Experimental and Theoretical Aspects of Cigarette Smoke Filtration

7

Removal of Trace Contaminants from the Air Downloaded from pubs.acs.org by UNIV OF MASSACHUSETTS AMHERST on 05/29/17. For personal use only.

Experimental Smoke

and

Theoretical

Aspects

of

Cigarette

Filtration

CHARLES H . KEITH Celanese Fibers Co., Box 1414, Charlotte, N. C. 28232

A cigarette f i l t e r is a seemingly simple f i l t r a t i o n device which has come to be a major component i n the tobacco industry i n the past twenty-five years. Like many filter systems, it u t i l i z e s well established fiber f i l t r a t i o n mechanisms to remove particles from the smoke stream, and it has an adsorbative capacity for some of the vaporous smoke components. However, i t s small and defined size - a cylinder of about 8mm diameter and 15 to 30mm length, and its extensive use as a consumer item - about five hundred billion filter cigarettes are consumed i n the U. S. each year - impose special requirements. The most obvious of these requirements is that a cigarette f i l t e r must be a p a r t i a l f i l t e r , as very few i f any cigarette smokers are interested i n brands with no appreciable taste or impact. At the present time, most popular brands of c i g arettes are equipped with f i l t e r s which remove 40 to 50% of the smoke stream. A further consumer acceptance requirement is that the draw resistance or pressure drop of the f i l t e r cigarette be low, as brands with total pressure drops i n excess of one hundred and f i f t y millimeters of water have been found to be unacceptable by the public. The filter pressure drop should thus not exceed one hundred millimeters of water at a standard flow of 17.5 ml/sec. Consumer preference also dictates that the filter should have a smooth, uniform appearance and this as well as manufacturing constraints make i t necessary that the filter be reasonably firm, both initially and throughout the smoking process. Because of the large numbers of f i l t e r s manufactured each year, the f i l t e r rod-making process and the combining process must be done at speeds up to three to four thousand units per minute. This requires that the filter material be taken from a bale, fluffed or opened, and made into cylindrical paper-wrapped rods at speeds of up to four hundred meters per minute. The material thus has to be reasonably strong, uniform, and readily handeable. These stringent requirements severely limit the choice of cigarette filter materials. The net result is that the vast majority of f i l t e r s i n this country and increasingly throughout the world cons i s t of a tow or bundle of crimped cellulose acetate continuous 79


80

REMOVAL

O F TRACE

CONTAMINANTS F R O M

T H E AIR

filaments. Before d i s c u s s i n g the p r o p e r t i e s o f t h i s and some other types of f i l t e r s , i t i s d e s i r a b l e t o consider whether a f i l t e r i s e f f e c t i v e from a h e a l t h standpoint, s i n c e t h i s question i s always p r e sent. The f i l t e r does remove s u b s t a n t i a l q u a n t i t i e s o f the compLex mixture o f components i n tobacco smoke, commonly c a l l e d t a r , and i t removes considerable amounts o f p h y s i o l o g i c a l l y a c t i v e components such as n i c o t i n e and phenol. E p i d e m i o l o g i c a l s t u d i e s by Bross ( l ) and Wynder (2) have shown t h a t f i l t e r c i g a r e t t e smokers run a s i g n i f i c a n t l y l e s s e r chance (50 t o 60%) o f c o n t r a c t i n g some r e s p i r a t o r y diseases a s s o c i a t e d with smoking. Dontenwill ( 3) has r e c e n t l y shown t h a t acetate f i l t e r s provide a s i g n i f i c a n t degree of p r o t e c t i o n i n i n h a l a t i o n experiments i n hamsters. Thus i t i s apparent that a f i l t e r does reduce the amount o f smoke ingested, and i t i s e f f e c t i v e t o a degree i n the context o f p u b l i c h e a l t h . As i n d i c a t e d p r e v i o u s l y , the usual c i g a r e t t e f i l t e r c o n s i s t s of a c y l i n d r i c a l bundle o f crimped acetate f i b e r s as i l l u s t r a t e d i n Figure 1. In general, there are some ten t o f i f t e e n thousand f i l a m e n t s present i n the bundle which extend through the f i l t e r p a r a l l e l t o the a x i s o f the c y l i n d e r and d i r e c t i o n o f smoke flow. The f i b e r s have ten t o twenty gsig-zag crimps per i n c h , so that on the average the f i b e r s are o r i e n t e d a t a t h i r t y - t o forty-degree angle t o the flow d i r e c t i o n . The packing o f crimped f i b e r s i s such that the volume f r a c t i o n occupied by f i b e r s i s about .1, and there are a number o f contact p o i n t s between f i b e r s . Under normal smoking c o n d i t i o n s which c o n s i s t o f 35ml p u f f s o f two seconds d u r a t i o n taken once a minute, the flow i s laminar with a Reynolds number o f l e s s than 1. With such a laminar flow regime, i t i s p o s s i b l e t o t h e o r e t i c a l l y p r e d i c t the pressure drop and p a r t i c l e removal e f f i c i e n c y u s i n g an i d e a l i z e d f i l t e r model such as that diagrammed i n the previous f i g u r e . In the case o f pressure drop, the Happel ( 4 ) s o l u t i o n s o f the Navier-Stokes equations f o r flow p a r a l l e l and perpend i c u l a r t o the f i b e r can be e f f e c t i v e l y combined t o provide an o v e r a l l pressure drop equation f o r f i b e r s o r i e n t e d a t any angle t o the flow d i r e c t i o n . T h i s model assumes that the pressure drop i s generated by drag o f the moving gas stream on the s t a t i o n a r y f i b e r s , and that the f i b e r s have a r e g u l a r o r i e n t a t i o n a t the average crimp angle. I t allows f o r the n o n c i r c u l a r c r o s s - s e c t i o n of the f i b e r s and f o r f i b e r touching by means o f c o r r e c t i o n s determined from s u r f a c e area data and microscopic counts o f f i b e r bundles. The complete pressure drop equation i s g i v e n i n F i g u r e 2 with a d e s c r i p t i o n o f the v a r i a b l e s i n c l u d e d . F i g u r e 3 presents a comparison o f observed and c a l c u l a t e d pressure drops f o r a wide v a r i e t y o f f i l t e r rods, which are s i x f i l t e r s l o n g . The agreement i s e x c e l l e n t as the p o i n t s are c l o s e l y c l u s t e r e d about the 1:1 l i n e . The standard d e v i a t i o n i s 23mm o r 6.5% f o r a mid-range r o d of 350mm pressure drop. From use o f the pressure drop equation, i t i s found t h a t pressure drop i n c r e a s e s l i n e a r l y with flow r a t e , f i l t e r length,


7.

Cigarette

KEITH

Smoke

Filtration

81

7 . 5 - 8 . 5 mm

Figure

7

·

3 4 x 1

4 m7r

°

F

W h e r e : oC ^

Ρ'

μ

Q

· S • b . S

W

1.

Model

L

of a cigarette

filter

S

' * ( Ζ * - 1 - l n o C ) + (4o( - 3 o C - 2 - l n < ) · c o s e 2

Q

S

2

= 3 36x10°

Q

F-L-p

cos©

=

'

T

9xl0 '

ί^ρ*··

5

Δ ρ

-

P r e s s u r e D r o p ( m m H ^ O @ 17. 5 c m / s e c

^

-

D e n s i t y of t h e f i b e r

=

V i s c o s i t y of the fluid (poise)

Q

=

V o l u m e t r i c flow rate

L

=

F i l t e r length (cm)

=

S p e c i f i c s u r f a c e a r e a of the fiber

°C

-

V o l u m e f r a c t i o n o c c u p i e d by fiber

F

-

C r o s s - s e c t i o n a r e a of the f i l t e r ( c m )

£

-

F i b e r denier per filament

b

=

Agglomeration

=

S p e c i f i c s u r f a c e a r e a of equivalent c y l i n d r i c a l fiber

Θ

-

A v e r a g e c r i m p angle

W

=

F i b e r weight

Τ

=

T o t a l d e n i e r of the f i l t e r = £ x no. of f i l a m e n t s

S

S

x

Q

3

polymer

L

W

flow

(g/cm^)

(cm^/sec)

(cm^/g) (unitless) 2

factor

(g/9xl0^cm)

(unitless)

= n o . of f i l a m e n t s /no. of f i l a m e n t b u n d l e s (cm^/g)

(degrees)

(g)

Figure

2.

Fressure drop

equation

(g/9/xl0 cm) 5


82

R E M O V A L OF

TRACE CONTAMINANTS F R O M T H E

AIR

and f i b e r surface area. Increasing f i b e r packing or weight g i v e s a more than l i n e a r increase i n pressure drop and i s the most important v a r i a b l e i n determining f i l t e r pressure drop. Increments i n c r o s s - s e c t i o n a l f i l t e r area, non-uniformity of f i b e r d i s t r i b u t i o n and f i b e r denier decrease pressure drop i n a h y p e r b o l i c f a s h i o n with the f i r s t two v a r i a b l e s having a g r e a t e r e f f e c t than f i b e r denier. Increasing crimp angle i n c r e a s e s pressure drop and has l e s s e f f e c t than the other v a r i a b l e s . A l l of these f i n d i n g s are c o n s i s t e n t with experimental observations. Turning now to f i l t r a t i o n theory, the s i z e of smoke p a r t i c l e s and t h e i r n e u t r a l or l i g h t l y charged character reduce the number of important f i l t r a t i o n mechanisms t o t h r e e . These are i l l u s t r a t e d i n F i g u r e 4 and c o n s i s t s s i m p l y o f d i f f u s i o n a l c a p t u r e , d i r e c t i n t e r c e p t i o n of the 0.1 to 1 micron p a r t i c l e s by the 20 t o 30 micron f i b e r s , and t o a l e s s e r extent i n e r t i a l impaction. The technique that we have used t o compute p a r t i c l e removal e f f i c i e n c i e s i s to compute the f r a c t i o n of p a r t i c l e s of a g i v e n s i z e which w i l l be i n t e r c e p t e d by or touch a s i n g l e f i b e r by the combination o f the three f i l t r a t i o n mechanisms. These are then i n t e g r a t e d over the range of p a r t i c l e s i z e s i n smoke to determine the s i n g l e f i b e r f i l t r a t i o n e f f i c i e n c y . The f i n a l step i s t o s u i t a b l y sum the s i n g l e f i b e r e f f i c i e n c i e s over a l l the f i b e r s along the l e n g t h of the f i l t e r t o o b t a i n an o v e r a l l p a r t i c l e removal e f f i c i e n c y . The equation f o r the s i n g l e f i b e r e f f i c i e n c y , d e r i v e d from the Happe1 s o l u t i o n s f o r a p a r t i c l e approaching the f i b e r at r i g h t angles i s g i v e n i n Figure 5. To u t i l i z e t h i s equation i t i s necessary to determine two v a r i a b l e s , the d i f f u s i o n r a d i u s and the e f f e c t i v e f i b e r r a d i u s . The l a t t e r can be approximately d e t e r mined s e v e r a l ways. The simplest but l e a s t s a t i s f a c t o r y method i s to assume t h a t the f i b e r s are c y l i n d e r s , which i s u s u a l l y not the case i n c i g a r e t t e f i l t e r s , and d i r e c t l y compute the r a d i u s from the f i b e r denier and d e n s i t y . A b e t t e r approximation i s obtained by c o n s i d e r i n g that the f i b e r has a r e g u l a r , n o n - c i r c u l a r geometric c r o s s - s e c t i o n such as a Y, X, or I beam. The e f f e c t i v e r a d i u s i s then considered to be the r a d i u s of the c i r c l e circums c r i b e d around the geometric shape, and may again be r e a d i l y computed. The b e s t method u t i l i z e s pressure drop data and the pressure drop equation p r e v i o u s l y given. S o l v i n g t h i s equation n u m e r i c a l l y f o r the volume f r a c t i o n occupied by f i b e r leads t o an e f f e c t i v e r a d i u s by d i v i s i o n by the number o f f i l a m e n t s present. T h i s technique has an advantage i n t h a t i t compensates f o r nonu n i f o r m i t i e s i n f i b e r c r o s s - s e c t i o n , f i b e r d i s t r i b u t i o n , and touching f i b e r s . As shown i n Figure 6, the e f f e c t i v e or f i l t r a t i o n r a d i u s can be considered as a circumscribed c i r c l e around the f i b e r . P a r t i c l e s reaching t h i s c i r c l e are considered to be i r r e v e r s i b l y c o l l e c t e d e i t h e r by c o n t a c t i n g one of the p r o t r u d i n g f i b e r lobes or by e n t e r i n g and remaining i n the stagnant a i r spaces spaces between the lobes. T h i s e f f e c t was c l e a r l y demons t r a t e d r e c e n t l y i n the work of Morie, Sloan and Peck ( 5) who showed that the primary d e p o s i t i o n i n normal c i g a r e t t e f i l t e r s i s

7.

600


Cigarette

KEITH

Smoke

83

Filtration

ι-

Ο X

2

400

'300

cP ο ο

cx> οχ ' oo

100

200

600

300

Calculated Pressure (MM H 0 )

Drop

2

Figure 3.

Observed vs. calculated pressure drop of acetate filter rods. Length, 120 mm; circumference, 23.8-27.2 mm; denier, 3-8.

84-

84

REMOVAL

(1 + I) ( r + r ) p

OF TRACE CONTAMINANTS F R O M

T H E AIR

2

d

/ι + , ÎLISLL. I n * ) y

4

a-*)


2

Where:

C Q I = A n inertial parameter = 2/9 · — £ • ' (· A a. F- (1 -cL) ef = Single fiber filtration efficiency

2 2

4

+ *)

(unitless)

rp = Smoke particle radius (microns) rd = Radial distance that the particle diffuses during its travel around the fiber (microns) oC -

Volume fraction occupied by fiber (unitless)

a

= Effective fiber radius (microns)

c

= Cunningham slip flow correction (unitless)

ρ

-

Density of the particle (g/cm^)

Q = Volumetric flow rate (cm^/sec) F A

2

= Cross-section area of the filter ( c m ) = Viscos ity of the fluid (poise) Figure 5.

Single fiber efficiency

equation

Figure 6.

Effective

radius of a fiber


7.

KEITH

Cigarette

Smoke

Filtration

85

at the t i p s o f the f i b e r lobes, but i n s p e c i a l f i l t e r s with f i b e r s o r i e n t e d p a r a l l e l to the smoke stream, the d e p o s i t i o n was mainly i n the v a l l e y s between the lobes. Knowing the e f f e c t i v e r a d i u s , i t i s p o s s i b l e t o approximately c a l c u l a t e the time that i t takes f o r a p a r t i c l e t o t r a v e l around the f i b e r . Using Langmuir's (6) equation f o r the average d i s placement of a p a r t i c l e undergoing Brownian motion, the d i f f u s i o n r a d i u s can be c a l c u l a t e d and u t i l i z e d t o f u r t h e r compute s i n g l e and o v e r a l l p a r t i c l e removal e f f i c i e n c i e s . To demonstrate the agreement between t h e o r e t i c a l l y derived p a r t i c l e removal e f f i c i e n c i e s and experimentally measured f i l t r a t i o n e f f i c i e n c i e s , the n i c o t i n e removal e f f i c i e n c y (NRE) of a wide range of f i l t e r s was measured and compared with the computed e f f i c i e n c i e s as shown i n Figure 7. N i c o t i n e was chosen f o r t h i s a n a l y s i s because i t i s a r e a d i l y measurable component of tobacco smoke and i n normal c i g a r e t t e s about 90% of i t i s found i n the p a r t i c u l a t e phase. In Figure 7, the p l o t t e d p o i n t s f a l l somewhat below and t o the r i g h t of the 1:1 c o r r e l a t i o n l i n e , and have a r e g r e s s i o n l i n e as i n d i c a t e d . There are two reasons f o r t h i s departure. The f i r s t i s that not a l l of the n i c o t i n e i s present i n the p a r t i c l e s and that t h i s vapor phase component can be adsorbed by the acetate f i l t e r m a t e r i a l . Lipp (7) has found that acetate f i l t e r s remove some 70% of vapor phase n i c o t i n e , so that the 10% of the n i c o t i n e i n the vapor phase would be more thoroughly removed f o r most of the f i l t e r s t e s t e d , thereby i n c r e a s i n g the o v e r a l l NRE values. At h i g h p a r t i c l e removal e f f i c i e n c i e s t h i s e f f e c t becomes l e s s important and c l o s e r agreement i s observed. The second reason i s t h a t the p a r t i c l e removal e f f i c i e n c i e s were c a l c u l a t e d using f i bers p e r p e n d i c u l a r l y o r i e n t e d t o the smoke stream. Although not t r a c t a b l e mathematically, f i b e r s o r i e n t e d a t other angles would be expected to increase the d i f f u s i o n a l f i l t r a t i o n by i n c r e a s i n g the t r a v e l time around the f i b e r without g r e a t l y a f f e c t i n g the other f i l t r a t i o n mechanisms. Thus i t would be expected that the p a r t i c l e removal e f f i c i e n c i e s should be somewhat higher. In t h i s r e gard, i t i s found from the equations that d i f f u s i o n a l capture i s the most important f i l t r a t i o n mechanism, accounting f o r about 65% to 68% of the t o t a l f i l t r a t i o n . D i r e c t i n t e r c e p t i o n accounts f o r 30 to 35$ and i n e r t i a l impaction i s r e l a t i v e l y unimportant, cont r i b u t i n g only 1 to 1.5? t o the o v e r a l l f i l t r a t i o n p r o c e s s . There are s e v e r a l measures of o v e r a l l f i l t e r performance besides n i c o t i n e removal e f f i c i e n c y and pressure drop. One of these i s smoke removal e f f i c i e n c y (SRE), which i s defined as the amount of weighable smoke captured by the f i l t e r d i v i d e d by the sum of m a t e r i a l c o l l e c t e d by the f i l t e r and an e f f i c i e n t , standard c o l l e c t i o n trap p l a c e d betv/een the f i l t e r c i g a r e t t e and the smoking machine. Another i s a t a r removal e f f i c i e n c y (TRE), which i s s i m i l a r l y computed from the c o l l e c t e d and d e l i v e r e d weights of a t a r r y residue d e f i n e d i n t h i s country as being the weight o f c o l l e c t a b l e smoke l e s s the analyzed weights of water and n i c o t i n e .


86

REMOVAL OF TRACE CONTAMINANTS F R O M

IX 0

I

I

I

TO

20

30

Nicotine Removal

Figure

7.

Correlation

1 4 Efficiency

I 0

5

0

T H E AIR

I

1

60

70

(%)

of computed particle removal efficiencies tine removal efficiencies

with experimental

nico-


7.

KEITH

Cigarette

Smoke

Filtration

87

Each of these e f f i c i e n c i e s i s d i f f e r e n t because o f the complex p h y s i c a l and chemical nature o f smoke and i t s i n t e r a c t i o n w i t h f i l t e r media. For example, smoke contains 15 t o 25% water which i s d i s t r i b u t e d between the p a r t i c u l a t e and vapor phases. Since most f i l t e r i n g m a t e r i a l s r e a d i l y adsorb water, t h e i r SEE values which i n c l u d e water are c o n s i d e r a b l y h i g h e r than t h e i r n i c o t i n e or t a r removal e f f i c i e n c i e s . T h i s i s i l l u s t r a t e d i n Figure 8 which shows the e f f e c t of f i b e r s i z e on removal e f f i c i e n c y over a p r a c t i c a l range of f i l t e r pressure drops. For the smaller f i b e r , 2 dpf f i l t e r SHE changes from 49 to 59% over a 40 t o 100mm p r e s sure drop range. TRE, which excludes water but i n c l u d e s other v o l a t i l e and s e m i - v o l a t i l e smoke components goes from 42 t o 53%, while NRE which i s e s s e n t i a l l y a measure o f mechanical e f f i c i e n c y goes from 34 to 40$. S i m i l a r changes at lower e f f i c i e n c i e s are observed f o r a coarser f i v e denier per f i l a m e n t f i l t e r . A further demonstration of t h i s e f f e c t and i l l u s t r a t i o n of the considerable e f f e c t of f i l t e r l e n g t h on performance i s g i v e n i n F i g u r e 9. Here doubling the f i l t e r l e n g t h at any given pressure drop i n c r e a s e s the SHE by 15 to 19 u n i t s . The e f f e c t on NRE i s l e s s pronounced because o f the water a d s o r p t i o n e f f e c t on SRE but i s s t i l l subs t a n t i a l a t 10 t o 13 u n i t s . Other f i l t e r v a r i a b l e s besides f i l t e r length, pressure drop and f i b e r s i z e have r e l a t i v e l y l i t t l e e f f e c t on removal e f f i c i e n c y . V a r i a b l e s such as circumference, f i b e r c r o s s - s e c t i o n , and f i b e r o r i e n t a t i o n and packing, a l l s t r o n g l y a f f e c t pressure drop, but when t h i s i s h e l d constant, there i s l i t t l e or no e f f e c t on removal e f f i c i e n c y . With a g e n e r a l understanding of the mechanical f i l t r a t i o n pro cess i n conventional f i l t e r s , some of the p r o p e r t i e s of other types o f c i g a r e t t e f i l t e r s can be b r i e f l y d i s c u s s e d . Another f a i r l y common type o f c i g a r e t t e f i l t e r c o n s i s t s of a f l u t e d or corrugated sheet of c e l l u l o s e paper which i s bundled i n t o a c y l i n d r i c a l rod with the corrugations along the a x i s o f the f i l t e r . This type o f f i l t e r operates mechanically very much l i k e a convent i o n a l f i l t e r , but the f o l d e d corrugations provide channels which reduce the o v e r a l l pressure drop of the f i l t e r t o reasonable l e v e l s without s e r i o u s l y d e t e r i o r a t i n g i t s p a r t i c l e removal e f f i c i e n c y . To be e f f e c t i v e , t h i s type of f i l t e r must have an essent i a l equivalence between the pressure drop along a channel and that between channels. Without such a balance i n pressure drops, i t would e i t h e r be l i k e a random array of f i b e r s with a high pressure drop o r l i k e a s e r i e s of c a p i l l a r i e s with r e l a t i v e l y impermable w a l l s which would have a low pressure drop and a very low removal e f f i c i e n c y . Another type of f i l t e r employs an i n t r i c a t e c o n s t r u c t i o n t o channel the smoke across dense f i b e r mats with face areas l a r g e r than the c r o s s - s e c t i o n of the c y l i n d r i c a l c i g arette. Others u t i l i z e v e n t i l a t i o n e i t h e r b e f o r e , i n , or a f t e r , a f i b r o u s f i l t e r p l u g to d i l u t e the smoke stream and thereby reduce the t a r d e l i v e r y of the c i g a r e t t e . A f i n a l type o f f i l t e r u t i l i z e s g r a n u l a r or powdered adsorbents such as c h a r c o a l i n conjunct i o n with f i b e r s or i n separate chambers to adsorb gaseous and

P r e s s u r e D r o p ( M M H^O)

Figure 9.

Effect of filter length on removal efficiency—2.0 dpf fiber, 24.8 mm circumference



7.

KEITH

Cigarette

Smoke

Filtration

89

vaporous components from the smoke stream. Except f o r t h i s l a s t type of f i l t e r , the f i l t r a t i o n p r o p e r t i e s and mechanisms are s i m i l a r t o those of c o n v e n t i o n a l acetate f i l t e r s . In the preceeding d i s c u s s i o n , the f i l t r a t i o n p r o p e r t i e s f o r removing a e r o s o l p a r t i c l e s from the smoke stream have been o u t l i n ed from a t h e o r e t i c a l and experimental viewpoint. As has been mentioned, f i l t e r s a l s o remove condensible gases and vapors from the smoke stream by p h y s i c a l or chemical a d s o r p t i o n . I t i s t h i s property which g i v e s r i s e t o s e l e c t i v e f i l t r a t i o n of some com ponents of the smoke mixture. I f , as a l l evidence a v a i l a b l e i n d i c a t e s , the f i l t r a t i o n of p a r t i c l e s i s a n o n - r e v e r s i b l e process i n which the captured p a r t i c l e s are not r e i n t r o d u c e d i n t o the smoke stream, then the g r e a t e r or l e s s e r removal of some smoke components must depend on vapor t r a n s f e r processes, i . e . conden s a t i o n , d i s t i l l a t i o n , and p h y s i c a l and chemical a d s o r p t i o n . That t h i s does occur has a l r e a d y been shown i n the comparison of removal e f f i c i e n c i e s , where the e l e v a t i o n o f SRE and TRE values above the NRE values i n d i c a t e s a s e l e c t i v e removal of water and v o l a t i l e components o f the t a r through a d s o r p t i o n on the f i l t e r i n g material. In f i b r o u s f i l t e r s , i t has been found that there i s a considerable p o t e n t i a l f o r s e l e c t i v e f i l t r a t i o n of m a t e r i a l s w i t h b o i l i n g p o i n t s between about 100 and 300°C which are s o l u b l e or p a r t i a l l y s o l u b l e i n the f i l t e r i n g m a t e r i a l . As examples of t h i s type of s e l e c t i v e f i l t r a t i o n i t has been found that acetate f i l t e r s remove 70 t o 80% o f phenol from c i g a r e t t e smoke, 60 to 75% of water, and 70 t o 75% of c r e s o l s from the smoke stream while only removing about 35 t o φ% of the p a r t i c u l a t e matter(β). This c a p a c i t y f o r s e l e c t i v e removal of h i g h b o i l i n g , s o l u b l e components can be enhanced or r e t a r d e d by the a d d i t i o n of p l a s t i c i z e r s and r e a c t i v e chemicals to the f i l t e r . For example the removal o f phenol by a c e t a t e f i l t e r s can be i n c r e a s e d to about 90$ by the a d d i t i o n of g l y c e r o l t r i a c e t a t e and polymeric g l y c o l s t o the s u r face o f the f i l t e r m a t e r i a l . In the case of n i c o t i n e , the a d d i t i o n of bases to the f i l t e r m a t e r i a l decreases i t s removal e f f i c i e n c y from about 35% t o 25% or l e s s . This occurs because the b a s i c a d d i t i v e r e a c t s w i t h the n i c o t i n e s a l t s i n the captured smoke p a r t i c l e s l i b e r a t i n g f r e e n i c o t i n e which d i s t i l l s back i n t o the smoke stream. As i n d i c a t e d p r e v i o u s l y , some types of f i l t e r s c o n t a i n added adsorbents such as c h a r c o a l . These types of f i l t e r s are designed to remove another range of v o l a t i l e smoke components, g e n e r a l l y c o n s i s t i n g of condensible vapors of m a t e r i a l s with b o i l i n g p o i n t s between 0 and 100 C. A number of i r r i t a t i n g and p o t e n t i a l l y harmful smoke components such as acetaldehyde, a c r o l e i n , hydrogen cyanide, and formaldehyde are thus s e l e c t i v e l y removed by this type of adsorbent f i l t e r . These components, which are l a r g e l y u n a f f e c t e d by conventional f i l t e r s , can be removed with e f f i c i e n c i e s of 60 t o 95% depending on the component, the amount and k i n d of adsorbent, and the presence o f surface treatments on the adsorbent. By combination of adsorbents w i t h f i b r o u s f i l t e r s a

R E M O V A L OF TRACE CONTAMINANTS


90

FROM THE

AIR

broad range of m a t e r i a l s can be s e l e c t i v e l y removed from the smoke stream( 8 ) . In conclusion, we f i n d that a c i g a r e t t e f i l t e r , which i s probably the l a r g e s t volume use of f i l t e r i n g m a t e r i a l i n i n d u s t r y , i s an e f f e c t i v e means of modifying tobacco smoke. I t removes about one h a l f o f the smoke stream by a t h e o r e t i c a l l y t r a c t a b l e combination o f f i l t r a t i o n mechanisms. In a d d i t i o n , conventional and s p e c i a l l y f a b r i c a t e d f i l t e r s show a degree o f s e l e c t i v i t y f o r various v o l a t i l e smoke components. Literature Cited 1.

Bross, I.D.J., Nat. Cancer I n s t . Monograph (1968) 28,pp.35-40.

2.

Wynder, E.L., Mabuchi, K. and B e a t t i e J r . , E . J . J.A.M.A. (1970) 213, pp. 2221-2228.

3.

Dontenwill,W., Chevalier,H.J., Harke,H.P., Lafrenz, U., Reckzeh,G., and Schneider,B., J . Nat. Cancer I n s t . (1973) 51 pp. 1781-1807.

4.

Happel,J., Α. I . Ch. E. Jour. (1959) 5, pp. 174-177.

5.

Morie,G.P., 7, 99-104.

6.

Langmuir,I., 0SRD Report 865 (1942), c f . Chen,C.Y., Chem.Rev. (1955) 55, pp. 595-623.

7.

Lipp,G., B e i t r . Tabakforsch (1965) 3, pp. 109-127.

8.

George,W. and Keith,C.H., "Tobacco and Tobacco Smoke" pp.577622, Academic Press, New York, 1967; c f . K e i t h , C.H., "The Chemistry o f Tobacco Smoke", pp. 149-166, Plenum Press, New York, 1972.

Sloan,C.H. and Peck,V.G., B e i t r . Tabakforsch(1972)

Experimental and Theoretical Aspects of Cigarette Smoke Filtration

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