Beitr3ge zur Tabakforschung International · Volume 11 · No. 4 · August 1982
DOI: 10.2478/cttr-2013-0512
Variation of Sidestream Gas Formation
during the Smoking Cycle*
by Richard R. Baker
Group Research and Development Centre, British-American Tobacco Co. Ltd.,
Southampton, England
INTRODUCTION
In a previous study (1), the regions in which gases were
formed inside a cigarette combustion coal, the conditions
under whic:h they were formed, and how these regions
varied during the smoking cycle, were determined. The
gases formed in the coal distribute themselves between
the mainstream and sidestream smoke. In the present
study, the gas temperature and concentrations outside
the coal have been measured, in order to determine
where the permanent gases enter the sidestream plume,
and how the sidestream formation varies during the
smoking cycle. In addition, calculations have been performed in order to clarify certain aspects of the formation of the sidestream smoke plume.
EXPERIMENTAL DETAILS
The gas temperature and concentration distributions external to the coal have been obtained concurrently using
exactly the same type of cigarette, sampling probe I
thermocouple, experimental system, and computer programs as previously described (1). The cigarette was
smoked in an atmosphere of 21 O/o (v/v) oxygen /790/o
(v/v) argon inside a cubic perspex chamber of 140 mm
side. The gas mixture was passed vertically through the
chamber at a flow rate of 250 crn1 s·•, or a linear velocity of 12.8 mm s· 1 past the cigarette, equivalent to a
Beaufort force 0 air condition, i.e. "calm air" (2). In
successive experiments the sampling tip of the probe was
positioned directly above the coal, and at angles of 45°,
90° and 180° (i.e. directly below the coal) from a vertical line through the axial centre of the coal, and for
1 to 15 mm from the surface of the cigarette, and at
>1-
R«eind: 16th February 1982- accepud: 15tb June 1982.
distances of between -10 and +10 mm from the line
of paper burn at the start of the puff (Table 1). At least
four replicate experiments were performed for each position of the probe.
RESULTS
External temperature and concentration contours at
various stages during the smoking cycle are shown in
Figures 1 to 16. These contours are very sensitive to
movements in the external atmosphere, as is the visible
sidestream smoke. The external contours in the present
study were obtained with the oxygen/argon smoking
atmosphere flowing vertically past the cigarette with a
linear velocity of 12.8 mm s· 1 • This imposed velocity is
small compared to the total velocity in the sidestream
plume of 330 mm s·t, as measured by Neurath and co·
workers (3). Thus the imposed flow should only have a
minimal effect on the plume.
Taking the characteristic dimension of the smoking
chamber as the length of its side (140 mm), the Reynolds
number for the imposed flow of 12.8 mm s·1 through
the chamber is 116. Thus the flow conditions past the
cigarette out to be laminar. In confirmation of this, the
sidestream smoke plume was oberserved to rise from the
coal in a very straight column (Figure 17).
The 95% confidence limits (mean of four replicates) of
temperature at each point are about ± 25 °C, and are
about ± 10'/o of the mean values for the gas concentrations. These confidence limits are due predominantly to
slight variations in the cigarettes used, and the inabilityto mount the probe into identical positions in the sidestream plume for each replicate experiment, rather than
any variation in the thermocouple temperature or mass·
spectrometric concentration measurements. A more detailed account of the errors involved in these types of
measurements has been given previously (1).
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181
Figure 1.
Gas temperature (OC) distribution outside the
coal, 1.0 seconds after the start of the puff.
= angle of contours from
9
+16
Figure 3.
Oxygen f/o (v/v)) distribution outside the coal,
at the start of the puff (0 seconds).
9 = angle of contours from the vertical.
the vertical.
9 = 00
+12
J
+ 8
9 - 00
+16
I(
+12
+
'E+ 4
g
8
e+ 4
g
!"'
·u"'
Cl
0
-8
-12
0
Cl
9
+12
= 45°
'
-8
_g -12
0
0
+4
+8
+12
"'
u
g.:l
~
1:
"'
9
= goo
CD
.~
-12
'
-8
·-12
Distance from paper burn line
-8
-4
0
+4
01
·l:;~~~:~'
:0
~=1~0
~a18
~
•
-4
-4
'
0
+4
-;;;
"'
c
+8
+12
(mm)
+8
+12
~~~~
- 12
- 8
- 4
0
+4
+8
+ 12
u
c
·:j·---~~
I
+4
-12
- 8
-12
-8
- 4
0
+4
Distance from paper burn line
or
-4
-==:::::::
0
+8
+12
(mm)
+4
+8
. ·~·
= 18.
20
+12
I
9 •180°
+4
Figure 2.
Gas temperature (°C) distribution around the
coal, +3 mm from the paper burn line, at (a) 0 s and
(b) 1.0 s from the start of the puff.
(a)
Figure 4.
Oxygen (Ofo (v/v)) distribution around the coal,
+3 mm from the paper burn line, at (a) 0 s and (b) 1.0 s
from the start of the puff.
(b)
150
182
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Figure 5.
Carbon monoxide (Ofo (v/v)) distribution outside the coal, 0 seconds from the start of the puff.
e = angle of contours
+16
Figure 8.
Carbon 111onoxlde (Ofo (v/v)) distribution outaide the coal, 1.0 second from the start of the puff.
from the vertical.
e-oo
e- oo
+16
\
+12
+12
+
+ 8
8
-e+ 4
.§.
i.
.S!
0
-12
+12
-8
.. +:1
..
CD
u
'1:
0
e- 45°
~
::J
E
.
I
-12
-8
-4
0
+4
+8
.,u
::J
I
+12
~~~~:~
-4
0
+4
Distance from paper burn line
-8
,,..
+4
.,
u
I
-12
-8
-4
0
+4
+8
I
+12
c
-8
.,. -
,g
1il
i5
I
-12
E
.
] ·---12
. +I~w
..
~0.5-.:............
'1:
CD
u
c
+12
"ij
0
1il
i5
+8
<a
01
"'
"ij
,g
a;
-4
0
+4
+8
I
+12
+4[="0
0~0.5~
-12
-8
(mm)
~~
~
-4
0
+4
Distance from paper burn line
+8
I
+12
-12
-8
0
+4 t e 1ao~
-4
I
0
+4
+8
+12
+8
-+12
(mm)
-==~·
0.2
0.3
=
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183
Figure 7.
Carbon monoxide (Dfo (v/v)) distribution outside the coal, 2.0 seconds from the start of the puff.
+16
Figure 8.
Carbon monoxide (Dfo (v/v)) distribution outside the coal, 2.5 seconds after the start of a two-second
puff.
+16
9-0°
+12
+12
1.0
1.0
e-+4
s
9-00
e+4
s
~-------------
rr-~.
.,
0
c
~
Q
I
+4
Distance from paper burn line
-12
o·le . .
-6
-4
0
<:::::!::: 0.3 -
+4
I
+12
-8
·r-·.
-12
-8
(mm)
so
2.0
-4
0
+8
+12
0.4 :> ~
180"
Figure 9.
Carbon monoxide (Dfo (v/v)) distribution around
the coal, +3 mm from the paper burn line, at (a) 0 a and
(b) 1.0 a from the start of the puff.
-12
-8
+6
+12
~1.0~
-4
~
0
+4
+8
-4
0
C::::::::::o.S'_o.4
ole
I
+4
Distance from paper burn line
+4
184
-12
1.5~
~3.0~
+12
(mm)
+4
+8
~
+12
==180"
+4
Figure 10.
Carbon monoxide (Dfo (v/v)) distribution around
the coal, +3 mm from the paper burn line, at (a) 2.0 sand
(b) 2.5 s from the start of the two-second puff.
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I
Figure 11.
Carbon dioxide (0/o (v/v)) distribution outside
the coal, 0 seconds from the start of the puff.
+18
Figure 12.
Carbon dioxide (Ofo (v/v)) distribution outside
the coal, 1.0 second from the start of the puff.
e. oo
e- oo
+18
+12
+12
+
8
+ 8
e+4
.§.
e-+4
.§.
+12
&~!~
-4
0
+4
+8
~
I
\
2.0
2.0
0~----~----~~L---~~~~~L-~~L----­
o; -12
·c:;"'
0
-8
+12
.,+4~=450.
~
'
+12
1.0-----
~0~2.0~~
_g.,
-12
-8
-4
0
+4
+8
+12
u
c
~
~~
~
-4
0
+4
Distance from paper burn line
-12
0
+4
1e
-8
-1~
-4
0
+4
+8
+12
c
c:=
+40C----------1.0
-12
-8
-12
0
-8
(mm)
+8
~1.5~
1.0
~!:~=~=-------4
0
+4
Distance from paper burn line
+12
'
!~ •
-4
0
~1.0
+4
I
+8
+12
. +8
+12I
(mm)
-====-=-==--
1aoo
+4
Figure 13.
Carbon dioxide (Ofo (v/v)) distribution around
the coal, + 3 mm from the paper burn line, at (a) 2.0 s and
(b) 2.5 s from the start of the puff.
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185
Figure 14. Hydrogen (0/o (v/v)) distribution outside the
coal, 0 Mconds from the start of the puff.
+12
I
0.2
1\
0.4
0.4
+ 8
-e+ 4
e-+4
.§.
.§.
~
I
+12
\
0.5
0.2
8
+
e. oo
+1&
e -oo
+18
Figure 15.
Hydrogen (Dfo (v/v)) distribution outside the
coal, 1.0 Mcond from the start of the puff.
0.5
0~----~----~~--~~~-L~~~~~----~~
.. -12
·;;
-8
""
-4
+4
+8
+12
~;;~,
0
-4
0
+4
+8
+12
.k;;~
-4
+4
0
Distance from paper burn line
-12
0
]
-8
1
-4
+4
0
+8
+8
~--·-0.2~
e- 180°
+12
- 12
- 4
0
+4
Distance from paper burn line
+12
-12
0
+4
+4
- 8
(mm)
\
-8
-4
0
+12
+4
+8
+12
~0.4~
0.2
e- 180°
Figure 16.
Hydrogen (Dfo (v/v)) distribution outside the
coal, + 3 mm from the paper burn line, at (a) 2.0 s and
(b) 2.5 s from the start of the two-second puff.
(a)
186
+8
(mm)
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1
Figure 17.
Photograph of a sides-tre-a m smoke column,
1.0 ,.cond a fte r IIIo atar1 or a two-second pull.
smoke curves back slightly, immtdiattly oboYe the cigartue, and away from the ash. Comparison with the temperature ~ontours in Fig-ure 1 indicates that the tem·
p<rature of the smoke is loss than 150 •c. The Yisibie
sidcstream smoke column does not change significantly
as the puff is taken. Tt d~, ho.,•e,•tr, deptnd critically
on the movement of the sunounding air.
DISCUSSIO N
A comparison of the internal oxrgen distribution in Fig·
ure 2(b) o f reference 1 wit h t he exttrnal distribution in
Figure 3 shows t hat the two seu of contours ma tdt up
fairly well. The con tours just inside che coal are as low
as 6°/o (v/v) at a bout 2 mm in front of the paper bur•
line, whereas just outside the co:tl at the same point the
oxygen concentrat ion is 85
(v/v) above the coal,
12 1 /o (v/ ,·) at the side of the coal, and as high a.s IS 1/ o
(vh) immediately below the coal. Examination of the
oxygen contours at other times during 1he smoking C).-cle
always sho~•.-s similar trends. Thu~ the le,·els of oxygen
outside the coaJ are not primarily determined by any
diHtUion gradient set up by the oxyg<n deficient coal,
sim:e then the externa l oxygen levels near coal would
be symmetrical a nd the $ame as the levels ju.n inside the
coal. Ra ther, the externa l levels are d~termined partly
by convect ive currents set up by the hot coal, and in fluentcd by buoyancy effects. The external g>S temperature distribution s depicted in Figure 2 are very similar
to lhe isotherms observed around a hot horiumtal cyl·
inder in a free-convection flow of ~ir a.s recorded by an
lnterfcrometer (S). Thus there is a natural convection
flow of air to the hot coal in an upwards direction. The
oxygen in the con"·ec:rion stream reacts on the hot sur·
face of the coal, and the convtcted air is transported
round th< coal (rather th•n <hrough it), so that the residual ox.ygen in the st.r eam is reduc.e d fun~r as it pro·
gresses round the coal.
The exter~l oxygen-ddic;ienr column rising abo,·e the
coal. centering a t about 3 mm in front of the paper burn
line (Figur« 3 and 4) remains largely uneh•nged during
the smoking c ycle, although the oxygen levels close to
the surface of the coa l do increase slightly during the
puff. Thus the natural convection stream is only a ffect·
ed sligh tly by the in flux o f air d uring the puff. The
combustion processes occurring on the surface of the coal
due eo t he convection stream proceed indeptndently of
those inside the coal.
The external car bon monoxide and dioxide contours are
affected appreciably by the occurrence of the puff. These
<hanges are not necessarily the nme as the changes that
occur to the internal contours. The external and inte-rn•l
contour di-Stributions for the oxides o£ carbon match up
fairly consiSiently btfore the puff (e.g. Figure 18, conStructed by superimposing: external contours from th-e
present study on the internal contours obtained for tb~
same type of cigarette a nd identical conditions in a pre
vious study (1)). H o weYer, in both cases the level o f t he
gas immediately above the coa l is higher chan t hat just
'I•
Prior to the puff, under well defined laminar atmos·
pheric conditions, hydrogen, carbon monoxide and car·
bon dioxide rise from the coal in a fairly wtll dtfintd
column of gas which cem.res on a position about 3 mm
in front of the paper bur-n line. The e-xternal tempuuure
and OX)'gtn·defident contours folio~· a s-imilar column.
The tem~rature distributions in Figures 1 1nd 2 :a.re
very similar to chose obtained "'·it h tbe cigarette smoked
under continuous dra '9.' condit ions ( +), and those report·
ed by Nturath et a l. (3) above tht su, face o f a smouldtr·
ing c igarene.
At a ll stages during the smok ing cycle, the oxygen contours behind che paper burn line become virtually par·
allel to the dgarette axis revealin g t he concentration
graditnt through which oxygen d iffu.ses into the oxygendeficient tob>cco rod, and on into the back of the coal
during smoulder (e.g. Figure 3). Thus the smoulder rate
of the cigarme dtpends partly on t..be rate of diffusion
of oxyg<n into the back of coal and it is observed (4)
that the cigar~ne smoulder rate is linarly related to the
difftUion coeHidtnt of oxygen in n itrogen through the
pap<r.
Litcle mange occurs tO the externa l oxygen and te"nl·
perawre contours when the pu ff is t<tken. Hov.·e . .·er,
during the puff the carbon dio xide lenls above the co•l
o f the cigarette de<rease continuously, l he h}•droge,,
levels increase, while t he carbon mon oxide levels remain
roughly constant (Figu res 5 to 16). T he concentr•t ion
gradients bthind the burn line beoome a lmost porallel
to the cigareue ax_is during the puff (e.g. Figures 6, 1.
12 and 15). This results from the ga ses diffusing out of
the tob.ac~o rod, and is a dire.c;t observation that such
diffusion OCCUN during the puff. When th< puff tnds
there is an efflux of g~ and the excern2.l g.l.SCS auain
their pre·puff diJuibutjons withjn 3-S seconds.
Figure 17 shows a photograph of the sideuream smoke
le.tving the dgar<tte half-way through the puff. The
photog.r aph was processed. to giYe maxlmum oonltan, in
order eo clarify the smoke as much as possib!e. During
smoulder, the sidestream smoke rises from the cigarette
in a column about 3 mm in d iameter, cemering at about
3 mm behind the line of p•per b urn. T he sideweam
4
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187
inside the coal at the same point (e.g. 3.00/o (v/v) carbon
monoxide above the cigarette between about 3 and 4 mm
in front of the paper burn line, and only 2.00/o inside
the coal at the same point). Thus, a proportion of the
external carbon oxides must be formed on the surface
of the coal from the oxygen being convected across the
coal, rather than entirely formed inside the coal and then
diffusing outwards. During and immediately following
the puff, the levels of the carbon oxides above the coal
are substantially lower than those just inside the coal
(e.g. 2.5°/o (v/v) carbon monoxide above the coal between 0 and 4 mm half-way through the puff, and up
to 6 1/o (v/v) just inside the coal (Figure 19)). Clearly,
the processes leading to the internal and external carbon
oxides are proceeding independently, at least in the region of the coal in front of the paper burn line.
As the convection stream travels round the coal, the
external carbon oxide concentrations increase as the
convective mass transfer effects enhance the levels produced on the surface and from outward diffusion. This
results in the non-symmetrical distribution of the gases
round the coal.
On the other hand, the external hydrogen distribution
is less influenced by the buoyancy effects in the convection stream (e.g. Figure 16(b)). The external hydrogen
levels are probably more influenced by diffusion out of
the coal than are the levels of carbon monoxide and
dioxide. In general, the levels of hydrogen just inside
the coal are higher than those just outside the coal,
especially in the region of the paper burn line. The
oxygen levels in this region are high (typically 10-150/o
(v/v)), and the gas temperatures just inside the cigarette
are between about 300 and 500 °C (1, 6), and about
100-200°C just outside the cigarette (Figure 1). These
conditions favour the rapid oxidation of hydrogen (7,
8), so that mudl of the outward diffusing hydrogen
would be oxidised to the water found in the sidestream.
In fact, water is an important combustion product, rivalling carbon monoxide and carbon dioxide on a mass
basis. However, unlike the carbon oxides, water is delivered almost entirely to the sidestream, and the incorporation of atmospheric oxygen in sidestream water
is much higher than in mainstream water (9, 10). Thus,
water in the sidestream gases is a secondary product
formed by oxidation of outward diffusing hydrogen.
During the smoulder period, all the gases investigated
rise from the coal in a fairly well defined column which
centres at about 3 mm in front of the burn line. During
the puff, the level of gases above the coal decreases, and
the contours are pulled down towards the drawing end
of the cigarette, revealing the symmetrical external concentration gradients behind the paper burn line, resulting from the outward diffusion of the gases out of the
tobacco rod through the cigarette paper. On the other
hand, the visible sidestream smoke column lies between
about 0 and 4 mm behind the paper burn line, becoming
visible at temperatures below about 150 °C, This is approximately the position at which pyrolysis products are
formed inside the tobacco rod, although a distinct pyrolysis region is only observed at particular times during
188
the smoking cycle (1). The mainstream smoke aerosol is
most probably formed in this region inside the cigarette,
since the gas temperatures (1, 6) are below the aerosol
threshold limit of 300°C (11). Thus it is probable that
the substances entering mainstream and sidestream smoke
particles are formed in about the same region behind the
coal.
Formation of Sidestream Smoke
Based on the incorporation of radioactive products into
smoke, it has previously been asserted (12) that the mainstream particulate phase is formed largely in the peripheral zone of the formation region, and the sidestream particulate phase is formed in both the axial and
peripheral zones. The range of aerosol particle sizes
found in sidestream smoke is mudl smaller than the
range in mainstream smoke. The majority of sidestream
particles have diameters in the range 0.01 to 0.1 run. and
the majority of mainstream particles have diameters in
the range 0.1 to 1.0 v.m (13-17}, although the particle
median diameters in the two smoke streams are much
closer together. Even though the open spaces between
the cellulose fibres of cigarette paper are 1-2 v.m, calculations described below using a mathematical model
of diffusion inside a cigarette show that the sidestream
particles are not formed inside the cigarette and diffuse
through the paper. Rather, a vapour formed in the
pyrolysis/distillation region behind the coal diffuses out
of the cigarette, where it condenses to form the side··
stream smoke particles. The vapour leaving the pyrolysis/distillation region to the outside of the cigarette is
subjected to gre;tter dilution and faster temperature
losses than that pulled down the tobacco rod to form the
mainstream smoke. The former conditions favour the
formation of smaller aerosol particles as is observed in
~idestream smoke.
This outward diffusion of the vapour occurs in the region immediately behind the coal, through cigarette
paper at temperatures of 300 °C and above. It is known
that paper permeability remains approximately constant
up to temperatures in the region of 270 °C, above which
disintegration of the cellulose structure starts and results in a very high permeability for all conventional
cigarette papers (18). Thus there will be little or no impedance by the cigarette paper to the outward diffusion
of the vapour from its formation zone, through cigarette
paper whose structure has partially disintegrated. The
same minimal resistance to the outward diffusion will
be offered by different conventional cigarette papers of
differing initial air permeabilities. Thus it is observed
(19) that the sidestream total particulate matter delivery
is independent of paper permeability.
Calculation of Diffusion Rates of
Sidestream Precursors
For the calculation of diffusion rates out of the cigarette,
a mathematical model of the mass transfer processes
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Figure 18. Comparison of carbon monoxide f/o (v/v))
distribution• lnalde and above the cigarette, 0 seconds
from the start of the puff.
1.0
Figure 19. Comparison of carbon monoxide (Ofo (v/v))
distributions Inside and above the cigarette, 1.0 second
from the start of the puff.
\
1.0
above
cigarette
above
cigarette
T
E
E
Inside
cigarette
...
l
-8
0
+4
+8
Distance from line of paper burn
(mm)
-4
+12
occurring inside the porous structure of a cigarette has
been used. The model is a mathematical extension of
that described previously (20). It includes diffusion
through the tobacco bed, through the cigarette paper, and
away from the outer surface of the paper into the atmosphere, and utilises the calculated pressure and gas
velocity distributions throughout the cigarette. The calculated gas velocities take into account the non-Darcy
gas flow through the tobacco bed (21-23), which is
given by:
!_
L
2
=
_g_
e + {_g_}
A
A
e'
where
P
is the pressure difference across the porous solid
[cm water, where 1 cm water = 98 N m-2 ],
Q
is the flow rate through the porous solid [cm3 s- 1 ],
L
is the length of the porous solid [cm],
A
is the area of cross-section [ cm2 ],
e
is the impedance due to viscous forces
[cm waters cm-2],
and
e' is the impedance due to inertial forces
[cm water s2 cm-3 ].
The model calculates, inter alia, the isothermal steadystate diffusive loss of a substance from the cigarette for
a given set of conditions and cigarette parameters. The
diffusion coefficient of the substance through the tobacco bed and cigarette paper are input parameters of
the model.
r
Inside·
cigarette
E
E
...
L
-8
-4
0
+4
Distance from line of paper burn
+8
+12
(mm)
For the present calculation, the sidestream particles or
their precursor vapour are assumed to_ be formed in a
region of the cigarette 4 mm long, with a constant temperature of 300 °C throughout that region. Dimensions
and parameters of a typical English cigarette are used
in the calculation, viz. radius 4 mm, paper thickness
36 J.Ull, e = 2.27 X 10-2 cm waters cm- 2 and e' = 1.51
X 10-4 cm water s2 cm-3 (measured at room temperature). A typical flow out of the cigarette during the puff
of 17.5 cm8 s- 1 is used (measured at room temperature),
while a convective flow rate of 1.0 cm3 s-1 through the
pyrolysis/distillation zone during smoulder is assumed.
Two types of calcultion have been undertaken: one in
which aerosol particles diffuse out of the cigarette, and
one in which a vapour diffuses. The effect of temperature
on the parameters used in the model must also be calculated.
An outlet flow from the cigarette of 17.5 cm3 s- 1 at
room temperature during a puff is equivalent to 33.6
cm3 s-1 at 300 °C, for an ideal gas mixture. Independent
experiments have shown that the viscous impedance (e) of
the tobacco rod is proportional to the viscosity (!l) of
the flowing gas. The viscosity of a gas is approximately
proportional to '!0·6 • Thus a typical viscous impedance
of 2.27 X 10-2 cm waters cm-2 at room temperature
would be 3.15 X 10-2 cm waters cm-2 at 300°C.
The inertial impedance (e') of the tobacco rod is proportional to the density (Q) of the flowing gas, and Q is
proportional to T- 1 • Thus e' is proportional to T-1, so
that a typical inertial impedance of 1.51 X 10-4 cm
water s2 cm-3 at room temperature would be 7.85 X
10-5 cm water s2 cm-3 at 300 °C.
The cigarette paper surrounding the pyrolysis/distillation
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189
region behind the combustion coal has a temperature
approaching 300 °C (6). It is known that the air permeability of conventional cigarette papers remains approximately constant up to temperatures in the region
of 270 °C, above which disintegration of the cellulose
structure results in a very high permeability (18). The
air permeability of the partially degraded paper surrounding the pyrolysis/distillation zone at 300 °C has a
typical value of 100 cm min-1 (10 cm watert1•
For an air permeability of 100 cm min- 1 (10 cm water)-\
the diffusion coefficient of a binary mixture of carbon
monoxide in nitrogen through such a paper (Op) is typically 0.018 cm2 s-1 (20). The diffusion coefficient of carbon monoxide I nitrogen through the unrestricted gas
phase (Dg) is 0.21 cm2 s-1 at room temperature (24), so
that Op/Og is 0.086 for carbon monoxide I nitrogen.
Similarly, Dt!Dg is 0.40, where Dt is the diffusion coefficient for carbon monoxide I nitrogen through a typical tobacco bed. It is assumed that these ratios apply to
other gas mixtures at room temperature. Furthermore,
diffusion through the porous structure of cigarettes obeys
Graham's Law (25-27). Thus, the diffusion coefficient
of any gas mixture through cigarette paper and a tobacco bed can be calculated. Similarly, since diffusion
coefficients are proportional to Tl. 75 (28), the room temperature values of Op and Dt can be converted to their
values at 300 °C. Some examples are given in Table 2.
The diffusion coefficients for spherical particles of unit
density and diameters of 0.01 and 0.1 11m are 5.24 X to-4
and 6.82 X to-o cm2 s-1 respectively in air at room temperature and pressure (29). It is assumed that values of
Dt and Op at room temperature can be calculated using
the above ratios. The variation of the diffusion coefficient of particles suspended in a fluid (D) with temperature T [K] is given by:
D
=
kTB
(a) aerosol particles of diameters 0.01 and 0.1 !J.M cannot diffuse out of the pyrolysis/distillation region
behind the coal, and
(b) a vapour having a molecular weight of 50-1000
can diffuse out of the region behind the coal, the
exact amount of diffusion decreasing as the molecular weight of the vapour increases.
Thus, these calculations suggest that sidestream smoke
is formed by the outward diffusion of a vapour from
the pyrolysis/distillation zone behind the coal. This
vapour must condense to form the aerosol particles once
it is outside the cigarette.
Table 1.
Distance (l}
from central
Ang Ie
axis of
(9)*
cigarette
(mm)
[cm2 s-1 ]
where
Initial positions of probe for external contour
distributions.
90o
k is Boltzmann's constant, 1.38 X 10-18 g cm2 s-2 K- 1,
Initial axial** distance (mm)
from paper burn line
5
-10 -6 -4 -2 0 2 4 6 8 10
6
-10 -6 -4 -2 0 2 4 6 8
7
-10 -6 -4 -2 0 2 4 6 8
8
-10 -6 -4 -2 0 2 4 6
10
-10 -6 -4 -2 0 2 4 6
12
-10 -6 -4 -2 0 2 4 6
14
-10 --6 -4 -2 0 2 4 6
19
-10 --6 -4 -2 0 2 4 6
5
-10 --6 -4 -2 0 2 4 6
6
-4-20246
7
-2 0 2 4
8
0
10
0
5
-10 --6 -4 -2 0 2 4 6 8
6
-10 --6 -4 -2 0 2 4 6
and
7
-10 --6 -4 -2 0 2 4
B is mobility of the particles, [s g- 1 ].
8
-4 -2 0 2
0
9
B is given by (30):
c
B=-3 1t11d
where
C is the Cunningham correction factor (dimensionless),
180°
5
-4-20246
• The angle (9) between the sampling tip of the probe and a vertical line through the axial centre of the cigarette.
Probe
11 is the viscosity of the fluid (poise, i.e. [g cm- 1 s-1 ]),
and
d is the particle diameter [cm].
Thus, since 11 for a gas is proportional to To.&, D is proportional: to Tu, and so the room temperature values
of Op and Dt for the aerosol particles can be converted
to their values at 300 °C (given in Table 2).
The calculated results, given in Table 2, show that for
both puffing and smouldering conditions of the cigarette:
190
•• Axial positions in the unburnt tobacco are given as negative
distances from the paper burn line, positions in the coal and ash
are given as positive distances, and the paper burn line is given
the axial position of zero.
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Table 2.
Diffusing
substance
Calculation of proportion of material behind combustion coal that diffuses Into
sldestream.
Molecular
weight of
vapour
Diameter
of aerosol
particle
(!lm)
Diffusion coefficients at
300°C (cm2 s-1)
Dt*
I
Per cent of substance that
diffuses into sidestream
Dp**
during
puff
I
during
smoulder
50
2.0 X 10-1
4.2 X 10-2
14.8
81
Vapour
100
1.4 X 10-1
3.0 X 10-2
11.9
72
Vapour
150
1.1 X 10-1
2.4 X 10-2
10.4
66
Vapour
200
9.9 X 10-2
2.1 X 10-2
9.4
62
Vapour
250
8.8 X 10-2
1.9 X 10-2
8.6
59
Vapour
300
8.0 X 10-2
1.7 X 10-2
8.1
57
Vapour
400
7.0 X 10-2
1.5 X 10-2
7.2
53
Vapour
500
6.2 X 10-2
1.3 X 10-2
6.6
51
5.6
46
43
Vapour
Vapour
750
5.1 X 10-2
1.1 X 10-2
Vapour
1000
4.4 X 10-2
9.4 X 10-3
5.0
2.9 X 10""4
6.2 X 10-s
0.0
0.4
3.8 X 10-6
10-7
0.0
0.0
Particles
Particles
0.01
0.1
8.0 X
* Diffusion coefficient through tobacco bed.
** Diffusion coefficient through paper.
SUMMARY
External contour distributions for gas temperatures and
for the concentrations of carbon monoxide, carbon
dioxide, hydrogen and oxygen are reported at successive
times in the smoking cycle.
The sidestream gases leave the coal in a vertical column
centering at about 3 mm in front of the paper burn line.
The levels of oxygen and the carbon oxides outside the
coal are not primarily determined by any diffusion
gradient originating in the inner coal. Rather, the external levels are determined partly by a ·convective current set up by the hot coal, and influenced by buoyancy
effects. The oxygen in the convection stream forms the
oxides of carbon on the hot surface of the· coal, and the
convected air is transported around the coal in an upwards direction. On the other hand, the hydrogen levels
outside the coal result from diffusion from the inner
coal, although much of this outward diffusing hydrogen
is oxidised to water on the surface of the coal.
The visible sidestream smoke column is about 3 mm in
diameter and its centre is about 5 mm behind the sidestream g-as column, becoming visible when the temperatures in the gas column are below about 150 °C. The
vapour which eventually condenses to form mainstream
and sidestream smoke is released in the general pyrolysis/
distillation region of the cigarette, just behind the paper
burn line. Some of this vapour diffuses to the outside .
of the cigarette, through the partially degraded paper,
and it condenses to form the sidestream smoke particles
once outside the coal.
ZUSAMMENFASSUNG
Es wird iiber die Verteilung der Temperatur und der
Konzentrationen an Kohlenmonoxid, Kohlendioxid,
Wasserstoff und Sauerstoff auBerhalb des Verkohlungsbereichs der Cigarette zu verschiedenen Zeitpunkten wahrend des Abrauchzyklus berichtet.
Die Gase des Nebenstromrauches entweichen am Glutkegel in Form einer senkrecht aufsteigenden Saule, deren
Mitte ungefahr 3 mm vor der Brennlinie des Papiers
liegt. Die auBerhalb der Glutzone befindlichen Mengen
an Kohlenoxiden und Sauerstoff werden im wesentlichen
nicht von einem aus dem Inneren des Glutkegels wirkenden Diffusionsgradienten bestimmt, sondern vielmehr
von einer von der heiBen Kohle ausgehenden Konvektionsstromung; auBerdem stehen sie unter dem EinfluB
von Auftriebskraften. Der Sauerstoff in dem Konvektionsstrom erzeugt die Kohlenoxide auf der heiBen Oberflache des Glutkegels, die erwarmte Luft unterliegt der
Konvektionsstromung und wird im Umkreis der Glutzone nach oben transportiert. Andererseits ist die Menge
des auBerhalb der Glutzone befindlichen Wasserstoffes auf
die aus dem Inneren der Kohle heraus sich vollziehenden
Diffusion zuriic:kzufiihren, wenngleich viel von dem nach
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191
auBen diffundierenden Wasserstoff auf der OberfHiche
der Kohle zu Wasser oxidiert.
Die Raudtsiule des Nebenstromraud!.es mit einem Durchmesser von ungefahr 3 mm hat ihre Mine in ihrem sichtbaren Teil etwa 5 mm hinter der unsid!.tbaren Nebenstromgassliule, denn die Raudu:aule wird erst sichtbar,
wenn ihre Temperatur unter etwa 150°C absinkt. Das
Dampf/Gas-Gemisch, das teilweise kondensien und
Haupt- und Nebenstromrauch bildet, wird unmittelbar
hinter der Brennlinie des Papiers im Pyrolyse/Destillations-Bereich freigesetzt. Dieses Gasgemisch diffundiert
durch das teilweise abgebrannte Papier nach au6en und
wird, sobald es sich au6erhalb des Glutkegels befindet,
zu den Partikeln des Nebenstromraud!.es kondensiert.
R£SUM£
Cet article traite de la distribution en fonction du temps
au cours du cycle de fumage des temp~ratures gazeuses
et des concentrations en CO, COr, Hr, 0 2 •
Les gaz du courant secondaire s'&:happent du c6ne de
combustion sous forme d'une colonne ascendante verticale dont la base se situe a environ 3 mm en avant de la
ligne de combustion du papier. Les quantit~s des oxydes
de carbone et d'oxygene qui se trouvent en dehors du
c6ne de combustion ne sont pas d~terminks principalement par l'effet d'un gradient de diffusion provenant
de l'intC:rieur du cOne de combustion. Elles sont plut6t
d~termin~es partiellement par un courant de convection
cree par le charbon briUant et, par ailleurs, par !'influence des effets de poussC:e. L'oxygene du courant de
convection forme les oxydes de carbone a la surface
brlilante du c6ne et en direction ascendante, D'autre
part, la quantitC: d'hydrogfne a l'exterieur du cOne
rCsulte de la diffusion
partir de l'interieur du c6ne,
quoique la plus grande partie de celui-ci s'oxyde en eau
son arrivee la surface du c6ne de combustion.
La colonne gazeuse visible du courant secondaire a
environ 3 mm de diamhre et son centre est environ
5 mm derriere la colonne invisible du courant secondaire:
elle devient visible lorsque les temperatures clans la eotonne gazeuse sont au-dessous de 150 °C. La vapeur qui
s~ condense eventuellement pour former le courant principal et le courant secondaire est libC:rC:e clans la zone
gC:nC:rale de pyrolyse-distillation juste derrihe la ligne de
combustion du papier. Une partie de cette vapeur diffuse
l'extC:rieur de la cigarette, travers le papier en cours
de degradation et elle se condense pour former la phase
particulaire du flux secondaire une fois sortie du c6ne
de combustion.
a
a
a
a
a
a
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192
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Acknowledgements
The technical assistance of Mr. B. G. Bunn and the computing assistance of Mr. J. M. Davey are gratefully
acknowledged.
Author's address:
Group Research and Development Centre,
British-American Tobacco Co. Ltd.,
Regent's Park Road,
Southampton, S09 JPE,
England.
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193