PARAMETRIC EVALUATION OF VOC CONVERSION VIA CATALYTIC INCINERATION

PARAMETRIC EVALUATION OF VOC CONVERSION VIA CATALYTIC INCINERATION

G. Kaskantzis Neto ¹ and J.C. Moura ²

¹Prof. DTQ/ST/UFPR, CP 19011, CEP 81531-970, Curitiba, PR, Brazil.

²Prof. FEQ/DPQ/UNICAMP, CP 6066, CEP 13081-970, Campinas, SP, Brazil.

(Received: October 30, 1996; Accepted: April 2, 1997)

ABSTRACT - A pilot-scale catalytic incineration system was used to investigate the effectiveness of catalytic incineration as a means of reducing volatile organic compound (VOC) air pollutants. The objectives of the study were: 1) to investigate the effects of operating and design variables on the reduction efficiency of VOCs; and 2) to evaluate reduction efficiencies for specific compounds in different chemical classes. The study results verified that the following factors affect the catalyst performance: inlet temperature, space velocity, compound type, and compound inlet concentration. Tests showed that reduction efficiencies exceeding 98% were possible, given sufficiently high inlet gas temperatures for the following classes of compounds: alcohols, acetates, ketones, hydrocarbons, and aromatics.

KEYWORDS: Volatile organic compound (VOC), catalytic incineration.

INTRODUCTION

Historically, volatile organic compounds (VOCs) have been regarded as important to human health only through their role in the formation of photochemical oxidants (Davis and Cornwell, 1991). The emission of VOCs from industrial plants significantly increases ambient levels of photochemical oxidants, ozone, and smog. Control of industrial VOC emissions, therefore, can result in substantial environmental benefits.

For most VOC emitting processes, several technologies are available. These include carbon adsorption, absorption, condensation, and thermal and catalytic incineration (Jennings, 1984). All of these techniques, with the exception of process modifications, are add-on control devices. Carbon adsorption, absorption and condensation allow the VOC to be recovered for reuse in the process. Catalytic and thermal incineration are VOC destruction devices.

Catalytic incineration is a combustion control technique in which the oxidation reaction occurs at lower temperatures than thermal incineration, with the help of a catalyst, allowing oxidation reactions to occur rapidly in the temperature range from 350 to 400 ° C. By contrast, a range from 800 to 1200 ° C is required for practical oxidation rates in thermal incinerators (Vaart and Vatavuk, 1991). Significant energy savings are possible using catalytic incineration. A major attraction of catalytic incinerators is its reduced fuel costs.

The basic elements of a catalytic incinerator are the preheat burner, combustion chamber, catalyst element, and an optional heat recovery system. The oxidation reaction that occurs at the surface of the catalyst produces the same products, carbon dioxide and water, and release the same heat of combustion as a thermal incinerator would.

A catalyst bed in commercial units is typically a metal mesh mat, ceramic honeycomb, or other ceramic matrix structure with a surface coating of finely divided platinum or other platinum-family metals, such as chromium, palladium, nickel, or cobalt (Keith, 1990).

The performance of catalytic incinerators is strongly affected by several factors. These factors are operating temperature, space velocity, VOC concentration, catalyst characteristics, and heat recovery (Beld and Westerterp, 1994). These factors are interrelated through the mechanism of the catalytic oxidation reaction and must be considered collectively when designing a catalytic incineration system .

The empirical observations (Frost et al., 1991) suggest that there are two rate limiting processes operating together in the catalytic reaction systems: 1) a mass transfer of reactant hydrocarbon from the bulk gas stream to the external catalyst surface and 2) an effective chemical reaction within the catalyst. At the high temperatures employed in combustion, the catalytic reaction rate is very high and the overall reaction becomes limited by the external mass transfer rate.

The objectives of the study were to investigate the effects of operating and design variables on the reduction efficiency of VOCs, and evaluate reduction efficiencies for specific compounds in different chemical classes.

MATERIAL AND METHODS

A schematic diagram of the pilot-scale catalytic incineration system is shown in Figure 1. The test system for this study consisted of a fixed-bed reactor, a vapour generation system, an analytic system and auxiliary equipment. The system was equipped with a blower, a heat exchanger, a preheater, valves, and temperature controls.

The reactor made of stainless steel was thermal isolated with rock wool. The fixed- bed reactor was filled with palladium-supported alumina catalyst. The bed was cylindrical in shape with a 12 cm diameter and a length of 4 cm, giving a total catalyst volume of 452.39 cm³. There were J type thermocouples for measuring axial and radial temperature profiles along the catalyst bed. The characteristics of the catalyst are summarised in Table 1.

The preheater of the system consisted of four electrical resistance elements controlled by a PID controller. Instrumentation of the pilot unit included controllers for the gas inlet temperature and vaporisation system. Other indicators provided measurements of gas outlet, preheater and heat exchange temperatures, and catalyst bed pressure drop.

Figure 1: Flow diagram of the experimental apparatus (Kaskantzis, 1995).

Type Pd supported on Al 2 O 3 Loading * 0.125 % wt as Pd Form spherical pellets Nominal size 3 mm BET surface area 248 m2/g Thermal stability * 650 ° C Specific density * 0.8 kg / l

Type	Pd supported on Al 2 O 3
Loading *	0.125 % wt as Pd
Form	spherical pellets
Nominal size	3 mm
BET surface area	248 m2/g
Thermal stability *	650 ° C
Specific density *	0.8 kg / l

Table 1: Characteristics of catalyst used in the fixed-bed reactor for VOC incineration

* data obtained from supplier

The vapour generation system provided an air stream with various concentrations of organic vapours for incineration. The air flowrate was measured with an orifice gas meter calibrated by Merian Flow Meter. The VOC injection, vaporisation, and mixing all occurred in a stainless steel vaporiser. The unit contained a ¼ ¢ ¢ o.d. VOC injection part set into the main line inlet at a 90° angle. This injection design was believed to prevent blowback of VOCs into the injection port.

The analytic system was a gas chromatograph with a flame ionization detector, a 6-port valve equipped with a fixed 2mL gas sample loop, and a Migrator model electronic integrator used to determine and record concentrations of individual species and integration parameters.

A more detailed description of the experimental apparatus, safety considerations, reactor assembly, and analytical procedures are given in the work of Kaskantzis (1995).

The experiments with ethanol, toluene, acetone, ethyl acetate, and n-hexane in air diluted were conducted in the range of inlet gas temperatures from 200 to 380 ° C, of gas flowrates from 0.20 to 0.70 Nm³/min and VOC concentrations from 300 to 3000 ppmv. The space velocities, based on total catalyst volume bed and standard gas flowrate, were varied from 30000 to 85000 h^-1. The reduction efficiency was defined as the conversion of VOC to carbon dioxide and water.

The effect of enthalpy variation due to reaction rate was leved using an energetic base common to all compounds, which allows a comparasion of the results. This energetic base can be calculated from:

, (1)

where BE is the energetic base, M_VOC is the mass flowrate of VOC, PCI_VOC is the heat of combustion of VOC, and V_g is the standard gas flowrate.

RESULTS AND DISCUSSION

Results from the pilot-scale catalytic incineration tests verified the effects of a number of operating and design parameters on catalytic incineration performance. Parameters found to have an effect on the reduction efficiency of VOCs include gas inlet temperature, space velocity, inlet concentration, and compound type.

The capability of catalytic incineration to achieve reduction efficiencies in the 98 to 99 percent range was also verified for VOCs in five different chemical classes. Table 2 shows some of the experimental results of the incineration of several VOCs at different operational conditions.

VOC s (h -1 ) TE (° C) Xvoc (%) C (ppmv) BE (kcal/Nm 3 ) ethanol 29308 252 97.56 2930 38.56 69657 300 98.15 2882 37.93 85427 310 96.54 2915 38.37 toluene 69345 251 96.44 954 38.06 69345 269 99.17 950 37.89 85427 321 99.10 964 38.45 acetone 69657 320 96.10 2183 37.93 69030 321 92.41 1134 19.69 85427 340 96.52 2193 38.10 85427 380 90.78 1063 18.74 29308 310 95.80 2193 38.10 ethyl acetate 29308 330 98.86 1706 38.39 69030 360 98.96 1725 38.80 85427 380 98.89 1713 38.53 n-hexane 69030 370 98.92 838 18.86 85427 403 98.99 849 19.10 85610 382 99.10 853 37.90

VOC	s (h -1 )	TE (° C)	Xvoc (%)	C (ppmv)	BE (kcal/Nm 3 )
ethanol	29308	252	97.56	2930	38.56
	69657	300	98.15	2882	37.93
	85427	310	96.54	2915	38.37
toluene	69345	251	96.44	954	38.06
	69345	269	99.17	950	37.89
	85427	321	99.10	964	38.45
acetone	69657	320	96.10	2183	37.93
	69030	321	92.41	1134	19.69
	85427	340	96.52	2193	38.10
	85427	380	90.78	1063	18.74
	29308	310	95.80	2193	38.10
ethyl acetate	29308	330	98.86	1706	38.39
	69030	360	98.96	1725	38.80
	85427	380	98.89	1713	38.53
n-hexane	69030	370	98.92	838	18.86
	85427	403	98.99	849	19.10
	85610	382	99.10	853	37.90

Table 2: Experimental results of reduction efficiencies for VOCs at various reactor conditions

Temperature and Space Velocity Effects

The effects of temperature and space velocity on reduction efficiencies of VOCs are discussed in this section for a fixed volume of catalyst. Figures 2 and 3 show the effects of inlet gas temperature and space velocity on conversion of VOCs, respectively.

The test results show an increasing trend in system reduction efficiency with increasing gas inlet temperature. The effect of gas inlet temperature on system conversion appears to be highest at the higher space velocities, and for system reduction efficiencies below approximately 95 percent. System reduction efficiencies of 98 percent or greater were observed at one or more test conditions for all compounds.

The experimental data for the catalytic combustion of toluene indicates that the effect of space velocity on conversion is higher at low inlet gas temperatures. For higher conversion level, the effect of space velocity on conversion is not so pronounced, as shown in Figure 3.

Figure 2: Conversion vs. inlet gas temperature for (U ) ethanol, (u ) toluene, (n ) acetone, (l ) ethyl acetate, and (D ) n-hexane, at s » 30000 h^-1 and BE » 38 kcal/Nm³.

Figure 3: Conversion vs. inlet gas temperature for toluene at (u ) 30000, (n ) 70000 and (l ) 85000 h^-1 at BE » 38 kcal/Nm³.

Inlet Concentration Effect

The effect of system inlet concentration on n-hexane reduction efficiencies is shown in Figure 4 for several inlet gas temperatures. Results at lower inlet gas temperatures show increases in component conversion with increasing concentration. For higher inlet gas temperatures, the increases of inlet concentration show a slight increase in the reduction efficiency of VOCs.

Based on experimental data, higher reduction efficiencies can be expected at higher inlet concentrations, with the greatest effect occurring at lower gas inlet temperatures. At higher gas inlet temperatures for the volume of catalyst used in the experiments, the effect of inlet concentrations on conversion is not so pronounced. This finding is in agreement with the observed effect of space velocity already discussed.

Compound Type Effect

Objectives of this study include the collection of data on a large number of compounds to allow: 1) an assessment of the capability of catalytic incineration to achieve high reduction efficiencies for a wide variety of compounds and 2) an estimate of the relative reduction of the various classes of compounds.

Based on the experimental data presented in Figure 2, catalyst reduction efficiencies of 98 percent or higher should be achievable for all compounds at sufficiently low space velocity and/or high enough gas inlet temperatures. The relative reducibility of selected compounds was verified by comparing the experimental reaction rate in a common energetic base, as shown in Table 3.

Figure 4: Effect of inlet concentration on reduction efficiencies for (n ) 852 ppmv and (l ) 423 ppmv of n-hexane at s » 85000 h^-1 .

r exp at conditions specified above. s » 30000 h -1 s » 70000 h -1 s » 85000 h -1 Compound TE = 280 ° C TE = 300 ° C TE = 300 ° C TE = 320 ° C Ethanol 1.201E-6 2.800E-6 3.391E-6 --- Acetone 8.680E-7 2.084E-6 2.430E-6 2.571E-6 Ethyl Acetate 4.772E-7 1.256E-6 1.354E-6 1.648E-6 Toluene --- --- 1.157E-6 1.160E-6 n - Hexane 2.284E-7 6.167E-7 6.230E-7 8.794E-7

	r exp at conditions specified above.
	s » 30000 h -1	s » 70000 h -1	s » 85000 h -1
Compound	TE = 280 ° C	TE = 300 ° C	TE = 300 ° C	TE = 320 ° C
Ethanol	1.201E-6	2.800E-6	3.391E-6	---
Acetone	8.680E-7	2.084E-6	2.430E-6	2.571E-6
Ethyl Acetate	4.772E-7	1.256E-6	1.354E-6	1.648E-6
Toluene	---	---	1.157E-6	1.160E-6
n - Hexane	2.284E-7	6.167E-7	6.230E-7	8.794E-7

Table 3: Experimental reaction rate of VOCs at BE » 38 kcal /Nm ³

The results show the same order of increasing ease of reducibility based on reaction rate for all test conditions. As shown in Table 3, n-hexane appears to be the most difficult to convert, ethyl acetate also appears to be somewhat harder to convert than other compounds, and ethanol appears to be the most easily converted of the compounds tested.

Mass Transfer Effect

Smith (1981) shows that when both diffusion and reaction resistance are significant, and for a first order reaction the expression for the reaction rate in terms of bulk concentration C_b is

, (2)

where r is the rate expressed per unit mass of pellet, k_o is the overall rate constant, C_b is the concentration in the bulk gas, k is the reaction rate, k_m is the mass-transfer coefficient between bulk gas and solid surface, and a_m is the external surface area per unit mass of pellet.

When the external diffusion is arbitrarily neglected, an apparent activation energy E could then be calculated from Arrhenius equation

, (3)

where A is the apparent frequency factor. This result would give an erroneous value for E if external diffusion were a significant resistance. In fact, the data points for different temperatures would not form a straight line, but would give a curve. The Figure 5 shows the Arrhenius plot for the ethyl acetate system.

The Figure 5 shows that at low temperature a straight line is obtained on the Arrhenius plot. As the temperature increases, a curve which ultimately flattens to a nearly horizontal line is obtained, and it is possible that external diffusion resistances are important.

CONCLUSIONS

Results from this study verified that the following factors affect the performance of catalytic incinerators: gas inlet temperature, space velocity, compound type, and inlet VOC concentration. In addition, the testing verified that reduction efficiencies in the 98 to 99 percent range are achievable with catalytic incineration for the following compounds or classes at sufficiently low space velocities and/or high enough gas inlet temperature: alcohols, acetates, ketones, hydrocarbons, and aromatics. The order of increasing ease of reducibility of VOCs for all test conditions is ethanol > acetone > ethyl acetate > toluene > n-hexane. The external diffusion effects is observed as the temperature increase, and at high temperature is possible that external mass-tranfer resistances are important. Currently there is one area where information on catalytic incineration technology could be expanded with additional testing. This area involves the types of compounds for which catalytic incineration is applicable. While this study investigates a large number of compound classes, further data on the applicability of catalytic incineration to control halogenated hydrocarbons would be useful to the synthetic organic chemicals manufacturing industry.

Figure 5: Plot of Arrhenius equation for ethyl acetate system at s » 85000 h^-1.

ACKNOWLEDGMENTS

The authors thank CNPq and CAPES for scholarships provided to one of the authors, FAEP and TERMOQUIP for their financial support, and BASF for supplying the catalyst.

NOMENCLATURE

BE Energetic base, kcal /Nm3

BL Blower

C Concentration, ppmv

CR Chromatograph

HE Heat exchanger

IT Electronic integrator

M Manometer

M_VOC Mass flowrate of VOC, kg/min

OP Orifice Plate

PCI_VOC Heat of combustion of VOC, kcal/kg

PH Preheater

ppmv Parts per million in volume

r_exp Experimental reaction rate of VOC, mol voc / g cat. s

RO Rotameter

RT Reactor

s Space velocity, h-1

T Thermocouple

TE Inlet gas temperature, ° C

TQ Tank

V Valve

Vg Volumetric flowrate of gas stream, Nm³/min

VP Vaporizator

VOC (s) Volatile Organic Compound, s

Xvoc Conversion of VOC, %

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• Jennings, S.M., Catalytic Incineration for Control of Volatile Organic Compound Emission, 1st. ed., Noyes Publication, New Jersey, USA (1984).
• Kaskantzis, G.N., Estudo da Incineraçăo Catalítica de Compostos Voláteis Orgânicos em Planta Piloto, Tese de Doutorado, DPQ/FEQ/ UNICAMP, Campinas, Săo Paulo, Brazil (1995).
• Keith, J.H., Catalyst for Volatile Organic Compounds Control, Incineration Conference, San Diego, California, May 14 - 18 (1990).
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Publication Dates

• Publication in this collection
  09 Oct 1998
• Date of issue
  June 1997

History

• Received
  30 Oct 1996
• Accepted
  02 Apr 1997