Pneumatic System Air Drying by Pressure Swing Adsorption - ACS

56 Pneumatic System A i r Drying by Pressure Swing

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch056

Adsorption J. P. A U S I K A I T I S Molecular Sieve Department, Linde Division, Union Carbide Corp., Tarrytown, N.Y. 10591

ABSTRACT Removing water from air i s the most basic a p p l i c a t i o n f o r molecular sieves. Regeneration of molecular sieves by the use of a pressure swing c y c l e and i n c o r p o r a t i n g a d d i t i o n a l isothermal purge to the d e p r e s s u r i z a t i o n step adds a degree of complexity to the application. T h i s paper describes the i n c o r p o r a t i o n of t h i s c y c l e in small-to-moderate s i z e pneumatic systems, develops the p e r t i nent design theory and equations, defines the key process parameters, and demonstrates t h e i r e f f e c t on drying performance.

Introduction A l l equipment r e q u i r i n g compressed a i r i s s u s c e p t i b l e t o the problems o f c o r r o s i o n and damage o f a i r a c t u a t e d components by the passage o f m o i s t u r e , o i l and d u s t . More severe damage can occur i n c o l d weather when m o i s t u r e i n the compressed a i r may condense and r e s u l t i n f r e e z e - u p s . The term a i r precleanup i s used t o i n c l u d e the removal and r e j e c t i o n o f a l l the p r e v i o u s l y mentioned contaminants from the compressed a i r system. The b e n e f i t s o f t h i s precleanup s t e p a r e : (a) m i n i m i z i n g the f r e quency o f r e p a i r , (b) r e d u c t i o n o f r o u t i n e maintenance, and (c) most i m p o r t a n t l y , t o decrease downtime l o s s e s . Source o f Problem As the temperature o f a i r i n c r e a s e s i t can s u b s e q u e n t l y c o n t a i n more water vapor b e f o r e i t becomes s a t u r a t e d . The o p p o s i t e i s t r u e f o r i n c r e a s i n g p r e s s u r e s . As a i r i s compressed above atmospheric p r e s s u r e , i t w i l l hold l e s s water a t s a t u r a t i o n . F i g u r e 1 shows the e f f e c t s o f p r e s s u r e on the s a t u r a t e d water content o f a i r . Most pneumatic equipment r e q u i r e s a i r a t 100 t o 150 p s i t o operate a i r a c t u a t e d components. A s i n g l e - s t a g e compressor i s 681 Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


682

MOLECULAR SIEVES—II

o f t e n used t o p r o v i d e the compressed a i r . T h i s compression i s done i n an a d i a b a t i c manner; and the compressed a i r u s u a l l y passes t o a primary r e s e r v o i r exposed t o ambient temperatures. The compressed a i r may o n l y approach ambient temperature t o w i t h i n 20 t o 50°F before e n t e r i n g the r e s e r v o i r . The hot compressed a i r c o u l d c a r r y - o v e r a l l o f the water as vapor and, as the a i r c o o l s , l i q u i d water w i l l accumulate i n the primary r e s e r v o i r . The s i g n i f i c a n t ambient c o n d i t i o n s which a f f e c t the magnitude o f t h i s water accumulation are ambient temperature and r e l a t i v e h u m i d i t y . Routine maintenance such as "blowing down" the primary r e s e r v o i r w i l l not circumvent t h i s problem. Furthermore, t h i s procedure cannot l i m i t downstream condensation i f the compressed a i r i s subsequently used i n pneumatic equipment a t temperatures below those o f the r e s e r v o i r . Conventional Drying Devices In r e c e n t y e a r s , the pneumatic equipment user has been s u p p l i e d w i t h numerous types o f precleanup equipment, each c l a i m ing t o do a l l o r p a r t o f the t o t a l precleanup j o b r e q u i r e d . These d e v i c e s can be c l a s s i f i e d as mechanical s e p a r a t o r s , c e n t r i f u g a l s e p a r a t o r s , heat exchangers, and/or a f t e r c o o l e r s . Although these d e v i c e s may be c a l l e d by one p a r t i c u l a r name, a l l take advantage o f heat exchange and mechanical s e p a r a t i o n t o p a r t i a l l y condense and separate e n t r a i n e d water, o i l and dust. U n i t s employing these two precleanup c l a s s i f i c a t i o n s can a c h i e v e m o i s t u r e removal t o l e v e l s no lower than ambient temperature dew p o i n t s a t p r e s s u r e , even o p e r a t i n g a t 100% e f f i c i e n c y . Such d e v i c e s a r e l o o s e l y termed as d r y e r s . However, can any o f the p r e v i o u s l y mentioned d r y e r s even be c l a s s i f i e d as d r y e r s a t a l l ? Or, a r e they s i m p l y d e v i c e s which hasten the r a t e a t which the compressed a i r seeks i t s new e q u i l i b r i u m c o n d i t i o n . Thus the u l t i m a t e performance o f any o f these u n i t s i s o n l y t h a t f i n a l c o n d i t i o n t h a t the compressed a i r would have o b t a i n e d by i t s e l f g i v e n time. These mechani c a l d e v i c e s are not u s e l e s s o f course because the a i r system i s u s u a l l y o p e r a t i n g i n a dynamic mode and t h e r e f o r e entrainment w i l l occur along w i t h condensation o f water i n the a i r l i n e s . These d e v i c e s are adequate f o r r e d u c i n g the amount o f l i q u i d water t h a t w i l l reach the primary r e s e r v o i r . P r e s s u r e Swing D e s i c c a n t Dryers The usual drawback o f u s i n g p r e s s u r e swing f o r d e h y d r a t i o n i s t h a t very low dew p o i n t s are d i f f i c u l t t o a t t a i n . In a d d i t i o n , the energy requirement per pound o f water removed i s u n f a v o r a b l e when compared t o a long c y c l e thermal swing p r o c e s s . However, bone dry a i r i s not r e q u i r e d i n many pneumatic a p p l i c a t i o n s . The o n l y requirement i s t h a t the a i r system be c o m p l e t e l y f r e e o f l i q u i d water which i n e v i t a b l y i s a r e s u l t o f condensation. S i n c e such equipment i s exposed t o the environment, the compressed a i r

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


56. AusiKAiTis

Pneumatic

System Air

Drying

683

w i l l have a maximum temperature change imposed by normal d a i l y temperature c y c l e s . T h e r e f o r e , i f t h e dew p o i n t o f t h e compressed a i r i s f a r below t h e e x i s t i n g ambient temperature, no condens a t i o n w i l l occur when t h e ambient temperature f a l l s . Depending upon t h e geographic l o c a t i o n , t h i s dryness requirment may be q u a n t i f i e d as a 20 t o 60°F dew p o i n t d e p r e s s i o n . To a t t a i n t h i s degree o f dryness i t i s necessary t o remove water vapor i n a d d i t i o n t o e n t r a i n e d and condensed l i q u i d water. T h i s i s where a p r e s s u r e swing d e s i c c a n t d r y e r t a k e s over where o t h e r d e v i c e s leave o f f . A d e s i c c a n t d r y e r can t a k e f u l l advantage o f t h e normal i n t e r m i t t e n t s e r v i c e o f a i r compressors by r e g e n e r a t i n g the d e s i c c a n t immediately a f t e r t h e c o m p l e t i o n o f t h e duty c y c l e . In a d d i t i o n , t h i s type o f d r y e r has t h e p h y s i c a l s i z e and, when the proper d e s i c c a n t i s used, t h e mechanical s t r e n g t h r e q u i r e d f o r use on equipment which imposes r i g o r o u s demands on such units. F i g u r e 2 shows t h e r e l a t i v e performance o f a d e s i c c a n t d r y e r o p e r a t i n g a t a 60°F ( P o i n t A) and a 30°F ( P o i n t B) dew p o i n t d e p r e s s i o n versus t h e performances o f an a f t e r c o o l e r which a t t a i n s a 20°F approach t o ambient temperature ( P o i n t E) and a heat exchanger which a t t a i n s a 10°F approach t o ambient temperat u r e ( P o i n t D). As can be seen from t h e f i g u r e , i f t h e compressed a i r temperature f a l l s back t o ambient temperature ( P o i n t C) from P o i n t E o r D, t h e r e i s s t i l l p o t e n t i a l f o r f u r t h e r water t o condense. The performance i s based on t h e t o t a l percentage o f i n l e t water t o t h e compressor r e j e c t e d versus s e v e r a l ambient r e l a t i v e h u m i d i t i e s a t a temperature o f 75°F and a t o t a l p r e s sure o f 125 p s i . Operation o f a P r e s s u r e Swing D e s i c c a n t Dryer F i g u r e 3 i s a f u n c t i o n a l diagram o f t h e i n t e r n a l s o f a t y p i c a l d e s i c c a n t d r y e r . When t h e p r e s s u r e i n t h e primary r e s e r v o i r f a l l s below i t s minimum a l l o w a b l e l e v e l , t h e compressor engages and begins t o pump up t h e r e s e r v o i r . A i r from t h e compressor e n t e r s t h e a n n u l a r s e c t i o n o f t h e concent r i c c y l i n d e r s which i s made up o f an e x t e r n a l housing and t h e i n t e r n a l d e s i c c a n t chamber. A i r s w i r l s around and down a l o n g the cool s u r f a c e o f t h e e x t e r n a l h o u s i n g , condensing water and c o a l e s c i n g o i l d r o p l e t s i n t h e p r o g r e s s . While t h e l i q u i d water and o i l a r e c o l l e c t e d i n t h e sump, t h e a i r changes d i r e c t i o n 180 degrees and begins t o f l o w upflow through t h e bed o f d e s i c cant. The a i r passes through t h e bed and i s d r i e d on t h e way. D r i e d a i r e x i t s t h e t o p o f t h e d r y e r and f l o w s t o t h e primary r e s e r v o i r . While t h e primary r e s e r v o i r i s being pumped up w i t h dry a i r , a f r a c t i o n o f t h i s a i r i s d i v e r t e d and saved f o r t h e purge volume. When the primary r e s e r v o i r i s pumped up t o i t s maximum o p e r a t i n g p r e s s u r e , t h e compressor disengages. The compressor u n l o a d i n g a c t u a t e s t h e blowdown v a l v e and t h e p r e s s u r e w i t h i n t h e d e s i c c a n t chamber i s r a p i d l y reduced t o


684

MOLECULAR SIEVES—JJ 10-


1

Figure 1. (right) Water content of saturated air vs. temperature at various total pressures Figure 2. (below) Relative performance of a pressure swing desiccant dryer vs. a heat exchanger and an aftercooler as a function of relative humidity

T E M P E R A T U R E (DEW POINT). ° F

COMPRESSED AIR DEW POINT, °F


56.

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System Air

Drying

685


atmospheric p r e s s u r e . The a i r from the purge volume i s expanded through a r e s t r i c t i o n , such as an o r i f i c e , i n a d i r e c t i o n count e r c u r r e n t t o a d s o r p t i o n . T h i s d r i e d purge a i r flows over the d e s i c c a n t and p i c k s up water u n t i l i t e x i t s the d r y e r as wet a i r v i a the blowdown v a l v e i n the sump. As compressed a i r i s used, the a i r pressure i n the r e s e r v o i r begins to f a l l u n t i l the lower pressure l i m i t i s reached, a t which time the compression and d r y i n g c y c l e repeat. B a s i c Pressure Swing Theory A pressure swing a d s o r p t i o n process adsorbs water a t high t o t a l pressures and desorbs water a t low t o t a l p r e s s u r e s . In so d o i n g , a d s o r p t i o n i s c a r r i e d out at the h i g h e s t water p a r t i a l p r e s s u r e , r e s u l t i n g i n h i g h e r l o a d i n g s ; and d e s o r p t i o n i s c a r r i e d out a t the lowest water p a r t i a l p r e s s u r e , r e s u l t i n g i n lower l o a d i n g s . Water i s c a r r i e d out o f the bed by u s i n g a f r a c t i o n of the d r i e d h i g h e r pressure product a i r expanded t o the lower d e s o r p t i o n p r e s s u r e . Because a i r has a h i g h e r s a t u r a t ed water content a t the lower p r e s s u r e s , o n l y a f r a c t i o n o f the d r i e d compressed a i r needs t o be used t o remove an e q u i v a l e n t amount o f water from the adsorbent. A t y p i c a l pneumatic a i r system might r e q u i r e 600 SCFH o f dry a i r a t 125 p s i g . To p r o v i d e t h i s volume of a i r an a d d i t i o n a l volume o f d r i e d a i r must be produced f o r purging d u r i n g the d e s o r p t i o n s t e p . In a d d i t i o n , the compressor i s o p e r a t i n g i n t e r m i t t e n t l y which a l l o w s f o r the d e s o r p t i o n s t e p . T h e r e f o r e , a 24 SCFM compressor would be s u i t a b l e t o p r o v i d e the r e q u i r e d volume o f a i r . T y p i c a l l y , t h i s system would operate on approximately one minute c y c l e s , d u r i n g which the compressor would operate 60% o f the time. The compressor would thus d e l i v e r 14.4 SCF o f compressed a i r per c y c l e . To dry t h i s a i r a two pound bed o f m o l e c u l a r s i e v e i s adequate. S a t u r a t e d a t 75°F and 125 p s i g , t h i s volume o f a i r would c o n t a i n o n l y 0.00216 pounds o f water. T h e r e f o r e , the a v a i l a b l e a d s o r p t i o n c a p a c i t y f o r the two pounds o f m o l e c u l a r s i e v e would need t o be 0.108 weight percent based on e q u i l i b r i u m c o n s i d e r a t i o n s . Thus the displacement i n v o l v e d along the isotherm i s very s m a l l . Theref o r e i t might be expected t h a t the shape o f the isotherm may not be n e a r l y as important t o the o v e r a l l performance o f the d e s i c c a n t d r y e r as the mass t r a n s f e r r e s i s t a n c e o f the adsorbent. In f a c t , i t i s the mass t r a n s f e r zone t h a t c o n t r o l s the u l t i m a t e product dew p o i n t t h a t the d r y e r w i l l achieve. The f u n c t i o n o f the adsorbent, which i s i n excess o f the mass balance r e q u i r e ments, i s c o n t a i n i n g the mass t r a n s f e r zone. F i g u r e 4 d e p i c t s the c h a r a c t e r i s t i c shape o f a mass t r a n s f e r zone. The c r o s s hatched area between the a d s o r p t i o n mass t r a n s f e r f r o n t and the d e s o r p t i o n mass t r a n s f e r f r o n t i s the d i f f e r e n t i a l amount o f water r e j e c t e d per c y c l e . Instead o f the small d i f f e r e n t i a l amount o f water being r e j e c t e d i n s t a n t a n e o u s l y by a small amount


MOLECULAR SIEVES-

686 DRY AIR SUPPLY TO PRIMARY RESERVOIR Qo,Yo(Ti,Po) PURGE VOLUME Qp.Yo (Ti.Po)

EXPANSION VALVE

CHECK VALVE

PURGE VOLUME Q ,Yo(Ti,Po) D


Qp.Y

0

(T ,P

COMPRESSORQi,Yi (Ti.Pi)

p

atm

)

DESORPTION

EXTERNAL. HOUSING

APPROXIMATELY 2 TO 3 POUNDS OF DESICCANT • * • SCREEN-I-»'CS5SS IS3

ADSORPTION

INTERNAL -DESICCANT CHAMBER

,

SUMP-

COMPRESSOR ACTUATED BLOWDOWN V A L V E

BLOWDOWN WASTE Op,Y ( T p . P ^ ) p

Figure 3.

Schematic flow diagram of a pressure swing desiccant dryer

©

LOW MASS TRANSFER RATE

DESORPTION

ADSORPTION

( Outlet

Figure 4. Characteristic shape of adsorption and desorption mass transfer fronts in a pressure swing dehydrator

-BED

LENGTH-

©

- LOADING IN EQUILIBRIUM WITH INLET COMPRESSED AIR

(2)

- LOADING IN EQUILIBRIUM WITH PRODUCT OR OUTLET AIR


Pneumatic


56. AusiKAiTis

System Air

687

Drying

o f adsorbent (as assumed i n a mass b a l a n c e ) , t h e water i s spread out over a l a r g e amount o f adsorbent due t o t h e f i x e d r a t e s a t which i t i s removed. The shape o f t h e zones i s determined by t h e mass f l o w r a t e o f a i r , temperature, p r e s s u r e , adsorbent type and adsorbent p a r t i c l e s i z e . The l o c a t i o n o f t h e zones i s determined by a f l o w balance between t h e a d s o r p t i o n s t r o k e and t h e desorpt i o n s t r o k e . I n c r e a s i n g t h e amount o f incoming water d u r i n g t h e a d s o r p t i o n s t r o k e tends t o push t h e f r o n t towards t h e o u t l e t end which r e s u l t s i n high water c o n c e n t r a t i o n s i n t h e product a i r . Too much purging w i t h a d r i e d product tends t o push t h e desorpt i o n f r o n t out the i n f l u e n t end and thereby reduces t h e y i e l d o f d r i e d product a i r per c y c l e . Thus the volume o f purge gas used must be determined by a t r a d e o f f between dryness and t h e volume o f d r i e d a i r produced. The amount o f adsorbent r e q u i r e d f o r such r a p i d c y c l e systems must, be determined e m p i r i c a l l y . F o r a s p e c i f i c l e v e l o f d r y n e s s , t h e adsorbent requirements may be c o n s i d e r e d n e a r l y p r o p o r t i o n a l t o t h e volume o f a i r e n t e r i n g per c y c l e as long as t h e y i e l d remains c o n s t a n t . E m p i r i c a l Performance Model Knowing t h e t h e o r e t i c a l l i m i t a t i o n s o f the performance o f a p r e s s u r e swing d e s i c c a n t d r y e r , i t i s p o s s i b l e t o develop a simple mathematical model t o s i m u l a t e t h e o p e r a t i o n o f such u n i t s under v a r i o u s c o n d i t i o n s . A r i g o r o u s dynamic model o f t h e system i s not warranted because o f t h e v a r i a b l e nature o f the system. The v a r i a b l e s which a f f e c t performance, such as compressed a i r temperature, a i r f l o w r a t e , and ambient temperature, c o u l d e a s i l y be changing from c y c l e t o c y c l e . The simple mass balance model which f o l l o w s w i l l serve t h e purpose b e t t e r . The amount o f compressed a i r e n t e r i n g the primary r e s e r v o i r per c y c l e i s expressed a s : Ql

where,

= Qo + Qp

Q = d r i e d o u t l e t a i r , SCF/cycle Qj = wet i n l e t a i r , SCF/cycle Qp purge a i r , SCF/cycle 0

=

By mass b a l a n c e , t h e amount o f water adsorbed by t h e d e s i c c a n t per a d s o r p t i o n c y c l e i s expressed a s : AX = QjYj - (Q + Q ) Y A

ft

p

n x 1 0 0=

Q j ( Y i - Y ) 100 ft

WD

WO

b

B

x

where, AX = d i f f e r e n t i a l l o a d i n g d u r i n g a d s o r p t i o n , l b A

H 0/100 l b s . adsorbent 2

Y = water c o n c e n t r a t i o n o f i n l e t a i r , l b . H2O/SCF a i r Y = water c o n c e n t r a t i o n o f o u t l e t a i r , l b s . H2O/SCF a i r Wg = weight o f adsorbent bed, l b s . adsorbent i

Q



688

The d i f f e r e n t i a l amount o f water r e j e c t e d from t h e d e s i c c a n t per c y c l e i s expressed a s : A X = Q p ( p " o ) X 100 B Y

Y

D

W

where,

A X

= d i f f e r e n t i a l loading during desorption, l b . H2O/IOO l b s . adsorbent Y = water c o n c e n t r a t i o n o f purge a i r , l b . H 0/SCF a i r D

p


2

S i n c e t h e d i f f e r e n t i a l amount o f water r e j e c t e d per c y c l e d i c t a t e s t h e d i f f e r e n t i a l amount o f water adsorbed per c y c l e a t steady s t a t e , A X^ = A Xrj

Therefore, Q l ( Y l - Y ) = Qp(Y - Y ) 0

p

0

By rearrangement, t h e o u t l e t water c o n t e n t o f t h e compressed a i r can be expressed a s : Y o p

where, n = overall e f f i c i e n c y , dimensionless However, t h e r e i s another c o n s t r a i n t imposed upon t h e maximum water content o f t h e purge a i r e x i t i n g t h e d e s i c c a n t per c y c l e . T h i s c o n s t r a i n t r e s u l t s from t h e f a c t t h a t t h e amount o f water r e j e c t e d per c y c l e cannot be g r e a t e r than t h e amount o f water e n t e r i n g t h e d e s i c c a n t per c y c l e a t steady s t a t e . F i g u r e 5 g r a p h i c a l l y demonstrates t h e d e f i n i t i o n o f e f f i c i e n c y and i t s i n t e r r e l a t i o n s h i p w i t h t h e mass b a l a n c e . A f u n c t i o n which has t h i s proper double a s s y m p t o t i c c o n s t r a i n t and i s s u i t a b l e f o r t h i s model was found t o be: Q p Y = (nQpYpO) tanh p

(Jj*^)"

T h e r e f o r e , by s u b s t i t u t i o n , t h e o u t l e t water c o n t e n t o f t h e product a i r from t h e d e s i c c a n t d r y e r can be c a l c u l a t e d a s : Y

Q

Y

Y

- i i - (^P p°)

t a n n

( QiYi ) 'TQpYp 0

(1)

The l a s t requirement f o r t h e model i s n e c e s s i t a t e d by the a c t u a l mechanical o p e r a t i o n o f t h e d e s i c c a n t and s u r r o u n d i n g hardware.


56.

Pneumatic

AUSIKAITIS

System Air

Drying

689

The model as p r e v i o u s l y w r i t t e n s t a t e s t h a t the purge volume i s pumped up t o P before any a i r reaches the primary r e s e r v o i r . T h i s , i n a c t u a l i t y , i s not the case because both volumes r e c e i v e a i r s i m u l t a n e o u s l y . T h e r e f o r e , Q becomes a f u n c t i o n o f Qj a t low values o f Q-j. Depending on the r e l a t i v e volumes o f the two r e s e r v o i r s and the f l o w r e s t r i c t i o n s i n t r a n s i t t o each, the maximum Q-j t o i n s u r e Qp has achieved i t s maximum c o n s t a n t l e v e l may be two t o f i v e times Qp. For the purpose o f t h i s model, f o u r purge volumes w i l l be assumed t o be the minimum SCF o f a i r f e d per c y c l e t o a c h i e v e complete p r e s s u r i z a t i o n o f the purge volume. Thus f o r 0


p

Qi 1 4 Q

P

e q u a t i o n (1) holds and f o r Qi < 4 Q

p

the e q u a t i o n below i s used: (-QiYi_) v

Q i Y i - ( n QpYpo) tanh n Q Y o p

Y

o

=

0.75 Q

p

i

Experimental Data The performance o f any p r e s s u r e swing a d s o r p t i o n u n i t i s not o n l y a s t r o n g f u n c t i o n o f the adsorbent used but o f the mechanical d e s i g n . There are a v a r i e t y o f commercial p r e s s u r e swing d e s i c cant d r y e r s a v a i l a b l e f o r which the designs a r e f i x e d . T h e r e f o r e , a p r e s s u r e swing dehydrator was c o n s t r u c t e d t o p r o v i d e the f o l lowing i n f o r m a t i o n : 1) t o i n v e s t i g a t e the key v a r i a b l e s i n the d e s i g n , 2) t o c o n f i r m the mathematical model w i t h experimental d a t a , and 3) to determine the performance o f d i f f e r e n t d e s i c c a n t s i n o r d e r t o p r o v i d e the optimal adsorbents f o r t h i s a p p l i c a t i o n . E f f e c t o f Length To Diameter R a t i o . F i g u r e 6 shows the r e s u l t s o f the f i r s t mechanical d e s i g n parameter i n v e s t i g a t e d . T h i s parameter was the l e n g t h t o diameter r a t i o o f the cnamber c o n t a i n i n g the d e s i c c a n t . As the L/D o f the adsorbent bed i n c r e a s e s , the e f f i c i e n c y as determined by the computer model i n c r e a s e s . T h i s e f f i c i e n c y was determined as being the maximum e f f i c i e n c y a t t a i n a b l e w i t h o t h e r d e s i g n parameters, e x c l u d i n g L/D, o p t i m i z e d . A l s o as the average s i z e o f the Linde M o l e c u l a r S i e v e Beads a r e reduced from 4X8 t o 8X12 mesh, the e f f i c i e n c y i s a l s o s l i g h t l y improved. Thus any f a c t o r s which improve f l o w d i s t r i b u t i o n a l s o improve the o p e r a t i n g e f f i c i e n c y . However, the p e n a l t y t h a t i s i n c u r r e d by going t o a l a r g e L/D bed i s t w o f o l d . The f i r s t i s t h a t the p r e s s u r e drop a c r o s s the


MOLECULAR SIEVES—U


690

Figure 6. Efficiency and pressure drop at fixed experimental conditions as a function of length to diameter ratio of the desiccant bed


56.

AUSIKAITIS

Pneumatic

System Air

Drying

691


d e s i c c a n t bed i n c r e a s e s w i t h i n c r e a s i n g L/D. The second p e n a l t y i s t h a t t h e p h y s i c a l dimensions o f a l a r g e L/D u n i t s e v e r e l y l i m i t t h e o v e r a l l compactness o f t h e d e s i c c a n t d r y e r . E f f e c t o f Purge Flowrate. The second c r i t i c a l mechanical design parameter i n v e s t i g a t e d was purge f l o w r a t e . F i g u r e 7 shows t h e r e s u l t o f testwork on two adsorbent beds. Both bed c o n f i g u r a t i o n s demonstrate t h e e x i s t e n c e o f an optimum purge f l o w r a t e . A t low purge f l o w r a t e s t h e r e l a t i v e l y small amount o f a i r i s i n s u f f i c i e n t t o produce a uniform d i s t r i b u t i o n o f a i r . At high f l o w r a t e s , t h e r a t e a t which t h e purge gas passes through the bed may exceed t h e r a t e f o r which maximum water r e j e c t i o n occurs. E f f e c t o f Purge Volume. F i g u r e 8 shows t h e e f f e c t s o f both Yp and on t h e dew p o i n t d e p r e s s i o n a t 80°F and 120 p s i g . The experimental data c o r r e l a t e s q u i t e w e l l t o t h e purge volume used when expressed as a f u n c t i o n o f t h e amount o f a i r d r i e d per c y c l e . I t i s expected t h a t most commercial d e s i c c a n t d r y e r s would be o p e r a t i n g somewhere between t h e two l i n e s . Thus i t i s apparent, t h a t product dryness i s achieved o n l y a t t h e expense o f product y e i l d . However, t h e expense o f approximately 15 t o 20% o f feed a i r t o use as purge t o achieve a 40 t o 65°F dew p o i n t d e p r e s s i o n i s a s i g n i f i c a n t gain i n p r o t e c t i o n f o r t h e compressed a i r system. E f f e c t o f Temperature. The t h i r d and most s i g n i f i c a n t o p e r a t i n g v a r i a b l e i s temperature. Operation a t higher ambient temperatures r e s u l t s i n o n l y a small change i n performance. Because t h e performance parameter, "Dew P o i n t Depression", i s based on t h e r e l a t i o n s h i p below; Dew P o i n t Depression = Ambient Temperature - Dew P o i n t of Effluent A i r . The i n l e t a i r temperature does not e n t e r i n t o t h i s e x p r e s s i o n and w i l l o n l y a f f e c t t h e f i n a l dew p o i n t o f t h e a i r . In o r d e r t o have the most e f f i c i e n t water r e j e c t i o n system, t h e hot a i r from t h e compressor should be allowed t o cool as much as p o s s i b l e t o a l l o w the d e s i c c a n t d r y e r t o suppress t h e o u t l e t a i r dew p o i n t f a r below ambient temperature. In F i g u r e 9, t h e ambient temperature was assumed t o be f i x e d a t 80°F and t h e i n l e t a i r i s s a t u r a t e d a t t h e e l e v a t e d temperatures. T h i s i s t h e worst case s i n c e t h e hot a i r may not c o n t a i n enough water t o become s a t u r a t e d (see F i g u r e 1 ) . The performance based on isothermal o p e r a t i o n a t t h e e l e v a t e d i n l e t temperature i s d e p i c t e d i n t h e f i g u r e as t h e s o l i d l i n e s . T h i s shows a very dramatic e f f e c t on performance. However, t h i s i s n o t t o say t h a t l i t t l e d r y i n g i s done. For a s a t u r a t e d i n l e t a i r temperature o f 160°F f o r which a 10°F Dew P o i n t Depression i s



692 MOLECULAR SIEVES—II



56.

AUSIKAITIS

Pneumatic

System Air

Drying

693

a t t a i n e d , the dew p o i n t o f the product a i r has dropped 10°F from the ambient temperature o f 80°F, which i s 90°F from 160°F. I f the p r e s s u r e swing d e s i c c a n t d r y e r was assumed t o be a poor t o f a i r heat exchanger, a c h i e v i n g o n l y a 40% approach t o ambient temperature, the performance improves markedly as r e p r e s e n t e d by the dashed l i n e on F i g u r e 9. I f the d r y e r were an average heat exchanger, which i s a good assumption, then an 80% approach to ambient temperature i s not e x t r a v a g e n t . For t h i s case the i n l e t a i r temperature has o n l y a moderate e f f e c t on the p e r f o r mance. E f f e c t o f D e s i c c a n t S e l e c t i o n . The f i n a l area o f i n t e r e s t i s i n the s e l e c t i o n o f the d e s i c c a n t i t s e l f . An o p e r a t i n g parameter which may be o p t i m i z e d , even i n an e x i s t i n g u n i t i s the d e s i c c a n t i t s e l f . In o r d e r t o demonstrate t h i s concept t h r e e d e s i c c a n t s were e v a l u a t e d a t s i m i l a r o p e r a t i n g c o n d i t i o n s . The t e s t s were c a r r i e d out a t an ambient temperature o f 80°F and two i n l e t s a t u r a t e d a i r temperatures were e v a l u a t e d f o r each d e s i c c a n t . In a d d i t i o n , each d e s i c c a n t was r e t e s t e d a f t e r i t was t h o r o u g h l y soaked w i t h compressor o i l . In o r d e r t o approach the worst thermal c o n d i t i o n s , the d e s i c c a n t d r y e r housing was i n s u l a t e d from the s u r r o u n d i n g s t o prevent temperature l o s s e s d u r i n g the d r y i n g c y c l e . Thus the z e r o percent approach t o ambient temperature case o f F i g u r e 9 i s being approximated. The d e s i c c a n t s t e s t e d were S i l i c a G e l , A c t i v a t e d Alumina and M o l e c u l a r S i e v e i n the form o f 4X8 beads. The m o l e c u l a r s i e v e used was e s p e c i a l l y s e l e c t e d f o r t h i s a p p l i c a t i o n because of high o v e r a l l water c a p a c i t y , h i g h water mass t r a n s f e r r a t e s , good mechanical s t r e n g t h , and s e l e c t i v i t y f o r water i n the presence o f o i l . F i g u r e 10 shows the r e s u l t a n t performance o f Linde MOLSIV versus S i l i c a Gel and A c t i v a t e d Alumina. S i n c e the u n i t was o n l y i n s u l a t e d and not heated, condensation r e s u l t s and t h e r e b y d i s t o r t s some o f the greater-than-ambient-temperature dew p o i n t data. The performance o f the d e s i c c a n t s i n the "as r e c e i v e d " s t a t e are compared by d e t e r m i n i n g the e f f i c i e n c y which b e s t f i t s the d a t a . T h i s i n d i c a t e s t h a t by s e l e c t i n g the o p t i m a l d e s i c c a n t an o v e r a l l g a i n i n o p e r a t i n g e f f i c i e n c y i s r e a l i z e d at f i x e d mechanical o p e r a t i n g and d e s i g n c o n d i t i o n s . The performance l o s s f o r the " o i l soaked" c a s e , i s l a r g e l y a t t r i b u t a b l e t o an i n c r e a s e i n mass t r a n s f e r r e s i s t a n c e due t o t h e l a y e r of o i l which impedes the r a t e a t which water vapor i s t r a n s f e r r e d from the gas phase t o the d e s i c c a n t s u r f a c e . For the S i l i c a Gel and Alumina t h e r e i s a l s o expected t o be a l o s s i n e q u i l i b r i u m c a p a c i t y s i n c e the o i l i s not excluded from the micropores due to " m o l e c u l a r - s i e v i n g " a c t i o n . Summary The amount o f d r y i n g a c h i e v e d by a p r e s s u r e swing d e s i c c a n t


694



EXPERIMENTAL CONDITIONS: Ambient Temperature=80 F Yi=Sat'dat T j . T O AP=0, L/D=2.6 P =120 psig Qj=7.2 SCF/Cycle V =250 cu. in. (Qp=1.26 SCF) Desiccant Size=4x8 Beads o

CONDENSATION

p

•

fe^ Computer Simulation Using the Given Efficiency and Assuming Isothermal Operation at the Inlet Air Temperature _ Symbol Desiccant

Figure 10. Effects of elevated temperature and oil on the performance of selected desiccants at a fixed experimental operating cycle

Condition

O • A

Molecular Sieve Fresh Silica Gel Fresh Act. Alumina Fresh

• •

Molecular Sieve Oil Soaked Silica Gel Oil Soaked Act. Alumina Oil Soaked

A

100

110

INLET AIR TEMPERATURE, °F



56.

AUSIKAITIS

Pneumatic

System Air

Drying

695

d r y e r compared t o o t h e r d e v i c e s , when c o n s i d e r i n g the percentage of i n l e t water r e j e c t e d , c l e a r l y shows t h a t the d e s i c c a n t d r y e r has a s i g n i f i c a n t advantage over a f t e r c o o l e r s , heat exchangers, and s e p a r a t o r s . In o r d e r t o p r e d i c t the performance o f p r e s s u r e swing d e s i c c a n t d r y e r s over a v a r i e t y o f o p e r a t i n g c o n d i t i o n s , and duty c y c l e s , a s i m p l i f i e d mathematical model was developed. In o r d e r t o account f o r t h e i n t r i n s i c mechanical c h a r a c t e r i s t i c s o f most commercial d e s i c c a n t d r y e r s , an e f f i c i ency parameter was d e f i n e d t o account f o r these v a r i a t i o n s from i d e a l i t y i n the model. As c o n f i r m a t i o n o f t h i s mathematical model, experiments were performed and mechanical v a r i a b l e s were o p t i m i z e d w i t h r e s p e c t t o t h e p r e d i c t e d performance. Although i t may not always be p r a c t i c a l t o d e s i g n a p r e s s u r e swing d e s i c c a n t d r y e r t o operate a t a l l o f these optimum v a l u e s , i t was a l s o demonstrated t h a t the s e l e c t i o n o f t h e d e s i c c a n t i s the s i n g l e most important d e s i g n c o n s i d e r a t i o n . M o l e c u l a r s i e v e was determined t o have s i g n i f i c a n t advantages over s i l i c a gel and a c t i v a t e d alumina over a broad range o f c o n d i t i o n s .


Pneumatic System Air Drying by Pressure Swing Adsorption - ACS

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