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Air pollution control by adsorption
Adsorption and its Applications in Industryand EnvironmentalProtection Studies in Surface Science and Catalysis,Vol. 120 A. Dabrowski(Editor) 9 1998Elsevier Science B.V. All rights reserved.
807
Air pollution control by adsorption
W.M.T.M. Reimerink, D. v.d. Kleut NORIT Nederland B.V., P.O. Box 105, 3800 AC Amersfoort, The Netherlands 1. I N T R O D U C T I O N The pressure on industry to decrease the emission of pollutants to the air is increasing. The importance to industry is to keep the costs as low as possible. A broad spectrum of techniques is available and is developed to control air pollution. The choice of a technique is determined by the type of pollution and the process conditions. In relation to price/performance, physical adsorption is one of the most important techniques to control air pollution. Both organic and inorganic molecules can be removed from a gas stream by physical adsorption. However, the adsorption affinity increases as the molecules become larger. As a consequence the adsorption capacity of an adsorbent is higher for large molecules t h a n for small molecules. For this reason physical adsorption is extremely suited for adsorption of organic compounds from gas, air, water and liquid streams. 2. A D S O R B E N T S In actual practice only the following adsorbents are applied: Activated carbon Carbon molecular sieves Polymers Silica Alumina - Zeolites Activated carbon can adsorb a broad range of pollutants with varying dimensions by its broad pore distribution of micro- and small meso pores. Activated carbon can adsorb a large a m o u n t of pollutants due to its large pore volume. Due to its hydrophobic character adsorption takes place at high relative humidity. The usability of activated carbon for air pollution control is limited by the risk of ignition at high temperature. Much research has been done to develop polymer adsorbents.In the market polymers as polyad [1,2] are applied for adsorption of high boiling compounds. At -
808 high relative h u m i d i t y these adsorbents have a tendency to swell. The polymers cannot be used at high t e m p e r a t u r e s due to deformation. A great disadvantage of polymer adsorbents is the low adsorption capacity on a volume basis. Alumina and silica [1,2] are meso porous and are not suited for adsorption of small organic molecules by physical adsorption. Unmodified silica and a l u m i n a are hydrophilic, so a high relative humidity disturbs the adsorption of organic pollutants to a large extent. Silica is suitable as adsorbent of water by chemisorption. Zeolites [1,2] and carbon molecular sieves [3-7] have a narrow pore distribution. The pore distribution determines which adsorbate adsorbs well and which adsorbate adsorbs to a less extent. These adsorbents can be applied in purifications of well defined gas s t r e a m s such as are present in gas separation applications. Compared with activated carbon the usability of these adsorbents is limited. Zeolites can be made hydrophobic by increasing the A1 content. These hydrophobic zeolites are suited to purify gas s t r e a m s at high relative h u m i d i t y and high t e m p e r a t u r e . Of the above mentioned adsorbents activated carbon is the most convenient for air pollution control for a broad range of compounds and for a large variation in process conditions. Compared to carbon molecular sieves and hydrophobic zeolites, activated carbon is a relatively cheap adsorbent. 3. A C T I V A T E D C A R B O N Activated carbons are micro porous carbonaceous materials. The activated carbons available in the m a r k e t differ in pore distribution, in form and in chemical composition. To decrease the emission an optimal carbon and system should be chosen dependent of the kind of molecules to be removed and the process conditions. The differences between activated carbons types are a consequence of the choice of activation process, the activation conditions and to some extend the choice of raw material. Activated carbons are produced from raw materials such as peat, wood, lignite, anthracite, fruit pits and shells. The raw materials are converted in activated carbon by steam or chemical activation With steam activation [8] the raw material is carbonised and/or oxidized depending on the carbonisation degree. Activation takes place above 900~ with steam. Process variations as residence time in the kiln, the activation temperature, the type of kiln and other conditions, allow carbonised materials to develops small micro pores which are enlarged up to large micro pores or small meso pores. Activated carbons suitable for gas and air purification are micro porous. When the gas stream contains a low concentration of pollutants, lesser activated carbons with a large a m o u n t of small micro pores exhibiting a high adsorption capacity at low relative pressure are applied. These carbons are produced by steam activation in a rotary kiln after a relatively short residence time. In the case which the gas stream contains a high concentration of pollutants higher activated carbons are applied. These carbons show a higher adsorption capacity at high relative pressures t h a n lower activated carbons. At high concentrations the service time of a filter is relatively short. As a consequence the activated carbon has to be regenerated insitu,
809 otherwise the carbon consumption will be too high. Higher activated carbons have a greater proportion of larger micro pores and small meso pores. These larger pores easily desorb their adsorbate. High activated carbons are also produced by steam activation in a rotary kiln. Only the residence time is longer than for the production of carbons with small micro pores. With chemical activation[9] an activating chemical, normally phosphoric acid, is mixed with a young carbonaceous vegetable material, carbonised at about 500~ followed by recovery of the activation chemical by water washing. The activated carbons produced on this way have less micro pores and more meso pore compared to steam activated carbons and are suited for adsorption of larger molecules such as are present in decolorization steps in the chemical, pharmaceutical and food industries. However special types of chemical activated carbons are suited for a small part of gas phase applications with insitu regeneration. By the choice of the raw material and by modifications in the activation process, more small meso pores and large micro pores can be produced. In Figure 1 the benzene adsorption isotherm of steam activated carbons suitable for gas phase purification with insitu regeneration (SORBONORIT 3) and for gas purification on throw away basis (NORIT R 2030) are given. In this figure a chemical activated carbon for gas phase purification with insitu regeneration has also been involved (NORIT GF 45).The adsorption capacity is given as weight of adsorbate per unit volume of activated carbon since in most applications, adsorbents are compared on performance in an existing filter (fixed volume). IBenzene
o
adsorption
isotherms
I
........i ....iiitil .........tttitii'!t ........tti titil .........i .....iitifl .........i'.i~ ""ti "'_ i i i iiiiii i i i iiiiii i i i iiiiii i i i ii~ill ,.T ~
r
......... i..... iTTIiYYi ......... YYYIYiIIY ........
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i i i iiiiii : .~It] iiiiii i i i iiiiii i i ii iiili : : : :::::: .y.,V": :::::: : : : :::::: : : : :::::: ......... i ..... !"" .~"i'~. "i~ .~ .......... ~'" .~"i" .~'i'i'i ~. ........ .~'"" .~'"i'" .~'!" .~ ~. i.~ ......... i ..... ........ i...i..6.66661
0.00001
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0.001 rel. p r e s s u r e
0.01
i iiiiii
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i i i iiili : : : ::::: ......... i ..... i...6..i.6.i.i6
0.1
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(%)
1~ SORBONORIT 3 4,. NORIT R 2 0 3 0
-T. NORIT GF 45
Figure 1. Adsorption isotherm of different carbon types for gas phase applications.
810 With the help of the Kelvin equation the relative pressure can be converted into pores dimensions. Thus Figure 1 shows the indirect relationship between adsorption capacity and pore distributions of different carbon types. The following carbon physical forms are utilised dependent upon the application: - extrudated carbons - granular carbon - powdered carbons - fibres. Extrudates and granular carbons are mostly applied in fixed bed systems. The particle size is chosen dependent on the allowed pressure drop. Especially in gas phase recovery systems, activated carbon is exposed to large pressure differences for long times and is transported on a regular basis to sieve the carbon. In that case the hardness and attrition are important qualities Extrudates especially are extremely hard. Recently in contrast to fixed bed systems, powder injection systems have been applied in gas/air purification. In recovery systems with a large flow and a relatively low concentration loosely woven fibre systems are used. Activated carbons ignite at high temperature in air. The ignition depends of the activation process and the purity of the activated carbon and varies from 200~ for a chemical activated carbon to 500~ for steam activated carbon produced from peat without additives as potassium. In most gas/air applications activated carbons are used at low temperatures (up to 200~ In this case danger of ignition does not exist. Without modification, activated carbon can show chemical interaction with adsorbates or can be catalytically active. This chemical interaction and the catalytic activity can be desired or not. For example, in the recovery of ketones the catalytic activity is undesired. The chemical interaction and the catalytic activity are connected to the presence of functional groups and ash components. Steam activated carbons contain as a consequence of exposing to air after activation, a limited number of varying functional groups, which give the carbon basic qualities. Chemically activated carbons possess by virtue of the production process a much larger number of varying functional groups, which give the carbon acid qualities. The ash content is dictated by the used raw material and can be diminished by washing. For very small molecules, the physical adsorption capacity can be low. Thus for a gas stream with a mixture of very small and larger molecules the low adsorption capacity for the small molecules can dictate the performance of the filter to a large extent. In that case the activated carbon can be modified to increase the removal efficiency of the small molecules by chemisorption or catalytic conversion. For this reason activated carbons are modified.
811 4. GAS P H A S E A D S O R P T I O N ISOTHERMS 4.1. I n t r o d u c t i o n Gas phase adsorption isotherms describe the relationship between the relative pressure (or the concentration) of a component in the gas phase and the maximum loading capacity, that is the loading capacity at equilibrium. A great number of equations have been developed to describe the equilibrium adsorption. For activated carbon as adsorbent none of these equations describe the measured isotherm for all the concentration ranges. For gas phase adsorption the following equations are used in practice [10] - equations based on the theory of Dubinin - the Langmuir equation - the Freundlich equation - the Henry equation - the BET equation. For gas and air purification isotherms based on the equation of Dubinin yield good results and give the most possibilities to predict the adsorption capacity for different compounds and temperatures [11]. 4.2. The a d s o r p t i o n i s o t h e r m of D u b i n i n and R a d u s h k e v i c h The adsorption isotherms of Dubinin and Radushkevich are based on the potential theory of Polanyi and assume filling of the pore volume by means of liquefaction of the gas by physical adsorption. The equation has been modified by a large number of investigators. Investigations carried out by Van Soelen [11] show that these modified equations hardly show an improvement for predicting the adsorption capacity. The equation of Dubinin-Radushkevich, has the following form
in (Av)- ln(W. d ) - B.
0//nl /
9log
(1)
Av
Equilibrium adsorption in terms of weight per volume unit of activated carbon (g/cm 3) W and B Carbon constants d Density of the adsorbate(g/cm 3) T Temperature (K) b Affinity constant of the adsorbate p/po Relative pressure n Exponent, varies from 1 to 3 For n=2 the equation has been suited to micro porous activated carbons. J. Reussien [12] shows that the equation with n=l can be applied for the meso porous part of the pores structure. The carbon constants W and B can be calculated from the intercept and the slope by plotting ln(Av) against [T/b'log(p/p0)] n. To use the
812 above equations, the temperature must be below the critical temperature. For temperatures above the critical temperature an adjusted equation must be applied [12]. Activated carbon is used for the adsorption of a broad range of adsorbates. The advantage of the use of the theory of Dubinin and Radushkevich is the possibility to predicts the adsorption capacity of all kinds of adsorbates on a carbon type using the carbon constants W and B calculated from the adsorption isotherm of a standard adsorbate. The adsorption isotherm of other adsorbates can then be calculated by substituting the liquid density of the adsorbate at the adsorption temperature and the affinity constant. The affinity constant can be calculated from the parachor or from the surface tension as given in standard tables. For gas streams with more than 1 component the adsorption capacities of the combined components are calculated by combining the isotherms of the pure components [13].
5. T H E F I X E D B E D A D S O R P T I O N 5.1.
PROCESS
Introduction
In a fixed bed system a polluted gas stream is passed through a bed of activated carbon. After the start of the adsorption process the activated carbon at the inlet side is loaded. Only after a certain time does the inlet side of the bed reach equilibrium because adsorption in pores does not takes place directly and is subject to transport limitations. Within the bed a mass transfer zone develops (MTZ). After a certain time the MTZ boundary reaches the outlet side of the bed and the emission concentration increases up to the allowed value, when the adsorption process is stopped. For adsorbent - adsorbate systems with a convex adsorption isotherm the length of the MTZ is constant and independent of the bed height [14]. In Figure 2 the course of the MTZ through various bed heights is given. The adsorption capacity of a filter is determined by the equilibrium adsorption and the length of the MTZ. A large equilibrium adsorption capacity and a small MTZ means a long service time of the filter and a low carbon consumption. In the most gas phase applications the MTZ is relatively small, certainly at low relative humidity. Thus the adsorption capacity of a filter is largely determined by the equilibrium adsorption. NORIT has developed an empirical model to calculate service time and carbon consumption on the basis of a standard carbon analysis and process conditions. The carbon analysis used are: the adsorption isotherm of a standard adsorbate the particle size. The process conditions used are: temperature concentration -
flow
-
adsorbate qualities.
813
Mass Transfer zone in a carbon bed 1.2
oo ~
0.8
o~
0.6
start loading
8 N 0
end loading 0.4
0.2
0 0
0.2
0.4
0.6
0.8
1
1.2
bed height Figure 2. The MTZ as a function of the bed height.
5.2. Adsorption kinetics Adsorption of gas molecules does not takes place instantaneously. The t r a n s p o r t is limited by: - axial dispersion - external transport - internal transport I n t e r n a l t r a n s p o r t characteristics are affected by: - pore diffusion - K n u d s e n diffusion a limited adsorption velocity surface diffusion. D e p e n d i n g of the process conditions, the type of a d s o r b a t e a n d the adsorbent qualities one or more steps are dominant. For exact d e t e r m i n a t i o n of the MTZ a set of differential equations h a v e to be solved. To e s t i m a t e the MTZ an empirical equation can be used which is derived from m e a s u r e d d a t a w i t h i n the m a t r i x of process conditions, which exist in practical situations. For activated carbon systems u n d e r relative dry conditions this equation is:
814 MTZ : e S T . (F)"0"054 9(Ci)0"133 9(Dp)1"549 9log Ci-Co Co with MTZ Mass transfer zone (cm) constant CST F Flow (cma/min) inlet concentration (g/cm 3) Ci outlet concentration (g/cm 3) Co particle diameter (cm) Dp
(2)
In solvent recovery applications when the activated carbon is wet after regeneration and cannot be dried during adsorption, the MTZ is 3 times larger.
5.3. The service time and the c a r b o n c o n s u m p t i o n Figure 2 shows that the MTZ curve is symmetrical. Thus half of the MTZ part of the bed can be considered to be in equilibrium with the inlet concentration and half of the MTZ part of the bed can be considered to be completely empty. For a fixed bed with an cross section area S (cm2)and a bed height L(cm) the dynamic loading Aa (g) A d - A v ( L - - - M T Z / $ 2"
(3)
At a flow F and an inlet concentration Ci the service time tb(min) of a bed with volume L'S is equal to tb-
Ad Ci.F
(4)
The carbon consumption CS (cm3/min) is CS- Ci'F'L'S Ad
(5)
5.4. F i x e d bed in insitu r e g e n e r a t i o n s y s t e m s 5.4.1. I n t r o d u c t i o n Insitu regeneration can be applied to gas streams with a high component concentration. In this case, the carbon consumption is too high for throw away basis operation. In insitu-regeneration systems the carbon is loaded in the same way as in fixed bed adsorption system up to break through. After break through the adsorbate is desorbed. Desorption may be followed by a (partly) drying step. During insitu regeneration the adsorbate is desorbed by pressure swing or temperature swing action. With pressure swing, desorption takes place at lower pressure than is present during adsorption. Pressure swing is applied for gas
815 separation. In t e m p e r a t u r e swing, desorption takes place at a higher t e m p e r a t u r e t h a n is present during adsorption.Temperature swing is mostly applied for solvent recovery. S t e a m or inert gas such as nitrogen is used as carrier gas. The benefit of the use of s t e a m is t h a t the installation, including the activated carbon bed,is w a r m e d up very quickly. Inert gas regeneration is applied for components which desorb at relatively high t e m p e r a t u r e and for components which decompose by oxidation with the activated carbon acting as a catalyst. In the last case steam activation is only possible by the use of activated carbon with a low catalytic activity such as the SORBONORIT K4. The motive to recover solvents can be the value of the recovered solvent. Recovery of solvents can also be a method to fulfill emission requirements. In some cases, recovery of a mixture of solvents can be used to effectively concentrate emissions to allow incineration.
5.4.2. The c a l c u l a t i o n of the s t e a m c o n s u m p t i o n With s t e a m regeneration, the solvent is recovered at high t e m p e r a t u r e with steam as the carrier gas. The rest loading on the carbon Ar as a function of the steam consumption is m e a s u r e d to determine the universal steam curve. For calculation of the adsorption capacity of an adsorbent in solvent recovery the effective loading Aef is an i m p o r t a n t factor. The effective loading is the difference between the dynamic loading Ad and the rest loading on the carbon after regeneration. In Figure 3 a steam curve has been given. Figure 3 shows that the recovery of the same a m o u n t of solvent costs much more in steam when starting from a lower dynamic
I
5
Steam curve
i
...........................................................................................................................................................................................................................
••••••••••••
5
0 0
200
! 400 steam
Figure 3. The steam curve.
I 600 volume
800
1000
816 loading t h a n starting from a higher dynamic loading. In most cases, desorption is stopped before all the adsorbates have been desorbed. By using activated carbons with special pore distribution, desorption can be made more effective.
5.5. F i x e d b e d i n s t a l l a t i o n s In designing an installation the m a x i m u m linear velocity (cm/sec) should be about 100 times the particle diameter, thus preventing fluidisation. The m i n i m u m linear velocity is a few cm/sec preventing axial dispersion. Thus the cross section of an installation is mainly determined by these conditions. The bedheight is mainly determined by the desired service time and the allowed pressure drop. Absorbers with a small bed height are applied for the removal of low concentrations of pollutants (< 1 rag/m3). The bed height is normally about 2 to 5 cm. The contact time is the order of 0.05 to 0.2 sec. Examples of this kind of filter are cylinders and thin rectangle boxes, divided into compartments. In this type of application carbon bounded in sheets such as N O R I T H E N E can be applied. I m p o r t a n t applications for these kind of filters are: - air conditioning - concentration peak smoothing. For higher concentrations (1 mg/m 3 up to 1 g/m 3) larger absorbers with a bed height of 25 up to 50 cm are used. The contact time in this kind of filter is about 0.2 to 2 seconds. Examples of this type of filter are simple steel drums provided with an inlet, an outlet and a base (aeropure filter), rectangular carbon absorbers and vertical as horizontal cylindrical absorbers. Important applications for these kind of filters are: - emission prevention in the chemical and food industries - paint spray installations - sewage air purification. For still higher concentrations (1 up to 50 g/m 3) recovery installations are applied with a bed height of 50 up to 150 cm. The contact time is about 2-4 seconds. Such recovery installations are of m i n i m u m 2 absorbers, one in loading and one in regeneration. Most installations comprise of a large number of absorbers. I m p o r t a n t applications of these kind of filters are: - solvent recovery in printing industry - dry cleaning. 6. I N S T A L L A T I O N S WITH P O W D E R S In a recent development air pollution control systems with injection of powder carbon in the gas stream can be applied. To keep a system in equilibrium, sufficient carbon m u s t be dosed t h a t emission concentration is in equilibrium with the equilibrium adsorption of the carbon. So in gas streams with a pollutant inlet concentration Ci and an emission concentration Co, Z g/cm 3 has to be adsorbed on to the activated carbon.
817 Z = C i - Co At the emission concentration of Co the equilibrium adsorption of the activated carbon Av can be calculated on basis of the theory of Dubinin as shown in 4.2. The carbon consumption CS is CS -
Z.F
(6)
Av The m a x i m u m loading of an activated carbon in such a system may be low compared to the m a x i m u m loading of an activated carbon in a fixed bed system, but by the use of very small particles, the kinetic effect is much faster, an a d v a n t a g e in processes determined by kinetics. Powder injection is applied in gas streams with a high debit and it can be built into a purification train. The system can be relatively cheap compared to fixed bed systems and is flexible concerning carbon dosage. Powder injection systems are used on a large scale at the purification of the flue gas of waste incineration plants [15]. In this way dioxins, dibenzofurans and heavy metals are removed from the flue gas. Typical dosing rates are 50 up to 200 g/m 3. Recently, impregnated powders have also been applied for special applications such as the removal of high concentrations of mercury. Powdered activated carbons have been tested excessively on explosion risks and are considered safe for flue gas conditions.
7. MODIFIED ACTIVATED CARBON The physical adsorption capacity for very small molecules can be low. Thus for a gas stream with a mixture of very small and larger molecules, the low adsorption capacity for the small molecules can dictate the performance of the filter to large extent. In this case the activated carbon m a y be modified to increase the removal efficiency of the small molecules by chemisorption or catalytic conversion. All possible impregnations with metal salts and with organic molecules as well as the modification of the functional groups are mentioned in the literature. To reduce air pollution only a few types of impregnations and modifications are of commercial interest. These i m p r e g n a n t s and the application are given in Table 1.
818 Table 1 Impregnation/modification commercial available activated carbons Component
Impregnant
Application
H2S, methyl mercaptan
- KI - Fe(OH)3 - complexes of transition metals (i.e:Cu,Cr) KOH
sewage air chemical industry
KOH - Na(OH) - KeCO3
sewage air chemical industry air conditioning
- CuO ZnSO4
chemical industry
Hg
-S -KI
purification methane prod. of batteries waste incineration
COS
complexes of transition metals (i.e:Cu,Cr)
chemical industry
HCN, C1CN
complexes of Cu,Cr,Zn and TEDA
gasmasks chemical industry
-
SO2
-
NH3
-
ASH3, PH3
Cu and Cr complexes
radioactive iodide
R
E
F
E
R
E
N
C
E
- TEDA -KI
chemical industry nuclear power plants
S
1. TNO (IMET), Alternatieve Adsorbentia voor het reinigen van koolwaterstoffen bevattende luchtstromen, (1991) (Dutch). 2. Y. Cohen (ed.), Novel adsorbents and their environmental applications, American Institute of Chemical Engineers, (1995). 3. Carbon containing molecular sieves, US Patent No. 3801513 (1971/1974). 4. Carbon containing molecular sieves, US Patent No. 3979330 (1974/1976). 5. Kohlenstoffhaltige Molekularsiebe, German Offenlegungschrift 2305435 (1973/1974). 6. Verfahren zur Gewinning von Stickstoffreichen Gasen aus neber Ne wenigstens 02 enthaltenden Gasen, wie zB. Luft, German Offenlegungsschrift 2441447 (1974/1976).
819 7. Kohlenstoffhaltige Adsorptionsmittel mit einstellbarem unterschiedlichen Porensystem, German Offenlegungsschrift 2624663 (1976/1977). 8. T. Wigmans, Fundamentals and practical implications of activated carbon production by partial gasification of carbonaceous material, NATO ASI series E 105, 559. 9. A. Cameron and J.D. MacDowall, The pore structure of wood based activated carbons, from: Principles and applications of pore structural characterization. Proc. R.I.L.E.M./C.N.R. Symp., Milan, Italy, (1983). 10. R.C. Bansal, J.P. Donnet and F. Stoeckli, Active carbon, New York and Basel
(~988). 1 I. A.C.D. van Soelen, Gas-fysisorptie-isothermen van aktieve kool. RU Utrecht (1991). 12. J.G.J. Reussien, De standaard-benzeenadsorptieisotherm aan actieve kool als een basisgegeven voor de berekening van terugwinnings- en luchtzuiveringsinstallaties, NORIT N.V., (1973). 13. D.M. Ruthven, Principles of adsorption and adsorption processes, New York, (1984). 14. Kel'tsev, Translation chapter 6 and 8 (Dutch), (1984). 15. B.v.d. Akker, D.v.d. Kleut and W.M.T.M. Reimerink, 16th Symposium on Chlorinated dioxins and related compounds, DIOXIN 96, Amsterdam (1996).
807
Air pollution control by adsorption
W.M.T.M. Reimerink, D. v.d. Kleut NORIT Nederland B.V., P.O. Box 105, 3800 AC Amersfoort, The Netherlands 1. I N T R O D U C T I O N The pressure on industry to decrease the emission of pollutants to the air is increasing. The importance to industry is to keep the costs as low as possible. A broad spectrum of techniques is available and is developed to control air pollution. The choice of a technique is determined by the type of pollution and the process conditions. In relation to price/performance, physical adsorption is one of the most important techniques to control air pollution. Both organic and inorganic molecules can be removed from a gas stream by physical adsorption. However, the adsorption affinity increases as the molecules become larger. As a consequence the adsorption capacity of an adsorbent is higher for large molecules t h a n for small molecules. For this reason physical adsorption is extremely suited for adsorption of organic compounds from gas, air, water and liquid streams. 2. A D S O R B E N T S In actual practice only the following adsorbents are applied: Activated carbon Carbon molecular sieves Polymers Silica Alumina - Zeolites Activated carbon can adsorb a broad range of pollutants with varying dimensions by its broad pore distribution of micro- and small meso pores. Activated carbon can adsorb a large a m o u n t of pollutants due to its large pore volume. Due to its hydrophobic character adsorption takes place at high relative humidity. The usability of activated carbon for air pollution control is limited by the risk of ignition at high temperature. Much research has been done to develop polymer adsorbents.In the market polymers as polyad [1,2] are applied for adsorption of high boiling compounds. At -
808 high relative h u m i d i t y these adsorbents have a tendency to swell. The polymers cannot be used at high t e m p e r a t u r e s due to deformation. A great disadvantage of polymer adsorbents is the low adsorption capacity on a volume basis. Alumina and silica [1,2] are meso porous and are not suited for adsorption of small organic molecules by physical adsorption. Unmodified silica and a l u m i n a are hydrophilic, so a high relative humidity disturbs the adsorption of organic pollutants to a large extent. Silica is suitable as adsorbent of water by chemisorption. Zeolites [1,2] and carbon molecular sieves [3-7] have a narrow pore distribution. The pore distribution determines which adsorbate adsorbs well and which adsorbate adsorbs to a less extent. These adsorbents can be applied in purifications of well defined gas s t r e a m s such as are present in gas separation applications. Compared with activated carbon the usability of these adsorbents is limited. Zeolites can be made hydrophobic by increasing the A1 content. These hydrophobic zeolites are suited to purify gas s t r e a m s at high relative h u m i d i t y and high t e m p e r a t u r e . Of the above mentioned adsorbents activated carbon is the most convenient for air pollution control for a broad range of compounds and for a large variation in process conditions. Compared to carbon molecular sieves and hydrophobic zeolites, activated carbon is a relatively cheap adsorbent. 3. A C T I V A T E D C A R B O N Activated carbons are micro porous carbonaceous materials. The activated carbons available in the m a r k e t differ in pore distribution, in form and in chemical composition. To decrease the emission an optimal carbon and system should be chosen dependent of the kind of molecules to be removed and the process conditions. The differences between activated carbons types are a consequence of the choice of activation process, the activation conditions and to some extend the choice of raw material. Activated carbons are produced from raw materials such as peat, wood, lignite, anthracite, fruit pits and shells. The raw materials are converted in activated carbon by steam or chemical activation With steam activation [8] the raw material is carbonised and/or oxidized depending on the carbonisation degree. Activation takes place above 900~ with steam. Process variations as residence time in the kiln, the activation temperature, the type of kiln and other conditions, allow carbonised materials to develops small micro pores which are enlarged up to large micro pores or small meso pores. Activated carbons suitable for gas and air purification are micro porous. When the gas stream contains a low concentration of pollutants, lesser activated carbons with a large a m o u n t of small micro pores exhibiting a high adsorption capacity at low relative pressure are applied. These carbons are produced by steam activation in a rotary kiln after a relatively short residence time. In the case which the gas stream contains a high concentration of pollutants higher activated carbons are applied. These carbons show a higher adsorption capacity at high relative pressures t h a n lower activated carbons. At high concentrations the service time of a filter is relatively short. As a consequence the activated carbon has to be regenerated insitu,
809 otherwise the carbon consumption will be too high. Higher activated carbons have a greater proportion of larger micro pores and small meso pores. These larger pores easily desorb their adsorbate. High activated carbons are also produced by steam activation in a rotary kiln. Only the residence time is longer than for the production of carbons with small micro pores. With chemical activation[9] an activating chemical, normally phosphoric acid, is mixed with a young carbonaceous vegetable material, carbonised at about 500~ followed by recovery of the activation chemical by water washing. The activated carbons produced on this way have less micro pores and more meso pore compared to steam activated carbons and are suited for adsorption of larger molecules such as are present in decolorization steps in the chemical, pharmaceutical and food industries. However special types of chemical activated carbons are suited for a small part of gas phase applications with insitu regeneration. By the choice of the raw material and by modifications in the activation process, more small meso pores and large micro pores can be produced. In Figure 1 the benzene adsorption isotherm of steam activated carbons suitable for gas phase purification with insitu regeneration (SORBONORIT 3) and for gas purification on throw away basis (NORIT R 2030) are given. In this figure a chemical activated carbon for gas phase purification with insitu regeneration has also been involved (NORIT GF 45).The adsorption capacity is given as weight of adsorbate per unit volume of activated carbon since in most applications, adsorbents are compared on performance in an existing filter (fixed volume). IBenzene
o
adsorption
isotherms
I
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0.00001
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0.01
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0.1
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(%)
1~ SORBONORIT 3 4,. NORIT R 2 0 3 0
-T. NORIT GF 45
Figure 1. Adsorption isotherm of different carbon types for gas phase applications.
810 With the help of the Kelvin equation the relative pressure can be converted into pores dimensions. Thus Figure 1 shows the indirect relationship between adsorption capacity and pore distributions of different carbon types. The following carbon physical forms are utilised dependent upon the application: - extrudated carbons - granular carbon - powdered carbons - fibres. Extrudates and granular carbons are mostly applied in fixed bed systems. The particle size is chosen dependent on the allowed pressure drop. Especially in gas phase recovery systems, activated carbon is exposed to large pressure differences for long times and is transported on a regular basis to sieve the carbon. In that case the hardness and attrition are important qualities Extrudates especially are extremely hard. Recently in contrast to fixed bed systems, powder injection systems have been applied in gas/air purification. In recovery systems with a large flow and a relatively low concentration loosely woven fibre systems are used. Activated carbons ignite at high temperature in air. The ignition depends of the activation process and the purity of the activated carbon and varies from 200~ for a chemical activated carbon to 500~ for steam activated carbon produced from peat without additives as potassium. In most gas/air applications activated carbons are used at low temperatures (up to 200~ In this case danger of ignition does not exist. Without modification, activated carbon can show chemical interaction with adsorbates or can be catalytically active. This chemical interaction and the catalytic activity can be desired or not. For example, in the recovery of ketones the catalytic activity is undesired. The chemical interaction and the catalytic activity are connected to the presence of functional groups and ash components. Steam activated carbons contain as a consequence of exposing to air after activation, a limited number of varying functional groups, which give the carbon basic qualities. Chemically activated carbons possess by virtue of the production process a much larger number of varying functional groups, which give the carbon acid qualities. The ash content is dictated by the used raw material and can be diminished by washing. For very small molecules, the physical adsorption capacity can be low. Thus for a gas stream with a mixture of very small and larger molecules the low adsorption capacity for the small molecules can dictate the performance of the filter to a large extent. In that case the activated carbon can be modified to increase the removal efficiency of the small molecules by chemisorption or catalytic conversion. For this reason activated carbons are modified.
811 4. GAS P H A S E A D S O R P T I O N ISOTHERMS 4.1. I n t r o d u c t i o n Gas phase adsorption isotherms describe the relationship between the relative pressure (or the concentration) of a component in the gas phase and the maximum loading capacity, that is the loading capacity at equilibrium. A great number of equations have been developed to describe the equilibrium adsorption. For activated carbon as adsorbent none of these equations describe the measured isotherm for all the concentration ranges. For gas phase adsorption the following equations are used in practice [10] - equations based on the theory of Dubinin - the Langmuir equation - the Freundlich equation - the Henry equation - the BET equation. For gas and air purification isotherms based on the equation of Dubinin yield good results and give the most possibilities to predict the adsorption capacity for different compounds and temperatures [11]. 4.2. The a d s o r p t i o n i s o t h e r m of D u b i n i n and R a d u s h k e v i c h The adsorption isotherms of Dubinin and Radushkevich are based on the potential theory of Polanyi and assume filling of the pore volume by means of liquefaction of the gas by physical adsorption. The equation has been modified by a large number of investigators. Investigations carried out by Van Soelen [11] show that these modified equations hardly show an improvement for predicting the adsorption capacity. The equation of Dubinin-Radushkevich, has the following form
in (Av)- ln(W. d ) - B.
0//nl /
9log
(1)
Av
Equilibrium adsorption in terms of weight per volume unit of activated carbon (g/cm 3) W and B Carbon constants d Density of the adsorbate(g/cm 3) T Temperature (K) b Affinity constant of the adsorbate p/po Relative pressure n Exponent, varies from 1 to 3 For n=2 the equation has been suited to micro porous activated carbons. J. Reussien [12] shows that the equation with n=l can be applied for the meso porous part of the pores structure. The carbon constants W and B can be calculated from the intercept and the slope by plotting ln(Av) against [T/b'log(p/p0)] n. To use the
812 above equations, the temperature must be below the critical temperature. For temperatures above the critical temperature an adjusted equation must be applied [12]. Activated carbon is used for the adsorption of a broad range of adsorbates. The advantage of the use of the theory of Dubinin and Radushkevich is the possibility to predicts the adsorption capacity of all kinds of adsorbates on a carbon type using the carbon constants W and B calculated from the adsorption isotherm of a standard adsorbate. The adsorption isotherm of other adsorbates can then be calculated by substituting the liquid density of the adsorbate at the adsorption temperature and the affinity constant. The affinity constant can be calculated from the parachor or from the surface tension as given in standard tables. For gas streams with more than 1 component the adsorption capacities of the combined components are calculated by combining the isotherms of the pure components [13].
5. T H E F I X E D B E D A D S O R P T I O N 5.1.
PROCESS
Introduction
In a fixed bed system a polluted gas stream is passed through a bed of activated carbon. After the start of the adsorption process the activated carbon at the inlet side is loaded. Only after a certain time does the inlet side of the bed reach equilibrium because adsorption in pores does not takes place directly and is subject to transport limitations. Within the bed a mass transfer zone develops (MTZ). After a certain time the MTZ boundary reaches the outlet side of the bed and the emission concentration increases up to the allowed value, when the adsorption process is stopped. For adsorbent - adsorbate systems with a convex adsorption isotherm the length of the MTZ is constant and independent of the bed height [14]. In Figure 2 the course of the MTZ through various bed heights is given. The adsorption capacity of a filter is determined by the equilibrium adsorption and the length of the MTZ. A large equilibrium adsorption capacity and a small MTZ means a long service time of the filter and a low carbon consumption. In the most gas phase applications the MTZ is relatively small, certainly at low relative humidity. Thus the adsorption capacity of a filter is largely determined by the equilibrium adsorption. NORIT has developed an empirical model to calculate service time and carbon consumption on the basis of a standard carbon analysis and process conditions. The carbon analysis used are: the adsorption isotherm of a standard adsorbate the particle size. The process conditions used are: temperature concentration -
flow
-
adsorbate qualities.
813
Mass Transfer zone in a carbon bed 1.2
oo ~
0.8
o~
0.6
start loading
8 N 0
end loading 0.4
0.2
0 0
0.2
0.4
0.6
0.8
1
1.2
bed height Figure 2. The MTZ as a function of the bed height.
5.2. Adsorption kinetics Adsorption of gas molecules does not takes place instantaneously. The t r a n s p o r t is limited by: - axial dispersion - external transport - internal transport I n t e r n a l t r a n s p o r t characteristics are affected by: - pore diffusion - K n u d s e n diffusion a limited adsorption velocity surface diffusion. D e p e n d i n g of the process conditions, the type of a d s o r b a t e a n d the adsorbent qualities one or more steps are dominant. For exact d e t e r m i n a t i o n of the MTZ a set of differential equations h a v e to be solved. To e s t i m a t e the MTZ an empirical equation can be used which is derived from m e a s u r e d d a t a w i t h i n the m a t r i x of process conditions, which exist in practical situations. For activated carbon systems u n d e r relative dry conditions this equation is:
814 MTZ : e S T . (F)"0"054 9(Ci)0"133 9(Dp)1"549 9log Ci-Co Co with MTZ Mass transfer zone (cm) constant CST F Flow (cma/min) inlet concentration (g/cm 3) Ci outlet concentration (g/cm 3) Co particle diameter (cm) Dp
(2)
In solvent recovery applications when the activated carbon is wet after regeneration and cannot be dried during adsorption, the MTZ is 3 times larger.
5.3. The service time and the c a r b o n c o n s u m p t i o n Figure 2 shows that the MTZ curve is symmetrical. Thus half of the MTZ part of the bed can be considered to be in equilibrium with the inlet concentration and half of the MTZ part of the bed can be considered to be completely empty. For a fixed bed with an cross section area S (cm2)and a bed height L(cm) the dynamic loading Aa (g) A d - A v ( L - - - M T Z / $ 2"
(3)
At a flow F and an inlet concentration Ci the service time tb(min) of a bed with volume L'S is equal to tb-
Ad Ci.F
(4)
The carbon consumption CS (cm3/min) is CS- Ci'F'L'S Ad
(5)
5.4. F i x e d bed in insitu r e g e n e r a t i o n s y s t e m s 5.4.1. I n t r o d u c t i o n Insitu regeneration can be applied to gas streams with a high component concentration. In this case, the carbon consumption is too high for throw away basis operation. In insitu-regeneration systems the carbon is loaded in the same way as in fixed bed adsorption system up to break through. After break through the adsorbate is desorbed. Desorption may be followed by a (partly) drying step. During insitu regeneration the adsorbate is desorbed by pressure swing or temperature swing action. With pressure swing, desorption takes place at lower pressure than is present during adsorption. Pressure swing is applied for gas
815 separation. In t e m p e r a t u r e swing, desorption takes place at a higher t e m p e r a t u r e t h a n is present during adsorption.Temperature swing is mostly applied for solvent recovery. S t e a m or inert gas such as nitrogen is used as carrier gas. The benefit of the use of s t e a m is t h a t the installation, including the activated carbon bed,is w a r m e d up very quickly. Inert gas regeneration is applied for components which desorb at relatively high t e m p e r a t u r e and for components which decompose by oxidation with the activated carbon acting as a catalyst. In the last case steam activation is only possible by the use of activated carbon with a low catalytic activity such as the SORBONORIT K4. The motive to recover solvents can be the value of the recovered solvent. Recovery of solvents can also be a method to fulfill emission requirements. In some cases, recovery of a mixture of solvents can be used to effectively concentrate emissions to allow incineration.
5.4.2. The c a l c u l a t i o n of the s t e a m c o n s u m p t i o n With s t e a m regeneration, the solvent is recovered at high t e m p e r a t u r e with steam as the carrier gas. The rest loading on the carbon Ar as a function of the steam consumption is m e a s u r e d to determine the universal steam curve. For calculation of the adsorption capacity of an adsorbent in solvent recovery the effective loading Aef is an i m p o r t a n t factor. The effective loading is the difference between the dynamic loading Ad and the rest loading on the carbon after regeneration. In Figure 3 a steam curve has been given. Figure 3 shows that the recovery of the same a m o u n t of solvent costs much more in steam when starting from a lower dynamic
I
5
Steam curve
i
...........................................................................................................................................................................................................................
••••••••••••
5
0 0
200
! 400 steam
Figure 3. The steam curve.
I 600 volume
800
1000
816 loading t h a n starting from a higher dynamic loading. In most cases, desorption is stopped before all the adsorbates have been desorbed. By using activated carbons with special pore distribution, desorption can be made more effective.
5.5. F i x e d b e d i n s t a l l a t i o n s In designing an installation the m a x i m u m linear velocity (cm/sec) should be about 100 times the particle diameter, thus preventing fluidisation. The m i n i m u m linear velocity is a few cm/sec preventing axial dispersion. Thus the cross section of an installation is mainly determined by these conditions. The bedheight is mainly determined by the desired service time and the allowed pressure drop. Absorbers with a small bed height are applied for the removal of low concentrations of pollutants (< 1 rag/m3). The bed height is normally about 2 to 5 cm. The contact time is the order of 0.05 to 0.2 sec. Examples of this kind of filter are cylinders and thin rectangle boxes, divided into compartments. In this type of application carbon bounded in sheets such as N O R I T H E N E can be applied. I m p o r t a n t applications for these kind of filters are: - air conditioning - concentration peak smoothing. For higher concentrations (1 mg/m 3 up to 1 g/m 3) larger absorbers with a bed height of 25 up to 50 cm are used. The contact time in this kind of filter is about 0.2 to 2 seconds. Examples of this type of filter are simple steel drums provided with an inlet, an outlet and a base (aeropure filter), rectangular carbon absorbers and vertical as horizontal cylindrical absorbers. Important applications for these kind of filters are: - emission prevention in the chemical and food industries - paint spray installations - sewage air purification. For still higher concentrations (1 up to 50 g/m 3) recovery installations are applied with a bed height of 50 up to 150 cm. The contact time is about 2-4 seconds. Such recovery installations are of m i n i m u m 2 absorbers, one in loading and one in regeneration. Most installations comprise of a large number of absorbers. I m p o r t a n t applications of these kind of filters are: - solvent recovery in printing industry - dry cleaning. 6. I N S T A L L A T I O N S WITH P O W D E R S In a recent development air pollution control systems with injection of powder carbon in the gas stream can be applied. To keep a system in equilibrium, sufficient carbon m u s t be dosed t h a t emission concentration is in equilibrium with the equilibrium adsorption of the carbon. So in gas streams with a pollutant inlet concentration Ci and an emission concentration Co, Z g/cm 3 has to be adsorbed on to the activated carbon.
817 Z = C i - Co At the emission concentration of Co the equilibrium adsorption of the activated carbon Av can be calculated on basis of the theory of Dubinin as shown in 4.2. The carbon consumption CS is CS -
Z.F
(6)
Av The m a x i m u m loading of an activated carbon in such a system may be low compared to the m a x i m u m loading of an activated carbon in a fixed bed system, but by the use of very small particles, the kinetic effect is much faster, an a d v a n t a g e in processes determined by kinetics. Powder injection is applied in gas streams with a high debit and it can be built into a purification train. The system can be relatively cheap compared to fixed bed systems and is flexible concerning carbon dosage. Powder injection systems are used on a large scale at the purification of the flue gas of waste incineration plants [15]. In this way dioxins, dibenzofurans and heavy metals are removed from the flue gas. Typical dosing rates are 50 up to 200 g/m 3. Recently, impregnated powders have also been applied for special applications such as the removal of high concentrations of mercury. Powdered activated carbons have been tested excessively on explosion risks and are considered safe for flue gas conditions.
7. MODIFIED ACTIVATED CARBON The physical adsorption capacity for very small molecules can be low. Thus for a gas stream with a mixture of very small and larger molecules, the low adsorption capacity for the small molecules can dictate the performance of the filter to large extent. In this case the activated carbon m a y be modified to increase the removal efficiency of the small molecules by chemisorption or catalytic conversion. All possible impregnations with metal salts and with organic molecules as well as the modification of the functional groups are mentioned in the literature. To reduce air pollution only a few types of impregnations and modifications are of commercial interest. These i m p r e g n a n t s and the application are given in Table 1.
818 Table 1 Impregnation/modification commercial available activated carbons Component
Impregnant
Application
H2S, methyl mercaptan
- KI - Fe(OH)3 - complexes of transition metals (i.e:Cu,Cr) KOH
sewage air chemical industry
KOH - Na(OH) - KeCO3
sewage air chemical industry air conditioning
- CuO ZnSO4
chemical industry
Hg
-S -KI
purification methane prod. of batteries waste incineration
COS
complexes of transition metals (i.e:Cu,Cr)
chemical industry
HCN, C1CN
complexes of Cu,Cr,Zn and TEDA
gasmasks chemical industry
-
SO2
-
NH3
-
ASH3, PH3
Cu and Cr complexes
radioactive iodide
R
E
F
E
R
E
N
C
E
- TEDA -KI
chemical industry nuclear power plants
S
1. TNO (IMET), Alternatieve Adsorbentia voor het reinigen van koolwaterstoffen bevattende luchtstromen, (1991) (Dutch). 2. Y. Cohen (ed.), Novel adsorbents and their environmental applications, American Institute of Chemical Engineers, (1995). 3. Carbon containing molecular sieves, US Patent No. 3801513 (1971/1974). 4. Carbon containing molecular sieves, US Patent No. 3979330 (1974/1976). 5. Kohlenstoffhaltige Molekularsiebe, German Offenlegungschrift 2305435 (1973/1974). 6. Verfahren zur Gewinning von Stickstoffreichen Gasen aus neber Ne wenigstens 02 enthaltenden Gasen, wie zB. Luft, German Offenlegungsschrift 2441447 (1974/1976).
819 7. Kohlenstoffhaltige Adsorptionsmittel mit einstellbarem unterschiedlichen Porensystem, German Offenlegungsschrift 2624663 (1976/1977). 8. T. Wigmans, Fundamentals and practical implications of activated carbon production by partial gasification of carbonaceous material, NATO ASI series E 105, 559. 9. A. Cameron and J.D. MacDowall, The pore structure of wood based activated carbons, from: Principles and applications of pore structural characterization. Proc. R.I.L.E.M./C.N.R. Symp., Milan, Italy, (1983). 10. R.C. Bansal, J.P. Donnet and F. Stoeckli, Active carbon, New York and Basel
(~988). 1 I. A.C.D. van Soelen, Gas-fysisorptie-isothermen van aktieve kool. RU Utrecht (1991). 12. J.G.J. Reussien, De standaard-benzeenadsorptieisotherm aan actieve kool als een basisgegeven voor de berekening van terugwinnings- en luchtzuiveringsinstallaties, NORIT N.V., (1973). 13. D.M. Ruthven, Principles of adsorption and adsorption processes, New York, (1984). 14. Kel'tsev, Translation chapter 6 and 8 (Dutch), (1984). 15. B.v.d. Akker, D.v.d. Kleut and W.M.T.M. Reimerink, 16th Symposium on Chlorinated dioxins and related compounds, DIOXIN 96, Amsterdam (1996).