Reactors Effects on Microwave Decontamination of Oily Wastes in a

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Reactors Effects on Microwave Decontamination of Oily Wastes in a Multimode Cavity Hui Shang, Samuel W. Kingman,* Colin E. Snape, and John P. Robinson School of Chemical, EnVironmental and Mining Engineering, UniVersity of Nottingham, UniVersity Park, Nottingham, NG7 2RD, United Kingdom

Three reactors were designed and used for the microwave remediation of oil-polluted wastes in a high-power microwave multimode cavity; a pressure model was also built based on reactor shapes and sizes. It was found that the reactor shape and size can extremely affect the electric field strength and distribution and, therefore, the efficiency of oil removal from the wastes. The reactor size would affect the pressure produced during treatment and thus affect the boiling point of the oil. The sweet gas distribution is also a factor for oil removal, although the flow rate has little effect on the oil removal when a uniform distribution is applied with a specific reactor. It was verified that multimode cavities are best-suited for big volume samples, in the case of uniform heating. 1. Introduction Microwave dielectric heating is a well-established procedure, not only for the domestic preparation of meals, but also for its widespread industrial use in the processing of food and industrial materials.1-3 The heating effect results from the interaction of the electric field component of the wave with charged particles in the material.4,5 If a material has a dipolar molecule in the structure, such as water, the dipolar polarization will occur in the electromagnetic field. Dipolar polarization is the phenomenon responsible for the majority of microwave heating effects that are observed in polar systems. The dipole is sensitive to external electric fields, and it will attempt to align with them via rotation, with the energy for this rotation being provided by the field. The microwave frequency is low enough that the dipoles have time to respond to the alternating field (and, therefore, to rotate), but also high enough that the rotation does not precisely follow the field. As the dipole orientates to align itself with the field, the field is already changing and a phase difference exists between the orientation of the field and that of the dipole. This phase difference causes energy to be lost from the dipole in random collisions and results in dielectric heating.6-8 Materials that absorb microwave radiation are called dielectrics; thus, microwave heating is also referred to as dielectric heating.9 The use of microwave energy for processing materials has the potential to offer similar advantages, such as reduced processing times and energy savings based on the mechanism of microwave heating. In conventional thermal processing, energy is transferred to the material through convection, conduction, and radiation of heat from the surfaces of the material. In contrast, microwave energy is delivered directly to materials through molecular interaction with the electromagnetic field.10 The internal temperature distribution of a material subject to conventional heating is limited by its thermal conductivity, whereas microwave heating results in all the individual elements of the material being heated individually.11 1.1. Microwave Applicators. The term “applicator” is used in microwave heating to refer to a device into which a material is inserted for processing. The temperature distribution within * To whom correspondence should be addressed. E-mail address: [email protected].

the material undergoing microwave heating is inherently linked to the distribution of the electric fields within the applicator.5,10 For processing materials, resonant applicators, such as singlemode and multimode applicators, are most common, because of their high field strengths.12 The mode is the field pattern that exists inside the cavity. As the name implies, single-mode cavities sustain only one mode; multimode cavities, on the other hand, sustain many modes and is used for bulkier items and for batch or continuous processes. Multimode cavities are the most commonly used form of microwave heating applicators, comprising well over 50% of all industrial applications and all domestic ones.13 In principle, a multimode cavity consists of a metal box at least several wavelengths long in two dimensions. Multimode cavity has advantages such as low cost, simplicity of construction, and versatility,14 whereas the design of the single-mode cavity is based on the solution of Maxwell equations to support only one mode at the source frequency.10 The multimode cavity is best-suited to large volume loads (>50 vol %); small loads give a nonuniform temperature profile, because of the uneven field distribution. The single-mode applicator has very high power density, which makes high heating rates possible and can heat low-loss-factor materials. For the single-mode cavity, the electromagnetic field can be determined using analytical or numerical techniques; the areas of high and low electromagnetic field then can be known, through properly designed single-mode applicators can be used to focus the microwave field at a given location.15,16 The main disadvantages are that only relatively small-sized objects can be treated and the material to be processed requires careful placement for optimum heating. The electric field distribution and the power densities within the single-mode cavity can be found in another paper.17 This paper will draw the attention to the influence of the reactor on the oil removal in a multimode cavity. 1.2. Reactor Design. Various dimensions and types of cavities require appropriate reactors to be determined, in which samples can be well-treated. There are five main criteria to be considered, according to microwave heating theory: (1) The reactor material should be transparent to microwaves and be able to withstand appreciable temperature (e.g., glass, quartz). (2) No oxygen should be present within the reactor, which asks the reactor to be easily fitted and makes the system closed.

10.1021/ie070124l CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007

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(3) No metallic components should be included, because, for metals, because of the free electrons, most of the microwave energy does not penetrate the surface of the material, but rather is reflected. However, the colossal surface voltages that may still be induced are responsible for the arcing that is observed from metals under microwave irradiation.8 (4) The removed oil should be condensed and collected. (5) The reactor should be easily placed inside or taken out of the cavity. Based on these considerations, three types of reactors were designed and used in a high-power multimode cavity. The samples should be put inside the reactor and heated by microwaves. Glass and quartz were used as materials of construction, because they are relatively transparent to microwaves and, hence, do not induce any appreciable power loss.11 The microwave heating technique for the treatment of oilcontaminated wastes has been shown to be a potentially important and high-efficiency approach to achieve the required environmental discharge limit.17-19 This study assesses the effects of different reactors on the oil removal in a multimode cavity. 1.3. Numerical Techniques. Numerical techniques are required to provide valuable information on the parameters of the electric and magnetic fields and the power absorbed by the load. The starting point for the simulation is the Maxwell equations, which describe steady-state, sinusoidal time-varying fields and electromagnetic phenomena.10,20 All macroscopic electromagnetic phenomena that occurs in practice can be mathematically described with the complete set of Maxwell equations, a form of which is shown in eqs 1.

I∂AE ds ) -

dA ∫∫A ∂B ∂t

+ J) dA ∫∂AH ds ) ∫∫A (∂D ∂t II

(∂D∂t + J) dA ) 0 IIB‚dA ) 0

(1a) (1b) (1c) (1d)

where E is the electric field intensity, D the electric displacement density, H the magnetic field intensity, B the magnetic displacement density (in units of Wb/m2), and J the current density. In a uniform isotropic medium, there exist simple relationships between B, H, D, E, and J:

B ) µH

(2a)

D ) ′E

(2b)

J ) σE

(2c)

where µ is the magnetic permeability, ′ the dielectric constant, and σ the electrical conductivity. This set of equations can be solved only for particular configurations of simple geometry, e.g., rectangular or spherical structures. For more-complicated shapes, numerical methods must be used. Essentially, the Maxwell equations are solved by finding solutions to match the requirements of the field intensities, which must exist at the boundaries of the structure. The electric field must terminate normally on the conductor (i.e., the tangential component of the electric field must be zero). This is to be expected, because, for a perfect conductor, there can be no potential difference between two points. This condition is often written as

Etan ) 0

(3)

However, the component of the electric field normal to a conducting surface generally has a nonzero value. The component of the magnetic field normal to a conducting surface is zero, because it is not possible to create a magnetic loop, i.e.,

Hnorm ) 0

(4)

The multidisciplinary approach required to integrate numerical modeling with microwave heating applications means that this has rarely been reported in the past, and it has never been published for the application of contaminated solids. The numerical method used for the field simulation in this work is the Finite Integration Technique (FIT), which was first proposed by Weiland in 1977.21 FIT generates exact algebraic analogues to the Maxwell equations, which guarantee that the physical properties of fields are maintained in the discrete space and lead to a unique solution. The electric field intensity changes with time and frequency, as shown in eq 5:

Ei(t) ) E0 sin(2πft)

(5)

where Ei(t) and E0 are the field intensities at time t and t ) 0, f is the frequency, and t is the time. The time domain is used to describe the analysis of mathematical functions, or real-life signals, with respect to time. In the time domain, the signal or function value is known at various discrete time points (or for all real numbers, for the case of continuous time). The time domain FIT is the most efficient scheme for many microwave applications.22 The leapfrog algorithm is used for the discretization of the time derivative, and this scheme is shown in Figure 1. The leapfrog algorithm is very memory-efficient and has the advantage that the calculation of some unknowns at each time step only requires one matrix-vector multiplication (linear complexity). The input parameters required for the numerical simulations are the microwave power and frequency, the sample geometry, and the geometry of the microwave cavity. Also needed are the dielectric constant and the dielectric loss factor of the sample to be studied, which were determined experimentally using a cavity perturbation technique. 2. Experimental Apparatus and Materials 2.1. Experimental Apparatus. A 3-15 kW microwave generator was used, and the schematic of the apparatus is shown in Figure 2. Three different reactorssnamed M1, M2, and M3s were investigated. 2.2. Experimental Procedure. The well-mixed oil-contaminated waste samples were placed inside specifically designed glass and quartz reactors, which were positioned within the multimode cavities. A vacuum pump and a flow of nitrogen were used to aid the removal of oil and water vapor, as well as maintain an anaerobic atmosphere. The gas flow rate was maintained such that the gas velocity in the reactor was not sufficient to fluidize the waste samples. Before microwaves were applied to the sample, the pressure and gas flow rates were set for several minutes to equilibrate to the desired values. After microwave treatment, the waste samples were removed from the reactor and the oil and water content were analyzed. The water content was measured using the Dean and Stark method, while oil was extracted by dichloromethane. The oil/ water removed from the samples was condensed and then

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Figure 1. Schematic depiction of the leapfrog algorithm.

separated, and the oil composition was determined using gas chromatography (GC) or gas chromatography/mass spectroscopy (GC/MS) techniques. 2.3. Materials Characterization. The sample materials selected and used are called drill cuttings, which were obtained from North Sea drilling operations. Drill cuttings are the waste materials removed from wells drilled to explore, evaluate, and produce oil and gas. The cuttings are rock fragments produced by the drill bit. The particle size of the cuttings range in diameter from tens of micrometers to 1-2 cm, depending largely on the nature of the rock.23 The compositions of the cuttings used for this research are sandstone rocks, oil, moisture, mud, and a mixture of chemical additives that are added into the mud developed to perform a variety of functions during drilling; the main groups include additives for density control, viscosity control, formation stability, emulsifiers, wetting agents, and corrosion inhibitors.24 The cuttings samples used within this research have a particle size range of 300 µm, while, when with oil and moisture, the particles agglomerated with the average diameter of the order of 5 mm. The oil inside the cuttings samples is primarily based on low aromatic oil (in the range of C8-C16) with an end boiling point of 250 °C. The dielectric properties are physical properties of materials that influence the response of the material to the applied microwave field. The dielectric constant and loss factor quantify the capacitive and conductive components of the dielectric response.11 The value of the dielectric properties of a material are dependent on their chemical composition, the frequency of the applied electric field, the temperature of the material, the density and physical structure of the material, and the moisture content.11,25 Previous studies have shown that water within the cuttings sample is a good microwave absorber, which has the

Figure 3. Schematic of reactor M1 (borosilicate glass flask, capacity of 1 L), used in the 3-15 kW multimode cavity.

critical role for oil removal. The material used within this research has a dielectric constant of 8.2 and a loss factor of 2.0. 2.4. Reactors Designed for Microwave Thermal Treatment of Oil-Polluted Waste in a Multimode Cavity. Three types reactors, labeled M1-M3, were designed for the multimode cavities. Reactor M1 was composed of very basic components, including a 1-L conical flask, which is a common flask and can be found in almost any chemical analysis laboratories. Figure 3 shows a schematic of reactor M1. The flask volume was 1 L and is suitable for ∼200 g of cuttings sample to be treated. Nitrogen was introduced in the

Figure 2. Schematic of the microwave treatment of oil-contaminated waste in a 3-15 kW multimode cavity.

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Figure 4. Schematic of reactor M2, used with the 3-15 kW multimode cavity.

Figure 6. Effects of specific power and treatment times on the residual oil levels. Table 2. Residual Oil Levels for “Wet” and “Dry” Areas in Reactor M1 Residual Oil (%)

Figure 5. Schematic of reactor M3, used for the multimode cavity. Table 1. Experimental Conditions with Reactors M1, M2, and M3 reactor

power (kW)

sample mass (g)

nitrogen flow rate (L/min)

absolute pressure (mm Hg)

M1 M2 M3

10-15 10-15 5-15

200 40 80-700

1.4 1.4 1.4

360 360 360

middle of the sample bed, to act as a sweep gas during the experiments. To obtain a more homogeneous nitrogen distribution, reactor M2 was designed with the sample resting on a sinter, through which the nitrogen gas passes; this reactor can accommodate ∼40 g of sample. Figure 4 shows a schematic of reactor M2. To handle larger samples, reactor M3 has been designed to accommodate quantities of up to 2 kg. Glass or quartz wool was used to support the sample, and the apparatus is shown schematically in Figure 5. All dimensions are given in millimeters. 3. Results and Discussions 3.1. Effects of Reactors on the Residual Oil Levels. The experimental conditions used with each reactor are highlighted in Table 1.

power (kW)

time (s)

in “wet” areas

in “dry” areas

wet area (%, w/w)

10 10 15 15

22 37 15 22

13 16 15 12

6 2 6 3

24 14 21 8

The residues obtained using reactor M1 were extremely heterogeneous, with distinct dry (light) and wet (darker) areas being evident. This is an example of the nonuniform heating. The wet and dry areas were separated to determine the residual oil contents, and these are shown in Table 2. The wet area percentages in Table 2 were obtained by separating and weighing the wet area and then dividing by the total sample weight. Table 2 presents the residual oil levels of the “wet” and “dry” areas with power levels and treatment times in reactor M1 with the 3-15 kW multimode cavity. At 10 kW, by increasing the treatment time from 22 s to 37 s, the “wet” area reduced from 24% to 14%, for the same treatment time (22 s). By increasing the power level from 10 kW to 15 kW, the “wet” area was reduced to 8%, and, in addition, the residual oil levels both in “wet” and “dry” areas were decreased. Increasing the residence time or the microwave power increased the ratio of the dry area. It is suspected that the heterogeneous nature of the waste samples after microwave treatment results from varying regions of electric field strength (which will be discussed in the later section) or poor distribution of the sweep gas. To improve the mass transfer of volatiles out of the sample, reactor M2 was used to give an improved sweep gas distribution. Reactor M2 was designed with the sample resting on a sinter, through which the nitrogen passes. Although only small samples (∼40 g) can be used in this reactor, the initial tests demonstrated that more-homogeneous samples can be obtained (from observation). An example of the results obtained with this reactor is shown in Figure 6. The reflected power is difficult to record and quantify over time, because the electric field configuration changes as volatiles are lost from the sample. Therefore, the specific power in Figure

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Figure 8. Indication of the difference in sample geometry inside reactors M2 and M3.

Figure 7. Comparison of residual oil levels obtained with reactors M2 and M3 in the multimode cavity.

6 is based on the forward power alone, which is important from an economic point of view. At the same specific power, e.g., at 250 kW/kg, by increasing treatment time from 10 s to 40 s, the residual oil levels can be reduced from 2.7% to 1.0%. Oil levels were reduced as the specific power increased; for example, increasing specific power from 250 kW/kg to 375 kW/kg for 22 s allowed the residual oil levels to be reduced from 1.3% to as low as 0.5%. Note that Table 2 and Figure 6 indicate that high power levels have a greater effect than greater treatment time. This is because, at low power, the heating rate is low, and, hence, heat losses through conduction are likely to be significant. Therefore, much of the applied microwave energy is dissipated into the surrounding media before sufficient temperatures are achieved to remove the oil. At higher microwave powers, the heating rate is high, such that sufficient temperatures are achieved for volatilization to occur without significant heat losses to the surroundings. Furthermore, it is the water within the samples that absorbs the microwave energy. The oil itself is relatively microwave transparent, so the energy to remove the oil must be supplied, in some way, by the water. It is feasible that, in areas of high electric field intensity, the heating rates could be such that the water will superheat and, hence, provide the temperatures necessary for volatilization of the oil. When the heating rate is lower, the water may vaporize at temperatures below the boiling range of the oil, inhibiting its removal. Reactor M3 can accommodate up to 2 kg of oil-contaminated drill cuttings (OCDC) samples. Despite the large sample size, residual oil contents of 10 kW and long treatment times (>50 s). The results from reactors M2 and M3 are compared in Figure 7, which shows the evidence of the relationship between the specific energy input and the residual oil levels with reactors M2 and M3. The specific energy was calculated using eq 6, based on the forward power:

E)

Pt m

(6)

where E is the specific energy, based on the forward power (expressed in units of kJ/kg); P the applied power level (given in kilowatts); t the treatment time (in seconds); and m the sample mass (in kilograms). Figure 7 shows the residual oil levels versus specific energy input for reactors M2 and M3. The energy required in reactor M3 was less than that in reactor M2 to achieve the same residual oil level. For example, when a specific energy of 2500 kJ/kg

was applied, the residual oil levels were reduced to ∼1%, using reactor M2. However, with reactor M3 and the same specific energy input, the residual oil level was 6%, with 5500 kJ/kg required to reduce the oil levels to 1%. The addition of more energy seems to be of little aid in the removal of the last 1% of oil; i.e., after the residual oil level was reduced to 1%, there was no significant reduction by increasing the specific energy input. This is because almost all of the moisture that can absorb microwave energy is lost with heating and it is difficult to couple more energy into the sample, with almost all the energy being reflected instead of being absorbed into the sample. To reduce the residual oil level further, dielectric refreshment or higher heating rate is required, which has been shown in previous papers.17-19 3.2. Discussions. From the aforementioned results, the oil removal efficiency is highly affected by the nitrogen and electric field distributions. The results in Table 2 and Figure 6 above clearly show that reactors M2 and M3 are more effective than reactor M1. This may be due to the fact that the gas distributions in reactors M2 and M3 are more uniform than in reactor M1, and, hence, mass transfer is greatly improved. Reactor M2 was then used for the effects of nitrogen flow rate and the results showed that the nitrogen flow rate did not have a significant effect on the oil removal.19 Therefore, the difference of oil removal efficiency, when a uniform nitrogen distribution exists, may be caused by the reactor geometry, which is related to the penetration depth and the gas pressure inside the reactor, as well as the electric field distributions. 3.2.1. Effect of Reactor Geometry on Penetration Depth. An indication of the difference when the same amount of sample is sitting in reactors M2 and M3 is shown in Figure 8. The sample (same mass) placed in reactor M2 has a greater height and smaller diameter than when loaded into reactor M3, because of the configuration of these two reactors (see Figures 4 and 5). These loads will correspond to areas with different electrical distributions and will influence the electric field pattern. For a similar experimental setup, Di et al.26 showed a large temperature difference along the depth of the sample column early in the heating process. The absorbed power decays with the propagation/penetration distance in an exponential form. As the heating time increases, such an effect diminishes because the dielectric loss factor changes with time in the sample and thus induces the change in power density. Therefore, the shallow sample profile in M3 is less prone to penetration depth effects than the cylindrical profile in reactor M2 and goes some way to explain the improved efficiency. Similarly, the configuration in reactor M3 is more favorable for mass transfer and removal of volatiles, which may also contribute to the difference in efficiency. 3.2.2. Geometry Effects on the Inside Pressure of the Reactor. A further complication with the different reactor

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geometries is the possible pressure buildup inside the reactors during microwave treatment, which may impact upon the boiling point, and, hence, the removal of volatiles. A dynamic massbalance model was developed for the calculation of vapor pressure inside the multimode reactor, and this model was developed starting with eq 7:

m˘ generation ) m˘ + maccumulation

(7)

where m˘ is the mass flow out of the reactor. The generation term refers to the rate of generation of vapor from the waste sample, and the accumulation term represents the gases that remain inside the reactor. The mass flow of generated gas leaving the reactor can be calculated using eq 8, and the theory is based on the gas flow through a nozzle:

m˘ ) AP

xRTγ M [1 + ( γ -2 1) M ] 2

-(γ+1)/[2(γ-1)] (8)

where A is the area of the reactor neck (expressed in square meters), P the pressure inside the reactor (in pascal), M the Mach number (which is the value of gas velocity divided by the speed of sound), R the universal gas constant (8.314 Pa m3/(kg K)), T the temperature (in Kelvin), and γ the specific heat ratio (Cp/ Cv; here, this value is assumed to be equal to 1.4). Assuming ideal gas behavior:

P)

nRT V

(9)

Based on eq 8 and 9, eq 7 can be rewritten as

P)

-kAt G 1 - exp kA Vb

[

(

)]

(10)

where

k)

x

γ γ - 1 2 - γ+1 2(γ-1) M ( )/[ ] M 1+ RT 2

[ (

b)

) ]

1 RT

(11) (12)

G is the generation rate (expressed in terms of kg/s). Equation 10 then can be used as a pressure model to calculate the vapor pressure in a reactor. Based on eq 10, the mass generation rate, the Mach number, the volume of the reactor, and the temperature can affect the pressure; of this set of factors, the mass generation rate and Mach number are the two most important. Generation rates are estimated from experimental data; however, this cannot be done for Mach numbers. The Mach number is assumed to be 0.005 for this discussion, which equates to a realistic gas velocity of 1.7 m/s. A higher Mach number of 0.01 is also considered, to evaluate the sensitivity of the model to this parameter. Figure 9 shows the relationship between pressure and time for reactors M2 and M3. Figure 9 indicates that there is some difference in pressure buildup with reactors M2 and M3 for the same Mach number. Because the boiling point of a material varies with pressure, the ranges of the boiling point of the oil in reactors M2 and M3 are therefore different. However, the pressures achieved within the reactor are relatively small, and they are not likely to have a significant impact on the boiling range in either case; therefore, the electromagnetic field within the sample is believed to have a key role in oil removal.

Figure 9. Pressure buildup with treatment time for reactors M2 and M3 in the multimode cavity.

3.2.3. Effects on Electric Field. There are interactions between the electric field and materials inserted into the cavity. Different sample positions result in various electric field strength and distributions; therefore, a small sample has the chance in a very low or high electric field area. Although the heating seems much more uniform, because of small sample size, the results can be quite different if the sample cannot be set in exactly the same position for each test. When a large sample amount and a big reactor are used, the sample can occupy as many modes that allow the sample to be heated more homogeneously and the results would be more reliable. Materials are heated by interacting with microwaves. When a material is inserted in a microwave cavity, the electric field pattern consequently changes with the material’s physical properties, such as dielectric properties, conductivities, and geometries. The electric field distributions can be simulated using the proper technique to sort out the Maxwell equations. In this study, the electric field patterns were simulated using FIT, which was first proposed by Weiland.21 FIT generates exact algebraic analogues to the Maxwell equations, which guarantee that the physical properties of fields are maintained in the discrete space and lead to a unique solution. The simulated electric field that is observed when reactor M3 is used with 1 kg of OCDC sample and reactor M1 is used with 200 g of sample are shown in Figures 10 and 11. The electric field patterns in Figures 10 and 11 have significant differences, which verified the fact that the sample size and sample geometry each have an obvious influence of the electric field. The electric field strength within the sample area of reactor M3 is much higher than that with reactor M2. Because of the fact that, from the experiments, a presence of