LIMITING FLUX IN ULTRAFILTRATION OF MACROMOLECULAR SOLUTIONS

The mass transfer equation describing the process of gel polarized ultrafiltration is solved using an integral method. A concentration profile is assumed to be an nth degree polynomial. In conventional integral method, “n”is arbitrarily assigned an integer value. In this paper we have taken the first moment of the convective diffusion equation to determine “n” as a function of the system parameters. The agreement between the closed form integral method solution and the exact numerical solution is excellent while the widely used film theory deviates considerably.     

LIMITING FLUX IN ULTRAFILTRATION OF MACROMOLECULAR SOLUTIONS

The mass transfer equation describing the process of gel polarized ultrafiltration is solved using an integral method. A concentration profile is assumed to be an nth degree polynomial. In conventional integral method, “n”is arbitrarily assigned an integer value. In this paper we have taken the first moment of the convective diffusion equation to determine “n” as a function of the system parameters. The agreement between the closed form integral method solution and the exact numerical solution is excellent while the widely used film theory deviates considerably.     

SHORT AND LONG TERM FLUX DECLINE ANALYSIS IN ULTRAFILTRATION

Flux decline during stirred UF over a long time of operation can be classified into short and long time domains. In the short time, the decline in flux is basically osmotic pressure limited and this decline is extremely rapid. The limiting flux for the osmotic pressure controlled domain is obtained by successive substitution technique. This technique includes the variations of properties with concentrations. A modified Sherwood number correlation for determining the mass transfer coefficient is proposed for laminar flow in a stirred cell. The long term flux decline can be modeled semi-empirically by a two parameter model. One of these, the steady state polarized layer resistance, can be correlated to the operating conditions while the other constant of the model is found to be a characteristic of the solute only. The simplistic model developed here to account the flux decline during UF may be useful to the process engineers for design purposes.     

ON THE LIQUID FLOW REVERSAL IN BUBBLE COLUMNS

The maximum range of the radial position within which liquid flow reversal can be expected to occur in zero net liquid flow bubble columns is predicted. It is shown that existing models, that employ this position as an input parameter for predicting the liquid velocity profile, are intrinsically valid only when the flow reversal dimensiontess radius is confined to ihe narrow range of 0.644-0.707. It is demonstrated that radial positions outside this range are unacceptable on physical grounds. Guidelines for evaluating the appropriate location of the flow reversal point for typical bubble column operating conditions are proposed.     

PERMEATE FLUX BEHAVIORS UNDER VARIOUS PRESSURE DECLINES IN MEMBRANE ULTRAFILTRATION MODULE

The effects of operating conditions on the permeate flux for ultrafiltration of aqueous solution of PVP-360 in hollow-fiber membrane modules, have been investigated based on the exponential model with the consideration of the declines of transmembrane pressure along the axial direction in the hollow fibers. It is seen that the exponential model qualitatively correlates the experiment. The correlation prediction may be improved if the exact but complicated pressure distribution is used for the derivation of presented model.     

COMPARATIVE LIMITING FLUX ANALYSIS OF BLACK LIQUOR AND POLYETHYLENE GLYCOL IN ULTRAFILTRATION

Ultrafiltration of black liquor was carried out using an asymmetric membrane and the results were compared with that of polyethylene glycol, a standard macromolecule, in a stirred batch cell. The effects of system parameters, e.g., pressure, concentration and stirrer speed on permeate flux and solute rejection were studied extensively for both the solutes.

An osmotic pressure model was used to analyze experimental results. To take into account the phenomena of concentration polarization, an extra resistance term, called the polarization layer resistance (R_p), was incorporated into the model. The polarization layer resistance was found to be a function of flow regime and concentration at the membrane. To correlate these, the following two types of relationship were examined and tested with the experimental results,

and

where a, b, c, a₁, b₁, are constants.     

CRITICAL FLUX DETERMINATION IN ULTRAFILTRATION OF WASTEWATER FROM A FOOD INDUSTRY BY OPTIMIZATION METHOD

Two ultrafiltration membranes with different geometries (spiral polymeric and tubular ceramic) but similar cutoffs were used to treat wastewater from a food industry. Hydrodynamic conditions were optimized by statistical methods as a strategy to get more accurate values of the critical parameters and then to produce higher water flux and minimization of membrane fouling. The validation of the optimization method was obtained by experimental critical flux determination at critical parameters. Membrane fluxes revealed significant differences during filtration. The polymeric membrane showed an optimal flux of 45.60 Lh^?1 m^?2 at 3.21 bar while operating at a stable time of 11.61 h, whereas optimal flux of the ceramic membrane was 32.43 Lh^?1 m^?2 at 3.98 bar for 16.03 h. Experimental critical flux values were only slightly lower than optimal fluxes for both membranes, showing the validity of the statistics models applied. Negligible osmotic pressure was found on the two membranes at critical flux parameters, indicating irreversible fouling for both cases. The polymeric membrane revealed strong fouling behavior and the ceramic membrane showed a weak form; the flux decline occurred first in the polymeric membrane, whereas the ceramic membrane exhibited high stability during the filtration operations. A high degree of purification of wastewater was obtained by this membrane at critical flux conditions.     

MASS TRANSFER ENHANCEMENT BY SMALL FLOW OBSTACLES IN ELECTROCHEMICAL CELLS

Mass transfer enhancement by small obstacles attached to the cathode in electrolytic flow cells of 5x5 mm cross-section and 500 mm length was investigated. Double beam laser interferometry was used to observe the local mass transfer boundary layer thicknesses preceding and following rod-shaped dielectric obstacles placed normal to the direction of electrolyte flow. Flow patterns have been visualized by use of suspensions of small inert particles and dark field photography. For the evaluation of the effectiveness of mass transport enhancement, pressure drops, and limiting currents for the reduction of ferricyanide have been measured in the range of Reynolds Number 80 to 3200. The degree of enhancement increases with decreasing obstacle spacing until an optimal spacing of approximately 15 times the obstacle size is reached. A three to five-fold increase in the average mass transfer coefficient is achieved by the use of obstacles with a small fraction of the pumping power required to obtain the same limiting current by increasing the flow rate in the unobstructed channel. Small obstacles produce efficient mixing near the electrode surface, and corresponding improvement in uniformity and magnitude of mass transport rates, without increasing the energy dissipation in the bulk fluid.     

MASS TRANSFER ENHANCEMENT BY SMALL FLOW OBSTACLES IN ELECTROCHEMICAL CELLS

Mass transfer enhancement by small obstacles attached to the cathode in electrolytic flow cells of 5x5 mm cross-section and 500 mm length was investigated. Double beam laser interferometry was used to observe the local mass transfer boundary layer thicknesses preceding and following rod-shaped dielectric obstacles placed normal to the direction of electrolyte flow. Flow patterns have been visualized by use of suspensions of small inert particles and dark field photography. For the evaluation of the effectiveness of mass transport enhancement, pressure drops, and limiting currents for the reduction of ferricyanide have been measured in the range of Reynolds Number 80 to 3200. The degree of enhancement increases with decreasing obstacle spacing until an optimal spacing of approximately 15 times the obstacle size is reached. A three to five-fold increase in the average mass transfer coefficient is achieved by the use of obstacles with a small fraction of the pumping power required to obtain the same limiting current by increasing the flow rate in the unobstructed channel. Small obstacles produce efficient mixing near the electrode surface, and corresponding improvement in uniformity and magnitude of mass transport rates, without increasing the energy dissipation in the bulk fluid.     

合成甲醇反应器流向变换强制周期操作特性

实验研究了合成甲醇固定床反应器流向变换强制周期操作的非定态特性 ,考查了流向变换周期、进料流速和进料组成对床层轴向温度分布、热波在床层内的爬行速度以及反应器流出物组成的影响 .研究发现 ,即使进料中∑CO_x 总浓度降低至 0 .33 % (mol) ,合成甲醇的非定态反应过程也能自热进行     

CATALYTIC COMBUSTION OF NATURAL GAS IN A FIXED BED REACTOR WITH FLOW REVERSAL

A new application of the fixed bed catalytic reactor with flow reversal for combustion of natural gas is investigated by mathematical modeling and computer simulation. Comparison between the results obtained for this new reactor and those for a classic catalytic fixed bed is made. Inexpensive perovskite type catalysts containing no noble metals were used. It is shown that an appropriate choice of operating parameters (concentration and temperature of input gas mixture, superficial gas velocity, size and shape of catalyst and inert material, volumetric ratio between catalyst and inert material in the bed) allows for a methane combustion at must lower temperatures in the reactor with flow reversal than in a classic catalytic reactor. Under such a low temperature combustion, no nitrogen oxides are produced.     

ENHANCEMENT OF STRAUSS' SECONDARY FLOW MIXING IN THIN FILM COMMERCIAL REACTORS

Experiments involving both viscous mixing and flow visualization in complex geometries simulating thin film commercial reactors were performed. Highly complex recirculation flows of elastic polymers were reproduced and studied on a TV screen by injecting small volumes of dye (representing process catalyst) into polyvinyl acetate melt, on a blade simulator developed by us recently. High resolution motion pictures taken at 30 frames per second demonstrate that secondary flows, as predicted by Strauss' equations, fail to correct the tendency of the fluid-fluid interface to remain parallel to the velocity streamlines. This problem is further aggravated by the presence of stagnant zones at the center of the vortices. This situation often led to reaction excursions in poorly mixed regions. Specially designed notches were developed to enhance the mixing process, and these increased the interfacial area for mass transfer several fold and also reduced viscous heat dissipation at the gap between the stationary blade and the moving plate. Commercial scale tests with the new blade design corroborate the laboratory findings.

A simplified apparatus has been developed to simulate the secondary flow behavior that occurs in thin film commercial reactors. The extension of Strauss' mathematical model to the case of viscoelastic fluid enabled reactor performance improvement through high speed cinematographic studies using the simulator. The rate of recirculation in the secondary flow was shown to be proportional to the speed of the impeller; however, the center of the secondary flows was noted to be a zone of poor mixing. This was corrected through a special impeller design that destroys stagnation zones in the reactor.     

VOLUMETRIC FLUX RATE ENHANCEMENT AND REDUCTION IN CONICAL VISCOUS FLOWS WITH MULTIPLY CONNECTED CROSS SECTIONS

Axisymmetric viscous flow in a conical channel is considered for geometrical systems in which the cross section is multiply-connected. The volumetric flux rates are compared for a 2-flow region and a 1 -flow region of equal cross sectional area, and same mean radial pressure gradient. It is found that for various configurations of the 2-flow system there can be both flux rate enhancement and reduction. As part of an introduction some similar results are presented for Poiseuille flow, and flow between non parallel plane walls the former of which satisfies the exact Navier-Stokes equations.     

ULTRAFILTRATION FLUX AND REJECTION CHARACTERISTICS OF BLACK LIQUOR AND POLYETHYLENE GLYCOL

Comparative experimental studies were carried oul on ultrafiltration with black liquor, and polyethylene glycol as a standard molecule, using an asymmetric membrane in a stirred batch cell. Effects of pressure, concentration, and stirrer speed were studied on flux and salute rejection characteristics for both the solutes.

The experimental results were analysed using the osmotic pressure limitation model to account for concentration polarization. It was found that, apart from the rise in osmotic pressure with pressure and concentration, there was a uniform rise in the polarization layer resistance (R_p) with membrane surface concentration (c_m). The results were correlated by the equation:

where ∝ and βt are constants. The membrane was characterized by determining the solution permeability (P_m) and reflection coefficient (a). The value of a was found to be close to unity—a representative value for high rejection membranes.     

人工湿地反硝化碳源补充研究进展

碳源作为反硝化过程的电子供体,是影响人工湿地反硝化过程的主要因素.人工湿地的反硝化碳源主要来自于进水,但是由于进水中的溶解性有机碳浓度很低,并且大部分为难降解有机碳,因此需要考虑使用外加碳源提供反硝化电子供体.研究中用于人工湿地反硝化碳源主要有污水、低分子碳水化合物和植物生物质等.植物生物质作为人工湿地反硝化碳源有其独特的优势,不仅价格低廉,来源充足,而且解决了湿地植物的处置问题,还不会增加系统的能耗和二氧化碳的排放量.本文结合现有人工湿地反硝化碳源补充的相关研究,描述了不同外加碳源对反硝化过程的作用,并对不同外碳源的效能进行了对比.     

曝气强度对重力出水式膜组件膜通量的影响

采用重力出水式膜生物反应器,在不同曝气强度下,研究两种不同膜组件的膜通量变化情况,同时找出两组膜组件的经济曝气强度.研究结果表明,膜组件A的经济曝气强度约为50m3·m-2·h-1,平均膜通量为8.91 L·m-2·h-1;膜组件B经济曝气强度约为40 m3·m-2.h-1,平均膜通量为3.73 L·m-2:h-1.长期运行结果表明,膜组件B相对膜组件A可能较快发生膜污染现象.     

MIXING ENHANCEMENT BY FREQUENCY-SELECTIVE CHAOTIC ADVECTION IN A 3-D TIME-PERIODIC STOKES FLOW

A new 3-D periodic Stokes flow has been imagined and realized experimentally. It consists of axial Poiseuille flow superimposed on the 2-D tangential motion between two confocal ellipses that glide circumferentially so that the geometry is invariant. Chaotic streak lines obtained experimentally are compared to numerical simulations of this time-periodic flow. We next turn our attention to the problem of determining how to move the boundaries in order to obtain the most efficient mixing. Using a numerical experiment to study the advection of a passive scalar, we show that for a given 3-D mixer geometry and flow rate there is an optimum modulation frequency of the boundary displacement protocol for which the mixing process is most efficient. Furthermore, it is shown that chaotic advection can be regarded as a frequency-selective amplifier. This behavior is similar to that of fluid instability where external perturbations are amplified for a certain frequency range. For values above or below this range, perturbations are damped and the system is stable.     

MIXING ENHANCEMENT BY FREQUENCY-SELECTIVE CHAOTIC ADVECTION IN A 3-D TIME-PERIODIC STOKES FLOW

A new 3-D periodic Stokes flow has been imagined and realized experimentally. It consists of axial Poiseuille flow superimposed on the 2-D tangential motion between two confocal ellipses that glide circumferentially so that the geometry is invariant. Chaotic streak lines obtained experimentally are compared to numerical simulations of this time-periodic flow. We next turn our attention to the problem of determining how to move the boundaries in order to obtain the most efficient mixing. Using a numerical experiment to study the advection of a passive scalar, we show that for a given 3-D mixer geometry and flow rate there is an optimum modulation frequency of the boundary displacement protocol for which the mixing process is most efficient. Furthermore, it is shown that chaotic advection can be regarded as a frequency-selective amplifier. This behavior is similar to that of fluid instability where external perturbations are amplified for a certain frequency range. For values above or below this range, perturbations are damped and the system is stable.     

汽-水换热器内流体诱导振动强化传热试验

提出利用换热器内流体诱导振动实现强化传热的方法 .在利用传热表面振动提高对流换热系数的同时 ,利用振动变形减少积垢 ,降低污垢热阻 ,实现了复合强化传热 .对管内外流体流动诱导弹性管束的振动特性、强化传热特性和污垢特性进行了试验研究