Design and Numerical Simulation of a High-efficiency Microwave Applicator for Industrial Processes


Table 3: Determination of Energy Efficiency of Microwave Cavity Containing a Water Sample at 300w Set Power and 60 Seconds Intervals (Frequency 2.45GHz).


Deflector Reflector Forward Reflected Initial (w) (ºC) Final (ºC) Energy


Angle Position Power Power Temp. Temp. Efficiency (Degree) (mm) (w) A B


(%) 90 C Figure 4: Bench-top Pilot-scale Microwave Reactor Facility 291 9 22.4 45.5 80


View port


Waveguide


The cavity is highly resonant and supports nearly perfect standing waves. The electric field radiation pattern was uniform with high intensity. The animation of electric field (animation figures are available as supplementary materials or from the authors) also confirmed the excellent field uniformity with no hot spots observed inside the cavity. Furthermore, a network analyser was used to determine the lowest reflected power of real samples (i.e. water). The values of S parameters were obtained from the screen of the network analyser. Reflected power values using different reflector plate positions and various inserted deflector plate angles are shown in Table 2. Finally, optimised results from simulations and reflected power measurements by the network analyser are used to determine the energy efficiency of water samples in the newly designed microwave cavity (see Table 3).


Reflector plate


Online micro gas chromatograph


Microwave generator


Water load


Microwave-assisted Production of Ethylene from Ethane We have undertaken a research programme aimed at assessing the viability of using microwaves as a processing tool in the industrial sector. More specifically we have an interest in assessing and validating applications for the petrochemical and petroleum sectors with the aim of reducing energy intensity and greenhouse gases emissions. Ethylene production is an important industrial activity in the petrochemical industry. NOVA Chemicals, Canada’s largest producer of ethylene, has a publicly reported production capacity of over 2.5 billion kg per year. The process used is very simple:7


de-embedded again. The rounded slots are created by picking the inner edges and applying the blend edges (1mm). The inclined angles of slots are investigated from θ = 0º (non-radiating longitudinal slot) to a maximum value for θ = 90º (transverse slot) in 5º steps. The slot lengths are investigated from 49 to 59.96mm in 1mm steps. The slot widths are investigated from 1 to 15mm in 1mm steps.


CST Microwave Studio (version 2006B) software was used to characterise slotted waveguides. The slot lengths, widths and angles are important for obtaining a uniform and high electric field inside the cavity. The slot length 50mm, slot width 6mm and slot angle 45º results show uniform and high electric field intensity.


Energy Efficiency of Microwave Applicator A system is usually considered well-designed and properly operational if <10% of the energy delivered to the cavity is lost due to all kinds of reflections. Taking control over this characteristic is important: the reflections could be high for a particular configuration of the cavity and parameters of load. These factors may remarkably decrease the efficiency of the whole system. Preliminary empirical validation of the cavity showed reflection of 36%. Such a large reflection would void any positive impact the microwaves may have from an energy standpoint. Such a nominal value for the reflection pointed to a mechanical fabrication flaw. A small adjustment into the slot configuration, made to correct for the longitudinal imperfection of the waveguide itself, led to reflections of less than 2%. To enhance energy efficiency of the microwave cavity, various angles of deflector plates are inserted through the waveguide inlet side (see Figure 2).


Figure 3 shows the electric field pattern of a water sample inside of the cavity. The electric field strength inside the cavity and electric field inside the sample is determined by using CST simulations (see Table 1).


74


the ethane is fed into long tubular


reactors at temperatures of roughly 850 to 900°C and at pressures slightly over ambient. Once the materials leave the reactor it must be cooled to 450°C within 50 milliseconds to prevent degradation of the ethylene into side products.


One of the main problems with the conventional heating process is the overheating of some zones within the reactors that lead to ‘coking’: a process that usually results in significant losses in production and reduced efficiency. To mitigate this problem, the process makes use of injected steam to reduce the partial vapour pressure of ethane and to


Microwaves have been shown to be effective in many applications in areas of oil production, from underground extraction to upgrading and refining.


‘clean’ the surface of the reactor and minimise the potential for hot spots along the tubular structure. In fact, about 30% of the mixture is steam. It is believed that thermal control can be improved using microwaves because microwaves do not inherently suffer from thermal inertia. In fact, a large amount of the energy used in the process is due to the requirement to generate the steam.


Water has a heat capacity roughly double that of ethylene, thus accounting for almost 50% of the energy needed in that process unit. Removing or reducing the amount of steam required would have a significant effect on the energy requirements of the process. A pressing concern is that scaling up a cavity with uniform electrical field quality


HYDROCARBON WORLD – VOLUME 6 ISSUE 1


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