Table Of ContentANTEC
A REVOLUTION
IN PLASTICS
May 1-5, 2005
Hynes Convention Center
Boston, Massachusetts
CONFERENCE PROCEEDINGS
Volume
3 - Special Areas
Sponsered by
SOCIETY OF PLASTICS ENGINEERS
SIMPLE TECHNIQUE FOR ABATEMENT OF NOISE GENERATED
FROM AN AIR RING OF A LARGE POLYETHYLENE FILM-BLOWING
EQUIPMENT
K. K. Botros, J. Geerligs, J. Marler and B. Rehlau
NOVA Chemicals Research & Technology Centre.
Calgary, Alberta, Canada.
Abstract Subsequent measurements were taken with a
handheld OSPL device, when the airflow to the Outer
This paper presents a simple technique to suppress a Bubble Cooling ring was set at maximum. These
high level single-tone sound pressure generated from the measurements were taken during operation simulations
air ring of a large polyethylene film blowing machine (cooling air blower operating, but no film being blown)
when operating close to its high output range. SPL and indicated a maximum noise level of 107 dBA at the
suppression obtained was in the order of 27-29 dBA. operator’s platform on the Macro Unit. The 107 dBA
Analysis showed that the shallow cavity created between noise was distinguished as a discrete tone character, while
the adjustable chimney inner geometry and the fixed other noise generated during different operating
chimney of the upper air ring is responsible for a discrete- conditions was of a broadband type. The Inner Bubble
tone noise around 119 dB. Two positions of the Cooling (IBC) air was examined and did not generate any
adjustable chimney were found to be responsible for this noise level of concern.
high single-tone noise. The suppression technique was
based on a perforated sliding collar placed around the
chimney which has the effects of reducing the initial Relevant Data
perforation on the adjustable chimney.
Figure 1 shows a schematic of the subject Macro
Film Line, where the outer Bubble Cooling air is diverted
Introduction into two channels (lower chamber and upper chamber).
The main components of the upper air chamber are: the
deflector lip, the fixed chimney and the adjustable
A concern was raised that a high overall sound
chimney. The adjustable chimney is used to direct the
pressure level (OSPL), in the order of 104 dBA, was
airflow relative to the film bubble so as to maintain the
recorded around the Macro Film Blowing Line (Model
frost line on the bubble at a reasonably low location along
D10-1) when it was operating close to its high output.
the bubble length. Maximum air mean flow velocity in
Area and personnel noise samples taken indicated that an
the gap between the lip of the fixed chimney and the
actual personal exposure over 6 hours reached a level of
deflector ring was measured by a handheld anemometer
93.1 dBA and a projected exposure over 8 hours reached
and was found to be around 40 m/s at a blower speed
94.5 dBA. The latter translates to 893% dose based on
setting of 52%.
the Alberta Occupational Exposure Limit of 85 dBA over
an 8-hour period. This high OSPL has also caused
Discrete-tone noise is generated when the lower
discomfort to personnel in offices adjacent to the area of
chamber was throttled to minimum gap and most of the
the equipment, even when adjacent firewalls were closed.
OSPL measurements taken in an area of 90 m2 air is diverted to the upper chamber. Table 1 gives the
relevant data for the cooling air at this setting and the
surrounding the Macro Line also confirmed that the OSPL
blower speed of 52%.
was around 90 - 104 dBA, which is quite high for
continuous exposure. The actual OSPL at source would
The axi-symmetric cavity created by the inner space
be considerably higher than that measured away from
of the adjustable chimney and the top surface of the fixed
source according to the distance rule {20 log (R/Ro)},
chimney (see Fig. 2) is characterized by the stream-wise
where R and Ro are radii at measured and source
length ‘L’, and an average depth ‘D’ in the radial
locations, respectively.
direction. The ratio ‘L/D’ is an important parameter for
discrete-tone generation as will be seen later.
Measurements sustaining oscillations. Successful semi-empirical
prediction of the frequencies of oscillation has been given
Acoustic measurements were taken using a Bruel & by Rockwell [2] Rossiter [6] Heller and Bliss [9] and
Kjaer 2148 Dual Channel Portable Signal Analyzer. A Block [10]. In all of these investigations, the prediction
type 4134 B&K microphone was used with type 2669 techniques have the same general form and require two
cable connected to a ZN 0664 dual channel input module. empirical constants. They are based on a simple
The microphone was firmly mounted on a stationary description of the downstream movement of the shear-
structure with its sensing point located approximately layer vortices and the internal cavity acoustic field.
0.35 m horizontally from centerline of the Macro Line Heller and Bliss [9] provide the most widely used formula
and 0.35 m vertically above the plane of the deflector lip. for peak Strouhal number (defining the frequency at peak
At this location, the microphone is approximately 0.50 m SPL) as follows:
diagonally away from the center of the upper air ring. f L n−C
St = = 1
m U [ k−1 ]
Measurements tests were taken with the air ring set to M / 1+ M2 +1/C
∞ 2 ∞ 2
apply maximum throttle to the lower ring, directing the
The parameter (n) in the above equation refers to the
majority of the airflow to the upper ring. Operation of the
mode number, where n = 1, 2, 3, 4 etc. The values of C
film line with the air ring at this setting produced the 1
and C depend on the shear layer momentum thickness,
highest detected noise level. The results of measurements 2
flow Mach number, and L/D, among other parameters.
taken at different positions of the adjustable chimney
Rockwell [2] has found values of C =0.25 and C =0.56
starting with the lower-most position (L = 0 mm) to the 1 2
at high values of L/D (e.g. >2).
upper-most position (L = 46 mm) are shown in Figure 2
and a summary is given in Table 2.
Based on the above theory, the experimentally
determined peak Strouhal number of the self-sustaining
The above test results are summarized in Fig. 3 in
oscillations within the Macro air ring assembly are plotted
terms of peak SPL as function of L/D. It indicates that
versus the positions of the adjustable chimney where the
there are two positions of the adjustable chimney where a
peak SPL occurred (Fig. 3). The theoretical predictions
peak discrete tone develops. This is strikingly similar to
of the same (by Rockwell [2] and Heller and Bliss [9]) are
that associated with self-excited oscillations of cavity
also shown as solid lines on the same Figure for the first
flows where the excitation mechanism is coupled to an
four modes. It indicates that the peak SPL observed
acoustical resonance mode, a phenomenon that has been
around L = 21 mm corresponds to mode n=2, while the
extensively studied [1-10]. Detailed account of this
peak SPL observed around the second position of L = 40
phenomenon applied to the air ring has been analyzed by
mm corresponds to mode n=4. The aforementioned
Botros et al [11] together with details of the flow field by
observations indicate that the shear-layer/acoustic field
computational fluid dynamics (CFD).
coupling mechanism is different for 21 mm and 40 mm
positions of the adjustable chimney. Information about
the flow field over the cavity was needed, to discern the
Analysis
nature of the difference between these two distinct
positions, prompting a CFD simulation to be performed.
Self-sustaining cavity oscillation is a phenomenon Detailed CFD analysis and results were given in [11].
that results from a strong coupling between the resonant
wave effects within the cavity and the flow field at the
mouth of the cavity created by the shear layer separating
Testing of Noise Abatement Technique
the primary flow over the cavity from the secondary flow
inside the cavity. When L/D >1, the cavity is termed a
Analysis mentioned in [11] concluded with the following:
shallow cavity. Small perturbations at the leading edge of
the cavity are amplified as they travel downstream into
1. The lower position is akin to self-excited
vortical structures producing organized pressure and
oscillations coupled to acoustic resonance of a
velocity fluctuations. These fluctuations are drastically
system comprising the cavity, holes, outer guides
self-enhanced at selected frequencies if an impingement
and gaps between the guides.
edge exists downstream of the flow separation (e.g. the
trailing edge of the cavity). The resulting pressure
2. The upper position is akin to a conventional self-
oscillations can cause high vibration or noise problems as
sustaining oscillation created by flow passing a
is shown in the present paper. Numerous investigations
shallow cavity and a feedback mechanism of
(experimental and numerical) have been reported [1-10]
impinging jet on the downstream lip of the cavity.
which explain the mechanism and conditions for the self-
At chimney positions not matching the above two machining techniques. In order to suppress the high level
conditions, self-excited oscillations are not sustained, ofnoise when the chimney is at the lower position (L ≈21
resulting in lower SPL levels as shown in Fig. 4. In order mm), partially sealed holes resulted in substantial noise
to suppress the noise of the upper position, the literature suppression without any noticeable bubble instability.
offers various methods and techniques, which are
elegantly summarized by the review of Rockwell [2]. Acknowledgment
These noise suppression methods encompass variations in
the leading or trailing edge geometry of the cavity to
The authors wish to thank Gil Arnould and Phil
destroy the feedback mechanism of the self-sustaining
Edwards for valuable discussion during the course of this
oscillation phenomena to occur. In the present
study. Thanks are also due to Randi Raynard for
investigation, it was found that roughening the leading
identifying the problem and for taking initial
edge of the cavity (i.e. the lip of the fixed chimney) by
measurements. Thorough fabrication of the perforated
means of pasting a Velcro tape on the lip to create stream- collar by Dave Lamb is greatly appreciated.
wise grooves on the edge, as shown in Fig. 5. This results
Permission to publish this paper by NOVA Chemicals
in noise reduction of the order of 26.5 dBas demonstrated
Corporation is hereby acknowledged.
by the comparison in Fig. 6.
References
The disruption of the noise mechanism at the lower
position of the chimney was rather challenging, requiring
1. Rockwell, D. and Naudascher, E. “ Review – Self-
variations of the system geometry in an attempt to
Sustaining Oscillations of Flow Past Cavities”,
interrupt the acoustic characteristics of the system and
ASME Journal of Fluids Engineering, Vol. 100, June
destroy its coupling to the separated flow at the lip.
1979, pp.152- 165.
Spoiling the fixed chimney lip with stream-wise grooves
created by the Velcro tape did not produce any noise
2. Rockwell, D.: ”Prediction of Oscillation Frequencies
suppression at the lower position. This reinforces the
for Unstable Flow Past Cavities”, ASME Journal of
supposition that the discrete frequency excitation
Fluids Engineering, Vol. 99, 1997, pp. 294-300.
observed is caused by the separated jet oscillation
coupled to an acoustic mode occurring outside the
3. Rockwell, D.: ”Oscillations of Impinging Shear
cavity. When the two rows of holes were partially
Layers”, AIAA Journal, Vol. 21, No. 5, May 1983,
sealed by an adjustable perforated collar slid on the outer
pp.645-664.
surface of the adjustable chimney, the system comprising
the cavity, holes and outer guide was disrupted,
4. Tam, K.W.T.: “The Acoustic Modes of a Two-
destroying the acoustic mode responsible for sustaining
Dimensional Rectangular Cavity”, Journal of Sound
the jet instability and/or amplification. This theory is
and Vibration, 49(3), 1976, pp.353-364.
confirmed with the test results of Fig. 7 with actual film
blow tests with two resin grades. A significant noise
5. Tam, K.W.T. and Block, P. J. W.: “On the Tones and
reduction of the order of 19 dB (with resin #1) and 26dB
Pressure Oscillations Induced by Flow Over
(with resin #2), is demonstrated by closing the holes down
Rectangular Cavities”, J. Fluid Mechanics, Vol. 89,
to 15% open as shown. Film bubble stability was
part 2, 1978, pp.373-399.
observed to be not affected by the perforated collar even
at a closing position of 15% open. However, it is
6. Bilanin, A and Covert, E.: “Estimation of Possible
recognized that bubble stability is a subjective parameter
Excitation Frequencies for Shallow Rectangular
that requires a method be developed to establish it in a
Cavities”, AIAA Journal, Vol. 11, No. 3, March
more quantitative manner.
1973, pp.347-351.
7. Hardin, J.C and Mason, J.P: “Broadband Noise by a
Summary
Vortex Model of Cavity Flow”, AIAA Journal Vol.
15, No. 5, May 1977, pp.632-637.
The above argument, supported by measurements,
clearly support that spoiling the leading edge of the cavity
8. Komerath, N.M., Ahuja, K.K and Chambers, F.W.:
by roughening the lip of the fixed chimney of the Big
“Prediction and Measurement of Flows Over Cavities
Mac Film Line is effective in noise abatement at the upper
– A Survey”, AIAA 25th Aerospace Science
position (L ≈ 40 mm). The development of a spoiler on
Meeting, Reno, Nevada, January 12-15, 1987.
the leading edge of the cavity could be easily achieved
using standard single-path machine knife tools, EDM
(Electrical Discharge Machining) methods or other
9. Heller, H.H. and Bliss, D.B.: “The Physical
Mechanism of Flow Induced Pressure Fluctuations in
Cavities and Concepts for Their Suppression”,
AIAAPaper 75-491, Hampton, VA., March 1975.
10. Block, P. J. W.: “Noise Response of Cavities of
Varying Dimensions at Subsonic Speeds”, N.A.S.A.
Tech Note D-8351, 1976.
11. Botros, K.K., Clavelle, E., Geerligs, J., Marler, J.,
Raynard, R., Cust, D., and Rehlau, B., “Noise
Suppression Spoiler for the Air Ring of Polyethylene
Film Blowing Equipment”, Proc. of 2004 TAPPI
PLACE Conference, Aug 29 – Sept 2, 2004,
Indianapolis, IN, 2004.
Figure 1: Schematic of the Air Ring at Normal Position.
Key Words
Noise suppression, film blowing, polyethylene, air ring. 140
L=0mm
L=5mm
BA) 120 LL==1129 mmmm
d L=21 mm
s- L=23 mm
m
(r
e
ur 100
ss
e
Pr
d
n
Table 1: Relevant Data at Blower Speed = 52%. ou 80
S
Air Temperature (oC) 15
Air Density (kg/m3) 1.086 600 1000 2000 3000 4000 5000
Speed of Sound (m/s) 339.4
Frequency - Hz
Air Velocity at gap (m/s) 38
Diameter of Deflector Lip (mm) 330
140
Air Viscosity (Pa.s) 1.79E-05
Average Reynolds number (based on 130 L=31 mm
gap width between Fixed Chimney and L=39 mm
DMeefalenc Atoirr SLitpre oafm 1 0M macmh) Number 230,.101572 ms-dBA) 111200 CLLh==im4460n emmym mRemoved
(r
e
ur 100
s
Table 2: Summary of Peak SPL at Different Positions of Pres 90
the Adjustable Chimney. nd
ou 80
S
Adjustable Average Frequency Strouhal 70
Chimney Cavity Peak rms- at Peak
L/D Number
Position L Depth D dBA Amplitude (fL/U) 60
(mm) (mm) (Hz) 0 1000 2000 3000 4000 5000
0 20 0.00 86.88 1664 0 Frequency - Hz
5 19 0.26 86.43 3200 0.421053
12 18 0.67 86.85 2112 0.666947
19 17 1.12 116.23 2048 1.024
Figure 2: Measurements of SPL Spectra at Different
21 16 1.31 118.51 1888 1.043368
23 15 1.53 117.03 1760 1.065263 Positions of the Adjustable Chimney.
31 14 2.21 92.8 1728 1.409684
39 13 3.00 116.77 1952 2.003368
40 12 3.33 118.08 1920 2.021053
46 12 3.83 108.11 1664 2.014316
Removed no cavity N/A 85.65 2592
2.5
140
n=4 L=39 mm (with Closed Holes and Lip Spoiler)
U) 2.0 L=39 mm (Original Design)
(fL/ BA) 120
mber 1.5 n=3 ms-d
Nu e(r
Strouhal 1.0 n=2 dPressur 100
Peak 0.5 n=1 Soun 80
0.0 60
0 1 2 3 4 5 6 0 1000 2000 3000 4000 5000
Effective Cavity Length/Depth (L/D) Frequency - Hz
Figure 3: Comparison Between Measured Peak SPL
Figure 6: Measured SPL Spectrum at L = 39 mm With a
Theory.
Lip Spoiler.
130
100
A) Resin #1
s-dB 120 BA) 90 HHoolleess ((1150%0%) OOppeenn)
m d
essure(r 110 ACdhuimstnaebyle ure(rms- 80
ndPr 100 Removed Press 70
Sou und
eak 90 So 60
P
80 50
0 1 2 3 4 5 6 0 1000 2000 3000 4000 5000
Cavity Length/Depth (L/D) Frequency - Hz
Figure 4: Measured Peak SPL vs. Position of Adjustable
Chimney (L/D). 110
Hoels (100% Open)
A) 100 Resin #2 Holes (15% Open)
B
d
s-
(rm 90
e
ur
SSaaww TTeeeetthh LLiipp Press 80
d
n
u
o
S 70
60
0 1000 2000 3000 4000 5000
Frequency - Hz
Figure 7: Measured SPL Spectra for Two Film Resin
Grades, at L = 21 mm With Holes Partially Closed by the
Figure 5: Roughened Leading Edge (Fixed Chimney) of Perorated Collar.
the Cavity.
A NOVEL SUSPENSION POLYMERIZATION PROCESS WITHOUT
MECHANICAL AGITATION, YIELDING MONO-SIZED BEADS
K.C. Bleijenberg,
NOVA Chemicals, Styrenics Technology, 400 Frankfort Road, Monaca, PA, US
G. Petela,
NOVA Chemicals, 2928-16th Street NE, Calgary, AB, T2E 7K7, Canada
distribution (e.g. for average bead diameter in range 1.3-
1.6mm, standard deviation σ =0.25-0.35mm). Only part of
the beads is within the targeted size range and the rest of the
Abstract
population becomes an off-size product which has to be sold
at lower value or wasted.
The concept of a novel suspension polymerization
technology without mechanical agitation is presented. It This paper presents the concept of a novel technology,
aims to yield a practically monosized product, allows the as an alternative to a traditional stirred tank process, with
inclusion of a variety of components in the polymer the aim to improve bead size distribution to a practically
matrix with a potential for broader range of products, “mono-size” product and with a potential for a broader
compared to a conventional stirred-tank process. The product range compared to a conventional suspension
technology is comprised of two stages: dispersion of an polymerization process. The technology concept is
organic phase into droplets with the required size, and the comprised of two independent, sequential process stages:
subsequent polymerization of the droplets, while
1) the dispersion of the organic phase which can be pre-
preserving their initial size distribution. The dispersion
polymerized monomer or monomer with dissolved polymer,
method combines pressure atomization of the organic
to achieve the desired droplet diameter and very narrow size
phase below free surface of a stabilizing continuous
distribution, and
phase, with the controlled pressure pulsation imposed on
the flowing stream, and yields mono-sized droplets for 2) subsequent polymerization of the dispersed phase,
both Newtonian and Non-Newtonian viscoelastic liquids. preserving the initial size of the droplets, to yield a mono
The atomized droplets are next polymerized in low shear, size final bead product.
low turbulence flow pattern created within the continuous
Both process stages are discussed below including the
phase, by injecting an inert immiscible gas at selected
results of lab-scale experiments which were carried out to
reactor locations. The process was validated on lab-scale;
validate the technology concept.
the results are presented and discussed.
Background Mono-Size Dispersion of Organic Phase by
Pulse-Pressure Atomization
A traditional process of suspension polymerization
takes place in a batch stirring tank where styrene
A method was developed to disperse a Non-Newtonian
monomer is poured in an already present immiscible
viscoelastic liquid as mono-size droplets, at the very
continuous phase comprising water and stabilizer.
beginning of polymerization process. The method is based
Monomer is broken into droplets in the continuous phase
on liquid-into-liquid pressure atomization and an organic
by mechanical impellers. When heat is transferred,
phase which is styrene monomer pre-polymerized to
styrene polymerization starts as the dissolved initiator
different conversion levels, (10%-35%), and is atomized in
decomposes and the reaction progresses for several hours.
the reactor into a continuous viscous liquid phase
All this time mechanical agitation is applied and
comprising water with stabilizing agents.
monomer droplets undergo continuously through
breakups and coalescence. As droplets polymerize, their Although atomization is a known technique, publications
viscosity and density increase. At a certain conversion on atomization of Non Newtonian liquids are very limited
level the droplets become elastic and sufficiently resilient and are mainly related to such industrial aspects as
to retain their shape and identity and are not affected by production of paint sprays [5], fine metal powders [3] and
impeller action any more. Their size distribution remains dispersion of heavy oil fuels for combustion, [2]. In these
unchanged and will translate into the final size of the bead processes, liquids are atomized into air, either as emulsions
product. Generally, the obtained sizes of polystyrene or as sprays with the small droplet size, and a narrow size
beads can be represented by a relatively broad Gaussian distribution is not a requirement.
Mechanism of atomization of a viscoelstic liquid is The pulse-atomization of a styrene pre-polymerized to
complex, as those liquids posses specific properties, such low conversion level (10%-15%) which had relatively low
as extensional viscosity which determines the stretching shear viscosity of 0.020-0.300 Ns/m2, (20-300cP),
rate and which may exceed a thousand times shear interfacial surface tension (5-10)10-3N/m, (5-10 dynes/cm),
viscosity. A Non-Newtonian viscoelastic liquid can and a behavior similar to a Newtonian liquid, into a
sustain large extension and develop large normal stresses stabilizing continuous phase, yielded excellent droplets. The
without breaking up. It inhibits dispersion into droplets mono-size, (for the average droplet diameter d_av=0.8mm-
and is in contrast to a Newtonian liquid behavior, [6]. 1.6mm, standard deviation σ=0.04mm-0.06mm), dual size
or "custom designed" distributions of the droplets could be
Therefore, to disperse a pre-polymerized styrene into
obtained by the proper selection of the properties of both
a narrow droplet size distribution, a new technique was
phases and the process parameters. Spray pattern was a
developed which combined pressure atomization with
“string” type, in a low-jetting regime, before critical point
low-frequency, low-amplitude pressure pulsations
was reached. Flow rates and corresponding pressure levels
imposed on the flowing organic phase, upstream of the
of the atomized pre-polymerized styrene were relatively low
atomizer inlet. The imposed low-frequency and low-
(flow rate <~0.25ml/s, p< ~800 kPa). Decisive parameters
amplitude pulsation had amplified a primary instability
affecting droplet formation during atomization were
generated by the atomizer orifice and caused jet breakup
viscosity of both phases (dispersed and continuous) and
into droplets with a well controlled size distribution.
interfacial surface tension. The example of the spray of the
Atomization (and a subsequent polymerization styrene pre-polymerized to 10% level which was atomized
process) were carried out in the lab experimental setup into water- stabilizer solution is shown in Fig.2, including
shown in Fig. 1. The organic phase was poured into a the enlargement of the atomized droplets and the resulting
steel plunger and pushed out by a piston through the size distribution.
transportation line to the atomizer located in the bottom of
Atomization of highly pre-polymerized (to 30%-35%
a jacketed reaction vessel filled with a continuous phase.
conversion level) styrene was more difficult to control. The
Speed of a piston advancement determined a volumetric
liquid was highly viscoelasitc, had a shear viscosity in range
flow rate of the to-be-dispersed and pressure of a flowing
of 1.0-8.0 Ns/m2, (1000-8000cP), at ambient temperature
liquid and was controlled. To reduce viscosity of organic
and only slightly higher interfacial tension of (10-15)10-
phase, it was heated as it flows through a transportation
3N/m, (10-15 dynes/cm). Interaction between viscoelastic
line to the atomizer, in a heat exchanger located
properties and interfacial tension created flow anomalies
downstream the plunger. Exiting the heat exchanger, the
and the dispersion required order of magnitude higher
flowing organic phase was subjected to pressure pulsation
pressure, a careful optimization of frequency and amplitude
with the controlled frequency and amplitude, to yield
of the imposed pulsation and pre-heating of the flow in the
mono-sized droplet dispersion. Measurements of
transportation line to the atomizer. Stabilizers had to be
temperature and static pressure of the flowing liquid, as
dissolved at high concentrations (>3%wt) in a continuous
well as the FFT (Fast Fourier Transform) data of the
phase, to increase its viscosity, reduce buoyancy velocity of
imposed pulsation, including temperature, instantaneous
the atomized droplets and to cause mutual droplet repulsion
pressure and a pressure spectrum, were taken for every
preventing coalescence. The examples of the atomized
atomization experiment. Quality of atomization was
sprays obtained with styrene pre-polymerized to 35%
quantified by photographing and analyzing the samples of
conversion are shown in Fig.3. The spray structures differ
the dispersed droplets.
particularly in the stream part just downstream of the
It has to be mentioned that application of mechanical atomizer exit, but eventually all streams were broken into
vibration for a liquid atomization was known as the almost mono-size droplets, (for d_av=1.8mm-3.2mm,
method to improve size distribution of low viscosity σ=0.10mm-0.15mm).
Newtonian liquids. Mechanical or piezoelectric vibrators
were used to pulsate low-viscosity flow at frequencies of
30,000-210,000 Hz, dispersing the stream into air as fine Polymerization of the Atomized Droplets -
droplets of size below 50 microns which were moving
Preservation of Initial Size Distribution
with velocity of several meters/sec, [4]. In contrast,
pressure pulsation applied in the presented dispersion
method, was in a very low frequency range and after
dispersion the organic droplets were moving with a low The obtained mono-size organic droplets dispersed by
velocity through the surrounding continuous phase. Only pulse-pressure atomization were subsequently polymerized
pulsation at low frequency could propagate without for 5-8 hours at temperature of 90oC, in conditions which
attenuation in a viscoelastic liquid and, eventually, to would ensure preservation of their initial size distribution
break the stream into droplets. into the final polystyrene product. During polymerization
droplets can be destroyed by a number of factors: interfacial
surface tension between dispersed and continuous phase terms. The model comprises equations of conservation and
is significantly reduced due to high temperature of the force equilibrium for dispersed, continuous and gas phases.
reaction and due to presence of stabilizers dissolved in The solution gives relations between the process parameters
continuous phase, making droplets prone to breakups and gas properties and calculates the maximum diameter d
during early stage of polymerization; further, droplets of a styrene droplet which remains submerged in a
initially have density lower than density of the continuous continuous phase stirred by gas bubbles as:
phase and to prevent them to float to free surface and
agglomerate the buoyancy has to be forcefully overcome d= 3 w2ψ/[g(1−ρ /ρ )]
w disp cont
and, finally, as the viscosity of the droplets dramatically 4
increases with progressing polymerization, they tend to where: w - velocity of a descending continuous phase
w
agglomerate if they remain in contact with each other. stream; ψ - drag coefficient calculated as
( )
Therefore, preservation of the initial droplet size ψ=a+b(lnRe/Re)+c/Re+dlnRe/Re2 ; a, b, c, d -
during polymerization required a strategy different from constants, (a=0.35, b=5.74, c=25.66, d=-1.27); Re -
the conventional stirred tank suspension process. A flow Reynolds number, Re =w dρ /η .
w cont cont
pattern in a reactor had to be created with a sufficiently
low shear and turbulence levels to minimize breakups or The model indicated that a diameter of a completely
agglomeration of droplets but, simultaneously, to ensure submerged droplet increases with higher interfacial tension
efficient heat transfer, to overcome droplet buoyancy and continuous phase viscosity, with the increase in
forces and to keep droplets uniformly distributed within densities of dispersed phase and injected gas, with diameter
the reactor volume. of gas bubbles and with a smaller size of the reactor.
Such a flow pattern was created by a low-pressure The series of experiments were carried out to verify
continuous injection of an inert immiscible gas into feasibility of the proposed concept of polymerization
reactor. Nitrogen was selected because it has no adverse without mechanical agitation. Styrene pre-polymerized to
effect on polymerization process. different levels (10% - 35%) was pulse-atomized into
droplets and polymerization of the atomized droplets was
Due to interaction of interfacial surface tension,
carried in the experimental setup shown in Fig.1, with no
drag and buoyancy forces, the injected nitrogen formed a
mechanical agitation but with nitrogen injected to the
train of large bubbles which rose upward towards the free
reactor bottom parts to stir and to distribute droplets while
surface, stirred the continuous phase and generated the
they polymerized. The size distribution of the obtained
desired vertical vortex in the reactor. The gas flow rate,
polystyrene beads was compared with the initial size of the
geometry and locations of injection gas ports determine
atomized droplets.
geometry and intensity of this vortex. The organic
Experiments showed that the flow pattern created by a
droplets remained suspended in a slow circular motion in
gas injection, provided a uniform and well controlled
the vortex and homogeneously distributed within the
distribution of the dispersed phase within a reactor volume,
reactor volume. Gas flow rate could be adjusted during
and effectively suspended both buoyant and sinking
polymerization process and, at a later stage, might even
droplets.
be applied periodically. Optionally, gas exiting from the
reactor can be recycled back to the injection ports. Low The initial pre-polymerization level of the atomized
pressure gas injection was found to create an effective styrene was decisive for the preservation of the droplets
stirring for organic droplets during entire polymerization during polymerization: the higher was the initial pre-
process, in spite of their density changing with the polymerization of the atomized droplets, the better their size
advancing conversion. was preserved during polymerization process.
To establish basic gas injection parameters, flow Consequently, droplets atomized from styrene which
pattern created in a reactor was simulated with CFD had been initially pre-polymerized to very low conversion,
(Computational Fluid Dynamics). Commercially available generally did not preserve their shape during polymerization
software was run to calculate a 3-D case of nitrogen and disintegrated into extremely small beads with d<0.2mm.
entering through the central port and two tangential ports
In contrast, droplets atomized from the styrene which
in the reactor bottom part. Results, shown in Fig.4,
had been initially pre-polymerized to 30%-35% level,
indicated that the strongly buoyant flow of the gas
preserved their integrity during polymerization process very
bubbles upwards to the free surface created an upward
well and polymerized into beads with a very narrow size
rising central core of liquid which was next descending
distribution, similar to the initial one. Important parameters
down the walls, on either side of the upward flowing
affecting preservation of their size were flow pattern in the
region.
reactor, interfacial surface tension, stabilizer type and
In addition, a simplified analytical model was derived concentration, and dispersed/continuous phase ratio. The
to describe the mechanism of gas injection in qualitative example of size distributions of the initially atomized
droplets of 35% pre-polymerized styrene and the Acknowledgement
polystyrene beads obtained as a result of their
NOVA Chemicals permission to publish is acknowledged.
polymerization is shown in Fig.5.
Special thanks to J. O’Blenes and J. Geerligs for
Experimental results demonstrated that the proposed
experimental designs and to E. Clavelle for CFD simulation.
polymerization technology is feasible on a lab-scale, if the
substrate for the process is a monomer pre-polymerized to References
a sufficiently high (~35%) level .
1. Abu-Qudais, Moh'd; Okasha, Gassan, Diesel fuel and
Conclusion olive-cake slurry: Atomization and combustion
performance. Appl. Energy (1996), 54(4), 315-326
2. Ballester, J. M.; Dopazo, C., Experimental study of the
The novel suspension polymerization concept was influence of atomization characteristics on the
verified on a lab scale: dispersion of the organic phase by combustion of heavy oil. Combust. Sci. Technol.
pulse pressure atomization produced mono-size droplets (1994), 103(1-6), 235-63
even for highly viscoelastic Non Newtonian liquids such 3. Chun, Jung Hoon; Passow, Christian H., A study of
as styrene pre-polymerized to 35% conversion level; spray forming using uniform droplet sprays. Adv.
subsequent polymerization of the atomized droplets, Powder Metall. Part. Mater. 1992, Vol. 1, Powder
carried without mechanical agitation, preserved well their Production and Spray Forming), 377-91
initial size and yield the polystyrene bead product with a 4. Frohn A., Roth N., Dynamics of droplets. Berlin,
narrow size distribution. Springer Verlag, 2000.
5. Geckler, Samuel C.; Sojka, Paul E., High mass flow
If successfully scaled up, the technology is material and
rate effervescent atomization of viscoelastic fluids.
energy efficient; narrowing bead size distribution reduces
HTD (Am. Soc. Mech. Eng.) (1993), Volume Date
the unwanted product; elimination of mechanical agitation
1993, 270, 109-15
lowers energy consumption and increases mechanical
6. Mansour, Adel; Chigier, Norman, Air-blast atomization
reliability of the process; product performance can be
of non-Newtonian liquids. J. Non-Newtonian Fluid
improved by inclusion of different additives which, if
Mech. (1995), 58(2/3), 161-194.
mixed with styrene and atomized into droplets, will be
retained in the polystyrene matrix of the bead product.
N2
Temperature Controlled Temperature Controlled
Circulating Oil Bath Circulating Water Bath
90.3 90.3
Jacketed Atomized Droplets
Reactor
Vessel Nitrogen Bubbles
PC Equipped with
Data Acquisition
and GPIB
Inlet of water jacket
micucer Atomizer N2
RAcotdu aStcorrew Plunger SHheealtl Tube Static and DynaPressure Transd Thermocouple PistoOnPG -RRueoinldnsg eaSretiaaoltnor
Servo-Motor
Exchanger _
P t Linear
10..508003 00..000000 Pre-polymerized P~ Actuator
Styrene
Motor Controller
Fig.1 Experimental setup for pulse-pressure atomization and polymerization without mechanical agitation of highly pre-
polymerized styrene
