::: Publication
Concentration measurement of
acetic acid/water mixtures in
real time in the production of
ketene
Relevant for:
DPRn 427, SPRn, DSRn
by R. Bismark, Th. List
(translated from Chemie Technik, 31. Jahrgang, Nr. 10
The following report has been published in
German in "Chemie & Technik" in
September 2002 (31, Nr. 10).
to an accuracy of 1 % abolute. Even a small
change in the concentration in the return
pipe can lead to side reactions.
The
Wacker
process
manufacturing ketene
As this is a continuous and dyamic system,
the concentration monitoring should occur
„online“ in real time. This avoids the delays
or inaccuracies of laboratory methods such
as titration.
for
The Wacker process is the catalytic
breakdown of acetic acid into ketene. It is a
key technology because ketene is used as
a starting material for a variety of other
company products.
Vaporous acetic acid is cracked into ketene
and water in a gas-heated cracking
furnace. The cracking process is very
endothermic (140 KJ/mol) and occurs at
700 degC and 500 mbar (abs). TEP (tri
ethyl phosphate) is used as the catalyst.
The reaction product (downstream)
contains unreacted acetic acid, acetic
anhydride, carbon dioxide, methane,
propadiene (allene), hydrogen, diketene,
carbon monoxide and ethylene as well as
conversion products of the TEP catalyst
and the inhibitor ammonia.
The unreacted acetic acid and the water
produced are important variables for the
performance of the cracking furnace and
the condensation.
This current is called “dilute acid” and is
pumped away for processing.
The temperature of the condensate
changes due to fluctuations in the added
acetic acid concentration, the cooling
agents, the outside temperature and the
oven capacity.
The
necessity
of
concentration control
online
To control the reaction, it is essential to
regulate both the intake of acetic acid
before the reaction and the dilute acid itself
www.anton-paar.com
However, the different concentration
ranges of dilute acid and cracked acetic
acid cannot be represented by a single
physical property because the density
reaches a maximum value between 85 and
95 % acetic acid concentration. After this
point, the density decreases with increasing
concentration (see Fig. 2).
An
unambiguous
concentration
determination via density measurement is
therefore not possible in this range.
When looking for a suitable method for the
rapid (< 1 sec) and reasonably priced
concentration determination of acetic acid
in this range we considered velocity of
sound measurement. The velocity of sound
of acetic acid changes by 400 m /sec in the
50 to 100 % range and is therefore wellsuited for concentration determination (see
Fig. 1).
However, the anomaly of the supporting
material (water) changes the temperature
coefficients depending on the concentration
and this makes a polynomial description of
the relationships necessary.
Dilute acid monitoring via ultrasound is not
possible because the 30 to 40 %
concentration range falls in the eak of the
curve. Here, density can be used: high
resolution and with an exact temperature
compensation because the relationships
here are also polynomial.
This guarantees an accuracy of better than
0.1 m% absolute.
The necessity of exact temperature
compensation
The aim of instrument manufacturers in
recent years has been to develop
instruments with increasingly higher
physical resolution to meet the increasing
demands of customers.
An important factor in process analysis
measuring technology is the rapid
temperature changes and the high degree
of temperature dependency of density and
velocity of sound.
Figs. 1 and 2 show the velocity of sound
and density for acetic acid over the entire
concentration range.
Rapid and accurate determination of the
supporting material temperature and the
simultaneous mathematical compensation
of the temperature influence on the
measured properties are essential for
accurate concentration determination in
real time.
In the 85 to 95 % range, velocity of sound
measurement is the most suitable method
for monitoring the intake of acetic acid. The
curve in Fig. 1 falls in the upper
concentration
range,
whereby
the
steepness of the curve indicates an
attainable accuracy of better than 0.1 m%
absolute.
Fig. 3 shows the density and temperature of
dilute acid over 3 days. Two instruments
were tested: The DPRn 427 density sensor
from Anton Paar (highly accurate
temperature compensation, repeatability 1
x 10–5 g/cm3) and a coriolis instrument (no
temperature compensation, repeatability 1
x 10 –3 g/cm3).
Different
physical
measuring
properties – different possibilities
Page 1
Fig. 1
Velocity of sound of acetic acid
Fig. 2
Fig. 3
Density and temperature curves for dilute acid during the process
Density of acetic acid
If no temperature compensation is carried
out, the value of the measured density
decreases, even if the concentration does
not change.
Both instruments react to the changes in
concentration. The considerable scattering
by the coriolis instrument is partly due to its
considerably lower resolution and therefore
lower accuracy. The values determined by
the density sensor were confirmed by
laboratory titration.
Outlook
The more stable measuring values
determined by the DPRn density cell are
due to the recorded and calibrated
temperature compensation.
It is clear that it is essential to determine the
temperature simultaneously and highly
accurately under process conditions in
www.anton-paar.com
order to achieve maximum accuracy and
therefore
correct
values
for
the
concentration determination.
A
highly
accurate
temperature
measurement
and
temperature
compensation is the keystone of every
method in process analysis.
Page 2