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ATELIER INTER UNIVERSITAIRE DE GENIE DES PROCEDES

BATCH

DISTILLATION

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1. pilot plant description

The pilot can be controlled, either conventionally in manual mode, or

in automatic mode, via the supervision software installed on a

microcomputer in the control room.

The mixture to be separated is the ternary acetone-ethanol-water with the

composition: - around 8% mass for acetone

- around 15% mass for ethanol

The main parts of the pilot, presented on the general diagram on figure 1,

are:

- a boiler (referred 3 on the diagram), heated by a steam boiler,

(coaxial tube external heat exchanger (referred 25). The pressure of the

vapour is approximatively 3.5 bars.

- a plate column for gas liquid contact (referred 2), composed of two

identical sections of 10 perforated plates (Oldershue type);

- a condenser (cold water coil) (referred 1);

- a subcooler (referred 6), to store the distillate at a moderate

temperature;

- 5 graduated recovery vessel;

- 5 pneumatic valves to control the admission of the distillate in the

recovery vessel;

- a flowmeter to measure the cooling water flow;

- a differential pressure measuring device (referred 4) , to control the

flow of vapour in the boiler exchanger, in respect to the vapour flow

through the column;

- several thermocouples (noted TI1, TI2...), to measure the temperatures

at different levels of the column :

* in the boiler,

* on the plates (every two plates),

* at the input and the output of the cooling water at the

condenser.

Thermodynamic data and physical properties of the charge (acetone-

methanol-water mixture) are provided in appendix 1.

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Figure 1: diagram of the pilot

2. Characteristics of the installation

The boiler is made of glass and has a maximum capacity of 50

L. A metallic rule allows measuring the liquid level (graduated in litres) and

liquid sampling is possible. The boiling of the charge to be separated is

carried out by means of an exchanger working by steam boiler.

The distillation column is made of glass and comprises two

sections of 10 perforated plates, as detailed on figure 2. Its diameter is 100

mm and total height 2 metres. It is operated under atmospheric pressure.

Perforated plates are also made of glass. Their spacing is 100 mm. Each one

presents 1150 circular perforations and a 15 mm diameter central

downcomer. This geometry ensures a good distribution of the liquid and a

maximum liquid hold up of 25 cm3/plate

Figure 2 : Figure 3 :

Column diagram Plate diagram

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The condenser is made of glass and is operated under atmospheric pressure.

Its dimensions are 150 mm diameter and 600 mm length, with 0,75m2

cooling coil, where cold water is circulated)

The glass recovery vessels are graduated in litres. Their maximum

capacity is 10 litres.

The cooling water flow circulating in the condenser is measured by means of

the flowmeter H250 M9.

3. Description of the supervision

The supervision software, named WINCC, installed on the PC of the

control room, allows, (when the buttons of the electric control panel of the

pilot are in position AUTO):

- the start up (or the stop) of the cooling water flow. The valve

of admission of water is automatically programmed to be open at 85% of its

maximum value, which corresponds to a flow rate of approximately 700 L/h;

- admission (or the stop) of the heating vapour, (HEATING,

On/Off), by the opening of the pneumatic valve for vapour admission

- permanent visualization of the different temperatures and

pressure drop through the column. On the general synoptic, “Temp”

permits to know in real time the evolution of the temperatures along the

column. “Histo1” and “Histo2” permit to follow the time evolution of

temperatures, cooling water flow and the pressure drop in the column.

- adjustment and the start up (or the shutdown) of the reflux in

the column. In “Time Management” the button “Cycle Timer” permits to

launch (CYCLE) or to stop (STOP) the reflux. Two options are proposed:

* a fixed reflux (“Timer” regulated on FIXED), whose

value is regulated by imposing the reflux duration (in seconds) and the

duration of withdrawal (in seconds);

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𝑅𝑒𝑓𝑙𝑢𝑥 = 𝑟𝑒𝑓𝑙𝑢𝑥 𝑑𝑢𝑟𝑎𝑡𝑖𝑜𝑛

𝑑𝑢𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑤𝑖𝑡ℎ𝑑𝑟𝑎𝑤𝑎𝑙

* a automatically controlled reflux by the software, and

controlled either from the top temperature, or with from the temperature of a

specific plate (sensitive plate). This option, available via “Time

Management” makes it possible to optimize the evolution of the

concentration of the distillate along time, as explained more in detail in

paragraph 5.2.4.

- opening (or closing) of the different valves controlling the

admission of the distillate in the recovery vessels. Two options are again

proposed:

* valves can be opened or closed directly by the

operator via the software, by clicking on “V2”, “V3”

… on the general synoptic

* or, in automatic mode, they can be controlled from the

variation of the top temperature, via the “Valves Management”, as

explained in paragraph 5.2.5.

Let us specify that the majority of these operations can also be carried

out in Manuel mode directly on the installation.

4. Sampling

Liquid sample can be carried out on the level:

- of the boiler, thanks to the vertical pricking

(which must be purged before sampling);

- of the different plates along the column, by

means of syringes (only one plate every two plates is equipped);

- of the distillate, via the valve noted V1 on the

electric control panel near to the installation, (button on Manuel to open

the valve and 0 to close it);

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- of the recovery vessel, by the valve in the bottom

of each vessel.

The sample composition is analysed by gas chromatography (see

annex 3).

5. Experiment

5.1. Start up (Always in the presence of a teacher)

- start the steam boiler ;

- start software WINCC on the computer;

- open the valves for water cooling and air near the pilot

- Check that the orders, on the electric control panel, near

column, are in AUTO mode;

- Check at the flowmeter that cooling water flow is about 700

l/h.

- if necessary discharge liquid from recovery vessels in the

boiler;

- Check that:

* the liquid volume in the boiler is aproximatively 35 L

(if not add mixture);

* acetone and ethanol concentrations are

respectively approximately 8% for acetone and 15% for

ethanol, from chromatographic analysis;

- start the column operation the total reflux configuration.

To do so, in the software WINCC, in menu “Time Management”, put a

total reflux (duration of withdrawal = 0, reflux duration unspecified,

Cycle timer on CYCLE, et Timer on FIXE);

- When the pressure of the vapour is close to 3.5 bars, start

heating via the software.

- wait for the stabilization of the temperatures, (and hence of

the concentrations) along the column.

5.2. Experiments to be done

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5.2.1. Total reflux operation

This procedure is the compulsory starting point procedure for

any other operation at fixed reflux rate. It allows, on the one hand to initiate

the distillation process (filling of the column and enrichment of the top

plate), and on the other hand, to homogenize concentrations all along the

column, between two experiments.

When steady state is reached, sampling can be done in the boiler and in

various column plates.

It is also possible to estimate the thermal losses of the column, in assessing

the power to the boiler via the electric meter of the oil heater.

5.2.2. Fixed reflux operation (reflux between 0.1 and 5)

- when steady state is reached for total reflux, set the desired

value of the reflux. For example, to obtain R=1, in Time Management set

durations at 25s / 25s, then set the Cycle timer on CYCLE, the Timer

being always on FIXE

- note the liquid volume in the boiler, the boiler composition,

and the boiler and top of column temperatures.

- every 10 minutes, take a sample in the boiler and the

distillate, and analyse the methanol composition by chromatography .

- take samples along the column and analyse their methanol

composition by chromatography.

- define a criterion for the end of experiment, which may be a

given value of the average composition of the receipt, or the temperature of

top column...

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- At the end of the experiment, note the liquid volume in the

boiler, the boiler composition, the liquid volume in the recovery vessel, the

distillate composition.

5.2.3. Manual variable reflux operation

The aim of this experiment is first to maintain given acetone

composition in the distillate (this the most volatile component)

- when steady state is reached for total reflux, reflux ratio will

be varied, according to the variation of the top column temperature. Start

with small value of reflux, lower than 1, and define the aimed composition

of acetone in the distillate (75% for example)

- at initial time, note the liquid volume in the boiler, the boiler

composition, and boiler and top column temperatures.

- every 10 minutes, take a sample in the boiler and the

distillate, and analyse the methanol composition by

chromatography .

- take samples along the column and analyse the methanol

composition by chromatography .

- when the value of the reflux becomes high (R>10), note the

liquid

volume in the boiler, the volume in the recovery vessel, take a

sample in the boiler, in the distillate and in the recovery vessel

and analyse composition by chromatography .

We can continue the experiment in order to maintain now a given

methanol composition in the distillate.

-open the valve of the next recovery vessel and close the recovery

vessel containing the acetone.

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- reflux ratio will be varied, according to the variation temperature of

top column. Start with small reflux, lower or equal to 1, and now aim

at a given methanol composition of 85% at the distillate

- every 10 minutes, take a sample in the boiler and the distillate, and

analyse the methanol composition by chromatography .

- at the end of the experiment (reflux ratio equal to 10), note the liquid

volume in the boiler, the boiler composition, the volume in the

recovery vessel, the distillate composition .

5.2.4. Automatic variation of reflux ratio from top column

temperature

As in the last paragraph, the aim is to maintain a given acetone

composition in the distillate along time, but in an automatic way, thanks to

the software of supervision.

- when the steady state is reached with total reflux, in Time

Management, click on Timer in order to set it on REGULATED and

choose the reference temperature for control, while clicking on TIC1/TIC3-

TIC7 or TIC3/TIC7

- click then on TIC1 (or TIC3 or TIC7) to define the

considered plate temperature. This temperature can be set by clicking on the

value noted in red. In green, the actual value is posted, and in blue the

instantaneous percentage of opening of the valve of distillate racking.

- repeat then the operational protocol of the preceding experiments.

Similarly, it is now possible to do a second selection, to

maintain a given composition constant in the distillate along time, but in an

automatic way, thanks to the software of supervision.

- open the next recovery vessel and close the vessel containing the

acetone.

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- click on TIC1 (or TIC3 or TIC7) to modify the regulation

temperature instruction.

- repeat then the operational protocol of the preceding experiments

5.2.5 Automatic control of the opening of the recovery vessel

valves at various temperatures in the column:

Software WINCC can automatically control the opening and the

closing of valves of the 5 recovery vessels from the value of the top column

temperature, in order to distribute the distillate in different vessels.

In Valves Management, the values of the temperatures, at which

different recovery vessels open, can be defined. Automatic control is set

while clicking on AUTO V2, or AUTO V3, ...

Opening and closing of the valves can also directly be done by the operator,

on the synoptic General, while simply clicking on V2, V3 …

Repeat the operational protocol of the preceding experiments.

5.3. Shutdown of the installation

- Set reflux at total reflux.

- Stop vapour feed on the software.

- After complete cooling of the column, stop the cooling water

circulation at the condenser and close the valves of water and air flow.

6. Theoretical studies

During the experiments, it is possible to study:

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a) evolution of the phenomena along time:

- different plate temperature,

- boiler and distillate concentration

- concentration profile along the column.

b) establishment of global and partial mass balances

- list the necessary assumptions,

- calculate average values for densities and distillate

concentration.

- determine the cause of the possible experimental errors.

c) simulation of the experiments with the ProSimBatch software for

example.

d) Mac Cabe and Thiele graphical Constructions

e) Analysis of the influence of reflux on the various parameters

characterizing the functioning of the column (number of theoretical plates,

efficiency, temperature, distillate purity, distillate flow rate …)

Additional subjects of reflexion:

- What are the consequences of the thermal losses on the column?

- Does the reflux fixed on the computer really correspond to the actual reflux

in the column?

- It is also possible to approach deviation from perfect operation of the

column, by using the temperatures and/or the concentrations at the different

plates, and the liquid vapour equilibrium diagram. Comments.

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APPENDIX 1

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Thermodynamic data and physical properties of

mixture

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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

Frac

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mas

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vap

eu

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Fractions massiques en acétone liquide

Courbe d'équilibre Acétone - Ethanol

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55

60

65

70

75

80

0 0,2 0,4 0,6 0,8 1

Tem

pé

ratu

re (°C

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Fractions massiques en acétone liquide

Courbe de bulle et de rosée Acétone - Ethanol

bulle

rosée

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0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

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0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Frac

tio

ns

mas

siq

ue

s Et

han

ol v

ape

ur

Fractions massiques Ethanol liquide

Courbe d'équilibre Ethanol - Eau

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75

80

85

90

95

100

0 0,2 0,4 0,6 0,8 1

Tem

pé

ratu

re (°C

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Fractions massiques en Ethanol liquide

Courbe de bulle et de rosée Ethanol - Eau

bulle

rosée