Fractional Distillation Report Example

Introduction

Fractional distillation is a method used to separate miscible liquids having different boiling points. The liquids dissolve on one another, therefore, the process involves repeated condensation and distillation (Sorensen and Gorak 101). The equipment used in fractional distillation process include fractionating column, distilling flask, the source of heat, thermometer, condenser, distillation adapter and the receiving flask.

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A fractionating column is a long glass tube having a wide bore, filled with either glass beads, small stones or porcelain rings. The function of the glass beads is to improve the process of distillation. The column can be blown to form shapes with numerous pear bulbs or spheres. The column is always insulated to allow the constant temperature in the column (Sorensen and Gorak). Fractional distillation for commercial purpose use theoretical plates instead of glass beads and they function similarly to the glass beads. Fractionating column core function is to increase the surface area for liquid cooling and to create a barrier to the vapor rising and the descending liquid. The distillation flask is round-bottomed shaped and used to hold the distilled liquid. The flask should be half or two-thirds filled to prevent liquid foaming in the receiving flask before vaporizing (Sorensen and Gorak 117). The silicon carbide chips or porous clay plate are added to the distillate before heating to allow even boiling of the distillate. The distillation adapter also is known as distillation head act as a connection between the distillation flask, thermometer, and the condenser. The glass joints need to be tightly mated and lined up to prevent vaporized liquid leaking. Thermometer measures the temperature of the boiling liquid the distillation flask. The thermometer needs to be positioned below the adapter and vapors from the liquid need to surround and be in contact with the thermometer for accurate readings (Perini, Silvestre and Agostini). The condenser is used to cool the vapor so as to dissolve into liquid and directed to the receiving flask. The condenser is connected to water supply water in and out with rubber hoses. The receiving flask receives the liquefied vapor, the flask may be a bottle, round-bottom flask, Erlenmeyer flask or graduated cylinder. The receiver connects to the condenser through an adapter which directs the liquid to the receiving flask. The heat source is used to heat the liquid in the distillation flask until its boiling points (Sorensen and Gorak 98). Bunsen burner is mostly used in laboratories other forms of heat can be used in industrial distillation.

Analysis of Fractional Distillation Columns

Heat Transfer

The efficiency of distillation columns and other separation systems are normally determined based on the first law of thermodynamics. However, the inability to transform thermal power completely into work makes the accounting of thermal energy a necessity (Demire 3901). The rate of producing heat is positive and determined as a factor of the fluxes and thermodynamic pressures acting within the system. The general equation for the balance of energy is

H = QR + QC (1)

Where QR is positive coefficient and QC is a negative coefficient

Depending on the provided information, the balances between the reboiler and the condenser can augment the equation.

On the rectifier section, the heat transfer equation is given by

H = QC(2)

On the stripping section, the transfer equation takes the form

H = QR(3)

Mass Transfer

According to Tiwari and Tripathi (165), the pillar of every separation process is based on the principle of mass transfer between phases in equilibrium as described by Raoult's law, which gives

yip = xiP S i (T) (4)

Where yi - mole fraction of component i in the vapor phase, xi- mole fraction of component i in the liquid phase, p- total pressure and P S i (T)- vapor pressure of component i, that is temperature dependent. In the case of the vapor pressure of component I, the empirical correlation below is used. With Ai, Bi, and Ci are constants, T, being the temperature in oC and PiS the vapor pressure in mmHg (Demire).

log10 (P i S (T)) = Ai Bi /T + Ci(5)

From the figure above representing the distillation column the coefficients F, D and B refer to the flow rates in the feed, distillate, and bottoms. Li and Vi are the internal fluid and vapor streams with i representing the number of stages the streams are originating. An overall material balance can thereby be derived from the column where xi is the lighter mixture component in the separator.

F = D + B

F zF = DxD + BxB (6)

The reboiler and condenser equations can also be derived from the main equation (6) above. With equation (7) being the condenser equation and (8) being the reboiler equation.

V1 = L0 + D

V1y1 = L0xD + DxD

V1H1 + Qc = L0h0 + DhD

y1 = f(xD)(7)

Ln = Vn+1 + B

Lnxn = Vn+1yn+1 + BxB

Lnhn = Vn+1 Hn+1 + BhB

y1 = f(xD)(8)

The above analysis can be repeated for each stage in the stripping (4) and rectifying (5) stage of a distillation colomn as below

V 'k + 1 = L 'k

V 'k + 1yk+1 = L' k xk

V 'k + 1Hk+1 = L' k hk

yk+1 = f(xk)(9)

Vj = Lj1

Vjyj = Lj1xj1

VjHj = Lj1hj1

yj = f(xj1)(10)

The analysis as above is exceptional and does not hold for the feed stage in cases where the feed has to be included in determining the balances (Tiwari and Tripathi 168). From equation (9) and (10), it is notable that the liquid and vapor flow rates vary from one stage to another. By assuming the flow rates of the liquid and vapor in one stage of the column to be constant, it is easier to derive the working lies of the column. Where V and L are rectifier section constants and L' and V' are condenser constants.

y = L/Vx + (1-L/V)xD(11)

y = L'/V'x + (1- l'/ V')xB(12)

Applications of Fractional Distillation

Crude Oil Separation

Fractional distillation is used to separate numerous crude oil components such as diesel oil, gasoline, paraffin, lubricating oil, fuel oil, kerosene, and naphtha. Gasoline is used as petrol for car fueling, naphtha is used to manufacture various chemicals, kerosene or paraffin fuel oil stoves and jet engines. Diesel oil is used for fueling diesel engines while fuel oil is essential for ship fueling and used in home heating (Yost 25). The residue from the crude oil distillation is necessary for making bitumen, which is used for road surfacing.

Water Purification

Natural source water contains various impurities and minerals which can be removed by fractional distillation. Use of distilled water is prevalent in circumstances where certain equipment efficiency are reduced by minerals present in natural water (Sorensen and Gorak 111). Some individuals drink distilled water since they are avoiding minerals present in tap water or due to its nice taste. Distilled water is often used by most parents in preparation of infant formula. Also, desalination plants employ fractional distillation technique in turning seawater into clean drinking water.

Alcoholic Beverages

The fractional distillation process is used in the production of various alcoholic beverages like brandy, whiskey, and rum. As plants and fruit substances ferment, dilute component of ethyl alcohol is formed. Separating the fermented substance purifies and produces a concentrated ethanol. Various components such as esters, water, and other alcohol types are collected using fractional distillation process bringing unique flavors in different alcoholic spirits (Perini, Silvestre and Agostini 1398).

Separation of Pure Gases From Mixture Of Gases

Fractional distillation is used to separate oxygen and nitrogen gases and other gases from the air. The air is cooled to -200 degrees Celsius hence becoming liquid. Water vapor condenses and removed using filters, freezing of carbon dioxide occurs at 79 degrees Celsius and the remaining air contains oxygen and nitrogen (Yost 12). The fractional distillation process is used to separate the gases. The liquefied air is passed to the bottom of the fractionating column where liquid nitrogen boils and gaseous nitrogen rise upwards hence collected. Liquid oxygen remains at the bottom of the column and is separated with argon using another fractionating column since their boiling points are close.

Separation of A Mixture Of Acetone And Water

Acetone is a volatile liquid having a boiling point of 56 degrees Celsius while water is non-volatile liquid it boils at 100 degrees Celsius. The solution can be separated using fractional distillation process considering differences in their boiling points (Yost 17). Acetone vapor distills out at 56 degrees Celsius faster than water, which is then cooled in the condenser and collected in the receiving flask. Water is then distilled out at 100 degree Celsius and collected in a different receiving flask.

Works Cited

Demire, Y. "Thermodynamic Analysis Of Separation Systems." Papers in Thermal Mechanics (2004): 3898-4024.

Tiwari, G. and Rajesh Tripathi. "Study of heat and mass transfer in indoor conditions for distillation." Desalination Volume 154, Issue 2 (2003): 161-169.

Perini, J. F., et al. "Fractioning of Orange (Citrus sinensis L.) Essential Oil Using Vacuum Fractional Distillation." Separation Science and Technology (2017): 1397-1403.

Sorensen, Eva, and Andrzej Gorak. "Distillation Fundamentals and Principles." London, UK: Oxford Publishers, 2014. 97-125.

Yost, Roy W. "The Application of Modern Distillation Equipment in Analytical Chemistry." Separation and Purification Methods Journal Volume 4, 1975 - Issue 1 (2006 ): 1-21.