Znd. Eng. Chem. Res. 1995,34, 1630-1640
1630
Control of Volatile Organic Compounds by the Catalytic Reverse Process? Yurii Sh. Matros” Department of Chemical Engineering, Washington University, St. Louis, Missouri 63130
Grigori A. Bunimovich Matros Technologies, 2080 Concourse Drive, St. Louis, Missouri 63146-4119
The catalytic reverse process employs a fixed catalyst bed placed between two beds of noncatalytic material. The gas flow direction in such a “sandwich” is periodically switched from one end to the other. Using a mathematical model of this unsteady-state process, this work discusses the efficiency of the catalytic reverse process for the purification of industrial off-gases containing various volatile organic compounds. A method of process economics optimization is considered. Advantages of the catalytic reverse process in comparison with the traditional steady-state catalytic and regenerative incineration processes of volatile organic compound control are discussed. Potential application of the reverse process and first steps made in this direction by industry are discussed.
Introduction The catalytic reverse process (RP) emerged in the mid-1970s (Matros, 1977; Boreskov et al., 1977) as a new type of catalytic process featuring periodic reversal of the gas flow direction through a packed bed of catalyst. The original process arrangement (Matros, 1977) includes a reactor with one catalyst bed and a switching valve for gas flow reversal. The catalyst bed is used both in a traditional way, for acceleration of a chemical reaction, and as a regenerative heat exchanger. The reverse process technology is accomplished as follows. Prior to start-up, the catalyst bed is preheated to the initial temperature (i.e., 250-350 “C)at which the catalytic reaction proceeds at a reasonable rate. Inlet temperature of the gas mixture, Ti,,is usually low. Part of the bed close to the gas inlet cools down as it heats up the incoming gas. In the central part of the catalyst bed there is intensive heat release due to the chemical conversion of the reactants. As a result of periodical reversals of the gas flow, the outlet (in the previous semicycle) part of the bed cools down. The former inlet part of the bed begins to warm due to hot gases coming from the central part of the bed. After several flow reversals, repeating temperature and concentration fields are established. Since the reaction rate over the cold bed ends is usually very low, replacement of catalyst by an inexpensive inert material is rational. This material acts as regenerative heat exchanger along with the catalyst bed. A flow diagram of the unit is shown in Figure la. According to the literature, RP is among the most economical and least complex processes for the purification of volatile organic compounds (VOC) from industrial flue gases and air. It is equally successful for carrying out the oxidation of SO2 to so3 for sulfuric acid
* Author to whom correspondence should be addressed. Present address: Matros Technologies, 2080 Concourse Drive, St. Louis, MO, 63146-4119. Fax: (314)462-6226. ’Paper presented a t the Symposium on Catalytic Reaction Engineering for Environmentally Benign Processes, San Diego ACS Meeting, March 13-18, 1994. [Other papers from this symposium appear in the December 1994 issue of Industrial & Engineering Chemistry Research (Znd. Eng. Chem. Res. 1994,33(121, 2885-3069).] 0888-588519512634-1630$09.00/0
production, synthesis of ammonia and methanol, reduction of NO,, and other catalytic processes (Boreskov and Matros, 1983a,b; Matros, 1985, 1989; Matros et al., 1991). The aim of this article is to discuss the fundamental basis, practical features, and advantages of RP for catalytic decontamination of VOC-laden gases. Substantial attention will be paid to comparison of RP t o alternatives such as regenerative thermal oxidation (or incineration) system (RTO),and recuperative catalytic oxidation system (RC). In RTO, noncatalytic homogeneous combustion of VOCs is carried out at high temperatures. The reactor configuration for RTO includes the beds of ceramic packing where the heat is captured, stored, and reused. The direction of flow in these beds is periodically reversed. At low VOC concentrations a burner is installed in the space between the ceramic beds. In this space, a so-called combustion or retention chamber, the major fraction of VOCs is oxidized. The burner is used t o preheat the system a t start-up and t o keep the temperature inside the combustion chamber sufficiently high. Often RTO systems include more than two ceramic packed beds (Figure lb). Each bed is periodically purged by air to remove unreacted VOCs collected in the low-temperature zone of oxidizer and ceramics during part of the cycle. The same purge system can be installed for catalytic regenerative oxidizer too. Recuperative catalytic systems (RC) include the catalyst bed and shell-and-tubeheat exchanger (Figure IC). Temperature at the catalyst bed inlet is maintained sufficiently high usually by means of a burner, and the process is performed in steady-state regime. The heat obtained during the catalytic combustion is used to preheat the incoming cool gas stream by means of an indirect gas-gas heat exchange. Also known is the catalytic process (Figure Id) combining periodical flow reversal in a regenerative ceramics bed and unidirectional flow in a catalyst bed. This process will be discussed also in comparison with the catalytic reverse process. In this article no comparison is being made to other known methods of VOC control such as flaring, condensation, adsorption, absorption, biofiltration, membrane separation, and ultraviolet oxidation for areas of their
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1631
I I
f0 1
I
I
)
JJ -&
3
I
Figure 1. Flow diagram of the various VOC control systems. (a) Catalytic reverse process; (b) regenerative thermal oxidizer; (c) recuperative catalytic oxidizer; (d) system combining unidirectional flow in catalytic reactor and flow reversals in ceramic beds. I, cold gas to purification; 0, purified gas; 1,catalyst bed; 2, bed of inert material; 3, switching valves; 4, fuel burner; 5, shell-and-tube heat exchanger; 6, preheater.
practical applications are usually different than those of thermal or catalytic incineration.
of equations
Ae,xx> = wwy) w(eyy>= p(r - X )
Mathematical Description In this paper, the simplest one-dimensionaltwo-phase mathematical model for unsteady-state process in a fxed bed reactor is used. This model is adequate for illustration of the main characteristics of the reverse process, while understanding that in reality a more complex approach can be used. The model used here takes into account (a) occurrence of an exothermic reaction complicated by the intraparticle diffusion resistance, (b) transport of mass and energy in the fluid volume of the bed by convection, (c) external heat and mass transfer from fluid phase to the surface of catalyst and ceramic packing, and (d) axial heat dispersion through the packing. The most important dynamic factor is the heat capacity of the solid phase that determines the time scale of reactor dynamics. Temperature in the gas flow phase and gas composition can be assumed to be in a quasi-steady state related to the temperature in the solid. These assumptions as well as the model parameters had been previously discussed by Matros (1985,1989), Eigenberger and Nieken (1988), and Matros et al. (1993). The equations are:
(4)
where ,8 = /3dSp. A single irreversible exothermic reaction is usually assumed to occur in accordancewith the first order rate equation
W(0y) = KO exp(-E,/Re)(l
-y )
(5)
If the reactor is designed according to Figure l a , its mathematical description is given by the system of equations (1)-(4) with parameters depending on the axial position along the length of the packed bed. In the beds of inert material placed within the intervals 0 5 1 I 11 and 12 5 1 I L, where 11 and 12 are the coordinates of the catalyst bed interfaces, the rate of chemical conversion is equal to zero, i.e., WCy,f3) = 0. In addition, parameters cg, A,, es, and a can be different for the catalyst and ceramic packing. The interface conditions on the boundaries between catalyst and ceramics take into account equality of heat fluxes through the solid phase: a t 1 = li (i = 1, 2):
Tll-o= Tll+o,
a t I = I,:
x=o
At the boundary of the reactor the conditions are as follows:
(3) where a = @sp. Function f determines the rate of chemical reaction taking into account the mass transfer limitation between the external surface of catalyst particle and bulk of fluid. It is found from the system
at 1 = 0: at1 =L:
ae
T=Ti, and A,-=O ai
(7)
ae
il,-=O
ai
Assuming that a picture of the moving temperature and
1632 Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995
concentration profiles will be cyclically repeated after a number of flow reversals, the following expression for cycle average heat balance can be derived (at cg = constant):
Tin+ ATa@= Tout
(9)
where ATad = (-AH)Cd@&g;3 and Toutare cycle average conversion and outlet temperature, respectively; ATad is adiabatic temperature rise of VOC oxidation. At low adiabatic temperature rise the oxidation unit diagram can incorporate the heater installed in the reactor center between two identical packing sections filled with catalyst and ceramics. In this case the total temperature rise of gas mixture will be
AT = ATa&
+ AT,,,,
(10)
where ATh,l is the increase of the gas temperature due t o fuel burning or electric heating. The profiles under reversals are numerically calculated by several solutions of the system (1)-(9) along the time intervals 0 I t I tJ2. The first solution (or iteration) is determined a t initial conditions:
e(l)(o,z)= do)
(11)
where @O) is given initial temperature of the beds, which is specified sufficiently high to provide a reasonable rate for the chemical reaction. The initial data for subsequent iterations, Oo+l)(O,l), depends on the preceding:
eVf1)(o,z)
= e%jz,~-z)
Thermal efficiency (7)related to recovery of the heat of oxidation reaction can be defined as
(12)
where j = 1, 2, ... is a number of iteration. The calculations are finished when a heat balance (9) is satisfied. Numerical solution of the system (1)-(8) can be performed by different methods (see, for example, Dieterich et al., 1992). Some difficulties, resulting in a large amount of computational time, have been discussed in the literature (Bhatia, 1991; Gupta and Bhatia, 1991). They are due to steep temperature or concentration gradients and high number of cycles required for stationary profiles to be attained. Our numerical procedure includes analytical solutions of linear equations (2) and (3) on a space interval (Matros et al., 1988; Young et al., 1993), so that the system (1)(4)can be reduced t o one parabolic partial differential equation and algebraic equations for the gas temperature and conversion profiles. At high flow reversal frequency, obeying the condition t, -:1 -V
e(t)x
1)
+
-+1+Tin ATagout(2I;H 2
(A15)
+
where 2 = 1, (l/&). For positive 1, and Ct the square root, is between zero and unity, which leads t o the estimation of the temperature profile:
m,
The form (A16), written as an equation, is an equivalent to the solution (A121, but for simpler pseudo-). From the form (A16) homogeneous model (at we derived the estimation of the catalyst bed space time (zct) required to obtain the maximum temperature Tmax:
-
The same estimation has been earlier obtained for the pseudo-homogeneousmodel by Boreskov et al. (1983). The maximum temperature T,, can be roughly estimated from (17) accounting for the oxidation kinetics for specific catalyst and VOCs. If the inert packing is used a t the reactor boundaries, and the heater is installed into the reactor center the temperature profiles will be estimated as
+
( +2inJ
+
a t t < t,: e(t) Tin (ATa,pout ATfuel)1
(A18)
L
at -> t >
2u
t,:
x
+
+
Tin (ATa,pout
when subscript "inr" denotes inert ceramic packing; z1 = 11KJ is the space time for one ceramic bed. Equation A18 can be used for the estimation of the space time in
Aerov, M. E.; Todes, 0. M.; Narinskii, D. A. Columns with Packed Bed; Chimia: Leningrad, 1979 (in Russian). Bhatia, S. K. Analysis of catalytic reactor operation with periodical flow reversal. Chem. Eng. Sci. 1991,46,361-367. Boreskov, G. K.; Matros, Yu. Sh. Flow Reversal of Reaction Mixture in a Fixed Catalyst Bed-a Way to Increase the Efficiency of Chemical Processes. Appl. Catal. 1983a,5, 337. Boreskov, G. K.; Matros, Yu. Sh. Unsteady-State Performance of Heterogeneous Catalytic Reactions. Catal. Rev.-Sci. Eng. 1983b,25, 551-569. Boreskov, G. K.; Matros, Yu. Sh.; Kiselev, 0. V.; Bunimovich, G. A. Catalytic processes under unsteady-state conditions. Dokl. &ad. Nauk SSSR 1977,237,160-164 (in Russian). Boreskov, G. K.; Bunimovich, G. A,; Matros, Yu. Sh.; Zolotarskii, I. A.; Kiselev, 0.V. Cyclic operation in fixed catalyst bed reactor under periodical flow reversal. Dokl. Acad. Nauk SSSR 1983, 268,647-653 (in Russian). Combu-Changer. Principe du Combu-Changer. In SOCREMATIQUE l'air propre; Combu-Changer: B.P. 756-95004 Cregy Pontoise Cedex, France, 1992. Dieterich, E.; Soresku, G.; Eigenberger, G. Numerical methods for simulation of chemical engineering processes. Chem.-1ng.-Tech. 1992,64, 136. Eigenberger, G.; Nieken, U. Catalytic Combustion with Periodic Flow Reversal. Chem. Eng. Sci. 1988,43,2109-2115. Eigenberger, G.; Nieken, U. Catalytic cleaning of polluted air: reaction engineering problems and new solutions.Chem.-1ng.Tech. 1991,63,781-791. Eigenberger, G.; Nieken, U. Catalytic cleaning of polluted air: reaction engineering problems and new solutions. Int. Chem. Eng. 1994,34,4-16. Gupta, V. K.; Bhatia, S. K. Solution of cyclic profiles in catalytic reactor operation with periodic flow reversal. Comput. Chem. Eng. 1991,15,229-237. Matros, Yu. Sh. Perspectives of Using of Unsteady-State Processes in Catalytic Reactors. Zh. Vses. Khim. Ova im D. I. Mendeleeva (J. Mendeleev' Chem. Soc.) 1977,22, 576-580 (in Russian). Matros, Yu. Sh. Unsteady-State Processes in Catalytic Reactors; Elsevier: Amsterdam, 1985. Matros, Yu. Sh. Catalytic Processes under Unsteady-State Conditions; Elsevier: Amsterdam, 1989. Matros, Yu.,Sh.; Ivanov, A. G.; Gogin, L. L. Unsteady-state process of high-potential heat recovery from low concentrated gases and fuels. Teor. Osn. Khim. Tekhnol. 1988,22,481-487 (in Russian). Matros, Yu. Sh.; Noskov, A. S.; Chumachenko, V. A. Catalytic Neutralization of Waste Gases after Industrial Plants; Nauka, Sibirskoe Otdelenie: Novosibirsk, 1991 (in Russian). Matros, Yu. Sh.; Noskov, A. S.; Chumachenko, V. A. Progress in Reverse Process Application to Catalytic Incineration Problems. Chem. Eng. Process. 1993a,32,89-98. Matros, Yu. Sh.; Bunimovich, G. A.; Noskov, A. S. The Decontamination of Gases by Unsteady-State Catalytic Method. Theory and Practice. Catal. Today 1993b,17,261-274. Matros, Yu. Sh.; Noskov, A. S.; Zagoruiko, A. N.; Goldman, 0. V. Comparison of Two Types of Catalytic Reverse-Flow Reactor. Submitted for publication in Teor. Osn. Khim. Tekhnol. (Theor. Fundam. Chem. Technol.) 1994a,28 (in Russian). Matros, Yu. Sh.; McCombs, D. E.; Strots, V. 0.;Bunimovich, G. A.; Roach, C. Catalytic Reverse Process for VOC Control: Experimental Data and Reactor Simulation. In Proceedings of the International Petroleum Environmental Conference, March 2-4, 1994, Houston, TX,1994b, in press. Mulina, T. V.; Popovskii, V. V.; Bakaev, A. Ya. Scientific Fundamentals of Catalysts Preparation: Summaries of All-Union
1640 Ind. Eng. Chem. Res., Vol. 34,No.5, 1995 Meeting; Preprint; Institute of Catalysis SO AN USSR: Novosibirsk, 1983 (in Russian). Schack, A. Industrial Heat-Transfer; John Wiley & Sons, Inc.: New York, 1965. Smith Engineering Company. Smith regenerative thermal oxidizers for VOC emissions and odor control; Brochure by Smith Engineering Company, Smith Environmental Corporation, 2837 East Cedar Street, Ontario, CO 91761, 1990. Van den Beld, L.; Westerterp, K. R. Purification of Waste Air in a Reverse Flow Reactor. In Preprint Volume of First Topical Conference on Industrial Chemical Engineering Technology; Cropley, J. B., Coordinator; St. Louis Annual Meeting ofAIChE; AIChE: New York, 1993; pp 186-191. Vanin, G. V.; Noskov, A. S.; Popova, G. Ya.; Andrushkevich, N. V.; Matros, Yu. Sh. The Industrial Plant for Unsteady State
Purification of Flue Gases from Acrylonitrile and Cyanic Acid. Catal. Today 1993,17, 251-259. Young, B.; Hildebrandt, D.; Glasser, D. Analysis of a n exothermic reversible reaction in a catalytic reactor with periodic flow reversal. Chem. Eng. Sci 1992,47, 1825-1837.
Received for review March 25, 1994 Revised manuscript received October 12, 1994 Accepted February 17, 1995@ IE940189L ~~
~
~
~~~
Abstract published i n Advance ACS Abstracts, April 1, 1995. @
