Study on regeneration of spent activated carbon by using a clean technology

2.1 Experimental materials

The spent catalyst of vinyl acetate synthesis was obtained from a chemical plant in Yunnan province, China, and its composition is presented in Table 1.

The X-ray diffraction (XRD, Rigaku Company, Japan) pattern of the spent catalyst is displayed in Figure 1, which shows the samples used in this work, mainly composed of C₄₉H₆₆O₃₃, C₄H₆O₄Zn·2H₂O, and C₁₉H₂₂O₆.

Figure 1:

XRD pattern of the spent catalyst.

SEM (XL30ESEM-TMP, Philips Company, Holland) equipped with an EDS (GENESIS, EDAX Company, USA) was used in analyzing the internal morphology and the element distribution of the spent catalyst, shown in Figure 2. The microstructure of the spent catalyst is shown in Figure 2A. It is also shown that two substances exist in the waste zinc acetate-activated carbon catalyst, the carbon material and the filler material in pore. To obtain more information on the element distribution characteristics in the spent catalyst, X-ray EDS line scanning was characterized, as shown in Figure 2B and C. The EDS line scanning from A to B shows that the major ingredients of the filler material in the pore are C, O, and Zn, and it seemed that C, O, and Zn formed the C₄H₆O₄Zn·2H₂O and the other organic impurities combined in the XRD analysis.

Figure 2:

SEM/EDS pattern of the spent catalyst.

2.2 Experimental setup and methods

2.2.1 Test device for dielectric parameters:

The dielectric parameter-measuring device scheme is shown in Figure 3. The dielectric parameter tester (Dielectric kit for Vials) is supplied by the German Püschner company. The device consists of a microwave power source, a directional coupler, a microwave receiver, and a cavity resonator. The microwave signal receiver of AD-8320 integrated circuit can detect the signal amplitude and phase. The resonator was used to hold in the analyzing cavity. The test control unit was via a USB data cable connected to the computer that calculates the dielectric parameters.

Figure 3:

Dielectric constant measurement device scheme.

2.2.2 Microwave equipment:

A 3-kW box-type microwave reactor developed by the Key Laboratory of Unconventional Metallurgy in the Ministry of Education of Kunming University of Science and Technology was utilized for experimentation. The experimental device connection diagram is shown in Figure 4.

Figure 4:

Experiment equipment for microwave regeneration.

The microwave heating frequency is 2450 MHz, while the power can be varied from 0 kW to 3 kW, continuously adjustable, and a thermocouple was used to measure the temperature. A mullite crucible, with an inner diameter of 90 mm, height of 120 mm, having good wave-transparent and heat shock properties, was used. The smoke soot absorption system was composed of a suction bottle, two water bottles, a surge flask, and an aspirator pump. The flue dust and the exhaust were collected and absorbed in the experimental process.

3.1 Dielectric property test results and analysis

Interaction of microwaves with materials depends on their dielectric properties, which were determined by heating a material subjected to electromagnetic fields [26]. Dielectric properties [27] consisted of dielectric constant (ε′), dielectric loss (ε″), and loss tangent (tan δ). The dielectric constant is a measure of the ability of a material to store electromagnetic energy, and dielectric loss is a measure of the ability of a material to convert electromagnetic energy to heat, while loss tangent used to describe how well a material absorbs microwave energy, is the ratio of dielectric loss factor and the dielectric constant (tan δ=ε″/ε′) [28]. The heating rate of a material under a microwave field is closely related to loss tangent; a material with a higher loss tangent will heat faster compared to a lower loss tangent.

The dielectric parameters (ε′, ε″, and tan δ) of H₂O, polytetrafluoroethylene, C₄H₆O₄Zn, ZnO, and spent catalyst were measured at room temperature at 2.45 GHz by the cavity perturbation method, and the results are presented in Table 2.

As the results shown in Table 2, the system error of the dielectric coefficient was estimated to be 3%–5% during the perturbation method tests. The dielectric constants of deionized water (80.4 F/m) [29] and polytetrafluoroethylene (2.08 F/m) [30] were used as standards, resulting in 79.79 and 2.04 F/m, respectively, 0.76% and 1.92% differences. Thus, the results of this evaluation system were credible, which had certain valuable feasibility and applicability. Also, it was found that the loss tangent of the spent catalyst was higher than those of C₄H₆O₄Zn and ZnO. Therefore, the spent catalyst could be easily heated under a microwave field.

3.2 Temperature rising of spent catalyst

In the microwave field, the temperature-rising characteristics of materials are closely related to the amount of materials and the microwave power.

The spent catalyst quality affecting heating behavior is shown in Figure 5A under the microwave output power of 900 W. The relationship between the temperature of the spent catalyst sample and the time with different masses of 50 g, 100 g, and 150 g, respectively, and the empirical equations are shown in Eqs. (1)–(3):

Figure 5:

(A) Spent catalyst heating rate (900 W) curve at a different quality; (B) spent catalyst heating rate (100 g) curve at a different microwave power.

The results showed that the spent catalyst average heating rates were 40°C, 36.5°C, 26°C/min for different masses of 50 g, 100 g, and 150 g, respectively, and the temperature reached 920°C, 840°C, and 600°C in 23 min. Thus, the smaller the quality of the spent catalyst, the faster is the apparent heating rate. The greater spent catalyst mass has a smaller heating rate, which is consistent with the experimental results. On one hand, the material quality increased, when the unit mass microwave power density ratio was decreased. Meanwhile, the materials increase the contact area and heat the external environment; on the other hand, when the microwave power is constant, the greater the amount of soot material, the harder is the microwave penetrating uniformly into the inside of the material. Therefore, during the experimental range, with the increasing smoke quality, the microwave ability of the spent catalyst under microwave heating is weakened, and the heating rate is decreased.

The heating curve of 100 g of spent catalyst at microwave powers of 700 W, 900 W, and 1100 W, respectively are shown in Figure 5B. The empirical formula of the sample temperature and time could be seen from Eqs. (4) to (6),

It can be seen from Figure 5B that the main influence of the microwave power on the temperature of the materials was that the microwave power increased, the apparent average heating rate of the spent catalyst increased, and the time to attain the same temperature was shortened. The conclusion is as follows: within a certain range, there is an increase in the material temperature with increasing microwave output power.

In addition, the unit volume of the spent catalyst absorbed the microwave power, or the microwave energy dissipated power in smoke and could be expressed as the following equation [31],

In Eq. (7), P is the microwave energy of the material absorption, ƒ is the microwave frequency, ε″ is the dielectric loss factor, which is a function of temperature, and E is the electric field strength.

Equation (7) showed that to improve the power of the microwave heating means to increase the electric field intensity in the case wherein other conditions remained unchanged. With the electric field strength (E) increasing, the microwave energy could be better and uniformly penetrates into the interior of the material. With constant smoke penetrating the microwave deeper, smoke absorbed more microwave power and temperature increased. Therefore, increasing the microwave heating power properly shortened the heating time and improved the smoke apparent average heating rate.

3.3 Regeneration of spent activated carbon

In order to evaluate the reactivation efficiency of the RAC, the effects of the reactivation temperature and time on iodine adsorption capacity and the yield of the RAC were investigated. The iodine adsorption value of the activated carbon had been tested according to the National Standard Testing Methods of P. R. China (GB/T12496.8-1999), and the yield was calculated by Eq. (8),

where M₀ is the weight (g) of the spent catalyst activated carbon, and M is the weight (g) of the RAC.

The experimental research results are shown in Figure 6A and B. As can be seen in Figure 6A, the iodine adsorption value of the RAC increased (from 368.55 mg/g to 930.55 mg/g) when the temperature increases from 25°C to 1000°C, and the percentage yield of the RAC decreases continuously from 100% to 65.8%.

Figure 6:

Effects of the temperature (A) and time (B) on iodine adsorption value and yield.

The effects of regeneration time on the iodine adsorption values and the yield of the regenerated activated carbons are shown in Figure 6B. As can be seen, the regeneration time had great influence on the development of the iodine adsorption values and the yield. At the initial stage, as the hold time increased from 0 to 40 min, the iodine adsorption values increased significantly from 368.55 mg/g to 880.62 mg/g, and the yield of the RAC decreased from 100% to 70.55% with the regeneration time increasing from 0 to 1 h, indicating that the pores were formed and enlarged simultaneously in this stage. With the activation time going on, the iodine adsorption values and the yield of the RAC gradually became slow and remained constant.

The iodine adsorption value and yield under different regeneration methods are shown in Table 3. The experiment results in Table 3 shows that the iodine adsorption value, the yield of the spent catalyst, and the RAC by microwave regeneration method (MRM) were higher than those prepared with thermal regeneration method (TRM) by muffle furnace. At the same time, under the constant activation temperature, the microwave regeneration time was significantly shorter, and the iodine adsorption value and yield of RAC by MRM were significantly improved.

3.4 Characterization of the reactivated activated carbon

3.4.1 Pore structures analysis

For the structural and chemical characteristics of the spent catalyst, the RAC prepared by MRM and TMR was characterized by nitrogen adsorption isotherm, cumulative pore volume distribution, and pore size distribution, as shown in Figure 7A–C, respectively. Nitrogen isotherm analysis showed that the RAC prepared by MRM has great higher surface area, total pore volume, and a relatively smaller average pore width compared to that of the RAC prepared by TRM and the spent catalyst as shown in Figure 7 and Table 4. This result clearly indicates that the pore structure can be significantly improved with microwave regeneration treatment.

Figure 7:

(A) Nitrogen adsorption isotherms on spent catalyst and RAC; (B) total pore volume distribution for spent catalyst and RAC; (C) pore size distributions for spent catalyst and RAC.

Table 4:

Textural parameters of spent catalyst and regenerated activated carbon.

Material	BET surface areas SBET (m2/g)	Pore volume (cm3/g)	Average pore size (nm)
Spent catalyst	264.1	0.22	33.58
MRM (40 min)	743.6	0.54	28.83
TRM (40 min)	628.5	0.49	30.88

1. BET, Brunauer–Emmett–Teller.

3.4.2 XPS analysis

The surface chemical structures of the spent catalyst and the RAC were also determined with XPS (Thermo ESCALAB 250Xi, Thermo Fisher Scientific Company, USA). XPS was applied to further investigate the state of zinc, and the spectrums of the spent catalyst and the RAC by MRM at 900°C are shown in Figure 8. Significant intensities of the C1s (284.0 eV), O1s (531.0 eV), and the F1s (684 eV) levels for the spent catalyst are shown in Figure 8A, while the observed peaks at 1044.0, 1021.0, and 88.0 eV correspond to the Zn2p1/2, Zn2p3/2, and Zn3p levels, respectively. The relation was reported by Mekhalif et al. [32]. However, the peaks of F1s, Zn2p, and Zn3p line for the RAC in Figure 8C were not found.

Figure 8:

XPS spectra of the spent catalyst (A, B) and the RAC (C, D).

To better understand the characteristics of the Zn substrates, The C1s spectra of the Zn substrates for the spent catalyst and the RAC were studied and are shown in Figure 8B and D, respectively. Figure 8B and D shows a common main peak at 284.6 eV, which is a characteristic of aliphatic carbons [32]. The rather important feature of Figure 8B is the concentrate at 288.0 eV, and it comprises contributions of oxidized carbon species (O-C=O) [32], [33]. This is corroborated by the presence of oxidized carbon species (C₄H₆O₄Zn·2H₂O and other organic impurities) noted for the spent catalyst; however, these oxidized carbon species have been completely decomposed by microwave regeneration treatment, as shown in Figure 8D.

3.4.3 XRD analysis

The characterization of the RAC prepared by MRM was performed after microwave roasting. The XRD pattern of the RAC by MRM at 900°C was shown in Figure 9B. It was matched with that of the spent catalyst XRD pattern (Figure 9A). The results showed that the peak for carbon (C) is enhanced significantly, and the peaks for both C₄₉H₆₆O₃₃, C₄H₆O₄Zn·2H₂O, and C₁₉H₂₂O₆ disappeared. At the same time, the ZnO formants were found.

Figure 9:

XRD patterns of the spent catalyst (A) and the RAC (B).

3.4.4 SEM-EDS analysis

In order to determine the separation mechanism that is associated with activated carbon and impurities, the structural characteristics of the spent catalyst and the RAC at different temperatures were studied with SEM-EDS, and the results are shown in Figure 10.

Figure 10:

SEM patterns of the spent catalyst and the RAC at different temperatures. (A) Spent catalyst, (B) 400°C, (C) 500°C, (D) 600°C, (E) 700°C, (F) 800°C, (G) 900°C, (H) bright white particles in 900°C.

Figure 10A and B shows that the organic impurities were observed at the surface of the activated carbon after activation. The patterns of RAC at 400°C were characterized by SEM-EDS, as shown in Figure 11. The EDS line scanning from point a to point b shows that the major substances of the surface materials in activated carbon are organic compounds that contain C, O, and Zn. It is a process wherein the organic impurities were aggregated from the pore internal migration to the surface under microwave irradiation (compared with Figure 2).

Figure 11:

SEM/EDS patterns of the RAC at 400°C.

The pyrolysis creates gradual porosity (Figure 10B–G) as the temperature increases from 400°C to 900°, and the porosity is more developed. Macropores were formed after activation for 40 min at 800°C (Figure 10F), and the pores of the spent catalysts were still saturated with some by-products at 800°C, which may have remained inside the micropore network of the spent activated carbon and will not decompose until reaching a high temperature. However, more macropores were obtained after activation for 40 min at 900°C (Figure 10G). The comparison of Figure 10F and G indicates clearly that a regular macroporosity and a rather homogeneous surface are obtained by activation at 900°C for 40 min, and the organic impurities were completely decomposed. In addition, when the activated carbon and the organic impurities were separated, simultaneously, some bright white by-products were obtained. These bright white particles were analyzed by SEM-EDS, and the results are shown in Figure 12. The results show that these by-products are ZnO, and their structure is a hexagonal crystal.

Figure 12:

SEM/EDS patterns of the by-products.