ACT IVATED CARBON ADSORPTION FOR TREA T REAT T MENT OF VOC EMIS EMISSION IONS Pres Presented at at t he 13th A nnual En vir oEx po, Boston M ass assachuse achusett tt s—M ay 20 01 Austin Shephe hepherd, rd, P.E., C.I .H . Vice Pres President, ident, Technic Technical al D irector, irector, CA RBT RO L C orporation e-mai l: a.shephe a.shepherd rd @carbt ro l.com
A review review of the characte characteri ri sti cs of acti acti vated vated carb carb on and i ts applicabilit y t o emiss emission contr ol of V O C’ s. D esign and and
EVALUATION OF ALTERNATIVE TREATMENT TREATMENT PROCESSES PROCESSES
cost cost s of carbo n syst syst ems are als al so di scussed. cussed. INTRODUCTION
The principal use of vapor phase activated ca rbon in the environmental environmental field field is for the removal of volatile orga org a n ic compounds such as hydrocarbons, solvents, toxic gases and organic based odors. In addition, chemically impregimpregnated activated carbons can be used to co n t rol certain inorga nic polluta polluta nts such as hydrogen sulfide, mercu mercury, or radon. When When properly applied, th e adsorption process will remove pollutants for which it is designed, to virtually nondetectable levels. In fact one of the first la rge- scale uses of activated carbon w as in military gas masks w here here complete plete conta minant removal is essential. essential. Carbon adsorption is equally effe eff ect iv e on single component emissions emissions as w ell ell as complex mixtures of pollutants. In the industrial area, the most common a ppl pplications ications of activated carbon a re for process process off -gases, ta nk vent emissions, emissions, w ork area air purificatio n, and o dor control, either either w ithin the plant plant or relat relat ed to plant exhausts. exhausts. Ad d it io n a lly, activated carbon is used in the hazardo us w aste reme remediation diation a rea rea to treat off-gases from air strippers and f rom soil vapor extra ction ction rem remed ia t io n pro ject s.
ACTIVATED CARBON
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THERMALOXIDATION SCRUBBERS
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PARTICULATE FILTERS CATALYTIC CATALYTIC OXIDATION OXIDATI ON
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TABLE I
AP P L IC ATION CONSIDERATI CONSIDERATIO N S
One of the major issues that must first be addressed when evaluating a specific en vironmental ronmental VOC problem problem is what t reatment technology to consider. For a given situation there are likely a number of treatment alternatives that appear to have some utility. The first step in this evaluation is to effectively characterize the application. You will nee need t o know at least least the following informa informa t io n :
Flow Rate - Continuos vs intermittent Contaminants Pres Present – individual contaminants, concentration concentration a nd va r ia b ilit y Temperature - Average and maximum Falamibility - Upper and low er explosive limits
Once you have characterized your p roblem, each technology can be consid ered for its ability to deal with the conditions identified. As an example, Table I lists some of the more common technologies used to control industrial vapor phase pollutants, and the conditions under which they might be most favorably applied. I can’t stress enough the importance of this rev re view, as this is w h ere most technical solutions fail. If you solve the w rong problem or pick a technical solution that does not re spo n d to a ll the variables variables of your a ppl pplication, ication, poor perfo perfo rmance will likely res result .
H O W IT WO R K S
In the a dsorption process, process, mo lecule leculess of a contaminated gas are attracted to and accumulate on the surface of the activated carbon. Carbon is a commonly used adsorbent due to its very large larg e surface area. It can b e mad mad e from a variety of base materials including coal, w ood a nd coconut shells, shells, and is manuf a ct ured or activated in a high temperat ure contro lled lled oxidat ion process. A pound of highly activated carbon has a surface area approaching 140 acres acre s. CR OSS SECT SECTION ION O F CARBON
This Fi gu re I presents an arti art ist ’s ren ren d ition of the cross cross section section of an a ctivated ctivated carbon particle. particle. N ote that almost a ll of the surface surface area a vailable for a dsorption is associat associat ed w ith its interna interna l pore strucstruct ure. Also note the relative change in po re diameters, going from very large at the granule surface boundary, to much smaller w ithin the par ticle interior. interior. Balancing of the large and small pore volumes during the activation process is w hat makes makes individual individual activated carbons perfo rf o rm diffe differen ren t ly. Molecules of a contaminant tend to a dsorb most strongly in a reas where the pore diameter of the adsorbent is close to the molecular diameter of the compound. While most organic compounds will adsorb on activated carbon to some d eg ree, ree, the ad sorption pr ocess ocess is most effective on higher higher molecular w eight eight a nd high boiling point compounds.
FIGUR FIGUR E I
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L I ST O F O R G AN I C C O M P O U N D S
RELATIVE RELATIVE ADSORBTION ADSORBTION RATE MOLECU MOLECULAR LAR WEIGHT WEIGHT
BOILIN BOILING G POINT POINT
CARBON CARBON CAPACITY CAPACITY %
NITROBENZENE
123
211 C
51
TETRACHLOROETHANE
166
147 C
40
TETRACHLOROETHYLENE
16 5
121 C
35
STYRENE
10 4
14 5 C
25
XYLENE
10 6
138 C
21
NAPATHYLENE
128
21 7 C
20
TOLUNE
92
111 C
20
BENZENE
78
80 C
12
MTBE
88
55 C
12
HEXANE
86
68 C
7
10 0
57 C
5
DIDHLOROETHANE
99
99 C
7
METHYL ETHYLKET ONE
72
80 C
4
METHYLENE CHLORIDE
84
40 C
2
ACRILONITRILE
53
74 C
2
ACETONE
58
56 C
0.8
V I N YL C H L O R I D E
62
CHLOROETHANE
64
12 C
14 9
neg 58 C
0.1 3
16
neg 161 C
0.0003
ETHLY ACRILATE
BROMOTRI FLOROMETHANE METHANE
neg 14 C
0 .7 0.5
TABLEII
Compounds having a molecular weight over 50 and a boiling point greater than 50 degrees degrees centigrad centigrad e are good candidates for a dsorption. dsorption. T A B L E I I presents a rep representative list of orga org a nic compounds and their relative adsorption strength. strength. O rganic contaminants are often classified classified a s w eakly, eakly, mod erat ely, ely, or strongly adsorbed. You will note that a compound such as nitrobenzene having a molecular weight of 123 and a boiling point of 211 C is characterized as a very strong adsorber. On the other hand a compound such such as methane w hich hich has a molecul molecular ar w eight ight of 16 and a boiling point of –161 C is a very w eakly adsorbed compound. compound. In fact, a t this capacity, for all practical purposes, methane methane removal removal w ith activated carbon would not be cost effe eff ect ive.
Activated Activated Carbon Adsorption for Treatme Treatment nt of VO C Emiss Emissions
AB SO R P TI O N C APA PAC C ITY
Physical adsorption is dependant on the characteristics of the contaminant to be adsorbed, the temperature of the gas st ream to be processed, and the concentration of the contaminant contaminant in the gas st ream. The The adsorption capacity f or a pa rticular rticular conta minant rep re presents the amount of the contaminant that can be adsorbed on a unit unit w eight eight of a ctivated carbon consumed at the conditions present sent in the applicatio n. Typical Typical a dsorption capacities for moderately adsorbed compounds range from 5 to 30 perce perc en t of the w eight of the carbon.
TRICH LORO ET ETHY HY LENE ISOTHERM ISOTHERM
The adsorptio n isotherm plot show show s the influence of concentration on adsorption ca pa cit y. Fi gu re I I presents an adsorption isotherm used to predict adsorption capacity for trichloroethylene. Note how the adsorption capacity va ries ries from 20 to 65 percent over the concentration range of 10 to 10000 ppm in the gas st rea rea m .
VAPOR PHASE ISOTHERM
VAPORPHASE ISOTHERM 100
10
1
0.1
0.01 trichloroethylene
A series of isotherms at differing temper a t ures shows the influence of tempera t ure on adsorption capacity. In Figure III you can see the effect of temperature on the same trichloroethylene compound. At 100ppm the capacity of activated carbon for trichloroethylene varies f rom 17 to 40 percent as the temperature chan ges fro m 140 to 32 degrees degrees F. F. Fo rt u n a t ely, most carbon suppliers have developed isotherms for a range of en vironmental contaminants. At C a r b t rol w e have built built a computerize computerized d database of adsorption isotherms so that we can easily model most environenviro nmental applications. By supplying supplying to us the gas flow rate, the contaminant concentrat centrat ion and the temper temperature ature of t he gas stream, a carbon usage pred pre d ict io n can be made. D ESIG N C O N SID ER AT ATIIO N S
0.001 0.00 1
0.01
0 .1
1
10
1 00
10 00
1 00 00
CARBTROL
concentration - ppmv
FIGURE II
VAPOR VA POR PHASE ISOTHERM ISOTHERM VAPOR PHASE ISOTHERM 100
10
1
0.1
TRICH TRICHLOR LOROETH OETHYLENE YLENE 32F
0.01
TRICH TRICHLOR LOROETH OETHYLENE YLENE140 140 F 0.001 0 .0 0 1
0 .0 1
0 .1
1 10 concentration - ppmv
100
1000
10 0 0 0
CARBTROL CORP.
FIGURE III
Outlet
Inlet
Activated Carbon Bed
Activated carbons used in the air pollution control field are normally supplied in a granular form with a particle size ranging from 1 to 5 millimeters. In the granular form activated carbon can easily be packed into a containment device through which a contaminated gas stream stream can be processe processed d f or purificat ion.
section of a Fi gu re IV show s the cross section typical fixed bed vapor phase adsorber. An adsorpt ion system in its simplest simplest f o rm is made of a containment device (d rum or vessel), distribution and collection lection d evices evices to effect proper circ ulation of the gas stream stream through the activated carbon bed, and a means for moving the gas stream through the bed (such (such as a fa n, a blow er, er, o r pres pressur iz ed gas displacement). Packed activated carbon beds can be conveniently config-
Drain Distributor CARBTROL G-4 ADSORBER FIGURE IV
Activated Activated Carbon A dsorption dsorption for Tr eatment atment of V O C Emissions Emissions
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ured into small transportable drums or tanks, or into large fixed contacting devices devices depending depending o n the a pplication. Adsorber Adsorber sizing for a particular a ppl ppliication is governed primarily by bed surface loading rate. With a standard 4-foot carbon bed depth, a maximum gas-loading gas-loading rat e of 100 cfm per per squa re foot of bed surface should be maintained. This insures adequate gas conta ct and sufficien sufficientt time to r ea ch adsorption equilibrium. Higher gas flows are handled by increasing the carbon bed surface (larger (larger a dsorbers) dsorbers) or adding multiple beds in parallel. One of the chief advantages of granular activated carbon adsorption is its simplic simplicity ity of application, Pr e-en g in eered fixed bed adsorbers can be purchased chased a nd installed on mo st existing existing exhaust systems systems with a minimum minimum of capital expenditure. Tra Tr a n spo rt a b le ad sorbers from 100 to 5000 pounds a re readily available as standard supply f rom several several ma nufactures. nufactures. Air flow s for these stock units go up to as much as 5000 5000 CFM . Larger systems systems up to 20,000 CFM or higher are available on a custom custom b asis. O nce installe installed d these systems systems operate operate for the most part unattended until the carbon becomes spent and req re q uires replacement. The freq fre q uency of r eplacement placement w ill vary for each application but may range from weekly to annually. Another Another signifi significant cant a dvanta ge of a n adsorption process is that it prov pro vid es on line res reserve capacity on a passive basis. The system is, in effect, available on a continuos continuos basis to handle vary in g loads but only consumes carbon when contaminants are present in the exhaust st ream. Other processes may have significant fuel or chemical operating costs even when no contaminants are being t rea rea t ed .
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R EAC TIVAT VATII O N
Once the activated carbon has become spent it must be removed from servi serv ice and replace replaced d w ith fresh fresh carbon in ord er to maintain the effectiveness of tre a tment. The spent carbon can be disposed of and replaced with virgin carbon, o r the spe spent nt carbon can be ret ret urned to the supplier for rea re a ct iva t io n and reuse. Reactivation res re st o res most of the original original carbo n a dsorption dsorption capa city and avoids expensive disposal costs. Most carbon adsorption applications rely on offsite reactivation to support their activated carbon supply. The spent carbon can be vacuumed f rom fixed adsorption vessels and shipped to the reactivation center in bulk, or ret ret urned to the rea re a ct iva t io n facility in a tra nsportable adsorber that serves both as an adsorption vessel and a UN shipping container. C AP ITAL AND OPERATING COSTS
Adsorption system capital and operating costs can vary widely but depend primarily on the gas volume to be trea tre a t ed and the amount of carbon consumed. A simple100 simple100 cubic foo t per minute adso rption system system consisting of tw o 200 pound canisters and a blower would req req uire about 20 sq ft of area are a and cost a bo ut 2-$3,000. 2-$3,000. When When spent spent these smaller adsorption units can often be exchanged exchanged for new units units w ith the original supplier. L a rger custom carbon adsorption systems can cost from several thousand, up to several hundred thousand dollars, and are usually supported with off site reactivation servi services. A 5000 5000 cfm system system with tw o adsorbers and an exhaust blower would have a capital cost in the 30-$40,000 range and if skid mounted mounted w ould take up an a rea of about 100sq ft. The C ustom reactiva tion service service to suppo rt this system usually costs about $2.00 per pound of carbon proc pro cessed including including transportation.
Activated Activated Carbon Adsorption for Treatme Treatment nt of VO C Emiss Emissions
AC TIVATED CARBON QUALITY
You should be aware that there is a significant nificant d iffe iff erence in adsorption capacity among the various commercial activated carbon products available. When procuring activated carbons it is important to recognize that the value of the activated carbon product that you are purchasing is related to its adsorption ca p a cit y, and not its w eight ight or volume. volume. The carbon manufa cturing industry, industry, in conjunction with ASTM, has developed seve severa ra l standa rd t ests ests that enable comparison of the relative adsorption capacities of various a ctivated car bons. These These tests tests can b e used used to a ssess ssess the quality o f a virgin or reactivated carbon prod prod uct and to predict its cost effe eff ect iven ess. The industry standard quality tests for VAPOR PHASE activated carbons is the CARBON TETRACHLORIDE ADSORPTION ADSORPTION CAPAC CAPAC ITY, or ASTM D 3467. Typica Typica l vapor pha se virgin activated activated carbons have C ARBON TETRACH ETRACH LOR IDE ADSORPTION ADSORPTION C APACITIES ranging from 45 to 70 percent by weight. When procuring either virgin or rea rea ctivated carbon products, make sure the a p propriate adsorption number is specified. Then Then compare t he alterna altern a t ive product specifications to insure you are getting the best activated carbon value. SU M M ARY
In summary, activated carbon has been shown to be applicable applicable for t reatment reatment of a w ide variety variety of environmental nvironmental conta minants. It is a proven technology that is simple to install and easy to operate and maintain. C apital costs are among the least expensive expensive for m ost a lternative trea trea tment technologies. Operating costs are primarily related to the amount of activated carbon consumed in the adsorption pro cess.
