Using Low Energy-Consuming Microwave Technology to Regenerate High Carbon-Containing Pt Catalyst

In this research, the high carbon-containing (coke, >20 wt%) Pt catalyst wasted from a full-scale petrochemical plant are regenerated in quartz and borax glass reactors by applying 250 W microwave irradiation for 18 hours. When irradiated with microwave substances of various dielectric characteristics will consume the microwave energy; the quantity of energy consumed is proportional to dielectric constant. Because borax glass has higher dielectric constant than quartz glass (e.g. 4.6 vs. 3.6 F/M), it causes higher energy loss than quartz glass. Laboratory results reveal that the removal of coke is smaller for the sample placed in the borax glass reactor than in the quartz glass container (17.7 vs. 25.9 %). On the other hand, higher MW output power leads to stronger electrical field to influence the mutual reactions between microwave and dielectric materials (e.g. the absorption of microwave energy), and the overall energy efficiency. The laboratory results also show that when the microwave energy is increased from 250 to 450 W, the carbon removal rate is raised from 25.9 to 38.1% with the sample placed in quartz glass reactor. Using the microwave energy to regenerate high carbon-containing (coke) Pt-catalyst as studied in this research is an innovated technology.


Introduction
Prosperous global economic development (D'Ippolitoa et al., 2010;Hu & Kao, 2007) leads to continually increasing demand of energy that gradually depletes reserves of petrochemical energy (Veziroğlu & Şahin, 2008;Tonna & Peretz, 2007;Bahadori & Vuthaluru, 2010;Ismail & Mehta, 2011).One solution to alleviate this problem is to renew and improve the petrochemical and petroleum processes for improving the product quality and increasing the product add-on value that will stimulates the rapid development of petrochemical industries.These measures include: recombining heavy naphtha to raise the gasoline octane value (Marafi & Stanislaus, 2008), and cracking naphtha into olefins to adjust the supply of gasoline and increase the fuel value (Mazzieri et al., 2008).
The catalyst used in petrochemical and petroleum industry are converted to coke or carbon products during the process of hydrocarbon cracking or condensation; the carbon blocks the catalyst particle surface pores thus causing the catalyst to lose its activity that adversely affects the quantity and quality of products and by-products as well as the objective of waste reduction (Rajaei et al., 2012).For example the loss of activity for Pt-based catalysts is caused by either deposit of carbon or sintering of metal particles, and the formation of coke is regarded as the major reason for loss of catalyst activity (Nagaokaet al., 2001).Additionally, when the catalyst oxidation temperature is raised to 400~550 o C in the absence of chlorine gas, Pt sintering may also contribute to the loss of catalyst activity (Arteaga et al., 1999).For industrial application, the coke deposit is usually removed with oxidation in a fluidized catalyst unit operation to regenerate the catalyst for maintaining the catalyst high reactivity (Dimitriadis et al., 1998).However, during this regenerating process, hot spots may be formed in the catalyst bed, and some catalyst sintering may occur.Hence, various oxidizing agents such as oxygen, air, CO 2 , H 2 and N 2 have been used in order to reduce as much as possible the catalyst sintering problem (Lee et al., 2012).Pieck et al. pointed out that the complete oxidation of metallic coke occurs between 280 and 400 o C, whereas complete oxidation of the coke associated with Al 2 O 3 , or the acidic position occurs between 280 to 400 o C (Dimitriadis et al., 1998;D'Ippolitoa et al., 2010).Johnsson and Maryland indicated that treatment of the coke catalyst in inert gas is important prior to measuring the burning rate because the burning results are valid only after all volatile substances have been removed (Dimitriadis et al., 1998).
The microwave energy is converted into heat energy directly through mutual reactions between the molecules to be heated and the electromagnetic field.When a substance is subject to microwave irradiation, the rapid molecular movement causes the temperature to rise, and the temperature increasing rate is proportional to the dissipation factor (tangent δ) that is defined as the ratio between dielectric loss factor (ε″) and dielectric constant (ε′) (Appleton et al., 2005;Ku et al., 2001;Venkatesh & Raghavan, 2004): The dielectric constant is a measurement of the capacity of the substance being irradiated to resist the microwave radiation whereas the dielectric loss factor measures the capacity of the substance to dissipate the energy.In other words, when microwave penetrates a sample, the energy absorbing rate is dependent of the dissipation factor (δ); substances with higher dissipation factor absorbs more microwave energy with higher temperature increase so that they are easier to be decomposed.The microwave radiation may penetrate substances and accumulates energy in the substance being irradiated (Ku et al., 2001), and the energy transmission is dependent of surface dissipation more than geometric related parameters; thick samples can be rapidly and evenly heated with microwave energy as thinner samples (Venkatesh & Raghavan, 2004).Hence, this unique feature for microwave radiation to generate and transfer heat has been used extensively as an effective method for decomposing chlorine-containing organic substances.For example, methods using microwave-induced nano-scale zero-valence iron to decompose chlorobenzene (Jou et al., 2011(Jou et al., , 2010;;Lee & Jou 2011), tetrachloroethylene, and pentachlorophenol (Lee et al ., 2012), and microwave-included nano-scale zero-valence copper to decompose benzene are quite efficient with many advantages.Additionally, the microwave-induced waste cordierite combined with nano-scale zero-valence iron, titanium oxide, and copper/iron bi-metal are also effective in decomposing chloro-containing organic substances (Lee & Jou, 2013, 2012;Lee et al., 2010).
With increasing applications of catalysts, the need for regenerating the wasted catalyst becomes more important.
Industries have used batch or continuous-flow methods to carry out the regenerating process for extending the useful life of catalysts in order to reduce the production costs.The occurrence of hot spot and sintering of some catalyst in the catalyst bed makes it necessary to replace some catalyst in the bed especially for catalysts containing more than 20% coke.The replaced coke needs to be regenerated and reused to alleviate environmental impacts.The wasted Pt catalyst with high coke content has high dielectric constant so that it is a perfect media to absorb microwave radiation and convert it into heat energy.Such characteristics can be used to regenerate the wasted high coke-containing Pt catalyst using microwave irradiation for more efficient with less adverse environmental impacts.In this research, regenerating the wasted Pt catalyst that contains high percentage of coke using microwave irradiation is studied to test the feasibility of this regeneration method.The success of this study will be developed into a novel concept that is different from the conventional method in both theory and practice for regenerating wasted Pt catalyst.

Experimental Equipment
The microwave oven used in the laboratory study is equipped with PID to control the output power; its frequency is 2.45 GHz with the maximum output of 750 W. All reactors are custom-made 80-mL borax and quartz ground glass containers with wide mouth connected to a tail gas collecting pipe made of the same material.

Experimental Method
The Pt regenerating experiment was conducted batch-wise operations with each batch using 30 grams of coke that contains 30-35% Pt catalyst.Each batch consisting of 5 reactors each containing 6 grams of the coke/Pt catalyst samples were prepared in laboratory, and then subject to either 250 W or 450W microwave irradiation.
The exposure time for each batch is was set at 5 durations of 0, 4, 8, 13 and 18 hours.The microwave treated samples were analyzed using an impedance analyzer (Agilent Co, 4291B) with 2.45 GHz at 25 o C. Samples of the wasted Pt-containing coke were found to have 11.7 F/m dielectric constant, and 13.7 F/m dielectric loss coefficient.

Influence of Reactor Material on the Removal of Coke from the Wasted Pt Catalyst
Irradiation of a substance by microwave will cause loss of energy, and the loss is proportional to the dielectric constant of the substance.In this research, the Pt catalyst that contains high percentage of coke is put in a reactor, and the reactor is placed in the microwave oven to be irradiated for carrying out the regeneration experiment.If the reactor has a high dielectric constant that leads to high energy loss, the microwave energy that passes through the reactor wall to reach the Pt catalyst will be proportionally reduced.The loss of microwave energy when the microwave radiation passes through the container will reduce the quantity of microwave energy to reach the sample being regenerated and the amount of the microwave energy to penetrate the sample media.The microwave penetration is defined as when the microwave energy that reaches the sample has been reduced to 36.8% of its original level; Where: dp = penetration depth (cm); λ o = Microwave free space wavelength (12.2 cm) On the other hand, when microwave passes through the coke contained in Pt-catalyst, the Pt-catalyst/coke internal energy instantaneously rises, and the stored energy is subsequently released to cause molecular polarization and electronic vibration, these movements lead to high temperature due to friction.When subject to 250-W microwave irradiation for 4, 8, 13 and 18 hours in quartz-glass reactor and borax-glass reactor, the Pt-catalyst coke shows higher coke removal efficiency in quartz-glass reactor than in borax glass reactor (25.9 vs. 17.7%)(Figure 2).The dielectric coefficient and dielectric loss coefficient for borax glass are 4.6 and 0.017 F/M, respectively; they are higher than those (3.6 and 0.0018 F/M, respectively) for quartz glass so that more loss of microwave energy is observed for borax-glass reactor than quartz-glass reactor.This observation is also confirmed by the results calculated using Equation (2).

Influence of Microwave Sources on Coke Removal from Wasted Pt Catalyst
Microwave energy is directly transmitted though the mutual reaction between molecule and electromagnetic filed to convert the electromagnetic energy into heat energy.The high electrical field frequency causes the metal contained in the catalyst to absorb more microwave energy thus leading to stronger electro-potential attraction.This causes the electrons to move rapidly to produce more friction heat.On the other hand, the microwave output power will affect the electrical field intensity inside the material; a larger output power leads to stronger electrical field so that the mutual reaction between microwave and media (absorption of microwave energy), and the overall energy efficiency are affected.When Pt-catalyst containing coke is placed in quartz glass reactor to be irradiated with 250 W and 450 W microwave for 0, 4, 8, 13, and 18 hours.Results shown in Figure 3 reveal that the carbon removal efficiencies increase from 25.9% for 250 W to 38.1% for 450 W.
The high coke-containing Pt-catalyst has been removed from the catalyst removed from petrochemical industrial processes, it still contains volatile organic matter; the VOC is found with laboratory measurement to release 70 ppm at 23 o C and 170 ppm at 55 o C. Because the laboratory studies conducted in this research were carried out under anaerobic conditions, during the initial stage of microwave irradiation, all VOC's contained in the Pt-catalyst undergo pyrolysis to attach on the catalyst particle surface that adversely impact the efficiency of carbon removal.The calculation using Eq. ( 2) shows that the penetration depth for microwave is 4.9 mm.Using X-ray photoelectron spectroscopy (XPS) to analyze the Pt-catalyst, the resulting the X-ray photoelectron spectra (Figure 4) demonstrate that the surface substance is PtCl 4 .
When the Pt-catalyst containing more than 20 wt% coke is used in the process, the vigorous activation of coke produces high temperature thus leading to the sintering of catalyst to form lumps with coke that may affect the transportation of the catalyst and even cause accidents.Hence, such regenerated Pt-catalyst coke needs to be replaced with new Pt-catalyst; this catalyst replacement is costly.However, the replaced Pt-catalyst coke can be regenerated using microwave energy to re-activate Pt-catalyst.Hence, the wasted Pt-catalyst can be recycled and reused.The use of microwave regeneration method is a new technology allowing the petrochemical industry to achieve economic sustainable development for the petrochemical industry.

Conclus
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