Investigating the Effect of Reflux Ration on the Separation of Ethanol and Water

SUMMARY

A packed column distillation process experiment was conducted with the goal of to operate vapor – liquid separation process using a packed column distillation unit, to analyze the effect of reflux ratio on separation of ethanol and water system and to determine the number stages by using McCabe Thiele method. Liquid separation, analyzing the sample for the top and bottom product by refractometer to obtain the refractive index (RI) in order to determine their respective composition and also to obtain the time when the vapour

Liquid separation is nearly finish. In this experiment 30 litres of 10% v/v (volume/volume) mixture of Ethanol in water was prepared and filled in a reboiler vessel. The mixture was allowed to boiled until almost all component with low boiling point has been vaporized from the mixture. Temperature readings of all Temperature Indicator, samples from the Reboiler and the overhead from the Sampling Port were observed and recorded for every 5 minutes. The concentration of the samples drawn was measured using refractometer. After analyzing the data collected from this experiment, it was found that the refractive index obtained for top product is 1.3541 and for bottom product is 1.331 thus, the top product was Ethanol while the bottom product was water. The time obtained for the vapour.

Liquid separation is nearly finish was 30 minute because the temperature reading was almost constant at 20-30 minutes after boiled. This led to a conclusion that this method used for the separation of Ethanol-water mixture can be considered a reasonably reliable method. When the reflux ratio is raised to 2, the mole fraction of top and bottom product turns out to be and respectively. As we know, the number of stages that occur in the process of reflux with 1 ratio is 2 stages while the other produced 1 stages. We can see the difference of stages when different reflux ratio is used which means that reflux ratio plays a crucial role in separation process.

OBJECTIVE

To operate vapor and liquid separation process using a packed column distillation unit

To analyze the effect of reflux ratio on separation of ethanol and water system.

To determine the number of stages by using McCabe Thiele method.

INTRODUCTION

Distillation is an ancient unit operation and has been widely practiced for years.The idea of the distillation came out from the first application used crude vaporization and condensation equipment that usually used for concentrating the alcoholic content of beverages.This distillation process is also known as an effective separation technique for crude oil into various products(Henry,1992,p. 2).Thus,it has been improved through years and majority of chemical processes apply this application.Distillation separates two or more liquid components in a mixture using the principle of relative volatility or boiling points.The greater the relative volatility the greater the nonlinearity and more easier for the mixture to separate.Due to that difference in relative volatility or boiing points,it can be said the vapour is rich in light and the liquid is rich in heavy components.The distillation happen by heating the mixture until it boils in a still and removal of vapour from the still through condensation.Condensation is the process of a substance in gaseous state convert into liquid state by a change in temperature and pressure of the substance.

There are two types of distillation column which are continuous and batch.Continous distillation explains that the liquid mixture(feed) which is to be separated into its components is fed into the column at one or more points along the column.Liquid flows down due to gravity while vapour flows up the column.Before reaching the bottom of the column,liquids will be partially evaporated to produce vapour and the remaining liquid is withdrawn from the column as bottom product .At the top,the vapour will be partially or fully condensed.Part of the condensed liquid is refluxed to the column while the remainder is withdrawn as the distillate product.The column section above the feed tray rectifies the vapour stream with light components which terms as rectifying section while the column section below the feed tray strips heavy components from the vapour stream to the liquid streams is termed as stripping section(I.M.Mujtaba,2004,p.4).

Besides,batch distillation is known as the oldest operation used for separation of liquid mixtures where two liquids with different boiling points are heated,then vaporized and cooled to gain new product which one of the liquid has higher concentration than the other one. A batch distillation system might contain a heating vessel or still, a simple distillation column or a complex column with trays or packing, a liquid condenser that uses water or air cooling and the final product collection lines and storage. This process runs where the still heats the liquid mixture, which then travels up through the column to the condenser. Water typically is used to cool the condenser and some or all of the liquid condensate is permitted to fall back through the column which known as a technique of reflux. Batch systems that return all of the condensed liquid to the column are referred to as total reflux systems.

THEORY

This experiment has used packed column in the distillation process to enhance contact between vapour and liquid.The packing in the column is known as rachig rig glass or also called as raschig glass.Packing column is more preferable because it offers easy trade off between capacity and separation,economical to minimize pressure drop and has smaller liquid holdup of packed columns where the higher percentage of liquid can be ecovered as the top product(Henry,1992,p.518-519).Packing column are also used in gas absorption and liquid-liquid extraction.This distillation process involves the separation of vapour- liquid where the liquid that will be used are ethanol and deionized water. Separation of components from a liquid mixture via distillation depends on the differences in boiling points of the individual components and also depending on the concentrations of the components present which the liquid mixture will have different boiling point characteristics. Therefore, distillation processes depends on the vapour pressure characteristics of liquid mixtures.This distillation column also contains a condenser which located at the top of the distillation column and removes energy from the distillation column. The purpose of the condenser is to condense the vapor leaving the top tray of the column.A reboiler can be regarded as the heat exchanger that required enough energy to bring the liquid at the bottom column to boiling point.

The feed mixture which are ethanol and water contains of different volatilities or boiling points, and enters the column approximately at the middle.Theoritically,the boiling points of ethanol and water are 78.5ºC and 100ºC.Furthermore,the higher the relative volatility of the liquid the more easier for the liquid to separate.In this case,the relative volatility of ethanol is more than water,thus ethanol will evaporates first than water.In distillation process, the liquid flows downward through the packing, and the vapour flows upward through the column.Because there is a difference in concentration so the least volatile liquid will transfer from the vapour phase to liquid phase .The packing increases the time of contact, which increases the separation efficiency. The exiting vapour contains the most volatile components, while the liquid product stream contains the least volatile components. Distillation column also has many factors affecting its operation,one of those are reflux ratio. Reflux is a distillation technique involving the condensation of vapours and the return of this condensate to the system from which it originated. As mentioned, distillation columns are designed using VLE data for the mixtures to be separated. The vapour-liquid equilibrium characteristics which is the equilibrium line of the mixture will determine the number of stages and the number of trays required for the separation. This is illustrated clearly by applying theMcCabe-Thiele method to design a binary column.

Test tube Volume of Ethanol(ml) Mole of Ethanol (mol) Volume of Deionized-Water(ml) Refractive Index(RI) mole mole fraction of ethanol

A1 0 0 20 1.33 1.1111 0

A2 1 0.0172 19 1.332 1.0556 0.016

A3 2 0.0343 18 1.3341 1 0.0331

A4 3 0.0515 17 1.3362 0.9444 0.0517

A5 4 0.0686 16 1.3373 0.8889 0.0436

A6 5 0.0858 15 1.3405 0.8333 0.0602

A7 6 0.1029 14 1.3416 0.7778 0.0712

A8 7 0.12 13 1.3426 0.7222 0.1425

A9 8 0.1372 12 1.3437 0.6667 0.1707

A10 9 0.1544 11 1.3458 0.6111 0.2017

A11 10 0.1715 10 1.348 0.5556 0.2359

A12 11 0.1887 9 1.353 0.5 0.274

A13 12 0.2058 8 1.354 0.444 0.3167

A14 13 0.2239 7 1.355 0.3889 0.3654

A15 14 0.2391 6 1.356 0.3333 0.4177

A16 15 0.2565 5 1.357 0.2778 0.4801

A17 16 0.2739 4 1.358 0.2222 0.5521

A18 17 0.2913 3 1.358 0.1667 0.636

A19 18 0.3089 2 1.358 0.1111 0.7355

A20 19 0.3261 1 1.359 0.0556 0.8543

A21 20 0.3435 0 1.359 0 0

Table 1

Table of Results

Reflux ratio = 1.0

Rotameter reading R1 (L/hr) = 2

Rotameter reading R2 (L/hr) = 2

Temperature T4 (oC) = 98

Temperature T2 (oC) = 98

Time, t

(min) TOP PRODUCT BOTTOM PRODUCT

Refractive index (RI) Mole fraction Refractive index (RI) Mole fraction

0 1.35514 0.37 1.3362 0.06

5 1.35514 0.37 1.3320 0.02

10 1.35414 0.32 1.3310 0.01

15 1.35414 0.32 1.3310 0.01

20 1.35414 0.32 1.3310 0.01

Reflux ratio = 2.0

Rotameter reading R1 (L/hr) = 2

Rotameter reading R2 (L/hr) = 1

Temperature T4 (oC) = 101

Temperature T2 (oC) = 101

Time, t

(min) TOP PRODUCT BOTTOM PRODUCT

Refractive index (RI) Mole fraction Refractive index (RI) Mole fraction

0 1.35212 0.26 1.3300 0

5 1.35112 0.24 1.3300 0

10 1.35213 0.26 1.3300 0

15 1.35213 0.26 1.3300 0

DISCUSSION

Distillation is a method of separating mixtures based on differences in volatility of components in a boiling liquid mixture. It is one of the most common equipment in industry for vapour-liquid separation process. It can be run in a continuous or batch system. The compound that has higher volatility will vapourize and condensed before collected while the compound that has lower volatility will remain in the column.

Batch distillation plant was used in this experiment, it refer to a mixture is distilled to separate it into its component fractions before the distillation still is again charged with more mixture and the process is repeated. In this experiment, packed column distillation was used for vapour-liquid separation. It has pack tower contain layer of device called packing. The tower of the packed column has section filled with cylindrical ring known as raschig ring. The packing breaks up the liquid so that it flows over a large amount of surface area. This exposes more liquid to the vapor and increase heat transfer from the vapor to the liquid.

The mixture of ethanol and distilled water was used as the sample, it was prepared by mixing 3L of ethanol with 27 L of distilled water and then it was boiled. The temperature readings was observed and recorded for every 5 minutes after the equipment achieve thermal equilibrium. During record the temperature, the sample of bottom product and top product was taken and being analyzed using refractometer to obtain the refractive index of both samples.

The calibration curve of ethanol-water mixtures can be obtained by preparing 21 test tubes that will be filled with certain amount of ethanol and water. The total volume in each test tube is 20 mL. The mixture of solution will be stirred and ready to obtained its refractive index by using refractometer.

Based on the result it was found that, the refractive index for 21 test tubes were increased with volume of ethanol. From the calibration graph that has been plotted;refractive index on y-axis against mole fraction of ethanol on the x-axis, the mole fraction of ethanol for the top and bottom product for each reflux can be obtained.

For a given separation, increasing the reflux ratio will increase the gradient of the rectifying operating line and move its intercept with the y-axis downwards, moving it away from the equilibrium line and therefore decreasing the number of stages required to achieve the separation. Alternatively for a fixed number of stages (real column) adjusting the reflux ratio will control the purity of the distillate, the zhigher the reflux ratio the purer the distillate.

It can It also show that water has lower volatility because it remain in the column while Ethanol has high volatility due to it vapourized and condensed. For the refractive index obtained the constant reading for reflux ratio 1.0, the top product is 1.35414 and for bottom product is 1.331. It also found that the top product was Ethanol while the bottom product was water. The time obtained for the vapour-liquid separation nearly finish was 20 minute because the refractive index number was almost constant at minute 15-25. For the reflux ratio 2.0, the top product is 1.35213 and for bottom is 1.3300 and nearly finish at 15 minute because the number of refractive index constant at minute 5-15.

In order to minimize the error to improve during the experiment. , we must make sure not to spill the water as it will reduce the exact amount of the substance thus affecting the result obtained later on during the transfer process of mixture ethanol-deionized water into the reboiler vessel. Next, we should always observe the reflux ratio valve to ensure that the reflux ratio is maintained at its given values which will then contribute to accuracy of the amount of the top product produced. After that, we must always observe the temperature of the reboiler to prevent the distillation to occur at faster rate. Finally, in order to reduce parallax, we must make sure the eye level is perpendicular to the reading during recording the temperature and the rotameter readings.

Calculation for feed line molar fraction, XF

Total volume of 10% ethanol water mixture = 30L

Ethanol

Volume ethanol = 10/100 x 30L = 3L = 0.003m3

Molecular weight ethanol = 0.046 kg/mol

ρ of ethanol = 789 kg/m

3

n = mass/MW = ρv/MW

= 789 kg/m

3 x 0.003m3 x (1 mol)/(46 kg/mol)

= 0.0515 mol

Volume ethanol = 90/100 x 30L = 27L = 0.027m3

Water

Molecular weight water = 18 kg/mol

ρ of water = 1000 kg/m

3

n = mass/MW = ρv/MW

= 1000 kg/m

3 x 0.027m3 x (1 mol)/(18 kg/mol)

= 1.5 mol

Mole fraction water feed, XFA = 1.5/(1.5+0.0515)

=0.967 mol %

Mole fraction ethanol feed, XFB = 0.0515/(1.5+0.0515)

=0.033 mol %

XDA = 0.32 mol %

XDB = 0.68 mol %

XFA = 0.033mol %

XFB = 0.967 mol %

XBA = 0.01 mol %

XBB = 0.99 mol %

OVERALL MATERIAL BALANCE

F = D + B

2 L/hr = D +B

B = 2 L/hr – D ----------- (1)

COMPONENT MATERIAL BALANCE

FXF = DXD + BXB

Ethanol

2(0.033) = D (0.32) + B (0.01) --------- (2)

Subs (1) into (2)

0.066 = D (0.32) + (2– D) (0.01)

0.066 = D (0.32) + 2(0.01) – 0.01D

D = 0.1484 L/hr

Insert D into (1)

B = 2 L/hr – 0.1484 L/hr

B = 1.8516 L/hr

B = R = Ln/D

Ln = RD

Ln = 1(0.1484 L/hr)

Ln = 0.1484 L/hr

Vn+1 = Ln + D

Vn+1 = 0.1484 L/hr+ 0.1484 L/hr

Vn+1 = 0.2968 L/hr

Conclusion

In this experiment, we found that the mole fraction for both top and bottom product for reflux with the ratio of 1 was 0.32 and 0.01 respectively. When the reflux ratio is raised to 2, the mole fraction of top and bottom product turns out to be 0.26 and 0.00 respectively. As we know, the number of stages that occur in the process of reflux with 1 ratio is 2 stages while the other only 1 produced stages. We can see the difference of stages when different reflux ratio is used which means that reflux ratio plays a crucial role in separation process. In conclusion, when reflux ratio is high, the stages obtained will be less. Less stages means that the mixture is easier to separate. All in all, the three objectives of this experiment are said to be successfully achieved

Recommendation

A few of recommendations have been suggested in order to minimize the error from occuring during the experiment. The first recommendation to reduce the errors in this experiment is during the transfer process of mixture ethanol-deionized water into the reboiler vessel, we must make sure not to spill the water as it will reduce the exact amount of the substance thus affecting the result obtained later on. Next, we should always observe the reflux ratio valve to ensure that the reflux ratio is maintained at its given values which will then contribute to accuracy of the amount of the top product produced. After that, we must always observe the temperature of the reboiler to prevent the distillation to occur at faster rate. Finally, in order to reduce parallax, we must make sure the eye level is perpendicular to the reading during recording the temperature and the rotameter readings.