The Effect of the Pressure Relief Valve on the
Deaeration Process
Sofie Hillåker
8th of January 2012
Tetra Pak Processing Systems AB & Department of Chemical Engineering, Lund Institute
of Technology
Abstract
In this study the main objective was to examine how the pressure relief valve affects the
efficiency of the deaeration process. The experiments showed that the flow pattern after the
valve consists of a jet containing liquid droplet in gas rather than bubbles in liquid as was
thought before. A high pressure drop over the valve leads to a much more efficient deaeration
process and most of the oxygen is removed immediately after the valve. It is also here the
biggest divergence in efficiency between the different pressure drops is. This is probably due to
a combination of cavitation and diffusion. During cavitation new gas bubbles are formed, which
leads to more turbulence and the split of bubbles decreasing the distance and increasing the
surface for mass transport of oxygen.
Introduction
Tetra
Pak
develops
designs
and
manufactures equipment for processing of
milk, juice, soups and many other liquid
foods.
An important part of the food
processing is the reduction of the air
content in the products. Air is not desired in
the product mainly because it causes
various problems in the process line, among
others it increases the fauling on heating
surfaces in pasteurisers, it causes reduced
skimming efficiency in separators and it
increases the capability to create bubbles
which facilitates cavitation. The oxygen
also affects the products negatively by
causing oxidation and shortening of their
lasting quality [Bylund, 2003].
To reduce the level
of oxygen, vacuum deaeration is being
used. Because of the vacuum the saturation
level of the oxygen is very low and the
dissolved oxygen forms bubbles that are
separated away. The design of today’s
deaeration is using flash boiling, meaning
that that the vacuum is adjusted to a level
below the boiling point of the pre-heating
temperature, which consumes a lot of
energy. Therefore Tetra Pak wants to design
a new deaertaion concept that consumes
less energy. A negative flash means that the
pressure in the deaeration vessel is above
the vapour pressure at the current
temperature. Previous examinations of
the process have indicated that one key
mechanism is the pressure release stage
over the valve [Arvinius & Komorin, 2011].
By achieving a better understanding of the
cavitation and bubble formation at this
stage, a more efficient deaeration concept
could be developed.
Results and Discussion
There is a strong connection between the
oxygen concentration and the pressure
drop over the valve. As seen in figure 1 the
oxygen concentration in the product is
significantly lower when higher pressure
drop over the valve. When far away from
O2 Concentration Depending on ΔP at
different ΔT Flash
7
6
O2 Concentration (mg/kg)
For measurements NI LabView was used,
which was the program to log the data from
the pressure transmitters, the oxygen
transmitters
and
the
temperature
transmitters. In addition to the pressure
measurements from the transmitters,
pressure differences along the pipe were
measured to examine the pressure profile
along the pipe more closely. These
measurements was made using plastic
tubes connected to the pipes in pares
forming water columns corresponding to
the pressure difference.
To examine the effect of the
pressure relief valve on the deaeration and
to visualise the process, a SPC-2 valve was
manufactured in acrylic plastic. The flow
profile after the valve was visualised for five
different pressure drops over the valve; 1
bar, 2 bar, 3 bar, 4 bar and 5 bar. For each
pressure drop different levels of vacuum in
the vessel was tested in the range of a
pressure corresponding to 35 °C from flash
(referred to as -35 °C flash) to a pressure
corresponding to 1 °C above flash (referred
to as +1°C flash). The temperature and the
flow rate were held constant for all the runs
at 35°C and 6000 l/h respectively.
The structure of the flow was
closely investigated and photos were taken
to improve the understanding of the effect
of the valve. The oxygen concentration after
the valve was measured at two different
points (18 cm and 32 cm) after the valve. In
these measurements the original SPC-2
valve in steel was used to be able to
measure the concentration immediately
after the valve.
flash the difference in oxygen concentration
is bigger between a high pressure drop and
a low pressure drop over the valve.
5
4
3
2
1
0
0
1
2
3
4
5
ΔP (bar)
Figure 1: Oxygen concentration in product at
different ΔP and different degree of flash. The blue
line represent -35°C flash, red -25°C flash, green -15°C
flash, purple -4°C flash and turquoise +1°C flash.
Oxygen Concentration at different
Distance from the Valve
The result from the experiment where the
oxygen concentration was measured along
the pipe at different pressure drop over the
valve is presented in figure 2. Most of the
oxygen is removed in the beginning of the
pipe, near the valve. The difference in
oxygen removal between different pressure
drops over the valve is largest in the
beginning. The deaeration nearer the vessel
seems less affected by the pressure drop
and the lines are rather parallel here.
10
O2 Concentration (mg/kg)
Materials and Methods
O2 Concentration at different Distance from
the Valve at different ΔP
8
6
4
2
0
0
15 30 45 60 75 90 105 120 135 150
Distance from the Valve (cm)
Figure 2: O2 Concentration at different distance from
the valve at different ΔP. The blue line represent 1
bar, red 2 bar, green 3 bar, purple 4 bar and turquoise
5 bar pressure drop over the valve.
The results from the experiments where the
oxygen concentration was measured along
the pipe at different degree of flash is
presented in figure 3. Most of the oxygen
has been removed in the beginning of the
pipe near the valve. The pressure in the
vessel doesn’t seem to affect the oxygen
removal near the valve except when the
pressure in the vessel is set to a level
corresponding to -35°C and -30°C to a
smaller extent. This indicates that the
removal in the beginning doesn’t depend
on the pressure in the vessel, and the better
oxygen removal in the product at low
pressures in the vessel is a result of a more
efficient removal along the pipe.
O2 Concentration (mg/kg)
10
O2 Concentration at different Distance from
the Valve and at different ΔT Flash
8
6
4
2
0
0
25
50
75
100
125
Distance from the Valve (cm)
150
Figure 3: O2 concentration at different distance from
the valve at 3 bar pressure drop and different degree
of flash. The blue line represent -35°C flash, red -30°C
flash, green -20°C flash, purple -15°C flash, turquoise 4°C flash and orange +1°C flash.
Structure of the Jet
The flow profile immediately after the valve
was examined at different pressure drop
over the valve. At higher pressure drop over
the valve the jet is being more defined and
reaches the walls of the pipe further away
from the valve compared to when lower
pressure drop, see figure 4.
1 bar
3 bar
5 bar
Figure 4: Structure of the jet at 1 bar, 3 bar and 5 bar
pressure drop over the valve, at -4°C flash.
The flow profile immediately after the valve
was also examined at different degree of
flash (-35°C-1°C). This revealed that the flow
profile is transformed from that the pipe is
filled with liquid to a jet with no contact
with the walls when between 35°C and 25°C
from flash, see figure 5. When the jet has
formed, there is not a defined alteration of
the structure of the jet when changing the
degree of flash.
-35°C
-25°C
+1°C
Figure 5: Structure of the jet at -35°C, -25°C and +1°C
flash, at 3 bar pressure drop over the valve.
Pressure Profile
When varying the pressure drop over the
valve the measurements indicated that
there is a low pressure immediately after
the valve, and then the pressure is
increasing to a peak. At high pressure drop
over the valve the minimum pressure is
lower, and the pressure at the peak is lower
compared with low pressure drop over the
valve, see figure 6.
Pressure Profile along the Pipe at different ΔP
Pressure (bar)
0.12
0.11
0.10
0.09
0.08
0.07
0
15 30 45 60 75 90 105 120 135 150
Distance from the Valve (cm)
Figure 6: The pressure profile along the pipe at
different pressure drop over the valve. The blue line
represent 1 bar, green 2 bar, red 3 bar, orange 4 bar
and dark blue 5 bar pressure drop over the valve.
The results from the pressure measurement
that was done when varying the degree of
flash at 3 bar pressure drop over the valve
is presented in figure 7. The measurements
indicate that the pressure after the valve is
very low even when the pressure in the
vacuum vessel is far over flash.
Pressure Profile along the Pipe at different
Degree of Flash
0.35
Pressure (bar)
0.3
0.25
the valve is more defined and reaches
further in the pipe when the pressure drop
over the valve is large.
The pressure immediately after the
valve is very low, both the pressure profiles
and the photos indicate that flash seems to
occur here even when the pressure in the
vessel is far away from flash. The pressure is
increasing to a peak probably because the
jet is reaching the walls of the pipe and the
velocity is there for decreased, increasing
the pressure. The pressure is lower at high
pressure drop over the valve both the
pressure immediately after the valve and
the peak is lower compared with lower
pressure drop over the valve. This could be
an explanation for the more efficient
oxygen removal at high pressure drop.
The pressure in the deaeration
vessel doesn’t affect the oxygen removal
that occurs immediately after the valve
since flash occurs here independently of the
pressure in the vessel. The difference in the
efficiency of the oxygen removal is very
small when 4°C from flash compared to
when +1°C flash. Most of the oxygen is
removed in the beginning of the pipe
immediately after the valve. This is probably
due to a combination of cavitation and
diffusion. During cavitation new gas bubbles
are formed, and it also leads to more
turbulence and the split of bubbles
decreasing the distance and increasing the
surface for mass transport of oxygen.
0.2
0.15
0.1
0.05
0
15 30 45 60 75 90 105 120 135 150
Distance from the Valve (cm)
Figure 7: Pressure profile along the pipe at different
degree of flash. The blue line represent -35°C flash,
green -30°C flash, dark blue -20°C flash, orange -15°C
flash, red -4°C flash and grey +1°C flash
Conclusions
The flow pattern after the valve consists of
a jet with liquid droplets in gas rather than
bubbles in liquid. The jet that is formed in
References
Bylund, Gösta (2003) Deaerators. Dairy
processing handbook second revised
edition, 149-151
Arvinius, Emelie & Komorin, Ilia (2011) New
Deaeration Concep – The deaeration
mechanisms and experimental results.
Master Thesis, LTH Chemical Engineering
Department &Tetra Pak Processing Systems
AB.