Destruction of volatile organic compounds by catalytic oxidation

359

Catalysis Today, 17 (1993) 359-366 Elsevier Science Publishers B.V., Amsterdam

Destruction E. Noordally,

of Volatile Organic Compounds by Catalytic Oxidation J.R. Richmond and S.F. Tahir

Chemical and Environmental Catalyst Group, UOP Limited, Jeffreys Road, Brimsdown, Enfield, EN3 7PN, (UK). Tel : (+44-81) 804 8232 Fax : (+44-81)

443 3055

Abstract Three categories of UOPR UNIDOXTM precious metal supported catalysts on washcoated cordierite monolith have been studied for the oxidation of VOCs and halogenated VOCs. These catalysts are classified as low-temperature oxidation catalyst (LTC), hightemperature oxidation catalyst (HTC) and halocarbon oxidation catalyst (HOC). The catalysts LTC and HTC showed high destructive efficiencies and excellent stability when treating CO and various hydrocarbon emissions to form CO2 and H,O. HOC is stable and highly active in destroying halocarbons to form C02, H,O, and halogen acid. The temperatures required for complete oxidation depended on the organic component class present and its contact time.

INTRODUCTION Volatile organic compounds (VOCs) have long been a major source of air pollution, and in many cases, legislation has already been introduced to reduce their emission. These VOC contaminated gas streams are vented from a variety of industrial and commercial processes, including those involving printing, metal decorating, paint drying, metal degreasing, manufacturing of organic compounds and polymers, food processing, and air stripping units associated with ground water and contaminated soil cleanup. In this context the term VOC refers to solvents, organic vapours, and odours. The desired reaction is the complete oxidation of the VOCs to CO;, and H,O. In the case of chlorinated VOCs, the products are HCl, CO2 and H,O. Catalytic combustion for pollution abatement is being used in a variety of industrial processes. It offers high destructive efficiency that generally exceeds 99%) lower operating temperatures than thermal combustion systems, lower capital cost, and smaller units. Several types of catalyst formulation are commercially available and they fall broadly into two categories: base metal oxides, such as Cu, Cr, and Mn, and supported noble metal catalysts (Pt, Pd). The base metal oxide catalysts are available mostly in granular form, and the precious metal ones are available in granular and monolithic forms or as foams. A comprehensive review on the subject of catalytic oxidation of VOCs was carried out by Spivey in 1987 [ 11. One of the most significant advantages of precious metal catalysts for the control of VOCs and halogenated solvents is their ability to form complete combustion products.

0920-5861/93/$6.00 0 1993Elsevier Science Publishers B.V. All rights reserved.

360

The work described in this paper was undertaken to demonstrate that catalysts are available for use in three VOC emission control areas in which catalytic oxidation can be applied: Low temperature oxidation catalysts (LTC) which have lower fuel requirement and improved economics. High temperature oxidation catalysts (HTC), which have the ability for sustained operation & 550°C at high hydrocarbon loadings. Halocarbon oxidation catalysts (HOC) which have no trace of by-product formation at reasonable residence times and temperatures. The choice of catalyst is always dependant upon several factors (a) types of VOC (b) concentration of VOC (c) any inlet temperature constraints. The LTC, HTC and HOC catalysts between them cover the great majority of chemical types and process conditions likely to be experienced. The results presented in this paper are for these three groups of catalyst, in monolithic form, using both bench and pilot rigs and, under conditions and compositions that simulate actual industrial emissions. Catalyst efficiency, stability, and lifetime are the properties that were studied. EXPERIMENTAL The catalysts (LTC, HTC, and HOC) under study consisted of precious metals deposited on washcoated cordierite monolith having 200 and 400 cells per square inch (cpi). The bench rig is shown in Figure 1. The monolith catalyst sample (10 mL, cylindrical) is held and sealed in a staihless-steel cup, which in turn is fixed in a stainless steel tube reactor. The whole assembly is placed inside a horizontal furnace and heated in a continuous stream of the reactant gas mixture at atmospheric pressure. The reaction is carried out at a space velocity ranging between 15,000 and 65,000 hi’. The temperature of the reactor is controlled by a programmer, which raises the temperature from ambient at a constant rate of 5°C or lOWminute and is capable of a maximum temperature of 900°C. The inlet and exit temperatures are monitored by thermocouples. A portion of the exit gas is passed through an on-line gas chromatography-flame ionization detector (GC-FID) to determine the conversion by measuring the total unreacted hydrocarbons. In addition, samples of the reactant and the product gas are analysed during the run using a GC (Carlo Erba Instruments 6000 Vega Series 2) equipped with flame ionization detector (FID), thermal conductivity detector (TCD) and electron capture detector (ECD) to detect the formation of any products resulting from partial oxidation. In the case of halogenated VOCs, TeflonTM tubes and fittings are used to avoid corrosion, and the haloacid is trapped by passing the effluent gas through a bubbler containing caustic solution. For each catalyst sample, a plot of the extent of conversion of VOC vs. temperature was constructed; in each case, the plot was an S-shaped curve. The first inflection point on the curve is the light-off temperature, the temperature at which 99% conversion was achieved was readily determined from the curve. Figure 2 illustrates the pilot-scale combustion rig, where the catalyst in monolith form is tested under varying conditions of temperature and pressure (up to 20 barg) to determine the optimum operating conditions and the long-term stability of the catalyst. In most cases, the reactant feed is synthetic gas mixtures. For the high-boiling-point hydrocarbons, liquid was pumped into the reactor at a rate that yielded a desired air concentration.

361

FLOWMETERS

(YONOLITH)

I I

HOLDER

TEMP. )EUP.

PROBE OUT

PROBE IN

Figure 1. Schematic diagram of bench rig.

0 -

300

psi Prasnure Relief Valve

,,

N

I

(Air

Mixture

From

Simco

iawooo

Figure 2. Schematic diagram of pilot rig.

Burette

362

RESULTS AND DISCUSSION Low Temuerature Oxidation Catalvst (LTQ The effect of temperature on the conversion of a feed of fixed concentration of VOCs over the LTC was conducted on the bench rig and compared with a standard Pd-Pt-based commercial VOC oxidation catalyst (designated CC). The specific customer requirement was to achieve 95 % destruction of VOCs at a catalyst inlet temperature limited to 213°C by heat exchange capacity. Figure 3 shows total conversion-temperature curves for the LTC and the CC (both in 200 cpi monolithic form) for the combustion of a synthetic gas stream resembling the actual emissions composition containing 0.1% C,H6, 0.6% C,H,, 4% 02, 8% CO,, and Nz as balance at 1.0 bara and at a GHSV of 42,000 hr.‘. The two catalysts showed similar lightoff temperatures at about 150°C with more than 99 % conversion occurring at 390°C for LTC and at 420°C for CC. For the LTC catalysts, ethylene is first oxidised at the lower temperatures and the steep slope is characteristic of the spontaneous oxidation of the ethylene. The heat evolved which assisted the almost immediate oxidation of ethane, and brought the oxidation to completion at 390°C is equivalent to a temperature rise ( T) of 120°C. A comparison of the performance of LTC and CC (Figure 3) shows that at 213°C LTC gives 95% conversion and CC gives only 73% conversion. In this type of reaction, the oxygen consumption is stoichiometric.

0

50

100

150

200

250

300

350

400

450

TEMPERATURE%

Figure 3. Comparison mixture

of LTC and CC for combustion

of ethane and ethylene

500

363

Both activity and stability of the two catalysts were further rig at near industrial conditions. The results of this investigation obtained in the previous experiments. The process conditions were inlet temperature of 2lO”C, and GHSV of 110,000 hr-‘. The results

Table 1 Total conversion

investigated in the pilot confirmed the ranking pressure of 13.6 barg, are shown in Table 1.

of ethane and ethylene mixture.

Catalyst

Total Conversion,

%

T, “C

LTC

96.1

200 - 230

cc

90.5

190 - 217

The final products of combustion were basically CO, and H,O. The CC-designated catalyst also yielded a carbon monoxide concentration of about 150 ppm; the LTC yielded less than 3 ppm. The LTC has been on-stream for a continuous period in excess of 1,200 hrs without any decline in activity. The used LTC was then tested in a second simulated commercial application (2.6% CO, 0.02% CzH4, 0.2% &HI<,, 0.6% CO,, 3.2% O* and N, as balance). At air inlet temperature of 15&C, pressure of 1 barg, and GHSV 40,000 hr.‘, the used LTC gave 98% conversion in the pilot plant. Catalyst activity was monitored for 120 hrs continuously, and no decline in activity was observed. In such gas mixtures, the CO and ethylene are readily oxidised, and the heat evolved helped to oxidise the butane at the low inlet temperature of 150°C. The same used catalyst was subjected to the combustion of other trace quantities of hydrocarbons individually, namely paraffinic, aromatic, alcohol, and amine compounds. Figure 4 shows the conversion-temperature curves for n-butylamine, n-hexane, methanol, and toluene, which were tested at concentrations of 500 ppm in air, pressure of 1 barg, and GHSV of 40,000 hr.‘. The catalyst achieved 99% conversion at 328, 355, 190, and 23O”C, respectively. The stability of this catalyst is clearly demonstrated in these experiments: after running for 240 hrs each component exhibited 99% plus conversion throughout. The bench-rig and pilot-scale tests indicate that the LTC is a well-designed VOC abatement catalyst for the combustion of CO and hydrocarbons. The LTC gives high conversion at relatively low temperature and at high space velocity.

364

80 70 P 5 z g Fj 0

60 50 40 -Methanol 30

+

20

#G n-Butylamine 8

10

Toluene

n-Hexane

1

0 100

150

250

200 CATALYST

Figure 4. Combustion

INLET

300 TEM~.

of methanol, toluene, n-butylamme

350

%

and n-hexane

Hiph TemDerature Oxidation Catalyst Typical precious-metal-on-alumina VOC oxidation catalysts exhibit deactivation at operating temperatures much in excess of 575 to 600°C. UOP manufactures a precious metal catalyst with significantly improved thermal stability at temperatures of 650°C and above. The stability of this HTC was investigated and compared to commercial VOC catalyst (CC) in the pilot rig in a simulated industrial application (ethane off gas destruction), where a high inlet temperature (465°C) and large spikes of VOC can combine to push catalyst temperature above 650°C for extended periods. The composition of the actual emission was 0.1% C,H,, 4% O,, 8% CO, and N2 as balance and the process conditions were: pressure of 13.6 barg, inlet temperature of 465°C and GHSV of 110,000 hi’. The results are shown in Table 2. Table 2 Catalyst efficiency

for the combustion

of C,H,.

Catalyst

* **

Conversion

%

Fresh Catalyst

Catalyst air aged at 750°C for 168 hr then re-test at 465°C

HTC

97.0*

97.0**

cc

88.4

Conversion Conversion

% after running for 720 hrs % after running for 100 hrs

365 In all cases, the only products of combustion were CO, and H,O. These results indicate that HTC is a high-temperature-stabilised catalyst with enhanced activity toward paraffinic (ethane) hydrocarbons.

Halocarbon

Oxidation Catalvst (HOC)

UOP has developed a stable, noble metals catalyst that will oxidise organo-halogen compounds forming CO,, H,O, and HCl, HF, or HBr. Figure 5 shows the effect of temperature on the conversion for the HOC of a humid (2.5% H,O) air stream containing 400 ppm of dichloromethane at 1.O bara and at a GHSV of 12,000 hr.’ using the bench rig. The conversion increased with increasing temperature and reached more than 99 % at 472°C. The HOC was on-stream in the pilot rig for a period of 480 hrs at inlet temperature 465”C, pressure of 1 barg and GHSV of 12,000 hr’ and gave 95% steady conversion without any sign of die off. Dichloromethane (methylene chloride) is one of the more refractory halocarbons. Compounds with carbon-carbon linkages or increasing numbers of halogen atoms can be destroyed at lower temperatures than this material. Comparable conversion tests were run on trichloroethylene (500 ppm), 1,2 dichloroethane (500 ppm) and fluorobenzene (500 ppm). The results are presented in Figure 5. The catalyst has been in operation in excess of 1,000 working hours without any sign of deactivation.

-fc I,2 dichloroethane 20

0 150

200

250 CATALYST

Figure 5. Oxidation of 1,2 dichloromethane in humid air.

-H 8

trichloroethylene fluorobenzene

-3

dichloromethane

/

I

I

1

300

350

400

450

INLET

dichloroethane,

TEMPERATURE

500

%

trichloroethylene,

fluorobenzene,

and

366

In all cases, exit gas was screened for partial oxidation or decomposition product within the sensitivity limits of the electron capture detectors (< 0.1 ppb). We have not seen any evidence of trace by-product formation. Only species in the exit line were CQ, H,O, halogen acid, or unreacted halocarbon. Figure 6 shows the conversion results for the oxidation of trichloroethylene (500 ppm) as a function of temperature at different space velocities. The results show that the conversion of trichloroethylene increased with decreasing space velocity.

100 i

80-

150

200

260

300

350 TEMPERATURE

400

Space

Velocity

+-

6,000 hi’ 12,000

hi’

+t+

18,000

hi’

450 ’

I

1

500

650

J

600

C

Figure 6 Effect of space velocity on the temperature-conversion curve for the oxidation of trichloroethylene (500 ppm) in humid air (2.5% H,O) over HOC at 1 bara

CONCLUSIONS The results from the bench-and pilot-plant activity presented in this paper showed that the UOPR UNIDOXTM catalysts have high destructive efficiencies and high stability. These catalysts form the basis for UOP’s recommended catalysts for industrial application. ACKNOWLEDGMENTS We thank A.R. Boon and M. Shykles for their technical support. REFERENCES 1 J.J. Spivey, Ind. Eng. Chem. Res., 26 (1987) 2165.