Showing papers on "Countercurrent exchange published in 2004"

Modeling wood gasification in a countercurrent fixed‐bed reactor

Abstract: A one-dimensional, unsteady mathematical model is presented of fixed-bed countercurrent wood gasifiers, which couples heat and mass transport with wood drying and devolatilization, char gasification, and combustion of both char and gas-phase species. The model is used to simulate the structure of the reaction fronts and the gasification behavior of a laboratory-scale plant as the reactor throughput and the air-to-wood (or char) weight ratio are varied. It is observed that a wide zone, acting essentially as a countercurrent heat exchanger, separates combustion/gasification from devolatilization/ drying. Moreover, the former zone presents interesting dynamic patterns driven by the highly variable solid/gas heat transfer rates. For a constant air-to-wood weight ratio, the gasification process is improved by increasing the reactor throughput as a result of higher temperatures, in spite of the simultaneous reduction in the amount of char generated from wood devolatilization. In fact, an increase in the air-to-wood (or char) ratio always lowers the efficiency of the gasification process. Finally, good agreement is obtained between predictions and experiments for the axial temperature profiles and the composition of the producer gas. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2306 –2319, 2004

Method and apparatus for acquiring heat from multiple heat sources

Abstract: The present invention relates to systems and methods for implementing a closed loop thermodynamic cycle utilizing a multi-component working fluid to acquire heat from two or more external heat source stream in an efficient manner utilizing countercurrent exchange. The liquid multi-component working stream is heated by a first external heat source stream at a first heat exchanger and is subsequently divided into a first substream and a second substream. The first substream is heated by the first working stream at a second external heat source stream at a second heat exchanger. The second substream is heated by the second working stream at a third heat exchanger. The first substream and the second substream are then recombined into a single working stream. The recombined working stream is heated by the second external heat source stream at a fourth heat exchanger.

Scale-up of reactive distillation columns with catalytic packings

Abstract: The scale-up of reactive distillation columns with catalytic packings requires the knowledge of reaction kinetics, phase equilibrium and packing characteristics. Therefore, pressure drop, liquid holdup and separation efficiency have been determined for the catalytic packing MULTIPAK®. A new hydrodynamic model that describes the countercurrent gas-liquid flow for the whole loading range and considers the influence of the column diameter has been implemented into a rate-based column model. Simulation results for the methyl acetate synthesis are compared with pilot plant experiments that cover a wide range of different process conditions. The experiments are in good agreement with the simulation results and confirm the applicability of the modelling approach for reactive distillation processes with catalytic packings.

Effect of vapor flow on the falling-film heat and mass transfer of the ammonia/water absorber

Abstract: Falling-film heat and mass transfer in an absorber can be influenced by the motion of the surrounding refrigerant vapor. In this study, the effect of the vapor flow direction on the absorption heat and mass transfer has been investigated for a falling-film helical coil absorber which is frequently used in the ammonia/water absorption refrigerators. The heat and mass transfer performance was measured for both parallel and countercurrent flow. The experiments were carried out for three different solution concentrations (3, 14, and 30%). The vapor in equilibrium with the solution is supplied to the test section. It is found that the falling-film heat and mass transfer is deteriorated in the countercurrent flow if the specific volume of the vapor solution is large. For the countercurrent flow, the high velocity of the vapor due to large specific volume seems to cause the unfavorable distribution of falling-film and reduce the heat and mass transfer performance of the ammonia absorber. The effect of vapor flow direction decreased with increasing concentration of ammonia solution since the specific volume of the ammonia vapor which is in equilibrium with the solution becomes smaller and the vapor velocity becomes lower.

Flooding of gas-solids countercurrent flow in fluidized beds

Abstract: Flooding experiments were carried out in a semicircular pilot-scale cold model of Syncrude's two fluid cokers. This unit circulates solids downward through a stripper zone and externally back to the top of the vessel, facilitating countercurrent gas/solid contacting. The onset of flooding for FCC particles was determined by analysis of differential pressure signals, supported by visual observations. The solids circulation flux at flooding increased with decreasing superficial gas velocity, with increasing fractional open area, with increasing slot width between adjacent baffles, and with decreasing baffle top included angle. A semiempirical model gives predictions of the onset of flooding that are in good agreement with both cold-flow model results and available commercial data.

Matrix-fracture transfer through countercurrent imbibition in presence of fracture fluid flow

Abstract: Although a lot of research has been done in modeling the oil recovery from fractured reservoirs by countercurrent imbibition, less attention has been paid to the effect of the fracture fluid velocity upon the rate of oil recovery. Experiments are conducted to determine the effect of fracture flow rate upon countercurrent imbibition. A droplet detachment model is proposed to derive the effective water saturation in a thin boundary layer at the matrix–fracture interface. This effective boundary water saturation is a function of fluid properties, fluid velocity in the fracture and fracture width. For a highly water–wet porous medium, this model predicts an increase in the boundary water saturation with increase in fracture fluid velocity. The increase in boundary water saturation, in turn, increases the oil recovery rate from the matrix, which is consistent with the experimental results. The model also predicts that the oil recovery rate does not vary linearly with the boundary water saturation.

Dynamic model of a countercurrent packed column operating at high pressure conditions

Abstract: A dynamic simulation model of a countercurrent packed column operating at supercritical fluid (SCF) conditions is presented. The model was developed and applied to a case study involving the fractionation of a binary mixture of squalene and methyl oleate using supercritical carbon dioxide. The purpose of the separation process is to remove the methyl oleate from the solution and concentrate squalene in the raffinate phase. The final model comprises the differential material balances in the packed column and algebraic equations describing the thermodynamic phase equilibrium, mass transfer and the hydrodynamics of the two countercurrent phases. The model was validated by carrying out a series of experiments in a lab-scale continuous SCF extraction unit. A good agreement was obtained between measured and predicted composition profile of the outlet streams over time.

Hot Coal-Gas Desulfurization with Calcium-Based Sorbents in a Pressurized Moving-Bed Reactor

Abstract: The performance of a pressurized moving bed on hot coal-gas desulfurization was analyzed for the countercurrent and cocurrent configurations. The behavior of several calcium-based sorbents, two limestones, and a dolomite was determined at temperatures in the range of 1073−1273 K and a pressure of 1 MPa. The limestones were used only under calcining conditions, whereas the dolomite was used under both calcining and noncalcining conditions. The desulfurization level and sorbent conversion, as well as the longitudinal H2S concentration profiles in the transference zone, were determined as a function of the main design and operating variables, including the Ca/S molar ratio, the bed height, the type of flow configuration (countercurrent or cocurrent), the temperature, the H2S concentration, and the particle size. The length of the transference zone, and then the reactor length necessary for full desulfurization, was highly dependent on the particle size and the solids velocity. In both countercurrent and cocu...

Bubbling behavior in gas-liquid countercurrent bubble column bioreactors

Abstract: Characteristics of bubbling behavior and bubble properties were investigated and diagnosed in a gas-liquid countercurrent bubble column bioreactor which is 0.152 m in inside diameter and 3.5 m in height. The effects of gas and liquid velocities and bubble distribution mode (even, wall-side, central or asymmetric distribution) on the bubble properties such as chord length, frequency, rising velocity, holdup and bubbling behavior were examined. For the analysis of resultant bubbling behavior, pressure fluctuation signals were measured and analyzed by adopting the concept of the chaos theory; the signals were interpreted by means of an attractor in the phase space portraits and correlation-dimension. It was found that the resultant bubbling behavior could be detected effectively and quantitatively in terms of the phase space portraits and correlation dimension of pressure fluctuations in the bubble column bioreactor. The bubble size, frequency and holdup increased with increasing gas (U G ) or liquid velocity (U L ). The rising velocity of bubbles increased with increasing U G , whereas decreased with increasing U L . The uniformity of bubble size distribution and bubble holdup decreased when the distribution mode of bubbles at the gas distributor was changed from even to wall-side, central or asymmetric. The central distribution of bubbles was better than the asymmetric mode but worse than wall-side distribution, on considering the bubble holdup and uniformity of distribution. The bubble holdup and size were well correlated in terms of correlation dimension of pressure fluctuations elucidating the bubbling phenomena in the gas-liquid countercurrent bubble column bioreactor.

Situation and Developing Trend of Rare-Earth Countercurrent Extraction Processes Control

Abstract: On the basis of the description of the rare-earth countercurrent extraction process, the on-line detecting method and equipments of rare-earth elements and the application in the process of the rare-earth countercurrent extraction are summarized. The procedure simulation of the computer, the automation control method and its current application are also mentioned in the process of rare-earth countercurrent extraction. The method of soft sensor is proposed. Optimal control method based on object-oriented rare-earth countercurrent extraction process and integrated automation system composed of process management system and process control system are presented, which are the developing direction of the automation of rare-earth countercurrent extraction process.

A two-phase computational fluid dynamics model for ozone tank design and troubleshooting in water treatment

Abstract: A computational fluid dynamics (CFD) tool was employed to design and study ozone contactors The emphasis was to achieve the desired flow distribution The Eulerian-Eulerian multiphase model was used with the standard k-ϵ turbulent model The water surface was slip wall boundary and was specified as a sink to remove ozone bubbles For a single-column contactor with side entry, the flow pattern was found to be crucially dependent on both the direction and magnitude of the entry velocity from the inlet pipe It was difficult to achieve uniform gas concentration over the contactor volume In a multicompartment contactor, the countercurrent flow resulted in a mixed flow condition and the mixing increased with a higher gas rate For the cocurrent flow, water was accelerated by the gas and the plug flow pattern was achieved The flow distribution in each compartment can be significantly different even though the overall residence time distribution curves are similar

Countercurrent flow limitations in horizontal stratified flows of air and water

Abstract: During a postulated Loss-Of-Coolant-Accident (LOCA) in a Pressurized Water Reactor (PWR) it is of vital importance that the reactor core remains properly cooled. The Emergency Core Cooling System (ECCS) in German PWRs compensates the loss of coolant with injection of additional coolant into the cold legs as well as into the hot legs. While the coolant is injected in the cold legs through nozzles, the hot leg injection is performed by means of a secondary pipe placed at the bottom of the pipe of the primary circuit. The subject of this thesis concerns the latter case. The liquid injected into the hot leg flows directly into the core from its upper part and constitutes a rapid delivery of coolant into the reactor core at high mass flow rate. However, saturated steam is generated in the reactor core due to depressurization of the primary system and flows out of the Reactor Pressure Vessel (RPV) into the hot leg. Therefore, a countercurrent stratified flow of injected coolant and saturated steam occurs along one and a half meter inside the hot leg before the coolant reaches the RPV. This horizontal stratified countercurrent flow of coolant and steam is only stable for a certain range of coolant and steam mass flow rates. Even if the coolant is injected at very high velocities and high Froude numbers, there is always a threshold steam velocity above which the cooling of the reactor core can be reduced or complete interrupted. This phenomenon is known in two-phase flow science as Countercurrent Flow Limitation (CCFL), since there is a limitation of liquid delivery due to the presence of a gas phase flowing countercurrently to the liquid phase. CCFL in reflux condensation cooling was more investigated than in ECC in the hot leg. For this purpose, the test facility WENKA was built at Forschungszentrum Karlsruhe GmbH (Germany) to investigate for which flow conditions CCFL poses a safety risk during hot leg injection and to provide experimental data to support the analysis of such an accident scenario with CFD - Codes. The WENKA test facility models a simplified PWR hot leg geometry including the secondary pipeline placed at the bottom of the main coolant line. The countercurrent flow of coolant and saturated steam during injection by means of the ECCS was investigated with air and water in a rectangular test section. The fluid dynamics of the injection process was reproduced for a wide range of flow conditions to identify flow regimes and to derive 1-dimensional models to predict the limits of coolant delivery. On the other hand, a data base of local flow parameters was established to enhance CFD - Codes performance. Experimental local velocities of the liquid film were obtained by means of Particle Image Velocimetry and the liquid film morphology was analyzed depending on the flow regimes. Flow regime maps were obtained for inlet liquid depths ranging from 3 to 15 mm. Depending on the superficial velocities of liquid and gas and on the Froude number of the liquid film, a stratified countercurrent flow, a partially reversed flow or a totally reversed flow were experimentally observed. The CCFL occurred as a breakdown of the stable countercurrent stratified two-phase flow: The liquid begun to be carried over by the gas and to flow partially or totally in the air flow direction.

Liquid phase residence time distribution for a two-phase countercurrent flow in a packed column with a novel internal

Abstract: The liquid phase residence time distribution (RTD) for gas–liquid countercurrent flow in a packed column with a novel internal was measured by conductivity measurements and an air–water system. The RTD of a liquid tracer is well represented by the ADM and PDE models. At lower gas flow rates, the Peclet number of the liquid in the packed column with the internal is lower than that without the internal; at higher gas flow rates, it is vice versa, especially with an internal with a higher volume fraction. The distribution of the liquid RTD can be improved by using suitable geometric parameters of the internal to give a larger volume fraction and a lower stage height.

Multi-chamber supercavitation reactor

Abstract: The invention relates to a device for the molecular integration or disintegration of solid, liquid and/or gaseous flowing, entrained and/or countercurrent components by means of cavitation in order to modify, build or disintegrate molecular compounds. The invention allows to obtain stable mixtures from immiscible or difficult-to-mix components or to separate such mixtures. The supercavitation molecular reactor allows to build up or disintegrate or modify, with low expenditure in terms of energy, even complex compounds that so far have not been accessible to modification and/or production or only by very extensive multiple processes and a large amount of technical complexity.

Analysis of the Reverse Flow Chromatographic Reactor

Abstract: A reverse flow chromatographic reactor with side feed (RFCR) can be used to improve the conversion and selectivity for irreversible and reversible reaction systems beyond the values obtained from conventional steady-state reactors in the case where the reactants are more strongly adsorbed than the products. It is also simpler than conventional chromatographic, moving, and simulated moving beds and other systems that combine reaction and adsorption separation. The results of the study show the robustness and stability of the system, as well as its capacity to significantly improve conversion and selectivity for several reaction systems, including consecutive reactions. Simulation results of this mathematical model for the hydrogenation of 1,3,5-trymethylbenzene (MES) compare well with previous studies, using countercurrent moving-bed reactors and simulated countercurrent bed reactors. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2266–2275, 2004

Counterflow heat exchanger

Abstract: A countercurrent heat exchanger includes a pair of heat exchanger cores (1, 2) with multiple tubes (11, 21) and fins (12, 22) which are arranged alternatively and next to each other in its depth direction. One end sides of the tubes (11, 21) of the inflow-side heat exchanger core (1) and the outflow-side heat exchanger core (2) is connected with a U-turn intermediate tank (3), and the other end sides are connected with the inflow-side tank (4) and the outflow-side tank (5) which are separated from each other. The inflow-side tank (4), outflow-side tank (5) and intermediate tank (3) are attached to a vehicle body side so that the heat exchanger cores (1) and (2) can expand and contract with respect to the intermediate tank (3).

Modelling of a Countercurrent Hydrogenation Process

Abstract: Countercurrent operation of trickle-bed hydrogenation reactors has advantages over the commonly used co-current operation mode in some specific cases. The advantages evolve from one of the basic practices applied in chemical engineering, i.e. the countercurrent operation utilizes the available mass transfer driving force between gas and liquid phases to its maximum and thus drives the conversion to its limit. In reactors this also provides the means to strip light components from the liquid phase. These light compounds might inhibit the reaction or poison the catalyst. The modelling of hydrogenation processes is a challenging task not only because of the various hydrodynamic aspects involved but also in respect of the mass transfer processes between liquid and vapour, liquid and catalyst and sometimes even inside the catalyst particles. Countercurrent operation is mathematically complex. The solution is an iterative process and setting boundary values can be a problem whereas co-current operation can be solved easily as a group of partial differential equations with known initial values. In this paper we utilize the so-called rate-based approach to countercurrent hydrogenation reactors and show some of its features. Comparison with co-current process has been done additionally and the efficiency of countercurrent operation in removing of light compounds has been shown.

Dynamic holdup in a countercurrent gas - flowing solids - packed bed contactors

Abstract: Equations for the prediction of the holdup of dynamic solids in countercurrent gas - flowing solids - packed bed contactors are presented in this paper. The correlations do not require the use of parameters that need to be determined by experimental measurements in the actual system of interest. They could be used for a wide range of operational conditions, different packing types and a variety of flowing solids materials. The equations are compared with all available experimental data from the literature.

A new principle for creation of packings operating at extremely low liquid superficial velocity

Abstract: The lack of packings able to operate at extremely low liquid superficial velocities does not allow to carry out mass transfer processes at full countercurrent flow at too low concentrations of the component to be absorbed. For this reason, the full countercurrent flow in many important processes is replaced by division of the packing into layers and re-circulation of the absorbent in each of them. The latter increases the expenses for the construction of the apparatus with parallel decreasing the process driving force. A method for creation of packings with effective surface equal to their total surface even at liquid superficial velocities that tend towards zero is described. The construction of such effective packing is given. The volumetric mass transfer coefficient when the mass transfer is controlled by the gas-side boundary layer has been received from the results of the absorption of sulfur dioxide into a solution of sodium hydroxide. Data for the pressure drop and liquid hold-up of the packing are also presented. The comparison of the new packing with other types of highly efficient packings shows its great advantages.

Countercurrent chromatographic separation process for reducing fixed phase

Abstract: The present invention relates to countercurrent chromatographic process for separating active component from plant extractive. The present invention features the separated compounding of fixed phase and flowing phase constituting the solvent system. The fixed phase is one of alkane, chlorine-containing organic solvent, ethyl acetate, fatty alcohol and fatty ketone or their combination; and the flowing phase is one of chlorine-containing organic solvent, fatty alcohol and water or their combination; with the fixed phase and the flowing phase being of different matter. The present invention has shortened active component monomer peak displaying time, improves the peak shape of the active component monomer, and may be used in industrial production to raise the yield of active component monomer and raise production efficiency.

Performance of fractionating reactors in the absence of rate limitations

Abstract: A fractionating reactor for equilibrium-limited reactions is studied theoretically. Reactant A is fed in the center of the countercurrent fractionating system. Product P is effectively transported with the auxiliary phase, while product Q is effectively transported with the main phase, in which the reaction takes place. Model calculations were based on partition and reaction equilibrium at all stages. These show that if the initial reactant concentration and the flow rates are properly selected, the extent of conversion will significantly exceed the corresponding batch conversion. To approach complete conversion in the fractionating reactor, and to recover both products in a pure form, net transport of reactant in either of the countercurrent directions should be prevented. However, irrespective of the number of equilibrium stages, this situation cannot be fully reached when the reactant feed stream is too large (compared with the main and auxiliary streams). Nonetheless, one of the two products may be recovered in a pure form even for such large feed streams. Copyright © 2004 Society of Chemical Industry

Research and Application Progress in Countercurrent Solvent Extraction

Abstract: In this article, the recent progress made by Peking University is briefly introduced. Based on Theory of Countercurrent Extraction established by Prof. Guangxian Xu, the static equilibrium and dynamic process for two- and three-outlet countercurrent extractions, and designed the optimized parameters for the real cascades to separate different rare earth minerals, which can be scaled-up to the industrial process without any further experimental verification were systematically investigated. In order to stabilize the quality of products and improve the capability of automation for extraction process, we have also established an 241Am stimulated X-ray fluorescent energy dispersive method to detect the elemental composition in both organic and aqueous phases, which can be used as an on-line analysis method in practice. Furthermore, the expert system and the control software with open- and close-loop models have been set up. Combined with the detected data from on-line analysis, the flow-rates of extracting, feeding, scrubbing and stripping solutions can be controlled in an optimum status for the industrial cascades. In addition, we have developed a new multi-input and multi-output countercurrent extraction process for separating multi-component rare earth mixture with lower chemical cost and pollution.