Batch Distillation Packed

8/6/2019 Batch Distillation Packed

1/12

1

Packed Distillation Colunm

ChE 1007

8-1-01


2/12

2

Location

Benedum SB-92, Manual is available

Introduction

The purpose of the lab is to introduce batch distillation using a packed column.

Batch distillation is a technique used for separating two or more miscible liquid or vapor

mixtures that are separated into their component fractions of desired purity. This is

accomplished by the application and removal of heat. The separation is based on the

boiling points of the mixture components. You will operate this column at total reflux,

periodically taking small distillate samples, and determine the number of theoretical trays

in the column.

Distillation is the most common separation technique, however it does suffer from

some disadvantages (Refer to Table 1 for applications). It usually takes a large amount of

heat, both in terms of heating and cooling, to run a distillation apparatus. This heat

requirement can contribute to more than 50% of plant operating cost. The best way to cut

down on operating costs is to improve the distillation units efficiency and operation via

process control and optimization.

Table 1. Industrial applications of distillation columns.

Industry Application

Food Concentrating alcoholic content of beverages

Petrochemical Separation of crude oil into various products.

Chemical Hydrocarbon Processing

Chemical Solvent Recycling

Chemical Radioactive Waste Reduction

Chemical Monomer Purification


3/12

3

Theory

Distillation is a process in which miscible liquids are separated based on their

physical properties, specifically, relative volatilities. A liquid can be classified as volatile

when it is readily vaporized at a relatively low temperature. The boiling of the more

volatile components of the mixture drives the distillation process. When the vapor is

cooled, the more volatile material condenses in a greater proportion than the less volatile

material.

The two types of distillations utilized in industry are batch and continuous. Batch

distillation is desirable when small quantities of high valued chemicals need to be

separated. The biggest advantage to using a batch column is its flexibility. This allows

one to deal with unknowns in the feed or product specifications. In a batch system, the

column can handle different mixtures by simply changing its operating conditions. The

main disadvantage to using a batch system is that the longer the components are exposed

to high temperatures, the better the chances that the components are broken down via

thermal degradation. Along with this, the energy requirements are usually higher for a

batch system. A column is built for separating a specific mixture in continuous

distillation. Therefore, the distillation column apparatus needs to be modified for each

new mixture that is to be separated.

A batch distillation apparatus consists of a distillation column, a condenser, and a

reboiler. The distillation column provides an environment where the gas and liquid

phases of each component can approach equilibrium. A column can contain either

packing or trays. In both types of columns, an increase in surface area allows for better

contact between the liquid and vapor phases. In a column containing trays there is a

discrete distribution of surface area, whereas in a packed column the distribution of area

is continuous. The continuous distribution found in a packed column maximizes the

surface area available for mass transfer, therefore allowing for a more efficient

separation. In order to provide the highest contact area, a column is filled with packing

that has a large volumetric area and that has high porosity. The liquid trickles down the

column and through the packing as small droplets. The gas is sent through the column in


4/12

4

the upward direction. This countercurrent flow of liquid and vapor exists only in a

packed column. Ideally, the porosity of the packing should not hinder the gas flow

through the column. In this lab, the packing in the column is glass sand. The distillation

column also contains a condenser, which cools and condenses the vapor leaving the top

of the column. A reboiler is connected to the bottom of the distillation apparatus and it

provides the reboil heat that is necessary for distillation.

Reboiler

Condenser

Figure 1. Schematic of a simple, packed distillation unit.

A useful way to determine a columns effectiveness is to limit its operating

conditions. One way to accomplish this is to run the column at total reflux. In total

reflux, all of the overhead vapor (reflux) and all of the bottoms liquid (boilup) is returned

to the column. Total reflux conditions allow for assumptions that make calculations

easier and allow the student an easy way to graphically evaluate the column.

Packing


5/12

5

Temperature vs. MeOH concentration

60

65

70

75

80

85

9095

100

105

0 0.2 0.4 0.6 0.8 1

x,y

Temperature

Dew Point Curve

Bubble Point Curve

Figure 2. Boiling Point Diagram of a Binary Mixture at a Specific Pressure.

The Tx diagram shows how the equilibrium compositions of the components in a

mixture vary with temperature at the column pressure. The dew point is the temperature

at which the saturated vapor starts to condense. The bubble point is the temperature in

which the liquid starts to boil. The Txy diagram for methanol in water can be produced

using the UNIFAC method in Aspen. Data for this system is in the Appendix.

Boiling point diagrams can help aid in the construction of a vapor-liquid-

equilibrium (VLE) curve, Figure 3. The VLE plot shows the bubble point and dew point

at constant pressure. The equilibrium line describes the compositions of the liquid and

vapor in equilibrium at some fixed pressure.


6/12

6

Figure 3. Representation of Vapor-Liquid Equilibrium Curve at a Specific Pressure.

A packed distillation column allows for continuous contact of liquid and vapor,

however it is convenient to analyze the column as if it were discontinuous (a staged

tower). The packing in the column can be divided up into segments that are of equal

height. Each of these segments can be looked at as a stage. It can be assumed that

each stage allows the vapor and liquid to leave the stage in equilibrium with each other.

This method of assuming that the packing can be broken down into stages is not

physically accurate, but can be used for calculations. The following equation relates

column height and the number of equilibrium stages to the height equivalent to a

theoretical plate (HETP).

HETP is defined as the height of packing needed to obtain the change in composition

obtained with one theoretical contact. HETP is measured experimentally and usually can

range from one to four feet. A small HETP indicates a small column and more efficient

packing. To measure the HETP, the compositions of the top and bottoms streams must

be found at total reflux and the number of equilibrium stages must be calculated. The

following equation shows the relationship between the HETP, the height of packing and

the number of theoretical stages.


7/12

7

There are two methods to find the number of theoretical stages in the column.

Both techniques use the McCabe-Thiele analysis, which is a graph of vapor composition

(mole fraction) verses liquid composition (mole fraction). The system operating line and

the equilibrium line for the system are plotted. The operating line for a batch system with

total reflux is y=x. This is true because the flow rate of liquid must be the same as the

Methanol x-y diagram

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Methanol in liquid

Methanolinvapor

flow rate of vapor. McCabe-Thiele analysis can be performed in two ways. One method

calculates theoretical stages numerically by expressing the equilibrium and operating

lines mathematically and finding the number of vertical and horizontal intersections

(stages) required to reach the desired separation. The other method allows the student to

manually draw the stages on the VLE graph. The first method is more accurate because

of the numerical calculations, however it is much more time consuming. Regardless of

Figure 4. Total Reflux McCabe-Thiele with Theoretical Number of Stages.

Reboiler

1

2

3 4


8/12

8

the method used, the graph obtained will look similar to Figure 4, which can be produced

using Aspen.

The HETP is dependant upon the packing type and size, the gas flow rate and the

chemicals being separated. The higher the HETP the lower the efficiency of the packing.

If the gas flow rate is low, the HETP will generally be higher because the packing is not

completely wet. The HETP can be calculated as explained in the above method, however

literature values are usually much more accurate. One approximation that can be used is

to set the HETP equal to the column diameter.

Lab Objectives

The student should understand the theory behind a distillation column. The student should gain a working knowledge of liquid separation using a packed

batch distillation column. The student will carefully measure the amount and

composition of the material charged to the vessel.

The student should be able to quantify the composition of distillate samples (the

column should be run at total reflux) using mixture density at a specifiedtemperature (typically, cool the samples to ambient temperature).

The student should be able to understand the principles of batch distillation at

total reflux.

The student should understand the principles involved in vapor/liquid equilibrium. The student should be able to determine the number of theoretical plates along

with HETP in the packed column operating at total reflux. The lab result can be

represented on a figure such as Figure 4.

The student should be able to use analytical methods such as density (usingpychnometers and/or densimeters) to determine the composition of a sample of

distillate.

Lab Tasks

The task associated with this lab is to separate methanol from water using a packed batch

column operating at total reflux. The separation should be measured by the purity of the

distillate sample. From the initial concentration of the feed and the concentration of thedistillate sample the number of theoretical stages and HETP for the column can be found.

Composition-density and/or composition refractive index calibration curves can be

generated to assist in determining the composition of the distillate sample.


9/12

9

Equipment Schematic

2

1

3

4

5

6

7

8

Height

of Packing


10/12

10

Equipment Description

1) Heat Source

2) Rebolier: vaporizes the liquid that is sent to it

3) Feed Line: Transports the feed to the distillation unit

4) Packing (Glass Sand) : Provides maximum surface area for mass transfer

5) Reflux Tube: Place where overhead vapor is returned to column

6) Condenser: Cools and condenses the vapor leaving the top of the column

7) Water Line

8) Distillate Release Valve: allows sample to be taken from column. Also controls the

reflux ratio.

Example Procedure

1) Open the water line valve.

2) Fill the still pot with the desired concentration of methanol and water mixture. A

concentration of 5 mole % is recommended.

3) Turn heat source on.

4) Let the column run for approximately one to two hours to reach steady state.

5) After the column has reached steady state, collect a distillate sample by opening

the distillate release valve.6) Turn the heat source and the water line off.

7) Allow the sample to cool to room temperature.

8) Analyze the sample using a densimeter, a refractometer, or a pychnometer in

order to determine the distillate composition. Take multiple measurements on the

same sample (if possible) in order to determine the precision of the measurments.

Be very careful to make sure the densimeter tube is clean and dry before loading

the sample.

9) Allow the still pot and contents to cool. Drain the excess liquid from the still pot.

10) Calibration curves must be created for each analytical instrument.


11/12


12/12

12

Txy data, x and y in mol fractions, T in oC

T oC x y

100 0 0

92.39 0.05 0.2797

87.53 0.10 0.427784.01 0.15 0.5233

81.48 0.20 0.5870

79.48 0.25 0.6352

77.90 0.30 0.6723

76.56 0.35 0.7036

75.36 0.40 0.7317

74.19 0.45 0.7592

73.16 0.50 0.7834

72.20 0.55 0.8065

71.29 0.60 0.8287

70.45 0.65 0.849669.58 0.70 0.8718

68.69 0.75 0.8946

67.83 0.80 0.9167

66.97 0.85 0.9387

66.14 0.90 0.9597

65.31 0.95 0.9806

64.50 1.00 1.00

Kojima, Tochigi, Seki, Watase; Kagaku Kogaku 32, 149 (1969)

With Updates: 1/9/02

Batch Distillation Packed

Documents

Transcript of Batch Distillation Packed