Measurement, Analysis, and Control of Cotton Dust - ACS Symposium

5 Measurement, Analysis, and Control of Cotton

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Dust JOSEPH G. MONTALVO, JR., DEVRON P. THIBODEAUX, and ALBERT BARIL, JR. Southern Regional Research Center, New Orleans, LA 70179 The OSHA standard on occupational exposure to cotton dust defines "cotton dust" as "the dust present during the handling or processing of cotton, which may contain a mixture of substances including ground-up plant matter, f i b e r , b a c t e r i a , fungi, soil, pesticides, non-cotton plant matter and other contaminants which may have accumulated during the growing harvesting, and subsequent processing or storage periods" (1). The standard presents OSHA's determination that exposure to cotton dust presents a significant health hazard to employees and establishes permissible exposure limits f o r selected processes i n the cotton industry and for non-textile industries where there i s exposure to cotton dust. The cotton dust standard also provides for employee exposure monitoring, engineering controls and work practices, respirators, employee t r a i n i n g , medical surveillance, signs and record keeping. In order to provide a healthier, safer work environment for cotton textile workers, the Cotton Textile Processing Laboratory at the Southern Regional Research Center, New Orleans, LA, i s conducting research on agricultural particulates, including the measurement, analysis and control of cotton dust generated i n the handling and processing of cotton. This paper presents a review of research that includes determining the effectiveness of the standard vertical e l u t r i a t o r f o r measuring airborne cotton dust, an impaction precutter dust sampler, and i n situ cotton particulate assays. Mathematical modeling includes predicting the internal flow patterns and e l u t r i a t o r errors associated with magnitude and direction of ambient a i r currents, and property relationships and errors associated with establishing particulate burden i n cotton particulate reference

This chapter not subject to U.S. copyright. Published 1981 American Chemical Society

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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m a t e r i a l s . Research and development o f devices f o r removal and capture o f dust i n c l u d e a wet wall e l e c t r o i n e r t i a l p r e c i p i t a t o r and a f l u i d e l e c t r o d e p r e c i p i t a t o r .


Measurement and A n a l y s i s o f Airborne Cotton Dust The current c r i t e r i a f o r l i m i t i n g a i r b o r n e dust concent r a t i o n s i n t e x t i l e m i l l s are based on the e p i d e m i o l o g i c a l s t u d i e s o f Merchant e t a].. {2). The sampling device used f o r studying the dose-response c h a r a c t e r i s t i c s o f t e x t i l e m i l l workers f o r cotton dust i n that study was the Lumsden-Lynch v e r t i c a l e l u t r i a t o r (VE) as d e s c r i b e d by Lynch (3)· The VE was s e l e c t e d as the device f o r measuring airborne cotton f i b e r s because o f the widely-held b e l i e f that b y s s i n o s i s i s a s s o c i a t e d with the n o n - l i n t , r e s p i r a b l e f r a c t i o n o f cotton dust i n the a i r of c o t t o n t e x t i l e m i l l s . NIOSH (£) u l t i m a t e l y recommended the VE because i t i s designed t o c o l l e c t cotton dust i n an upward laminar flow o f a i r with an average v e l o c i t y equal t o the s e t t l i n g v e l o c i t y o f a u n i t d e n s i t y 15 ym diameter sphere. The c o t t o n dust standards promulgated by OSHA f o r cotton dust c o n c e n t r a t i o n i n the t e x t i l e i n d u s t r y are as f o l l o w s QJ : 1. 200 ug/m3 o r l e s s i n yarn manufacturing 2. 750 yg/m^ o r l e s s i n s l a s h i n g and weaving 3. 500 yg/rn^ o r l e s s elsewhere i n the cotton industry. Since i t s adoption as a cotton dust sampler, the VE has been recognized as a device which sampled other than merely l i n t - f r e e r e s p i r a b l e p a r t i c l e s . Bethea and Morey (5) reported that t h e VE o p e r a t i n g under standard c o n d i t i o n s ( 7 . 4 Lpm) d i d c o l l e c t a p o r t i o n o f l i n t . Several other researchers i n c l u d i n g Neefus ( 6 ) , Matlock and P a r n e l l (_7), and Claassen (8) have a l s o reported c o l l e c t i o n s o f l i n t and other p a r t i c u l a t e s g r e a t e r than 15 ym diameter. A l l o f these problems have l e d researchers at the SRRC t o concentrate on three areas o f airborne cotton dust measurements. These i n c l u d e : a) a n a l y t i c a l ( t h e o r e t i c a l ) modeling o f the performance c h a r a c t e r i s t i c s o f the VE; b) experimental measurements o f the VE's aerodynamic flow c h a r a c t e r i s t i c s ; and c ) development o f an a l t e r n a t e a i r sampling method capable o f a c c u r a t e l y measuring l i n t - f r e e , a i r b o r n e , r e s p i r a b l e cotton dust. A n a l y t i c a l Models. A r e a l i s t i c semi-empirical model (coded VELUT) o f the i s o k i n e t i c sampling e f f i c i e n c y o f t h e VE has been developed a t SRRC by Robert (9). Robert deduced that the a i r flow i n t h e VE i s not laminar as assumed by i t s o r i g i n a t o r s , but i s c h a r a c t e r i z e d by flow s e p a r a t i o n at i t s i n l e t accompanied by the shedding o f t u r b u l e n t v o r t i c e s , reverse flow, and l o c a l upward flow v e l o c i t i e s s i g n i f i c a n t l y l a r g e r than the average v e l o c i t y c a l c u l a t e d assuming laminar o r p a r a b o l i c flow. The


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model c a l c u l a t e s the sampling e f f i c i e n c y (E) o f the VE f o r c o l l e c t i n g a i r b o r n e cotton dust as a f u n c t i o n o f the s i z e d i s t r i b u t i o n c h a r a c t e r i s t i c s o f the p a r t i c l e s . The modeling a l g o r i t h m i s based upon the assumptions o f two physical processes a c t i n g i n s e r i e s t o capture d u s t . These are an aerodynamic s e p a r a t i o n o f l a r g e r p a r t i c l e s based on e l u t r i a t i o n ( s e t t l i n g ) i n the lower VE chamber and the capture of the p a r t i c l e s upon a semi-permeable membrane f i l t e r . For a given p a r t i c l e diameter (s) the e f f i c i e n c y , e(s) can be expressed i n t h i s model a s : ( s ) = Pa · f Pa · Ο " Pf) 0) where p and p f are the p e n e t r a t i o n p r o b a b i l i t i e s o f the aerodynamic c o l l e c t o r and the f i l t e r , r e s p e c t i v e l y , and where r f i s the r e t e n t i o n p r o b a b i l i t y o f the f i b e r . The average e f f i c i e n c y Ε may be computed by i n t e g r a t i n g c(s) as f o l l o w s : e


Cotton Dust

ET AL.

r

=

a

(2) where the f u n c t i o n F i s the log-normal d i s t r i b u t i o n f u n c t i o n , MMAD the mass median aerodynamic diameter, GSD the geometric standard d e v i a t i o n , and C i s the t r u e t o t a l mass c o n c e n t r a t i o n in sampled a i r given by C = M/V = m/E-V (3) where m i s the measured and M the actual mass o f dust contained in volume V. The e v a l u a t i o n o f Eq. 2 r e q u i r e s e i t h e r knowledge of o r assumption o f the d i s t r i b u t i o n F and the e v a l a u t i o n o f B ( S ) as a f u n c t i o n o f p and r f . The determination o f p i s made d i f f i c u l t because a n a l y s i s o f the continuum mechanics i n v o l v e d i n d i c a t e s that the a i r w i l l separate from the w a l l s o f the VE c l o s e t o i t s i n l e t as shown i n Figure 1. This i s accompanied by regions o f backflow and r e c i r c u l a t i o n w i t h i n the sampler, which reduces the e f f e c t i v e diameter o f the VE and c o n c u r r e n t l y increases i t s c e n t e r l i n e v e l o c i t y . This phenomenon w i l l c e r t a i n l y promote the t r a n s p o r t o f p a r t i c l e s l a r g e r than the nominal 15 \m c u t - o f f t o be captured by the f i l t e r . In order to t e s t the v a l i d i t y o f the VELUT model i t was used t o generate p r e d i c t e d i s o k i n e t i c values of VE d i f f e r e n t i a l sampling e f f i c i e n c y f o r values o f MMAD and GSD corresponding t o experimental values reported i n the l i t e r a t u r e . A comparison o f the experimental data obtained by Carson and Lynch (10) u s i n g mono-disperse a e r o s o l s o f d i o c t y l p h t h a l a t e , with the p r e d i c t i o n s of VELUT f o r the same c o n d i t i o n s i s given i n Table I . This l i m i t e d data i s i n e x c e l l e n t agreement with the p r e d i c t i o n s o f VELUT. a


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TABLE I Comparison o f Experimental and T h e o r e t i c a l Values of I s o k i n e t i c Sampling E f f i c i e n c y


MMAD (pm)

Efficiency Experiment B e s t - F i t Theory Carson and Lynch QO) Program VELUT

GSD

0.95 0.72 0.49 0.34

1.13 1.05 1.10 1.16

6.7 15.4 19.2 24.5

+ + + +

0.923 0.653 0.507 0.320

0.08 0.07 0.035 0.05

The model has been used t o p r e d i c t the sampling e f f i c i e n c y of t h e VE f o r a wide range o f MMAD and GSD values t y p i c a l o f what might be encountered i n cotton t e x t i l e p r o c e s s i n g . These parameter values are f o r the actual s i z e d i s t r i b u t i o n o f p a r t i c l e s i n the sampled a i r , and not f o r those c o l l e c t e d on the membrane f i l t e r . These r e s u l t s are summarized i n Table I I . A remarkable f e a t u r e o f t h i s model i s that i t p r e d i c t s that the VE w i l l c o l l e c t s i g n i f i c a n t amounts o f p a r t i c l e s with aerodynamic diameters g r e a t e r than 30 ym. TABLE II Absolute Values o f I s o k i n e t i c Sampling E f f i c i e n c y f o r Various Cotton Dust D i s t r i b u t i o n s MMAD (Micrometers) GSD 05

1

3

6

10

15

1.5 0.82 0.94 0.98 0.92 0.80 0.62 2.0 0.81 0.91 0.95 0.88 0.74 0.58 2.5 0.79 0.89 0.92 0.83 0.70 0.56 3.0 0.78 0.87 0.89 0.80 0.67 0.55 5.0 0.74 0.80 0.78 0.70 0.61 0.53 10.0 0.69 0.70 0.67 0.61 0.55 0.49

20

25

40

60

0.46 0.46 0.46 0.46 0.46 0.45

0.32 0.36 0.38 0.39 0.41 0.42

0.10 0.02 0.18 0.08 0.22 0.12 0.25 0.16 0.31 0.23 0.35 0.29

Most r e c e n t l y the VELUT model has been expanded t o handle the problem o f s i m u l a t i n g the p a r t i c l e c o l l e c t i o n e f f i c i e n c y o f a VE sampling dust-laden a i r under n o n - i s o k i n e t i c c o n d i t i o n s (11). These cases i n c l u d e sampling from quiescent a i r as well as from a i r moving with both h o r i z o n t a l ( c r o s s f l o w ) and v e r t i c a l


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(upflow or downflow) v e l o c i t y components with respect to the VE a x i s . Robert's c o n c l u s i o n s from t h i s study were that unless the c o n d i t i o n s of l o c a l a i r f l o w i n the v i c i n i t y of the VE i n l e t are c a r e f u l l y monitored, the experimental r e s u l t s obtained can not be considered to have an accuracy of b e t t e r than a f a c t o r of two r e l a t i v e to the t r u e dust c o n c e n t r a t i o n s . Experimental Measurements. Besides t h e o r e t i c a l modeling of the performance of the VE as a cotton dust sampler, the SRRC has a l s o conducted experimental research on the d e v i c e . Claassen (8) has c a r r i e d out extensive s t u d i e s of the a i r f l o w c h a r a c t e r i s t i c s of the VE and, i n g e n e r a l , his r e s u l t s are i n q u a l i t a t i v e agreement with the p r e d i c t i o n s of the VELUT model. Observations of a vapor cloud as i t entered the transparent e l u t r i a t o r revealed that the a i r f l o w separated from the VE wall w i t h i n about 2.5 centimeters of the entrance, c o n f i r m i n g the p r e d i c t i o n s from VELUT. As the flow proceeded upward through the VE, t u r b u l e n t v o r t i c e s of the order of one-inch diameter were formed i n the lower cone and propagated upward. Also i n v e r i f i c a t i o n of the p r e d i c t i o n o f VELUT, regions o f r e c i r c u l a t i o n and backflow were i d e n t i f i e d i n the v i c i n i t y of the w a l l s with no v i s i b l e damping of the turbulence i n the straight section. The second experimental technique i n v o l v e d the i n t r o d u c t i o n of f l o w - t r a c i n g smoke i n j e c t e d r a d i a l l y through small holes d r i l l e d i n t o an a x i a l 1y i n s e r t e d smoke i n j e c t o r tube. Studies of m u l t i p l e exposure photographs lead to several c o n c l u s i o n s concerning the flow c h a r a c t e r i s t i c s of the VE. Included among these are the f o l l o w i n g : a) l a r g e v a r i a t i o n s i n both the magnitude and d i r e c t i o n of the v e l o c i t y was observed with strong i n d i c a t i o n s of downflow at l o c a t i o n s near the c e n t e r l i n e ; b) d e f i n i t e i n d i c a t i o n s of non-axisymmetric flow c o n d i t i o n s with upflow at some r a d i a l l o c a t i o n s and. downflow at other r a d i a l p o s i t i o n s ; c) evidence of a high v e l o c i t y j e t of a i r along the c e n t e r l i n e of the VE; and d) observations of high r a d i a l d i f f u s i o n of the cloud at the higher portions of the VE i n the v i c i n i t y of the upward converging cone. These q u a l i t a t i v e observations are c o n s i s t e n t with the p r e d i c t i o n s of VELUT. A l t e r n a t e Sampling Methods. The t h i r d area of SRRC's research on a i r b o r n e cotton dust has concentrated on developing an a l t e r n a t e a i r sampling method f o r a c c u r a t e l y measuring l i n t - f r e e r e s p i r a b l e cotton dust. The device chosen f o r f u r t h e r study was o r i g i n a l l y developed by B a t t e l l e Memorial I n s t i t u t e while under c o n t r a c t t o USDA, SRRC (22). This instrument, c a l l e d a " p r e c u t t e r , " u t i l i z e d the impaction of dust on a moving s t r i p of tape as a method of d i s c r i m i n a t i n g against l a r g e r p a r t i c l e s i n an a i r s t r e a m . A sharp 15 pm aerodynamic c u t o f f was observed. As shown i n Figure 2, the sampled a i r flows through a U-shaped geometry with p a r t i c l e s l a r g e r than a f i x e d s i z e being


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BOUNDARY


ill )\

Figure J.

U SEPARATION A

Y

E

R

Boundary layer separation in inlet section of the vertical elutriator

Figure 2. Schematic illustration of the SRRC precutter sampler



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c o l l e c t e d on the tape and the smaller p a r t i c l e s being c o l l e c t e d by a f i l t e r mounted i n a c a s s e t t e attached to the sampler's outlet. An e v a l u a t i o n of the p r e c u t t e r was performed by a cooperat i v e research program between SRRC and the North C a r o l i n a State U n i v e r s i t y School of T e x t i l e s . The c h i e f o b j e c t i v e of t h i s research was t o determine i f the p r e c u t t e r had dust c o l l e c t i o n c h a r a c t e r i s t i c s which were e q u i v a l e n t to the VE. The approach followed was to t e s t a prototype of the p r e c u t t e r along s i d e of a VE i n the N.C.S.U. model card room where cottons having a wide v a r i e t y of p r o p e r t i e s were being processed. Results from t h i s study (13) showed that when the p r e c u t t e r was operated under c o n d i t i o n s corresponding t o 50% c o l l e c t i o n e f f i c i e n c y at 15 prn, i t sampled c o n s i d e r a b l y less mass of material than the VE operated i n c l o s e p r o x i m i t y (18 i n . away). The c h i e f d i f f e r e n c e s between the two samplers was that the VE c o l l e c t e d an a p p r e c i a b l e amount of l i n t and l i n t fragments while the p r e c u t t e r sampled e s s e n t i a l l y f i n e r e s p i r a b l e p a r t i c l e s . F o r t h i s reason the p r e c u t t e r gave readings which were of the order of 30% t o 40% l e s s than the corresponding VE measurements. I t was found that in order to make the two samplers s t a t i s t i c a l l y e q u i v a l e n t i t was necessary to enlarge the e x i t j e t diameter of the p r e c u t t e r . Under these c o n d i t i o n s the p r e c u t t e r has a nominal 29 \m aerodynamic c u t o f f diameter and w i l l a l l o w the c o l l e c t i o n of l i n t and l i n t fragments on the sampling f i l t e r . These r e s u l t s f u r t h e r enforce the p r e d i c t i o n s of VELUT and r a i s e the question of the s u i t a b i l i t y of the VE as a l i n t - f r e e a i r sampler. The standard, however, i s based on environmental data obtained by a VE. At t h i s w r i t i n g , i t i s not known i f m o d i f i c a t i o n of the standard w i l l be recommended by 0SHA. Measurement and A n a l y s i s of Trash and Dust i n Cotton Measurement and a n a l y s i s of t r a s h and dust i n cotton i s important i n the p r e d i c t i o n of dust generation p o t e n t i a l , i n r e l a t i o n t o b y s s i n o s i s (14) and r o t o r s p i n n i n g performance (15). By d e f i n i t i o n , t r a s h i s the n o n l i n t p a r t i c l e s i n c l u d i n g dust which s e t t l e under the i n f l u e n c e of g r a v i t y i n the p r o c e s s i n g of c o t t o n . Dust i s the f i n e r material capable of being d i s p e r s e d in a i r . P a r t i c u l a t e burden i s the combined t r a s h and dust l e v e l in c o t t o n . Techniques are a v a i l a b l e to measure p a r t i c u l a t e and dust burden i n c o t t o n . U n f o r t u n a t e l y , the accuracy a s s o c i a t e d with any given method i s unknown. Cotton p a r t i c u l a t e and dust reference m a t e r i a l s , needed to asses accuracy and c o m p a t a b i l i t y of measurement systems between concerned l a b o r a t o r i e s , are a l s o not a v a i l a b l e . To overcome t h i s dilemma, theory and approaches are being developed to v e r i f y p a r t i c u l a t e and dust burdens i n bulk q u a n t i t i e s of cotton stock designated i n advance as reference


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m a t e r i a l s . The cotton reference m a t e r i a l s w i l l , i n t u r n , be used to assess accuracy of the s t a t e - o f - t h e - a r t measurement systems, and f a c i l i t a t e material balances and mathematical modeling of ginning and t e x t i l e s t u d i e s . P a r t i c u l a t e burden v e r i f i c a t i o n i s discussed i n the f o l l o w i n g s e c t i o n . Theory and o p e r a t i o n of dust v e r i f i c a t i o n techniques w i l l be reported el sewhere. In S i t u Native Standard Method. A fundamental approach t o v e r i f i c a t i o n of p a r t i c u l a t e burden i n cotton reference m a t e r i a l s i s under e v a l u a t i o n (16) based on a null h y p o t h e s i s . The hypothesis s t a t e s that upon rendering a cotton f r e e of f o r e i g n m a t e r i a l , the recoverable p a r t i c u l a t e s - l i n t i t h property con stant λ-j ( f o r example, c o l o r ) of the s y n t h e s i z e d mixture i s equal to that f o r the i n s i t u p a r t i c u l a t e constant, ψ ·. The experimental scheme to t e s t the hypothesis i s as f o l l o w s . Cotton i s mechanically cleaned by c y c l i n g the l i n t through a mechanical c l e a n e r such as a S h i r l e y a n a l y z e r Q7) or SRRC n o n l i n t t e s t e r (18). P a r t i c u l a t e s recoverable from c l e a n i n g are added back to the cleaned l i n t and the i t h property measured t o f a c i l i t a t e computation o f . Cleaned l i n t spiked with recoverable p a r t i c u l a t e s c o n s t i t u t e s a n a t i v e standard. Assum ing that λ.| = Ψ-j, an apparent p a r t i c u l a t e c o n c e n t r a t i o n i n the stock m a t e r i a l i s computed by measuring the i n s i t u property v a l u e . The s p i k i n g process i s repeated, and d i f f e r e n t proper t i e s measured. Means of group apparent i n s i t u p a r t i c u l a t e l e v e l s are compared to decide which, i f any, d i f f e r because o f r e j e c t i o n of the null hypothesis. P a r t i c u l a t e burden i n the reference material i s the average of the accepted group means. The method i s demonstrated on ginned c o t t o n . Two equations were d e r i v e d to p r e d i c t τ% the i n s i t u p a r t i c u l a t e burden e r r o r a n t i c i p a t e d f o r a s p e c i f i c i t h property measurement under a l t e r n a t i v e hypothesis c o n d i t i o n s , Xj φ ψ·,·. This e r r o r or bias may be expressed as a f u n c t i o n o f Ί

9

TA =

or

τ

(4) (5)

? = 100

λ.,· and ψ-j, Equation 4 or as a f u n c t i o n of % t , λ ·, and Equation 5 where % t = the percent n o n l i n t t r a s h ( r e l a t i v e to the i n s i t u p a r t i c u l a t e burden) recovered from mechanical c l e a n i n g f o r s p i k i n g purposes and θ · = other p a r t i c u l a t e s i t h property constant ( p r o p o r t i o n a l i t y constant between i n s i t u nonrecoverable p a r t i c u l a t e s and the i t h property value) n

Ί

n

Ί


5.

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73

The two e r r o r equations q u a n t i f y the model p r e d i c t i o n s , Figure 3, where i i n s i t u p a r t i c u l a t e burden based on the i t h property measurement and εj = i t h property value f o r the uncleaned stock cotton Table III gives p r e d i c t e d e r r o r f o r operational l i m i t i n g % t and v a l u e s . The projected upper l i m i t of recoverable n o n l i n t t r a s h f o r s p i k i n g purposes i s about 70-90% of the i n s i t u p a r t i c u l a t e burden. Assuming then that % t = 9 0 % and θί = Xi + 0.5 Xj (6) then Xj/9j v a r i e s within the a r b i t r a r i l y chosen l i m i t s 0.67 < λ -/θ · i 2.0 (7) so that τ* w i l l be within the range -3.33 to 10.0%. For a l a r g e number of mutually independent p r o p e r t i e s measured the p r o j e c t e d average e r r o r would be 3.34%. T

=

n


n

Ί

Ί

TABLE III %

Dependence of τ* on Operational L i m i t i n g X /6 i

θ

Μ/ 1

i

and %

n t

Values

*nt

*1

1.000

1.750 0.571

90.0 80.0 70.0

1.044 1.093 1.147

-4.29 -8.58 -12.87

1.000

1.500

90.0 80.0 70.0

1.034 1.071 1.111

-3.33 -6.67 -9.99

1.000

1.000 1.000

1.000

0.500

2.000

90.0 80.0 70.0

0.909 0.833 0.769

+10.0 +20.0 +30.0

1.000

0.250

4.000

90.0 80.0 70.0

0.769 0.625 0.526

+30.0 +60.0 +90.0

0.667

v a r i a b l e 1.000

0

D e v i a t i o n from λ ·/θ · = l may be due to changes i n the r e l a t i v e amounts of l e a f , b r a c t , stem, e t c . found i n the recover able t r a s h as opposed to the nonrecoverable p a r t i c u l a t e s . It i s 1

1


74

CHEMICAL

HAZARDS IN T H E

WORKPLACE

doubtful that the spread of λ · and θ · values would approach the maximum range suggested by Equations 6 and 7 unless % t i s q u i t e small. P r e d i c t e d τ-j e r r o r s may be computed from Equations 4 or 5 or the graphic a n a l y s i s presented below. A l l three methods give e q u i v a l e n t e r r o r values, which lends credence to the d e r i v a t i o n s . F i g u r e 4 shows a graphic a n a l y s i s of the i t h property values under the c o n s t r a i n t s i n d i c a t e d . Curve A λ-j = θ = ψ · = 1.000 would r e s u l t i n a 0% e r r o r f o r τ|, regardless of percent n o n l i n t t r a s h recoverable f o r s p i k i n g , thus, Δ-j = τ-,· at point ( a ) , see a l s o the model i n Figure 3. For λ-j/θ · = 2, curves Β and C, p o i n t (c) on the curve corresponds t o Δ·,· > τ,·. The e r r o r i n i s equal t o the l i n e segment ac. Note that curve C, slope = ψ · = 0.090, i s the t r u e i n s i t u slope f o r = 2 and % = 90. For = 0.67, curves D and E, point (e) on the curve corresponds to Δ-j < T J . The e r r o r i s equal to the l i n e segment ae. The actual i n s i t u slope would be = 1 .034. To demonstrate f e a s i b i l i t y , the i n s i t u n a t i v e standard method i s i l l u s t r a t e d on a ginned cotton by measuring t h r e e mutually independent p r o p e r t i e s : c o l o r , conductance, and pH. Color was measured on the s o l i d samples by r e f l e c t a n c e s p e c t r o s copy, conductance and pH on aqueous suspensions. An example o f the c a l c u l a t i o n protocol based on c o l o r measurements i s shown i n Table IV. Xj=] through Xj=3 denote j = 3 r e p l i c a t i o n s of meter readings on each sample. Xcolor the average value f o r the j j r e p l i c a t i o n s . Nonlint t r a s h added values are regressed on Xcolor* A x - | i s the apparent p a r t i c u l a t e value based on c o l o r measurement. Ί

Ί

n


1

1

Ί

η

nt

1 S

co

or



MONTALVO E T A L .

Figure 3.

CottOH Dust

Model of the in situ native standard method


CHEMICAL

HAZARDS IN T H E

WORKPLACE


76


5.

MONTALVO E T A L .

Cotton

Dust

11

TABLE IV Apparent In S i t u P a r t i c u l a t e Burden Based on C o l o r i m e t r i c Measurement


N o n l i n t Trash(nt) Added, Percent

Meter Reading

Y

xj-i

X

1.06 2.06 3.00 4.00 5.00 Stock

62.9 58.7 51.0 46.2 43.2 50.0

63.3 60.0 51.7 47.0 44.1 50.2

A

color

a

*color

Xcolor

j = 2

col or

+

Bcolor

-0.988: s l o p e , 50.0; assume

correlation coefficient, r _ = i n t e r c e p t , 3_ = J 2 . 8 4 1 ; ε = c o l o r at σ = ε

63.4 59.4 51.6 46.8 43.8 50.0

63.9 59.6 52.2 47.3 44.0 49.9

λ = λ =

-0.1853; ψ, evaluate

A x

T

A color

- ^^color

A x

=

color

β

e

color

(-.1853) · (50)

+

+

&color 12.84

=

3.58%

Results i n Table V are t a b u l a t e d f o r the three p r o p e r t i e s s t u d i e d . One hundred e i g h t y - f o u r property measurements were performed i n o b t a i n i n g the data. The apparent p a r t i c u l a t e (Ατ-,·) value c a l c u l a t e d by measuring pH, s e t 4, i s an obvious o u t l i e r and i s reported but not included i n the s t a t i s t i c a l c a l c u l a t i o n s . Group mean apparent p a r t i c u l a t e (Ατ ·) value ranged from a low o f 3.46 t o a high o f 3.51%. Set standard d e v i a t i o n (s) ranged from 0.150 t o 0.259%. C o e f f i c i e n t _ o f v a r i a t i o n (CV) ranged from 4.27 t o 7.47%. V a r i a t i o n o f ATJ values was i n the second decimal p l a c e . The pooled Ατ · variance was only 0.04%. Ί

Ί


78

CHEMICAL

HAZARDS IN T H E

WORKPLACE

TABLE V Results τ

Percent Apparent In S i t u P a r t i c u l a t e s (Α ·) Ί


Set Number

Color

Conductance

pjj

2 3 4

3.58 3.31 3.21 3.78

3.50 3.66 3.33 3.34

3.39 3.47 3.68 (4.74)

s,% CV,*

3.47 0.259 7.47

3.46 0.156 4.50

3.51 0.150 4.27 τ

PERCENT IN SITU PARTICULATES ( ) Ύ

= 0.01 l e v e l of s i g n i f i c a n c e

τ

= (3.47

+ 3.46 + 3.51)/3 = 3.48% Ύ

Based on a l e v e l of s i g n i f i c a n c e , , of 0.01, there i s no reason to b e l i e v e the c a l c u l a t e d values d i f f e r ( s t a t i s t i c a l d e c i s i o n i s based on the f o l l o w i n g degrees of freedom: t o t a l , 11; property groups, 3; and set o b s e r v a t i o n s , 11 - 3 = 8 ) . T h e r e f o r e , the i n s i t u p a r t i c u l a t e burden i n the stock i s the average of a l l three A i v a l u e s , 3.48% based on the a v a i l a b l e d a t a . The data suggests the absence of a s i g n i f i c a n t a l t e r n a t i v e hypothesis e r r o r , ? , i n t h i s t r i a l run and i s i n e x c e l l e n t agreement with the model p r e d i c t i o n s . Refinement o f measurement procedures to reduce variances and development o f a d d i t i o n a l systems to i n c r e a s e degree of confidence i n the n u l l hypothesis i s on-going. T

τ

Control o f Cotton Dust A wide v a r i e t y of systems are c u r r e n t l y being used t o c l e a n the a i r i n t e x t i l e m i l l s p r o c e s s i n g l i n t c o t t o n . Most of these are custom designed, and use p r o p r i e t o r y equipment such as bag f i l t e r s , r o t a r y drum f i l t e r s , condensers, V c e l l s , e t c . (19), but many systems have evolved as a r e s u l t of needed improve ments. The b e t t e r systems have several stages, i n c l u d i n g a waste separator t h a t can reduce dust content of the a i r by some form of f i l t r a t i o n , and a f i n e dust separator capable o f remov ing r e s p i r a b l e dust ( l e s s than 15 urn). The e f f e c t i v e c o n t r o l and removal of dust in t h i s s i z e range i s q u i t e d i f f i c u l t and


5.

MONTALVO E T A L .

Cotton

Dust

79


expensive. P r e s e n t l y , there are few a i r handling systems in the t e x t i l e i n d u s t r y capable o f economically and e f f i c i e n t l y c o l l e c t i n g and removing r e s p i r a b l e dust. For the past several y e a r s , our l a b o r a t o r y has sponsored and c a r r i e d out research on the systems o u t l i n e d below. Wet Wall E l e c t r o i n e r t i a l P r e c i p i t a t o r . The wet wall e l e c t r o i n e r t i a l p r e c i p i t a t o r (WWEP) a i r c l e a n e r was developed t o meet the requirements o f high operating e f f i c i e n c y , low maintenance, low operating c o s t , and s i m p l i c i t y of d e s i g n . To achieve high e f f i c i e n c y with low pressure l o s s , an e l e c t r o s t a t i c p r e c i p i t a t i o n was the best candidate method. A wet wall was d e s i r a b l e t o f l u s h the p r e c i p i t a t e away and to minimize maintenance. I n e r t i a l e f f e c t s were added t o a s s i s t i n moving m a t e r i a l toward the wall and to improve the c l e a n i n g a c t i o n o f the water on the wall (20). These f e a t u r e s were best i n c o r p o r a t e d i n the design shown i n F i g u r e 5. The u n i t c o n s i s t s of a c o n c e n t r i c wire-in-tube p r e c i p i t a t o r . The charging wire i s l o c a t e d a x i a l l y i n a s t a i n l e s s steel tube i n a v e r t i c a l o r i e n t a t i o n . A i r enters t a n g e n t i a l 1 y at the upper end of the tube (through a duct with a f l a t t e n e d cross s e c t i o n ) and acquires a r o t a t i n g movement. When high v o l t a g e i s a p p l i e d to the wire, an i o n i z i n g coronal d i s charge charges the dust i n the a i r . As the a i r flows through the tube, the charged dust i s d r i v e n to the wall by the r a d i a l e l e c t r i c a l f i e l d . Water from the upper water i n l e t flows down the wall and f l u s h e s the p r e c i p i t a t e d dust i n t o the water o u t l e t at the lower end o f the tube. The r o t a t i o n a l movement o f the a i r a l s o induces r o t a t i o n a l flow of the water, which a s s i s t s i n uniformly wetting the surface o f the tube. Clean a i r i s e x p e l l e d from the tube's lower end. Tests of the WWEP were conducted with 4 i n , 8 i n and 16 i n diameter u n i t s at flow rates ranging from 100 to 4000 f t f y m i n . P a r t i c l e c o l l e c t i o n e f f i c i e n c y of these u n i t s was measured with atmospheric dust, AC f i n e t e s t dust (mass mean diameter (MMD) 12 urn), a r t i f i c i a l cotton dust (MMD = 4 . 0 ym), and cotton dust drawn from the processing area of a card (MMD - 3.0 \m). Tests conducted with the 4 i n diameter WWEP provided guidel i n e s t o determine the e f f e c t s of the tube l e n g t h , a i r flow r a t e , and voltage p o l a r i t y on e f f i c i e n c y o f p a r t i c l e c o l l e c t i o n . Tube lengths o f 1, 2, and 4 f t were evaluated. Shearing o f water d r o p l e t s from the wall at higher v e l o c i t i e s made the p r a c t i c a l upper l i m i t on flow rate about 200 f t / m i n . Results of the e f f i c i e n c y t e s t s with the 2 f t u n i t are i n c l u d e d i n Table VI (water flow at 0.25 g a l / m i n ) . Runs made at zero p o t e n t i a l and zero c u r r e n t with the r a t h e r course AC dust show e f f i c i e n c i e s i n the 80% range due merely to c e n t r i f u g a l f o r c e s . Increasing the voltage t o -30 kV r a i s e s the e f f i c i e n c y t o 99%, which diminishes to 98.5% when +33 kV i s a p p l i e d . Tests with 3


CHEMICAL

80

HAZARDS

IN T H E W O R K P L A C E

Lucite Air

tube

inlet


Insulator

>— Water i n l e t

S t a i n l e s s s t e e l tube

Corona wire

I

I

Insulator

i f f Brass tube

^

Water o u t l e t Air

Figure 5.

exhaust

Wet wall electroinertial precipitator


5.

MONTALVO E T A L .

81

CottOH Dust

the f i n e a r t i f i c i a l cotton dust i n d i c a t e a somewhat lower e f f i c i e n c y (98.1 and 96.1% r e s p e c t i v e l y ) f o r 100 and 200 ft^/min. TABLE VI Total Mass E f f i c i e n c y of the 4 i n Wet Wall E l e c t r o i n e r t i a l P r e c i p i t a t o r


Test Dust

AC f i n e AC f i n e AC f i n e AC f i n e AC f i n e Cotton dust Card trash Cotton dust

Circulation Rate Potential (ft3/min) (kV)

Current (mA)

Dustfeed Pressure E f f i Rate Drop c i e n c y (g/min) (in H 0) (%) 2

100 200 100 200 100

0 0 -30 -30 +33

0 0 2.4 2.4

1.0 2.0 0.2 0.2 1.0

1.25 3.75 1.25 1.25 1.25

81.2 87.6 99.0 99.0 98.5

100

-30

2.2

0.83

1.25

98.1

200

-30

2.2

0.67

3.75

99.7

200

-30

2.1

0.62

3.75

96.1

Under a memorandum of understanding r e l a t i v e to the research and development of the wet wall e l e c t r o i n e r t i a l p r e c i p i t a t o r as a commercial product, a 16 i n diameter, 60 i n long u n i t capable of handling up t o 4000 CFM was i n s t a l l e d at SRRC f o r t e s t and e v a l u a t i o n . Runs made at zero p o t e n t i a l with dust from operating cards, p r e f i l t e r e d by a V - c e l l , showed e f f i c i e n c i e s i n the range of 70-80% due to the c e n t r i f u g a l f o r c e s . Applying a p o t e n t i a l of 100 kV r a i s e d the e f f i c i e n c y up to 95%, with an a i r flow ranging from 2900 to 3700 CFM. Under another agreement f o r the commercialization of the p r e c i p i t a t o r , a 23 i n diameter, 72" long u n i t was developed capable of handling up to 12,000 CFM. This u n i t was powered by a pulsed power supply to meet allowed ozone l e v e l s . Several of these models were b u i l t and are c u r r e n t l y undergoing e v a l u a t i o n . I t i s a n t i c i p a t e d that these u n i t s w i l l be commercially a v a i l a b l e by mid 1980. F l u i d E l e c t r o d e P r e c i p i t a t o r . In g e n e r a l , the best way to remove f i n e dust from an a i r stream i s t o use conventional e l e c t r o s t a t i c p r e c i p i t a t o r s . Although the e f f i c i e n c i e s of these devices are q u i t e high, there are inherent drawbacks that do not make them acceptable i n c o n t i n u o u s l y operating a i r systems. The c o l l e c t i o n p l a t e s e v e n t u a l l y become saturated with the deposited



82

CHEMICAL

HAZARDS IN T H E

WORKPLACE

dust and the u n i t has t o be cleaned. Most of the c l e a n i n g methods r e q u i r e that the u n i t be shut down i n order f o r the c l e a n i n g c y c l e to occur. The development of the f l u i d e l e c t r o d e p r e c i p i t a t o r i s an attempt to a l l e v i a t e t h i s problem (21J. The f l u i d e l e c t r o d e p r e c i p i t a t o r i l l u s t r a t e d i n F i g u r e 6 c o n s i s t s o f an array o f e l e c t r o d e s in a casing to charge, d i r e c t , and c o l l e c t f i n e dust. Tungsten wires serve as discharge e l e c t r o d e s f o r a l l u n i t s t e s t e d , a u x i l i a r y d r i v i n g e l e c t r o d e s are charged metal p l a t e s o r screens that can be i n s e r t e d between the f l u i d e l e c t r o d e s , and the c o l l e c t i o n e l e c t r o d e s are formed by v e r t i c a l tubes that r e l e a s e a laminar flow of water f a l l i n g through the height o f the c a s i n g . The p r e c i p i t a t o r operation i s q u i t e simple. The f a l l i n g columns o f grounded water act as c y l i n d e r s i n cross f l o w and c r e a t e v o r t i c i e s f o r enhanced p a r t i c l e c o l l e c t i o n . A u x i l i a ry e l e c t r o d e s and discharge e l e c t r o d e s can be p o s i t i o n e d i n arrays to d i r e c t charged dust i n t o the grounded f l u i d . Two s e r i e s of t e s t s , one a t low c a p a c i t y and one a t h i g h , were performed with cotton dust from d i f f e r e n t sources. For low c a p a c i t y t e s t s , a i r was drawn through a model-size cotton c a r d i n g machine running at 2 Ib/hr. The a i r was p r e f i l t e r e d t o remove l i n t and l a r g e p a r t i c l e s and was then passed through the t e s t tunnel (9 i n χ 9 i n s t r a i g h t tunnel 9 f t long c o n t a i n i n g t r a v e r s a b l e sampling probes l o c a t e d upstream or downstream of the p r e c i p i t a t o r t e s t s e c t i o n ) . In the high c a p a c i t y t e s t s , f i n e dust r e l e a s e d by mechanically tapping a V - c e l l nonwoven f i l t e r was used. The f i l t e r was p r e v i o u s l y loaded with l i n t and dust from two production cotton c a r d i n g machines. The a i r drawn through the f i l t e r and passed through the t e s t tunnel contained f i n e dust that was s i m i l a r to the p r e f i l t e r e d a i r from the model card. Samples of dust from each of the two dust sources were c o l l e c t e d f o r t e s t i n g . They were d i s p e r s e d i n an e l e c t r o l y t e and analyzed f o r s i z e on a volume b a s i s . Model card dust had a mass mean diameter o f 3.6 ym with a geometric standard d e v i a t i o n of 1.6. The cotton dust r e l e a s e d by tapping the loaded f i l t e r v a r i e d from 4.5 t o 6.7 ym mass mean diameter with a geometric standard d e v i a t i o n of about 2. A l l t e s t c o n c e n t r a t i o n s were i n the range of 0.5 to 1.5 mg/m , which are t y p i c a l of cotton m i l l f i n e dust c o n c e n t r a t i o n . Table VII c o n t a i n s the o p e r a t i o n a l data and c o l l e c t i o n efficiency for a typical configuration tested. Collection e f f i c i e n c y i s expressed i n terms o f p a r t i c l e and s i z e e f f i c i e n c y . 3



MONTALVO E T A L .

CottOïl Dust

Figure 6.

Fluid electrode precipitator


84

CHEMICAL HAZARDS IN THE WORKPLACE

TABLE V I I Efficiency of Typical Fluid Electrode Precipitator Configuration Field Strength


Velocity Efficiency ( f t / m i η) (kV/cm) 250 350 500

6.2 5.9 4.9

Dust Supply Concentration (mg/m3) 0.8 0.5 0.8

Collection Efficiency Particle Size (1.5 μΐη10 ym) Mass (%) (%) 75 81 60

86 93 78

Once properly adjusted an array of f l u i d electrodes (up to 45 in length) can operate with l i t t l e maintenance to continuously remove dust. Dust entrained in the f l u i d can continue to circulate through the system until i t s accumulation warrants a f l u i d cleaning cycle. Fluid cleaning would not require percipitator shutdown. Applied to t e x t i l e m i l l s , these precipitators could be incorporated via parallel or series configurations into existing duct work where continuous fine cotton dust removal of 85% or better would greatly reduce levels of fine airborne dust. Literature Cited 1.

2. J.

3. 4. 5. 6. 7. 8. 9.

Occupational Health and Safety Administration, U.S. Depart ment of Labor: "Occupational Exposure to Cotton Dust, Final Mandatory Occuaptional Safety and Health Standards." Federal Register, Part I I I , June 23, 1978, 27350-27463. Merchant, J. Α.; Lumsden, J. C.; Kilburn, Κ. H.; O'Fallon, W. M . ; Ujda, J. R.; Germino, V. H.; Hamilton, J. D. Occup. Med., 1973, 15, 22-230. Lynch, J. R. Trans. Nat. Conf. Cotton Dust Health, Charlotte, N.C., 1970, 33-43. National Institute for Occuaptional Safety and Health. U . S . Dept. Health Educ. Welf. Pub. 75-118, 1974, 117. Bethea, R. M . ; Morey, P. R. Am. Conf. Gov. Ind. Hyg. Nat. Cotton Dust Symp. 1974, Atlanta, G a . , 285-327. Neefus, J. D. Am. Ind. Hyg. Assoc. J., 1975, 37, 475. Matlock, S. W.; P a r n e l l , C. B. Am. Soc. Mech. Engrs. 76-Tex-10, 1976. Claassen, B. J., Jr. Am. Ind. Hyg. Assoc. J., 1979, 40, 993-941. Robert, K. Q., Jr. Am. Ind. Hyg. Assoc. J., 1979, 40, 535-542.


5.

MONTALVO ET AL.

CottOYl Dust


10.

85

Carson, G. Α . ; Lynch, J. R. "Calibration of the Vertical E l u t r i a t o r Cotton Dust Sampler." Presented at the Annual Meeting, A.I.H.A., Boston, Mass. 1973. (Also: personal communication to K. Q. Robert). 11. Robert, K. Q., Jr. Relationship between Crossflow, Iso k i n e t i c , and Calm-Air Sampling with a Vertical E l u t r i a t o r , Proc. 1980 Beltwide Cotton Prod. Res. Conf. (Special Session on Cotton Dust), 86-96. 12. Reif, R. B . ; Albrechtson, L . R.; Neville, F. E.; Thompson, W. B . ; Hanks, C. L.; McCrady, P. E.; Gieseke, J. Α.; Schmidt, E. W.; Miga, L . W. Final Report. 1977, Battelle Memorial Institute, Contract No. 12-14-7001-365. 13. Batra, S. K., Shang, P. P . ; Hersh, S. P . ; Robert, K. Q., Jr. Proc. 1980 Beltwide Cotton Prod. Res. Conf. (Special Session on Cotton Dust) 97-102. 14. Ayer, Η. E . and Kilburn, K. H, CRC Crit. Rev. Environ. Control. 1971, 2, 207-241. 15. Langley, E . D., Text. Res. J., 1979, 49, 455-457. 16. Montalvo, J. G., Jr.; DeLuca, L . B . ; Segal, L.; Proc. 1980 Beltwide Cotton Prod. Res. Conf. (Special Session on Cotton Dust) 70-71. 17. American Society for Testing and Materials, Annu. Book ASTM Stand. 1977, Part 33, 576-583. 18. Rusca, R. Α . ; Little, H. W.; Gray, W. H., Text. Res. J., 1964, 34, 61-68. 19. Barr, H. S.; Hocutt, R. H.; Smith, J. B., U.S. Dept. Health Educ. Welf. Rep. HSM 99-72-44, March 1974. 20. Thibodeaux, D. P.; Baril, Α., Jr.; Reif, R., IEEE Trans. Ind. Applic. January/February 1980, Vol 1A-16, No. 1, 80-86. 21. Claassen, B. J., Jr., Conf. Proc. 70th Annu. Meet. Air P o l l u t . Control Assoc. Toronto, June 20-24, 1977, 1-15. RECEIVED October 27,

1980.


Measurement, Analysis, and Control of Cotton Dust - ACS Symposium

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