas stages respectively. And the subscripts A and B show the MVC and the LVC respectively. This table shows the results for the distillate: xA 0.34 0.29 0.31

yA 0.58 0.56 0.59

xB 0.66 0.71 0.69

yB 0.42 0.44 0.41

xB 0.91 0.91 0.91

yB 0.57 0.57 0.57

This tanle shows the results for the bottoms xA 0.09 0.09 0.09

yA 0.43 0.43 0.43

Using the formula : (ya/xa)/(yb/xb) = alpha The relative volatilities can be calculated: (alpha)distillate = 3.00 (alpha)bottoms = 7.62 The average relative volatility is calculated by square rooting the product of the two above answers: = 4.78 Using the Fenske’s formula the number of theoretical plates is –1 (rounding up as whole plates are required) So the efficiency of the of the tower is: -1/8 x 100 = -12.5%

Conclusion: From the results it is clear there is an increasing pressure drop as the boil-up rate in increased. This would be caused by the energy loses through increased thermal radiation and frictional energy with an increased vapour velocity through the tower. There was a large error in one of the results for the pressure drop. This may have been caused by a fluctuation in the wattmeter, which would lead to a warmer tower, or a misreading of the manometer. As experiments were only conducted once with no repeats for each power setting, it is likely that the results would not be very accurate. The experiment should have been conducted with four different power settings, this report only covers the usage of three power setting. It was not possible to complete all four due to running out of distillate as too much was taken out at a time for analysis. With even one extra data point, the boil-up/ pressure drop graph would have been a lot more accurate. References: Perry’s Chemical Engineering Handbook 7th ed. 13-12 Physical Chemistry – Peter Atkins Chemical Engineering Vol 1 – Coulson and Richardson Chemical Engineering Laboratories Handbook 08-09