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Schematic diagram of the experimental apparatus for catalytic decomposition of VOCs is shown in Fig. 1. Mixed standard gas of VOCs was induced into electric furnace using mass flow-meter. The gas was sampled at inlet and exit ports of the furnace (KOYO KTF030N). Experimental conditions were as follows: mixed VOC gas; IPA+ethyl acetate+MEK+toluene (total VOC concentration: 1,368 ppmC) in O₂ (7.45%) /N2, reaction tube; 21 mmi.d.×530mm(quartz), flow-rate; 2 L/min, catalyst; Pt or Pd/honeycomb, SV=17,000 h^-1, heating temperature range; 100-400°C.

Fig. 1. 
Schematic diagram of the small-scale experimental apparatus for catalytic decomposition of VOCs.

Total VOC concentrations were measured by Shimadzu VMS-1000F VOC meter (FID method). CO₂ was analyzed by Riken Keiki RI-215D. CO was measured by Testo 350M. The each VOC component in sample gas was analyzed by Shimadzu GC-17A.

The temperature changes of inlet of SiC honeycomb, outlet of the honeycomb and exhaust gas from the proto-type apparatus were measured at the condition of 24 A of max electric current. The electrical circuit including SiC honeycomb was shown in Fig. 2. The scale of one SiC block was 92 mm×92 mm×30 mm, and 9 block×2 layer were set in the apparatus. The air flow in the test was 76m³/h.

Fig. 2. 
Arrangement plan of SiC honeycomb and electricity method.

A proto-type apparatus for VOC decomposition in small-scale factories was produced. The outline of the VOC control system was shown in Fig. 3. The flow-rate planned for the control of exhaust gas including VOCs is 100 m³/h. The sample gas passes through the SiC honeycomb layer and is heated in this layer. The gas after heating flows into the catalytic layer and VOC components in the gas decomposes efficiently. Finally, the controlled gas is exhausted to atmosphere.

Fig. 3. 
Schematic diagram of the proto-type apparatus for control of exhaust involving VOCs.

The SiC filter system was same as mentioned in 2.2. The catalytic layer was shown in Fig. 4. The scale of one catalytic block is 150 mm×150 mm×50 mm. The SiC honeycomb and the catalytic portion are covered by insulator material. The electric current is controlled up to max 24 A, and the voltage is 200 V. The temperatures at gas inlet portion, outlet of SiC layer and exhaust portion are monitored using thermocouples.

Fig. 4. 
Outward appearance of catalytic layer.

The decomposition of VOCs exhausted from a printing factory in Gifu prefecture, Japan was demonstrated using the proposed proto-type apparatus. Pd or Pt catalyst was used in the VOC decomposition test. Total VOCs and VOC components were measured by the procedure described in 2.1.

VOC decomposition over Pd or Pt catalyst was investigated using the small-scale electric furnace. The results were shown in Fig. 5. The removal of each component, i.e., IPA, ethyl acetate, MIBK or toluene, over Pt catalyst was superior to that over Pd catalyst under 250°C, and that was up to ca. 90% over 300°C in each catalyst. The total VOCs conversion and the selectivity of CO₂ were both ca. 90% in the condition of upper 300°C. These results show that ca. 90% of the VOC components were oxidized completely to CO₂ under the condition of >300°C.

Fig. 5. 
Decomposition results of VOC components and total VOCs and selectivity of CO₂ over Pt or Pd catalyst using small-scall electric furnace. (a) IPA, (b) ethyl acetate, (c) MIBK, (d) toluene, (e) selectivity of CO₂ for total VOCs, (f) conversion of total VOCs.

SiC is a semiconducting material and generates heat when applying electric current, so sintered silicon carbide is commercially applied for endurable heaters in the atmospheric condition. Unlike nickel-chrome heating elements, SiC has a negative temperature coefficient of electric resistance. To adjust the heating temperature, a power compensating device is needed against the change in electrical resistance and electrical exothermic characteristics of the SiC honeycomb should be known experimentally.

The changes of the temperature before and after SiC honeycomb (Fig. 2) were monitored when the maximum electric current was set at 24 A. The results were shown in Fig. 6. Though the inlet temperatures were 6-7°C, the outlet temperatures of SiC honeycomb were over 300°C after 5 min from the starting of applying electrical current. Exothermic performance of the honeycomb was sufficient for the catalytic decomposition of VOC by catalytic filter. That is, the combination system of an electrical exothermic SiC honeycomb and a catalytic filter is assumed to be effective for warming the inlet gas and then decomposing the involved VOC components. This linear combination system is very useful to the catalytic decomposition of VOCs, because the sample gas involving VOCs is well heated in the SiC honeycomb and then flows into the catalytic filter.

Fig. 6. 
Temperature changes at inlet and outlet of SiC honeycomb in the electricity-exothermic test.

The emission of VOCs from the printing factory was investigated. The results were shown in Table 1. In the printing factory selected for the performance of the VOC decomposition test, the total VOCs emitted from the printing process were 458-8,210 ppmC. 73-76 ppmC of total VOCs were emitted from the drying process. The main VOC components were recognized to be IPA, MEK, ethyl acetate and toluene.

The proto-type apparatus as shown in Fig. 7 for the demonstrative decomposition of VOCs emitted from the printing process was set in a printing factory in Gifu prefecture, Japan.

Fig. 7. 
Proposed proto-type apparatus for VOCs control.

The inlet temperature of catalytic filter in this apparatus was achieved to be 289-309°C at the conditions of 73-85m³ h^-1 of gas flow and at 33-41°C of exhaust gas temperature. Therefore, the heating of inlet gas involving VOCs using the electrical exothermic SiC honeycomb was performed sufficiently for the catalytic degradation of VOC components in the proposed apparatus.

The results of VOC decomposition test using Pd and Pt catalyst were shown in Table 2 and 3. The decomposition of total VOCs using Pt catalyst was superior to that using Pd catalyst. In the degradation test of each component over Pd catalyst, the decomposition of IPA or MEK was each 70%, and that of ethyl acetate or toluene was 85-90%. On the other hand, the decomposition of each component, IPA, MEK, ethyl acetate or toluene, over Pt catalyst was 86-89%. That is, the decomposition efficiency of each VOC component using Pt catalyst was relatively higher and stable. As shown in Table 4, the average decomposition rate of total VOCs using Pt catalyst was 85% at SV=19,000 and that using Pd was 77% at SV=17,000. The proposed combination system of an electrical exothermic SiC honeycomb and a catalytic filter was demonstrated to be very effective for VOC decomposition. This system is also very safe because of not using combustion.