Narrow Residence Time Distribution Research Articles

Modular Coiled Flow Inverter with Narrow Residence Time Distribution for Process Development and Production

AbstractThe ideal plug flow reactor is a common model with narrow residence time distribution in reaction engineering. This contribution explains a design strategy of a coiled flow inverter (CFI) with narrow residence time and flexible adjustable process conditions. The design space diagram illustrates the design limits of inner diameter and coil diameter on the base of dimensionless numbers. Standard coil tube diameters allow for rational design of a limited number of devices. The reactor performance diagram enables the facile scale‐up and adaption of new process conditions with the CFI with economic boundary conditions.

Characterization of Residence Time Distribution in a Plug Flow Reactor

AbstractOne 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.

Virus study for continuous low pH viral inactivation inside a coiled flow inverter.

Continuous processing for the production of monoclonal antibodies (mAb) gains more and more importance. Several solutions exist for all the necessary production steps, leading to the possibility to build fully continuous processes. Low pH viral inactivation is a part of the standard platform process for mAb production. Consequently, Klutz et al. introduced the coiled flow inverter (CFI) as a tool for continuous low pH viral inactivation. Besides theoretical calculations of viral reduction, no viral clearance study has been presented so far. In addition, the validation of continuous viral clearance is often neglected in the already existing studies for continuous processing. This study shows in detail the development and execution of a virus study for continuous low pH viral inactivation inside a CFI. The concept presented is also valid for adaptation to other continuous viral clearance steps. The development of this concept includes the technical rationale for an experimental setup, a valid spiking procedure, and finally a sampling method. The experimental results shown represent a viral study using xenotropic murine leukemia virus as a model virus. Two different protein A (ProtA) chromatography setups with varying pH levels were tested. In addition, one of these setups was tested against a batch experiment utilizing the same process material. The results show that sufficient low pH viral inactivation (decadic logarithm reduction value >4) was achieved in all experiments. Complete viral inactivation took place within the first 14.5 min for both continuous studies and the batch study, hence showing similar results. This study therefore represents a successful virus study concept and experiment for a continuous viral inactivation step. Moreover, it was shown that the transfer from batch results to the continuous process is possible. This is accomplished by the narrow residence time distribution of the CFI, showing how close the setup approaches the ideal plug flow and with that batch operation.

Two phase gas-liquid stratified laminar flows in tubular reactors sustaining liquid phase reactions

In this work, we develop a mathematical model for analyzing reactions with mass transfer in two phase gas-liquid stratified flow tubular reactors. The reaction occurs in the liquid phase and the flow is assumed laminar. The hydrodynamics, Residence Time Distribution (RTD) and mass transfer in stratified gas-liquid flows with different liquid holdups is analyzed using a semi-analytical approach. The bipolar cylindrical coordinate system is employed for an elegant description of the boundaries and the gas-liquid interface.A novel contribution of this work is the model developed for analyzing the RTD of partially filled tubular reactors with different liquid holdups. Towards this, the velocity profile obtained is used to solve the diffusion-free species transport equation with a pulse tracer input and the evolution of the outlet concentration is obtained. The model developed is validated with RTD experiments carried out in a circular glass channel using 0.5 N NaOH as the tracer. The model developed for RTD only describes mixing in an ideal laminar flow reactor. In order to model and predict conversion in realistic systems where radial diffusion is significant, the convection-diffusion-reaction equation is solved next. Two cases are studied where the (i) reactant is present only in the liquid phase and when (ii) the reactant is present in both the phases and it transfers from the gas phase to the liquid phase as it gets consumed. It is found that the product formation is enhanced in the latter case. By controlling the liquid holdup and the reactant concentration in the gas phase, product formation is enhanced while maintaining a narrow RTD.

Residence time distribution on flow characterisation of multichannel systems: Modelling and experimentation

Residence Time Distribution (RTD) is a frequently used tool in conventional process equipment and it provides internal flow characterisation by simple tracer tests. In this paper, we explore the feasibility of using RTD to identify fluid distribution uniformity in millimetric multichannel devices. Both theoretical modelling and experimental implementation are conducted to 16-channel systems. Theoretical modelling confirms the effectiveness of non-intrusive RTD measurements in evaluating flowrate distribution uniformity. Different influencing factors, such as channel corrosion, blockage, distributor structure, channel length or width variation, etc., can be reflected by RTD response curve. The experimental setup consists of a lab-developed RTD test platform coupling a fast camera and two miniflowcells capable to quantify rapid tracer concentration evolution through carbon ink visualization. The platform is particularly powerful for very narrow RTD measurement with residence time down to 1 s. With the platform, we investigate the RTD characteristics of a multichannel device under several flow conditions. Model correlation of the experimental data gives valuable information such as fluid distribution, plug-flow ratio and perfectly mixed volume.

Numerical Studies of Novel Inwardly Off-Center Shearing Jet-Stirred Reactor

A novel inwardly off-center shearing jet-stirred reactor is proposed and examined computationally. The inwardly off-center shearing jet-stirred reactor is compared with two traditional jet-stirred reactor designs, the outward cross-injector jet-stirred reactor and the concentric inward and outward jet-stirred reactor. The results show that the present inwardly off-center shearing jet-stirred reactor has significant improvement in terms of mixture uniformity and residence time distribution. Numerical results show the distributions of residence time in the two traditional jet-stirred reactors are wide and long tailed because of the formation of large and stable vortices, and the corresponding mean residence time of the two classical jet-stirred reactors deviates from the theoretical value by about 20%, while the new inwardly off-center shearing jet-stirred reactor has a much narrower residence time distribution and reduces the deviation to 8%, mainly attributed to the smaller vortices generated by an optimized jet arrangement. Moreover, the new inwardly off-center shearing jet-stirred reactor provides a fully optic-accessible platform for kinetic studies of alternative and real jet fuels.

Numerical and experimental analysis of particle residence times in a continuously operated dual-chamber fluidized bed

Fluidized beds that consist of multiple chambers are used in various processes including granulation, drying or coating. As overly short or long particle residence times inside the vessel are undesirable, it is of high interest to measure and detect the influences of variations of the operational parameters on the particle residence times so that they can be adjusted accordingly to gain narrow particle residence time distributions. For this purpose, an ultra-high frequency (UHF) radio frequency identification system (RFID) has been developed, that is able to detect multiple particles without a direct line-of-sight at different positions inside the vessel. Furthermore, coupled discrete element method (DEM)/computational fluid dynamics (CFD) simulations have been carried out to study a continuously operated laboratory scale dual-chamber fluidized bed. To examine the influences of operational and particle parameters 10 different variations have been evaluated both numerically and experimentally. Results show good agreement between simulation and experiment for most cases. The highest change of the residence times could be achieved by changing the particle mass inflow and the fluidization velocity. The results for non-spherical particles showed some deviations in the longer residence times, as certain particle/wall effects were not fully recreated in the simulation. Nonetheless, it was proven that coupled DEM-CFD simulations can be a useful tool in the prediction of residence time which can greatly help to optimize continuously operated fluidized processes.

Process intensification of honeycomb fractal micro-reactor for the direct production of lower olefins from syngas

Inspired by the nature fractal structure, a novel honeycomb fractal micro-reactor was designed and applied to intensify both heat and mass transfer in the exothermic conversion of syngas to olefins. The results show that the honeycomb fractal micro-reactor provides more even distribution of temperature profiles, a narrower reactant residence time distribution, and relative smaller pressure drop per unit length, compared with parallel straight microchannel reactor and mini-fixed bed reactor under the same reaction conditions. Owning to the reinforcement of flow separation and convergence in the honeycomb structure, the contact between reactants and catalyst particles were enhanced, leading to a better heat and mass transfer. Under the current range of temperature, pressure and standard GHSV, the honeycomb fractal micro-reactor leads to supreme CO conversion and lower olefins yield.

Continuous Drying of Pharmaceutical Powders Using a Twin-Screw Extruder

Continuous manufacturing of pharmaceuticals offers such benefits as production flexibility, reduced drug product costs, and improved product quality. Moving toward continuous manufacturing requires suitable small-scale equipment, either by development of new equipment or optimization of existing equipment. In primary manufacturing, particle properties are often altered during crystallization and have to be restored during subsequent processing. Drying a crystallized product is one of the most challenging steps, especially since attrition and agglomeration can occur. To that end, we investigated the drying behavior of a crystalline model compound with moisture levels of up to 10 wt % in a corotating twin-screw extruder. The feed mass flows on a piece of small-scale equipment used for pharmaceutical production varied between 0.5 and 2.0 kg/h. Experiments were conducted to evaluate the drying performance in various process settings. Because of a very narrow and consistent residence time distribution, extrusi...

Spinning Disc Technology – Residence Time Distribution and Efficiency in Textile Wastewater Treatment Application

The spinning disc (SD) technology has received increased attention in the last years due to its enhanced fluid flow features resulting in improved property transfers. The actual study focuses on characterization of the flow within a spinning disc system based on experimental data used to establish the residence time distribution (RTD) and its dependence on the feeding liquid flowrate and the disc rotational speed. To obtain these data, an inert tracer (sodium chloride) was injected as a pulse input in the liquid stream entering the disc and the salt concentration of the liquid leaving the disc was continuously recorded. The obtained data indicate that an increase in the liquid flowrate from 10 L/h to 30 L/h determines a narrower RTD function. Also, at rotational speed of 200 rpm, the residence time distribution is broader than that for 500 rpm and 800 rpm. The RTD data suggest that depending on the needed flow characteristics, one can choose a certain flowrate and rotational speed domain for its application. Also, the SD technology was used to process textile wastewater treated with bentonite (as both coagulation and discoloration agent) in order to investigate whether the quality indicators such as the total suspended solid content, turbidity and discoloration, can be improved. The experimental results are promising since the discoloration and the removals of suspended solids attained values of over 40%, and respectively, 50 %, depending on the effluent flowrate (10 l/h and 30 L/h), and the disc rotational speed (200 rpm, 550 rpm and 850 rpm) without any other addition of chemicals, or initiation of other simultaneous treatment processes (e.g., advanced oxidative, or reductive, or biochemical processes). This recommends spinning disc technology as a suitable and promising tool to improve different wastewater characteristics.

Cuboid Packed-Beds as Chemical Reactors?

Columns are widely used as packed-bed or fixed-bed reactors in the chemical process industry. Packed columns are also used for carrying out chemical separation techniques such as adsorption, distillation, extraction and chromatography. A combination of the variability in flow path lengths, and the variability of velocity along these flow paths results in significant broadening in solute residence time distribution within columns, particularly in those having low bed height to diameter ratios. Therefore, wide packed-column reactors operate at low efficiencies. Also, for a column of a particular bed height, the ratio of heat transfer surface area to reactor volume varies inversely as the radius. Therefore, with wide columns, the available heat transfer area could become a limiting factor. In recent papers, box-shaped or cuboid packed-bed devices have been proposed as efficient alternatives to packed columns for carrying out chromatographic separations. In this paper, the use of cuboid packed-beds as reactors for carrying out chemical and biochemical reactions has been proposed. This proposition is primarily supported in terms of advantages resulting from superior system hydraulics and narrower residence time distributions. Other potential advantages, such as better heat transfer attributes, are speculated based on geometric considerations.

Continuous emulsion copolymerization processes at mild conditions in a 3D-printed tubular bended reactor

A polylactic acid tubular bended reactor, printed by fused deposition modeling with a short and narrow residence time distribution, air-cooled, and with dead volumes below 5 vol% was integrated into a modular reaction system enabling product changes in the running system. Continuous, redox-initiated emulsion copolymerizations of styrene - butyl acrylate and of vinyl acetate - neodecanoic acid vinyl ester were successfully performed with 20 and 40 wt% monomer in the feed. Copolymerizations had an ambient starting temperature with a mean residence time of 5 and 15 min, conversions were between 81 and 99%. The resulting emulsions were stable with Zeta potential values up to –132.0 mV and no product adhesion to the reactor wall was found. A stable steady state of the reaction plant was also obvious from thermal imaging, showing an invariant temperature profile along the 3D-printed reactor. The high versatility of the reactor system was shown by a grade transition between the copolymer systems with 20 wt% monomer content.

Design of a novel continuous flow reactor for low pH viral inactivation.

Insufficient mixing in laminar flow reactors due to diffusion-dominated flow limits their use in applications where narrow residence time distribution (RTD) is required. The aim of this study was to design and characterize a laminar flow (Re 187.7-375.5) tubular reactor for low pH viral inactivation with enhanced radial mixing via the incorporation of curvature and flow inversions. Toward this aim, the reactor described here, Jig in a Box (JIB), was designed with a flow path consisting of alternating 270° turns. The design was optimized by considering the strength of secondary flows characterized by the Dean No., the corresponding secondary flow development length, and the reactor turn lengths. Comprehensive CFD analysis of the reactor centerline velocity profile, cross-sectional velocity, and secondary flow streamlines confirmed enhanced radial mixing due to secondary flows and changes in flow direction. For initial CFD and experimental studies the reactor was limited to a 16.43 m length. Pulse tracer studies for the reactor were computationally simulated and experimentally generated to determine the RTD, RTD variance, and minimum residence time for the tracer fluid elements leaving the reactor, as well as to validate the computational model. The reactor was scaled length wise to increase incubation time and it was observed that as the reactor length increases the RTD variance increases linearly and the dimensionless RTD profile becomes more symmetrical and tighter about the mean residence time.

Towards the design of an intensified coagulator

This study compares the hydrodynamics in three millimeter-scale continuous reactor geometries that can be easily used in laboratories and industries – a straight tube, a coiled tube and a Dean-Hex reactor – via numerical simulations and analyses the data in a way that is specifically relevant to coagulation processes, thereby offering insights for engineers to develop new coagulation reactors. A numerical approach based on Lagrangian particle tracking is presented to better understand the impact of the geometry and flow on properties that influence coagulation. The results show that the Dean-Hex meandering geometry provides narrower residence time and shear rate distributions, as well as higher mean average shear rates and Camp number distribution than the other geometries. This is attributed to the generation of transverse flows and radial mixing in the Dean-Hex reactor and suggests that a faster and more homogenous coagulation can be expected.

Process Design for Size-Controlled Flame Spray Synthesis of Li4Ti5O12 and Electrochemical Performance

Abstract Inexpensive synthesis of electroceramic materials is required for efficient energy storage. Here the design of a scalable process, flame spray pyrolysis (FSP), for synthesis of size-controlled nanomaterials is investigated focusing on understanding the role of air entrainment (AE) during their aerosol synthesis with emphasis on battery materials. The AE into the enclosed FSP reactor is analysed quantitatively by computational fluid dynamics (CFD) and calculated temperatures are verified by Fourier transform infrared spectroscopy (FTIR). Various Li4Ti5O12 (LTO) particle compositions are made and characterized by N2 adsorption, electron microscopy and X-ray diffraction while the electrochemical performance of LTO is tested at various charging rates. Increasing AE decreases recirculation in the enclosing tube leading to lower reactor temperatures and particle concentrations by air dilution as well as shorter and narrower residence time distributions. As a result, particle growth by coagulation - coalescence decreases leading to smaller primary particles that are mostly pure LTO exhibiting high C-rate performance with more than 120 mAh/g galvanostatic specific charge at 40C, outperforming commercial LTO. The effect of AE on FSP-made particle characteristics is demonstrated also in combustion synthesis of LiFePO4 and ZrO2.

Experimental characterization of axial dispersion in coiled flow inverters

Narrow residence time distributions (RTDs) are desirable in many chemical engineering processes. However, when a system operates in the laminar flow regime, significant fluid dynamic dispersion takes place. This problem is often encountered in micro and millifluidic devices. Exploiting the beneficial effects of secondary flow and chaotic advection, so-called coiled flow inverters (CFIs) are a promising solution for the reduction of fluid dynamic dispersion. These devices, however, have not been extensively used due to the lack of experimental data and of correlations relating the design parameters and operating conditions to the amount of axial dispersion. In this work, we investigated RTDs in micro and millifluidic devices using step input injection and UV–vis inline spectroscopy for the detection of the concentration of a tracer. Experiments were performed for different operating conditions and geometries. Helically coiled tubes (HCTs) were similarly characterized. Dispersion data were expressed in terms of an axial dispersion coefficient and an empirical correlation was derived. The experimental results show that lower axial dispersion is achieved in CFIs as compared to HCTs and straight tubes.

CFD Analysis of a Helically Coiled Tube in Tube Heat Exchanger

A helical coil tube heat exchanger is generally applied in industrial applications due to its compact structure, larger heat transfer area and higher heat transfer capability etc. The importance of compact heat exchangers has been recognized in many industrial applications ranging from chemical and food industries, power production, electronics, environmental engineering, manufacturing industry, air conditioning, waste heat recovery, cryogenic processes and space applications for the last six decades. However, flow and helical coils are extensively used as heat exchangers and reactors due to higher heat and mass transfer coefficients, narrow residence time distributions and compact structure.

Single phase mixing in coiled tubes and coiled flow inverters in different flow regimes

The present study focuses on the measurement of residence time distribution (RTD) in coiled tubes and coiled flow inverters (CFI) over a wide range of Reynolds numbers (NRe), in contrast to earlier studies that have reported the RTD over a very narrow range, mainly restricted to the laminar region. The analysis of our experimental data reveals the existence of three distinct flow regimes in coiled structures, viz. laminar flow with prominent Dean circulation, laminar flow with less contribution of Dean circulation, and fully developed turbulent flow. Even though the turbulent transition is known to be delayed in coiled geometries, good radial mixing brought in by Dean circulation results in a narrow RTD of the exiting fluid even when the flow is clearly laminar. For coiled flow inverters, the 90° inversion allows further enhanacement in overall mixing, albeit at the cost of extra pressure drop. The effect of different design parameters on the role of secondary flow for coiled geometries of varying number of inversions (n) and curvature ratios (λ) has been investigated. Finally, a master plot is developed between the dispersion number (D/Udt) and Reynolds number (NRe) showing the marked improvements in cross-sectional mixing in coiled flow, over that in straight tubes. Based on this, a correlation is presented.

Separation Units and Equipment for Lab-Scale Process Development

For complete chemical processes, downstream operation steps are essential, but on a miniaturized scale, they are not so far developed as the microreactors. This contribution presents three different unit operations for phase and component separation. Liquid—liquid extraction is often performed in columns, which were miniaturized for higher separation efficiency and flow rates suitable for processes in flow chemistry. Two-phase mass transfer processes in capillaries benefit from rapid final phase separation, which can be performed in an in-line phase splitter based on different surface wetting behavior. Crystallization is often a final purification step, which is performed in a continuously operated helical tube setup with narrow residence time distribution. For all unit operations, design criteria are shown with typical applications. The methodology of downscaling of known equipment and employing typical microscale phenomena such as good flow control, laminar flow, or dominant surface forces leads to succ...

Experimental investigation of wall film renewal in liquid–liquid slug flow

Liquid–liquid slug flow offers the unique characteristics of high heat and mass transfer combined with a narrow residence time distribution in continuous flow and has thus attracted considerable attention in the field of microfluidics. To exploit its advantages in the successful design and operation of micro-reactors, a precise understanding of the mass transfer processes is essential. In the present work, the role of the thin continuous liquid film formed on the capillary wall in mass transfer is investigated. Fluorescence microscopy is used to determine the exchange between wall film and continuous phase segments to determine if the film is continuously renewed and can therefore be considered to contribute interfacial area available for mass transfer. The distinct wetting properties of different capillary materials are utilized in the experimental set-up to achieve a reproducible and non-invasive release of tracer. The degree of wall film mass transfer as a function of velocity, interfacial area and wall-film thickness is established.