Laboratory Course - UTRM - Ruhr

RUHR-UNIVERSITÄT BOCHUM
Lehrstuhl für Fluidverfahrenstechnik
Laboratory Course
Experiment: “Continuous Distillation”
Summer Semester 2016
Advisors:
Mrs. M.Sc. Iris Rieth
Mrs. M.Sc. Carolin Stegehake
Room:
IC 3/099
IC 3/109
Phone Number:
+49 (0)234 / 32 – 25 849
+49 (0)234 / 32 – 26 344
E-Mail:
[email protected]
[email protected]
Laboratory:
IDN 02 / 751
1
Inhaltsverzeichnis
1. Introduction ....................................................................................................................... 3
2. Operating Principle of a Continuous Distillation Column .................................................... 3
2.1. Forming a two-phase System...................................................................................... 3
2.2. Mass Transfer across the Phase Boundary................................................................. 4
2.3. Separation of the two phases ...................................................................................... 4
3. Design of a Continuous Distillation Column ....................................................................... 4
4. Thermodynamic Fundamentals ......................................................................................... 5
4.1. Boiling und Equilibrium Diagram of Binary Systems .................................................... 5
4.2. Classification of Real Systems .................................................................................... 7
4.2.1. Ideal Mixtures ....................................................................................................... 7
4.2.2. Mixtures with Negative Deviation .......................................................................... 8
4.2.3. Mixtures with Positive Deviation ........................................................................... 9
5. Equilibrium-Stage Operations ...........................................................................................10
5.1 Concept of Equilibrium-Stage Operation .....................................................................10
5.2 Packed Columns .........................................................................................................11
6. McCabe-Thiele Design Method ........................................................................................11
6.1 Determination of the Equilibrium-Stages by Using the McCabe-Thiele-Method ...........12
6.1.1. Operating Line for the Rectification Section .........................................................12
6.1.2. Operating Line for the Stripping Section ..................................................................14
6.1.3. The Intersection Line/Feed Line ..........................................................................16
6.1.4. Construction of the McCabe-Thiele Diagram .......................................................19
7. Instructions .......................................................................................................................20
7.1. Preparation of the Experimental Setup .......................................................................20
7.2. Operation of the Software ..........................................................................................20
7.3. Experimental Procedure.............................................................................................23
8. Task Formulation ..............................................................................................................23
9. Special Safety Instructions ...............................................................................................24
10. Short Questions ..............................................................................................................24
11. Appendix ........................................................................................................................25
12. Literature ........................................................................................................................32
2
1. Introduction
Continuous distillation is used widely in the chemical process industries where large
quantities of homogeneous liquids have to be distilled. This unit operation leads to high
purity of products, even if the system is difficult to separate. [1] Continuous distillation
is used when a simple distillation does not achieve a purity that is high enough.
The task of the experiment in this laboratory is to investigate a continuous distillation
of the system ethanol/water.
2. Operating Principle of a Continuous Distillation Column
The principle of continuous distillation is the same as the principle of normal distillation.
It consists of three steps:
1. Forming a two-phase system
2. Mass transfer across the phase boundary
3. Separation of the two phases
1. Step
2. Step
3. Step
Figure 1: Principle of continuous distillation [7]
2.1. Forming a two-phase System
To separate a mixture of substances into its components, a second phase is required.
The separating component transfers into the second phase. The second phase is
generated by partially boiling the initial phase – this means to supply heat in the
vaporizer. Thereby, the mixture of substances is separated into a vapor and a liquid
phase.
3
2.2. Mass Transfer across the Phase Boundary
Mass transfer between the two phases is caused by disequilibrium of the substances
– a difference in the intensive variables. Because of the different boiling temperatures
of the pure substances, the thermodynamic properties can be influenced such that the
components which have to be separated accumulate in different phases. This effect is
enhanced by intensive contact of the two phases and a large surface caused by
packing material.
2.3. Separation of the two phases
Mass transfer between the liquid and the vapor phase leads to separation of the
mixture into the vapor and the liquid phase. The composition of the two phases is
different: the vapor phase mainly consists of light boilers and the liquid phase of high
boilers. By condensing the vapor phase, there are two (liquid) output fractions in the
distillation: the first one at the upper part of the column with light boilers of high purity
(depending on the separation efficiency of the column) and the second one mainly with
Condenser
high boilers at the bottom.
3. Design of a Continuous Distillation
Column
Distinguished between the types of packing
material, the Continuous Distillation Columns
are called plate column, distillation tower or
distillation column. [2]
These designs can be operated as a batch or,
Rectification
section
overhead
Feed
Reflux
reflux divider
overhead
product
Feed tray
distillation. Continuous distillation columns
consist of a bottom part with a reboiler, a
separation column and a head part with a reflux
Stripping
section
like in this laboratory, as a continuous
Reboiler
condenser and a “reflux divider”. The lower part
of the column is called stripping section and the
upper part is called rectification section. The
feed section with a preheater is located in
bottom
bottoms product
Figure 2: Continuous distillation column [3]
4
between the upper and the lower part. The schematic of a Continuous Distillation tower
is presented in Figure 2.
In the bottom part (sump) of the column, a section of the liquid is extracted and the rest
of it is partially vaporized and supplied to the stripping section again. In the stripping
section the reduction of the light boilers in the condensate phase takes place. In the
rectification section the enrichment of the light boilers in the vapor phase takes place
which is partly removed after condensation as the head product and partly recycled to
the column. The reflux liquid gets in contact with the vapor and leads to mass transfer
again. The ratio between reflux and overhead product is controlled by a reflux divider.
With high reflux ratio, the residence time is increased. Residence time is the average
amount of time that the substances spend in the column so that mass transfer takes
place. The purity of the head product increases with increasing residence time.
4. Thermodynamic Fundamentals
To analyze the separation efficiency of a Continuous Distillation Column it is important
to understand some particular diagrams. The diagrams are based on theoretical
principles.
4.1. Boiling und Equilibrium Diagram of Binary Systems
In a mixture of a binary system there are specific intensive properties, e.g. composition
of liquids xi, composition of vapor yi and boiling temperature Ti. It is common to refer
the compositions of each phase to the light boiler i. The drawing of the related
properties into a diagram results in the boiling point diagram and the vapor-liquid
equilibrium diagram.
The boiling point diagram (Figure 3) shows how the equilibrium compositions of the
components in a liquid mixture vary with temperature at a fixed pressure. The boiling
point diagram consists of the bubble-point curve (bubble-point is the temperature at
which the liquid starts to boil) and the dew-point curve (dew point is the temperature at
which the saturated vapor starts to condense). With the boiling point diagram it is
possible to determine the composition of the liquid phase xi and the composition of the
vapor phase yi at different temperatures at a fixed pressure. Besides, the boiling point
and the dew point of a specific composition can be investigated.
5
vapor
dew-point curve
two-phase
bubble-point
curve
liquid
Figure 3: Boiling Point diagram [9]
The vapor-liquid-equilibrium diagram (Figure 4) expresses the bubble-point and the
dew-point of a binary mixture at a constant pressure. By using this diagram, the amount
of equilibrium stages (theoretical trays) of the column can be determined.
equilibrium line
Figure 4: Vapor-Liquid-Equilibrium Diagram [9]
6
4.2. Classification of Real Systems
4.2.1. Ideal Mixtures
Systems of ideal mixtures do not have any deviations from the ideal behavior. In this
case Raoult’s law is applicable – the partial vapor pressure pi of component i of an
ideal mixture of liquids is equal to the vapor pressure of the pure component 𝑝𝑖0
multiplied by its mole fraction 𝑥𝑖 in the mixture: [3]
𝑝𝑖 = 𝑝𝑖0 ∙ 𝑥𝑖
(1)
Besides, Dalton’s law is applicable. It states that in a mixture of non-reacting gases,
the total pressure exerted is equal to the sum of the partial pressures of the individual
gases. [4]
𝑝𝐺𝑒𝑠 = ∑ 𝑝𝑖
(2)
Moreover, the partial pressures of the components of the mixture can be calculated by
the concentration of the component of the vapor phase and the total pressure:
𝑝𝑖 = 𝑦𝑖 ∙ 𝑝𝐺𝑒𝑠
(3)
The linear behavior of the partial pressures for ideal systems is shown in the vapor
pressure diagram (Figure 5, left). The bubble point diagram (Figure 5, middle) features
the typical boiling point curve. This diagram shows the dependence of phase
transitions of a mixture on temperature and composition. The vapor-liquid-equilibrium
diagram (Figure 5, right) is typically determined experimentally but for ideal binary
mixtures it can be calculated by knowing the separation factor α. The separation factor
characterizes the separation behavior of an ideal binary system and describes the ratio
of the ratios of the vapor-liquid-composition of the pure components A and B:
7
𝑦𝐴
𝑦𝐴
𝑝𝐴
⁄𝑥𝐴 𝑥𝐴 ∙ 𝑝𝐺𝑒𝑠
⁄𝑥𝐴 𝑝𝐴0
𝛼=𝑦
=𝑦
=𝑝
= 0
𝐵⁄
𝐵⁄
𝐵
𝑥𝐵 𝑝𝐵
𝑥𝐵 𝑥 ∙ 𝑝𝐺𝑒𝑠
𝐵
(4)
The vapor-liquid-equilibrium curve for ideal mixtures is determined by Dalton’s law,
Raoult’s law and the separation factor:
𝑦𝐴 =
𝛼 ∙ 𝑥𝐵
1 + 𝑥𝐴 ∙ (𝛼 − 1)
(5)
Many real component systems deviate from the ideal behavior. These systems are
divided into two categories, depending on the trend of deviation.
4.2.2. Mixtures with Negative Deviation
If the real partial pressures of the components in the mixture are lower than the (ideal)
partial pressures calculated by Raoult’s law, the mixture is called “mixture with negative
deviation” (Figure 5).
This behavior results in a vapor pressure minimum in the vapor-pressure diagram and
in a temperature maximum in the boiling point curve. The composition of the mixture
in the extreme value is called azeotrope: the boiling temperature of the azeotrope of a
mixture with temperature maximum is higher than the boiling temperature of the single
components. For the continuous distillation it means in fact that the lighter boilers
vaporize first (mainly). The temperature increases to the boiling point of the azeotrope
and then the remaining mixture vaporizes as an azeotrope. Thereby the liquid and
vapor phase have the same composition – the phases cannot be more purified by a
simple distillation.
8
4.2.3. Mixtures with Positive Deviation
For mixtures with positive deviation there are the same considerations applied as for
the ones with negative deviation. The effects concerning maximum and minimum are
reverse. The boiling temperature of the azeotrope is lower than the boiling temperature
of the single components. The azeotrope vaporizes first (mainly). A complete
separation of this mixture is not possible by using a simple distillation.
vapor pressure diagram boiling point diagram
equlibrium diagram
ideal mixtures
negative
deviation
positive
deviation
Figure 5: vapor pressure diagram, boiling point diagram, equilibrium diagram for the three
classifications of real systems [9]
9
5. Equilibrium-Stage Operations
The number of equilibrium-stages and the reflux ratio (equation 10) of a distillation
column for a certain separation task are important characteristics to estimate
investment and operation costs.
5.1 Concept of Equilibrium-Stage Operation
The definition of equilibrium-stages was once established for columns that used trays
as packing material. Mass transfer in tray columns takes place spatially separated on
each tray and not steady over the whole column. The highest enrichment of a
component on an ideally operated tray is characterized by the equilibrium-stage, if the
following conditions are fulfilled:
1. Ideal mixing of the liquid
2. Ideal heat and mass transfer between vapor and liquid
3. Vapor is dry saturated – no liquid droplets are drawn along.
condenser
Reflux
3. Tray
re-cooler
2. Tray
distillate
F4
1. Tray
Figure 6: Vapor and liquid Hold-up on the different trays [3]
The number of equilibrium stages is determined by using the McCabe-Thiele method
with the required purity of the products as described in section 6.
10
5.2 Packed Columns
Packing material is used to minimize pressure losses in case of high separation effects.
To transfer the theory of equilibrium-stages to packed columns, the HETP-value
(height equivalent to one theoretical plate) is defined [6]:
𝐻𝐸𝑇𝑃 =
𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑝𝑎𝑐𝑘𝑖𝑛𝑔 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑙𝑎𝑦𝑒𝑟
𝑛𝑢𝑚𝑏𝑒𝑟𝑠 𝑜𝑓 𝑒𝑞𝑢𝑖𝑙𝑖𝑏𝑟𝑖𝑢𝑚 − 𝑠𝑡𝑎𝑔𝑒𝑠
liquid
vapor
Figure 7: examples of packing material
6. McCabe-Thiele Design Method
The McCabe-Thiele method is used to examine graphically the function of a distillation
column for binary systems, such as the one in the laboratory (ethanol and water). This
method is based on the vapor-liquid-equilibrium data and it assumes constant molar
overflow which implies that: [5]
-
Molar heats of vaporization of the components are roughly the same
-
Heat effects (heats of solution, heat losses to and from column, etc.) are
negligible
-
For every mole of vapor condensed, 1 mole of liquid is vaporized
11
(6)
6.1 Determination of the Equilibrium-Stages by Using the McCabe-ThieleMethod
The design procedure is simple. Given the vapor-liquid-equilibrium diagram of the
binary mixture, operating lines are drawn first. Operating lines define the mass balance
relationships between the liquid and vapor phases in the column. There is one
operating line for the bottom (stripping) section of the column, and one for the top
(rectification or enriching) section of the column. The use of the constant molar
overflow assumption also ensures the operating lines are straight lines. [6]
6.1.1. Operating Line for the Rectification Section
The operating line for the Rectification Section is based on the following relations.
The mass balance of the total flow and of the flow of the lighter boiler component lead
to the following relations according to Figure 8:
𝐺̇ = 𝐿̇ + 𝐾̇
(7)
𝐺̇ ∙ 𝑦 = 𝐿̇ ∙ 𝑥 + K̇ ∙ 𝑥𝐾
(8)
Figure 8: mass-balance rectification section [7]
Solving equation (8) for 𝑦 results in equation (9):
𝑦=
𝐿̇
𝐾̇
∙ 𝑥 + ∙ 𝑥𝐾
𝐺̇
𝐺̇
12
(9)
The reflux ratio 𝑣 – the ratio of the reflux flow 𝑅̇ to the distillate flow 𝐾̇ – leads to
equation (10):
𝑣=
𝑅̇
𝐾̇
(10)
Substituting equation (7) in equation (9) and expanding the fractions results in equation
(11):
𝐿̇⁄
𝐾̇⁄
̇
𝐾
𝐾̇
𝑦=
∙𝑥+
∙ 𝑥𝐾 =
∙𝑥+
∙ 𝑥𝐾
𝐿̇ + 𝐾̇
𝐿̇ + 𝐾̇
𝐿̇⁄ + 𝐾̇⁄
𝐿̇⁄ + 𝐾̇⁄
𝐾̇
𝐾̇
𝐾̇
𝐾̇
𝐿̇
𝐾̇
(11)
Substituting equation (10) in equation (11), in consideration of 𝐿̇ ≈ 𝑅̇ results in the
operating line for the rectification section (12):
𝑦=
𝑣
1
∙𝑥+
∙𝑥
𝑣+1
𝑣+1 𝐾
(12)
A complete reflux (𝐾̇ = 0) is expressed in equation (13) based on equation (8).
𝐺̇ ∙ 𝑦 = 𝐿̇ ∙ 𝑥
(13)
Using the relation of the incoming amount of vapor to be equal to the outcoming
amount of liquid, the operating line for complete reflux is according to equation (14):
𝑦=𝑥
(14)
The operating line for complete reflux is equivalent to the diagonal line of the vaporliquid-equilibrium diagram.
The operating line for the rectification section with a certain reflux ratio is constructed
as follows. First the desired top product composition is located on the vapor-liquidequilibrium diagram, and a vertical line produced until it intersects the diagonal line. A
line from this intersection point is drawn to the y-axis intercept according to equation
(16), as shown in Figure 9. [6]
𝑥 = 𝑥𝐾
𝑦=
𝑥𝐾
𝑣+1
13
(15)
(16)
6.1.2. Operating Line for the Stripping Section
The operating line for the stripping section is constructed in a similar manner.
equilibrium
line
rectification
line
Figure 9: rectification line in vapor-liquid-diagram
The mass balance of the total flow and of the lighter component for the system
boundary of the stripping section results in equations (17) and (18) according to Figure
10. 𝑆̇ is the extracted liquid in the bottom part of the distillation column.
𝐿̇∗ = 𝐺̇ ∗ + 𝑆̇
(17)
𝐿̇∗ ∙ 𝑥 ∗ = 𝐺̇ ∗ ∙ 𝑦 ∗ + 𝑆̇ ∙ 𝑥𝑆
(18)
Figure 10: mass balance stripping section [7]
14
Analogous to the operating line for the refraction section results the operating line for
the stripping section in equation (19):
∗
𝑦 =
𝐿̇∗
𝐿̇∗ − 𝑆̇
∙ 𝑥∗ −
𝑆̇
𝐿̇∗ − 𝑆̇
∙ 𝑥𝑆
(19)
The starting point for the construction is the desired bottom product composition
(equation 20) on the x-axis. A vertical line is drawn from this point to the diagonal line,
and a line of slope β according to equation (21) as illustrated in the diagram in Figure
11.
𝑥 = 𝑥𝑆
tan 𝛽 =
stripping
line
𝐿̇∗
𝐿̇∗ − 𝑆̇
(20)
(21)
equilibrium
line
Figure 11: stripping line in vapor-liquid-diagram
Depending on the state of the feed the slope of the operation line for the stripping
section and the intersection point with the operation line for the rectification section
changes.
15
6.1.3. The Intersection Line/Feed Line
A separation of a mixture by continuous distillation is possible if the operation line for
the rectification section and the stripping section intersect in a point below the vaporliquid-equilibrium line. The thermal state of the feed influences the liquid and the vapor
flow in the feed-section of the column which results in the intersection point of the
operating lines. [6]
However, if the feed composition is saturated liquid, the vapor flow does not change
but the liquid flow in the lower part of the column increases. If the feed composition is
saturated vapor, the vapor flow increases. [5] Figure 12 shows the influence of the
thermal state of the feed on the internal flows of the column.
a)
b)
c)
d)
e)
subcooled liquid e>1
saturated liquid e=1
mix of liquid and vapour 0<e<1
saturated vapour e=0
superheated vapour e<0
Figure 12: influence of the condition of feed on vapor and liquid flows in feed section [5]
The condition of the feed can be deduced by the slope of the feed line or e-line. The
e-line is that drawn between the intersection of the operating lines, and where the feed
composition lies on the diagonal line. e is the caloric factor and is the ratio of the feed
that flows downward the column as a liquid. (1-e) is the ratio of the feed that flows
upward the column as vapor. [8]
The heat balance around the feed section results in equation (22):
ℎ𝐹′ − ℎ𝐹
𝑒 =1+
∆ℎ𝑣
(22)
ℎ𝐹 is the molar enthalpy of the feed, ℎ𝐹′ is the molar enthalpy of the boling liquid and
∆ℎ𝑣 the molar evaporation enthalpy.
16
The intersection line is determined by using the caloric factor and the heat balance
around the feed section:
𝑦=
𝑒
1
∙𝑥−
∙𝑥
𝑒−1
𝑒−1 𝐹
(23)
Figure 13 shows the vapor-liquid-equilibrium diagram with the intersection line and the
resulting intersection with the rectification line and stripping line.
equilibrium
line
compound of vapor
intersection
line
stripping
line
rectification
line
compound of liquid
Figure 13: Intersection line in vapor-liquid-diagram [8]
The intersection line intersects the diagonal line according to equation (24):
𝑥 = 𝑥𝐹
(24)
The intersection of the intersection line with the x-axis results in equation (25):
𝑥=
𝑥𝐹
𝑒
17
(25)
The intersection lines for the various feed conditions are shown in Figure 14.
Table 1: Examples of various feed conditions [2]
Example
state of feed
e-factor
1
superheated vapour
<0
2
saturated vapour
=0
3
mix of liquid and vapour
0<e<1
4
saturated liquid
=1
5
subcooled liquid
>1
Figure 14: Insection line in depency on
thermical feed conditions (qualitative). [8]
18
6.1.4. Construction of the McCabe-Thiele Diagram
The McCabe-Thiele method assumes the equilibrium between the liquid on a tray and
the vapor above it. Using the constructed operation line for the used reflux ratio and
the thermal state of the feed for a certain separation as described above, the required
number of theoretical stages is graphically determined as a stage-construction, shown
in Figure 15. The construction begins with a horizontal line from the intersection point
of rectification line and diagonal line to the vapor-liquid-equilibrium line. A vertical line
follows to the operation line. The construction of horizontal and vertical lines between
operation line and vapor-liquid-equilibrium line repeated until the vertical line intersects
the diagonal line. The resulting trays are equivalent to the necessary trays of a
distillation column with respect to the number and composition of liquid and vapor
phase. [6]
compound of vapor
equilibrium
line
rectification
line
compound of liquid
Figure 15: Stage construction in McCabe-Thiele-Diagram
19
7. Instructions
This chapter describes the procedure of the experimental setup. A flow chart and
process picture of the distillation column are shown in the appendix.
7.1. Preparation of the Experimental Setup
The following aspects have to be denoted to startup the distillation column:
1. Switch on the Refractrometer
The weight proportions of the head and bottom product are determined by the
refractive index. The refractive index is measured by a refractrometer.
2. Open the coolant valve
The cooling water of the whole plant is run in a closed loop. The water tap is on
the wall behind the distillation column. The coolant flow has to be high to ensure
a sufficient cooling capacity. Inside of the stainless steel tray below the plant,
there is a flow controller to prove the flow.
3. Switch on the Main Switch
The power supply of the plant is ensured by the box on the left of the plant. The
Main Switch has to be turned on.
4. Turn on the Computer
Temperature and pump settings are controlled by a software on the computer.
The computer is located on the left of the power box. After booting the computer,
log in with the user name “user” and the password “user”.
7.2. Operation of the Software
The software opens automatically after logging in on the computer. By clicking the left
mouse button of the user interface, the process picture (Figure 22) shows up. Each
required numerical value has to be confirmed by pressing “Enter”. Activated buttons
are flashing green.
20
1. Voltage Supply
By pressing the button „230 V” of the control panel
(Figure 16), the voltage supply of the pumps and of the
heaters are released. Not until the release occurred,
heater and pumps can be started. The status of the
emergency shut-off which is located on the power box is
symbolized by a small triangle.
Figure 16: Voltage
Abbildung
1:
supply
Allgemein
2. Sump Temperature
The sump of the column is heated by
two quartz crystal heater. The desired
temperature is entered by a left-click
on “Feld 1” in the operating field
“Regelung Sumpftemperatur” (Figure
17). By pressing the button “Temp.
Sumpf” the heater of the sump is
Figure 17: Sump Temperature
turned on and the button flashes green.
The automatic control of the sump temperature may have oscillations.
Therefore, a manual control of the sump temperature is recommended. To
activate the manual control, the button “Manuell” has to be clicked and the
desired power in % has to be entered in “Feld 2” (Figure 17).
The heating of the sump switches off automatically, if the filling level of the sump
tank B01 is dropping below or above the minimum or maximum height.
3. Sump Pump
To keep the filling of the sump tank constant during operation
and to determine the concentration of the components in the
sump, a sample has to be taken. The flow of the sump is
controlled by the gear pump P02. The power of the pump is Figure 18: Sump
entered in “Feld 3” (Figure 18). The pump is turned on by
pump
pressing the button “Start P02” so that the liquid is pumped into tank B04.
21
4. Feed Temperature
The temperature of the feed is controlled by the preheater.
The preheater is heated by a quartz crystal heating rod. The
power is controlled, appropriate to the power of the sump
pump, automatically or manually in the operating field
“Regelung Vorheizer” (Figure 19). The automatic control is
started by pressing the button “Start Temp.”. The desired
temperature can be entered in “Feld 4” (figure 18). The
Figure 19: Feed
manual control is activated by pressing the button “Manuell”. Temperature
The power of heating can be entered in “Feld 5”. The current
temperature is shown in “Feld 4”. Note that the preheater is active just if the feed
pump P01 is switched on!
5. Feed Pump
The feed is supplied to the distillation column by a
reciprocating pump. The unit of the pumping capacity is
1/min. The maximum pumping capacity is 180 1/min. By
pressing the button “Start P01” (Figure 20), the pump is
Figure 20: Feed
started. The desired pumping capacity is entered in „Feld 6“ pump
(Figure 20).
6. Reflux Separator
The reflux separator works electrically and is
controlled by the operation field „Rücklaufteiler“
(Figure 21). If the reflux separator is turned off,
the condensate flows back into the column
completely (reflux = infinity). Activating the Figure 21: reflux separator
button “Automatik” turns on the automatic control of the reflux separator, which
switches between supply pipe and return pipe. The value in “Feld 7” shows the
time in which the condensate (as a product) flows from the column into the tank
B03 (Figure 23). „Feld 8“ shows the time in which the condensate is fed back to
the column. The example in Figure 21 expresses a reflux ratio of 𝑣 = 5⁄1. If the
reflux separator is turned on, the button „Automatik“ flashes green. The
22
extraction of condensate with infinite reflux occurs with activating the button
„Abnahme“.
7.3. Experimental Procedure
After preparation of plant and refractrometer, as shown in chapter 7.1, it is possible to
start the experiment. First, the feed has to be prepared by using a balance. The
refractive index and the mass fractions of the solution have to be determined. Then the
solution is filled into the column and the desired sump temperature has to be set. The
sump heater cannot be turned on if the fluid level is too low or too high. In these cases
the sump tank has to be filled or flushed. As soon as the vapor starts condensing, the
supply feed can flow, the desired reflux ratio can be set and the reflux separator can
be turned on. Depending on the parameters and the mass fractions of the feed, the
distillation process is steady-state (equilibrium state) after 40-45 minutes. The samples
of the head product are taken from tank B03. The samples of the sump product are
taken from tank B04 (Figure 23). The refractive indices have to be determined directly
after extraction because of the easily volatilizing ethanol. It is necessary to take three
samples of each operating condition at intervals of seven minutes. Finally, pumps and
heater are turned off. The coolant is not turned off until the column is cooled down.
8. Task Formulation
The number of equilibrium stages is supposed to determine for the following
operating conditions:
Sump Temperature: 92- 93°C
Feed Flow: 60 min-1
Reflux Ratios: (∞); (5:1) ; (5:2) ; (2:5)
Feed Composition:
1. Mass fraction of Ethanol in Feed:
- 30%
Thermical State of Feed::
- Subcooled Liquid (23°C)
23
9. Special Safety Instructions

During operation the plant gets hot. There is danger of burning!

There is risk of shock because of the electrical parts of the plant. Avoid liquids
in electronics!

Ethanol is highly flammable – avoid ignition sources!

Operate at atmospheric pressure! Negative pressure caused by vacuum pump
P03 (Figure 23) may lead to implosion. The vacuum pump must not be used
during the experiment.
10. Short Questions
1. Draw the schematic layout of a distillation column!
2. Explain the functional principle of a distillation column!
3. What diagram shows the composition of the vapor and liquid phase (at
constant pressure)?
4. How are mixtures classified?
5. Which laws describe the behavior of these mixture?
6. What is an azeotrope?
7. What kind of column types do exist?
8. How are the different column types designed?
9. Explain the McCabe-Thiele method for the determination of the number of
equilibrium stages by using the x,y-diagram!
10. Does the feed state have an influence on the distillation?
11. Which function does the reflux ratio in a distillation column have?
12. Which property is measured by a refractrometer?
24
11. Appendix
Figure 22: Process picture of the used distillation column
25
Figure 23: Flow sheet of the used distillation column
26
boiling-point diagram Ethanol-Water
100°C
bubble-point curve
dew-point curve
temperature
95°C
90°C
85°C
80°C
75°C
0%
10%
20%
30%
40%
50%
mass percent Ethanol
27
60%
70%
80%
90%
100%
refracting index Ethanol-Water
1,3650
1,3600
refracting index
1,3550
1,3500
1,3450
1,3400
1,3350
1,3300
0%
10%
20%
30%
40%
50%
mass percent Ethanol
28
60%
70%
80%
90%
100%
Table 2: refracting index at various mass percent ethanol at 25°C
Mass percent Ethanol
Refracting index at 25°C
000%
1,3324
010%
1,3389
020%
1,3458
030%
1,3518
040%
1,3560
050%
1,3594
060%
1,3614
070%
1,3629
080%
1,3631
085%
1,3629
090%
1,3623
095%
1,3611
100%
1,3591
Table 3: enthalpie
temperature
pressure
[°C]
[bar]
23,0
84,0
86,0
95,7
Enthalpie
30 mass% Ethanol in Water
[kJ/kg]
0063,16
1
0310,53
0595,00
2198,92
29
Azeotropic point
xETOH=95,6%
equilibrium diagramm Ethanol-Water
1
0,9
Azeotropic point
xETOH=95,6%
0,8
mass percent Ethanol in vapor
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
0
0,1
0,2
0,3
0,4
0,5
0,6
mass percent Ethanol in liquid
30
0,7
0,8
0,9
1
Table 4: vapor-liquid equilibrium for the system ethanol-water
pressure [bar]
temperature [°C] Mass percent ethanol in water
liquid
1
vapor
99,6288479
0
0
96,7856255
0,025
0,2328148
94,4820698
0,050
0,3689599
92,5789585
0,075
0,4582788
90,9818473
0,100
0,5213259
89,6255695
0,125
0,5681531
88,4629583
0,150
0,6042613
87,4587360
0,175
0,6329184
86,5875692
0,200
0,6561343
85,8251803
0,225
0,6753756
85,1555691
0,250
0,6915564
84,5647358
0,275
0,7053575
84,0409024
0,300
0,7172811
83,5743468
0,325
0,7277075
83,1564579
0,350
0,7369322
82,7801245
0,375
0,7451902
82,4388467
0,400
0,7526721
82,1271245
0,425
0,7595370
81,8400134
0,450
0,7659200
81,5730689
0,475
0,7719388
81,3224578
0,500
0,7776987
81,0845689
0,525
0,7832957
80,8563466
0,550
0,7888201
80,6350688
0,575
0,7943585
80,4184022
0,600
0,7999961
80,2043466
0,625
0,8058186
79,9913466
0,650
0,8119140
79,7781799
0,675
0,8183741
79,5641799
0,700
0,8252969
79,3491799
0,725
0,8327880
31
79,1337354
0,750
0,8409632
78,9190687
0,775
0,8499508
78,7073465
0,800
0,8598947
78,5019576
0,825
0,8709582
78,3077353
0,850
0,8833278
78,1312909
0,875
0,8972188
77,9797353
0,900
0,9128930
77,8699575
0,925
0,9306173
77,8160686
0,950
0,9507564
77,8406798
0,975
0,9737210
77,9763464
1
1
12. Literature
[1] R. Perry und D. Green, Perry's Chemical Engineers' Handbbok, McGraw-Hill, 1984.
[2] H. Bockhorn, „Versuchsbeschreibung zum Chemisch-Technischen Grundpraktikum,“
Universität Karlsruhe.
[3] P. Perrot, A to Z of Thermodynamics, Oxford University Press, 1998.
[4] M. Silberberg, Chemistry_ the molecular nature of matter and change, Boston: McGrawHill, 2009.
[5] M. Baerns, Technische Chemie, Wiley-VCH, 2006.
[6] M. T. Tham, „Distillation,“ 1997. [Online]. Available:
http://www.istitutofermiverona.it/LEZIONI/distil/distildes.htm. [Zugriff am 22 02 2016].
[7] J. Apelt, „Thermische Verfahrenstechnik,“ Hochschule Fulda, 2009.
[8] D. Christen, Praxiswissen der chemischen Verfahrenstechnik, Springer, 2009.
[9] „Praktikum Grundoperationen,“ Universität Paderborn.
32