RECOVERY

FROM SMALL IN DUSTRIAL

~ ~ L ~ ~ l ~ ~ PREVENT1 NORTH CAROLINA DEPARTMENT OF ENVIRONMENT, HEALTH, AND NATURAL RESOURCES

James G. Martin Governor

William W. Cobey, Jr Secretary, EHNR

Recovery of Volatile Organic Compounds from Small

Industrial Sources

Prepared by James J. Spivey

Research Triangle Institute

Prepared for North Carolina Pollution Prevention Pays Program

Department of Natural Resmrces and Community Development

DISCLAIMER

This p r o j e c t w a s supported by t h e North Caro l ina P o l l u t i o n Prevent ion Pays Program. views and p o l i c i e s of t h e S ta te of North Carol ina, nor does mention of t r a d e names o r commercial products c o n s t i t u t e endorsement o r recommendation f o r use.

This does n o t s i g n i f y t h a t t h e con ten t s n e c e s s a r i l y r e f l e c t t h e

Copyright September 1986

North Carol ina P o l l u t i o n Prevent ion Pays Program Department of Natura l Resources and Community Development

Pos t Of f i ce Box 27687 Raleigh, Plorth Carol ina 27611-7687

(919) 733-7015

Prepared by

James J . Spivey Research Tr i ang le I n s t i t u t e

Pos t O f f i c e Box 12194 Research Tr i ang le Park, North Caro l ina 27709

THE POLLUTION PREVENTlON PROGRAM

The Pollution Prevention Program provides free technical assistance to North Carolina industries and municipalities on ways to reduce, recycle and prevent wastes before they become pollutants. This non-regulatory program, located in the Division of Environmental Management) addresses water and air quality, toxic materials, and solid and hazardous waste. Designated as the lead agency in waste reduction, the Program works in cooperation with the Solid and Hazardous Waste Management Branch and the Governor ' s Waste Management Board. The services and assistance available fall into the following categories:

Information Clearinghouse. An information data base provides access to literature sources, contacts, and case studies on waste reduction techniques for specific industries or waste streams. Information is also available through customized computer literature searches. Waste reduction reports published by the Program are also available.

Specific Information Packages. The staff can prepare facility or waste-stream-specific waste reduction reports for industries and communities. Information provided by the facility is used to identify cost-effective waste reduction options. A short report detailing these options is provided along with references, case studies, and contacts.

On-site Technical Assistance. The staff can provide comprehensive technical assistance through facility visits. During an on-site visit, detailed process and waste stream information is collected. The information is analyzed, and a series of waste reduction options are identified. A report is prepared detailing these options and includes literature, contacts, case studies, and vendor information.

Outreach. The staff can give presentations on pollution prevention to industries, trade associations, professional organizations, and citizen groups. Depending on the audience, these programs range from an overview of the State's Pollution Prevention Program to in-depth discussions of technologies for specific industries.

Challenge Grants. A matching grant program provides funds for the cost of personnel, materials, or consultants needed to undertake pollution prevention projects. Projects eligible for grant funds range from characterizing waste streams in order to identify pollution reduction techniques to conducting in-plant and pilot-scale studies of reduction technologies.

For information or technical assistance contact:

Pollution Prevention Program Division of Environmental Management N.C. Department of Natural Resources & Community Development Post Office Box 27687 Raleigh, North Carolina 27611-7687

Telephone: 919/733-7015

Table of Contents

Sect i o n

Introduction ............................................. 1.0 Process Description ......................................

1.1 Adsorption .......................................... 1.2 Absorption .......................................... 1.3 Condensation ........................................

2.0 Technical Considerations ................................. 2.1 Adsorption ..........................................

2.1 .1 Sorbents ..................................... 2.1.2

2.2 Absorption .......................................... 2.2 .1 Absorbent .................................... 2.2.2 Process Configuration ........................

2.3 Condensation ........................................

Typical Design and Performance Parameters ....

3.0 Economic Considerations .................................. 3.1 Adsorption .......................................... 3.3 Condensation ........................................ 3.2 Absorption ..........................................

4.0 Vendors .................................................. 4.1 Adsorption .......................................... 4.2 Absorption .......................................... 4.3 Condensation ........................................

5.0 Example Cases ............................................ 5.2 Absorption .......................................... 5.3 Condensation ........................................ 5 .1 Adsorption ..........................................

6.0 References ............................................... Appendix A . Vendor Worksheets for Carbon ............... Adsorption Systems .......................................

1

9 9 9

14 19 20 20 22

25 25 25 26

30 30 30 32

33 33 38 40

43

43

L i s t of Figures

Ne,

1

2

3

4

&2.&

1

2

3

4

General process f low diagram of an adsorp t ion process f o r VOC recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

L%s

3

General process f low diagram of a condensation process f o r VOC recovery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

General process f low diagram of an adsorp t ion process f o r VOC recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

General ranges of VOC concent ra t ion and gas 10 f low r a t e f o r s e l e c t e d recovery techniques. ..............

Adsorption isotherms f o r two VOCs ....................... 12

Heat Exchanger Cos ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Cost of I n d u s t r i a l Re f r ige ra t ion ......................... 27

L i s t of Tables

P r a c t i c a l Guidel ines f o r t h e Use of Act ivated Carbon f o r VOC Recovery .................................. 15

Complicating Fac tors i n Carbon Adsorption Design ......... 1 7

Advantages and Disadvantages of Various S t r ipp ing Agents ......................................... 23

Summary of Costs f o r Condensation and Fixed-Bed Carbon Adsorption ........................................ 29

5 L i s t of Vendors of Activated Carbon Systems €or VOC Recovery. ........................................ 31

6 Information t h e Buyer Must Supply ........................ 34

INTRODUCTION

This monograph descr ibes t h e t h r e e gener ic technologies--adsorpt ion,

absorp t ion , and condensat ion--avai lable f o r t h e recovery of v o l a t i l e

organic compounds ( V O C s ) from gas streams generated by small

sources . Adsorption is t h e contac t of t h e VOC-containing gas

s o l i d sorbent followed by regenera t ion of t h e spent so rben t ,

c y c l i c process . Absorption is t h e con tac t of t h e gas stream

i n d u s t r i a l

stream wi th a

u s u a l l y i n a

wi th a

nonvo la t i l e l i q u i d i n which t h e VOC is so lub le followed by regenera t ion of

t h e l i q u i d , u sua l ly cont inuously. Condensation is simply t h e cont inuous

cool ing of t h e gas stream t o condense t h e VOC d i r e c t l y .

Because it is t h e most economical way t o rermve VOCs from a wide range

of streams encountered i n t h e f i e l d , adsorp t ion is by f a r t h e most

widespread technology for VOC recovery. By choosing an appropr i a t e sorbent

and process conf igu ra t ion , many chemical types of VOCs can be recovered.

Absorption i s used where t h e VOC concent ra t ion i s r e l a t i v e l y h igh and when

an appropr i a t e inexpensive absorbent l i q u i d is a v a i l a b l e . Condensation can

be energy i n t e n s i v e and is gene ra l ly used f o r gas streams wi th h igh VOC

conten t .

T rea t ing a VOC-containing gas stream t o recover t h e V O C ( s ) i s only one

Other a l t e r n a t i v e s t h a t may be of s e v e r a l broad a l t e r n a t i v e s t o consider .

more economical inc lude process modi f ica t ions t o reduce or e l imina te t h e

VOC conten t of t h e gas s t ream and us ing an u l t i m a t e d e s t r u c t i o n technique.

A t e c h n i c a l and economic eva lua t ion of a v a i l a b l e a l t e r n a t i v e s ( recovery ,

process modi f ica t ion , or u l t ima te d e s t r u c t i o n ) is a p r e r e q u i s i t e t o making

t h e r i g h t choice f o r VOC con t ro l . I t is t h e purpose of t h i s monograph t o

d i scuss only recovery and t o provide genera l gu ide l ines f o r eva lua t ion of

t h e a v a i l a b l e recovery techniques f o r s p e c i f i c cases.

not consider VOC c o n t r o l techniques t h a t a r e p r imar i ly means of u l t i m a t e

d e s t r u c t i o n of t h e VOC, e .g. , i n c i n e r a t i o n or c a t a l y t i c ox ida t ion .

This monograph does

1

1.0 PROCESS DESCRIPTION

1.1 Bdsorption

Adsorption is t h e process of adhesion of a very t h i n l a y e r of molecules

from a gas or l i q u i d t o t h e s u r f a c e of a s o l i d a s a r e s u l t of phys i ca l o r

chemical f o r c e s between t h e s o l i d and t h e adhering molecules. For t h e

recovery of V O C s , we a r e i n t e r e s t e d exc lus ive ly i n gas-phase molecules

adsorbing on a s o l i d m a t e r i a l as a r e s u l t of phys i ca l f o r c e s a lone , which

pe rmi t s t h e adsorbed molecule t o be recovered chemical ly unchanged.

Adsorption processes are extremely f l e x i b l e f o r VOC recovery. By

choosing an appropr i a t e adsorbent ( t h e s o l i d i n t h e above d e f i n i t i o n ) and

process ing cond i t ions , a wide range of VOCs can be recovered c o s t

e f f e c t i v e l y . For t h i s reason, numerous commercially a v a i l a b l e processes

have been developed f o r t h i s purpose. For r e l a t i v e l y small i n d u s t r i a l VOC

sou rces , systems are a v a i l a b l e t h a t r e q u i r e a minimum of maintenance and

ope ra to r a t t e n t i o n . These systems a r e usua l ly p r a c t i c a l f o r small f low

r a t e s of VOC-containing gas (from s e v e r a l hundred t o t e n s of thousands of

f t3 /min ) and low VOC concen t r a t ions (usua l ly s e v e r a l hundred up t o s e v e r a l

thousand ppm) . Figure 1 is a gene ra l schematic diagram of how adso rp t ion is used f o r

VOC recovery. Although t h e s p e c i f i c d e t a i l s of con tac t ing between t h e gas

s t ream and t h e adsorbent may d i f f e r among vendors of va r ious systems, i n

p r i n c i p l e a l l c y c l i c adso rp t ion systems f o r VOC recovery can be represented

by t h e process shown." The VOC-containing gas stream is f i r s t condi t ioned

by a p a r t i c u l a t e f i l t e r and perhaps a cooler ldehumidi f ie r .

contac ted wi th a bed of t h e sorbent m a t e r i a l , u s u a l l y a f ixed bed of

granular a c t i v a t e d carbon. The VOC is p r e f e r e n t i a l l y adsorbed from t h e gas

It is t hen

*While a l l subsequent d i scuss ion of VOC recovery by adso rp t ion is based on t h e type of sys tem shown i n Figure 1, two o t h e r types of adso rp t ion VOC recovery systems a r e a v a i l a b l e . One involves adso rp t ion under p r e s s u r e wi th r egene ra t ion accomplished by p res su re reduct ion . Such "p res su re swing" systems a r e not g e n e r a l l y used f o r VOC recovery, bu t i n p r i n c i p l e could be ( see r e fe rence 1). The second involves cont inuous con tac t of t h e gas and sorbent s t r eam i n a f l u i d i z e d bed conf igu ra t ion . Such systems a r e not p r a c t i c a l f o r t h e r e l a t i v e l y low flow rates considered h e r e i n , b u t should be eva lua ted for l a r g e r flows ( s e e r e fe rence 4 , p. 7 0 1 ) .

2

.

b Y-

C m Y- O

m E

3

s t ream and t h e c l e a n gas leaves t h e sorbent bed. When t h e bed i s

exhausted, i . e . , when t h e VOC concen t r a t ion i n t h e o u t l e t gas s t ream

exceeds a maximum acceptab le l e v e l ( t h i s is c a l l e d "breakthrough") , t h e bed

is taken o f f - l i n e f o r regenera t ion and t h e VOC-containing gas s t ream is

d i v e r t e d t o a f r e s h ( regenera ted) sorbent bed. For continuous VOC removal,

t h i s impl ies t h e need f o r a t least t w o , and perhaps more, sorbent beds

ope ra t ing i n p a r a l l e l . Regeneration of t h e spent sorbent can be done on-

s i t e ( a s shown) or o f f - s i t e by t h e sorbent vendor or by a f o r - f e e

r egene ra t ion s e r v i c e .

p re s su re (-15 p s i g ) steam. Other regenera t ing gases a r e sometimes used i n

s p e c i a l cases ( s e e Sec t ion 2.1). I f steam is used, hea t r e l eased by

condensat ion of t h e steam causes t h e VOC t o desorb from t h e sorbent and t h e

The most common method of r egene ra t ion is by low-

r e s u l t i n g vapor mixture is condensed downstream of t h e sorbent bed. The

condensed l i q u i d is allowed t o s e p a r a t e i n t o two phases. The recovered VOC

is then decanted and i s a v a i l a b l e f o r re-use or s a l e . ( I n t h i s ca se , t h e

block i n F igure 1 l abe led "VOC recovery" would c o n s i s t of a condenser and

d e c a n t e r . ) The aqueous phase can then be s e n t d i r e c t l y t o wastewater

t r ea tmen t , s u b j e c t t o r e s t r i c t i o n s d iscussed i n Sec t ion 2.1.

1 .2 Absorption

Absorption i s t h e t r a n s f e r of one component of a gas mixture t o a

l i q u i d due t o p r e f e r e n t i a l s o l u b i l i t y of t h e gas i n t h e l i qu id . " For VOC

recovery, t h i s process involves t h e t r a n s f e r of t h e VOC from a gas s t r eam

t o a s u i t a b l e l i q u i d i n which t h e VOC i s so lub le .

I n p r i n c i p l e , almost a l l VOCs can be recovered from any gas mixture by

s e l e c t i o n of an appropr i a t e l i q u i d absorbent and ope ra t ing cond i t ions .

Although abso rp t ion is probably t h e most important gas-phase p o l l u t i o n

c o n t r o l ope ra t ion7 (for removal of a c i d s t a c k gases , f o r example), i t s use

f o r VOC recovery from small i n d u s t r i a l sources is not widely p rac t i ced .8

I ts r e l a t i v e l y complex ope ra t ion and t h e consequent c o s t account for i t s

l i m i t e d use .

*Absorption i s sometimes accompanied by chemical r e a c t i o n or complexation of t h e absorba te ( t h e s o l u b l e gas i n t h i s d e f i n i t i o n ) wi th , or i n , t h e absorbent . This process is not a p p l i c a b l e t o VOC recovery, however.

6

Figure 2 i s a general process f low diagram of an abso rp t ion process f o r

VOC recovery. The VOClcontaining gas stream i s f i r s t contacted

cont inuously and coun te rcu r ren t ly wi th an absorbent l i q u i d i n a column

containing e i t h e r packing o r t r a y s . The VOC absorbs i n t h e l i q u i d , which

then e x i t s t h e bottom of t h e tower, w i th t h e c l e a n gas l eav ing t h e t o p of

t h e column. The absorbent/VOC mixture is t hen contacted wi th a s t r i p p i n g

agent of some s o r t . This can be steam, a i r , o r another l i q u i d . The * purpose of t h i s s t r i p p i n g agent is t o remove t h e VOC from t h e absorbent so

t h a t t h e absorbent can be r e c i r c u l a t e d t o t h e abso rp t ion column. This

s t r i p p i n g is t h e exact oppos i t e of t h e abso rp t ion process and involves t h e

t r a n s f e r of t h e VOC from t h e absorbent t o t h e s t r i p p i n g agent .

used, h e a t t r a n s f e r occurs simultaneously and causes t h e s o l u b i l i t y of t h e

VOC i n t h e absorbent t o decrease, r e s u l t i n g i n vapor i za t ion of t h e VOC.

The s t e a m l V O C mixture is then condensed and decanted i n t h e VOC recovery

I f steam is

s e c t i o n i n Figure 2 , much as i n t h e case of carbon adso rp t ion (Sec t ion

2 .1) .

m u s t be sepa ra t ed t o recover t h e VOC. One d i r e c t way of doing t h i s is by

condensation, as discussed i n Sec t ions 1.3 and 2.3. If another l i q u i d i s

used, t h e VOC is e x t r a c t e d from t h e absorbent and m u s t be subsequent ly

recovered by d i s t i l l a t i o n o r some o t h e r ope ra t ion . Also no te t h a t t h e

s t r i p p i n g column of Figure 2 could be operated as a convent ional

d i s t i l l a t i o n column with t h e a d d i t i o n of a r e b o i l e r a t t h e bottom of t h e

column and t h a t t h e VOC could be recovered by t h i s convent ional technique.

I f a i r is used, t h e a i r / V O C mixture e x i t i n g t h e t o p of t h e column

1.3

Condensation i s t h e process of cool ing a gas t o a l i q u i d form by

t r a n s f e r of hea t . For t h e recovery of VOCs by condensation, t h i s process

involves cool ing t h e VOC-containing gas stream t o a temperature below t h e

dew p o i n t of t h e VOC and c o l l e c t i n g t h e condensed VOC d i r e c t l y as a l i q u i d .

Condensation has been used f o r VOC recovery by small i n d u s t r i a l u s e r s

because of i t s simple ope ra t ion , u s u a l l y f o r low gas f low rates.

equipment gene ra l ly involved i s a h e a t exchanger, which can be placed

d i r e c t l y a t t h e po in t of discharge and r equ i r e s l i t t l e , i f any, o p e r a t o r

a t t e n t i o n . Since cool ing water i s o f t e n t h e most r e a d i l y a v a i l a b l e coo lan t

f o r condensation, and s i n c e i t s i n l e t temperature i s normally a t o r above

The only

5

i

6

ambient temperature , VOC recovery by condensation using cool ing water is

l imi t ed t o h igh VOC concent ra t ions and h igher b o i l i n g VOCs. To recover

low-boi l ing, low-molecular-weight VOCs r equ i r e s r e f r i g e r a t e d cool ing , which

can be expensive.

Figure 3 i s a gene ra l process f low diagram of VOC recovery by

condensation. The VOC-containing gas is f i r s t f i l t e r e d t o remove any

p a r t i c u l a t e s t h a t could f o u l t h e hea t exchange s u r f a c e over time. Heat i s

removed from t h e gas by i n d i r e c t con tac t wi th t h e cooled s u r f a c e of t h e

condenser, and t h e VOC is recovered d i r e c t l y , e i t h e r by simply c o l l e c t i n g

drops a s they a r e formed, or by convent ional demis te rs or cyclones which

he lp coa lesce the VOC d r o p l e t s as they a r e formed.

7

0 0

t v) U w

S

d

I 0 0 > b rc

S m rc 0 E E a m m

8

2.0 TECHNICAL CONSIDERATIONS

Figure 4 shows approximate ranges of VOC concen t r a t ions and f low r a t e s

t o which t h e t h r e e technologies considered i n t h i s "graph can be

app l i ed .

and economic, and a r e discussed i n Sec t ions 2.0 and 3.0.

Limits of a p p l i c a t i o n of t h e s e technologies are both t e c h n i c a l

2 .1 Adsorption

To eva lua te s p e c i f i c adsorpt ion processes f o r a given VOC recovery

need, t h e sorbent i t s e l f and t h e o v e r a l l process design parameters must be

considered.

2.1.1 Sorbents

I n p r i n c i p l e , many so rben t s could be used f o r VOC recovery. These

include a c t i v a t e d carbon, molecular sieves, a c t i v a t e d alumina, and

ed i n commercial VOC recovery systems

ce for a c t i v a t e d carbon de r ives from i t s

, high s u r f a c e area, high s o r p t i v e

c o s t . Although o t h e r s o r b e n t s

o p e r t i e s may e x i s t , t hey have no t y e t

stems for VOC recovery.

a so rben t such as a c t i v a t e d carbon

u a t i o n t h a t expresses t h e weight of

adsorbed p e r u n i t mass of so rben t at

i l i b r i m n ( W i , i n u n i t s of l b of a d s o r b a t e l l b of s o r b e n t ) as a f u n c t i o n

of t h e p a r t i a l p re s su re of t h e VOC i n t h e gas stream ( P i , i n u n i t s of m

Hg) i n a form known as t h e Freundlich isotherm:

where n i s g r e a t e r than 1. Both n and a are empi r i ca l ly determined

c o n s t a n t s , both of which gene ra l ly decrease with inc reas ing temperature.

This suggests t h a t a given sorbent w i l l absord less of a given adso rba te

( a t equ i l ib r ium) a t a higher temperature. This p r i n c i p l e i s used t o

regenerate a spent sorbent by hea t ing i t , with steam f o r example. The

values of n and a a r e a l s o unique t o a given sorbent-adsorbate system.

Because a unique r e l a t i o n s h i p e x i s t s between a given adsorbate ( e . g . , a

9

100,000 ’

S .- E *-

oi

%

r

+ m cT 10,000 - g u. % 0

I

I I I 100 1,000 10,000 100,000 1,000

10 VOC Concentration, ppm

Figure 4. General ranges of VOC concentration and gas flow rate for selected recovery techniques.

VOC) and a sorbent ( e .g . , a c t i v a t e d carbon) a t a given temperature ,

equat ion (1) is c a l l e d an isotherm. Often, vendors w i l l perform t h e

necessary t e s t s on a given VOC gas s t ream t o determine t h e gene ra l shape of

t h e isotherm i f they do not a l ready have experience wi th t h e p a r t i c u l a r gas

composition. Kovach5 g ives a procedure f o r c a l c u l a t i n g t h e i so therm a t any

temperature knowing i ts shape a t one temperature is known.

Equation (1) i s u s e f u l f o r any sorbent-adsorbate system and has found

widespread use i n p r a c t i c e . Figure 5 shows isotherms f o r two d i f f e r e n t

VOCs on a c o m e r c i a l a c t i v a t e d carbon sorbent . Note t h a t equat ion (1)

sugges ts t h a t t h e s e log-log isotherms should be l i n e a r over a l l VOC p a r t i a l

p re s su res . Although t h e a c t u a l isotherms a r e not t r u l y l i n e a r , equa t ion

(1) i s an adequate approximation, e s p e c i a l l y s i n c e VOC recovery systems

would normally be designed f o r a f a i r l y low range of VOC p a r t i a l p re s su res ,

u sua l ly about 300 t o 5,000 ppmv (0.23 t o 3 . 8 mm Hg a t 1 atm t o t a l

p r e s s u r e ) .

Equation (1) and Figure 5 represent t h e q u a n t i t y of VOC t h a t can be

adsorbed a t equi l ibr ium. This q u a n t i t y is o f t e n c a l l e d t h e " s a t u r a t i o n

capac i ty" by system vendors and is a func t ion of temperature , t h e chemical

and phys ica l na tu re of t h e VOC of i n t e r e s t , and t h e VOC concen t r a t ion of

t h e gas i n con tac t wi th a given carbon. This va lue f o r most a c t i v a t e d

carbons and VOC adsorba tes is about 20-40%; which means t h a t t h e carbon can

adsorb about 20-40% of i t s own weight a t equ i l ib r ium ( a t a VOC

concen t r a t ion of 300 t o 5,000 ppmv and a t y p i c a l working temperature of 70

t o 90'F).

50% of t h e " s a t u r a t i o n capac i ty" , o r roughly 8 t o 15% of i ts own weight,

because not a l l t h e VOC i s e a s i l y desorbed a t p r a c t i c a l ope ra t ing

cond i t ions . This 8-152 adsorba te loading is c a l l e d t h e "working capac i ty"

and i s t h e b a s i s f o r a l l r e a l system design.

I n p r a c t i c e , t h e carbon is designed t o adsorb only about 25 t o

The use r should a l s o be aware t h a t because a c t i v a t e d carbons a r e made

from a wide v a r i e t y of carbonaceous m a t e r i a l s , inc luding c o a l , coconut

s h e l l s , wood, p e a t , and petroleum coke, phys i ca l and chemical p r o p e r t i e s

can vary s u b s t a n t i a l l y from vendor t o vendor as w e l l as from ba tch t o ba tch

even from t h e same vendor. This v a r i a t i o n does not r e f l e c t improper

process ing by the manufacturer but is inherent i n t h e precursor m a t e r i a l s

and should be kept i n mind by t h e use r .

11

d e 5: D

Y

Y

io0 80 60 40

20

io 8 6 4

2

I 01 i o io I00 1000

Pressure, m m ng

(a) Methyl eth? 1 ketone adsorption iwthenns on Union Cahide 45 cart)on

Figure 5. Adsorption isotherms for two VOCS (from Kovach,J.L.,in Handbook of Separation Techniques for Chemical Engineers, P.A. Schweitzer ed., McGraw Hill, 1979, p. 3-14).

12

In practice, it has been shown that in spite of the wide variety of activated carbons available and the wide range of VOCs to which this sorbent has been applied, there are limits to the types of VOCs that can be recovered. A review of technical and vendor information has shown that activated carbons are suitable for the following range of VOCs:

(1) Those with molecular weights between roughly 50 and 200, corresponding to boiling points between about 68'F and 350'F

All aliphatic and aromatic hydrocarbons, subject to (1) above, i.e., carbon number between roughly C4 and C14

Most c o m n halogenated solvents (subject to (1) above) including CCl4, ethylene dichloride, methylene chloride, perchlorethylene, and trichloroethylene

Most common ketones (acetone, methyl ethyl ketone) and some esters (butyl and ethyl acetate)

Common alcohols (ethanol, propanol, butanol).

However, there are several types of compounds that are not suitable to

activated carbon adsorption. These include compounds that react with the

carbon itself or with the steam normally used for regeneration, those that polymerize on the carbon, or those that are difficult to remove in any

practical regeneration step (e.g., high-molecular-weight compounds).

Examples include:

Reactive compounds . Organic acids (e.g., acetic acid) Aldehydes (e.g., formadehyde) . Some ketones (e.g., cyclohexanone) Some easily hydrolyzed esters

. Some halogenated hydrocarbons (which react with steam, e.g., methyl acetate)

(if they hydrolyze easily, e.g., ethyl chlo r ide )

High-molecular-weight compounds . Plasticizers . Resins . Hydrocarbons >C14 . Phenols, glycols . Amines. Caution should be taken before ruling out carbon adsorption even for

the above compounds, however. Process modifications can be made to permit

13

economical recovery of t hese VOCs by carbon adsorp t ion . As an example, t h e

carbon can be regenerated wi th a ho t i n e r t gas such as n i t rogen ( i n s t e a d of

steam) t o e l imina te t h e hydro lys is r e a c t i o n of some e s t e r s and halogens

wi th steam. Vendors (Sec t ion 4 . 1 ) should be consul ted f o r guidance on

i n d i v i d u a l problems.

2.1.2 Typical System Design and Performance Parameters

Years of p r a c t i c e i n t h e use of a c t i v a t e d carbon adso rp t ion f o r VOC

recovery have l e d t o gene ra l gu ide l ines f o r system des ign and performance.

The purpose of t h i s s e c t i o n i s t o summarize t h e s e gu ide l ines , f i r s t i n

terms of o v e r a l l performance and second i n terms of each major process s t e p

shown i n Figure 1.

Overa l l Performance--Table 1 summarizes t h e major c h a r a c t e r i s t i c s of a

VOC-containing gas s t ream t o which a c t i v a t e d carbon adso rp t ion can be

p r a c t i c a l l y appl ied .

As a r u l e , a well-designed carbon adsorp t ion system f o r VOC recovery

is s u i t a b l e f o r handl ing gases wi th VOC concen t r a t ions of s e v e r a l hundred

t o about 5,000 ppm. The upper l i m i t on VOC conten t is due t o hea t t r a n s f e r

limits and s a f e t y . Because adso rp t ion is always an exothermic process

(wi th a h e a t of adso rp t ion roughly equal t o t h e hea t of condensat ion,

B tu / lb V O C ) , t h e higher t h e VOC conten t of t h e i n l e t gas s t r eam (e .g . , l b

VOCllb incoming g a s ) , t h e h igher t h e t o t a l hea t evolved when t h e VOC i s

adsorbed ( B t u / l b incoming gas ) . As t h e VOC conten t of t h e gas i n c r e a s e s ,

hea t i s evolved f a s t e r t han it can be removed i n a f ixed-sorbent bed and

t h e bed temperature may r i s e t o unsafe l e v e l s .

can a l s o be l i m i t e d by t h e explos ive limits of t h e gas ( u s u a l l y a i r ) / V O C

mixture. One EPA r epor t s t a t e s t h a t insurance companies l i m i t i n l e t gas

concen t r a t ion t o <25% of t h e lower explos ive l i m i t (LEL) , un le s s s p e c i a l

c o n t r o l s a r e added ( i n which case up t o 40-50% of the(LEL) can be

t o l e r a t e d ) . 2 The lower l i m i t on VOC conten t is u s u a l l y economic.

VOC l e v e l s , it i s not c o s t - e f f e c t i v e t o pay f o r t h e VOC recovery equipment

( s e e Sec t ion 3.1) because t h e bed wi l l gene ra l ly not become exhausted f o r

q u i t e some t ime, i n which case o f f - s i t e r egene ra t ion of t h e sorbent is more

economical. Act ivated carbon adsorp t ion w i l l s t i l l remove t h e VOCs even a t

i n l e t l e v e l s of s e v e r a l ppm; c o s t a lone determines whether VOC recovery a t

t h e s e l e v e l s i s reasonable . A l e v e l of s e v e r a l hundred ppm has been

repor ted t o be a p r a c t i c a l economic 1imit.374

The upper VOC concen t r a t ion

A t l o w

14

TABLE 1. PRACTICAL GUIDELINES FOR THE USE OF ACTIVATED CARBON FOR VOC RECOVERY

_ ~ _

ITEM PRACTICAL LIMIT(S) COMMENTS REFERENCE

VOC concen t r a t ion 300-5,000 ppm Limited by h e a t 2 ,4 of adso rp t ion (upper l i m i t ) and economics (lower l i m i t )

I n l e t gas < 100- 120'F I n l e t 2 temperature t o so rben t temperature bed can be lowered

b e f o r e t h e so rben t

bed, i f necessary Flow r a t e 200-100,000 May be h ighe r 2

f t 3 ( ~ ~ ~ ) / m i n i n s p e c i a l cases

Relative humidity <50X RH If h ighe r , a 2 condenser can be used i n the "gas condi t ion ing" process (see Sec t ion 2.1.1.1)

15

Temperatures a t t h e i n l e t t o t h e adsorp t ion bed i t s e l f a l s o have

p r a c t i c a l limits.

t o 120"F, t h e hea t of adsorp t ion i n t h e bed may r e s u l t i n i n s u f f i c i e n t

removal of t h e VOC ( t h i s h igher temperature corresponds t o an i so therm a t

which t h e p a r t i a l p re s su re of t h e VOC i n t h e gas above t h e carbon is

h ighe r , as shown i n Figure 5 f o r two VOCs).

r a t e s i s a v a i l a b l e i n commercial systems,2 from s e v e r a l hundred t o about

1,000,000 f t 3 (STP)/min. The upper l i m i t is 'one of p r a c t i c a l s i z e f o r a

s i n g l e given system, but t h e r e i s no reason t h a t mu l t ip l e systems could

not be used t o handle any conceivably l a r g e r f low r a t e . The lower l i m i t i s

P r a c t i c e has shown t h a t a t temperatures above about 100

A q u i t e wide range of f low

aga in economic because o n - s i t e VOC recovery would be i m p r a c t i c a l l y

expensive f o r very low f low r a t e s , a l though i n p r i n c i p l e such a system

would work.

The r e l a t i v e humidity of t h e i n l e t stream t o t h e sorbent bed i t s e l f

(downstream of any gas-condi t ioning process) must gene ra l ly be less than

50%.

of t h e carbon p a r t i c l e s and g r e a t l y reduce t h e carbon's working capac i ty .

Higher humidi t ies may r e s u l t i n blockage of t h e small i n t e r n a l pores

Severa l f a c t o r s can complicate t h e des ign of a carbon absorptJ.on

system.3 These f a c t o r s , t h e i r e f f e c t s , and p o s s i b l e s o l u t i o n s a r e shown i n

Table 2.

The major process components shown i n Figure 1 w i l l now be d iscussed

b r i e f l y i n t u r n , and p r a c t i c a l des ign and performance parameters given.

Gas ConcUkrhg-Most commercial VOC recovery systems make p rov i s ions

f o r gas condi t ion ing t o p r o t e c t t h e sorbent bed. Depending on t h e

composition of t h e raw i n l e t gas s t ream, t h e gas-condi t ioning process may

c o n s i s t of any o r a l l of t h e fol lowing ( t h e s e processes would l o g i c a l l y be

c a r r i e d out i n t h e o rde r shown below):

F i l t r a t i o n . The removal of p a r t i c l e s down t o 3 t o 5 microns is f a i r l y s t r a igh t fo rward . f i l t e r s .

F ine r p a r t i c l e s may r e q u i r e s p e c i a l

* Dehumidification. This i s done t o reduce t h e r e l a t i v e humidity t o <50% and can be done with des i ccan t s such a s alumina o r s i l i c a g e l .

* Cooling. This may be done by r e f r i g e r a t e d c o i l hea t exchangers (which can a l s o accomplish some measure of dehumidi f ica t ion) t o reduce t h e temperature of t h e gas t o 70 t o 90.F.

Demisting. This i s t h e removal of ae roso l s ( f i n e l i q u i d d r o p l e t s ) and is accomplished wi th convent ional demis te rs .

16

TABLE 2. COMPLICATING FACTORS I N CARBON ABSORPTION DESIGN

FACTORS EFFECTS POSSIBLE SOLUTIONS

Presence of more than one VOC i n t h e i n l e t gas s t ream

(1) High- molecular- Inc rease sorbent bed weight components s i z e d i s p l a c e lower mo 1 e cu 1 ar we i gh t components, poss ib ly

( 2 ) Recovered VOCs Use d i s t i l l a t i o n o r wi l l not be pure some o t h e r f r a c t i o n - components i f steam a t i o n process t o is used f o r regen- p u r i f y recovered e r a t i o n . Decanting VOCs; u se regen- may be d i f f i c u l t e r a t i n g gas o t h e r i f one VOC i s heavier t han steam than water and one is l i g h t e r

Presence of water- VOC w i l l not s e p a r a t e Use d i s t i l l a t i o n o r s o l u b l e organics by simple decant ing o r some o t h e r

i f steam is used. f r a c t i o n a t i o n process t o p u r i f y recovered VOCs; use r egene ra t ing gas o t h e r than s team

Presence of r e a c t i v e Carbon working Remove t h e r e a c t i v e gases i n t h e i n l e t capac i ty w i l l gases i n t h e gas gas steam degenerate condi t ion ing

i r r e v e r s i b l y and process sometimes r ap id ly

Presence of co r ros ive gases (1) Carbon working i n t h e i n l e t gas steam capac i ty w i l l

degenerate sometimes r a p i d l y and i r r e v e r s i b l y

( 2 ) Sorbent v e s s e l and p ip ing may corrode

Remove t h e r e a c t i v e gases i n t h e gas condi t ion ing

Use l i n e d sorbent v e s s e l s or cor ros ion - r e s i s t a n t alloys

Presence of p a r t i c u l a t e s Carbon p a r t i c l e s may F i l t e r t h e incoming o r ae roso l s i n t h e i n l e t become coated wi th s t ream. gas s t ream small p a r t i c l e s ,

i r r e v e r s i b l y des t roying t h e i r adso rp t ive p r o p e r t i e s

17

Sorbent Bed--The sorbent bed i t s e l f is gene ra l ly capable of

a c e o q l i s h i n g an o v e r a l l VOC removal caf at l e a s t 95% and u s u a l l y 99% o r

b e t t e r . 2 9 3 ~ ~

r o u t i n e l y and 10 t o 20-ppm l e v e l s can be reached f o r many compounds.

adso rp t ion capac i ty of t h e bed, being a d i r e c t func t ion of t h e equ i l ib r ium

capac i ty a s shown i n Figure 5, is g r e a t e r a t lower temperatures . Thus, t h e

lower t h e temperature of t h e i n l e t gas , t h e h igher t h e working capac i ty of

t h e bed. A l s o , a s a gene ra l r u l e , unsa tura ted compounds ( a lkenes , a lkynes ,

a romat ics ) a r e more s t r o n g l y adsorbed than s a t u r a t e d compounds of s i m i l a r

carbon number.

unsa tura ted compounds a t given condi t ions .

Ou t l e t VOC concent ra t ions of 50 t o 100 ppm can be achieved

The

Thus h igher removal e f f i c i e n c i e s w i l l be obta ined f o r t h e s e

The VOC concen t r a t ion i n t h e o u t l e t s t ream w i l l g r adua l ly i n c r e a s e

wi th t ime a f t e r an i n i t i a l break-in per iod of s e v e r a l days.

adequate removal e f f i c i e n c y r equ i r e s p e r i o d i c replacement of a t l e a s t a

p o r t i o n of t h e carbon bed.

bed l i f e of up t o 5 years . This is accounted f o r e x p l i c i t l y i n

Sec t ion 3.1, where t h e economics of carbon adso rp t ion VOC recovery a r e

d iscussed .

Maintaining

Proper system des ign should r e s u l t i n a carbon

The carbon bed depth wi th in t h e sorbent v e s s e l i s normally f ixed by

experience.

inches being t y p i c a l , t o achieve adequate mass t r a n s f e r of t h e VOC t o t h e

pores of t h e ind iv idua l carbon p a r t i c l e s a t t y p i c a l l i n e a r gas v e l o c i t i e s

through t h e bed of 30 t o 100 f t l m i n (wi th 50 f t l m i n being a good average

v a l u e 3 f 4 ) .

a normal granular a c t i v a t e d carbon, of about 0.5 inch H20 pe r inch bed

depth.

Usually, t h e bed depth must be between 1 and 3 f e e t , 4 wi th 18

This f low r a t e r e s u l t s i n a pres su re drop through t h e bed, f o r

Beds a r e u s u a l l y s i z e d f o r an on - l ine "cycle" t ime of s e v e r a l hours.

Trr i s i s based on t h e t i m e needed t o desorb t h e VOC and dry t h e sorbent bed

( i f steam i s used) i n a two-bed system.

Steam usage ( @ 15 p s i g ) f o r r egene ra t ion of mos t carbonlVOC

combinations v a r i e s between 0.25 and 0.35 l b s t eaml lb carbon.

YOC Recovery--After t h e VOC has been adsorbed by carbon, it must be

desorbed by a r egene ra t ing gas .

steam, though o f f - the - she l f systems us ing hot a i r , n i t rogen , o r o t h e r gases

a r e a v a i l a b l e f o r s p e c i a l ca ses (d iscussed above).

The most widely used r egene ra t ing gas is

18

I f steam is used, t h e VOC recovery process of Figure 1 c o n s i s t s of a

I f condenser and a decanter f o r g r a v i t y s e p a r a t i o n of t h e VOC and water .

t h e VOC i s water - inso luble , t h i s s epa ra t ion is s imple and t h e VOC i s

recovered f o r reuse while t h e water is simply discharged without f u r t h e r

t rea tment or i s re-used f o r b o i l e r makeup. I f t h e VOC is water s o l u b l e and

steam is used a s t h e regenerant gas , t hen t h e VOC must be sepa ra t ed from

t h e water , u sua l ly by d i s t i l l a t i o n o r e x t r a c t i o n . Commercial systems a r e

a v a i l a b l e t o do t h i s (See Sec t ion 4 . 1 ) .

removal of organics i n t h e water t o acceptab le l e v e l s for discharge . Note

t h a t t h e r e a r e s t r i c t limits t o t h e organic conten t of water acceptab le t o

many municipal sewage t rea tment systems; u sua l ly , "percent" l e v e l s of

organics a r e unacceptable . Very low l e v e l s must be achieved i f t h e o rgan ic

is t o x i c . One vendor has s t a t e d t h a t t h e presence of water -so luble VOCs i n

t h e gas s t ream i s probably t h e s i n g l e b igges t disadvantage t o t h e use of

carbon adsorp t ion f o r VOC r e ~ o v e r y . ~

Care must be taken t o a l low f o r

I f an i n e r t gas is used t o regenera te t h e bed, t hen t h e VOC may be

recovered by condensation of t h e VOC from t h e presumably concent ra ted

regenera t ing gas s t ream, o r may be inc ine ra t ed to recover i t s f u e l va lue

( t h i s is not c o s t e f f e c t i v e f o r halogenated so lven t s because they have low

hea t ing va lue , a r e co r ros ive t o i n c i n e r a t o r i n t e r n a l s , and probably r e q u i r e

c o s t l y f l u e gas t r ea tmen t ) . Both types of systems are commercially

a v a i l a b l e ( s e e Sec t ion 4 . 1 ) .

2.2 -tion

While absorp t ion i s not used f o r VOC recovery from small i n d u s t r i a l

sources (EPA r e p o r t s "...no known systems o f fe red by equipment

manufacturers"' f o r a t l e a s t some s o l v e n t s s ) , gas absorp t ion i s an extremely

u s e f u l and widespread u n i t ope ra t ion i n t h e chemical process i n d u s t r i e s .

k; a r u l e , t h i s process i s used f o r removal or recovery of nonVOC gases and

is appl ied within a chemical process as opposed t o end-of-pipe " p o l l u t i o n

c o n t r o l . "

a p p l i c a t i o n s i s t h e absorba te concent ra t ion , which is r e l a t i v e l y h igh i n

chemical processing and r e l a t i v e l y low f o r p o l l u t i o n c o n t r o l .

The p r i n c i p a l ope ra t ing d i f f e r e n c e between t h e s e two

For any absorp t ion process , s e v e r a l gene ra l cons ide ra t ions a r e

important i n t h e design and eva lua t ion . These inc lude t h e absorbent and

t h e conf igu ra t ion of t h e process .

19

2.2.1 Absorbent

For any VOC recovery process based on absorp t ion , t h e absorbent should

have t h e fol lowing p r o p e r t i e s :

Low v o l a t i l i t y t o minimize absorbent loss t o , and contaminat ion o f , t h e e x i t gas s t ream

* High s o l u b i l i t y of t h e VOC t o maximize t h e amount of VOC recovered pe r volume of absorbent

e Low c o s t

Low f lammabil i ty t o ensure s a f e ope ra t ion because a i r is u s u a l l y t h e gas from which t h e VOC is removed i n t h e abso rp t ion column

Low w a t e r s o l u b i l i t y t o minimize absorbent loss i f steam is used i n s t r i p p i n g .

2.2.2 Process Conf igura t ion

There are s e v e r a l t e x t s t h e reader may wish t o consu l t on t h e gene ra l

t o p i c of abso rp t ion , inc luding T r e ~ b a l , ~ Buonicore and Theordore,

Crawford, lo and Marchellol

some s p e c i f i c examples. Ind iv idua l po r t ions 0 7 t h e process a r e considered

b r i e f l y below.

which provide gene ra l des ign procedures and

BbsorIrfipn C o b - - I n t h i s column, t h e VOC is t r a n s f e r r e d from t h e gas

phase t o t h e absorbent .

between t h e s e phases over e i t h e r packing or t r a y s .

equat ions f o r t h e s e two types of columns d i f f e r somewhat, t h e r a t e of

t r a n s f e r of t h e VOC t o t h e absorbent can gene ra l ly be expressed i n t h e

fol lowing form:

This t r a n s f e r is completed by promoting con tac t

Although t h e des ign

(21 * Flux = ka (Pvoc- Pvoc)

where Flux = r a t e of mass t r a n s f e r of t h e VOC t o t h e absorbent , l b / h r

k = mass t r a n s f e r c o e f f i c i e n t , l b lh r - a tm- f t2

a = i n t e r f a c i a l a r ea , f t 2

Pvoc = p a r t i a l p re s su re of t h e VOC, atm.

= equ i l ib r ium p a r t i a l p re s su re of t h e VOC, a t m . * pvoc

20

Simple a s t h i s expression may be, t h e design and eva lua t ion of an

absorp t ion column can r ap id ly become complex because k is a func t ion of

temperature , gas and l i q u i d f low r a t e , packing or t r a y type , and v e r t i c a l and

r a d i a l p o s i t i o n i n t h e column; a is a func t ion of gas and l i q u i d f low r a t e ,

packing o r t r a y type; and * Pvoc and Pvoc are func t ions of temperature .

This express ion shows t h a t t h e h igher t h e equ i l ib r ium s o l u b i l i t y of t h e VOC * i n t h e absorbent ( i . e , , t h e g r e a t e r t h e d i f f e r e n c e between Pvoc and Pvoc) ,

t h e f a s t e r t h e r a t e of absorpt ion. Also, note t h a t t h e o u t l e t concen t r a t ion

of t h e VOC cannot be reduced below t h e p a r t i a l p re s su re of t h e VOC i n

equ i l ib r ium wi th t h e absorbent a t t h e top of t h e column. I n p r a c t i c e , t h e

VOC o u t l e t l e v e l can only approach t h i s l e v e l . The equ i l ib r ium s o l u b i l i t y i s

t hus extremely important i n s e l e c t i n g an absorbent . I t also t u r n s o u t , a s

one would expec t , t h a t var ious types of packing and t r a y s as we l l as s p e c i f i c

ope ra t ing condi t ions have a g r e a t i n f luence on k; gene ra l ly , t h e g r e a t e r t h e

degree of con tac t between t h e gas and t h e l i q u i d , t h e f a s t e r t h e VOC

absorp t ion . However, t h i s g r e a t e r degree of con tac t is u s u a l l y a t t h e

expense of p re s su re drop, which m u s t a l s o be accounted f o r .

Obtaining t h e needed equi l ibr ium and equipment-specif ic d a t a t o use i n

design equat ions , which a r e u l t i m a t e l y based on t h e express ion above, is not

simple. Often t h e da t a f o r a given VOC absorbent may not e x i s t ; t h i s i s

e s p e c i a l l y t r u e f o r multicomponent systems.

As a gene ra l r u l e , absorp t ion columns cannot t o l e r a t e wide ranges of gas

and l i q u i d f low r a t e s and s t i l l ope ra t e e f f i c i e n t l y . Thus, t h i s means of VOC

recovery i s not s u i t a b l e f o r ope ra t ions i n which t h e VOC-containing gas f low

r a t e v a r i e s .

because once b u i l t , a given absorber must ope ra t e near i t s des ign cond i t ions .

This l i m i t a t i o n a l s o implies t h a t proper des ign is c r i t i ca l

Although most absorp t ion systems a r e designed as columns (wi th packed

towers t h e most p r e v a l e n t ) , o the r conf igu ra t ions can a l s o be used. These

inc lude v e n t u r i s and s p r a y towers. Ventur is a r e f r equen t ly used t o

accomplish simultaneous p a r t i c u l a t e and chemical removal, bu t g e n e r a l l y

r e q u i r e high p res su re drops f o r e f f i c i e n t opera t ion . Spray towers have l o w

pres su re drops, but a r e i n e f f i c i e n t con tac t ing devices . Both t h e s e devices

a r e u s e f u l when t h e incoming gas s t ream con ta ins s i g n i f i c a n t p a r t i c u l a t e

loading.

2 1

IVOC Recoverv--In the stripping column, a specific

stripping agent is contacted with the absorbentfVOC mixture from the

absorption column. Stripping can be accomplished by air, steam, heat (i.e.,

by distillation), liquid extraction, or some combination thereof. Steam is

frequently used, provided the absorbent and VOC are water insoluble.

advantages and disadvantages of various stripping agents are given in

Table 3 .

2.3 Condensation

Some

Condensation is the simplest of the three VOC recovery techniques

discussed in this monograph.

process, which is discussed in numerous standard texts, including Kern13

and Perry.14

It is a straightforward heat exchange

The governing equation for all heat exchange processes is:

Q = U A A T ( 3 )

where Q = rate of heat transfer, Btu/hr U = overall heat transfer coefficient, Btulhr-ft2-.F

A = heat exchange surface area, ft2

AT = temperature difference, 'F. Although the above expression is simple, much information is needed to

design or evaluate a condenser for a specific VOC recovery application. This information includes physical properties of the VOC-containing gas and

the coolant as well as the heat exchanger geometry.

Specifically for VOC recovery, the gas stream is usually air and the

cooling fluid of economic choice is cooling water. Design inlet

temperatures for industrial cooling water, depending on season and

location, are usually 80 to lOO'F. Fromthe above equation, if the VOC

condenses at a temperature less than this, it cannot be recovered by

condensation unless another coolant is used. Such coolants and related

equipment are readily available, but add to the capital and operating

costs.

22

TABLE 3 . ADVANTAGES AND DISADVANTAGES OF VARIOUS STRIPPING AGENTS STRIPPING AGENT ADVANTAGES DISADVANTAGES

Air Simple operation VOC concentration in the exit gas may be small

VOC recovery by condensation may be expensive

Steam Simple operation Additional recovery steps needed if VOC is water soluble

Stripping is rapid or if a multicomponent VOC is because of re c ove red simultaneous heat/ mass transfer

Heat (distillation) Multicomponent VOC Design may be more complex than streams can be for other stripping agents purified, resulting in higher value for Operation is energy intensive the recovered VOC

Liquid extraction A specific VOC may Suitable extracting agent must be recovered from a be identified m l t icomponent absorbentlVOC Additional recovery steps needed mixture

23

VOC concentration in the inlet gas is also important. Since the entire

gas stream must be cooled to condense the VOC, energy costs can be

prohibitive if the VOC concentration is low. Below about 5,000 ppm,

recovery by condensation is not usually practicale2 This limit is purely

economic, however, because in principle almost any VOC could be recovered

from a given inlet gas stream.

If the dew point of the inlet gas stream is higher than the coolant temperature, water will condense along with the VOC. This can cause two

problems-corrosion and low VOC purity. Corrosion will result if the VOC hydrolyzes easily or forms any corrosive compound on contact with liquid

water, such as some chlorinated VOCs do. If the VOC is water soluble or toxic, condensation of water simultaneously with the VOC will require subsequent VOC-water separation or wastewater treatment to remove the toxic

VOC from any discharged water, respectively. One way to avoid this problem

is to use a desiccant bed (molecular sieves, alumina, or silica gel)

upstream of the condenser (see Kohl and Rei~enfeld,~ p. 630). VOC removal efficiencies by condensation can be fixed at any level

desired, the only constrairt being cost. If greater than 90% removal must be achieved, coolants other than cooling water must generally be used,2

especially if low-molecular-weight VOCs are present.

For inlet gas streams containing more than one VOC, it is possible,

though not normally practical, to design partial condensers in series, each

of which condenses one VOC. This could become practical if the VOCs

differed widely in boiling points, say by at least 75 to 100.F. Finally, the condensed VOC may form either an aerosol or entrained

droplets in the heat exchanger at practical gas velocities. If either occurs, the VOC must be recovered by either demisters or a collection

device such as a cyclone. The overall efficiency of the VOC recovery

process (condensation and collection) may be significantly reduced if the

aerosolldroplet size is much less than about 5 pm. This can be solved by

limiting the gas velocity t.rrough the condenser.

24

3.0 ECONOMIC CONSIDERATIONS

3.1 Adsorption

The c a p i t a l and ope ra t ing c o s t s of a carbon adsorp t ion s y s t e m f o r VOC

a r e b e s t obtained by completing a s p e c i f i c a t i o n shee t provided by t h e

vendor. This i s f a i r l y s imple; an example s h e e t i s provided i n Appendix A.

The s i n g l e most important f a c t o r a f f e c t i n g t h e c a p i t a l c o s t of a carbon

adso rp t ion system i s t h e volumetr ic throughput, f t3 lmin .

c o s t s f o r convent ional steam-regenerated systems a r i $15-20/ft3/min. 3. When

t h e VOC concent ra t ion i n t h e i n l e t gas s t ream i s l e s s t han about 300 ppm,

one vendor r e p o r t s t h e use of a " t h i n bed" (4 t o 18-inch bed depth) t o

lower t h i s c a p i t a l c o s t t o about $lO/f t3/min f o r systems g r e a t e r than

10,000 f t3 /min .3

dehumidi f ica t ion t o <50% RH may add about $ l / f t 3 / m i n t o t h e $15-20/ft3/min

above.3

recovery inc reases very r a p i d l y as t h e VOC mass f low r a t e decreases t o

below 100 l b of VOClhr.5

concen t r a t ion of 1,000 ppm, t h i s mass f low r a t e corresponds t o a t o t a l gas

f low r a t e of 6,500 f t3 /min a t 1 a t m p re s su re and 75.F.

Typica l c a p t i a l

The only o t h e r c a p i t a l c o s t s a v a i l a b l e show t h a t

One q u a l i t a t i v e observa t ion is repor ted by DuPont: t h e c o s t of VOC

For a VOC of molecular weight 100 a t a

Operating c o s t s inc lude steam, e l e c t r i c i t y , and condenser water f o r t h e

convent ional system considered here . Steam usage is repor ted t o be 0.25 t o

0.35 l b steam ( @ 15 p s i g ) / l b carbon3 o r about 6 l b s team/ lb VOC.4

E l e c t r i c i t y usage w i l l be about 2.9 t o 4.5 kW/1,000 f t 3 1 m i n . 3 ~ ~

water is used a t about 12 gal lmin 100 l b steam.3

Condenser

Replacement carbon, i f

needed, c o s t s about $3-6/ lb , wi th a t y p i c a l carbon l i f e being up t o 5

year^.^,^ VOC per h r p e r yea r .4

3.2 Absorption

This r e s u l t s i n carbon replacement c o s t s of roughly $7 per l b

Because absorp t ion c o s t s depend s t r o n g l y on t h e VOC-absorbent

equ i l ib r ium r e l a t i o n s h i p , choice of absorbent , and choice of s t r i p p i n g

agent , r e p r e s e n t a t i v e economics f o r VOC recovery from small i n d u s t r i a l

sources a r e d i f f i c u l t t o determine.

Vatavuk and Neveri l have descr ibed a procedure t o e s t ima te c a p i t a l and

ope ra t ing c o s t s f o r absorp t ion processes f o r p o l l u t i o n c o n t r o l . The

c o s t of t h e gas absorp t ion column i t s e l f i s given a s a func t ion of column

25

weight and t h e c o s t of p e r i p h e r a l support equipment is a l s o given.

However, no s p e c i f i c examples a r e shown.

Zenz g ives a d e t a i l e d design procedure f o r abso rp t ion towers , bu t does

not d i scuss economics i n d e t a i l . 1 7

and T imerhaus (pp. 768-776) f o r absorp t ion equipment c o s t s . 6

The reader should a l s o consu l t P e t e r s

3 . 3 Condensation

Because condensation is a s imple h e a t t r a n s f e r process , t h e r e a r e two

major c o s t elements--the hea t exchanger (condenser) i t s e l f and t h e

r e f r i g e r a t i o n u n i t , i f needed.

comparison t o system c o s t s f o r carbon adsorp t ion . Figure 6 shows s h e l l and

tube hea t exchanger c o s t s as a func t ion of s u r f a c e a r e a , as given by

P e t e r s and T imerhaus (pp . 668-671) .6

t h e hea t exchanger can be made of carbon s t e e l and designed f o r modest

coolan t p re s su res ( coo lan t is u s u a l l y on t h e tube s i d e of a s h e l l and tube

condenser) and near-atmospheric s h e l l - s i d e pressures . Costs f o r o t h e r

m a t e r i a l s of cons t ruc t ion such as s t a i n l e s s , Monel, o r Has te l loy a r e

cons iderably h igher t han f o r carbon s t e e l .

Heat exchanger c o s t s are very modest by

For most VOC recovery a p p l i c a t i o n s ,

The r e f r i g e r a t i o n u n i t c o s t depends on t h e hea t duty (Btu /hr ) and, t o a

l e s s e r though s i g n i f i c a n t e x t e n t , on t h e temperature of t h e coolan t a t t h e

i n l e t t o t h e condenser.

For t y p i c a l VOC recovery a p p l i c a t i o n s , coolan t i n l e t temperatures of

-10 t o +10*F a r e adequate t o condense VOCs w i th b o i l i n g p o i n t s down t o

about 60 t o 80'F. Figure 7 shows a curve f o r r e f r i g e r a t i o n purchased

equipment c o s t s a s a func t ion of h e a t duty from P e t e r s and T imerhaus

(p . 886) .6

and Neveril16 of 120% of purchased equipment c o s t s , t h e t o t a l c a p i t a l c o s t

f o r a u n i t d e l i v e r i n g 2.42 x lo6 Btu/hr a t 20'F coolan t i n l e t temperature

is $330,000. This va lue compares t o an independently repor ted c a p i t a l c o s t

of $310,000 f o r t h e same s i z e d un i t8 .

Including d i r e c t and i n d i r e c t i n s t a l l a t i o n c o s t s from Vatavuk

Operat ing c o s t s w i l l be p r imar i ly f o r e l e c t r i c i t y , especial1.r i f a

r e f r i g e r a t i o n u n i t i s requi red . P u r c e l l and Shareef (p . 5-27) c i t e a

t y p i c a l va lue of 1.5 kW of e l e c t r i c i t y p e r t o n of r e f r i g e r a t i o n (1 t o n

r e f r i g e r a t i o n = 12,000 B t u / h r ) . 2

ope ra t ing r a t e of 8,000 h r l y r , annual e l e c t r i c a l c o s t s would be:

A t a c o s t of $O.O6/kWh and a t an annual

26

Figure 6. Heat exchanger costs as a function of surface area (from Peters and Timmerhaus, p. 670)6

6 Cost of industrial refrigeration (from Peters a n d Timmerhaus, p. 886)

Figure 7.

27

= $720/yr/ ton r e f r i g e r a t i o n .

Although it is not w i th in t h e scope of t h i s monograph t o p re sen t

d e t a i l e d s p e c i f i c comparisons of VOC recovery techniques , t h e r e s u l t s of

one such s t u d y 8 comparing carbon adsorp t ion and condensat ion are presented

i n Table 4 . These r e s u l t s support t h e gene ra l comparisons of Figure 4 .

S p e c i f i c a l l y , t h i s s tudy shows t h a t condensation is not economical a t l o w

VOC concent ra t ion , t h a t condensation i s more c a p i t a l i n t e n s i v e than

adsorp t ion , and t h a t a t t h e h ighes t VOC concen t r a t ion examined (8,000 ppm),

condensation i s competi t ive wi th adsorp t ion . A l s o no te t h e two op t ions for

use of t h e recovered VOC--either as a so lven t or a f u e l . Re-use as a

so lven t i s considerably more economical i n a l l cases. This s tudy a l s o

concludes t h a t only carbon adsorp t ion is capable of p r a c t i c a l removal

e f f i c i e n c i e s of 95% or g r e a t e r (condensat ion is assumed t o achieve 90%

t y p i c a l l y ) .

28

TABLE 4. SUMMARY OF COSTS FOR CONDENSATION AND FIXED-BED CARBON ADSORPTION^,^

N CD

S t a c k Gas E m i s s i o n s C o n c e n t r a t i o n (ppm v o I ume) C o s t I t e m

CONDENSATION FIXED-BED CARBON ADSORPTION

R e c o v e r e d R e c o v e r e d R e c o v e r e d So I v e n t R e c o v e r e d S o I v e n t

S o l v e n t Used a s So I v e n t Used a s Re-used Fue I Re-used Fue I

200

3,000

8,000

T o t a l c a p i t a l c o s t ( S )

Annua l d i r e c t c o s t ( S / y r )

T o t a l c a p i t a l c o s t ( 3 )

A n n u a l d i r e c t c o s t ( S / y r )

T o t a l c a p i t a l c o s t ( S )

A n n u a l d i r e c t c o s t ( S / y r )

-161,900

706,300

-9,700

140,000

-24,708

140,008

691,600 691,600 140,000 140,008

-147,600

706,300

-36,200 +228,600 +21,808 +175,800

613,600 686,700 269,600 269,608

+687,200 +16% ,000 +634,800 +91,808

. N e g a t i v e a n n u a l d i r e c t c o s t i n d i c a t e s e x p e n d i t u r e ; p o s i t i v e a n n u a l d i r e c t c o s t i n d i c a t e s e a r n e d r e v e n u e .

b T h i s t a b l e i s f r o m r e f e r e n c e 8, p . 126.

cThe V O C - c o n t a i n i n g g a s s t r e a m e v a l u a t e d a s : 5,000 s t a n d a r d f t 3 / m i n f l o w r a t e , 300OF i n l e t t e m p e r a t u r e ( t h i s i m p l i e s t h e need f o r p r e c o o l i n g f o r t h e c a r b o n a d s o r p t i o n c a s e ) , and 6% RH. The V O C i s a com- p l e x h y d r o c a r b o n m i x t u r e assumed t o h a v e t h e e m p i r i c a l f o r m u l a Cl0XS0.

4.0 VENDORS

This s e c t i o n is a noncomprehensive l i s t of some vendors of systems f o r

VOC recovery. The reader should consul t chemical equipment ca t a logs f o r a

more complete l i s t i n g .

4 .1 Adsorption

Table 5 provides a l i s t of vendors of a c t i v a t e d carbon systems f o r VOC

recovery.

4.2 Absorption

There a r e no known vendors of absorp t ion systems f o r VOC recovery from

small i n d u s t r i a l sources . However, absorp t ion is an extremely widespread

technology f o r ac id gas removal and o t h e r p o l l u t i o n c o n t r o l a p p l i c a t i o n s ,

The i n t e r e s t e d reader should consul t a d i r e c t o r y of chemical equipment

vendors under t h e heading "Absorbers" o r "Scrubbers. Some vendors i n

t h e s e l i s t i n g s include:

Croll-Reynolds Co., Inc. 751 Cen t ra l Ave. P.O. Box 668 Westf ie ld , NJ 07901 201-232-4200

D i s t i l l a t i o n Engineering Co., Inc. 105 Dorsa Ave. Livingston, NJ 07039 201-992-9600

Koch Engineering Co,, Inc. 1 7 1 Kel ley Ave. P.O. Box 109 Akron, OH 44309 216-724-1277

O t t o Pork Co. 4 1 I n t e r v a l e Rd. P.O. Box 3100 Parsipany, NJ 07054 201-299-9200

30

TABLE 5 . LIST OF VENDORS OF ACTIVATED CARBON SYSTEMS FOR VOC RECOVERYa SYSTEM CONFIGURATION

Company Name/Address Adsorb e r RegenerationlVOC Recovery

Baron Blakeslee Inc. 2001 N. Janice Ave. Melrose Park, IL 60160 3121450-3900

HOYT The Clean Air Co. 251 Forge Rd. Westport, MA 02790-0217 617 /636-8811 800 /343-9411

Met-Pro Corporation Systems Division 160 Cassell Rd. Box 144 Harleysville, PA 19438 215/723-675 1

RaySoLv Inc. 225 Old New Brunswick Rd. Piscataway, NJ 08854 201/981-0500 TWX 710 997-9666

TIGG Corporation Box 11661 Pittsburgh, PA 15228 412/563-4300

VARA International Inc. 1201 19th Place Vero Beach, FL 32960 305 /567-1320

Vic Manufacturing Co. Field Sales Office 152 Main St. Nantucket, MA 02554 6171228-3464 800 /824-7888 ext M 3162

Fixed bad

Fixed bed

Small fixed beds (drums )

Fixed bed

Carbon fibers

Fixed bed

Small fixed beds (drums )

Fixed bed

Fixed bed

Steamidecanting

Steam/decanting Steamldistillation

Off site regeneration

Steamldecanting Steam/decanting-air strippingb

Steamldecanting

Steamldecanting Inert gaslcondensation Steamldistillation

Offsite regeneration only

Steamldecanting

Steam/decanting

aInformation in this table is based on general process descriptions provided by each company upon request. explore all process modifications that can be made by each company on a custom basis. For example, the fact that inert gas regeneration is not shown for one company above does not mean they cannot supply it. The reader is encouraged to contact each vendor to explore special needs.

No attempt has been made to

this case, the vendor reports that the decanted water from the steam/ VOC condensation step can be air-stripped to remove any residual organics, with the resulting VOCImoist air stream being recycled to the adsorption bed inlet (see also Kenson, R.E., Env. Prog., 4 ( 3 ) , August 1985 , 1 6 1 - 1 6 4 ) .

31

4.3 Condensation

Condensation systems include both the condenser itself and often a

refrigeration system. Vendors listed below are able to supply both.

Alloy Fab, Inc. 200 Ryan St. P.O. Box 898 S. Plainfield, NJ 07080 201-753-9393

Croll-Reynolds Co., Inc. 751 Central Ave. P.O. Box 668 Westfield, NJ 07901 201-232-4200

Edwards Engineering 101 Alexander Ave. Pompton Plains, NJ 07444 201-835-2800

Pfaudler Co. 1000 West Ave. Box 1600 Roch?-ster, NY 14692 716-235-1000

32

5.0 EXAMPLE CASES

5.1 Adsorption

The purpose of t h i s s e c t i o n i s to provide t h e reader w i th an example of

t h e a p p l i c a t i o n of t h e design and t e c h n i c a l d i scuss ion of Sec t ion 2.1 t o a

s p e c i f i c case of VOC recovery by a c t i v a t e d carbon adsorpt ion. The r eade r

should a l s o r e f e r t o App i x A. This appe x con ta ins examples of some

vendor-supplied work s h e e t s which they ask t h e buyer t o complete i n o r d e r

t o conduct a prel iminary eva lua t ion of t h e buyer 's needs, u s u a l l y a t no

c o s t . Vendors w i l l normally supply prel iminary quotes and s p e c i f i c a t i o n s

based on t h i s information.

For any VOC recovery need, t h e buyer must know or o b t a i n c e r t a i n

information about t h e VOC-containing gas stream and t h e s i t e .

information has been summarized from t h e work s h e e t s of Appendix A and

s p e c i f i c va lues have been s e l e c t e d f o r t h e example exercise h e r e i n (see

Table 6 ) .

This

From t h i s information t h e buyer can make t h e following judgments and

c a l c u l a t i o n s about t h e VOC recovery system under cons ide ra t ion :

Gas Condir:.Qning--From t h e d i scuss ion i n Sec t ion 2.1, t h e gas-stream .

temperature, r e l a t i v e humidity, and t h e presence of d u s t i n d i c a t e t h e need

f o r gas condi t ioning upstream of t h e carbon beds.

m u s t accomplish p a r t i c u l a t e removal ( v i a a f i l t e r ) and coo l ing /

dehumidif icat ion t o approximately 80 t o lOO'F and <SOX RH ( v i a a

condenser /heat exchanger).

This gas cond i t ion ing

Sorbent Bed--The lower explosive l i m i t (LEL) for t o luene i n a i r is

12,000 ppm,2 thus t h e r e i s no need f o r d i l u t i o n of t h e gas stream f o r

s a f e t y reasons because up t o 25 t o 50% LEL can be t o l e r a t e d with some

s p e c i a l c o n t r o l s . The approximate so rben t bed s i z e can be c a l c u l a t e d from

t h e gene ra l gu ide l ines of Sec t ion 2.1 of 100 f t l m i n s u p e r f i c i a l v e l o c i t y

and t h e throughput of 20,000 f t 3 (STP)/min a t , s ay , 90'F:

Carbon bed c ross - s e c t i o n a l a r e a = r m 2 - 1 (460 + 90°R/492'R)

1 100 f t / m i n J

= 224 f t 2 .

33

TABLE 6. INFORMATION THE BUYER MUST SUPPLY

m

Flow rate of VOC-containing 20,000 ft3 ( ~ ~ ~ ) / m i n gas stream

VOC content of the gas stream 1,200 ppmtoluene in air

Gas-stream temperature 110.F

Impurities in the gas dust (trace amount)

stream

Relative humidity of the gas 75% stream

Value of recovered solvent $8.86/lb

Available utilities Steam 15 psig Electricity Sufficient (some vendors ask for

Cooling water Sufficient (some vendors ask for specific volts, amps)

specific supply temperature, pressure, and even water hardness)

Any space limitations None Operating schedule 8,000 hr/yr

34

From an isotherm of to luene adsorp t ion one a c t i v a t e d ~ a r b o n , ~ a t 70'F

and 1,200 ppm i n t h e VOC-containing gas , t h e " s a t u r a t i o n c a p a c i t y , " o r

adsorp t ion of to luene i s 30% ( i . e . , 1 l b of t h i s p a r t i c u l a r . I .

carbon w i l l adsorb 0.3 l b of to luene a t 70'F a t equi lbr ium).

a r b i t r a r y conserva t ive va lue f o r t h e "working capac i ty" of 25% of t h e

s a t u r a t i o n capac i ty f o r a t y p i c a l l -h r on- l ine ~ y c l e , ~ , ~ t h e working

capac i ty is:

Using an

Working capac i ty = (0 .25)(0.3 l b t o l u e n e l l b carbon)

= 0.075 l b t o l u e n e l l b carbon.

For s i n g l e VOCs o t h e r than to luene , t h e s a t u r a t i o n capac i ty a t i n l e t

temperatures of 70 t o 90'F, <50% RH, and 300 t o 5,000 ppm VOC w i l l vary

between roughly 20 and 40 wt percent ( i .e. , 1 l b carbon w i l l adsorb between

0.2 and 0.4 lb VOC & & b r i m a t these cond i t ions ) . Thus, us ing 25%

of t h i s s a t u r a t i o n capac i ty a s t h e working capac i ty , a r u l e of thumb

working capac i ty ( i n t h e absence of any o t h e r information) would be 5 t o 10

w t percent . B e cautioned t h a t it w i l l always be b e t t e r t o have

experimental d a t a f o r t h e VOC of i n t e r e s t , however.

The mass f low r a t e of to luene is:

Mass f low rate = rX00 x 1QZ l b - d e to luene i r 92 1 b 1 of t o luene 1 lb-mole gas J Llb-mole toluene]

369 l b to luene lh r - - Thus, t h e a c t u a l carbon requirement f o r a l - h r cyc le t i m e w i l l be:

Actual carbon - requirement 1 h r J l 1 1 0.075 l b toluene]

1369 l b tolueng1 1 1 hr1 r l b carbon 1 -

= 4,920 lb carbon.

Knowing t h a t t h e bulk dens i ty of mst granular a c t i v a t e d carbon is

about 30 l b l f t , 3 and t h a t t h e bed s u r f a c e area is 224 f t 2 , t hen t h e bed

depth i s c a l c u l a t e d t o be

35

Bed depth - - r-ncarbonl L-l-.-l t 30 l b / f t 3 J 1224 f t 2 J

= 0.73 f t . This bed depth is t o o shal low t o adsorb any b u t t h e smallest l e v e l s of

VOCs (such sha l low beds are, however, recommended f o r VOC concen t r a t ions i n

t h e range of 10 t o 100 ppm3). To o b t a i n a more appropr i a t e bed depth of a t

least 18 inches3, and p re fe rab ly 2 t o 3 f e e t , it w i l l be necessary i n t h i s

example t o inc rease t h e amount of carbon on - l ine from 4,920 l b

f t bed depth) t o :

(u s ing a 2-

Carbon needed - - ( 2 f t ) (24 f t 2 ) r30 ~b 1 on- l ine a t any t ft31 one t i m e

= 13,440 l b carbon.

This would probably be done with a th ree -vesse l system, wi th each

v e s s e l con ta in ing 6,720 l b carbon and wi th two vessels on - l ine a t any one

time. Such an arrangement would reduce t h e t o t a l amount of carbon needed

from a t o t a l of 26,880 l b carbon f o r a two-bed system (two 13,440-1b beds)

t o a t o t a l of 20,160 l b carbon ( t h r e e 6,720-1b beds) .

The steam usage, from Sec t ion 2.1 and based on a l - h r cyc le , w i l l be

approximately:

Steam usage = r-i r-i 1 l b carbon J L h r J

= 202 J.k steam h r .

The blower horsepower w i l l be:

Blower hp - - 1 (22400 f t3 /min )

134 hp. - -

VOC Reco very--Because steam is u s e d as t h e r egene ra t ing gas , t h e VOC

recovery s e c t i o n w i l l c o n s i s t of a condenser and a decan te r .

The condenser water needs w i l l be ( f o r a l -h r c y c l e ) :

36

Condenser water = r 1 (202 l b steam) f low 1100 l b s t e a d

- - 24 gal /min

One p o i n t of cau t ion is t h a t f o r t h i s p a r t i c u l a r case, t h e aqueous

s o l u b i l i t y of t h e VOC i n water is neg l ig ib l e . Thus the VOC can be

recovered d i r e c t l y and re-used o r s o l d and the decanted w a t e r is probably

s u i t a b l e f o r b o i l e r o r condenser feed. This may not be t r u e i n some

a p p l i c a t i o n s and t h e buyer should be c a r e f u l t h a t t h e recovered VOC is not

unduly contaminated wi th water , nor t h e w a t e r with VOC.

such cross-contaminat ion, even a t f a i r l y small l e v e l s , can d r a s t i c a l l y

a f f e c t t h e economics of VOC recovery.

The presence of

Economics--Based on t h e rough guidance of Sec t ion 3.1, t h e c a p i t a l c o s t

of t h e e n t i r e system is about

C a p i t a l c o s t = F S ~ O 1 (22,400 f t3 /min) lf t3lminJ

$44 5,000. - -

Carbon c o s t ( u u d e d . i n t h e above c a p i t a l c o s t ) w i l l be ( a t $ 4 / l b ) :

Carbon c o s t = ($4/1b)(20,160 l b carbon)

$80 , 6 4 0 . - -

A 5 t o 10-year t y p i c a l carbon l i f e implies a 10 t o 20%/year carbon

replacement c o s t , o r $8,100 t o $16,20O/yr.

Annual e l e c t r i c a l c o s t s a t $O.O6/kWh and 4 kW/1,000 f t3 /min and 8,000

h r / y r ope ra t ion w i l l be:

Annual e l e c t r i c i t y = r8000 hrl rs0.06 1 r 4 kw 1(22,400 f t3/mir . ) c o s t 1 yrJ 1 kWh J 11000 ft3/minJ

- - $43,000.

Annual steam c o s t ( a t $l/lOOO l b ) w i l l be:

Steam c o s t s = r202 1 ~ 1 r8000 h r l r SI 1 I hrJ 1 y r J 11000 1bJ

37

Annual condenser water c o s t ( a t $0.15/1000 g a l ) w i l l be:

Annual condenser = 124 sal1 f6-1 F snnn_hrl f $u 1 water c o s t 1 min J 1 h r J 1 y r J 11000 galJ

= $1,700.

There w i l l be a c r e d i t f o r t h e recovered VOC as w e l l . Assuming 95%

o v e r a l l e f f i c i e n c y and a va lue of $8.861100 I b f o r t o luene ,6 t h i s annual

c r e d i t w i l l be :

VOC recovery = fS8.86 1 f0.951 f369 b o 0 0 h r l c r e d i t 1100 1bJ 1 J L h r J 1 y r J

= $248,500.

I n summary f o r t h i s case, t h e c o s t s w i l l be roughly:

C a p i t a l c o s t $448,000 Annual ope ra t ing c o s t

Carbon replacement 8,100 - 16,200 E l e c t r i c i t y 43,000 Steam I , 600 Condenser w a t e r 1,700

Annual ope ra t ing c r e d i t $248,500.

To t h e annual ope ra t ing c o s t s must be added d i r e c t and i n d i r e c t l a b o r

and r e l a t e d i n d i r e c t annual expenses ( t a x e s , insurance, e t c . ) . The c a p i t a l

c o s t must be amortized and a f i n a l c o s t (or c r e d i t ) i n $ /y r determined.

This e x e r c i s e i s s p e c i f i c t o each small i n d u s t r i a l u s e r and is not completed

he re in .

f o r such an eva lua t ion .

However, t h e above should provide t h e use r w i th a s t a r t i n g po in t

5.2 Absorption

Although VOC recovery by abso rp t ion is not widely p r a c t i c e d , some

examples of i t s use are similar t o VOC recovery. One such example is t h e

recovery of aromatics from coke-oven gas us ing a heavy hydrocarbon absorbent

known as wash oil. S t r i p p i n g i s done by s t e a m . This o p e r a t i o n da res back

t o t h e 1880s i n t h e United S t a t e s and was used t o recover t h e r e l a t i v e l y

va luab le benzene, t o luene and xylene from t h e c o a l coking processes a s i n

s t e e l p l a n t s .

38

The example below i s summarized from S i l v e r and Hopton12 and a

d i scuss ion of t h e i r work by Kohl and Reisenfeld.5 Unfortunately, t h e

absorber and s t r i p p i n g column are not designed f o r coupled opera t ion .

However, t h i s example provides some idea of t h e major des ign and ope ra t ion

parameters of one well-proven VOC absorp t ion process .

I n l e t gas

VOC concent ra t ion Flow ra te Veloc i ty Temperature

11,000 ppm 418,000 f t 3 / h r 192 f t / m i n 73*F

Column

Diameter 9 f t Packing Wooden boards Surface area of packing O i l f low r a t e 83 gal lmin Removal e f f i c i e n c y 98.6X Operating p res su re 1 a t m G/L

110,000 f t 2

521 f t 3 g a s / f t 3 wash o i l - Column diameter 38 inches Wash o i l f low rate

Packing 1.4-inch r ings Wash o i l temperature 253.F Operating pressure 1 p s i g Steam f low r a t e 13.2 f t l m i n

14 gal lmin 1.8 g a l / m i n / f t 2

Other example cases are given i n T r e ~ b a l . ~ Costs can be es t imated from

a simple procedure given by Vatavuk and Neveril16, which involves t h e

c a l c u l a t i o n of t h e he ight and number of t r a n s f e r u n i t s us ing the VOC-

absorbent equi l ibr ium p r o p e r t i e s , gas and l i q u i d mass f low rates, and

phys ica l p r o p e r t i e s of t h e packing and column. From t h e des ign

c a l c u l a t i o n s , c o s t s a r e es t imated based on column s i z e , packing c o s t s , and

support equipment c o s t s . I n genera l , absorp t ion systems are more l abor and

c a p i t a l i n t e n s i v e than o the r VOC recovery techniques, a l though absorp t ion

system c o s t s can be competi t ive i n c e r t a i n app l i ca t ions .

39

5.3 Condensation

The following axample is taken from Purcell and Shareef (p. 4.7-lf.)

and demonstrates the condensation of a VOC from a stream typical of one to

which this technique can be applied--relatively low flow rate and high VOC

concentration. 2

For this case, we assume the following:

Inlet gas flow rate Inlet gas temperature 90'F voc styrene VOC concentration 13,000 ppm Moisture content negligible" Condenser operating pressure 1 atm

2000 ft3 (STP)/min

Removal efficiency required 90%

If the dew point of the inlet gas were close to the dew point of the VOC, then a desiccant might be necessary upstream of the condenser (see Section 2.3).

*

The key variable above is the required removal efficiency. To achieve

90% removal (i.e., an outlet concentration of 1,300 ppm), the temperature

of the gas stream must be reduced at least below that at which the

equilibrium vapor pressure of styrene in air at 1 atm is (1,300 x (760

mm Hg), or 1.0 mm Hg. This temperature is 2O'F for styrene. (In general,

knowledge of the equilibrium vapor pressure-temperature behavior for the

VOC(s) of interest is required.)

Because a coolant temperature of at most 2O'F is needed, refrigerated coolant fluid i s used. This is usually a "brine" solution, which can be

simply water with an appropriate antifreeze added (CaC12 is selected for

tbe example case2).

The condenser heat load is calculated by a straightforward heat balance

on the inlet and exit gas.

Condenser heat = 241,000 Btu/hr. load

The condenser surface area is given by the basic design equation of

Section 2.3 ( Q = U A AT). Assuming a countercurrent shell and tube

40

design, dT is the log mean temperature of the coolant and gas of the inlet minus the same difference at the outlet. This is somewhat arbitrary, but

often an "approach" temperature of 15 to 25*F is specified, meaning that the outlet gas temperature is specified to be 15 to 25*F higher than the

coolant temperature at the condenser outlet.

specifying that the coolant temperature rise be limited to 25'F (this may

be limited by the refrigeration equipment), and further assuming the inlet

coolant temperature to be 5*F, the log mean temperature difference, AT is

Using an approach of 15*F and

AT = (go* - 30 *F) (30 * - 5*F) In [ (90* - 30*F) /(ZOO - 5*F) J

= 32*F.

Calculation of U, the overall heat transfer coefficient, can be an

extremely time-consuming process, but is nevertheless necessary.

ely, some short-cut methods can result in considerable error, and

the reader is urged to go through the procedure of Kern13 or others to

obtain a reasonable value.

ft2 - *F as ""conservatively assumed" and cite Ludwiglf as a reference. Using this value, the heat exchanger area is:

Purcell and Shareef2 use a value of 20 Btu/hr-

A = Q I U A T

= (241,000 Btu/hr) (20 Btu/hr-ft2-*F) ( 3Z0F)

= 377 ft2.

The coolant flow rate, after calculating a coolant heat capacity of

0.65 Btu/lb-*F and recalling the specified temperature rise of 5'F tp 3Q0F,

is :

Coolant flow rate = fU.000 Btul r Ib-OF 1 f 1 1 1 hr 110.65 BtuJ LO'F - 5'FJ

= 14,800 lblhr.

The amount of styrene recovered at 90% efficiency will be:

41

A”-it of =

r ecove r ed

(0.90) mi,x!-&f+3(STPLi Fa-mole pas 1 TX.000 x 1Q-6 lb-mole s t y r enel styrene 1 min J 159 ft3 (STP)J 1 lb-mole gas J

rl“ I r u i llb-mole styreneJ 1 hr J

- - 407 lb styrenelhr.

The purchased equipment cost of a carbon steel shell and tube condenser

can be found to be about:

Purchase cost = $8,000. of condenser

To this must be added direct cost and indirect installation costs,

which are roughly 1202 of the condenser,16 to obtain the total capital cost

of the condenser of about $17,600. From Peters and Timmerhaus (p. 881),

the purchased equipment cost of the refrigeration unit is about $80,000. Assuming the same installation costs of 120%, the refrigeration unit capital cost will be $176,000, or 10 times that of the heat exchanger.

Operating costs include electricity for refrigeration unit and cooling

water pumps.

hr/yr operation and $O.O6/kWh (using a value of 1.5 kW per ton of refrigeration,2 with a 1 ton refrigeration equal to 12,000 Btulhr) to be:

Electricity for refrigeration can be estimated assuming 8,000

Electrical costs = fSO.061 fSann_hrl f241.00Q BrJal 1 kWh J 1 yr J L hr J

- f hr-ton 1 r1.5 kW1 112,000 BtuJ 1 ton J

-

= $14,50O/yr.

This may be a significant cost, and points out the economic reasons for

judicious use of condensation as a VOC recovery technique. The reader

should also refer to Chandrasekhar and Poulin ( p.126) where a similar

conclusion is implied.

42

6.0 REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

9.

10 *

11.

12.

13.

14.

15.

16.

17.

Kennedy, P.E., and J.C. McGill, "Adsorption Systems for Air Pollution Control in the Refining and Petrochemical Industries," Spring National AICE Meeting, Houston, TX, March 27-31, 1983.

Purcell, R.Y., and G.S. Shareef, "Evaluation of Control Technologies for Hazardous Air Pollutants," EPA-600/7-86-009a,b, February 1986.

Vic Mfg. Co., "Carbon Adsorption/Emission Control," (no date) Minneapolis, MN 55413.

Kohl, A.L., and F.C. Riesenfeld, Gas Purification, 4th ed., Gulf Publishing, Houston, 1985.

Kovach, J.L., in Handbook of Separation Techniques for Chemical Engineers, P.A. Schweitzer ed., McGraw Hill, 1979.

Peters, M.S., and K.D. Timmerhaus, Plant Design and Economics for Chemical Engineers, 3rd ed., McGraw Hill, 1980.

Buonicore, A.J., and L. Theodore, Industrial Control Equipment for Gaseous Pollutants, vol. I, CRC Press, 1975.

Chandresekhar, R., and E. Poulin, Control of Hydrocarbon Emissions from Cotton and Synthetic Textile Finishing Plants, report preparo,d for EPA by Foster-Miller, Inc., Waltham, MA, under contract 68-02-3134, May 1983.

Treybal, R.E., Mass Transfer Operations, 2nd ed., McGraw Hill, 1968.

Crawford, M., Air Pollution Control Theory, McGraw Hill, 1976.

Marchello, J.M., Control of Air Pollution Sources, Marcel Dekker, 1976.

Silver, L., and G.U. Hopton, J. SOC. Chem. Ind. (London), 61, March 1942, p. 37.

Kern, D . Q . , Process Heat Transfer, McGraw Hill, 1950.

Perry J.H., ed., Chemical Engineer's Handbook, 5th ed., McGraw Hill, 1973.

Ludwig, E.E., Applied Process Design for Chemical and Petrochenical Plants, vol. 111, Gulf Publishing, Houston, 1965.

Vatavuk, W.M., and R.B. Neveril, Capital and Operating Cost of Selected Air Pollution Control Systems, EPA-450/5-80-002, 1980. This report was also published in parts in Chemical Engineering, beginning in the October 6, 1980 issue.

Zenz, F.A., in Handbook of Separation Techniques for Chemical Engineers, P.A. Schweitzer, ed., McGraw Hill, 1979.

43

Appendix A

VENDOR WORKSHEETS FOR CARBON ADSORPTION SYSTEMS

44

DATE,

SOLVENT RECOVERY DATA SHEET

COMPANY

ADDRESS

,

NAME PHONE

PROCESS I N WHICH SOLVENT IS VAPORIZED

NORMAL OPERATION: HOURS PER D A Y DAYS PER WEEK

SOLVENT COMPOSITION (Give % of Components if mixture, and specify by weight or volume)

BOILING TEMPERATURE OF

MAXIMUM LOSS (Ibs./hr.)

HOW MUCH VARIATION OCCURS

MINIMUM LOSS (IbsJhr.)

VOLUME OF EXHAUST AIR: CFM TEMP. O F HUMIDITY

F A N STATIC PRESSURE REQUIRED -EXTERNALLY

CONTAMINANTS IN SOLVENT LADEN AIR: DUST

TAR/OILS ACID RESIN OTHER

UTILITIES AVAILABLE: STEAM (PSIG & BHP) COMPRESSED AIR (PSIG & SCFM)

ELECTRIC: (Vo l tage , Cycle, Phase)

COOLING WATER AVAILABLE: TEMP O F GPM PS I

DEGREE OF SEPARATION REQUIRED O F RECOVERED SOLVEVT MIXTURE

APPROXIMATE VALUE OF SOLVENT (Per Ib. or Gal.)

TYPE O F QUOTATION REQUIRED: BUDGET FINAL

PROJECT STATUS: IMMEDIATE 6-12 MONTHS OTHER

P lease Return to: HOYT CORP. Forge Road Westport, Massachusetts 02790

Toll Free 1-800-343-941 1 IN MASSACHUSETTS (617) 636-881 1 ATTENTION: Industrial Environmental Division SR 7 ?

160 CASSELL ROAD BOX 144 HARLEYSVILLE, PA 19438-9990 (215) 723-6751 TELEX 846-424

PROPOSAL REQUEST-PROJECT DATA SHEET

I. General:

A. Company

Address

Telephone ( 1 Ext . Date

Contact/ T i t l e

Location f o r Proposed Equipment '

B. P re fe r r ed Treatment: Best Choice O r

Concentration Followed By Adsorption O r I n c i n e r a t i o n

Adsorption w/ Steam

Thermal O r C a t a l y t i c I n c i n e r a t i o n With m e c u p e r a t i v e Heat Exchange

O r With 1b.j hr I Steam Generation a t PSIG

O r Vacuum Regeneration

Design Solvent Removal E f f i c i ency Desired

Performance Solvent Removal E f f i c i ency Desired %

%

C. Budget o r Firm Quotation Requested. Quotation Required By (da t e )

E s t i m a t e Date of Purchase Company RFQ No.

Estimated Date of Ins t a f l a t i on

11. Process:

A. Descr ipt ion

B. Volme of Exhaust: SCFM ( m X 1 (min)

Solvent Loading: lb./Hr. (-XI (min 1 Operating Pressure t o Proposed System: In . of water

Operacing Temperature t o Proposed System:

Re la t ive Humidity of P lan t Location % a t " F

--

O F

Operating A l t i tude i f above 2, 000 Ft . F t .

---- Duct S ize and Shape:

C . Operation : H r / Day Days/ Wk wks/ Yr

Continuous O r Batch

D . Contaminants : 1b.l fir. V O l . % ppm (Vol.) Mol. W t . B o i l P t . . 1.

2 .

D. Contaminants: l b . / h r . Vol. % ppm (Vol.) Mol. W t . B o i l P t .

4 .

5 .

-- E. P a r t i c u l a t e Present : Yes/No lb./ hr . % Combustible

._ Composition P a r t i c u l a t e Size D i s t r i b u t i o n

Phys ica l P r o p e r t i e s

F. Aerosol Present : Yes/No l b ./ h r . %Combustible

Composition

Phys ica l P r o p e r t i e s

G. U t i l i t i e s Avai lable:

E l e c t r i c vo It Cycle Phase

Compressed Air PSIG; Nitrogen PSIG

Cooling Water gpm P s i O F Allowable

Chi 1 led Water @m p s i OF Allowable

Steam lb/ h r p s i g O F

H. Insurance Coverage: FIA FM Other

I. I n s t a l l a t i o n P re fe r r ed :

Ground Indoor Outdoor Roof

J. Cont ro l Panel Location: On Unit O r Remote F t . From Unit

O r Both ; Indoors Out doors

K. Space Avai lab le f o r Equipment I n s t a l l a t i o n L W --- H

L. Should Equipment By-Pass be Provided

111. Carbon Adsorption:

A.

B. Intended use f o r recovered s o l v e n t :

C. P u r i t y r equ i r ed : Solvent Water

D. Value of recovered so lven t : $/Gal. E . Disposa l : Method

F. Disposal Cost: $/Gal.

Is recovered so lven t f o r r euse o r d i sposa l?

I V . Cat a l y t i c / Therma 1 Inc ine ra t i o n :

a t BTU/CF and A . Fuel Avai lab le : Type of Gas

p s i KO. o i l a t BTU/ Gal. and

p s i g : %su l fu r

B. Heat Recovery: YeslNo R e genera t ive A i r make-up

C . Steam Generat ion: YeslNo l b s . / h r PSIG

REQUEST FOR QUOTATION

1 Company Name Date

Address ci ty State Z i p

Person Supplying Information Phone

Furnish Quotation t o the Attent ion O f :

T i t l e Department

2 Purpose of System: A i r Po l lu t ion Control? (Requirement)

Reuse o f Solvent? Reuse o f A i r ? Other

Type o f operation i n which solvent was vaporized

Hours per s h i f t Sh i f ts d a i l y Days per week - Days per year

o . . . o . o ~ ~ o . . o o ~ . ~ o o o o ~ o ~ o o ~ o . . . ~ o . ~ . o o o o o ~ -

3 Vapor Compost t i o n X Vol. o r X W t .

a.

b.

C.

Maximum amount (lbs.) of solvent vaporized per hour

d. Is evaporation loss constant? I f not, explain haw much var ia t ion takes place

eels t h i s fnfonnation based on Usage o r stock analysfs

4 Cubic fee t per minute o f a i r handled Temperature o f a i r OF

I s t h i s an estimate or actual measurement

Maximum temperature of solvent laden a i r OF. Humidity

Impuri t ies present: Dust? Resins? O i l Vapor?

Paint? Acid Fumes? L i n t ? Other?

OVER

5 Cooling water temperature (max.) Water pressure P.S.I. Min,

Air pressure available P.S,I ,

Electrical power available V o l t s Cycles Phase,

Starter Full voltage Other

Avai lab4 7 i ty o f steam i s pounds/hour a t P.S. I.G.

6 Fieldman Recomnendations

Quotation Fonat: Budget Letter Long Form

FIELDMAN

V-APC 5187 12/82

Pollution Controls Division Vara International, Inc.

1201 19th Place, Vero Beach, FL. 32960 Telephone (305) 567-1520

Telex: 803460

ENGINEERING DATA SHEET SOLVENT RECOVERY SYSTEMS

NAME OF FIRM: DATE:

ADDRESS: TEL. NO. ( 1

ATTENTION:

ORIGIN OF THE SOLVENTS BRIEF DESCRIPTION OF PROCESS FROM WHICH SOLVENT IS EMITTED

I

IF MIXED SOLVENTS, WILL THE MIXTURE RECOVERED HAVE TO BE SEPARATED? El Yes [Ii No WHAT DEGREE O F SEPARATION IS NECESSARY?

COST OF SOLVENT(S)

WHATIS PERMISSIBLE WATER CONTENT

AIR OR GAS DATA DEGREE OF DUST

OR FLUFF OR VOLUME TEMPERATURE WATER PARTICULATES IS THERE ANMHING IN THE PROCESS LIABLE TO INTRODUCE

(cfm) ( O F ) CONTENT IN AIR STREAM A CORROSIVE GAS. ACID OR A COMPOUND?

I

PLANT OPERATIONS NORMAL HRS. DAY -- . - DAYS PER YEAR

UTlLITIES AVAILABLE STEAM PRESSURE SUPERHEAT

COOLING WATER TEMPERATURE HARDNESS

ELECTRICAL CHARACTERISTICS COMPRESSED AIR

DETAILS OF SPACE A V ~ L A B L E FOR RECOVERY PLANT INDOORS OR OUTDOORS? APPROX. HOW FAR IS PROPOSED SITEFROM PROCESS EMlSSlON OR S.L.A. SOURCE? -

ADD PAGES FOR ADDITIONAL INFORMATION OR REMARKS

C 0 ii;l P A N Y

&; O h 2 C .

a subsidiary of Purex Industries, Inc. R E P L Y T O

ZOO1 NORTH JANICE AVENUE MELROSE PARK ILLINOIS 601 60 AREA CODE 3 12-450 3900 CABLE PRX--BARB-MLPK TELEX 25-4701

C A R B O N A D S O R P T l O N D A T A S H E E T

D A T E

A D D R E S S

P R O C E S S F R O M VfHlCH S O L V E N T V A P O R IS E M I T T E D ,

H O U R S P E R D A Y D A Y S P E R WEEK D A Y S P E R Y E A R

P R E S E N T E X H A U S T : CFAl ; TEMP. O F ; HUliillDlTY %R H

S O L V E N T . (SI TO B E A D S O R B E D ( I F M O R E T H A N O N E G l V E P E R C E N T A G E O F EACH BY

\'!EIGHT OR V O L U M E - SPECIFY)

VOLUs*.lE I N A I R S T R E A i l l : PPtl : L B / t 1 R : O T t i E R .,

I : . lP[JRiTIES I N A I R STREA7.1 ( D U S T , O I L S , E T C . )

S T E A h l ( L B / H R , P S I G , B H P )

C O M P R E S S E D A I R ( P S I G , C U B I C F E E T h l l N U T E )

WATER ( T E M P E R A T U R E , G A L L O N S PER M I N U T E , PSIG)

IS Q U O T A T I O N R E Q U I R E D ? T Y P E : B U D G E T F O R M A L

P R O J E C T S T A G E : I h l M E D I A T E 3-6 M O N T H S 6-12 M O N T H S

C O M M E N T S :

PLEASE R E T U R N T O : B A R O N - B L A K E S L E E , -lNC. 3 4 M A N C H E S T E R D-RIVE W E S T F I E L D , N E W J E R S E Y 07030

A T T N : D I C K S E L Z N t C K .