Continuous Thermal Processing Research Articles

Computer-aided engineering: Quantification of the heating non-uniformity and distribution of the thermal load occurring during continuous ohmic and conventional thermal food sterilization

The characterization of continuous thermal processing (CTP) is a crucial aspect in the design and selection of technologies for the production of safe products with optimal quality retention after the thermal stress. Conventional methods for CTP characterization are constrained in their capacity to comprehensively capture the complex dynamics of fluid flow through pipelines and the different heating principles that contribute to local temperature variations. These methods rely on isolated local temperature measurements, which fail to account for the intricate interactions between temperature, heat transfer phenomena and fluid dynamics. In view of the aforementioned limitations, this study presents the implementation of a computational fluid dynamic digital model as a toolbox for the characterization and comparison of a conventional heating (CH) ultra-high temperature (UHT) sterilization process and ohmic heating (OH) UHT sterilization. This serves as a tool for an extensive and accurate comparison of the two processes. The model enabled the estimation of each technology's thermal load through the F0 value, thereby providing a more comprehensive assessment than local temperature measurements alone. Furthermore, this approach accounted for the flow behavior throughout the sterilization process. This strategy demonstrated that 2.5 % of the food product is exposed to an up to 75.4 times the average thermal load, whereas for OH treatments only 0.4 % of the product was exposed up to 5.1 times the average thermal load. This is due to the volumetric heating feature of OH, which leads to a 54.7 % reduction in the cooking grade of the product. Furthermore, the computer-aided comparison revealed no statistically significant difference (p-value of 0.6) between the two technologies in their capacity to inactivate Geobacillus stearothermophilus spores in terms of thermal load. This study highlights the importance of computer-aided engineering methodologies for the technological assessment of food sterilization processes prior to industrial transfer.

Characterization and optimization of continuous ohmic thermal sterilization based on the development of a predictive computational toolbox

Continuous thermal processing (CTP) is a common method for sterilizing food. However, it can result in an uneven temperature distribution, which can lead to a varying degree of processing intensity. Ohmic heating (OH) can be advantageous in this regard, as it enables volumetric heating for more homogenous treatments. However, evaluating the processing intensity distribution inside the equipment for OH is challenging due to the complex interaction between electrical, mechanical and thermal phenomena. Furthermore, the comparison of OH and conventional heating treatments often lack a profound basis of comparable treatment intensity considerations. To gain a deeper mechanistic understanding of the technology, a numerical computational fluid dynamics model for the OH sterilization of a clear carrot juice from the heating region to the cooling process was developed. The model was validated with thermal and electrical measurements and showed an error rate below 2.5% in its prediction capacities. Moreover, the model was implanted for the validation of the products sterilization and compared to a conventional validation approach, reviling a 33.3% underestimation of the thermal load by conventional manners, which can lead to faulty sterilization of the food product. Additionally, the model was expanded to also be able to predict the microbial inactivation ratio of the system with an average error of 1.10±0.74%. In addition, results indicate that the numerical calculation of the F0 values and their validation with the microbial inactivation ratio have a notable potential for localization and evaluation of hotspots in OH simulations. Therefore, it can be seen as a promising step for establishing a foundation for computer-assisted optimization of CTP and targeted processing.

Post-annealing-free BaOxFy/LiF-based stack electron-selective contacts for high efficiency crystalline silicon solar cells featuring ultra-low contact resistivity

In silicon heterojunction and tunnel oxide passivating contact crystalline silicon (c-Si) solar cells based on successful passivating contacts technologies, an annealing activation step is indispensable to achieve a suitably low contact resistivity (ρc), which is a costly and non-ideal process. Here, combined with a calcium aluminium (Ca:Al) alloy electrode, a low work function barium oxide and lithium fluoride (BaOxFy/LiF) stack electron-selective contact (ESC) without an annealing activation step on c-Si solar cells clinches an ultra-low ρc of 1.4 mΩ·cm2 delivering a power conversion efficiency (PCE) of 20.5 %. The BaOxFy/LiF/Ca:Al stack structure is built by BaOx/LiF/Ca/Al hybrid structure via a facile continuous thermal evaporation process. As a proof of concept, the stacked BaOxFy/LiF/Ca:Al/Al contact is applied to the rear side of c-Si solar cells, achieving a PCE of 20.5 % with an open-circuit voltage of 625.7 mV, an impressive fill factor of 83.5 % and a short-circuit current density of 39.2 mA/cm2. With the aid of a transient photovoltage setup, the carrier recombination mechanism of the stacked BaOxFy/LiF/Ca:Al structure is explored. This work demonstrates an alternative idea for undoped ESCs, showing potential for next-generation cost-effective ultrathin-slice c-Si solar cells (∼80 μm) with high efficiency, and low-parasitic light absorption losses.

A dual-solver CFD model for conjugate heat transfer in continuous thermal processing

High temperature heat treatment of powders is a common operation in the manufacturing of a range of commercial products. Often, powders are heated or cooled in discrete quantities while being conveyed through a continuous furnace or oven, where a certain temperature–time profile is required for product quality reasons. For powders that are contained and conveyed in oven-ware (saggars), heat-transfer occurs between the surrounding gas phase and the powder material and oven-ware, as well as between oven ware that is vertically stacked.The large physical size and long transit times through typical industrial furnaces (up to 40 m length) means that a full transient CFD solution for both gas and solid phases is computationally very expensive . In this study, a novel CFD simulation model is developed for conjugate heat transfer in the cooling section at the outlet of a continuous furnace. A particular novel feature in this model is the inclusion of heat conduction in multiple solid domains. A dual-solver approach enables the convective and radiant heat-transfer processes in the gas phase are modelled using a steady-state conjugate solver, while the heat conduction processes within the powders and saggars are modelled using a time-dependent conjugate solver that solves for heat conduction only. Using only steady-state solution for the gas phase enables a “snap-shot” of the thermal performance of the system to be generated in a reasonable time frame, which has practical implications for modelling of large-scale industrial equipment. In the “dual-solver” approach, heat fluxes at solid boundaries from the steady-state model are applied as boundary conditions to the transient solver, while the temperature field resulting from the solution of the transient solver is then applied to the steady-state model. Radiative cooling effects are accounted for using a discrete-ordinates model.

A Dual-Solver Cfd Model for Conjugate Heat Transfer in Continuous Thermal Processing

A Dual-Solver Cfd Model for Conjugate Heat Transfer in Continuous Thermal Processing

A numerical model for studying the thermal denaturation-aggregation of whey proteins under continuous thermal processing

A computational fluid dynamics model was designed to study the problem of thermal processing of a liquid food product containing whey proteins within a heat exchanger consisting of heating, holding and cooling tubular sections. This physical problem is associated with strong coupling between the phenomena of fluid flow, heat transfer, and thermal denaturation-aggregation of whey proteins. Our primary objective was to investigate the two-way coupling between these phenomena within the heat exchanger. This was carried out by analyzing the model predictions of velocity, temperature and product properties at both axial and radial directions. Attention was focussed on the whey proteins present in a cream cheese formulation. The thermal denaturation-aggregation kinetics was supposed to follow that of the beta-lacto-globulin, which plays a major role in fouling when milk derivatives are submitted to thermal processing in heat exchangers. Model predictions demonstrated that the apparent viscosity of the liquid product exhibited a complex behavior along the processing unit: in addition to its dependence on local temperature, it was affected by the local degree of denaturation of whey proteins – and hence on the product history previous to this position in the heat exchanger. The numerical model was structured into a sequence of computational domains; its versatility was illustrated by changing the length of the holding section and then assessing the impact on the final degree of denaturation of the whey proteins present in the liquid product.

Comparative Study of the Continuous and Batch Thermal Processing of MgB2 Wires

The last stage of the manufacturing process requires complex reactive diffusion formation process of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in the presence of SiC nanoparticles. Continuous thermal processing was adopted to produce long length MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in situ wires with a homogeneous mixture of micron-sized Mg, nanosized B, as well as SiC dopant powders. This process has enabled the formation of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting compound in a relatively short time. Traditional superconductor batch processing requires the wire batch to be heat treated in dedicated large furnaces. Additionally, such a batch process requires controllable slow heating-up, dwelling, and cooling down procedures to ensure uniformity of the superconducting properties along the wire length. Such a prolonged reactive diffusion process does require lower dwelling temperature and can potentially prevent full utilization of the doping materials, resulting in less effective pinning centers formation. On the other hand, continuous wire thermal processing enables rapid formation of the doped MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> with full utilization of the dopant. Also, in the continuous process, the moving thermal front brings complex dynamics to Mg-B, C-B, Mg-Si interaction during MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> formation processes. The manuscript presents a comparative study of the reactive diffusion kinetics, the microstructural formation of the doped MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> compound, and their J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (B, T) characteristics.

On Thermal Solitons during Wave Heat Transfer in Restricted Areas

This paper simulates wave heat transfer based on an analysis of the dynamics of an isolated heat wave (thermal soliton). An isolated thermal wave arises under the action of a thermal impulse that acts for a short time and moves along a cold region. Unlike a continuous thermal process in a nonequilibrium state, when the moving front of a traveling heat wave occurs in a semi-infinite body, a thermal soliton has two fronts, anterior and posterior. There is a temperature distribution noted between them over the spatial variable. First-order gaps in the temperature distribution are observed on these fronts with decreasing amplitude due to the dissipation of thermal energy. Reaching the opposite boundary, the soliton is not reflected in the same way as a mechanical wave. At first, the vicinity of the opposite boundary heats up to a certain level, and the soliton then moves along cold space in the opposite direction with decreasing amplitude. The results of an analytical solution of the wave heat transfer problem on the basis of the hyperbolic-type heat conduction equation with allowance for relaxation phenomena are presented.

The 2D RGO-NiS2 dual co-catalyst synergistic modified g-C3N4 aerogel towards enhanced photocatalytic hydrogen production

The two-dimension RGO (reduced graphene oxide)-NiS2 dual co-catalyst synergistic modified g-C3N4 nanosheets aerogel is synthesized via the continuous thermal oxidation etching-hydrothermal method-freeze drying process. The results of scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) imply that the RGO-NiS2 dual co-catalyst is introduced into the aerogel system successfully. The photocatalytic activity of the RGO-NiS2 synergistic modified g-C3N4 aerogel remarkably exhibits an enhancement of 67 times than that of simplicial g-C3N4. Further, the photocatalytic process and the mechanism of the photocatalytic hydrogen production enhancement are studied, which is ascribed to RGO-NiS2 dual co-catalyst synergistic modification, including the Pt-like behavior of the NiS2 and the high conductivity and large specific surface area of the RGO.

Computation fluid dynamics design of a plate fin heat exchanger with different transverse vortex generators for food processing

AbstractIn this work, the performance of a plate heat exchanger with rectangular, rhombus, and elliptical vortex generators with 3, 5, and 7 mm width and 3, 6, and 9 mm height were investigated by means of computation fluid dynamics (CFD). Moreover, seven flow rates (from 0.01 to 1 ms−1) were tested and 196 CFD models were obtained. Heat transfer coefficient, pressure drop, Colburn j, and Fanning f factor were considered to evaluate the performance of the heat exchanger. The best heat exchange was obtained for rhombus wings and the highest Colburn j factor was obtained for 7 mm width and 6 mm height. The lowest pressure drop was obtained for 3 mm width and 3 mm height elliptical wings. Regarding surface reduction, the best result for all the geometries was with 5 mm width and 9 mm height, allowing a possible reduction of 25, 26, and 23% for rectangular, rhombus, and elliptical geometry.Practical applicationsPlate heat exchangers are extensively used for continuous thermal processing of liquid foods without particles in suspension and with low viscosity. In the last decades, due to the increasing demand of high quality foods and energy saving, increasing the overall heat transfer performances of food processes became crucial for the sector. Computation fluid dynamics (CFD) technique can contribute to understanding food fluid flows inside heat exchangers and in supporting equipment design. Product and process quality can be improved with minimum cost and time investment. In this work CFD was used for studying the performances of a plate heat exchanger with different passive vortex generators presenting different shapes and dimensions. The simulated data allowed identifying the best geometry with the best balance between heat performance improvements and pressure losses. The proposed approach successfully applied to Newtonian fluid could be extended also to non‐Newtonian liquid foods and to different plate heat exchanger design.

Using chaotic advection to enhance the continuous heat-hold-cool sterilisation process

Abstract In a conventional continuous sterilisation process, the food product flows steadily through a heat-hold-cool system, but viscous flow poses a serious challenge as heat transfer is controlled by thermal conduction which leads to a wide radial temperature distribution and slow heating of the core region of the flow. We use a validated Computational Fluid Dynamics (CFD) model to show that the superimposition of transverse mechanical oscillations on the heat-hold-cool sterilisation process of a viscous single-phase Newtonian fluid, creates a strong oscillatory-perturbed or chaotic advection flow which leads to significant improvements in thermal processing uniformity and product quality compared with a conventional process with or without an inline static mixer fitted. Chaotic advection flow produces processing conditions which are more in line with the high temperature for short time (HTST) assumption. Results show that the vibrated process leads to faster nearly-uniform heating and cooling, thus, achieving much higher levels of sterility and product quality in a much shorter process. Industrial Relevance Technological solutions are needed to help solve the long-standing problem of wide variation of product sterility and nutritional quality across the tube in continuous thermal processing of viscous fluids, where much of the product has to be over-processed to ensure sterility throughout, thus, often violating the high temperature for short time (HTST) processing assumption. Superimposing a secondary chaotic flow on the process leads to high levels of sterility, processing uniformity and product quality over relatively short process lengths. This represents an enhanced process which surpasses conventional steady flow sterilisation even when an usually undesirable static mixer is fitted.

Unusual positive temperature coefficient effect of polyolefin/carbon fiber conductive composites

The high-density polyethylene/carbon fiber (HDPE/CF) and isotactic polypropylene/carbon fiber (iPP/CF) conductive composites were prepared by melting blending and unusual positive temperature coefficient (PTC) effects were observed. For HDPE/CF composites, the sharp increase of resistivity happened in an extremely narrow temperature range and hardly any negative temperature coefficient (NTC) effect was observed when the temperature increased. Especially, the conductive network became more sensitive with increasing thermal cycles which manifested as obviously ever-increasing PTC intensity with the increase of thermal cycles. For the iPP/CF composites, the PTC transition temperature decreased with increasing thermal cycles. In addition, the onset resistivity of each thermal cycle obviously increased during the continuous thermal processes.

The effect of processing conditions on the stability of fructooligosaccharides in acidic food products

The effect of processing conditions (temperature and degree of polymerisation, DP) on the stability of short-chain fructooligosaccharides (sc-FOS) was investigated in three reaction media (sodium citrate buffer and orange and tomato juices) in a kinetic study at pH 3.5. In addition, kinetic equations as a function of temperature and pH were developed, using published data.Pentasaccharides were more stable to heat treatment than were trisaccharides under all of the conditions tested. In addition, the sc-FOS were more stable in orange juice, followed by tomato juice and citrate buffer. The results showed that, in addition to temperature and pH, the DP and food matrix, including the type of pasteurisation, must be considered when processing foods enriched with sc-FOS. Furthermore, the continuous thermal processing simulation for each of the equivalent processes at 90°C revealed that the percent retention of sc-FOS is greater than 95% at temperatures above 95°C.

Continuous HTST pasteurization of liquid foods with plate heat exchangers: Mathematical modeling and experimental validation using a time–temperature integrator

The continuous thermal processing of liquid foods with a three-section plate heat exchanger (PHE), heat integration and a non-isothermal holding tube was modeled to derive the temperature history of the product and the lethality distribution. Sequential linear temperature changes were assumed along the process and plug-flow was considered with three options for residence time: space time (bulk velocity), experimental mean residence time and minimum residence time (theoretical or experimental). In order to validate and test the model, it was used to simulate the operation of a laboratory-scale plate pasteurizer processing an enzymic time–temperature integrator (TTI) at four temperature conditions (70, 75, 80 and 85°C). Previous studies of residence time distribution and heat transfer in the equipment provided important parameters for the simulation. Predicted results of temperature distribution were in excellent agreement with experimental values and the simulated process lethality was close to the value assessed with the TTI. A case study showed the effect of the adopted residence time and the over-processing associated with a non-isothermal holding tube with tubular connections.

Modeling of continuous thermal processing of a non-Newtonian liquid food under diffusive laminar flow in a tubular system

The classic conservative approach for thermal process design can lead to over-processing, especially for laminar flow, when a significant distribution of temperature and of residence time occurs. In order to optimize quality retention, a more comprehensive model is required. A model comprising differential equations for mass and heat transfer is proposed for the simulation of the continuous thermal processing of a non-Newtonian food in a tubular system. The model takes into account the contribution from heating and cooling sections, the heat exchange with the ambient air and effective diffusion associated with non-ideal laminar flow. The study case of soursop juice processing was used to test the model. Various simulations were performed to evaluate the effect of the model assumptions. An expressive difference in the predicted lethality was observed between the classic approach and the proposed model. The main advantage of the model is its flexibility to represent different aspects with a small computational time, making it suitable for process evaluation and design.

Residence Time Distribution Models Derived from Non‐Ideal Laminar Velocity Profiles in Tubes

AbstractThe theoretical E‐curve for the laminar flow of non‐Newtonian fluids in circular tubes may not be accurate for real tubular systems with diffusion, mechanical vibration, wall roughness, pipe fittings, curves, coils, or corrugated walls. Deviations from the idealized laminar flow reactor (LFR) cannot be well represented using the axial dispersion or the tanks‐in‐series models of residence time distribution (RTD). In this work, four RTD models derived from non‐ideal velocity profiles in segregated tube flow are proposed. They were used to represent the RTD of three tubular systems working with Newtonian and pseudoplastic fluids. Other RTD models were considered for comparison. The proposed models provided good adjustments, and it was possible to determine the active volumes. It is expected that these models can be useful for the analysis of LFR or for the evaluation of continuous thermal processing of viscous foods.

Moving towards an energy neutral WWTP – the positive impact of ExelysTM continuous thermal hydrolysis in achieving this goal

There is an increasing need for large wastewater treatment plants to generate as much electricity as possible from biogas. This not only ensures significant operational cost savings, but also improves the environmental profile of the plant. Enhanced digestion through the use of the ExelysTM continuous thermal hydrolysis process has the potential to help a large wastewater treatment plant move towards electrical energy neutrality. This article aims to use Csepel waste water treatment plant (WWTP) (3.5 million PE) in Budapest, Hungary as a case study to investigate the potential advantages an ExelysTM-DLD process could provide. The results indicate that despite already having an effective and efficient digestion process, through implementation of the ExelysTM-DLD process, Csepel has the potential to cover 65% of its total electrical needs from biogas. This is 33% more than the current situation. This also leads to a significantly improved NPV for the ExelysTM-DLD scenario, an important factor for the operator of a large wastewater treatment plant. When looking to the future, through the optimisation of the wastewater treatment process and the application of the ExelysTM-DLD, Csepel WWTP has the potential to be completely electrical energy neutral. This would be a significant financial advantage as electrical prices rise and carbon taxes are enforced.

Case studies in food engineering: learning from experience

Section 1: Processes Based Largely on Physical Operations.- Dry Mixing.- Snacks and Baking.- Breakfast Cereals.- Pet Foods.- Fruit and Vegetable Juice Processing.- Membrane Processing.- Freeze Drying.- Section 2: Processes Based on Biochemical Reactions and Thermal Treatment.- Continuous Thermal Processing.- Retort Pouch Foods.- Ice Cream.- Sausages and Other Meat Products.- Non-thermal Processing.- Section 3: A few broader topics.- Economic Evaluation.- Design of a New Facility.- How to Tour a Food Plant.- Build New, Expand, or Upgrade?.- Developing Processes.

Physical, mechanical, thermal and electrical properties of cooked red bean ( Phaseolus vulgaris L.) for continuous ohmic heating process

Due to their complex composition and properties, the continuous thermal processing of solid–liquid mixtures (e.g. suspension of fragile particle in viscous carrier fluids) remains an empirical and random operation as compared to canning. Alternative technologies (e.g. ohmic heating) may achieve high-temperature treatment in a short time (HTST) but requires a perfect knowledge of thermo-physical and electrical properties of both particles and carrier fluid. Food properties are needed and play a significant role to predict and define the quality and behaviour of solid–liquid mixture. The properties of red beans ( Phaseolus vulgaris L.) and a model non-Newtonian carrier fluid were studied throughout the duration of the process. Physical (rheological behaviour, density, shape and dimensions), mechanical (elasticity modulus, maximal deformation and stress) and thermal (heat capacity, thermal conductivity, thermal diffusivity) properties as a function of water content ranging from 11.6 to 67.4% w/w are reported. The electrical conductivity (electrical properties) was described as a function of the temperature and the solid concentration by a semi-empirical equation. The limiting factors to succeed a HTST for heterogeneous products in continuous thermal process were identified and discussed in the light of the properties of the foods involved. The large dispersion of particle mass volume had a simultaneous incidence of the suspension flow and the heat transfer. The volume expansion of particle (+22% between blanched and cooked bean) and the important loss of mechanical properties (−68% for elastic properties) constitutes unavoidable limiting factors inducing mechanical degradation and sometimes plugging of the duct. The electrical conductivity is strongly affected by a combined effect of temperature and solid concentration, which will induces irreversible heat treatment heterogeneity between particles. This work stresses that the continuous conventional or ohmic heating of these cooked dishes will be hard to achieve on an industrial scale.

The principle of recycling and optimization as applied to batch-operated thermal processing plants

The efficiency of continuous and batch-operated thermal processing is analyzed in terms of fuel rate and heat-treatment process control. The principle of recycling of the working medium is proposed for optimization of batched-operated plants. The recycling ratio is proposed as an optimization criterion.