Continuous Flow Microwave System Research Articles

Continuous-flow microwave heating system with high efficiency and uniformity for liquid food

Continuous-flow microwave heating has good potential in liquid food processing. Efficiency and uniformity of continuous-flow heating are critical to processing results. During continuous-flow heating, the dielectric constant of the liquid varies with increasing temperature, and the efficiency and uniformity of continuous-flow heating are compromised under significant dynamic variations in the dielectric constant of the liquid. To address these issues, this paper proposes a continuous-flow microwave heating system based on a coaxial inner conductor structure. First, a multiphysics model was developed, the parameter of the continuous-flow heating system is optimised so that the load effectively absorbs electromagnetic waves, and the heating uniformity can be enhanced due to the uniform cross-sectional electric field distribution of TEM mode in the coaxial structure. Second, experiments were performed to validate the proposed model, and the results were in good agreement with the simulation. The heating efficiency under varying dielectric constants (from 10 to 80) and loss angle tangents (from 0.1 to 0.5) of the liquid was above 90%. Then, the proposed system is compared with multimode cavity and system without waveguide coaxial conversion structure, and the proposed system is more efficient for liquid with different dielectric constants. Next, a section of metal bar was added to the center of the glass tube and the influence of the metal bar on heating effect was analysed. With the addition of the metal bar, the efficiency of the system in heating a low-dielectric-constant liquid could be effectively improved, and the heating uniformity could be increased by more than 30%. Finally, the proposed system was applied to heat liquid food and the results indicated that all of liquid food materials' efficiency larger than 90%. The continuous-flow microwave system provides the possibility for large-scale industrial production.

Multicomponent Synthesis of Potentially Biologically Active Heterocycles Containing a Phosphonate or a Phosphine Oxide Moiety

Several multicomponent synthetic approaches were elaborated for plenty of novel nitrogen or oxygen heterocycles containing a phosphonate or a phosphine oxide moiety. All multicomponent reactions were optimized through a model reaction in respect of the heating mode, molar ratio of the starting materials, atmosphere, catalyst, temperature, reaction time and solvent applied, and then, the extended preparation of small libraries of structurally-related compounds was performed. Most of the reactions could be considered as "green syntheses", as they were carried out in the absence of any catalyst and/or solvent using microwave (MW) irradiation or even at ambient temperature. The scaling-up of a MW-assisted synthesis was also elaborated in a continuous flow MW system. Altogether more than 150 heterocyclic organophosphorus compounds were synthesized, among them several derivatives showed moderate or promising activity against the HL-60 cell line and Bacillus subtilis bacteria.

Steam replacement strategy using microwave resonance: A future system for continuous-flow heating applications

This study proposed the replacement of the steam heat exchanger by a novel continuous-flow microwave system. The current microwave heating system has two main drawbacks for continuous-flow applications: higher reflection losses, which reduce the effective microwave power and pose potential safety risks, and an unsteady heating rate, which leads to insufficient heating in a limited time and lower energy conversions. Therefore, we introduced the conceptual design of a novel continuous-flow microwave system for overcoming the specific drawbacks to the largest extent possible. A numerical model of this designed system was developed based on a waveguide-type cavity resonator after fully considering the properties of energy conversion and absorption. Rather than the previous systems, the combined input of waveguides on the E- and H-planes can not only improve over 20% energy utilization, but also facilitate the heating effect of the fluids. Noteworthily, the 45°-tilted waveguides can significantly reduce the reflection losses on the both input direction by enhanced microwave resonance effect, which can further promote the energy efficiency. In addition, the stub tuner was used to match impedance for reducing the application risks. Compared with the traditional steam heating methods, this novel continuous-flow microwave system with higher energy efficiency design is attractive for the dairy industry due to its outstanding electromagnetic responses and cleaner processing.

Continuous flow microwave system with helical tubes for liquid food heating

Abstract The structural parameters of tubes (straight or helical tubes) might affect the heating liquid foods in continuous flow microwave system. Therefore, the objectives were to build a mathematical model for continuous flow microwave heating system with helical tubes for liquid foods heating. A finite element calculation was used for the model, and distilled water, as a representative of liquid foods, was used to validate experimentally and numerically. This model was employed to analyze the liquid heating efficiency with different pitch circle diameters (PCDs), tube diameters, and screw pitches of helical tubes. The heating efficiency was affected by electromagnetic power density distribution, secondary flow intensity, and dielectric properties of heated fluid. Furthermore, the optimal heating efficiency with these variations were given. This study is expected to provide advice to enhance the heating efficiency of liquid foods in design and optimization studies for continuous flow microwave system.

Performance of combined technologies for the inactivation of Saccharomyces cerevisiae and Escherichia coli in pomegranate juice: The effects of a continuous‐flow UV‐Microwave system

AbstractDuring the past decade, the application of new technologies such as high pressure, ultrasound, pulsed electric fields, short‐wave ultraviolet C (UVC) light, and microwave (MW) has been thoroughly investigated for food processing. Combinations of UVC light with different technologies have been previously investigated, with promising results when combined with mild thermal treatments. The objective of this work was to study the inactivation of Saccharomyces cerevisiae and Escherichia coli inoculated in pomegranate juice within a unit of UVC‐coiled (254 nm) equipment and the continuous‐flow MW (876 W energy output). Individual and combined treatments were evaluated at two flow rates through the tested systems. The combined technologies notably improved the inactivation of the target microorganisms. Our findings indicated a synergistic effect between the systems, respectively yielding 6.1 and 5.5 log‐cycle reductions in the yeast and bacteria for the UVC–MW treatment with a flow rate of 400 ml/min. When increasing the flow rate, different results were observed, depending on the temperature pattern. A dynamic model was obtained for MW temperature profiles and residence time distributions for the studied systems to better explain the obtained results.Practical applicationsNowadays, alternative food‐processing technologies are under research to obtain safe, fresh‐tasting, and nutritive foods using mild temperatures or no heat. This work aimed to evaluate a configuration wherein a unit of UVC‐coiled equipment was coupled with a continuous‐flow MW system. Processing times for treating juices can be significantly reduced by combining a UVC coiled equipment with a continuous flow microwave system, because of the enhanced effect of UVC at mild temperatures and the faster heating due to the MW. Its potential implementation will allow the food industry to offer minimally processed products with clean labels and at an affordable cost.

Development of large-scale oxidative Bromination with HBr-DMSO by using a continuous-flow microwave system for the subsequent synthesis of 4-Methoxy-2-methyldiphenylamine

An efficient procedure of continuous-flow microwave reactor, for oxidative bromination with HBr (hydrogen bromide) -DMSO (dimethyl sulphoxide) in large scale is described herein. The reaction is carried out in a metal-free manner, with the use of water as solvent under microwave irradiation conditions. The productivity of this continuous step attains 60 g/min under the optimal conditions, implying a theoretical productivity of 8.6 kg/day. The reaction mixture is screened by thermal insulation accelerometer to ensure the safe process of this reaction in these conditions. 4-Bromo-3-methylanisole is further reacted with formanilide in the presence of cuprous iodide (CuI) and potassium carbonate (K2CO3) to give 4-methoxy-2-methyldiphenylamine, a common intermediate in fine chemical industry such as dye and paper chemistry, in an isolated yield of 65.2%. with more than 200 g scale.

Improved canola oil expeller extraction using a pilot-scale continuous flow microwave system for pre-treatment of seeds and flaked seeds

Continuous flow microwave (MW) pre-treatment of canola seeds and flaked canola seeds increases the canola oil extraction yield on the expeller compared with conventional thermal pre-treatment using steam heating. Flaked seeds were “cooked” (heat-treated) with steam or using microwave heating in the temperature range of 70 °C–110 °C before expeller pressing. Microwave cooking at 100 °C resulted in the highest increase in the pressed oil yield which is an increase of 4.2% (w/w) on a pressed oil basis or 9.2% (oil in seed basis) compared with steam cooking. Whole canola seeds conditioning was conducted with microwaves or steam, in the temperature range of 40 °C–60 °C, followed by microwave or steam cooking at 100 °C to evaluate the effects on the expeller oil yield. The use of a continuous microwave process for combined conditioning of whole seeds at 50 °C and subsequent cooking of flaked seeds at 100 °C gave a 4.3% increase in expeller oil yield, compared with steam conditioning and cooking at the same temperature. The effects of steam and MW treatments on the dielectric properties of flaked seeds (dielectric constant, loss factor and loss tangent) were investigated at a microwave frequency of 2450 MHz at three temperatures (70, 90, and 100 °C). The results showed that the dielectric properties of MW treated flaked seeds were higher than those of samples treated with steam. The lipid classes of the extracted oil which included triacylglycerols (TAG), diacylglycerols (DAG), free fatty acids (FFA), and phospholipids (PL) and their fatty acid compositions from both the steam and continuous microwave processes were analysed. The contents of polyunsaturated fatty acids (PUFA), monounsaturated fatty acids (MUFA) and saturated fatty acids (SFA) after control and continuous MW cooking were not significantly different, nor was there any significant difference in the oil quality parameters such as FFA content, peroxide value (PV), phosphorus and chlorophyll contents between MW and conventional steam heating.

Scalable Microwave-Assisted Johnson–Claisen Rearrangement with a Continuous Flow Microwave System

We demonstrated the rapid Johnson–Claisen rearrangement of allyl alcohol and triethyl orthoacetate with a continuous flow apparatus combined with a microwave reactor. The reaction could be carried out without solvent, and only a catalytic amount of acetic acid was sufficient to promote the reaction under microwave irradiation conditions. To confirm the optimal reaction conditions found experimentally, we performed Design of Experiments (DoE) by the Nelder–Mead method and a least-squares method regarding the amount of acetic acid and the flow rate. Consequently, the highest yield of the desired γ,δ-unsaturated ester was obtained, and the productivity at the reaction step of the continuous process was 89.5 g/h under the optimal conditions, suggesting that 2.1 kg of the product would be theoretically obtained in 1 day. We also investigated the Johnson–Claisen rearrangement using other allylic alcohols, and the corresponding products were obtained in good to high yields per unit of time.

Development of continuous flow microwave and hot water bath system for destruction of spoilage microorganisms in food

A continuous pasteurization system was designed based on a domestic microwave oven. Broth was pumped through helical coils of glass tubing placed in the center of the oven cavity. Inactivation of two selected spoilage microorganisms, Bacillus cereus and Saccharomyces cerevisiae in broth were evaluated under continuous flow microwave heating conditions and compared with conventional batch heating in a well stirred hot water bath. Inoculated broth was heated in a microwave oven (700 W, 2450 MHz) under continuous flow conditions to selected exit temperatures of 90°C for B. cereus and 60°C for S. cerevisiae at five power levels (210, 280, 350, 420, 490 W) and five time intervals (1, 2, 3, 4, 5 minute). Broth treated in hot water bath at 90˚C for B. cereus and 60˚C for S. cerevisiae was taken as control. There was a decrease in B. cereus and S. cerevisiae count after microwave and hot water bath treatment (control) with increasing treatment time. Higher microbial inactivation was observed at lower power levels. For all the microwave power levels, higher inactivation of B. cereus and S. cerevisiae was observed in comparison to control, this may be due to some non thermal effects associated with microwave. Heating rate and flow rate also increased with the increasing power level with decrease residence time to kill the contaminants. In future, this system may be useful for effective pasteurization of liquid foods e.g. sugarcane juice and soymilk without affecting the taste of processed juices.

高温高圧フロー型マイクロ波応用有機合成装置の開発

高温高圧フロー型マイクロ波応用有機合成装置の開発

Briefing: H2O2 dosing strategy on microwave treatment of sewage sludge

A H2O2 dosing strategy for microwave enhanced advanced oxidation process treatment of sewage sludge was formulated. The timing of hydrogen peroxide introduction in a continuous-flow 915-MHz system affected the treatment efficiency in terms of solids destruction, nutrient release and volatile fatty acid (VFA) production. Introducing hydrogen peroxide into the treatment set at 60°C was found to be more effective than adding it at the beginning. The treatment set with hydrogen peroxide introduced at 60°C resulted in higher soluble chemical oxygen demand concentrations and VFAs, higher soluble total Kjeldahl nitrogen and soluble total phosphate and also improved dewaterability and settling. However, there were no differences in terms of ammonia and orthophosphate release between the two treatment schemes. It is an advantage to introduce hydrogen peroxide into the treatment set at 60°C in a continuous-flow microwave system.

Separation of Oil/Water Emulsions in Continuous Flow Using Microwave Heating

This work studies a continuous flow microwave system to enhance gravity settling of water-in-oil emulsions. Settling times were found to be dependent upon the applied power, flow rate, and energy input. Power and energy input are linked to liquid flow rate within the flow system used in this study, so a key objective of this work was to understand the effect of turbulence on the heating and separation of the flowing emulsion. At high flow rates (9–12 L/min), it was found that turbulence dominates, with settling times largely independent of energy input. At lower flow rates (1–6 L/min), when turbulence was decreased, it was found that the settling time decreased as the power density was increased. Settling times have the potential to be less than half that of untreated emulsions and can be reduced further if turbulence can be minimized between the microwave heating zone and the settling zone in the process equipment.

Microwave-assisted continuous flow synthesis on industrial scale

A continuous flow microwave system based on a transmission line short-circuited waveguide reactor concept is described. The continuous flow reactor is capable of operating in a genuine high-temperature/high-pressure process window (310°C/60 bar), applying a microwave transparent and chemical resistant cylindrical γ-Al 2 O 3 tube as reaction zone. The system can be operated in an extremely energy efficient manner, utilizing 0.6–6 kW microwave power (2.45 GHz). The application of the reactor for processing four chemical transformations in a high-temperature/high-pressure regime with a throughout of 3.5–6.0 l/h is demonstrated.

COMSOL Multiphysics model for continuous flow microwave heating of liquids

A numerical model was developed to simulate temperature profiles in Newtonian as well as non-Newtonian fluids during continuous flow microwave heating by iterative coupling of electromagnetism, fluid flow and heat transport in COMSOL Multiphysics. The model was validated by using extensive experimental data for carboxymethyl cellulose solution and tap water flowing at two different flow rates (1 and 2 l/m) through a 915 MHz continuous flow microwave system at 4 kW of power. The electromagnetic power generation (18–20% error) and average numerical temperature were in fairly good agreement with the average experimental temperatures for carboxymethyl cellulose ( R 2 = 0.89 at 1 l/m and R 2 = 0.91 at 2 l/m) and tap water ( R 2 = 0.90 at 1 l/m and R 2 = 0.81 at 2 l/m).

Microwave Assisted Extraction of Biodiesel Feedstock from the Seeds of Invasive Chinese Tallow Tree

Chinese tallow tree (TT) seeds are a rich source of lipids and have the potential to be a biodiesel feedstock, but currently, its invasive nature does not favor large scale cultivation. Being a nonfood material, they have many advantages over conventional crops that are used for biodiesel production. The purpose of this study was to determine optimal oil extraction parameters in a batch-type and laboratory scale continuous-flow microwave system to obtain maximum oil recovery from whole TT seeds using ethanol as the extracting solvent. For the batch system, extractions were carried out for different time-temperature combinations ranging from 60 to 120 degrees C for up to 20 min. The batch system was modified for continuous extractions, which were carried out at 50, 60, and 73 degrees C and maintained for various residence times of up to 20 min. Control runs were performed under similar extraction conditions and the results compared well, especially when accounting for extremely short extraction times (minutes vs hours). Maximum yields of 35.32% and 32.51% (by weight of dry mass) were obtained for the continuous and batch process, respectively. The major advantage of microwave assisted solvent extraction is the reduced time of extraction required to obtain total recoverable lipids, with corresponding reduction in energy consumption costs per unit of lipid extracted. This study indicates that microwave extraction using ethanol as a solvent can be used as a viable alternative to conventional lipid extraction techniques for TT seeds.

Experimental study of the effect of dielectric and physical properties on temperature distribution in fluids during continuous flow microwave heating

Temperature data was collected at multiple locations in tap water, saltwater, and carboxymethylcellulose (CMC) solutions heated in a continuous flow microwave system by use of custom made temperature measurement system employing a single fiber optic probe. Tap water, 3% saltwater and 0.5% CMC solution were pumped through a 915 MHz continuous flow microwave system operating at 4 kW at three flow rates of 1 lit/m, 1.6 lit/m and 2 lit/m. Saltwater absorbed most power (3940 W) out of the 4000 W incident power, followed by CMC solutions (2690 W) and tap water (2626 W). Cross-sectional temperature distribution patterns showed that saltwater had the most uniform temperature distribution followed by tap water; CMC exhibited a non-uniform temperature distribution due to viscosity changes and thermal runaway effects. The study was very useful in enhancing the understanding of continuous flow microwave heating process for a variety of material properties and flow rates.

Continuous Flow, Circulating Microwave System and Its Application in Nanoparticle Fabrication and Biodiesel Synthesis

The current research is an attempt to combine reactions in a domestic microwave oven (DMO) with a circulating pump in order to create a continuous circulating flow microwave system. This system allows the reagents and products to circulate in and out of the microwave reactor until the end of the synthesis. In fact, this is the first attempt of its kind to build a continuous flow microwave system for production of nanosized particles. The continuous DMO system has several advantages over batch microwave ovens, such as its ability to work with large quantities of reagents, the possibility to withdraw samples of the product without stopping the process, and the constant stirring of the reagents during the reaction. Various nanoparticles with different particle sizes were prepared. Another reaction that was tested in this study was the synthesis of biodiesel. Complete conversion of commercially available vegetable oils to fatty-acid esters and glycerol was accomplished in the newly designed system.

Thermophysical and Dielectric Properties of Salsa Con Queso and its Vegetable Ingredients at Sterilization Temperatures

Aseptic processing of a low-acid multiphase food product using continuous flow microwave heating system can combine the advantages of an aseptic process along with those of microwave heating. The objective of this study was to determine the thermophysical and dielectric properties of salsa con queso and its vegetable ingredients (tomatoes, bell peppers, jalapeno peppers, and onions) at a temperature range of 20 to 130°C to design a safe process for aseptic processing of salsa con queso using a continuous flow microwave system. The influence of temperature on apparent viscosity of salsa con queso was described by an Arrhenius-type relationship. Second order polynomial correlations for the dependence of thermophysical and dielectric properties (at 915 MHz) of salsa con queso and its vegetable ingredients on temperature were developed. The results showed that the dielectric constant decreased with an increase in temperature and the dielectric loss factor increased with an increase in temperature.

Dielectric Properties of Sweet Potato Purees at 915 MHz as Affected by Temperature and Chemical Composition*

A process for rapid sterilization and aseptic packaging of sweet potato puree using a continuous flow microwave system operating at 915 MHz has been successfully developed. In microwave processing, dielectric properties have a major role in determining the interaction between purees and the electromagnetic energy. The objective of this research was to determine how dielectric properties are affected by temperature and chemical composition of purees derived from thirteen sweet potato cultivars with varying flesh colors. Results indicated that temperature, moisture, sugar and starch content had a pronounced effect (p < 0.001) on dielectric properties measured from 15°C to 145°C at 915 MHz. Dielectric constant decreased with increasing temperature, while dielectric loss factor increased quadratically. Power penetration depth of all cultivars decreased with increasing temperature. Predictive equations were developed for dielectric constant (R2 = 0.82) and dielectric loss factor (R2 = 0.90) as a function of temperature, moisture, sugar, and starch. The predictive equations would be useful in determining the dielectric properties of sweet potato purees for the microwave processing technology.

Overcoming issues associated with the scale-up of a continuous flow microwave system for aseptic processing of vegetable purees

Continuous flow microwave heating is a promising alternative to conventional heating for aseptic processing of low-acid vegetable purees. However, non-uniform temperature distribution and control of processing parameters are the major hurdles in the implementation of continuous flow microwave heating. This study was undertaken to overcome issues associated with the scale-up of a continuous flow microwave system from pilot plant scale to industrial scale and to conduct extended run times of 8 h based on the procedure developed. Dielectric properties and cross-sectional temperature profiles were measured during processing of green pea puree and carrot puree from 20 to 130 °C in a 5-kW continuous flow microwave system. During processing of green peas, cross-sectional temperature differences of 8.6 and 5 °C were observed at the outlet for center temperatures of 50 and 130 °C respectively. These temperature differences were 32.9 and 3.6 °C for carrot puree. For process scale-up, green pea puree and carrot puree were processed in a 60-kW microwave system with the objective of successful operation for at least 8 h. Static mixers, installed at the exit of each of the microwave applicators, improved temperature uniformity for both purees. Successful completion of processing the purees for 8 h in the 60-kW microwave system showed the potential for the scale-up of a continuous flow microwave system from pilot plant scale to industrial scale.