Characterization of the performances of an innovative heat-exchanger/reactor

The use of heat exchanger/reactors (HEX/reactors) is a promising way to overcome the barrier of poor heat transfer in batch reactors. However to reach residence time long enough to complete the chemistry, low Reynolds number has to be combined with both a plug flow behaviour and the intensification of heat and mass transfers. This work concerns the experimental approach used to characterize an innovative HEX/reactor. The pilot is made of three process plates sandwiched between five utility plates. The process stream flows in a 2 mm corrugated channel. Pressure drop and residence time distribution characterizations aim at studying the flow hydrodynamics. Identified Darcy correlations point out the transition between laminar and turbulent flow around a Reynolds number equal to 200. Moreover the flow behaves like a quasi-plug flow (Pe > 185). The heat transfer and mixing time have also been investigated. The ratio between the reaction kinetics and the mixing time is over 100 and the intensification factor ranges from 5000 to 8000 kW m⁻³ K⁻¹. As a consequence, no limitations were identified which allows the implementation of an exothermic reaction. It has been successfully performed under severe temperature and concentration conditions, batchwise unreachable. Thus, it highlights the interest of using this continuous HEX/reactor.     

Numerical study of the flow pattern and heat transfer enhancement in oscillatory baffled reactors with helical coil inserts

Oscillatory baffled reactors (OBRs) are a means of process intensification as they allow processes with long residence time to be converted from batch to continuous processing. Helically baffled OBRs have only been developed at “mesoscale” so far, but at this scale have displayed significant advantages in terms of the increased range of conditions over which plug flow is achieved. Scale-up studies are underway to determine whether this is replicated at larger scales. This paper reports fluid mechanical modeling of a helically baffled oscillatory flow for the first time. Time-dependent flow structures induced in tubular reactors have been analyzed on the basis of periodic, laminar flow numerical simulation. A reversing swirled core flow and its interaction with the unsteady mechanism of vortex shedding downstream of the wires has been described. This has allowed greater understanding of the flow structures, which will underpin optimal design and scale-up. The potential for heat transfer enhancement is discussed, considering the compound effect of oscillatory motion and helical coil inserts. The results show that the heat transfer for the helical baffled tube could be enhanced by a factor of 4 compared to a smooth tube in the tested range of oscillation conditions.     

Characterization of Residence Time Distribution in a Plug Flow Reactor

One of the biggest advantages of plug flow reactors lies in their narrow residence time distribution. The pulse experiment, as a common method on acquiring that distribution, relies on the tracer injection being a perfect pulse. A deviation from a perfect pulse leads to erroneous results if not taken into account. With a numerical analysis of experimental data, this effect is quantified in turbulent and laminar flow regime and the results are compared to an analytical method. Significant deviations occur mostly in the turbulent regime, which has the greatest technical relevance.     

Comparative Thermohydraulic Efficiency of Processes in Channels with Chaotic Packing

A new approach to calculation of the thermohydraulic efficiency of channels with different methods of intensification including small-size chaotic packings was considered, and the corresponding expressions were obtained. For this, the Kirpichev, Antufiev, and Spalding energy coefficients, thermal efficiency of the process in the channel, and cell model of the flow structure were used. Expressions were given for calculating the temperature profiles, thermal efficiency, hydraulic resistance of channels with chaotic elements, and average Nusselt number. The results of calculations agreed with the experimental data obtained on an experimental stand with a tube-in-tube heat exchanger and oil–water heat transfer agents. Examples of calculations were given for several types of metal packing, and conclusions were drawn about the most effective designs. It was recommended to use metal chaotic packing in channels for liquids with increased viscosity because the laminar flow of the heat transfer agent changes to the turbulent one, and the heat transfer coefficient substantially increases in this case.     

Numerical analysis of the lethality and processing uniformity in a double-pipe helical heat exchanger

A numerical study of a double-pipe helical heat exchanger was performed to ascertain the residence time, temperature, and processing uniformity for food processing applications. A range of laminar flow rates were used, with both parallel flow and counterflow configurations. Both heating and cooling in the inner tube were studied. Heating/cooling uniformity was estimated by using a first-order kinetics model for sterilization. Process uniformity is important in the quest for high quality product and this report is a first study for the uniformity in double-pipe helical heat exchangers.     

Augmentation of heat transfer performance in coiled flow inverter vis-à-vis conventional heat exchanger

In the present work, primarily two studies were carried out to ascertain the performance of coiled flow inverter (CFI) as heat exchanger at pilot plant scale. In the first study, performance of CFI heat exchanger has been compared with conventional heat exchangers, i.e. shell and tube heat exchanger (SHE) and plate type heat exchanger (PHE) under identical heat transfer area and process conditions. Experiments were conducted with water flowing under laminar flow regime within the flow range of 30–300 kg/h in the tube side of SHE and PHE. Friction factor and Nusselt number calculated from present experimental study in SHE and PHE were compared with the experimental data previously reported for CFI heat exchanger (Kumar et al., 2007). The Number of Transfer units (NTU) calculated in the present study for CFI was nearly 3.7–7.5 times higher as compared to SHE and 2–2.5 times higher as compared to PHE. In the second part of the study, experiments were performed first time to investigate the pressure drop and heat transfer of compressed air flowing under turbulent flow condition in CFI heat exchanger at pilot plant scale. Hot air at elevated pressures (10–30 kg/cm²) in the tube side of CFI heat exchanger with flow range 3×10⁴<N_Re<1.4×10⁵ was cooled by either cooling water or ambient air. The friction factor and Nusselt number values for compressed air flowing in the CFI were also compared with the experimental data reported in the literature for coiled tube at ambient conditions. On the basis of experimental results, new correlations for friction factor and Nusselt number of compressed air flowing under turbulent flow condition in CFI heat exchanger have been developed.     

板式换热器在高粘度流体强化传热的工程研究

通过对高粘度流体强化传热的工程研究,确定传热系数与流量、流体阻力与流速的关联式,实验证明采用板式换热器代替反应釜夹套撤热是行之有效的,高效换热器在高粘度流体的工程应用上可达到强化换热的效果。     

Macroscopic fluid flow conditions in spiral-wound membrane elements

Reverse osmosis (RO) finds increasing applications as separation technique in chemical and environmental engineering where desalination, selective separations in the agro-industrial processes or wastewater purification are well-established examples. To fully evaluate the potential of RO and facilitate scale-up procedures, the modelling of the process is an important tool and literature models analyze the separation efficiency in terms of mass transfer with material balances, pressure drop through the module and mass transfer coefficients as dominant parameters. Important underlying factors are the geometry of the module and the hydrodynamic flow regime since mass transfer and pressure drop are a function of these factors, as witnessed by several publications. Since the concentrate-side (which is also the feed side) of the membrane plays the key-role, measurements should focus on the concentrate-channel of the spiral-wound membrane element. The determination of the channel flow regime and hydrodynamics of spiral-wound RO-channels can be carried out through the measurement of the residence time distribution (RTD). The present paper describes our experimental investigations on RTD through the application of a step change in tracer concentrations and relates the RTD-response curves to the regime of flow through the concentrate channels. This tracer-technique is common in characterizing flow regimes in chemical reactors. Its application to RO flow channels is innovative and results obtained stress its applicability to this specific geometrical layout. Results indeed demonstrate that the experimental average residence time of the concentrate channel is smaller than the theoretically calculated residence time, with differences between both values gradually decreasing with increasing liquid flow rate. This observation corresponds with findings for traditional packed bed applications where the presence of dead volumes reduces the real average residence time. The presence of dead zones in the spiral-wound membrane element is therefore evident. Since in our experimental procedure, residence time distributions are measured both between the tracer injection point and respectively the inlet and outlet of the spiral-wound element, the nett RTD-contribution of the membrane element itself can only be determined by eliminating the influence of the inlet flow equalization zone. This is performed by applying the convolution principle. Experimental and calculated RTD-curves for values of the Péclet-number of approximately 20 are in very good agreement. In analogy with fixed bed applications, the interpretation of the findings corresponds to a laminar flow profile with a limited dispersion. The definition of this flow profile in the concentrate channel is important in the use of transport models to characterize the membrane performance, as will be shown in a further paper.     

Amelioration du transfert convectif de chaleur par l'écoulement pulse dans un fluide visqueux

Pulsed perturbation effects on convective heat transfer in the laminar flow of a viscous fluid were studied in a co-axial cylindrical tube heat exchanger over the experimental temperature range 30 to 45°C. The fluid studied here, whose viscosity is twelve times larger than water at room temperature, is an aqueous solution of glycerol (60 wt %), and the Reynolds numbers of the steady flow are between 150 and 1000. The pulses significantly increase the thermal transfer at constant dissipated mechanical power. Indeed, enhancement of the heat transfer coefficient by more than 300% was obtained with strong pulsed perturbations. Therefore, pulses superimposed to a steady flow are a simple and efficient manner to improve thermal exchanges in viscous fluids that are usually pumped in the laminar regime. A correlation based on our experimental data on pulsatile flow is used to evaluate the Nusselt number with an average error of 10%.     

Flow effects on the kinetics of a second-order reaction

Flow effects on the kinetics of an isothermal, equimolar, second-order reaction taking place in a channel were investigated using a Lagrangian numerical method. The reactants were released instantaneously from the two opposite walls of the channel into fully developed turbulent or laminar flow. The overall conversion, the residence time and reactor length required to achieve 80% conversion, and the effective reaction rate coefficient were calculated. A correlation of the efficiency ratio, defined as the effective rate coefficient divided by the reaction rate constant, with the flow parameters was found.     

Flow visualization and modelling of a filter-press type electrochemical reactor

A study of flow visualization and residence time distribution is provided in order to model the flow between two electrodes in a commercial filter-press reactor, the ElectroSynCell® from Electrocell AB. Flow visualization indicates that both axial and lateral dispersion phenomena occur and a global plug flow behaviour is observed. The flow distribution is asymmetric due to the design of the inlet system in the active zone. The flow throughout the cell is described by a dispersed plug flow model for which the mean residence time and the Pe´clet number are determined. The reaction area and the inlet system are separately analysed by locating conductimetric probes inside the electrochemical cell. The reaction area is also well described by a dispersed plug flow model, and characterized by high dispersion. The inlet system is, respectively, described by a dispersed plug flow model and by a cascade of continuous stirred tank reactors. The high number of reactors in the cascade denotes a quasi plug flow behaviour. The results are confirmed by two cascades of continuously stirred tank reactors in series. The dispersion coefficients obtained throughout the reaction area of the cell are not constant. This shows that the flow is not well established at the entrance of the reaction zone and depends on the entrance conditions.     

Residence Time Distribution in Tubular Joule Effect Heaters with and without Geometric Modifications

In the food industry, heat treatment of highly viscous fluids in continuous processes is becoming more and more common, and the process should perform as a homogenous thermal treatment, in order to ensure quality and safety of the final product. To improve treatment homogeneity, geometric modifications can be used even in the laminar regime, to induce flow perturbation and mixing. The objectives of this work include: (i) Investigation of the residence time distribution (RTD) for industrial indirect Joule effect heaters (JEH), with smooth (ST) and modified (MT) tubes, (ii) Demonstration and quantification of the efficiency of the geometrical modifications, and (iii) Proposition of a single semi‐empirical model including the flow regime (10 < Re < 2000) and tube diameters (18 and 23 mm). The results obtained confirm that the simple Dispersed Plug Flow (DPF) model is not adaptable to small Reynolds numbers. Further analysis demonstrates that certain geometrical modifications improve the treatment homogeneity by increasing the plug flow contribution and reducing the value of the reduced variance. These beneficial effects increase when the Reynolds number is increased, the nominal diameter is reduced, and modified tubes are used. The proposed model enables the prediction of the RTD in JEH with an accurate degree of confidence.     

涡强化扁管管片散热器流动与传热的数值模拟

以空气为介质(Pr=0.698),通过数值模拟的方法,在Re=300～1800的范围内对涡强化扁管管片散热器初始段层流状态下的流动与传热进行了模拟分析,说明了涡产生器横向位置,即交错系数Sr改变时对局部Nu_local和横断面上的平均Nu_b的影响,并通过实验数据验证了数值模拟方法的正确性,证实了通过数值模拟的方法研究、开发换热板芯的可行性.     

Effect of Thermal Asymmetry on Laminar Forced Convection Heat Transfer in a Porous Annular Channel

The effect of thermal asymmetry on laminar forced convection heat transfer in an annular porous channel with a Darcy dissipation of fluid kinetic energy was investigated numerically. The cylindrical surfaces making the channel boundaries were kept at constant but different temperatures. The thermal asymmetry thus imposed on the system results in an asymmetric temperature field and different heat fluxes across the channel boundaries. Depending on the Darcy, Péclet and Reynolds numbers, the thermal asymmetry may lead to a reversal of the heat flux along the channel at least at one of the channel walls. The corresponding Nusselt number becomes zero and subsequently experiences a discontinuity, thereby jumping from infinite negative to infinite positive, or vice versa. This feature is observed in the region of thermal development. In the fully developed heat transfer region, the Nusselt numbers can be positive or negative for the same inlet conditions, depending on the heat source strength. In the case of a plug flow, the analytical expressions for the Nusselt numbers have been obtained.     

Casson流体层流反应器中连串反应的产品分布

引　言目前化工流体力学涉及的范围已由传统的牛顿流体发展到非牛顿流体 .常见的非牛顿流体包括幂律流体、Bingham流体和Casson流体 .有许多高聚物反应物系和生化反应物系属于非牛顿流体 .特别是某些生化反应系统 ,所处理的物系常由各种天然物质组成 ,因而表现出典型的非牛顿流     

Residence time distribution measurements in a pulsed baffled tube bundle

Experimental flow pattern and associated residence time distribution measurements are reported for a tube bundle where periodic baffles and fluid oscillation may be present. When there is no fluid oscillation, high Reynolds number flow conditions are required to give sharp residence time distributions. When baffles are present, fluid oscillation can give sharp residence time distributions for modest low net flow Reynolds numbers. These observations extend our previous results and also show the viability of the system, for example in use as a reactor or heat exchanger where a multi-tube configuration might be required.     

The residence time distribution for laminar flow in helically coiled tubes

Sizable errors exist in previously published studies on the residence time distribution for ideal laminar flow in a helically coiled tubes. The numerical methods used to give corrected results are generally useful for flow situations where the projections of the streamlines on the channel cross-section are closed curves.An asymptotic solution, valid at long residence times, has been obtained. This solution confirms the numerical observation that the tail of the distribution is similar to that for laminar flow in a straight tube. If molecular diffusion is ignored, this form of asymptotic behavior is shown to be a general characteristic of all flow systems involving a fixed wall. When diffusion is considered, the asymptotic residence time distribution will be a decaying exponential in time. This confirms the widespread experimental observation that residence time distributions have exponential tails.     

双螺旋结构螺旋折流板换热器试验研究

螺旋折流板换热器中壳程的流动方式与单弓形结构下具有很大的差别,在采用扇形板拼接而成的螺旋折流板结构中采用双螺旋结构来布置更多的折流板,减少流体在扇形板拼接处的漏流,使壳程流体流动更接近于平推流.分别以重柴油和水作为壳程介质,对普通螺旋折流板以及双螺旋结构螺旋折流板的传热性能、阻力性能进行试验研究,发现双螺旋结构在相同Re时,阻力提高9.9%和6.15%,Nu提高14.12%和11.72%,同时可以增大单位压降的Nu.     

Wärmeübergang in kurzen,wandgängig gerührten Apparaten

Scraped surface heat exchanger are often used in industrial settings for the treatment of systems with solid particle formation or higher viscosities. In reactors with a small length‐diameter ratio, the backmixing has thereby a significant influence on the heat transfer. In the experiments presented here, it can be shown that the heat transfer coefficient strongly depends on the flow pattern within the laminar flow regime, and that, depending on the running conditions, an increased rotor speed may have a negative effect on the heat transfer rate.     

Prediction of rates of heat or mass transfer in complex situations by interpolating between simpler limiting cases

This paper emphasizes the generality of a simple algebraic procedure suggested in 1962 by the author to predict the rate of heat or mass transfer in complex situations. It consists in replacing the terms in the convective diffusion equation by expressions involving scaling quantities multiplied by constants. Thus the convective-diffusion equation is replaced by an algebraic equation for the mass transfer coefficient containing a number of constants. The constants are determined from the solutions available for some simpler limiting cases. To illustrate the procedure a variety of examples are given which include: penetration theory for diffusion with chemical reaction, laminar flow along a plate accompanied by a chemical reaction, diffusion with chemical reaction in a turbulent liquid, mass transfer from a drop to a continuous phase, unsteady heat transfer to a liquid in steady laminar flow along a plate and free convection superimposed on forced convection. Numerous other complex situations have been or can be treated by interpolation between some simpler limiting cases.