Showing papers on "Countercurrent exchange published in 1994"

Stretch and the adiabatic burning velocity of methane- and propane-air flames

Abstract: Results of measured adiabatic burning velocities of methane- and propane-air premixed laminar flames are presented, as obtained by the Heat Flux method. A comparison is made with results of the Counterflow method. The effect on the Counterflow method results of a non-linear instead of a linear correction for stretch is estimated. With a non-linear correction the agreement between both methods is good. It can be concluded that non-linear correction for stretch on the results of the Counterflow method is necessary in general. However, its effect may vary with varying fuel gas.

Modeling of multicomponent countercurrent gas permeators

Abstract: Modeling of gas permeation in hollow-fiber or spiral wound modules necessitates considering the effect of permeate pressure variation along the module length which could have a significant effect on the prediction of the exit compositions and membrane area requirements depending on the membrane characteristics and module geometry The transport equations governing the permeator performance are a set of coupled nonlinear differential equations The complexity of the solution procedure for these equations increases with the number of components in the mixture and consideration of pressure variation Thus, there is a need for simplified solution methodologies which could reduce the computational efforts This paper presents a solution methodology to solve the multicomponent gas permeator transport equations in a countercurrent flow pattern, taking the permeate pressure variation into consideration The present method yields analytical expressions for flow rates, permeate pressure, membrane area, and compositions along the length of the permeator

Pervaporation by countercurrent condensable sweep

Abstract: There is described a process for the removal of at least one component of a liquid mixture, the basic process comprising directing a liquid mixture against the feed side of a membrane, directing a condensable vapor sweep stream past the permeate side of the membrane in a manner such that the flow of the condensable vapor sweep is substantially countercurrent to the flow of the liquid mixture, thereby transporting at least a portion of at least one component of the liquid mixture from the feed side to the permeate side of the membrane to form a combined permeate side mixture of condensable vapor and at least one transported component.

A Unified Model for Countercurrent Vapor/Liquid Packed Columns. 2. Equations for the Mass Transfer Coefficients, Mass Transfer Area, the HETP, and the Dynamic Liquid Holdup

Abstract: In part I a model for the two-phase pressure drop in a packed column was derived from an analogy to electrical percolation on a conductor/insulator lattice. In part II we extend that development to include the mass-transfer coefficients, the area available for mass transfer, the height of packing equivalent to a theoretical plate, and the dynamic liquid holdup

Process for Removing Elemental Sulfur from a Gas Stream

Abstract: The invention relates to a process for removing elemental sulfur which is present in a gas in the form of vapor and/or entrained particles, in which process the gas to be treated is cooled. The gas to be treated is introduced into a heat exchanger at the lower end thereof and with the aid of the temperature and/or the flow velocity of the cooling medium it is ensured that the wall of the heat exchanger has a temperature below the solidification point of sulfur and above the dew point of water, if any, present in the gas. The deposited sulfur is removed under the influence of gravity countercurrent to the gas to be treated.

Cryogenic rectification system for lower pressure operation

Abstract: A double column cryogenic rectification system wherein lower pressure column (42) bottoms undergo additional rectification within a once-through downflow reflux condenser (41) by countercurrent direct contact flow with vapor generated by condensing higher pressure column shelf vapor (70) enabling the higher pressure column (40) to operate at a reduced pressure thus reducing feed compression power requirements.

Ion exchange methods and systems incorporating regenerant recycling

Abstract: An ion exchanger (10) of the countercurrent type includes a pressure vessel (12) for supporting a resin bed (14). During a normal regeneration cycle, the vessel (12) exhibits both cocurrent and countercurrent operation.

A Unified Model for Countercurrent Vapor/Liquid Packed Columns. 1. Pressure Drop

Abstract: A model for the two-phase pressure drop in a packed column is developed from percolation concepts. Specifically, the macroscopic flood point for countercurrent liquid/vapor flow in a packed column is assumed to play the identical role that the incipient percolation threshold plays for the conductor/insulator transition on an electrical lattice. The flow passages making up the void space of the packing are taken to the conducting when they are open to vapor flow and insulating when they are choked off by liquid (i.e., localized flooding). The Wallis equation, developed to describe flooding for countercurrent flow in vertical tubes, is used to describe the flood point in the column. By an appropriate renormalization to the flood point, a simple equation for the pressure drop is derived

Catalytic cleaning of polluted air: reaction engineering problems and new solutions

Abstract: Integration of regenerative heat exchange into a catalyst bed enables auto· thermal operation of catalytic purification of polluted air with a low con­ tent of combustible pollutants. Concentrations corresponding to an adia­ batic temperature rise of less than 20 °C can be handled without introduction of additional fuel. For higher concentrations, a technique involving side-stream withdrawal allows utilization of the total heat of combustion at the highest reactor temperature. Heat recovery by integrated heat exchange gives rise to an unusual behavior of the reactor. Based upon an analogy to fIXed-bed reactors with integrated countercurrent heat ex­ change, simple equations are derived for reactor design and operation. Substantial reduction in pressure loss and in the volume of the packed bed can be obtained by replacing conventional catalyst packings by monolithic catalysts. The corresponding relationships are briefly discussed.

Limitation of Falling Water in Countercurrent Two-Phase Flow in Vertical Circular Tubes.

Abstract: The mechanism of countercurrent flow limitation (CCFL) was investigated analytically and quantitatively for the existing experimental results for vertical circular tubes, adopting a criterion that the CCFL condition is given by maximizing falling water mass velocity with respect to void fraction for the whole flow channel. Through the analysis, it was found that significant factors in the CCFL condition were the interfacial friction factor between liquid and gas based on the relative velocity, wall friction factors for laminar, transition and turbulent liquid flows and the effects of tube diameter and length. Analytical results gave very good predictions of the experimental results for vertical circular tubes of 19 to 140mm in diameter and 12.7 to 1520mm in length, under atmospheric pressure.

Effect of hydrodynamic forces on the pressure-difference equation

Abstract: Because of the influence of hydrodynamic forces, the difference in macroscopic pressure which exists, at static equilibrium, between two immiscible phases located in a porous medium may be different from that which pertains during flow. In this paper, the concept of relative pressure difference, together with a new pressure-difference equation, is used to investigate the impact that the hydrodynamic forces have on the difference in macroscopic pressure which pertains when two immiscible fluids flow simultaneously through a homogeneous, water-wet porous medium. This investigation reveals that, in general, the equation defining the difference in pressure between two flowing phases must include a term which takes proper account of the hydrodynamic effects. Moreover, it is pointed out that, while neglect of the hydrodynamic effects introduces only a small amount of error when the two fluids are flowing cocurrently, such neglect is not permissible during steady-state, countercurrent flow. This is because failure to include the impact of the hydrodynamic effects in the latter case makes it impossible to explain the pressure behaviour observed in steady-state, countercurrent flow. Finally, the results of this investigation are used as a basis for arguing that, during steady-state, countercurrent flow, saturation is uniform, as is the case of steady-state, cocurrent flow.

Acid concentration control in SO3 absorption

Abstract: The concentration of sulfuric acid exiting a countercurrent SO 3 absorber is controlled by controlled addition of water vapor to the SO 3 -containing process gas stream entering the absorber. The novel means of concentration control is useful for control of the acid concentration gradient in an SO 3 absorption process and apparatus for high temperature absorption, so as to achieve high temperature absorption heat recovery while minimizing corrosion and formation of acid mist.

Multicomponent separation using a column‐switching chromatographic method

Abstract: A cyclic chromatographic process based on the method of column switching was used to separate a three-component carbohydrate mixture. It permits continuous introduction of the feed and is designed to produce higher product concentration compared to a normal elution cyclic process. A fixed-bed model incorporating axial dispersion and a linear driving force for mass transfer was successfully used to estimate optimum operating conditions (switch times and fluid-flow rates). Its experimental setup can be easily constructed by modifying the apparatus of a simulated countercurrent process since both systems comprise segmented columns equipped with on-off valves for fluid-flow control. This allows one to conduct either multicomponent separation based on elution operation or binary separation based on countercurrent operation using essentially the same set of experimental apparatus with minor modification.

Physical modeling and parameter identification of a heat exchanger

Abstract: Proposes a model and the experimental identification of the double pipe countercurrent water-to-water heat exchanger of the Control Laboratory of the Conservatoire National des Arts et Metiers, Paris, France. First a bond graph model is proposed, representing the heat conduction and heat convection occurring in the heat exchanger. Secondly, the identifiability of the obtained set of structural dynamic equations is discussed with respect to the nonlinear model and its linearization. Finally the identification of the physical parameters is performed by using the prediction error method with the linearized model and the distributions of the simulated and measured temperatures along the pipes are compared. >

Analytical Study of Critical Heat Flux under Countercurrent Flow Limitation in Vertical Channels

Abstract: A quantitative analysis of critical heat flux (CHF) under countercurrent flow limitation (CCFL) or flooding was successfully carried out using the existing experimental results for the vertical circular, rectangular and annular channels, applying the criteria, which was proposed by the author, that the CCFL condition could be determined by maximizing the water mass flux falling down in the vertical channels with respect to the water film thickness and the model of dryout of water film flow at the CHF point. It was clarified that the analytical results give good predictions of the existing experimental results of vertical channels, identifying the factors determining the CHF under the CCFL conditions.

An Experimental Study of Gravity-Driven Countercurrent Two-Phase Flow in Horizontal and Inclined Channels

Abstract: Countercurrent two-phase flow in horizontal and inclined channels, connecting a sealed liquid-filled reservoir to the atmosphere, is experimentally studied. This type of gravity-driven countercurrent two-phase flow can occur during the operation of passive safety coolant injection systems of advanced reactors. It can also occur in the pressurizer surge line of pressurized water reactors during severe accidents when the hot leg becomes voided. Four distinct flow regimes are identified: (a) stratified countercurrent, which mainly occurs when the channel is horizontal; (b) intermittent stratified-slug; (c) oscillating, which occurs when the angle of inclination is [>=]30 deg; and (d) annular countercurrent. The characteristics of each regime and their sensitivity to important geometric parameters are examined. The superficial velocities in the stratified countercurrent and oscillating regimes are empirically correlated.

Countercurrent jet device

Abstract: The flow features of a jet device mounted at the bottom of a rectangular channel and issuing a plane wall jet are studied both by computational and experimental means. It is found that the countercurrent jet is able to lift the water dynamically, and that the difference of water levels in the up‐ and downstream portions depends mainly on the slot Froude number and the relative height of slot. From the practical point of view, two applications may be considered: (1) The case in which a difference in water level should be created without usual hydraulic structures such as gates or overflow structures; and (2) the case in which excess energy must be dissipated. In case 2 the slot Froude number should be large, combined with a low tailwater depth. The maximum efficiency of dissipation occurs for a transition from subcritical upstream to supercritical downstream flow, beyond which a conventional secondary stilling basin may be used. These features are shown to be analogous to the classical hydraulic jump.