Real Time Measurement of Particle Size Distribution of Airborne

9 R e a l T i m e M e a s u r e m e n t of P a r t i c l e S i z e D i s t r i b u t i o n of A i r b o r n e C o t t o n D u s t by

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L i g h t Scattering S. P. HERSH, S. K. BATRA, and W.-W. LEE North Carolina State University, School of Textiles, Raleigh, NC 27650 The particle size distribution (PSD) of airborne cotton dust has been measured in real time by light scattering while processing cotton in a model card room. The light sensing particle counter was coupled to a multichannel analyzer. With this instrumentation it is possible to identify components of the dust as originating from vegetable matter in the cotton, mineral matter from humidifiers, or background dust. The influence of various dust control techniques such as adding dust suppressing lubricants to the cotton or using an electrostatic precipitator as a second stage filter was also studied. For example, it has been found that dust in the size range from 2 to 4.5μmis more efficiently removed (95-98%) by the use of a dust suppressant additive than is larger (~90%) or smaller (~70%) size dust. Measurement o f the p a r t i c l e s i z e d i s t r i b u t i o n (PSD) o f airborne cotton dust by t r a d i t i o n a l methods has been a rather tedious and time consuming undertaking. Manually s i z i n g and counting magnified images o f dust p a r t i c l e s c o l l e c t e d on a f i l t e r i s , by nature, wearisome. With image analysers the task becomes much simpler, but t h i s technique s t i l l r e q u i r e s dust t o be c o l l e c t e d on a f i l t e r over an optimum p e r i o d o f time. Fractionation with cascade impactors i s unsuitable since l i n t , because o f i t s aerodynamic c h a r a c t e r i s t i c s , i s c o l l e c t e d on a l l stages. The mass of the l i n t f r e q u e n t l y overwhelms that o f the f r a c t i o n a t e d equant dust p a r t i c l e s . Coulter counters s u f f e r from the problem o f having t o remove dust from the f i l t e r and the attendant unknown e f f e c t s o f agglomeration and d i s p e r s i o n when doing so. A l s o many of the dust components are s o l u b l e i n the e l e c t r o l y t e s used. In c o n t r a s t t o the techniques j u s t enumerated, measurement o f the PSD by l i g h t s c a t t e r i n g o f f e r s many advantages. Foremost o f these i s the a b i l i t y to measure the p a r t i c l e s while they are a i r -

0097-6156/82/0189-0123$06.00/0 © 1982 American Chemical Society

Montalvo; Cotton Dust ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

124

COTTON

DUST

borne and thereby to e l i m i n a t e the problems of aggregation and d i s p e r s i o n . In a d d i t i o n , measurements can be made q u i c k l y , g e n e r a l l y w i t h i n 60 seconds. The u t i l i t y of the technique i s demonstrated by the development of s e v e r a l cotton dust mass sampl e r s which u t i l i z e l i g h t s c a t t e r i n g for the sensing mechanism (1 ,_2). Measuring

Instrumentation

The b a s i c instrumentation i n the present work i s a Royco Model 225/518 High Concentration P a r t i c l e Counter. The l o c a t i o n of the a i r i n l e t and l i g h t sensing u n i t of the instrument i n the card room has been described p r e v i o u s l y (3). The i n l e t was f i t t e d with a v e r t i c a l e l u t r i a t o r preseparator designed to prevent p a r t i c l e s >15 un aerodynamic diameter from e n t e r i n g the l i g h t sensor. Thus the c o l l e c t i o n e f f i c i e n c y of t h i s instrumentation as a funct i o n of p a r t i c l e s i z e should be s i m i l a r to that of the V e r t i c a l E l u t r i a t o r Cotton Dust Sampler. The counting u n i t of t h i s instrument i s l i m i t e d to f i v e channels. Although i t i s p o s s i b l e to d e r i v e the PSD of the sampled dust from only f i v e channels, i t has been shown that a histogram c o n t a i n i n g at l e a s t seven c e l l s i s necessary to o b t a i n s a t i s f a c t o r y agreement between PSD's measured by l i g h t s c a t t e r i n g and those measured by manually counting dust p a r t i c l e s c o l l e c t e d simultaneously on f i l t e r s (4). To o b t a i n t h i s amount of informat i o n on the standard instrument, i t i s necessary to f i r s t take a one-minute sample and then r e c a l i b r a t e the instrument for a second set of diameters. A second sample i s then measured assuming the dust PSD does not change during the 15-minute i n t e r v a l r e q u i r e d for the r e c a l i b r a t i o n . To e l i m i n a t e the need for r e c a l i b r a t i o n during a measurement and to o b t a i n a d d i t i o n a l information, the Royco instrument was supplemented with a Nuclear Data ND-60 Multichannel Analyser (MCA). The a m p l i f i e d s i g n a l of the Royco 225 (which i s proport i o n a l to the amount of l i g h t s c a t t e r e d from each p a r t i c l e ) was connected to the input of the MCA which can count and c l a s s i f y pulsed input s i g n a l s i n t o as many as 2048 channels and d i s p l a y the r e s u l t s on a cathode ray tube (CRT). T h i s number of c e l l s i s o f course much more than r e q u i r e d to determine the PSD. The data were t h e r e f o r e grouped i n t o eleven c e l l s whose l i m i t s were c o n s i s tent with those used e a r l i e r (_5), and the counts i n these c e l l s were then p r i n t e d on a Texas Instruments 743 KSR Data Terminal i n t e r f a c e d with the MCA. In p r e l i m i n a r y e v a l u a t i o n s a Nuclear Data ND-PSA P a r t i c l e S i z i n g A m p l i f i e r (PSA) was i n t e r f a c e d between the Royco and the MCA. The PSA, which could serve as e i t h e r a l i n e a r or l o g a r i t h m i c a m p l i f i e r , was operated i n the l o g mode to develop an output s i g n a l which would be more n e a r l y p r o p o r t i o n a l to the log of the p a r t i c l e diameter and spread the small-diameter p a r t i c l e count over a l a r g e r number of channels. The d i s t r i b u t i o n d i s p l a y e d on


9.

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ET

AL.

125

Particle Size Distribution of Airborne Cotton Dust

the CRT would have been i n that case more n e a r l y comparable with the normal g r a p h i c a l r e p r e s e n t a t i o n of the PSD. I t was found, however, that the PSA merely introduced a d d i t i o n a l noise to the Royco s i g n a l and d i d not increase the r e l i a b i l i t y of the count o f small-diameter p a r t i c l e s . For these reasons, use of the PSA was discontinued.

Experimental

Procedure

Dust Sampling and PSD A n a l y s i s . One minute samples taken at a flow rate of 0.1 f t / m i n at i n t e r v a l s ranging from f i v e to f i f t e e n minutes were analysed. Sampling began as the card was s t a r t e d for each run but about 30 minutes before cotton was fed and the Pneumafil l i n t capture system was turned on. I t ended when processing of the c o t t o n was stopped. ( D e t a i l s of the card room layout, equipment and operating procedures have been described p r e v i o u s l y (6).) A surge i n p a r t i c l e count u s u a l l y occurred as cotton entered the card and the Pneumafil system s t a r t e d c i r c u l a t i n g the p r e v i o u s l y s e t t l e d dust. In f i v e to ten minutes, however, the count would f a l l to a reasonably steady l e v e l . The r e s u l t s reported below are for the r e l a t i v e l y steadystate c o n d i t i o n . (Detailed d e s c r i p t i o n s of the changes i n dust concentration which occur during and a f t e r processing have been reported elsewhere (.4*7).) The PSD*s measured were expressed i n terms of the mean d i a meter and standard d e v i a t i o n of the l o g normal d i s t r i b u t i o n c a l c u l a t e d i n two d i f f e r e n t ways. F i r s t they were c a l c u l a t e d a l g e b r a i c a l l y as the geometric mean diameter (on a number basis) d g ^ and as the geometric standard d e v i a t i o n a . Second, they were ' determined from cumulative l o g normal p r o b a b i l i t y p l o t s using w e l l documented standard procedures (see, for example, references 8, 9J. The a l g e b r a i c c a l c u l a t i o n s of the d i s t r i b u t i o n parameters included a l l c e l l s of the histogram while those obtained from the cumulative p l o t s were sometimes based on only a p a r t of the c e l l s . When the d i s t r i b u t i o n was multimodal, the l a t t e r c a l c u l a t i o n s included only the values from the d i s t r i b u t i o n with the smaller diameter, and no attempt was made to adjust the percentage count for the number of p o i n t s i n the larger diameter d i s t r i b u t i o n ( s ) . Other f a c t o r s reported are the number of p a r t i c l e s per 0.1 f t , the dust concentration measured with the V e r t i c a l E l u t r i a t o r Cotton Dust Samplers (VE), the nature of the cumulative p l o t s , and for multimodal d i s t r i b u t i o n s , the percent of p a r t i c l e s i n the d i s t r i b u t i o n with the smaller diameter. 3

n

g

3

Mock Runs—No Cotton Processed. In a d d i t i o n to the measurements described above, a set of experiments was c a r r i e d out to assess the amount of background dust i n the card room. A c o n s i d -


126

COTTON

DUST

érable amount of the r e s p i r a b l e dust measured i n the card room has been reported to come from the s o l i d s contained i n the water supp l i e d to the atomizers used to maintain the r e l a t i v e humidity (10)• The spray drying of the atomized water d r o p l e t s form p a r t i c l e s of s o l i d residue which a l s o are c o l l e c t e d by the samplers. To determine the PSD of the dust c o n t r i b u t e d by the atomizers, mock runs were made with (A) the h u m i d i f i e r s and the Pneumafil l i n t capture system (PN) o f f , (B) only the PN on, (C) only the h u m i d i f i e r s on, and (D) with both on. During these mock runs the a i r conditioner and card were operated, but no cotton was processed.

Results and D i s c u s s i o n

Mock runs. Results of a l l the measurements are summarized i n Table I, and histograms of the d i s t r i b u t i o n s observed for two of the mock runs are shown i n Figures 1 and 2. The corresponding cumulative d i s t r i b u t i o n curves are shown i n Figure 3. The geometric mean diameters (number basis) d for the mock runs c a l c u l a t e d a l g e b r a i c a l l y (including a l l c e l l s ; range from 1.30 to 1.44 um. The highest value was obtained when the _ atomizers were on and the PN was not. The s l i g h t increase i n d ^ over that observed when the PN was running (1.44 compared with 1.32 urn) was expected since i t has been shown p r e v i o u s l y that the PN system s e l e c t i v e l y removes smaller diameter p a r t i c l e s from the card room (4,J7) The cumulative d i s t r i b u t i o n curves i n d i c a t e the d i s t r i b u t i o n s to be bimodal except for the mock run made with only the PN on. The small diameter end of the l a t t e r curve was concave down suggesting that smaller p a r t i c l e s were being removed s e l e c tively. The mean diameters d o f the smaller diameter d i s t r i b u t i o n s c a l c u l a t e d from the cumulative curves are about 0.1 urn greater than those c a l c u l a t e d a l g e b r a i c a l l y . Because the higher diameter d i s t r i b u t i o n s contain no more than 3% of the p a r t i c l e s counted (and sometimes l e s s than 0.05%), no attempt was made to resolve the two d i s t r i b u t i o n s present. Applying a c o r r e c t i o n for the p a r t i c l e s i n the upper d i s t r i b u t i o n would have a n e g l i g i b l e e f f e c t on d and a c a l c u l a t e d from the cumulative curve for the smaller diameter d i s t r i b u t i o n . Although the s i x c e l l s with diameters >3.17 urn c o n t a i n only a small f r a c t i o n of the t o t a l number of p a r t i c l e s counted, the numbers i n the c e l l s are remarkably c o n s i s t e n t as noted i n Table I I . The p o s s i b l e s i g n i f i c a n c e of t h i s observation w i l l be discussed l a t e r . n

g

w

g

n


n


H K M B

Footnotes on next page.

minus minus minus minus

1.57 1.60 1.63 1.37

1.96 1.78 1.89 1.31

G J L D

1.66 1.69

1.82 1.71

Cotton 4, ESP o f f Cotton 4, ESP on

L M

1.62 1.09

1.59 1.40

1.64 1.27

Cotton 3 0.60% Milube N-32

J K

1.44 1.28 1.26

1.85 1.45 1.44

1.58 1.48 1.44

1.88 1.44 1.43

Cotton 2 0.48% Texpray 1.09% Spraycot

G H I

Linear Bimodal

1.84 1.30

1.63 1.08

1.76 1.31

1.63 1.04

Cotton 1 PET Staple

c

3

#

660 219 870 1754 441 166

304,819 122,281 475,107 324,146 182,538 158,865

X X 65.5 X

Nearly Nearly

Bimodal Nearly Linear Linear Linear

1.58 1.60 1.40 1.62 1.66 1.22

1.80 1.65 1.90 1.85 1.90 1.43

-

1931 177 433,791 109,645 99.95 96.0

Nearly Linear Bimodal

1.48 1.56

Linear Linear

1234 364 187 554,638 79,482 89,226 99.56 90.3 91.92

Nearly Linear Bimodal Bimodal

47 82 376 248

n

25,730 16,058 436,376 174,923

o

Dust

1403 377

-

97.1 X 99.96 99.29

C

ivg/m )

f

Particles

o

168,249 262,464

b

^

98.9

c

c

Nature Bimodal Bimodal Bimodal Bimodal

g

a

, Cumulative P l o t

1.22 1.28 1.28 1.24

1.42 1.45 1.52 1.42

1.40 1.50 1.39 1.39

1.30 1.39 1.44 1.32

(Mm)

°g

(pm)

E F

o f f , PN o f f o f f , PN on on, PN o f f on, PN on

a

H H H H

Conditions

A B C D

Run

D i s t r i b u t i o n Parameters Algebraic Cumulative d~~^ ~d~^

TABLE I Parameters o f P a r t i c l e S i z e D i s t r i b u t i o n s Measured by L i g h t S c a t t e r i n g

Montalvo; Cotton Dust ACS Symposium Series; American Chemical Society: Washington, DC, 1982. distribution.

a p p r o x i m a t e value because cumulative d i s t r i b u t i o n curve for the lower diameter curved concave to the diameter a x i s .

P e r c e n t of observed p a r t i c l e s contained i n the smaller diameter

b

distribution

was

= h u m i d i f i e r , PN = Pneumafil l i n t capture system, PET » poly(ethylene t e r e p h t h a l a t e ) , ESP « electrostatic precipitator. P h y s i c a l p r o p e r t i e s o f cottons 1, 2, 3, and 4 are: C l a s s 43, 51, 82, 43, M i c r o n a i r e 4.4, 4.1, 3.0, 5.5; S h i r l e y Analyzer t o t a l t r a s h 6.1%, 4.9%, 7.7%, 2.4%.

H

a

TABLE I Footnotes.

9.

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ET AL.

129

LJ O CO

3 •40K 8 88 f O CM

j§20K

—

7088

0 0.5 0.8 1.0

L

•4—-

-M

2.0 3.04.06.0 10.0

PARTICLE DIAMETER (/im) Figure 1. Particle size histogram of dust collected during mock run (no cotton being processed) with humidifier off and Pneumafil lint capture system on. Conditions: dgn , nonlinear; 16,059 particles; VE, 80 fig/m . 3

t

83,064

pf,ow

en m i^- ro -

-

CM —

&377 -4—1

0.5 0.81.0

2.0

4.0 6.0 10.0

PARTICLE DIAMETER (/im) Figure 2. Particle size histogram of dust collected during mock run (no cotton being processed) with humidifier and Pneumafil lint captive system on. Conditions: d , 1.32 fim; n

g


130

COTTON

DUST

PARTICLE DIAMETER (fim) Figure 3.

Cumulative distribution curves of distributions shown in Figures 1 and 2. Key: humidifier on, PN on; O, humidifier off, PN on.


9.

HERSH

E T

AL.


TABLE I I P a r t i c l e Counts i n S i x Largest Diameter Histogram (number counted i n 0.1 f t sample)

131

Cells

3

Run B D F l

a

C e l l Midpoint (Diameter, pm) 3.6 4.5 5.7 7.1 9.0 11.3 263 293 382

145 95 154 74 199 125

44 36 40

14 11 14

13 12 10

P a r t i c l e Count Upper 6 c e l l s No. Total (%) 16,059 174,923 262,464

3.57 0.33 0.30

574 580 780

See Table I f o r processing c o n d i t i o n s .

Processing o f Cotton and PET Staple F i b e r . The histograms obtained while processing 100% cotton and 100% poly(ethylene terephthalate) staple f i b e r (PET) are shown i n Figure 4. The cotton was c l a s s 43 ( s t r i c t low m i d d l i n g ) , 4.4 micronaire and contained 6.1% trash as measured by the S h i r l e y Analyser. The PET was type 310 F o r t r e l , 1.5 denier, and 1.5 inch s t a p l e length. Cotton with a r e l a t i v e l y high trash content was s e l e c t e d d e l i b e r a t e l y as a c o n t r o l i n experiments designed t o evaluate techniques for reducing dust generation. The r e s u l t s reported i n Table I (runs E and F) show that although the c o n c e n t r a t i o n o f dust engendered while processing the cotton was 271% greater than when processing the__PET, the number o f p a r t i c l e s present was 36% l e s s . The diameters d„ „ o f dust from the cotton and PET were 1.63 and g ,n 1.04 um r e s p e c t i v e l y . The observed a f o r the cotton was a l s o g r e a t e r , which suggests that many more larger diameter p a r t i c l e s are generated when the cotton i s processed. This c o n c l u s i o n i s further supported by the histograms shown i n Figure 4. The presence o f a large number o f small-diameter p a r t i c l e s during the PET run i s a t l e a s t p a r t i a l l y a t t r i b u t a b l e to the use of four atomizers (instead o f the normal two) t o maintain^ a high enough humidity to permit the PET t o be processed. The d of the dust measured during the PET run i s c o n s i d e r a b l y smaller than that observed during the mock run, a f a c t which suggests that the PSD o f aerosols generated by water atomizers i s q u i t e v a r i a b l e . Indeed, the diameter o f the p a r t i c l e s would be expected t o vary with the concentration o f s o l i d s i n the water and the s i z e o f the d r o p l e t s formed. These f a c t o r s could change c o n s i d e r a b l y over time and with changing h u m i d i f i c a t i o n requirements based on outside ambient c o n d i t i o n s . Even though the scale o f the histograms shown i n Figure 4 suggest that no p a r t i c l e s are present i n the PET run with diameter >2.6 urn, p a r t i c l e s i n t h i s s i z e range were indeed present. The a c t u a l p a r t i c l e count, shown i n Table I I , i s remarkably c l o s e t o that reported f o r the mock runs. The agreement between the counts g

n


COTTON

J

DUST

IOOKI-

PARTICLE

DIAMETER (fitn)

Figure 4. Particle size histograms of dust collected while processing cotton and polyethylene terephthalate) (PET). Key: g§, 100% PET; VE, 377 ug/m ; N, 262,464; • , 100% cotton; VE, 1400 fig/m ; N, 168,249. 3

3


9.

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ET AL.


133

in the s i x highest diameter c e l l s , representing i n one case only 0.3% o f a count o f 262,464 (that for the PET run), suggests that the r e s o l u t i o n o f the counting and s i z i n g system employed i s remarkably good. The cumulative d i s t r i b u t i o n curve o f the cotton histogram i s l i n e a r through the e n t i r e s i z e range (up t o 99.94% o f the p a r t i c l e s counted). The PET curve, on the other hand, i s bimodal as shown i n Figure 5. When the bimodal d i s t r i b u t i o n i s resolved i n t o i t s two components using the method described by Taylor (11), the cumulative d i s t r i b u t i o n p l o t o f the l a r g e r diameter d i s t r i b u t i o n , p a r t o f which i s shown i n Figure 5, i s l i n e a r . The f a c t that the number o f p a r t i c l e s included i n t h i s d i s t r i b u t i o n (2051) i s o n l y 0.78% o f the t o t a l p a r t i c l e s counted and that the cumulative curve for these p a r t i c l e s i s l i n e a r again shows the s e n s i t i v i t y o f the instrumentation i n measuring the p a r t i c l e count and s i z e . The mean and standard d e v i a t i o n o f the upper d i s t r i b u t i o n diameter i s 2.9 un and 1.58, r e s p e c t i v e l y . The histograms o f the resolved d i s t r i b u t i o n s , p l o t t e d on a log scale to show the number o f p a r t i c l e s i n the high diameter c e l l s , i s shown i n Figure 6. E f f e c t o f Dust Suppressing L u b r i c a n t s . The authors have studied the i n f l u e n c e o f dust suppressing a d d i t i v e s on the dust generating c h a r a c t e r i s t i c s o f cotton (12). These s t u d i e s were concerned p r i m a r i l y with the c o n c e n t r a t i o n o f the dust generated during p r o c e s s i n g . In t h i s paper the PSD o f the dust obtained from a few o f these runs w i l l be examined. The e f f e c t s o f applying 0.48% o f Texspray Compound and 1.09% of Spraycot 8853 are summarized i n Table I. In both cases there i s a marked drop i n the number o f p a r t i c l e s (an average o f 84.8%) and i n dust concentration (70.5% for the Texspray and 84.8% for the Spraycot). The d c a l c u l a t e d a l g e b r a i c a l l y decreases from 1.88 un for the c o n t r o l cotton t o 1.44 ym for the two l u b r i c a t e d c o t t o n s . The cumulative p r o b a b i l i t y curves o f the dust emitted from the cottons c o n t a i n i n g a d d i t i v e s become bimodal. These changes i n the nature o f the cumulative d i s t r i b u t i o n curves suggest that the f r a c t i o n o f p a r t i c l e s removed i s not constant f o r a l l diameters. Table I a l s o i l l u s t r a t e s the e f f e c t o f adding 0.60% o f Milube N-32 t o a low grade, low micronaire and high trash content c o t t o n . The histograms and cumulative d i s t r i b u t i o n curves o f the c o n t r o l and treated cottons are shown i n Figures 7 and 8. The r e s u l t s are s i m i l a r t o those found for the other cottons c o n t a i n i n g a d d i t i v e s in that there i s a drop o f 74.7% i n p a r t i c l e count and of__90.8% i n dust c o n c e n t r a t i o n . The presence o f the a d d i t i v e lowers d from 1.62 to 1.09 un (based on the cumulative p l o t s ) . The cumulative d i s t r i b u t i o n o f the stock cotton i s l i n e a r (except for the lowest diameter c e l l ) over a range i n c l u d i n g 99.95% o f the p a r t i c l e s and that o f the a d d i t i v e - t r e a t e d cotton i s bimodal. The smaller n

n


134

COTTON

4.0r

1.0

DUST

01

2.0 3.0 4.0 60 aOIO.O PARTICLE DIAMETER (/xm)

Figure 5. Cumulative distribution curves of dust collected while processing 100% PET. Key: O, original bimodal curve; d— 1.08 ^m, 4 fxm.

05

06 1.0

20

3.0 4.0

60 aO 100

RARTICLE DIAMETER (ftm) Figure 6. Particle size histograms of dust collected while processing cotton control and PET. Logarithmic particle count. Key: B, 100% PET, d — 2.9 ^m,

Real Time Measurement of Particle Size Distribution of Airborne

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